HAND-BOOK OF PHYSIOLOGY. NEW EDITIONS. THE ? QUIZ-COMPENDS ? A NEW SERIES OF MANUALS FOR THE USE OF STUDENTS AND PHYSICIANS. Price of each, Cloth, $1.00. Interleaved, for taking Notes, $i 25. These Compends are based on the most popular text books, and the lectures of prominent professors, and are kept constantly revised, so that they may thoroughly represent the present state of the subjects upon which they treat. The Authors have had large experience as Quiz Masters and attaches of col- leges, and are well acquainted with the wants of students. They are arranged in the most approved form, thorough and concise, containing 231 illustrations, inserted wherever they could be used to advantage. Jt£g~ Can be used by students of any college. Xfca* They contain information nowhere else collected in such a condensed, practical shape. . . . 1 Size is such that they may be easily carried in the pocket, and the price is low. Jtktr They will be found very serviceable to physicians as remembrancers. LIST OF VOLUMES. No. 1. HUMAN ANATOMY. Fourth Revised and Enlarged Edition. Including Visceral Anatomy, formerly published separately. 117 Illustrations. By Samuel O. L. Potter, m.d., Professor of the Practice of Medicine, Cooper Medical College, San Francisco, late A. A. Surgeon, U. S. Army. Revised and enlarged. Index. No. 2. PRACTICE OF MEDICINE. Part I. Third Edition. Revised and Enlarged. By Dan'l E. Hughes, m.d., Demonstrator of Clinical Medicine, Jef- ferson College, Philadelphia. No. 3 PRACTICE OF MEDICINE. Part II. Third Edition. Revised and Enlarged Same author as No 2. No. 4. PHYSIOLOGY. Fourth Edition, with Illustrations and a table of Physiological Constants. Enlarged and R evised. By A. P. Brubaker, m.d., Professor of Phys- iology and General Pathology in the Pennsylvania College of Dental Surgery; Demonstrator of Fhysiology, Jeiferson Medical College, Philadelphia. Index. No. 5. OBSTETRICS. Third Edition. Enlarged. By Henry G. Landis, m.d., Professor of Obstetrics and Diseases of Women and Children, Starling Medical College, Columbus, Ohio. Illustrated. No. 6. MATERIA MEDICA, THERAPEUTICS, AND PRESCRIPTION WRITING. Fifth Revised Edition. Enlarged. By Samuel O. L. Potter, m.d.. Professor of Practice, Cooper Medical College, San Francisco ; late A. A. Surgeon U. S. Army. Index. No. 7. GYNAECOLOGY. A Compend of Diseases of Women. By Henry Morris, m.d , Demonstrator of Obstetrics, Jefferson Medical College, Philadelphia. No. 8. DISEASES OF THE EYE, AND REFRACTION, including Treatment find Surgery. By L. Webster Fox, m.d , Chief Clinical Assistant. Ophtbalmo- logical Department, Jefferson Medical College Hospital, and George M. Gould, a.b. With 71 Illustrations Second Edition. No. 9. SURGERY. Third Edition. Enlarged and Improved. By Orville Hor- witz, bs., m.d , Demonstrator of Anatomy, Jefferson College, Chief of the Out- Patient Surgical Department, Jefferson College Hospital, late Resident Physician Pennsylvania Hospital, Philadelphia. With many Formulae and 90 Illustrations. Index. No. 10. CHEMISTRY. Inorganic and Organic. For Medical and Dental Students. By Henry Leffmann, m.d.. Professor of Chemistry in Penn'a College of Dental Surgery, Phila. No. 11. PHARMACY. Second Edition. Based upon Prof. Remington's Text-book of Pharmacy. By F. E. Stewart, m. d., ph. g., Quiz Master in Pharmacy and Chemistry, Philadelphia College of Pharmacy; Lecturer at the Medico-Chirurgical College, and Woman's Medical College, Philadelphia. Second Edition, carefully revised. Others in preparation. Price, each. Cloth, $1.00. Interleaved, for taking Notes, $1.25- P. BLAKISTON, SON & CO., Medical Publishers and Booksellers. 1012 WALNUT STREET, PHILADELPHIA. KI IKES' HAND-BOOK OF PHYSIOLOGY. HAND-BOOK OF P H Y S10 L 0 G Y. BY W. MORRANT BAKJ1R, F.R.C.S., SURGEON TO ST. BARTHOLOMEW'S HOSPITAL; MEMBER OF THE COURT OF EXAMINERS OF THE ROYAL COLLEGE OF SURGEONS EXAMINER IN SURGERY IN THE UNIVERSITY OF LONDON AND AT THE ROYAL COLLEGE OF PHYSICIANS ; LATE LECTURER ON PHYSIOLOGY AT ST BARTHOLOMEW'S HOSPITAL. AND VINCENT DORMER HARRIS, M.D. Lond., FELLOW OF THE ROYAL COLLEGE OF PHYSICIANS; EXAMINER IN ELEMENTARY PHYSIOLOGY AT THE CONJOINT BOARD OF THE ROYAL COLLEGES OF PHYSICIANS AND SURGEONS; DEMONSTRATOR OF PHYSIOLOGY AT ST. BARTHOLOMEW'S HOSPITAL; PHYSICIAN TO THE VICTORIA PARK HOSPITAL FOR DISEASES OF THE CHEST. TWELFTH EDITION. REARRANGED, REVISED AND REWRITTEN, AND WITH FIVE HUNDRED ILLUSTRATIONS. PHILADELPHIA: P. BLAKISTON, SON & CO., 1012 WALNUT STREET. 1889. PREFACE TO THE TWELFTH EDITION. It has been found necessary to make a considerable number of alterations in the Twelfth Edition of Kirkes' Physiology. Many of these have been in the earlier chapters, and in the sections on The Blood, The Heart, and The Mus- cular System; while the chapters on The Nervous System, on The Reproductive Organs, and on Development, have been re arranged and to a great extent re-written. About fifty new illustrations have been added. In those chapters which treat of the subjects of which the junior student is expected to exhibit a knowledge at his first examination, some details which may be omitted on first reading \\qnq been printed in smaller type; they must not, however, be con- sidered for this reason to be unimportant. Without attempting to enumerate all the important text-books on Physiology and monographs on physiological subjects of which use has been made in preparing the present edition, and to the authors of which we beg to record our obligations, we would mention especially those of Drs. Gaskell, Gowers, Halliburton, and Wooldridge; Landois and Stirling's Text book, and the works of the late Prof. F. M. Balfour. Dr. Gowers has kindly allowed us to copy several of the diagrams from his works on the Nervous System. VI PREFACE. Our thanks are due to Dr. T. W. Shore, who has kindly helped us in revising and seeing through the press certain sections, particularly those relating to biological questions. Mr. Daniellson has undertaken, as in the two previous editions, the drawings upon wood and the engraving of all the new illustrations, and has carried out the work with much skill. W. MORRANT BAKER. VINCENT D. HARRIS. In the preparation of the present edition it seems only right to state, that while I am responsible with my colleague, Dr. Vincent D. Harris, for the general supervision of the work in its passage through the press, he has undertaken the labour of investigation and the arrangement of the details. Many parts, moreover, he has re-written. And to him has fallen in chief part the difficult task of selecting from the many new facts and observations which have been published within the last few years such as can fitly find a place in a handbook for students. W. MORRANT BAKER. September, 1888. CONTENTS. CHAPTER I. PAGE The Phenomena of Life i CHAPTER II. The Structure of the Elementary Tissues . . . . 16 Cells il). Nucleus . . 18 Intercellular Substance . . . . *1 C«E®1V. . 10 Fibres Tubules ' ( • • • * ?7a Epithelium k N. . . \ . 21 Connective Tissues . . . X • • • 33 The Fibrous Tissues . . . , 36 Cartilage A Y 45 Bone r-~-.- . . 49 The Blood 65 Quantity of Blood Coagulation of the Blood 67 Conditions affecting Coagulation ....... 75 The Blood-Corpuscles ......... 79 Physical and Chemical Characters of Red Blood-Cells . . . 80 The White Corpuscles, or Blood-Leucocytes 83 Chemical Composition of the Blood 87 The Serum 88 Gases contained in the Blood 92 Blood-Crystals 9- Derivatives of Haemoglobin 97 Development of the Blood 102 Uses of the Blood I06 CHAPTER III. VIII CONTENTS. PAGE Circulation op the Blood106 The Systemic, Pulmonary, and Portal Circulations . . . 108 The Heart109 Structure of the Heart and its Valves . . . . • • 109 Stucture of the Arteries, Capillaries, and Veins . . 118 Physiology of the Heart . . I31 Physiology of the Arteries152 Physiology of the Capillaries174 Physiology of the Veins178 Velocity of the Circulation 180 Velocity of the Blood in the Arteries181 „ „ „ „ Capillaries182 „ „ „ „ Veins183 Velocity of the Circulation as a whole . . . • . . . ib. Peculiarities of the Circulation in Different parts . 184 Circulation in the Brain . . . . . . . . . iJ. Circulation in the Erectile Structures 186 Agents concerned in the Circulation 188 Discovery of the Circulation ib. Proofs of the Circulation of the Blood . . . . . . 189 CHAPTER IV. Respiration190 Position and Structure of the Lungs . . . . . . . 191 Structure of the Trachea and Bronchial Tubes .... 192 Structure of the Lungs and Pleura . . . . ' . . . 198 Mechanism of Respiration . . . . . . . . 202 Respiratory Movements 203 Quantity of Air respired . 210 Vital or Respiratory Capacityib. Force exerted in Respiration . . . . . . . .212 Changes of the Air in Respiration213 Changes produced in the Blood by Respiration . . . .219 Mechanism of various Respiratory Actions . . . . 220 Influence of the Nervous System in Respiration .... 224 Effects of Vitiated Air-Ventilation •. •227 Effect of Respiration on the Circulation 228 Apnoea-Dyspnoea-Asphyxia . . . . . . . . 233 CHAPTER V. CONTENTS. IX CHAPTER VI. PAGE Food and Diet 237 Classification of Foods239 Foods containing chiefly Nitrogenous Bodies .... ib. „ „ „ Carbohydrate Bodies . . . . 242 >, fI „• Fatty Bodies . .... ib. Substances supplying the Saltsib. Liquid Food243 Effects of Cookingib. ■ Effects of an Insufficient Diet . . •244 Starvation246 Effects of Improper Food247 Effects of too much Food248 Diet Scale 249 CHAPTER VII. Digestion Passage of Food through the Alimentary Canal . . ib. Mastication . . . . . . . . . . ~. ib. The Teeth . . '/j. Insalivation 263 The Salivary Glands and the Salivaib. Structure of the Salivary Glandsib. The Saliva . 267 Influence of the Nervous System on the Secretion of Saliva . . 270 The Pharynx 275 The Tonsils276 The (Esophagus or Gullet277 Swallowing or Deglutition279 Digestion of Food in the Stomach 280 Structure of the Stomach281 Gastric Glands284 The Gastric Juice287 Functions of the Gastric Juice288 Movements of the Stomach . . . . . . . . 291 Influence of the Nervous System on Gastric Digestion . . 293 Digestion of the Stomach after Death294 Vomiting Digestion in the Intestines297 Structure of the Small Intestine . •. . . • . . . ib. Structure of the Large Intestine304 X CONTENTS. PAGE Digestion in the Intestines-continued. The Pancreas and its Secretion 309 Structure and Functions of the Liver313 The Bile . . . . . . . . . . . . 320 The Liver as a Blood-elaborating Organ328 Succus Entericus332 Summary of the Changes which take place in the Food during its Passage through the Small Intestine 333 Summary of the Process of Digestion in the Large Intestine . . 336 Movements of the Intestines337 Influence of the Nervous System on Intestinal Digestion . . 338 Defaecation . 339 Gases contained in the Stomach and Intestines . . . . 341 CHAPTER VIII. Absorption341 The Lacteal and Lymphatic Vessels and Glands . . . . ib. Properties of Lymph and Chyle353 Absorption by the Lacteal Vessels 355 Absorption by the Lymphatic Vessels ib. Absorption by Blood-vessels . . . . . . . . ib. CHAPTER IX. Animal Heat361 Variations in Bodily Temperature . . . . . . . ib. Sources of Heat364 Loss of Heat365 Production of Heat368 Inhibitory Heat-centre370 CHAPTER X. Secretion371 The Structure and Functions of the Skin . . . . 388 CHAPTER XI. CHAPTER XII. The Structure and Functions of the Kidneys . . . 402 Structure of the Kidneysib. Structure of the Ureter and Urinary Bladder . . . .410 The Urine412 Micturition431 CONTENTS. XI PAGE Che Vascular Glands 436 CHAPTER XIII. Che Muscular System 449 Causes and Phenomena of Motion ib. Plain or Unstriped Muscle 449 Striated Muscle 450 Chemical composition of Muscle 456 Physiology of Muscle at rest 459 „ „ in activity 462 Rigor Mortis 477 Actions of the Voluntary Muscles 479 „ „ Involuntary Muscles . . . . . . 484 Electrical Currents in Nerves 485 CHAPTER XIV. Futrition : The Income and Expenditure of the Human Body 490 Nitrogenous Equilibrium and Formation of Fat .... 494 CHAPTER XV. CHAPTER XVI. he Voice and Speech 496 CHAPTER XVII. he Nervous System 511 Elementary Structures of the Nervous System ib. Functions of Nerve Fibres 519 Laws of Conduction in Nerve-Fibres 520 Functions of Nerve-Centres 529 CHAPTER XVIII. 5REBR0-SPINAL NERVOUS SYSTEM 535 The Spinal Cord and its Nerves ib. Functions of the Spinal Cord , . 544 XII CONTEXTS. PAGE Cerebro-Spinal Nervous System-continued. The Medulla Oblongata 556 Structure and Distribution of the Fibres of the Medulla Oblongata ib, Functions of the Medulla Oblongata . . . ... 560 Pons Varolii ........... 564 Crnra Cerebri 565 Corpora Quadrigemina 568 The Cerebrum 568 Structure of the Cerebrum 569 Functions of the Cerebrum . 578 The Cerebellum 592 Structure and Functions of the Cerebellum . . . . . 592 CHAPTER XIX. Physiology of the Cranial Nerves 598 CHAPTER XX. The Senses 616 The Sense of Touch 620 The Sense of Taste 627 The Sense of Smell 635 The Sense of Hearing ......... 640 The Sense of Sight . • 660 CHAPTER XXI. The Sympathetic Nervous System 707 CHAPTER XXII. The Reproductive Organs 716 CHAPTER XXIII. Development 740 The Changes in the Ovumib. Development of Organs 766 CHAPTER XXIV. Cn the Relation of Life to other Forces 809 CONTEXTS. XIII APPENDIX. PAGE The Chemical Basis of the Human Body 827 APPENDIX B : Anatomical Weights and Measures848 Measures of Weight „ „ Length?7-. Sizes of various Histological Elements and Tissues . . . . 849 Metrical System of Weights and Measures compared with the Common Measures/j Classification of the Animal Kingdom .... 850 INDEX 853 •Table for converting Degrees of the FAHRENHEIT Ther- MEASUREMENTS. FRENCH INTO ENGLISH. mometer Scale CENTIGRADE. into Degrees Fahrenheit. 500° Centigrade. 260° LENGTH. 205 392 383 374 200 1 m&tre ) 195 10 decimetres 100 centimetres = 39'37 English 190 inches (or 1 yard and in.) 180 347 338 329 320 3" 302 284 275 175 170 165 1,000 millimetres 1 decimetre 160 ■ = 3'937 inches (nr nearly 4 inches) 155 150 10 centimetres 100 millimetres 140 135 1 centimetre 130 - = '3937 or about (nearly g inch.) 248 239 230 212 120 10 millimetres 115 110 1 millimetre = nearly a inch. 100 203 194 176 167 140 122 95 90 80 CAPACITY. 75 1,000 cubic decimetres 1,000,000 cubic centimetres 60 - = 1 cubic metre 50 45 "3 105 104 40'54 1 cubic decimetre 40 - 1 litre 37-8 or or 1,000 cubic centimetres (35i fluid oz., 983 95 36-9 35 or rather less than an English quart) 77 68 25 WEIGHT. 20 50 4i 32 23 14 + 5 - 4 -13 - 22 10 1 gramme 'i 5 10 decigrammes 100 centigrammes 1,000 milligrammes . = 15'432349 grs. (or nearly rs$) Zero O - 5 -10 - 15 -20 1 decigramme 10 centigrammes -25 > = rather more -30 -40 -40 -60 100 milligrammes than ii grain - 76 1 centigramme 10 decigrammes I degree Fahr. 1'8 n ,, 3'6 „ » = -54° C. = 1° C. = 2° C. = rather more than a grain 4-5 „ „ = 2'5° C. 5'4 » » = 3° C. • Modified from Townes' Chemistry. 1 milligramme = rather more than 20o grain XIV Measure of i decimetre, or io centimetres, or 100 millimetres. 12345 6 7 8 9 io EPIGASTRIC LUMBAR UMBILICAL LUMBAR Highest point of Crest of the Ilium. HYPOCASTRiC Anterior Su- perior Spine of the Hium. Symphysis Pubis. DIAGRAM OF THORACIC AND ABDOMINAL REGIONS. A. Aortic Valve. J A Mitral Valve. P. Pulmonary Valve. 2'. Tricuspid Valve. Cranium. 7 Cervical Vertebrae. Clavicle. Scapula. 12 Dorsal Vertebra?. Humerus. 5 Lumbar Vertebrae. Ilium. Ulnar. Radius. Pelvis. Bones of the Carpus. Bones of the Meta- carpus. Phalanges of Fingers. Femur. Patella. Tibia. Fibula. Bones of the Tarsus. Bones of the Meta- tarsus. Phalanges of Toes. iseliium J?n1>c» THE SKELETON (after Holden). HANDBOOK OF PHYSIOLOGY. CHAPTER I. THE PHENOMENA OF LIFE. Human physiology is that part of animal physiology which treats of man-of the way in which he lives and moves and has his being. It teaches how man is begotten and born ; how he attains maturity, and how he dies. As, however, man is a member of the animal kingdom, although separated and specialised no doubt to a remarkable degree, he during life manifests certain characteristics-possesses certain pro- perties and performs certain functions-in common with all living animals, even the very lowest, and these may be called essentials of animal life. If we go a step further we find that most of these characteristic properties and functions are possessed also by the very lowest vegetable structures, and are in fact the characters by which we distinguish living from not-living matter; they are essentials or phenomena of life in general. Thus we see that as human physiology, which treats of man only, is a part of animal physiology, which treats of the functions and organisation of animals in general, so is animal physiology but a part of the wider science of Biology, which embraces the organisation and manifestations of all living things. Before entering upon the study of Human physiology, therefore, it is useful and even necessary to devote our attention for a little while to the investigation of what are the properties and functions common to all living matter, and how they are manifested, since it would be unwise to attempt to comprehend the working of the complex machine of the life of man without some knowledge of the motive power in its simplest form. 2 THE PHENOMENA OF LIFE. [chap. I. Living matter, in its most elementary form, is found to consist of a jelly-like substance which is now generally known under the name of Protoplasm. This substance, in its most primitive form, and in minute masses, is found undifferentiated and perfectly homogeneous, and constitutes the lowest types both of animal and vegetable life that can be observed under the microscope. Tt is this substance, too, which forms the cells, of which even the most complex organism lias been proved to be made up and from which it has been de- veloped. Thus, the human body can be shown by dissection to consist of various dissimilar parts, bones, muscles, brain, heart, lungs, intestines, &c., and these on more minute examination are found to be composed of different tissues, such as epithelial, connec- tive, nervous, muscular, and the like. Each of these tissues is made up of cells or of their altered equivalents. Again, we are taught by Embryology, the science which treats of the growth and struc- ture of organisms from their first coming into being, that the human body, made up of all these dissimilar structures, com- menced its life as a minute cell or ovum about -j-joth of an inch in diameter, consisting of a spherical mass of protoplasm in the midst of which was contained a smaller spherical body or germinal vesicle. The phenomena of life then are exhibited in cells, whether existing alone or developed into the organs and tissues of animals and plants. It must be at once evident, therefore, that a correct knowledge of the nature and activities of the cell, forms the very foundation of physiology. Cells are, in fact, physiological no less than morphological units. The prime importance of the ceil as an element of structure was first established by the researches of Schleiden, and his conclusions, drawn from the study of vegetable histology, were at once extended by Schwann to the animal kingdom. The earlier observers defined a cell as a more or less spherical body limited by a membrane, and containing a smaller body termed a nucleus, which in its turn encloses one or more nucleoli. Such a definition applied admirably to most vegetable cells, but the more extended investigation of animal tissues soon showed that in many cases no limiting membrane or cell-wall could be demonstrated. The presence or absence of a cell-wall, therefore, was now regarded as quite a secondary matter, while at the same time the cell-substance came gradually to be recognised as of primary importance. Many of the lower forms of animal life, e.<j., the Rhizopoda, were found to consist almost entirely of matter very similar in appearance and chemical composition to the cell-substance of higher forms : and this from its chemical resemblance to flesh was termed Sarcode by Dujardin. When recognised in vegetable CHAP. I.] THE PROPERTIES OF PROTOPLASM. 3 cells it was called Protoplasm by Mulder, while Ueinak applied the same name to the substance of animal cells. As the presumed formative matter in animal tissues it was termed Blastema, and in the belief that, wherever found, it alone of all substances has to do with generation and nutrition, Beale has named it Germinal matter or Bioplasm. Of these terms the one most in vogue at the present day, as we have already said, is Protoplasm, and inasmuch as all life, both in the animal and vegetable kingdoms, is associated with protoplasm, we are justified in describing it, with Huxley, as the " physical basis of life," or simply " living matter." A cell may now be defined as a nucleated mass of protoplasm,* of microscopic size, which possesses sufficient individuality to have a life-history of its own. Each cell goes through the same cycle of changes as the whole organism, though doubtless in a much shorter time. Beginning with its origin from some pre- existing cell, it grows, produces other cells, and finally dies. It is true that several lower forms of life consist of non-nucleated protoplasm, but the above definition holds good for all the higher plants and animals. Hence a summary of the manifestations of cell life is really an account of the vital activities of protoplasm. Protoplasm.-Physically, protoplasm is viscid, varying from a semi-fluid to a strongly coherent consistency. Chemically, living protoplasm is an extremely unstable albuminoid substance, insoluble in water. It is neutral or weakly alkaline in reaction. It undergoes heat stiffening or coagulation at about 130° F. (54'5° C.), and hence no organism can live when its own tempera- ture is raised beyond this point. Many, of course, can exist for a time in a much hotter atmosphere, since they possess the means of regulating their own temperature. Besides the coagulation produced by heat, protoplasm is coagulated and therefore killed by all the reagents which pro- duce this change in albumen (see Appendix). If protoplasm be subjected to chemical analysis, the chief substances of which it is found to consist belong to the class of bodies called Proteids or albumins. These are bodies made up of the chemical elements C. H. N. 0. and S., in certain slightly varying propor- tions. They are essential to the formation of protoplasm, for * In the human body the cells range from the red blood-cell in.) to the ganglion-cell in.). 4 THE PHENOMENA OF LIFE. [chap. i. without one or more of them, protoplasm cannot exist. Indeed some would put this still more shortly, and say that protoplasm is living proteid. Associated with proteids as an essential, is a certain amount of water ; but there are other bodies, non-essential, frequently present, and varying under different circumstances ; such as glycogen, starch, cellulose, chlorophyll, fats, and the like. The protoplasmic substance of cells may undergo more or less essential modifications ; thus, in fat cells we may have oil, or fatty crystals, occu- pying nearly the whole cell: in pigment cells we find granules of pigment; in the various gland cells the elements of their secretions. Moreover, the original protoplasmic contents of the cell may undergo a gradual chemical change with advancing age ; thus the protoplasmic cell-substance of the deeper layer's of the epidermis becomes gradually converted into keratin as the cell approaches the surface. So, too, the original protoplasm of the embryonic blood-cells is infiltrated with the haemoglobin of the mature coloured blood- corpuscle. The vital or physiological characters of protoplasm are seen in the performance of its functions. Many of these qualities are exceed- ingly well illustrated in the microscopic animal called the Amoeba, which is a monocellular organism found chiefly in fresh water, but also in the sea and in damp earth. Under the same term no doubt more than one kind of organism is included, but at any rate in each most of the vital properties of protoplasm may well be studied. They are as follows :- 1. The power of spontaneous movement.--When an amoeba is observed with a sufficiently high power of the microscope, it is found to consist of an irregular mass of protoplasm distinguished into an outer dense layer and an inner more fluid mass. If watched for a minute or two an irregular projection or pseudo- ptodium is seen to be gradually thrust out from the main body and retracted : a second mass is then protruded in another direction, and gradually the whole protoplasmic substance is, as it were, drawn into it. The Amoeba thus comes to occupy a new position, and when this is repeated several times we have locomotion in a definite direction, together with a continual change of form. These movements, when ob- served in other cells, such as the colourless blood-corpuscles of Fig. i.-Amoeba?. CHAP. I.] AMCEBOID MOVEMENT. 5 higher animals (fig. 2), in the branched cornea cells of the frog and elsewhere, are hence termed amoeboid. Fig. 2.-Human colourless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.) Other illustrations of amoeboid movement.