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A TEXT BOOK 
 
 OF 
 
 PHYsioLoay 
 
A TEXT BOOK 
 
 C)F 
 
 PHYSIOLOGY 
 
 BY 
 
 M. FOSTEK, M.A., M.D., F.RS. 
 
 PRELECTOR IN PHYSIOLOGY AND FELLOW OF TRINITY COLLEGE, 
 CAMBRIDGE. 
 
 WITH IL L USTRA TIONS. 
 
 FOURTH EDITION, REVISED. 
 
 ?[onlron: 
 MACMILLAN AND CO. 
 
 1883 
 
 \The Right of Translation is reserved.] 
 

 
 PBINTED BY C. J. CLAY, M.A. & SON, 
 AT THE UNIVERSITY PEESS, 
 
PREFACE TO THE FOURTH EDITION. 
 
 In previous Editions of this work I endeavoured, by the 
 use of smaU and large print, to distinguish between the 
 more important and stable portions of Physiology, which 
 ought to be made known to every one engaged in a 
 serious study of the science, and the less settled, often 
 controverted views which should be attacked by the more 
 advanced students only. Experience however has taught 
 me that the advantages of such a plan are more than 
 counterbalanced by its disadvantages. I especially felt 
 that the amount of space which I could fairly allow to 
 the small print paragraphs was wholly insufficient to 
 permit me to do justice to the conflicting views which 
 I strove, in them, to expound. 
 
 In this Edition accordingly I have made no attempt 
 at any such distinction, and have used small print almost 
 exclusively for the description of methods and apparatus. 
 This step involving, as it necessarily did, the transference, 
 into the body of the work, of some of the statements 
 which previously had found their place in the small print 
 
vi PREFACE. 
 
 portions, has given the volume, at first sight, the appear- 
 ance of having heen largely altered. This however is 
 not the case. For good or for bad, the book remains 
 very much as it was ; and though I have done my best 
 to remove some of the many defects present in previous 
 editions, I have been encouraged, by the favour with 
 which those editions have been successively received, to 
 persevere in the views which I have always held as to 
 which are the parts of physiology most to be insisted on, 
 and which may be lightly touched or wholly omitted ; 
 and though I would still most strenuously repudiate 
 the idea, put forward by some, that there is such a 
 thing as a physiology for medical men, different from 
 that physiology which is a part of science, I have 
 tried to make this volume especially useful to medical 
 students. 
 
 My decision to do away with the small print portions 
 of former editions has been largely determined by the 
 fact that my former pupils, now my colleagues at Cam- 
 bridge, have undertaken to join with me in treating 
 these higher or advanced parts of physiology in a more 
 extended and satisfactory form. And the hope that the 
 result of their labours will soon appear has led me, in 
 this volume, to omit all references, and to use as little as 
 possible the personal authority of the names of investi- 
 gators. The fondness of students for the use of names 
 of persons is as marked as the pertinacity with which 
 they use them wrongly ; and if any observer may feel 
 aggrieved at his name being absent from an ordinary text- 
 book, he may at least have the satisfaction of reflecting 
 that the omission of all names does something to prevent 
 others receiving the credit of his labours. 
 
I'll E FACE. vii 
 
 I cannot say liow niiicli 1 ;mi indebted to the con- 
 tinued help of those friends \vho assisted me in former 
 editions ; and I have also to acknowledt^e with gratitude 
 the aid afforded me by Prof. C. IS. Hoy, to whose kindness 
 I owe several of the new illustrations. 
 
 The appendix on chemical matters, as in former 
 editions, has been under the care of Mr Sheridan Lea; 
 in this, which stands on a somewhat different footing 
 from the rest of the work, references and names of authors 
 have been retained. 
 
 Trinity College, Cambridge, 
 Fehruart/, 1883. 
 
CONTENTS. 
 
 PAOE 
 
 INTRODUCTORY 1 
 
 BOOK I. 
 
 BLOOD. THE TISSUES OF MOVEMENT. 
 THE VASCULAR MECHANISM. 
 
 CHAPTER I. 
 
 Blood, pp. 11 — 34. 
 
 Seel. The Coagulation of Blood 13 
 
 Sec. 2. The Chemical Composition of Blood .2-1 
 
 Sec. 3. The History of the Corpuscles 2S 
 
 Sec. 4. The Quantity of Blood, and its distribution in the body ... 33 
 
 CHAPTER II. 
 The Contractile Tissues, pp. 35 — 103. 
 
 Sec. 1. The Phenomena of Muscle and Nerve 37 
 
 Muscular and Nervous Lritability, p. 87. The Phenomena of a 
 simple muscular contraction, p. 39. Tetanic contractions, p. 48. 
 
 Sec. 2. The Changes in a Muscle during Muscular Contraction ... 54 
 The change in form, p. 54. Electrical changes, p. 58. Chemical 
 changes, p. 64. Thermal changes, p. 70. The changes in a Nerve 
 during the passage of a Nervous Impulse, p. 72. 
 
 Sec. 3. The Nature of the Changes through which an Electric Current is able 
 
 to generate a Xercous Impulse ....... 75 
 
 The action of the Constant Current, p. 75. Electrotonus, p. 77. 
 Electrotonic Currents, p. 80. 
 
X CONTENTS. 
 
 PAGE 
 
 Sec. 4. The Muscle-Nerve Preparation as a Machine 83 
 
 The nature aud mode of application of the Stimulus as affecting 
 the amount and character of the Contraction, p. 83. The influence 
 of the Load, p. 87. The influence of the Size and Form of the 
 Muscle, p. 88. The work done, p. 89. 
 
 Sec. 5. The Circumstances which determine the Degree of Irritability of 
 
 Muscles and Nerves ......... 90 
 
 The effects of severance from the Central Nervous System, p. 91. 
 The Influence of Temperature, p. 93. The Influence of Blood 
 Supply, p. 94. The Influence of Functional Activity, p. 95. 
 
 Sec. 6. The energy of Muscle and Neire, and the nature of Muscular and 
 
 Nervous Action .......... 98 
 
 Sec. 7. Other forms of Contractile Tissue ....... 101 
 
 Unstriated Muscular Tissue, p. 101. Cardiac Muscles, p. 102. 
 Cilia, p. 102. Migrating Cells, p. 103. 
 
 CHAPTER III. 
 
 The Fundamental Properties of Nervous Tissues, pp. 104 — 114, 
 
 Automatic actions, p. 107. Reflex actions, p. 109. Actions of 
 sporadic ganglia, p. 112. Inhibition, p. 113. 
 
 CHAPTER IV. 
 
 The Vascular Mechanism, pp. 115 — 230. 
 
 I. The Physical Phenomena of the Cieculation .... 116 
 
 Seel. Main general facts of the Circulation ...... 117 
 
 The Capillary Circulation, p. 117. The flow in the Arteries, p. 120. 
 The flow iu the Veius, p. 127. Hydraulic principles of the Circu- 
 lation, p. 128. 
 
 Sec. 2. The Heart 135 
 
 The Phenomena of the Normal Beat, p. 135. The Mechanism of 
 the Valves, p. 142. The Sounds of the Heart, p. 143. On the 
 relative duration and special characters of the cardiac events, 
 p. 146. The work done, p. 157. Variations in the Heart's Beat, 
 p. 159. 
 Sec. 3. The Pulse . ". 161 
 
 n. The Vital Phenomena of the Cieculation .... 176 
 
 Sec. 4. Changes in the Beat of the Heart 178 
 
 The Mechanism of the normal beat, p. 180. Inhibition of the 
 beat, p. 184. The effects on the circulation of changes in the 
 heart's beat, p. 194. 
 
CONTJjJNTS. xi 
 
 PAGK 
 
 Sec. 5. Changes in the calibre of the minute arteries. Vaso-viotor actions . 1^7 
 Vaso-molor Nerves, p. 1J9. Vaso-motor Centres, p. 212. The 
 effects of local vascular constriction or dilution, p. 21G. 
 
 Sec. G. Changes in the Capillary Districts 21'J 
 
 Sec. 7. Changes in the Quantity of Blood 224 
 
 Sec. 8. The Mutual Jielations and the Co-ordination of the Vascular Factors 111 
 
 BOOK 11. 
 
 THE TISSUES OF CHEMICAL ACTION WITH THEIR RESPECTIVE 
 MECHANISMS. NUTRITION, 
 
 CHAPTER I. 
 The Tissoes and Mechanisms of Digestion, pp. 233 — 311. 
 
 Seel. The Properties of the Digestive Juices 234 
 
 Saliva, p. 234. Gastric juice, p. 239. Bile, p. 247. Pancreatic 
 juice, p. 250. Succus entericus, p. 255. 
 
 Sec. 2. The act of secretion in the case of the Digestive Juices and the Nervous 
 
 Mechanisms which regulate it 256 
 
 Sec. 3. The Muscular Mechanisms of Digestion 281 
 
 Mastication, p. 281. Deglutition, p. 282. Movements of the 
 CEsopbagns, p. 284. Movements of the stomach, p. 285. Move- 
 ments of the small intestine, p. 286. Movements of the large 
 intestine, p. 288. Defsecation, p. 288. Vomiting, p. 290. 
 
 Sec. 4. The Changes which the Food undergoes in the Alimentary Canal . 292 
 
 Sec. 5. Absorption of the Products of Digestion 301 
 
 The Lymphatics, p. 301. Entrance of the chyle into the lacteals, 
 p. 303. Movements of the chyle, p. 304. Lymph-hearts, p. 306. 
 The course taken by the several products of digestion, p. 306. 
 
 CHAPTER II. 
 The Tissues and Mechanisms of Respiration, pp. 312 — 383. 
 
 Seel. The Mechanics of Pulmonary Respiration ...... 314 
 
 The Ebythm of Respiration, p. 316. The Eespiratory Move- 
 ments, p. 319. Facial and LaryTigeal Eespiration, p. 324. 
 
 Sec. 2. Changes of the Air in Eespiration 326 
 
 Sec. 3. The Respiratory Changes in the Blood . . . . . . 329 
 
 The relations of oxygen in the blood, p. 332. Hemoglobin ; its 
 properties and derivatives, p. 333. Colour of venous and arterial 
 blood, p. 338. The relations of the carbonic acid in the blood, 
 p. 342. The relations of the nitrogen in the blood, p. 343. 
 
 Sec. 4. Tlte Respiratory Changes in the Lungs 344 
 
 The entrance of oxygen, p. 344. The exit of carbonic acid, p. 
 346, 
 
xii CONTENTS. 
 
 PAOE 
 
 Sec. 5. The Respiratonj Changes in the Tissues 348 
 
 Sec. 6. The Nervous Mechanism of Respiration ...... 358 
 
 Sec. 7. The Effects of Respiration on the Circulation 364 
 
 Sec. 8. The Effects of Changes in the Air breathed ..... 375 
 The effects of deficient air. Asphyxia. Phenomena of asphyxia, 
 p. 375. The circulation in asphyxia, p. 378. The effects of an 
 increased supply of air. Apnoea, p. 380. The effects of changes 
 in the composition of the air breathed, p. 380. The effects of 
 changes in the pressure of the air breathed, p. 381. 
 
 Sec. 9. Modified Respiratonj Movements 382 
 
 Sighing, Yawning, Hiccough, Sobbing, Coughing, Sneezing, 
 Laughter and Crying, p. 382, 
 
 CHAPTER III. 
 
 Secretion by the Skin, pp. 384 — 391. 
 
 The nature and amount of Perspiration, p. 385. Cutaneous 
 Eespiration, p. 386. The Secretion of Perspiration, p. 387. The 
 Nervous Mechanism of Perspiration, p. 388. Absorption by the 
 Skin, p. 390. 
 
 CHAPTER IV. 
 
 Secretion by the Kidneys, pp. 392 — 414, 
 
 Sec. 1. The Composition of Urine 893 
 
 Sec. 2. The Secretion of Urine 397 
 
 The relation of the secretion of urine to arterial pressure, p. 398. 
 Secretion by the renal epithelium, p. 404. 
 
 Sec. 3. Micturition • . . 410 
 
 CHAPTER V. 
 
 The Metabolic Phenomena of the Body, pp. 415 — 478. 
 
 Sec. 1, Metabolic Tissues 416 
 
 The History of Glycogen, p. 416. Diabetes, p. 424, The History 
 of Fat, Adipose Tissue, p. 426. The Mammary Gland, p. 429, 
 The Spleen, p. 432. 
 
 Sec. 2. The History of Urea and its allies 436 
 
 Sec, 3, The Statistics of Nutrition 443 
 
 Compariso-n of Income and Output, p. 446, Nitrogenous Meta- 
 bolism, p. 449. The effects of Fatty and of Carbohydrate Food, 
 p. 453. 
 
 Sec. 4, The Energy of the Body 457 
 
 The income of energy, p. 457. The expenditure, p. 458. The 
 sources of Muscular Energy, p, 459, The sources and distribution 
 of Heat, p. 461. Eegulation by variations in loss, p. 464. Regu- 
 lation by variations in production, p. 465, 
 
 Sec. 5, The Influence of the Nervous System on Nutrition .... 470 
 
 Sec. 6. Dietetics . . . 474 
 
COS TEXTS. xiii 
 
 BOOK III. 
 
 THE CENTRAL NERVOUS SYSTEM AND Tl'S INSTRUMENTS. 
 
 CHAPTER I. 
 
 Sensory Nkkves, pp. 181 — 4iS9. 
 
 CHAPTER II. 
 
 SiGUT, pp. 490—555. 
 
 PAGE 
 
 Sec. 1. Dioptric Mechanisins 491 
 
 The Formation of the Image, p. 491. Accommodation, p. 493. 
 Movements of the Pupil, p. 500. Imperfections in the Dioptric 
 apparatus, p. 506. 
 
 Sec. 2. Visual Sensations . . . , 511 
 
 The origin of Visual Impulses, p. 511. Simple Sensations, p. 519. 
 Colour Sensations, p. 524. 
 
 Sec. 3. Visual Perceptions .......... 535 
 
 Modified Perceptions, p. 536. 
 
 Sec. 4. Binocular Vision 541 
 
 Corresponding or identical points, p. 511. Movements of the eye- 
 balls, p. 542. The Horopter, p. 547. 
 
 Sec. 5. Visual Judgments 549 
 
 Sec. 6. The Protective Mechanisms of the Eye 554 
 
 CHAPTER III. 
 Hearing, Smell, and Taste, pp. 556 — 571. 
 
 Sec. 1. Hearing 556 
 
 The acoustic apparatus, p. 557. Auditory Sensations, p. 559. 
 Auditory Judgments, p. 565. 
 
 Sec. 2. Smell 567 
 
 Sec. 3. Taste 570 
 
xiv CONTENTS. 
 
 CHAPTER IV. 
 
 Feeling and Touch, pp. 572 — 584. 
 
 PAGE 
 
 bee. 1. General Sensibility and Tactile Perceptions 572 
 
 Sec. 2. Tactile Sensations 575 
 
 Sensations of Pressure, p. 575. Sensations of Temperature, 
 
 p. 576. 
 
 Sec. 3. Tactile Perceptions and Judgments 579 
 
 Sec. 4. The Muscular Sense 982 
 
 CHAPTER V. 
 
 The Spinal Coed, pp. 585 — 607. 
 
 Sec. 1. As a Centre or Group of Centres of Reflex Action .... 585 
 Inhibition of Reflex Action, p. 590. The Time required for Eeflex 
 Actions, p. 513. 
 Sec. 2. As a Centre or Group of Centres of Automatic Action . . . 595 
 Sec. 3. As a Conductor of Afferent or Efferent Impulses .... 598 
 
 CHAPTER VI. 
 
 The Beain, pp. 608—650. 
 
 Sec. 1. On the Phenomena exhibited by an animal deprived of its Cerebral 
 
 Hemispheres ........... 608 
 
 Sec. 2. The Mechanisms of Co-ordinated Movements 614 
 
 Forced Movements, p. 620. 
 
 Sec. 3. The Functions of the Cerebral Convolutions ; . . . . 623 
 
 Sec. 4. The Functions of other parts of the Brain 636 
 
 Corpora striata and optic thalami, p. 636. Corpora quadrigemina, 
 p. 638. Cerebellum, p. 640. Crura cerebri and pons Varolii, 
 p. 641. Medulla oblongata, p. 642. 
 
 Sec. 5. The Rapidity of Cerebral Operations ...... 643 
 
 Sec. 6. The Circulation in the Brain 645 
 
 Sec. 7. The Cranial Nerves 648 
 
 CHAPTER VII. 
 
 Special Musculae Mechanisms, pp. 651 — 664. 
 
 Sec. 1. The Voice 651 
 
 Sec. 2. Speech . 657 
 
 Vowels, p. 657. Consonants, p. 658. 
 
 Sec. 3. Locomotor Mechanisms ....■•••• 662 
 
CONTENTS. 
 BOOK IV. 
 
 THE TISSUES AND MECHANISMS OF REPRODUCTION. 
 
 CHAPTER I. 
 
 Menstruation, pp. 669 — 671. 
 
 CHAPTER II. 
 Impregnation, pp. 672 — 673. 
 
 CHAPTER III. 
 The Nutrition of the Embryo, pp. 674 — 680. 
 
 CHAPTER IV. 
 
 Parturition, pp. 681 — 683. 
 
 CHAPTER V. 
 
 The Phases of Life, pp. %%i — 694-. 
 
 CHAPTER VI. 
 Death, pp. 695 — 696. 
 
 APPENDIX. 
 On the Chemical Basis op the Animal Body, pp. 697 — 770. 
 
 INDEX, pp. 771—785. 
 
INTRODUCTORY. 
 
 Among the simpler organisms kuowu to Biologists, perhaps the 
 most simple as well as the most common is that which has received 
 the name of Amoeba. There are many varieties of Amoeba, and 
 probably many of the forms which have been described are, in 
 reality, merely amoebiform phases in the lives of certain animals or 
 plants ; but they all possess the same general characters. Closely 
 resembling the white corpuscles of vertebrate blood, they are 
 wholly or almost wholly composed of undifferentiated protophism, 
 in the midst of which lies a nucleus, though this is sometimes absent. 
 In many a distinction may be observed between a more solid external 
 layer or ectosarc, and a more fluid granular interior or endosarc ; 
 but in others even this primary differentiation is wanting. By 
 means of a continually occurring flux of its protoplasmic substance, 
 the amoeba is enabled from moment to moment not on!}' to change 
 its form but also to shift its position. By flowing round the sub- 
 stances which it meets, it, in a way, swallows them ; and having 
 digested and absorbed such parts as are suitable for food, ejects or 
 rather flows away from the useless remnants. It thus lives, moves, 
 eats, grows, and after a time dies, having been during its whole 
 life hardly an) thing more than a minute lump of protoplasm. 
 Hence to the Pliysiologist it is of the greatest interest, since 
 in its life the problems of physiology are reduced to their simplest 
 forms. 
 
 Now the study of an amoeba, with the help of knowledge 
 gained by the examination of more complex bodies, enables us to 
 state that the undifferentiated protoplasm of which its body is so 
 largely composed exhibits certain fundamental phenomena which 
 we may .speak of as ' vital.' 
 
 F. 1 
 
2 PHENOMENA OF PROTOPLASM. 
 
 1. It is contractile. There can be little doubt that the 
 changes in the protoplasm of an amoeba which bring about its 
 peculiar ' amoeboid ' movements, are identical in their fundamental 
 nature with those which occurring in a muscle cause a contraction : 
 a muscular contraction is essentially a regular, an amoeboid move- 
 ment an irregular flow of protoplasm. The substance of the 
 amoeba may therefore be said to be contractile. 
 
 2. It is irritable and automatic. When any disturbance, 
 such as contact with a foreign body, is brought to bear on the 
 amoeba at rest, movements result. These are not passive move- 
 ni^ats, the effects of the push or pull of the disturbing body, 
 proportionate to the force employed to cause them, but active 
 manifestations of the contractility of the protoplasm ; that is to 
 say, the disturbing cause, or ' stimulus,' sets free a certain amount 
 of energy previously latent in the protoplasm, and the energy set 
 free takes on the form of movement. Any living matter which, 
 when acted on by a stimulus, thus suffers an explosion of energy, 
 is said to be ' irritable.' The irritability may, as in the amoeba, 
 lead to movement ; but in some cases no movement follows the 
 application of the stimulus to irritable matter, the energy set free 
 by the explosion taking on some other form than movement, 
 ex. gr. heat. Thus a substance may be irritable and yet not 
 contractile, though contractility is a very common manifestation 
 of irritability. 
 
 The amoeba (except in its prolonged quiescent stage) is rarely 
 at rest. It is almost continually in motion. The movements 
 cannot always be referred to changes in surrounding circumstances 
 acting as stimuli; in many cases the energy is set free in conse- 
 quence of internal changes, and the movements which result are 
 called spontaneous or automatic movements. We may therefore 
 speak of the protoplasm of the amoeba as being irritable and 
 automatic. 
 
 3. It is receptive and assimilative. Certain substances 
 serving as food are received into the body of the amoeba, and there 
 in large measure dissolved. The dissolved portions are subse- 
 quently converted from dead food into new living protoplasm, and 
 become part and parcel of the substance of the amoeba. 
 
 4. It is metabolic and secretory. Pari passu with the re- 
 ception of new material, there is going on an ejection of old 
 material, for the increase of the amoeba by the addition of food is 
 not indefinite. In other words, the protoplasm is continually 
 undergoing chemical change (metabolism), room being made for 
 the new protoplasm by the breaking up of the old protoplasm into 
 products which are cast out of the body and got rid of. These 
 products of metabolic action have, in many cases at all events, 
 subsidiary uses. Some of them, for instance, we have reason to 
 think, are of value for the purpose of dissolving and effecting other 
 
ISTHoDlCTonr. 3 
 
 preliminary clianges iu the raw food iiitrodnccd into the body of 
 the amu3ba; and hence are retained within the body for some little 
 time. Such products are generally spoken of as 'secretions.' 
 Others which pass more rapidly away are generally called 'ex- 
 cretions.' The distinction between the two is an uiiimp(jrtaut and 
 freipiontly accidental one. 
 
 The energy expended in tlic movements of the amceba is 
 supplied by the chemical changes going on in the protoplasm, by 
 the breaking up of bodies possessing much latent energy into 
 bodies possessing less. Thus the metabolic changes which the 
 food (as distinguished from tlie undigested stuff mechcinically 
 lodged for a while in the body) undergoes in passing through the 
 protoplasm of the amaba are of three classes : those preparatory 
 to and culminating in tiie conversion of the food into protoplasm, 
 those concerned iu the discharge of energy, and those tending to 
 economise the immediate products of the second class of changes 
 by rendering them more or less useful in carrying out the first. 
 
 5. It is respiratory. Taken as a whole, the metabolic changes 
 are pre-eminently processes of oxidation. One article of food, i.e. 
 one substance taken into the body, viz. oxygen, stands apart from 
 all the rest, and one product of metabolism peculiarly associated 
 with oxidation, viz. carbonic acid, stands also somewhat apart from 
 all the rest. Hence the assumption of oxygen and the excretion 
 of carbonic acid, together with such of the metabolic proces.ses as 
 are more especially oxidative, are frequently spoken of together as 
 constituting the respiratory processes. 
 
 G. It is reproductive. The individual amceba represents a 
 unit. This unit, after a longer or shorter life, havincr increased iu 
 size by the addition of new protoplasm in excess of that which it 
 is continually using up, may, by fission (or by other means) lesolve 
 itself into two (or more) parts, each of which is capable of living 
 as a fresh unit or individual. 
 
 Such are the fundamental vital qualities of the protoplasm of 
 an amoeba ; all the facts of the life of an amoeba are manifesta- 
 tions of these protoplasmic qualities in varied sequence and sub- 
 ordination. 
 
 The higher animals, we learn from morphological studies, may 
 be regarded as groups of amoebae peculiarly associated together. 
 All the physiological phenomena of the higher animals are simi- 
 larly the results of these fundamental qualities of protoplasm 
 peculiarly associated together. The dominant principle of this 
 association is the physiological division of labour corresponding to 
 the morphological differentiation of structure. Were a larger or 
 'higher' animal to consist simply of a colony of undifferentiated 
 amoebae, one animal differing from another merely in the number 
 of units making up the mass of its body, without any differences 
 between the individual units, progress of function would be an 
 
 1—2 
 
4 THE FUNDAMENTAL TISSUES. 
 
 impossibility. The accumulation of units would be a hindrance to 
 welfare rather than a help. Hence, in the evolution of living 
 beings through past times, it has come about that in the higher 
 animals (and plants) certain groups of the constituent amoebiform 
 units or cells have, in company with a change in structure^ been 
 set apart for the manifestation of certain only of the fundamental 
 properties of protoplasm, to the exclusion or at least to the 
 complete subordination of the other properties. 
 
 These groups of cells, thus distinguished from each other at 
 once by the differentiation of structure and by the more or less 
 marked exclusiveness of function, receive the name of 'tissues.' 
 Thus the units of one class are characterized by the exaltation of 
 the contractility of their protoplasm, their automatism, metabolism 
 and reproduction being kept in marked abeyance. These units 
 constitute the so-called muscular tissue. Of another tissue, viz. 
 the nervous, the marked features are irritability and automatism, 
 with an almost complete absence of contractility and a great 
 restriction of the other qualities. In a third group of units, the 
 activity of the protoplasm is largely confined to the chemical 
 changes of secretion, contractility and automatism (as manifested 
 by movement) being either absent or existing to a very slight 
 degree. Such a secreting tissue, consisting of epithelium-cells, 
 forms the basis of the mucous membrane of the alimentary canal. 
 In the kidney, the substances secreted by the cells, being of no 
 further use, are at once ejected from the body. Hence the renal 
 tissue may be spoken of as excretory. In the epithelium-cells of 
 the lungs, the protoplasm plays an altogether subordinate part in 
 the assumption of oxygen and the excretion of carbonic acid. 
 Still we may perhaps be permitted to speak of the pulmonary 
 epithelium as a respiratory tissue. 
 
 In addition to these distinctly secretory or excretory tissues, 
 there exist groups of cells specially reserved for the carrying on of 
 chemical changes, the products of which are neither cast out of the 
 body, nor collected in cavities for digestive or other uses. The 
 work of these cells seems to be of an intermediate character ; they 
 are engaged either in elaborating the material of food that it may 
 be the more easily assimilated, or in preparing used-up material 
 for final excretion. They receive their materials from the blood 
 and return their products back to the blood. They may be called 
 the metabolic tissues par excellence. Such are the fat-cells of 
 adipose tissue, the hepatic cells (as far as the work of the liver 
 other than the secretion of bile is concerned), and probably many 
 other cellular elements in various regions of the body. 
 
 Each of the various units retains to a greater or less degree 
 the power of reproducing itself, and the tissues generally are 
 capable of regeneration in kind. But neither units nor tissues 
 can reproduce other parts of the organism than themselves, much 
 less the entire organism. For the reproduction of the complex 
 
iXTUonrcToi:)-. -, 
 
 irKlivklual, certain units are .sot apart in tlio torin of ovarv an<l 
 testis. In tliesc all the properties of protopla.sm arc distinctly 
 subordinated to the work of growth. 
 
 Lastly, there are certain groups of units, certiiin tissues, whidi 
 are of use to the body of which they form a part, not by reason of 
 their manifesting any of the fundamental qualities of protoplasm, 
 but on account of the physical and mechanical properties of certain 
 substances which their protoplasm lias been able by virtue of its 
 metabolism to manufacture and to deposit. Such tissues are bone, 
 cartilage, connective tissue in large part, and the greater portion 
 of the skin. 
 
 We may therefore consider the complex body of a higher 
 animal as a compound of so many tissues, each tissue correspond- 
 ing to one of the fundamental cpialities of protoplasm, to the 
 development of which it is specially devoted by the division of 
 labour. It must however be remembered that there is probably 
 a distinct limit to the division of labour. In each and every 
 tissue, in addition to its leading quality, there are more or less 
 pronounced remnants or at least some traces of all the other 
 protoplasmic qualities. Thus, though we may call one tissue par 
 excellence metabolic, all the tissues are to a greater or less extent 
 metabolic. The energy of each, whatever be its particular mode, 
 has its source in the breaking-up of the protoplasm. Chemical 
 changes, including the assumption of oxygen and the production 
 complete or partial of carbonic acid, and therefore also entailing 
 a certain amount of secretion and excretion, must take place in 
 each and every tissue. And so Avith all the other fundamental 
 properties of protoplasm ; even contractility, which for obvious 
 mechanical reasons is soonest reduced where not wanted, is present 
 in many other tissues besides muscle. And it need hardly be said 
 that each tissue retains the power of assimilation. However 
 thoroughly the material of food be prepared by digestion and 
 subsequent metabolic action, the last stages of its conversion into 
 living protoplasm are etfected directly and alone by the tissue of 
 which it is about to form a part. 
 
 Bearing this qualification in mind, we may draw up a physio- 
 logical classification of the body into the following fundamental 
 tissues : — 
 
 1. The Giuiuently contractile: the muscles. 
 
 2. „ ,. iiritable and automatic: the nervous system. 
 
 3. „ „ secretory, or excretory : digestive, urinary, 
 
 and pulmonary, &c., epithelium. 
 
 4. „ „ metabolic : fat-cells, hepatic cells, lymphatic 
 
 and ductless glands, \k.c. 
 J. „ ,, reproductive : ovary, testis. 
 
 (5. The indifferent or mechanical : cartilage, bone, &c. 
 
 X\\ these separate tissues, with their indiviilual characters, arc 
 
G INTEGRATION^. 
 
 however but parts of one body; and in order that they may bo 
 true members working harmoniously for the good of the whole, 
 and not isolated masses each serving its own ends only, they need 
 to be boimd together by coordinating bonds. Some means of 
 communication must necessarily exist between them. In the 
 mobile homogeneous body of the amoeba, no special means of com- 
 munication are required. Simple diffusion is sufficient to make 
 the material gained by one part common to the whole mass, and 
 the native protoplasm is physiologically continuous, so that an 
 explosion set up at any one point may be immediately propagated 
 throughout the whole irritable substance. In the higher animals, 
 the several tissues are separated by distances far too great for the 
 slow process of diffusion to serve as a sufficient means of commu- 
 nication, and their primary physiological continuity is broken by 
 their being imbedded in masses of formed material, the product of 
 the indifferent tissues, which being devoid of irritability, present 
 an effectual barrier to the propagation of molecular explosions. It 
 thus becomes necessary that in the increasing complexity of animal 
 forms, the process of differentiation should be accompanied by a 
 corresponding integration, that the isolated tissues should be made 
 a whole by bonds uniting them together. These bonds moreover 
 must be of two kinds. 
 
 In the first place there must be a ready and rapid distribution 
 and interchange of material. The contractile tissues must be 
 abundantly supplied with material best adapted by previous elabo- 
 ration for direct assimilation, and the waste products arising from 
 their activity must be at once carried away to the metabolic or 
 excretory tissues. And so with all the other tissues. There must 
 be a free and speedy intercourse of material between each and all. 
 This is at once and most easily effected by the regular circulation 
 of a common fluid, the blood, into which all the elaborated food is 
 discharged, from which each tissue seeks what it needs, and to 
 which each returns that for which it has no longer any use. The 
 carrying on such a circulation of fluid, being in large measure a 
 mechanical matter, needs a machinery, and calls forth an expendi- 
 ture of energy. The machinery is supplied by a special con- 
 struction of the primary tissues, and the energy is arranged for 
 by the presence among these of contractile and irritable matter. 
 Thus to the fundamental tissues there is added, in the higher 
 animals, a vascular bond in the shape of a mechanism of 
 circulation. 
 
 In the second place, no less important than the interchange of 
 material is the interchange of energy. In the amoeba the irritable 
 surface is physiologically continuous with the more internal proto- 
 plasm, while each and every part of the body has automatic 
 powers. In the higher animal, portions only of the skin remain 
 as eminently irritable or sensitive structures, while automatic 
 actions are chiefly confined to a central mass of irritable nervous 
 
lyTRODUCTOKY. 7 
 
 matter. Botli forms of irriUihk! mattur ure .separated, by lonj; 
 tracts of iiulitfereiU material, from those contractile tissues tliroiigli 
 which they chieHy manifest the changes going on in themselves. 
 Hence the necessity for long strands of eminently irritable ti.ssue 
 to connect the skin antl contractile tissues as well with each other 
 ;\8 with the automatic centres. Similar sirands are also needed, 
 though perhaps less urgently, to connect the other tissues with 
 these and with each other. To the vascular bond there must be 
 added an irritable bond, along the strands of which, impulses set 
 up by changes in one or another part, may travel in determinate 
 courses for the regulation of the energy of distant spots. In other 
 words, part of the irritable tissues must be specially arranged to 
 form a coordinating nervous system. 
 
 Still further complications have yet to be considered. In the 
 life of a minute homogeneous amoeba, possessing no special form 
 or structure, there is little scope for purely mechanical operations. 
 As however we trace out the gradual development of the more 
 complex animal forms, we see coming forward into greater and 
 greater prominence the arrangement of the tissues in definite 
 ways to secure mechanical ends. Thus the entire body acquires 
 particular shapes, and parts of the body are built up into mechan- 
 isms, the actions of which are to the advantage of the individual. 
 Into the composition of these mechanisms or 'organs' the active 
 fundamental tissues, as well as the passive or indifferent tissues, 
 enter; and the working of each mechanism, the function of each 
 organ, is dependent partly on the mechanical conditions offered by 
 the passive elements, partly on the activity of the active elements. 
 The vascular mechanism, of w4iich we have just spoken, is such 
 a mechanism. Similarly the urgent necessity for the access of 
 oxygen to all parts of the body, has given rise to a comj^licated 
 respiratory mechanism ; and the needs of copious alimentation, to 
 an alimentary or digestive mechanism. 
 
 Further, inasmuch as muscular movement is one of the chief 
 ends, or the most important means to the chief ends, of animal 
 life, we find the animal body abounding in motor mechanisms, 
 in which the prime mover is muscular contraction, while the 
 machinery is supplied by complicated arrangements of muscles 
 wnth such indifferent tissues as bone, cartilage, and tendon. In 
 fact, the greater part of the animal body is a collection of mus- 
 cular machines, some serving for locomotion, others for special 
 manoeuvres of particular members and parts, others as an assist- 
 ance to the senses, and yet others for the production of voice, and 
 in man, of speech. 
 
 Lastly, the simple automatism of the amoeba, wuth its simple 
 responses to external stimuli, is replaced in the higher animals by 
 an exceedingly complex volition affected in multitudinous ways 
 by influences from the world without; and there is a correspond- 
 ingly complex central nervous .system. And here we meet with 
 
8 CENTRAL NERVOUS MECHANISM. 
 
 a new form of dilfereutiation unknown elsewhere. Wiiile the 
 contractility of the amoebal protoplasm differs bnt slightly from 
 the contractility of the vertebrate striated muscle, there is an 
 enormous difference between the simple irritability of the amoeba 
 and the complex action of the vertebrate nervous system. Except- 
 ing the nervous or irritable tissues, the fundamental tissues have 
 in all animals the same properties, being, it is true, more acute 
 and perfect in one than in another, but remaining fundamentally 
 the same. The elementary muscular fibre of a mammal is a 
 mass of differentiated protoplasm, forming a whole physiologically 
 continuous, and in no way constituting a mechanism. Each fibre 
 is a counterpart of all others ; and the muscle of one animal 
 differs from that of another in such particulars only as are 
 wholly subordinate. In the nervous tissues of the higher animals, 
 on the contrary, we find properties unknown to those of the lower 
 ones ; and in proportion as we ascend the scale, we observe an 
 increasing differentiation of the nervous system into unlike parts. 
 Thus we have, what does not exist in any other tissue, a mechanism 
 of nervous tissue itself, a central nervous mechanism of complex 
 structure and complex function, the complexity of which is due 
 not primarily to any mechanical arrangements of its parts, but to 
 the further differentiation of that fundamental quality of irrita- 
 bility and automatism which belongs to all irritable tissues, and to 
 all native protoplasm. 
 
 In the following pages I propose to consider the facts of phy- 
 siology very much according to the views which have been just 
 sketched out. The fundamental properties of most of the ele- 
 mentary tissues will first be reviewed, and then the various special 
 mechanisms. It will be found convenient to introduce early the 
 account of the vascular mechanism, and of its nervous coordinating 
 mechanism, while the mechanisms of respiration and alimentation 
 will be best considered in connection with the respiratory and 
 secretory tissues. The description of the purely motor mechanisms 
 will be brief; and, save in a few instances, confined to a statement 
 of general principles. The special functions of the central nervous 
 system, including the senses, must of necessity be considered by 
 themselves. The tissues and mechanism of reproduction and the 
 phenomena of the decay and death of the organism, will naturally 
 form the subject of the closing chapters. 
 
BOOK I. 
 
 BLOOD. THE TISSUES OF MOVEMENT. THE 
 VASCULAE MECHANISM. 
 
CHAPTER I. 
 
 BLOOD. 
 
 Blood, -wlien tlowiiig in a normal condition through the blood- 
 vessels, consists of an almost colourless fluid, the plasma, in which 
 are suspended a number of more solid bodies, the red and white 
 corpuscles. Were Ave anxious to give a formal completeness to the 
 classification of the various parts of the body into tissues, we might 
 speak of the blood as a tissue of which the corpuscles are the 
 essential cellular elements, while the plasma is a liquid matrix. 
 We might compare it to a cartilage, the firm matrix of which had 
 become completely liquefied so that the cartilage-corpuscles were 
 perfectly free to move about. 
 
 In regarding blood as tissue, however, v/e come upon the 
 difificulty that it, unlike all the other tissues, possesses no one 
 characteristic property. The protoplasm of the white corpuscles 
 is native undifferentiated protoplasm, in no respect fitted for any 
 special duty ; and though, as we shall see, the red corpuscles have 
 a definite respiratory function, inasmuch as they are carriers of 
 oxygen from the lungs to the several tissues, still this respiratory 
 woi k is only one of the very many labours of the blood. It will 
 be therefore far more profitable, indeed necessary, to treat of the 
 blood, not as a tissue by itself, but as the great means of com- 
 munication of material between the tissues properly so called. Its 
 real usefulness lies not so much in any one property of either its 
 corpuscles or its plasma, as in its nature fitting it to serve as the 
 great medium of exchange between all parts of the body. The 
 receptive tissues pour into it the material which they have received 
 from without, the excreting tissues withdraw from it the things 
 which are no longer of any use, and the irritable, the contractile, 
 and indeed all the tis.sues, seek in it the substances (including 
 
12 BLOOD AN INTERNAL MEDIUM. [Book i. 
 
 oxygen) which they need for the manifestation of energy or for 
 the storing up of differentiated material, and return to it the waste 
 products resulting from their activity. All over the body every- 
 where there is so long as life lasts a double current, here rapid, 
 there slow, of material from the blood to the tissues, and from the 
 tissues to the blood. 
 
 It, together with lymph (whether in the lymph-canals or in the 
 interstices of the tissues), may. as Bernard has suggested, be 
 resfarded as an internal medium bearincr the same relations to the 
 constituent tissues that the external medium, the world, does to 
 the whole individual. Just as the whole organism lives on the 
 things around it, its air and its food, so the several tissues live on 
 the complex fluid by which they are all bathed and which is to 
 them their immediate air and food. 
 
 From this it follows, on the one hand, that the composition and 
 characters of the blood must be for ever varying in different parts 
 of the body and at different times ; and on the other hand, that 
 the united action of all the tissues must tend to establish and 
 maintain an average uniform composition of the whole mass of 
 blood. The special changes which blood is known to undergo 
 while it passes through the several tissues will best be dealt with 
 when the individual tissues and organs come under our considera- 
 tion. At present it will be sufficient to study the main features, 
 which are presented by blood, brought so to speak into a state of 
 equilibrium by the common action of all the tissues. 
 
 Of all these main features of blood, the most striking if not 
 the most important is the property it possesses of clotting or 
 coaofulating when shed. 
 
SECT. 1. THE COAGULATION OF BLOOD. 
 
 Blood, \vhen shed from the blood-vessels of a living body, is 
 perfectly fluid. In a short time it becomes viscid; it flows less 
 readily from vessel to vessel. The viscidity increases rapidly until 
 the whole mass of blood under observation becomes a complete 
 jelly. The vessel into which it has been shed, can at this stage be 
 inverted without a drop of the blood being spilt. The jelly is of 
 the same bulk as the previously fluid blood, and if forcibly removed, 
 presents a complete mould of the interior of the vessel. If the 
 blood in this jelly stage be left untouched in a glass vessel, a few 
 drops of an almost colourless fluid soon make their apjDearance on 
 the surface of the jelly. Increasing in number, and running 
 together, the drops after a while form a superficial layer of pale 
 stra\v-coloui-ed fluid. Later on, similar layers of the same fluid are 
 seen at the sides and finally at the bottom of the jelly, which, 
 shrunk to a smaller size and of firmer consistency, now forms a 
 clot or cvassamentum, floating in a perfectly fluid serum. 'J'he 
 shrinking and condensation of the clot, and the corresponding 
 increase of the serum, continue for some time. The upper surface 
 of the clot is generally cupped. A portion of the clot examined 
 under the microscope is seen to consist of a feltwork of fine granular 
 fibrils, in the meshes of which are entangled the red and white 
 corpuscles of the blood. In the serum nothing can be seen but a 
 few stray corpuscles. The fibrils are composed of a substance 
 called fibrin. Hence we may speak of the clot as consisting 
 of fibrin and corpuscles ; and the act of clotting or coagulation is 
 obviously a conversion of the naturally fluid portion of the blood 
 or plasma into fibrin and serum, followed by separation of the 
 fibrin and corpuscles fi'om the serum. 
 
14 COAGULATION OF BLOOD. [Book i. 
 
 In man, blood when shed becomes viscid in about two or 
 three minutes, and enters the jelly stage in about five or ten 
 minutes. After the lapse of another few minutes the first drops 
 of serum are seen, and coagulation is generally complete in from 
 one to several hours. The times however will be found to vary 
 according to the condition of the individual, the temperature of 
 the air, and the size and form of the vessel into which the blood 
 is shed. Among animals the rapidity of coagulation varies ex- 
 ceedingly in different species. The blood of the horse coagulates 
 with remarkable slowness ; so slowly indeed that many of the red 
 corpuscles (these being specifically heavier than the plasma) have 
 time to siuk before viscidity sets in. In consequence there ap- 
 pears on the surface of the blood an upper layer of colourless 
 plasma, containing in its deeper portions many colourless cor- 
 puscles (which are lighter than the red). This layer clots like 
 the other parts of the blood, forming the so-called ' buffy coat.' 
 A similar buffy coat is sometimes seen in the blood of man, in 
 inflammatory conditions of the body. 
 
 This buify coat makes its appearance in horse's blood even at 
 the ordinary temperatiire of the air. If a portion of horse's blood 
 be surrounded by a cooling mixture of ice and salt, and thus kept 
 at about 0° C, coagulation may be almost indefinitely postponed. 
 Under these circumstances a more complete descent of the cor- 
 puscles takes place, and a considerable quantity of colourless 
 transparent plasma free from blood-corpuscles may be obtained. 
 A portion of this plasma removed from the freezing mixture 
 clots exactly as does the entire blood. It first becomes viscid and 
 then forms a jelly, which subsequent!)'' separates into a colourless 
 shrunken clot and serum. This shews that the corpuscles are not 
 an essential part of the clot. 
 
 If a few cubic centimetres of the same plasma be diluted with 
 50 times its bulk of a 0'6 p.c. solution of sodium chloride^ coagu- 
 lation is much retarded, and the various stages may be more 
 easily watched. As the fluid is becoming viscid, fine fibrils of 
 fibrin will be seen to be developed in it, especially at the sides 
 of the containing vessel. As these fibrils multiply in number, the 
 fluid becomes more and more of the consistence of a jelly and at 
 the same time somewhat opaque. Stirred or pulled about with 
 a needle, the fibrils shrink up into a small opaque stringy mass ; 
 and a very considerable bulk of the jelly may by agitation be 
 resolved into a minute fragment of shrunken fibrin floating in a 
 quantity of what is really diluted serum. If a specimen of such 
 diluted plasma be stirred from time to time, as soon as coagulation 
 begins, with a needle or glass rod, the fibrin may be removed 
 piecemeal as it forms, and the jelly stage may be altogether done 
 away with. When fresh blood which has not yet had time to 
 
 ^ A solution of sodium chloiide of tLis strength will hereafter be spoken of as 
 'normal saline solution.' 
 
CiiAP. I.J BLOOD. 15 
 
 coagulate is stirred or wliippcd with a bundle of rods (or anything 
 presenting a large amount of rough surface), no jelly-like coagu- 
 lation takes place, but the rods become covered with a mass of 
 shrunken tibrin. Blood thus whipped until fibrin ceases to be 
 deposited, is found to have entirely lost its power of coagulation. 
 
 Putting all these facts together, it is very clear that the 
 phenomena of the coagulation of blood are caused by the appear- 
 ance in the plasma of fine fibrils of fibrin. As long as these are 
 scanty, the blood is simply viscid. When they become sufficiently 
 numerous, they give the blood the firmness of a jelly. Soon after 
 their formation they begin to shrink ; and in their shrinking en- 
 close in their meshes the corpuscles, but squeeze out the fluid 
 parts of the blood. Hence the appearance of the shrunken 
 coloured clot and the colourless serum. 
 
 Fibrin, whether obtained by whipping freshly-shed blood, or 
 by washing either a normal clot, or a clot obtained from colourless 
 plasma, exhibits the same general characters. It belongs to that 
 class of complex unstable nitrogenous bodies called jjroteids which 
 form a large portion of all living bodies and an essential part of 
 all protoplasm \ It gives the ordinary proteid reactions. It is 
 insoluble in water and in dilute saline solutions ; and though it 
 swells up in dihite hydrochloric acid, it is not thereby appreciably 
 dissolved''. 
 
 Coagulation then is brought about by the appearance in the 
 blood-plasma of a substance, fibrin, which previously did not exist 
 there as such. Such a substance must have antecedents, or an 
 antecedent — what are they, or what is it ? 
 
 If blood be received direct from the blood-vessels into one- 
 third its bulk of a saturated solution of some neutral salt such as 
 magnesium sulphate, and the two gently but thoroughly mixed, 
 coagulation, especially at a moderately low temperature, ^vill be 
 deferred for a very long time. If the mixture be allowed to stand, 
 the corpuscles will sink, and a colourless plasma will be obtained 
 similar to the plasma gained from horse's blood by cold, except 
 that it contains an excess of the neutral salt. The presence of 
 the neutral salt has acted in the same direction as cold : it has 
 prevented the occurrence of coagulation. It has not destroyed the 
 tibrin; for if some of the plasma be diluted with from five to 
 ten times its bulk of water, it will coagulate speedily in quite a 
 normal fashion, with the pnjduction of quite normal fibrin. 
 
 If some of the colourless transparent plasma, obtained either 
 by the action of neutral salts from any blood, or by the help of cold 
 from horse's blood, be treated with sjme solid neutral salt, such as 
 sodium chloride, to saturation, a white flaky somewhat sticky 
 precipitate will make its appearance. If this precipitate be re- 
 moved, the fluid is no longer coagulable (or very slightly so), 
 even though the neutral salt present be removed by dialysis, or 
 
 ' See Appendix. ^ For further details ?ee Appendix. 
 
IG COAGULATION OF BLOOJJ. [Book i. 
 
 its influeuce lessened by dilution. With the removal of the 
 substance precipitated, the plasma has lost its power of coagu- 
 lating. 
 
 If the precipitate itself, after being washed with a saturated 
 solution of the neutral salt (in which it is insoluble) so as to get 
 rid of all serum and other constituents of the plasma, be treated 
 with a small quantity of water, it readily dissolves \ and the solution 
 rapidly filtered gives a clear colourless filtrate, which is at first 
 perfectly fiuid. Soon however the fluidity gives way to viscidity, 
 and this in turn to a jelly condition, and finally the jelly shrinks 
 into a clot floating in a clear fluid; in other words, the filtrate 
 clots like plasma. Thus there is present in cooled plasma, and 
 in plasma kept from clotting by the presence of neutral salts, 
 a something, precipitable by saturation with neutral salts, a some- 
 thing which, since it is soluble in very dilute saline solutions, 
 cannot be fibrin itself, but which in solution speedily gives rise to 
 the appearance of fibrin. To this substance its discoverer, Denis, 
 gave the name of plasmine. We are justified in saying that the 
 coagulation of blood is the result of the conversion of plasmine 
 or some part of plasmine into fibrin. 
 
 But there are reasons for thinking that plasmine is a mixture 
 of at least two bodies. If sodium chloride be carefully added to 
 plasma to an extent of about 13 per cent, a white flaky viscid 
 precipitate is thrown down very much like plasmine. If after 
 the removal of the first precipitate more sodium chloride, and 
 especially if magnesium sulphate, be added a second precipitate 
 is thrown down, less viscid and more granular than the first. 
 The name fibrinogen is given to the former, paraglohulin to the 
 latter. Both are proteids belongiog to the globulin family^ the 
 members of which while insoluble in distilled water are readily 
 soluble in dilute solutions of neutral salts. According to some 
 authors solutions of fibrinogen are characterized by their being 
 precipitated, and coagulated ^ at a teinperature of about 55° — 60" 
 while solutions of paraglobulin are not so changed till the tempe- 
 rature rises to 68° — 70°. There are also other differences (see 
 Apj)endix). 
 
 Both these substances are thro■^^'n down when plasma is 
 saturated with sodium chloride so that the plasmine of Denis 
 appears to be a mixture of fibrinogen and paraglobulin, and the 
 question arises. Are both these concerned in the formation of fibrin ? 
 
 Paraglobulin not only occurs as a constituent of plasma, but 
 is found in considerable quantity in the serum left after clotting ; 
 it forms as we shall see a large portion of the proteids present in 
 
 1 The substance itself is not soluble iu distilled water, but a quantity of the 
 neutral salts always clings to the precipitate, and thus the addition of water ^■il•tually 
 gives rise to a dilute saline solution, in which the substance is readily soluble. 
 
 '^ See Appendix. 
 
 3 See Appendix for the distinction between the coagulation of proteids by heat, 
 and the coagulation due to the appearance of fibiin. 
 
I'liAi-. I 1 BLOOD. 17 
 
 scruni. Now the addiliun of serum will olteii bring about coji"u- 
 latiou in Huitls wliicli, left to themselves, will not coagulate, the 
 clot so formed bi-in;^' composed of I'lhiin with normal characters, and 
 the artificial coai;ulati<>n thus inducetl bein<( in all other respects 
 exactly like a natural clottiiij,'. Thus for instance hydrocele iluid, 
 carefully removed without admixture of bK)od from a hydrocele, 
 will in most cases remain fluid without any disposition to clot'. 
 So also the serous fluid removed from the pericardial, pleural, or 
 peritoneal cavities some liours after death in most cases shews no 
 disposition to clot*. But these fluids, hychocelo or pericardial, 
 though they do not clot spontaneously, will generally, upon the 
 addition of serum or a little whipped blood, clot in a most unmis- 
 takeable manner ^ Now libriuogen is certainly present in these 
 fluids, and may be thrown down from them by the addition of 
 sodium chloride or by other means ; and, since serum contains 
 paraglobulin, it was at first thought that the absence of spontaneous 
 coagulation in the untouched hydrocele or pericardial fluid was 
 due to the absence of paraglobulin, which as we have seen is 
 present with fibrinogen iu the spontaneously coagulable plasma of 
 blood, and that the coagulating effect of the addition of the serum 
 was due to the paraglobulin it contained, the jDaraglobulin and 
 fibrinogen acting in some way or other upon each other to produce 
 fibrin. And this view was supported by the fact that paraglobulin 
 precipitated from serum was, like the entire serum, efficacious in 
 giving rise to a coagulation in fibrinogenous pericardial, or hydrocele 
 fluids. 
 
 It was soon found however that certain specimens of pericardial 
 and even hydrocele fluid did not need the addition of the para- 
 globulin to make them coagulate ; that though they would not 
 coagulate spontaneously they might be made to coagulate by 
 adding to them a constituent of serum which was not paraglobulin 
 but something else. Thus if serum, or indeed whipped blood, be 
 mixed with a large quantity of alcohol and allowed to stand some 
 days, the proteids present are in time so changed by the alcohol 
 as to become insoluble in water. Hence if the copious precipitate, 
 after long standing, be separated by filtration from the alcohol, 
 dried at a low temperature not exceeding 40', and extracted with 
 distilled water, the aqueous extract contains very little proteid 
 matter, indeed very little organic matter at all. Nevertheless 
 even a small quantity of this aqueous extract added alone to certain 
 specimens of hydrocele fluid will bring about a speedy coagulation. 
 The same aqueous extract has also a remarkable eSect in hastening 
 the coagulation of fluids which though they Avill eventually clot, 
 do so very slowly. Thus plasma may, by the careful addition of a 
 
 ^ In some specimens, however, a spontaneous coagulation, generally slight, but 
 in exceptional cases massive, may be observed. 
 
 - If it be removed immediately after death it generally clots readily and firmly, 
 giving a colourless clot consisting of librin and white corpuscles only. 
 
 3 In a few cases no coagulation can thus be induced. 
 
 F. 2 
 
18 FIBRIN-FERMEXT. [Book i. 
 
 certain quantity of neutral salt and water, be reduced to such a 
 condition that it coagulates very slowly indeed, taking perhaps 
 days to complete the process. The addition of a small quantity of 
 the aqueous extract we are describing will however bring about a 
 coagulation which is at once rapid and complete. 
 
 The active substance, whatever it be, in this aqueous extract 
 exists in small quantity only, and its coagulating virtues are at 
 once and for ever lost when the solution is boiled. Further, there 
 is no reason to think that the active substance actually enters 
 into the formation of the fibrin to which it gives rise ; it seems, 
 without undergoing changes in itself, to act in some way or other 
 on the actual fibrin factors (fibrinogen and paraglobulin or one of 
 them) and to convert them or part of them into fibrin. It appears 
 to belong to a class of bodies playing an important part in 
 physiological processes and called ferments, of which we shall have 
 more to say hereafter. We may therefore speak of it as ih.Q fibrin- 
 ferment, the name given to it by its discoverer Alex. Schmidt. 
 
 Fibrin-ferment appears to make its appearance in blood soon 
 after it has been shed, and like other ferments is apt to be 
 entangled in and carried down by any precipitates which occur 
 in blood. It is carried down by the plasmine, and hence solutions 
 of plasmine coagulate spontaneously. 
 
 It exists in serum, and is carried down with paraglobulin when 
 that substance is precipitated. And hence arises the serious 
 question whether the coagiilating effects of serum or prepared 
 paraglobulin on hydrocele or pericardial fluid are not, after all, due 
 to the ferment present rather than to the paraglobulin. So that 
 two views may be taken of the nature of coagulation. One^ teaches 
 that fibrin arises from some mutual action of fibrinogen and para- 
 globulin induced by the fibrin ferment ; the other^ that fibrin is 
 formed through the conversion of fibrinogen alone by the agency 
 of the ferment, paraglobulin either having nothing to do with the 
 matter, or merely assisting by its presence in some indirect way. 
 
 There can be no doubt that fibrinogen is an essential factor, 
 that coagulation cannot take place without it and that it or some 
 part of it actually becomes fibrin. There is equally no doubt that 
 the presence of the fibrin-ferment is absolutely necessary. It is 
 also more than probable that fibrin does not result from the union 
 of fibrinogen and paraglobulin, since the quantity of fibrin formed 
 is not greater than that of either of these two substances used to 
 produce it. But we still need further light as to the exact nature 
 of the change produced by the ferment, the true characters of the 
 ferment itself, and the part played by paraglobulin. 
 
 In favour of the view that paraglobulin is not concerned in 
 the matter, it is asserted, that fibrinogen cautiously precipitated 
 from plasma by small quantities of sodium chloride so as to obtain 
 
 1 That of Alexander Schmidt, and his pupils and others. 
 ^ That of Hammarsten, Fredericq and others. 
 
c'liAi-. I.J J:lj)ul>. 10 
 
 it apart from piirai^lobuliM, and tlum iret'il fruiii A-riiK-nt hy repeated 
 washing, will yieKl a solution not spuntanemisly coagulable, but 
 clotting freely on the adtlition of ferment only. In f;iv(jur of 
 the view that the presence of paraglobuliu is essential may be 
 quoted the striking fact that certain specimens of hydrocele liuid 
 may be met with which will not coagulate either spontaneously or 
 nj)on the addition of ferment alone, but will coagulate upon the 
 addition of paranlubulin and ferment. Such fluids may be supposed 
 to contain fibrinogen only. And it has been argued that two 
 substances have been confused under the name of tibrinogen : oiu; 
 coagulating at the same temperature as paraglobuliu, and needing 
 the cooperation of paraglobuliu to form fibrin ; and another body, 
 which may be thrown down from solutions of plasmine or from 
 blood at the temperature of 55" — GO" (the fluids thereby losing the 
 power of coagulating), and which is fibrinogen already on its way 
 to become fibrin, in fact a sort of nascent fibrin, capable of becoming 
 actual fibrin in the total absence of paraglobuliu. Lastly the 
 presence of a neutral salt, such as sodium chloride, appears to be 
 essential to the process, coagulation not occurring even where all 
 three factors are present, if no neutral salt accompanies them. 
 
 Awaiting further investigation we may for the present conclude 
 that fibrin is formed by the conversion, through the agency of 
 a ferment, of a substance fibrinogen, which forms part of the 
 plasmine spoken of above, but the exact nature of that conversion 
 and whether paraglobuliu has any share in the matter, and if so 
 what, must remain as yet undecided. 
 
 This conception of coagulation as a chemical process between 
 certain factors renders easy of comprehension the influence of 
 various conditions on the coagulation of blood. The quickening 
 influence of heat, the retarding efiect of cold, the favourable action 
 of motion and of contact with surfaces, and hence the results 
 of whipping and the influence exerted by the form and surface of 
 vessels, become intelligible. The greater the number of points, 
 that is the larger and rougher the surface presented by the vessel 
 into which blood is shed, the more quickly coagulation comes on, 
 for contact with surfaces favours chemical union. So also the 
 presence of spongy platinum, or of an inert powder like charcoal, 
 quickens the coagulation of tardily clotting fluids, such as many 
 specimens of pericardial fluid. 
 
 Having thus arrived at an approximative knowledge of the 
 nature of coagulation, we are in a better position for discussing the 
 question. Why does blood remain fluid in the vessels of the living 
 body and yet clot when shed ? 
 
 The older views may be at once summarily dismissed. The 
 clotting is not due to loss of tempei-ature, for cold retards coagu- 
 lation, and the blood of cold-blooded animals behaves just like that 
 of warm-blooded animals in clotting when shed. It is not due to 
 loss of motion, for motion favours coagidation. It is not due to 
 
20 INFLUENCE OF THE LIVING BLOOD-VESSELS. [Book i. 
 
 exposure to air, whereby either an increased access of oxygen or an 
 escape of volatile matters is facilitated, for on the one hand the 
 blood is fully exposed to the air in the lungs, and on the other 
 shed blood clots when received, without any exposure to the 
 atmosphere, in a closed tube over mercury. 
 
 All the facts known to us point to the conclusion, that when 
 blood is contained in healthy living blood-vessels, a certain relation 
 or equilibrium exists between the blood and the containing vessels 
 of such a nature that as long as this eqviilibrium is maintained the 
 blood remains fluid, but that when this equilibrium is disturbed by 
 events in the blood or in the blood-vessels or by the removal of the 
 blood, the blood undergoes changes which result in coagulation. 
 The most salient facts in support of this conclusion are as follows. 
 
 1. After death, when all motion of the blood has ceased, the 
 blood renaains for a long time fluid. It is not till some time 
 afterwards, at an epoch when post-mortem changes in the blood 
 and in the blood-vessels have had time to develope themselves, 
 that coagulation begins. Thus some hours after death the blood in 
 the great veins may be found perfectly fluid. Yet such blood has 
 not lost its power of coagulating ; it still clots when removed from 
 the body, and clots too when received over mercury without 
 exposure to air, shewing that the fluidity of the highly venous 
 blood is not due to any excess of carbonic acid or absence of oxygen. 
 Eventually it does clot even within the vessels, but perhaps 
 never so firmly and completely as when shed. It clots first in the 
 larger vessels, but remains fluid in the smaller veins, for a very long 
 time, for many hours in fact, since in these the same bulk of blood 
 is exposed to the influence of, and reciprocally exerts an influence 
 on, a larger surface of the vascular walls than in the larger veins. 
 
 2. If the vessels of the heart of a turtle (or any other cold- 
 blooded animal) be ligatured, and the heart be cut out and 
 placed in favourable circumstances so that it may continue to beat 
 for as long a period as possible, the blood will remain fluid within 
 the heart as long as the pulsations go on, i. e. for one or two days 
 (and indeed for some time afterwards), though a portion taken away 
 at any period of the experiment will clot very speedily. 
 
 3. If the jugular vein of a large animal, such as an ox oi" 
 horse, be ligatured when full of blood, and the ligatured portion 
 excised, the blood in many cases remains perfectly fluid, along the 
 greater part of the length of the piece, for twenty-four or even 
 forty-eight hours. The piece so ligatured may be suspended in 
 a framework and opened at the top so as to imitate a living 
 test-tube, and yet the blood will often remain long fluid, though 
 a portion removed at any time into another vessel will clot in 
 a few minutes. If two such living test-tubes be prepared, the 
 blood may be poured from one to the other without coagulation 
 taking place. 
 
 The above facts illustrate the absence of coagulation in intact 
 
Chap, i.] JlLOUl). 21 
 
 or slightly altered living blooj-vessols ; tliu following shew that 
 coagulation may take plaee even in the living vessels. 
 
 4. ]f a needle or piece of wire or thread he introduced into 
 the living blood-vessel of an animal, cither during life or imme- 
 diately after death, the piece will be found encrusted with fibrin. 
 
 5. If in a living animal a blood-vessel be ligatured, the 
 ligature being of such a kind as to injure the inner coat, coagu- 
 lation takes place at the ligature and extends for some distaiic(; 
 from it. Thus if the jugular vein of a rabbit be ligatured roughly 
 in two places, clots will in a few hours be found in the ligatured 
 portion, reaching upwards and downwards from the ligatures, the 
 middle portion being the least coagulated. Clots Avill also l)e 
 found on the far side of each ligature. The clots will still appear 
 if the ligature be removed immediately after being applied, 
 provided that in the process the inner coat has been wounded. 
 If the ligatures bo applied in such a way as not to injure the 
 inner coat, coagulation will not take place, though the blood may 
 remain for many hours perfectly at rest between the ligatures. 
 
 So also when an artery is ligatured a conspicuous clot is formed 
 on the cardiac side of the ligature. The clot is largest and firmest 
 in the immediate neighbourhood of the ligature, gradually thinning 
 away from thence and reaching usually as far as where a branch 
 is given off. Between this branch and the ligature there is stasis ; 
 the walls of the artery suffer from the want of renewal of blood, 
 and thus favour the propagation of the coagulation. On the distal 
 side of the ligature where the artery is much shrunken, the clot 
 which is formed, though naturally small and inconspicuous, is 
 similar*. 
 
 6. Any injury of the inner coat of a blood-vessel causes a 
 coagulation at the spot of injury. Any treatment of a blood-vessel 
 tending to injure its normal condition causes local coagulation. 
 
 7. Disease involving the inner coat of a blood-vessel causes a 
 coagulation at the part diseased. Thus inflammation of the lining 
 membrane of the valves of the heart in endocarditis is frequently 
 accompanied by the deposit of fibrin. In aneurism the inner coat 
 is diseased, and layers of fibrin are commonly deposited. So also 
 in fatty and calcareous degeneration without any aneurismal 
 dilation there is a tendency to the formation of clots. 
 
 Similar phenomena are seen in the case of serous fluids which 
 coagulate spontaneously. If, as soon after death as the body is 
 cold and the fat is solidified, the pericardium be carefully removed 
 from a sheep b}'- an incision round the base of the heart, 
 the pericardial fluid may be kept in the pericardial bag as in 
 a living cup for many hours without clotting, and yet a small 
 portion removed with a pipette clots at once, and a thread left 
 hanging into the fluid soon becomes covered with fibrin. 
 
 The only interpretation which embraces these facts is that 
 so long as a certain normal relation between the linintr surfaces of 
 
22 SOURCES OF THE FIBRIN FACTORS. [Book i. 
 
 the blood-vessels and the blood is maintained, coagulation does not 
 take place ; but when this relation is disturbed by the more or 
 less gradual death of blood-vessels, or by their more sudden disease 
 or injury, or by the presence of a foreign body, coagulation sets in. 
 Two additional points may here be noticed. 1, Stagnation of 
 blood favours coagulation within the blood-vessels, apparently 
 because the blood-vessels, like other tissues, demand a renewal 
 of the blood on which they depend for the maintenance of their 
 vital powers. 2. The influence of surface is seen even in the 
 coaofulation within the vessels. In cases of coas^ulation from 
 gradual death of the blood-vessels, as in the case of an excised 
 jugular vein, the fibrin, when its deposition is sufficiently slow, is 
 .seen to appear first at the sides, and from thence gradually, 
 frequently in layers, to make its way to the centre. So in 
 aneurism, the deposit of fibrin is frequently laminated. In cases 
 where coagulation results from disease of the lining membrane, the 
 rougher the interior, the more speedy and complete the clotting. 
 So also a rough foreign body, presenting a large number of surfaces 
 and points of attachment, more readily produces a clot when intro- 
 duced into the living blood-vessels than a perfectly smooth one. 
 
 We may perhaps go a step further, for there are certain weighty 
 reasons for believing that in normal circulating blood all the fibrin- 
 iactors are not present in the plasma, and that a disturbance of the 
 equilibrium between the blood and the blood-vessels gives rise to 
 coagulation by inducing changes in certain corpuscles, either the 
 ordinary white corpuscles or corpuscles of a special kind, whereby 
 one or more of the fibrin-factors are discharged into the plasma. 
 
 1. When blood is received direct from the blood-vessels into 
 alcohol, the aqueous extract of the precipitate contains little or no 
 fibrin-ferment. If the blood be allowed to stand a little while before 
 being thrown into alcohol some ferment makes its appearance ; and 
 the longer, up to clotting, that the blood stands before being treated 
 with alcohol, the more efficacious is the aqueous extract of the 
 precipitate. Fibrin-ferment therefore seems to make its appearance 
 in blood after being shed. 
 
 2. When blood, kept from clotting by exposure to cold 
 or through being retained by ligatures in a living blood-vessel, is 
 allowed to stand till the corpuscles have sunk, the upper layers of 
 the plasma, free from both red and white corpuscles, exhibit when 
 removed very little power of coagulation and, upon examination, are 
 found to contain a very small quantity only of fibrin-ferment. 
 
 3. We have reasons for thinking that when blood is shed, 
 a certain number of corpuscles, which we may speak of as 
 white corpuscles, leaving it for the present uncertain whether 
 they are to be regarded as a special kind of corpuscles or not, 
 are broken up and disappear. 
 
 Putting these facts together we are led to think that normal 
 blood plasma circulating in the normal blood-vessels contains 
 
CiiAi'. i] JllJJOl). 23 
 
 no fihriu-fiTinont, but that wlion tlio equilibrium of blood is 
 (listurbL'iJ, either by the shedding of the 1)1()()(1 or by injury to the 
 blood-vessels or by the introduction of foreign Ixjdies, fibrin-ferni<jnt 
 is discharwd into the plasma, as the result of changes takinir 
 place in certain corpuscles. 
 
 With regard to the other fibrin-factors our knowledge is at 
 present delicient. As wc shall have to state presently, paraglo- 
 bulin apparently exists in serum and therefore in plasma, in very 
 considerable (piantity; and to say nothing of the doubt as to 
 whether paragiobulin has any share in forming libriii, it seems 
 extremely unlikely that the whole of this large quantity could 
 have come from disintegrating corpuscles. Fibrinogen is generally 
 supposed to be pre-existent in the p'asma ; but there do not 
 appear to be adequate reasons for this view; and it is quite 
 possible that it too, like the ferment, comes from the corpuscles. 
 But this is aliiKJst tantamount to saying that the whole fibrin 
 comes from the corpuscles, and indeed it has been argued that the 
 Avhite corpuscles are in part bodily converted into fibrin. 
 
 The Avhole matter needs further investigation, and Avhen 
 Ave remember that fibrin-ferment and even masses of white 
 corpuscles injected into the living blood-vessels do not necessarily 
 bring about coagulation, it is clear that wc have much yet to 
 learn. Moreover we have reason, as we shall see, to think that 
 corpuscles are continually dying in the body, and therefore continu- 
 ally setting free fibrin-factors; and these, unless Ave suppose that 
 a certain quantity of fibrin can exist scattered so to speak in the 
 blood, must be made aAvay with or at least prevented from giving 
 rise to clots. 
 
SEC. 2. THE CHEMICAL COMPOSITION OF BLOOD. 
 
 As we have already urged, the chief chemical interests of blood 
 are attached to the changes "which it undergoes in the several 
 tissues, and which will be considered in connection with each tissue 
 at the appropriate place. Nevertheless a brief summary of the 
 main characters of blood as a whole may be introduced here. 
 
 The average specific gravity of human blood is 1055, varying 
 from 1045 to 1075 within the limits of health. The reaction of 
 blood as it flows from the blood-vessels is found to be distinctly 
 though feebly alkaline. 
 
 If the corpuscles be supposed to retain the amount of water 
 proper to them, blood may, in general terms, be considered as 
 consisting by weight of from about one-third to somewhat less than 
 one-half of corpuscles, the rest being plasma. As will be insisted 
 on presently, the number of corpuscles in a specimen of blood 
 is found to vary considerably, not only in different animals and in 
 different individuals, but in the same individual at different times. 
 Conspicuous and striking as are the results of coagulation, 
 massive as appears to be the clot which is formed, it must be remem- 
 bered that by far the greater part of the clot consists of corpuscles. 
 The amount by weight of fibrin required to bind together a number 
 of corpuscles in order to form even a large firm clot is exceedingly 
 small. Thus the average quantity by weight of fibrin in human 
 blood is said to be "2 p. c, but the amount which can be obtained 
 from a given quantity of plasma varies extremely; the variation 
 being due not only to circumstances affecting the blood, but also to 
 the method employed. 
 
 The difficulties indeed of acquiring an exact knowledge of the 
 chemical constitution of the plasma, which as we have seen from the 
 
Cu.U". i] JiLUUD. 2."5 
 
 foreguiiig .section is piubably uiulcigoingdiiinges fiuni tliu inoincnt 
 of being .shed, are very great; our information conccrnin<' tlie 
 eompo.sition of tiie serum and of tlic corpuscles i.s much more tru.st- 
 worthy. 
 
 Compositiou of S3riim. In 10) pints of serum there are in 
 round munbers 
 
 Water 90 parts 
 
 Proteid Substances iS to !J 
 
 Fats, Extractives*, and Saline Matters 2 to 1 ,, 
 The proteiil substances present in serum are"'': — (1) The so-called 
 scrum albumin, (2) paragiobulin. The paraglobulin, as has been 
 stated in the preceding section, may be removed from the serum in 
 several wa3's: viz. by pas.sing carbonic acid through, or by cautiously 
 adding dilute acetic acid to, the diluted serum, or more completely 
 by saturating the undiluted serum with magnesium sulphate. 
 When the whole of the paragiobulin has been removed a con- 
 siderable quantity of joroteid material is still left in the scrum 
 in the form known as serum albumin, distinguished from 
 paragiobulin among other characters by its being soluble in 
 distilled water, and therefore not requiring for its solution the 
 presence of a neutral salt^ From the researches of Hammar- 
 sten it Avould appear that, owing to imperfect methods the 
 amount of paragiobulin in serum has been much underrated. 
 According to him, the quantity though varying in different animals, 
 is at times equal to and sometimes even greater than that of the 
 serum albumin. Even if we were to accept as definitely proved 
 the view that paragiobulin in some form or other is in some 
 way associated with the formation of fibrin, it seems hardiv 
 probable that the whole of this large quantity of paragiobulin 
 present in serum is fibrinoplastic, i.e. capable of taking part in 
 the formation of fibrin. We cannot at present however attach 
 any definite functions to the paragiobulin and serum albumin 
 respectively, nor do we know much as to what extent they vary 
 in quantity, though the interesting observation has been made 
 that in snakes the serum albumin disappears during starvation, 
 while the paragiobulin is fairly constant. When serum, after the 
 cautious addition of acetic acid in order to neutralize its alkalinity, 
 is heated to about 75" C. both the serum albumin and paragiobulin 
 are thrown down in the form known as coagulated jJroteids, sub- 
 stances characterized by their great insolubility. This 'coagulation' 
 by heat of these and other proieids is, it perhaps need hardly be 
 repeated, not to be confounded with the coagulation of plasma due 
 to the appearance of fibrin. 
 
 ^ This word is usclI to denote substances of varied origin and nature, occurring 
 iu small quantities, and therefore requiring to be ' extracted ' by special mpan.s. 
 
 ^ There seems no longer any reason to distinguish a serum-casein from paragio- 
 bulin, see Appendix. 
 
 ^ For further details see Appcmlix. 
 
26 COMPOSITION OF THE RED CORPUSCLES. [Book i. 
 
 The fats, wliicli are scanty, except after a meal or in certain 
 pathological conditions, consist of the neutral fats, stearin, pal- 
 raitin, and olein, Avith a certain quantity of their respective alkaline 
 soaps. Lecithin^ and cholesterin occur in very small quantities only. 
 Among the extractives present in serum may be put down all the 
 nitrogenous and other substances which form the extractives of the 
 body and of food, such as urea, kreatin, sugar, lactic acid, &c. A 
 very large number of these have been discovered in the blood 
 under various circumstances, the consideration of "which must be 
 left for the present. The peculiar odour of blood-serum is pro- 
 bably due to the presence of volatile bodies of the fatty acid series. 
 The faint yellow colour of serum is due to a special yellow pigment. 
 The most characteristic and important chemical feature of the 
 saline constitution of the serum is the preponderance of sodium 
 salts over those of potassium. In this respect the serum offers a 
 marked contrast to the corpuscles (see below). Less marked, but 
 still striking, is the abundance of chlorides and the poverty of 
 phosphates in the serum as compared with the corpuscles. The 
 salts may in fact briefly be described as consisting chiefly of 
 sodium chloride, v/ith some amount of sodium carbonate, or more 
 correctly sodium bicarbonate, and potassium chloride, with small 
 quantities of sodium sulphate, sodium phosphate, calcium phos- 
 phate, and magnesium phosphate. And of even the small quantity 
 of phosphates found in the ash, part of the phosphorus exists in 
 the serum itself not as a phosphate but as phosphorus in some 
 organic body. 
 
 Composition of the red corpuscles. The corpuscles contain less 
 water than the serum, the amount of solid matter being variously 
 estimated at from 80 to 40 or more p. c. The solids are almost 
 entirely organic matter, the inorganic salts in the corpuscles 
 amounting to less than 1 p. c. Of the organic matter again by far 
 the larger part consists of hoBmoglobin. In 100 parts of the dried 
 organic matter of the corpuscles of human blood, Hoppe-Seyler 
 and Judell found, as the mean of two observations, 
 
 Haemoglobin 90'5-i Lecithin '54 
 
 Proteid Substances 8'C7 Cholesterin "25 
 
 There are reasons for believing that not only may tlie number 
 of red corpuscles vary, but also the quantity of ha-moglobin present 
 in tlie corpuscles differ under different circumstances. Malassez, 
 by comparing the tint of a quantity of blood the numbers of whose 
 corpuscles had been estimated, with that of a graduated solution 
 of picrocarminate of ammonia, has been able to estimate the 
 amount of hemoglobin present in the corpuscles under different 
 circumstances. He finds that in anaemia the poverty of the 
 
 ^ For detailed accounts of the characters of the several chemical substances 
 mentioned in this and succeeding chapters consult the Appendix under the appro- 
 priate headings. 
 
Chap, i.] HLOOD. 27 
 
 corpuscles in lucninnlubiii is even inoro striking tlian tlic scanti- 
 ness of the coipuscles, and is sooiiCr affected by the administration 
 of iron. 
 
 The composition and properties of hoomogloLin will be con- 
 sidered in connection "with respiration. 
 
 Of the proteid substances which form tlio stroma of the red 
 corpuscles this much may be said, that they appear to belong to 
 tlie globulin family; their exact nature need not be considered 
 here. As regards the inorganic constituents, the corpuscles are 
 distinguished by the relative abundance of the salts of potassium 
 and of phosphates. This at least is the case in man ; the relative 
 ([uantities of sodium and potassium in the corpuscles and serum 
 lespectively appear however to vary in different animals; in some 
 tiic sodium salts are in excess even in the corpuscles. 
 
 Composition of the white corpuscles. Our knowledge of the 
 exact nature of the proteid matrix of the white corpuscles is at 
 present too uncertain to enable any definite or useful statements 
 to be made, and the probable relation of the corpuscles to coagula- 
 tion has already been spoken of. The corpuscles are found to 
 contain in addition to proteid material, lecithin or protagon, gly- 
 cogen, extractives and inorganic salts, there being in the ash a 
 preponderance of potassium salts and of phosphates. The nuclei 
 contain nuclein. Upon the death of the corpuscle the glycogen 
 appears to be converted into sugar. 
 
 Both the corpuscles and the plasma (or serum) contain gases. 
 These will be considered in connection with respiration. 
 
 The main facts of interest then in the chemical composition of 
 the blood are as follows. The red corpuscles consist chiefly of 
 l)ffiraoglobin. The organic solids of serum consist partly of serum- 
 albumin, and partly of paroglobulin. The serum or plasma 
 contrasts in man at least, with the corpuscles, inasmuch as the 
 former contains chiefly chlorides and sodium salts while the latter 
 are richer in jDhosphates and potassium salts. The extractives of 
 the blood are remarkable rather for their number and variability 
 than for their abundance, the most constant and important being 
 perhaps urea, kreatin, sugar, and lactic acid. 
 
SEC. 3. THE HISTORY OF THE CORPUSCLES. 
 
 In the living body red blood-corpuscles are continually being 
 destroyed, and new ones as continually being produced. The proofs 
 of this are, 
 
 1, Tlie number of the red corpuscles in the blood at any given 
 time varies much. 
 
 The mimber of corpuscles in a specimen of blood is determined by 
 mixing a small but carefully measured quantity of the blood with a large 
 quantity of some indifferent fluid, and then actually counting the 
 corpuscles in a known minimal bulk of the mixture. 
 
 This may be done either by Vierordt's plan (somewhat modified by 
 Gowers), in which a minimal quantity of the diluted blood, measured 
 in a fine capillary tube, is spread on a surface marked out in sqiiaie 
 areas, and the number of corpuscles in each square area counted under 
 the microscope; or by that of Malassez, in wliich the diluted blood 
 is drawn into a capillary tube with flattened sides, and the number 
 of corpuscles counted in situ in the tube by means of an ocular 
 marked out in squares, the microscope being so adjusted that each 
 area of the ocular corresjionds to a certain capacity of the capillary tube. 
 
 The average number of red corpuscles in mammals generally 
 ranges from 3 to 18 millions;yin human blood it is about 5 millions 
 in a cubic millimetre; The number is increased by meals, and dimi- 
 nished by fasting ; of course, the number of corpuscles present in any 
 given bulk of blood being merely the expression of the proportion 
 of corpuscles to the amount of plasma, variations in the number 
 counted might and in certain cases are probably caused by an 
 increase or decrease in the quantity of plasma, occurring while the 
 actual number of corpuscles is stationary. But many of the 
 variations cannot be so accounted for ; they must be due to an in- 
 
Chap, i] IILOOI). -J'j 
 
 crease or decrease of the toUil miniber of" corj)Uficlus in the body. 
 After a very hirgc reduction of the total number of red corpuscles, 
 as by hajuiorrhage or disease (anaBniia), the normal proportion may 
 be regained even within a very short time. 
 
 2. There are reasons for thinking that the urinary and bilc- 
 pigments are derivatives of ha3moglobin. If this be so, an immense 
 number of corpuscles must be destroyed daily (and replaced by 
 new ones) in order to give rise to the amount of urinary and bile- 
 pigment discharged daily from the body, 
 
 3. When the blood of one animal is injected into the vessels 
 of another (ex. gr. that of a bird into a mammal), the corpuscles of 
 the first may for some time be recognised in blood taken from the 
 second; but eventually they wholly disappear. This of course is 
 no strong evidence, since the destruction of foreign corpuscles 
 might take place even though the proper ones had a permanent 
 existence. 
 
 That the M-hite corpuscles or leucocytes also are continually being 
 destroyed and replaced is similarly probable from the fact that they 
 vary extremely in number at different times and under various 
 circumstances. Most observers agree that they are very largely 
 increased by taking food. Thus during fasting they may be seen 
 in a drop of blood to bear to the red the proportion of 1 in 800 
 or 1000. After a meal this proportion may rise to 1 in 300 or 400. 
 
 The mode of origin of the red corpuscles is .so fully dealt wiih 
 in histological treatises and at the same time the subject of so 
 many conflicting opinions, that it will be sufficient to remind the 
 reader that the facts at present in our possession seem to shew 
 that in the adult the generation of new corpuscles takes place 
 chieOy in the red medulla of bones, but also, at all events in young 
 animals, and especially after great loss or destruction of red 
 corpuscles, in the spleen and possibly in other places. In the 
 peculiar capillary mesh-work of the red medulla are found certain 
 corpuscles which differ, among other characters, from the normal 
 red corpuscles (in mammals) by possessing a nucleus, and from the 
 ordinary leucocytes by having their protoplasm impregnated with 
 a certain quantity of haemoglobin. These peculiar intermediate 
 corpuscles appear to be transformed into normal red corpuscles, 
 but the exact mode of transformation, whether for instance the 
 nucleus is bodily extruded from the cell, or broken up within the 
 cell, or whether indeed, as some think, the nucleus and not the 
 v/hole cell becomes the red corpuscle, is not yet wholly cleared up. 
 Nor are we at present sure whether these peculiar corpuscles 
 themselves arise by a metamorphosis of ordinary leucocytes, or 
 as Bizzozero urges, represent a special class of cells, whose 
 continual existence is ensured by their continually undergoing 
 division. Intermediate cells of this description (which must 
 not be confounded with smaller cells described by Hayem, and 
 called by him hsematoblasts, but whose nature is doubtful) have 
 
30 HISTORY OF THE CORPUSCLES. [Book i. 
 
 been seen in circulating and even shed blood by various observers, 
 and it is this kind of corpuscle which Alex. Schmidt believes to 
 break up so largely and disappear, with the production of fibrin- 
 factors, when blood is shed. Making every allowance for contro- 
 verted points, we may conclude, that the red-medulla of bones has 
 an important, function in giving rise to new red corpuscles, and 
 that after unusual loss or destruction of these bodies, the normal 
 activity of this tissue at least is greatly increased. 
 
 When we come to treat of respiration, we shall bring forward 
 evidence that the red corpuscles, by virtue of haemoglobin, have a 
 most important use in carrying oxygen from the lungs to the 
 several tissues. It is through the red corpuscles that the tissues 
 themselves breathe, at least as far as breathing is the taking in of 
 oxygen. We do not know what wear and tear the red corpuscles 
 undergo in this respiratory function ; nor have we any evidence as 
 to any other work which they perform in the economy, and which 
 would tend to their being used up. But, as we have already urged, 
 we have reason to think that they are being constantly destroyed, 
 and apparently one place at least where this destruction goes on is 
 the spleen. 
 
 In this organ may be seen, as Kolliker long since pointed out, 
 large protoplasmic cells in which are included a number of red 
 corpuscles : and these red corpuscles may be observed in various 
 stages of apparent disintegration. Moreover the serum of tlie 
 blood of the splenic vein, unlike that of blood in general, is said to 
 be tinged with hasmoglobin. It would seem therefore probable that 
 a certain amount of haemoglobin is set free in the spleen from 
 disintegrating red corpuscles, and carried, in part at least, from 
 thence through the portal circulation to the liver. Whether any 
 large amount of destruction of red corpuscles goes on elsewhere we 
 do not know. 
 
 Since the serum of blood, with the exception of that from the 
 splenic vein, contains no dissolved haemoglobin, it is clear that the 
 haemoglobin of the broken-up corpuscles must speedily be trans- 
 formed into some other body. Into what other body ? In old 
 blood- clots (as in those of cerebral hsemorrhage) there are fre- 
 quently found minute crystals of a body free from iron, which has 
 received the name hceniatoidin. There can be no doubt that the 
 hsematoidin of these clots is a derivative from the haemoglobin of 
 the escaped blood. We know^ that hemoglobin contains, besides 
 a proteid residue, a residue not proteid in nature, called hasmatm. 
 We know further that hasmatin may lose the iron which it contains 
 (and which appears to be loosely attached), and yet remain a 
 coloured body. So that there is no difficulty in the passage from 
 the proteid-and-iron-containing haemoglobin to the proteid-and- 
 iron-free hasmatoidin. But haematoidin, not only in the form and 
 appearance of its crystals, but also, as far as can be ascertained by 
 1 See Chapter on Changes of Blood in Eesjiii-ation. 
 
CiiAiv 1.) BLOOD. 31 
 
 the analysis of tlie fcinall (iuaiititi(;s at disposal, in its cliemical 
 composition, is identical with bilirubin, the j)nniary j)ijfnK'nt of 
 bile. Moreover, according to some observers the injection of 
 haemoglobin, or of dissolved red corpuscles, into the vessels of a 
 living animal, gives rise to a large amount of bilo-pigment in the 
 urine, and at the same time increases enormously the relative 
 quantity of bilirubin in the bile. Thus though no one has yet 
 succeeded in producing bilirubin artificially from haemoglobin, and 
 the actual identity of the two cannot as yet perhaj)s be regarded 
 as settled, facts, and especially peihaps the presence of ha-mo- 
 globin, in the serum of the splenic vein, and its disaj)pearance after 
 the blood has passed through the liver, point very strongly to the 
 view that the red corpuscles are used up to supply bile-pigment. 
 
 Our knowledge of urinary pigments is so imperfect that little 
 can be said as to their relation to haemoglubin. We cannot at 
 present definitely trace the normal urinary pigment back to 
 haemoglobin, however probable such a source may seem. 
 
 As regards the -white corpuscles of the blood, using this 
 term without prejudice or as to the question Avhether or no 
 there be more than one distinct kind, these as we have seen 
 also come and go. 
 
 The fact that in the lymphatic glands, and other adenoid 
 structures, corpuscles, similar to if not identical with white blood- 
 corpuscles, are to be seen of very various sizes, many Avith double 
 nuclei and some indeed actually dividing into two corpuscles, 
 suggests that these organs are the birth-places of the white 
 corpuscles. The lymph is continually pouring into the blood a 
 crowd of white corpuscles, which, since they for the most part 
 make their appearance in the lymph-vessels after the latter have 
 traversed the lyuiphatic glands, probably take origin from those 
 bodies. 
 
 At the same time it is open for us to suppose that any 
 proliferating tissue may give rise to nevr corpuscles; and Klein 
 states that he has seen them budded off from the reticulum of the 
 spleen. The ■white corjjuscles have also been observed to divideV 
 
 We may conclude therefore that the white corpuscles probably 
 arise, by division chiefly, from the corpuscles of adenoid tissue, but 
 that other sources may exist. 
 
 While we are able to attribute to the numerous red corpuscles 
 an important respiratory function, we are at present at all 
 events unaware of any special work carried on by the scantier 
 white corj)UScles while they are being hurried along in the blood 
 cuiTent. As far as our present knowledge goes they seem to taiTy 
 in the blood only on then- way either to be broken up or to pass 
 into the tissues. 
 
 We have already referred to the jirobablo view that it is not the 
 ordinary white corpuscle but a special kind of corpuscle which is 
 1 Klein, Hdb. Phys. Lab., p. 8. 
 
32 HISTORY OF THE CORPUSCLES. [Book i. 
 
 transformed into the red corpuscle ; if this is the case the keeping 
 up a supply of red corpuscles cannot, as was once thought, be an 
 important end of the existence of white corpuscles in general. We 
 have already (p. 22) dwelt on the probability that the coagulation 
 of shed-blood is due to white corpuscles breaking up and discharg- 
 ing certain fibrin-factors into the plasma ; but it is uncertain in 
 the first place whether this function is to be attributed to all white 
 corpuscles or to a special kind only, and in the second place 
 whether in normal conditions of the economy any appreciable 
 amount of fibrin-factors are in this way habitually discharged into 
 the blood, and as constantly got rid of without fibrin being formed. 
 It is quite possible that normal circulating plasma may always 
 contain a certain stock for instance of fibrinogen, which is 
 continually being drawn upon for the nourishment of the tissues, 
 and as continually replaced by the destruction of corpuscles. But 
 there are no facts at present which absolutely contradict the view 
 that fibrinogen is normally absent from intact circulating plasma, 
 and that the arrangements for the manufacture of fibrin exist only 
 for the purpose of meeting the contingency of fibrin being required 
 under circumstances which may be considered abnormal. 
 
 On the other hand we know that in an inflamed area the white 
 corpuscles migrate in large numbers into the extravascular portions 
 of the tissues, and it has been maintained that not only the pus 
 corpuscles and ' exudation' corpuscles which are the common pro- 
 ducts of inflammation, but even the new tissue elements (connec- 
 tive-tissue cells and fibres), which make their appearance as the 
 result of the so-called 'productive' inflammations, are the descen- 
 dants, immediate, or remote, of such migratory corpuscles. But 
 a discussion of this question would lead us too far away from the 
 purpose of this work. 
 
 It would appear therefore that with the exception of the respira- 
 tory function of the red corpuscles, the physiological interest of the 
 blood is attached rather to the plasma than to the corpuscles. 
 The work, done by the corpuscles, even when it is fully under- 
 stood, will, with the exception of the carrying of oxygen by the 
 red corpuscles, always appear insignificant compared with the 
 incessant labours of the plasma, which is for ever busy as the 
 middle-man between the several tissues, bringing to each tissue 
 what it needs and taking from it that which is useless or even 
 injurious to itself but necessary to the well-being of some other 
 part. 
 
SEC. 4. THE QUANTITY OF BLOOD, AND ITS 
 DISTRIBUTION IN THE BODY. 
 
 The total quantity of blood present in an animal body is 
 estimated in the following way. As much blood as possible is 
 allowed to escape from the vessels ; this is measured directly. 
 The vessels are then washed out with water or normal saline 
 solution, and the washings carefully collected, mixed and measured. 
 A known quantity of blood is diluted with water or normal saline 
 solution until it possesses the same tint as a measured specimen of 
 the washings. This gives the amount of blood (or rather of 
 haemoglobin) in the measured specimen, from which the total 
 quantity in the whole washings is calculated. Lastly, the whole 
 body is carefully minced and washed free from blood. The 
 washings are collected and filtered, and the amount of blood in 
 them estimated as before by comparison with a specimen of diluted 
 blood. The quantity of blood in the two washings, together wdth 
 the escaped blood, gives the total quantity of blood in the body. 
 
 The method is not free from objections, the most serious 
 of which perhaps are attached to the difficulty of obtaining 
 infusions of the minced tissues clear enough to have their tint 
 accurately estimated, and to the fact that the animal must be 
 killed for the purpose; but other methods, for instance those in which 
 the quantity is calculated from the proportion of red corpuscles to 
 plasma before and after either diminution of the plasma by sweat- 
 ing or increase by the injection of serum or other fluids free from 
 corpuscles, are open to still gi-aver objections. 
 
 F. 3 
 
34 DISTRIBUTION OF BLOOD. [Book i. 
 
 From the result of a few observations on executed criminals it 
 has been concluded that the total quantity of blood in the human 
 body is about j^th of the body weight. But in various animals, 
 the proportion of the weight of the blood to that of the body has 
 been found to vary very considerably ; and probably this holds good 
 for man also, at all events within certain limits. 
 
 The blood is in round numbers distributed as follows : 
 
 About one-fourth in the heart, lungs, large arteries and veins, 
 ;, „ liver, 
 „ „ „ „ skeletal muscles, 
 
 „ ^ „ „ „ other organs. 
 
 Since in the heart and great blood-vessels the blood is simply 
 in transit, without imdergoing any great changes (and in the lungs, 
 as far as we know, the changes are limited to respiratory changes), 
 it follows that the changes which take place in the blood passing 
 through the liver and skeletal muscles far exceed those which take 
 place in the rest of the body. 
 
CHAPTER 11. 
 
 THE CONTRACTILE TISSUES. 
 
 The greater number of the movements of the complex animal 
 body are carried on by means of the skeletal striated muscles. A 
 skeletal muscle when subjected to certain influences contracts, i. e. 
 shortens, bringing its two ends nearer together ; and the shortening, 
 acting through various bony levers or by help of other mechanical 
 arrangements, produces a movement of some part of the body. The 
 striated tissue of which the skeletal muscles are composed is the 
 chief contractile tissue. The pecuUar muscular tissue of the heart 
 is another contractile tissue; under certain influences the fibres 
 into which it is arranged, shorten and thus give rise to the beat of 
 the heart. A similar shortening or contraction of the fusiform 
 fibre cells of plain muscular tissue, gives rise to movements such as 
 changes of calibre &c. of the alimentary canal, the urinary bladder, 
 the uterus, the arteries and the hke. 
 
 At first sight 'contraction' of any one of these forms of differen- 
 tiated muscular tissue seems wholly unlike an amoeboid movement 
 of an amoeba or of a white blood corpuscle. And yet the transition 
 from the one to the other is very shght. A typical amoeba may be 
 regarded as spherical in form, and when it is executing its niove- 
 ments the pseudopodic bulging of its protoplasm may be seen 
 to occur now on this now on that part of its circumference and to 
 take now this and now that direction. The fibre cell of plain 
 muscular tissue is a nucleated protoplasmic mass of a distinctly 
 fusiform shape, and when it executes its movements, i. e. contracts, 
 the bulging of its protoplasm is always a lateral bulging in a 
 direction at right angles to the long axis of the fibre cell. Since as 
 
 3—2 
 
.36 MUSCULAR COE TRACTION. ■ [Book i. 
 
 we shall see there is no change of total bulk, this thickening of the 
 fibre by means of the lateral bulging is necessarily accompanied 
 by a shortening of its length. The contraction of muscular tissue 
 is in fact a limited and definite amoeboid movement in which 
 intensity and rapidity are gained at the expense of variety. 
 
 Besides these movements which are carried out in the body by 
 means of differentiated muscular tissue, there are others brought 
 about by the peculiar structures known as cilia, among which we 
 may include the motile tails of spermatozoa ; and ordinary amoeboid 
 movements are not wanting, being conspicuously shewn by the 
 so-called migrating cells. We may include both these under the 
 heading of contractile tissues. 
 
 Of all these various forms of contractile tissue the skeletal 
 striated muscles, on account of the more complete development of 
 their functions, will be better studied first ; the others, on account 
 of their very simplicity, are in many respects less satisfactorily 
 understood. 
 
 All the ordinary striated skeletal muscles are connected with 
 nerves. We have no reason for thinking that their contractility is 
 called into play, under normal conditions, otherwise than by the 
 agency of nerves. 
 
 Muscles and nerves being thus so closely allied, and having 
 besides so many properties in common, it will conduce to clearness 
 and brevity if we treat them together. 
 
SEC. 1. THE PHENOMENA OF MUSCLE AND NERVE. 
 Muscular and Nervous Irritability. 
 
 The skeletal muscles of a frog, the brain and spinal cord of 
 which have been destroyed, do not exhibit any spontaneous move- 
 ments or contractions, even though the ner^^es be otherwise quite 
 intact. Left untouched the whole body may decompose without 
 any contraction of any of the muscles having been witnessed. 
 Neither the skeletal muscles nor the nerves distributed to them 
 possess any power of automatic action. 
 
 If however a muscle be laid bare and be more or less \4olently 
 disturbed, if for instance it be pinched, or touched with a hot wire, 
 or brought in contact with certain chemical substances, or subjected 
 to the action of galvanic currents, it will contract whenever it is 
 thus disturbed. Though not possessing any automatism, the 
 muscle is (and continues for some time after the general death ot 
 the animal to be) irritable. Though it remains quite quiescent 
 when left untouched, its powers are then dormant only, not absent. 
 These require to be roused or ' stimulated' by some change or 
 disturbance in order that they may manifest themselves. The 
 substances or agents which are thus able to evoke the activity of 
 an irritable muscle are spoken of as stimuli. 
 
 But to produce a contraction in a muscle the stimulus need 
 not be apphed directly to the muscle ; it may be applied indirectly 
 by means of the nerve. Thus, if the trunk of a nerve be pinched, 
 or subjected to sudden heat, or dipped in certain chemical 
 substances, or acted upon by various galvanic currents, contrac- 
 tions are seen in the muscles to which branches of the nerve are 
 distributed. 
 
38 MUSCULAR CONTRACTION. |Booki. 
 
 The nerve like the muscle is irritable, it is thrown into a state 
 of activity by a stimulus ; but unhke the muscle it does not itself 
 contract. The changes set up in the nerve by the stimulus are not 
 visible changes of form; but that changes of some kind or other 
 are set up and propagated along the nerve down to the muscle is 
 shewn by the fact that the muscle contracts when a part of the 
 nerve at some distance from itself is stimulated. Both nerve 
 and muscle are irritable, but only the muscle is contractile, ^. e. 
 manifests its irritability by a contraction. The nerve manifests its 
 irritability by transmitting along itself, -without any visible alteration 
 of form, certain molecular changes set up by the stimulus. We shall 
 call these changes thus propagated along a nerve, 'nervous impulses.' 
 
 We have stated above that the muscle is irritable in the sense 
 that it may be thrown into contractions by stimuli applied directly 
 to itself But it might fairly be urged that the contractions so pro- 
 duced are in reality due to the fact that, although the stimulus is 
 apparently applied directly to the muscle, it is, after all, the fine 
 nerve-branches, so abundant in the muscle, which are actually 
 stimulated. The following facts however go far to prove that 
 the muscular fibres themselves are capable of being directly stimu- 
 lated without the intervention of any nerves. When a frog (or other 
 animal) is poisoned with urari, the nerves may be subjected to 
 the strongest stimuli without causing any contractions in the 
 muscles to which they are distributed ; yet even ordinary stimuli 
 applied directly to the muscle readily cause contractions. If before 
 introducing the urari into the system, a ligature be passed under- 
 neath the sciatic nerve in one leg, for instance the right, and drawn 
 tightly round the whole leg to the exclusion of the nerve, it is 
 evident that the urari when injected into the back of the animal, 
 will gain access to the right sciatic nerve above the ligature, but 
 not below, while it will have free access to the whole left sciatic. If, 
 as soon as the urari has taken effect, the two sciatic nerves be 
 stimulated, no movement of the left leg wiU be produced by stimu- 
 lating the left sciatic, whereas strong contractions of the muscles of 
 the right leg below the ligature will follow stimulation of the right 
 sciatic, whether the nerve be stimulated above or below the ligature. 
 Now since the upper parts of both sciatics are equally exposed to 
 the action of the poison, it is clear that the failure of the left nerve 
 to cause contraction is not attributable to any change having taken 
 place in the upper portion of the nerve, else why should not the 
 right, which has in its upper portion been equally exposed to the 
 action of the poison, also fail ? Evidently the poison acts on some 
 parts of the nerve lower down. If a single muscle be removed from 
 the circulation (by ligaturing its blood-vessels), previous to the 
 poisoning with urari, that muscle will contract when any part of the 
 nerve going to it is stimulated, though no other muscle in the body 
 will contract when its nerve is stimulated. Here the whole nerve 
 right down to the muscle has been exposed to the action of the 
 
Chap, u.j THE CONTRACTILE TISSUES. 39 
 
 poison ; aud yet it has lost none of its power over the muscle. On 
 the other hand, if the muscle be allowed to remain in the body, and 
 so be cx})Osed to the action of the poison, but tlie nerve be divided 
 hiorh up and the part connected with the muscle gently lifted ujj 
 before the urari is introduced into the system, so that no blood 
 flows to it and so that it is protected from the influence of the 
 poison, stimulation of the nerve will be found to produce no con- 
 tractions in the muscle, though stimuli applied directly to the 
 muscle at once cause it to contract. From these facts it is clear 
 that urari poisons the ends of the nerve within the muscle long 
 before it affects the trunk, and it is exceedingly probable that it 
 is the very extreme ends of the nerves (possibly the end-plates, for 
 urari poisoning, at least when profound, causes a slight but yet 
 distinctly recognisable effect in the microscopic appearance of these 
 structures) which are affected. The phenomena of urari poisoning 
 therefore go far to prove that muscles are capable of being made 
 to contract by stimuli applied directly to the muscular fibres them- 
 selves ; and there are other facts which support this view. 
 
 This question of 'independent muscular irritability' was once thoxight 
 to be of importance. In old times, the swelling of a muscle during con- 
 traction was held to be caused by the animal spirits descending into it 
 along the nerves; and when the doctrine of 'spirits' was given up, it 
 was still taught that the vital activity of the muscle was something 
 bestowed upon it by the action of the nerve, and not properly belonging 
 to itself. We owe to Haller the establishment of the truth, that the 
 contraction of a muscle is a manifestation of the muscle's own energy, 
 excited it may be by nervous action, but not caused by it. Haller spoke 
 of the muscle as possessing a vis insita, while he called the nervous 
 action, which excites contraction, the vis nervosa. He used the word 
 irritabiUty as almost synonymous with contractiHty, a meaning which is 
 still adopted by many authors. In this work we have used it in the 
 wider sense, first employed by Glisson, which includes other manifesta- 
 tions of energy than the change of form which constitutes a contraction. 
 
 The Phenomena of a simple Muscular Contraction. 
 
 If the far end of the nerv^e of a muscle-nerve preparations 
 Figs. 1 and 2, be laid on the electrodes of an induction-machine ^, 
 
 1 By this is meant a muscle dissected out ■svith some length of nerve attached to it, 
 both being ia a living condition, i.e. still irritable. The muscle generally used is the 
 gastrocnemius of the frog, the attachment to the femur and a portion of the tendo 
 AchiUis, together with a considerable length of the sciatic nerve, being carefully 
 preserved. 
 
 - It may perhaps be worth whUe to remind the reader of the following facts. 
 
 In a galvanic battery, the substance (plate of zinc for instance) which is acted 
 upon and used up by the liquid is called the positive element, and the sub- 
 stance which is not so acted upon and used up (plate <S:c. of copper, platinum, or 
 carbon, &c.) is called the negative element. A galvanic action is set up when the 
 positive (zinc) and the negative (copper) elements are connected outside the battery 
 
40 MUSCULAR CONTRACTION. [Book i. 
 
 the passage of a single induction-shock, which may be taken as a 
 convenient form of an almost momentary stimulus, will produce no 
 visible change in the nerve, but the muscle will give a short sharp 
 contraction, i.e. will for an instant shorten itself, becoming thicker 
 the while, and then return to its previous condition. If one end of 
 the muscle be attached to a lever, while the other is fixed, the 
 
 by some conducting material, such as a wire, and the current is said to flow in a 
 circuit or circle from the zinc or positive element to the copper or negative element 
 inside the battery and then from the copper or negative element back to the zinc or 
 positive element through the wire outside the battery. If the conducting wire be cut 
 through, the current ceases to flow, but if the cut ends be brought into contact, 
 the current is re-estabhshed and continues to flow so long as the contact is good. 
 The wires or the ends of the wires, which may be fashioned in various ways, are 
 called electrodes. When the electrodes are brought into contact or are connected 
 by some conductiog material, galvanic action is set up, and the current flows 
 through the battery and wires ; this is spoken of as " making the current " or 
 •'completing or closing the circuit." When the electrodes are drawn apart from each 
 other, or when some non-conducting material is interposed between them, the 
 galvanic action is arrested; this is spoken of as "breaking the current" or "opening 
 the circuit." The current passes from the electrode connected with the negative 
 (copper) element in the battery to the electrode connected with the positive (zinc) 
 element in the battery ; hence the electrode connected with the copper (negative) 
 element is called the 'positive electrode, and that connected with the zinc (positive) 
 element is called the negative electrode. 
 
 In an "induction machine " the wire connecting the two elements of a battery 
 is twisted at some part of its course into a close spiral, called the "primary coil. 
 Thus in Fig. 1 the wire x'" connected with the copper or negative plate e.p. of 
 the battery, E, joins the primary coil pr. c, and then passes on as y'", through 
 the "key" F, to the positive (zinc) plate z.p. of the battery. (In Fig. 9. p. 51) 
 the direction of the current from x io y through the primary coil P is shewn by 
 arrows; but in this figure complications are introduced which will be explained 
 hereafter.) Over this primary coil, but quite unconnected with it, slides another 
 coil, the secondary coil, s.c. ; the ends of the wire forming this coil, y" and x", are 
 continued on in the arrangement illustrated in the figure as y' and y, and as x' 
 and X and terminate in electrodes. If these electrodes are in contact or connected 
 with conducting material, the circuit of the secondary coil is said to be closed ; 
 otherwise it is open. 
 
 In such an arrangement it is found that at the moment when the_ primary 
 circuit is closed, i.e. when the primary current is "made" a secondary "induced" 
 current is, for an exceedingly brief period of time, set up in the secondary coU. 
 Thus in Fig. 1 when by moving the "key" F, y'" and x'" previously not in con- 
 nection with each other, are put into connection and the primary current thus 
 made, at that instant a current appears in the wires y" x" &c., but almost 
 immediately disappears. A similar almost instantaneous current is also developed 
 when the primary current is "broken," but not till then. So long as the primary 
 current flows with uniform intensity, no current is induced in the secondary coil. 
 It is only when the primary current is either made or broken, or suddenly varies in 
 intensity that a current appears in the secondary coil. In each case the current 
 is of very brief duration, gone in an instant almost, and may therefore be spoken of 
 as " a sliock," an induction shock ; being called a "making shock" when it is caused 
 by the making, and a "breaking shock" when it is caused by the breaking, of the 
 primary circuit. The direction of the current in the making shock is opposed 
 to that of the primary current; thus in the figure while the primary current flows 
 from x'" to y'", the induced making shock flows from y to x. The current of the 
 breaking shock on the other hand flows in the same direction as the primary current 
 from X to y, and is therefore in dhection the reverse of the making shock. 
 
 When the primary current is repeatedly and rapidly made and broken, the 
 secondary current being developed with each make and with each break, a rapidly 
 recurring series of alternating currents is developed in the secondary coil and passes 
 through its electrodes. We shall frequently speak of this as the interrupted induction 
 current, or more briefly the interrupted current. 
 
OlIAP. 11. J 
 
 77/ A' COXTRACTILE TTS^VES 
 
 41 
 
 ^rrcrrry^^ 
 
42 
 
 THE MUSCLE CURVE. 
 
 [Book i. 
 
 Fig. 1. 
 
 Diagram illustrating Apparatus arranged for Experiments with 
 Muscle and Nerve. 
 
 A. The moist chamber containing the muscle-nerve preparation. (The muscle- 
 nerve and electrode-holder are shewn on a larger scale in Fig. 2.) The muscle 
 m, supported by the clamp cl,, which firmly grasps the end of the femur/, is 
 connected by means of the S hook s and a thread with the lever i, placed below 
 the moist chamber. The nerve n, with the portion of the spinal column nl still 
 attached to it, is placed on the electrode-holder el, in contact with the wires 
 X, y. The whole of the interior of the glass case gl. is kept saturated with 
 moisture, and the electrode-holder is so constructed that a piece of moistened 
 blotting-paper may be placed on it without coming into contact with the nerve. 
 
 B. The revolving cylinder bearing the smoked paper on which the lever writes. 
 
 C. Du-Bois Eeymond's key arranged for short-circuiting. The wires x and y of 
 the electrode-holder are connected through binding screws in the floor of the 
 moist chamber with the wires x', y', and these are secured in the key, one on 
 either side. To the same key are attached the wires x" y" coming from the 
 secondary coils s. c. of the induction-machine D. This secondary coil can be 
 made to shde up and down over the primary coil^r. c, with which are con- 
 nected the two wires x!" and y'". x'" is connected directly with one pole, for 
 instance the copper pole c. p. of the battery E. y'" is carried to a binding 
 screw a of the Morse key F, and is continued as y^ from another binding 
 screw h of the key to the zinc pole z. p. of the battery. 
 
 Supposing everything to be arranged, and the battery charged, on depressing the 
 handle lia, of the Morse key F, a current will be made in the primary coil pr. c, 
 passing from c. p. through x'" to pr. c, and thence through y'" to a, thence to b, 
 and so through y"' to z. p. On removing the finger from the handle of F, a spring 
 thrusts up the handle, and the primary circuit is in consequence immediately 
 broken. 
 
 At the instant that the primary current is either made or broken, an induced 
 current is for the instant developed in the secondary coil s. c. If the cross bar h in 
 the du-Bois Eeymond's key be raised (as shewn in the thick line in the figure), the 
 wires x", x', x, the nerve between the electrodes and the wires y, y', y" form the 
 complete secondary circuit, and the nerve consequently experiences a making or 
 breaking induction-shock whenever the primary current is made or broken. If the 
 cross bar of the du-Bois Eeymond key be shut down, as in the dotted line h' in the 
 figure, the resistance of the cross bar is so slight compared with that of the nerve 
 and of the wires going from the key to the nerve, that the whole secondary (induced) 
 current passes from x" to y" (or from y" to as"), along the cross bar, and practically 
 none passes into the nerve. The nerve being thus "short-circuited," is not affected 
 by any changes in the current. 
 
(■IIAl'. II.] 
 
 THE CONTRACTILE TISSUES. 
 
 43 
 
 Fio. 2. Tho musclc-ncrvo preparation of Fi},'. 1, with the clamp, elcctrodcB, and 
 elcctrode-holdcr, arc licrc sliewu on a larger Bcalo. The letters as in l-'ig. 1. 
 
 The apparatus figured in V\'^^. 1 and 2 is intended merely to illustrate the general 
 method of studying muscular contraction; it is not to be supposed that the 
 details here given are universally adopted or indeed tho best for all purposes. 
 
 lever will by its movement indicate the extent and duration of the 
 shortening. It' the })oint of the lever be brought to bear on some 
 rapidly travelling surface, on which it leaves a mark (being lor this 
 purjjose armed with a pen and ink if the surface be plain paper, or 
 with a bristle or needle if the surface be smoked glass or paper), so 
 long as the muscle remains at rest the lever will describe an even 
 line. When, however, a contraction takes place, as when a single 
 induction-shock is sent through the nerve, some such curve as that 
 shewn in Fig. 3 will be described, the lever rising with the shorten- 
 
 Fio. 3. A Muscle-curve obtained bt means of the Pendulum MYooRAPn. 
 To be read from left to right. 
 
 o indicates the moment at which the induction-shock is sent into the nerve, h the 
 commencement, c the maximum, and d the close of the contraction. 
 Below the muscle-curve is the curve drawn by a tuning-fork making 180 double 
 vibrations a second, each complete curve representing therefore ;-i^ of a second. 
 It will be observed that the plate of the myograph was travelling more rapidly 
 towards the close than at the beginning of the contraction, as shewn by the greater 
 length of the vibration- curves. 
 
 ing of the muscle, and descending as the muscle returns to its natural 
 length. This is known as the ' muscle-curve.' In order to make 
 the ' muscle-curve ' complete, it is necessary to mark on the record- 
 ing surface the exact time at which the induction-shock is sent 
 into the nerve, and also to note the speed at which the recording 
 surface is travelling. These points are best effected by means of 
 the pendulum myograph, Fig. 4. 
 
 In this instrument a smoked glass plate, on which a lever writes, forms 
 the bob of a pendulum and consequently s\vings with it. The pendulum 
 with the glass plate attached being raised up, is suddenly let go. It swings 
 of courye to the opposite side, the glass plate travels through an arc of a 
 circle, and, the lever being stationary, the pomt of the lever describes an 
 arc on the glass plate. The rate at which the glass plate travels, i.e. tlie 
 time it takes for the lever-point to describe a line of a given length on 
 
44 
 
 THE MUSCLE CURVE. 
 
 [Book i. 
 
 Fig 4 The Pendulum Myograph. 
 
PiiAP. II.] THE CONTRACT ILK TlS.'iUES. 45 
 
 bearings nt G. The coutrivnuco3 by which tlio glftSB plate can he removed and 
 rophiced at pleasure arc not bIiowu. A Kccond glasH plate ho arranged that the 
 lirst glass plato may bo moved up and duwn without altering the Hwing of the 
 pendulum is also omitted, lieforo commencing an experiment the pendulum is 
 raised up (in the liguro to the right), and is kept in that position by the tooth a 
 catching on the si)riug-cateh b. On depressing tlie catch h the glass plate is set 
 free, swings into the new position indicated by the dotted lines, and is held in that 
 position by the tooth a' catching on the catch U. In the course of its swing the 
 tooth a' coming into contact with the projecting steel rod c, knocks it on one side 
 into the position indicated by the dotted lino c'. The rod c is in electric continuity 
 with the wire x of the primary coil of an induction-machine. The screw d is 
 similarly in electric continuity with the wire y of the same primary coil. The 
 screw il and the rod c arc armed with platinum at the points in which they are in 
 contact, and both are insulated by means of the ebonite block e. As long as c and d 
 are in contact the circuit of the primary coil to which x and y belong is closed. 
 When in its swing the tooth a' knocks c away from d, at that instant the circuit is 
 broken, and a 'breaking' shock is sent through the electrodes connected with the 
 secondary coil of the machine, and so through the nerve. The lever I, the end only 
 of which is shewn in the tigure, is brought to bear on the glass plate, and when at 
 rest describes a straight line, or more exactly an arc of a circle of large radius. The 
 tuuiug-fork /, the ends only of the two limbs of which are shewn in the figure 
 placed immediately below the lever, serves to mark the time. 
 
 the glass plate, may be calculated from the length of the pendulum, but it 
 is simpler and easier to place a vibrating tuning-fork immediately under 
 the ])oint of the lever. If the vibi-ations of the tuning-fork are known, 
 then the number of vibrations which are marked on the plate between any 
 two points on the line described by the lever gives the time taken by the 
 lever in passing from one point to the other. An easy aiTangement 
 permits the exact time at which the shock is sent through the nei-ve to 
 be marked on the line of the lever. To avoid the confusion of too 
 many markings on the plate the pendulum after describing an arc is 
 caught by a spring catch on the opposite side. 
 
 A complete muscle-curve, such as that shewn in Fig. 3, taken 
 from the gastrocnemius of a frog, teaches us the following facts : 
 
 1. That although the passage of the induced current from 
 electrode to electrode is practically instantaneous, its effect, measured 
 from the entrance of the shock into the nerve to the return of the 
 muscle to its natural length after the shortening, takes an appreci- 
 able time. In the figure, the whole curve from a to d takes up 
 about the same time as eighteen double vibrations of the tuning- 
 fork. Since each double vibration here represents j^ of a second, 
 the duration of the whole curve is ^^y sec. 
 
 2. In the first portion of this period, from a to 6, there is no 
 visible change, no shortening of the muscle, no raising of the lever. 
 
 91 
 
 3. It is not until h, that is to say after the lapse of :j^ 
 
 i.e. about 7^7- sec, that the shortening begins. The shortening as 
 shewn by the cur\e is at first slow, but soon becomes more rapid, 
 and then slackens again until it reaches a maximum at c ; the 
 whole shortening occupying about -^ sec. 
 
 4. Arrived at the maximum of shortening, the muscle at once 
 begins to relax, the lever descending at first slowly, then very 
 
46 THE MUSCLE CURVE. [Book i. 
 
 rapidly, and at last more slowly again, until at d the muscle has 
 reo-ained its natural length ; the whole return from the maximum 
 of contraction to the natural length occupying ■^, i.e. about -^ sec. 
 Thus a simple muscular contraction, a simple spasm or twitch as 
 it is sometimes called, produced by a momentary stimulus, such as 
 a single induction-shock, consists of three main phases : 
 
 1. A phase antecedent to any visible alteration in the muscle. 
 This phase, during which invisible preparatory changes are taking 
 place in the nerve and muscle, is called the ' latent period '. 
 
 2. A phase of shortening or, in the more strict meaning of the 
 word, contraction. 
 
 8. A phase of relaxation or return to the original length. 
 
 In the case we are considering, the electrodes are supposed to be 
 applied to the nerve at some distance from the muscle. Consequently 
 the latent period of the curve comprises not only the preparatory 
 actions going on in the muscle itself, but also the changes necessary 
 to conduct the immediate effect of the iiiduction-shock from the 
 part of the nerve between the electrodes, along a considerable 
 length of nerve down to the muscle. It is obvious that these latter 
 changes might be eliminated by placing the electrodes on the 
 muscle itself or on the nerve close to the muscle. If this were 
 done, the muscle and lever being exactly as before, and care were 
 taken that the induction-shock entered into the nerve at the new 
 spot, at the moment when the point of the lever had reached 
 
 Fig. 5. Curves illustrating the measueement of the Velocity of a Nervous 
 Impulse. (Diagranunatic.) To be read from left to right. 
 
 The same muscle-nerve preparation is stimulated (1) as far as possible from the 
 muscle, (2) as near as possible to the muscle; both contractions are registered by 
 the pendulum myograph exactly in the same way. 
 
 In (1) the stimulus enters the nerve at the time indicated by the line a, the con- 
 traction, shewn by the dotted line, begins at V ; the whole latent period therefore is 
 indicated by the distance from a to V . 
 
 In (2) the stimulus enters the nei-ve at exactly the same time a ; the contraction, 
 shewn by the unbroken Line, begins at h ; the latent period therefore is indicated by 
 the distance between a and h. 
 
 The time taken up by the nervous impulse in passing along the length of nerve 
 between 1 and 2 is therefore indicated by the distance between h and V, which may 
 be measured by the tuning-fork curve below. N.B. No value is given in the figure 
 for the vibrations of the tuning-fork, since the figure is diagrammatic, the distance 
 between the two curves, as compared with the length of either, having been purposely 
 exaggerated for the sake of simplicity. 
 
CiiAi'. 11. j THE COXTRACTILE TISSUES. 47 
 
 exactly the same point of the travellinfy surface as before, a curve 
 like that shewn by the plain line in Fig. 5 would be gained. It 
 resembles the first curve (indicated in the figure by a dotted line) 
 iu all points, except that the latent period is shortened : the con- 
 traction begins rather earlier. From this we learn two facts : 
 
 1. The greater part of the latent period is taken up by changes 
 in the muscle itself, preparatory to the actual visible shortening, 
 for the two latent periods do not differ much. Of course, even in 
 the second case, the latent period includes the changes going on in 
 the short piece of nerve still lying between the electrodes and the 
 muscular fibres. To eliminate this with a view of determining the 
 latent period in the muscle itself, the electrodes might be placed 
 directly on the muscle poisoned mth urari. If this were done, it 
 would still be found that the latent period was chiefly taken up 
 by changes in the muscular as distinguished from the nervous 
 elements. 
 
 2. Such diflerence as does exist indicates the time taken up by 
 the propagation, along the piece of nerve, of the changes set up at 
 the far end of the nerve by the induction-shock. These changes 
 we shall hereafter speak of as constituting a nervous impulse ; and 
 the above experiment shews that it takes some appreciable time 
 for a nervous impulse to travel along a nerve. In the figure the 
 difference between the two latent periods, the distance between h 
 and 6', seems almost too small to measure accurately ; but if a long 
 piece of nerve be used for the experiment, and the recording 
 surface be made to travel very fast, the diiference between the 
 duration of the latent period w'hen the induction-shock is sent in 
 at a point close to the muscle, and that when it is sent in at a 
 point as far away as possible fi'om the muscle, may be satisfactorily 
 meas.ured in fractions of a second. If the length of nerve between 
 the two points be accurately measured, the rate at which a nervous 
 impulse travels along the nerve to a muscle can thus be easily 
 calculated. This has been found to be in the frog about 28, and in 
 man about 33 metres per second. 
 
 Thus when a momentary stimulus, such as a single induction- 
 shock, is sent into a nerve connected with a muscle, the following 
 events take place : 
 
 1. The generation at the spot stimulated of a nervous impulse, 
 and the propagation of the impulse along the nerv'e to the muscle. 
 The time taken up by this varies according to the length of the 
 nerve but is always very short. 
 
 2. The setting up of certain molecular changes in the muscle, 
 unaccompanied by any visible alteration in its form, constituting 
 the latent period, and occupying on an average about y^th sec. 
 
 3. The shortening of the muscle up to a maximum, occupying 
 about Y^ sec. 
 
48 TETANIC CONTRACTIONS. [Book i. 
 
 4. The return of a muscle to its former length, occupying about 
 
 We have given what may be considered the average duration ^ 
 of each phase chiefly for the sake of shewing their relative propor- 
 tions. But it must be borne in mind that the duration of a 
 contraction diiffers in different animals and in different muscles of 
 the same animal ; in the rabbit the more deeply coloured so-called 
 "red" muscles have in their contraction a longer period than 
 have the pale muscles. The duration may also differ in the same 
 muscle under different conditions; moreover the duration of the 
 several phases may vary independently. Temperature has a marked 
 effect in varying the length of the muscle-curve, a high temperature 
 shortening, and a low temperature prolonging, the contraction, and 
 especially the third phase or relaxation. Fatigue also lengthens 
 the contraction as do also to a remarkable extent certain poisons 
 such as veratrin. An increase in the load which the muscle is 
 lifting, shortens the descending or return part of the curve and 
 increases the length of the latent period. All such influences will 
 be better studied when we come to speak more in detail of the 
 changes which take place in a muscle during contraction. Their 
 effects are only mentioned now in order that the reader may 
 thus early learn to conceive of even a simple muscular contraction 
 as a complex act, the several parts of which are variable, so that 
 many differing forms of a muscle-curve may be obtained under 
 different circumstances. 
 
 Tetanic Contractions. 
 
 If a single induction-shock be followed at a sufficiently short 
 interval by a second shock of the same strength, the first simple 
 
 Fig. 6. Teacing op a Double Muscle-Cueve. To be read from left to right. 
 
 While the muscle^ was engaged in the first contraction (whose conaplete course, 
 had nothing intervened, is indicated by the dotted line), a second induction-shock 
 was thrown in, at such a time that the second contraction began just as the first 
 was beginning to decline. The second curve is seen to start from the first, as does 
 the first from the base-line. 
 
 ^ The curve described in the previous text happened to have a rather long latent 
 period, and the lengthening to be of shorter instead of longer duration than the 
 shortening. 
 
 ^ In this and the other curves of this section the tracings figured were taken 
 from frog's muscle. 
 
ClIAP. II.] 
 
 THE (JO^ TRACTILE TISSUES. 
 
 49 
 
 contractiun or spasm will be followed by a second spasm, the two 
 bearing some such relation to each other as that shewn by the 
 curve in Fig. 6, where the interval between the two shocks was 
 just long enough to allow the first spasm to have passed its maxi- 
 nmra before the latent period of the second was over. It will be 
 observed that the second cui^ve is almost in all respects like the 
 first except that it starts, so to speak, from the first curve instead 
 of from the base-line. The second nervous impulse has acted on 
 the already contracted muscle, and made it contract again just as 
 it would have done if there had been no first impulse and the 
 muscle had been at rest. The two contractions are added together 
 and the lever raised nearly double the height it would have been 
 by either alone. A more or less similar result would occur if the 
 second contraction began at any other phase of the first. The 
 combined effect is, of course, gi'eatest when the second contraction 
 begins at the maximum of the first, being less both before and 
 afterwards. If in the same way a third shock follows the second 
 at a sufficiently short interval, a third curve is piled on the top of 
 the second. The same with a fourth, and so on. 
 
 Fig. 7. Muscle thrown into Tetanus, when the Primary Current of an In- 
 duction-machine IS REPEATEDLY BROKEN AT INTERVALS OF SIXTEEN IN A SECOND. 
 
 To be read from left to right. 
 
 The upper hue is that described by the muscle. The lower marks time, the 
 intervals between the elevations indicating seconds. The intermediate Hne shews 
 when the shocks were sent in, each mark on it corresponding to a shock. The lever, 
 which describes a straight line before the shocks are allowed to fall into the nerve, 
 rises almost vertically (the recording surface travelling in this case slowly) as soon 
 as the first shock enters the nerve at a. Having risen to a certain lieight, it begins 
 to fall again, but in its fall is raised once more by the second shock, and that to a 
 greater height than before. The third and succeeding shocks have similar effects, 
 the muscle continuing to become shorter, though the shortening at each shock is 
 less. After a while the increase in the total shortening of the muscle, though the 
 individual contractions are stiU visible, almost ceases. At h, the shocks cease to be 
 sent into the nerve ; the contractions almost immediately disappear, and the lever 
 forthwith commences to descend. The muscle being hghtly loaded, the descent is 
 very gradual ; the muscle had not regained its natural length when the tracing was 
 stopped. 
 
50 TETANIC CONTRACTIONS. [Book i. 
 
 When however repeated shocks are given it is found that the 
 height of each contraction is rather less than the preceding one, 
 and this diminution becomes more marked the greater the number 
 of shocks. Hence after a certain number of shocks, the succeeding 
 impulses do not cause any further shortening of the muscle, any 
 further raising of the lever, but merely keep up the contraction 
 already existing. The curve thus reaches a maximum, which it 
 maintains, subject to the depressing effects of exhaustion, so long 
 as the shocks are repeated. When these cease to be given, the 
 muscle returns, in the usual way, at first very rapidly, and then 
 more slowly, to its natural length. When the shocks do not succeed 
 each other too rapidly, the individual contractions may readily be 
 traced along the whole curve, as is seen in Fig. 7, where the 
 primary current of the induction-machine was repeatedly broken 
 at intervals of sixteen in a second. When the shocks succeed 
 each other more rapidly, the individual contractions, visible at first, 
 
 Fig. 8. Tetanus produced with the okdinaby Magnetic Inteeruptob of an In- 
 duction-machine. (Eecording surface travelling slowly.) To be read from left to right. 
 
 The interrupted current being thrown in at a the lever rises rapidly, but at 6 the 
 muscle reaches the maximum of contraction. This is continued -till c, when the 
 current is shut off and relaxation commences. 
 
 may become fused together and lost to view as the tetanus con- 
 tinues and the muscle becomes tired. When the shocks succeed 
 each other still more rapidly (the second contraction beginning in 
 the ascending portion of the first), it becomes difficult or impossible 
 to trace out the single contractions. The curve then described by 
 the lever is of the kind shewn in Fig. 8, where the primary current 
 of an induction-machine was rapidly made and broken by the 
 magnetic interruptor, Fig. 9. The lever, it will be observed, rises 
 at a after the latent period (which is not marked), first rapidly, 
 and then more slowly, in an apparently unbroken line to a maxi- 
 mum at about h, maintains the maximum so long as the shocks 
 continue to be given, and when these cease to be given, as at c, 
 gradually descends to the base-line. This condition of muscle, 
 brought about by rapidly repeated shocks, this fusion of a number 
 of simple spasms into an apparently smooth, continuous effort, is 
 known as tetanus, or tetanic contraction. The above facts are most 
 clearly shewn when induction-shocks, or at least galvanic currents 
 in some form or other, are employed. They are seen, however, 
 whatever be the form of stimulus employed. Thus in the case of 
 
CUAI'. II.] 
 
 THE CONTRACTILE TISSUES. 
 
 51 
 
 mechanical stimuli, while a single blow may cause a single spasm, 
 a pronounced tetanus may be obtained by rapidly striking suc- 
 cessively fresh portions of a nerve. With chemical stimulation, as 
 when a nerve is dipped in acid, it is impossible to secure a 
 momentary application ; hence tetanus, generally irregular in 
 character, is the normal result of this mode of stimulation. In the 
 living body, the contractions of the striated muscles, brought about 
 either by the will or by reflex action, are generally tetanic in 
 character. Even very short sharp movements, such as a sudden 
 jerk of the limbs, are in reality examples of tetanus of short 
 duration. 
 
 Fig. 9. The Magnetic Intebbcpxob, 
 
 The figure is introduced to illustrate the action of this instrument as commonly 
 used by physiologists. 
 
 The two wiies x and y from the battery are connected with the two brass pillars 
 a and d by means of screws. Directly contact is thus made the current, iacQcated 
 in the figure by the thick iaterrupted hne, passes in the direction of the arrows, up 
 the pillar a, along the steel spring h, as far as the screw c, the point of which, 
 armed with platinum, is in contact with a small platinum plate on b. The current 
 passes from b through c and a connecting wire into the primary coil p. Upon its 
 entering into the primary coU, an induced (making) current is for the instant de- 
 veloped in the secondary coil (not shewn in the figure). From the primary coil^ 
 the current passes, by a connecting wire, through the double spiral, m, and, did 
 nothing happen, would continue to pass from m by a connecting wire to the pillar d, 
 and so by the wire y to the battery. The whole of this course is indicated by the 
 thick interrupted hne with its arrows. 
 
 As the current howeyer passes through the spirals m, the iron cores of these are 
 made magnetic. They in consequence draw down the iron bar e, fixed at the end of 
 the spring b, the flexibility of the spring allowing this. But when e is drawn down, 
 the platinum plate on the upper surface of h is also drawn away from the screw c, 
 and a similar platinum plate on the under surface of b is brought into contact with 
 the platinum-armed point of the screw /, the screws being so arranged that this 
 takes place. In consequence of this change the current can no longer pass from h to 
 
 -t— 2 
 
52 TETANIC CONTRACTIONS. [Book i. 
 
 c. Ou the contrary, it passes from b to /, and so down the pillar d, in the direction 
 mdicated by the thin interrupted line, and out to the battery by the wire y. Thus 
 the current is 'short-circuited' from the primary coil; and the instant that the 
 current is thus cut off from the primary coU, an induced (breaking) current is for the 
 moment developed in the secondary coil. But the current is cut off not only from 
 the primary coil, but also from the spirals m ; in consequence their cores cease to be 
 magnetised, the bar e ceases to be attracted by them, and the spring 6, by virtue of 
 its elasticity, resumes its former position in contact with the screw c. This return 
 of the spring however re-establishes the current in the primary coil and in the 
 spirals, and the spring is drawn down, to be released once more in the same manner 
 as before. Thus as long as the current is passing along x, the contact of h is 
 constantly alternating between c and/, and the current is constantly passing into 
 and being shut off from p, the periods of alternation being determined by the periods 
 of vibration of the spring b. With each passage of the current into, or withdrawal 
 from the primary coil, an induced (making and, respectively, breaking) shock is de- 
 veloped in a secondary coil. 
 
 When it has once been realized that an ordinary tetanic 
 muscular movement is essentially a vibratory movement, that 
 the apparently rigid and firm muscular mass is really the subject 
 of a whole series of vibrations, a series namely of simple spasms, it 
 will be readily understood why a tetanized muscle, like all other 
 vibrating bodies, gives out a sound. That a contracting (tetanized) 
 muscle does give out a sound, the so-called muscular sound, is 
 easily proved by listening with a stethoscope to a contracted 
 biceps, or by stopping the ears and listening to the contractions 
 of one's own masseter and temporal muscles. 
 
 When a muscle is thrown into tetanus by interrupted shocks 
 applied directly to the nerve or to the muscle, the note is the same 
 as that of the interrupter determining the number of the shocks. 
 This is naturally the case, since the note of the muscle-sound is 
 determined by the rapidity of the spasms or vibrations which go to 
 make up the tetanus, and these are determined by the rapidity 
 with which the stimulus is repeated. 
 
 When a muscle is thrown into tetanus by the will or by reflex 
 action or by direct stimulation of the spinal cord, in fact, in any 
 way through the action of the central nervous system, the same 
 note is always heard, viz. one of 36 to 40 vibrations per second, 
 which however is probably a harmonic of a lower note indicating 
 that the muscle is really vibrating 19 or 20 times a second. 
 
 It need hardly be said that a single muscular contraction, a 
 single vibration, cannot cause a muscular sound. 
 
 The general observations which have been described in this 
 section may, when proper precautions are taken, be carried out on 
 a muscle-nerve preparation from a frog for a very considerable 
 time after its removal from the body. After some hours however, 
 or it may be days, the length of time varying according to 
 circumstances, it will be found that no stimulus, however powerful, 
 will cause any contraction, when applied either to the nerve or to 
 the muscle. Both muscle and nerve are then said to have lost 
 their irritability ; and a short time afterwards the muscle may be 
 observed to pass into a peculiar condition known as rigor mortis, 
 
OiiAP. II.] rUK aONTRACTILK TJHSUES 53 
 
 in which it loses all the suppleness and extensibility characteristic 
 of the living irritable nuiscle. The causes of this loss of irritability 
 as well as the features and nature of this rigor mortis we shall 
 study in detail presently. 
 
 The muscles and nerves of a mammal, or indeed of any warm- 
 blooded animal, lose their irritability, and the muscles become 
 rigid in a very short time (it may be a few minutes) after removal 
 from the body. Hence these are less suitable for experiments 
 than the muscles and nerves of the frog, though their general 
 phenomena are exactly the same. 
 
 We must now attempt to study in greater detail the changes 
 which take place in a muscle and nerve during the contraction of 
 the former and the passage of an impulse along the latter, with a 
 view to the better understanding of both events. 
 
SEC. 2. THE CHANGES IN A MUSCLE DURING 
 MUSCULAR CONTRACTION. 
 
 The Change in Form. 
 
 We have seen that at the close of the latent period the muscle 
 shortens, that is, each fibre shortens, at first slowly, then more 
 rapidly, and lastly more slowly again. The shortening (which in 
 severe tetanus may amount to three-fifths of the length of the 
 muscle) is accompanied by an almost exactly corresponding thicken- 
 ing, so that there is hardly any actual change in bulk. If a muscle 
 be placed horizontally, and a lever laid upon it, the thickening of 
 the muscle will raise up the lever, and cause it to describe on a 
 recording surface a curve exactly like that described by a lever 
 attached to the end of the muscle. There appears to be a minute 
 diminution of bulk not amounting to more than one thousandth. 
 
 If a long muscle of parallel fibres, poisoned with urari, so as to 
 eliminate the action of its nerves, be stimulated at one end, the 
 contraction may be seen, almost with the naked eye, to start from 
 the end stimulated, and to travel along the muscle. If two levers 
 be made to rest on, or be suspended from, two points of such a 
 muscle placed horizontally, the points being at a known distance 
 from each other and from the point stimulated, the progress of the 
 contraction may be studied. It is found that the contraction 
 starting from the spot stimulated, passes along the muscle in the 
 form of a wave diminishing in vigour as it proceeds. The velocity 
 with which this contraction wave travels in the muscles of the 
 frog is about 3 or 4 metres a second ; and since it takes, in round 
 numbers, from about 0".5 to "1 sec. for the contraction to pass over 
 any point of the fibre, the wave-length of the contraction wave 
 must be from about 200 to 400 mm. 
 
 In the muscles of a mammal laid bare for the purposes of 
 experiment the velocity does not seem to be very different from 
 that in the frog ; but in the intact muscles in their normal con- 
 
Chak II. ] 
 
 THE COS TRACT ILK TISSUES 
 
 55 
 
 dition in tlie livinor body, it is probably .somewhat greater, aud the 
 wave also ])rubably travels with undiminished velocity and vigour 
 to the end ol the fibre. In general, the velocity with which the 
 contraction wave travels, like the duration and character of th<- con- 
 traction, varies under different circumstances, being much influenced 
 by temperature, by the action of drugs, and especially by those 
 complex intrinsic changes which we speak of as fatigue or ex- 
 haustion. 
 
 Seeing that the extreme limit of the length of a muscular fibre 
 is about 30 or 40 mm., it is evident that even when the stimulation 
 begins at one end and the wave travels at the more rapid rate, 
 the whole fibre is not only in a state of contraction at the same 
 time, but almost in the same phase of the contra>ction wave. In 
 an ordinary contraction occurring in the living body the stimulus 
 is never applied to one end of the fibre ; the nervous impulse 
 which in such cases acts as the stimulus to the muscle, falls into 
 the fibre at about its middle, where the nerv^e ends in an end-plate, 
 and the contraction wave starting from the end-plate travels along 
 the muscular fibre in both directions. In such a case therefore, 
 still more even than in the urarised muscle stimulated artificially 
 at one end, must the whole fibre be occupied at the same time by 
 the wave of contraction. 
 
 Changes in microscopic stmctnre. When portions of living 
 irritable muscle are examined under the microscope, contraction 
 waves similar to those just described, but feebler and of shorter 
 
 Fig. 10. MCSCULAB fibre rNTDEEGOIXO COXTRACTION. 
 
 The muscle is that of Telephoms melanurus treated -with osmic acid. The fibre 
 at c is at rest, at a the contraction begins, at b it has reached its maximum. The 
 right-liand side of the figure shews the same fibre as seen in polarized light. (After 
 Engeknann.) 
 
56 THE CHANGE OF FORM. [Book i. 
 
 length, may be observed passing along tbe fibres. By appropriate 
 treatment with osmie acid or other reagents, these short contraction 
 waves may be fixed, and the structure of the contracted portion 
 compared at leisure with that of the portions of the fibre at rest. 
 In Fig. 10, representing a fibre of the muscle of an insect (in which 
 these changes can be more satisfactorily studied than in vertebrate 
 muscle), the contraction wave begins near a, and has reached 
 about its maximum at h, while at c the fibre is at rest, the 
 contraction wave not having reached it (or having passed over 
 it, for the beginning and end of the wave are exactly alike). It 
 wiU be seen that at h, each disc of the fibre is shorter and broader 
 than at c. Further, while at c the dim band x is conspicuous, and 
 the light band y, with its accessory markings y , is together lighter 
 than the dim band x, at h in the fully contracted part of the fibre 
 the dim band appears light as compared with the black line y 
 occupying the middle of the previously light band. In the 
 contracted muscle then there is a reversal of the state of things 
 in the resting muscle, the light band (or part of the light band) 
 of the latter in contracting becomes dark, and the dim band of 
 the latter becomes by comparison light. Between rest and full 
 contraction there is an intermediate stage, as at d, in which the 
 distinction between dim and bright bands seems to be largely lost. 
 The subject however is one offering peculiar difficulties in the way 
 of investigation, and while most, though not all, observers agree in 
 the broad facts which have just been stated, there is great diversity 
 of opinion concerning further details and especially as to the 
 interpretation of the various appearances observed. The accessory 
 markings in the middle of the light band have, in particular, been 
 the subject of controversies into which we cannot enter here. 
 
 When the fibre is examined in polarised light it is seen that 
 the dim band is anisotropic, and the light band isotropic. This 
 is the case during all the phases of the contraction. At no period 
 is there any confusion between the anisotropic and isotropic material ; 
 these maintain their relative positions, both become shorter and 
 broader; but it will be observed that the isotropic substance 
 diminishes in height to a much greater extent than does the 
 anisotropic substance. The latter in fact appears to increase in 
 bulk at the expense of the former. 
 
 Kelaxatiou. The shortening as we have seen is followed by a 
 relaxation, the muscle returning to its original length. When an 
 appropriate weight is attached to the muscle this return is generally 
 complete, the curve speedily rejoining, as shewn in Fig. 3, the base 
 line from which it started; but when no load is used and the 
 muscle therefore is acted upon by its own weight and that of a 
 very light lever only, the return is incomplete ; the curve, though 
 descending near to, fails to touch the base line and runs nearly 
 parallel to it for some considerable distance. The relaxation 
 is therefore obviously assisted by the extendiag force of the load ; 
 
CnAP. II.] THE CONTRACTILE TISSUES. 57 
 
 but, nevertheless, is iii the main the result of intrinsic processes 
 going on in the muscle, the reverse of those leading to the shorten- 
 ing. The return of the muscle to its elongated condition, is 
 not a mere passive stretching, after the causes leading to the 
 shortening have passed away ; it like the shortening itself is a 
 manifestation of activity. And hence we find that the complete- 
 ness of the relaxation is dependent on the complex changes which 
 we speak of as the nutrition of the muscle. Thus in their natural 
 position in the living body, muscles, owing to their vigorous nu- 
 trition, assisted by tlie fact that their anatomical disposition keeps 
 them always on the stretch, return completely to their original 
 length, after even powerful and prolonged contractions. In a 
 muscle out of the body, on the other hand, even when loaded, re- 
 peated successive contractions frequently result in the failure to 
 achieve complete relaxation becoming very conspicuous ; and the 
 tetanus cur\'es. Figs. 6 and 7, shew very strikingly this shortcoming, 
 which is often spoken of as the 'contraction remainder.' 
 
 We may speak of the relaxation as the result of an elastic 
 reaction, but only in the sense that the elastic qualities of the 
 muscle, at any moment, are the expression of deep-seated and con- 
 tinually varying molecular changes going on in the muscular sub- 
 stance. And in this connection attention may be called to a peculiar 
 physical character of contracting muscle. Living muscle at rest 
 is very extensible, but when stretched returns after the extending 
 cause has been removed, rapidly and completely to its former 
 length. In physical language muscle is spoken of as possessing 
 slight but perfect elasticity. It might be imagined that during a 
 contraction this extensibility would be diminished in order that none 
 of the resistance which the muscle had to overcome, no part of the 
 weight for instance which had to be Kfted, should be employed in 
 stretching the muscle itself and thus lead to an apparent waste of 
 energy. On the contrary we find that during a contraction there 
 is an increase of extensibility ; thus if a muscle at rest be loaded 
 \\-ith a given weight, say 50 gi-ammes, and its extension observed, 
 and be then while unloaded thrown into tetanus, and the load 
 applied during the tetanus, the extension in the second case will 
 be distinctly greater than in the first. During the contraction 
 there is so to speak a greater mobility of the muscular molecules, 
 and though this greater mobility may have its advantages, the 
 loaded muscle has in contracting to overcome its own increased 
 tendency to lengthen on extension before it can produce any effect 
 on the weight which it has to lift. 
 
 The elasticity and extensibility of the muscular substance is how- 
 ever a complicated and difficult subject, and it will be sufficient to 
 reassert that it is essentially a vital property, being dependent, like 
 the irritability of the muscular substance, on certain nutritive factors. 
 As the muscular substance becomes weary with too much work or 
 impoverished by scanty nutrition, its elasticity suffers pan passu 
 
58 
 
 MUSCLE CURRENTS. 
 
 [Book i. 
 
 with its irritability. The exhausted muscle when extended does 
 not return so readily to its proper length as the fresh active muscle, 
 and, as we shall see, the dead muscle does not return at all. 
 
 Electrical Changes. 
 
 Muscle-currents. If a muscle be removed in an ordinary 
 manner from the body, and two non-polarisable electrodes \ con- 
 
 ^Mjjuu^^ 
 
 Fig. 11. NoN-PoLAEisABLE Electbodes. 
 
 a, the glass tube; z, the amalgamated zinc slips connected with their respective 
 wires; z. s., the zinc sulphate solution; cli. c, the plug of china clay; c', the portion 
 of the china-clay plug projecting from the end of the tube ; this can be moulded into 
 any required form. 
 
 nected with a delicate galvanometer of many convolutions, be 
 placed on two points of the surface of the muscle, a deflection of 
 the galvanometer will take place indicating the existence of a 
 current passing through the galvanometer from the one point of 
 the muscle to the other, the direction and amount of the deflection 
 varying according to the position of the points. The 'muscle- 
 currents' thus revealed are seen to the best advantage when the 
 muscle chosen is a cylindrical or prismatic one with parallel fibres, 
 and when the two tendinous ends are cut off by clean incisions at 
 right angles to the long axis of the muscle. The muscle then 
 presents a (artificial) transverse section at each end and a longi- 
 tudinal surface. We may speak of the latter as being divided 
 into two equal parts by an imaginary transverse line on its surface 
 
 1 These (Fig. 11) consist essentially of a slip of thoroughly amalgamated zinc 
 dipping into a saturated solution of zinc sulphate, which in turn is brought into 
 connection with the nerve or muscle by means of a plug or bridge of china-clay 
 moistened with normal sodium chloride solution ; it is important that the zinc should 
 be thoroughly amalgamated. This form of electrodes gives rise to less polarisation 
 than do simple platinum or copper electrodes. The clay affords a connection be- 
 tween the zinc and the tissue which neither acts on the tissue nor is acted on by the 
 tissue. Contact of any tissue with copj^er or platinum is in itself sufficient to de- 
 velope a current. 
 
Oh A I'. II ] 
 
 THE CONTRACTILE TT.'^^UES. 
 
 59 
 
 called the 'equator ' containincy all the points of the surface midway 
 between the two e ids. Fi<:j. 12 is a diap^ainmatic representation of 
 such a muscle, the line ah being the equator. In such a mu.scle the 
 development of the muscle-currents is found to be as follows. 
 
 The greatest deflection is observed when one electrode is placed 
 
 Fig. 12. Diagram illustrating the electric currents of nerve and muscle. 
 
 Being purely diagrammatic, it may serve for a piece either of nerve or of muscle, 
 except that the currents at the transverse section cannot be shewn in a nerve. The 
 arrows shew the direction of the current through the galvanometer. 
 
 ah the equator. The strongest currents are those shewn by the dark lines, as 
 from a, at equator, to x ov to ij at the cut ends. The current from a to c is weaker 
 than from a to y, though both, as shewn by the arrows, have the same direction. A 
 current is shewn 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 shewn at gh. From « to b or from 
 X to y there is no current, as indicated by the dotted Unes. 
 
 at the mid-point or equator of the muscle, and the other at either 
 cut end ; and the deflection is of such a kind as to shew that posi- 
 tive currents are continually passing from the equator through the 
 galvanometer to the cut end, that is to say, the cut end is negative 
 relatively to the equator. The currents outside the muscle may be 
 considered as completed by currents in the muscle from the cut end 
 to the equator. In the diagram Fig. 12, the aiTows indicate the 
 direction of the currents. If the one electrode be placed at the 
 equator ah, the effect is the same at whichever of the two cut ends cc 
 or y the other is placed. If, one electrode remaining at the equator, 
 the other be shifted from the cut end to a spot c nearer to the 
 equator, the current continues to have the same direction, but is of 
 less intensity in proportion to the nearness of the electrodes to each 
 other. If the two electrodes be placed at unequal distances e and/, 
 one on either side of the equator, tliere will be a feeble current from 
 the one nearer the equator to the one farther off, and the current will 
 be the feebler, the more nearly they are equidistant from the equator. 
 
60 MUSCLE CURRENTS. [Book i. 
 
 If they are quite equidistant, as for instance when one is placed on 
 one cut end x, and the other on the other cut end y, there will be 
 no current at all. 
 
 If one electrode be placed at the circumference of the transverse 
 section and the other at the centre of the transverse section, there 
 will be a current through the galvanometer from the former to the 
 latter; there will be a current of similar direction but of less intensity 
 when one electrode is at the circumference g of the transverse section 
 and the other at some point li nearer the centre of the transverse 
 section. In fact, the points which are relatively most positive and 
 most negative to each other are points on the equator and the two 
 centres of the transverse sections ; and the intensity of the current 
 between any two points will depend on the respective distances 
 of those points from the equator and from the centre of the 
 transverse section. 
 
 Similar currents may be observed when the longitudinal surface 
 is not the natural but an artificial one ; indeed they may be witnessed 
 in even a piece of muscle provided it be of cylindrical shape and 
 composed of parallel fibres. 
 
 These 'muscle-currents' are not mere transitory currents dis- 
 appearing as soon as the circuit is closed ; on the contrary they 
 last a very considerable time. They must therefore be maintained 
 by some changes going on in the muscle, by continued chemical 
 action in fact. They disappear as the irritability of the muscle 
 vanishes, and are connected with those nutritive, so-called vital 
 changes which maintain the irritabihty of the muscle. 
 
 Muscle-currents such as have just been described, may, we repeat, 
 be observed in any cylindrical muscle suitably prepared, and similar 
 currents, with variations which need not be discussed here, may be 
 seen in muscles of irregular shape with obliquely or otherwise ar- 
 ranged fibres. And du Bois-Keymond, to whom chiefly we are 
 indebted for our knowledge of these currents, has been led to re- 
 gard them as essential and important properties of living muscle. 
 He has moreover advanced the theory that muscle may be con- 
 sidered as composed of electro-motive particles or molecules, each 
 of which like the muscle at large has a positive equator and 
 negative ends, the whole muscle being made up of these molecules 
 in somewhat the same way, (to use an illustration which must not 
 however be strained or considered as an exact one) as a magnet 
 may be supposed to be made up of magnetic particles each with its 
 north and south pole. 
 
 There are reasons however for thinking that these muscle-currents 
 have no such fundamental origin, that they are in fact of surface 
 and indeed of artificial origin. Without entering largely into the 
 controversy on this question, the following important facts may be 
 mentioned. 
 
 1. When a muscle is examined while it still retains untouched 
 its natural tendinous terminations, the currents are much less than 
 
Cmap. ii] the contractile TISSUES. Gl 
 
 when artiticial transverse sections have been made ; the natural 
 tendinous end is less negative than the cut surface. But the 
 tendinous end becomes at once negative when it is di])ped in 
 water or acid, indeed when it is in any way injured. The less 
 roughly in fact a muscle is treated the less evident are the muscle- 
 ciu-rents, and it has been maintained that if adequate care be 
 taken to maintain a muscle in an absolutely natural condition no 
 such currents as those we have been describing exist at all. 
 
 2. Englemann has shewn that the surface of the uninjured in- 
 active* ventricle of the frog's heart is isoelectric, i. e. that no current 
 is obtained when the electrodes are placed on any two points of the 
 surface. If however any part of the surface be injured, or if the 
 ventricle be cut across so as to expose a cut surface, the injured spot 
 or the cut surface becomes at once most powerfully negative towards 
 the uninjured surface, a strong current being developed which passes 
 through the galvanometer from the uninjured surface to the cut 
 surface or to the injured spot. The negativity thus developed in 
 a cut surface passes off in the course of some hours, but may be 
 restored by making a fresh cut and exposing a fresh surface. 
 
 Now, when a muscle is cut or injured the substance of the fibres 
 dies at the cut or injured surface. And many physiologists, among 
 whom the most prominent is Hermann, have been led by the above 
 and other facts to the conclusion that muscle-currents do not exist 
 naturally in untouched muscles, that the muscular substance is 
 naturally, when living, isoelectric, but that whenever a portion of 
 the muscular substance dies, it becomes while dying negative to the 
 living substance, and thus gives rise to currents. They explain the 
 typical currents (as they might be called) manifested by a muscle with 
 a natural longitudinal surface and artificial transverse sections, by 
 the fact that the dying cut ends are negative relatively to the rest 
 of the muscle. 
 
 Du Bois-Reymond and those vn.\\\ him offer special explanations 
 of the above facts and of other objections which have been urged 
 against the theory of naturally existing electro-motive molecules. 
 Into these we cannot enter here. We must rest content with the 
 statement that in an ordinary muscle currents such as have been 
 described may be mtnessed, but that strong arguments may be 
 adduced in favour of the view that these currents are not ' natural ' 
 phenomena but essentially of artificial origin. It will therefore be 
 best to speak of them as ' currents of rest.' 
 
 Negative variation of the Muscle-current. The controversy 
 whether the " currents of rest " observable in a muscle be of natural 
 origin or not, does not affect the truth or the importance of the 
 fact that an electrical change takes place in a muscle whenever it 
 enters into a contraction. When currents of rest are observable in 
 a muscle these are found to undergo a diminution at the onset of a 
 
 ^ The necessity of its being inactive will be seen subsequently. 
 
62 NEGATIVE VARIATION. [Book i. 
 
 contraction, and this diminution is spoken of as ' the negative 
 variation ' of the currents of rest. The negative variation may be 
 seen when a muscle is thrown into a single contraction, but is most 
 readily shewn when the muscle is tetanized. Thus if a pair of 
 electrodes be placed on a muscle, one at the equator, and the 
 other at or near the transverse section, so that a considerable 
 deflection of the galvanometer needle, indicating a considerable 
 current of rest, be gained, the needle of- the galvanometer will, 
 when the muscle is tetanized by an interrupted current sent 
 through its nerve (at a point too far from the muscle to allow 
 any escape of the current into the electrodes connected with the 
 galvanometer), swing back towards zero ; it returns to its original 
 deflection when the tetanizing current is shut off. 
 
 Not only may this negative variation be shewn by the galvano- 
 meter, but it, as well as the current of rest, may be used as a 
 galvanic shock and so employed to stimulate a muscle, as in the 
 experiment known as ' the rheoscopic frog.' For this purpose the 
 muscles and nerves need to be very irritable and in thoroughly 
 good condition. Two muscle-nerve preparations A and B having 
 been made and each placed on a glass plate for the sake of insulation, 
 the nerve of the one B is allowed to fall on the muscle of the 
 other A in such a way that one point of the nerve comes in 
 contact with the equator of the muscle, and another point with 
 one end of the muscle or with a point at some distance from the 
 equator. At the moment the nerve is let fall and contact made, a 
 current, viz. the 'current of rest' of the muscle A, passes through 
 the nerve ; this acts as a stimulus to the nerve, and so causes 
 a contraction in the muscle connected with the nerve. Thus the 
 muscle A acts as a battery, the completion of the circuit of which 
 by means of the nerve of B serves as a stimulus, causing the muscle 
 B to contract. 
 
 If while the nerve of B is still in contact with the muscle of A, 
 the nerve of the latter is tetanized with an interrupted current, 
 not only is the muscle of A thrown into tetanus but also that of 
 B; the reason being as follows. At each spasm of which the 
 tetanus of A is made up, there is a negative variation of the 
 muscle-current of A. Each negative variation in the muscle- 
 current of A serves as a stimulus to the nerve of B, and is hence 
 the cause of a spasm in the muscle of B ; and the stimuli following 
 each other rapidly, as being produced by the tetanus of A they must 
 do, the spasms in B to which they give rise are also fused into 
 a tetanus in B. B in fact contracts in harmony with A. This 
 experiment shews that the negative variation accompanjdng the 
 tetanus of a muscle, though it causes only a single swing of the 
 galvanometer, is really made up of a series of negative variations, 
 each single negative variation corresponding to the single spasms 
 of which the tetanus is made up. 
 
 But an electrical change may be manifested even in cases when 
 
Chap, ii] THE COyTRACTlLK TISSUES. 63 
 
 110 currents of rest exist. We have stated (p. Gl) that the surface 
 of the iininjurt-'d inactive ventricle of the frog's heart is isijeh-ctric, 
 110 currents being observed when the electrodes of a galvanometer 
 are placed on two points of the surface. Nevertheless a most 
 distinct current is developed whenever the ventricle contracts. 
 This may be shewn either by tlie galvanometer or by the rheos- 
 copic frog. If the nerve of an irritable muscle-nerve preparation 
 be laid over a pulsating ventricle, each beat is responded to by 
 a spasm of the muscle of the preparation. In the case of ordinary 
 muscles too instances occur in w hich it seems impossible to regard 
 the electrical change manifested during the contraction as the 
 mere diminution of a preexisting current. 
 
 Accordingly Hermann and those who with him deny the 
 existence of ' natural ' muscle-currents speak of a muscle as de- 
 veloping during a contraction a ' current of action,' occasioned as 
 they believe by the muscular substance as it is entering into the 
 state of contraction becoming negative towards the muscular 
 substance which is still at rest, or has returned to a state of 
 rest. In fact, they regard the negativity of muscular substance 
 as characteristic alike of beginning death and of a beginning 
 contraction. So that in a muscular contraction a wave of 
 negativity, starting from the end-plate when indirect, or from 
 the point stimulated when direct stimulation is used, passes 
 along the muscular substance to the ends or end of the fibre. 
 We cannot however enter more fully here into a discussion of this 
 difficult subject. 
 
 Whichever view be taken of the nature of these muscle-currents, 
 and of the electric change dunng contraction, whether we regard 
 that change as a ' negative variation ' or as a ' current of action,' 
 it is important to remember that it takes place entirely during the 
 latent period. It is not in any way the result of the change of 
 form, it is the forerunner of that change of form. Just as a 
 nervous impulse passes down the nerve to the muscle without any 
 visible changes, so a molecular change of some kind, unattended 
 by any visible events, known to us, at present, only by an electrical 
 change, runs along the muscular fibre from the end-plates to the 
 terminations of the fibre, preparing the way for the visible change of 
 form which is to follow. This molecular invisible change is the work 
 of the latent period, and careful obsersations have shewn that it, 
 like the visible contraction which follows at its heels, travels along 
 the fibre from a spot stimulated towards the ends of the fibres, in 
 the form of a w ave having about the same velocity as the contrac- 
 tion, viz. about 3 metres a second^ 
 
 1 In the muscles of the fiog; but as we have seen ha\-ing probably a higher 
 velocdty in the intact mammalian muscles, within the hving body, and var}"ing 
 according to circumstances. 
 
64 CHEMICAL CHANGES. [Book i. 
 
 Chemical Changes. 
 
 Before we attack the important problem, What are the chemical 
 changes concerned in a muscular contraction ? we must study in 
 some detail the chemical features of muscle at rest. And here we 
 are brought face to face with the chemical differences between 
 living and dead muscles. All muscles, within a certain time after 
 removal from the body, or while still within the body, after 
 'general' death of the body, lose their irritability. The loss of 
 irritability, even when rapid, is gradual, but is succeeded by au 
 event which is somewhat more sudden, viz. the entrance into the 
 condition known as rigor mortis, the occurrence of which is marked 
 by the following features. The muscle, previously possessing a 
 certain translucency, becomes much more opaque. Previously very 
 extensible and elastic, it becomes much less extensible and at the 
 same time loses its elasticity; the muscle now requires considerable 
 force to stretch it, and when the force is removed, does not, as 
 before, return to its natural length. To the touch it has lost much 
 of its former softness, and becomes firmer and more resistent. The 
 entrance into rigor mortis is characterised by a shortening or con- 
 traction, which may, under certain circumstances, be considerable. 
 The energy of this contraction is not great, so that when opposed, 
 no actual shortening takes place. When rigor mortis has been fully 
 developed, no muscle-currents whatever are observed. The onset 
 of this rigidity may be considered as the token of the death of the 
 muscle itself. As we shall see, the chemical features of the dead 
 rigid muscle are strikingly different from those of the living 
 muscle. 
 
 If a dead muscle, from which all fat, tendon, feiscia, and con- 
 nective tissue have been as much as possible removed, and which 
 has been freed from blood by the injection of saline solution, be 
 minced and repeatedly washed with water, the washings will 
 contain certain forms of albumin and certain extractive bodies, 
 of which we shall speak directly. When the washing has been 
 continued until the wash-water gives no proteid reaction, a large 
 portion of muscle will still remain undissolved. If this be treated 
 with a 10 p. c. solution of a neutral salt, ammonium chloride being 
 the best, a large portion of it will become imperfectly dissolved 
 into a viscid fluid which filters with difficulty. If the viscid 
 filtrate be allowed to fall drop by drop into a large quantity of 
 distilled water, a white flocculent matter will be precipitated. 
 This flocculent precipitate is myosin. It is a proteid, giving the 
 ordinary proteid reactions, and having the same general elementary 
 composition as other proteids. It is soluble in dilute saline solutions, 
 especially those of ammonium chloride, and may be classed in 
 the globulin family, though it is not so soluble as paraglobulin. 
 Dissolved in saline solutions it readily coagulates when heated, i. e. 
 
Chap, ii.] THE CONTRACTILE TISSUES. 05 
 
 is converted into coagulated proteid", and it is worthy of notice 
 that it coac,nilates at a lower temperature, viz. 5o' — GO'C, than 
 does soruni-albuniin, paraglobulin and many other proteids; it is 
 precipitated and after long action coagulated by alcohol, and is 
 precipitated by an excess of sodium chloride. By the action 
 of dilute acids it is very readily converted into what is called 
 syntonin or acid-albumin'^ by the action of dilute alkalis into 
 alkali-albumin. Speaking generally it may be said to be inter- 
 mediate in its character between fibrin and globulin. On keeping, 
 and especially on drying, its solubility is much diminished. 
 
 Of the substances which arc left in washed muscle, from which 
 the myosin has thus been extracted by ammonium chloride solution, 
 little is known. If washed muscle be treated directly with dilute 
 hydrochloric acid, the greater part of the material of the muscle 
 passes at once into syntonin. The quantity of syntonin thus 
 obtained may be taken as representing the quantity of myosin 
 previously existing in the muscle. The portion insoluble in dilute 
 hydrochloric acid consists in part of the substance of the sarco- 
 Icmma, of the nuclei, and of the tissue betAveen the bundles, and in 
 part probably of certain structural elements of the fibres themselves. 
 
 If living contractile frog's muscle, freed as much as possible 
 from blood, be frozen^, and while frozen, minced, and rubbed 
 up in a mortar with four times its weight of snow containing 
 1 p. c. of sodium chloride, a mixture is obtained which at 
 a temperature just below 0" C. is sufiiciently fluid to be filtered, 
 though with difficulty. The slightly opalescent filtrate, or muscle- 
 plasma as it is called, is at first quite fluid, but Avill when exposed 
 to the ordinary temperature become a solid jelly, and afterwards 
 separate into a clot and sei'um. It will in fact coagidate like 
 blood-plasma, with this difference, that the clot is not firm and 
 fibrillar, but loose, granular and flocculent. During the coagidation 
 the fluid, which before was neutral or slightly alkaline, becomes 
 distinctly acid. 
 
 The clot is myosin. It gives all the reactions of myosin obtained 
 from dead muscle. 
 
 The serum contains ordinary serum-albumin, one or more pe- 
 culiar proteids* coagidating at a lower temperature than does serum- 
 albumin, and extractives. Such muscles as are red also contain a 
 small quantity of haemoglobin, to which indeed theu' redness is 
 due. 
 
 Thus while dead muscle contains myosin, serum-albumin, and 
 other proteids and extractives with certain insoluble matters and 
 certain gelatinous elements not referable to the muscle-substance 
 
 ' See Appendix. • Ibid. 
 
 3 Since, as we shall presently see, a muscle may be frozen and thawed again 
 without losing any of its ^-ital powers, we are at liberty to regard the frozen muscle 
 as a still Hving muscle. 
 
 * See Appendix. 
 
 F. 5 
 
66 RIGOR MORTIS. [Book i. 
 
 itself, living muscle contains no myosin, but some substance or 
 substances which bear somewhat the same relation to myosin that 
 the fibrin factors do to fibrin, and which give rise to myosin upon 
 the death of the muscle. 
 
 We may in fact speak of rigor mortis as characterised by a 
 coagulation of the muscle-plasma, comiparable to the coagulation of 
 blood-plasma, but differing from it inasmuch as the product is not 
 fibrin but myosin. The rigidity, the loss of suppleness, and the 
 diminished translucency appear to be at all events largely, though 
 probably not wholly, due to the change from the fluid plasma to the 
 solid myosin. We might compare a living muscle to a number of 
 fine transparent membranous tubes containing blood-plasma. When 
 this blood-plasma entered into the 'jelly' stage of coagulation, the 
 system of tubes would present many of the phenomena of rigor 
 mortis. They would lose much of their suppleness and translucency, 
 and acquire a certain amount of rigidity. 
 
 There is however one very marked and important difference 
 between rigor mortis of muscle and the coagulation of blood : blood 
 during its coagulation undergoes only a slight change in its reaction ; 
 but muscle during the onset of rigor mortis becomes distinctly acid. 
 
 A living muscle at rest is in reaction neutral, or, possibly from 
 some remains of lymph adhering to it, faintly alkaline. If on the 
 other hand the reaction of a thoroughly rigid muscle be tested, it 
 will be found to be most distinctly acid. This development of an 
 acid reaction is witnessed not only in the solid untouched fibre but 
 also in expressed muscle-plasma ; it seems to be associated in some 
 way with the appearance of the myosin. 
 
 The exact causation of this acid reaction has not at present 
 been clearly worked out. Since the coloration of the litmus pro- 
 duced is permanent, carbonic acid, which as Ave shall immediately 
 state, is set free at the same time, cannot be regarded as the active 
 acid, for the reddening of litmus produced by carbonic acid speedily 
 disappears on exposure. On the other hand it is possible to ex- 
 tract from rigid muscle a certain quantity of lactic acid, or rather 
 of a variety of lactic acid known as sarcolactic acid^; and it has 
 been thought that the appearance of the acid reaction of rigid 
 muscle is due to a new formation or to an increased formation of 
 this sarcolactic acid. But there is considerable doubt whether any 
 such increase of sarcolactic acid does actually take place in rigor 
 mortis. Hence though there can be no doubt that an acid reaction 
 is established, we are not yet in a position to affirm positively the 
 exact manner in which that reaction is produced, the complex 
 nature of the muscular substance suggesting to the chemist several 
 ways in which it might come about. 
 
 Coincident with the appearance of this acid reaction, though 
 as we have said, not the direct cause of it, a large development of 
 carbonic acid takes place when muscle becomes rigid. Irritable 
 
 ^ See Appendix. 
 
r'liAr. 11.] THE COXTHACTILE TISSUES. 0? 
 
 living muscular substance like all living jirotoplasni is continually 
 respirinuf, continually consuiniuL,' ()xyL,n-n an<l •,nvin<( out carbonic 
 acid. In the body, the arterial blood going to the nuiscle gives up 
 some of its oxygen, and gains a quantity of carbonic acid, thus 
 becoming venous as it passes through the muscular capillaries. 
 Even alter removal froni the body, the living muscle continues to 
 take up from tlie surrounding atmos])here a certain rpiantity of 
 o.xygen and to give out a certain quantity of carbonic acid. 
 
 At the onset of rig(H' mortis there is a very large and sudden 
 increase in this production of carbonic acid, in fact an outburst as it 
 were of that gas. This is a phenomenon deserving special attention. 
 Knowing that the carbonic acid which is the outcome of the res- 
 piration of the whole body is the result of the oxidation of carbon- 
 holding subtances, we might very naturally suppose that the in- 
 creased production of carbonic acid attendant on the development 
 of rigor mortis is due to the fact that during that event a certain 
 quantity of the carbon-holding constituents of the muscle are 
 suddenly oxidized. But such a view is negatived by the following 
 facts. In the first place, the increased production of carbonic acifl 
 during rigor mortis is not accompanied by any corresponding in- 
 crease in the consumption of oxygen. In the second jDlace, a 
 muscle (of a frog for instance) contains in itself no free or loosely 
 attached oxygen : when subjected to the action of a mercurial air- 
 pump it gives off no oxygen to a vacuum, offering in this respect 
 a marked contrast to blood ; and yet, when placed in an atmosphere 
 free from oxygen, it will not only continue to give off carbonic 
 acid while it remains alive, but will also exhibit at the onset of 
 rigor mortis, the same increased production of carbonic acid that 
 is shewn by a muscle jilaced in an atmosphere containing oxygen. 
 It is obvious that in such a case the carbonic acid does not arise 
 from the direct oxidation of the muscle substance, for there is no 
 oxygen present at the time to can-y on that oxidation. We are 
 driven to suj^pose that during rigor mortis, some complex body, 
 containing in itself ready formed carbonic acid so to sj^eak, is split 
 up, and thus carbonic acid is set free, the process of oxidation by 
 which that carbonic acid was formed out of the carbon-holding 
 constituents of the muscle having taken place at some anterior date. 
 
 Living resting muscle then, is alkaline or neutral in reaction, 
 and the substance of its fibres contains a coagulable j^lasma. Dead 
 rigid muscle on the other hand is acid in reaction, and no longer 
 contains a coagulable plasma, but is laden with the solid myosin. 
 Further, the change from the living irritable condition to that of 
 rigor mortis is accompanied by a large and sudden development of 
 carbonic acid. 
 
 It is found moreover that there is a certain amount of i")arallel- 
 ism between the intensity of the rigor mortis, the degi-ee of acid 
 reaction and the quantity of carbonic acid given out. If we 
 suppose, as we fairly may do, that the intensity of the rigidity is 
 
 5—2 
 
68 CHEMICAL CHANGES. [Book i. 
 
 dependent on the quantity of myosin deposited in the fibres, and 
 the acid reaction to the development if not of lactic acid, at least 
 of some other substance, the parallelism between the three products, 
 myosin, acid-producing substance, and carbonic acid, would suggest 
 the idea that all three are the results of the splitting-up of the 
 same highly complex substance. But we have not at present 
 succeeded in isolating or in otherwise definitely proving the exist- 
 ence of such a body, and though the idea seems tempting, it may 
 in the end prove totally erroneous. 
 
 We may now return to the question, What are the chemical 
 changes which take place when a living resting muscle enters into 
 a contraction ? These changes are most evident after the muscle 
 has been subjected to a prolonged tetanus ; but there can be no 
 doubt that the chemical events of a tetanus are, like the physical 
 events, simply the sum of the results of the constituent single 
 contractions. 
 
 In the first place, the muscle becomes acid, not so acid as in 
 rigor mortis, but still sufficiently so, after a vigorous tetanus, to 
 turn blue litmus distinctly red. The cause of the acid reaction 
 like that of rigor mortis is doubtful ; but is in all probability the 
 same in both cases. 
 
 In the second place, a considerable quantity of carbonic acid is 
 set free ; and the production of carbonic acid in muscular contrac- 
 tion is altogether similar to the production of carbonic acid during 
 rigor mortis. It is not accompanied by any corresponding increase 
 in the consumption of oxygen. This is evident even in a muscle 
 through which the circulation of blood is still going on, for though 
 the blood passing through a contracting muscle gives up more 
 oxygen than the blood passing through a resting muscle, the increase 
 in the amount of oxygen taken up falls below the increase in the 
 carbonic acid given out ; but it is still more markedly shewn in a 
 muscle removed from the body. For in such a muscle both the 
 contraction and the increase in the production of carbonic acid will 
 go on in the absence of oxygen. A frog's muscle suspended in an 
 atmosphere of nitrogen will remain irritable for some considerable 
 time, and at each vigorous tetanus an increase in the production of 
 carbonic acid may be readily ascertained. 
 
 Moreover there seems to be a coiTespondence between the 
 energy of the contraction and the amount of carbonic acid and 
 the degree of acid reaction produced, so that, though we are now 
 treading on somewhat uncertain ground, we are naturally led to the 
 view that the essential chemical process lying at the bottom of a 
 muscular contraction as of rigor mortis is the splitting up of some 
 highly complex substance. But here the resemblance between rigor 
 mortis and contraction ends. We have no evidence of the formation 
 during a contraction of any body like myosin. Now the contracted 
 and rigid muscle differ essentially in the fact that while the former, 
 as compared with living resting muscle, increases in extensibility and 
 
Chap, ii.] THE COXTUACTILE TISSUES. C9 
 
 loses none of its translucency, the latter becomes less extensible, less 
 clastic, and loss translucent. Corresponding to this marked differ- 
 ence, wo find myosin f<jrnied in the rigid muscle, but we cannot 
 find it in the contracted muscle. 
 
 The other chemical changes in muscle during a contraction 
 have not yet been clearly made out. Indeed our whole information 
 concerning the other chemical constituents of muscle is at present 
 imperfect. 
 
 The bodies which we have called extractives are numerous and 
 varied. Among the nitrogenous crystalline extractives the most 
 important is krcatin, which occurs to the extent of about "2 to 
 •3 p. c, is an invariable constituent of muscle, and is found elsewhere 
 only in nervous tissue, the kidney, and to a slight extent in the 
 blood. As we shall hereafter see, great interest is attached to 
 this body inasmuch as it readily splits up into urea, and sarcosin, 
 and accordingly has been regarded as one at least of the antecedents 
 of urea, which body is conspicuous by its absence from muscular 
 tissue. The alkaline kreatinin into which kreatin is converted by 
 the action of acids, and which appears in the urine, is apparently 
 absent from muscle. The other nitrogenous cr}'stalline bodies, 
 which need not detain us now, are kamin, hypoxanthin (or sarkin), 
 xanthin, inosinic acid, taurin and possibly uric acid\ 
 
 Fats are present in considerable quantities both in the adipose 
 tissue between the bundles of fibres and also as constituents of the 
 muscular substance within the sarcolemma. 
 
 The peculiar starch-like body, glycogen, of which we shall have 
 to speak more fully in a later part of this work, is especially 
 abundant in the muscles of the embr3'o at an early period, and 
 besides, is so continually met ■v\"ith in the muscles of the adult that 
 it may fairly be considered as a normal constituent of muscle to a 
 variable extent, possibly from 'b to 1 p. c. A dextrin-like body has 
 also been found, and at times glucose or an allied sugar. The 
 cardiac muscular tissue contains the peculiar sugar, inosit. 
 
 The ashes of muscle, like those of the red corpuscles, are cha- 
 racterised by the preponderance of potassium salts and of phos- 
 phates ; these form in fact nearly SO p. c. of the whole ash. 
 
 The general composition of human muscle is she\Ma in the 
 foUowincr table of v. Bibra. 
 
 Water 
 
 ... 744-5 
 
 Solids 
 
 
 Myosin and other matters, elastic ele- 
 
 
 ments, &c., insoluble in water . . . 
 
 lo5*4 
 
 Soluble proteids 
 
 19-3 
 
 Gelatin 
 
 20-7 
 
 Extractives and Salts ... 
 
 37-1 
 
 Fats 
 
 230 
 
 
 2o5"5 
 
 1 See Appendix. 
 
 
70 THERMAL CHANGES. [Book i. 
 
 Concerning the functional importance of these various bodies 
 Ave have very little exact knowledge. 
 
 Helmholtz shev>^ed long ago that the effect of long continued 
 contraction is to diminish the substances in muscle which are 
 soluble in water, but to increase those which are soluble in 
 alcohol. In other words, during contraction some substance or 
 substances soluble in water are converted into another or other 
 substances insoluble in water but soluble in alcohol. During or 
 after rigor mortis, glycogen is converted into sugar, and it has been 
 contended that a similar change takes place during contraction; 
 but we are not, at present at all events, in a position to affirm that 
 such a conversion is a necessary and integral part of the chemical 
 transformations which lie at the bottom of a muscular contraction. 
 
 We shall have occasion to treat more fully and from a different 
 point of view, of the relations between muscular exercise and the 
 quantity of urea discharged by the kidneys. Meanwhile we may 
 state that not only does this all-important nitrogenous crystalline 
 body appear to be absent from normal muscle, both during rest 
 and after contraction, but we have as yet no adequate evidence 
 that the contraction of a muscle is followed by the appearance in 
 the substance of the muscle or in the blood passing through it of 
 any new nitrogenous product, or by any increase in any of the 
 nitrogenous extractives which we have mentioned as normally 
 present in muscle. In fact all we know at present is that a 
 contraction is followed by an increase in the discharge of carbonic 
 acid, and by certain changes v/hich lead to an acid reaction. 
 Beyond this we are in the dark. 
 
 Thermal Changes. 
 
 The view hoAvever that chemical changes lie at the bottom of a 
 muscular contraction, that the energy which takes on the form of 
 muscular work arises from a metabolism of the muscular substance, 
 is supported by a variety of considerations and especially perhaps 
 by the fact, that the develoj)ment of energy as muscular work, is 
 accompanied by a development of energy as heat. 
 
 Though we shall have hereafter to treat this subject more fully, 
 the leading facts may be given here. Whenever a muscle contracts, 
 its temperature rises, indicating that heat is given out. When a 
 mercury thermometer is plunged into a mass of muscles, such as 
 those of the thigh of the dog, a rise of the mercury is observed 
 upon the muscles being throAvn into a prolonged contraction. 
 More exact results however are obtained by means of a thermopile, 
 by the help of which the rise of temperature caused by a few 
 repeated single contractions, or indeed by a single contraction, may 
 be observed and the amount of heat given out approximatively 
 measured. 
 
Chap, ii.] THE COXTUACTJ LK TISSUES. 71 
 
 Tlio tlu'nnoj)il<! iiiiiy consist either of a siiiji^lo junction in tliu form of 
 a iiceiUo plunged into the substance of the muscle ; or of several junctions 
 either in the shape of a ihit surface carefully opposed to the surface of 
 muscle (Heidenhain) the pile being l)alanced so as to move with the con- 
 tracting muscle, and thus to keep the contact exact; or in the shape of a 
 thin wedge (Fick) the edge of which comprising the actual junctions is 
 thrust into a mass of muscles and held in position by them. In all ca.ses 
 the fellow junction or junctions must be kept at a constant temperature. 
 
 Fick calculated that the greatest heat given out by the muscles 
 of the thicjh of a froof in a sincjle contraction was 3'1 micro-units of 
 heat' for a gramme of muscle, the result being obtained by dividing 
 by live the total amount of heat given out in live succcs.sive single 
 contractions. It will however be safer to regard these figures as 
 illustrative of the fact that the heat given out is considerable 
 rather than as data for elaborate calculations. Moreover we have 
 no satisfactory quantitative determinations of the heat given out 
 by the muscles of warm-blooded animals, though there can be no 
 doubt that it is much greater than that given out by the muscles 
 of the frog. 
 
 There can hardly be any doubt that the heat thus set free is 
 the product of chemical changes within the muscle, changes, which 
 though they cannot for the reasons given above be regarded as 
 simple and direct oxidations, may be spoken of in general terms as 
 a combustion. So that the muscle may be likened to a steam- 
 engine, in which the combustion of a certain amount of material 
 gives rise to the development of energy in two forms, as heat and 
 as movement, there being certain quantitative relations between 
 the amount of energy set free as heat and that giving rise to move- 
 ment. We must however carefully guard ourselves against pressing 
 this analogy too closely. In the steam-engine, Ave can distinguish 
 clearly between the fuel which through its combustion is the sole 
 source of energ}^, and the machinery, Avhich is not consumed to 
 provide energy and only suffers wear and tear. In the muscle we 
 can make no such distinction ; though the whole matter is not 
 fully worked out, we have reason to think that the muscular fibre 
 is not to be regarded as a machine which takes so to speak a charge 
 of certain substances from the blood, and by inducing an explosion 
 of these substances in itself gives rise to the energy of heat and 
 movement. On the contrary the evidence goes to shew that it is 
 the living contractile substance as a whole which is continually- 
 breaking down in an explosive decomposition and as continually 
 building itself up again out of the material supplied by the blood. 
 In a steam-engine only a certain amount of the total potential 
 energy of the fuel issues as work, the rest being lost as heat, the 
 proportion varying, but the work rarely exceeding one-tenth of the 
 total energy. In the case of the muscle wo are not at present in a 
 position to draw up an exact equation between the latent energy 
 
 ^ The micro unit being a milligramme of water raised one degree centigrade. 
 
72 CHANGES DURIXG A NERVOUS IMPULSE. [Book i. 
 
 on the one hand and the two forms of actual energy on the other. 
 We have reason to think that the proportion between heat and 
 work varies considerably under different circumstances, the work 
 sometimes rising as high as one-fourth, sometimes possibly sink- 
 ing as low as one twenty-fourth of the total energy, and obser- 
 vations seem to shew that the greater the resistance which the 
 muscle has to overcome, the larger the proportion of the total 
 energy expended which goes out as work done. The muscle in 
 fact seems to be so far selt-regulating, that the more work it has to 
 do, the greater, within certain limits, is the economy with which 
 it works. 
 
 Lastly it must be remembered that the giving out of heat by 
 the muscle is not confined to the occasions when it is actually con- 
 tracting. When, at a later period, we treat of the heat of the body 
 generally, evidence will be brought forward that the muscles even 
 when at rest are giving rise to heat, so that the heat given out at 
 a contraction is not some wholly new phenomenon, but a temporary 
 exaggeration of what is going on, continually, at a more feeble 
 rate. 
 
 The Changes in a Nerve daring the passage of a Nervous 
 
 Impulse. 
 
 The change in the form of a muscle during its contraction is a 
 thing which can be seen and felt; but the changes in a nerve 
 during its activity are invisible and impalpable. We stimulate one 
 end of a nerve, and since we see this followed by a contraction of 
 the muscle attaehed to the other end, we know that some changes 
 or other, constituting a nervous impulse, have been propagated 
 along the ner^^e ; but these are changes which we cannot see. Nor 
 have we satisfactory evidence of any chemical events or of any pro- 
 duction of heat, accompanying a nervous impulse. We may fairly 
 suppose that some chemical changes form the basis of a nervous 
 impulse, and that these changes set free a certain amount of heat; 
 but these if they occur are too slight to be recognized satisfactorily 
 by the means at present at our disposal. In fact, beyond the 
 terminal results, such as a muscular contraction in the case of 
 a nerve going to a muscle, or some affection of the central 
 nervous system in the case of a nerve still in connection with 
 its nervous centre, there is one event and one event only which 
 we are able to recognize as the objective token of a nervous 
 impulse, and that is the so-called negative variation of the nerve- 
 current. For a piece of nerve removed from the body exhibits 
 nearly the same electric phenomena as a piece of muscle. It has 
 an equator which is electrically positive as compared to its two cut 
 ends. In fact the diagram Fig. 12, and the description which it 
 was used on p. 59 to illustrate, may be applied to nerve .as well as 
 
CUAP. II.] THE COSTllACTILE TISSUES. l?, 
 
 to muscle, except that tlio currtnts arc in all cases much more 
 feeble in the case of nerves tlian of muscles, and the special 
 currents from the circumference to the centre of the transverse 
 sections cannot well be shewn in a slender nerve; indeed it is 
 doubtful if they exist at all. 
 
 During the passage of a nen'ous impulse the ' natural norvo- 
 current' undergoes a negative variation, just as the 'natural muscle- 
 current' imdergoes a negative variation during a contraction. 
 There arc however difficulties in the case of the nerve similar to 
 those in the case of the muscle, concerning the pre-cxistence of 
 any such ' natural ' currents. It is maintained by many that 
 a nerve in an absolutely natural condition is like a muscle, 
 iso-electric ; hence we may say that in a nerve during the passage 
 of a nervous impulse, as in a muscle during a musculai' contraction, 
 a ' current of action ' is developed. 
 
 This 'current of action' or 'negative variation' may be shewn 
 either by the galvanometer or by the rheoscopic frog. If the nerve 
 of the 'muscle-nerve preparation' B (see p. 62) be placed in an 
 appropriate manner on a thoroughly irritable nerve A (to which of 
 course no muscle need be attached), i.e. touching say the equator 
 and one end of the nerve, then single induction-shocks sent into 
 the far end of A will cause single spasms in the muscle of B, while 
 tetanization of A, i.e. rapidly repeated shocks sent into A, will 
 cause tetanus of the muscle of B. 
 
 That this current, whether it be regarded as an independent 
 ' current of action ' or as a negative variation of a ' pre-existing ' 
 current, is an essential feature of a nerv'ous impulse is shewn by 
 the fact that the degree or intensity of the one varies with that of 
 the other. They both travel too at the same rate. In describing 
 the muscle-curve, and the method of measuring the muscular latent 
 period, we have incidentally sheAvn (p. 47) how the velocity of the 
 nervous impulse is measured also, and stated that the rate in the 
 nerves of a frog is about 28 metres a second. Bernstein by means 
 of a special and somewhat complicated apparatus finds that the 
 current of action travels along an isolated piece of ners-e at the 
 same rate. He also finds that it, like the molecular change in a 
 muscle preceding the contraction, and indeed like the contraction 
 itself, passes over any given spot of the nerve in the form of a 
 wave, rising rapidly to a maximum and then more gradually 
 declining again. He has been able to measure the length of the 
 wave, and this he finds to be about 18 mm., taking '0007 sec. to 
 pass over any one point. 
 
 When an isolated piece of nerve is stimulated in the middle, the 
 current of action is propagated equally well in both directions, and 
 that whether the nerve be a chiefly sensory or a chiefly motor nerve, 
 or indeed if it be a nerve-root composed exclusively of motor or 
 of sensory fibres. Taking the current of action as the token of a 
 nervous impulse, we infer from this that when a nerve-fibre is 
 
74 CHANGES DURIXG A NERVOUS IMPULSE. [Book r. 
 
 stimulated artificially at any part of its course, the nervous 
 impulse set going travels in both directions. 
 
 We used just now the phrase ' tetanization of a nerve,' meaning 
 the application to a nerve of rapidly repeated shocks such as would 
 produce tetanus in the muscle to which the nerve was attached, 
 and we shall have frequent occasion to employ the phrase. It will 
 however of course be understood that there is in the nerve as far 
 as we know no summation of nervous impulses comparable to the 
 summation of muscular contractions. The matter perhaps needs 
 fuller investigation, but as far as we know at present, we may 
 say that the series of shocks sent in at the far end of the nerve 
 start a series of impulses; these travel down the nerve and 
 reach the muscle as a series of distinct impulses; and the 
 first changes in the muscle, the molecular latent-period changes, 
 also form a series the members of which are distinct. It is 
 not until these molecular changes become transformed into visible 
 changes of form that any fusion or summation takes place. 
 
 Putting together the facts contained in this and the preceding 
 sections, the following may be taken as a brief approximate history 
 of what takes place in a muscle and nerve when the latter is 
 subjected to a single induction-shock. At the instant that the 
 induced current passes into the nerve, changes occur, of whose 
 nature we know nothing certain except that they cause a ' current 
 of action' or 'negative variation of the natural' nerve-current. 
 These changes propagate themselves along the nerve in both 
 directions as a nervous impulse in the form of a wave, having 
 a wave-length of about 18 mm., and a velocity (in frog's nerve) of 
 about 28 m. per sec. Passing down the nerve-fibres to the muscle, 
 flowing along the branching and narrowing tracts, the wave at last 
 breaks on the end-plates of the fibres of the muscle. Here it is 
 transmuted into a muscle-impulse, with a shorter steeper wave, 
 and a greatly diminished velocity (about 3 m. per sec). This 
 muscle-impulse, of which Vv^o know hardly more than that it 
 is marked by a current of action, travels from each end-plate 
 in both directions to the end of the fibre, where it appears to 
 be lost, at all events we do not know what becomes of it. As 
 it leaves the end plate it is followed by an explosive decomposition 
 of material, leading to a discharge of carbonic acid, to the appearance 
 of some substance or substances with an acid reaction, and probably 
 of other unknown things, with a considerable development of heat. 
 This explosive decomposition gives rise to the visible contraction- 
 wave, which travels behind the invisible muscle-impulse at about 
 the same rate, but with a vastly increased wave-length. The fibi-e, 
 as the wave passes over it, swells and shortens, bringing its two ends 
 nearer together, its molecules during the change of form arranging 
 themselves in such a way that the extensibility of the fibre is 
 increased. 
 
SEC. 3. THE NATURE OF TPIE CHANGES THROUGH 
 WHICH AN ELECTRIC CURRENT IS ABLE TO GENE- 
 RATE A NERVOUS IMPULSE. 
 
 Action of tJie Constant Current. 
 
 In the preceding account, the stimulus applied in order to give 
 rise to a nervous impulse has always been supposed to be an 
 induction shock, single or repeated. This choice of stimulus has 
 been made on account of the almost momentary duration of the 
 induced current. Had we used a current lasting for some consider- 
 able time, the problems before us would have become more com- 
 plex in consequence of our having to distinguish between the 
 events taking place Avhile the current was passing through the 
 nen-e from those which occurred at the moment when the cuiTent 
 was thrown into the nerve or at the moment when it was shut 
 ofif from the nerve. These complications do arise when instead of 
 emplo}'ing the induced current as a stimulus, we use a constant 
 current, i.e. when v.e pass through the ner\'e (or muscle) a current 
 direct from the battery -without the intervention of any induction- 
 coil. 
 
 Before making the actual experiment, we might perhaps 
 naturally suppose that the constant current would act as a 
 stimulus throughout the whole time during which it was applied, 
 that, so long as the current passed along the nerve, nervous 
 impulses would be generated and thus the muscle thrown into 
 something at all events like tetanus. And under certain conditions 
 this does take place ; occasionally it happens that at the moment 
 the current is thrown into the nerve, the muscle of the muscle- 
 nerv'e preparation falls into a tetanus which is continued until the 
 current is shut off. But such a result is exceptional. In the vast 
 
76 ACTION OF THE CONSTANT CURRENT. [Book i. 
 
 majority of cases what happens is as follows. At the moment that 
 the circuit is made, the moment that the current is thrown 
 into the nerve, a single spasm, a simple contraction, the so-called 
 making contraction, is witnessed ; but after this has passed away 
 the muscle remains absolutely quiescent in spite of the current 
 continuing to pass through the nerve, and this quiescence is 
 maintained until the circuit is broken, until the current is shut 
 off from the nerve, when another simple contraction, the so- 
 called breaking contraction, is observed. The mere passage of a 
 constant current of uniform intensity through a nerve does not 
 under ordinary circumstances act as a stimulus generating a 
 nervous impulse ; such an impulse is only set up when the 
 current either falls into or is shut off from the nerve. It is 
 the entrance or the exit of the current, and not the continuance of 
 the current, which is the stimulus. 
 
 The quiescence of the nerve and muscle during the passage of 
 the current is however dependent on the current remaining 
 uniform in intensity or at least^ not being suddenly increased 
 or diminished. Any sufficiently sudden and large increase or 
 diminution of the intensity of the current, will act like the 
 entrance or exit of a current, and by generating nervous impulses 
 give rise to contractions. If the intensity of the cuiTent however 
 be very slowly and gradually increased or diminished, a very wide 
 range of intensity may be passed through without any contraction 
 being seen. It is the sudden change from one condition to another, 
 and not the condition itself, which causes the nervous impulse. 
 
 In many cases, both a ' making ' and a ' breaking ' contraction, 
 each a simple spasm, are observed, and this is perhaps the 
 commonest event ; but when the current is very weak, and again 
 when the current is very strong either the breaking or the making 
 contraction may be absent, i.e. there may be a contraction only 
 when the current is thrown into the nerve or only when it is 
 shut off from the nerve. 
 
 Under ordinary circumstances the contractions witnessed with 
 the constant current either at the make or at the break, are of the 
 nature of a ' simple ' contraction, but, as has already been said, the 
 application of the current may give rise to a very pronounced 
 tetanus. Such a tetanus is seen sometimes when the current 
 is made, lasting during the application of the current, sometimes 
 when the current is broken, lasting some time after the current has 
 been wholly removed from the nerve. The former is spoken of as 
 a ' making,' the latter as a ' breaking ' tetanus. But these excep- 
 tional results of the constant current need not detain us now. 
 
 The great interest attached to the action of the constant 
 current lies in the fact, that during the passage of the current, 
 in spite of the absence of all nervous impulses and therefore 
 of all muscular contractions, the nerve is for the time both between 
 and on each side of the electrodes profoundly modified in a most 
 
Chap, n.] THE COXTRACriLE TISSUES. 77 
 
 peculiar manner. This modification, important both for the light 
 it throws on the generation of nervous impulses and for its practical 
 applications, is known under the name of electrotonus. 
 
 Electrotonus. The marked feature of the electrotonic con- 
 dition is that the nerve though apparently quiescent is changed in 
 respect to its irritability ; and that in a differenf way in the 
 neighbourhood of the two electrodes respectively. 
 
 Suppose that on the ner\-e of a muscle-nene preparation are 
 placed two (non-polarizable) electrodes (Fig. 18, a, k) connected 
 with a battery and ananged with a key so that a constant current 
 can at pleasure be thrown into or shut off from the nerve. 
 This constant current, whose effects we are about to study, may be 
 called the ' polarizing current.' Let a be the positive electrode or 
 anode, and k the negative electrode or kathode, both placed at 
 some distance from the muscle, and also with a certain interval 
 between each other. At the point x let there be applied a pair of 
 electrodes connected with an induction-machine. Let the muscle 
 further be connected with a lever, so that its contractions can 
 be recorded, and their amount measured. Before the polarizing 
 current is thrown into the nerve, let a single induction-shock 
 of kno^^'n intensity (a weak one being chosen, or at least not 
 one which would cause in the muscle a maximum contraction) be 
 thrown in at x. A contraction of a certain amount will follow. 
 
 A. 
 
 Fig. 13. Muscle-xekve Peeparatioxs, with the nerve exposed in .J to a descendiug 
 and in £ to an ascending constant cunent. 
 
 In each a is the anode, k the kathode of the constant current, x represents the 
 spot where the induction-shocks used to test the irritabihty of the nerve are sent in. 
 
 That contraction may be taken as a measure of the irritability of 
 the ner\'e at the point x. Now let the polarizing current be 
 thrown in, and let the direction of the current be a descending 
 one, with the kathode or negative pole nearest the muscle. 
 
78 KLECTROTONUS. [Book i. 
 
 as in Fig. loA. If while the current is passing, the same 
 induction-shock as before be sent through x, the contraction which 
 results will be found to be greater than on the former occasion. 
 If the polarizing current be shut off, and the point x after a 
 short interval again tested with the same induction-shock, the 
 contraction will be no longer greater, but of the same amount, 
 or perhaps not so great, as at first. During the passage of 
 the polarizing current, therefore, the irritability of the nerve at 
 the point x has been temporarily increased, since the same shock 
 applied to it causes a greater contraction during the presence than 
 in the absence of the current. But this is only true so long as the 
 polarizing current is a descending one, so long as the point x lies on 
 the side of the kathode. On the other hand, if the polarizing 
 current had been an ascending one, with the anode or positive pole 
 nearest the muscle, as in Fig. loB, the irritability of the nerve at 
 X would have been found to be diminished instead of increased by 
 the polarizing current. That is to say, when a constant current is 
 applied to a nerve, the irritability of the nen^e between the polar- 
 izing electrodes and the muscle is, during the passage of the current, 
 increased when the kathode is nearest the muscle (and the polar- 
 izing current descending) and diminished when the anode is nearest 
 the muscle (and the polarizing current ascending). The same 
 result, mutatis mictandis, and with some qualifications which we 
 need not discuss, would be gained if x were placed not between 
 the muscle and the polarizing current, but on the fa,r side of the 
 latter. Hence it may be stated generally that during the passage 
 of a constant current through a nerve the irritability of the nerve 
 is increased in the region of the kathode, and diminished in 
 the region of the anode. The changes in the nerve which give 
 rise to this increase of irritability in the region of the kathode 
 are spoken of as katelectrotonus, and the nerve is said to be 
 in a katelectrotonic condition. Similarly the changes in the 
 region of the anode are spoken of as anelectrotonus, and the nerve 
 is said to be in an anelectrotonic condition. It is also often usual 
 to speak of the katelectrotonic increase, and anelectrotonic decrease 
 of irritability. 
 
 This law remains true whatever be the mode adopted for 
 determining the irritability. The result holds good not only 
 with a single induction-shock, but also with a tetanizing inter- 
 rupted current, with chemical and with mechanical stimuli. It 
 further appears to hold good not only in a dissected nerve-muscle 
 preparation but also in the intact nerves of the living body. The 
 increase and decrease of u'ritability are most marked in the 
 immediate neighbourhood of the electrodes, but spread for a 
 considerable distance in either direction in the extrapolar regions. 
 The same modification is not confined to the extrapolar region, 
 but exists also in the intrapolar region. In the intrapolar region 
 there must be of course an indifferent point, where the katelectro- 
 
Chap, ii ] 
 
 rilE COST n ACT HE TrsSUE.^. 
 
 tonic increase merges into the aneloctrot<jnic decrease, and where 
 therefore the irritability is unchanged. When the polarizing 
 current is a weak one, this indifferent point is nearer the anode 
 than the kathode, but as the polarizing current increases in 
 intensity, draws nearer and nearer the kathode (see Fig. 14). 
 
 The amount of increase and decrease is dependent : (1) On the 
 strength of the current, the stronger current up to a certain limit 
 producing the greater effect. (2; On the irritability of the nerve, 
 the more irritable, better conditioned nerve being the more affected 
 by a current of the same intensity. 
 
 In the experiments ju.st described the increase or decrease of 
 irritability is taken to mean that the same stimulus starts in the one 
 case a larger or more powerful and in the other case a smaller or 
 less energetic impulse ; but we have reason to think that the mere 
 propagation or conduction of impulses started elsewhere is affected 
 by the electrotonic condition. At all events anelectrotonus appears 
 to offer an obstacle to the passage of a nervous impulse. 
 
 These variations of irritability at the kathode and anode re.spec- 
 tively must be the result of molecular changes, brought about by 
 the action of the constant current. They are interesting theoreti- 
 cally because the}' shew that the generation of a nervous impulse 
 as the result of the makino^ or breakinof of a constant current is 
 dependent on the change of a nerve from its normal condition into 
 either katelectrotonus or anelectrotonus, or back asrain from one 
 of these phases into its normal condition. And certain results as 
 to the occuiTence or absence of a contraction at the make or at 
 the break, according as the current is strong or weak, ascending, 
 or descending (results which need not detain us here but which 
 have been formulated as the co-called "law of contraction") 
 
 Fig. 1-1. Diagram iLLCSTnATixo th:-: Variations of Irritability Drr.iNo Electro- 
 Toxcs, vain Polarizixo Currents of Increasing Intensity (from Pfliiger). 
 
 Tkp anode is supposed to be placed at A, the kathode at B; AB is consequently 
 the inTrapolar district. In each of the three curves, the portion of the curve below 
 the base line represents dimiuisbeJ irritability, that above, increased irritability, 
 i/j represent? the effect of a weak current; the indifterent point a-j is near the 
 anode A. In _?/„, a stronger current, the indifferent point x., is nearer the kathode 
 B, the diminution of irritability in anelectrotonus and the increase in katelectro- 
 tonus being greater than in Vi ; the effect also spreads for a greater distance along 
 the estrapolar regions in both directions. In y.^ the sime events are seen to be still 
 more marked. 
 
80 ELECTROTONIC CURRENTS. [Book i. 
 
 go far to shew that a nervous impulse is generated only when 
 a nerve passes suddenly from a normal condition into the phase of 
 katelectrotonus (making contra,ction) or returns from the phase of 
 anelectrotonus to a normal condition (breaking contraction), in 
 other words, when it passes suddenly from a phase of lower to 
 a phase of higher irritability. 
 
 The phenomena of electrotonus are also interesting practically 
 in as much as they shew that in the constant current appropriately 
 applied we have the means of changing at will the irritability of 
 this or that nerve, decreasing it when we wish to lessen pain or 
 spasm, increasing it when we wish to heighten sensibility or 
 muscular action. For the increase or decrease is observed in the 
 case of nervous impulses passing towards the central nervous system 
 as well as in those passing to muscles. 
 
 Electrotonic Currents. During the passage of a constant current 
 through a nerve, variations in the electric currents of the^nervc analogous 
 in some respects to the variations of the irritability of the nerve may be 
 witnessed. Thus if a constant current supplied by the battery P 
 (Fig. 15) be applied to a piece of nerve by means of two non-polarizable 
 electrodes j), V i ^he " currents of rest " obtainable from various points of 
 the nerve v/ill be different during the passage of the polarizing current 
 from those which were manifest before or after the current was applied; 
 and, moreover, the changes in the nerve-currents produced by the polariz- 
 ing current will not be the same in the neighbourhood of the anode (p) 
 as those in the neighbourhood of the kathode (j)'). Thus let G and // be 
 two galvanometers so connected with the two ends of the nerve as to 
 obtain good and clear evidence of the "currents of rest." Before 
 the polarizing current is thrown into the nerve, the needle of H will 
 occupy a position indicating the passage of a current of a certain 
 intensity from A to W through the galvanometer (from the positive 
 longitudinal surface to the negative cut end of the nerve), the circuit 
 being completed by a current in the nerve from li to A, i.e. the current 
 will flow in the direction of the arrow. Similarly the needle of G will 
 by its deflection indicate the existence of a current flowing from g to g 
 through the galvanometer, and from g to g through the nerve, in the 
 direction of the arrow. 
 
 At the instant that the polarizing current is thrown into the nerve 
 at irp\ the currents at gg ^ hh' will suffer a "current of action" correspond- 
 ing to the nervous impulse, which, at the making of the polarizing 
 current, passes in both directions along the nerve, and may cause a 
 contraction in the attached muscle. The current of action is, as we 
 have seen, of extremely short duration, it is over and gone in a small 
 fraction of a second. It therefore must not be confounded with a 
 permanent effect which, in the case we are dealing with, is observed in 
 both galvanometers. This effect, which is dependent on the direction 
 of the polarizing current, is as follows : Supposing that the polarizing 
 current is flowing in the direction of the arrow in the flgure, that is, 
 passes in the nerve from the positive electrode or anode p to the negative 
 electrode or kathode p', it is found that the current through the 
 galA'anometer G is increased, while that through H is diminished. We 
 
Chap, n.] 
 
 Till-: COXTILiCTlLK TISSUES. 
 
 81 
 
 may cxphiin this result hy saying that tlio polarizinj^ current has caused 
 the appearance in the nerve outside the ehictrodes of a new current, 
 the 'eloctrotonic' current, having the same direction as itself, wliich 
 adds to, or takes away from, the natural nerve-current or " current of 
 rest" according as it is flowing in the same or in an opposite direction. 
 
 / 
 
 .'{ 
 
 jj ' «» — * p 
 
 ______ - 
 
 K 
 
 Fig. 15. Diagraii illustrating Electrotoxic Ccrrekts. 
 
 P the polarizing battery, with A; a key, p the anode, and p' the kathode. At the left 
 end of the piece of nerve the natural current flows through the galvanometer G 
 from Q to (J , in the direction of the arrows ; its direction therefore is the same 
 as that of the polarizing current; consequently it appears increased, as indicated 
 by the sign + . The current at the other end of the piece of nerve, from h to h' , 
 thi-ough the galvanometer R, flows in a contrary direction to the polarizing 
 current ; it consequently appears to be diminished, as indicated by the sign — . 
 
 N.B. For simpUcity's sake, the polarizing current is here supposed to be thrown 
 in at the middle of a piece of nerve, and the galvanometer placed at the two ends. 
 Of course it will be understood that the former may be thrown in anywhere, and the 
 latter connected with any two pairs of jjoints which will give currents. 
 
 The strength of the electrotonic current is dependent on the strength 
 of the polarizing current, and on the length of the intrapolar region 
 which is exposed to the polarizing current. When a strong polarizing 
 current is used, the electromotive force of the electrotonic current may 
 be much gi-eater than that of the natural nerve-current. 
 
 The strength of the electrotonic current varies with the irritability, 
 or vital condition of the nerve, being greater -with the more irritable 
 nerve; and a dead nerve will not manifest electrotonic currents. More- 
 over, the propagation of the current is stopped by a ligature, or by 
 crushing the nerve. 
 
82 ELECTROTONIC CURRENTS. [Book i. 
 
 We may speak of the conditions wliich give rise to tliis electrotonic 
 current as a pAysica? electro tonus analogous to that 2y^i?ysiolocp'cal electro- 
 tonus -which is made known by variations in irritability. The physical 
 electrotonic current is probably due to the escape of the polarizing 
 current along the nerve under the peculiar conditions of the living 
 nerve ; but we -must not attempt to enter here into this disputed 
 and difficult subject or into the allied question as to the exact con- 
 nection between the physical and the physiological electrotonus, though 
 there can be little doubt that the latter is dependent on the former. 
 
 An induction-sliock is a current of very short duration developed 
 very suddenly and disappearing more gradually. Hence, when it 
 falls into a nerve, the nerve undergoes a sudden transition from its 
 normal condition to the katelectrotonic phase, and consequently a 
 nervous impulse giving rise to a contraction is the result. The- 
 return from the anelectrotonic phase to the normal condition 
 appears from a number of considerations to be less effective as 
 a generator of nervous impulses than the change from the normal 
 condition to the katelectrotonic phase. Hence in the induced 
 current we have to deal with a 'making' contraction only, the 
 breaking contraction being absent. This is true whether the induced 
 current be generated by the making or by the breaking of a con- 
 stant current. 
 
 The constant current applied directly to a muscle from which 
 the purely nervous element has been eliminated by urari poisoning, 
 has effects similar to and yet somewhat different from those which 
 it has upon a nerve. The efficacy of the rise of katelectrotonus 
 and the fall of anelectrotonus respectively in producing contraction 
 is the same as in a nerve. In one respect the muscle is more 
 striking, and offers a support of the hypothesis mentioned above. 
 The making contraction may under favourable circumstances be 
 seen to start from the kathode and the breaking contraction from 
 the anode. Besides the make and break spasm a partial tetanus 
 during the whole time of the passage of the current through a 
 muscle is very often seen. Another marked difference between 
 muscle and nerve is that in muscle the current must act for a 
 much longer time upon the tissue before it can call forth a con- 
 traction. This is what we might expect from the more sluggish 
 nature of the muscular impulse-wave. Hence muscular tissue 
 which has lost its nervous elements or does not possess them, is 
 far less readily affected by the almost momentarj^ induction-shocks 
 than are nerves. 
 
SEC. 4. THE MUSCLE-NERVE PREPARATION AS A 
 
 MACHINE. 
 
 The facts described in the forec^oiiio- sections shew that a 
 muscle "With its nerve may be' justly regarded as a machine which, 
 Avhen stimulated, "will do a certain amount of work. But the 
 actual amount of work which a muscle-nerve preparation will do is 
 found to depend on a large number of circumstances, and conse- 
 quently to vary within very wide limits. These variations will be 
 largely determined by the condition of the muscle and nerv-e in 
 respect to their nutrition ; in other words, by the degree of irrita- 
 bility manifested by the muscle or by the nerxe or by both. But 
 quite apart from the general influences affecting its nutrition and 
 thus its irritability, a muscle-nerve preparation is affected as 
 regards the amount of its work by a variety of other circumstances, 
 which we may briefly consider here, reserving to a succeeding 
 section the study of variations in irritability. 
 
 TTie nature and mode of application of the stimulus as affecting 
 the amount and character of the contraction. 
 
 We have seen that a nervous impulse is a molecular disturbance 
 travelling along the nerve in the form of a wave. We saw further 
 that the velocity with which this Avave travels is in the frog about 
 28 inches per sec, and in the mammal someAvhat higher, but that 
 it varies according to cii'cumstances, being especially dependent on 
 temperature. The wave-length, that is, the total length of nerve 
 along which the disturbance is at any one instant taking place, 
 from the jDoint nearer the muscle which the disturbance has just 
 reached, to the point farther from the muscle which the disturbance 
 has just left, may we have seen be put down (in the frog) as IS mm.; 
 but possibly this too varies somewhat. The greatest and most 
 important variations however are those of the energy of the nervous 
 impulse, of the amount of disturbance which takes place in the 
 nerve or in the nerve fibre as the wave of the nervous impulse 
 passes over it ; this we might designate as the height of the wave. 
 
 G— 2 
 
84 THE MUSCLE-NERVE MACHINE. [Book i. 
 
 Thus a weak stimulus gives rise to a small disturbance, that is a 
 weak nervous impulse, and a strong stimulus gives rise to a large 
 disturbance, that is a powerful nervous impulse. 
 
 We are not in a position at present to speak definitely as to 
 the occurrence of other differences in the characters of nervous 
 impulses. As far as we know at present, nervous impulses what- 
 ever their origin are alike in nature ^ ; the impulses generated, in 
 a natural way, by the brain or spinal cord, or produced artificially 
 by mechanical stimuli, as by cutting or pinching, or by thermal 
 stimuli, as by touching the nerve Avith a red-hot wire, or by 
 chemical stimuli, or by electrical stimuli, may differ in intensity, and 
 in the rapidity with which they succeed each other, but, as far as 
 we know at present, not otherwise. Thus a drop of acid placed on 
 a nerve gives rise to tetanus in the muscle which differs from the 
 tetanus produced by repeated induction shocks applied to the nerve, 
 only so far as the tetanus is generally irregular, the individual 
 nervous impulses generated by the acid forming an irregular series, 
 not following each other at equal intervals and not being all of the 
 same intensity, whereas the impulses generated by the 'interrupted 
 current ' are generally of the same intensity and follow each other 
 at equal intervals. So also we are led at present to believe that 
 when muscles are thrown into action in a natural way in the living 
 body by the agency of the spinal cord, what goes on in the nerve 
 differs from wdiat goes on in the same nerve when the interrupted 
 current is brought to bear on it, only in so far as in the former case 
 the impulses folloAv each other at a fixed rate (nineteen a second), 
 whereas in the latter, the rate of repetition varies according to 
 the rapidity with which in the induction-machine the shocks 
 follow each other; the individual impulses as far as we know at 
 present have the same characters in the two cases save only that 
 they may differ in intensity. 
 
 Supposing that the irritability of a nerve-muscle preparation 
 remains for the period of the experiment fairly constant, care 
 being taken to avoid the effects of exhaustion, and that the 
 stimulus be applied to the same part of the nerve, we find that the 
 intensity of the nervous impulse generated (as measured by the 
 muscular contraction) varies up to a certain limit according to 
 what we may call the strength of the stimulus. Thus taking a 
 single induction shock as the most manageable stimulus, we find 
 that if, before we begin, we slide the secondary coil (Fig. 1, 5C.) 
 a certain distance from primary coil ]w. c, no visible effect at all 
 follows upon the discharge of the induction shocks. The passage 
 of the momentary weak current is either unable to produce any 
 nervous impulse at all, or the weak nervous impulse to which it 
 
 1 It will be observed that we are speaking now exclusively of the nerve of a 
 muscle-nerve preparation, i.e. of what we shall hereafter term a motor nerve. 
 Whether sensory impulses differ essentially from motor impulses will be considered 
 later on. 
 
Chap. n. ] THE COSTltACTI LK TISSUES. ar, 
 
 Ljivi'S rise is unablo to stir tlio sluo-gish muscular sulj.staiici! to a 
 visible contraction. As wc slide the secondary coil towards tliu 
 primary, sending in an induction shock at each new position, we 
 tind that at a certain distance between the secondary and primary 
 coils, the muscle rt'si)onds to each induction shock' with a con- 
 traction wdiich makes itself visible by the slightest possible rise 
 of tlie attached lever. This position of the coils, the battery 
 remaining the same and other things being equal, marks the 
 minimal stimulus giving rise to the minimal contraction. As the 
 secondary coil is brought nearer to the primary, the contractions 
 increase in height corresponding to the increase in the intensity 
 of the stimulus. Very soon however an increase in the stimulus 
 caused by continuing to slide the second ai'y coil over the y)rimary 
 fails to cause any increase in the contraction. This indicates 
 that the maximal stimulus giving rise to the maximal contraction 
 has been reached ; though the shocks increase in intensity as 
 the secondary coil is pushed further and further over the primary, 
 the contractions remain of the same height, until fatigue lowers 
 them. Sometimes however, after the contractions have for some 
 time remained of the same height, in spite of the stimulus, at 
 each fresh stimulation, being increased in strength, a point is 
 reached at wdiich, with a further increase in the strength of the 
 stimulus, a new increase of contraction sets in; but we must not 
 attempt to explain here this paradoxical super-maximal contraction 
 as it is called. 
 
 With single induction shocks then the muscular contraction, 
 and by inference the nervous impulse, increases with an increase in 
 the intensity of the stimulus, between the limits of the minimal 
 and maximal stimuli; and this dependence of the nervous impulse 
 and so of the contraction on the strength of the stimulus may be 
 observed not only in electric but in all kinds of stimuli. 
 
 It may here be remarked that in order for a stimulus to 
 be effective, a certain abruptness in its action is necessary. Thus 
 we have seen that the constant current when it is passing through 
 a nerve with uniform intensity does not give rise to a nervous 
 impulse and that it may be increased or diminished to almost any 
 extent without generating nenous impulses, provided that the 
 change be made gradually enough ; it is only when there is a 
 sudden change tliat the cuiTcnt becomes effective as a stimulus. 
 The current which is induced in the secondar}'^ coil of an induction- 
 machine at the bi-eaking of the primary circuit, is more rapidly 
 developed, and has a steeper rise than the current which appears 
 when the primary circuit is made ; and accordingly we find that 
 the breaking induction shock is more potent as a stimulus than 
 the making shock. Similarly a shai-p tap on a nerve will produce 
 
 ^ In these experiments either the breaking or making shock must be used, not 
 sometimes one and sometimes the other, for tlie two kinds of shock differ in efficiency, 
 tlie breaking being the most potout. 
 
86 THE MUSCLE-NERVE MACHINE. [Book i. 
 
 a contraction, when a gradually increasing pressure will fail to do 
 so ; and in general the efficiency of a stimulus of any kind will 
 depend in part on the suddenness or abruptness of its action. 
 
 A stimulus, in order that it may be effective, must have an 
 action of a certain duration, the time necessary to produce an effect 
 varying according to its strength and being different in nerve from 
 what it is in muscle. It would appear that an electric current 
 applied to a nerve must have a duration of at least about '0015 sec. 
 to cause any contraction at all, and needs longer than this to 
 produce its full effect. When the current is applied directly to a 
 muscle, whose nervous elements are placed hors de combat by 
 the action of urari, or by degeneration of the nerve-fibres this 
 period of necessary duration seems to be still longer, and to be 
 especially increased by deficient nutrition. And this may be 
 offered as an explanation of the well-known clinical fact that in 
 various cases of paralysis, muscles which have by degeneration 
 of their nerves, lost their ner^^ous supply, more readily respond 
 to the break and make of the constant current than to induction 
 shocks, the duration of the former as stimuli beinof much greater 
 than that of the latter. 
 
 In the case of electric stimuli, the strength of the contrac- 
 tion, and by inference of the nervous impulse, depends on the 
 manner in which the current flows into the nerve. Though the 
 matter has been disputed, it appeal's that the current must 
 pass along some appreciable length of nerve-fibre in order to 
 produce an effect : a current which passes through a nerve in 
 an absolutely transverse direction being powerless to generate 
 impulses ; and further there is a connection between the etficiency 
 of the current and the angle at which it falls into the nerve. 
 
 It would also appear, at all events up to certain limits, and as 
 a general iTile, that the longer the piece of nerve through which 
 the current jjasses, the greater is the effect of the stimulus. 
 
 When two pairs of electrodes are placed on the nerve of a long 
 and perfectly fresh and successful nerve-preparation, one near to 
 the cut end, and the other nearer the muscle, it is found that the 
 same stimulus produces a greater contraction when applied through 
 the former pair of electrodes than through the latter. Two inter- 
 pretations of this result are j)0ssible. Either the nerve at the part 
 farther away from the muscle is more irritable, i.e. that the 
 stimulus gives rise at the sjjot stimulated to a larger nervous 
 impulse; or the impulse started at the farther electrodes gathers 
 strength, like an avalanche, in its progress to the muscle. The 
 latter view has been strongly urged by Pfliiger, and is generally 
 kno'svn under the name of the ' avalanche theory ', Against it 
 may be urged that as far as we know, the progress of the current 
 of action along a nerve is marked by no such increase. It is 
 probable that the larger contraction produced by stimulation of 
 the portions of the nerve near the spinal cord is due to the 
 
C'liAi'. 11.] THK COXrnACTlLK TISSi'KS. 87 
 
 stimulus srttinsj^ livo a lar^^ur inipiilsc, i.e. to this part of tho 
 nerve boin<if im)ro irritable. It is possible that the irritability 
 of a nerve may vary considerably at ditierent ])oints of its course. 
 
 \Vt' have in a ])recedini,^ section discussed at length the manner 
 in which a stimulus repeated sutiiciently rajjidly produces a com- 
 plete and uniform tetanus, during -which the constituent single 
 contractions cannot be recognized either by the appearance of the 
 muscle itself or by any features in the cur\-e which it may be 
 made to describe, though the ' muscular sound ' shews that the 
 muscle is really in a state of vibration. If the frequency of 
 the stimulus be reduced the tetanus becomes incomplete and 
 a flickering of the muscle becomes obvious, and upon further 
 reduction of the frequency the flickeiing gives place to a rhythmic 
 series of single contractions. Since the height to which the lever 
 is raised, i.e. the amount of total shortening resulting from 
 any second conti'action, is greater when that contraction starts 
 from the summit of the preceding cun-e than when it starts from 
 the decline, it is obvious that the amount of total contraction will 
 up to a certain limit increase with the frequency of repetition 
 of the stimulus. Thus a stimulus repeated rapidly will produce a 
 tetanus, shortening the muscle and raising the weight to a greater 
 extent than will the same stimulus less rapidly repeated. The 
 exact frequency of repetition required to produce complete tetanus 
 varies according to the condition of the muscle and is not the same 
 for all muscles, being dependent on the rapidity with which 
 the muscle executes each single contraction. In those animals 
 which possess two kinds of skeletal muscles, red and pale, the 
 red muscles (the single contractions of which are slow and long- 
 drawn) are thrown into complete tetanus 'W'ith a repetition of much 
 less frequency than that required for the pale muscles. Thus, 
 ten stimuli in a second are quite sufficient to throw the red 
 muscles of the rabbit into complete tetanus, while the pale muscles 
 require at least twenty stimuli in a second. 
 
 When the stimulus is repeated more frequently than is 
 required to bring about a complete tetanus the constituent 
 contractions are still proportionately increased in frequency. This 
 is shewn by the increased pitch of the muscular sound. How 
 far the increase in the frequency of the constituent contractions 
 can be carried by increasing the frequency of the stimulus is 
 a question which presents considerable difficulties, and cannot be 
 discussed here. 
 
 The value of the muscle as a machine is also in jjart dependent 
 on the Load. It might be imagined that a muscle, which, when 
 loaded with a given weight, and stimulated b}^ a current of a given 
 intensity, had contracted to a certain extent, would only contract 
 to half that extent when loaded with twice the weight and stimu- 
 lated with the same stimulus. Such however is not the case; the 
 height to which the weight is raised may be in the second instance 
 
88 TEE. MUSCLE-NERVE MACHINE. [Book i. 
 
 as great, or even greater, than in the first. That is to say, the 
 resistance offered to the contraction actually augments the con- 
 traction,^ the tension of the muscular fibre increases the facility 
 with which the explosive changes resulting in a contraction take 
 place._ And it has been observed by Heidenhain that the degree 
 of acid reaction, the amount of carbonic acid given off and the 
 rise of temperature are greater in a muscle contracting against 
 resistance than when the resistance is removed ; that is to say, 
 the tension increases the metabolism. There is, of course, a 
 limit to this favourable action of the resistance. As the load con- 
 tinues to be increased, the height of the contraction is diminished, 
 and at last a point is reached at which the muscle is unable (even 
 when the stimulus chosen is the strongest possible) to lift the load 
 at all. 
 
 In a muscle viewed as a machine we have to deal not merely 
 with the height of the contraction, that is with the amount of 
 shortening, but with the work done. And this is measured as the 
 height to which the load is raised multiplied into the weight of 
 the load. Hence it is obvious from the foregoing observations 
 that the work done must be largely dependent on the weio-ht 
 itself Thus there is a certain weight of load with which in any 
 given muscle, stimulated by a given stimulus, the most work will 
 be done. 
 
 Since mere tension afTects the changes going on in the muscular 
 fibres, it is desirable in experiments in which muscles are loaded, that 
 the weight should not bear upon the lever until the contraction actually 
 begins. This is easily managed by interposing between the end of the 
 muscle and the weight a lever with a support so arranged that, before 
 contraction takes place, the weight only extends the muscle to the 
 length natural to it during rest; but that the muscle directly it shortens 
 at once begins to pull on the weight. The muscle is then said to 
 be after-loaded^. 
 
 The value of a muscle as a inachine is further determined by the 
 Size and Form of the Muscle. Since all known muscular fibres are 
 much shorter than the wave-length of a contraction, it is obvious 
 that the longer the fibre, the greater the height of the contraction 
 with the same stimulus. Hence in a muscle of parallel fibres, the 
 height to which the load is raised as the result of a given stimulus 
 applied to its nerve, will depend on the length of the fibres, while 
 the maximum weight of load capable of being lifted will depend 
 on the number of the fibres, since the load is distributed among 
 them. Of two muscles therefore of equal length (and of the same 
 quality) the most work will be done by that which has the greater 
 sectional area; and of two muscles with equal sectional areas, 
 the most work will be done by that which is the longer. If 
 the two muscles are unequal both in length and sectional area, 
 
 ^ This is perhaps the best equivalent of the German iihcrlastet. 
 
CiiAP. II.] Tin: COXTRACTIfJ: tissues. 80 
 
 flic work done will lie llio frvfatcr in tho one which has thti 
 lari^^er bulk, whieli contains the i^acatci' number of cubic units. 
 In s[)eakiiiLj thcivlbrc of tho work which can bo done by a musclo, 
 wu may use us a standard a cubic unit of l)ulk, or, the specific 
 gravity of tho muscle being tho same, a unit of w(!ight. 
 
 Absolute power of a mancle. Wo have scon that with a given 
 weight a stimulus (induction shock) may bo chosen of such a 
 strength that a contraction is only just visible. In such a case a 
 very slight increase of the weight Avould prevent even that 
 minimal contraction. Upon increasing the stimulus the minimal 
 contraction would reappear and vanish again ujDon a further 
 increase of tho Aveight. Increasing the stimulus and weight in 
 this way wo should bo able to find out the weight which, Avith a 
 maximal stimulation, is just sufficient to prevent any visible con- 
 traction from taking place, a very slight diminution of weight at 
 once allowing a minimum contraction to make its appearance. 
 Such a weight is taken as the measure of what is called the 
 'absolute power' of the muscle; and from what has been said in the 
 pre\dous paragraph, it is obvious that this Avill depend on the 
 number of fibres in, or more correctly, on the sectional area of, the 
 muscle. The absolute power of a square centimetre of a frog's 
 muscle has been in this way estimated at about 2800 to 3000 grms.: 
 of a square centimetre of human muscle at GOOO to 8000 grms. 
 
 It may be worth while to mention in this connection the 
 following interesting fact. 
 
 If the weight be determined Avhich will stop a contraction 
 when applied directly the contraction begins, and also that which 
 stops any further contraction when applied at a moment Avhon the 
 contraction is already partly accomplished, it Avill be found that 
 the second weight is much less than the first. It appears, in fact, 
 that the forces which cause the change in the form of the muscle 
 are at their maximum at the beginning of the shortening, and 
 thenceforwards decline until they become nothing when the short- 
 ening is complete. 
 
 [Ihe work done. We learn then from the foregoing jDaragraplis 
 that the Avork done, i.e. the Aveight of the load multiplied into the 
 height of the lift. Avill depend, not only on the actiAdty of the nerve 
 and muscle as determined b}'- their OAvn irritability, but also on the 
 character and mode of application of the stimulus, on the kind 
 of contraction (whether a single spasm, or a sloAvly repeated tetanus 
 or a rapidly repeated tetanus) on the load itself, and on the size 
 and form of the muscle. Taking the most favourable circum- 
 stances, A'iz. a Avell nourished, lively preparation, a maximum 
 stimulus causing a rapid tetanus and an appropriate load, avo may 
 determine the maximum Avork done by a given Aveight, say one 
 gramme, of muscle. This in the case of the muscles of the frog 
 has been estimated at about four gi'am-metres for one gramme 
 of muscle. 
 
SEC. 5. THE CIRCUMSTAKCES WHICH DETERMINE 
 THE DEGREE OF IRRITABILITY OF MUSCLES AI^D 
 KERVES. 
 
 A muscle-nerve preparation, at the time that it is removed 
 from the body, possesses a certain degree of irritability, it responds 
 by a contraction of a certain amount to a stimulus of a certain 
 strength, applied to the- nerve or to the muscle. After a while, 
 the exact period depending on a variety of circumstances, the 
 same stimulus produces a smaller contraction, i.e. the irritability 
 of the preparation has diminished. In other Vv^ords, the muscle 
 or nerve or both have become partially 'exhausted'; and the 
 exhaustion subsequently increases, the same stimulus producing 
 smaller contractions until at last all irritability is lost, no stimulus 
 however strong producing any contraction whether applied to the 
 nerve or directly to the muscle ; and eventually the muscle, as we 
 have seen, becomes rigid. The progress of this exhaustion is more 
 rapid in the nerves than in the muscles ; for some time after the 
 nerve-trunk has ceased to respond to even the strongest stimulus, 
 contractions may be obtained by applying the stimulus directly to 
 the muscle. It is much more rapid in the warm-blooded than in 
 the cold-blooded animals. The muscles and nerves of the former 
 lose their irritability, when removed from the body, after a period 
 varying according to circumstances from a few minutes to two or 
 three hours ; those of cold-blooded animals (or at least of an 
 amphibian or a reptile) may under favourable conditions remain 
 irritable for two, three, or even more days. The duration of 
 irritability in warm-blooded animals may however be considerably 
 prolonged by reducing the temperature of the body before death. 
 
Chai'. II.] THE coy rn ACT ILK TISSUES. 91 
 
 If with soino thin body a shiirp blow Ije struck across a rnuscle wliicli 
 has entered into the later stages of exhaustion, a wjieal lasting for 
 several seconds is developed. This wheal appears to bo a contructiou 
 wave limited to the part struck, and disappearing very slowly, without 
 extending to the neighbouring muscular substance. It has been called 
 an 'idio-muscular' contraction, because it may be brought out even when 
 ordinary stimuli have ceased to produce any effect. It may however be 
 accompanied at its beginning by an ordinary contraction. It is readily 
 produced in the living body on the pectoral and otlior muscles of persons 
 sufl'ering from phthisis and other exJaausting diseases. 
 
 This natural exhaustion and diminution of irritability in 
 muscles and nerves rcmo\ed from the body may be modified both 
 in the case of the muscle and of the nerve, by a variety of circum- 
 stances. Similarly, wliile the nerve and muscle still remain in the 
 body, the irritability of the one or of the other may be modified 
 either in the way of increase or of decrease by various events. 
 We have already seen (p. 78) how the constant current produces 
 the variations in irritability known as katelectrotonus and anelec- 
 trotonus. We have now to study the effect of more general 
 influences, of which the most important are, severance from the 
 central nervous system, and variations in temperature, in blood- 
 supply, and in functional activity. 
 
 The Effects of Severance from the Central Xervous System. 
 
 When a nerve, such for instance as the sciatic, is divided in 
 situ, in the living body, there is first of all observed a slight 
 increase of irritability, noticeable especially near the cut end ; but 
 after a while the irritability diminishes, and gradually disappears. 
 Both the slight initial increase and the subsequent decrease begin 
 at the cut end and advance centrifugally towards the peripheral 
 terminations. This centrifugal feature of the loss of irritability is 
 often spoken of as the Ritter-Valli law. In a mammal it may be 
 two or three days, in a frog, as many, or even more weeks, before 
 irritability has disappeared from the nerve-trunk. It is maintained 
 in the small (and especially in the intramuscular) branches for still 
 longer periods. 
 
 This centrifugal loss of irritability is the forerunner in the 
 peripheral portion of the divided nerve of structural changes which 
 proceed in a similar centrifugal manner. The medulla suffers 
 changes similar to those seen in nerve-fibres after removal from the 
 body. Its double contour and its characteristic indentations be- 
 come more marked, it breaks up into small irregular fragments, or 
 drops, a separation apparently taking place between its proteid and 
 its fatty constituents. The latter are soon absorbed, but the 
 former remain for a longer time within the sheath of Schwann, 
 being in some cases scarcely, if at all, to be distinguished from the 
 
92 YARIATIOXS OF IRRITABILITY. [Book i. 
 
 swollen axis-cylinder. Meanwhile the nuclei which occur, one in 
 each segment of the nerve between each two nodes of Ranvier, 
 divide and multiply rapidly. Lastly the axis-cylinder breaks up 
 and disappears so that nothing remains of the original fibre 
 but the sheath of Schwann enclosing a proteid mass with many 
 nuclei. If no ' regeneration takes place these nuclei eventually 
 disappear. 
 
 In the central portion of the divided nerve similar changes may 
 be traced as far only as the next node of Eanvier. Beyond this 
 the nerve usually remains in a normal condition. 
 
 Regeneration, when it occurs, is apparently carried out by the 
 peripheral growth of the axis-cylinders of the intact central 
 portion. When the cut ends of the nerve are close together the 
 axis-cylinders growing out from the central portion run into and 
 between the sheaths of Schwann of the peripheral portion ; but 
 much uncertainty still exists as to the exact parts which the 
 proliferated nuclei referred to above, the proteid remnants of the 
 medulla, and the old axis-cylinders of the peripheral portion 
 respectively play in giving rise to the new structures of the 
 regenerated fibre. 
 
 This degeneration may be obser^^ed to extend down to the very 
 endings of the nerve in the muscle, including the end-plates, but 
 does not at first affect the muscular substance itself. The muscle, 
 though it has lost all its nervous elements, still remains irritable 
 towards stimuli appalled dkectly to itself: an additional proof of 
 the existence of an independent muscular irritability. 
 
 For some time the irritability of the muscle, as well towards stimuli 
 applied directly to itself as towards those applied througii the impaired 
 nerve, seems to be diminished; but after a while a peculiar condition 
 (to Vidiich we have already alluded on p. 86) sets in, in which the muscle 
 is found to be not easily stimulated by single induction shocks but to 
 respond readily to the make or break of a constant current. In fact it 
 is said to become even more sensitive to the latter mode of stimulation 
 than it was when its nerve was intact and functionally active. At the 
 same time it also becomes more irritable towards direct mechanical 
 stimuli, and very frequently fibrillar contractions, more or less rhythmic 
 and apparently of spontaneous origin, though their causation is ob- 
 scure, make their appearance. This phase of heightened sensitiveness 
 of a muscle, especially to the constant current, appears to reach its 
 maximum, in man at about the seventh week after nervous impulses, 
 from injury to the nerves or nervous centre, have ceased to reach the 
 muscle. 
 
 If the muscle thus deprived of its nervous elements be left to 
 itself its irritability however tested sooner or later diminishes, but 
 if the muscle be periodically thrown into contractions by artificial 
 stimulation with the constant current, the decline of irritability 
 and attendant loss of nutritive power may be postponed for some 
 considerable time. But as far as our experience goes at present 
 
("'iiAP. II.] Tin: C(K\TRACT1LE TISSUES. 03 
 
 the artilicitil .stiiiuihition caiiiiot lully replace: the natural one and 
 sooner or later the muscle like the nerve suffers degeneration, loses 
 all irritability and ultimately becomes rci^laced by connective 
 tissue. 
 
 TJie Influence of Temperature. 
 
 We have ah-eady seen that sudden heat applied to a limited 
 part of a nerve or muscle, as when the nerve or muscle is touched 
 with a hot Avire, Avill act as a stimulus, and the same might 
 he said of cold Avhen sufficiently intense. It is however much 
 more ditticult to generate nervous or muscular impulses by ex- 
 posing a Avhole nerve or muscle to a gradual rise of temperature. 
 Thus according to most observers a nerve belonging to a muscle' 
 may be either cooled to 0'^ C. or below, or heated to 50" or even 
 100" C, without discharging any nervous impulses, as sheAATi by the 
 absence of coutracti(jn in the attached muscle. The contractions 
 moreover may be absent even when the heating has not been very 
 gradual. 
 
 A muscle may be cooled to 0" C. or below without any contrac- 
 tion being caused ; but when it is heated to a limit, which in the- 
 case of frog's muscles is about 45", of mammalian muscles about 
 50", a sudden change takes place : the muscle falls, at the limiting 
 temperature, into a rigor mortis, which is initiated by a forcible 
 contraction or at least shortening. The rigor mortis thus brought 
 about by heat is often spoken of as rigor caloris. 
 
 Moderate warmth, ex. r/r. in the frog an increase of temperature 
 up to somev/hat below 45" C, favours both muscular and nervous 
 irritability. All the molecular processes are hastened and facili- 
 tated : the conti'actiou is for a given stimulus greater and more 
 rapid, i.e. of shorter duration, and nervous impulses are generated 
 more readily by slight stimuli. Owing to the quickening of the 
 chemical changes, the suppl}^ of ncAv material may prove insuffi- 
 cient ; hence muscles and nerves removed from the body lose their 
 iiTitability more rapidly at a high than at a low temperature. 
 
 The gradual application of cold to a nerve, especially when the 
 temperature is thus brought near to 0", slackens all the molecular 
 processes, so that the wave of nervous impulse is lessened and pro- 
 longed, the velocity of its passage being much diminished, e.g. from 
 28 m. to 1 m. per sec. At about 0" the irritability of the nerve 
 disappears altogether. 
 
 When a muscle is exposed to similar cold, ex. r/r. to a tempera- 
 ture very little above zero, the contractions are remarkably pro- 
 longed ; they are diminished in height at the same time, but not 
 in proportion to the increase of their duration. Exposed to a 
 temperature of zero or below, muscles soon lose their imtability, 
 
 ^ The action of cold and heat on sensory nerves will be conr-iclcred in the later 
 portion of the woi!:. 
 
94 VARIATIONS OF IRRITABILITY. [Book i. 
 
 without however undergoing rigor mortis. After an exposure of 
 not more than a few seconds to a temperature not much below 
 zero, they may be restored, by gradual Avarmth, to an irritable con- 
 dition, even though they may appear to have been frozen. When 
 kept frozen however for some few minutes, or when exposed for a 
 less time to temperatures of several degrees below zero, their 
 in-itability is permanently destroyed. When thawed, they enter 
 into rigor mortis of a most pronounced character. 
 
 The Influence of Blood-Supply. 
 
 When a muscle still within the body is deprived by any means 
 of its proper blood-supply, as when the blood-vessels going to it are 
 ligatured, the same gradual loss of irritability and final appearance 
 of rigor mortis are observed as in muscles removed from the body. 
 Thus if the abdominal aorta be ligatured, the muscles of the lower 
 limbs lose their irritability and finally become rigid. So also in 
 systemic death, when the blood-supply to the muscles is cut off by 
 the cessation of the circulation, loss of irritability ensues, and rigor 
 mortis eventually follows. In a human corpse the muscles of the 
 body enter into rigor mortis in a fixed order : first those of the jaw 
 and neck, then those of the trunk, next those of the arms, and 
 lastly those of the legs. The rapidity with which rigor mortis 
 comes on after death varies considerably, being determined both by 
 external circumstances and by the internal conditions of the body. 
 Thus external warmth hastens and cold retards the onset. After . 
 great muscular exertion, as in hunted animals, and w^hen death 
 closes wasting diseases, rigor mortis in most cases comes on rapidly. 
 As a general rule it m-ay be said that the later it is in making its 
 appearance, the more pronounced it is, and the longer it lasts ; but 
 there are many exceptions, and v/hen the state is recognized as 
 being fundamentally due to a coagulation, it is easy to understand 
 that°the amount of rigidity, i.e. the amount of thecoagulum, and 
 the rapidity of the onset, i.e. the quickness with which coagulation 
 takes place, may vary independently. The rapidity of onset after 
 muscular exercise and wasting disease is apparently dependent on 
 an increase of acid reaction, being produced under those circum- 
 stances in the muscle, for this seems to be favourable to the coagu- 
 lation of the muscle plasma. When rigor mortis has once become 
 thoroughly established in a muscle through deprivation of blood, it 
 cannot be removed by any subsequent supply of bipod. Thus 
 where the abdominal aorta has remained ligatured until the lower 
 limbs have become completely rigid, untying the ligature will not 
 restore the muscles to an irritable condition ; it simply hastens the 
 decomposition of the dead tissues by supplying them with oxygen 
 and, in the case of the mammal, with warmth also. A muscle 
 however may acquire as a whole a certain amount of rigidity on 
 account of some of the fibres becoming rigid, while the remainder, 
 
Chap, ii.] THE COXTHACTl LK TL'SSUES. O.') 
 
 tho\i^li tliey have lost their irritability, have not yet advanced into 
 rigor mortis. At siu-h a juncture a renewal of" the blood-stream 
 may restore the irritability of those iibres which were not yet rigid, 
 and thus appear to do away with rigor mortis ; yet it appears tiiat 
 in such cases the fibres which have actually become rigid never 
 regain their irritability, but undergo degeneration. 
 
 Mere loss of irritability, even though complete, if stopping short 
 of the actual coagulation of the muscle-substance, may be with 
 care removed. Thus if a stream of blood be sent artificially 
 through the vessels of a separated (mammalian) muscle, the irrita- 
 bility may be maintained for a very considerable time. On stopping 
 the artificial circulation, the irritability diminishes and in time 
 entirely disappears ; if however the stream be at once resumed, 
 the irritability will be recovered. By regulating the fiow, the 
 irritability may be lov/ered and (up t<^ a certain limit) raised at 
 pleasure. From the epoch however of interference with the nor- 
 mal blood-stream there is a gi'adual diminution in the responses 
 to stimuli, and ultimately the muscle loses all its irritability and 
 becomes rigid, however w^ell the artificial circulation be kept up. 
 This failure is probably in great part due to the blood sent through 
 the tissue not being in a perfectly normal condition ; but we have 
 at present very little information on this point. Indeed with 
 respect to the quality of blood thus essential to the maintenance 
 or restoration of irritability, our knowledge is definite with regard 
 to one factor only, viz. the oxygen. If blood deprived of its oxygen 
 be sent through a muscle removed from the body, irritability, so 
 far from being maintained, seems rather to have its disajopearance 
 hastened. In fact, if venous blood continues to be driven through 
 a muscle, the imtability of the muscle is lost even more rapidly 
 than in the entire absence of blood. It would seem that venous 
 blood is more injurious than none at all. If exhaustion be not 
 carried too far, the muscle may however be revived by a proper 
 supply of oxygenated blood. 
 
 The influence of blood-supply cannot be so satisfactorily studied 
 in the case of nerves as in the case of muscles ; there can however 
 be little doubt that the effects are analosrous. 
 
 Tlie InJIuence of Functional Activity. 
 
 This too is more easily studied in the case of muscles than of 
 nerves. 
 
 When a muscle within the body is unused, it wastes ; when 
 used it (within certain limits) grows. Both these facts shew that 
 the nutrition of a muscle is favourably affected by its functional 
 activity. Part of this may be an indirect effect of the in.creased 
 blood-supply which occurs when a muscle contracts. When a 
 nerve going to a muscle is stimulated, the blood-vessels of the 
 muscle dilate. Hence at the time of the contraction more blood 
 
96 VARIATIONS OF IRRITABILITY. [Book i. 
 
 flows throi.igh the muscle, and this increased flow continues for 
 some little while after the contraction of the muscle has ceased. 
 But, apart from the blood-supply it is probable that the ex- 
 haustion caused by a contraction is immediately followed by a 
 reaction favourable to the nutrition of the muscle ; and this is a 
 reason, possibly the chief reason, why a muscle is increased by use, 
 that is to say, the loss of substance and energy caused by the 
 contraction is subsequently more than made up for by increased 
 metabolism during the following period of rest. 
 
 Whether there be a third factor, whether muscles for in- 
 stance are governed by so-called trophic nerves which affect their 
 nutrition directly in some other way than by influencing either 
 their blood-supply or their acti^dty, must at present be left 
 undecided. 
 
 A muscle, even within the body, after prolonged action is 
 fatigued, i.e. a stronger stimulus is required to produce the same 
 contraction; in other words, its irritability may be lessened by 
 functional activity. "Whether functional activity therefore is in- 
 jurious or beneficial depends on its amount in relation to the 
 condition of the muscle. It may be here remarked that as a 
 muscle becomes more and more fatigued, stimuli of short duration, 
 such as induction shocks, sooner lose their efficacy than do stimuli 
 of longer duration such as the break and make of the constant 
 current. 
 
 The sense of fatigue of v/hich, after prolonged or unusual exertion, 
 we are conscious in our own bodies, is probably of complex origin, 
 and its nature, like that of the normal muscular sense of which we 
 shall have to speak hereafter, is at present not thoroughly under- 
 stood. It seems to be in the first place the result of changes in 
 the muscles themselves, but is possibly also caused by changes in 
 nervous apparatus concerned in muscular action, and especially 
 in those parts of the central nervous system which are concerned 
 in the production of voluntary impulses. In any case it cannot be 
 taken as an adequate measure of the actual fatigue of the muscles; 
 for a man who says he is absolutely exhausted may under excite- 
 ment perform a very large amount of work with his already weary 
 muscles. The will in fact rarely if ever calls forth the greatest 
 contractions of which the muscles are capable. 
 
 Absolute (temporary) exhaustion of the muscles, so that the 
 strongest stimuli produce no contraction, may be produced even 
 v/ithin the body by artificial stimulation; recovery takes place 
 on rest. Out of the body absolute exhaustion takes place readily. 
 Here also recovery may take place. Whether in any given case it 
 does occur or not, is determined by the amount of contraction 
 causing the exhaustion, and by the previous condition of the 
 muscle. In all cases recovery is hastened by renewal (natural or 
 artificial) of the blood-stream. The more rapidly the contractions 
 follow e€<.ch other, the less the interval between any two con- 
 
Chap, ii.] TJIK COSTUACTll.E TISSUES. 97 
 
 tractions, the more rapid the exhaustion. A certain number of 
 single induction-shocks repeated rapidly, say every second or 
 oftener, brinj.^ about exhaustive loss of irritability more rapidly 
 than the Sixme number of shocks repeated less rapidly, for instance 
 every 5 or 10 seconds. Hence tetanus is a ready means of pro- 
 ducing exhaustion. 
 
 In exhausted muscles the elasticity is much dinunished ; the 
 tired muscle returns less readily to its natural length than does the 
 fresh one. 
 
 The exhaustion due to contraction may be the result : — Either 
 of the consumption of the store of really contractile material 
 present in the muscle. Or of the accumulation in the tissue 
 of the products of the act of contraction. Or of both of these 
 causes. 
 
 The restorative influence of rest may be explained by supposing 
 that during the repose, either the internal changes of the tissue 
 manufacture new explosive material out of the comparatively raw 
 material already present in the fibres, or the directly hurtful pro- 
 ducts of the act of contraction undergo changes by which they are 
 converted into comparatively inert bodies. A stream of fresh 
 blood may exert its restorative influence not only by quickening 
 the above two events, but also by carrying off the immediate waste 
 products while at the same time it brings new raw material. It is 
 not known to what extent each of these parts is played. That the 
 products of contraction are exhausting in their effects, is she's^^l by 
 the facts that the injection of a solution of the muscle-extractives 
 into the vessels of a muscle produces exhaustion and that exhausted 
 muscles are recovered by the simple injection of inert saline 
 solutions into their blood-vessels ; moreover lactic acid and indeed 
 other acids injected into a muscle cause rapid exhaustion ; and 
 we may suppose that carbonic acid, with the other substances 
 which after a contraction tend to give rise to an acid reaction, 
 when generated too rapidly to be neutralized by the alkaline 
 lymph in which the fibres are bathed, in part at least determine 
 the exhaustion. But the matter has not yet been fully worked out. 
 
 One important element brought by fresh blood is oxygen. This, 
 as we have seen, is not necessary for the canying out of the actual 
 contraction, and yet is essential to the maintenance of irritability. 
 It is probably of iise as what may be called "intramolecular 
 oxygen " in preparing the explosive material whose decomposition 
 gives rise to the carbonic acid, and other products of contraction. 
 
 r. 
 
SEC. 6. THE ENERGY OE MUSCLE AXD NERYE, AND THE 
 NATURE OF MUSCULAR AND NERYOUS ACTION. 
 
 We may briefly recapitulate some of the chief results arrived at 
 in the preceding pages as follows. 
 
 A muscular contraction itself is essentially a translocation of 
 molecules, a change of form not of bulk. We cannot say however 
 anything definite as to the nature of this translocation or as to 
 the way in which it is brought about. Though it would appear 
 that the dim doubly refractive bands increase in bulk at the 
 expense of the bright singly refractive bands, we cannot satis- 
 factorily explain the connection between the striation of a muscular 
 fibre and a muscular contraction. Nearly all rapidly contracting 
 muscles are striated, and we must suppose that the striation is 
 of some use ; but it is not essential to the carrying out of a 
 contraction, for many muscles are not striated. But whatever 
 be the exact way in which the translocation is effected, it is 
 fundamentally the result of a chemical change, of an explosive 
 decomposition of certain parts of the muscle-substance. The 
 energy which is expended in the mechanical work done by the 
 muscle has its source in the latent energy of the muscle-substance 
 set free by that explosion. Concerning the nature of that ex- 
 plosion we only know at present that it results in the production 
 of carbonic acid and in an increase of the acid reaction, and that 
 heat is set free as well as the specific muscular energy. There is 
 a general parallelism between the extent of metabolism taking 
 place and the amount of energy set free. The greater the 
 development of carbonic acid, the larger is the contraction and 
 the higher the temperature. 
 
 It has not been possible hitherto to draw up a complete equa- 
 tion between the latent energy of the material and the two forms 
 of actual energy set free. The proportion of energy given out 
 as heat to that taking on the form of work probably varies 
 under different circumstances; and it would appear that on the 
 whole a muscle would be no more economical than a steam-engine 
 in respect to the conversion of chemical action into mechanical 
 
CiiAi'. II.] THE COXTnArriLf: 77 SSf'ES. 99 
 
 work, were it not that in warm-blooded animals the heat given 
 out is not, as in the steam-engine, mere loss, but by keeping up the 
 animal temperature sei*ves many subsidiary purposes. It might be 
 supposed that when in a contraction work is actually done, the 
 increase of temperature is less than when the same contraction 
 takes jilace without doing actual work, that is to sa}', that the 
 mechanical work is done at the expense of energy which otherwise 
 wouUl go out as heat. Probable as this may seem it has not 
 yet been experimentall}^ verified. 
 
 Of the exact nature of the chemical changes which underlie a 
 muscular contraction we know very little, the most important fact 
 being, that the contraction is not the outcome of a direct oxidation, 
 but the splitting up or explosive decomposition of some complex 
 substance. The muscle does consume oxygen, and the products of 
 muscular metabolism are in the end products of oxidation, but the 
 oxygen appears to be introduced not at the moment of explosion 
 but at some earlier date. There is no evidence of nitrogenous 
 products being given off as waste ; such nitrogenous crystalline 
 bodies as ai"e present in muscle, kreatin, &c., may be regarded 
 rather as the Avear-and-tear of the machine than as products of 
 the material consumed in the work. Yet it is hardly consonant 
 with what Ave know elscAvhere, to suppose that the contraction 
 of a muscular fibre has for its essence the decomposition of a 
 non-nitrogenous substance ; and Ave ma}^ suppose that the explosion 
 does involve some nitrogenous products, Avhich however are re- 
 tained Avithin the tissue, and used up again. We may even go so 
 far as to entertain Avith Hermann the vioAv that a single complex 
 substance, an hypothetical inogen, splits up partly into nitrogenous, 
 partly into non-niti'ogenous factors, the former, possibly of the 
 nature of myosin, being rapidly built up again into neAV inogen, 
 Avhile the latter, such as the cai'bonic acid, are discharged at once 
 from the muscle. But our knoAvledge of these matters is not yet 
 ripe enough for the construction of an adequate and Avholly 
 satisfactory theory. It may be worth Avhile to point out that 
 during even the most complete repose muscle is undergoing chemi- 
 cal changes, Avhich, as far as Ave knoAV, are the same in kind, and 
 only dift'er in degree from those chai'acteristic of a contraction. 
 Thus carbonic acid is constantly being produced, as are probably 
 other substances, all being got rid of as they form, just as they are 
 got rid of in larger quantities during the repose Avhich folloAvs 
 contraction. Supposing the existence of a substance Avhich splits 
 up into these vaiious products, and Avhich Ave may speak of as the 
 true contractile material, it is evident that this material being thus 
 constantly used up, must be as constantly repaired. Thus a stream 
 of chemical substances may be conceived of as floAving through 
 muscle, the raAv material brought by the blood being gradually 
 converted into true contractile stuff, the breaking-doAAm again of 
 Avhich IS gentle and gradual so long as the muscle is at rest, but 
 
100 MUSCULAR AND NERVOUS ACTION. [Book i. 
 
 becomes excessive and violent when a contraction takes place. 
 When rigor mortis sets in, the whole remaining contractile material 
 is decomposed. 
 
 While in muscle the chemical events ai'e so prominent that we 
 cannot help considering a muscular contraction to be essentially a 
 chemical process, with electrical changes as attendant phenomena 
 only, the case is different with nerves. Here the electrical pheno- 
 mena completely overshadow the chemical. Our knowledge of the 
 chemistry of nerves is at present of the scantiest, and the little we 
 know as to the chemical changes of nervous substance is gained by 
 the study of the central nervous organs rather than of the nerves. 
 We find that the irritability of the former is closely dependent on 
 an adequate supply of oxygen, and we may infer from this that in 
 nervous as in muscular substance a metabolism, of in the main an 
 oxidative character, is the real cause of the development of energy; 
 and the axis-cylinder (which is probably the active element of a 
 nerve-fibre, the medulla being useful for its nutrition and protec- 
 tion only,) undoubtedly resembles in many of its chemical features 
 the substance of a muscula.r fibre. But we have as yet no sa,tis- 
 factory experimental evidence that the passage of a nervous impulse 
 along a nerve is the result, like the contraction of a muscular fibre, 
 of chemical changes, and like it accompanied by an evolution of 
 heat. 
 
 On the other hand, the electric phenomena are so prominent 
 that some have been tempted to regard a nervous impulse as 
 essentially an electrical change. But it must be remembered that 
 the actual energy set free in a nervous impulse is so to speak in- 
 significant, so that chemical changes too slight to be recognized by 
 the means at present at our disposal would amply suffice to provide 
 all the energy set free. On the other hand, the rate of transmission 
 of a nervous impulse, putting aside other features, is alone sufficient 
 to prove that it is something quite different from an ordinary 
 electric current. 
 
 The curious disposition of the end-plates, and their remarkable 
 analogy with the electric organs which are found in certain animals, 
 has suggested the view that the passage of a nervous impulse from 
 the nerve-fibre into the muscular substance is of the nature of an 
 electric discharge. But these matters are too difficult and too 
 abstruse to be discussed here. 
 
 It may however be worth while to remind the reader that in 
 every contraction of a muscular fibre, the actual change of form is 
 preceded by invisible changes propagated all over the fibre and 
 occupying the latent period, and that these changes resemble in 
 their features the nervous impulse of which they are so to speak the 
 continuation rather than the contraction of which they are the 
 forerunners and to which they give rise. So that a muscle, even 
 putting aside the visible terminations of the nerve, is funda- 
 mentally a muscle and a nerve besides. 
 
SEC. 7. OTHER FOR:\rS OF CONTRACTILE TISSUE. 
 
 Unstnated Muscular Tissue. Our knowledge of the phenomena 
 of these structures is very imperfect since (in vertebrates) they do 
 not exist in isolated masses like the striated muscles, but occur as 
 constituents of complex organs, such as the intestine, ureter, 
 uterus, &c. They undergo rigor mortis : and what little informa- 
 tion we do possess concerning their chemical and physical features 
 leads us to believe that the processes which take place in them are 
 fundamentallv identical with those occurring in striated muscle, 
 the two ditfering in degi'ee rather than in kind. When stimulated, 
 they contract. If a stimulus, mechanical or electrical, be applied 
 to the intestine or ureter of a maiiimal, a circular contraction is 
 seen to take place at the spot stimulated. The contraction, which 
 is preceded by a very long latent period, lasts a very considerable 
 time, in fact "several seconds, after which relaxation slowly takes 
 place. That is to say, over the circularly dispersed fibres of the 
 intestine (or ureter) at the spot in question there has passed a con- 
 traction-wave remarkable for its long latent period and for the slow- 
 ness of its development. From the spot so directly stimulated, the 
 contraction may pass as a wave (with a length of 1 cm. and a 
 velocity of from 20 to 30 millimetres a second in the ureter), along 
 the circular coat both upwards and downwards. The longitudinal 
 fibres at the spot stimulated are also thrown into contractions of 
 altogether similar character, and a wave of contraction may also 
 travel longitudinally along the longitudinal coat both upwards and 
 do^^-nward's. It is evident however that the wave of contraction of 
 which we are now speaking is in one respect different from the 
 wave of contraction treated of in dealing with striated muscle. In 
 the latter case the contraction-wave is a simple wave propagated 
 
102 UNSTRI ATE D MUSCLE. [Book i. 
 
 along the individual fibre ; in the case of the intestine or ureter, 
 the wave is complex, being the sum of the contraction-waves of 
 several fibres engaged in different phases and is propagated from 
 fibre to fibre, both in the direction of the fibres, as when the whole 
 circumference of the intestine is eng-aged in the contraction, or 
 when the wave travels longitudinally along the longitudinal coat, 
 and also in a direction at rig-ht angles to the axes of the fibres, as 
 when the contraction-wave travels lengthways along the circular 
 coat of the intestine, or when it passes across a breadth of the 
 longitudinal coat. Moreover, it is obvious that the contraction- 
 wave which passes along a single unstriated fibre differs from that 
 passing along a striated fibre, in the very great length both of its 
 latent period and of the duration of its contraction. 
 
 Waves of contraction thus passing along the circular and longi- 
 tudinal coats of the intestine constitute what is called peristaltic 
 action. 
 
 Like the skeletal muscles, whose nervous elements have been 
 rendered functionally incapable (p. 86), unstriated muscles are 
 much more sensitive to the making and breaking of a constant 
 current than to induction-shocks. 
 
 The unstriated muscles seem to be remarkably susceptible to 
 the influences of temperature. Thus the unstriated muscles of 
 the trachea are said not to contract at a temperature below 12"C., 
 and are most active at a temperature above 21° C. So also the 
 movements of the intestine cease at a temperature below 19°C. 
 
 In striking contradistinction to what takes place in the striated 
 muscles, automatic movements are exceedingly common in struc- 
 tures built up of non-striated muscles ; these moreover exhibit a 
 great tendency to rhythmic action. Thus the peristaltic action of 
 the intestine and ureters, and the corresponding movements of the 
 uterus, are at once rhythmic, and largely automatic. What share 
 the nervous elements take in the automatism and the rhythm is 
 uncertain. 
 
 Cardiac Muscles. The most important features of this form of 
 contractile tissue will be studied when we come to deal with the 
 heart. It will be seen that they are intermediate between ordinary 
 skeletal and non-striated muscles. 
 
 Cilia. Ciliary movement consists in the rapid flexion (into a 
 sickle or hook-form) of the cilium and its less rapid return to its 
 previous straight form. The diminished velocity of the return 
 leads to the force of the ciliary action being exerted in the same 
 direction as the flexion. The cause of the flexion seems to be the 
 contraction of the cilium, and that of the return, an elastic reaction. 
 In the lower animals however many varieties in the mode of move- 
 ment of cilia may be observed. 
 
 Various attempts to explain the movement by the presence of 
 special mechanisms at the base of the cilia have hitherto failed. 
 Some authors have attributed the movement to a protojDlasmic 
 
Chap, ii.] COXT/iACTr/J: TISSUKS. 103 
 
 contraction of the cell itself, the ciliura acting merely as a minute 
 elastic rod ; and some such view as this is supported by the fact 
 that no movement has ever been observed in an isolated ciliiun. 
 Tt is difficult however to understand how the peculiar sickle-like 
 ih'xion of the cilium can be brought about unless the contractile 
 material is continued up into the cilium itself; and the tail of a 
 spermatozoon, which is practically a single cilium, may contract 
 even when separated from the head. 
 
 Ciliary movement appears therefore to differ from ordinary 
 muscular contraction chicHy in the size of the apparatus concerned. 
 The movement is rapid : thus Engelmann has estimated that 
 in the frog the flexions are repeated at least twelve times in a 
 second. The movement in fact is too rapid to be visible ; it 
 can only be seen at a time when exhaustion and coming death 
 have begun to retard the action ; Engelmann found that he was 
 first able to count them when their rajjidity declined to eight in a 
 second. 
 
 In the vertebrate animal, cilia are, as far as we know, wholly 
 independent of the nervous system, and their movement is pro- 
 bably ceaseless. In such animals however as Infusoria, Hydrozoa, 
 &c. the movements in a ciliary tract may often be seen to stop 
 and go on again, to be now fast now slow, according to the needs 
 of the economy, and, as it almost seems, according to the will 
 of the creature ; indeed in some of these animals the ciliary move- 
 ments are clearly under the influence of the nervous system. 
 
 Obsen-ations ^^dth galvanic currents, constant and inteiTupted, 
 have not led to any satisfactory results, and, as far as we know at 
 present, ciliary action is most affected by changes of temperature and 
 chemical media. Moderate heat quickens the movements, but a 
 rise of temperature beyond a certain limit (about 40'C. in the case 
 of the pharyngeal membrane of the frog) becomes injurious ; cold 
 retards. Very dilute alkalis are favourable, acids are injurious. 
 An excess of carbonic acid or an absence of oxygen diminishes or 
 arrests the movements, either temporarily or permanently, according 
 to the length of the exposure. Chloroform or ether in slight doses 
 diminishes or suspends the action temporarily, in excess kills and 
 disorganises the cells. 
 
 Migrating Cells. We have already (p. So) urged the view that 
 an amoeboid movement of a white corpuscle is essentially a form 
 of contraction. 
 
 All the circumstances which affect muscular contraction, heat, 
 absence or presence of oxygen and carbonic acid, &c., also affect 
 protoplasmic movements. The white coii^uscles, like muscular 
 fibres, suffer rigor mortis, in which state they become spherical. 
 
CHAPTEK III. 
 
 THE FUNDAMENTAL PROPERTIES OF NERVOUS 
 
 TISSUES. 
 
 In its simplest, and probably earliest form, a nerve is nothing more 
 tban a tbin strand of irritable protoplasm, forming the means of 
 
 Fig. 16. Diagram to illustrate the Simplest Forms op a Nervous System. 
 
 A. An ectoderm cell e.c, •with its muscular process m.'p., as in Hydra. 
 
 B. The ectoderm cell e.c. is connected with the muscle cell mx. by means of the 
 primary motor nerve m.n. 
 
 C. The differentiated sensitive cell s.c. is connected by means of the sensory 
 nerve s.n. with the central cell c.c, which is again connected by means of 
 the motor nerve m.n. with the muscle cell mx. 
 
L'liAi". Ill ] PRO PERT IKS OF XKRVOCS TISSl'HS. lOj 
 
 vital coinmunieation botwcin a sc-nsitlvi' fctodeniiic cell exposed 
 to extrinsic aceidcuts, and a muscular, hif^ddy contractile coll (or a 
 muscular process of the same cell) buried at some distance from the 
 surface of the body, and thus less susceptible to external infiueuces. 
 (Fig. 10, A, B.) If in Hydra, we imagine the junction of the 
 ectodermic nmscidar process with the body of its cell to be drawn out 
 into a thin thread (as is said to be the case in some other Hydrozoa), 
 we should have just such a primary nerve. Since there would be 
 no need for such a means of communication to be contractile and 
 capable of itself changing in form, but on the other hand an ad- 
 vantage in its remaining immobile, and in its dimensions being 
 reduced as much as possible consistent with the maintenance of 
 irritability, the primary ner\-e Avould in the process of development 
 lose the property of contractility in proportion as it became more 
 imtable, i.e. more apt in the propagation of the waves of disturb- 
 ance arising in the ectodermic cell. 
 
 We have already seen that automatism, i.e. the power of initiat- 
 ing disturbances or vital impulses, independent of any immediate 
 disturbing event or stimulus from w'ithout, is one of the fundamen- 
 tal properties of protoplasm. In simpler but less exact language, 
 such a mass of protoplasm as an amoeba, though susceptible in the 
 highest degree to influences from without, ' has a will of its own ;' 
 it executes movements which cannot be explained by reference to 
 any changes in suiTounding circumstances at the time being. A 
 hydra has also a will of its own ; and seeing that all the constituent 
 cells (beyond the distinction into ectoderm and endoderm) are ahke, 
 we have no reason for thinking that the will resides in one cell 
 more than in another, but are led to infer that the protoplasm of 
 each of the cells (of the ectoderm at least) is automatic, the will 
 of the individual being the co-ordinated wills of the component 
 cells. In both Hydra and Amoeba the processes concerned in 
 automatic or si^ontaneous impulses, though in origin independent 
 of, are subject to and largely modified by, influences proceeding 
 from Avithout. Indeed the great value of automatic processes in a 
 living body depends on the automatism being affected by external 
 influences, and on the simj^le effects of stimulation being profoundly 
 modified by automatic action. 
 
 The next step of development beyond Hydra, is evidently to 
 differentiate the single (ectodermic) cell into tw^o cells, of which 
 one, by division of labour, confines itself chiefly to the simple de- 
 velopment of impulses as the result of stimulation, leaving to the 
 other the task of automatic action, and the more complex trans- 
 formation of the impulses generated in itself. The latter, which 
 we may call the eminently automatic cell (though much of the 
 work which it has to do is of the kind we shall presently speak of 
 as reflex action), will naturally be withdrawn from the surface of 
 the body, while the other, which we may call the eminently sensitive 
 cell, will still retain its superficial position, so that it may most 
 
106 SEFSORY AND MOTOR NERVES. [Book i. 
 
 readily be affected by all changes in the world without, Fig. 16 C. 
 And just as a primary motor nerve arises as a retained thread of 
 communication between a sensitive cell and its muscular process, 
 so a primary sensory nerve may be conceived of as arising as a 
 thread of communication between an eminently sensitive cell and 
 its twin the eminently automatic or central cell. By this arrange- 
 ment the sensitive cell, relieved of the heavy burden of spontaneous 
 action, is enabled to devote itself with greater vigour to the re- 
 ception of external influences ; while the automatic cell, no longer 
 hampered by the physical necessities of being which are imposed 
 on the superficial cell, exposed as this is to every wind and wave, 
 but secure in its internal retreat, is able with similar increased 
 energy, to devote itself either to the production of spontaneous 
 impulses, or to profoundly modifying the impulses which it receives 
 from the sensitive cell. Naturally the muscular process or muscu- 
 lar fibre would on the splitting of the original single cell remain in 
 connection with the more eminently automatic. We thus arrive 
 at that triple fundamental arrangement of a nervous system, in 
 its simplest form, viz. a sensitive cell on the surface of the body 
 connected by means of a sensory nerve with the internal automatic 
 central nervous cell, which in turn is connected by means of a 
 motor nerve with the muscular fibre-cell. 
 
 We have already seen that the physiology of the motor ner^^e 
 cannot without inconvenience be separated from that of the mus- 
 cular fibre. In the same way the physiology of the sensory nerve 
 cannot well be separated from those modifications of superficial 
 sensitive cells which constitute the organs of sense. We may add 
 that the special physiology of the central nervous cells can only 
 profitably be studied in connection with the sensory organs. In 
 the present chapter, therefore, we purpose to confine ourselves to 
 the consideration of the simplest and most general properties ot 
 the central nervous cells. 
 
 These are arranged in the vertebrate body in two great systems 
 the cerebro-spinal axis, and the various ganglia scattered over the 
 body ; we shall deal with such properties only as are more or less 
 common to the tw^o systems. We may premise that as far as our 
 knowledge at present goes, the processes which are concerned in 
 the propagation of nervous impulses along a sensory nerve-trunk 
 are identical with those which take place in a motor nerve-trunk. 
 The phenomena of the natural nerve current, of the currents of 
 action during the passage of an impulse and of electrotons (and 
 these facts mark out, as v/e have seen, the limits of our information 
 on this matter,) are exactly the same, whether the piece of nerve- 
 trunk experimented on be a mixed nerve-trunk, or an almost 
 purely motor, or an almost purely sensory nerve-trunk, or an an- 
 terior or posterior nerve-root, or the special sensory nerve of a 
 particular sense, such as the optic nerve. In both sensory and 
 motor nerves the changes accompanying a nervous impulse are 
 transmitted equally well in both directions. 
 
ciiAi'. III.] /'/!() /'/:/rn/:s of xicin'ocs tissues. lu: 
 
 We seem justitied in eoncludiiiL': lluit the events which occur in 
 a sensory nerve \vhen it is an instriniK'nt of sensation, diflftT from 
 those ^vhit■h take ])laee in a motor nerve M'hen that is an instru- 
 ment of movement, only so far as tlie sensory impulses are gener- 
 ated by particular processes which bear the stamp of the sensory 
 cell in which they originated, while the motor impulses are gener- 
 ated by particular processes which bear the stamp of the central 
 nervous cells in which they in turn originated. All sensory im- 
 pulses appear to be tetanic in nature, i. e. to be composed of a 
 series of constituent simple impulses ; and it is probable that 
 while the motor impulses which proceed from the central nerN'ous 
 system to the muscles are composed of simple impulses repeated 
 with the same rapidity, and thus giving rise to the same muscular 
 note (p. 52), the sensory impulses which proceed from the periph- 
 eral sense organs to the central nervous system vary exceedingly as 
 to the way in which their constituent simple impulses are com- 
 bined. It is indeed possible that the complex sensory impulses 
 which give rise, for instance, to sight and touch respective!}", may 
 ditier only in the wave-length, so to speak, of their constituent 
 simple impulses, much in the same way as red light differs from 
 blue light. 
 
 In the scheme sketched out above, the same central nei'vous 
 cell is supposed to be engaged at once, both in originating auto- 
 matic actions and in modifying sensory impulses (i.e. impulses 
 proceeding from the superficial sensitive cells) previous to these 
 being passed on to the muscular fibre. It is evident that, where 
 two or more central nervous cells occur together, a further differen- 
 tiation would be of advantage : a ditferentiation into cells which, 
 though still susceptible of being influenced from without, should be 
 more especially restricted to automatic action, and into cells which 
 should forego their automatism for the sake of being more efficient 
 in modifpng sensory impulses, with a view of transmuting them 
 into motor impulses, and so of giving rise to appropriate move- 
 ments. ^Ve thus gain the fundamental and primary differentiation 
 of the work of a central nervous system into automatic and into 
 reflex operations. These are very clearly manifested by the brain 
 and spinal cord, and probably also, though this is less certain, by 
 the sjDoradic ganglia. 
 
 Automatic Actions. In the vertebrate animal the highest form 
 of automatism, individual volition, with which conscious intelli- 
 gence is associated, is a function of certain parts of the brain. 
 There are evidences of the existence in the brain of other fonns of 
 automatism. All these will be considered in detail hereafter. 
 
 In the spinal cord separated from the brain by section of the 
 medulla oblongata, it becomes difficult to draw a line between 
 purely automatic and reflex actions. Thus, when we come to deal 
 with respiration, we shall see that while there can be no doubt that 
 
108 AUTOMATIC ACTIONS. [Book :. 
 
 the muscular respiratory apparatus is kept at work by impulses 
 proceeding, in a rhythmic manner, from a group of nerve-cells, or 
 respiratory nervous centre, in the medulla oblongata it is an open 
 question whether those impulses, whose generation is certainly 
 modified by centripetal impulses passing to the centre along 
 various nerves, are absolutely automatic: i.e. whether they can 
 continue to make their appearance when no influences whatever 
 from without are brought to bear upon the centre. Similar doubts 
 hover round other automatic functions of the spinal cord. We 
 shall see hereafter reasons for speaking of the existence in the 
 medulla oblongata of a vaso-motor centre, that is of a group 
 of nerve-cells, whence impulses habitually proceed along the so- 
 called vaso-motor nerves to the muscular coats of tbe small 
 arteries, and keep these vessels in a state of semi-contraction or 
 tone. Here too it is doubtful whether these motor or efferent 
 impulses can be generated in the absence of all sensory or afferent 
 impulses. The posterior lymphatic hearts of the frog are connected 
 by the small tenth pair of spinal nerves with the grey matter of the 
 termination of the spinal cord, in such a manner that destruction of 
 that part of the spinal cord or section of the tenth nerves apparently 
 puts an end to the rhythmic pulsations of the lymphatic hearts. Here 
 it would seem as if rhythmic impulses were automatically generated 
 in the lower end of the cord, and proceeded along the efferent 
 nerves to the hearts, thus determining their rhythmic pulsations. 
 But if it be true, as asserted, that the rhythmic pulsations, though 
 arrested for a time by severance of the nerves, or destruction of the 
 lower end of the cord, are after a while resumed, then these too, 
 can be no longer counted among the automatic phenomena of the 
 cord. And so in other instances which we shall meet with in the 
 course of this book. The existence of automatism, then, even of 
 this comparatively simple character, is at least doubtful. That all 
 higher automatism comparable at least to that of the cerebral hemi- 
 spheres is absent, may be regarded as certain. 
 
 In the sporadic ganglia the evidence of automatic action seems 
 more clear, and yet is by no means absolutely decisive. The beat 
 of the heart is a typical automatic action : and, since the heart will 
 continue to beat for some time when isolated from the rest of the 
 body (that of a cold-blooded animal continuing to beat for hours, 
 or even days), its automatism must lie in its ovm structures. 
 When, however, we come to discuss the beat of the heart in detail, 
 we shall find that it is still an open question whether the automa- 
 tism is confined to the ganglia (either of the sinus venosus, auricles, 
 or auriculo-ventricular boundary), or shared in by the muscular 
 tissue : Vv^hether, in fact, the automatism is a muscular automatism 
 like that of a ciliated cell, or the automatism of a differentiated 
 nerve-ceil. And yet the heart is the case where the automatism of 
 the ganglia seems clearest. 
 
 The peristaltic contractions of the alimentary canal are auto- 
 
riiAP. 111.] AirOMATIC ACTIOXS. 109 
 
 matio movciuouts ; wo cannot speak of them as being simply excited 
 by the presence of food in the canal, any more than we can say that 
 the beat of the heart is caused by the jjresence of blood in its 
 cavities. \Vhen absent they may be set agoing, and when present 
 may be stopped without any change in the contents of the canal. 
 They ma}', of course, be influenced by the contents, just as the beat 
 of the heart is influenced by the quantity of blood in its cavities. 
 Throughout the intestines are found the nerve plexus of Auerbach 
 and that of ^leissner ; to each or both of these the automatism of the 
 peristaltic movements has been referred. Yet in the ureter, whose 
 peristaltic waves of contraction closely resemble that of the intes- 
 tine, automatism is evident in the middle third of its length even 
 when completely isolated ; in which region (in the rabbit at 
 least), according to Engelmann, ganglia, and indeed nerve-cells, are 
 entirely absent. 
 
 Thus, while in the spinal cord there is doubt whether purely 
 automatic, as stringently distinguished from reflex, actions take 
 place, in the case of the sporadic ganglia the uncertainty is whether 
 the clearlv automatic movements of the organs with which the 
 ganglia are associated are due to the nerve-cells of the ganglia, or 
 to the muscular tissue itself, 
 
 Reflex Actions. The spinal cord offers the best and most 
 numerous examples of reflex action. In fact, reflex action may be 
 said to he, par excellence, the function of the spinal cord; and the 
 grey matter of the spinal cord may be broadly considered as a 
 multitude of reflex centres. We have here to consider the cord 
 merely in its general aspects ; and must postpone the special con- 
 sideration of the particular forms of reflex action which it exhibits, 
 as they come before us in various connections, or until we have to 
 deal with it as part of the gi'eat central nervous machinery. 
 
 In its simplest form a reflex action is as follows. All the ma- 
 chinery it demands is (a) a sentient surface (external or internal), 
 connected by {h) a sensory, or — to adopt the more general and 
 better term — afterent ner\-e, with (c) a central ner%-e-cell or group 
 of connected nerve-cells, which is in relation by means of {d) a 
 motor, or efferent, nerve, or nerves, with {e) a muscle, or muscles, 
 or some other irritable tissue-elements, capable of responding by 
 some change in their condition, to the advent of efferent impulses. 
 The afferent impulses started in a, passing along h, reach the 
 centre c, are there transmuted into efferent impulses, w^hich, 
 passing along d, finally reach e, and there produce a cognisable 
 effect. The essence of a reflex action consists in the transnuitation, 
 by means of the irritable protoplasm of a nei^'e-cell, of afterent into 
 efferent impulses. As an approach to a knowledge of the nature 
 of that transmutation, we may lay down the following propositions. 
 
 Tlie inimher, intensity, character and distnbution of the efferent 
 iinpulses are determined chiefiij b>/ the events luhich talce place in the 
 
110 REFLEX ACTIONS. [Book i. 
 
 protoplasm of the reflex centre. It is not that the afferent impulse 
 is simply reflected in the nerve-cell, and so becomes with but little 
 change an efferent impulse. On the contrary, an afferent impulse 
 passing along a single sensory fibre may give rise to efferent im- 
 pulses passing along many motor nerves, and call forth the most 
 complex movements. An instance of this disproportion of the 
 afferent and efferent impulses is seen in the case where the contact 
 with the glottis of a foreign body so insignificant as a hair causes a 
 violent fit of coughing. Under such circumstances a slight contact 
 with the mucous membrane, such as could not possibly give rise to 
 anything more than few and feeble impulses, may cause the 
 discharge of so many efferent impulses along so many motor 
 nerves, that not only all the respiratory muscles, but almost all the 
 muscles of the body, are brought into action. Similar though less 
 striking instances of how incommensurate are afferent and efferent 
 impulses may be seen in most reflex actions. In fact, the afferent 
 impulse when it reaches the protoplasm of the nerve produces there 
 a series of changes, of explosive disturbances, which, except that the 
 nerve-cell does not in any way change its form, may be likened to 
 the explosive changes in a muscle on the arrival of an impulse along 
 its motor nerve \ The changes in a nerve-cell during reflex action, 
 we might say during any form of activity, far more closely resemble 
 the changes during a muscular contraction than those wdiich 
 accompany the passage along a nerve of either an afferent or 
 efferent impulse. The simple passage along a nerve is accompanied 
 by little expenditure of energy ; it neither gains nor loses force to 
 any great extent as it progresses. The transmutation in a nerve- 
 cell is most probably (though the direct proofs are perhaps wanting) 
 accompanied by a large expenditure of energy, and a simple nervous 
 impulse in suffering the transmutation in a central nervous organ 
 may accumulate in intensity to a very remarkable extent, as in the 
 case of strychnia poisoning. 
 
 The nature of the efferent impulses is, however, determined also 
 by the nature of the afferent impulses. The nerve-centre remaining 
 in the same condition, the stronger or more numerous impulses will 
 give rise to the more forcible or more comprehensive movements. 
 Thus if the flank of a brainless frog be very lightly touched, 
 the only reflex movement which is visible is a slight twitching of 
 the muscles lying immediately underneath the spot of skin 
 stimulated. If the stimulus be increased, the movements will 
 spread to the hind-leg of the same side, which frequently will 
 execute a movement calculated to push or wipe av/ay the stimulus. 
 By forcibly pinching the same spot of skin, or otherwise increasing 
 the stimulus, the resulting movements may be led to embrace the 
 fore-leg of the same side, then the opposite side, and finally, almost 
 all the muscles of the body. In other words, the disturbance 
 
 ^ The question as to how far these processes in the central cells are connected 
 with the development of consciousness is here purposely passed over. 
 
CiiAi«. Ill ] REFLEX ACTIONS. . Ill 
 
 set going in the cfiitral luTve-cells, confined ulion tlic stimulus is 
 slight to a few nei"ve-cells and to a few nerve-fibres, overjiaws, so to 
 speak, when the stimulus is increased, on to a number of adjoining 
 and (we must conclude) connected cells, and thus throws impulses 
 into a larger and larger number of efferent nerves. 
 
 Certain relations may he observed between the sentient spot 
 stimulated and the resulting movement. In the simplest cases of 
 retlcx action this relation is merely of such a kind that the muscles 
 thrown into action are those governed by a motor nerve which is the 
 fellow of the sensory nerve, the stimulation of which calls forth the 
 movement. In the more complex reflex actions of the brainless frog, 
 and in other cases, the relation is of such a kind that the resulting 
 movement bears an adaptation to the stimulus: the foot is with- 
 drawn from the stimulus, or the movement is calculated to push or 
 wipe away the stimulus. In other words, a certain purpose is 
 evident in the reflex action. 
 
 Thus in all cases, except, perhaps the very simplest, the move- 
 ments called forth by a reflex action are exceedingly complex, com- 
 pared with those which result from the dii-ect stimulation of a 
 motor trunk. When the peripheral stump of a divided sciatic 
 nerve is stimulated with the interrupted current, the muscles of 
 the leg are at once thrown into tetanus, continue in the same rigid 
 condition during the passage of the current, and relax immediately 
 on the current being shut off. When the same current is applied 
 for a second only, to the skin of the flank of a brainless frog, the 
 leg is drawn up and the foot rapidly swept over the spot irritated, 
 as if to wij)e aw^ay the irritation ; but this movement is a complex 
 one, requiring the contraction of particular muscles in a definite 
 sequence, with a carefully adjusted jDroportion between the amounts 
 of contraction of the individual muscles. And this complex move- 
 ment, this balanced and aiTanged series of contractions, may be 
 repeated more than once as the result of a single stimulation of the 
 skin. When a deep breath is caused b}- a dash of cold water, the 
 same co-ordinated and carefully arranged series of contractions is 
 also seen to result, as part of a reflex action, from a simple stimulus. 
 And many more examples might be given. 
 
 In such cases as these, part of the complexity may be due to 
 the fact that the stimulus is applied to terminal sensor}^ organs 
 and not directly to a nerve-trunk. As we shall see in speaking of 
 the senses, the impulses which are generated by the application of 
 a stimulus to a sensory organ are more complex than those which 
 result from the direct stimulation of a sensory nen'e-trunk. Never- 
 theless, reflex actions of gi-eat if not of equal complexity may be 
 induced by stimuli apjDlied directly to a nerve-trunk. We are 
 therefore obliged to conclude that in a reflex action, the processes 
 which are originated in the central nerv^e-cells by the arrival of even 
 simple impulses along afferent nen^es may be highly complex; and 
 that it is the constitution and condition of the nerve-cells which 
 
112 GAXGL/A. [Book I. 
 
 determine the complexity and character of the movements which 
 are affected. In other words, the central nerve-cells concerned in 
 reflex actions are to be regarded as constituting a sort of molecular 
 machinery, the character of the resulting movements being deter- 
 mined by the nature of the machinery set going and its condition 
 at the tim.e being, the character and amount of the afferent 
 impulses determining exactly what parts of and how far the central 
 machinery is thrown into action. 
 
 Actions of Sporadic Ganglia. Seeing that in the spinal cord 
 the nerve-cells undoubtedly are the central structures concerned in 
 the production of reflex action, it is only natural to infer that the 
 nerve-cells of the sporadic ganglia possess similar functions. Yet 
 the evidence of this is at present of very limited extent. With 
 regard to the ganglia on the posterior roots of the spinal nerves, all 
 the evidence goes to shew that these possess no power Vvdiatever of 
 reflex action. Of the larger ganglia visible to the naked eye, such 
 as the ciliary, otic, &c., we have indications of reflex action in 
 one only, viz. the submaxillary, and these indications are, as we 
 shall see in treating of the salivary glands, disputed. We have no 
 exact proof that the ganglia of the sympathetic chain, or of 
 the larger sympathetic plexuses, are capable of executing reflex 
 actions. 
 
 In fact, in searching for reflex actions in ganglia, we are 
 reduced to the small microscopic groups of cells buried in the 
 midst of the tissues to which they belong, such as the ganglia of 
 the heart, of the intestine, the bladder, &c. When a quiescent 
 frog's heart is stimulated by touching its surface, a beat takes 
 place. This beat is, as v/e shall see, a complex, co-ordinated move- 
 ment, very similar to a reflex action brought about by means of 
 the spinal cord; and in its production it is probable that the 
 cardiac ganglia are in some way concerned. When a quiescent 
 intestine is touched or otherwise stimulated, peristaltic action is 
 set up. Here again the ganglia present in the intestinal walls 
 may be supposed to play a part ; but this movement is much more 
 simple than the beat of the heart, and as regards it, and more 
 especially as regards the similar peristaltic action of the ureter, it 
 becomes difficult to distinguish between a movement governed by 
 ganglia, and one produced by direct stimulation of the muscular 
 fibres. We have seen that the gi-eat distinction between a reflex 
 action and a movement caused by direct stimulation of a nerve or 
 of a muscle lies in the greater complexity of the former ; and we 
 may readily imagine, that by continued simplification of the central 
 nervous machinery, the two might in the end become so much 
 alike as to be almost indistinguishable. 
 
 In the vertebrate animal then the chief seat of reflex action 
 is the spinal cord and brain. We say 'and brain' because, as 
 we shall see later on, the brain, in addition to its automatism, 
 is as busy a field of reflex action as the spinal cord. 
 
Chap, mi.] 1*R0J'ERTIES OF NERVOUS TISSUES. 113 
 
 Inhibition. I u speaking of reflex action, wo took it for granted 
 that the spinal cord was, at the moment of the arrival of the 
 afferent impulses at the central nerve-cells, in a quiescent state ; 
 that the nerve-cells themselves were not engaged in any auto- 
 matic action. We were justified in doing so, because as far as 
 the muscles generally of the body are concerned, the spinal cord 
 is in a brainless frog perfectly quiescent ; an afferent impulse 
 reaching an ordinary nerve-cell of the spinal cord does not find 
 it preoccupied in discharging efferent impulses to the muscles with 
 which by means of nerve-fibres it is connected. But what happens 
 when afferent impulses reach a nerve-cell or a group of nen^e-cells 
 already engaged in automatic action ? 
 
 We have already referred to an automatic respiratory' centre 
 in the medulla oblongata. We may here premise, what we shall 
 shew more in detail hereafter, that the pneumogastric nerve is 
 peculiarly associated as an afferent nerve with this respiratory 
 centre. Now if the central end of the divided pneumogastric be 
 stimulated at the time when the respiratory centre is engaged 
 m its accustomed rhythmic action, sending out complex co- 
 ordinated impulses of inspiration (and of expiration) at regular 
 intervals, one of two things may happen, the choice of events 
 being determined by circumstances which need not be considered 
 here. 
 
 The most striking event, and the one which interests us now, 
 is that the respiratory rhythm is slowed or stopped altogether. 
 That is to say, afferent impulses which, under ordinaiy con- 
 ditions, would, on reaching a quiescent nervous centre, give rise 
 to movement, may, under certain conditions, when brought to 
 bear on an already active automatic nervous centre, check or stop 
 movement by interfering with the production of efferent impulses 
 in that centre. This stopping or checking an already present 
 action is spoken of as an ' inhibition ; ' and the effect of the 
 pneumogastric in this way on the respiratory centre is spoken of 
 as ' the inhibitory action of the pneumogastric on the respiratory 
 centre.' 
 
 The other event is that the respiratory rhythm is accelerated. 
 We shall hereafter discuss the explanation of the two events. We 
 may however state that according to one view the pneumo- 
 gastric contains among its afferent fibres two sets, which are either 
 of a different nature from each other, or are so differently connected 
 with the respiratory centre, that impulses arriving along one stop, 
 while those arriving along the other quicken, the action of that 
 centre. Hence, the one set are called ' inhibitory,' the other ' ac- 
 celerating' or 'augmenting' fibres. But we are concerned at present 
 only with the fact that the stimulation of a nerve may produce 
 either inhibitory or augmentative effects. 
 
 Similarly the vaso-motor centre in the medulla may, by im- 
 pulses arriving along various afferent tracts, be inhibited, during 
 
 V. 8 
 
114 INHIBITION. [Book i. Chap. hi. 
 
 which the muscular walls of various arteries are relaxed ; or 
 augmented, whereby the tonic contraction of various arteries is 
 increased. 
 
 The most striking instance of inhibition is offered by the heart. 
 If when the heart is beating well and regularly, the pneumogastric 
 be divided, and the peripheral portion be stimulated even for a very 
 short time with an interrupted current, the heart is immediately 
 brought to a standstill. Its beats are aiTested, it lies perfectly 
 flaccid and motionless, and it is not till after some little time that 
 it recommences its beat. Here again it is usually said that the 
 pneumogastric contains efferent cardio-inhibitory fibres, impulses 
 passing along which from the medulla stop the automatic actions 
 of the cardiac ganglia; the respiratory inhibitory fibres of the 
 same nerve are afferent, i.e. impulses pass along them up to the 
 medulla. 
 
 Though inhibition is most clearly seen in the case of automatic 
 actions, other actions may be similarly inhibited. Thus, as we 
 shall see later on, the reflex actions of the spinal cord may, by 
 appropriate means, be inhibited. 
 
 To sum up, then, the most fundamental properties of nervous 
 tissues. 
 
 Nerve-fibres are concerned in the propagation only, not in the 
 origination or transformation, of nervous impulses. As far as is at 
 present known, impulses are propagated in the same manner along 
 both sensory and motor nerves. Sensory impulses differ from 
 motor impulses inasmuch as the former are generated in sensory 
 organs and pass up to the central nervous cells, while the latter 
 pass from the central nervous cells to the muscles or to some 
 other peripheral organs. 
 
 The operations of the nerve-cells are either automatic or reflex. 
 In both an automatic and a reflex action, the diversity and the 
 co-ordination of the impulses are determined by the condition of 
 the nerve-cells. During the passage of an impulse along a nerve- 
 fibre, there is no augmentation of energy ; in passing through a 
 nerve-cell, the augmentation may be, and generally is, most con- 
 siderable. 
 
 When afferent impulses reach a centre already in action, the 
 activity of that centre may, according to circumstances, be either 
 depressed or exalted, may be ' inhibited ' or ' augmented.' 
 
CHAPTER IV. 
 
 THE VASCULAE MECHANISM. 
 
 In order that the blood may be a satisfactory medium of com- 
 munication between all the tissues of the body, two things are 
 necessary. In the first place, there must be through all parts of 
 the body a flow of blood, of a certain rapidity and general con- 
 stancy. In the second place, this flow must be susceptible of both 
 general and local modifications. In order that any tissue or organ 
 may readily adapt itself to changes of circumstances (action, 
 repose, &c.), it is of advantage that the quantity of blood passing 
 to it should be not absolutely constant, but capable of variation. 
 In order that the material equilibrium of the body may be main- 
 tained as exactly as possible, it is desirable that the loading of the 
 blood with substances proceeding fi'om the unwonted activity of 
 any one tissue, should be accompanied by a gi-eater flow of blood 
 through some excretory or metabolic tissue by which these 
 substances may be removed. Similarly it is of advantage to 
 the body that the general flow of blood should in some circum- 
 stances be more energetic, and in others less so, than normal. 
 
 The first of these conditions is dependent on the mechanical 
 and physical properties of the vascular mechanism ; and the 
 problems connected with it are almost exclusively mechanical or 
 physical problems. The second of these conditions depends on 
 the intervention of the nervous system ; and the problems con- 
 nected with it are essentially physiological problems. 
 
 8—2 
 
116 PHYSICAL PHENOMENA OF CIRCULATION. [Book i, 
 
 I. The Physical Phenomena of the Circulation. 
 
 The apparatus concerned in the Maintenance of the Normal 
 Flow is composed of the following factors : 
 
 1. The heart, beating rhythmically by virtue of its contractility 
 and intrinsic mechanisms, and at each beat discharging a certain 
 quantity of blood into the aorta. [For simplicity's sake we omit 
 for the present the pulmonary circulation.] 
 
 2. The arteries, highly elastic throughout, with a circular mus- 
 cular element increasing in relative importance as the arteries 
 diminish in size. It must not be forgotten that the muscular 
 element is also elastic. 
 
 When an artery divides, the united sectional area of the 
 branches is, as a rule, larger than the sectional area of the stem. 
 Thus the collective capacity of the arteries is continually (and 
 rapidly) increasing from the heart towards the capillaries. If all 
 the arterial branches were fused together, they would form a 
 funnel, with its apex at the aorta. The united sectional area of 
 the capillaries has been calculated by Vierordt to amount to 
 several (eight ?) hundred times that of the aorta. 
 
 3. The capillaries, channels of exceedingly small but variable 
 size. Their walls are elastic (as shewn by their behaviour during 
 the passage of blood-corpuscles through them), exceedingly thin 
 and permeable. They are permeable both in the sense of allowing 
 fluids to pass through them by osmosis, and also in the sense of 
 allowing white and red corpuscles to traverse them. The small 
 arteries and veins, which gradually pass into and from the capil- 
 laries properly so called, are similarly permeable, the more so, the 
 smaller they are. 
 
 4. The veins, less elastic than the arteries (the difference being 
 especially marked when both sets of vessels become distended) and 
 with a very variable muscular element. The united sectional area of 
 the veins diminishes from the capillaries to the heart, thus resem- 
 bling the arteries ; but the united sectional area of the venae cavse 
 at their junction with the right auricle is greater than that of the 
 aorta at its origin. (The proportion is nearly two to one.) The 
 total capacity of the veins is similarly much greater than that of 
 the arteries. The veins alone can hold the total mass of blood 
 which in life is distributed over both arteries and veins. Indeed 
 nearly the whole blood is capable of being received by what is 
 merely a part of the venous system, viz. the vena portsa and 
 its branches. Such veins as are for various reasons liable to a 
 reflux of blood from the heart towards the capillaries are provided 
 with valves. 
 
SEC. 1. MAIN GENERAL FACTS OF THE CIRCULATION. 
 
 The Capillary Circulation. 
 
 If the web of a frog's foot be examined with a microscope, 
 the blood, as judged of by the movements of the corpuscles, is seen 
 to be passing in a continuous stream from the small arteries 
 through the capillaries to the veins. The velocity is greater in the 
 arteries than in the veins, and greater in both than in the 
 capillaries. In the arteries faint pulsations, sjmchronous with the 
 heart's beat, are occasionally visible ; and not unfrequently varia- 
 tions in velocity and in the distribution of the blood, due to 
 causes which will be hereafter discussed, are Nvitnessed from time 
 to time. 
 
 The flow through the smaller capillaries is very variable. 
 Sometimes the corpuscles are seen passing through the channel 
 in single file with great regularity; at other times, they may 
 be few and far between. Sometimes the corjiuscle may remain 
 stationary at the entrance into a capillary, the channel itself being 
 for some little distance entirely free from corpuscles. Any one 
 of these conditions readily passes into another; and, especially with 
 a somewhat feeble circulation, instances of all of them may be seen 
 in the same field of the microscope. It is only when the vessels 
 of the web are unusually full of blood that all the capillaries can be 
 seen equally filled with corpuscles. The long oval red corpuscle 
 moves with its long axis parallel to the stream, frequently rotating 
 on its long axis and sometimes on its short axis. The flexibility and 
 elasticity of a corpuscle are well seen when it is being driven into a 
 
118 THE CAPILLARY CIRCULATION. [Book i. 
 
 capillary narrower than itself, or when it becomes temporarily lodged 
 at the angle between two diverging channels. The small mam- 
 malian corpuscles rotate largely as they are driven along. 
 
 In the web of the frog's foot the average velocity with which the 
 corpuscles move may be put down as about half a millimetre in a 
 second. In the human retina, the velocity of the capillary flow 
 has, by indirect methods, been estimated at '75 mm. per sec. The 
 movement of the blood in the capillaries is very slow, compared 
 with that in the arteries or even in the veins. 
 
 In the larger capillaries, and especially in the small arteries and 
 veins which permit the passage of several corpuscles abreast, it is 
 observed that the red corpuscles run in the middle of the channel, 
 forming a coloured core, between which and the sides of the 
 vessel all round is a layer, which has been called the ' inert layer,' 
 or better the 'plasmatic layer,' containing no red corpuscles. This 
 division into a plasmatic layer and an axial stream is due to the 
 fact that in any stream passing through a closed channel the 
 friction is greatest at the immediate sides, and diminishes towards 
 the axis. The corjDuscles pass where the friction is least, in the 
 axis, A quite similar axial core is seen when any fine particles 
 are driven with a sufficient velocity in a stream of fluid through 
 a narrow tube. As the velocity is diminished the axial core 
 becomes less marked and disappears. In the plasmatic layer, 
 especially in that of the veins, are frequently seen white corpuscles, 
 sometimes clinging to the sides of the vessel, sometimes rolling 
 slowly along, and in general moving irregularly, and often in jerks. 
 The greater the velocity of the flow of blood, the fewer the white 
 corpuscles in the plasmatic layer, and with a very rapid flow they, 
 as well as the red corpuscles, may be all confined to the axial 
 stream. The presence of the white corpuscles in the plasmatic 
 layer has been attributed to their being specifically lighter than 
 the red corpuscles, it being affirmed that when fine particles of two 
 kinds, one lighter than the other, are driven through a narrow tube, 
 the heavier particles flow in the axis and the lighter in the more 
 peripheral portions of the stream. This however has been disputed, 
 and the phenomenon explained by the white corpuscles being dis- 
 tinctly more adhesive than the red, as is seen by the manner in 
 which they become fixed to the glass slide and cover-slip when 
 a drop of blood is mounted for microscopical examination. By 
 reason of this adhesiveness which possibly may vary with the 
 varying nutritive conditions of the corpuscles and of the blood- 
 vessels, the white corpuscles, it is urged, become temporarily 
 attached to the walls of the vessel, and consequently appear in 
 the plasmatic layer. 
 
 The resistance to the flow of blood thus caused by the Mction 
 generated in so many minute passages, is one of the most important 
 physical facts in the circulation. In the large arteries the friction 
 is small ; it increases as they divide, and receives a very great 
 
Chap, iv.] THE VASCULAR MIX If AX ISM. 
 
 119 
 
120 THE FLOW IN THE ARTERIES. [Book i. 
 
 Fig, 17. Apparatus for investigating Blood-pbessuee. 
 
 At the tipper right-hand comer, is seen, on an enlarged scale, the carotid artery, 
 clamped by the forceps hd, with the vagus nerve v lying by its side. The artery 
 has been ligatured at V and the glass cannula c has been introduced into the artery 
 between the ligature V and the forceps hd, and secured in position by the ligature I. 
 The shrunken artery on the distal side of the cannula is seen at ca'. 
 
 p.h. is a box containing a bottle holding a saturated solution of sodium car- 
 bonate or a solution of sodium bicarbonate of sp. gr. 1083, and capable of being 
 raised or lowered at pleasure. The solution flows by the tube p. t regulated by the 
 clamp c" into the tube t. A syringe, with a stop-cock, may be substituted for the 
 bottle, and attached at c". This indeed is in many respects a more convenient plan. 
 The tube t is connected with the leaden tube t, and the stopcock c with the mano- 
 meter, of which m is the descending and m' the ascending limb, and s the support. 
 The mercury in the ascending limb bears on its surface the float fl, a long rod 
 attached to which is fitted with the pen p, writing on the recording surface ?•. The 
 clamp cl. at the end of the tube t has an arrangement shewn on a larger scale 
 at the right hand upper corner. 
 
 The descending tube m of the manometer, and the tube t being completely filled 
 along its whole length with fluid to the exclusion of all air, the cannula c is filled 
 with fluid, slipped into the open end of the thick- walled india-rubber tube i, until it 
 meets the tube t (whose position within the india-rubber tube is shewn by the dotted 
 lines), and is then securely fixed in this position by the clamp cl. 
 
 The stopcocks c and c" are now opened, and the pressure-bottle raised or fluid 
 driven in by the syringe until the mercury in the manometer is raised to the 
 required height. The clamp c" is then closed and the forceps bd removed from the 
 artery. The pressure of the blood in the carotid ca. is -in consequence brought to 
 bear through t upon the mercury in the manometer. 
 
 addition in the minute arteries and capillaries. We may speak of 
 it therefore as the 'peripheral friction' and the resistance which 
 it offers as the 'peripheral resistance.' It need perhaps hardly 
 be said that this peripheral friction not only opposes the flow of 
 blood through the capillaries themselves, but, working backwards 
 along the whole arterial system, has to be met by the heart at 
 each systole of the ventricle. 
 
 It is well known that when any portion of the skin is pressed 
 upon, it becomes pale and bloodless; this is due to the pressure 
 driving the blood out of the capillaries and minute vessels and 
 preventing any fresh blood entering into them. By carefully 
 investigating the amount of pressure necessary to prevent the 
 blood entering the capillaries and minute arteries of the web of the 
 frog's foot, or of the skin beneath the nail in man or elsewhere, the 
 internal pressure which the blood is exercising on the walls of the 
 capillaries and minute arteries and veins may be approximately 
 determined. In the frog's web this has been found to be equal to 
 about 7 or 11 mm, mercury. 
 
 2. The Flow in the Arteries. 
 
 When an artery is severed, the flow from the proximal section 
 is not equable, but comes in jets, which correspond to the heart- 
 
fJn.M'. IV. 1 THE I'ASCL'LAA' MFJ'HAMSM. 121 
 
 beats, though the flow does not cease between the jets. The blood 
 is ejected with considerable force; thus, in l)r Stephen Hales' 
 experiments, when the crural artery of a mare was severed, the jet, 
 even alter much loss of blood, rose to the height of two feet. The 
 larger the artery and the nearer to the heart, the greater the force 
 with whieh the blood issues, and the more marked tne intcrmittencc 
 of the tlow. The How from the distal section may be very slight, 
 or may take place with consiilerablc force and marked intermittence, 
 according to the amoimt of collateral communication. 
 
 Arterial pressure. If a mercury (or other) manometer. 
 Fig. V7 m, in', be connected with a large artery, e.g. the carotid, in 
 such a way that while the blood is allowed to flow uninterruptedly 
 along the artery, there is free communication between the interior 
 of the artery and the proximal (descending) limb of the manometer, 
 the following facts are observed. 
 
 Immediately that communication is established between the 
 interior of the artery and the manometer, blood rushes from the 
 former into the latter, driving some of the mercury from the de- 
 scending limb into the ascending limb, and thus causing the level 
 of the mercury in the ascending limb to rise rapidly. This rise is 
 marked by jerks corresponding with the heart-beats. Having 
 reached a certain level, the mercury ceases to rise any more. It 
 does not, however, remain absolutely at rest, but undergoes oscilla- 
 tions ; it keeps rising and falling. Each rise, which is very slight 
 compared with the total height to which the mercury has risen, 
 has the same rhythm as the systole of the ventricle. Similarly, 
 each fall corresponds with the diastole. 
 
 If a float, swimming on the top of the mercury in the ascending 
 limb of the manometer, and bearing a brush or other marker, be 
 brought to bear on a travelling surface, some such tracing as that 
 represented in Fig. 18 will be described. Each of the smaller 
 
 r r 
 
 Fig. 18. Tracing of Arterial Pressure with a Mercury Manometer. 
 
 The smaller curves p p are the pulse-curves. The space from r to r embraces 
 a respiratory undulation. The tracing is taken from a clog, and the irregularities 
 visible in it are those frequently met with in this animal. 
 
 curves ( p, p) corresponds to a heart-beat, the rise corresponding to 
 the systole and the fall to the diastole of the ventricle. The larger 
 undulations (r, r) in the tracing, which are respiratory in origin, 
 
122 ARTERIAL PRESSURE. [Book i. 
 
 will be discussed hereafter. This observation teaches us that the 
 blood, as it is passing along the carotid artery, is capable of support- 
 ing a column of mercury of a certain height (measured by the 
 difference of level between the mercury in the descending limb, 
 and that in the ascending limb, of the manometer), when the 
 mercury is placed in direct communication with the side of the 
 stream of blood. In other words, the blood, as it passes through 
 the artery, exerts a lateral pressure on the sides of the artery, equal 
 to so many millimeters of mercury. In this lateral pressure we 
 have further to distinguish between the slighter oscillations corre- 
 sponding with the heart-beats, and a mean pressure above and 
 below which the oscillations range. A similar mean pressure with 
 similar oscillations is found, when any artery of the body is 
 examined in the same way. In all arteries the blood exerts a 
 certain pressure on the walls of the vessels which contain it. This 
 is generally spoken of as arterial pressure or arterial tension, and the 
 pressure in the aorta of any animal is usually spoken of as its blood- 
 pressure. 
 
 Description of Experiment, The carotid, or other vessel, is laid 
 bare, clamped in two places and divided between the clamps. Into the 
 cut ends is inserted a hollow |— piece of the same bore as the artery, the 
 cross portion forming the continuation of the artery. The other portion 
 is connected by means of a non-elastic flexible tube with the descending 
 limb of the manometer. In order to avoid loss of blood, fluid is in- 
 jected into the flexible tube until the mercury in the manometer stands 
 a very little below what may be beforehand guessed at as the probable 
 mean pressure. The fluid chosen is a saturated solution of sodium car- 
 bonate or a solution of sodium bicarbonate of sp. gr. 1083, with a view to 
 hinder the coagulation of the blood in the tube. When the clamps are 
 removed from the artery the blood rushes through the cross of the |— piece. 
 Some passes into the side limb of the H piece and continues to do so 
 until the mean pressure is quite reached. Thenceforward there is no more 
 escape ; but the pressure continues in the interior of the |— piece, is 
 transmitted along the connecting tube to the manometer, and the mercury 
 continues to stand at a height indicative of the mean pressure with 
 oscillations corresponding to the heart's beats. Practically the use of 
 the I— piece is found inconvenient. Accordingly the general custom is 
 to ligature the artery, to place a clamp on the vessel on the proximal 
 side of the ligature, and to introduce a straight cannula. Fig. 17 c, 
 connected with the manometer, into the artery between the ligature and 
 the clamp, and to secure it in that position. In this case, on loosing the 
 clamp, the whole column of blood in the artery is brought to bear on 
 the manometer, and the tracings taken illustrate the lateral pressure 
 not of the artery in which the cannula has been placed, but of the vessel 
 (aorta &c. as the case may be) of which it is itself a branch. 
 
 Tracings of the movements of the column of mercury in the mano- 
 meter may be taken either on a smoked surface of a revolving cylinder 
 (Fig. 1), or by means of a brush and ink on a continuous roll of paper, 
 as in the more complex kymograph (Fig. 1 9). 
 
CHAI'. IV.] 
 
 THE VASCULAR MECHANISM. 
 
 123 
 
 In such a inorcury luunometer, the in«rtia of tlio mercury obscuros 
 many of tho features of the minor curves caused by the heart^beats. 
 When therefore tlu-se, rath(^r than variations in the mean pressure, arc 
 bcin<^ studied, other methods have to be adopted. 
 
 The average pressure of the blood in the same body is greatest 
 in the hxrgest arteries, and diminishes as the arteries get less; but 
 the fall is a very gradual cue until the smallest arteries are reached, 
 in which it becomes very rapid. In the carotid of the horse, the 
 mean arterial pressure varies from 150 to 200 mm. of mercury ; of 
 the dog from 100 to 175 ; of the rabbit from 50 to 90. In the 
 carotid of man it probably amounts to 150 or 200. 
 
 Fig. 19. Labge Kymograph with continuous roll of papek. 
 
 The clock-work machinery, some of the details of which are seen, unrolls the 
 paper from the roU C, carries it smoothly over the cylinder B, and then winds it up 
 into the roll A. 
 
 Two electromagnetic markers are seen in the position ia which they record their 
 movements on the paper as it travels over B. The manometer, or any other 
 recording instrument used, can be fixed either in the notch immediately in front of 
 B or in any other position that may be desired. 
 
 Since in all arteries the blood is pressing on the arterial walls 
 with some considerable force, all the arteries must be in a state of 
 permanent distension, so long as blood is flowing through them 
 from the heart. When the blood-current is cut off, as by a 
 ligature, this expansion or distension disappears. 
 
 Not only is there a permanent expansion corresjaonding to the 
 mean pressure, but just as the mercury in the manometer rises 
 above the level of mean pressure at each systole of the heart, and 
 
124 THE VELOCITY OF THE FLOW. [Book i. 
 
 falls below it at each diastole, so at any spot in the artery there is 
 for each heart-beat a temporary expansion succeeded by temporary 
 contraction, the diameter of the artery in its temporary expansions 
 and contractions oscillating, in correspondence with the oscillations 
 of the manometer, beyond and within the diameter of permanent 
 expansion. These temporary expansions constitute what is called 
 the pulse, and will be discussed more fully hereafter. 
 
 The velocity of the flow. When even a small artery is severed 
 a considerable quantity of blood escapes from the proximal cut end 
 in a very short space of time. That is to say, the blood moves in 
 the arteries from the heart to the capillaries, with a very con- 
 siderable velocity. By various methods, this velocity of the blood- 
 current has been measured at different parts of the arterial system; 
 the results, owing to imperfections in the methods employed, cannot 
 be regarded as satisfactorily exact, but may be accepted as approxi- 
 mately true. The velocity of the arterial stream is greatest in 
 the largest arteries, and diminishes from the heart to the capillaries, 
 •pari passu with the increase of the width of the bed, i.e. with the 
 increase of the united sectional area. 
 
 Methods. The Hsemadromometer of Yolkmann. An artery, e.g. a 
 carotid, is clamped in two places, and divided between the clamps. Two 
 cannulse, of a bore as nearly equal as possible to that of the artery, or of 
 a known bore, are inserted in the two ends. The two cannulse are con- 
 nected by means of two stop-cocks, which work together, with the two 
 ends of a long glass tube, bent in the shape of a U, and filled with normal 
 saliae solution, or with a coloured innocuous fluid. The clamps on the 
 artery being released, a turn of the stop-cocks permits the blood to enter 
 the proximal end of the long U tube, along which it courses, driving the 
 fluid out into the artery through the distal end. Attached to the tube is 
 a graduated scale, by means of which the velocity with which the blood 
 flows along the tube may be read off. Even supposing the cannulse to 
 be of the same bore as the artery, it is evident that the conditions of 
 the flow through the tube are such as will only admit of the result thus 
 gained being considered as an approximative estimation of the real 
 velocity in the artery itself. 
 
 The Hheometer (Stromuhr) of Ludwig. This consists of two glass 
 bulbs A and B, Fig. 20, communicating above with each other and with 
 the common tube C by which they can be filled. Their lower ends are 
 fixed in the metal disc Z>, which can be made to rotate, through two 
 right angles, round the lower disc E. In the upper disc are two holes 
 a and b continuous with A and B respectively, and in the lower disc are 
 two similar holes a and b', similarly continuous with the tubes H and 0. 
 Hence, in the position of the discs shewn in the figure, the tube G is 
 continuous through the two discs with the bulb A and the tube H with 
 the bulb B. On turning the disc D through two right angles the tube G 
 becomes continuous with B instead of A, and the tube H with A instead 
 of B. There is a further arrangement, omitted from the figure for the 
 sake of simplicity, by which when the disc D is turned through one 
 
Chap. iv.J THE VASCULAR MKCIIAXISM. 125 
 
 instead of two right angles from either of the above positions, G becomes 
 directly continuous with //, both being completely shut off from the 
 bulbs. 
 
 Fig. 20. Dugbammatic Representation of Lutwio's Stkomuhr. 
 
 The ends of the tubes H and G are made to fit exactly into two 
 cajinulse inserted into the two cut ends of the artery about to be experi- 
 mented upon, and having a bore as nearly equal as possible to that of 
 the artery. 
 
 The method of experimenting is us follow^s. The disc D, being placed 
 in the intermediate position, so that a and h are both cut ofi" from 
 a and h' , the bulb A is filled with pure olive oil up to the mark o:, and 
 the bulb B, the rest of A, and the junction C, with defibrinated' blood; 
 and C is then clamped. The tubes H and G are also filled ^ith de- 
 fibrinated blood, and G is inserted into the cannula of the central, U 
 into that of the peripheral, end of the artery. On removing the clamps 
 from the artery the blood flows through G to H, and so back into the 
 artery. The observation now begins by turning the disc D into the 
 position shewn in the figure ; the blood then flows into A , driving the 
 oil there contained out before it into the bulb B, in the direction of the 
 arrow, the defibrinated blood pre^dously present in B passing by H into 
 the artery, and so into the system. At the moment that the blood is 
 seen to rise to the mark x, the disc D is with all possible rapidity turned 
 through two right angles: and thus the bulb B, now largely filled with 
 oil, placed in communication with G. The blood-stream now drives the 
 oil back into A, and the new blood in ^-1 through If into the artery. As 
 soon as the oil has wholly returned to its original position, the disc is 
 again turned round, and A once more placed in communication with G, 
 and the oil once more driven from A to B. And this is repeated several 
 times, indeed generally until the clotting of the blood or the admixture 
 of the oil Avith the blood puts an end to the experiment. Thus the flow 
 of blood is used to fill alternately with blood or oU the space of the bulb ^, 
 whose ca\ity as far as the mark x has been exactly measured; hence if 
 the number of times in any given time the disc J) has to be turned round 
 be kno-«Ti, the number of times A has been filled is also known, and 
 thus the quantity of blood which has passed in that time through the 
 
126 MEASUREMENTS OF VELOCITY OF FLOW. [Book i. 
 
 cannula coimected with the tube G is directly measured. For instance, 
 supposing that the quantity held by the bulb A when filled up to the 
 mark £c is 5 c.c, and supposing that from the moment of allowing the 
 first 5 c.c. of blood to begin to enter the tube to the moment when the 
 escape of the last 5 c.c. from the artery into the tube was complete, 100 
 seconds had elapsed, dui'ing which time 5 c.c. had been received 10 
 times into the tube from the artery (all but the last 5 c.c. being returned 
 into the distal portion of the artery), obviously "5 c.c. of blood had 
 flowed from the proximal section of the artery in one second. Hence 
 supposing that the diameter of the cannula (and of the artery, they 
 being the sam.e) were 2 mm., with a sectional area therefore of 3-14: 
 square mm., an outflow through the section of -5 c.c. or 500 c.mm. in a 
 second would give (l-yl-), a velocity of about 159 mm. in a second. 
 
 The Heematachometer of Vierordt is constructed on the principle of 
 measuring the velocity of the current by observing the amount of devia- 
 tion undergone by a pendulum, the free end of which hangs loosely in 
 the stream. A square or rectangular chamber, one side of which is of 
 glass and marked with a graduated scale in the foi'm of an arc of a circle, 
 is connected by means of two short tubes with the two cut ends of an 
 artery; the blood consequently flows from the proximal (central) 
 portion of the artery through the chamber into the distal portion of the 
 artery. Within the chamber and suspended from its roof is a short 
 pendulum, which when the blood-stream is cut ofi" from the chamber 
 hangs motionless in a vertical position, but when the blood is allowed to 
 flow through the chamber, is driven by the force of the current out of 
 its position of rest. The pendulum is so placed that a marker attached 
 to its free end travels close to the inner surface of the glass side along 
 the arc of the gTaduated side. Hence the amount of de^dation from a 
 vertical position may easily be read off on the scale from the outside. 
 The graduation of the scale haAong been carried out by experimenting 
 with streams of known velocity, the velocity can at once be calculated 
 from the amount of deviation. 
 
 An instrument based on the same principle has been invented by 
 Chauveau and improved by Lortet. In this the part which corresponds 
 to the pendulum in Yierordt's instrument is prolonged outside the 
 chamber, and thus the portion within the chamber is made to form the 
 short arm of a lever, the fulcrum of which is at the point where the waU 
 of the chamber is traversed and the long arm of which projects outside. 
 A somewhat wide tube, the wall of which is at one point composed of 
 an india-rubber membrane, is introduced between the two cut ends of an 
 artery. A long light lever pierces the india-rubber membrane. The 
 short expanded arm of this lever projecting within the tube is moved on 
 its fulcrum in the india-rubber ring by the current of blood passing 
 through the tube, the greater the velocity of the current, the larger being 
 the excursion of the lever. The movements of the short arm give rise 
 to corresponding movements in the opposite direction of the long arm 
 outside the tube, and these, by means of a marker attached to the end 
 of the long ai-m, may be directly inscribed on a recording surface. This 
 instrument is very well adapted for observing changes in the velocity of 
 the flow. In determining actual velocities, for which purpose it has to 
 be experimentally graduated, it is not so useful. 
 
Chap. iv.J THE VASCULAR MECHANISM. 127 
 
 in the horse, Volkinanu found the velocity of the stream 
 to bo ill tlic caiotitl artery about 300 iiiiii., in the maxillary artery 
 165 mm., and in the metatarsal artery 50 mm. in the .second. 
 Chauveau determined the velocity in the carotid of the horse to 
 vary from 520 to 150 mm. per sec. at each beat of the heart, flow- 
 ing at the former rate during the height of each pulse-expansion, 
 and at the latter in the interval between each two beats. Ludwig 
 and Dogiel found the velocity in the dog and in the rabbit to vary 
 within very wide limits, not only in different arteries, l)ut in the 
 same artery under different circumstances. Thus while in the 
 carotid of the rabbit it may be said to vary from 100 to 200 mm. 
 per sec, and in the carotid of the dog from 200 to 500 mm. per sec, 
 both these limits were frequently passed. 
 
 3. The Flow in the Veins. 
 
 When a vein is severed, the flow from the distal cut end {i.e. the 
 end nearest the capillaries) is continuous, the blood is ejected with 
 comparatively little force, and with no great velocity. 
 
 When a vein is connected with a manometer, the lateral pressure 
 is found to be very small ; it is greater in the veins farther from 
 the heart than in those nearer the heart. In the former it is much 
 less than that of the small arteries, and in the latter amounts only 
 to a few millimetres of mercury. Indeed in the immediate neigh- 
 bourhood of the heart the pressure may (during the inspii-atory 
 movement) become negative, i.e. when the manometer is brought 
 into connection with the interior of the vein, the mercury in the 
 distal limb falls, instead of, as in the case of an artery, rising. 
 
 In the case of most veins, under ordinary circumstances the 
 mercury of a manometer connected with a vein does not shew any 
 of those pulse-oscillations which are so striking in the arteries. As 
 a general rule the pulse is seen on the arterial side only of the 
 capillaries, though in special cases, under conditions which we shall 
 study presently, it may make its way through the capillaries from 
 the arteries to the small veins ; and it is probable that in general 
 a slight impulse does make its way right through the capil- 
 laries, but so feeble that it cannot be recognised by ordinary in- 
 struments save in special cases. Moreover, in the great veins near 
 the heart, under certain circumstances at all events, the movements 
 of that organ may make themselves felt as a so-called 'venous pulse' 
 transmitted in a backward direction along the veins from the heart. 
 But these exceptional instances and these recuiTent oscillations do 
 not invalidate the truth of the general statement that the pulse is 
 absent from the veins. The exact determination of venous pressure 
 is attended ^vith great ex]")crimental difficulties, and our knowledge in 
 
128 THE FLOW IN THE VEINS. [Book i. 
 
 this direction is very incomplete ; but in all probability the pressure 
 in a vein varies within much wider limits than does the pressure in 
 the corresponding artery. 
 
 In the small veins the velocity of the current, measured in the 
 same way as in the case of the arteries, is very slight. It increases 
 in the larger veins, corresponding to the diminution of the area of 
 ' the bed ' ; it is about 200 mm. per sec. in the jugular vein of the 
 dog. 
 
 Thus the flow in the veins presents strong contrasts with that 
 in the arteries. In the arteries, even in the smallest branches, 
 there is a considerable mean pressure. In the veins, even in the 
 small veins where it is largest, the mean pressure is very slight. 
 In other words, there is always a difference of pressure tending to 
 make the blood flow continuously from the arteries into the veins, 
 A pulse is present in the arteries, but, with certain exceptions, 
 absent in the veins. The velocity of the stream of blood in the 
 arteries is considerable ; in the small veins it is much less, but it 
 increases in the larger trunks ; for in both arteries and veins it 
 corresponds with the area of the bed, diminishing in the former 
 from the heart to the capillaries, and increasing in the latter from 
 the capillaries to the heart. 
 
 Hydraulic Principles of the Circulation. 
 
 All the above phenomena are the simple results of an intermit- 
 tent force (like that of the systole of the ventricle) working in a 
 closed circuit of branching elastic tubes, so arranged that while the 
 individual tubes first diminish (from the heart to the capillaries) 
 and then increase (from the capillaries to the heart), the area of 
 the bed first increases and then diminishes, the tubes together 
 thus forming two cones placed base to base at the capillaries, with 
 their apices converging to the heart. To this it must be added 
 that the friction in the small arteries and capillaries, at the junction 
 of the bases of the cones, offers a very great resistance to the flow 
 of the blood through them. It is this peripheral resistance (in the 
 minute arteries and capillaries, for the resistance offered by the 
 friction in the larger vessels may, when compared with this, be 
 practically neglected), reacting through the elastic walls of the 
 arteries upon the intermittent force of the heart, which gives the 
 circulation of the blood its peculiar features. 
 
 Circmnstances determining the character of the flow. When 
 fluid is driven by an intermittent force, as by a pump, through a 
 perfectly rigid tube (or system of tubes), there escapes at each 
 stroke of the pump from the distal end of the system just as much 
 fluid as enters it at the proximal end. The escape moreover takes 
 place at the same time as the entrance, since the time taken up by 
 
CiiAi'. IV.] 77/ A" VASCi'LMt MKCIIAMSM. Il>9 
 
 the transmission of the shock is so small, tli;it it may bo neglectod. 
 This result remains the same when any resistance to the tiow is 
 introduced into the system. The force of the pump remaining the 
 same, the introduction of the resistance undoubtedly lessens the 
 quantity issuing at the distal end at each stroke, but it does so 
 simply by lessening the (juantity entering at the proximal end ; 
 the income and outgo remain equal to each other, and occur at 
 almost the same time. And what is true of the two ends, is also 
 true of any part of the course of the system, so iar, at all events, as 
 the following proposition is concerned, that in a system of rigid 
 tubes, either with or without an intercalated resistance, the tiow 
 caused by an intermittent force is, in every part of the tubes, 
 intermittent synchronously with that force. 
 
 In a system of elastic tubes in which there is little resistance to 
 the progress of the fluid, the flow caused by an intermittent force 
 is also intermittent. The outgo being nearly as easy as the 
 income, the elasticity of the walls of the tubes is scarcely at all 
 called into play. These behave practically like rigid tubes. When, 
 however, sufficient resistance is introduced into any part of the 
 course, the fluid, being unable to pass by the resistance as rapidly 
 as it enters the system from the pump, tends to accumulate on the 
 proximal side of the resistance. This it is able to do by expanding 
 the elastic walls of the tubes. At each stroke of the pump a 
 certain quantity of fluid enters the system at the proximal end. 
 Of this only a fraction can pass through the resistance during the 
 stroke. At the moment when the stroke ceases, the rest still 
 remains on the proximal side of the resistance, the elastic tubes 
 having expanded to receive it. During the interval between this 
 and the next stroke, the distended elastic tubes, striving to return 
 to their natural undistended condition, press on this extra quantity 
 of fluid which they contain and tend to drive it past the resistance. 
 Thus in the rigid system (and in the elastic system without 
 resistance) there issues, from the distal end of the system, at each 
 stroke, just as much fluid as enters it at the proximal end, while 
 between the strokes there is perfect quiet. In the elastic system 
 with resistance, on the contrary, the quantity which passes the 
 resistance is only a fraction of that which enters the system from 
 the pump, the remainder or a portion of the remainder continuing 
 to pass during the inter\al between the strokes. In the former 
 case, the system is no fuller at the end of the stroke than at the 
 besrinninof ; in the latter case there is an accumulation of fluid 
 between the pump and the resistance, and a corresponding dis- 
 tension of that part of the system, at the close of each stroke — an 
 accumulation and distension, however, which go on diminishing 
 until the next stroke comes. The amount of fluid thus remaining 
 after the stroke will depend on the amount of resistance in relation 
 to the force of the stroke, and on the distensibility of the tubes ; 
 and the amount which passes the resistance before the next stroke 
 
130 INTERMITTENT FLOW. [Book i. 
 
 will depend on the degree of elastic reaction of which the tubes 
 are capable. Thus, if the resistance be very considerable in 
 relation to the force of the stroke, and the tubes very distensible, 
 only a small portion of the fluid will pass the resistance, the 
 greater part remaining lodged between the pump and the re- 
 sistance. If the elastic reaction be great, a large portion of this 
 will be passed on through the resistance before the next stroke 
 comes. In other words, the greater the resistance (in relation to 
 the force of the stroke), and the more the elastic force is brought 
 into play, the less intermittent, the more nearly continuous, will be 
 the flow on the far side of the resistance. 
 
 If the first stroke be succeeded by a second stroke before its 
 quantity of fluid has all passed by the resistance, there will be an 
 additional accumulation of fluid on the near side of the resistance, 
 an additional distension of the tubes, an additional strain on their 
 elastic powers, and, in consequence, the flow between this second 
 stroke and the third will be even more marked than that between 
 the first and the second, though all three strokes were of the same 
 force, the addition beino- due to the extra amount of elastic force 
 called into play. In fact, it is evident that, if there be a sufficient 
 store of elastic power to fall back upon, by continually repeating 
 the strokes a state of things will be at last arrived at, in which the 
 elastic force, called into play by the continually increasing dis- 
 tension of the tubes on the near side of the resistance, will be 
 sufiicient to drive through the resistance, between each two strokes, 
 just as much fluid as enters the near end of the system at each 
 stroke. In other words, the elastic reaction of the walls of the 
 tubes will have converted the intermittent into a continuous flow. 
 The flow on the far side of the resistance is in this case not the 
 direct result of the strokes of the pump. All the force of the 
 pump is spent, first in getting up, and afterwards in keeping up, 
 the over-distension of the tubes on the near side of the resistance ; the 
 cause of the continuous flow lies in the over-distension of the tubes 
 which leads them to empty of themselves into the far side of the 
 resistance, at such a rate, that they discharge through the resistance 
 during a stroke and in the succeeding interval just as much as 
 they receive from the pump by the stroke itself. 
 
 This is exactly what takes place in the vascular system. The 
 friction in the minute arteries and capillaries presents a considerable 
 resistance to the flow of blood through them into the small veins. 
 In consequence of this resistance, the force of the heart's beat is 
 spent in maintaining the whole of the arterial system in a state 
 of over-distension, as indicated by the arterial pressure. The over- 
 distended arterial system is, by the agen3y of its elastic walls, con- 
 tinually emptying itself by overflowing through the capillaries into 
 the venous system, overflowing at such a rate, that just as much 
 blood passes from the arteries to the veins during each systole 
 and its succeeding diastole as enters the aorta at each systole. 
 
CiiAiv IV.] TIIK I'ASrr/.A/i M/'X'I/AX/SM. 131 
 
 It cannot be too much insisted upon that the whole arterial 
 system is over-distended. This is what is meant by the high 
 arterial pressure. On the other hand, the veins are much les.s 
 distended. This is shewn by the low venous pressure. The dis- 
 tended arteries are continually striving to pass their suq>lus in 
 a continuous stream through the capillaries into the vein.s, so as 
 to bring both venous and arterial pressure to the same level. As 
 continually the heart by its beat is keeping the arteries distended, 
 and thus maintaining the ditference between the arterial and 
 venous pressure, and thus preserving the steady capillary stream. 
 When the heart ceases to beat, the arteries do succeed in emptying 
 their suqilus into the veins, and when the pressure on both sides of 
 the capillaries is thus equalized, the flow through the capillaries 
 ceases. 
 
 In the facts just discussed, it makes no essential difference 
 whether the outflow on the far side of the resistance be an open 
 one, or whether, as is the case in the vascular system, the fluid be 
 returned to the pump, provided only that the resistance offered to 
 that return be sufficiently small. We shall see, in speaking of the 
 heart, that, so far from there being any resistance to the flow of 
 blood from the great veins into the auricle, the flow is favoured by 
 a variety of circumstances. We have seen moreover that, besides 
 the very sudden decrease in the immediate neighbourhood of the 
 capillaries, there is in passing along the whole vascular system from 
 the aorta to the venae cav£e a gradual fall of pressure. A little 
 consideration shews that this must be the case. After what has 
 been said it is obvious that the movement of the blood may be 
 compared to that of a body of fluid, driven by pressure from the 
 ventricle through the vessels to its outflow in the auricle. Were the 
 pressure a continuous one, and were there no peripheral resistance, 
 there would be a gradual fall of pressure, from the part farthest 
 from the outfall, viz. the aorta, to the part nearest the outfall, viz. 
 the veme cavte. The introduction of the peripheral resistance and 
 its attendant phenomena gives rise to the feature of a very sudden 
 and marked fall in the capillar}' region, but leaves untouched the 
 gradual character of the fall in the rest of the course, from the 
 aorta to the minute arteries, and from the minute veins to the 
 vena3 cava?. 
 
 To recapitulate : there are three chief factors in the mechanics 
 of the circulation, (I) the force and frequency of the heart-beat, (2) 
 the peripheral resistance, (3) the elasticity of the arterial walls. 
 These three factors, in order to produce a normal circulation, must 
 be in a certain relation to each other. A disturbance of these 
 relations brings about abnormal conditions. Thus, if the peripheral 
 resistance be reduced beyond certain limits, while the force and 
 frequency of the heart remain the same, so much blood passes 
 through the capillaries at each stroke of the heart that there is 
 not sufficient left behind to distend the arteries, and bring their 
 
 9—2 
 
132 VARIATIONS IN VELOCITY. [Book i. 
 
 elasticity into play. In this case the intermittence of the arterial 
 flow is continued on into the veins. An instance of this is seen in 
 the experiments on the sub-maxillary gland, where sometimes the 
 resistance oflered by the minute arteries of the gland is so much 
 lowered, that the pulse is carried right through the capillaries, and 
 the blood in the veins of the gland pulsates^ A like result occurs 
 when, the peripheral resistance remaining the same, the frequency 
 of the heart's beat is lowered. Thus the beats may be so 
 infrequent that the whole quantity sent on by a stroke has time to 
 escape before the next stroke comes. Lastly, if, while the heart's 
 beat and the peripheral resistance remain the same, the arterial 
 walls become more rigid, the arteries will be unable to expand 
 sufficiently to retain the surplus of each stroke or to exert sufficient 
 elastic reaction to carry forward the stream between the strokes ; 
 and in consequence more or less intermittence will become manifest. 
 
 Circumstances determining the velocity of the flow. We 
 
 have seen that the velocity of the blood-stream diminishes from 
 the aorta to the capillaries, and increases from the capillaries to 
 the great veins. Thus in the dog the velocity in the great arteries 
 may be stated at from 300 to 500 mm., in the capillaries at less 
 than 1 mm. ('5 to "75 mm.), and in the large veins at about 200 mm. 
 in a sec. In fact, the greater part of the time of the circuit is 
 taken up in the capillary region. An iron salt, injected into the 
 jugular vein of one side of the neck of a horse, makes its appear- 
 ance in the blood of the jugular vein of the other side in about 
 30 seconds. 
 
 Hering's mean result in the horse was 27*6 sees. In the dog 
 Vierordt found it to be 15*2 sees.; in the rabbit 7 sees. 
 
 Without laying too much stress on this experiment, it may be 
 taken as a fair indication of the time in which the whole circuit 
 may be completed. It takes about the same time to pass through 
 about 20 mm. of capillaries. Hence, if any corpuscle had in its 
 circuit to pass through 10 mm. of capillaries, half the whole time 
 of its journey would be spent in the narrow channels of the 
 capillaries. Since, however, the average length of a capillary 
 is about "5 mm., about one second is spent in the capillaries. In- 
 asmuch as the purposes served by the blood are chiefly carried out 
 in the capillaries, it is obviously of advantage that its stay in them 
 should be prolonged. 
 
 The local differences in the velocity of the stream are directly 
 dependent on the area of the 'bed.' When a fluid is driven 
 by a uniform pressure through a narrow tube with an enlarge- 
 ment in the middle, the velocity of the stream diminishes in 
 the enlargement, but increases again when the tube once more 
 narrows. So a river slackens speed in a ' broad' but rushes on 
 
 ^ See Book i. cap. i. sec. 2, on the Secretion of the Digestive Juices. 
 
Chap iv] TlIK V ASCI' LA R MhV//A.V/S.]/. |:33 
 
 rapidly again when the banks close in. Exactly in the same way 
 the velocity of the blood-stream slackens from the aorta to the 
 capillaries corresponding with the increased total ])ed, but hurries 
 on again as the numerous veins are gathered into the smaller bed 
 of the vena? cavie. The loss of velocity in the caj)illaries, as com- 
 pared with the arteries, is not due to there being so much more 
 friction in the narrow channels of the former than in the wide 
 canals of the latter. For the peripheral resistance caused by the 
 friction in the capillaries and small arteries is an obstacle not only 
 to the tiow of blood through these small vessels where the resist- 
 ance is actually generated, but also to the escape of the blood from 
 the large into the small arteries, and indeed from the heart into 
 the large arteries. It exerts its influence along the whole arterial 
 tract. And it is obvious that if it were this peripheral resistance 
 which checked the flow in the capillaries, there could be no re- 
 covery of velocity along the venous tract. The rapidity of the flow 
 in arteries, capillaries, and veins, is in each case determined by the 
 total sectional area of the channels. There is, however, a loss of 
 velocity on the whole course. At each stroke as much blood enters 
 the right auricle as issues from the left ventricle ; but the sectional 
 area of the vense cavge is greater than that of the aorta, so that 
 even if the auricle were tilled in exactly the same time as the 
 ventricle is emptied, the blood must pass more rapidly through the 
 nan"ow aorta than through the broad venae cavse, in order that the 
 same quantity of blood should pass each in the same time. The 
 diastole of the auricle, however, is distinctly longer than the systole 
 of the ventricle; the time during which the auricle is being filled is 
 greater than that during which the ventricle is being emptied, and 
 hence the velocity of the venous flow into the auricle must be still 
 less than that of the arterial blood in the commencinof aorta. 
 
 The temporary variations of the velocity of the stream in any 
 given channel, and these we have already (p. 127) seen to be very 
 considerable in the case of the arteries at least, are dependent on a 
 variety of circumstances. In a tube of constant calibre, the velo- 
 city with which fluid flows from one point to another, for instance 
 from the point a to the point h, will be in main dejiendent on the 
 difierence between the pressures existing at a and h. The loAver 
 the pressure at h as compared with a the gi-eater the rapidity with 
 which the fluid flows from a to h. And temporary variations of 
 pressures form undoubtedly the main cause of the temporarj^ varia- 
 tions observable in the velocity of the arterial flow. Thus with 
 each systole of the ventricle there is an increase of velocity in the 
 whole arterial flow folloAved by a diminution during the diastole. 
 So also if the peripheral resistance in the minute arteries into 
 which a larger artery divides be suddenly lowered (by the action of 
 vaso-motor nerves, in a manner which we shall presently discuss), 
 without tlie calibre of the larger artery itself being changed, the 
 pressure on the distal (peripheral) side of the artery may be much 
 
134 VARIATIONS IN VELOCITY. [Book i. 
 
 diminished, while the pressure on the proximal (cardiac) side re- 
 mains at first unaltered ; and this would necessarily cause an increase 
 in the rapidity of the stream through that artery. But, as we shall 
 see later on, from the complications of the vascular machinery 
 such problems as these become very intricate; and the results 
 of observations on variations in arterial velocity are not altogether 
 intelligible. It has been suggested that varying conditions of the 
 blood, by affecting the amount of adhesion between the blood and 
 the walls of the vessels, may be an important factor in determining 
 the variations in the velocity of the stream. 
 
SEC. 2. THE HEART. 
 
 The heart is a pump, the motive power of which is supplied 
 by the contraction of its muscular fibres. Its action consequently 
 presents problems which are partly mechanical, and partly vital. 
 Regarded as a pump, its effects are determined by the frequency of 
 the beats, by the force of each beat, by the character of each beat 
 — whether, for instance, slow and lingering, or sudden and sharp — 
 and by the quantity of fluid ejected at each beat. Hence, with a 
 given frequency, force, and character of beat, and a given quantity 
 ejected at each beat, the problems which have to be dealt with are 
 for the most part mechanical. The vital problems are chiefly con- 
 nected with the causes which determine the frequency, force, and 
 character of the beat. The quantity ejected at each beat is governed 
 more by the state of the rest of the body, than by that of the 
 heart itself 
 
 Tlie Phenomena of the Normal Beat. 
 
 The visible movements. When the chest of a mammal is 
 opened and artificial respiration kept up, a complete beat of the 
 whole heart, or cardiac cycle, may be observed to take place tus 
 follows. 
 
 The great veins, inferior and superior vense cavse and pulmonary 
 veins, are seen, while full of blood, to contract in the neighbourhood 
 of the heart: the contraction runs in a peristaltic wave towards the 
 auricles, increasing in intensity as it goes. Anived at the auricles, 
 which are then full of blood, the wave suddenly spreads, at a rate 
 too rapid to be fairly judged by the eye, over the whole of those 
 organs, which accordingly contract with a sudden sharp systole. In 
 the systole, the walls of tlie auricles press towards the auriculo- 
 
136 MOVEMENTS OF THE HEART. [Book i. 
 
 ventricular orifices, and the auricular appendages are drawn inwards, 
 becoming smaller and paler. During the auricular systole, the ven- 
 tricles may be seen to become more and more turgid. Then follows, 
 as it were immediately, the ventricular systole, during which the 
 ventricles become more conical. Held between the fingers they are 
 felt to become tense and hard. As the systole progresses, the aorta 
 and pulmonary arteries expand and elongate, and the heart twists 
 slightly on its long axis, moving from the left and behind towards 
 the front and right so that more of the left ventricle becomes dis- 
 played. As the systole gives way to the succeeding pause or 
 diastole, the ventricles resume their previous form, the aorta and 
 pulmonary artery contract and shorten, the heart turns back to- 
 wards the left, and thus the cycle is completed. 
 
 A more exact determination of the changes in the form and 
 position of the heart during a beat is attended with considerable 
 difficulties. The following experiment has been made with the 
 view of studying these changes without opening the chest and thus 
 without depriving the heart of its natural supports. If, in the un- 
 opened chest of a rabbit or dog, three needles be inserted through 
 the chest-wall so that their points are plunged into the substance 
 of the ventricle, one (B) at the base, close to the auricles, another 
 (A) through the apex, and a third (M) at about the middle of the 
 ventricle, all three needles will be observed to move at each beat of 
 the heart. The head of B wdll move suddenly upwards, shewing 
 that the point of the needle plunged in the ventricle moves down- 
 wards, whereas A will only quiver, and move neither distinctly up- 
 wards nor downwards. M wall move upwards (and therefore its 
 point dowTiwards), but not to the same extent as B. The nearer 
 to B, M is, the more it moves : the nearer to A, the less. After 
 the death of the animal, the needles, if properly inserted at first, 
 perpendicular to the chest, will be found with all their heads 
 directed downwards, indicating that the Avhole ventricle has been 
 drawn up by the contraction of the empty aorta and pulmonary 
 artery. 
 
 The behaviour of the needles during the beat has been in- 
 terpreted as follows. At the systole the whole heart is thrust 
 downward by the elongation of the aorta and pulmonary artery. 
 The needle A at the apex however does not move its place, 
 because this downward movement is compensated by an upward 
 movement due to a shortening, during systole, of the longitudinal 
 diameter of the ventricle. The base in which the needle B is 
 plunged, moves downwards and draws closer to A, i. e. to the apex, 
 partly by the downward thrust from the elongation of the great 
 arteries and partly fi^om the shortening of the ventricle itself. 
 Naturally the behaviour of the needle M is intermediate in 
 character, its downward movement being the more conspicuous 
 the nearer it is to B, The experiment then is taken to 
 prove that during the systole the ventricle shortens in its 
 
riiAP. IV.] THE VAsrrLAi: mhciiamsm. 1:57 
 
 longitudiiiiil diameter, but that tlie apex ri'inains statifjuary on 
 account of the compensating downward thrust of the whtjle 
 ventricle. It has been urged however that this method is untrust- 
 worthy, and that simihxr movements of needles thus placed might 
 be produced by the twisting of the heait on its l«jng axis, com- 
 bined with an a])j)r<)ximation of the heart to the chest-wall. And 
 tlitierent cunclusions have been arrived at by taking ]ila.ster of 
 Paris models on the one hand of a dog's heart, which, while having 
 ceased beating but not yet become rigid, has been filled with blood 
 at a moderate pressure, and on the other hand of a heart of the 
 same size in which a condition simulating systolic contraction has 
 been brought about by immersing the empty heart in a saturated 
 solution of ]i()tassium bichromate at 50" C The former is taken to 
 represent the diastolic and the latter the systolic form of the heart; 
 and the results are checked by measurements taken between marks 
 placed on various points of the surface of the heart as well as by 
 sections of a heart filled wdth blood and hardened in a cold solu- 
 tion of potassium bichromate and of one emptied and hardened in 
 the same solution warmed to 50^ A comparison of the two hearts 
 in these ditierent conditions tends to shew that while both the 
 right-to-left and autero-posterior diameters are diminished during 
 systole, especially in the plane of the ostia venosa (whereby the 
 auriculo-ventricular orifices become narrowed) the longitudinal 
 diameter, at all events of the left ventricle, is not lessened, the 
 distance between the apex and the auriculo-ventricular groove 
 remaining unchanged. The right ventricle, the change of form of 
 which is complicated, does shorten to a certain extent, and there 
 is during systole a downward movement of the conus arteriosus 
 upon the plane of the ventricular base (which possibly may explain 
 the movement of the needle B in the above mentioned experiment) 
 so that the distance between the apex and the u])per border of the 
 conus is less duiing systole than during diastole. This method 
 also confirms the view" that the left ventricle in systole turns on 
 its long axis, towards the right, the movement increasing from the 
 base downwards so that the groove between the two ventricles 
 forms a closer spiral than during diastole. 
 
 Objections may be brought against this method also, and it 
 seems impossible to explain the movements of a lever placed upon 
 the heart unless wo admit that during systole, the antero-posterior 
 diameter, of the middle portion of the ventricle at least is increased 
 instead of lessened. We may however probably go so far as to 
 conclude that as far as the ventricles are concerned tTie chief change 
 during systole is one from a roughly hemispherical to a more 
 conical form, effected without any marked diminution of the 
 distance between the apex and the ventricular base. 
 
 Cardiac Impulse. If the hand be placed on the chest, a 
 shock or impulse will be felt at each beat, and on examination 
 
138 CARDIAC IMPULSE. [Book t. 
 
 this impulse, 'cardiac impulse,' will be found to be synchronous with 
 the systole of the ventricle. In man, the cardiac impulse may be 
 most distinctly felt in the fifth costal interspace, about an inch 
 below and a little to the median side of the left nipple. The same 
 impulse may be felt in an animal by making an incision through 
 the diaphragm from the abdomen, and placing the finger between 
 the chest-wall and the apex. It then can be distinctly recognized 
 as the result of the hardening of the ventricle during the systole. 
 And the impulse which is felt on the outside of the chest is the 
 same hardening of the stationary portion of the ventricle in contact 
 with the chest- wall, transmitted through the chest- wall to the 
 finger. In its flaccid state, during diastole, the apex is (in a 
 standing position at least) at this point in contact with the chest- 
 wall, Ijang between it and the tolerably resistant diaphragm. 
 During the systole, while being brought even closer to the chest- 
 wall, by the movement to the front and to the right of which 
 we have already spoken, it suddenly grows tense and hard. The 
 ventricles, in executing their systole, have to contract against 
 resistance. They have to produce within their cavities, tensions 
 greater than those in the aorta and pulmonary arteries, respectivel}^ 
 This is, in fact, the object of the systole. Hence, during the swift 
 systole, the ventricular portion of the heart becomes suddenly 
 tense, just as a bladder full of fluid would become tense and hard 
 when forcibly squeezed. The sudden onset of this hardness gives 
 an impulse or shock both to the chest- wall and to the diaphragm, 
 which may be felt readily both on the chest-wall, and also through 
 the diaphragm when the abdomen is opened, and the finger 
 inserted. If the modification of the sphygmograph (see section 
 on Pulse), called the cardiograph, be placed on the spot where the 
 impulse is felt most strongly, the lever is seen to be raised during 
 the systole of the ventricles, and to fall again as the systole passes 
 away, very much as if it were placed on the heart directly. A 
 tracing may thus be obtained, of which we shall have to speak 
 more fully immediately. If the button of the lever be placed, 
 not on the exact spot of the impulse, but at a little distance 
 from it, the lever will be depressed during the systole. While 
 at the spot of impulse itself the contact of the ventricle 
 is increased during systole, away from the spot the ventricle 
 retires from the chest-wall (by the diminution of its right-to-left 
 diameter), and hence, by the mediastinal attachments of the peri- 
 cardium, draws the chest-wall after it. 
 
 Endo-cardiac events. In order to study more fully the 
 changes going on in the heart during the cardiac cycle, it becomes 
 necessary to know something of what is taking place in the interior 
 of the cavities of the heart. Chauveau and Marey, by introducing 
 into the right auricle and ventricle respectively of the horse, through 
 the jugular vein, small elastic bags, each communicating with a 
 
Chat iv. 
 
 77//; IM.SVTA.I/; M /:('// A. \/S.]/ 
 
 139 
 
 recording tambour, were enabled to take simultaneous tracings 
 of changes occurring in the two cavities. These results are 
 embodied in Fig. 21, of which the upper curve is a tracing taken 
 
 Fig. 21. Simultaneous tracings from the interior of the right Auricle, from 
 
 THE interior OF THE RIGHT VENTRICLE, AND OF THE CaRDIAC ImPULSE, IN THE 
 
 HoBSE. (After Chadveau AND Maret.) To be read from left to rights. 
 
 The upper curve represents changes taking place -within the auricle, the middle 
 curve changes within the ventricle. The lower curve represents the variations of 
 pressure transmitted to a lever outside the chest and constituting the cardiac 
 impulse. A complete cardiac cycle, beginning at the close of the ventricular 
 systole, is comprised between the thick vertical lines I and II. The thin vertical 
 lines represent tenths of a second. The explanation of the letters is given in 
 the text. 
 
 from the auricle, the middle curve a similar tracing taken from 
 the ventricle, while the lower curve is a cardiographic tracing 
 of the cardiac impulse. All these curves were taken simultaneously 
 on the same recording surface. 
 
 Method. A tube of appropriate curvature is furnished with two 
 small elastic bags, one at the extreme end and the other at such a 
 distance that when the former is within the cavity of the ventricle the 
 latter is in the cavity of the auricle ; sucli an instrument is spoken of 
 as a ' cardiac sound.' Each bag (Fig. 22 A) or 'ampulla' communicates 
 by a separate air-tight tube with an air-tight tambour (Fig. 22 B) on 
 which a lever rests so that any pressure on either bag is communicated 
 to the cavity of its respective tambour, the lever of which is raised in 
 
 ' It must be remembered that the curves in the diagram are intended merely 
 to illustrate the changes occurring at different times in the same chamber, 
 or to shew what changes in the one chamber are coincident in point of time with 
 changes in the other. They in no way indicate the amount of pressure exerted in the 
 auricle as compared with that in the ventricla 
 
140 
 
 KN DO-GARB I AC EVENTS. 
 
 [Book i. 
 
 proportion. The writing points of all three levers are brought to bear 
 on the same recording surface exactly underneath each other. The 
 tube is carefully introduced through the right jugular vein into the 
 right side of the heart until the lower (ventricular) bag is fairly in the 
 
 Fig. 22. SIabey's Tambouk, with Cardiac Sound. 
 
 A. A simple cardiac sound such as may be used for exploration of the left 
 ventricle. The portion a of the ampulla at the end is of thin india-rubber, stretched 
 over an open framework with metallic supports above and below. The long tube h 
 serves to introduce it into the cavity which it is desired to explore. 
 
 B. The Tambour. The metal chamber m is covered in an air-tight manner 
 with the india-rubber c, bearing a tliin metal plate vi to which is attached the lever I 
 moving on the hinge /;. The whole tambour can bo placed by means of the clamp 
 cl at any height on the upright s . The india-rubber tube t serves to connect the 
 interior of the tambour either with the cavity of the ampulla of A or with any other 
 cavity. Supposing that the tube t were connected with b, any pressui'e exerted on a 
 would cause the roof of the tamboar to rise and the point of the lever would be pro- 
 portionately raised. 
 
 cavity of the right ventricle, and consequently the upper (auricular) 
 bag in the cavity of the right auricle. Changes of pressure on either 
 ampulla then cause movements of the corresponding lever. When the 
 pressure, for instance, on the ampulla in the auricle is increased, the 
 auricular lever is raised and describes on the recording surface an 
 ascending curve ; when the pressure is taken off the curve descends ; 
 and so also with the ventricle. 
 
 The ' sound ' may in a similar manner be readily introduced through 
 the carotid artery into the left ventricle and the changes taking place 
 in that chamber also explored; these are found to be very similar to 
 those of the right ventricle. 
 
 We may employ these curves as giving a general and usefiil 
 view of the sequence of events in the interior of the heart; but we 
 must bear in mind exactly what they mean. The tracings given 
 
CiiAr. iv.j Till': VASCrLAl; MF.CIIAMSM. \\\ 
 
 by till' auricuhu" iiud vciitriciihir levi-rs really rcprcsoiit variiilions 
 in the pressure exerted ou the respective uni})ullie, and so far are 
 instructive; but they must not be taken as representing variations 
 in the pressure exerted on the blood in the several cavities. For 
 we can easily conceive that, in the systole of the ventricle for 
 instance, the contraction of the muscular walls might continue 
 after all the blood contained in the ventricle had been driven out. 
 In such a case the ventricle would continue to press upon the 
 ampulla, and this continued pressure would be transmitted to the 
 lever, and indicated on the curve; but we should be in error in 
 interpreting this part of the curve as meaning that the ventricle 
 was still continuing to exert pressure on the blood as yet remain- 
 ing in its cavity. With this caution, and with the remark that the 
 tracing of the cardiac impulse is very unlike the usual cardio- 
 graphic tracings taken from man, we may use the curves to deduce 
 the following conclusions. 
 
 A complete cardiac cycle is comprised between the vertical 
 lines I and II. The recording surface was travelling at such a 
 rate that the intervals between any two of the thin vertical lines 
 coiTesponds to one-tenth of a second. Hence in this case (the 
 heart being that of a horse) the whole cardiac cycle occupied 
 about f§ths of a second. Any point in the cycle might of course 
 be taken as its commencement. In the figure, the cycle is 
 supposed to begin shortly after the end of the ventricular systole, 
 and the beginning of the diastole. 
 
 On examining the three curves we see, at a, a steady rise of the 
 auricular, accompanied by similar gradual ascents of the ventricular 
 and also of the cardiograph lever. These may be interpreted as 
 indicating that the blood is pouring from the great veins into the 
 auricle, increasing the pressure there, and at the same time 
 passing on into the ventricle, increasing the internal pressure 
 there as well, a, and also by distending the ventricle, causing 
 it to press somewhat on the chest-wall and thus to raise the 
 cardiograph lever, a". This continues for about j*gths of a second, 
 and is then followed by the sudden rise of auricular pressure h due 
 to the auricular systole, followed by a sudden fall as the blood 
 escapes into the ventricle and the systole ceases. The sudden 
 entrance of blood into the ventricle causes a sudden increase of the 
 pressure in the ventricle as indicated by the ventricular lever h', and 
 a sudden increase in the pressure on the chest-wall h". The 
 auricular systole is followed immediately by the sudden strong 
 ventricular systole c', the lever rising very abruptly. Owing to 
 the presence of the tricuspid valves, the pressure exerted by the 
 ventricular systole is kept off the auricle almost altogether; but 
 the chest-wall, as shewn by the tracing at c", feels the sudden 
 increase of the pressure of the ventricle against it. The most 
 important points concerning this rise of ventricular pressure are 
 that it is sudden in its onset and also rapid in its decline, and 
 
14-2 THE If EC II A y ISM OF THE VALVES. f Boo re i. 
 
 that it lasts for a comparatively long time; in the figure this 
 part of the curve embraces more than four-tenths of a second. These 
 features, the sudden rise, the long duration, and the rapid fall 
 of the pressure exerted by the ventricle are seen in all tracings of 
 the ventricles engaged in a cardiac beat whatever be the method 
 employed. They mean of course that the muscular contractions 
 which constitute the ventricular systole come on suddenly, that 
 they last altogether a considerable time, and that relaxation is 
 also rapid. With the end of the ventricular systole the cycle 
 represented in figure ends, and a new cycle begins, repeating 
 the same changes. The meaning of the features on the curves 
 marked e and d, &c., as well as a more complete discussion of 
 the changes thus briefly described, we must defer till we have 
 spoken of 
 
 The Mechanism of the Valves. 
 
 The auriculo-ventricular valves present no difficulty. As the 
 blood is being driven by the auricular systole into the ventricle, a 
 reflux current is probably set up, by which the blood, passing along 
 the sides of the ventricle, gets between them and the flaps of the 
 valve (whether tricuspid or mitral). As the pressure of the 
 auricular systole diminishes, the same reflux current floats the 
 flaps up, until at or immediately after the close of the systole they 
 meet, and thus the oriflce is at once and firmly closed, at the very 
 beginning of the ventricular beat. The increasing intraventricular 
 pressure serves only to render the valve more and more tense, and 
 in consequence more secure, the chordae tendinese (the slackening 
 of which through the change of form of the ventricle is probably 
 obviated by a regulative contraction of the papillary muscles) at 
 the same time preventing the valve from being inverted or even 
 bulging into the auricle, and indeed, according to some observers, 
 keeping the valvular sheet actually convex to the ventricular 
 cavity, by which means the complete emptying of the ventricle 
 is more fully effected. Since the same papillary muscle is in 
 many cases connected by chordye with the adjacent edges of two 
 flaps, its contraction also serves to keep these flaps in more 
 complete apposition. Moreover the extreme borders of the valves, 
 outside the attachments of the chordse, are excessively thin, so 
 that when the valve is closed, these thin portions are pressed flat 
 together back to back ; hence while the tougher central parts of 
 the valves bear the force of the ventricular systole, the opposed 
 thin membranous edges, pressed together by the blood, more 
 completely secure the closure of the orifice. 
 
CiiAi'. iv.j Tin: VAsccLMi Mi':('iiAxis.]r. 113 
 
 The semilunar valves are, durint^'tlic ventricular systole, pressed 
 outwards towards but not close to the arterial walls, reflex currents 
 probably keepinrj them in an intermediate position, their orifice 
 formiufif an eijuilateral triangle with cun-ed sides ; they thus offer 
 little obstacle to the escape of blood from the cavities of the 
 ventricles. The ventricle propels the blood with great force and 
 rapidity into the aorta and the whole contents arc s})eedily ejected. 
 ISow, when in a closed channel a rapid cun-ent suddenly ceases, 
 a negative pressure makes its appearance in the rear of the fluid, 
 and sets up a reflux cuiTent. So when the last portions of blood 
 leave the ventricle a negative pressure makes its appearance behind 
 them in the ventricle, and leads to a reflux current from the aorta 
 towards the ventricle. This alone would tend to bring the 
 valves together; but in all probability it is not till a short 
 (variable) time afterwards, that upon the commencing diastolic 
 relaxation of the ventricle, the elastic rebound of the arterial walls 
 completely fills and renders tense the pockets, causing their fi-ee 
 margins to come into close and firm contact, and thus entirely 
 blocking the way. The corpora Arantii meet in the centre, and the 
 thin membranous festoons or lunulse are brought into exact apposi- 
 tion. As in the tricuspid valves, so here, while the pressure of the 
 blood is borne by the tougher bodies of the several valves, each two 
 thin adjacent lunulse, pressed together by the blood acting on both 
 sides of them, are kept in complete contact, without any strain 
 being put upon them ; in this way the orifice is closed in a 
 most efficient manner. 
 
 The ingenious view put forward by Briicke that during the ven- 
 tricular systole, the flaps are pressed back flat against the arterial walls, 
 and in the case of the aorta completely cover up the orifices of the 
 coronary arteries, so that the flow of blood from the aorta into the 
 coronary arteries can take place only during the ventricular diastole 
 or at the very beginning of the systole, and not at all during the systole 
 itself, has been disproved. 
 
 Tlie Sounds of the Heart. 
 
 When the ear is applied to the chest, either directly or by 
 means of a stethoscope, two sounds are heard, the first a com- 
 paratively long dull booming sound, the second a short sharp 
 sudden one. Between the first and second sounds, the internal 
 of time is very short, too short to be measurable, but between the 
 second and the succeeding first sound there is a distinct pause. 
 The sounds have been likened to the pronunciation of the syllables, 
 lubb, dup, so that the cardiac cycle, as far as the sounds arc 
 concerned, might be represented by : — lubb, du]\ pause. 
 
144 THE SOUNDS OF THE HEART. [Book r. 
 
 The second short sharp sound presents no difficulties. It is 
 coincident in point of time with the closure of the semilunar 
 valves, and is heard to the best advantage over the second right 
 costal cartilage close to its junction with the sternum, i. e. at the 
 point where the aortic arch comes nearest to the surface. Its 
 characters are such as would belong to a sound generated by the 
 sudden tension of valves like the semilunar valves. It is obscured 
 and altered, replaced by ' murmurs ' when the semilunar valves are 
 affected by disease, the alteration being most manifest to the ear 
 at the above-mentioned spot when the aortic valves are affected. 
 When the aortic valves are hooked up by means of a wire intro- 
 duced down the arteries, the second sound is obliterated and 
 replaced by a murmur. These facts prove that the second sound 
 is due to the sudden tension of the aortic (and pulmonary) semi- 
 lunar valves. 
 
 The first sound, longer, duller, and of a more ' booming ' 
 character than the second, heard with greatest distinctness at the 
 spot where the cardiac impulse is felt, presents many difficulties 
 in the way of a complete explanation. It is heard distinctly when 
 the chest-walls are removed. The cardiac impulse therefore can 
 have little or nothing to do with it. In point of time, and in the 
 position in which it may be heard to the gi'eatest advantage (at 
 the spot of the cardiac impulse where the ventricles come nearest 
 to the surface), it corresponds to the closure of the auriculo-ven- 
 tricular valves. In point of character it is not such a sound as one 
 would expect from the vibration of membranous structures, but 
 has, on the contrary, many of the characters of a muscular sound. 
 In favour of its being a valvular sound, may be urged the fact that 
 it is obscured, altered, replaced by murmurs, when the tricuspid or 
 mitral valves are diseased ; and according to some authors clamp- 
 ing the great veins so as to shut off the blood supply stops the 
 sound though the beat continues. The first argument may be met 
 by the consideration that a murmur though itself undoubtedly of 
 valvular origin, might largely or completely hide a sound occurring 
 at the same time as the closure of the valves but due to other 
 causes ; and the second is directly contradicted by an experiment 
 of Ludwig and Dogiel. These observers tied in succession, in the 
 order of the flow of blood, the great veins and arteries of the heart 
 of a dog so as to completely deprive the heart of blood, and 
 listened to the heart both within the body and after removal. 
 For the short time that the heart continued to beat, the first sound 
 was heard, feeble but with its main characters recognisable. From 
 this they inferred that the sound was of muscular origin. But 
 there is a great difficulty in regarding the sound as a muscular 
 one, for a muscular sound is the result of a tetanic contraction, 
 the height of the note produced varying with the rate of re- 
 petition of the simple contractions which go to make up the 
 
Chat. IV.] TlIK VAScri.Ml Ml-CIf.WfS.V. 145 
 
 tetanus, A simple contraction or spasm cannot possibly produce 
 a sound liaviiif^ tlic cluiractcrs of the iirst cardiac sound. And tho 
 evidence, tliougli perlia|)s not conclusive, fjoes to shew that the beat 
 of the heart is a slow long-continued single sjjasm, interracdiato 
 between the contraction of an ordinary striated and that of an 
 unstriated muscle, and not a tetanic contraction. We cannot, it 
 is true, now rely in support of this view on the fact that when the 
 nerve of a rheoscopic muscle-nerve pi-ejiaration is placed on the 
 beating ventricle, each beat is followed by a single spasm of the 
 muscle, and not by a tetanus ; for we now know that many forms 
 of tetanus {o.rj. those caused by the constant current, by strychnia, 
 and probably all natural voluntary contractions) give rise, in a 
 rheoscopic muscle-nerve preparation, to a single initial spasm and 
 not to a tetanus. But the general features of the beat, its long 
 latent period and the gradation of the ventricular systole through 
 the auricular systole into the rhythmic contractions of the un- 
 striated fibres of the Avails of the great veins, render it difficult 
 to suppose that the beat is really a tetanus. Moreover the long 
 duration of the ventricular systole is readily explained by the 
 wave of contraction passing in a complicated peristaltic manner 
 over the different fibres in succession. But if the beat be a simple 
 contraction, it cannot give rise to a muscular sound, unless we 
 suppose that this sequence of simple contractions over various 
 parts of the ventricle in succession is adequate to produce such a 
 sound. This, however, does not seem very satisfactory. 
 
 On the other hand, if we reject the distinctly muscular origin of 
 the sound, we are almost driven to suppose that the abnipt systole 
 is able even in the absence of blood to produce such a sudden 
 tension of the valves, and of the ventricular v.-alls, as to give rise to 
 a note. On such a view, the sound ought to vary in character 
 according as the ventricle is more or less filled, beinsr low and 
 booming when it is full, and high and sharp v.dien the contents are 
 scanty. And such is said to be the case. But the matter does not 
 at present seem ripe for any dogmatic statement. 
 
 In the normal state of things, the beats of the two ventricles 
 are so far synchronous with each other that practically onh^ one 
 first sound and one second sound is heard. It sometimes happens 
 hov/ever that the synchronism fails to such an extent and the 
 closure of the pulmonary and aortic valves respectively are sej^a- 
 rated by such an interval as to give the second sound a double 
 character. 
 
 10 
 
146 SPECIAL CARDIAC PHASES. [Book i. 
 
 On the relative duration and special characters of the 
 Cardiac events. 
 
 We may now return to a more detailed study of what is taking 
 place in the heart during a beat. We have already spoken of the 
 conclusions which may be drawn from Chauveau and Marey's curves, 
 and have incidentally (p. 138) referred to the cardiograph. 
 
 Various forms of cardiograph have been used to record the 
 cardiac impulse. In some the pressure of the impulse as in the 
 sphygmograph is transmitted directly to a lever which writes upon 
 
 Fia. 23. Caediogeaphic tbacing of Caeduc Impulse in Man (from Landois). 
 
 An entire beat occurs between a and /. Tbe auricular systole is marked by 6, 
 the end of tbe Tentricular relaxation by/. At c, the highest point of the curve, the 
 blood begins to be propelled from the ventricle, d and e are considered by some to 
 indicate the closure of the aortic and semilunar valves respectively, see text. Five 
 cardiac beats are represented ; the convex curve which their base line forms is due to 
 the respiratory movements. 
 
 a travelling surface. In others the impulse is, by means of an 
 ivory button, brought to bear on an air-chamber, connected by 
 a tube with a tambour as in Fig. 22 ; the pressure of the cardiac 
 impulse compresses the air in the air-chamber, and through this 
 the air in the chamber of the tambour by which the lever is raised. 
 In such delicate and complicated movements as those of the 
 heart however, the use of long tubes filled with air is liable to 
 introduce various errors. A cardiographic tracing of ordinary 
 characters is given in Fig. 23. 
 
 Curves of the variations in internal pressure may be obtained 
 by passing a tube connected with a mercurial manometer (as in the 
 investigation of arterial pressure, p. 122) into the right ventricle 
 through the jugular vein or into the left ventricle through the 
 carotid artery. But this method, though useful for the purpose of 
 investigating generally the pressure exerted by the cardiac walls, 
 is, by reason of the inertia of the mercury, unsuitable for detecting 
 rapid and small changes. 
 
Chap, iv.] 
 
 77/ /; I 'A SC UL A R M ECU . I \ IS .1/. 
 
 HT 
 
 Tracings of the movements of the ventricles themselves, coitc- 
 sponding to the cardiac impulse and so to a certain extent to the 
 variations of internal pressure, may also be taken directly by 
 bringing a light lever to bear on the outside of the ventricles, the 
 chest having been previously opened and artificial respiration kept up. 
 A cune* taken bv this method is shewn in Fig. 24. 
 
 id &, jh la 
 
 Fig. 24. Normal heart curve shewing changes in the antero-posterior diameter of 
 the ventricle obtained from the cat by a light recording lever moved by a button 
 which pressed gently on the anterior surface of the ventricle. The time curve gives 
 50 double vibrations per sec. and lines have been drawn to shew the duration of the 
 different phases of the ventricular movement, a to 6 corresi;onds to the distension of 
 the ventricle including the auricular systole, the wave-like rise during this period being 
 due to the increase in the diameter of the ventricle resulting from the entrance into it 
 of the contents of the auricle. The period from 6 to c corresponds to the time from 
 the commencement of the ventricular contraction to the moment when the or^'an 
 has completed its change in shape from a flattened to a more rounded form. The 
 highest part of the cur^'e corresponds also in time with the opening of the semilunar 
 valves as well as the firm closure of the auriculo-ventr!cular valves. The duration of this 
 
 ^ The majority of cardiographic, sphygmographic and other tracings shew certain 
 points which can be understood at a glance, but many characteristics can only 
 be learned by "measuring out the curve" as it is termed. This is done as foUows. 
 
 Every tracing ought to bear on it an abscissa line, marked by a point which 
 remains motionless while the recording surface is travelling. Moreover, either 
 before or after taking a curve, while the paper or recording surface is at rest, the 
 point of the lever should be always moved up and down so as to describe a segment 
 of a circle of which the axis of the lever is the centre. 
 
 The tracing thus prepared, when it has to be measured, is pinned out on aboard, 
 and, by means of a pair of compasses, the distance between whose points has 
 previously been made equal to the distance between the axis and the point of 
 the lever used in making the experiment, the centre of the circle of which the 
 curved lines previously made as directed are segments is found and marked on the 
 paper. Through this centre, which of course corresponds to the position of 
 the axis of the lever, a horizontal line is drawn parallel to the abscissa line. 
 
 Keeping one of the compass points on this line, segments of circles are drawn in 
 succession through various points of the cnr^-e, the distance between the points of 
 the compass being fixed, but the centre of the circle described being shifted 
 backwards and forwards along the horizontal line. The points where these 
 segments cut the horizontal line are marked upon it, and the distances between them 
 measured as, for example, in Fig. 29, p. 166. If the curve of a tuning-fork, the 
 point of whose recording style was carefully placed on the same vertical line as the 
 point of the lever, be also present, the segments of circles may be continued until 
 they cut this, and the time corresponding to distances between them (as, for 
 instance, in Fig. 24 the intervals between a. h. c, rf,) thus directly mep.=nred off. 
 
 10—3 
 
148 SPECIAL CARDIAC PHASES. [Book i. 
 
 period in this case is only about 3-50tlis of a sec. The period from c to tf is that 
 during which the ventricle having grasped its contents is emptying its cavity and 
 remaining contracted. It can be seen that only during the first half of this period 
 is there any marked descent of the lever point ; in other words the antero-posterior 
 diameter does not continue to diminish during the whole period of the systole, 
 indicating that little or no blood was thrown out during the second half of this 
 period, the ventricle remaining simply contracted after having emptied its cavity. 
 The period from ci to a is that during which the ventricular muscle is relaxing. 
 Here, as is frequently the case, there is no period of pause between the close of 
 the relaxation of the ventricle and the commencement of the succeeding distension. 
 The tracing gives no evidence as to the time of closure of the semilunar valves. 
 
 The chief mterest and the chief difficulties are attached to the 
 systole of the ventricles. In order to understand this, the most 
 important of the cardiac events, it must be borne in mind that, 
 as we have already seen, the pressure of the blood in the aorta 
 is always considerable. This pressure closes and keeps closed the 
 semilunar valves ; and it is not till the pressure in the ventricle 
 becomes greater than the pressure in the aorta that these valves 
 open to allow of the escape of the ventricular contents. The blood 
 therefore does not begin to pass from the left ventricle into the 
 aorta until some time, and that a variable time, after the commence- 
 ment of the systole of the ventricle ; and the same may be said of 
 the right ventricle and pulmonary artery, it being understood that 
 the arterial pressure on the right side is less than on the left. In 
 Fig. 24 the ventricular lever reaches its maximum c at once^ 
 gradually declining afterwards till the more sudden fall begins, 
 and we may suppose that the escape of blood from the ventricle 
 begins at the moment when the maximum is attained; and this 
 view is confirmed by carefully comparing a tracing of the expansion 
 of an artery with the cardiac tracing. It is quite possible however 
 to conceive that owing to circumstances, such as an increasing con- 
 traction of the ventricular fibres or deficient expansion of the 
 arteries, the pressure might continue to increase even after blood 
 was escaping from the cavity of the ventricle. And indeed in some 
 curves, the ventricular lever after the first sudden leap continues 
 to rise gradually and does not reach the maximum point until 
 afterwards. In such cases the summit of the first rise must be 
 taken as marking the beginning of the flow from the ventricle. 
 
 By the sudden systole the blood is ejected with considerable 
 force and rapidity from the ventricle, and as the ventricle becomes 
 empty a negative pressure, as we have seen, makes its appearance 
 behind the column of blood which leaves the cavity and leads to 
 the closure of the semilunar valves. Much dispute has taken place 
 as to the exact condition of the ventricle at the moment of closure 
 of the semilunar valves. The slight rise e in Chauveau and 
 Marey's curves (Fig. 21) in the ventricular curve, seen also in the 
 auricle at e and in the cardiac impulse at e", and which has been 
 taken to indicate the shutting of the semilunar valves, appears quite 
 at the close of the descent of the ventricular lever. This would 
 mean that at the moment of the closure of the valves the ventricle 
 
Chap, iv] THE VASCULAR MI-ClIAXISSf. 149 
 
 hatl not only completed its contraction but was far advanced in 
 relaxation. Such a view is not only d pi^iori improbable but i.s 
 directly contradicted by the fact that when we compare a tracing 
 obtained by ])laeing a lever directly on the heart or indeed a tracing 
 of the cardiac impulse with a pulse tracing, that is a tracing of the 
 expansion of an artery, we find that the ventricle continues con- 
 tracted after its contents have entirely k-ft the cavity. That is to 
 say, the actual How of blood takes place only during the middle 
 portion of the time during which the muscular fibres of the ven- 
 tricle are contracting and engaged in carry ino- on the systole. 
 During the first part, pressure is being got up, during the second 
 the blood is being propelled, during the third the ventricle continues 
 to remain empty and contracted. By this means the complete 
 emptying of the ventricle is eftectually secured. And others have 
 urged that the closure of the semilunar valves, being entirely due 
 to the reflux spoken of above, follows close upon the emptjing of 
 the ventricle ; in other words that it takes place while the ven- 
 tricle is still contracted. It is very difficult to point out indications 
 on the ventricular cur\-e which indubitably coiTespond to this event. 
 In tracings of the cardiac impulse, and in tracings taken by a lever 
 placed directly on the heart, a notch, followed by a rise, is some- 
 times observed in that part of the curv^e which intervenes between 
 the first large rise and the final sudden fall ; and this secondary 
 rise has been taken to indicate the closure of the semilunar valves; 
 but, if this be the case, the time during which the ventricle 
 remains contracted after the closure of the valves forms a very con- 
 siderable fraction of the whole period of the systole ; and this 
 presents difficulties. Sometimes two such notches and peaks are 
 seen, and the occurrence of the two has been attributed to a want of 
 sjTichronism in the closure of the pulmonar}' and aortic semilunar 
 valves, the latter closing some little time before the former. But 
 it is by no means clear that these notches and peaks are thus due 
 to the closure of the valves ; they may possibly have another 
 origin, they are not always present, and the attempt to fix the time 
 of the closure of the semilunar valves by them cannot be regarded 
 as satisfactory. On the other hand, the second sound of the heart 
 is undoubtedly due to the complete closure and sudden tension of 
 the semilunar valves ; and not only is this second sound separated 
 from the first sound by a distinctly appreciable interval (from which 
 we may infer either that the systole of the ventricle ceases before 
 the complete closure and sudden tension of the semilunar valves or 
 that the first sound does not last so long as the systole itself and 
 is therefore not a muscular sound) but the time elapsing between 
 the beginning of the first sound and the second sound is, as we shall 
 see, remarkably constant. Now we have reason to believe that the 
 quantity of blood expelled at any one beat, and hence the time 
 taken up in its escape, does vary very considerably ; whereas the 
 duration of the actual systole is probably much more constant. 
 
11)0 ENDO-CAEDIAC PRESSUBE. [Book i. 
 
 Hence we may infer, and the conclusion may be supported by other 
 arguments, that at the actual closure of the semilunar valves, 
 giving rise to the second sound, the ventricle has just finished its 
 systole and is beginning to relax. If this view be correct the time 
 of the closure of the valves is not indicated on the cardiographic 
 tracing by any special mark, but coincides with the commencement 
 of the more sudden and final fall of the lever as at d in Fig. 24. 
 
 Marey thought that the oscillations seen at cV in his curves and 
 obvious in the auricle and cardiac impulse as well, were due to 
 oscillations of the auriculo-ventricular valve, but in that case they 
 would be inverted in the auricular curve ; whereas they are not. 
 It is difiicult to say what gives rise to them. "We may repeat that 
 many of the details of these curves vary considerably even with the 
 same method of investigation and when the same apparatus is 
 employed. In all probability the character and sequence of the 
 events are modified by various circumstances, such as the rate and 
 rapidity of the beat, the qua,ntity of blood flowing into the heart, 
 and the pressure obtaining in the arteries. 
 
 Amount of Pressure. Although the instrument of Chauveau 
 and Marey may be experimentally graduated and has been used 
 to measure the amount of pressure in the several cavities of the 
 heart, it is, as we have said, open to objections. Better results may 
 be gained bypassing through the jugular vein into the right auricle 
 and thence into the right ventricle, or through the carotid artery 
 into the left ventricle, a tube open at the end introduced into the 
 heart and connected at the other end with a manometer. Varia- 
 tions of pressure in the cardiac cavities are thus transmitted di- 
 rectly to the mercury column of the manometer in the same way 
 as those of an artery when arterial pressure is measured. The 
 inertia of the mercury column however prevents an exact response 
 to the rapid movements of the heart, and obscures the results ; 
 though by using maximum and minimum manometers, the 
 maximum and minimum pressures of the several cavities may be 
 determined. 
 
 The prmciple o£ the maximum manometer, Fig. 25, consists in the 
 introduction into the tube leading from the heart to the mercury 
 column, of a (modified cup-and-ball) valve, opening, like the aortic 
 semilunar valves, easily from tlie heart, but closing firmly when fluid 
 attempts to return to the heart. By reversing the direction of the 
 valve, the manometer is converted from a maximum into a minimum 
 instrument. When an ordinary manometer is connected with a ven- 
 tricular cavity, the movements of the mercury do not follow exactly the 
 rapid variations of pressure of the cavity, and the height of the column 
 fails to indicate both the highest and the lowest pressures. 
 
 In this way in the dog a maximum pressure has been observed 
 
 ■ in the left ventricle of about 140 mm. (mercury), in the right 
 
 ventricle of about 60 mm. and in the right auricle of about 20 mm. 
 
Chap, iv] 
 
 77/ A' VASCULAR MECHANISM. 
 
 161 
 
 Marey luul ])reviously, by means of his own instrument, determined 
 the pressure in the liorse to bo in tlie left ventricle about 150 rnm., 
 in the riii^lit ventricle only about *iO mm., while that of the right 
 auricle he estiniatiMl at not more than a IVw mm. 
 
 Fig. 25. The Maximum Manometer of Goltz and Gaule. 
 
 At c a connection is made with the tube leading to the heart. When the screw 
 damp k is closed, the valve v comes into action, and the instrument, in the position 
 of the valve shewn in the figure, is a maximum manometer. By reversing the 
 direction of v it is converted into a minimum manometer. When k is 02:)ened, the 
 variations of pressure are conveyed along a, and the instrument then acts like an 
 orcUuary manometer. 
 
 It is interesting to observe that the minimum pressure may 
 fall below that of the atmosphere : thus in the left ventricle (of 
 the dog) a minimum pressure varying from — 52 to — 20 mm. may 
 be reached, the minimum of the right ventricle being from — 17 to 
 — 16 mm., and of the right auricle from — 12 to — 7 mm.^ Part of 
 this diminution of pressure in the cardiac cavities may be due, as 
 will be explained in a later part of this work, to the aspiration of 
 the thorax in the resj)iratory movements. But even when the 
 thorax is opened, and artificial respiration kept up, under which 
 circumstances no such aspiration takes place, the pressure in the 
 left ventricle may still sink as low as — 2-i mm. The minimum ma- 
 nometer, which shews most distinctly the existence of this negative 
 pressure, obviously gives no information as to the exact phase of 
 the beat in which it occurs ; and there is some difference of opinion 
 as to the exact time at which it takes place. Goltz and Gaule, to 
 
 ^ These numbers are to be considered merely as instances which have been 
 observed, and not as averages drawn from a large number of cases. 
 
152 DURATION OF THE CARDIAC PHASES. [Book i. 
 
 \vliom "we are indebted for the maximum and minimum manometer, 
 l)elieved that the negative pressure appeared at the beginning of 
 the diastole and indeed that it was caused by the expansion of the 
 ventricle. Were this the case, the ventricle might be regarded not 
 only as a force pumj) driving blood into the arteries, but also as a 
 suction pump drawing blood from the auricles and great veins. 
 
 Others however find great difficulties in supposing that the 
 ventricular walls can, either by virtue of the elasticity of their 
 fibres, or by the contraction of special dilating fibres, or by 
 becoming suddenly injected with blood through the coronary 
 arteries, actually expand so as to exert any such suction power. 
 And they maintain that the negative pressure seen in the ventricle 
 is merely that same negative pressure due to the sudden emptying 
 of the ventricle which we have already described as serving to 
 close the semilunar valves. When the minimum manometer is 
 used, the lowest limit of negative pressure is not reached until after 
 several beats, indicating that its duration in any single beat must 
 be very brief The negative pressure due simply to the cessation 
 of the flow is in fact almost immediately made away with by the 
 ventricular walls, in theu' continued contraction coming into com- 
 plete contact ; it passes off therefore before any blood can enter into 
 the ventricle from the auricle, and hence can exert no suction power. 
 
 Admitting this, however, it is still open for us to suppose that 
 after this negative pressure has passed away, a second negative 
 pressure is caused by the expansion of the ventricle in diastole ; 
 and that this, though also brief, does exert a suction power. And 
 indeed the view that the ventricle in expanding can produce such 
 a negative pressure is one which cannot as yet be regarded as 
 definitely disproved. 
 
 The dnration of the several phases. The time-measurements 
 given in Fig. 21 afford a general idea of the relative duration of 
 the several events in the slowly beating heart of the horse. Thus 
 it is obvious that the longest phase (viz. about yq sec.) is that 
 occurring between the end of the ventricular systole at e to 
 the beginning of the auricular systole at h ; this is often spoken of 
 as the diastole, or as the "passive interval," since during this time 
 both auricles and ventricles are in diastole. The next longest phase 
 is the systole of the ventricles (viz. rather more than -^^ sec), and the 
 shortest (viz. rather less than j^^ sec.) is the systole of the auricles. 
 
 When we desire to arrive at more complete measurements, we 
 are obliged to make use of calculations based on various data ; 
 and these give only approximate results. Naturally the most 
 interest is attached to the duration of events in the human heart. 
 
 The datum which perhaps has been most largely used is th3 
 interval between the beginning of the first and the occurrence of 
 the second sound. This may be determined with approximative 
 correctness, and according to Donders varies from "301 to '327 sec, 
 
C'liAi'. iv] TIIK VASCULAR MECHANISM. 153 
 
 occupying from 40 to 46 p. c. of the whole period; and being fairly 
 constant for different rates of heart-beat. 
 
 The observer, listening to the sounds of the heart, made a signal at 
 each event on a recording surface, the diflference in time between tlie 
 marks being measured by means of tlie vibrations of a tuning fork 
 recorded on the same surface. By practice it was found possible 
 to reduce the errors of observation within very small limits. 
 
 Now whatever be the exact causation of the first sound, 
 it is undoubtedly coincident with the systole of the ventricles, 
 though possibly the actual commencement of its becoming audible 
 may be slightly behind the actual beginning of the muscular con- 
 tractions. Similarly the occurrence of the second sound due to the 
 closure of the semilunar valves may, as we have seen, be taken to 
 mark the close of the ventricular systole. And thus the inten-al 
 between the beginning of the first and the occurrence of the second 
 sound has been regarded as indicating approximatively the duration 
 of the ventricular systole, i.e. the period during which the ventri- 
 cular fibres are contracting. If however we accept the \dew that 
 the ventricle still remains contracted for a brief period after the 
 valves are shut, then the second sound does not mark the end of 
 the systole, and the duration of the systole is rather longer than 
 the "3 sec. given above. 
 
 The propulsion of the blood into the aorta leads to an expansion 
 of the aorta walls, known as the pulse, which we shall study more 
 fully immediately. This pulse travels, as we shall see, along 
 the arteries at a certain rate : it is later at arterial points 
 more distant from the heart than at points nearer the heart. We 
 can calculate with approximative coiTectness the time it takes for 
 the expansion to travel from the aortic valves to the radial artery 
 at the wrist, for example. Now when we record, as we may do on 
 the same recording surface, the exact moment at which the first 
 sound begins, or at which the lever of the cardiograph begins to 
 rise in the ventricular systole, and also the exact moment at which 
 the expansion of the corresponding pulse at the wrist begins, and 
 measure the interval of time between them, we find that the interval 
 is greater than is required for the expansion of the pulse-wave to 
 travel from the heart to the wrist. The difference gives the measure 
 of the time during which the ventricle by its contraction is getting 
 up an adequate pressure upon its contents, and during which, as yet, 
 blood has not escaped from the ventricular cavity and begun to ex- 
 pand the aorta : the measure in fact of what we called, a little while 
 ago, the first period of the ventricular systole. This may also be 
 estimated by directly measuring the time taken up by the upstroke 
 of the cardiographic tracing, and has been said to be on an average 
 about '085 sec. These measurements however are approximative 
 only and there can be no doubt that the time varies very largely, 
 being dependent on the quantity of blood in the ventricle, on the 
 blood-pressure in the aorta and on the condition of the heart. 
 
154 DURATION OF THE CATtDIAC PHASES. [Book i. 
 
 During the expansion of the artery and probahly for some httle 
 time beyond, viz. up to the occurrence of what in speaking of the 
 pulse-wave we shall call the dicrotic notch, blood is being propelled 
 from the ventricle. By measuring this time or by deductions from 
 the curve of the cardiac impulse, it has been concluded that the time 
 during which blood is escaping from the ventricle or the duration 
 of the second phase of the ventricular systole, amounts to about 
 0"1 sec. 
 
 Deducting these two periods from the total period of 0'3 sec.-j 
 there would be left a period of O'llo sec, marking the third phase 
 of the systole, during which the ventricle, though empty, is con- 
 tinuing its contractions. Upon the view however that the closure 
 of the valves does not mark the end of the systole, this j^hase 
 must be taken as still longer. 
 
 In a heart beating 72 times a minute, which may be taken as 
 the normal rate, each entire cardiac cycle would last about 0"8 sec, 
 and taking 0"3 sec. as the duration of the systole, the deduction of 
 this would leave 0"5 sec for the whole diastole of the ventricle 
 including its relaxation. 
 
 At the close of this period, there occurs the systole of the 
 auricles, the exact duration of which it is difficult to determine, it 
 being hard to say when it really begins, but which perhaps may 
 be taken as lasting on an average O'lsec The systole of the 
 ventricle follows so immediately upon that of the auricles, that 
 practically no interval exists between the two events. 
 
 We may sum up therefore the details of the duration of the more 
 important phases of the cardiac cycle in the following tabular form. 
 
 sees. sees. 
 
 Systole of ventricular previous to 
 
 opening of semilunar valves . O'OSo 
 Escape of blood into aorta . . OlOO i- 
 Continued contraction of the 
 
 emptied ventricle . . . 0"115j 
 Total systole of the ventricle . . 0'3 
 
 Diastole of both auricle and ven- "j 
 
 tricle or "passive interval" . O'-iOO Y 
 
 Systole of auricle . . . 0-lOOj 
 
 Sum of above two, making the 
 diastole of ventricle or "pause" 
 between second and first sound . 0'5 
 
 Total Cardiac Cycle .... 08 
 
 Or selecting only the important facts out of the -f^ sec. occupying 
 the whole cardiac cycle, f^ sec. or possibly rather more are taken up 
 by the systole, and -f^ sec. or possibly rather less by the diastole of 
 the ventricle. 
 
 The following diagram may be useful as giving in a graphic 
 form a general idea of the sequence and duration of the seveM 
 
 ^%c.^^ 'im\"_ 
 
f'llAl'. IV.] 
 
 77/ A' VASCULAR MECIUy/SAf. 
 
 155 
 
 cardiac events. It will bo understood of course that the diagram i& 
 intended to sliew merely the ^'eneral relations of" the several events 
 and not to represent exact measurements. 
 
 Fio. 26. 
 
 DiAGRAilllATIC EePRESENTATIOX OF THE MOVEMENTS AKD SoUNDS OF TUE 
 
 Heart during a Cardi.4.c Period. (After Dr Sharpey.) 
 
 We may repeat that the details given above are at the best 
 approximative only, and, we may add, to a certain extent hypo- 
 thetical. We have given them at such length not on account 
 of their intrinsic importance, or because they are trustworthy data 
 for further calculations, but because the study of them may help 
 the reader in forming a more vivid image in his mind of what is 
 taking place in the heart during a beat. Moreover it must be 
 remembered that the figures quoted are those belonging to what 
 may be considered a normal rate of heart beat. The rate how- 
 ever at which the heart beats varies, as we shall see, under the 
 influence of circumstances, within very wide limits. With regard 
 to the duration of the several phases at different rates of heart 
 beat, the most important fact is perha^js that the pause varies 
 much more than does the systole of the ventricles. A quickly 
 beating heart differs from a slowly beating heart by reason of the 
 pause being shortened, much more than by each systole being of 
 less duration. 
 
 We may briefly recapitulate the main facts connected with 
 the passage of blood through the heart as follov\s. The right 
 auricle during its diastole, by the relaxation of its muscular fibres, 
 and by the fact that all pressure from the ventricle is removed by 
 the tension of the tricuspid valves, offers but little resistance to 
 the ingress of blood from the veins. On the other hand, the blood 
 in the trunks, of both the superior and inferior vena cava, is under a 
 pressure, which diminishing towards the heart and becoming within 
 
156 DURATION OF THE CARDIAC PHASES. [Book i. 
 
 the thorax actually negative (as we shall see in speaking of 
 respirations), remains higher than the pressure obtaining in the 
 interior of the auricle; the blood in consequence flows into the 
 empty auricle, its progress in the case of the superior vena cava 
 being assisted by gravity. At each inspiration, this flow is favoured 
 by the increased negative pressure in the heart and great vessels 
 caused by the respiratory movements. Before this flow has gone on 
 very long, the diastole of the ventricle begins, its cavity dilates, 
 the flaps of the tricuspid valve fall back, and blood for some little 
 time flows in an unbroken stream from the venoB cavse into the 
 ventricle. In a short time, however, probably before much blood 
 has had time to enter the ventricle, the auricle is full, and forth- 
 with its sharp sadden systole takes place. Partly by reason of the 
 onward pressure in the veins, which increases rapidly from the 
 heart towards the capillaries, partly from the presence of valves in 
 the venous trunks and at the mouth of the inferior vena cava, but 
 still more from the fact that the systole begins at the great veins 
 themselves and spreads thence over the auricle, the force of the 
 auricular contraction is spent in driving the blood, not back into 
 the veins, but into the ventricle, where the pressure is still ex- 
 ceedingly low. Whether there is any backward flow at all into the 
 great veins or whether by the progressive character of the systole 
 the flow of blood continues, so to speak, to follow up the systole 
 without break so that the stream from the veins into the auricle is 
 really continuous, is at present doubtful ; though a slight positive 
 wave of pressure synchronous with the auricular systole, travelling 
 backward along the great veins has been observed at least in cases 
 where the heart is beating vigorously. 
 
 The ventricle thus being filled by the auricular systole, the 
 play of the tricuspid valves described above comes into action, 
 the auricular systole is followed by that of the ventricle and the 
 pressure within the ventricle, cut off from the auricle by the 
 tricuspid valves, is brought to bear entirely on the conus arteriosus 
 and the pulmonary semilunar valves. As soon as by the rapidly 
 increasing shortening of the ventricular fibres the pressure within 
 the ventricle becomes greater than that in the pulmonary artery, 
 the semilunar valves open and the still continuing systole discharges 
 the contents of the ventricle into that vessel. 
 
 As the ventricle thus rapidly and forcibly empties itself, a 
 transient negative pressure makes its appearance in the rear of the 
 ejected column of blood. This in return leads to a reflux of blood 
 towards the ventricle. The first act of this reflux however is, as 
 we have seen, to close the semilunar valves, and even if it be 
 urged that the exit of the ventricular contents does not always end 
 with sufficient abruptness to cause a negative pressure adequate 
 to produce this result, the elastic rebound of the arteries, upon 
 their receiving no fresh blood, has the same effect of closing the 
 semilunar valves, and thus of shutting off the blood in the over- 
 
PHAr. IV.] THE VASCrLAR M/XI/A XIS.U. i:.7 
 
 distended arteries from the emptied ventricle. Coincidcntly with 
 this closure, the systole as we have seen probably ends and 
 relaxation begins ; then once more the cavity of the ventricle be- 
 comes unfolded and finally distended by the influx of blood from 
 the auricle. 
 
 During the whole of this time the left side has with still 
 greater cnerg}' been executing the same manoeuvre. At the same 
 time that the vena; cav33 arc tilling the right auricle, the pulmonary- 
 veins are filling the left auricle. At the same time that the right 
 auricle is contracting, the left auricle is contracting too. The 
 systole of the left ventricle is synchronous A\'ith that of the right 
 ventricle, but executed with gi-cater force ; and the flow of blood 
 is guided on the left side by the mitral and aortic valves in the 
 same way that it is on the right by the tricuspid valves and those 
 of the pidmonary artery. 
 
 The Work done. 
 
 "We can measure ^^'ith ajDproximativc exactness the intraven- 
 tricular pressure, the length of each sj'stole, and the number of 
 times the systole is repeated in a given period, but perhaps the 
 most important factor of all in the determination of the work of 
 the vascular mechanism, the quantity ejected from the ventricle 
 into the aorta at each systole, cannot be accurately determined ; 
 we are obliged to fall back on calculations having many sources 
 of error. The mean result of these calculations gives about 180 
 grms. (G oz.) as the quantity of blood which is driven from each 
 ventricle at each systole in a full-groMTi man of average size and 
 weight. It is evident that exactly the same quantity must issue at 
 a beat from each ventricle ; for if the right ventricle at each beat 
 gave out rather less than the left, after a certain number of beats 
 the whole of the blood would be gathered in the systemic circu- 
 lation. Similarly, if the left ventricle gave out less than the right, 
 all the blood would soon be crowded into the lungs. The fact that 
 the pressure in the right ventricle is so much less than that 
 in the left (probably 30 or 40 mm. as compared with 200 mm. 
 of mercury), is due, not to differences in the quantity of blood in 
 the cavities, but to the fact that the peripheral resistance which 
 has to be overcome in the lungs is so much less than that in the 
 rest of the body. 
 
 Various methods have been adopted for calculating the average 
 amount of blood ejected at each ventricular systole. It has been 
 calculated from the capacity of the recently removed and as yet not 
 rigid ventricle, filled with blood under a pressure equal to the calculated 
 average prcs-sure in the ventricle. This method of cour?;c presupposes 
 
158 THE WORK DONE BY THE HEART. [Book i. 
 
 that the whole contents of the ventricle are ejected at each systole. 
 Volkmann measured the sectional area of the aorta, and taking an 
 average velocity of the blood in the aorta (a very uncertain datum), 
 calculated the quantity of blood which must pass through the sectional 
 area in a given time. The number of beats in that time then gave 
 him the quantity flowing through the area, and consequently ejected 
 from the heart, at each beat. The mean of many experiments on 
 diflferent animals came out '0025 p. c. of the body weight, which in 
 a man of 75 kilos would be 187'5 grms. Yierordt measured the mean 
 velocity and the sectional area in the carotid, and thence, from a 
 measurement of the sectional area of the aorta, and from a calculation 
 of the blood's mean velocity in it, based on the supposition that the 
 mean velocity in an artery was inversely as its sectional area, arrived 
 at the quantity flowing through the aortic sectional area in a given 
 time, and thus at the quantity passing at each beat. Both these 
 calculations are vitiated by the fact that the variations of velocity 
 in the aorta are so great, that any mean has really but little positive 
 value. 
 
 rick by means of calculations based partly on the data gained by 
 observing the increase of the volume of the whole arm at each cardiac 
 systole, arrived at results much less than either of the above. In one 
 case he estimated the quantity ejected from the heart at each beat 
 at 53 grm., and in a second case at 77 grm. 
 
 It must be remembered that though it is of advantage to speak 
 of an average quantity ejected at each stroke, it is more than 
 probable that that quantity may vary within very wide limits. 
 Taking, however, 180 grms. as the quantity, in man, ejected at 
 each stroke at a pressure of 250 mm.^ of mercury, which is equiva- 
 lent to 3'21 metres of blood, this means that the left ventricle is 
 capable at its systole of lifting 180 grms. 3'21 m. high, i. e. it does 
 578 gram-metres of work at each beat. Supposing the heart to 
 beat 72 times a minute, this Avould give for the day's work of the 
 left ventricle, nearly 60,000 kilogram-metres ; calculating the work 
 of the right ventricle at one-fourth that of the left, the work of the 
 whole heart would amount to 75,000 kilogram-metres, which is 
 just about the amount of work done in the ascent of Snowdon by 
 a tolerably heavy man. A calculation of more practical value is 
 the following. Taking tho quantity of blood as -^^ of the body 
 weight, the blood of a man weighing 75 kilos would be about 
 5,760 grms. If 180 grms. left the ventricle at each beat, a 
 quantity equivalent to the whole blood would pass through the 
 heart in 32 beats, i.e. in less than half a minute. 
 
 1 A high estimate is purijosely taken here. 
 
Chap. IV.] THE VASCCLAIi M/CCJ/AX/S.U. 159 
 
 Variations in the Heart's heat. 
 
 These are for the most juirt in reality vital iihcnomena, i.e. 
 brought about by events tk-pcnding on chanrrcs in the vital 
 properties of some or other of the tissues of the body. It will 
 be convenient, however, briefly to review them here, though the 
 discussion of their causation must be deferred to its appropriate 
 place. 
 
 The frequency of the heart, i.e. the number of beats in any 
 given time, may vary. The average rate of the human pulse or 
 heart -beat is 72 a mmute. It is quicker in children than in adults, 
 but quickens again a little in advanced age. It is quicker in the 
 adult female than in the adult male, in persons of short stature 
 than in tall people. It is increased by exertion, and thus is 
 quicker in a standing than in a sitting, and in a sitting than 
 in a lying posture. It is quickened by meals, and while varying 
 thus from time to time during the day, is on the whole quicker 
 in the evening than in early morning. It is said to be on the 
 whole quicker in summer than in winter. Even independently of 
 muscular exertion it seems to be quickened by great altitudes. It 
 is profoundly influenced by mental conditions. 
 
 The length of the systole may vary, indeed we have reason 
 to think that it does vary considerably, though as a general and 
 broad rule it may be stated that a frequent differs from an 
 infrequent pulse chiefly by the length of the diastole. Donders 
 found the length of the systole as measured by the interval 
 between the fli'st and second sounds to be for ordinary pulses 
 remarkably constant in different persons, varying not more than 
 from '327 to "oOl sec, and being therefore relatively to the whole 
 cardiac period less in slow than in quick pulses. 
 
 The force of the heat may vary ; the ventricular systole may be 
 weak or strong. When the rate of beat is suddenly increased 
 there is a tendency for the individual beats to be diminished in 
 force, and on the other hand to be increased in force when the 
 rate is diminished. But there is no necessary connection between 
 rate and strength ; both a frequent and an infrequent pulse may 
 be either weak or strong. 
 
 The character of the heat ma}' vary; the systole may be sudden 
 and sharp, rapidly reaching a maximum and rapidly declining, or 
 slow and lengthened, reaching its maximum only after some time 
 and declining very gradually; the latter being the slow pulse 
 (pulsus tardus) as distinguished from the infrequent pulse (pulsus 
 varus). The jDulse is also sometimes spoken of as being slapping, 
 and sometimes as heaving. But, as we shall see immediately, the 
 features of the pulse are dependent not only on the heart beat but 
 also on the condition of the arteries. 
 
160 YARIATIOXS IN THE HEARTS BEAT. [Book i. 
 
 The rhythm may be intermittent or in-egular. Thus in an 
 intermittent pulse, a beat may be so to speak dropped : the hiatus 
 occurring either regularly or irregularly. In an irregular rhythm 
 succeeding beats may differ in length, force, or character. 
 
SEC. 3. THE PULSE. 
 
 When the finger is placed on an arten', such as the radial, an 
 intermittent pressure on the finger, coming and going Avith the 
 beat of the heart, is felt. "When a light lever such as that of the 
 sphygmograph is placed on the artery, the lever is raised at each 
 beat, falling between. The pressure on the finger, and the raising 
 of the lever, are expressions of the expansion of the elastic artery, 
 of the temporary additional distension which the artery undergoes 
 at each systole of the ventricle. This intermittent expansion is 
 called the pulse ; it corresponds to the intermittent outflow of 
 blood from a severed artery, being present in the arteries only, 
 and except under particular circumstances, absent from the veins 
 and caj)illaries. The expansion is frequently \T.sible to the eye, 
 and in some cases, as where an artery has a bend, may cause a 
 certain amount of locomotion of the vessel. 
 
 All the more important phenomena of the pulse may be 
 witnessed on an artificial scheme. 
 
 If two levers be placed on the arterial tubes of an artificial' 
 scheme, one near to the pump, and the other near to the peripheral 
 resistance, with a considerable length of tubing between them, and 
 both levers be made to write on a recording surface, one im- 
 mediately below the other, so that their curves can be more easily 
 compared, the follo^^-ing facts may be observed, when the pump is 
 set to work regidarly. 
 
 ^ By this is simply meant a system of tubes, along which fluid can be driven by a 
 pump worked at regular intervals. In the course of the tubes a (variable) resistance 
 is introduced in imitation of the peinpheral resistance. The tubes on the proximal 
 side of the resistance consequently represent arteries ; those on the distal side, veins. 
 
 F. 11 
 
162 
 
 THE PULSE. 
 
 [Book i. 
 
 1. With eacli stroke of the pump, each lever (Fig. 27, 1, and II.) 
 
 rises to a maximum, la, 2a, and then falls again, thus describing a 
 curve, — the pulse-curve. This shews that the expansion of the 
 
 5oAAAAAAAAAAAA/\AAAy 
 
 Fig. 27. Pulse-curves described by a series of sphygmograpliic levers placed at 
 intervals of 20 cm. frora each other along an elastic tube into which fluid is forced 
 by the sudden stroke of a pump. The pulse--wave is traveUing fi-om left to right, as 
 indicated by the arrows over the primary (a) and secondary {b, c) pulse-waves. The 
 dotted vertical lines drawn from the summit of the several primary waves to the 
 tuning-fork curve below, each complete vibration of which occupies Jjj-sec., allow the 
 time to be measured which is taken up by the wave in passing along 20 cm. of the 
 tubing. The waves a are waves reflected from the closed distal end of the tubing; 
 this is indicated by the direction of the arrows. It wUl be observed that ia the 
 more distant lever YI. the reflected wave, having but a shght distance to travel, 
 becomes fused with the prhnaiy wave. (From Marey.) 
 
 tubing passes the point on which the lever rests in the form of 
 a wave. At one moment the lever is quiet: the tube beneath 
 it is simply distended to the normal permanent amount indicative 
 
CiiAi'. iv.) 77//; iM.sY7v..i/t' Mi:('iiAM,s.\r. ic;; 
 
 of the mean arterial pressure; at tlic next nionieiit the j)uls(! 
 cx]>ansiou reaches the lever, and the lever begins to rise, and 
 continues to do so until the top of the wave reaches it, after which 
 it falls attain until it is once more at rest, the wave havinjr 
 completely passed by. 
 
 The rise of each lever is somewhat sudden, but the fall is more 
 gradual, and is generally marked with some irregularities. The 
 suddenness of the rise is due to the suddenness with which the 
 sharp stroke of the pump expands the tube ; the fall is more 
 gradual because the elastic reaction of the walls, whereby the tube 
 returns to its former condition after the expanding power of the 
 pump has ceased, is gradual in its action. 
 
 2. The size and form of each curve depend in part on the 
 amount of pressure exerted by the levers on the tube. If the 
 levers only just touch the tube in its expanded state, the rise 
 in each will be insignificant. If on the other hand they be pressed 
 doAvn too firmly, the tube beneath will not be able to expand 
 as it otherwise would, and the rise of the levers will be proportion- 
 ately diminished. There is a certain pressure, depending on the 
 expansive power of the tubing, at which the tracings are best 
 marked. 
 
 8. If the points of the two levers be placed exactly one under 
 the other on the recording surface, it is obvious that, the levers 
 being alike except for their position on the tube, any difference in 
 time between the movements of the two levers will be shewn by 
 an interval between the beginnings of the curves they describe, if 
 the recording surface be made to travel sufficiently rapidly. 
 
 If the movements of the two levers be thus compared, it will be 
 seen that the far lever (Fig. 27, II.) commences later than the near 
 one (Fig. 27, I.), the farther apart the two levers are, the greater is 
 the interval in time between their curves. Comj)are the series 
 I. to VI. (Fig. 27). This means that the wave of expansion, the 
 pulse-wave, takes some time to travel along the tube. By exact 
 measurement it would similarly be found that the rise of the near 
 lever began some fraction of a second after the stroke of the pump. 
 
 The velocity with which the pulse-wave travels depends chietly 
 on the amount of rigidity possessed by the tubing. The more 
 extensible (N\dth corresponding elastic reaction) the tube, the slower 
 is the wave ; the more rigid the tube becomes, the faster the wave 
 travels. The width of the tube is of much less influence, though ac- 
 cording to some observers the wave travels more slowly in the wider 
 tubes. 
 
 The rate at which the normal pulse-wave travels in the human 
 body has been variously estimated at from 10 to 5 metres per 
 second. In all probability the lower estimate is the more correct 
 one ; but it must be remembered that in all probability the rate 
 varies very considerably under different conditions. According to 
 all observers the velocity of the wave in passing from the gi'oin to 
 
 11—2 
 
1G4 THE rULSE. [Book i. 
 
 the foot is greater than tliat in passing from the axilla to the 
 wrist (6 m. against 5 m.). This is probably due to the fact that 
 the femoral artery with its branches is more rigid than the axil- 
 lary. So also in the arteries of children, the wave travels more 
 slowly than in the more rigid arteries of the adult ; and the velocity 
 appears to be increased by circumstances which heighten and 
 decreased by those which lessen the mean arterial pressure, since 
 ■with increasing or diminishing pressure the arterial walls become 
 more or less rigid. 
 
 4. When two curves taken at different distances from the 
 pump are compared with each other, the far curve will be found to 
 be shallower, with a less sudden rise, and with a more rounded 
 summit than the near curve : compare 5a with la. Fig. 27. In 
 other words, the pulse-wave as it travels onward becomes diminished 
 and tiattened out. If a series of levers, otherwise alike, were 
 placed at intervals on a piece of tubing sufficiently long to convert 
 the intermittent stream into a continuous flow, the pulse-wave 
 might be observed to gradually flatten out and grow less until it 
 ceased to be visible. 
 
 Care must be taken not to confound the progression of the 
 pulse-wave with the progression of the fluid itself. The pulse- 
 wave travels over the moving blood somewhat as a rapidly moving 
 natural wave travels along a sluggishly flowing river, the velocity 
 of the pulse-wave being 9 metres per sec, while that of the current 
 of blood is not more than half a metre per sec. even in the large 
 arteries, and diminishes rapidly in the smaller ones. 
 
 Taking the duration of the pulse-wave, that is the time taken 
 by any point in the arterial tract, in expanding and returning to 
 its former calibre, so low as -^^ of a second, it is evident that the 
 pulse-wave started by any one systole, even if it travels so slowly 
 as 5 m. per sec, will before it is completed have reached a 
 point Y^ of 5m. = 2 m. distant from the ventricle. But even in the 
 tallest man the tips of the toes are not 2 m. distant from the heart. 
 In other words, the length of the pulse-wave is much greater 
 than the whole length of the arterial system, so that the beginning 
 of each wave has become lost in the small arteries and capillaries 
 some time before the end of it has finally passed away from the 
 beginning of the aorta. 
 
 The general causation of the pulse may then be summed up 
 somewhat as follows. The systole of the ventricle drives a 
 quantity of blood into the already full aorta. The sudden injection 
 of this quantity of blood expands the portion of the aorta next 
 to the heart, and thus gives rise to the sudden up-stroke of the 
 pulse-curve. The rapidity of the flow from the ventricle being 
 greatest at its beginning, the maximum of expansion is soon 
 reached, and the aortic walls, even while for a short time blood is 
 still, with diminishing rapidity, issuing from the ventricle, tend by 
 virtue of their elasticity to return to their former calibre. This 
 
Chap, iv.] 77//; VASCL'LAA' M Ec II A .\ IS.\f. 1G5 
 
 ivturn continues after the flow lias ceased, and the aortic valves 
 soon becoming closed, the elastic force thus brought into play serves 
 to drive the blood onward. The elastic recoil being slower than the 
 initial expansion, the down-stroke of the pulse-curve is more gradual 
 than the up-stroke. Of this jiortion of the aorta, which actually 
 receives the blood cgcoted from the heart, the part immediately 
 adjacent to the semilunar valves begins to exjiand first, and the ex- 
 pansion travels thence on to the end of this portion. In the same 
 way it travels on from this portion through all the succeeding portions 
 of the arterial system. For the total expansion required to make room 
 for the new quantity of blood cannot be provided by that portion 
 alone of the aorta into which the blood is actually received ; it is 
 supjdied by the whole arterial system : the old quantity of blood 
 which is rejilaced by the new in this first portion has to find room 
 for itself in the rest of the arterial space. As the expansion 
 travels onward, hoAvever, the increase of pressure which each 
 portion transmits to the succeeding portion will be less than that 
 which it received from the preceding portion. For the whole 
 increase of pressure due to the systole of the ventricle has to 
 be distributed ov^er the whole of the arterial system, and a fraction 
 of it must therefore be left behind at each stage of its progress ; 
 that is to say, the expansion is continually gro\Wng less, as the 
 pulse travels from the heart to the capillaries ; hence the diminished 
 height of the pulse-curve in the more distant arteries, and its 
 disappearance in the capillaries. 
 
 Secondary Waves and Dicrotism, In nearly all pulse tracings, 
 the curve of the expansion and contraction of the artery is broken 
 
 Fig. 28. Pclse-tracing feom cuiotid abtery of healthy man^ (from Moens). 
 
 X, commencement of expansion of the artery. A, summit of the first rise. 
 
 C, dicrotic secondary wave. B, predicrotic secondary wave, p notch preceding this. 
 
 D, succeeding secondary wave. The curve above is that of a tuning-fork with ten 
 double vibrations in a second. 
 
 1 It will be understood that in the case of this and the succeeding sphygmo- 
 graphic tracings (for the latter I am indebted to Dr Galabin and Dr Roy) 
 comparisons between the several curves can only be made in a limited 
 manner and with precautions, since the tracings are taken ^^-ith different 
 amplifications, pressures, Ac. — and are some from man, others fi'om animals. They 
 are introduced simply to illustrate points treated of successively in the text. 
 
lGt3 
 
 THE PULSE. 
 
 [Book l 
 
 by two, three, or several smaller elevations and depressions : 
 secondary waves are imposed upon the fundamental wave. In the 
 sphygmographic tracing from the carotid and radial reproduced in 
 Figs. 28 and 29 and in many of the other tracings given, these 
 secondary elevations are marked as B, C, D. When one such 
 
 Fig. 29. Pulse-cdbve fkom radial of man. 
 
 Taken with extra vascular pressure of 70 mm. mercury. The vertical curved line 
 L, gives the tracing which the recording lever made when the blackened paper was 
 motionless. The horizontal line forms the abscissa of the tracing. The curved 
 interrupted lines shew the distance from one another in time of the chief phases of 
 the pulse wave. x = commencement and A close of expansion of artery, p, predi- 
 crotic notch, d, dicrotic notch. C, dicrotic crest. D, post-dicrotic crest. /, the 
 post-dicrotic notch. 
 
 secondary elevation only is conspicuous, so that the pulse-curve 
 presents two notable crests only, the primary crest and the second- 
 ary one, the pulse is said to be "dicrotic"; when two secondary 
 crests are prominent, the pulse is often called "tricrotic," where 
 
 \ 
 
 Fig. 30. Anacrotic tulse-tracing from the carotid of rabbit. 
 
 several "polycrotic." As a general rule, the secondary elevations 
 appear only on the descending limb of the whole wave as in most 
 of the curves given, and the curve is then spoken of as "katacrotic." 
 Sometimes, however, the first elevation or crest is not the highest but 
 
CUAl'. IV.] 
 
 THE VASCULMi MKCIIAMSM. 
 
 167 
 
 appears on the ascending portion of the main curve as in Fig. 30 
 and Fig. 33 : such a curve is spoken of as "anacrotic." 
 
 Of these secondary elevations, the most frequent, conspicuous 
 and important is the one wliich appears some way do^\Ti on the 
 descending limb and is marked C on most of the curves. It is 
 more or less distinctly visible on all sphygmographic tracings and 
 may be seen in sphygmograms of the aorta aa well as of other 
 arteries. Sometimes it is so slight as to be hardly discemiljle ; at 
 other times it may be so marked as to give rise to a really double 
 pulse (Fig. 31), i.e. a pulse which can be felt as double by the finger ; 
 hence it has been called the dicrotic elevation or the dicrotic wave, 
 the notch preceding the elevation being spoken of as the "dicrotic 
 
 \i 
 
 Fig. 31. Two gk.m^es of makked hicrgtism in radlvl pulse of man. 
 (Typlioicl Fever.) 
 
 notch." Neither it nor any other secondary elevations can be recog- 
 nised in the tracings of blood-pressure taken with a manometer. 
 This may be explained by the fact that the movements of the 
 mercury column are too sluggish to reproduce these finer variations; 
 but dicrotism is also conspicuous by its absence in the tracings 
 given by more delicately responsive instruments. Moreover, when 
 the normal pulse is felt by the finger, most persons find themselves 
 unable to detect any dicrotism. Hence some have been led to 
 
 mwmffifSfmmHummmmivmHmmmmmmmmMmhmmmmm^mmfmM 
 
 Fig. 32. Noeiial pulse-curve in the aorta from the dog. 
 
 maintain that this and the other secondary elevations do not really 
 exist in the normal pulse. But it seems difficult to maintain this 
 view in face of the experiment of Landois, in which the tracing 
 obtained by allo^ving the blood to spirt directly from an opened 
 small artery, such as the dorsalis pedis, upon a recording surface, 
 shewed in an unmistakeable manner the existence of the dicrotic 
 wave. 
 
168 THE PULSE. [Book i. 
 
 Less constant and conspicuous than the dicrotic wave but yet 
 appearing in most sphygmograms is an elevation which appears 
 higher up on the descending limb of the main wave; it is marked 
 B on some of the curves and is frequently called the 'predicrotio 
 
 Fig. 33. Axaceotic sphygmograph tracing fkom the ascending aorta (Aneumm). 
 
 wave ; it may become very jDrominent. Sometimes other secondary 
 waves are seen following the dicrotic wave as at D in Fig. 28; but 
 these are very inconstant and usually even when present incon- 
 sjdIcuous. 
 
 When tracings are taken from several arteries or from the same 
 artery under different conditions of the body, these secondary 
 waves are found to vary very considerably, giving rise to many 
 characteristic forms of pulse-curve. Moreover in the same arteiy. 
 
 Fig. C4. Pulse-teacing from the dorsalis pedis. 
 
 and with the same instrument, the form and even the special features 
 of the curve vary according to the amount of pressure (expressed 
 either in ounces or in mm. of mercury) with which the lever is 
 pressed upon the artery. Figs. 35, 36 shew a series of changes 
 thus brought about by varying the pressure of the lever ; and Fig. 37 
 shews the effect of this extra vascular pressure on the form of a 
 fully dicrotic pulse. This effect of pressure in fact varies according 
 to the condition of the vascular system. 
 
 Were we able with certainty to trace back the several features 
 of the curves to their respective causes, an adequate examination 
 of sphygmographic tracings would undoubtedly disclose much 
 valuable information concerning the condition of the body pre- 
 senting them. Unfortunately the problem of the origin of these 
 secondary waves is a most difficult and complex one ; so much so 
 that the detailed intei'pretation of a sphygmographic tracing is still 
 in most cases extremely uncertain. 
 
ClIAT. IV.] 
 
 THE VASCrLAU MIX'llAMSM. 
 
 109 
 
 Various causes liavo been suggested as bringing about the 
 secondary waves, and much discussion has arisen especially cou- 
 ceniing the dicrotic wave. When the tube of the artiticial scheme 
 bearing two levers is blocked just beyond the far lever, the primary 
 wave is seen to be accompanied by a second wave, which at the i'ar 
 lever is seen close to, and often fused into, the primary wave 
 
 cTX.' 
 
 ^O 
 
 \ 
 
 Fig. 35. 
 
 IXFLUEKCE OF CHANGES IN THE PHESSUKE ATPLIEC TO THE EXTERIOR OF TUE 
 VESSEL ON THE FOUH OF THE CURVE. 
 
 a. From the Kii. radialis of healtliy man of 27 years of age with an extra arterial 
 pressure equal in a to 70 mm., in a' to 50 mm., in a" to 30 mm. mercafy. 
 
 (Fig. 27, VI. a), but at the near lever is at some distance from it 
 (Fig. 27, I. a), being the farther from it, the longer the interval 
 between the lever and the block in the tube. The second wave is 
 evidently the primarj^ wave reflected at the block and travelling 
 backwards towards the pump. It thus of course passes the far 
 lever before the near one. And it has been argued that the 
 dicrotic wave of the pulse is really such a reflected wave, started 
 either at the minute arteries and capillaries, or at the points 
 of bifurcation of the larger arteries, and travelling backwards to 
 the aorta. But if this were the case the distance between the 
 primary crest and the dicrotic crest ought to be less in arteries 
 more distant from the heart than in those nearer, just as in 
 the artificial scheme the reflected wave is fused with a primary 
 
170 
 
 THE PULSE. 
 
 [Book i. 
 
 wave near the block, but becomes more and more separated 
 from it, the farther back we trace it. Now this is not the case 
 with the dicrotic wave. Careful measurements shew that the 
 distance between the primary and dicrotic crests is either greater 
 
 *s* *• 
 
 Fig. 36. Normal pulse-curve feom carotid op rabbit ; 
 
 shewing influence on height and form of curve of changes in the extra vascular 
 pressui-e which was in a 20 mm., in h 30 mm., in c 40 mm., and d 50 mm. of mer- 
 cury. 
 
 or certainly not less in the smaller or more distant arteries than in 
 the larger or nearer ones. This feature indeed proves that the 
 dicrotic wave cannot be in any way a retrograde wave. Again, the 
 more rapidly the primary wave is obliterated or at least diminished 
 on its way to the periphery the less conspicuous should be the 
 dicrotic wave. Hence increased extensibility and increased elastic 
 reaction of the arterial walls which tend to use up rapidly the 
 primary wave, should also lessen the dicrotic wave. But as a 
 matter of fact these conditions are favourable to the prominence of 
 
('ii\r. i\.J 
 
 THE VASCVLMl MKCllAMSM. 
 
 the dicrotic wave. Besides the imiltiludinons peri] hcr.d division 
 would render one large pcripherically reflected wave inipiissible. 
 
 But in addition to retlccted waves, other waves whii;li may be 
 called "waves of oscillation," make their appearance when a fluid is 
 driven through a system of tubes, by means of an intennitteut 
 force. And dilVerent origins have been assigned to secondary waves 
 of this description. 
 
 \ 
 
 vi 
 
 ^' ^V^\. V'V' ' 
 
 v.^Jv^->\^'■ 
 
 Fig. 37. Dicrotic pulse-curve due to loss of blood. 
 
 From carotid of rabbit vritb extra-vascular pressures in a of 50 mm., h of -iO mm. 
 c of 20 mm., and d of 10 mm. of mercury. 
 
 Thus when the rapid flow of a fluid along a tube is suddenly 
 checked at a point of its course the inertia of the fluid ^^ill carry the 
 column of fluid still forwards so as to leave behind it a diminution 
 of pressure. This diminution will appear on a graphic record of 
 the pressure as a depression or notch ; and will be followed by a 
 secondary rise as a reflux of fluid takes place towards the point 
 where the pressure has become diminished. Both the depression 
 and the secondary rise will travel as a wave along the tube, being 
 frequently followed by other smaller waves of similar character and 
 similar origin. Waves thus originating have been appealed to as 
 explaining the secondary waves of the pulse-curve. Thus at the 
 moment when the ventricle, ha\dng emptied itself, ceases to throw 
 any more blood into the aorta, the blood which was last ejected 
 being carried forward by its inertia gives rise to a diminution of 
 pressure in the ventricle and at the root of the aorta. The aortic 
 w^alls forth-\\ith contract upon this diminished pressui-e, and a reflux 
 of blood towards the semilunar valves takes place, leading to the 
 appearance of a depression or notch in the pulse-curve, which is 
 jiropagated forwards along the aorta. This reflux closes the semi- 
 lunar valves and at the same time leads to a recovery of pressure 
 
172 THE PULSE. [Book i. 
 
 which similarly appears on the pulse-curve as an elevation succeed- 
 ing the notch. 
 
 Then again it has been argued that in any section of the 
 arterial tract, the inertia of the walls and of the contained blood, 
 in each expansion of the section, carries them on in their movement 
 of expansion some little time after the actual expanding force has 
 ceased to act. This leads to a falling back or contraction, which 
 again by reason of the same inertia overshoots its mark, and thus 
 through a series of oscillations, of which the first is the most 
 conspicuous, the artery settles down to its normal calibre before the 
 next expansion reaches it. The extent of such oscillations is 
 determined, not only by the character of the walls but by the 
 specific gravity of the contained fluid. In the artificial scheme 
 with the same elastic tubing the secondary waves thus caused are 
 much greater with mercury than with water, and disappear almost 
 wholly when air is employed. Such waves of oscillation may be 
 supposed to be generated in different degrees, in each and every 
 section of the arterial tract ; the waves due to a cessation of the 
 flow are on the contrary generated at the point where the inter- 
 mittence is effected, and may be seen in rigid as well as in elastic 
 tubes ; but these latter waves also are profoundly modified by the 
 nature of the walls of the tubes along which they are transmitted. 
 
 Lastly, it has been maintained that these secondary waves are 
 of active not passive origin; that is, that they are caused by a 
 rapid muscular contraction of the arterial walls following up so to 
 speak the arterial beat. 
 
 We have dwelt at so great a length on these secondary waves 
 of the pulse-curve because of the importance attached to them in 
 clinical medicine ; but it would be hardly profitable to enter more 
 fully into the discussion of these several contending views. As an 
 instance of the difficulty of the subject and the insufficiency of our 
 knowledge, we may point out that observers are not yet agreed as 
 to which part of the curve corresponds to the closure of the 
 semilunar valves. Thus some maintain that this event corresponds 
 to and indeed is indicated by the dicrotic wave, the dicrotic notch 
 representing the reflux towards the ventricle, and the dicrotic 
 elevation a new forward movement reflected from the closed 
 valves. But under this view, though it seems the more probable, 
 the predicrotic wave presents a difficulty; and indeed others 
 maintain that the moment of closure of the semilunar valves is 
 indicated by this the predicrotic, and not by the dicrotic wave. 
 Until this and other points are finally settled, all interpretations of 
 modifications of the pulse-curve must remain uncertain and un- 
 satisfactory. 
 
 The following facts however may be borne in mind as not only 
 of practical importance, but as necessary data for any judgments 
 concerning the pulse-curve. 
 
CiiAi'. IV.] 77/ A' WlSCrLAJ: MKCUAMSM. 173 
 
 1. \\'luiti'ver (lie origin of the dicrotic wave, its features may 
 1)1' modified by changes taking ]»hice in the peripheral (arterial) 
 ilistricts without any alteration in the central (cardiac) events. 
 Thus dicrotism may become conspicuous in one artery while re- 
 maining indistinct in others. 
 
 2. The prominence of the dicrotic wave, though favoured by 
 a sudden strong ventricular systole, is especially assisted by a 
 diminution of blood-pressure. Thus it is a marked characteristic 
 of the pulse in many cases of fever (Fig. 31) where blood-pressure 
 is low. So also it may be brought on at once in an artery in which 
 it was previously insignificant by sudden lowering of the blood- 
 pressure as is shewn in Fig. 88. It may similarly be induced by 
 
 A. ,1 
 
 Mf^AMM 
 
 Fig. 33. Tracing fuom b.u>ial in max ; 
 
 shewing change in form of pulse-curve accompanying a sudden fall in tlie lilood- 
 pressure. The pulse, at first not markedly dicrotic, rapidly hecomes so, and then 
 passes on into the condition known as hyperdicrotism, where the dicrotic notch 
 leaches a level lower than that from which tlie primary rise started. 
 
 section of the vaso-motor nerves belonging to the branches of the 
 artery; this, as we shall presently see, diminishes the peripheral 
 resistance, through an expansion of the minute arteries, and so 
 leads to a lowering of the blood-pressure in the main arteries. The 
 prominence of the dicrotic wave is further dependent on the amount 
 of extensibility and elastic reaction of the arterial Avails, Hence 
 the dicrotic wave is not well marked in arteries which have become 
 rigid by disease or old age. 
 
 "We may add that an anacrotic pulse, in which a crest followed 
 by a notch is visible on the ascending portion of the curve, before 
 the maximum of expansion is reached, though it may sometimes 
 be produced temporarily in healthy persons, is generally associated 
 with diseased conditions, usually such in which the arteries arc abnor- 
 mally rigid. It has been interpreted as due to the pressure in the 
 aorta rising even after the first rapid rush from the ventricle. Under 
 
174 TUE PULSE. [Book i. 
 
 normal conditions, as "we have already seen, the maximum expansion 
 is soon reached, but in cases where the arterial walls are unusually 
 rigid and the heart at the same time not abnormally weak, the 
 ventricle may continue to empty itself against a resistance which 
 increases rapidly, with the amount of blood passing into the aorta, 
 so that in spite of the diminishing rapidity with w^hich the blood 
 is leaving the ventricle the insufficient distensibility of the vessels 
 causes the pressure in their interior to continue to rise until nearly 
 the end of the outflow from the heart. An anacrotic pulse also 
 frequently accompanies hypertrophy and dilation of the left 
 ventricle. 
 
 The pulse then is the expression of two sets of conditions : one 
 pertaining to the heart, and the other to the arterial system. The 
 arterial conditions remaining the same, the characters of the pulse 
 may be modified by changes taking place in the beat of the heart ; 
 and again, the beat of the heart remaining the same, the pulse may 
 be modified by changes taking place in the arterial walls. Hence 
 the diagnostic use of the pulse-characters. It must however be 
 remembered that arterial changes may be accompanied by com- 
 pensating cardiac changes, to such an , extent, that the same 
 features of the pulse may obtain under totally diverse conditions, 
 provided that these conditions affect both factors in compensating 
 directions. 
 
 Venous Pulse. Under certain cu-cumstances the pulse may be 
 carried on from the arteries through the capillaries into the veins. 
 Thus when the salivary gland is actively secreting, the blood may 
 issue from the gland through the veins in a rapid pulsating stream. 
 The nervous events which give rise to the secretion of saliva, lead 
 at the same time, by the agency of vaso-motor nerves, of which we 
 shall presently speak, to a dilation of the small arteries of the gland. 
 This dilation of the small arteries diminishes the peripheral resist- 
 ance by allowing more blood to pass through them with less friction; 
 in consequence the elasticity of the arterial walls is brought into play 
 to a less extent than before, and this may in certain cases go so 
 far, that as in the case of the artificial apparatus, where the elastic 
 tubing has an open end (see p. 129), not enough elasticity is brought 
 into action to convert the intermittent arterial flow into a con- 
 tinuous one. A similar venous pulse is also sometimes seen in 
 other organs. 
 
 Careful tracings of the great veins in the neighbourhood of the 
 heart shew elevations and depressions, which appear due to the 
 variations of intracardiac pressure, and which may perhaps be 
 spoken of as constituting a "venous pulse" ; but at present they 
 need further elucidation. In cases of insufficiency of the tricuspid 
 valves, the systole of the ventricle makes itself felt in the great 
 veins ; and a distension travelling backwards from the heart be- 
 
Chap. iv.J THE VASCULAll MPXIIAMSM. 175 
 
 comes very visible in the veins of the neck. This is sometimes 
 spoken of as a venous pulse. 
 
 Variations of pressure in the great veins due to the respiratory 
 movements are also sometimes spoken of as a venous pulse ; the 
 nature of these variations will be explained in treating of respi- 
 ration. 
 
11. THE VITAL PHENOMENA OF THE CIRCULATION. 
 
 So far the facts with which we have had to deal, with the ex- 
 ception of the heart's beat itself, have been simply physical facts. 
 All the essential phenomena which we have studied may be re- 
 produced on a dead model. Such an unvarying mechanical 
 vascular system would however be useless to a living body whose 
 actions were at all complicated. The prominent feature of a living 
 mechanism is the power of adapting itself to changes in its in- 
 ternal and external circumstances. In such a system as we have 
 sketched above there would be but scanty power of adaptation. 
 The well-constructed machine might work with beautiful regu- 
 larity; but its regularity would be its destruction. The same 
 quantity of blood would always flow in the same steady stream 
 through each and every tissue and organ, irrespective of local and 
 general wants. The brain and the stomach, whether at work and 
 needing much, or at rest and needing little, would receive their 
 ration of blood, allotted with a pernicious monotony. Just the 
 same amount of blood would pass through the skin on the hottest 
 as on the coldest day. The canon of the life of every part for the 
 whole period of its existence would be furnished by the inborn 
 diameter of its blood-vessels, and by the unvarying motive power 
 of the heart. 
 
 Such a rigid system however does not exist in actual living 
 beings. The vascular mechanism in all animals which possess one 
 is capable of local and general modifications, adapting it to local 
 and general changes of circumstances. These modifications fall 
 into two great classes : 
 
 1. Changes in the heart's beat. These, being central, have of 
 course a general effect. 
 
 2. Changes in the peripheral resistance, due to variations in 
 the calibre of the minute arteries, brought about by the agency of 
 their contractile muscular coats. These changes may be either 
 locol or general. 
 
CiiAP. IV.) rill-: vasci'i.m: mi:(:iia.\ism. 177 
 
 To flirsi' may he addi'il as subsidiary iiiudirvini,' fvnits: 
 
 W. C'liau^t'S ii» tlu' pcriitlu'ral rcsistauco of th(! capillai-ics duo 
 to alterations in tho adlu'sivcness of the capillary walls or to 
 «)ther inHuences arisiiiLj out of the as yet obscure relations existing 
 betw(>en the blood within and the tissue without the thin per- 
 meable capillary walls, and depending on the vital conditions of the 
 one or of the other. Such changes causing an increase of ])ori- 
 I)heral resistance arc seen to a marked degree in the patholoo-ical 
 condition known as stasis. 
 
 4. Changes in the quantity of blood in circulation. 
 
 The tw^o fii-st and chief classes of events (and probably the 
 third) are directly under the dominion of the nervous system. It 
 is by means of the nervous system that the heart's beat and the 
 calibre of the minute arteries are brought into relation with each 
 other, and with almost every part of the body. It is by means of 
 the nervous system acting either on the heart, or on the small 
 arteries, or on both, that a change of circumstances affecting either 
 the Avhole or a part of the body is met by compensating or 
 regulative changes in the flow of blood. It is by means of the 
 nervous system that an organ has a more full supply of blood when 
 at work than when at rest, that the stream of blood through the 
 skin rises and ebbs with the rise and fall of the temperature of the 
 air, that the work of the heart is tempered to meet the strain of 
 overfull arteries, and that the arterial gates open and shut as the 
 force of the central pump waxes and w^anes. Each of these vital 
 factors of the circulation must therefore be considered in connection 
 with those parts of the nervous system which are concerned in 
 its action. 
 
 12 
 
SEC. 4. CHANGES IN" THE BEAT OF THE HEART. 
 
 We have already discussed the more purely mechanical pheno- 
 mena of the heart. We have therefore in the present section only 
 to inquire into the nature and working of the mechanism (chiefly 
 at least nervous) by which the beat of the heart is maintained, 
 varied, and regulated. 
 
 In studying closely the phenomena of the beat of the heart it 
 becomes necessary to obtain a graphic record of various movements. 
 
 1. In the frog or other cold-blooded animal, a light lever may be 
 placed directly on the ventricle (or on an auricle, &c.) and changes of 
 form, due either to distension by the influx of blood, or to the systole, 
 will cause movements of the lever, which may be recorded on a 
 travelling surface. The same method may be applied to the mam- 
 malian heart, but difficulties are introduced by the locomotion of the 
 heart caused by the movements of the lungs. 
 
 2. Or, as in Gaskell's method, the heart may be fixed by a clamp 
 carefully adjusted round the auriculo-ventricular groove while the apex 
 of the ventricle and some portion of one auricle are attached by threads 
 to horizontal levers placed respectively above and below the heart. 
 The auricle and the ventricle each in its systole pulls at the lever 
 attached to it; and the times and extent of the contractions may thus 
 be recorded. 
 
 3. A record of mtracardiac pressure may be taken in the frog or 
 tortoise, as in the mammal, by means of an appropriate manometer. 
 And in these animals at all events it is easy to keep up an artificial 
 circulation. A cannula is introduced into the sinus venosus and another 
 into the ventricle through the aorta. Serum or dilute blood (or any 
 other fluid which it may be desired to employ) is driven by moderate 
 pressure through the former; to the latter is attached a tube connected 
 by means of a side piece with a small mercury manometer. So long 
 as the exit tube is open at the end, fluid flows freely through the 
 heart and apparatus. Upon closing the exit tube at its far end, the 
 force of the ventricular systole is brought to bear on the manometer. 
 
ClIAI". IV. 
 
 Till': V ASCI' I. A U mi;/' II AX ISM. 
 
 179 
 
 tlio iiiiK'X of wliicli registers in tlio usual way the movenmuts of 
 tho mercury column. J>Ie\voll Martin has suocoedcd in applying a 
 luoililicatioii of this method to tho mammalian heart. 
 
 4. Tho movements of tho ventricle may be registered by introducing 
 into it through the auriculo-ventricular oritico a so-called 'perfusion' 
 cannula^ Fig. 39 i. with a double tube, one inside the other, and tying the 
 ventricle on to tho cannula at the auriculo-ventricular groove, or at aiiv 
 level below that which may be desired. The blood or other fluid is 
 driven at an adequate pressure through the tube a, enters the ventricle, 
 and returns by the tube b. If 6 be connected with a manometer as in 
 method 3, the movements of the ventricle may be registered. 
 
 5, In the apparatus of Hoy, Fig. 39 li., the exit tube is free but the 
 ventricle (the same method may be adopted for the whole heart) is 
 placed in an air-tight chamber tilled with oil or partly with normal 
 saline solution and partly with oil. By means of the tube b the interior 
 
 Fig. 39. Purely dugrammatic figures of 
 
 I. Perfusion cannula tied into frog's ventricle, a. entrance, b, exit, tube; 
 a, wall of ventricle ; /3, ligature. 
 
 II. Eoy's apparatus modified by GaskcU. a, chamber filled with saline solution 
 and oU, containing the ventricle a tied on to perfusion cannula/, b, tube leixding 
 to cylinder c, in which moves piston d, working the lever e. 
 
 of the chamber a is continuous with that of a small cylinder c in which 
 a piston d secured by thin flexible animal membrane works up and 
 down. The piston again bears on a lever e by means of which its 
 movements may be registered. When the ventricle contracts, and by 
 contracting diminishes in volume, there is a lessening of pressure in the 
 interior of the chamber, this is transmitted to the cylinder, and the 
 piston correspondingly rises, carrying with it the lever. As the 
 ventricle subsequently becomes distended the pressure in the chamber 
 is increased, and the piston and lever sink. In this way variations in 
 the ^'olume of the ventricle may be recorded, without any interference 
 with the flow of blood or fluid through it. 
 
 JO 2 
 
180 THE CARDIAC MUSCLES. [Book i. 
 
 The Mechanism of the JS'ormal Beat. 
 
 The cardiac Muscles. When a frog's heart which has ceased 
 to beat spontaneously is stimulated by touching it with a blunt 
 needle, a beat is frequently called forth ; this artificial beat differs 
 in no obvious characters from a natural beat. The latent period of 
 such an artificial beat is remarkably long, the length varying within 
 very wide limits. Thus the cardiac contraction is more like that 
 of an unstriated than of a striated muscle. The beat is in fact a 
 modified or peculiar form of peristaltic contraction. In the hearts 
 of some animals, the ventricle fonns a straight tube ; and in these 
 the peristaltic character of the beat is obvious ; but in a twisted 
 tube like that of the vertebrate ventricle, ordinary peristaltic 
 action would be impotent to drive the blood onward, and is 
 accordingly so far modified that the peristaltic character of the beat 
 is recognised only when the action of the heart becomes slow and 
 feeble. 
 
 The cardiac, like the skeletal muscular fibre, after a contraction 
 returns by relaxation to its previous shape, and the whole ventricle 
 (or whole heart) regains after a beat the form natural to its qui- 
 escent state. This diastolic expansion, though increased by, is not 
 dependent on, the influx of fluid into the cavities of the heart. 
 Thus the cavity of the empty quiescent mammalian left ventricle, 
 though smaller than when it is distended with blood as in its 
 normal action, is larger than when it is in systole or when rigor 
 mortis has set in ; moreover if its dimensions be artificially less- 
 ened, as when it is squeezed with the hand, it returns by an elastic 
 reaction to its former volume when the pressure is removed. 
 
 The cardiac muscles in a healthy condition are, like the skeletal 
 muscles, very elastic. Their elasticity is however soon interfered 
 with by imperfect nutrition ; and a 'contraction remainder' (p. 57) 
 under certain circumstances is readily developed. 
 
 Under the influences of certain poisons, veratrin, digitalin, &c., 
 the length of the beat is enormously prolonged, and the ventricle 
 is eventually thrown into a remarkable contracted condition, the 
 exact nature of which is perhaps not thoroughly understood, 
 though it is believed by many to be due to a deficiency of elastic 
 reaction. 
 
 One great feature of the cardiac beat produced by artificial 
 stimulation is the absence of that relationship between the strength 
 of the stimulus employed and the amount of contraction evoked 
 which is so striking in a skeletal muscle (p. 85). The beat with 
 which a heart responds to a stimulus, e.g. a single induction shock, 
 is, if there be any response at all, equally large when a feeble as 
 when a strong stimulus is used, though the strength of the beat 
 evoked either by a strong or a weak stimulus may vary con- 
 siderably ■v\dthin even a very short period of time. 
 
Chap. IV. I /'//A' VASCCLA U M i:c IIA MSM. \^\ 
 
 When a si'foiul iiKliicti(»n shock is st.-iit in at a certain interval 
 after a first, the beat due to the second shock is often lar^o-r tlian 
 the first, the benetieial elleets of a contraction (see p. Ut!) beinj^ 
 even still more manifest in the heart than in an ordinary skeletal 
 muscle. Frequently by successive shocks of equal intensity a 
 'staircase' of beats of successively increasing amplitude may be 
 proibiced. 
 
 When a second induction shock follows upon the first too 
 rapidly, it is apparently without elTect; no second beat is produced. 
 So also when a series of rapidly repeated induction shocks are sent 
 in, a certain niuubcr of them are thus 'inetiectual'; the application 
 of the ordinary interrupted current gives rise not to a tetanus but 
 to a rhythmic series of beats. The ' refractory period,' which is so 
 brief in the skeletal muscle (see p. 87), is very prolonged in the 
 cardiac muscle. So also in a spontaneously beating heart, induc- 
 tion shocks sent in at a certain phase of a cardiac cycle, e.rj. the 
 commencement of the systole, are ineffectual, though they produce 
 forced beats when sent in at the other phases of the cycle. 
 
 As we shall immediately see, the beat of the heart, and even of 
 a part of the heart such as the ventricle, is not a mere muscular 
 contraction but a complex act, in which both nervous and muscular 
 elements intervene ; and it is difficult in all cases to distinguish 
 the action of the one from that of the other. It is probable how- 
 ever that many of the features which we have just described are 
 due to peculiarities of the cardiac muscle. 
 
 Nervous mechanism of the Beat. The heart of a mannnal or 
 of a wanu-blot ide'd animal ceases to beat almost immediately after 
 being removed from the body in the ordinary way ; and though by 
 special precautions and by means of an artificial circulation of blood, 
 an isolated mammalian heart may be preserved in a pulsating con- 
 dition for a considerable time, our knowledge of the exact nature 
 and of the causes of the cardiac beat is as yet almost entirely 
 based on the study of the hearts of cold-blooded animals, Avhich 
 will continue to beat for hours, or under favourable circumstances 
 even for days, after they have been removed from the body with 
 only ordinary care. M^e have reason to think that the mechanism 
 by which the beat is carried on, varies in some of its secondary 
 featui'es in even the cold-blooded animals: that the hearts, for 
 instance, of the snake, the tortoise and the frog, differ as to the 
 exact manner of canying out the beat, both from each other and 
 from the bird and the mammal ; but we may, at first at all 
 events, take the heart of the frog as illustrating the main and 
 important truths concerning the causes and mechanism of the 
 beat. 
 
 The heart of the frog, as we have just said, will continue to 
 beat for hours after removal from the body; and the beats are in 
 all important respects identical with the beats executed by the 
 
182 NERVOUS MECHANISM OF THE BEAT. [Book i. 
 
 heart in its normal condition within the living body. Hence we 
 may infer that the beat of the heart is an automatic action : the 
 muscular contractions which constitute the beat are caused by 
 impulses which arise spontaneously in the heart itself. 
 
 The beat goes on even after the cavities have been cleared of 
 blood, and indeed when they are almost empty of all fluid, A beat 
 cannot therefore be, as was once thought, a reflex act excited by 
 the entrance of blood into the cavities of the heart. 
 
 In the frog's heart, as in that of the mammal, there is a distinct 
 sequence of events. First comes the beat of the sinus venosus, 
 preceded by a more or less peristaltic contraction of the large veins 
 leading into it, next follows the sharp beat of the two auricles 
 together, then comes the longer beat of the ventricle, and lastly 
 the beat of the bulbus arteriosus completes the cycle. If the 
 incisions, by which the heart is removed, be made carefully, so as 
 not to injure at all the sinus venosus, the beats will continue after 
 a very short pause, or sometimes without any real interruption, 
 with great vigour for a very considerable time. In order that 
 the frog's heart may beat after removal from the body with the 
 nearest approach in rapidity, regularity and endurance to the 
 normal condition, the removal must be carried out so as to leave 
 the sinus venosus intact. 
 
 When the incision is carried through the auricles so as to leave 
 the sinus venosus behind in the body, the result is different. The 
 sinus venosus beats forcibly and regularly, having suffered hardly 
 any interruption from the operation. The excised heart, however, 
 remains, in the majority of cases, for some time motionless. 
 Stimulated by a prick or an induction-shock, it will give one, two 
 or several beats, and then come to rest But it will in the majority 
 of cases, the animal having previously been in a vigorous condi- 
 tion, recommence after a while its spontaneous beating, the systole 
 of the ventricle following that of the auricles ; but the rhythm of 
 beat will not necessarily be the same as that of the sinus venosus 
 left in the body, and the beats will not continue to go on for so 
 long a time as will those of a heart still retaining the sinus 
 venosus. 
 
 When the incision is carried through the auriculo- ventricular 
 groove, so as to leave the auricles and sinus venosus within the 
 body, and to isolate the ventricle only, the results are similar but 
 more marked. The sinus and auricles beat regularly and vigor- 
 ously, with their proper sequence, but the ventricle generally re- 
 mains for a long time quiescent. When stimulated however the 
 ventricle will give one, two or several beats, and after a while, in 
 many cases at least, will eventually set up a spontaneous pulsation 
 with an independent rhythm; and this may last for some consider- 
 able time, but the beats are not so regular and will not go on for 
 so long a time as will those of a ventricle to which the auricles are 
 still attached. 
 
CiiAP. IV.] TlIK VASCl'LAU MEClIANIi^M. J.s.'i 
 
 If a transvcrso incision be carried through tho ventricle at 
 about its u])i)er third, leaving the base of the ventricle still 
 attached to the auricles, the jjortion of the heart left in the body 
 will go on ])ulsating regularly, with the ordinary sequence of 
 sinus, auricles, ventricle, but the isolated lower two-thirds of tlie 
 ventricle will not beat spontaneously at all however long it be 
 watched. Moreover in response to a single stimulus such as an 
 iiuluction-shock or a gentle ])rick it gives, not aa in the case of the 
 entire ventricle or of the ventricle to which the auricles are attached, 
 a series of beats, but a single beat. 
 
 Lastly, to complete the story we may add, that when the 
 heart is bisected longitudinally, each half continues to beat 
 spontaneously, with an independent rhythm, so that the beats of 
 the two halves are not necessarily synchronous, and this continuance 
 of spontaneous pulsations after longitudinal bisection may be seen 
 in the conjoined auricles and ventricle, or in the isolated auricles, 
 or in the isolated but entire ventricle. Moreover the auricles 
 may be divided in many ways and yet many of the segments 
 will continue beating ; small pieces even may be seen under 
 the microscope pulsating, feebly it is true but distinctly and 
 rhythmically. 
 
 The various parts of the frog's heart thus fonn, as regards 
 the power of spontaneous pulsation, a descending series : sinus 
 venosus, auricles, entire ventricle, lower portions of ventricle, 
 the last exhibiting under ordinary circumstances no spontaneous 
 pulsations at all. 
 
 Now ganglia, containing nerve cells, are found in great abun- 
 dance in the sinus venosus, are seen in various parts of the auricles, 
 and occur as the so-called Bidder's ganglia at the junction of the 
 auricles and ventricle, from whence they also spread into the 
 upper part of the ventricle ; in the lower two-thirds of the 
 ventricle they are entirely wanting. It is natural to infer from 
 this that the ganglia are in some way the agents of the spontaneous 
 pulsation. 
 
 The uncertainty, and in most cases temporary character of the 
 pulsations, occurring with seeming spontaneity, in the auricles or 
 ventricle separated from the sinus venosus, have led many to the 
 opinion that these are not really spontaneous, but of the nature of 
 reflex acts, induced by some obscurely acting stimuli, and that 
 really spontaneous pulsations proceed only from the sinus venosus. 
 And a view has been generally adopted which teaches that the 
 spontaneous beats of the frog's heart are due to rhythmic nervous 
 impulses started in the ganglia of the sinus venosus and spreading 
 thence to other parts, the ganglia of the auricles and of the 
 auriculo-ventricular groove acting in subordination to those of 
 the sinus, or behaving under certain circumstances independently 
 as reflex centres, or performing other functions which we shall 
 have to speak of immediately as of a restraining or inhibitory 
 
184 INHIBITION OF THE BEAT. [Cook i. 
 
 character. And the same view with possibly some slight modi- 
 fications has been supposed to hold good for the hearts of all 
 vertebrate animals. 
 
 Facts however are met with which appear to oppose this con- 
 ception. If the " perfusion" cannula previously described be 
 introduced into a frog's ventricle and secured by a ligature 
 carried round the ventricle some little distance below the base, the 
 lower part of the ventricle remains motionless and free from 
 pulsations in the same way as when it has been removed by 
 an incision. If however the cavity be regularly supplied with 
 serum or diluted blood (that of the rabbit being practically the 
 most useful), after a longer or shorter time, this portion of the 
 ventricle begins to pulsate with a more or less regular rhythm 
 and will continue these apparent spontaneous beats for an almost 
 indefinite time. It is usual to explain these pulsations, which 
 may be witnessed even when only the extreme tip of the ventricle 
 is tied on to the cannula, as not really spontaneous but as excited 
 by the senim or dilute blood, supplied under pressure, acting as a 
 stimulus ; such an explanation is however hardly satisfactory. 
 Then again, though it is quite true that the beats of an isolated 
 frog's ventricle are uncertain and temporary, so much so as 
 perhaps to justify the view that they are not really spontaneous, 
 the isolated ventricle of the tortoise beats with such regularity and 
 for so long a time, that it seems almost impossible to avoid the 
 conclusion that in this animal, at all events, the ventricle by itself 
 possesses a real power of spontaneous pulsation. Moreover even 
 in the frog, section at various points, of the nerves with which the 
 ganglia are connected, may be effected and indeed Bidder's ganglia 
 carefully extirpated, without the natural sequence of beat of the 
 several parts being changed. And careful investigation has dis- 
 closed many other facts, which we cannot discuss here but which 
 go far to shew that the generation of the beat of the heart is 
 a very complex matter indeed. While we must admit that the 
 ganglia of the sinus venosus (in the frog, or what corresponds to 
 these in other animals) are prepotent in the work of producing 
 the beat, our knowledge will not at present allow us to make 
 a definite and consistent statement as to what it is they exactly 
 do, or as to the share in generating and carrying out the beat, 
 which is taken by the other ganglia, and their respective nerves, or 
 by the muscular fibres themselves. 
 
 Inhibition of the Beat. The beat of the heart may be 
 stopped or checked, i.e. may be inhibited by efferent impulses 
 descending the vagus nerve. 
 
 If while the beats of the heart of a frog are being care- 
 fully registered (Fig. 40) an interrupted current of moderate 
 strength be sent through one of the vagi, the heart is seen to stop 
 beating. It remains for a time in diastole, perfectly motionless 
 
CUAI'. IV. 
 
 Till-: VASCULAi: .Uh'( 'I/AX ISM. 
 
 185 
 
 iiiul thuviti, 11" the (luniliuii of the curn-iit be short an<l the 
 streu_i;th of the current j^reat, the staiulslill may continue aflm- the 
 current has been sliut oH"; the beats when they reappear are 
 generally at tirst feeble and infrequent, but soon reach or even go 
 beyond their previous vigour and frequency. A wholly similar 
 inhibition may bo seen in the mammal, and indeed in man: 
 C'/ermak, by pressing his vagus against a small osseous tumour 
 in his neck, and thus mechanically stimulating the nerve, was able 
 to stop at will the beating of his own heart; it need hardly be 
 added that such an experiment is a dangerous one. 
 
 The effect is not produced instantaneously; if on the curve the 
 point be exactly marked as at a (Fig. 4<0), when the current is 
 
 .WJI'^JWLMjUjl., 
 
 «f 
 
 Fic. iO. Inhhution of Fkog's Hkakt by stimulation of the Vagus. 
 The contractious of the ventricle are registered by means of a simple lover, so 
 lliat each rise of the lever corresponds to a beat. The interrn]ited current was 
 thrown in at a, and shut off at b. It will be seen that one beat occurred after «, and 
 that the pause continued for some time after b. To be read from rif,dit to left. 
 
 thrown in it will frequently be found that one beat at least occurs 
 after the current has passed into the nerve. In other wonls, the 
 iidiibitory action of the vagus has a long latent period; this has 
 been estimated by Bonders to last in the rabbit '16 sec. The in- 
 hibitory eft'ect is at a maximum soon after the moment of applica- 
 tion of the current, and diminishes gradually onward; so much so 
 is this the case, that when the current is applied for more than a 
 very short time the heart recommences beating before the current 
 is removed. 
 
 It is obvious that the normal beat of the heart may be in- 
 terfered with in two distinct ways : on the one hand the systole of 
 the auricles and ventricle (or of either) may be diminished in 
 \ igour, on the other hand the diastole or passive interval may be 
 prolonged. The vagus is able to act upon the heart in both these 
 directions; and sometimes the one, sometimes the other etifect 
 is most prominent. Thus at times, as in the instance shewn in 
 Fig. 40, the most conspicuous result is the total suppression for 
 some time, of all visible contractions of the ventricle ; and the 
 beats, whi'U Ihev ap|icar again, are separated by tliastolic intervals 
 
186 INHIBITION OF THE BEAT. [Book i. 
 
 not much larger than the normal. At other times stimulation of 
 the vagus does not cause any disappearance of the beats, but the 
 intervals between the beats are much prolonged, so that the 
 rhythm is for a while very slow. It is possible that these two 
 different effects are brought about by more or less distinct 
 mechanisms. 
 
 We said just now that after the stimulation of the vagiis lias 
 ceased the beats may go beyond their previous vigour and f/e- 
 ([uency. This is sometimes remarkably the case. We might be 
 tempted to speak of it as a reaction, were it not that no necessary 
 relation obtains between the amount of slowing or weakening aud 
 the amount of succeeding acceleration or augmentation. Indeed 
 the latter effect may make its appearance without any previous 
 inhibition; that is to say, under certain circumstances stimulation 
 of the vagus may produce not inhibition but either augmentation 
 of the beats, or quickening of the rhythm, or both. 
 
 During the standstill, direct stimulation of the heart, as by 
 touching the auricle or ventricle, will produce a single beat; 
 though spontaneous pulsations are absent, the mechanism for the 
 production of a beat is capable of being put into action. 
 
 The stimulus need not be an interrupted current; mechanical 
 and chemical stimulation of the vagus also produces inhibition, 
 though less readily. 
 
 The stimulus may be applied at any part of the course of 
 either vagus (though it frequently happens in some animals, as in 
 the frog, that one vagus is more efficient than the other); but 
 perhaps the most marked effects are produced, when the electrodes 
 are placed on the boundary-line between the sinus venosus and the 
 auricles. 
 
 The effects of various poisons in reference to this inhibitory 
 action are very interesting. After atropin, even in a minute dose, 
 has been injected into the blood, stimulation of the vagus even 
 with the most powerful currents produces no inhibition whatever. 
 The heart continues to beat as if nothing were happening ; atropin 
 in some way or other does away with the normal inhibitory action 
 of the vagus. 
 
 In slight urari poisoning, the inhibitory action of the vagus 
 is still present; in the profounder stages it disappears, but even then 
 inhibition may be obtained by applying the electrodes to the sinus. 
 In order to explain this result it has been supposed that what 
 we may call the inhibitory fibres of the vagus terminate in an in- 
 liibitory mechanism (probably ganglionic in nature), seated in the 
 heart itself, and that the urari, while in large doses it may paralyse 
 the terminal fibres of the vagus, leaves this inhibitory mechanism 
 intact and capable of being thrown into activity by a stimulus ap- 
 plied directly to the sinus. After atropin has been given, inhibition 
 cannot be brought about by stimulation either of the vagus fibres 
 or of the sinus, or indeed of any part of the heart. Hence it is iu- 
 
Chap, iv] THE VASCULAlt MECIIANL'SM. 1S7 
 
 ferrcd that atropin, unlike urari, })aralysc'S this intrinsic inhibitory 
 mechanism itself. 
 
 After the application of muscaiin' or pilocarpin, the heart st^jps 
 beatin<]f, and remains in diastole in perfect standstill. Its aj)- 
 pearance is then exactly that of a heart inhibited by profound and 
 lasting vagus stimulation. This effect is not hindered by urari. 
 The application however of a small dose of atropin at once restores 
 the beat. These facts are interpreted as meaning that muscarin 
 (or pilocarpin) stimulates or excites the inhibitory apparatus 
 S})oken of above, which atropin paralyses or places hors de combat. 
 
 There are many other eifects of various poisons which have been 
 appealed to as thro^^ing light on the action of the heart; but we 
 must not enter into the discussion of these here. We may however 
 in this connection call attention to a remarkable experiment 
 known as that of Stannius. If a ligature be drawn tightly 
 round the junction of the sinus venosus with the auricles, or if 
 the auricles be separated from the sinus by an incision carried 
 along the boundary-line between them, a standstill is produced 
 closely resembling a very prolonged vagus inhibition. Quiescence 
 thus induced may last a very considerable time. During the 
 standstill, a pulsation may be induced by a stimulus applied 
 directly to the heart, a whole series of beats beiug evoked when a 
 mechanical stimulus, such as the prick of a needle, is applied over 
 the seat of Bidder's ganglia at the junction of the auricles \dt\\ the 
 ventricles, or to the ganglia in the auricles or to those in the 
 bulbus ; and when the ventricle is separated by an incision from 
 the auricles, the former will recommence beating, while the latter 
 remain as quiescent as before. The condition of the heart in 
 this experiment so closely resembles the standstill produced by 
 vagus stimulation, that the effect might be supposed to be caused 
 by the ligature (or section) stimulating the vagus fibres or the 
 inhibitory mechanism at the sinus; but this view is clearly dis- 
 proved by the fact that the experiment succeeds perfectly well 
 after atropin has been given. Another explanation attributes the 
 standstill to the section depriving the heart of the prepotent 
 ganglia in the sinus, and the recommencement of pulsation in the 
 ventricle after separation by incision or ligature from the auricles 
 to the incision or ligature acting as a stimulus to the ventricle but 
 not to the auricle. The experiment in fact is brought forward in 
 support of the views enunciated on p. 183. But these, as we have 
 said, are not satisfactory, and an adequate interpretation of the 
 experiment has yet to be supplied. Indeed, did it seem profitable, 
 we might relate many other puzzling results which have been 
 obtained in experimenting on the heart. We have already warned 
 the reader that the problem of the causes of the normal spon- 
 taneous beat is as yet far from being solved, and until we get 
 
 - The poisonous effects of many mushrooms arc probably iu large measure due 
 to a similar actiou on the ht-ui t. 
 
188 REFLEX INHIBITION. [Book i. 
 
 clearer views as to that main event we cannot expect to under- 
 stand exactly how inhibition is brought about. The conception 
 of an inhibitory mechanism, in which certain of the fibres of the 
 vagus end, must be regarded as a temporary hypothesis, useful 
 only until we gain further light; and we have ventured to 
 dwell on so obscure a topic at so great a length only because 
 inhibition of the heart through the vagus is not only a factor 
 of immense importance in the general operations of the economy, 
 and jilays so prominent a part in the action of many drugs, but 
 because it is a type of other inhibitory processes in the nervous 
 system and elsewhere, which, perhaps even more than itself, con- 
 tribute to render the working of the complicated machine of the 
 animal body, at once both uniform when regularity is required 
 and delicately responsive when variety is needed. 
 
 E/Sflex inhibition. For it must not be thought that cardiac 
 inhibition by means of the vagus nerve is a mere experiment of 
 the laboratory; we have reason to think that it is an incident 
 continually recurring in daily life. For we have evidence 
 that the inhibitory action of the vagus may be brought about by 
 reflex action. If the abdomen of a frog be laid bare, and the 
 intestine be struck sharply, as with the handle of a scalpel, the 
 heart will stand still in diastole with all the phenomena of vagus 
 inhibition. If the nervi mesenterici or the connections of these 
 nerves with the sympathetic chain be stimulated with the inter- 
 rupted current, cardiac inhibition is similarly produced. If in 
 these two experiments both vagi are divided, or the medulla 
 oblongata destroyed, inhibition is not produced, however much 
 either the intestine or the mesenteric nerves be stimulated. This 
 shews that the phenomena are caused by impulses ascending along 
 the mesenteric nerves to the medulla, and so affecting a portion of 
 that organ as to give rise by reflex action to impulses which 
 descend the vagi as inhibitory impulses. The portion of the 
 medulla thus mediating between the afferent and efferent impulses 
 may be spoken of as the cardio-inhihitory centre. Reflex in- 
 hibition through one vagus may be brought about by stimulation 
 of the central end of the other. 
 
 If the peiitoneal surface of the intestine be inflamed, very 
 gentle stimulation of the inflamed surface will produce marked 
 inhibition; and in general the alimentary tract seems in closer 
 connection with the cardio-inhibitory centre than other parts of 
 the body : the injurious, sometimes fatal effects of a violent blow 
 on the stomach are known to all. But apparently stimuli if suffici- 
 ently powerful will through reflex action produce inhibition from 
 whatever be the part of the body to which they are applied. Thus 
 crushing a frog's foot will stop the heart. In ourselves the fainting 
 from emotion or from severe pain is the result of a reflex inhibition 
 of the heart, the afferent impulses in the one case at least, and 
 
CiiAi'. IV. J THE VASCULAU M ECU AX ISM. IRO 
 
 ])rol)al)ly in l>*»th cases, reaching the niechilhi from the; hrain. In 
 succeethng pages wc sliall have occasion more than onc(! in discuss- 
 ing the erti'cts of stimuhiting a given nerve, to consider liow far 
 tliose eilects are due to a reflex inhibition of the heart; and probably 
 there are few events taking phice in the body which have not a 
 tendency thus to aflect the central vascular pump, though in many 
 cases the tendency is counteracted by interfering agencies. ]jut 
 we nuist be careful to avoid falling into the error of supposing 
 that every arrest, or slowing or weakening of the heart, is due to 
 impulses descending the vagus fibres. In many instances cardinc 
 troubles are due to events originating in the heart it.self, so far 
 indei)endent of the inhibitory processes which we are studying 
 now, that they are in no way whatever counteracted by atropin. 
 
 Direct stinudation of the cardio-inhibitory centre itself, such as 
 occurs during the destniction of or results from injury to the 
 medulla, also produces inhibition. 
 
 And the question naturally arises, Has this cardio-inhibitory 
 centre any constant automatic action ? 
 
 In the dog, and also, though to a far less extent, in the rabbit, 
 section of botli vagi is followed by a quickening of the heart's beat. 
 This result may be inteqireted as shewing that the centre in the 
 medulla exercises a permanent restraining influence on the heart ; 
 that organ in fact being habitually curbed. The argument that 
 the effects of an artificial stimulation of the vagus soon wear off, 
 and that therefore a permanent stimulation of the vagi, leading to 
 permanent inhibitory action, would be impossible, may be met by 
 the reflection that a natural stimulation is, possibly, not wholly 
 identical with artificial stimulation, and its eSects need not 
 necessarily wear off. 
 
 We need not now stay to discuss the question whether this 
 central action is really automatic, i.e. kept up by molecular pro- 
 cesses originating in certain nerve cells, or reflex, that is, maintained 
 by nervous impulses reaching it along certain or various afferent 
 nen'es. Granting, however, the existence of a centre in the 
 medulla, which either automatically or otherwise is in permanent 
 action, it is obviously open to us to speak of reflex inhibition as 
 being brought about by influences which augment the action of 
 that centre. But we have seen that active nervous centres are 
 subject, not only to augmentative, but also to inhibitory infiuences. 
 Hence the cardio-inhibitory centre might itself be inhibited by 
 impulses reaching it from various quarters. In other words, the 
 beat of the heart might be quickened by a lessening of the normal 
 action of the inhibitory centre in the medulla. It is in fact 
 probable, that many cases of quickening of the heart's beat are 
 produced in this way ; though the matter requires further investi- 
 gation. 
 
 Accelerator nerves. The heart's beat may in the mammal be 
 quickened, even after division of both vagi, by direct stimulation 
 
190 
 
 A CCEL ERA TO R iVER VES. 
 
 [Book i. 
 
 of the cervical spinal cord. The effects produced, however, are 
 very complex, and led, on their first being made known, to much 
 discussion, one outcome of which was the discovery of certain 
 nerves of a very peculiar character, which pass from the cervical 
 spinal cord, frequently along the nerve accompanying the vertebral 
 artery, and reach the heart through the last cervical and first 
 thoracic ganglia ; these have been called the ' accelerator nerves.' 
 Their course is different in the rabbit and in the dog, see Figs. 41 
 and 42, and indeed varies even in the same kind of animal. 
 Stimulation of these nerves with the interrupted current causes a 
 quickening of the heart's beat, in which what is gained in rate is 
 lost in force, for the blood-pressure is not necessarily increased, but 
 may remain the same, or even be diminished ; apparently not only 
 
 sym.f^or. 
 
 n.t'a£^. 
 
 Fig. 41. The last cervical and first thoracic ganglia in the Eabeit. (Left 
 side.) (Somewhat diagrammatic, many of the various branches being omitted.) 
 
 Tracli. Trachea. Ca. carotid artery. s5. subclavian artery, n. Vag. the 
 vagus trunk, n. rec. the recurrent laryngeal, sym. the cervical sympathetic nerve 
 ending in the inferior cervical ganglion, gl. cerv. inf. Two roots of the ganglion 
 are shewn, racL, the lower of the two accompanying the vertebral artery, A. vert., 
 being the one generally possessing accelerator properties, gl. thor. pr. the first 
 thoracic ganglion. Its two branches communicating with the cervical ganglion 
 surround the subclavian artery forming the annulus of Vieussens. sym. thor. the 
 thoracic sympathetic chain, n. dep. depressor nerve, which, though running by the 
 side of the sympathetic, is really a branch of vagus, from wliich it separates higher 
 up. This is joined in its course by a branch from tlie lower cervical ganglion, there 
 being a small ganglion at their junction, from which proceed nei-ves to form a plexus 
 over the arch of the aorta. It is this branch from the lower cervical ganglion which 
 possesses accelerator properties— hence the course of the accelerator fibres is 
 indicated in the figure by the arrows. 
 
UllAl'. IV. ] 
 
 THE VASCi'LAU M i:C II A MS.]f. 
 
 191 
 
 is the dia-stolo diminisliod but the systole is actually shortened. 
 Our knowledge of these 'accelerator' nerves is however too im- 
 perfect to be dwelt uj)on here. 
 
 Other modiiying agents. The beat of the heart may also be 
 moditied by inHuences bearing directly on the nutrition of the 
 lieart. The tissues of the heart, like all other tis.sues, need an 
 adequate supply of blood of a proper quality; if the blood vary 
 in quality or (piantity the beat of the heart is correspondingly 
 alTected. The excised frog's heart, as we have seen, continues to 
 beat for some considerable time, though apparently empty of blood. 
 
 r. .lym. 
 
 / sym.^ioreic. 
 
 Via. 42. The last certic.\l and first thoracic ganglia in the Dog. 
 
 The cardiac nerves of the Dog. The figure is largely diagramiiiatic, and represents 
 the left side. 
 V. sym. the united vagus and cervical sympathetic nerves, gl. cerv. i. the 
 mferior cervical ganglion, n. v. the continuation of the trunk of the vagus. 
 aim. V. the two branches forming the anuulus of Vieussens round the subclavian 
 artery, art. suhcl., and joining gl. th. pr., the first thoracic or stellate ganglion (the 
 branch running in front of the artery is considered by Schmiedeberg to be an especial 
 channel of accelerator fibres), sym tltorac. the sympathetic trunk in the thorax. 
 r. vert, communicating branches from the cervical nerves running alongside the 
 vertebral artery, the rami vertebrales. n. ree. the recurrent laryngeal, n. c. cardiac 
 branches from the lower cervical ganglion, accelerator nerves of Schmiedeberg. n'. c'. 
 cardiac branches from the first thoracic ganglion, accelerator nerves of Cyon. n". c". 
 cardiac branch fi'om recurrent nerve, r. rcc. branch from lower ceiTical ganglion to 
 the recurrent nerve, often containing accelerator fibres. 
 
 After a while however the beats diminish and disappear; and their 
 disappearance is greatly hastened by washing out the heart vnih a 
 normal saline solution, which when allowed to flow through the 
 cavities of the heart readily permeates the tissues on account of 
 the peculiar construction of the ventricular walls. If such a 
 
192 MODIFYING AGENTS. [Book i. 
 
 'washed out' quiescent heart be fed in the manner described at 
 p. 179, with dihited blood (of the rabbit, sheep, &c.) it may be 
 restored to functional activity. A similar but less complete resto- 
 ration may be witnessed if serum be used instead of blood ; and a 
 heart fed regularly with fresh supplies of blood or even of serum 
 may be kept beating for a very great length of time. In treating 
 of the skeletal muscles we saw that in their case the exhaustion 
 following upon withdrawal of the blood-stream might be attributed 
 either to an inadequate supply of new nutritive material and 
 oxygen, or to an accumulation in the muscular substance of the 
 products of muscular metabolism, or to both causes combined. 
 And the same considerations hold good for the nervous and 
 muscular structures of the heart, though the subject has not yet 
 been sufficiently well Vv^orked out to permit any very definite 
 statements to be made. It seems probable however that an 
 important factor in the matter is the accumulation in the muscular 
 fibres and in the surrounding lymph of carbonic acid, and of the 
 substances which give rise to the acid reaction. 
 
 When the frog's heart is thus 'fed' with various substances 
 the interesting fact is brought to light that some substances, 
 such, for instance as very dilute lactic acid, lead to increased 
 expansion, and others, such for instance as very dilute solutions of 
 sodium hydrate, to diminished expansion, or to continued con- 
 traction of the quiescent ventricle. It would appear that the 
 muscular fibres of the ventricle over and above their rhythmic 
 contractions are capable of varying in length, so that at one time 
 they are longer, and the ventricle when pressure is applied to it 
 internally dilates beyond the normal, while at another time they 
 are shorter, and the ventricle, with the same internal pressure is 
 contracted beyond the normal. Further, in the frog at least, when 
 the pause between two beats is lengthened the relaxation of the 
 ventricle goes on increasing, so that apparently the ventricle when 
 beating normally is already somewhat contracted when a new 
 beat begins. In other words, the ventricle possesses what we shall 
 speak of in reference to arteries as tonicity or tonic contraction, 
 and the amount of this tonic contraction, and in consequence the 
 capacity of the ventricle, varies according to circumstances. 
 
 When the frog's ventricle is thus artificially fed with serum or 
 even with blood, the beats, whether spontaneous or provoked by 
 stimulation, are apt to become intermittent and to arrange them- 
 selves into groups. This intermittence is possibly due to the 
 serum or blood being unable to carry on nutrition in a completely 
 normal manner, and to the consequent production of abnormal 
 chemical substances ; and it is probable that cardiac intermittences 
 seen during life have often a similar causation. Various chemical 
 substances in the blood, natural or morbid, may thus affect the 
 heart's beat by acting on its muscular fibres, or its nervous 
 elements, or both, and that probably in various ways, modifying in 
 
CuAP. IV.] THE VASCULAR MFCI/AXISJf. 193 
 
 different directions the rliytlnn, or the individual contractions, or 
 both. 
 
 The physical or mechanical circumstances of the heart also 
 affect its beat ; of these perhaps the most important is the amount 
 of the distension of its cavities. The contractions of cardiac 
 muscle, like those of ordinary muscle (see p. 87), are increased up 
 to a certain limit by the resistance which they have to overcome ; 
 a full vcntriele will, other things being equal, contract more 
 vigorously tlian one less full ; though, as in ordinary muscle, the 
 limit at which resistance is beneficial may be passed, and an over- 
 full ventricle will cease to beat at all. 
 
 Under normal conditions the ventricle probably empties itself 
 completely at each systole. Hence an increase in the quantity of 
 blood in the ventricle would augment the work done in two ways ; 
 the quantity thrown out would be greater, and the increased 
 quantity would be ejected with greater force. Further, since the 
 distension of the ventricle is (at the commencement of the systole 
 at all events) dependent on the auricular systole, the work of the 
 ventricle (and so of the heart as a whole) is in a measure governed 
 by the auricle. 
 
 The relation of the heart's beat to blood-pressure. When 
 the blood-pressure is high, not only is the resistance to the 
 ventricular systole increased, but, other things being equal, more 
 blood flows (in the mammalian heart) through the coronary artery. 
 Both these events would increase the activity of the heart, and 
 we might expect that the increase would be manifest in the rate of 
 the rhythm as well as in the force of the individual beats. As a 
 matter of fact, however, we do not find this. On the contrary, as 
 Marey has insisted, the relation of heart-beat to pressure may 
 be put almost in the form of a law, that "the rate of the beat 
 is in inverse ratio to the arterial pressure;" arise of pressure being 
 accompanied by a diminution, and fall of pressure with an increase 
 of the pulse-rate. This however only holds good if the vagi 
 be intact. If these be pre\dously divided, then in whatever 
 way the blood-pressure be raised — whether by injecting blood 
 or clamping the aorta, or increasing the peripheral resistance, 
 through that action of the vaso-motor ner^^ es which we shall have 
 to describe directly — or in whatever way it be lowered, no such 
 clear and decided inverse relation between blood-pressure and 
 pulse rate is observed. It is inferred therefore that increased 
 blood-pressure causes a slowing of the pulse, when the vagi 
 are intact, because the cardio-inhibitory centre in the medulla 
 is thereby stimulated, and the heart in consequence to a certain 
 extent inhibited. 
 
 F. 
 
 13 
 
194 
 
 INHIBIT 10 F AND BLOOD-PRESSURE. [Book i. 
 
 The Effects on the Circulation of Changes in the Heart's Beat. 
 
 Any variation in tlie heart's beat directly affects the blood- 
 pressure unless some compensating influence be at work. The 
 most extreme case is that of complete inhibition. Thus if, while a 
 tracing of arterial pressure is being taken, the beat of the heart be 
 suddenly arrested, some such curve as that represented in Fig. 43 
 will be obtained. It will be observed that immediately after 
 the last beat, there is a sudden rapid fall of the blood pressure. 
 
 — v.^^ 
 
 Fig. 43. Tracing, shewing the influence of Cardiac Inhibition on Blood-Pkes- 
 STJRE. Proii a Babbit. 
 
 The current was thrown into the vagus at a and shut off at h. It will be 
 observed that one beat is recorded after the commencement of the stimulation. 
 Then follows a very rapid faU, continuing after the cessation of the stimulus. 
 With the returning beats, the mercury rises by leaps until the normal pressure 
 is regained. 
 
 At the pulse due to the last systole, the arterial system is at its 
 maximum of distension; forthwith the elastic reaction of the 
 arterial walls propels the blood forward into the veins, and 
 there being no fresh fluid injected from the heart, the fall of 
 the mercury is unbroken, being rapid at first, but slower afterwards, 
 as the elastic force of the arterial walls is more and more used up. 
 With the returning beats, the pressure correspondingly rises in 
 successive leaps until the normal mean pressure is regained. The 
 size of these returning leaps of the mercury may seem dispro- 
 portionately large, but it must be remembered that by far the 
 greater part of the force of the first few strokes of the heart is 
 expended in distending the arterial system, a small portion only 
 of the blood which is ejected into the arteries passing on into the 
 veins. As the arterial presstire rises, more and more blood passes 
 at each beat through the capillaries, and the rise of the pressure at 
 each beat becomes less and less, until at last the whole contents 
 
C'llAl'. I\.j 
 
 THE VASCVLAli MI-JJUAMSM. 
 
 lo; 
 
 of the vcutriclo ]);iss at each stroke into tlic veins, and the 
 mean arterial pressure is established. To tliis it may be added, 
 that, as we have seen, the force of the individual ])eats may he 
 soniewhat <:^reater after than before iidiibition. Besides, when the 
 mercury nianoineter is used, the inertia of the mercury tends to 
 magnify tlie ettects of the initial beats. 
 
 ST(M0L/1T/0N V/tCL/S 
 
 Fig. 44. Vagus Stimulation. 
 
 Pulse-tracing from the carotid of rabbit, talien by a modification of tlie 
 siihygmograph. The period of Vagus stimulation is marked by the line below. One 
 beat occurs after stimulation has bcgim. Shews the fall of blood-pressure, and the 
 character of the first recommencing beats. 
 
 Complete an-est of the heart -beats is not necessary to produce a 
 fall of pressure. As is seen in Fig. 45, mere slowing of the beats 
 will lower the mean pressure. And, speaking generally, we may say 
 
 \f^vYvw^wm^^h 
 
 Fig. 45. Stimulation of Vagus. 
 
 Blood-pressure curve taken with mercury manometer. The effect is to slow the 
 rhythm rather than to bring about complete standstill. With the slow pulse the 
 pressure still continues to fall. The beginning of stimulation is marked by a, 
 
 that if while the force of the individual beats remains constant the 
 frequency is increased or diminished, and vice versa, if while the 
 
 13—2 
 
196 INHIBITION AND BLOOD-PRESSURE. [Book i. 
 
 frequency remains the same the force is increased or diminished, the 
 result in both cases is that the pressure is proportionately increased 
 or diminished. This clearly must be the case ; but obviously it is 
 quite possible that the beats might, while more frequent, so lose in 
 force, or while less frequent, so increase in force, that no difference 
 in the mean pressure should result. And this indeed is not 
 unfrequently the case. So much so, that variations in the heart- 
 beat must always be looked upon as a far less important factor of 
 blood-pressure than variations in the peripheral resistance. 
 
 An increase in the quantity of blood ejected at each beat must 
 necessarily augment, and a decrease diminish, the blood-pressure, 
 other things remaining the same. But the quantity sent out 
 at each beat, on the supposition that the ventricle always empties 
 itself at each systole, will depend on the quantity entering into the 
 ventricle during each diastole, and that will be determined by the 
 circumstances not of the heart itself, but of some other part or 
 parts of the body. 
 
SEC. 5. CHANGES IN THE CALIBRE OF THE MINUTE 
 ARTERIES. VASO-MOTOR ACTIONS. 
 