-The remarkable motions of pigment-granules observed in the branched pigment-cells of the frog's skin by Lister are probably due to amoeboid movement. These granules are seen at one time distributed uniformly through the body and branched pro- cesses of the cell, while under the action of various stimuli (e.g., light and electricity) they collect in the central mass, leaving the branches quite colourless. Ciliary action must be regarded as only a special variety of the general motion with which all protoplasm is endowed. The grounds for this view are the following: In the case of the Infusoria* which move by the vibration of cilia (microscopic hair-like processes pro- jecting from the surface of their bodies) it has been proved that these are simply processes of their protoplasm protruding through pores of the invest- ing membrane, like the oars of a galley, or the head and legs of a tortoise from its shell: certain reagents cause them to be partially retracted. More- over, in some cases cilia have been observed to develop from, and in others to be transformed into, amoeboid processes. In the hairs of the stinging-nettle and Tradescantia and the cells of Vallisneria and Chara, the movement of protoplasm can be marked by the movement of the granules nearly always imbedded in it. For example, if part of a hair of Trade- scantia (fig. 3) be viewed under a high magnifying power, streams of protoplasm containing crowds of granules hurrying along, like the foot- passengers in a busy street, are seen flowing steadily in definite directions, some coursing round the film which lines the interior of the cell- wall, and others flowing towards or away from the irregular mass in the centre of the cell-cavity. Many of these streams of proto- I'1#- 3--Veil of Tradescantia drawn at success'"* intervals of two minutes.-The cell-contents consist of a central mass connected hy many irregular processes to a peripheral film: the whole forms a vacuolated mass of proto- ship".' (sSeid.)continually chansw 6 THE PHENOMENA OF LIFE. [chap. 1. plasm run together into larger ones, and are lost in the central mass, and thus ceaseless variations of form are produced. 2. Irritability and the power of response to stimuli.-Although the movements of the amoeba have been described above as spontaneous, yet they may be increased under the action of various stimuli, and if the movement have ceased for a time, as is the case if the temperature be lowered beyond a certain point, it may be set up by raising the temperature. Again, contact with foreign bodies, gentle pressure, certain salts, and electricity, if applied to the amoeba, produce or increase the move- ment. It is, therefore, sensitive or irritable to stimuli, and shows its irritability by movement or contraction of its mass. The effects of some of these stimuli may be thus further detailed :- 1. Changes of temperature.-Moderate heat acts as a stimulant : this is readily observed in the activity of the movements of a human colourless blood-corpuscle when placed under conditions in which its normal tempera- ture and moisture are preserved. Extremes of heat and cold stop the motions entirely. 2. Mechanical stimuli.-When gently squeezed between a cover and object- glass under proper conditions, a colourless blood-corpuscle is stimulated to active amoeboid movement. 3. Nerve influence.-By stimulation of the nerves of the frog's cornea, contraction of certain of its branched cells has been produced. 4. Chemical stimuli.-Water generally stops amoeboid movement, and by imbibition causes great swelling and finally bursting of the cells. In some cases, however (myxomycetes) protoplasm can be almost entirely dried up, and is yet capable of renewing its motions when again moistened. Dilute salt-solution and many dilute acids and alkalies, stimulate the move- ments temporarily. Ciliary movement is suspended in an atmosphere of hydrogen or carbonic acid, and resumed on the admission of air or oxygen. 5. dectrical.-Weak currents stimulate the movement, while strong currents cause the corpuscles to assume a spherical form and become motion- less. 3. Nutritive powers.-The power of taking in food, modifying it, building up tissue by assimilating it, and rejecting what is not assimilated. All these processes take place in the amoeba. They are effected by its simply flowing round and enclosing within itself minute organisms such as diatoms and the like, from which it extracts what it requires, and then rejects or excretes the re- mainder, which has never formed part of the body, by with- drawing itself from it. The assimilation which goes on in the body of the amoeba, is to replace waste of its tissue consequent upon manifestation of energy. CHAP. I.] WASTE AND REPAIR. 7 The two processes of waste and repair, then, go on side by side, and as long as they are equal the size of the animal remains stationary. If, however, the building up exceed the waste, then the animal grows; if the waste exceed the repair, the animal decays; and if decay go on beyond a certain point, life becomes impossible, so the animal dies. Growth, or inherent power of increasing in size, although essential to our idea of life, is not confined to living beings. A crystal of common salt, or of any other similar substance, if placed under appropriate conditions for obtaining fresh material, will grow in a fashion as definitely charac- teristic and as easily to be foretold as that of a living creature. It is, there- fore, necessary to explain the distinctions which exist in this respect between living and lifeless structures ; for the manner of growth in the two cases is widely different. Differences between living and lifeless growth.-(i.) The growth of a crystal, to use the same example as before, takes place merely by additions to its outside ; the new matter is laid on particle by particle, and layer by layer, and, when once laid on, it remains unchanged. In a living structure, on the other hand, as, for example, a brain or a muscle, where growth occurs, it is by addition of new matter, not to the surface only, but through- out every part of the mass. (2.) All living structures are subject to constant decay ; and life consists not, as once supposed, in the power of preventing this never-ceasing decay, but rather in making up for the loss attendant on it by never-ceasing repair. Thus, a man's body is not composed of exactly the same particles day after day, although to all intents he remains the same individual. Almost every part is changed by degrees ; but the change is so gradual, and the renewal of that which is lost so exact, that no difference may be noticed, except at long intervals of time. A lifeless structure, as a crystal, is subject to no such laws; neither decay nor repair is a necessary condition of its existence. That which is true of structures which never had to do with life is true also with respect to those which, though they are formed by living are not themselves alive. Thus, an oyster-shell is formed by the living animal which it encloses, but it is as lifeless as any other mass of inorganic matter; and in accordance with this circumstance its growth takes place layer by layer, and it is not subject to the constant decay and reconstruction which belong to the living. The hair and nails are examples of the same fact. (3.) In connection with the growth of lifeless masses there is no alteration in the chemical composition of the material which is taken up and added to the previously existing mass. For example, when a crystal of common salt grows on being placed in a fluid which contains the same material, the properties of the salt are not changed by being taken out of the liquid by the crystal and added to its surface in a solid form. But the case is essen- tially different in living beings, both animal and vegetable. A plant, like a crystal, can only grow when fresh material is presented to it; and this is absorbed by its leaves and roots ; and animals for the same purpose of get- ting new matter for growth and nutrition, take food into their stomachs. But in both these cases the materials are much altered before they are finally assimilated by the structures they are destined to nourish. 8 THE PHENOMENA OF LIFE. [chap. I. (4.) The growth of all living things has a definite limit, and the law which governs this limitation of increase in size is so invariable that we should be as much astonished to find an individual plant or animal without limit as to growth as without limit to life. 4. Reproductive powers.-The amoeba, to return to our former illustration, when the growth of its protoplasm has reached a certain point, manifests the power of reproduction, by splitting up into (or in some other way producing) two or more parts, each of which is capable of independent existence. The new amoeba) manifest the same properties as their parent, perform the same functions, grow and reproduce in their turn. This cycle of life is being continually passed through. In more complicated structures than the amoeba, the life of Fig. 4.-Diagram of an ovum («) undergoing segmentation.-In (6) it has divided into two ; in (c) into four ; and in (d) the process has ended in the production of the so-called "mulberry mass." (Frey-.) individual protoplasmic cells is probably very short in comparison with that of the organism they compose: and their constant decay and death necessitate constant reproduction. The mode in which this takes place has long been the subject of great controversy. It is now very generally believed that every cell is descended from some pre-existing (mother-) cell. This derivation of cells from cells takes place by (i) gemmation, or (2) fission or division. (1) Gemmation.-This method has not been observed in the human body or the higher animals, and therefore requires but a passing notice. It consists essentially in the budding off and separating of a portion of the parent cell. (2) Fission or Division.-As examples of reproduction by fission, we may select the ovum, the blood cell, and cartilage cells. In the frog's ovum (in which the process can be most readily observed) after fertilization has taken place, there is first some amoeboid movement, the oscillation gradually increasing until a permanent dimple appears, which gradually extends into a furrow running completely round the spherical ovum, and deepening until the entire yelk-mass is divided into two hemispheres of CHAP. I.] ORIGIN OF CELLS. 9 protoplasm each containing a nucleus (fig. 4, 6). This process being repeated by the formation of a second furrow at right angles to the first, we have four cells produced (c): this subdivision is carried on till the ovum has been divided by segmentation into a mass of cells (mulberry-mass) (<Z) out of which the embryo is developed. Segmentation is the first step in the development of all the higher animals, including man. Multiplication by fission has been observed in the colourless blood-cells of many animals. In some cases (fig. 5), the process Fig. 5.-Blood-corpuscle from a young deer embryo, multiplying by fission. (Frey.) has been seen to commence with the nucleolus which divides within the nucleus. The nucleus then elongates, and soon a well- marked constriction occurs, rendering it hour-glass shaped, till finally it is separated into two parts, which gradually recede from each other: the same process is repeated in the cell-substance, and at length we have two cells produced which by rapid growth soon attain the size of the parent cell (direct division). Tn some Fig. 6.-Diagram of a cartilage cell undergoing fusion within its capsule.-The process of division is represented as commencing in the nucleolus, extending to the nucleus, and at length involving the body of the cell. (Frey.) cases there is a primary fission into three instead of the usual two cells. In cartilage (fig. 6), a process essentially similar occurs, with the exception that (as in the ovum) the cells produced by fission remain in the original capsule, and in their turn undergo division, so that a large number of cells are sometimes observed within a common envelope. This process of fission within a capsule has 10 THE PHENOMENA OF LIFE. [chap. 1. been by some described as a separate method, under the title "endogenous fission," but there seems to be no sufficient reason for drawing such a distinction. It is important to observe that fission is often accomplished with great rapidity, the whole process occupying but a few minutes, hence the comparative rarity with which cells are seen in the act of dividing. Indirect cell division.-In certain and numerous cases the division of cells does not take place by the simple constriction of their nuclei and surround- ing protoplasm into two parts as above described (direct division), but is preceded by complicated changes in their nuclei (karyokinesis). These Fig. 7.-Karyokinesis. a, ordinary nucleus of a columnar epithelial cell; b, c, the same nucleus in the stage of convolution ; n, the wreath or rosette form ; e, the aster or single star; f, a nuclear spindle from the Descemet's endothelium of the frog's cornea ; g, n, 1, diaster ; k, two daughter nuclei. (Klein.) changes consist in a gradual re-arrangement of the intranuclear network of each nucleus (see p. 19), until two nuclei are formed similar in all respects to the original one. The nucleus in a resting condition, i.e., before any changes preceding division occur, consists of a very close meshwork of fibrils, which stain deeply in carmine, embedded in protoplasm, which does not possess this property, the whole nucleus being contained in an envelope. The first change consists of a slight enlargement, the disappearance of the envelope, and the increased definition and thickness of the nuclear fibrils, which are also more separated than they were and stain better. This is the stage of con- volution (fig. 7, B, c). The next step in the process is the arrangement of the fibrils into some definite figure by an alternate looping in and out around a central space, by which means the rosette or wreath stage (fig. 7, d) is reached. The loops of the rosette next become divided at the periphery, and their central points become more angular, so that the fibrils, divided into portions of about equal length, are, as it were, doubled at an acute angle, and radiate V-shaped from the centre, forming a (aster) or wheel Cl FAP. I. J PLANTS AND ANIMALS. 11 (fig. 7, e), or perhaps from two centres, in which case a double star (diaster) results (fig. 7, G, h, and l). After remaining almost unchanged for some time, the V-shaped fibres being first re-arranged in the centre, side by side (angle outwards), tend to separate into two bundles, which gradually assume position at either pole. From these groups of fibrils the two nuclei of the new cells are formed (daughter nuclei) (fig. 7, K), and the changes they pass through before reaching the resting condition are exactly those through which the original nucleus (mother nucleus) has gone, but in a reverse order, viz., the star, the rosette, and the convolution. During or shortly after the formation of the daughter nuclei the cell itself becomes constricted and then divides in a line about midway between them. 5. Decay and death of cells.-There are two chief ways in which the comparatively brief existence of cells is brought to an end : (1) Mechanical abrasion, (2) Chemical transformation. 1. The various epithelia (p. 22) furnish abundant examples of mechanical abrasion. As it approaches the free surface the cell becomes more and more flattened and scaly in form and more horny in consistence, till at length it is simply rubbed off. Hence we find epithelial cells in the mucus of the mouth, intestine, and genito-urinary tract. 2. In the case of chemical transformation the cell-contents undergo a degeneration which, though it may be pathological, is very often a normal process. Thus we have (a.) fatty metamorphosis producing oil-globules in the secretion of milk, fatty degeneration of the muscular fibres of the uterus after the birth of the foetus, and of the cells of the Graafian follicle giving rise to the " corpus luteum." (See chapter on Generation.) (b.) Pigmen- ta ry degeneration from deposit of pigment, as in the epithelium of the air- vesicles of the lungs, (c.) Calcareous degeneration, which is common in the cells of many cartilages. Differences between Plants and Animals. Having now considered somewhat at length the vital properties of protoplasm, as shown in cells of vegetable as well as animal organisms, we are now in a position to discuss the question of the differences between plants and animals. It might at the outset of our enquiry have seemed an unnecessary thing to recount the very great distinctions which exist between an animal and a vegetable, but, however great these may be between the higher animals and plants, yet in the lowest of them the distinctions are much less obvious. (i.) Perhaps the most essential distinction is the power which vegetable protoplasm possesses of being able to build up new albuminous material out of such chemical bodies as ammonium salts, carbonic acid gas and water, together with mineral sulphates and phosphates. By means of their green colouring matter, 12 THE PHENOMENA OF LIFE. [chap. 1. chlorophyl-a substance almost exclusively confined to the vege- table kingdom-plants are capable of decomposing the carbonic acid gas, which they absorb by their leaves. The result of this chemical action, which occurs only under the influence of light, is, so far as the carbonic acid is concerned, the fixation of carbon in the plant structures and the exhalation of oxygen. The carbon thus obtained becomes combined with the elements of water ab- sorbed by the roots, to form starch. By the re-arrangement of the elements composing this body, with the addition of nitrogen and sulphur derived from nitrates and sulphates of the soil, vegetable protoplasm can construct albumen. Animal protoplasm is incapable of thus using such substances and never exhales oxygen as a pro- duct of decomposition. Ft must have ready-formed albuminous food in order to live. The power of living upon albuminous as well as non-albuminous matter is less decisive of an animal nature ; inasmuch as fungi and some other parasitic plants derive their nourishment in part from the former source. (2.) There is, commonly, a difference in general chemical com- position between vegetables and animals, even in their lowest forms; for associated with the protoplasm of the former is a considerable amount of cellulose, a substance closely allied to starch and containing carbon, hydrogen, and oxygen only. The presence of cellulose in animals is much more rare than in vege- tables, but there are many animals in which traces of it may be discovered, and some, the Ascidians, in which it is found in con- siderable quantity. The presence of starch in vegetable cells is very characteristic, though not distinctive, and a substance, glycogen, nearly allied in composition to cellulose, is very common in the organs and tissues of animals. (3.) Inherent power of movement is a quality which we so commonly consider an essential indication of animal nature, that it is difficult at first to conceive it existing in any other. The capability of simple motion is now known, however, to exist in so many vegetable forms, that it can no longer be held as an essential distinction between them and animals, and ceases to be a mark by which the one can be distinguished from the other. Thus the zoospores of many of the Cryptogamia exhibit ciliary or amoeboid movements (p. 4) of a like kind to those seen in amoebae ; and even among the higher orders of plants, many, e.g., Dionoea Muscipula (Venus's fly-trap), and Mimosa sensitiva (Sensitive CHAP. I.] PLANTS AND ANIMALS. 13 plant), exhibit such motion, either at regular times, or on the application of external irritation, as might lead one, were this fact taken by itself, to regard them as sentient beings. Inherent power of movement, then, although especially characteristic of animal nature, is, when taken by itself, no proof of it. (4.) The presence of a digestive canal is a very general mark by which an animal can be distinguished from a vegetable. But the Fig. 8.-'(a). Young vegetable cells, showing cell-cavity entirely filled with granular proto- plasm enclosing a large oval nucleus, with one or more nucleoli. (b). Older cells from same plant, showing distinct cellulose-wall and vacuolation of protoplasm. lowest animals are surrounded by material that they can take as food, as a plant is surrounded by an atmosphere that it can use in like manner. And every part of their body being adapted to absorb and digest, they have no need of a special receptacle for nutrient matter, and accordingly have no digestive canal. This distinction then is not a cardinal one. It would be tedious as well as unnecessary to enumerate the chief distinctions between the more highly developed animals and vegetables. They are sufficiently apparent. In passing, it may be well to point out the main distinctions between animal and vegetable cells. It has been already mentioned that in animal cells an envelope or cell-wall is by no means always present. In adult vegetable cells, on the other hand, a well-defined cellulose wall is highly characteristic ; this, it should be remembered, is non-nitrogenous, and thus differs chemically as well as structurally from the con tained mass. Moreover, in vegetable cells (fig. 8, b), the protoplastic contents of the cell fall into two subdivisions : (i) a continuous film which lines the interior of the cellulose wall; and (2) a reticulate mass 14 THE PHENOMENA OF LIFE. [chap. I. containing the nucleus and occupying the cell-cavity ; its inter- stices are filled with fluid. In young vegetable cells such a distinction does not exist; a finely granular protoplasm occupies the whole, cell-cavity (fig. 8, a). Another striking difference is the frequent presence of a large quantity of intercellular substance in animal tissues, while in vegetables it is comparatively rare, the requisite consistency being given to their tissues by the tough cellulose walls, often thickened by deposits of lignin. As an example of the manner in which this end is attained in animal tissues, may be mentioned the deposition of lime-salts in a matrix of intercellular substance in ossification. Morphological Development and Division of Functions. As we proceed upwards in the scale of life from monocellulai organisms, we find that another phenomenon is exhibited in the life history of the higher forms, namely, that of Development. An amoeba comes into being derived from a previous amoeba; it manifests the properties and performs functions of life which have been already enumerated ; it grows, it reproduces itself, whereby several amoebae result in place of one, and it dies, but it can scarcely be said to develop unless the formation of a nucleus can be so con- sidered. In the higher organisms, however, it is different; they, indeed, begin as a single cell, but this cell on its division and sub- division does not form so many different organisms, but possesses the material from which, by development, the completed and perfected whole is to be derived. Thus, from the spherical ovum, or germ, which forms the starting-point of animal life, and which consists of a protoplasmic cell with a nucleus and nucleolus (see fig. 4), in a comparatively short time, by the process of segmenta- tion which has been already mentioned, a complete membrane of cells, polyhedral in shape from mutual pressure, called the blasto- derm, is formed, and this speedily divides into two and then into three layers, chiefly from the rapid proliferation of the cells of the first single layer. These layers are called the epiblast, the meso- blast, and the hypoblast. It is found in the further development of the animal that from each of these layers is produced a very definite part of its com- pleted body. For example, from the cells of the epiblast, are derived, among other structures, the skin and the central nervous CHAP. I.] DEVELOPMENT. 15 system; from the mesoblast is derived the flesh or muscles of the body, and from the hypoblast, the epithelium of the alimentary canal and some of the chief glands. From the epiblast are ultimately developed the superficial skin or epider- mis and its various appendages, also the central or cerebro-spinal nerve centres, the sensorial epithelium of the organs of special sense (the eye, the ear, the nose), and the epithelium of the mouth and salivary glands. From the hypoblast is developed the epithelium of the whole digestive canal, together with that lining the ducts of all the glands which open into it ; also the glandular parenchyma of the glands (e.g., liver and pancreas) connected with it, and the epithelium of the respiratory tract. Fig. 9.-Transverse section through embryo chick (26 hrs.), a, epiblast; b, mesoblast; c, hypo- blast ; d, central portion of mesoblast, which is here fused with epiblast; e, primitive groove ; f, dorsal ridge. (Klein.) From the mesoblast are derived all the tissues and organs of the body inter- vening between these two, the whole group of the connective tissues, the muscles and the cerebro-spinal and sympathetic nerves, with the vascular and genito-urinary systems, and all the digestive canal, with its various appendages, with the exception of the lining epithelium above mentioned. It is obvious that these tissues and organs exhibit in a varying degree the primary properties of protoplasm. The muscles, for example, derived from certain cells of the mesoblast are highly contractile and respond to stimuli readily, but they have little to do with digestion except indirectly, and again, the cells of the liver, although doubtless contractile to a certain extent, yet have secretion and digestion for their chief functions. Thus we see development in two directions going on side by side. It speedily becomes necessary for the organism to depute to different groups of cells, or their equivalents (i.e., to the tissues or organs to which they give rise), special functions, so that the various functions which the original cell may be supposed to discharge, and the various properties it may be supposed to possess, are divided up among various groups of resulting cells. 16 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. 11. The work of each group is specialised. As a result of this divi- sion of labour, as it is called, these functions and properties are, as might be expected, developed, and made more perfect, and the tissues and organs arising from each group of cells are developed also, with a view to the more convenient and effective exercise of their functions and employment of their properties. It would be out of place here to discuss the question as to the exact manner in which a property or function, rudimentary in a low form of animal life, is found to be highly developed as we pass up the series ; neither is it our province to discuss the very com- plicated subject of the relationship of man to other animals, and of these to one another. Having now briefly indicated the close connection which exists between Human physiology and Biology in general, we are better prepared to commence the study of the former as constituting a part of a great whole. The next two chapters will be devoted to a consideration of the minute structure, or the histology (lo-tos, a tissue or web) of epithelium and the connective tissues. CHAPTER II. THE STRUCTURE OF THE ELEMENTARY TISSUES. The cells of the body are described in various ways; for example, according to their shape, situation, contents, origin and functions. («.) Their shape varies :-Starting from the spherical or spheroidal (fig. io, a) as the typical form assumed by a free cell, we find this altered to a polyhedral shape when the pressure on the cells in all directions is nearly the same (fig. io, 6). Of this, the primitive segmentation-cells may afford an example. The discoid shape is seen in blood-cells (fig. io, c), and the scale- like form in superficial epithelial cells (fig. io, df Some cells have a jagged outline (prickle-cells) (fig. 27). Cylindrical, conical, or prismatic cells occur in the deeper layers OHAP. II.] CELLS AND THEIR NUCLEI. 17 of laminated epithelium, and the simple cylindrical epithelium of the intestine and many gland ducts. Such cells may taper off at one or both ends into fine processes, in the former case being- caudate, in the latter fusiform (fig. 11). They may be greatly elongated so as to become fibres. Ciliated cells (fig. io, d) must Fig. io.-Various forms of cells, a. Spheroidal, showing nucleus and nucleolus ; b. Poly- hedral ; c. Discoidal (blood cells); d. Scaly or squamous (epithelial cells). a- b c d be noticed as a distinct variety: they possess, but only on their free surfaces, hair-like processes (cilia). These vary im- mensely in size, and may even exceed in length the cell itself. Finally, we have the branched or stellate cells, of which the large nerve-cells of the spinal cord, and the connective tissue corpuscle Fig. ii.- Various forms of cells, a. Cylindrical or columnar; ft. Caudate; c. Fusiform; <Z. Ciliated (from trachea); e. Branched, stellate. are typical examples (fig. n, e). In these cells the primitive branches by secondary branching may give rise to an intricate network of processes. (6.) According to their situation in the tissues cells are known as epithelial, connective tissue cells, blood cells, glandular, and the like. (c.) According to their contents, they are called fat cells when their protoplasm contains an excess of fat, pigment cells when it contains pigment; coloured, when their protoplasm is infiltrated with a colouring matter, as haemoglobin. 18 STRUCTURE OF THE ELEMENTARY TISSUES, [chap. ii. (<7.) According to their functions, they are called secreting, protective, sensitive, contractile, Ac. (e.) According to their wigin, they are named epiblastic, mcsoblastic, and hypoblastic. Nearly all cells at some period of their existence possess nuclei. As has been incidentally suggested the origin of a nucleus in a cell is the first trace of the differentiation of protoplasm. The existence of nuclei was first pointed out in the year 1833 by Robert Brown, who observed them in vegetable cells. They are A B Fig. 12.-(a). Colourless blood-corpuscle showing intra-cellular network of Heitzmann, and two nuclei with intra-nuclear network (Klein and Noble Smith). (b.) Coloured blood-corpuscle of newt showing intra-cellular network of fibrils (Heitzmann). Also oval nucleus composed of limiting membrane and fine intra-nuclear network of fibrils, x 800. (Klein and Noble Smith.) either small transparent vesicular bodies containing one or more smaller particles (nucleoli), or they are semi-solid masses of pro- toplasm always in the resting condition bounded by a well-defined envelope. In their relation to the life of the cell they are certainly hardly second in importance to the protoplasm itself, and thus Beale is fully justified in comprising both under the term " germinal matter." They exhibit their vitality by initiating, in the majority of cases, the process of division of the cell into two or more cells (fission) by first themselves dividing. Distinct observations have been made, showing that spontaneous changes of form may occur in nuclei as also in nucleoli. Histologists have long recognised nuclei by two important characters :- (i.) Their power of resisting the action of various acids and alkalies, particularly acetic acid, by which their outline is more clearly defined, and they are rendered more easily visible. This indicates some chemical difference between the protoplasm of the cell and nuclei, as the former is destroyed by these reagents. CHAI*. II.] MINUTE STRUCTURE OF CELLS. 19 (2.) Their quality of staining in solutions of carmine, haema- toxylin, &c. Nuclei are most commonly oval or round, and do not generally conform to the diverse shapes of the cells ; they are altogether less variable elements than cells, even in regard to size, of which fact one may see a good example in the uniformity of the nuclei in cells so multiform as those of epithelium. But some- times nuclei appear to occupy the whole of the cell, as is the case in the lymph corpuscles of lymphatic glands, and in some- small nerve cells. Their position in the cell is very variable. In many cells, especially where active growth is progressing, two or more nuclei are present. Minute structure of cells.--The protoplasm which forms the body as well as that which forms the nuclei of cells has been shown in many varieties of cells, e.g., the colourless blood-corpuscles, epithe- lial cells, connective-tissue corpuscles, nerve-cells, to be made up of a network of very fine fibrils, the meshes of which are occupied by a hyaline interstitial substance (Heitzmann's network) (fig. 12). At the nodes, where the fibrils cross, are little swellings, and these are the objects described as granules by the older observers : but in the body of some cells, e.g., colourless blood-corpuscles, there are real granules, which appear to be quite free and unconnected with the intra-cellular network. Modes of connection.-Cells are connected together to form tissues in various ways. (1) By means of a cementing intercellular substance. This is probably always present as a transparent, colourless, viscid, albu- minous substance, even between the closely apposed cells of epithelium, while in the case of cartilage it forms the main bulk of the tissue, and the cells only appear as imbedded in, not as cemented by, the intercellular substance. This inter- cellular substance may be either homogeneous or fibrillated. In many cases (e.g. the cornea) it can be shown to contain a number of irregular branched cavities, which communicate with each other, and in which branched cells lie : through these branch- ing spaces nutritive fluids can find their way into the very remotest parts of a non-vascular tissue. As a special variety of intercellular substance must be mentioned the basement membrane (membrana propria) which is found at the base of the epithelial cells in most mucous membranes, and especially as an investing tunic of gland follicles which determines 20 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAP. II. their shape, and which may persist as a hyaline saccule after the gland-cells have all been discharged. (2) By anastomosis of their processes. This is the usual way in which stellate cells, c.<?. of the cornea, are united : the individuality of each cell is thus to a great extent lost by its connection with its neighbours to form a reticulum: as an example of a network so produced we may cite the stroma of lymphatic glands. Sometimes the branched processes breaking up into a maze of minute fibrils, adjoining cells are connected by an intermediate reticulum : this is the case in the nerve-cells of the spinal cord. Derived tissue-elements.-Besides the Cell, which may be termed the primary tissue-element, there are materials which may be termed secondary or derived tissue-elements. Such are Inter- cellular substance, Fibres and Tubules. a. Intercellular substance is probably in all cases directly derived from the cells themselves. In some cases (e.g. cartilage), by the use of re-agents the cementing intercellular substance is, as it were, analysed into various masses, each arranged in con- centric layers around a cell or group of cells, from which it was probably derived (fig. 46). /3. Fibres.-In the case of the crystalline lens, and of muscle both striated and non-striated, each fibre is simply a metamor- phosed cell: in the case of the striped fibre the elongation being accompanied by a multiplication of the nuclei. The various fibres and fibrillae of connective tissue result from a gradual transformation of an originally homogeneous intercellular substance. Fibres thus formed may undergo great chemical as well as physical transformation: this is notably the case with yellow elastic tissue, in which the sharply defined elastic fibres, possessing great power of resistance to re-agents, contrast strikingly with the homogeneous matter from which they are derived. y. Tubules, such its the capillary blood-vessels, which were originally supposed to consist of a structureless membrane, have now been proved to be composed of flat, thin cells, cohering along their edges. With these simple materials the various parts of the body are built up; the more elementary tissues being, so to speak, first compounded of them ; while these tissues are variously mixed and interwoven to form more intricate combinations. Thus are constructed epithelium and its modifications, the con- chap. ir. ] EPITHELIUM. 21 nective tissues, the fibres of muscle and nerve, &c.; and these, again, with the more simple structures before mentioned, are used as materials wherewith to form arteries, veins, and lym- phatics, secreting and vascular glands, lungs, heart, liver, and other parts of the body. Tn this chapter the leading characters and chief modifications of the first two of the great groups of tissues-the Epithelial and Connective-will be described; while the others will be appro- priately considered in the chapters treating of their physiology. Epithelium. The term epithelium is applied to the cells covering the skin, the mucous and serous membranes, and to those forming a lining to other parts of the body as well as entering into the formation of glands. For example :- Epithelium clothes (i) the whole exterior surface of the body, forming the epidermis with its appendages-nails and hairs; becoming continuous at the chief orifices of the body-nose, mouth, anus, and urethra-with the (2) epithelium which lines the whole length of the (3) respiratory, alimentary and genito-urinary tracts, together with the ducts of their various glands. Epithe- lium also lines the cavities of (4) the brain, and the central canal of the spinal cord, (5) the serous and synovial membranes, and (6) the interior of all blood-vessels and lymphatics. Epithelial cells possess an intracellular and an intranuclear net- work (p. 19). They are held together by a clear, albuminous, cement substance. The viscid semi-fluid consistency both of cells and intercellular substance permits such changes of shape and arrangement in the individual cells as are necessary if the epithe- lium is to maintain its integrity in organs the area of whose free surface is so constantly changing, as the stomach, lungs, etc. Thus, if there be but a single layer of cells, as in the epithelium lining the air vesicles of the lungs, the stretching of this membrane causes such a thinning out of the cells that they change their shape from spheroidal or short columnar, to squamous, and vice versd, when the membrane shrinks. Epithelial tissues are non-vascular, but in some varieties minute channels exist between the cells of certain layers through which they may be supplied with nourishment from the subjacent blood-vessels. Nerve fibres are supplied to the cells of many epithelia. 22 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. II. Epithelial tissue is classified according as the cells composing it are arranged in a single layer when it is simple, or in several layers when it is called stratified or laminated, or in two or three layers occupying a position between the other two forms, when it is termed transitional. Of each form when there are several varieties they are named according to the shape of the cells composing it. A. Simple.-(i.) Squamous, scaly, pavement or tesselated ; (2.) Spheroidal or glandular ; (3.) Columnar, cylindrical, conical or goblet- shaped ; (4.) Ciliated. B. Transitional. C. Stratified. A. Simple.-Squamous Epithelium (fig. 13).-Arranged as a . v - Fig. 13.-Squamous epithelium scales from the inside of the mouth. X 260. (Henle.) Fig. 14.-Pigment cells from the retina. A, cells still cohering, seen on their surface; «, nu- cleus indistinctly seen. In the other cells the nucleus is concealed by the pigment granules. B, two cells seen in pro tile; «, the outer or posterior part containing scarcely any pigment. X 370. (Henle.) single layer, this form of epithelium is found as (a) the pigmentary layer of the retina, and forms the lining of (6) the interior of the serous and synovial sacs, (c) the alveoli of the lungs, and (<Z) of the heart, blood- and lymph-vessels. It consists of cells, which are flattened and scaly, with a more or less irregular outline. In the pigment cells of the retina, there is a deposit of pigment in the cel]-substance. This pigment consists of minute molecules of melanin, imbedded in the cell-substance and almost concealing the nucleus, which is itself transparent (fig. 14). In white rabbits and other albino animals, in which the pig- ment of the eye is absent, this layer is found to consist of colour- less pavement epithelial cells. chap. 11.] EPITHELIAL CELLS. 23 The squamous epithelium which is found as a single layer lining the alveoli of the lungs, the serous membranes, and the interior of blood- and lymphatic-vessels, is generally called by a distinct n ame -End otheliu m. The presence of endothelium may be demonstrated by staining the part lined by it with silver nitrate. When a small portion of a perfectly fresh serous membrane for Fig. 15.-ParZ of the omentum of a cat, stained in silver nitrate, X 100. The tissue forms a "fenestrated membrane," that is to say, one which is studded with holes or windows. In the figure these are of various shapes and sizes, leaving trabeculee, the basis of which is fibrous tissue. The trabecula; are of various sizes, and are covered with endothelial cells, the nuclei of which have been made evident by staining with haema- toxylin after the silver nitrate has outlined the cells by staining the intercellular sub- stance. (V. D. Harris.) example, as the mesentery or omentum (fig. 15), is immersed for a few minutes in a quarter per cent, solution of silver nitrate, washed with distilled water and exposed to the action of light, the silver oxide is precipitated in the intercellular cement substance and the endothelial cells are thus mapped out by fine dark and generally sinuous lines of extreme delicacy. The cells vary in size and shape, and are as a rule irregular in outline; those lining the interior of blood-vessels and lymphatics being spindle-shape with a very wavy outline. They enclose a clear, oval nucleus, which, when the cell is viewed in profile, is seen to project from its surface. The nuclei arc not however evident unless the tissue which has been 24 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAP. II. already stained in silver nitrate, is placed in another dye, such as htematoxylin, which has the property of picking out its nuclei. T ig. 16.-Abdominal surface of centrum tendineumof diaphragm of rabbit, showing'the general polygonal shape of the endothelial cells: each is nucleated. (Klein.) x 300. Endothelial cells may be ciliated, e.g., those in the mesentery of frogs, especially about the breeding season. Fig. 17 .-Silver-stained preparation of great omentum of dog, winch shows, amongst the flat endothelium of the surface, small and large groups of germinating endothelium be- tween which numbers of stomata are to be seen. (Klein.) x 300. Besides the ordinary endothelial cells above described, there are found on the omentum and parts of the pleura of many animals, CHAV. IT.] ENDOTHELIUM. 25 little bud-like, processes or nodules, consisting of small polyhedral granular cells, rounded on their free surface, which multiply very rapidly by division (fig. 17). These constitute what is known as " germinating endothelium." The process of germination doubt- less goes on in health, and the small cells which are thrown off in succession are carried into the lymphatics, and contribute to the number of the lymph corpuscles. The buds may be enormously increased both in number and size in certain diseased conditions. On those portions of the peritoneum and other serous mem- Fig. 18.-Peritoneal surface of septum ciste.ruce lymphatiae magna of frog. The stomata, some of which are open, some collapsed, are surrounded by germinating endothelium.' (Klein.) x 160. branes in which lymphatics abound (fig. 18), apertures are found surrounded by small more or less cubical cells. These apertures are called stomata. They are particularly well seen in the anterior wall of the great lymph sac of the frog (fig. 18), and in the omentum of the rabbit. These are really the open mouths of lymphatic vessels or spaces, and through them lymph-corpuscles, and the serous fluid from the serous cavity, pass into the lymphatic system. They should be distinguished from smaller and more numerous apertures between the cells which are not lined by small cells, although the surrounding cells seem to radiate from them, filled up by intercellular substance or by processes of the cells underneath. These are called pseudo-stomata (fig. 16). In the neighbourhood of the stomata, the cells often manifest indications of germinating. They may be either large with two or more nuclei, or about half the size of the generality of cells. 26 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAP. II. Germinating cells of this kind or of the kind above described, are generally very granular. 2. Spheroidal epithelial cells are the active secreting agents in most secreting glands, and hence are often termed glandular; they are generally more or less rounded in outline : often polygonal from mutual pressure. Excellent examples are to be found in the liver, in the secreting Fig-. 19.-Glandular epitheliu m. A, small lobule of a mucous gland of the tongue showing nucleated glandular spheroidal cells. B. Liver cells. X 200. (V. D. Harris.) tubes of the kidney, and in the salivary and gastric glands (fig. 19). 3. Columnar epithelium (fig. 21, a and 6) as a single layer, lines (a.) the mucous membrane of the stomach and intestines, from the cardiac orifice of the stomach to the anus, and (b.) wholly or in part the ducts of the glands opening on its free surface ; also (c.) many gland-ducts in other regions of the body, e.g., mammary, salivary, &c. Columnar epithelium consists of cells which are cylindrical or prismatic in form, and contain a large oval nucleus. They vary in size and also in shape to a certain extent, the outline being often irregular from pressure of neighbouring cells, but speaking generally one end of the cell is narrower than the other, and by this end the cell is attached to the membrane beneath. The inter- cellular and inter-nuclear network are well developed. The columnar epithelial cells of the alimentary canal possess a structureless layer on their free surface : such a layer, appearing CHAP. II.] COLUMNAR EPITHELIUM. 27 striated when viewed in section, is termed the " striated basilar border" (fig. 20, a, a). Columnar cells may undergo a curious transformation, and from the alteration in their shape are termed " goblet-cells " (fig. 20, Fig. 20.-A. Vertical section of a villus of the small intestine of a cat. a. Striated basilar border of the epithelium, b. Columnar epithelium, c. Goblet cells, d. Central lymph-vessel, e. Smooth muscular fibres, f. Adenoid stroma of the villus in which lymph corpuscles lie. B. Goblet-cells. (Klein ) a, c, and b). These are hardly ever seen in a perfectly fresh speci- men : but if such a specimen be watched for some time, little knobs are seen gradually appearing on the free surface of the epithelium, and are finally detached; these consist of the cell- d c Fig. 21.-Columnar epithelial cells from the intestinal mucous membrane of a cat.- a and b, small cells of the lowest layer; superficial layer; d, goblet cells. (Cadiat.) e (1 h Fig. 22.-Columnar ciliated cells from the human nasal mem- . brane: magnified 300 diame- ters. fSharpey.) contents which are discharged by the open mouth of the goblet, leaving the nucleus surrounded by the remains of the protoplasm in its narrow stem. This transformation is a normal process which is continually 28 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAP. II. going on during life, the discharged cell-contents contributing to form mucus, the cells being supposed in many cases to recover their original shape. It is an example of secretion. 4. Ciliated cells are generally cylindrical (fig. 23, b), but may be spheroidal or even almost squamous in shape (fig. 23, a). This form of epithelium lines-(a.) the whole of the respiratory tract from the larynx, except over the vocal cords, to the finest sub-divisions of the bronchi, also the lower parts of the nasal passages, the nasal duct, and the lachrymal sac. Tn part of this tract, however, the epithelium is in several layers, of which only the most superficial is ciliated, so that it should more accurately Fig. 23.-A. Spheroidal ciliated cells from the mouth of the frog. X 300 diameters. (Sharpey.) B. a. Ciliated columnar epithelium lining a bronchus, b. Branched connective-tissue corpuscles. (Klein and Noble Smith.) be termed transitional (p. 29) or stratified, (b.), some portions of the generative apparatus in the male, viz., lining the "vasa efferentia " of the testicle, and their prolongations as far as the lower end of the epididymis; in the female (c.) commencing about the middle of the neck of the uterus, and extending throughout the uterus and Fallopian tubes to their fimbriated extremities, and even for a short distance on the peritoneal surface of the latter, (d.) The ventricles of the brain and the central canal of the spinal cord are clothed with ciliated epithelium in the child, but in the adult this epithelium is limited to the central canal of the cord. The Cilia, or fine hair-like processes which give the name to this variety of epithelium, vary a good deal in size in different classes of animals, being very much smaller in the higher than among the lower orders, in which they sometimes exceed in length the cell itself. The number of cilia on any one cell ranges from ten to thirty, and those attached to the same cell are often of different lengths. CHAP. II.] TRANSITIONAL EPITHELIUM. 29 When living ciliated epithelium, e.g., from the gill of a mussel, or oyster, or from the mouth of the frog, or from a scraping from a polypus from the human nose, is examined under the microscope, the cilia are seen to be in constant rapid motion; each cilium being fixed at one end, and swinging or lashing to and fro. The general impression given to the eye of the observer is very similar to that produced by waves in a field of corn, or swiftly running and rippling water, and the result of their movement is to produce a continuous current in a definite direction, and this direction is invariably the same on the same surface, being always, in the case of a cavity, towards its external orifice. Ciliary Motion.-Ciliary, which is closely allied to amoeboid and muscular motion, is alike independent of the will, of the direct influence of the nervous system, and of muscular contraction. It continues for several hours after death or removal from the body, provided the portion of tissue under examination be kept moist. I ts independence of the nervous system is shown also in its occurrence in the lowest invertebrate animals apparently unprovided with anything analogous to a nervous system, in its persistence in animals killed by prussic acid, by narcotic or other poisons, and after the direct application of narcotics, such as morphia, opium, and belladonna, to the ciliary surface, or of electricity through it. The vapour of chloroform arrests the motion ; but it is renewed on the discontinuance of the application (Lister). The movement ceases when the cilia are deprived of oxygen, but is revived on the admission of this gas. Carbonic acid stops the movement. The contact of various substances, e.g., bile, strong acids, and alkalies, will stop the motion altogether; but this seems to depend chiefly on destruc- tion of the delicate substance of which the cilia are composed. Temperatures above 45° C., and below 0° C., stop the movement, but moderate heat and dilute alkalies are favourable to the action and revive the movement after temporary cessation. As a special sub-division of ciliary action may be mentioned the motion of spermatozoa, which may be regarded as cells with a single cilium. B. Transitional Epithelium.