 The middle coat of all arteries contains circularly disposed 
 plain muscular fibres. As the arteries become smaller, the mus- 
 cular element becomes more and more prominent as compared 
 with the elastic element, until, in the minute arteries, the middle 
 coat consists entirely of a series of plain muscular fibres wrapped 
 round the elastic internal coat. Nerve-fibres belonging to the 
 sympathetic system are distributed largely to blood-vessels, but 
 their terminations have not as yet been clearly made out. By 
 galvanic, or still better by mechanical stimulation, this muscular 
 coat may, in the living artery, be made to contract. During 
 this contraction, which has the slow character belonging to 
 the contractions of all plain muscle, the calibre of the vessel 
 is diminished. 
 
 If the web of a frog's foot be examined under the microscope, 
 any individual small artery will be found to vary in calibre, being 
 sometimes narrowed and sometimes dilated. During the narrowing, 
 which is obviously due to a contraction of the muscular coat of the 
 artery, the attached capillary area and the corresponding veins 
 become less filled with blood, and paler. During the stage of 
 dilation, which corresponds to the relaxation of the muscular coat, 
 the same parts are fuller of blood and redder. It is obvious that, 
 the pressure at the entrance into any given artery remaining the 
 same, more blood will enter the artery when relaxation takes place 
 and consequently the resistance offered by the artery is lessened, 
 and less when contraction occurs and the resistance is consequently 
 increased. The blood always flows in the direction of least 
 resistance. 
 
198 VASO-MOTOR NERVES. [Book i. 
 
 The small arteries frequently manifest what may be called 
 spontaneous variations in their calibre, and these variations are 
 very apt to take on a distinctly rhythmical character. If a small 
 artery in the web of the frog be carefully watched, it will be seen 
 from time to tinie to vary very considerably in width, without any 
 obvious change taking place in the heart's beat or any events 
 occurring in the general vaso-motor system. Similar variations may 
 be witnessed in the vessels of the mesentery of a mammal. The 
 most striking and most easily observed instance of rhythmical 
 constriction and dilation is to be found in the median artery of the 
 ear of the rabbit. If the ear be held up before the light, it will be 
 seen that at one moment the artery appears as a delicate hardly 
 visible pale streak, the whole ear being at the same time pallid. 
 After a while the artery slowly widens out, becomes thick and red, 
 the whole ear blushing, and many small vessels previously invisible 
 coming into view. Again the artery narrows and the blush fades 
 away; and this may be repeated at somewhat irregular intervals 
 several times a minute. The extent and regularity of the rhythm 
 are usually markedly increased if the rabbit be held up by the ears 
 for a short time previous to the observation. Similar rhj'^thmic 
 variations in the calibre of the arteries have been observed in 
 several places, ex. gr. in the saphena artery of the rabbit, in 
 the axillary artery of the tortoise, and in the small arteries 
 of the muscles of the frog ; probably they are widely spread. They 
 may be compared with the rhythmic movements of the veins in the 
 bat's wing and of the caudal vein of the eel. 
 
 The extent and intensity of the constriction or dilation which 
 may be observed in the frog's web are found to vary very largely. 
 Irregular variations of slight extent occur even when the animal 
 is apparently subjected to no disturbing causes; while as the 
 result of experimental interference the arteries may become 
 either constricted, in some cases almost to obliteration, or dilated 
 until they acquire double or more than double their normal 
 diameter. This constriction or dilation may be brought about 
 not only by treatment applied directly to the web, but also by 
 changes affecting the nerve of the leg. Thus section of the 
 sciatic nerve is generally followed by a dilation which may 
 be slight or which may be very marked, and which is sometimes 
 preceded by a passing constriction ; wdiile stimulation of the 
 peripheral stump of the divided nerve by an interrupted current 
 of moderate intensity generally gives rise to constriction, often 
 so great as almost to obliterate some of the minute arteries. 
 
 These facts shew that the contractile elements of the minute 
 arteries of the web of the frog's foot are capable by contraction or 
 relaxation of causing constriction or dilation of the calibre of the 
 arteries ; and that this condition of constriction or dilation may be 
 brought about through the agency of nerves. 
 
CuAi'. IV. ] THE VASCULAR MKC llAMSM, 199 
 
 Vaso-motor nerves. In Avarm-bluudeJ auiumls, though we 
 cannot readily, as in the frog, watch the circulation under the 
 microscope, we have abundant evidence of tlie influence of the 
 nervous system on the calibre of the arteries. Thus in the 
 mammal, division of the cervical sympathetic on one side of the 
 neck causes a dilation of the minute arteries of the head on the 
 same side, shewn by an increased su])])ly of blood to the parts. 
 If the experiment be performed on a rabbit, the effect on the 
 circulation in the ear is very striking. The whole ear of the side 
 operated on is nmch redder than normal, its arteries are obviously 
 dilated, its veins unusually full, innumerable minute vessels before 
 invisible come into view, and the temperature may be more than a 
 degree higher than on the other side. 
 
 Division of the sciatic nerve in a mammal causes a similar 
 dilation of the small arteries of the foot and leg. Where the 
 condition of the circulation can be readily examined, as for instance 
 in the hairless balls of the toes, especially when these are not 
 pigmented, the vessels are seen to be dilated and injected; and a 
 thermometer j^laced between the toes shews a rise of temperature 
 amounting, it may be, to several degi'ees. 
 
 The quantity of blood present in the blood-vessels of the mammal 
 may sometimes be observed directly, but has frequently to be determined 
 indirectly. The temperature of passive structures subject to cooling in- 
 Huencos, such as the skin, is largely dependent on the supply of blood, 
 the more abundant the supply the warmer the part. Hence in these 
 parts variations in the quantity of blood may be inferred from varia- 
 tions of temperature ; but in dealing with more active structures there 
 are obviously sources of error in the possibility of the treatment 
 adopted, such as the stimulation of a nerve, giving rise to an increase of 
 temperature due to increased metabolism, independent of vai'iations in 
 blood supply. 
 
 The quantity of blood may also be determined by the plethysmograph. 
 In this instrument, a part of the body, such as the arm is introduced into 
 a closed chamber tilled with fluid, ex. gr, a large glass tube, the opening 
 by which the arm is mtroduced being secured with a stout caoutchouc 
 membrane. An increase or decrease of blood sent into the arm 
 will lead to an increase or decrease of the volume of the arm, and 
 this will make itself felt by an increase or diminution of pressure in the 
 fluid of the closed chamber, which may be registered and measured in 
 the usual way. We shall have to speak again of a modification of this 
 instrument when we are dealing with the kidney. 
 
 Division of the brachial plexus produces a similar dilation of 
 the blood-vessels of the front limb. Division of the splanchnic 
 nerve produces a dilation of the blood-vessels of the intestines and 
 other abdominal viscera. Division in the mammal of the hypo- 
 glossal nerve on one side causes a dilation of the vessels in the 
 corresponding half of the tongue. Division of a nerve supplying 
 a muscle causes a large and sudden increase in the venous flow 
 
200 YASO-MOTOR NERVES. [Book i. 
 
 from the muscle, indicating that the muscular arteries have 
 become dilated; and in the frog this dilation, consequent on 
 section of the nerve, may be actually observed by placing a thin 
 muscle such as the mylo-hyoid under the microscope, and watching 
 the calibre of the small arteries and the circulation of the blood 
 through them while the nerve is being cut. 
 
 We find in fact that in almost all parts of the body certain 
 ' vascular areas ' stand in such a relation to certain nerves that the 
 division of one of these nerves causes a dilation of the minute 
 arteries in, and consequently an increased supply of blood to, a 
 corresponding vascular area. We may speak of these nerves as 
 'vaso-motor' nerves, or more correctly, since in the vast majority 
 of cases the nerves in question have other functions than that of 
 governing arteries, as containing vaso-motor fibres, much in the 
 same way as an ordinary spinal nerve is spoken of as containing 
 sensory and motor fibres ; and from what has been said above it is 
 evident that these vaso-motor fibres are found sometimes in 
 sympathetic, sometimes in cerebro-spinal nerves. 
 
 Since division of a vaso-motor nerve, or nerve containing vaso- 
 motor fibres, leads to the dilation of the arteries of its appropriate 
 vascular area, it is obvious that previous to that division these arte- 
 ries were in a state of permanent constriction, due to a permanent 
 contraction of their muscular coats. This permanent constriction, 
 which may vary considerably in degree (the dilating effects of 
 section of the vaso-motor nerve correspondingly varying in a- 
 mount), is spoken of as ' tone,' ' arterial tone.' Axteries in such a 
 state of permanent constriction as under ordinary circumstances is 
 normal to arteries whose vaso-motor fibres have not been divided 
 and which are otherwise in a normal condition, are said to ' possess 
 tone.' When, as after division of the vaso-motor fibres, the constric- 
 tion gives place to dilation the arteries are said to have 'lost tone;' 
 and when, under various circumstances which we shall study 
 hereafter, the constriction becomes greater than normal, their tone 
 is said to be increased. 
 
 A very little consideration will shew that this arterial tone is 
 a most important factor in the circulation. In the first place the 
 whole flow of blood in the body is adapted to and governed 
 by what we may call the general tone of the arteries of the body at 
 large. In a normal condition of the body, if not all, at least 
 the great majority of the minute arteries of the body are in a 
 state of tonic, i.e. of moderate, constriction, and it is the narrowing 
 due to this constriction which forms a large item of that peripheral 
 resistance which we have seen (p. 129) to be one of the two great 
 factors of blood-pressure. The normal general blood-pressure, and 
 therefore the normal flow of blood, is in fact dependent on the 
 'general tone' of the minute arteries. In the second place, changes 
 in local tone, i.e. the tone of any particular vascular area, have very 
 decided effects on the circulation. These effects are both local 
 and general, as the following considerations will shew. 
 
Cii.vi'. IV.] 77/ A" VASCl'LAn }n:(JIAXISM. 201 
 
 Lot us sup])oso that the artery ^1 is in a cundition of nonnal 
 tone, is niitlway betwcLMi extreme constriction antl dilation. Tlie 
 How tliruiif(h ^-l is dctcnnintd by the resistance in A and in the 
 vascular tract whicli it supplies, in relation to the mean arterial 
 pressure, which again is dependent on the way in which the heart 
 is beating and on the peripheral resistance of all the small arteries 
 and capillaries, A included. If, while the heart and the rest of 
 the arteries remain unchanged, A be constricted, the peripheral 
 resistance in A will increase, and this increase of resistance will 
 lead to an increase of the general arterial pressure. This increase 
 of pressure will tend to cause the blood in the body at large to 
 How mure rapidly from the arteries into the veins. The con- 
 striction of A however will prevent any increase of the flow 
 through it, in fact will make the flow through it less than before. 
 Hence the whole increase of discharge from the arterial into the 
 venous system must take place through channels other than A. 
 Thus, as the result of the constriction of any artery there occur, 
 (1) diminished flow through the artery itself, (2) increased general 
 arterial pressure, leading to (3) increased flow tlirough the other 
 arteries. If, on the other hand, A be dilated, while the heart and 
 other arteries remain unchanged, the peripheral resistance in ^ is 
 diminished. This leads to a lowering of the general arterial 
 pressure, which in turn causes the blood to flow less rapidly from 
 the arteries into the veins. The dilation of A however permits, 
 even with the lowered pressure, more blood to pass through it 
 than before. Hence the diminished flow tells all the more on the 
 rest of the arteries. Thus, as the result of the dilation of any 
 artery, there occur (1) increased flow of blood through the artery 
 itself, (2) diminished general pressure, and (3) diminished flow 
 through the other arteries. Where the artery thus constricted or 
 dilated is small, the local effect, the diminution or increase of flow 
 through itself, is much more marked than the general effects, the 
 change in blood-pressure and the flow through other arteries. 
 When, however, the area the arteries of which are affected is large, 
 the general effects are very striking. Thus if while a tracing of 
 the blood-pressure is being taken by means of a manometer 
 connected with the carotid artery, the splanchnic nen-es be divided, 
 a conspicuous but steady fall of pressure is observed, very similar 
 to that which is seen in Fig. 46. The section of the splanchnic 
 nen-es causes the mesenteric and other abdominal arteries to 
 dilate, and these being very numerous, a large amount of peripheral 
 resistance is taken away, and the blood-pressure falls accordingly ; 
 a large increase of flow into the portal veins takes place, and the 
 supply of blood to the face, arms, and legs is proportionally 
 diminished. It will be obsen-ed that the dilation of the arteries 
 is not instantaneous but somewhat gradual, the pressure sinking 
 not abruptly but with a gentle curve. 
 
 Arterial tone then, both general and local, is a powerful 
 
202 VASO-MOTOR NERVES. [Book i. 
 
 instrument for determining the flow of blood to the various organs 
 and tissues of the body, and thus becomes a means of indirectly 
 influencing their functional activity. We should accordingly ex- 
 pect to find that the vaso-motor nerves were connected with, and 
 arterial tone regulated by, the central nervous system, in order 
 that the calibre of the arteries of, and the supply of blood sent to, 
 this or that vascular area might be varied according to the varying 
 needs of the economy. And experiment proves this to be the 
 case. 
 
 We stated that section of the cervical sympathetic in the neck 
 causes dilation or loss of tone in the blood-vessels of the head and 
 face. This is true at whatever point of the course of the nerve 
 from the upper to the lower cervical ganglion, both included, the 
 section be made. No such dilation of the vessels of the head and 
 face takes place when the thoracic sympathetic chain is divided 
 anywhere below the upper thoracic ganglion; but dilation does 
 occur after division of certain of the rami communicantes connect- 
 ing the spinal cord with the cervical sympathetic through the 
 lower cervical or upper thoracic ganglion. Hence it is clear that 
 the normal tone of the arteries of the head and face is maintained 
 by influences (whose exact nature we shall study presently) pro- 
 ceeding from the central nervous system, passing through certain 
 rami communicantes (the exact path being somewhat uncertain or 
 possibly not constant) into the cervical sympathetic, and ascending 
 to the head and face by that nerv^e. In other w^ords, the vaso- 
 motor fibres of the vessels of the head and face may be traced 
 down the sympathetic to the lower cervical ganglion, and thence 
 by rami communicantes into the spinal cord. 
 
 In a similar manner the vaso-motor fibres of the splanchnic 
 nerves governing the mesenteric and other abdominal arteries can 
 also be traced into the spinal cord, as may also those of the sciatic 
 governing the blood-vessels of the hind limb and of the brachial 
 nerves governing those of the fore limb. In fact all the vaso- 
 motor fibres (with certain special exceptions which will be discussed 
 presently) may thus be traced into the spinal cord ; they are all 
 connected with the central nervous system. There is at present 
 some uncertainty in certain cases as to the exact manner in which 
 the fibres pass from the spinal cord to this or that nerve, as, for 
 instance, along which nerve-roots the vaso-motor fibres eventually 
 joining the sciatic trunk run, whether they all pass on their way 
 into the abdominal sympathetic or no, and the like ; but these are 
 questions which need not delay us now; in whichever way they 
 may be settled, they do not affect the important fact that in some 
 way or other all vaso-motor fibres spring from the central nervous 
 system, and that (with certain special exceptions) what we have 
 called the normal tone of the various vascular areas is maintained 
 by influences proceeding from the central nervous system. 
 
 Far more important however than the maintenance of a normal 
 
Ciww IV.] 71/!^ VASCULAR M fX'IIAXTSir. 203 
 
 tone, ■wliicli iiuU'cil inijj^lit Lo at once and for ever arranged fur by 
 the ])roper natural calibre of the elastic blood-vessels, is the power 
 which the central nervous system possesses of varyinj^ the tone of 
 this or that artery or f^roup of arteries, of increasing it or of 
 diminishing it, of ])roducing constriction or dilation in those arteries, 
 and thus, as we have seen, of effecting changes in general or 
 local blood-j)ressure or in both, and consequently of determining a 
 ilow of blood in this or that direction, according to the needs of the 
 economy. And the exercise of this carefully arranged manipulation 
 of the muscular walls of the arteries may be called forth in either 
 direction, in the way of constriction, or in the way of dilation (or 
 of both at the same time, one in one area and the other in others), 
 by means of nervous impulses either originating in the central 
 nervous system itself or stalled by afterent impulses passing up to 
 the central nervous system from some sentient surface. 
 
 BlushinGT is a familiar instance of vascular dilation brought 
 about by the action of the central nervous system. Nervous im- 
 pulses started in some parts of the brain by an emotion produce 
 certain changes in the central nervous system (the exact nature 
 and locality of these changes we shall discuss presently) which 
 have in turn an effect on the vaso-motor fibres of the cervical 
 sympathetic almost exactly the same as that produced by section 
 of the nerve. In consequence the muscular walls of the arteries of 
 the head and face relax, the arteries dilate and the w^hole region 
 becomes suffused. Sometimes an emotion gives rise not to blushing, 
 but to the opposite, viz. to pallor. In a great number of cases this 
 has quite a different cause, being due to a sudden diminution or 
 even temporary arrest of the heart's beats; but in some cases 
 it may occur wdthout any change in the beat of the heart, and is 
 then due to a condition the very converse of that of blushing, that 
 is, to an increased arterial constriction; and this increased con- 
 striction, like the dilation of blushing, is eff"ected through the 
 agency of the central nervous system and the cervical sympathetic. 
 These are familiar examples, but we have in abundance exact 
 experimental evidence of the effect of afferent impulses in inducing 
 through the central nervous system vaso-motor changes and thus 
 bringing about sometimes constriction, sometimes dilation, some- 
 times the two together. The action of the so-called depressor 
 nerve is a striking instance of reflex dilation as it may be called. 
 
 If in the rabbit while the pressure in an artery such as the 
 carotid is being registered, the depressor nerve, which is a branch 
 of the vagus running alongside the carotid artery and sympathetic 
 nerve (Fig. 41, n. dep.), be divided, and its central end (?'. e. the one 
 connected with the brain) be stimulated with the interrupted 
 current, a gradual but marked fall of pressure in the carotid 
 is observed, lasting, where the period of stimulation is short, 
 some time after the removal of the stimulus (Fig. 46). Since the 
 beat of the heart is not markedly changed, the fall of pressure 
 
204 
 
 VA SO -MO TOR NEE VES. 
 
 [Book r. 
 
 must be due to the diminutiou of iDeripheral resistance occasioned 
 by the dilation of some arteries. And there is evidence that the 
 
 /X/X.-V^, 
 
 "xy^ 
 
 Fig. 46. TEAcrxo, shewing the Effect on Blood-peessuee of stimulating the 
 
 CENTBAL END OF THE DePEESSOB NeRYE IN THE EaBBIT. 
 
 (To be read from right to left. ) 
 
 T indicates the rate at which the recording surface was travelling ; the intervals 
 marked corresponds to seconds. C the moment at which the current was thrown 
 into the nerve ; the moment at wliicli it was shut off. The effect is some 
 time in developing and lasts after the current has been taken off. The larger 
 undulations are the respii'atory curves ; — the id ulse- oscillations are very small. 
 
 arteries thus dilated are chiefly if not exchisively those arteries of 
 the abdominal viscera which are governed by the splanchnic 
 nerve. For if both the splanchnic nerves are divided previous to 
 the experiment, the fall of pressure when the depressor is stimulated 
 is very small, in fact almost insignificant. The inference from this 
 is clear ; the afferent impulses passing along the depressor have so 
 affected some part of the central nervous system that the influences 
 which, in a normal condition of things, passing along the splanchnic 
 nerves keep the minute arteries of the abdominal viscera in 
 a state of moderate tonic constriction, fail altogether, and those 
 arteries in consequence dilate just as they do when the splanchnic 
 nerves are divided, the effect being possibly increased by the 
 similar dilation of other smaller vascular areas. 
 
 The condition of the splanchnic or other vascular areas may 
 moreover be changed, and thus the general blood-pressure modi- 
 lied, by afferent impulses jjassing along other nerves than the 
 depressor, the modification taking on, according to circumstances, 
 the form either of decrease or of increase. 
 
 Thus, if in an animal (dog) placed under the influence of urari 
 the central stump of the divided sciatic nerve be stimulated, an 
 increase of blood-pressure, almost exactly the reverse of the de- 
 crease brought about by stimulating the depressor, is obsen^ed. 
 
Chat, iv] 
 
 77//; VAScuLAi: Mi:ciiAXis.\r. 
 
 205 
 
 Tliu curve of the l^lood-pressure, after a latent period (luring which 
 no chanf,fe.s are visible, rises steadily without any correspondin;^ 
 change in the heart's beat, reaches a maxinmrn and after a whih; 
 slowly falls at^^ain, the fall sometimes beginning to aj)pear before 
 the stimulus has been removed. There can be no doubt that the 
 rise of ])ressure is due to the constriction of certain arteries ; the 
 arteries in question being those of the splanchnic area certaiidy, 
 and possibly of other vascular areas as well. The effect is not 
 confined to the sciatic; stimulation of any nerve containing af- 
 ferent fibres may produce the same rise of pressure, and so constant 
 is the result that the experiment has been made use of as a 
 method for determining the existence of afferent fibres in any 
 given nerve and even the paths of centripetal impulses through 
 the sjunal cord. 
 
 If, on the other hand, the animal be under not urari but 
 chloral, instead of a rise of blood-pressure a fall, quite similar to 
 that caused by stimulating the depressor, is observed when an 
 afferent nerve is stimulated. The condition of the central nervous 
 system seems to determine whether the reflex effect on the vaso- 
 motor fibres is in the direction of constriction leading to a rise, or 
 of dilation leading to a fall of blood-pressure. 
 
 Fio. 47. EisE OF Blood-pressure from stimulation of Nostril with smoke. 
 
 The respiration and cardiac rhythm are at the same time rendered more slow. 
 The mark s indicates the time of stimulation. 
 
 Stimulation of a sentient surface in many cases causes a similar 
 rise in blood-pressure as shewn in Fig. 47, where a rise of blood - 
 pressure follows irritation of the nostrils. In this case however the 
 rise in blood-pressure is accompanied by changes in respiration and 
 in the cardiac rhvthm. 
 
206 VASO-MOTOR NERVES. [Book i. 
 
 In the instances just quoted, the effect of the stimulation of 
 the afferent nerve may be spoken of as a general one ; it is the 
 general blood-pressure which is diminished or increased ; though 
 in the case of the depressor at all events it is chiefly in the 
 splanchnic area that the constriction or dilation takes place. 
 
 There are however some remarkable cases where a local effect 
 can be readily distinguished from the general effect, because the 
 two are in opposite directions. Thus if in a rabbit under urari, the 
 central stump of the auricularis magnus nerve or of the auricularis 
 posterior be stimulated, the rise of general pressure which is 
 caused by the stimulation of this as of any other afferent nerve, is 
 accompanied by a dilation of the artery of the ear. That is to say, 
 the afferent impulses passing along the auricular nerve while 
 affecting the central nervous system in an ordinary way, so as to 
 cause constriction of many of the arteries of the body (but chiefly 
 probably the splanchnic vessels), at the same time so affect some 
 particular part of the central nervous system, more especially 
 connected "s^dth the vaso-motor fibres governing the artery of the 
 ear, as to lead to the dilation of that vessel. 
 
 So also in the same animal stimulation of branches of the 
 tibial nerve causes dilation of the saphena artery, together with 
 constriction of other arteries, as shewn by the concomitant rise of 
 pressure. And there are probably innumerable instances of the 
 same kind of action going on in the body during life, for it is 
 evident that the object of the local dilation, viz. the increased flow 
 of blood to the organ, must be assisted if a general constriction 
 is at the same time taking place in other regions. 
 
 The general effect may not always be obvious, may perhaps be 
 absent, so that the local dilation or constriction, as the case may 
 be, is the only obvious result of the vaso-motor action. When the 
 ear of the rabbit is gently tickled, the effect that is seen is a 
 blushing of the ear, and though this may be in part due, as we 
 shall see, to the action of a local mechanism, the case we have just 
 cited shews that the central nervous system must be largely 
 engaged. When the right hand is dipped in cold water, the 
 temperature of the left hand falls, on account of a reflex con- 
 striction of the vessels of the skin of that hand caused by the 
 stimulus applied to the other. Many more instances might be 
 quoted, and we shall again and again come upon examples. The 
 numerous pathological phenomena classed under sympathetic 
 action, such as the affection of one eye by disease in the other, 
 are probably in part at least the results of reflex vaso-motor 
 action. 
 
 We have said enough to shew that the calibre of the small 
 arteries, which by determining the peripheral resistance forms one 
 important factor regulating the flow of blood, is subject to in- 
 fluences proceeding from all parts of the body, and that these 
 influences reach the arteries in a reflex manner by means of the 
 
Chap, iv.l THE VASCl'LAn MIU'IIAXIS.U. 207 
 
 conlral nervous system, the afferent ini])nls('s bciii^' for tlio most priil 
 cai'ried by onlinary sensory nerves, while the etil'erent impulses i)ass 
 along special vaso-motor fibres, whieh, though the centre of the 
 rellex action lies in the cerebro-spinal axis, have a great tondency 
 to run in sympathetic tracts. 
 
 The atlerent impulses of course need not start from the peri- 
 pheral nerve-endings. They may for instance arise in the brain. 
 Thus, as wo have seen, an emotion originating in the cerel^rum 
 may by vaso-motor action give rise either to blusliing or to pallor. 
 Nay more, changes may be induced in the central nervous system 
 itself without the need of any impulses reaching it from without. 
 When we come to discuss the relations of respiration to the 
 circulation, we shall see reason to think that the vaso-motor action 
 of the central nervous s^^stem may be directly ati'ected by the 
 condition of the blood passing through it, so that if the quantity of 
 oxygen in the blood be reduced, a general arterial constriction 
 takes place, and a rise of blood-pressure follows ; while with a 
 retuiTi of oxygen to the blood, the vessels dilate and pressure falls. 
 And it is more than probable that many substances introduced 
 into the blood, or arising in the blood from natural or morbid 
 changes, may affect blood-pressure by acting directly on the 
 centres in the central nervous system. They may also however act 
 on the jDcripheral structures. We shall return to these phenomena 
 later on. 
 
 In many ways then, and to a varying degree and extent, the 
 central nervous system can bring about arterial constriction or 
 dilation, general or local. We have now to study the question, 
 What is more exactly the nature of the nervous influences 
 which lead to constriction and dilation respectively ? How do 
 those which cause constriction differ from those which cause 
 dilation ? 
 
 In the fundamental experiment of the cervical S3"mpathetic, 
 when arterial dilation has followed upon section of the nerve, if the 
 perijDheral stump of the divided nerve be stimulated, the dilation 
 gives place to constriction, the blush is replaced by pallor. If the 
 stimulus be veiy strong the constriction is greater than normal, but 
 by carefully adjusting the strength of the stimulus, the cii'culation 
 may be brought to quite a normal condition, the ' loss of tone ' 
 consequent on the severance of the vaso-motor fibres from the 
 central nervous system may be replaced, and not more than 
 replaced, by an artificial tone generated by the action of the 
 stimulus on the sympathetic nerve. The most natural interpreta- 
 tion therefore of the vaso-motor action in this case is to suppose 
 that the nomial tone of the arteries of the face is maintained by 
 'tonic' constrictive impulses of a certain intensity which pass 
 from the central nervous system along the sympathetic, and that 
 the dilation of the same arteries is due simply to a diminution or 
 absence of these constrictive impulses, an increased constriction or 
 
208 VASO-MOTOR JERVES. [Book i. 
 
 pallor being similarly due to an increase beyond what is normal of 
 these same impulses. In other words, the nervous influences 
 leading to arterial dilation and constriction differ in degree only, 
 not in kind, and may be considered as being merely jDhases (of 
 decrease or of increase as the case may be) of the same action. 
 And if we turn to the splanchnic nerve we find a similar interpre- 
 tation equally valid. Stimulation of the splanchnic nerv'e causes 
 constriction of the arteries governed by that nerve, apparently 
 because the stimulation supplies artificially the constrictive im- 
 pulses which, so long as the nerve is intact, pass down it from the 
 central nervous system, giving the requisite tone to its vascular 
 area, and the loss of which by division of the nerve gives rise to 
 dilation. So that were we to stop our inquiries at this point, our 
 explanation of vaso-motor action would be very simple. We might 
 speak of constrictive impulses as passing from the central nervous 
 system to the various vascular areas, to such an extent as to 
 constitute normal tone, but as being susceptible either of in- 
 hibition, complete or partial, thus leading to gi-eater or less 
 arterial dilation, or of augmentation, thus leading to excessive con- 
 striction. 
 
 But this simple view appears insufficient Avhen we push our 
 studies further. 
 
 In the first place such a conception does not cover all the facts 
 connected even with the two nerves just mentioned. For the 
 dilation or loss of tone which follows upon section of the cervical 
 sympathetic (and the same is true of the splanchnic) is not 
 permanent ; after a while, it may be not until after several days, it 
 may be sooner, the dilation disappears and the arteries regain 
 their usual calibre. This recovery is not due to any regeneration 
 of vaso-motor fibres in the sympathetic, for it may be observed 
 when the whole length of the nerve including the superior cervical 
 ganglion is removed. When recovery of tone has thus taken place, 
 dilation or increased constriction may be occasioned by local treat- 
 ment : the ear may be made to blush or to pale by the application 
 of heat or cold, by gentle stroking or rough handling and the like ; 
 but neither the one nor the other condition can be brought about 
 by the intervention of the central nervous system. So also the 
 spontaneous rhythmic variations in the calibre of the arteries of 
 the ear of which we spoke on p. 198, though they cease for a time 
 after division of the cervical sympathetic, eventually reapj)ear, even 
 if the superior cer\dcal ganglion be removed. And the analogous 
 rhythmic variations of the veins of the bat's wing have been proved 
 experimentally to go on vigorously when all connection with the 
 central nervous system has been severed; they may continue in 
 fact in isolated pieces of the wing. From this it is clear that what 
 we have spoken of as the tone of the vessels of the face, though 
 influenced by and in a measure dependent on the central nervous 
 system, is not simply the result of an effort of that system. The 
 
CiiAr. IV.] THE VASCULAR MECHANISM. 209 
 
 muscular walls of the arteries arc not mere passive instruments 
 worked by the cercbro-spinal axis through the cerv'ical syinpa- 
 thetic; obviously they have an intrinsic tone of their own, de- 
 })endent possibly on some local nervous inechauism, though in 
 the car at least no such mechanism has yet been found ; and it 
 seems natural to suppose that when the central ner\'ous system 
 causes dilation or constriction of the vessels of the face, it makes 
 use, in so doing, of this intrinsic local tone. But if so, then the 
 simple view entertained above, that arterial dilation and constric- 
 tion are simply determined by the decrease or increase of tonic 
 constrictive impidses passing directly from the central nervous 
 system, is not a comijlete re23resentation of the facts. 
 
 In the second place, if wc turn from the sympathetic or 
 splanchnic to other nerv^es containing vaso-motor fibres, we meet 
 ^vith still gi-eater difficulties. To take, for instance, a nerve sup- 
 plying a muscle, such as that going, in the frog, to the mylo-hyoid 
 muscle. Here, as in the cervical sympathetic, section of the nerve 
 produces dilation, but that dilation is even more transient than in 
 the case of the sympathetic ; the vessels speedily return to their 
 former calibre. And then it is found that stimulation of whatever 
 strength of the peripheral portion of the divided ner^'e brings about 
 not constriction but dilation. A similar dilation is seen when the 
 nerve of a mammalian muscle is stimulated, and probably occurs in 
 the case of all muscular nerves. There are therefore in the body 
 ners^es, stimulation of which, as w^ell as mere section, always brings 
 about arterial dilation. 
 
 There are other nerves in the body of a mixed character, 
 intermediate between the cervical sympathetic on the one hand, 
 and the muscular nerves on the other, stimulation producing now 
 constriction, now^ dilation. Such a nerve is the sciatic of a mam- 
 mal. We have already seen that section of this nen'e produces 
 dilation of the vessels of the foot ; but the dilation so caused after 
 a few days disappears ; the foot on the side on which the nerve was 
 divided becomes not only as cool and pale, but frequently cooler 
 and paler than the foot on the sound side. If the peripheral por- 
 tion of the divided nerve be stimulated with an interrupted cmTent, 
 immediately or very shortly after division, the dilation due to the 
 division gives place to constriction ; the sciatic acts then quite like 
 the cervical sympathetic, except perhaps that this artificial con- 
 striction cannot be maintained for so long a time, and is very apt 
 to be followed by increased dilation. If however the stimulation 
 be deferred for some days, until the dilation has given place to a 
 returning constriction, the effect is not constriction but dilation; 
 the nerve then acts, as far as its vaso-motor fibres are concerned, 
 like a muscular nerve and not Kke the cervical sympathetic. In 
 fact, by variations in the attendant circumstances, and in the mode 
 of stimulation, into the details of which we cannot enter now, 
 stimulation of the divided sciatic may at the will of the experi- 
 F. 14 
 
210 , VASO-MOTOR NERVES. [Book i. 
 
 menter be made to produce either arterial dilation or arterial con- 
 striction. 
 
 In all the above cases section of the nerve produces dilation, 
 whether the subsequent stimulation causes constriction or dilation; 
 the dilation after section may be sometimes not very marked, but 
 is always present to some extent or other. But there are certain 
 nerves, section of which produces no marked changes in the vascular 
 areas to which they are distributed, and yet stimulation of which 
 brings about dilation often of an extreme character. A striking 
 example of this is seen in the so-called nervi erigentes. The 
 erection of the penis is, putting aside the subsidiary action of 
 muscular bands in restraining the outflow through the veins, chiefly 
 due to the dilation of branches of the pudic arteries, whereby a 
 large quantity of blood is discharged into the venous sinuses. 
 Erection may in the dog be artificially produced by stimulating the 
 peripheral ends of the divided nervi erigentes, which are branches 
 from the first and second and sometimes from the third sacral nerve 
 passing across the pelvis. On applying the interrupted current to 
 the peripheral ends of these nerves, the corpora cavernosa at once 
 become turgid. And yet simple section of these nervi erigentes 
 will not in itself give rise to erection. 
 
 A similar case is presented by the submaxillary gland. As will 
 be explained more in detail in treating of secretion, this gland 
 is supplied by two nerves, by branches of the chorda tympani 
 reaching it along its duct, and by branches of the cervical sym- 
 pathetic reaching it along its arteries. Neither section of the 
 chorda tympani nor section of the cervical sympathetic produces 
 any very marked effect in the circulation of the gland. Yet stimu- 
 lation of the former will bring about a most striking dilation, of 
 the latter a no less striking constriction, of the arteries of the gland. 
 
 How can we construct a view of the action of vaso-motor nerves 
 which will be consistent with all these various facts ? 
 
 In the first place, we must admit the existence of a local tone 
 in the several vascular areas, independent of the central nervous 
 system. In such cases as the corpora cavernosa of the penis, and 
 the submaxillary gland, this independence is umnistakeable ; in 
 other regions it is not at first sight so apparent, but, as we have 
 already urged, must be admitted even for these. 
 
 In the second place, as is strikingly shewn by the case of the 
 submaxillary gland, there are nerves which, since stimulation of 
 them always causes dilation, may be called vaso-dilator nerves, and 
 nerves which, since stimulation of them always causes constriction, 
 may be called vaso-constrictor nerves. Examples of the first are^ 
 seen in the nervi erigentes, the chorda tympani, the nerves of 
 muscles, &c. ; of the second, in the cervical sympathetic, the 
 splanchnic, &c. Or to be more exact, we may say that the vaso- 
 motor fibres of the former are vaso-dilator, of the latter, vaso-con- 
 strictor. 
 
HnAr. IV.] Till-: VASCULAR MECHANISM. 211 
 
 In the tliinl ])hice, llic cases of tlic corpora cavernosa of the 
 penis and the submaxillary gland sugt^^cst the idea that dilation is 
 the result of the complete or partial loss of local tone, that in fact 
 vaso-dilators act by iuhibitiiif^, and vaso-constrictors by augmenting, 
 the activity of the local mechanism (whatever it be) which gives rise 
 to the local tone. The erection of the penis which follows stimula- 
 tion of the nervi erigentes, and the injection of the submaxillary 
 gland which follows stimulation of the chorda tympani, present a very 
 close analogy to the inhibition of the heart by stimulation of the 
 vagus. Just as the rhythmic contraction of the cardiac fibre is 
 stopped by the vagus, so th-e tonic contraction of the arterial fibre 
 (and this tonic contraction is indeed at bottom an obscure rhythmic 
 contraction) is stopped by the chorda or the nervi erigentes. And 
 it seems to be very natural to draw the conclusion that dilation is in 
 all cases mere inhibition, and constriction in all cases mere augmen- 
 tation, of local tone. But tempting as this view is, and useful perhaps 
 as it may be as a working hypothesis, it must not be regarded as 
 definitely proved. It is Cjuite possible that dilation may be brought 
 about in different w^ays in different cases ; and so also with con- 
 striction. 
 
 Further, the occm'rence of dilation after simple section of a 
 nerve raises an interesting question. Do the arteries in such a case 
 dilate because the veiy section of the nerve acts as a stimulus to 
 vaso-dilator fibres, or because the local tone is insufficient to keep 
 up an adequate arterial constriction unless it be supplemented by 
 additional tonic impulses reaching the local mechanism from the 
 central nervous system, which supplement is lost by section of the 
 nerve ? Obviously, if mere section behaves as a stimulus to vaso- 
 dilator fibres of such a potency as to give lise to a dilation lasting 
 hours or it may be days, all evidence of 'tonic' impulses proceeding 
 from the central nervous system is done away with. We can then 
 only speak of dilation and constriction as being the result of the action 
 of vaso-dilator and vaso-constrictor fibres respectively, both worked 
 in a reflex manner by the central nervous system. Into the dis- 
 cussion whether such an interpretation of the effects of simple 
 section is justified by facts or not, and into the allied controversy 
 concerning the reason why the vaso-motor effects of stimulating the 
 efferent fibres of the sciatic and other nerves vary so much under 
 different circumstances, we cannot enter here. We must content 
 ourselves with the general conclusion that though local tone may 
 exist independently of the central nervous system, the condition of 
 the various vascular areas, in the living bod}' in a normal condition, 
 is arranged and modified to meet passing or permanent needs, by 
 the central nervous system, through the agency of vaso-motor nerves, 
 and that these vaso-motor nei'\'es in some cases, since they are used 
 to give rise to dilation only, may be spoken of as vaso-dilator nerves, 
 or as containing vaso-dilator fibres, in other cases may similarly be 
 called vaso-constrictor, and in yet a third class of cases be regarded 
 
 U— 2 
 
212 VASO-MOTOE CENTRES, [Book i. 
 
 as mixed in character, since according to circumstances they give 
 rise either to dilation or to constriction. 
 
 The course of vaso-motor fibres. Leaving out of consideration 
 local vaso-motor mechanisms, such as those which may be sup- 
 posed to exist in the submaxillary gland, we may make the general 
 statement that vaso-motor influences may be traced back to the 
 spinal cord. The exact paths taken by the vaso-motor fibres have 
 not however as yet been fully worked out. 
 
 Most observers are agreed that the fibres leave the spinal cord 
 by the anterior roots of the spinal nerves; but in the majority of 
 cases at all events as far as the mammal is concerned, the fibres do 
 not run in a direct course to their destination in company with the 
 ordinary motor fibres passing to the same structures as themselves. 
 Thus the vaso-motor fibres of the hind limbs do not pass directly 
 with the anterior roots into the sciatic nerve but, largely at all 
 events, turn aside, to join through the rami communicantes the 
 abdominal sympathetic ; and it is only after they have traversed a 
 certain length of sympathetic nerve that they again return to the 
 spinal nerves, enter into the sciatic plexus, and thus become 
 part of the nerves of the leg. So also the vaso-motor fibres for the 
 forelimb pass in large measure from the anterior roots of the upper 
 dorsal nerves to the thoracic sjnupathetic chain and thence by the 
 first thoracic ganglion to the brachial plexus and so on to the fore- 
 limb. And we have already seen that the vaso-motor fibres for the 
 head and face, pass from the lower cervical or l^^r-dorsal spinal 
 cord to the first thoracic or to the last cervical ganglion and by the 
 cervical sympathetic upwards. 
 
 When, as in the case of the submaxillary gland, the presence of 
 distinct and antagonistic vaso-constrictor and vaso-dilator nerves is 
 conspicuous in the same organ, the dilator fibres are generally found 
 running in a cerebro-spinal and the constrictor fibres in a sympa- 
 thetic nerve, but we cannot at present say that such a contrast is 
 invariable. We cannot as yet trace out such distinct courses for 
 the dilator and constrictor fibres of either the fore or hind limb ; 
 and in the tongue while dilator fibres run into the lingual nerve, 
 constrictor fibres appear in the hypoglossal which is no less clearly 
 a spinal nerve than the fifth of which the lingual is a branch. 
 
 Vaso-motor centres. There remains the important question, 
 What part of the central nervous system is it which intermediates 
 as a nervous vaso-motor centre or centres either of purely reflex or 
 of partly reflex and pa^rtly automatic action, between various affer- 
 ent impulses and the efferent vaso-motor impulses leading either to 
 dilation or constriction ? 
 
 We have seen that stimulation of the central stump of the 
 divided sciatic gives rise, in an animal under urari, to an increase 
 of general blood-pressure, brought about chiefly, if not entirely, by 
 an augmentation of constrictive impulses passing along the splanch- 
 nic nerves. This increase of blood-pressure is manifested, with 
 
 tA^y^ 
 
CiiAr. iv.J THE VASCULAR MECHAXISM. 213 
 
 (in satisfactory exporimcnts) undimiiiisliod inttinsity, even wlien the 
 whole of the brain, down to a certain limit in the medulla oblon^^ata, 
 has been removed. But it" the removal be carrietl beyond this 
 limit, or if a small area of the medulla oblongata lying above the 
 calamus scriptorius be removed, the effect on the general blood- 
 prcssm-e of stimulating the central stump of the sciatic — we might 
 add, of any otlier aftcrcut nerve — is comparatively insignificant. The 
 simi>k'st view to take of these facts is to suppose that this small 
 portion of the medulla oblongata acts as a vaso-motor centre, by the 
 action of which ordinary afferent impulses coming from the sciatic 
 or any other afferent ner\'e, arc transformed into vaso-motor 
 impulses of constrictive (or as in the case of an animal under 
 chloral, of dilating) effect and so discharged along the splanchnic 
 nerves. 
 
 The lower limit of this region -which wc may call the medullary 
 vaso-motor centre has been placed in the rabbit at a horizontal 
 line drawn about 4 or 5 mm. above the point of the calamus 
 scriptorius, and the upper limit at about 4mm. higher up, i.e. 
 about 1 or 2 mm. below^ the corpora quadrigemina. When trans- 
 verse sections of the brain are cai'iied successively lower and 
 lower down, an effect on blood-j)ressure in the way of lowering it 
 and also of diminishing the rise of blood-pressure resulting from 
 stimulation of the sciatic, is first obsen-ed when the upper limit 
 is reached. On carrying the sections still lower, the effect of 
 stimulating the sciatic becomes less and less, until when the 
 lower limit is reached no effects at all are observ'ed. The centre 
 appears to be bilateral, the halves being placed not in the middle 
 line but more sideways and rather nearer the anterior than the 
 posterior surface. It may perhaps be more closely defined as a 
 small prismatic space in the forward prolongation of the lateral 
 columns after they have given off their fibres to the decussating 
 pyramids. This space is largely occupied by a mass of gi^ey 
 matter, called by Clarke the antero-lateral nucleus, and containing 
 large multipolar cells. 
 
 Whether this medullary vaso-motor centre has any distinct 
 automatic action, whether it may be regarded as continually gene- 
 rating out of its own molecular oscillations, and discharging along 
 the vaso-motor fibres, impulses whereby the general arterial tone 
 is maintained, is a question w^hich, like the allied question mooted 
 on p. 188, need not be discussed here. Granting even the existence 
 of such automatic functions, they must be of secondary importance. 
 As w^e have already urged, the great use of the whole vaso-motor 
 system is not to maintain a general arterial tone, but to modify 
 according to the needs of the economy the condition of this or 
 that vascular area. 
 
 The impulses passing down the vaso-motor fibres of the cervical 
 sympathetic and of many other nerves may similarly be traced back 
 to this same reerion of the medulla oblongata. Whether all vaso- 
 
214 YASO-MOTOR CENTRES, [Book i. 
 
 motor fibres are actually in functional connection with it may perhaps 
 be doubted; but at all events the fibres passing to so many vascular 
 areas, and those of such magnitude and importance, are by means 
 of it brought into functional relationship with so many afferent 
 nerves of the body, that it may fairly be spoken of as the general 
 vaso-motor centre. 
 
 But the use of this phrase must not be understood to imply 
 that this small portion of the medulla oblongata is the only part of 
 the central nervous system which can act as a vaso-motor centre. 
 In the frog reflex vaso-motor effects may be obtained by stimu- 
 lating various afferent nerves after the whole medulla has been 
 removed, and indeed even when only a comjDaratively small portion 
 of the spinal cord has been left intact and connected, on the one 
 hand, with the afferent nerve which is being stimulated and, on the 
 other, with the efferent nerves in which run the vaso-fibres v>'hose 
 action is being studied. In the mammal such effects do not so 
 readily appear, but may with care and under special conditions be 
 obtained. Thus in the dog, when the spinal cord is divided in the 
 dorsal region, the arteries of the hind limbs and hinder part of the 
 body become dilated. This one Vv^ould naturally expect as the 
 result of their severance from the general medullary vaso-motor 
 centre. But if the animal be kept in good condition for some time, 
 a normal or nearly normal arterial tone is after a while re-esta- 
 blished ; and the tone thus regained may be modified in the direc- 
 tion certainly of dilation, and possibly, l3ut this is by no means so 
 certain, of constriction by afferent impulses reaching the lumbar 
 cord. Erection of the penis through the nervi erigentes may then 
 be still brought about by suitable stimulation of sensory surfaces, 
 and dilation of various vessels of the limbs readily produced by 
 stimulation of the central stump of one or another nerve. 
 
 These remarkable results, which though they are most striking 
 in connection with the lumbar cord hold good apparently for the 
 dorsal cord also and indeed for all parts of the spinal cord, naturally 
 suggest a doubt whether the explanation just given above of the 
 effects of section of the medulla oblongata, is a valid one. When 
 we come to study the central nervous system, we shall again and 
 again see that the immediate effect of operative interference with 
 these delicate structures is a temporary suspension of nearly all 
 their functions. This is often spoken of as ' shock ' and may be 
 regarded as an extreme form of inhibition. And the question may 
 fairly be put whether the effects of cutting and injuring the 
 structures which we have spoken of as the medullary vaso-motor 
 centre, are not in reality simply those of shock. The case of the 
 dog with the divided dorsal cord, and other similar cases, clearly 
 prove that parts of the spinal cord, other than the particular 
 region of the medulla oblongata of which we are speaking, may 
 act as vaso-motor centres. And we may very fairly at least put 
 forward the view, that the vascular dilation which follows upon 
 
CiiAP. iv.J THE VASCULAR MKClIAyiSM. 215 
 
 sections of the so-called medullary vaso-motor centre, comes about 
 because section of or injury to this region exercises a strong 
 iidiibitory intluenee on all the vaso-niotor centres situated in the 
 spinal cord below. Owing to the special function of the medulla 
 oblongata in carrying on the all-important work of respiration, a 
 mammal whose medulla has been divided cannot be kept alive for any 
 length of time. Wo cannot therefore put the matter to the simple 
 experimental test of extirpating the supposed medullary vaso-motor 
 centre and seeing what happens when the animal has completely 
 recovered from the effects of the operation : we have to be guided 
 in our decision by more or less indirect arguments. We must not 
 attempt to discuss the matter fully here, but may say that, after all 
 due weight has been attached to the play of inhibitory impulses, 
 there still remains a balance of evidence in favour of the view 
 that the region of the medulla of which we are speaking does act 
 as a general vaso-motor centre. It is not however to be regarded 
 as the .single vaso-motor centre, whither afferent impulses from all 
 parts of the body must always travel before they can start vaso- 
 motor impulses along this or that nerve. We are rather to suppose 
 that the spinal cord along its whole length, contains, interlaced 
 with the reflex and other mechanisms by which the skeletal muscles 
 are governed, vaso-motor centres and mechanisms of varied com- 
 plexity, the details of whose functions and topography have yet 
 largely to be worked out. As in the absence of the sinus venosus 
 the auricles and ventricle of the frog's heart may still continue to 
 beat, so in the absence of the medulla oblongata, these spinal vaso- 
 motor centres provide for the vascular emergencies which arise. 
 As however in the normal entire frog's heart, the sinus, so to speak, 
 gives the word and governs the work of the whole organ, so the 
 medullary vaso-motor centre rules and co-ordinates the lesser 
 centres of the cord, and through them presides over the chief 
 vascular areas of the body. It is possible moreover that the me- 
 dullary centre is specially connected with the splanchnic nerves and 
 thus with the capacious vascular area of the abdominal viscera, and 
 in consequence possesses an additional importance. By means of 
 these vaso-motor central mechanisms, by means of the head centre 
 in the medulla, and the subsidiary centres in the spinal cord, the 
 delicate machinery of the circulation, which determines the blood 
 supply, and so the activity of each tissue and organ, is able to 
 respond by narrowing or widening arteries to the ever-varying 
 demands and to meet by compensating changes the shocks and 
 strains of daily life. 
 
 Vaso-motor nerves of the Veins. Although the veins are 
 provided with muscular fibres, and are distinctly contractile and 
 although rhythmic variations of calibre due to contractions may 
 be seen in the great veins opening into the heart, in the veins of 
 
216 VASCULAR CONSTRICTION AND DILATION. [Book i. 
 
 the bat's wing, and elsewhere, and similar rhytlimic variations, also 
 possibly due to active rhythmic contractions, but possibly also of 
 an entirely passive nature, have been observed in the portal veins, 
 very little is known of any nervous arrangements governing the 
 veins. When in the frog the brain and spinal cord are destroyed, 
 very little blood comes back to the heart as compared with the 
 normal supply, and the heart in consequence appears almost blood- 
 less and beats feebly. This has been interpreted as indicating the 
 existence of a normal tone in the veins dependent on the central 
 nervous system. When the latter is destroyed, the veins become 
 abnormally distended and a large quantity of blood becomes lodged 
 and hidden as it were in them. 
 
 The Effects of Local Vascular Constriction or Dilation. 
 
 Whatever be determined ultimately to be the modus oj^erandi 
 of vaso-motor mechanisms, the following fundamental facts remain 
 of prime importance. 
 
 The tone of any given vascular area may be altered, positively 
 in the direction of augmentation (constriction), or negatively in 
 the way of inhibition (dilation), quite independently of what is 
 going on in other areas. The change may be brought about by (1) 
 a stimulus applied to the spot itself, and acting either directly on 
 some local mechanism, or indu-ectly by reflex action through the 
 general central nervous system ; (2) by a stimulus applied to some 
 other sentient surface, and acting by reflex action through the 
 central nervous system ; (3) by a stimulus (chemical, arising in or 
 carried by the blood) acting directly on the central nervous system; 
 (4) by some part of the central nervous system acting on the vaso- 
 motor centre, as in emotions. 
 
 The effects of local dilation are local and general. 
 
 The local effects are as follows. The arteries in the area being 
 dilated, offer less resistance than before to the passage of blood. 
 Consequently, more blood than usual passes through them, filling 
 up the capillaries and distending the veins. Owing to the diminu- 
 tion of the resistance, the fall of pressure in passing from the 
 arteries to the veins will be less marked than usual ; that in the 
 small arteries themselves will be lowered ; that in the corresponding 
 veins heightened. The lowering of the pressure in the arteries 
 means that their elastic coats are not put to the stretch as much 
 as usual ; i.e. their elasticity is not called into play to the same 
 extent as before. Now, as has been seen, every portion of the 
 arterial wall has its share in destroying the pulse by converting the 
 
Chap. lY.] THE VASC'l'LAR MJ:CnAXIS}r. 217 
 
 intorniittcnt into a continuous flow, rienco, tho dilated arteries, 
 their elasticity not bein(( called into jjlay so much as before, will 
 not contribute their usual share towards destroying the ptdsations 
 which reach them at the cardiac side. The ])ulsations will travel 
 thniu(,di them less chanj^fed than before, and may, in certain cases, 
 j)ass rii^dit on into the veins. This is frefiuently seen in tho sub- 
 maxillary gland, when the chorda tympani is stimulated. The 
 channels being wider, resistance being less, and the force of the 
 heart behind remaining the same, more blood than before passes 
 through the area in a given time ; or, put diflierently, the same 
 quantity of blood passes through the area in a shorter time. The 
 blood, consequently, as it passes into the veins is less changed than 
 in the normal condition of the area. Usually the flow is so rapid 
 that the oxy-hffimoglobin of the corpuscles is deoxidised to a much 
 less extent than usual, and the venous blood still possesses an 
 arterial hue. On the other hand, since more blood passes in 
 a given time, there is an opportunity for an increase in the total 
 interchange between the blood and the tissue. Thus the total 
 work may be greater, though the share borne by each quantity of 
 blood is less. 
 
 The general effects of dilation are briefly these. Supposing 
 that the total quantity of blood issuing from the ventricle remains 
 the same, that is to say, supposing that the quantity of blood put 
 into circulation is constant, the sui^plus passing through the dilated 
 area must be taken away from the rest of the circulation. Con- 
 sequently the fulness of the dilated area wtlII lead to an emptying 
 of the other areas. This is seen very clearly when the dilated area 
 is a capacious one. At the same time, local dilation causes a local 
 diminution of peripheral resistance. This in turn causes a lower- 
 ing of the general arterial pressure ; to this we have already called 
 attention. 
 
 The effects of local constriction, similarly local and general, are 
 naturally the reverse of those of dilation. In the vascular area 
 directly affected, less blood passes through the capillaries in a given 
 time, and in consequence less total interchange between the blood 
 and the tissues takes place, though each unit volume of blood which 
 does pass through is more deeply affected. The blood-pressure in 
 the corresponding arteries is increased, and, if the area be large, tho 
 pressure in even distant arteries may be heightened. 
 
 Thus, to indicate results in a general manner, local dilation en- 
 courages a copious flow of blood through the area where the 
 dilation is taking place, and, by reducing the blood-pressure, 
 hinders the flow of blood into other areas. Local constriction, on 
 the other hand, lessens the flow of blood in the particular area, and 
 by heightening the blood-pressure tends to throw the mass of the 
 blood on to other areas. Hence the great regidative value of the 
 vaso-motor system. By augmenting or inhibitory influences (con- 
 strictor or dilating) applied either to peripheral mechanisms or to 
 
218 VASCULAR CONSTRICTION AND DILATIOX. [Book i. 
 
 cerebro-spinal centres, and called forth by stimuli either intrinsic 
 and acting through the blood, or extrinsic and acting through 
 nervous tracts, the supply of blood to this or that organ or tissue 
 may be increased or reduced : the surplus or deficit being carried 
 away to, or brought up from, either the rest of the body generally 
 or some other special organ or tissue. 
 
SEC. C. CHANGES IN THE CAPILLARY DISTRICTS. 
 
 We have already seen (p. 116) that the capillary channels vary 
 very much in width from time to time ; but the capillaries do not, 
 like the arteries, jDossess a distinct muscular coat, and the mechanism 
 by which they are brought now to a dilated now to a constricted 
 condition has not been worked out so thoroughly as in the case of 
 the arteries. On the one hand there can be no doubt that the 
 changes in their calibre are in part of a passive nature. They are 
 expanded when a large supply of blood reaches them through the 
 supplying arteries, and, by virtue of their elasticity, shrink again 
 when the supply is lessened or withdrawn. 
 
 On the other hand there is an increasing amount of evidence 
 that the capillary walls are really contractile. The constituent 
 epithelioid cells have been seen to change their form under the 
 inflvience of stimuli ; and there is much reason for believing that 
 the calibre of a capillary canal may var}', quite independently of 
 the arterial supply or the venous outflow, in consequence of changes 
 in form of the epithelioid cells, allied to the changes in a muscle- 
 fibre or muscle-cell which constitute a contraction. Though the 
 matter requires further investigation, it is probable that these 
 active changes play an important part in determining the quant it}'' 
 of blood passing through a capillary area ; but there is as yet no 
 evidence that they, like the coiTesponding changes in the arteries, 
 are governed by the nervous system. 
 
 Over and above these changes of form, the capillaries and 
 minute vessels also possess other active properties, which cause 
 them to play an important part in the work of the circulation. 
 They are concerned in assisting to maintain a vital equilibrium 
 between the intra-vascular blood and the extra-vascular tissue, an 
 
220 CHANGES IN THE CAPILLARIES. [Book r. 
 
 equilibrium which is the central fact of a normal capillary circula- 
 tion, of a normal interchange between the blood and the tissue, and 
 thus of a normal life of the tissue. The existence of this equi- 
 librium is best shewn when it is overthrown or modified, as in 
 inflammation and allied conditions. 
 
 If an irritant, such as a drop of chloroform or a little diluted 
 oil of mustard, be applied to a small portion of a frog's web, a frog's 
 tongue, or some other transparent tissue, the following changes may 
 be observed under the microscope. The first effect that is noticed 
 is a dilation of the arteries, accompanied by a quickening of the 
 stream. The capillaries become filled with corpuscles, and many 
 passages, previously invisible or nearly so on account of their con- 
 taining no corpuscles, novtf" come into view. The veins at the same 
 time appear enlarged and full. The increase of width is most 
 marked in the arteries, next so in the veins, and least of all in 
 the capillaries. If the stimulus be very slight, this may all pass 
 away, the arteries gaining their normal constriction, and the capil- 
 laries and veins returning to theu' normal condition ; in other 
 words, the effect of the stimulus in such a case is simply a tem- 
 porary blush. Unless however the chloroform or mustard be applied 
 with especial care the effects are much more profound and lasting. 
 In the case of the frog's web a condition is set up known under the 
 name of stasis. This has been considered as merely a phase of in- 
 flammation, since in the frog's v^eb in which inflammation has been 
 largely studied, the agents which produce inflammation frequently 
 produce stasis. But in the frog's tongue and elsewhere true inflam- 
 mation may be set up and produce all its results without any stasis 
 making its appearance ; and though the two conditions are in 
 several respects similar, they appear to be distinct: stasis being the 
 result of the profounder action of the irritant and the forerunner of 
 local death or necrosis. 
 
 It is this stasis which particularly illustrates the points to which 
 we wish to call attention. Y/hen as the result of the irritant, the 
 initial blush passes into stasis, the following events may be observed. 
 The quickening of the stream gives way to a slackening; this is 
 not due to any returning constriction of the arteries, for they still 
 continue dilated. It will further be observed that the red corpuscles, 
 instead of being in the larger capillaries and smaller arteries and 
 veins confined to the axial stream, are diffiised and indeed crowded 
 over the whole vndth of the channels. The capillaries and veins 
 get more and more crowded with corpuscles, the white corpuscles 
 being scattered UTegularly among the more numerous red ones ; 
 and though the channels get wider and wider, becoming frequently 
 even enormously distended, the stream becomes slower and slower, 
 until at last the movement of the blood in the affected area ceases 
 altogether. The phase of accelerated flow has given place to stasis. 
 The capillaries, veins and small arteries are choked with corpuscles, 
 and it may now be remarked that the red corpuscles seem to run 
 
Chap, iv.] THE VASCULAR MICCIIAXISM. 221 
 
 to^C,fetlier, so that their outlines arc no l()n;(er (li.stinpfuisliablc; they 
 aj)pcar to have become fused into a homogeneous mass. Kxcejit in 
 cases where the stimulus proiluees permanent mischief, this peculiar 
 condition after a while subsides. The outlines of the corpuscles 
 become once more distinct, those on the venous side of the block 
 gradually drop awa}' into the ucighbouring currents, little by little 
 the whole obstruction is removed, the current through the area is 
 re-established, and though the arteries and capillaries remaiu 
 dilated for some considerable time, they eventually return to their 
 normal calibre. 
 
 The stasis, the arrest of the current hero seen, is not due to any 
 lessening of the heart's beat; the arterial pulsations, or at least the 
 arterial flow, may be seen to be continued down to the affected 
 area, and there to cease very suddenly. It is not due to any 
 increase of peripheral resistance caused by constriction of the small 
 arteries, for these continue dilated rather than constricted. It must 
 therefore be due to some new and unusual resistance occurrino- in 
 the capillary area itself. The increase of resistance is not caused 
 by any change confined to the corpuscles themselves ; for if after a 
 temporary delay one set of corpuscles has managed to pass away 
 from the affected area, the next set of corpuscles is subjected to the 
 same delay and the same apparent fusion. The cause of the resist- 
 ance must therefore lie in the capillary walls, or in the tissue 
 of Avhich they form a part. We are driven to conclude that the 
 walls of the capillaries (and of the other vessels) exert in health a 
 certain attraction on the corpuscles, maintain a certain adhesiveness 
 between them and themselves, thereby determining the normal flow, 
 with its axial stream and plasmatic layer, and offering a normal 
 resistance to the pressure of the arterial system; and that, in stasis, 
 for reasons which we cannot as yet explain, this attraction, this 
 adhesiveness is largely and progressively increased. Hence the 
 early disappearance of the distinction between the axial stream and 
 plasmatic layer, the tarrying of the corpuscles in spite of the widen- 
 ing of theu^ path, and finally their agglomeration and fusion in the 
 even enormously distended channels. 
 
 That the increased adhesion is due to the vascular walls and not 
 primarily to the cor[Dusclcs themselves is further shewn by the fact 
 that if in the frog, an artificial blood of normal saline solution to 
 which milk has been added be substituted for normal blood, a 
 stasis may by initants be induced in which oil-globules play the 
 part of corpuscles, and by their aggregation bring about an arrest 
 of the flow through the capillaries. 
 
 In true inflammation the course of events is different. The 
 vessels become dilated, but the loss of distinction between the axial 
 stream and the jDlasmatic layer does not occur. On the contrary 
 the plasmatic layer appears even more striking on account of the 
 large number of white corpuscles which gather in it and become 
 adherent to the inner surface of the walls of the veins and venous 
 
222 CHAXGES IX THE CAPILLARIES. [Book i. 
 
 capillaries. In the normal circulation only a few white corpuscles 
 are from time to time seen in this situation slowly moving on 
 in jerks ; but now the walls of the veins seem to be more and 
 more thickly lined with white corpuscles, which are at first com- 
 pletely stationary. At the same time white corpuscles become 
 also very abundant in the capillaries. Very soon these white 
 corpuscles may be seen, either through stomata at the junctions of 
 the epithelioid cells forming the lining of the vessels, or by 
 temporary breaches which are rapidly repaired, making their way 
 through the walls of the veins and capillaries, and escaping into 
 the surrounding tissues. Through the walls of the capillaries and 
 smaller veinleis, red coi-puscles pass as well as white. And this 
 takes place to such an extent that very soon the tissue around the 
 veins and capillaries becomes crowded with white corpuscles, and to 
 a less extent "svith red corpuscles which have made their way out 
 of the vessels. At the same time a large quantity of coagulable 
 lymph, which since it appears also to have passed from the blood- 
 stream through the walls of the blood-vessels is spoken of as exu- 
 dation, makes its appearance in the interstices of the inflamed 
 tissue. While however these changes are going on there is not, as 
 in stasis, a delay and final arrest of the blood-stream. On the 
 contrar}^ the flow through the Avidened channels continues during 
 the whole time to remain accelerated. By comparing the outflow 
 from the veins of the inflamed foot of a dog, with the outflow from 
 the veins of the healthy foot, it has been ascertained that a larger 
 quantity of blood passes through the inflamed foot than through 
 the healthy foot in the same time. 
 
 We must not however pursue this subject of inflammation any 
 further. We have simply brought it forward as affording another 
 illustration of the action of the walls of the blood-vessels; for, 
 though the matter is perhaps not definitely settled, it seems pro- 
 bable that the aggTegation, in inflammation, of the white corpuscles 
 upon the lining surface of the vessels is due to a special attraction 
 which the blood-vessels exert on the white corpuscles, vdthout pro- 
 ducing that general adhesion of all the corpuscles which is the mark 
 of stasis, and that the migration of the corpuscles is also at least 
 facilitated by similar intrinsic changes in the vascular walls. 
 
 We cannot say at present whether the vascular walls are also 
 capable of modif}dng the passage of the fluid parts as distrag-uished 
 from the corpuscular elements of the blood, though we know by 
 experiment that the flow of fluid through capillary tubes may be 
 modified on the one hand by changes in the substance of which the 
 tubes are composed, and on the other hand by changes iu the 
 chemical nature (even independent of the specific gravity) of the 
 fluid which is used. We have said enough to shew that the peri- 
 pheral resistance in the capillaries (and consequently all that depends 
 on that peripheral resistance) is not merely a matter of the mechani- 
 cal friction of the blood arainst the smooth walls of the blood-vessels, 
 
CiiAi'. iv.J rilE VASCULAR M IX'IIA X ISM. 223 
 
 but is concerned with the vital condition of the tissues. Wlien the 
 tissue is in health, a certain resistance is offered to the passage of 
 blood through the capillaries, and the whole vascular mechanism is 
 adapted to overcome this resistance to such an extent that a 
 normal circulation can take place. When the tissue becomes 
 affected, the disturbance of the equilibrium between the tissue 
 and the blood may as in inflammation so modify the flow as to lead 
 to the abnormal escape from the blood of various constituents, or as 
 in stasis so augment the resistance that the passage of the blood 
 becomes difficult or impossible. And it is quite open to us to 
 suppose tliat there are conditions the reverse of stasis, in which the 
 resistance may be lowered below the normal, and the circulation 
 in the area quickened. 
 
 Thus the vital condition of the tissue becomes a factor in the 
 maintenance of the circulation ; and it is possible, though not yet 
 proved, that these vital conditions are directly under the dominion 
 of the nervous system. 
 
 It is perhaps hardly necessary to observe that the considerations 
 urged above are quite distinct from what is sometimes spoken of under 
 the name of 'capillary' force, as an agent of the circulation. If by 
 capUlary force it is intended to refer to the rise of fluids in capillary 
 tubes, it is e^"ident that since such phenomena are the results of 
 adhesion, capillarity can only be a greater or less hindrance to the flow 
 of blood, seeing that this is propelled by a force (the heart's beat) 
 which has been proved by experiment to be equal to the task of 
 dri%-ing the blood from ventricle to auricle through the capillary regions. 
 If by capdlary force it is meant that the tissues have some vital power 
 of withdrawing the fluid parts of the blood from the small arteries and 
 thus of assisting an onward flow, it becomes necessary also to assume 
 that they have as well the power of returning the fluid parts to the 
 veins. Both these assumptions are imnecessary and without foundation. 
 
SEC. 7. CHANGES IN THE QUANTITY OF BLOOD. 
 
 In an artificial scheme, changes in the total quantity of fluid in 
 circulation will have an immediate and direct eftect on the arterial 
 pressure, increase of the quantity heightening and decrease 
 diminishing it. This effect will be produced partly by the pump 
 being more or less filled at each stroke, and partly by the j)eri- 
 pheral resistance being increased or diminished by the greater 
 or less fulness of the small peripheral channels. The venous 
 pressure will under all circumstances be raised with the increase of 
 fiuid, but the arterial pressure will be raised in proportion only so 
 long as the elastic walls of the arterial tubes are able to exert their 
 elasticity. 
 
 In the natural circulation, the direct results of change of quan- 
 tity are obscured by compensatory arrangements. Thus experi- 
 ment shews that v/hen an animal with normal blood-pressure 
 is bled from one carotid, the pressure in the other carotid sinks so 
 long as the bleeding is going on^ and remains deioressed for 
 a brief period after the bleeding has ceased. In a short time how- 
 ever it regains or nearly regains the normal height. This recovery 
 of blood-pressure, after haemorrhage, is witnessed so long as the 
 loss of blood does not amount to more than about 3 per cent, of 
 
 1 Chiefly in consequence of free opening in tlie vessel from wlaicli the bleeding 
 is going on, cutting oH a great deal of the peripheral resistance, and so leading to a 
 general loweiing of the blood-pressure. 
 
Chap, iv.] TIIK VASCULAR MECIIAKIHM. 225 
 
 the Ixnly-weight. Bovoiul lli.il, a lurj^^' ;iii(l IVcipiciilly :i sudilcn 
 (liinLTfrous jxTUKiiu'iit dfjUH'Ssioii is obsrrvi'd. 
 
 Tlio R'stonition ot" the jiressure after the cessation of thn 
 bleedin*,' is too r;i|)id to peniiit lis to snp))ose that tlie (luaiitity of 
 thiiil ill the blootl-vessels is repaired by tlie withdrawal of lyiiijili 
 from the extra-vaseiilar elements of the tissues. In all probability 
 the result is gained by an increased action of the vaso-motor 
 nerves, increasing the peri|)heral resistance, the vaso-motor centres 
 being tlirown into increased action by the diminution of th<ir 
 blood-supplv. When tlie loss of Ijlood has gone beyond a certain 
 limit, this vaso-motor action is insufficient to compensate the 
 diminished (piautity (possibly the vaso-motor centres in part 
 become exhausted), and a considerable depression takes place ; but 
 at this epoch the loss of blood frequently causes anaemic con- 
 vulsions. 
 
 Similarly when an additional quantity of blood is injected into 
 the vessels, no marked increase of blood-pressure is observed so 
 lonjj as the vaso-motor centre in the medulla obloncrata is intact. 
 If however the cervical spinal cord be divided previous to the in- 
 jection, the pressure, which on account of the removal of the 
 medullary vaso-motor centre, is very low, is permanently raised by 
 the injection of blood. At each injection the pressure rises, falls 
 somewhat afterwards, but eventually remains at a higher level than 
 before. This rise continues until the amount of blood in the 
 vessels above the normal quantity reaches from 2 to 3 per cent, 
 of the body-weight. Beyond this point there is no further rise of 
 pressure. 
 
 These facts shew, in the first place, that when the volume of 
 the blood is increased, compensation is effected by a lessening of 
 the peripheral resistance by means of a vaso-dilator action of the 
 vaso-motor centres, so that the normal blood-pressure remains con- 
 stant. They further shew that a much greater quantity of blood 
 can be lodged in the blood-vessels than is normally present in 
 them. That the additional quantity injected does remain in the 
 vessels is proved by the absence of extravasations, and of any con- 
 siderable increase of the extra-vascular lymphatic fluids. It has 
 already been insisted that, in health, the veins and capillaries nuist 
 be regarded as being far from filled, for were they to receive all the 
 blood which they can, even at a low pressure, hold, the whole quan- 
 tity of blood in the body would be lodged in them alone. In these 
 cases of large addition of blood, the extra quantity appears to 
 be lodged in the small veins and capillaries (especially of the 
 internal organs), which are abnormally distended to contain the 
 surplus. 
 
 We leani from these facts the two practical lessons, first, that 
 blood-pressure cannot be lowered directly by bleeding, unless the 
 quantity removed be dangerously large, and secondly, that there is 
 
 F. 15 
 
226 CHANGES IN THE QUANTITY OF BLOOD. [Book i. 
 
 110 necessary connection between a high blood -pressure and fuhiess 
 of blood or plethora, since an enormous quantity of blood may be 
 driven into the vessels without any marked rise of pressure. 
 
SEC. 8. THE MUTUAL RELATIONS AND THE CO-ORDI- 
 NATION OF THE VASCULAR FACTORS. 
 
 The foregoiDg considerations shew how complicated, and 
 sensitive, and therefore how useful, is the vascular mechanism. 
 It may be worth while briefly to summarize the relations of the 
 different factors, and to point out the manner in which they are 
 made to work in harmony for the good of the body. 
 
 Two facts stand out prominent above all others: (1) the heart's 
 beat may be made slow by vagus inhibition, and, on the other 
 liand, quickened either by withdrawal of the constant inhibitory 
 influence exercised by the cardio-inhibitory centre, or by the direct 
 action of accelerating mechanisms. (2) The peripheral resist- 
 ance may be increased or diminished, the increase and decrease 
 being due either to increased or diminished action of the vaso- 
 motor centres which preside over arterial tone, or to the action of 
 special constrictor or dilator fibres. 
 
 These two facts are, by the mediation of the nervous system, 
 placed in mutual regulative dependence on each other. Thus, if 
 with a given peripheral resistance, and proportionate blood-pressure, 
 the heart begins to beat violently, afferent impulses passing up the 
 depressor nerves diminish peripheral resistance (by opening the 
 splanchnic flood-gates), and prevent the rise of blood-pressure 
 which would other^^dse take place. In this way a delicate organ, 
 such for instance as the retina, is sheltered from the turbulence of 
 the heart by the flow of blood being diverted to the less noble 
 
 15—2 
 
228 RELATIONS OF THE VASCULAR FACTORS [Book i. 
 
 organs of the abdomen. Conversely, if peripheral resistance be in 
 any area increased, the general blood-pressure is prevented from 
 rising too high, by reason of the actual increase of blood-pressure 
 so affecting the medulla, that inhibitory impulses descend the 
 vagus, and, by producing a less frequent, possibly a weaker pulse, 
 tone down the distension of the arteries. 
 
 The more we learn of the working of the body, the more aware 
 we become of the fact that it is crowded with regulative and 
 compensating arrangements no less striking and exquisite than the 
 two we have just described. Some of these will be seen in the 
 following almost tabular statement of the various modifications of 
 the vascular factors, and of their causes. 
 
 A. The Beat of the Heart is affected 
 
 1. By the amount of distension of the ventricular cavities pre- 
 ceding the systole. This will depend on 
 
 a. The quantity of blood reaching the heart and passing into 
 its cavities during the diastole. This in turn is determined by 
 the flow of blood through the veins, the flow itself being influenced 
 by the arterial pressure, resj)iratory movements, &c. &c. 
 
 h. The force of the auricular contractions, 
 
 c. The amount of resistance which has to be overcome by the 
 systole. This is determined by the mean arterial pressure, and is 
 influenced by everything which influences that. 
 
 2. By the quantity of the blood passing through the coronary 
 arteries. In the frog the thin walls of the auricle and the spongy 
 texture of the ventricle permit the nourishment of the cardiac sub- 
 stance to be carried on by direct contact with the blood in the 
 cavities. In mammals this mode of nutrition must be insignificant. 
 In them the condition of the cardiac muscles and nervous ap- 
 pendages depends almost exclusively on the blood distributed by 
 the coronary arteries. The coronary circulation however is peculiar 
 and is largely determined by the action of the heart itself. 
 
 3. By the quality of the blood passing through the coronary 
 arteries, and acting upon simply the muscular tissue, or upon the 
 various nervous mechanisms, or upon both. This is illustrated by 
 the action of poisons. The quantitative relations of the normal, 
 and the presence of abnormal, constituents of the blood must of 
 necessity profoundly affect the heart's beat. 
 
 4. Through the inhibitory fibres of the vagus. 
 
 a. By the blood directly stimulating the endings of the vagus 
 fibres. This -is only seen in the case of poisons. 
 
 h. By the blood directly affecting the cardio-inhibitory centre 
 
Cn.vr. IV.] 77/ A' \'Asrf/.A/:.\//:C//AX/S.\f. 220 
 
 in the medulla oblongata, either positively by augmenting the 
 normal inhibitory intiui-nees and so slowing the heart, or negatively 
 by dtjircssing those intlutuees and so quiekcning the heart. 
 
 c. By rctlex stimulation of the same centre. Cases of exalta- 
 tion through reriex stimulation have already been quoted. In- 
 stanees of depression leading to (juickening of tho lujart's beat are 
 not so clear. The atierunt impulses may be started in any part of 
 the body ; but, as we have seen, there seems to be a special 
 connection between this centre and the alimentary canal. 
 
 5. By the accelerator nerves. We have however, at present, 
 no very satisfactory evidence of the natural activity of these nerves. 
 
 B. The PevipJieral Iiesi!stance is atiected 
 
 1. By the vital i,e. the nutritive condition of the tissue of the 
 .part. This is again intluenced by 
 
 «. The quality (and quantity?) of the blood brought to it. 
 
 h. Through the agency of the ners^ous system, as is seen in 
 cases of inflammation caused by nervous influences. 
 
 Both these points are very obscure. 
 
 2. By the varjdng calibre (constriction, dilation) of the minute 
 aiteries, brought about 
 
 a. By the blood or other stimulus acting directly on the peri- 
 pheral vaso-motor mechanism. 
 
 h. By the blood or other stimulus acting directly on the vaso- 
 motor centres in the central nervous system. 
 
 c. By reflex stimulation of the vaso-motor centres. 
 
 d. By the quantity of blood supplied to the vaso-motor centre, 
 this being in turn dependent on the blood-pressure in the arteries 
 supplying the centre. Thus a regidative mechanism is established 
 for cases when the quantity of blood, as distingiushed from its 
 quality, is changed (see p. 225). 
 
 Through these intricate ties it comes to pass that an event 
 which takes place in one part of the body is felt, to a greater or 
 less extent, by all parts. To take a simple instance : a change in 
 the condition of the skin at any one spot, such as that produced by 
 the application of cold or heat, may lead, 
 
 a. By direct local action to a constriction or dilation of the 
 vessels of the part, giving rise to local pallor or sufi'usion. 
 
 yS. By reflex action through the central nervous system, to an 
 increase of the same local eft'ects, and in addition to a change in 
 
230 RELATIONS OF THE VASCULAR FACTORS [Book i. 
 
 the calibre of the blood-vessels in other parts. This distant reflex 
 change may be of the same or the opposite nature as the local 
 change. 
 
 7. By reflex action to a quickening or slowing of the heart's 
 beat, though the heart is in this respect less intimately connected 
 with the skin than with other parts. 
 
 Out of these primary effects there may arise secondary effects ; 
 the constriction or dilation produced locally will affect the general 
 blood-pressure, which in turn will produce all its effects. 
 
 The modifications of the heart-beat will not only affect the general 
 blood-pressure, but in a reflex manner may affect the peripheral 
 resistance, and hence the flow of blood in particular areas {e.g. the 
 splanchnic area). The modifications of the flow through the area 
 directly, and also through those secondarily, affected, will influence 
 the temperature and chemical changes of the blood, and variations 
 in these will in turn produce their effects everywhere. And so on. 
 
 On the other hand, the turbulence which would be the natural 
 outcome of all these events is softened down, by the compensating 
 eftects of which we have spoken, into the smoothness which we call 
 health. Still, the greatness of the possibilities of change which lie 
 hidden in the body are clearly enough shewn by the violence of 
 disease, when compensation fails of accomplishment. 
 
BOOK 11. 
 
 THE TISSUES OF CHEMICAL ACTION WITH THEIR 
 RESPECTIVE MECHANISMS. NUTRITION. 
 
CHAPTER I. 
 
 THE TISSUES AND MECHANISMS OF DIGESTION. 
 
 The food in passing along the alimentary canal is subjected to the 
 action of certain juices which arc produced by the secretory activity 
 of the epithcliura-cclls lining the canal itself or forming part of its 
 glandular appendages. These juices (viz. saliva, gastric juice, 
 bile, pancreatic juice, and the secretions of the small and large 
 intestines), poured upon and mingling with the food, produce 
 in it such changes, that from being largely insoluble it becomes 
 largely soluble, or otherwise modify it in such a way that the 
 larger part of what is eaten 25asses into the blood, either directly 
 by means of the capillaries of the alimentary canal or indirectly 
 by means of the lacteal system, while the smaller part is discharged 
 as excrement. 
 
 We have therefore to consider — First, the properties of the 
 various juices, and the changes they bring about in the food eaten. 
 Secondly, the nature of the processes by means of which the 
 various epithelium-cells of the various glands and various tracts of 
 the canal are able to manufacture so many various juices out of 
 the common source, the blood, and the manner in which the 
 secretory activity of the cells is regulated and subjected to the 
 needs of the economy. Thirdly, the mechanisms, here as elsewhere 
 chiefly of a muscular nature, by which the food is passed along the 
 canal, and most efficiently brought in contact with successive 
 juices. Fourthly and lastly, the means by which the nutritious 
 digested material is separated from the undigested or excremental 
 material, and absorbed into the blood. 
 
SEC. 1. THE PROPERTIES OF THE DIGESTIVE JUICES, 
 
 Saliva. 
 
 Mixed saliva, as it appears in the mouth, is a thick, glairy, 
 generally frothy and turbid fluid. Under the microscope it is 
 seen to contain, besides the molecular debris of food (and fre- 
 quently cryptogamic spores), epithelium-scales, mucus-corpuscles 
 and granules, and the so-called saliva corpuscles. Its reaction in 
 a healthy subject is alkaline, especially when the secretion is 
 abundant. When the saliva is scanty, or when the subject suffers 
 from dyspepsia, the reaction of the mouth may be acid. Saliva 
 contains but little solid matter, on an average probably about 
 ■5 p. c, the specific gravity varying from 1*002 to I"006. Of these 
 solids, rather less than half, about "2 p. c, are salts (including a 
 small quantity of potassium sulphocyanate). The organic bodies 
 which can be recognised in it are chiefly mucin, with small 
 quantities of globulin and serum-albumin. 
 
 The chief purpose served by the saliva in digestion is to 
 moisten the food, and to assist in mastication and deglutition. In 
 some animals this is its only function. In other animals and in 
 man it has a specific solvent action on some of the food-stuffs. 
 Such minerals as are soluble in slightly alkaline fluids are dis- 
 solved by it. On fats it has no effect save that of producing a 
 very feeble emulsion. On proteids it has also no action. Its 
 characteristic property is that of converting starch into some form 
 of sugar. 
 
f'livr. I.] DIGESTION. 2.35 
 
 Action of Saliva on Starch. 1 1' t.> a (|ii;iiit ity of boiled .stmvh. 
 
 ■which is always iiion- or less viscid and somewhat o))afiiU' or tiu'bid, 
 n small (quantity of saliva be atkhui, it will be found after a short 
 time that an important change has taken place, inasmnch as the 
 mixture has lost its previous viscidity aiul become thinner and 
 more transparent. In order to understand this change, the reader 
 must bear in mind the existence of the following bodies (described 
 more fully in the Appendix) all belonging to tlie class of carbo- 
 hydrates : 1. Starch, which forms with Avater not a true solution 
 but a more or less viscid mixture, and gives a characteristic l)lvie 
 colour with iodine. 2. Dextrin, differing from starch in forming a 
 clear solution and in giving a red colour with iodine. -3. Dextrose, 
 also called glucose or grape-sugar, giving no colouration with iodine, 
 but characterised by the power of reducing cupric and other 
 metallic salts; thus, when dextrose is boiled with a fluid often 
 known as Fehling's fluid, which is a solution of cupric suljihate Avith 
 an excess of sodium hydrate, the cupric salt is reduced and a red 
 or yellow deposit of cuprous oxide is thrown down. This reaction 
 serves with others as a convenient test for dextrose. Neither starch 
 nor dextrin, nor that commonest form of sugar known as cane-sugar, 
 give this reaction. 4. Maltose, very similar to dextrose, and like it 
 capable of reducing cupric salts. Besides having a slightly different 
 formula, it differs from dextrose, chiefly in its smaller reducing 
 power, i.e. a given quantity will not convert so much cupric oxide 
 into cuprous oxide as will the same weight of dextrose, and in having 
 a stronger rotatory action on rays of light (see Appendix). Besides 
 the above we may mention the peculiar body, achroodextrin, which 
 differs from dextrin in giving no colouration at all with iodine; and 
 the so-called soluble starch, which like dextrin forms a clear solution 
 with water, but unlike dextrin gives a blue colour with iodine. 
 
 Hence when a quantity of starch is boiled with water we may 
 recognize in the viscid imperfect solution, on the one hand the 
 presence of starch, by the blue colour which the addition of iodine 
 gives rise to, and on the other hand the absence of sugar (dextrose, 
 maltose), by the fact that when boiled with Fehling's fluid no 
 reduction takes place and no cuprous oxide is precipitated. 
 
 If however the boiled starch be submitted for a while to the 
 action of saliva, especially at a somewhat high temperature such as 
 35° or 40*0., it is found that the subsequent addition of iodine 
 gives no blue colour at all, or very much less colour, shewing that 
 the stai-ch has disappeared or diminished ; on the other hand the 
 mixture readily gives a precipitate of cuprous oxide when boiled 
 with Fehling's fluid, shewing that dextrose or maltose is present. 
 That is to say the saliva has converted the starch into dextrose 
 or maltose; and there are reasons, which we need not enter into 
 liere, for thinking that while some dextrose is formed the gi'eater 
 part of the sugar which appears is in the form of maltose. As 
 the conversion of the starch by the saliva is going on the addition 
 
236 SALIVA. [Book ii. 
 
 of iodine frequently gives rise to a red or violet colour instead of 
 a pure blue, but when the conversion is complete no colouration 
 at all is observed. The appearance of this red or violet colour 
 indicates the presence of dextrin. 
 
 The temporary appearance of dextrin shews that the action of 
 the saliva on the starch is somewhat complex; and this is still 
 further proved by the fact that even when the saliva has completed 
 its work the whole of the starch does not reappear as dextrose or 
 maltose. There are probably several other bodies formed out of 
 the starch besides these, the relative proportions varying according 
 to circumstances. The change therefore, though perhaps we may 
 speak of it in a general way as one of hydration, cannot be exhibited 
 under a simple formula, and we may rest content for the present 
 with the statement that starch when subjected to the action of 
 saliva is converted chiefly into the sugar known as maltose with a 
 comparatively small quantity of dextrose, dextrin appearing tempo- 
 rarily in the process, and other bodies on which we need not dwell 
 being formed at the same time. 
 
 Eaw unboiled starch undergoes a similar change but at a much 
 slower rate. This is due to the fact that in the curiously formed 
 starch grain the true starch, or gramdose, is invested with coats 
 of cellulose. This latter material, which requires previous treat- 
 ment with sulphuric acid before it will give the blue reaction, 
 on the addition of iodine, is apparently not acted upon by saliva. 
 Hence the saliva can only get at the granulose by traversing 
 the coats of cellulose, and the conversion of the former is thereby 
 much hindered and delayed. 
 
 The conversion of starch into sugar, and this we may speak of as 
 the amylolytic action of saliva, will go on at the ordinary tempera- 
 ture of the atmosphere. The lower the temperature the slower the 
 change, and at about 0° C. the conversion is indefinitely prolonged. 
 After exposure to this cold for even a considerable time the action 
 recommences v\^hen the temperature is again raised. Increase of 
 temperature up to about 35°— 40", or even a little higher, favours 
 the change, and the greatest activity is said to be ^vitnessed at 
 about 40°. Much beyond this, however, increase of temperature 
 becomes injurious, markedly so at 60° or 70° ; and saliva which 
 has been boiled for a few minutes not only has no_ action on starch 
 while at that temperature, but does not regain its powers on 
 cooling. By being boiled, the amylolytic activity of saliva is per- 
 manently destroyed. 
 
 The action of saliva on starch needs for its developraent a 
 slightly alkaline or at least a neutral reaction pf the mixture ; 
 it is hindered or arrested by a distinctly &cid reaction. Indeed the 
 presence of even a very small quantity of free acid, at all events 
 of hydrochloric acid, at the temperature of the body not only 
 suspends the action but speedily leads to permanent abolition of 
 the activity of the juice. The bearing of this will be seen later on. 
 
CiiAi'. I.] DIOESTIOX. 237 
 
 The action of saliva is hampered by the presen<!e in a concon- 
 tratocl state of the product of its own acti^ni, that is, of su^^ar. If 
 a small (piantil y of saliva be added to a thick mass of boiled starch, 
 the action willaCler a while slacken, and eventually come to almost 
 a staml-still ionL,^ before all the starch has been converted. On 
 dilutintj^ the nnxturo with water, the action will recommence. If 
 the proilucts of action be removed as soon as they are formed, a 
 small quantity of saliva will, if sufficient time be allowed, convert 
 into sugar a very larg'c, one might almost say an indefinite, 
 ([uantity (^f starch. Whether the particular constituent on which 
 the at'tivity of saliva dei)onds is at fill consumed in its action has 
 not at jtrescnt been definitely settled. 
 
 On what constituent do the amylolytic virtues of saliva depend 1 
 
 If saliva, filtered and thus freed from mucus and other formed 
 constituents, be treated with ten or fifteen times its bulk of alcohol, 
 a precipitate takes place containing besides other substances all 
 the proteid matters. Upon standing under the alcohol for some 
 time (several days, or, better, weeks), the proteids thus precipitated 
 become coagulated and insoluble in water. Hence, an aqueous 
 extract of the precipitate, made after this interval, contains very 
 little proteid material, and yet is exceedingly active. Moreover 
 by other more elaborate methods there may be obtained from 
 saliva solutions wdiich ajipear to be almost entirely free from 
 l^roteids and yet are intensely amylolytic. But even these probably 
 contain other bodies besides the really active constituent. AVhatever 
 the active substance be in itself, it exists in such extremely small 
 quantities, that it has never yet been satisfactorily isolated; and 
 indeed the only evidence w^e have of its existence is the manifesta- 
 tion of its peculiar powers. 
 
 The salient features of this body, which we may call pti/alin, are 
 then 1st, its presence in minute and almost inappreciable quantity. 
 L^nd, the close dependence of its activity on temperature. 3rd, its 
 permanent and total destruction by a high temperature and by 
 various chemical reagents. 4th, the want of any clear proof that it 
 itself undergoes any change during the manifestation of its powers; 
 that is to say, the energy necessary for the transformation which 
 it effects dues vot come out of itself; if it is at all used up m 
 its action, the loss is rather that of simple w^ear and teai- of a 
 machine, than that of a substance expended to do work. 5th, the 
 action wdiich it induces is probably of such a kind (splitting up 
 of a molecule w^ith assumption of water) as is effected by the agents 
 called catalytic, and by that particular class of catalytic agents 
 called hydrolytic. 
 
 These features mark out the amylolytic active body of saliva 
 as belonging to the class of ferments^- and we may henceforward 
 speak of the amylolytic ferment of saliva. 
 
 ^ Ferments may, for the present at least, be divided into two classcR, commonly 
 called organised an.l niwrga]iis>ed. Of the former, yeast may be taken as a well- 
 
238 SALIVA. [Book ii. 
 
 Mixed saliva, whose properties we have just discussed, is the 
 result of the mingling in various proportions of saliva from the 
 parotid, submaxillary, and sublingual glands with the secretion 
 from the buccal glands. These constituent juices have their own 
 special characters, and these are not the same in all animals. 
 Moreover in the same individual the secretion differs in composition 
 and properties according to circumstances ; thus, as we shall see in 
 detail hereafter, the saliva from the submaxillary gland secreted 
 under the influence of the chorda tympani nerve is very different 
 from that which is obtained from the same gland by stimulating 
 the sympathetic nerve. 
 
 In man pure parotid saliva may easily be obtained by introducing a 
 fine cannula into the opening o£ the Stenonian duct, and submaxillary 
 saliva, or rather a mixture of submaxillary and sublingual saliva, by 
 similar catheterisation of the Whartonian duct. In animals the duct 
 may be dissected out and a cannula introduced. 
 
 Parotid saliva in man is clear and limpid, not viscid ; the reaction 
 of the first drops secreted is often acid, the succeeding portions, 
 at all events when the flow is at all copious, are alkaline ; that is 
 to say the natural secretion is alkaline, but this may be obscured 
 by acid changes taking place in the fluid which has been retained 
 in the duct. On standing, it becomes turbid from a precipitate of 
 calcic carbonate, due to an escape of carbonic acid. It contains 
 globulin and some other forms of albumin, with little or no mucin. 
 Potassium sulphocyanate may also sometimes be detected, but 
 structural elements are absent. 
 
 Submaxillary saliva, in man and in most animals, differs from 
 parotid saliva in being more alkaline and, from the presence of 
 mucus, more viscid; it contains, often in abundance, salivary 
 corpuscles, and amorphous masses of proteid material. The so- 
 called chorda saliva in the dog, of which we shall presently speak, is 
 under ordinary circumstances thinner and less viscid, contains less 
 mucus, and fewer structural elements, than the so-called sympa^ 
 thetic saliva, which is remarkable for its viscidity, its structural 
 elements, and for its larger total of solids. 
 
 Sublingual saliva is more viscid, and contains more mucin and 
 more total solids (in the dog 2'75 p. c), than even the submaxillary 
 saliva. 
 
 The action of saliva varies in intensity in different animals. 
 Thus in man, the jjig, the guinea-pig, and the rat, both parotid 
 and submaxillary and mixed saliva are amylolytic; the sub- 
 known example. The fermentative activity of yeast which leads to the conversion 
 of sugar into alcohol, is dependent on the life of the yeast-cell. Unless the yeast- 
 cell be living and functional, fermentation does not take place; when the yeast- 
 cell dies fermentation ceases ; and no substance obtained from yeast, by precipita- 
 tion with alcohol or otherwise, will give rise to alcoholic fermentation. The salivary 
 ferment belongs to the latter class; it is a substance, not a living organism 
 like yeast. 
 
CiiAi-. i] DIGESriOX. 2:?0 
 
 uiaxilliiry saliva boing in most cases more active than the parotid. 
 Jii the rabbit, ^vhilo the submaxillary saliva has scarcely any 
 action, that ut' the parotid is encri^etic. Tlu! saliva of the cat is 
 much less active than the above, and that of the do(( still less; 
 indeed the jjarotid saliva of the dog is wholly inert. In the hor.se, 
 sheep, and ox, the amylolytic powers of either mixed saliva, or (jf 
 any one of the constituent juices, are extremely feeble. 
 
 Where the saliva of any gland is active, an aqueous infusion of 
 the same gland is also active. The importance and bearing of this 
 statement will be .seen later on. From the aqueous infusion of 
 the gland, as from saliva itself, the ferment may be approximately 
 i.solated. In some cases at least some ferment may be extracteil 
 from the gland even when the secretion is itself inactive. 
 
 The readiest method indeed of preparing a highly amylolytic 
 liquid tolerably free from proteid and other impurities, is to mince 
 finely a gland known to have an active secretion, such for instance 
 as that of a rat, dehydrate it by allowing it to stand under absolute 
 alcohol for some days, and then, having poured off most of the 
 alcohol, and removed the remainder by evaporation at a low tempe- 
 rature, to cover the pieces of gland with strong glycerine. A 
 mere drop of such a glycerine extract rapidly converts starch into 
 su£far. 
 
 Gastric Juice. 
 
 There is no difficulty in obtaining what may fairly be con- 
 sidered as a normal saliva ; but there ai'e many obstacles in the 
 way of determining the normal characters of the secretion of the 
 stomach. WTien no food is taken the stomach is at rest and no 
 secretion takes place. When food is taken, the characters of the 
 gastric juice secreted are obscured by the food with which it is 
 mingled. The gastric membrane may it is true be artificially 
 stimulated, by touch for instance, and a secretion obtained. This 
 we may speak of as gastric juice, but it may be doubted whether 
 it ought to be considered as normal gastric juice. And indeed as 
 we shall see even the juice, which is poured into the stomach 
 during a meal, varies as digestion is going on. Hence the charac- 
 ters which we shall give of gastric juice must be considered as 
 having a general value only. 
 
 Gastric juice, obtained by artificial stimulation from the healthy 
 stomach of a fasting dog, by means of a gastric fistula, is a thin 
 almost colourless fluid with a sour taste and odour. 
 
 In the operation for gastric fistula, an incision is made througli the 
 abdominal walls, along the linea alba, the stomach is opened, and the 
 lips of the gastric wound securely sewn to those of the incision in the 
 abdominal walls. Union soon takes place, so that a permanent opening 
 
240 GASTRIC JUICE. [Book ii. 
 
 from tlie exterior into the inside of the stomach is established. A tube 
 of proper construction, introduced at the time of the operation, becomes 
 firmly secured in place by the contraction of healing. Through the 
 tube the contents of the stomach can be received, and the mucous 
 membrane stimulated at pleasure. 
 
 When obtained from a natural fistula in man, its specific 
 gravity has been found to differ little from that of water, varying 
 from 1001 to I'OIO, and the amount of solids present to be 
 correspondingly small. In animals, pure gastric juice seems to be 
 equally poor in solids, the higher estimates which some observers 
 have obtained being probably due to admixture with food, &c. 
 
 Of the solid matters present about half are inorganic salts, chiefly 
 alkaline (sodium) chlorides, with small quantities of phosphates. 
 The organic material consists of pepsin, a body to be described 
 immediately, mixed with other substances of undetermined nature. 
 In a healthy stomach gastric juice contains a very small quantity 
 only of mucus, unless some submaxillary saliva has been swallowed. 
 
 The reaction is distinctly acid, and the acidity is normally due 
 to free hydrochloric acid. This is shewn by various proofs, among 
 v/hich we may mention the fact that the amount of hydrochloric 
 acid is more than can be neutralized by the bases, and the excess 
 corresponds to the quantity of free acid present. Lactic and 
 butyric and other acids when present are secondary products, 
 arising either by their respective fermentations from articles of 
 food, or from the decomposition of their alkaline or other salts. In 
 man the amount of free hydrochloric acid in healthy juice may 
 be stated about '2 per cent., but in some animals it is jDrobably 
 higher. 
 
 On starch gastric juice has per se no effect whatever ; indeed 
 the acidity of the juice tends to weaken, or may be sufficient to 
 arrest and even destroy, the amylolytic action of any saliva with 
 which it may be mixed. 
 
 On dextrose healthy gastric juice has no effect. And its power 
 of inverting cane-sugar seems to be less than that of hydrochloric 
 acid diluted to the same degree of acidity as itself In an un- 
 healthy stomach however containing much mucus, the gastric 
 juice is very active in converting cane-sugar into dextrose. This 
 power seems to be due to the presence in the mucus of a special 
 ferment, analogous to, but quite distinct from, the ptyalin of 
 saliva. An excessive quantity of cane-sugar introduced into 
 the stomach causes a secretion of mucus, and hence provides for 
 its own conversion. 
 
 On fats gastric juice has at most a limited action. When 
 adipose tissue is eaten, the chief change which takes place in the 
 stomach is that the proteid and gelatiniferous envelopes of the 
 fat-cells are dissolved, and the fats set free. Though there is 
 experimental evidence that emulsion of fats to a certain extent 
 
Cn.w. I.] DIGESTIOX. 211 
 
 docs take placi' in IIk' s(niii;icli, llic n"rc;it. mass of the fat oC ;i iMc.'il 
 is nol i-o i'haiiL,^Ml. 
 
 Such luiiuTals as arc solulilc in fnu; liydnirliloiic ariil ai-i; for 
 tlio most part ilissolved ; though there is a dilTcreiicc in this and in 
 some other respects bi'tween gastric juice and simple free liydnj- 
 ehloric acid dihitetl with Avater to the same degree of acidity as the 
 juice, the presence cither of the pepsin or other bodies apparently 
 modifying the solvent action of the acid. 
 
 The essential property of gastric juice is the power of dissolving 
 proteiil matters, and of converting them into a substance called 
 peptone. 
 
 Action of gastric juice on proteids. The results are essen- 
 tially the same whether natural juice obtained by means of a 
 fistula or artiticial juice, i.e. an acid infusion of the mucous mem- 
 brane of the stomach, be used. 
 
 Artificial gastric juice may bo prepared hi any of the following 
 ways. 
 
 1. The mucous membrane of a pig's or dog's stomacli is removed 
 from the muscular coat, finely minced, rubbed in a mortar with 
 pounded glass and exti'acted with water. The aqueous extract filtered 
 and acidulated (it is in itself somewhat acid), until it has a free acidity 
 corresponding to "2 p. c. of hydrochloric a.cid, contains but little of the 
 products of digestion such as peptone, but is fairly potent. 
 
 2. The mucous membrane similarly prepared and minced, allowed 
 to digest at 35" C. in a large quantity of hydrochloric acid diluted 
 to -2 p. c. The greater part of the membrane disappears, shreds only 
 being left, and the somewhat opalescent liquid can be decanted and 
 filtered. The filtrate has powerful digestive (peptic) properties, but 
 contains a considerable amount of the products of digestion (peptone, 
 ifec), arising from the digestion of the mucous membrane itself. 
 
 3. From the mucous membrane, similarly pi-epared and minced, 
 the superfluous moisture is removed with blotting paper,. and the pieces 
 are thrown into a comparatively large quantity of concentrated 
 glycerine, and allowed to stand. The membrane may be previously 
 dehydrated by being allowed to stand under alcohol, but this is not 
 necessary. The decanted clear glycerine, in which a comparatively 
 small quantity of the ordinary proteids of the mucous membrane are 
 dissolved, if added to hydrochloric acid of '2 p. c. (about 1 c.c. of 
 glycerine to 100 c.c. of the dilute acid are sutficient), makes an artificial 
 juice tolerably free from ordinary proteids and peptone, and of remark- 
 able potency, the presence of the glycerine not interfering with the 
 results. 
 
 If a few shreds of fibrin, obtained by wliipping blood, after 
 being thoroughly washed and boiled, be thrown into a quantity of 
 gastric juice, and the mixture be exposed to a temperature of from 
 
 ^ TliGse liov.-evcr may be removed by conccutiation at 10" C, and subsequent 
 dialysis. 
 
 F. IG 
 
242 GASTRIC JUICE. [Book ii. 
 
 35" to 40" C, the fibrin will speedily, in some cases in a few 
 minutes, be dissolved. The shreds first swell up and become 
 transparent, then gradually dissolve, being especially liable to fall 
 to pieces into flakes when the vessel containing them is shaken, 
 and finally disappear with the exception of some granular debris, 
 the amount of which, though generally small, varies according to 
 circumstances. 
 
 If small morsels of coagulated albumin, such as white of egg, be 
 treated in the same way, the same solution is observed. The 
 pieces become transparent at their surfaces ; this is especially seen 
 at the edges, which gradually become rounded down; and solution 
 steadily pro^i^sses from the outside of the pieces inwards. 
 
 If any other form of coagulated albumin {e.g. precipitated 
 acid- or alkali-albumin, suspended in water and boiled) be treated 
 in the same way, a similar solution takes place. The readiness 
 with which the solution is effected, will depend, ceteris paribus, 
 on the smallness of the pieces, or rather on the amount of surface 
 as compared with bulk, which is presented to the action of the juice. 
 
 Gastric juice then readily dissolves coagulated proteids, which 
 otherwise are insoluble, or soluble only, and that with difficulty, in 
 very strong acids. 
 
 Nature of the change as shewn by the products of the action. 
 
 If raw white of egg, largely diluted with water and strained, be 
 treated with a sufficient quantity of dilute hydrochloric acid, the 
 opalescence or turbidity which appeared in the white of egg on 
 dilution, and which is due to the precipitation of various forms of 
 globulin, disappears, and a clear mixture results. If a portion of 
 the mixture be at once boiled, a large deposit of coagulated albu- 
 min occurs. If, however, the mixture be exposed to 50° or 55" C. 
 for some time, the amount of coagulation which is produced by 
 boiling a specimen becomes less, and, finally, boiling produces no 
 coagulation whatever. By neutralisation, however, the whole of 
 the albumin (with such restrictions as the presence of certain 
 neutral salts may cause) may be obtained in the form of acid- 
 albumin or syntonin, the filtrate after neutralisation containing no 
 proteids at all (or a very small quantity). Thus the whole of the 
 albumin present in the white of egg is converted, by the simple 
 action of dilute hydrochloric acid, into acid-albumin or syntonin. 
 
 If the same white of egg be treated with gastric juice instead of 
 simple dilute hydrochloric acid, the events for some time seem the 
 same. Thus after a while boiling causes no coagulation, while 
 neutralisation gives a considerable precipitate of a proteid body, 
 which, being insoluble in water and in dilute sodium chloride 
 solutions, and soluble in dilute alkali and acids, at least closely 
 resembles syntonin. But it is found that only a portion of the 
 proteids originally present in the white of egg can thus be regained 
 by precipitation. A great deal is still retained in the filtrate after 
 
CiiAP. i] DIGEST/OX. 2\?, 
 
 neutralisation, in {\n\ lona of what is calloil peptone, aw], on thn 
 whole, the loiiLfcr the (iii^estion is ciirriucl on, the greater is the; 
 ])roj)ortion borne by the ])eptune to the i)recipitato thrown df)wn 
 on neutralisation; indeed, in some cases at all events, all the 
 protcids are brought into the condition of"pej)tono. 
 
 Peptone is a protcid, having the same approximate elementary 
 composition as other protcids, and giving most of the usual protoid 
 reactions. 
 
 It is distinguislu'd Thmu oilier protcids by the f(jllowing marked 
 features : 
 
 1st. Though soluble in distilled Avater and in neutral saline 
 solutions, even the most dilute, and therefore not precipitated 
 from its acid or alkaline solutions by neutralisation, it is not, like 
 the other similarly soluble protcids, coagulated by heat. 
 
 2nd. It is ditfusible, passing through membranes with consider- 
 able ease. The diffusion is not so rapid as that of salts, sugar, and 
 (.'ther similar substances, but is very marked as compared Avith that 
 of other protcids ; these pass through membranes with the greatest 
 ditliculty. (For the other less important reactions see Appendix.) 
 
 The neutralisation precipitate resembles, in its general charac- 
 ters, acid-albumin or syntonin. Since, hoAvever, it probably is 
 distinguishable from the body or bodies produced by the action of 
 simple acid on muscle or white of egg, it is best to reserve for it 
 the name of piarapeptone. Thus the digestion by gastric juice of 
 Avhite of egg results in the conversion of all the protcids present 
 into peptone and parapeptone, of Avhich the former must be 
 considered as the final and chief product, the latter a bye product 
 or initial j^roduct of variable occurrence and importance. The 
 gastric digestion of fibrin, either raw or boiled, and of all foims of 
 coagulated albumin, gives rise to the same products, peptone and 
 parajjeptone. Milk Avhen treated Avith gastric juice is first of all 
 coagulated or curdled. This is the result partly of the action of 
 the free acid but chiefly of the special action of a particular 
 constituent of gastric juice, of Avhich Ave shall speak hereafter. 
 The coagulum consists of a protcid, casein, and of fat ; and the casein 
 is subsequently dissolved Avith the same appearance of peptone and 
 parapeptone as in the case of other protcids. In fact, the digestion 
 by gastric juice of all the varieties of protcids consists in the con- 
 version of the protcid into peptone, Avith the concomitant a^jpear- 
 ance of a certain variable amount of parapeptone. 
 
 Circumstances affecting gastric digestion. The solvent action 
 of gastric juice on protcids is modified by a variety of circum- 
 stances. The nature of the proteid itself makes a difference, 
 though this is determined probably by physical rather than by 
 chemical characters. Hence in making a series of comparative 
 
 IC— 2 
 
244 GASTRIC JUICE. [Book ir. 
 
 trials the same proteid should be used, and the form of proteid 
 most convenient for the purpose is fibrin. If it be desired simply 
 to ascertain whether any given specimen has any digestive powers 
 at all, it is best to use boiled fibrin, since raw fibrin is eventually 
 dissolved by dilute hydrochloric acid alone, probably on account of 
 some pepsin present in the blood becoming entangled with the fibrin 
 during coagulation. But in estimating quantitatively the peptic 
 power of two specimens of gastric juice under different conditions, 
 raw fibrin prepared by Griitzner's method is the most convenient. 
 
 Portions of well washed fibrin are stained with carmine and again 
 washed to remove the superfluous colouring matter. A fragment of 
 this coloured fibrin thrown into an active juice on becoming dissolved, 
 gives up its colour to the fluid, and if the same stock of coloured fibrin 
 be used in a series of experiments, the amount of fibrin dissolved may 
 be fairly estimated by the depth of tint given to the fluid. Fibrin thus 
 coloured with carmine may be preser^-ed in ether. 
 
 Since, if sufficient time be allowed, even a small quantity of 
 gastric juice will dissolve at least a very large if not an indefinite 
 quantity of fibrin, we are led to take, as a measure of the activity 
 of a specimen of gastric juice, not the quantity of fibrin which it 
 will ultimately dissolve, but the rapidity with which it dissolves a 
 given quantity. 
 
 The greater the surface presented to the action of the juice, the 
 more rapid the solution ; hence minute division and constant move- 
 ment favour digestion. And this is probably, in part at least, the 
 reason why a fragment of spongy filamentous fibrin is more readily 
 dissolved than a solid clump of boiled white of egg of the same size. 
 Neutralisation of the juice wholly arrests digestion ; fibrin may be 
 submitted for an almost indefinite time to the action of neutralised 
 gastric juice without being digested. If the neutralised juice be 
 properly acidified, it may again become active ; in gastric juice 
 however which has been made alkaline, and kept at a temperature 
 of So**, the solvent powers are not only suspended but actually 
 destroyed. Digestion is most rapid with dilute hydrochloric acid 
 of "2 p.c. (the acidity of natural gastric juice). If the juice con- 
 tains much more or much less free acid than this, its activity is 
 visibly impaired. Other acids, lactic, jDhosphoric, &c. may be sub- 
 stituted for hydrochloric ; but they are not so effectual, and the 
 degree of acidity most useful varies with the different acids. The 
 presence of neutral salts, such as sodium chloride, in excess is 
 injurious. The action of mammalian gastric juice is most rapid at 
 35" — 40" C. ; at the ordinary temperature it is much slower, and at 
 about 0° C. ceases altogether. The juice may be kept however at 
 0"G. for an indefinite period without injury to its powers. The 
 gastric juice of cold-blooded vertebrates is relatively more active 
 at low temperatures than that of warm-blooded mammals or birds. 
 
 At temperatures much above 40" or 45" the action of the juice 
 
CiiAi'. 1.] DIGESTIOX. 245 
 
 is impaired. By boiling for a few minutes the activity of the most 
 powerful juice is irrevocably destroyed. The presence in a cou- 
 centratetl form of the ])roducts of dif(estion hinders the prr)cess. 
 If a large quantity of fibrin be placed in a small quantity of juice, 
 digestion is soon arrested; on dilution with the normal hvdro- 
 chloric acid (2 p.c), or if the mixture be submitted to dialysis to 
 remove the peptones formed, and its acidity be kept up to the 
 normal, the action recommences. By removing the products of di- 
 gestion as fiist as they arc formed, and by keeping up the acidity 
 to the normal, a given amount of gastric juice may be made to 
 digest a very large quantity of proteid material. Whether the 
 quantity is really unlimited is disputed ; but in any case the ener- 
 gies of the juice are not rapidly exhausted by the act of digestion. 
 
 Nature of the action. All these facts go to shew that the 
 digestive action of gastric juice on proteids, like that of saliva on 
 starch, is a ferment-action ; in other words, that the solvent action 
 of gastric juice is essentially due to the presence in it of a ferment- 
 body. To this ferment-body, which as yet has been only ap- 
 proximately isolated, the name of pepsin has been given. It is 
 present not only in gastric juice but also in the glands of the 
 gastric mucous membrane, especially in certain parts, and under 
 certain conditions which we shall study presently. The glycerine 
 extract of gastric mucous membrane, especially of that which has 
 been dehydrated, contains a minimal quantity of proteid matter, 
 and yet is intensely active. Other methods, such as the elaborate 
 one of Briicke, crive us a material which, thousfh containinsf nitrogen, 
 exhibits none of the ordinary proteid reactions, and yet in concert 
 with normal dilute hydrochloric acid is peptic in the highest 
 degree. We seem therefore justified in asserting that pepsin is not 
 a proteid, but it would be hazardous to make any dogmatic state- 
 ment concerning a substance, obtained in small quantity only, 
 probably mixed with other bodies, and the chemical characters of 
 which we know as yet very little. A± present the manifestation 
 of peptic powers is our only safe test of the presence of pepsin. 
 
 In one important respect pepsin, the ferment of gastric juice, 
 differs from ptyalin, the ferment of saliva. Saliva is active in a per- 
 fectly neutral medium, and there seems to be no special connection 
 between the ferment and any alkali or acid. In gastric juice, 
 however, there is a strong tie between the acid and the ferment, 
 so strong that some writers speak of pepsin and hydrochloric 
 acid as forming together a compound, popto-hydrochloric acid. 
 
 In the absence of exact knowle- Ige of the constitution of 
 proteids, we cannot state distinctly what is the precise nature of 
 the change into peptone. Judging from the analogy with the 
 action of saliva on starch, we may fairly suppose that the process 
 is at bottom one of hydration ; but we have no exact proof of 
 this, and it is at least quite as probable that peptone arises by a 
 
;246 GASTRIC JUICE. [Book ii. 
 
 simple splitting up of larger proteid molecules. Peptone closely 
 .resembling, if not identical with, that obtained by gastric di- 
 gestion, may be obtained by the action of strong acids, by the 
 prolonged action of dilute acids especially at a high temperature, 
 ox simply by digestion with super-heated water in a Papin's 
 digester. The role of pepsin therefore is only to facilitate a change 
 which may be effected without it. 
 
 All proteids, so far as we know, are converted by pepsin into 
 peptone. Of its action on other nitrogenous substances not truly 
 proteid in nature, we need only say that mucin, nuclein, and the 
 chemical basis of homy tissues are wholly unaffected by it, but 
 that the gelatiniferous tissues are dissolved and changed into a 
 substance so far analogous with peptone, that the characteristic 
 property of gelatinisation is entirely lost. Chondrin and the 
 elastic tissues are also dissolved. 
 
 Action of gastric juice on milk. It has long been known that 
 an infusion of calves' stomach, called rennet, has a remarkable 
 effect in rapidly curdling milk, and this property is made use of in 
 the manufacture of cheese. Gastric juice has a similar effect ; 
 milk when subjected to the action of gastric juice is first curdled 
 and then digested. If a few drops of gastric juice be added to a 
 little milk in a test tube, and the mixture exposed to a tempera- 
 ture of 40°, the milk will curdle into a complete clot in a very 
 short time. If the action be continued the curd or clot will be 
 ultimately dissolved and digested. Milk contains, besides albumin, 
 fats, milk, sugar and various salines, a peculiar proteid called 
 casein^, a body allied to the so-called alkali-albumin. In natural 
 milk casein is present in solution, and ' curdling ' consists essen- 
 tially in the casein becoming insoluble and being precipitated in a 
 solid form, a great deal of the fat being generally carried down 
 with it. Now casein is readily precipitated from milk upon the 
 addition of a small quantity of acid, and it might be supposed that 
 the curdling effect of gastric juice was due to its acid reaction. 
 But this is not the case, for neutralized gastric juice, or neutral 
 rennet, is equally efficacious. Moreover the substance thrown down 
 by an acid is not quite exactly the same as that which ajDpears in 
 curdling. 
 
 The effect is closely dependent on temperature, being like the 
 peptic action of gastric juice favoured by a rise of temperature up 
 to about 40". Moreover the curdling action is destroyed by 
 previous boiling of the juice or rennet. These facts suggest that a 
 ferment is at the bottom of the matter; and indeed, all the 
 features of the action support this view. The ferment however is 
 not pepsin but some other body; and the two may be separated 
 by cautiously adding magnesium carbonate to gastric juice or to 
 an infiision of calves' stomach. The clear fluid, left above the pre- 
 
 1 See Appendix. 
 
UiiAP. 1.] DIGESTION. 217 
 
 ripitatc thus fi)rmod, readily curdles milk, but even when acidified 
 li;is MO pej)tic action on proteids, shewint,' that the precipitate caused 
 by the addition ot" the niaf^nesiuin carbonate has carried down all 
 tlie pe})sin but left behind at least a good deal of the rennet-fcniieiit. 
 
 llennet-fermcnt seems to be present in variable quantity in the 
 givstric juice of most animals, and may also be obtained from the 
 gastric mucous membrane of many though not all animals. It is 
 especially abundant in the stomach of the calf. 
 
 It has been suggested that the ferment might act by inducing 
 a fermentation in the sugar of milk, giving rise to lactic acid, 
 which precipitates the casein by virtue of its being an acid. But 
 this view is disproved not only by the difference in the product 
 mentioned above, but also by the fact that casein precipitated 
 from milk by neutral salts, Avashed free from milk sugar and 
 redissolvcd, forms a fluid which is readily curdled by rennet like 
 natural milk. It seems probable that the ferment really acts on the 
 casein, converting it in some way from a soluble to an insoluble form. 
 
 Bile. 
 
 The quality of bile varies much, not only in different animals, 
 but in the same animal at different times. It is moreover affected 
 by the length of the sojourn in the gall-bladder ; bile taken direct 
 from the hepatic duct, especially when secreted rapidly, contains 
 little or no mucus; that taken from the gall-bladder, as of 
 slaughtered oxen or sheep, is loaded with mucus. The colour of 
 the bile of carnivorous and omnivorous animals, and of man, is a 
 bright golden red : of graminivorous animals, a golden green, or a 
 bright green, or a dirty green, according to circumstances, being 
 much modified by retention in the gall-bladder. The reaction is 
 alkaline. The following may be taken as the average composition 
 of human bile (Frerichs).Ji;^ 
 
 lu 1000 parts. 
 
 Water 859-2 
 
 Solids : — 
 Bile Salts 91-4 
 
 Fats, &c. ... 
 Cholesterin 
 Mucus and Pigment 
 Inorganic Salts 
 
 9-2 
 
 2-6 
 
 29-8 
 
 7-8 
 
 140-8 
 
 The entire absence of proteids is a marked feature of bile. 
 With regard to the inorganic salts, the points of interest are the 
 presence of a large quantity of sodium chloride (-2 to "27 per cent.), 
 the presence of phosphates, of iron (about -006 p. c), manganese, 
 and occasionally, at all events, of copper. The ash contains soda in 
 
 
 02. 
 
248 BILE. [Book ii. 
 
 a very large amount, and also sulphates, both coming from the 
 bile-salts. The peculiar body cholesterin is conspicuous by its 
 quantity and constancy, but its physiological functions are obscure. 
 The constituents which deserve chief attention are the pigments 
 and the bile-salts. 
 
 Pigments of Bile. The natural golden red colour of normal 
 human or carnivorous bile, is due to the presence of Bilirubin. 
 This, which is also the chief pigmentary constituent of gall-stones, 
 and occurs largely in the urine of jaundice, may be obtained in the 
 form either of an orange-coloured powder, or of well-formed 
 rhombic tablets and prisms. Insoluble in water, and but little 
 soluble in ether and alcohol, it is readily soluble in chloroform, and 
 in alkaline fluids. Its composition is CjgHjgN^Og. Treated with 
 oxidizing agents, such as nitric acid yellow with nitrous acid, it 
 displays a succession of colours in the order of the spectrum. The 
 yellowish golden red becomes green, this a greenish blue, then 
 blue, next violet, afterwards a dirty red, and finally a pale yellow. 
 This characteristic reaction of bilirubin is the basis of the so-called 
 Gmelin's test for bile-pigments. Each of these stages represents a 
 distinct pigmentary substance. An alkaline solution of bilirubin, 
 exposed in a shallow vessel to the action of the air, turns green, 
 becoming converted into Biliverdin (Cj^H^gNgO. or Cj^Hj^NgO^ 
 Maly), the green pigment of herbivorous bile. Biliverdin is also 
 found at times in the urine of jaundice, and is probably the body 
 which gives to bile which has been exposed to the action of gastric 
 juice, as in biliary vomits, its characteristic green hue. It is the 
 first stage of the oxidation of bilirubin in Gmelin's test. Treated 
 with oxidizing agents biliverdin runs through the same series of 
 colours as bilirubin, with the exception of the initial golden red. 
 
 The bile-salts. These consist, in man and many animals, of 
 sodium glycocholate and taurocholate : the proportion of the two 
 varying in different animals. In man both the total quantity of 
 bile-salts and the proportion of the one bile-salt to the other seem 
 to vary a good deal, but the glycocholate is said to be always the 
 more abundant. In ox-gall, sodium glycocholate is abundant, and 
 taurocholate scanty. The bile-salts of the dog, cat, bear, and other 
 camivora, consist exclusively of the latter. 
 
 Insoluble in ether but soluble in alcohol and in water, the 
 aqueous solutions having a decided alkaline reaction, both salts 
 may be obtained by crystallisation in fine acicular needles. They 
 are exceedingly deliquescent. The solutions of both acids have a 
 dextro-rotatory action on polarized light. 
 
 Preparation, Bile, mixed with animal charcoal, is evaporated to 
 dryness and extracted with alcohol. If not colourless, the alcoholic 
 filtrate must be further decolorized with animal charcoal, and the 
 
Cu.vr. I.J DIGESTION. 2i0 
 
 alcohol distilh'd olF. The dry vositluo is treated with absolute alcohol, 
 and to the alcoholic liltrato anhydrous ether is added as lonj^ as any 
 precipitate is formed. On stiindin;^ the cloudy precij)itate become.s 
 transformed into a crystalline mass at the bottom of the vessel. If the 
 alcohol be not absolute, the crystals are very apt to be changed into a 
 thick syrupy iluid. Tliis mass of crystals has been often spoken of as 
 b'din. Both salts are thus precipitated, so that in such a l)ile as that of 
 the ox or man bilin consists both of sodium glycocholate and sodium 
 taurocholate. The two may be separated l)y precipitation from their 
 aqueous solutions with sugar of lead, which throws down the former 
 much more readily than the latter. The acids may be separated from 
 their respective salts by dilute sulphuric acid, or by the action of lead- 
 acetate and suljjhydric acid. 
 
 On boiling with dilute acids (sulphuric, hydrochloric), or caustic 
 potash, or baryta -water, glycocholic acid is split up into cholalic 
 (cholic) acid and glycin. Taurocholic acid may similarly be split 
 up into cholalic acid and taurin. Thus 
 
 glvcocLolic acid cholalic acid glycin 
 
 C.H„NO., + HP = C,,H,„0, + C ANO, 
 
 taurocholic acid cholalic acid taurin 
 
 C,,H,,NSO, + HP = a,H,p, -f C,H,NS03. 
 
 Both acids contain the same non-nitrogenous acid, cholalic acid ; 
 but this acid is in the first case associated or conjugated with the 
 important nitrogenous body glycin, or amido-acetic acid, that is a 
 compound formed out of ammonia, and one of the series of fatty 
 acids viz. acetic ; and in the second case with taurin, or amido- 
 isethionic acid, that is a compound formed out of ammonia, a 
 member of the ethyl group, and sulphuric acid. The decom- 
 position of the bile acids into cholalic acid and taurin or glycin 
 respectively takes place naturally in the intestine, the glycin and 
 taurin being absorbed, so that from the two acids, after they have 
 served their purpose in digestion, the two ammonia compounds 
 are returned into the blood. Either of the two acids, or cholalic 
 acid alone, when treated with sulphuric acid and cane-sugar, gives 
 a magnificent purple colour (Pettenkofer's test) with a character- 
 istic spectrum. A similar colour may often be produced by the 
 action of the same bodies on albumin, amyl alcohol, and some 
 other organic bodies. 
 
 Action of Bile on Food. In some animals at least bile contains 
 a ferment capable of converting starch into sugar ; but its action 
 in this respect is wholly subordinate. 
 
 On proteids bile has no direct digestive action whatever, but 
 since it is at least often alkaline it tends to neutralise the acid 
 contents of the stomach as they pass into the duodenum and so 
 prepares the way for the action of the pancreatic juice. To peptic 
 action it is distinctly antagonistic; the presence of a sufficient 
 quantity of bile renders gastric juice inert toAvards proteids. ^More- 
 
250 PANCREATIC JUICE. [Book ii. 
 
 over when bile, or a solution of bile-salts, is added to a fluid con- 
 taining the products of gastric digestion, a precipitate takes place, 
 consisting of parapeptone, peptone and bile salts. The precipitate 
 however is redissolved in an excess of bile or solution of bile-salts. 
 Concerning the purpose of this precipitation, which actually takes 
 place in the duodenum, we shall speak hereafter. 
 
 With regard to the action of bile on fats, the following state- 
 ments may be made : 
 
 Bile has a slight solvent action on fats, as seen in its use by 
 painters. It has by itself a slight but only slight emulsifjdng 
 power: a mixture of oil and bile separate after shaking rather 
 less rapidly than a mixture of oil and water. With free fatty acids, 
 bile forms soaps. It is moreover a solvent of solid soaps, and it 
 would appear that the emulsion of fats is under certain circum- 
 stances at all events facilitated by the presence of soaps in solution. 
 Hence bile is probably of much greater use as an emulsion agent 
 when mixed with pancreatic juice than when acting by itself alone. 
 To this point we shall return. Lastly, the passage of fats through 
 membranes is assisted by wetting the membranes with bile, or 
 with a solution of bile-salts. Oil passes with considerable ease 
 through a filter-paper kept wet with a solution of bile-salts, where- 
 as it passes with extreme difficulty through one kept constantly 
 wet with distilled water. 
 
 Lastly bile possesses so-called antiseptic qualities. Out of the 
 body its presence hinders various putrefactive processes ; and when 
 it is prevented from flowing into the alimentary canal, the contents 
 of the intestine undergo changes different from those which take 
 place under normal conditions, and leading to the appearance of 
 various products, especially of ill-smelling gases. 
 
 These various actions of bile seem to be dependent on the bile 
 salts and not on the pigmentary or other constituents. 
 
 Pancreatic Juice. 
 
 Natural healthy pancreatic juice obtained by means of a tem- 
 porary pancreatic fistula differs from the preceding fluids in the 
 comparatively large quantity of proteids which it contains. Its 
 composition varies according to the rate of secretion, for, with the 
 more rapid flow, the increase of total solids does not keep pace 
 with that of the water, though the ash remains remarkably 
 constant. 
 
 By an incision through the linea alba the pancreatic duct (or ducts) 
 can easily be found either in the rabbit or in the dog, and a cannula se- 
 cured in it. There is no difficulty about a temporary fistula; but 
 
Chap, i.] DHJESTIOS. 251 
 
 Uornarcl found tli;it witli permanent fistuhi! tlio sccrclioii alLcrod in 
 nature, ami lost many of its characteristic properties. Otliers, however, 
 have succeeded in ohtainiiig permaneiit listuhe without any impairment 
 of the secretion. 
 
 Healthy pancreatic juice is a clear viscid lluiil, frothing when 
 sliaken. It has a very decided alkaline reaction, and contains few 
 or no structural constituents, 
 
 The average amount of solids in the pancreatic juice of the dog 
 when obtained I'roni a temporary fistula is about 8 to 10 p. c. ; but 
 in the thoroughly active secretion from a permanent fi.stula it is 
 not more than about 2 to 5 p. c, *8 being inorganic matter, 
 and this is probably the normal amount. The important con- 
 stituents are albumin, a peculiar form of casein or alkali-albumin, 
 (precipitable by saturation with magnesium sulphate) peptone, 
 leucin and tyrosin, a small amount of fats and soaps, and a com- 
 paratively large quantity of sodium carbonate, to which the alkaline 
 reaction of the juice is due, and which seems to be peculiarly- 
 associated with the albumin. 
 
 Since, as we shall presently see, pancreatic juice contains a 
 ferment acting energetically on proteid matters in an alkaline 
 medium, it rajjidly digests itself; and, when kept, speedily changes 
 in character. Perfectly fresh juice appears to contain a substance 
 not unlike myosin giving rise to a sort of coagulation, but the 
 coagulum is soon dissolved. Perfectly fresh juice is also said to be 
 almost entirely free from leucin, tyrosin and peptone, which also 
 seem to be the products of self-digestion. 
 
 Action on food-stnfifs. On starch, raw or boiled, pancreatic 
 juice acts with great energy, rapidly converting it into sugar 
 (chiefly maltose). All that has been said in this respect con- 
 cerning saliva might be repeated in the case of pancreatic juice, 
 except that the activity of the latter is far greater than that of the 
 former. Pancreatic juice and the aqueous infusion of the gland 
 are always capable of converting starch into sugar, whether the 
 animal from which they were taken be starving or well fed. 
 From the juice, or, by the glycerine method, from the gland itself, 
 an amylolytic ferment may Idc approximately isolated. 
 
 On pi'oteids pancreatic juice also exercises a solvent action, so 
 far similar to that of gastric juice that by it proteids are converted 
 into peptone. If a few shreds of fibrin arc thrown into a small 
 (quantity of pancreatic juice, they speedily disappear, especially at 
 a temperature of 35° C., and the mixture is found to contain 
 peptone. The activity of the juice in thus converting jDroteids 
 into peptone, is favoured by increase of temperature up to 40° or 
 thereabouts, and hindered by low temperatures ; it is permanently 
 destroyed by boiling. The digestive powers of the juice in fact 
 depend, like those of gastric juice, on the presence of a ferment ; to 
 this ferment the name trypsin has been given. A glycerine extract 
 
252 PANCREATIC JUICE. [Book ii. 
 
 of pancreas, prepared in the same method as that of the gastric 
 mucous membrane, is (under appropriate conditions) active on 
 proteids, like the native juice. 
 
 The appearance of fibrin undergoing pancreatic digestion is 
 however different from that undergoing peptic digestion. In the 
 former case the fibrin does not swell up, but remains as opaque as 
 before, and appears to suffer corrosion rather than solution. But 
 there is a still more important distinction between pancreatic and 
 peptic digestion of proteids. Peptic digestion is essentially an 
 acid digestion ; we have seen that the action only takes place in 
 the presence of an acid, and is arrested by neutralisation. Pan- 
 creatic digestion, on the other hand, may be regarded as an alkaline 
 digestion ; the action is most energetic when some alkali is present; 
 and the activity of an alkaline juice is hindered or delayed by 
 neutralisation and arrested by acidification at least with mineral 
 acids. The glycerine extract of pancreas is under all circumstances 
 as inert in the presence of free mineral acid as that of the stomach 
 in the presence of alkalis. If the digestive mixture be supplied 
 with sodium carbonate to the extent of 1 p. c, digestion proceeds 
 rapidly, just as does a peptic mixture when acidulated with hydro- 
 chloric acid to the extent of "2 p. c. Sodium carbonate of 1 p. c. 
 seems in fact to play in pancreatic digestion a part altogether 
 comparable to that of hydrochloric acid of '2 p.c. in gastric di- 
 gestion. And just as pepsin is rajDidly destroyed by being heated 
 to about 40" ^vith a 1 p.c. solution of sodium carbonate, so trypsin 
 is rapidly destroyed by being similarly heated with dilute hydro- 
 chloric acid of "2 p.c. Alkaline bile, which arrests peptic digestion, 
 seems, if anything, favourable to pancreatic digestion. 
 
 Corresponding to this difference in the helpmate of the ferment, 
 there is in the two cases a difference in the nature of the products. 
 In both cases peptone is produced, and such differences as can be 
 detected between pancreatic and gastric peptones are comparatively 
 slight ; but in pancreatic digestion the bye-product is not, as in 
 gastric digestion, a kind of acid-albumin, but a body having more 
 analogy with alkali-albumin. Before solution has actually taken 
 place the fibrin becomes altered in character. It is soluble not 
 only in dilute acids and alkalis, but also in a 10 per cent, solution 
 of sodium chloride, and the solutions obtained by the latter reagent 
 are coagulable on boiling and on the addition of strong nitric acid. 
 The first action of the pancreatic juice therefore seems to be to 
 convert the proteid under digestion into a body intermediate 
 between alkali-albumin and ordinary native albumin. 
 
 But though the general characters of pancreatic and gastric 
 digestion are on the surface similar, it is more than probable 
 that profound differences do exist between them. This is shewn 
 by the appearance, in the pancreatic digestion of proteids, of two 
 remarkable nitrogenous crystalline bodies, leucin and tyrosin. 
 When fibrin (or other proteid) is submitted to the action of 
 
Oiivr. i] DIGKSTIOX. 253 
 
 jniiK-n'atif juii-o, tlio amount ftf poptono which can be recovered 
 iVom the mixture lulls far short of the orif]^inal amount of proteids, 
 much more so than in the case of gastric juice ; and the longer the 
 digestive action, the greater is this aj)parent loss. If a pancreatic 
 digestion mixture be freed from the alkali-albumin by neutral- 
 isation, and after concentration by evaporation be treated with 
 excess of alcohol, most of the peptone will be precipitated. The 
 alcoholic liltrato when concentrated, gives, on cooling, crystals of 
 tyrosin, and the mother liquor from these crystals will afford 
 abundance of crystals of leucin. Thus by the action of the pan- 
 creatic juice a considerable amount of the proteid, which is being 
 digested, is so broken up as to give rise to products which are no 
 longer proteid in nature. From this breaking up of the proteid 
 there arise leucin, tyrosin, and j^robably several other bodies, such 
 as fatty acids and volatile substances. 
 
 As is well known, leucin and tyrosin are the bodies which 
 make their appearance when proteids or gelatin are acted on by 
 dilute acids, alkalis, or various oxidising agents. Now leucin is 
 amido-caproic acid, and thus belongs distinctly to the fatty bodies ; 
 while tyrosin is a member of the aromatic group, being closely 
 related to benzoic acid. So that in pancreatic digestion we have 
 the large complex proteid molecule split up into its constituent 
 fatty acid and aromatic molecules, and into its other less distinctly 
 kno\N'n components. In gastric digestion such a profound destruc- 
 tion of proteid material occurs to a much less extent or not at all ; 
 neither leucin nor tyrosin can at present be considered as natural 
 products of the action of pepsin. 
 
 Among the supplementary products of pancreatic digestion 
 may be enumerated a body Avhich gives a violet colour with 
 chlorine water (this reaction is often seen in the juice itself), and 
 indol, to which apparently the strong and peculiarly faecal odour 
 which makes its appearance during pancreatic digestion is 
 due. Indol, how^ever, unlike the leucin and tyrosin, is not a 
 product of pure pancreatic digestion, but of an accompanying 
 decomposition due to the action of organised ferments. A pan- 
 creatic digestive mixture soon becomes swarming with bacteria, in 
 spite of careful precautions, when natural juice or an infusion of 
 the gland is used. When isolated ferment is used, and atmospheric 
 germs are excluded, or when pancreatic digestion is canied on in 
 the presence of salicylic acid, which prevents the develoj^ment of 
 bacteria and like organisms but permits the action of the trypsin, 
 no odour is perceived, and no indol is produced. 
 
 After long-continued digestion, especially when accompanied by 
 putrefactive decomposition, the amount of proteids which are 
 carried beyond the peptone stage and broken up, may be very 
 great. 
 
 On the gelatiniferous elements of the tissues in their normal 
 condition pancreatic juice appears to have no solvent action. In 
 
254 PANCREATIC JUICE. [Book ii. 
 
 this respect it affords a striking contrast to gastric juice. But 
 when tliey have been previously treated with acid or boiled so 
 as to become converted into actual gelatine, trypsin is able to 
 dissolve them, apparently changing them much in the same way 
 as does pepsin. Trypsin unlike pepsin, will dissolve mucin. Like 
 pepsin, it is inert towards nuclein, horny tissues, and the so-called 
 amyloid matter. 
 
 On Fats pancreatic juice has a twofold action: it emulsifies 
 them, and it splits up neutral fats into their respective acids and 
 glycerine. If hog's lard be gently heated till it melts and be then 
 mixed with pancreatic juice before it solidifies on cooling, a creamy 
 emulsion, lasting for almost an indefinite time, is formed. So also 
 when olive oil is shaken up with pancreatic juice, the separation 
 of the two fluids takes place very slowly, and a drop of the mixture 
 under the microscope shews that the division of the fat is very 
 minute. An alkaline aqueous infusion of the gland has similar 
 emulsifying powers. If perfectly neutral fat be treated with 
 pancreatic juice, especially at the body-temperature, the emulsion 
 speedil}^ takes on an acid reaction, and by appropriate means not, 
 only the corresponding fatty acids but glycerine may be obtained 
 from the mixture. When an alkali is present, the fatty acids thus 
 set free form their corresponding soaps. Pancreatic juice contains 
 fats, and is consequently apt after collection to have its alkalinity 
 reduced; and an aqueous infusion of a pancreatic gland (which 
 always contains a considerable amount of fat) very speedily becomes 
 acid. 
 
 Thus pancreatic juice is remarkable for the power it possesses 
 of acting on all the food-stuffs, on starch, fats and proteids. 
 
 The action on starch and on proteids, and the splitting up of 
 fatty acids appear to be due to the presence of three distinct 
 ferments, and methods have been suggested for isolating them. 
 The emulsifying power, on the other hand, is connected with the 
 general composition of the juice (or of the aqueous infusion of the 
 gland), being probably in large measure dependent on the alkali- 
 albumin present. The proteolytic ferment trypsin as ordinarily 
 prepared seems to be proteid in nature and capable of giving rise, 
 by digestion to peptones; but it may be doubted, as in the case of 
 pepsin &c. whether the pure ferment has yet been isolated. There 
 are no means of distinguishing the amylolytic ferment of the 
 pancreas from ptyalin. The term pancreatin has been variously 
 applied to many different preparations from the gland, and its use 
 had perhaps better be avoided. 
 
 The action of pancreatic juice, or of the infusion or extract of 
 the gland, on starch, is seen under all circumstances, whether the 
 animal be fasting or not. The same may probably be said of the 
 action on fats. On proteids the natural juice, when secreted in a 
 normal state, is always active. The glycerine extract or aqueous 
 infusion of the gland, on the contrary, differs at different times; 
 
Chap. j.J DIGESTION. 255 
 
 prepared from an animal some 4 to 10 hours after food has been 
 taki-n, it is viay powerful; prepared from a fasting animal, it is 
 said to exhibit searcely any action at all. To this point howcvt-r 
 we shall return immediately. 
 
 Succus Entenciis. 
 
 When, in a living animal, a portion of the small intestine is 
 ligatured, so that the secretions coming down from above cannot 
 enter its canal, while yet the blood-supply is maintained as usual, 
 a small amount of secretion collects in its interior. This is spoken 
 of as the succus entericus, and is supposed to be furnished by the 
 glands of Liebcrklihn. We h-ave no exact knowledge however as 
 to the extent to which such a secretion takes place under normal 
 circumstances; and the statements with regard to its action are 
 conflicting. Probably it has no direct action on either fats or 
 proteids; but is amylolytic in some animals, though not in all. 
 
 A small quantity of fluid free from bile, gastric or pancreatic 
 juice, and wliich may be considered as pure succus entericus, may 
 also be obtained by the following method known as that of Tliiry. 
 The small intestine is divided in two places at some distance apart. 
 By fine sutures the lower end of the upper section is united with the 
 upper end of the lower section, thus as it were cutting out a whole piece 
 of the small intestine from the alimentary tract. In successful cases, 
 union between the cut surfaces takes place, and a shortened but other- 
 wise satisfactory canal is re-established. Of the isolated piece the lower 
 end is carefully closed by sutures, while the upper is brought to the 
 wound in the abdominal wall and secured there. A fistula is thus 
 formed, leading into a short piece of intestine quite isolated from tlie 
 rest of the alimentary canal. 
 
 Succus entericus has also been said to change cane- into grape- 
 sugar, and by a fermentative action to convert cane-sugar into 
 lactic acid, and this again into butyric acid with the evolution of 
 carbonic acid and free hydrogen. 
 
 Of the possible action of other secretions of the alimentary 
 canal, as of the caecum and large intestine, we shall speak when we 
 come to consider the changes in the alimentary canal. 
 
 Concerning the secretion of Brunner's glands our information 
 is at present imperfect. The cells of the glands resemble the 
 central cells of the gastric glands; and an extract of the gland 
 is said to digest fibrin in an acid solution, but to have no distinct 
 amylolytic action. 
 
SEC. 2. THE ACT OF SECRETION IN THE CASE OF THE 
 
 DIGESTIVE JUICES AND THE NERVOUS MECHANISMS 
 
 WHICH REGULATE IT. 
 
 The various juices whose properties we have just studied, 
 though so different from each other, are all drawn ultimately from 
 one common source, the blood, and they are poured into the ali- 
 mentary canal, not in a continuous flow, but intermittently as 
 occasion may demand. The epithelium cells which supply them 
 have their periods of rest and of activity, and the amount and 
 quality of the fluids which these cells secrete are determined by 
 the needs of the economy as the food passes along the canal. We 
 have therefore to consider how the epithelium cell manufactures 
 its special secretion out of the materials supplied to it by the 
 blood, and how the cell is called into activity by the presence of 
 food at some distance from itself, or by circumstances which do 
 not bear directly on itself. In dealing with these matters in 
 connection with the digestive juices, we shall have to enter at 
 some length into the physiology of secretion in general. 
 
 The question which presents itself first is : By what mechanism 
 is the activity of the secreting cells brought into play ? 
 
 While fasting, a small quantity only of saliva is poured into the 
 mouth; the buccal cavity is just moist and nothing more. When 
 food is taken, or when any sapid or stimulating substance, or 
 indeed a body of any kind, is introduced into the mouth, a flow is 
 induced which may be very copious. Indeed the quantity secreted 
 in ordinary life during 24 hours has been roughly calculated at as 
 much as from 1 to 2 litres. An abundant secretion in the absence 
 
Ciivr. I.] DIUESTIOX. 257 
 
 of I'ood in tliL' mouth may be called luiLli Ijy an emotion, as when 
 tho mouth waters at the sight of food, or by a smell, or by events 
 occurring in the stomach, as in some cases of nausea, l^videutly 
 in these cases some uervous mechanism is at work. In studying 
 the action of this uervous mechanism, it will be of advantage to 
 conPme our attention at first to the submaxillary gland. 
 
 The submaxillary gland (Fig. 48) is supplied with nerves from 
 two sources : from the cervical sympathetic along the su]>maxillary 
 arteries, and from the seventh or facial ncr\e by fibres, which, run- 
 ning in the chorda tympani, join the lingual branch of the fifth 
 nerve, from Avhich they diverge under the lower jaw, and run as a 
 small nerve close beside the duct to the gland. 
 
 If a tube be placed in the duct, it is seen that when sapid 
 substances are placed on the tongue, or the tongue is stimulated 
 in any other way, or the lingual nerve is laid bare and stimulated 
 with an interrupted current, a copious flow of saliva takes place. 
 If the sympathetic be divided, stimulation of the tongue or lingual 
 nerve still produces a flow. But if the small chorda nerve spoken 
 of above be divided, stimulation of the tongue or lingual nerve 
 produces no flow. 
 
 Evidently the flow of saliva is a nervous reflex action, the 
 lingual nerve sei'ving as the channel for the afferent and the small 
 chorda nerve for the efferent impulses. If the trunk of the 
 lingual be divided above the point where the chorda leaves it, as 
 at n. I' Fig. 48, stimulation of the tongue produces, under ordinary 
 circumstances, no flow. This shews that the centre of the reflex 
 action is higher up than the 2:»oint of section ; it lies in fact in 
 the brain. 
 
 In the angle between the lingual and the chorda, where the latter 
 lea^•es the former to pass to the gland, lies the small submaxillary gan- 
 glion (represented diagrammatically in Fig. 48 sm. gh), from which 
 branches pass to the lingual on the one hand and to the chorda on the 
 other ; branches may also be traced towards the ducts and glands and 
 towards the tongue. It has been much debated whether this ganglion 
 can act as a centre of reflex action, but no conclusive e^ddence that it 
 does so act has as yet been shewn. 
 
 Stimulation of the glossopharyngeal is even more effectual 
 than that of the lingual. Probably this indeed is the chief 
 afferent nerve in ordinary secretion. Stimulation of the mucous 
 membrane of the stomach (as by food introduced through a 
 gastric fistula) or of the vagus also jDroduces a flow of saliva, as 
 indeed may stimulation of the sciatic, and probably of many other 
 afferent nerves. All these cases are instances of reflex action, the 
 cerebro-spinal system acting as a centre. We may further define 
 the centre as a part of the medulla oblongata, apparently not far 
 removed from the vaso-motor centre. When the brain is removed 
 down to the medulla obloncrata, that orran being left intact, a flow 
 
258 
 
 SECRETION OF SALIVA. 
 
 [Book ii. 
 
 of saliva may still be obtained by adequate stimulation of various 
 afferent nerves ; when the medulla is destroyed no such action is 
 possible. And a flow of saliva may be produced by direct stimu- 
 lation of the medulla itself. When a flow of saliva is excited by 
 ideas, or by emotions, the nervous processes begin in the higher 
 parts of the brain, and descend thence to the medulla before they 
 
 '. ST/Tn. 
 
 1'J 
 
 n-.st/m.sm'. 
 
 c7L:t" 
 
 Fig. 48. Diageammatic Eepresentation of the Submaxillaet Gland of the Dog 
 WITH ITS Nef.yes and Blood-Yessels. 
 
 (This is not intended to illustrate the exact anatomical relations of the several 
 structures.) 
 
 sm. gld. The submaxillary gland, into the duct (.sm. tZ.) of which a cannula has 
 been tied. The sublingual gland and duct are not shewn. 
 
 n.l.,n.l'. The lingual branch nerve. ch.t.,ch. t'. The chorda tympani, pro- 
 ceeding from the facial nerve, becoming conjoined with the lingual at n.V and after- 
 wards diverging and passing to the gland along the duct. 
 
 sm. fjl. The submaxillary ganglion with its several roots. 71. 1. The lingual 
 proceeding to the tongue. 
 
 a. car. The carotid artery, two branches of which, a.srn. a. and r. sm. p., pass to 
 the anterior and posterior parts of the gland, v. sm. the anterior and posterior veins 
 from the gland, falling into v. j. the jugular vein. 
 
 V. sym. The conjoined vagus and sympathetic trunks. 
 
 gl.cer.s. The super-cervical ganglion, two branches of which forming a plexus 
 (a.f.) over the facial artery, are distributed {n.sym.sm.) along the two glandular 
 arteries to the anterior and joosterior portions of the gland. 
 
 The arrows indicate the direction taken by the nervous impulses during reflex 
 stimulation of the gland. They ascend to the brain by the lingual and descend by 
 the chorda tympani. 
 
 give rise to distinctly efferent impulses ; and it would appear that 
 these higher parts of the brain are called into action when a flow 
 of saliva is excited by distinct sensations of taste. 
 
Chap, i.] DWESTlUX. 2:>9 
 
 ('(Hisiiloring {\\v\\ tlie How of .siiliva as a rellex act the centre 
 of which lies in the medulla oblongata, we may imagine the 
 efferent impulses passing from that centre to the gland, either by 
 the chorda tymi)ani or by the sympathetic nerve. Although it 
 would perhaps be rash to say that in this relation the sympathetic 
 nerve never acts as an efterent channel, as a matter of fact we 
 have no satisfactory experimental evidence that it docs so ; and we 
 may therefore state that, practically, the chorda tympani is the 
 sole efferent nerve. Section of that nerve, either where the fibres 
 pass from the lingual nerve and the submaxillary ganglion to the 
 gland, or where it runs in the same sheath as the lingual, or 
 in any part of its course from the main facial trunk to the lingual, 
 jHits an end, as far as we know, to the possibility of any flow being 
 excited by stimuli applied to the sensory nerves or sentient 
 surfaces of the mouth, or of other parts of the body. 
 
 The natural reflex act of secretion may be inhibited, like the 
 reflex action of the vaso-motor nerves, at its centre. Thus 
 when, as in the old rice ordeal, fear parches the mouth, it is 
 probable that the afferent impulses caused by the presence of foo;! 
 in the mouth cease, through emotional inhibition of their reflex 
 centre, to give rise to efferent impulses. 
 
 In life, then, the flow of saliva is brought about by the advent 
 to the gland along the chorda tympani of efferent impulses, started 
 chiefly by reflex actions. The inquiry thus narrows itself to the 
 question : In what manner do these efferent impulses cause the in- 
 crease of flow ? 
 
 If in a dog a tube be introduced into Wharton's duct, and the 
 chorda be divided, the flow, if any be going on, is from the lack of 
 efferent impulses arrested. On passing an interrupted current 
 through the peripheral portion of the chorda, a copious secretion 
 at once takes place, and the saliva begins to rise rapidly in the 
 tube ; a very short time after the appHcation of the current the 
 flow reaches a maximum which is maintained for some time, and 
 then, if the current be long continued, gradually lessens. If the 
 current be applied for a short time only, the secretion may last for 
 some time after the current has been shut off. The saliva thus 
 obtained is but slightly viscid, and contains few salivary corpuscles 
 or protoplasmic lumps. If the gland itself be watched, while its 
 activity is thus roused, it Anil be seen that its arteries are dilated, 
 and its capillaries filled, and that the blood flows rapidly through 
 the veins in a full stream and of bright arterial hue, frequently 
 with pulsating movements. If a vein of the gland be opened, this 
 large increase of flow, and the lessening of the orcUnary deoxy- 
 genation of the blood consequent upon the rapid stream, \vill be 
 still more evident. It is clear that excitation of the chorda largely 
 dilates the arteries ; the nerve acts energetically as a dilator nerve, 
 probably from acting on some local vaso-motor centre in the gland. 
 
 Thus stimulation of the chorda brings about two events : a 
 
 17—2 
 
260 ACTION OF CHORDA TYMPANI. [Book ii. 
 
 dilation of the blood-vessels of the gland, and a flow of saliva. 
 The question at once arises, Is the latter simply the result of the 
 former or is the flow caused by some direct action on the secreting 
 cells, apart from the increased blood-supply? In support of the 
 former view we might argue that the activity of the epithelial 
 secreting cell, like that of any other form of protoplasm, is 
 dependent on blood-supply. When the small arteries of the gland 
 dilate, while the pressure in the arteries on the side towards the 
 heart is as we have seen in the last chapter correspondingly 
 diminished, the pressure on the far side in the capillaries and 
 veins is increased ; hence the capillaries become fuller, and more 
 blood passes through them in a given time. From this we might 
 infer that a larger amount of nutritive material would pass away 
 from the capillaries into the surrounding lymph-spaces, and so 
 into the epithelium cells, the result of which must be to quicken 
 the processes going on in the cells, and to stir these up to greater 
 activity. But even admitting all this it does not necessarily follow 
 that the activity thus excited should take on the form of secretion. 
 It is quite possible to conceive that the increased blood-supply 
 should lead only to the accumulation in the cell of the constituents 
 of the saliva, or of the raw materials for their construction, and not 
 to a discharge of the secretion. A man works better for being fed, 
 but feeding does not make him work in the absence of any stimulus. 
 The increased blood-supply therefore, while favourable to active 
 secretion, need not necessarily bring it about. Moreover, the 
 following facts are distinctly opposed to such a view. When a 
 cannula is tied into the duct and the chorda is energetically 
 stimulated, the pressure acquired by the saliva accumulated in 
 the cannula and in the duct may exceed for the time being the 
 arterial blood-pressure, even that of the carotid artery ; that is to 
 say, the pressure of fluid in the gland outside the blood-vessels is 
 greater than that of the blood inside the blood-vessels. This 
 must, whatever be the exact mode of transit of nutritive material 
 through the vascular walls, tend to check that transit. Again, 
 if the head of an animal be rapidly cut off, and the chorda 
 immediately stimulated, a flow of saliva takes place far too copious 
 to be accounted for by the emptying of the salivary channels 
 through any suj)|)osed contraction of their walls. In this case 
 secretion is excited in the absence of blood-supply. Lastly, if a 
 small quantity of atropin be injected into the veins, stimulation of 
 the chorda produces no secretion of saliva at all, though the 
 dilation of the blood-vessels takes place as usual. These facts 
 prove that the secretory activity is not simply the result of vascular 
 changes, but may be called forth independently; they further 
 lead us to suppose that the chorda contains two sets of fibres, 
 one secreting fibres, acting directly on the epithelium cells only, 
 and the other vaso-motor or dilating fibres, acting on the blood- 
 vessels only, and further that atropin, Avhile it has no efiect on 
 
Chap, i.] DIGESTIOX. 2GI 
 
 the latter, paralyses the foriiit-r just aa it paraly.'^c.s the inhibitory 
 fibres of the vagais. Hence when the chorda is stimulated, there 
 pass d<i\vn the nerve, in addition to impulses affecting the blood- 
 suj)ply, impulses atVecting directly the protoplasm of tiic secreting 
 cells, and calling it into action, just as similar impulses call into 
 action the contractility of the protoplasm of a muscular fibre. 
 Indeed the two things, secreting activity and contracting activity, 
 are quite parallel. \Vc know that when a muscle contracts, its 
 blood-vessels dilate; and just as by atropin the secreting action of 
 the gland may be isolated from tiic vascular dilation, so by urari 
 muscular contraction may be removed, and leave dilation of the 
 blood-vessels as the only effect of stimulating the muscular nerve. 
 In both cases the greater fiow of blood may be an adjuvant to, but 
 is not the exciting cause of, the activity of the protoplasm. 
 
 Since the chorda acts thus directly on the secreting cells, we 
 should expect to find an anatomical connection between the cells 
 and the nerve ; and some authors have maintained that the nerve- 
 fibres may be traced into the cells. But, save perhaps in the case of 
 certain glands of invertebrates (so called salivary glands of Blatta), 
 the evidence is as 3'et not convincing. 
 
 When the cervical sympathetic is stimulated, the vascular 
 effects are the exact contrary of those seen when the chorda 
 is stimulated. The small arteries are constricted, and a small 
 quantity of dark venous blood escapes by the vein. Sometimes, 
 indeed, the flow through the gland is almost an-ested. The sym- 
 pathetic therefore acts as a constrictor nerve, and in this sense is 
 antagonistic to the chorda. We have already referred to the 
 probable existence of a local vaso-motor centre situated in the 
 gland itself, in which indeed there are found ganglionic cells in 
 abundance. The fact that section of the cervical s}Tnpathetic does 
 not cause complete dilation of the vessels of the gland — the dilating 
 effects of stimulation of the chorda being fully evident after pre- 
 vious section of the sjniipathetic — affords additional support to this 
 view. We may accordingly suppose that, while the chorda tympani 
 inhibits, the sjTnpathetic exalts, the action of this local centre. 
 
 As concerns the flow of saliva brought about by stimulation of 
 the sympathetic, in the case of the submaxillary gland of the dog 
 the effects are very peculiar. A slight increase of flow is seen, but 
 this soon passes off, and so much saliva as is secreted is remarkably 
 viscid, of higher specific gra\'ity, and richer in corpuscles and proto- 
 plasmic lumps, and is said to be more active on starch than the 
 chorda saliva. This action of the sympathetic is said not to be 
 affected by atropin. 
 
 In the submaxillary gland of the dog then the contrast between 
 the effects of chorda stimulation and those of s}Tnpathetic stimu- 
 lation are very marked: the former gives rise to vascular dilation 
 Avith a copious flow of limpid saliva, the latter to vascular 
 constriction with a scanty flow of viscid saliva. And in other 
 
262 SECRETION OF GASTRIC JUICE. [Book ii. 
 
 animals a similar contrast prevails, though with minor differences. 
 Thus in the rabbit both chorda saliva and sympathetic saliva are 
 limpid and free from mucus, and in the cat, chorda saliva is more 
 viscid than sympathetic saliva; but in both these cases, as in the 
 dog, stimulation of the chorda causes a copious flow with dilated 
 blood-vessels, and stimulation of the sympathetic, a scanty flow 
 with vascular constriction. We shall return again presently to 
 these different actions of the two nerves ; meanwhile we have seen 
 enough of the history of the submaxillary gland to learn that 
 secretion in this instance is a reflex action, the efferent impulses of 
 which directly affect the secreting cells, and that the vascular phe- 
 nomena may assist, but are not the du-ect cause of, the flow. We 
 have dwelt long on this gland because it has been more fruitfully 
 studied than any other. But the nervous mechanisms of the other 
 secretions are in the main features similar. 
 
 Thus the secretion of the parotid gland, like that of the sub- 
 maxillary, is governed by two sets of fibres: one of cerebro-spinal 
 origin, running along the auriculo-temporal branch of the fifth 
 nerve but originating either in the glosso-pharyngeal or the facial, 
 and the other of sympathetic origin coming from the cervical 
 sympathetic. Stimulation of the cerebro-spinal fibres produces a 
 copious flow of limpid saliva, free from mucus, the secretion 
 reaching in the dog a pressure of 118 mm. mercury; stimulation of 
 the cervical sympathetic gives rise in the rabbit to a secretion free 
 from mucus but rich in organic matter and of greater amylolytic 
 pov/er than the cerebro-spinal secretion, but in the dog little or no 
 secretion is produced, though, as we shall see later on, certain 
 changes are brought about in the gland itself In both animals 
 the cerebro-spinal fibres are vaso-dilator and the sympathetic 
 fibres vaso-constrictor in action. Stimulation of the central end of 
 the glosso-i3har}Tigeal produces by reflex action a secretion of the 
 parotid, but that of the lingual is said to be w^ithout effect. 
 
 Gastric juice. Though a certain amount of gastric juice may 
 sometimes be found in the stomachs of fasting animals, it may be 
 stated generally that the stomach, like the salivary glands, remains 
 inactive, yielding no secretion, so long as it is not stunulated by 
 food or otherwise. The advent of food into the stomach however at 
 once causes a copious flow of gastric juice ; and the quantity secreted 
 in the twenty-four hours is probably very considerable, but we have 
 no trustworthy data for calculating the exact amount. So also when 
 the gastric mucous membrane is stimulated mechanically, as with a 
 feather, secretion is excited: but to a very small amount even when 
 the whole interior surface of the stomach is thus repeatedly stimu- 
 lated. The most efficient stimulus is the natural stimiilus, viz. food; 
 though dilute alkalis seem to have unusually powerful stimulating 
 effects ; thus the swallowing of saliva at once provokes a flow of 
 gastric juice. During fasting the gastric membrane is of a pale 
 grey colour, somewhat dry, covered with a thin layer of mucus, aud 
 
CiiAi'. I J DldKSTIoX. 2G3 
 
 thrown into folds; durini^ di^^ustion it boconios red, fiushcd, and 
 tuniiil, the folds disappear, and minute drops of Uuid appearing at 
 the mouths of tlie glands, speedily run together into small streams. 
 When the secretion is very active, the hlood flows from the 
 capillaries into the veins in a rapid stream without losing its bright 
 ai'terial hue. The secretion of gastric juice is in fact accompanied 
 by vascular dilation in the same way as is the secretion of saliva, 
 but the vascular mechanism lias not yet been fully worked out, 
 though there is evidence of the vagi nerves being concerned in 
 the matter. 
 
 Seeing that, unlike the case of the salivary secretion, food is 
 brought into the immediate neighbourhood of the secreting cells, it 
 is exceedingly probable that a great deal of the secretion is the 
 result of the working of a local mechanism; and when a mechanical 
 stimulus is applied to one spot of the gastric membrane the secre- 
 tion is limited to the neighbourhood of that spot and is not excited 
 in distant parts. This local mechanism may be nervous in 
 nature or the effect of the stimulus may perhaps be conveyed 
 directly from cell to cell, from the mouth of the gland to its 
 extreme base without the intervention of any nervous elements ; 
 but the vascular changes at least would seem to imply the presence 
 of a nervous mechanism. 
 
 The importance of this local mechanism and the subordinate 
 value of any connection between the gastric membrane and the 
 central nervous system is further shewn by the fact that a secretion 
 of quite normal gastric juice will go on when both vagi, or when 
 the sympathetic ner\'es going to the stomach are di\-ided, 
 or indeed w^hen all the nervous connections of the stomach 
 are severed. And all attempts to provoke or modify gastric 
 secretion by the stimulation of the nerves going to it, have 
 hitherto failed. On the other hand, in cases of gastric fistula, where 
 by complete occlusion of the oesophagus stimulation by the descent 
 of saliva has been avoided, the mere sight or smell of food has 
 been seen to provoke a lively secretion of gastric juice. ThLs 
 must have been due to some nervous action ; and the same may 
 be said of the cases where emotions of grief or anger suddenly 
 arrest the secretion or prevent the secretion which would otherwise 
 have taken place as the result of the presence of food in the 
 stomach. So that much has yet to be learnt in this matter. 
 
 The contrast presented between the scanty secretion resulting 
 from mechanical stimulation and the copious flow which actual 
 food induces is interesting because it seems to shew that the 
 secretory activity of the cells is heightened by the absorption 
 of certain jDroducts derived fi'om the portions of food first 
 digested. This is well illustrated by the following experiment of 
 Heidenhain. This observer, adopting the method employed 
 for the intestine (see p. 255), succeeded in isolating a portion 
 of the fundus from the rest of the stomach; that is to say, he cut 
 
264 SECRETION OF BILE. [Book ii. 
 
 out a portion of the fundus, sewed together the cut edges of the 
 main stomach, so as to form a smaller but otherwise complete 
 organ, while by sutures he converted the excised piece of fundus, 
 into a small independent stomach opening on to the exterior by a 
 fistulous orifice; When food was introduced into the main stomach 
 secretion also took place in the isolated fundus. This at first sight 
 might seem the result of a nervous reflex act; but it was observed 
 that the secondary secretion in the fundus, was dependent on actual 
 digestion taking place in the main stomach. If the material intro- 
 duced into the main stomach were indigestible or digested with 
 difficulty, so that little or no products of digestion were formed 
 and absorbed into the blood, such ex gr. as pieces of ligamentum 
 nuchae, very little secretion took place in the isolated fundus. We 
 quote this now as bearing on the question of a possible nervous 
 mechanism of gastric secretion, but we shall have to return to it 
 under another aspect. 
 
 Bile. When the acid contents of the stomach are poured over 
 the orifice of the biliary duct, a gush of bile takes place. Indeed, 
 stimulation of this region of the duodenum with a dilute acid at 
 once calls forth a floAv, whereas alkaline fluids so applied have 
 little or no effect. This, probably, is a reflex action leading to the 
 contraction of the muscular walls of the gall-bladder and ducts, 
 accompanied by a relaxation of the sphincter of the orifice ; 
 it refers therefore to the discharge rather than to the secretion 
 of bile. 
 
 When the secretion of the bile is studied by means of a biliary 
 fistula (which, however, probably induces errors by the total with- 
 drawal from the body of the bile which should naturally flow into 
 the intestine), it is seen to rise rapidly after meals, reaching its 
 maximum in from four to ten hours. There seems to be an im- 
 mediate, sudden rise when food is taken, then a fall, followed 
 subsequently by a more gradual rise up to the maximum, and ending 
 in a final fall to the lowest point ; but it must be remembered that 
 the lowest point is not zero, since the secretion of the bile, unlike 
 that of the saliva and gastric juice, is continuous and even in 
 a fasting animal does not cease. It may be that these variations 
 are due to the action of the nervous system, but experiments have 
 hitherto failed to demonstrate clearly the existence of any distinct 
 nervous mechanism. 
 
 The pressure under which the bile is secreted is in general very 
 low. When a water manometer is connected with the gall-bladder 
 of a guinea-pig, the ductus choledochus being ligatured, the fluid 
 may rise in the manometer to about 200 mm. (equivalent to about 
 16 mm. mercury), but not much beyond. This is of course much 
 less than the arterial pressure in the same animal ; but it must not 
 be forgotten that the liver receives its chief blood-supply from a 
 venous source, viz. from the portal vein; and it would appear from 
 
ClIAl'. I.] 
 
 DIGESTIoy. 
 
 J (35 
 
 oxpoiiint-'iits oil clogs that the pressui-e at which tlic bile Ls secreted 
 exceods that of the blood in the mesenteric veins going to form the 
 portal vein. Hence, the limit of pressure, thoui^h so (litfen^nt from 
 that of the salivary glands, resembles it in this fundamental fact 
 that it exceeds the pressure of the blood in the capillaries of the 
 oro-an. The same peculiar vascular supply of the liver renders it 
 ditiicult to draw any comparison between its vascular condition 
 during active secretion and that of the salivary glands, though 
 durino- digestion the liver is swollen and increased in weight, 
 apparently from an increase in the blood-supply. 
 
 The quantity of bile secreted in man in the twenty-four hours 
 has been estimated to be exceedingly great, but the calculations are 
 based on very imperfect data. 
 
 Pancreatic juice. In the .dog the secretion of pancreatic juice 
 after food has been taken, follows the curve given in Fig. 49. There 
 is a sudden maximum rise immediately after food has been taken. 
 This at all events suggests very strongly some nen^ous action. 
 Then follows a fall, after which there is, as in bile, a secondary 
 rise, the causation of which may, or may not, be nervous in nature. 
 In the dog, there may be, during fasting, a complete cessation of 
 
 2|3l4i5!6!7 8l9|l0'll|l2;i3!l4'l5 16! I '2.314! 5 6 7.8 9'lo| 
 
 Fig. 49. Diacram iLLrsTRAXixo the influence op Food on the Secretion of 
 Pancreatic Jcice. (N. 0. Bernstein.) 
 
 The abscissa; represent hours after taking food ; the ordinates represent in c.c. 
 the amount of secretion in 10 min. A marked rise is seen at B immediately aft€r 
 food ^as taken, with a secondary rise between the 4th and 5th hours afterwards. 
 Where the line is dotted the observation was interrupted. On food being again 
 given at C, another rise is seen, followed in turn by a depression and a secondary 
 rise at the 4th hour. A very similar curve would represent the secretion of bile. 
 
2G6 SECRETION OF PANCREATIC JUICE. [Book it. 
 
 secretion. The quantity secreted in 24 hours by man has been 
 calculated at 300 c.c. Like the salivary glands, the j)ancreas 
 while secreting is flushed, through dilation of its blood-vessels. 
 
 The secretion if present may be increased, or if absent may be 
 called forth, by stimulation of the medulla oblongata, and when 
 going on may be arrested by stimulation of the central end 
 of the vagus through a reflex act, the efferent channels of 
 which have not yet been made out; probably the arrest of the 
 secretion which is said to be caused by nausea or vomiting is thus 
 brought about by stimulation of the vagus endings. These facts 
 shew that the secretion is under the influence of the central 
 nervous system ; but we have no such satisfactory knowledge of the 
 exact working of the nervous mechanism as in the case of the 
 salivary glands. 
 
 Succns entericus. AYith regard to the secretion furnished by 
 the intestine itself our information is very limited. The secretion 
 of the isolated intestine appears to be not a constant one, but to 
 need for its production some stimulus (mechanical or other) which 
 probably acts in a reflex manner. After section of the nerves 
 going to a piece of intestine isolated after Thiry's method, a 
 copious flow of a dilute intestinal juice is said to take place. 
 
 Thus, while the influence of the nervous system is in the case 
 of the submaxillary gland tolerably clear, in the case of the other 
 secretions we have yet much to learn, and we must rest rather 
 on analogy with the submaxillary gland, than on any kno^vn facts. 
 We cannot, however, go far wrong, if we conclude that in all cases 
 secretion is essentially due to a du-ect activity of the epithelium 
 cells, and that variations in the blood- supply have a secondary 
 effect only. 
 
 We may now pass on to the second problem. What is the 
 exact nature of the activity which is thus called forth ? 
 
 Towards the solution of this jDroblem much progress has been 
 made by the study of the microscopical changes in secreting glands 
 during various stages of activity and rest. And these are perhajDS, 
 in some respects, best shewn in the pancreas. 
 
 It is possible, by special precautions, to examine with even 
 high powers of the microscope the pancreas of an animal such 
 as the rabbit, while still alive with the cumulation intact; and 
 thus to watch the changes going on both when the animal has 
 been deprived of food for some time and the gland is therefore 
 at rest, and when the animal has been recently fed so that digestion 
 is going on and the pancreas in consequence is engaged in j)ouring 
 its secretion into the duodenum. In the former case, i. e. when the 
 pancreas is at rest and little or no secretion is being poured out, 
 the following appearances may be recognised. The outlines of 
 the individual cells forming an alveolus (Fig. oO A) are very in- 
 distinct, and each cell is loaded vrith a number of small highly 
 
CiiAP. I.] DIGKSTIOX. 207 
 
 refractive granules. These however arc crowded towards tlie 
 iniu'r side ot" the tell abutting on the lumen of the alveolus, 
 leaving tlu,' outer jiait of the crll next to the basement nieuibrMK' 
 
 B. 
 
 Fig. 50. A poktion of the Pancreas of the Rabbit (Kcune and Sheridan Lea), 
 A at rest, iJ in a state of activity. 
 
 a the inuer granular zone, which in A is larger, and more closely stadded with 
 line granules, than in B, in which the granules are fewer and coarser. 
 
 h tlie outer transparent zone, small in A, larger in B, and in the latter marked 
 with faint strije. 
 
 c the lumen, very obvious in B, but indistinct in A. 
 
 d an iudcutation at the juuctiou of two cells, seen in B, but not occurring in A. 
 
 clear and hyaline. We can in fact distinguish in each cell two 
 zones, a smaller outer zone, j&'ee from granules, and a larger or 
 broader inner zone thickly studded -svith granules. At the same 
 time it may be remarked that the lumen of the alveolus is 
 narrow and very obscure; the blood-supply moreover is scanty, 
 the small arteries being constricted and the capillaries imperfectly 
 tilled with corpuscles. 
 
 If, however, the same pancreas be examined while it is in a 
 state of acti\ity, either from the presence of food in the stomach, 
 or from the injection of some stimulating drug such as pilocarpin, 
 a very diflerent state of things is seen. The individual cells 
 (Fig. 50 B) have become smaller and much more distinct in outline 
 and the lumen of the alveolus is now wider and more conspicuous. 
 In each cell the gi-anules have become much fewer in number and 
 as it were have retreated to the inner margin, so that the inner 
 gi'anular zone is much naiTower and the outer transparent zone 
 much broader than before ; the latter too is frequently marked at 
 its inner part by delicate striae nmning into the inner zone. At 
 the same time the blood-vessels are largely dilated and the stream 
 of blood through the capillaries is full and rapid. 
 
 These things, the disappearance of gi-anules during activity 
 leading to a diminution of the inner granular zone and a widening 
 of the outer transparent zone, and the appearance of new granules 
 during rest leading to a restoration of the inner zone and its 
 consequent encroachment on the outer zone, may be witnessed in 
 the living pancreas of the rabbit, and the changes from the one 
 
268 HISTOLOGICAL CHANGES. [Book ii. 
 
 condition to the other successively observed. And sections of the 
 prepared and hardened gland of this or of any other animal tell nearly 
 the same tale. Thus in the pancreas of a dog which has been 
 fasting for about 30 hours, each secreting cell is found to consist 
 of two zones : an inner zone, studded with fine granules, and a 
 smaller outer zone, which is homogeneous or marked with delicate 
 striffi, the nucleus being placed partly in the one and partly in 
 the other zone. When however the pancreas of an animal in full 
 digestion (about six hours after food) is examined, though the 
 whole cell is smaller, the outer homogeneous zone is found to be 
 relatively much wider, the granular inner zone being narrower, 
 and in some cases actually disappearing. If the pancreas be ex- 
 amined at the end of digestion, when its activity has once more 
 ceased, and it has entered into a state of rest, the outer zone is 
 again found to be narrow, the granular inner zone occupying 
 the greater part of the cell, which has once more become larger. 
 Carmine stains the outer zone easily, the inner zone with difficulty. 
 Hence when, as during activity, the outer zone is relatively large, 
 the cell as a whole seems more deeply stained than when, as during 
 rest, the outer zone is small. During activity the nucleus is large 
 and round; during rest it often appears irregular, o\axig to its 
 being in such a condition that it shrinks under the influence of the 
 reagents employed. 
 
 Leaving aside for the present the changes in the nucleus, and 
 the matter of staining, we may say that the results of the two 
 methods are identical. 
 
 Before, however, v/e attempt to explain what these results mean, 
 it will be well to pay attention for a while to another type of 
 secreting gland, the so-called mucous glands. We have already 
 seen that some salivary glands, such as the submaxillary of the 
 dog, secrete a thick viscid saliva, the viscidity being due to the 
 presence of the body mucin (see Appendix), the essential con- 
 stituent of the so-called mucus ; while other salivary glands, such 
 as the parotid of most animals, secrete a thin limpid saliva free from 
 mucin. Glands of the latter kind, from the nature of their secre- 
 tion, receive the name of 'serous' glands. Glands, however, which 
 give rise to a viscid mucin-holding secretion, always contain a 
 certain number of cells of a distinct type. These cells are called 
 'mucous cells;' and the glands in which they are found are called 
 'mucous glands.' Sometimes the mucous cells are abundant form- 
 ing a large part of many or most of the alveoli; sometimes they are 
 scanty. Each ' mucous ' cell when examined in a fresh and natural 
 condition is loaded throughout with somewhat large granules; 
 but when treated with alcohol or other hardening reagents 
 (Fig. 51 A) appears to consist of two parts: of a small quantity of 
 what we may speak of as ordinary protoplasm, readily staining 
 with carmine, &c. and gathered round the nucleus, which is 
 placed towards the outside of the cell, generally close to the base- 
 
ClIAP. 1.1 
 
 DIUKSTIOX. 
 
 2')9 
 
 ment moinbrane ; ami of another diffcrout substance which occu- 
 pies the gi-eatcr part of the cell. This latter substance i.s 
 conijiosed of a loose network of fine fibres, the spaces of which 
 are occupied by a transparent material which docs not stain 
 readily with carmine ; and upon examination is found to consist 
 
 e 
 
 A. 
 
 B. 
 
 Fig. 51. Sectiox of a 'Mucous' Glasd, ^ in a state of rest, B after it has been for 
 some time actively secreting. (After Lavdowsky.) 
 
 a demilune cells, c leucocytes lying in the inter-alveolar spaces. Tire darker 
 shading in both figures is intended to indicate the amount of stai.iing. 
 
 largely of a material which is readily transformed into mucin, 
 and which may be spoken of as mucinogen or by abbre\'iation 
 mucigen. So that the ordinary mucous cell of a mucous gland 
 may be said to consist of a smaller portion of ordinary protoplasmic 
 substance and of a larger portion of a mucigenous substance. 
 
 Such a condition of things exists however only in a mucous 
 cell at rest. When the gland is actively secreting, or rather after 
 the gland has for some time been actively secreting, as for 
 instance after the submaxillary gland of the dog has been subjected 
 to long and powerful stimulation of the chorda, a different state of 
 things presents itself when prepared sections of the hardened gland 
 are examined. The alveolus is then found to be made up of 
 smaller cells (Fig. bl B) almost Avholly formed of protoplasmic 
 substance readily staining A\ith carmine. In extreme cases hardly 
 a trace is left of the mucigenous substance spoken of above ; in 
 cases of moderate activity cells may be seen in which the muci- 
 genous substance has diminished, with an increase of the ordinary 
 protoplasmic substance, but has not entirely disappeared. 
 
 How are we to interpret these results ? obviously in this way. 
 The mucigenous basis is manufactured at the exjaense of the 
 ordinary protoplasm of the cell ; the latter by its metabolism 
 produces the former and deposits it in the meshes of its own 
 framework, becoming as it were pregnant with mucigen. This 
 during the resting phase of the gland may go on to such an 
 extent, that only a small quantity of protoplasm is left to carry 
 
270 HISTOLOGICAL CHANGES. [Book ir. 
 
 the large c[uantity of mucigen to which it has given rise ; that 
 is to say, the growth of new protoplasm does not keep pace with 
 the manufacttire of protoplasm into mucigen. During activity 
 the mucigen is used up to provide the mucus of the saliva, 
 being probably converted into mucin and so discharged from the 
 cell, while at the same time the protoplasm takes a fresh start and 
 grows apace ; and thus a fresh supply of new, deeply-staining 
 protoplasm takes the place of the mucigenous matrix which has 
 been lost. We may remark incidentally that this rejuvenescence 
 of the protoplasm is marked as in the corresponding phase of the 
 j^ancreas, by the nucleus becoming round and conspicuous, whereas 
 when the mucigen is abundant it is of such a nature as to become 
 irregular in outline when acted upon by hardening reagents. 
 
 We have reason to think that in certain cases, v/here the activity 
 of the cell is long-continued and vehement, the whole cell may 
 disappear, and its place be taken by an entirely new cell supplied 
 by the so-called demilune cells lying on the outside of the alveolus 
 beneath the basement membrane. But in ordinary cases the 
 same cell probably, for a while at all events, continues to form and 
 discharge successive quantities of mucin without actually itself 
 disappearing. 
 
 In any case we see that in the mucous cell what takes place 
 in secretion is as follows. As the result of a period of rest there ac- 
 cumulates in the cell a quantity of mucigen, which is a product of 
 the metabolism of the protoplasm of the cell. During the active 
 phase, that is while the secretion is being poured forth, the 
 mucigen is converted into mucin and discharged from the cell. 
 A loss consequently accrues to the cell, but this is at once partly 
 made up by the protoplasm being stirred to a more active growth. 
 Subsequently during the succeeding rest the new protoplasm is 
 transformed into new mucigen, the cell wholly regains its former 
 dimensions and features and so the cycle is completed. 
 
 What relation do these changes in the mucous gland bear to 
 those of the j^ancreas ? To answer this question we must bring 
 the reader back to a statement made on p. 255, that in order to 
 obtain an actively proteolytic aqueous pancreatic extract, the 
 animal should be killed during full digestion. This statement now 
 requires modification. 
 
 If the pancreas of an animal, even in full digestion, be treated, 
 luhile still ivarm from the body, with glycerine, the glycerine 
 extract, as judged of by its action on fibrin in the j)resence_ of 
 sodium carbonate, is inert or nearly so as regards proteid bodies. 
 If, however, the same pancreas be kept for 24 hours before being 
 treated with glycerine, the glycerine extract readily digests fibrin 
 and other proteids in the presence of an alkali. If the pancreas, 
 while still warm, be rubbed up in a mortar for a few minutes 
 with dilute acetic acid, and then treated with glycerine, the 
 glycerine extract is strongly proteolytic. If the glycerine extract 
 
Chap, i.] DlUE.'STluy. I'Tl 
 
 obtained witlioul aciil from tlid warm j)ancivas, and llieivfuio iiiort, 
 1)0 dilutod lar^a'ly with watm-, and kc])t at 3')" C for some time, 
 it becomes active. If treated with aciihdated instead of distilled 
 water, its activity is much sooner developed. If the inert glyco- 
 rino extract of warm pancreas be precipitated with alcohol in 
 excess, the precipitate, inert as a proteolytic ferment when fresh, 
 becomes active when exposed for some time in an aqueous solution, 
 rapidly so Avhen treated with acidulated water. These facts shew 
 that a pancreas taki>n fresh from the body, even during full 
 digestion, contains hut Utile ready-made ferment, tliough there is 
 ])resent in it a body which, by some kind of decomposition, gives 
 birth to the ferment. We may remark incidentally that though the 
 presence of an alkali is essential to the proteolytic action of the 
 actual ferment, the formation of the ferment out of its forerunner 
 is favoured by the jiresence of a small quantity of acid. To this 
 body, this mother of the ferment which has not at present been 
 .satisfactorily isolated, the name of zymogen has been applied. 
 But it is better to reserve the term zymogen as a generic name 
 for all such bodies as not being themselves actual ferments, may 
 by internal changes give rise to ferments, for all 'mothers of 
 ferment' in fact; and to give to the particular mother of the 
 pancreatic proteolytic ferment, the name trypsinogen. 
 
 The pancreatic cell then contains trj-psinogen ; and now comes 
 the important observation that the amount of trypsinogen in a 
 pancreas at any given time rises and sinks pari jmssu with the 
 granular inner zone, i.e. w^ith the amount of granular substance in 
 the cell. The ^vider the inner zone and the more abundant the 
 granules the larger the amount, the narrower the zone and the 
 fewer the gi'anules the smaller the amount, of try]3sinogen ; and in 
 the cases of old-established fistulre, where the secretion is wholly 
 inert on proteids, the inner granular zone is absent from the 
 cells. 
 
 We have no corresponding satisfactory information concerning 
 the history of any zymogen which may be sujjposed to belong to 
 the amylohlic ferment of the pancreas or to the ferment which 
 acts upon fats. Nor on the other hand are we in a position to say 
 that the granules are wholly composed of trj^Dsinogen ; but it seems 
 clear that they contain trypsinogen, and that their abundance or 
 scarcity afford a measure of the quantity of that substance present 
 in the cell. 
 
 Hence we may draw a parallel between the mucous cell and 
 the pancreatic cell. Just as the protoplasm of the former b}^ its 
 metabolism manufactures mucigen, so the jDrotoplasm of the latter 
 by its metabolism manufactures trjqisinogen, and just as the 
 mucigen gives rise to mucin which escapes from the cell to form 
 part of the actual secretion, so also the trj^isinogen gives rise to 
 trypsin, which similarly forms part of the pancreatic juice. Just as 
 with the disappearance of the mucigen the protoplasm grows with 
 
272 HISTOLOGICAL CHANGES. [Book ir. 
 
 renewed vigour, so in the pancreas with the disappearance of the 
 granules from the inner zone, there is a rejuvenescence of the pro- 
 toplasm, to be followed both in the one case and the other by a 
 subsequent conversion of the protoplasm into a product, viz. 
 mucigen and trypsinogen respectively. In both cases the product 
 of the protoplasmic metabolism is deposited in the inner parts of 
 the cell, though the line of demarcation between the inner and 
 outer zone is much more distinct in the pancreas than in the 
 mucous gland. In the former abundance of granules is identical 
 with a broad inner zone, scarcity of granules with a broad outer 
 zone; and similarly the growth of the new protoplasm is most 
 obvious as an increase of the outer zone. In the mucous cell too 
 the mucigen appears on the inner side of the cell. This distinction, 
 however, between an inner and outer zone is not an essential 
 feature of the matter, though probably the growth of new proto- 
 plasm naturally tends to take place at a greater rate on the side 
 of the cell most exposed to the blood-stream, i.e. on the outer side 
 towards the basement membrane, and the deposition of zymogen or 
 mucigen tends to be greatest on the other side nearer to the lumen, 
 into which its products are about to be discharged. 
 
 When we come to study other glands, such as the serous 
 salivary glands, the glands of the stomach, and the hepatic cells, 
 we have evidence that in these also the same essential processes 
 are going on. Certain special features however are in various 
 instances met with, and, these becoming exaggerated by particular 
 modes of preparation, are apt to obscure the normal series of 
 events. 
 
 Thus in the case of the glands of the stomach, if we were 
 to trust exclusively to the indications given by sections of glands 
 hardened in alcohol, we should be led to make the following state- 
 ment. In an animal previous to taking a meal, the central or ' chief 
 (as distinguished from the ovoid, 'border,' or 'peptic') cells of the 
 gastric glands are pale, finely gTanular, and do not stain readily 
 with carmine and other dyes. During the early stages of gastric 
 digestion, the same cells are found somewhat swollen, but turbid 
 and more coarsely granular ; they stain much more readily. At a 
 later stage they become smaller and shrunken, but are even more 
 turbid and granular than before, and stain still more deeply. 
 This is true, not only of the central cells in the so-called peptic 
 glands, but also of the cells of which the glands of the pyloric end 
 of the stomach are built up. The ovoid or border cells appear 
 swollen during digestion, and project more on the outside of the 
 gland, but otherwise seem unchanged. This series of events is 
 different from that which we have seen to take place in the pancreas, 
 inasmuch as the cells appear to become more granular instead of less 
 granular during activity. But we have reason to think that the 
 granular character of the gastric cells thus seen during digestion 
 is due to some special material precipitated by the alcohol, where- 
 
ClIAP. I.] 
 
 jjiaj'JSTWX. 
 
 273 
 
 by cliau^i's really comparable to thoseof the i:)ancrcas are ob.sciinKl. 
 For \vc tiiul that in the newt the cells, when examined in a living 
 condition, arc granular throughout when at rest but during activity 
 developc a clear outer zone, the granules becoming restricted to 
 the inner zone. And iu many mammals similar changes may be 
 demonstrated by the use of osmic acid (Fig. 52). In some mammals 
 
 Fig. 52. Gastric gland of MAiniAL (Mole) dciung activity (Langley). 
 
 c, the month of tlie gland Nvith its C3-lindrical cells. 
 
 71, the neck, containing conspicuous ovoid cells, with their coarse protoplasmic 
 network. 
 
 /, the body of the gland. The granules are seen in the central cells to be limited 
 to the inner portions of each cell, the round nucleus of which is conspicuous. 
 
 no very obvious difference between rest and activity can be made 
 out; and it is possible that in these a regeneration of granules 
 takes place during activity as 'well as during rest and that in pro- 
 portion as granules are being used up, so that the amount of 
 granules remains fau'ly constant. 
 
 Moreover wc have evidence of the existence in the gastric 
 membrane of a zymogen, a mother of pepsin, a pepsinogen; 
 though owdng to the facility with which apparently the conversion 
 of peiDsinogen into pepsin takes place, the matter is not so clear as 
 in the analogous case of trv-psinogen ; and it would appear that the 
 amount of pepsinogen and the abundance of visible gi'anules in fresh 
 living cells run parallel to each other with considerable regularity. 
 
 F. IS 
 
274 
 
 MUCOUS AND SEROUS GLANDS. 
 
 [Book ir. 
 
 Ill the case of the serous glands also the results are somewhat 
 different according as use is made of preparations hardened in 
 alcohol, or the gland is studied in a living state. Thus in the 
 parotid of the. rabbit, which is a serous gland, even when a most 
 copious secretion has been called forth b j stimulation of the auriculo- 
 temporal nerve, alcohol specimens shew an almost complete absence 
 of structural changes. When however the cervical sympathetic is 
 stimulated, either in the rabbit or the dog, very marked changes, 
 quite similar to those witnessed in the central cells of the gastric 
 glands, may be seen in the parotid hardened by alcohol, even though, 
 as occurs in the dog, no saliva whatever may be secreted. During 
 rest the cells of the parotid as seen in sections of the gland 
 hardened in alcohol (Fig. 53 A) are pale, transparent, staining with 
 difficulty, and the nuclei possess irregular outlines as if shrunken 
 by the reagents employed. After stimulation of the sympathetic, 
 the protoplasm of the cells becomes turbid (Fig. 53 B), and stains 
 much more readily, while the nuclei are no longer irregular in 
 outline but round and large, with conspicuous nucleoli, the whole 
 cell at the same time, at least after prolonged stimulation, becoming 
 
 ^ I 
 
 A 
 
 Fig. 53. Section of a 'Seeous' Gland: the Paeotid of tee Eabbit. A at 
 rest, B after stimulation of the cervical sympathetic. (After HeiJenhain.) 
 
 distinctly smaller. When however we study ^the gland in a living 
 state, we find that the changes which take place during activity are 
 quite comparable to those of the pancreas. During rest (Fig. 54 A), 
 the cells are large, their outlines very indistinct, in fact almost 
 invisible and the protoplasm of the cell is studded wath granules. 
 
 Fig. 5i. Changes in the Parotid during Secretion (Langlej'). 
 The figure which is somewhat diagrammatic represents the microscopic changes 
 which may be observed in the living gland. A. During rest. The obscure outlines 
 of the cells are introduced to shew the relative size of the cells, they could not be 
 readily seen in the specimen itself. B. After moderate stimulation. C. After 
 prolonged stimulation. The nuclei are diagrammatic, and intro:luced to shew their 
 appearance and position. 
 
f'MAP. i] hlCESTinX. 275 
 
 Durinfj activity (Fig. 54 li), the cells become smaller, their outlines 
 more distinct, and the grannies disappear especially from the outer 
 portions of each cell. After prolonged activity, as in Fig. 54 G, the 
 cells are still smaller with their outlines still more distinct, ami the 
 granules have disappeared almost entirely, a few oidy lieing left at 
 the extreme inner margin of eaeh cell abutting upon the conspicuous 
 almost gaping lumen of the alveolus. And upon special examina- 
 tion it is found that the nuclei are large and round. In fact we 
 might almost take the parotid as thus studied, to be more truly 
 typical of secretory changes than even the pancreas. For, as wc 
 have already stated, the demarcation of an inner and outer zone is not 
 a necessary feature of the cell at rest. What is essential is that the 
 protoplasm manufactures granules, which for a while, that is during 
 rest, are deposited in the cell, and during activity these granules 
 are used up, their disappearance being earliest and most marked at 
 the outer portions of each cell, and progressing inwards towards the 
 lumen, the whole cell becoming smaller and as it were shrunken. 
 
 It would liardly bo profitable to enter more fully into the 
 discussion of this matter, and especially of the differences, to which 
 we have just called attention, as occurring in different glands ; 
 enough has been seen to justify us in the conclusion, which 
 further study will be found to strengthen, that the act of secretion 
 is not a mere filtration from the blood but a complicated business, 
 which we may picture to ourselves somewhat as follows. 
 
 The protoplasm of the secreting cell lives upon its 'internal 
 medium,' the lymph filling the l}Tnph spaces by which the 
 alveolus is surrounded ; this Ipnph being constantly renewed from 
 the blood-stream. We have no reason to think that the main 
 nutritive constituents of the l3Tiiph in the interstices of a gland 
 are different from those in the interstices of a muscle ; but are led 
 to believe that the same substances are built up in the one 
 case into muscular and in the other into glandular protoplasm by 
 the specific acti\'ity of the already existing protoplasm which is 
 different in the one case and the other. The cell substance which 
 has thus built itself up out of the hinph materials sooner or later 
 breaks down again : the constructive metabolism is inevitably 
 followed by a destructive metabolism. In this downward path are 
 probably many steps, two of which become conspicuous : the 
 formation of some intermediate product or 'mesostate,' as we may 
 call it, such as zymogen or mucigen, and the conversion of the 
 z}Tnogen into an actual ferment or of the mucigen into mucin, that 
 is of the mesostate into the final product, which is discharged as a 
 constituent of the seci'etion. 
 
 In what we may consider the common or typical case where 
 periods of rest alternate with periods of secretory activity, the 
 do^^^lward metabolism stops short at the formation of zymogen, 
 which becomes deposited, commonly in the form of granules in the 
 meshes of the protoplasm, the constructive metabolism or growth 
 
 IS -2 
 
276 THEORY OF SECRETION. [Book ii. 
 
 of the latter languishing as the storage increases. Then generally 
 as the result of stimulation, changes takes place in the cell 
 by which the zymogen is converted into actual ferment, and this 
 ejected from the cell. This is the process which we sometimes 
 speak of as the act of secretion, and it obviously has many 
 analogies with a muscular contraction. Coincident with the dis- 
 turbances which thus give rise to the ejection of ferment, the 
 constructive metabolism of the cell is excited to greater activity, 
 and for a while there is an accumulation of new protoplasm in 
 great excess of zymogen. Soon, however, but slowly rather than 
 suddenly, this new protoplasm again breaks down into zymogen, 
 which in turn is stored up in the cell, and so the cycle is completed. 
 
 Such may be considered the more common mode of procedure ; 
 and in such a case we are enabled, as in the pancreas or mucous 
 gland, to watch the accumulation and disappearance of the zymo- 
 gen or mucigen, because this is alternately in excess of or less 
 than the actual protoplasm. But we can easily imagine a case in 
 which all the various stages of the upward and downward 
 metabolism keep pace with each other, in which for instance when 
 any quantity of zymogen is converted into ferment which leaves 
 the cell, just that quantity of zymogen is replaced by a destruction 
 of protoplasm, and a new quantity of protoplasm appears just 
 sufficient to replace the old which has been broken down. In 
 such an instance of continuous changes it Avould be impossible, Avith 
 our present means at least to trace out the series of events, though 
 those at bottom would be identical with those where the changes 
 were discontinuous. And indeed it is obvious that this same plan of 
 secretion, if we may so call it, might be made to produce very 
 varied results, by variations in the proportions and rates of the 
 several steps. 
 
 Admitting, however, this view of what we may call the proto- 
 plasmic aspect of secretion, another feature has to be considered. 
 The juice secreted by any gland consists not only of the specific 
 ferments, trypsin etc. as the case may be, found only in it, but also 
 of a large quantity of water, and of various saline or other soluble 
 substances common to it and other juices. And the question arises,- 
 Is this water, or are these salts and soluble substances furnished by 
 the same act as that which supplies the specific constituents ? 
 
 To this we may reply, that the very water is discharged by the 
 activity of the cell, and is not a mere filtration from the blood- 
 vessels. For, as we have seen in the case of the salivary glands, 
 when atroj)in is given, not only do the specific constituents cease 
 to be ejected in spite of the vessels becoming dilated, but the 
 discharge of water is also arrested: no saliva at all leaves the 
 gland. And what is true of the salivary glands probably holds 
 good with the other glands. Assuming then that even the escape 
 of water is the result of the activity of the cell, we cannot but feel 
 an increased interest in the fact mentioned some time ago, that in 
 
CiiAi>. i] DIGESTION-. 277 
 
 the submaxillary gland of the dog, stimulation of the chorda 
 tympani produces a cojiious flow of thin limpid saliva, and stimu- 
 lation of the cervical sympathetic a scanty flow of thick viscid 
 saliva. That is to say, stimulation of the chorda afl'ects chicfli/ tlio 
 discharge of water, which carries away with it various soluble 
 matters, while stimulation of the sympathetic chiefly aftccts the 
 c(.)nversion of nuicigen into mucin. To this avg may add the case 
 of the parotid of the dog. In this stimulation of a cercbro-spinal 
 ncr^•o, the auriculo-tcmporal, produces a copious flow of limpid 
 saliva, while stimulation of sympathetic produces itself little or no 
 secretion at all ; but ailer previous stimulation of the S3'mpathetic, 
 the saliva which flows upon stimulation of the ccrebro-spinal nerve 
 is much richer in solid and especially in organic matter. And we 
 have already seen that while the microscoijic changes after cerebro- 
 spinal stimulation are inappreciable, those following upon sym- 
 pathetic stimulation are vcry'consjDicuous. The latter gives rise to 
 certain constituents, while the former, so to speak, washes them 
 away into the duct. 
 
 These and other facts, on which we need not now dwell, have 
 led to the conception that the act of secretion consists of two 
 parts, both distinct efforts of the cell, which in one case may 
 coincide, in another may take place apart or in different pro- 
 portions. On the one hand, there is the discharge of water 
 carrying with it common soluble substances ; on the other, the 
 escape of specific substances resulting from the profound meta- 
 bolism of the cell protoplasm. And it has been supposed that 
 two kinds of nerve fibres exist, one governing the former process 
 and preponderating in the chorda tymi^ani, for instance, the other 
 governing the latter and preponderating in the branches of the 
 cervical sympathetic. Further h}^otheses have been put forward 
 to explain the modus operandi of the discharge of water, such 
 as the existence of substances in the cell which absorb water from 
 the blood or lymph on the one side and give it ujj on the other 
 side into the lumen of the alveolus. But these matters are not 
 yet ripe for any distinct assertion, and though we have thought it 
 right to bring the matter before our readers, we must not pursue 
 the discussion any further. Whether there be two sets of fibres or 
 no, whether the two processes be absolutely distinct or merely 
 variations of the same fundamental changes, the proi^osition on 
 which we ha\'e so long dwelt — that the flow of juice from a 
 secreting gland is essentially the outcome of the activity of the 
 secreting cell — remains equally true. 
 
 Before we leave the mechanism of secretion there arc one 
 or more accessory points which deserve attention. 
 
 In treating just now of the gastric glands we spoke as if 
 pepsin were the only important constituent of gastric juice, 
 whereas, as we have previously seen, the acid is equally essential. 
 The formation of the free acid of the gastric juice is very 
 
278 THEORY OF SECRETION. [Book ii. 
 
 obscure and many ingenious but unsatisfactory views have been 
 put forward to explain it. It seems natural to suppose that 
 it arises in some way from the decomposition of sodium chloride 
 drawn from the blood; and this is supported by the fact that 
 when the secretion of gastric juice is actively going on, the 
 amount of chlorides leaving the blood by the kidney is pro- 
 portionately diminished; but nothing definite can at present be 
 stated as to the mechanism of that decomposition, though an 
 organic acid such as lactic, which as we have seen appears in the 
 juice, might under certain conditions succeed in decomposing 
 chlorides. And even admitting that the sodium chloride of the 
 body at large is the ultimate source of the chlorine element of 
 the acid, it appears more likely that that element should be set 
 free in the stomach by the decomposition of some highly complex 
 and unstable chlorine compound previously generated, than that it 
 should arise by the direct splitting-up of so stable a body as 
 sodium chloride, at the time when the acid is secreted. 
 
 In the frog, while pepsin free from acid is secreted by the 
 glands in the lower portion of the oesophagus, an acid juice is 
 afforded by glands in the stomach itself, which have accordingly 
 been called oxyntic {o^vveiv to sharpen, acidulate) glands ; but 
 these oxyntic glands appear also to secrete pepsin. In the 
 mammal the isolated pylorus secretes an alkaline juice; in fact 
 the appearance of an acid juice is limited to those portions 
 of the stomach in which the glands contain both 'chief or 
 'central,' and 'ovoid' or 'border' cells. Now there can be no 
 doubt that the chief cells do secrete pepsin. During life the 
 granules visible in the living chief cells abound or are scanty 
 according as pepsin is about to be or has been secreted, and 
 after death they contain pepsin (or pepsinogen), and that in 
 proportion to their richness in granules. No such correspondence 
 can be seen in the ' border ' or ' ovoid ' cells. Hence it has been 
 inferred that the border cells secrete acid; but the argument is one 
 of exclusion only, there being no direct proofs of these cells actually 
 manufacturing the acid. 
 
 The rennet ferment appears to be formed by the same cells 
 which manufacture the pepsin, that is, by the chief cells of the 
 fundus generally and to some extent by the cells of the pyloric 
 glands. We may add that we have evidence of the existence of a 
 zymogen of the rennet ferment analogous to the zymogen of pepsin 
 or trypsin. 
 
 The mucus which is present as a thin layer over the surface 
 of the fasting stomach, and which especially in herbivorous animals 
 is increased during digestion, comes from the mucous cells which line 
 the mouths of the several glands and cover the intervening surfaces. 
 
 We previously called attention to the fact that in the case of 
 the stomach the absorption of the products of digestion largely 
 increased the activity of the secreting cells. This has led to .the 
 
Chap. i.J DiaKSTIOX. 27 D 
 
 idea that one effect of food is to 'charge' the gastric cells with 
 pepsinogen, and that certain articles of food might be ccjnsidered 
 as especially peptogenous, i.e. conducive to the formation of pep.sin. 
 Such a view is tein{)ting, but needs as yet to be more fully 
 supported by facts. 
 
 Seeing the great solvent ])ower of both gastric and pancreatic 
 juice, the question is naturally suggested, Why does not the 
 stomach digest itself? After death, the stomach is frequently 
 found partially digested, viz, in cases when death has taken place 
 suddenly on a full stomach. In an ordinary death, the membrane 
 ceases to secrete before the circulation is at an end. That there 
 is no special virtue in living things which prevents their being 
 digested is shewn by the fact, that the legs of a frog or the ear 
 of a rabbit introduced into a stomach through a fistula are readily 
 digested. It has been suggested that the blood-current keeps up an 
 alkalinity sufficient to neutralize the acidity of the juice in the 
 region of the glands themselves; but this will not explain why 
 the pancreatic juice, which is active in an alkaline medium, does 
 not digest the j^roteids of the pancreas itself, or why the digestive 
 cells of the bloodless actinozoon or hydrozoon do not digest them- 
 selves. We might add, it does not explain why the amoeba, Avhile 
 dissohning the protoplasm of the swallowed diatom, does not dis- 
 solve its owTi protoplasm. We cannot answer this question at all 
 at present, any more than the similar one, why the delicate 
 protoplasm of the amoeba resists during life all osmosis, while a few 
 moments after it is dead, osmotic effects become abundantly evident. 
 
 The secretion of bile needs a few additional w^ords. The analogy 
 of the other glands and what we already know of the microscopic 
 changes in the hepatic cells, leads us to believe that the secretion 
 of even such a complex fluid as the bile is in the main the result of 
 the direct metabolic activity of the protoplasm of the hepatic cells. 
 And this view is supported by the fact that after extirpation of the 
 liver, no accumulation of the biliary constituents is observed to take 
 place during the few hours of life remaining to the animal after the 
 operation. Still the great complexity of the secretion introduces 
 several very important considerations. In the first place, the liver, 
 unlike the other digestive glands, has a double supply of blood; and 
 vain attempts have been made to settle by direct experiment the 
 question whether the hepatic artery or the vena portse is the more 
 closely concerned in the production of bile. Ligature of the 
 hepatic artery has sometimes had no effect on the secretion, some- 
 times has interfered with it. Sudden ligature of the vena portre at 
 once stops the flow of bile; but gradual obliteration may be effected 
 Avithout either causing death or even interfering with the secretion, 
 anastomotic branches forming a collateral circulation, and thus 
 maintaining an efficient flow of blood through the liver. The 
 problem, which is probably a barren one, cannot be settled in this way. 
 
 In the second place, the hepatic cells not only secrete bile, but. 
 
280 SECRETION OF BILE. [Book it. 
 
 as we shall see later on, take an active part in other operations of 
 even greater importance. The consideration of the question in 
 what way these several functions of the hepatic cells are related to 
 each other must be deferred for the present. 
 
 In the third place, even if we maintain that the chief constitu- 
 ents of the bile are manufactured in the hepatic cells, and not 
 simply drained off from the blood, we are not thereby precluded 
 from admitting that the hepatic cells may avail themselves of 
 certain half-made materials, the arrival of which in the blood may, 
 so to speak, lighten their labours, or that they may even boldly seize 
 iipon and pass off as their own handiwork any wholly manufactured 
 constituents which may be offered to them. Thus we have already 
 seen reasons for thinking that the bile-pigments are not made de novo 
 in the hepatic cells, but spring from haemoglobin, the change in the 
 liver being one of comparatively simple transformation. So also it is 
 quite possible, though not proved, that much if not all of the choles- 
 terin of bile is merely withdrawn by the liver from the body at large. 
 And even with the central components of bile, the bile-salts, we know 
 that in the case of taurocholic acid, taurin is normally present in 
 certain tissues, and that in the case of glycocholic acid, glycin, if not 
 a normal constituent of any tissue, is present in the body, since the 
 body can convert benzoic into hippuric acid, as we shall see in a 
 succeeding section ; so that the formation of these bodies by the 
 hepatic cells may be limited to the production of cholalic acid and 
 its conjugation with one or other of the above amido-acids. More- 
 over as a matter of fact, we find that the flow of bile from a biliary 
 fistula is much increased by the injection of bile into the small 
 intestine. This experiment renders it possible that some of the 
 bile which in natural digestion is poured into the intestine is re- 
 absorbed, and carried back to the liver to do duty over again. 
 
 In medical practice, distinction is drawn between jaundice by 
 suppression of the secreting functions of the liver and jaundice by 
 retention, brought about by an obstruction existing in some part of 
 the biliary passages. The gravity of the symptoms in the first class 
 of cases shews that an arrest or a too great diminution of the 
 normal functions of the hepatic cells is at least accompanied by the 
 presence in the blood of substances injurious to life ; but how far 
 the presence of those substances is due to a failure of the manufac- 
 ture of bile and the accumulation in the system of the materials for 
 the formation of bile, or to a failure of other functions of the hepatic 
 cells, must be regarded as at present undetermined. The presence 
 of the bile-pigment in this form of jaundice would seem to indicate 
 that the formation of the pigment, i.e. the transformation of haemo- 
 globin into bilirubin, in contrast to the formation of bile acids, re- 
 quires but little labour on the part of the cell, and may be carried 
 on even when the nutrition of the cell is highly deranged. 
 
SEC. 3. THE MUSCULAR MECHANISMS OF DIGESTION". 
 
 From its entrance into the mouth until such remnant of it as is 
 undigested leaves the body, the food is continually subjected to 
 movements having for their object the trituration of the food as in 
 mastication, or its more complete mixture with the digestive juices, 
 or its forward progress through the alimentary canal. These various 
 movements may briefly be considered in detail. 
 
 Mastication. Of this it need only be said that in man it con- 
 sists chiefly of an up and down movement of the lower jaw, com- 
 bined, in the grinding action of the molar teeth, with a certain 
 amount of lateral and fore-and-aft movement. The lower jaw is 
 raised by means of the temporal, masseter, and internal pterygoid 
 muscles. The slighter effort of depression brings into action chiefly 
 the digastric muscle, though the mylohyoid and geniohyoid probably 
 share in the matter. Contraction of the external pterygoids pulls 
 forward the condyles, and thrusts the lower teeth in front of the 
 upper. Contraction of the pterygoids on one side will also throw 
 the teeth on to the opposite side. The lower horizontally placed 
 fibres of the temporal serve to retract the jaw. 
 
 During mastication the food is moved to and fro, and rolled 
 about by the movements of the tongue. These are effected by the 
 muscles of that organ governed by the hypoglossal nerve. 
 
 The act of mastication is a voluntary one, guided, as are so 
 many voluntary acts, not only by muscular sense but also by contact 
 
282 DEGLUTITION. [Book ii. 
 
 sensations. The motor fibres of the tifth cranial nerve convey 
 motor impulses from the brain to the muscles ; but paralj^sis of the 
 sensory fibres of the same nerve renders mastication difficult by 
 depriving the Avill of the aid of the usual sensations. 
 
 Deglutition. The food when sufficiently masticated is^ by the 
 movements of the tongue, gathered up into a bolus on the middle 
 of the upper surface of that organ. The front of the tongue being 
 raised — partly by its intrinsic muscles, and partly by the stylo- 
 glossus — the bolus is thrust back between the tongue and the 
 palate through the anterior pillars of the fauces or isthmus faucium. 
 Immediately before it arrives there, the soft palate is raised by the 
 levator palati, and so brought to touch the posterior wall of the 
 pharynx, which, by the contraction of the upper margin of the 
 superior constrictor of the pharynx, bulges somewhat forward. The 
 elevation of the soft palate causes a distinct rise of pressure in the 
 nasal chambers ; this can be shewn by introducing a water mano- 
 meter into one nostril, and closing the other just j)revious to 
 swallowing. By the contraction of the palato-pharyngeal muscles 
 which lie in the posterior pillars of the fauces, the curved edges of 
 those pillars are made straight, and thus tend to meet in the middle 
 line, the small gap between them being filled up by the uvula. 
 Through these manoeuvres, the entrance into the posterior nares is 
 blocked, while the soft palate forms a sloping roof, guiding the bolus 
 down the pharynx. By the contraction of the stylo-pharyngeus 
 and palato-pharyngeus, the funnel-shaped bag of the pharynx is 
 brought up to meet the descending morsel, very much as a glove 
 may be drawn up over the finger. 
 
 Meanwhile in the larynx, as shewn by the laryngoscope, the 
 arytenoid cartilages and vocal cords are approximated : the latter 
 being also raised so that they come very near to the false vocal 
 cords : the cushion at the base of the epiglottis covers the rima 
 glottidis, while the epiglottis itself is depressed over the larynx. 
 The thyroid cartilage is now, by the action of the laryngeal muscles, 
 suddenly raised up behind the hyoid bone, and thus assists the 
 epiglottis to cover the glottis. This movement of the thyroid can 
 easily be felt on the outside. Thus, both the entrance into the 
 posterior nares and that into the larynx being closed, the impulse 
 given to the bolus by the tongue can have no other effect than to 
 propel it beneath the sloping soft palate, over the incline formed 
 by the root of the tongue and the epiglottis. The palato-glossi or 
 constrictores isthmi faucium, which lie in the anterior pillars of the 
 fauces, by contracting, close the door behind the food which has 
 passed them. 
 
 When the bolus of food is large, it is received by the middle 
 and lower constrictors of the pharynx which, contracting in 
 sequence from above downwards, thrust it into the oesophagus, 
 along which it is driven by a similar series of successive con- 
 
CiiAi'. 1.] DKJESTIOX. 283 
 
 tractions which we shall speak of iininediati'ly a.s peristaltic 
 action. This comparatively slow descent of the food from the 
 pharjnx into the stomach, may be readily seen if animals with 
 lon<T necks such as horses and dogs be watched while swallowing. 
 Recent observations however seem to shew that when the morsel 
 is not large and especially when the substance swallowed is liquid, 
 the movement of the back jiart of the tongue is sufhcient not 
 merely to introduce the food into the grasp of the constrictors 
 of the pharynx, but even to propel it rapidly, to shoot it in fact, 
 along the lax oesophagus before the muscles of that organ have 
 time to contract. In such a mode of swallowing the middle and 
 lower constrictors take little or no pail in driving the food 
 onward, though they and the oesophagus appear to contract from 
 above downwards after the food has passed by them, as if to 
 complete the act and to ensure that nothing has been left behind. 
 Deglutition in this fashion Still remains possible after the con- 
 strictors have become paralysed by section of their motor nerves. 
 
 Deglutition therefore, though a continuous act, may be regarded 
 as divided into three stages. The first stage is the thrusting of the 
 food through the isthmus faucium ; this may be either of long or 
 short duration. The second stage is the passage through the upper 
 part of the pharynx. Here the food traverses a region common 
 both to the food and to respiration, and in consequence the move- 
 ment is as rapid as possible. The third stage is the descent through 
 the grasp of the constrictors. Here the food has passed the respira- 
 tory orifice, and in consequence its passage may again become com- 
 paratively slow, or, as we have seen, may continue to be rapid. 
 
 The first stage in this complicated process is undoubtedly a 
 voluntary action. The raising of the soft palate and the approxi- 
 mation of the posterior pillars may also be, at times, voluntary^ 
 since they have been seen, in a case where the pharjTix was laid 
 bare by an operation, to take place before the food had touched 
 these parts ; but the movement may take place without any 
 exercise of the will or presence of consciousness. And indeed the 
 second stage taken as a whole, though some of the earlier com- 
 ponent movements are, as it were, on the borderland between the 
 voluntar}' and involuntaiy kingdoms, must be regarded as a reflex 
 act. The third and last stage, -whatever be the exact form which it 
 takes, is undoubtedly reflex ; the will has no power whatever over 
 it and can neither originate, stop, nor modify it. 
 
 Deglutition in fact as a whole is a reflex act ; it cannot take 
 place unless some stimulus be applied to the mucous membrane of 
 the fauces. When Ave voluntarily bring about swallowing move- 
 ments with the mouth empty, we supply the necessarv' stimulus 
 by forcing with the tongue a small quantity of saliva into the 
 fauces, or by touching the fauces with the tongue itself. 
 
 In the reflex act of deglutition the aS'erent impulses originated 
 in the fauces arc carried up chiefly by the glosso-pharj-ngeal, but 
 
284 MOVEMENTS OF THE (ESOPHAGUS. [Book ii. 
 
 also by branches of the fifth, and by the pharyngeal branches 
 of the superior laryngeal division of the vagus. The efferent 
 impulses descend the hypoglossal to the muscles of the tongue, and 
 pass down the glosso-pharyngeal, the vagus through the pharyngeal 
 plexus, the fifth and the facial, to the muscles of the fauces and 
 pharynx : their exact paths being as yet not fully known, and 
 probably varying in different animals. The laryngeal muscles are 
 governed by the laryngeal branches of the vagus. 
 
 The centre of the reflex act lies in the medulla oblongata. 
 Deglutition can be excited, by tickling the fauces, in an animal 
 rendered unconscious by removal of the brain, provided the 
 medulla be left. If the medulla be destroyed, deglutition is 
 impossible. The centre for deglutition lies higher up than that 
 of respiration, so that the former act is frequently impaired or 
 rendered impossible while the latter remains untouched. It is 
 probable that, as is the case in so many other reflex acts, the 
 whole movement can be called forth by stimuli affecting the 
 centre directly, and not acting on the usual afferent nerves. 
 
 Movements of the CEsophagus. As we have already said, in 
 certain cases at all events, the food is carried down from the 
 pharynx to the stomach in a comparatively slow manner, by the 
 action of the muscular coat of the oesophagus itself. Contractions of 
 the circular fibres occur in succession from above downwards, driving 
 the food before them, very much as a fluid may be driven along a 
 tube by squeezing it. The movement is probably assisted by 
 a similarly progressive contraction of the longitudinal muscular 
 coat ; but the exact manner in which this acts is uncertain. Such 
 a progressive movement, of v/hich we have already spoken on 
 p. 101, and which is much more pronounced in the small intestine 
 than in other parts of the alimentary canal, is spoken of as "peristaltic 
 action." These peristaltic movements of the oesophagus may, like 
 those of the intestine, be seen after removal of the organ from the 
 body ; and indeed may continue to appear upon stimulation, for an 
 unusual length of time. Nevertheless, in the intact body, the 
 movements of the oesophagus seem to be much more closely 
 dependent on the central nervous system than do those of the 
 intestines; the contractions are not, as in the latter case, trans- 
 mitted from section to section of the tube, but afferent impulses 
 started in the pharynx and passing to the medulla oblongata, give 
 rise to reflex efferent impulses which descend along nervous tracts 
 to successive portions of the organ. If the oesophagus be cut 
 across some way down, or if a portion of the middle region be 
 excised, stimulation of the pharynx will produce a peristaltic 
 contraction, which travelling downwards will not stop at the 
 section but will be continued on into the lower disconnected 
 portion by means of the central nervous system. And it is stated 
 that ordinary peristaltic contractions of the lower part of the 
 
CiiAr. I.] ])iai:STIOX. 285 
 
 (i'S(tplui<,ais can be readily excited Ly stimulation of the pharynx, 
 but not by stimuli a}»plied to its own mucous membrane. In 
 the reflex act which thus brings about the peristaltic contraction 
 of the oesophagus the afferent nerves are those of the jjharynx, 
 viz, the superior laryngeal nerve, branches of the fifth, and in 
 some animals at least branches of the gl()sso])]iaryngeal, but chiefly 
 the flrst. The centre lies in the medulla oblongata, being a p)art 
 of the general deglutition centre ; and the efferent impulses pass 
 along flbres of the vagus, reaching the upper part of the oesophagus 
 by the recurrent laryngeal nerves and the lower part through the 
 plexuses over the root of the lungs and the stomach, to Avhich the 
 vagus gives origin. Section of the trunk of the vagus renders 
 diflieult the passage of food along the oesophagus, and stimulation 
 of the peripheral stump causes oesophageal contractions. The 
 force of this movement in the oesophagus is considerable ; thus 
 Mosso found that in the dog a ball pulling by means of a pulley 
 against a weight of 250 grammes was readily carried down from 
 the phar}Tix to the stomach. 
 
 The junction of the oesophagus with the stomach remains in a 
 more or less p)ermanent condition of tonic or obscurely rhythmic 
 contraction, more particularly when the stomach is full of food, and 
 thus serves as a sphincter to prevent the return of food from the 
 stomach into the oesophagus. During the passage of the food 
 from the oesophagus into the stomach this sphincter becomes 
 relaxed, probably by a mechanism which will be described in 
 treating of vomitino-. 
 
 o o 
 
 Movements of the Stomach. These are at bottom peristaltic 
 in nature, though largely modified by the peculiar arrangement of 
 the gastric muscular fibres. When food first enters the stomach, 
 the movements are feeble and slight, but as digestion goes on 
 they become more and more vigorous, giving rise to a sort of 
 churning within the stomach, the food travelling from the c_ardiac 
 orifice along the greater curvature to the pylorus, and returning by 
 the lesser curvature, wdiilc at the same time subsidiary currents 
 tend to carry the food which has been passing close to the mucous 
 membrane tow^ard the middle of the stomach, and vice versa. _At 
 the pyloric end strong circular contractions are set up, by Tvhich 
 portions of food, more especially the dissolved parts, but also small 
 solid pieces, are carried through the relaxed sphhicter into the 
 duodenum. As digestion proceeds, more and more material leaves 
 the stomach, which is thus gi'adually emptied, the last portions 
 which are carried through being those matters which are least 
 digestible, and foreign bodies which happen to have been swallowed. 
 The piresence of food then leads to the development of obscurely 
 peristaltic rhythmic movements, the stomach when empty being 
 contracted, but quiescent; but evidently it is not the mere 
 mechanical repletion of the organ which is the cause of the move- 
 
286 MOVEMENTS OF THE SMALL INTESTLNE. [Book ir. 
 
 ments, since the stomach is fullest at the beginning when the 
 movements are slight, and becomes emptier as they grow more 
 forcible. The one thing which does increase jiari j)assu with the 
 movements is the acidity, which is at a minimum when the 
 (generally alkaline) food has been swallowed, and increases steadily 
 onwards. It has not however been definitely shewn that the 
 increasing acidity is the efficient stimulus, giving rise to the 
 movements. 
 
 The nervous mechanism of the gastric movements is at present 
 very obscure. The stomach receives its nervous supply from the 
 vagi and also from the solar plexus, with which the splanchnics are 
 connected. When the vagi are divided, a spasmodic constriction of 
 the cardiac orifice takes place; in other words the tonic action of the 
 sphincter is increased, and food is thus prevented, for a time at least, 
 from leaving the oesophagus. In addition, the natural movements 
 of the stomach itself cease, or become uncertain and irregular, even 
 if food be present. Incomplete movements may be induced by 
 stimulation of the peripheral stumps of the vagi, when the stomach 
 is full, but not so readily if it be empty. The effects of section or 
 stimulation of the splanchnics or of the branches from the solar 
 plexus are uncertain. Nor do we know the exact mechanism by 
 which the pyloric sphincter is used to strain off gradually the 
 more digested portions of the food. The movements of even 
 a full stomach are said to cease during sleep. 
 
 Movements of the small Intestine. Though peristaltic move- 
 ments occur along the whole length of the alimentary canal, from 
 the oesophagus to the rectum, they are more pronounced in the 
 small intestine than elsewhere. When the intestines are watched, 
 after opening the abdomen, circular contractions, that is con- 
 tractions of the circular coat, may be seen travelling lengthways 
 'along the intestine and often upwards as Avell as downwards. 
 Similarly longitudinal contractions, that is contractions of the longi- 
 tudinal coat, may also be seen to travel lengthways. The circular 
 coat being much thicker and stouter than the longitudinal coat, is 
 the more important of the two, and it is by the contractions 
 of the circular coat that in the normal state of things the 
 contents of the intestine are driven along toward the ileo-csecal 
 valve. The contractions of the longitudinal coat appear to be 
 chiefly of use in producing peculiar oscillating movements of the 
 pendent loops in which the intestine is arranged. The rh}i:hmic 
 occurrence of these circular and to-and-fro movements, together 
 with the passive movements caused by the entrance of the fluid 
 contents into or their exit from the various loops, brings about the 
 peculiar writhing of the intestines which has given rise to the 
 phrase peristaltic action. 
 
 The movements, as we have said, take place from above down- 
 wards, and a wave beginning at the pylorus may be traced a long 
 
('II \r. I.] DIGESTIOX. 287 
 
 way down. But contractions nuiy, and in all probability oc- 
 casionally do, begin at various jioints alonr^ the length of the 
 intestine. In the living b(nly the intestines have periods of rest, 
 alternating with perioils of activity, the occurrence of the periods 
 depemling on various circumstances. 
 
 With regard to the causation of the peristaltic movements of 
 the intestine, this much may be affirmed that they may occur, as 
 in a piece of intestine cut out from the body, wholly indepen- 
 dently of the central nervous system; and the only nervous elements 
 which can be regarded as essential to their development are 
 the ganglia of Auerbach or those of Meissner in the intestinal 
 walls. Though the movements can readily be excited by stimuli, 
 applied either to the outside, or, more especially, to the inside of 
 the intestine, they are probably at bottom automatic. The 
 presence of food, especially of food in motion, may at times act 
 as a stimulus, and may in all cases be a condition afifecting the 
 nature and extent of the movement; but cannot be regarded as 
 the real cause of the action. "When any body is introduced into 
 the intestine, a contraction at first occurs, but soon passes off as 
 the intestine becomes accustomed to the presence of the body. 
 There is no reason Avhy the intestine should not become equally 
 accustomed to the presence of food; and, as a matter of fact, 
 peristaltic movements are often absent when the intestines are 
 full. The presence of food bears about the same relation to the 
 movements of the intestine, that the presence of blood bears to the 
 beat of the heart. Both are favouring but not indispensable 
 conditions : in both cases the action can go on without them. 
 We may add that just as the tension of a muscle increases up 
 to a certain extent the amount of its contraction, and a full heart 
 beats more strongly than an empty one, so distension of the 
 intestine largely increases peristaltic action. Hence in cases of 
 obstruction of the bowels, the movements become distressing by- 
 their violence. 
 
 Among the chief circumstances affecting peristaltic action 
 may be mentioned in the first place the condition of the blood. 
 A lack of oxygen or an excess of carbonic acid in the blood excites 
 powerful movements. This is well seen in asph}-xia, and the 
 powerful post-mortem peristaltic movements witnessed on opening 
 a recently-killed animal, as well as those which firequently occur 
 when in the living body, the blood-stream is cut off by com- 
 pression of the aorta, are probably due to the deficiency of oxygen 
 <^r the accumulation of carbonic acid in the blood and tissues of the 
 intestinal walls. Conversely, saturation of the blood with oxygen, 
 as in the peculiar condition known as apncea (see chapter on 
 Respiration), tends to check peristaltic movements. 
 
 In the second place, peristaltic action is largely influenced by 
 nervous influences passing along the splanchnic and vagus nerves. 
 The movements yv^\ go on after section of both these nerves ; but 
 
288 MOVEMENTS OF THE LARGE INTESTINE. [Book ii. 
 
 as a general rule, while stimulation of the splanchnic tends to 
 check, that of the vagus tends to excite them ; but much has 
 probably yet to be learnt about the exact manner in which these 
 nerves act. It is probably through the vagus that peristaltic 
 movements can be effected in an indirect manner, as in that in- 
 crease of the movements of the intestine in consequence of 
 emotions, which has given rise to the phrase ' my bowels yearned.' 
 
 When the vagus is stimulated, peristaltic contraction is seen to 
 begin at the pylorus of the stomach and so to descend along the 
 intestine. When however the duodenum is mechanically stimu- 
 lated, both a peristaltic and an antiperistaltic wave, that is, a 
 wave of contraction passing upwards instead of downwards, may 
 be observed, the former passing downward and ceasing at the 
 ileo-csecal valve if not before, the latter passing up and ceasing 
 at the pylorus. And when in the exposed intestines a wave, as 
 occasionally happens, begins spontaneously in the duodenum, it 
 may sometimes be seen to pass both upwards and downwards. It 
 is worthy of notice that stimulation of the small intestine is said 
 not to cause movement either in the stomach or large intestine, 
 and stimulation of the large intestine or of the stomach causes 
 no movement of the small intestine, the ileo-csecal valve and the 
 pylorus barring the progress of the waves. 
 
 Certain drugs, such as nicotin, induce strong peristaltic action ; 
 the modus operandi of these and of the more specific purgative 
 drugs is at present uncertain. 
 
 Movements of the large Intestine. These are fundametally 
 the same as those of the small intestine, but distinct in so far as 
 the latter cease at the ileo-coecal valve, at which spot the former 
 normally begin. They are said, moreover, not to be inhibited by 
 stimulation of the splanchnics. 
 
 The faeces in their passage through the colon are lodged in the 
 sacculi during the pauses between the peristaltic waves. Arrived 
 at the sigmoid flexure, they are supported by the bladder and the 
 sacrum, so that they do not press on the sphincter ani. 
 
 Befaecation. This is a mixed act, being superficially the result 
 of an effort of the will, and yet carried out by means of an involun- 
 tary mechanism. Part of the voluntary effort consists in produ- 
 cing a pressure-effect, by means of the abdominal muscles. These 
 are contracted forcibly as in expiration, but the glottis being 
 closed, and the escape of air from the lungs prevented, the whole 
 force of the pressure is brought to bear on the abdomen itself, and 
 so drives the contents of the descending colon onward into the 
 rectum. The sigmoid flexure is by its position sheltered from this 
 jjressure ; a body introduced per anum into the empty rectum is 
 not affected by even forcible contractions of the abdominal walls. 
 
 The anus is guarded by the sphincter ani, which is habitually 
 in a state of normal tonic contraction, capable of being increased or 
 
Chap, i.] DICESTIOX. 281) 
 
 tliiniiiislu'd by a stimulus ii])plic<l, cither iutt-nially or cxtcnially, 
 to the anus. The tonic contraction is in j)art at least duo Xo the 
 action of a nervous centre situated in the lumbar spinal idid. If 
 the nervous connexion of the sphincter with the sj)inal c(jnl be 
 broken, relaxation takes place. If the spinal cord be divided in 
 the dorsal n^tj^ion, the sphincter, after the depressing effect of the 
 operation, which may last several days, has passed off, still main- 
 tains its tonicity, she\vin<^^ that the centre is not placed hif,dier up 
 than the lumbar region of tlie cord. The increased or diminished 
 contraction following on local stimulation is probably due to reflex 
 augmentation or inhibition of the action of this centre. The 
 centre is also subject to influences proceeding from higher regions 
 of the cord, and from the brain. By the action of the will, 
 by emotions, or by other nervous events, the lumbar sphincter 
 centre may be inhibited, and thus the sphincter itself relaxed ; or 
 augmented, and thus the sphincter tightened. A second item 
 therefore of the voluntary process in defsecation is the inhibition of 
 the lumbar sphincter centre, and consequent relaxation of the 
 sphincter muscle. Since the lumbar centre is wholly efficient when 
 separated from the brain, the paralysis of the sphincter which occurs 
 in certain cerebral diseases is probably due to inhibition of this 
 centre, and not to paralysis of any cerebral centre. 
 
 Thus a voluntary contraction of the abdominal walls, accom- 
 panied by a relaxation of the sphincter, might press the contents 
 of the descending colon into the rectum and out at the anus. 
 Since however, as we have seen, the pressure of the abdominal 
 walls is warded off the sigmoid flexure, such a mode of defsecation 
 would always end in leaving the sigmoid flexure full. Hence the 
 necessity for these more or less voluntary acts being accompanied 
 by an entirely involuntary augmentation of the peristaltic action 
 of the large intestine and sigmoid flexure. Or rather, to describe 
 matters in their proper order, defsecation takes place in. the 
 following manner. The sigmoid flexure and large intestine be- 
 coming more and more full, stronger and stronger peristaltic action 
 is excited in their walls. By this means the fseces are driven 
 against the siDhinctcr. Through a voluntary act, or sometimes at 
 least by a simple reflex action, the lumbar sphincter centre is 
 inhibited and the sphincter relaxed. At the same time the 
 contraction of the abdominal muscles presses firmly on the descend- 
 ing colon, and thus the contents of the rectum are ejected. 
 
 It must however be remembered that, while in a]ipealing to 
 our own consciousness, the contraction of the abdominal walls and 
 the relaxation of the sphincter seem purely voluntary cflibrts, the 
 whole act of defsecation, including both of these seemingly so 
 voluntarj'- components, may take place in the absence of conscious- 
 ness, and indeed, in the case of the dog at least, after the complete 
 severance of the lumbar from the dorsal cord; In such cases the 
 
290 VOMITING. [Book ii. 
 
 whole act must be purely reflex, excited by the presence of fseces 
 in the rectum. 
 
 Vomiting'. In a conscious individual this act is preceded by 
 feelings of nausea, during which a copious flow of saliva into the 
 mouth takes place. This being swallowed carries down with it a 
 certain quantity of air, the presence of which in the stomach, 
 by assisting in the opening of the cardiac sphincter, subsequently 
 facilitates the discharge of the gastric contents. The nausea is 
 generally succeeded at first by ineffectual retching in which a deep 
 inspiratory effort is made, so that the diaphragm is thrust down as 
 low as possible against the stomach, the lower ribs being at 
 the same time forcibly drawn in; since during this inspiratory 
 effort the glottis is kept closed, no air can enter into the lungs ; 
 but some is drawn into the pharynx, and thence probably descends 
 by a swallowing action into the stomach. In actual vomiting this 
 inspiratory effort is succeeded by a sudden violent expiratory 
 contraction of the abdominal walls, the glottis still being closed, so 
 that the whole force of the effort is spent, as in defsecation, in 
 pressure on the abdominal contents. The stomach is therefore 
 forcibly compressed from without. At the same time, or rather im- 
 mediately before the expiratory effort, by a contraction of its longi- 
 tudinal fibres the oesophagus is shortened and the cardiac orifice 
 of the stomach brought close under the diaphragm, while ap- 
 parently by a contraction of the fibres which radiate from the end 
 of the oesophagus over the stomach, the cardiac orifice, which is 
 normally closed, is somewhat suddenly dilated. This dilation opens 
 a Avay for the contents of the stomach, Avhich, pressed upon by the 
 contraction of the abdomen, and to a certain but probably only to 
 a slight extent by the contraction of the gastric walls, are driven 
 forcibly up the oesophagus, their passage along that channel being 
 possibly assisted by the contraction of the longitudinal muscles. 
 The mouth being widely open, and the neck stretched to afford as 
 straight a course as possible, the vomit is ejected from the body. 
 At this moment there is an additional expiratory effort which 
 serves to prevent the vomit passing into the larynx. In most 
 cases too the posterior pillars of the fauces are approximated, 
 in order to close the nasal passage against the ascending stream. 
 This however in severe vomiting is frequently ineffectual. 
 
 Thus in vomiting there are two distinct acts ; the dilation of 
 the cardiac orifice and the extrinsic pressure of the abdominal walls 
 in an expiratory effort. Without the former the latter, even when 
 distressingly vigorous, is ineffectual. Without the latter, as in 
 urari poisoning, the intrinsic movements of the stomach itself are 
 rarely sufficient to do more than eject gas, and, it may be, a very 
 small quantity of food or fluid. Pyrosis or waterbrash is probably 
 brought about by this intrinsic action of the stomach. 
 
 During vomiting the pylorus is generally closed, so that but 
 little material escapes into the duodenum. When the gall-bladder 
 
Chat, i] DIGESTIOK. 291 
 
 is full, a copious flow of bile into the duodeuum accompanies the 
 act of voniitin«T. Part of this may find its way into the stomach, as 
 in bilious vomiting, the pylorus then having evidently been opened. 
 
 The nervous mechanism of vomiting is complicated and in 
 many aspects obscure. The efferent impulses which cause the 
 expiratory effort must come from the respiratory centre in the 
 medulla; with these we shall deal in speaking of respiration. The 
 dilation of the cardiac orifice is caused, in part at least, by efferent 
 impidses descending the vagi, since when these are cut real 
 V(»miting with discharge of the gastric contents is difficult, through 
 •want of readiness in the dilation. The sympathetic abdominal 
 nerves coming from the ca'liac ganglia and the splanchnic nerves 
 seem to have no share in the matter. The efferent impulses 
 which cause the flow of saliva in the introductory nausea descend 
 the facial along the chorda tjTnpani branch. These various 
 impulses may best be considered as starting from a vomiting 
 centre in the medulla, having close relations with the respiratory 
 centre. This centre may be excited, may be throAATi into action, 
 in a reflex manner, by stimuli applied to peripheral nerves, as 
 •when vomiting is induced by tickling the fauces, or by irritation 
 of the gastric membrane, or by obstruction due to ligature, hernia, 
 etc., of the intestine. That the vomiting in the last instance is 
 due to nervous action, and not to any regurgitation of the intestinal 
 contents, is shewn by the fact that it -svill take place when the 
 intestine is perfectly empty and may be prevented by section of 
 the mesenteric nerves. The vomiting attending renal and biliary 
 calculi is apparently also reflex in origin. The centre however 
 may be affected directly, as probably in the cases of some poisons, 
 and in some instances of vomiting from disease of the medulla 
 oblongata. Lastly, it may be thro\\-n into action by impulses 
 reaching it from parts of the brain higher up than itself, as in 
 cases of vomiting, produced by smells, tastes and emotions, and by 
 the memory of past occasions, and in some cases of vomiting due 
 to cerebral disease. 
 
 Many emetics, such as tartar emetic, appear to act directly on 
 the centre, since, introduced into the blood, they will produce 
 vomiting even Avhen a bladder is substituted for the stomach. 
 Others again, such as mustard and water, act in a reflex manner 
 by irritation of the gastric mucous membrane. With others, again, 
 which cause vomiting by developing a nauseous taste, the reflex 
 action involves parts of the brain higher than the centre itself. 
 
 19—2 
 
SEC. 4. THE CHANGES WHICH THE FOOD UNDERGOES 
 IN THE ALIMENTARY CANAL. 
 
 Having studied the properties of the digestive juices, and the 
 various mechanisms by means of which the food is brought under 
 their influence, we have now to consider what, as matters of fact, 
 are the actual changes which the food does undergo in passing 
 along the alimentary canal, what are the steps by which the 
 food is converted into fgsces. 
 
 In the mouth the presence of the food, assisted by the move- 
 ments of the jaw, causes, as we have seen, a flow of saliva. By 
 mastication, and by the addition of mucous saliva, the food is 
 broken into small pieces, moistened, and gathered into a convenient 
 bolus for deglutition. In man some of the starch is, even during 
 the short stay of the food in the mouth, converted into sugar ; for 
 if boiled starch free from sugar be even momentarily held in the 
 mouth, and then ejected into water (kept boiling to destroy the 
 ferment), it will be found to contain a decided amount of sugar. 
 In many animals no such change takes place. The viscid saliva of 
 the dog serves almost solely to assist in deglutition; and even the 
 longer stay which food makes in the mouth of the horse is in- 
 sufficient to produce any marked conversion of the starch it may 
 contain. During the rapid transit through the oesophagus no 
 appreciable change takes place. 
 
 In the stomach, the arrival of the food, the reaction of which 
 is either naturally alkaline, or is made alkaline, or at least is 
 
CiiAr. I.] DIGESTIOX. 293 
 
 it\luood in aciility, by tlio addition of saliva, causes a flow of 
 ij^astric juice. This, alri'ady coninicucing while the food is its yet 
 111 the mouth, iucreases as the food accumulates in the st<Miia(;h, 
 and as, by the churning gastric movements, unchanged particles 
 are continually being brought into contact with the mucous 
 membrane. Ab)reover, the absorption of the earlier digested 
 jiortions gives rise to a further increase of secretion and especially 
 of pi'psin. The percentage of pepsin in the gastric juice (in 
 the dog) varies considerably, actually sinking during the earlier 
 stages but rising rapidly afterwards and attaining a maximum 
 at about the fourth or fifth hour. The secretion of acid appears 
 to continue at a fairly constant rate ; and consequently, unless 
 neutralized by fresh alkaline food, the reaction of the gastric 
 contents becomes more and more distinctly acid as digestion 
 proceeds. It would appear that in man, sometimes at least, the 
 contents of the stomach do not at first contain any free acid and 
 during this period the conversion of starch into sugar can still 
 go on. When the contents become acid, the conversion is arrested, 
 and indeed the amylolytic ferment probably destroyed. The fats 
 themselves probably remain in great measiire unchanged ; though 
 it would appear that in the dog at least a certain amount of fat 
 can be digested, that is emulsionized, or even partly split up 
 into fatty acids, by the action of the gastric juice, and absorbed. 
 Moreover even in man, through the conversion of proteids into 
 peptone, not only are the more distinctly proteid articles of food, 
 such as meat, broken up and dissolved, but the proteid framework, 
 in which the starch and fats are frequently imbedded, is loosened, 
 the starch-gi-anules are set free, and the fats, melted for the most 
 part by the heat of the stomach, tend to run together in large 
 drops, which in turn are more or less apt to be broken up into 
 an imperfect emulsion. The collagenous tissues are dissolved ; 
 and hence the natural bundles of meat and vegetables fall asunder; 
 the muscular hbre splits up into discs, and the protoplasm is 
 dissolved from the vegetable cells. Milk is at once curdled by the 
 rennet ferment and the clotted casein subsequently dissolved. 
 Since peptone and the other products of artificial digestion with 
 gastric juice have been found in the contents of the stomach, 
 we have every reason to believe that natural digestion in the 
 stomach agrees with the results of laboratory experiments described 
 in a previous section. While these changes are proceeding, the 
 thick turbid greyish liquid or chyme, formed by the imperfectly 
 dissolved food, is from time to time ejected through the j^ylorus, 
 accompanied by even large morsels of solid less-digested matter. 
 Tliis may occur -within a few minutes of food having been taken; 
 but the larger escape from the stomach probably does not in man 
 begin till from one to two, and lasts from four to five hours, after 
 the meal, becoming more rapid tow-ards the end, and such pieces 
 as most resist the gastric juice being the last to leave the stomach. 
 
294 CHANGES OF FOOD IN THE STOMACH. [Book ii. 
 
 The time taken up in gastric digestion probably varies not only 
 with different articles of food but also with varying conditions of 
 the stomach and of the body at large. In different animals it 
 varies very considerably, being from 12 to 24 hours in the dog, 
 while the stomachs of rabbits are never empty but always remain 
 largely filled with food. 
 
 In a dog fed on an exclusively meat diet, nearly the whole of 
 the digestion is carried out by the stomach, very little work 
 apparently being left for the intestines. In man, especially on 
 a mixed diet, the case in all probability is different, a considerable 
 portion of the proteids as well as the greater part of the fats and 
 carbohydrates passing but little changed through the pylorus. 
 But our information on this matter is imperfect being chiefly 
 drawn from the study of cases of gastric or duodenal fistula, in 
 which probably the order of things is not normal or being in 
 large measure deductions from experiments on dogs, whose economy 
 in this respect must be largely different from our o^vn. 
 
 In the presence of healthy gastric juice, and in the absence of 
 any nervous interference, the question of the digestibility of any 
 food is determined chiefly by mechanical conditions. The more 
 finely divided the material, and the less the proteid constituents 
 are sheltered by not easily soluble envelopes, such as those of 
 cellulose, the more rapid the solution. So also pieces of hard- 
 boiled Qgg, which have to be gradually dissolved from the outside, 
 are less easily digested than the more friable muscular fibre, the 
 repeated transverse cleavage of which increases the surface exposed 
 to the juice. Unboiled white of egg again, unless thoroughly 
 beaten up and mixed with air, is less digestible than the same 
 boiled. The unboiled white forms a viscid clotted mass, of low 
 diffusibility, into which the juice permeates with the greatest 
 difficulty. And so with the other instances. Beyond this me- 
 chanical aspect of digestibility, it is to be remembered that 
 different substances may differently affect the gastric membrane, 
 promoting or checking the secretion of the juice. Hence a 
 substance, the mass of which is readily dissolved by gastric juice, 
 and which offers no mechanical obstacles to digestion, may yet 
 prove indigestible by so affecting the gastric membrane through 
 some special constituent (or possibly in other ways) as to inhibit 
 the secretion of the juice. 
 
 That substances can be absorbed from the cavity of the stomach 
 into the circulation is proved by the fact that food when introduced 
 disappears very largely from the stomach of an animal, the pylorus 
 of which has been ligatured. But we cannot speak with certainty 
 as to what extent in ordinary life gastric absorption takes place, or 
 by what mechanism it is carried out. The presumption is, that 
 peptone and the diffusible sugars pass by osmosis direct into the 
 capillaries, and so into the gastric veins. In a dog fed on meat the 
 quantity of peptone present at any one time in the stomach has 
 
TiiAP. i] DIGKSTIOX. 29.1 
 
 been found fairly constant. From this it may fairly Ix; inftrr<«l 
 that the peptone is absorbed in proporti(jn as it is fornu-d. 
 
 In the act of swallowing, no inconsiderable quantity of air is 
 carried down into the stomach, entangled in the saliva, or in the 
 fooil. This is returned in eructations. When the gas of eructation 
 or that obtained directly from the stomach is examined, it is found 
 to consist chietly of nitrogen and carbonic acid, the oxygon of the 
 atmospheric air having been largely absorbed. In most cases the 
 carbonic acid is derived by simple ditfusion from the blood, or from 
 the tissues of the stomach, which similarly take up the oxygen. 
 In many cases of flatulency, however, it may arise from a fermen- 
 tative decomiDosition of the sugar which has been taken as such in 
 food, or which has been produced from the starch, the gas being 
 either formed in the stomach or passing upwards from the intestine 
 through the pylorus. 
 
 The enormous quantity of gas which is discharged through the 
 mouth in cases of hysterical flatulency, even on a 2:)erfectly empty 
 stomach, and which seems to consist largely of carbonic acid, 
 presents difiiculties in the Avay of explanation ; it is possible that it 
 may be simply diffused from the blood. 
 
 In the small intestine, the semi-digested acid food, or chjTue, 
 as it passes over the biliary orifice, causes gushes of bile, and at the 
 same time, as we have seen (p. 265), the pancreatic juice, which 
 flowed freely into the intestine at the taking of the meal, is 
 secreted again -with renewed vigour, when the gastric digestion is 
 completed. These two alkaline fluids tend to neutralize the 
 acidity of the chyme, but the contents of the duodenum do not 
 become distinctly alkaline until some distance from the pylorus is 
 reached. Even in the lower part of the ileum the chyme may be 
 acid ; possibly however in such cases it has been reacidified in con- 
 sequence of acid fermentations taking place in the intestinal 
 contents. The reaction of these contents appears to vary in fact 
 according to the nature of the food, the changes which it under- 
 goes, and other circumstances. Moreover it is probably not the 
 same in all animals. In a dog fed on starch and fat, the contents 
 of the intestine may remain acid throughout. 
 
 The conversion of starch into sugar, which as we have seen is 
 probably arrested in the stomach, is resumed with great activity 
 and indeed completed by the pancreatic juice, possibly assisted by 
 the succus entericus ; portions however of still undigested starch 
 may be found in the large intestine and even at times in the faeces. 
 
 The pancreatic juice, as we have seen, emulsifies fats, and also 
 splits them into their respective fatty acids and glycerine. The 
 fatty acids thus set firee become converted by means of the alkaline 
 contents of the intestine into soaps ; but to what extent saponifica- 
 tion thus takes place is not exactly known. Undoubtedly soaps 
 have to a small extent been found both in portal blood and in the 
 
296 DIGESTION OF FATS. [Book ii. 
 
 thoracic duct after a meal ; but there is no proof that any large 
 quantity of fat is introduced in this form into the circulation. On 
 the other hand, the presence of neutral fats, both in portal blood, 
 and especially in the lacteals, is a conspicuous result of the diges- 
 tion of fatty matters ; and in all probability saponification in the 
 intestine is a subsidiary process, intended rather to facilitate the 
 emulsion of neutral fats than to introduce soaps as such into the 
 blood. For the presence of soluble soaps favours the emulsion of 
 neutral fats. Thus a rancid fat, i.e. a fat containing a certain 
 amount of free fatty acid, forms an emulsion with an alkaline fluid 
 more readily than does a neutral fat. A drop of rancid oil let fall 
 on the surface of an alkaline fluid, such as a solution of sodium 
 carbonate of suitable strength, rapidly forms a broad ring of emul- 
 sion, and that even without the least agitation. As saponification 
 takes place at the junction of the oil and alkaline fluid currents 
 are set up, by which globules of oil are detached from the main 
 drop and driven out in a centrifugal direction. The intensity of 
 the currents and the consequent amount of emulsion depend on 
 the concentration of the alkaline medium and on the solubility of 
 the soaps which are formed ; hence some fats such as cod-liver oil 
 are much more easily emulsionized in this way than others. Now 
 the bile and pancreatic juice supply just such conditions as the 
 above for emulsionizing fats : they both together afford an alkaline 
 medium, the pancreatic juice gives rise to an adequate amount of 
 free fatty acid, and the bile in addition brings into solution the 
 soaps as they are formed. So that we may speak of the emulsion 
 of fats in the small intestine as being carried on by the bile and 
 pancreatic juice acting in conjunction ; and as a matter of fact the 
 bile and pancreatic juice do largely emulsify the contents of the 
 small intestine, so that the greyish turbid chyme is changed into a 
 creamy-looking fluid, which has been sometimes called chyle. It 
 is advisable however to reserve this name for the contents of the 
 lacteals. 
 
 This mutual help of bile and pancreatic juice in producing an 
 emulsion, explains to a certain extent the controversy which long 
 existed between those who maintained that the bile and those who 
 maintained that the pancreatic juice was necessary for the diges- 
 tion and absorption of fatty food. That the pancreatic juice does 
 produce in the intestine such a change as favours the transference 
 of neutral fats from the intestine into the lacteals, is shewn by the 
 fact that in diseases affecting the pancreas, much fatty food 
 frequently passes through the intestine undigested, and great 
 wasting ensues ; but it cannot be maintained that the pancreatic 
 juice is the sole agent in this matter, since in animals in which the 
 pancreatio ducts have been successfully ligatured chyle is still 
 found in the lacteals. On the other hand, that the bile is of use in 
 the digestion of fat is shewn by the prevalence of fatty stools in 
 cases of obstruction of the bile-ducts ; and though the operation of 
 
f'liAP. I.] DfdESTIOX. 297 
 
 ligaturiuf,' the bile-ducts, and leading all the bile externally through 
 a tistula ul' the gall bladder, is open to objection, since it so 
 exhausts the animal as indirectly to affect digestion, still the 
 results of Bidder and Schmidt, in which the resorption of fat was 
 distinctly lessened (the quantity of fat in the lacteals falling from 
 32 to '02 p.c.) by the ligature and tistula, obviously point to the 
 same conclusion. That in man the succus entericus possesses a 
 wholly insutticient emulsifpng power is she-vvn by the observation 
 of Busch, in a case where the duodenum opened on the surface by 
 a tistula in such a way that the lower part of the intestine could 
 be kept free from the contents of the upper part containing the 
 bile and pancreatic juice and matters proceeding from the stomach. 
 Fats introduced into the lower part, where they could not be acted 
 upon either by the bile or by the jDancreatic juice were but slightly 
 digested. Without denWng the possible assistance of the succus 
 entericus, or even of gastric juice, we may conclude that the diges- 
 tion of fat is in the main carried out by the conjoint action of bile 
 and pancreatic juice. 
 
 We have seen that the addition of bile to a digesting mixture 
 gives rise to a precipitate consisting of parapeptone, and bile salts 
 Anth some pepsin, but that on the further addition of bile this pre- 
 cipitate is redissolved. In the upper part of the duodenum the 
 inner surface, if examined while digestion is going on, is found to be 
 lined by a coloured granular material, which is probably a precipi- 
 tate thus formed ; but the purpose of its formation does not seem 
 clear. It is more important to remember that not only is bile anta- 
 gonistic to peptic digestion, but apparently pepsin is destroyed by 
 tripsin in an alkaline medium, so that with the flow of bile and 
 pancreatic juice into the duodenum the processes which have been 
 going on in the stomach come to an end. In fact it would seem 
 that the juices of the various districts of the alimentary canal are 
 mutually destructive ; thus, Avhile pej^sin in an acid solution de- 
 stroys the active constituents of saliva, and of pancreatic juice (pro- 
 bably also those of the succus entericus), it is in its turn antagonized 
 or destroyed by the bile and the other alkaline juices of the intes- 
 tine. Hence pancreatic juice introduced through the mouth must 
 lose its powers in the stomach and can only be of use as an alkaline 
 medium containing certain proteid matters. On the other hand if, 
 as we have reason to believe, the contents of the stomach as they 
 issue from the pylorus still contain a large quantity of undigested 
 proteids, these must be digested by the pancreatic juice (^^ith or 
 ^\^thout the assistance of the succus entericus), the action of which 
 seems to be assisted or at least not hindered by bile. To what 
 stage the pancreatic digestion is carried, whether peptone is chiefly 
 formed, and when formed at once absorbed, or whether the pan- 
 creatic juice in the body, as out of the body, carries on its work 
 in the more destructive form, whereby the proteid material sub- 
 jected to it is broken dowTi largely int<i k-ucin and tyrosin, is 
 
298 DIGESTION OF FATS. [Book it. 
 
 at present not exactly known. Leucin and tyrosin have been 
 found in the intestinal contents, and may therefore be formed 
 during normal digestion, but whether a large quantity or a small 
 quantity of the proteid material of food is thus hurried into 
 a crystalline form cannot be definitely stated. The extent to 
 which the action is carried is probably different in different 
 animals, and varies also according to the nature of the meal and 
 the condition of the body. Possibly when a large and unnecessary 
 quantity of proteid material is taken at a meal together mth other 
 substances, no inconsiderable amount of the proteids undergo this 
 profound change, and, as we shall see, rapidly leave the body as 
 urea, without having been used by the tissues, their contribution 
 to the energy of the body being limited to the heat given out 
 during their formation. To this apparently wasteful use of 
 proteids we shall return in speaking of what is called the ' luxus 
 consumption ' of food. 
 
 Possibly also, in the intestines as in the laboratory, this 
 pancreatic digestion of proteids in excess is accompanied by a 
 considerable development of bacteria and other organized bodies, 
 which create trouble by inducing fermentative changes in the 
 accompanying saccharine constituents of the chyme. That fer- 
 mentative changes may occur in the small intestine is indicated 
 by the facts that the gas present there may contain free hydrogen, 
 and that chyme after removal from the intestine continues at 
 the temperature of the body to produce carbonic acid and hydrogen 
 in equal volumes. This suggests the possibility of the sugar of 
 the intestinal contents undergoing the butjTic acid fermentation 
 (during which, as is well known, carbonic anhydride and hydrogen 
 are evolved) and thus, so to speak, put on its way to become fat ; 
 and we shall see hereafter that sugar is somewhere in the body 
 converted into fat. Moreover it is probable that by other fermen- 
 tative changes a considerable quantity of sugar is converted into 
 lactic acid, since this acid is found in increasing quantities as the 
 food descends the intestine. 
 
 Thus during its transit through the small intestine, by the 
 action of the bile and pancreatic juice, assisted possibly to some 
 extent by the succus entericus, the proteids are largely dissolved 
 and converted into j)eptone and other products, the starch is 
 changed into sugar, the sugar possibly being in part further con- 
 verted into lactic acid, and the fats are largely emulsified, and 
 to some extent saponified. These products, as they are formed, 
 pass into either the lacteals or the portal blood-vessels, so that the 
 contents of the small intestine, by the time they reach the ileo- 
 cascal valve, are largely but by no means wholly deprived of their 
 nutritious constituents. As far as water is concerned, the secretion 
 into the small intestine is about equal to the absorption from it, so 
 that the intestinal contents at the end of the ileum, though much 
 more broken up, are about as fluid as in the duodenum. 
 
CiiAi'. I.] DKJKSTIOX. 299 
 
 In the largfe intestine, the contents become once more distinct- 
 ly iicid. This, however, is not caused by any acid secretion from 
 the mucous membrane : the reaction of the intestinal walls in the 
 large as in the small intestine is alkaline. It must therefore arise 
 from acid fermentations going on in the contents themselves; and 
 that fermentations do go on is shewn by the appearance of marsh 
 gas as well as hydrogen in this portion of the aiiinontary canal. 
 The character and amount of fermentation pr(jl)ably depend 
 largely on the nature of the food and probably also vary in 
 ditierent animals. 
 
 Of the particular changes which take place in the large 
 intestine we have no definite knowledge ; but it is exceedingly 
 probable that in the voluminous ctecum of the herbivora, a large 
 amount of digestion of a peculiar kind goes on. We know that in 
 herbivora a considerable quantity of cellulose disappears in passing 
 through the alimentary canal, and even in man some is probably 
 digested. It seems probable that this cellulose digestion is carried 
 on in the large intestine, though we know nothing of the nature of 
 the agency by which it is effected, and possibly the conversion may 
 take place elsewhere as well ; indeed recent evidence goes to shew 
 that in ruminants the change takes place in part in the stomach 
 and that it is effected by the saliva. The other digestive changes 
 are probably of a fermentative kind. 
 
 Be this as it may, whether digestion, properly so called, is all 
 but complete at the ileo-coecal valve, or whether important changes 
 still await the ch}Tne in the large intestine, one great characteristic 
 of the work done in the colon is absorption. By the abstraction of 
 all the soluble constituents, and especially by the withdrawal of 
 water, the liquid ch}'nie becomes as it approaches the rectum con- 
 verted into the fii-m solid fseces, and the colour shifts from the 
 bright orange, which the grey chyme gradually assumes after 
 admixture with bile, into a darker and duller brown. 
 
 In the faeces there are found in the first place the indigestible 
 and undigested constituents of the meal : shreds of elastic tissue, 
 hairs and other corneous elements, much cellulose and chlorophyll 
 from vegetable, and some connective tissue from animal food, frag- 
 ments of disintegrated muscular fibre, fat-cells, and not unfre- 
 quently undigested starch-corpuscles. The amount of each must of 
 course vary very largely, according to the nature of the food, and 
 the digestive powers, temporary or permanent, of the individual. 
 In the second place, to these must be added substances, not 
 introduced as food, but arising as part of, or as products of, the 
 digestive secretions. The fieces contain a ferment similar to 
 pepsin, and an amylolytic ferment similar to that of saliva or 
 pancreatic juice. They also contain mucus in variable amount, 
 sometimes albumin, cholesterin, but}Tic and other fatty acids, hme 
 and magnesia soaps, excretin (a non-nitrogenous crystalline body, 
 
300 CHANGES OF FOOD IN THE FJECES. [Book il 
 
 containing sulphur, obtained by Marcet), colouring matters, and 
 salts, especially those of magnesia. Cholalic acid (and dyslysin) 
 are found in very small quantities only, thus indicating that the 
 bile-salts have been in part at least destroyed (they may have 
 been in part reabsorbed, see p. 280), the less stable taurocholic 
 acid (of the dog) disappearing more readily than the glycocholic 
 acid (of the cow). The fact that the fseces become ' clay-coloured ' 
 when the bile is cut off from the intestine shews that the bile- 
 pigment is at least the mother of the faecal pigment ; and a special 
 pigment, which has been isolated and called stercobilin, is said to 
 be identical with the substance called urobilin, which may be 
 formed from bilirubin \ We have already seen that during artificial 
 pancreatic digestion, a distinctly fsecal odour due to the presence of 
 indol is generated ; and the fact that the presence of bacteria, or 
 other similar organisms, is essential to the production of this body, 
 does not preclude the possibility of it (or of the allied body skatol, 
 having an evil fsecal odour, formed after prolonged putrefaction of 
 the pancreas and present in human excrement) being the chief 
 cause of the natural odour of faeces, for undoubtedly bacteria may 
 exist throughout the whole length of the intestinal canal. At the 
 same time it is quite possible, that specific odoriferous substances 
 may be secreted directly from the intestinal wall, especially from 
 that of the large intestine. 
 
 1 See Appendix. 
 
SEC. 5. ABSORPTION OF THE PRODUCTS OF DIGESTION. 
 
 We have seen that absorption does, or at least may, take place 
 from the stomach. We have also stated that a large absorption, 
 especially of water, occurs along the whole large intestine. We 
 may add that absorption from the large intestine after injection 
 per anum or through a fistula has been observed not only in the 
 case of soluble peptone and sugar, but also in that of starch, white 
 of egg, and casein, though the exact changes undergone by the 
 latter previous to absorption are as yet unknown. 
 
 Nevertheless the largest and most important part of the digested 
 material passes away from the canal, during the transit of food 
 along the small intestine, partly into the lacteals, partly into the 
 portal vessels. The portal vessels are simply parts of the general 
 vascular system, but the lacteals, into which we may at once say 
 the greater part of the fat passes, need special attention. 
 
 The Lymphatics, 
 
 Characters of Chyle. In a fasting animal the contents of the 
 thoracic duct are clear and transparent ; shortly after a meal they 
 become milky and opaque, the change being entirely due to a 
 difference in the quality and quantity of the fluid brought to 
 the duct by the lacteals, that fluid also being, as seen by inspection 
 
302 CHYLE, [Book ii. 
 
 of the mesentery, transparent during fasting, and becoming milky 
 and opaque after a meal, especially after one containing much fat. 
 The contents of the thoracic duct therefore after a meal may be 
 taken as illustrative of the nature of the chyle present in the 
 lacteals, though strictly speaking the chyle of the thoracic duct 
 is mixed with lymph coming from the intestines and from the rest 
 of the body. During fasting the contents of the lacteals agree in 
 their general character with lymph obtained from other structures. 
 
 The contents of the thoracic duct may. be obtained by laying bare 
 the junction of tlie subclavian and jugular veins and introducing a 
 cannula into the duct as it enters into the venous system at that point. 
 The operation is not unattended with diificulties. 
 
 Chyle obtained from the thoracic duct, after a meal, is a white 
 milky-looking fluid, which after its escape coagulates, forming a 
 not very iirm clot. The nature of the coagulation seems to be 
 exactly the same as that of blood. The surface of the clot after 
 exposure to air becomes pink, even though no blood be artificially 
 mixed with the chyle during the operation ; the colour is due 
 to immature red corpuscles proper to the chyle. Examined micro- 
 scopically, the coagulated chyle consists of fibrin, a large number 
 of white corpuscles, a small number of developing red corpuscles, 
 an abundance of oil-globules of various sizes but all small, and 
 a quantity of fatty granules, too minute to be recognised under the 
 microscope as fatty in nature, forming the so-called ' molecular 
 basis.' Each oil-globule is invested with an albuminous envelope ; 
 this may be dissolved by the aid of alkalis, whereupon the globules 
 run together. The fibrin and white corpuscles are very scanty 
 (and the red corpuscles entirely absent) in lymph or chyle taken 
 from peripheral vessels; but they increase in quantity as the 
 lymph passes through the lymphatic glands. 
 
 The composition of chyle varies considerably not only in differ- 
 ent animals but in the same animal at different times. The average 
 percentage of solids may perhaps be put down as about 9, that of 
 proteid material as about 4 or 5, and that of fat as about 3 or 4 
 (though the latter may sometimes rise as high as 14), the remainder 
 being extractives and salts. The fats occur chiefly in the form of 
 neutral fats, though some soaps or fatty acids are present. Some 
 amount of lecithin, and cholesterin in considerable quantity, are 
 also frequently present. 
 
 The proteids consist chiefly of serum-albumin, with a globulin, 
 probably paraglobulin, and a variable but small quantity of fibrin. 
 Among the extractives have been found sugar, urea, and leucin ; 
 since these are found in lymph as well as chyle they cannot be 
 regarded as derived exclusively from the intestinal contents. The 
 ash is remarkable for the abundance of sodium chloride and the 
 scantiness of phosphates. Iron is present in greater quantity than 
 can be accounted for by the presence of red corpuscles. 
 
CiiAP. I.] DiaKSTIOX. 303 
 
 The nature of the fiit is supposed to vary with that of the food, 
 but this has not been conclusively shewn. 
 
 The lymph taken from the duct during fasting differs chiefly 
 from that taken after a meal, in the much smaller quantity of fat, 
 the microscope shewing besides the white corpuscles only very few 
 oil-globules, and in the almost entire absence of the molecular basis. 
 Lymph in fact is, broadly sjieaking, blood minus its red corpuscles, 
 and chyle is lymph plus a very large quantity of minutely divided 
 neutral fat. 
 
 It has been calculated that a quantity equal to that of the 
 whole blood may pass through the thoracic duct in 24 hours, and of 
 this it is supposed that about half comes from food through the 
 lacteals and the remainder from the body at large ; but these cal- 
 culations are based on uncertain data. 
 
 Entrance of the Chyle into the Lacteals. The lacteal begins 
 as a club-shaped (or bifurcate) lymphatic space lying in the centre 
 of the villus, and connected with the smaller lymphatic spaces of the 
 adenoid tissue around it; it opens below into the submucous lymph- 
 atic plexus from which the lacteal vessels spring. The adenoid 
 tissue of the surrounding crypts of Lieberklihu is by its lymphatic 
 spaces connected with the same lymphatic plexus. That the finely- 
 divided fat does pass from the intestine, through the epithelial 
 envelope of the villus, into the adenoid tissue, and so into the 
 lacteal chamber, is certain, but much discussion has arisen as to the 
 exact mechanism of the transit. Most observers agree that after a 
 meal the epithelium cells of the villus are loaded with fat and that 
 this fat is derived from the intestinal contents. Since the striation 
 of the hyaline border of the cells is not due to pores, as was once 
 thought, the particles must have entered into the cells very much 
 as foreign particles enter the body of an amosba. The epithelium 
 may thus be said to eat the fat, and subsequently to pass it on 
 into the lymphatic spaces of the adenoid tissue of the villus and so 
 into the central lymphatic chamber. There would thus be a stream 
 of fatty particles through the cell from without inwards, a stream 
 in the causation of which the cell took an active part. In fact, 
 under this view, absoi-ption by the cell might be regarded as a sort 
 of inverted secretion, the cell taking much material from the chyme 
 and secreting it, with little or no change, into the villus. Other 
 observers however believe that the fat passes not through but be- 
 tween the epithelium-cells, being taken by the inter-epitliclial pro- 
 cesses of the peculiar epitheloid-cells, described as forming a con- 
 tinuous protoplasmic reticulum, connecting the surface of the villus 
 Avith the central chamber.. Along this reticulum the fat is sup- 
 posed to travel, the epithelium cells themselves having no active 
 share in absorption. 
 
 The passage is probably assisted by the movements of the in- 
 testine, though even in the contractions of strong peristaltic move- 
 
304 MOVEMENTS OF CHYLE AND LYMPH. [Book ii. 
 
 ments the pressure within the intestine is never very great. Of 
 more obvious use is the contraction of the villus itself. The longi- 
 tudinal muscular fibre-cells, in contracting, pull down the villus on 
 itself; the contents of the lacteal chamber are thus forced into the 
 underlying lymphatic plexus. When the fibre-cells relax, the 
 empty lacteal chamber is expanded; the chyle cannot flow back 
 from the lymphatic channels by reason of the valves present in 
 them, and in consequence the lacteal chamber is filled from the 
 substance of the villus, and thus the entrance into the villus of 
 material from the intestine is facilitated. The villus in fact acts as 
 a kind of muscular suction-pumj). 
 
 Movements of the Chyle. Having reached the lymphatic chan- 
 nels the onward progress of the chyle is determined by a variety of 
 circumstances. Putting aside the pumping action of the villi, the 
 same events which cause the movement of the lymph generally also 
 further the flow of the chyle ; and these are briefly as follows. In 
 the first place, the wide-spread presence of valves in the lymphatic 
 vessels causes every pressure exerted on the tissues in which they 
 lie to assist in the propulsion forward of the lymph. Hence a 1 
 muscular movements increase the flow. If a cannula be inserted 
 in one of the larger lymphatic trunks of the limb of a dog, the dis- 
 charge of lymph from the cannula will be more distinctly increased 
 by movements, even passive movements, of the limb than by any- 
 thing else. In addition to the valves along the course of the 
 vessels, the embouchement of the thoracic duct into the venous 
 system is guarded by a valve, so that every escape of lymph or chyle 
 from the duct into the veins becomes itself a help to the flow. In 
 the second place, we have already seen that the blood-pressure in 
 the capillaries and minute vessels is considerably greater than that 
 in the large veins, such as the jugular ; in fact this difl"erence of 
 pressure is the cause of the flow of blood from the capillaries to the 
 heart. Now the lymph in the lymphatic spaces outside the capil- 
 laries and minute vessels undoubtedly stands at a lower pressure 
 than the blood inside the capillaries ; otherwise the transudation 
 from the blood into the tissues would be checked ; but the differ- 
 ence is probably not great. ' So that the lymph in the lymphatic 
 spaces of the tissues may still be considered as standing at a higher 
 pressure than the blood in the venous trunks, for instance in the 
 jugular vein. That is to say the lymphatic vessels as a whole form 
 a system of channels leading from a region of higher pressure, viz. 
 the lymphatic spaces of the tissues, to a region of lower pressure, viz. 
 the interior of the jugular and subclavian veins. This difference of 
 pressure will, as in the case of the blood-vessels, cause the lymph to 
 flow onward in a continuous stream. Further, this flow, caused by 
 the lowness of the mean venous pressure at the subclavian, will be 
 assisted at every respiratory movement, since at every inspiration 
 the pressure in the venous trunks becomes negative, and thus 
 
Ohap. i] digestion. 306 
 
 lymph \n\\ be sucked in from the thoracic duct, while the increase 
 of pressure in the great veins during expiration is warded off from 
 the duct by the valve at its opening. In the third place, the flow 
 may possibly be increased by rhythmical cuntractions of the 
 muscular walls of the lymphatics themselves; but this is doubtful, 
 since it is not clear whether the rhythmic variations which have 
 been observed in the lacteals of the mesentery of the guinea-pig are 
 active or simply passive, i.e. caused by the rhythmic peristaltic 
 action of the intestine, each contraction of the intestine filling the 
 lymph-channels more fully. Lastly, it is quite open for us to sup- 
 pose that just as osmosis may give rise to increased pressure on one 
 side of a diffusion septum, so the diffusion of substances from the in- 
 testines into the lacteals, or from the tissues into the lymphatics, 
 may be itself one of the causes of the flow of lymph. We have 
 at least, under all circumstances, one or other of these causes at 
 work promoting a continual flow from the lymphatic roots to the 
 great veins. We have no very satisfactory evidence that the flow of 
 lymph is in any way directly governed by the nervous system. We 
 cannot prove any direct connection between the nervous system 
 and absorption, though the phenomena of disease render such a 
 connection at least probable. 
 
 That the nervous system does exert an influence on absorption 
 is shewn by the follo\ving experiment, though probably in this 
 case the influence is an indirect one carried out through the medi- 
 ation of the vascular system. Of two frogs placed under the in- 
 fluence of urari so as to do away with muscular movements and 
 the action of the lymph-hearts, the brain and spinal cord are de- 
 stroyed in the one but in the other are left intact. Both animals 
 are suspended by the lower jaw ; chloride of sodium solution ('75 
 per cent.) is poured into the dorsal lymphatic sacs of both ; and in 
 both the aorta is cut across. In the one where the nervous system 
 is intact, absorption from the lymphatic sac takes place copiously 
 and the heart pumps out large quantities of fluid by the aorta. 
 In the other, absorption does not occur; the heart, though beating, 
 remains empty, and the skin becomes dry. The experiment 
 probably shews the influence of the nei'vous system in maintaining 
 the tonicity of the blood-vessels and keeping up the connection of 
 the heart with the peripheral vessels, rather than any direct con- 
 nection between absorption projDer and the nervous system. 
 When the nervous system is destroyed, dilation of the splanchnic 
 vascular area causes all the blood to remain stagnant in the portal 
 vessels, and probably these as well as other veins are rendered 
 unusually lax, so that the blood is largely retained in the venous 
 system, and very little reaches the heart ; and with the enfeebled 
 circulation the absoriDtion from the lymphatic sac is slight. So 
 long as the nervous system is still intact this stagnation does not 
 occur, the blood reaches the heart as usual, and with the more 
 vigorous circulation absorption from the lymphatic sac goes on 
 
 K. 20 
 
306 LYMPH HEARTS. [Book ii. 
 
 rapidly. As the blood is pumped away its place is renewed by the 
 lymph, supplied by the fluid in the sac, and thus the heart may be 
 made for a long time to pump away the fluid poured into the sac. 
 
 Lymph hearts. In frogs and some other animals the centri- 
 petal flow of lymph from the limbs is assisted by rhythmically 
 pulsating muscular lymph hearts, which present many curious 
 analogies with the blood-heart. In the frog, in which they have 
 been chiefly studied, their action as we have already stated (p. 108) 
 is in a measure dependent on the spinal cord. The posterior lymph 
 hearts belonging to the hind limbs are connected by means of the 
 delicate tenth pair of spinal nerves, with a region of the cord 
 opposite the sixth or seventh vertebra, in such a way that section 
 of the nerve or destruction of the particular region of the cord 
 suspends or destroys their activity. The anterior pair are similarly 
 connected "with a region of the spinal cord opposite the third 
 vertebra. Each pair therefore seems to have a 'centre' in the spinal 
 cord; but it is probable, though observers are not wholly agreed, 
 that the hearts, after destruction of their spinal centre, ultimately 
 resume their rhythmic beats, so that the dependence of their 
 activity on the spinal centre, like the similar dependence of the 
 blood heart on the ganglia of the sinus venosus, is not an absolute 
 one. Like the blood heart, the lymph hearts may be inhibited, 
 and that in a reflex manner, the inhibition centre being moreover 
 in the medulla oblongata. If a frog be carefully observed, the 
 activity of the lymph hearts Avill be found to vary largely, and these 
 variations appear to be in part due to nervous influences ; so that 
 in this way the movement of lymph, and hence the processes of 
 absorption, are in this animal directly dependent on the nervous 
 system. 
 
 'Tlie course taken hy the several products of digestion. 
 
 Digestion being, broadly speaking, the conversion of non- 
 diffusible proteids and starch into highly diffusible peptone and 
 sugar, and the emulsifying, or division into minute particles, of 
 various fats, it is natural to suppose that the diffusible peptone 
 and sugar pass by osmosis into the portal vessels and so dfrectly 
 into the blood, and that the emulsified fats pass into the lacteals 
 and so indirectly into the blood. That a large part of the fat 
 which enters the body from the intestine does pass through the 
 lacteals, there can be no doubt ; and there can be but little doubt 
 that a considerable quantity of peptone and sugar does pass into 
 the portal blood. But the question as to how far the fat in its 
 diflicult passage into the lacteal is accompanied by soluble peptone. 
 
Chap, i.] DIGESTION. .307 
 
 or by less diffusible forms of proteids arising as subsidiary products 
 of proteolytic digestion or by carbohydrate products, deserves 
 attention. 
 
 It cannot be a matter of indifference which course is taken by 
 the particular iligestivc products. For if they pass by the portal 
 vein they fall into the general blood-current after having undergone 
 only such changes as they may experience in the lymphatic system ; 
 while if they pass into the portal vein they are subjected to the 
 powerful intluences of the liver before they find their way to the 
 right side of the heart. What those influences are we shall study 
 in a future chapter. 
 
 Fats. As we have seen, a special mechanism is provided for 
 the passage of fats into the lacteals. On the other hand, it is 
 ditficult to suppose that solid particles of fat can pass into the 
 interior of the blood capillaries. So that we are led d priori to 
 the view that the whole of the fat takes the course of the lacteals. 
 But we cannot say that this is definitely proved. On the contraiy, 
 a large deficit is observed when the quantity of fat disappearing 
 after a meal from the alimentary canal is compared ^^dth that 
 flo-s\ing out through a cannula placed in the end of the thoracic 
 duct; and if it be true, as is stated, that the blood of the portal 
 vein contains dui'ing digestion more fat than the general venous 
 blood, some of this deficit may be explained by the fat passing 
 into the blood capillaries, difficult as that passage may appear. 
 The portal blood, moreover, during digestion contains a small but 
 appreciable quantity of soaps. It may be however that the deficit 
 observed is due to some of the fat disappearing in some way, in 
 the glands for instance, fi-om the interior of the vessels in its 
 transit. 
 
 The fat thus entering the blood either directly or indirectly is 
 rapidly got rid of in some way or other, for from experiments on 
 dogs it would appear that the percentage of fat in the blood 
 after a meal rich in fat, does not, after the lapse of 20 hours 
 from the swallowing of the food, differ materially whether the 
 fat has been during the whole time shut oft' from the blood by 
 being allowed to flow out of a cannula placed in the thoracic duct, 
 or has been allowed to pass into the venous system in the usual 
 way. 
 
 Proteids. The question as to the course taken by the digested 
 proteids is complicated by the insufficiency of our knowledge con- 
 cerning the exact stages to which the digestion of proteids is 
 naturally carried in the alimentary canal. If we take it for 
 granted that the proteids taken as food are reduced to the 
 condition of soluble and difiusible peptone, it seems easy to 
 suppose that the proteids of food pass by diffusion as peptone into 
 the blood capillaries which as is well kno^vn are placed in the villus 
 between the epithelium and the lacteal chamber; though even 
 
 20—2 
 
308 ABSORPTION OF PROTEIDS. [Book ii. 
 
 on this view it is open for us to imagine that all the peptone 
 which passes through the epithelium is not intercepted by the 
 blood capillaries, but that some reaches and passes away by the 
 more centrally placed lacteal. It is difficult to imagine how 
 proteids in any other form than that of diffusible peptone can 
 pass through the walls of the blood capillaries ; though perhaps the 
 difficulty is not insurmountable, seeing that our conceptions of 
 nutrition are based on the assumption that the natural proteids of 
 the blood plasma pass from the interior of the vessels into the 
 extravascular elements of the tissues ; and we might imagine that 
 an accumulation of proteids in the same extravascular spaces 
 might cause a reversal of the proteid current, and thus lead 
 to proteids other than peptone passing through the vascular walls. 
 On the other hand it is at least open for us to ask the question, If 
 solid particles of fat can pass from the interior of the alimentary 
 canal into the lacteals, why should not various forms of proteids 
 pass in the same way into the lacteals, either in solution or even 
 as solid particles? 
 
 It would thus seem possible for some of the proteids to pass 
 into the lacteals and so into the system in a less digested form 
 than peptone; and it is further possible that the proteids thus 
 entering into the system in different forms may play different parts 
 in the nutritive labours of the economy. 
 
 But in all these considerations the fact must be borne in mind 
 that the intestinal walls undoubtedly possess a selective power of 
 absorption, which overrides the laws of diffusion and solubility. 
 This is shewn for instance by an observation made on a dog, in 
 which such fairly soluble and diffusible salts as sodium tauro- 
 cholate and glycocholate were found not to be absorbed by the 
 duodenum and upper jejunum even at a time when fat was being 
 rapidly absorbed in those regions, but to disappear in the ileum or 
 lower jejunum, the glycocholate apparently being absorbed by both 
 the ileum and lower jejunum, while the taurocholate passed away 
 in the ileum alone. 
 
 We cannot judge therefore of the course taken by the proteids, 
 or of the form in which they are absorbed, by deductions based on 
 solubility and diffusion. The problems we are discussing can only 
 be satisfactorily settled by direct experiment. And here we meet 
 with difficulties. If all proteids are converted into peptone, and 
 so pass into the lacteals or into the blood capillaries, we might 
 expect to find a quantity of peptone in the chyle or in portal 
 blood or in both after a proteid meal. Now all observers are 
 agreed that peptone is absent from chyle or at least that its 
 presence cannot be satisfactorily proved, in spite of the possi- 
 bility of its entering into the lacteals together with the fat. And 
 while some have succeeded in finding peptone in the blood after 
 food, but not during fasting, many have failed to demonstrate the 
 presence of peptone in the blood either of the portal vein or of the 
 
Ohap. i] digestion. 309 
 
 vessel> at large even after a meal containing large quantities 
 of pruteids. Of course the quantity of peptone passing into the 
 portal blood at any moment might be small, and yet a considerable 
 (quantity might so pass during the hours of digestion. We may 
 suppose moreover that that which does pass is immediately con- 
 verteil, possibly by some ferment action, into one or other of the 
 natural protoids of the blood, or otherwise disposed of; and indeed 
 peptone injected carefully into a vein disappears from the blood, 
 though little or even none passes out by the kidney. And the 
 view that peptone is so changed, possibly in the very act of 
 absorption, is supported not only by the fact that peptone may be 
 found in the walls of the intestine even when it appears to be 
 absent from the blood, but also and especially by the following 
 obsen^ation. If an artificial circulation of blood be kept up in the 
 mesenteric arteries supplying a loop of intestine removed from the 
 body, the loop may be kept alive for some considerable time. 
 During this survival a considerable quantity of peptone placed in 
 the ca^-ity of the loop, -svill disappear, i.e. will be absorbed, but 
 cannot be recovered from the blood which is being used for the 
 artificial circulation, and which escapes from the veins after 
 traversing the intestinal capillaries. The disappearance is not 
 due to any action of the blood itself, for peptones introduced into 
 the blood before it is driven through the mesenteric arteries in the 
 experiment may be recovered from the blood as it escapes from 
 the mesenteric veins. It would seem as if the peptone were 
 changed before it actually gets into the capillaries. 
 
 But the argument that the absence of peptone from the blood 
 is no proof that peptones are not absorbed into the blood may also 
 be applied to the chyle. We have however an indirect proof that 
 peptones do not pass into the chyle. We shall see hereafter that 
 the quantity of urea passing by the kidney may, with certain pre- 
 cautions, be taken as a measure of the quantity of proteid material 
 taken into the body. Now when a cannula is placed in the thoracic 
 duct of a dog so that all the chyle passes away and is lost to the 
 blood, the amount of urea leaving the body by the kidney does 
 not materially differ from the amount which, \vith the same food, 
 is passed, when all the chyle flows into the blood. Did any 
 large quantity of peptone (or proteid) pass by the chyle we 
 should expect to find the urea much diminished. Hence except 
 on the very improbable view that proteids absorbed into the 
 lacteals of the villi escape from the IjTuphatic system before they 
 reach the thoracic duct, we must accept the view which seems to 
 follow legitimately from the results of artificial digestion, that 
 proteid food is converted into peptone and so passes from the 
 alimentary canal into the blood. And we know that artificially- 
 formed peptone is available for nutrition ; for dogs fed on peptone 
 and non-nitrogenous food may actually put on flesh and gain in 
 weight. 
 
310 ABSORPTION BY DIFFUSION. [Book ii. 
 
 Sugar. With regard to the path taken by the sugar, carefiil 
 inquiries shew that the percentage of sugar both in chyle and in 
 general blood is fairly constant, being to no marked extent in- 
 creased by even amylaceous meals ; but that a meal of sugar or 
 starch does temporarily increase the quantity of sugar in the 
 portal blood. From this we may infer that such portions of the 
 sugar of the intestinal contents as are absorbed as sugar pass 
 exclusively by the portal vein. But it must be remembered that 
 at present we have no accurate information as to how large a 
 proportion of the sugar resulting from a meal passes in this way 
 unchanged until it reaches the liver, and how much undergoes the 
 lactic acid or analogous fermentation. Nor do we know as yet 
 how much of the starch taken as food is removed from the 
 alimentary canal in the form not of sugar but of dextrin. 
 
 When a solution of sugar is injected into an empty isolated 
 loop of intestine a large quantity disappears, without the contents 
 of the loop becoming acid. In such a case it may fairly be 
 inferred that the sugar is directly absorbed without undergoing 
 any change. And where sugar is introduced in large quantities 
 into the alimentary canal, the percentage of sugar in the blood 
 may be temporarily increased; to such an extent indeed that 
 sugar may appear in the urine. But neither of these facts prove 
 that the sugar of an ordinary meal, passing as it does along the 
 intestine with the other portions of the food, and products of 
 digestion, and appearing as it does in most cases in comparatively 
 small quantities at a time owing to the more or less gradual con- 
 version of the starch of the meal, is similarly absorbed unchanged ; 
 while in order that the marked acidity of the contents of the 
 lower intestine should be kept up, a considerable quantity of 
 sugar must suffer lactic acid fermentation, if the acidity be due as 
 stated to lactic acid. 
 
 To sum up, the evidence is distinctly in favour of the fats 
 passing largely by the chyle, and of the proteids and sugar passing 
 largely by the portal vein ; but there still remains much doubt as 
 to the course and fate of a not inconsiderable portion of the fat, 
 and the question as to the exact form in which proteids and 
 carbohydrates leave the alimentary canal, cannot be answered in a 
 perfectly definite manner. 
 
 Absorption by diflftision. It is evident, from the discussion 
 just concluded, that simple diffusion is far from explaining the 
 whole transit of the digested food from the intestine into the 
 blood. Nevertheless, it must not be supposed that the great 
 and general property of diffusion does not make itself felt in the 
 process of absorption, however much it may, in the case of various 
 substances, be subordinated and held in check by more potent 
 influences. Thus the passage of water from the alimentary cavity 
 into the blood, or from the blood into the alimentary cavity, and 
 
OiiAP. I.] niGESTWy. Ml 
 
 the behaviour of various inorganic salts, when taken as t'uod or 
 medicine, iUustrate very clearly the influence of osmosis. When 
 the intestine contains a large quantity of watery matter, the 
 surplus water passes by diffusion into the blood, just as it passes 
 through the membrane of a dialyser, \ni\\ blood or serous fluid on 
 the one side, and water on the other. When an albuminous fluid 
 of the spcciflc gravity of blood-serum is exposed in a dialyser to 
 water, about 200 parts of Avater pass through the membrane of the 
 dialyser from the water into the albuminous fluid for every one 
 part of the albumin which passes from the fluid into the water. 
 i\Ioreover, in the living body, the blood in the mesenteric capillary, 
 thus diluted by diffusion from th(j intestinal contents, is con- 
 tinually being replaced by fresh blood concentrated by its passage 
 through the skin, lung, or kidney. By the help of the circulation 
 an almost unlimited quantity of water can be absorbed from the 
 alimentary canal. 
 
 It is a matter of common experience that such inorganic and 
 organic salts as are readily diffusible, pass with great rapidity into 
 the blood (and thus into the urine) when taken by the mouth ; and 
 the rapidity with which they are absorbed is in large measure pro- 
 portionate to their diffusibility. Of course, coincident mtli this 
 passage of the salt from the intestine into the blood, there is a 
 proportionate current of water in the contrary direction from the 
 blood into the intestine ; but this, though opposed to, is, under 
 ordinary circumstances, too small to diminish to any serious extent 
 the passage of water fi'om the intestine into the blood, of which we 
 spoke just now, as caused by the osmotic influence of the albuminous 
 constituents of the blood. But, under certain circumstances, the 
 former may overcome the latter. Thus, when a concentrated solu- 
 tion of a highly diffusible salt, such as magnesium sulphate, is in- 
 troduced into the alimentary canal, the flow of water from the blood 
 into the intestine accompanying the osmotic transit of the salt from 
 the intestine into the blood, is so great as largely to exceed the 
 current in the contrary direction ; and the intestine becomes filled 
 Avith water at the expense of the blood. This is probably the cause of 
 the purgative action of large doses of many saline substances. And 
 even the purgative action of more dilute solutions may be explained 
 in the same way, since in the case of some salts at least the transit 
 of water as compared with the transit of the salt is relatively more 
 rapid Avith very dilute solutions than Avith more concentrated solu- 
 tions. Salts such as these, which, when introduced into the intes- 
 tine, produces diaiThoea, bring about a contrary condition when 
 injected directly into the blood ; and magnesium sulphate, with its 
 higher endosmotic equivalent, is more purgative in its action than 
 sodium chloride with its lower equivalent. 
 
CHAPTEE 11. 
 THE TISSUES AND MECHANISMS OF RESPIRATION. 
 
 We have already seen (Introduction, p. 3) that one particular item 
 of the body's income, viz. oxygen, is peculiarly associated with one 
 particular item of the body's waste, viz. carbonic acid, the means 
 which are applied for the introduction of the former being also used 
 for the getting rid of the latter. Both are gases, and in consequence 
 the ingress of the one as well as the egress of the other is far more 
 dependent on the simple physical process of diti'usion than on any 
 active vital processes carried on by means of tissues. Oxygen 
 passes from the air into the blood mainly by diffusion, and mainly 
 by diffusion also from the blood into the tissues ; in the same way 
 carbonic acid passes mainly by diffusion from the tissues into the 
 blood, and from the blood into the air. Whereas, as we have seen, 
 in the secretion of the digestive juices the epithelium-cell plays an 
 all-important part, in respu-ation the entrance of oxygen from the 
 lungs into the blood, and from the blood into the tissue, and the 
 passage of carbonic acid in the contrary direction, are affected, if at 
 all, in a wholly subordinate manner, by the behaviour of the pul- 
 monary, or of the capillary epithelium. What we have to deal with 
 in respiration then is not so much the vital activities of any par- 
 ticular tissue, as the various mechanisms by which a rapid inter- 
 change between the air and the blood is effected, the means by 
 which the blood is enabled to carry oxygen and carbonic acid to and 
 
On A F. 11.] RKSPIRA TION. 3 1 3 
 
 firom the tissues, and the maimer in which the several tissues take 
 oxygen fvom and give carbonic acid up to the blood. We have 
 reasons for tliinkliig that oxygen can be taken into the blood, not 
 only from the hnigs, but al.so from the .skin, and, as we have .seen, 
 occasionally from the alimentary canal also; and car]>onic acid cer- 
 tainly passes away from the skin, and through the various secretions, 
 as well as by the lungs. Still the lung.s are so eminently the chan- 
 nel of the interchange of gases between the body and the air, that 
 in dealing at the present with respiration, we shall confine ourselves 
 entirely to pulmonary respiration, leaving the consideration of the 
 subsidiary respiratory processes till we come to study the secretions 
 of which they respectively form part. 
 
SEO. 1. THE MECHANICS OF PULMONARY RESPIRATION. 
 
 The lungs are placed, in a semi-distended state, in the aii-tight 
 thorax, the cavity of which they, together with the heart, great 
 blood-vessels and other organs, completely fill. By the contraction 
 of certain muscles the cavity of the thorax is enlarged ; in conse- 
 quence the pressure of the air within the lungs becomes less than 
 that of the air outside the body, and this difference of pressure 
 causes a rush of air through the trachea into the lungs until an 
 equilibrium of pressure is established between the air inside and 
 that outside the lungs. This constitutes inspiration. Upon the 
 relaxation of the inspiratory muscles (the muscles whose contraction 
 has brought about the thoracic expansion), the elasticity of the 
 lungs and chest-walls, aided perhaps to some extent by the contrac- 
 tion of certain muscles, causes the chest to return to its original 
 size ; in consequence of this the pressure within the lungs now 
 becomes greater than that outside, and thus air rushes out of the 
 trachea until equilibrium is once more established. This consti- 
 tutes expiration ; the inspiratory and expiratory act together form- 
 ing a respiration. The fresh air introduced into the upper part of 
 the pulmonary passages by the inspiratory movement contains more 
 oxygen and less carbonic acid than the old air previously present in 
 the lungs. By diffusion the new or tidal air, as it is frequently 
 called, gives up its oxygen to, and takes carbonic acid from, the old 
 or stationary air, as it has been called, and thus when it leaves the 
 chest in expiration has been the means of both introducing oxygen 
 
Chap. II.] RESPIRATIoy. 315 
 
 into the chest and ot" removing curbuuic acid from it. In thi.s way, 
 by the ebb and tlow of the tidal air, and by diffusion between it 
 and the stationary air, the air in the lungs is being constantly 
 renewed through the alternate expansion and contraction of the 
 chest. 
 
 In ordinary respiration, the expansion of the chest never reaches 
 its maximum ; by more forcible muscular contraction, by what is 
 called laboured inspiration, an additional thoracic expansion can be 
 brought about, leading to the inrush of a certain additional quantity 
 of air before equilibrium is established. This additional quantity 
 is often spoken of as complemental air. In the same way, in 
 ordinary respiration, the contraction of the chest never reaches its 
 maximum. By calling into use additional muscles, by a laboured 
 expiration, an additional quantity of air, the so-called reserve or 
 supplemental air, may be driven out. But even after the most 
 forcible expiration, a considerable quantity of air, the residual air, 
 still remains in the lungs. The natural condition ( f the lungs in 
 the chest is in fact one of partial distension. The elastic pulmonary 
 tissue is always to a certain extent on the stretch ; it is always, so 
 to speak, striving to pull asunder the pulmonary from the parietal 
 pleura ; but this it cannot do, because the air can have no access to 
 the pleural cavity. When however the chest ceases to be air-tight, 
 when by a puncture of the chest-wall or diaphragm, air is introduced 
 into the pleural chamber, the elasticity of the lungs pulls the pul- 
 monary away from the parietal pleura, and the lungs collapse, 
 driving out by the windpipe a considerable quantity of the residual 
 air. Even then, however, the lungs are not completely emptied, 
 some air still remaining in the air-cells and passages. It need 
 hardly be added that when the pleura is punctured, and air can 
 gain free admittance from the exterior into the pleural chamber, 
 the effect of the respiratory movements is simply to drive air in 
 and out of that chamber, instead of in and out of the lung. There 
 is in consequence no renewal of the air \rithin the lungs under 
 those circumstances. 
 
 In man the pressure exerted by the elasticity of the lungs alone 
 amounts to about 5 mm. of mercury. This is estimated by t}-ing a 
 manometer into the windpipe of a dead subject and observing the 
 rise of mercury which takes place when the chest-walls are punc- 
 tured. If the chest be forcibly distended beforehand, a much larger 
 rise of the mercury is observed, amounting, in the case of a dis- 
 tension corresponding to a very forcible inspiration, to 30 mm. In 
 the living body this mechanical elastic force of the lungs is assisted 
 by the contraction of the plain muscular fibres of the bronchi ; the 
 pressure however which can be exerted by these probably does 
 not exceed 1 or 2 mm. 
 
 When a manometer is introduced into a lateral opening of the 
 windpipe of an animal, the mercury will fall, indicating a negative 
 pressure as it is called, during inspiration, and rise, indicating a 
 
316 
 
 RHYTHM OF RESPIRATION. 
 
 [Book ii. 
 
 positive pressure, during expiration, both fall and rise being slight 
 and varjdng according to the freedom with which the air passes in 
 and out of the chest. When a manometer is fitted with air-tight 
 closure into the mouth, or better, in order to avoid the suction- 
 action of the mouth, into one nostril, the other nostril and the mouth 
 being closed, and efforts of inspiration and expiration are made, the 
 mercury falls or undergoes negative pressure with inspiration, and 
 rises, or undergoes positive pressure during expiration. It has been 
 found in this way that the negative pressure of a strong inspiratory 
 effort may vary from 30 to 74 mm., and the positive pressure of a 
 strong expiration from 62 to 100 mm. 
 
 The total amount of air which can be given out by the most 
 forcible expfration following upon a most forcible inspiration, that 
 is, the sum of the complemental, tidal and reserve airs, has been 
 called 'the vital capacity;' 'extreme differential capacity' is a better 
 phrase. It may be measured by a modification of a gas-meter called 
 a spirometer ; and though it varies largely, the average may be put 
 down at 3—4000 c.c. (200 to 2.50 cubic inches). 
 
 Of the whole measure of vital capacity, about 500 c.c. (30 c. 
 inch) may be put down as the average amount of tidal air, the 
 remainder being nearly equally divided between the complemental 
 and reserve airs. The quantity left in the lungs after the deepest 
 expiration amounts to about 1400 — 2000 c.c. 
 
 Since the respiratory movements are so easily affected by various 
 circumstances, the simple fact of attention being directed to the breath- 
 ing being sufficient to cause modifications both of the rate and depth of 
 the respiration, it becomes very difficult to fix the volume of an average 
 breath. Thus various authors have given figures varying from 53 c.c. 
 to 792 c.c. The statement made above is that given by Vierordt as the 
 mean of observations varying from 177 to 699 c.c. 
 
 The Rhythm of E/espiration. If the movements of the column 
 of tidal air, or the movements of expansion and contraction, or the 
 
 Fig. 55. Teacing of Thokacic Eespieatokt Movements obtained by means of 
 Mabey's Pneumatogeaph. (To be read from left to right.) 
 
 A whole respiratory phase is comprised between a and a ; inspiration, during which 
 the lever descends, extending from a to h, and expiration from 6 to a. The 
 undulations at c are caused by the heart's beat. 
 
(^ii.M'. 11.] RESPIRATION. 317 
 
 fall and rise of the diaphragm, be registered, curves are obtained, 
 which, while diflbring in dcluil, exhibit the same general features, 
 and more or less resemble the curve shewn in Fig. 55. 
 
 The movements of the column of air may be I'ecorded by introducing 
 a T piece into the trachea, one cross piece being left open or connected 
 with a piece of india-rubber tubing open at the end, and tlie other con- 
 nected with a !Marey's tambour or with a receiver which in turn is con- 
 nected with a tambour. Fig. 22, p. 140, and Fig. 56. The movements of 
 the column of air in the trachea are transmitted to the tambour, the 
 consequent expansions and contractions of which are transmitted to the 
 i-ecording drum by means of a lever resting on it. The movements of the 
 chest-walls may be recorded by means of the recording stethometer of 
 Burdon-Sandei'son. This consists of a rectangular framework constructed 
 of two rigid parallel bars joined at right angles to a cross piece. The free 
 ends of the bars, the distance between which can be regulated at pleasure, 
 are armed, the one with a tambour, the other simply with an ivory button. 
 The tambour also bears on the metal plate of its membrane (Figs. 22 
 and 56, m',) a small ivory button {in place of the lever shewn in 
 Figs. 22 and 56). When it is desired to record the changes occurring in 
 any diameter of the chest, e.g. an antero-posterior diameter from a point 
 in the sternum to a point in the back, the instrument is made to encircle 
 the chest somewhat after the fashion of a pair of callipers, the ivory 
 button at one free end being placed on the spine of a vertebra behind 
 and the tambour at the other on the sternum in front in the line of the 
 diameter which is being studied. The distance between the free ends of 
 the instrument being carefully adjusted so that the button of the tambour 
 presses slightly on the sternum, any variations in the length of the 
 diameter in question will, since the framework of the tambour is im- 
 mobile, give rise to variations of pressure within the tambour. These 
 variations of the 'i-eceivmg' tambour as it is called are conveyed by a 
 flexible tube containing air to a second or 'recording' tambour similar 
 to that shewn in Figs. 22 and 56, the lever of which records the varia- 
 tions on a travelling surface. For the purpose of measuring the extent 
 of the movements the instrument must be experimentally graduated. 
 In Marey's pneumatogi'aph, a long elastic chamber is used as a pectoral 
 girdle. When the chest expands, the girdle is elongated, and the air 
 within it raretied, and the lever of the tambour connected with it de- 
 pressed : and conversely, when the chest contracts, the lever is elevated. 
 The pncumatograph of Fick is somewhat similar. The movements of the 
 diaphragm may be registered by means of a needle, which is thrust 
 through the sternum so as to rest on the diaphragm, the head of the 
 needle being connected with a lever. Various moditications of these 
 several methods have been adopted by different observers. 
 
 As is shewn in Fig. 55, inspiration begins somewhat suddenly 
 and advances rapidly, being followed immediately by expii-ation 
 which is carried out at hrst rapidly, but afterwards more and 
 more slowly. Such pauses as are seen occur between the end of 
 expii-ation and the beginning of inspiration. In normal breathing, 
 hardly any such pause exists, but in cases where the respiration 
 becomes infrequent, pauses of considerable length may be observed. 
 
318 
 
 RHYTHM OF RESPIRATION. [Book ii. 
 
Chap. II. 1 RESPIRATION. 319 
 
 Jj'io. 50. Arr.\RATOs for taking Tn.\ciN03 of the Movements of the 
 Column of Aib in Respiration. 
 
 The recording apparatus shewn is the ordinary cylinder recording apparatus. The 
 cylinder A covered with smoked paper is by means of the friction-plate B put into 
 revolution by the spring clock-work in C reguhitod by Foucault'.s regulator D. By 
 means of the screw E, the cylinder can be raised or lowered, and by means of the 
 screw F its speed may be increased or diminished. 
 
 The tracheotomy tube ( fixed in the trachea of an animal is connected by india- 
 rubber tubing a with a glass T piece inserted into the large jar G. From the other 
 end of the T piece proceeds a second piece of tubing h, the end of which can be either 
 closed or partially obstructed at pleasure by means of the screw clamp c. From the 
 jar proceeds a third piece of tubiug d, connected with a Marey's tambour m (see 
 Fig. 22, p. 140), the lever of which I writes on the recording sui-face. When the tube 
 h is open the animal breathes freely through this, and the movements in the air of 
 G and consequently in the tambour are slight. On closing the clamp c, the animal 
 breathes only the air contained in the jar, and the movements of the lever of the 
 tambour become consequently much more marked. 
 
 Below the lever is seen a small time-marker n connected with an electro-magnet, 
 the current through which coming from a battery by the wires x and y is made and 
 broken by a clock-work or metronome. 
 
 lu what maybe considered as normal breathing, the respiratory 
 act is repeated about 17 times a minute ; and the duration of the 
 inspiration as compared with that of the expiration (and such pause 
 as may exist) is about as ten to twelve. 
 
 The rate of the respiratory rhythm varies very largely, and in 
 this as in the volume of each breath it is very difficult to fix 
 a satisfactory average, the figures given varying from 20 to 13 a 
 minute. It varies according to age and sex. It is influenced by 
 the position of the body, being quicker in standing than in Ipng, 
 and in lying than in sitting. Muscular exertion and emotional 
 conditions affect it deeply. In fact, almost every event which 
 occurs in the body may influence it. We shall have to consider in 
 detail hereafter the manner in which this influence is brought to 
 bear. 
 
 When the ordinary respiratory movements prove insufficient to 
 effect the necessary changes in the blood, their rh}-thm and 
 character become changed. Normal respiration gives place to 
 laboured respiration, and this in turn to dyspnoea, which, unless 
 some restorative event occurs, terminates in asj^hyxia. These 
 abnormal conditions we shall study more fully hereafter. 
 
 The Respiratory Movements. 
 
 When the movements of the chest during normal breathing ai-e 
 watched, it is seen that during respiration an enlargement takes 
 place in the antcro-posterior diameter, the sternum being thrown 
 forwards, and at the same time mo^dng upward. The lateral width 
 of the chest is also increased. The vertical increase of the cavity 
 is not so obvious fiom the outside, though when the movements of 
 
320 MOVEMENTS OF INSPIRATION. [Book ii. 
 
 the diaphragm are watched by means of an inserted needle, the 
 upper surface of that organ is seen to descend at each inspiration, 
 the anterior walls of the abdomen bulging out at the same time. 
 In the female human subject, the movement of the upper part of 
 the chest is very conspicuous, the breast rising and falling with 
 every respiration ; in the male, however, the movements are almost 
 entirely confined to the lower part of the chest. In laboured 
 respiration all parts of the chest are alternately expanded and 
 contracted, the breast rising and falling as well in the male as in 
 the female. We have now to consider these several movements in 
 greater detail, and to study the means by which they are carried 
 out. 
 
 Inspiration. There are two chief means by which the chest is 
 enlarged in normal inspiration, viz. the descent of the diaphragm 
 and the elevation of the ribs. The former causes that movement 
 in the lower part of the chest and abdomen so characteristic of 
 male breathing, which is called diaphragmatic ; the latter causes 
 the movement of the upper chest characteristic of female breath- 
 ing, which is called costal. These two main factors are assisted by 
 less important and subsidiary events. 
 
 The descent of the diaphragm is effected by means of the 
 contraction of its muscular fibres. When at rest the diaphragm 
 presents a convex surface to the thorax ; when contracted it 
 becomes much flatter, and in consequence the level of the chest- 
 floor is lowered, the vertical diameter of the chest being pro- 
 portionately enlarged. In descending, the diaphragm presses on 
 the abdominal viscera, and so causes a projection of the flaccid ab- 
 dominal walls. From its attachments to the sternum and the false 
 ribs, the diaphragm, while contracting, naturally tends to pull the 
 sternum and the upper false ribs downwards and inwards, and the 
 lower false ribs upwards and inwards, towards the lumbar spine. 
 In normal breathing, this tendency produces little effect, being 
 counteracted by the accompanying general costal elevation, and by 
 certain special muscles to be mentioned presently. In forced 
 inspiration however, and especially where there is any obstruction 
 to the entrance of air into the lungs, the lower ribs may be 
 so much drawn in by the contraction of the diaphragm, that the 
 girth of the trunk at this point is obviously diminished. 
 
 The elevation of the ribs is a much more complex matter than 
 the descent of the diaphragm. If we examine any one rib, such as 
 the fifth, we find that while it moves freely on its vertebral arti- 
 culation, it inclines when in the position of rest in an oblique 
 direction from the spine to the sternum ; hence it is obvious that 
 when the rib is raised, its sternal attachment must not only 
 be carried upward, but also thrown forwards. The rib may in fact 
 be regarded as a radius, moving on the vertebral articulation as a 
 centre, and causing the sternal attachment to describe an arc of a 
 
CiiAi'. 11. 1 RESPIRATION. 321 
 
 circle in the vertical plane of the body; as the ril) is carried 
 upwards from an obli([iie to a more horizontal position, the sti-i-nal 
 attachment must of necessity be canied farther away in front of 
 the spine. Since all the ribs have a downward slanting direction, 
 they must all tend, when raised towards the horizontal position, to 
 thnist the sternum forward, some more than others according to 
 theii" slope and length. The elasticity of the sternum and costal 
 cartilages, together with, the articulation of the sternum to the 
 clavicle above, permit the front surface of the chest to be thus 
 thrust forwards as well as upwards, Avhen the ribs are raised. 
 By this action, the antero-posterior diameter of the chest is 
 enlarged. 
 
 Since the ribs form arches which increase in their sweep as 
 one proceeds from the first doAvnwards as far at least as the seventh, 
 it is evident that when a lower rib such as the fifth is elevated so 
 as to occupy or to approach towards the position of the one above 
 it, the chest at that level will become wider from side to side, 
 in proportion as the fifth arch is wider than the fourth. Thus 
 the elevation of the rib increases not only the antero-posterior but 
 also the transverse diameter of the chest. Further, on account 
 of the resistance of the sternum, the angles between the ribs and 
 their cartilages are, in the elevation of the ribs, somewhat opened 
 out, and thus also the transverse as well as the antero-posterior 
 diameter, somewhat increased. In several ways, then, the elevation 
 of the ribs enlarges the dimensions of the chest. 
 
 The ribs are raised by the contraction of certain muscles. Of 
 these the external intercostals are the most important. Even in the 
 case of two isolated ribs such as the fifth and sixth, the contraction 
 of the external intercostal muscle of the intervening space raises 
 the two ribs, thus bringing them towards the position in wbich the 
 fibres of the muscle have the shortest length, viz. the horizontal 
 one. This elevating action is further favoured by the fact that 
 the first rib is less moveable than the second, and so atfords a 
 comparatively fixed base for the action of the muscles between the 
 two, the second in turn supporting the third and so on, while the 
 scaleni muscles in addition serve to render fixed, or to raise, the 
 first two ribs. So that in normal respiration, the act begins 
 probably by a contraction of the scaleni. The first two ribs 
 being thus fixed, the contraction of the series of external inter- 
 costal muscles acts to the gi-eatest advantage. 
 
 While the elevating i.e. inspiratory action of the external 
 intercostals is admitted by all authors, the function of the internal 
 intercostals has been much disputed. Haller may be regarded 
 as the leader of those who . regard the internal intercostals as 
 inspiratory, while Hamberger was the first who successfully ad- 
 vocated the perhaps more commonly adopted view that while 
 those parts of them which lie between the sternal cartilages act 
 like the external intercostals as elevators, i.e. as inspiratory in 
 
 F. 21 
 
322 LABOURED INSPIRATION. [Book ii. 
 
 functiou, those parts which lie between the osseous ribs act as 
 depressors, i.e. as expiratory in function. 
 
 In the well-known model invented by Bernoulli and adopted 
 by Hamberger, consisting of two rigid bars, representing the ribs, 
 moving vertically by means of their articulations with an upright 
 representing the spine and connected at their free ends by a piece 
 representing the sternum, it is undoubtedly true that stretched 
 elastic bands attached to the bars in such a way as to represent 
 respectively the external and internal intercostals, viz. sloping in 
 the one case downwards and forwards and in the other downwards 
 and backwards, do, on being left free to contract, in the former 
 case elevate and in the latter depress the ribs. Such a model 
 however does not fairly represent the natural conditions of the 
 ribs, which are not straight and rigid, but peculiarly curved and of 
 varying elasticity, capable moreover of rotation on their own axes, 
 and having their movements determined by the characters of 
 their vertebral articulations. The mechanical conditions in fact 
 of these muscles are so complex, that a deduction of their actions 
 from simple mechanical principles, or from the dfrection of the 
 fibres, must be exceedingly difficult and dangerous. Actual experi- 
 ments on the cat and dog tend to shew that in these animals the 
 contraction of the internal intercostals, along their whole length, 
 takes place, in point of time, alternately with that of the 
 diaphragm, and thus afford an argument in favour of these muscles 
 being expiratory in function. 
 
 Next in importance to the external intercostals come the 
 levatores costarum, \vhich, though small muscles, are able, from 
 the nearness of their costal insertions to the fulcrum, to produce 
 considerable movement of the sternal ends of the ribs. The 
 external intercostals and the levatores costarum with the scaleni 
 may fairly be said to be the elevators of the ribs, i.e. the chief 
 muscles of costal inspiration in normal breathing. 
 
 Additional space in the transverse diameter is afforded probably 
 by the rotation of the ribs on an antero-posterior axis; but this 
 movement is quite subsidiary and unimportant. When the chest 
 is at rest, the ribs are somewhat inclined with their lower borders 
 directed inwards as well as downwards. When they are dra-\vn up 
 by the action of the intercostal muscles, thefr lower borders are 
 everted. Thus their flat sides are presented to the thoracic cavity, 
 which is thereby slightly increased in width. 
 
 Laboured Inspiration. When respiration becomes laboured, 
 other muscles are brought into play. The scaleni are strongly 
 contracted, so as to raise or at least give a very fixed support 
 to the first and second ribs. In the same way the serratus 
 posticus superior, which descends from the fixed spine in the lower 
 cervical and upper dorsal regions to the second, third, fourth, and 
 fifth ribs, by its contractions raises those ribs. In laboured breath- 
 
Cii.vr. ti.] lil-Sl'fh'ATlOX. 323 
 
 ing a, t'unctioii ol" the lower false ribs, not very noticeable in ea^y 
 breathing, comes into play. They are depressed, retracted, and 
 fixed, thereby giving increased support to the diaphragm, and 
 directing the whole energies of that muscle to the vertical enlarge- 
 ment of the ehest. In this way the serratus posticus inferior, which 
 passes upward from the lumbar aponeurosis to the last four ribs, by 
 depressing and fixing those ribs becomes an adjuvant inspiratory 
 muscle. The quadratus lumhoriuii and lower portions of the sacro- 
 lumbalis may have a similar function. 
 
 All these muscles may come into action even in breathing 
 which, deeper than usual, can hardly perhaps be called laboured. 
 When however the need for greater inspiratory otibrts becomes 
 urgent, all the muscles which can, from any fixed point, act in 
 enlarging the chest, come into play. Thus the arms and shoulder 
 being fixed, the serratus magnus passing from the scapula to the 
 middle of the first eight or nine ribs, the pectoralis minor passing 
 from the coracoid to the front parts of the third, fourth, and fifth 
 ribs, the pectoralis major passing from the humerus to the costal 
 cartilages, from the second to the sixth, and that portion of the 
 latissimus dor si which passes from the humerus to the last three 
 ribs, all serve to elevate the ribs and thus to enlarge the chest. 
 The stemo-mastoid and other muscles passing from the neck to the 
 sternum, are also called into action. In fact, every muscle which 
 by its contraction can either elevate the ribs or contribute to the 
 fixed support of muscles which do elevate the ribs, such as the 
 trapezius, levator anguli scapulae and rhomboidei by fixing the 
 scapula, may, in the inspiratory efforts which accompany dyspnoea, 
 be brought into play. 
 
 Expiration. In normal easy breathing, expiration is in the 
 main a simple eftect of elastic reaction. By the inspiratory effort 
 the elastic tissue of the lungs is put on the stretch ; so long as 
 the inspii-atory muscles continue contracting, the tissue remains 
 stretched, but directly those muscles relax, the elasticity of the 
 lungs comes into play and drives out a portion of the air contained 
 in them. Similarly the elastic sternum and costal cartilages are by 
 the elevation of the ribs put on the stretch : they are driven into a 
 position which is unnatural to them. When the intercostal and 
 other elevator muscles cease to contract, the elasticity of the ster- 
 num and costal cartilages causes them to return to their previous 
 position, thus depressing the ribs, and diminishing the dimensions 
 of the chest. When the diaphragm descends, in pushing do^\^^ the 
 abdominal viscera, it puts the abdominal walls on the stretch : and 
 hence, when at the end of inspu-ation the diaphragm relaxes, the 
 abdominal walls return to their place, and by pressing on the ab- 
 dominal viscera, push the diaphragm up again into its position of 
 rest. Expiration then is, in the main, simple elastic reaction ; but 
 there is probably some, though possibly in most cases, a very 
 
 21—2 
 
324 FACIAL AND LARYNGEAL BESFIBATION. [Book ii. 
 
 slight, expenditure of muscular energy to bring the chest 
 more rapidly to its former condition. This is, as we have 
 seen, supposed by many to be afforded by the internal inter- 
 costais acting as depressors of the ribs. If these do not act in 
 this way, we may suppose that the elastic return of the abdominal 
 walls is accompanied and assisted by a contraction of the abdominal 
 muscles. The triangularis sterni, the effect of whose contraction is 
 to pull down the costal cartilages, may also be regarded as an ex- 
 piratory muscle. 
 
 When expiration becomes laboured, the abdominal muscles 
 become important expiratory agents. By pressing on the contents 
 of the abdomen, they thrust them and therefore the diaphragm 
 also up towards the chest, the vertical diameter of which is 
 thereby lessened, while by pulling down the sternum and the 
 middle and lower ribs they lessen also the cavity of the chest in its 
 antero-posterior and transverse diameters. They are in fact the 
 chief expiratory muscles, though they are doubtless assisted by 
 the serratus posticus inferior and portions of the sacro-lumbalis, 
 since when the diaphragm is not contracting, the depression of the 
 lower ribs which the contraction of these muscles causes, serves 
 only to narrow the chest. As expiration becomes more and more 
 forced, every muscle in the body which can either by contracting 
 depress the ribs, or press on the abdominal viscera, or afford fixed 
 support to muscles having those actions, is called into play. 
 
 Facial and Laryngeal Respiration. The thoracic respiratory 
 movements are accompanied by associated respiratory movements 
 of other parts of the body, more particularly of the face and of the 
 glottis. 
 
 In normal healthy respiration the current of air which passes 
 in and out of the lungs, travels, not through the mouth but through 
 the nose, chiefly through the lower nasal meatus. The ingoing air, 
 by exposure to the vascular mucous membrane of the narrow and 
 vidnding nasal passages, is more efficiently warmed than it would be 
 if it passed through the mouth ; and at the same time the mouth 
 is thereby protected from the desiccating effect of the continual 
 inroad of comparatively dry air. 
 
 During each inspiratory effort the nostrils are expanded, pro- 
 bably by the action of the dilatores naris, and thus the entrance of 
 air facilitated. The return to their previous condition during expi- 
 ration is effected by the elasticity of the nasal cartilages, assisted 
 perhaps by the compressores naris. This movement of the nostrils, 
 perceptible in many people, even during tranquil breathing, becomes 
 very obvious in laboured respiration. 
 
 When the mouth is closed, the soft palate which is held some- 
 what tense, is swayed by the respiratory current, but entirely in a 
 passive manner, and it is not until the larynx is reached by the in- 
 going air that any active movements are met with. When the 
 
CiiAP. 11. 1 RESPIRATION. 326 
 
 laryiix is oxaminod with the laryngoscope, it is frequently seen that, 
 while during inspiration the glottis is widoly open, with each exjji- 
 ratiou the arytenoid cartilages approach each other so as to narrow 
 the glottis, the cartilages of Santorini projecting inwards at the 
 same time. Thus, synchronous with the respiratory expansion and 
 contraction of the chest, and the respiratory elevation and depres- 
 sion of the ala3 nasi, there is a rhythmic widening and narrowing 
 of the glottis. Like the movements of the nostril, this respiratory 
 action of the glottis is much more evident in laboured than in 
 tranquil breathing. Indeed in the latter case it is frequently 
 absent. The manner in which this rhythmic opening and narrow- 
 ing is effected will be described when we come to study the pro- 
 duction of the voice. Whether there exists a rhythmic contraction 
 and expansion of the trachea and bronchial passages effected by 
 means of the plain muscular tissue of those organs and synchronous 
 with the respiratory movements of the chest, is uncertain. 
 
SEC. 2. CHANGES OF THE AIR IN RESPIRATION. 
 
 During its stay in the lungs, or rather during its stay in the 
 bronchial passages, the tidal air (by means of diffusion chiefly) 
 effects exchanges with the stationary air ; in consequence the ex- 
 pired air differs from inspired air in several important particulars. 
 
 1. The temperature of expired air is variable, but under 
 ordinary circumstances is higher than that of the inspired air. At 
 an average temperature of the atmosphere, for instance at about 
 20" C, the temperature of expired air is, in the mouth 23'9\ in the 
 nose SB'S". When the external temperature is low, that of the ex- 
 pired air sinks somewhat, but not to any great extent, thus at 
 — G'S" C. it is 29*8° C. When the external temperature is high, the 
 expired air may become cooler than the inspired, thus at il'Q" it 
 was found by Valentin to be SS'l". The exact temperature in fact 
 depends on the relative temperatures of the blood and inspired air, 
 and on the depth and rate of breathing. 
 
 2. The expired air is loaded with aqueous vapour. The point 
 of saturation of any gas, that is, the utmost quantity of water 
 which any given volume of gas can take up as aqueous vapour, 
 varies with the temperature, being higher with the higher 
 temperature. For its own temperature expired air is according 
 to most observers saturated with aqueous vapour. 
 
 3. When the total quantity of tidal air given out at any expira- 
 tion is compared with that taken in at the corresponding inspi- 
 
Chap. II.] RE-^PI RATIOy. 327 
 
 ration, it is found that, both being dried and measured at the 
 same pressure, the expired air is less in vohimo than the inspired 
 air, the dittcrence amounting to about 7j'(jth or j}Qt\\ of the volume 
 of the latter. Hence, when an animal is made to breathe in 
 a contined space, th(! atmosphere is absolutely diminished, as 
 was observed so long ago as 1674 by Mayow. The approximate 
 equivalence in volume between inspired and expired air arises 
 from the fact that the volume of any given quantity of carbonic 
 acid is equal to the volume of the oxygen consumed to produce it; 
 the slight falling short of the expired air is due to the circum- 
 stance that all the oxygen inspired does not reappear in the 
 carbonic acid expired, some having formed other combinations. 
 
 4. The expired air contains about 4 or 5 p.c. less oxygen, and 
 about 4 p.c. more carbonic acid than the inspired air, the quantity 
 of nitrogen sufFerinq- but little change. Thus 
 
 oxygen. 
 
 nitrogen. 
 
 carbonic acid. 
 
 Inspired air contains 20*81 
 
 79-15 
 
 -04 
 
 Expired „ „ 16-033 
 
 79-587 
 
 4-380 
 
 The quantity of nitrogen in the expired air is sometimes found 
 to be slightly greater, as in the table above, but sometimes less, 
 than that of the inspired air. 
 
 In a single breath the air is richer in carbonic acid (and poorer 
 in oxygen), at the end than at the beginning. Hence the longer 
 the breath is held, the greater the pause between inspiration and 
 expiration, the higher the percentage of carbonic acid in the 
 expired air. Thus by increasing the interval between two ex- 
 pii-ations to 100 seconds, the percentage may be raised to 7-5. When 
 the rate of breathing remains the same, by increasing the depth 
 of the breathing the percentage of carbonic acid in each breath is 
 low^ered, but the total quantity of carbonic acid expired in a given 
 time is increased. Similarly, when the depth of breath remains 
 the same, by quickening the rate the percentage of carbonic acid 
 in each breath is lowered, but the quantity expired in a given 
 time is increased. 
 
 Taking, as we have done, at 500 c.c. the amount of tidal aii- 
 passing in and out of the chest of an average man, such a person 
 will expire about 22 c.c. of carbonic acid at each breath; this, 
 reckoning the rate of breathing at 17 a minute, would give over 
 500 litres of carbonic acid for the day's production. By actual 
 experiment, however, Pettenkofer and Voit, of whose researches we 
 shall have to speak hereafter, determined the total daily excretion 
 of carbonic acid in an average man to be 800 grms., i.e. rather 
 more than 400 litres (406), containing 218-1 grms. carbon, and 
 581-9 grms. oxygen, the oxygen actually consumed at the same 
 time being about 700 grms. This amount represents the gases 
 given out and taken in, not by the lungs only, but by the whole 
 
328 CHANGES IN THE AIR. [Book ii. 
 
 body; but the amount of carbonic acid given out by the skin is, as 
 we shall see, very slight (lU grms. or even less), so that 800 grms. 
 may be taken as the average production of carbonic acid by an 
 average man. The quantity however, both of oxygen consumed 
 and of carbonic acid given out, is subject to very wide variations ; 
 thus in Pettenkofer and Voit's observations, the daily quantity of 
 carbonic acid varied from G86 to 1285 grms., and that of the 
 oxygen from 594 to 1072 grms. These variations and their causes 
 will be discussed when we come to deal with the problems of 
 nutrition. 
 
 5. Besides carbonic acid, expired air contains various im- 
 purities, many of an unknown nature, and all in small amounts. 
 Traces of ammonia have been detected in expired air, even in that 
 taken directly from the trachea, in which case its presence could 
 not be due to decomposing food lingering in the mouth. When 
 the expired air is condensed by being conveyed into a cooled 
 receiver, the aqueous product is found to contain organic matter, 
 and rapidly to putrefy. The organic substances thus shewn to be 
 present in the expired air are the cause in part of the odour 
 of breath. It is probable that many of them are of a poisonous 
 nature; for an atmosphere containing simply 1 p.c. of carbonic 
 acid (with a corresponding diminution of oxygen) has very little 
 effect on the animal economy, whereas an atmosphere in which the 
 carbonic acid has been raised to 1 p.c. by breathing, is highly 
 injurious. In fact, air rendered so far impure by breathing that 
 the carbonic acid amounts to '08 p.c. is distinctly unwholesome, 
 not so much on account of the carbonic acid, as of the accom- 
 panying impurities. Since these impurities are of unknown 
 nature and cannot be estimated, the easily determined carbonic 
 acid is usually taken as the measure of their presence. We have 
 seen that the average man loads, at each breath, 500 c.c. of air 
 with carbonic acid to the extent of 4 p.c. He will accordingly at 
 each breath load 2 litres to the extent of 1 p.c; and in one hour, 
 if he breathe 17 times a minute, will load rather more than 2000 
 litres to the same extent. At the very least then a man ought to 
 be supplied with this quantity of air hourly; and if the air is to be 
 kept fairly wholesome, that is with the carbonic acid reduced 
 below '1 p.c, he should have more than ten times as much. 
 
SEC. 3. THE RESPIRATORY CHANGES IN THE 
 BLOOD. 
 
 While the air in passing in and out of the lungs is thus robbed 
 of a portion of its oxygen, and loaded with a certain quantity of 
 carbonic acid, the blood as it streams along the pulmonary capil- 
 laries undergoes important correlative changes. As it leaves the 
 right ventricle it is venous blood of a dark purple or maroon 
 colour; when it falls into the left auricle, it is arterial blood of a 
 bright scarlet hue. In passing through the capillaries of the body 
 from the left to the right side of the heart, it is again changed from 
 the arterial to the venous condition. We have to inquire, What are 
 the essential differences between arterial and venous blood, by 
 Avhat means is the venous blood changed into arterial in the lungs, 
 and the arterial into venous in the rest of the body, and what 
 relations do these changes in the blood bear to the changes ia the 
 air which we have already studied? 
 
 The facts, that venous blood at once becomes arterial on being 
 exposed to or shaken up with air or oxygen, and that arterial 
 blood becomes venous when kept for some little time in a closed 
 vessel, or when submitted to a current of some indifferent gas 
 such as nitrogen or hydrogen, prepare us for the statement that 
 the fundamental difference between venous and arterial blood is in 
 the relative proportion of the oxygen and cai'bonic acid gases 
 contained in each. From both, a certain quantity of gas can be 
 extracted by means which do not otherwise materially alter the 
 constitution of the blood ; and this gas when obtained from arterial 
 
330 CHANGES IN THE BLOOD. [Book ii. 
 
 blood is found to contain more oxygen and less carbonic acid than 
 that obtained from venous blood. This is the real differential 
 character of the two bloods ; all other differences are either, as we 
 shall see to be the case with the colour, dependent on this, or are 
 unimportant and fluctuating. 
 
 If the quantity of gas which can be extracted by the mercurial 
 air-pump from 100 vols, of blood be measured at {)°C., and a 
 pressure of 760 mm., it is found to amount, in round numbers, to 
 60 vols. 
 
 The vacuum produced by the ordinary mechanical air-pump is in- 
 sufficient to extract all the gas from blood. Hence it becomes necessary 
 to use either a large Torricellian vacuum or a Sprengel's pump. In the 
 former (Fig. 57) case two large globes of glass, one fixed and the other 
 moveable, are connected by a flexible tube; the fixed globe is made to 
 communicate by means of air-tight stopcocks alternately with a receiver 
 containing the blood, and with a receiver to collect the gas. When the 
 moveable globe filled with mercury is raised above the fixed one, the 
 mercury from the former runs into and completely fills the latter, the 
 air previously present being driven out. After adjusting the cocks, the 
 moveable globe is then depressed thirty inches below the fixed one, 
 in which the consequent faU of the mercury produces an almost 
 complete vacuum. By turning the proper cock this vacuum is put into 
 connection with the receiver containing the blood, which thereupon be- 
 comes proportionately exhausted. By again adjusting the cocks and 
 once more elevating the moveable globe, the gas thus extracted is 
 driven out of the fixed globe into a receiver. The vacuum is then 
 once more estabhshed and the operation repeated as long as gas continues 
 to be given off from the blood. This form of pump, introduced by 
 Ludwig, or a modification of it, with drying apparatus, employed by 
 Pfluger, or a similar form introduced by French observers, is the one 
 which has been hitherto most extensively used ; but a Sprengel's pump 
 is preferred by some. 
 
 The average composition of this gas in the two kinds of blood 
 is, stated in round numbers, as follows: 
 
 From 100 vols. may be obtained 
 
 Of oxygen, of carbonic acid, of nitrogen. 
 
 Of Arterial Blood, 20 vols, 40 vols. 1 to 2 vols. 
 
 Of Venous Blood, 8 to 12 vols. 46 vols. 1 to 2 vols, 
 
 all measured at 760 mm, and 0° C, 
 
 That is to say, venous blood, as compared with arterial blood, 
 contains 8 to 12 p.c. less oxygen and .6 p.c. more carbonic acid. 
 But the gases of venous blood are much more variable than those of 
 arterial blood. 
 
 It will be convenient to consider the relations of each of these 
 gases separately. 
 
Chap, ii.j 
 
 331 
 
 Fig. 57. Diagr.vmmatic Illustration of LcDw^G's Mebcubial Gas Pump. 
 
 A and B are two glass globes, connected by strong india-rubber tubes, a and b, 
 with two similar glass globes A' and B'. A is farther connected by means of the 
 stopcock c -with the receiver C containing the blood (or other fluid) to be analysed, 
 and B by means of the stopcock d and the tube e with the receiver D for recei\ing 
 the gases. A and B are also connected with each other by means of the stopcocks 
 / and g, the latter being so arranged that B also communicates with B' by the 
 passage g'. A' and B' being full of mercury and the cocks A-, /, g, and d being open 
 but c and g' closed, on raising A' by means of the pulley p the mercury of A' tills A, 
 driving out the air contained in it, into B, and so out through e. WTien the mercury 
 has risen above g, f is closed, and g' being opened, B' is in turn raised till B is 
 completely filled with mercury, all the air previously in it being driven out through e. 
 Upon closing d, and lowering B', the whole of the mercury in B falls in B', and a 
 vacuum consequently is established in B. On closing g', but opening g, f, and k and 
 lowering A', a vacuum is similarly estabhshed in A and in the junction between 
 A and B. If the cock c be now opened the gases of the blood in C escape into the 
 vacuum of A and B. By raising A', after the closure of c, and opening of d, the 
 gases so set free are driven from A into B, and by the raising of B' from B, through 
 e into the receiver D, standing over mercury. 
 
332 CHANGES IN THE BLOOD. [Book il 
 
 The relations of Oxygen in the Blood. 
 
 When a liquid sucli as water is exposed to an atmosphere con- 
 taining a gas such as oxygen, some of the oxygen will be dis- 
 solved in the water, that is to say will be absorbed from the 
 atmosphere. The quantity which is so absorbed will depend on 
 the quantity of oxygen which is in the atmosphere above ; that is 
 to say on the pressure of the oxygen ; the greater the pressure of 
 the oxygen, the larger the amount which will be absorbed. If on 
 the other hand water, already containing a good deal of oxygen dis- 
 solved in it, be exposed to an atmosphere containing little or no 
 oxygen, the oxygen will escape from the water into the atmosphere. 
 The oxygen in fact which is dissolved in the water is in a state of 
 tension, the degree of tension depending on the quantity dissolved; 
 and when water containing oxygen dissolved in it is exposed to any 
 atmosphere, the result, that is whether the oxygen escapes from the 
 water into the atmosphere, or passes from the atmosphere into the 
 water, depends on whether the tension of the oxygen in the water is 
 greater or less than the pressure of the oxygen in the atmosphere. 
 Hence when water is exposed to oxygen, the oxygen either escapes 
 or is absorbed until equilibrium is established between the pressure 
 of the oxygen in the atmosphere above and the tension of the 
 oxygen in the water below. This result is, as far as mere absorption 
 and escape are concerned, quite independent of what other gases 
 are present in the water or in the atmosphere. Suppose a half- 
 litre of water were lying at the bottom of a two-litre flask, and that 
 the atmosphere in the flask above the water was one-third oxygen; 
 it would make no difference, as far as the absorption of oxygen by 
 the water was concerned, whether the remaining two-thirds of the 
 atmosphere was carbonic acid, or nitrogen, or hydrogen, or whether 
 the space above the water was a vacuum filled to one-third with 
 pure oxygen. Hence it is said that the absorption of any gas 
 depends on the 'partial pressure of that gas in the atmosphere to 
 which the liquid is exposed. This is true not only of oxygen and 
 water, but of all gases and liquids which do not enter into chemical 
 combination with each other. Different liquids will of course 
 absorb different gases with differing readiness; but, with the same 
 gas and the same liquid, the amount absorbed will depend directly 
 on the partial pressure of the gas. It should be added that the 
 process is much influenced by temperature. Hence, to state the 
 matter generally, the absorption of any gas by any liquid, will 
 depend on the nature of the gas, the nature of the liquid, the pres- 
 sure of the gas,' and the temperature at which both stand. 
 
 Now it might be supposed, and indeed was once supposed, that 
 the oxygen in the blood was simply dissolved by the blood. If this 
 were so, then the amount of oxygen present in any given quantity 
 of blood exposed to any given atmosphere, ought to rise and fall 
 
Chap. 11. ] RESPIRATION. 333 
 
 steadily and regularly as the partial pressure of oxygen in that 
 atmosphere is increased or diminished. But this is found not to 
 be the case. If we expose blood containing little or iio oxygen to 
 a succession of atmospheres containing increasing quantities of 
 oxygen, we find that at first there is a very rapid absorption of the 
 available oxygen, and then this somewhat suddenly ceases or 
 becomes very small ; and if on the other hand we submit arterial 
 blood to successively diminishing pressures, we find that for a long 
 time very little is given oft', and then suddenly the escape becomes 
 very rapid. The absorption of oxygen by blood does not follow the 
 general law of absorption according to pressure. The phenomena 
 on the other hand suggest the idea that the oxygen in the blood is 
 in some particular combination with a substance or some sub- 
 stances present in the blood, the combination being of such a kind 
 that dissociation readily occurs at certain pressures and certain 
 temperatures. What is that substance or what are those sub- 
 stances ? 
 
 If serum, free from red corpuscles, be used in such absoi-ption 
 experiments, it is found that as compared with the entire blood, 
 very little oxj'^gen is absorbed, about as much as would be absorbed 
 by the same quantity of water ; but such as is absorbed does follow 
 the law of pressures. In natural arterial blood the quantity of 
 oxygen which can be obtained from serum is exceedingly small ; it 
 does not amount to half a volume in one hundred volumes of the 
 entire blood to which the serum belonged. It is evident that the 
 oxygen which is present in blood is in some way or other peculiarly 
 connected with the red corpuscles. Now the distinguishing feature 
 of the red corpuscles is the presence of haemoglobin. We have 
 already seen (p. 26) that this constitutes 90 per cent, of the dried 
 red coi-jDuscles. There can be a priori little doubt that this must 
 be the substance with which the oxygen is associated ; and to the 
 properties of this body we must therefore direct our attention. 
 
 Hcemoglobin ; its properties and derivatives. 
 
 When separated from the other constituents of the serum, 
 hemoglobin appears as a substance, either amorphous or crystal- 
 line, readily soluble in water (especially in warm Avater) and in 
 serum. 
 
 Since hsemoglobin is soluble in serum, and since the identity of the 
 crystals observed occasionally Mn.thin the corpuscles witli those obtained 
 in other ways shews that the hfemoglobin as it exists in the corpuscle is 
 the same thing as that which is artificially prepared from blood, it is 
 evident that some peculiar relationship between the stroma and the 
 haemoglobin must, in natural blood, keep the latter from being dissolved 
 by the serum. Hence in preparing haemoglobin it is necessary first of 
 
334 
 
 HEMOGLOBIN. 
 
 [Book ii. 
 
 Fig. 58. (After Preyer and Gamgee). The spectra of Oxy-H.emoglobin in 
 
 ijIFFEEENT GKADES OF CONCENTBATION, OF (bEDUCED) HEMOGLOBIN AND OF CaEBONIC- 
 OxiDE-H.5EiIOGLOBIN. 
 
 1. Solution of Oxy-HEemoglobin containing less than -01 p.c. 
 
 2. „ ,, ,, containing "09 p.c. 
 
 3. „ „ „ „ -37 p.c. 
 
 4. ,, ,, ,, ,, '8 p.c. 
 
 5. ,, (reduced) Heemoglobin containing about •2 p.c. 
 
 6. ,, carbonic oxide HtBmoglobiu. 
 
(^HAP. n] RESPIRATION. 335 
 
 bi each ot ibe six oases the layer brought before the spectrosuope was 1 cm. in 
 tliickncss. 
 
 Tho Ijettcrs (A, a Ac.) indicate Fraucnhofer's lines, and the figures wave-lenglhs 
 expressed iu lUO, 000th of a millimbtre. 
 
 all to bnuk up tho corpuscles. Tliis may bo done by the addition of 
 water, of other, of chloroform or of bile salts, or by repeatedly freezing 
 and thawing. It is also of advantage previously to remove the alkaline 
 soium as much as possible so as to operate only on the rod corpuscles. 
 The corpuscles being thus broken up, a solution of htemogloliin is 
 the result. The alkalinity of the solution, when present, being re- 
 duced by the cautious addition of dilute acetic acid, and the solvent 
 power of the aqueous medium being diminished by the addition of one 
 fourth its bulk of alcohol, the mixture, set aside in a temperature 
 of 0° C. in order still further to reduce the solubility of the haemo- 
 globin, readily crystallizes, when the blood used is that of the dog, cat, 
 horse, rat, guinea-pig, ikc. In the case of the dog indeed it is simply 
 sufficient to add ether to the blood and then to let it stand in a cool 
 place; the mixture soon becomes a mass of crystals. The crystals may 
 be separated by filtration, redissolved in water and re-crystallized. 
 
 Heemoglobin from the blood of the rat, guinea-pig, squirrel, 
 hedgehog, horse, cat, dog, goose, and some other animals, crystal- 
 lizes readily, the crystals being generally slender four-sided prisms, 
 belonging to the rhombic system, and often appearing quite 
 acicular. The crystals from the blood of the guinea-pig are octa- 
 hedral, but also belong to the rhombic system; those of the 
 squirrel are six-sided plates. The blood of the ox, sheep, rabbit, 
 pig, and man, crystallizes with difficulty. Why these differences 
 exist is not knoAvn ; but the composition, and the amount of water 
 of crystallization, vary somewhat in the crystals obtained from 
 different animals. In the dog, the percentage composition of the 
 crystals is, according to Hoppe-Seyler, C. 53'85, H. 7o2, N. 1617, 
 0. 21-84, S. 0-39, Fe. '43, with 3 to 4 per cent, of water of crystal- 
 lization. It wdll thus be seen that baBmoglobin contains, in 
 addition to the other elements usually present in proteid sub- 
 stances, a certain amount of iron; that is to say the element iron 
 is a distinct part of the haBmoglobin molecule: a fact w^hich of 
 itself renders haemoglobin remarkable among the chemical sub- 
 stances i^resent in the animal body. 
 
 The crystals, when seen in a sufficiently thick layer under the 
 microscope, have the same bright scarlet colour as arterial blood 
 has to the naked eye ; when seen in a mass they naturally appear 
 darker. An aqueous solution of haemoglobin, obtained by dis- 
 solving purified crystals in distilled w^ater, has also the same 
 bright arterial colour. A tolerably dilute solution placed before 
 the spectroscope is found to absorb certain rays of Ught in a 
 pecuhai- and characteristic manner. A portion of the red end of 
 the spectrum is absorbed, as is also a much larger portion of the 
 blue end; but w^hat is most striking is the presence of two 
 
336 HAEMOGLOBIN. [Book ii. 
 
 strongly marked absorption bands, lying between tbe solar lines D 
 and E. (See Fig. 58.) Of these the one towards the red side, 
 sometimes spoken of as the band a, is the thinnest, but the 
 most intense, and in extremely dilute solutions (Fig. 58 1) is the 
 only one visible; its middle lies at some little distance to the blue 
 side of D. Its position may be more exactly defined by expressing 
 it in wave-lengths. As is well known the rays of light which 
 make up the spectrum differ in the length of their waves, diminish- 
 ing from the red end where the waves are longest to the blue end 
 where they are shortest. Thus Frauenhofer's line D corresponds 
 to rays having a wave-length of 589'4 millionths of a millimetre. 
 Using the same unit, the centre of this absorption band a of hsemo- 
 globin corresponds to the wave-length 578 ; as may be seen in 
 Fig. 58, where however the numbers of the divisions of the scale 
 indicate only 100,000 of a millimetre. The other, sometimes called 
 ^, much broader, lies a little to the red side of E, its blue-ward 
 edge, even in moderately dilute solutions (Fig. 58 2) coming close 
 up to that line; its centre corresponds to about wave-length 539. 
 Each band is thickest in the middle, and gradually thins away at 
 the edges. These two absorption bands are extremely characteristic 
 of a solution of hsemoglobin. Even in very dilute solutions both 
 bands are visible (they may be seen in a thickness of 1 cm. in a solu- 
 tion containing 1 gTm. of hemoglobin in 10 litres of water), and 
 that when scarcely any of the extreme red end, and very little of the 
 blue end, is cut off. They then appear not only faint but narrow. 
 As the strength of the solution is increased, the bands broaden, and 
 become more intense ; at the same time both the red end, and still 
 more the blue end, of the whole spectrum, are encroached upon 
 (Fig. 58 3). This may go on until the two absorption bands become 
 fused together into one broad band (Fig. 58 4). The only rays 
 of light which then pass through the hsemoglobin solution are 
 those in the green between the blueward edge of the united bands 
 and the general absorption which is now rapidly advancing from 
 the blue end, and those in the red between the united bands 
 and the general absorption at the red end. If the solution be still 
 further increased in strength, the interval on the blue side of the 
 united bands becomes absorlDed also, so that the only rays which 
 pass through are the red rays lying to the red side of D ; these are 
 the last to disappear, and hence the natural red colour of the solu- 
 tion as seen by transmitted light. Exactly the same appearances 
 are seen when crystals of hsemoglobin are examined with a micro- 
 spectroscope. They are also seen when arterial blood itself 
 (diluted with saline solutions so that the corpuscles remain in as 
 natural condition as possible) is examined with the spectroscope, as 
 well as when a drop of blood, which fr'om the necessary exposure to 
 air is always arterial, is examined with the micro-spectroscope. In 
 fact, the spectrum of hsemoglobin is the spectrum of normal 
 arterial blood. 
 
C'iiAi< ii] HESPJRATIOX. 337 
 
 When crystals of lunnogloLin, prepared in the way described 
 above, arc subjected to the vacuum of the mercurial air-pump, they 
 give oft" a certain quantity of oxygen, and at the same time they 
 change in colour. The quantity of oxygen given oflf Ls definite, 
 1 gnn. of the crystals giving oft" l"o9 c.cm. of oxygen. In other 
 words, the crystals of haemoglobin over and above the oxygen 
 ■which enters intimately into their composition, (and which alone 
 is given in the elementary composition stated on p. 335), contain 
 another quantity of oxygen, which is in loose combination only, 
 and which may be dissociated from them by subjecting them to a 
 sufficiently low pressure. The change of colour which ensues when 
 this loosely combined oxygen is removed, is characteristic; the 
 crystals become darker and more of a pui'ple hue, and at the same 
 time dichroic, so that while the thin edges appear green, the 
 thicker ridges are purj^le. 
 
 An ordinary solution of hsemoglobin, like the crystals from 
 which it is foniied, contains a definite quantity of oxygen in 
 a similarly peculiar loose combination; this oxygen it also gives up 
 at a sufficiently low pressure, becoming at the same time of 
 a purplish hue. This loosely combined oxygen may also be 
 removed by passing a stream of hydrogen or other indifi"erent gas 
 through the solution, whereby dissociation is effected. It may 
 also be got rid of by the use of reducing agents. Thus if a few 
 drops of ammonium sulphide or of an alkaline solution of ferrous 
 sulphate, kept from precipitation by the presence of tartaric acid, 
 be added to a solution of haemoglobin, or even to an unpurified 
 solution of blood corpuscles such as is afforded by the washings 
 from a blood clot, the oxygen in loose combination "svith the 
 hcemoglobin is immediately seized upon by the reducing agent. This 
 may be recognised at once, by the characteristic change of colour; 
 from a bright scarlet the solution becomes of a purplish claret colour, 
 when seen in any thickness, but green when sufficiently thin : the 
 colour of the reduced solution is exactly like that of the crj-stals 
 from which the loose oxygen has been removed by the air-pump. 
 
 Examined by the spectroscope, this reduced solution, or solution 
 of reduced haemoglobin as we may now call it, offers a spectrum 
 (Fig. 58. 5) entirely different from that of the unreduced solution. 
 The two absorption bands have disappeared, and in their place 
 there is seen a single, much broader, but at the same time much 
 fainter band whose middle occupies a position about midway be- 
 tween the two absorption bands of the unreduced solution, though 
 the red-ward edge of the band shades away rather farther towards 
 the red than does the other edge towards the blue; its centre 
 corresponds to about wave length 555. At the same time the 
 general absorption of the spectrum is different from that of the 
 umreduced solution ; less of the blue end is absorbed. Even when 
 the solutions become tolerably concentrated, many of the bluish- 
 green rays to the blue side of the single band still pass through. 
 
 F. 22 
 
338 HMMOGLOBIN. [Book ii. 
 
 Hence the difference in colour between lisemoglobin which retains 
 the loosely combined oxygen \ and haemoglobin which has lost its 
 oxygen and become reduced. In tolerably concentrated solutions, 
 or tolerably thick layers, the former lets through the red and the 
 orange-yellow rays, the latter the red and the bluish-green rays. 
 Accordingly, the one appears scarlet, the other purple. In dilute 
 solutions, or in a thin layer, the reduced hsemoglobin lets through 
 so much of the green rays that they preponderate over the red, and 
 the resulting impression is one of green. In the unreduced hsemo- 
 globin or oxyh;3emoglobin, the potent yellow which is blocked out 
 in the reduced hsemoglobin makes itself felt, so that a very thin 
 layer of oxyhsemoglobin, as in a single corpuscle seen under the 
 microscope, appears yellow rather than red. 
 
 When the hsemoglobin solution (or crystal) which has lost its 
 oxygen by the action either of the air-pump or of a reducing agent 
 or by the passage of an indifferent gas, is exposed to air containing 
 oxygen, an absorption of oxygen at once takes place. If sufficient 
 oxygen be present, the whole of the hsemoglobin seizes upon its 
 complement, each gramme taking up in combination 1*59 c.cm. of 
 oxygen ; if there be an insufficient quantity of oxygen, a part only 
 of the hsemoglobin gets its allowance and the remainder continues 
 reduced. If the amount of oxygen be sufficient, the solution (or 
 crystal), as it takes up the oxygen, regains its bright scarlet colour, 
 and its characteristic absorption spectrum, the single band being 
 replaced by the two. Thus if a solution of oxyhsemoglobin in a 
 test-tube after being reduced by the ferrous salt, and shewing the 
 purple colour and the single band, be shaken up with air, the 
 bright scarlet colour at once returns, and when the fluid is placed 
 before the spectroscope, it is seen that the single faint broad band 
 of the reduced hsemoglobin has wholly disappeared, and that in its 
 place are the two sharp thinner bands of the oxyhsemoglobin. If 
 left to stand in the test-tube the quantity of reducing agent still 
 present is generally sufficient again to rob the hsemoglobin of the 
 oxygen thus newly acquired, and soon the scarlet hue fades back 
 again into the purple, the two bands giving place to the one. 
 Another shake and exposure to air will however again bring back 
 the scarlet hue and the two bands ; and once more these may dis- 
 appear. In fact, a few drops of the reducing fluid will allow this 
 game of taking oxygen from the air and giving it up to the reducer 
 to be played over and over again, and at each turn of the game the 
 colour shifts from scarlet to purple, and from purple to scarlet, 
 while the two bands exchange for the one, and the one for the two. 
 
 Colour of venous and arterial Blood. Evidently we have in 
 these properties of hsemoglobin an explanation of at least one-half 
 
 1 For brevity's sake we may call the hasmoglobin containing oxygen in loose 
 combination, oxyhcemoglobin, and the hemoglobin from -which this loosely combined 
 oxygen has been remoyed, reduced hsemoglobin or simply hasmoglobin. 
 
Chap, ii] RESPIRATIOX. 330 
 
 of the great respiratory process, aud they teach us the meaning of 
 the change of colour which takes place when venous blood becomes 
 juierial or arterial venous. In venous blood, as it Issues from the 
 right ventricle, the oxygen present is insuthcient to satisfy the 
 whole of the ha-moglobin of the red corpuscles ; much reduced hae- 
 moglobin is present, hence the purple colour of venous blood. 
 
 When ordinary venous blood, diluted without access of oxygen, 
 is brought before the spectroscope, the two bands of oxyhsemo- 
 globin are seen. This is explained by the fact that in a mixture 
 of oxyha^moglobin and (reduced) haemoglobin, the two shaqD bands 
 of the former are always much more readily seen than the much 
 fainter band of the latter. Now in ordinar}- venous blood there is 
 always some loose oxygen, removable by diminished pressure or 
 otherwise ; there is always some, indeed a considerable quantity, of 
 oxyhaemoglobin as well as (reduced) hsemoglobin. It is only in 
 the very last stages of asphyxia that all the loose oxygen of the blood 
 disappears ; and then the two bands of the oxyhemoglobin vanish 
 too. So distinct are the two bands of even a small quantity of 
 oxyhaemoglobin in the midst of a large quantity of hDemoglobin 
 that a solution of (completely reduced) hiemoglobin may be used 
 as a test for the presence of oxygen. 
 
 As the blood passes through the capillaries of the lungs, this 
 reduced haemoglobin takes from the pulmonaiy air its complement 
 of oxygen, all or nearly all the haemoglobin of the red corpuscles 
 becomes oxyhaemoglobin, and the pm-ple colour forthwith shifts 
 into scarlet. 
 
 The haemoglobin of arterial blood is saturated or nearly satu- 
 rated with oxygen. By increasing the pressure of the oxygen, 
 an additional quantity may be driven into the blood, but this, 
 after the haemoglobin has become completely saturated, is eftected 
 b}' simple absorption. The quantity so added is extremely small 
 compared with the total quantity combined with the haemoglobin, 
 but its physiological importance is increased by its being present 
 at a high tension. 
 
 Passing from the left ventricle to the capillaries, some of the 
 oxyhaemoglobin gives up its oxygen to the tissues, becomes 
 reduced haemoglobin, and the blood in consequence becomes once 
 more venous, with a purple hue. Thus the red corpuscles by 
 virtue of their haemoglobin are emphatically oxygen-carriers. 
 Undergoing no intrinsic change in itself, the haemoglobin combines 
 in the lungs with oxygen, which it carries to the tissues; these, 
 more greedy of oxygen than itself, rob it of its charge, and the 
 reduced haemoglobin hun-ies back to the lungs in the venous 
 blood for another portion. The change from venous to arterial 
 blood is then in part (for as we shall see there are other events as 
 well) a peculiar combination of haemoglobin with oxygen, while 
 the change from arterial to venous is, in part also, a reduction of 
 oxyhaemoglobin; and the ditference of colour between venous and 
 
340 COLOUR OF VENOUS AND ARTERIAL BLOOD. [Book ii. 
 
 arterial blood depends almost entirely on the fact that the reduced 
 hgemoglobin of the former is of purple colour, while the oxy- 
 hsemoglobin of the latter is of a scarlet colour. 
 
 There may be other causes of the change of colour, but these 
 are wholly subsidiary and unimportant. When a corpuscle swells, 
 its refractive power is diminished, and in consequence the number 
 of rays which pass into and are absorbed by it are increased at the 
 expense of those reflected from its surface ; anything therefore 
 which swells the corpuscles, such as the addition of water, tends to 
 darken blood, and anything, such as a concentrated saline solution, 
 which causes the corpuscles to shrink, tends to brighten blood. 
 Carbonic acid has apparently some influence in swelling the 
 corpuscles, and therefore may aid in darkening the venous blood. 
 
 We have spoken of the combination of hsemoglobin with 
 oxygen as being a peculiar one. The peculiarity consists in the 
 facts that the oxygen may be associated and dissociated, without 
 any general disturbance of the molecule of hsemoglobin, and that 
 dissociation may be brought about very readily. Haemoglobin 
 combines in a wholly similar manner with other gases. If car- 
 bonic oxide be passed through a solution of hsemoglobin, a change 
 of colour takes place, a peculiar bluish tinge making its ap- 
 pearance. At the same time the spectrum is altered ; two bands 
 are still visible, but on accurate measurement it is seen that they 
 are placed more towards the blue end than are the otherwise 
 similar bands of oxyhsemogiobin (see Fig. 58. G) ; their centres corre- 
 sponding respectively to about wave-lengths 572, and 533, while 
 those of oxyhsemogiobin as we have seen correspond to 578 and 
 539. When a known quantity of carbonic oxide gas is sent through 
 a hsemoglobin solution, it will be found on examination that a 
 certain amount of the gas has been retained, an equal volume of 
 oxygen appearing in its place in the gas which issues from the 
 solution. If the solution so treated be crystallized, the crystals will 
 have the same characteristic colour, and give the same absorption 
 spectrum as the solution; when subjected to the action of the 
 mercurial pump, they will give off a definite quantity of carbonic 
 oxide, 1 grm. of the crystals yielding 1"59 c.cm. of the gas. In 
 fact, hsemoglobin combines loosely with carbonic oxide just as it 
 does with oxygen ; but its affinity with the former is greater than 
 with the latter. While carbonic oxide readily turns out oxygen, 
 oxygen cannot so readily turn out carbonic oxide. Indeed, 
 carbonic oxide has been used as a means of driving out and 
 measuring the quantity of oxygen present in any given blood. 
 This property of carbonic oxide explains its poisonous nature. 
 When the gas is breathed, the reduced and the unreduced hsemo- 
 globin of the venous blood unite with the carbonic oxide, and 
 hence the peculiar bright cherry-red colour observable in the blood 
 and tissues in cases of poisoning by this gas. The carbonic oxide 
 hsemoglobin, however, is of no use in respiration ; it is not an 
 
Chap. II.] RESPJRATIOX. 341 
 
 oxygeu-canicr, nay more, it will not readily, though it does so 
 slowly and eventually, give up its carbonic oxide for oxygen, when 
 the poisonous gas ceases to enter the chest and is replaced by pure 
 air. The organism is killed by suffocation, by want of oxygen, in 
 spite of the blood not assuming any dark venous colour. As 
 Bernard phrased it, the corpuscles are paralysed. 
 
 Haemoglobin similarly forms a compound, having a character- 
 istic specti-um, with nitric oxide, more stable even than that with 
 carbonic oxide. 
 
 It has been supposed by some that the oxygen thus associated 
 with hx'moglobin is in the condition know-n as ozone ; but the 
 arguments urged in support of this view are inconclusive. 
 
 Although a crj'stalline body, haemoglobin diffuses with great 
 difficulty. This arises from the fact that it is in part a proteid 
 body; it consists of a colourless proteid, associated with a coloured 
 compound named hcematin. All the iron belonging to the haemo- 
 globin is in reality attached to the hsematin. A solution of 
 hsemooflobin, when heated, coagulates, the exact degi-ee at which 
 the coagulation takes place depending on the amount of dilution ; 
 at the same time it turns brown from the setting free of the 
 hoematin. If a strong solution of haemoglobin be treated with 
 acetic (or other) acid, the same brown colour, from the appearance 
 of haematin, is observed. The proteid constituent however is not 
 coagulated, but by the action of the acid passes into the state of 
 acid-albumin. On adding ether to the mixture, and shaking, the 
 haematin is dissolved in the supernatant acid ether, which it colours 
 a dark red, and which, examined with the spectroscoj)e, is found to 
 possess a well-marked sjjectrum, the spectrum of the so-called acid 
 haematin of Stokes. The proteid in the water below the ether 
 appears in a coagulated form owing to the action of the ether. In 
 a somewhat similar manner alkalis split up haemoglobin into a 
 proteid constituent and haematin. 
 
 The exact nature of the proteid constituent of haemoglobin 
 has not as yet been clearly determined. It was supposed to 
 be globulin, (hence the name hsematoglobulin contracted into 
 haemoglobin), but though belonging to the globulin family, has 
 characters of its own; it is possibly a mixture of two or more 
 distinct proteids. It has been provisionally named glohhi and is 
 said to be free from ash. Haematin when separated from its 
 proteid fellow, and purified, appears as a dark-brown amorphous 
 jjowder, or as a scaly mass with a metallic lustre, having the 
 probable composition of Cgj, H,^, N^, Fe, O^. It is fairly soluble 
 in dilute acid or alkaline solutions, and then gives characteristic 
 spectra. 
 
 An interesting feature in haematin is that its alkaline solution 
 is capable of being reduced by reducing agents, the spectrum 
 changing at the same time, and that the reduced solution will, like 
 the haemoglobin, take up oxygen again on being brought into 
 
342 CAEBOXIC ACID IX THE BLOOD. [Book ii. 
 
 contact with air or oxygen. This would seem to indicate that the 
 Gxygen-holding power of hasmoglobin is connected exclusively 
 with its hsematin constituent. By the action of strong suljjhuric 
 acid hsematin • may be robbed of all its iron. It still retains the 
 feature of possessing colour, the solution of iron-free hsematin 
 being a dark rich brownish red ; but is no longer capable of com- 
 bining loosely with oxygen. This indicates that the iron is iD 
 some way associated with the peculiar respiratory functions of 
 hsemoglobin ; though it is obviously an error to suppose, as was 
 once supposed, that the change from venous to arterial blood 
 consists essentially in a change from a ferrous to a ferric salt. 
 
 Though not crystallizable itself, hsematin forms with hydro- 
 chloric acid a compound, occurring in minute rhombic ci-ystals, 
 known as hamin crystals. 
 
 In conclusion, the condition of oxygen in the blood is as 
 follows. Of the whole quantity of oxygen in the blood, only a 
 minute fraction is simply absorlDed or dissolved, according to the 
 law of pressures (the Henry-Dalton law). The gi'eat mass is in a 
 state of combination with the hsemoglobin, the connection being of 
 such a kind that while the hsemoglobin readily combines with the 
 oxygen of the air to which it is exposed, dissociation readily occurs 
 at low pressures, or in the presence of indifferent gases, or by the 
 action of substances having a greater affinity for oxygen than has 
 haemoglobin itself The difference between venous and arterial 
 blood, as far as oxygen is concerned, is that while in the latter 
 there is an insignificant quantity of reduced hsemoglobin, in the 
 former there is a great deal ; and the characteristic colours of 
 venous and arterial blood are in the main due to the fact that the 
 colour of reduced hiemoglobin is purple, while that of oxyhsemo- 
 globin is scarlet. 
 
 The relations of the Carhonic Acid in the Blood. 
 
 The presence of carbonic acid in the blood appears to be detei - 
 mined by conditions more complex in their nature and at present 
 not so well understood as those which determine the presence of 
 oxygen. The carbonic acid is not simply dissolved in the blood; its 
 absorption by blood does not follow the law of pressures. It exists 
 in association with some substance or substances in the blood, and 
 its escape from the blood is a process of dissociation. We cannot 
 however speak of it as being associated, to the same extent as is 
 the oxygen, with the hsemoglobin of the red corpuscles. So far 
 from the red corpuscles containing the great mass of the carbonic 
 acid, the quantity of this gas which is present in a volume of serum 
 
CiiAr. II.] RESPIHATIOX. 343 
 
 is according to some observers actually greater than that which is 
 ])resent in an equal volume of blood, i.e. an equal volume of mixed 
 corpuscles and serum. 
 
 When serum is subjected to the mercurial vacuum, by far the 
 greater part of the carbonic acid is given otf ; but a small additional 
 (piantity (2 to 5 vols, per cent.) may be extracted by the subsequent 
 addition of an acid. This latter portion may be spoken of as 'fixed' 
 carbonic acid in distinction to the larger 'loo.se' portion which is 
 given off" to the vacuum. When however the whole blood is subjected 
 to the vacuum, all the carbonic acid is given off", so that when 
 serum is mixed with corpuscles all the carbonic acid may be spoken 
 of as 'loose'; and it is stated that the excess of carbonic acid in 
 serum over that present in entire blood, corresponds to the fixed 
 portion in serum which has to be diiven off by an acid. Moreover, 
 even those who maintain that the quantity of carbonic acid in blood 
 is less than that in an equal volume of serum, admit that the 
 tension of the carbonic acid in blood is greater than in serum. 
 
 If these statements be accepted it seems probable that the 
 carbonic acid exists associated with some substance or substances 
 in the sei'um, but that the conditions of its association (and therefore 
 of its dissociation) are determined by the action of some substance or 
 substances present in the corpuscles. It is further probable that 
 the association of the carbonic acid in the serum is with sodium as 
 sodium bicarbonate, and it is even possible that the hsemoglobin of 
 the corpuscles plays a part in promoting the dissociation of the 
 sodium bicarbonate or even the carbonate, and thus keeping up the 
 carbonic acid tension of the entire blood. Other observers however 
 maintain that the serum does not hold this exclusive possession of 
 the carbonic acid, but that a considerable quantity of this gas is in 
 some way associated with the red corpuscles. Indeed further 
 investigations are necessary before the matter can be said to 
 have been placed on a satisfactory footing. 
 
 The relations of the Nitrogen in the Blood. 
 
 The small quantity of this gas which is present in both arterial 
 and venous blood seems to exist in a state of simple solution. 
 
SEC. 4. THE EESPIRATORY CHANGES IN THE LUNGS. 
 
 The entrance of Oxygen. 
 
 We have already seen that the blood in passing through the 
 lungs takes up a certain variable quantity (from 8 to 12 vols, p. c.) 
 of oxygen. We have further seen that the quantity so taken 
 up, putting aside the insignificant fraction simply absorbed, 
 enters into direct but loose combination with the haemoglobin. 
 In drawing a distinction between the oxygen simply absorbed 
 and that entering into combination with the haemoglobin, it 
 must not be understood that the latter is wholly independent 
 of pressure. On the contrary all chemical compounds are in 
 various degrees subject to dissociation at certain pressures and 
 temperatures ; and the existence of the somewhat loose compound of 
 oxygen and haemoglobin is dependent on the partial pressure of 
 oxygen in the atmosphere to which the haemoglobin is exposed. A 
 solution of haemoglobin or a quantity of blood will either absorb 
 oxygen and thus undergo association or will undergo dissociation 
 and give off oxygen according as the partial pressure of oxygen 
 in the atmosphere to which it is exposed is high or low, and the 
 amount taken up or given off will depend on the degree of the 
 partial pressure. But the law according to which absorption or 
 escape thus takes place is quite different from that observed in the 
 simple absorption of oxygen by liquids. The association or dis- 
 
Chap. M.] liKSPIRAriOX. 345 
 
 sociation is furtlior especially deiKiultiit on temperature, a hi^^li 
 temperature lavouriug dissuciatiuu so that at a high temijerature 
 less oxygen is taken up than would be taken up (or more given off 
 as the case may be than would be given off) at a lower 
 tcmperaturt', the partial pressure of the oxygen in the atmosphere 
 remaining the same. 
 
 Hence the question arises, Are the conditions in which haemo- 
 globin and oxygen exist in ordinary venous blood as it flows to the 
 lungs, of such a kind that the venous blood in passing through the 
 ])ulmonary capillaries will find the partial pressure of the oxygen 
 in the pulmonary alveoli sufiicient to bring about the association of 
 the additional quantity of oxygen whereby the venous is converted 
 into arterial blood ? 
 
 The oxygen of expired air contains (in man) as we have seen 
 about 16 p. c. of oxygen. The air in the pulmonary alveoli must 
 contain rather less than this, since the expired air consists of tidal 
 air mixed by diffusion with the stationaiy air. How much less 
 it contains we do not exactly know, but probably the difference 
 is not very great. The question therefore stands thus, Will venous 
 blood, exposed at the temperature of the body to a partial pressure 
 of less than 16 p. c. of oxygen take up sufficient oxygen (from 8 to 
 12 vols. p. c.) to convert it into arterial blood ? Numerous experi- 
 ments have been made (chiefly on the dog) to determine on the one 
 hand the oxygen-tension of both arterial and venous blood {i.e. the 
 partial pressure of oxygen in an atmosjDhere exposed to which the 
 arterial blood neither gives up nor takes in oxygen, and the same 
 for venous blood) and on the other hand the behaviour at the 
 temperature of the body or at ordinar}' temperatures of blood or of 
 solutions of haemoglobin (for the two behave in this respect very 
 much alike) towards an atmosphere in which the partial pressure 
 of oxygen is made to vary. Without going into detail, we may state 
 that these experiments shew that the partial pressure of oxygen 
 in the lungs is amply sufficient to bring about, at the temperature 
 of the body, the association of that additional amount of oxygen by 
 which venous blood becomes arterial. When a solution of haemo- 
 globin or when blood is successively exposed to increasing oxygen 
 pressures, as the partial pressure of oxygen is gradually increased, 
 the curve of absorption rises at first very rapidly but afterwards more 
 slowly, that is to say, the later additions of oxygen at the higher 
 pressures are proportionately less than the earlier ones, at the 
 lower pressures. And this is consonant with what appears to be 
 the fact that the haemoglobin of arterial blood though nearly 
 saturated with oxygen, i.e. associated with almost its full comple- 
 ment of oxygen, is not quite saturated. "WTien arterial blood 
 is thoroughly exposed to air, it takes up rather more than 1 vol. 
 p. c. of oxygen ; and that appears to represent the difference 
 between exposing blood to air as it enters the mouth in inspiration 
 and exposing blood to the air as it exists in the pulmonary alveoli. 
 
346 EXIT OF CARBOXIC ACID. [Book ii. 
 
 The greater relative absorption at the lower pressures has a 
 beneficial effect in as much as it still permits a considerable quantity 
 of oxygen to be absorbed even when the partial pressure of oxygen 
 in the air in the lungs is largely reduced, as in ascending to great 
 heights. 
 
 The statements made so far refer to ordinary breathing, but 
 the question may be asked, What happens when the renewal of 
 the air in the pulmonary alveoli ceases, as when the trachea 
 is obstructed ? In such a case the oxygen in the alveoli is found 
 to diminish rapidly so that the partial pressure of oxygen in them 
 soon falls below the oxygen tension of ordinary venous blood. But 
 in such a case the blood is no longer ordinary venous blood ; instead 
 of containing a comparatively small amount, it contains a large and 
 gradually increasing amount, of reduced hsemoglobin. And as the 
 reduced hsemoglobin increases in amount, the oxygen tension of the 
 venous blood decreases ; it thus keeps below that of the air in the 
 lungs. Hence even the last traces of oxygen in the lungs are 
 taken up by the blood, and carried away to the tissues ; so that 
 with the last heart's beat, when the oxygen in the lungs has sunk 
 to a mere fraction, the bands of oxy hsemoglobin may still, it is said, 
 be detected for a moment in the blood of the left side of the heart 
 showing that oxygen has even still been absorbed. 
 
 The exit of Carbonic Acid. 
 
 It seems natural to suppose that the carbonic acid would 
 escape by diffusion from the blood of the alveolar capillaries 
 into the air of the alveoli. But in order that diffusion should 
 thus take place, the carbonic acid tension of the air in the 
 pulmonary alveoli must always be less than that of the venous 
 blood of the pulmonary artery and indeed ought not to exceed 
 that of the blood of the pulmonary vein. There are however 
 many practical difficulties in the way of an exact determi- 
 nation of the carbonic acid tension of the pulmonary alveoli (for 
 though it must be greater than that of the expired _ air, it is 
 difficult to say how much greater), and of the carbonic acid tension 
 of the blood at the same time, so as to be in a position to_ compare 
 the one with the other. In the case of oxygen, there is always 
 present in the lungs a surplus of the gas, a portion only being 
 absorbed at each breath ; in the case of carbonic acid, the whole 
 quantity comes direct from the blood, and any modifications in^ 
 breathing seriously affect the amount given out. Thus when the 
 breath is held for some time the percentage of carbonic acid in the 
 expired air reaches 7 or 8 p.c, but we cannot take this as a measure 
 of the normal percentage of carbonic acid in the pulmonary alveoli, 
 
Chap, h] RESPIRATIOX. 347 
 
 since by the mere holding of the breath the carbonic acid in the 
 blood and in the pulmonary alveoli is increased beyond the normal. 
 The dithculties of the problem seem however to have been 
 overcome by an ingenious experiment in which there is introduced 
 into the bronchus of the lung of a dog a catheter, round which is 
 arranged a small bag ; by the inflation of this bag the bronchus, 
 whenever desii'ed, can be completely blocked up. Thus, without any 
 disturbance of the general breathing, and therefore without any 
 change in the normal proportions of the gases of the blood, the ex- 
 perimenter is able to stop the ingress of fresh air into a limited 
 portion of the lung. At the same time he is enabled by means of 
 the catheter to withdraw a sample of the air of the same limited 
 portion, and, by analysis to determine its carbonic acid tension. 
 The blood passing through the alveolar capillaries of this limited 
 portion of the lung naturally possesses the same carbonic acid 
 tension as the rest of the venous blood flowing through the 
 pulmonary arters^ a tension which, though varying slightly 
 from moment to moment, will maintain a normal average. On 
 the supposition that carbonic acid passes simply by diffusion 
 from the pulmonary blood into the air of the alveoli, because the 
 carbonic acid tension of the latter is normally lower than that of 
 the former, one would expect to find that the air in the occluded 
 portion of the lung would continue to take up carbonic acid 
 until an equilibrium was established between it and the carbonic 
 acid tension of the venous blood. Consequently if after an 
 occlusion, say of some minutes (by which time the equilibrium 
 might fairly be assumed to have been established), the carbonic 
 acid tension of the air of the occluded portion were determined, it 
 ought to be found to be equal to, and not more than equal to, the 
 carbonic acid tension of the venous blood of the pulmonary artery. 
 And this is the result which has been arrived at ; it has been 
 found that the tensions of the carbonic acid of the occluded air and 
 of the venous blood of the right side of the heart are just about 
 equal, that of the occluded air being, if anything, slightly less than 
 that of the venous blood. So that the evidence so lar as it goes is 
 distinctly in favour of the view that the escape of carbonic acid 
 from the blood into the pulmonary alveoli is simply due to diffusion, 
 and that there is no need to seek for any further explanation. 
 There is for instance no necessity to suppose that the epithelium 
 of the pulmonary alveoli has any specific secretory power of 
 discharging carbonic acid from the blood independently of or 
 in antagonism to the influence of pressures. 
 
SEC. 5. THE RESPIRATORY CHANGES IN THE TISSUES. 
 
 In passing through the several tissues the arterial blood 
 becomes once more venous. A considerable quantity of the oxy- 
 hsemoglobin becomes reduced, and a quantity of carbonic acid 
 passes from the tissues into the blood. The amount of change 
 varies in the various tissues, and in the same tissue may vary at 
 different times. Thus in a gland at rest, as we have seen, the 
 venous blood is dark, shewing the presence of a large quantity of 
 reduced hasmoglobin ; when the gland is active, the venous blood 
 in its colour, and in the amount of hsemoglobin which it contains, 
 resembles closely arterial blood. The blood therefore which issues 
 from a gland at rest is more 'venous' than that from an active 
 gland ; though owing to the more rapid flow of blood which, as we 
 saw in an earlier section, accomjDanies the activity of the gland, the 
 total quantity of carbonic acid discharged into the blood from the 
 gland in a given time may be greater in the latter. The blood, on 
 the other hand, which comes from an active i.e. a contracting 
 muscle, is, in spite of the more rapid flow, not only richer in 
 carbonic acid, but also, though not to a corresponding amount, 
 poorer in oxygen than the blood which flows from a muscle 
 at rest. 
 
 In all these cases the great question which comes up for our 
 consideration is this: Does the oxygen pass from the blood into the 
 tissues, and does the oxidation take place in the tissues, giving rise 
 to carbonic acid, which passes in turn away from the tissues into 
 
Chap, ii] RESPIRATIOX. 319 
 
 the blood ? or do certain oxidizable rodaciug substances pass from 
 the tissues into the blood, and there become oxidized into carbonic 
 acid and other products, so that the chief oxidation takes place in 
 the blood itself? 
 
 There are, it is true, reducing oxidizable substances in the 
 blood, but these are small in amount, and the quantity of carbonic 
 acid to which they give rise when the blood containing them is 
 agitated with air or oxygen, is so small as scarcely to exceed the 
 en-ors of observation. 
 
 On the other hand, it will be remembered that in speaking of 
 muscle, Ave drew attention to the fact that a frog's muscle removed 
 from the body (and the same is true of the muscles of other animals) 
 contains no free oxygen whatever; none can be obtained from 
 it by the mercurial air-pump. Yet such a muscle will not only 
 when at rest go on producing and discharging a certain quantitv, 
 but also when it contracts evolve a very considerable quantity, of 
 carbonic acid. Moreover this discharge of carbonic acid will go on 
 for a certain time in muscles under circumstances in which it is 
 impossible for them to obtain oxygen from without. Oxygen, it is 
 true, is necessary for the life of the muscle : when venous instead 
 of arterial blood is sent through the blood-vessels of a muscle, the 
 irritability speedily disappeai'S, and unless fresh oxygen be ad- 
 ministered the muscle soon dies. The muscle may however, 
 during the interval in w^hich irritability is still retained after the 
 supply of oxygen has been cut off, continue to contract vigorouslv. 
 The supply of oxygen, though necessary for the maintenance of 
 irritability, is not necessary for the manifestation of that irritability, 
 is not necessary for that explosive decomposition which developes 
 a contraction. A frog's muscle wall continue to contract and to 
 produce carbonic acid in an atmosphere of hydrogen or nitrogen, 
 that is in the total absence of free oxygen both from itself and 
 from the medium in which it is jjlaced. 
 
 Thus on the one hand the muscle seems to have the property 
 of taking up and fixing in some way or other the oxygen to which 
 it is exposed, of converting it into intra-molecular oxygen, in which 
 condition it cannot be removed by simple diminished pressure, so 
 that the tension of oxygen in the muscular substance may be con- 
 sidered as always nil ; while on the other hand the muscular sub- 
 stance is always undergoing a decomposition of such a kind that 
 carbonic acid is set free, sometimes, as when the muscle is at rest, 
 in small, sometimes, as during a contraction, in large quantities. 
 But if the oxygen tension of the muscular tissue be thus always 
 nil, the oxygen of the blood-corpuscles, in which it is at a com- 
 paratively high tension, will be always passing over, through the 
 plasma, through the capillary walls, the lymph spaces and the 
 sarcolemma, into the muscular substance, and as soon as it arrives 
 there will be hidden away as intra-molecular oxygen, leaving the 
 oxvgen tension of the muscular substance once more nil. Con- 
 
350 CHANGES IX THE TISSUES. [Book n. 
 
 versely, the carbonic acid produced by the decomposition of the 
 muscular substance will tend to raise the carbonic acid tension of 
 the muscle until it exceeds that of the blood ; whereupon it will 
 pass from the muscle into the blood, its place in the muscular 
 substance being supplied by freshly generated carbonic acid. 
 There will always in fact be a stream of oxygen from the blood to 
 the muscle and of carbonic acid from the muscle to the blood. 
 The respu'ation of the muscle then does not consist in throwing 
 into the blood oxidizable substances there to be oxidized into car- 
 bonic acid and other matters; but it does consist in the assumption 
 of oxygen (as intra-molecular oxygon), in the building up by help 
 of that oxygen of explosive decomposable substances, and in the 
 occurrence of decompositions whereby carbonic acid and other 
 matters are discharged first into the substance of the muscle and 
 subsequently into the blood. We cannot as yet trace out the steps 
 taken by the oxygen from the moment it slips into its intra- 
 molecular position to the moment when it issues united with carbon 
 as carbonic acid. The whole mystery of life lies hidden in the 
 story of that progress, and for the present we must be content 
 with simply knowing the beginning and the end. 
 
 Our knowledge of the respiratory changes in muscle is more 
 complete than in the case of any other tissue; but we have no 
 reason to suppose the phenomena of muscle are exceptional. On 
 the contrary, all the available evidence goes to shew that in all 
 tissues the oxidation takes place in the tissue, and not in the 
 adjoining blood. It is a remarkable fact, that lymph, serous fluids, 
 bile, urine, and milk contain a mere trace of free or loosely 
 combined oxygen, and saliva or pancreatic juice a very small quan- 
 tity only, while the tension of carbonic acid in peritoneal fluid and 
 probably in the tissues of the intestinal walls is higher than that 
 of venous blood, and in bile and urine is still greater. The tension 
 of carbonic acid in lymph, while higher than that of arterial blood, 
 is lower than that of the general venous blood ; but this jDrobably 
 is due to the fact that the lymph in its passage onwards is largely 
 exposed to arterial blood in the connective tissues and in the 
 lymphatic glands, where the production of carbonic acid is slight 
 as compared to that going on in muscles. All these facts point to 
 the conclusion, that it is the tissues, and not the blood, which 
 become primarily loaded with carbonic acid, the latter simply 
 receiving the gas from the former by diffusion, except the (pro- 
 bably) small quantity which results from the metabolism of the 
 blood-corpuscles ; and that the oxygen which passes from the 
 blood into the tissues is at once taken up in some combination, so 
 that it is no longer removable by diminished tension. 
 
 In further support of this view may be urged the fact that if, in 
 a frog, the whole blood of the body be replaced by normal saline solu- 
 tion, the total metabolism of the body is, for some time, unchanged. 
 The saline medium is able owing to the low rate of metabolism, 
 
TiiAP. II.] nESPIRATIoy. 351 
 
 nnd large respiratory surlace of the animal, to .suj)|)ly the tissues 
 with all the oxygen they need, and to remove all the earbonic acid 
 they produce. It is ditiicult to believe that, in such an experiment, 
 the oxidation t<K)k ])lace in the saline solution itself ^vhile cir- 
 culating in the blood-vesscLs and tissue-spaces of the animal. 
 
 We may add, that the oxiilativc power which the blood itself 
 removed from the body is able to exert on substances which are 
 undoubtedly oxidized in the body is so small that it may be 
 neglected in the present considerations. If grape-sugar be added 
 to blood, or to a solution of haemoglobin, the mixture may be kept 
 for a long time at the temperature of the body, without undergoing 
 oxidation. Even within the body a slight excess of sugar in the 
 blood over a certain percentage wholly escapes oxidation, and is 
 discharged unchanged. Many easily oxidized substances, such as 
 pyrogallic acid, pass largely through the blood of a living body 
 without being oxidized. Thi3 organic acids, such as citric, even 
 in combination with alkaline bases, are only partially oxidized; 
 when administered as acids, and not as salts, they are hardly 
 oxidized at all. It is of course quite possible that the changes 
 which the blood undergoes when shed might interfere with its 
 oxidative action, and hence the fact that shed blood has little 
 or no oxidizing power, is not a satisfactory proof that the un- 
 changed blood within the living vessels may not have such a 
 power. But did oxidation take place largely in the blood itself, 
 one would expect even highly diffusible substances to be oxidized 
 in their transit ; whereas if we suppose the oxidation to take place 
 in the tissues, it becomes intelligible why such diffusible substances 
 as those which the tissues in general refuse to take up largely, 
 should readily j^ass unchanged from the blood through the secreting 
 organs. 
 
 We have seen that in muscle the production of carbonic acid 
 is not directly dependent on the consumption of oxygen. The 
 muscle produces carbonic acid in an atmosphere of hydrogen. What 
 is true of muscle is true also of other tissues and of the body 
 at large. Spallanzani and W. Edwards shewed long ago that 
 animals might continue to breathe out carbonic acid in an 
 atmosphere of nitrogen or hydrogen ; and more recently Pfluger 
 has shewn, by a remarkable exj^eriment, that a frog kept at a 
 low temperature will live for several hours, and continue to 
 produce carbonic acid, in an atmosphere absolutely free from 
 oxygen. The carbonic acid produced during this period was made 
 by help of the oxygen inspired in the hours anterior to the com- 
 mencement of the experiment. The oxygen then absorbed was 
 stowed away from the haemoglobin into the tissues, it was made 
 use of to build up the explosive comjDounds, whose explosions later 
 on gave rise to the carbonic acid. Or, to adopt Pfliiger's simile, the 
 oxygen helps to wind up the vital clock; but once wound up the 
 clock will go on for a jicriod without further winding. The frog 
 
352 CHANGES IN THE TISSUES. [Book ii. 
 
 \^^.l^ continue to live, to move, to produce carbonic acid for a while 
 without any fresh oxygen, as we know of old it will without any 
 fresh food; it will continue to do so till the explosive compounds 
 which the oxygen built up are exhausted ; it will go on till the 
 vital clock has run down. 
 
 To sum up, then, the results of respiration in its chemical 
 aspects. As the blood passes through the lungs, the low oxygen 
 tension of the venous blood permits the entrance of oxygen from 
 the air of the pulmonary alveolus, through the thin alveolar wall, 
 through the thin capillary sheath, through the thin layer of blood- 
 plasma, to the red corpuscle, and the reduced haemoglobin of the 
 venous blood becomes wholly, or all but wholly, oxyhsemoglobin. 
 Hurried to the tissues, the oxygen, at a eomparatively high tension 
 in the arterial blood, passes largely into them. In the tissues, the 
 oxygen tension is always kept at an exceedingly low pitch, by the 
 fact that they, in some way at present unknown to us, pack away 
 at every moment into some stable combination each molecule of 
 oxygen which they receive from the blood. With much but not 
 all of its oxyhsemoglobin reduced, the blood passes on as venous 
 blood. How much hemoglobin is reduced will depend on the 
 activity of the tissue itself. The quantity of hjEmoglobin in the 
 blood is the measure of limit of the oxidizing power of the body 
 at large ; but within that limit the amount of oxidation is deter- 
 mined by the tissue, and by the tissue alone. 
 
 We cannot trace the oxygen through its sojourn in the tissue. 
 We only know that sooner or later it comes back combined in 
 carbonic acid (and other matters not now under consideration). 
 Owing to the continual production of carbonic acid, the tension 
 of that gas in the extravascular elements of the tissue is always 
 higher than' that of the blood ; the gas accordingly passes from 
 the tissue into the blood, and the venous blood passes on not only 
 with its haemoglobin reduced, i.e. with its oxygen tension decreased, 
 but also with its carbonic acid tension increased. Arrived at the 
 lungs, the blood finds the pulmonary air at a lower carbonic acid 
 tension than itself. The gas accordingly streams through the thin 
 vascular and alveolar walls, till the tension without the blood-vessel 
 is equal to the tension within. At the same time the blood finds 
 in the air of the pulmonary alveoli a supply of oxygen, more than 
 adequate to convert the greater part of the reduced haemoglobin 
 back again to • oxyhaemoglobin. Thus the air of the pulmonary 
 alveoli, having given up oxygen to the blood and taken up carbonic 
 acid from the blood, having a higher carbonic acid tension and a 
 lower oxygen tension than the tidal air in the bronchial passages, 
 mixes rapidly with this by diffusion. The mixture is further assisted 
 by ascending and descending cun-ents; and the tidal air issues from 
 the chest at the breathing out poorer in oxygen and richer in 
 carbonic acid than the tidal air which entered at the breathing in. 
 
SEC. 6. THE NERVOUS MECHANISM OF RESPIRATION. 
 
 Breathing is an involuntary act. Though the diaphragm and 
 all the other muscles employed in respiration are voluntary muscles, 
 i.e. muscles which can be called into action by a direct eflbrt of the 
 will, and though respiration may be modified within very wide 
 limits by the will, yet we habitually breathe without the interven- 
 tion of the will : the normal breathing may continue, not only in 
 the absence of consciousness, but even after the removal of all 
 the parts of the brain above the medulla oblongata. 
 
 We have already seen how complicated is even a simple respira- 
 tory act. A very large number of muscles are called into play. 
 Many of these are very far apart from each other, such as the 
 diaphragm and the nasal muscles ; yet they act in harmonious 
 sequence in point of time. If the lower intercostal muscles con- 
 tracted before the scaleni, or if the diaphragm contracted alternately 
 with the other chest-muscles, the satisfactory entrance and exit of 
 air would be impossible. These muscles moreover are coordinated 
 also in respect of the amount of their several contractions ; a gentle 
 and ordinary contraction of the diaphragm is accompanied by gentle 
 and ordinary contractions of the intercostals, and these are preceded 
 by gentle and ordinary contractions of the scaleni. A forcible con- 
 traction of the scaleni, followed by simply a gentle contraction of 
 the intercostals, would perhaps hinder rather than assist inspiration, 
 and at all events would be waste of power. Further, the whole com- 
 plex inspiratory effort is often followed by a less marked but still 
 complex expiratory action. It is impossible that all these so 
 
 p. 23 
 
354 NERVOUS MECHANISM. [Book ii. 
 
 carefully coordinated muscular contractions should be brought 
 about in any other way than by coordinate nervous impulses 
 descending along efferent nerves from a coordinating centre. By 
 experiment ^ye find this to be the case. 
 
 When in a rabbit the trunk of a phrenic nerve is cut, the dia- 
 phragm on that side remains motionless, and respiration goes on 
 without it. When both nerves are cut, the whole diaphragm 
 remains quiescent, though the respii'ation becomes excessively 
 laboured. 
 
 When an intercostal nerve is cut, no active respiratory move- 
 ment is seen in that space, and when the spinal cord is divided 
 below the origin of the seventh cervical spinal nerve, costal 
 resj)iration ceases, though the diaphragm continues to act and 
 that with increased vigour. When the cord is di\-ided just below 
 the medulla, all thoracic movements cease, but the respiratory 
 actions of the nostrils and glottis still continue. These however 
 disappear when the facial and recurrent laryngeal are divided. We 
 have already stated that after removal of the brain above the 
 medulla, respiration still continues very much as usual, the modifi- 
 cations which ensue from loss of the brain being unessential. 
 Hence, putting all these facts together, it is clear that the 
 respii^atory movements are, as we suggested, brought about by 
 coordinated impulses which, originated in the medulla, find their 
 way thence along the several efferent nerves. The proof is completed 
 by the fact that the removal or extensive injury of the medulla 
 alone is, save in exceptional cases, at once followed by the cessation 
 of all respiratory movements, even though every muscle and every 
 nerve concerned be left intact. Nay more, if only a small portion 
 of the medulla, a tract whose limts are not as yet exactly fixed, 
 but which lies below the vaso-motor centre, between it and the 
 calamus scriptorius, be removed or injured, respiration ceases, 
 and death at once ensues. Hence this portion of the nervous 
 system was called by Flourens the vital knot, or ganglion of life, 
 ncBiid vital. We shall speak of it as the respu-atory centre. 
 
 The nature of this centre must be exceedingly complex ; for 
 while even in ordinary respiration it gives rise to a whole group of 
 coordinate nervous impulses of inspiration followed in due sequence 
 by a smaller but still coordinate group of expiratory impulses, in 
 laboured respiration fresh and larger impulses are generated, 
 though still in coordination with the normal ones, the expiratory 
 events being especially augmented; and in the cases of more 
 extreme dyspnoea and asphyxia impulses overflow, so to speak, 
 from it in all directions, though only gi-adually losing their co- 
 ordination, until almost every muscle in the body is thrown into 
 contractions. 
 
 We must not however conceive of this centre as one of such a 
 kind that the imjoulses leave it fully coordinated and equipped so 
 that nothing remains for them but to travel, unchanged, along the 
 
Chap, ii] liESPIliATION. 355 
 
 several efferent nerve-fibres to their several muscular destinations. 
 On the contrary we have reason to think that the respiratory motor 
 nerves, like other special nerves as they arc about to issue from the 
 spinal cord, are connected with a nervous ganf^lionic machinery, — 
 a point which we shall consider more fully in treating of the spinal 
 Corel ; and that the respiratory impulses pass into and are modified 
 by such spinal nervous machinery immediately before they issue 
 along the motor nerve-roots. Indeed recent observations shew 
 that under particular conditions, and especially in young animals, 
 respiratory movements may be carried out in the entire absence of 
 the medulla oblongata. Thus in a kitten, after removal of the 
 medulla, if the excitability of the spinal cord be heightened by 
 small doses of strychnia, not only may respiratory movements of 
 the chest be induced, in a reflex manner, by pinching or by 
 blowing on the skin, but even transient spontaneous efforts of 
 breathing may with care be observed. These are the excep- 
 tional instances mentioned above ; and they shew that the respi- 
 ratory nervous mechanism is not confined, as was once thought, 
 to the centre in the medulla, but also embraces other subsidiary 
 centres in the spinal cord below. The respiratory nervous s}i:em 
 seems in fact in many ways analogous to the vaso-motor nervous 
 system, with its head centre in the medulla, and secondary centres 
 elsewhere, and to the cardiac nervous system with its potent 
 ganglia in the sinus, and its secondary ganglia in the auricles, and 
 auriculo-ventricular groove. The matter is not at present thorough- 
 ly worked out, but we shall probably not greatly err in continuing 
 to speak of the centre in the medulla as being "the respiratory 
 centre" while admitting that it works through other nerv'ous 
 machinery placed lower down in the spinal cord, and that this 
 subordinate machinery may, in exceptional cases, carr}' out, though 
 inadequately, the work of the chief centre. 
 
 Admitting then the existence of this medullary respiratory 
 centre the question naturally arises, Are we to regard its rhythmic 
 action as due essentially to changes taking place in itself, or as due 
 to afferent nervous impulses cr other stimuli which affect it in a 
 rhythmic manner from "v\'ithout ? In other words, Is the action of 
 the centre automatic or purely reflex ? We know that the centre 
 may be influenced by impulses proceeding from -w-ithout, and that 
 the breathing may be affected by the action of the will, or by an 
 emotion, or by a dash of cold water on the skin, or in a hundred 
 other ways ; but the fact that the action of the centre may be thus 
 modified from without, is no proof that the continuance of its 
 activity is dependent on extrinsic causes. 
 
 In attempting to decide this question we naturally turn to the 
 pneumogastric as being the nerve most likely to serve as the 
 channel of afferent impulses setting in action the respiratory centre. 
 If both vagi be divided, respiration still continues though in a 
 modified form. This proves distinctly that afferent impulses 
 
 23 -2 
 
356 NERVOUS MECHANISM. [Book ii. 
 
 ascending those nerves are not the efficient cause of the respiratory 
 movements. We have seen that when the spinal cord is divided 
 below the medulla, the facial and laryngeal movements still 
 continue. This proves that the respiratory centre is still in 
 action, though its activity is unable to manifest itself in any 
 thoracic movement. But when the cord is thus divided, the 
 respiratory centre is cut off from all sensory impulses, save those 
 which may jDass into it from tlie cranial nerves ; and the division 
 of these cranial nerves by themselves, when the medulla and 
 spinal cord are left intact, does not destroy respiration. Hence 
 we may infer that the respiratory impulses proceeding from the 
 respiratory centre are not simply afferent impulses reaching the 
 centre along afferent nerves and transformed by reflex action in 
 that centre. They evidently start de novo from the centre itself, 
 however much their characters may be affected by afferent impulses 
 reaching that centre at the time of their being generated. The 
 action of the centre is automatic, not simply reflex. 
 
 Among the afferent impulses which affect the automatic action 
 of the centre, the most important are those which ascend along 
 the vagi. If one vagus be divided, the respiration becomes slower; 
 if both be divided, it becomes very slow, the pauses between 
 expiration and inspiration being excessively prolonged. The 
 character of the respiratory movement too is markedly changed ; 
 each respiration is fuller and deeper, so much so indeed that, 
 according to some observers, what is lost in rate is gained in extent, 
 the amount of carbonic acid produced and oxygen consumed in a 
 given period remaining after division of the nerves about the 
 same as when these were intact. Without insisting too much on 
 the exactness of this compensation we may at least conclude from 
 the effects of section of the vagi, in the first place, that during life 
 afferent impulses are continually ascending the vagi and modifying 
 the action of the respiratory centre, and in the second place, that 
 the modification bears chiefly on the distribution in time of the 
 efferent respiratory impulses, and not so much on the amount to 
 which they are generated. 
 
 These afferent impulses are probably started in the lungs by 
 the condition of the blood in the pulmonary capillaries acting as a 
 stimulus to the peripheral endings of the nerves, though possibly 
 the altered air in the air-cells may also act as a stimulus to the 
 nerve-endings. It has further been suggested that the mere 
 movements of expansion and contraction may also serve as a 
 stimulus. Thus when air is mechanically driven into the chest, an 
 expiratory movement follows, and when air is drawn out, an in- 
 spiratory; and this not only with atmospheric air but with in- 
 different gases, such as nitrogen; when both vagi are cut, these 
 effects do not appear. So also, when in an animal, after division 
 of the spinal cord below the medulla, artificial respiration is kept 
 up, the resi^iratory movements of the nostrils follow the rhythm 
 
CiiAP. n] nSSPI RATION. 357 
 
 of tlu> artificiiil respiration so loiii,' us tlic va;,n arc intact ; wlicn 
 these are dividi'd the movements of the nostrils exhibit a rliythm 
 independent of those of the chest. From tliis it is inferred that the 
 mere mechanical expansion of the lungs transmits along the vagus 
 an impidsc tending to inhibit inspiration and to generate an ex- 
 piration, and the mechanical contraction of the lungs an impulse 
 tending to inhibit ex])iration and to generate an inspiration. That 
 is to say tiie very ex])ansion of the lungs, Avhich is the natural 
 ctTect of an inspiration, tends of itself to cut short that inspiration 
 and to inaugurate the sequent expiration, and similarly the 
 contraction of an expiration promotes the following inspiration. 
 The lungs in fact may be spoken of as being so far self-regulating. 
 
 The influence of the vagus is further shewn by the following 
 experiment. If the medulla oblongata be carefully divided in the 
 middle line respiration may continue to go on in quite a normal 
 fashion, indicating that the centre is composed of two lateral 
 halves placed one on each side of the median line. If however 
 one vagus be then divided, the respiratory movements both costal 
 and diaphragmatic, on the side of the body on which division of 
 the vagus has taken place, become slower than those on the other 
 side, so that the two sides arc no longer synchronous. Obviously 
 the vagus influences primarily the respiratory centre of its owti side; 
 though under normal conditions the two halves of the centre work 
 in harmony and sjTichronism. 
 
 When after division of both vagi, the medulla being intact, the 
 central stump of one vagus is stimulated with a gentle interrupted 
 cun-ent, the respii"ation, which from the division of the nerves had 
 become slow, is quickened again; and with care, by a proper 
 application of the stimulus, the normal respiratory rhythm may 
 for a time be restored. Upon the cessation of the stimulus, the 
 slower rhythm returns. If the current be increased in strength, 
 the rhythm may in some cases be so accelerated that at last the 
 diaphragm is brought into a condition of prolonged tetanus, and 
 a standstill of respiration in an extreme inspiratory phase is the 
 result. 
 
 If the central end of the superior lar\'ngeal branch of the vagus 
 be stimulated, whether the main trunk of the nerve be severed or 
 not, a slov^'ing of the respiration takes place, and this may by 
 proper stimulation be earned so far that a complete standstill of 
 respiration in the phase of rest is brought about, i.e. the respiratory 
 apparatus remains in the condition which obtains at the close of 
 an ordinary expiration, the diaphragm being completely relaxed. 
 In other words, the superior lar}-ngeal nerve contains fibres, the 
 stimulation of which produces afterent impulses whose effect is to 
 inhibit the action of the respiratory centre; while the main trunk 
 of the vagus contains fibres, the stimulation of which produces 
 atTerent impulses whose effect is to accelerate or augment the 
 action of the respiratory centre. In some cases stimulation of the 
 
358 NERVOUS MECHANISM. [Book ii. 
 
 main trunk of the vagus also causes a slowing or even standstill of 
 the respiration, as for instance in deep chloral narcosis or when 
 the nerve has become exhausted by j)revious stimulation. Stimu- 
 lation of the superior laryngeal frequently produces not only a 
 complete cessation of all inspiratory movements, as indicated by 
 the perfectly lax diaphragm, but also contractions of the abdominal 
 muscles indicating an expiratory effort; and it is obvious that the 
 commencement of an expiration must be preceded by a cessation of 
 inspiratory effects, just as similarly insj^iration must be preceded 
 by the cessation of expiration. Hence the influences which inhibit 
 inspiration are often spoken of as expiratory though they may not 
 go so far as to produce an actual expiration. 
 
 Corresponding to these antagonistic influences we may suppose 
 the existence of separate fibres, augmentative or inspiratory fibres, 
 the stimulation of which leads to inspiratory movements, and 
 inhibitory or expiratory fibres the stimulation of which checks 
 inspiration and subsequently gives rise to expiration. But it 
 must be remembered that the existence of these fibres is hypo- 
 thetical, and that some other explanation may eventually be given 
 of the facts which we have just described. Indeed we are not able 
 at present to give a wholly consistent and satisfactory explanation of 
 the nature and working of the respiratory centre. Apparently we 
 must conceive of its consisting of two parts, an inspiratory and 
 an expiratory: and direct stimulation of the medulla produces 
 sometimes inspiration, sometimes expiration ; but the two parts 
 must be considered as co-ordinated in such a way as to act 
 alternately. Of the two the inspiratory centre is in ordinary life 
 the more important, the more sensitive and the more active, since 
 in normal breathing active expu-atory effects are scanty, and the 
 empt3dng of the chest is chiefly the result of the cessation of 
 inspiration. Under conditions, however, which we shall speak of 
 presently under the name of dyspnoea, the expiratory centre 
 comes distinctly into play, since actual expiratory efforts come to 
 the front and, as we shall see, the greater the difiiculty of breathing 
 the more and more prominent they become. We may picture to 
 ourselves, as Eosenthal has done, that the inspiratory centre is the 
 seat of two conflicting processes, one tending to the discharge of 
 inspiratory impulses and the other offering resistance to that 
 discharge, the former gathering head during a period of rest and so 
 at last overcoming the latter, and effecting an actual discharge. 
 After this the accumulation of inspiratory processes once more 
 begins, and once more terminates in a discharge, thus leading to 
 the rhythm of respiration. We may further suppose that the aug- 
 mentative impulses ascending the vagi, produce their effect by 
 diminishing the processes of resistance, and thus bring about move- 
 ments which are at once quicker and less ample. But we have to add 
 to this conception some view as to the relation of the expiratory to 
 the inspiratory centre in order to explain why the impulses inhibitory 
 
Chap, ii] JiESriliATIOy. 359 
 
 to the latter sluiuU be augmentative to the former. Indeed the 
 whule matter becomes too com])licated to be discussed any further 
 hero ; and we have introduced the view not because we regard it as 
 an adetjuate i^xphination of the plienomena, but l)ecausc it alfurds a 
 useful gra[)hic concei)tion of the molecular activity of these and 
 other automatic nen'ous centres. We may be at present content with 
 the knowledge that, as far as the vagus is concerned, the respiratory 
 centre as a whole may be intluenced by augmentative or inspiratory 
 impulses which run chieHy in the trunk of the nerve and by inhibi- 
 tory or ex])iratory imjnilses which run ccrtairdy in the superior 
 laryngeal, apparently also in the recurrent laryngeal, and to a certain 
 extent in the trunk also ; in the latter case, however, their presence 
 is manifested under certain conditions only. And while, from the 
 results of sim})le section of the main trunk, it is clear that the ac- 
 celerating intiucnces are continually at work, it is not so clear that 
 the inhibitory influences are always in action, since section even of 
 both superior laryngcals docs not necessarily quicken respiration. 
 
 This double or alternate respiratory action of the vagi may bs 
 taken as in a general way illustrative of the manner in which other 
 afferent nerves and various parts of the cerebrum are enabled to 
 influence respiration. As we know from daily experience, of all 
 the apsychical nervous centres, the respiratory centre is the one 
 which is most frequently and most deeply affected by nervous im- 
 pidses from various quarters. Besides the changes brought about 
 by the will (and when Ave breathe vokmtarily we probably make 
 use to some extent of the normal nervous machinery of respiration, 
 w'orking through this, rather than sending independent volitional 
 impulses dii'ect to the diajahragm and other respii'atory muscles), 
 we hnd that emotions, and painful sensations alter profoundly 
 the character of the respiratory movements. Sometimes the 
 breathing thereby becomes quicker and Hatter, sometimes it is 
 deepened as well as hurried; at other times it may be slowed 
 or for a w^hile stopped altogether, while occasionally expiratory 
 efforts are made prominent. And though these effects may 
 be partly indirect, the emotion modifying the heart-beat, and 
 so influencing the tlow of blood through the respiratory centre, they 
 are chiefly due to the direct actions of nervous impulses reaching 
 that centre from higher parts of the brain. So also impulses from 
 almost every sentient surface, or passing along almost every 
 sensory nerve, may modify respiration in one direction or another, 
 the slighter feebler impulses tending apparently to quicken, and 
 the stronger larger impulses to arrest or inhibit the respiratory 
 discharges. The influence in this way of stimuli applied to the 
 skin is well known to all; but perhaps next to the vagus the 
 nerve most closely connected with the respiratory centre is the 
 fifth nerve, branches of which guai'd the nasal respii'atory channels; 
 the slightest stimulation of the nostrils at once atiecting the 
 breathing and most frequently arresting it. Thus the working of 
 
360 NERVOUS MECHANISM. [Book ii. 
 
 the resj)iratory centre is made to respond delicately to the varying 
 needs of the economy. 
 
 Besides these nervous influences, however, there is another 
 circumstance which perhaps above all others affects the respiratory 
 centre, and that is the condition of the blood in respect to its 
 respiratory changes ; the more venous (less arterial) the blood, the 
 greater is the activity of the respiratory centre. When by reason 
 either of any hindrance to the entrance of air into the chest, or of a 
 greater respiratory activity of the tissues, as during muscular 
 exertion, the blood becomes less arterial, more venous, i.e. with a 
 smaller charge of oxy-hsemoglobin and more heavily laden with 
 carbonic acid, the respiration from being normal becomes laboured. 
 We may speak of normal breathing as eupncea, and say that this, 
 when the blood is insufficiently arterialized, passes into dyspncea, 
 an intermediate stage in which the respiratory movements are 
 simply exaggerated being known as hyper pncea. This effect of 
 deficient arterialization of blood is very different from that of 
 section of the vagi: it is no mere change in the distribution of 
 impulses ; the breathing is quicker as well as deeper, there is an 
 increase in the sum of efferent impulses proceeding from the centre, 
 and the expiratory impulses, which in normal respiration are very 
 slight, acquire a pronounced importance. As the blood becomes, 
 in cases of obstruction, less and less arterial, more and more 
 venous, the discharge from the respiratory centre becomes more 
 and more vehement, and instead of confining itself to the usual 
 tracts, and j)assing down to the ordinary respiratory muscles, over- 
 flows into other tracts, puts into action other muscles, until there is 
 perhaps hardly a muscle in the body which is not made to feel its 
 effects. And this discharge may, as we shall see in speaking 
 of asphyxia, continue till the nervous energy of the respiratory 
 centre is completely exhausted. The effect of venous blood then is 
 to augment these natural explosive decompositions of the nerve- 
 cells of the respiratory centre which give rise to respiratory 
 impulses; it increases their amount, and also quickens their 
 rhythm. The latter change however is much less marked than 
 the former, the respiration being much more deepened than 
 hurried, and the several respiratory acts are never so much 
 hastened as to catch each other up, and so to produce an in- 
 spiratory tetanus like that resulting from stimulation of the vagus. 
 On the contrary, especially as exhaustion begins to set in, the 
 rhythm becomes slower out of proportion to the weakening of the 
 individual movements. 
 
 On the other hand, the blood may be made not more but less 
 venous than usual. When we attempt to hold our breath, we find 
 that we can only do this for a limited time ; sooner or later a 
 breath must come ; but the time during which we can remain 
 without breathing may be much prolonged, if we first of all take a 
 series of deep breaths. By this increased ventilation we bring our 
 
Chap, u.] nh'S/'/h\i770\. 361 
 
 ros|tii-utury centre into snt-h a condition that it takes a much 
 lunger time for the succeeding respiratory impulses to become 
 irresistible. A similar but even more pronounced condition may 
 be brought about in an animal by making it inspire oxygen, 
 or breathe ordinary air more raj)itlly and more forcil»ly than the 
 needs of the economy require. If in a rabbit artificial respiration 
 is carried on very vigorously for a while, and then suddenly 
 stopped, the animal does not immediately begin to breathe. For 
 a variable period no respiration takes place at all, and when it does 
 begin occurs gently and normally, only passing into dyspnoea if the 
 animal is unable to breathe of itself; and even then the transition is 
 quite gradual. E\ idently during this period the respiratory centre 
 is in a state of comjjlete rest, no explosions are taking place, no 
 respiratory impulses are being generated, and the quiet transition 
 from this condition to that of normal respiration shews that the 
 subsequent generation of impulses is attended by no great dis- 
 turbance. The cause of this state of things, which is known as 
 that of apnoea, is to be sought for in the condition of the blood. 
 By the increased vigour of the artificial respiratory movements the 
 ha-moglobin of the arterial blood, which in normal breathing is not 
 quite saturated, becomes almost completely so, and the quantity of 
 oxygen simply dissolved is increased, its tension being largely 
 augmented. Respiration is arrested because the blood is more 
 highly arterializcd than usual. Thus we have in apnoea the 
 converse to dyspnoea; and both states jDoint to the same con- 
 clusion, that the activity of the respiratory centre is dependent on 
 the condition of the blood, being augmented when the blood is less 
 arterial and more venous, being depressed when it is more arterial 
 and less venous than usual. 
 
 The question now arises, Does this condition of the blood affect 
 the respiratory centre directly, or does it produce its effect by 
 stimulating the peripheral ends of afferent nerves in various j^arts 
 of the body, and, by the creation there of afferent impulses, 
 indirectly modify the action of the centre ? Without denying the 
 possibility that the latter mode of action may help in the matter, 
 as regards not only the vagi, but all afferent nerves, it is clear from 
 the following reasons that the main effect is produced by the 
 direct action of the blood on the respiratory centre itself. If the 
 spinal cord be divided below the medulla oblongata, and both vagi 
 be cut, want of proper aeration of the blood still produces an 
 increased acti\'ity of the resjTiratory centre, as shewn by the 
 increased vigour of the facial respiratory movements. If the 
 supply of blood be cut off from the medulla by ligature of the 
 blood-vessels of the neck, dyspnoea is jiroduced, though the opera- 
 tion produces no change in the blood generally, but simply affects 
 the respiratory condition of the medulla itself, by cutting off 
 its blood-supply, the immediate result of which is an accumulation 
 of carbonic acid and a paucity of available oxygen in the proto- 
 
362 NERVOUS MECHANISM. ' [Book ii. 
 
 plasm of the nerve-cells in that region. If the blood in the carotid 
 artery in an animal be warmed above the normal, dyspnoea is at 
 once produced. The overwarm blood hurries on the activity of 
 the nerve-cells, of the respiratory centre, so that the supply of 
 blood, even though greater than normal owing to the blood-vessels 
 of the medulla becoming dilated by the increased temperature, is 
 yet insufficient for their needs. The condition of the blood then 
 affects respiration by acting directly on the respiratory centre itself. 
 Deficient aeration |)roduces two effects in blood : it diminishes 
 the oxygen, and increases the carbonic acid. Do both of these 
 changes affect the respiratory centre, or only one, and if so, which ? 
 When an animal is made to breathe an atmosphere containing 
 nitrogen only, the exit of carbonic acid by diffusion is not affected, 
 and the blood, as is proved by actual analysis, contains no excess 
 of carbonic acid. Yet all the phenomena of dyspnoea are present. 
 In this case these can only be attributed to the deficiency of 
 oxygen. On the other hand, if an animal be made to breathe an 
 atmosphere rich in carbonic acid, but at the same time containing 
 abundance of oxygen, though the breathing becomes markedly 
 deeper and also somewhat more frequent, there is no culmination 
 in a convulsive asphyxia, even when the quantity of carbonic acid 
 in the blood, as shewn by direct analysis, is very largely increased. 
 On the contrary the increase in the respiratory movements after a 
 while passes off", the animal becoming unconscious, and appearing 
 to be suffering rather from a narcotic poison than from simple 
 dyspnoea. It does not seem certain that the increased respiratory 
 movements seen at first are the direct result of the action of the 
 carbonic acid on the respiratory centre; it is possible that the 
 carbonic acid may affect the resjDiratory centre in an indirect way, 
 by stimulating the respiratory passages, or by its action on higher 
 parts of the brain; and in all cases there is a marked contrast 
 between the slow development and evanescent character of the 
 hyperpnoea of carbonic acid poisoning, and the rapid onset and 
 speedy culmination in convulsions and death of the dyspnoea due 
 to the absence of oxygen. There can in fact be no doubt that the 
 action of deficiently arterialized blood on the respiratory centre, as 
 manifested in an augmentation of the respiratory explosions, is due 
 primarily to a want of oxygen, and in a secondary manner only to 
 an excess of carbonic acid. 
 
 Cheyne-Stokes Respiration. A remarkable abnormal rhythm 
 of respiration, first observed by Cheyne but afterwards more fully 
 studied by Stokes and hence called by their combined names, 
 occurs in certain pathological cases. The respiratory movements 
 gradually decrease both in extent and rapidity until they cease 
 altogether, and a condition of apnoea, lasting it may be for several 
 seconds, ensues. This is followed by a feeble respiration, succeeded 
 in turn by a somewhat stronger one, and thus the respiration 
 
Chap, ii.] RESPIRATIOX. 3C3 
 
 returns f^n-adually to the normal, or may even rise to hyperpnoea or 
 slight dyspna'a after ^vhich it again deelines in a similar manner. 
 A secondary rhythm of respiration is thus developed, periods of 
 normal or slightly dyspna'ic respiration alternating by gradual 
 transitions with periods of apna^a. The cau.sc of the phenomena 
 is iK^t thoroughly undei-stood. Stokes connected it with a fatty 
 condition of the heart, but it has been met with in various maladies. 
 Closely similar phenomena have been observed during sleep, under 
 perfectly normal conditions. It presents a striking analogy with 
 the 'groups' of heart-beats so frequently seen in the frog's ventricle 
 placed under abnormal circumstances. 
 
SEC. 7. THE EFFECTS OF RESPIRATION ON THE 
 CIRCULATION. 
 
 We have seen, v/hile treating of the circulation, that the blood- 
 pressure curves are marked by undulations, which, since their 
 rhythm is synchronous with that of the respiratory movements, are 
 evidently in some way connected with respiration. 
 
 Fig. o9. Comparison of Blood-Pkessuke curve -with curve of Intea-thoracic 
 PRESSURE. To be read from left to right. 
 
 a is the blood-pressure curve, with its respiratory undulations, the slower beats 
 on the descent being very marked, b is the curve of iutra-thoracic pressure 
 obtained by connecting one limb of a manometer with the pleural cavity. In- 
 spiration begins at i, expiration at e. The intra-thoracic pressure rises very 
 rapidly after the cessation of the inspiratory effort, and then slowly falls as tlie air 
 issues from the chest ; at the beginning of the inspiratory effort the fall becomes 
 more rapid. 
 
Chat, ii.j 
 
 RESPIRATIOX. 
 
 3C5 
 
 When these unduUitions of the blood-prossurc curve are com- 
 pared carefully with the res])irat(jry iiKnemonts or with the 
 variations of iutra-thoracic pressure, it is seen tliat wliile in f(cneral 
 the blood-pressure rises during inspiration and falls during expiration 
 neither the rise nor the fall of the former is exactly synchronous 
 with either inspiration or expiration. Fig. 59 shews two tracings 
 from a dog taken at the same time, one, a, being the ordinary 
 blood-pi'cssure curve from the carotid, and the other, h, representing 
 the condition of the intra-thoracic pressure as obtained by carefully 
 bringing a manometer into connection with the pleural cavity. 
 On comparing the two curves, it is evident that neither the 
 maximum nor the minimum of arterial pressure coincides exactly 
 cither with inspiration or with expiration. At the beginning of 
 inspiration (z) the arterial pressure is seen to be falling ; it soon 
 however begins to rise, but does not reach the maximum until 
 some time after expiration (e) has begun ; the fall continues during 
 the remainder of expiration, and jiasses on into the succeeding 
 inspiration. 
 
 This suggests the idea that, while inspiration tends to increase 
 and expiration to diminish the blood-pressure, there are causes 
 
 Fig. 60. Influence of respiration on tbe form of the pulse- wave and the medium 
 blood-pressure. The upper curve gives the radial pulse from a healthy man of 
 27 years of age and with an extra-arterial pressure of 70 mm. mercury. The lower 
 curve gives the movements of the chest wall, the points of the recording levers of 
 both instrimicnts being in the same vertical line. It can be seen that during 
 inspiration the pulse-waves diminish in height and become more dicrotic, while 
 during expiration the height of the pulse-waves is increased and their form tends 
 more towards that of the pulse- wave with a raised arterial pressm-e. 
 
366 EFFECTS ON CIRCULATION. [Book ii. 
 
 at work which in each case delay the effect. Extended observa- 
 tions however shew that such a relation as that shewn in 
 the figure though frequent is not constant. In fact the 
 effects of the respiratory movements on blood-pressure are 
 found to vary very widely according as the respiration is 
 quick or slow, easy and shallow or laboured and deep, and 
 especially as the air enters into the chest readily or -with 
 difficulty. A similar variety of effect is seen in sphygmographic 
 tracings, and these further shew that the respiratory movements 
 bring about changes not only in the mean blood-pressure but in 
 the form and characters of each pulse-wave. In Fig. 60, which 
 represents a state of things frequently occurring, but which must 
 not be considered as illustrating what always takes place, it will 
 be seen that during the greater part of expiration the pulse-wave 
 is high and the dicrotic wave is not very prominent, while during 
 inspiration the height of the wave is diminished, and dicrotism 
 becomes much more marked ; the former indicates a higher and 
 the latter a lower blood-pressure. 
 
 These variations in the exact relations of the respiratory 
 undulations to the respiratory movements themselves will prepare 
 the reader for the statement that the causation of the undulations 
 is complex. Apparently the respiratory act affects the vascular 
 system in several different ways, and the general effect varies 
 according as one or other infl.uence is predominant. These several 
 actions are sufficiently interesting and important to deserve dis- 
 cussion. 
 
 When the brain of a living mammal is exposed by the removal 
 of the skull, a rhythmic rise and fall of the cerebral mass, a 
 pulsation of the brain, quite distinct from the movements caused 
 by the pulse in the arteries of the brain, is observed; and upon 
 examination it will be found that these movements are synchronous 
 with the respiratory movements, the brain rising up during ex- 
 piration and sinking during inspiration. They disappear when the 
 arteries going to the brain are ligatured, or when the venous 
 sinuses of the dura mater are laid open so as to admit of a free 
 escape of the venous blood. They evidently arise from the ex- 
 piratory movements in some w^ay hindering and the inspiratory 
 movements assisting the return of blood from the brain. We have 
 already (p. 127) stated that during inspiration the jDressure of blood 
 in the great veins may become negative, i. e. sink below the pressure 
 of the atmosphere; and a puncture of one of these veins may 
 cause immediate death by air being actually drawn into the vein 
 and thus into the heart during an inspiratory movement. When 
 the veins of an animal are laid bare in the neck and watched, 
 the so-called indsus venosus may be observed in them, that is, they 
 swell up during expiration and diminish again during inspiration. 
 And indeed a little consideration will shew that the expansion and 
 contraction of the chest must have a decided effect on the flow of 
 
CiiAi'. II.] RESPIRATION. 3G7 
 
 Motnl throuc,'h the thoracic portion of, and thus indirectly on tliut 
 thnniL,di the whole of, the vascular system. 
 
 The heart ami great blood-vessels are, like the lungs, placed in 
 the air-tight thoracic cavity, and are subject like the lungs to the 
 pumping action of the respiratory movements. Were tlie lungs 
 entirely absent from the chest, the whole force of the expansion of 
 the thorax in inspiration would be directed to drawing blood from 
 the extra-thoracic vessels towards the heart, and conversely the 
 effect of the contraction of the thorax in expiration would be to 
 drive the blood back again from the heart towards the extra-thoracic 
 vessels. In the i:)resence of the lungs however the free entrance of 
 air into the interior of the chest tends to maintain the pressure 
 around the heart and great vessels within the thorax equal to the 
 ordinary atmospheric pressure on the vessels of the rest of the body 
 outside the thorax ; but it is unable completely to equalise the two 
 pressures. Did the air enter- as freely into the lungs as it does into 
 the pleural cavities when wide openings are made in the thoracic 
 walls, the respiratory movements would have very little effect 
 indeed on the How of blood to and from the heart, just as when 
 such free openings exist they are ineffectual in promoting the 
 entrance and exit of air to and from the lungs. But the air does 
 not pass into the pulmonary alveoli as freely as it would do into a 
 pleural cavity through an opening in the thoracic wall. Before the 
 inspired air can fill a pulmonary alveolus, the walls of the alveolus 
 have to be distended at the expense of the pressure which causes the 
 inspired air to enter. Part of the atmospheric pressure in fact 
 which causes the entrance of the air into the lung is spent in over- 
 coming the elasticity of the pulmonary passages and cells. Conse- 
 quently, any structure lying within the thorax but outside the 
 lungs, is never, even at the conclusion of an inspiration when the 
 lungs are filled ^vith air, subject to a pressure as great as that of the 
 atmosphere. The pressure on such a structure always falls short of 
 the pressure of the atmosphere by the amount of pressure necessary 
 to counterbalance the elasticity of the pulmonary passages and cells. 
 And, since the fraction of the atmospheric pressure which is thus 
 spent in distending the lungs increases as the lungs become more 
 and more stretched, it follows that the fuller the inspiration the 
 greater is the difference between the pressure on structures within 
 the thorax but outside the lungs and the ordinary pressure of the 
 atmosphere. Now we have seen that the pressure necessary 
 to counterbalance the elasticity of the lungs, when they are com- 
 pletely at rest (in the pause between expiration and inspiration), is 
 in man about 5 to 7 mm. of mercury, and that when the lungs are 
 fully distended, as at the end of a forcible inspiration, the pressure 
 rises to as much as 30 mm. of mercury. Hence at the height of a 
 forcible inspiration the pressure exerted on the heart and great 
 vessels within the thorax is 30 mm. less than the ordinary atmo- 
 spheric pressure of 760 mm., and even when the chest is completely 
 
368 EFFECTS ON CIRCULATION. [Book ii. 
 
 at rest, at the end of an expiration, tlie pressure on the heart and 
 great vessels is slightly (by about 5 mm. mercury) below that of the 
 atmosphere. 
 
 During an inspiration then the pressure around the heart and 
 great blood-vessels becomes considerably less than that of the atmo- 
 sphere on the vessels outside the thorax. During expiration this 
 pressure returns towards that of the atmosphere, but in ordinary 
 breathing never quite reaches it. It is only in forcible expiration 
 that the pressure on the thoracic vascular organs exceeds that of 
 the atmosphere. But if during inspiration the pressure bearing on 
 the right auricle and the vense cavoe becomes less than the pressure 
 which is bearing on the jugular, subclavian, and other veins out- 
 side the thorax, this must result in an increased flow from the 
 latter into the former. Hence, during each inspiration a larger 
 Cj[uantity of blood enters the right side of the heart. This probably 
 leads to a stronger stroke of the heart, and at all events causes 
 a larger quantity to be ejected by the right ventricle; this 
 causes a larger quantity to escape from the left ventricle, and thus 
 more blood is thrown into the aorta, and the arterial tension 
 proportionately increased. During expiration the converse takes 
 place. The pressure on the intra-thoracic blood-vessels returns to 
 the normal, the flow of blood from the veins outside the thorax 
 into the vense cavae and right auricle is no longer assisted, and in 
 consequence less blood passes through the heart into the aorta, 
 and arterial tension falls again. During forced expiration, the 
 intra-thoracic pressure may be so great as to afford a distinct 
 obstacle to the flow from the veins into the heart. 
 
 The effect of the respiratory movements on the arteries is 
 naturally different from that on the veins. During inspiration the 
 diminution of pressure in the thorax around the aortic arch tends 
 to draw the blood from the arteries outside the thorax back to the 
 arch of the aorta, or, in other words, tends to check the onward 
 flow of blood. At the same time, and this is the point to which 
 we wish to call attention, the aortic arch itself tends to expand ; in 
 consequence the pressure of blood within it, ^. e. the arterial tension, 
 tends to diminish. During expiration, the increase of pressure 
 outside the aortic arch of course tends to increase also the blood- 
 pressure within it, acting in fact just in the same way as if the 
 coats of the aorta themselves contracted. Thus as far as arterial 
 blood-pressure is concerned the effects of the respiratory move- 
 ments on the great veins and great arteries respectively are 
 antagonistic to each other ; the effect on the veins being to increase 
 arterial tension during inspiration and to diminish it during 
 expiration, while the effect on the arteries is to diminish arterial 
 tension during inspiration and to increase it during expiration. 
 But we should naturally expect the effect on the thin- walled veins 
 to be greater than that on the stout thick-walled arteries, so that 
 the total effect of inspiration would be to increase, and the total 
 
Chap, ii.] HES PI RATION. ;}09 
 
 effect of expiration to diminish, arterial tension. That is to say, 
 wc should expect the blood-pressure to rise dunii^ inspiration 
 and to lull during ex})iratiun. This as we ha\ e seen is i'lcquently the 
 case, anil indeed when the breathing is deep and laboured the 
 influence in this direction on the blood-pressure curve of the 
 pumping action of the chest is unmistakeable. 
 
 In addition to the influence thus exerted by the thoracic move- 
 ments on the great veins leading to, and the great arteries leading 
 from the heai't, we have to consider the behaviour of the pulmonary 
 vessels themselves under the varying thoracic pressure. These, 
 like the venaD cava; and aorta, tend to expand under the influence 
 of the inspiratory expansion of the chest, and thus to become fuller 
 of blood, very much as they would if the whole lung were placed 
 under a large cupping-glass. The first effect of this increased filling 
 of the pulmonary vessels would be to retain for a while a certain 
 quantity of blood in the lungs and thus to lessen the amount falling 
 into the left auricle. But this would be temporary only ; and the 
 widening of the pulmonary vessels would sjjeedily produce an 
 exactly contrary effect, namely, an increased flow through the lungs 
 due to the diminished resistance offered by the widened passages. 
 Conversely the first effect of expiration would be an increased flow 
 into the left auricle due to the additional quantity of blood driven 
 onwards by the partial collapse of the pulmonary vessels, followed 
 by a more significant diminished flow caused by the greater resis- 
 tance now offered by the naiTower vascular channels. Thus the 
 effect of inspiration in this way would be fii'st to diminish the flow 
 into the left auricle and so into the left ventricle and thus to 
 diminish for a brief initial period the blood-pressure in the aorta, 
 but afterwards, for the rest of the inspiration until the beginning of 
 expiration, to increase the flow into the ventricle and thus to raise 
 the arterial pressure ; while conversely the effect of expiration would 
 be fii'st, for a brief period, to increase and afterwards, during the rest 
 of the movement, to diminish the flow of blood into the left ven- 
 tricle, and through that the amount of arterial pressure. Further, 
 while this may be considered as the effect on the pulmonary vessels, 
 large and small taken altogether, the influence both of the 
 thoracic negative pressure during inspiration, and the return 
 in a positive dii'ection during exjDiration, will bear more on 
 the thin-walled pulmonary veins than on the stouter pulmonary 
 artery ; that is to say, as inspiration becomes established, 
 there will be a diminution of pressure in the pulmonary veins 
 gi-eater than that in the pulmonary artery, and this will be an 
 additional influence favouring the flow into the left ventricle ; during 
 ex2m'ation a similar difterence of effect mil be felt in the contrary 
 direction. The general effect then of the movements of the chest on 
 the pulmonar}'' vessels will be during the beginning of inspiration to 
 continue the loweiing of arterial pressure which was taking place 
 during expiration but subsequently to raise the arterial pressure; and 
 
 F. 24 
 
370 EFFECTS ON CIRCULATION. [Book ii. 
 
 conversely at the beginning of expii-ation to continue the rise of 
 arterial pressure which was taking place during inspiration but sub- 
 sequently to lower arterial pressure. In ordinary breathing, as we 
 have seen, what may be considered as the normal relations of blood- 
 pressure to the respii'atory movements are precisely of this kind ; 
 and it seems exceediagly probable that they are, to a large extent 
 at least, produced in this way. 
 
 In attempting however to estimate the action of the thorax, we 
 must bear in mind what is taking place in the abdomen. In easy 
 inspiration the descent of the diaphragm compresses the abdominal 
 viscera, and so, while at the very first it drives a quantity of 
 blood onward along the inferior vena cava, subsequently hinders 
 the upward flow from the abdomen and lower limbs ; at the same 
 time by compressing the abdominal aorta, it tends to raise 
 the pressure in the thoracic aorta and its branches. The effect 
 of easy expiration would be the converse of this ; but in forced 
 expiration the jDressure of the contracting abdominal muscles would, 
 as in inspiration, first tend to drive the blood onward along the vena 
 cava but subsequently hinder the flow both along the vena cava and 
 the aorta. The effect of the abdominal movements therefore is 
 naixed and variable, and the influence on the blood-pressure in the 
 femoral artery must be different from that on the radial artery or 
 other branch of the thoracic aorta. It is difficult to predict what 
 in all cases the effect would be, but it is stated that section of the 
 phrenic nerves, leading to quiescence of the diaphragm, largely 
 dimiaishes, and sometimes causes the total disappearance of, even 
 the normal respiratory undulations. 
 
 Effects of the respiratory movements are seen not only in 
 natural but also in artificial respiration. When, for instance, in an 
 animal under urari, artificial is substituted for natural respiration, 
 undulations of the blood-pressure curve, sjmchronous ^vith the 
 respiratory movements, are still observed (Fig. 61), though generally 
 less in extent than those seen under natural conditions. Now in 
 artificial respii'ation, the mechanical conditions, under which the 
 thoracic ^dscera are placed as regards pressure, are the exact 
 opposite of those existing during natural respiration ; for when air 
 is blown into the trachea to distend the lungs, the pressure within 
 the chest is increased instead of diminished. 
 
 Under these circumstances we should expect to find that 
 while the first effect of an artificial inspiration would be to drive 
 an additional quantity of blood out of the lungs into the left 
 ventricle, and thus to raise arterial pressure, this would be in turn 
 followed by a fall of arterial pressure due to the increased re- 
 sistance offered both to the passage of blood through the lungs 
 and to the entrance of blood through the venge cavge into the 
 right auricle. Conversely the effect of the succeeding expiration 
 would be an initial continuance of the fall of arterial pressure 
 succeeded by a rise. In other words we should expect to find in 
 
Chap, ii.] RESriRATWN. 37l 
 
 artificial respiration effects exactly the reverse of those which we 
 find in normal respiration ; and indeed in many curves of blood- 
 pressure taken during artificial respiration this is the case ; but 
 here as in natural respiration the features of the blood-pressure 
 curve vary according as the breathing is hurried or slow, shallow 
 or deep. 
 
 We may add that another explanation than those given above 
 has been offered of these effects of the respii-atory movements. 
 It has been suggested that when the lung is expanded, the 
 increase in the area of the wall of each pulmonary alveolus tends 
 to stretch and elongate the capillaries lying in the alveolar walls, 
 and in elongating them necessarily narrows them, just as an 
 india-rubber tube is narrowed when it is stretched lengthways. 
 This narrowing of the capillaries would present an obstacle to 
 the passage of blood through them ; and hence the expansion 
 of the alveoli in inspiration, other things being equal, would 
 be unfavourable to the flow of blood through the lungs. It has been 
 further suggested that the first effect of the expansion of the alveolus 
 and narrowing of the capillaries would be to drive out suddenly the 
 blood at the moment contained in them and thus for the instant to 
 produce a passing increase of flow; and conversely that the first effect 
 of the collapse of the alveolus and consequent widening of the 
 capillaries w^ould be to find room for an extra quantity of blood, and 
 thus for a moment to check the flow. Whether in each case the 
 first or the second phase becomes predominant would depend on 
 the rate and depth of the breathing. There are difficulties however 
 in accepting this view and the one previously given seems to be 
 the more valid one. 
 
 From what has been said it is clear that the influences of 
 the respiratory movements are not only many but conflicting, and 
 that the exact effect at any one moment will vary according 
 as circumstances render one or other factor predominant. It will 
 hardly be profitable to make any further attempt to unravel 
 the complexity of the several cases. 
 
 The relations between respii'ation and circulation which M'e 
 have just discussed are of a mechanical nature, but there are also 
 ties of a nervous kind between the two systems. One striking 
 featm'e of the respirator}^ undulation in the blood-pressure curve 
 of the dog* is the fact that the pulse-rate is quickened during the 
 rise of the undulation and becomes slower during the fall. The 
 quickening of the beat might be considered as itself partly 
 accounting for the rise, or on the other hand it might be urged 
 that the increased tioAv of blood which causes the rise, at the same 
 time leads to the quickening, were it not for one fact, viz. that the 
 difference is at once done away with, without any other essential 
 
 1 In the rabbit, the respiratory undulations, though well marked, present a very 
 small difference of pulse-rate in the rise and fall. 
 
 21—2 
 
372 EFFECTS ON CIRCULATION. [Book ii. 
 
 change in the undulations, by section of both vagi. Evidently the 
 slower pulse i luring the fall is caused by a coincident stimulation of 
 the cardio-inhibitory centre in the medulla oblongata, the quicker 
 pulse during the rise being due to the fact that, during that 
 interval, the centre is comparatively at rest. We have here most 
 important indications that, while the respiratory centre in the 
 medulla oblongata is at work, sending out rh}^hmic impulses of 
 inspiration and expiration, the neighbouring cardio-inhibitory 
 centre is, as it were by sympathy, thrown into an activity of such a 
 kind that its influence over the heart waxes and wanes with each 
 respiratory movement. 
 
 But if the cardio-inhibitory centre is thus synchronously 
 affected, ought we not to expect that the vaso-motor centre should 
 also be influenced ? We have indeed evidence that the action of 
 the vaso-motor centre is largely dependent on the respiratory 
 changes of blood. 
 
 When artificial respiration is stopped, a very large but steady 
 rise of pressure is observed. This may be in part due to the 
 increased force of the cardiac beat, caused by the increasingly 
 venous character of the blood ; but only in part, and that a small 
 part. The rise so witnessed is very similar to that brought about 
 by powerfully stimulating a number of vaso-constrictor nerves ; and 
 there can be no doubt that it is due to the venous blood stimu- 
 lating the vaso-motor centre in the medulla, and thus causiug 
 constriction of the small arteries of the body, particularly perhaps 
 those of the splanchnic area. We say ' stimulating the medullary 
 vaso-motor centre,' because, though we must admit that, since arise 
 of pressure follows upon dyspnoea when the spinal cord has been 
 previously divided below the medulla, the venous blood may 
 stimulate other vaso-motor centres in the spiual cord and possibly 
 even act directly on local peripheral mechanisms, or on the 
 muscular coats of the small arteries themselves, yet the fact 
 that the rise of pressure is much less under these circum- 
 stances shews that the medullary centre plays the chief part. 
 Upon the cessation of the artificial respiration, the respiratory 
 undulations cease also, so that the blood-pressure curve rises 
 at first steadily in almost a straight line ; yet after a while new 
 undulations, the so-called Traube or Traube-Hering curves, make 
 their appearance (Fig. 61. 2, 3), very similar to the previous ones, 
 except that their curves are larger and of a more sweeping 
 character. These new undulations, since they appear in the 
 absence of all thoracic or pulmonary movements, passive or active, 
 and are witnessed even when both vagi are cut, must be of vaso- 
 motorial origin; the rhythmic rise must be due to a rhythmic 
 constriction of the small arteries, and this probably is caused 
 by a rhythmic discharge from vaso-motor centres and especially 
 from the medullary vaso-motor centre. The undulations are main- 
 tained as long as the blood -pressure continues to rise. With the 
 
CnAi«. II.] UESPIUATION. 373 
 
 increasing vcnosity of tho blood, however, hoth the vaso-m(jtor 
 centres and the heart become exliausted; the undulations disappear, 
 and the blood-pressure rapidly sinks. 
 
 ^ A A A ^ /^ A /^ A i^ A 
 
 
 Fir,. 61. Thaube-Heeing Curves. To be read from left to right. 
 
 The curves 1, 2, 3, 4, 5 are portions selected from one long continuous tracing 
 forming the record of a prolonged observation, so that the several curves represent 
 successive stages of the same experiment. Each curve is placed in its proper position 
 relative to the base Hue, which, to save space, is omitted ; and it is ob\ious that, 
 starting from the stage represented by 1, the blood -pressure rises in stages 
 2, 3, and 4, but falls again in stage 5. Curve 1 is taken from a period 
 when artificial respiration was being kept up, and the undulations visible 
 are those the nature of vrhich have been discussed; the vagi having been 
 cut the pulsations on the ascent and descent of the undulations do not differ. 
 When the artificial respiration was suspended these undulations for a while 
 disappeared, and the blood-pressure rose steadily while the heart-beats became 
 slower. Soon, as shewn in curve 2, new undulations appeared. A little later, the blood- 
 pressure was stUl rising, the heart-beats still slower, but the undulations still more 
 obvious (curve 3). Still later (curve 4), the pressure was stUl higher, but the heart- 
 beats were quicker, and the undulations flatter. The pressure then began to fall 
 rapidly (curve 5), and continued to fall until some time after artificial respiration 
 was resumed. 
 
 The appearance of these Traube-Heriug curves is not however 
 dependent on the cessation of the respiratory movements, and on 
 an abnormally venous condition of the blood. They sometimes 
 (Fig. G2) are seen in an animal whose breathing is fairly normaL 
 We need not discuss them any further nov/, and have introduced 
 them chielly to illustrate the fact that the vaso-motor nervous 
 system is apt to fall into a condition of rhythmic activity. It has 
 
374 EFFECTS ON CIRCULATION. [Book ii. 
 
 been suggested that the normal respiratory undulations may be due 
 to a rhythmic rise and fall of the activity of the vaso-motor centre, 
 synchronous, like that of cardio-inhibitory centre, with the respi- 
 
 I 
 
 
 Fig. 62. Blook-Peesstjee cueve of a Babbit, recoeded on a slowly moving 
 
 SURFACE, TO SHEW TeAUBE-HeEING CuEVES. 
 
 In each heart-beat the upward and downward stroke are very close 
 together but may be easily distinguished by the help of a lens. The undula- 
 tions of the next order are those of resjpiration. The wider sweeps are the 
 Traube-Hering curves, of which two complete curves and portions of two others are 
 shewn. Each Traube-Hering curve comprises about nine respiratory curves, and 
 each respiratory curve about the same number of heart-beats. 
 
 ratory movements. A review of all the evidence however goes to 
 shew that the respiratory variations in blood-pressure are due to 
 the mechanical conditions discussed above, and that vaso-motor 
 influences intervene but little if at all. 
 
SEC. 8. THE EFFECTS OF CHA:N'GES IN THE 
 AIR BREATHED. 
 
 The Effects of deficient Air. Asphyxia. 
 
 Wten, on account of occlusion of the trachea, or by breathing 
 in a confined space, a due supply of air is not obtained, normal 
 respiration gives place, through an intermediate phase of dyspnoea, 
 to the condition known as asphyxia; this, unless remedial measures 
 be taken, rapidly proves fatal. 
 
 Phenomena of Asphyxia. As soon as the oxygen in the 
 arterial blood sinks below the normal, the respiratory movements 
 become deeper and at the same time more frequent; both the 
 inspiratory and expiratory' phases are exaggerated, the supple- 
 mentary muscles spoken of at p. 322 are brought into play, and the 
 rate of the rhythm is hurried. In this respect, dyspnoea, or hy- 
 perpnoea as this first stage has been called, contrasts very strongly 
 with the pecuhar respiratory condition caused by section of the 
 vagi, in which the respiratory movements, while much more pro- 
 found than the normal, are diminished in frequency. 
 
 As the blood continues to become more and more venous the 
 respiratory movements continue to increase both in force and fre- 
 quency, a larger number of muscles being called into action and 
 that to an increasing extent. Very soon, however, it may be 
 observed that the expiratory'' movements are becoming more 
 marked than the inspiratory. Every muscle which can in any way 
 
376 ASPHYXIA. [Book ii. 
 
 assist in expiration is in turn brought into play; and at last almost 
 all the muscles of the body are involved in the struggle. The 
 orderly expiratory movements culminate in expiratory convulsions, 
 the order and sequence of which are obscured by their violence and 
 extent. That these convulsions, through which dyspnoea merges 
 into asphyxia, a,re due to a stimulation of the medulla oblongata 
 by the venous blood, is proved by the fact that they fail to make 
 their appearance when the spinal cord has been previously divided 
 below the medulla, though they still occur after those portions of 
 the brain which lie above the medulla have been removed. It is 
 usual to speak of a 'convulsive centre' in the medulla, the stimu- 
 lation of which gives rise to these convulsions ; but if we accept 
 the existence of such a centre we must at the same time admit 
 that it is connected by the closest ties with the normal expiratory 
 division of the respiratory centre, since every intervening step 
 may be observed between a simple slight expiratory movement 
 of normal respiration and the most violent convulsion of asphyxia. 
 An additional proof that these convulsions are carried out by the 
 agency of the medulla is afforded by the fact that convulsions of 
 a wholly similar character are witnessed when the supply of 
 blood to the medulla is suddenly cut off by ligaturing the blood- 
 vessels of the head. In this case the nervous centres, being no 
 longer furnished with fresh blood, become rapidly asphyxiated 
 through lack of oxygen, and expiratory convulsions quite similar to 
 those of ordinary asphyxia, and preceded like them by a passing 
 phase of dyspnoea, make their appearance. Similar 'ansemic' con- 
 vulsions are seen after a sudden and large loss of blood from the 
 body at large, the medulla being similarly stimulated by the lack of 
 arterial blood. 
 
 Such violent efforts speedily exhaust the nervous system ; and 
 the convulsions after being maintained for a brief period suddenly 
 cease and are followed by a period of calm. The calm is one of 
 exhaustion; the pupils, dilated to the utmost, are unaffected by 
 light ; touching the cornea calls forth no movement of the eyelids, 
 and indeed no reflex actions can anywhere be produced by the 
 stimulation of sentient surfaces. All expiratory active movements 
 have ceased ; the muscles of the body are flaccid and quiet ; and 
 though from time to time the respiratory centre gathers sufficient 
 energy to develope respiratory movements, these resemble those of 
 quiet normal breathing, in being, as far as muscular actions are 
 concerned, almost entirely inspiratory. They occur at long intervals, 
 like those after section of the vagi; and like them are deep 
 and slow. The exhausted respiratory centre takes some time to 
 develope an inspiratory explosion; but the impulse when it is 
 generated is proportionately strong. It seems as if the resistance 
 which had in each case to be overcome was considerable, and 
 the effort in consequence, when successful, productive of a large 
 effect. 
 
Chap, ii.] RKSPIRATWN. 'Ml 
 
 As time goes on, these inspiratory efforts become less freqiiout ; 
 their rlij'thni becomes irregular ; long pauses, each one of which 
 seems a liual one, are succeeded by several somewhat rapidly re- 
 peated inspirations. The pauses become longer, and the inspiratory 
 movements shallower. Each inspiration is accompanied by the 
 contraction of accessory muscles, especially of the face, so that 
 each breath becomes more and more a prolonged gasp. The in- 
 spiratory gasps spread into a convulsive stretching of the whole 
 body ; and with extended limbs, and a straightened trunk, with 
 the head thrown back, the mouth widely open, the face drawn, and 
 the nostrils dilated, the last breath is taken in. 
 
 Thus we are able to distinguish three stages in the phenomena 
 \vhich result from a continued deficiency of air : — (1) A stage of 
 dyspnoea, characterized by an increase of the respiratory movements 
 both of inspiration and expiration. (2) A convulsive stage, charac- 
 terized by the dominance of the expiratory efforts, and culminating 
 in general convulsions. (3) A stage of exhaustion, in which lin- 
 gering and long-drawn inspirations gradually die out. When 
 brought about by sudden occlusion of the trachea these events 
 run through their course in about 4 or 5 minutes in the dog, and 
 in about 3 or 4 minutes in the rabbit. The first stage passes 
 gradually into the second, convulsions ajapearing at the end of the 
 first minute. The transition from the second stage to the third 
 is somewhat abrupt, the convulsions suddenly ceasing early in the 
 second minute. The remaining time is occupied in the third 
 stage. 
 
 The duration of asphyxia varies not only in different animals 
 but in the same animal under different circumstances. Newly 
 born and young animals need much longer immersion in water 
 before death by asphyxia occurs than do adults. Thus while in 
 a full-grown dog recovery from drowning is unusual after 1^ 
 minutes, a new-born puppy has been kno"svn to bear an immersion 
 of as much as 50 minutes. The cause of the difference lies in the 
 fact that in the quite young or rather just born animal the re- 
 spiratory changes of the tissues are much less active. These con- 
 sume less oxygen, and the general store of oxygen in the blood 
 has a less rapid demand made upon it. The respiratory activity of 
 the tissues may also be lessened by a deficiency in the circulation ; 
 hence bodies in a state of syncope at the time when the deprivation 
 of ox3^gen begins can endure the loss for a much longer period 
 than can bodies in which the circulation is in full swing. There 
 being the same store of oxygen in the blood in each case, the 
 quicker circulation must of necessity bring about the speedier 
 exhaustion of the store. In many cases of drowning, death is 
 hastened by the entrance of water into the lungs. 
 
 By training, the respiratory centre may be accustomed to bear 
 a scanty supply of oxygen for a much longer time than usual 
 before dysj^noea sets in, as is seen in the case of divers. 
 
378 ASPHYXIA. [Book ii. 
 
 The phenomena of slow asphyxia, where the supply of air is 
 gradually diminished, are fundamentally the same as those result- 
 ing from a sudden and total deprivation. The same stages are 
 seen, but their development takes place more slowly. 
 
 The circulation in Asphyxia. If the carotid or other artery 
 of an animal be connected with a manometer during the develop- 
 ment of the asphyxia just described, the following facts may be 
 observed. During the first and second stages the blood-pressure 
 rises rapidly, attaining a height far above the normal. During the 
 third stage it falls even more rapidly, repassing the normal and 
 becoming nil as death ensues. The respiratory undulations of the 
 pressure-curve are abrupt and somewhat irregular, the inspiratory 
 movements being accompa,nied by a fall of pressure. When the 
 animal has been previously placed under urari, so that the respira- 
 tory impulses cannot manifest themselves by any muscular move- 
 ments, the rise of the pressure curve, as we have already said, is at 
 first steady and unbroken, but after a variable period Traube's 
 curves make their appearance. As during the third stage the 
 pressure sinks, these undulations pass away. 
 
 The heart-beats are at first somewhat quickened, but speedily 
 become slow, while at the same time they acquire great force ; so 
 that the pulse-curves on the tracing are exceedingly bold and 
 striking, Fig. 61. Even while the blood-pressure is sinking, the 
 pulse-curves still maintain somewhat these characters ; and the 
 heart continues to beat for some seconds after the respiratory 
 movements have ceased, the strokes at last rapidly failing in 
 frequency and strength. 
 
 If the chest of an animal be opened under artificial respiration, 
 and asphyxia brought on by cessation of the respiration, it will be 
 seen that the heart during the second and third stages becomes 
 completely gorged with venous blood, all the cavities as well as 
 the large veins being distended to the utmost. If the heart be 
 watched to the close of the events, it will be seen that the feebler 
 strokes which come on towards the end of the third stage are quite 
 unable to empty its cavities ; and when the last beat has passed 
 away its parts are still choked with blood. The veins spirt out 
 when pricked : and it may frequently be observed that the beats 
 recommence when the over-distension of the heart's cavities is 
 relieved by puncture of the great vessels. When rigor mortis 
 sets in after death by asphyxia, the left side of the heart is more 
 or less emptied of its contents ; but not so the right side. Hence 
 in an ordinary post-mortem examination in cases of death by 
 asphyxia, while the left side is found comparatively empty, the 
 right appears gorged. 
 
 These various phenomena of asphyxia are probably brought 
 about in the following way. 
 
 The increasingly venous character of the blood augments the 
 
('MAI'. II.] HESriRATIOy. .379 
 
 action of the general vaso-motor centre, and thus leads to a general 
 constriction of the small arteries This is the cause of the 
 markedly increased blood-pressure ; though, as we have already 
 said, the venous blood may also act directly on the other spinal 
 vaso-motor centres and possibly on peripheral vaso-motor me- 
 chanisms or on the muscular arteiial coats, or may even aftect the 
 peripheral resistance by modifying the changes in the capillary 
 regions. 
 
 This increased peripheral resistance, while indirectly help- 
 ing to augment the force of the heart's beat, is a direct obstacle 
 to the heart emptying itself of its contents. On the other hand, 
 the increased respiratory movements favour the flow of venous 
 blood towards the heart, which in consequence becomes more and 
 more full. This repletion is moreover assisted by the marked 
 infrequency of the beats. This in turn depends in part on the 
 cardio-inhibitory centre in the medulla being stimulated by the 
 venous blood ; since when the vagi are di\ided the infrequency is 
 much less pronounced. It does not however disappear altogether ; 
 and we are therefore driven to suppose it is in part due to the 
 venous blood acting in an inhibitory manner directly on the heart 
 itself. The increased resistance in front, the augmented supply 
 from behind, and the long pauses between the strokes, all concur 
 in distending the heart more and more. 
 
 When the large veins have become full of blood the inspiratory 
 movements can no longer have their usual effect in increasing the 
 blood-pressure. The whole force of the chest movement, as far as 
 the circulation is concerned, is spent in diminishing the pressure 
 around the large arteries ; and hence the sinking of the blood- pres- 
 sure during each inspiratory movement. 
 
 The distension of the cardiac cavities, at first favourable to the 
 heart-beat, as it increases becomes injurious. At the same time 
 the cardiac tissues, which at first probably are stimulated, after a 
 while become exhausted by the action of the venous blood ; and 
 the strokes of the heart become feebler as well as slower. 
 
 On account of this increasing slowness and feebleness of the 
 heart's beat, the blood-pressure, in spite of the continued arterial 
 constriction, begins to fall, since less and less blood is pumped into 
 the arterial system ; the boldness of the pulse-curves at this stage 
 being chiefly due to the infrequency of the strokes. As the 
 quantity which passes from the heart into the arteries becomes 
 less second by second, the pressure gets lower and lower, the 
 descent being assisted by the exhaustion of the vaso-motor centre, 
 until almost before the last beats it has sunk to zero. Thus at the 
 close of asphyxia, while the heart and venous system are distended 
 with blood, the arterial system is less than normally full. 
 
380 APNCEA. [Book ii. 
 
 The Effects of an increased supply of Air. Apnoea. 
 
 We have already (p. 361) seen that when artificial respiration 
 is carried on too vigorously, a condition of peculiar breathlessness 
 known as "apnoea^" is brought about. We have further seen that 
 the essential feature of apnoea consists in the blood containing 
 for the time being more oxygen than usual. In consequence of 
 this a longer time is needed before the deficiency of oxygen in the 
 blood of the capillaries of the medulla oblongata, or rather in the 
 nerve-cells constituting the respiratory centre, reaches the limit 
 which determines the discharge of a respiratory impulse. As we 
 have seen, the molecular processes of these cells are so arranged, 
 that whenever the oxygen which is available for their use sinks 
 below a certain level, respiratory explosions occur whereby a fresh 
 supply of oxygen is gained. We must suppose that the 
 changes going on in these cells, like those taking place in other 
 cells and tissues, are oxidative in character; but they possess this 
 peculiar feature, that the absence or diminution of oxygen acts as 
 it were as a stimulus, leading to an explosive decomposition. The 
 facts previously (p. 361) discussed lead us to adopt this view, though 
 we cannot explain why oxygen has this remarkable effect on these 
 particular cells. By increasing their available oxygen, the ex- 
 plosive action of the cells is deferred and diminished; that is, 
 apnoea is established. Similarly when the supply of oxygen is 
 diminished, the explosions are hastened and increased, that is, 
 dyspnoea is brought about. The different conditions of the respi- 
 ratory centre during apnoea, normal breathing or eupnoea, and 
 dyspnoea, are well shewn by the different effects produced by 
 stimulating the afferent fibres of the trunk of the vagus with the 
 same stimulus during the three stages. If the current chosen be 
 of such a strength as will gently increase the rhythm of normal 
 breathing, it will be found to have no effect at all in apnoea, while 
 in dyspnoea it may produce almost convulsive movements. Indeed 
 in well-marked apnoea even strong stimulation of the vagus may 
 produce no effect whatever. 
 
 The Effects of changes in the Composition of the Air breathed. 
 
 We have already discussed the effects of such changes as are 
 produced by the act of respiration itself, viz. a deficiency of oxygen 
 and an excess of carbonic acid. We have only to add, that the 
 result of an excess of oxygen, except in the cases of extreme 
 pressure to be mentioned immediately, is simply apnoea, and that 
 
 1 It is to be regretted that this name is used by some medical authorities in a 
 sense almost identical with asphyxia. In its physiological sense, as here used, it is 
 the very opposite of asphyxia. 
 
Chap, ii.] RESPIHA TION. 3«1 
 
 variations in amouut ut iiitru<(uu have ut themselves no effect, this 
 gas being eminently an indifferent gas as far as physiological pro- 
 cesses ai-e concerned. 
 
 Poisonous gases. Carbonic oxide produces the same effects as 
 deticieucy of oxygen, inasmuch as it preoccupies the haemoglobin 
 and so prevents the blood from becoming properly oxygenated, see 
 p. 340. Sulphuretted hydrogen produces similar effects, but in a 
 different manner ; it acts as a reducing agent, see p. 337. Some 
 gases are irrespirable, on account of their causing spasm of the 
 glottis, and this is said to be, to a certain extent, the case with 
 cai'bonic acid. 
 
 The Effects of changes in the Pressure of the Air breathed. 
 
 Gradual Diminution of Pressure. The symptoms are those of 
 deticieucy of oxygen ; the animals die of asphyxia. The blood con- 
 tains less and less oxygen as the pressure is reduced, the quantity 
 present in the arterial blood soon becoming less than that in 
 normal venous blood. The quantity of carbonic acid in the blood 
 is also diminished. The increasing dyspnoea is accompanied by 
 great general feebleness ; and convulsions though frequent are not 
 invariable. The occurrence of these seems to depend on the 
 suddenness with which the oxygen of the blood is diminished. 
 
 Sudden Diminution. Death in these cases ensues from the 
 liberation of gases within the blood-vessels and the consequent 
 mechanical interference with the circulation. The gas which is 
 found in the blood-vessels on examination after death consists 
 chiefly of nitrogen. 
 
 Increase of Pressure. Up to a pressure of several atmospheres 
 of air, tho only symptoms which present themselves are those 
 somewhat resembling narcotic poisoning. At a pressure however 
 of 4 atmospheres of oxygen, corresponding to 20 atmospheres of air, 
 and upwards, a very remarkable phenomenon presents itself. The 
 animals die of asphyxia and con\Tilsions, exactly in the same way 
 as when oxygen is deficient. Corresponding with this it is found 
 that the production of carbonic acid is diminished. That is to say, 
 when the pressure of the oxygen is increased beyond a certain 
 limit, the oxidations of the body are diminished, and with a still 
 further increase of the oxygen are arrested altogether. The oxida- 
 tion of phosphorus is quite analogous ; at a high pressure of oxygen 
 phosphorus will not bum. Not only animals but plants, bacteria, 
 and organised ferments, are similarly killed by a too great pressure 
 of oxygen. 
 
SEC. 9. MODIFIED EESPIRATORY MOVEMENTS. 
 
 The respiratory mechanism with its adjuncts, in addition to its 
 respiratory function, becomes of service, especially in the case of 
 man, as a means of expressing emotions. The respiratory column 
 of air, moreover, in its exit from the chest, is frequently made use 
 of in a mechanical way to expel bodies from the upper air- 
 passages. Hence arise a number of peculiarly modified and more 
 or less compHcated respiratory movements, sighing, coughing, 
 laughter, &c. adapted to secure special ends which are not dis- 
 tinctly respiratory. They are all essentially reflex in character, the 
 stimulus determining each movement, sometimes affecting a 
 peripheral afferent nerve as in the case of coughing, sometimes 
 working through the higher parts of the brain as in laughter and 
 crying, sometimes possibly, as in yawning and sighing, acting on 
 the respiratory centre itself Like the simple respiratory act, they 
 may with more or less success be carried out by a direct effort of 
 the will. 
 
 Sighing is a deep and long-drawn inspiration chiefly through 
 the nose followed by a somewhat shorter, but correspondingly large 
 expiration. 
 
 Yawning is similarly a deep inspiration, deeper and longer con- 
 tinued than a sigh, drawn through the widely open mouth, and 
 accompanied by a pecuHar depression of the lower jaw and 
 frequently by an elevation of the shoulders. 
 
 Hiccough consists in a sudden inspiratory contraction of the 
 diaphragm, in the course of which the glottis suddenly closes, so 
 
CiiAK It.] RESPIRATION. 383 
 
 that the further entrance of air into the chest is prevented, while 
 the impulse of tiie column of air just entering, as it strikes upon 
 the closed glottis, gives rise to a well-known accompanying sound. 
 The afferent impulses of the reliex act are conveyed })y the gastric 
 branches of the vagus. The closure of the glottis is carried out by 
 means o^ tiie inferior laryngeal nerve. Sec Voice. 
 
 In sobbing a series of similar convulsive inspirations follow each 
 other slowly, the glottis being closed earlier than in the case of 
 hiccough, so that little or no air enters into the chest. 
 
 Coiigldng consists in the first place of a deep and long-drawn 
 inspiration by which the lungs are well filled with air. This is fol- 
 lowed by a complete closure of the glottis, and then comes a 
 sudden and forcible expiration, in the midst of which the glottis 
 suddenly opens, and thus a blast of air is driven through the upper 
 respiratory passages. The afferent impulses of this reflex act are 
 in most cases, as when a foreign body is lodged in the larynx or by 
 the side of the epiglottis, conveyed by the superior laryngeal ner^'e ; 
 but the movement may arise from stimuli applied to other afferent 
 branches of the vagus, such as those supplying the bronchial 
 passages and stomach and the auricular branch distributed to 
 the meatus externiis. Stimulation of other nerves also, such as 
 those of the skin by a draught of cold air, may develope a cough. 
 
 In sneezing the general movement is essentially the same, 
 except that the opening from the pharj^nx into the mouth is 
 closed by the contraction of the anterior pillars of the fauces and 
 the descent of the soft palate, so that the force of the blast is 
 driven entirely through the nose. The afferent impulses here 
 usually come from the nasal branches of the fifth. When sneezing 
 however is produced by a bright light, the optic nerve would seem 
 to be the afferent nerve. 
 
 Laughing consists essentially in an inspiration succeeded, not 
 by one, but by a whole series, often long continued, of short spas- 
 modic expirations, the glottis being freely open during the whole 
 time, and the vocal cords being thrown into characteristic vibrations. 
 
 In crying, the respiratory movements are modified in the same 
 way as in laughing ; the rhythm and the accompanying facial 
 expi'essions are however different, though laughing and crying 
 frequently become indistinguishable. 
 
CHAPTEE III. 
 SECRETION BY THE SKIN. 
 
 We have traced the food jfrom the alimentary canal into the blood, 
 and, did the state of our knowledge permit, the natural course of 
 our study would be to trace the food from the blood into the 
 tissues, and then to follow the products of the activity of the 
 tissues back into the blood and so out of the body. This how- 
 ever we cannot as yet satisfactorily do ; and it will be more con- 
 venient to study first the final products of the metabolism of the 
 body, and the manner in which they are eliminated, and after- 
 wards to return to the discussion of the intervening steps. 
 
 Our food consists of certain food-stuffs, viz. proteids, fats and 
 carbohydrates, of various salts, and of water. In their passage 
 through the blood and tissues of the body, the proteids, fats and 
 carbohydrates are converted unto urea (or some closely allied body), 
 carbonic acid and water, the nitrogen of the urea being furnished 
 by the proteids alone. Many of the proteids contain sulphur, and 
 also have phosphorus attached to them in some combination or 
 other, and some of the fats taken as food contain phosphorus; 
 these elements ultimately suffer oxidation into phosphates and 
 sulphates, and leave the body in that form in company with the 
 other salts. 
 
 Broadly speaking then, the waste products of the animal 
 economy are urea, carbonic acid, salts and water. Of these a large 
 
Chap, mi.) CUTANEOUS SECRETION. 385 
 
 portion of the carbonic acid, and a considerable quantity of water, 
 leave the body by the lung.s in respiration ; wliile all (or nearly all) 
 the urea, the greater portion of the salts, and a large amount of 
 water, with an insignilieant quantity of carbonic acid, pass away 
 by the kidneys. The work therefore of the remaining excretory 
 tissue, the skin, is confined to the elimination of a comparatively 
 small quantity of salts, a little carbonic acid, and a variable but 
 on the whole large quantity of water in the fonn of perspiration. 
 The actual excretion by the bowel, that is to say, that portion of 
 the fasces which is not simply undigested matter, we have seen to 
 be very small. 
 
 The Nature and Amount of Perspiration. 
 
 The quantity of matter which leaves the human body by way 
 of the skin is very considerable. Thus it has been estimated that, 
 while 7 grains pass away through the lungs per minute, as much 
 as 11 grains escape through the skin. The amount however varies 
 extremely ; it has been calculated, from data gained by enclosing the 
 arm in a caoutchouc bag, that the total amount of perspiration 
 from the whole body in 24 hours might range from 2 to 20 kilos ; 
 but such a mode of calculation is obviously open to many sources 
 of error. 
 
 Of the whole amount thus discharged, part passes away at 
 once as watery vapour mixed with volatile matters, while part may 
 remain for a time as a fluid on the skin ; the former is frequently 
 spoken of as insensible, the latter as sensible perspiration. The 
 proportion of the insensible to the sensible perspiration will depend 
 on the rapidity of the secretion in reference to the dryness, tem- 
 perature, and amount of movement, of the surrounding atmosphere. 
 Thus, supposing the rate of secretion to remain constant, the drier 
 and hotter the air, and the more rapidly the strata of air in contact 
 with the body are renewed, the gi-eater is the amount of sensible 
 perspiration which is by evaporation converted into the insensible 
 condition; and conversely when the air is cool, moist, and stagnant, 
 a large amount of the total perspiration may remain on the skin 
 as sensible sweat. Since, as the name implies, we are ourselves 
 aware of the sensible perspiration only, it may and frequentl}^ does 
 happen that we seem to ourselves to be perspiring largely, when in 
 reality it is not so much the total perspiration which is being in- 
 creased as the relative proportion of the sensible perspiration. 
 The rate of secretion may however be so much increased, that no 
 amount of dryness, or heat, or movement of the atmosphere, is 
 sufficient to carry out the necessary evaporation, and thus the 
 sensible perspiration may become abundant in a hot dry air. And 
 practically this is the usual occurrence, since certainly a high 
 
 F. 25 
 
386 CUTANEOUS RESPIRATION. [Book ii. 
 
 temperature conduces, as we shall point out presently, to an in- 
 crease of the secretion, and it is possible that mere dryness of the 
 air has a similar effect. 
 
 The total amount of perspiration is affected not only by the 
 condition of the atmosphere, but also by the nature and quantity 
 of food eaten, by the amount of fluid drunk, and by the amount of 
 exercise taken. It is also influenced by mental conditions, by 
 medicines and poisons, by diseases, and by the relative activity of 
 the other excreting organs, more particularly of the kidney. 
 
 The fluid perspiration, or sweat, when collected, is found to be 
 a clear colourless fluid, with a strong and distinctive odour varying 
 according to the part of the body from which it is taken. Besides 
 accidental epidermic scales, it contains no structural elements. 
 The reaction of the secretion of the sweat glands, apart from that 
 of the sebaceous glands, appears to be alkaline. This is well seen 
 when the sweat becomes abundant. An admixture of sebaceous 
 secretion may, when the sweat itself is scanty, give rise to an acid 
 reaction, probably from the sebaceous fats becoming converted into 
 fatty acids. The average amount of solids is about 1'81 p.c, of 
 which about two-thirds consist of organic substances. The chief 
 normal constituents are: (1) Sodium chloride with small quantities 
 of other inorganic salts. (2) Various acids of the fatty series, 
 such as formic, acetic, butyric, with probably propionic, caproic, 
 and caprylic. The presence of these latter is inferred from the 
 odour ; it is probable that many various volatile acids are present 
 in small quantities. Lactic acid, which Berzelius reckoned as a 
 normal constituent, is stated not to be present in health. (3) 
 Neutral fats, and cholesterin ; these have been detected even in 
 places, such as the palms of the hand, where sebaceous glands are 
 absent. (4) Though some observers seem to have found a consider- 
 able quantity of urea (calculated at 10 grms. in the 24 hours for 
 the whole body) in sweat, the evidence goes to shew that neither 
 urea nor any ammonia compound exists in the normal secretion to 
 any extent ; apparently some small amount of nitrogen leaves the 
 body by the skin, but this is probably supplied by the epidermis. 
 
 In various forms of disease the sweat has been found to contain, 
 sometimes in considerable quantities, blood, albumin, urea (par- 
 ticularly in cholera), uric acid, calcium oxalate, sugar, lactic acid, 
 indigo, bile and other pigments. Iodine and potassium iodide, 
 succinic, tartaric, and benzoic (partly as hippuric) acids have been 
 found in the sweat when taken internally as medicines. 
 
 Cutaneous Respiration. 
 
 A frog, the lungs of which have been removed, will continue to 
 live for some time ; and during that period will continue not only 
 to produce carbonic acid, but also to consume oxygen. In other 
 
OiiAP in] CUTANEOUS SECRETION. 387 
 
 words, the frog is able to breathe wthout lungs, respiration being 
 carried on etiieiently by means of the skin. In mammals and in 
 man this cutaneous rt-spiration is, by reason of the thickness of the 
 epidennis, restricted to within very narrow limits; nevertheless, 
 when thf body remains for some time in a closed chamber to 
 which the air passing in and out of the lungs has no access (as 
 when the body is enclosed in a large air tight bag fitting tightly 
 round the neck, or where a tube in the trachea carries air to and 
 from the lungs of an animal placed in an air-tight box), it is found 
 that the air in the chamber loses oxygen and gains carbonic acid. 
 The amount of carbonic acid which is thus thrown off by the skin 
 of an average man in 24 hours amounts to about 10 grms., or accord- 
 ing to some observers to (no more than) about 4 grms., increasing 
 with a rise of temperature, and being very markedly augmented by 
 bodily exercise. It is stated that the amount of oxygen consumed 
 is about equal in volume to that of the carbonic acid given off, but 
 some observers make it rather less. It is evident that the loss 
 which the body suffers through the skin consists chiefly of water. 
 
 When an animal, such as a rabbit, is covered over with an 
 impermeable varnish such as gelatine, so that all exit or entrance 
 of gases or Kquids by the skin is prevented, death shortly ensues. 
 This result cannot be due, as was once thought, to arrest of 
 cutaneous respiration, seeing how insignificant is the gaseous inter- 
 change by the skin as compared with that by the lungs. Nor are 
 the sjTuptoms those of asphyxia, but rather of some kind of 
 poisoning, maiked by a very great fall of temperature, which 
 however does not seem to be the result of diminished production 
 of heat, since it is said to be coincident with an actual 
 increase of the discharge of heat from the surface. The 
 animal may be restored, or at all events its life may be prolonged 
 with abatement of the symptoms, if the great loss of heat which is 
 evidently taking place be prevented by covering the body thickly 
 with cotton wool, or keeping it in a warm atmosphere. The 
 symptoms have not as yet been clearly analysed, but they seem to 
 be due in part to a pyrexia or fever possibly caused by the reten- 
 tion within or re-absorption into the blood of some of the con- 
 stituents of the sweat, or by the products of some abnormal meta- 
 bolism, and in part to a dilation of the cutaneous vessels which 
 causes an abnormally large loss of heat, even through the varnish. 
 
 The Secretion of Perspiration. 
 
 The skin contains, besides the ordinary sudoriparous glands, the 
 sebaceous glands, and the special odoriferous glands of the axilla, 
 anus, and other regions. With regard to the various volatile and 
 odoriferous substances peculiar to sweat, and especially with regai'd 
 
 lio— 2 
 
388 NERVOUS MECHANISM. [Book ii. 
 
 to those peculiar to the sweat of particular regions of the skin, 
 there can be no doubt that these are secreted by the epithelium of 
 the appropriate glands. There can be equally no doubt that the 
 fats which come to the surface of the skin from the sebaceous 
 glands arise from a metabolism of the cells of those glands. And 
 we shall probably not go far wrong in regarding the sweat as a 
 whole as supplied by the sweat-glands alone. For though it seems 
 evident that some amount of fluid must pass by simple transuda- 
 tion through the ordinary epidermis of the portions of skin inter- 
 vening between the mouths of the glands, yet on the whole it is 
 probable that the portion which so passes is a small fraction only 
 of the total quantity secreted by the skin ; and direct experiment 
 shews that even the simple evaporation of water is much greater 
 from those parts of the skin in which the glands are abundant than 
 from those in which they are scanty. 
 
 The nervous mechanism of Perspiration. The secreting ac- 
 tivity of the skin, like that of other glands, is usually accompanied 
 and aided by vascular dilation. In one of Bernard's early experi- 
 ments on division of the cervical sympathetic, it was observed that 
 in the case of the horse, the vascular dilation of the face on the side 
 operated on was accompanied by increased perspiration. Indeed 
 the connection between the state of the cutaneous blood-vessels 
 and the amount of perspiration is a matter of daily observation. 
 When the vessels of the skin are contracted, the secretion of the 
 skin is diminished ; when they are dilated it becomes abundant. 
 And in this way, as we shall later on point out, the temperature of 
 the body is largely regulated. When the surrounding atmosphere 
 is warm, the cutaneous vessels are dilated, the amount of sweat 
 secreted is increased, and the consequently augmented evaporation 
 tends to cool down the body. On the other hand, when the 
 atmosphere is cold, the cutaneous vessels are constricted, perspira- 
 tion is scanty, and less heat is lost to the body by evaporation. 
 
 The analogy with the other secreting organs which we have 
 already studied leads us however to infer that there are special 
 nerves directly governing the activity of the sudoriparous glands, 
 independent of variations in the vascular supply. And not only is 
 this view supported by many pathological facts, such as the profuse 
 perspiration of the death agony, of various crises of disease, and 
 of certain mental emotions, and the cold sweats occurring in 
 phthisis and other maladies, in all of which the skin is anaemic 
 rather than hypersemic ; but we have direct experimental evidence 
 of a nervous mechanism of perspiration as complete as the vaso- 
 motor mechanism. 
 
 If in the cat^ the peripheral stump of the divided sciatic nerve 
 1 The cat sweats freely in the hairless soles of the feet but not on any part 
 of the body covered with hahs. The dog also sweats in the same regions but 
 not so freely as the cat. Eabbits and other rodents appear not to sweat at all. The 
 snout of the pig sweats freely ; and the often profuse sweating of the horse is known 
 to all. 
 
Chap. III.] CUTAXWUS SECRir/'fOiV. .3R9 
 
 bo stimulated with the intcrrupteil current, a protuso sweat may 
 readily be observed to break out in the hairless sole of the loot on 
 that side. Not only may the secretion be observed when the 
 cutaneous vessels arc thrown into a state of constriction by the 
 stimulus, but it also appears when the aorta or cniral artery is 
 clamped previous to the stimulation, or indeed when the leg is am- 
 putated. Moreover when atropin has been injected, tlie stimula- 
 tion produces no sweat, though vaso-motor effects follow as usual. 
 The analogy between the sweat-glands of the foot and such a gland 
 as the submaxillary is in fact very close, and we are justified in 
 speaking of the sciatic nerve as containing secretory fibres di.stri- 
 buted to the sudoriparous glands of the foot. Similar results 
 may be obtained with the nerves of the fore limb. And in our- 
 selves a copious secretion of sweat may be induced by tetanizing 
 through the skin the nerves of the limbs or the face. 
 
 If a cat in which the sciatic nerve has been divided on one side 
 be exposed to a high temperature in a heated chamber, the limb 
 the nei*ve of which has been divided remains dry, while the feet of 
 the other limbs sweat fireely. This result shews that the sweating 
 which is caused by exposure of the body to high temperatures is 
 brought about not by a local action on the sweat-glands but by the 
 agency of the central nervous system. A high temperature up to 
 a certain limit increases the irritability of the epithelium of the 
 sweat-glands as it does that of other forms of protoplasm : thus 
 stimulation of the sciatic in the cat produces a much more abundant 
 secretion in a limb exposed to a temperature of 35° or somewhat 
 above, than in one which has been exposed to a distinctly lower 
 temperature, and in a limb which has been placed in ice-cold 
 water hardly any secretion at all can be gained ; but apparently 
 mere rise of temperature without nerve-stimulation will not give 
 rise to a secretory activity of the glands. The sweating caused 
 by a dyspnceic condition of blood, and such appears to be the 
 sweat of the death agony, is similarly brought about by the 
 agency of the central nervous system. When an animal with the 
 sciatic nerve divided on one side is made dyspnceic, no sweat 
 appears in the hind limb of that side, though abundance is seen in 
 the other feet. 
 
 Sweating may be brought about as a reflex act. Thus when 
 the central stump of the divided sciatic is stimulated sweating is 
 induced in the other limbs, and in ourselves the introduction 
 of pungent substances into the mouth vnll frequently give rise to a 
 copious perspiration over the side of the face. We are thus led to 
 speak of sweat centres, analogous to the vaso-motor centres, as 
 existing in the central nen-ous system ; and as in the case of vaso- 
 motor centres, a dispute has arisen as to whether there is a 
 dominant sweat centre in the medulla oblongata or whether such 
 centres are more generally distributed over the whole of the spinal 
 cord. 
 
390 ABSORPTION BY THE SKIN. [Book ii. 
 
 It does not at present appear certain whether the sweating 
 caused by heat is carried out by direct action on the sweat centres, 
 or by the higher temperature affecting the skin and so producing 
 its effect in a reflex manner ; but in the case of dyspnoea at least 
 we may fairly suppose that the action of the venous blood is 
 chiefly if not exclusively on the nerve centres. Some drugs, such 
 as pilocarpin, which cause sweating, appear to produce their effect 
 chiefly by a local action on the glands since the action continues 
 after the division of the nerves (though pilocarpin at least has as 
 well some action on the nerve centres), and the antagonistic action 
 of atropin is similarly local. Nicotin appears to produce its 
 sweating action chiefly by acting on the central nervous system. 
 
 The sweat-fibres for the hind foot (in the cat) appear to leave 
 the spinal cord by the roots of the last dorsal and first two lumbar 
 or last two dorsal and first four lumbar nerves, pass along the rami 
 commumcantes to the abdominal sympathetic, and thus reach the 
 sciatic nerve. Similarly the sweat-nerves for the fore foot leave 
 the spinal cord by the roots of the fourth (or fourth, fifth, and sixth) 
 dorsal nerves, pass into the thoracic sympathetic, thence into the 
 ganglion stellatum, and thus join the brachial plexus ; the course to 
 the foot is finally along the median and ulnar nerves respectively. 
 In the horse and pig the sweat-fibres for the side of face and snout 
 appear to run in branches of the fifth and not in the facial. 
 
 Absorption by the Skin. 
 
 Although under normal circumstances the skin serves only as 
 a channel of loss to the body, it has been maintained that 
 it may, under particular circumstances, be a means of gain. 
 Cases are on record where bodies are said to have gained in weight 
 by immersion in a bath, or by exposure to a moist atmosphere 
 during a given period, in which no food or drink was taken, or to 
 have gained more than the weight of the food or drink taken ; 
 the gain in such cases must have been due to the absorption of 
 water by the skin. Direct experiments however throw doubt on 
 these statements, for they shew that under ordinary circumstances 
 such a gain by the skin is slight, being apparently due to mere imbibi- 
 tion of water by the epidermis. It is uncertain whether substances in 
 aqueous solution can be absorbed by the skin when the epidermis 
 is intact, the evidence on this point being contradictory. In the 
 case of the sound human skin the balance of conflicting evidence 
 is in favour of the view that soluble non-volatile substances are 
 not absorbed, and that volatile substances such as iodine which 
 may be detected in the system after a bath containing them are 
 absorbed not by the skin but by the mucous membrance of the 
 respiratory organs, the substance making its way to the latter by 
 
Chap, hi.] CUTANEOUS SECRETION. 391 
 
 volatilisation from the surface of the bath. In the case of the 
 skin of the frog an absorption of water and of various soluble 
 substances would c'>jrtainly appear to take place. The lymphatics 
 in the skin of a newborn infant have been found crowded with the 
 particles of the peculiar fatty secretion which covers the skin at 
 birth ; and solid particles rubbed into even the sound skin may, 
 especially when applied in a fatty vehicle, as ex. gr. in the well- 
 known mercury-ointment, find their way into the underlying 
 lymphatics. So that possibly absorption to a certain extent may 
 ordinarily take place in this way. By abraded surfaces, where the 
 dermis is laid bare and covered only by the lowest layers of 
 epidermis, absorption takes place very readily. 
 
CHAPTER IV. 
 SECRETION BY THE KIDNEYS. 
 
 The epithelium of the kidney, like that of the alimentary canal, 
 is a secreting tissue. The protoplasmic cells which line at least a 
 large portion of the tuhuli uriniferi elaborate from the blood, in a 
 manner which we shall presently discuss, certain substances, and 
 discharge them into the channels of the tubules. Besides these 
 distinctly active secreting structures, however, the kidney exhibits 
 in its Malpighian bodies an arrangement very analogous to that 
 which obtains in the lungs. Just as in the latter the functions of 
 the alveolar epithelium are reduced to a minimum, and the en- 
 trance and egress of the gases of respiration are mainly carried on 
 by diffusion, so in the former the epithelium covering the glomerulus 
 has probably but little secreting activity, and the passage of 
 material from the interior of the convoluted blood-vessels into the 
 cavity of the tubule is chiefly carried on by processes which 
 more closely resemble ordinary filtration. What substances 
 pass in this way, and what substances are secreted by the 
 direct action of the epithelium of the secreting tubules, we 
 shall shortly consider. The various substances passing, in company 
 with a large amount of water, in either the one or the other way, 
 into the ducts of the gland, constitute the secretion called urine. 
 And since none of the substances so thrown out are of any further 
 use in the economy, but are at once carried away, urine is generally 
 spoken of as an excretion. 
 
SEC. 1. COMPOSITION OF URINE. 
 
 The healthy urine of man is a clear yellowish slightly fluorescent 
 fluid, of a peculiar odour, saline taste, and acid reaction, having a 
 mean specific gravity of 1*020, and generally holding in suspension a 
 little mucus. The normal constituents may be arranged in several 
 classes. 
 
 1. Water. 
 
 2. Inorganic salts. These for the most part exist in urine in 
 natural solution, the composition of the ash almost exactly cor- 
 responding with, the results of the direct analysis of the fluid ; in 
 this respect urine contrasts forcibly Avith blood, the ash of which is 
 largely composed of inorganic substances, which previous to the 
 combustion existed in peculiar combination with proteid and other 
 complex bodies. In the ash of urine there is rather more sulphur 
 than corresponds to the sulphuric acid directly determined ; this 
 indicates the existence in urine of some svilphur-holding complex 
 body. And there are traces of iron, pointing to some similar iron- 
 holding substance. But otherwise, all the substances found in the 
 ash exist as salts in the natural fluid. The most abundant and 
 important is sodium chloride. There are found in smaller quanti- 
 ties, calcium chloride, potassium and sodium sulphates, sodium, 
 calcium and magnesium phosphates, wdth traces of silicates. 
 Alkaline carbonates are frequently found, and nitrates in small 
 quantity are also said to be sometimes present. 
 