-This term has been applied to cells, which are neither arranged in a single layer, as is the case with simple epithelium, nor yet in many superimposed strata as in laminated; in other words, it is employed when epithelial cells are found in two, three, or four superimposed layers. The upper layer may be either columnar, ciliated, or squamous. 30 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. II. When the upper layer is columnar or ciliated, the second layer consists of smaller cells fitted into the inequalities of the cells above them, as in the trachea (fig. 24, 6). The epithelium which is met with lining the urinary bladder and ureters is, however, the transitional par excellence. In this variety there are two or three layers of cells, the upper being Fig. 24.-Epithelium of the bladder, a, one of the cells of the first row ; b, a cell of the second row; c, cells in situ, of first, second, and deepest layers. (Obersteiner.) Fig. 25. - Transitional epithelial cells from a scraping of the mucous membrane of the bladder of the rabbit. (V. D. Harris.) more or less flattened according to the full or collapsed condition of the organ, their under surface being marked with one or more depressions, into which the heads of the next layer of club- shaped cells fit. Between the lower and narrower parts of the second row of cells, are fixed the irregular cells which constitute the third row, and in like manner sometimes a fourth row (fig. 24). It can be easily understood, therefore, that if a scraping of the mucous membrane of the bladder be teased, and examined under the microscope, cells of a great variety of forms may be made out (fig. 25). Each cell contains a large nucleus, and the larger and superficial cells often possess two. C. Stratified EpitheliumThis term is employed when the cells forming the epithelium are arranged in a considerable number of superimposed layers. The shape and size of the cells of the different layers, as well as the number of the layers, vary in different situations. Thus the superficial cells are as a rule of the squamous, or scaly variety, and the deepest of the columnar form. The cells of the intermediate layers are of different shapes, but those of the middle layers are more or less rounded. The superficial cells overlap by their edges (fig. 26); they are broad (fig. 13). Their chemical composition is different from that of the underlying cells, as they contain keratin, and are therefore horny in character. CHAP. II.] STRATIFIED EPITHELIUM. 31 The nucleus is often not apparent. The really cellular nature of even the dry and shrivelled scales cast off from the surface of the epidermis, can be proved by the application of caustic potash, which causes them rapidly to swell and assume their original form. Fig. 26.-Vertical section of the stratified epithelium of the Rabbit's cornea, a. Anterior epithe- lium, showing the different shapes of the cells at various depths from the free surface. b. Portion of the substance of cornea. (Klein.) The squamous cells exist in the greatest number of layers in the epidermis or superficial part of the skin ; and the most superficial of these are being continually removed by friction, and new cells from below supply the place of those cast off. The intermediate cells approach more to the flat variety the nearer they are to the surface, and to the columnar as they approach the lowest layer. There may be considerable inter- cellular intervals ; and in many of the deeper layers of epithelium in the mouth and skin, the out- line of the cells is very irregular, in consequence of processes pass- ing from cell to cell across these intervals. Such cells (fig. 27) are termed " ridge and furrow," " cogged " or " prickle " cells. These " prickles " are prolongations of the intra-cellular network which run across from cell to cell, thus joining them together (Martyn),, the interstices being filled by the transparent intercellular cement substance. When this increases in quantity in inflammation, the cells are pushed further apart, and the connecting fibrils or " prickles " elongated, and therefore more clearly visible. Fig. 27.-Jogged cells of the middle layers of pavement epithelium, from a verti- cal section of the gum of a newborn infant. (Klein.) 32 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. II. The columnar cells of the deepest layer are distinctly nucleated ; they multiply rapidly by division ; and as new cells are formed beneath, they press the older cells forwards to be in turn pressed forwards themselves towards the surface, gradually altering in shape and chemical composition until they are cast off from the surface. Stratified epithelium is found in the following situations : - (x.) Forming the epidermis, covering the whole of the external surface of the body; (2.) Covering the mucous membrane of the tongue, mouth, pharynx, and oesophagus; (3.) As the conjunctival epithelium, covering the cornea; (4.) Lining the vaginal part of the cervix uteri. Functions of Epithelium.-According to function, epithelial cells may be classified as :-(1.) Protective, e.g., in the skin, mouth, blood-vessels, &c. (2.) Protecti ve and moving-ciliated epithelium. (3.) Secreting-glandular epithelium ; or, Secreting formed elements-epithelium of testicle secreting spermatozoa. (4.) Protective and. secreting, e.g., epithelium of intestine, (5.) Sensorial, e.g., olfactory cells, rods and cones of retina, organ of Corti. Epithelium forms a continuous smooth investment over the whole body, being thickened into a hard, horny tissue at the points most exposed to pressure, and developing various appendages, such as hairs and nails, whose structure and functions will be considered in a future chapter. Epithelium lines also the sensorial surfaces of the eye, ear, nose, and mouth, and thus serves as the medium through which all impressions from the external world- touch, smell, taste, sight, hearing-reach the delicate nerve-endings, whence they are conveyed to the brain. The ciliated epithelium which lines the air-passages serves not only as a protective investment, but also by the movements of its cilia promotes currents of the air in the bronchi and bronchia, and is enabled to propel fluids and minute particles of solid matter so as to aid their expulsion from the body. In the case of the Fallopian tube, this agency assists the progress of the ovum towards the cavity of the uterus. Of the purposes served by cilia in the ventricles of the brain nothing is known. The epithelium of the various glands, and of the whole intestinal tract, has the power of secretion, i.e., of chemically transforming certain materials of the blood ; in the case of mucus and saliva this has been proved to involve the transformation of the epithelial cells themselves ; the cell-substance of the epithelial cells of the intestine being discharged by the rupture of their envelopes, as mucus. Epithelium is likewise concerned in the processes of transudation, diffusion, and absorption. It is constantly being shed at the free surface, and reproduced in the deeper layers. The various stages of its growth and development can be well seen in a section of any laminated epithelium such as the epi- dermis. CHAP. II.] CLASSIFICATION OF CONNECTIVE TISSUES. 33 The Connective Tissues. This group of tissues forms the Skeleton with its various con- nections-bones, cartilages, and ligaments-and also affords a supporting framework and investment to the various organs com- posed of nervous, muscular, and glandular tissue. Its chief function is the mechanical one of support, and for this purpose it Fig. 28.-Horizontal preparation of cornea of frog, stained in gold chloride ; showing the network of branched cornea corpuscles. The ground substance is completely colour- less. x 400. (Klein.) is so intimately interwoven with nearly all the textures of the body, that if all other tissues could be removed, and the con- nective tissues left, we should have a wonderfully exact model of almost every organ and tissue in the body, correct even to the smallest minutiae of structure. Classification of Connective Tissues.- The chief varieties of connective tissues may be thus classified :- I. The Fibrous Connective Tissues. X.-Chief Forms. a. White fibrous. b. Elastic. c. Areolar. B.-Special Varieties. a. Gelatinous. b. Adenoid or Retiform. c. Neuroglia. d. Adipose. II. Cartilage. III. Bone. 34 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAP. II. Structure of Connective Tissues. All of the varieties of connective tissue are made up of two elements, namely, cells and intercellular substance. (A.) Cells.-The cells are of two kinds. (a.) Fired.-These are cells of a flattened shape, with branched processes, which are often united together to form a network : they can be most readily observed in the cornea, in which they are arranged, layer above layer, parallel to the free surface. They lie in spaces, in the intercellular or ground substance, which are of the same shape as the cells they contain, but rather larger, and which form by anastomosis a system of branching canals freely ■communicating (tig. 28). To this class of cells belong the flattened tendon corpuscles which are arranged in long lines or rows parallel to the fibres <fig- 34). These branched cells, in certain situations, contain a number of pigment-granules, giving them a dark appearance : they form one variety of pigment-cell. Branched pigment-cells of this kind are found in the outer layers of the choroid (fig. 29). In many of the lower animals, such as the frog, they are found widely distributed, not only in the skin, but also in in- ternal parts, e. g., the mesentery and sheaths of blood-vessels. In the web of the frog's foot such cells may be seen, with pigment-granules evenly distributed throughout the body of the cell, and its processes ; but under the action of light, elec- tricity, and other stimuli, the pig- ment-granules become massed in the body of the cell, leaving the processes quite hyaline; if the stimulus be removed, they will gradually be distributed again throughout the processes. Thus the skin in the frog is some- times uniformly dusky, and sometimes quite light-coloured, with isolated dark spots. In the choroid and retina the pigment-cells absorb light. (6.) Amoeboid cells, of an approximately spherical shape : they have a great general resemblance to colourless blood corpuscles Fig. 29.-Humified piymeut-cells, from the tissue of the choroid coat of the eye. x 350. «, cell with pig- ment ; I, colourless fusiform cells. (Kolliker.) CHAP. II.J INTERCELLULAR SUBSTANCE. 35 (fig. 2), with which some of them are probably identical. They consist of finely granular nucleated protoplasm, and have the property, not only of changing their form, but also of moving about, whence they are termed migratory. They are readily distinguished from the branched connective-tissue corpuscles by their free condition, and the absence of processes. Some are much larger than others, and are found especially in the sub- lingual gland of the dog and guinea pig, and in the mucous membrane of the intestine. A second variety of these cells called plasma cells (Waldeyer) are larger than the amoeboid cells, appa- rently granular, less active in their movements. They are chiefly to be found in the intermuscular septa, in the mucous and sub-mucous coats of the intestine, in lymphatic glands, and in the omentum. (B.) Intercellular sub- stance.-This may be fibril- lar, as in the fibrous tissues, and in certain varieties of car- tilage ; or homogeneous, as in hyaline cartilage. The fibres composing the former are of two kinds- (a.) White fibres. (6.) Yellow elastic fibres. («.) White Fibres.-These are arranged parallel to each other in wavy bundles of various sizes : such bundles may either have a parallel arrangement (fig. 31), or may produce quite a felted texture by their interlacement. The individual fibres composing these fasciculi are homogeneous, un- branched, and of the same diameter throughout. They can readily be isolated by macerating a portion of white fibrous tissue (c. g., a small piece of tendon) for a short time in lime, or baryta- water, or in a solution of common salt, or of potassium permanga- nate : these reagents possess the power of dissolving the cementing interfibrillar substance (which is nearly allied to syntonin), and of thus separating the fibres from each other. By prolonged boiling the fibres yield gelatin. (6.) Yellow Elastic Fibres (fig. 32) are of all sizes, from ex- Fig. 30.-Flat, pigmented, branched connective tissue cells from the sheath of a large blood- vessel of frog's mesentery: the pigment is not distributed uniformly through the substance of the larger cell, consequently some parts of the cell look blacker than others (uncontracted state). In the two smaller cells most of the pigment is with- drawn into the cell-body, so that they appear smaller, blacker, and less branched. X 350. (Klein and Noble Smith.) 36 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAP. II. cessively fine fibrils up to fibres of considerable thickness: they are distinguished from white fibres by the following characters :- (1.) Their great power of resistance even to the prolonged action of chemical reagents, e. g., Caustic Soda, Acetic Acid, Ac. (2.) Fig. 31.-Fibrous tissue of cornea, showing bundles of fibres with a few scat- tered fusiform cells lying in the inter-fascicular spaces, x 400. (Klein and Noble Smith.) Fig. 32.-Elastic fibres from the ligamenta subflava. x 200. (Sharpey.) Their well-defined outlines. (3.) Their great tendency to branch and form networks by anastomosis. (4.) They very often have a twisted corkscrew-like appearance, and their free ends usually curl up. (5.) They are of a yellowish tint, and very elastic. These fibres yield a gelatinous substance called elastin. Varieties of Connective Tissue. A.-Chief Forms.-(a.) White Fibrous Tissue. Distribution.-Typically in tendon; in ligaments, in the perios- teum and perichondrium, the dura mater, the pericardium, the sclerotic coat of the eye, the fibrous sheath of the testicle ; in the fascise and aponeurosis of muscles, and in the sheaths of lymph- atic glands. Structure.-To the naked eye, tendons and many of the fibrous membranes, when in a fresh state, present an appearance as of watered silk. This is due to the arrangement of the fibres in wavy parallel bundles. Under the microscope, the tissue appears I. Fibrous Connective Tissues. CHAP. II.] THE FIBROUS TISSUES. 37 to consist of long, often parallel, bundles of fibres of different sizes. The fibres of the same bundle now and then intersect each other. The cells in tendons (fig. 34) are arranged in long chains in the ground substance separating the bundles of fibres, and are more or less regularly quadrilateral with large round nuclei containing nucleoli, which are generally placed so as to be contiguous in two cells. The cells consist of a body, which is thick, from which pro- cesses pass in various directions into, and partially filling up the spaces between the bundles of fibres. The rows of cells are Fig. 33.-a. Mature white fibrous tissue of tendon, consisting mainly of fibres with a few scattered fusiform cells. (Stricker.) Fig. 34.-Caudal tendon of young rat, showing the arrangement, form, and structure of the tendon cells, x 300. (Klein). separated from one another by lines of cement substance. The cell spaces can be brought into view by silver nitrate. The cells are generally marked by one or more lines or stripes when viewed longitudinally. This appearance is really produced by the laminar extension either projecting upwards or downwards. The branched character of the cells is seen in transverse section in fig. 35. (6.) Yellow Elastic Tissue. Distribution.-In the ligamentum nuchae of the ox, horse, and many other animals; in the ligamenta subflava of man; in the arteries, constituting the fenestrated coat of Henle ; in veins; in the lungs and tracheaJ in the stylo-hyoid, thyro-hyoid, and crico-thyroid ligaments ; in the true vocal cords; and in areolar tissue. Structure.-Elastic tissue occurs in various forms, from a struc- 38 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. II. tureless, elastic membrane to a tissue whose chief constituents arc bundles of fibres crossing each other at different angles ; when seen in bundles elastic fibres are yellowish in colour, but indi- vidual fibres are not so distinctly coloured. The varieties of the tissue may be classified as follows :- (<z.) Fine elastic fibrils, which branch and anastomose to form a network : this variety of elastic tissue occurs chiefly in the skin and mucous membranes, in subcuta- neous and submucous tissue, in the lungs and true vocal cords. (6.) Thick fibres, sometimes cylin- drical, sometimes flattened like tape, which branch, anastomose and form a network: these are seen most typically in the ligamenta subflava and also in the ligamentum nuclue of such animals as the ox and horse, in which it is largely developed (fig. 32). (c.) Elastic membranes with perfora- tions, c.y., Henle's fenestrated mem- brane : this variety is found chiefly in the arteries and veins. (<Z.) Continuous, homogeneous elastic membranes, e.y., Bowman's anterior elastic lamina, and Descemet's posterior elastic lamina, both in the cornea. A certain number of flat connective tissue cells are found in the ground substance between the elastic fibres which make up this variety of connective tissue. (c.) Areolar Tissue. Distribution.-This variety has a very wide distribution, and constitutes the subcutaneous, subserous and submucous tissue. It is found in the mucous membranes, in the true skin, and in the outer sheaths of the blood-vessels. It forms sheaths for muscles, nerves, glands, and the internal organs, and, penetrating into their interior, supports and connects the finest parts. Structure.-To the naked eye it appears, when stretched out, as a fleecy, white, and soft meshwork of fine fibrils, with here and there wider films joining in it, the whole tissue being evi- dently elastic. The openness of the meshwork varies with the Fig. 35.-Transverse section of ten- don from a cross section of the tail of a rabbit, showing sheath, fibrous septa, and branched con- nective-tissue corpuscles. The spaces left white in the drawing represent the tendinous fibres in transverse section, x 250. (Klein. CHAP. II.] SPECIAL FIBPtOUS TISSUES. 39 locality from which the specimen is taken. Under the microscope it is found to be made up of fine white fibres, which interlace in a most irregular manner, together with a variable number of elastic fibres. On the addition of acetic acid, the white fibres swell up, and become gelatinous in appearance (fig. 36) ; but as the elastic fibres resist the action of the acid, they may still be seen arranged in various directions, sometimes appearing to pass in a more or less circular or spiral manner round a small gela- tinous mass of changed white fibres. The cells of the tissue are not arranged in a very regular manner, as they are contained in Fig. 36.-Magnified view of areolar tissue {from different parts) treated with acetic acid. The white filaments are no longer seen, and the yellow or elastic fibres with the nuclei come into view. At c, elastic fibres wind round a bundle of white fibres, which, by the effect of the acid, is swollen out between the turns. Some connective-tissue corpuscles are indistinctly represented in c. (Sharpey.) the spaces (areolse) between the fibres. They communicate, how- ever, with one another by branched processes, and also with the cells forming the walls of the capillary blood-vessels in their neighbourhood. The fibres are connected together with a certain amount of albuminous cement substance. B.-Special Forms.-(a.) Gelatinous Tissue. Distribution.-Gelatinous connective tissue forms the chief part of the bodies of jelly fish ; it is found in many parts of the human embryo, but remains in the adult only in the vitreous humour of the eye. It may be best seen in the last-named situation, in the " Whartonian jelly" of the umbilical cord, and in the enamel organ of developing teeth. Structure.-It consists of cells, which in the vitreous humour 40 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. II. are rounded, and in the jelly of the enamel organ are stellate, imbedded in a soft jelly-like inter-cellular substance which forms the bulk of the tissue, and which contains a considerable quantity of mucin. In the umbilical cord, that part of the jelly immediately surround- ing the stellate cells shows marks of obscure fibrillation (fig- 37)- (6.) Adenoid or Retiform. Distribution.-It composes the stroma of the spleen and lymphatic glands, and is found also in the thymus, in the tonsils, in the follicular glands of the tongue, in Peyer's patches and in the solitary glands of the intestines, and in the mucous membranes generally. Structure. - Adenoid or retiform tissue consists of a very delicate network of minute fibrils, formed ori- ginally by the union of pro- cesses of branched connec- tive-tissue corpuscles the nuclei of which, however, are visible only during the early periods of develop- ment of the tissue (fig. 38). The nuclei found on the fibrillar meshwork do not form an-essential part of it. The fibrils are neither white fibres nor elastic tissue, as they are insoluble in boiling- water, although readily soluble in hot alkaline solu- tions. The lymphoid cor- puscles found in the interstices of the tissue are small round cells, the protoplasm of which is practically occupied by their spherical nuclei. Fig. 37.-Tissue of the jelly of Wharton from umbilical cord, a, connective-tissue cor- puscles ; b, fasciculi of connective tissue; c, spherical formative cells. (Frey.) Fig. 38.-Part of a section of a lymphatic yland, from which the corpuscles have been for the most part removed, showing the adenoid reti- culum. (Klein and Noble Smith.) CHAP. II.] DEVELOPMENT OF FIBROUS TISSUES. 41 (c.) Neuroglia.-This tissue forms the support of the Nervous elements in the Brain and Spinal cord. It consists of a very fine meshwork of fibrils, said to be elastic, and with nucleated plates which constitute the connective-tissue corpuscles imbedded in it. Development of Fibrous Tissues.-In the embryo the place of the fibrous tissues is at first occupied by a mass of roundish cells, derived from the " mesoblast." These develop either into a network of branched cells, or into groups of fusiform cells (fig. 39). The cells are imbedded in a semi-fluid albuminous substance derived either from the cells themselves or from the neighbouring blood-vessels ; this after- Fig'. 39.-Portion of submucous tissue, of gravid uterus of sow. a, branched cells, more or less spindle-shaped; b, bundles of connective tissue. (Klein.) wards forms the cement substance. In it fibres are developed, either by part of the cells becoming fibrils, the others remaining as connective-tissue corpuscles, or by the fibrils being developed from the outside layers of the protoplasm of the cells, which grow up again to their original size and remain embedded among the fibres. The process gives rise to fibres arranged in the one case in interlacing networks (areolar tissue), in the other in parallel bundles (white fibrous tissue). In the mature forms of purely fibrous tissue not only the remnants of the cell-substance, but even the nuclei may disappear. The embryonic tissue, from which '.elastic fibres are developed, is composed of fusiform cells, and a structureless intercellular substance by the gradual fibrillation of which elastic fibres are formed. The fusiform cells dwindle in size and eventually disappear so completely that in mature elastic tissue hardly a trace of them is to be found : meanwhile the elastic fibres steadily increase in size. Another theory of the development of the connective-tissue fibrils sup- poses that they arise from deposits in the intercellular substance and not from the cells themselves ; these deposits, in the case of elastic fibres, appear- ing first of all in the form of rows of granules, which, joining together, form long fibrils. It seems probable that even if this view be correct, the cells themselves have a considerable influence in the production of the deposits outside them. Functions of Areolar and Fibrous Tissue.-The main function of connective tissue is mechauicai rather than vital: it fulfils the subsidiary but important use of supporting and connecting the various tissues and organs of the body. 42 STRUC'/'UltE OF THE ELEMENTARY TISSUES. [chap. II. In glands the trabeculae of connective tissue form an interstitial frame- work in which the parenchyma or secreting gland-tissue is lodged : i n muscles and nerves the septa of connective tissue support the bundles of fibres which form the essential part of the structure. Elastic tissue, by virtue of its elasticity, has other important uses : these, again, are mechanical rather than vital. Thus the ligamentum nuchae of the horse or ox acts very much as an India-rubber band in the same position would. It maintains the head in a proper position without any muscular exertion ; and when the head has been lowered by the action of the flexor muscles of the neck, and the ligamentum nuchae thus stretched, the head is brought up again to its normal position by the relaxation of the flexor muscles which allows the elasticity of the ligamentum nuchse to come again into play. (cZ.) Adipose Tissue. Distribution.-In almost all regions of the human body a larger or smaller quantity of adipose or fatty tissue is present; the chief Fig. 40.-Ordinary fat cells of a fat tract in the omentnm of a rat. (Klein.) Fig. 41.-<T?