394 COMPOSITION OF URINE. [Book ii. 
 
 The phosphates are derived partly from the phosphates taken 
 as such in food, partly from the phosphorus or phosphates peculiarly 
 associated with the proteids, and partly from the phosphorus of 
 certain complex fats such as lecithin. When urine becomes alkaline, 
 the calcic and magnesic phosphates are precipitated, the sodium 
 phosphates remaining in solution. The sulphates are derived 
 partly from the sulphates taken as such in food and partly from 
 the sulphur of the proteids. The carbonates, when occurring in 
 large quantity, generally have their origin in the oxidation of such 
 salts as citrates, tartrates, &c. The bases present depend largely 
 on the nature of the food taken. Thus with a vegetable diet, the 
 excess of the alkalis in the food reappears in the urine ; with an 
 animal diet, the earthy bases in a similar way come to the front. 
 
 3. Nitrogenous crystalline bodies, derivatives of the metabo- 
 lism of the proteids of the body and food. First and foremost 
 come urea and its immediate ally, uric acid. These will be con- 
 sidered in detail hereafter; they are the typical products of the 
 metabolism of proteids. Existing in much smaller quantities are a 
 number of bodies more or less closely related to urea, which may for 
 the most part be regarded as less-completely oxidised products of 
 metabolism. Such are : kreatinin, xanthin, hypoxanthin, and 
 occasionally allantoin. To these may be added hippuric acid, 
 ammonium oxalurate, and, at times, taurin, cystin, leucin, and 
 tyrosin. These too we shall have to consider in dealing with the 
 metabolism of the body. 
 
 4. Non-nitrog'enous bodies. These exist in very small quan- 
 tities, and many of them are probably of uncertain occurrence. 
 They are organic acids, such as lactic, succinic, formic, oxalic^ phe- 
 nylic, &c. It has been maintained that minute quantities of sugar 
 are invariably present in even healthy urine ; this however has not 
 as yet been placed beyond all doubt. 
 
 5. Pigments. These are at present very imperfectly under- 
 stood. Whether the natural yellow colour of urine be due to a 
 single pigment, or to more than one, and what is the exact nature 
 of these pigments, must be left undecided. As was stated above 
 (p. 300), the urine frequently contains urobilin ; and the peculiar 
 red colour of some rheumatic urines is due to the presence of a 
 body called purpurin or uroerythrin. The urine of many animals, 
 especially of the dog, and occasionally of man, contains indican, 
 which under certain circumstances may give rise to the production 
 of indigo-blue. 
 
 6. Other bodies. When urine is treated with many times its 
 volume of alcohol, a granular or flocculent precipitate is thrown 
 down, consisting of phosphates, some substance or substances giving 
 proteid reactions and probably other bodies in small quantities. 
 An aqueous solution of the precipitate is both amylolytic and 
 
CiiAi'. iv.l UESAL SECRETION. 395 
 
 proteolytic, from wliicli it appears probiiMu that some of the 
 iurmoiits of the salivary L,^lands, paiieroas, stomach, &c., having done 
 their work, escape from the body by the urine. 
 
 7. Gases. Those gases which can be extracted fron\ urine by 
 the morcurial pump are chiefly nitrogen and carbonic acid, oxygen 
 occurring in very small (|uantitirs or being wholly absent. 
 
 The quantities in which these nuiltil'aricms constituents are pre- 
 sent vary within very wide limits, being dependent on the nature 
 of the food taken, and on the conditions of the body. These 
 points will be considered in the succeeding chapter. What may be 
 called the average composition of human urine is shewn in the fol- 
 lowing table. 
 
 AMOUNTS OF THE SEVERAL URINARY CONSTITUENTS PASSED IN 
 TWENTY-FOUR HOURS. (After Pakkes.) 
 
 
 By an average 
 
 Per 1 lulo 
 
 
 man of 66 kilos. 
 
 of Body Weight. 
 
 Water 1 500-000 
 
 grammes 
 
 28-0000 grammes 
 
 Total Solids 
 
 72-000 
 
 
 1-1000 
 
 Urea 
 
 33180 
 
 
 •5000 
 
 Uric Acid 
 
 •555 
 
 
 •0084 
 
 Hijjpuric Acid 
 
 •400 
 
 
 •0060 
 
 Eo-eatinin 
 
 •910 
 
 
 •0140 
 
 Pigment, and 
 
 
 
 
 other substances 1 0*000 
 
 
 •1510 
 
 Sulphuric Acid 
 
 2-012 
 
 
 •0305 
 
 Phosphoric Acid 
 
 3-164 
 
 
 •0480 
 
 Chlorine 
 
 7-000 
 
 (8-21) 
 
 •1260 
 
 Ammonia 
 
 -770 
 
 
 
 Potassium 
 
 2-500 
 
 
 
 Sodium 
 
 11-090 
 
 
 
 Calcium 
 
 •260 
 
 
 
 Magnesium 
 
 •207 
 
 
 
 Acidity of Urine. The healthy urine of man is acid, owing 
 to the presence of acid sodium phosphate, the absence of free acid 
 being shewn by the fact that sodium hy]Dosulphite gives no pre- 
 cipitate. The amount of acidity is about equivalent to 2 grms. 
 of oxalic acid in twenty-four hours, but the degree of acidity at any 
 one time varies much during the day, being in an inverse ratio to 
 the amount of acid secreted by the stomach ; thus it decreases 
 after food is taken, and increases as gastric digestion becomes 
 complete. It varies with the nature of the food ; with a vege- 
 table diet the excess of alkalis secreted leads to alkalinity, or 
 at least to diminished acidity, whereas this effect is wanting -with 
 an animal diet, in which the earthy bases preponderate. Hence 
 the urine of carnivora is generally very acid, while that of herbivora 
 
396 CONSTITUENTS OF URINE. [Book ii. 
 
 is alkaline. The latter, when fasting, are for the time being 
 carnivorous, living entirely on their own bodies, and hence their 
 urine becomes under these circumstances acid. 
 
 The natural acidity increases for some time after the urine has 
 been discharged^ owing to the formation of fresh acid, apparently 
 by some kind of fermentation. This increase of acid frequently 
 causes a precipitation of urates, which the previous acidity has 
 been insufficient to throw down. After a while however the acid 
 reaction gives way to alkalinity. This is caused by a conversion 
 of the urea into ammonium carbonate through the agency of a 
 specific ferment. This ferment as a general rule does not make its 
 appearance except in urine exposed to the air ; it is only in un- 
 healthy conditions that the fermentation takes place within the 
 bladder. 
 
 Abnormal constituents of Urine. The structural elements 
 found in the urine under various circumstances are blood, pus and 
 mucus corpuscles, epithelium from the bladder and kidney, and 
 spermatozoa. Serum-albumin, fibrin (frequently as ' casts '), alkali- 
 albumin, globulin, a peculiar form of albumin (the so-called hemi- 
 albumose), fats, cholesterin, sugar, leucin, tyrosin, oxalic acid, bile 
 acids and bile pigment, may be enumerated as the most important 
 metabolic products abnormally present in urine. Besides these the 
 urine serves as the chief channel of elimination for various bodies, 
 not proper constituents of food, which may happen to have been 
 taken into the system. Thus various minerals, alkaloids, salts, 
 pigmentary and odoriferous matters, may be passed unchanged. 
 Many substances thus occasionally taken suffer changes in passing 
 through the body ; the most important of these will be considered 
 in a succeeding chapter. 
 
SEO. 2. THE SECRETION OF URINE. 
 
 We have already called attention to the fact that the kidney, 
 unlike the other secreting organs which we have hitherto studied, 
 consists of two parts so distinct in structure that it seems im- 
 possible to resist the conclusion that their functions are different 
 and that the mechanism by which the urine is secreted is of 
 a double kind. On the one hand the tubuli urinifen with theu- 
 characteristic epithelium seem obviously to be actively secretmg 
 structures comparable to the secreting alveoli of the salivary and 
 other glands. On the other hand the Malpighian _ capsules with 
 their glomeruli are organs of a peculiar nature with an almost 
 insignificant epithelium, and their structure irresistibly suggests 
 that they act rather as a filtering than as a truly secreting 
 mechanism. Hence the view put forward by Bowman long ago, 
 that certain constituents only of the urine are secreted after the 
 fashion of other secreting glands by the tubuli urmiferi, and that 
 the rest of the constituents, including a great deal of the water 
 with such highly soluble and diffusible salts as preexist m con- 
 siderable quantity in the blood, are as it were filtered by the glome- 
 ruli of the Malpighian capsules. This view is moreover, as we 
 shall presently see, supported by direct experimental e%adence. 
 Assuming for the present the truth of it, we may remark that 
 the passage of fluids and dissolved substances through membranes 
 being in large part directly dependent on pressure, the extent and 
 rapidity of that part of the whole process of the secretion of urme, 
 
398 
 
 RELATIONS TO BLOOD-PRESSURE. [Book ii. 
 
 whicli is a kind of filtration, will be directly affected by the amount 
 of arterial pressure in the renal arteries, while the effect of varia- 
 tions of arterial pressure on that part of the process which is a real 
 active secretion will be an indirect one only. Hence, the discharge 
 of urine by the kidneys must be to a much greater extent than is 
 the case with the secretion of saliva or of gastric juice a mere 
 matter of pressure ; and it will consequently be of advantage 
 to study the relations of urinary secretion to blood-pressure before 
 we enter upon the discussion of the active secretion itself. 
 
 The Relation of the Secretion of Urine to Arterial Pressure. 
 
 Recent observations have shewn experimentally that the kidney 
 is supplied with a vaso-motor mechanism as well developed perhaps 
 as that of any other part of the body. 
 
 By means of a modification of the plethysmograph, we can readily 
 observe the variations which take place in the volume of the 
 kidney and the same method can be applied also to other internal 
 organs. 
 
 Fig. 63. Eenal Oncometer. Seen in section (semi-diagrammatic). JK. kidney, 
 V. vessels and nerves imbedded in fat, &c. entering hilus of organ, O.C. and I.C. 
 outer and inner metal capsules screwed together by the screw S, and holding between 
 
ClIAl*. IV.] 
 
 RENAL iSECHETlUN. 
 
 399 
 
 them tho cJgo of the membrane it/ which api)He3 itself to the surfuco of tJio kidney, 
 and forms with tho metal capsule two chambers a and ii, one of which (//) is clo'ud 
 by a pluy filling the opening /i, while the other (a) communicates by a tube T with 
 tho recording instrument. The other opening C (which i.s closed by a small taj)) is 
 for the purpose of filling the chamber a with warm oil, after the kidney has been 
 placid in the box, tho other chamber B having been previously partly filled, the 
 quantity introduced into it depending upon the size of the kidney. 
 
 ■ntn^nt 
 
 Fio. 64. SEin-DiAGKAiiMATic SECTIONAL VIEW OF ONCOGRAPH. Half natui-al size. 
 K tube connecting instrument with oncometer. D jnston floating on oil contained 
 in the cavity J/ ; the oil is prevented from escaping by the side of the piston, by the 
 delicate flexible membrane £, which does not interfere with the movements of the 
 piston. K, recording lever connected with the piston by a needle G passing through 
 the guides F, F . The screw G is for the purpose of clamping the edge of the membrane 
 between the two ring-shaped surfaces at jV, while the side tube L is for the purpose 
 of filling the instrument. 
 
 The instrument consists of two parts, one of which (Fig. 63) called 
 by Dr C. S. Roy, who introduced it, an oncometer ', is applied to the 
 organ about to be studied, while the other (Fig. 64), called the 
 oncograph, is the recording part of the apparatus. Any diminution 
 in the volume of the organ (Fig. 63, if), kidney, spleen, kc. as the case 
 may be, causes a diminution of the quantity of fluid in the 
 chamber a; this is transmitted through the tube T, continuous 
 with the tube K (Fig. 64) to the chamber Ji; the piston Z> accordingly 
 falls and with it the lever H. Similarly an increase in the volume of 
 the organ causes the lever to rise. 
 
 The volume of the kidney may be increased by a swelling of its 
 constituent cells and other structural elements, by an accumulation 
 of lymph in its lymph spaces and by a distension of its blood- 
 vessels. Compared Avith the third, the two former causes are in 
 health so insignificant and problematical that they may be disre- 
 garded. Further the distension of the blood-vessels will in general 
 
 ' From oncos, bulk. 
 
400 RELATIONS TO BLOOD-PRESSURE. [Book ji 
 
 depend on the constriction or dilation of the renal arteries and their 
 ramifications, for distension due to venous obstruction will only 
 occur in special cases. Hence variations in the volume of the 
 kidney may be taken as a measure of variations in its vascular 
 supply, increase of volume indicating dilated renal vessels, and 
 decrease of volume indicating constriction of the renal vessels. 
 
 When by means of the instrument just described a tracing is 
 taken of the volume of a kidney in what may be considered 
 a normal condition, some such result as that shewn in Fig. 65 is 
 obtained. 
 
 BLOOD PRESSURE 
 
 KIDNEY CURVE 
 
 Fig. 65. Blood-peessuee tbacing, and cdeve feom Eenal Oncometee. Natural 
 size. The blood- pressure abscissa line has been raised 2 '75 cm. (the actual medium 
 blood-pressure having been 115 mm. Hg.). The time-curve gives interruptions re- 
 curring every three seconds. 
 
 The volume of the kidney is seen to be so delicately responsive 
 to changes in the mean arterial pressure that the curve reproduces 
 almost exactly a blood-pressure curve, shewing not only the respira- 
 tory undulations, but even the rise and fall due to the individual 
 heart beats. With each rise of mean arterial pressure more blood 
 is driven into the renal vessels and the kidney swells: with each fall 
 of pressure less blood enters and the kidney shrinks. On other 
 tracings taken in the same way may often be seen the wider 
 variations corresponding to the Traube-Hering curves; but it will 
 be observed that in these the kidney shrinks with the rise of 
 pressure and swells with the fall. For as we have seen (p. 373) the 
 rise in the Traube-Hering undulation is due to an augmentation of 
 peripheral resistance caused by the constriction of minute arteries ; 
 and this constriction occurs in the kidney as elsewhere ; the renal 
 arterioles take their share in producing the result, and in consequence 
 of their constriction the kidney shrinks. Similarly the relaxation 
 of the renal vessels contributes to bring about the sequent fall. 
 
 Other variations in the volume of the kidney are seen to arise 
 from various influences. When respiration is stopped the in- 
 creasingly venous blood, acting on the medullary or spinal vaso- 
 motor centres, leads to constriction of the renal as well as of other 
 
CiiAi-. iv.] RENAL SECRETION. 401 
 
 arteries, as shewn by the shrinking of the kidney. Stinuilatioii of 
 the niediiUa oblongata causes a very marked shrinking of the 
 kichiey, indicating powerful constriction of its arteries, as does also 
 stimulation of the splanchnic nerve; the effect when the splanchnic 
 on one side is stinmlated frequently affects the kidney on the 
 opposite side as much as that on the same side. Stimulation of 
 a sensory nerve causes shrinking of the kidney, in spite of a rise 
 of mean pressure, Avhich in itself would tend to swell the kidney, 
 taking place at the same time ; this is an instance of reflex con- 
 striction as that of stimulation of the splanchnic nerve is of direct 
 constriction. A direct constriction may also be brought about by 
 stimulation of the renal nerves. When all the renal nerves 
 are divided (an operation by no means easy), stimulation of nerves 
 in other parts of the body does not cause a constriction but an 
 expansion of the kidney., since it gives rise to an increase of 
 blood-pressure, through which the renal vessels are passively filled 
 to a greater extent. 
 
 The same method further shews that the vaso-motor mechanism 
 of the kidney is remarkably sensitive to changes in the chemical 
 constitution of the blood. The injection into the blood of even 
 a small cpantity of water causes a shrinking of the kidney followed 
 by a more lasting expansion. The injection of urea and some 
 other diuretics produces the same effect to a more marked degree, 
 while the injection of normal saline solution and especially 
 of such diuretics as sodium acetate causes an expansion from the 
 very first, the primary shrinking being absent. It is moreover 
 worthy of note that these effects of diuretics and of chemical 
 changes in blood appear even after all the renal nerves have 
 apparently been completely severed, indicating that these bodies 
 induce vascular changes by acting either upon some peripheral 
 vaso-motor mechanism, or, even more directly, on the blood-vessels 
 themselves. It may be added that they Avill produce considerable 
 effects in the kidney itself without appreciably modifying the 
 general blood-pressure. 
 
 As yet this method has not disclosed any distinct vaso-dilator 
 fibres passing to the kidney from other parts, positive dilation 
 having been observed only as the result of chemical agents. But, 
 even if these prove eventually to be really absent, enough has been 
 said to shew that the kidney has an ample and well-developed 
 vaso-motor supply. In many of the observations referred to above, 
 the flow of urine was determined at the same time as the volume 
 of the kidney, by measuring the escape from the ureter of the kidney 
 experimented upon through a cannula tied into it. And it was 
 found that, unless special causes intervened, expansion of the 
 kidney was accompanied by an increase and contraction by a 
 decrease in the flow of urine. But before we attempt to illustrate 
 the working of the vaso-motor mechanism just described, it will be 
 as well to call attention to the fact that, as far as filtration is 
 
402 RELATIONS TO BLOOD-PRESSURE. [Book ii. 
 
 concerned, the chief circumstance of the vascular condition of the 
 kidney which we have to consider is the extent of pressure present 
 in the small vessels of the renal glomeruli. The more the pressure 
 of the blood in these exceeds the pressure of the fluid in the 
 channels of the uriniferous tubules, the more rapid and extensive 
 will be the filtration from the one into the other. 
 
 This local blood-pressure in the small vessels of the glomeruli 
 may be increased — 
 
 1. By an increase of the general blood-pressure, brought about 
 — {a) by an increased force, frequency, &c. of the heart's beat, 
 (6) by the constriction of the small arteries supplying areas other 
 than the kidney itself. 
 
 2. By a relaxation of the renal artery, which, as we have pre- 
 viously pointed out (p. 216), while diminishing the pressure in the 
 artery itself, increases the pressure in the capillaries and small 
 veins which the artery supplies. It need hardly be added that 
 this local relaxation must either be accompanied by constriction in 
 other vascular areas, or at all events must not be accompanied by a 
 sufficiently compensating dilation elsewhere. 
 
 The same local pressure may similarly be diminished — 
 
 1. By a constriction of the renal artery and its branches, which, 
 while increasing the pressure on the cardiac side of the artery, 
 diminishes the pressure in the capillaries and veins which are 
 supplied by the artery. This again must either be accompanied by 
 dilation in other vascular areas, or at least not accompanied by a 
 compensating constriction. 
 
 2. By a lowering of the general blood-pressure, brought about 
 — (a) by diminished force, &c. of the heart's beat, (b) by a general 
 dilation of the small arteries of the body at large, or by a dilation 
 of vascular areas other than the kidneys. 
 
 Bearing these facts in mind, it becomes apparently easy to 
 explain many of the instances in which an increase or diminution 
 of urine is produced by natural or artificial means. Thus section 
 of the spinal cord below the medulla causes a great diminution, and 
 indeed in many cases a complete or almost complete arrest, of the 
 secretion of urine. This operation, as we have seen in discussing 
 the vaso-motor system, leads to a very general vascular dilation, in 
 consequence of which there ensues a great fall of the general blood- 
 pressure. At present it seems uncertain whether the renal arteries 
 really possess a normal tone like that of most other arteries ; and 
 we do not know whether they in consequence of the operation 
 share in the general dilation. Even if they do, then expansion 
 apparently is insufficient to compensate the great diminution of 
 general blood-pressure. It has been stated that the effect of section 
 of the medulla is so marked and constant that when, in the dog, the 
 blood-pressure sinks at least below 30 mm. mercury the secretion of 
 urine is invariably arrested. It would appear however that this is 
 not always the case, and that secretion is sometimes observed to 
 
OiiAi'. IV. j RENAL SFAIRKTION. 403 
 
 continue when the blood-pressure sinks even below this point. 
 Section of the spinal cord in tlu; dorsal region similarly depresses 
 the general blood-] tressure and similarly arrests or diminishes the 
 secrt>tiou of urine. This is an operation however from which an 
 animal may, if duly tended, recover, and live for a long time with 
 the lumbar spinal cord cpiite separated i'rom the brain and upper 
 pai'ts of the spinal cord. In such a case the secretion of urine is 
 soon re-established ; but the general blood-pressure is also re- 
 established, so that this condition of things also illustrates the 
 connection between blood-pressure and the secretion of urine. 
 
 Stimulation of the spinal cord below the medulla, though acting 
 in the converse direction, brings about the same result, arrest of 
 the secretion. By the stimulation the action of the vaso-motor 
 nerves is augmented, and constriction of the renal arteries as well 
 as of other arteries in the body is brought about. The increase of 
 general blood-pressure thus produced is insufficient to compensate 
 for the increased resistance in the renal arteries ; and as a con- 
 sequence the flow of blood into the glomeruli is largely reduced. 
 We have seen that under these circumstances the kidney shrinks, 
 and indeed on inspection it is seen to become during the stimu- 
 lation pale and bloodless. 
 
 Section of the renal nerves is followed by a most copious 
 secretion, by what has been called hydruria or polyuria. The 
 section of the nerves, by interrupting the vaso-motor tracts, even if 
 it does not act in the way of destroying a normal tone (the existence 
 of which seems doubtful), prevents the advent of ordinary con- 
 stricting impulses, and thus indirectly leads to dilation of the renal 
 arteries, and so to increased pressure in the small vessels of the 
 glomeruli. If, after section of the renal nerves, the cord be divided 
 below the medulla, the polyuria disappeai's ; for the diminution of 
 general blood-pressure thus produced more than compensates for 
 the special dilation of the renal arteries. Conversely, if after 
 section of the renal nerves the cord be stimulated, the flow of urine 
 is still further increased, since the rise of general blood-pressure due 
 to the general arterial constriction caused by the stimulation tends 
 to throw still more blood into the renal arteries, on which, owing 
 to the division of their nerves, the spinal stimulation is powerless. 
 The section of the renal nerves sometimes leads to the appear- 
 ance of albumin in the urine, but this is probably due to some 
 other effects than those of variations in blood-pressure simply. 
 
 Section of the splanchnic nerves produces also an increased flow 
 of urine. But the augmentation in this case is smaller and less 
 certain than in the case of section of the renal nerves themselves, 
 partly because the vaso-motor tracts from the spinal cord to the 
 kidneys do not run exclusively in the splanchnic nerves but reach 
 the kidney along some other path or paths, and partly because the 
 splanchnic nerves govern the whole splanchnic area, and hence a 
 large portion of the increased supply of blood is diverted from the 
 
 26—2 
 
404 ACTIVITY OF THE EPITHELIUM. [Book ii. 
 
 kidney to other abdominal organs. On the other hand, stimulation 
 of the splanchnic nerves is able to arrest the flow of urine by- 
 producing constriction of the renal arteries. 
 
 The experimental phenomena recorded above are thus seen to 
 receive a fairly satisfactory explanation when they are referred 
 exclusively to variations in blood-pressure. And many of the 
 natural variations in the flow of urine may be interpreted in the 
 same way. No fact in the animal economy is oftener or more 
 strikingly brought home to us than the correlation of the skin and 
 the kidney as far as their secretions are concerned ; and this seems 
 to be, in part at least, maintained by means of the vaso-motor 
 nervous mechanism. Thus when the skin is cold, its blood-vessels 
 are, as we know, constricted. This, by causing an increase of 
 general blood-pressure, will augment the flow through the kid- 
 neys, and conversely, the dilated condition of the arteries of a 
 warm skin, with the consequent diminution of general blood- 
 pressure, will give rise to a diminished renal discharge. It is 
 probable, however, that a more direct connection exists between 
 the skin and the kidneys, so that a warm skin leads to constriction 
 and a cold skin to dilation of the renal vessels ; and it is further 
 possible that the one may react on the other in another way, viz., 
 by changes induced in the blood. The effects of emotions may 
 possibly also be explained as essentially vaso-motor phenomena. 
 
 Secretion by the Renal Epithelium. 
 
 While thus recognising the importance of the relations of the 
 flow of urine to blood-pressure, we must not be led into the error 
 of supposing that the work of the kidney is wholly a matter of 
 filtration. The glomerular mechanism, so specially fitted for filtra- 
 tion, is after all a small portion only of the whole kidney, and the 
 epithelium over a large part of the course of the tubuli uriniferi 
 bears most distinctly the characters of an active secreting epi- 
 thelium. These facts would lead us a priori to suppose that the 
 flow of urine is in part the result of an active secretion comparable 
 to that of the salivary or other glands which we have already 
 studied. And we have experimental and other evidence that such 
 is the case. 
 
 In the first place a flow of urine may be artificially excited 
 even when the natural flow has been arrested by diminution of 
 blood-pressure. Thus if, when the urine has ceased to flow in 
 consequence of a section of the medulla oblongata, certain sub- 
 stances, such as urea, sodium acetate, &c,, be injected into the 
 blood, a more or less copious secretion is at once set up. This 
 secretion is, or at least may be, unaccompanied by any rise of 
 
Chap, iv.] RENAL SECRETION. 405 
 
 blootl-pressiirc sufficient to awouut i'ur the llow ou the filtration 
 hypothesis. A very similar result is illustrated by the common 
 expi'ricnce that the flow of urine is largely increased after taking 
 thuds especially in large quantities. We cannot explain this by a 
 reference to blood-pressure, siucc we have seen (p. 225) that the 
 qvumtity of blood may be increased largely without raising the 
 blood-pressure. On the other hand, observations with the on- 
 cometer have shewn us that the kidney is remarkably sensitive 
 to changes in the chemical constitution of the blood, an expansion, 
 preceded or not by a passing constriction, being caused by the 
 injection into tiie blood of even small quantities of water, sodium 
 chloride and other substances. We ha\e further seen that the 
 expansion of the kidney, or rather the dilation of the renal vessels 
 which is the cause of that expansion, brought about in this way, 
 is dependent on a local peripheral action of some kind or other, 
 since it will take place after complete severance of all the renal 
 nerves. It is of course open for us to suppose that this very 
 dilation of the renal vessels is the cause of the increased flow at 
 the same time, since, the general blood-pressure remaining the 
 same, it will lead to increased pressure in the glomeruli. So that 
 the acti^dty of the kidney which follows upon food and drink or 
 upon the injection of nrea and other substances into the blood may 
 be taken as really illustrating the dependence of the flow of urine 
 on blood-pressure, though the vascular mechanism concerned is 
 limited to the kidney itself and variations of the general blood- 
 pressure play no part in the matter. But it is also open for us to 
 suppose that the presence of these substances in the blood excites 
 the renal epithelium cells to an unwonted activity, causing them 
 to pour into the interior of the tubules a copious secretion, just 
 as the presence of pilocai-pin in the blood will cause the salivary 
 cells to pour forth their secretion into the lumen of their ducts ; 
 and that this activity of the epithelium cells is accompanied, also 
 as in the case of the submaxillary and other glands, by a vascular 
 dilation which, though adjuvant and beneficial, is not the distinct 
 cause of the activity. That this latter view is probably the true 
 one is shewn by the following remarkable experiment, from which 
 we learn that of the various substances finding their way into the 
 blood, some pass into the urine through the glomeruli while others 
 are distinctly secreted by the tubuli uriniferi, their discharge being 
 accompanied by an activity of the secreting cells indicated by the 
 flow of water taking place at the same time. 
 
 In the amphibia, the kidney has a double vascular supply : it 
 receives arterial blood from the renal artery, but thei'e is also 
 poured into it venous blood from another som'ce. The femoral 
 vein divides at the top of the thigh into two branches, one of 
 which runs along the front of the abdomen to meet its fellow 
 in the middle line and form the anterior abdominal vein, while the 
 other passes to the outer border of the kidney and branches in the 
 
406 ACTIVITY OF THE EPITHELIUM, [Book ii. 
 
 substance of that organ, forming the so-called renal portal system. 
 Now the glomeruli are supplied exclusively by the branches of the 
 renal artery, the renal vena portse only serving to form the ca- 
 pillary plexus around the tubuli uriniferi which is also sup- 
 plied by the ' efferent vessels of the glomeruli. From this it 
 is obvious that if the renal artery be tied, the blood is shut 
 off entirely from the glomeruli, actual observation of the kidney 
 of the newt having shewn that under these circumstances there 
 is no reflux jfrom the capillary network surrounding the tubules 
 back to the glomeruli ; thus the kidney by this simple operation is 
 transformed into an ordinary secreting gland devoid of any special 
 filtering mechanism. We owe to Nussbaum the ingenious use of 
 such a kidney to ascertain what substances are excreted by the 
 glomeruli, and what by the tubules in some other part of their 
 course. It is found that sugar and peptones, which injected into 
 the blood readily pass through the untouched kidney and appear 
 in the urine, do not pass through a kidney the renal arteries 
 of which have been tied. These substances therefore are excreted 
 by the glomeruli. Urea on the other hand, injected into the 
 blood, gives rise to a secretion of urine, when the renal arteries are 
 tied ; this substance therefore is secreted by the epithelium of the 
 tubules, and in being so secreted gives rise at the same time to a 
 flow of water through the cells into the interior of the tubules. 
 
 Additional evidence in favour of the activity of the epithelium 
 cells is afforded by an observation for which we are indebted to 
 Heidenhain. Into the veins of animals in which the urinary flow 
 had been arrested by section of the spinal cord below the medulla, 
 this observer injected a quantity of colouring material known as 
 sodium sulphindigotate^. By killing the animals at appropriate 
 times after the injection of the material and examiaing the 
 kidneys microscopically and otherwise, he was enabled to ascer- 
 tain that the pigment so injected passed from the blood into 
 the renal epithelium, and from thence into the channels of the 
 tubules, where it was precipitated in a solid form. There being 
 no stream of fluid through the tubules, owing to the arrest of 
 urinary flow by means of the preliminary operation, the pigment 
 travelled very little way down the interior of the tubules, and 
 remained very much where it was cast out by the epithelium cells. 
 There were no traces whatever of the pigment having passed 
 by the glomeruli ; and the cells which could be seen distinctly 
 to take up and eject it, were those lining such portions of the 
 tubules {viz. the so-called secreting tubules, intercalated tubules 
 and portions of the loops of Henle) as from their microscopic 
 features have been supposed to be the actively secreting portions 
 of the entire tubules. By varying the quantity injected and the 
 time which was allowed to elapse between the injection and subse- 
 
 ^ Sometimes called indigo -cannine, though this name is more properly appHed 
 to a crude impure preparation of potassium sulphindigotate. 
 
Chap, iv.] RENAL SECRETION. t07 
 
 quent inspection, Hcideuhain was ablo to trace the material step 
 by step into the cells, out of the cells into the interior of the 
 tubules, and for some little distance along the tubules. The 
 advantage of the absence of a large flow of urine is obvious ; had 
 this been present, the pigment immediately that it issued from the 
 cells would have been rapidly washed away down the channels of the 
 tubules. One observation he made of a peculiarly interesting 
 character. After injecting a certain quantity of pigment, and 
 allowing such a time to elapse as he knew from previous experi- 
 ments would suffice for the passage of the material through the 
 epithelium to be pretty well completed, he injected a second 
 quantity. He found that the excretion of this second quantity 
 was most incomplete and imperfect. It seemed as if the cells were 
 exhausted by their previous efforts, just as a muscle which has 
 been severely tetanized -svill not respond to a renewed stimulation. 
 
 This obser\"ation may be objected to on the ground that this 
 colouring matter does not occur as a constituent of the blood either 
 in health or disease, and especially that the absence of any con- 
 comitant discharge of fluid from the cells excites suspicion that the 
 process observed was not really one of secretion ; for the injection 
 of such substances as urea or urates into the blood does cause 
 a copious flow of fluid, and indeed thus prevents the microscopic 
 tracking out of their passage, which in the case of urates mightbedone 
 much in the same way as with the sodium sulphindigotate. Moreover 
 other observers have maintained that the sodium sulphindigotate 
 does like ordinary carmine pass through the glomeruli ; but in the 
 case of the amphibian kidney when sodi\im sulphindigotate is injected 
 after ligature of renal arteries, no urine is found in the bladder, 
 but the pigment can be traced, through the epithelium of the 
 secreting portions of the tubuli. "Without insisting too much on 
 the value of the sodium sulphindigotate experiments, they may 
 be taken as fairly supporting the view we are considering. 
 
 Experimental evidence then justifies the conception which the 
 structure of the kidney led us to adopt. The secretion of urine by 
 the kidney is a double process. It is partly a process of filtration, 
 whose object is to remove as rapidly as possible a quantity of 
 water from the body, and this part of the work of the kidney is 
 directly dependent on blood-pressure. It is also however a process 
 of active secretion by the epithelium of the tubuli, and this part of 
 the work of the kidney is, in an indirect manner only, dependent 
 on blood-pressure. Both processes may give rise to a discharge of 
 water from the blood, and both may give rise to the presence 
 of the solid constituents of the urine, in solution in that water. In 
 the first process the discharge of water is the primary object, and 
 the solid matters which escape at the same time are of secondary 
 importance ; in the second process the excretion of the soKd 
 substance is the primary object, and the accompanying water 
 of secondary importance. The first process is governed (mainly at 
 
408 ACTIVITY OF THE EPITHELIUM. [Book ii. 
 
 least) by the vaso-motor nervous system; the second process is 
 excited, as far as we know at present, by substances in the blood 
 acting directly as chemical stimuli to the epithelium ; but future 
 researches may disclose the existence of a secretory nervous me- 
 chanism analogous to that of other secretory glands. It must also 
 be left for further inquiries to determine exactly which of the two 
 processes, or to what relative extent each of them, is concerned in 
 bringing about the presence in the urine of its several constituents. 
 
 In one respect the kidney as a secreting organ differs markedly 
 from such a gland as the salivary. In the case of the latter, we 
 have seen that the saliva as it flows may cause a pressure in the 
 duct greater than the mean arterial pressure ; in the case of the 
 former when a manometer is connected with a cannula tied into the 
 ureter of a dog, the mercury may rise to 60 mm.,but not much beyond, 
 and often becomes stationary at lower level, shewing that the urine 
 cannot be secreted at a pressure greater than that probably 
 obtaining in the renal vessels ; or, at least, if secretion does 
 take place it is counterbalanced by an absorption taking place at 
 the same time. But in this respect the kidney has its fellow in 
 another secreting organ, the liver, for in this, as we have seen, the 
 secretion of bile is arrested when the pressure is raised too high. 
 
 One or two words of caution are necessary. In speaking of the 
 glomerulus as a filtering apparatus, it must not be understood that 
 it is thereby really compared to an ordinary filter made of dead 
 material, and that when filtration through it is spoken of, a process 
 exactly like that which takes place in the laboratory is meant. In 
 the glomerulus the elements of the blood have to pass through the 
 living wall of the capillary, and the covering layer of epithelium 
 cells ; and the transit must be affected by the condition of these 
 living structures. By virtue of their constitution they allow cer- 
 tain things to pass and not others; and when they become changed 
 the passage of material is changed also. The possible influence 
 of a mere layer of squamous epithelium is shewn by experiments on 
 the cornea, which acts absolutely differently as a filter according 
 as the epithelium of Descemet is retained or removed ; and in 
 speaking of the circulation we dwelt on the importance of the 
 physiological condition of the capillary walls. The nature of the 
 filtration taking place through the glomerulus will depend therefore 
 on the condition of the capillary walls and their epithelial invest- 
 ment. This is illustrated by the phenomena of albuminuria (or 
 the passage of albumin into the urine), especially as seen in the 
 following interesting experiment by Nussbaum on the artificial 
 production of albuminuria in the frog. The renal arteries being 
 tied, an injection of urea (1 cm. of a 10 p.c. solution) into the blood 
 gave rise to a flow of urine which was free from albumin. Upon 
 loosing the ligatures so as to re-establish the flow of blood through 
 the glomeruli, the urine at once became albuminous. The arrest 
 of the circulation through the glomeruli had damaged the capillary 
 
Chap. iv.| RENAL SECRKTION. 409 
 
 walls, aiul so allowed tho passapfo throuf^di them into the interior of 
 the iMalpit^hian capsules ol' tho natural protcids of the blood, which 
 in a normal condition of the capilhiries cannot effect such a 
 passage. The injury liowover was tenii)orary only; in a short time 
 the capillary walls were restored to health and the urine ceased to 
 be albuminous. 
 
 We may further quote as shewing the peculiar nature of 
 the filtration that ligature of the renal veins arrests the secretion 
 of urine. Apparently the effect which it should produce by 
 increasing the pressure in the glomeruli is more than counter- 
 balanced by other influences. Upon removal of the ligatures, the 
 urine is usually albuminous, shewing that in the interval the 
 glomeruli have become changed. 
 
 One consideration, of quite secondary importance in the glands 
 which have been previously studied, acquires great prominence 
 when the kidney is being. studied. In studying the pancreas and 
 gastric glands, we concluded without much discussion that the 
 zymogen and pepsinogen were formed in the epithelium cells; for 
 no great manufacture of these substances is going on in other parts 
 of the body. The kidney however is emphatically an excreting 
 organ : its great function is to get rid of substances produced by 
 the activity of other tissues ; its work is not to form but to eject. 
 There can be no doubt, to put forward a strong instance, that with 
 regard to urea it would be absurd to suppose that the whole series 
 of changes from the proteid condition to the urea stage is carried 
 on by the kidne3^ But there still remains the question, Are any 
 of the stages carried on in the kidney, and if so, w^hat ? Is the 
 secreting acti%dty of the renal epithelium confined to picking out the 
 already formed urea from the blood ? Or does the secreting cell of 
 the tubule receive from the blood some antecedent of urea, and in 
 the laboratory of its protoplasm convert that antecedent of urea 
 into urea itself? and if so, what is that antecedent which comes to 
 the kidney in the blood of the renal artery ? And so Avith many 
 other of the urinary constituents. 
 
 In order to complete our study of renal activity, this question 
 ought to be considered now ; but for many reasons it will be more 
 convenient to defer the matter to the succeeding chapter, in which 
 we deal with the metabolic events of the body in general. 
 
SEC. 3. MICTURITION. 
 
 The urine, like the bile, is secreted continuously ; the flow may 
 rise and fall, but, in health, never absolutely ceases for any length 
 of time. The cessation of renal activity, the so-called suppression 
 of urine, entails speedy death. The minute streams passing con- 
 tinuously, now more rapidly now more slowly, along the collecting 
 and discharging tubules, are gathered into the renal pelvis, whence 
 the fluid is carried along the ureters partly by pressure and gravity 
 and partly by the peristaltic contractions of the muscular walls 
 of those channels (see p. 101) into the urinary bladder. When a 
 ureter is divided in an animal, and a cannula inserted, the urine 
 may be observed to flow from the cannula drop by drop, slowly or 
 rapidly according to the rate of secretion. Frequently, after a 
 series of single drops at long intervals, several drops follow in 
 rapid succession, apparently urged by a peristaltic wave. In the 
 urinary bladder, the urine is collected, its return into the ureters 
 being prevented by the oblique entrance into the bladder and 
 valvular nature of the orifices of those tubes ; and its discharge from 
 thence in considerable quantity is effected from time to time by 
 a somewhat complex muscular mechanism, of the nature and 
 working of which the following is a brief account. 
 
 The involuntary muscular fibres forming the greater part of the 
 vesical walls are arranged partly in a more or less longitudinal 
 direction forming the so-called detrusor urinae, and partly in a 
 circular manner, the circular fibres being most developed round 
 the neck of the bladder and forming there the so-called sphincter 
 
Chap. IV. J RENAL SECRETION. 411 
 
 vesicae. After it has been emptied the bladder is contracted and 
 thrown into folds; as the nrine frradually collects, the bladder 
 becomes more and more distended. The escape of the fluid is 
 however prevented by the nisistanco offered by the elastic fi]>r(js of 
 the urethra which keep the urethra channel closed. Some 
 maintain that a tonic contraction of the sphincter vesicae aids in or 
 indeed is the chief cause of this retention. The continuity of the 
 sphincter vesicae with the rest of the circular fibres of the bladder 
 suggests that it prubably is not a sphincter, but that its use lies in 
 its contracting alter the rest of the vesical fibres, and thus finish- 
 ing the evacuation of the bladder. On the other hand, the fact 
 that the neck of the bladder can withstand a pressure of 20 inches 
 of water so long as the bladder is governed by an intact spinal 
 cord, but a pressure of 6 inches only when the lumbar spinal cord 
 is destroyed or the vesical nerves are severed, affords very strong 
 evidence in favour of the view that the obstruction at the neck of 
 the bladder to the exit of urine depends on some tonic muscular 
 contraction maintained by a reflex or automatic action of the 
 lumbar spinal cord. 
 
 When the bladder has become full, we feel the need of making 
 water, the sensation being heightened if not caused by the 
 trickling of a few drops of urine from the full bladder into the 
 urethra. We are then conscious of an effort ; during this effort the 
 bladder is thrown into a long-continued contraction of an obscurely 
 peristaltic nature, the force of which is more than sufficient to 
 overcome the elastic resistance of the urethra, and the urine issues 
 in a stream, the sphincter vesicae, if it act as a sphincter, being at 
 the same time either relaxed after the fashion of the sphincter ani, 
 or at least overcome. In its passage along the urethra, the exit 
 of the urine is forwarded by irregularly rhythmic contractions of 
 the bulbo-cavernosus or ejaculator urinae muscle, and the whole 
 act is further assisted by pressure on the bladder exerted by 
 means of the abdominal muscles, very much the same as in defse- 
 cation. 
 
 We said just now, "when the bladder has become full," but this 
 must not be understood to mean, "when the bladder has received a 
 certain quantity of fluid." On the contrary, it is a matter of 
 common experience that we feel the desire to make water some- 
 times when a large quantity and sometimes when a small quantity 
 of urine has accumulated in the bladder. We have e\ddence that 
 the bladder possesses to a very high degree that obscure con- 
 tinuous contraction which we speak of as 'tone' ; and further that 
 the amount of its tone is exceedingly variable, the organ, quite 
 independently of distinct efforts at mictmition, being at one time con- 
 tracted and at another flaccid and distended. When it is in a 
 conti'acted state, a small quantity of fluid may exert the same 
 pressure on the vesical walls as a larger quantity when the bladder 
 is flaccid. Hence the determining cause of the desii'e to make 
 
412 MICTURITION. [Book ii. 
 
 water is the pressure of the urine upon the vesical walls, the 
 quantity needed to produce fulness being dependent on the 
 amount of tonic contraction of the muscular fibres existing at the 
 time. 
 
 Micturition as sketched above seems at first sight, and espe- 
 cially when we appeal to our own consciousness, a purely voluntaiy 
 act. A voluntary effort throws the bladder into contractions, an 
 accompanying voluntary effort throws the ejaculator and abdominal 
 muscles also into contractions, and, the resistance of the urethra 
 being thereby overcome, the exit of the urine naturally follows. If 
 we adopt the view of a sphincter vesicse being relaxed at the same 
 time, we have to add to the above simple statement the supposition 
 that the will, while causing the detrusor urin^ to contract, also 
 lessens the tone of the sphincter, probably by inhibiting its centre 
 in the lumbar cord. 
 
 There are facts however which prevent the acceptance of so 
 simple a view. In the first place, in cases of urethral obstruction, 
 where the bladder cannot be emptied when it reaches its ac- 
 customed fulness, the increasing distension sets up fruitless but 
 powerful contractions of the vesical walls, contractions which are 
 clearly involuntary in nature, which wane or disappear, and 
 return again and again in a rhythmic manner, and which may 
 be so strong and powerful as to cause great suffering. It seems 
 that the fibres of the bladder, like all other muscular fibres, have 
 their contractions augmented in proportion as they are subjected 
 to tension. Just as a previously quiescent ventricle of a frog's 
 heart may be excited to a rhythmic beat by distending its cavity 
 vrith blood, so the quiescent bladder may, quite independent of 
 the will, be excited, by the distention of its cavity, to a peristaltic 
 action which in normal cases is never earned beyond a first effort, 
 since with that the bladder is emptied and the stimulus is removed, 
 but which in cases of obstruction is enabled clearly to manifest its 
 rhythmic nature. 
 
 In the second place it has been shewn that quite normal mictu- 
 rition may take place in a dog in which the lumbar region of the 
 spinal cord has been completely and permanently separated by 
 section from the dorsal region. In such a case there can be no 
 exercise of volition, and the whole process appears as a reflex 
 action. When under these circumstances the bladder becomes 
 full (and otherwise apparently the act fails) any slight stimulus, 
 such as sponging the anus or slight pressure on the abdominal 
 walls, causes a complete act of micturition : the bladder is entirely 
 emptied, and the stream of urine towards the end of the act 
 undergoes rhythmical augmentations due to contractions of the 
 ejaculator urinee. These facts can only be interpreted on the view 
 that there exists' iu the lumbar cord (of the dog) what we may 
 speak of as a micturition centre capable of being thrown into 
 action by appropriate afferent impulses, the action of the centre 
 
UiiAi'. IV.] RFNAL SECRETION. 413 
 
 being such as to cause a contractiou of the walls of the bladder and 
 of the ejaculator urinas, and possibly at the same time to suspend 
 the tone of the sphincter vesicae. 
 
 Moreover we have, in the case both of man and of other 
 animals, oxperi mental and other evidence that contraction of the 
 blailder is freciuontly brought about by reflex action. Thus the 
 pressure within the bladder when observed for any length of time 
 is found to bo subject to considerable and manifold variations. 
 Over and above passive changes in pressure due to the respi- 
 ratory movements, when the bladder is pressed upon at each 
 descent of the diaphragm, active contractions, of a strength in- 
 adequate to bring about micturition, are from time to time 
 observed. These in some instances appear to be spontaneous, or 
 be the result of emotions, but they may be readily induced in a 
 reflex manner, by stimulating various sentient surfaces or sensory 
 nerves. And common experience aftords many instances where 
 vesical contractions thus brought about in a reflex manner acquire 
 strength adequate to empty the bladder. 
 
 Observations of vesical pressure may be most conveniently carried 
 out by introducing into the bladder a catheter connected either with an 
 oncograph, or some other similar registering apparatus, so arranged as to 
 allow fluid to be driven into or received from the bladder at pleasure. 
 
 Involuntary micturition obviously of reflex nature has frequently 
 been observed in cases of paralysis from disease or injury of the 
 spinal cord ; and the involuntary micturition which is common in 
 children, as the result of irritation of the pelvis and genital 
 organs, and which sometimes occurs in the adult as the result 
 of emotions, or at least sensory impressions, appears to be the 
 result of reflex action. In these several cases w^e may fairly 
 suppose that the centre in the lumbar cord is affected by afferent 
 impulses reaching it along various sensory nerv^es or descending 
 from the brain. Hence we are led to the conception that wdien we 
 make water by a conscious effort of the will, what occurs is not a 
 direct action of the will on the muscular walls of the bladder, but 
 that impulses started by the will descend from the brain after 
 the fashion of afferent impulses and thus in a reflex manner throw 
 into action the micturition centre in the lumbar spinal cord. 
 Nor is this view negatived by the fact that paralysis of the 
 bladder, or rather inability to make water either voluntarily or in 
 a reflex manner, is a common symptom of cerebral or spinal disease 
 or injury. Putting aside the cases in which the reflex act is not 
 called forth because the appropriate stimulus has not been applied, 
 the failure in micturition under these circumstances may be 
 explained by supposing that the shock of the spinal injury or 
 some extension of the disease has rendered the lumbar centre 
 unable to act. 
 
 The so-called incontinence of urine in children is simply an 
 
414 MICTURITION. [Book ii. Chap. iv. 
 
 easily excited and frequently repeated reflex micturition. In 
 cases of cerebral or spinal disease a form of incontinence is frequently 
 met with which seems to be of a different nature. The bladder 
 becoming full, but, owing to a failure in the mechanism of voluntary 
 or reflex micturition, being unable to empty itself by a complete 
 contraction, a continual dribbling of urine takes place through the 
 urethra, the fulness of the bladder being sufficient to overcome the 
 elastic resistance at the neck of the urethra. It is probable how- 
 ever that even in these cases the flow is partly caused by obscure, 
 unfelt, intrinsic contractions of the bladder. 
 
CHAPTER V. 
 
 THE METABOLIC PHENOMENA OF THE BODY. 
 
 We have followed the food through its changes in the alimentary 
 canal, and have seen it enter into the blood, either directly or by the 
 intermediate channel of the lacteals, in the form of peptone (or other- 
 wise modified albumin), sugar (lactic acid), and fats, accompanied 
 by various salts. We have further seen that the waste products 
 which leave the body are urea, carbonic acid and salts. We have 
 now to attempt to connect together the food and the waste pro- 
 ducts ; to trace out as far as we are able the various steps by which 
 the one is transformed into the other, and to inquire into the 
 manner in which the energy set free in this transformation is 
 distributed and made use of 
 
 The master tissues of the body are the muscular and nervous 
 tissues ; all the other tissues may be regarded as the servants of 
 these. And we may fairly presume that, besides the digestive and 
 excretory tissues which we have already studied, many parts of the 
 body are engaged either in further elaborating the comparatively 
 raw food which enters the blood, in order that it may be as- 
 similated with the least possible labour by the master tissues, or in 
 so modifying the waste products which arise from the activity of 
 the master tissues that they may by removed from the bod\' as 
 speedily as possible. There can be no doubt that manifold inter- 
 mediate changes of this kind do take place in the body ; but our 
 knowledge of the matter is at present very imperfect. In one or 
 two instances only can we localize these metabolic actions and 
 speak of distinct metabolic tissues. In the majority of cases we 
 can only trace out or infer chemical changes, without being able 
 to say more than that they do take place somewhere ; and in 
 consequence, perhajDs somewhat loosely, speak of them as taking 
 place in the blood. 
 
SEC. 1. METABOLIC TISSUES. 
 
 The History of Glycogen. 
 
 The best known and most carefully studied example of 
 metabolic activity is the formation of glycogen in the hepatic cells. 
 
 Claude Bernard, in studying the history of sugar in the economy, 
 was led to compare the relative quantities of sugar in the portal 
 and hepatic veins, expecting to find that the sugar possibly 
 diminished during the passage of the blood through the liver; he was 
 astonished to discover that, on the contrary, the quantity appeared 
 to be greatly increased. He found, and anyone can make the obser- 
 vation, that when an animal living under ordinary conditions is killed, 
 the hepatic blood after death contains a considerable amount of sugar 
 (grape-sugar), even when there is little or none in the portal 
 blood; moreover a simple aqueous infusion of the liver is rich 
 in sugar. Not only so, but the sugar continues to be present in 
 the liver when all blood has been washed out of the organ by a 
 stream of water driven through the portal vein, and goes on 
 increasing in amount for some hours after death. Only one 
 interpretation of these facts is possible; so far from the liver 
 destroying or converting the sugar brought to it by the portal 
 vein, it is clearly a source of sugar ; the hepatic tissue evidently 
 contains some substance capable of giving rise to the presence 
 of sugar. Bernard further found that when the liver was removed 
 from the body immediately after death, and, after being divided 
 into small pieces, was thrown into boiling water, the infusion or 
 
Chap, v.] NUTRITIOX. 417 
 
 decoctiou contained very little sugar, and that the small quantity 
 which was present did not increase even when the decoction was 
 allowed to stand for sonic time. The decoction, however, was 
 
 Seculiarly opalescent, indeed milky in appearance; whereas the 
 ecoction of a liver which had been allowed to remain exposed to 
 warmth for some time after death, before being boiled, and which 
 accordingly contained a large amount of sugar, w^is quite clear. 
 On adding saliva, or other amylolytic ferment, to the opalescent, 
 sugarless, or nearly sugarless, decoction and exposing it to a gentle 
 warmth (35° — 40"), the opalescence disappeared ; the fluid became 
 clear, and was then found to contain a considerable quantity of 
 sugar. Here again the explanation was obvious. The opalescence 
 of the decoction of boiled liver is due to the presence of a body 
 which is capable of being converted by the action of a ferment 
 into sugar, and is therefore of the nature of starch. At the 
 moment of death the liver must contain a considerable quantity 
 of this substance, which after death becomes gradually converted 
 into sugar, either through the action of some amylolytic ferment 
 present in the hepatic cells or in the blood of the hepatic vessels 
 or possibly by some special agency. Hence the post-mortem 
 appearance of a continually increasing quantity of sugar. By 
 precipitating the opalescent decoction with alcohol, by boiling the 
 precipitate with alcohol containing potash, whereby the proteid 
 impurities clinging to it were destroyed, and by removing adherent 
 fats by ether, Bernard was able to obtain this sugar-producing or 
 glycogenic substance in a jjure state as a white amorphous powder, 
 Avith a composition of C^HjoO,,, and therefore evidently a kind of 
 starch. Its most striking differences from ordinary starch were 
 that it gave a deep red and not a blue colour with iodine, and that 
 when dissolved in water it formed a milky fluid. He gave to it the 
 name o^ glycogen. 
 
 Since Bernard's discoverv crlvcocren has been recognized as a 
 normal constituent, vaiiable in quantity, of hepatic tissue both in 
 vertebrate and in\ertebrate animals. That it is present in the 
 hepatic cells, and not simply contained in the hef)atic blood, is 
 shewn by the fact that it remains in the liver after all blood has 
 been washed out of that organ. It has also been found in muscle, 
 of which indeed it is almost a constant constituent, in the placenta, 
 ■white corpuscles, testes, brain, and in other jDarts of the body; the 
 tissues of the embryo at an early stage, especially before the liver 
 has become functionally active, are particularly rich in it. 
 
 We have some reasons for thinking that there are several 
 varieties of glycogen, and that the gly(5ogen which exists in muscle is 
 not quite identical with that which occurs in the liver. Indeed there 
 seem to be intermediate stages between glycogen and starch or 
 dextrin. The physiological value of these differences has not yet 
 however been clearly determined, and, with this caution, we shall in 
 the discussions which follow, speak of glycogen as a single substance. 
 
 F. 27 
 
418 - GLYCOGEN. [Book ii. 
 
 Formation and Uses of Glycogen. The amount of glycogen 
 present in the liver of an animal at any one time is largely 
 dependent on the amount and nature of the food previously taken. 
 When all food is withheld from an animal, the glycogen in the 
 liver diminishes, rapidly at first, but more slowly afterwards. 
 Even after some days' starvation a small quantity is frequently 
 still found; but in rabbits, at ail events, the whole may eventually 
 disappear. 
 
 If an animal, after having been starved until its liver mav 
 be assumed to be free or almost free from glycogen, be fed on 
 a diet rich in carbohydrates or on one consisting exclusively of 
 carbohydrates, the liver will in a short time be found to 
 contain a very large quantity of glycogen. Obviously the 
 presence of carbohydrates in food leads to an accumulation of 
 glycogen in the liver; and this is true both of starch and of dex- 
 trin and of the various forms of sugar, cane, gi'ape and milk 
 sugar. The effect may be quite a rapid one, for glycogen has 
 been found in the liver in considerable quantity Anthin a few 
 hours after the introduction of sugar into the alimentary canal of 
 a starving animal. 
 
 If an animal, similarly starved, be fed on an exclusively meat 
 diet a certain amount of glycogen is found in the liver. This 
 appears to be especially the case with dogs (probably with other 
 carnivorous animals also) ; and in his earlier researches Bernard 
 was led to regard the constant presence of glycogen in the livers of 
 dogs fed on meat, as an important indication of the conversion 
 Avithin the body of nitrogenous into non-nitrogenous material. 
 But in the first place, the quantity of glycogen thus stored up in 
 the liver as the result of a meat diet, is much less than that Avhich 
 follows upon a carbohydrate diet; and in the second place, ordinary 
 meat, especially horse-flesh on Avhich dogs are ordinarily fed, 
 contains in itself a certain amount either of glycogen or some 
 form of sugar. Moreover Avhen animals are fed not on meat but 
 on purified proteid, such as fibrin, casein or albumin, the quantity 
 of glycogen in the liver becomes still smaller, though according to 
 most observers remaining greater than during starvation. We may 
 infer therefore that part of the glycogen Avhich appears in the liver 
 after a meat diet is really due to carbohydrate materials present in 
 the meat. Part hoAvever Avould appear to be the result of the actual 
 proteid food and Ave have similar cAidence that gelatine taken as 
 food leads to the formation of some glycogen in the liver. But in 
 this respect these nitrogenous substances fall very far short indeed 
 of carbohydrate material. 
 
 With regard to fats, all observers are agreed that these lead to 
 no accumulation of glycogen in the liver; an animal fed on an 
 exclusively fatty diet has no more glycogen in its liver than a 
 starving animal. 
 
 Hence of the three great classes of food-stuffs, the carbohydrates 
 
CiiAi- v.] XUTRITION. 419 
 
 stand out prominently as the substances which taken as food lead 
 to an accunnilation of glycop^cn in the liver. As far as we know 
 at present the glyeoi^en which thus appears in the liver as the 
 result of feeding either with any of the various forms of carbo- 
 hydrates, or with proteids, or with other substances, is of the same 
 kind and presents the same characters ; at least we have no 
 evidence to the contrary. 
 
 The question naturally arises, What is the use and purpose of 
 this hepatic glycogen ? What ultimately becomes of the glycogen 
 thus for a while stored up in the liver ? 
 
 One view which has been ))ut forward is as follows. We have 
 evidence, as we shall presently learn, that a great deal of the fat of 
 the body is not taken as such in the food, but is constnictcd anew 
 in the body out of other substances. Both carbohydrates and 
 proteids, taken in excess or under certain circumstances, lead to an 
 jiccumulation of fat, and we have reason to believe that carbo- 
 hydrates on the one hand and the carbon-holding portions of 
 various proteids, may by some process or other be converted into 
 fat. And it has been suggested that the glycogen in the liver is a 
 phase of a constructive fatty metabolism, that it is material on its 
 way to become fat. 
 
 The positive evidence in favour of this view is very scanty; it 
 is almost limited to the facts that fat, sometimes in very large 
 quantity, is found in the hepatic cells, that while fat itself taken 
 as food leads to no increase in the hepatic glycogen, carbo- 
 hydrates, which are especially fattening, are most active producers 
 of glycogen, and that the fat present in the hepatic cells seems 
 to be increased by such diets as naturally increase the glycogen 
 in the liver. No evidence has been offered as to the several 
 steps of the conversion of glycogen into fat, nor indeed has 
 it been suggested what those steps are. The view indeed is 
 almost exclusively based on the supposed proof that the blood 
 of hepatic vein contains during life no sugar, or at least not 
 more than does the general blood or even the blood of the portal 
 vein. From this it is infeiTed that the glycogen in the liver is not 
 lost to the liver by becoming converted into sugar and so discharged 
 into the liepatic blood and therefore must be converted into some 
 other substance whieli substance is presumably fat. Bernard both 
 in his earlier and later researches maintained that the blood of 
 the hepatic vein under normal conditions is richer in sugar than 
 the blood of the portal vein or indeed of any other part of the vas- 
 cular system ; this he regarded as an indication that the liver is 
 always engaged in discharging a certain quantity of sugar into the 
 hepatic veins; and his views have been accepted by many obsen'ers. 
 On the other hand others maintain that the blood in the hepatic 
 vein, if care be taken to keep the animal in a perfectly normal 
 condition, contains no more sugar than does the blood of the right 
 auricle or of the portal vein, and indeed that the liver itself, if 
 
420 GLYCOGEN, [Book ii. 
 
 examined before any post-mortem changes have had tuxie to de- 
 velope themselves, is absolutely free from sugar. 
 
 Normal hepatic blood was obtained by Pavy, by means of an in- 
 genious catheterisation. He introduced through the jugular vein, into 
 the superior, and so into the inferior vena cava, a long catheter, con- 
 structed in such a manner that he could at pleasure plug up the vena 
 cava below the embouchement of the hepatic veins, and draw blood ex- 
 clusively from the latter ] or vice versa. 
 
 Now the quantitative determination of sugar in blood by any 
 of the methods as yet suggested is open to many sources of error. 
 And when the quantity of blood which is continually flowing- 
 through the liver is taken under consideration, it is obvious that 
 an amount of sugar, which in the specimen of blood taken for 
 examination fell \vithin the limits of errors of observation, might 
 when multiplied by the wliole quantity of blood, and by the 
 number of times the blood passed through the liver in a certain 
 time, reach dimensions quite sufficient to account for the conversion 
 into sugar of the whole of the glycogen present in the liver at any 
 given time. Hence we may safely conclude that the comparative 
 analysis of hepatic and portal blood, if they do not of themselves 
 prove that the liver is either continually or at intervals converting 
 some of its glycogen into sugar and discharging this sugar into the 
 general system, are at least not sufficiently trustworth}- to disprove 
 the possibility of such a discharge of sugar being one of the normal 
 functions of the liver. 
 
 We may therefore regard the view that glycogen is simj)ly a 
 stage in the formation of fat as not proved ; and indeed we shall 
 presently see reason to believe that fat is formed elsewhere. 
 
 Another view makes use of the formation of fat for the pur- 
 poses of analogy only. Seeing that adipose tissue serves as a 
 storehouse of fat which is not wanted by the body at the moment 
 but may be wanted presently, the question readily presents itself. 
 May not the hepatic glycogen have an analogous function ? May we 
 not regard the presence of glycogen in the liver as in large measure 
 due to the fact that it is deposited there simply as a store of carbohy- 
 drate material, being accumulated whenever amylaceous material is 
 abundant in the alimentary canal, and being converted into sugar 
 and so drawn upon by the body at large to meet the general 
 demands for carbohydrate material during the intervals when food 
 is not being taken ? And we can accept this view without being 
 able to say definitely what becomes of the sugar thus thrown into 
 the hepatic blood. Bernard believed that this sugar underwent an 
 immediate and direct oxidation, but we have already dwelt (p. 851) 
 on the objections to such a view. It is sufficient for us at the 
 present to admit that the sugar is made use of in some way or 
 other. 
 
 Now, many considerations lead us to believe that a certain 
 
Chap, v.] ^UtRITlO.W 421 
 
 avcraj^c composition is necessary for that great internal nicdiuin 
 the blood, in order that the several tissues may thrive upon it 
 to the best advantac^e, one clement of that composition being a 
 certain percentage of sugar. It would appear that some at least if 
 not all of the tissues arc continually drawing upon the blood for 
 suf^ar, and that hence a certain supply must be kept up to meet 
 this demand. On the other hand an excess of sugar in the blood 
 itself Avould be injurious to the tissues. And as a matter of fact 
 we find the quantity of sugar in blood is small but constant ; it 
 remains about the same when food is being taken as in the 
 inter^-als between meals. If sugar be injected into the jugular 
 ^•ein in too large quantities or too rapidly a certain quantity 
 appears in the urine, indicating an effort of the system to throw 
 otf the excess and so bring back the blood to its average con- 
 dition. The maintenance of such a constant percentage of sugar 
 would obviously be provided for or at least largely assisted by 
 the liver acting as a structure where the sugar might at once 
 and without much labour be packed away in the form of the 
 less soluble glycogen, at those times when, as during an amylaceous 
 meal, sugar is rapidly passing into the blood, and there is a 
 danger of the blood becoming loaded with far more sugar than 
 is needed for the time being ; and it may be incidentally noted 
 that a larger quantity of sugar may be injected into the jDortal 
 than into the jugular vein without any reappearing in the urine, 
 apparently because a large portion of it is in such a case retained 
 in the liver as glycogen. When on the other hand sugar ceases to 
 pass into the blood from the alimentary canal, we may suppose 
 that the average percentage in the blood is maintained by the 
 glycogen previously stored up becoming reconverted into sugar, 
 and slowly discharged into the hej^atic blood. 
 
 ]\Ioreover, this view, that the glycogen of the liver is a reserve 
 fund of carbohydrate material, is strongly supported by the analogy 
 of the migi-ation of starch in the vegetable kingdom. We know 
 that the starch of the leaves of a plant, whether itself having 
 pre\iously passed through a glucose stage or not, is normally 
 converted into sugar, and earned down to the roots or other parts, 
 where it frequently becomes once more changed back again into 
 starch. 
 
 A similar argument may be draAra from the relations of 
 glycogen to muscle; that is to say, the glycogen in the muscle 
 may be regarded as a subsidiary store of carbohydrate material 
 laid up for the private use so to speak of the muscle. So fre- 
 quently is glycogen found in muscle that it may be regarded as 
 an ordinary though not an invariable constituent of that tissue ; 
 indeed it may almost be considered as a constituent of all con- 
 tractile tissues. The quantity varies very largely both in the dif- 
 ferent muscles of the same animal and in coiTesponding muscles of 
 different animals. It disappears readily upon starvation, even 
 
422 GLYCOGEN. [Book ii. 
 
 before the hepatic glycogen is exhausted ; at least this is the case 
 with most muscles. It is said to be increased in quantity when 
 the nerve of the muscle is divided, and the muscle thus brought 
 into a state of quiescence. On the other hand it diminishes or 
 even disappears when the muscle enters into rigor mortis. Some 
 have maintained that it diminishes during tetanus, but this 
 appears doubtful; and certainly muscles may be fully alive and 
 contractile from which glycogen is wholly absent. From this we 
 may infer, not that glycogen is a necessary chemical factor of 
 muscular metabolism, but that it can furnish materials for that 
 metabolism, and hence is stored up in the muscle so as to be ready 
 at hand for use. 
 
 Accepting then the view that the hejiatic glycogen is simply 
 store glycogen, waiting to be converted into sugar little by little as 
 the needs of the economy demand, and not glycogen on its way to 
 take part, through the agency of the hepatic protoplasm, in the 
 formation of some more complex compound, such as fat, we have 
 next to deal with the question what is the exact origin of the 
 hepatic glycogen ? By what steps is it formed and what are 
 its immediate antecedents ? We have already seen that the 
 presence of glycogen in the liver is especially favoured by a 
 carbohydrate diet. Hence, if the use of the glycogen be such as 
 we have supposed, it seems only reasonable to conclude that the 
 glycogen which makes its appearance in the liver after an amy- 
 laceous meal arises from a direct conversion of the sugar carried 
 to the liver by the portal vein, the sugar becoming through some 
 action of the hepatic protoplasm dehydrated into starch, by a 
 process the reverse of that by which in the alimentary canal starch 
 is hydrated into sugar through the action of the salivary and 
 pancreatic ferments. Vegetable protoplasm can undoubtedly con- 
 vert both starch into sugar and sugar into starch ; and there are 
 no Ob -priori arguments or positive facts which would lead us to 
 suppose that the activity of animal protoplasm cannot accomplish 
 the latter as well as the former of these changes. Again, as we 
 have incidentally mentioned, sugar injected into the jugular vein 
 readily gives rise to sugar in the urine ; but a very considerable 
 quantity can be slowly injected into the portal vein without any 
 appearing in the urine. This suggests the idea that the liver, so 
 to speak, catches the sugar as it is passing through the hepatic 
 capillaries and at once dehydrates it into glycogen. 
 
 Upon such a view, the carbohydrate taken as food would be 
 converted in glycogen by the agency of the hepatic cell, without at 
 any time becoming an integral part of the protoplasm of the cell. 
 Such a view may be the true one ; but it is open for us to look at 
 the matter in another light. We may conceive of the hepatic cells 
 as being continually engaged in giving rise to carbohydrate 
 material, in the form either of sugar or of some other body, as a 
 product of the metabolism of their own protoplasm ; and we may 
 
CiiAr. v.] NUTRITION. 423 
 
 suppose tli:it iindor certaiu chcuiustaiicL'S, as iu the absence ot" 
 ailetpiate totnl, the carbohydrate material thus formed is at once 
 (HscharsifiMl into the hepatic blood, for the general use of the body, 
 but that under other circumstances as when an amylaceous meal 
 has bei-n taken, the immediate wants of the economy being covered 
 bv the earbohydrati's of the meal, the carbohydrate products of the 
 hepatic metabolism are stored up as glycogen. Under such a view 
 the sugar of the meal is used up somewhere in the body and the 
 glycogen to which it gives rise comes direct from the hepatic 
 proto]>la-<m. An argument against such a view is afforded by the 
 behaviour of the substance glycerine. This substance when taken 
 into the alimentary canal, or when injected into the portal vein, 
 gives rise to an increase of glycogen in the liver, but when it is 
 injected into the general venous system no such increase is ob- 
 served. But if the formation of glycogen were due to the glycerine 
 covering in some way or other the carbohydrate expenditure 
 of the body and thus sparing the products of hepatic metabolism, 
 we should expect to find an increase of gl3'cogen in the latter case 
 as well as in the two former cases. From this not taking place 
 we infer that glycerine gives rise to glycogen by being in some 
 way or other transformed into glycogen by the agency of the 
 hepatic cell, or at all events by the glycerine producing changes in 
 the hepatic cell. And the small quantity of glycogen Avhich 
 results from proteid food is probably in a similar way the product 
 of the direct action of the proteid on the hepatic protoplasm. 
 From these and similar cases we may conclude that sugar also and 
 the other carbohydrates give rise to glycogen, not by covering the 
 general carbohydrate expenditure, but by producing changes in the 
 liver itself How far the sugar reaching the hepatic cell by the 
 portal capillaries enters into the upward and downward series of 
 the protoplasm of the cell, whether it is actually built up into the 
 protoplasm before its elements reappear in the course of the 
 destructive metabolism of the complex protoplasm as glycogen, 
 formed so to speak afresh, or whether the protoplasm simply 
 dehydrates the sugar as we just now suggested, while it is still 
 outside itself, Ave have not at present sufficient evidence to decide. 
 Though the former method seems to entail an unnecessary labour 
 there may be reasons why it should be adopted. 
 
 We have said that glycogen is readily converted by ferments 
 into sugar, and that, after death, a conversion into sugar goes on in 
 most cases \di\\ considerable rapidity and energy. An amylolytic 
 ferment may be extracted from the hepatic tissue and it seems pro- 
 bable that the post-mortem appearance of sugar in the liver is due to 
 the uncontrolleil action of such a ferment. If this be the case, 
 the question may be asked. How is it possible for the glycogen 
 to remain as glycogen and become stored up in larger quantities 
 in the presence of such a ferment ? We can only answer that the 
 solution of this problem is of the same kind as that of the 
 
424 DIABETES. [Book ii. 
 
 problems, why blood does not clot in the living blood-vessels, why 
 the living muscle does not become rigid, and why the living 
 stomach or pancreas does not digest itself. It might be added, 
 bearing in mind the history of the fibrin ferment, that even ad- 
 mitting the presence of an amylolytic ferment after death, we have 
 no proof that such a ferment exists in the hepatic cells during life. 
 It is possible that the ferment which can be extracted after death 
 only makes its aj)pearance as the result of post-mortem changes 
 which have taken jilace in the protoplasm of the hepatic cells. 
 Moreover it is stated that the sugar w"hich makes its appearance 
 in the liver is dextrose Avhereas the sugar into which glycogen is 
 converted by the ordinary amylolytic ferments of saliva and pan- 
 creatic juice, is, as in the case of starch, largely maltose. This 
 would indicate that the conversion which takes place in the liver 
 is of a peculiar nature; but the matter requires further investi- 
 gation. 
 
 Diabetes. Natural diabetes is a disease characterized by the 
 appearance of a large quantity of sugar in the urine. Into the 
 pathology of the various forms of this disease it is impossible to 
 enter here ; but a temporary diabetes, the appearance for a while 
 of a large quantity of sugar in the urine, may be artificially pro- 
 duced in animals in several ways. If the medulla oblongata of a 
 well-fed rabbit be punctured in the region which we have pre- 
 viously described (p. 212) as that of the vaso-motor centre (the 
 area marked out as the "diabetic area" agreeing very closely with 
 that defined as the vaso-motor area), though the animal need not 
 necessarily be in any other way obviously affected by the operation, 
 its urine will be found, in an hour or two, or even less, to be in- 
 creased in amount and to contain a considerable quantity of sugar. 
 A little later the quantity of" sugar will have reached a maximum, 
 after which it declines, and in a day or two, or even less, the urine 
 will be again j)erfectly normal. The better fed the animal, or, 
 more exactly, the richer in glycogen the liver, at the time of the 
 operation, the greater the amount of sugar. If the animal be 
 previously starved so that the liver contains little or no glycogen, 
 the urine will after the operation contain little or no sugar. It 
 is clear that the urinary sugar of this form of artificial diabetes 
 comes from the glycogen of the liver. The puncture of the 
 medulla causes such a change in the liver that the previously 
 stored-up glycogen disappears, and the blood becomes loaded with 
 sugar, much if not all of which passes away by the urine. In 
 the absence of any proof to the contrary, we may assume that 
 in this form of artificial diabetes the glycogen previously present 
 in the liver becomes converted into sugar, just as we know that 
 it does become so converted by post-mortem changes. The glyco- 
 genic function of the liver is therefore subject to the influence 
 of the nervous system, and in particular to the influence of a 
 
OnAi>. v.] NUTRITION. 425 
 
 region of the cerebro-spinal centre which we already know as 
 the vaso-niotor centre, or at least of a part of that region. The 
 path of the intluence may be traced along the cervical spinal cord 
 (and not along the vagi, though the roots of these nerves lie so 
 close to the diabetic spot), as far down as (in rabbits) the level of 
 the third or fourth dorsal vertebra, or even a little lower, from 
 the spinal cord to the first thoracic ganglion, and from thence to 
 the liver by some channel or channels at present undetermined. 
 We cannot at present define clearly the nature of that influence. 
 We cannot say whether the temporary diabetes is a simple effect 
 of a dilation of the hepatic arteries which accompanies the diabetic 
 puncture or of some direct action of the nerves on the metabolic 
 activity of the hepatic protoplasm, though the latter view seems 
 the more probable one. 
 
 Artificial diabet<?s is also a prominent symptom of urari poison- 
 ing. This is not due to the artificial respiration, which is had 
 recourse to in order to keep the urarized animals alive ; because, 
 though disturbance of the respii'atory functions sufficient to inter- 
 fere Avith the hepatic circulation may produce sugar in the urine, 
 artificial respiration may with care be carried on without any 
 sugar making its appearance. Moreover, urari causes diabetes in 
 frogs, although in these animals respiration can be satisfactorily 
 carried on without any pulmonary respiratory movements. The 
 exact way in which this form of diabetes is brought about has not 
 yet been clearly made out. 
 
 A very similar diabetes is seen in carbonic oxide poisoning ; 
 and is one of the results of a sufficient dose of moi-phia or of amyl 
 nitrate. 
 
 There can be no doubt that in diabetes, arising from whatever 
 cause, the sugar appears in the mine because the blood contains 
 more sugar than usual. The system can only dispose (either by 
 oxidation, or as seems more probable in other ways) of a certain 
 quantity of sugar in a certain time. Sugar injected into the 
 jugular vein reappears in the urine, whenever the injection becomes 
 so rapid that the percentage of sugar in the blood reaches a certain 
 (low) limit. Sugar in the urine means an excess of sugar in the 
 blood. How in natural diabetes that excess arises, has not at 
 present been clearly made out. It may be that some forms of 
 diabetes resemble the artificial diabetes just described as resulting 
 from puncture of the medulla, and arise from a too rapid con- 
 version of the hepatic glycogen or from carbohydrate material 
 failing to be stored up as glycogen. All forms of diabetes how- 
 ever cannot be satisfactorily explained in this way; and it has 
 been suggested, though adequate proof has not yet been supplied, 
 that the sugar of diabetes is of a peculiar nature and accumulates 
 in the blood because it is unable to undergo those changes, what- 
 ever they be, which befall the normal sugar of the blood. We 
 must not pursue the subject any further; but there is much to 
 
426 , FAT. [Book 11. 
 
 be said in favour of the view that the sources of the excess of 
 sugar in the blood may be various, and hence that several dis- 
 tinct varieties of diabetes may exist. In one among many points, 
 the clinical history of diabetes throws light on the possible 
 sources of glycogen. While in many, especially of the less severe 
 cases of diabetes, withdrawal of all amylaceous food is followed 
 by a disappearance of sugar from the urine, in many instances 
 the sugar continues to be discharged even though the diet be 
 perfectly free from carbohydrates ; and in many other cases the 
 sugar in the urine is far in excess of the quantity which might be 
 derived from the food. In these cases the sugar must have some 
 non-amylaceous source ; from this we infer that glycogen also may 
 have a similar origin. And the fact that the urea is increased 
 (and that too in some cases in ratio with the sugar) in diabetes, 
 suggests that the sugar may arise from proteids which have been 
 split up into a nitrogenous (urea) and a non-nitrogenous moiety, 
 and so points out the way in which proteids may be a source of 
 glycogen. 
 
 As a sort of converse of diabetes we may mention that the 
 administration of arsenic in sufficient doses or for an adequate time 
 prevents an accumulation of glycogen in the liver and aj)parently 
 in the body generally, whatever be the diet used. 
 
 Tlie History of Fat. Adipose Tissues. 
 
 Of all the tissues of the body adipose tissue is the most fluctu- 
 ating in bulk; within a very short space of time a large amount of 
 adipose tissue may disappear, and within an almost equally short 
 time the quantity present in a body may be several times multi- 
 plied. Histological inquiries teach us that when an animal is fatten- 
 ing the minute drops or specks of fat normally present in certain 
 connective-tissue corpuscles (either of a special kind, or certain 
 individuals of the ordinary kind) are seen to increase in number, 
 the protoplasm enlarging at the same time. As these specks 
 increase they coalesce into drops, which by similar coalescence 
 form larger drops, until, the protoplasm first ceasing to increase 
 and then diminishing, the original connective-tissue corpuscle is 
 transformed into a fat-cell, with a remnant only of protoplasm 
 gathered about the nucleus and forming an imperfect envelope 
 round the enlarged contents. When, on the contraiy, an animal is 
 fasting, the fat seems in some way or other to escape from the cell, 
 which it may leave as a bag either filled with serous fluid or 
 empty and collapsed around the nucleus. These facts point to 
 the conclusion that the fat of adipose tissue is not simply and 
 mechanically collected in the cell, but is formed by the active 
 agency of the cell, being apparently the result of a breaking up of 
 
Chap, v.] NUTRITION. 427 
 
 the protoplasm ; when tbrined, liowover, it ap])ear.s to be dis- 
 char<,f('(l from the cell in a more or less mechanical maiuier, as the 
 ueeds vi the economy demand. And this view is snpported bv 
 the tact that jirotoplasm, ^vherever occnrring', bcjth durini( life and 
 alter ileatii (when it could not possibly be supplied with fat from 
 without), is subject to fatty degeneration, in which the fat evi- 
 dently arises, in large part at least, from the breaking up of 
 proteid compounds. 
 
 On the other hand, we have traced the fats taken as food, and 
 found that they pass with comparatively little change from the 
 alimentary canal, chiefly through the intermediate passage of the 
 lacteals, into the blood. We might infer from this that an excess 
 of fat thus entering the blood would naturally be simply stored up 
 in the available adipose tissue, without any further change, the 
 connective-tissue corpuscles eating the fat brought to them after 
 the fashion of an amoeba but not digesting it, simply keeping it 
 in store till it was wanted elsewhere. 
 
 Which of these views is the true one, or how^ far are both 
 these operations carried on in the animal body ? In the first 
 place, it is e\ident that in an animal fattened on ordinary fattening 
 food, only a small fraction of the fat stored up in the body can 
 possibly come direct from the fat of the food. Long ago, in 
 opposition to the views of Dumas and his school, who taught that 
 all construction of organic material, that all actual manufacture of 
 protoplasm or even of its organic constituents, Avas confined to 
 vegetables and unknown in animals, Liebig shewed that the butter 
 present in the milk of a cow was much greater than could be 
 accounted for by the scanty fat present in the grass or other fodder 
 she consumed. He also urged, as an argument in the same 
 direction, that the w-ax produced by bees is out of all proportion to 
 the fat contained in their food, consisting as this does chiefly of 
 sugar. And Lawes and Gilbert have shewn by direct analysis that 
 for every 100 parts of fat in the food of a fattening pig, 472 parts 
 were stored up as fat during the fattening period. It is clear that 
 fat is formed in the body out of something Avhich is not fat. 
 
 There are two possible sources of this manufactured fat. In 
 treating of digestion (p. 298), we refeired to the possibility of 
 digested carbohydrates becoming by fermentation converted into 
 butyric acid; and we may imagine that when a member of 
 the fatty acid series had thus been formed, higher and more 
 complex members of the same series might be obtained out of it. 
 There can be no doubt indeed that a carbohydi'ate diet is most 
 efficacious in producing an accumulation of fat in the body : sugar 
 or starch, in some form or other, is always a large constituent of 
 ordinary fattening foods. 
 
 Another source of fat is to be found in the proteids. We have 
 seen that the urea of the urine practically represents the whole of 
 the nitrogen which passes through the body. Noav in any given 
 

 Carbon. 
 
 Hydrogen. 
 
 Oxygen. 
 
 Nitrogen, 
 
 Urea 
 
 20-00 
 
 6-QQ 
 
 26-67 
 
 46-67 
 
 Proteid 
 
 53 
 
 7-30 
 
 23-04 
 
 15-53 
 
 428 FAT. [Booii li. 
 
 quantity of urea the amount of carbon is far less than that found 
 in the quantity of proteid containing the same amount of nitrogen. 
 Thus the percentage composition of the two being respectively. 
 
 Sulphur. 
 
 1-13 
 
 100 grms. of urea contain about as much nitrogen as 300 grms. of 
 proteid; but the 300 grms. of proteid contain 139 grms. (159 — 20) 
 more carbon than do the 100 grms. urea. Hence the 300 grms. of 
 proteid in passing through the body and giving rise to 100 grms. 
 of urea, would leave behind 139 grms. of carbon, in some combi- 
 nation or other ; and this surplus of carbon, if the needs of the 
 economy did not demand that it should be immediately converted 
 into carbonic acid and thrown off from the body, might be deposited 
 somewhere in the form of fat. We have already seen, in treating 
 of the action of the pancreatic juice (j). 252), that there is evidence 
 of a fatty element (viz. leucin, which is amido-caproic acid, and 
 so belongs to the fatty acid series) being thrown off from the 
 complex proteid compound in the very process of digestion. 
 
 It is clear that a construction of fat does occur in the body 
 somewhere. What limits can we place on the degree to which 
 this construction is carried? In reference to this point it is worthy 
 of notice that the composition of fat varies in different animals. 
 The fat of a man differs from the fat of a dog, even if both feed on 
 exactly the same food, fatty or otherwise. Were the fat which is 
 taken as food stored up as adipose tissue directly and without 
 change, recourse being had to other sources of food for the con- 
 struction of fat only in cases where the fat in the food was de- 
 ficient, we should expect to find that the constitution of the fat of 
 the body would vary greatly with the food. So far from this being 
 the case, direct experiment shews that the fat of the dog is, as far 
 as composition is concerned, almost entirely independent of the 
 food, that the normal constituents of fat make their appearance as 
 usual, though some of them, such as stearin or olein, may wholly be 
 absent in the food, and that abnormal fats such as spermaceti 
 presented as food are not to be found in the fat which is stored up 
 in the body as a consequence of a large supply of that food. 
 
 Of course it is quite possible that in such cases as these, though 
 the stearin, or the olein, when absent from the food, was in some 
 way or other constructed anew, yet at the same time those con- 
 stituents which were present were simply stored up ; but it is also 
 open for us to suppose that all the fat taken as food was in some 
 way or other disposed of, and that all the new fat which made its 
 appearance was constructed anew. And the latter view is supported 
 by the histological facts just mentioned, as well as by other con- 
 siderations, which we shall presently have to urge. At the pre- 
 
Cmap. v.] NUTIUTIOX. 429 
 
 sent, however, we may be content with the foUowhif^ conclusions. 
 1. Fat is actiuiUy Ibnned in th(! animal body, and is nc^t merely 
 stored \\\^ from the fat of the food. 2. The carbon elements of 
 the uewly-formed fat may be supplied either from amylaceous 
 food, or from the carbon surplus of proteid food, or from fats 
 taken as food which are not the natural constituents of the body- 
 fat. 3. The fat stored up appears as fat granules or drops de- 
 posited in the protoplasm of certain cells, and the increase of the 
 iat in the cells is accompanied first by a growth, and subserpiently 
 by a decay of the protoplasm; but as in the analogous case 
 of glycogen there is no complete evidence to shew whether 
 the fat-granules which appear are simply deposited by the proto- 
 plasm in a more or less mechanical manner, -without their forming 
 an integral portion of it, the chief stages of the manufacture 
 of the fat having been gone through elsewhere, or whether they 
 arise from a breaking up, a functional metabolism of the proto- 
 plasm of the fat-cell itself; the latter view is on the whole 
 hoAvever the most probable. 
 
 The Mammary Gland. 
 
 Since milk is a secretion, and indeed an excretion, the mam- 
 mary gland ought not to be classed as a metabolic tissue, in the 
 limited meaning we are now attaching to those Avords. Yet the 
 metabolic i^henomena giving rise to the secretion of milk are so 
 marked and distinct, and have so many analogies with the purely 
 metabolic events in adipose tissue, that it will be more convenient 
 to consider the matter here, rather than in any other connection. 
 
 Human milk has a specific gravity of from 1"028 to 1'03-i, and 
 when quite fresh possesses a slightly alkaline reaction. It speedily 
 becomes acid, and cow's milk, even when quite fresh, is sometimes 
 slightly acid, the change of reaction taking place during the 
 stagnation of the milk in the mammary ducts. 
 
 The constituents of milk are : 
 
 1. Protcids, viz. casein^, and an albumin, agi^eeing in its gene- 
 ral features with ordinary serum-albumin. The casein may be sepa- 
 rated by curdling with rennet (p. 246) ; it may also be thrown down 
 by the careful addition of acetic acid, jjut a more complete precipita- 
 tion is effected by first adding to the milk a slight quantity of 
 acetic acid, and then passing through it a stream of carbonic acid. 
 From the filtrate the serum-albumin, which is present in small and 
 variable quantities, may be obtained by coagulation with heat, 
 or by precipitation with potassium ferrocyanide, &c. 
 
 1 Or, if we restiict the woid casein to the substance which appears in a solid 
 foiTO in cnrciling, or which uiay be precipitated by acids, an antecedent of casein. 
 
430 MILK. [Book ii. 
 
 2. Fats, These are, in human milk, palmitin, stearin, and 
 olein. But other fats are also present in small quantities ; and the 
 composition of the fats of milk differ in different animals. 
 
 3. Milk-sugar, the conversion of which into lactic acid gives 
 rise to many of the features of milk. 
 
 4. Extractives, including, according to some observers, lurea, 
 and salts. The last consists chiefly of potassium phosphate, with 
 calcium phosphate, potassium chloride, small quantities of mag- 
 nesium phosphate, and traces of iron. 
 
 The following is the composition of 1000 parts of 
 
 
 Human Milk. 
 
 Cow's Milk. 
 
 Casein 
 
 39-24 
 
 48-28 
 
 Albumin 
 
 — 
 
 5-76 
 
 Fat 
 
 26-66 
 
 43-05 
 
 Sugar 
 
 43-64 
 
 40-37 
 
 Salts 
 
 1-38 
 
 5-48 
 
 Total Solids 
 
 110-92 
 
 142-94 
 
 Water 
 
 889-08 
 
 857-06 
 
 Milk is an emulsion, the fats existing in the form of globules of 
 various but minute size, each protected by a thin envelope of 
 casein or albumin. It is this condition of the fat which gives to 
 milk its peculiar white colour. The colostrum, or secretion of the 
 mammary gland at the beginning of lactation, differs from milk in 
 being very deficient in casein and proportionately rich in albumin. 
 It is said that the milk at the end of a long lactation again 
 becomes poor in casein and rich in albumin. Milk on standing 
 turns sour and curdles. This is generally due to the milk-sugar 
 becoming converted by a fermentative process into lactic acid, which 
 in turn precipitates the casein. Curdling may however as we have 
 already seen (p. 246) take place by the action of rennet ferment 
 quite independently of the production of any acid. 
 
 Milk, like the other secretions which we have studied, is the 
 result of the activity of certain protoplasmic secreting cells forming 
 the epithelium of the mammary gland. As far as the fat of milk 
 is concerned, the jjrocesses taking place in the gland are very 
 instructive, since the fat can be seen to be gathered in the 
 epithelium-cell, in the same way as in a fat-cell of the adipose 
 tissue, and to be discharged into the channels of the gland, either 
 by breaking away from the cell, or by a contractile extrusion very 
 similar to that which takes place when an amoeba ejects its digested 
 food. All the evidence Ave possess goes to prove that the fat 
 is formed in the cell through a metabolism of the protoplasm. 
 The microscopic history is thoroughly supported by other facts. 
 Thus the quantity of fat present in milk is largely and directly 
 increased by proteid, but not increased, on the contrary diminished, 
 by fatty food. This is quite intelligible when we know, as will be 
 
Cii.vr. v.] NUTRITION. 431 
 
 shewn ill a siu'coe(lin<j section, that protcid food incroa.ses, and 
 fatty food diniiiiishes, the niotabolisni of the 1)ody ; and wc; havi- 
 already discussed the manner in wliicli proteid material may give 
 rise to iat. A hitch ffd on meat for a ;^iven ])eriod gave off more 
 fat in her milk than she could possibly have taken in her food, and 
 that too while she was gaining in weight, so that she could not 
 have supplied the mannnary gland Avith fat at the expense of 
 fat previously existing in her body; she apparently obtained it 
 ultimately IVom the j)roteids of her food. In the 'ripening' of 
 cheese we have a similar conversion of proteids into tat, though 
 this appears to be etiected by the agency of certain fungi. 
 We have also in<lications that the casein is, like the fat, fonned in 
 the cells of gland, and not simply separated from the blood. When 
 the action of the cell is imperfect, as at the beginning or end of 
 lactation, the albumin in milk is in excess of the casein ; but as 
 long as the cell possesses its proper activity the foimation of casein 
 becomes prominent. When milk is kept at 35" C. out of the body 
 the casein is said to be increased at the expense of the albumin, 
 but the substance thus formed out of albumin is probably not real 
 casein but ordinary alkali-albumin, produced by the action of the 
 alkalis of the milk on the albumin. It has been suggested 
 that the casein may be formed by a splitting up of albumin by 
 some fermentative process, and a ferment capable of effecting this 
 is said to have been isolated. That the milk-sugar also is formed in 
 and by the protoplasm of the cell, is indicated by the facts that 
 it is found in no other part of the body, and that its presence in 
 milk is not dependent on carbohydrate food, for it is main- 
 tained in abundance in the milk of camivora when these are fed 
 exclusively on meat, as free as possible from any kind of sugar or 
 glycogen. We thus have evidence in the mammary gland of the 
 formation, by the direct metabolic activity of the secreting cell, of 
 the representatives of the three great classes of food-stufis, proteids, 
 fats and carbohydrates, out of the comprehensive substance proto- 
 plasm. And what we see taking place in the mammary cell is 
 probably a picture of what is going on in all protojilasmic bodies. 
 If the fat of the milk were not ejected from the mammary cell, the 
 mannnary gland would become a mass of adipose tissue, especially 
 if, by a slight change in the metabolism, the production of fat were 
 exalted at the exi^ense of the production of casein or milk-sugar. If, 
 again, by a similar slight change the milk-sugar were accumulated 
 rather than the iat or proteid, we should ha\e a result which, by an 
 easy step, would bring us to glycogenic tissue. And, lastly, if the 
 proteid accumulation Avere greater than the fatty, or the saccharine, 
 these being carried off in some Avay or other, we should have an 
 image of the nutrition of an ordinary nitrogenous tissue. 
 
 That both the secretion and ejection of milk are under the 
 control of the nervous system is shewn by common experience, 
 but the exact nervous mechanism has not 3'et been fully worked 
 
432 THE SPLEEN. [Book ii. 
 
 out. While erection of the nipple ceases when the spinal nerves 
 which supply the breast are divided, the secretion continues, and 
 is not arrested even when the sympathetic as well as the spinal 
 nerves are cut. 
 
 The Spleen. 
 
 The Spleen may be wholly removed from an animal without 
 any obvious changes in the economy taking place : the functions of 
 the rest of the body appear to go on unimpaired. We are obliged 
 to assume that some comi^ensating actions take place ; but what 
 those actions are we do not know, and we are left at present by 
 these experiments almost completely in the dark as to the functions 
 of the spleen. The most that has been observed is a slight increase 
 in the lymphatic glands, and in the activity of the medulla of bones 
 (see p. 29), but even this is doubtful. 
 
 After a meal the spleen increases in size, reaching its maximum 
 about five hours after the taking of food ; it remains swollen for 
 some time, and then returns to its normal bulk. In certain diseases, 
 such as in the pyrexia attendant on fevers or inflammations, and 
 more especially in ague, a similar temporary enlargement takes 
 place. In prolonged ague a permanent hypertrophy of the spleen, 
 the so-called ague-cake, occurs. 
 
 The turgescence of the spleen seems to be due to a relaxation 
 both of the small arteries and of the muscular bands of the 
 trabeculse ; to be, in fact, a vaso-motor dilation accompanied by a 
 local inhibition of the tonic contraction of the other plain muscular 
 fibres entering into the structure of the organ, the latter, at all 
 events in some animals, being probably the more important of the 
 two. And the condition of the spleen, like that of other vascular 
 areas, appears to be regulated by the central nervous system, 
 the digestive turgescence being altogether comparable to the 
 flushed condition of the pancreas and of the gastric membrane 
 during their phases of activity. 
 
 The application of the plethysmographic method to the spleen, 
 carried out in the way which we described in sjjeaking of the 
 kidney (p. 398), has revealed certain interesting phenomena. 
 
 A ' spleen curve ' Fig. 66 taken in the same way as a ' kidney 
 curve' brings to light the following facts. The volume of the 
 spleen does not vary, as does that of the kidney with each pulse 
 wave. The kidney curve as we have seen, p. 400, gives clear 
 indications of each heart beat, but the spleen curve shews only 
 gentle undulations, obviously due to the respiratory movements. 
 This difference corresponds to the difference in the vascular 
 
Ciivi'. v.] KUTRITIOX. 433 
 
 arrangements of tlio two organs, lu the kidney tlie small arteries 
 arc relatively numerous, and a large portion of the blood in the 
 kidney is contained in them ; in tlie spleen the small arteries are 
 relatively few, and the great bulk of the blood is contained in the 
 capillarit'S and in the meshes of the peculiar splenic tissue. Con- 
 sequently the blood-flow through the spleen is of a more even 
 character than tliat through the kidney, and the effects of variations 
 in the distension of the arteries, and of vaso-motor iiiiluences gene- 
 rally are less directly felt. 
 
 'i0i0f^ 
 
 Fig. G6. Nokhal spleen cdrve fkom Dog. 
 
 Tlie npper curve is the spleen curve shewing the rhythmic contractions and 
 expansions; the smaller waves are due to the respiratory movements. The lower 
 curve is the blood-pressure curve and the point a of the spleen curve corresponds in 
 time to the point b of the blood-pressure curve. The marks on the time curve 
 below indicate seconds. 
 
 Besides the respiratory undulations the spleen cun-e usually 
 shews, as seen in the figure, large slow variations of volume. 
 Rhythmic contractions and expansions, though not always present, 
 frequently make their appearance, each contraction with its fellow 
 expansion lasting in the cat and dog about a minute, and recurring 
 with great regularity for a long time. There can be little doubt but 
 that these variations in volume are due to rhv-thmic contractions, 
 with intervening relaxations, of the muscular trabeculse and capsule. 
 In many animals the contractility of the splenic tissue is shewn by 
 the white lines of constriction which appear when the electrodes 
 of an induction machine in action are drawn over its surface ; and 
 similar lines may be produced by mechanical stimulation with the 
 point of a needle. So that the spleen may be considered as a 
 muscular organ, now expanding to receive a larger quantity of 
 blood and now contracting to drive the blood on to the liver. We 
 have evidence moreover that the muscular activity of the spleen is 
 under the dominion of the nervous system. A rapid contraction of 
 the spleen may be brought about in a direct manner by stimulation 
 of the splanchnic or vagus nerves, or in a reflex manner by stimu- 
 lation of the central ends of a sensory nen'e ; it may also be 
 p. 28 
 
434 THE SPLEEN. [Book ii. 
 
 caused by stimulation of the medulla oblongata with a galvanic 
 current or by means of asphyxia. Though the matter has not yet 
 been fully worked out, we have already sufficiently clear indi- 
 cations that the flow of blood through the spleen is, through 
 the agency of the nervous system, varied to meet changing needs. 
 At one time a small quantity of blood is passing through the 
 organ, and the blood is at such times probably confined to the 
 well-established capillary passages, the metabolic changes which 
 it undergoes in the transit being comparatively slight. At 
 another time a larger quantity of blood enters the organ, and then 
 probably is let loose, so to speak, into the splenic pulp, there 
 to undergo more profound changes, and afterwards to be ejected by 
 the rhythmic contractions of the muscular trabeculse. 
 
 Indeed, when the peculiar arrangements of the blood-vessels of 
 the spleen, with their large open venous networks, are borne in 
 mind, it seems in the highest degree probable that metabolic 
 events of great importance (possibly associated in some way with 
 the destruction or metamorphosis of the blood-corpuscles) take 
 place in the spleen, though at present we are unable to follow 
 them. And this view is supported by the somewhat peculiar 
 chemical characters of the sj)leen-pulp, which, in spite of its con- 
 taining a very large number of blood-corpuscles, differs markedly in 
 its chemical composition from either blood or serum. Thus a 
 special proteid of the nature of alkali-albumin holding iron in some 
 way peculiarly associated with it seems to be present. The occur- 
 rence of this ferruginous proteid, accompanied as it is by several 
 peculiar but at present little understood pigments, rich in carbon; 
 bears out the histological conclusions (see p. 30) concerning the dis- 
 appearance of the red corpuscles. The "^^inorganic salts of the 
 spleen, or at least those of its ash, are remarkable for the large 
 amount of both soda and phosphates, and the small amount of 
 potash and chlorides which they contain, thus differing from blood- 
 corpuscles on the one hand, and from blood-serum on the other. 
 But perhaps the most striking feature of the spleen-pulp is its rich- 
 ness in the so-called extractives. Of these the most common and 
 plentiful are succinic, formic, acetic, butyric and lactic acids (these 
 may arise in part from the decomposition of haemoglobin), inosit, 
 leucin, xanthin, hypoxanthin and uric acid. Tyrosin apparently is 
 not present in the perfectly fresh spleen, though leucin is : both 
 are found when decomposition has set in. The constant presence 
 of uric acid is remarkable, especially since it has been found even 
 in the spleen of animals, such as the herbivora, whose urine 
 contains none. No less suggestive is the fact that the increase of 
 iiric acid in the urine during ague, and during ordinary pyrexia, 
 seems to run parallel to the turgescence, and therefore presumably 
 to the activity, of the spleen. But these facts are at present 
 suggestive only ; they point to an active metabolism, associated in 
 some way with digestion, taking place in the spleen ; exact informa- 
 
Chap, v.] NUTRiriOX. 435 
 
 tion as to the nature of tho metabolism is liowevor wantinfr. Tho 
 thyroid and tliyinus bodies, often in descriptions associated with tho 
 spleen, though diilerent in structure, the former absolutely so, re- 
 semble the spleen somewhat, as far as their extractives are 
 concerned. The thymus contains leucin, xanthin and hypoxanthin, 
 ■with lactic and succinic acids; uric acid seems to be absent. The 
 extractives of the thyroid are scanty, but ap])arontlv of the same- 
 uaturo. 
 
SEC. 2. THE HISTORY OF UREA AND ITS ALLIES. 
 
 We may now return to the questions wliich Ave left unanswered 
 at p. 409. Where is urea formed ? what are its immediate ante- 
 cedents ? what are the various chemical links between it and the 
 proteid material of which it is the excretory representative ? 
 
 We have seen, p. 69, that the muscular tissues contain kreatin, 
 together with smaller quantities of allied nitrogenous crystalline 
 bodies, such as xanthin, hypoxanthin, &c. ; and we cannot go far 
 wrong in supposing that these bodies are in some way or other the 
 products of muscular metabolism. We do not know in what 
 quantities they are formed; but since they are such bodies as 
 would readily be carried away from the muscle by the blood- 
 stream, and yet are always to be found in the muscle, we infer that 
 they are continually being formed, and as continually being con- 
 verted into some other bodies and carried away. And we may 
 further say, that since kreatin exists in muscle to the extent 
 of '2 or "4 p.c, and since muscle forms so large a portion of the 
 whole body, and must be continually undergoing some nitrogenous 
 metabolism, even if the energy of the muscle (see p. 99) have a 
 non-nitrogenous source, it is at least possible, if not probable, that 
 a considerable amount of kreatin passes within twenty-four hours 
 into the blood, on its way to become transformed by other tissues 
 into urea, or into some stage nearer to urea than itself. The urine, 
 it is true, contains a certain amount (•9grm. in 24 hours) of 
 kreatinin, into which kreatin is easily converted ; but this can be Hiil 
 
 A 
 
 ^y 
 
€UAP. v.] NUTRITION. 137 
 
 considered as the iionual lurm hi which the kreatin of the muscles 
 j)asses out ot" the body. For tlie urinary krcatiniu is cxceodiiiL,dy 
 viuuablo iu quantity, vaniyhing during starvation, and, though not at 
 all increased by exercise, is largely augmented by a liesh-diet ; and 
 kreatin injected into the blood, even in small quantities, reappears 
 jis kreatinin in the urine. Without laying too much stress on the 
 last tact, wo are led to conclude that the kreatinin or kreatin in 
 lu-ino has an origin quite independent of that which is present in 
 the muscles, being ])robably derived directly from the food. 
 
 Of the metabolism of the nervous tissues we know little ; but 
 kreatin is found in the brain, in some cases in not inconsiderable 
 quantity. Moreover the bodies of the nerve-cells are undoubtedly 
 composed of protoplasm ; the axis-cylinders of the nerve-fibres are 
 also prot<'»plasmic in nature, and it is at least possible that much of 
 the peculiar matrix of the cerebral and cerebellar convolutions, and 
 of the grey matter generally, is also in reality protoplasmic. Hence 
 we may, with a certain amount of reason, supijose that the 
 nervous, like the muscular tissues, are continually, but to a 
 much less extent, sujjplyiug an antecedent to urea in the form of 
 kreatin. 
 
 Lastly, the spleen contains a considerable quantity of kreatin, 
 as well as of xanthin, &c. ; and these are present also in various 
 glandular organs. 
 
 We thus have evidence of a continual formation of kreatin, 
 possibly in large quantities, in various parts of the body. On the 
 other hand, urea is certainly not present in muscle (save in certain 
 exceptional cases) and its presence in nervous tissue is extremely 
 doubtful. It is absent from the spleen (of the occurrence of urea 
 in the Kver we shall speak presently), the thymus, and thyroid 
 bodies, and from the lymphatic glands, though uric acid, as we 
 have seen, appears to be a normal constituent of the spleen. It 
 seems very tempting to jump at once from these facts to the 
 conclusion that kreatin is the natural antecedent of urea, and that 
 as far as nitrogenous excretion is concerned the labom' of the 
 kidney is confined to the simple transformation of kreatin into 
 urea. We have only to suppose that the kreatin passes from these 
 several tissues into the blood, iu which it may be found, and 
 while circulating in the blood is seized upon by the renal epithelium 
 and converted into urea. And in support of this view it has been 
 urged that while ligature of the ureters leads to an accumulation 
 of urea in the various tissues and fiuids of the body, kreatin takes 
 the place of urea when the kidneys are wholly extirpated, the ex- 
 planation given of the ditferent results being that in the former 
 case the kidneys continue to perform their functions, and to manu- 
 facture urea out of kreatin, and the urea thus formed is thrown 
 back upon the blood, whereas in the latter case the kreatin- 
 converting organs are absent. Further observations however have 
 clearly shewn that whether the kidneys be wholly extirpated or the 
 
438 UEEA. [Book ii. 
 
 ureter simply ligatured, tiie result in both cases is the same, an 
 accumulation in the tissues and fluids of urea, or, in the case of 
 birds and snakes, &c., of uric acid. And indeed the ligature of the 
 ureters soon leads to such trouble in the renal epithelium as to 
 arrest their functional activity, so that the distinction between ex- 
 tirpation of the kidney and ligature of the ureters is an illusory 
 one. Hence, though we need not go so far perhaps as to say 
 that no part of the urea of urine is furnished by a transformation 
 of kreatin by the kidney itself, it is obvious that we cannot speak 
 of the main mass of urea, which passes from the body, as having 
 such an origin, and as having undergone such a transformation in 
 the kidney. Clearly the great part of the urea of the urine reaches 
 the kidney either as urea existing in the blood and has simply to be 
 passed through the epithelium cells, the healthy kidneys usually 
 performing their work so well a^ to leave behind in the blood only 
 such a slight amount of urea as to be with difficulty detected by the 
 means at present at our command, or as some substance, which 
 though not actually urea itself, is so nearly allied as to be capable 
 of being readily transformed into urea, and accumulated as urea 
 in the tissues and fluids, when the excreting power of the kidneys 
 is lost. 
 
 The kreatin of muscle and other tissues may, before it reaches 
 the kidney, be a source of this urea or antecedent of urea in the 
 blood, having undergone transformations whose seat and nature 
 are hidden from us or, as we have suggested, it may not be. There 
 are however other possible sources of urea besides the kreatin 
 formed in muscle and elsewhere. We have seen that one result of 
 the action of the pancreatic juice is the formation of considerable 
 quantities of leucin and tyrosin. In dealing with the statistics of 
 nutrition, our attention will be drawn to the fact that the intro- 
 duction of proteid matter into the alimentary canal is followed 
 by a large and rapid excretion of urea, suggesting the idea that a 
 certain part of the total quantity of the urea normally secreted 
 comes from a direct metabolism of the proteids of the food, without 
 these really forming a part of the tissues of the body. We do not 
 know to what extent normal pancreatic digestion has for its product 
 leucin, and its companion tyrosin; but if, especially when a meal 
 rich in proteids has been taken, a considerable quantity of leucin 
 is formed, we can perceive an easy and direct source of urea, 
 provided that the metabolism of the body is capable of converting 
 leucin into urea. That the body can effect this change is shewn by 
 the fact that leucin, when introduced into the _ alimentary canal in 
 even large quantities, does reappear in the urine as urea; thatis, 
 the urine contains no leucin, but its urea is proportionately in- 
 creased ; and the same is possibly the case with tyrosin, though 
 this is disputed. Now the leucin formed in the alimentary canal 
 is probably carried by the portal blood straight to the liver; and 
 the liver, unlike other glandular organs, does, even in a perfectly 
 
Chap, v.] M'TA'/TIOX. 439 
 
 normal state of things, contain una. W'c are thus led to the \io\v 
 that among the numerous metabolic events which occur in the 
 hepatic cells, the formation of urea out of leucin or out of other 
 antcccilents may be ranked as one. And in support of this view it 
 may be urged that a large quantity of urea seerns to be present in 
 the liver of mammals, and of urates in the liver of birds. More- 
 over when a stream of fresh blood is passed several times through 
 the liver of an animal recently killed, the percentage of urea in 
 the blood so used is found to be decidedly increased. This however 
 is not conclusive, for the increased quantity in the blood which had 
 been circulated might have been simply urea which had been 
 washed out from the liver, where it had previously been sta}-ing. 
 Probable, therefore, as this view may seem, it has not as yet been 
 established as a fact. A strong presumption however in favour of 
 urea arising through the hepatic metabolism, from leucin as an 
 antecedent, is afforded by the fact that in cases of acute atrophy of 
 the liver, where the hepatic cells lose their functional activity, the 
 urea of the urine is replaced by leucin and t}Tosin. And, lastly, it 
 may be remarked that not only are leucin and tyrosin found in 
 nearly all the tissues after death, especially in the glandular tissues, 
 but they also appear with striking readiness in almost all de- 
 compositions of proteids, and leucin is also a product of decompo- 
 sition of gelatiniferous substances. 
 
 The view that leucin is tranforraed into urea lands us however 
 in very considerable difficulties. Leucin, as we know, is amido- 
 caproic acid ; and, with our present chemical knowledge, we can 
 conceive of no other way in which leucin can be converted into 
 urea than by the complete reduction of the former to the ammonia 
 condition (the caproic acid residue being either elaborated into a 
 fat or oxidized into carbonic acid) and by a reconstruction of 
 the latter out of the ammonia so formed. We have a somewhat 
 parallel case in glycin. This, which is amido-acetic acid, when 
 introduced into the alimentary canal, also reappears as urea ; here 
 too a reconstruction of urea out of an ammonia phase must take 
 place. Moreover when ammonium chloride is given to a dog a 
 very large portion reappears as urea, i.e. there is an increase in the 
 urea of the urine corresponding to a large portion of the nitrogen 
 contained in the ammonium chloride. And there is a certain 
 amount of evidence into which we cannot enter here, leading to 
 the conception that the immediate antecedent of urea is ammonium 
 carbamate which by dehydration (and this it is stated may be 
 effected by electrolysis with rapidly alternating currents) is trans- 
 formed into urea. Or the antecedent which is dehydrated may be not 
 ammonium carbamate but ammonium carbonate (see Appendix); and 
 on the other hand, seeing how readily ammonium cyanate is trans- 
 formed into urea, it may be that the immediate antecedent is some 
 cyanogen compound. Leaving these matters for the present, and 
 indeed we have ventured to call attention to them, chieflv because 
 
440 URIC ACID. [Book ii. 
 
 they serve as a warning not to neglect the possible synthetic 
 operations of the animal body, we may sum up our imperfect 
 knowledge concerning the history of urea as follows. We have 
 evidence, not exactly complete but fairly satisfactory, that a part 
 at least of the urea is simply withdrawn from the blood by the 
 renal epithelium. The activity of the protoplasm of the secreting 
 cells must therefore, as far as this part of the urea is concerned, 
 be confined to absorbing the urea from the renal blood, and 
 to passing it on into the cavities of the renal tubules. The 
 mechanism by which this is effected we cannot at present 
 fathom, but it seems more comparable to a selection of food than 
 to anything else ; the cells appear to treat urea much in the same 
 way as they treat sodium sulphindigotate. The antecedents of the 
 urea in the blood are, we may at present suppose, partly the 
 kreatin formed in muscle and elsewhere, partly the leucin and 
 other like bodies formed in the alimentary canal as well as in 
 various tissues. The transformation of these bodies into urea may 
 take place in the liver and possibly in the spleen, but we have no 
 exact proof of this, nor can we say exactly in what way the 
 transformation is effected. There is no proof of any body existing 
 in the blood capable of effecting this transformation ; and we may 
 probably rest assured that in this, as in other metabolic events, 
 the activity exercised in the change comes from some tissue, and 
 cannot be manifested by simple blood plasma. 
 
 Lastly, it is possible that the kidney may, besides the simpler 
 duty of withdrawing ready formed urea from the blood, be exercised 
 in transforming various nitrogenous crystalline bodies to serve as 
 part of the supply of urea which passes from it. 
 
 Uric Acid. This, like urea, is a normal constituent of urine, 
 and, like urea, has been found in the blood, and in the liver 
 and spleen ; we have already, p. 434, referred to its relations with 
 this latter organ. In some animals, such as birds and most reptiles, 
 it takes the place of urea. In various diseases the quantity in the 
 urine is increased ; and at times, as in gout, uric acid accumulates 
 in the blood, and is deposited in the tissues. By oxidation a 
 molecule of uric acid can be split up into two molecules of urea, 
 and a molecule of mesoxalic acid. It may therefore be spoken of 
 as a less oxidised product of proteid metabolism than urea; but 
 there is no evidence whatever to shew that the former is a 
 necessary antecedent of the latter; on the contrary, all the facts 
 known go to shew that the appearance of uric acid is the result of 
 a metabolism slightly diverging from that leading to urea. And 
 we have no evidence to prove that the cause of the divergence lies 
 in an insufficient supply of oxygen to the organism at large ; on 
 the contrary, uric acid occurs in the rapidly breathing birds, as 
 well as in the more torpid reptiles. Nor can the fact that in the 
 frog urea again replaces uric acid be explained by reference to that 
 animal having so large a cutaneous in addition to its pulmonary 
 
CiiAP. v.] NUTRITION. 411 
 
 respiration. The final causes of the divergence are to be sought 
 rather in the fact that urea is the form adapted to a fluid, and uric 
 acid to a more solid excrement. 
 
 Hippuric Acid. In the urine of herbivora uric acid is for 
 the most part absent, being replaced by hippuric acid. In the 
 urine of omnivorous man, both acids may be present together. 
 The history of the hippuric acid of urine is very instructive ; for 
 though at first sight its presence might appear to indicate that the 
 metabolism of the herbivora is in some points fundamentally 
 different from that of carnivora, there can be little doubt that the 
 liippuric acid which appears in the urine of herbivora comes 
 directly from the ingested food. Hippuric acid is a compound of, 
 or rather a result of, the union or conjugation of benzoic acid and 
 glycin ; and when benzoic acid is introduced into the stomach of 
 an animal, whether herbivorous or not, it reappears not as benzoic 
 but as hippuric acid. It evidently meets, somewhere in the body, 
 with glycin ; and uniting with this becomes hippuric acid, in which 
 form it passes out by the urine. Nitrobenzoic acid in a similar 
 way becomes nitrohippuric acid ; and many other bodies of the 
 aromatic class, by a like assumption of glycin, become cocjugated 
 in their passage through the body. 
 
 The knowledge of the fact that benzoic acid is thus converted 
 into hippuric acid naturally suggested the idea that the food of 
 herbivora might contain either benzoic acid, or some allied body, 
 and that the presence of hippuric acid as a normal constituent of 
 urine might be thus accounted for. And it would appear 
 that all the hippuric acid of herbivorous urine is in reality 
 due to the presence in ordinary fodder (hay) of a jDarticular 
 constituent containing a benzoic residue ; when this constituent is 
 withdrawn, the hippuric acid disappears from the urine. 
 
 The transformation or conjugation appears to take place chiefly 
 in the kidneys, for when blood containing benzoic acid is driven 
 through the vessels of a fresh, that is of a living, kidney, it is found 
 after the transit to contain hippuric acid. And indeed the same 
 change may be effected by simply mixing benzoic acid with 
 portions of fresh and still warm kidney, broken to pieces and as it 
 were mashed up, the mixture being exposed to the temperature of 
 the body. If the kidney be kept some time before the benzoic 
 acid is submitted to its action, the transformation fails, indicating 
 that the change is effected by certain elements, probably by the 
 renal epithelium cells, which retain this power so long only as they 
 remain alive after removal from the body. 
 
 A similar transformation of benzoic into hippuric acid is said 
 to take place in some animals at least in the liver, benzoic acid 
 injected into the portal vein reappearing as hippuric acid in the 
 blood of the hepatic vein. In both these cases it is worthy of note, 
 and the observation bears on the formation of urea just discussed, 
 that the change takes place even if no glycin be added with the 
 
442 HIPPURIC ACID. [Book ir. 
 
 benzoic acid. Glycin must therefore be present in the kidney or 
 liver, and its presence in the liver is further shewn by the formation 
 of glycocholic acid from cholalic acid. But singularly enough, glycin, 
 though a common j^roduct of the decomposition of proteid and 
 gelatiniferous substances, has hitherto not been found as such in 
 any part of the living body. 
 
 Of the meaning of the appearance in the tissues of such bodies 
 as xanthin, &c., and of the exact nature of the metabolism which 
 they undergo, we know nothing. We cannot say whether they are 
 simply the accidental bye-products of nitrogenous metabolism, the 
 result of imperfect chemical machinery ; or whether they, though 
 small in quantity, serve some special ends in the economy. 
 
SEC. 3. THE STATISTICS OF N [ITRITION. 
 
 The precediug sections have shewn us how wholly impossible 
 it is at present to master the metabolic phenomena of the body by 
 attempting to trace out forwards or backwards the several changes 
 undergone by the individual constituents of the food, the body or 
 the waste products. Another method is however open to us, the 
 statistical method. We may ascertain the total income and the 
 total expenditure of the body during a given period, and by com- 
 paring the tw^o may be able to draw conclusions concerning the 
 changes which must have taken place in the body while the 
 income was being converted into the output. Many researches 
 have of late years been carried out by this method ; but valuable 
 as are the results which have been thereby gained, they must be 
 received with caution, since in this method of inquiry a small error 
 in the data may, in the process of calculation and inference, lead to 
 most wrong conclusions. The great use of such inquiries is to 
 suggest ideas, but the vieAs-s to which they give rise need to be 
 verified in other ways before they can acquire real worth. 
 
 Composition of the Animal Body. The tirst datum we require 
 is a knowledge of the composition of the body, as far as the rela- 
 tive proportion of the various tissues is concerned. In the human 
 body the proportions by weight of the chief tissues are probably 
 somewhat as follows : 
 
 
 Adult maa. 
 
 Newborn baby. 
 
 Skeleton 
 
 1.5-9 p.c. 
 
 177 p. c. 
 
 Muscles 
 
 41-8 „ 
 
 22-9 „ 
 
 Thoracic viscera 
 
 1-7 „ 
 
 30 „ 
 
 Abdominal viscera 
 
 7-2 „ 
 
 11-5 „ 
 
 Fat 
 
 18-2 „ 
 
 .[ 20-0 ., 
 
 Skin 
 
 6-9 „ 
 
 Brain 
 
 1-9 „ 
 
 15-8 „ 
 
Hi THE STATISTICAL METHOD. [Book ii. 
 
 An analysis of a cat has given the following result : 
 
 Muscles and tendons 45'0 p. c. 
 
 Bones 147 
 
 Skin 12-0 
 Mesentery and adipose tissue 3"8 
 
 Liver 4'8 
 
 Blood (escaping at death) 6"0 
 
 Other organs and tissues 13'7 
 
 One point of importance to be noticed in these analyses is that 
 the skeletal muscles form nearly half the body; and we have 
 already seen (p. 34) that about a quarter of the total blood in the 
 body is contained in them. We infer from this that a large 
 part of the metabolism of the body is carried on in the muscles. 
 Next to the muscles we must place the liver, for though far less in 
 bulk than them, it is subject to a very active metabolism, as 
 shewn by the fact that it alone may hold about a quarter of the 
 whole blood. 
 
 The Starving' Body. Before attempting to study the influence 
 of food, it will be useful to ascertain what changes occur in a body 
 when all food is withheld. A cat was found to lose in a hunger 
 period of 13 days 734 grammes of solid material, of which 248'8 
 were fat and 118-2 muscle, the remainder being derived from the 
 other tissues. The percentages of dry solid matter lost by the more 
 important tissues during the period were as follows : 
 
 Adipose tissue 97"0 p. c. 
 
 Spleen 63"1 „ 
 
 Liver 56'6 „ 
 
 Muscles 30-2 „ 
 
 Blood 17-6 „ 
 Brain and spinal cord 00 „ 
 
 Thus the loss during starvation fell most heavily on the fat, indeed 
 nearly the whole of this disappeared. Next to the fat, the gland- 
 ular organs, the tissues which we have seen to be eminently 
 metabolic, suffered most. Then come the muscles, that is to say, 
 the skeletal muscles, for the loss in the heart was very trifling; 
 obviously this organ, on account of its importance in carrying on 
 the work of the economy, was spared as much as possible : it was 
 in fact fed on the rest of the body. The same remark applies to 
 the brain and spinal cord ; in order that life might be prolonged 
 as much as possible, these important organs were nourished by 
 material drawn from less noble organs and tissues. The blood 
 suffered proportionally to the general body-waste, becoming gra- 
 dually less in bulk but retaining the same specific gravity; of 
 the total dry proteid constituents of the body 17'3 p. c, was lost, 
 
Chap, v.] NUTRITION. 4 tn 
 
 which agi'ces very closely witli the 17"G p. c. lost by the blood. It 
 is -svortliy of remark that the tissues in general became more 
 ■watery than in health. We might infer from these data the con- 
 clusions that metabolism is most active first in the adipose tissue, 
 next in such metabolic tissues as the hepatic cells and .spleen-pulp, 
 then in the muscles, and so on; but these conclusions must be 
 guarded by the reflection that because the loss of cardiac and 
 nervous tissue was so small, we must not therefore infer that their 
 metabolism was feeble; they may have undergone rapid metabolism, 
 and yet have been preserved from loss of substance by their 
 drawing upon other tissues for their material. 
 
 During this starvation-period, the urine contained in the form 
 of urea (for, as we shall see, the other nitrogenous constituents of 
 urine may for the most part be disregarded) 277 grammes of 
 nitrogen. Now the amount of muscle which was lost during the 
 period contained about 15"2 of nitrogen. Thus, more than half 
 the nitrogen of the output during the stan^ation-period must 
 have come ultimately from the metabolism of muscular tissue. 
 This is an important fact of which we shall be able to make use 
 hereafter. Bidder and Schmidt came to the conclusion, from their 
 observations on a starving cat, that the quantity of urea excreted 
 per diem, in all but the earlier days of the inanition period, bore a 
 fixed ratio to the body-weight. In the first two or three days of the 
 period, the daily quantity of urea was much in excess of this ratio. 
 They were thus led to distinguish two sources of urea : a quantity 
 arising from the functional activity of the whole body, and therefore 
 bearing a fixed ratio to the body-Aveight, and continuing until near 
 the close of life ; and a quantity arising from the amount of surplus 
 nitrogenous or proteid material which happened to be stored up in 
 the body at the commencement of the period, and which was 
 rapidly got rid of The latter they regarded as not entering 
 distinctly into the composition of the tissues, but as, so to speak, 
 floating capital, upon which each or any of the tissues could draw. 
 They spoke of its direct metabolism as a luxus consumjjtion. More 
 extended observations however have she'\ATi that though the urea 
 of the first two or three days much exceeds that of the subsequent 
 days of a starvation-period, no such fixed relation of urea to body- 
 weight as that suggested obtains. To the question of a luxus con- 
 sumption we shall have again to refer. 
 
 The Normal Diet. What is the proper diet for a given animal 
 under given circumstances, can only be determined when the laws 
 of nutrition are known. Meanwhile it is necessary to gain an 
 approximate idea of Avhat may be considered as the normal diet 
 for a body such as that of man imder ordinary circumstances. 
 This may be settled either by taking a very large average, or by 
 determining exactly the conditions of a particular case. In the 
 table below is given both the average result obtained b}^ Moleschott 
 from a large number of public diets, and the diet on which an 
 
445 THE ST A TISTICAL METHOD. [Book ii. 
 
 observer (Ranke) found himself in good health, neither losing nor 
 gaining weight. 
 
 Tublic Diet (Moleschott). Eanke. 
 
 Proteids 80 100 
 
 Fat 84 100 
 
 Amyloids 404 240 
 
 Salts 30 25 
 
 Water 2800 2600 
 
 Of these two diets, which agree in many respects, that of 
 Ranke is probably the better one, since in public diets, the cheaper 
 carbohydrates are used to the exclusion of the dearer fats. 
 
 Comjjarison of Income and Output. 
 
 Method. We have now to inquire how the elements of such a 
 diet are distributed in the excreta, in order that, from the manner 
 of the distribution, we may infer the nature of the intermediate 
 stages which take place within the body. By comparing the 
 ingesta with the excreta, we shall learn what elements have been 
 retained in the body, and what elements appear in the excreta 
 wdiich were not present in the food ; from these we may infer the 
 changes which the body has undergone through the influence of 
 the food. 
 
 In the first place, the real income must be distinguished from 
 the apparent one by the subtraction of the feeces. We have seen 
 that by far the greater part of the fseces is undigested matter, i. e. 
 food which, though placed in the alimentary canal, has not really 
 entered into the body. The share in the faeces taken up by matter 
 which has been excreted from the blood into the alirnentary canal, 
 is so small that it may be neglected; certainly with regard to 
 nitrogen, the whole quantity of this element, which is present in 
 the fyeces, may be regarded as indicating simply undigested nitro- 
 genous matter. 
 
 The income, thus corrected, will consist of so much nitrogen, 
 carbon, hydrogen, oxygen, sulphur, phosphorus, saline matters, and 
 water, contained in the proteids, fats, carbohydrates, salts, and 
 water of the food, together with the oxygen absorbed by the lungs, 
 skin, and alimentary canal. The output may be regarded as 
 consisting of (1) the respiratory products of the lungs, skin, and 
 alimentary canal, consisting chiefly of carbonic acid and water, 
 v^ith small quantities of hydrogen and carburetted hydrogen, these 
 two latter coming exclusively from the alimentary canal; (2) of 
 perspiration, consisting chiefly of water and salts, for the dubious 
 excretion (see p. 386) of urea by the skin may be neglected and 
 
Chap, v.] KUTRITIO.y. 417 
 
 tlic other organic constituents of sweat amount to very little ; and 
 ('?>) of the urine, Nvhich is assumed to contain all the nitrogen really 
 excn^ted by the hody, besides a large (juantity of saline matters, 
 and of water. Where great accuracy is re(iuiretl the total nitrogen 
 of the urine ought to be determined; it is maintained, however, 
 that no errors of serious importance arise when the urea alone, as 
 determined by Licbig's method, is taken as the measure of the 
 total quantity* of nitrogen in the urine, since, in this method, other 
 nitrogenous bodies besides urea are precijiitated, and so contribute 
 to the quantitative result. It has been and indeed still is debated 
 whether the body may not suffer loss of nitrogen by other channels 
 than by the urine and faces, Avhether nitrogen may not leave the 
 body by the skin or indeed in a gaseous state by the lungs. The 
 balance of the conflicting evidence seems however in favour of the 
 view that no sucli loss takes place. It would appear that though 
 nitrogen, the pivot, so to speak, of the chemical changes of living 
 beings, forms so large a portion of the atmosphere and moreover is 
 physically diffused through the bodies of both plants and animals, 
 free nitrogen is of no chemical use to either of them. It enters 
 into and remains in their bodies as an inert substance, and the 
 nitrogen which leaves a plant or animal, in a gaseous state, is simply 
 a part of the same inert supply and does not come from the break- 
 ing up of the nitrogenous substances of the body or of food. 
 
 Of these elements of the income and outiDut, the nitrogen, the 
 carbon, and the free oxygen of respiration are by far the most 
 important. Since water is of use to the body for merely mechanical 
 purposes, and not solely as food in the strict sense of the w^ord, the 
 hydrogen element becomes a dubious one ; the sulphur of the 
 proteids, and the phosphorus of the fats, are insignificant in 
 amount ; while the saline matters stand on a wholly different 
 footing from the other parts of food, inasmuch as they are not 
 sources of energy, and pass through the body with comjDaratively 
 little change. The body-weight must of course be carefully 
 ascertained at the beginning and at the end of the period, cor- 
 rection being made where possible for the faeces. 
 
 It Avill be seen that the labour of such inquiries is considerable. 
 The urine, wdiich must be carefully kept separate from the faeces, 
 requii'es daily measurement and analysis. Any loss by the skin, 
 either in the form of sweat, or, in the case of woolly animals, of 
 hair, must be estimated or accounted for. The food of the period 
 must be as far as possible uniform in character, in order that the 
 analyses of specimens may serve faithfully for calculations involving 
 the whole quantity of food taken ; and this is especially the case 
 when the diet is a meat one, since portions of meat differ so much 
 from each other. But the greatest difficulty of all lies in the 
 estimation of the carbonic acid produced and the oxygen con- 
 sumed. In some of the earlier researches, this factor Avas 
 neglected and the variations occurring were simply guessed at, 
 
448 TEE STATISTICAL METHOD. [Book ii. 
 
 through which very serious errors were introduced. No comparison 
 of income and output can be considered satisfactory unless at least 
 the carbonic acid produced be directly measured by means of a 
 respiration chamber. And in order that the comparison should be 
 really complete, the water given off by the skin and lungs must be 
 directly measured also ; but this seems to be more difficult than 
 the determination of the carbonic acid. 
 
 In the plan originally adopted by E-egnault and Reiset and folio-wed 
 by some other observers, the animal experimented on is allowed to 
 breathe a limited and measured atmosphere. The carbonic acid, as fast 
 as it is formed, is fixed and removed by a strong solution of caustic potash, 
 and the normal percentage of oxygen in the atmosphere is maintained 
 by a supply of this gas from a gasholder. In this way both the oxygen 
 consumed and the carbonic acid produced are directly determined, while 
 the continual supply of fresh oxygen prevents any evil eifects due to 
 breathing a confined portion of air. In order however to avoid all 
 possible errors arising from a too restricted atmosphere a difierent method 
 has been adopted by Pettenkofer and Voit. Their apparatus consists 
 essentially of a large chamber, capable of holding a man comfortably. 
 By means of a steam-engine a current of pure air, measured by a gaso- 
 meter, is drawn through the chamber. Measured portions of the out- 
 going air are from time to time withdrawn and analysed ; and from the 
 data afibrded by these analyses, the amounts of carbonic acid (and other 
 gases) and of water given ofi" by the occupant of the chamber during a 
 given time are determined. The oxygen consumed may also be determined 
 by a comparison of the ingoing and outgoing air ; besides if the total 
 amounts of carbonic acid and of water given out by the lungs and skin 
 are thus ascertained and the amount of urine and fjBces known, the 
 quantity of oxygen is determined by a simple calculation. For evidently 
 the difference between the terminal weight plus all the egesta and the 
 initial weight plus all the ingesta can be nothing else than the weight of 
 the oxygen absorbed during the period. This method in turn however is 
 also open to objections, since minute errors in the sample determinations 
 acquire by multiplication considerable dimensions. It seems moreover 
 undesirable to leave the quantity used of so important an element as 
 oxygen to be determined by indirect calculations. 
 
 Let us imagine, then, an experiment of this kind to have been 
 completely carried out, that the animal's initial and terminal 
 weights have been accurately determined, the composition of 
 the food satisfactorily known to consist of so much proteid, fat, 
 carbohydrates, salts, and water, and to contain so much nitrogen 
 and carbon, the weight of the faeces and the nitrogen they contain, 
 ascertained, the nitrogen of the urine determined, the carbonic 
 acid and water given off by the whole body carefully measured, 
 and the amount of oxygen absorbed calculated — what interpreta- 
 tion can be placed on the results ? 
 
 Let us suppose that the animal has gained v) in weight during 
 the period. Of what does w consist ? Is it fat or proteid material 
 which has been laid on, or simply water which has been retained, 
 
Ch.u'. v.] xrrniTiox. 449 
 
 or some of u\w ainl cC llu- dtlnr^ Let us riirtlu r siij)])o.se tlwiL the 
 iiitro^oii of the uriiii- ]iassiMl duriiig the period is less, say by x 
 «,'raiiunes, tliaii the nitroL,^'!! in the food taken, of course after 
 (leihution of tlie nitroi(eii in the fiT'ces. 'J'his means tliat x 
 _i,Mammes of nitro«,'en have been retained in the body; and we niay 
 with reason infer that they liave been retained in the form of 
 proteid material. Wc may even go farther and say that they are 
 retained in the form of flesh, i.e. of muscle. In this inference we 
 are going somewhat beyond our tether, for the nitrogen might be 
 stored up as hepatic, or s])lenic, or any other form of protoplasm. 
 Indeed it might be for the while retained in the form of some 
 nitrogenous crystalline body; but this last event is unlikely; and 
 if we use the word 'flesh' to mean protophtsm of any kind, 
 contractile or metabolic, or of any other kind, we may without fear 
 of any great cn*or reckon the deficiency of x grammes nitrogen 
 as indicating the storing up of a grammes tlesh. There still 
 remain w — a grammes of increase to be accounted for. Let us 
 suppose that the total carbon of the egesta has been found to be 
 y gi-ammes less than that of the ingesta ; in other words, that if 
 grammes of carbon have been stored up. Some carbon has been 
 stored up in the flesh with the nitrogen just considered ; this we 
 must deduct from y, and wc shall then have y' grammes of carbon 
 to account for. Now there are only two principal foinis in which 
 carbon can be stored up in the body : as glycogen or as fat. The 
 former is even in most favourable cases inconsiderable, and we 
 therefore cannot err greatly if Ave consider the retention of y' 
 grammes carbon as indicating the laying on of h grammes fat. 
 If a + Z> are found equal to lu, then the whole change in the 
 economy is known; \i iu — {a + h) leaves a residue c, we infer that 
 in addition to the laying on of flesh and fat some water has been 
 retained in the s^'stem. If w — (a + h) gives a negative f[uantity, 
 then water must have been given off at the same time that flesh 
 and fat were laid on. In a similar way the nature of a loss of 
 weight can be ascertained, whether of flesh, or fat, or of Avater, and 
 to Avhat extent of each. The careful comparison, the debtor and 
 creditor account of income and output, enables us, Avith the 
 cautions rendered necessary by the assumptions just noAv men- 
 tioned, to infer the nature and extent of the bodily changes. The 
 results thus gained ought of course, if an account is kept of the 
 Avater taken in and given out, to agree AA'ith the amount of oxygen 
 consumed, and also to tally Avith the conclusions arrived at con- 
 cerning the retention or the reverse of AA'ater. 
 
 Having thus studied the method and seen its Avcakness as well 
 as its strength, Ave may briefly re\'ieAV the results Avhicli have been 
 obtained by its moans. 
 
 Nitrogenous Metabolism. When a diet of lean meat, as free 
 as possible from fat, is given to a dog, Avhich has previously been 
 deprived of food for some time, and Avhose body therefore is greatly 
 F. 29 
 
450 NITROGEXUUS METABOLISM. [Book ii. 
 
 deficient in flesli, it might be expected that the great mass of food 
 would be at once stored up, and only a small quantity be imme- 
 diately worked off as an additional quantity oi' urea, occasioned 
 by the increased labour thrown on the economy by the very 
 presence of the food. This however is not the case ; the larger 
 portion passes off as urea at once, and only a comparatively small 
 quantity is retained. If the diet be continued, and we are sup- 
 posing the meals given to be ample ones, the proportion of the 
 nitrogen which is given off in the form of urea goes on increasing 
 until at last a condition is established in which the nitrogen of 
 the cgesta exactly equals that of the ingesta. This condition, 
 which is sj)oken of as nitrogenous equilibrium, is attained in dogs 
 with an exclusively meat diet only when large quantities of food 
 are given, and is not easily maintained for any length of time. 
 The exact quantity of meat required to attain nitrogenous equi- 
 librium varies with the previous condition of the dog ; it is fre- 
 quently seen when 1500 or 1800 grms. of meat are given daily. 
 Thus the most striking effect of a purely nitrogenous diet is largely 
 to increase the nitrogenous metabolism of the body. This result 
 has been explained by supposing that with the meat diet the 
 consumption of oxygen is largely increased ; in other words, that 
 the oxidizing activity of the body is directly augmented by a meat 
 diet. This in turn may be due in part to the fact that proteid 
 food largely increases the number of the red corpuscles, and so 
 augments the amount of oxygen with which the tissues are 
 supplied ; but as we have already urged more than once the 
 oxidative activity of the tissues is determined by the tissues 
 themselves rather than by the mere abundance of oxygen at their 
 disposal ; and probably other agencies are at work. 
 
 When nitrogenous equilibrium is established, it does not mean 
 that a body-equilibrium is established, that the body-weight 
 neither increases nor diminishes. On the contrary, when the meal 
 necessary to balance the nitrogen is a large one, the body may 
 gain in weight, and the increase is proved, both by calculation 
 from the income and output, and by actual examination of the 
 body, to be due to the laying on of fat. The amount so stored up 
 may be far greater than can possibly be accounted for by any fat 
 still adhering to the meat given as food. We are therefore driven 
 to the conclusion that the proteid food is split into a urea moiety 
 and a fatty moiety, that the urea moiety is at once discharged, and 
 that such of the fat as is not made use of directly by the body is 
 stored up as adipose tissue. And this disrujDtion of the proteid 
 food at the same time explains why the meat diet so largely and 
 immediately increases the urea of the egesta. We have already 
 pointed out that possibly this disruptive metabolism of proteids is 
 largely carried on in the alimentary canal itself by the aid of the 
 pancreatic juice ; whether or to what extent other organs share in 
 the action we do not at present know. 
 
UiiAi'. v.] NUTIUTIOX. 451 
 
 Tlic charactcri.stio iuctal)i)lic t-Hrcts of proteid fcjoil an; slunvn 
 not only by tlu-sc calculations ot" \vhat is supposed to take; place 
 in the body, but also by direct analysis. Lawes and Gilbert 
 laboriously analysing the body of a pig, which had been fed on a 
 known diet, and conij^aring the analysis with that of another pig of 
 the same litter, killed at the time when the first was put on the 
 fixed diet, found that of the dry nitrogenous material of the food 
 only 734 p. c. was laid up as dry i)roteid material during the 
 fattening ])eriod, though the amount of proteid food was low; in 
 the sheep the increase was only 4"1-Jl< p. c. 
 
 It may be worth while to consider briefly here what is exactly 
 meant by the proteid metabolism of which we are speaking. 
 In the first place, in dealing with the changes taking place in the 
 body we may distinguish between morphological and physiological 
 destruction and renewal. We know that an ejiithelium cell, as 
 notably in the case of the skin, may be bodily cast off and its 
 place filled by a new cell ; and probably a similar disappearance of 
 and renewal of histological units takes place in all the tissues of 
 the body to a variable extent. But, in the adult body, these 
 histological transformations are, in the cases of most of the tissues, 
 slow and infrequent. A muscle for instance may suffer very 
 considerable wasting and recover from that wasting without any 
 loss or renewal of its elementary fibres. And it is ob\dous that 
 the metabolism of which we are now speaking does not involve any 
 such shifting of histological units. On the other hand we find 
 these histological imits, the muscle-fibre or the gland-cell for 
 instance, living on their internal medium the blood, or rather on 
 the lymph Avhich is the middleman between themselves and the 
 actual blood flowing in the vascular channels. Now we have 
 previously insisted at length on the view that no oxidative 
 changes on a large scale take place, as was once thought, in the 
 blood. The proteid metabolism which we are now considering or 
 rather the destructive part of that metabolism (and to avoid the 
 introduction of a new word we may venture, in using the word 
 metabolism, to leave the context to explain, whether the whole 
 series of changes constructive and destructive, or the constructive 
 changes alone, or the destructive changes alone, are intended) is 
 fundamentally oxidative in character; and we may therefore assume 
 that the large proteid metabolism which we are considering does 
 not go on in the blood. In other words, the metabolism of proteids 
 and the reduction of their nitrogenous residues into urea or into 
 immediate antecedents of urea is carried out by the agency of the 
 elements of the tissues. In a tissue unit however, such as a muscle- 
 fibre or gland-cell, we must distinguish between the actual living 
 protoplasm or modified protoplasm, the morphological framework so 
 to speak, and the material or substances, solid or in solution, which 
 are lodged in the spaces of the framework, which are not part of the 
 living unit but rather form a sort of internal medium to the unit 
 
 29—2 
 
452 NITROGENOUS METABOLISM. [Book ii. 
 
 itself. And we may readily conceive of the living unit effecting 
 changes, and even profound changes, in this its internal medium, 
 without the substances thus changed ever becoming an integral part 
 of the living unit itself. Moreover we can also conceive of the unit 
 as a whole producing changes in the lymph surrounding it, much 
 in the same way, as, according to some observers, the yeast-cell 
 produces changes in the molecules of sugar which surround it, bvit 
 which never become part of itself. We may therefore, in the case 
 of proteids, follow Voit and others in distinguishing between the 
 proteids on the one hand which actually become j^art of living 
 units and which may be called " tissue-proteids " or " morphotic 
 proteids," and those on the other hand which are found in the 
 internal meshes of the unit or in the surrounding Ij^mph or in the 
 blood, and which, since they probably pass readily from one medium 
 to the other, may be spoken of as "circulating proteids." By 
 older physiologists at a time when the energy of bodily movements, 
 of which we shall speak directly, was supposed to come from the 
 direct metabolism of the morphotic proteids of muscle, the increase 
 of urea due to food independent of exertion Avas regarded as simply 
 arising from proteids metabolized in the blood, and so cast out as 
 useless ; hence the phrase, to which we have already referred, of 
 luxus-consumption. We now know however, as will presently be 
 pointed out, that the energy of bodily movements does not come 
 from the metabolism of the proteids of muscle, and we have 
 already seen that oxidations on a large scale do not take place 
 in the blood. Hence this view of a luxus-consumption is no 
 longer tenable. There still remains, however, the difficulty of 
 silpposing that every grain of urea which passes from the body 
 after a rich proteid meal is the issue of the metabolism of a 
 quantity of proteid previously existing as an integral part of some 
 tissue unit ; in other words, it seems unlikely that, simply as the 
 result of such a meal, the actual living proteid framework of the 
 body should be so largely renewed. Moreover the contrast between 
 that part of the daily urea which is variable and fluctuating and 
 that part which is more constant has to be explained. Hence has 
 arisen the view that the sources of urea are twofold, corresponding 
 to the metabolism of two distinct categories of the proteids of the 
 body. On the one hand, part of the urea, especially that which 
 appears as the immediate result of food, is supposed to be derived 
 from the metabolism of what we defined above as "circulating 
 proteids;" while, on the other hand, a certain (presumably smaller) 
 portion is rea,lly due to the metabolism of the "tissue-proteids," 
 i. e. of the actual living framework of the body. 
 
 We must not attempt to discuss this view at length, and indeed 
 our knowledge is inadequate for the purpose. We have not as yet 
 a measure of the rate at which the metabolism of the living unit 
 does or may take place, and the arguments in favour of a metabolism 
 of cii'culating proteids are, at present, of a very indu-ect character. 
 
Cmm'. V.J MrniTiox. 4.^3 
 
 iMon-ovcT it is pdsslhli' that tli<' \\\\n^\ |intU'i(l lactabuli.sm iiuli -at.- I 
 by the givat increase ot" urea whii-li follows upon a meal rich in 
 jn'oteids, may be, as we have hinteil, merely a destructive di^^estiou 
 of the proteiils while they still are retained in the alimentary 
 canal. 
 
 We may however call attention to a possible analogy between 
 the iiistory of proteids anil that of fats and carbohydrates. The 
 uniform comj)osition of the ])loo(,l, which tlie body seems ever 
 striving to maintain, probably applies to its proteids as well as 
 to its other constituents. We have seen that a surplus of non- 
 nitrogenous materials in the blood is withdrawn from the circula- 
 tion and stored uj) as fat or glycogen, and it is possible that an 
 excess of proteids might similarly l)e stored up in some tissue or 
 tissues, though i'rom the facts i)reviously mentioned it is obvious 
 that tile power of storage is far less than in the case of fats 
 and carbohydrates. Such a store of protcid matter would re- 
 present a sort of circulating protcid, but nevertheless for its final 
 metabolism might have to form an integral part of some living 
 tissue unit. 
 
 The Eflfects of Fatty and of Carbohydrate Pood. Unlike 
 those of proteid food, the effects of fats antl carbohydrates cannot 
 be studied alone. When an animal is fed simply on non-nitro- 
 genous food, death soon takes place ; the food rapidly ceases to be 
 digested, and starvation ensues. We can therefore only study the 
 dietetic etiects of these substances when they arc taken together 
 with proteid material. 
 
 When a small quantity of fat is taken, in company with a fixed 
 moderate quantity of proteid material, the whole of the carbon, of 
 the food reappears in the egcsta. No fat is stored up ; some even 
 of the previously existing fat of the body may be consumed. As 
 the i'at of the meal is increased, a point is soon reached at which 
 carbon is retained in the body as fat. So also with starch or sugar; 
 when the quantity of this is small, there is no retention of carbon; 
 as soon however as it is increased beyond a certain limit, carbon is 
 stored up in the form of fat cr, to a smaller extent, as glycogen. 
 Fats and carbohydrates therefore difTer markedly from proteid food 
 in that they are not so distinctly provocative of metabolism. This 
 is exceedingly well shewn in the results of Lawes and Gilbert, for 
 in the pig jfreviously mentioned 472 parts of fat were laid on for 
 every 100 parts of fat taken as such in the food (which consisting 
 of barley-meal, &c. contained a very small amount of actual fat), 
 while of every 100 paits of the total dry non-nitrogenous food 
 including fat, starch, cellulose, tSrc. no less than 21'2 parts were 
 retained in the body in the form of fat. No clearer proof 
 than this could be afforded that fat is formed in the body out of 
 something which is not fat. 
 
 As one might imagine, the presence of fat or carbohydrates in 
 the food is found to decrease the amount of proteid metabolism 
 
454 FATTY AXD CAltBOHYDRATE FOOD. [Book ii. 
 
 necessary to establish nitrogenous equilibrium. For instance, with 
 a diet of 800 grms. meat and 1-50 gTms. fat, the nitrogen in the 
 egesta became equal to that in the ingesta in a dog, in whose case 
 1800 gi-ms. meat had to be given to i^roduce the same result 
 in the absence of fats or carbohydrates. 
 
 On the other hand, it was found, that vath a fixed quantit}^ of 
 fatty or carbohydrate food, an increase of the accompanjong proteid 
 led not to a storing up of the surplus carbon contained in the extra 
 quantity of proteid, but to an increase in the consumption of 
 carbon. Proteid food increases not only proteid but also non- 
 nitrogenous metabolism. This explains how an excess of proteid 
 food may, by the increase of metabolism, actually reduce the fat of 
 the body. 
 
 There can be no doubt then that both a proteid diet and a 
 carbohydi'ate diet may give rise to the formation of fat within the 
 body. And the question which we have already (p. 427) partly 
 discussed comes again before us, In what way is this fat so 
 formed ? Is the sugar, arising during digestion from the carbo- 
 hydrate, converted by a series of fermentative changes into fat ? Or 
 is the sugar directly consumed by the tissues in oxidative changes, 
 by which means the fatty derivatives of the metabolized proteids 
 are sheltered from oxidation and stored up as fat ? This is a vexed 
 question which has been hotly debated. Many observers hold 
 strongly to the latter view, and hence contend that all fattening 
 food must contain a supply of proteids adequate to provide, by 
 their decomposition, the carbon of the fat which it is desired to lay 
 up. The balance of evidence, however, seems to be in favour of 
 the view that carbohydrates may be, in some way, directly converted 
 into fat and that therefore fattening foods need not necessarily 
 contain any such definite proportion of proteids. 
 
 We have at present no exact information concerning the 
 nutritive differences between fats and carbohydrates, beyond the 
 fact that in the final combustion of the tu'O, while carbohydrates 
 require sufficient oxygen to combine with their carbon only, there 
 being ah-eady sufficient oxygen in the carbohydrate itself to form 
 water ^^'ith the hydrogen present, fats require in addition oxygen 
 to burn off some of theii' hydrogen. Hence in herbivora a larger 
 portion of the oxygen consumed reappears in the carbonic acid of 
 the egesta, than in carnivora, where more of it leaves the body as 
 formed water : the proj)ortions of the oxygen in the carbonic acid 
 expired to the oxygen consumed being on an average 90 p. c. in 
 the former and 60 jd. c. in the latter. When a herbivorous animal 
 starves, it feeds on its own fat, and under these circumstances the 
 oxygen proportion in the expired carbonic acid falls to the car- 
 nivorous standard. The carbohydrates are notably more digestible 
 than the fats, but on the other hand the fats contain more potential 
 energy in a given weight. As to the dietetic or rather metabolic 
 difference between starch and sugar, we know nothing very definite ; 
 
CiiAi". V.J NUTUlTlo.W 455 
 
 it lius biLii tlmuglit liiiwrvL-r tliiit (.aiic-sut^ar is rutlicr iiKjru 
 fattening than starch. 
 
 The Effects of Gelatine Food. Jt is a nialtt-r of common 
 cxpeiifiKf that nflniin,' will not siijjply the place of proteids as a 
 constituent of food. Animals fed on gelatine together with fat or 
 carbohydrates die very nuieli in the same way as when they are fed 
 on non-nitrogenous material alone. Nevertheless it would appear, 
 as miglit 1)0 expected, that tlic presence of gelatine in food is not 
 without efieet. Thus nitrogenous Cfpiilibrium is estahlislied at a 
 lower level of real proteid iood when gelatine is added. In a dog, 
 moreover, fed on a diet of gelatine and fat the excess of nitrogen 
 in the excreta over that in the ingosta is less, than Avhen the same 
 dog is fed on a diet of fat alone ; that is to say, the gelatine has 
 sheltered from metabolism some proteid constituents of the body ; 
 and the consumption of fat also seems to bo lessened by the presence 
 of gelatine. These facts become intelligible if ^ve suppose that 
 gelatine is rapidly split up into a urea and a fat moiety, in the 
 same way that Ave have seen a certain quantity of proteid material 
 to be. It is this direct destructive metabolism of proteid matter 
 which gelatine can take up ; it seems however unable to imitate 
 the other function of proteid matter, and to take part in the 
 formation of living protojilasm. What is the cause of this differ- 
 ence, we cannot at present say. 
 
 The Effects of Salts as Food. All food contains, besides the 
 potential substances which we have just studied, certain saline 
 matters, organic and inorganic, having in themselves little or no 
 latent energy, but yet either absolutely necessary or highly bene- 
 ficial to the bod}'. These must have important functions in 
 directing the metabolism of the body : the striking distribution of 
 them in the tissues, the preponderance of sodium and chlorides in 
 blood-serum and of potassium and i^hosphates in the red coi-puscles 
 for instance, must have some meaning ; but at present Ave are in 
 the dark concerning it. The element phosphorus seems no less 
 important from a biological point of vieAv than carbon or nitrogen. 
 Jt is as absolutely essential for the gi-oAvth of a lowly being like 
 Penicillium as for man himself. We find it probably plapng an 
 important part as the conspicuous constituent of lecithin, Ave find 
 it peculiarly associated Avith the proteids ; but Ave cannot explain its 
 role. The element sulphur, again, is only second to phosphonis, and 
 Ave find it as a constituent of nearly all proteids ; but Ave cannot 
 tell Avhat exactly Avould happen to the economy if all the sulphur 
 of the food Avere Avithdi-aAvn. We knoAv that the A'arious saline 
 matters are essential to health, that Avhen they are not present in 
 proper proportions, nutrition is affected, as is sheAvn by certain 
 forms of scurvy ; aa'O are also aAvare that the properties and 
 reactions of various proteid substances are closely dependent on the 
 presence of certain salts ; but beyond this Ave know very little. 
 
456 GELATINE AS FOOD. [Book ii. 
 
 Lastly, water has an effect on metabolism, as shewn by the fact 
 that when the water of a diet is increased, the urea is increased to 
 an extent beyond that v/hich can be explained by the increase of 
 fluid increasing' the facilities of mere excretion. 
 
SEC. 4. THE ENERGY OF THE BODY. 
 
 Broadly speaking, the animal body is a machine for converting 
 potential into actual energy. The potential energy i.s supplied by 
 food ; this the metabolism of the body convert.s into the actual 
 energy of heat and mechanical labour. We have in the present 
 section to study what is known of the laws of this conversion, and 
 of the distribution of the energy set free. 
 
 TJie Income of Energy. 
 
 Neglecting all subsidiary and unimportant sources of cuerg}', 
 we may say that the income of animal energy consists in the 
 oxidation of food into its waste products, viz. the oxidation of 
 proteids, fats and carbohydrates into urea, carbonic acid and water. 
 A principle laid down by the chemist teaches that the potential 
 energy of any body, considered in relation to any chemical change 
 in it, is the same when the final result is the same, whether that 
 result be gained at one leap or by a series of steps ; that, for 
 instance, the energy set free by the oxidation of 1 grm. of fat into 
 carbonic acid and water is the same, whatever the changes forwards 
 or backwards which the fat undergoes before it finally reaches the 
 stage of carbonic acid and water ; and similarly, that the energy- 
 available for the body in 1 grm. of dry jsroteid is the energy given 
 out by the complete combustion of that 1 grm., less the energy 
 given out by the complete combustion of that quantity of urea to 
 which the 1 gi-m. of proteid gives rise in the body. Taking this as 
 
458 THE EXPENDITURE OF ENERGY. [Book ir. 
 
 our guide we may easily calculate the total energy of any diet. The 
 following determinations, expressed both in gramme-degree (centri- 
 grade) units of heat, and kilogramme-metre units of work, may 
 serve as data. 
 
 The direct oxiclatiou of the 
 following, dried at 100" C. 
 
 1 grm. Beef-fat 
 1 grm. Butter 
 
 gives : 
 
 iis9 to 
 
 gram.-deg. 
 
 kilo.-met, 
 
 9069 
 
 3841 
 
 7264 
 
 3077 
 
 3912 
 
 1657 
 
 5103 
 
 2161 
 
 2206 
 
 934 
 
 1 grm. Arrowroot 
 
 1 grm. Beef-muscle purified with ether 
 
 1 grm. Urea 
 
 Supposing that all the nitrogen of j)i'oteid food goes out as 
 urea, 1 grm. of dry proteid, such as dried beef-muscle, would give 
 rise to about J grm. of urea ; hence 
 
 gram.-deg. kilo.-met. 
 1 grm. Proteid 5103 2161 
 
 less 
 I grm. Urea 735 311 
 
 would give as 
 Available energy of Proteid 4368 1850 
 
 In a normal diet, such as Ranke's, p. 446, would be found : 
 
 gram.-deg. kilo.-met. 
 
 100 grm. Proteid 436800 185000 
 
 100 grm. Fat 906900 384100 
 
 240 grm. Starch 938880 397680 
 
 Total Income 2281580 966780 
 
 or, in round numbers, one million kilogramme-metres. 
 
 The Expenditure. 
 
 There are only two ways in which energy is set free from the 
 body : mechanical labour and heat. The body loses energy in 
 producing muscular work, as in locomotion, in all kinds of labour, 
 in the movements of the air, in respiration and speech, and, though 
 to a hardly recognizable extent, in the movements of the air or 
 contiguous bodies by the pulsations of the vascular system. The 
 body loses energy in the form of heat by conduction and radiatioir, 
 by respiration and persj)iration, and by the warming of the urine 
 and faeces. All the internal work of the body, all the mechanical 
 labour of the internal muscular mechanisms with their accompany- 
 ing friction, all the molecular labour of the nervous and other 
 tissues, is converted into heat before it leaves the body. The 
 
cuAi'. v.) NVTh'rnux. 4r,y 
 
 most intiMisc niL-iital iiftion, iiiiacniiiijtaiiii'il hyaiiy imisciilar mani- 
 I'cstatioiis, tho most oncrgi'tic action of tlu; lu'art or of the bowels, 
 with the sliglit exceptions mentioned above, the busiest activity (jf 
 the secreting or metabolic tissues, all tliese end simply in aug- 
 menting the expenditure of income in the form of heat. 
 
 A normal daily expenditure in the way of mechanical labour 
 can bo easily determined b}' observation. Wlicthcr the work take 
 on the form of walking, or of driving a machine, or of any kind 
 of nniscular toil, a good day's work may be put down at about 
 150,000 kilogrammc-mctrcs. The normal daily expenditure in 
 the way of heat cannot be so readily determined. Direct calori- 
 metric observations arc attended with this difficulty, that the body 
 while within the calorimeter is placed in abnormal conditions, 
 which produce an abnormal metabolism. Hence results arrived at 
 by this method arc of little value unless they be accompanied by a 
 comparison of the egesta and ingesta, so that the rate and nature 
 of the metabolism going on may be known. Many attempts have 
 been made to calculate the amount in an indu-ect manner. As 
 trustworthy as any is the plan of simply subtracting the normal 
 daily mechanical exj)enditure from the normal daily income. Thus, 
 150,000 k.-m. subtracted from one million k.-m. gives 850,000 k.-ni. 
 as the daily expenditure in the form of heat; i.e. between one-fifth 
 and one-sixth of the total income is expended as mechanical labour, 
 the remaining four-fifths or five-sixths leaving the body in the 
 form of heat. 
 
 The Sources of Muscular Energ-y. Liebig, satisfied with 
 having proved that the animal body was constructive as far as the 
 formation of fat was concerned, still held to the distinction between 
 nitrogenous or plastic and non-nitrogenous or respiratory food. 
 Put broadly, his -sdew was that all the nitrogenous food went to 
 build up the proteid tissues, the muscular flesh, and other forms 
 of protoplasm, and that the nitrogenous egesta arose solely from 
 the functional metabolism of these tissues, while the non- 
 nitrogenous food was used with equal exclusiveness for respiratory 
 or calorific purposes, being either directly oxidized in the blood or, 
 if present in excess, stored up as fatty tissue. According to him 
 the two classes of income corresponded exactly to the two forms of 
 expenditure. We have already urged several objections against 
 this view. We have seen that in the blood itself very little 
 oxidation takes place, that it is the active tissue, and not the 
 passive blood-plasma, which is the seat of oxidation. We have 
 further seen that proteid food may undoubtedly be in Liebig's 
 sense respiratory, and incidentally give rise to the storing-up of 
 fat. One division of Liebig's view is thereby overthro\m. We 
 have now to inquire whether the other division holds good, whether 
 muscle or other protoplasm is fed exclusively on the proteid 
 material of food, and whether muscular energy comes exclusively 
 
460 THE EXPENDITURE OE ENERGY. [Book ii. 
 
 from the metabolism of the proteid constituents of muscle. We 
 have already seen (p. 70) that when the muscle itself is examined, we 
 find no proof of nitrogenous waste, but, on the other hand, clear 
 evidence of the production of non-nitrogenous bodies, such as 
 carbonic acid. And when we ask the question. Does muscular 
 exercise increase the urea given off by the body as a whole ? for 
 this, according to Liebig's theory, it certainly ought to do, the 
 evidence we can obtain, though somewhat conflicting, gives on the 
 whole a decidedly negative answer. Exercise, even severe, appears 
 not necessarily to increase the urea of the urine. 
 
 More than this, the experience of Fick and Wislicenus lands us 
 in an absurdity if we suppose the whole energy of muscular work 
 to arise from proteid metabolism. These observers performed a 
 certain amount of work (an ascent of the Faulhorn) on a non- 
 nitrogenous diet, and estimated the amount of urea passed during 
 the period. Assuming the urea to represent the oxidation of so 
 much proteid matter, which oxidation represented in turn so much 
 energy set free, they found that whereas the actual work done 
 amounted to 120 026 and 148"656 kilogram .-kilometres, for each 
 respectively, the total energy available from proteid metabolism 
 during the period was in the case of the first 68"G9, and of the 
 second 68'376 kilogram.-kilometres. That is to say, the energy 
 set free by the proteid metabolism of the muscles engaged 
 in the work was at the most far less than that necessary to 
 accomplish the work actually done, besides having to provide 
 for the movements of respiration and circulation. Their muscular 
 energy therefore must have had other sources than proteid meta- 
 bolism. 
 
 That on the contrary the production of carbonic acid is at once 
 and largely increased by muscular exercise is beyond all doubt. 
 One hour's hard labour will increase fivefold the quantity of 
 carbonic acid given off within the hour. And in an experiment 
 directed to this point it was found that a man in 24 hours con- 
 sumed 954 grms. oxygen and produced 1284 gims. carbonic acid 
 when doing v/ork, as against 708 grms. oxygen consumed and 
 911 grms. carbonic acid produced when remaining at rest, the 
 quantity of urea secreted being in the first case 37 grms., in the 
 second 37'2 grms. 
 
 It is evident that the conclusions arrived at by the statistical 
 method entirely cori-oborate those gained by an examination of 
 muscle itself, viz. that during muscular contraction an explosive 
 decomposition takes place, the non-nitrogenous products of which 
 alone escape from the muscle and from the body, any nitrogenous 
 products which result being retained within the muscle. We must 
 therefore reject the second as well as the first division of Liebig's 
 view; not only is the muscle not fed exclusively on proteid 
 material, but also its energy does not arise from an exclusively 
 proteid metabolism. 
 
Cii.vr. v.] yrTUITloX. 101 
 
 The Sources and Distribution of Heat. \\'«' liav(- jiln;uly 
 seen th;it iho conception ot iIh' non-nitro^cnous jiortions of food 
 being solely caloritiicioiit or nspiratory proves to 1)0 unfounded 
 when we attempt to trace the history of the food on its way 
 throtit^h the body. The same view is still more strikin;,dy shewn 
 to he inadecpiate when we study the manner in which the heat 
 of the body is produced. Wc may indeed at once alHrm tliat 
 the heat of the body is generated by the oxidation, not of any 
 particular substances, but of the tissues at large. Wherever 
 metabolism of protoplasm is going on, heat is being set free. In 
 growth and in rejiair, in the deposition of new material, in the 
 transformation of lifeless pabuhnn into living tissue, in the con- 
 structive metabolism of the body, lieat may be umloubtedly to 
 a certain extent absorbed and rendered latent: the energy of the 
 construction may be, in part at least, sui)plied by the heat present. 
 But all this, and more than this, viz. the heat present in a potential 
 form in the substances themselves so built up into the tissue, is lost 
 to the tissue during its dcstnictivc metabolism; so that the whole 
 metabolism, the whole cycle of changes from the lifeless pabulum 
 through the living tissue back to the lifeless products of vital 
 action, is eminently a source of heat. 
 
 Of all the tissues of the body the muscles not only from their 
 bulk, forming as they do so large a jjortion of the whole frame, but 
 also from the characters of their metabolism, must bo regarded as 
 the chief sources of heat. 
 
 In treating (p. 70) of the thermal changes in muscle we have 
 seen that in the total energy expended in a muscular contraction, 
 the ratio of that which appears as heat to that which ap})ear3 
 as external work is variable. If Ave take what is somewhat below 
 the mean result and assume that the energy involved in the work 
 done in a muscular contraction is about one-tenth of the total 
 energy expended, the rest going out as heat, then, upon the 
 calculation that the total external Avork of the body is about 
 one-fifth of the total energy set free in the body, it is clear that the 
 heat given out by the muscles, even at those times only Avhen they 
 are contracting, must form a very large part of the total heat 
 given out by the body. But the skeletal muscles, though 
 frequently, are not continually contracting; they have j^^riods, 
 at times long periods, of rest ; and during these periods of rest, 
 metabolism, of a subdued kind it is time, but still a metabolism 
 involving an expenditure of energy, is going on. This quiescent 
 metabolism must also give rise to a certain amount of heat ; and if 
 we add this amount, Avhich in the present state of our knoAvledgc 
 we cannot exactly gauge, to that gi\cn out during the movements 
 of the body, it is very clear, even in the absence of exact data, that 
 the metabolism of the muscles must supply a very large proportion 
 of the total heat of the body. They are par excellence the thermo- 
 genic tissues. 
 
462 ANIMAL HEAT. [Book ii. 
 
 Next to the muscles in importauce come the various secreting 
 glands. In these the protojDlasm, at the periods of secretion at all 
 events, is in a state of metabolic activity, which activity as else- 
 where must give rise to heat. In the case of the salivary gland of 
 the dog the temperature of the saliva secreted during stimulation 
 of the chorda has been found to be as much as 1° or 1".5° higher 
 than that of the blood in the carotid artery at the same time, and 
 in all probability the investigation of other secreting glands would 
 lead to similar results. Of all these various glands, the liver 
 deserves special attention on account of its size and large supply 
 of blood, and because it appears to be continually at work. We 
 find indeed that the blood in the hepatic veins is the warmest 
 in the body. Thus in the dog a temperature of 40'73'' C. has been 
 observed in the hepatic vein, while that of the vena cava mferior 
 was 38-3o° to 39-58, and that of the right heart 377. The fact 
 that the blood of the hepatic vein is warmer than that of either 
 the portal vein or the aorta, shews that the increased temperature 
 is not due simply to the liver being far removed from the surface of 
 the bod}^ 
 
 The brain too may be regarded as a source of heat, since its 
 temperature is higher than that of the arterial blood with which it 
 is supplied; though from the smaller quantity of blood passing 
 through its vessels it cannot in this respect compare with either the 
 liver or the muscles as a source of heat to the body. 
 
 The blood itself cannot be regarded as a source of any 
 considerable amount of heat, since, as we have so frequently 
 urged, the oxidations or other metabolic changes taking place 
 in it are comparatively slight. The heat evolved by the in- 
 different tissues such as bone, cartilage and connective tissue 
 may be passed over as insignificant ; and we cannot even regard 
 the adipose tissue as a seat of the production of heat, since the fat 
 of the fat-cells is in all probability not oxidized in situ but simply 
 carried av/ay from its place of storage to the tissue which stands in 
 need of it, and it is in the tissue that it undergoes the metabolism 
 by Avhich its latent energy is set fi'ee. Some amount of heat is also 
 produced by the changes which the food undergoes in the ali- 
 mentary canal before it really enters the body. 
 
 Hence taking a survey of the whole body we may conclude 
 that since metabolism is going on to a greater or less extent every- 
 where, heat is everywhere being generated ; but that, looked at 
 from a quantitative point of view, the muscles and the glandular 
 organs must be regarded as the main sources of the heat of the 
 body, the muscles being in all probability the more important of 
 the two. 
 
 But heat, whila being thus continually produced, is as con- 
 tinually being lost, by the skin, the lungs, the urine and the fseces. 
 The blood passing from one part of the body to the other, and 
 carrying warmth from the tissues where heat is being rapidly 
 
Chai'. v.] NUrniTloX. 4G3 
 
 oreMcriitnl, lo tlif tissurs it i>rL;;uis wliurc heat is bciiij,' lost by 
 radiation, comluctiou or cvajjuratiuii, tends to ('(jualizu the tenijx-ra- 
 turo of the various parts, and thus maintains a "constant bodily 
 temperature." 
 
 When the j»rti(hictii>n of lu-at is not f,a-rat as compared with the 
 loss there is no Lfroat aceunudation of iieat within the body, the 
 temperature of whirli consetjuently is but sli^ditly raised above 
 that of surrounding^ objects. Thus the temperature of the frog, for 
 instance, is rarely more than "O-i" to "Oo" C. above that of the atmo- 
 sphere, though in the breeding season the difference may amount 
 to 1". Such animals, and they comprise all classes except birds 
 and mammals, are sjwken of as cold-blooded. Exceptions among 
 them are not uncommon. Some tish, such as the tunny, are warmer 
 than the water in which tliey live, and in a species of Python {P. 
 bicitfatus) a difterence of as much as 12" C. has been observed. 
 Hliber found that in a beehive the temperature rose at times as 
 much as to 40'' C. In the so-called warm-blooded animals, birds 
 and mannnals, the loss and production of heat are so balanced 
 that the temperature of the body remains constant at, in round 
 numbers, 35" or 40" C, whatever be the temperature of the air. 
 The temperature of man is about ST'G" C ; in some birds it is as 
 high as 44" C. (Hirundo) and in the "wolf it is said to be as low as 
 35-24' C. 
 
 This temperature is vdih slight variations maintained through- 
 out life. After death the generation of heat rapidly diminishes, 
 and the body isp3edily becomes cold; but for some short time 
 immediately following upon systemic death, a rise of temperature 
 may be observed, due to the fact that, while the metabolism of the 
 tissues is still going on, the loss of heat is somewhat checked by 
 the cessation of the circulation. The onset of pronounced rigor 
 mortis causes a marked accession of heat, and when occurring after 
 certain diseases may give rise to a very considerable elevation of 
 temperature. 
 
 This mean bodily temperature of warm-blooded animals is, 
 during health, maintained, with slight variations of which we shall 
 presently speak, -within a very narrow margin, a rise or indeed a 
 fall of much more than a degree above or below the limit given 
 above being indicative of some failure in the organism, or of some 
 unusual influence being at work. It is evident, therefore, that the 
 mechanisms which co-ordinate the loss with the production of heat 
 must be exceedingly sensitive. It is obvious, moreover, that these 
 mechanisms may act when the bodily temperature is tending to 
 rise, by either checking the production or by augmenting the loss 
 of heat ; and when the bodily temperature is tending to fall, by 
 either increasing the production or by diminishing the loss of heat. 
 As the regulation of temperature by variations in the loss of heat 
 is better known than regulation by vai'ia,tion§ in production, it will 
 be best to consider this first. 
 
464 AXniAL HEAT. [Book ii. 
 
 Regnlation by variations in loss. Heat is lost to the body 
 by the warming of the feces and of the urine, by the Avarming of 
 the expired air, by the evaporation of the water of respiration, by 
 conduction and radiation from the skin, and by the evaporation 
 of the water . of perspiration. It has been calculated that the 
 relative amounts of the loss by these several channels are as 
 follows : In warming the faeces and urine about 3, or according to 
 others 6 per cent. By respiration about 20, or according to others 
 about 9 only per cent., leaving 77, or alternatively 85, per cent, for 
 conduction and radiation and evaporation by the skin. 
 
 The two chief means of loss then, which are at all susceptible 
 of any great amount of variation, and which can be used to regu- 
 late the temperature of the body, are the skin and the lungs. 
 
 The more air passes in and out of the lungs in a given time, 
 the greater will be the loss in warming the expired air, and in 
 cvajDorating the water of respiration. And in such animals as the 
 dog, which do not perspire freely by the skin, respiration is a most 
 important means of regulating the . temperature. The changes 
 which give rise to this loss take place before the inspired air 
 reaches the pulmonary alveoli ; both the warming and the evapo- 
 ration being effected in the nasal and pharyngeal, and to some 
 extent in the bronchial passages. Some observers have maintained 
 that the left side of the heart is warmer than the right, and hence 
 argued that chemical changes leading to a considerable develop- 
 ment of heat take place in the pulmonary capillaries. It would 
 appear however that the right ventricle, owing to its lying 
 nearer to the liver, the high temperature of which has already 
 been mentioned, is in reality rather hotter than the left. And 
 indeed we have no satisfactory evidence of any large amount of 
 heat being produced by any pulmonary metabolism. 
 
 The great regulator however is undoubtedly the skin. The 
 more blood passes through the skin the greater will be the loss of 
 heat by conduction, radiation, and evaporation. Hence, any action 
 of the vaso-motor mechanism which, by causing dilation of the 
 cutaneous vascular areas, leads to a larger flow of blood through 
 the skin, will tend to cool the body; and conversely, any vaso- 
 motor action which, by constricting the cutaneous vascular areas, 
 or by dilating the splanchnic vascular areas, causes a smaller flow 
 through the skin, and a larger flow of blood through the abdominal 
 viscera, will tend to heat the body. Besides this the special 
 nerves of perspiration will act directly as regulators of temperature, 
 increasing the loss of heat Vv'hen they promote, and lessening the 
 loss when they cease to promote, the secretion of the skin. The 
 working of this heat-regulating mechanism is well seen in the case 
 of exercise. Since every muscular contraction gives rise to heat, 
 exercise must increase for the time being the production of heat ; 
 yet the bodily temperature rarely rises so much as a degree centi- 
 grade, if at all. By the exercise the respiration is quickened, and 
 
CiiAP. v.] NUTRITION. 465 
 
 the loss of lioat by the lungs incroasod. The circulation of blood 
 is also quickeueil, and the cutaneous vascular areas becoinin<^ 
 dilated, a lar<,'er amount of blood passes through the skin. Add«.-(1 
 to this, the skin persinres freely. Thus a large amount of heat is 
 lost to the body, sutticient to neutralise the addition caused by the 
 muscular contraction, the increase \Yhich the more rapid flow of 
 blood through the abdominal organs might tend to bring about 
 being more than suthciently counteracted by their smaller supply 
 for the time. The sense of warmth which is felt during exercise 
 in consequence of the Hushing of the skin, is in itself a token that 
 a regulative cooling is being carried on. In a similar way the ap- 
 plication of external cold or heat, either partially or completely, 
 defeats its own ends. Under the influence of external cold the 
 cutaneous vessels are constricted, and the splanchnic vascular 
 areas dilated, so that the blood is Avithdrawn from the colder and 
 cooling regions to the hotter and heat-producing organs. This 
 vascular change may be used to explain the fact that stripping 
 naked in a cold atmosphere often gives rise to a distinct increase in 
 the mean temperature of the blood, as indicated by a thermometer 
 placed in the mouth, though possibly the effect may be partly 
 due to an actual increase of the production of heat. Under the 
 influence of external warmth, on the other baud, the cutaneous 
 vessels are dilated, a rapid discharge of heat takes place ; and if 
 the circumstances be such that the body can perspire freely, and 
 the perspiration be readily evaporated, the temperature of the 
 body may remain very near to the normal, even in an excessively 
 hot atmosphere. Thus, more than a century ago, Drs Fordyce and 
 Blagden were able to remain Avith impunity in a chamber heated 
 even to 127'' (260" Fahr.), and with ease in one so hot, that it 
 became painful for them to touch the metal buttons of their 
 clothing. It is unnecessary to give any more examples of this 
 regulation of temperature by variations in the loss of heat ; they 
 all readily explain themselves. 
 
 E/egidation by variations in production. It is not however 
 solely by variations in the loss of licat that the constant tempera- 
 ture of the warm-blooded animal is maintained. A^'ariations in the 
 amount of heat actually generated in the body constitute an 
 important factor not only in the maintenance of the normal 
 temperature, but also probably in the production of the abnormally 
 high or low temperatures of various diseases. Many considerations 
 have long led physiologists to suspect the existence of a nervous 
 mechanism by which afferent impulses arising in the skin or 
 elsewhere might through the central nersous system originate 
 efferent impulses Avhose effect would be to increase or diminish the 
 metabolism of the muscles or other organs and thus to increase or 
 diminish the amount of heat generated for the time being in the 
 body. The existence in fact of a metabolic or thermogenic ucn-ous 
 mechanism, comparable in many respects to the vaso-motor 
 
 F. 30 
 
466 ANIMAL HEAT. [Book ii. 
 
 mechanism or to the various secreting nervous mechanisms, seems 
 in itself d iDriori probable. And we have experimental evidence 
 that such a mechanism does really exist. 
 
 The warm-blooded animal is distinguished from the cold-blooded 
 animal by the fact that when it is exposed to cold or heat, it does not 
 like the latter become colder or hotter, as the case may be, but, 
 within certain limits, maintains its normal temperature. If the 
 maintenance of the temperature of the warm-blooded animal during 
 exposure to cold is assisted by an increased production of heat and 
 is not due simply to a diminished loss, we ought to find evidence of 
 an increased metabolism during that exposure. We ought to find 
 under these circumstances an increased production of carbonic 
 acid, and an increased consumption of oxygen, since it is to these 
 products, rather than to the nitrogenous factors, on the peculiar- 
 ities of which as uncertain signs of metabolism we have already 
 insisted, we must look for indications of the rise or fall of metabolic 
 activity. Of these two, the production of carbonic acid and the 
 consumption of oxygen, the latter is the more important and 
 trustworthy measure of metabolism, especially when observations 
 are made for short periods only at a time ; for as we have seen in 
 treating of respiration the exit of carbonic acid is more closely 
 dependent on the action of the respiratory mechanism than is the 
 income of oxygen, and carbonic acid can be retained in loose 
 combination and so temporarily stored up by various constituents 
 of the body. 
 
 Taking then the consumption of oxygen, and though with less 
 confidence the production of carbonic acid, as a measure of metabolic 
 activity and so of heat-production, Pfliiger, and his pupils, as well as 
 other observers, have shewn that a marked contrast in this respect 
 exists between cold-blooded and warm-blooded animals exposed to 
 changes of temperature. In the cold-blooded animal, cold dimin- 
 ishes and heat increases the metabolic activity of the body ; as the 
 temperature to which the animal is subjected rises or falls, so the 
 consumption of oxygen and production of carbonic acid is increased 
 or lessened. The body of a cold-blooded animal behaves in this 
 respect like a mixture of dead substances in a chemist's retort : 
 heat promotes and cold retards chemical action in both cases. 
 Very different is the behaviour of a warm-blooded animal. In 
 this case, within a lower and a higher limit, cold increases and heat 
 diminishes the bodily metabolism, as shewn by the increased or 
 diminished consumjotion of oxygen and production of carbonic 
 acid as the temperature falls or rises. In these animals there is 
 obviously a mechanism of some kind, counteracting and indeed 
 overcoming those more direct effects, which alone obtain in cold- 
 blooded animals. And that this mechanism is of a nervous nature, 
 is indicated by the following facts. 
 
 When an animal is poisoned by urari, the temperature falls 
 and the metabolism, measured by the consumption of oxygen and 
 
CiiAi'. v.] NUTRITION. 4G7 
 
 the j^rocluetion of carbonic acid, sinks also ; and that the latter is 
 the cause not the elVect of the former is shewn by the fact that 
 the niet;ii)oli.sni continues to fall thouf^h loss of heat be prevented 
 by surroundint; the animal with wrappin^js of cotton wool. In 
 such a urarized animal, exposure to hit(her temperatures augments 
 and exjiosurc to lower temperatures diminishes metabolism ; the 
 urarized warm-blooded animal in fact behaves like a cold-blooded 
 animal. Similar, but perhaps not such striking results arc gained 
 by division of the medulla oblongata. After this operation the 
 temperature of the body sinks, and the fall, though partly due to 
 increased loss of heat by the skin, caused by the dilated condition of 
 the cutaneous vessels, is also accompanied by diminished metabo- 
 lism and is therefore in part due to diminished production of heat. 
 And when an animal is in this condition, exposure to higher tem- 
 peratures increases and exposure to lower temperatures diminishes 
 the bodily metabolism. We can best explain these results by 
 supposing that, under normal conditions, the muscles, which as we 
 have seen contribute so largely to the total heat of the body, are 
 placed, by means of their motor nerves and the central nervous 
 system, in some special connection with the skin, so that a lowering 
 of the temperature of the skin leads to an increase, while a heigh- 
 tening of the temperature of the skin leads to a decrease, of the 
 muscular metabolism. Further, though the matter has not yet 
 been fully worked out, the centre of this thermotaxic reflex mecha- 
 nism appears to be placed above the medulla oblongata, possibly in 
 the region of the pons varolii. When urari is given, the reflex 
 chain is broken at its muscular end ; when the spinal cord is divided 
 the break is nearer the centre. Whether we should conclude that 
 the working of this reflex mechanism is of such a kind that cold to 
 the skin excites the centre to a heat-producing activity, or of such 
 a kind that warmth to the skin inhibits a previously existing auto- 
 matic activity of the centre, may be left for the present undeter- 
 mined. 
 
 We ma}^ add that the muscular metabolism which thus helps 
 to regulate temperature need not involve visible muscular con- 
 tractions, though the heat given out by a muscle will be temporarily 
 increased at every contraction; and that the regulative nervous 
 mechanism may apparently be overborne by an exjDosure to too 
 great heat or cold. When for instance the cold to which the 
 animal is exj^osed becomes excessive, the reaction of the thermo- 
 taxic nervous system is powerless against the depressing direct 
 effects, and the metabolism, together with the temperature, sinks. 
 
 Lastly, we have increasing evidence that the jihenomena of 
 fever are due, not merely to a derangement of the regulation by 
 loss, though this may be a factor, but also, and indeed chiefly, 
 to an increased production of heat ; for in fever, the production of 
 carbonic acid, and the consumption of oxygen, that is to say, the 
 metabolic changes of the tissues, are increased. 
 
 30—2 
 
468 ANIMAL HEAT. [Book ii. 
 
 We may regard it then as established that such a thermotaxic 
 nervous mechanism does exist, and the importance of such a 
 mechanism in explaining not only the maintenance of the nor- 
 mal temperature but the abnormal variations of temperature in 
 disease can hardly be exaggerated. Much however still requires 
 to be learnt before we can speak with full confidence as to its 
 exact nature, or expound with certainty the details of its work. 
 
 By regulative mechanisms of this kind the temperature of the 
 warm-blooded animal is maintained within very narrow limits. In 
 ordinary health the temperature of man varies between 36" and 
 88", the narrower limits being 36'25" and 37'5**, when the thermo- 
 meter is placed in the axilla. In tha mouth the reading of the 
 thermometer is somewhat (•25" to I'o®) higher; in the rectum it is 
 still higher (about "O") than in the mouth. The temperature of 
 infants and children is slightly higher and much more susceptible 
 of variation than that of adults, and after 40 years of age the 
 average maximum temperature (of health) is somewhat lower than 
 before that epoch. A diurnal variation, independent of food or 
 other circumstances, has been observed, the maximum ranging 
 from 9 A.M. to 6 p.m. and the minimum from 1 1 P.M. to 3 A.M. 
 Meals cause sometimes a slight elevation, sometimes a slight 
 depression, the direction of the influence depending on the nature 
 of the food : alcohol seems alwaj'S to produce a fall. Exercise and 
 variations of external temperature, within ordinary limits, cause 
 very slight change, on account of the compensating influences 
 which have been discussed above. The rise from even active 
 exercise does not amount to V C. ; when labour is carried to 
 exhaustion a depression of temperature may be observed. In 
 travelling from very cold to very hot regions a variation of less 
 than a degree occurs, and the temperature of tropical inhabitants 
 is practically the same as of those dwelling in arctic regions. 
 
 When external cold or warmth passes certain limits, or when 
 during the application of these agents the regulative mechanisms 
 are interfered with, the temperature of the body may be lowered 
 or raised until death ensues. When the cold or warmth applied is 
 not very great, as in cold and warm baths, it has been noticed that 
 the temperature is more easily raised by warmth than depressed 
 by cold. Death ensues from extreme cold by a depression of the 
 activities of all the tissues, more especially of the nervous; as- 
 phyxia is produced in animals when the fall of temperature is 
 rapid. Puppies can be recovered after the temperature in the 
 rectum has fallen to about 4" or 5" C, and hybernating mammals 
 may be cooled with impunity down to nearly freezing point. 
 W^hen external warmth is brought to bear on a mammal in such a 
 way as to cause a rise of temperature in the body, death ensues 
 when an elevation of about 6" or 7" C. above the normal is reached. 
 The exact cause of the death has not been as yet sufficiently ex- 
 
CiiAi'. v.] NUTRITIOX. 469 
 
 plained. It cannot be tine, as has liecn suggested, to the niuscle.s 
 eiiteiiug into rigor caloris, for the animals frequently succumb 
 hi'foro this takes place. A high temperature makes the heart 
 irregular, and iinally stops its beat, but probal)ly other tissues aro 
 aJso injuriously atleeted, so that death cannot be attributed to the 
 sto])iiago of the heart alone. 
 
 One of the most marked phenomena of starvation is the fall of 
 temperature, ^Yhich becomes very rapid during the last days of life. 
 Indeed the low temperature of the body is a powerful factor in 
 bringing about death, for life may be much prolonged by wrapping 
 a starving animal in some bad conductor so as to economise the 
 boililv heat. 
 
SEC. 5. THE INFLUENCE OF THE NERVOUS SYSTEM 
 ON NUTRITION. 
 
 In tlie preceding sections we had more tlian once to refer to 
 the possibihty of the nervous system having the poAver of directly 
 affecting the metabolic actions of the body, apart from any 
 irritable, contractile, or secretory manifestations. Thus the phe- 
 nomena of diabetes cannot, at present at all events, be satisfactorily 
 explained as a purely vaso-motor effect, and the production of heat 
 is, as we have seen, under the special guidance of the nervous 
 system. In the case of the salivary glands Ave meet with the 
 striking fact that when all the nerves of a gland have been divided 
 the gland enters into a peculiar condition during which it pours 
 forth a continuous, so-called 'paralytic' secretion, while ultimately 
 the tissue of the gland degenerates. This result differs perhaps 
 from the wasting of a muscle which follows upon severance of its 
 motor nerve, since this may be, partly at all events, explained by 
 the fact that the muscle is no longer functional ; and indeed, if the 
 muscle is rendered functional, if it is directly stimulated for 
 instance from time to time with a galvanic current, the atrophy 
 may be for a Avhile at least postponed, though as we have seen 
 (p. 92) the postponement is probably not indefinite. But the 
 salivary gland at all events in the case in question is functional, it 
 does go on secreting; nevertheless in the absence of its usual 
 
Chap, v.] NUTRITIOX. J7I 
 
 nervous p^uiihiucc its nutrition bt-conics profoundly affucted. Wc 
 are not justilied in sayint^ that in this case tlic nutrition of thci 
 salivary cell is directly dependent on the nervous system, because 
 all biological studies teach us that the growth, repair, and repro- 
 duction of protoplasm may go on cjuite independently of any 
 nervous system, and the nutrition of the nervous system itself 
 cannot be dependent on the action of that system on itself; but 
 we may go so far as to infer that the nutrition of the salivary cell 
 is in the complex animal body so arranged to meet the constantly 
 recurring influences brought to bear on it by the nervous system, 
 that, when those influences are permanently withdrawn, it is thrown 
 out of equilibrium ; its molecular processes, so to speak, run loose, 
 since the bit has been removed from their mouths. And we might 
 expect that similar instances would be met with where nutrition 
 became abnormal after the removal of wonted nervous influences. 
 Such instances indeed are not uncommon. And there are many 
 pathological phenomena, inflammation itself to begin with, which 
 seem inexplicable, except when regarded as the result of nervous 
 action. As examples Ave might mention the rapid and peculiar 
 degeneration of and loss of contractility in the skeletal muscles in 
 certain affections of the spinal cord, the changes in the muscles 
 being more raj)id and profound than in the nerves ; the pheno- 
 mena of bed-sores, especially the so-called acute bed-sores of 
 cerebral apoplexy ; some at least of the cases of vesical affections 
 attendant on spinal or cerebral diseases or injuries ; the more 
 rapid atrophy and loss of contractility in muscles which follow 
 upon contusions of nerves as compared Avith the effects of simple 
 section of nerves ; the occurrence of certain eruptions, such as 
 lichen, zona, ecthyma, &c., in various spinal or cerebral diseases, 
 frequently accompanied, as in maladies affecting the posterior 
 comua, Avith intermittent pains ; and indeed the general phe- 
 nomena, and especially the topogi-aphy of the eruption, of a 
 large number of cutaneous diseases. In all these cases, hoAvever, 
 there are many attendant circumstances to be considered before 
 AA'e can feel justified in speaking of any direct influence of the 
 nervous system on nutrition, of any specific action of what have 
 been called 'trophic' nerves. Perhaps the instance Avhich has 
 been best Avorked out is the connection of the nutrition of the eye 
 and face with the fifth or trigeminal nerve. When in a rabbit the 
 trigeminus is divided in the skull there is loss of sensation in those 
 jjarts of the face of Avhich it is the sensory nerve. Very soon, 
 AA'ithin twenty-four hours, the cornea becomes cloudy ; and this is 
 the precursor of an inflammation Avhich may involve the Avhole eye 
 and end in its total disorganization. At the same time the nasal 
 chambers of the same side are inflamed, and very frequently ulcers 
 make their appearance on the lips and gums. Seeing hoAv delicate 
 a structure the eye is, and hoAV carefully it is protected by the 
 
472 TROPHIC NERVES. [Book ii. 
 
 mechanisms of the eyelids and tears, it seems reasonable to 
 suppose that the inflammation in question might simply be the 
 result of the irritation caused by dust and contact mth foreign 
 bodies, to -which the eye, no longer guided and protected by 
 sensations, these being destroyed by the section of the nerve, 
 became subject. In the same way the ulcers on the lips and 
 gums might be explained as injuries inflicted by the teeth on 
 those stnictures in their insensitive condition. And some ob- 
 servers maintain that the inflammation of the eye may be gTeatly 
 lessened or altogether prevented if the organ be carefully covered 
 up and in all possible ways protected from the irritating influences 
 of foreign bodies. Other observers however have failed to prevent 
 the inflammation in spite of every care. This negative result is in 
 itself no strong argument, but the question cannot yet be con- 
 sidered as entirely cleared up. 
 
 In a mammal division of both vagi is followed by pneumonia 
 (inflammation of the lungs) ending in death. This has been ad- 
 duced as an instance of the trophic action on the pulmonary tissues 
 of certain fibres of the vagi ; but the real explanation seems to be 
 that, owing to a paralysis of the oesophagus and larjmx caused by 
 section of the vagi, food accumulating in the pharynx passes into 
 the air-passages and so sets up the pneumonia. In birds death 
 follows, sometimes from pneumonia of a similar causation, but more 
 frequently from inanition on account of the food not being able to 
 enter the stomach. The immediate cause of death however appears 
 in many cases at all events, both in birds and mammals, to be a 
 paralysis of the heart, and the histological changes (acute fatty 
 degeneration) observable in the cardiac muscles are of such a 
 character as to suggest a trophic action of the vagus fibres on that 
 tissue. 
 
 Other instances of nerves manifesting even a doubtful trophic 
 action as the result of experimental interference are rare; yet there 
 seems to be no reason why the fifth nerve or the vagus should be 
 conspicuous in possessing trophic fibres. When the sciatic nerv^e of 
 the frog is divided, no nutritive alterations beyond those explicable 
 as the result of loss of function are observed ; and indeed the ma- 
 jority of the effects on growth and nutrition resulting from the 
 section of nerves, or from paralysis, can be refen-ed to the absence 
 of the usual functional acti\T.ty, accompanied in some cases Arith an 
 altered vascular supply. It must be remembered however that 
 functional activity is itself the result of metabolic and therefore 
 nutritional changes ; and in cases of inhibition, as for instance in 
 the action of the vagus on the heart, we seem to have illustrations 
 of a nerve producing metabolic changes leading not to the exercise 
 but to the arrest of functional activity. 
 
 Taking all things into consideration, we may venture to say 
 that the numerous phenomena of disease, joined to the facts men- 
 
CuAr. v.] NUTRITION. 473 
 
 tioned above, turn tlic balance of evidence in favour of the view 
 that sonic more or less direct influence of the nervous system on 
 metabolic actions, and so on nutrition, will be established by future 
 iiu^iiiiics. 
 
SEC. G. DIETETICS. 
 
 We may sum up the main results of the previous sections some- 
 what in the following way. Although the body consists, like the 
 food, of proteids, fats and carbohydrates, yet the conversion of the 
 one into the other is not dii'ect. Assimilation does not proceed in 
 such a way that the proteids of the food all become the proteids of 
 the body, the fats of the food the fats of the body, and the starch 
 and sugar of the food the glycogen, dextrin, and sugar of the body. 
 We cannot even say that the non-nitrogenous food supplies alone 
 the non-nitrogenous parts of the body, and that the nitrogenous food 
 remains as the sole source of the nitrogenous tissues. We have 
 seen that under all circumstances a certain quantity of proteid 
 food is immediately metabolized, probably while still within the 
 alimentary canal, and that an excess of proteid food may lead to 
 the accumulation of bodily fat. On the other hand, we find that 
 a large proportion of the carbonic acid of the egesta comes from 
 the metabolism taking place in nitrogenous tissues, such as 
 muscle ; and we have had proof that the energy set free by mus- 
 cular contraction may be far greater than could be supplied by the 
 proteid food taken, and that therefore the non-nitrogenous factors of 
 the metabolism which set free the energy must have ultimately come 
 from non-nitrogenous food. We have abundant evidence that the 
 various food-stuffs become more or less metabolized, and their ele- 
 ments more or less rearranged and mixed, before they appear as 
 constituents of the bodily tissues. 
 
CiiAi'. v.] NUTlilTIOX. 475 
 
 W'ii luiM' si'i'ii that the uxkUitiuiis ul' tlic btnly arc, as in tlie 
 case of muscle, of a peculiar character, and carried on by the tissues 
 themselves. "While at present we should be hardly justified in 
 denying that any oxidations at all take place in the blood-plasma, 
 such as do occur nuist be sli_i(l\t in ammmt as compared with those 
 going on in the tissues. A\'e might also say that one body only, 
 viz. lactic acid, presents itself as a substance likely to be directly 
 oxidised in the blood itself; and even with regard to this the evi- 
 dence is as much against as for any such direct oxidation taking 
 place. The great mass of the oxidation of the body is of an 
 indirect kind, determined by the activity of the several tissues. 
 The blood serves as an oxygen carrier for the tissues ; and it is m^t 
 itself the large combustion agent it Avas once thought to be. The 
 tendency of all recent inquiries is to shew that the body cannot be 
 compared, either as a whole, or in its parts, to a furnace for the 
 direct combustion of combustible food. On the contrary, we are 
 driven nearer and nearer to the conclusion that all food which has 
 become absorbed into the blood must become tissue before it be- 
 comes waste product, and only becomes waste product through a 
 metabolism of the tissue. When w^e say "become tissue" we must 
 leave it at present Avholly undecided how far the constant meta- 
 bolism which this view demands affects the so-called structural 
 elements of the more highly organized tissues ; it is quite open 
 however, as we have already suggested, for us to imagine that in 
 muscle, for instance, there is a framework of more stable material, 
 giving to the muscular fibre its histological features, and under- 
 going a comparatively slight and slow metabolism, while the 
 energy given out by muscle is supplied at the expense of more 
 fluctuating molecules which fill up so to speak the interstices 
 of the more durable frame-work, and the metabolism of which 
 alone is large and rapid. 
 
 The characteristic feature of proteid food is that it increases the 
 oxidative, metabolic activity of the tissues, leading to a rapid con- 
 sumption, not only of itself, but of non-nitrogenous Ibod as well. 
 Where therefore a rapid renewal of the tissues is sought for, an 
 excess of proteid food may be desirable. But it must be borne in 
 mind that by the very nature of its rapid metabolism, proteid food 
 must tend to load the body with the so-called extractives, i.e. with 
 nitrogenous crystalline bodies. How far these are of use to the 
 body, and what part they play, is at present unknown to us. That 
 they are of some use is suggested by the beneficial efiects of the 
 extractum carnis when taken as food in conjunction wath non-nitro- 
 genous material, though it is possible that the dietetic value of 
 this preparation may be due to the small amount of non-crystalline 
 extractives which it contains. That when in excess these nitro- 
 genous i^roducts may be highly injurious is indicated by the little 
 we know of the connection between the symptoms of gout and the 
 
476 DIETETICS. [Book ii. 
 
 presence of uric acid. A large meal of proteid material must tax 
 the system to the utmost in getting rid of or stowing away the 
 nitrogenous crystalline bodies arising through changes either in 
 the alimentary canal or in the liver. 
 
 One value of fats and carbohydrates lies in their being sources 
 of energy, more than three-fourths of the normal income of poten- 
 tial energy coming from them (p. 458); and, as we have seen, they 
 are ultimate sources of muscular energy as well as of heat. But 
 their great characteristic is that they do not, like proteid food, excite 
 the metabolic activity of the body. Hence, to a far greater extent 
 than is the case with proteid food, they can be retained and stored 
 up in the body with comparative ease. The digested elements of 
 fatty or carbohydrate food which go to form the protoplasm of 
 adipose tissue, become part and parcel of a substance which can 
 perform its metabolism without any explosive expenditure of 
 energy, and which therefore, instead of giving rise to bodies de- 
 manding immediate excretion from the system, can deposit its 
 metabolic products as apparently little, but as we have seen in 
 reality greatly, changed fat. In this way the non-nitrogenous 
 food of to-day is rendered available for future and even far distant 
 wants. 
 
 In comparing feits with carbohydrates, we can only point to the 
 much greater potential energy of the former than of the latter, 
 weight for weight (see p. 458). 
 
 A diet may be chosen either for the simple maintenance of 
 health, or for the sake of muscular energy, or for fattening purposes. 
 For the first purpose there is, we may suppose, a normal diet ; and 
 in the case of man, instinct and experience have probably not erred 
 far in choosing some such proportions as those given on p. 446. If, 
 as we have urged, all food becomes tissue before it leaves the body 
 as waste product, the dominant principle of all nutrition, and the 
 ultimate tribunal of all questions of diet, must be the individual 
 character of the tissue, the idiosyncrasy of the body. The same 
 mysterious qualities which cause the same blood-plasma to become 
 here a muscle, and there a secreting cell, convert ,the same food into 
 the body of a man or of a sheep. All the simpler and more general 
 laws of metabolism are made subservient to more intricate and 
 special laws of protoplasmic construction. We can only speak of a 
 normal diet in the same way that we speak of the average intelli- 
 gence of man. 
 
 In seeking to supply such a normal diet out of ordinary 
 articles of food, we must bear in mind that the nutritive value 
 of any substance, estimated in terms of the potential energy of 
 the proteids, fats or carbohydrates it contains, must of course 
 be corrected by its digestibility. One gramme of cheese has, as 
 far as potential energy is concerned, an exceedingly high value ; 
 but the indigestibility of cheese brings its nutritive value to a 
 
CiiAP. v.] NUTRITION. 177 
 
 very low levi-l. Here too the llietor ol' idiusyucrasy makes itself 
 exceedingly felt. 
 
 In feeding for fattening purposes the comparatively cheap 
 carbohytlrates arc of course chietly depended on. If the view 
 mentioned on p. 4r)4 be correct, that the fat really stored up all 
 comes from ])roteid metabolism, an equivalent of this food-stuff 
 must always be gi\en. If, as seems probable, this view is a too 
 hurried generalisation, there still remains the possibility that for 
 economical fattening, with the least waste, a certain proportion 
 between the nitrogenous and non-nitrogenous foods must always bo 
 maintained. 
 
 From what has been previously said it is evident that proteid 
 food is not the only food-st\ifY to be regarded in selecting a diet for 
 nuiseular labour. We should however equally err in the ojiposite 
 direction if we selected exclusively non-nitrogenous food on which 
 to do work, since, as we have seen, there is no evidence that the 
 fats or carbohydrates are the direct, though they may be in part 
 the ultimate source, of muscular energy. Considering how comi^lex 
 a thing strength is, how much it depends on the vigour of parts of 
 the body other than the muscles, a normal diet, calculated to 
 develope equally all parts of the body, is probably the best diet for 
 active labour. It is possible however that an excess of proteid 
 food, by reason of the renewal of tissue caused by its metabolic 
 activity, may be, in such cases, of service. 
 
 Lastly, the several saline matters, including the extractives of 
 animal and vegetable food, are no less essential elements of a diet 
 than proteids, fats, or carbohydrates. Of use, not for the energy 
 they themselves possess, but by reason of then- regulating the 
 energy of the food-stuffs more strictly so called, they are necessary 
 to life : the body in their absence fails to carry out its usual 
 metabolism, and disease if not death follows. 
 
 The dietetic superiority of fresh meat and vegetables depends 
 in part on their still retaining these various saline and extractive 
 matters. A diet from which phosphorus (or even possibly jahos- 
 phates), or chlorides, or potash, or soda salts are absent, is, as soon 
 as the store of the substance in the body is exhausted, useless 
 for nutritive purposes. Calcium and magnesia may, to a certain 
 extent, be replaced by bases closely allied to them ; but the 
 metabolic role of phosjohorus or of sulphur cannot be taken up 
 by an analogous body; and, as is illustrated by their distribution 
 in the body, the physiological functions of potash and soda are 
 widely different if not antagonistic, closely allied as are these two 
 alkalis when regarded from a chemical point of view. Like 
 medicines and poisons — and indeed they are in a manner natural 
 medicines — the action of these bodies depends in part on their 
 dose. Indispensable as are potash salts to the economy, a large 
 dose of them is injurious ; and a dog fed on nothing but Liebig's 
 
478 DIETETICS. [Book ii. 
 
 extract dies sooner than a dog not fed at all^ on account of the 
 potash salts of the extract exerting their deleterious influence in 
 the absence of the food whose metabolism their function is to 
 direct. 
 
BOOK III. 
 
 THE CENTRAL NERYOUS SYSTEM AND ITS 
 INSTRUMENTS. 
 
CHAPTER I. 
 
 SENSORY NERVES. 
 
 In studying the phenomena of motor nerves we are greatly assisted 
 by two facts : — First, that the muscular contraction by which wc 
 judge of what is going on in the nerve is a comparatively simple 
 thing, one contraction differing from another only by such features 
 as amount, rapidity, and frequency of repetition, and all such 
 differences being capable of exact measurement. Secondly, that 
 when we apply a stimulus du-ectly to the nerve itself, the effects 
 diff"er in degree only from those which result when the nerve is set 
 in action by natural stimuli, such as the will. When we come, on 
 the other hand, to investigate the phenomena of afferent nerves, 
 our labours are for the time rendered heavier, but in the end 
 more fruitful, by the facts : — First, that wc can only judge of 
 Avhat is going on in an afferent nerve by the effects it produces 
 in some central nervous organ, in the way of exciting or modifying 
 reflex action, or modifying automatic action, or affecting conscious- 
 ness ; and we are consequently met on the very threshold of every 
 incpiiry by the difficulty of clearly distinguishing the events which 
 belong exclusively to the afferent nerve from those which belong to 
 the central organ. Secondly, that the effects of applpng a stimulus 
 to the peripheral end-organ of an afferent nerve are very diff"erent 
 from those of applying the same stimulus directly to the nerve- 
 trunk. This may be shewn by the simple experience of comparing 
 the sensation caused by the contact of any shai-jj body with a 
 ner^'c laid bare by a wound with that caused by contact of an intact 
 skin \\-ith the same body. These differences reveal to us a com- 
 plexity of impulses, of which the phenomena of motor nerves gave 
 us not so much as a hint ; but for the time being they increase the 
 difficulties of our study. 
 
 F. 31 
 
482 SENSORY NERVES. [Book hi. 
 
 An afferent impulse passing along an afferent nerve may in 
 certain cases simply produce a change in our consciousness 
 unaccompanied by any visible bodily movements ; in other cases 
 it may give rise to reflex movements, or modify existing reflex 
 or automatic actions without causing any change in consciousness ; 
 in still other cases it may bring about both results at the same 
 time. An afferent nerve the stimulation of which gives rise to 
 a sensation, and so leads to a modification of consciousness, may be 
 more closely defined as a ' sensory ' nerve. There is however no 
 distinct proof, having regard to the difficulties just mentioned, 
 that the afferent fibres which in the body are commonly used to 
 cause or affect reflex action differ at all in kind from those whose 
 function it is to modify consciousness. On the contrary, such 
 evidence as we have goes to shew that an appropriate stimulus 
 of the same fibre may give rise to one or other or both events ; and 
 that whether the one or the other, or both, events occur depends 
 on the condition of the central organ, and on the relation of its 
 several parts to the afferent nerve. The stimulation of the same 
 nerve (and there are no positive facts which would preclude us 
 from saying ' of the same fibre ') may under certain circumstances, 
 as for instance when the brain has been removed, simply cause 
 a reflex action and under other circumstances give rise merely to a 
 sensation. Hence an afferent nerve is frequently spoken of as 
 a sensory nerve even under circumstances where there is no 
 evidence of consciousness being actually affected, because by a 
 slight change of circumstances the same stimulation of the same 
 nerve might give rise to a distinct sensation ; the substitution of 
 the specific for the general term being justified by the convenience 
 of the former. 
 
 All the spinal nerves are mixed nerves, composed of afferent 
 and efferent, of motor and sensory fibres. When a spinal nerve 
 is divided, stimulation of the peripheral portion causes muscular 
 contraction, of the central portion, a sensation (or a reflex action). 
 At the junction of the nerve with the spinal cord the sensory fibres 
 are gathered into the posterior and the motor fibres into the 
 anterior root. The proof of this, which was first made known 
 by Charles Bell and Majendie, their discoveries forming the founda- 
 tion of modern nervous physiology, is simply as follows. 
 
 When the anterior root is divided, the muscles supplied by the 
 nerve cease to be thrown into contractions either by the will, or by 
 reflex action, while the structures to which the nerve is distributed 
 retain their sensibility. During the section of the root, or when the 
 proximal stump, that connected with the spinal cord, is stimulated, 
 no sensory effects are produced. When the distal stump is stimu- 
 lated, the muscles supplied by the nerve are thrown into contractions. 
 When the posterior root is divided, the muscles supplied by the 
 nerve continue to be thrown into action by an exercise of the will 
 or as part of a reflex action, but the structures to which the nerve 
 
CiiAi". I.] SENSORY NERVES 483 
 
 is distribntotl lose the sensibility whicli they previously possessed. 
 Duriii^i,' the section of the root, and when the jji-oxinial stump is 
 stimulated, sensory effects are jiroduced. When the distal stump 
 is stimulated no movements are called forth. These facts demon- 
 strate that sensory imi)ulses pass exclusively by the posterior r(jot 
 from the peripheral to the central organs, and that motor impulses 
 pass exclusively by the anterior root from the central to the 
 peripheral organs. 
 
 An exception must be made to the above general statement, 
 on account of the so-called recurrent sensibility which is witnessed 
 in conscious mammals, under certain circumstances. It often 
 happens that when the peripheral stump of the divided anterior 
 root is stimulated, signs of pain are witnessed. These are not 
 caused by the concuiTcnt muscular contractions or cramp Avhich 
 the stimulation occasions, for they remain if the Avhole trunk of 
 the nerve be divided some little way below the union of the roots 
 above the origins of the muscular branches, so that no contractions 
 take place. They disappear if the posterior root be also cut, and 
 they are not seen if the mixed nerve-trunk be divided close to the 
 union of the roots. The phenomena are probably due to the fact, 
 that bundles of sensory fibres of the posterior root after running a 
 short distance down the mixed trunk turn back and run upwards 
 in the anterior root, and by this recurrent course give rise to the 
 recurrent sensibility. 
 
 Concerning the ganglion on the posterior root, we may say defi- 
 nitely that it is neither a centre of reflex nor of automatic action. 
 Our knowledge concerning its function is almost limited to the 
 fact that it is in some way intimately connected mth the nutrition 
 of the nerve. When a mixed nerve-trunk is divided, the peripheral 
 portion degenerates from the point of section do-wnwards towards 
 the periphery. The central portion does not so degenerate, and if 
 the length of nerve removed be not too great, the central portion 
 uniting with the degenerating peripheral portion may gi'ow down- 
 wards, and thus regenerate the nerve. This degeneration is 
 observed when the mixed trunk is divided in any part of its 
 course from the periphery to close up to the ganglion. When the 
 posterior root is divided between the ganglion and the spinal cord, 
 the portion attached to the spinal cord degenerates, but that 
 attached to the ganglion remains intact. When the anterior root 
 is divided, the proximal portion in connection with the spinal cord 
 remains intact, but the distal portion between the section and the 
 junction ^ith the other root degenerates; and in the mixed nerve- 
 trunk many degenerated fibres are seen, which, if they be carefully 
 traced out, are found to be motor fibres. If the posterior root be 
 divided carefully between the ganglion and the junction with the 
 anterior root, the posterior root above the section remains intact, 
 but in the mixed nerve-tnmk are seen mnnerous degenerated 
 fibres, which when examined are found to have the distribution of 
 
 31—2 
 
484 ROOTS OP SPINAL NERVES. [Book hi. 
 
 sensory fibres. Lastly, if the posterior ganglion be excised, the 
 whole posterior root degenerates, as do also the sensory fibres of 
 the mixed nerve-trunk. Putting all these facts together, it would 
 seem that the growth of the motor and sensory fibres takes j)lace 
 in opposite directions, and starts from different nutritive or 
 'trophic' centres. The sensory fibres grow away from the ganglion 
 either towards the periphery, or toAvards the spinal cord. The 
 motor fibres grow outwards from the s]3inal cord towards the 
 periphery. This difference in their mode of nutrition is frequently 
 of great help in investigating the relative distribution of motor 
 and sensory fibres. When a posterior root is cut beyond the 
 ganglion, or the ganglion excised, all the sensory nerves degenerate, 
 and the sensory fibres, by their altered condition, can readily be 
 traced in the mixed nerve-branches. Conversely, when the an- 
 terior roots are cut, the motor fibres alone degenerate, and can be 
 similarly diagnosed in a mixed nerve-tract. When the anterior 
 root is divided some few fibres in it do not, like the rest, degene- 
 rate, and when the posterior root is divided, a few fibres in the 
 anterior root are seen to degenerate like those of the posterior 
 root ; these appear to be the fibres which give to the anterior root 
 its "recurrent sensibility." By the same means in a mixed nerve 
 like the vagus, the fibres which spring from the real vagus root 
 may be distinguished from those proceeding from the spinal 
 accessory, by section of the vagus and spinal accessory roots 
 respectively ; and in the mixed vago-sympathetic trunk, met with 
 in many animals, the vagus fibres may be distinguished from the 
 sympathetic, since, after a section of the mixed trunk, the former 
 degenerate from above downwards, whereas the latter degenerate 
 in an upward direction from the inferior cervical ganglion below to 
 the superior cervical ganglion above; for the ganglia of the 
 sympathetic behave in this respect like the spinal ganglia of the 
 posterior roots. This method of diagnosis is often spoken of as 
 the Wallerian method, after A. Waller, to whom we are indebted 
 for the discovery of most of these facts. 
 
 In the cranial nerves the motor and sensory tracts are far less 
 mixed than in the spinal nerves. The olfactory, optic and acoustic 
 nerves are j)urely sensory nerves. The fifth, glosso-pharyngeal 
 and vagus are mixed nerves ; and it is stated that in the dog the 
 afferent and efferent fibres of the vagus are gathered into two 
 bundles so distinct that they may be separated by the knife, 
 the afferent bundle lying to the outside of the efferent bundle. 
 The facial and hypoglossal are for the most part motor (efferent) 
 nerves, but contain sensory (afferent) fibres. The third, fourth, 
 sixth and spinal accessory are exclusively motor (efferent) nerves. 
 These statements refer to what are commonly looked upon 
 as the trunks of the respective nerves. More exactly speaking, 
 the sensory fibres of the facial come from the fifth, pneumo- 
 gastric and glosso-pharyngeal nerves, so that the facial proper is in 
 
Chap, i.] SENSORY NERVES. 485 
 
 reality a puivly motor iiorvo. So likewise is the hjiioglossul, its 
 sensory fibres coming from the fifth, pneumogastric, and three 
 upper cervical nerves. The fifth is a mixed nerve entirely on the 
 ])lan of a sj)inal nerve, having distinct motor and sensory roots. 
 The glosso-j)haryngeal seems to be essentially a sensory nerve, its 
 motor filaments springing from the fifth and facial nerves. Con- 
 cerning the vagus some have maintained that the pneumogastric 
 I'oot proper is entirely sensory (afterent), and that all the ctiferent 
 functions of the vagus are dependent on the fibres of the spinal 
 accessory Avhich join it. To this point we shall return when we 
 come to consider briefiy the special functions of the several nerves. 
 
 We have already stated (p. lOG) that isolated pieces of motor 
 and of sensory nerves behave exactly alike as far as all the physical 
 manifestations attendant on the passage of a nervous impulse are 
 concerned ; the current of action makes its appearance in the same 
 way and seems to have the same characters in both kinds of 
 nerves. The same is also truCj as far as we know, of nerves within 
 the body. 
 
 Moreover, the rate at which nei'vous impulses travel appears to 
 be about the same in motor and sensory nerves ; at least we have 
 no evidence of any fundamental difference in this resjaect between 
 the two. We have seen that the velocity of a nervous impulse in 
 the motor-nerve of a frog is about 28 metres per sec. The velocity 
 of a motor impulse in man, as judged by the difference of the 
 latent period of the contraction of the thumb-muscles, when stimu- 
 lation is brouo-ht to bear on the motor-nerve at the wrist, or hiofh 
 up m the arm, is about 33 metres per sec. In warm-blooded 
 animals, however, the rate of transmission of motor impulses is 
 very variable, being in particular closely dej)endent on temperature, 
 and probably also on other circumstances. Thus, it may range 
 from as low as 30 m, when the nerve is cooled to as high as 90 m. 
 when it is warmed. The velocity of a sensory impulse is estimated 
 by measuring the time taken between a stimulus being brought 
 to bear on some sentient surface, as the skin, and the maldng of a 
 signal by the individual experimented on at the instant that he 
 feels the stimulus. The time taken up in the sensory impulse 
 becoming converted into a sensation after reaching the nervous 
 central organs, in the mental operation of determining to make 
 the signal, and in the effort of making the signal, corresponds in a 
 way to the purely muscular portion of the latent period in the 
 experiment for determining the velocity of a motor impulse. The 
 application of the stimulus and the making of the signal {ex. gr. 
 closing a galvanic circuit) being both recorded on a rapidly 
 travelling surface, the time taken up in the whole operation 
 can be easily measured ; and the difference between the time taken 
 when the stimulus is applied to some spot separated from the 
 central nervous system by a short piece of nerve, ex. gr. the top of 
 the thigh, and that taken when a long piece of nen-e intervenes, ex. 
 
486 AFFERENT AND EFFERENT NERVE FIBRES. [Book uv. 
 
 gr, when the stimukis is applied to the toe, will give the time 
 required for the sensory impulse to pass along a piece of sensory 
 nerve as long as the difference of length between the above two 
 nerves ; from which the velocity can be calculated. Observations 
 carried on in this way have led to most discordant results, varying 
 from 26 metres to 94 metres, or even more, per sec. The difference 
 here is far too great to allow any value to be attached to an 
 averao-e. When it is remembered how complex are all the central 
 nervous operations in these instances, as compared with the changes 
 going on in a muscle during the latent period of its contraction, 
 and how these central operations might vary according as one or 
 other spot of skin was stimulated, quite independently of the 
 length of nerve between the centre and the spot stimulated, these 
 discrepancies will not be wondered at; and it may fairly be 
 concluded that the velocity of a sensory impulse does not materially 
 differ from that of a motor impulse. 
 
 There are, however, certain phenomena which might at first 
 sight be intei-preted as indicating that afferent and efferent nerve- 
 fibres behave differently towards stimuli. We have already (p. 93) 
 stated that according to most observers, when an ordinary motor 
 nerve, such as a nerve supplying a muscle, is heated, no indications 
 of the generation of nervous impulses, no contractions of the 
 muscle for instance, are observed. The heat does not act as a 
 stimulus; it may increase the irritability of the nerve for the 
 time being, but apparently cannot originate the explosive dis- 
 charge which we call an impulse. We have also seen that during 
 the passage of a constant current along the nerve of a muscle- 
 nerve preparation no contractions are visible, no impulses, save 
 in certain particular cases, are generated, so long as the current 
 is not suddenly varied in strength. But it has been found 
 that when afferent nerve-fibres, such as those in the central 
 stump of the divided sciatic or in the central stump of the 
 vagus, are heated to 45° or 50° events occur, clearly proving that 
 impulses are generated in the afferent fibres by the elevation of 
 temperature. In the case of the sciatic the animal shews sign 
 of pain, the blood-pressure is affected, &c. ; and in the case of the 
 vagus the heart is slowed by reflex inhibitory impulses passing 
 down the other, intact, vagus, though heating the peripheral 
 instead of the central stump of the divided vagus has no effect 
 whatever on the heart. Similarly when the same nerves or other 
 nerves containing afferent fibres are submitted to the action of the 
 constant current, there are like evidences of the continued genera- 
 tion of nervous impulses during the whole time of the passage of 
 the current, even though it be kept as uniform in strength as 
 possible. On the other hand many chemical substances which 
 act as powerful stimuli to motor nerves are ineffectual towards 
 afferent fibres. These results, however, until the contrary is 
 proved by further inquiries into the phenomena attending the 
 
CiiAi'. I.] SEN.iORY NERVES. 487 
 
 gcnt'nitiou and transmission of norvoiis impulses, may be taken 
 as indieatinijf not so much that the atlen-nt and efferent lil)res 
 are themselves acted upon in a different way by heat or by the 
 constant current as that the molecular disturbances generated in 
 both eases lune different effects according as they impinge u])oji a 
 central or a ])eri})heral meehanisni. We can readil}' imagine that 
 molecular disturbances which would be impotent to stir the 
 sluggish muscular substance to a contraction, and thus so to speak 
 be lost upon the muscle, might produce a \'ery great effect on the 
 more sensiti\e and mobile material of the central nervous system. 
 We may for the present therefore conclude that there is no distinct 
 proof of an absolute difference between afferent and efferent fibres, 
 but we must at the same time be cautious not to consider the 
 gi'osser phenomena, presented by a muscle-nerve preparation, as 
 a satisfactory test of all the changes which may take place in a 
 nerve-fibre. The necessity of this caution will be almost im- 
 mediately illustrated from another point of view. 
 
 The apparent identity in function between afferent and efferent 
 fibres, taken into consideration with the facts just mentioned con- 
 cerning the regeneration of nerves, suggests the inquiry whether by 
 a change of the j^eripheral or central organs a motor nerve can be 
 converted into a sensory nerve, or vice versa. Experiments made 
 A\dth a view of obtaining a functional union between purely motor 
 and sensory nerves have, in the hands of most observers, failed. 
 And though an apparent union between the central portion of a 
 divided lingual (sensory) nerve and the peripheral portion of a 
 divided hypoglossal (motor) nerve has been accomplished, with the 
 result that stimulation of the lingual trunk produced movements in 
 the tongue, the case breaks dovsoi upon examination. In the first 
 place though the nerves appeared to have united, there was no 
 actual union between the lingual and hypoglossal fibres, but 
 degeneration of the latter, and a growth downward of the former ; 
 in the second place the movements of the tongue when the lingual 
 trunk was stimulated appear to have been brought about by stimu- 
 lation not of the sensory, true lingual, fibres, but of motor (chorda 
 tympani) fibres running in the lingual trunk. 
 
 We have already seen (p. 106) that a sensory ner\-e in its 
 simplest form may be regarded as a strand of eminently iri'itable 
 protoplasm, forming a link between a superficial cell which alone is 
 subject to extrinsic stimuli, and a central (reflex or automatic) cell 
 Avhich receives stimuli, chiefly in the form of nervous impulses, pro- 
 ceeding from the former along the connecting strand. In the 
 earliest stages of the developement of a sensory nervous system, the 
 superficial sensory cell is susceptible of stimuli of all. kinds, pro- 
 vided they are sufficiently strong ; and probably all the imjaulses 
 which it transmits to the central cell resemble each other very 
 closely, differing only in degree. It is obvious however that the 
 economy would gain by a further division of labour, by a differeu- 
 
488 SPECIAL SENSES. [Book hi. 
 
 tiation of the simple uniform superficial cell into a number of cells, 
 each of which was more susceptible to particular stimuli than its 
 fellows. Thus one cell, or rather one group of cells, would become 
 eminently susceptible to the influence of light : in them the impact 
 of rays of light would give rise to nervous impulses more readily 
 than in the other groups ; another gi-oup would develope a sensi- 
 tiveness to Avaves of sound, and so on. In this way the primary 
 homogeneous bodily surface would be differentiated into a series of 
 seme- org cms, disposed and arranged among ectodermic cells, the 
 purpose of the latter being simply protective, and therefore not 
 demanding the existence of any direct connection with the central 
 nervous system. Similar but less highly marked differentiations 
 would be established in the endings of the afferent nerves connect- 
 ing the central nervous system with the internal surfaces and parts 
 of the body. 
 
 Moreover it is obvious that the sensory impulses transmitted to 
 the central nervous system by these differentiated sense-organs will 
 probably be themselves largely differentiated. Just as the impulses 
 which pass along a motor nerve differ according to the nature of the 
 stimulus which is applied to the nerve (whether, for instance, the 
 stimulus be a single induction-shock, or several shocks repeated 
 slowly, or several shocks repeated rapidly, and so on, the effect on 
 the muscle being in each case a different one), so also and even to a 
 much greater degree do the impulses generated by light in a visual 
 sense-organ in all probability differ from those generated by simple 
 pressure in a tactile sense-organ. 
 
 And since these various sensory impulses have much work to 
 jDerform on arriving at the central nervous system, in the way of 
 influencing the multitudinous molecular oj)erations going on in the 
 central cells, and of affecting consciousness, this differentiation of 
 sensory organs and sensory impulses will naturally be accompanied 
 by a corresponding differentiation of those central cells which the 
 impulses first reach on arriving at the central organ. Those cells, 
 for instance, of the central nervous system, which first receive the 
 particular nervous impulses coming from the visual sense-organs, 
 will be set apart for the task of so modifying and preparing those 
 impulses as to adapt them in the best possible way for the work 
 which they have to do. Hence each j^et'ipheral sense-organ will be 
 united by means of its nerve with a corresponding ceiUral sense- 
 organ, the former being able to affect various parts of the central 
 nervous system only through the medium of the latter. And we 
 have evidence, at least as far as relates to all the central nervous 
 operations in which consciousness is concerned, that such central 
 sense-organs do really exist. For of the total characters which 
 belong to an affection of consciousness by means of any of the 
 sense-organs, i.e. which belong to any special sensations, we find 
 that while some are gained during the rise of the sensory impulses 
 in the peripheral sense-organ, others first appear in the centra] 
 
Chap. I.] SKXSORY yERVES. 489 
 
 sen.so-or<,'aii in the course of the changes through wliich the sensory 
 inij)ulsi's give rise to a sensation. Thus a stiniuhis of any kind 
 a|»j»liL'd to the optic nerve along any part of its course, if it is able 
 to start any impulses at all, gives rise to a .sensation of liglit, and 
 j)reeisely the .same .stinudus applied to the acoustic nerve along any 
 j)art of its course gives rise to a sensation of sound; and so on. All 
 the evidence we possess goes against the view that a piece of optic 
 nerve, deprived of both its peri})heral and central endings, differs in 
 function from a similarly isolated piece of acoustic nerve; such facts 
 as are within our knowledge go to shew that the disturbances 
 generated in a piece of optic nerve by a galvanic cuiTcnt are the 
 same as those generated in a piece of acoustic nerve. We arc 
 therefore driven to the conclusion that the difference which ajijiears 
 when the central endings are intact arises in the central organs. 
 
 In all these differentiated sensory mechanisms, or special senses 
 as they are called, avc have then to deal with two elements : the 
 peripheral sense-organ, in which we have to study how the special 
 physical agent gives rise to special sensory impulses ; and the 
 central sense-organs, in which our study is confined to the manner 
 in which these special impulses modify the 02:)erations of the 
 central nervous system. Inasmuch as in a normal body the 
 peripheral organ remains in connection with the central organ, and 
 our study of the special senses is carried on chiefly by subjective 
 observations in which we make use of our o'wai consciousness, it 
 frequently becomes very difficult to distinguish in any given 
 sensation the peripheral from the central element. The two 
 become more distinct, the more complex the sense and the more 
 highly organised the sense-organs. For this reason it will be most 
 convenient to commence our study of the .special senses \r\i\\ the 
 sense of vision. 
 
CHAPTER II. 
 SIGHT. 
 
 A RAY of light falling on the retina gives rise to what we call a 
 sensation of light ; but in order that distinct vision of any object 
 may be gained, an image of the object must be formed on the 
 retina, and the better defined the image the more distinct will be 
 the vision. Hence in studying the physiology of vision, our first 
 duty is to examine into the arrangements by which the formation 
 of a satisfactory image on the retina is effected; these we may 
 call briefly the dioptric mechanisms. We shall then have to inquire 
 into the laws according to which rays of light impinging on the 
 retina give rise to sensory impulses, and those according to which 
 the impulses thus generated give rise in turn to sensations. Here 
 we shall come upon the difficulty of distinguishing between the 
 unconscious or physical and the conscious or psychical factors. 
 And we shall find our difficulties increased by the fact, that in 
 appealing to our ovm consciousness we are apt to fall into error by 
 confounding primary and direct sensations with states of conscious- 
 ness which are produced by the weaving of these primary sen- 
 sations with other operations of the central nervous system, or, 
 in familiar language, by confounding what we see with what we 
 think we see. These two things we Avill briefly distinguish as visual 
 sensations and visual judgments ; and we shall find that both in 
 vision with one eye, but more especially in binocular vision, visual 
 judgments form a very large part of what we frequently speak of 
 as our sight. 
 
SEC. 1. DIOFTRIC MECHANISMS. 
 
 The Formation of the Image. 
 
 The eye is a camera, consisting of a series of lenses and media 
 arranged in a dark chamber, the iris serving as a diaphragm ; and 
 the object of the apparatus is to fomi on the retina a distinct 
 image of external objects. That a distinct image is formed on 
 the retina, may be ascertained by remoA^ng the sclerotic from the 
 back of an eye, and looking at the hinder surface of the transparent 
 retina while rays of light proceeding from any external object axe 
 allowed to fall on the cornea. 
 
 A dioptric apparatus in its simplest form consists of two 
 media separated by a (spherical) surface ; and the optical proper- 
 ties of such an apparatus depend upon (1) the cun-ature of the 
 surface, (2) the relative refractive power of the media. The eye 
 consists of several media, bounded by surfaces which are approxi- 
 mately spherical but of different cunature. The surfaces are all 
 centred on a line called the optic axis, which meets the retina at a 
 point somewhat above and to the inner (nasal) side of the fovea 
 centralis. In passing from the outer surface of the cornea to the 
 retina the rays of light traverse in succession the cornea, the 
 aqueous humour, the lens and the vitreous humour. Refraction 
 takes place at all the surfaces bounding these several media, but 
 particularly at the anterior surface of the cornea, and at both the 
 anterior and posterior surfaces of the lens. Since the anterior and 
 posterior surfaces of the coniea are parallel, or very nearly so, the 
 rays of light would suflfer little or no change of direction in passing 
 through the cornea, if it were bounded on both sides by the same 
 
492 SIGHT. [Book hi. 
 
 medium. The direction of the rays of light in the aqueous 
 humour would therefore remain the same if the cornea were made 
 exceedingly thin, if in fact its two surfaces were made into one, 
 forming a single anterior surface to the aqueous humour; or, 
 which comes to the same thing in the end, since the refractive 
 power of the substance of the cornea is almost exactly the same 
 as that of the aqueous humour, the refraction at the posterior 
 surface of the cornea may be neglected altogether. Thus the two 
 surfaces of the cornea are practically reduced to one. The lens 
 varies in density in different parts, the refractive power of the 
 central portions being greater than that of the external layers; 
 but the refractive power of the whole may, without any serious 
 error, be assumed to be uniform. The refractive power of the 
 vitreous humour is almost exactly the same as that of the aqueous 
 humour. 
 
 Thus the apparently complicated natural eye may be simplified 
 into a 'diagrammatic eye,' in which the refracting surfaces are re- 
 duced to three, viz. (1) the anterior surface of the cornea, (2) the 
 anterior surface of the lens separating the lens from the aqueous 
 humour, and (3) the posterior surface of the lens separating the 
 lens from the vitreous humour. The media will similarly be 
 reduced to two ; the substance of the lens, and the aqueous or 
 vitreous hum.our. This 'diagrammatic eye' is of great use in the 
 various calculations which become necessarj^ in studying physio- 
 looical oiDtics : for the magnitudes which are derived by calculation 
 from it rej)resent the cori'espondmg magnitudes m an average 
 natural eye with sufficient accuracy to serve for all practical 
 purposes. The values adopted by Listing for the constants of this 
 'diagrammatic eye,' and to him we are indebted for the intro- 
 duction of it, are as follow : 
 
 Eadius of curvature of cornea 8 mm. 
 
 „ „ of anterior surface of lens 10 „ 
 
 „ „ of posterior „ „ 6 „ 
 
 Refractive index of aqueous or vitreous humour ^^ 
 
 Mean refractive index of lens yf 
 
 Distance from anterior surface of cornea to anterior 
 
 surface of lens 4 mm. 
 
 Thickness of lens 4 ,, 
 
 The calculated position of the j)rind]^al i^osterior focus, i.e. the 
 point at which all rays falling on the cornea parallel to the optic 
 axis are brought to a focus, is in the diagrammatic eye 14"6470 mm. 
 behind the posterior surface of the lens, or 22-6470 mm. behind the 
 anterior surface of the cornea. That is to say, the fovea centralis 
 must occupy this position in order that a distinct image of a 
 distant object may be formed upon it. It must be understood 
 that these values refer to the eye when at rest, i.e. when it is not 
 undergoing any strain of accommodation. 
 
CiiAi'. ii.| i^IGUT. WS 
 
 Accommodation. 
 
 When an object, a lens, and a .screen to receive the image, are 
 so arranged in reference to each other, that the image falls upon 
 the screen in exact focus, the rays of light proceeding from each 
 luminous point of the object are brought into focus on the screen 
 in a point of the image coiTCsponding to the point of the object. 
 If the object be then removed farther away from the lens, the rays 
 proceeding in a pencil from each luminous point will be brought to 
 a focus at a point in front of the screen, and, subsequently diverg- 
 ing, Avill fall upon the screen as a circular patch composed of a 
 series of circles, the so-called diffusion circles, arranged concentri- 
 cally round the principal ray of the pencil. If the object be 
 removed, not farther, but nearer the lens, the pencil of rays "vvill 
 meet the screen before they have been brought to focus in a point, 
 and consequently will in this case also give rise to diffusion circles. 
 When an object is placed before the eye, so that the image falls 
 into exact focus on the retina, and the pencils of rays proceeding 
 from each luminous point of the object arc brought into focus in 
 points on the retina, the sensation called forth is that of a distinct 
 image. When on the contrary the object is too far away, so that 
 the focus lies in front of the retina, or too near, so that the focus 
 lies behind the retina, and the pencils fall on the retina not as 
 points, but as systems of diffusion circles, the sensation produced 
 is that of an indistinct and blurred image. In order that objects 
 both near and distant may be seen with equal distinctness by the 
 same dioptric apparatus, the focal arrangements of the apparatus 
 must be accommodated to the distance of the object, either by 
 changing the refractive power of the lens, or by altering the 
 distance between the lens and the screen. 
 
 That the eye does possess such a power of accommodation is shewn 
 by every-day experience. If two needles be fixed upright some tAvo 
 feet or so apart, into a long piece of ■wood, and the wood be held 
 before the eye, so that the needles are nearly in a line, it will be 
 found that if attention be directed to the far needle, the near one 
 appears bluiTcd and indistinct, and that, conversely, when the near 
 one is distinct, the far one appears bluiTcd. By an effort of the 
 will we can at pleasure make either the far one or the near one 
 distinct; but not both at the same time. ^NTien the eye is 
 arranged so that the far needle appears distinct, the image of 
 that needle falls exactly on the retiiia, and each pencil from each 
 luminous point of the needle unites in a point upon the retina; 
 but when this is the case, the focus of the near needle lies behind 
 the retina, and each pencil from each luminous point of this needle 
 falls upon the retina in a series of diffusion cii'cles. Similarly, 
 when the eye is arranged so that the near needle is distinct, 
 
494 ACCOMMODATION. • [Book iii. 
 
 the image of that needle falls upon the retina in such a way, 
 that each pencil of rays from each luminous point of the needle 
 unites in a point on the retina, while each pencil from each 
 luminous point of the far needle unites at a point in front of the 
 retina, and then diverging again falls on the retina, in a series 
 of diffusion circles. If the near needle be gradually brought nearer 
 and nearer to the eye, it will be found that greater and greater 
 effort is required to see it distinctly, and at last a point is reached 
 at which no effort can make the image of the needle appear any- 
 thing but blurred. The distance of this point from the eye marks 
 the limit of accommodation for near objects. Similarly, if the 
 person be short-sighted, the far needle may be moved away from 
 the eye, until a point is reached at which it ceases to be seen 
 distinctly, and appears blurred. In the one case, the eye, with all 
 its power, is unable to bring the image of the needle sufficiently 
 forward to fall on the retina : the focus lies permanently behind 
 the retina. In the other, the eye cannot bring the image suf- 
 ficiently backward to fall on the retina: the focus lies permanently 
 in front of the retina. In both cases the pencils of rays from the 
 needles strike the retina in diffusion circles. 
 
 The same phenomena may be shewn with greater nicety by 
 what is called Scheiner's Experiment. If two smooth holes be 
 pricked in a card, at a distance from each other less than the 
 diameter of the pupil, and the card be held up before one eye, 
 with the holes horizontal, and a needle placed vertically be looked 
 at through the holes, the following facts may be observed. When 
 attention is directed to the needle itself, the image of the needle 
 appears single. Whenever the gaze is directed to a more distant 
 object, so that the eye is no longer accommodated for the needle, 
 the image appears double and at the same time blurred. It also 
 appears double and blurred when the eye is accommodated for a 
 distance nearer than that of the needle. When only one needle 
 is seen, and the eye therefore is properly accommodated for the 
 distance of the needle, no effect is produced by blocking up one 
 hole of the card, except that the whole field of vision seems 
 dimmer. When, however, the image is double on account of the 
 eye being accommodated for a distance greater than that of the 
 needle, blocking the left-hand hole causes a disappearance of the 
 right-hand or opposite image, and blocking the right-hand hole 
 causes the left-hand image to disappear. When the eye is ac- 
 commodated for a distance nearer than that of the needle, blocking 
 either hole causes the image on the same side to vanish. The 
 following diagram will explain how these results are brought about. 
 
 Let a (Fig. 67) be a luminous point in the needle, and ae, aj 
 the extreme right-hand and left-hand rays of the pencil of rays 
 proceeding from it, and passing respectively through the right-hand 
 e, and left-hand/, holes in the card. (The figure is supposed to be 
 a horizontal section of the eye.) When the eye is accommodated 
 
Chap, ii.] 
 
 SIGHT. 
 
 495 
 
 for (I, tlio rays e and / meet together in tlio ptnnt c, the rutina 
 oocupyin<^ the position ot" the piano nn; the hiniinuus point appears 
 as one p(»int, anil tlio needli; will ajjpcar as one needle. When the 
 eye is aeotnnniodated tor a distance beyond a, the retina may be 
 considered to lie ' no lon<fer at nn, bnt nearer the lens, at mm for 
 example ; the rays ae will cut this plane at p, and the rays af at 
 q; hence the hnninous point will no longer appear single, but will 
 
 Fig. G7. DiAonASi of Scheinek's Expebimknt. 
 
 be seen as two points, or rather as two systems of diffusion circles, 
 and the single needle will appear as two blurred needles. The 
 rays passing through the right-hand hole e, will cut the retina at 
 p, i.e. on the right-hand side of the optic axis ; but, as we shall see 
 in speaking of the judgments pertaining to vision, the image on 
 the right-hand side of the retina is referred hy the mind to an 
 object on the left-hand side of the person ; hence the affection 
 of the retina at p, produced by the rays ae falling on it there, gives 
 
 * Of course, in the actual eye, as we shall see, accommodation is effected by a 
 change in the lens, and not by an alteration in the position of the retina ; but for 
 convenience sake, we may here suppose the retina to be moved. 
 
496 ACCOMMODATION. [Bookiii. 
 
 rise to an image of the spot a at P, and similarly the left-hand 
 spot q corresponds to the right-hand Q. Blocking the left-hand 
 hole, therefore, causes a disappearance of the right-hand image, 
 and vice versa. Similarly when the eye is accommodated for a 
 distance nearer than the needle, the retina may be supposed to 
 be removed to II, and the right-hand ae and left-hand of rays, after 
 uniting at c, will diverge again and strike the retina at p' and q. 
 The blocking of the hole e will now cause the disappearance of the 
 image q" on the left-hand side of the retina, and this Avill be 
 referred by the mind to the right-hand side, so that Q will seem to 
 vanish. 
 
 If the needle be brought gradually nearer and nearer to the eye, 
 a point will be reached within which the image is always double. 
 This point marks "with considerable exactitude the near limit of 
 accommodation. With short-sighted persons, if the needle be 
 removed farther and farther away, a point is reached beyond 
 which the image is ahvays double; this marks the far limit of 
 accommodation. 
 
 The experiment may also be performed with the needle placed 
 horizontally, in which case the holes in the card should be vertical. 
 
 The adjustment of the eye for near or far distances may be 
 assisted by using two needles, one near and one far. In this case 
 one needle should be vertical, and the other horizontal, and the 
 card turned round so that the holes lie horizontally or vertically 
 according to whether the vertical or horizontal needle is being 
 made to appear double. 
 
 In what may be regarded as the normal eye, the so-called emme- 
 tropic eye, the near limit of accommodation is about 10 or 12 cm. 
 and the far limit may be put for practical pui'poses at an infinite 
 distance. The 'range of distinct vision' therefore for the emme- 
 tropic QjQ is very great. In the myopic, or short-sighted eye, the 
 near limit is brought much closer (5 or 6 cm.) to the cornea ; and 
 the far limit is at a variable but not very great distance, so that 
 the rays of light proceeding from an object not many feet away 
 are brought to a focus, not on the retina, but in the vitreous 
 humour. The range of distinct vision is therefore in the myopic 
 eyQ very limited. In the hypermetropic, or long-sighted eye, the 
 rays of light coming from even an infinite distance are, in the 
 passive state of the eye, brought to a focus beyond the retina. 
 The near limit of accommodation is at some distance off, and a far 
 limit of accommodation does not exist. The presbyopic eye, or the 
 long sight of old people, resembles the hypermetropic eye in the 
 distance of the near point of accommodation, but differs from 
 it inasmuch as the former is an essentially defective condition 
 of the accommodation mechanism, whereas in the latter the power 
 of accommodation may be good and yet, firom the internal arrange- 
 ments of the eye, be unable to bring the image of a near object on 
 to the retina. When a nonnal eye becomes presbyopic, the far 
 
CiiAi-. 11.] SKIHT. 497 
 
 limit iiiiiv niuaiii tin- saiiic, Itiit siiict' the jxiwcr of ;i(('MiiiiinMl;iting 
 ft)r near objiM-ts is wcakeiii'd or Ittst, the chaiit^c is distiiHtly a 
 roductiou ut" the raiii^jc of distinct vision. In the normal emme- 
 tropic eye, when no I'tfort of accommodation is made, the principal 
 focus of the eye lies on the retina, in the myopic eye in front of it, 
 and in the hypermetri>pic eye behind it. 
 
 Mechauism of Accommodation. In din't-tin^^ our attention 
 from a far to a very near object we are conscious of a distinct effort, 
 and feel that some change has taken place in the eye ; when we 
 turn from a very near to a far object, if we are conscious of any 
 change in the eye, it is one of a ditferent kind. The former is the 
 sense of an active accommodation for near objects ; the latter, 
 when it is felt, is the sense of relaxation after exertion. 
 
 Since the far limit of an emmetropic eye is at an infinite 
 distance, no such thing as active accommodation for far distances 
 need exist. The only change that will take place in the eye in 
 turning from near to far objects Avill be a mere passive undoing 
 of the accommodation previously made for the near object. And 
 that no such active accommodation for far distances takes place 
 is sheA\Ti by the facts — that the eye, when opened after being 
 closed for some time, is found not in medium state but adjusted 
 for distance ; that when the accommodation mechanism of the eye 
 is paralysed by atropin or nervous disease, the accommodation for 
 distant objects is unaffected ; and that we are conscious of no 
 effort in turning from moderately distant to far distant objects. 
 The sense of effort often spoken of by myopic persons as being felt 
 when they attempt to see things at or beyond the far limit of their 
 range seems to arise from a movement of the eyelids, and not 
 from any internal changes taking place in the eye. 
 
 What then are the changes which take place in the eye, when 
 we accommodate for near objects ? It might be thought, and 
 indeed once was thought, that the curvature of the cornea was 
 chancjed, becominof more convex, with a shorter radius of cun^ature, 
 for near objects. Young, however, shewed that accommodation 
 took place as usual when the eye (and head) is immersed in water. 
 Since the refractive powers of aqueous humour and water are very 
 nearly alike, the cornea, with its parallel surfaces, placed between 
 these two fluids, can have little or no effect on the direction of 
 the rays passing through it when the eye is immersed in water. 
 And accurate measurements of the dimensions of an image on 
 the cornea have shewn that these undergo no chanofc durincr 
 accommodation, and that therefore the curvature of the cornea 
 is not altered. Nor is there any change in the fonn of the bulb ; 
 for any variation in this would necessaril}' produce an alteration 
 in the curvature of the cornea, and pressure on the bulb would 
 act injuriously by rendering the retina anemic and so less 
 sensitive. In fact, there arc only two changes of importance 
 F. 32 
 
'4:08 ACCOMMODATION. [Book iii. 
 
 •which can be ascertained to take place in the eye during accommo- 
 dation for near objects. 
 
 ■ One is that the pupil contracts. When we look at near objects, 
 ihe pupil becomes small ; when W3 turn to distant objects, it 
 .dilates. This however cannot have more than an indirect influence 
 on the formation of the image; the chief use of the. contraction of 
 the pujjil in accommodation for near objects is to cut off the more 
 
 divergent circumferential rays of light. ~ - 
 
 The other and really efficient change is that the anterior 
 surface of the lens becomes more convex. If a light be held 
 before the eye, three reflected images may, with care and under 
 proper precautions, be seen by a bystander : one a very bright 
 ,one caused by the anterior surface of the cornea, a second less 
 bright, by the anterior surface of the lens, and a third very dim, 
 by the posterior surface of the lens ; when the images are those 
 of an object, such as a candle, in which a top and bottom can 
 be recognized, the two former images are seen to be erect, but 
 the third inverted. When the eye is accommodated for near 
 objects, no change is observed in either the first or the third of 
 these images ; but the second, that from the anterior surface 
 of the lens, is seen to become distinctly smaller, shewing that 
 the surface has become more convex. When, on the contrary, 
 vision is directed from near to far objects, the image from the 
 anterior surface of the lens grows larger, indicating that the 
 convexity of the surface has diminished, while no change takes 
 place in the curvature either of the cornea or of the posterior 
 surface of the lens. And accurate measurements of the size of 
 the image from the anterior surface of the lens have shewn that 
 the variations in curvature which do take place, are sufficient to 
 account for the power of accommodation which the eye possesses. 
 
 The observation of these I'eflected images is facilitated by the simple 
 instrument introduced by Helmholtz and called a Phakoscope. It 
 consists of a small dark chamber, with apertures for the observed and 
 observing eyes ; a needle is fixed at a short distance in front of the 
 former, to serve as a near object, for Avhich accommodation has to be 
 made ; and a lamp or candle is so disposed as to throAv an image on 
 each of the three surfaces of the observed eye. Since the distance 
 between two images is more readily appreciated than is a simple change 
 of size of a single image, two prisms are employed so as to throw a 
 double image of the lamp on each of the three surfaces. When the 
 anterior surface of the lens becomes more convex the two images 
 reflected from that surface approach each other, when it becomes less 
 convex they retire from each other. 
 
 These observations leave no doubt that the essential change by 
 which accommodation is effected, is an alteration of the convexity of 
 the anterior surface of the lens. And that the lens is the agent of 
 accommodation, is further shewn by the fact that after removal of 
 the lens, as in the operation for cataract, the power of accommo- 
 
('M.vr. n.] SKllIT. 499 
 
 (Uition is lost. Ill tliu c'as(>s whiili have boon recorded, wlierc eyes 
 IVoiii wliuh tlie lens luid been removed seemed still to possess 
 some aeeommodiition, we must su])i)ose that no real accommodation 
 took place, but that the })U])il contracted wlien a near object wa,s 
 looked at, and so assisted in making vision more distinct. 
 
 This increase of the convexity of the lens has been supposed 
 to be due to a com})ression of the circumference of tlie lens by a 
 contraction of the iris; but this is disproved by the fact that 
 acconnnodatiou may take place in eyes from whicli the iris is con- 
 genitally absent. It has also been attributed to vaso-motor 
 changes, to increased fulness of the vessels of the iris or ciliary 
 processes, surrounding the lens ; but this also is disproved by the 
 fact that accommodation may be effected, after death in an eye 
 •which is practically bloodless, by stimulating the ciliary ganglion 
 or ciliary nerves with an interrupted current or b}^ other means. 
 The real nature of the mechanism seems to be as follows. 
 
 The lens when examined after removal from the eye is found 
 to be a body of considerable elasticity. When the curvature of 
 the anterior surface of the lens is determined, as may be dono by 
 appropriate means, in its natural jDosition in the eye at rest, and 
 then again determined, after the lens has been removed from the 
 eye, the anterior surface is found to be more convex in the latter 
 than in the former case. There seems to be, in the eye in its 
 natural condition, some agency at work, keeping the anterior 
 surface of the lens somewhat flattened. The suspensory liga- 
 ment, attached to the choroid and ciliary processes behind, and 
 passing over the front of the lens, is just such a structure as 
 would produce this eftect. In the natural position of the choroid 
 this ligament is tense, and tends to flatten the front of the lens. 
 When the choroid is pulled forward, the ligament becomes slack 
 and the lens bulges out forward. Further the ciliary muscle 
 attached on the one hand to a fairly fixed region, the junction of 
 the sclerotic and cornea, and on the other to the looser and more 
 moveable choroid, would naturally, when thrown into contraction, 
 pull forward the choroid and so slacken the suspensory ligament, 
 and hence permit the elastic lens to bulge out forwards. And Ave 
 have experimental evidence, carried out on lower animals, that 
 stimulation of the ciliary ganglion or of its so-called radix brevis, 
 does lead on the one hand to a contraction of the ciliary muscle 
 and pulling forward of the choroid, and on the other hand to an 
 increased curvature of the anterior surface of the lens. Hence we 
 may conclude that accommodation for near objects consists essen- 
 tially in a contraction of the ciliary muscle, which, by j^ulling 
 forward the choroid coat and the ciliary process, slackens the sus- 
 pensory ligament, and allows the lens to bulge forward by virtue 
 of its elasticity, and so to increase the convexity of its anterior 
 surface. 
 
 Accommodation is in most cases a voluntary act ; since, however, 
 
 32—2 
 
500 MOVEMENTS OF THE PUPIL. [Book hi. 
 
 the change in the lens is always accompanied by movements in 
 the iris, it will be convenient to consider the latter, before we 
 discuss the nervous mechanism of the whole act. 
 
 Movements of the Pupil. Though by making the efforts 
 required for accommodation we can at pleasure contract or dilate 
 the pupil, it is not in our power to bring the will to act directly on 
 the iris by itself. This fact alone indicates that the nervous 
 mechanism of the pupil is of a peculiar character, and such indeed 
 we find it to be. The pupil is contracted (1) when the retina (or 
 optic nerve) is stimulated, as when light falls on the retina, the 
 brighter the light the greater being the contraction, (2) when we 
 accommodate for near objects. The pupil is also contracted when 
 the cA^eball is turned inwards, when the aqueous humour is deficient, 
 in the early stages of poisoning by chloroform, alcohol, &c.; in nearly 
 all stages of poisoning by morphia, physostigmin, and some other 
 drugs, and in deep slumber. The pupil is dilated (1) when stimu- 
 lation of the retina (or optic nerve) is diminished or arrested as in 
 passing from a bright into a dim light or into darkness, (2) when 
 the eye is adjusted for far objects. Dilation also occurs when there 
 is an excess of aqueous humour, during dyspnoea, during violent 
 muscular efforts, as the result of a stimulation of sensory nerves, as 
 an effect of emotions, in the later stages of poisoning by chloroform, 
 &c. and in all stages of poisoning by atropin and some other drugs. 
 
 Contraction of the pupil is caused by contraction of the circular 
 fibres or sphincter of the iris. Dilation is caused by contraction of 
 the radial fibres of the iris; for though the existence of radial fibres 
 has been denied by many observers, the preponderance of evidence 
 is clearly in favour of their being really present. 
 
 Considering how vascular the iris is, it does not seem unreason- 
 able to interpret some of the variations in the condition of the pupil 
 as the results of simple vascular turgescence or of depletion brought 
 about by vaso-motor action or otherwise, the small or contracted 
 pupil corresponding to the dilated and filled, and the large or 
 dilated pupil to the constricted and emptied condition of the blood- 
 vessels. Thus slight oscillations of the jjupil may be observed 
 synchronous with the heart-beat and others synchronous with the 
 respiratory movements. But the variations in the pupil seem too 
 marked to be merely the effects of vascular changes, and indeed that 
 constriction of the pupil cannot be wholly the result of turgescence, 
 nor dilation wholly the result of depletion of the vessels of the iris, 
 is shewn by the facts that both these events m.ay be witnessed in a 
 perfectly bloodless eye, and that the movements of the pupil when 
 brought about by agents which also affect the blood-vessels, begin 
 some time before the changes in the calibre of the blood-vessels, 
 and indeed may be over before these have arrived at their maxi- 
 mum. Moreover the fibres of the sympathetic, which as we shall 
 see are concerned in causing dilation of the pupil, run a somewhat 
 
Cm A.-. II.] si(;nr. 501 
 
 (lifftTt'iit c'uiirsc tVuin tliusc wliii li ^'•umiu tile l)lu«)<l-vessel.s of the eye. 
 Wt- iiKiv tlu'ivlurt' iullii-n- to tln' vu*\v that thf iiuiiu chuiii^'i'S of tht- 
 pupil in the diiL-ctiou of narrowiiji,' and wick-ning an- l)nMit(ht about 
 by contractions of the plain muscular fibres in the iris. 
 
 Muscular ct>ntractions leadinj.^ to chanf,'es of the pupil may be 
 observed in the eye removed from the body, and indeed in the 
 extirpateil iris. The plain muscular fibres of the iris like other 
 plain nuiscular fibres are remarkably sensitive to variatitjns in tem- 
 perature. Besides this there seems to be in certain animals at 
 least a coimection within the eye between the iris and retina of 
 .such a kind, that light falling into an extirpated eye will lead to a 
 narrowing of the pu|)il. Putting a.side however such exceptional 
 t'vents we may lay down the broad principle that contraction of the 
 pupil, brought about by light falling on the retina, is a refiex act, 
 of which the optic is the afferent nerve, the third or oculo-motor 
 the efferent nerve, and the centre some portion of the brain lying 
 below the corpora quadi'igemina in the front part of the floor of the 
 atpieduct of Sylvius. This is proved by the following facts. When 
 the optic nerve is divided, the falling of light on the retina no 
 longer causes a contraction of the pupil. When the third ner^'e is 
 divided, stimulation of the retina or of the optic nerve no longer 
 cau.ses contraction ; but direct stimulation of the peripheral portion 
 of the divided third nerve causes extreme contraction of the pupil. 
 If the region of the brain spoken of above as a centre be carefully 
 stimulated contraction of the pupil will take place even in the 
 absence of light and after division of the optic nerve. After 
 removal of the same centre stimulation of the retina is ineffectual 
 in narrowing the pupil. But if the centre and its connections with 
 the optic nerve and third nerve be left intact and in thoroughly 
 sound condition, contraction of the pupil will occur as a result of 
 light falling on the retina, though all other nervous parts be 
 removed. 
 
 The nervous centre is not a double centre with two completely 
 independent halves, one for each eye; there is a certain amount 
 of functional communion between the two sides, so that when one 
 retina is stimulated both pupils contract. It might be imagined 
 that this cerebral centre acted as a tonic centre, whose action was 
 simply increased not originated by the stimulation of the retina ; 
 but this is disproved by the fact that, if the optic nerve be 
 divided, subsequent section of the third ners'e produces no fiirther 
 dilation. 
 
 In considering the movements of the pupil, however, Ave have 
 to deal not only with a naiTowing of the pupil thus brought about, 
 in a reflex way by contraction of the circular sphincter fibres, and 
 with the absence of such a narrowing, but also with active dilation 
 due to a contraction of the radial dilator fibres, and this renders the 
 whole matter much more complex than might be supposed to be the 
 case from the simple statement just made. 
 
502 
 
 MOVEMENTS OF THE PUPIL. 
 
 [Book in. 
 
 The iris is supplied, in common with the ciliary muscle and 
 choroid, by the short ciliary nerves (Fig. 68 s.c) coming from the 
 ophthalmic or lenticular (ciliary) ganglion {l.c) which is connected 
 by its roots with the third nerve (r.h.), the cervical sympathetic nerve 
 (sym.), and with the nasal branch of the ophthalmic division of the 
 
 Fig. 68. Diagrammatic eepeesektation op the Neiives goverxixg the pupil. 
 
 II. Optic nerve, l.g. lenticular ganglion, r.b. its short root from 121. oc.in. third 
 or oculo-motor nerve. syni. its sympathetic root. r.L its long root from 
 V. ophthm. the nasal branch of the ophthalmic division of the fifth nerve. 
 s.c. the short ciliary nerves from the lenticular ganglion, l.c. the long cHiary 
 nerve from the nasal branch of the ophthalmic division of the fifth nerve. 
 
 fifth nerve (?'.Z.) The short ciliary nerves are, moreover, accompanied 
 by the long ciliary nerves (I.e.) coming from the same nasal branch 
 of the ophthalmic division of the fifth nerve. What are the uses' 
 of these several nerves in relation to the pupil ? 
 
 If the cervical sympathetic in the neck be divided, all other 
 portions of the nervous mechanism being intact, a contraction of 
 the pupil (not always very well marked) takes place, and if the 
 peripheral portion {i.e. the upper portion still coimected with the 
 
Vuw. 11. j SlUllT. Wi 
 
 luad) bi' stiiMiihitt'tl, ;i \vcll-ilL'Vt'li>|)(;(l (Illation is tin- r<'Siilt. Tho 
 syiujHitlu'tic Ikis, it will bo observed, an elVeet on tho iris, tliu 
 opptisitt' ot" that which it cxcreises (Hi the blood-vessels; when it is 
 stimulatrtl the pupils are dilatt'd while the blood-vessels are con- 
 stricted. This diluting,' intliienee of the syinj)athetic may, as in the 
 case of the vaso-nmtor action of the same ncrvi;, be traced back 
 down the neck to tlie uj)per thoracic •^'-an^^dion and thence along tlie 
 rami conimunicantes ami roots of the lower cer\ical and first dorsal 
 or two first dorsal spinal nerves, to a region in the lower cervical 
 and vipper dorsal cord (called by some authors the centrum cilio- 
 spiiuile inferitis), and from thence np through the medulla oblongata 
 to a centre, which appears lobe plai-ed in the floor of the front part- 
 of the acjueduct of Sylvius not far from and apparently on either 
 side of the centre for contraction of the pupil. 
 
 The dilation of the pupil which is witnessed in dyspna-a, and that 
 which results from stimulation of sensory nerves and from emotions, 
 appears to be brought about by the action of the sympathetic, the 
 venous blood, or the sensory impulses or tho emotional impulses so 
 affecting the dilating centre as to augment the dilating impulses 
 proceeding from it along the sympathetic. The existence of the 
 subordinate centre in the cervical or dorsal cord, spoken of just 
 now, is supposed to be indicated by the fact that after division of 
 the medulla oblongata, and consequent severance of the efferent 
 paths from the centre in the aqueduct of Sylvius, dilation of the 
 pui)il may still be brought about, in some animals at least, by 
 dyspnoea or by adecj[uate stimulation of sensory nerves. A 
 question is raised here in fact somewhat similar to that raised in 
 connection with the medullaiy respiratory centre (p. 355); and 
 here as there we may probably conclude that the independent 
 action of such a spinal centre is of subordinate importance. 
 
 The pupil then seems to be under the dominion of two antago- 
 nistic mechanisms : one a contracting mechanism, reflex in nature, 
 the third nerve serving as the efferent, and the optic as the afferent 
 tract ; the other a dilating mechanism, apparently tonic in nature, 
 but subject to augmentation from various causes, and of this the 
 cervical sympathetic is the efferent channel. Hence, when the 
 third or optic nerve is divided, not only does contraction of the 
 pupil cease to be manifest, but active dilation occurs, on account of 
 the tonic cUlating influence of the symj^athetic being left free to 
 work. When, on the other hand, the sympathetic is cUvided, this 
 tonic dilating influence falls away, and contraction results. When 
 the optic or third nerve is stimulated, the dilating effect of the 
 sympathetic is overcome, and contraction results; and when the 
 sympathetic is stimulated, any contracting influence of the third 
 nerve which may be present is overcome, and dilation ensues. 
 
 But there are considerations which shew that the matter is still 
 more complex than this. A small quantity of atropin introduced 
 into the eye or into the system causes a dilation of the pupil. This 
 
504 MOVEMENTS OF THE PUPIL. [Book hi. 
 
 might be attributed to a paralysis of the third nerve, and indeed it 
 is found that after atropin has produced its effects the falHng of 
 light on the retina no longer causes contraction of the pupil. A 
 difficulty however is introduced by the fact that when the third 
 nerve is divided, and when therefore the contracting effects of 
 stimulation of the retina are placed entirely on one side, and there 
 is nothing to prevent the sympathetic producing its dilating effects 
 to the utmost, dilation is still further increased by atropin. When 
 physostigmin is introduced into the eye or system, contraction of 
 the pupil is caused, whether the third nerve be divided or not ; and 
 when the dose is sufficiently strong the contraction is so great that 
 it cannot be overcome by stimulation of the sympathetic. The 
 dilation Avhich is caused by a sufficient dose of atropin may be 
 greater than that which can ordinarily be produced by stimulation 
 of the sympathetic, and the contraction caused by a sufficient dose 
 of physostigmin may be greater than that which is ordinarily 
 produced in a reflex manner by stimulation of the optic nerve, 
 or even than that produced by direct stimulation of the third 
 nerve. Evidently these drugs act either directly on the plain 
 muscular fibres of the iris or on some local mechanism, the 
 one in such a way as to cause dilation, the other in such a w^ay 
 as to cause contraction. Such a local mechanism cannot how- 
 ever lie in the ophthalmic ganglion, for both drugs continue to 
 produce these effects in a most marked degree after the ganglion 
 has been excised. We must suppose therefore that the mechanism 
 if it exists is situated in the iris itself or in the choroid, where 
 indeed ganglionic nerve-cells are abundant. The movements of 
 the iris in the extirpated eye, spoken of just now, may perhaps 
 be attributed to the same local mechanism. Further it is stated 
 that with stimulation of the sympathetic, the latent period, ?.e. 
 the period intervening between the beginning of stimulation and 
 the beginning of the movement of the iris, is much greater than 
 with stimulation of the. third nerve, indicating that the former 
 acts through a local mechanism but the latter more directly on 
 the muscular fibres. The whole question however of this local 
 mechanism, and of the exact mode of action of the various drugs 
 and of the changes in the body which lead to contraction or 
 dilation respectively of the pupil, needs fuller discussion than 
 we can afford to give to it here. We may add that the local 
 action of atropin in contrast to any action on the cerebral centre 
 is well illustrated by applying atropin to one eye locally. The 
 pupil of that eye dilates widely ; in consequence more light falls 
 on the retina, and this so affects the cerebral centre, which as 
 we have seen is not strictly unilateral but in communion with 
 its fellow, that increased constricting impulses pass from both 
 centres, and these, though ineffectual in the atropinized eye, 
 lead in the untouched eye to an increased narrowing of the pupil. 
 The share of the fifth nerve in the work of the iris seems to be 
 
i'u.w. \i.\ siciiT. .-)0:. 
 
 ill |tart a si-usury one ; tlif iris is sciisit ivc, ami the sensory impulses 
 which aiv <(eiierati'(l in it pass tVuin it alonj; tlif libn-s of the tit'tli 
 nt-rvf. Moivuver the titth is peeuliurly related to the dilating etii-cts 
 of the syinpathetio. For though the ophthalniie ganglion does 
 receivi' tihres directly from the cavernous plexus of the sympathe- 
 tic, the dilating action of the sympathetic would seem to be carried 
 out not by tlu-se tibn-s but by lilin-s joining the fifth iicrvr, and 
 j>assing to tlu' iris not by the ganglion l)ut by the ophthalmic 
 branch and the long ciliary nerves. The vasomotor fibres of the 
 sympathetic, and those wliich dilate the iris, after running to- 
 gether in the main cervical sympathetic chain, part company 
 liigher iip, the latter ]jassing to the CJasserian ganglion, and thus 
 reaching the nasal branch of the oplithalmic division of the fifth 
 nerve. Some observers maintain that in addition to these dilating 
 fibres of the sympathetic joined to it, the filth contains fibres of its 
 own which also are able to dilate the pupil. 
 
 We may sum uj) the nervous mechanism of the pupil then 
 somewhat as follows. The salient and most frequently repeated 
 event, the contraction of the pujiil, upon exposure to light, is a 
 reflex act, the centre of which is placed in the brain ; and the 
 correlative widening of the pupil upon diminution of light is due 
 to the tonic action of the sympathetic making itself felt upon the 
 waning of its antagonist. The contraction of the pupil in the 
 earlier stages of the action of alcohol and chloroform and in 
 slumber is probably due to an increased action of the contracting 
 centre, but the narrow pupil caused by such drugs as morphia 
 and physostigmin is due, cliiefiy at least, to a local action. The 
 dilating effects of such drugs as atropin are also largely due to a 
 local action, but in the widened pupil of the later stages of alcohol 
 poisoning and of dyspnoea we can probably trace the eflfects (f 
 an exhaustion of the cerebral contracting centre, assisted possibly 
 by an increased activity of the dilating centre. 
 
 There remains a word to be said concerning the contraction of 
 the pupil which takes place when the eye is accommodated for near 
 objects, and when the pupil is turned inwards (the two being 
 closely allied, since the eyes converge to see near objects), and the 
 return to the more dilated condition Avhen the eye returns to rest 
 and regains the accommodation for far objects. These are instances 
 of what are called " associated movements." Two movements are 
 thus spoken of as " associated " when the special central nervous 
 mechanism employed in carrying out the one act is so connected 
 by nervous ties of some kind or other with that employed in 
 carrying out the other, that when we set the one mechanism 
 in action we unintentionally set the other in action also. The 
 ciliary muscles which bring about accommodation are governed in 
 this action by fibres which may be traced, through the ciliary 
 nerves and lenticular ganglion, along the third or ocido-motor 
 nerve, to a centre which lies (in dogs) in the hind part of the floor 
 
506 DIOPTPdC IMPERFECTIONS. [Book in. 
 
 of the third ventricle, and which is especially connected with the 
 most anterior bundles of the roots of the third nerve. This centre 
 is under the command of our will : when we Avish to accommodate 
 for near objects we throw it into action, and it, when in action, calls 
 also into action by ' association ' the centre for the contraction of 
 the pupil ; when the action of the accommodation centre ceases and 
 the eye falls back to the condition of rest, in which it is accommo- 
 dated for far objects, the action of the pupil-contracting centre 
 ceases also, and the pupil therefore widens. 
 
 The mechanism of accommodation may also be affected in a 
 local manner. And the drugs which have a special action on the 
 pupil, such as atropin and calabar bean, also affect the mechanism 
 of accommodation. Atrojjin 23aralyses it, so that the eye remains 
 adjusted for far objects; and physostigmin throws the eye into a 
 condition of forced accommodation for near objects. This double 
 action has been explained by the supposition that while atropin 
 paralyses, physostigmin throws into tonic or tetanic contraction, 
 on the one hand the circular muscles of the iris and on the other 
 the ciliary muscles ; but the phenomena, on inquiry, appear too 
 complicated to be explained in so simple a manner. 
 
 We can accommodate at will ; but few persons can effect the 
 necessary change in the eye unless they direct their attention to 
 some near or far object, as the case may be, and thus assist their 
 Avill by visual sensations. By practice, however, the aid of external 
 objects may be dispensed with; and it is when this is achieved 
 that the pupil may seem to be made to dilate or contract at 
 pleasure, accommodation being effected without the eye being 
 turned to any particular object. 
 
 Imperfections in the Dioptric Apparatas. 
 
 The emmetrojaic eye may be taken as the normal eye. The 
 myopic and hypermetropic eyes may be considered as imperfect 
 eyes, though the former possesses certain advantages over the 
 normal eye. An eye might be myopic from too great a convexity 
 of the cornea, or of the anterior surface of the lens, or from per- 
 manent spasm of the accommodation-mechanism, or from too great 
 a length of the long axis of the eyeball. The last appears to 
 be the usual cause. Similarly, most h3rpermetropic eyes possess 
 too short a bulb. Moreover in the strongly marked myopic eye 
 there is frequently hypertrophy of the longitudinal (meridional) 
 fibres of the ciliary muscle, often spoken of exclusively as the 
 ciliary muscle, and atrophy or absence of the circular fibres ; in the 
 hypermetropic eye on the other hand the circular fibres are well 
 developed and the meridional fibres scanty. The presbyopic eye 
 
c'liAi'. II.) xiaiir. -,07 
 
 is, us wo luivi' scfii, an e}'^ iinnnally e<>Tistitii<i'<l In which \\v 
 power (»t" aeoonuuodatioii has been lost or is failing through in- 
 crcasinor weakuL'ss of the ciliary muscle or a loss ol' elasticity in 
 the lens, or through the parts becoming rigid. 
 
 Spherical Aberration. In a spherical lens the rays which 
 impinge on tiic circumi'crencc are brought to a focus sooner tlian 
 those which pass nearer the centre, and the rays proceeding from 
 a luminous point are no longer brought to a single focus at one 
 point but form a number of foci at different distances. Hence when 
 rays are aili»wed to i'all on the wIkjIo of the lens, the image formed 
 on a screen placed in the focus of the more central rays is bluired 
 by the diftusion-circles caused by the circumferential rays Avhich 
 have been brought to a premature focus. In an ordinary optical 
 instrument spherical aberration is obviated by a diaphragm which 
 .sliuts off the more circumferential rays. In the eye tlie iris is an 
 adjustable diaphragm ; and when tl;e pupil contracts in near vision 
 the more dixergcnt rays proceeding irom a near object, which tend 
 to fall on the circumferential parts of tlie lens, are cut off. As, 
 however, the refractive power of the lens does not increase regu- 
 larly and progi-essively from the centre to the circumference, but 
 varies most irregularly, the purpose of the narrowing of the pupil 
 cannot be simi^ly to obviate spherical aberration ; and indeed the 
 other optical imperfections of the eye are so great, that such 
 spherical aberrations as arc caused by the lens produce no obvious 
 effect on vision. 
 
 Astig'matism. We have hitherto treated the eye as if its 
 dioptric surfaces were all parts of perfect spherical surfaces. In 
 reality this is rarely the case, either with the lens or with the 
 cornea. Slight deviations do not produce any marked effect, but 
 there is one deviation, knowai as regular astigmatism, which, present 
 to a certain extent in most eyes, very largely developed in some, 
 frequently leads to very imperfect vision. This defect is due to the 
 dioptric surface being not spherical but more convex along one 
 meridian than another, more convex, for instance, along the vertical 
 than along the horizontal meridian. When this is the case the rays 
 proceeding from a luminous point are not brought to a single focus 
 at a point, but possess two linear foci, one nearer than the normal 
 focus and coiTcsponding to the more convex surface, the other 
 farther than the normal and corresponding to the less convex 
 surface. If the vertical meridians of the surface be more convex 
 than the hoiizontal, then the nearer linear focus will be horizontal 
 and the farther linear focus Avill be vertical, and vice versa. (This 
 can be shewn much more effectually on a model, than in a diagram 
 in which we are limited to two dimensions.) Now, in order to see 
 a vertical line distinctly, it is much, more imjDortant that the raj'S 
 which diverge from the line in a series of horizontal planes should 
 be brought to a focus pro|)erly than those which diverge in the 
 
508 DIOPTRIC IMPERFECTIONS. [Book hi. 
 
 vertical plane of the lino itself; and similarly, in order to see a 
 horizontal line distinctly it is much more important that the rays 
 whch diverge from the line in a series of vertical planes should be 
 brought to a focus properly than those which diverge in the 
 horizontal plane of the line itself. Hence a horizontal line held 
 before an astigmatic dioptric surface, most convex in the vertical 
 meridians, will give rise to the image of a horizontal line at the 
 nearer focus, the vertical rays diverging from the line being here 
 brought to a linear horizontal focus. Similarly, a vertical line 
 held before the same surface will give rise to an image of a vertical 
 line at the farther focus, the horizontal rays diverging from the 
 vertical line being here brought to a linear vertical focus. In 
 other words, with a dioptric surface most convex in the vertical 
 meridians, horizontal lines are brought to a focus sooner than are 
 vertical lines. 
 
 Most eyes are thus more or less astigmatic, and generally with 
 a greater convexity along the vertical meridians. If a set of 
 horizontal or vertical lines be looked at, or if the near point of 
 accommodation be determined by Scheiner's experiment (p. 494), 
 for the needle placed first horizontally and then vertically, the 
 horizontal lines or needle will be distinctly visible at a shorter 
 distance from the eye than the vertical lines or needle. Similarly, 
 the vertical line must be farther from the eye than a horizontal 
 one, if both are to be seen distinctly at the same time. The cause 
 of astigmatism is, in the great majority of cases, the unequal 
 curvature of the cornea ; but sometimes the fault lies in the lens, 
 as was the case with Young. 
 
 When the curvature of the cornea or lens differs not in two 
 meridians only but in several, irregular astigmatism is the result. 
 A certain amount of irregular astigmatism exists in most lenses, 
 thus causing the image of a bright point, such as a star, to be not 
 a circle but a radiate hgure. 
 
 Chromatic Aberration. The different rays of the spectrum 
 are of different refrangibility, those towards the violet end of the 
 spectrum being brought to a focus sooner than those near the red 
 end. This in optical instruments is obviated by using compound 
 lenses made up of various kinds of glass. In the eye we have no 
 evidence that the lens is so constituted as to correct this fault ; 
 still the total dispersive power of the instrument is so small, that 
 such amount of chromatic aberration as does exist attracts little 
 notice. Nevertheless some slight aberration may be detected by 
 careful observation. When the spectrum is observed at some 
 distance the violet end will not be seen in focus at the same time 
 as the red. If a luminous point be looked at through a nan-ow 
 orifice covered by a piece of violet glass, which while shutting out 
 the yellow and green allows the red and blue rays to pass through, 
 there will bo seen alternately an image having a blue centre with 
 
ClIAl'. 11.] 
 
 SI CUT. 
 
 509 
 
 a ivil i'liii^fr, or ;i rid centn' with ;i blue Iriiigc, acconling us the 
 image of the point looked at is thrown on one side or other ol' the 
 true focus. Thus supposing / (Fig. (jO) to he tho plane of the 
 
 Fig. 69. Dl^gkam iliasti;atixg Chkoji.'.tic AnEniiAxioN. 
 
 hh is the dioptric surface, hv represents the Uuc, and lir the red ra;s; I' is tlic focal 
 plane of the blue, R of the red rays. 
 
 mean focus of A, the \ioletrays will be brouglit to a locus in the 
 plane Y, and the red rays in the plane R. If the rays be supposed 
 to fall on the retina between V and /', the diverging or blue rays 
 will form a centre surrounded by the still converging red rays ; 
 whereas if the rays fall on the retina between /' and U, the con- 
 verging red rays will form a centre with the still diverging blue 
 rays forming a fringe round them. If the rays fall on the retina at 
 f, the two kinds of rays will be mixed together ; as will be seen 
 from the figure, the circumferential still converging red ray hr as 
 it cuts the plane of the retina is, in ordinary vLsion, accompanied 
 b}^ the diverging violet ray kv, and thus by a sort of compensation, 
 we see together even the rays which differ most in refraction. 
 
 Entoptic Phenomena. The various media of the eye are not 
 unilbrmly transparent ; the rays of light in passing through them 
 undergo local absorption and refraction, and thus various shadows 
 are thrown on the retina, of which we become conscious as im- 
 perfections in the field of vision, especially when the eye is directed 
 to a unifonnly illuminated surface. These are spoken of as 
 entoptic phenomena, and are very varied, many forms having been 
 described. 
 
 The most common are those caused by the presence of floating 
 bodies in the vitreous humour, the so-called vniscce voUtantes. 
 These are readily seen when the eye is turned towards a uniform 
 surface, and are frequently very troublesome in looking through a 
 microscope. They are especially obvious when divergent rays fall 
 upon the eye. The}^ assume the form of rows and groups of 
 beads, of single beads, of streaks, patches and gi*anules, and may 
 be recognised by their almost continual movement, especially when 
 the head or eye is moved up and down. AVhcn an attempt is 
 made to fix the vision upon them, they immediately float away. 
 Tears on the cornea, temporary unevenness on the anterior surface 
 of the cornea after the eyelid has been pressed on it, and imper- 
 fections in the lens or its capsule, also give rise to visual images. 
 
510 DIOPTRIC IMPERFECTIONS. [Book iii. 
 
 Not unfrequentlya radiate figure corresponding to the arrangement 
 of the fibres of the lens makes its appearance. 
 
 Imjoerfections in the margin of the pupil appear in the shadow 
 of the iris which bounds the field of vision ; and the movements 
 of the iris in one eye may be rendered visible by looking at a bright 
 point or luminous surface through a pin-hole in a card placed close 
 in the front of the eye, in the anterior focus in fact, and then 
 alternately closing and opening the other eye ; the field of the 
 first may be observed to contract when light enters, and to expand 
 when the light is shut off from the second. The media of the eye are 
 fluorescent : a condition which favours the perception of the ultra- 
 violet rays. If a white sheet or white cloud be looked at in day- 
 light through a Nicol's prism, a somewhat bright double cone or 
 double tuft, with the apices touching, of a faint blue colour, is 
 seen in the centre of the field of vision, crossed by a similar double 
 cone of a somewhat yellow darker colour. These are spoken of 
 as Haidinger's brushes ; they rotate as the prism is rotated, and 
 are supposed to be due to the unequal absorption of the polarized 
 light in the yellow spot. The prism must be frequently rotated, 
 as Avhen the prism remains at rest the phenomena fade. Lastly, 
 the optical arrangements have a further imperfection in that the 
 dioptric surfaces are not truly centred on the optic axis. 
 
SEC. 2. VISUAL SENSATIONS. 
 
 Light falling on the retina excites sensorij impulses, and these 
 pa.ssing up the optic nerve to certain parts of the brain, produce 
 changes in certain cerebral structures, and thus give rise to what 
 WQ. call a sensation. In a sensation we ought to be able to dis- 
 tinguish between the events through which the impact of the rays 
 of light on the retina is enabled to generate sensory impulses, and 
 the events, or rather series of events, through Avhich these sensor\- 
 impulses (for, judging by the analogy of motor nencs, we have no 
 reason to think that they undergo any fundamental changes in 
 passing along the optic nerve), by the agency of the cerebral 
 arrangements, develope into a sensation. Such an analysis, how- 
 ever, is, at present at least, in most particulars, quite beyond our 
 power; and we must therefore treat of the sensations as a whole, 
 distinguishing between the peripheral and central phenomena, on 
 the rare occasions when vrc are able to do .so. 
 
 The On'r/in of Visual Impulses. 
 
 Of primary importance to the understanding of the way in 
 which luminous undulations give lise to those ner\-ous changes 
 which pass along the optic nerve as ^-isual impulses, is the fact that 
 the rays of light produce their effect by acting not on the optic 
 nerve itself but on its terminal organs (see p. 488). They pa.ss 
 through the anterior layers of the retina apparently without induc- 
 ing any effect; it Is not till they have re'ached the region of the 
 rods and cones that they set up the changes concerned in the 
 
512 VI.'SUAL SEXSATIONS. [Book iii. 
 
 generation of visual impulst^ ; and the impulses here generated 
 travel back to the layer of fibres in the anterior surface of the 
 retina and thence pass along the optic nerve. That the optic fibres 
 are themselves insensible to light and that visual impulses begin in 
 the region of rods and cones is shewn by the phenomena of the 
 blind spot and of Purkinje's figures respectively. 
 
 Blind Spot. There is one part of the retina on which rays of 
 light falling give rise to no sensations ; this is the entrance of the 
 optic nerve, and the coiTesponding area in the field of vision is 
 called the blind spot. If the visual axis of one eye, the right for 
 instance, the other being closed, be fixed on a black spot in a white 
 sheet of |)aper, and a small black object, such as the point of a quill 
 pen dipped in ink, be moved gradually sideways over the paper 
 away to the outside of the field of vision, at a certain distance the 
 black point of the quill wdll disappear from view. On continuing 
 the movement still farther outward the point will again come into 
 view and continue in sight until it is lost in the periphery of the 
 field of vision. If the pen be used to make a mark on the paper at 
 the moment when it is lost to view, and at the moment when it 
 comes into sight again ; and if similar marks be made along the 
 other meridians as well as the horizontal, an irregular outline Avill 
 be drav\'n circumscribing an ai^ea of the field of vision within which 
 rays of light produce no visual sensation. This is the blind spot. 
 The dimensions of the figure drawn vary of course with the distance 
 of the paper from the eye. If this distance be known, the size as 
 well as the position of the area of the retina corresponding to the 
 blind spot may be calculated from the diagrammatic eye (p. 492). 
 The position exactly coincides with the entrance of the optic nerve, 
 and the dimensions (about Vo mm. diameter) also correspond. 
 While drawing the outline as above directed the indications of the 
 large branches of the retinal vessels as they diverge from the 
 entrance of the ner\-e can frequently be recognised. The existence 
 of the blind spot is also shewn by the fact that an image of light, 
 sufificiently small, thrown upon the optic nerA"e by means of the 
 ophthalmoscope, gives rise to no sensations. 
 
 The existence of the blind spot proves that the optic fibres 
 themselves are insensible to light ; it is only through the agency 
 of the retinal expansion that these can be stimulated by luminous 
 vibrations. 
 
 PurMllje's Figures. If one enters a dark room with a candle, 
 and while looking at a plain (not parti-coloured) w^all, moves the 
 candle up and down, holding it on a level with the eyes by the side 
 of the head, there will appear in the field of vision of the eye of the 
 same side, projected on the wall, an image of the retinal vessels, 
 quite similar to that seen on looking into an eye with the ophthal- 
 moscope. The field of vision is illun^inated with fi glare, and on 
 
L'llAI'. II 
 
 su;nr. 
 
 bVi 
 
 tins tl"H' luaiirhi'd iftiiiul \fsscl.s apjicjir iis sluidows. In this iinidr 
 of fxperiin('ntin<^ tin- lit^dit ciittTS the eye through th(.' cornea, ami 
 an image ot" the candle is t'onned on the n:isal side of the retina; 
 and it is the light emanating from this image which throws shadows 
 of th<> retinal vessels on to the rest of the retina. A far better 
 method is tor a second person to concentrate the rays of light, with 
 a lens of low power, on to the outside of the sclerotic just behind 
 the cornea; the light in this case emanates from the illuminated 
 spot on the sclerotic and passing straight through the vitreous 
 humour throws a direct shadow of the vessels on to the retina. 
 Thus the rays passing through the sclerotic at b, Fig. 70, in the 
 direction hv, will throw a sliadow of the vessel v on to the retina at 
 /9; this will appear as a dark line at B in the glare of the field of 
 vision. This jiroves that the structures in which visual impuLses 
 originate must lie behind the retinal vessels, otherwise the shadows 
 of these could not be perceived. 
 
 B A 
 
 l-'iG. 70. Diagram ii.LrsrnAriNG the Formation of PuKKixjii's Figcues when xua 
 Illcmixatid:," is kirkcted through the Sclerotic. 
 
 If the light be moved from h to a, the shadow on the retina will 
 move from fi to a, and the dark line in the iield of vision will move 
 from B to A. If the distance BA be measured v.hen the whole 
 image is projected at a known distance, A-B from the eye, k being 
 the optical centre', then, knowing the distance k^ in the diagram- 
 matic eye, the distance /3a can be calculated. But if the distance 
 /Sa be thus e.stimated, and the distance ha be directly measured, 
 the distances /3i', nv, hv, av can be calculated, and if the appearance 
 in the field of vision is really caused by the shadow of v falling on 
 
 ^ For the properties of the optical centre, we must refer the rcxder to the various 
 treatises on optics. The optical centre of a lens is the point through which all the 
 principal rays, of the various pencils of rays falling on the lens, pass. The diagram- 
 matic eye of Listing (p. 4;^t2) has two optical centre?, but the. e may, without serious 
 error, be further rcluced for practical purposes to one lying in the lens near its 
 posterior surface, at about 15 mm. distance from the retina. 
 
 F. 33 
 
514 VISUAL SEXSATIONS. [Book iii. 
 
 ft, these distances ought to correspond to the distances of the retinal 
 vessels v from the sclerotic h on the one hand, and from that part 
 of the retina p where visual impressions begin, on the other. H. 
 Miiller found that the distance ftu thus calculated corresponded to 
 the distance of the retinal vessels fi-om the layer of rods and cones. 
 Thus Purkinje's figures prove in the first place that the sensory 
 impulses which form the commencement of visual sensations origi- 
 nate in some part of the retina behind the retinal vessels, i. e. some- 
 where between them and the choroid coat; and H. Miiller's calcula- 
 tions go far to shew that they originate at the most posterior 
 or external part of the retina, viz. the layer of rods and cones. 
 It must be admitted however that H. Miiller's results were not 
 sufficiently exact to allow any gTeat stress to be placed on this 
 argument. 
 
 In the second method of exiaerimenting, the image always moves 
 in the same direction as the light, as it obviously must do. In the 
 first method, where the light enters through the cornea, the image 
 moves in the same direction as the light when the light is moved 
 from right to left, j)rovided the movement does not extend beyond 
 the middle of the cornea, but in the oj)posite direction to the light 
 when the latter is moved up and down. In Fig. 71, which repre-. 
 sents a horizontal section of an eye, if a be moved to a, h will move 
 to ft, the shadow on the retina c to 7, and the image d to h. If on 
 the other hand a be supposed to move above the plane of the paper, 
 h will move below, in consequence c will move above, and d will 
 appear to move beloA\^, i.e. d will siiil^ as a rises. 
 
 Fig. 71. Diagram illusxkating the Formation of Purkinje's Figures when the 
 Illumination is directed through the Cornea. 
 
 It is desirable in these cases to move the light to and fro, espe- 
 cially in the first method, as the retina soon becomes tired, and the 
 image fades away. Some observers can recognise in the axis of 
 vision a faint shadow corresponding to the edge of the depression 
 of the fovea centralis. 
 
 The retinal vessels may also be rendered visible by looking 
 through a small orifice such as a pin-hole in a card placed close 
 to the eye, at a bright field such as the sky, and moving the 
 
CiiAi-. ii.j ,si(;jiT. 01.') 
 
 orificT vtTv rai>i(lly from side to side or up and down. If the 
 inovoinoiit be I'roin side to sitle, the vessels which run vertical will 
 be seen; if up and down, the horizontal vessels. The tine eajtillary 
 vessels are seen more easily in this way than by Purkinje's method. 
 The same appearances may also be produced by looking through 
 a microscope from which the objective has been removed and the 
 eye-piece only left (or in which at least there is no object distinctly 
 in focus in the field), and moving the head rapidly from side to side 
 or backwards and forwards. Or the microscope itself may be 
 moved ; a circular movement of the field will then bring both 
 the vertically and horizontally directed vessels into view at the 
 same time. 
 
 The Photocliemistry of the Retina. In seeking to understand 
 how it is that rays of light falling upon the region of the rods and 
 cones can give rise to sensory, visual, impulses in the optic nerve, we 
 may adopt one or other of two views. On the one hand we may 
 suppose that the ^^b^ations of the ether are able, through the means 
 of the retinal apparatus of the rods and cones for example, to give 
 rise in some way or other to molecular vibrations Avhich are the 
 beginning of the nen'ous impulses in the optic nerve. Xo satis- 
 factory explanation of how such a change can be brought about has 
 been offered, and indeed the difficulties of such a conception are 
 very great. On the other hand we may more naturally turn to a 
 chemical explanation. We are familiar with the fact that rays of 
 light are able to bring about the decomposition of vers' many 
 chemical substances ;. and we accordingly speak of these substances 
 as being sensitive to light. All the facts dwelt on in this book 
 illustrate the great complexity and coiTesponding instability of the 
 composition of protoj^lasm. And we might reasonably suppose that 
 protoplasm itself Avould be sensitive to light ; that is to say that 
 rays of light falling on even undifferentiated protoplasm might set 
 up a decomposition of that protoplasm and so inaugurate a mole- 
 cular disturbance ; in other words, that light might act as a direct 
 stimulus to protoplasm. As a matter of fact, however, such evi- 
 dence as we at present possess goes to shew that native undifferen- 
 tiated protoplasm is as a rule not sensitive to light (that is, to those 
 [•articular waves which when they fall on our retina give rise in us 
 to the sensation of light), though in the case of some lowh' organ- 
 isms whose protoplasm exhibits very little differentiation and in 
 particular contains no pigment, a sensitiveness to light has been 
 observed. Nor can we be suqnised at this indifference of proto- 
 plasm when we reflect that what we may call pure protoplasm is 
 remarkable for its transijarency, that is to say the rays of light pass 
 through it with the slightest possible absorption. But in order 
 that Ught may produce chemical effects, it must be absorbed ; it 
 must be spent in doing the chemical work. Accordingly the first 
 step towards the formation of an organ of vision is the differentia- 
 
 33—2 
 
bl6 VL'SUAL SEXSATIONS. [Book iii. 
 
 tion of a portion of protoplasm into a pigment at once capable of 
 absorbing light, and sensitive to light, i.e. undergoing decomposition 
 upon exposure to light. An organism, a portion of whose proto- 
 plasm had thus become differentiated into such a pigment, would 
 be able to react toAvards light. The light falling on the organism 
 would be in part absorbed by the pigment, and the rays thus 
 absorbed would produce a chemical action and set free chemical 
 substances which before were not present. We have only to sup- 
 pose that the chemical substances are of such a nature as to act as 
 a stimulus to the protoplasm of other parts of the organism, (and 
 we have manifold evidence of the exquisite sensitiveness of proto- 
 l^lasm in general to chemical stimuli,) in order to see how rays of 
 light falling on the organism might excite movements in it, or 
 modify movements which were being carried on, or might other- 
 wise affect the organism in whole or in part. 
 
 Such considerations as the foregoing may be applied to even 
 the complex organ of vision of the higher animals. If we suppose 
 that the actual terminations of the optic nerve are surrounded by 
 substances sensitive to light, then it becomes easy to imagine how 
 light falling on these sensitive substances should set free chemical 
 bodies possessed of the j)ropei"ty of acting as stimuli to the actual 
 nerve-endings and thus give rise to visual impulses in the optic 
 fibres. We say "easy to imagine," but we are, at present, far from 
 being able to give definite proofs that such an explanation of the 
 origin of visual impulses is the true one, probable and enticing as 
 it may appear. 
 
 One of the most striking features in the stracture of the retina 
 is the abundance of black pigment in the retinal, or as it is some- 
 times called choroidal, epithelium. It is difficult to suppose that 
 the sole function of this pigment is to absorb the superfluous rays 
 of light, and that the rays thus absorbed are put to no use but 
 simply wasted. And indeed it has been shewn that the pigment is 
 sensitive to light; but the changes in it induced by light are ex- 
 cessively slow. Moreover its presence cannot be of fundamental 
 importance, since vision is not only possible but fairly distinct Avith 
 albinos in which this pigment is absent. 
 
 Then again, in the vast majority of vertebrate animals, the 
 outer limbs of the rods are suffused with a purplish-red pigment, 
 the so-called visual purple, which is so eminently sensitive to light 
 that images of external objects may by appropriate means be 
 photographed in it on the retina. When the eye of a frog or of a 
 rabbit is examined in an ordinary Avay, with full exposure to light, 
 the retina appears colourless. But if the eye be kept in the 
 dark for some time before it is examined, the retina, if removed 
 rapidly, Avill be found to be of a beautiful jxirplish-red colour. 
 Upon exposure to light the colour changes to yellow and then 
 fades away, leaving however the retina, not only white but more 
 opaque than it was before. Upon examination with the microscope 
 
Chap, ii.] SIC NT. .".17 
 
 it is toiijul tlmt the ))nrj)lt' ctilour is roiiHncd exclusively t(j the 
 riKlsand to the outer limits of the rods, the inner limbs being wholly 
 devoid ot" it. 
 
 The colour of the rods is dui' to the presence of n distinct 
 pigment, the "visual ])uri)le," ditt'used thiout^di the sidxstunce of the 
 outer lindis; and this maybe extracted from the rods by dissolving 
 tliose in an atpieous solution of bile salts. A clear pur])Io .solution 
 is thus obtained, uhich is capable of being bleached by the action 
 of light, and in its general features and behaviour is similar to the 
 pigment a,s it naturally exists in the retina. 
 
 Visual purple is found as we have said exchisively in the outer 
 limbs of the rods ; it has never yet been found in the cones, 
 and it is accordingly absent from the retinas (such as those of 
 .*«nakes) which are compo.sed of cones only, and from the macula 
 lutea and fovea centralis of the retinas of man and the ape. The 
 intensity of the colouration varies in different animals, and the 
 retinas even of some animals possessing rods (bat, dove, hen) seem 
 to be "wholly devoid of the visual purple ; it is generally well 
 marked in retinas in -which the outer limbs of the rods are well 
 developed. Its absence or presence is not dependent on nocturnal 
 habits, since the intense colour of the retina of the owl is in strong 
 contrast to the absence of colour in the bat. It has been found in 
 the retina of the embryo. 
 
 The visual purjDle is bleached not only by white but also by 
 monochromatic light. Of the various prismatic rays the most 
 active are the greenish-yellow rays, those to the blue side of the.se 
 coming next, the least active being the red. Now it is precisely 
 the gi-eenish-yellow rays which are most readily absorbed by the 
 colour itself. A natural coloured retina or a solution of visual 
 purple gives a ditl'use spectrum without any defined absorption 
 bands, and according to the amount of colouring material through 
 which the light passes, absor])tion is seen either to be limited to 
 the greenish-yell(5w part of the spectrum or to spread thence 
 towards the blue and, to a much less extent, towards the red. 
 Thus the various prismatic rays produce a photochemical effect on 
 the visual purple in proportion as they are absorbed by it. Under 
 the action of light the visual purple, whether in solution, or in its 
 natural condition in the rods, passes through a purplish orange to 
 a yellow, and finally becomes colourless ; and we appear to be 
 justified in speaking of a "vi.sual yellow' and "visual white" as 
 products of the photochemical changes undergone by the visual 
 purple. 
 
 For the restoration of the visual purple, after it has been 
 destroyed by light, the maintenance of the circulation of the blood 
 through the tissues of the eye is not essential. The choroidal 
 epithelium has by itself, provided that it still retains its tissue life, 
 the power of regenerating the jDui-ple. If a portion of the retina 
 of an excised eye be raised from its epithelial bed, bleached, and 
 
518 VISUAL SENSATIONS. [Book ni. 
 
 then carefully restored to its natural position, the purple will 
 return if the eye be kept in the dark. The choroidal epithelium 
 may in fact be spoken of as a 'purpurogenous' membrane. 
 
 If the image of some bright object such as a lamp or a window 
 be thrown on to the retina, either of an eye in its natural position 
 or of one recently excised, care having been taken to keep the 
 retina for some time previous away from any rays of light, the 
 portion of the retina on which the rays have fallen will be found 
 to be bleached, the rest of the retina remaining purple. In fact 
 an " optogram " of external objects may thus be obtained ; and 
 if the retina be removed and treated with a 4p.c. solution of 
 potash alum before the choroidal epithelium has had time to 
 obliterate the bleaching effects, the retina may remain permanently 
 in that condition : the photochemical effect may, as the photo- 
 graphers say, be " fixed." 
 
 It seemed very tempting, especially upon the first discovery 
 of it, to suppose that this visual pui'ple is directly concerned in 
 vision. If we suppose that visual purple itself is inert towards 
 the endings of the optic nerve, but that either visual yellow or 
 visual white, i.e. some product of the action of light on visual 
 purple, may act as a stimulus to those endings, the way seems 
 opened to understanding how rays of light can give rise to sensory 
 impulses in the optic nerve. Unfortunately visual purple is absent 
 from the cones, and from the fovea centralis which, as we shall 
 see, is the region of distinct vision ; it is further entirely wanting 
 in some animals which undoubtedly see very well ; and lastly 
 animals, such as frogs naturally possessing the pigment, continue 
 to see very well and even apparently to see colours when their 
 visual purple has been absolutely bleached, as it may be by 
 prolonged exposure of the eyes to strong light. We cannot there- 
 fore, at present at least, explain the origin of visual impulses by 
 the help of visual purple. At the same time its history suggests 
 that some substances, sensitive like it to light, but unlike it, 
 colourless and therefore escaping observation, may exist, and by 
 photochemical changes be the means of exciting the optic nerves. 
 And, as we shall see later on, one theory of colour vision is based on 
 the assumption that vision is carried on in some way or other by 
 changes in what may be called visual substances present in the 
 retina, these substances being used up and regenerated as vision is 
 going on. 
 
 But even admitting as probable the existence of these sensitive 
 visual substances, the changes in which lead to stimulation of the 
 real endings of the retinal nervous mechanism, we cannot at 
 present state anything definite concerning those nerve endings or 
 the manner of their stimulation. It may be that even the outer 
 limbs of the rods and cones, in spite of the apparent break of 
 continuity between the outer and inner limbs, are really nervous 
 in nature. It may- be on the other hand that the outer limbs 
 
riiAi'. II.) siaiir. nio 
 
 are citlar purely (linptric iu tuiutiun, or urn associatc-d witli tin' 
 sensitive visual substaneos in such a way that the purely nervous 
 structures must be considered as extending no further at least than 
 the inner limbs. We cannot as yet make any definite statement 
 in the oiu; direction or the other. 
 
 In connection with the oriy^in of \isiial iin})ul.ses we may 
 perhaps call attention to the remarkable chaiitrcs which the cells 
 of the retinal pigment epithelium undergo under the influence of 
 light. When an eye has been shut off from all light for some 
 little time the pigment is concentrated in the bodies of the 
 cells, and the remarkable filamentous processes of the cells, with 
 the pigment granules or crystals which they carry, extend a 
 slight distance only between the limbs of the rods and cones 
 (about one-third down the length of the outer limbs of the rods). 
 Under the influence of light these processes loaded with pigment 
 thrust themselves a much longer way down towards the external 
 limiting membrane ; in consequence a considerable quantity of 
 pioment is found massed between the outer and even the inner 
 limbs of the rods and cones ; indeed the outer limbs of the rods 
 swelling at the same time become jammed as it were between the 
 masses of pigment, causing the epithelial layer to adhere very 
 closely to the layer of rods and cones. 
 
 The retina and optic nerve like other nervous structures de- 
 velope electric currents which may be spoken of as currents of 
 rest and currents of action. They may be shewn by placing one 
 electrode on the retina of a bisected eye, or on the cornea of 
 a Avhole one, and the other on the optic nerve, or hind part of the 
 eye ball or even on some distant part of the body. They are also 
 manifested by the isolated retina itself. The phenomena appear 
 somewhat complicated by the appearance now of positive, now of 
 negative variations ; but this fact comes out clearly that the in- 
 cidence of light on the irritable retina developes an electric 
 change, the magnitude of which is to a certain extent propor- 
 tionate to the intensity of the light acting as a stimulus. The 
 changes accordingly diminish and cease to appear as the retina 
 gradually loses its irritability after death. We may add that these 
 electric phenomena appear to be quite independent of the con- 
 dition of the visual purple. 
 
 Simple Sensations. 
 
 Relations of the Sensation to the Stimulus. If we put aside 
 for the present all (piestii:)ns of colour, we may say that light, 
 viewed as a stimulus atYecting the retina, varies in intensity, that 
 is, in the energy of the luminous vibrations as manifested by theii' 
 amplitude, and in duration, that is, in the length of time a succes- 
 sion of waves continues to fall upon the retina. The effect of the 
 
520 VISUAL SEXSATTONS. [Book m. 
 
 light will also depend on the extent of retinal surface exposed to 
 the luminous vibrations at the same time. Taking a luminous 
 point, in order to eliminate the latter circumstance, we may make 
 the following statements. 
 
 The sensation has a duration much greater than that of the 
 stimulus, and in this respect is comparable to a muscular contrac- 
 tion caiised by such a stimulus as a single induction shock. The 
 sensation of a flash of light for instance lasts for a much longer time 
 than that during- Avhich luminous vibrations are falling on the 
 retina. Hence when two stimuli, such as two flashes of light, 
 follow each other at a sufficiently short interval, the two sensations 
 are fused into one ; and a luminous point moving rapidly round in 
 a circle gives rise to the sensation of a continuous circle of light. 
 This again is quite comparable to muscular tetanus. The interval 
 at which fusion takes place, that is the interval between successive 
 stimuli which must be exceeded in order that successive distinct 
 sensations may be produced, varies according to the intensity of the 
 light, being shorter Avith the stronger light ; Avith a faint light it is 
 about -^ sec, with a strong light -^\ or Jjy sec. This may be shewn 
 by rotating rapidly before the eye a disc arranged v/ith alternate 
 black and vrhite sectors of equal width. With a faint illumination, 
 the fliclcerincr indicative of the successive sensations from the white 
 sectors not being completely fused, ceases when the rotation be- 
 comes so rapid that each pair of black and white sectors takes 
 only -J^sec. in passing before the eye. When a brighter illumina- 
 tion is used the rapidity must be increased before the flickering 
 disappears. That j^art of the sensation which is recognised as 
 lasting after the cessation of the stimulus is frequently spoken of 
 as the 'after-image.' 
 
 Though the duration of the after-image is longer with the 
 stronger light (that caused by looking even momentarily at the 
 sun lasting for some time) the commencement of the decline of 
 the sensation begins relatively earlier, hence the greater difficulty 
 in the complete fusion of successive sensations wdth the stronger 
 light. The interval at which fusion takes place differs wuth different 
 colours, being shortest with yellow, intermediate with red, and 
 longest with blue. 
 
 The duration of a stimulus necessary to call forth a saasation is 
 exceedingly short ; thus the shortest possible flash, such as that 
 of an electric spark, gives rise to a sensation of light. 
 
 Objects in motion when illuminated by a single electric spark 
 appear motionless, the stimulus of the light reflected from them 
 ceasing before they can make an appreciable change in their posi- 
 tion. When a moving body is illuminated by several rapid flashes 
 in succession, several distinct images coiTCsponding to the positions 
 of the body during the several flashes are generated : the images of 
 the body corresponding to the several flashes fall on different parts 
 of the retina. 
 
CiiAi' II. I SIC JIT. 521 
 
 Thr intensity ol llic si-nsatinn varies with the lunilnous inten- 
 sity (if the object; a wax candle a]jjjears bii^'litcr than a rushli^'ht. 
 Tlic ratio, however, of the sensation to the stinmhis is not a simple 
 one. If tlie luminosity of an object be gradually incretused iVom a 
 very feebli> stage to a very In'ight one, it will be iound that though 
 the corresjjonding sensations likewise gradually increase, th(! incre- 
 ments of the sensations due to increments of tiie luniinosity 
 gradiially diminish ; and at last an increase of the Iumin<jsity 
 produces no appreciable increase of sensation ; a light, when it 
 reaches a certain brightness, appears so bright that we cannot tell 
 when it becomes any brighter. Hence it is much easier to distin- 
 guish a slight ditference of brightness between two feeble lights 
 than the same dilierenct' between two bright lights; we can easily 
 tell the ditTereuce between a nishliglit and a wax candle ; but two 
 suns, or e\en two bright lamps one of which (.lifiered from the 
 other merely by just the number of luminous rays which a wax 
 candle emits in addition to those sent forth by a rushlight, w'ould 
 appear to us to have exactly the same brightness. In a darkened 
 room an object placed before a candle will throw what we consider 
 a deep shadow on a sheet of pajDcr, or any wdiite surface. If, 
 however, sunlight be allowed to fall on the paper at the same 
 time from the opposite side, the shadow is no longer visible. The 
 difference between the total light reflected from that part of the 
 paper where the shadow was, and which is illuminated by the sun 
 alone, and that reflected from the rest of the paper which is 
 illuminated by the candle as well as by the sun, remains the same; 
 yet we can no longer appreciate that difference. 
 
 On the other hand, if using two rushlights we throw two 
 fhadows on a white surface and move one rushlight away until the 
 shadow caused by it ceases to be visible ; and, having noted the 
 distance to which it had to be moved, repeat the same experiment 
 with two wax candles ; we shall find that the wax candle has to 
 be moved just as far as the rushlight. In fact, it is found by 
 careful observation, that within tolerably wide limits, the smallest 
 ditlerence of light Avhich we can appreciate by visual sensations 
 is a constant fraction (about yrnrth) of the total luminosity emjjloyed. 
 The same law holds good with regard to the other senses as well. 
 The smallest ditference in length we can detect between two lines, 
 one an inch long and the other a little less than an inch, is the 
 same fraction of an inch, that the smallest difference in length 
 we can detect between a line a foot long and one a little less than 
 a foot, is of a foot. Put in a more general form then, the law, 
 which is often called Weber's law \ is as follows : When a stimulus 
 is continually increased, the increase of stimulus necessary to call 
 Ibrth the smallest appreciable increase of sen.sation always bears 
 the same proportion to the whole stimulus. 
 
 ' From whic'.i Fechner, by au assumption, obtained a mathematical cxprcEsion 
 cr formula, which is sometimes incorrectly spoken of as Fechner's law. 
 
52:2 VISUAL SENSATIONS. [Book in. 
 
 Distinction and Fusion of Sensations. When light falls on a 
 large portion of the retina the total sensation produced is greater 
 in amount than when a small portion only of the retina is affected; 
 a large piece of white paper produces a greater total effect on our 
 consciousness. than a small one, though, if the surfaces be uniformly 
 and equally illuminated, the intensvty of the sensation is in each 
 case the same ; the small piece of paper appears as bright or as 
 ' white ' as the large one. If the images of two luminous objects 
 fall on the retina at sufficient distances apart, the consequent 
 sensations are distinct, and the intensity of each sensation will 
 depend solely upon the luminosity of the corresponding object. If 
 however the two objects are made to approach each other, a point 
 will be reached at which the two sensations are fused into one. 
 When this occurs the intensity of the total sensation produced will 
 be greater than that of either of the sensations caused by the 
 single objects. A number of luminous points scattered over a wide 
 surface would appear each to have a certain brightness ; each would 
 give rise to a sensation of a cei'tain intensity. If they were all 
 gathered into one spot, that spot would appear far brighter than 
 any of the previous points ; the intensity of the sensation would be 
 greater. We may therefore suppose the retina to be divided into 
 areas corresponding to sensational units. If the images from two 
 luminous objects fall on separate visual areas, if we may so call 
 them, two distinct sensations will be produced ; if, on the contrary, 
 they both fall on the same visual area, one sensation only will be 
 produced. Where the sensations are separate, the intensity of the 
 one (with exceptions hereafter to be mentioned) is not affected by 
 the presence of the other ; but where they become fused the 
 intensity of the united sensations is greater than either of, though 
 not eqiial to the sum of, the single sensations. The existence of 
 these sensational units is the basis of distinct vision. When we 
 speak of the smallest size visible or distinguishable, we are referring 
 to the dimensions of the retinal areas corresponding to these sen- 
 sational units. The retinal area must be carefully distinguished 
 from the sensational unit, for the sensation is, as we have seen, a 
 process whose arena stretches from the retina to certain parts of 
 the brain, and the circumscription of the sensational unit, though 
 it must bec^in as a retinal area, must also be continued as a 
 cerebral area in the brain, the latter corresponding to, and bemg 
 as it were the projection of, the former. With most people two 
 stars appear as a single star when the distance between them sub- 
 tends an angle of less than 60 seconds ; and the best eyes generally 
 fail to distinguish two parallel white streaks when the distance 
 between the two, measured from the middle of each, subtends an- 
 aiigle of less than 73 seconds. Some however can distinguish 
 objects 50 seconds distant from each other. An angle of 73 seconds 
 in an object corresponds in the diagrammatic eye (see p. 492) to the 
 
Chap. ii.J 
 
 SIGHT. r.2.3 
 
 length (»f 0-8(5 /x in the ri-tinal imago', luul one of 50 seconds to 
 305^. 
 
 In the luuiuiu eye oO cones may be couutud along a line ot 
 200 /A in length drawn through the centre of the yellow spot; this 
 wunld give 4^ for the distance between the centres of two ad- 
 joining'c-unes in the yellow spot, the average diameter of a cone at 
 its widest part being 3 jx and there being slight intervals between 
 neighbouring cones. Hence if we take tlu; centre of a cone as the 
 centre of an anatomical retinal area, these anatomical areas cor- 
 respond very fairly to the physiological visual areas as determined 
 above. That is to say, if two points of the retinal image are less 
 than 4 yu. apart, thoy may both lie within the area of a single cone ; 
 and it is just when they arc less than about 4/i apart that they 
 cease to give rise to two distinct sensations. It must be remem- 
 bered, however, that the fusion or distinction of the sensations is 
 ultimately determined by the brain and not by the retina. Two 
 points of the retinal image less than 4/a apart might lie both 
 within the area of a single cone ; but the reason why, under such 
 circumstances, they give rise to one sensation only is not because 
 one cone-fibre only is stimulated. Two points of a retinal image 
 might lie, one on the area of one cone and another on the area of 
 an adjoining cone, and still be less that 4^ apart ; in such a case 
 two cone-fibres would be stimulated, and yet only one sensation 
 would be produced. So also in the less sensitive peripheral parts 
 of the retina two points of the retinal image might stimulate two 
 cones a considerable distance apart, and yet give rise to one sensa- 
 tion only. 
 
 In the case where the two points lie entirely within the area of 
 a single cone, it is exceedingly probable that, even if the adjacent 
 cones or cone-fibres in the retina are not at the same time stimu- 
 lated, impulses radiate from the cerebral ending of the excited cone 
 into the neighbouring cerebral endings of the neighbouring cones ; 
 in other words, the sensation-area in the brain does not exactly 
 correspond to and is not sharply defined like the retinal area, but 
 gradually fades away into neighbouring sensation-areas. We may 
 imagine two points of the retinal image so far apart that even the 
 extreme margins of their respective cerebral sensation-areas do not 
 touch each other in the least ; in such a case there can be no doubt 
 about the two points giving rise to two sensations. AVe might, 
 however, imagine a second case where two points were just so far 
 apart that their respective sensation-areas should coalesce at their 
 margins, and yet that, in passing from the centre of one sensation- 
 area to the centre of the other, we should find on examination 
 a considerable fall of sensation at the junction of the two areas ; and 
 in a third case we might imagine the two centres to be so close to 
 each other that in passing from one to the other no appreciable 
 diminution of sensation could be discovered. In the last case there 
 1 By fi is meant one-thousandth of a millimetre. 
 
524 COLOUR SENSATIONS. [Book in. 
 
 would be but one sensation, in the second there might still be two 
 sensations if the marginal fall were great enough, even though the 
 areas partially coalesced. Thus, though the mosaic of rods and 
 cones is the basis of distinct vision, the distinction or fusion of 
 two visual impulses is ultimately determined by the disposition 
 and condition of the cerebral centres. Hence the possibility of 
 increasing by exercise the faculty of distinguishing two sensations, 
 since by use the cerebral sensation-areas become more and more 
 differentiated. This however is even more strikingly shewn in 
 touch than in sight. 
 
 Colour Sensations. 
 
 When we allow sunlight reflected from a cloud or sheet of 
 paper to fall into the eye, we have a sensation which we call a 
 sensation of white light. When we look at the same light through 
 a prism, and allow different parts of the spectrum to fall in 
 succession into the eye, we have sensations which we call respec- 
 tively sensations of red, orange, yellow, green, blue, violet, &c. light. 
 In other words, rays of light falling on the retina give rise to different 
 sensations, according to the wave-lengths of the rays. Though 
 we speak of the spectrum as consisting of a few colours, such as 
 red, orano-e, &c., there are an almost infinite number of intermediate 
 tints in the sjjectrum itself; and we perceive in external nature a 
 large number of colours, such as purple, brown, grey, &c., which do 
 not correspond to any of the colour sensations gained by regarding 
 the successive parts of the spectrum. We find however, on exami- 
 nation, that certain distinct colour sensations, not corresponding to 
 any of the colours of the spectrum, may be obtained by the fusion 
 of the sensations caused by two or more of the prismatic colours. 
 Thus purple, which is not present in the spectrum, may be at once 
 produced by fusing the sensations of blue and red in proper 
 proportions. Moreover many of the various tints and shades 
 of nature may be imitated by fusing a particular colour sensation 
 with the sensation of white, or by alloAving a certain quantity of 
 light of a particular colour to fall sparsely over the area of the retina, 
 which is at the same time protected from the access of any other 
 light, i.e. as we say, by mixing the colour with black. Thus the 
 browns of nature result from various admixtures of yellow, red, 
 white, and black ; and a small quantity of white light, scattered 
 over a large area of the retina, i.e. white lai'gely mixed with black, 
 forms a grey. In fact, the qualities of a colour depend (1) on the 
 nature of the prismatic colour or colours, i.e. on the wave-lengths of 
 the constituent rays, falling on a given area of the retina ; (2) on 
 the amount of this coloured light which falls on the area of 
 the retina in a given time ; and (3) on the amount of white light 
 
Vhxv. II.] SiaUT. 525 
 
 f'alliiii,^ oil the saiiu- uiva at the saiiu; tiiiu'. When rays corro- 
 sjK)U(liii<,' to a j)risiiiatif colour tall upon thu retina unaccoinpanicd 
 by any wiiitc light, the colour is said to be 'saturated'; an<l 
 a colour is spoken of as more or less saturated according jus 
 it is mixed with less or more white light. When we are led 
 to describe a colour as being of such a tint or hue, we are guidt-d 
 by the first of the above conditio »ns. But we have no common 
 phrases by which we distinguish the second of the above conditions 
 from the third. The word 'pale,' it is true, is most frefpiently used 
 to expreb's a colour very slightly saturated; but the words 'rich' or 
 'deep' are used sometimes as meaning highly .saturated, sometimes 
 as meaning simply that a large quantity of light of the particular 
 hue is passing into the eye. So also with the phrase 'bright'; this 
 we often n.se when a large amount of coloured and white light fall 
 at the same time on the same retinal area, but we sometimes also 
 use it to express the mere intensity of the sensation. 
 
 The best method of fusing colour sensations is that adopted by 
 Maxwell, of allowing two different parts cf tlie spectrum to fall on the 
 same part of the retina at the same time. The use of tlie pure prismatic 
 colours eliminates errors which arise when pigments, the colours of which 
 are not pure, but mixed, are employed. And where pigments are used^ 
 it is the sensations to which the pigments give rise which must be mixed 
 and not the pigments themselves. Thus while the sensations gained by 
 looking at gamboge yellow and indigo respectively when fused give ris<; 
 to a sensation of white, gamboge and indigo themselves when mixed 
 appear gi-een. The colour of the mixed pigment is due to the fact that 
 the rays which reach the eye from the mixture are those which are least 
 absorbed by the two pigments. The gamboge absorbs the blue rays very 
 largely, but the gTcen to a much less extent ; while the indigo absorbs 
 the red and yellow rays very largely, but also absorbs very little of the 
 green. Hence green is the predominant hue of the mixture. When 
 pure pigments, i.e. pigments corresponding as closely as possible to the 
 prismatic colours, are used, satisfactory results may be gained, either by 
 using the reflected image of one pigment, and arranging so that it falls 
 on the retina at the same spot as the direct image of the other pigment, 
 or by allowing the image of one pigment to fall on the retina before the 
 sensation produced by the other has passed away. The tirst result is 
 easily reached by Helmholtz's simple method of placing two pieces of 
 coloured paper a little distance apart on a table, one on each side of a 
 glass plate inclined at an angle. By locking with one eye down on the 
 glass plate the reflected image of the one paper may be made to coincide 
 with the direct image of the other, the angle which the gla.ss plate makes 
 with the table being adjusted to the distance between the pieces 
 of paper. In the second method, the 'colour top' is used; sectors of 
 the colours to be investigated are placed on a disc made to rotate very 
 rapidly, and the image of one colour is thus brought to bear on the 
 retina so soon after the image of another, that the two sensations 
 arc fused into one. 
 
 When the sensations corresponding to the several prismatic 
 
526 COLOUR SENSATIONS. [Book hi. 
 
 colours are fused together in various combinations, the following 
 remarkable results are brought about. 
 
 1. When red and yellow in certain proportions are mixed 
 together the result is a sensation of orange, quite indistinguishable 
 from the orange of the spectrum itself. Now the latter is produced 
 by rays of certain wave-lengths, whereas the rays of red and of 
 yellow are respectively of quite different wave-lengths. The orange 
 of the spectruvi cannot be made up by any mixture of the 
 red and the yelloiu of tJie spectrum in the sense that the red 
 and yellow rays can unite together to form rays of the same wave- 
 lengths as the orange rays ; the three things are absolutely different. 
 It is simply the mixed sensation of the red and yellow which is so 
 like the sensation of orange ; the mixture is entirely and absolutely 
 a physiological one. In the same way we may by appropriate 
 mixtures produce the sensations corresponding to other parts of the 
 spectrum. Now we must suppose that rays of different wave- 
 lengths give rise to different sensory impulses, that, for instance, the 
 sensor}^ impulses generated by orange rays are different from those 
 generated by red and by yellow rays. Hence we are led by 
 the fact of mixed sensations being identical with other apparently 
 simple sensations to infer that the sensory impulses which any 
 ray originates are either themselves of a complex character, or in 
 becoming converted into sensations give rise to complex or mixed 
 sensations ; that, for instance, the impulse or sensation which a ray 
 in the middle of the orange gives rise to, is not a simple impulse or 
 sensation answering exclusively to the colour of that ray, but 
 that the ray gives rise either to a complex impulse which becomes 
 converted into a complex sensation, or to a simple impulse which 
 eventually developes into a mixed or complex sensation, into the 
 composition of which in each case other orange tints and shades 
 of red and yellow enter. 
 
 2. When certain colours are mixed together in pairs in certain 
 definite proportions, the result is white. These colours are 
 
 Eed (near a)^ and Blue-Green (near F), 
 
 Orange (near C), and Blue (between F and G), 
 
 Yellow (near D), and Indigo-Blue (near G), 
 
 Green- Yellow (near E), and Violet (between G and H), 
 
 and are said to be 'complementary' to each other. To these might 
 be added the peculiar non-prismatic colour purple, which with 
 green also gives white. 
 
 8. If we select arbitrarily any three colours corresponding to 
 any three parts of the spectrum sufficiently far apart, say for 
 instance red, green, and blue, we can, by a proper adjustment of 
 the proportions of each, produce white. Further, these three 
 
 1 These letters refer to Fraueuhofer's lines. 
 
CiiAi'. II.] SK.Iir. jJ7 
 
 colours can lu- takiii in snrli innjuiit imis as \vitli a ]»i()])ci' a<ltlitiun, 
 it" necessary, of white to produce the sensations of all other C(jlours'. 
 That is to say, given three standard sensations, uU the other 
 sensations may be gained by the j>roi)er mixture of these. 
 
 It is obvious from the foregoing that our real colour sensations 
 are much fewer in number than those which we a])|)ear to have 
 when we look on the colours of the spectrum or of nature; that 
 rays of light awake in us ct-rtain simple sensations, Avhich mixed in 
 various ])roportions reproduce all our sensations. And the question 
 arises, what is the nature or what are the characters of these sini2)le 
 sensat'ons ? 
 
 When we examine our own sen.sations of light we find that 
 certain of these seem to be quite distinct in nature from each 
 other, so that each is something sui generis, whereas we easily 
 recognise all other sensations as various mixtures of these. Thus 
 red and yellow are to us quite distinct: we do not recognise any 
 thing common to the two; but orange is obviously a mixture of red 
 and yellow. The sensations caused by different kinds of light 
 which thus appear to us distinct, and which we may speak of as 
 'fundamental sensations,' are white, black, red, yellow, green, blue. 
 Each of these seems to us to ha"sc nothing in common with any of 
 the others, whereas in all other colours we can recognise a mixture 
 of two or more of these. 
 
 This result of connnon experience suggests the idea that these 
 fundamental sensations are the primary or simple sensations, 
 spoken of above as those out of which all other sensations may be 
 supposed to be compounded. And a theory has been proposed to 
 reconcile the various facts of coloui' vision, with the supposition 
 that we possess these six fundamental sensations. This theory, 
 known as that of Hering, is somew^hat as follows. The six sensa- 
 tions readily fall into three pairs, the members of each ya\y having 
 anal<.)gous relations to each other. White and black naturally go 
 together, the one being the antagonistic or correlative of the other. 
 There is a similar connection between red and gi'een, the one being 
 the com2:)lementary of the other, and between yellow^ and blue 
 which are similarly complementary. We saw reason, a short time 
 back (p. 518), for believing that vision originates in the changes 
 taking place in certain visual substances (or a visual substance) in 
 the retina. And the theory of which we are speaking supposes that 
 there exist in the retina, or at least somewhere in the visual appa- 
 ratus, three distinct visual substances which are continually under- 
 going a double metabolism, one constructive, of assimilation or 
 building tip, and the other destructive, of dissimilation or breaking 
 down. One of these substances is further of such a nature that 
 
 1 A few liighlj' saturated colours cannot be so reproduced, but a mixture of any 
 one of them with white can. We may perhaps therefore speak of these saturated 
 colours as being reproduced by a proper combination of the three arbitrarily selected 
 colours, with the subtraction of white. 
 
528 COLOUR SENSATIONS [Book hi. 
 
 Avhen dissimilatiou is in excess of assimilation, we have a sensation 
 of white, and when assimilation is in excess a sensation of black. 
 With a second substance excess of dissimilation provokes red, of 
 assimilation green ; and with the third substance, yellow and blue 
 respectively. When in the latter two substances dissimilation and 
 assimilation are exactly equal, no effect is produced ; but with the 
 first substance, this condition produces in us the effect of grey. 
 Further these substances are of such a kind that while the first or 
 white-black substance is influenced by rays along the whole range 
 of'the spectrum, the two other substances are differently influenced 
 by rays of different wave-length. Thus in the part of the spectrum 
 which we call red, the rays promote a rapid dissimilation of the 
 red-green substance with comparatively slight effect in either di- 
 rection on the yellow-blue substance ; hence our sensation of red. 
 In that part of the spectrum which we call yellow the rays effect 
 a marked dissimilation of the yellow-blue substance but their 
 action on the red-green substance is equal in the direction of both 
 assimilation and dissimilation ; hence our sensation of yellow. 
 The green rays, again, promote assimilation of the red-green 
 substance, leaving the assimilation of the yellow-blue substance 
 equal to the dissimilation, and similarly blue rays cause assimila- 
 tion of the yellow-blue substance, and leave the red-green sub- 
 stance neutral. Finally at the extreme blue end of the spectrum, 
 the rays once more provoke dissimilation of the red-green sub- 
 stance. When orange rays fall on the retina, there is an excess of 
 dissimilation of both the red-green and the yellow-blue substance ; 
 when greenish-blue rays are perceived there is an excess of assimi- 
 lation of both these substances; and other intermediate tints 
 correspond to variable amounts of dissimilation or assimilation of 
 two or more of these substances. 
 
 When all the rays together fall on the retina, the red-green and 
 yellow-blue substance remain in equilibrium, but the white-black 
 substance is violently dissimilated ; and we say the light is white. 
 
 Another theory (known as the Young-Helmholtz theory, be- 
 cause it was introduced by Young and more fully elaborated by 
 Helmholtz) strives to reduce the matter to still further simplicity. 
 Starting from the fact mentioned a short time since, that all colour 
 sensations, including the sensation of white, may be obtained by 
 the appropriate mixture of three standard sensations, this theory 
 teaches that our visual apparatus is so constituted as, when excited, 
 to give rise to three primary sensations, and that these primary 
 sensations are called forth in dilTerent degrees by different rays of 
 light, so that each ray gives rise to a different mixture of the three. 
 Several sets of three such primary sensations might be chosen, 
 which would satisfy the conditions of giving rise, by appropriate 
 mixture, to all sensations of colour including white ; but for reasons, 
 into which we cannot enter fully here, the sensations which may 
 thus be taken as primary sensations appear to correspond to our 
 
Chat, h.] 
 
 smiiT. 
 
 529 
 
 sensations ul" red, green, and blue or violet. Such a view (jf three 
 primary colour sensations is represented in the diagram (Fig. 72). 
 Thus the red primary sensation, excited to a certain extent by the 
 rays at the extreme red end, is most powerfully affected by the 
 rays at a little distance from the end, the rays from this point 
 onwards towards the blue end producing less and less effect. The 
 curve of the green primary sensation begins later and reaches its 
 maximum in the green of the spectrum, -while the blue or violet 
 primary sensation is still later and only reaches its maximum 
 towards the blue end of the spectrum. Each ray calls forth each 
 sensation but to a different degree, and the total result of each ray, 
 or of each group of rays, is determined by the proportionate amount 
 of the three sensations. Thus the sensation of orange (0 in the 
 figure) is brought about by a mixture of a great deal of the primary 
 red with much less of the primary green, and hardly any of the 
 primary blue ; the orange sensation is converted into a yellow 
 sensation by diminishing the primary red and largely increasing 
 the primary green, the primary blue undergoing also some slight 
 increase. And similarly with all the other sensations. When 
 each of the primary sensations is excited to a maximnm, as when 
 ordinary light falls on the retina, the result is a sensation of white. 
 According to this theorv^, black is simply the absence of sensation, 
 from the visual ajDparatus. 
 
 1 ''■' 
 
 
 
 ~"^~ 
 
 "-^-- 
 
 ~~" ~~" — — ,. 
 
 
 
 
 ^^^ ^'"^^ 
 
 ^^-.^^ 
 
 
 
 
 ^--^ 
 
 
 ""^-^^ 
 
 
 
 
 
 . 
 
 _/^^-N 
 
 \ 
 
 a 
 
 ffr. 
 
 St. 
 
 Fig. 72. Di.ujkam of Tur-ze Pkimaky Colour Sensations. 
 
 1 is tlie so-called 'red,' 2 'green,' and 3 'violet' primary colour sensation. R,0,Y,&c. 
 represent the red, orange, yellow, &c., colour of the spectrum, and the diagram 
 shews, by the height of the curve in each case, to what extent the several 
 primary colour sensations are respectively excited by vibrations of different 
 wave-lengths. 
 
 In the view, as originally put forward by Young, the three 
 primary sensations were supposed to be represented by three sets of 
 iibres, each set of fibres being differently affected by diiferent rays of 
 light, and the impulses passing to the brain along each set aAvaken- 
 ing a distinct sensation. No such distinction of fibres can be found 
 in the retina ; but an anatomical basis of this kind is not necessary 
 for the theory ; we can easily conceive of the same fibre trans- 
 V. 34 
 
530 COLOUR SENSATIONS. [Book iii. 
 
 mitting three distinct kinds of imjiulses ; or we may suppose that 
 the visual substances are three in number instead of six, the 
 changes in each substance provoking a primary sensation. 
 
 Such are the two main theories of colour vision ; and much may 
 be said in favour of both of them ; at the same time both of them 
 present many difficulties. To discuss them fully, is a task beyond 
 the limits of this book, and to discuss them in any but a full manner 
 would be unsatisfactory. We must be satisfied therefore with the 
 foregoing simple statement of the two views. Independently of any 
 theory, however, we may remember (1) that all the sensations 
 which we experience under the action of light of whatever kind 
 may be reduced to six, white, black, red, yellow, green and blue ; 
 and (2) that these may be all reproduced by various mixtures of 
 three standard sensations, if black be allowed to indicate the 
 absence of all sensation. These are matters of fact : what is at 
 present debated is whether the six fundamental sensations are 
 the outcome of three primary sensations or whether they represent 
 six distinct conditions of the visual apparatus. 
 
 Colour Blindness. Persons vary much in their power of 
 appreciating and discriminating colour, i.e. in the intensity and 
 accuracy of their colour sensations ; and some people regard 
 as similar, colours which to most people are glaringly distinct; 
 these latter are said to be ' colour blind.' The most common form 
 of colour blindness is that of persons unable to distinguish green 
 and red from each other. As in the case of Dalton, they tell a red 
 gown lying on a green grass plot, or a red cherry among the gTeen 
 leaves, by its form, and not by its colour. They confound not only 
 red, green, and certain forms of brown, but also rose, purple, 
 and blue. Such persons are often spoken of as ' red blind.' On the 
 Hering theory they lack the red-green visual substance ; hence, 
 all the colour sensations they possess must be those of yellow and 
 blue freed from all mixture of red or green ; and such accounts as 
 have been given of their sensations by those persons who are 
 'red-blind' in one eye, but possess normal vision with the other, 
 accord with this conclusion. On the Young-Helmholtz theory, 
 such persons lack the primary red sensation; and hence the 
 sensations which they have must be mixtures of green and blue 
 alone, our yellow appearing to them a bright green, and our green- 
 blue a kind of grey. 
 
 All such red-blind people ought, on either theory, to be 
 less affected than are persons with normal eyes, by the red end 
 of the spectrum : this ought with them to be shortened and obscure. 
 In a certain number of persons who confound red and green, this is 
 the case ; but in some instances no such lack of appreciation of the 
 red end of the spectrum can be ascertained. Such cases have been 
 supposed to be green-blind, that is lacking the primary sensation 
 of green. According to the Hering theory green blindness apart 
 
(hiAT. II. I sicirr. 631 
 
 iVom rrd hliiuliu'ss is iinpossihlt.', the only two puy.siblc culoiir 
 defects being red-green and blue-yellow blindness. And the 
 existence ot" distinct green blindness has been held to contradict 
 that theory. On the other hand tlu; Hering theory admits the 
 possibility of total colour blindness, i.e. the inal)ility t<^see anything 
 but white and black ; and this, on the Young-Hehnholtz theory, is 
 inipossibk', since lor vision to exist at all, one ot" the three primary 
 sensations must be present ; a man to see at all must see things 
 in various shades of either red, or of green, or of violet, though he 
 may confound this single-coloured vision with the normal vision of 
 white of ditierent intensities. But indeed a full examination of 
 colour blindness rather increases than diminishes the difficulties of 
 deciding between the two rival theories. 
 
 Influence of the pigment of the yellow spot. In the macula 
 lutea, which part of the rethia wc use chiefly for vision, images 
 falling on other parts of the retina being said to give rise to 
 'indirect vision,' the yellow pigment absorbs some of the greenish- 
 blue rays. Hence the sensation which we receive from objects 
 which we are in the habit of calling white is that which, if this 
 pigment were absent, we should receive from objects more or less 
 yellow. We may use this feature of the yellow spot for the purpose 
 of making the spot, so to speak, visible to ourselves, by an experi- 
 ment suggested by Maxwell. A solution of chrome alum, Avhich 
 only transmits red and greenish-blue rays, is held up between the 
 eye and a white cloud. The greenish-blue rays are absorbed by the 
 yelloAv spot, and here the light gives rise to a sensation of red ; 
 whereas in the rest of the iield of vision, the sensation is that 
 ordinarily produced by the purplish solution. The yellow spot is 
 consequently marked out as a rosy patch. This very soon however 
 dies away. 
 
 In speaking of sensation as a function of the stimulus, p. 520, 
 we referred to white light only ; but the different colours are 
 unequal in the relations borne by the intensity of the stimulus, 
 to the amount of sensation produced. Thus the more refrangible 
 blue rays produce a sensation more readily than the yellow or red 
 rays. Hence in dim lights, as those of evening and moonlight, 
 the blues prejjonderate, and the reds and yellows are less obvious. 
 So also when a landscape is viewed through a yellow glass, 
 the yellow hue suggests to the mind bright sunlight and summer 
 weather, although the actual illumination which reaches the eye is 
 diminished by the glass. Conversely when the same landscape is 
 viewed through a blue glass the idea of moonlight or winter 
 is suggested. 
 
 The theory of three primary colour sensations may be used 
 to explain why any coloured light, if made sufficiently intense, 
 appears white. Thus a violet light of moderate intensity appears 
 violet because it excites the primary sensation of violet much more 
 
 34—2 
 
532 COLOUR SE^SATIOiYS. [Book hi. 
 
 than those of green and red. If the stimulus be increased the 
 maximum of violet stimulation will be reached, while the stimula- 
 tion of green will continue to be increased and even that of red to 
 a slight degree. The result will be that the light appears violet 
 mixed with green, that is blue. If the stimulus be still further 
 increased Avhile the green and violet are both excited to the 
 maximum, the red stimulation may be increased until the result is 
 violet, green, and red in the proportions which make white light. 
 And so with light of other colours. 
 
 After-Images. We have already seen that in vision the 
 sensation lasts much longer than the stimulus. Under certain 
 circumstances, such as particular conditions of the eye, an intense 
 stimulus, &c., the sensation is so prolonged, that it is spoken of as 
 an after-image. Thus, if the eye be directed to the sun, the image 
 of that body is present for a long while after; and if, on early 
 Avaking, the eye be directed to the window for an instant and then 
 closed, an image of the window with its bright panes and darker 
 sashes, the various parts being of the same colour as the object, will 
 remain for an appreciable time. These images, which are simply 
 continuations of the sensation, are spoken of as 2^ositive after- 
 images. They are best seen after a momentary exposure of the eye 
 to the stimulus. 
 
 When, however, the eye has been for some time subject to a 
 stimulus, the sensation which follows the withdrawal of the sti- 
 mulus is of a different kind ; what is called a negative after-image, 
 or negative image, is produced. If, after looking stedfastly at a 
 white patch on a black ground, the eye be turned to a white 
 ground, a grey patch is seen for some little time. A black patch 
 on a white ground similarly gives rise on a grey ground to a 
 negative image in the form of a white patch. This may be 
 explained as the result of exhaustion. When the white patch 
 has been looked at steadily for some time, that part of the retina 
 on which the image of the patch fell becomes tired; hence the 
 white light, coming from the white ground subsequently looked 
 at, which falls on this part of the retina, does not produce so much 
 sensation as in other parts of the retina; and the image, con- 
 sequently, appears grey. And so in the other instance, the whole 
 of the retina is tired, except at the patch; here the retina is 
 for a while most sensitive, and hence the white negative image. 
 
 When a red patch is looked at, the negative image is a green 
 blue, that is, the colour of the negative image is complementary to 
 that of the object. Thus also orange produces a blue, green a 
 pink, yellow an indigo-blue, negative image ; and so on. This too 
 can be explained as a result of exhaustion on either hypothesis of 
 colour vision. When the coloured patch is looked at, one of the 
 three primary colour sensations is much exhausted, and the other 
 two less so, in varying proportions, according to the exact nature of 
 
Chap. ii.J SICIIT. 833 
 
 the colour of tliG patch; and the less exhausted sensations become 
 ])romineut in the af'tt'r-iina^e. Thus, the red patch exhausts the 
 red sensatit)U, and the negative image is made up chietly of given 
 and blue sensations, that is, aijpears to be greenish blue, or bluish 
 green, according to the tint of the red. On the other hyj)othesi3, 
 we may su2)pose that, owing to the continued effect of lo(jkiug at 
 the red ])atch, dissimilation of the red-given substance becomes 
 less and less, leading to a j)rominencc and indeed to an actual 
 increase of the process of assimilation of the same substance; 
 hence the sensatii>n of green dominating in the negative image. 
 
 Similarly, when the eye, after looking at a coloured patch, is 
 turned to a coloured ground, the effects may easily be explained 
 by reference to the comparative exhaustion of the colour sensations 
 excited by the patch and the ground respectively ; if a yellow 
 ground be chosen after looking at a given object, the negative 
 image will appear of a reddish yellow, and so on. 
 
 The theory of three primaiy sensations does not so readily 
 explain why negative images should make their apj^earance with- 
 out any subsequent stimulation of the retina. When the eyes are 
 shut and all access of light, even through the eyelids, carefully 
 avoided, the field of vision is not absolutely dark; there is still a 
 sensation of light, the so-called 'proper light' of the retina. If a 
 white patch on a black ground be looked at for some time, and the 
 eyes then shut, a negative (black) image of the spot will be seen 
 on the ground of the 'proper light' of the retina, having in its 
 immediate neighbourhood a specially bright corona. So also, if a 
 window be looked at and the eyes then closed, the positive after- 
 image with bright j^anes and dark sashes gives rise to a negative 
 after-image with bright sashes and dark panes ; and similar effects 
 apjjear with colours. These and similar facts have been largely 
 used in support of the Hering tlieory. When the eye has been 
 looking at red, and so has caused dissimilation of the red-green 
 substance mere rest, as on shutting the eyes, favours assimilation of 
 the same substance and thus leads to a sensation of green. And 
 the rhythmic oscillations from one colour to its correlative and 
 back again, frequently observed under these conditions and which 
 point to assimilation and dissimilation alternately gaining the 
 iq^per hand, are not without analogies in other common instances 
 of protoplasmic metabolism. 
 
SEC. 3. VISUAL PERCEPTIONS. 
 
 Hitherto we have studied sensations only, and have considered 
 an external object, such as a tree, as simply a source of so many 
 distinct sensations, dififering from each other in intensity and kind 
 (colour). In the mind these sensations are coordinated into a per- 
 ception. We are not only conscious of a number of sensations of 
 bright and dim lights, of green, brown, black, &c., but these sen- 
 sations are so related to each other and by virtue of cerebral pro- 
 cesses so fashioned into a whole, that we 'see a tree.' We some- 
 times, in illustration of such an effect, speak of an image or picture 
 in the mind corresponding to the physical image on the retina. 
 
 When we look upon the external world, a variety of images 
 are formed at the same time on the retina, and give rise to a 
 number of contemporaneous visual sensations. The sum of these 
 sensations constitutes 'the field of vision,' which varies of course 
 with every movement of the eye. This field of vision, being in 
 reality an aggregate of sensations, is of course a subjective matter; 
 but we are in the habit of using the same phrase to denote the 
 sum of external objects which give rise to the aggregate of visual 
 sensations; in common language the field of vision is 'all that we 
 can see' in any position of the eye, and we have a field of vision 
 for each eye separately and for the two eyes combined. 
 
 Using for the present the words in their subjective sense, we 
 may remark, that we are able to assign to each constituent 
 sensation its place among the aggregate of sensations constituting 
 the field of vision ; we can, as we say, localise the sensation. We 
 can say whether it belongs to (what we regard as) the right-hand 
 or left-hand, the upper or the lower part, of the field of vision. We 
 are able to distinguish the relative positions of any two distinct 
 
CiiAi'. 11.] SWllT. .',35 
 
 sensations ; and the relative positions, together with the rehitive 
 intfiisities and tiualities (colour) of the sensations arising from any 
 object determine our j)ereeption of the ohject. It need hardly he 
 remarktHi that this loealisation is purely subjective. Wo simply 
 determine the position of the sensation in the field of vision 
 (which is itself a wholly subjective matter) ; we do not determine 
 the position of the object. The connection between the position of 
 the object in the external world and the position of the sensation 
 in the field of vision, caimot be determined by visual observation 
 alone. All the information ^vhich can be gained by the eye is 
 limited to the field of vision, and provided that the relative position 
 of the sensations in the field of vision remained the same, the 
 actual position of external objects might, as far as vision is con- 
 cerned, be changed without our being aw^are of it. 
 
 As a matter of fact the field of vision in one important par- 
 ticular does not correspond to the field of- external objects. The 
 image on the retina is inverted ; the rays of light proceeding from 
 an object which by touch we know to be on what we call our right 
 hand, foil on the left-hand side of the retina. If therefore the field 
 of vision con-esponded to the retinal image, the object would be 
 seen on the left hand. We how^ever see it on the right hand, 
 because we invariably associate right-hand tactile localisation with 
 left-hand visual localisation ; that is to say, our field of vision, when 
 interpreted by touch, is a re-inversion of the retinal image. 
 
 The dimensions of the field of vision of a single eye are about 
 145° for the horizontal and 100° for the vertical meridian, the 
 former being distinctly greater than the latter. The horizontal 
 dimension of the field of vision for the two eyes is about 180°. 
 By movements of the eyes, however, even apart from those of the 
 head, the extent may be considerably increased. 
 
 The satisfactory perception of external objects requires distinct 
 vision ; and of this, as we have already said, the formation of a 
 distinct image on the retina is an essential condition. We can 
 ri'ceive visual sensations of all kinds with the most imperfect 
 dioptric apparatus, but our perception of an object is precise in 
 proportion to the clearness of the image on the retina. 
 
 Region of Distinct Vision. If we take two points, such as 
 two black dots, only just so far ajjart that they can be seen dis- 
 tinctly as two w^hen placed near the axis of vision, and then, 
 keeping the axis fixed, move the two points out into the circum- 
 ferential parts of the field of vision, it will be found that the two 
 soon appear as one. The two sensations become fused, as they 
 would do if brought nearer to each other in the centre of the field. 
 The farther away from the centre of the field, the farther apart 
 must two points be in order that they may be seen as two. ^ In 
 other words, vision is much more distinct in the centre of the field 
 than towards the circumference. Practically the region of distinct 
 
536 VISUAL PERCEPTIONS. [Book hi. 
 
 vision may be said to be limited to the macula lutea, or even to 
 the fovea centralis ; by continual movements of the eye we are 
 constantly bringing any object which we wish to see in such a 
 position that its image falls on this region of the retina. 
 
 The dimmution of distinctness does not take place equally from 
 the centre to tlie circumference along all meridians. The outline 
 described by a line uniting the points where two spots cease to be 
 seen as two when moved along different radii from the centre, is a 
 very irregular figure. 
 
 The sensations of colour are much more distinct in the centre 
 of the retina, than towards the circumference. If the visual axis 
 be fixed and a piece of coloured paper be moved towards the out- 
 side of the field of vision, the colour undergoes changes and is 
 eventually lost, red disappearing first, and blue last, the object re- 
 maining visible, though wdth very indistinct outlines, when its 
 colour can be no longer recognised. A purple colour becomes 
 blue, and a rose colour a bluish white. In fact, there seems to be 
 a certain amouut of red-blindness in the peripheral parts of all 
 retinas. 
 
 Modified Perceptions. 
 
 Since our perception of external objects is based on the dis- 
 tinctness of the sensations which go to form the perception, it 
 might be expected that when an image of an object is formed on 
 the retina the sensory impulses would corresjDond to the retinal 
 image, the sensations con^espond to the sensory impulses and the 
 perception correspond to the sensations, and that therefore the 
 mental condition resulting from our looking at any object or view 
 would correspond exactly to the retinal image. We find, however, 
 that this is not the case. The sensations and probably even the 
 simple sensory impulses produced by an image react upon each 
 other, and these reactions modify our perceptions, independently of 
 the physical conditions of the retinal image. There arise certain 
 discrepancies between the retinal image and the perception, some 
 having their source in the retina, some in the brain, and others 
 being of such a nature, that it is difficult to say where the 
 irrelevancy is introduced. 
 
 Irradiation. A white patch on a dark ground appears larger, 
 and a dark patch on a white ground smaller, than it really is. 
 This is especially so when the object is somewhat out of focus, and 
 
Chap. II. J SIC I IT. :^?,1 
 
 may, in this case, be partly explainctl Ijy the dil'lusiun circli^n wliicli, 
 in each case, encroach from the white upon the dark. V>\\\. over 
 ami bi'voml this, any sciisatiuii, cominfj Irom a fjiven retinal an-a, 
 Kcciipifs a iar^'cr sluire of the licld of vision, wlicn the rest of the 
 U'tina and central visnal apparatns arc at rest, than -when they are 
 sinudtanoonsly excited. It is as if the m.'i^h])ouiini^f, oitlior retinal 
 or cerebral, structnres were sympathetically tlnown into acti<jn at 
 the same time. 
 
 Contrast. If a wliite strip be placed between two black strips, 
 the edi^es of the white strip, near to the black, Avill appear whiter 
 than its median portion ; and if a white cro.ss be placed on a black 
 background, the centre of the cross will appear sometimes so dim, 
 compared with the parts close to the black, as to seem shaded. 
 This occurs even when the object is well in focus; the increased 
 sensation of light which causes the apparent greater whiteness of 
 the borders of the cross is the result of the 'contrast' with the 
 black placed immediately close to it. Still more curious results 
 are seen with coloured objects. If a small piece of grey paper be 
 placed on a sheet of green paper, and both covered with a sheet of 
 thin tissue paper, the grey paper will appear of a pink colour, the 
 complementary of the green. This effect of contrast is far less 
 striking, or even wholly absent, when the small piece of paper is 
 white instead of grey, and generally disappears Avhen the thin 
 covering of tissue paper is removed. It also vanishes if a bold 
 broad black line be drawn round the small piece of paper, so as to 
 isolate it from the gi'ound colour. If a book, or pencil, be jjlaced 
 vertically on a sheet of white paper, and illuminated on one side 
 by the sun, and on the other by a candle, two shadows' will be 
 produced, one from the sun which will be illuminated by the 
 yellowish light of the candle, and the other from the candle which 
 will in turn be illuminated by the white light of the sun. The 
 former naturally appears yellow ; the lattei", however, appears not 
 white but blue ; it assumes, by contrast, a colour complementary 
 to that o"f the candle-light which surrounds it. If the candle be re- 
 moved, or its light shut off by a screen, the blue tint disappears, 
 but returns when the candle is again allowed to j)roduce its shadow. 
 If, before the candle is brought back, a vision be directed through a 
 narrow blackened tube at some part falling entirely within the 
 area of what will be the candle's shadow, the area, which in the 
 absence of the candle appears white, will continue to appear white 
 when the candle is made to cast its shadow, and it is not until the 
 direction of the tube is changed so as to cover part of the ground 
 outside the shadow, as well as part of the shadow, that the latter 
 assumes its blue tint. 
 
 Filling up the Blind Spot. Though, as we have seen, that 
 part of the retina which corresponds to the entrance of the optic 
 
"538 VISUAL PERCEPTIONS. [Book hi. 
 
 nerve is quite insensible to light, we are conscious of no blank in the 
 field of vision. When in looking at a page of print we fix the 
 visual axis so that some of the print must fall on the blind spot, no 
 gap is perceived. We could not expect to see a black patch, be- 
 cause what we call black is the absence of the sensation of light 
 from structures which are sensitive to light ; we must have visual 
 organs to see black. But there are no visual organs in the blind 
 spot, and consequently we are in no way at all affected by the 
 rays of light which fall on it. There is in our subjective field of 
 vision no gap corresponding to the gap in the retinal image. We 
 refer the sensations coming from two points of the retina lying on 
 opposite margins of the blind spot to two points lying close 
 together, since we have no indication of the space which separates 
 them. Concerning the effects which are produced when an object 
 in the field of view passes into the region of the blind spot there " 
 has been much discussion. In ordinary vision of course, the exist- 
 ence of the blind spot is of little moment since it is outside the 
 region used for distinct vision, and besides the image of an object 
 does not fall on the blind spots of both eyes at the same time. 
 
 Ocular Spectra, So far from our perceptions exactly corre- 
 sponding to the arrangements of the luminous rays which fall on 
 the retina, we may have visual sensations and perceptions in the 
 entire absence of light. Any stimulation of the retina or of the 
 optic nerve sufficiently intense will give rise to a visual sensation. 
 Gradual pressure on the eyeball causes a sensation of rings of colour- 
 ed light, the so-called phosphenes ; a sudden blow on the eye causes 
 a sensation of flashes of light, and the seeming identity of the 
 visual sensations so brought about with visual sensations produced 
 by light is well illustrated by the statement once gravely made in 
 a"^German court of law, by a witness who asserted that on a pitch 
 dark night he recognised an assailant by help of the flash of light 
 caused by the assailant's hand coming in violent contact with his 
 eye. Electrical stimulation of the eye or optic nerve will also give 
 rise to visual sensations. 
 
 The sensations which may arise without any light falling on the 
 retina need not necessarily be undefined; on the contrary they 
 may be most clearly defined. Complex and coherent visual images 
 or perceptions may arise in the brain without any corresponding 
 objective luminous cause. These so-called ocular spectra or phan- 
 toms, which are the result of an intrinsic stimulation of some 
 (probably cerebral) part of the visual apparatus, have a distinctness 
 which gives them an apparent objective reality quite as striking as 
 that of ordinary visual perceptions. They may occasionally be 
 seen with the eyes open (and therefore while ordinary visual per- 
 ceptions are being generated) as well as when the eyes are closed. 
 They sometimes become so frequent and obtrusive as to be dis- 
 
CuAp. ii.| SIGHT, 539 
 
 Ircssiu^', uiul lorm ;iu iinj)urtaiit elcinciil in .suiuc kiu<_l.s ut' tlcliriuiu, 
 such as delirium tnnuiis. 
 
 Appreciation of apparent size. Jiy the eye alone we can only 
 estimate the apparent h^ize otan object, we can only tell what space 
 it takes in the field ot" vision, "\ve can only perceive the dimensions 
 of the retinal inia|,^', and therefore have a right only to s|)eak of 
 the anijle 'which tiie diameter of the ohject subtends. The real 
 size of an object must be determined by other means. But our 
 perception of even the apparent size of an object is so modified by 
 concurrent circumstances that in many cases it cannot be relied 
 on. The apparent size of the moon must be the same to every 
 eye, and yet while some persons will be found ready to comimre 
 the moon in mid heavens with a threepenny piece, others will 
 liken it to a cart-wheel ; that is to say, the angle subtended by the 
 moon seems to the one to be about equal to that subtended by a 
 threepenny piece held at the distance from the eye at which it is 
 most commonly looked at, and to the other about equal to that sub- 
 tended by a cart-wheel similarly viewed at the distance at which 
 it is most commonly looked at. If a line such as AG, Fig. 73, be 
 divided into two equal parts AB, BC, and AB he divided by 
 distinct mai'ks into several parts, as is shewn in the figure, while 
 BG be left entire, the distance AB will always appear greater than 
 GB. So also, if two equal squares be marked, one with hoiizontal 
 and the other with vertical 
 
 alternate dark and light bands, ' "^" '^' 
 
 the former will appear higher, a * * 
 
 and the latter broader, than it 
 
 really is. Hence short persons affect dresses horizontally striped 
 in order to increase their apparent height, and very stout persons 
 avoid longitudinal stripes. Two perfectly parallel lines or bauds, 
 each of which is crossed by slanting parallel short lines, will appear 
 not parallel, but diverging or converging according to the direction 
 of the cross-lines. 
 
 Again, when a short person is placed side by side with a tail 
 person, the former appears shorter and the latter taller than each 
 really is. The moon on the horizon appears larger than when at 
 the zenith, because in the first position it can be most easily com- 
 pared with teiTcstrial objects. The absence of comparison may, 
 however, contribute to an opposite effect, as when a person looks 
 larger in a fog ; being seen indistinctly, he is judged to be farther 
 off than he really is, and so appears larger than he naturally would 
 do at the distance at which he is supposed to be. So, conversely, 
 distant mountains when seen distinctly in a clear atmosphere 
 appear small, because on account of their distinctness they are 
 judged to be nearer than they really are. Indeed, our daily life is 
 full of instances in which our direct percejotion is modified by 
 circumstances. Among those circumstances previous experience is 
 
540 VISUAL rEECEPTIONS. [Book hi. 
 
 one of the most potent, and thus simple perceptions become 
 mingled with what are iu reality judgments, though frequently 
 made unconsciously. But this intrusion of past experience into 
 present perceptions and sensations is most obvious in binocular 
 vision, to which. we now turn. 
 
SEC. 4. BINOCULAR VISION. 
 
 Corresponding or Identical Points. 
 
 Though wc have t\YO eyes, and must therefore receive from 
 every object two sets of sensations, our perception of any object is 
 under ordinary circumstances a single one ; "\ve see one object, not 
 two. By putting either eye into an unusual position, as by sc^uint- 
 ing, we can render the jDerception double ; we see two objects 
 ■where one only exists. From which it is evident that singleness of 
 perception depends on the image of the object falling on certain 
 parts of each retina at the same time, these parts being so related 
 to each other, that the sensations from each are blended into one 
 perception ; and it is also evident that the movements of the eye- 
 balls are adapted to bring the image of the object to fall on these 
 ' corresponding ' or ' identical ' parts, as they are called, of each 
 retina. 
 
 "When Ave look at an object with one eye the visual axis of that 
 eye is directed to the object, and when we use two eyes the visual 
 axes of the two eyes converge at the object, the eyeballs moviug 
 accordingly. The corresponding points of the two retinas are those 
 on which the two images of the object fall when the visual axes 
 converge at the object. Thus in Fig. 74, if Cc, Cc^ be the two 
 visual axes, c, c, being the centres of the foveoe centrales of the two 
 eyes, then, the object ACB being seen single, the point a on the 
 one retina will 'correspond' to or be 'identical' with the point a^ 
 on the other, and the point b in the one to the point h^ in the other. 
 
542 
 
 BINOCULAR VISION. 
 
 [Book iii. 
 
 Hence a point lying anywhere on tlie right side of one retina, has 
 its corresponding point on the right side of the other retina, and 
 the points on the left of one correspond with those on the left of 
 
 .T- R 
 
 Fig. 74. Diagram illustrating Corresponding Points. 
 I, the left, R the right eye, K the optical centre, a^, /;,, Cj are points in the 
 right eye corresponding to the points a, h, c in the left eye. The two figures below 
 are projections of L the left and R the right retina. It will be seen that a on the 
 malar side of L corresponds to a^, on the nasal side of R. 
 
 the other. Thus, while the upper half of the retina of the left eye 
 corresponds to the upper half of the retina of the right eye, and the 
 lower to the lower, the nasal side of the left eye corresponds with 
 the malar side of the right, and the malar of the left with the 
 nasal side of the right. 
 
 The blending of the two sensations into one only occurs when 
 the two images of an object fall on these corresponding points of 
 the two retinas. Hence it is obvious that in single vision with two 
 eyes the ordinary movements of the eyeballs must be such as to 
 bring the visual axes to converge at the object so that the 
 two images may fall on corresponding points. When the visual 
 axes do not so converge, and when therefore the images do not fall 
 on corresponding points, the two sensations are not blended into 
 one perception and vision becomes double. 
 
 Movements of the Eyeballs. 
 
 The eye is virtually a ball placed in a socket, the bulb and the 
 orbit forming a ball and socket-joint. In its socket-joint the optic 
 ball is capable of a variety of movements, but it cannot by any 
 voluntary effort be moved out of its socket. It is stated that by a 
 very forcible opening of the eyelids the eyeball may be slightly 
 
Chap, ii.] HIG11T. h\\\ 
 
 protnulcd; Imt ihis trilliiii,' locomotion may be neglected. I3y 
 (lisousi\ however, the position of the eyeball in the socket may be 
 inateiially ohanged. 
 
 Each eyeball is capable of rotating round an immobile centre of 
 rotation, which has been found to be placed a little (r77mm.) be- 
 hind the centre of the eye ; but the movements of the eye round 
 the centre are limited in a peculiar way. The shoulder-joint is 
 also a ball and socket-joint; and we know that we can not only 
 move the arm up and down round a horizontal axis passing through 
 the centre of rotation of the head of the humerus, and from side 
 to side round a vertical axis, but we can also rotate it round its own 
 longitudinal axis. When, however, we come to examine closely 
 the movements of the eyeball we find, that though we can move it 
 up and down round a horizontal axis, as when with fixed head we 
 direct our vision to the heavens or to the ground, and from side to 
 side, as when we look to left or right, and though by combining 
 these two movements we can give the eyeball a variety of in- 
 clinations, we cannot, by a voluntary effort, rotate the eyeball 
 round its longitudinal visual axis. The arrangement of the muscles 
 of the eyeball will permit of such a movement, but we cannot by 
 any direct effort of will bring it about by itself. In certain move- 
 ments of the eye, rotation of the eyeball does take place ; and by 
 bringing about these movements, -vve can indirectly cause rotation ; 
 but we cannot rotate the eyeball except thus indirectly as a part 
 of these movements. 
 
 If, when vision is dii'ected to any object, the head be moved 
 from side to side, the eyes do not move mth it; they appear 
 to remain stationary, very much as the needle of a ship's compass 
 remains stationary when the head of the ship is turned. The 
 change in the position of the ^•isual axes to which the movement 
 of the head would naturally give rise is met by compensating 
 movements of the eyeballs; were it not so, steadiness of vision 
 would be imjDossible. 
 
 There is one position of the eyes which has been called the 
 'primarij jjosition. It corresponds to that which may be attained 
 by looking at the distant horizon vdth the head vertical and the 
 body upright; but its exact determination requires special pre- 
 cautions. The visual axes are then parallel to each other and to 
 the median plane of the head. All other positions of the eyes are 
 called secondary positions. 
 
 Muscles of the Eyeball. The eyeball is moved by six muscles, 
 the recti inferior, superior, ifdej-nus, and externus, and the ohliqui 
 inferior and supenor. It is found by calculation from the attach- 
 ments and directions of the muscles, and confirmed by actual obser- 
 vation, that the six muscles may be considered as three pairs, each 
 pair rotating the eye round a particular axis. The relative attach- 
 ments and the axes of rotation are diagrammatically shewn in 
 
Mi 
 
 BiyOG ULA R VISION. 
 
 [Book in. 
 
 Fig. 75. The rectus superior and the rectus inferior rotate the eye 
 round a horizontal axis, which is directed from the upper end of 
 the nose to the temple; the obliquus superior and obliquus in- 
 ferior round a horizontal axis du'ected from the centre of the eye- 
 
 obt.suj}. 
 
 S71JK 
 
 r. ihf 
 
 ?:ext. 
 
 Fig. 75. Dugeam of the Attachments of the Muscles of the Eye, and of their 
 Axes of Eotation, the latter being represented by dotted lines. The axis of 
 rotation of the rectus externus and internus, being perpendicular to the plane 
 of the paper, cannot be she^vn. (After Fick.) 
 
 ball to the occiput; and the rectus internus and rectus externus 
 round a vertical axis (which, being at right angles to the plane of 
 the paper, cannot be shewn in the diagTam), passing through 
 the centre of rotation of the eyeball parallel to the median jDlane 
 of the head when the head is vertical. Thus the latter pair acting 
 alone would turn the eye from side to side, the other straight pair 
 acting alone would move the eye up and down, while the oblicpie 
 muscles acting alone would give the eye an oblique movement. 
 The rectus externus acting alone would turn the eye to the malar 
 side, the internus to the nasal side, the rectus superior upwards, 
 the rectus inferior downwards, the oblique superior downwards and 
 outwards, and the inferior upwards and outwards. The recti 
 superior and inferior in moving the eye up and dowm also turn it 
 somewhat inward and at the same time give it a slight amount of 
 rotation; but this is corrected if the oblique muscles act at the 
 same time; and it is found that the rectus superior acting Avith the 
 obliquus inferior moves the eye upwards, and the rectus inferior 
 with the obliquus superior doA^mwards in a vertical direction. In 
 oblique movements also, the obliqui are alwaj^s associated with the 
 recti. Hence the various movements of the eyeball may be ar- 
 ranged as follows : 
 
CiiAi-. u.] SIGHT. 545 
 
 bo fl 
 
 Elevation. Rectus superior and obliquus inferior 
 
 Depression. Rectus inferior and obliquus superior. 
 
 Adduction to j^^^^^g .^^^^^g_ 
 
 nasal side. 
 Adduction to 
 
 malar side. 
 
 Rectus extcrnus. 
 
 ^ I 
 o % 
 
 Elevation with Rectus superior and intemus with obliquus 
 
 adduction. inferior. 
 
 Depression Rectus inferior and intemus with obliquus 
 
 with adduction. superior. 
 
 Elevation with Rectus superior and externus with obliquus 
 
 abduction. inferior. 
 
 Depression Rectus inferior and externus with obliquus 
 
 with abduction. superior. 
 
 Coordination of Visual Movements. Thus even in the move- 
 ments of a single eye, a considerable amount of coordination takes 
 place. When the eye is moved in any other than the vertical and 
 horizontal meridians, impulses must descend to at least three 
 muscles, and in such relative energy to each of the three as to pro- 
 duce the required inclination of the visual axis. But the co- 
 ordination observed in binocular vision is more striking still. If 
 the movements of any person's eyes be watched it will be seen that 
 the two eyes move alike. If the right eye moves to the right, so 
 does also the left; and, if the object looked at be a distant one, 
 exactly to the same extent; if the right eye looks up, the left eye 
 looks up also, and so in every other direction. Very few persons 
 are able by a direct effort of the will to move one eye inde- 
 pendently of the other; though some, and among them one 
 distinguished both as a physiologist and an oculist, have acquired 
 this power. In fact, the movements of the two eyes are so arranged 
 that in the various movements the images of any object should fall 
 on the corresponding points of the two retinas, and that thus single 
 vision should result. We cannot by any direct eJBEbrt of our will 
 place our eyes in such a position that the rays of light proceeding 
 from any object shall be brought to a focus on parts of the two 
 retinas which do not correspond, and thus give rise to two distinct 
 visual images. We can bring the visual axes of the two eyes from 
 a condition of parallelism to one of great convergence, but we 
 cannot, without special assistance, bring them from a condition 
 of parallelism to one of divergence. The stereoscope will enable us 
 to create a divergence. If in a stereoscopic picture the distance 
 between the pictures be increased very gradually so as carefully to 
 maintain the impression of a single object, the visual axes may be 
 brought to diverge. Similarly if a distant object be looked at 
 with a prism before one eye, and the image of the object be kept 
 carefully single, while the prism is turned very slowly up or down, 
 then on suddenly removing the prism a double image is for a 
 p. 35 
 
546 BINOCULAR VISION [Book hi. 
 
 moment seen; shewing that the eye before which the prism was 
 placed had moved in disaccordance with the other. The double 
 image however in a few seconds after the removal of the prism 
 becomes single, on account of the eyes coming into accordance. 
 
 It is only when loss of coordination occurs, as in various 
 diseases and in alcoholic or other poisoning, that the movements of 
 the two eyes cease to agree with each other. It is evident then 
 that when we look at an object to the right, since we thereby 
 abduct the right eye and adduct the left, we throw into action the 
 rectus externus of the right eye and the rectus intemus of the left; 
 and similarly when we look to the left we use the rectus externus 
 of the left and the rectus intemus of the right eye. On the other 
 hand when we look at a near object, and therefore converge 
 the visual axes, we use the recti interni of both eyes ; and when we 
 look at a distant object, and bring the axes from convergence 
 towards parallelism, we use the recti extemi of both eyes. In the 
 various movements of the eye there is therefore, so to speak, 
 the most delicate picking and choosing of the muscular instruments. 
 Bearing this in mind, it cannot be wondered at that the various 
 movements of the eye are dependent for their causation on visual 
 sensations. In order to move our eyes, we must either look at or 
 for an object; when we wish to converge our axes, we look at some 
 near object real or imaginary, and the convergence of the axes 
 is usually accompanied by all the conditions of near vision, such as 
 increased accommodation and contraction of the pupil. And so with 
 other movements. The close association of the movements of the 
 eye may be ihustrated by the following case. Suppose the eyes, to 
 start with, directed for the far distance, and that it is desired 
 to direct attention to a nearer point lying in the visual line of the 
 right eye. In this case no movement of the right eye is required ; 
 all that is necessary is for the left eye to be turned to the right, 
 that is, for the rectus intemus of the left eye to be thrown 
 into action. But in ordinary movements the contraction of 
 this muscle is always associated with either the rectus externus of 
 the right eye, as when both eyes are turned to the right, or 
 the rectus intemus of that eye, as in convergence ; the muscle 
 is quite unaccustomed to act alone. This would lead us to suppose 
 that in the case in question the contraction of the rectus internus 
 of the left eye is accompanied by a contraction of both recti externus 
 and intemus of the right eye, keeping that eye in lateral equi- 
 librium. And the peculiar oscillating movements seen in the right 
 eye, as well as the sense of efforts in the right eye which is felt by 
 the person, shew this to be the case. 
 
 Such a complex coordination requires for its carrying out 
 a distinct nervous machinery; and we have reasons for thinking 
 that such a machinery exists in certain parts of the corpora quadri- 
 gemina or in the underlying structures. In the nates, there 
 appears to be a common centre for both eyes, stimulation of the 
 
Chap, ii.] 
 
 SKIIIT. 
 
 547 
 
 right sidi' pro(lu('inf( movciiients of b(ith eyes to th(! left, of ihf loft 
 silk- movcnuiits to tlu' ri_t;ht ; whiU; stinnilation iu the middle lino 
 bohiiid causes a downward movement of both eyes with convergence 
 oi" the axes, and iu the front an upward movement with return to 
 parallelism, both accompanied by the naturally associated move- 
 ments of the pupil. Stimulation of various parts of the nates causes 
 various movements, depending on the position of the spot stimu- 
 lated. After an incision in the middle line, stimulation of the 
 nervous centre on one side produces movements in the eye of the 
 same side only. 
 
 The Horopter. 
 
 WTien we look at any object we direct to it the visual axes, so 
 that when the object is small, the ' corresponding ' parts of the two 
 
 Fig. 76. Diagram illustkating a simple HoRoriEE. 
 
 ^^'he^ the visual axes converge at C, the images a a of any point A on the 
 eii'cle diawn through C and the optical centres fc h, will fall on corresponding points. 
 
 retinas, on which the two images of the object fall, lie in their 
 respective fovere centrales. But while we are looking at the 
 particular object the images of other objects surrounding it fall 
 on the retina surrounding the fovea, and thus go to form what 
 is called indirect vision. And it is obviously of advantage 
 that these images also should fall on ' coiTesponding ' parts in 
 the two eyes. Now for any given position of the eyes there exists 
 in the field of vision a certain line or surface of such a kind 
 that the images of the points in it all flill on corresponding points 
 of the retina. A line or surface having this property is called 
 a Horopter. The horopter is in fact the aggregate of all those 
 points in space which are projected on to corresponding points 
 of the retina; hence its determination in any pai'ticular case 
 is simply a matter of geometrical calculation. In some instances it 
 
 35—2 
 
548 BINOCULAR- VISION. [Book hi. 
 
 becomes a very complicated figure. The case whose features are 
 most easily grasped is a circle drawn in the plane of the two 
 visual axes through the point of the convergence, of the axes and 
 the optic centres of the two eyes. It is obvious from geometrical 
 relations that in Fig. 76 the images of any point in the circle will 
 fall on corresponding points of the two retinas. When we stand 
 upright and look at the distant horizon the horopter is (approxi- 
 mately, for normal emmetropic eyes) a plane drawn through 
 our feet, that is to say, is the ground on which we stand; the 
 advantage of this is obvious. 
 
SEC. 5. VISUAL JUDGMENTS. 
 
 Binocular vision is of use to us inasmuch as the one eye is able 
 to fill up the gaps and imperfections of the other. For example, 
 over and above the monocular filling up of the blind spot, of which 
 we spoke in page 537, since the two blind spots of the two eyes, being 
 each on the nasal side, are not 'corresponding' parts, the one eye 
 supplies that part of the field of vision which is lacking in the other. 
 And other imperfections are similarly made good. But the great use 
 of binocular vision is to afford us means of forming visual judgments 
 concerning the form, size, and distance of objects. 
 
 Judgment of Distance and Size. The perceptions which we 
 gain simply and solely by our field of vision, concern two dimensions 
 only. We can become aware of the apparent size of any part of the 
 field coiTesponding to any jDarticular object, and of its topographical 
 relations to the rest of the field, but no more. Had we nothing 
 more to depend on, our sight would be almost valueless as far as any 
 exact information of the external world was concerned. By 
 association of the visual sensations with sensations of touch, and 
 Avith sensations derived from the movements of the eyeballs 
 required to make any such part of the field as corresponds to 
 a particular object distinct, we are led to form judgments, i.e. to 
 draw conclusions concerning the external world by means of 
 an interpretation of our visual perceptions. Looking before us, we 
 say we see a certain object of a certain colour neai'ly in front of us, 
 or much on our right hand or much on our left ; that is to say, we 
 judge such an object to be in such a position because from 
 
550 VISUAL JUDGMENTS. [Book hi. 
 
 the constitution of our brain, strengthened by all our experience, 
 we associate such a part of our field of vision with such an object. 
 The subjective visual complex sensation or perception is to us 
 a symbol of the external object. 
 
 Even with one eye we can, to a certain extent, form a judgment, 
 not only as to the position of the object in a plane at right angles 
 to our visual axis, but also as to its distance from us along the 
 visual axis. If the object is near to us, we have to accommodate 
 for near vision; if far from us, to relax our accommodation 
 mechanism so that the eye becomes adjusted for distance. The 
 muscular sense (of which we shall speak presently) of this effort 
 enables us to form a judgment whether the object is far or near. 
 Seeing the narrow range of our accommodation, and the slight 
 muscular effort which it entails, all monocular judgments of distance 
 must be subject to much error. Everyone who has tried to thread 
 a needle without using both eyes, knows how great these errors 
 may be. When, on the other hand, we use two eyes, we have still 
 the variations in accommodation, and in addition have all the 
 assistance which arises from the muscular effort of so directing the 
 two eyes on the object that single vision shall result. When the 
 object is near, we converge our visual axes ; when distant, we bring 
 them back towards parallelism. This necessary contraction of the 
 ocular muscles affords a muscular sense, by the help of which we form 
 a judgment as to the distance of the object. Hence, when by any 
 means the convergence which is necessary to bring the object into 
 single vision is lessened, the object seems to become more distant, 
 when increased, to move towards us : as may be seen in the 
 stereoscope. 
 
 The judgment of size is closely connected with that of distance. 
 Our perceptions, gained exclusively from the field of vision, go no 
 farther than the apparent size of the image, i. e. of the angle sub- 
 tended by the object. The real size of the object can only be 
 gathered from the apparent size of the image when the distance of 
 the object from the eye is known. Thus perceiving directly the 
 apparent size of the image, we judge the distance of the object 
 giving the image, and upon that come to a conclusion as to its 
 size. And conversely, when we see an object, of whose real size 
 we are otherwise aware, or are led to think we are aware, our 
 'judgment of its distance is influenced by its apparent size. Thus 
 when in our field of vision there appears the image of a man, 
 knowing otherwise the ordinary size of a man, we infer, if the 
 image be very small, that the man is far off. The reason of the 
 image being small may be because the man is far off, in which 
 case our judgment is correct ; it may be, however, because the 
 image has been lessened by artificial dioptric means, as when the 
 man is looked at through an inverted telescope, in which case 
 our judgment becomes a delusion. So also an image on a screen 
 when gradually enlarged seems to come forward, when gradually 
 
Chap, ii.] 
 
 siaiir. 
 
 r.5i 
 
 diminished seems to recede. In these cases the influence; on our 
 judgment of the muscular sense of binocular adjustment, or 
 monocular accommodation, is thwarted by the more direct influence 
 of the association between size and distance. 
 
 Judgment of Solidity. When we look at a small circle all 
 parts of tlie circle are at the same distance from us, all parts are 
 equally distinct at the same time, whether we look at it with one 
 eye or with two eyes. When, on the other hand, we look at a 
 sphere, the various parts of which are at different distances from 
 
 Fig. 77. 
 
 \ 
 
 
 / 
 
 
 \ 
 \ 
 
 / 
 
 
 \ 
 
 
 y 
 
 
 
 
 
 B 
 
 
 
 
 /^ 
 
 
 \ 
 
 / 
 
 \ 
 
 / 
 
 
 \ 
 
 R 
 
 US, a sense of the accommodation, but much more a sense of the 
 binocular adjustment, of the convergence or the opposite of the two 
 eyes, required to make the various parts successively distinct, 
 makes us aware that the various parts of the sphere are unequally 
 distant; and from that we form a judgment of its solidity. As 
 with distance of objects, so with solidity, which is at bottom a 
 matter of distance of the parts of an object, we can form a judg- 
 ment with one eye alone ; but our ideas become much more exact 
 and trustworthy when two eyes are used. And we are much 
 assisted by the effects produced by the reflection of light fi'om the 
 various surfaces of a solid object ; so much so, that raised surfaces 
 may be made to appear depressed, or vice versa, and flat surfaces 
 either raised or depressed, by appropriate arrangements of shadings 
 and shadow. 
 
 Binocular \asion, moreover, affords us a means of judging of the 
 solidity of objects, inasmuch as the image of any solid object which 
 falls on to the right eye cannot be exactly like that which falls on 
 the left, though both are combined in the single perception of the 
 two eyes. Thus, when we look at a truncated pyramid placed in 
 the middle line before us, the image which falls on the right eye is 
 of the kind represented in Fig. 77 R, while that which falls on the 
 left eye has the form of Fig. 77 L; yet the perception gained 
 from the two images together corresponds to the form of which 
 Fig. 77 B is the projection. Whenever we thus combine in one 
 perception two dissimilar images, one of the one, and the other of 
 the other eye, we judge that the object giving rise to the images is 
 solid. 
 
 This is the simple principle of the stereoscope, in which two 
 slightly dissimilar pictures, such as would correspond to the \asion 
 of each eye separately, are, by means of reflecting mirrors, as in 
 
552 VISUAL JUDGMENTS. [Book hi. 
 
 Wheatstone's original instrument, or by prisms, as in the form 
 introduced by Brewster, made to cast images on corresponding 
 parts of the two retinas so as to produce a single perception. 
 Though each picture is a surface of two dimensions only, the 
 resulting perception is the same as if a single object, or group of 
 objects, of three dimensions had been looked at. 
 
 It might be supposed that the judgment of solidity which 
 arises when two dissimilar images are thus combined in one per- 
 ception, was due to the fact that all parts of the two images cannot 
 fall on corresponding parts of the two retinas at the same time, and 
 that therefore the combination of the two needs some movement 
 of the eyes. Thus, if we superimpose E on L (Fig. 77), it is 
 evident that when the bases coincide the truncated apices will not, 
 and vice versa; hence, when the bases fall on corresponding parts, 
 the apices will not be combined into one image, and vice versa; in 
 order that both may be combined, there must be a slight rapid 
 movement of the eyes from the one to the other. That, however, 
 no such movement is necessary fm^ each particular case is shewn 
 by the fact that solid objects appear as such when illuminated by 
 an electric spark, the duration of which is too short to permit of 
 any movements of the eyes. If the flash occurred at the moment 
 that the eyes were binocularly adjusted for the bases of the pyra- 
 mids, the two apices not falling on exactly corresponding parts 
 would give rise to two perceptions, and the whole object ought to 
 appear confused. That it does not, but, on the contrary, appears a 
 single solid, must be the result of cerebral operations, resulting in 
 what we have called a judgment. 
 
 Struggle of the two Fields of Vision. If the images of two 
 surfaces, one black and the other white, are made to fall on corre- 
 sponding parts of the eye, so as to be united into a single percep- 
 tion, the result is not always a mixture of the two impressions, that 
 is a grey, but, in many cases, a sensation similar to that produced 
 when a polished surface, such as plumbago, is looked at : the surface 
 appears brilliant. The reason probably is because when we look at 
 a polished surface the amount of reflected light which falls upon 
 the retina is generally different in the two eyes ; and hence we as- 
 sociate an unequal stimulation of the two retinas with the idea of 
 a polished surface. So also when the impressions of two colours 
 are united in binocular vision, the result is in most cases not a 
 mixture of the two colours, as when the same two impressions are 
 brought to bear together at the same time on a single retina, but a 
 struggle between the two colours, now one, and now" the other, be- 
 coming prominent, intermediate tints however being frequently 
 passed through. This may arise from the difficulty of accommo- 
 dating at the same time for the two diflFerent colours (see p. 508) ; 
 if two eyes, one of which is looking at red, and the other at blue, 
 be both accommodated for red ravs, the red sensation will over- 
 
CnAP. II.] SIGHT. 553 
 
 power the blue, and nee versa. It may be however that the ten- 
 dency to rhythmic action, so manifest in other simpler manifesta- 
 tions of proto])l;ismic activity, makes its appearance also in the 
 higher cerebral labours of binocular vision. 
 
SEC. 6. THE PROTECTIVE MECHANISMS OF THE EYE. 
 
 The eyeball is protected by the eyelids, which are capable of 
 movements called respectively opening and shutting the eye. The 
 eye is shut by the contraction of the orbicularis muscle, carried out 
 either as a reflex or voluntary act, by means of the facial nerve. 
 The eye is opened chiefly by the raising of the upper eyelid, 
 through the contraction of the levator palpebrae carried out by 
 means of the third nerve. The upper eyelid is also raised and the 
 lower depressed, the eye being thus opened, by means of plain 
 muscular fibres existing in the two eyelids and governed by the 
 cervical sympathetic. The shutting of the eye as in winking is in 
 general effected more rapidly than the opening. 
 
 The eye is kept continually moist partly by the secretion of the 
 glands in the conjunctiva, and of the Meibomian glands, but chiefly 
 by the secretion of the lachrymal gland. Under ordinary circum- 
 stances the fluid thus formed is carried away by the lachrymal 
 canals into the nasal sac and thus into the cavity of the nose. 
 When the secretion becomes too abundant to escape in this way it 
 overflows on to the cheeks in the form of tears. 
 
 If a quantity of tears be collected, they are found to form 
 a clear faintly alkaline fluid, in many respects like saliva, contain- 
 ing about I p. c. of solids, of which a small part is proteid in nature. 
 Among the salts present sodium chloride is conspicuous. 
 
 The nervous mechanism of the secretion of tears, in many 
 respects, resembles that of the secretion of saliva. A flow is usually 
 brought about either in a reflex manner by stimuli applied to the 
 
CiiAP. ii.J SIGHT. 555 
 
 conjunctiva, tho nasal nmcons mcinl)rano, tongue, optic nerve, &c. 
 or more directly by emotions. A^<>nous congestion of the head 
 is also said to cause a How. The efferent m.-rves belong either 
 to the cercbro-spinal system, (the lachrymal and orbital branches 
 of the fifth nerve,) or arise from the cervical sympathetic, the 
 afferent nerves varying according to the exciting cause. 
 
 The act of blinking undoubtedly favours the passage of tears 
 through the lachrymal canals into the nasal sac, and hence when 
 the orbicularis is paralysed tears do not pass so readily as usual 
 into the nose ; but the exact mechanism by which this is efifected 
 has been much disputed. According to some authors, the con- 
 traction of the orbicularis presses the fluid onwards out of the 
 canals, which, upon the relaxation of the orbicularis, dilate and 
 receive a fresh ([uantity. Others maintain that a special arrange- 
 ment of muscular fibres keeps the canals open even during the 
 closing of the lids, so that the pressure of the contraction of the 
 orbicularis is able to have full effect in driving the tears through 
 the canals. 
 
CHAPTEE III. 
 HEARING, SMELL, AND TASTE. 
 
 SEC. 1. HEAPJXG. 
 
 As in the eye, so in the ear, we have to deal first with a nerve of 
 special sense, the stimulation of which gives rise to a special sensa- 
 tion; secondly with terminal organs through which the physical 
 changes proper to the special sense are enabled to act on the nerve; 
 and thirdly with subsidiary apparatus, by which the usefulness of 
 the sense is increased. The central connections of the auditory 
 nerve are such that whenever the auditory fibres are stimulated, 
 whether by means of the terminal organs in the usual way or by 
 the direct application of stimuli, electrical, mechanical, &c., the 
 result is always a sensation of sound. Just as stimulation of the 
 optic fibres produces no other sensation than that of light, so 
 stimulation of the auditor}^ fibres produces no other sensation than 
 that of sound \ The terminal organs of the auditory nerve are of 
 two kinds : the complicated organ of Gorti in the cochlea, and the 
 epithelial arrangements of the maculae and cristse acusticse in other 
 parts of the labyrinth. Waves of sound falling on the auditory 
 nerve itself produce no eftect whatever; it is only when by the 
 medium of the endolymph they are brought to bear on the delicate 
 and peculiar epithelium cells which constitute the peripheral ter- 
 minations of the nerve, that sensations of sound arise. Such 
 deUcate structures are for the sake of protection naturally ^vith- 
 drawn from the surface of the body where they would be subject to 
 injury. Hence the necessity of an acoustic apparatus, forming the 
 middle and external ear, by which the waves of sound are most 
 advantageously conveyed to the terminal organs. 
 
 ^ It will be seen later on that there are reasons for thinking that impulses 
 passing along the auditory nerve may give rise to other effects than auditory 
 sensations. 
 
Ohap. iii.J HEARING, SMELL, AND TASTE. 657 
 
 TliG Acoustic Apparatus. 
 
 Waves of sound can and do roach the cndolymph of the 
 labyiinth by direct conduction through the skull. Since however 
 sonorous vibrations are transmitted with great difficulty from tlie 
 air to solids and liquids, and most sounds come to us through the 
 air, some special apparatus is required to transfer the aerial \'ibra- 
 tions to the liquids of the internal ear. This apparatus is supplied 
 by the tyuipauum and its appendages. 
 
 The concha. The use of this, as far as hearing is concerned, is 
 to collect the waves of sound coming in various direction.s, and to 
 direct them on to the membraua tympani. In ourselves of moderate 
 service only, in many animals it is of great importance. 
 
 The membrana tympani. It is a characteristic property of 
 stretched membranes that they are readily thrown into vibration by 
 aerial waves of sound. The membrana tympani, from its peculiar 
 conformation, being funnel-shaped with a depressed centre sur- 
 rounded by sides gently convex outwards, is peculiarly susceptible 
 to sonorous vibrations, and is most readily thrown into correspond- 
 ing movements when waves of sound reach it by the meatus. It 
 has moreover this useful feature, that unlike other stretched 
 membranes, it has no marked note of its own. It is not thrown into 
 vibrations by waves of a particular length more readily than 
 by others. It answers equally well "svithin a considerable range, to 
 vibrations of very different wave-lengths. Had it a fundamental 
 tone of its own, we should be distracted by the j^rominence of this 
 note in most of the sounds we hear. When sounds impinge on the 
 solids of the head, as when a watch is held between the teeth, the 
 membrana tympani is still functional. Vibrations are conveyed 
 from the temporal bone to it and hence pass in the usual way, 
 in addition to those transmitted directly from the bone to the peri- 
 lymph. 
 
 The auditory ossicles. The malleus, the handle of which de- 
 scending forwards and inwards, is attached to the membrana tym- 
 pani, and the incus, whose long process is connected by means of its 
 OS orbiculare or lenticular process and the stapes to the fenestra 
 ovalis, form together a body which rotates round an axis, passing 
 through the short process of the incus, the bodies of the incus and 
 malleus, and the processus gracilis of the malleus. When the mal- 
 leus is carried inwards, the incus moves inwards too, and when the 
 malleus returns to its position, the incus returns ■\\dth it, the peculiar 
 saddle-shaped joint ■with its catch teeth permitting this movement 
 readily, but preventing the stapes being pulled back when the mem- 
 brana t}Tnpani with the malleus is, for any reason, pushed outwards 
 more than usual ; the joint then gapes, so as to permit the malleus 
 to be moved alone. Various ligaments, the superior or suspensory, 
 
558 THE TYMPANUM. [Book hi. 
 
 anterior, and external, also serve to keep the malleus in place. The 
 whole series of ossicles may be regarded as a single-armed lever, 
 moving on the ligamental attachment of the short process of 
 the incus to the posterior wall of the tympanum, the weight 
 being brought to bear at the end of the long process of the incus, 
 and the power at the end of the handle of the malleus. The 
 long, malleal arm of this lever is about 9| mm., the short, stapedial, 
 6|- mm. in length ; hence the movements of the stapes are less 
 than those of the tympanum '; but the loss in amplitude is made up 
 by a gain of force, which is in itself an obvious advantage. 
 
 Thus every movement of the tympanic membrane is trans- 
 mitted through this chain of ossicles to the membrane of the 
 fenestra ovalis, and so to the perilymph of the labyrinth; the 
 vibrations of the tympanic membrane are conveyed with increased 
 intensity, though with diminished amplitude, to the latter. That 
 the bones thus move en masse has been proved by recording their 
 movements in the usual graphic method. A very light style 
 attached to the incus or stapes is made to write on a travelling 
 surface; when the membrana tympani is thrown into vibrations by 
 a sound, the curves described by the style indicate that the chain 
 of bones moves with every vibration of the tympanum. On the 
 other hand, the comparatively loose attachments of the several 
 bones is an obstacle to the molecular transmission of sonorous 
 vibrations through them. Moreover, sonorous vibrations can only 
 be transmitted to or pass along such bodies as either are very long 
 compared to the length of the sound-waves, or, as in the case 
 of membranes and strings, have one dimension very much smaller 
 than the others. Now the bones in question are not especially 
 thin in any one dimension, but are in all their dimensions ex- 
 ceedingly small compared with the length of the vibrations of even 
 the shrillest sounds we are capable of hearing ; hence they must 
 be useless for the molecular propagation of vibrations. 
 
 The tensor tympani muscle even in a quiescent state is of use 
 in preventing the membrana tympani being pushed out far. When 
 it contracts it renders the membrana tympani more tense and 
 hence has been supposed to act as a damper lessening the 
 amount of vibration of the membrane in the case of too powerful 
 sounds; it is said to be readily thrown into contraction at the 
 commencement of a sound or noise, but to return to rest during 
 the continuance of a musical note. Efferent impulses reach it 
 through fibres of the fifth nerve, and its activity is regulated by 
 a reflex action. In some persons the muscle seems to be partly 
 under the dominion of the will, since a peculiar crackling noise 
 which these persons can produce at pleasure appears to be caused 
 by a contraction of the tensor tympani. 
 
 The so-called laxator tympani is considered to be not a muscle 
 at all, but a part of the ligamentous supports of the malleus. 
 
Chap. III.] IIKA1UNG\ SMELL, AND TASTE. 559 
 
 The stapedius muscle is .supposed to n-jrulate the movements 
 ot" the stapes, and especially to pnn'eiit its l)ase beiu<^ driven too far 
 into tlie fenestra ovalis (hiring large or sudden movements of the 
 membrana tym})ani. It is governed by fibres from the facial 
 nerve. 
 
 The Eustachian tube. Ti\is serves to maintain an ecpii- 
 librium of pressure between the external air and that within the 
 tyntpaniim, and to serve as an exit for the secretions of that cavity. 
 Were the tympanum permanently closed the vibrations of the 
 membrana tympani would be injuriously affected by variations of 
 pressure occurring either inside or outside. The Eustachian tube 
 is undoubtedly open during swallowing, but it is still dis])uted 
 whether it remains permanently open, or is opened only at in- 
 tervals ; probably it is, at most times, neither widely open nor 
 closely shut. 
 
 Auditory Sensations. 
 
 Each vibration communicated by the stapes to the perilymph 
 travels as a wave over the vestibule, the semicircular canals, and 
 other parts of the labyrinth; and from the perilymph is transmitted 
 through the membranous w^alls to the endolymph. From the 
 vestibule it passes on into the scala vestibuli of the cochlea, and 
 descending the scala tympani, ends as an impulse against the 
 membrane of the fenestra rotunda. In the regions of the maculae 
 and cristse the vibrations of the endolymph are supposed to throw 
 into corresponding vibrations the so-called auditory hairs. In the 
 cochlea the vibrations of the perilymph are supposed to throw into 
 vibrations the basilar membrane with the superimposed organ of 
 Corti, consisting of the rods of Corti with the inner and outer hair- 
 cells. The vibrations thus transmitted to these structures give rise 
 to nervous impulses in the terminations of the auditory nerves, and 
 these impulses reaching certain parts of the brain produce what we 
 call auditory sensations. "VVe are accustomed to divide our auditory 
 sensations into those caused by noises and those caused by musical 
 sounds. It is the characteristic of the latter that the vibrations 
 which constitute them are periodical; they occur and recur at regular 
 intervals. When no marked periodicity is present in the vibrations, 
 when the repetition of the several vibrations is irregular, or the 
 period so complex as not to be readily appreciated, the sensation 
 produced is that of a noise. There is however no abrupt line be- 
 tween the two. Between a pure and simple musical sound pro- 
 duced by a series of vibrations each of which has exactly the same 
 wave-length, and a harsh noise in which no consecutive vibrations 
 may be alike, there are numerous intermediate stages. 
 
 In both noises and musical sounds we recognise a character 
 which we call loudness. This is determined by the amplitude of 
 
560 AUDITORY SENSATIONS. [Book iii. 
 
 the vibrations; the greater the disturbance of the air (or other 
 medium) the louder the sound. In a musical sound we recognise 
 also a character which we call pitch. This is determined by the 
 wave-length of the vibrations; the shorter the wave-length, the 
 larger the number of consecutive vibrations which fall upon the ear 
 in a second, the higher the pitch. We are able to speak of a whole 
 series of tones or musical sounds of different pitch, from the lowest 
 to the highest audible tone. And even in many noises we can, to 
 a certain extent, recognise a pitch, indicating that among the 
 multifarious vibrations there is a periodicity of certain groups of 
 vibrations. 
 
 Lastly, we distinguish musical sounds by their quality; the 
 same note sounded on a piano and on a violin produce very 
 different sensations, even when a series of vibrations having in each 
 case the same period of repetition is set going. This arises from 
 the fact that the musical sounds generated by most musical in- 
 struments are not simple but compound vibrations. When the 
 note C in the treble for instance is struck on the piano, and we 
 analyse the total sound, we find that it can be resolved partly into 
 a series of vibrations with a period characteristic of the pure tone 
 of the treble C, and partly into other series of vibrations with 
 periods characteristic of the C in the octave above, of the G above 
 that, of the C in the next octave, and of the E above that. And 
 the sensation which we associate with the sound of the treble C on 
 the piano is determined by the characters of the complex vibration 
 arising out of these several constituent simple vibrations. Almost 
 all musical sounds are thus composed of what is called a 'funda- 
 mental tone' accompanied by a number of 'overtones.' And the 
 overtones varying in number and relative prominence in different 
 instruments, give rise to a difference in the sensation caused by the 
 whole tone. So that while the fundamental tone determines the 
 pitch of the sound, the quality of the sound is determined by the 
 number and relative prominence of the overtones. In a somewhat 
 similar way we distinguish the quality of noises, such as a banging, 
 crackling, or rustling noise, by an appreciation of sudden or 
 irregular changes in the amplitude and period of the constituent 
 vibrations. 
 
 Since we have a very considerable appreciation, capable by 
 exercise of astonishing enlargement, of the loudness, pitch, and 
 quality of a wide range of noises and musical sounds, it is clear 
 that, within the limits of hearing, each vibration or series of 
 vibrations must produce its effect on the auditory nerves, according 
 to the measure of its intensity and period. Out of those effects, out 
 of the sensory impulses to which the several vibrations thus give 
 rise, are generated our sensations of the noise or of the sound. 
 
 The vibrations of a musical sound (and since noises are so im- 
 perfectly understood, we may, with benefit, chiefly confine ourselves 
 to musical sounds), as they pass through the air (or other medium) 
 
OiiAi-. Ill] IIEARINa, SMELL, AND TASTE. 561 
 
 are not discrete ; the vibrations corresponding to the fundamental 
 tone and overtones do not travel as so many separate waves ; they 
 all together form one complex disturbance of the medium; and it 
 is as one composite wave that the sound falls on the membrana 
 tympani, and ])fissing through the auditory ai)paratus, breaks on 
 the terminations of the auditory nerve. And wiien two or more 
 musical sounds are heard at the same time, the same fusion of the 
 waves occurs. Since we can distinguish several tones reaching our 
 ear at the same time, it is clear that we must possess in our minds 
 or in our ears some means of analysing these composite waves 
 of sound which fall on our acoustic organs, and of sorting out their 
 constituent vibrations. 
 
 There is at hand a simple and easy physical method of analysing 
 composite sounds. If a person standing before an open piano sings 
 out any note, it "will be observed that a number of the strings of 
 the piano will be thrown into vibration, and on examination it mil 
 be found that those strings which are thus set going correspond in 
 pitch to the fundamental tone and to the several overtones of the 
 note sung. The note sung reaches the strings as a complex wave, 
 but these strings are able to analyse the wave into its constituent 
 vibrations, each string taking up those vibrations and those vibra- 
 tions only which belong to the tone given forth by itself when 
 struck. If we suppose that each terminal fibril of the auditory 
 nerve is connected with an organ so far like a piano-string that it 
 will readily vibrate in response to a series of vibrating impulses of 
 a given period and to none other, and that we possess a number of 
 such terminal organs sufficient for the analysis of all the sounds 
 which we can analyse, and that each terminal organ so affected by 
 particular vibrations gives rise to a sensory impulse and thus to a 
 sensation of a distinct chai'acter — if we suppose these organs to 
 exist, our appreciation of sounds is in a large measure explained. 
 In the organ of Corti we find structures the arrangement of Avhich 
 irresistibly suggests to us that these are the organs we are seeking. 
 We have only to suppose that of the long series of rods of Corti, 
 varying regularly as these do from the bottom to the top of the 
 spiral, in length and in the span of their arch, each pair will 
 vibrate in response to a particular tone, and the whole matter 
 seems explained. But the more the subject is inquired into, the 
 more complex and difficult it appears ; and we are obliged to 
 conclude that the part played by the rods of Corti is only a sub- 
 ordinate part of the function of the whole organ of Corti. 
 
 In the first place, it is difficult to see how the rods of Corti, 
 even if they are thrown into vibration, can originate sensoiy 
 impulses, for the fibrils of the auditory nei've terminate in the 
 inner and outer hair-cells, and it is in these cells, and not along the 
 course of the fibrils as they pass under and between the rods 
 of Corti, that the sensory impulses must begin. In the second 
 place, the variation in length of the fibres along the series is 
 
 F. 3G 
 
562 AUDITORY SENSATIONS. [Book in. 
 
 insufficient for the work assigned to them. Moreover, they ap- 
 pear not to be elastic. Lastly, they are wholly absent in birds, 
 who very clearly can appreciate musical sounds. This last fact 
 proves indubitably that the rods in question are not absolutely 
 essential for the recognition of tones. In the face of these diffi- 
 culties it has been suggested that the basilar membrane, which is 
 present in birds as well as in mammals, and which, being tense 
 radially but loose longitudinally, i. e. along the spiral of the cochlea, 
 may be considered as consisting of a number of parallel radial 
 strings, each capable of independent vibrations, is the sought-for 
 organ of analysis ; for it may be shewn mathematically that a 
 membrane so stretched in one direction only is capable of vibrating 
 in such a manner. And the radial dimensions of the basilar 
 membrane give a much greater range of difference than do the 
 rods of Corti, diminishing in man downwards from "495 mm. at 
 the hamulus to •04125 mm. near the bottom of the spiral, whereas 
 the difference in length of the latter is simply that between '048 
 and '085 mm. for the inner, and between '019 and "085 for the outer 
 fibres. According to this view, a particular simple vibration 
 reaching the scala tympani of the cochlea throws into sympathetic 
 vibrations a small portion of the basilar membrane, the vibrations 
 of which in turn so affect the structures overlying it, that sensory 
 impulses are generated. These sensory impulses reaching the brain 
 give rise to a corresponding sensation of a particular tone. 
 
 The remarkable reticular membrane which has such peculiar 
 relations with the hair-cells, and through them with the basilar 
 membrane, must, one might imagine, have some special function ; 
 but it is impossible at present to assign to it any satisfactory duty. 
 The structural arrangements seem, if anything, to indicate that when 
 a segment of the basilar membrane is thrown into vibrations, the 
 overljring hair-cells, reticular membrane, and rods of Corti vibrate 
 en masse together with it. But this renders the whole matter still 
 more difficult. Indeed the whole subject is in the highest degree 
 obscure, and the most we can say is that the organ of Corti as 
 a whole seems to be in some way connected with the appreciation 
 of tones, but that at present it is very hazardous to attempt to 
 explain how it acts, or to assign particular functions to particular 
 parts. The distinction between the inner and outer hair-cells 
 seems to be very parallel to that between the rods and the cones of 
 the retina ; but even this analogy may be a fallacious one. 
 
 It has been observed that among the auditory hairs of the 
 Crustacea, some will vibrate to particular notes ; but the auditory 
 hairs of the mammal are far too much of the same length to permit 
 the supposition that they can act as organs of analysis. 
 
 If the organ of Corti is the means by which we appreciate 
 tones, it is evident that by it also we must be able to estimate 
 loudness, for the quality of a musical sound is dependent on the 
 relative intensity, as well as on the nature, of the overtones. And 
 
CiiAi'. Ill] lli:.\RI.\(;, SMICLL, AX I) TASTK. .JO.'i 
 
 since noise is at best but contused music, the cochlea must be a 
 means of appreciating noises iis well as sounds. Jiut this would 
 leave nothiug whatever for the rest of the labyrinth to do in 
 respect to the appreciation of sound save so far as the difference in 
 structure between the hair-cells of Corti, with their short thick 
 rods, and the hair-bearing structures in the maculae and crista; 
 with their thin delicate hairs, may possibly indicate a difference of 
 function, the latter being more susceptible to the irregular vibra- 
 tions of noises. That the vestibule and semicircular canals are 
 however concerned in hearing is shewn by its being the only audi- 
 tory organ in the ichthyopsida, unless we suppose that in the 
 higher vertebrates its function has been wholly transfeiTed to the 
 cochlea. That the semicircular canals may have duties apart from 
 hearing we shall shew later on. 
 
 Concerning the function of the other parts of the internal ear 
 we know very little. The otoliths have been supposed to intensify 
 the vibrations of the endolymph ; but since apparently they are 
 lodged in a quantity of mucus it is probable that they really act as 
 dampers. A similar damping action has been suggested for the 
 membrane of Corti {inemhrmia tectoria) overhanging the fibres and 
 hair-cells ; and some ^vriters have supposed that muscular fibres 
 present in the planum semilunare may by tightening the basilar 
 membrane serve as a sort of accommodation mechanism. 
 
 It must however be borne in mind that even making the fullest 
 allowance for the assistance afforded us by the organ of Corti, the 
 appreciation of any sound is ultimately a mental act. The analysis 
 of the vibrations by the fibres of Corti or the basilar membrane is 
 simply preliminary to a synthesis of the sensory impulses so gene- 
 rated into a complex sensation. We do not receive a distinct 
 series of specific auditory impulses resulting in a specific sensation 
 for every possible variation in the wave-length of sonorous vibra- 
 tions any more than we receive a distinct series of specific visual 
 impulses for every possible wave-length of luminous vibrations. 
 In each case we have probably a number of primary sensations, 
 from the various mingling of which, in different proportions, our 
 varied complex sensations arise; the difference between the eye 
 and the ear being that whereas in the former the number of 
 primary sensations appears to be limited to three or at least to six, 
 in the latter, thanks to the organ of Corti, the number is very 
 large ; what the exact Jiumber is we cannot at present tell. Our 
 appreciation of a sound is at bottom an appreciation of the com- 
 bined effect produced by the relative intensities to which the 
 primary auditory sensations are, with the help of the organ of 
 Corti, excited by the sound. 
 
 Whatever be the explanation of the manner in which our 
 distinct auditory sensations arise, the range and precision of our 
 appreciation of musical sounds is very great. Vibrations with a 
 
 36—2 
 
564 AUDITORY SENSATIONS. [Book in. 
 
 recurrence below 30 a second^ are unable to produce a sensation of 
 sound ; if the waves are powerful enough we may feel them, but 
 we do not hear them if the vibrations are simple, and such as 
 would give rise to a pure tone ; if the fundamental tone is accom- 
 panied by overtones we may hear these, and are thus apt to say we 
 hear the former when in reality we only hear the latter. The note 
 of the 16-feet organ pipe, 33 vibrations a second, gives us the sen- 
 sation of a droning sound. A tone of 40 vibrations is however 
 quite distinct. In the other direction it is possible to hear a note 
 caused by 38,000 vibrations a second, though the limit for most 
 persons is far lower, about 16,000. Some persons hear grave 
 sounds more easily than high ones, and vice versa. This may be so 
 pronounced as to justify the subjects being spoken of as deaf to 
 gTave or high tones respectively. The range in different animals 
 is very different. 
 
 The power of distinguishing one note from another varies, as is 
 well known, in different individuals, according as they have or have 
 not a ' musical ear.' A well-trained ear can distinguish the differ- 
 ence of a single or even of a half vibration a second, and that 
 through a long range of notes. The range of an ordinary apprecia- 
 tion of tones lies between 40 and 4000 vibrations a second, i.e. be- 
 tween the lowest bass C (Cj 33 vibrations) and the highest treble 
 C (C° 4224 vibrations) of the piano ; tones above and below these, 
 even when audible, being distinguished from each other with great 
 difficult3^ 
 
 When two consecutive sounds follow each other at a sufficiently 
 short interval the sensations are fused into one. In this respect 
 auditory sensations are of shorter duration than ocular sensations. 
 When ocular sensations are repeated ten times in a second they 
 become fused (p. 520), whereas the ticks of a pendulum beating 
 100 in a second are readily audible as distinct sounds. When two 
 tuning-forks not quite in tune are struck together the interference 
 of the vibrations gives rise to an alternating rise and fall of the 
 sound, known as 'beats.' When the beats follow each other as 
 rapidly as 132 in a second they cease to be recognised, that is to 
 say, the sensations which they cause become fused. Before 
 they disappear they give a peculiar disagreeable roughness to the 
 sound. The pleasure given by musical sounds depends largely ou 
 the absence of this incomplete fusion of sensations. 
 
 Corresponding to entoptic phenomena there are various entotic 
 phenomena, sensations or modifications of sensations originating in 
 the tympanum or in the labyrinth ; moreover sensations of sound 
 may rise in the auditory nerve or in the brain itself, without any 
 vibration whatever falling on the labyrinth. 
 
 1 By some authors the limit is placed as low as 24 or even 15 a second. 
 
Chap, hi.] HEARING, SMELL, AND TASTE. 56'. 
 
 Auditory Judgments. 
 
 In seekiug fur the cause of our visual sensations we invariably 
 refer to the external world. The sensation caused by a direct 
 stimulation of the optic nerve or retina by a blow or a galvanic 
 current, we identify with that caused by a fiash of light. A sensa- 
 tion arising from any stimulation of the left side of our retina we 
 regard as caused by some object on the right-hand side of our ex- 
 ternal visible world. In a similar way, but to a less extent, we 
 project our auditory sensations into the world outside us, and when 
 the auditory nerve is affected we seek the cause in vibrations start- 
 ing at a greater or less distance from us. We do not think of the 
 soimd as originating in the ear itself. 
 
 This mental projection of the sound is much more complete 
 when the ear is stimulated by vibrations reaching it through the 
 membrana tympani than when the vibrations are conducted by the 
 solids of the head dii'ectly to the perilymph of the labyrinth. 
 When the meatus extemus is filled with fluid and the vibrations 
 of the membrana tympani are in consequence interfered with, the 
 apparent outwardness of sounds is to a very large extent lost ; 
 sounds, however caused, seem under these circumstances to arise in 
 the ear. Hence it would seem that the vibrations of the mem- 
 brana tympani, or possibly the action of the muscles attached 
 to the ossicula, give rise to obscure sensations of which, by them- 
 selves, we are not distinctly conscious, but which nevertheless lead 
 us to judge that the sounds heard by means of the tympanum 
 come from outside the ear. 
 
 Our judgment of the distance of sounds is very limited. A 
 sound whose characters we know appears to us near when it is loud, 
 and far off when it is faint. A blindfold person \\n\[ be unable to 
 distinguish between the difference of intensity produced on the 
 one hand by a tuning-fork being held before him, first "with the 
 broad edge of the fork toward him and then mth the naiTow edge, 
 and the difference on the other hand caused by the removal of the 
 tuning-fork to a distance. We can on the whole better appreciate 
 the distance of noises than of musical sounds. 
 
 Our judgment of the direction of sounds is also very limited. 
 Our chief aid in this is the position in which we have to place the 
 head in order that we may hear the sound to the best advantage. 
 If a tuning-fork be held in the median vertical plane over the 
 head, though it is easy to I'ecognise it as being in the median plane, 
 it becomes very difficult when the eyes are shut to say what is its 
 position in that plane, i.e. whether it is more towards the front or 
 
566 AUDITORY SENSATIONS. [Book in. 
 
 back of the head. In this respect, too, our appreciation is more 
 accurate in the case of noises than of musical sounds, with the 
 exception of those given out by the human voice, the direction of 
 which can be judged better than even that of a noise. 
 
SEC. 2. SMELL. 
 
 Odorous particles present in the inspired air passing through 
 the lower nasal chambers difiuse into the upper nasal chambers, 
 and falling on the olfactory epithelium produce sensory impulses 
 which, ascending to the brain, give rise to sensations of smell. "We 
 rnay presume that the sensor}^ impulses are originated by the con- 
 tact of the odorous particles with the peculiar rod-shaped olfactor}- 
 cells described by I^Iax Schultze ; but we are as much in the dark 
 about this matter as about the development of visual sensor}' im- 
 pulses in the rods and cones or of auditory sensory impulses in the 
 organ of Corti. 
 
 The subsidiary apparatus of smell is exceedingly meagre. By 
 the forced nasal inspiration, called sniffing, we draw air so forcibly 
 through the nostrils that currents pass up into the upper as well as 
 the lower nasal chambers ; and thus a more complete contact of the 
 odorous particles with the olfactory membrane than that supplied 
 by mere diffusion is provided for. 
 
 We have every reason to think that any stimulus applied to the 
 olfactory nerve will produce the sensation of smell ; but the proof 
 of this is not so clear as in the case of the optic and auditory 
 nerves. We are, however, subject to sensations of smell not caused 
 by objective odours. The olfactory membrane is the only part of 
 the body in which odours as such can give rise to any sensations ; 
 
568 SMELL. [Book in. 
 
 and the sensations to which they give rise are always those 
 of smell. The mucous membrane of the nose is however also an 
 instrument for the development of afferent impulses other than the 
 specific olfactory ones. Chemical stimulation of the olfactory 
 membrane by pungent substances such as ammonia gives rise to a 
 sensation distinct from that of smell, a sensation which affords us 
 no information concerning the chemical nature of the stimulus, 
 and which is indistinguishable from the sensations produced by 
 .chemical stimulation of other parts of the nasal membrane as well 
 as of other surfaces equally sensitive to chemical action. It is 
 probable that these two kinds of sensations thus arising in the ol- 
 factory membrane are conveyed by different nerves, the former by 
 the olfactory, the latter by the fifth nerve. 
 
 For the developement of smell it appears necessary that the 
 odorous particles should be conveyed to the nasal membrane in a 
 gaseous medium, or at least that the surface of the membrane 
 should not be exposed at the same time to the action of fluids. 
 Thus, when the nostril is filled with rose-water, the odour of roses 
 is not perceived ; and simply filling the nostrils with distilled water 
 suspends for a time all smell, the sense returning gradually after 
 the water has been removed ; the water apparently acts injuriously 
 on the delicate olfactory cells. 
 
 Each substance that we smell causes a specific sensation, and 
 we are not only able to recognise a multitude of distinct odours, 
 but also to distinguish individual odours in a mixed smell. 
 
 As in the previous senses, we project our sensation into the 
 external world ; the smell appears to be not in our nose, but some- 
 where outside us. We can judge of the position of the odour how- 
 ever even less definitely than we can of that of a sound. 
 
 The sensation takes some time to develope after the contact of 
 the stimulus with the olfactory membrane, and may last very long. 
 When the stimulus is repeated the sensation very soon dies out : 
 the sensory terminal organs speedily become exhausted. Mental 
 associations cluster more strongly round sensations of smell than 
 round any other impressions we receive from without. And reflex 
 effects are very frequent, many people fainting in consequence of 
 the contact of a few odorous particles with their olfactory cells. 
 
 Apparently the larger the surface the more intense the sen- 
 sation; animals with acute scent having a proportionately large 
 area of olfactory membrane. The quantity of material required to 
 produce an olfactory sensation may be, as in the case of musk, 
 almost immeasurably small. 
 
 When two different odours are presented to the two nostrils, an 
 oscillation of sensation similar to that spoken of in binocular vision 
 (p. 652) takes place. 
 
 The assertion that the olfactory nerve is the nerve of smell has 
 been disputed. Cases have been recorded of persons who appeared 
 to have possessed the sense of smell, and yet in whom the olfactory 
 
Chap, hi.] HEARING, SMELL, AND TASTE. neo 
 
 lobes were t'ouml after death to be absent. Direct experiments on 
 animals however sliew that lo.ss of the olfactory lobes entails lo.ss of 
 smell. On the other hand, it is stated that section or injury of the 
 fifth nerve causes a loss of smell though the olfactory nerve remains 
 intact; but in these cases it has not been shewn that the olfactory 
 membrane remains intact, and it is quite possible that, as in the 
 case of the eye, changes may take place in the nasal membrane as 
 the result of the injury to the fifth nerve, sufficient to prevent its 
 performing its usual functions. 
 
SEC. 3. TASTE. 
 
 The word taste is frequently used when the word smell ought 
 to be employed. We speak of 'tasting' odoriferous substances, such 
 as an onion, wines, &c., when in reality we only smell them as we 
 hold them in our mouth; this is proved by the fact that the 
 so-called taste of these things is lost when the nose is held, or the 
 nasal membrane rendered inert by a catarrh. 
 
 The terminal organs of the sense of taste thus more strictly de- 
 fined, are the endings of the glossopharyngeal and lingual nerves in 
 the mucous membrane of the tongue and palate, those nenes serving 
 as the special nerves of taste, ^^^lether the so-called gnistatoiy 
 buds can be regarded as specific organs of taste, appears doubtful. 
 The subsidiary apparatus is confined to the tongue and lips, which 
 by their movements assist in bringing the sapid substances into 
 contact with the mucous membrane of the mouth. 
 
 Though we can hardly be said to project our sensation of taste 
 into the external world, we assign to it no subjective localisation. 
 When we place quinine in our mouth, the resulting sensation of 
 taste gives us no information as to where the quinine is, though we 
 may learn that by concomitant general sensations arising in the 
 buccal mucous membrane. 
 
 We recognise a multitude of distinct tastes, which may be 
 broadly classified into acid, saline, bitter and sweet tastes. Sapid 
 substances have the power of producing these sensations by virtue 
 of their chemical nature. But other stimuli will also give rise to 
 sensations of taste. When the tongue is tapped, a taste is felt; 
 and when a constant current is passed through the mouth, an 
 alkaline or, in some persons, a bitter metallic taste is developed 
 when the anode, and an acid taste when the kathode, is placed on 
 the tongue. It is probable that in these cases the terminal organs 
 are indirectly affected by the current. When hot or pungent 
 substances are introduced into the mouth, sensations of general 
 feeling are excited, which obscure any strictly gustatory sensations 
 which may be present at the same time. 
 
 Though analogy would lead us to suppose that a stimulus 
 applied to any part of the course of the real gustatory fibres of 
 
Chap, hi.] HEARING, SMELL, AND TASTE. 571 
 
 oitht'v the glossophar}Tigeal or lingual nerves, would give rise to a 
 sensation of ta^^to and nothing else, the proof is not forthcoming ; 
 since both these nerves are mixed nerves containing other afferent 
 fibres as well as those of taste. 
 
 When the constant current is used as a means of exciting taste, 
 gustatory sensations are f^und to be developed in the back, edges 
 and tip of the tongue, the soft palate, the anterior pillar of the 
 fauces, and a small tract of the posterior part of the hard palate. 
 They are absent from the anterior and middle dorsal, and under 
 surface of the tongue, the front portion of the hard palate, the 
 posterior pillars of the fauces, the gums and the lips. Sapid 
 substances are unsuitable as a test for this purpose, on account of 
 their rapid diffusion. Bitter substances produce most effect when 
 placed on the back of, and sweet substances when placed on the tip 
 of the tongue ; but the tasting power of the tip of the tongue varies 
 very much in different individuals and in many seems almost 
 entirely absent. It is said that acids are best appreciated by the 
 edsre of the tongue. 
 
 It is essential for the developement of taste, that the substance 
 to be tasted should be dissolved ; and the effect is increased by 
 friction. The larger the surface the more intense the sensation. 
 The sensation takes some time to develope, and endures for a long 
 time, though this may be in part due to the stimulus remaining in 
 contact with the terminal organs. A temperature of about 40" is 
 the one most favourable for the production of the sensation. At 
 temperatures much above or below this, taste is much impaired. 
 The nerves of taste are, as we have said, the glossopharyngeal and 
 the lingual or gustatory. The former suppHes the back of the 
 tongue, and section of it destroys taste in that region. The latter 
 is distributed to the front of the tongue, and section of it similarly 
 deprives the tip of the tongue of taste. There is no reason for 
 doubting that the gustatoiy fibres in the glossopharyngeal are 
 proper fibres of that nerve ; but it has been urged by many, that 
 the gustatory fibres of the lingual are derived fi'om the chorda 
 tympani, and that those fibres of the lingual which come from the 
 fifth are employed exclusively in the sensations of touch and 
 feeling ; the evidence in favour of this view is however incon- 
 clusive. 
 
CHAPTER IV. 
 FEELING AND TOUCH. 
 
 SEC. 1. GENERAL SENSIBILITY AND TACTILE 
 PERCEPTIONS 
 
 We have taken the foregoing senses first in the order of discussion 
 on account of their being eminently specific. The eye gives us 
 only visual sensations, the ear only auditory sensations. The sen- 
 sations are produced in each case by specific stimuli : the eye 
 is only affected by light and the ear only by sound. Moreover, the 
 information they afford us is confined to the external world; they 
 tell us nothing about ourselves. The various visual sensations 
 which arise in our retina are referred by us not to the retina itself, 
 but to some real or imaginary object in the world without (in- 
 cluding as part of the external world such portions of our own 
 bodies as are visible to ourselves). Such also with diminishing 
 precision is the information gained by hearing, taste and smell. 
 
 All the other afferent nerves of the body, centripetal impulses 
 along which are able to affect our consciousness, are the means of 
 conveying to us information concerning ourselves. The sensations, 
 arising in them from the action of various stimuli, are referred by 
 us to appropriate parts of our own body. When any body comes 
 in contact with our finger, we know that it is our finger which has 
 been touched ; from the resultant sensation we not only learn the 
 existence of certain qualities in the object touched, but we also are 
 led to connect the cognizance of these qualities with a particular 
 pa»rt of our own body. 
 
 Like the more specific senses previously studied, the sensations 
 of which we are now speaking, and which may be referred to under 
 the name of touch, using that word for the present in a wide 
 
Ohap. IV ] FEELING AND TOUCH. 573 
 
 meanini^, rcniuiie for their production terminal organs; and the 
 chief but not cxchisive organ of touch is to be found in the cpidor- 
 mis of the skin and certain underlying nenous structures. For 
 the developement of specific tactile sensations these terminal organs 
 are as essential as are the terminal organs of the eye for sight or of 
 the ear for hearing. Contattt of the skin with a hard or with 
 a hot body gives rise to a distinct sensation, whereby we recognise 
 that we have touched a hard or a hot body. But the application of 
 either body or of any other stimulus to a nerve-tnmk gives rise to 
 a sensation of general feeling only, corresponding to the simple 
 sensation of light which is produced by direct stimulation of the 
 optic nerve. We have no more tactile 'perception of a body which 
 is in contact with a nerve-trunk than we could have visual per- 
 ception of any luminous object, the rays proceeding from which 
 were strong enough to excite sensory impulses when directed on to 
 the optic nerve instead of on to the retina, supposing such a thing 
 to be possible. It is further characteristix; of these ordinary nerves 
 of general feeling, that the sensations caused by any stimulation of 
 them beyond a certain degree develope that state of consciousness 
 which we are in the habit of speaking of as 'pain.' Putting aside 
 the general feeling which many parts of the eye possess, a very 
 strong luminous stimulation of the retina is required to produce a 
 sensation of pain, if indeed it can be at all brought about ; whereas 
 a ver}- moderate stimulation of the skin, and almost exery stimu- 
 lation of an ordinary nerve-trunk, is said by us to be painful. 
 
 Though the skin is the chief organ of touch, the mucous mem- 
 brane lining the various passages of the body also serves as an 
 instrument for the same sense, but only for a short distance from 
 the respective orifices. We can recognise hard or hot bodies "s\dth 
 our lips or mouth, but a hot liquid, when it has reached the oeso- 
 phagus or stomach, simply gives rise to a sensation of pain: we 
 cannot distinguish the sensation caused by it from the sensation 
 caused by a draught of a too acid fluid. 
 
 From parts and tissues of the body other than the skin and the 
 portions of mucous membrane just mentioned we have obscure 
 sensations of general feeling, by which we are made vaguely 
 aware of the general condition of our body, though our judgments 
 in this matter are chiefly influenced by what we shall have to 
 speak of directly as a muscular sense. In all parts of the body, 
 however, on occasions all too frequent, this general feeling may 
 become prominent as pain. 
 
 The stimuli which, when applied to the skin, give rise to tactile 
 perceptions are of two kinds only : (1) mechanical, that is, the con- 
 tact of bodies exerting varying degrees of pressure; and (2) thermal, 
 i.e. the raising or lowering of the temperature of the skin by the 
 approach or contact of hot or cold bodies. We can judge of the 
 weight and of the temperature of a body, because we can, through 
 touch, perceive how much it presses when allowed to rest on 
 
574 TACTILE SENSATIONS. [Book hi. 
 
 our skin or how hot it is. But we can through touch derive 
 no other perceptions and form no other judgments. An electric 
 shock sent through the skin will give rise to a sensation, but 
 the sensation is an indefinite one, because the electric current acts 
 not on the terminal organs of touch, but on the fine nerve-branches 
 of the skin. We cannot distinguish the sensation so caused from a 
 mechanical prick of similar intensity, we cannot perceive that the 
 sensation is caused by an electric current. Similarly certain 
 chemical substances such as a strong acid will give rise to a 
 sensation, but we cannot perceive the acid, we can form no judg- 
 ment of its nature such as we could if we tasted it ; and if the acid 
 does not permeate the skin so as to act directly and chemically on 
 the fine nerve-fibres, we cannot distinguish the acid from any 
 other liquid giving rise to the same simple contact impressions. 
 The terminal organs of the skin are such as are only affected 
 by pressure or by temperature. Conversely pressure or a variation 
 in temperature brought to bear on a nerve-trunk, instead of on the 
 terminal organs, produces no specific tactile sensations of pressure 
 or temperature, but merely general sensations of feeling rapidly 
 rising into pain. 
 
SEC. 2. TACTILE SENSATIONS. 
 
 Sensations of Pressure. 
 
 As with visual, so with tactile and indeed with all other sen- 
 sations, the intensity of the sensation maintains that general 
 relation to the intensity of the stimulus which we spoke of at 
 p. 521 as being formulated under Weber's law. We can distinguish 
 the difference of pressure between one and two grammes as readily 
 as we can that between ten and twenty or one hundred and two 
 hundred. 
 
 When two sensations follow each other in the same spot at a 
 sufficiently short interval they are fused into one ; thus, if the 
 finger be brought to bear lightly on a rotating card having a series 
 of holes in it, the holes cease to be felt as such when they follow 
 each other at a rapidity of about 1500 in a second. The vibrations 
 of a cord cease to be appreciable by touch when they reach the 
 same rapidity. T^Tien sensations are generated at points of the 
 skin too close together they become fused into one ; but to this 
 point we shall return presently. 
 
 The sensation caused by pressure is at its maximum soon after 
 its begimiing, and thenceforward diminishes. The more suddenly 
 the pressure is increased, the greater the sensation ; and if the 
 increase be sufficiently gradual, even very great pressure may be 
 applied without giving rise to any sensation. A sensation in any 
 spot is increased by contrast when the sun-ounding areas are not 
 subject to pressure. Thus if the finger be dipped into mercury the 
 pressure will be felt most at the surface of the fiuid ; and if the 
 finger be drawn up and down, the sensation caused \\\\[ be that of 
 a ring moving along the finger. 
 
576 TACTILE SENSATIONS. [Chap. iv. 
 
 All parts of the skin are not equally sensitive to pressure; small 
 differences of simple pressure are more readily appreciated when 
 brought to bear on the palmar surface of the finger, or on the 
 forehead, than on the arm or on the sole of the foot. In making 
 these determinations all muscular movements should be avoided in 
 order to eliminate the muscular sense of which we shall speak 
 presently; and the area stimulated should be as small and the 
 surfaces in contact as uniform as possible. In a similar manner 
 small consecutive variations of pressure, as in counting a pulse, are 
 more readily appreciated by certain parts of the skin than by 
 others ; and the minimum of pressure which can be felt differs in 
 different parts. In all cases variations of pressure are more easily 
 distinguished when they are successive than when they are simul- 
 taneous. 
 
 Sensations of Temperature. 
 
 When the temperature of the skin is raised or lowered in any 
 spot we receive sensations of heat and cold respectively ; and by 
 these sensations of the temperature of our own skin we form judg- 
 ments of the temperature of bodies in contact with it._ Bodies of 
 exactly the same temperature as the region of the skin to which 
 they are applied produce no such thermal sensations, though we 
 can, from the very absence of sensations, form a judgment as 
 to their temperature; and good conductors of heat appear re- 
 spectively hotter and colder than bad conductors raised to the same 
 temperature. 
 
 We may consider the skin as having at any given time and in 
 any given spot a normal temperature at which the sensation 
 of temperature is at zero ; for under ordinary circumstances we ar-e 
 not directly conscious of the temperature of our skin ; it is only 
 when the normal temperature at the spot is raised or lowered that 
 we have a sensation of heat or cold respectively. This normal 
 temperature may be at the same time different in different parts of 
 the body ; thus at a time when neither the forehead nor the hand 
 are giving rise to any sensation of temperature, we may, by putting 
 the hand to the forehead, frequently feel the former hot or cold 
 because the normal temperatures of the two parts differ. The 
 normal temperature in any spot may also vary from time to time. 
 Thus when the hand is placed in a warm medium for some time, 
 the sensation of warmth ceases ; a new normal temperature^ is 
 established with the zero of sensation at a higher level, a depression 
 or elevation of this new temperature giving rise however as before 
 to sensations of heat and cold respectively. That it is the changed 
 condition, and not the change itself, of which we are consciousis 
 shewn by the fact that when a portion of skin is cooled, by brief 
 contact mth a cold metal for instance, we are still conscious of the 
 
CnAi- IV.] FEELING AND TOUCH. 577 
 
 spot being cold attLT the cooling agent has been removed, that is 
 at a time when the cooled spot is in reality being heated by the 
 surrounding warmer tissues. 
 
 The change in temperature of the skin necessary to produce a 
 sensation must have a certain rapidity ; and the more gradual the 
 change the less intense the sensation. The repeated dipping of the 
 hand into hot water produces a greater sensation than when the 
 hand is allowed to remain all the time in the water, though in the 
 latter case the temperature of the skin is most affected. The 
 effects of contrast are also seen in these sensations as in those of 
 pressure. 
 
 We can with some accuracy distinguish variations of tempera- 
 ture, especially those lying near the normal temperature of the 
 skin. These sensations, in fact, follow Weber's law, thouorh 
 apparently sensations of slight cold are more vivid than those 
 of slight heat, the range of most accurate sensation seeming to lie 
 between 27° and 33°. 
 
 The regions of the skin most sensitive to variations in tempera- 
 ture are not identical ^vith those most sensitive to variations 
 in pressure. Thus the cheeks, eyelids, temples and lips, are more 
 sensitive than the hands. The least sensitive parts are the legs, 
 and front and back of the trunk. 
 
 The simplest view which can be taken with regard to the 
 distinction between pressure sensations and temperature sensations, 
 and which is suggested by the facts just mentioned, is to suppose 
 that two distinct kinds of terminal organs exist in the skin, one of 
 which is affected only by pressure, and the other only by variations 
 in temperature ; and that the two kinds of peripheral organs are 
 connected with different parts of the central sensory organs by 
 separate nerve-fibres. Certain pathological cases have been quoted 
 as shewing not only that this is the case, but that the two sets of 
 fibres pursue different courses in the spinal cord. Thus in certain 
 diseases or injuries to the brain or spinal cord, hypersesthesia 
 as regards temperature has been observed unaccompanied by an 
 augmentation of sensitiveness to pressure ; and conversely instances 
 have been seen where the patient could tell when he was touched, 
 but could not distinguish between hot and cold. On the other 
 hand there are facts which shew a close dependence between the 
 sensations of pressure and temperature. When each stimulus is 
 brought to bear on a very limited area, the two sensations are fre- 
 quently confounded, especially in those regions of the body where 
 sensations are not acute. So also a penny cooled down nearly 
 to zero and placed on the forehead wall be judged by most 
 people to be as heavy or even heavier than two pennies of the 
 temperature of the forehead itself; and conversely a body warmer 
 than the skin will often appear heavier than a body of the same 
 weight but of the same temperature as the skin. JMoreover cases 
 have been recorded where a hot body, such as a heated spoon, 
 
 F- 37 
 
578 SENSATIONS OF TEMPERATURE. [Book iii. 
 
 was felt, though the application of the same spoon at the 
 temperature of the body produced no sensations, and yet the 
 heated spoon was not recognised as a hot body but appeared 
 to be simply something touching the skin. It may be argued 
 that these instances shew nothing more than the changes in 
 the skin whatever they be, which give rise to sensations of 
 pressure, are modified by the temperature of the skin for the time 
 being, whereby the judgment as to the pressure which is being- 
 exerted is rendered faulty ; but they may also be taken to indicate 
 that variations in pressure and temperature affect the same 
 terminal organs, and the same nerve-fibres, though affecting them 
 in a different way and generating nervous impulses so far different 
 that the}^ give rise to different sensations. And we may here note 
 that we certainly cannot speak of nerves of warmth in the same 
 sense in which we speak of nerves of sight or of hearing. A 
 stimulus (of whatever kind) applied to an optic or auditory 
 nerve, if adequate, gives rise, as we have seen, to a sensation of 
 light or of sound; a stimulus, on the other hand, applied to the 
 trunk of a cutaneous nerve gives rise only to general feeling or 
 pain; though the nerve certainly contains fibres by which sen- 
 sations of pressure and of temperature reach the brain, the general 
 feeling which stimulation of the trunk causes is akin neither to 
 sensations of pressure nor to those of warmth. 
 
 The rapidity with which hot or cold bodies brought into contact 
 with the skin give rise to sensations of temperature, suggests that the 
 terminal apparatus for generating these sensations, whatever be its 
 nature, is placed in the epidermis, and indeed as near as possible to 
 the surface. Pressure on the other hand can be readily transmitted 
 through even a thick layer of skin. And those who maintain the 
 existence of different terminal organs for pressure and temperature, 
 regard the nerve-endings in the epidermis as the latter and the 
 cor^Duscula tactus, end-bulbs and allied organs as the former. But 
 the evidence we possess concerning this matter is at present incon- 
 clusive. 
 
SEC. 3. TACTILE PERCEFTIONS AND JUDGMENTS. 
 
 When a body presses on. any spot of our skin, or when the tem- 
 perature of the skin at that spot is raised, we are not only conscious 
 of pressure or of heat, but perceive that a particular part of our 
 body has been touched or heated. We refer the sensations to 
 their place of origin, and we thus by touch perceive the relations to 
 ourselves of the body which gives rise to the tactile sensations, in 
 the same way as in our visual perception of external objects we 
 refer to external nature the sensations originating in certain parts 
 of the retina. When we are touched on the finger and on the back 
 we refer the sensations to the finger and to the back respectively, 
 and when we are touched at two places on the same finger at the 
 same time we refer the sensations to two points of the finger. In 
 this way we can localize our sensations, and are thus assisted 
 in perceiving the space relations of objects with which we come in 
 contact. 
 
 This power of localizing pressure-sensations varies in difi'erent 
 parts of the body. The following table from Weber gives the 
 distance at which two points of a pair of compasses must be held 
 apart, so that when the two points are in contact with the skin, 
 the two consequent sensations can be localized with sufficient 
 accuracy to be referred to two points of the body, and not 
 confounded together as one. 
 
 Tip of tongue 
 
 Palm of last phalanx of finger 
 
 Palm of second „ „ 
 
 Tip of nose 
 
 White part of lips ... 
 
 Back of second phalanx of finger 
 
 Skin over malar bone 
 
 Back of hand 
 
 Forearm 
 
 Sternum 
 
 Back 
 
 37—2 
 
 
 1-1 m 
 
 
 2-2 „ 
 
 
 4-4 „ 
 
 
 6-6 „ 
 
 
 8-8 „ 
 
 
 111 „ 
 
 
 15-4 „ 
 
 
 29-8 „ 
 
 
 39-6 „ 
 
 
 44-0 „ 
 
 
 66-0 „ 
 
580 TACTILE PERCEPTIONS. [Book hi. 
 
 And an analogous distribution has been observed in reference to 
 the localisation of sensations of temperature. As a general rule it 
 may be said that the more mobile parts are those by which we can 
 thus discriminate sensations most readily. The lighter the pressure 
 used to give rise to the sensations, the more easily are two sen- 
 sations distinguished ; thus two points which, when touching the skin 
 lightly, appear as two, may, when firmly pressed, give rise to one 
 sensation only. The distinction between the sensations is obscured 
 by neighbouring sensations arising at the same time. Thus two 
 points brought to bear within a ring of heavy metal pressing on 
 the skin, are readily confused into one. And it need hardly be 
 said that these tactile perceptions, like all other perceptions, are 
 immensely increased by exercise. 
 
 Our 'field of touch,' if we may be allowed the expression, 
 is composed of tactile areas or units, in the same way that our field 
 of vision is composed of visual areas or units. The tactile sensation 
 is, like the visual sensation, a symbol to us of some external event, 
 and we refer the sensation to its appropriate place in the field of 
 touch. All that has been said (p. 523) concerning the subjective 
 nature of the limits of visual areas, applies equally well, mutatis 
 mutandis, to tactile areas. When two points of the compasses are 
 felt as two distinct sensations, it is not necessary that two and only 
 two nerve-fibres should be stimulated ; all that is necessary is that 
 the two cerebral sensation-areas should not be too completely 
 fused together. The improvement by exercise of the sense of 
 touch must be explained not by an increased development of the 
 terminal organs, not by a grov^h of new nerve-fibres in the skin, 
 but by a more exact limitation of the sensational areas in the 
 brain, by the development of a resistance which limits the radiation 
 taking place from the centres of the several areas. 
 
 By a multitude of simultaneous and consecutive tactile sensa- 
 tions thus converted into perceptions we are able to make ourselves 
 acquainted with the form of external objects. We can tell by 
 variations of pressure whether a surface is rough or smooth, plane 
 or curved, what variations of surface a body presents, andhow far 
 it is heavy or light; and from the information thus gained we 
 build up judgments as to the form and nature of objects, judgments 
 however which are most intimately bound up with visual judg- 
 ments, the knowledge derived by one sense correcting and com- 
 pleting that obtained by the other. As in other senses so in this, 
 our sensations may mislead us and cause us to form erroneous 
 judgments. This is well illustrated by the so-called experiment of 
 Aristotle. It is impossible in an ordinary position of the fingers to 
 bring the radial side of the middle finger and the ulnar side of the 
 ring finger to bear at the same time on a small object such 
 as a marble. Hence when with the eyes shut we cross one finger 
 over the other, and place a marble between them so that it touches 
 the radial side of the one and the ulnar side of the other, we 
 
Chat, iv.] FEELING AND TOUCH. 581 
 
 recognise that the object is such as could not under ordinary 
 conditions be touched at the same time by these two portionb 
 of our skin, and therefore judge tliat we are touching not one but 
 two marbles. Upon repetition however we are able to correct our 
 judi^ment and the illusion disappears. 
 
 Distinct tactile sensations are, as we have seen, produced only 
 when a stimulus is applied to a terminal organ. When sensations or 
 afteotions of general sensibility other than the distinct tactile sen- 
 sations arc developed in the termination of a nerve, we are still able, 
 though with less exactitude, to refer the sensation to a particular 
 part of the body. Thus when we are pricked or burnt, we can feel 
 where the prick or burn is. When a sensory nerve-tnmk is 
 stimulated, the sensation is always referred to the peripheral 
 terminations of the nerve. Thus a blow on the ulnar nerve at the 
 elbow is felt as a tingling in the little and ring lingers correspond- 
 ing to the distribution of the nerve, and sensations started in the 
 stump of an amputated limb are referred to the absent member. 
 When cold is applied to the elbow it is felt as cold in the skin 
 of the elbow ; but a cooling of the ulnar nerve at this spot, since 
 stimulation of a nerve-trunk gives rise to general sensations only, 
 simply gives rise to pain which is referred to the ulnar side of 
 the hand and arm. 
 
SEC. 4. THE MUSCULAE SENSE. 
 
 When we come into contact with external bodies we are 
 conscious not only of the pressure exerted by the object on our 
 skin, but also of the pressure which we exert on the object. If we 
 place the hand and arm flat on a table, we can estimate the 
 jjressure exerted by bodies resting on the palm of the hand, and 
 so come to a conclusion as to their weights ; in this case we are 
 conscious only of the pressure exerted by the body on our skin. If 
 however we hold the body in the hand, we not only feel the 
 pressure of the body, but we are also aware of the muscular 
 exertion required to supjjort and lift it. We possess a muscular 
 sense ; and we find by experience that when we trust to this 
 muscular sense as well as to sensations of pressure, we can form 
 much more accurate judgments concerning the weight of bodies than 
 when we rely on sensations of pressure alone. When we want to tell 
 haw heavy a body is, we are not in the habit of allowing it simply to 
 press on the hand laid flat on a table ; we hold it in our hand and 
 lift it up and down. We appeal to our muscular sense to inform 
 us of the amount of exertion necessary to move it, and by help of 
 that, judge of its weight. And in all the movements of our body 
 we are guided, even to an astonishing degree of accuracy, as is well 
 seen in the discussions concerning vision, by an appreciation, more 
 or less distinctly conscious, of the amount of the contraction to 
 which we are putting our muscles. In some way or other we are 
 made aware of what particular muscles or groups of muscles are 
 being thrown into action, and to what extent that action is being 
 
CiiAi'. IV. 1 FEELINU AND TOUCH. 583 
 
 carried. We are also conscious of the varying condition of our 
 nniscles, even when they are at rest ; the tired and especially the 
 paralysed limb is said to ' feel ' heavy. In this way the state of our 
 muscles largely determines our general feeling of health and 
 vigour, of weariness, ill health and feebleness. 
 
 It has been suggested that since muscle possesses little or no 
 general sensibility, comparatively little pain being felt for instance 
 when muscles are cut, our muscular sense is chiefly derived from 
 the traition of the contracting muscle on its attachments ; and 
 undoubtedly in many instances of cramp, the pain is chiefly felt 
 at the joints ; and, as we know, Pacinian bodies are abundant 
 around the joints. Afferent nerves, however, having a different 
 disposition from the ordinary motor nerves which terminate in 
 end-plates, have been described as present in muscle ; and analogy 
 would lead us to suppose that these afierent fibres, though possess- 
 ing a low general sensibility, might be easily excited in a specific 
 manner by a muscular contraction ; but further investigations are 
 necessary before these can be accepted as the true nerves of the 
 muscular sense. 
 
 In favour of the view that the muscular sense is peripheral and 
 not central in origin, may be urged the fact that the sense is felt 
 when the muscles are thrown into contraction by direct galvanic 
 stimulation instead of by the agency of the will. Many authors, 
 even while admitting the existence of a muscular sense of peripheral 
 origin, contend that we also possess and are very largely guided in 
 our movements by w^hat might be called a ' neural ' sense of central 
 origin. That is to say, the changes in the central nervous system 
 involved in initiating and carrying out a movement of the body, so 
 affect our consciousness, that we have a sense of the effort itself 
 
 It has been observed that when the posterior roots are divided, 
 movements become less orderly, as if they lacked the guidance of a 
 muscular sense ; and although the impaii'ment of the movements 
 may be due in part to the coincident loss of tactile sensations, it is 
 probable that it is increased by the loss of the muscular sense. 
 There is a malady or rather a condition attending various diseased 
 states of the central nervous system called locomotor ataxy, the 
 characteristic feature of which is that, though there is no loss of 
 direct power over the muscles, the various bodily movements are 
 effected imperfectly and with difficulty, from want of proper 
 co-ordination. In such diseases the pathological mischief is fre- 
 quently found in the posterior columns of the spinal cord and the 
 posterior roots of the spinal nerves, that is in distinctly afferent 
 structures ; and the phenomena seem in certain cases at least to be 
 due to inefficient co-ordination caused by the loss both of the 
 muscular sense and of ordinary tactile sensations. The patients 
 walk with difficulty, because they have imperfect sensations both of 
 the condition of their muscles and of the contact of their feet wdth 
 the ground. In many of theii' movements they have to depend largely 
 
584 MUSCULAR STRENGTH. [Book hi. 
 
 on visual sensations ; hence when their eyes are shut, they become 
 singularly helpless. In other cases again ataxy may be present 
 without any impairment of touch; but a discussion of the varied 
 phenomena of this class of maladies cannot be entered into here. 
 
CHAPTER V. 
 THE SPINAL CORD. 
 
 SEC. 1. AS A CENTRE OR GROUP OF CENTRES OF 
 REFLEX ACTION. 
 
 Of the several functions of the spinal cord perhaps the most 
 striking aud important is that of carrying out reflex actions. As 
 we have already said, the sj)inal cord is par excellence the organ 
 of reflex action; in by far the greater number of the reflex move- 
 ments of the body, the centre is supphed by some part of the spinal 
 cord. We have already (Book i. Chap, ni.) touched on the 
 general features of reflex actions, and elsewhere have incidentally 
 dwelt on particular instances ; we may therefore confine ourselves 
 now to certain points of special interest. 
 
 Keflex movements are perhaps best studied in the frog and 
 other cold-blooded aniaials, where the phenomena are less ob- 
 scured by the working of the other so-called higher parts of the 
 central nervous system. They obtain however in the warm-blooded 
 mammal also, but in these special precautions are necessary to 
 secure their full development. 
 
 In the fi'og the shock which follows upon division of the spinal 
 cord, and which, as we shall presently see, for a while inhibits reflex 
 activity, soon passes away ; within a very short time after the 
 medulla oblongata for instance has been divided the most com- 
 plicated reflex movements can be carried on by the frog's sjDinal 
 cord when the appropriate stimuli are applied. With the mammal 
 the case is very different. For days even after division of the 
 spinal cord the parts of the body supplied by nerves springing 
 from the cord below the section exhibit very feeble reactions only. 
 
586 REFLEX ACTIONS. [Book hi. 
 
 In the dog, for instance, after division of the spinal cord in the 
 lower dorsal region, the hind limbs hang flaccid and motionless, 
 and pinching the hind foot evokes as a response either slight 
 irregular movements or none at all. Indeed were our observations 
 limited to this period we might infer that the reflex actions of the 
 spinal cord in the mammal were but feeble and insignificant. If 
 however the animal be kept alive for a longer period, for weeks or 
 better still for months, though no union or regeneration of the 
 spinal cord takes place, reflex movements of a powerful, varied and 
 complex character manifest themselves in the hind limbs and 
 hinder parts of the body ; a very feeble stimulus applied to the 
 skin of these regions promptly gives rise to extensive and yet co- 
 ordinate movements. Compared with the reflex actions of the 
 frog, the movements carried out by the lower portion of the spinal 
 cord of the mammal while they are more energetic have hitherto 
 been regarded as being less definite and complete and less 
 purposeful ; but it would be dangerous to insist on this, for recent 
 -experience tends to shew that, in the case of most mammals, the 
 powers of the spinal cord have been unduly underrated. It is 
 worthy of attention that the reflex phenomena in mammals vary 
 very much not only in different species but also in difi^erent in- 
 dividuals and in the same individual under different circumstances. 
 Eace, age, and previous training, seem to have a marked effect in 
 determining the extent and character of the reflex actions which 
 the spinal cord is capable of carrying out ; and these seem also to 
 be largely influenced by passing circumstances, such as whether food 
 has been recently taken or no. And it is asserted that the spinal 
 cord of the rabbit, which has been the subject of so many experi- 
 ments, is, as compared with that of the dog and many other 
 mammals, singularly deficient in the power of carrying out comj^lex 
 reflex movements. 
 
 Both in the cold-blooded and warm-blooded animals the sahent 
 featui-e of ordinary reflex actions is their purposeful character, 
 though every variety of movement may be witnessed, from a simple 
 spasm to a most complex manoeuvre ; and in all reflex movements, 
 both simple and complex, we can recognise certain determining 
 causes, the influences of which more or less directly contribute 
 to the shaping of this purposeful character. 
 
 Thus the features of any movement taking place as part of a 
 reflex action are in part determined by the nature of the afi"erent 
 impulses. Simple nervous impulses generated by the direct 
 stimulation of afferent nerve-fibres generally evoke as reflex 
 movements merely irregular spasms in a few muscles ; whereas 
 the more complicated differentiated sensory impulses generated 
 by the application of the stimulus to the skin, readily give rise 
 to large and purposeful movements. It is easier to produce a 
 complex reflex action by a slight pressure on the skin than by 
 even a strong single induction-shock applied directly to a nerve- 
 
C'liAi'. v.] Til K SPINAL CORD. 587 
 
 trunk. If, in a bruinloss frog, the area of skin supplied by om- of 
 the dorsal eutaneous nerves be sejjarated by S(!etion from the rest 
 of the skin of the back, the nerve beino;- left attached to the piece 
 of skin and can-fully protected from injury, it will be found that 
 slight stimuli applied to the surface of the ])iece of skin easily 
 evoke reflex actions, whereas the trunk (jf the; nerve may be stimu- 
 lated with even strong currents without jiroducing anything more 
 than irregular movements. In ordinary mechanical and chemical 
 stimulation of the skin it is a series of impulses and not a single 
 impulse Avhich passes upwards along the sensory nerve, the changes 
 in which may be compared to the changes in a motor nerve during 
 tetanus. In every retlex action, in fact, the central mechajiism 
 may be looked upon as being thrown into activity through a 
 summation of the afferent impulses reaching it. Hence while a 
 retlex action is readily called forth by even feeble single induction- 
 shocks applied to the skin if they be repeated sufficiently rapidly, 
 a solitaiy induction-shock is ineffectual unless it be strong enough 
 to cause profound changes in the skin or nerves. 
 
 When a muscle is thrown into contraction in a reflex action, 
 the note which it gives forth does not vary with the stimulus, but 
 is constant, being the same as that given forth by a muscle thrown 
 into contraction by the will. From wdiich we infer that in a reflex 
 action the afferent impulses do not simply pass through the centre 
 in the same way that they pass along afferent nen'es, but are 
 profoundly modified. And this explains why a reflex action takes 
 always a considerable time, and frequently a very long time, for its 
 development. When the toes of a brainless frog are dipped in 
 dilute sulphuric acid, several seconds may elapse before the feet 
 ai-e withdrawn. Making every allow^ance for the time needed for 
 the acid to develope sensor}^ impulses in the peripheral endings of 
 the afierent nerve, a very large fraction of the period must be 
 taken up by the molecular actions going on in the nerve-cells. In 
 other words, the interval between the advent at the central organ 
 of afferent, and the exit from it of efferent impulses, is a -busy time 
 for the nerve-cells of that organ; during it many processes, of 
 which we have at present very little exact knowledge, are being 
 carried on. 
 
 The character of the movement forming part of a reflex 
 action is also influenced by the intensity of the stimulus. A 
 slight stimulus, such as gentle contact of the skin with some 
 body, will produce one kind of movement ; and a strong stimulus, 
 such as a sharp prick ai5]tlied to the same spot of skin, will 
 call forth quite a different movement. When a decajiitated 
 snake or newt is suspended and the skin of the tail lightly 
 touched with the finger the tail bends towards the finger; when 
 the skin is pricked or burat, the tail is turned aw^ay from the 
 offending object. And so in many other instances. Further we 
 have already pointed out (p. 110) that while the effects of a weak 
 
588 REFLEX ACTIONS. [Book hi. 
 
 stimulus applied to an afferent nerve are limited to a few, those of 
 a strong stimulus may spread to many efferent nerves. Granting 
 that any particular afferent nerve is more especially associated with 
 certain efferent nerves than with any others, so that the reflex 
 impulses generated by afferent impulses entering the cord by the 
 former, pass with the least resistance down the latter, we must 
 evidently admit further that other efferent nerves are also, though 
 less directly, connected with the same afferent nerve, the passage 
 into the second efferent nerve meeting with an increased but not 
 insuperable resistance. When a frog is poisoned with strychnia, 
 a slight touch on any part of the skin may cause convulsions of the 
 whole body; that is to say, the afferent impulses passing along any 
 single afferent nerve may give rise to the discharge of efferent im- 
 pulses along any or all of the efferent nerves. This proves that a 
 physiological if not an anatomical continuity obtains between 
 all the nerve-cells of the spinal cord which are concerned in reflex 
 action, that the nerve-cells with their processes form a functionally 
 continuous protoplasmic network. This network however we must 
 suppose to be marked out into tracts presenting greater or less re- 
 sistance to the progress of the impulses into which afferent impulses, 
 coming from this or that afferent nerve, are transformed on their 
 advent at the network; and accordingly the path of any series 
 of impulses in the network will be determined largely by the 
 energy of the afferent impulses. And the action of strychnia may 
 be in part explained by supposing that it reduces and equalises the 
 normal resistance of this network, so that even weak impulses 
 travel over all its tracts with great ease. 
 
 Further, the movement forming part of a reflex action varies 
 in character, according to the particular area of the skin or 
 the locality of the body to which the stimulus is applied. 
 Pinching the folds of skin surrounding the anus of the frog 
 produces different effects from those witnessed when the flank 
 or toe is pinched; and, speaking generally, the stimulation 
 of a particular spot calls forth particular movements. In the 
 case of the simpler reflex movements, it appears to be a 
 general rule that a movement started by the stimulation of a 
 sensory surface or region on one side of the body, is developed 
 on the same side of the body, and if it spreads to the other side, 
 still remains most intense on the same side ; the movement on the 
 other side moreover is symmetrical with that on the same side. It 
 has been maintained that 'crossed' or diagonal reflex movements, 
 as where stimulation of one fore-foot leads to movements of the 
 opposite hind-limb, do not occur unless some portion of the medulla 
 oblongata be left attached to the spinal cord. Seeing that loco- 
 motion in four-footed animals is largely effected by diagonal move- 
 ments of the limbs, one would rather have expected to find the 
 spinal cord itself provided with mechanisms to assist in carrying 
 them out ; and indeed it is affirmed that in the case of cold- 
 
Chap, v.] THE SPINAL CORD. 589 
 
 blooded animals and of many young mammals, after division of the 
 
 spinal cord below the medulla, a gentle stimulation will provoke a 
 diagonal movement, slight pressure on one fore-l(jot lor example 
 giving rise to movements in the opposite hind-leg; a strong 
 stimulus however will produce an ordinary one-sided movement. 
 
 From these and similar phenomena we may infer that the proto- 
 plasmic network spoken of above is, so to speak, mapped out into 
 nervous mechanisms by the establishment of lines of greater or less 
 resistance, so that the disturbances in it generated by certain 
 afferent impulses are directed into certain efferent channels. But 
 the arransfement of these mechanisms is not a fixed and rigid one. 
 We cannot always predict exactly the nature of the movement 
 which vAW result from the stimulation of any particular spot, be- 
 cause the result will vary according to the condition of the spinal 
 cord, especially in relation to the strength of the stimulus. More- 
 over, under a change of circumstances a movement quite different 
 from the normal one may make its appearance. Thus when a 
 drop of acid is placed on the right flank of a frog, the right foot is 
 almost invariably used to rub off the acid ; in this there appears 
 nothing more than a mere 'mechanical' reflex action. If however 
 the right leg be cut oft', or the right foot be other^vise hindered 
 from rubbing off the acid, the left foot is, under the exceptional 
 circumstances, used for the purpose. This at first sight looks like 
 an intelligent choice. A choice it evidently is; and were there 
 many instances of choice, and were there any evidence of a 
 variable automatism, like that of a conscious volition, being mani- 
 fested by the spinal cord of the frog, we should be justified in 
 supposing that the choice was determined by an inteUigence. 
 It is however, on the other hand, quite possible to suppose that the 
 lines of resistance in the spinal protoplasm are so arranged as to 
 admit of an alternative, though still mechanical, action ; and seeing 
 how few and simple are the apparent instances of choice Avitnessed 
 in a brainless irog, and how absolutely devoid of spontaneity 
 or irregular automatism is the spinal cord of the frog, this seems 
 the more probable ^^.ew. Moreover this conclusion is supported by 
 the behaviour of other animals. Thus similar vicarious reflex 
 movements may be witnessed in mammals, though not perhaps to 
 such a striking extent as in frogs. In dogs, in which partial removal 
 of the cerebral hemispheres has apparently heightened the reflex 
 excitability of the spinal cord, the remarkable scratching move- 
 ment s of the hind leg which are called forth by stimulating a particu- 
 lar spot on the loins or side of the body, are executed by the leg 
 of the opposite side, if the leg of the same side be gently held. 
 In this case the vicarious movements are ineffectual and can 
 hardly be considered as betokening intelligence. Again the 
 'mechanical' nature of reflex actions is well illustrated by the 
 behaviour of a decapitated snake. When the body of the animal 
 in this condition is brought into contact at several places with any 
 
590 REFLEX ACTIONS. [Book hi. 
 
 object such as an arm or a stick, complex reflex movements 
 are excited, the obvious purpose as well as effect of which is to 
 twine the body round the object. A decapitated snake will how- 
 ever with equal and fatal readiness twine itself round a red-hot bar 
 of iron. 
 
 It may be added that the movements evoked by even a 
 segment of the cord may be purposeful in character; hence we 
 must conclude that every segment of the protoplasmic network is 
 mapped out into mechanisms. 
 
 Lastly, the characters of a reflex movement are, as we need 
 hardly say, dependent on the condition of the cord. The action of 
 strychnia just alluded to is an instance of an apparent augmen- 
 tation of reflex action best explained by supposing that the 
 resistances in the cord are lessened. There are probably however 
 cases in which the explosive energy of the nerve-cells is positively 
 increased above the normal. Conversely, by various influences of 
 a depressing character, as by various anaesthetics or other poisons, 
 reflex action may be lessened or prevented ; and this again may 
 arise either from an increase of resistance, or from a diminished 
 action of the nerve-cells themselves. 
 
 In actual life reflex movements, in by far the greater number 
 of instances, are occasioned by stimulation of the skin or of the 
 mucous membrane. They may however occur as the result of 
 stimulation of the organs of special sense. A sound or a flash 
 of light readily produces a start, a bright light causes many 
 persons to sneeze, and reflex movements may even result from a 
 taste or smell. 
 
 InMbition of Reflex Action. The reflex actions of the spinal 
 cord, like other nervous actions, may be totally or partially inhibited, 
 that is may be arrested or hindered in their developement by 
 impulses reaching the centre while it is already in action. Thus 
 if a decapitated snake be suspended, slow rhythmic pendulous 
 movements, which appear to be reflex in nature, soon make their 
 appearance, and these may be for a while arrested by slight 
 stimulation, as by gently stroking the tail. We have already seen 
 that the action of such nervous centres as the respiratory and vaso- 
 motor centres, which frequently at all events is of a reflex nature, 
 may be either inhibited or augmented by afferent impulses. The 
 micturition centre in the mammal, which is also largely a reflex centre, 
 may be easily inhibited by impulses passing downward to the lumbar 
 cord from the brain, or upward along the sciatic nerves. In the case 
 of dogs, whose spinal cord has been divided in the dorsal region, 
 micturition set up as a reflex act by simple pressure on the 
 abdomen, or by sponging the anus, is at once stopped by sharply 
 pinching the skin of the leg. And it is a matter of common 
 experience that micturition may be suddenly checked by an 
 emotion or other cerebral event. The erection centre in the 
 
Chap. V. I TllK Sl'lXAL COIU). 591 
 
 lumbar ctncl also, in large; lueaRuro a reflex centre, is .similarly 
 su.sceptible of" heiiii^f inhibited by impulses reaching it from various 
 sources. And indeed many sinnlar instances of the inhibition 
 of rcHex movements might readily be quested. 
 
 Several apparent instances of the inhibition of reflex acts are 
 not really such : in these cases all the nervous processes of the 
 act may take place in their entirety and yet fail to produce their 
 effect on account of a failure in the nniscular part of the act. 
 Thus ^vhen we ourselves sto}) or inhibit the reflex movements 
 which otherwise would be produced by tickling the soles of the 
 feet, we achieve this to a large extent by throwing voluntarily 
 into action certain muscles, the contractions of which antagonise 
 the action of the muscles engaged in carrying out the reflex 
 movements. But it may be doubted even in the.se cases, whether 
 inhibition is always or wholly to be explained in this way ; and 
 certainly in very many instances of reflex inhibition, no such 
 muscular antagonism is present, and the reflex act is checked at 
 its nervous centre. 
 
 It is a remarkable fact that when the brain of a frog is re- 
 moved, reflex actions are developed to a much greater degree than 
 in the entire animal. This suggests the idea that there must be 
 in the brain some mechanism or other for preventing the normal 
 developement of the spinal reflex actions. And we learn by experi- 
 ment that stimulation of certain parts of the brain has a remark- 
 able effect on reflex action. If a frog, from which the cerebral 
 hemispheres only have been removed (the optic thalami, optic 
 lobes, medulla oblongata and spinal cord being left intact), be 
 suspended by the chin, and the toes of the pendent legs be from 
 time to time dipped into very dilute sulphuric acid, a certain 
 average time Avill be found to elapse between the dipping of the 
 toe and the resulting withdrawal of the foot. If, however, the 
 optic lobes or optic thalami be stimulated, as by putting a crystal 
 of sodium chloride on them, it will be found on repeating the 
 experiment while these structures are still under the influence 
 of the stimulation, that the time intervening between the action of 
 the acid on the toe and the withdrawal of the foot is very much 
 prolonged. That is to say, the stimulation of the optic lobes has 
 caused impulses to descend to the cord, which have there so inter- 
 fered with the action of the nerve-cells engaged in reflex action as 
 gi'eatly to retai-d the generation of reflex impulses ; in other words, 
 the stimulation of the optic lobes has inhibited the reflex action of 
 the cord. And similar results may be obtained in mammals by 
 stimulating certain parts of the corpora quadrigemina, which 
 bodies are analogous to the optic lobes of frogs. From this it has 
 been infen-cd that there is present in this part of the brain a 
 special mechanism for inhibiting the reflex actions of the spinal 
 cord, the impulses descending from this mechani.-^ni to the 
 various centres of reflex action being of a specific inhibitory 
 
592 REFLEX ACTION'S. [Book m. 
 
 nature. But, as we have already seen, impulses of an ordinary 
 kind, passing along ordinary sensory nerves, may inhibit reflex 
 action. We have quoted instances where a slight stimulus, as in 
 the pendulous movements of the snake, and where a stronger 
 stimulus as in the case of the micturition of the dog, may produce 
 an inhibitory result ; we may add that adequately strong stimuli 
 applied to any afferent nerve will in the frog inhibit, i.e. will retard 
 or even wholly prevent reflex action. If the toes of one leg are 
 dipped into dilute sulphuric acid at a time when the sciatic of the 
 other leg is being powerfully stimulated with an interrupted 
 current, the period of incubation will be found to be much pro- 
 longed, and in some cases the reflex withdrawal of the foot will 
 not take place at all. And this holds good, not only in the 
 complete absence of the optic lobes and medulla oblongata, but 
 also when only a portion of the spinal cord, sufficient to carry out 
 the reflex action in the usual way, is left. There can be no 
 question here of any specific inhibitory centres, such as have been 
 supposed to exist in the optic lobes. 
 
 Hence it is clear that inhibition may be brought about by 
 impulses which are not in themselves of a specific inhibitory 
 nature, and accordingly we may hesitate to accept the view that 
 a special inhibitory mechanism in the sense of one giving rise to 
 nothing but inhibitory impulses is present in the optic lobes of frogs. 
 Nor is there adequate proof that the exaltation of reflex actions 
 which is manifest in decapitated animals is due to the withdrawal 
 of such a specific inhibitory mechanism. We shall have occasion 
 again to return to these inhibitory phenomena of the central nervous 
 system. We have seen enough to shew that the spinal cord, and 
 the same holds good, as we shall see, for the whole central nervous 
 system, may be regarded as an intricate mechanism in which the 
 direct effects of stimulation or automatic activity are modified and 
 governed by the checks of inhibitory influences. Seeing that in the 
 ordinary actions of life the spinal cord is to a large extent a mere 
 instrument of the cerebral hemispheres, we may readily expect that 
 among the many impulses passing from the latter to the former, 
 some under certain circumstances should result in an inhibition of 
 spinal activity, while others, or the same under different circum- 
 stances, should lead to an exaltation of the same spinal activity. 
 The experiments quoted above shew that the optic lobes when 
 stimulated are especially prone to give rise to inhibitory results; 
 but we have as yet much to learn before we can speak with 
 certainty as to the exact manner in which such an inhibition is 
 brought about. 
 
Chap, v.] the SPINAL CORD. 593 
 
 The Time required for Reflex Actions. 
 
 When one eyelid is stimulated with a sharp electrical shock, 
 both eyelids blink. Hence, if the length of time intervening 
 between the stimulation of the right eyelid and the movement of 
 the left eyelid be measured, this will give the total time required for 
 the various processes which make up a reflex action. It has been 
 found to be from "0662 to 0578 sec. Deducting from these figures 
 the time required for the passage of afferent and efferent impulses 
 along the fifth and facial nerves to and from the medulla, and for 
 the latent period of the contraction of the orbicularis muscle, 
 there would remain "OoSo to '0471 sec. for the time consumed in 
 the central operations of the reflex act. The calculations, however, 
 necessary for this reduction, it need not be said, are open to sources 
 of error. Blinking thus produced is a reflex act of the very simplest 
 kind ; but as we have seen in the preceding pages, reflex acts 
 differ very widely in nature and character; and we accordingly 
 find, as indeed we have incidentally mentioned, that the time 
 taken up by a reflex movement varies very largely. This indeed 
 is seen in the blinking itself When the blinking is caused not by 
 an electric shock applied to the eyelid, but by a flash of light 
 falling on the retina, in which case complex visual processes are 
 involved, the time is exceedingly prolonged ; moreover the results 
 in different experiments of such a kind are not nearly so uniform 
 as when the blinking is caused by stimulation of the eyelid. 
 
 In general it may be said that the time required for any 
 reflex act varies very considerably with the strength of the 
 stimulus employed, being less for the stronger stimuli; this we 
 should expect, seeing that the efferent impulses of the reflex act 
 are not simply afferent impulses transmitted through the central 
 organ, but result from internal changes in the central organ started 
 by the afferent impulse or impulses ; and these internal changes 
 will naturally be more intense and more rapidly effective when the 
 afferent impulses are strong. It is stated that when the movement 
 induced is on the same side of the body as the surface stimulation 
 of which starts the act, the time taken up is less than when the 
 movement is on the other side of the body, allowance being made 
 for the length of central nervous matter involved in the two cases ; 
 that is to say the central operations of a reflex act are propagated 
 more rapidly along the cord than across the cord. The rapidity 
 of the act varies of course with the condition of the spinal 
 
 F. 38 
 
594 RAPIDITY OF REFLEX ACTIONS. [Book iii. 
 
 cord, being greatly prolonged when the cord becomes exhausted. 
 The time thus occupied by purely reflex actions must not be 
 confounded with the interval required for mental operations; of 
 the latter we shall speak presently. 
 
SEC. 2. AS A CENTEE OR GROUP OF CENTRES OF 
 AUTOMATIC ACTION. 
 
 Irregular automatism, i.e. a spontaneity comparable to our o^vn 
 volition, is wholly absent from the spinal cord. A brainless frog 
 placed in a condition of complete equilibrium in which no stimulus 
 is brought to bear on it — protected from sudden passing changes in 
 temperature, from a too rapid evaporation and the like — remains 
 perfectly motionless till it dies. Such apparently sjjontaneous 
 movements as are occasionally witnessed are so few and seldom, 
 that we can hardly do otherwise than attribute them to some 
 stimulus, internal or external, Avhich has escaped observation. In 
 the mammal (dog) after division of the spinal cord in the dorsal 
 region regular and apparently spontaneous movements may be ob- 
 served in the parts governed by the lumbar cord. When the 
 animal has thoroughly recovered from the operation the hind limbs 
 rarely remain at rest for any long period ; they move restlessly in 
 various ways ; and when the animal is suspended by the upper 
 part of the body, the pendent hind limbs are continually being 
 drawn up and let down again with a monotonous rhythmic regu- 
 larity, highly but perhaps falsely suggestive of automatic rhythmic 
 discharges from the central mechanisms of the cord. In the newly 
 bom mammal too, after removal of the brain, apparently spon- 
 taneous movements are frequently observed. This greater prone- 
 ness to activity is how^ever just what might be expected, when 
 we take into consideration the more rapid metabolic changes and 
 the consequent greater molecular mobility of the whole nervous 
 systiem of the mammal. The movements, even when most highly 
 developed, are wholly different from the movements irregular in 
 their occurrence, but orderly and purposeful in then- character, 
 which result from the working of volition. 
 
 38—2 
 
596 AUTOMATIC ACTION. [Book m. 
 
 Of the various regular automatic centres, both the numerous 
 ones in the medulla oblongata, such as the vaso-motor, respiratory, 
 &c., and the more sparse ones in other regions of the cord, such as 
 those connected with micturition, defsecation, erection, parturition, 
 and so on, we have treated or shall have to treat so fully in 
 reference to their respective mechanisms, and discussed how far 
 they are purely automatic, or in reality merely reflex in nature, 
 that nothing more need be said here. 
 
 It has been much disputed whether the spinal cord exercises 
 over the skeletal muscles a tonic action comparable to that of the 
 vaso-motor centres over the smooth muscles of the arteries. The 
 arguments which were once brought forward as proving the exist- 
 ence of such a tone are invalid. It is true that when a muscle is cut 
 across in the living body, the section gapes ; but this is because all 
 the muscles of the body are slightly stretched beyond their normal 
 length. Again, when one side of the face is paralysed the mouth 
 is drawn to the opposite side, not because the paralysed muscles 
 have lost tone, but because there are on the paralysed side no con- 
 tractions to antagonise the effect of the continually repeated con- 
 tractions of the sound side. And indeed the existence of such a 
 tone seems distinctly disproved by the fact that, according to most 
 observers, when in the living body the nerve going to a muscle is 
 cut no permanent lengthening of the muscle is caused. On the 
 other hand, when the sciatic plexus of one leg of a brainless frog is 
 cut, and the animal is suspended, that leg hangs down more help- 
 lessly than the other ; that is to say the sound leg is rather more 
 flexed than the other. The difference which is sometimes marked, 
 but sometimes hardly visible, disappears entirely when the whole 
 cord is destroyed. But the same flaccidity is observed in a leg 
 in which the posterior roots only of the sciatic plexus have been 
 divided. Hence it is to be regarded as an instance not so much of 
 automatic tone, as of feeble reflex action occasioned by afferent 
 impulses. 
 
 Though however the view of a real tone lacks adequate support, 
 several considerations favour the idea that the condition of a 
 muscle, apart from its being in a state of contraction or at rest, is 
 closely dependent on influences proceeding from the spinal cord. 
 We saw, in treating of muscle and nerve (p. 92), that the irritability 
 of a muscle is markedly affected by the section of its nerve, i.e. 
 by severance from the central nervous system ; and more recently, 
 (p. 471) in speaking of the so-called trophic action of the nervous 
 system, we referred to changes in the nutrition of muscles oc- 
 casioned by diseases of the nervous system. An instance of a 
 similar action is afforded by the so-called ' tendon-phenomena." It 
 is well known that when the leg is placed in an easy position, as 
 when resting on the other leg, a sharp blow on the patellar tendon 
 will cause a sudden jerk forward of the leg, brought about by a 
 contraction of the quadriceps femoris. Similarly the muscles of the 
 
Chap, v.] 77/ A' SPINA L CORD. DO? 
 
 calf may bo throwu into action by tappin/^ the tondo Achillis ; and 
 in somo cases the same muscles may be made to execute a aeries of 
 rhythmic contractions, by suddenly pressinj^ back the sole of the 
 foot so JUS to put them on the stretch. These;, and other instances 
 of a liki- kind, at iirst siglit ajipear to be, and indci-d it has been 
 maintained tliat they are, cases of reflex action, duo to affi-rent 
 imjiulses started in the tendon; hence the}' have been frequently 
 spoken of as * tendon-retlex.' But the evidence, on the whole, 
 shews that they are not reHex, but due to direct stimulation 
 of the muscles. Nevertheless, and this is the interesting point, 
 they are closely dependent on the integrity of the spinal cord, and 
 of the connections between the cord and the muscles. In the case 
 of animals they disappear when the spinal cord is destroyed, or the 
 nerves going to the muscles are severed or even when the posterior 
 roots only are divided. And in the case of man they are diminished 
 or wanting in certain diseases of the spinal cord (locomotor ataxy), 
 and exaggerated in others ; so much so indeed that they have be- 
 come of practical clinical importance as a means of diagnosis. With- 
 out discussing the matter any further, we may say that such pheno- 
 mena indicate that the nutrition and the irritability of a muscle 
 are in some way governed by influences of one kind or another 
 which proceed from the spinal cord, and which, in certain cases at 
 all events, are the result of, or are determined by, influences of a 
 similarly obscure nature reaching the cord from the muscles by the 
 posterior roots of the spinal nerves. 
 
SEC. 3. AS A CONDUCTOR OF AFFERENT OR EFFERENT 
 
 IMPULSES. 
 
 When we feel something touching our foot, or when we move 
 our foot, afferent or efferent impulses must evidently pass along the 
 whole length of the spinal cord on their way to and from the brain. 
 We may say at once that it is impossible that sensory impulses 
 should be conveyed straight along a fibre from the periphery to 
 the sensorium, and volitional impulses straight along another 
 fibre from the 'organ of the will' to the muscular fibre; the 
 number of fibres in the cord is wholly insufficient for such a 
 purpose. Moreover not only anatomical but physiological con- 
 siderations shew that conduction along the cord is not simple, but 
 carried out by a more or less intricate system of relays. 
 
 The phenomena of reflex action have shewn us that the cord 
 contains a number of more or less complicated mechanisms capable 
 of producing, as reflex results, coordinated movements altogether 
 similar to those which are called forth by the will. Now it must 
 be an economy to the body, that the will should make use of these 
 mechanisms already present, rather than that it should have 
 recourse to a special apparatus of its own of a similar kind. It is 
 therefore a priori probable that when the foot is pricked the 
 sensory impulses so generated, on reaching the cord pass into the 
 grey matter there to undergo a certain amount of transformation 
 and thence to be transmitted, either by direct and sindple, or by 
 indirect and complicated, paths to the brain. Similarly, we may 
 suppose that when the leg is moved by an effort of the will, 
 volitional impulses starting from the brain, pass by a more or less 
 direct path to certain portions of grey matter in the lumbar cord. 
 
Hmap. v.] 
 
 77/ A' .'<r/NAL CORD. 
 
 599 
 
 with which the motor fibres of the uerves of the leg are specially 
 connected , and induce such changes in this grey matter as to lead 
 to the discharge of the appropriate impulses along those motor fibres. 
 And such a view is strongly sup])orted by the anatomical fact, as 
 illustrated by Figs. 78—^80, that along the length of the spinal cord, 
 the amount of grey matter varies according to the immber of fibres 
 passing into the cord, indicating that the fibres as they pass into the 
 cord have a certain amount of grey matter allotted to them. More- 
 over, though the course which the fibres of the posterior roots take 
 immediately upon entering the cord has perhaps yet not been 
 satisfactorily determined, the fibres of the anterior root have been 
 definitely traced to the nerve cells of the anterior comu; and 
 according to recent observations, in the frog at all events, the 
 cells of the anterior comu are equal in number to the fibres of the 
 anterior root. 
 
 V IV III II I V IV III II I XII XI X IX VIII VII VI V IV III II I VIII VII VI V IV III II I 
 
 Fig. 78. Dugbam shewing the EELATrvE sectional abeas of the Spinal Nerves, 
 
 AS THEY JOIN THE SpINAL CoRD. 
 
 V IV III II I V IV III II I XI XI X IX VIII VII VI V IV III II I VIII VII VI V IV III II I 
 
 Fig. 79. Diagbam shewing the united sectional areas of the Spinal Nerves, 
 proceeding fboil below upwards. 
 
 V IV III II I V IV ui II I xii XI X IX vai vii vi v iv iii ii i v,-:! vii vi v iv ;ii ii i 
 
 Fig. 80. Dugram shewing the vabutions in the sectional area of the grey 
 
 matter OF THE SpINAL CoRD, ALONG ITS LENGTH. 
 
 All three figures are to read from left (the bottom of the cord) to right (top of 
 the cord), the numerals indicating successively the sacral, lumbar, dorsal and 
 cer\ical nerves. The figures are not drawn to the same scale. 
 
600 CONDUCTION OF IMPULSES. [Book hi. 
 
 But admitting this, there still remains the question, How do 
 volitional and sensory impulses travel along the cord, between the 
 brain on the one hand and the grey matter belonging to this 
 or that nerve root on the other ? 
 
 Our information concerning the conduction of impulses along 
 the spinal cord is derived partly by anatomical deduction, partly 
 from experiment and partly from pathological observation. These 
 several methods have their advantages and disadvantages. We 
 have just now brought forward a very general anatomical de- 
 duction. More detailed inferences are afforded by the Wallerian 
 method (see p. 484). When the spinal cord is diseased or injured at 
 any point, tracts of degenerated fibres may at times be traced on 
 the one hand upwards towards the brain, or on the other down- 
 wards in a peripheral direction. The former may be taken as 
 being sensory or afferent, and the latter, together with tracts of 
 degeneration in the cord which are found associated with disease of 
 the brain, as motor or efferent. Again, when the development of 
 the spinal cord is studied, it is found that the fibres of different 
 tracts assume their medullary sheaths at different times; and by 
 this means the longitudinal fibres of the cord may be differentiated 
 into tracts having different terminal connections, some tracts being 
 thus traced into the crura cerebri, others into the cerebellum, while 
 others appear to terminate in the medulla oblongata, or both to 
 begin and end in the cord itself These different distributions 
 obviously suggest different functions. But all such anatomical de- 
 ductions must here, as elsewhere, be received with caution. ^ 
 
 When experiments are used as a means of inquiry, we 
 are met with the danger of confounding the immediate and 
 temporary effects of the operation, such as those produced by 
 shock, with the more real and lasting effects. It is difficult too in 
 such cases to determine the existence of sensations, and to dis- 
 tinguish between reflex and purely voluntary movements. The 
 difficulty of recognizing, and especially of quantitatively estimating 
 the value of, signs of sensation has however been met by an 
 ingenious use of variations in blood-pressure. We have seen that, 
 at all events in an animal under urari, afferent impulses occasion a 
 rise of blood-pressure. If, having determined the amount of rise 
 due to a definite stimulation of a sensory nerve, such as the 
 sciatic, we make an incision into the spinal cord of the dorsal 
 region, dividing for instance part of the lateral column on one side, 
 and afterwards find that the same stimulus applied in the same 
 way to the sciatic nerve, leads to a greatly diminished rise of 
 blood-pressure, we are justified in inferring that the afferent 
 impulses affecting blood-pressure are largely conducted through the 
 part of the lateral column which has been divided. We may 
 thus obtain a definite measure, in millimetres of mercury, of the 
 effect produced by the injury. On the other hand, the value of 
 this precise measurement is diminished by the doubt whether we 
 
Chap, v.] THE SPINAL CORD. 601 
 
 have a right to conclude that the afferent impulses wliich affect 
 bloo(l-j)ressure take the same course as those which give rise to 
 sensations, and affect consciousness. And further, in all experi- 
 nu'utal results on animals we must bear in mind the probability 
 that the functions of the spinal cord ma} vary in different animals, 
 possibly to a very considerable extent. 
 
 In pathological cases we have the advantage of being able 
 clearly to define sensation and volition, but this is frequently more 
 than counterbalanced by the diffuse nature of the injury or disease, 
 and the want of exact anatomical verification. When these facts 
 are borne in mind, it will easily be understood that in no part of 
 physiology are the statements of investigators more conflicting and 
 unsatisfactory. 
 
 One salient fact comes out in all observations, whether ex- 
 perimental or pathological, viz. that between the brain, where 
 volitional impulses are started, or where conscious sensations are 
 perfected, and the muscle which carries out the movement or the 
 sentient surface where the sensory impulses begin, there is a 
 complete crossing or decussation of all impulses whether sensory or 
 motor. When the right side of the brain is injured or diseased, 
 when for instance damage is done to the right corpus striatum and 
 optic thalamus, it is on the left side of the body, in the left limbs, 
 and in the left face, that the paralysis and loss of sensation appear ; 
 it is on the left side that sensory impulses fail to affect conscious- 
 ness, it is on the left side that the muscles can no longer be 
 reached by volitional impulses. Results other than these indicate 
 complications involving the other side of the brain. 
 
 Further, all observers are agreed that as far as the spinal 
 nerves are concerned (and since we are now treating of the spinal 
 cord we may for the present leave out of consideration the cranial 
 nerves), the decussation is complete at about the level of the 
 upper part of the medulla oblongata and pons Varolii when the 
 paths are traced upwards ; that is to say, all the sensory impulses 
 coming from, and all the volitional impulses passing to, the left 
 side of the body, make their way along the right crus cerebri. 
 Nearly all observers again are agreed that the sensory impulses 
 cross over lower down in the spinal cord than do the volitional 
 impulses ; but opinions differ as to the exact difference in the 
 paths of the two kinds of impulse. Experiments conducted by 
 some observers have seemed to shew that transverse division of the 
 lateral half of the cord in any part of its course below the medulla 
 oblongata is foUowed on the same side, below the injury, by loss of 
 voluntary movement, accompanied by no loss of sensation, but 
 even by increased sensitiveness or hypersesthesia, and on the 
 opposite side by loss of sensation without any affection of voluntary 
 movement. From these and other experiments these authors 
 conclude that sensoiy impulses entering into the cord at a posterior 
 root immediately cross to the other side of the cord and so ascend 
 
602 CONDUCTION OF IMPULSES. [Book hi. 
 
 to the brain, whereas efferent impulses of volition cross wholly in 
 the region of the medulla oblongata, and afterwards keep to the 
 same side of the cord along its whole length. Other observers, and 
 these perhaps are deserving of the greater confidence, find that a 
 section of a lateral half of the cord affects both volitional and 
 sensory impulses of both sides, though to different degrees, the 
 loss both of sensation and motion being greater on the side operated 
 on than on the other. Hence they maintain that the decussation 
 of both kinds of impulses is gradual, extending some distance 
 along the cord. Thus they hold that the volitional impulses 
 for the hind limb, while crossing over largely in the upper part 
 of the cord, continvie to cross over right down to the lumbar 
 region, and similarly that the sensory impulses from the hind limb, 
 while crossing largely in the lumbar region of the cord, continue to 
 cross in the dorsal or even in the cervical region. 
 