-oup offal cells (fc) wi<A capillary vessels (<•). (Noble Smith.) exceptions being the subcutaneous tissue of the eyelids, penis, and scrotum, the nymphse, and the cavity of the cranium. Adipose tissue is also absent from the substance of many organs, as the lungs, liver, and others. Fatty matter, not in the form of a distinct tissue, is also widely present in the body, e.y., in the liver and brain, and in the blood and chyle. Adipose tissue is almost always found seated in areolar tissue, and forms in its meshes little masses of unequal size and irregular shape, to which the term lobules is commonly applied. Structure.-Under the microscope adipose tissue is found to consist essentially of little vesicles or cells which present dark, CHAP. II.] ADIPOSE TISSUE. 43 sharply-defined edges when viewed with transmitted light: they arc about or of an inch in diameter, each composed of a structureless and colourless membrane or bag, filled with fatty matter, which is liquid during life, but in part solidified after death (fig. 40). A nucleus is always present in some part or other of the cell-protoplasm, but in the ordinary condition of the cell it is not easily or always visible. This membrane and the nucleus can generally be brought into view by staining the tissue: it can be still more satisfactorily Fig. 42.-Blood-vessels of adipose tissue, a. Minute flattened, fat-lobule, in which the vessels only are represented, a, the terminal artery; v, the primitive vein ; b, the fat-vesi- cles of one border of the lobule separately represented, x 100. b. Plan of the arrange- ment of the capillaries (c) on the exterior of the vesicles; more highly magnified. (Todd and Bowman.) demonstrated by extracting the contents of the fat-cells with ether, when the shrunken, shrivelled membranes remain behind. By mutual pressure, fat-cells come to assume a polyhedral figure 4i )• The ultimate cells arc held together by capillary blood-vessels (fig. 42) ; while the little clusters thus formed are grouped into small masses, and held so, in most cases, by areolar tissue. The oily matter contained in the cells is composed chiefly of the compounds of fatty acids with glycerin, which are named olein, stearin, and palmitin. Development of Adipose Tissue.-Fat-cells are developed from con- nective-tissue corpuscles: in the infra-orbital connective-tissue cells may 44 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. II. be found exhibiting every intermediate gradation between an ordinary branched connective-tissue corpuscle and a mature fat-cell. The process of development is as follows: a few small drops of oil make their appearance in the protoplasm : by their confluence a larger drop is produced (fig. 43) : this gradually increases in size at the expense of the original protoplasm of the cell, which becomes correspond- ingly diminished in quantity till in the mature cell it only forms a thin crescentic film, closely pressed against the cell-wall, and with a nucleus imbedded in its substance (figs. 43 and 44). Under certain circumstances this process may be reversed and fat- cells may be changed back into connective-tissue corpuscles. (Kol- liker, Virchow.) Vessels and Nerves.-A large number of blood-vessels are found in adipose tissue, which subdivide until each lobule of fat contains a fine meshwork of capillaries en- sheathing each individual fat- globule (fig. 42). Although nerve fibres pass through the tissue, no nerves have been demonstrated to terminate in it. The Uses of Adipose Tissue. -Among the uses of adipose tissue, these are the chief:- a. It serves as a store of combustible matter which may be re-absorbed Fig. 43.-A lobule of developing adipose tissue from an eight months' fretus. a. Sphe- rical or, from pressure, polyhedral cells with large central nucleus, surrounded by a finely reticulated substance staining uniformly with haematoxylin, b. Similar cells with spaces from which the fat has been removed by oil of cloves, c. Simi- lar cells showing how the nucleus with enclosing protoplasm is being pressed towards periphery, d. Nucleus of endo- thelium of investing capillaries. (McCar- thy.) Drawn by Treves. Fig. 44.-Branched connective-tissue corpuscles, developing into fat-cells. (Klein.) into the blood when occasion requires, and, being burnt, may help to pre- serve the heat of the body, b. That part of the fat which is situate beneath the skin must, by its want CHAP. II.] CARTILAGE. 45 of conducting power, assist in preventing undue waste of the heat of the body by escape from the surface. c. As a packing material, fat serves very admirably to fill up spaces, to form a soft and yielding yet elastic material wherewith to wrap tender and delicate structures, or form a bed with like qualities on which such structures may lie, not endangered by pressure. As good examples of situations in which fat serves such purposes may be mentioned the palms of the hands and soles of the feet, and the orbits. rZ. In the long bones, fatty tissue, in the form known as yellow marrow, fills the medullary canal, and supports the small blood-vessels which are distributed from it to the inner part of the substance of the bone. Structure of Cartilage.-All kinds of cartilage are composed of cells imbedded in a substance called the matrix: and tho apparent differences of structure met with in the various kinds of cartilage are more due to differences in the character of the matrix than of the cells. Among the latter, however, there is also con- siderable diversity cf form and size. With the exception of the articular variety, cartilage is invested by a thin but tough firm fibrous membrane called the perichon- drizim. On the surface of the articular cartilage of the foetus, the perichondrium is represented by a film of epithelium ; but this is gradually worn away up to the margin of the articular surfaces, when by use the parts begin to suffer friction. Nerves are probably not sup- plied to any variety of cartilage. Cartilage exists in three dif- ferent forms in the human body, viz., i, Hyaline cartilage, 2, Yel- low elastic-cartilage, and 3, White fibro-cartilage. 1. Hyaline Cartilage. Distribution.-This variety of cartilage is met with largely in the human body-investing the articular ends of bones, and form- ing the costal cartilages, the nasal cartilages, and those of the larynx with the exception of the epiglottis and cornicula laryngis, as well as of those of the trachea and bronchi. II. Cartilage. n ... . rig. 45.- Ordinary hyaline cartilage from trachea of a child. The cartilage cells are enclosed singly or in pairs in a capsule of hyaline substance. x T5° diams. (Klein and Noble Smith.) 46 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAI'. II. Structure.-Like other cartilages it is composed of cells imbedded in a matrix. The cells, which contain a nucleus with nucleoli, are irregular in shape, and generally grouped together in patches (fig. 45). The patches are of various shapes and sizes, and placed Fig. 46.-b'renh cartilage from the Triton. (A. Itollett.) at unequal distances apart. They generally appear flattened near the free surface of the mass of cartilage in which they are placed, and more or less perpendicular to the surface in the more-deeply seated portions. The matrix of hyaline cartilage has a dimly granular appearance like that of ground glass, and in man and the higher animals has no apparent structure. In some carti- lages of the frog, however, even when examined in the fresh state, it is seen to be mapped out into poly- gonal blocks or cell-territories, each containing a cell in the centre, and representing what is generally called the capsule of the cartilage cells (fig. 46). Hyaline cartilage in man has really the same structure, which can be demonstrated by the use of certain reagents. If a piece of human hyaline cartilage be macerated for a long time in dilute acid or in hot water 950- -costal cartilage from an adult chap, ii.] CARTILAGE. 47 ii3° F. (350 to 450 C.), the matrix, which previously appeared quite homogeneous, is found to be resolved into a number of con- centric lamellae, like the coats of an onion, arranged round each cell or group of cells. It is thus shown to consist of nothing but a number of large systems of capsules which have become fused with one another. The cavities in the matrix in which the cells lie are connected together by a series of branching canals, very much resembling those in the cornea : through these canals fluids may make their way into the depths of the tissue. In the hyaline cartilage of the ribs, the cells are mostly larger than in the articular variety, and there is a tendency to the development of fibres in the matrix (fig. 47). The costal cartilages also frequently become calcified in old age, as also do some of those of the larynx. Fat-globules may also be seen in many cartilages (fig. 47). In articular cartilage the cells are smaller, and arranged ver- tically in narrow lines like strings of beads. Temporary Cartilage.-In the foetus, cartilage is the material of which the bones are first constructed ; the " model " of each bone being laid down, so to speak, in this substance. In such cases the cartilage is termed temporary. It closely resembles the ordinary hyaline kind ; the cells, however, are not grouped together after the fashion just described, but arc more uniformly distributed throughout the matrix. A variety of temporary hyaline cartilage which has scarcely any matrix is found in the human subject and in the higher animals generally, in early foetal life, when it constitutes the chorda dorsalis. Nutrition.-Hyaline cartilage is reckoned among the so-called structures, no blood-vessels being supplied directly to its own substance ; it is nourished by those of the bone beneath. When hyaline cartilage is in thicker masses, as in the case of the cartilages of the ribs, a few blood-vessels traverse its substance. The distinction, however, between all so-called vascular and 7?o«- vascular parts, is at the best a very artificial one. 2. Yellow Elastic Cartilage. Distribution.-In the external ear, in the epiglottis and cornicula laryngis, and in the Eustachian tube. Structure.-The cells are rounded or oval, with well-marked nuclei and nucleoli (fig. 48). The matrix in which they are seated 48 STRUCTURE OF THE ELEMENTARY TISSUES, [chap. ii. is composed almost entirely of fine elastic fibres, which form an intricate interlacement about the cells, and in their general characters are allied to the yellow variety of fibrous tissue : a small and variable quantity of hyaline intercellular substance is also usually present. A variety of elastic cartilage, sometimes called cellular, may be obtained from the external ear of rats, mice, or other small mam- mals. It is composed almost en- tirely of cells (hence the name), which are packed very closely, with little or no matrix. When present the matrix consists of very fine fibres, which twine about the cells in various directions and enclose them in a kind of network. Elastic cartilage seldom or never ossifies. 3. White Fibro-Cartilage. Distribution.-The different situations in which white fibro- cartilage is found have given rise to the following classification:-- 1. Inter-articular fibro-cartilage, e.g., the semilunar cartilages of the knee-joint. Fig. 48. -Section of the epiglottis. (Baly.) Fig. 49.-Transverse section through the intervertebral cartilage of tail of mouse, showing lamellae of fibrous tissue with cartilage cells arranged in rows between them. The cells are seen in profile, and being flattened, appear staff-shaped. Each cell lies in a capsule. X 350. (Klein and Noble Smith.) 2. Circumferential or marginal, as on the edges of the aceta- bulum and glenoid cavity. 3. Connecting, e.g., the inter-vertebral fibro-cartilages. 4. In the sheaths of tendons, and sometimes in their substance. In the latter situation, the nodule of fibro-cartilage is called a sesamoid fibro-cartilage, of which a specimen may be found in the tendon of the tibialis posticus, in the sole of the foot, and usually in the neighbouring tendon of the peroneus longus. Structure.-White fibro-cartik £ e (fig. 49), which is much more CHAP. II.] FUNCTIONS OF CARTILAGE. 49 widely distributed throughout the body than the foregoing kind, is composed, like it, of cells and a matrix; the latter, however, being made up almost entirely of fibres closely resembling those of white fibrous tissue. In this kind of fibro-cartilage it is not unusual to find a great part of its mass composed almost exclusively of fibres, and de- riving the name of cartilage only from the fact that in another portion, continuous with it, cartilage cells may be pretty freely distributed. By prolonged boiling, cartil- age yields a gelatinous sub- stance called chondrin- white fibro-cartilage yields gelatin as well. Functions of Cartilage.-Car- tilage not only represents in the foetus the bones which are to be formed (temporary cartilage), but also offers a firm, but more or less yielding, framework for certain parts in the developed body, possessing at the same time strength and elasticity. It maintains the shape of tubes as in the larynx and trachea. It affords attachment to muscles and ligaments ; it binds bones together, yet allows a certain degree cf move- ment, as between the vertebrae ; it forms a firm framework and protec- tion, yet without undue stiffness or weight, as in the pinna, larynx, and chest walls ; it deepens joint cavities, as in the acetabulum, without unduly restricting the movements of the bones. Development of Cartilage.-Cartilage is developed out of an embryonal tissue, consisting of cells with a very small quantity of intercellular sub- stance : the cells multiply by fission within the cell-capsules (fig. 6) ; while the capsule of the parent cell becomes gradually fused with the surrounding intercellular substance. A repetition of this process in the young cells causes a rapid growth of the cartilage by the multiplication of its cellular elements and corresponding increase in its matrix. Thus we see that the matrix of cartilage is chiefly derived from the cartilage cells. Kg so z,,,.0.carftWe from an inter-vertebral ligament. (Klein and Noble Smith.) III. Bone. Chemical Composition.-Bone is composed of earthy and animal matter in the proportion of about 67 per cent, of the former to 33 per cent, of the latter. The earthy matter is composed chiefly of calcium phosphate, but besides there is a small quantity (about 50 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. it. 11 of the 67 per cent.) of calcium carbonate and fluoride, and magnesium phosphate. The animal matter is resolved into gelatin by boiling. The earthy and animal constituents of bone are so intimately blended and incorporated the one with the other, that it is only by chemical action, as, for instance, by heat in one case and by the action of acids in another, that they can be separated. Their close union, too, is further shown by the fact that when by acids the earthy matter is dissolved out, or, on the other hand, when the animal part is burnt out, the shape of the bone is alike preserved. The proportion between these two constituents of bone varies in different bones in the same individual, and in the same bone at different ages. Structure.-To the naked eye there appear two kinds of struc- ture in different bones, and in different parts of the same bone, namely, the dense or compact, and the spongy or cancellous tissue. Thus, in making a longitudinal section of a long bone, as the humerus or femur, the articular extremities are found capped on their surface by a thin shell of compact bone, while their interior is made up of the spongy or cancellous tissue. The shaft, on the other hand, is formed almost entirely of a thick layer of the compact bone, and this surrounds a central canal, the medullary cavity-so called from its containing the medulla or marrow. In the flat bones, as the parietal bone or the scapula, one layer of the cancellous structure lies between two layers of the compact tissue, and in the short and irregular bones, as those of the carpus and tarsus, the cancellous tissue alone fills the interior, while a thin shell of compact bone forms the outside. Marrow.-There are two distinct varieties of marrow'-the red and yellow. died marrow is that variety which occupies the spaces in the cancellous tissue; it is highly vascular, and thus maintains the nutrition of the spongy bone, the interstices of which it fills. It contains a few fat-cells and a large number of marrow'-cells, many of which are undistinguishable from lymphoid corpuscles, and has for a basis a small amount of fibrous tissue. Among the cells are some nucleated cells of very much the same tint as coloured blood-corpuscles. There are also a few' large cells with many nuclei, termed " giant-cells " (myeloplaxes), which are derived from over-growth of the ordinary marrow-cells (fig. 51). CHAP. II.] PERIOSTEUM. 51 Yelloiv marrow fills the medullary cavity of long bones, and consists chiefly of fat-cells with numerous blood-vessels; many of its cells also are in every respect similar to lymphoid corpuscles. Fig. Cells of the red marrow of the guinea pig, highly magnified, a, a large cell, the nucleus of which appeal's to be partly divided into three by constrictions ; b, a cell, the nucleus of which shows an appearance of being constricted into a number of smaller nuclei; c, a so-called giant cell, or myeloplaxe, with many nuclei; d, a smaller myelo- plaxe, with three nuclei; e-i, proper cells of the marrow. (E. A. Schafer.) From these marrow-cells, especially those of the red marrow, are derived, as we shall presently show, large quantities of red blood- corpuscles. Periosteum and Nutrient Blood-vessels.-The surfaces of bones, except the part covered with articular cartilage, are clothed by a tough, fibrous membrane, the periosteum ; and it is from the blood-vessels which are distributed in this membrane, that the bones, especially their more compact tissue, are in great part supplied with nourishment,-minute branches from the periosteal vessels entering the little foramina on the surface of the bone, and finding their way to the Haversian canals, to be immediately described. The long bones are supplied also by a proper nutrient artery which, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are distributed to the interior of the bone. Other small blood-vessels pierce the articular extremities for the supply of the cancellous tissue. Microscopic Structure of Bone.-Notwithstanding the differences of arrangement just mentioned, the structure of all bone is found under the microscope to be essentially the same. Examined with a rather high power its substance is found to 52 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. ir. contain a multitude of small irregular spaces, approximately fusiform in shape, called lacunce, with very minute canals or canaliculi, as they are termed, leading from them, and anasto- mosing with similar little prolongations from other lacunte (fig. 52). In very thin layers of bone, no other canals than these may be Fig. 52.-Transverse section, of compact bony tissue (of humerus). Three of the Haversian canals are seen, with their concentric rings ; also the corpuscles or lacunee, with the canaliculi extending from them across the direction of the lamellee. The Haversian apertures had got filled with debris in grinding down the section, and therefore appear black in the figure, which represents the object as viewed with transmitted light. The Haversian systems are so closely packed in this section, that scarcely any interstitial lamellee are visible, x 150. (Sharpey.) visible; but on making a transverse section of the compact tissue as of a long bone, e.y., the humerus or ulna, the arrangement shown in fig. 52, can be seen. The bone seems mapped out into small circular districts, at or about the centre of each of which is a hole, and around this an appearance as of concentric layers-the lacunae and canaliculi following the same concentric plan of distribution around the small hole in the centre, with which, indeed, they communicate. On making a longitudinal section, the central holes are found to be simply the cut extremities of small canals which run lengthwise through the bone, anastomosing with each other by lateral branches (fig. 53), and are called Haversian canals, after the name of the physician, Clopton Havers, who first accurately CHAP. II.] MINUTE STRUCTURE OF BONE. 53 described them. The Haversian canals, the average diameter of which is of an inch, contain blood-vessels, and by means of them blood is conveyed to all, even the densest parts of the bone; the minute canali- cidi and lacuna) absorbing nutrient matter from the Haversian blood-vessels, and conveying it still more intimately to the very sub- stance of the bone which they traverse. The blood-vessels enter the Haversian canals both from without, by travers- ing the small holes which exist on the surface of all bones beneath the perios- teum, and from within by means of small channels which extend from the medullary cavity, or from the cancellous tissue. The arteries and veins usually occupy separate canals, and the veins, which are the larger, often present, at irregular intervals, small pouch-like dilata- tions. The lacunce are occupied by branched cells (bone- cells, or bone-corpuscles) (fig- 54), which very closely resemble the or- dinary branched connec- tive-tissue corpuscles; each of these little masses of protoplasm ministering to the nutrition of the bone immediately Fig. 53.-Longitudinal section of human ulna, show- ing Haversian canal, lacuna*, and canaliculi. (Rollett.) Fig. 54.-Bone corpuscles with their processes as seen in a thin section of human hone. (Rollett.) 54 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAP. II. surrounding it, and one lacunar corpuscle communicating with another, and with its surrounding district, and with the blood- vessels of the Haversian canals, by means of the minute streams of fluid nutrient matter which occupy the canaliculi. It will be seen from the above description that bone is essentially connective-tissue impregnated with lime salts : it bears a very close resemblance to what may be termed typical connective-tissue such as the substance of the cornea. The bone-corpuscles w ith their processes, occupying the lacuna? and canaliculi, correspond exactly to the cornea-corpuscles lying in branched spaces; while the finely fibrillated structure of the bone-lamellte, to be presently described, resembles the fibrillated substance of the cornea in which the branching spaces lie. Lamellae of Compact Bone.-In the shaft of a long bone three distinct sets of lamella? can be clearly recognised. (i.) General or fundamental lamella?; which are most easily traceable just beneath the periosteum, and around the medullary cavity, forming around the latter a series of concentric rings. At a little distance from the me- dullary and periosteal surfaces (in the deeper portions of the bone) they are more or less interrupted by (2.) Special or Haversian lamellae, which are concentrically arranged around the Haversian canals to the number of six to eighteen around each. (3.) Interstitial lamellae, which connect the systems of Haversian lamellae, filling the spaces between them, and consequently attaining their greatest development where the Haversian systems are few', and vice versa. The ultimate structure of the lamellae appears to be reticular. If a thin film be peeled off the surface of a bone, from which the earthy matter has been removed by acid, and examined with a high power of the microscope, it will be found composed of a finely reticular structure, formed apparently of very slender fibres decussating ob- liquely, but coalescing at the points of intersection, as if here the fibres were fused rather than woven together (fig. 55). (Sharpey.) Fig. 55.-Thin layer peeled off from a softened bone. This figure, which is intended to represent the reticular structureof a lamellae,gives a better idea of the object when held rather farther off than usual from the eye. x 400. (Sharpey.) CHAP. IT.] DEVELOPMENT OF BONE. 55 In many places these reticular lamella? are perforated by taper- ing fibres {Clavicnli of Gagliardi), resembling in character the ordinary white or rarely the elastic fibrous tissue, which bolt the neighbouring lamella? to- gether, and may be drawn out when the latter are torn asunder (fig. 56). These perforating fibres originate from ingrowing processes of the periosteum, and in the adult still retain their connection with it. Development of Bone. -From the point of view of their development, all bones may be subdivided into two classes. (a.) Those which are ossi- fied directly in membrane or fibrous tissue, e.g., the bones forming the vault of the skull, parietal, frontal. (6.) Those whose form, previous to ossification, is laid down in hyaline cartil- age, e.g., humerus, femur. The process of development, pure and simple, may be best studied in bones which are not preceded by cartilage-" membrane- bones " {e.g., parietal); and without a knowledge of this process (ossification in membrane), it is impossible to understand the much more complex series of changes through which such a structure as the cartilaginous femur of the foetus passes in its transformation into the bony femur of the adult (ossification in cartilage). Ossification in Membrane.-The membrane, afterwards forming the periosteum, from which such a bone as the parietal is developed, consists of two layers-an external fibrous, and an internal cellular or osteo-genetic. The external one consists of ordinary connective-tissue, being composed of layers of fibrous tissue with branched connective- tissue corpuscles here and there between the bundles of fibres The internal layer consists of a network of fine fibrils with a large Fig. 56.-ianieZte torn off from a decalcified human parietal hone at some depth from the surface. a, a lamellae, showing reticular fibres; b, b, darker part, where several lamellae are super- posed ; c, perforating fibres. Apertures through which perforating fibres had passed, are seen especially in the lower part, a, a, of the figure. (Allen Thomson.) 56 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. II. number of nucleated cells, some of which are oval, others drawn out into a long branched process, and others branched : it is more richly supplied with capillaries than the outer layer. The rela- tively large number of its cellular elements, which vary in size and shape, together with the abundance of its blood-vessels, clearly mark it out as the portion of the periosteum which is immediately concerned in the formation of bone. In such a bone as the parietal, the deposition of bony matter, which is preceded by increased vascularity, takes place in radiat- ing spiculse, starting from a " centre of ossification," and shooting out in all directions towards the periphery. While the bone in- creases in thickness by the deposition of succes- sive layers beneath the periosteum, in-growths of the osteogenetic layer of the periosteum take place, and it is by the action of theirosteoblasts that bone is secreted at a centre of ossification. The osteoblasts, being in part retained within the primary bone trabe- culae thus produced,form- ing bone corpuscles. It is doubtful what part the finely fibrillar part of the osteogenetic in-growth takes in the formation of the trabeculae, piobably it supplies the reticular matrix of the new-formed bone. On the bony trabeculae first formed, fresh layers of cells (osteo- blasts) from the osteo-genetic layer are developed side by side, lining the irregular spaces like an epithelium (fig. 57, 6). Lime-salts are deposited in the circumferential part of each osteoblast, and thus a ring of osteoblasts gives rise to a ring of bone with the remaining uncalcified portions of the osteoblasts imbedded in it as bone corpuscles, as in the first formation. Thus, the primitive spongy bone is formed, whose irregular branching spaces are occupied by processes from the osteogenetic layer of the periosteum with numerous blood-vessels and osteo- blasts. Portions of this primitive spongy bone are re-absorbed ; Fig. 57.- Osteoblasts from the parietal bone of a human embryo, thirteen weeks old. a, bony septa with the cells of the lacunae; b, layers of osteo- blasts ; c, the latter in transition to bone cor- puscles. Highly magnified. (Gegenbaur.) CHAP. II.] DEVELOPMENT OF BONE. 57 the osteoblasts being arranged in concentric successive layers and thus giving rise to concentric Haversian lamellae of bone, until the irregular space in the centre is reduced to a well-formed Haversian canal, the portions of the primitive spongy bone between the Haver- sian systems remaining as interstitial or ground-lamellae (p. 54). The bulk of the primitive spongy bone is thus gradu- ally converted into compact bony-tissue with Haversian canals. Those portions of the in-growths from the deeper layer of the perios- teum which are not con- verted into bone remain in the spaces of the can- cellous tissue as the red marrow. Ossification in Cartil- age.-Under this heading, taking the femur as a typi - cal example, we may con- sider the process by which the solid cartilaginous rod which represents it in the foetus is converted into the hollow cylinder of compact bone with expanded ends of cancellous tissue which forms the adult femur; bearing in mind the fact that this foetal cartilaginous femur is many times smaller than the medullary cavity even of the shaft of the mature bone, and, therefore, that not a trace of the original cartilage can be present in the femur of the adult. Its purpose is indeed purely temporary; and, after its calcification, it is gradually and entirely absorbed as will be presently explained. The cartilaginous rod which forms the foetal femur is sheathed in a membrane termed the perichondrium, which so far resembles the periosteum described above, that it consists of two layers, in the deeper one of which spheroidal cells predominate and blood- 1'ig. 58.-Jf'rom a transverse section through part of foetal jaw near the extreme periosteum, in the state of spongy bone, p, fibrous layer of periosteum; 1>, osteogenetic layer of perios- teum ; o, osteoblasts; c, osseous substance, containing many bone corpuscles. X 300. (Schofield.) 58 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. II. vessels abound, while the outer layer consists mainly of fusiform cells which are in the mature tissue gradually transformed into fibres. Thus, the differences between the foetal perichondrium and the periosteum of the adult are such as usually exist between the embryonic and mature forms of con- nective-tissue. Between the hyaline car- tilage of which the foetal femur consists and the bony tissue forming the adult femur, two intermediate stages exist-viz., calcified cartilage, and embryonic spongy bone. These tissues, which successively occupy the place of the foetal cartil- age, are in succession en- tirely absorbed, and their place taken by true bone. The process by which the cartilaginous is transformed into the bony femur may be divided for the sake of clearness into the following six stages:- Stage 1.-Vascularisa- tion of the Cartilage.- Processes from the osteoge- netic or cellular layer of the perichondrium containing blood-vessels grow into the substance of the cartilage much as ivy insinuates it- self into the cracks and crevices of a wall. This begins at the " centres of ossification," from which the blood-vessels spread chiefly up and down the shaft, &c. Thus the substance of the cartilage, which previously contained no vessels, is traversed by a number of branched anastomosing channels formed by the enlarge- Fig. 59--Ossifying cartilage showing loops of blood-vessels. CHAP. II.] STAGE OF CALCIFICATION. 59 ment and coalescence of the spaces in which the cartilage-cells lie, and containing loops of blood-vessels (fig. 59) and spheroidal-cells which will become osteoblasts. Stage 2.-Calcification of Cartilaginous Matrix.-Lime salts are next deposited in the form of fine granules in the hyaline matrix of the cartilage, not yet vascularised, which thus becomes gradually transformed into a number of calcified trabecula) (fig. 61, 5), enclosing alveolar spaces (primary areolae) which contain cartilage cells. By the absorption of some of the trabecula; larger spaces are de- veloped, which contain cartilage- cells for a very short time only, their places being taken by the so-called osteogenetic layer of the perichon- drium (before referred to in Stage 1) which constitutes the primary mar- row. The cartilage-cells, gradually enlarging, become more transparent and finally undergo disintegration. Stage 3.-Substitution of Em- bryonic Spongy Bone for Carti- lage.-The cells of the primary marrow arrange themselves as a con- tinuous layer like epithelium on the calcified trabeculae and deposit a layer of bone, which ensheathes the calcified trabecula): these calcified trabecula), encased in their sheaths of young bone, become gradually ab- sorbed, so that finally we have trabe- cula) composed entirely of spongy bone, all trace of the original calci- fied cartilage having disappeared. It is probable that the large multinucleated giant-cells termed "osteoclasts" by Kolliker, whicli are derived from the osteoblasts by the multiplication of their nuclei, are the agents by which the absorption of calcified cartilage, and subsequently of embryonic spongy bone, is carried on (fig. 62,0). At any rate, they are almost always found wherever absorption is in progress. Fig. 60.-Longitudinal section of ossi- fying cartilage from the humerus of a foetal sheep. Calcified trabecuhe are seen extending between the columns of cartilage cells, c, car- tilage cells, x 140. (Sharpey.) 60 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. it. Stages 2 and 3 are precisely similar to what goes on in the growing shaft of a bone which is increasing in length by the advance of the process of ossification into the intermediary carti- lage between the diaphysis and epiphysis. In this case the cartilage-cells become flattened and, multiplying by division, are Fig. 61.-Transverse section of a portion of a metacarpal bone of a fatus, showing-i, fibrous layer of periosteum; 2, osteogenetic layer of ditto; 3, periosteal bone; 4, cartilage with matrix gradually becoming calcified, as at 5, with cells in primary areolse; beyond 5 the calcified matrix is being entirely replaced by spongy bone, x 200. (V. D. Harris.) grouped into regular columns at right angles to the plane of calcification, while the process of calcification extends into the hyaline matrix between them (figs. 59 and 60). Stage 4. - Substitution of Periosteal Bone for the Primary Embryonic Spongy Bone.-The embryonic spongy bone, formed as above described, is simply a temporary tissue occupying the place of the foetal rod of cartilage, once representing the femur; and the stages 1,2, and 3 show the successive changes chap, n.] FORMATION OF COMPACT BONE. 61 which occur at the centre of the shaft. Periosteal bone is now deposited in successive layers beneath the periosteum, i.e., at the circumference of the shaft, exactly as described in the section on " ossification in membrane," and thus a casing of periosteal bone is formed around the embryonic endochondral spongy bone: this casing is thickest at the centre, where it is first formed, and thins out towards each end of the shaft. The embryonic spongy bone is absorbed, its trabecuke becoming gradually thinned and its Fig. 62.-A small isolated mass of bone next the periosteum of the lower jaw of human foetus, a, osteogenetic layer of periosteum. G, multinuclear giant cells, the one on the left acting here probably like an osteoclast. Above c, the osteoblasts are seen to become surrounded by an osseous matrix. (Klein and Noble Smith.) meshes enlarging, and finally coalescing into one great cavity- the medullary cavity of the shaft. Stage 5.-Absorption, of the Inner Layers of the Perios- teal Bone.-The absorption of the endochondral spongy bone is now complete, and the medullary cavity is bounded by periosteal bone : the inner layers of this periosteal bone are next absorbed, and the medullary cavity is thereby enlarged, while the deposi- tion of bone beneath the periosteum continues as before. The first-formed periosteal bone is spongy in character. Stage 6.-Formation of Compact Bone.-The transforma- tion of spongy periosteal bone into compact bone is effected in a manner exactly similar to that which has been described in con- nection with ossification in membrane (p. 55). The irregularities in the walls of the areolae in the spongy bone are absorbed, while 62 STRUCTURE OF THE ELEMENTARY TISSUES. [CHAP. II. the osteoblasts which line them are developed in concentric layers, each layer in turn becoming ossified till the comparatively large space in the centre is reduced to a well-formed Haversian canal Fig. 63.-Transverse section through the tibia of a foetal kitten, semi-diagrammatic. X 60. P, Periosteum. O, osteogenetic layer of the periosteum showing the osteoblasts arranged side by side, represented as pear-shaped black dots on the surface of the newly-formed bone. B, the periosteal bone deposited in successive layers beneath the periosteum and ensheathing E, the spongy endochondral bone ; represented as more deeply shaded. Within the trabeculre of endochondral spongy bone are seen the remains of the calcified cartilage trabeculre represented as dark wravy lines. C, the medulla, with V, V, veins. In the lower half of the figure the endochondral spongy bone has been completely absorbed. (Klein and Noble Smith.) (fig. 64). When once formed, bony tissue grows to some extent interstitially, as is evidenced by the fact that the lacunae are rather further apart in fully-formed than in young bone. From the foregoing description of the development of bone, it CHAP. II.] CENTRES OF OSSIFICATION. 63 will be seen that the common terms " ossification in cartilage " and "ossification in membrane " are apt to mislead, since they seem to imply two processes radically distinct. The process of ossification, however} is in all cases one and the same, all true bony tissue being formed from membrane (perichondrium or periosteum); but in the development of such a bone as the femur, which may be taken as the type of so-called " ossification in cartil- age,'' lime-salts are first of all deposited in the cartilage; this calcified cartilage, however, is gradually and entirely re-absorbed, being ulti- mately replaced by bone formed from the perios- teum, till in the adult structure nothing but true bone is left. Thus, in the process of " ossifi- cation in cartilage," calci- fication of the cartilagi- nous matrix precedes the real formation of bone. We must, therefore, clearly distinguish be- tween calcification and ossification. The former is simply the infiltra- tion of an animal tissue with lime-salts, and is, therefore, a change of chemical composition rather than of structure ; while ossification is the formation of true bone-a tissge more complex and more highly organized than that from which it is derived. Centres of Ossification.-In all bones ossification commences at one or more points, termed " centres of ossification." The long bones, e.g., femur, humerus, &c., have at least three such points -one for the ossification of the shaft or diaphysis, and one for each articular extremity or epiphysis. Besides these three primary centres which are always present in long bones, various secondary u f 9 d b d- f Fig. 64.-Transverse section of femur of a human embryo about eleven weeks old. a, rudimentary Haversian canal in cross section; b, in longi- tudinal section; c, osteoblasts ; d, newly formed osseous substance of a lighter colour ; e, that of greater age; f, lacuna- with their cells ; g, a cell still united to an osteoblast. (Frey.) 64 STRUCTURE OF THE ELEMENTARY TISSUES. [chap. it. centres may be superadded for the ossification of different pro- cesses. Growth of Bone.-Bones increase in length by the advance of the process of ossification into the cartilage intermediate between the diaphysis and epiphysis. The increase in length indeed is due entirely to growth at the two ends of the shaft. This is proved by inserting two pins into the shaft of a growing bone : after some time their distance apart will be found to be unaltered though the bone has gradually increased in length, the growth having taken place beyond and not between them. If now one pin be placed in the shaft, and the other in the epiphysis, of a growing bone, their distance apart will increase as the bone grows in length. Thus it is that if the epiphyses with the intermediate cartilage be removed from a young bone, growth in length is no longer possible ; while the natural termination of growth of a bone in length takes place when the epiphyses become united in bony continuity with the shaft. Increase in thickness in the shaft of a long bone, occurs by the deposition of successive layers beneath the periosteum. If a thin metal plate be inserted beneath the periosteum of a growing bone, it will soon be covered by osseous deposit, but if it be put between the fibrous and osteogenetie layers, it will never become enveloped in bone, for all the bone is formed beneath the latter. Other varieties of connective tissue may become ossified, e.g., the tendons in some birds. Functions of Bones.-Bones form the framework of the body ; for this they are fitted by their hardness and solidity together with their compara- tive lightness ; they serve both to protect internal organs in the trunk and skull, and as levers worked by muscles in the limbs ; notwithstanding their hardness they possess a considerable degree of elasticity, which often saves them from fractures. CHAP. III.] THE BLOOD. 65 CHAPTER HI. THE BLOOD. The blood of man, as indeed of the great majority of vertebrate animals, is a more or less viscid red fluid. The exact shade of red is variable, for whereas that taken from the arteries, from the left side of the heart and from the pulmonary veins, is of a bright scarlet hue, that obtained from the systemic veins, from the right side of the heart, and from the pulmonary artery, is of a much darker colour, and varies from bluish-red to reddish-black. At first sight, the red coloui- appears to belong to the whole mass of blood, but on further examination this is found not to be the case. In reality blood consists of an almost colourless fluid, called Plasma or Liquor Sanguinis, in which are suspended numerous minute rounded masses of protoplasm, called Blood Corpuscles, which are, for the most part, coloured, and it is to their presence in the fluid that the red colour of the blood is due. Even when examined in very thin layers blood is opaque, on account of the different refractive powers possessed by its two constituents, viz., the plasma and the corpuscles. On treatment with chloroform and other reagents, however, it becomes trans- parent, and assumes a lake colour, in consequence of the colouring matter of the corpuscles having been discharged into the plasma. The average specific gravity of blood at 6o° F. (150 C.) is 1055, the extremes consistent with health being 1045-1062. The re- action of blood is faintly alkaline. Its temperature varies slightly, the average being ioo° F. (37'8° C.). The blood stream is warmed by passing through the muscles, nerve centres, and glands, but is somewhat cooled on traversing the capillaries of the skin. Recently drawn blood has a distinct odour, which in many cases is characteristic of the animal from which it has been taken. It may be further developed also by adding to blood a mixture of equal parts of sulphuric acid and water. Quantity of the Blood.-The quantity of blood in any animal under normal conditions bears a pretty constant relation to the body weight. The methods employed for estimating it are not so simple as might at first sight be thought. For 66 THE BLOOD. [chap. in. example, it would not be possible to get any accurate informa- tion on the point from the amount obtained by rapidly bleeding an animal to death, for then an indefinite quantity would remain in the vessels, as well as in the tissues ; nor, on the other hand, would it be possible to obtain a correct estimate by less rapid bleeding, as, since life would be more prolonged, time would be allowed for the passage into the blood of lymph from the lym- phatic vessels and from the tissues. In the former case, therefore, we should under-estimate, and in the latter over-estimate the total amount of the blood. Of the several methods which have been employed, the most accurate appears to be the following. A small quantity of blood is taken from an animal by venesection ; it is defibrinated and measured, and used to make standard solutions of blood. The animal is then rapidly bled to death, and the blood which escapes is collected. The blood vessels are next washed out with water or saline solutions until the washings are no longer coloured, and these are added to the previously withdrawn blood ; lastly the whole animal is finely minced with water or saline solution. The fluid obtained from the mincings is carefully filtered, and added to the diluted blood previously obtained, and the whole is measured. The next step in the process is the comparison of the colour of the diluted blood with that of standard solutions of blood and water of a known strength, until it is discovered to what stan- dard solution the diluted blood corresponds. As the amount of blood in the corresponding standard solution is known, as well as the total quantity of diluted blood obtained from the animal, it is easy to calculate the absolute amount of blood which the latter contained, and to this is added the small amount which was withdrawn to make the standard solutions. This gives the total amount of blood which the animal contained. It is contrasted with the weight of the animal, previously known. The result of many experiments shows that the quantity of blood in various animals averages ~ to ~ of the total body weight. An estimate of the quantity in man which corresponded nearly with this proportion, was made some years ago from the following data. A criminal was weighed before and after decapitation; the difference in the weight representing, of course, the quantity of blood which escaped. The blood-vessels of the head and trunk were then washed out by the injection of water, until the fluid which escaped had only a pale red or straw colour. This fluid was then also weighed ; and the amount of blood which it repre- sented was calculated by comparing the proportion of solid matter contained in it with that of the first blood which escaped on decapitation. Two experiments of this kind gave precisely similar results. (Weber and Lehmann.) CHAP. III.] COAGULATION OF THE BLOOD. 67 It should be remembered, in connection with these estimations, that the quantity of the blood must vary, even in the same animal, very considerably with the amount of both the ingesta and egesta of the period immediately preceding the experiment; and it has been found, indeed, that the quantity of blood obtainable from the body of a fasting animal rarely exceeds a half of that which is present soon after a full meal. Coagulation of the Blood.-One of the most characteristic properties which the blood possesses is that of clotting or coagulating, when removed from the body. This phe- nomenon may be ob- served under the most favourable con- ditions in blood which has been drawn into an open vessel. In about two or three minutes, at the ordi- nary temperature of the air, the surface of the fluid is seen to become semi-solid or jelly-like, and this change takes place, in a minute or two afterwards, at the sides of the vessel in which it is contained, and then extends throughout the entire mass. The time which is required for the blood to become solid is about eight or nine minutes. The solid mass occupies exactly the same volume as the previously liquid blood, and adheres so closely to the sides of the containing vessel that if it be inverted none of its contents escape. The solid mass is the crassamentum or clot. If the clot be watched for a few minutes, drops of a light, straw- coloured fluid, the may be seen to make their appearance on the surface and, as they become more and more numerous, to run together, forming a complete superficial stratum above the solid clot. At the same time the fluid begins to transude at the sides and at the under surface of the clot, which in the course of an hour or two floats in the liquid. The first drops of serum appear on the surface about eleven or twelve minutes after the blood has Fig. 65.-Reticulum, of fibrin, from a drop of human blood, after treatment with rosanilin. (Ranvier.) 68 THE BLOOD. [chap. tit. been drawn ; and the fluid continues to transude for from thirty- six to forty-eight hours. The clotting of blood is due to the development in it of a sub- stance called fibrin, which appears as a mesh work (fig. 65) of fine fibrils. This meshwork entangles and encloses within it the blood corpuscles, as clotting takes place too quickly to allow them to sink to the bottom of the plasma. The first clot formed, therefore, includes the whole of the constituents of the blood in an apparently solid mass, but soon the fibrinous mesh work begins to contract, and the serum which does not belong to the clot is squeezed out. When the whole of the serum has transuded, the clot is found to be smaller, but firmer and harder, as it is now made up of fibrin and blood corpuscles only. It will be noticed that coagulation rearranges the constituents of the blood according to the following scheme, liquid blood being made up of plasma and blood-corpuscles, and clotted blood of serum and clot. Liquid Blood. Plasma. i Corpuscles. Serum. Fibrin. Clot. Under ordinary circumstances coagulation occurs, as we have mentioned above, before the red corpuscles have had time to subside; and thus from their being entangled in the meshes of the fibrin, the clot is of a deep red colour throughout, somewhat darker, it may be, at the most dependent part, from accumulation of red corpuscles, but not to any very marked degree. When, however, coagulation is delayed from any cause, as when blood is kept at a temperature of 320 F. (o° C.), or when clotting is normally a slow process, as in the case of horse's blood, or, lastly, in certain diseased conditions of the blood in which clotting is naturally delayed, time is allowed for the coloured corpuscles to sink to the bottom of the fluid. When clotting does occur, the upper layers of the blood, being free of coloured corpuscles and consisting chiefly of fibrin, form a superficial stratum differing in appearance from the rest of the clot, in that it is of a grayish yellow colour. This is known as the "buffy coat.'' I Clotted Blood. chap, in.] THE FORMATION OF FIBRIN. 69 When the huffy coat has been produced in the manner just described, it commonly contracts more than the rest of the clot, on account of the absence of coloured corpuscles from its meshes, and because contraction is less interfered with by adhesion to the interior of the containing vessel in the vertical than the horizontal direction. This produces a cup-like appearance of the huffy coat, and the clot is not only buffed but cupped on the surface. The buffed and cupped appearance of the clot is well marked in certain states of the system, especially in inflammation, where the fibrin- forming constituents are in excess, and it is also well marked in chlorosis where the corpuscles are deficient in quantity. Formation of Fibrin.