 Admitting this latter view as the one most in accordance with 
 facts, we have still to ask what are the exact paths taken by the 
 volitional and sensory impulses in their respective courses, that is 
 to say. What are the particular parts of the spinal cord which serve 
 to conduct volitional impulses on the one hand and sensory 
 impulses on the other ? Upon the discovery of the distinctive 
 functions of the anterior and posterior roots, it seemed natural to 
 conclude that the anterior columns with which the anterior roots 
 are more directly connected, should serve as the path for volitional 
 impulses, and similarly that the posterior columns should afford a 
 path for sensory impulses. But this view was soon found to be 
 imtenable ; and it became modified into the conception that sensory 
 impulses pass along the posterior columns and the grey matter, 
 while volitional impulses descend in the antero-lateral columns. 
 Further, this somewhat general statement has been reduced to 
 greater definiteness by some authors in the following way. They 
 hold that the impulses which when they reach the brain give rise 
 to feelings of general sensibility only or of pain, and the impulses 
 which form part of the chain of a reflex act, as when the fore leg or 
 hind leg moves in response to a stimulus applied to the hind leg or 
 fore leg respectively, are transmitted by the grey matter, and by all 
 parts of the grey matter and that in any direction. Distinct tactile 
 sensations on the other hand, they contend, travel exclusively by 
 the posterior columns, and distinct volitional impulses by the antero- 
 lateral columns. That these several kinds of impulse should travel 
 by separate paths, does not in itself seem improbable, and is to a 
 certain extent suggested by pathological experience. For carefully 
 observed cases have been recorded of disease of the cord, and ap- 
 parently of the cord alone, in which the patient could appreciate 
 even a slight touch but felt no pain when a needle was thrust 
 into the skin, or when the skin was otherwise treated in a way 
 which, under normal circumstances, would give rise to pain. Con- 
 versely the sense of touch has been found to be absent, while pain 
 
Chap, v.] THE SPINAL CORD. 003 
 
 was still felt. Similarly, as was stated on p. 577, cases have been 
 recorded whore the sensation of pressure was retained but that of 
 temperature imjjaired or lost, and vice versa. Such cases however 
 liave not as yet afforded any clear insight as to what are the actual 
 paths of the respective sensory impulses. While they do not oppose 
 they do not distinctly confirm the view we are speaking of as to the 
 particular paths of these several kinds of impulse. Nor indeed can 
 this view, either in its more general or in its more elaborate form. 
 be considered as adequately supported by experimental evidence. 
 
 For the investigations of other observers, especially the more 
 recent ones, including those conducted by the blood-pressure 
 method, largely concur in shewing that along the cord, at all 
 events in the dorsal region, both volitional and sensory impulses, 
 indeed we might say impulses of all kinds passing between the 
 brain and various parts of the spinal cord, or between distinct 
 parts of the cord itself, run in the lateral columns. This view is 
 further supported by the anatomical facts that the lateral 
 columns (Fig. 81) increase in bulk from below upwards to a much 
 greater degree than do either the anterior or posterior columns 
 (Figs. 82, 83), and that after certain diseases or injuries of the brain 
 
 eoJ 
 
 ^ 
 
 j V IV III II 1 V IV III II I XII yi X IX VIII VII vi v iv iii ii i viii vii vi v iv iii n i 
 Fig. 81. Diagram shewing the variations in the sectional area of the lateral 
 
 COLUMNS OF THE SpIN.AX CoRD, ALONG ITS LENGTH. 
 
 V IV III 11 I V IV III II I y.w XI X IX Vlll VII VI V IV III II I VIII VII VI V IV III II | 
 
 Fig. 82. Diagram shewing the variations in the sectional area of the anterior 
 
 COLUMNS OF THE SpINAL CoRD, ALONG ITS LENGTH. 
 
 V IV in II I V IV III II I xii XI X IX v:ii VII VI v iv in n i vin vii vi v iv in ii i 
 
 Fig. 83. Diagram shewing the variations in the sectional area of the portehior 
 
 COLUMNS OF THE SpINAL CoRD, ALONG ITS LENGTH. 
 
604 CONDUCTION OF IMPULSES. [Book iii. 
 
 or cord tracts of degeneration travel downwards and upwards re- 
 spectively in the lateral columns (though accompanied by degenera- 
 tion in other parts); moreover the method of development spoken 
 of at p. 600 teaches that a large part of each lateral column is asso- 
 ciated with the pyramids (and hence sometimes called pyramidal 
 tracts) and so with the crura cerebri and the brain. It may 
 be added that the decussation of the pyramids in the medulla 
 oblongata is chiefly a decussation of the lateral columns, though 
 obviously, from what has been said before, decussation of impulses 
 does not take place exclusively here. And such pathological evidence 
 as is forthcoming also to a certain extent supports this same view. 
 For while disease of the posterior cornua and posterior columns 
 seems to affect the sensory impulses passing along the nerves which 
 pass into the cord at the diseased part, and disease of the anterior 
 cornua and (though this is less clear) of the anterior columns is 
 similarly confined in its action to motor impulses passing out by 
 the nerves belonging to the diseased part, disease of the lateral 
 columns seems to affect chiefly the transmission of impulses along 
 the length of the cord and especially the transmission of volitional 
 impulses, for the evidence as to the interference in the conduction of 
 sensations by disease other than of the posterior columns and 
 cornua is by no means large or conclusive. 
 
 Accepting this view provisionally we may form some such 
 conception as follows of the conduction of volitional and sensory 
 impulses. "Volitional impulses cross, to a considerable extent in 
 the medulla oblongata, but continue to cross probably to a less 
 and less extent all the way down. In either case they appear to 
 travel along the cord in the lateral columns. Eventually they 
 become connected (possibly through part at least of the anterior 
 columns) with the grey matter of the anterior cornua, where 
 they join the local nervous mechanisms of which we have spoken 
 above. From the grey matter the impulses proceed by the anterior 
 roots to the appropriate muscles. Similarly sensory impulses make 
 their way, with the intervention possibly of the posterior columns, 
 first into the grey matter of the posterior cornua, and thence 
 into the lateral columns, and so up to the brain, decussation being 
 effected at first largely, and afterwards to a less extent, though 
 continued upwards for some distance. 
 
 It must be remembered however that such a conception 
 can only be regarded as provisional. And indeed continued 
 experimental investigations teach us that this view also is in 
 turn beset with many difficulties. We have already called and 
 shall have occasion again to call attention to the importance 
 of distinguishing between the immediate and the more permanent 
 effects of any operation on the central nervous system. Now 
 cases have been recorded where section of the lateral columns on 
 both sides in the dorsal region, has had for its immediate effect 
 loss of sensibility and voluntary power in the hind legs, but where 
 
THAI', v.] THE SPINAL CORD. nOf) 
 
 without a!iy rL't^eneration in the divided tracts, recovery hus ulti- 
 mately taken place, the return of voluntary power being nearly if 
 not absolutely complete. So also cases have been recorded in which 
 the section nt' a lateral (say right) halt' of the lower dorsal cord has 
 led to an inipairnient of sensibility and voluntary power in the hind 
 legs, most nuxrked on the same side, but in which the impairment 
 has in the course of time completely or nearly completely dis- 
 appeared without any regeneration of the cord taking place, and 
 further in which a second lateral section higher up in the cord, and 
 this time of the other (let\) half, has again led to impairment, again 
 to be followed by eomplete or nearly complete recovery. Cases of 
 this kind, carefully conducted and observed by competent persons, 
 place us in considerable difficulties. If we admit that the imme- 
 diate effects of the operation are largely due to 'shock' and inhibi- 
 tory processes, and that after the lateral section of the right half of 
 the cord, sensory volitional . impulses passed to and from the hind 
 legs by the left half, then the recovery from the second operation 
 shews that these impulses must have crossed between the two 
 sections from the left to the right side. From which we might infer 
 that impulses travelling along the cord were continually passing in 
 a zigzag fashion from one lateral half to the other ; and an analo- 
 gous series of experiments in which the anterior and posterior 
 halves were divided at different heights would lead us to infer in 
 addition that the impulses also crossed in a similar zigzag fashion 
 from front to back and back to front. But then the serious 
 question is stai-ted, Is this serpentine path the normal one, or an 
 artificial one forced upon the impulses by the abnormal condition 
 of the cord ? Finding their usual path blocked, did they make for 
 themselves new tracts ? But if such alternative passages be pos- 
 sible how can we trust to either experiment or disease to shew us 
 the normal paths, or what right have we to speak of normal paths 
 at all ? 
 
 It will be seen from the foregoing that the time is not yet ripe 
 for making any dogmatic statement concerning the conduction of 
 impulses along the cord ; and indeed the controversies concerning 
 it have perhaps acquu-ed a factitious importance. If we might 
 venture to deduce any distinct lesson from all the various conflicting 
 statements and results, it would be that the complexity and per- 
 fection of the nervous mechanisms of the spinal cord itself has been 
 underrated rather than overrated. We spoke at the beginning of 
 this section of a system of relays ; we insisted on the existence of 
 mechanisms with which the anterior and posterior roots were 
 respectively in immediate connection; and the results of experi- 
 ment as well as of pathological experience seem to shew that im- 
 pulses, whether of volition or of sensation, work their v,-ay along 
 the cord through a whole series of such and similar mechanisms, 
 rather than through simple direct straightforward tracts of con- 
 tinuous fibres whether in the lateral columns or elsewhere. 
 
606 CONDUCTION OF IMPULSES. [Book hi. 
 
 In connection with this, a curious apparent contradiction 
 between the results of pathological observation and experimental 
 investigation may be mentioned. On the one hand pathological 
 observation clearly teaches that a limited disease of the cord 
 affects the conduction of volitional impulses much more distinctly 
 than it does that of sensory impulses. A segment of the cord may be 
 very largely diseased, causing a large or complete block to volitional 
 impulses, and may yet serve to conduct sensory impulses, which 
 however are in that case generally retarded as if the impulses were 
 making progress upwards by a roundabout and difficult route. On 
 the other hand, in the case of experiments, after various sections of 
 the cord, the recovery of voluntary power is generally more speedy 
 and more complete than that of distinctly conscious sensations, even 
 when both are primarily affected by the operation. The contradic- 
 tion may partly perhaps be explained by the difficulty, in the case 
 of animals, of any objective quantitative determination of sensation, 
 but not wholly. Both facts point to the possibility of both sensory 
 and volitional impulses making their way by changed paths under 
 changed circumstances. 
 
 We may conclude our observations on this difficult but probably 
 pregnant topic by the following statement. While we appear to have 
 evidence that sensory and motor impulses are connected, at their 
 entrance into and exit from the cord with complicated mechanisms, 
 in which the grey matter undoubtedly, and possibly portions of the 
 posterior and anterior columns, are involved, the paths along the 
 cord are not clearly known. We have some reason to think that 
 they pass largely along the lateral columns, but probably not in a 
 direct straightforward manner, the whole cord being functionally a 
 series of mechanisms, for which the white matter supplies com- 
 missural connections. And the view that the paths may shift 
 according to circumstances is not without a certain support. 
 
 As was stated above, after unilateral section of the spinal cord, 
 the sensation on the same side below the injury, so far from being 
 diminished or lost has in a certain number of cases, though by no 
 means always, been observed to be increased ; the parts are then 
 said to suffer from hypersesthesia. Since the hypersesthesia appears 
 immediately after the operation, it cannot be due to any inflam- 
 matory process. Nor can it be explained as simply the result of 
 the increased supply of blood to the peripheral terminations of 
 the sensory nerves, caused by the section involving vaso-motor 
 tracts ; since the simple section of a vaso-motor tract, as when the 
 cervical sympathetic is divided, does not give rise to hypersesthesia. 
 Nor can we explain it as due to a one-sided hyperhasmia of the 
 spinal cord itself, for we have no evidence that such a state of 
 things is brought about. Since it lasts for a very considerable 
 time it cannot be due to any passing exciting effect of the operation. 
 It has been suggested that the section in such cases has removed 
 previously existing influences which descending the cord exercised 
 
CiiAi'. v.] THE 8FINAL CORD. G07 
 
 an iuliibitory action on the generation of sensory impulses, more 
 ptiiticuliiily of those more com|)h'x impulses which we have 
 supposed to arise in the local mechanism of gn^y matter with 
 which the posterior roots are connected. In other words, this 
 one-sided exaltation of sensation may be compared to the general 
 increase of reflex action which occurs in the spinal cord after 
 removal of the brain. But we cannot enter into the full discussion 
 of this matter here. 
 
 Much discussion has arisen on the question whether the spinal 
 cord itself is ii'ritable towards stimuli other than nervous, that 
 is whether it can be excited by electric and other stimuli applied 
 directly to it. Undoubtedly, the cord, as a whole, is irritable ; 
 if two electrodes be plunged into it, and a current sent through 
 it, muscular movements, arterial constriction, and other results, 
 follow. But in such a case, the current may fall into nerve- 
 roots, which are as irritable, at least, as the nerve-trunks. But 
 even if the nerve-roots be eliminated, the white matter at 
 least is irritable ; for it has been found that movements result 
 when the anterior columns are isolated for some way down and 
 stimulated with an electric current. With regard to the grey 
 matter it has been maintained that though it will convey both 
 motor and sensory imjDulses, it cannot originate them. It has 
 accordingly been spoken of as kinesodic and cesthesodic, as simply 
 affording paths for motor and sensory impulses. But the argu- 
 ments urged in support of this view cannot be regarded as 
 conclusive. 
 
CHAPTER VI. 
 THE BRAIN. 
 
 SEC. 1. ON THE PHENOMENA EXHIBITED BY AN ANIMAL 
 DEPRIVED OF ITS CEREBRAL HEMISPHERES. 
 
 A FROG from which the cerebral lobes have been removed, even 
 though all the rest of the brain has been left intact, seems to 
 possess no volition. The apparently spontaneous movements which 
 it executes are so few and seldom that it is much more rational 
 to attribute those which do occur to the action of some stimulus 
 which has escaped observation, than to suppose that they are the 
 products of a will acting only at long intervals and in a feeble 
 manner. 
 
 By the application however of appropriate stimuli, such an 
 animal can be induced to perform all the movements which an 
 entire frog is capable of executing. It can be made to swim, to 
 leap, and to crawl. When placed on its back, it immediately 
 regains its natural position. When placed on a board, it does not 
 fall from the board when the latter is tilted up so as to displace 
 the animal's centre of gravity : it crawls up the board until it 
 gains a new position in which its centre of gravity is restored to 
 its proper place. Its movements are exactly those of an entire 
 frog except that they need an external stimulus to call them forth. 
 They inevitably follow when the stimulus is applied ; they come 
 to an end when the stimulus ceases to act. By continually varying 
 the inclination of a board on which it is placed, the frog may 
 be made to continue crawling almost indefinitely; but directly 
 the board is made to assume such a position that the body of 
 the frog is in equilibrium, the crawling ceases ; and if the position 
 be not disturbed the animal will remaia impassive and quiet for 
 an almost indefinite time. When thrown into water, the creature 
 
Cm A p. VI.] THE /ihWfX. 609 
 
 begins at once to swim about in the most regular manner, ami will 
 continue to swim till it is exhausted, if there be nothing present on 
 which it can come to rest. If a small piece of wood be placed on 
 the water the frog will when it comes in contact with the wood 
 crawl upon it, and so come to rest. Such a frog, if it.s Hanks be 
 gently stroked, will croak; and the croaks follow so regularly and 
 surely upon the strokes that the animal may almost be played up- 
 on like a musical instrument. Moreover, the movements of the 
 animal apjiear to be influenced by light ; if it be urged to move in 
 any particular direction, it seems to avoid in its progress objects 
 casting a strong shadow. In fact, even to a careful observer the 
 differences between such a frog and an entire frog which was 
 simply very stupid or very obstinate, would appear slight and 
 unimportant except in one point, viz. that the animal without its 
 cerebral hemispheres was obedient to every stimulus, and that 
 each stimulus evoked an appropriate movement, whereas with the 
 entire animal it would be impossible to predict whether any result 
 at all, and if so what result, would follow the application of this or 
 that stimulus. Both are machines ; but the one is a machine and 
 nothing more, the other is a machine governed and checked by a 
 dominant volition. 
 
 Now such movements as crawling, leaping, swimming, and in- 
 deed, to a greater or less extent, all bodily movements, are carried 
 out by means of coordinate nervous motor impulses, influenced, 
 arranged, and governed by coincident sensory or afferent impulses. 
 We have already seen that muscular movements are determined 
 by the muscular sense ; they are also directed by means of sensory 
 impulses passing centripetally along the sensory nerves of the skin, 
 the eye, the ear, and other organs. Independently of the afferent 
 impulses, which acting as a stimulus call forth the movement, all 
 manner of other afferent impulses are concerned in the generation 
 and coordination of the resultant motor impulses. Every bodilv 
 movement such as those of which we are speaking is the work of a 
 more or less complicated nervous mechanism, in which there are 
 not only central and efferent, but also afferent factors. And, 
 putting aside the question of consciousness, with which we have 
 here no occasion to deal, it is evident that in the frog deprived of 
 its cerebral hemispheres all these factors are present, the afferent 
 no less than tiie central and the efferent. The machinery for all 
 the necessary and usual bodily movements is present in all its 
 completeness. The share therefore which the cerebral hemispheres 
 take in executing the movements of which the entire animal is 
 capable, is simply that of putting this machinery into action. The 
 relation which the higher nervous changes concerned in volition 
 bear to this machinery is not unlike that of a stimulus. We 
 might almost speak of the will as an intrinsic stimulus. Its 
 operations are limited by the machinery at its command. The 
 cerebral hemispheres in their action can only give shape to a 
 
 ir. 39 
 
610 THE BRAINLESS FROG. [Book iii. 
 
 bodily movement by throwing into activity particular parts of the 
 nervous machinery situated in the lower encephalic structures; 
 and precisely the same movement may be initiated in their 
 absence, by applying such stimuli as shall throw precisely the same 
 parts of that machinery into the same activity. 
 
 Very marked is the contrast between a frog which, though de- 
 prived of its cerebral hemispheres, still retains the optic lobes, cere- 
 bellum and medulla oblongata, and one which possesses a spinal 
 cord only. The latter when placed on its back makes no attempt 
 to regain its normal position ; in fact, it may be said to have com- 
 pletely lost its normal position, for even when placed on its feet it 
 does not stand with its fore feet erect, as does the other animal, 
 but lies flat on the ground. When thrown into water, instead of 
 swimming it sinks like a lump of lead. When pinched, or other- 
 wise stimulated, it does not crawl or leap forwards; it simply 
 throws out its limbs in various ways. When its flanks are stroked 
 it does not croak ; and when a board on which it is placed is 
 inclined sufficiently to displace its centre of gravity it makes no 
 effort to regain its balance, but falls off the board like a lifeless 
 mass. Though, as we have seen, there is in all parts of the spinal 
 cord of the frog a large amount of coordinating machinery, it is 
 evident that a great deal of the more complex machinery of this 
 kind, especially all that which has to deal with the body as a 
 whole, and all that which is concerned with equilibrium and is 
 specially governed by the higher senses, is seated not in the spinal 
 cord but in the brain including the medulla oblongata; and 
 apparently a great deal of this more complex machinery is con- 
 centrated in the optic lobes. The point however to which we wish 
 now to call special attention is that the nervous machinery re- 
 quired for the execution, as distinguished from the origination, of 
 bodily movements even of the most complicated kind, is present 
 after complete removal of the cerebral hemispheres, though these 
 movements are such as to require the cooperation of highly 
 differentiated afferent impulses. 
 
 Our knowledge of the phenomena presented by the bird or 
 mammal from which the cerebral hemispheres have been removed 
 is not so exact as in the case of the frog. Under such circum- 
 stances movements apparently spontaneous in character are more 
 common with the bird or mammal than with the frog. This 
 might be expected, seeing that the more complicated brain of the 
 former affords, even in the absence of the cerebral hemispheres, 
 much more opportunity for the origination of stimuli within the 
 nervous system itself, and for the play of stimuli however origi- 
 nating, than does that of the latter. It would be hazardous to 
 regard such apparently spontaneous movements as indications of 
 volition, and indeed it seems a priori improbable that the will 
 should be confined to the cerebral hemispheres in the frog, and yet 
 so to speak diffused among other parts of the brain in the more 
 
("iiAj'. VI.] THE JiliAIX. 611 
 
 highly difrorentiate(l bird or mammal. On thu other hand, when 
 the cerebral hemi.spheres are bodily removed by the knife, the 
 portions of the brain left behind are so profoundly atlected by the 
 'shock' of the operation, are for a while so obviously in an ab- 
 normal eontlition, that no just deductions can then be made as to 
 what are their normal functions. And the animals generally die 
 before they have entirely recovered from these immediate effects of 
 the operation. 
 
 In the case of the bird, it has been found possible to keep the 
 creature alive for mouths, after apparently complete removal of 
 the hemispheres, and the following phenomena have then been ob- 
 served. The bird is able to maintain a completely normal posture, 
 and will balance itself on one leg, after the fashion of a bird which 
 has in a natural way gone to sleep. In fact, its appearance and be- 
 haviour are strikingly similar to those of a bird sleepy and stupid. 
 Left alone in perfect quiet, it will remain impassive and motionless 
 for a long, it may be for an almost indefinite, time. When stiired 
 it moves, shifts its position ; and then on being left alone returns 
 to a natural, easy posture. Placed on its side or its back it will 
 regain its feet ; thrown into the air, it flies with considerable pre- 
 cision for some distance before it returns to rest. It frequently 
 tucks its head under its Avings, and at times may be seen to 
 clean its feathers and to pick up corn or to drink water presented 
 to its beak. It may be induced to move not only by ordinary 
 stimiili applied to the skin, but also by sudden sharp sounds, or 
 flashes of light ; and it is evident that its movements are to a 
 certain extent guided by visual sensations, for in its flight it will, 
 though imperfectly, avoid obstacles. Save that all clear signs of 
 distinct volition are absent, that all satisfactory indications of 
 intelligence are wanting, and that the movements are on the 
 whole clumsy, resembling rather those of a stupid drowsy bird 
 than those of one quite wide awake, there is very little to dis- 
 tinguish such a bird from one in full possession of its cerebral 
 hemispheres. 
 
 In a mammal, during the few hours which intervene between 
 the sudden removal of the whole of both hemispheres and death, 
 very much the same phenomena may be observed. The rabbit, or 
 rat, operated on can stand, run and leap; placed on its side or 
 back it at once regains its feet. Left alone, it remains as motion- 
 less and impassive as a statue, save now and then when a passing 
 impulse seems to stir it to a sudden but brief movement. Such a 
 rabbit will remain for minutes together utterly heedless of a can-ot 
 or cabbage-leaf placed just before its nose, though if a morsel be 
 placed in its mouth it at once begins to gnaw and eat. When 
 stirred, it will with perfect ease and steadiness run or leap forward ; 
 and obstacles in its course are very frequently, with more or less 
 success, avoided. It will often follow by movements of the huad a 
 bright light held in front of it (provided that the optic nerves and 
 
 39—2 
 
612 THE BRAINLESS MAMMAL. [Book hi. 
 
 tracts have not been injured during the operation), and starts 
 when a shrill and loud noise is made near it. When pinched it 
 cries, often with a long and seemingly plaintive screarn. Evidently 
 its movements are guided and may be originated by tactile, visual, 
 and auditory sensations^. But there is no satisfactory evidence 
 that it possesses either visual or other perceptions, or that the 
 sensations it experiences give rise to ideas. Its avoidance of 
 objects depends not so much on the form of these as on their inter- 
 ference with light. No image, whether pleasant or terrible, 
 whether of food or of an enemy, produces an effect on it, other 
 than that of an object reflecting more or less light. And though 
 the plaintive character of the cr}^ which it gives forth when 
 pinched suggests to the observer the existence of passion, it is pro- 
 bable that this is a wrong interpretation of a vocal action ; the cry 
 appears plaintive simply because, in consequence of the complete- 
 ness of the reflex nervous machinery and the absence of the usual 
 restraints, it is prolonged. The animal is able to execute all its 
 ordinary bodily movements, but in its performances nothing is ever 
 seen to indicate the retention of an educated intelligence. With 
 the removal of that part of the brain which lies between the 
 hemispheres and the medulla a large number of these coordinate 
 movements disappear. The animal can no longer balance itself, it 
 lies helpless on its side, and though various movements of a 
 complex character, including cries, may be produced by appropriate 
 stimuli, they are much more limited than when these cerebral 
 structures are intact. 
 
 When in a dog, the cerebral convolutions are removed piece- 
 meal at several operations, the animal may be kept alive and in 
 good health for a long time, many months at least, though these 
 parts of the brain have been reduced to very small dimensions. 
 In such a case the indications of volition are much more prominent 
 and numerous. We do not wish now to discuss whether the residues 
 of volition and of intelligence then observed are to be ascribed to 
 the small portion of the cerebral hemispheres still left, or whether 
 they result from the working of other parts of the brain. To do this 
 we should have to attempt to define with greater exactness than we 
 
 ' Here we come upon a difficulty, wliicli we shall meet with again in the i3resent 
 chapter. Are we justified in speaking of 'sensation' in cases where we have reason 
 to think that consciousness is absent, or where, as in the present instance, we have 
 no evidence to shew whether consciousness is present or not ? In treating of the 
 senses we called attention to the fact, that we must suppose in the case, for instance, 
 of vision, the visual peripheral organ to be connected witli a visual central organ in 
 such a v.ay that the sensory impulses originating in the former become modified in 
 the latter before they affect consciousness. In the peripheral organ and along the 
 nerve of sense, the affection of the nervous tissue may be spoken of as a sensory 
 impulse; but after the affection has traversed the central organ and become modified 
 it is no longer a simple sensory impulse. We must then either call it a sensation 
 irrespective of whether any change of consciousness intervenes or no, or we must 
 give it a new name. Not wishing to introduce a new name, we have ventured to use 
 the word ' sensation ' in a sense which neither affirms nor denies the coexistence of 
 consciousness. 
 
Chap, vi.] THE BRAIN. r,l3 
 
 have hitherto done, the mcaninL,' of the words ' vulition ' and ' in- 
 telligenee ;' and shouKl probably in the end come to the conclusion 
 that the discussion is a barren one. The more we study the pheno- 
 mena exhibited by animals possessing a part cmly of their brain, 
 thi' closer we are pushed to the conclusion that no sharp line can 
 be drawn between vulition and the lack of volition, or between the 
 possession and absence of intelligence. Between the muscle-nerve 
 preparation at the one limit, and our conscious ^villiug selves at 
 the other, there is a continuous gradation without a break ; we 
 cannot fix on an}'' linear barrier in the brain or in the general 
 nervous system, and say ' beyond this there is volition and in- 
 telligence but up to this there is none.' 
 
 This however is not the question with which we are now deal- 
 ing. What we want to point out is that in the higher animals, 
 including mammals, as in the frog, after the removal of the cerebral 
 hemispheres (or rather of the cerebral convolutions, for interference 
 with the corpora striata and optic thalarai is apt to induce disorders 
 of Avhich we shall speak pi'esently), even though volition and in- 
 telligence appear to be largely, if not entirely, lost, the body is 
 still capable of executing all the ordinaiy movements which the 
 animal in its natural life is wont to perform, in spite of these 
 movements necessitating the cooperation of various afferent 
 impulses ; and that therefore the nervous machinery for the 
 execution of these movements lies in some part of the brain 
 other than the cerebral hemispheres. We have reasons for 
 thinking that it is situated in the structures forming the middle 
 or hind brain . 
 
SEC. 2. THE MECHANISMS OF COORDHSTATED 
 
 MOVEMENTS. 
 
 When in a pigeon the horizontal membranous circular canal 
 of the internal ear is cut through, the bird is observed to be 
 continually moving its head from side to side. If one of the 
 vertical canals be cut through, the movements are up and down. 
 The peculiar movements may not be witnessed when the bird is 
 perfectly quiet, but they make their appearance whenever it is 
 disturbed, or attempts in any way to stir. When one side only 
 of the head is operated on, the condition after a while passes away. 
 When the canals of both sides have been divided, it becomes much 
 exaggerated, lasts longer, and sometimes remains permanently. 
 And it is then found that these peculiar movements of the head 
 are associated with what appears to be a complete want of co- 
 ordination of all bodily movements. If the bird be thrown into 
 the air, it flutters and falls down in a helpless and confused 
 manner; it appears to have totally lost the power of orderly 
 flight. If placed in a balanced position, it may remain for some 
 time quiet, generally with its head in a peculiar posture ; but 
 directly it is disturbed, the movements which it attempts to 
 execute are irregular and fall short of their purpose. It has great 
 difficulty in picking up food and in drinking; and in general 
 its behaviour very much resembles that of a person who is exceed- 
 ingly dizzy. 
 
 It can hear perfectly well, and therefore the symptoms cannot 
 be regarded as the result of any abnormal auditory sensations, such 
 as ' a roaring ' in the ears. Besides, any such stimulation of the 
 auditory nerve as the result of the section, would speedily die away, 
 whereas these phenomena may be permanent. 
 
CirAP. VI.] THE BRAIN. 615 
 
 The movements arc not occa.sioiicd by any partial paralysis, by 
 any want of power in particular muscles or group of muscles. Nor 
 on the other hand are they tlue to any uncontrollaljle impulse; 
 a very gentle pressure of th(( hand suffices to stop the movements 
 of the head, and the hand in doing so experiences no strain. The 
 assistance of a very slight support enables movements otherwise 
 impossible or most difficult, to be easily executed. Thus, though 
 when left alone tlie bird has great difficulty in drinking or picking 
 up corn, it will continue to drink or eat with ease if its beak be 
 plunged into watei-, or into a heap of barley ; the slight support of 
 the water or of the grain being sufficient to steady its movements. 
 In the same way, it can, even without assistance, clean its feathers 
 and scratch its head, its beak and foot being in these operations 
 guided by contact with its own body. 
 
 In mammals (rabbits) section of the canals produces a loss of 
 coordination similar to that witnessed in birds ; but the movements 
 of the head are not so marked, peculiar oscillating movements of 
 the eye-balls (nystagmus), differing in direction and character 
 according to the canal or canals operated upon, becoming however 
 very prominent. In the frog no deviations of the head are seen, 
 but there is, as in other animals, a loss of coordination in the 
 movements of the body. 
 
 Injury to the bony canals alone is insufficient to produce the 
 symptoms ; the membranous canals themselves must be divided or 
 destroyed. 
 
 How are we to explain these remarkable phenomena ? Let us 
 for a while turn aside to ourselves and examine the coordination of 
 the movements of our own bodies. When we appeal to our own 
 consciousness we find that our movements are governed and guided 
 by what we may call a sense of equilibrium, by an appreciation of 
 the position of our body and its relations to space. When this 
 sense of equilibrium is disturbed Ave say we are dizzy, and we then 
 stagger and reel, being no longer able to coordinate the move- 
 ments of our bodies or to adapt them to the position of things 
 around us. What is the origin of this sense of equilibrium? By 
 what means are we able to appreciate the position of our body ? 
 There can be no doubt that this appreciation is in large measure 
 the product of visual and tactile sensations; we recognize the 
 relations of our body to the things around us in great measure by 
 sight and touch ; we also learn much by our muscular sense. But 
 there is something besides these. Neither sight nor touch nor 
 muscular sense would help us when, placed perfectly flat and at 
 rest on a horizontal rotating table, Avith the eyes shut and not a 
 muscle stirring, we attempted to determine whether the table 
 and we with it were moved or no, or to ascertain how much it 
 and we were turned to the right or to the left. Yet under such 
 circumstances we are not only conscious of a change in our position 
 but some observers have been able to pass a tolerably successful 
 
616 THE SEMI-CIRCULAR CANALS. [Book rir. 
 
 judgment as to the angle through which they have been moved. 
 What are the data on which such a judgment can be formed ? 
 It is possible that the mere displacement of blood or of the more 
 fluid parts of the tissues in various regions of the body, by giving 
 rise to affections of general sensibility, may contribute to these 
 data ; but the peculiar features of the semi-circular canals suggest 
 that these are special agents in this matter. The three canals are, 
 as we know, placed in the head in planes nearly at right angles to 
 one another. Hence the pressure of the endolymph on the walls 
 of the canal (including the maculae of the ampullae) in any given 
 position of the head, and variations of that pressure due to move- 
 ments of the head, or the movements of the endolymph within the 
 canals accompanying movements of the head, would be different 
 in the three canals; a sonorous wave on the other hand would 
 affect all the ampullse equally. If Ave suppose that the pressure of 
 the endolymph or variations in that pressure, or the movements of 
 the endolymph can give rise to afferent impulses which, though 
 passing up to the brain along the auditory nerve, are not of 
 the nature of auditory impulses, we appear to have the data for 
 which we are seeking; for it is quite possible to conceive that 
 the impulses thus generated in the ampullse by movements of 
 the head, should by becoming transformed into sensations enter 
 into the judgment which we form of the movements which ha.ve 
 given rise to them. 
 
 But if ampullar sensations, if we may so call them, thus enter 
 into our appreciation of the position of our body and thus form, in 
 part, the basis of our sense of equilibrium, it is obvious that when 
 these are absent or deranged, the sense of equilibrium will be 
 affected and the coordination of movements interfered with. And 
 it has been urged that the phenomena attendant on injury to the 
 semi-circular canals are due either to the absence of normal or to 
 the influence of abnormal ampullar sensations. There are how- 
 ever difficulties in the v/8.y of giving a satisfactory explanation of 
 these phenomena. If, as some observers state, both auditory 
 nerves may be completely and permanently severed, without any 
 effect on the coordination of movements, it is obvious that the 
 incoordination v/hich follows upon section of the auditory canals is 
 due to some irritation set up by the operation and not to the 
 absence of any normal impulses passing up from those organs to the 
 brain, to the lack of what we called just now ampullar sensations. 
 But if the effects are those of irritation, it is difficult to understand 
 how they can, as according to certain observers the}^ certainly do, 
 become permanent. It has however been strongly urged that in 
 such cases of permanent incoordination, the operation has set up 
 secondary mischief in the brain, in the cerebellum for instance, and 
 that the permanent effects are really due to the disease going on here; 
 and we have reason as we shall see to think that the cerebellum 
 is concerned in the coordination of movements. But the matter is 
 
Chap, vi.] THE BRAIN. r,17 
 
 one on whicli it does not as yet seem possible to make a dogniatio 
 statement. 
 
 We compared the condition of a pigeon after injury to the 
 semi-circular canals to that of a person who is dizzy, and indeed 
 one gix>at characteristic of vertigo or dizziness is an iuability on the 
 part of the subject to maintain a due equilibrium ; he cannot co- 
 ordinate his movements properly or adapt them to the circum- 
 stances aroun<l him, and in consequence staggers and reels. Vertigo 
 may be brought about in various ways. It may be the result 
 simply of unusual and powerful visual sensations, such as those 
 produced by water falling rapidly from a great height or 1)y objects 
 moving swiftly across the field of vision. It may arise from 
 changes taking place in the brain itself, and is a common symptom of 
 many maladies and of the action of many poisons. As is well known, 
 a most severe vertigo ma}' be at once produced by rapidly rotating 
 the body ; and a very curious form may be induced by passing an 
 electric constant current of adequate strength through the head 
 from ear to ear. All cases of vertigo, however produced, have this 
 common subjective feature, that one or more of the sets of sensa- 
 tions which form the basis of our appreciation of the relation of our 
 body to external things disagi'ee, and are in conflict with, the rest 
 of the sensations which go to make up the same appreciation. 
 Thus in the vertigo after rapid rotation of the body, while we seem 
 to see the whole world whirling round us, this conclusion is contra- 
 dicted by other sensations. Corresponding to this subjective feature 
 of vertigo is the objective feature of the failure of motor coordina- 
 tion ; and there can be no doubt that the two are connected 
 together as cause and eflfect. The exact manner in which the 
 vertigo is developed, i.e. the sequence and relation of the various 
 factors of it, will naturally vary according to the nature of the 
 exciting cause, and the course of events appears to be not only 
 different in different forms, but in many cases complex. When 
 vertigo comes on from rapidly rotating the body with the eyes 
 open, an element of discord is introduced by the eye-balls not 
 keeping pace with the movements of the head but following ir- 
 regularly, executing the oscillatory movements known as nystagmus, 
 movements which continue after the body has come to rest, and 
 then give rise to the false sensation that external objects are 
 mo\'ing rapidly. But in this vertigo of rotation there are other 
 factors at work, for the dizziness comes on, though less readily, 
 when the eyes are kept shut all the time. It has been suggested 
 that false ampullar sensations arise from the rotation of the body 
 exciting the semi-circular canals ; and the form of vertigo^ which is 
 the salient symptom of the so-called Meniere's malady, has been 
 ascribed to disease of the semi-circular canals. But it must be re- 
 membered that the canals are frequently diseased without any 
 vertigo appearing; and if, as some observers state, vertigo by rotation 
 may be readily induced in rabbits after section of both auditory 
 
618 VERTIGO. [Book in. 
 
 nerves, it is clear that the semi-circular canals can have little share 
 in this form of vertigo. And indeed, even admitting this as a contri- 
 bution to the total effect, it seems probable that changes in the brain 
 due to the displacement of the blood or even of the brain-substance 
 itself caused by the too rapid rotation, are at work. It is difficult 
 otherwise to explain the unconsciousness which may ensue if the 
 rotation be rapid and long-continued ; and the vertigo resulting 
 from various poisons seems to be distinctly of central origin. 
 
 Whether we accept the view of ampullar sensations just dis- 
 cussed or not, and whatever be the exact share which false ampul- 
 lar sensations take in the causation of vertigo, this at all events is 
 clear, that afferent impulses of various kinds so far contribute to 
 the building up of the coordinating mechanisms that changes in 
 these impulses tend to throw the mechanisms into disorder, or at 
 least to impau' their proper working. It is not necessary that 
 these afferent impulses should directly affect consciousness (or, to 
 speak more correctly, should affect that complete consciousness 
 which is associated with volition), and so develope into distinct 
 perceptions. We have seen that a bird from which the cerebral 
 hemispheres have been removed is perfectly able to fly ; and that 
 therefore the coordinating nervous mechanism necessary for flight 
 is situated in the parts of the brain lying behind the cerebral 
 hemispheres. We have also dwelt on the fact that all the chief 
 coordinating mechanisms of the frog lie in the hind parts of the 
 brain ; yet in the frog, as in the bird, and we may add, as in the 
 mammal, injury to the hinder parts of the brain produces loss 
 of coordination whether the hemispheres be present or not. Now, 
 we have no satisfactory reasons for either asserting or denying that 
 what we call consciousness, i.e. a distinct consciousness similar to 
 our own consciousness, exists in animals deprived of their cerebral 
 hemispheres. When signs of volition are present, we may safely 
 take these signs as indications of consciousness also ; but we are not 
 justified in saying that all consciousness is absent when satisfactory 
 signs of volition We wanting. We cannot form any just judgment 
 on the matter without some more trustworthy and objective tokens 
 of consciousness than we at present possess. But what we may 
 safely assert is, that the coordinating mechanism, the retention of 
 which is so striking a feature of an animal deprived of its cerebral 
 hemispheres, is constructed out of divers afferent impulses of 
 various kinds arriving at the coordinating centre from various 
 parts of the body, that in fact the coordination taking place at the 
 centre is the adjustment of efferent to afferent impulses. Many, if 
 not all, of these afferent impulses are such that in the presence of 
 consciousness they would give rise to perceptions and ideas ; but 
 we have no reason for thinking that the complete development of 
 the afferent impulse into a perception or an idea is always necessary 
 to the carrying out of coordination. We may say that we have a 
 sense of equilibrium by means of the semi-circular canals, and 
 
fiiAi'. VI.] THE BRAIN. 019 
 
 when that sense is deranged, wc feel giddy and cannot stand. We 
 have no reason, however, for thinking that the failure to keep 
 upright is due to the feeling of gidcHness, in the .sen.se of being 
 a direct result of the condition of the consciousness. On the con- 
 traiT, since the peculiar movements characteristic of vertigo may 
 take place in the absence of consciousness without the vertigo 
 being actually felt, we may with security assert that the failure to 
 stand upright and the feeling of giddiness are both concomitant 
 effects of the same disarrangement of the coordinating mechanism. 
 
 It cannot be too much insisted upon that for every bodily 
 movement of any complexity afferent impulses are as essential as 
 the executive efferent impulses. Our movements, as we have 
 ahreadv urged, are guided not only by the muscular sense, but also 
 by contact sensations, auditory sensations, visual sensations, and 
 visual perceptions (for the remarks made above concerning the 
 relations of the coordinating mechanism to consciousness do not 
 exclude the possibility of consciousness affecting the mechanism, 
 indeed not only may perceptions enter into the causation of vertigo, 
 but even an imaginary idea may be the sole exciting cause of this 
 condition); and when we say 'they are guided,' we mean that with- 
 out the sensations the movements become impossible. In studying 
 vision we saw repeatedly that the movements of the eyes were 
 directly dependent on vision, and every ball-room affords abundant 
 evidence of the ties between sensations of sound and motions of the 
 limbs. So essential, in fact, are afferent impulses to the develop- 
 ment of complex bodily movements, that we are almost justified in 
 considering every such movement in the light of a reflex action 
 made up of afferent and efferent impulses and central actions, and 
 set going by the influence of some dominant afferent impulse, or 
 by the direct action of those nervous changes, whose psychical cor- 
 relative is what we call the will, on the centre itself. All day long 
 and every day multitudinous afferent impulses, from eye, and ear, 
 and skin, and muscle, and other tissues and organs, are streaming 
 into our nervous system ; and did each afferent impulse issue as its 
 correlative efferent motor impulse, our life would be a prolonged 
 convulsion. As it is, by the checks and counterchecks of cerebral 
 and spinal activities, all these impulses are drilled and marshalled, 
 and kept in hand in orderly array till a movement is called for ; and 
 thus w^e are able to execute at will the most complex bodily 
 manoeuvres, knowing only why, and unconscious or but dimly 
 conscious how, we carry them out. 
 
 We have ventured to use the phrase ' coordinating centre,' but 
 it must be understood that we have no right to attach more than a 
 general meaning to the words. We cannot, at present at least, 
 define such a centre in the same way that we can the vaso-motor 
 or respiratory centre. When the optic lobes as well as the cerebral 
 hemispheres are removed from the frog, the power of balancing 
 itself is lost ; when such a frog is thrown off" its balance by inclining 
 
620 COORDINATING MECHANISMS. [Book in. 
 
 the plane on which it is placed, it falls down. The special coordi- 
 nating mechanism for balancing must therefore in this animal be 
 situated in the optic lobes ; but after removal of these organs, the 
 animal is still capable of a great variety of coordinate movements : 
 unlike a frog retaining its spinal cord only, it can swim and leap, 
 and when placed on its back immediately regains the normal posi- 
 tion. The cerebellum of the frog is so small, and in removing it 
 injury is so likely to be done to the underlying parts, that it becomes 
 difficult to say how much of the coordination apparent in a frog 
 possessing cerebellum and medulla is to be attributed to the former 
 or to the latter ; probably, however, the part played by the former 
 is small. In the mammal, as we have stated, removal of the whole 
 middle and hind brain does away with the most marked of these 
 coordinating mechanisms. Removal of the pons Varolii alone has 
 the same effect. Injury to, or disease of, the more superficial parts 
 of the corpora quadrigemina or of the cerebellum, does not appear 
 to influence the movements of the body at large to any striking 
 extent ; but there are many pathological cases, as well as experi- 
 mental observations, tending to associate the coordinating mechan- 
 isms of which we are speaking with the deeper parts of the cere- 
 bellum. It would be hazardous, in the present state of our know- 
 ledge, to make any definite statement concerning the share taken 
 by these several cerebral structures in the various coordinations. 
 
 Forced Movements. 
 
 All investigators who have performed experiments on the brain, 
 have observed as the result of injury to various parts of it remark- 
 able compulsory movements. One of the most common forms is 
 that in which the animal rolls incessantly round the longitudinal 
 axis of its OAvn body. This is especially common after section of 
 one of the crura cerebri, more particularly of the external and 
 superior parts, or after unilateral section of the pons Varolii, but 
 has also been witnessed after injury to the medulla oblongata and 
 corpora quadrigemina. Sometimes the animal rotates towards and 
 sometimes away from the side operated on. Another form is that 
 in which the animal executes 'circus movements,' i.e. continually 
 moves round and round in a circle, sometimes towards and some- 
 times away from the injured side. This may be seen after several 
 of the above-mentioned operations, but is perhaps particularly com- 
 mon after injuries to the coi-pora striata and optic thalami. There 
 is a variety of the circus movement said to occur frequently after 
 lesions of the nates, in which the animal moves in a circle, with the 
 longitudinal axis of its body as a radius, and the end of its tail for 
 
Chat, vi] TIIK lUiAlX. 621 
 
 a centre. And thia lorni ;i|L;ain may easily p;iss into a simply rolling 
 movement. In yet another form i\iv animal rotates over the trans- 
 verse axis of its body, tnniblcs head over heels in a series of somer- 
 saults; or it may run incessantly in a straight line backwards or 
 forwards until it is stopped by some obstacle. These latter forms 
 of forced movements are frequently seen after injury to the corpora 
 striata even when a very limited portion of their grey matter is 
 affected. Lastly, mam', if not all, these various forced movements 
 may result fi-om injuries which appear to be limited to the cerebral 
 cortex. 
 
 Attempts have been made to explain the rotatory movements 
 by reference to unilateral paralysis or to spasm of various muscles 
 of the body caused by the cerebral injury ; and in the case of the 
 'circus' movements with partial hemiplegia, which follow upon 
 injury to the corpora striata or other parts, the explanation that 
 the animal in progressing forward naturally bears on its paralysed 
 or weak side seems a valid one; but the movements may frequently 
 be -witnessed in the complete absence of either paralysis or spasm, 
 and cannot therefore be always so explained. On the other hand, 
 if the views urged just now concerning the nature of the coordina- 
 ting mechanisms of the brain are true, it is evident that they afford 
 a general explanation of the phenomena, though our present know- 
 ledge will not permit us to explain the genesis of each particular 
 kind of movement. Such gross injuries as are involved in dividing 
 cerebral structures or m injecting corrosive substances into the 
 midst of cerebral organs, must of necessity, either by irritation or 
 otherwise, seriously ajffect the transmission not only of afferent 
 impulses in their cerebral course, but also of central impulses, in- 
 hibitory and the like, passing from one part of the brain to another; 
 and must therefore seriously affect the due working of the general 
 coordinating mechanisms. The fact that an animal can, at any 
 moment, by an eftort of its own will, rotate on its axis or run 
 straight forwards, shews that the nen-ous mechanism for the execu- 
 tion of those movements is ready at hand in the brain, waiting only 
 to be discharged; and it is easy to conceive how such a discharge 
 might be affected either by the substitution of some potent intrinsic 
 afferent impulse for the will or by some misdirection of the volitional 
 impulses. Persons who have experienced similar forced movements 
 as the result of disease report that they are frequently accompanied, 
 and seem to be caused, by disturbed visual or other sensations ; 
 thus when they suddenly fall forward they say that they do 
 so because the ground in front of them appears to sink away 
 beneath their feet. Without trusting too closely to the interpreta- 
 tions the subjects of these disorders give of their o-^ti feelings, we 
 may at least conclude that the disorderly movements are in many 
 cases due, not to any paralytic or other failing of the simple muscu- 
 lar instruments of the nervous system, but to a disorder of the 
 coordinating mechanism, which in many cases is itself the result of 
 
622 FORCED MOVEMENTS. [Book hi. 
 
 disordered sensory impulses. And this view is supported by the 
 fact that many of these forced movements are accompanied by a 
 peculiar and wholly abnormal position of the eyes, which alone 
 might perhaps explain many of the phenomena. 
 
SEC. 3. THE FUNCTIONS OF THE CEREBRAL 
 CONVOLUTIONS. 
 
 Using the word cerebral hemisphere for brevity's sake to 
 denote the cortical substance of the cerebrum (for we have no 
 reason to think that the fibres of the white matter serve for 
 any other than conducting functions, and it is advisable to keep 
 apart the consideration of the corpora striata) we have endeavoured 
 to shew that in respect to function a broad line separates these 
 structures from the rest of the brain. We have seen reason to 
 think that will and intelligence are associated with the former, 
 while the latter are concerned in elaborating and coordinating 
 afferent and efferent impulses in such a way as to furnish a com- 
 plicated nervous machinery of which the former makes use in 
 can-ying out the voluntary and intelligent movements of the body. 
 It is not uncommon to speak of the cerebral hemispheres as 'the 
 seat of will and intelligence. Such a phrase is liowever open to 
 objection. It suggests that if the cerebral hemispheres could be 
 kept alive quite isolated from the rest of the brain, the processes of 
 volition and intelligence, though unable to manifest themselves by 
 any outward show, would still go on. But we are not in a position 
 to accept, without hesitation, such an assumption. All we know 
 is that the existence of volition and intelligence are dependent 
 on the connection of the cerebral cortex with the rest of the 
 brain. When that connection is broken they disappear; and it 
 may be that what we call volition and intelligence are the product 
 of both the cerebral hemispheres and other parts of the brain 
 working together by virtue of their connections and ceasing so to 
 work when the connections fail ; on the other hand it may 
 be that they are generated in the former alone, and that the 
 connections only serve to allow the former to make use of the 
 machinery of the latter. Our present knowledge will not allow us 
 
624 CEREBRAL CONVOLUTIONS. [Book hi. 
 
 to decide between these two views ; meanwhile it will be well not 
 to consider the latter as the only possible one. 
 
 With this preliminary caution we may now proceed to inquire 
 whether we can attribute different functions to different parts 
 of the cerebral cortex. All the older observers, Flourens and others, 
 agreed that when the cerebral cortex was gradually removed, 
 piece by piece or slice by slice, no obvious effects manifested 
 themselves, either in the intelligence or volition of the animal, 
 when the first small portions were taken away ; but that, as the 
 removal was repeated, the animal became more and more dull and 
 stupid, until at last both intelligence and volition seemed to be entirely 
 lost. It has been frequently observed that in case of wounds of 
 the skull large portions of the brains of men might be removed 
 without any marked effect on the psychical condition of the 
 patients. The brain when exposed was found not to be sensitive ; 
 and ordinary stimuli applied to the surface of the convolutions of 
 animals failed in the hands of most experimenters to produce any 
 clearly recognizable effect. Hence it became very common to deny 
 the existence of any localization of functions in the convolutions of 
 the hemisphere, and to speak of the brain as ' acting as a whole,' 
 whatever that might mean. On the other hand, pathological 
 observation seemed clearly to shew that diseased conditions limited 
 to different areas of the convolutions, produced different effects. It 
 was found that in the case of circumscribed lesions confined to 
 parts of the cerebral cortex, the effects whether in the way of 
 paralysis or of convulsive movements, were frequently confined to 
 certain regions of the body or were even limited to particular 
 groups of muscles. One set of phenomena in particular spoke very 
 strongly in favour of definite convolutions, i.e. definite parts of the 
 cerebral cortex having definite functions. In certain cases of cere- 
 bral disease the patient is unable to speak at all or speaks imper- 
 fectly or incorrectly. It is obvious that the failure to speak or to 
 speak properly may be brought about in various ways; the fault 
 may be simply in the tongue or hypoglossal nerve, or it may lie in 
 one or other of the series of central and cerebral processes which 
 issue in coordinate impulses being sent to the organs of speech. 
 Using the word aphasia, as is usually done, in its general sense to 
 denote the partial or complete loss of articulate speech, due to 
 cerebral causes, we may say that aphasia was found to be so closely 
 associated "with disease of a definite part of the brain, viz. the 
 posterior portion of the third frontal convolution, Fig. 86 (9) (10), 
 as to afford almost irresistible proof that this particular part of 
 the brain must be specially connected with the faculty of speech. 
 Moreover the disease occurs in so great a majority of cases on one 
 side of the brain only, namely the left (aphasia being frequently a 
 symptom accompanying right hemiplegia, or paralysis of the right 
 side of the body, the disease in such cases affecting other parts of 
 the brain as well), as to suggest the idea that in the act of speech. 
 
ClIAl'. VI.] 
 
 THE BRAIN. 
 
 625 
 
 oue side of the brain only is used. On this point however wo 
 must not dwell now; we cannot here discuss the question of 
 unilateral cerebral activity, or the exact nature of the failings 
 which K'ad to the loss of, or to errors in, sjieech, or with what 
 particular link or links in the chain of cerebral events leading 
 to speech, whether mainly atlereut or mainly efferent ones, this 
 portion of the cortex is associated ; we simply quote these cases of 
 aphakia, as aflbrding proof of the localization of function in the 
 cerebral cortex. 
 
 Fig. 84. The .Vkeas of tue Cerebkal Convolctions of the Dog, accobdino to 
 
 HiTZIG AND FbITSCH. 
 
 (1) A The area for the muscles of the neck. (2) -^ The area for the extension 
 and adduction of the fore limb. (3) + The area for the flexion and rotation of the 
 fore limb. (4) ft The area for the hind hmb. Eunnin^' transversely towards and 
 separating (1) and (2) from (3) and (4) is seen the crucial sulcus, (o) Q The facial 
 area. 
 
 For a while then the teachings of pathology and experiment were 
 contradictory ; but continued experimental inquiry shewed that the 
 former were in the right. Bitzig and Fritsch were the fii'st to shew 
 that the local application of the constant galvanic current to par- 
 ticular convolutions and to particular parts of convolutions gave rise 
 to definite coordinate movements of various groups of muscles. Thus 
 while the stimulation of one spot (Fig. 84) caused movements in the 
 muscles of the neck, another caused extension with adduction of 
 the fore leg, a third movements of the hind leg, a fourth movements 
 of the eye and other parts of the face. In fact, they and Ferrier, 
 who using chiefly the interrupted or faradaic current, repeated and 
 extended their observations, were able to map out the convolutions 
 of the front and middle parts of the hemisphere of the dog (Figs. 84, 
 85), cat, monkey (Figs. 86, 87), and other animals, into a number 
 
 F. 40 
 
626 
 
 CEREBRAL CONVOLUTIONS. 
 
 [Book hi. 
 
 iici I 
 
 Fig. 85. The Areas oe the Cebebbal Convolutions of the Dog, accobding 
 
 TO Feebiee. 
 
 0. The Olfactory Lobe. A. The Fissure of Sylvius. B. The Crucial Sulcus. 
 
 Stimulation by the interrupted current of the areas indicated by the several 
 circles produces the following results. 
 
 (1) The hind leg is advanced as in walking. (3) Lateral or wagging motion of 
 the tail. (4) Ketraction and adduction of the opposite forelimb. (5) Elevation of 
 the shoulder of, and extension forwards of, the opiJosite forelimb. (7) Closure of 
 the opposite eye caused by combined action of the orbicular and zygomatic muscles. 
 (8) Eetraction and elevation of the opposite angle of the mouth. (9) The mouth is 
 opened and the tongue moved, sometimes barking is produced. (11) Eetraction of 
 the angle of the mouth. (12) Opening of the eyes and dilation of the pupils; the 
 eyes and then the head turning to the opposite side. (13) The eyeballs move to the 
 opposite side. (14) Pricking or sudden retraction of the opposite ear. (15) Torsion 
 of the nostril on the same side. (16) Elevation of the lip and dilation of the 
 nostiil (?). 
 
 of precisely limited areas, the stimulation of each area producing a 
 distinct and limited movement, while stimulation of a large surface 
 produced general convulsions. The movements were so precise that 
 they answered each to the spot stimulated almost as completely as 
 a note answers to a key struck on the piano. A somewhat similar 
 relationship has also been obser\^ed between various regions of the 
 cortex and the secretion of saliva, the beat of the heart, the con- 
 dition of the pupil, the action of vaso-motor nerves, and other 
 organic functions. 
 
 These experiments, which have not only been confirmed by 
 many observers, but may, with due care, be successfully repeated 
 by any one, clearly shew, in spite of some discordance among 
 various authors as to the exact position and extent of the several 
 'areas,' that there is a connection between electric stimulation of 
 certain areas of the brain-surface and certain bodily movements. The 
 areas in question have been spoken of by some authors as 'motor 
 centres.' Such a term is however undesirable, since it suggests 
 that the brain-surface in a given area is largely occupied in giving 
 rise to the coordinate nervous impulses which carry out the 
 movement resulting from stimulation of the area; whereas, as 
 we have already seen from the behaviour of an animal deprived of 
 
(."UAT. VI. J 
 
 TIJE BRAiy. 
 
 627 
 
 Fig. 86. 
 
 Figs. 86 and 87. Side and Uppek Views of ihe Bkajn ok Mak, with the 
 Abeas of the Cerebrax, Convolutions, according to Ferbier. 
 
 The figures are constructed by viarking on the brain of man, in their respective 
 situations, the areas of the train of the monkey as determined by experiment, 
 and the description of the effects of stimulating the various areas refers to tlie brain of 
 the moniey. 
 
 (1) (On the postero-parietal [superior parietal] lobule). Advance of the opposite 
 hind limb as in walking. (2), (3), (4) (Around the upper extremity of the fissure of 
 Rolando). Complex movements of the opposite leg and arm, and of the trunk, as in 
 swrimmiug. (a), {h), {c), (d) (On the postero-parietal [posterior central] convolution). 
 Individual and combined movements of the fingers and wrist of the opposite hand. 
 Prehensile movements. (5) (At the posterior extremity of the superior frontal con- 
 volution). Extension fonvard of the opposite arm and hand. 
 
 (6) (On the upper part of the antero-parietal or ascending frontal [anterior 
 central] convolution). Supination and flexion of the opposite forearm. (7) (On 
 the median portion of the same convolution). Eetraction and elevation of the 
 opposite angle of the mouth by means of the zygomatic muscles. (S) (Lower 
 down on the same convolution). Elevation of the ala nasi and upper hp with de- 
 pression of the lower lip, on the opposite side. (9), (10) (At the inferior extremity 
 of the same convolution, Broca's convolution). Opening of the mouth with (9) pro- 
 trusion and (10) retraction of tbe tongue ; region of Aphasia, bilateral action, 
 
 (11) (Between (10) and the inferior extremity of the postero-parietal convolution). 
 Eetraction of the opposite angle of the mouth, the head turned slightly to one side. 
 
 (12) (On the posterior portions of the superior and middle frontal convolutions). 
 The eyes open widely, the pupils dilate, and the head and eyes turn towards the 
 opposite side. (13), (13') (On the supra-marginal lobule and angular g}Tus). The 
 eyes move towards the opposite side with an upward (13) or downward (13') 
 deviation. The pupils generally contracted. (Centre of vision.) (14) (On the 
 
 40—2 
 
628 
 
 GEBEBBAL CONVOLUTIONS. 
 
 [Book hi. 
 
 Fig. 87. 
 
 infra-marginal or superior [first] temporo-sphenoidal convolution). Pricking of the 
 opposite ear, the head and eyes turn to the opposite side, and the pupils dilate 
 largely. (Centre of hearing.) Ferrier moreover places the centres of taste and 
 smell at the extremity of the temporo-sphenoidal lobe, and that of touch in the 
 gyrus uncinatus and hippocampus major. 
 
 its cerebral hemispheres, coordination is effected in parts of the 
 brain other than the surface of the cerebral hemispheres; and all 
 that the areas in question do is to make use in some way or other 
 of these lower coordinating mechanisms. 
 
 As will be seen from an inspection of the figures the areas, 
 stimulation of which gives rise to definite movements, are dis- 
 tributed over a part only of the surface of the hemispheres. 
 Over large tracts of the surface electric stimulation gives rise to no 
 movements at all. It has been supposed that the stimulation 
 of these parts gives rise to various psychical states, of such a nature 
 that they do not manifest themselves by any movements as do the 
 psychical states brought about by stimulation of the so-called 
 motor areas; and hence these tracts have been supposed to be 
 
Chap, vi.] THE BRAIN. 629 
 
 composed of so-called 'sensory' centres, a term even more objecti( in- 
 able than 'motor' centres. 
 
 Confining ourselves for the present to the areas, stimulation of 
 which produces movements, the ([uestion naturally presents itself. 
 Do events of importance take place in the grey matter itself 
 when stinudated, or is it that either by stimulation of the fibres of 
 the white matter, or by simple escape downwards of the current 
 employed as a stinndus, the parts below, such as the corpora striata, 
 are stimulated, and that the shaping of the movements according 
 to the locality operated on is effected in these lower parts and not 
 at the surface itself? On this point considerable controversy 
 h;vs taken place. On the one hand it seems clear that localization 
 of function, that is to say the occurrence of definite movements as 
 the result of stimulation of definite parts, may take place in the 
 regions of the brain below the cortex, since the appropriate move- 
 ments follow upon stimulation of the several cortical areas, when 
 the grey matter has been removed, or rendered functionally incapa- 
 ble by treatment with acids and the like, or separated functionally 
 from the white matter below by an incision parallel with the surface; 
 and definite movements have been even obtained by stimulation 
 of definite parts of the surface of the corpora striata. On the 
 other hand, we have adequate evidence that when movements in a 
 muscle or group of muscles are produced by stimulation of an area of 
 the surface of the cerebrum, the movements dififer in character, for 
 instance in the length of the latent period, the form of the muscle 
 curves, &c. according as the stimulus is applied to the intact cortex 
 or to the underlying white matter. Further the irritability of the 
 grey matter, falling and rising with the condition of the animal, 
 is much more variable than is that of the white matter ; and while 
 under certain circumstances stimulation of the grey matter is apt 
 to give rise to general epileptiform convulsions, stimulation of the 
 wliite matter rarely if ever produces such an effect. Without 
 discussing the matter any more fully we may say that the preponde- 
 rating evidence seems clearly in favour of the view that when the 
 grey matter is stimulated, some events of an important kind do 
 take place in the grey matter itself; in other words, we have 
 evidence of a localization of function in the cortex, inasmuch as 
 when a given area of the cortex is stimulated the movements in a 
 definite group of muscles which result are in part at least due to 
 changes taking place in the grey matter itself of that area. 
 
 If such is the case, if events of importance having an especial 
 connection vdth certain muscles result from the stimulation of 
 a given area, it is only reasonable to conclude that in actual life 
 more or less similar events, having similar relations to the muscles, 
 take place in the area from time to time. Further it seems also 
 reasonable to suppose that these events are of such a kind that the 
 area may be regarded as a ' point d'appui ' by which will and 
 intelligence are brought to bear on the muscles corresponding 
 
630 CEREBRAL CONVOLUTIONS. [Book iii. 
 
 to the area. We may very fairly imagine that when a clog wills to 
 extend the forelimh, the cerebral changes are of such a kind that 
 eventually processes are set up in the grey matter of the area for 
 extension of the forelimb similar to those which arise from stimu- 
 lation of the area. But if this be the case, then removal of the 
 area ought permanently to remove also, from the dominion of the 
 will, the muscles employed in extension of the limb; the chain of 
 events leading down from the inception of the voluntary effort to 
 the actual contraction of the appropriate muscles ought to be 
 broken in the link constituted by the events occurring in the 
 cortical area; the dog ought thereafter to be unable to extend his 
 forelimb by any direct effort of the will. The results of experiment 
 however shew that this is not the case. Immediately after the 
 operation by which the area is removed, more or less paralysis it is. 
 true may be observed in the corresponding muscles. But this soon 
 passes away, and complete power over the muscles may be re- 
 gained. The temporary paralysis seems to be a sort of inhibitory 
 effect due to the injury caused by the operation; and, though the 
 experiment confirms the view that some special connection exists 
 between the cortical area and the appropriate group of muscles, it 
 disproves the view that the area serves as a direct instrument 
 of the will. Nor can we take refuge in the idea that some other 
 area has taken up vicariously the duties of the lost area. This is 
 disproved by the observations of many inquirers which go to shew 
 that not only many different parts of the brain, but a very large 
 portion of the whole brain, may be removed without any clear and 
 definite paralysis of any group of muscles being occasioned. In the 
 experiments of Goltz, which we shall mention directly, the loss of a 
 large mass of the cerebral convolutions diminishes the movements 
 of the body inasmuch as it curtails the general action of the 
 intelligent vdll which brings them about, but does not withdraw 
 any particular sets of muscles from the influence of the so to speak 
 shortened will which is left to the animal. Indeed we have reason 
 to think that when in such operations directed solely to the 
 removal of the cerebral convolutions, paralysis of any particular 
 group of muscles occurs, mischief must unwittingly have been done 
 to other parts of the brain. 
 
 What then can we conclude as to the nature of the events 
 which take place in the several cortical areas ? To this question 
 unfortunately no clear answer can yet be given; for the results 
 of different inquirers are so far irreconcilably opposed. 
 
 On the one hand, one observer (Munk) states that the re- 
 moval of a certain area in the posterior lobes produces no other 
 effect whatever but blindness. He further states that removal 
 of small portions of the area leads to partial blindness, that is 
 to the formation so to speak of artificial blind spots in the field of 
 vision corresponding to the spots of cerebral cortex removed; so 
 that the retinal image may be conceived of as projected as it were 
 
Cbav. VI. 1 THE BR A IK 631 
 
 on to the visual area' of the cerebral convolutions. Munk huvS 
 moreover been led, for reasons which we cannot enter into here, to 
 believe that removal of part of this area (the circumferential part; 
 leads to what may be called 'absolute blindness,' i.e. the inability 
 to gain conscious sensations of the images falling on the retina, 
 whereas renun al of another part (the central part) leads to what may 
 be called 'psychical blindness,' i.e. the inability to form an intelligent 
 comprehension of the visual impressions received. The latter, he 
 maintains, may eventually be recovered from by processes which 
 may be crudely spoken of as the deposition of new visual experiences 
 in the visual area. We cannot discuss these results in detail here, 
 but we may add that Munk has similarly been led from his experi- 
 ments to conclude that the rest of the cerebral surface may be 
 parcelled out into auditory, olfactory, tactile, &c. areas, in fact that 
 all the sensory impulses which stream upon the living body are 
 projected as sensations on to the cerebral convolutions in definite 
 order, being there elaborated into perceptions and experiences, this 
 reception and elaboration being the special work of the cerebral 
 cortex. And one author (Schiff) has from the first maintained 
 that the cortical ' motor ' areas associated with movements in 
 particular regions of the body, have to do with the tactile 
 sensations arising in those regions, and that the movements 
 arising from stimulation of the areas are tactile reflex actions 
 started in the cortex of the brain instead of in the periphery 
 of the body. 
 
 On the other hand another observer (Goltz) maintains that the 
 whole of the posterior lobes may be removed without affecting 
 vision any more directly than does the removal of the anterior or 
 middle lobes. In fact this author in his latest, as in his earlier 
 researches, insists most strongly that he can no more obtain distinct 
 evidence of localization in reference to vision or other sensations 
 than in reference to movements. When in a dog the lesions are 
 sKght the recovery from imperfections of vision, of the other senses, 
 and of general sensibility which follow immediately on the opera- 
 tion, may be complete. When a larger portion of brain is removed, 
 whatever be the region of the hemisphere acted on, certain 
 peculiar imperfections of sight and other sensations, corresponding 
 to the psychical blindness spoken of just now as observed by Munk, 
 become striking, and may remain permanent. In the case of 
 vision the salient character of this imperfection is that though the 
 animal evidently can see, and uses his sight successfully in avoiding 
 obstacles and guiding his movements, yet what he sees does not 
 produce its usual effect on him; he obviously fails to recognize 
 many things, and has become indifferent to scenes which formerly 
 affected him strongly. Thus a dog from which portions of the 
 cerebral hemispheres have been removed, fails to recognize his 
 food by sight; when he is threatened vrith the whip, he is not 
 cowed; when the hand is held out for his paw he makes no 
 
632 CEREBRAL CONVOLUTIONS. [Book hi. 
 
 response ; and though before the operation he became violently 
 excited when the laboratory servant dressed in a fantastic garb was 
 presented to him, he remains after the operation perfectly in- 
 different to the same image. Another striking character of this 
 imperfection of vision is that recovery from it to a considerable 
 extent is, under certain circumstances, possible by means of 
 educational exercise; the dog, which at first could not recognize 
 his food by sight and was indifferent to the whip, learns after 
 a while to know the one and to respect the other. It would 
 be hazardous however to insist upon the view that in such a case 
 the failure was of distinctly psychical origin, due to the want 
 of intelligent power to fully appreciate the crude sensations. It 
 might be that the sensations were themselves imperfect, and 
 unable to give rise to sufficiently definite perceptions, all things 
 possibly appearing to the dog as if seen through a gauze with 
 all their colours washed out. With such an imperfect vision a dog 
 might readily fail to recognize meat by sight and might easily 
 regard with unconcern any figure however fantastically dressed. 
 
 According then to the views advocated by Goltz, barring some 
 possible difference in the extent to which the intellect or the 
 emotions are respectively affected according to the part of the 
 brain operated on, removal of the brain gives no evidence as to 
 any part of the mind (using that word in a wide sense) being 
 connected with any particular part of the cerebral cortex. 
 According to the views advocated by Munk, all the sensations 
 which form the basis of psychical activity, are very definitely 
 associated with distinct areas of the convolutions ; and nearly 
 all those, Ferrier and others, who have urged the doctrine of 
 localization of function in the cerebral cortex have been led to 
 entertain conceptions more or less similar to those of Munk. 
 The time is not yet ripe to decide dogmatically between these 
 conflicting views, though it appears to us that of the two, the 
 former one is the nearer the truth. All the more so since there 
 are some reasons for thinking that in these operations which appear 
 to be confined to the cortex and the white matter immediately 
 beneath this, damage is not unfrequently done to more central 
 parts of the brain, such as the corpora striata and optic thalami ; 
 and it is quite possible that where blindness becomes a prominent 
 symptom after these operations, the immediate cause of that blind- 
 ness is not in the cortex but in the optic thalami. 
 
 But if we accept Goltz's conclusions there still remains at least 
 an apparent contradiction between these and the conclusions 
 we reached just now concerning the results of stimulation of the 
 surface ; but this we must leave for further inquiries to clear up. 
 
 Before leaving the subject of the cerebral convolutions we wish 
 to call attention to certain remarkable results which have been 
 observed to follow upon stimulation of the cerebral cortex, under 
 various circumstances, more particularly in different stages of the 
 
CuAV. VI. ] 77//; BRAIN. 033 
 
 influonce of narcotic drugs such as morphia. In certain stages of 
 narcotism by morphia, the dose re(piir('{l varying' a(tc(jrding to the 
 indivi(hial, hut gonerally being hirgt^, tlie irritability of the cortex 
 is diniinishcd ; currrnts wliii'h jneviously readily produced contrac- 
 tions in the muscles corresponding to the area stimulated, now 
 produce little or none at all ; and indeed the cortex may be thus 
 brought to such a condition that even very strong currents produce 
 no movements at all. In such cases, movements may, at times at all 
 events, be brought about by removing the cortex and applying the 
 electrodes directly to the underlying white matter, thus shewing 
 that the morphia produces its effects, in part at least, by acting 
 directly on the cortex itself. From this we gain an additional ar- 
 gument in favour of the independent irritability of the cortex. 
 
 On the other hand in certain stages of the action of morphia 
 the irritability of the cortex is not diminished, but on the contrary 
 increased. It is well known that an animal imder the influence of 
 morphia frequently manifests an increase of reflex excitability, 
 being for instance remarkably sensitive, and readily responding to 
 the stimuli of sounds and noises ; and a similar exaltation has been 
 observed in reference to the influence of electric stimuli applied to 
 the cortical areas. At times this increased excitability may become 
 so developed that the application of even a moderate stimulus 
 leads to epileptiform convulsions lasting for some considerable 
 time. Not unfrcquently indeed, experiments of this kind have 
 to be suspended on account of the appearance of these convulsions. 
 When any particular 'motor area' is being stimulated the con- 
 \ailsive contractions generally appear first in the appropriate group 
 of muscles and thence spread first over the same side and then over 
 the other side of the body vmtil sometimes the whole frame is 
 convulsed. When the cortex is removed, and the electrodes are 
 applied directly to the subcortical white matter, these convulsions 
 are not nearly so readily produced, and when they appear are not 
 exactly of the same character, being generally limited to one side of 
 the body. It would thus appear that the con\mlsions, though carried 
 out by the nervous machinery of the lower parts of the brain and 
 more especially perhaps by the so-called 'convulsive centre' in the 
 medulla oblongata, originate and to a large extent are fashioned by 
 changes in the cerebral cortex ; and, though this is a matter into 
 which we must not go more fully here, pathological and clinical 
 observations similarly tend to shew that epilepsy itself, in certain 
 cases at all events, is the product of an abnormal action of the 
 cerebral convolutions. 
 
 From what has been said in previous sections, more particularly 
 in reference to the reflex actions of the spinal cord (p. 592) and co- 
 ordinating mechanisms (p. 619), the reader Avill be prepared for the 
 obsen-ation that the phenomena of these convulsions suggest the 
 idea that they arise not so much from a positive increase in the 
 explosive, discharging energy of the central nen'ous mechanisms as 
 
634 CEREBRAL CONVOLUTIONS. [Book in. . 
 
 from a withdrawal of certain normal restraining inhibitory influences. 
 We have already more than once insisted that almost any event in 
 the central nervous system is to be regarded not as the result of 
 the activity of some one isolated nervous machine, but as the out- 
 come of various, conflicting processes, some positive, tending to bring 
 out the event, others negative, offering a resistance or bringing 
 inhibitory influences to bear. And it would seem that this is even 
 more true perhaps of the cerebral convolutions than of any other 
 part of the nervous system. Such a view is at all events strongly 
 supported by some observations lately made by Heidenhain. If in 
 an animal under morphia, the contractions in a muscle resulting 
 from the stimulation of the appropriate ' motor area,' by a current 
 of known strength, be recorded, the sciatic nerve then divided or 
 torn or otherwise irritated, and the motor area again stimulated 
 with the same strength of current, the contractions vdll be much 
 less in height, and the latent period will be much longer. This of 
 course is nothing more than an instance of somewhat ordinary inhi- 
 bition. But, in certain stages of the influence of morphia, the fol- 
 lowing remarkable result makes its appearance. If a subminimal 
 stimulus be found, that is a current of such intensity that ap- 
 plied to a motor area it will produce no movement, but if 
 increased ever so slightly will give a feeble contraction of the 
 appropriate muscles, it may be observed that a slight stimulus, 
 such as gently stroking the skin over the muscles in question, 
 or indeed some other part of the body, will render the previous 
 subminimal stimulus effective, and so call forth a movement. 
 Thus if the area experimented on be that connected with the 
 lifting of the forepaw, and the subminimal stimulus be applied to 
 the area at intervals, after several ineffective applications, a gentle 
 stroke or two over the skin of the paw will lead to the paw being 
 lifted the next time the stimulus is applied to the area. On the 
 other hand, in certain other stages of the influence of morphia, the 
 convolutions and the rest of the nervous system are in such a 
 condition that the application of even a momentary stimulus to an 
 area leads to a long-continued tonic contraction of the appropriate 
 muscles. Under these circumstances, a gentle stimulus, such as 
 stroking the skin, or blowing on the face, applied immediately 
 after the application of the electric stimulus to the area, suddenly 
 cuts short the contraction, and brings the muscles at once to rest 
 and normal flaccidity. Thus according to the condition of the 
 central nervous system (and in these instances the effect appears 
 to be dependent largely though not wholly on the condition of the 
 cerebral cortex) the same kind of stimulus, and indeed we might 
 almost say the same stimulus, will lead now to exaltation, now to 
 inhibition of a nervous action. We must not dwell on these matters 
 any further, though we might point out the interesting even if 
 partial light which they throw on the phenomena known as Hyp- 
 notism. We have introduced them chiefly to emphasize the view 
 
Chap, vi.] Till': ISh'AIX. (J3r, 
 
 that i\\v nervous system is uot to be considered aa a collcctiim of 
 isolated org^ans each fulfilling its functions independently of its 
 fellow, hut as a large maehine, integrated into a whole, the consti- 
 tuent parts of which are in almost all its actions acting and react- 
 ing on each other in various ways. 80 much so does this aj)pcar to 
 be the case, that the secondary influences, often of the kind called 
 inhibitory, of outlying parts prove as important to the due function 
 of any |iarticular structure as its own more direct action. 
 
SEC. 4, THE FUNCTIONS OP OTHER PARTS OF THE 
 
 BRAIN. 
 
 If the views just expressed are true then it is clear that the 
 proper method to study the brain is to trace out a cerebral 
 operation along its chain of events rather than to seek to attach 
 readily definable functions to the cerebral anatomical components. 
 
 We may therefore be permitted to summarise very briefly what 
 can be fairly placed under this heading. 
 
 Corpora Striata and Optic Thalami. These two bodies, often 
 spoken of as 'the basal ganglia/ are undoubtedly the great means 
 of communication between the cerebral hemispheres on the one 
 hand and the crura cerebri on the other. Though some fibres appear 
 to pass from the crura by or through the ganglia to the cerebral 
 convolutions without being connected with the nerve-cells of those 
 ganglia, the great mass of the peduncular fibres are probably con- 
 nected with the superficial grey matter of the hemispheres in an 
 indirect manner only, the lower or anterior fibres (criista) passing 
 first into the corpora striata, and the upper or posterior fibres (teg- 
 mentum) into the optic thalami. This anatomical disposition would 
 lead us to suppose that these bodies have important functions in 
 mediating between the psychical operations of the cerebral convo- 
 lutions on the one hand, and the sensori-motor machinery of the 
 middle and hind brain on the other; and the separate courses taken 
 by the peduncular fibres would further lead us to expect that the 
 functions of the corpora striata differ fundamentally from those of 
 the optic thalami. 
 
 When in the human subject a lesion occurs involving both 
 these bodies, on one side of the brain, the result is a loss of sensa- 
 tion in, and voluntary power over, the opposite side of the body 
 and face, a so-called hemiplegia, which may be absohitely complete 
 without any impairment whatever of the intellectual faculties. 
 The will and the psychical power to receive impressions are present 
 in their entirety, but neither efferent nor afferent impulses can 
 
CiiAP. VI.] THE /ih'A/.V. G37 
 
 mako thoir way to or IVoui tlio puriphcial organs and tlio cerebral 
 convolutions. The injury to the basal ganglia blocks the way. In 
 the great majority of cases, the ana'sthesia (or loss of sensation) 
 and akinesia (or loss of movement) arc absolutely confined to the 
 opjiosite side of the body ; and the cases in which a lesion of the 
 basal ganglia of one side of the brain affects the same side of the 
 body or both sides, must be regarded as exceptional, and explicable 
 as the results of the action of one side of the brain on the other 
 side either of the brain or of some region of the cerebro-spinal axis. 
 The results of experiments on animals agree entirely with the 
 general experience of pathologists, that lesions of the corpora 
 striata and optic thalami produce their effect on the opposite side 
 of the body. Whatever be the view taken concerning the de- 
 cussations of sensory and motor impulses in the spinal cord (see 
 p. 601), it must be admitted that both kinds of impulses cross 
 over completely somewhere during their transmission to and from 
 the basal ganglia and the peripheral organs. 
 
 When however we have admitted that these bodies act, as it 
 were, the part of middlemen between the cerebral convolutions 
 and the rest of the brain, we have gone almost as far as facts will 
 support us. We are not at present in a position to state dogmati- 
 cally what is the nature of the mediation which either body 
 respectively effects. A very tempting hypothesis is one which 
 suggests that the corpora striata are concerned in the downward 
 transmission and elaboration of efferent volitional impulses, and 
 the optic thalami in a similar upward transmission and elaboration 
 of afferent sensory impulses ; and there are many facts which may 
 be urged in favour of this view. 
 
 The evidence in this matter afforded by pathology is perhaps the 
 most consistent, but not wholly so. A number of cases may be cited 
 to shew not only that lesions of a corpus striatum may be accom- 
 panied by akinesia without aucesthesia, but that lesions of an optic 
 thalamus may cause anaesthesia without actual akinesia, that is 
 without any further interference with the execution of voluntary 
 movements than is occasioned by the loss of the coordinating 
 sensations. Of these two classes of cases, the latter is the more 
 valuable, since all clinical experience shews that any lesion more 
 readily interferes with volitional movements than with the reception 
 of sensory impressions. Convulsions are not common when the lesions 
 are confined to these bodies; but when witnessed they can generally 
 be refeiTed to the corpora striata rather than to the optic thalami ; 
 like the paralysis, the convulsions are generally limited to the 
 opposite side of the body, though feeble movements may occasion- 
 ally be seen on the same side as well. But it would be dangerous 
 to trust too much to evidence of this kind ; for numerous cases 
 have been recorded where an injury apparently confined to one 
 corpus striatum has had as part of its results anaesthesia of the 
 opposite side of the body ; and others where disease apparently 
 
638 CORPORA QUADRIGEMINA. [Book hi. 
 
 confined to an optic thalamus has caused loss of movement as well 
 as of sensation. 
 
 The evidence obtained by means of experiments on animals is 
 still more discordant. Some observers have found that stimulation, 
 either mechanical or electrical, of the corpora striata gives rise to 
 convulsive movements, while stimulation of the optic thalami does 
 not ; and have seen in these results a confirmation of the view we 
 are discussing. Such a confirmation is, at best, a feeble one, and 
 moreover is not supported by the results of all observers. Some 
 observers again have found that removal or destruction, by the 
 injection of corrosive substances, of both nuclei lenticulares (the 
 extra-ventricular portions of the corpora striata) leads to a sup- 
 pression of voluntary movements almost as complete as if both 
 hemispheres were removed, whereas after removal or destruction of 
 both nuclei caudati (intra- ventricular portions of the same bodies) 
 voluntary movements still persist ; and it has been affirmed that 
 the removal or destruction of the optic thalami may with care be 
 effected without the animal appearing any the worse. In the 
 absence of more exact knowledge it is useless to attempt to form 
 any clear judgment ; and the view we stated above as to the motor 
 functions of the corpora striata and sensory functions of the optic 
 thalami may be allowed to stand as neither definitely disproved 
 nor satisfactorily proved, and as in any case affording an inadequate 
 expression of the part played by these masses in the general work 
 of the brain. Two points we may venture to call attention to, 
 which as far as they go may be used as arguments in support of 
 the above view. Almost all observers agree that after injuries 
 to the corpora striata, more particularly after one-sided or after 
 partial injuries, and especially after injuries of the nuclei caudati, 
 forced movements such as those of which we spoke on p. 620 are 
 very apt to make their appearance. With regard to the optic 
 thalami on the other hand there is an agreement both of experi- 
 mental and pathological evidence in favour of the view (which as 
 the very name of the bodies shews is an old one) that these 
 structures are in some way or other concerned in vision. Where 
 the optic thalami are directly involved in an injury to or disease of 
 the brain, blindness or at least some imperfection of vision is a 
 frequent result ; and there are reasons for thinking that in some at 
 all events of the cases where blindness has resulted from removal 
 of the cerebral cortex of the hinder part of the hemispheres, the 
 optic thalami have been either directly or secondarily affected. 
 
 Corpora Quadrigemina. We have already seen that the centre 
 of coordination for the movements of the eyeballs (p. 546), and that 
 for the contraction of the pupil (p. 501), lie in the neighbourhood of 
 the upper or anterior pair of the corpora quadrigemina. These 
 two centres are associated together in such a way that when the 
 eyeballs are voluntarily directed inwards and downwards, as for near 
 
Chap, vi.] THK BRA IX. 639 
 
 vision, the pupils juc at the sunie time contracted ; und when the 
 eyeballs are diret-ted upwards, and return to parallelism, the pupils 
 are dilated to a corresponding extent ; when both eyeballs are 
 moved to^^othrr sideways the pupils remain unchanged. We have 
 Seen (p. 540) that the various movements of the eyeballs may be 
 brought about by direct stimulation of particular parts of the 
 anterior corpora quadrigemina, and are then also accompanied by 
 the appropriate changes in the pupils. The association therefore 
 of the movements of the pupil and of the ocular muscles is not 
 simply psychical in nature but is dependent on the close connection 
 of their respective centres. From the fact of the movements of 
 the eyeball and pupil being so readily and variously excited by 
 stimulation of the anterior corpora quadrigemina it has been 
 inferred that the centres for these movements lie in those bodies : 
 it would appear however that what may be called the real or im- 
 mediate centres of these movements lie beneath the corpora quad- 
 rigemina, in the front part of the floor of the aqueduct of Sylvius, 
 and therefore are affected in an indirect manner only when the 
 corpora quadrigemina are stimulated. 
 
 It w^as long ago observed that unilateral extirpation of the 
 corpora quadrigemina in mammals or of the optic lobes in birds 
 produced blindness in the opposite eye ; and the same result has 
 been gained by many subsequent observers. We have seen more- 
 over that both frogs, birds, and mammals continue to receive and 
 within limits to react upon visual impressions after the total 
 removal of the cerebral hemispheres. From these facts we infer 
 that visual sensory impulses become transformed into visual sensa- 
 tions in the corpora quadrigemina : or, in other words, that these 
 nervous structures are centres of sight. But they are so in a limited 
 sense only. We have seen that destruction or injury of the cerebral 
 hemispheres profoundly affects vision ; even admitting that in such 
 cases the results may be in part at least due to concomitant failures 
 in the optic thalami, we may still venture to say that in the absence 
 of the cerebral convolutions, a crude vision, devoid of distinct visual 
 perceptions, is probably all that is possible. The processes consti- 
 tuting distinct and perfect vision, in fact, begin in the retina, are 
 partially elaborated in the corpora quadrigemina and further de- 
 veloped in the optic thalami, but do not become perfected until the 
 cerebral convolutions have been called into operation. Anatomical 
 considerations lead us to suppose that the anterior pair of the 
 corpora quadrigemina are alone connected with the optic tract, 
 and so with the external corpus geniculatum and optic thalamus. 
 Hence we may infer that it is the anterior pair alone which are 
 thus concerned in vision and that the posterior paii' have some 
 other function. 
 
 In those animals (ar. (/?•. rabbits) in which unilateral destruction 
 of the corpora quadrigemina entails blindness of the opposite eye, 
 and yet does not atfect at all the visual sensory impulses originating 
 
640 CEREBELLUM. [Book hi. 
 
 in the eye of the same side, it is obvious that complete decussation 
 of the sensory impulses must take place before the centre is 
 reached. The question however whether the decussation of fibres 
 (and consequently of impulses) in the optic chiasma is complete or 
 incomplete, whether the optic tract of one side is the continuation 
 of the fibres in the optic nerve of the opposite side exclusively or 
 whether it is composed of representatives of the optic nerves of both 
 sides, is one which has been much debated, both from an anatomical 
 and a physiological standpoint. In the case of mammals the evidence 
 goes to shew that in some kinds of animals (rabbits) the decussation 
 is complete, but in others (dogs) more or less incomplete. In man 
 a peculiar affection of vision, which may be spoken of under the 
 general name of hemiopia, is a frequent symptom of diseases of the 
 brain. In this affection portions of the field of vision are wanting ; 
 thus a patient sometimes can see nothing in the right half or the 
 upper half of the fields of vision of both eyes ; looking at a man or a 
 house he can only see half the object, the left half or the lower half 
 as the case may be. Hemiopia of both eyes has been observed in 
 cases in which disease was apparently limited to one side of the 
 brain; and these cases added to other evidence lead to the con- 
 clusion that in man the decussation is incomplete. 
 
 Many observers have noticed that injury or removal of the 
 corpora quadrigemina on one side frequently caused forced move- 
 ments, and that removal of the whole mass led to great want of co- 
 ordination. These results are quite in harmony with the fact men- 
 tioned above (p. 610) concerning the coordinating functions of the 
 optic lobes in frogs. But at present we have no exact knowledge 
 concerning the nature of the coordination, and what relations are 
 borne in this respect by the corpora quadrigemina to the cere- 
 bellum, crura cerebri, and pons Varolii. 
 
 Various observers have witnessed as the result of stimulation of 
 the corpora quadrigemina movements of the several parts of the 
 alimentary canal, and of the urinary bladder, changes in blood- 
 pressure, and alterations in the working of the respiratory mecha- 
 nism, indicating that these bodies have a special connection with 
 the centres (in the medulla oblongata and spinal cord respectively) 
 concerned in carrying on these movements. 
 
 Cerebellum. We have already referred to the cerebellum as 
 being probably concerned in the coordination of movements. It 
 was long ago observed that when a small portion of the cerebellum 
 was removed from a pigeon, the animal's gait became unsteady ; 
 when larger portions were taken away its movements became much 
 more disorderly, and when the whole of the organ was removed an 
 almost total loss of coordination supervened. When the portion 
 removed was small, the disorderly movements which at first ap- 
 peared eventually vanished, but when a large portion was re- 
 moved the loss of coordination became permanent. Subsequently 
 
Chap, vi.] 77/ A' liRAlN. 641 
 
 observers have obtained similar results in other animals ; and it has 
 in general been found that lateral or unsymuietrical lesions and 
 incisions produce a greater ed'ect than those which are mc-dian or 
 symmetrical. Section of the middle peduncle on one side almost 
 invariably gives rise to a iorced movement, the animal rolling 
 rapidly round its own longitudinal axis; the rotation is generally 
 though not always towards the side operated on; and is accompanied 
 by nystagmus, i.e. by peculiar rolling movements of the eyes sug- 
 gestive of vertigo ; fret^uently one eye is moved in one direction, ex. 
 gr. inwards and downwards, and the other in a different or opposite 
 direction, ex. gr. outwards and upwards. As we have already said 
 the permanent effects which follow upon injury to the semicircular 
 canals, have been attributed by some to secondary mischief being 
 set up in the cerebellum. The clinical evidence is discordant, for 
 though unsteadiness of gait has been frequently mtnessed in cases 
 of cerebellar disease, many histories have been recorded in which 
 extensive disease, amounting at times to almost complete destruc- 
 tion, of the cerebellum has existed without any obvious disturbance 
 of the coordination of movements. Still the experimental evidence 
 is so strong, that we must consider the cerebellum as an important 
 organ of coordination, though we are unable at present to define its 
 functions more exactly. 
 
 In this connection we may observe that the history of the 
 developement of the spinal cord (see p. 600) tends to connect a 
 definite portion of the lateral columns of the sj)inal cord with the 
 cerebellum ; but the meaning of this connection is obscure. 
 
 Attempts have been made to connect the cerebellum with the 
 sexual functions ; but there is no satisfactory evidence of any such 
 relation. As we shall see later on, the nervous centres connected 
 with the sexual and generative organs are seated, in the case of 
 dogs at least and probabl}' of all animals, in the lumbar spinal cord; 
 and all or nearly all sexual phenomena may be witnessed in animals, 
 in which the lumbar spinal cord has been isolated by section 
 from the rest of the cerebro-spinal system. Galvanic stimulation 
 of the cerebellum produces no change in the generative organs, and 
 when erection of the penis is caused by emotions, the tract con- 
 necting the cerebral convolutions with the erection-centre in the 
 spinal cord must be supposed to pass straight along the crura cerebri 
 and medulla, for it has been observed that stimulation of these parts 
 in the dog will produce erection. 
 
 Crura Cerebri and Pons Varolii. Though from the grey 
 matter abundant in both these organs we may infer that they 
 possess important functions, we hardly know more concerning 
 them than that the former serve as the great means of communi- 
 cation between the spinal cord and the higher parts of the brain, 
 and that both are intimately connected with the coordination of 
 movements, since either forced or disorderly movements are the 
 
 F. 41 
 
642 MEDULLA OBLONGATA. [Book hi. 
 
 frequent results of section of either of them ; and as we have seen, 
 the possession of these parts, in the absence of the cerebral 
 hemispheres, and even of the corpora striata and optic thalami, is 
 sufficient to cai'ry out the most complex bodily movements. 
 
 Since the paralysis of the face seen in cases of hemiplegia from 
 disease of one corpus striatum is on the same side as that of the 
 limbs, it follows that the impulses proceeding along the cranial 
 nerves cross over like those of the spinal nerves ; and when the 
 nucleus of origin of such a nerve as the facial is stimulated on one 
 side, the movements which result are on the opposite side. Hence 
 when paralysis of the face occurs on the opposite side to that of the 
 body, it may be inferred that the injury or disease has affected 
 the cranial nerve (or nerves) in a part of its course before 
 decussation has taken place ; and pathological observations support 
 this view, unilateral disease or injury of the pons Varolii not 
 unfrequently involving the facial nerve of the same side in its 
 comparatively superficial couise before decussation has taken place, 
 and so causing paralysis of the muscles of the same side of the 
 face as the disease, and the opposite side to the paralysis of the 
 limbs. It is probable that the decussation which we have seen 
 to take place partly in the spinal cord, is gradually completed as 
 the impulses pass through the medulla and pons Varolii. There 
 does not appear to be adequate support for the view of those who 
 maintain that volitional impulses cross suddenly and completely at 
 the decussation of the pyramids. 
 
 Medulla Oblongata. We have so often spoken of this link 
 between the brain and the spinal cord, that it is hardly necessary 
 here to do more than recall the fact, that the majority of the 
 ' centres ' for various organic functions are situated in it. 
 
 These we may briefly recapitulate as follows : The respiratory 
 centre with its neighbouring convulsive centre. The vaso-motor 
 centre. The cardio-inhibitory centre. The diabetic centre, or 
 centre for the production of artificial diabetes. The centre for 
 deglutition. The centre for the movements of the oesophagus 
 and stomach, with its allied vomiting centre. The centre for 
 reflex excitation of the secretion of the saliva, with which may 
 be associated the centre through which the vagus influences the 
 secretion of pancreatic juice, and possibly of the other digestive 
 juices. 
 
 In the frog, as we have urged, p. 610, the medulla is undoubt- 
 edly largely concerned in the coordination of movements, and it is 
 exceedingly probable that in the mammal also a considerable 
 portion of work of this kind falls to its lot. 
 
SEC. 5. ON THE RAPIDITY r)F CEREBRAL OPERATIONS. 
 
 We have already seen (p. 593) that a considerable time is 
 taken up in a purely reflex act, such as that of winking, though 
 this is perhaps the most rapid form of reflex movement. _ When 
 the movement which is executed in response to a stimulus involves 
 mental operations a still longer time is needed ; and the interval 
 between the application of the stimulus and the commencement of 
 the muscular contraction varies according to the nature of the 
 mental labour involved. 
 
 The simplest case is that in which a person makes a signal 
 immediately that he perceives a stimulus, ex. gr. closes or opens a 
 galvanic circuit the moment that he feels an induction shock 
 applied to the skin, or sees a flash of light, or hears a sound. By 
 arrangements similar to those employed in measuring the velocity 
 of nervous impulses, the moment of the application of the stimulus 
 and the moment of the making of the signal are both recorded 
 on the same travelling surface, and the interval between them 
 is carefully measured. This interval, which has been called ' the 
 reaction period,' consists of three portions: (1) the passage of 
 afferent impulses from the peripheral sensory organ to the central 
 nervous system, including the possible latent period of the gene- 
 ration of the impulses in the sensory organ ; (2) the transformation, 
 by the operations of the central nervous system, of the afferent into 
 efferent impulses ; and (3) the passage of the efferent impulses to 
 the muscles, including the latent period of the muscular contractions. 
 If the time required for the first and third of these events be 
 deducted from the whole, the ' reduced reaction period,' as it may be 
 called, gives the time taken up exclusively by the operations going 
 on in the central nen'ous system. 
 
 4i— 2 
 
644 RAPIDITY OF CEREBBAL OPERATION'S. [Book iii. 
 
 The reaction period, both reduced and unreduced, varies 
 according to the nature and disposition of the peripheral organs 
 stimulated. The reaction period of vision has long been known 
 to astronomers. It was early found that when two observers 
 were watching the appearance of the same star, a considerable 
 discrepancy existed between their respective reaction periods ; and 
 that the difference, forming the basis of the so-called 'personal 
 equation,' varied from time to time according to the personal 
 conditions of the observers. 
 
 In general it may be said that tactile sensations produced by 
 the stimulus of an electric shock applied to the skin, are followed 
 by a shorter reaction period than are auditory sensations, while the 
 period of these is in turn shorter than that of visual sensations 
 produced by luminous objects; on the other hand, the shortest 
 period of all is that of visual sensations produced by direct electrical 
 stimulation of the retina. Roughly speaking we may say that the 
 reaction period or physiological time is for feeling |th, for hearing 
 ith, and for sight ith of a second. But even with the same 
 stimulus, the reaction period will vary according to circumstances, 
 such as the time of year, weather, &c., and according to the 
 condition of the individual, previous practice, fatigue, and the like. 
 
 The calculations involved in ' reducing ' the reaction period are 
 obviously open to much error; in general the reduced reaction 
 period may be said to be less than yLth of a second, that is to say 
 an intelligent person takes about this time to perceive and to will. 
 
 The reaction period just given belongs to cases where a 
 single stimulus is used, and all that the person experimented 
 on has to do is to perceive the stimulus, and to make an 
 effort in accordance. If, however, the stimulus, instead of being 
 applied to a part of the body determined by previous arrange- 
 ment, as for instance to the left foot, were applied either 
 to the left or the right foot, without the person being told 
 which it was to be, and it was arranged that he should make 
 a signal when the left foot, but not when the right foot was 
 stimulated, additional mental exertions would be necessary; and 
 it is found that in such a case the reaction period is considerably 
 prolonged. The difference between a simple reaction period, and 
 one in which a mental decision has to be carried out before the 
 voluntary effort to make the signal is initiated, gives the time 
 required for a person to ' make up his mind ' in accordance with 
 the nature of the sensation which he receives ; this is found to be, 
 roughly speaking, from 4 to -^ of a second. 
 
SEC. 6. THE CIROULATION IN THE BRAIN. 
 
 The supply of blood to the brain seems at first sight not to 
 cori'espond to the importance of this the chief organ of the body. 
 In the rabbit it would appear that not much more than one per 
 cent, of the total quantity of the blood of the body is present at 
 any one time in the brain, a quantity distinctly less than that 
 which is found in the kidneys ; and of the total weight of the 
 organ, the weight of blood in the brain at any one time amounts to 
 about five per cent., being about the same as in the muscles, whereas 
 in the kidney it amounts to nearly twelve per cent, and in the liver 
 to as much as nearly thii'ty per cent. Making every allowance for 
 the relative small size and functional importance of the rabbit's brain, 
 the blood-supply of even the human brain must still be small. In 
 other words, the metabolism of the brain-substance is of importance, 
 not so much on account of its quantity as of its special qualities. 
 
 We have seen (p. 366) in speaking of respiration that when 
 the brain is exposed the quantity of blood in the brain and so the 
 total volume of the brain rises and falls, in a conspicuous manner, 
 with the respiratory movements. And observations by the plethys- 
 mographic method, a portion of the skull being removed for the 
 purpose or advantage being taken of a natural deficiency, have shewn 
 the existence of more rapidly repeated movements, of a swelling 
 and shrinking synchronous with and due to the beats of the heart, 
 as well as of variations, larger and slower than the respiratory 
 undulations, and brought about by various causes such as the 
 position of the head in relation to the trunk, movements of the 
 limbs, modifications of the respiratory movements, and apparently 
 phases of activity of the brain itself, as in waking and sleeping 
 
646 THE CIRCULATION IN THE BRAIN. [Book hi. 
 
 In certain respects tlie circulation in the brain is peculiar. 
 The skull forms a fairly complete inextensible envelope, presenting 
 a strong contrast to the extensible elastic capsules which invest 
 such organs as the spleen and kidney. As a consequence of this, 
 when at any time an extra quantity of blood is sent from the 
 heart to the brain, room must be made for it by the increased exit 
 of the fluids already present. For any pressure on the brain- 
 substance beyond a certain limit is injurious to its welfare and 
 activity, as is seen in certain maladies, where blood passing by 
 rupture of the blood-vessels out of its normal channels remains 
 effused on the surface of the brain or elsewhere, and thus taking 
 up the room of the proper brain-substance leads, by ' compression ' 
 as it is called, to paralysis, loss of consciousness, or death. Within 
 the limits of the normal cerebral circulation, the characteristic venous 
 sinuses serve to regulate the internal pressure ; they form temporary 
 reservoirs from which a comparatively large quantity of blood can 
 be rapidly discharged from the cranium, the flow from the sinuses 
 being greatly assisted by the inspiratory movements of the chest. 
 
 The arterial supply of the brain as a whole is undoubtedly 
 regulated by vaso-motor nerves, and in all probability the special 
 distribution of blood to the various parts of the brain is determined 
 by the same agents. When the head is suddenly shifted from the 
 erect to a hanging position, there must be a tendency for the blood 
 to accumulate in the cranial cavity, and conversely when the head 
 is suddenly shifted from a hanging to an erect position, there must 
 be a tendency for the supply of blood within the cranium to be for 
 a while less than normal. Either change of position, and especially 
 perhaps the latter, would thus lead to cerebral disturbances 
 which would in ourselves be revealed by affections of our con- 
 sciousness. That a perfectly healthy, and especially young organism 
 whose vaso-motor mechanisms are at once effective and delicately 
 responsive, can pass swiftly from one position of the head to the 
 other without inconvenience, whereas those in whom the vaso- 
 motor mechanisms by age or otherwise have become imperfect are 
 giddy when they attempt such rapid changes, is in itself adequate 
 evidence of the importance of the vaso-motor arrangements of the 
 brain. But our information concerning this matter, is at present of 
 a very vague and general character. As yet we have no detailed 
 knowledge, and are especially ignorant as to how far special 
 parts of the brain are supplied with independent vaso-motor 
 mechanisms. 
 
 Many writers have insisted on the mechanical importance of 
 the cerebro-spinal fluid. By the foramen of Majendie at the apex 
 of the roof of the fourth ventricle, the fluid within the various 
 ventricular cavities of the brain is continuous with the fluid in the 
 subarachnoid labyrinth of the spinal cord. And it has been argued 
 that when an extra quantity of blood is driven into the skull, the 
 transference of a corresponding quantity of cerebro-spinal fluid 
 
Chap, vi.] THE BRAIN. 647 
 
 through the foramen of Majendie, from the cranium into the spinal 
 cniial, the walls of which arc less rigidly complete, prevents any 
 injurious intracranial compression. Experimental evidence h(jw- 
 ever, as far as it goes, does not lend any very great support to this 
 view ; anel though removal of the fluid by aspiration is said to lead 
 to hemorrhage from the pia mater and to various nervous disorders, 
 the value of the cerebro-spinal fluid depends in all probability more 
 on its physiological properties as lymph, than on its mechanical 
 properties as a mere fluid. 
 
SEC. 7. THE CRANIAL NERVES. 
 
 Though we have incidentally dwelt on the functions of all these 
 nerves, it may be as well to recapitulate them in a tabular form. 
 
 1. Olfactory. Nerve of smell. 
 
 2. Optic. Nerve of sight. 
 
 3. Oculo-motor. Motor nerve to the levator palpebrae superi- 
 oris and all the muscles of the eye, except the obliquus superior 
 and the rectus externus. Efferent nerve for the contraction of the 
 pupil and for the muscles of accommodation. Hence when the 
 nerve is divided or otherwise paralysed the upper eyelid falls 
 (ptosis) ; the eye, which is turned outwards, is capable of partial 
 movements only, viz. such as can be produced by the rectus 
 externus and obliquus superior ; when the head is moved, the eye 
 moves with it, the inferior oblique not being able to execute the 
 usual compensating movements of the eyeball ; the pupil is 
 dilated, and the eye cannot accommodate for near distances. The 
 root of the nerve shews recurrent sensibility, due to fibres from 
 the fifth, but is otherwise a purely motor nerve. 
 
 4. Trochlear or Pathetic. Motor nerve to the obliquus 
 superior. When the nerve is paralysed, no marked difference is 
 observed in the position of the eye, but the patient sees double 
 when he attempts to look straight forward or towards the paralysed 
 side ; the images however coalesce when he turns his head to the 
 sound side. Vfhen the head is moved from side to side the eye 
 moves with it, the usual compensating movement of the eye which 
 accompanies the movements of the head failing in consequence of 
 the superior oblique not acting. It is a purely motor nerve, but 
 receives recurrent fibres from the fifth. 
 
Chap. VI.] THE BRAIN. Gt9 
 
 5. Trigeminus. A mixed efferent and afferent nerve, with 
 distinct motor and sensory roots, the latter bearing the gan<^lion of 
 Gasser. 
 
 Efferent Fibres. Motor fibres to the muscles of mastication, 
 temporal, masseter, two pterygoids (mylo-hyoid, anterior belly of 
 digastric), to the tensor palati, and tensor tympani ; vaso-motor 
 fibres to various parts of the head and face ; secretory fibres to the 
 lachrymal gland, and according to some authors to the parotid and 
 submaxillary glands by fibres joining the facial. Trophic (?) 
 fibres to eye, nose, and other parts of the face. Efferent fibres for 
 the dilatitm of the pupil. 
 
 Aftereut Fibres. General nerve of sensation of the skin of 
 head and face, and of the mucous membrane of the mouth, except 
 the back part of the tongue, the posterior pillars of the fauces, and 
 a large part of the pharynx, these parts being supplied by the 
 glossopharyngeal and vagus ; the back of the head is chiefly 
 supplied by branches from the cervical nerves, and the external 
 meatus and concha are supplied chiefly by the auricular branch of 
 the vagus. Nerve of special sense of taste for the front part of 
 the tongue. 
 
 6. Abducens. Motor nerve to the rectus extemus. When 
 the nerve is divided or otherwise paralysed, the eye is turned 
 inwards. It probably receives recurrent sensory fibres from the fifth. 
 It is also joined by fibres coming from the cervical sympathetic; 
 when this latter nerve is divided in the neck, the action of the 
 muscle is said to be weakened. 
 
 7. Facial. Motor nerve to the muscles of the face; hence 
 nerve of expression. Supplies also styloh^^oid, posterior belly of 
 the digastric, buccinator, stapedius, muscles of the external ear, 
 platysma, some muscles of the palate, viz. the levator palati and 
 probably others. Secretory nerve of submaxillar}'- and parotid 
 gland. Receives afferent, possibly efferent, fibres from trigeminus 
 and also from vagus. It is said b}^ some to contain vaso-motor 
 fibres for the tongue and side of the face. The effects of paralysis 
 of the facial, from the inability of the orbicularis to close the eye, 
 the dra^ving of the face to the sound side, and the smoothness of 
 the paralysed side, are very striking. 
 
 8. Auditory Nerve. Special nerve of hearing; afferent ner\'e 
 for impulses other than auditory proceeding from the semi-circular 
 canals. 
 
 9. Glosso-pharyngeal. Motor nerve for levator palati, azygos 
 uvulae, stylo-pharyngeus, constrictor faucium medius; the motor 
 functions of this nerve have been disputed. Special nerve of taste 
 for the back of the tongue. General nerve of sensation for the 
 root of the tongue, the soft palate, the pharynx (being hei-e 
 associated with the vagus), the Eustachian tube and the tympanum. 
 
650 GRANT AL NERVES. [Book hi. 
 
 10. Pneumogastric. Vagus. 
 
 Efferent Fibres. Motor nerve for the muscles of the pharynx, 
 for the movements of the oesophagus, of the stomach, of the 
 intestines, for the muscles of the larynx, possibly for the plain 
 muscular fibres of the trachea and bronchial divisions. Vaso- 
 motor fibres for lungs. Inhibitory nerve of the heart. Trophic 
 fibres for lungs and heart. 
 
 Afferent Fibres. Sensory nerve of the respiratory passages, 
 and of the pharynx, oesophagus and stomach. Afferent nerve, 
 augmenting and inhibiting, of the respiratory centre, afferent 
 inhibitory nerve (depressor branch) of the medullary vaso-motor 
 centre, afferent nerve producing salivary secretion, inhibiting 
 pancreatic secretion. It is stated that in the rabbit the vagus may 
 be easily dissected into two strands, an outer one containing the 
 afferent, and an inner one containing the efferent fibres. 
 
 11. Spinal accessory. Motor nerve to the sterno-mastoid and 
 trapezius muscles. It receives recurrent sensory fibres from the 
 cervical nerves. Part of the spinal accessory blends with the 
 pneumogastric, and the efferent effects (such as the movements of 
 the larynx, pharynx, &c., and cardiac inhibition) of the united 
 trunk seem to be largely due to the spinal accessory fibres con- 
 tained in them. It is stated however that division of the spinal 
 accessory before it joins the pneumogastric, does not entirely do 
 away with either swallowing or the movements of the larynx. In 
 the movements of the oesophagus and stomach, brought about by 
 the vagus acting as an efferent nerve, the accessory fibres seem to 
 have no share. The cardiac inhibitory fibres seem to be distinctly 
 of accessory origin. 
 
 12. Hypoglossal. Motor nerve for the muscles of the tongue, 
 and for all the muscles connected Math the hyoid bone except the 
 digastric, stylo-hyoid, mylo-hyoid, and middle constrictor of the 
 pharynx ; it also supplies the sterno-thyroid. It receives sensory 
 fibres from the fifth and vagus, and is also connected with the 
 three upper cervical nerves as well as with the sympathetic. 
 
CHAPTER Vri. 
 
 SPECIAL MUSCULAR MECHANISMS. 
 
 SEC. 1. THE VOICE. 
 
 A BLAST of air, driven by a more or less prolonged expiratory 
 movement, throws into vibrations two elastic membranes — the 
 chordce vocales. These impart their vibrations to the column of 
 air above them, and so give rise to the sound which we call the 
 voice. Since the sound is generated in the vocal cords, we may 
 speak of them and of those parts of the larynx which decidedly 
 atfect their condition as constituting the essential vocal apparatus ; 
 while the chamber above the vocal cords, comprising the ventricles 
 of the larynx with the false vocal cords, the pharynx and the 
 ca\-ity of the mouth, the latter varying much in form, constitute a 
 subsidiary apparatus of the nature of a resonance-tube, modifying 
 the sound originating in the vocal cords. In the voice, as in other 
 sounds, we distinguish : (1) Loudness. This depends on the 
 strength of the expiratory blast. (2) Pitch. This depends on the 
 length and tension of the vocal cords. Their length may be 
 regarded as constant, or varying only with age. It consequently 
 determines the range only of the voice, and not the particular note 
 given out at any one time. The shrill voice of the child is 
 determined by the shortness of the cords in infancy, and the 
 voices of a soprano, tenor and baritone are all dependent on the 
 respective lengths of their vocal cords. Their tension is on the 
 contrary variable; and the chief problems connected with the 
 voice refer to variations in the tension of the vocal cords. {'6) 
 
652 
 
 THE VOICE. 
 
 [Book hi. 
 
 Quality. This depends on the number and character of the over- 
 tones accompanying any fundamental note sounded, and is deter- 
 mined by a variety of circumstances, chief among which is the 
 physical quality of the cords. 
 
 The vocal cords, attached in front to the thyroid cartilage, end 
 behind in the processus vocales of the arytenoid cartilages. Hence 
 a distinction has been drawn between the rima vocalis, i.e. the 
 
 Fig. 88. The Laeynx a.s seen by means of the Laryngoscope in difeebent 
 ' CONDITIONS OF THE Glottis. (From Quain's Anatomy after Czermak.) 
 
 A while singing a high note ; B in quiet breathing ; G during a deep inspiration. 
 The corresponding diagrammatic figures A', B', C, illustrate the changes in 
 position of the arytenoid cartilages, and the form of the rima vocalis and 
 rima respiratoria in the above three conditions. 
 
 I the base of the tongue ; e the upper free part of the epiglottis ; e' the 
 tubercle or cushion of the epiglottis ; ph. part of the anterior wall of the 
 pharynx behind the larynx; iv swelling in the aryteno-epiglottidean fold 
 caused by the cartilage of Wrisberg ; s swelhng caused by the cartilage of 
 Santorini ; a the summit of the arytenoid cartilage ; cv the true vocal cords ; 
 CVS the false vocal cords ; tr the trachea with its rings ; h the two bronchi at 
 their commencement. 
 
 opening bounded laterally by the vocal cords, and the rima 
 respiratoria, or space between the arytenoid cartilages behind the 
 processus vocales; these names however are not free from ob- 
 jections. In quiet breathing (Fig. 88 B) the two form together a 
 V-shaped space, which, as we have seen (p. 325), m deep mspiration 
 
Chap, vii.] SPECIAL MUSCULAR MECHANISMS. G53 
 
 is widened into a rhomboidal opening by the divergence of the 
 processus vocalcs (Fig. 88 C). When a note is about to be uttered, 
 the vocal cords arc by the approximation of the processus vocales 
 brought into a position parallel to each other, and the whole rima 
 is narrowed (Fig. 88 A). By their parallelism and by the narrow- 
 ness of the interval between them the cords are rendered more 
 susceptible of being thrown into vibration by a moderate blast of 
 air. The problems we have to consider are, first, by what means 
 are the cords brought near to each other or drawn asunder as 
 occasion demands ; and secondly, by what means is the tension of 
 the cords made to vary. We may speak of these two actions as 
 naiTowing or widening of the glottis, and tightening or relaxation 
 of the vocal cords. 
 
 Narrowing of the Glottis. The change of form of the glottis is 
 best understood when it is borne in mind that each arytenoid 
 cartilage is, when seen in horizontal section (Fig. 88), somewhat of 
 the form of a triangle, with an internal or median, an external, 
 and a posterior side, the processus vocalis being placed in the 
 anterior angle at the junction of the median and external sides. 
 When the cartilages are so placed that the processus vocales are 
 approximated to each other, and the internal surfaces of the 
 cartilages nearly parallel, the glottis is narrowed. When on the 
 contrary the cartilages are wheeled round on the pivots of their 
 articulations, so that the processus vocales diverge, and the inter- 
 nal surfaces of the cartilages form an angle vnih. each other, the 
 glottis is widened. 
 
 There are several muscles forming together a group, which has 
 been called by Henle the sphincter of the larynx. These are (1) 
 the thi/ro-ary-epiglotticus, proceeding from the inner surface of the 
 thyroid cartilage and from the arytenoid epiglottidean ligament, 
 and sweeping round the outer ridge of the ar^-tenoid cartilage of 
 its own side to be inserted into the processus muscularis of the 
 arytenoid cartilage of the other side : (2) the thyro-arytenoideiis 
 extemus, passing from the reentrant angle of the thjToid cartilage 
 to be inserted into the outer edge of the arytenoid cartilage of the 
 same side : (3) the thyro-arytenoideus internus, passing from the 
 angle of the thyroid cartilage to the processus vocalis and outer 
 side of the ar}'tenoid cartilage : (4) the arytenoideus (j)osticus), 
 passing transversely from one arytenoid cartilage to another. All 
 these muscles, when they act together, grasp round the glottis and 
 tend to close it up : and each of them, acting alone, has, with the 
 exception of the last-named (arytenoideus), the same effect. In 
 addition to these, the crico-arytenoideus lateralis, which passes 
 from the lateral border of the cricoid cartilage upwards and back- 
 wards to the outer angle of the arytenoid, by pulling this outer 
 angle forwards throws the processus vocalis inwards, and so also 
 narrows the glottis. 
 
654 THE VOICE. [Book hi. 
 
 Widening of the Glottis. The crico-arytenoideus posticus 
 passing from the posterior surface of the cricoid cartilage to the 
 outer angle of the arytenoid cartilage behind the attachment of 
 the lateral crico-arytenoideus, pulls back this outer angle, and so 
 causing the processus vocalis to move outwards, widens the glottis. 
 The arytenoideus posticus, acting alone, has a similar effect. 
 
 Tightening of the Vocal Cords. The crico-thyroideus pulls 
 the thjrroid downwards and forwards, and so increases the distance 
 between that cartilage and the arytenoids when the latter are 
 fixed. Supposing then the arytenoideus and crico-arytenoideus 
 posticus to fix the arytenoids, the effect of the contraction of the 
 crico-thyroideus would be to tighten the vocal cords. 
 
 Slackening of the Vocal Cords. This is effected by the whole 
 sphincter group just mentioned, but more especially by the thyro- 
 arytenoidei externus and internus ; these acting alone, supposing 
 the arytenoid cartilages to be fixed, would pull the thyroid car- 
 tilage upwards and backwards, and so shorten the distance between 
 the processus vocales and that body. 
 
 Thus almost every movement of the larynx is effected not by 
 one muscle only but by several, or at least by more than one, 
 acting in concert. The movements which give rise to the voice 
 are preeminently combined and coordinate movements. When 
 we remember how a very slight variation in the tension of the 
 vocal cords must give rise to a marked difference in the pitch of 
 the note uttered, and yet what a multitude of fine differences of 
 pitch are at the command of a singer of even moderate ability, it 
 appears exceedingly probable that the various muscular com- 
 binations required to produce the possible variations in pitch are 
 of such a kind that frequently a part only, possibly a few fibres 
 only, of a particular muscle, may be thrown into contraction, while 
 all the rest of the muscle remains quiet. Taking into view 
 moreover the great range of pitch possessed by even common 
 voices, as compared with the possible variations of tension of 
 which the vocal cords in their natural length are capable, it has 
 been suggested that some of the fibres of the thyro-arytenoideus 
 internus, which passing either from the thyroid or from the 
 arytenoid, appear to end in the vocal cords themselves, may, by 
 fixing particular points of the cords, so to speak, 'stop' them; and 
 by thus artificially shortening the length actually thrown into 
 vibration, produce higher notes than the cords in their natural 
 length are capable of producing. It has been also suggested that 
 the processus vocales may overlap each other, and thereby shorten 
 the length of cord available for vibration. 
 
 These various muscles are supplied by the vagus nerve, or 
 rather by spinal accessory fibres running in the vagus trunk. The 
 superior laryngeal is the afferent nerve supplying the mucous 
 membrane, but it also contains the motor fibres distributed to the 
 
Chap, vii] SPICCIAL MUSCULAR MECJJAN'/fiAfS. (iuf) 
 
 orico-thyn)itl nmsdi' ; lionco when (his nerve is dividecl on one 
 side the corresponding vocal cord is relaxed and high notes become 
 imjiossiblo. It is worthy of notice that this, the chief tensor, and 
 (hori'fore the most important, muscle of the larynx, has a separate 
 and distinct nervous supply. According to some authors the 
 arytcnoideus posticus also receives its nervous supply from this 
 nerve ; but this is denied by others. 
 
 The inferior laryngeal or recurrent branch supplies all the other 
 muscles. When this nerve is divided the voice is lost, since the 
 approximation and parallelism of the vocal cords can no longer be 
 oftccted. When in a living animal both recurrent nerves are 
 divided, the glottis is seen to become immobile and partially 
 ililated, the vocal cords assuming the position in which they are 
 found in the body after death, and which may be considered as the 
 condition of equilibrium between the dilating and constricting 
 muscles. During forcible inspiration the glottis passes from this 
 condition in the direction of more complete dilation ; during 
 forcible expiration, the change is one of constriction. When the 
 peripheral portion of one recurrent nerve is stimulated, the vocal 
 cord of the same side is approximated to the middle line ; when 
 both nerves are stimulated, the vocal cords are brought together 
 and the glottis is narrowed. Though the nerve is distributed to 
 both dilating and constricting muscles, the latter overcome the 
 former when the nerve is artificially stimulated. In the complete 
 closure of the glottis, which is so important a part of the act of 
 coughing (p. 383), the group of muscles which we have spoken of 
 as constituting a sphincter is thrown into forcible contractions by 
 the recurrent laryngeal nerve. 
 
 Though fundamentally a voluntary act, the utterance of a given 
 note is not effected by the direct passage of simple volitional im- 
 pulses down to the laryngeal muscles. So complex and coordinate 
 a movement as that of sounding even a simple and natural note, 
 requires a coordinating nervous mechanism in which, as in other 
 complex muscular actions, afferent impulses play an important part. 
 Auditory sensations, if not as important for an accurate manage- 
 ment of the voice as are visual sensations for the movements of the 
 eye, are yet of prime importance. This is I'eeognized when we say 
 that such and svich a one whose power over his laryngeal muscles is 
 imperfect, 'has no ear.' 
 
 A person may speak or sing in two kinds of voice. In the 
 one the sounds are full and strong, and the resonance chamber which 
 is supplied by the trachea, bronchi and indeed by the whole chest, 
 is thi'own into powerful and palpable vibrations ; hence this voice 
 is spoken of as the chest-voice. The other kind of voice, called the 
 falsetto, is thin and poor, deals chiefly vdth high notes, and is not 
 accompanied by the same conspicuous vibrations of the chest. 
 Much controversy has taken place as to the exact manner in which 
 these two voices are respectively produced. The prevailing opinion 
 
656 THE VOICE. [Book in. 
 
 teaches that in the chest-voice the vocal cords are somewhat thick, 
 their substance being thrust inward towards the median line by 
 the contraction of the thyro-arytenoidei externi muscles, and the 
 opening between them, sometimes so narrow as to be almost linear, 
 extends along, their whole length. In the falsetto voice, on the other 
 hand, the vocal cords are said to be thin and membranous, and the 
 note to be given forth by a vibration, not of the whole width of the 
 cords as in the chest voice, but of the extreme edges only, the lateral 
 parts though not absolutely at rest vibrating with a different rhythm. 
 Though the whole larynx in the falsetto voice is stretched in the 
 antero-posterior direction and the vocal cords correspondingly elon- 
 gated, the rima vocalis does not extend along their whole length ; 
 at their posterior part the cords are in contact, and indeed according 
 to some authors, the high falsetto notes are produced by a sort of 
 'stopping' of the cords. The sense of effort which accompanies 
 the falsetto voice indicates that the changes in the larynx which 
 bring it about, are effected by some special muscular manoeuvres, 
 as is also suggested by the fact that the ease with which falsetto 
 notes can be uttered is readily increased by practice. The change 
 from the chest to the falsetto voice is an abrupt one, and the com- 
 bined range may be very extensive, as in the case of persons who 
 can carry on a duet, singing alternately, for instance, in a tenor 
 (chest) and a soprano (falsetto) voice. 
 
 The ventricles of Morgagni are apparently of use in giving the 
 vocal cords sufficient room for their vibrations, and perhaps supply 
 a secretion by which the vocal cords are kept adequately moist. 
 The purpose of the false vocal cords is not exactly known. Some 
 authors think that in the falsetto voice they are brought down into 
 contact with, and thus serve to stop, the true vocal cords. 
 
 At the age of puberty a rapid development of the larynx takes 
 place, leading to a change in the range of the voice. The peculiar 
 harshness of the voice when it is thus 'breaking' seems to be due 
 to a temporary congested and swollen condition of the mucous mem- 
 brane of the vocal cords accompanying the active growth of the 
 whole larynx. The change in the mucous membrane may come on 
 quite suddenly, the voice 'breaking' for instance in the course of a 
 night. 
 
SEC. 2. SPEECH. 
 
 Vowels. 
 
 Every sound, or every note (for all vocal sounds when considered 
 by themselves are musical sounds), caused by the vibrations of the 
 vocal cords, besides its loudness due to the force of the expiratory 
 blast, and its pitch due to the tension of the cords, has a quality of 
 its own, due to the number and relative prominence of the over- 
 tones which accompany the fundamental tone. Some of these 
 features which make up the quality are imposed on the note by 
 the nature of the vocal cords, but still more arise from various modi- 
 fications Avhich the relative intensities of the overtones undergo 
 through the resonance of the cavity of the mouth and throat. 
 Whenever we hear a note sounded by the larynx we are able to 
 recognize in it features which enable us to state that one or other 
 of the 'vowels' is being uttered. Vowel sounds are in fact only 
 extreme cases of quality, extreme prominence of certain overtones 
 brought about by the shape assumed by the buccal and pharyngeal 
 passages and orifices, as the vibrations pass through them. Each- 
 vowel has its appropriate and causative disposition of these parts. 
 When i (ee in feet) is sounded, the sounding-tube of the upper air 
 passages is made as short as possible, the larynx is raised and the 
 lips are retracted, the whole cavity of the mouth taking on the form 
 of a broad flask with a narrow neck. During the orivinor out of e 
 (a in fat) the shape of the mouth is similar, but somewhat longer. 
 For the production of a (as in father) the mouth is \\ddely open, so 
 that the buccal cavity is of the shape of a funnel with the apex at 
 the pharjTix. With o, the buccal cavity is again flask-shaped, with 
 F. 42 
 
658 SPEECH. [Book hi 
 
 the mouth more closed than in a, but the lips, instead of being re- 
 tracted as in % and e, are somewhat protruded, so that the sounding 
 tube is prolonged. The greatest length of the tube is reached in u, 
 (oo), in which the larynx is depressed and the lips protruded as 
 much as possible. While the two latter vowels are being uttered, 
 the general form of the buccal cavity is that of a flask with a 
 short neck and a small opening, the orifice being smaller for %i than 
 for 0. 
 
 Each of these various 'vowel' forms of the mouth possesses a 
 note of its own, one towards which it acts as a resonance chamber. 
 Thus if several tuning-forks of various pitch be held while sounding 
 before a mouth which has assumed the particular form necessary 
 for sounding U, it "will be found that the resonance will be particu- 
 larly great with the fork having the pitch of the bass 6-flat. Simi- 
 larly other and higher notes will be intensified when the mouth is 
 moulded to utter the other vowels. And it is the experience of 
 singers that each vowel is sung with peculiar ease on a note having 
 a prominent overtone corresponding to the tone proper to the 
 mouth when moulded to utter the vowel. The precise nature of 
 the vowel sounds is however still disputed. 
 
 As the vibrations are travelling through the pharyngeal and 
 buccal cavities, the posterior nares are closed by the soft palate ; 
 and it may be shewn, by holding a flame before the nostril, that no 
 current of air issues from the nose when a vowel is properly said or 
 sung. When the posterior nares are not effectually closed the 
 sound acquires a nasal character. The same happens when the 
 anterior nares are closed, as when the nose is held between the 
 fingers, the nasal chamber then forming a cavity of resonance. 
 
 Consonants. 
 
 Vowels are, as their name implies, the only real vocal sounds ; 
 it is only on a vowel that a note can be said or sung. Our speech 
 however is made up not only of vowels but also of consonants, i.e. 
 of sounds which are produced not by the vibrations of the vocal 
 cords but by the expiratory blast being in various ways interrupted 
 or otherwise modified in its course through the throat and mouth. 
 
 The distinction between the two is however not an absolute one, 
 since, as we have seen, the characters of the several vowels depend 
 on the form of the mouth, and in the production of some conso- 
 nants (B, D, M, N, &c.) vibrations of the vocal cords form a neces- 
 sary though adjuvant factor. 
 
 Consonants have been classified according to the place at which 
 
("iiAP. vii.l SPEC I A I. Ml\SC/LAR MECHANISMS. 059 
 
 the characteristic iutt'rriiptioii or modification takes place. Thus it 
 may occur, 
 
 1. At the lips, by the movement or position of the lips in 
 roforence to each other or to the teeth, giving rise to labial conso- 
 nants. 
 
 '1. At the teeth, by the movement or position of the front part 
 of the tongue in reference to the teeth or the hard palate, giving 
 rise to dental consonants. 
 
 :i. In the throat, by the movement or position of the root of 
 the tongue in reference to the soft palate or pharynx, giving rise to 
 guttural consonants. 
 
 Among the dentals again may be distinguished the dentals 
 commonly so called, such as T, the sibilants such as S, and the 
 lingual L, all differing in the relative position of the tongue, teeth, 
 and palate. 
 
 Consonants may also be classified according to the character of 
 the movements which give rise to them. Thus they may be either 
 explosive or continuous. 
 
 1. Explosives. In these the characters are given to the sound 
 by the sudden establishment or removal of the appropriate inter- 
 ruption. Thus, in uttering the labial P, the lips are first closed, 
 then an expiratory current of air is driven against them, and ujDon 
 their being suddenly opened, the sound is generated. Similarly, 
 the dental T is generated by the sudden removal of the interruption 
 caused by the approximation of the tip of the tongue to the front 
 of the hard palate, and the guttural K by the sudden removal of 
 the interruption caused by the approximation of the root of the 
 tongue to the soft palate. 
 
 The labial B differs from P, inasmuch as it is accompanied by 
 vibrations of the vocal cords (that is, a vowel sound is uttered at 
 the same time), and these vibrations continue after the removal of 
 the interruption. Hence B is often spoken of as being uttered with 
 voice and P without voice ; and D and G (hard) with voice bear 
 the same relation to T and K without voice. 
 
 The continuous consonants may further be divided into 
 
 2. Aspirates. In these the sound is generated by a rush of 
 air through a constriction formed by the partial closure of the lips, 
 or by the raising of the tongue against the hard or soft palate, &c. 
 Thus F is sounded when the lips are brought into partial, and not 
 as in P and B into complete approximation, and a current of air is 
 driven through the narrowed opening. F is uttered without any 
 accompanying vibration of the vocal cords, i.e. without voice. With 
 voice it becomes V. 
 
 The sibilant S is formed by a rush of air past an obstruction 
 caused by the partial closure of the teeth, the front of the tongue 
 
 42—2 
 
660 CONSONANTS. [Book hi. 
 
 being depressed at the same time ; and S accompanied with vibra- 
 tions of the vocal cords becomes Z. 
 
 In Sh the dorsal surface of the tongue is raised so as to narrow 
 the passage between that organ and the palate for a considerable 
 portion of its length. 
 
 Th is formed by placing the tongue between the two partially 
 open rows of teeth ; and the hard and soft Th bear to each other 
 the same relation as do P and B. 
 
 L is produced when the passage is closed in the middle by 
 pressing the tip of the tongue against the hard palate and the air 
 is allowed to escape at the sides of the tongue. 
 
 When the constriction in an aspirate is formed by the approxi- 
 mation of the root of the tongue to the soft palate, we have the 
 guttural CH (as in loch) without voice and GH (as in lough) with 
 voice. 
 
 3. Resonants or Nasals. In these, all of which must have 
 vibrations of the vocal cords as a basis, the usual passage through 
 the mouth is closed either in a labial, dental, or guttural, fashion 
 and the peculiar character is given to the sound by the nasal 
 chambers acting as a resonance cavity. Thus in M, the passage is 
 closed by the approximation of the lips, in N, by the approximation 
 of the tongue to the hard palate, and in NG by the approximation 
 of the root of the tongue to the soft palate. 
 
 4. The various forms of R are often spoken of as vibratory, the 
 characteristic sounds being caused by the vibration of some or 
 other of the parts forming a constriction in the vocal passage. 
 Thus the ordinary R is produced by vibrations of the point of the 
 tongue elevated against the hard palate, the guttural R by the 
 vibrations of the uvula or other parts of the walls of the pharynx ; 
 and in some languages there seems to be an R produced by the 
 vibrations of the lips. 
 
 H is caused by the rush of air through the widely open glottis. 
 When, in sounding a vowel, the sound coincides with a sudden 
 change in the position of the vocal cords from one of divergence to 
 one of approximation, the vowel is pronounced with the spiritus 
 asper. When the vocal cords are brought together before the 
 blast of air begins, the vowel is pronounced with the spiritus lenis. 
 The Arabic H is produced by closing the rima vocalis, the epi- 
 glottis and false vocal cords being depressed, and sending a blast of 
 air through the rima respiratoria. 
 
 On many of the above points however, there are great dif- 
 ferences of opinion, the discussion of which as well as of other more 
 rare consonantal sounds would lead us too far away from the 
 purpose of this book. The following tabular statement must 
 therefore be regarded as introduced for convenience only. 
 
CiiAi'. vii.l ;SPECIAL MUSCULAR MECHANISMS. 
 
 001 
 
 ExPLOSiVKS. Labials, without voice, P. 
 
 „ with voice, B. 
 
 Dentals, without voice, T. 
 
 „ with voice, D. 
 
 Gutturals, without voice, K (hard C). 
 
 „ with voice, G (hard). 
 
 Aspirates. Labials, without voice, F. 
 
 „ ^vith voice, V. 
 
 Dentals, \vithout voice, L. 
 
 S. (soft C), Sh,Th(hard). 
 
 „ with voice, Z, Zh (in a^ure, the 
 
 Frcnchj),Th(soft). 
 
 Gutturals, \Wthout voice, CH (as in loch). 
 
 „ with voice, GH (as in lou^h). 
 
 Resonants. Labial, M. 
 
 Dental, , N. 
 
 Guttural, NG. 
 
 Vibratory. Labial, not known in European speech. 
 Dental, R (common). 
 Guttural, R (guttural). 
 
 Whispering is speech without any employment of the vocal 
 cords, and is effected chiefly by the lips and tongue. Hence in 
 whispering the distinction between consonants needing and those 
 not needing voice, such as B and P, becomes for the most part lost. 
 
SEC. 3. LOCOMOTOR MECHANISMS. 
 
 The skeletal muscles are for the most part arranged to act on 
 the bones and. cartilages as on levers, examples of the first kind of 
 lever being rare, and those of the third kind, where the power 
 is appKed nearer to the fulcrum than is the weight, being more 
 common than the second. This arises from the fact that the 
 movements of the body are chiefly directed to moving com- 
 paratively light weights through a gTeat distance, or through a 
 certain distance with great precision, rather than to moving heavy 
 weights through a short distance. The fulcrum is generally 
 supplied by a (perfect or imperfect) joint, and one end of the 
 acting muscle is made fast by being attached either to a fixed 
 point, or to some point rendered fixed for the time being by 
 the contraction of other muscles. There are few movements of 
 the body in which one muscle only is concerned; in the majority 
 of cases several muscles act together in concert; nearly all our 
 movements are coordinate movements. Where gravity or the 
 elastic reaction of the parts acted on does not afford a sufficient 
 antagonism to the contraction of a muscle or group of muscles, 
 the return to the condition of equilibrium is pro\dded for by the 
 action either elastic or contractile of a set of antagonistic muscles ; 
 this is seen in the case of the face. 
 
 The erect j^osture, in which the weight of the body is borne by 
 the plantar arches, is the result of a series of contractions of the 
 muscles of the trunk and legs, having for their object the keeping 
 the body in such a position that the line of gravity fails within 
 the area of the feet. That this does requu^e muscular exertion 
 
CiiAi'. Yii] SPECIAL MUSCULAR MECHANISMS. 663 
 
 is shewn by tlio tacts, thai a pcrtsun when .sLauding perfectly at 
 rest in a completely balanced position falls when he becomes 
 unconscious, and that a dead body cannot bo set on its feet. 
 Tht! lino of gravity of the head falls in front of the occipital 
 articulation, as is shewn by the nodding of the head in sleep. 
 The centre of gravity of the combined head and trunk lies at 
 about the level of the ensiform cartilage, in front of the tenth 
 dorsal vertebra, and the line of gravity di-awn from it passes 
 behind a line joining the centres of the two hip-joints, so that the 
 erect botly would fall backward were it not for the action of the 
 muscles passing from the thighs to the pelvis assisted by the 
 anterior ligaments of the hip-joints. The line of gravity of the 
 combined head, tnink and thighs falls moreover a little behind 
 the knee-joints, so that some, though little, muscular exertion is 
 required to prevent the knees from being bent. Lastly, the line of 
 gravity of the whole body passes in front of the line drawn 
 between the two ankle-joints, the centre of gravity of the whole 
 body being placed at the end of the sacrum ; hence some exertion 
 of the muscles of the calves is required to prevent the body falling 
 forwards. 
 
 In walking, there is in each step a moment at which the body 
 rests vertically on the foot of one, say the right leg, while the other, 
 the left leg, is inclined obliquely behind with the heel raised and 
 the toe resting on the ground. The left leg, slightly flexed to avoid 
 contact ■s\ath the gi-ound, is then swung forward like a pendulum 
 the length of the swing or step being determined by the length of 
 the leg ; and the left toe ^ is brought to the ground. On this left 
 toe as a fulcrum, the body is moved forward, the centre of gravity 
 of the body describing a curve the convexity of which is upward 
 and the left leg necessarily becoming straight and rigid. As the 
 body moves forward, a point vdW be reached similar to that with 
 which we supposed the step to be started, the body resting 
 vertically on the left foot, and the right leg being directed behind 
 in an oblique position. The movement on the left foot however 
 caiiies the body beyond this point, and in doing so s-wings the 
 right leg forward until it is the length of a step in advance of its 
 previous position, and its toe in turn forms a fulcrum on which the 
 body, and Avith it the left leg, is again s\vung forward. Hence 
 in successive steps the centre of gravity, and with it the top of the 
 head, describes a series of consecutive curves, with their convexities 
 upwards, very similar to the line of flight of many birds. 
 
 Since in standing on both feet the line of gi-avity falls between 
 the two feet, a lateral displacement of the centre of gravity is 
 necessary in order to balance the body on one foot. Hence in 
 walking the centre of gravity describes not only a series of vertical, 
 
 ' This indicates perhaps what should be done rather than the actual practice; 
 most people put the heel to the ground first, the contact with the toe coming later. 
 
664 LOCOMOTOR MECHANISMS. [Eook iii. 
 
 but aiso a series of horizontal curves, inasmuch as at each step the 
 line of gravity is made to fall alternately on each standing foot. 
 While the left leg is swinging, the line of gravity falls within the 
 area of the right foot, and the centre of gravity is on the right side 
 of the pelvis. As the left foot becomes the standing foot, the 
 centre of gravity is shifted to the left side of the pelvis. The 
 actual curve described by the centre of gravity is therefore a 
 somewhat complicated one, being composed of vertical and 
 horizontal factors. The natural step is the one Avhich is de- 
 termined by the length of the swinging leg, since this acts as 
 a pendulum; and hence the step of a long-legged person is 
 naturally longer than that of a person with short legs. The 
 length of the step however may be diminished or increased by a 
 direct muscular effort, as when a line of soldiers keep step in spite 
 of their having legs of different lengths. Such a mode of marching 
 must obviously be fatiguing, inasmuch as it involves an unnecessary 
 expenditure of energy. 
 
 In slow walking there is an appreciable time during which, 
 while one foot is already in position to serve as a fulcrum, the 
 other, swinging, foot has not yet left the ground. In fast walking 
 this period is so much reduced, that one foot leaves the ground the 
 moment the other touches it ; hence there is practically no period 
 during which both feet are on the ground together. 
 
 When the body is swung forward on the one foot acting as a 
 fulcrum with such energy that this foot leaves the ground before 
 the other, swinging, foot has reached the ground, there being 
 an interval during which neither foot is on the ground, the person 
 is said to be running, not walking. 
 
 In jumping this propulsion of the body takes place on both feet 
 at the same time ; in hopping it is effected on one foot only. 
 
 The locomotion of four-footed animals is necessarily more com- 
 plicated than that of man. The simple walk, such as that of the 
 horse, is executed in four times, with a diagonal succession : thus, 
 right fore leg, left hind leg, left fore leg, right hind leg. In the 
 amble, such as that of the camel, the two feet of the same side are 
 put down at one and the same time, this movement being followed 
 by a similar movement of the other two legs ; it corresponds there- 
 fore very closely to human walking. In the trot, which corresponds 
 to human running, the two diagonally opposite feet are brought to 
 the ground at the same time, and the body is propelled forwards on 
 them. Concerning this however, as well concerning the still more 
 complicated gallop and canter obser^^ers are not agreed and much 
 discussion has arisen. 
 
 The other problems connected with the action of the various 
 skeletal muscles of the body are too special to be considered here. 
 
BOOK IV. 
 
 THE TISSUES AND MECHjWISMS OF REPRODUCTION. 
 
THE TISSUES AND MECHANISMS OF HEPRODUCTION. 
 
 Many of the individual constituent parts of the body are capable 
 of reproduction, i.e. they can give rise to parts like themselves; or 
 they are capable of regeneration, i.e. their places can be taken by 
 new parts more or less closely resembling themselves. The ele- 
 mentary tissues undergo during life a very large amount of re- 
 generation. Thus the old epithelium scales which fall away from 
 the surface of the body are succeeded by new scales from the 
 underlying layers of the epidermis ; old blood-corpuscles give place 
 to new ones ; worn-out muscles, or those which have failed from 
 disease, are renewed by the accession of fresh fibres; divided nerves 
 gi'ow again ; broken bones are united ; connective tissue seems to 
 disappear and appear almost without limit ; new secreting cells 
 take the place of the old ones which are cast off; in fact, ^\dth the 
 exception of some cases, such as cartilage, and these doubtful ex- 
 ceptions, all those fundamental tissues of the body, which do not 
 form part of highly differentiated organs, are, within limits fixed 
 more by bulk than by anything else, capable of regeneration. 
 That regeneration by substitution of molecules, which is the 
 basis of all life, is accompanied by a regeneration by substitution 
 of mass. 
 
 In the higher animals regeneration of whole organs and mem- 
 bers, even of those whose continued functional activity is not 
 essential to the well-being of the body, is never witnessed, though 
 it may be seen in the lower animals ; the digits of a neA\i: may be 
 restored by gi'owth, but not those of a man. And the repair 
 which follows even partial destruction of highly differentiated 
 organs, such as the retina, is in the higher animals very imperfect. 
 
668 TISSUES OF REPRODUCTION. [Book iv. 
 
 In the higher animals the reproduction of the whole individual 
 can be effected in no other way than by the process of sexual 
 generation, through which the female representative element or 
 ovum is, under the influence of the male representative or sperma- 
 tozoon, developed into an adult individual. 
 
 We do not purpose to enter here into any of the morphological 
 problems connected with the series of changes through which the 
 ovum becomes the adult being; or into the obscure biological 
 inquiry as to how the simple all but structureless ovum contains 
 within itself, in potentiality, all its future developments, and as to 
 what is the essential nature of the male action. These problems 
 and questions are fully discussed elsewhere ; they do not properly 
 enter into a work on physiology, except under the view that all 
 biological problems are, when pushed far enough, physiological 
 problems. We shall limit ourselves to a brief survey of the more 
 important physiological phenomena attendant on the impregnation 
 of the ovum, and on the nutrition and birth of the embryo. 
 
CHAPTEE I. 
 
 MENSTRUATION. 
 
 From puberty, which occurs at from 13 to 17 years of age, to the 
 climacteric, which arrives at from 45 to 50 years of age, the human 
 female is subject to a monthly discharge of ova from the ovaries, 
 accompanied by special changes, not only in those organs but also 
 in the Fallopian tubes and uterus, as well as by general changes in 
 the body at large, the whole constituting 'menstruation.' The 
 essential event in menstruation is the escape of an ovum from its 
 Graaffian follicle. The whole ovary at this time becomes con- 
 gested, and the ripe follicle bulges from its surface. The most 
 projecting portion of the wall of the follicle, which has previously 
 become excessively thin, is now ruptured, and the ovum, which 
 having left its earlier position, is Ipng close under the projecting 
 surface of the follicle, escapes, together with the cells of the discus 
 proligerus, into the Fallopian tube. How the entrance of the 
 ovum into the Fallopian tube is secured is not exactly known. 
 Some maintain that the ovaiy is grasped by the trumpet-shaped 
 fimbriated mouth of the Fallopian tube, itself turgid and con- 
 gested ; the movements necessary to bring this about being effected 
 by the plain muscular fibres present in the mouth of the tube. 
 Others, rejecting this view, and asserting that the turgescence 
 of the tube does not occur until after the ovum has become 
 safely lodged in the tube, suggests that the ovum is earned in the 
 proper dfrection by currents in the peritoneal cavity set up by the 
 action of the ciliated epithelium lining the tube, currents whose 
 direction and strength seem, as she^\'n by experiment, to be 
 adequate to carry into the uterus particles present in the perito- 
 neal fluid. Arrived in the tube, the ovum travels do^vnwards, ver^' 
 
670 MENSTRUATION. [Book iv. 
 
 slowly, by the action probably of the cilia lining the tube, though 
 possibly its progress may occasionally be assisted by the peristaltic 
 contractions of the muscular walls. The stay of the ovum in the 
 Fallopian tube may extend to several days. There is an effusion of 
 blood into the ruptured follicle, which is subsequently followed by 
 histological changes in the coats of the follicle resulting in a corpus 
 luteum. The discharge of the ovum is accompanied not only by a 
 congestion or erection of the ovary and Fallopian tube, but also by 
 marked changes in the uterus, especially in the uterine mucous 
 membrane. While the whole organ becomes congested and en- 
 larged, the mucous membrane, and especially the uterine glands, 
 arc distinctly hjrpertrophied. The swollen internal surface is 
 thrown into folds which almost obliterate the cavity ; and a 
 haemorrhagic discharge, often considerable in extent, constituting 
 the menstrual or catamenial flow, takes place from the greater 
 part of its surface. The blood as it passes through the vagina 
 becomes somewhat altered by the acid secretions of that passage, 
 and when scanty coagulates but slightly ; when the flow however is 
 considerable, distinct clots may make their appearance. The 
 swollen and hypertrophied mucous membrane then undergoes a 
 rapid degeneration, and is shed, passing away sometimes in distinct 
 masses, forming the latter part of the menstrual flow. The loss of 
 the mucous membrane is so complete, that the bases only of the 
 uterine glands are left, and from the epithelial cells lining these 
 the regeneration of the new membrane is said to take place. It is 
 not certain that menstruation, in the human subject at all events, 
 is always accompanied by a discharge of an ovum ; indeed cases 
 have been recorded in which menstruation continued' after what 
 appeared to be complete removal of both ovaries. And it seems 
 probable also that under certain circumstances, ex. gr. coitus, a dis- 
 charge of an ovum may take place at other times than at the 
 menstrual period. Since however the time during which both the 
 ovum and the spermatozoon may remain in the female passages 
 alive and fanctionally capable is considerable, probably extending 
 to some days, coitus effected either some time after or some time 
 before the menstrual escape of an ovum might lead to impregna- 
 tion and subsequent development of an embryo; hence the fact 
 that impregnation may follow upon coitus at some time after or 
 before menstruation is no very cogent argument in favour of the 
 view that such a coitus has caused an independent escape of an 
 ovum. The escape of the ovum is said to precede, rather than 
 coincide with or follow, the catamenial flow. If no spermatozoa 
 come in contact with the ovum it dies, the uterine membrane 
 returns to its normal condition, and no trace of the discharge of an 
 ovum is left, except the corpus luteum in the ovary. 
 
 It is obvious that in these phenomena of menstruation we have 
 to deal with complicated reflex actions affecting not only the 
 vascular supply but, apparently in a direct manner, the nutritive 
 
ri.vp. i.l MRNFiTRUATKhW G71 
 
 changes of the organs concerned. Our studies on tlio nervous 
 action of secretion render it easy for us to conceive in a general 
 way how the several events are brought about. It is no more 
 dilHcult to suppose that the stimulus of the enlargement of a 
 (Iraaffian fojlitjo causes nutritive as well as vascular changes in the 
 uterine nuuous nionibrane, than it is to suppose that the stimulus 
 of food in the aliiuiMitary canal causes those nutritive changes in 
 the salivary glands or pancreas which constitute secretion. In the 
 latter case we can to some extent trace out the chain of events ; in 
 the former case we hardly know more than that the maintenance 
 of the lumbar cord is sufficient, as far as the central nervous 
 system is concerned, for the carrying on of the worlc. In the case 
 of a dog in which the spinal cord had been completely divided in 
 the dorsal region while the animal was as yet a mere puppy, 'heat' 
 or menstruation took place as usual. 
 
CHAPTER II. 
 
 IMPREGNATION. 
 
 In coitus the discharge of the semen containing the spermatozoa is 
 most probably effected by means of the peristaltic contractions of 
 the vesiculse seminales and vasa deferentia. assisted by rhythmical 
 contractions of the bulbo-cavemosus muscle, the whole being a 
 reflex act, the centre of which appears to be in the lumbar spinal 
 cord. In the dog, emission of semen can be brought about by 
 stimulation of the glans penis after complete division of the spinal 
 cord in the dorsal region. The emission of semen is preceded by 
 an erection of the penis. This we have already seen, p. 210, is in 
 part at least due to an increased vascular supply brought about by 
 means of the nervi erigentes ; it is probable, however, that the 
 condition is farther secured by a compression of the efferent veins 
 of the corpora cavernosa by means of smooth muscular fibres 
 present in those bodies. The semen being received into the female 
 organs, which are at the time in a state of turgescence resembling 
 the erection of the penis, but less marked, the spermatozoa find 
 their way into the Fallopian tubes, and here (probably in its upper 
 part) come in contact with the ovum. In the case of some animals 
 impregnation may take place at the ovary itself The passage of 
 the spermatozoa is most probably effected mainly by their own 
 vibratile activity ; but in some animals a retr(3grade peristaltic 
 movement travelling from the uterus along the Fallopian tubes has 
 been observed ; this might assist in bringing the semen to the 
 ovum, but inasmuch as these movements are probably parts of the 
 act of coitus and impregnation may be deferred till some time after 
 that event, no great stress can be laid upon them. 
 
CnAi'. 11.] IMPREGNATION. 6t3 
 
 As the result of the uctiou ul the spuruiatozua on tlic ovum, the 
 latter, instead of" dying as when impregnation fails, awakes to great 
 nutritive activity accompanied by remarkable morphological 
 changes ; it enlargi-s and develops into an embryo. No sooner, how- 
 ever, have these changes begun in the ovum than correlative changes, 
 brought about probably by reliex action, but at present most 
 obscure in their causation, take place in the uterus. The mucous 
 membrane of this organ, whether the coitus resulting in im- 
 pregnation be coincident with a menstrual period or not, becomes 
 congested, and a ra{)id growth takes place, characterized by a rapid 
 proliferation of the epithelial and subepithelial tissues. Unlike the 
 case of menstruation, however, this new growth does not give way 
 to immediate decay and haemorrhage, but remains ; and may be 
 distinguished as a new temporary lining to the uterus, the so-called 
 decidua. Into this decidua the ovum, on its descent from the 
 Fallopian tube, in which it has probably already undergone some 
 developmental changes, is received ; and in this it becomes em- 
 bedded, the new growth closing in over it. Meanwhile the rest of 
 the uterine structures, especially the muscular tissue, become also 
 much enlarged ; as pregnancy advances a large number of new 
 muscular tibres are formed. As the ovum continues to increase in 
 size, it bulges into the cavity of the uterus, carrying with it the 
 portion of the decidua which has closed over it. Henceforward, 
 accordingly, a distinction is made in the now well-developed 
 decidua between the decidua reflexa, or that part of the membrane 
 w^hich covers the projecting ovum, and the decidua vera, or the 
 rest of the membrane lining the cavity of the uterus, the two being 
 continuous round the base of the projecting ovum. That part of 
 .the decidua which intervenes between the ovum and the nearest 
 uterine wall is frequently spoken of as the decidua serotina. As 
 the ovum developes into the foetus with its membranes, the decidua 
 reflexa becomes pushed against the decidua vera; about the end 
 of the third month, in the human subject, the two come into com- 
 plete contact all over, and ultimately the distinction between them 
 is lost. In the region of the decidua serotina the allantoic vessels 
 of the foetus dcvelope a placenta. For an account of the various 
 changes by which these events are brought about, as well as of the 
 history of the embryo itself, we must refer the reader to anatomical 
 treatises. 
 
 F. 43 
 
CHAPTER III. 
 
 THE NUTRITION OF THE EMBRYO. 
 
 During the development of the chick within the hen's egg the 
 nutritive material needed for the growth first of the blastoderm, 
 and subsequently of the embryo, is supplied by the yolk, while the 
 oxygen of the air j3assing freely through the porous shell, gains 
 access to all the tissues both of the embryo and yolk, either directly 
 or by the intervention of the allantoic vessels. The mammalian 
 embryo, during the period which precedes the extension of the 
 allantoic vessels into the cavities of the uterine walls to form the 
 placenta, must be nourished by direct diffusion, first from the con- 
 tents of the Fallopian tube, and subsequently from the decidua ; 
 and its supply of oxygen must come from the same sources. All 
 analogy would lead us to suppose that, from the very first, oxida- 
 tion is going on in the blastodermic and embryonic structures ; but 
 the amount of oxygen actually withdrawn from without is probably 
 exceedingly small in the early stages, seeing that nearly the whole 
 energy of the metabolism going on is directed to the building up 
 of structures, the expenditure of energy in the form of either heat 
 or external work being extremely small. The marked increase of 
 bulk which takes place during the conversion of the mulberry mass 
 into the blastodermic vesicle, shews that at this epoch a relatively 
 speaking large quantity of water at least, and probably of nutritive 
 matter, must pass from without into the ovum ; and subsequently, 
 though the blastoderm and embryo may for some time draw the 
 material for their continued construction at first hand from the 
 yolk-sac or umbilical vesicle, both this and they continue probably 
 until the allantois is formed to receive fresh material from the 
 mother by direct diffusion. 
 
 As the thin-walled allantoic vessels come into closer and fuller 
 
Chap, hi.] 77/ A' NUTRITION UF THE KMliUYO. 675 
 
 connection with the maternal uterine sinuses, until at last in the 
 fully tunned placenta the former are freely bathed in the blood 
 strcamini,' through the latter, the nutrition of the embryo bi-comes 
 more antl mori- confined t(j this special channel. The blood of the 
 foetus flowing along the umbilical arteries effects exchanges with 
 the venous blood of the mother, and leaves the placenta by the 
 umbilical vein richer in oxygen and nutritive material and poorer 
 in carbonic acid and excretory products than when it issued from 
 the feet U.S. 
 
 As far as the gain of oxygen and the loss of carbonic acid are 
 concerned these are the results of simple diffusion. Venous blood, 
 as we have already seen, always contains a quantity of oxyhemo- 
 globin, and the quantity of this substance present in the blood of 
 the uterine veins is sufficient to supply all the oxygen that the em- 
 bryo needs ; the blood of the foetus, containing less oxygen than 
 even the venous blood of the. mother ^^^ll take up a certain though 
 small quantity. The foetal blood travelling in the umbilical artery 
 must, in proportion to the extent of the nutritive changes going on 
 in the embryo, possess a higher carbonic tension than that in the 
 umbilical vein or uterine sinus; and by diffusion gets rid of this 
 surplus during its stay in the placenta. The blood in the umbilical 
 arteries and veins is therefore, relatively speaking, venous and arte- 
 rial respectively, though the small excess of oxyhsemoglobiii in the 
 blood of the umbilical vein is insufficient to give it a distinctly arterial 
 colour, or to distinguish it as sharply from the more venous blood 
 of the umbilical artery, as is ordinary arterial from ordinary venous 
 blood. Thus the foetus breathes by means of the maternal blood, 
 in the same way that a fish breathes by means of the Avater in which 
 it dwells. 
 
 The blood of the foetus is very poor in haemoglobin correspon- 
 ding to its low oxygen consumption. When the mother is asphyxi- 
 ated, the foetus is asphyxiated too, the oxygen of the latter passing 
 back again into the blood of the former ; and the asphyxia thus pro- 
 duced in the foetus is much more rapid than that which results 
 when the oxygen is used up by the tissues of the foetus alone, 
 as when the umbilicus is ligatured and the foetus not allowed to 
 breathe. 
 
 If oxygen and carbonic acid thus pass by diffusion to and from 
 the mother and the foetus, one might fairly expect that diffusible 
 salts, proteids, and carbohydrates would be conveyed to the latter, 
 and diffusible excretions carried away to the former, in the same 
 way; and if fats can pass directly into the portal blood during 
 ordinary dio^estion, there can be no reason for doubting that this 
 class of food-stuffs also would find its way to the foetus through the 
 placental structures. We do know from experiment that diffusible 
 substances will pass both from the mother to the foetus, and from 
 the foetus to the mother ; but we have no definite knowledge as to 
 the exact form and manner in which, during normal intra-uterine 
 
 43—2 
 
676 IFCETAL METABOLISM. [Book iV. 
 
 life, nutritive materials are conveyed to or excretions conveyed from 
 the growing young. The placenta is remarkable for the great de- 
 velopment of cellular structures, apparently of an epithelial nature, 
 on the border-land between the maternal and foetal elements ; and 
 it has been suggested that these form a temporary digestive and 
 secretory (excretory) organ. But we have no exact knowledge of 
 what actually does take place in these structures. From the coty- 
 ledons of ruminants may be obtained a white creamy-looking fluid, 
 which from many features of its chemical composition might be 
 almost spoken of as a 'uterine milk.' 
 
 Speaking broadly, the foetus lives on the blood of its mother, 
 very much in the same way as all the tissues of any animal live 
 on the blood of the body of which they are the parts. 
 
 For a long time all the embryonic tissues are 'protoplasmic' in 
 character ; that is, the gradually differentiating elements of the 
 several tissues remain still embedded, so to speak, in undifferen- 
 tiated protoplasm ; and during this period there must be a general 
 similarity in the metabolism going on in various parts of the body. 
 As diiferentiation becomes more and more marked, it obviously 
 would be an economical advantage for partially elaborated material 
 to be stored up in various foetal tissues, so as to be ready for 
 immediate use when a demand arose for it, rather than for a 
 special call to be made at each occasion upon the mother for 
 comparatively raw material needing subsequent preparatory changes. 
 Accordingly, we find the tissues of the foetus at a very early period 
 loaded with glycogen. The muscles are esj)ecially rich in this 
 substance, but it occurs in other tissues as well. The abundance 
 of it in the former may be explained partly by the fact that they 
 form a very large proportion of the total mass of the foetal body, and 
 partly by the fact that, while during the presence of the glycogen 
 they contain much undifferentiated protoplasm, they are exactly 
 the organs which will ultimately undergo a large amount of 
 differentiation, and therefore need a large amount of material for 
 the metabolism which the differentiation entails. It is not until 
 the later stages of intra-uterine life, at about the fifth month, 
 when it is largely disappearing from the muscles, that the glycogen 
 begins to be deposited in the liver. By this time histological 
 differentiation has advanced largely, and the use of the glycogen 
 to the economy has become that to which it is put in the ordinary 
 life of the animal ; hence we find it deposited in the usual place. 
 Besides being present in the foetal, glycogen is found also in the 
 placental structures ; but here probably it is of use, not for the 
 foetus, but for the nutrition and growth of the placental structures 
 themselves. We do not know how much carbohydrate material 
 finds its way into the umbilical vein; and we cannot therefore 
 state what is the source of the foetal glycogen ; but it is at least 
 possible, not to say probable, that it arises, in part at all events, 
 from a sphtting up of proteid material. 
 
Chap, iii-l TlIK NrTRITlON OF Till': EMBRYO. r,77 
 
 Concerning the rise and development of the functional activities 
 of the embryo, our knowlodf^c is almost a blank. We know 
 scarct'ly aiiythin<^ about the various steps by which the primary 
 fundamental (pialities of the protoplasm of the ovum arc differen- 
 tiated into the complex phenomena which we have attempted in 
 this book to expound. We can hardly state more than that while 
 muscular contractility becomes early developed, and the heart 
 probably, as in the chick, beats even before the blood-corpuscles 
 are formed, movements of the foetus do not, in the human subject, 
 become pronounced until after the fifth month ; from that time 
 forward they increase and subsequently become very marked. 
 They are often spoken of as reflex in character ; but only a pre- 
 conceived bias would prevent them from being regarded as largely 
 automatic. The digestive functions are naturally, in the absence 
 of all food from the alimentary canal, in abeyance. Though 
 pepsin may be found in the gastric membrane at about the fourth 
 month, it is doubtful whether a truly peptic gastric juice is 
 secreted during intra-uterine life ; trypsin appears in the pancreas 
 somewhat later, but an amylolytic ferment cannot be obtained 
 from that organ till after birth. The date however at which these 
 several ferments make their appearance in the embryo appears to 
 differ in different animals. The excretory functions of the liver 
 are developed early, and about the third month bile-pigment and 
 bile-salts find their way into the intestine. The quantity of bile 
 secreted during intra-uterine life, accumulates in the intestine and 
 especially in the rectum, forming, together with the smaller secre- 
 tion of the rest of the canal, and some desquamated epithelium, 
 the so-called meconium. Bile salts, both unaltered and variously 
 changed, the usual bile pigments, and cholesterin, are all present 
 in the meconium. The distinct formation of bile is an indication 
 that the products of foetal metabolism are no longer wholly carried 
 oft' by the maternal circulation ; and to the excretory function of 
 the liver there are now added those of the skin and kidney. The 
 substances escaping by these organs find their way into the 
 allantois or into the amnion, according to the arrangement of the 
 foetal membranes in different classes of animals; in both these 
 fluids urea or allied bodies have been found as well as the ordinary 
 saline constituents ; the latter may or may not have been actually 
 secreted. From the allantoic fluid of ruminants the body allan- 
 toin has been obtained, and human and other amniotic fluids have 
 been found to contain urea. It is maintained by some however 
 that the fluid in the amnion is secreted by the mother and that 
 hence the substances present in it are of maternal origin. 
 
 About the middle of intra-uterine life, when the foetal circula- 
 tion is in full development, the blood flo^ring along the umbilical 
 vein is carried chiefly by the ductus venosus into the inferior vena 
 cava and so into the right auricle. Thence it is dii'ected by the 
 valve of Eustachius through the foramen ovale into the left 
 
678 FCETAL CIRCULATION. [Book iv. 
 
 auricle, passing from which into the left ventricle it is driven into 
 the aorta. Part of the umbilical blood, however, instead of passing 
 directly to the inferior cava, enters by the jjortal vein into the 
 hepatic circulation, from which it returns to the inferior cava by 
 the hepatic veins. The inferior cava also contains blood coming 
 from the lower limbs and lower trunk. Hence the blood which 
 passing from the right auricle into the left auricle through the 
 foramen ovale is distributed by the left ventricle through the 
 aortic arch, though chiefly blood coming direct from the placenta, 
 is also blood which on its way from the placenta has passed 
 through the liver and blood derived from the tissues of the lower 
 part of the body of the foetus. The blood descending as foetal 
 venous blood from the head and limbs by the superior vena cava 
 does not mingle with that of the inferior vena cava, but falls into 
 the right ventricle, from which it is discharged through the ductus 
 arteriosus (Botalli) into the aorta, below the arch, whence it flows 
 partly to the lower trunk and limbs, but chiefly by the umbilical 
 arteries to the placenta. A small quantity only of the contents 
 of the right ventricle finds its way into the lungs. Now the blood 
 which comes from the placenta by the umbilical vein direct into 
 the right auricle is, as far as the foetus is concerned, arterial blood ; 
 and the portion of umbilical blood which traverses the liver 
 probably loses at this epoch very little oxygen during its transit 
 through that gland, the liver being at this period a simple ex- 
 cretory rather than an actively metabolic organ, Hence the blood 
 of the inferior vena cava, though mixed, is on the whole arterial 
 blood; and it is this blood which is sent by the left ventricle 
 through the arch of the aorta into the carotid and subclavian 
 arteries. Thus the head of the foetus is provided with blood 
 comparatively rich in oxygen. The blood descending from the 
 head and upper limbs by the superior vena cava is distinctly 
 venous ; and this passing from the right ventricle by the ductus 
 arteriosus is driven along the descending aorta, and together with 
 some of the blood passing from the left ventricle round the aortic 
 arch falls into the umbilical arteries and so reaches the placenta. 
 The foetal circulation then is so arranged, that while the most 
 distinctly venous blood is driven by the right ventricle back to the 
 placenta to be oxygenated, the most distinctly arterial (but still 
 mixed) blood is driven by the left ventricle to the cerebral struc- 
 tures, which have more need of oxygen than have the other tissues. 
 Contrary to what takes place afterwards, the work of the right 
 ventricle is in the foetus greater than that of the left ; and, ac- 
 cordingly, that greater thickness of the left ventricular walls, 
 so characteristic of the adult, does not become marked until close 
 upon birth. 
 
 In the later stages of pregnancy the mixture of the various 
 kinds of blood in the right auricle increases preparatory to the 
 changes taking place at birth. But during the whole time of 
 
Chap, in.] THE NUTRITION OF TllK EMBRYO. 079 
 
 mtra-uterino life the amount of oxygen in the blood passing from 
 the aortii- arch to the medulla oblongata is sufficient to prevent 
 any inspiratory impulses being originated in the medullary 
 resj)iratory centre. This during the whole period elapsing between 
 the date of its structural establishmiiit, or rather the conse([Uent 
 full d('vclo])niont of its inital)ility, and the epoch of birth, remains 
 dormant; the oxygen-su])ply to the protoplasm of its nerve-cells 
 is never brought so low a« to set going the respiratory molecular 
 explosions. As soon however as the intercourse between the 
 maternal and umbilical blood is interrupted by separation of the 
 placenta or by ligature of the umbilical cord, or when, as by the 
 dt'ath of the mother, the umbilical blood ceases to be replenished 
 with oxygen by the maternal blood, or when in any other way 
 blood of sufficiently arterial quality ceases to find its way by the 
 left ventricle to the medulla oblongata, the supply of oxygen in 
 the respiratory centre sinks, and when the fall has reached a 
 certain point an impulse of inspiration is generated and the foetus 
 for the first time breathes. This action of the respiratory centre 
 may be assisted by adjuvant impulses reaching the centre along 
 various afterent nerves, such as those started by exposure of the 
 body to the air, or to cold ; but these are subordinate, not essential. 
 A retarded first breath may be hunied on by dashing water on the 
 face of the new-born infant; but on the other hand, the foetus, 
 upon the cessation of the placental circulation, will make its first 
 respiratory movements while it is still invested wdth the intact 
 membranes and thus sheltered from the air and indeed from all 
 external stimuli. 
 
 Before this first breath is taken the pulmonary alveoli contain 
 no air, and the lungs when thrown into water sink at once ; they 
 are then said to be 'atelectatic' After the first breath, the alveoli 
 contain air and the lungs float Avhen thrown into water. A 
 striking difference however exists between the lungs of a new-bom 
 infant and those of an older person. When the pleural cavity of 
 the former is opened, the lungs do not collapse, no air is driven 
 out by the trachea; that partial distension of the lungs, and 
 negative thoracic pressure, which we studied (p. 367) in treating of 
 respiration, appears not to be established immediately upon birth. 
 That portion of the residual air (p. 315) in the lungs of the adult, 
 which, remaining after the most forcible expiration, is still driven 
 from the lungs upon the pleural cavity being laid open, and which 
 might be called 'collapse air,' is wanting in the new-born infant. 
 When the change from one condition to the other is effected is 
 not at present known ; it may possibly arise from the growth of 
 the chest outstripping that of the lungs. 
 
 When the first breath is taken, as under normal circumstances 
 it is, with fi-ee access to the atmosphere, and the lungs become 
 filled with air, the scanty supply of blood which at the moment 
 was passing from the right ventricle along the pulmonary artery 
 
680 FGETAL CIRCULATION. [Book iv. 
 
 returns to the left auricle brighter and richer in oxygen than ever 
 was the foetal blood before. With the diminution of resistance in 
 the pulmonary circulation caused by the expansion of the thorax, 
 a larger supply of blood passes into the pulmonary artery instead 
 of into the ductus arteriosus, and this derivation of the contents 
 of the right ventricle increasing with the continued respiratory 
 movements, the current through the latter canal at last ceases 
 altogether, and its channel shortly after birth becomes obliterated. 
 Corresponding to the greater flow into the pulmonary artery, a 
 larger and larger quantity of blood returns from the pulmonary 
 veins into the left auricle. At the same time the current through 
 the ductus venosus from the umbilical vein having ceased, the flow 
 from the inferior cava has diminished ; and the blood of the right 
 auricle finding little resistance in the direction of the ventricle, 
 which now readily discharges its contents into the pulmonary 
 artery, but finding in the left auricle, which is continually being 
 filled from the lungs, an obstacle to its passage through the fora- 
 men ovale, ceases to take that course. Any return of blood from 
 the now vigorous and active left auricle into the right auricle 
 is prevented by the valve which, during the latter stages of intra- 
 uterine life, has been growing up in the left auricle over the 
 foramen ovale. At birth the edge of this valve is to a certain 
 extent free so that, in case of an emergency, as when the pulmonary 
 circulation is obstructed, a direct escape of blood into the left 
 auricle from the over-burdened right auricle can take place. 
 Eventually, in the course of the first year, adhesion takes place, 
 and the separation of the two auricles becomes complete. With 
 its larger supply of blood and greater work the left ventricle 
 acquires the greater thickness characteristic of it during life. Thus 
 the fcetal circulation, in consequence of the respiratory movements 
 to which its interruption gives rise, changes its course into that 
 characteristic of the adult. 
 
CHAPTER IV. 
 
 PARTURITION. 
 
 In spite of the increasing distension of its cavity, the uterus remains 
 quiescent, as far as any marked muscular contractions are concerned, 
 until a certain time has been run. In the human subject the period 
 of gestation generally lasts from 275 to 280 days, i.e. about 40 
 weeks, the general custom being to expect parturition at about 
 280 days from the last menstruation. Seeing that, in many cases, 
 it is uncertain whether the ovum which developes into the embryo 
 left the ovary at the menstruation preceding or succeeding coitus, 
 or, as some have urged, independent of menstruation, by reason of 
 the coitus itself, an exact determination of the duration of preg- 
 nancy is impossible. 
 
 In the cow the period of gestation is about 280 days, in the mare 
 about 350, sheep about 150 days, dog about 60 days, rabbit about 30 
 days. 
 
 The extrusion of the foetus is brought about, partly by rhyth- 
 mical contractions of the uterus itself, and partly by a pressure 
 i-xerted by the contraction of the abdominal muscles, similar to that 
 described in defalcation. The contractions of the uterus are the 
 first to appear, and their first effect is to bring about a dilation of 
 the OS uteri ; it is not till the later stages of labour, while the foetus 
 is passing into the vagina, that the abdominal muscles are brought 
 into play. 
 
 The whole process of parturition may be broadly considered as 
 a refiex act, the nervous centre being placed in the lumbar cord. 
 In a dog, whose dorsal cord had been completely severed, parturi- 
 tion took place as usual : and the fact that, in the human subject, 
 
682 UTERINE CONTRACTIONS. [Book iv. 
 
 labour will progress quite naturally while the patient is unconscious 
 from the administration of chloroform, shews that in woman also 
 the whole matter is an involuntary action, however much it may be 
 assisted by direct volitional efforts. That the uterus is capable of 
 being thrown into contractions through reflex action, excited by 
 stimuli ap|)lied to various afferent nerves, is well known. The con- 
 traction of the uterus, which is so necessary for the prevention of 
 haemorrhage after delivery, may frequently be brought about by 
 exerting pressure or by dashing cold water on the abdomen, by the 
 introduction of foreign bodies into the vagina, and especially by 
 putting the child to the nipple. And we learn from experiments 
 on animals that rhythmic contractions of the uterus resembling at 
 least those of parturition may be brought about in a reflex manner 
 by stimulating various afferent nerves. Similar movements may 
 be induced by direct stimulation of the spinal cord along its whole 
 length, as well as of various parts of the brain ; but there are 
 reasons for thinking that in these cases, the impulses started in the 
 brain, and upper part of the spinal cord, produce their effects by 
 working upon what may be called a 'parturition' centre in the 
 upper lumbar regions of the cord. And it would appear that the 
 uterine contractions which are induced by such drugs as ergot as 
 well as those caused by asphyxia, are, at ail events in part, brought 
 about by the agency of the same lumbar centre. From this centre 
 the paths for the efferent impulses appear (in the dog) to be two- 
 fold : one along sympathetic tracts, by nerves passing from the 
 inferior mesenteric ganglion to the hypogastric plexus, and the 
 other along spinal tracts by branches of the sacral nerves to the 
 same plexus. It is stated that the characters of the movements 
 induced by stimulating these two tracts are somewhat different 
 and moreover that the sympathetic tract is vaso-constrictor and the 
 spinal tract vaso-dilator in nature; but the matter has not yet been 
 fully worked out. 
 
 We are however hardly justified in considering the rhythmical 
 contractions of the uterus during parturition as simple reflex acts 
 excited by the presence of the foetus. We are utterly in the dark 
 as to why the uterus, after remaining apparently perfectly quiescent 
 (or with contractions so slight as to be with difficulty appreciated) 
 for months, is suddenly thrown into action, and within it may be a 
 few hours or even less gets rid of the burden it has borne with 
 such tolerance for so long a time; none of the various hypotheses 
 which have been put forward can be considered as satisfactory. 
 And until we know what starts the active phase, we shall remain 
 in ignorance of the exact manner in which the activity is brought 
 about. The peculiar rhythmic character of the contractions, each 
 'pain' beginning feebly, rising to a maximum, then declining, and 
 finally dying away altogether, to be succeeded after a pause by a 
 similar pain just like itself, pain following pain like the tardy 
 long-drawn beats of a slowly beating heart, suggests that the cause 
 
CiiAi'. IV.] PARTURITION. 083 
 
 of the rh} thiiiic contraction is seated, likt^ that of the rhythinic l)cat 
 of the heart, in the organ itself. And this view is suiJjKjrted by the 
 fact that contractions of the uterus, similar to those of parturition, 
 have been observeil in animals even after complete destruction of 
 the spinal cord; and the movements induced by asphyxia seem in 
 part, and those caused by some drugs such as anniioni;i, seem to 
 be wholly due to an intrinsic action of the uterus itself. Neverthe- 
 less general evidence supports the conclusion that, in a normal 
 state of things at all events, the contractions of the uterus, like 
 those of the lymph-hearts, are largely dependent on the spinal 
 conl. 
 
 The occurrence of contractions in consequence of an asphyxiated 
 condition of the blood, explains why when pregnant animals are 
 asphyxiated, an extrusion of the foetus frequently takes place. There 
 is no evidence however that the onset of labour is caused by a 
 gradual diminution of oxygen in the blood, reaching at last to a 
 climax. Nor are there sufficient facts to connect parturition with 
 any condition of the ovary resembling that of menstruation. 
 
 The action of the abdominal muscles in parturition is, on the 
 other hand, obviously a reflex act earned out by means of the spinal 
 cord, the necessary stimulus being supplied by the pressure of the 
 foetus in the vagina, or by the contractions of the uterus. Hence 
 the whole act of parturition may with reason be considered as a 
 reflex one. 
 
 Whether it be wholly a reflex or partly an automatic one, the 
 act can readily be inhibited by the action of the central nei-vous 
 system. Thus emotions are a very frequent cause of the progress 
 of parturition being suddenly stopped; as is well known, the 
 entrance into the bedroom of a stranger often causes for a time the 
 sudden and absolute cessation of 'labour' jDains, which previously 
 may have been even violent. Judging from the analogy of mictu- 
 rition, between which and parturition there are many points of 
 resemblance, we may suppose that this inhibition of uterine con- 
 tractions is brought about by an inhibition of the centre in the 
 lumbar cord. 
 
 After the expulsion of the foetus, the foetal placenta separates 
 from the uterine walls, and is, together with the remnants of the 
 membranes, expelled after it. The uterus then falls into a firm 
 tonic contraction, similar to that of the emptied bladder, by Avhich 
 means ha^moiThage from the vessels torn by the separation of the 
 placenta is avoided. The lining membrane of the uterus is gradually 
 restored, the muscular elements are reduced by a rapid fatty de- 
 generation, and in a short time the whole organ has returned to its 
 normal condition. 
 
CHAPTEE V. 
 
 THE PHASES OF LIFE. 
 
 The child has at birth, on an average, rather less than one-third 
 the maximum length, and about one-twentieth the maximum 
 Aveight, to which in future years it will attain. 
 
 The composition of the body of the new-born babe, as compared 
 with that of the adult, will be seen from the following table, in 
 which the details are more full than those given on p. 443. 
 
 Weight of organ in 
 
 adult, as compared 
 
 with that of new-born 
 
 babe taken as 1. 
 
 i-7 
 3-7 
 12 
 12 
 13-6 
 15 
 
 20 
 
 20 
 26 
 28 
 60 
 
 It will be observed that the brain and eyes are, relatively to 
 the whole body-weight, very much larger in the babe than in the 
 adult, as is also, though to a less extent, the liver. This dispro- 
 portion is a very marked embryonic feature, and as far as the braiu 
 
 
 Weight of organ 
 
 in percentage 
 
 
 of Body- weight 
 
 A 
 
 
 New-born babe. 
 
 Adult, 
 
 Eye 
 
 •28 
 
 •028 
 
 Brain 
 
 14-34 
 
 237 
 
 Kidneys 
 
 •88 
 
 •48 
 
 Skin 
 
 113 
 
 6-3 
 
 Liver 
 
 4^39 
 
 2-77 
 
 Heart 
 
 •89 
 
 •52 
 
 Stomach and) 
 Intestine j 
 
 2-53 
 
 234 
 
 Lungs 
 
 2^16 
 
 201 
 
 Skeleton 
 
 16-7 
 
 15-35 
 
 Muscles, &c. 
 
 23^4 
 
 431 
 
 Testicle 
 
 •037 
 
 •8 
 
vum: v.] rnji PHASES of life. 685 
 
 aud t'yc are concerned at least, has a morphological or pliylo^aMiic, 
 us well as a physiological or teleological, si^uiticance. Inasmuch 
 as the smaller body hiis relatively the larger surface, the skin is 
 naturally proportionately greater in the babe. It is chiefly by the 
 accunuilution of muscle or llesh, properly so called, that the child 
 acquires the bulk and weight of the man, the skeletal framework, 
 in spite of its being s])eeitically lighter in its earlier cartilaginous 
 condition, maintaining throughout life about the same relative 
 weight. 
 
 The increase in stature is very rapid in early infancy, 
 proceeding however by decreasing increments. During or shortly 
 before puberty, there is again a somewhat sudden rise, with a 
 subsequent more steady but diminishing increase up to about 
 the twenty-fifth year. From thence to about fifty years of age the 
 height remains stationary, after which there may be a decrease, 
 especially in extreme old age. 
 
 The increase in weight is also very rapid at first, and proceed- 
 ing, like the height, with diminishing increments, may continue 
 till about the fortieth year. After the sixtieth year a decline 
 of variable extent is generally witnessed. It is a remarkable 
 fact, however, that in the first few days of life, so far from 
 there being an increase, there is an actual decrease of weight, 
 so that, even on the seventh day the weight still continues to be 
 less than at birth. 
 
 The saliva of the babe is active on starch, and its gastric juice, 
 unlike that of many neAV-born animals, has good peptic jDowers, 
 from wdiich we may infer that its digestive processes in general are 
 identical mth that of the adult ; but the feces of the infant con- 
 tain, besides considerable quantity of undigested food (fat, casein 
 &c.), unaltered bile-pigment, and undecomposed bile-salts. 
 
 The heart of the babe (see Table, p. 684) is, relatively to its 
 body-weight, larger than the adult, and the frequency of the heart- 
 beat much greater, viz. about 130 or 140 per minute, falling to 
 about 110 in the second year, and about 90 in the tenth year. 
 Corresponding to the smaller bulk of the body, the whole circuit of 
 the blood system is traversed in a shorter time than in the adult 
 (12 seconds as against 22); and consequently the renewal of 
 the blood in the tissues is exceedingly rapid. The respiration of 
 the babe is quicker than that of the adult, being at first about 35 
 ])er minute, falling to 28 in the second year, to 26 in the fifth 
 year, and so onwards. The respiratory work, while it increases 
 absolutely as the body grows, is, relatively to the body-weight, 
 greatest in the earlier years. It is worthy of notice, that the ab- 
 sorption of oxygen is said to be relatively more active than the 
 production of carbonic acid ; that is to say, there is a continued 
 accumulation of capital in the form of a store of oxygen-holding 
 explosive compounds (see p. 349). This, indeed, is the striking 
 feature of infant metabolism. It is a metabolism directed largely 
 
686 CHILDHOOD. [Book iv. 
 
 to constructive ends. The food taken represents, undoubtedly, so 
 much potential energy; but before that energy can assume a 
 vital mode, the food must be converted into tissue ; and, in such a 
 conversion, morphological and molecular, a large amount of energy 
 must be expended. The metabolic activities of the infant are 
 more pronounced than those of the adult, for the sake, not so much 
 of energies which are spent on the world without, as of energies 
 which are for a while buried in the rapidly increasing mass of flesh. 
 Thus the infant requires over and above the wants of the man, not 
 only an income of energy corresponding to the energy of the flesh 
 actually laid on, but also an income corresponding to the energy 
 used up in making that living sculptured flesh out of the dead amor- 
 phous proteids, fats, carbohydrates and salts, which serve as food. 
 Over and above this, the infant needs a more rapid metabolism to 
 keep up the normal bodily temperature. This, which is no less, 
 indeed slightly ("S") higher, than that of the adult, requires a 
 greater expenditure, inasmuch as the infant with its relatively far 
 larger surface, and its extremely vascular skin, loses heat to a 
 proportionately much greater degree than does the grown-up man. 
 It is a matter of common experience that children are more 
 affected by cold than are adults. 
 
 This rapid metabolism is however not manifest immediately 
 upon birth. During the first few days, corresponding to the loss 
 of weight mentioned above, the respiratory activities of the tissues 
 are feeble; the embryonic habits seem as yet not to have been 
 completely thrown off, and, as was stated on p. 377, new- born 
 animals bear with impunity a deprivation of oxygen, which would 
 be fatal to them later on in life. 
 
 The quantity of urine passed, though scanty in the first two 
 days, rises rapidly at the end of the first week, and in youth the 
 quantity of urine passed is, relatively to the body-weight, larger 
 than in adult life. This may be, at least in quite early life, partly 
 due to the more liquid nature of the food, but is also in part the 
 result of the more active metabolism. For not only is the quantity 
 of urine passed, but also the amount of urea and some other 
 urinary constituents excreted, relatively to the body-weight, 
 greater in the child than in the adult. The presence of uric,_ of 
 oxalic, and according to some, of hippuric acids in unusual quantities 
 is a frequent characteristic of the urine of children. It is stated 
 that calcic phosphates, and indeed the phosphates generally, are 
 deficient, being retained in the body for the building up of the 
 osseous skeleton. 
 
 Associated probably with these constructive labours of the 
 growing frame is the prominence of the lymphatic system. Not 
 only are the lymphatic glands largely developed and more active 
 (as is probably shewn by their tendency to disease in youth), but 
 the quantity of lymph circulation is greater than in later years. 
 Characteristic of youth is the size of the thymus body, which 
 
oiiAc. v.] rni': phases of life. 687 
 
 increases uj) to the socund year, and may then remain for a while 
 stationary, but gcuerully before puberty, has surt'ered a retro- 
 gressive metamorphosis, and fre([ucntly hardly a vestige of it 
 remains behind. Tlu^ thyroid body is also relatively greater 
 in the babe than in the adult; the spleen, on the other 
 hand, which grows rapitlly in early infancy, is not only absolutely, 
 but also relatively, greater in the adult. It need hardly be 
 said that the roeu])erative power of infancy and early youth is 
 very marketl. 
 
 It would be beyond the scope of this work to enter into the 
 psychical condition of the babe or the child, and our knowledge 
 of the details of the working of the nervous system in infancy 
 is too meagre to permit of any profitable discussion. It is hardly 
 of use to say that in the young the whole nervous system is 
 more irritable or more excitable than in later years; by Avhich 
 we probably to a great extent mean that it is less rigid, less 
 marked out into what, in preceding portions of this work, we 
 have spoken of as nervous mechanisms. It may be mentioned 
 that stimulation of the various cerebral areas, in new-born animals, 
 does not give I'ise to the usual localized movements. The sense 
 of touch, both as regards pressure and temperature, appears 
 well developed in the infant, as does also the sense of taste, 
 and possibly, though this is disputed, that of smell. The pupil 
 (larger in the infant than in the man) acts fully, and Bonders 
 observed normal binocular movements of the eyes in an infant 
 less than an hour old. The eye is (in man) from the outset 
 fully sensitive to light, though of course visual perceptions are 
 imperfect. As regards hearing, on the other hand, very little 
 reaction follows upon sounds, i. e. auditory sensations seem to be 
 dull during the first fe^v days of life ; this may be partly at least 
 due to absence of air from the tympanum and a tumid condition 
 of the tjrmpanic mucous membrane. As the child grows up his 
 senses rapidly culminate, and in his early years he possesses a 
 general acuteness of sight, hearing, and touch, which frequently 
 becomes blunted as his psychical life becomes fuller. Children 
 however are said to be less apt at distinguishing colours than in 
 sighting objects ; but it does not appear whether this arises from 
 a want of perceptive discrimination or from their being actually 
 less sensitive to variations in hue. A characteristic of the nervous 
 system in childhood, the result probably of the more active meta- 
 bolism of the body, is the necessity for long or frequent and deep 
 slumber. 
 
 Dentition marks the first epoch of the new^ life. At about seven 
 months the two central incisors of the lower jaw make their w^ay 
 through the gum, followed immediately by the con-esponding teeth 
 in the upper jaw\ The lateral incisors, first of the low^er and then 
 of the upper jaw^, appear at about the ninth month, the first molars 
 at about the twelfth month, the canines at about a year and a half, 
 
688 PUBERTY. [Book iv. 
 
 and the temporary dentition is completed by the appearance of the 
 second molars usually before the end of the second year. 
 
 About the sixth year the permanent dentition commences by 
 the appearance of the first permanent molar beyond the second 
 temporary molar; in the seventh year the central permanent 
 incisors replace their temporary representatives, followed in the 
 next year by the lateral incisors. In the ninth year the temporary 
 first molars are replaced by the first bicuspids, and in the tenth 
 year the second temporary molars are similarly replaced by the 
 second bicuspids. The canines are exchanged about the eleventh 
 or twelfth year, and the second permanent molars are cut about the 
 twelfth or thirteenth year. There is then a long pause, the third 
 or wisdom tooth not making its appearance till the seventeenth, or 
 even twenty-fifth year, or in some cases not appearing at all. 
 
 Shortly after the conclusion of the permanent dentition (the 
 wisdom teeth excepted) the occurrence of puberty marks the begin- 
 ning of a new phase of life ; and the difference between the sexes, 
 hitherto merely potential, now becomes functional. In both sexes 
 the maturation of the generative organs is accompanied by the 
 well-known changes in the body at large ; but the events are much 
 more characteristic in the typical female than in the aberrant male. 
 Though in the boy, the breaking of the voice and the rapid growth 
 of the beard which accompany the appearance of active spermato- 
 zoa, are striking features, yet they are after all superficial. The 
 curves of his increasing weight and height, and of the other events 
 of his economy, pursue for a while longer an unchanged course ; the 
 boy does not become a man till some years after puberty ; and the 
 decline of his functional manhood is so gradual that frequently it 
 ceases only when disease puts an end to a ripe old age. With the 
 occurrence of menstruation, on the other hand, at from thirteen to 
 seventeen years of age, the girl almost at once becomes a woman, 
 and her functional womanhood ceases suddenly at the climacteric 
 in the fifth decennium. During the whole of the child-bearing 
 period her organism is in a comparatively stationary condition. 
 While before the age of puberty up to about the eleventh or 
 twelfth year, the girl is lighter and shorter than the boy of the 
 same age, in the next few years her rate of growth exceeds his ; but 
 she has then nearly reached her maximum, while he continues to 
 grow. Her curve of weight from the nineteenth year onward to 
 the climacteric, remains stationary, being followed subsequently by 
 a late increase, so that while the man reaches his maximum of 
 weight at about forty, the woman is at her greatest weight about 
 fifty. 
 
 Of the statical differences of sex, some, such as the formation 
 of the pelvis, and the costal mechanism of respiration, are directly 
 connected with the act of child-bearing, while others have only an 
 indirect relation to that duty ; and indications at least of nearly all 
 the characteristic differences are seen at birth. The baby boy is 
 
Chap, v.] 77/ A' I'll ASKS OF Lll'K. C89 
 
 heavier aiul taller than tlio baby ^\v\, and lliu maiden of five 
 breathes with her ribs in tlic same way as docs the matron of forty. 
 The woman is liij^hter and sliorter than the man, the limits in the 
 case of the former beinL,^ from r444' to I'T-K) metres of hei^^ht and 
 froni .*31)"8 to 9;V8 kilos of weight, in the latter from 1"4G7 to 
 1890 of height, and from 49'1 to 98"5 kilos of weight. The muscu- 
 lar system and skeleton are both absolutely and relatively less in 
 woman, and her brain is lighter and smaller than that of man, 
 being about 1272 grammes to 1424. Her metabolism, as measured 
 by the respiratory and urinary excreta, is also not only absolutely 
 but relatively to the body-weight less, and her blood is not only 
 less in quantity but also of lighter specific gravity and contains a 
 smaller proportion of red corpuscles. Her strength is to that of 
 man as about 5 to 9, and the relative length of her step as 1000 to 
 1157. 
 
 From birth onward (and indeed from early intra-uterine life) 
 the increment of growth progressively diminishes. At last a point is 
 reached at which the curve cuts the abscissa line, and the increment 
 becomes a decrement. After the culmination of manhood at forty 
 and of womanhood at the climacteric, the prime of life declines 
 into old age. The metabolic activity of the body, which at first 
 was sufficient not only to cover the daily waste, but to add new 
 material, later on is able only to meet the daily wants, and at laist 
 is too imperfect even to sustain in its entirety the existing frame. 
 Neither as regards vigour and functional capacity, nor as regards 
 weight and bulk, do the turning-points of the several tissues and 
 organs coincide either with each other or w^ ith that of the body at 
 large. We have already seen that the life of such an organ as the 
 thymus is far shorter than that of its possessor. The eye is in its 
 dioptric prime in childhood, when its media are clearest and its 
 muscular mechanisms most mobile, and then it for the most part 
 serves as a toy ; in later years, when it could be of the greatest 
 service to a still active brain, it has already fallen into a clouded 
 and rigid old age. The skeleton reaches its limit very nearly at 
 the same time as the whole frame reaches its maximum of height, 
 the coalescence of the various epiphyses being pretty well completed 
 by about the twenty-fifth year. Similarly the muscular system in 
 its increase tallies with the Aveight of the wdiole body. The brain, 
 in spite of the increasing complexity of structure and function to 
 which it continues to attain even in middle life, early reaches its 
 limit of bulk and weight. At about seven years of age it attains 
 what may be considered as its first limit, for though it may increase 
 somewhat up to twenty, thirty, or even later years, its progress is 
 much more slow after than before seven. The vascular and digestive 
 organs as a whole may continue to increase even to a very late 
 period. From these facts it is obvious that though the phenomena 
 of old age are, at bottom, the result of the individual decline of the 
 several tissues, they owe many of their features to the disarrange- 
 F. 44 
 
690 OLD AGE. [Book iv. 
 
 ment of the whole organism produced by the premature decay or 
 disappearance of one or other of the constituent bodily factors. 
 Thus, for instance, it is clear that were there no natural intrinsic 
 limit to the life of the muscular and nervous systems, they would 
 nevertheless come to an end in consequence of the nutritive dis- 
 turbances caused by the loss of the teeth. And what is true of the 
 teeth is probably true of many other organs, with the addition that 
 these cannot, like the teeth, be replaced by mechanical contrivances. 
 Thus the term of life which is allotted to a muscle by virtue of its 
 molecular constitution, and which it could not exceed were it always 
 placed under the most favourable nutritive conditions, is, in the 
 organism, determined by the similar life-terms of other tissues; 
 the future decline of the brain is probably involved in the early 
 decay of the thymus. 
 
 Two changes characteristic of old age are the so-called cal- 
 careous and fatty degenerations. These are seen in a completely 
 typical form in cartilage, as, for instance, in the ribs; here the 
 protoplasm of the cartilage-corpuscle becomes hardly more than an 
 envelope of fat globules, and the supple matrix is rendered rigid 
 Avith amorphous deposits of calcic phosphates and carbonates, which 
 are at the same time the signs of past and the cause of future 
 nutritive decline. And what is obvious in the case of cartilage is 
 more or less evident in other tissues. Everywhere we see a dis- 
 position on the part of protoplasm to fall back upon the easier task 
 of forming fat rather than to carry on the more arduous duty of 
 manufacturing new material like itself; everywhere almost we see 
 a tendency to the replacement of a structured matrix by a deposit 
 of amorphous material. In no part of the system is this more 
 evident than in the arteries ; one common feature of old age is the 
 conversion by such a change of the supple elastic tubes into rigid 
 channels, whereby the supply to the various tissues of nutritive 
 material is rendered increasingly more difficult, and their intrinsic 
 decay proportionately hurried. 
 
 Of the various tissues of the body the muscular and nervous are 
 however those in which functional decline, if not structural decay, 
 becomes soonest apparent. The dynamic coefficient of the skeletal 
 muscles diminishes rapidly after thirty or forty years of life, and a 
 similar want of power comes over the plain muscular fibres also ; 
 the heart, though it may not diminish, or even may still increase 
 in weight, possesses less and less force, and the movements of the 
 intestine, bladder, and other organs, diminish in vigour. In the 
 nervous system, the lines of resistance, which, as we have seen, help 
 to map out the central organs into mechanisms, and so to produce 
 its multifarious actions, become at last hindrances to the passage of 
 nervous impulses in any direction, while at the same time the 
 molecular energy of the impulses themselves becomes less. The 
 eye becomes feeble, not only from cloudiness of the media and 
 presbyopic muscular inability, but also from the very bluntness of 
 
CuAP. v.] 77//; PHASES OF LIFK. CO I 
 
 the retina; the sensory and motor impulses j)us.s witli incre.asing 
 slowness to and from the eentral nervous syst(nn, and the brain 
 becomes a more and mon; rif^id mass of protoplasm, the molecular 
 lines of which rather mark the history of past actions than serve as 
 indications of present potency. The epithelial glandular elements 
 seem to be those whoso powers arc the longest preserved ; and 
 hence the man who in the prime of his manhood was a ' martyr to 
 dyspepsia' by reason of the sensitiveness of gastric nerves and 
 the retlex inhibitory and other results of their irritation, in his 
 later years, when his nerves are blunted, and when therefore his 
 peptic cells are able to pursue their chemical work undisturbed by 
 extrinsic nervous worries, eats and drinks with the courage and 
 success of a boy. 
 
 Within the range of a lifetime are comprised many periods of 
 a more or less frequent recurrence. In spite of the aids of a com- 
 plex civilisation, all tending to render the conditions of his life 
 more and more equable, man still shews in his economy the effects 
 of the seasons. Some of these are the direct results of varying 
 temperature, but some probably, such as the gain of w^eight in 
 winter and the loss in summer, are habits acquired by descent. 
 Within the year, an approximately monthly period is manifested in 
 the female by menstruation, though there is no exact evidence of 
 even a latent similar cycle in the male. The phenomena of recur- 
 rent diseases, and the marked critical days of many other maladies, 
 may be regarded as pointing to cycles of smaller duration than that 
 of the moon's revolution, unless we admit the view urged by some 
 authors that in these cases the recurrence is to be attributed rather 
 to periodical phases in the disease-producing germ itself, than to 
 variations in the medium of the disease. 
 
 Prominent among all other cyclical events is the fact that most 
 animals possessing a well-developed nervous system, must, night 
 after night, or day after day, or at least time after time, lay them 
 down to sleep. The salient feature of sleep is the cessation of the 
 automatic activity of the brain ; it is the diastole of the cerebral 
 beat. But the condition is not confined to the cerebral hemi- 
 spheres ; all parts of the body either directly or indirectly take 
 share in it. The phenomena of sleep are perhaps seen in their 
 simplest form in the winter-sleep or hybernation, to which especially 
 cold-blooded animals, but also to some extent warm-blooded animals, 
 are subject. In these cases the cold of winter slackens the vibra- 
 tions and lessens the explosions of the protoplasm, not only of 
 nervous but also of muscular and glandular structures ; indeed the 
 activity of the whole body is lowered, in some respects almost to 
 actual arrest. At the same time that the labour of the cerebral 
 molecules becomes insufficient to develop consciousness, the respi- 
 ratory centre is either wholly quiescent or discharges feeble im- 
 pulses at rare intervals, and the heart beats with a slow infrequent 
 stroke, not by reason of any inhibitory restraint, but because its 
 
 4i— 2 
 
692 SLEEP. [Book IV. 
 
 very substance in its slow molecular travail can gather head for 
 explosions only after long pauses of rest. And such few and distant 
 beats as do occur are amply sufficient to meet the needs of the 
 feeble metabolism of the several tissues. The sleep of every day 
 differs from the sleep of winter-cold chiefly because the slackening 
 of molecular activities is due in the former not to extrinsic but to 
 intrinsic causes, not to changes in the medium, but to exhaustion 
 of the subject, and because the phenomena are largely confined to 
 the cerebral hemispheres. It is true that the whole body shares in 
 the condition. The pulse and breathing are slower, the intestine 
 and other internal muscular mechanisms are more or less at rest, 
 the secreting organs are less active, some apparently being wholly 
 quiescent, and the sleeper on waking rubs his eyes to bring back 
 to his conjunctiva its needed moisture. Indeed the whole meta- 
 bolism and the dependent temperature of the body are lowered ; 
 but we cannot say at present how far these are the indirect results 
 of the condition of the nervous system, or how far they indicate a 
 partial slumbering of the several tissues. 
 
 Thoracic respiration is said to become more prominent than 
 diaphragmatic respiration during sleep, and the Cheyne-Stokes 
 rhythm of respiration (see p. 362) is frequently observed. During 
 sleep the pupil is contracted, during deep sleep exceedingly so ; and 
 dilation, often unaccompanied by any visible movements of the limbs 
 or body, takes place when any sensitive surface is stimulated ; on 
 awaking also the pupils dilate. The eye-balls have been generally 
 described as being during sleep directed ujDAvards and converging, 
 or according to some authors, diverging ; but others maintain that 
 in true sleep the visual axes are parallel and directed to the far 
 distance. The eyes of children have been described as continually 
 executing during sleep movements, often irregular and unsymme- 
 trical and unaccompanied by changes in the pupils. 
 
 We are not at present in a position to trace out the events 
 which culminate in this inactivity of the cerebral structures. It 
 has been urged that during sleep the brain is ansemic ; but even if 
 this anaemia is a constant accompaniment of sleep, it must, like the 
 vascular condition of a gland or any other active organ, be regarded 
 as an effect, or at least as a subsidiary event, rather than as a 
 primary cause. Nor can the view which regards sleep as the result 
 of a shifting of the mechanical arrangements of the cranial circu- 
 lation be considered as satisfactory. The explanation of the con- 
 dition is rather to be sought in purely molecular changes ; and the 
 analogy between the systole and diastole of the heart, and the 
 waking and sleeping of the brain, may be profitably pushed to a 
 very considerable extent. The sleeping brain in many respects 
 closely resembles a quiescent but still living ventricle. Both are 
 as far as outward manifestations are concerned at rest, but both 
 may be awakened to activity by an adequately powerful stimulus. 
 Both, though quiescent, are irritable, in both the quiescence will 
 
Chap, v.] THK rilASES OF LIFE. 093 
 
 ultimately give place to activity, and in both an appropriate 
 stimulus applied at the right time ^vill determine the change from 
 rest to action. Just as a single i)riek will under certain circum- 
 stances awake a ventricle, which for sumo seconds has been 
 motionless, into a rhythmic activity of many beats, so a loud noise 
 will start a man from sleep into a long day's wakefulness. And 
 just as in the heart the cardiac irritability is lowest at the beginning 
 of the diastole and increases onwards till a beat bursts out, so is 
 sleep deej)est at its commencement after the day's labour ; thence 
 onward .slighter and slighter stimuli are needed to wake the sleeper. 
 For juilging of the depth of ordinary nocturnal sleep by the inten- 
 sity of the noise recjuired to wake the sleeper, it may be concluded 
 that, increasing very rapidly at first, it reaches its maximum within 
 the first hour ; from thence it diminishes, at first rapidly, but after- 
 w^ards more slowly. 
 
 We cannot, however, at present make any definite statements 
 concerning the nature of the molecular changes which determine 
 this rhythmic rise and fall of cerebral irritability. The fact that 
 the products of protojalasmic activity when they accumulate within 
 the protoplasm appear to become in the end an obstruction to 
 that activity, has suggested the idea that the presence in the 
 cerebral tissue of an excess of the products of nerv^ous metabolism 
 is the cause of sleep. Indeed lactic acid, the increase of which 
 was supposed to be the cause of the acid reaction of muscular 
 and nervous tissues after exercise, has been especially pointed to 
 in this connection ; but, as we have seen, the acid reaction in 
 question appears not to be due to any increased production of 
 lactic acid. Besides, if the accumulation of metabolic products of 
 any kind were the cause of sleejD, it is not clear why we should 
 ever have any hope of waking. More may be said in favour of 
 the conception that during the waking hours the expenditure of 
 oxygen exceeds the income and that the quiescence, w^hich w^e call 
 sleep, comes from the exhaustion of the body's store of oxygen, 
 more especially of that 'intramolecular' oxygen of w^hich we spoke, 
 in dealing with the respiration of the tissues. But to this view 
 must be added some hypothesis, such as the byplay of some 
 inhibitory mechanism, whereby the respiratory centre is not roused 
 to increased activity by this lack of oxj^gen, for as we have seen 
 the breathing shares in the slumber of the body, though continuing 
 to play with an amount of energy, which permits a gi'adual 
 restoration of the lost store of oxygen and so finally brings on 
 the awakening which ends the sleep. And the necessity for such 
 a complication indicates that the explanation is, at present at 
 least, inadequate. 
 
 The phenomena of sleep shew very clearly to how large an 
 extent an apparent automatism is the ultimate outcome of the 
 effects of antecedent stimulation. When we wish to go to sleep we 
 withdraw our automatic brain as much as possible from the influ- 
 
694 SLEEP. [Book iv. 
 
 ence of all extrinsic stimuli ; and an interesting case is recorded of 
 a lad whose connection with the external world was, from a compli- 
 cated ansesthesia, limited to that afforded by a single eye and a 
 single ear, and who could be sent to sleep at will, by closing the 
 eye and stopping the ear. 
 
 The cycle of the day is however manifested in many other ways 
 than by the alternation of sleeping and waking, with all the in- 
 direct effects of these two conditions. There is a diurnal curve of 
 temperature (see p. 468), apparently independent of all immediate 
 circumstances, the hereditary impress of a long and ancient sequence 
 of days and nights. Even the i^ulse, so sensitive to all bodily 
 changes, shews, running through all the immediate effects of the 
 changes of the minute and the hour, the working of a diurnal in- 
 fluence which cannot be accounted for by waking and sleeping, by 
 working and resting, by meals and abstinence between meals. And 
 the same may be said concerning the rhythm of respiration, and 
 the products of pulmonary, cutaneous and urinary excretion. There 
 seems to be a daily curve of bodily metabolism, which is not the 
 product of the day's events. Within the day we have the narrower 
 rhythm of the respiratory centre with the accompanying rise and 
 fall of activity in the vaso-motor centres. And lastly, there stands 
 out the fundamental fact of all bodily periodicity, that alternation of 
 the heart's systole and diastole which ceases only at death. Though, 
 as we have seen, the intermittent flow in the arteries is toned down 
 in the capillaries to an apparently continuous flow, still the con- 
 stantly repeated cycle of the cardiac shuttle must leave its mark 
 throughout the whole web of the body's life. Our means of inves- 
 tigation are, however, still too gross to permit us to track out its 
 influence. Still less are we at present in a position to say how far 
 the fundamental rhythm of the heart itself, that rhythm which is 
 influenced, but not created, by the changes of the body of which 
 it is the centre, is- the result of cosmical changes, the reflection as 
 it were in little of the cycles of the universe, or how far it is the 
 outcome of the inherent vibrations of the molecules which make up 
 its substance. 
 
CHAPTER YI. 
 DEATH. 
 
 When the animal kingdom is surveyed from a broad stand-point, it 
 
 becomes ob^■ious that the ovum, or its correlative the spermatozoon, 
 is the goal of an individual existence : that life is a cycle beginning 
 in an ovum and coming round to an ovum again. The greater part 
 of the actions Avhich, looking from a near point of view at the 
 higher animals alone, we are apt to consider as eminently the pur- 
 poses for which animals come into existence, when ^'iewed from the 
 distant outlook whence the whole living world is sun-eyed, fade 
 away into the likeness of the mere b}^lay of ovum-bearing organ- 
 isms. The animal body is in reality a vehicle for ova ; and after 
 the life of the parent has become potentially renewed in the oft- 
 spring, the body remains as a cast-off envelope whose future is but 
 to die. 
 
 Were the animal frame not the complicated machine we have 
 seen it to be, death might come as a simple and gradual dissolution, 
 the 'sans everything' being the last stage of the successive loss of 
 fundamental powers. As it is, however, death is always more or 
 less violent ; the machine comes to an end by reason of the disorder 
 caused by the breaking down of one of its parts. Life ceases not 
 because the molecular powers of the whole body slacken and are 
 lost, but because a weakness in one or other part of the machinery 
 throws its whole working out of gear. 
 
 We have seen that the central factor of life is the circulation 
 of the blood, but we have also seen that blood is not only useless, 
 but injurious, unless it be duly oxygenated ; and we have further 
 seen that in the higher animals the oxygenation of the blood can 
 only be duly effected by means of the respirator}' muscular mechan- 
 ism, presided over by the medulla oblongata. Thus the life of a 
 complex animal is, when reduced to a simple form, composed of 
 three factors ; the maintenance of the circulation, the access of au- 
 to the hcemoglobin of the blood, and the functional activitv of the 
 
696 DEATH. [Book iv. 
 
 respiratory centre ; and death may come from the arrest of either 
 of these. As Bichat put it, death takes place by the heart or by 
 the lungs or by the brain. In reality, however, when we push the 
 analysis further, the central fact of death is the stoppage of the 
 heart, and the consequent arrest of the circulation ; the tissues then 
 all die, because they lose their internal medium. The failure of the 
 heart may arise in itself, on account of some failure in its nervous or 
 muscular elements, or by reason of some mischief affecting its me- 
 chanical working. Or its stoppage may be due to some fault in its 
 internal medium, such for instance as a want of oxygenation of the 
 blood, which in turn may be caused by either a change in the blood 
 itself, as in carbonic oxide poisoning, or by a failure in the mechan- 
 ical conditions of respiration, or by a cessation of the action of the 
 respiratory centre. The failure of this centre, and indeed that of 
 the heart itself, may be caused by nervous influences proceeding 
 from the brain, or brought into operation by means of the central 
 nervous system ; it may, on the other hand, be due to an imperfect 
 state of blood, and this in turn may arise from the imperfect or 
 perverse action of various secretory or other tissues. The modes of 
 death are in reality as numerous as are the possible modifications 
 of the various factors of life; but they all end in a stoppage of the 
 circulation, and the withdrawal from the tissues of their internal 
 medium. Hence we come to consider the death of the body as 
 marked by the cessation of the heart's beat, a cessation from which 
 no recovery is possible ; and by this we are enabled to fix an exact 
 time at which we say the body is dead. We can, however, fix 
 no such exact time to the death of the individual tissues. They 
 are not mechanisms, and their death is a gradual loss of power. 
 In the case of the contractile tissues, we have apparently in rigor 
 mortis a fixed term, by which we can mark the exact time of their 
 death. If we admit that after the onset of rigor mortis recovery 
 of irritability is impossible, then a rigid muscle is one permanently 
 dead. In the case of the other tissues, we have no such objective 
 sign, since the rigor mortis of simple protoplasm manifests itself 
 chiefly by obscure chemical signs. And in all cases it is obvious 
 that the possibility of recovery, depending as it does on the skill 
 and knowledge of the experimenter, is a wholly artificial sign of 
 death. Yet we can draw no other sharp line between the seemingly 
 dead tissue whose life has flickered down into a smouldering ember 
 which can still be fanned back again into flame, and the handful of 
 dust, the aggregate of chemical substances into which the decom- 
 posing tissue finally crumbles. 
 
 Moreover, the failure of the heart itself is at bottom loss of 
 irritability, and the possibility of recovery here also rests, as far 
 as is known at present, on the skill and knowledge of those who 
 attempt to recover. So that after all the signs of the death of the 
 whole body are as artificial as those of the death of the constituent 
 tissues. 
 
APPENDIX. 
 
APPENDIX. 
 
 ON THE CHEMICAL BASIS OF THE ANIMAL BODY. 
 
 The animal body, from a chemical point of view may be regarded 
 as a mixture of various representatives of three large classes of 
 chemical substances, viz. proteids, carbohydrates and fats, in associa- 
 tion with smaller quantities of various saline and other crystalline 
 bodies. By proteids are meant bodies containing carbon, oxygen, 
 hydrogen and nitrogen in a certain proportion, varying within narrow 
 limits, and having certain general features ; they are frequently spoken 
 of as albuminoids. By carbohydrates are meant starches and sugars 
 and their allies. "We have also seen that the animal body may be 
 considered as an assemblage of protoplasm under various modifications 
 and of numerous products of protoplasmic activity. We do not at 
 present know anything definite about the molecular composition of 
 active living protoplasm ; but when we submit protoplasm to chemical 
 analysis, in which act it is killed, we always obtain from it a considerable 
 quantity of the material spoken of as proteid. And many authors go 
 so far as to speak of protoplasm as being purely proteid in nature : they 
 regard the living protoplasm as proteid material, which in passing from 
 death to life, has assumed certain characters and presumably has 
 been changed in construction, but still is proteid matter ; they some- 
 times speak of protoplasm as ' living proteid ' or ' living albumin.' It is 
 worthy of notice however that even simple fonns of protoplasm, like that 
 constituting the body of a white corpuscle, forms of protoplasm which 
 we may fairly consider as native protoplasm, when they can be obtained 
 in sufiicient quantity for chemical analysis, are found to contain some 
 
700 CHEMICAL BASIS [App, 
 
 representatives of carbohydrates and fats as well as of proteids. We 
 might perhaps even go as far as to say, that in all forms of living proto- 
 plasm^ the proteid basis is found upon analysis to have some carbohydrate 
 and some kind of fat associated with it. Further, not only does the 
 normal food Avhich is eventually built up into protoplasm consist of all 
 three classes, but, as we have seen in the sections on nutrition, proto- 
 plasm gives rise by metabolism to members of the same three classes ; 
 and as far as we know at present, carbohydrates and fats, when formed 
 in the body out of proteid food, are so formed by the agency of living 
 protoplasm, by some living tissue. Hence there is at least some 
 reason for thinking it probable that the molecule of protoplasm, if 
 we may use such a phrase, is far more- complex than a molecule of 
 proteid matter, that it contains in itself residues so to speak not only of 
 proteid but also of carbohydrate and fatty material. 
 
 Be this as it may, for no dogmatic statement can at present be made, 
 when we examine the various tissues and fluids of the animal body from 
 a chemical point of view we find present in different places, or at different 
 times, seA^eral varieties and derivatives of the three chief classes ; we 
 find many forms of proteids, and bodies closely allied to proteids, in the 
 forms of mucin, gelatine, Jic. ; many varieties of fats ; and several kinds 
 of carbohydrates. 
 
 We find moreover many other bodies which we may regard as 
 stages in the constructive or destructive metabolism of both native and 
 difierentiated protoplasm, and which are important not so much from 
 the quantity in which they occur in the animal body at any one time as 
 from their throwing light on the nature of animal metabolism ; these 
 are such bodies as urea, other organic crystalline bodies, and the ex- 
 tractives in general. 
 
 In the following pages the chemical features of the more important 
 of these various substances which are known to occur in the animal 
 body will be briefly considered, such characters only being described as 
 possess or promise to possess physiological interest. The physiological 
 function of any substance must depend ultimately on its molecular 
 (including its chemical) nature ; and though at present our chemical 
 knowledge of the constituents of an animal body gives us but little 
 insight into their physiological properties, it cannot be doubted that 
 such chemical information as is attainable is a necessary preliminary to 
 all physiological study. 
 
Apr.] OF Till-: AMMAL IIODY. 701 
 
 rilOTElDS. 
 
 Tliesc form the principal solids of the muscular, nervous, and gland- 
 ular tissues, of the serum of blood, of serous fluids, and of lymph. In 
 a healthy condition, sweat, tears, bile and urine contain mere traces, if 
 any, of proteids. Their general percentage composition may be taken as 
 
 O. ir. N. C. S. 
 
 From 20-9 GO IT) -2 51-5 3 
 
 to 23-5 to 7-3 to 170 to 54-5 to 20 
 
 (Hoppe-Seyler'.) 
 
 These figures are obtained from a consideration of numerous analyses, slight 
 differences in the various results being immaterial, where the purity of the substance 
 operated upon cannot be definitely determined. 
 
 In addition to the above constituents, proteids leave on ignition a variable 
 quantity of ash. lu the case of egg-albumin the principal constituents of the ash 
 are chlorides of sodium and potassium, the latter greatly exceeding the former in 
 amount. The remainder consists of sodium and potassium, in combination with 
 phosphoric, suli^huric, and carbonic acids, and very small quantities of calcium, 
 magnesiimi aud iron, in union with the same acids. There is also a trace of silica^. 
 The ash of serum-albumin contains an excess of sodium chloride, but the ash of the 
 proteids of muscle contaius an excess of potash salts and phosphates. The nature 
 of the connection of the ash with the proteid is still a matter of obscurity. Globin 
 from haemoglobin is said to leave no ash on ignition. 
 
 Proteids as met with in the animal body are all amorphous ; some 
 are soluble, some insoluble in water, and all are for the most part insoluble 
 in alcohol and ether; they are all soluble in strong acids and alkalis, 
 but in becoming dissolved mostly undergo decomposition. Their solutions 
 possess a left-handed rotatory action on the plane of polarisation, the 
 amount depending on various circumstances, and being, with one ex- 
 ception, viz. peptones, changed by heating. 
 
 Crystals into whose composition certain proteid (especially globulin)' elements 
 enter were long since observed in the seeds of many plants; as yet they have not 
 been obtained sufiiciently isolated or in quantities large enough to permit any 
 accurate analysis to be made. A method of isolating iu quantity and recrystallizing 
 these substances has however* been indicated, and it seems probable that analysis 
 of these may lead to interesting information on the subject of the constitution and 
 combinations of proteids. 
 
 The presence of proteids may be determined by the following tests. 
 
 1. Heated with strong nitric acid, they or their solutions turn 
 yellow, and this colour is, on the addition of ammonia, or caustic soda 
 or potash, changed to a deep orange hue. (Xanthoproteic reaction.) 
 
 1 Hdb. Phys. Path. Chem. Anal., Ed. iv. (1875) S. 223. 
 
 - See Gmeliu, Hdb. Orrj. Chem., Bd. viii. S. 285. 
 
 3 Vines, Jl. oj Fhygiol.' \o]. iii. (1880) p. 98. 
 
 * Drechsel, Journ.f. prakt. Chem., N. V. Bd. xis. (1879) S. 331. 
 
702 PROTEIDS. [App. 
 
 3. With Millon's reagent they give, when present in sufficient 
 quantity, a precipitate, which turns red on heating. If they are only 
 present in traces, no precipitate is obtained, but merely a red colouration 
 of the solution. 
 
 3. If mixed with some concentrated solution of sodic hydrate, 
 and one or two drops of a solution of cupric sulphate, a violet colour is 
 obtained, which deepens in tint on boiling. 
 
 The above serve to detect the smallest traces of all proteids. The 
 two following tests may be used when there is more than a trace 
 present, but do not hold for every kind of proteid. 
 
 4. Render the fluid strongly acid with acetic or other acid, and 
 add a few drops of a solution of ferrocyanide of potassium ; a pre- 
 cipitate shews the presence of proteids. 
 
 5. Render the fluid, as before, strongly acid with acetic acid, add 
 an equal volume of a concentrated solution of sodic sulphate, and 
 boil. A precipitate is formed if proteids are present. 
 
 This last reaction is useful, not only on account of its exactness, but also 
 because the reagents used produce no decomposition of other bodies which may be 
 present; and hence after filtration the same fluid may be further analysed for other 
 ■substances. Additional methods of freeing a solution from proteids are : acidulating 
 with acetic acid and boiling, avoiding any excess of the acid ; precipitation by excess 
 of alcohol; in the latter case the solution must be neutral or faintly acid. Hoppe- 
 Seyler^ recommends the employment of a saturated solution of freshly precipitated 
 ferric hydrate, in acetic acid ; this is added to the solution, and on boiling the 
 whole of the proteids are precipitated as well as the ferric salt, the latter as a basic 
 acetate, Briicke's method of removing the last traces of proteids from glycogen 
 solutions is also of use (see glycogen). Precipitation of the last traces of proteids by 
 means of hydrated oxide of lead at a boiling temperature ^ may be also employed. 
 
 Proteids may be conveniently divided into classes. 
 
 Class I. Native Albumins. 
 Members of this class, as their name implies, occur in a natural 
 condition in animal tissues and fluids. They are soluble in water, are 
 not precipitated by very dilute acids, by carbonates of the alkalis, or 
 by sodium chloride. They are coagulated by heating in solution to a 
 temperature of about 70" C. If dried at 40" C, the resulting mass is of 
 a pale yellow colour, easily friable, tasteless, inodorous and soluble. 
 
 1. Egg-albumin. 
 
 Forms in aqueous solution a neutral, transparent, yellowish fluid. 
 From this it is precipitated by excess of strong alcohol. If the alcohol 
 be rapidly removed the precipitate may be readily redissolved in water; 
 
 1 Op. cit. S. 227. 
 
 - Hofmeister, Zeitsch. f. plnjsioh Chem., Ed. ii. (1878). S. 2S8, 
 
API'.] CHEMICAL HAS IS OF THE ASIMAL BODY. 703 
 
 if subjected to lengthier action a coagulation occurs, and the albumin is 
 then no longer thus soluble. Strong acids, especially nitric acid, cause 
 a coagulation similar to that produced by heat or by the prolonged 
 action of alcohol ; the albumin becomes profoundly changed by the 
 action of the acid and does not dissolve upon removal of the acid. 
 Mercuric chloride, argentic nitrate, and lead acetate, precipitate the 
 albumin, forming insoluble compounds of variable composition with it : 
 the precipitant may be removed by means of sulphuretted hydrogen and 
 the albumin again obtained, apparently unaltered, in solution. 
 
 Strong acetic acid in excess gives no precipitate, but when the 
 solution is concentrated the albumin is transformed into a transparent 
 jelly. A similar jelly is produced when strong caustic potash is added 
 to a concentrated solution of egg-albumin. In both these ca.ses the 
 substance is profoundly altered becoming in the one case acid — in the 
 other alkali-albumin. 
 
 The specific rotatory power of egg-albumin in aqueous solution is, 
 for yellow light, — 35 ■5". Hydrochloric acid, added until the reaction is 
 strongly acid, increases this rotation to — 37'7". Tlie formation of the 
 gelatinous compound with caustic potash is at first accompanied with an 
 increase, but this is followed by a decrease of rotation. 
 
 Preparation. Wliite of hen's eg^ is broken up with scissors into 
 small pieces, diluted with an equal bulk of water, and the mixture 
 .shaken strongly in a flask till quite fi'othy ; on standing, the foam rises 
 to the top, and carries all the fibres in whose meshwork the albumin 
 ■was contained. The fluid, from which the foam has been removed, is 
 strained, and treated carefully with dilute acetic acid as long as any 
 precipitate is formed; the precipitate is then filtered off, and the filtrate 
 after neutralisation concentrated at 40*^ to its original bulk. 
 
 2. Serum-albumin. 
 
 This form of albumin resembles, to a great extent, the one previously 
 described. The following may suftice as distinguishing features. 
 
 1. The specific rotation of serum-albumin is — 50"; that of egg- 
 albumin is - 35 o", both measured for yellow light. 
 
 2. Serum-albumin is not coagulated by being shaken up with ether, 
 egg-albumin is. 
 
 3. Serum-albumin is not very readily precipitated by strong hydro- 
 chloric acid, and such precipitate as does occur is readily redissolved on 
 further addition of the acid ; the exact reverse of these two features 
 holds good for egg-albumin. 
 
 4. Precipitated or coagulated serum-albumin is readily soluble, 
 egg-albumin is with difliculty soluble, in strong nitric acid. 
 
704 PROTEIDS. [App. 
 
 5. Egg-albumin if injected subcutaneously or into a vein, reappears 
 unaltered in the urine* ; serum-albumin similarly injected does not thus 
 normally pass out by the kidney. 
 
 Serum-albumin is found not only in blood-serum, but also in lymph, 
 both that contained in the proper lymphatic channels and that diffused 
 in the tissues ; in chyle, milk, transudations and many pathological 
 fluids. 
 
 It is this form in which albumin generally appears in the urine. 
 
 In addition to the above, Scherer^ has described two closely related bodies, to 
 which he gives the names Paralbumin and Metalbumin. The first he obtained from 
 ovarian cysts ; its alkaline solutions are remarkable for being very ropy. It seems 
 doubtful whether this body is a proteid; it differs sensibly in composition from 
 these. Haerlin^ gives as its composition, 0. 26*8, H. 6-9, N. 12-8, C. 51-8, S. 1-7 p.c. 
 It seems to be associated with some body like glycogen, capable of being converted 
 into a substance giving the reactions of dextrose. Metalbumin, found in a dropsical 
 fluid, resembles the preceding, but is not precipitated by hydrochloric acid, or by 
 acetic acid and ferrocyanide of potassium; it is precipitated, but not coagulated, by 
 alcohol ; its solution is scarcely coagulated on boiling. 
 
 Albumins are generally found associated with small but definite 
 amounts of saline matter. A. Schmidt* says that they may be freed 
 from these by dialysis, and that they are then not coagulated on boiling. 
 From this it might be inferred that the albumin and the saline matters 
 were peculiarly related, and that the latter played some special part during 
 the coagulation of the former by heat. Schmidt's observations however 
 have not been conclusively corroborated by subsequent observers. 
 
 Class II. Derived Albumins [Albuminates), 
 
 1. Acid-albumin. 
 
 When a native albumin in solution, such as serum-albumin, is treated 
 for some little time with a dilute acid such as hydrochloric, its proper- 
 ties become entirely changed. The most marked changes are (1) that 
 the solution is no longer coagulated by heat ; (2) that when the solution 
 is carefully neutralized the whole of the proteid is thrown down as a 
 precipitate; in other words, the serum-albumin which was soluble in 
 water, or at least in a neutral fluid containing only a small quantity of 
 neutral salts, has become converted into a substance insoluble in water 
 or in similar neutral fluids. The body into which serum-albumin thus 
 becomes converted by the action of an acid is spoken of as acid-albumin. 
 Its characteristic features are that it is insoluble in distilled water, and 
 
 ^ Stokvis, Rech. exp. sur les condit. pathol. de Valhuminurie, Bruxelles, 1867 ; also 
 Lehmann, Arch. f. path. Atiat. Bd. xxx. (1864), S. 593. 
 
 2 Ann. der Chem. und Pharni., Bd. 82, S. 135. 
 
 3 Chevi. Centralblatt, 1862, No. 56. 
 
 4 Pfluger's Archiv, xi. (1875), S. 1. 
 
Ait. I CHEMICAL HAS/S OF T IIE A S I M A L I'.oDY. 70.-) 
 
 in neutnil s:ilin(> solutions, suc-h ns those of sodic cliloridi-, (hat it 
 is readily solulik- in dilute acids or dilute alkalis, and that its solutions 
 in acids or alkalis are not coagulated by boiling. When suspended, in 
 the undissolved state, in water, and heated to 70" C, it becomes coagulated, 
 and is then undistinguishable from coagulated serum-albumin, or indeed 
 from any other form of coagulated proteid. It is evident that the sub- 
 stance when in solution in a dilute acid is in a different condition from 
 that in whicli it is when precipitated by neutralisation. If a quantity 
 of serum- or egg-albumin be treated with dilute hydrochloric acid, it 
 will be found that the conversion of the native albumin into acid- 
 albumin is gradual ; a specimen heated to 70" C immediately after the 
 addition of the dilute acid, will coagulate almost as usual ; and another 
 specimen taken at the same time will give hardly any precipitate on 
 neutralisation. Some time later, the interval depending on the pro- 
 portion of the acid to the albumin, on temperature, and on other 
 circumstances, the coagulation will be less, and the neutralisation 
 precipitate will be considerable. Still later the coagulation will be absent, 
 and the whole of the jiroteid will be thrown down on neutralisation. 
 
 If finely-chopped muscle, from which the soluble albumins have been 
 removed by repeated washing, be treated for some time with dilute 
 (•2 per cent.) hydrochloric acid, the greater part of the muscle is dis- 
 solved. The transparent acid filtrate contains a large quantity of 
 proteid material in a form which, in its general characters at least, 
 agrees with acid-albumin. The acid solution of the proteid is not 
 coagulated by boiling, but the whole of the proteid is precipitated on 
 neutralisation ; and the precipitate,, insoluble in neutral sodic chloride 
 solutions, is readily dissolved by even dilute acids or alkalis. The 
 proteid thus obtained from muscle has been called syntonin, but we 
 have at present no satisfactory test to distinguish the acid-albumin (or 
 syntonin) prepared from muscle from that prepared from egg- or serum- 
 albumin. When coagulated albumin or other coagulated proteid or 
 fibrin is dissolved in strong acids,, acid-albumin is formed ; and when 
 fibrin or any other proteid is acted upon by gastric juice, acid-albumin 
 is one of the first products ; and these acid-albumins cannot be dis- 
 tinguished from acid-albumin prepared fi-om muscle or native albumin. 
 Though hydrochloric acid is pei-haps the most convenient acid for 
 forming acid-albumin, other acids may also be used for the purpose of 
 preparing it. Acid-albumin is solul)le not only in dilute alkalis, but 
 also in dilute solutions of alkaline carbonates ; its solutions in these are 
 not coagulated by boiling. 
 
 If sodic chloride in excess is added to an acid solution of acid- 
 albumin, the acid-albumin is precipitated : this also occurs on adding 
 sodic acetate or phosphate. 
 
 F. 45 
 
706 PROTEIDS. [App. 
 
 As special tests of acid-albumin may be given : 1. Partial coagu- 
 lation of its solution in lime-water on boiling. 2. Further precipitation 
 of the same solution after boiling, on the addition of calcic chloride, 
 magnesic sulphate, or sodic chloride. 
 
 Dissolved in very dilute hydrochloric acid, acid-albumin (syntonin) 
 prepared from muscle possesses a specific Isevo -rotatory power of —72" for 
 yellow light, this being independent of the concentration^ On heating 
 the solution in a closed vessel in a water-bath, the rotatory power rises 
 to - 84-8°. 
 
 The body known as parapeptone which makes its appearance during 
 the peptic digestion of proteids is closely allied to the substances just 
 described. (See p. 243). 
 
 2. Alkali-albumin. 
 
 If serum- or egg-albumin or washed muscle be treated with dilute 
 alkali instead of with dilute acid, the proteid undergoes a change quite 
 similar to that which was brought about by the acid. The alkaline 
 solution, when the change has become complete, is no longer coagulated 
 by heat, the proteid is wholly precipitated on neutralisation, and the 
 precipitate, insoluble in water and in neutral sodic chloride solutions, 
 is readily soluble in dilute acids or alkalis. Indeed in a general way 
 it may be said that acid-albumin and alkali-albumin are nothing more 
 than solutions of the same substance in dilute acids and alkalis 
 respectively. When the precipitate obtained by the neutralisation of 
 a solution of acid- albumin in dilute acid is dissolved in a dilute alkali, 
 it may be considered to become alkali- albumin ; and conversely when 
 the precipitate obtained from an alkali-albumin solution is dissolved in 
 dilute acid, it may be regarded as acid-albumin. 
 
 It is stated ^ as a characteristic reaction of this modified or derived 
 albumin that it is not precipitated when its alkaline solutions are 
 neutralised in the presence of alkaline phosphates; solutions of acid- 
 albumin on the contrary are said to be precipitated on neutralisation in 
 the presence of alkaline phosphates, and this difierence is considered 
 to be a distinguishing feature of the two proteids. But doubt has been 
 cast on this statement ^ 
 
 Alkali-albumin may be prepared by the action not only of dilute 
 alkalis but also of strong caustic alkalis on native albumins as well as 
 on coagulated albumin and other proteids. The jelly produced by the 
 action of caustic potash on white of egg, spoken of in Class I. 1, is 
 alkali-albumin ; the similar jelly produced by strong acetic acid is acid- 
 
 1 Hoppe-Seyler, Hdl. Plnjs. Path. Chem. Anal., Ed. IV. (1875), S. 246. 
 
 ^ Hoppe-Seyler, loc. cit. S. 245. 
 
 3 Soyka, Pfliiger's Arch. Bd. xii. (1876), S. 347. 
 
Ai'i'.J CIIKMICAL HASIS OF THE AMMAL nODV. 707 
 
 nlliuiuiii. Olio of tlio most proiliu-tivc; iiirtluxls of obtaiiiiii/; alkali- 
 aU>uiniii is that introduocil hy Lii!l)frkuliii', and consists in adding a 
 strong solution of caustic potash to purified -whito of egg until the 
 abovc-montioned j<'lly is obtained. This is then cut into small pieces, 
 and dialysod until quite ■white. Tlie lumps are then dissolved hy 
 heating on the water-bath, and the alkali-albumin precipitated by the 
 careful addition of acetic acid. 
 
 IJoth alkali- and acid-albumin are with difficulty precipitated by 
 alcohol from their alkaline or acid solutions. The neutralisation pre- 
 cipitate however becomes coagulated under the prolonged action of 
 alcohol. 
 
 The body 'protein,' described by IMulder appears, if it exists at all, to be closely 
 connected with this body. All subscciueut observers have however failed to confirm 
 Ills views. 
 
 The rotatory power of alkali-albumin varies according to its source ; 
 thus when prepared by strong caustic potash from serum-albumin, the 
 rotation rises from - 56" (that of serum albumin) to— 86", for yellow 
 light. Similarly prepared from egg-albumin, it rises from — 38-5" to 
 — 47°, and if from coagulated white of egg, it rises to — 58-8". Hence 
 the existence of various forms of alkali-albumin is probable. 
 
 In addition to the methods given above, alkali-albumin may be also readily ob- 
 tained by shaking milk with strong caustic soda solution and aether, removing the 
 aetherial solution, precipitating the remaining fluid with acetic acid and washing the 
 precipitate with water, cold alcohol and aether. 
 
 The most satisfactory method of regarding acid- and alkali-albumin 
 is to consider them as respectively acid and alkali compounds of the 
 neutralisation precipitate. We have reason to think that when the 
 precipitate is dissolved in either an acid or an alkali, it does enter into 
 combination with them. The neutralisation precipitate is in itself 
 neither acid- nor alkali-albumin, but may become either, upon solution 
 in the respective reagent. 
 
 It is probable that several derived albumins exist", differing according to the 
 proteid from which they are fonned or possibly according to the mode of their pre- 
 paration, and that each of these may exist in its correlative fonns of acid- and alkali- 
 albumin; but the whole subject requu-es further investigation. 
 
 Acid-albumin, prepared by the direct action of dilute acids on native 
 albumins or on muscle-substance, contains sulphur, as shewn by the 
 brown colouration which appears when the precipitate is heated with 
 caustic potash in the presence of basic lead acetate. Alkali albumin, 
 at all events as prepared by the action of strong caustic potash or soda, 
 
 ^ Poggendorff's Annalen, Bd. lxxxvi. S. 118. 
 
 2 Morner. Pfliiger's Arch. Bd. xvii. (1878), S. 4G3. 
 
 45—2 
 
708 . P ROTE IDS. [App. 
 
 does not contain any sulphur ; and the acid-albumin, prepared by the 
 solution in an acid of the neutralisation precipitate from such an alkali- 
 albumin solution, is similarly free from sulphur. 
 
 3. Casein. 
 
 This is the well-known proteid existing in milk. When freed from 
 fat, and in the moist condition, it is a white, friable, opaque body. In 
 most of its reactions it corresponds closely with alkali-albumin ; thus it 
 is readily soluble in dilute acids and alkalis, and is re-precipitated on 
 neutralisation ; if, however, potassic phosphate is present, as is the 
 case in milkj the solution must be strongly acid before any precipitate 
 is obtained. 
 
 Various reactions have at different times been assigned to casein as distinguishing 
 it from the closely allied body alkali-albumin. Later researches have however in 
 most cases cast so much doubt on these differences that the identity or non-identity 
 of casein and alkali-albumin must still be left an open question, the discussion of 
 which would be out of place here. 
 
 Casein, as occurring in milk, has had several reactions ascribed to it, as character- 
 istic ; but these lose their importance on considering that milk contains, in addition 
 to casein, other substances such as potassic phosphate, and a number of bodies 
 which yield acids by fermentation. The presence of potassic phosphate has an 
 especial influence on the reactions of casein. In the entire absence of this salt, 
 acetic acid in the smallest quantities, as also carbonic anhydride, gives a precipitate; 
 but if this salt is present, carbonic anhydride gives no precipitate, and acetic acid 
 one only when ihe solution is acid from the presence of free acid, and not from 
 that of acid potassic phosphate i. 
 
 When prepared from milk by magnesic sulphate (see below), freed 
 by aether from fats, and dissolved in water, casein possesses a specific 
 lotatory power of - 80' for yellow light ; in dilute alkaline solutions, of 
 -76"; in strong alkaline solutions, of -91"; in dilute hydrochloric acid, 
 of -87". 
 
 Casein has been asserted to occur in muscle, in serous fluids, and in 
 blood-serum (Serum-casein). In many cases it has probably been 
 confounded with globulins (see Class III.); but blood-serum and muscle- 
 plasma undoubtedly contain an alkali-albumin in addition to whatever 
 globulin may be present, but the usual doubt exists as to the identity- 
 of this with true casein. Its presence may be shewn by adding dilute 
 acetic acid to blood-serum which has been freed from globulin by a 
 current of carbonic anhydride ; a distinct precipitate is thrown down. 
 A substance similar to casein has also been described as existing in 
 unstriated muscle and in the protoplasm of nerve-cells. 
 
 Preparation. Dilute milk with several (10 to 15) times its bulk of 
 water, add dilute acetic acid till a precipitate begins to appear, then pass 
 
 1 See Kiihne, Lehrh. d. Physiol. Chem., 18G8, S. oGo. 
 
AiT.] CHEMICAL BASIS OF THE AXIMAL IU)l)Y. 709 
 
 a current of carbonic anhydride, lilter, and wa-sh tlio precipiUito ^vith 
 watiT, alcohol and aether : the complete removal of the fat carried 
 down -with the casein presents some dilliculties. Mu^nusic sulphate 
 adtled to saturation also precipitates casein from milk ; the precipitate 
 as thus formed is readily soluble on the addition of water. 
 
 Ci.AS.s III. Globulins. 
 
 Besides the native albumins there are a number of native proteids 
 which difler from the albumins in not being soluble in distilled water; 
 they need for their solution the presence of an appreciable, though it rnay 
 be a small, quantity of a neutral saline body such as sodic cldoride. 
 Thus they resemble the albuminates in not being soluble in distilled 
 water, but differ from them in being soluble in dilute sodic chloride 
 or other neutral saline solutions. Their general characters may be 
 stated as follows. 
 
 They are insoluble in water, soluble in dilute (1 p.c.) solutions of 
 sodic chloride ; they are also soluble in dilute acids and alkalis, being 
 changed on solution into acid- and alkali-albumin respectively unless 
 the acids and alkalis are exceedingly dilute. The saturation with solid 
 sodic chloride of their solutions in dilute sodic chloride, precipitates 
 most members of this class. 
 
 1. Globulin {Cryslallin). 
 
 If the crystalline lens be rubbed up with fine sand, extracted with 
 water and filtered, the filtrate will be found to contain at least three 
 proteids. On passing a current of carbonic anhydride a copious precipi- 
 tate occurs; this is globulin. 
 
 The addition of dilute acetic acid to the filtrate from the globulin, gives a \}xg- 
 cipitate of alkali-albumin^ ; and the filtrate from this if heated gives a further pie- 
 cipitate, due to serum-albumin. 
 
 In its general reactions globulin corresponds almost exactly with 
 the next members of this class (paraglobulin and fibrinogen), but has no 
 power to form or promote the formation of fibrin in fluids containing 
 the above-mentioned bodies, and possesses the following special features. 
 1. According to Lehmann, its oxygenated, neutral solutions become 
 cloudy on heating to 73" C, and are coagulated at 93" C. 2. It is readily 
 precipitated on the addition of alcohol. According to Iloppe-Seyler, it 
 is not precipitated on saturation with sodic chloride, resembling 
 vitellin in this respect. 
 
 1 But see also PflUger's Arch. Bd. xin. (187C), S. C31. 
 
710 r BOTE IDS. [A pp. 
 
 According to Kiihne' and Eichwald^ a globulin witli properties identical ■with 
 those just given may be precipitated from dilute serum by the cautious addition of 
 acetic acid. This body is stated by WeyP to be the same as paraglobulin (fibrino- 
 plastin), the latter diEfeiiug from it only by a small admixture of fibrin-ferment. 
 
 2. Paraglobulin {Fibrlnojjlastin). 
 
 Preparation. Blood-serum is diluted ten-fold -with water, and a 
 brisk current of carbonic anhydride is passed through it. The first-formed 
 cloudiness soon becomes a flocculent precipitate, which is finally quite 
 granular, and may easily be separated by decantation and filtration : it 
 should be washed on the filter with water containing carbonic acid. 
 
 It has usually been stated that paraglobulin may be separated from 
 serum by saturation with sodic chloride. But Hammarsten* has shown 
 that this is only in part true, a considerable portion of the globulin 
 remaining nnprecipitated. The separation may however be completely 
 efiected by saturation with magnesic sulphate. When determined 
 by this method the amount of paraglobulin in serum is very con- 
 siderable, amounting, in some cases according to Hammarsten, to as 
 much as 4-565 p. c. (reckoned on 100 cc. of serum). The quantity 
 seems to vary in different animals, the precipitation being much more 
 complete in serum from ox-blood than in that from the blood of horses. 
 
 From its solution in dilute sodic chloride, paraglobulin may be pre- 
 cipitated by a current of carbonic anhydride or the addition of exceedingly 
 dilute (less than 1 pro mille) acetic acid. If the acid is strong, the 
 precipitated proteid becomes immediately changed into acid-albumin 
 (Class II. 1.). In pure water, free from oxygen, paraglobulin is insoluble, 
 but on shaking with air or passing a current of oxygen, solution readily 
 takes place ; from this it may be reprecipitated by a current of carbonic 
 anhydride. Very dilute alkalis dissolve this body without change ; if, 
 however, the strength of the alkali be raised even to 1 p. c. the para- 
 globulin is changed into alkali-albumin (Class II. 2.). 
 
 According to Kuhne and A. Schmidt the solutions of this body in 
 water containing oxygen or in very dilute alkalis are not coagulated on 
 heating. The sodic chloride solutions do however coagulate when 
 heated to 68°— 70"^, and if the substance itself be suspended in 
 water and heated to 70° C. it is coagulated. Although insoluble in 
 alcohol, its solutions are with difficulty precipitated by this reagent. 
 
 Paraglobulin occurs not only in blood-serum, but it is also found in 
 white corpuscles, in the stroma of red corpuscles (to some extent at 
 
 1 Lehrh. d. Physiol. Chem. 1868. S. 175. 
 
 2 Beitrdge zur Chem., d. gewehchild. SulM. Berlin, 1873. Hf. i. 
 
 3 Zeitschr.f. Physiol. Chem., Bd. i. (1878) S. 79. 
 
 4 Pfliiger's Archiv, Bd. xvii. (1878) S. 446. Bd. xviii. (1878), S. 38. 
 ^ Hammarsten, op. cit. 
 
Arr.] CHEMICAL BASIS OF THE AXIMAL IlODY. 711 
 
 least), in connective tissue, the cornea, aqueous humour, lymph, chylf, 
 and serous fluids. 
 
 For the occurrence of j^lobuliu in urine see Edlcfsen' and Senator'. 
 
 3. Fibrinogen. 
 
 The general reactions of this body are identical with those of 
 paraglobulin. The most marked difference between the two is 
 the point at which coagulation of their solutions takes place. 
 Ilannnarsteu^ has shewn that fibrinogen in a 1 — 5 p. c. solution of 
 sodic chloride coagulates at from 52" — 55" C, whereas, as stated above, 
 paraglobulin (fibrinoplastin) coagulates first at from 68° — 70° C. This 
 however is disputed by A. Schmidt who holds that the substance 
 coagulating at 52° — 55" C is not fibrinogen but a sort of nascent fibrin. 
 There is also a marked dillerence in the precipitability of the two bodies 
 by sodic chloride. (See below.) Other differences between the two 
 may be thus enumerated : — In precipitating fibrinogen by a current 
 of carbonic anhydride, the containing fiuid must be much more strongly 
 diluted, and the gas must pass for a much longer time. Tlie pre- 
 cipitate thus obtained differs from that of paraglobulin in that it 
 forms a viscous deposit, adhering more closely to the sides and bottom 
 of the containing vessel ; there is also no flocculent stage pre^•ious 
 to the viscous precipitate. 
 
 Fibrinogen occurs in blood, chyle, serous fluids, and in various 
 transudations. The relations of fibrinogen and paraglobulin to the 
 foi-mation of fibrin have been discussed in the text, p. 18. 
 
 Prejmrcdion*. Salted plasma, obtained by centrifugalising blood 
 whose coagulation is prevented by the addition of a certain proportion 
 of magnesic sulphate, is mixed with an equal volume of a saturated 
 (35-87 p.c. at 14° C.)', solution of sodic chloride; the fibrinogen is thus 
 precipitated whOe the paraglobulin remains in solution. The adhering 
 plasma may be removed by washing with a solution of sodic chloride 
 and the fibrinogen finally purified by being several times dissolved in 
 and reprecipitated by sodic chloride. 
 
 There is no proof that the wliole of the substance thrown do-svn by 
 carbonic anhydride from diluted blood-serum is fibrinoplastic, indeed we 
 know that a true globulin devoid of fibrinoplastic properties may be 
 prepared from serum^ "NVeyF considers that there is only one globulin 
 
 1 Ccntralblatt f. d. med. Wixs. Jahrg. 1870, S. 367. Also Arch, f, klin. iled. Bd. 
 VII., S. CO. 
 
 - Virchow's Archiv, Bd. lx., S. 476. 
 
 3 Ujysala Lrikareforenittfis jorhandUngar. Bd. xi. 1876. 
 
 -• See Hammars'ten, Nov. Act. Reg. Soc. Sci. Upsala, Ser. iii. Vol. x. (1875), 
 p. 31. Also Pfliiper's Archiv, Bd. xix. (1879) S. 563, and Bd. xxii. (1S80), S. 431. 
 
 5 PoRgiale, Ann. Chim. Phijs (3) Vol. vin. p. 469. 
 
 6 Kiiline and Eichwald, loc. cit. ^ Loc. cit. 
 
712 PROTEIDS. [App. 
 
 in serum, wliich lie characterises by the name of 'serum-globulin,' and 
 regards fibrinoplastin as a mixture of this body with a portion of fibrin- 
 ferment. We know for certain (see p. 16) that tlie whole of the fibrino- 
 plastic precipitate, used to cause the coagulation of a fibrinogenous fluid, 
 does not enter into the composition of the fibrin produced ; we also know 
 that such a precipitate may lose its fibrinoplastic powers without any 
 marked change in its general reactions. It would seem advisable 
 therefore to speak of the deposit produced by carbonic anhydride in 
 dilute serum, or by saturation with sodic chloride in undiluted serum, 
 as globulin, and to distinguish it as fibrinoplastic ^ globulin when it is 
 able to give rise to fibrin. Eibrinogen similarly might be spoken of as 
 fibrinogenous globulin. The name crystallin rather than globulin 
 might then be given to the substance obtained from the crystal- 
 line lens. 
 
 4. Myosin. 
 
 This is the substance which forms the chief proteid constituent 
 of dead, rigid muscle ; its general properties and mode of preparation 
 have been already described at p. G4. In the moist condition, it forms 
 a gelatinous, elastic, clotted mass ; dried, it is very brittle, slightly 
 transparent and elastic. From its solution in sodic chloride it is pre- 
 cipitated, either by extreme dilution, or by saturation with the solid 
 salt. When precipitated by dilution and submitted to the prolonged 
 action of water myosin loses its property of being soluble in solutions 
 of sodic chloride'. The sodic chloride solution, if exposed to a rising 
 temperature, becomes milky at 55" C, and gives a flocculent precipitate 
 at 60° C. This precipitate is however no longer myosin, for it is in- 
 soluble in a 10 p. c. sodic chloride solution, and does not, until after 
 many days' digestion, yield syntonin on treatment with hydrochloric 
 acid (-1 p. c). It is in fact coagulated proteid (see Class V.). 
 
 Myosin is excessively soluble in dilute acids and alkalis. Advan- 
 tage may be taken of its solubility in the former to extract it from 
 muscles". But if the reagents are at all concentrated, myosin under- 
 goes in the act of solution a radical change, becoming in the one case 
 acid-albumin or syntonin, in the other alkali-albumin (Class II.). 
 
 Like fibrin, it can in some cases decompose hydrogen dioxide, and oxidise guaiacum 
 with formation of a blue colour. 
 
 5. Vitellin. 
 
 As obtained from yolk of egg, of which it is the chief proteid con- 
 stituent, vitellin is a white granular body, insoluble in water, but very 
 soluble in dilute sodic chloride solutions ; it surpasses myosin in this 
 
 1 Weyl, Zeitschr.f. phijsiol. Chem. Ed. i. (1878) S. 77. 
 
 ^ Danilewsky, Zeitsch. f. physlol. Chem. Bd. v. (1881), S. 158. 
 
App.] chemical i}as/s of the axi.ual body. 71.3 
 
 respect, for the solution may be easily filtered. Its coagulation tempera- 
 ture is hij,'luT than that of myosin, lying according to Weyl', between 
 70" C. and 80" C. Suturatiuii with solid sodic chloride gives no precipi- 
 tate ; in this respect it differs from most other members of this class. 
 In yolk of egg vitellin is always associated with, and probably exists 
 in combination ■with, the peculiar complex body lecithin (see p. 7-43). 
 
 Denis, and after him, Hoppe-Seyler, have shewn that vitellin before the treatment 
 requisite to free it from kcitliin, possesses properties quite dillcrent from other 
 proteids. 
 
 A theory has been advanced that vitellin is really a complex body 
 like haemoglobin, and on treatment with alcohol splits up into coagulated 
 proteid and lecithin. When well purified it contains -75 p. c. sulphur, 
 but no phosphorus. Dilute acids or alkalis readily convert it in its un- 
 coagulated form into a member, of Class II. 
 
 Fremy and Valenciennes- have described a series of proteids, viz. ichthin, 
 ichthidin, drc., derived from fish and ampliibia. They appear to be either identical 
 with, or closely related to, vitellin. 
 
 Preparation. Yolk of egg is treated with successive quantities of 
 aether, as long as this extracts any yellow colouring matter; the residue 
 is dissolved in moderately strong (10 p. c.) sodic chloride solution, and 
 filtered. The filtrate on falling into a large excess of water is 
 precipitated. In this state it is mLxed with lecithin and nuclein, and in 
 order to free it from these it was usually treated with alcohol This, 
 as above stated, entirely changes the "vdtellin into a coagulated form. 
 It seems probable that the separation of vitellin from the other bodies 
 with which it is mixed in the yolk of e^^g^ may be effected by precipitat- 
 ing the sodic chloride solution by the addition of excess of water ; the 
 precipitate is then re-dissolved in 10 p. c. solution of sodic chloride and 
 the process repeated as rapidly as possible^. 
 
 G. Globin. 
 
 Globin, stated by Preyer^ to be the proteid residue of the complex body hfemo- 
 globin (see p. 341), ought probably to be considered as an outlying member of this 
 class. It is however not readily soluble either in dilate acids or sodic chloride 
 solutions. It is said to be absolutely free from ash. 
 
 Class IY. Fibrin. 
 
 Insoluble in water and dilute sodic chloride solutions; soluble 
 with difficulty in dilute acids and alkalis, and more concentrated 
 neutral saline solutions. 
 
 1 Op. cit. 2 Compt. Rend. T. xxxviu. , pp. 469 and 525. 
 
 3 Weyl, op. cit. S. 74. ■• Die Blutkri/^talk (1871), S. 166. 
 
714 PROTEIDS. [App. 
 
 Fibrin, as ordinarily obtaiaed, exhibits a filamentous structure, the 
 component threads possessing an elasticity much greater than that of 
 any other known solid proteid. 
 
 If allowed to form gradually in large masses, the filamentous structure is not so 
 noticeable, and it resembles in this form pure india-rubber. Such lumps of fibrin 
 are capable of being split in any direction, and no definite arrangement of parallel 
 bundles of fibres can be made out. 
 
 At ordinary temperatures fibrin is insoluble in water, being dissolved 
 only at very high temperatures, and then undergoing a complete change 
 in its characters. In hydrochloric acid solutions of 1 — 5 p. c. fibrin 
 swells up and becomes transparent, but is not dissolved \ In this 
 condition the mere removal of the acid by an excess of water, 
 neutralisation, or the addition of some salt, causes a return to the 
 original state. If, however, the acid be allowed to act for many days 
 at ordinary temperatures or for a few hours at 40° — 60° 0., solution takes 
 place, and the resulting proteid is syntonin. In dilute alkalis and 
 ammonia, fibrin is much more readily soluble, though in this case also 
 the solution is greatly aided by warming ; the resulting fluid contains 
 no longer fibrin, but alkali-albumin. This property is not distinctly 
 characteristic of fibrin, although it dissolves perhaps more readUy in 
 both dilute acids and alkalis than do coagulated proteids. None of 
 these solutions can be coagulated on heating, which is intelligible when 
 it is remembered that they no longer contain fibrin, but either acid or 
 alkali-albumin. In addition to the above, fibrin is soluble, though with 
 difiiculty and only after a considerable time, in 10 p. c. solutions of 
 sodic chloride, potassic nitrate or sodic sulphate, the solution being 
 often accompanied by putrefactive changes. These solutions may be 
 coagulated by a temperature of 60° C, and are precipitated by dilution 
 with water or saturation with solid sodic chloride ; in fact, by the 
 action of the neutral saline solutions the fibrin has become converted 
 into a body exceedingly like myosin or globulin ^ 
 
 On ignition of fibrin a residue of inorganic matter is always 
 obtained; it is, however, considered that sulphur is the only one of 
 these elements which enters essentially into its composition. In other 
 respects fibrin corresponds entirely in general composition with other 
 proteids. 
 
 Suspended in water and heated to 70° C, it loses its elasticity, and be- 
 comes opaque; it is then iadistinguishable from other coagulated proteids. 
 
 A peculiar property of this body remains yet to be mentioned, viz. its power of 
 decomposing hydrogen dioxide. Pieces of fibrin placed in this fluid, tbou^ them- 
 
 1 Complete solution may however take place if the fibrin, as is frequently the 
 case, contains any adherent pepsin. 
 
 2 Gautier, Compt. Rend. T. lxxix. (1874), p. 227. 
 
App.] chemical basis of tub animal body. 715 
 
 selves undergoing no change, soon become covered with bubbles of oxygen; and 
 guaittcum is turnoJ blue by libriu in picsuuco of hydrogen dioxide or ozonisjd tur- 
 peulino. 
 
 Preparation. By vigorously stirring lAood with a bundle of twigs 
 and tlicn washing with water until it is (juite white. If required per- 
 fectly pure and colourless it should be prepared from plasma free from 
 corpuscles. If the blood, before stirring, be diluted with an equal bulk 
 of water, the subsequent washing of the librin is much facilitated, and 
 it may readily be obtained quite white. Any adherent fats may be re- 
 moved by aether. 
 
 When globulin, myosin, and fibrin are compared each with the other, 
 it will be seen that they form a series in wljich myosin is intermediate 
 between globulin and fibrin. Globulin is excessively soluble in even 
 the most dilute acids and alkalis ; fibrin is almost insoluble in these ; 
 while myosin, though more soluble than fibrin, is less soluble than 
 globulin. Globulin again dissolves with the greatest ease in a very 
 dilute solution of sodic chloride. Myosin, on the other hand, dissolves 
 with dilliculty ; it is much more soluble in a 10 per cent, than in a one 
 per cent, solution of sodic chloride; and even in a 10 per cent, 
 solution the myosin can hardly be said to be dissolved, so viscid is the 
 resulting fluid and with such difficulty does it filter. Fibrin again 
 dissolves with great difficulty and very slowly in even a 10 per cent, 
 solution of sodic chloride, and in a one per cent, solution it is 
 practically insoluble. When it is remembered that fibrin and myosin 
 are, both of them, the results of coagulation, their similarity is in- 
 telligible. Myosin is in fact a somewhat more soluble form of fibrin, 
 deposited not in threads or filaments but in clumps and masses. 
 
 Class Y. Coagulated Proteids. 
 
 These are insoluble in water, dilute acids and alkalis, and neutral 
 saline solutions of all strengths. In fact they are really soluble only in 
 strong acids and strong alkalis, though prolonged action of even dilute 
 acids and alkalis will efiect some solution, especially at high tempera- 
 tures. During solution in strong acids and alkalis a destructive decom- 
 position takes place, but some amount of acid- or alkali-albumin is 
 always produced. 
 
 Very little is known of the chemical characteristics of this class. 
 They are produced by heating to 70" C., solutions of egg- or serum- 
 albumin, globulins suspended in water or dissolved in saline solutions ; by 
 boiling for a short time fibrin suspended in water or dissolved in saline 
 solutions, or precipitated acid- and alkali-albumin suspended in water. 
 They are readily converted at the temperature of the body into peptones, 
 
716 P ROTE IDS. [App. 
 
 by the action of gastric juice in an acid, or of pancreatic juice in an 
 alkaline medium. 
 
 All proteids in solutions are precipitated by an excess of strong 
 alcohol. If the precipitant be rapidly removed they are again soluble 
 in water, but if the precipitated proteids are subjected for some time to 
 the action of the alcohol they are, with the exception of peptones, 
 coagulated and lose their solubility. It appears however that the 
 proteids contained in the aleurone-grains of plants are exceedingly 
 resistant to this coagulating action of alcohol '. 
 
 It seems scarcely necessary to point out the distinction in the use of the word 
 ' coagulation ' as applied to blood- or muscle-plasma on the one hand and to the 
 action of heat and alcohol upon proteids on the other. The diiierence is obvious 
 when it is remembered that in the first case the coagulation leads to the formation 
 of fibrin (Glass iv.), or myosin (Class iii.), and that these bodies may then further 
 be coagulated by heat or alcohol as described above. 
 
 Class YI. Tei^tones. 
 
 Very soluble in water, and not precipitated from their aqueous 
 solutions by the addition of acids or alkalis, or by boiling. Insoluble 
 in alcohol, they are precipitated with difficulty by this reagent, and are 
 unchanged in the process ; they differ from all other proteids in not being 
 coagulated by prolonged exposure to alcohol. They are not precipitated 
 by cupric sulphate, ferric chloride, or, except in the instances to be 
 mentioned presently, by potassic ferrocyanide, and acetic acid. In 
 these points they differ from most other proteids. On the other hand, 
 precipitation is caused by chlorine, iodine, tannin, mercuric chloride, 
 nitrates of mercury and silver, and both acetates of lead ; also by bile- 
 acids in an acid solution. In common with all proteids, these bodies 
 possess a specific Itevo-rotatory power over polarised light ; but they 
 differ from all other proteids in the fact that boiling produces no change 
 in the amount of rotation. 
 
 A solution of peptones, mixed with a strong solution of caustic 
 potash, gives, on the addition of a mare trace of cupric sulphate, a finh 
 colour. An excess of the cupric salt gives a violet colour, which 
 deepens in tint on boiling, in fact the ordinary proteid reaction. Other 
 proteids simply give the violet colour. But the most characteristic 
 feature of peptones is their relatively great diffusibility, a property which 
 they alone, of all the proteids, may be said to possess, since all other forms 
 of proteids pass through membranes with the greatest difficulty, if at all. 
 
 The diffusibility of peptones is however absolutely small as compared with that 
 of crystalline bodies such as sodic chloride ; in fact solutions of peptones may be 
 freed from salts by dialysis, a process employed in their preparation. 
 
 1 See Vines, Jl. of Physiol. Vol. iii. p. 1C8. 
 
A n-.] CUKMK 'AL I', A SIS OF THE A X/.U. I /. JU) D )'. 717 
 
 Notwithstanding their i.iohal.lc fijnnulioii in liir.Lje quantities in the 
 stomach and intestine, to jud<,'e from the results of artificial digestion, a 
 very small quantity only can be found in the contents of these organs 
 They are probably absorbed as soon as formed. Another point of 
 interest is their reconversion into otlier forms of proteids, since this 
 must occur to a great extent in the body. We are however as yet 
 ignorant of the manner in whioli this reverse change is eficcted. 
 
 Proditctton. All proteids, with the exception of lardaceln (sec p. 720), 
 yield peptones (and other products) on treatment with acid gastric or 
 alkaline pancreatic juice, most readily at the temperature of the human 
 body. Peptones are likewise produced, in the absence of pepsin and 
 trypsin, by the action of dilute and moderately strong acids at medium 
 temperatures, also by the action of distilled water at high temperatures 
 under pressure. For various methods of preparing peptones, see Maly ' 
 Adamkiewicz^, Henninger^, and Pekelharing '. 
 
 It appears possible to reobtain ordinary coagulable proteids from peptones by 
 the action of either prolonged beating to l-lOO— 170" C. or of dehydrating agents^. 
 
 No difference in percentage composition between peptones and the 
 proteid from which they arc formed has, at present, been definitely 
 established. 
 
 We have used the phrase 'pci)tones' in the plural number because 
 we have reason to think that more than one kind of peptone exists. 
 Meissner" described three peptones, naming them respectively A- B- 
 and C-peptone. He distinguished them as follows. A- peptone is 
 precipitated from its aqueous solutions by concentrated nitric acid, and 
 also by potassic ferrocyanide in the presence of even weak acetic 
 acid. B-peptone is not precipitated by concentrated nitric acid, nor 
 will potassic ferrocyanide give a precipitate unless a considerable 
 quantity of strong acetic acid be added at the same time. C-peptone is 
 precipitated neither by nitric acid nor by potassic ferrocyanide and 
 acetic acid, whatever be the strength of the acetic acid. In jilace how- 
 ever of speaking of all these as peptones, it is better to consider C- 
 peptone as the only real peptone, and the A- and B-peptones as not 
 peptones at all. Nevertheless we have reason, from the researches of 
 Kiihne, to speak of more than one peptone, viz. of a hemipeptone which 
 is capable under the action of trypsin of being converted into leucin and 
 tyrosin, and of an antipeptone which resists such a decomposition. The 
 name antipeptone is given to the latter on account of this resistance which 
 
 1 Pfliiger's Arch. Bd. ix. (1874) S. 585. 
 - Die Natur u. Nahrwerth d. Peptons (1877), S. 33. 
 3 Dc la Nature et da Role physiologique des Peptones, Paris, 1878. 
 * PflUger's Arch., Bd. xxii. (1882), S. 185. 
 
 5 Henninger, loc. cit., Hofmeister, Zeitsch.f. physiol. Chem., Bd. ii. (1878), S. 206. 
 Pekelharing, loc. cit. 
 
 " Zcitschr. f. rat. Med., Bdc. vii., viii., x., xii. und xiv. 
 
718 P ROTE IDS. [A pp. 
 
 it offers towards trypsin ; the name hemipeptone, given to the former, 
 signifies that this peptone is the twin or correlative half of antipeptone. 
 
 We have seen (p. 243) that when any proteid is digested with 
 pepsin, what we may preliminarily call a bye-product makes its appear- 
 ance. This bye product, which has many resemblances to acid- 
 albumin or syntonin, appearing as a neutralisation precipitate soluble 
 in dilute acids and alkalis but insoluble in distilled water, is generally 
 spoken of as parapeptone. According to Finkler* this neutralisation 
 precipitate is especially abundant if the pepsin be previously modified 
 by exposure to a temperature of 40" to 60° 0. The pepsin thus modified 
 is spoken of by Finkler as 'isopepsin.' Many authors regard parapep- 
 tone, syntonin, and acid-albumin as being the same thing, Meissner 
 however gave the name parapeptone to a body, which need not and 
 probably does not make its appearance during normal natural digestion 
 or during artificial digestion with a thoroughly active pepsin, but which 
 is formed when proteids are subjected to the action of weak hydrochloric 
 acid, either alone or in company with an imperfectly-acting pepsin, and 
 which in certain characters is quite distinct from ordinary syntonin or 
 acid-albumin. Its distinguishing feature is that it cannot be changed 
 into peptone by the action of even the most energetic pepsin, though it 
 is readily so converted under the influence of trypsin ; otherwise it very 
 closely resembles syntonin. We have here an indication that the simple 
 characters by which we have described acid-albumin may be borne 
 by bodies having marked differences from each other. The researches 
 of Kiihne^ have thrown an important light on these differences. The 
 fundamental notion of Klihne's view is that an ordinary native albumin 
 or fibrin contains within itself two residues, which he calls respectively 
 an anti-residue and a hemi-residue. The result of either peptic or 
 tryptic digestion is to split up the albumin or fibrin, and to produce on 
 the part of the anti-residue antipeptone, and on the part of the hemi- 
 residue hemipeptone, the latter being distinguished from the former by 
 its being susceptible of further change by tryptic digestion into leucin, 
 tyrosin, &c. Antipeptone remains as antipeptone even when placed 
 under the action of the most powerful trypsin, provided putrefactive 
 changes do not intervene. 
 
 Before the stage of peptone (whether anti- or hemi-) is reached, 
 there is an intermediate stage corresponding to the formation of synto- 
 niru In both normal peptic and tryptic digestion antipeptone is 
 preceded by an anti-albumose, and hemipeptone by a he}ni-albumose. 
 Of these the anti-albumose is closely related to syntonin, and has 
 
 1 Pfliiger's Archiv, xiv. (1877), S. 128. 
 
 2 Only a short account of these has as yet been published. Verhand. d. Nature 
 hist-Med. Ver-Ueiddbg. Bd. i. Hf. 4, 1876. 
 
app.] chemical basis of the animal body. 719 
 
 hitherto been regarded as syntonin. The hcmi-albumoso lias not 
 been so frequently observed ; it was however isolated by Meissner ; it 
 is apparently the body called by him A-peptonc. It possesses several 
 peculiar features. If its solutions are heated they partially coagulate 
 at about GO — 65° C. ; the precipitate is soluble at about 70" C and is 
 rc-precii)itated as the temperature again falls. It also yields a precipitate 
 with nitric acid and potassic ferrocyanide and this also is soluble at the 
 higher temperature reprecipitating on cooling. In these respects it 
 closely resembles a proteid body observed by Bence-Jones in the urine 
 of osteomalacia. It approaches myosin in being readily soluble in a 
 10 per cent, solution of sodic chloride. 
 
 If however albumin be digested with insufficient or with imperfectly 
 active pepsin, or simply with dilute hydrochloric acid at 40" C, anti- 
 albumose is not formed, but in its place a body makes its appearance 
 which Kiiline calls anti-albumate'. Its characteristic property is that it 
 cannot be converted by peptic digestion into peptone, though it can be so 
 changed by tryptic digestion. It is in fact the parapeptone of Meissner. 
 
 It may perhaps be advisable, now that Meissner's parapeptone is 
 cleared up, to reserve the name parapeptone for the initial products of 
 both peptic and tryptic digestion, and to speak of anti-albumose and hemi- 
 albumose as being both parapeptones. But in this sense parapeptone 
 will be an intermediate and not a collateral product of digestion. 
 
 Meissner also described a particularly insoluble form of his parapep- 
 tone as dyspeptone, and another intermediate product as metapeptone ; 
 but further investigation of both these bodies, as well as of his B-peptone, 
 is necessary. Under the influence of dilute hydrochloric acid, anti- 
 albumate becomes changed into a body which Kiihne calls anti-albumid 
 and which seems identical with the very insoluble proteid described by 
 Schiitzenberger as 'hemiprotein,' and probably with Meissner's dyspep- 
 tone. The same body is produced at once in company with products 
 belonging to the hemi-group by the action of 3 to 5 per cent, sulphuric 
 acid on native albumin or fibrin. The following tables shew the 
 relations and genesis of the bodies we have just described. The several 
 products (antipeptone, kc.) are given in duplicate, on the h\-pothesis 
 (which though not proved is probable) that the changes of digestion are 
 essentially hydrolytic changes", accompanied by a deduplication; that 
 just as a molecule of starch splits up into at least two molecules 
 of dextrose, or as a molecule of cane-sugar splits up into a molecule of 
 dextrose and a molecule of levulose, so a molecule of antialbumose, for 
 instance, splits up into tAvo molecules of antipeptone, and so on. But 
 the whole scheme is of course only proA"isional. 
 
 1 An albumate must not be confounded with an albuminate, 
 
 2 Herminger, loc. cit. p. 49. 
 
•20 , PROTEIDS. [App. 
 
 Decompositiok of Proteids by Digestiox. 
 
 Albumin. .s 
 
 ^ I 
 
 gn ( Antialbumose. Hemialbumose. E-i 
 
 {I '■ ' ^~\ . I ' 1 
 
 Antipeptone. Antipeptone. Hemipeptone. Hemipeptone 
 
 I I 
 
 5=^ 
 
 Leucin. Tyrosin. Leucin. Tyrosin. 
 etc. etc. > 
 
 Decomposition by Acids. 
 
 1. 
 
 By -25 p. c. HCl at 40" C. 
 Albumin. 
 
 Antialbumate. HemialbumosCr 
 
 Antialbumid. Hemipeptone. Hemipeptone. 
 
 By 3—5 p. c. H^SO^ at 100" C. 
 
 Albumin. 
 
 i 
 
 I — ' 1 
 
 Antialbumid. Hemialbumose. 
 
 I 
 
 Hemipeptone. Hemipeptone. 
 
 1 . I . 
 
 Leucin. Tyrosin. etc. Leucin. Tyrosin. etc. 
 
 Class YIL Lardacein, or the so-called amyloid substance. 
 
 The substance to which, the above name is applied, is found 
 as a pathological deposit in the spleen and liver, also in numerous other 
 organs, such as the blood-vessels, kidneys, lungs, <fcc. 
 
 It is insoluble in water, dUute acids and alkalis, and neutral saline 
 solutions. 
 
 Li centesimal composition it is almost identical with other proteids', 
 
 viz. : — 
 
 0. and S. H. N. C. 
 
 24-4 7-0 150 53-6 
 
 The sulphur in this body exists in the oxidised state, for boiling 
 with caustic potash gives no sulphide of the alkali. The above results of 
 
 1 C. Schmidt, Ann. d. Chem. n. Pharm, Bd. ex., S. 250, and Friedreich and 
 Kekule, Yirchow's Archiv, Bd. xvi., S. 50. 
 
Apr.] CUI:M1C.\L basis of the AXIMAL nODY. 721 
 
 analysis woukl lead at once to the ranking of lardacciu as aproteid, and 
 this is strongly supportid by other facts. Strong hydrochloric acid 
 converts it into acid-albumin, and caustic alkalis into alkali-albumin. 
 On the other hand, it exhibits the following marked diflerences from other 
 proteids: — It wholly resists the action of ordinary digestive fluids; it is 
 coloured rod, not yellow, by iodine, and violet or pure blue by the joint 
 action of iodine and sulphuric acid. From these last reactions it ha.s 
 derived one of its names, 'amyloid,' though this is evidently badly chosen ; 
 for not only does it differ from the starch group in composition, but by 
 no means can it be converted into sugar : this latter is one of the crucial 
 tests for a true member of the carbohydrate group. According to 
 Heschl' and Cornil" anilin-violet (methyl-anilin) colours lardaceous 
 tissue rosy red, but sound tissue blue. 
 
 The colours mentioned above, as being produced by iodine and sulphuric acid, 
 are much clearer and brighter when the reagents are applied to the purified lardacein. 
 "SMicn the reagents are applied to the crude substance in its normal position in the 
 tissues, the coloius obtained are always dark and dirty-looking. 
 
 Purified lardacein is readily soluble in moderately dilute ammonia, 
 and can, by evaporation, be obtained from this solution in the form of 
 tough, gelatinous flakes and lumps ; in this form it gives feeble reactions 
 only with iodine. If the excess of ammonia is expelled, the solution 
 becomes neutral, and is precipitated by dilute acids. 
 
 Preparation. The gland or other tissue containing this body 
 is cut up into small pieces, and as much as possible of the surrounding 
 tissue removed. The pieces are then extracted several times with water 
 and dilute alcohol, and if not thus rendered colourless, are repeatedly 
 boiled with alcohol containing hydrochloric acid. The residue after 
 this operation is digested at 40° C, with good artificial gastric juice in 
 excess. Everything except lardacein, and small quantities of mucin, 
 nuclein, keratin, together with some portion of the elastic tissue, "will 
 thus be dissolved and removed ^ From the latter impurities it may be 
 separated by decantation of the finely-powdered substance. 
 
 The chief products of the decomposition of proteids are ammonia, car- 
 bonic anhydride, leucin and tyrosin. Several other bodies, for the most 
 part, like leucin, amidated acids, such as asjDartic acid, glutamic acid, 
 
 ' Wien. nicd. Wochemclir. No. 32, S. 714. 
 2 Compt. Rend. T. lxxx. (1875), p. 1288. 
 
 ' E-uhne und Eudnefif, Yirchow's Arch. Bd. xxxiii. (I860), S. 06. 
 F. 4(5 
 
722 PEOTEIDS. [App. 
 
 &c., have also been obtained; also by tryptic digestion, hypoxanthin 
 and perhaps xanthin. But urea has never yet been derived by direct 
 decomposition from proteid material, the statements to this effect 
 having been based on errors. In spite of numerous researches, we 
 cannot at present state definitely what is the real constitution of 
 a proteid, or in what manner these several residues are contained in the 
 undecomposed substance. It is unnecessary to give here any of 
 the formulae, nearly all empirical, which have been made to represent 
 a px"oteid ; they all give with eqvial exactitude the percentage compo- 
 sition, but beyond this they are untrustworthy. Of the various attempts 
 which have been made to assign to proteids some definite molecular 
 structure, none appear, at the present stage of information, sufficiently 
 reliable for general acceptance. 
 
 Among the most elaborate labours in this direction may be mentioned those of 
 Hlasiwetz and Haberman. In their first pubhcation^, starting from the general 
 similarity of the products of decomposition of the proteids and carbohydrates, they 
 tried to establish a definite relation between the two classes of bodies. In this they 
 were not successful, and in their second research^ they come to the conclusion 
 that the carbohydrates take no part in the formation of the proteids. 
 
 Other experiments in the same direction have been made by Schiitzenberger^. He 
 shews that albumin can be decomposed into carbonic anhydride and ammonia, and 
 that the ratio of these two is the same as though urea had been the body on which 
 he operated. From this he concludes that " the molecrde of albumin contains the 
 grouping of urea and represents a complex ureide." In his second pubhcation* he 
 confirms his preTious results, stating that the ammonia, carbonic anhydride and 
 oxahc acid, produced by the decomposition of proteids, are so connected quanti- 
 tatively as to be capable of derivation from varying proportions of urea and oxamide. 
 He also obtained from the decomposition of proteids a nitrogenous residue which 
 could be formulated as gi\'ing rise to all the amidated acids and other bodies spoken 
 of above. Thus according to him, albumin, built up as a complex ureide, decomposes 
 into ammonia, carbonic, oxalic, and acetic acids, and this nitrogenous body : this 
 last then gives rise to the other products of decomposition'^. 
 
 It will be noticed that in the general description of the various 
 proteids, distinctive reactions for each could not be given, but that 
 varying solubilities were the chief means at our disposal for distinguishing 
 them. They may be arranged according to their solubilities in the 
 following tabular form. 
 
 Soluble in distilled water : 
 
 Aqueous solution not coagulated on boiling Peptones. 
 
 Aqueous solution coagulated on boiling Albumins. 
 
 1 Ann. d. Chem. u. Pliarm. Bd. 159, S. 304. 
 
 2 Ihid. Bd. 169, S. 150. 
 
 3 Comptes Rendus, T. 80 (1875), p. 232. Bull, de la Soc. chim. xxiii. 161, 193, 
 216, 242, 385, 433, xxiv. 2 et 145. 
 
 4 Compt. Rend. T. 81, p. 1108. Bull, de la Soc. chim. xxv. 147. 
 
 e See also Schutzenberger, Ami. de Chem. et de Phys. T. xvi. (1879), p. 2S0. 
 
App.] ( UEMICAL JiAS/S OF THE AXIMAL UODY. 723 
 
 Insoluble in dhdlled waler : 
 
 Solul)l«' in NaCl solution 1 {ht cent Globulins. 
 
 InsoluMo ,, „ ,, 
 
 ( Acid- and Alkcdi- 
 
 SoluMf in IICI"! nor cent, in the cold ,, 
 
 * ( albumin. 
 
 Insoluble in IICl "1 per cent, in the cold, Imt) 
 
 soluble at GO" )^ Fibrin. 
 
 Insoluble in HCl •! per cent, at GU" ; soluble in strong acids. 
 
 Soluble in gastric juice Coagulated albumin. 
 
 Insoluble ,, ,, Lardacein. 
 
 Such a classification is however obviously a wholly artificial one, 
 useful for temporary purposes, but in no way illustrating the natural 
 relations of the several members. Nor is a division into 'native' 
 and 'derived' proteids much more satisfactory. It is true that we may 
 thus put together serum- and egg-albumin, with vitellin, myosin, 
 and fibrin, on the one hand ; and peptones, coagulated proteids, 
 and acid- with alkali-albumin, on the other. But in what light are we 
 to consider casein, seeing that though a natural product, it has so many 
 resemblances to alkali-albumin ? Moreover the system of classification 
 must be useless which would place fibi-inoplastic globulin and fibrinogen 
 in the same class as fibrin, and yet we can hardly speak of either of the 
 two former bodies as derived proteids. If the view be true that when 
 fibrin is converted into peptone the large molecule of the former is split 
 up, with assumption of water, into two smaller molecules of the latter, 
 one belonging to the 'anti' and the other to the 'hemi' group, we might 
 speculate on a possible classification of all proteids into hemi-proteids, 
 anti-proteids and holo-proteids. Thus serum- and egg-albumin, myosin, 
 and fibrin would be undoubtedly holo-proteids, peptones either anti- or 
 hemi-proteids, and we should have to distinguish probably in the 
 hetei'Ogeneous group of derived albumins both anti-, hemi- and holo- 
 proteid members. It is possible, moreover, that fibrinoplastic and 
 fibrinogenous globulin and casein may be natural hemi- or anti-proteids 
 and not holo-proteids. But we have at present no positive knowledge 
 on these points. 
 
 Nitrogenous Non-Ckystalline Bodies allied to Proteids. 
 
 These resemble the proteids in many general points, but exhibit 
 among themselves much greater differences than do the proteids. As 
 regards their molecular structure nothing satisfactory is known. Their 
 percentage composition approaches that of the proteids, and like these 
 they yield, under hydrolytic treatment, large quantities of leucin and in 
 some cases tyrosin. They are all amorphous. 
 
 46—2 
 
724 MUCIN, GELATIN, ETC. [A pp. 
 
 . Mucin. (O, 35-75. H, 6-81. N, 8-50. C, 48-94.)' 
 
 The characteristic component of mucus. Its exact composition is 
 not yet known, the figures given above being merely an approximation. 
 
 As occurring in the normal condition it gives to the fluids which 
 contain it the well-known ropy consistency, and can be precipitated 
 from these by acetic acid, alcohol, alum and mineral acids ; the latter, 
 if in excess, redissolve the precipitate, but this is not the case with 
 acetic acid. In its precipitated form it is insoluble in water, but swells 
 up strongly in it, and this effect is increased by the presence of many 
 alkali salts. Alkalis and alkaline earths dissolve it readily. Its 
 solutions do not dialyse ; they give the proteid reactions with Millon's 
 reagent and niti'ic acid, but not that with sulphate of copper, and are 
 precipitated by basic lead acetate only when neutral or faintly alkaline. 
 According to Eichwald^, when heated with dilute mineral acids, 
 mucin yields acid-albumin, and another body which in many of its 
 properties closely resembles a sugar, inasmuch as it reduces solutions 
 of cupric sulphate. Prolonged boiling with sulphuric acid gives leucin 
 and about 7 p. c. of tyrosin. 
 
 Preixiration^. Ox-gall or an aqueous extract of finely-chopped 
 submaxillary gland is acidulated with acetic acid ; the precipitated 
 mucin is then washed with water, dissolved in dilute sodic carbonate 
 and finally precipitated with acetic acid. It may also be obtained from 
 snails*. 
 
 Chondrin. (O, 31-04. H, 6-76. N, 13-87. C, 47-74. S, 
 -60p. c.)' 
 
 This is usually regarded as forming the essential part of the matrix 
 of hyaline cartilage, and is contained in the interstices of the fibres in 
 elastic cartilage. A similar substance can be prepared from the cornea. 
 Boiled with water, it dissolves slowly, forming an opalescent solution, 
 which is precipitated by acetic acid, lead acetate, dilute mineral acids, 
 alum, and salts of silver and copper ; an excess of the last four reagents 
 redissolves the precipitate. Solutions of this body gelatinise on 
 standing, even if very dilute ; the solid mass is insoluble in cold 
 water, readily soluble in hot water, alkalis and ammonia. 
 
 The aqueous and alkaline solutions of chondrin possess a left-handed 
 rotatory power on polarised light of— 213-5"; in presence of excess of 
 alkali this becomes - 552-0", both measured for yellow light". 
 
 1 Eichwald, Ann. d. Chem. u. Pharm. Bd. 134, S. 193. - Op. cit. 
 
 3 Eichwald, op. cit. and Chem. Centralb., 1866, No. 14. Staedeler, Ann. de Chem. 
 V. Pharm. Bd. Ill, S. 14. Landwelir, Zeitsch. f. physiol. Chem. Bd. v. (1881), 
 S. 871. 
 
 * Landwehr, Zeitsch. f. phydol. Chem. Bd. vi. (1882), S. 75. 
 
 5 I. V. Mering, Beitrag zur Chemie des Knorpels, 1873. 
 
 6 Hoppe-Seyler, Hdh. phys. path. chem. Anal, i Aufl. 1S75, S. 26-3, 
 
Aff.] rilKMICAL BASIS OF TIIK AX/MAL liODY. 723 
 
 It seems, according to the obsen'ations of iiiatiy, thut choudriu can, 
 by heating with hydrochloric acid, be converted into a body whose 
 reactions resemble those of syntonin, and another substance, which like 
 the similar product from mucin, so far resembles grape-sugar that it 
 reduces cupric salts in alkaline solution' ; it appf-ars however to contain 
 nitrogen. The existence of chondrin as a distinct substance has however 
 been denied* on the supposition that it is in all cases a mere mixture of 
 other bodies. It is stated that a substance having all the reactions of the 
 so-called chondrin, may at any time be produced by a mixture of mucin, 
 glutin and inorganic salts. The extreme similarity in the reactions of 
 chondrin and mucin point to a close relationship between the two. The 
 whole subject, however, requires more complete investigation. "With 
 alkalis or dilute sulphuric acid chondrin gives Icucin, but no tyrosin or 
 glycin. "Whether chondrin exists as such in cartilage is uncertain; 
 it seems probable that it does not, since its extraction from cartilage 
 requires an amount of boiling with water much greater than that 
 requisite to dissolve dried chondrin. 
 
 Preparation. From cartilage by extracting with water, and 
 precipitating with acetic acid- 
 Gelatin or Glutin\ (O, 23-21. H, 715. X, 18-32, C, 50-76. 
 S, -56 p. c.) 
 
 This is the substance which is yielded when connective tissue fibres 
 are heated for several days with very dilute acetic acid, at a temperature 
 of about 15° C, or by the prolonged action of water in a Papin's digester. 
 The elastic elements of connective tissue are unaffected by the above 
 treatment. 
 
 As obtained in this way glutin is when heated a thin fluid, solidi- 
 fying on cooling to the well-known gelatinous form. "When dried it is 
 a colourless, tran-sparent, brittle body, swelling up, but remaining undis- 
 solved in cold water; heating, or the addition of traces of acids or 
 alkalis, readily effects its solution. "When dissolved in water it 
 possesses a Icevo-rotatory power of -130', at 30' C; the addition of 
 strong alkali or acetic acid reduces this to — 112" or- 114', both 
 measured for yellow light*. Its solutions -will not dialyse. 
 
 Mercuric chloride and tannic acid are the only two reagents which 
 yield insoluble precipitates with this body. Its presence prevents the 
 action of Trommer's sugar test, since it readily dissolves up the 
 precipitated cuprous oxide. The proteid reactions of glutin are so 
 feeble that they are probably due merely to impurities. Heated with 
 sulphuric acid it yields ammonia, leucin and glycin, but no tyrosin. 
 
 1 De Bary, Hoppe-Seyler'a Untersuch. Hft. i. S. 71. 
 
 - ilorochowetz. Verhand. nalurhut. med. Ver. Heidelherfi. Bd. I. (1876) Hft. n. 
 
 2 Not to be confounded with the vegetable proteid 'glnten.' 
 
 * Hoppe-Seyler, Hhd. d. physt. path. chem. Anal. 4 .\nfl. 1873, S. 22-2. 
 
726 MUCIX, GELATIN, ETC. [App. 
 
 It appears improbable that glutin exists ready formed in connective 
 tissue fibres, since these do not s-n'ell np in water, and only yield glutin 
 after prolonged treatment with boiling water ; to which it may be added 
 that while glutin is acted upon by trypsin, the connective tissue fibres 
 in their natural condition resist its action (see p. 253). When glutin is 
 submitted for some time to the action of dilute hydrochloric acid, at 
 38° 0., and the change is brought about even more readily by the action 
 of pepsin, it loses its power of gelatinising and is now difi'usible through 
 porous membranes ; the name of gelatin-peptone has been given to the 
 product thus obtained \ 
 
 Elastin. (0,20-5. H, 74. N, 16-7. C, 55-5 p. c.) 
 
 This characteristic component of elastic fibres is left on the removal 
 of all the glutin, mucin, &c. from such tissues as " ligamentum nuchse," 
 advantage being taken of its not being altered when it is heated with 
 water, even xmder pressure, with strong acetic acid, or with dilute 
 alkalis. When moist it is yellow and elastic, but on drying becomes 
 brittle. It is soluble in strong alkalis at boiling temperatures, and 
 concentrated sulphuric and nitric acids dissolve it even in the cold ; it 
 is also dissolved by the action of papaya juice. It is precipitated from 
 solutions by tannic acid, but not by the addition of ordinary acids. 
 Notwithstanding that it closely approaches the proteids in its per- 
 centage composition, and gives distinct although feeble proteid reactions, 
 any very close relationship between the two appears improbable, since 
 elastin when treated with sulphuric acid, yields leucin (30 — 40 p.c.) 
 only and no tyrosin. 
 
 Hilger" has obtained a similar body from the shell membrane of 
 snakes' eggs. 
 
 Ke^atin^ (O, 20-7— 25-0. H, G4— 7-0. N, 16-2— 17-7. C, 
 50-3— 52-5. S, -7— 5-0 p.c.) 
 
 This body, though somewhat resembling the proteids in general 
 
 composition, differs from them and also from the preceding bodies so 
 
 widely in other properties, that its description is placed here for 
 
 convenience rather than anything else. Hair, nails, feathers, horn, and 
 
 epidermic scales consist for the most part of keratin. Heated with 
 
 water in a digester at 150° C. keratin is partially dissolved with evolution 
 
 of sulphuretted hydrogen ; the solution then gives with acetic acid and 
 
 ferrocyanide of potassium a precipitate soluble in excess of the acid. 
 
 Prolonged boiling with alkalis and acids, even acetic, dissolves keratin ; 
 
 the alkaline solutions evolve sulphuretted hydrogen on treatment with 
 
 1 Hofmeister, Zeitsch. f. physiol. Cliem. Bd. ii. (1878), S. 299. 
 '■i Ber. d. deutsch. chem. Gesellsch. 1873, S. 166. But see also next reference. 
 3 Lindwall, "Nagra bidragtill kann. om. Ker. Upasala Ldkarefs.f'drh. xvi. (1881), 
 p. 546. 
 
Ai'H.] CHEMICAL r.ASiS OF Till-: AMMAL J'.OhV. 727 
 
 acids. TliL' sulphur in k«'rutiii is evidently very loosely united to the 
 substance, and in all its reactions there appears to be a want of 
 similarity between kenitin and either proteids, mucin or gelatin. The 
 most common of its products of decomposition are leucin (10 p.c), and 
 tyrosin (3G p.c), and some aspartic acid; no glycin is formed. What 
 is generally known as keratin is probably a compound body, which has 
 not yet been resolved into its components. 
 
 Ewald and Kiihne' liave described a new body to which, siuce it occurs as a 
 constituent of nervous tissue (both of nerves and of the central nervous system), 
 and is yet closely identical with ordinary horny tissue, they give the name of neuro- 
 keratin. It is prepared in quantity from the brain by extracting this tissue with 
 alcohol and ather, and subjecting the residue to the action of pepsin and trj'psin. 
 Tlie final residue is neuro-keratin, and amounts to 15 — 20 p.c. of the original tissue. 
 
 Nuclein. Cg ll,^ N^, P3 0.> 
 
 Discovered by Mieschcr^ in the nuclei of pus corpuscles and in the 
 yellow corpuscles of yolk of egg. Other observers have subsequently 
 obtained it from yeast, from semen, from the nuclei of the red blood-cor- 
 puscles of birds and amphibia, from hepatic cells, and it is probably 
 present in all nuclei. 
 
 When newly prepared it is a colourless amorphous body, soluble to a 
 slight extent in water, readily soluble in many alkaline solutions; but 
 its solubilities alter on keeping. If added gradually in sufficient 
 quantity to a solution of caustic alkali it first neutralises the solution 
 and then renders it acid. It seems to possess an indistinct xantho- 
 proteic reaction, but gives no reaction with Millon's fluid. It yields 
 precipitates with se\eral salts, e.g. zinc chloride, argentic nitrate, and 
 cupric sulphate. 
 
 rreparation^. Since nuclein is very resistent to the action of 
 pepsin, it may be obtained from the granular residue consisting chiefly 
 of nuclei, which occurs after digesting pus with pepsin. The most 
 remarkable feature of this body is its large percentage of phosphorus, 
 9-59 per cent. Tliis phosphorus is readily separated by boiling with 
 strong hydi'ocliloric acid or caustic alkalis ; the same occurs when 
 solutions of nuclein are acidulated and allowed to stand. 
 
 Chitin. CjjH^NjO,/. 
 
 Although not found as a constituent of any mammalian tissue, this 
 substance coniposes the chief part of the exoskeleton of many inverte- 
 brates. It may j)robably be regarded as the animal analogue of the 
 
 ^ Verhand. naturhist. med. Ver Heidelberg. Bd. i. (1876), Heft 5. 
 
 = Med. Chem. Vntersnch. Hoppe-Seyler, Heft 4, 1872, S. 441 und 502. 
 
 3 See Kossel, Zeitsch. f. jjhijsiol. Cl'iem. Bd. iii. (1879), S. 284 iv. (1880), S. 290. 
 VII. (1883), S. 7. " Untersuch. iiber d. Nuclein u. ihre Spaltungsprod, " Strassb., 
 1881. 
 
 * Ledderhose, Zeitsch. f. physiol. Chem. Bd. 11. (1878), S. 213. 
 
728 CARBOHYDRATES. [App. 
 
 cellulose of plauts, and from this point of view it possesses considerable 
 morphological interest. Both cellulose and chitin appear to yield some 
 form of sugar when treated with strong acids. 
 
 "When purified, chitin is a white amorphous body, often retaining the 
 shape of the tissue from which it has been prepared. It is insoluble in 
 all reagents except strong mineral acids, the best solvents being sulphuric 
 or hydrochloric acids. The immediate addition of water to these solu- 
 tions reprecipitates the chitin in an unaltered form ; but the prolonged 
 action of sulphuric acid causes a decomposition resulting, according to 
 some observers, in the formation of an amorphous fermentible carbo- 
 hydi-ate ; and when hydrochloric acid is used an amidated carbohydrate 
 is obtained to which the name of glycosamin ' has been given. 
 
 Preparation ^ The cleaned exoskeleton of a lobster is thoroughly 
 extracted with dilute hydrochloric acid and then with caustic soda. To 
 purify it finally it is submitted to prolonged boiling with a solution of 
 potassic permanganate. 
 
 CAEBOHYDRATES. 
 
 Certain members only of this class occur in the human body; of 
 these, the most important and wide-spread are those known as glycogen 
 and the two sugars, grape-sugar or dextrose (glucose), with which 
 diabetic sugar seems to be identical^ and maltose. ISText to these comes 
 milk-sugar. Inosit is another body of this class, although it difiers in 
 many important points from the preceding two. 
 
 Sugars are often considered to be iDolyatomic alcohols. Several of them 
 stand in peculiar relation to mannit, and may be converted into that substance by 
 the action of sodium amalgam^. 
 
 1. Dextrose (Grape-sugar), C^ H^^ O^ + H^ O. 
 
 Occurs in the contents of the alimentary canal to a variable extent 
 dependent on the nature of the food taken. It is also a normal con- 
 stituent of blood, chyle, and lymph. Concerning its presence in the 
 liver, see p. 419. The amniotic fluid also contains this body. Bile 
 in the normal condition is free from sugar, so also is urine, though 
 this point has given rise to great dispute'. The disease diabetes is 
 characterized by an excess of dextrose in the fluids and tissues of the 
 body (see p. 424). 
 
 1 Ledderhose, loc. cit. Bd. iv. (18S0), S. 139. 
 
 2 Butschli, Arch. f. Anat. u. Physiol. Jahrg. 1874, S. 362. 
 
 3 The question, however, whether several varieties of sugar occurring in the 
 animal body have not been confounded together under the common name of dextrose 
 or glucose may be considered at present an open one. 
 
 ■* Lmnemann, Ann. d. Chem. u. Pharm. Bd. 123, S. 136. 
 6 See Seegen, Dcr Diabetes Mellitus, 2 Ed. S. 196. 
 
Ai'i'.J CHEMICAL n.iSlS OF THE A S I M A L liODY. 7l"J 
 
 Wlioii pure, cU'xtrose is colourless ami crystallises ivoxw ils aqueous 
 solution in six-siilod tables or prisms, often agglomerated into warty 
 lumps. The crystals will dissolve in their own weight of cold water, 
 requiring however some time for the process ; they are very readily 
 soluble in hot water. Dextrose is somewhat sparingly soluble in alcohol, 
 and crystallises from anhydrous alcohol in prisms free from water of 
 crystallisation ; it is moreover insoluble in a;thcr. 
 
 The freshly prepared cold aciueous solutiou of tlie crystals possesses a dextro- 
 rotatory power of -I- 101" for yellow light. This, quickly on heating, more slowly on 
 standing, falls to + 50", at which point it remains constant. 
 
 Dextrose readily forms compounds with acids and many salts ; the latter are verj' 
 unstable, decomi>ositiou rapidly ensuing on heating them. When its metallic com- 
 pounds arc decomposed the decomposition is in many cases accompanied by the 
 precipitation of the metals, e.g. silver, gold, mercury, bismuth. Caustic alkalis 
 readily decompose them, as also doc-;. ammonia. 
 
 Dextrose is readily and conqjletely precipitated by lead acetate and 
 ammonia. 
 
 An important property of this body is its power of undergoing fer- 
 mentations. Of these the two princii^al are: (1) Alcoholic. This is 
 produced in aqueous solutions of dextrose, under the influence of yeast. 
 The decomposition is the following : C^ H,^ 0^ = 2 C^ H^ O -f 2 CO^, 
 yielding (ethyl) alcohol and carbonic anhydride. Other alcohols of the 
 acetic series are found in traces, as also are glycerine, succinic acid and 
 probably many other bodies. The fermentation is most active at about 
 25"C. Below 5"C. or above 4:5''C. it almost entirely ceases. If the 
 saccharine solution contains more than 1 5 per cent, of sugar it will not 
 all be decomposed, as excess of alcohol stops the reaction. (2) Lactic. 
 This occui's in the presence of decomposing nitrogenous matter, especially 
 of casein, and is probably the result of the action of a specific ferment'. 
 The first stage is the production of lactic acid, C^ H|„ 0,, = 2 Cg H^ O^. 
 In the second butyric acid is formed with evolution of hydrogen and 
 carbonic anhydride: 2 C, H,0 = C,H 0„ -f 2 CO, + 2 H,. The above 
 changes, the first of which is probably undergone by sugar to a consider- 
 able extent in the intestine, are most active at 35"C. ; the presence of 
 alkaline carbonates is also favourable. It is moreover essential that the 
 lactic acid should be neutralized as fast as it is foiTued, othenvise the 
 presence of the free acid stops the process. 
 
 The preparation, detection and estimation of dextrose are so fully 
 given in \ arious books that they need not be detailed hei-e. 
 
 1 Lister, Path. Soc. Trans. Vol. for 1873, p. 4.23, tiho Quart. Jl. of Micros. 
 Science, Yol. xviii. (1878), p. 177. 
 
730 CARBOHYDRATES. [App. 
 
 2. Maltose. C,^H^,0,, + H„0. 
 
 This form of sugar was first described by Dubrunfaut' as a product 
 of the action of malt extract on starch. Its existence was for a long time 
 doubted until O'Sullivan^ repeated and confirmed the previous experi- 
 ments. According to him it crystallises in fine acicular crystals, 
 possesses a specific rotatory power of + 150° and a reducing power which 
 is only one-third as great as that of dextrose. It seems probable that 
 this is the chief sugar obtained by the action not only of diastase but of 
 ptyalin and pancreatic ferment upon starch and perhaps also upon 
 glycogen^; although some dextrose may at the same time be formed. 
 Musculus and Gruber* have shewn that maltose may also be formed by 
 the action of dilute sulphuric acid on starch, and that it is capable of 
 undergoing alcoholic fei"mentation. 
 
 Rreparation. See Musculus and Gruber {loc. cif.). 
 
 3. Milk-sugar. C^^ H„ 0^^ + H^ 0. 
 
 Also known as Lactose. It is found in milk, and is characteristic 
 of this secretion. It is said however to occur abnormally in the urine of 
 lying-in women ^ 
 
 It yields, when pure, hard colourless crystals, belonging to the 
 rhombic system (four-sided prisms). It is less soluble in water than 
 dextrose, requiring for solution six times its weight of cold, but only 
 two parts of boiling, water ; it is entirely insoluble in alcohol and sether. 
 It is fully precipitated from its solutions by the addition of lead acetate 
 and ammonia. 
 
 When freshly dissolved, its aqueous solution possesses a specific dextro-rotatory 
 power of -f 93-1'' for sodium light: this diminishes, slowly on standing, rapidly on 
 boUing, until it finally remains constant at +52-o''. The amount of rotation is 
 independent of the concentration of the solution. 
 
 Lactose unites readily with bases, fonning unstable compounds; from itsmetalHc 
 compounds the metal is precipitated in the reduced state on boiling ; it reduces 
 copper salts as readily as dextrose but to a less extent viz. in the ratio of 70 : 100. 
 
 Lactose is generally stated to admit of no direct alcoholic fermenta- 
 tion ; this may however sometimes be induced by the prolonged action 
 of yeast. By boiling with dilute mineral acids lactose is converted into 
 galactose, which readily undergo alcoholic fermentation and jjossess a 
 greater rotatory power than lactose. 
 
 It may be remarked here that though isolated lactose is incapable of direct 
 alcoholic fermentation, milk itself may be fermented ; Berthelot was unable in this 
 
 1 A7in. Chim. Phys. (3) xxi. (1847), p. 178. 
 
 2 Jl. Chem. Soc. Ser. 2, Vol. x. (1872), p. 579. 
 
 3 Musculus u. V. Mering, Zeitsch.f. phi/siol. Chem. Bd. ii. (1878), S. 403. 
 * Zeitschr. f. physiol. Chem. Bd. ii. (1878), S. 177. 
 
 6 Hofmeister, Ihid. Bd. i. (1877), S. 101. 
 
Api'.] CIIKMICAf. ]'>.\SIS OF rill-: AMMAL liODY. 731 
 
 direct alcitholio fernioiitutioii tu tlclict any intermediate change of the lactose into 
 iiiiy other fcrnieutablo bugar. 
 
 Lactoso is lin\vfv«M' (/Irertfi/ capiiUlo of uiulcr^oiiig the lactic and 
 butyric fermentations ; the circumstances and products are the sajiie as 
 in the case of dextrose (see above). The action is generally jiroductive 
 of a collateral small quantity of alcohol. 
 
 Lactose is thus distinguished from dextrose by its smaller solubility 
 in ^\;lter, insolubility in alcohol, crystalline form, lower cupric oxide re- 
 ducing power and its incapability of undergoing direct alcoholic fermen- 
 tation. 
 
 Preparation. After the removal of the casein and other proteids of 
 the milk, the mother liquor is evaporated to the crystallising point; the 
 crystals are purilied by repeated crystallisation from warm water. 
 
 4. Inosit. C^ H,„ O^ + 2H^ 0. 
 
 This substance occurs but sparingly in the human body ; it was 
 found originally by Scherer' in the muscles of the heart. Cloetta shewed 
 its presence in the lungs, kidneys, spleen and liver ^, and Miiller in the 
 brain '. It occurs also in diabetic urine, and in that of ' Bright's disease,' 
 and is found in abundance in the vegetable kingdom. 
 
 Pure inosit forms large efflorescent crystals (rhomliic tables) ; in 
 microscopic preparations it is usually obtained in tufted lumps of fine 
 crystals. Easily soluble in water, it is insoluble in alcohol and rether. 
 It possesses no action on polarised light, and does not reduce solutions 
 of metallic salts. 
 
 It admits of no direct alcoholic, but is capable of undergoing the 
 lactic fermentation ; according to Hilger^ the acid formed is sarcolactic. 
 It is unaltered by heating with dilute mineral acids. 
 
 Preparation. It may be precij)itated from its solutions by the action 
 of basic lead acetate and ammonia ; the lead is then removed by sulphu- 
 retted hydrogen and the inosit precipitated with excess of alcohol. 
 
 As a special test (Scherer's ) may be mentioned the production of a 
 bright \'iolet colour by careful evaporation to dryness on platinum foil, 
 with a little ammonia and calcium chloride. 
 
 5. Dextrin. C^H,„0^. 
 
 By boiling starch-paste with dilute acids, or by the action of fer- 
 ments, the starch is converted into an isomeric body, to which, from its 
 action on polarised light, the name dextrin has been given. It is soluble 
 
 > Ann. d. Chem. u. Pkarm. Bd. 73, S. 322. = Hid. Bd. 99, S. 289. 
 
 » Ibid. Bd. 103, S. 140. * Ibid. Bd. ICO, S. 333. 
 
732 CARBOHYDRATES. [App. 
 
 in water, but is precipitated by alcohol. It does not undergo alcoliolic 
 fermentation until after it has been changed into dextrose, nor can it 
 reduce metallic salts. It yields a reddish port-wine colour with iodine, 
 which disappears on warming and does not return on cooling. Further 
 action of acids. or of ferments converts dextrin into dextrose. Dextrin 
 is present in the contents of the alimentary canal after a meal contairdng 
 starch, and has also been found in the blood. 
 
 . There is not the least doubt that several modifications of dextrin 
 exist and may be obtained by the action of acids and ferments on starch. 
 Of these two of the best known are those described by Briickei under 
 the name of erythrodextrin and achroodextriu, the former giving a red 
 colour with iodine, the latter not yielding any colour at all. Erythro- 
 dextrin may be readily converted into a sugar by the action of ferments, 
 and thus is not found as a product of the complete action of ptyalin on 
 starch. Achroodextriu on the other hand is not thus converted by 
 ferments, and therefore remains in solution, together with the sugar 
 formed by the action of ptyalin on starch. Achroodextrin may be con- 
 verted into dextrose by boiling with dilute hydrochloric acid, 
 
 6, Glycogen. C,H,„0,. 
 
 Belongs to the starch division of carbohydrates. Discovered by 
 Bernard in the liver and other organs (see p. 416). 
 
 Glycogen is, when pure, an amorphous powder, colourless, and taste- 
 less, readily soluble in water, insoluble in alcohol and sether. Its 
 aqueous solution is generally though not alwa^ys strongly opalescent, but 
 contains no particles visible microscopically ; the opalescence is much 
 reduced by the presence of free alkalis. The same solution possesses, 
 according to Hoppe-Seyler, a very strong dextro-rotatory power, about 
 three times as great as that of dextrose^; it dissolves hydrated cupric 
 oxide ; but this is not reduced on boiling. 
 
 By the action of dilute mineral acids (except nitric) it is partially 
 converted into a form of sugar very closely resembling, though probably 
 differing somewhat from true dextrose, and the same conversion is also 
 readily effected by the action of amylolytic ferments. The sugar into 
 which the glycogen of the liver is naturally converted after death (see 
 p. 424), appears to be true dextrose^ ; so also the sugar of diabetes. 
 The result of the action of diastase, or salivary or pancreatic ferment, 
 upon glycogen is hoAvever according to Musculus and v. Mering* a 
 
 1 Sitzher. d. Wien. Akad. 1872, iii. Abth. Also Vorlesungen 2. Aiijl. 1875, Bd. i. 
 S. 224, 
 
 2 See Kiilz, PMger's Arch. Bd. xxiv, (1881), S. 85, 
 
 3 Pfliiger's Arch. Bd. xix. (1879), S. 106, and xxii. (1880), S. 206. Also Kiilz, 
 Ibid. Bd. XXIV. (1881), S. 52. 
 
 4 Zeitschr. f. physiol. Chem. Bd. ii. (1878), S. 40-3. 
 
Ait.] chemical basis of Tin: am mm. V.OhV. 733 
 
 mixture of achroodextrin and maltose ; tlio quantity of dextrose making 
 its appearance at the same time being very small. 
 
 OpalcsccMit solutions of glycogen usually become clear on the addition 
 of caustic alkiili : Vintschgau and Dietl' have shewn that this is accom- 
 panied on boiling by a change which converts a portion of the glycogen 
 into a substance to which they give the name of ^.-glycogen-dextrine. 
 (Kiihne* had previously described a body to which he gave the name gly 
 cogen-dextriii. That described by Vintschgau and Dietl diflers slightly 
 from Kiihno's body, hence the name.) According to these authors one- 
 fifth of the glycogen is at the same time changed into some other, at pre- 
 sent undetermined, substance. Normal lead acetate gives a cloudiness, 
 the basic salt a precipitate, in solutions of glycogen. 
 
 As tests for this body may be used the formation of a port-wine 
 colour with iodine ; this disappears on warming but returns on cooling. 
 The same colour is produced by the action of iodine on dextrin, but 
 this docs not reappear on cooling after its disappearance by warming. 
 
 Preparation of Glycogen. The following is Briicke's^ method. TTie 
 fdtered or simply strained decoction of perfectly fresh liver or other gly- 
 cogenic tissue is, when cold, treated alternately with dilute hydrochloric 
 acid, and a solution of the double iodide of potassium and mercury \ 
 as long as any precipitate occurs. In the presence of free hydrochloric 
 acid, the double iodide precipitates proteid matters so completely as to 
 render their separation by filtration easy. The proteids being thus got 
 rid of, the glycogen is precipitated from the filtrate by adding alcohol to 
 the extent of between 60 and 70 p. c. Too much alcohol is to be 
 avoided, since other substances as well as glycogen are thereby pre- 
 cipitated. The glycogen is now washed with alcohol first of 60 and 
 then of 95 per cent., afterwards wdth aether, and finally with absolute 
 alcohoL It is then dried over sulphuric acid. 
 
 Tunicin. {C^'R^^O^)^. 
 
 This body is regarded by many observers as identical with the true 
 cellulose of plants, while others have ascribed to it properties difieriug 
 from those of cellulose suflaciently to justify its receiving a distinct name. 
 It appears to be more resistent to the action of chemical reagents than 
 plant-cellulose. 
 
 It constitutes the chief part of the integument of the Ascidia or 
 
 1 Pflugei's Jrc/i. Bd. xvn. (1878), S. 134. 
 
 2 Lehrh. d. physiol. Chein. (1868), S. 63. 
 
 3 Sitzung.^ber. d. Wiener Akad. Bd. 63 (1871), ii. Abth. 
 
 * This may be prepared by precipitating potassic iodide with mercuric chloride 
 and dissolving the washed precipitate in a hot solution of potassic iodide as lonj; 
 as it continues to be taken up. On cooling, some amount of precipitate occur.~, 
 which must be filtered off ; the filtrate is then ready for u.se. 
 
734 FATTY ACIDS. [App. 
 
 Tunicata. As prepared from this source it is when pure quite white and 
 usually retains the shape of the tissue. It is unacted upon by any 
 reagent except strong acids and alkalis, and by the action of the former 
 it yields some form of sugar. 
 
 FATS, THEIR DERIVATIVES AND ALLIES. 
 
 The Acetic Acid Series. 
 
 General formula C^ H^^ O^ (monobasic). 
 
 This, which is one of the most complete homologous series of organic 
 chemistry, runs parallel to the series of monatomic alcohols. Thus formic 
 acid corresponds to methyl alcohol, acetic acid to ethyl (ordinary) 
 alcohol, and so on. The several acids may be regarded as being derived 
 from their respective alcohols by simple oxidation : thus ethyl alcohol 
 yields by oxidation acetic acid : — C„ H^ O + O^ r= C„ H^ O^ + H^ O. The 
 various members differ in composition by CH^, and the boiling points 
 rise successively by about 19° C. Similar relations hold good with regard 
 to their melting points and specific gravities. The acid properties are 
 strongest in those where n has the least value. The lowest members of 
 the series are volatile liquids, acting as powerful acids ; these succes- 
 sively become less and less fluid, and the highest members are colour- 
 less solids, closely resembling the neutral fats in outward appear- 
 ance. Consecutive acids of the series present but very small differences 
 of chemical and physical properties, hence the difficulty of separating 
 them : this is further increased in the animal body by the fact that 
 exactly those acids which present the greatest similarities usually occur 
 together. 
 
 The free acids are found only in small and very variable quantities 
 in various parts of the body ; their derivatives on the other hand form 
 most important constituents of the human frame, and will be considered 
 further on. 
 
 Formic acid. CHO . OH. 
 
 When pure is a strongly corrosive, fuming fluid, with powerful iiTi- 
 
 tating odour, solidifying at 0*^0., boiling at 100" C, and capable of 
 
 being mixed in all propoi^tions with water and alcohol. It has been 
 
 obtained from various parts of the body, such as the spleen, thymus, 
 
 pancreas, muscles, brain, and blood ; in the latter its presence may be 
 
 due to the action of acids on the haemoglobin. According to some 
 
 authors' it occurs also in urine. 
 
 1 Buliginsky, Hoppe-Seyler's Med. chem. Mittheilung. Heft. 2, S. 240. Thudichum, 
 Journ. of the Chem. Soc. Yol. 8, p. 400. 
 
Arp.] CHEMICAL BASIS OF Till': AM. UAL liOhV. 7;5.) 
 
 llcatod wiili sulphurii' ;uid it yi«^'l(ls carbonic oxido and water; with 
 caustic potash it gives liydrogeii and oxalic acid. 
 
 Acetic acid. O^H.O . OH. 
 
 Is distinguished by its charactoristic odour; its boiling point is 
 IIT'C. ; it solidifies at 5" and is lluid at all temperatures above 15° C. 
 It is soluble in all proportions in alcohol and water. 
 
 It occurs in the stomach as the result of fermentative changes in tho 
 food, and is frequently present in diabetic urine. In other organs and 
 fluids it exists only in minute traces. 
 
 With ferric chloride it yields a blood-red solution, decolourized by hydrocblorio 
 acid. (It differs in this last reaction from sulpliocyanido of iron.) Heated with 
 alcohol and sulphuric acid, the characteristic odour of acetic aether is obtained. It 
 does not reduce silver nitrate. 
 
 Propionic acid. CJI.O . Oil. 
 
 This acid closely resembles the preceding one. It possesses a very 
 sour taste and pungent odour; is soluble in water, boils at 141" C, 
 and may be separated from its aqueous solution by excess of calcic 
 chloride. 
 
 It occurs in small quantities in sweat, in the contents of the stomach, 
 and in diabetic urine when undergoing fermentation. It is similai'ly 
 produced, mixed however with other products, during alcoholic fermen- 
 tation, or by the decomposition of glycerine. It partially reduces silver 
 nitrate solution on boiling. 
 
 Butyric acid. C^ H, . OH. 
 
 An oily colourless liquid, with an odour of rancid butter, soluble in 
 ■water, alcohol, and aether, boiling at 162° C. Calcic chloride separates 
 it from its aqueous solution. 
 
 Found in sweat, the contents of the large intestine, fieces, and in 
 urine. It occurs in traces in many other fluids, and is plentifully 
 obtained when diabetic urine is mixed with powdered chalk and kept at 
 a temperature of 35" C. It exists, as a neutral fat, in small quantities 
 in milk. 
 
 This is the principal pi'oduct of the second stage of lactic fermenta- 
 tion, (See Dextrose.) 
 
 Valerianic acid. C^H^^O . OH. 
 
 An oily liquid, of penetrating odour and burning taste ; soluble in 
 30 parts of water at 12"'C., readily soluble in alcohol and aether. Boils 
 at 175° C. Possesses, in free and combined form, a feeble right handed 
 rotation of the plane of polarisation. 
 
C.H„0. 
 
 , OH. 
 
 CsH.O- 
 
 OH. 
 
 0,„H, O 
 
 . OH. 
 
 736 FATTY ACIDS. [App. 
 
 It is found ill the solid excrements, and is formed readily by the de- 
 composition, through putrefaction, of impure leucin, ammonia being at 
 the same time evolved; hence its occurrence in urine when that fluid 
 contains leucin, as in cases of acute atrophy of the liver. 
 
 Caproic acid. 
 Caprylic „ 
 Capric (Rutic) acid 
 
 These three occur together (as fats) in butter, and are contained in 
 varying proportions in the faeces from a meat diet. The first is an oily 
 fluid, slightly soluble in water, the others are solids and scarcely soluble 
 in water; they are soluble in all proportions in alcohol and sether. 
 They may be prepared from butter, and separated by the varying solu- 
 bilities of their barium salts. 
 
 Laurostearic acid. Cj^H^jO . OH. 
 Myristic „ C,,H,,0 . OH. 
 
 These occur as neutral fats in spermaceti, in butter and other fats. 
 They present no points of interest. 
 
 Palmitic acid. C^^ B.^^ . OH. 
 
 Stearic „ C,3H3,O.OH. 
 
 These are solid, colourless when pure, tasteless, odourless, crystalKne 
 bodies, the former melting at 62" C, the latter at 69-2° C. In water 
 they are quite insoluble ; palmitic acid is more readily soluble in cold 
 alcohol than stearic: both are readily dissolved by hot alcohol, tether, or 
 chloroform. Glacial acetic acid dissolves them in large quantity, the 
 solution being assisted by warming. They readily form soaps with the 
 alkalis, also with many other metals. The varying solubilities of their 
 barium salts afford the means of separating them when mixed' : this 
 may also be applied to many others of the higher members of this 
 series. 
 
 These acids in combination with glycerin (see below), together with 
 the analogous compound of oleic acid, form the principal constituents of 
 human fat. As salts of calcium they occur in the fseces and in 
 'adipocire,' and probably in chyle, blood and serous fluids, as salts of 
 sodium. They are found in the free state in decomposing pus, and in 
 the caseous deposits of tuberculosis. 
 
 The existence of margaric acid, intermediate to the above two, is not now 
 admitted, since Heintz ^ has shewn that it is really a mixture of palmitic and stearic 
 acid. Margaric acid possesses the anomalous melting point of 59-9" C. A mixture 
 of 60 parts stearic and 40 of palmitic acids, melts at GO-S". 
 
 1 Heintz, AnnaJ. d. PJujs. u. Chem. Bd. 92, S. 588. ' Op. cit. 
 
Apr.] CHEMICAL BASIS OF THE ANIMAL lUjUY. 737 
 
 Acids of the Olkic (A( kvlic) Series. II (<--', IIj,,_a) O^, (monobasic). 
 
 Many acids of this strios occur as glyciuino compounds iu various 
 fats. They arc very unstable, and readily absorb oxygen when exposed 
 to the air. The higher members are decomposed on attempting to 
 distil them. Their most peculiar property is that of being converted 
 by traces of NO^, into solid, stable metameric acids, capable of being 
 distilled. They bear an interesting relation to the acids of the acetic 
 series, breaking up when heated with caustic potash into acetic acid and 
 some other member of the same series : — thus, 
 
 Ok'ic ackl. I'otassic acetate. Potassic palmitate. 
 
 nC.3H33O, + 2KH0 = KC,H3 0, + KC.. H^. O, + H,. 
 
 Oleic acid. C,„H3p . OH. 
 
 This is the only acid of the series which is physiologically important. 
 It is found united with glycerin in all the fats of the human body. 
 
 When pure it is, at ordinary temperatures, a colourless, odourless, 
 tasteless, oily liquid, solidifying at 4"C. to a crystalline mass. Insoluble 
 in water, it is soluble iu alcohol and a'ther. It cannot be distilled 
 without decomposition. It readily forms with potassium and sodium 
 soaps, which are soluble in water; its compounds with most other bases 
 are insoluble. It may be distinguished from the acids of the acetic 
 series by its reaction with NO^ and by the changes it undei-goes when 
 exposed to the air. 
 
 The ISTEUTaAL Fats. 
 
 These may be considered as aethers formed by replacing the exchange- 
 able atoms of hydrogen in the triatomic alcohol glycerin (see below), by 
 the acid radicles of the acetic and oleic series. Since there are three 
 such exchangeable atoms of hydrogen iu glycerin, it is possible to form 
 three classes of these aethers; only those, however, which belong to the 
 third class occur as natural constituents of the human body : those of 
 the fii-st and .second are of theoretical importance only. 
 
 They possess certain general characteristics. Insoluble in water and 
 cold alcohol, they are readily soluble in hot alcohol, aether, chloroform, 
 «fec. ; they also dissolve one another. They are neutral bodies, colourless 
 and tasteless when pure; are not capable of being distilled without 
 undergoing decomposition, and yield as a result of this decomposition, 
 solid and liquid hydrocarbons, Avater, fatty acids, and a peculiar body, 
 acrolein. (Glycerin contains the elements of one molecule of acrolein, 
 and two molecules of water.) 
 
 They po.ssess no action on polarised light. 
 
 They may readily be decomposed into glycerin and their respective 
 fatty acids by the action of caustic alkalis, or of superheated steam. 
 F. 47 
 
'73S FATS, ETC. [App. 
 
 Palmitin (Tri-palmitin). J^|f q J O3. 
 
 The following reaction for the formation of this fat is typical for all 
 the others : 
 
 Glycerin. Palmitic acid. Palmitin. 
 
 Palmitin is slightly soluble in cold alcohol, readily so in hot alcohol, 
 or in eether; when pure it crystallises in fine needles; if mixed with 
 stearin, it generally forms shapeless lumps, although the mixture may at 
 times assume a crystalline form, and was then regarded as a distinct body, 
 namely margarin. It possesses three different melting points, according 
 to the previous temperatures to which it has been subjected. It solidifies 
 in all cases at 45° C. 
 
 Stearin (Tri-stearin). \^ ^ I 0, 
 
 Preparation. From palm oil, by removing the free palmitic acid 
 with alcohol, and crystallising repeatedly from sether. 
 
 (C,sH3,0,)| 
 
 This is the hardest and least fusible of the ordinary fats of the 
 body; is also the least soluble, and hence is the first to crystallise out 
 from solutions of the mixed fats. It crystallises usually in square 
 tables. It presents peculiarities in its fusing points similar to those of 
 palmitin. 
 
 Preparation. From mutton suet, its separation from palmitin and 
 olein being effected by repeated crystallisation from sether, stearin being 
 the least soluble. 
 
 Olein (Tri-olein). ^^^/.^^^^ O3 
 
 Is obtained with difficulty in the pure state, and is then fluid at 
 oi-dinary temperatures. It is more soluble than the two preceding ones. 
 It readily undergoes oxidation when exposed to the air, and is converted 
 by mere traces of NO, into a solid isomeric fat. Olein yields, on dry 
 distillation, a characteristic acid, the sebacic, and is saponified with much, 
 greater difficulty than are palmitin and stearin. 
 
 Prejjaration. From olive oil, either by cooling to 0" C. and pressing 
 out the olein that remains fluid; or by dissolving in alcohol and cooling, 
 when the olein remains in solution while the other fats crystallise out. 
 
 Glycerin *^|^'| O3. 
 
 This principal constituent of the neutral fats may, as above stated, 
 be looked upon as a triatomic alcohol. 
 
Ait.] chemical BASIS OF Till-: AMMAL hODY. 739 
 
 Whoa j)urc, glycerin is a vi.sciil, colourless liquid, of a well-known 
 sweet taste. It is soluble iu wat<!r and alcohol in all proportionH, 
 insoluble in a;ther. Exposed to very low temperatures it becomes almost 
 solid; it may be dislilled iu close vessels without decomposition, between 
 275"— 280" 0. 
 
 It dissolves the alkalis and alkaline earths, also many oxides, such 
 as those of lead and copper; many of the fatty acids are also soluble in 
 glycerin. 
 
 It possesses no rotatory power on polarised light. 
 
 It is easily recognized by its ready solubility in water and alcohol, 
 its insolubility in a?ther, its sweet taste, and its reaction with bases. 
 The production of acrolein is also characteristic of glycerin. 
 
 C3H303-2H,0 = C,H,0 (Acrolein). 
 
 Pn'paration. By saponification of the various oils and fats. It is 
 also formed in small quantities during the alcoholic fermentation of 
 sugar'. 
 
 Soaps. These may be formed by the action of caustic alkalis on fats. 
 The process consists in a substitution of the alkali for the radicle of 
 glycerin, the latter combining with the elements of water to form 
 glycerin. Thus 
 Tristeariu. Potassic stearate. Glycerin. 
 
 (""ff'-] o. . 3^} o = f^'Y] « * ""&:] °- 
 
 Pancreatic juice can split up fats into glycerin and free fatty acids (see p. 254), 
 and the bile is known to be capable of saponifying these fatty acids. The amount 
 of soaps formed in the alimentary canal is however small and unimportant. 
 
 Acids of the Glycolic Series. 
 
 Running parallel to the monatomic alcohols (C,^H..„+„0) is the series 
 of diatomic alcohols or glycols {CJA^^.D.,). Thus corresponding to ethyl 
 alcohol is the diatomic alcohol, ethyl-glycol. As from the monatomic 
 alcohols, so from the glycols, acids may be derived by oxidation ; from 
 the latter (glycols) however two series of acids can be obtauied, known 
 respectively as the glycolic and oxalic series. The first stage of oxi- 
 dation of the glycol gives a member of the glycolic series, thus : 
 
 Ethyl-glycol. Glycolic-acid. 
 
 aHgO. + O, = C„H,03 + H.,0, or more generally 
 GJl^^.O, + O, -. C.H^O, + H,0. 
 
 1 Pasteur, Ann. d. Chan. u. Pharm. Ed. 106, S. 338. 
 
740 
 
 LACTIC ACIDS. 
 
 [A.PP. 
 
 By further oxidation a member of the glycolic series can be converted 
 into a member of the oxalic series, thus : 
 
 Glycolic acid. Oxalic acid. 
 
 O2H4O3 + 02 = C2H2O4 + H.O, or more generally 
 C„H,,03 + O, = C„H,„_,0, + H2O. 
 The acids of the glycolic series are diatomic but monobasic ; those of 
 the oxalic series are diatomic and diabasic. 
 
 The following table may be given to shew the general relationships 
 of alcohols and acids : 
 
 Radicle. 
 
 Alcohol. 
 
 Acid. 
 
 Glycols. 
 
 Acid I. 
 
 Acid II. 
 
 
 
 Formic. 
 
 
 Carbonic. 
 
 
 Methyl (CII3) 
 
 CH3(0H) 
 
 HCHO2 
 
 )) 
 
 H..CO3 
 
 ,, 
 
 
 
 Acetic. 
 
 Ethyl-glycol 
 
 Glycolic. 
 
 Oxalic. 
 
 Ethyl (C2H5) 
 
 C2H5(0H) 
 
 HC2H3O2 
 
 C2H,(OH)2 
 
 HO,H303 
 
 H2C2O4 
 
 
 
 Propionic. 
 
 Propyl-glocol 
 
 Lactic. 
 
 Malonic. 
 
 Propyl (C3H7) 
 
 C3H,(0H) 
 
 HC3HSO2 
 
 G3H,(OH)2 
 
 HC3H,,03 
 
 H2C3H2O, 
 
 
 
 Butyric. 
 
 Butyl-glycol 
 
 Oxybutyric. 
 
 Succinic. 
 
 Butyl (C4H9) 
 
 C,H<,(OH) 
 
 HC.H^Oa 
 
 C,H«(0H)2 
 
 HC,H,03 
 
 HgCiH^O^ 
 
 Glycolic Acid Series. 
 
 Lactic acid. CsHgOa. 
 
 Next to carbonic acid, the most important member of this series, as 
 far as physiology is concerned, is lactic acid. 
 
 Lactic acid exists in four isomeric modifications, but of these only 
 three have been found in the human body. These three all form sirupy, 
 colourless fluids, soluble in all proportions in water, alcohol and sether. 
 They possess an intensely sour taste, and a strong acid reaction. "When 
 heated in solution they are partially distilled over in the escaping 
 vapour. They form salts with metals, of which those with the alkalis 
 are very soluble and crystallise with difficulty. The calcium and zinc 
 salts are of the greatest importance, as will be seen later on. 
 
 1. Ethylidene-lactic acid. This is the ordinary form of the 
 acid, obtained as the characteristic product of the well-known ' lactic fer- 
 mentation.' It occurs in the contents of the stomach and intestines. 
 According to Ileintz^ it is found also in muscles, and according to 
 Gscheidlen^ in the ganglionic cells of the grey substance of the brain. 
 In many diseases it is found in urine, and exists to a large amount in 
 this excretion after poisoning by phosphorus ^ 
 
 1 Ann. d. Chem. u. Pharvi. Bd. 157, S. 320. 
 
 2 Pfluger's Archiv, Bd. vni. (1873—74) S. 171. 
 
 * Schultzen and'Rieis, Ueber acute Phosp1iorvergiftu7ig. Chem. Ccntralb. 1869, S. 
 631. 
 
Ai'p.] CHEMICAL BASIS OF THE AXIMAL BODY. 741 
 
 It may be prepared by the general methods of slowly oxidising the corresponding 
 glycol or by acting on mouochloriuated propionic acid with moist silver oxide. 
 In obtaining it from the products of lactic fermentation, the crusts of zinc lactate 
 are purified by several crystallisations, and the acid liberated from the compound by 
 the action of snlpluuctted hydrogen. 
 
 2. Ethylene-lactic acid. This acid is found accompanying the 
 next to be described, in the watery extract of muscles. From this it is 
 separated by taking advautage of the different solubilities in alcohol 
 of the zinc salts of the two acids. It seems probable, however, that 
 it has not yet been prepared iu the pure state by this method. 
 
 Wislicenus first obtained this acid by heating hydroxycyanide of ethylene with 
 aqiieous solutions of the alkalis ^ 
 
 The same observer found it also in many pathological fluids. 
 
 3. Sarcolactic acid. This acid has not yet been procured syn- 
 thetically. As its name implies, it is that form of the acid which 
 chiefly occui-s in muscles, and hence exists in large quantities in Liebig's 
 ' extract of meat.' It is often found also in pathological fluids. This 
 is the only acid of the scries which possesses any power of rotating the 
 plane of polarised light; it is otherwise indistinguishable from the 
 preceding ethylidene-lactic acid, and is generally represented by the 
 same formula. The free acid has dextro-, the anhydride loevo-rotatory 
 action. The specific rotation for tlie zinc salt in solution is -l-%b° for 
 yellow light. 
 
 The zinc and calcium salts of sarcolactic acid are more soluble both 
 in water and alcohol, than those of ethylidene-lactic acid, but less so 
 than those of ethylene-lactic acid, and the same salts of ethylene-lactic 
 acid contain more water of crystallisation than those of the other two, 
 
 Heintz- has compared the above acids to the modifications capable of existing in 
 tartaric acid^. 
 
 Hydraerylic acid, the fourth in this series of lactic acids, is distinguished by the 
 nature of its decomposition on heating. It is never found as a constituent of 
 animal bodies. 
 
 Oxalic Acid Series, 
 
 Oxalic acid. H.aO,, 
 
 In the free state this acid does not occur in the human body. Cal- 
 cic oxalate, however, is a not luifrequent constituent of urine, and 
 enters into the composition of many urinary calculi, the so-called mul- 
 berry calculus consisting almost entirely of it. It may occur in fieces, 
 and in the gall bladder, though this is rarely observed. 
 
 1 Ann. d. Chem. u. Pharm. Bd. 128, S. 6. 
 
 2 Op. cit. 
 
 3 See further, TVislicenns, op. cit. Also Ann. d. Chem. v. Pharm. Bd. 166 S 3 
 Bd. 167, S. 302, and Zeitschr. f. Chem. Bd. xiii. S. 159. 
 
742 ACIDS OF THE OXALIC SERIES. [App. 
 
 As ordinarily precipitated from solutions of calcic salts by am- 
 nionic oxalate, calcic oxalate is quite amorphous, but in urinary 
 deposits it assumes a strong characteristic crystalline form, viz. that of 
 rectangular octohedi'a. In some cases it presents the anomalous forms 
 of rounded lumps, dumb-bells, or square columns with pyramidal ends. 
 It is insoluble in water, alcohol and sether, also in ammonia and acetic 
 acid. Mineral acids dissolve tliis salt readily, as also to a smaller extent 
 do solutions of sodic phosphate or urate. All the above characteristics 
 serve to detect this salt ; its microscopical appearance, however, is gene- 
 rally of most use for this purpose. 
 
 The pure acid is prepared either by oxidising sugar with nitric acid, 
 or decomposing ligneous tissue with caustic alkalis. 
 
 Succinic acid. H2C4H4O4. 
 
 This is the third acid of the oxalic series, being separated from oxalic 
 acid by the intermediate malonic acid, H2C3H2O4. It occui'S in the 
 spleen, the tliymiis, and thyroid bodies, hydrocephalic and hydrocele 
 fluids. 
 
 According to Meissner and Shepard^ it is found as a normal constituent of urine. 
 This is contested by Salkowski^, and also by von Speyer. It seems probable how- 
 ever that since vrines and fermented liquors contain succinic acid, and this latter 
 passes unchanged into the urine, that it may thus be occasionally present in this 
 excretion. 
 
 Succinic acid crystallises in large rhombic tables, also at times in the 
 form of large prisms : they are soluble in 5 parts of cold water, and 2*2 
 of boiling, slightly soluble in alcohol, and almost insoluble in sether. 
 The crystals melt at 180° C, and boil at 235° C, being at the same time 
 decomposed into the anhydride and water. The alkali salts of this acid 
 are soluble in water, insoluble in alcohol and sether. 
 
 Preparation. Apart from the synthetic methods, it may readily be 
 obtained by the fermentation of calcic malate, acetic acid being produced 
 simultaneon sly. 
 
 Its presence is recognised by the microscopic examination of its 
 crystals, and its characteristic reaction with normal lead acetate. With 
 this it gives a precipitate, easily soluble in excess of the precipitant, but 
 coming down again on warming and shaking^. 
 
 Cholesterix. (C^elljjO.) 
 
 This is the only alcohol which occurs in the human body in the free 
 state. (The triatomic alcohol glycerin is almost always found combined 
 
 ^ Untersuch. ilber d. Entsteh. d. Hippursaure. Hannover, 1866. 
 2 Pfliiger's Archiv, Bd. 11. (1869) S. 367, and Bd. iv. (1871) S. 95. 
 ^ For further particulars see Meissner, op. cit. and Meissuer aud Solly, Zeitszlir. 
 /. rat. Med. (3) Bd. xxiv. S. 97. 
 
An-.] CIIE.]fICAL BASIS OF THE ANIMAL lUjDY. 7J3 
 
 OS ill the fats; and cetyl alcohol, or letlial, is oUtiiiiitil only from Rperma- 
 ceti.) It is a white crystalline body, crystullisiiif^ in Hue noodles from 
 its solution in rothcr, chloroform or benzol ; from its hot alcoholic solu- 
 tions it is dopusitcd on cooling in rhombic tables. When dried it melts 
 at 145", and distils in closed vessels at 300° U. It is quite insoluble in 
 water and cold alcohol ; soluble in solutions of bile salts. 
 
 Solutions of cholesterin jjossess a left-handed rotatory action on 
 polarised light, of - 32" for yellow light, this being independent of con- 
 centration and of the nature of the solvent. 
 
 Heated with sti'ong sulphuric acid it yields a hydrocarbon ; with 
 concentrated nitric it gives cholesteric acid and other products. It is 
 capable of uniting with acid-s and forming compound aethers. 
 
 Cholesterin occurs in small quantities in the blood and many tissue.", 
 and is present in abundance in the white matter of the cerebro-spiiiid 
 axis and in nerves. It is a constant constituent of bile, forming fre- 
 quently nearly the whole mass of some gall-stones. It is found in many 
 pathological fluids, hydrocele, the fluid of ovarial cysts, itc. 
 
 Preparation. From gall-stones by simple extraction with boiling 
 alcohol, and treatment with alcoholic potash to free from extraneous 
 matter. 
 
 As tests for this substance may be given : — With concentrated sul- 
 phuric acid and a little iodine a violet colour is obtained, changing 
 through green to red or blue. This is applicable to the microscopic 
 crystals. After dissolving in chloroform a blood-red solution is formed 
 on the addition of an equal volume of concentrated sulphuric acid; this 
 solution if exposed to the air in an open dish turns, blue, green and 
 finally yellow; the sulphuric acid under the chloroform has a green 
 fluorescence. After evaporation to dryness with nitric acid, the residue 
 turns red on treating with ammonia. 
 
 This body is described here rather for tbe sake of convenience than from its 
 possessing any close relationship to the substances immediately preceding. 
 
 Complex Nitrogenous Fats. 
 
 Lecithin. C^HgoNPOg. 
 
 Occurs widely spread throughout the body. Blood, bile, and serous 
 fluids contain it in small quantities, while it is a conspicuous component 
 of the brain, nerves, yolk of Qg^, semen, pus, white blood-corpuscles, and 
 the electrical organs of the ray. 
 
 When pure, it is a colourless, slightly crystalline substance, which 
 can be kneaded, but often crumbles during the process. It is readily 
 
744 N-ITKOGENOUS FATS. [App. 
 
 soluble in cold, exceedingly so in hot alcohol ; aether dissolves it freely 
 though in less quantities, as also do chlorofoi-m, fats, benzol, carbon di- 
 sulphide, (fee. It is often obtained from its alcoholic solution, by eva- 
 poration, in the form of oily drops. It swells up in water and in this 
 state yields a flocculent precipitate with sodium chloride. 
 
 Lecithin is easily decomposed : not only does this decomposition set 
 in at 70° C, but the solutions, if merely allowed to stand at the ordinary 
 temperature, acquire an acid reaction, and the substance is decomposed. 
 Acids and alkalis, of course, effect this much more rapidly. If heated 
 with baryta water it is completely decomposed, the products being 
 neurin, glycerinphosphoric acid, and baric stearate. This may be thus 
 represented : — 
 
 Lecithin. Stearic acid. •' ^ P. , ^ Neurin. 
 
 C«H9„NPO9+3H,O = 20,3H3A + ail'pb^ + aH^^NO,. 
 When treated in an eethereal solution with dilute sulphuric acid, it is 
 merely split up into neurin and distearyl-glycerinphosphoric acid. Hence 
 Diakonow ' regards lecithin as the distearyl-glycerinphosphate of neurin, 
 two atoms of hydrogen in the glycerinphosphoric acid being replaced by 
 the radicle of stearic acid. It appears also that there probably exist 
 other analogous compounds in which the radicles of oleic and palmitic 
 acids take part. 
 
 Preparation. Usually from the yolk of egg, where it occurs in union 
 with vitellin. Its isolation is complicated, and tlie reader is referred to 
 Hoppe-Seyler '. 
 
 Glycerinphosphoric acid. CjHsPOg. 
 
 Occui's as a product of the decomposition of lecithin, and hence is 
 found in those tissues and fluids in which this latter is present : in 
 leukhsemia the urine is said to contain this substance. It has not been 
 obtained in the solid form. It has been produced synthetically by 
 heating glycerin and glacial phosphoric acid ; it may be regarded as 
 formed by the union of one molecule of glycerin with one of phosphoric 
 acid, with elimination of one molecule of water. It is a dibasic acid ; 
 its salts with barium and calcium are insoluble in alcohol, soluble in cold 
 water. Solutions of its salts are precipitated by lead acetate. 
 
 Protagon. {0,,,Yi.,,,^,VO,,^) 
 
 A crystalline body, containing nitrogen and phosphorus, obtained by 
 Liebreich* from the brain substance and regarded by him as its principal 
 
 1 Hoppe-Seyler's ilfed. Chem. Untersuch, Heft. ii. (1867), S. 221, Heft. iii. (1868), 
 3. 405. Centralb. f. d. vied. Wiss. (1868), Nr. 1. 7 u. 28. 
 
 2 Med. Chem. Untersuch. Heft. ii. (1867), S. 215. 
 
 3 Ann. d. Chem. w. Pharm. Bd. 134, S. 29. 
 
API'.] CHEMICAL BASIS OF THE AXI.UAL 110 1) Y. 745 
 
 constituent Tho researclu'S of lloppo-Soylrr luid Diukonow tended to 
 .shew that protagou was merely a mixture of lecithin and cerebrin. A 
 repetition of Liebreich's experiments has however led Gamgce and Blan- 
 kenhorn' to eonfirm the truth of his I'esults. Protagon appears to separate 
 out from warm alcoliol on gradual cooling in tho form of very small 
 needles, often arranged in groups : it is slightly soluble in cold, more 
 soluble in hot alcohol, and a^'ther. It is insoluble ia water, but swells 
 up and forms a gelatinous mass. It melts at 200'' C. and forms a 
 brown sirupy fluid. 
 
 Preparation. Finely divided brain substance, freed from blood ami 
 connective tissue, is digested at 45* C. with alcohol (85 p. c.) as long as 
 the alcohol extracts anything from it. The protagon which separates 
 out from the filtrate is well washed with scther to get rid of all 
 cholesterin and other bodies soluble in aether, and finally purified by 
 repeated crystallisation from warm alcohol. 
 
 Neurin (Cholin). C5H15NO0. 
 
 Discovered by Strecker^ in pig's-gall, then in ox-gall. It does not 
 occur in the free state except as a product of the decomposition of lecithin. 
 It is a colourless fluid, of oily consistence, possesses a strong alkaline re- 
 action, and forms with acids very deliquescent salts. The salts with 
 hydrochloric acid and the chlorides of platinum and gold are the most 
 impoi'tant. 
 
 Neurin is a most unstable body, mere heating of its aqueous solution 
 sufficing to split it up into glycol, trimethylamin and ethylene oxide. 
 
 Preparation. From yolk of egg. For this see Diakonow ^ 
 
 Wmtz* has obtaiued it syuthetically, first by the action of glycol hydrochloriJe 
 on trimethylamin, and then by that of ethylene oxide and water on the same 
 substance. The above, together with the mode of its decomposition, point to the 
 idea that neurin may be regarded as trimethyl-oxyethyl-ammonium hydrate, 
 N(CH3)3(C,,H50)OH. 
 
 Cerebrin. C'l.HsjNOs (?) 
 
 Is found in the axis cylinder of nerves, in pus coi'puscles, and 
 largely in the brain. In former times many names were given to the 
 substance when in an impure state ex.gr. cerebric acid, cerebrote, &c. 
 "W. Miiller* fii-st prepared it in the pure form, and constructed the 
 above formula from his analysis; the mean of these is O, 15 85. H, 
 11-2. N, 4"5. C, 68'45. Great doubts are however thrown upon 
 
 > Zeitschr. f. physiol. Chem. Bd. in. (1879) S. 260, and Jl. of Physiol. Vol. 11. 
 (1879) p. 113. 
 
 2 Ann. d. Chem. u. Pharm. Bd. 123, S. 353, Bd. 148, S. 7G. 
 •'' Op. cit. (sub. Locitliin). 
 
 * Ann. d. Chem. u. Phnrm. Sup. Bd. 6, S. IIG u. 127. 
 » Ann. d. Chem. u. Pharm. Bd. 10.5, S. 361. 
 
746 UREA AND ITS ALLIES. [App. 
 
 its pui-ity, by the researches of later observers. According to Liebreich' 
 and Diakonow^, it is a glucoside^. 
 
 Cerebrin is a light, colourless, exceedingly hygroscopic powder, which 
 swells up strongly in water, slowly in the cold, rapidly on heating. 
 When heated. to 80" C. it turns brown, and at a somewhat higher tempe- 
 rature melts, bubbles up and finally burns away. It is insoluble in cold 
 alcohol, or sether; warm alcohol dissolves it easily. Heated with 
 dilute mineral acids, cerebrin yields a sugar-like body, possessing left- 
 handed rotation, but incapable of fermentation. 
 
 Preparation. For this see W. Miiller*. 
 
 NITROGENOUS METABOLITES. 
 
 The Urea GPtOUP, Amides, and similar Bodies. 
 
 Urea. (NH^)^ CO. 
 
 The chief constituent of normal urine in mammalia, and some other 
 animals; the urine of birds also contains a small amount. Normal 
 blood, serous fluids, lymph and the liver, all contain the same body in 
 traces. It is not found in the muscles, as a normal constituent, but 
 may make its appearance there under certain pathological conditions. 
 
 When pure it crystallises from a concentrated solution in the form 
 of long, thin glittering needles. If deposited slowly from dilute 
 solutions, the form is that of four-sided prisms with pyramidal ends; 
 these are always anhydrous. It possesses a somewhat bitter cooling 
 taste, like saltpetre. It is readily soluble in water and alcohol, the 
 solutions being neutral. In anhydrous aether it is insoluble. The 
 crystals may be heated to 120° C. without being decomposed; at a 
 higher temperature they are first liquefied and then decompose, leaving 
 no residue. Heated with strong acids or alkalis, decomposition ensues, 
 the final products being carbonic anhydride and ammonia. The same 
 decomposition may also occur as the result of the action of a specific 
 ferment on urea in an aqueous solution ^ Nitrous acid at once decom- 
 poses it into carbonic anhydride and free nitrogen. It readily forms 
 compounds with acids and bases; of these the following are of im- 
 portance. 
 
 Nitrate of urea. (NH,), CO. HNO3. 
 
 Crystallises in six-sided or rhombic tables. Insoluble in sether and 
 nitric acid, soluble in water, slightly soluble in alcohol. 
 
 1 Arch. f. pathol. Anat. Bd. 39 (1867). 
 
 2 Centralb. f. d. med. Wiss. 1868, Nr. 7. 
 
 3 See also, Geogheghan, ZeiUch.f. physiol. Chem. Bd. in. (1879) S. 332. 
 
 5 Musculus, Pfliiger's Archiv, Bd. xii. (1876) S. 214. Jaksch. Zeitsch.f. pJiysiol. 
 Chem. Bd. v. (1881) S. 395. 
 
Ait.] chemical BASIS OF THE ANI}fAL BODY. 1M 
 
 Oxalate of urea. [(NIQ.CO],. IT,C,0, + H,0. 
 
 Often crystallises in long thin prisms, but under tlie microscope is 
 obtivinetl in a form closi'ly niscnibling tho nitrate; it is slightly soluble 
 in water, less so in alcohol. 
 
 With mercuric nitrate urea yields three salts, containing respectively, 
 4, 3 and 2 equivalents of mei'curic oxide to one of urea. The first is the 
 pi'ecipitate formed in Liebig's quantitative determination of urea and 
 may be represented by the formula 2N, H^ CO . Hg (NOa)^ 3HgO. The 
 exact constitution of these salts has not yet been determined. 
 
 Preparation. Ammonic sulphate and pota.ssic cyanate are mixed to- 
 gether in aqueous solution, and the mixture is evaporated to dryness. 
 The residue when extracted with absolute alcohol yields urea. From 
 urine, either by evaporating to dryness, having previously precipitated 
 the xirine with normal and basic lead acetate in succession and removed 
 the lead by sulphuretted hydrogen, and then extracting with alcohol; or 
 concentrating only to a syrup, and then forming the nitrate of urea; 
 this is washed with pure nitric acid and decomposed with baric 
 carbonate. 
 
 Detection in Solutions. In addition to the mici'oscopic appear- 
 ance of the crystals obtained on evaporation, the nitrate and oxalate 
 should be formed and examined. Another part should give a precipitate 
 with mercuric nitrate, in the absence of sodic chloride, but not in the 
 presence of this last salt in excess. A third portion is treated with 
 nitric acid containing nitrous fumes ; if iirea is present, nitrogen and 
 carbonic anhydi-ide will be obtained. To a fourth part nitric acid in 
 excess and a little mercury are added, and the mixture is warmed. In 
 presence of urea a colourless mixture of gases (N and 00^) is given off, 
 A fifth portion is kept melted for some time, dissolved in water, and 
 cupric sulphate and caustic soda are added; a red or violet colour, due 
 to biuret^ is developed. 
 
 Quantitative determination. For this some special manual 
 must be consulted '. It will suffice here to point out that the determina- 
 tion is made either with a solution of mercuric nitrate of known strensrth 
 (Liebig) ; by decomposing the urea by means of sodic hypobromite into 
 nitrogen, carbonic anhydride and water and measuring the nitrogen 
 (Knop) [N, H^ CO + SNaBrO = SNaBr + CO, + 2H, O + NJ or by heating 
 the urea with caustic baryta in a sealed tube, the urea being determined 
 by the weight of baric carbonate formed (Bunsen). 
 
 Urea is generally considered to be an amide of carbonic acid i.e. car- 
 bamide. The amide of an acid is formed when water is removed from the 
 ammonium salt of the acid ; if the acid be dibasic and two molecules of 
 1 Neubauer and Vogel, Analyse des Hams. viii. Aufl. 1881. S. 264. 
 
nS UREA AND ITS ALLIES. [A pp. 
 
 watei' be removed, the result is often spoken of as a diamide. Thus if 
 from ammonic carbonate, (^114)2003, two molecules of water, 2H2O, be 
 removed, carbonic acid being a dibasic acid, the result is urea; thus : 
 
 (NH,)^ CO3 - 2H2O = (ISrHa)^ CO, 
 which may be written either according to the ammonia type as 
 
 ^^ I fNHa 
 
 H l]Sr„ or as COiT^-rxT 
 
 two atoms of amidogen (NH^) being substituted for two atoms of hy- 
 droxy! (HO). 
 
 This connection between carbonic acid and urea is shewn by the fact 
 that ammonic carbonate may be formed out of urea by hydration, as 
 when urea is subjected to the specific ferment mentioned above. 
 Regarded then as a diamide of carbonic acid, urea may be spoken 
 of as carbamide. But the theoretical derivation of urea from ammonic 
 carbonate by dehydration cannot be realised in practice, whereas urea 
 can readily be formed from ammonic carbamate, and Kolbe is inclined 
 to regard it, not as the diamide of carbonic acid, but as the amide of 
 carbamic acid. Ammonium carbamate, COoNoHg minus HnO, gives urea, 
 CO, N2, H4 — which, if carbamic acid be writteii as CO, OH, NHg, may 
 be written as CO, NH2, NH^, one atom of amidogen being substituted 
 for one atom of hydroxyl, and not two, as when the substance is re- 
 garded as derived from carbonic acid. Drechsel's experiments indicate 
 a ready derivation of urea from ammonic carbamate. He has obtained 
 urea by the electrolysis of a solution of this salt with rapidly alternating 
 currents thus removing the elements of water from the carbamate by 
 such alternating processes of oxidation and I'eduction as may be supposed 
 to take place in the body. The reaction is expressed as follows : 
 
 i. NH . CO . O . NH4 + O = NH. . CO . O . NH^ + H^O. 
 ii. NH2 . CO . ONHo + H, = NH2 . CO . NH2 + H.O. 
 
 Wanklyn and Gamgee^ however, since urea when heated with a 
 large excess of potassic permanganate gives off all its nitrogen in 
 a free state and not in the oxidized form of nitric acid, as do all other 
 amides, conclude that it is not an amide at all, that it is isomeric only 
 and not identical with carbamide. 
 
 It is important to remember that urea is also isomeric with ammonic 
 
 (N . 
 
 cyanate, C \ r\-KrjT ^^^ indeed was first formed artificially by Wbhlei 
 
 (1828) from this body. We thus have three isomeric compounds, am- 
 monium cyanate, urea, and carbamide, related to each other in such a way 
 1 Arch.f. Physiol. 1880, S. 550. 2 Joura. Chem. Soc. 2, Vol. vi. p. 25. 
 
An'.] CHEMICAL BASL'S OF THE AMMAL llOUY. 7t9 
 
 that uiva may bu ulitaiin'il readily either fruin ainiuoiiiuni cyanaLc or from 
 ammonic carbamate, and may with the greatest ease be converted into 
 ammonio carbonate". Now iirca is a much more stable body than 
 ammonic cyaiiato, and in the tivmsfurniation of the latter into the 
 former, energy is set free; and it is worthy of notice that though the 
 presence of sulphocyanidea in the saliva probably indicates the existence 
 of cyanic rcsiilues in the body, the nitrogenous products of the decom- 
 position of proteids belong chiefly to the class of amides, cyanogen 
 compounds being rare among them. Pfliiger^ has called attention to the 
 great molecular energy of the cyanogen compounds, and has suggested 
 that the functional metabolism of jirotoplasm by which energy is set 
 free, may be compared to the conversion of the energetic unstable 
 cyanogen compounds into the less energetic and more stable amides. In 
 other words, ammonium cyanate is a type of living, and urea of dead 
 nitrogen, and the conversion of the former into the latter i.s an image of 
 the essential change which takes place when a living proteid dies. 
 
 Compound Ureas. The liydrogen atoms of urea can be replaced by alcohol and 
 acid radicles. The results are compound m-cas or ureides when the hydrogen is 
 replaced by an acid radicle. Many of them are called acids, since the hydrogen 
 from the amide group, if not all replaced as above, can be replaced by a metal. 
 Thus the substitution of oxalyl (oxalic acid) gives parabanic acid, 
 I CO 
 K. .' Ho or CO, KH., N . C„ 0^; 
 ' I C.Oo 
 of tartronyl (tartrouic acid), dialmic acid, CO, NH^, N.C3H0O3; of mesoxalyl 
 (mesoxalic acid), alloxan, CO, NH,, N . C3O3. These bodies are interesting as being 
 also obtained by the artificial oxidation of uric acid. (See below). 
 
 Uric acid. C5H,N,03. 
 
 The chief constituent of the urine in birds and rejDtiles; it occui's 
 only sparingly in this excretion in man and most mammalia. It is 
 normally present in the spleen, and traces of it have been found in 
 the lungs, muscles of the heart, pancreas, brain and liver. Urinary 
 and renal calculi often consist largely of this body, or its salts. In gout, 
 accumulations of uric acid salts may occur in various parts of the body, 
 forming the so-called gouty concretions. 
 
 It is when pure a colourless, crystalline powder, tasteless, and with- 
 out odour. The crystalline form is very variable, but usually tends to- 
 
 1 The following literatm-e is interesting in connection with the question of the 
 cyanic or amide origin of urea. Drecbsel: Ber. d. k. s. GeseU. d. iri',«. Leipzig : 
 Sitz. 25. Juli. 1875; Arch. f. Physiol., 1880, S. 550. v. Knieriem: Zt.f. Biol., Bd. 
 X. (1874), S. 263. Munk: Zt. f. jyhijsiol. Chem., Bd. 11. (1878), S. 29. E. Salkow- 
 ski: Centralb. f. d. Med. Wiss., 1875. No. 58; Ber. d. deittsch. Chem. Gesell., 1875, 
 S. 116; Zeitxch. f. phy^iol. Chem., Bd. i. (1877), Sn. 1. u. 374; Bd. iv. (1880), Su. 
 54. u. 103. Schmiedeberg: Arch. f. exp. Pathol., Bd. viii. (1877), S. 1. 
 
 2 Pliiiger's Archil; Bd. x. (1875) S. 337. 
 
750 UREA AND ITS ALLIES. [App. 
 
 wards tliat of rhombic tables^. When impure it crystallises readily, 
 but then possesses a yellowish or brownish colour. In water it is very 
 insoluble (1 in 14,000 or 15,000 of cold water); eether and alcohol 
 do not dissolve it appreciably. On the other hand, sulphuric acid takes 
 it up without- decomposition, and it is also readily soluble in many salts 
 of the alkalis, as in the alkalis themselves. Ammonia however scarcely 
 dissolves it. 
 
 Salts of Uric acid. Of these the most important are the acid 
 urates of sodium, potassium, and ammonium. The sodium salt crystallises 
 in many different forms, these not being characteristic, since they are 
 almost the same for the corresponding compounds of the other two 
 bases. It is very insoluble in cold water (1 in 1100 or 1200), more 
 soluble in hot (1 in 125). It is the principal constituent of several 
 forms of urinary sediment, and composes a large part of many calculi ; 
 the excrement of snakes contains it largely. The potassium resembles 
 the sodium salt very closely, as also does the compound with 
 ammonium; the latter occurs generally in the sediment from alkaline 
 urine. 
 
 Preiyaration. Usually from guano, or snake's excrement. From 
 guano by boiling with caustic potash (1 part alkali to 20 of water) as 
 long as ammonia is evolved. In the filtrate a precipitate of acid urate 
 of potassium is formed by passing a current of carbonic anhydride ; 
 this salt is then washed, dissolved in a caustic potash and decomposed 
 by carefully pouring its solution into an excess of hydrochloric acid. 
 
 The presence of uric acid is recognized by the following tests. The 
 substance having been examined microscopically, a portion is evaporated 
 carefully to dryness with one or two drops of nitric acid. The residue 
 will, if uric acid is present, be of a red colour, which on the addition of 
 ammonia turns to purple. This is the murexide test, and depends 
 on the presence of alloxan and alloxantin in the residue. Schiff" has 
 given a delicate reaction for uric acid. The substance is dissolved 
 in sodic carbonate, and dropped on paper moistened with a silver 
 salt. If uric acid be present a brown stain is formed, due to the 
 reduction of the carbonate of silver. An alkaline solution of uric acid 
 can, like dextrose, reduce cupric sulphate, with precipitation of the 
 cuprous oxide. 
 
 Uric acid resists very lai-gely the action of even strong acids and 
 alkalis, exhibiting in this respect a marked difierence from urea. It 
 might therefore perhaps be supposed that urea residues do not pre-exist 
 in uric acid ; nevertheless by oxidation uric acid does give rise not only 
 
 ^ See Ultzmann and K. B. Hoffman, Atlas der Harnsedimente, Wien, 1872. 
 2 Ann. d. Cheni. u, PMrm. Bd. 109, S. 65. 
 
An«.] CHEMICAL BASIS OF THE AMMAL EOUY. 751 
 
 to ordinary urea, Imt ulsd, and nl the same time, to the compound ureas 
 (ureides) spoken of above. Tims by oxidation with acids, 
 Uric atiJ. Alloxan. Urcn. 
 
 CJI,N,03 + ir,o + = C,N,,1I A + CN,H,0. 
 
 Now alloxan, as was stated above, is a compound urea, viz. mesoxalyl- 
 
 urea, and by hydration can be converted into mesoxalic acid and urea, 
 
 thus : 
 
 Alloxan. Mcsoxalic. Urea. 
 
 C,N5H.,0, + 2HaO = Ca H, O^ + CN„H,0 ; 
 
 and by the action of chlorine uric acid can be split up directly into a 
 
 molecule of mesoxalic acid and two molecules of urea : 
 
 Uric acid. Mesoxalic acid. Urea. 
 
 C^H.NA + C1.J + 4HaO = C3 Hj O5 + 2CN,H,0C + 2HC1. 
 
 By oxidation with alkalis, uric acid is converted into allantoin and 
 
 carbonic acid, 
 
 Uric acid. Allautoin. 
 
 C^H.NA + H,0 + = C,H«N,03 + CO, ; 
 and allantoin, by hydration, becomes allauturic or lantanuric acid and 
 
 urea, 
 
 Allantoin. Urea. AUanturic acid. 
 
 C.HeN.Oj + H,0 = CH,N„0 + CjH.NoO. 
 
 Kow allanturic acid is a compound urea, with a residue of glyoxylic 
 acid. By other oxidations of uric acid, parabanic acid (oxalyl-urea), 
 oxaluric acid (which is hydrated parabanic acid), and dialuric acid 
 (tartronyl-urea) are obtained. In fact all these decompositions of a 
 molecule of uric acid lead to the production of urea and of a carbon acid 
 of some kind or other. The relation of uric acid to urea as illustrated 
 by the above reactions is brought very prominently into view by the 
 synthesis of uric acid which has recently been performed'. It is 
 obtained by simply fusing together glycocine (amido-acetic acid,) and 
 urea at a temperature of 200° — 230" C. The converse formation of gly- 
 cocine from uric acid with the simultaneous production of ammonia and 
 carbonic anhydride has been known for some time. Since in this latter 
 reaction the ammonia and carbonic anhydride are in the proportions in 
 which they would be obtained from cyanic or cyanuric acid, uric acid 
 has been regarded as built up from residues of cyanuric acid and glycin, 
 just as hi])puric acid is foinied from glycin and benzoic acid. It was 
 also at one time supposed that uric acid might be regarded as tartronyl 
 cyanamide. 
 
 r C3H„03 
 N, (CN), 
 
 1 Horbaczewski, Ber. d. deutsch. chem. Gesell. Jahrg. 1882, S. 2678. 
 
752 UREA AND ITS ALLIES, [App. 
 
 If the existence of some cyanogen residue is thus assumed in the 
 molecule of uric acid, then it must be supposed that before urea can be 
 obtained from it a molecular change takes place by which a portion at 
 least of the nitrogen of the uric acid is converted into the same condition 
 as the rest of the nitrogen, viz., into the amide state. 
 
 If this be so, since the metabolism of the animals in which uric acid 
 replaces urea cannot be supposed to be fundamentally different from 
 that of the urea-producing animals, we may infer that the antecedent 
 of both uric acid and urea in the regressive metabolism of proteids 
 is, as we suo-gested above, a body containing some at least of its nitrogen 
 in the form of cyanogen \ 
 
 Kreatin. C^ H, N, 0,. 
 
 Occurs as a constant constituent of the juices of muscles, though 
 possibly it may be formed during the process of extraction by the 
 hydration of kreatinin. Kreatin is not a normal constituent of urine, 
 but it is said to occur in traces in several fluids of the body. When 
 found in urine its presence is probably due to the conversion of 
 kreatinin, a constant constituent of urine, into kreatin during its 
 extraction, since Dessaignes" has shewn that the more rapidly the 
 separation is effected, the less is the quantity of kreatin obtained, and 
 the greater the amount of kreatinin. 
 
 In the anhydrous form it is white and opaque, but crystallises with 
 one molecule of water in colourless transparent rhombic prisms. It 
 possesses a somewhat bitter taste, is soluble in cold, extremely soluble in 
 hot water, is less soluble in absolute than in dilute alcohol, and is soluble 
 in sether. 
 
 It is a very weak base, scarcely neutralising the weakest acids. It 
 forms crystalline compounds with sulphuric, hydrochloric and nitric 
 acids. 
 
 FrejMiration. From extract of muscle by precipitating completely 
 with basic lead acetate, and crystallising out the kreatin, mixed with 
 kreatinin. From this latter it is separated by the formation of the 
 zinc-salt of kreatinin, kreatin not i-eadily yielding a similar compound. 
 
 Kreatin may be converted into kreatinin under the influence of acids, the trans- 
 foi-mation being one of simple dehydration. 
 
 Kreatin may be decomposed into sarcosin (methyl-glycin) and urea : 
 
 C4H9N3O2 + H,0 = CsH^KO, + CH,>^,0 ; 
 
 1 See V. Knieriem, Zeitsch. f. Biol. Bd. xm. (1877), S. 36. Schroder, Zeitsch. f. 
 Physiol. Chem. Bd. ii. (1878), S. 228. 
 
 2 J. Pharm. (o) Bd. xxxii. S. 41. 
 
Ai'V.J CHEMICAL BASIS UF THE AMMAL BUDY. 753 
 
 it mnybe foniu'il syntliotically' by the action of sarcosin and cyanamidc: 
 
 cu r.NO, (- cir.N, - cji^NjO. 
 
 Sarcosin is i,'lyciM in wlikli one uIdiii of liydrogcu has been replaced by 
 tlie alcobol radicle methyl, thus; 
 
 Glycm -,/ ^ bccoini's t^r c^> 
 
 like glycin, .^aroosiu has not been found in a free state in the body. 
 
 Kreatinin. C.HyN,©. 
 
 This, which is simply a dehydrated form of krcatiu, occurs nonnally 
 «s a constant constituent of urine and of muscle extract. It crystallises 
 in colourless .shining prism.s, possessing a strong alkaline taste and 
 reaction. It is readily soluble in cold water (1 in 11 "5), also in alcohol, 
 but is scarcely soluble in ajtlier. It acts as a powerful base, forming 
 with acids and salts compounds wMch crystallise well. Of these the 
 most important is the salt with zinc chloride (C4H7N30).jZn CI2. It is 
 formed when a concentrated solution of the chloride is added to a not 
 too dilute solution of kreatinin. Since the compound is very little 
 soluble in alcohol, it is better to use alcoholic rather than aqueous 
 solutions. It crystallises in warty lumps composed of aggregated 
 masses of prisms, or fine needles. 
 
 Preparation. Either by the action of acids on kreatin, or from 
 human urine by concenti'ating, and precipitating with lead acetate; in 
 the filtrate from this, a second precipitate is caused by the addition of 
 mercuric chloride, and consists of a compound of this salt with kreatinin. 
 The mercury is removed by sulphuretted hydrogen, and the kreatinin 
 purified by the formation of the zinc salt, and washing with alcohol. 
 
 Kreatinin-zinc chloride may be converted into kreatiu, by tlae action of hydrateil 
 oxide of lead on its boiling aqueous solution. 
 
 Allantoin. C^H^N, 0,. 
 
 The cliaracteristic constituent of the allantoic fluid of the fcetus; it 
 occurs also in the urine of animals for a short period after their birth. 
 Traces of it are sometimes detected in this excretion at a later date. 
 
 It crystallises in small, shining, colourless prisms, which are taste- 
 less and odourless. They are soluble in 1 60 parts of cold, more soluble 
 in hot water, insoluble in cold alcohol and sether, soluble in hot alcohol. 
 Carbonates of the alkalis dissolve them, and compounds may be formed 
 of allantoin with metals bub not with acids. 
 
 Allantoin, as already stated, p. 750, is one of the products of the 
 oxidation of uric acid, and by further oxidation gives rise to urea. 
 1 Sit-ungsber. d. hiyerscJi. Akad, IPG?, Hft. 3, S. 172. 
 F. 48 
 
^rm UREA AND ITS ALLIES. [Arp. 
 
 Preparation. This is best cai-ried out by the careful oxidation of uric 
 acid either by means of potassic permanganate or ferrocyanide, or by 
 plumbic oxide. 
 
 Hypoxanthin or Sarkin. CgH^N^O. 
 
 Is a normal constituent of muscles, occurring also in the spleen, 
 liver and medulla of bones. In leukhsemia it appears in the blood and 
 urine. It crystallises in fine needles which are soluble in 300 parts of 
 cold more soluble in hot "vvater, insoluble in alcohol, soluble in acids and 
 alkalis. It forms crystalline compounds witb acids and bases. It is 
 precipitated by basic acetate of lead, the precipitate being soluble in a 
 solution of the normal acetate. Its preparation from muscle-extract 
 depends on its precipitation first by basic acetate of lead, and then by an 
 ammoniacal solution of silver nitrate after the removal of kreatin. 
 
 Both hypoxanthin and the next body, xanthin, can also be obtained from 
 proteids by the action of putrefactive changes, of water at boihng temperature, of 
 dilute hydrochloric acid (-2 p. c.) at 40" C, and by the action of gastric and pancreatic 
 ferments 1. Chittenden has noticed a pecuHar difference between fibrin and egg- 
 albumin when submitted to the above processes; he finds that the latter does not 
 yield hypoxanthin when treated with boiUng water, with dilute hydrochloric acid, or 
 gastric ferment, while the former does. Egg-albumin on the other hand yields 
 hypoxanthin by the action of pancreatic ferment in alkahue solution but not so 
 readily as fibrin does. 
 
 Xanthin. C.U^^^O.,. 
 
 Pirst discovered in a urinary calculus, and called xanthic oxide. 
 More recently it has been found as a normal, though scanty, constituent 
 of urine, muscles, and several organs, such as the liver, spleen, 
 thymus, &c. 
 
 When precipitated by cooling from its hot, saturated, aqueous 
 solution it falls in white flocks, but if the solution be allowed to 
 evaporate slowly it is obtained in small scales. When pure it is a 
 colourless powder, very insoluble in water, requiring 1500 times its 
 bulk for solution at 100" 0. Insoluble in alcohol and eether, it readily 
 dissolves in dilute acids and alkalis, forming crystallisable compounds. 
 
 Hypoxanthin by oxidation becomes xanthin. Both these bodies, as 
 well as the following, guanin and carnin, are evidently closely allied to 
 uric acid; indeed, uric acid by the action of sodium-amalgam may be 
 converted into a mixture of xanthin and hypoxanthin. 
 
 Preparation. It is obtained from urine and the aqueous extract of 
 muscle by a pi'ocess similar to that for hypoxanthin, and is then 
 
 1 Salomon, Zeitschr. f. plnjsiol. Chem. Bd. ii. (1878-1879), S. 60. Krause, Inaug. 
 Diss., Berlin, 1878. Chittenden, Journ. of Physiol. Vol. n. (1879), p. 28. See also 
 Drechsel, Ber. d. deutsch. Chem. Gesell. Jahrg. xiii. (1880), S. 240. Salomon, 
 Ibid. S. 1160. Kossel, Zeitsch.f. lohysiol. Chem. Bd. v. (1881), Sn. 152 u. 267. 
 
Ait] chemical BASIS OF Till- AXIMAL BODY. 7oo 
 
 separated from the latter by the action of dilute hydrochloric acid; this 
 separation depends on the different solubilities of the hydrochlorides of 
 the two bodies. For further information see Neubauer and VogeP. 
 
 Carnin. C, HgN^Oa. 
 
 Discovered by Weidel" in extract of meat, of which it constitutes 
 about one per cent. 
 
 It crystallises in white masses composed of very small irregular 
 crystals; it is soluble with difficulty in cold, more easily soluble in hot 
 water, insoluble in alcohol and aether. Its aqueous solution is not 
 precipitated by normal lead acetate, but is by the basic acetate of this 
 metal. It unites with acids and salts forming crystalline compounds. 
 
 Preparation. Is found in the precipitate caused in extract of meat 
 by basic acetate of leadl 
 
 This body possesses an interesting relation to hypoxanthin, into which it maybe 
 converted by the action either of nitric acid or, stUl better, of bromine. 
 
 Guanin. C^H^jS'sO. 
 
 First obtained from guano, but recently observ^ed as occurring in 
 small quantities in the pancreas, liver and muscle extract. 
 
 It is a white amorphous powder, insoluble in water, alcohol, aether 
 and ammonia. It unites with acids, alkalis and salts to form crystallis- 
 able compounds. 
 
 Preparation. From guano by boiling successively with milk of 
 lime and caustic soda, precipitating with acetic acid, and purifving by 
 solution in hydrochloric acid and precipitation by ammonia, 
 
 Guanin may, by the action of nitrous acid, be converted into xanthin. 
 By oxidation it can be made to yield principally guanidine and parabanic 
 acid, accompanied however by small quantities of urea, xanthin and 
 oxalic acicL Capranica has given several reactions characteristic of 
 this body*. 
 
 Its separation from hypoxanthin and xanthin depends on its in- 
 solubility in water and behaviour with hydrochloric acid. 
 
 Kynurenic acid. C,, H„ K O, + 2Ho O. 
 
 Found in the urine of dogs, and first described by Liebig". When 
 pure it crystallises in brilliant white needles, insoluble in cold, soluble 
 in hot alcohol. The only salt of this body which crystallises well is 
 
 1 Ham- Analyse, Ed. viii. (1881), S. 26. Also the literature quoted above on 
 hypoxanthin. 
 
 - Ann. d. Chem. v. Pharm. Bd. 158, S. 365. 
 
 3 See Weidel, op. cit. * Zeitsch.f. phtjsiol. Chem. Bd. iv. (1880), S. 240 
 
 « Ann. d. Chem. ti. Pharm. Bd. 86, S. 125, and Bd. 108, S. 354. 
 
 48 2 
 
756 UREA AND ITS ALLIES. [App. 
 
 that formed with barium. For preparation and other particulars see 
 Liebig^ and Schultzen and Schmiedeberg^ 
 
 Glycin. C2 Hg (NH^) (OH). Also called Glycocoll and Gljcocine. 
 
 Does not occur in a free state in the human body, but enters into 
 the composition of many important substances, ex. gr. hippuric and bile 
 acids. It ciystallises in large, colourless, hard rhombohedra, which are 
 easily soluble in water, insoluble in cold, slightly soluble in hot alcohol, 
 insoluble in sether. It possesses an acid reaction, but a sweet taste. It 
 has also the property of uniting with both acids and bases, to form 
 crystallisable compounds. In this it exhibits its amide nature, and that 
 it is an amide is i-endered evident from the methods of its synthetic 
 preparation; thus mono-chlor-acetic acid and ammonia give glycin and ' 
 ammonic chloride :— C.HaCl O, + 2NH3 = C,H„{NH,)0(OH) + NH^Cl. It 
 is amido-acetic acid. Heated with caustic baryta it yields ammonia 
 and methylamine. 
 
 Preparation. From glutin by the action of acids or alkalis; from 
 hippuric acid by decomposing it with hydrochloric acid at a boiling 
 ' temperature and removing by precipitation the simultaneously formed 
 benzoic acid, 
 
 Taurin. C.H.NO^S. 
 
 In addition to entering into the comjiosition of taurocholic acid (see 
 p. 763) taurin is found in traces in the juices of muscle and in the lungs. 
 
 It crystallises in colourless, regular, six-sided prisms; these are 
 readily soluble in water, less so in alcohol. The solutions are neutral. 
 It is a very stable compound, resisting temperatures of less than 240° 0; 
 it is not acted on by dilute alkalis and acids, even when boiled with 
 them. It is not precipitated by metallic salts. 
 
 Taurin is amido-isethionic acid; and may be synthetically prepared 
 from isethionic (ethyl-sulphuric) acid by the action of ammonia; thus: 
 
 ^^^^1 SO, + NH3 --= ^^1 j SO3 + H,0. 
 
 Pre]}aration. As a product of the decomposition of bile, and is 
 purified by removing any traces of bile acids by means of lead acetate, 
 and then successively crystallising from water. 
 
 Leucin. OgHisNOa. 
 
 Is one of the principal products of the decomposition of nitrogenous 
 matter, either under the influence of putrefaction or of strong acids and 
 alkalis. It occurs however normally in the pancreas, spleen, thymus, 
 
 1 Op. cit. 2 ^nn. d. Cherr.. u. Pharm. Bd. 164, S. 155. 
 
App.] chemical basis of the AXIMAL 1U>IjY. T')? 
 
 thyroid, salivary gliuuls, liver, etc., and is one of tli<; products of the 
 tryptic (pancreatic) digestion of proteids; in acuto atrophy of the liver 
 it is present in the urine in largo quantity, in company with tyrosin. 
 
 As usually obtained in an impure form it crystallises in rounded 
 lumps which are often collected together and sometimes exhibit radiating 
 striation. When puic, it forms very thin, white, glittering flat crystals. 
 These are easily soluble in hot water, less so in cold water and alcohol, 
 insoluble in a'ther. They feel oily to the touch, and are without smell 
 and taste. Acids and alkalis dissolve them readily, and crystallisable 
 compounds are formed. 
 
 Carefully heated to 170" C. it sublimes, btit at a liighcr temperature is decom- 
 posed, yielding amylamiu, carbonic aubydrido and ammonia. In the presence of 
 jjutrefying animal matter it splits up into valeric acid and ammonia. 
 
 Leucin is amido-caproic acid,, and may be represented thus : 
 
 C„H„0-, 
 NHJ *^- 
 
 Preparation. From horn shavings by boiling with sulphuric acid, 
 neutralising with baryta and separating from tyrosin by successive crys- 
 tallisation. See also Kiihne^, w'ho prepares it by tlie action of pancreatic 
 ferment (trypsin) on i)roteids. 
 
 Scherer has given the following test for leucin. The suspected sub- 
 stance is evaporated carefully to dryness with nitric acid; the residue, 
 if it is leucin, will be almost transparent and turn yellow or brown on 
 the addition of caustic soda. If this be again very carefully concen- 
 trated with the alkali an oily drop is obtained, which is quite character- 
 istic of this substance. Leucin if not too impui'e, may be easily recog- 
 nized by its subliming on being heated ; a characteristic odour of 
 amylamin is at the same time evolved. 
 
 Asparagine. C^HglSroOa. 
 
 Is not found as a constituent of the animal body but appears to be 
 formed by the decomposition of proteids, notably during the germinative 
 changes of the proteids in leguminous seeds -. It is a crystalline body, 
 and when boiled with acids or alkalis is readily converted into aspartic 
 acid. 
 
 Aspartic {or asparaginic) acid. C^H-XO^. 
 
 This acid has been obtained in small quantities among the products 
 of the pancreatic digestion of fibrin^ and vegetable glutin*, although not 
 
 1 Virchow's ArcMv, Bd. 39, S. 130. 
 - Landwirthsch. Versuchs Statiojien. Bd. xviir. 1. 
 
 3 Eadziejewski u. Salkowski, Ber. d. deutsch. chem. G<:$eJl. Jabrg. vii. (1874), 
 S. 1030. 
 
 * V. Knieriem, Zeitsch.f. Bio'.. Bu. xi. (1875), S. 108. 
 
758 UREA AND ITS ALLIES, [App. 
 
 occurring as a constituent of any animal tissue or secietion. It is on the 
 other hand found normally in plants, notably in beet-sugar molasses. It 
 arises also as a constant product of the action of alkalis and other reagents 
 on both vegetable and animal proteids, and of acids on gelatine^. It thus 
 possesses considerable interest in respect of its relation to the proteids 
 (see p. 721). It crystallises in rhombic prisms which are but sparingly 
 soluble in cold water or alcohol, readily soluble in boiling water. Its 
 acid solutions are dextrorotatory, its alkaline laevorotatory and reduce 
 Fehling's fluid. It forms a characteristic readily crystallisable compound 
 with copper. Nitrous acid converts it into malic acid. 
 
 Glutaminic acid. CgHgNO^. 
 
 The circumstances and conditions under which this body occurs are 
 in general the same as for the aspartic acid, and hence as a product of 
 proteid decomposition it acquires some importance. It has not however 
 as yet been obtained by the action of pancreatic ferments on proteids 
 and in this it differs from the preceding body. 
 
 It crystallises in rhombic tetrahedra or octahedra ; is not very soluble 
 in cold, but readily soluble in hot water ; insoluble in alcohol and 
 aether. Its acid solutions possess a strong dextrorotatory power and it 
 reduces Fehling's fluid. 
 
 Cystin. CsH^NSO^. 
 
 Is the chief constituent of a rarely occurring urinary calculus in men 
 and dogs. It may also occur in renal concretions, and in gravel, and is 
 occasionally found in urina 
 
 From calculi it is obtained, by extraction with ammonia, as coloui"- 
 less six-sided tables or rhombohedra, which are neutral and tasteless. 
 It is insoluble in water, alcohol and sether, soluble in ammonia and the 
 other alkalis, and also in mineral acids. The fact that this body is one 
 of the few crystalline substances, occurring physiologically, which 
 contain sulphur, renders its detection very easy. Apart from its 
 insolubility in water, &c., it yields with caustic potash and salts of 
 either silver or lead, a brown colouration due to the presence of the 
 sulphides of these metals. 
 
 According to Dewar and Gamgee^ cystin is amido-sulpho-pyruvic acid, and its 
 formula is C3H5NSO2 — pyruvic being lactic acid minus two atoms of hydrogen. 
 
 1 HorbaczewsM, Sitsh. d. k. Akad. d. Wiss. Wien, 18S0. 2 Abth. Juni-Heft. 
 
 2 Journ. of Anat. and Physiol., Nov. 1370, p. 143. 
 
App.] chemical JiASlS OF THE AX /.UAL L< )!))'. 750 
 
 Tin: AltoMVTIC SKRIES. 
 
 Benzoic acid. UC-. H.O,. 
 
 This is not found as a normal constituent of the body, but owes its 
 presence in urine to the fermentative decomposition of hippuric acid, 
 whereby glycin and benzoic acid are formed : 
 
 Hippuric acid. Glycin. Benzoic acid. 
 
 C, H, (CV Hj O) NOa + Ho - C, H^ KO, + CV IT„ 0,. 
 
 Tlie sublimed acid is generally crystallised in fine needles, which are 
 light and glistening; any odour they possess is not due to the acid, but 
 to an essential oil, with which they are mixed. When precipitated from 
 solution, the crystalline form is always indistinct. This acid is soluble 
 in 200 parts cold, or 25 parts, of boiling water, but is easily soluble 
 in alcohol or Kther. It sublimes readily at 145" C. ; it also passes off 
 in the vapoui-s arising from its heated solutions. 
 
 Preparation. Either as above from hippuric acid by fermentation, 
 by boiling the hippuric acid with acids or alkalis or by sublimation from 
 gum-benzoin. 
 
 Tyrosin. CjHjiiSrOs. 
 
 Generally accompanies leucin, and is perhaps found normally in small 
 quantities in the pancreas and spleen. It is also usually obtained in 
 large quantities by the decomposition of proteid matter, either by 
 putrefaction or the action of acids. 
 
 The researches of Eadziejewsky^ render it probable that tji'osin does not occur 
 normally in any part of the human organism, except as a product of pancreatic 
 digestion. 
 
 All attempts to syuthetise tyrosin were for some time fruitless, 
 although evidence was obtained sufficient to indicate the probable 
 existence in its molecule of some aromatic (phenyl) radicle^ More 
 recently the synthesis has been performed^ and we now have every 
 reason for regarding tyrosin as para-hydroxy-phenyl-a alanine. This 
 synthesis as well as that of uric acid, referred to above, is of considerable 
 imjjortance, since the more definite the knowledge which is possessed 
 of the true molecular structure of the products of proteid decomposition 
 the more reason is there for expecting that the synthesis of a proteid 
 itself may be realisable in the not very remote future. 
 
 1 Archiv. f. path. Anat. Bd. 36, S. 1. Zeitsch. f. anal. Chem. Bd. 5, S. 4G6. 
 
 2 Bartb., Chem. Centralb. 1805. S. 1029. 1869. S. 761. 1872. S. 830. Hiifner, 
 Ibid. 1.869. S. 159. Beilstein u. Kuhlberg, Ibid. 1872, S. 830. 
 
 3 Erlenmeyer u. Lipp., Ber. d. deutsch. Chem. Gesell. Jahrg. xt. (1882\ S. 1544. 
 
760 THE AROMATIC SERIES. [App. 
 
 Tyrosin crystallises in exceedingly fine needles wliich are usually 
 collected into feathery masses. The crystals are snow-white, tasteless and 
 odourless, almost insoluble in cold water, readily soluble in hot water, 
 acids and alkalis, insoluble in alcohol and sether. If crystallised from 
 an alkaline solution tyrosin often assumes the form of rosettes composed 
 of fine needles arranged radiately. 
 
 Tyrosin does not sublime by heating, but is decomposed with an 
 odour of phenol and nitrobenzol. On boiling with Millon's reagent 
 it gives a reaction almost identical with, but much more marked than, 
 that for proteids (Hofiman's test). If tyrosin is treated on a watch- 
 glass with 6ne or two drops of strong sulphuric acid, then diluted with 
 a little water, neutralised with calcic carbonate and the solution filtered, 
 a characteristic violet colour is obtained on the addition of a drop of 
 acid- free ferric chloride (Piria's test). 
 
 Preparation. By means similar to those employed for leucin, the 
 separation of the two depending on their widely differing solubilities. 
 According to Kuhne's method^ large quantities are easily obtained as 
 the result of pancreatic digestion. 
 
 Hippuricacid. C9H9NO3. Or Benzoyl-glycin. C^^^{CjIi.^O)EO^. 
 
 Is found in considerable quantities in the urine of herbivora, and 
 also, though to a much smaller amount, in the urine of man. It is 
 formed in the body by the union with dehydration of glycin and benzoic 
 acid, see p. 441. 
 
 Crystallised from a saturated aqueous solution, it assumes the form 
 of fine needles ; if from a more dilute solution, white, semitransparent 
 four-sided prisms are obtained. These when pure are odourless, with a 
 somewhat bitter taste. They are soluble in 600 parts of cold water, 
 readily soluble in boiling water, readily soluble in alcohol, less so in 
 sether. All the solutions redden litmus. 
 
 Hippuric acid is monobasic, and forms salts which are readily 
 soluble in water (except the iron salts) ; from these, if.in sufficiently 
 concentrated solutions, excess of hydrochloric acid precipitates the acid 
 in fine needles. When heated with concentrated mineral acids it is 
 resolved into benzoic acid and glycin. The same decomposition occurs 
 in presence of putrefying bodies. Strong nitric acid produces an odour 
 of nitrobenzol. 
 
 Preparation. Fresh urine of horses or cows is treated with milk of 
 lime, in order to form calcic hippurate and thus prevent the decom- 
 position of the hippuric acid, filtered, and the filtrate evaporated to a 
 
 ^ Op. cit, (sub Leucin). 
 
aph.j chemical basis of Tin: ammai nouv. roi 
 
 .suuiU bulk; the liii>i>iiiic acid is then i)reci|>itttted by adding un excess of 
 liydrochlovic acid; tlie acid is then i)Uiilii'd liy .several crystallisations 
 tVoni boiling water. 
 
 AVheu heated in a small tube, hij^puric acid gives a sublimate of 
 benzoic acid and amnionic beuzoate, accompanied by an odour like that 
 f)f new liay, while oily, red drojis are observed in the tube. Tin's is very 
 characteristic, and distinguishes it from benzoic acid. 
 
 Phenylic {CarhoUc) acid or Phenol. C„ Hg O. 
 
 This body is undoubtedly obtained as the result of the jjutrefactive 
 decomposition of proteids, notably in putrefactive pancreatic digestions ^ 
 It may be obtained from the distillate of such digestive mixtures. It 
 is also found in the contents of the alimentary canal under the same 
 conditions which give rise to indol. "When so occurring a portion of it 
 may be obtained from the fieces while the rest reappears in the urine ^ 
 
 Buligmskj' says the urine of many animals, of cows and horses alwaj-s, contains 
 a substance insoluble in alcohol, and not precipitated by lead acetate and ammonia, 
 which by the action of dilute mineral acids gives carbolic acid. The same acid 
 applied to the body externally or internally also passes into the iirinc^. Similarly 
 benzol (CgHg) when taken into the stomach appears as carbolic acid in the luine ^. 
 
 The pure acid crystallises in long, colourless prismatic needles ; they 
 melt at 35° C, and boil at 180° C. It is readily soluble in alcohol and 
 aether, slightly soluble in water (1 part in 20). In most cases it acta as 
 a weak acid, forming crystalline salts with the alkalis. With nitric acid 
 it yields picric acid. Its solutions reduce silver and mercury salts. 
 
 Prejyaration. By the dry distillation of salicylic acid, also from the 
 acid products of the distillation of coal. It is obtained in the last 
 portions of the distillate when prepai'ing indol, and is separated by 
 forming a compound with bromine C^ITsBraO. 
 
 The Bile Series. 
 
 Cholalic {or choUc) acid. H . C,,^ H-g O^ + H^ 0. 
 
 Occurs in traces in the small intestine, in larger quantities in the 
 contents of the large intestine, and the faeces of men, cows and dogs. 
 
 1 Baumann, Zeitsch. f. physioL Chem. Bd. i. (1877), S. GO. 
 
 - Salkowski, Ber. d. deutsch. Chem. GescU. ix. (1876), S. 1595. Centralb. f. d. 
 vied. IViss. 1876, S. 818. Bcr. d. deutsch. Chem. Gesell. x. (1877), S. 842. Virchow's 
 Arch. Bd. lxxii. (1878), S. 409. See also Centralb. f. d. vied. Wiss. 1878, Nos. 30, 
 31, 34, 42, and Zeitsch. f. phijsiol. Chem. Bd. ii. (1878), S. 241. 
 
 3 Hoppe-Seyler, Med. chem. Uvtersuch. Heft 2 (1867), S. 234. 
 
 * Almen, Neiies Jahrb. d. PMrm. Bd. 34, S. 111. Salkowski, Pfliiger's Archiv, 
 Bd. V. (1871-7-2), S. 335. 
 
 5 Schultzen and Naunvn, Eeicbeit u. Du-Bois Eej-mond's Archir, 1867, Heft 3. 
 S. 349. 
 
762 THE BILE SERIES. [App. 
 
 In icterus, the urine often contains traces of this acid. But its principal 
 interest lies in its being the starting point for the various bile acids 
 (see below). The pure acid may be amorphous, or crystalline, in 
 the latter case crystallising from hot alcoholic solutions in tetrahedra. 
 These crystals are insoluble in water and sether. In the amorphous 
 form, it is somewhat soluble in water and isther. Heated to 200'' C, it 
 is converted into water and dyslysin (Co^H^gO^). 
 
 This acid possesses, in the anhydrous condition, a specific rotatory 
 power of + 50° for the yellow light : when it crystallises with HgO, the 
 l-otation is + 35°. The rotatory power of the alkali salts is always less 
 than the above, and when in solution in alcohol, the rotation is 
 independent of the concentration. For the alcoholic solution of the 
 sodium salt, the rotation is + 31-4°. 
 
 Preparation. By the decompositions of bile acids by means of acids, 
 alkalis, or fermentative changes. 
 
 Bayer^ has examined the bile-acids obtained from human bile, and has pre- 
 pared from them cholalic acid. To this he assigns the formula CjgH^gO^. If 
 this be so, then cholalic acid of human bile would seem to be a body entirely 
 different from that obtained from ox bile, and analysed by Strecker. Bayer's 
 results however requn-e further confirmation. 
 
 Pettenlcofer's test ~. 
 
 This well-known test for bile acids depends on the reaction of 
 cholalic acid in presence of sugar and sulphuric acid. If to a solution of 
 the acid a little sugar be added, and then sulpluiric acid, keeping the 
 temperature below but not much below 70° C, a beautiful reddish purple 
 is obtained. If diluted with alcohol this solution gives a characteristic 
 spectrum with two absorption bands, one betw^een D and E, nearest to E, 
 the other close to F on the red side of F. 
 
 The reaction is much impeded by the presence of colouring matters; 
 moreover proteids, and other bodies easily decomposed by sulphuric 
 acid such as amyl-alcohol and oleic acid, give a similar result, the 
 colouring matter jDroduced from these bodies does not however give the 
 absoiption bands described above ^ 
 
 Glycocholic acid. C.6ll43lSrOe. 
 
 This body was first obtained in the crystalline form and desci-ibed by 
 Gmelin (1826), who gave it the name of 'cholic' acid. 
 
 1 Zeitschr. f. physiol. CJiem. Bd. ii. (1878-79), S. 358. 
 
 2 Pettenkofer, Annalen. d. Chem. u. Pharm. Bd. lii. (1844), S. 90. 
 
 2 For further information on this subject see: Bischoff, Zeitsch.f. rat. Med. Ser. 
 3, Bd. 21, S. 126. Schulze, Ann. d. Chem. u. Pharm. lsxi. (1849), S. 266. Schenk, 
 Anatom. physiol. Untersuch. Wlen, 1872, S. 47. Adamkiewicz, Pfliiger's Arch. Bd. 
 IX, (1874), S. 156. 
 
App.] chemical basis of the animal r.ODY. 7C3 
 
 To avoid confusion it is now best to use the term 'cholic' as a synonym for 
 'cholalic,' Dcnmrcay who lirst (1838) described the cholalic acid as a product of the 
 decomposition of bile acids having given it the name of cholic acid. Tlio name 
 cholalic is i)erhaps the best, since it indicates the method by which the bile acids are 
 split up, viz. by treatment with alkali. 
 
 This is the principal bile-acid of ox gall; it is also present in the 
 bile of man, but Las so far not been observed in that of cainivora. In 
 icterus, the mine may contain traces of this acid. 
 
 It crystallises in fine, glistening needles. These are slightly soluble 
 ill cold water, readily so in hot water and alcohol but insoluble in a^thei*. 
 They possess a bitter and yet sweet taste, and a strong acid reaction. 
 
 The salts of this acid are readily soluble in water and crystallise 
 well. The salts, as well as the free acid, exert right-handed polarisation 
 amounting to + 29"0" for the acid, and + 25-7* for the sodium salt, both 
 measured for yellow light. 
 
 Glycocholic acid is a compound of glycin and cholalic acid ; thus : 
 Cholalic acid. Glvcin. Glvcocholic acid. 
 
 Prolonged boUing with dilute mineral acids or caustic alkalis decomposes gly- 
 cocholic acid into glycin and cholalic acid ; if dissolved in concentrated sulphuric 
 acid and then warmed, glvcocholic acid by the removal of one molecule of water 
 yields cholouic acid, CogH^iXOg. The barium salt of this last acid is insoluble in 
 water, which fact is of importance, since cholonic acid possesses nearly the same 
 specific rotatory power as glycocholic acid. 
 
 rreparatlon. From ox gall, by evaporating to a syrup, decolorising 
 with animal charcoal, extracting with strong alcohol, and precipitating 
 by a large excess of aether. Its separation from taurocholic acid depends 
 on its precipitation by normal lead acetate, taurocholic acid not being 
 precipitated by this reagent. 
 
 Taurocholic acid. C^.^ H^-j NSO-. 
 
 Occurs also in ox-gall, but is found especially plentiful in human 
 bile and that of carnivora, notably of the dog. 
 
 It crystallises with difficulty in veiy fine needles which are exceed- 
 ingly deliquescent. "When dried it is an anioridious powder, with 
 pure bitter taste, easily soluble in water and alcohol, insoluble in sether. 
 All its salts are soluble in water, and are precipitated by basic leatl 
 acetate in the presence of free ammonia. The sodium salt dissolved 
 in alcohol has a specific rotatory jjower of -f- 24-5''; if dissiJved in water 
 this rotation is less, and in this respect it resembles glycocholic acid. 
 
 This acid is far more unstable than the preceding one, being decom- 
 posed if boiled with water. The products of decomposition are tauriu 
 and cholalic acid. 
 
764 BILE PIGMENTS. [App. 
 
 Taurocholic acid is a compound of tauriii and cholalic acid; thus: 
 Cholalic acid. Taurin. Taurocholic acid. 
 
 Preparation. From the bile of dogs by a process similar to that for 
 glycocholic acid. It is separated from traces of this latter and from 
 cholalic acid by precipitation with basic lead acetate and ammonia ^. 
 
 Bile Pigments. 
 These have been very briefly described on p. 248. 
 
 Bilirubin. CigHigNjOs. 
 
 It is found chiefly in the fresh bile of inan and carnivora, to which it 
 gives the characteristic dark golden-red coloui\ It frequently constitutes 
 a considerable part of some kinds of gall-stones, not however as free 
 bilirubin but as a compound with earthy matter, chiefly chalk : the gall- 
 stones of oxen and pigs often contain 40 p. c. of this compound ^ These 
 are therefore the best material from which to prepare bilirubin. 
 
 Preparation. The gall-stones are treated with strong acetic or 
 dilute liydi'ochloric acid to separate the earthy matter and the residue 
 is thoroughly washed with water and alcohol and dried. Fi'om this 
 residue the prolonged action of hot chloroforoa extracts the bilirubin, 
 which may either be obtained in the amorphous form by precipitation 
 with alcohol of its solution in chloroform, or as well-defined crystals by 
 the slow evaporation of the chloroform solution. 
 
 The most usual form of the crystals is that of rhombic prisms ; they 
 are readily soluble in chloroform and alkaline solutions only. 
 
 By treatment with oxidising agents such as nitrous acid bilirubin 
 takes up oxygen and becomes biliverdin, the colour at the same time 
 changing to green. The possible oxidation does not end here, and .if 
 continued a series of products are obtained each with a characteristic 
 colour as in the well known Gmelin's test^ Of these only the final 
 product of the oxidation has been obtained in a state of sufiicient purity 
 to enable any definite statements to be made of its characteristics*. 
 This is the body known as Choletelin (see below). 
 
 Biliverdin. CigHigN^O^.s 
 
 This product of the oxidation of bilirubin gives the characteristic 
 
 colour to the bile of herbivora, and to biliary vomits. It occurs also 
 
 probably at times in the urine of jaundice and in the pigmentary matter 
 
 1 Parke, Tubing. Med. -cliem. Unters. Bd. i., S. 160. 
 
 - Maly, Sitzber. d. Wien Akad. lvii. 1868, ii. Abth. Febr. Hft. 
 
 ^ Tiederaann und Gmelin, Die Verdauung, 1826, S. 79. 
 
 4 Heynsius und Campbell, Pfliiger's Arch. Bd. tv. (1871), S. 497. 
 
 6 Maly, Sitzb. d. Wien. Akad. lxx. (1874), in. Abth. 
 
Apr.] CHEMICAL JlASIS OF THK AMMAL liUDY. 7Gj 
 
 of the plaoenln. It is nut louiul, or occurs in traces oiilv, in gall- 
 stones. 
 
 Preparation. An impure product is obtained by precipitating 
 ordinary herbivorous bile witli baric chloride, wa.shin^' the jjrecipitato 
 with water and alcohol and decomposing it with hydrochloric acid. 
 The biliverdin thus obtained is washed with aether and dissolved in 
 alcohol. From its solution in the latter itis obtained as an amorphou.s 
 green powder by slow evaporation. Pure biliverdin is best prepared by 
 the slow oxidation in tlie air of bilirubin dissolved iit dilute caustic 
 soda. 
 
 It does not crystallise, and is insoluble in aether or chloroform ; readily 
 soluble in alcohol. When oxidised it gives the same play of colours a.s 
 does bilirubiu, with the formation of the same final and intermediate 
 products. 
 
 Neither this body nor bilirubin gives any characteristic absorption 
 bonds. 
 
 There seems now no reason for doubting that the bile pigments are 
 derived ultimately from the colouring matter of the blood. (See p. 30.) 
 
 Yirchow has described' the gradual changes in old blood-clots, 
 as of cerebral haemorrhage, which lead to the presence of the so-called 
 hjematoidin crystals. Though these have not been obtained in sufficient 
 quantities to enable their composition to be finally fixed by a chemical 
 analysis", still tho identity of their crystalline form with that of bilirubin 
 and the fact that they both give the same play of colours when oxidised, 
 as in Ginclin's test, justify the assumption that hsematoidin and bilirubin 
 are identical^ Moreover the balance of experimental evidence distinctly 
 supports the view that a liberation from the corpuscles of the colouring 
 matter of the blood in the blood-vessels by an injection of chloroform, 
 water ttc, leads generally to the appearance of bile-pigments in the urine^ 
 The occurrence of bilirubin crystals in the urine has frequently been ob- 
 served after the operation of transfusion of blood in man. The chemical 
 possibility of the conversion of hsemoglobin into biliverdin is readily 
 seen by a comparison of the formulae of haematin (see p. 311) and 
 bilirubin. The former has, according to Hoppe Seyler'', the composition 
 indicated by the formula 2 (C'3^H3, N^ Fe O,) while that of bilirubin is 
 CisHjgNoOg. Although the conversion has not as yet been directly 
 effected the following facts are significant. If bilii-ubin is treated with 
 sodium amalgam the substance known as hydrobilirubin (.see below) is 
 
 1 Arch. f. path. Anat. Bd. i., S. 383. 
 
 - Kobin, Ann. d. Chem. v. Pharin. Bd. cxvi. S. 89. 
 « But see also Preyer, Die Blutknjstalle, 1871, S. 187. 
 
 •» Tarchauoff, Pfliiger's Arch. Bd. ix. (1874), S. 53. See also Bd. x. (1875) 
 5. 208. ^ ' 
 
 2 Phi/siolociische Chemie, 1879, S. 395. 
 
760 BILE PIGMENTS. [A pp. 
 
 obtained. If hsematin is dissolved in caiistic soda and treated with 
 sodium amalgam oi" in hydrochloric acid solution with zinc dust, a 
 substance is obtained which is now recognised as identical with hydro - 
 bilirubin^ This is the most direct chemical evidence of the relation of 
 the colouring matters of the blood and biJe. 
 
 Choletelin. C^g H^g K Og (?) =. 
 
 This substance is obtained as the final product of the oxidation of 
 either bilirubin or biliverdin. It is best prepared by acting upon 
 bilirubin with nitrous acid in presence of alcohol ; the Tarious colours 
 of Gmelin's reaction are observed and the final reddish-yellow solution, 
 if poured into water, yields a precipitate of choletelin. It is not 
 crystalline and is soluble in alcohol, sether and chloroform. When 
 freshly prepared it seems to give an uncertain absorption band if 
 examined in an acid solution. On this account some observers^ have 
 been led to regard it as identical with hydrobilirubin (urobilin). There 
 is however no doubt that they are quite distinct bodies*. 
 
 Hydrobilirubin. C30 H^f, IST^ O,,. 
 
 This body was first described by Maly^ as resulting from the action 
 of sodium amalgam on an alkaline solution of bilirubin. When the 
 reaction is complete, the solution is precipitated Avith hydrochloric acid, 
 the precipitate dissolved in ammonia, again precipitated by acid, and the 
 substance thus finally obtained is washed with water. It is readily 
 soluble in alcohol, less so in ssther. Its alkaline solutions are yellow, 
 and these turn pink on the addition of acid. Both its acid and alkaline 
 solvitions, the latter especially on the addition of a few drops of chloride 
 of zinc, give a characteristic absorption band between b and F, ^ In the 
 colours of its alkaline and acid solutions and the greenish fluorescence 
 of its ammonia cal solution on the addition of chloride of zinc, and in its 
 absorption spectrum hydrobilirubin shews its close relation to urobilin 
 (see below), with which indeed it is now considered to be identical. It 
 is also identical with a body named stercobilin^ which had previously 
 been described, as a product of the alteration of the bile pigments in the 
 
 1 Hoppe-Sevler, Med.-chem. Untersuch. Hft. iv. 1871, S. 523. Ber. d. deutsch. 
 chem. Gesell. vii. (1874), S. 1065. 
 
 2 Maly, Sitzb. d. Wien A kad. Bel. lvii. (1868), 2 Abth. Febr. und Bd. lis. 1869, 
 2 Abth. April. Bee also Heynsius and Campbell, loc. cit. 
 
 3 Heynsius and Campbell, loc. cit. Stokvis, Centralb. f. d. vied. TFiss. No. 14 
 (1873), S. 211. 
 
 4 Maly, Centralb. f. d. vied. Wiss. No. 21 (1875), S. 321. Liebermann, Pfliiger's 
 Arch. Bd. xi. (1875), S. 181. 
 
 s Centralb. f. d. vied. Wiss. No. 54, 1871. Annal. d. Chem. Bd. clxiii. (1872), 
 S. 77. 
 
 6 Vierordt, Zeitsch. f. Biol. Bd. ix. (1873), S. 160. 
 
 ^ Vanlair and Masius, Centralb. f. d. med. IViss. No. 24, 1871. 
 
App.J CllKMlCAL iLiSlH OF Till: J XI MA L liOlj)'. 7iu 
 
 alimeutary canal, occurring in fiuccs. Tin;rc is no tlilliculty in seeing 
 how this change (liydrogenation) can be brought aboufc in the intestine 
 since it is known that a consiilcrable quantity of hydrogen may make 
 its appearance by fermentative processes in the intestine, and in its 
 nascent state might readily produce the simple change which is known 
 to occur when bilirubin is converted into hydrobilirubin. 
 
 Pigments of Uuixe. 
 
 Our knowledge of these bodies is at present limited and imperfect. 
 Most probably' they are numerous, but only two appear sufficiently well 
 characterised to deserve mention here. 
 
 Urobilin. C3.. H^q N^ 0-. 
 
 As stated above, this is now regarded as identical with hydrobilirubin. 
 It was first described by Jaffe" as a well-characterised normal uiinary 
 pigment and its identity with hydrobilirubin subsequently determined^ 
 
 JSTomial urine contains only small quantities of urobilin but there is 
 present a substance (chroraogen) which nnder the influence of acids, 
 with absorption of oxygen, yields urobilin. The nrine of fever fre- 
 quently contains a considerable amount of actual urobilin as such. 
 
 The properties described above for hydrobilirubin are identical with 
 those of urobilin. Its pi-eparation from urine is somewhat difficult and 
 for this some special manual must be consulted^. 
 
 Uroerythrin 
 
 Is considered to be the substance which gives to the urine of 
 rheumatism its characteristic colour. Very little is known of its 
 chemical properties'. It appears to be an amorphous reddish body with 
 an acid reaction, slowly soluble in water, alcohol and sether. When 
 treated with caustic alkali it turns green. Urine containing this body 
 takes on a characteristic reddish-yellow colour on the addition of con- 
 centrated hydrochloric acid. 
 
 Thndichum considers that normal lurine contains only one pigment, which he calls 
 iirochrome^. Maly is inclined to regard this as the same as urobilin^. More recently 
 Thudichum has upheld his former views 8. 
 
 1 Tierordt, Die quantitative Spectralanahjse, &c. Tiibingen, 1876, S. 81. 
 - Centralb. f. d. med. Wisi. 1868, S. 2i3. Yirchow's Arch. Bd. xlvii. (1869), 
 S. 405. 
 
 3 Maly, Ann. d. Chem.u. Pharm. Bd. clxiii. (1872), S. 77. 
 
 •• Vide Neubauer and Vogel. Harnanahjue, ed. viii. (1881), S. 81. 
 
 5 Heller's Archie. (2) Bd. in. (1854), S. 361. 
 
 « Brit. Med. Jl. N. S., No. 201, 1864, p. 509. 
 
 " Maly, Ann. d. Chem. u. Pharm., loc. cit. 1872. S. 90. 
 
 8 Jl. chem. Soc. Ser. 2. Vol. xiii. (1875), pp. 397, 401. 
 
768 THE INDIGO SERIES. [App. 
 
 The Ikdigo Series. 
 
 Indican. C8H,NS04. 
 
 A body was long ago described ^ as occurring in the urine and sweat 
 of men and other animals which yielded by the action of acids the 
 blue colouring matter indigo as one of the products of its decom- 
 position. Schunk considered this substance to be identical with the 
 indican known to occur in several plants (Indigofera, Isatis). Hoppe- 
 Seyler^ on the other hand, having regard to the greater ease with which 
 the indican from plants undergoes decomposition, regarded them as most 
 probably different substances. Baumann has shown^ that the two 
 ai-e really different, and has confirmed his earlier statements in a more 
 recent publication*. According to him, the indican obtained from 
 urine is not a glucoside (so also Hoppe-Seyler) and yields sulphuric 
 acid by the action of hydrochloric acid. He assigas to it the formula 
 CgHgN.O.SO^.OH, and regards it as indoxylsulphuric acid. The acid 
 itself is not yet known in the free state, but it yields stable salts such 
 as that of potassium, CgHgN.SO^K. It occurs largely in the urine 
 as the result of the presence of indol in the alimentary canal. In this 
 way Baumann and Brieger^ were enabled to obtain large quantities by 
 giving indol to a dog. For its preparation their original paper must be 
 consulted. 
 
 When treated in aqueous solution with hydrochloric acid in presence 
 of oxygen it yields indigo blue 
 
 2C8HgNS04K + 0,= 2C8H5KO + 2KHSO4. 
 It is always estimated in urine by conversion into indigo blue. 
 
 Indigo. CgH^NO. 
 
 It is formed, as stated above, from indican, and gives rise to the 
 bluish colour sometimes observed in sweat and urine. 
 
 It may, by slow formation from indican, be obtained in fine 
 crystals; these are insoluble in water, slightly soluble, with a faint violet 
 colour, in alcohol and sether. Chloroform also dissolves them to a slight 
 extent. Indigo is soluble in strong sulphuric acid, forming at the same 
 
 1 Schunk, Phil. Mag. Vol. x. p. 73; xiv. p. 228; xv. pp. 29, 117, 18S. Chevi. 
 Centralb. 1856, S. 50 ; 1857, S. 957 ; 1858, S. 225. Hoppe-Seyler, Arch. f. path. 
 Anat. Bd. xxvii. S. 388. Jaffe, Pfliiger's Arch. Bd. iii. (1870), S. 418. 
 
 2 Handb. d. path. chem. Anal. Ed. iv. (1875), S. 191. 
 
 3 Pfliiger's Arch. Bd. xiii. (1876), S. 301. Zeitschr. f. physiol. Chem. Bd. i. 
 (1877—78), S. 60. 
 
 4 Zeitschr. f. physiol. Chem. Bd. in. (1879), S. 254. 
 
 5 Zeitsch. f. physiol. Chem. Bd. in. (1879), S. 254. See also Ber. d. dnrtsch. 
 Chem. Gesell. xii. (1879\ Sn. 1098, 1192; 2166, and xiii. (1880), S. 408. 
 
App.] chemical basis of the AXIMAL noDY. 700 
 
 time two coiupouiuls with tliis acid; those are koIuMc in watir. ll 
 possesses a puie hlue colour; when pressed with a hard body a reddish 
 copper-eolouri'<l mark is left, and the crystals exhibit thi; same colour if 
 seen in reflected light. 
 
 The solidde et)nip()unds witii sulphuric acid give an aljsorption band 
 iu the spectrum which lies cluse to the \) line and to tlie red side of it. 
 This may be used to detect indigo. 
 
 Treated with reducing agents, indigo is decolorised, being reduced to 
 indigo-white. The latter contains two atoms more hydrogen than indigo. 
 
 Indol. U3H.N. 
 
 To this body the specific odour of the faeces is partly due. It is 
 obtained as the final product of the reduction of indigo; and also by the 
 distillation of proteid matter with caustic alkalis '. 
 
 It often occurs among the products of the action of pancreatic 
 ferment on proteids ; its presence in such cases appears however to be 
 due, not to the action of the trypsin, but to a simultaneous putrefaction 
 under the influence of bacteria, etc." If the pancreatic digestion be 
 carried on in the presence of salic^dic acid, indol does not make its 
 appearance. Indol is a crystalline body, soluble in boiling water, 
 alcohol and aether. It passes over in the steam when its aqueous 
 solution is boiled. It is characterised by the following reactions. A 
 strip of pine-wood moistened with hydrochloric acid is coloured bright 
 crimson when dipped into a solution of indol. Its alcoholic solution 
 turns red when treated with nitrous acid and its aqueous solution gives 
 a copious red precipitate with the same reagent. It also yields a 
 characteristic crystalline compound with picric acid. 
 
 Skatol. CgHsN (?). Noticed by Brieger^ as one of the products of 
 putrefactive changes in the small intestine. Secretan^ had previously 
 described a similar substance as arising from the putrefaction of 
 albumin. 
 
 Skatol is crystalline and contains nitrogen; it is more soluble in 
 water than indol and does not give rise to any red colouration with 
 nitrous acid. 
 
 Skatol readily passes into the urine when it occurs in the alimentary 
 canal, and then gives a violet-red reaction with strong hydrochloric acid. 
 
 V. Nencki^ prepai'es this substance by the putrefaction of a mixture 
 
 1 Kiihne, Ber. d. deut^ch. cJiein. Gesell. viii. (1875), S. 206. 
 
 2 Kiihne, Verhaiul. d. Heidlb. naturhist med. Ver. N.S. Bd. i. Hft. 3. Bericht 
 d. Deutschen chem. Ge.^elhcluu't, 1875, S. 206. 
 
 3 Ber. d. Deut:ich. chem. Gesell., Jabrg. x. (1877), S. 1027. 
 
 * Recherches sur putrefaction de Valbiunine. Geneva, 1876. 
 5 Centralb.f. d. med.'Wiss., 1878, S. 84y. 
 
 F 49 
 
770 THE INDIGO SERIES. [App. 
 
 of finely divided pancreas and muscle substance. After the addition of 
 acetic acid the mass is distilled, when the skatol readily passes over. 
 From the distillate it is precipitated by picric acid, and the precipitate 
 when again distilled with ammonia gives off pure skatol which may be 
 finally purified by crystallisation. 
 
INDEX. 
 
 Note : — References to the appendix have an italic a after them. 
 
 An asterisk attached to a number denotes that a diagram will be found 
 at the page indicated. 
 
 Abducens nerve, 6^9 
 Absorption of the products of digestion, 
 301 
 ,, from alimentary canal, 306 
 „ by diffusion, 310 
 
 by the skin, 390 
 Accelerator nerves, 189, 190 
 Accommodation, power of, possessed by 
 the eye, 493 
 ,, Hmits of, how determined, 
 
 „ mechanism of, 497 
 
 ,, associated with move- 
 
 ments of the pupil, 505 
 Acetic acid, 735a 
 Acetic acid series, 734a 
 Achroo-dextrin, 235 
 Acid-albumin, characters of, 704a 
 Acid, reaction in rigid muscle, 66 
 Acidity of gastric juice, 240 
 Acids, 
 
 ,, acetic series, 784a 
 „ bile series, 761a 
 „ glycohc series, 739a 
 ,, oleic series, 737« 
 ,, oxalic series, 741a 
 Acoustic apjiaratus, 556 
 Adipose tissue, 420 
 Adult, composition of, 684 
 After images, 532 
 Air in resph-ation, 326 
 ,, complemeutal, 315 
 ,, residual, 315 
 ,, stationary, 314 
 „ supplemental, 315 
 „ tidal, 314 
 
 „ changes of, in respiration, 326 
 . ,, inspired and expired, composition of, 
 327 
 ,, effects of deficient, 375 
 Air, effect of increased supply of, 380 
 ., effect of changes in composition 
 of, 330 
 
 Air, effects of changes in pressure of 
 
 breathed, 381 
 Albuminates, 704a 
 Albumin, tests for, 701a 
 Albumins, native, 702a 
 ,, derived, 704a 
 Alimentary canal, changes which the 
 
 food undergoes in the, 292 
 
 Alkali-albumin, characters of, 706a 
 
 Allantoin, 753a 
 
 Amceba, features of, 1 
 
 Amphibia, renal circulation in, 405 
 
 Anacrotic pulse, 166, 168, 173 
 
 Animal liody, composition of, 443 
 
 Anti-albumate, 719a 
 
 Auti-albumid, 719a 
 
 Auti-albumose, 719a 
 
 Anti-peptone, 718a 
 
 Antiseptic ti[ualities of bile, 250 
 
 Apucea, 361, 380 
 
 Apparent size, appreciation of, 539 
 
 Aristotle's exi^eriment, 580 
 
 A 4. • 1 ui J (colour of, 338 
 Arterial blood, |^^^^.^ ^^^ 33^ 
 
 Arterial flow, intermittent, converted 
 
 into a continuous, 128 
 Ai-terial i^ressure, tracing of, with a 
 ilerciu-y manometer, 
 121* 
 „ features of, 121 
 
 ,, methods of estimat- 
 
 ing, 122 
 „ amount of, in various 
 
 animals, 123 
 Arterial tone, 200 
 
 Arteries, as factors in circulation, 116 
 ,, blood velocity in, 124, 127 
 ,, changes in calibre of, 197 
 ,, rhythmic pulsations in, 197 
 Artificial respiration, effects on blood- 
 pressure, 370 
 Asparagine, 757a 
 Aspartic acid, 757a 
 
 49—2 
 
772 
 
 INDEX. 
 
 Asphasia, 674 
 
 Asphyxia, phenomena of, 375 
 ,, stages of, 876 
 ,, in new-born animal, 376 
 ,, effects of, on circulation, 378 
 Aspirates, 659, 661 
 Astigmatism, 507 
 Atropin, effect of, on heart, 186 
 
 ,, actioii on the pupil of the eye, 
 503 
 Anelectrotonus, 78 
 Auditory judgments, 565 
 ,, nerve, 649 
 ,, sensations, 559 
 Auriculo-ventricular valves, mechanism 
 
 of, 142 
 Automatic actions, 107 
 
 ,, ,, in spinal cord, 595 
 
 Avalanche, theory of nervous impulse, 
 86 
 
 Bacteria, in intestine, 298 
 
 Basilar membrane of the ear, uses of, 
 
 562 _ 
 Benzoic acid, 759a 
 Bile, composition of, 247 
 ,, pigments of, 248, 764a 
 ,, pigments, in connection with red 
 
 corpuscles, 31 
 „ action on food, 249 
 „ tests for, 249 
 ,, antiseptic qualities of, 250 
 ,, nervous mechanism of secretion of, 
 
 264 
 , , action of, in intestine, 297 
 ,, the nature of the act of secretion, 
 279 
 Bile-acid series, 761a 
 Bile-salts, 248 
 
 ,, ,, preparation of, 248 
 Bile-secretion, 279 
 Bilirubin, 31, 764a 
 
 ,, in bile, 248 
 Biliverdin, 248, 764a 
 Binocular vision, 541 
 Bladder urinary, tone of, 411 
 
 ,, ,, movements of, 413 
 
 Blind Spot, the, 512 
 
 „ filling up of, 537 
 Blindness, psychical, 631 
 Blood, 1—34 
 
 ,, an internal medium, 12 
 ,, phenomena of coagulation, 13 
 „ why fluid in living blood-vessels, 19 
 „ origin of fibrin-factors from cor- 
 puscles of, 22 
 „ comiDosition of, 24 
 „ the corpuscles, 26, 27, 28 
 ,, quantity in body, 33 
 „ distribution in body, 34 
 „ influence of supply on irritability 
 
 of nerve and muscle, 94 
 ,, changes in quantity of, 224 
 ,, respiratory changes in the, 329 
 „ venous and arterial, nature of, 380 
 „ gases of, 380 
 
 Blood, gases of, method of obtaining, 830 
 ,, relations of oxygen in, 832 
 ,, venous and arterial, colour of, 
 
 338 
 ,, causes of change of colour of, 338 
 ,, relations of carbonic acid in, 842 
 ,, ,, ,, nitrogen acid, 343 
 
 ,, entrance of oxygen into, 344 
 ,, oxygen in, relations to pressure, 
 
 344 
 ,, exit of carbonic acid from, 846 
 ,, oxidation in, 851 
 Blood-pressure, apparatus for investi- 
 gating, 120* 
 ,, ,, in arteries, 123 
 
 ,, „ in veins, 128 
 
 ,, ,, in capillaries, 120 
 
 ,, ,, relation of heart's-beat 
 
 to, 193 
 „ ,, effect of cardiac inhibi- 
 
 tion on, 194* 
 „ „ influence of stimulation 
 
 of sensory nerves on, 
 205 
 ,, ,, effects of changes in 
 
 quantity of blood on, 
 224 
 ,, ,, curve, respiratory undu- 
 
 lations of, 364 
 ,, ,, curve and curve of intra- 
 
 thoracic pressure, 864* 
 ,, ,, effects of artificial respi- 
 
 ration on, 370 
 Blushing, 203 
 
 Body, the metabolic phenomena of the, 
 415 
 ,, changes in, during starvation, 444 
 ,, energy of the, 457 
 ,, temperature of, 463, 465 
 Brain, visual areas in the, 523 
 ,, movements of, 644 
 ,, circulation in the, 645 
 ,, weight of, 684 
 Breath, causes leading to the first, 679 
 Brunner's gland, secretion of, 255 
 "Buffy coat" in blood, 14 
 Butyric acid, 735a 
 
 Calibre of the minute arteries, changes 
 
 in the, 197 
 Capillaries, as factors in circulation, 
 116 
 ,, velocity of flow in, 118 
 
 ,, plasmatic or inert layer, 118 
 
 ,, blood-pressure in, 120 
 
 ,, changes in, 219 
 
 Capillary walls, action of, 222 
 Capric acid, 736a 
 Caproic acid, 736a 
 Caprylic acid, 736a 
 Carbohydrate food, a source of fat, 472 
 
 ,, ,, effects of, 458 
 
 Carbohydrates, 728a 
 Carbolic acid, 761a 
 Carbonic acid in the blood, 342 
 ,, ,, exit from blood, 346 
 
INDEX. 
 
 773 
 
 Curbunic iicid tension of in l)looil and 
 
 luugH, 347 
 Carbonic oxido HiiemoKlobin, all) 
 
 „ „ poisonini,', iirtiticiiil diu- 
 
 betos in, 425 
 Cardiac events, general characters of, 
 14(i 
 ,, ,, ilnratlon of, \')'l, 154 
 
 ,, impulse, 137 
 
 „ inhibition, effect on circulation, 
 l'J4 
 Cardiac muscles, 102 
 
 ,, ,, characteristics of, 180 
 
 Cardiograjih, 138, 146 
 Cardiographic tracings, 146* 
 Cardio-inhibitory centre, 188 
 Carnin, 75oa 
 
 Casein, characters of, 708a 
 Central Sense-organ, 488 
 Cerebellum functions of, 640 
 Cerebral convolutions, functions of, 623 
 ,, ,, motor areas in, 
 
 625 
 „ „ of the dog, areas 
 
 of, 625*, 626* 
 „ „ of man, 627*, 
 
 628* 
 „ ,, sensory areas in, 
 
 628 
 „ ,, effects of stimu- 
 
 lation of, then- 
 nature, 629 
 „ ,, effects of re- 
 
 moval of mo- 
 tor areas of, 
 638 
 „ „ visual areas in 
 
 the, 631 
 ,, ,, varying irrita- 
 
 bility of, under 
 morphia, 632 
 „ ,, activity of, modi- 
 
 fied by gentle 
 stimuli, 634 
 Cerebral hemispheres, natme of the move- 
 ments of frog in 
 absence of, 609 
 „ ,, behaviour of bird 
 
 in absence of, 611 
 „ ,, behaviour of mam- 
 
 mal in absence 
 of, 611 
 ,, operations, rapidity of, 643 
 Cerebriu, 745rt 
 Cerebro-spinal fluid, 645 
 Chemical basis of the animal body, 699a 
 
 ,, changes in muscle, 64 
 Chest-movements, influence on flow of 
 blood to and from 
 the heart, 367 
 ,, ,, influence on flow of 
 
 blood from the 
 lungs, 369 
 Chest voice, 655 
 
 Cheyne- Stokes, respiration, 362 
 Chitin, 727a 
 
 Cholalic acid, 21'.) 
 Cholesterin, 742a 
 
 ,, in red corpuHclcs, 26 
 
 Choletelin, 766a 
 Cholic acid, 761a 
 Ciioliii, 745a 
 Chondrin, 724a 
 
 Cliorda saliva and sj-mpathetic saliva, 262 
 Cliorda Tyiupaiii, action of, on sub- 
 
 maxiUary gland, 259 
 Choroidal epithelium, behaviour towards 
 
 light, 519 
 Chromatic aberration, 508, 509* 
 Chyle, characters of, 301 
 
 ,, entrance of, into lacteals, 303 
 ,, movements of, 304 
 Chyme, formation of, 293 
 Cilia, 102 
 
 Circulating proteids, 452 
 Circulation, physical phenomena of, 116 
 ,, chief factors enumerated, 
 
 116 
 ,, phenomena of capillary, 117 
 
 „ hydrauhc principles of, 128 
 
 ,, vital phenomena of, 176 
 
 „ chief causes of modifications 
 
 of, 176 
 ,, effect of changes in the 
 
 heart's beat on, 194 
 ,, effect of cardiac inhibition 
 
 on, 194 
 ,, effects of respiration on, 364 
 
 ,, effects of asphyxia on, 378 
 
 ,, in the brain, 644 
 
 foetal, 177 
 ,, changes in, at birth, 679 
 
 Circus movements, 621 
 Coagulated proteids, 25 
 Coagulation of blood, 13 
 „ nature of, 18 
 
 „ (artificial), of serous fluids, 
 
 17 
 Cold increases metabolism of body, 467 
 Cold and Heat, lethal effects of, 468 
 Colostrum, 430 
 Colour Blindness, 530 
 Colour sensations, 524 
 
 ,, ,, methods of mixing, 
 
 525 
 ,, „ fundamental or pri- 
 
 mary, 527 
 ,, ,, diagram of three pri- 
 
 mary, 529* 
 Colour \ision, the Hering theoiy of, 527 
 ,, ,, Young-Helmholtz theory 
 
 of, 528 
 Colours, complementary, 526 
 
 ,, nomenclature of, 524 
 
 Complemental air, 315 
 Complementarj- colours, 526 
 Concha, use of, 557 
 Consonants, natme of, 658 
 Constant cunent, action on muscle and 
 
 nerve, 75 
 Constriction, effects of local vascular, 
 216 
 
774 
 
 INDEX. 
 
 Contractile Tissues, 35 
 
 Contraction, simple muscular, 39 
 
 Contraction of muscle affected by load, 
 87 
 
 Contrast, phenomena of, in vision, 537 
 
 Coordinated movements, mechanism of, 
 614 
 
 Coordinating nervous mechanisms, na- 
 ture and seat of, 618 
 
 Coordination of visual movements, ner- 
 vous machinery of, 546 
 
 Corpora quadrigemina, functions of, 638 
 
 Corpora striata, functions of, 686 
 
 Corpuscles of the blood, 11, 12, 22, 26, 
 27, 29 
 
 Corresponding points, 541, 542* 
 
 Corti, organ of, use in the analysis of 
 sounds, 561 
 
 Crassamentum, or blood clot, 13 
 
 Cranial nerves, 648 
 
 ,, ,, motor and sensory, 484 
 
 Crura Cerebri, functions of, 641 
 
 Crying, 383 
 
 Curdling of milk, 430 
 
 Cutaneous respiration, 386 
 
 Cyclical changes in body, 691 
 
 Cystin, 758a 
 
 Diabetes, 424 
 Diagrammatic Eye, 492 
 Diagrams : 
 
 Apparatus for experiments with mus- 
 cle and nerve, 42 
 The muscle-nerve preparation with 
 
 clamp, electrodes &c, 43 
 Muscle-curve obtained by means of 
 
 the pendulum myograph, 43 
 The pendulum myograph, 44 
 Curves illustrating the measurement of 
 the velocity of a nervous imiDulse, 46 
 Tracing of a double muscle curve, 48 
 Muscle thrown into Tetanus, 49 
 Tracing produced with the ordinary 
 magnetic interruptor of an induction 
 machine, 50 
 Magnetic interruptor, 51 
 Muscular fibre undergoing contraction, 
 
 55 
 Non-polarisable electrodes, 58 
 Electric currents of nerve and muscle, 
 
 59 
 Muscle-nerve preparation, 77 
 Variations of irritability during elec- 
 
 trotouus, 79 
 Electrotonic currents, 81 
 Simplest forms of a nervous system, 
 
 104 
 Apparatus . for investigating blood- 
 pressures, 120 
 Tracing of arterial pressure with a 
 
 mercury manometer, 121 
 Large kymograph with continuous 
 
 roll of paper, 123 
 Ludwig's stromuhr, 125 
 Simultaneous tracings from the in- 
 terior of the right auricle, from the 
 
 Diagrams : 
 
 interior of the right ventricle, and 
 of the carc^c impulse in the horse, 
 139 
 Marey's tambour, with cardiac sound, 
 
 140 
 Cardio-graphic tracing of cardiac im- 
 
 piilse in man, 146 
 Normal heart curve, 147 
 Goltz and Gaule's maximum mano- 
 meter, 151 
 Movements and sounds of the heart 
 
 during a cardiac period, 155 
 Pulse curves, 162 
 Pulse tracing from carotid artery of 
 
 healthy man, 165 
 Pulse-curve from radial of man, 166 
 Anacrotic pulse-tracing from the ca- 
 rotid of rabbit, 166 
 Dicrotism in radial pulse of man, 167 
 Normal pulse-wave in aorta of frog, 167 
 Anacrotic sphymograph tracing from 
 
 the ascending aorta, 168 
 Pulse tracing from the dorsal pedis, 
 
 168 
 Influence of changes in the pressure 
 applied to the exterior of the vessel • 
 on the form of the curve, 169 
 Normal pulse-curve from carotid of 
 
 rabbit, 170 
 Dicrotic pulse-curve due to loss of 
 blood, 171 
 ■ Tracing from radial of man, 173 
 Perfusion canula tied into frog's ven- 
 tricle; Eoy's apparatus modified by 
 Gaskell, 179 
 Inhibition of frog's heart by stimula- 
 tion of the vagus, 185 
 Last cervical and first thoracic ganglia 
 
 in the rabbit, 190 
 Last cervical and first thoracic ganglia 
 
 in the dog, 191 
 Influence of cardiac inhibition on 
 
 blood-pressure, 194 
 Vagus stimulation, 195 
 Effect on blood-pressure of stimu- 
 lating the depressor nerve in the 
 rabbit, 204 
 Else of blood-pressure from stimula- 
 tion of nostril with smoke, 205 
 Submaxillary gland of the dog with its 
 
 nerves and blood-vessels, 258 
 Influence of food on the secretion of 
 
 pancreatic juice, 265 
 Pancreas of the rabbit, 267 
 Section of a 'mucous' gland, 269 
 Gastric gland of mammal, 273 
 Section of a serous gland, the parotid 
 
 of the rabbit, 274 
 Changes in the parotid during secre- 
 tion, 274 
 Thoracic respiratory movements by 
 
 Marey's pneumatograph, 316 
 Apparatus for taking tracings of the 
 movements of the column of an- in 
 respiration, 319 
 
IXDMX 
 
 lib 
 
 DingiftinH : 
 
 IiU(l\vi>,''a morcurial gas piiiiip, U.U 
 Spoctm of Oxy-hii'moKl(il)iii, ."{.'M 
 Curves of blood irtid intru-thorncic 
 
 prt'ssiuv, ;{t>l 
 lutliU'iico of respiration on the form of 
 
 the pulse-wave, ;{(>."> 
 Traube-Heriug curves, 37:5 
 Blood-pressure, curve of a rabbit's, 371 
 Renal oncometer, 3',(-< 
 Semi-diagrnmmatic sectional view of 
 
 oni'Oj,'rap]i, 'M\) 
 Blood-piessure traciuK and curve with 
 
 renal oncometer, -100 
 Nornuil splcen-curvo from dog, 433 
 Schciner's experiment, 4'.)5 
 Nerves governiu},' the pupil, 502 
 Chromatic aberration, 509 
 Formation of Purkiuje's figures from 
 illumination through the sclerotic, 
 513 
 The same through the cornea, 514 
 Three primary colom- sensations, 529 
 Appreciation of apparent size, 539 
 Corresponding points, 542 
 Attachments of tlie muscles of the 
 eve, and of their axes of rotation, 544 
 The Horopter, 547 
 Judgment of solidity, 551 
 Relative sectional areas of the spinal 
 nerves, as they join the spinal cord, 
 599 
 United sectional areas of the spinal 
 nerves, proceeding from below up- 
 wards, 599 
 Variations in the sectional area of the 
 grey matter of the spinal cord, 599 
 Variations in the sectional area of 
 the lateral, anterior, and posterior 
 columns of the spinal cord, 603 
 Cerebral convolutions of the dog, Hitzig 
 
 and Fritsch, 625 
 Cerebral convolutions of the dog, 
 
 Ferrier, 026 
 Side view of brain of man, witli areas 
 of cerebral convolutions, Ferrier, 627 
 Upper view of the same, Ferrier, 628 
 Larynx as seen by the laryngoscope in 
 different conditions of the Glottis, 652 
 Death, 695 
 
 Decay of the body, 689 
 Decussation of sensory and volitional 
 
 impulses, 601 
 Defaecation. 288 
 Deglutition,' 282 
 
 ,, nervous mechanism of, 283 
 
 Dental consonants, 659, 661 
 Dentition, 687 
 Depressor nerve, 204 
 Derived albumins, 704a 
 Dextrin, 235. 731rt 
 Dextrose, 235, 728a 
 
 Diaphragm, action of, in inspiration, 321 
 Dicrotic pulse, 166, 167, 169, 171 
 Diet, the normal. 445 
 „ general featiues of proteid, 475 
 
 DictoticH, 474 
 
 Digestion, tissues and mcchaniBmH of, 
 233 
 ,, circumstances aiTccting gas- 
 
 tric, 243 
 muscular mechanisms of, 281 
 ,, absorption of the products 
 
 of, 301 
 ,, course taken by the several 
 
 products of, 30G 
 ,, decomposition of proteids by, 
 
 720« 
 Digestive juices, properties of the, 234 
 Dilation, effect of local vascular, 216 
 Dioptric apparatus, imperfections in the, 
 
 506 
 Dioptric mechanisms, 491 
 Distance and size, judgment of, 549 • 
 Distribution of blood in the body, 34 
 Diurnal changes in the body, 694 
 Dyspeptone, 719a 
 Dyspnoea, 360 
 
 Ear, organ of Corti, 561 
 
 ,, basilar membrane of the, 562 
 Egg-albumin, characters of and pre- 
 paration, 702a 
 Elastin, 726rt 
 
 Electric currents in retina, 519 
 Electrical phenomena attending nervous 
 
 impulse, 72 
 Electrodes, uon-polarisable, 58* 
 Electrotonic currents, 80, 81* 
 Electrotouus, 77 
 
 ,. variations of irritability 
 
 during, 79* 
 Embryo, nutrition of, 674 
 
 ,, respiration of, 674 
 Emmetro^Hc eye, its features, 496, 506 
 Emetics, action of, 291 
 Endo-cardiac events, 138 
 
 ,, pressure, amount of, 150 
 
 „ tracings, Chauveau and 
 
 Marey, 139* 
 Energy, muscular and nervous, 98 
 ,, income of, 457 
 ,, ,, how determined, 457 
 
 ,, expenditure of, 458 
 ,, ,, how determined 
 
 459 
 ,, soui-ces of muscular, 459 
 ,, muscular, not due to proteids 
 alone, 460 
 Entotic phenomena, 509, 564 
 Equihbrium, nitrogenous, 450 
 
 ,, sense of, how gained, G15 
 
 Erect posture, maintenance of, 662 
 Expiration, movements of, 323 
 
 ,, laboured, 324 
 
 Expired aii', composition of, 327 
 Explosive consonants, 652, 661 
 Extractives in serum, 25 
 ,, in muscle, 69 
 
 ,, in milk, 430 
 
 Eye, the, as a dioptric apparatus, 491 
 ,, ,, diagi'ammatic, 492 
 
776 
 
 INDEX. 
 
 Eye, the, diagicammatic values of, 492 
 ,, power of accommodation possessed 
 by the, 493 
 the emmetropic, 496, 506 
 the myopic, 496, 506 
 the hypermetropic, 496, 506 
 the presbyopic, 496, 506 
 protective mechanisms of the, 554 
 Eyeballs, moverhents of, 542 
 ,, rotation of, 543 
 ,, muscles of, their action, 543 
 Ethylene-lactic acid, 741a 
 Ethylidene-lactic acid, 740a 
 Eu23noea, 360 
 Eustachian tube, uses of, 559 
 
 Facial nerve, 649 
 
 ,, and Laryngeal respiration, 324 
 
 Fffices, characters of, 299 
 
 Fallopian tube, descent of ovum into 
 the, 669 
 
 Falsetto voice, 655 
 
 Fat, deposition in Adipose Tissue, 426 
 ,, formed from carbohydrate food, 427 
 ,, ,, ,, proteid food, 427 
 
 Fats in serum, 25 
 
 ,, action of gastric juice on, 240 
 „ action of bile on, 250 
 ,, action of pancreatic juice on, 254 
 ,, emulsificationof, in small intestine, 
 
 296 
 ,, absorption of, from alimentary 
 
 canal, 307 
 „ in milk, 430 
 ,, complex nitrogenous, 743a 
 „ their derivatives and allies, 734a 
 „ the neutral, 737a 
 
 Fatty food, effects of, 453 
 
 Feeling and Touch, 572 
 
 Ferment of pancreatic juice, 251 
 ,, in fresh pancreas, 271 
 
 Ferments, 18, 237 
 
 Fibrin, characters of, 13, 14, 713a 
 
 Fibrin factors, origin from corpuscles of 
 blood, 22 
 
 Fibrin Ferment, 18 
 
 Fibrinogen, 16, 711a 
 
 Field of Touch, 580 
 
 Fields of Vision, struggle of the two, 
 552 
 
 Foetus, characters of blood of, 675 
 ,, secretory functions of, 677 
 ,, circulation of, 677 
 
 Food, action of bile on, 249 
 
 ,, action of pancreatic juice on, 251 
 ,, and the secretion of pancreatic 
 
 juice, 265* 
 ,, changes in the mouth, 292 
 ,, ,, „ ,, oesophagus, 292 
 
 ,, ,, ,, ,, stomach, 292 
 
 ,, ,, ,, ,, small intestine, 295 
 
 ;, ,, „ „ large intestine, 299 
 
 ,, influence of, on deposition of 
 
 glycogen, 418 
 , , fatty and carbohydrate, effects of, 
 453 
 
 Food, effects of gelatine as, 455 
 
 ,, effects of salts as, 455 
 Forced movements, 620 
 Formic acid, 734a 
 "Frog, the rheoscopic," 62 
 Functional activity, influence of, on 
 
 irritability of nerve and muscle, 95 
 
 Ganglia, action of sporadic, 112 
 
 , , cervical and thoracic in Babbit 
 and Dog, 190*, 191* 
 Gases in Stomach, 295 
 Gases of blood, 330 
 
 „ poisonous, 381 
 Gastric digestion of proteids, nature of, 
 242, 245 
 ,, ,, circumstances affecting, 
 
 243 
 ,, ,, duration of, 294 
 
 ,, ,, conditions affecting, 294 
 
 ,, flstula, formation of, 239 
 ,, gland of mammal, 273* 
 ,, glands, histological changes dur- 
 ing activity, 272 
 ,, juice, nature of, 239 
 ,, ,, acidity of, 240 
 ,, ,, action of, on starch and 
 
 fats, 240 
 ,, ,, action of, on proteids, 241 
 ., ,, preparation of artificial, 
 
 241 
 ,, ,, action on milk, 246 
 ,, ,, nervous mechanism of se- 
 cretion of, 262 
 ,, ,, formation of acid of, 278 
 Gelatine or glutin, 725a 
 Gelatine as food, 455 
 General sensibility, 572 
 Gestation, period of, 681 
 Glands, gastric, 272, 273* 
 ,, mucous, 268 
 ,, oxyntic, 278 
 ,, parotid, 274* 
 ,, salivary, 268 
 
 serous, 268, 274* 
 Globin, characters of, 713a 
 Globulin, characters of, 709a 
 Globuhns, 709a 
 Glomeruli of Kidney, pressure in, how 
 
 varied, 402 
 Glomerulus as a filtering apparatus, 408 
 Glosso-pharyngeal nerve, 649 
 Glottis, narrowing of, 653 
 
 ,, widening of, 654 
 Glycerin, 738a 
 
 Glyeerinphosphoric acid, 744a 
 Glycin, 249, 756a 
 Glycocholic acid, 249, 762a 
 Glycogen, 416, 732a 
 
 ,, preparation of, 417 
 
 ,, influence of food on, 418 
 
 ,, deposition of, 418 
 
 ,, use of, 419 
 
 ,, the liver a storehouse of, 420 
 
 ,, in muscle, 421 
 
 ,, conversion after death, 423 
 
IXDHX 
 
 Tn 
 
 Glycogen, iircseiit in I'lulnynnie liHsufH, 
 
 676 
 (tlycolic ftcid s(<ri<>H, 73U« 
 (llntiiniiiiic; iicid, IMa 
 tJoltz ami Gaulc's maxinniin inaiiomotii , 
 
 ir.i* 
 (Irapcsugar, 7'2H(( 
 (irowth, arrest and ilwlino of, (IH'.I 
 (triitzncr's lui'thod of luoasuring peptic 
 
 powers, '211 
 (iuaniii, lii'ni 
 Guttural consonants, 059, 661 
 
 Htemadromometer of Volkmann, 1*24 
 Htematuclioiiu'ter of Vierordt, 126 
 Htematin, IMI 
 
 ,, reduction of, 341 
 HaBmoglobin, iu the corpuscles, 26, 
 333 
 spectra of, 334*, 385 
 ,, reduction, of 337 
 
 ,, reduced, spectrum of, 347 
 
 Hearing, 550 
 
 ,, power of distinguishing notes, 
 564 
 Heart, as a factor in circulation, 110 
 „ normal beat of, 135 
 ,, visible movements of, 130 
 ,, change of fonu during systole, 
 
 130 
 ,, sounds of, 143 
 ,, first sound, 144 
 ,, second sound, 144 
 ,, Sounds and movements during a 
 
 cardiac period, 155* 
 ,, work done by, 157 
 ,, means of recording beat of, 178 
 ,, mechanism of the normal beat of , 
 
 180 
 ,, characteristics of cardiac muscles, 
 
 180 
 ,, nervoiis mechanism of beat, 181 
 ,, of Frog, effect of various sec- 
 tions, 182 
 ,, inhibition of the heat of, 184 
 ,, of Frog, inhibition of, hj- stimula- 
 tion of the vagus, 185 
 ,, effect of atropin on, 180 
 ,, effect of urari on, 180 
 ,, effect of muscarin on, 187 
 ,, effect of pilocarpin on, 187 
 „ Stannius' experiment on beat of, 
 
 187 
 ,, reflex inhibition of, 188 
 „ accelerator nerves of, 189 
 ,, agents, modifying beat of, 191 
 Heart's beat, variations in, 159 
 
 ,, relation of blood-pressure 
 
 to, 193 
 ,, effects on the circulation of 
 
 changes in the, 194 
 ,, what affected by, 228 
 
 Heat of the Body, som'ces and distribu- 
 tion of, 401 
 „ ,, muscles, chief source 
 
 of, 461 
 
 IJeat of tile bocly, clmnneis of, Iohh of, 
 
 464 
 Heat and cold, lethal offectH of, 468 
 Heini-albunioso, 719rt 
 
 llellli-)ie|)t(jlie, 71H// 
 
 1 l(iiii|Ji(i(ein, 71'.l'( 
 
 lleriiig, tiieoiy of colour vision, 527 
 
 HieeoMKh, 3H2 
 
 liijipuric acid, 700*/ 
 
 ,, how formed in body, 441 
 
 Hamatoidin, 30 
 Horopter, the, 547* 
 Hydraulic principles of the circulation, 
 
 '128 
 Hydvobilirubin, 700« 
 Hypcraesthesia after section of spinal 
 
 cord, 006 
 Hypermetropic eye, its features, 496, 506 
 Hyperpncea, 300 
 Hypoglossal nerve, 050 
 Hypoxauthin, 754a 
 
 Identical points, 541 
 
 Image, formation of the retinal, 491 
 
 Impregnation of ovum, 072 
 
 Income and output, method of deter- 
 mining, 446 
 ,, ,, ,, deductions from, 
 
 448 
 
 Indican, 707a 
 
 Indigo series, 767a 
 
 Indol, 253, 709a 
 
 Induction Machine, 39, 41* 
 
 Inflammation, 221 
 
 Inhibition, general nature of, 113 
 
 Inliibition of the Heart's beat, 184 
 
 Inogen, 29 
 
 luosit, 731a 
 
 Insensible perspiration, 385 
 
 Inspiration, movements of, 320 
 ,, action of ribs in, 320 
 
 ,, action of diaphragm in, 320 
 
 ,, laboured, muscles employed 
 
 in, 322 
 
 Inspired air, composition of, 327 
 
 Intestine, bile action of, in, 297 
 ,, bacteria iu, 298 
 ,, large, movements of, 288 
 ,, large, changes in, 299 
 ,, small, movements of, 286 
 ,, ,, changes of food in, 295 
 
 ,, ,, emulsificationof fatsin, 
 
 296 
 ,, ,, absorption from, 298 
 
 Intra-molecular oxygen, 349 
 
 Ii'radiation in vision, 530 
 
 Irritability muscular and nervous, 37, 90 
 ,, independent muscular, 38 
 
 Isopepsin, 718a 
 
 Katacrotic pulse, 166 
 Katelectrotonus, 78 
 Keratin, 720a 
 Kidneys, secretion by the, 392 
 
 , , double character of function of, 
 397 
 
778 
 
 INDEX. 
 
 Kidneys, variations in volume of, how 
 measured, oncometer and 
 oncograph, 398 
 ,, variations in volume of, due to 
 
 variations in blood supply, 
 400 
 ,, how brought about, 401 
 
 ,, curve, 400 
 
 Kreatin, 752a 
 
 ,, its relation to urea, 436 
 
 Kreatiniu, 753a 
 Kymograph, with continuous roll of 
 
 paper, 123* 
 Kynurenic acid, 755a 
 
 Labial consonants, 659, 661 
 
 Lacteals, entrance of the chyle into the, 
 
 303 
 Lactic acid, 740a 
 Lactose, 730a 
 
 Lardacein, or the so-called amyloid sub- 
 stance, 728a 
 Laryngeal and facial respiration, 824 
 Larynx, as seen by the laryngoscope, 
 652* 
 ,, action of muscles of, 653 
 ,, nerves of, 654 
 Latent period, 46 
 Laughing, 383 
 Laurostearic acid, 736a 
 Leucin, 756a 
 
 ,, origin of urea from, 439 
 ,, and Tyrosin, products of pan- 
 creatic digestion, 252 
 Lecithin, 743a 
 
 , , in red corpuscles, 26 
 
 Liver, a storehouse of glycogen, 420 
 Living blood-vessels, blood fluid in, 19 
 Local constriction, 217 
 
 „ dUation, 216 
 Localization of pressm-e sensations, 579 
 Locomotor ataxy and muscular sense, 
 583 
 ,, mechanisms, 662 
 
 Ludwig's mercurial gas pump, 331* 
 
 ,, stromuhr, 125 
 
 Lungs, pressure exerted by elasticity of, 
 315 
 ,, resphatory changes in the, 344 
 ,, self-regulating action of, 356 
 ,, of new born infant, condition of, 
 
 679 
 ,, condition of foetal, 679 
 Lymph-hearts, 306 
 
 „ movements of, 304 
 ,, influence of nervous system on 
 movements of, 305 
 Lymphatics, the, 301 
 
 Magnetic interrupter, 51* 
 Maltose, 235, 730a 
 Mammary gland, 429 
 
 ,, ,, formation of milk in, 
 
 430 
 Manometer for arterial pressure, 122, 123 
 maximum, 151* 
 
 Marey's tambour, 150* 
 Mastication, 281 
 Mechanisms of digestion, 233 
 
 ,, of respiration, 312 
 
 Medulla oblongata, functions of, 641 
 Medullary vaso-motor centre, 213 
 Membrana tectoria, uses of, 563 
 Membrana tympani, uses of, 557 
 Meniere's malady, 617 
 Menstruation, 669 
 Mercurial gas pump, 331* 
 Metabohc phenomena of the body, 
 
 415 
 Metabolic tissues, 416 
 Metapeptone, 719a 
 MetaboUsm, nitrogenous, 449 
 
 ,, proteid, exact natm"e of, 451 
 
 ,, of the body, increased by 
 
 cold, 467 
 Micturition, 410 
 Migrating cells, 103 
 Milk, action of gastric juice on, 246 
 ,, action of rennet on, 246 
 ,, constituents of, 429 
 ,, formation in mammary gland, 430 
 ,, influence of nerves on secretion 
 and ejection of, 431 
 Milk-sugar, 480, 730a 
 Morphia, producing irritability of cere- 
 bral convolutions, 682 
 Motor cranial nerves, 484 
 
 , , nerves, behaviour towards stimuli 
 different from that of sensory 
 nerves, 487 
 ,, and sensory nerves, differences 
 
 between, 106 
 ,, and sensory nerves, union of, 487 
 Movements and soimds of the heart, 
 during a cardiac period, 
 155* 
 ,, of chest, their influence on 
 
 the flow of blood to and 
 from the Heart, 367 
 ,, ,, chest, influence on flow of 
 
 blood through the lungs, 
 369 
 ,, ,, the Brain, 644 
 
 Mucigen, 269 
 Mucin, 724a 
 Mucous glands, characters of, 268 
 
 ,, gland in a state of rest and 
 activity, 269* 
 Muscarin, effect of, on heart, 187 
 Muscle throwTi into tetanus, 49*, 52 
 „ change in form during contrac- 
 tion, 54 
 ,, change in microscopic structure 
 
 during contraction, 55 
 ,, relaxation after contraction, 56 
 ,, elasticity of, 57 
 ,, electric currents of, 59* 
 ,, neutral or alkahne reaction of 
 
 living, 66 
 ,, chemical changes during con- 
 traction, 68 
 ,, extractives of, 6.) ^ 
 
INDEX. 
 
 779 
 
 Mnsole, composition of, td) 
 
 „ thermal clmngcH in duriiig cou- 
 triiction, 70 
 iictiou of constant current upon, 
 75 
 ,, value of, as a machine, dependant 
 on the load, 87 ; further dcter- 
 mined by the size and form of 
 the muscle, 88 
 ,, absolute power of, 89 
 ,, work done by, 81) 
 „ iutluence of temperature on irri- 
 tability of, 93 
 „ inthunce of blood-supply on IitI- 
 
 tability of, '.)-l 
 ,, influence of functional activity 
 
 on irrits'bility of, 95 
 ,, energy of, 98 
 ,, i^roperties o*' unstriated, 101 
 „ glycogen in, -121 
 „ acid reaction of dead, 6G 
 Muscle-currents, 58 
 
 , , , , negative variation of, 61 
 
 Muscle-ciu've, 42*, 48* 
 Muscle nerve preparation, 77* 
 
 ,, ,, ,, as a machine, 83 
 
 Muscle-plasma, 05 
 Muscle and uerve, phenomena of, 37 
 Muscles, cardiac, 102 
 
 ,, characteristics of cardiac, 180 
 ,, employed in inspiration, 322 
 ,, chief source of heat of body, 
 461 
 of the eyebaU, 543, 544* 
 „ of the Larynx, 653 
 „ use of glycogen in foetal, 676 
 ,, weight of, 684 
 Muscular action, nature of, 98 
 
 ,, contraction compared with 
 amaboid move- 
 ment, 35 
 „ ,, phenomena of a 
 
 simple, 39 
 M ,, as affected by na- 
 
 ture of stimulus, 
 83 
 „ energy, 459 
 „ fibre undergoing contraction, 
 
 55* 
 „ iii-itabUity, 37 
 ,, mechanisms of digestion, 281 
 „ mechanisms, special, 651 
 „ sense, the, 582 
 Musical sounds, analysis of, 560 
 Myopic eve, its features, 496, 506 
 Myosin, 64, 712rt 
 Myiistie acid, 736a 
 
 Negative variation of the muscle current, 
 61 
 ,, ,, nerve current, 73 
 
 Nei-ve, electric current of, 59* 
 
 ,, changes during the passage of 
 
 a nervous impulse, 72 
 „ action of constant current upon, 
 75 
 
 Nerve changes caused by Bcverance from 
 central ncrvcnis HyHtem in a, 91 
 ,, inliiience of tcmi)oraturu on irri- 
 tability of, 93 
 ,, and muscle, influence of lilood 
 
 KUpjily on irritabihly of, 94 
 ,, and Uiuscle, influence of func- 
 tional activity on, 95 
 Nerve, influence of rest on, 97 
 ,, energy of, 98 
 
 „ and muKcle, phenomena of, 37 
 Nerves, circumstances determining irri- 
 tability of, 90 
 ,, differences Ijetween motor and 
 
 sensory, 106 
 ,, accelerator, 189 
 ,, vaso-motor, 199 
 ,, vaso-dilator, 210 
 ,, vaso-constrictor, 210 
 „ vaso-motor, of the veins, 215 
 ,, their influence on secretion and 
 
 ejection of milk, 413 
 ,, then- influence on the spleen, 
 
 433 
 ,, trophic, 471 
 ,, sensory, 481 
 ,, spinal, 482 
 ,, cranial, 484, 648 
 ,, fibres, degeneration of, after 
 
 section, 44e-f''s, . 
 ,, governing the '^lipil of the eye, 
 502 
 Nervi erigeutes, 210 
 Nervous action, natme of, 98 
 Nervous impulses, definition of, 88 
 
 ,, ,, rate of propagation, 
 
 47 
 ,, ,, curves illustrating 
 
 then- velocity, 46* 
 „ ,, electrical phenomena 
 
 attending, 72 
 „ ,, avalanche theory of, 
 
 86 
 Nervous u-ritability, 37 
 Nervous mechanism, of Heart beat, 181 
 ,, „ of secretion of 
 
 gastric juice, 262 
 ,, ,, of secretion of bile, 
 
 264 
 ,, ,, of respiration, 353 
 
 ,, ,, of perspiration, 388 
 
 „ „ nature and seat 
 
 of co-ordinating, 
 618 
 ,, system, simplest fonn of, 104* 
 „ ,, influence on nutrition, 
 
 470 
 Nervous tissues, fundamental properties 
 
 of, 104 
 Neurin, 745a 
 
 New-born infant, respiration of, 679 
 ,, ,, cu'culation of, 679 
 
 ,, „ composition of, 684 
 
 ,, ,, growth of, 685 
 
 ,, „ function of, 685 
 
 „ ,, metabolism of, 685 
 
780 
 
 INDEX. 
 
 New-bom infant, psj'chical characters of, 
 
 687 
 Nitrate of urea, 746a 
 Nitrogen in the blood, 343 
 Nitrogenous equilibrium, 150 
 Nitrogenous Fats, 743a 
 
 „ metabolism, 449 
 
 ,, metaboUtes, 746a 
 
 , , non-crystalline bodies allied 
 
 to Proteids, 723a 
 Non-polarisable electrodes, 58* 
 Nuclein, 727a 
 Nutrition, 233 
 
 ,, statistics of, 443 
 
 ,, influence of the nervous 
 system on, 470 
 
 Ocular spectra, 538 
 
 Oculo-motor nerve, 648 
 
 Oesophagus, movements of, 284 
 
 Old age, characteristics of, 689 
 
 Oleic acid series, 737a 
 
 Oleiu, 738a 
 
 Olfactory nerve and sensation of smell, 
 
 567 
 Olfactory nerve, 648 
 Oncograph, 399* 
 Oncometer, 398* 
 Optic axis, the, 491 
 
 ,, lobes, inhibitory of reflex action, 
 
 591 
 ,, nerve, 648 
 
 ,, Thalami, functions of, 636 
 Ossicles, auditory, uses of, 557 
 Otoliths, 563 
 
 OutjDut and income, method of deter- 
 mining, 446 
 Ovum, descent of, into Fallopian tube, 
 669 
 ,, impregnation of, 672 
 Oxalate of urea, 747a 
 Oxalic acid series, 741a 
 Oxidation in blood, 351 
 Oxygen in the blood, 332 
 
 „ entrance of, into blood, 344 
 
 ,, tension of, in blood and lungs, 
 
 845 
 ,, influences respiratory centre, 362 
 Oxyhsemoglobin, spectrum of, 335 
 
 ,, ,, and Hagmoglobin, dif- 
 
 ference in colour of, 
 338 
 Oxyntie glands, 278 
 
 Pain, 573 
 
 Palmitic acid, 736a 
 
 Palmitin, 738a 
 
 Pancreas, histological changes during 
 
 activity and rest, 267* 
 Pancreatic digestion, characters of, 252 
 „ juice, characters of, 250 
 action on food, 251 
 action on proteids, 251 
 ,, „ fats, 254 
 ,, ,, starch, 254 
 act of secretion of, 265 
 
 Paraglobulin, 16, 25; characters of, 
 
 710a 
 Parapeptone, 243, 719a 
 Parotid glands during secretion, 274* 
 Parturition, 681 
 Pathetic nerve, 648 
 Pendulum Myograph, 44* 
 Peristaltic contractions, 284, 286 
 Pepsin, action on proteids, 246 
 Pepsinogen, 273 
 Peptic action, theories of nature of, 
 
 720a 
 Peptic digestion, 252 
 Peptone, characters of, 243 
 
 ,, proteids converted into, by 
 
 l^epsin, 246 
 „ characters of, 716a 
 ,, theories as to nature of, 717a 
 Perfusion Cannula, 179* 
 Peripheral Eesistance, 118 
 
 „ ,, what affected by, 
 
 229 
 
 ,, sense-organ, 488 
 
 Perspiration, amount of, 385 
 
 „ sensible and insensible, 385 
 
 „ characters of, 386 
 
 ,, secretion of, 387 
 
 ,, nervous mechanism of, 388 
 
 Pettenkofer's test for Bile acids, 762a 
 Phakoscope, 498 
 Phases of Life, 684 
 Phenol, 769a 
 Phenylic acid, 761a 
 Photo-chemistry of the retina, 515 
 Physical electrotonus, 82 
 Physiological electrotonus, 82 
 Physical phenomena of circulation, 116 
 Pigments of Bile, 764a 
 
 ,, ,, Urine, 767a 
 Pilocarpin, effect of, on heart, 187 
 Placenta, respiratory function of, 675 
 
 ,, nutritive functions of, 676 
 
 Plasmine, 16 
 Pneumogastric nerve, 650 
 
 ,, ,, ill respiration, 355 
 
 Poisonous gases, 381 
 Poisons, effect of, on heart, 186, 187 
 Polycrotic pulse, 166 
 Pons Varolii, functions of, 641 
 Predicrotic pulse- wave, 168 
 Presbyopic eye, its features, 496, 506 
 Pressure of air breathed, effects of 
 
 changes in, 381 
 Pressure-sensations, characters of, 575 
 
 ,, ,, localization of, 579 
 
 Pressure and temperature sensations, 
 separate nerves and nerve endings for, 
 577 
 Propionic acid, 735a 
 Protagon, 744a 
 
 Proteid diet, general features of, 475 
 Proteids, in serum, 25 
 
 ,, ,, red corpuscles, 26 
 
 ,, action of gastric juice on, 241 
 
 ,, ,, ,, pancreatic juice on, 
 
 251 
 
lyjjEX. 
 
 781 
 
 Protoids, ftbBorption of, from nlimcntary 
 ciiiml, 3U7 
 ,, a souic'o of flit, 127 
 ,, ec)iiiiH)siti()n of, 128 
 ,, I'lmiiu'lt'is i>r, 701(/ 
 ,, coiiKuliiUcl, cliiinictors of, 715fl 
 ,, decompDsition of, by ilit^estion, 
 
 720(1 
 „ products of decomposition of, 
 
 721« 
 ,, theory of composition of, 
 722(/ 
 Protoplasm, fundamental riicnonuna 
 of, 1—3 
 „ composition of, G'JDn 
 
 Psychical blindness, (j31 
 
 ,, characters of new-born infant, 
 C87 
 rtyalin, 237 
 
 ruberty, phenomena attending on, 688 
 Puhnonary alveoli, determination of car- 
 bonic acid tension in, 3-i7 
 Pulse, ICl 
 
 ,, waves produced on artificial 
 
 scheme, 161 
 ,, rate at which it travels, 163 
 ,, ,, circumstances determining, 
 
 163 
 „ general causation of, 164 
 „ secondary waves in, 165, 171 
 ,, dicrotic, 166 
 ,, tricrotic, 166 
 „ polycrotic, 166 
 ,, anacrotic, 166, 173 
 ,, katacrotic, 166 
 ,, causes of dicrotic wave of, 167, 
 
 169 
 ,, predicrotic wave of, 168 
 ,, dicrotic wave, variations in, 173 
 ,, venous, 17-4 
 
 ,, rate, as iutiuenced by respiratory 
 movement, 371 
 Pulse-waves, sphygmographic tracings, 
 162*, 165*, 166*, 167*, 168*, 16y*, 
 170*, 171*, 173* 
 Pupil, movements of, 500 
 
 by what contracted, 500 
 ,, ,, dilated, 500 
 diagi-ammatic representation of 
 
 nerves governing the, 502* 
 nervous mechanism of contraction 
 
 of, 507 
 by what means dilated, 503 
 relations of cervical sympathetic 
 
 to, 503 
 action of atropiu on, 503 
 relation of tifth nerve to, 506 
 accommodation associated with 
 movements of the, 505 
 Purkinje's figures, 512, 513*, 514* 
 
 Quadrupeds, locomotion of, 664 
 
 Keaction, period, 644^ 
 
 ,, ,, reduced, 644 
 
 RecuiTent sensibility, 483 
 
 Ked corpuHclcs, composition, 20 
 
 ,, number in tlie blood, 28 
 
 ,, proportion to wliitecor- 
 
 pusclcri, 2'.) 
 ,, origin of, 2'.* 
 
 „ destruction of, 30 
 
 „ connection witli bilo 
 
 pigment, 31 
 Keflex actions, nature of detenninants 
 of, 10.) 
 „ dopeiulent on nature of 
 
 afferent impulses, 58G 
 ,, dependent on intensity 
 
 of stiiiiulus, 587 
 ,, varying with area of skin 
 
 stimulated, 588 
 ,, in the mammal, 586 
 
 ,, indicating choice, 589 
 
 ,, inhibition of, 500 
 
 ,, time taken up in, 593 
 
 Reflex inhibition of heart, 188 
 Regeneration of tissues and organs, 667 
 Reproduction, tissues and mechanisms 
 
 of, 667 
 Renal circulation in amphibia, 405 
 ,, epithelium, share in secretion of 
 
 urine, 4U4 
 ,, nerves, result of section of, 403 
 Rennet, action of, 246 
 Residual ah, 315 
 
 Resonant or Nasal consonants, 660, 661, 
 Respiration, nature of, 312 
 
 „ mechanisms of pulmonary, 
 
 314 
 rhythm of, 316 
 ,, Facial and Laryngeal, 324 
 
 ,, changes of air in, 326 
 
 „ chemical aspects of, 352 
 
 ,, nervous mechanism of, 353 
 
 , , effects of pneumogastric on, 
 
 355 
 ,, inhibition of, by superior 
 
 laryngeal nerve, 357 
 „ Cheyue-Stokes, 362 
 
 ,, effects of, on the circulation, 
 
 364 
 ,, influence on the pulse- wave 
 
 and median blood-pres- 
 sure, 365* 
 ,, effects of artificial, on blood- 
 
 pressure, 370 
 „ cutaneous, 386 
 
 ,, of new-born infant, 679 
 
 Respiratory centre, 354 
 
 ,, ,, theoiT of action of, 
 
 358 
 ,, ,, action of, modified by 
 
 various influences. 
 359 
 ,, ,, action of dependant 
 
 on condition of 
 blood, 300 
 ,, ,, influenced by loss of 
 
 oxygen, 362 
 Respiratory changes in the blood, 329 
 ,, in the lungs, 344 
 
782 
 
 INDEX. 
 
 Eespiratory changes in the tissues, 348 
 ,, movements, pressure exerted 
 
 by, 315 
 ,, „ recorded by 
 
 means of 
 
 Marey'sPneu- 
 matograph, 
 316* 
 „ ;, method of re- 
 
 cording, 316, 
 319* 
 ,, ,, characters of, 
 
 317, 319 
 ,, „ influencing pulse 
 
 rate, 371 
 modified, 382 
 East, influence of, on Nerve and Muscle, 
 
 97 
 Eetina, electric currents in, 519 
 ,, Photochemistry of, 515 
 ,, visual areas in the, 523 
 Eheometer of Ludwig, 124 
 "Eheoscopic frog," 62 
 Rhythm of respiration, 316 
 Ribs, action of, in inspiration, 321 
 Rigor Mortis, 64, 66 
 Rima respiratoria, 652 
 
 ,, vocalis, 652 
 Roots of spinal nerves, functions of, 482 
 Running, 664 
 Eutic acid, 736a 
 
 Saline matters in serum, 25 
 ,, „ uses of, 477 
 
 Saliva, properties of, 234 
 
 „ action of, on starch, 235 
 ,, parotid, 238 
 ,, submaxillary, 238 
 ,, sublingual, 238 
 „ mixed, 238 
 ,, secretion of, 256 
 ,, chorda and sympathetic, 262 
 Salivary secretion, nervous mechanism 
 of, 257 
 ,, ,, inhibition of, 259 
 
 J, ,, vascular changes dm'- 
 
 ing, 259 
 Salivary gland, histological changes 
 
 during activity of, 268 
 Salts as food, 455 
 Salts of uric acid, 750a 
 Sarcolactic acid, 741a 
 Sarkin, 754a 
 
 Scheiner's experiment, 494, 495* 
 Secretion, theory of the nature of, 275 
 „ by the skin, 384 
 
 ,, by the kidneys, 392 
 
 ,, of perspiration, 387 
 
 ,, of urine, 397 
 Semicircular canals, result of injury to, 
 
 614 
 Semilunar valves, 143 
 Sensations, visual, 511 
 
 ,, of pressure, 575 
 
 ,, of temperature, 576 
 
 Sense organs, 488 
 
 Sensible perspiration, 385 
 Sensory impulses, velocity of, 485 
 ,, cranial nerves, 484 
 ,, nerves, 481 
 
 ,, nerves, behaviour towards sti- 
 muli different from that of 
 motor nerves, 486 
 Sensory and motor nerves, differences 
 between, 106 
 ,, and motor nerves, union of, 
 487 
 Serum-albumin, 25 
 
 , , , , characters of 703a 
 
 Serous fluids, artificial coagulation of, 
 
 17 
 Serous glands, 268, 274* 
 Serum, composition of, 25 
 Sex, difference of bodily constants in, 
 
 688 
 Shock, 214 
 Sighing, 382 
 Sight, 490 
 
 Size and distance, judgment of, 549 
 Skatol, 769a 
 Skin, secretion by the, 384 
 
 , , absorption by, 390 
 Sleep, phenomena of, 691 
 
 ,, causes of, 692 
 Smell, 567 
 
 ,, olfactory nerve, and, 567 
 Sneezing, 383 
 Soaps, 739a 
 Sobbing, 383 
 Sodium glycocholate, 248 
 Sodium sulphindigotate, secretion of, by 
 
 epithelium of kidney, 406 
 Sodium taurocholate, 248 
 SoHdity, judgment of, 551 
 Soluble starch, 235 
 Sounds, judgment of direction of, 565 
 
 „ of the heart, 143, 144 
 Spectra of Hfemoglobin, 334* 
 Speech, 657 
 
 Spherical aberration, 507 
 Spinal cord, 585 
 
 ,, ,, of frog, evidences of choice 
 
 in, 589 
 ,, ,, automatic actions in, 595 
 ,, ,, tonic action of, 596 
 ,, „ conduction of impulses in, 
 
 598 
 ,, ,, relation of grey matter to 
 roots of spinal nerves, 599 
 ,, ,, decussation of sensory and 
 volitional impulses in, 
 601 
 ,, ,, sectional areas of the late- 
 ral, anterior and posterior 
 columns of the, 603* 
 ,, ,, paths in, as determined by 
 anatomical investigation, 
 600 
 ,, ,, transmission of impulses a- 
 long the grey matter, 
 lateral, anterior and pos- 
 terior columns, 602 
 
INDEX. 
 
 783 
 
 Spinnl OQid, vicarioiiH tinnsmiRsion of 
 impulses, CiO."! 
 ,, ,, li}i>t riestlii'siiv iiftc'i' Hc'utii)ii 
 
 ' of till', ('.(Mi 
 Spinal gnn^lin, functions of, -183 
 Spinal ncct'ssDiy mrvo, tJaO 
 Spinal nerves, function of roots, 182 
 
 ,, ,, sectional areas of, 599* 
 
 Spirometer, the, ."UO 
 
 Spleen, destruction of red corpusclea in 
 the, 30 
 ,, variations in size of, 432 
 , , curve, features of, -133* 
 ,, intluenco of nerves on, 433 
 ,, circulation in, -IS-l 
 ,, substances present in, 434 
 ,, presence of uric acid in, 434 
 Stannius' experiment, 187 
 Stapedius muscle, uses of, 559 
 Starch, action of saliva on, 235, 236 
 ,, action of gastric juice on, 240 
 ,, action of pancreatic juice on, 251, 
 254 
 Starvation, changes in bodv during, 444 
 Stasis, 220 
 Stationary' air, 314 
 Stearic acid, 736a 
 Stearin, 738rt 
 Stereoscope, the, 545 
 Stimulus, definition of, 37 
 Stimulus affecting muscular contraction, 
 
 83 
 Stomach, why it does not digest itself, 
 279 
 , , movements of, 285 
 
 ,, absorption from, 294 
 
 ,, gases in, 295 
 
 Submaxillary gland, nerves of, 257, 258* 
 ,, ,, action of chorda 
 
 tympani on, 259 
 ,, ,, action of cervical 
 
 s-vmpathetic on, 
 261 
 Succinic acid, 742rt 
 Succus entericus, 255 
 
 ,, ,, Thiry's method of 
 
 obtaining, 255 
 ,, ,, act of secretion of, 
 
 266 
 Sugar in saliva, 237 
 
 ,, action of succus entericus on cane, 
 
 255 
 ,, absorption of, from intestine, 310 
 Supplemental air, 315 
 Sympathetic and chorda saliva, 262 
 Sweat, characters of, 386 
 „ centres, 389 
 ,, nerves, course of, 390 
 
 Tactile judgments, 579 
 
 „ perceptions, 572 
 
 ,, sensations, 575 
 Taste, 570 
 Tam-m, 249, 756a 
 Taurocholic acid, 249, 763(j 
 Tears, 554 
 
 Tomporaturo, influence of, on irritability 
 (if nerve and niUKcle, 1)3 
 ,, normal eoiiHtant, of the 
 
 l)ody, 463 
 ,, of tiie body, regulated by 
 
 the skin, 464 
 ,, of the body, re;,'ulated by 
 
 variations in produc- 
 tion, 465 
 ,, sensations, 576 
 
 ,, and pressure sensations, 
 
 Rcparatc nerves and 
 nerve endings for, 577 
 Tendon phenomena, 596 
 Tension of oxygen in blood and lungs, 345 
 ,, of carbonic acid in blood and 
 lungs, 347 
 Tensor tympani muscle, uses of, 558 
 Tetanic contractions, 48, 50 
 Tetanus produced with ma^etic inter- 
 
 ruptor, 50* 
 Thermal changes in muscle during con- 
 traction, 70 
 Thinking and willing, time taken up by, 
 
 644 
 Tidal air, 314 
 Tissue-proteids, 452 
 Tissues, the fundamental, 5 
 Tissues and mechanisms of digestion, 233 
 ,. II ,, reproduc- 
 
 tion, 
 667 
 I, ,, „ of respi- 
 
 ration, 
 312 
 ,, respiratory changes in, 348 
 ,, absence of free oxygen from, 349 
 Tone of arteries, 200 
 Tonic action in spinal cord, 596 
 Touch, nerve endings essential to, 573 
 
 ,, field of, 580 
 Traube-Hering curves, 372, 373*, 374* 
 Tricrotic pulse, 166 
 Trigeminal nerve, 649 
 Trochlear on Pathetic nerve, 648 
 Trophic nerves, 471 
 Trj'iDsiu, 251 
 Trypsinogen, 271 
 Tryptic action, theories of nature of, 
 
 720a 
 Tunicin, 733a 
 Tyrosiu, 759a 
 
 ., product of pancreatic digestion, 
 252 
 
 Uustriated muscular tissue, properties 
 
 of, 101 
 Urari, effect of, on heart, 186 
 
 ,, poisoning, artificial diabetes, a 
 prominent symptom of, 426 
 Urea, composition of, 42s, 746a 
 ,, its relation to kreatiu, 436 
 ,, origin of, in body, 437 
 ,, ,, from leucin, 439 
 
 ,. secretion of, by epithelium of kid- 
 uev, 406 
 
784 
 
 INDEX. 
 
 Urea, nitrate of, 746a 
 ,, oxalate of, 747a 
 ,, detection in solutions, 747a. 
 ,, quantitative determination, 747a 
 Uric acid in the spleen, 434 
 „ origin in body, 440 
 „ , 749a 
 ,, salts of, 750a 
 Urine, composition of, 333 
 „ characters of, 393 
 ,, quantity of constituents passed 
 
 in 24 hours, 395 
 ,, acidity of, 395 
 ,, abnormal constituents of, 396 
 ,, secretion of, 397 
 ,, secretion, in relation to blood- 
 pressure, 398, 402 
 ,j „ effects of section of 
 
 spinal cord, and of 
 nerves upon, 403 
 ,, ,, by the renal epithe- 
 
 lium, 404 
 ,, incontinence of, 413 
 , , of infants, 686 
 ,, pigments, 767a 
 Urobilin, 767a 
 Uroerythrin, 767a 
 Uterus, movements of, 682 
 
 ,, changes in, after impregnation, 
 
 673 
 , , condition of, in menstruation ,670 
 
 Vagus nerve, 650 
 
 ,, stimulation, 195* 
 Valerianic acid, 735a 
 Valves, auriculo-veutricular, 142 
 
 ,, semilunar, 143 
 Vascular constriction and dilation, 199 
 J J ,, effects of local, 216 
 
 ,, factors, mutual relations of, 227 
 ,, mechanism, 115 
 Vaso-constrictor nerves, 210 
 Vaso-dilator nerves, 210 
 Vaso-motor actions, 197 
 
 ,, ,, nature of, 207 
 
 ,, centres, 212 
 
 ,, medullary centre, 213 
 
 ,, nerves, 199 
 
 ,, ,, course of, 212 
 
 ,, ,, of the veins, 215 
 
 Veins, as factors in circulation, 116 
 ,, blood-pressure in, 128 
 ,, velocity of blood in, 128 
 ,, vaso-motor nerves of the, 215 
 Velocity of Blood in arteries, how mea- 
 sured, 124 
 , ,, in various arteries, 
 
 127 
 „ ,, on what dependent, 
 
 124 
 „ „ in veins, 128 
 
 ,, ,, circumstances deter- 
 
 mining, 132 
 
 Venous blood, colour of, 338 
 
 ,, ,, nature of, 330 
 
 ,, pulse, 174 
 Vertigo, 617 
 
 Vibratory consonants, 660, 661 
 Vierordt's Hsmatachometer, 126 
 Vision, influence of yellow spot on, 531 
 
 ,, dimensions of field of, 535 
 
 ,, region of distinct, 535 
 
 „ irradiation in, 536 
 
 ,, contrast in, 537 
 
 ,, binocular, 541 
 
 ,, struggle of the two fields of, 552 
 Visual areas in the retina and in the 
 brain, 523 
 
 ,, ,, in the cerebral convohi- 
 tion, 631 
 Visual impulses, origin of, 511 
 Visual judgments, 549 
 Visual movements, co-ordination of, 545 
 Visual perceptions, 534 
 Visual purple, 517 
 Visual sensations, 511 
 
 ,, ,, duration of, 520 
 
 ,, ,, their relations to the 
 
 stimulus, 521 
 
 ,, „ fusion and distinction 
 
 of, 522 
 Visual white, 517 
 
 ,, yellow, 517 
 Vital phenomena of the circulation, 176 
 VitelUn, character of, 712a 
 Vocal cords, 652 
 
 , , , , how tightened, 654 
 
 ,, ,, ,, slackened, 654 
 Voice, 651 
 
 ,, characters of, on what dependent, 
 651 
 
 ,, breaking of, 656 
 Volkmann's Haemadromometer, 124 
 Vomiting, 290 
 Vowels, nature of, 657 
 
 Walking, 663 
 
 Wallerian Method, 484 
 
 Weber's law, 521 
 
 Weight of organs of the body, 684 
 
 Whispering, 661 
 
 White corpuscles, composition of, 27 
 
 ,, ,, proportion to red cor- 
 
 puscles, 29 
 
 ,, ,, origin of, 31 
 
 ., ,, use of, 31 
 
 Xanthin, 754a 
 
 Yawning, 382 
 
 Yellow spot, influence of, on vision, 531 
 
 Young-Helmholtz,theory of colourvision, 
 
 528 
 
 Zymogen, 271 
 
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