-That the clotting of blood is due to the gradual appearance in it of fibrin is universally acknowledged. It may be easily demonstrated. For example, if recently drawn blood be whipped with a bundle of twigs which presents numerous points of contact and so, as we shall presently see, facilitates coagu- lation, the fibrin may be withdrawn from the blood before it can entangle the blood corpuscles within its meshes, as it adheres to the twigs in stringy threads almost free from corpuscles; whereas the blood from which the fibrin has been withdrawn no longer exhibits the power of spontaneous coagulability. Although these facts have long been known, the closely associated problem as to the exact manner in which fibrin is formed is still only partially solved. It will be most convenient to treat of the question step by step. In the first place it appears that under the ordinary conditions of experiment, fibrin is chiefly, if not entirely to be obtained from plasma; for although the colourless corpuscles may be intimately connected with the process, as will be shown later on, yet the coloured corpuscles do not appear to take an active part in it. This statement does not exclude the possibility that fibrin may be derived from the coloured corpuscles under certain conditions. Indeed, this is more than probable, as experiments have shown that if a little defibrinated blood be added to serum, the haemoglobin leaves the stroma of the coloured cor- puscles of the blood, and a substance arises from it called stroma-fibrin, indistinguishable from ordinary fibrin, which produces clotting of the serum. This may be shown by experimenting with plasma free from coloured corpuscles. Plasma may be procured by delaying coagulation in blood by keeping it at a low temperature, 320 F. (o° C.), until the coloured corpuscles, which are of higher specific gravity than the other constituents of blood, have had time to sink to the bottom of the 70 THE BLOOD. [chap. hi. containing vessel, and to leave an upper stratum of colourless plasma, in the lower layers of which, however, are many colourless corpuscles. The blood of the horse is specially suited for the purposes of this experiment, as might have been expected from what has been stated as to its naturally slow coagulating power. A portion of the colourless plasma, if decanted into another vessel and exposed to the ordinary temperature of the air, will be seen to coagulate just as though it were the entire blood, producing a clot similar in all respects to blood clot, except that it is almost colourless from the absence of red corpuscles. But if some of the plasma be diluted with * neutral saline solution, coagulation is delayed, and the stages of the gradual formation of fibrin may be more conveniently watched. The viscidity which precedes the com- plete coagulation may be actually seen to be due to fibrin fibrils developing in the fluid-first of all at the circumference of the containing vessel, and gradually extending throughout the mass. If a further portion of plasma be whipped with a bundle of twigs, the fibrin may be obtained as a solid, stringy mass, just in the same way as from the entire blood, and the resulting fluid no longer retains its power of spontaneous coagulability. In these experiments, it is not necessary that the plasma shall have been obtained by the process of cooling above described, as plasma obtained in any other way, c.y., by allowing blood to flow direct from the vessels of an animal into a vessel containing a third or a fourth of its bulk of a saturated solution of a neutral salt (preferably of magnesium sulphate) and mixing carefully, will answer the purpose and, just as in the other case the coloured corpuscles will subside leaving the clear superstratum of (salted) plasma. In order that this plasma may coagulate, it is necessary to get rid of the salts by dialysis, or to dilute it with several times its bulk of water. Evidently, therefore, fibrin is as a rule derived from the plasma of blood. The second step in the investigation is to consider from what part of the plasma fibrin is formed, and to that we shall now turn our attention. If plasma be saturated with solid magnesium sulphate or sodium chloride, a white, sticky, precipitate called plasmine is thrown down, after the removal of which, by filtration, the plasma will not spontaneously coagulate. Plasmine is soluble in dilute neutral * Neutral saline solution commonly consists of a 6 to 75 solution of common salt (sodium chloride) in water. CHAP. III.] PLASMINE. 71 saline solutions, and the solution of it speedily coagulates, pro- ducing a clot composed of fibrin. Blood plasma therefore contains a substance without which it cannot coagulate, and a solution of w'hich is spontaneously coagulable. This substance is very soluble in dilute saline solutions, and is not, therefore, fibrin, which is insoluble in these fluids. We are, therefore, led to the belief that plasmine produces or is converted into fibrin, when clotting of fluids containing it takes place. There is distinct evidence that plasmine is a compound body made up of two or more substances, and that it is not mere soluble fibrin. This view is based upon the following observations :- There exists in all the serous cavities of the body in health, e.g., the pericardium, the peritoneum, and the pleura, a certain small amount of transparent fluid, generally of a pale straw colour, which in diseased conditions may be greatly increased. It somewhat resembles serum in appearance, but in reality differs from it, and is probably closely allied to plasma. This serous fluid is not, as a rule, spontaneously coagulable, but may be made to clot on the addition of serum, which is also a fluid which has no tendency of itself to coagulate. The clot produced consists of fibrin, and the clotting is identical with the clotting of plasma. From the serous fluid (that from the inflamed tunica vaginalis testis or hydrocele fluid is mostly used) we may obtain, by saturating it with solid magnesium sulphate or sodium chloride, a white viscid substance as a precipitate which is called fibrinogen. If fibrinogen be sepa- rated by filtration, it can be dissolved in water, as a certain amount of the neutral salt used in precipitating it is entangled with the precipitate, and is sufficient to produce a dilute saline solution in which fibrinogen, being a body of the globulin class, is soluble. The solution of fibrinogen has no tendency to clot of itself. The same body may also be obtained as a viscid precipitate from hydrocele fluid by diluting it with water, and passing a brisk stream of carbon dioxide gas through the solution. Now if blood-serum be added to a solution of fibrinogen, obtained in either of these ways, the mixture clots. On the other hand from blood-serum may be obtained another globulin very similar in properties to fibrinogen, if it be treated in either of the ways by which fibrinogen is obtained from hydro- cele fluid; this substance is called paraglobulin, and it may be separated by filtration and dissolved in a dilute saline solution in a manner similar to fibrinogen. 72 THE BLOOD. [CHAP. III. If the solutions of fibrinogen and paraglobulin be mixed, the mixture cannot be distinguished from a solution of plasmine, and in a great majority of cases firmly clots like that solution, whereas a mixture of the hydrocele fluid and serum, from which these bodies have been respectively taken, no longer manifests the like property. In addition to this evidence of the compound nature of plasmine, it may be further shown that, if sufficient care be taken, both fibrinogen and paraglobulin may be separately obtained from plasma : the one, fibrinogen, as a flaky precipitate, by adding carefully 13 per cent, of crystalline sodium chloride to it; and the other, paraglobulin may be precipitated, after the removal of fibrinogen by filtration, on the further addition to saturation of the same salt or of magnesium sulphate to the filtrate. It is evident, therefore, that both these substances must be thrown down together when plasma is at once saturated with sodium chloride or magnesium sulphate, and that the mixture of the two corresponds with plasmine. So far it has been shown that plasmine, the antecedent of fibrin, to the possession of which blood owes its power of coagulating, is not a simple body, but is composed of at least two factors-viz., fibrinogen and paraglobulin ; there is reason for believing that yet another body is associated with them in plasmine to produce coagulation ; this is what is known under the name of fibrin ferment (Schmidt). Let us now consider the evidence in favour of this view. It was at one time thought that the reason why hydrocele fluid coagulated, when serum was added to it, was that the latter fluid supplied the paraglobulin which the former lacked; this, however, is not the case, as hydrocele fluid does not lack this body, and, moreover, if paraglobulin, obtained from serum by the carbonic acid method, be added to it, it will not coagulate, neither will a mixture of solutions of fibrinogen and paraglobulin obtained in the same way. But if paraglobulin, obtained by the saturation method, be added to hydrocele fluid, it will clot, as will also, as we have seen above, a mixed solution of fibrinogen and paraglobulin, both obtained by the saturation method. From this it is evident that in plasmine there is something more than the two bodies above mentioned, and that this something is precipitated with the para- globulin by the saturation method, and is not precipitated by the carbonic acid method. ( HAP. til.] FIBRIN FERMENT. 73 The following experiments show that it is of the nature of a ferment. If defibrinated blood or serum be kept in a stoppered bottle with its own bulk of alcohol for some weeks, all the proteid matter is precipitated in a coagulated form ; if the precipitate be then removed by filtration, dried over sulphuric acid, finely powdered, and then suspended in water, a watery extract may be obtained by further filtration, containing extremely little, if any, proteid matter. Yet a little of this watery extract will produce coagulation in fluids, e.</., hydrocele fluid or diluted plasma, which are not spontaneously coagulable, or which coagulate slowly and with difficulty. It will also cause a mixture of fibrinogen and paraglobulin, both obtained by the carbonic acid method, to clot. The watery extract appears to contain the body which is precipi- tated with the paraglobulin by the saturation method. Its active properties are entirely destroyed by boiling. The amount of the extract added does not influence the amount of the clot formed, but only the rapidity of clotting, and moreover the active sub- stance contained in the extract evidently does not form part of the clot, as it may be obtained from the serum after blood has clotted. So that the third factor, which is contained in the aqueous extract of blood, appears to belong to that class of bodies which promote the union of, or cause changes in, other bodies, without themselves entering into union or undergoing change, i.e., ferments. The third substance has, therefore, received the name fibrin ferment. This ferment is developed in blood soon after it has been shed, and its amount appears to increase for some little time afterwards (p. 74). So far we have seen that plasmine is a body composed of three substances, viz., fibrinogen, paraglobulin, and fibrin ferment. The next question which presents itself is, are these three bodies actively concerned in the formation of fibrin I Here we come to a point about which two distinct opinions prevail, and which it will be necessary to mention. On the one hand Schmidt holds that fibrin is produced by the interaction of the two proteid bodies, viz., fibrinogen and para- globulin, brought about by the presence of a special fibrin ferment. Also, that when coagulation does not occur in serum, which contains paraglobulin and the fibrin ferment, the non-coagulation is accounted for by lack of fibrinogen, and that when it does not occur in fluids which contain fibrinogen, it is due to the absence of paraglobulin, or of the ferment, or of both. It will be seen 74 THE BLOOD. [chap. III. that, according to this view, paraglobulin has a very important fibrino-plastic property. On the other hand Hammersten holds that jyaraglobulin is not an essential in coagulation, or at any rate does not take an active part in the process. He believes that paraglobulin possesses the property in common with many other bodies of combining with- er decomposing, and so rendering inert-certain substances which have the power of preventing the formation or precipitation of fibrin, this power of preventing coagulation being well known to belong to the free alkalies, to the alkaline carbonates, and to certain salts; and he looks upon fibrin as formed from fibrinogen, which is either (i) decomposed into that substance with the pro- duction of some other substances ; oi' (2) bodily converted into it under the action of a ferment, which is frequently precipitated with paraglobulin. Hammersten's view as to the formation of fibrin from fibrinogen by the action of a second body possibly of the ferment class, is now', very generally held. The presence of a certain but small amount of salts, especially of sodium chloride, is necessary for coagulation, and without it, clotting cannot take place. Sources of the Fibrin Generators.-It has been previously remarked that the colourless corpuscles which are always present in smaller or greater numbers in the plasma, even when this has been freed from coloured corpuscles, have an important share in the production of the clot. The proofs of this may be briefly summarised as follow's:-(1) That all strongly coagulable fluids contain colourless corpuscles almost in direct proportion to their coagulability; (2) That clots formed on foreign bodies, such as needles projecting into the interior or lumen of living blood-vessels, are preceded by an aggregation of colourless corpuscles ; (3) That plasma in which the colourless corpuscles happen to be scanty, clots feebly ; (4) That if horse's blood be kept in the cold, so that the corpuscles subside, it will be found that the lowest stratum, containing chiefly coloured corpuscles, will, if removed, clot feebly, as it contains little of the fibrin factors; whereas the colourless plasma, especially the lower layers of it in which the colourless corpuscles are most numerous, will clot well, but if filtered in the cold will not clot so well, indicating that when filtered nearly free from colourless corpuscles even the plasma does not contain sufficient of all the fibrin factors to produce thorough coagulation ; (5) In a drop of coagulating blood, observed under CHAP. III.] CONDITIONS AFFECTING COAGULATION. 75 the microscope, the fibrin fibrils are seen to start from the colour- less corpuscles. Although the intimate connection of the colourless corpuscles with the process of coagulation seems indubitable, for the reasons just given, the exact share which they have in contributing the various fibrin factors still remains uncertain. It is generally believed that the fibrin-ferment at any rate is contributed by them, inasmuch as the quantity of this substance obtainable from plasma bears a direct relation to the numbers of colourless corpuscles which the plasma contains. Many believe that the fibrinogen too is wholly or in part derived from them, and also that they are the usual source of the paraglobulin present in plasma. According to this view all the fibrin factors are derived from the disintegration of the colourless corpuscles. We have seen that the coloured corpuscles may also under certain circum- stances take a share in producing the fibrin generators. Conditions affecting Coagulation.-The coagulation of the blood is hastened by the following means:- 1. Moderate warmth,-from about ioo° to 120° F- (37'8- 49° C.). 2. Rest is favourable to the coagulation of blood. Blood, of which the whole mass is kept in uniform motion, as when a closed vessel completely filled with it is constantly moved, coagulates very slowly and imperfectly. 3. Contact with foreign matter, and especially multiplication of the points of contact. Thus, as before mentioned, fibrin may be quickly obtained from liquid blood by stirring it with a bundle of small twigs; and even in the living body the blood will coagulate upon rough bodies projecting into the vessels. 4. The free access of air.-Coagulation is quicker in shallow than in tall and narrow vessels. 5. The addition of less than tivice the bulk of water. The blood last drawn is said, from being more watery, to coagu- late more quickly than the first. The coagulation of the blood is retarded, suspended, or prevented by the following means :- 1. Cold retards coagulation; and so long as blood is kept at a temperature, 320 F. (o° C.), it will not coagulate at all. Freezing the blood, of course, prevents its coagulation; yet it will coagulate, though not firmly, if thawed after being frozen ; and it will do so, 76 THE BLOOD. [( HAP. III. even after it has been frozen for several months. A higher tem- perature than 120° F. (49° C.) retards coagulation, by coagulating the albumen of the serum, and a still higher one above 56° C. prevents it altogether. 2. The addition of water in greater proportion than twice the bulk of the blood, also the addition of syrup, glycerine, and other viscid substances. 3. Contact with living tissues, and especially with the interior of a living blood-vessel. 4. The addition of neutral salts in the proportion of 2 or 3 per cent, and upwards. When added in large proportion most of these saline substances prevent coagulation altogether. Coagulation, however, ensues on dilution with water. The time during which blood can be thus preserved in a liquid state and coagulated by the addition of water, is quite indefinite. 5. Imperfect aeration,-as in the blood of those who die by asphyxia. 6. In inflammatory states of the system the blood coagulates more slowly although more firmly. 7. Coagulation is retarded by exclusion of the blood from the air, as by pouring oil on the surface, etc. In vacuo, the blood coagu- lates quickly; but Lister thinks that the rapidity of the process is due to the bubbling which ensues from the escape of gas, and to the blood being thus brought more freely into contact with the containing vessel. Receiving blood into a vessel, well smeared inside with oil, fat, or vaseline, is said also to retard or prevent coagulation. 8. The coagulation of the blood is prevented altogether by the addition of strong acids and caustic alkalies. 9. It has been believed, and chiefly on the authority of Hunter, after certain modes of death the blood does not coagulate ; he enumerates the death by lightning, over-exertion (as in animals hunted to death), blows on the stomach, fits of anger. He says, " I have seen instances of them all." Doubtless he had done so; but the results of such events are not constant. The blood has been often observed coagulated in the bodies of animals killed by lightning or an electric shock; and Gulliver has published instances in which he found clots in the hearts of hares and stags hunted to death, and of cocks killed in fighting. 10. The injection of peptones, or of various digestive ferments, e.g., trypsin or pepsin, into the vessels of an animal appears to prevent CHAP. III.] THE FLUIDITY OF LIVING BLOOD. 77 or stay coagulation of its blood if it be killed soon after. The secre- tion of the month of the leech, and possibly the blood squeezed out of its body when full, also prevents the clotting if added to blood. Cause of the fluidity of the blood within the living- body.-Very closely connected with the problem of the coagulation of the blood is the question,-why does the blood remain liquid within the living body ? We have certain pathological and experimental facts, apparently opposed to one another, which bear upon it, and these may be, for the sake of clearness, classed under two heads:- (1) Blood will coagulate within the living body under certain condi- tions-for example, on ligaturing an artery, whereby the inner and middle coats are generally ruptured, a clot will form within it, or by passing a needle through the coats of the vessel into the blood stream a clot will gradually form upon it. Other foreign bodies, e.g., wire, thread, etc., produce the same effect. It is a well-known fact that small clots are apt to form upon the roughened edges of the valves of the heart when the roughness has been produced by inflammation, as in endocarditis, and it is also equally true that aneurysms of arteries are sometimes spontaneously cured by the deposi- tion within them, layer by layer, of fibrin from the blood stream, which natural cure it is the aim of the physician or surgeon to imitate. (2) Blood will remain liquid under certain conditions outside the body, without the addition of any re-agent, even if exposed to the air at the ordinary temperature. It is well known that blood remains fluid in the body for some time after death, and it is only after rigor mortis has occurred that the blood is found clotted. It has been demonstrated by Hewson, and also by Lister, that if a large vein in the horse or similar animal be ligatured in two places some inches apart, and after some time be opened, the blood contained within it will be found fluid, and that coagulation will occur only after a considerable time. But this is not due to occlusion from the air simply. Lister further showed that if the vein with the blood contained within it be removed from the body, and then be carefully opened, the blood might be poured from the vein into another similarly prepared, as from one test-tube into another, thereby suffering free exposure to the air, without coagulation occurring as long as the vessels retain their vitality. If the endothelial lining of the vein, however, be injured, the blood will not remain liquid. Again, blood will remain liquid for days in the heart of a turtle, which continues to beat for a very long time after removal from the body. Any theory which aims at explaining the normal fluidity of the blood within the living body must reconcile the above apparently contradictory facts, and must at the same time be made to include all other known facts concerning coagulation. We may therefore dismiss as insufficient the following :-that coagulation is due to exposure to the air or oxygen ; that it is due to the cessation of the circulatory movement; that it is due to evolution of various gases, or to the loss of heat. Two theories, those of Lister and Briicke, remain. The former supposes that the blood has no natural tendency to clot, but that its coagulation out of the body is due to the action of foreign matter with which it happens to be brought into contact, and in the body to conditions of the tissues which cause them to act towards it like foreign matter. The latter, on the other 78 THE BLOOD. [chap. in. hand, supposes that there is a natural tendency on the part of the blood to clot, but that this is restrained in the living body by some inhibitory power resident in the walls of the containing vessels. The blood must contain all the substances from which fibrin is formed, and the re-arrangement of these substances must occur very quickly whenever the blood is shed, and so it is somewhat difficult to prevent coagulation. It seems more reasonable to hold, therefore, that the blood has a strong tendency to clot, rather than that it has no special tendency thereto. But it has been recently suggested that the reason why blood does not coagulate in the living vessels, is that the factors which are necessary for the formation of fibrin are not in the exact state required for its production, and that at any rate the fibrin ferment is not formed or is not free in the living blood, but that it is produced (or set free) at the moment of coagulation by the dis- integration of the colourless (and possibly of the coloured) cor- puscles. This supposition is certainly plausible, and if it be a true one, it must be assumed either' that the living blood-vessels exert a restraining influence upon the disintegration of the cor- puscles in sufficient numbers to form a clot, or that they render inert any small amount of fibrin ferment which may have been set free by the disintegration of a few corpuscles ; as it is certain firstly that corpuscles of all kinds must from time to time disintegrate in the blood without causing it to clot; and, secondly, that shed and defibrinated blood which contains blood corpuscles, broken down and disintegrated, will not, when injected into the vessels of an animal, under ordinary conditions, produce clotting. There must be a distinct difference, therefore, if only in amount, between the normal disintegration of a few colourless corpuscles in the living uninjured blood-vessels and the abnormal disintegration of a large number which occurs whenever the blood is shed without suitable precaution, or when coagulation is unrestrained by the neighbourhood of the living uninjured blood-vessels. The explanation of the clotting of blood which has been given in the pre- ceding pages and which depends chiefly upon the researches of Alex. Schmidt and Hammersten, supposes that it is one of the fermentative actions, so many of which are believed to go on in the living body. Wooldridge ably contests this view of the process. His laborious researches have led him to the belief that coagulation of the blood is a vital process, or rather that it is the last act of vitality displayed by blood plasma, which he considers to be during life, living protoplasm. Some of the results of his experiments may with advantage be here mentioned, as they correct and amplify the informa- chap, in.] THE BLOOD CORPUSCLES. 79 tion as to blood-clotting which has been hitherto given and received. Firstly, he has shown that plasma itself contains everything that is necessary for coagulation. Peptone plasma obtained by injecting a solution of peptone into the veins of an animal and bleeding it immediately after- wards was experimented with. The whole of the corpuscular elements were removed by repeated treatment with a centrifugal machine. The plasma thus obtained was shown to clot by the use of some simple mechanical means, e.g., filtering through a clay cell, or through filter paper, or on neutralisation with acetic acid, or carbonic acid, or by dilution with water or saline solution. Thus it would appear that if the colourless blood corpuscles aid coagulation, their influence is only secondary. Secondly, he has shown that the important precursor of clotting in this peptone plasma may be separated from it, as a precipitate, if the plasma be kept in ice for some time, and that after its removal the plasma contains only a little fibrinogen capable of clotting by the action of fibrin ferment. If the plasma be diluted with water or slightly acidulated, however, the fibrin ferment is able to produce a complete clotting. In peptone plasma, Wooldridge states that three coagulable bodies exist, which he calls A, B, and C fibrinogen, and which are closely allied to one another. C-fibrinogen is identical with the body which has been hitherto described as fibrinogen, is present in very small amount, and clots on addition of fibrin ferment. The coagulable matter present in greatest amount is B-fibrinogen, which clots on addition of lecithin, or of lymph corpuscles, but not on the addition of fibrin ferment, A-fibrinogen is separated from plasma by cooling, in minute regular rounded granules, from which, rounded distinctly biconcave discs arise, if watched under the microscope, quite indistinguishable from coloured blood corpuscles; it is not coagulated by fibrin ferment. Finally, he considers that when blood plasma dies, an action takes place between A- and B-fibrinogen, which are both compounds of proteid and lecithin. The essential of this action, is a loss of lecithin on the part of the former and a gain of lecithin on the part of the latter, with the result of the production of fibrin, a third proteid-lecithin compound, and the setting free of other substances contained in the serum, including fibrin ferment. Thus, fibrin ferment, a body which can convert C-fibrinogen into fibrin, is not present in living plasma, but is a result of its disorganisation or death. As the fibrinogen which can be clotted by the ferment is only present in minimal amounts in living plasma, injection of a solution of fibrin ferment or of shed blood does not produce intra-vascular clotting, whereas injection of lymph corpuscles from lymphatic glands or of lecithin, either of which will produce clotting of the other fibrinogens which form the bulk of the coagulable matter in living blood, leads to extensive intra-vascular clotting. The Blood Corpuscles. There are two principal forms of corpuscles, the red and the white, or, as they are now frequently named, the coloured and the colourless. In the moist state, the red corpuscles form about 45 per cent, by weight, of the whole mass of the blood. The proportion of colourless corpuscles is only as i to 500 or 600 of the coloured. 80 THE BLOOD. [ci|Al'._ 111. Red or Coloured Corpuscles.-Human red blood-corpuscles are circular, biconcave discs with rounded edges, from -30V0 to inch in diameter, and hich in thickness, becoming flat or convex on addition of water. When viewed singly, they appear of a pale yellowish tinge ; the deep red colour which they give to the blood being observable in them only when they are seen €» masse. They are composed of a colourless, structureless, and transparent filmy framework or stroma, infiltrated in all parts by a red colouring matter termed haemoglobin. The stroma is tough and elastic, so that, as the corpuscles circulate, they admit of elongation and other changes of form, in adaptation to the ves- sels, yet recover their natural shape as soon as they escape from compression. The term cell, in the sense of a bag or sac, although sometimes applied, is inapplicable to the red blood corpuscle ; and it must be considered, if not solid throughout, yet as having no such variety of consistence in different parts as to justify the notion of its being a membranous sac with fluid contents. The stroma exists in all parts of its substance, and the colouring-matter uniformly pervades this, and is not merely surrounded by and mechanically enclosed within the outer wall of the corpuscle. The red corpuscles have no nuclei, although, in their usual state, the unequal refraction of transmitted light gives the ap- pearance of a central spot, brighter or darker than the border, according as it is viewed in or out of focus. Their specific gravity is about 1088. Varieties.-The red corpuscles are not all alike, some being rather larger, paler, and less regular than the majority, and some- times flat or slightly convex, with a shining particle apparent like a nucleolus. In almost every specimen of blood may be also observed a certain number of corpuscles smaller than the rest. They are termed microcytes, and are probably immature corpuscles. It is necessary to take notice that much importance is attached to one form of these smaller corpuscles named blood plates by Bizzozero. They are small, more or less rounded or slightly oval granules, slightly if at all coloured, and about one third the size of ordinary coloured corpuscles. From them it is supposed the fibrin ferment is specially derived. Some go so far as to say that they are practically broken up into it alone. They rapidly undergo change in blood, after it has been drawn. They may form masses by coalescing. CHAP. III.] THE COLOURED BLOOD CORPUSCLES. 81 A peculiar property of the red corpuscles, which is exag- gerated in inflammatory blood, may be here again noticed, i.e., their great tendency to adhere together in rolls or columns, like piles of coins. These rolls quickly fasten to- gether by their ends, and cluster; so that, when the blood is spread out thinly on a glass, they form a kind of irregular network, with crowds of corpuscles at the several points corresponding with the knots of the net (fig. 66). Hence the clot formed in such a thin layer of blood looks mottled with blotches of pink upon a white ground, and in a larger quantity of such blood help, by the consequent rapid subsidence of the corpuscles, in the formation of the huffy coat already referred to. Action of Re-agents.-Considerable light has been thrown on the phy- sical and chemical constitution of red blood-cells by studying the effects produced by mechanical means and by various reagents : the following is a brief summary of these re-actions :- Pressure.-If the red blood-cells of a frog or man are gently squeezed, they exhibit a wrinkling of the surface, which clearly indicates that there is a superficial pellicle partly differentiated from the softer mass within ; again, if a needle be rapidly drawn across a drop of blood, several corpuscles will be found cut in two, but this is not accompanied by any escape of cell ■contents ; the two halves, on the contrary, assume a rounded form, proving clearly that the corpuscles are not mere membranous sacs with fluid contents like fat-cells. Fluids, i. Water.-When water is added gradually to frog's blood, the oval disc-shaped corpuscles become spherical, and gradually discharge their haemoglobin, a pale, transparent stroma being left behind ; human red blood- cells change from a discoidal to a spheroidal form, and discharge their cell-contents, becoming quite transparent and all but invisible. ii. Saline solution (dilute) produces no appreciable effect on the red blood-cells of the frog. In the red blood-cells of man the discoid shape is exchanged for a spherical one, with spinous projections, like a horse-chestnut (fig. 67). Their original forms can be at ■once restored by the use of carbonic acid. iii. Acetic acid (dilute) causes the nucleus of the red blood cells in Fig. 66.-Red corpuscles in rouleaux. At ", ", are two white corpuscles. Fig. 67. 82 THE BLOOD. [chap. nr. Fig. 68.-The above illustration is somewhat altered from a drawing by Gulliver, in the Proceed. Zool. Society, and exhibits the typical characters of the red blood-cells in the main divisions of the Vertebrata. The fractions are those of an inch, and represent the average diameter. In the case of the oval cells, only the long diameter is here- given. It is remarkable, that although the size of the red blood-cells varies so much in the different classes of the vertebrate kingdom, that of the white corpuscles remains comparatively uniform, and thus they are, in some animals, much greater, in others much less than the red corpuscles existing side by side with them. the frog to become more clearly defined ; if the action is prolonged, the nucleus becomes strongly granulated, and all the colouring matter seems to be concentrated in it, the surrounding cell-substance and outline of the cell becoming almost invisible ; after a time the cells lose their colour altogether. The cells in the figure (fig. 69) represent the successive stages of the change. A similar loss of colour occurs in the red cells of human blood, which, however, from the absence of nuclei, seem to disappear entirely. iv. Alkalies cause the red blood-cells to swell and finally disappear. v. Chloroform added to the red blood-cells of the frog causes them to part with their haemoglobin ; the stroma of the cells becomes Fig. 69. CHAP, in.] ACTION OF HE-AGENTS. 83 gradually broken up. A similar effect is produced on the human red blood-cell. vi. Tannin.-When a 2 per cent, solution of tannic acid is applied to frog's blood it causes the appearance of a sharply-de- fined little knob, projecting from the free surface (Robert's macula) : the colouring matter becomes at the same time concentrated in the nucleus, which grows more distinct (fig. 70). A somewhat similar effect is produced on the human red blood corpuscle. vii. Magenta, when applied to the red blood-cells of the frog, produces a similar little knob or knobs, at the same time staining the nucleus and causing the discharge of the haemoglobin. The first effect of the magenta is to cause the discharge of the haemoglobin, then the nucleus becomes suddenly stained, and lastly a finely granular matter issues through the wall of the corpuscle, becoming stained by the magenta, and a macula is formed at the point of escape. A similar macula is produced in the human red blood-cell. viii. Boracic acid.-A 2 per cent, solution applied to nucleated red blood- cells (frog) will cause the concentration of all the colouring matter in the nucleus ; the coloured body thus formed gradually quits its central position, and comes to be partly, sometimes entirely, protruded from the surface of the now colourless cell (fig. 71). The result of this experiment led Briicke to distinguish the coloured contents of the cell (zooid) from its colourless stroma (oecoid). When applied to the non- nucleated mammalian corpuscle its effect merely resembles that of other dilute acids. ix. Ammonia.-Its effects seem to vary according to the degree of con- centration. Sometimes the outline of the corpuscles becomes distinctly crenated ; at other times the effect resembles that of boracic acid, while in other cases the edges of the corpuscles begin to break up. Gases. Carbonic acid.-If the red blood-cells of a frog be first ex- posed to the action of water-vapour (which renders their outer pellicle more readily permeable to gases), and then acted on by carbonic acid, the nuclei immediately become clearly defined and strongly granulated ; when air or oxygen is admitted the original appearance is at once restored. The upper and lower cell in fig. 72 show the effect of carbonic acid ; the middle one the effect of the re-admission of air. These effects can be re- produced five or six times in succession. If, however, the action of the carbonic acid be much prolonged, the granulation of the nucleus becomes permanent; it appeal's to depend on a coagulation of the paraglobulin. Heat.-The effect of heat up to 120°-140° F. (50°- 6o° C.) is to cause the formation of a number of bud-like processes (fig. 73). Electricity causes the red blood-corpuscles to become crenated, and at length mulberry-like. Finally they re- cover their round form and become quite pale. The Colourless Corpuscles.-In human blood the white or colourless corpuscles or leucocytes are nearly spherical masses Fig. 70. Fig. 71. Fig. 72. Fig. 73- 84 THE BLOOD. [chap. hi. of granular protoplasm without cell wall. The granular appear- ance more marked in some than in others (vide infra), is due to the presence of particles probably of a fatty nature. In all cases one or more nuclei exist in each corpuscle. The size of the corpuscle averages °f an dich diameter. In health, the proportion of white to red corpuscles, which, taking an average, is about i to 500 or 600, varies considerably even in the course of the same day. The variations appear to depend chiefly on the amount and probably also on the kind of food taken; the number of leucocytes being very considerably increased by a meal, and diminished again on fasting. Also in young persons, during pregnancy, and after great loss of blood, there is a larger proportion of colour- less blood-corpuscles, which probably shows that they are more rapidly formed under these circumstances. In old age, on the other hand, their proportion is diminished. Varieties.-The colourless corpuscles pre- sent greater diversities of form than the red ones. Two chief varieties are to be seen in human blood ; one which contains a considerable number of granules, and the other which is paler and less granular. In size the variations are great, for in most specimens of blood it is possible to make out, in addition to the full-sized varieties, a number of smaller corpuscles, consisting of a large spherical nucleus surrounded by a variable amount of more or less granular protoplasm. The small corpuscles are, in all probability, the undeveloped forms of the others, and are derived from the cells of the lymph. Besides the above-mentioned varieties, Schmidt describes another form which he looks upon as intermediate between the coloured and the colourless forms, viz., certain corpuscles which contain red granules of haemoglobin in their protoplasm. The different varieties of colourless corpuscles are especially well seen in the blood of frogs, newts, and other cold-blooded animals. Amoeboid movement.-The remarkable property of the colourless corpuscles of spontaneously changing their shape was first demonstrated by Wharton Jones in the blood of the skate. Fig. 74. - A. Three coloured blood-cor- puscles. B. Three colourless blood- corpuscles acted on by acetic acid ; the nuclei are very clearly visi- ble. X 900. CHAP. III.] AM (EBOID MOVEMENT. 85 If a drop of blood be examined with a high power of the micro- scope on a warm stage, or, in other words, under conditions by which loss of moisture is prevented, and at the same time the temperature is maintained at about that of the blood within the walls of the living vessels, ioo° F. (37'8° C.), the colourless corpuscles will be observed slowly to alter their shapes, and to send out processes at various parts of their circumference. The amoeboid movement can be most conveniently studied in the newt's blood. The processes which are sent out from the corpuscle are either lengthened or withdrawn. If lengthened, the protoplasm of the whole corpuscle flow's as it wrere into its Fig. 75.-Human colourless blood-corpuscle, showing' its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.) process, and the corpuscle changes its position ; if withdrawn, protrusion of another process at a different point of the circum- ference speedily follows. The change of position of the corpuscle can also take place by a flowing movement of the whole mass, and in this case the locomotion is comparatively rapid. The activity both in the processes of change of shape and also of change in position, is much more marked in some corpuscles, viz., in the granular variety than in others. Klein states that in the newt's blood the changes are especially likely to occur in a variety of the colourless corpuscle, which consists of masses of finely granular protoplasm with jagged outline, containing three or four nuclei, or of large irregular masses of protoplasm containing from five to twenty nuclei. Another phenomenon may be observed in such a specimen of blood, viz., the division of the corpuscles, which occurs in the following way. A cleft takes place in the protoplasm at one point, which becomes deeper and deeper, and then by the lengthening out and attenuation of the connection, and finally by its rupture, two corpuscles result. The nuclei have previously undergone division. The cells so formed are remarkably active in their movements. Thus we see that the rounded form which the colourless corpuscles present in ordinary microscopic specimens must be looked upon as the shape natural to a dead corpuscle or to one whose vitality is dormant rather than as the shape proper to one living and active. 86 THE BLOOD. [chap. nr. Action of re-agents upon the colourless corpuscles.- Feeding the corpuscles.-If some fine pigment granules, e.g., powdered vermilion, be added to a fluid containing colourless blood-cor- puscles, on a glass slide, these will be observed, under the micro- scope, to take up the pigment. In some cases colourless corpuscles have been seen with fragments of coloured ones thus embedded in their substance. This property of the colourless corpuscles is especially interesting as helping still further to connect them with the lowest forms of animal life, and to connect both with the organized cells of which the higher animals are composed. The property which the colourless corpuscles possess of passing through the walls of the blood-vessels will be described later on. Enumeration of the blood-corpuscles.-Several methods are employed for counting the blood-corpuscles, most of them depending upon the same principle, i.e., the dilution of a minute volume of blood with a given volume of a colourless solution similar in specific gravity to blood plasma, so that the size and shape of the corpuscles is altered as little as possible. A minute quantity of the well-mixed solution is then taken, examined under the microscope, either in a flattened capillary tube (Malassez) or in a cell (Hayem & Nachet, Gowers) of known capacity, and the number of cor- puscles in a measured length of the tube, or in a given area of the cell is counted. The length of the tube and the area of the cell are ascertained by means of a micrometer scale in the microscope ocular ; or in the case of Gowers' modification, by the division of the cell area into squares of known size. Having ascertained the number of corpuscles in the diluted blood, it is easy to find out the number in a given volume of normal blood. Gowers' modification of Hayem & Nachet's instrument, called by him " Hemacyto- meter" appears to be the most convenient form of instrument for counting the corpuscles, and as such will alone be described (fig. 76). It consists of a small pipette (A), which, when filled up to a mark on its stem, holds 995 cubic millimetres. It is furnished with an india-rubber tube and glass mouth-piece to facilitate filling and emptying; a capillary tube (b) marked to hold 5 cubic millimetres, and also furnished with an india-rubber tube and mouth-piece ; a small glass jar (d) in which the dilution of the blood is performed ; a glass stirrer (E) for mixing the blood thoroughly, (f) a needle, the length of which can be regulated by a screw ; a brass stage plate (c) carrying a glass slide, on which is a cell one-fifth of a millimetre deep, and the bottom of which is divided into one-tenth millimetre squares. On the top of the cell rests the cover-glass, which is kept in its place by the pres- sure of two springs proceeding from the stage plate. A standard saline solution of sodium sulphate, or similar salt, of specific gravity 1025, is made, and 995 cubic millimetres are measured by means of the pipette into the glass jar, and with this five cubic millimetres of blood, obtained by pricking the finger with a needle, and measured in the capillary pipette (b), are thoroughly mixed by the glass stirring-rod. A drop of this diluted blood is then placed in the cell and covered with a cover-glass, which is fixed in chap, in.] CHEMICAL COMPOSITION. 87 position by means 'of the two lateral springs. The preparation is then examined under a microscope with a power of about 400 diameters, and focussed until the lines dividing the cell into squares are visible. After a short delay, the red corpuscles which have sunk to the bottom of the cell, and are resting on the squares, are counted in ten squares, and the Fig. 76.-Hamacytomelcr. number of white corpuscles noted. By adding together the numbers counted in ten (one-tenth millimetre) squares, and multiplying by ten thousand, the number of corpuscles in one cubic millimetre of blood is obtained. The average number of corpuscles per each cubic millimetre of healthy blood, according to Vierordt and Weicker, is 5,000,000 in adult men, and rather fewer in women. Chemical Composition of the Blood. Before considering the chemical composition of the blood as a whole, it will be convenient to take in order the composition of the various chief factors which have been set out in the table on p. 68, into which the blood may be separated, viz.:-(i.) The Plasma; (2.) The Serum ; (3.) The Corpuscles ; (4.) The Fibrin. (1.) Chemical Composition of Plasma.-The Plasma, or liquid part of the blood, in which the corpuscles float, may be 88 THE BLOOD. [chav. nr. obtained free from coloured corpuscles in either of the ways mentioned below. In it are the fibrin factors, inasmuch as when exposed to the ordinary temperature of the air it undergoes coagulation and splits up into fibrin and serum. It differs from the serum in containing fibrinogen, but in appearance and in reaction it closely resembles that fluid; its alkalinity, however, is less than that of the serum obtained from it. It may be freed from white corpuscles by filtration at a temperature below 410 F. (50 C.), or by the centrifugal machine. The chief methods of obtaining plasma free from corpuscles are : (1) by- cold, the temperature should be about o° C. and may be two or three degrees higher, but not lower. (2) The addition of neutral salts, in certain propor- tions, either solid or in solution, e.g., of sodium sulphate, if solid, 1 part to 12 parts of blood ; if a saturated solution 1 part to 6 parts of blood ; of magne- sium sulphate, of a 23%, or if saturated solution 1 part to 4 of blood. (3) A third -way is to mix frog's blood with an equal part of a 5% of cane sugar, and to get rid of the corpuscles by filtration ; or (4) by the injection of peptone into the veins of mammals, previous to bleeding them to death, and afterwards subjecting the plasma thus obtained to the action of a centrifugal machine. Salts of the plasma.-in 1000 parts of plasma there are :- Sodium chloride . . . . . . .5'546 Soda 1 532 Sodium phosphate . . . . . . -271 Potassium chloride ....... *359 „ sulphate -281 Calcium phosphate ....... '298 Magnesium phosphate . . . . . . -218 8-5O5 (2.) Chemical Composition of Serum.-The serum is the liquid part of the blood or of the plasma remaining after the separation of the clot. It is an alkaline, yellowish, transparent fluid, with a specific gravity of from 1025 to 1032. In the usual mode of coagulation, part of the serum remains in the clot, and the rest, squeezed from the clot by its contraction, lies around it. Since the contraction of the clot may continue for thirty-six or more hours, the quantity of serum in the blood cannot be even roughly estimated till this period has elapsed. There is nearly as much, by weight, of serum as there is clot in coagulated blood. Serum may be obtained from blood corpuscles by allowing blood to clot in large test tubes, and subjecting the test tubes to the action of a centrifugal machine for some time. CHAP III.] COMPOSITION OF SERUM. 89 In tabular form the composition may be thus summarised. In 1000 parts of serum there are :- Waterabout 900 Proteids : a. Serum-albumin . . . . . . j3. Paraglobulin Salts. Fats-including fatty acids, cholesterin, lecithin ; and some soaps Grape sugar in small amount .... Extractives-kreatin, krcatinin, urea, &c. . . Yellow pigment, which is independent of haemoglobin Gases-small amounts of oxygen, nitrogen, and car- bonic acid 8o 20 1000 a. Water.-The water of the serum varies in amount according to the amount of food, drink, and exercise, and with many other circumstances. b. Proteids.-a. Serum-albumin is the chief proteid found in serum. The proportion which it bears to paraglobulin, the other proteid, is as ron to r in human blood. Serum-album in has been shown by Halliburton to be a compound body, which may be called serine, made up of three proteids, which coagulate at different temperatures, a at 730 C., 3 at 770 C., and 7 at 85° C. The serine is entirely coagulated at 940 C., and also by the addition of strong acids, such as nitric and hydrochloric ; by long contact with alcohol it is precipitated. It is not precipitated on addition of ether, and so differs from the other native albumin, viz., When dried at 104° F. (40° C.) serum- albumin is a brittle, yellowish substance, soluble in water, possessing a laevo- rotary power of - 56°. It is with great difficulty freed from its salts, and is precipitated by solutions of metallic salts, <?.</., of mercuric chloride, copper sulphate, lead acetate, sodium tungstate, &c. If dried at a tem- perature over 350 C. the residue is insoluble in water, having been changed into coagulated proteid. Serum-albumin may be precipitated from serum, from which the paraglobulin has been previously separated by saturation with magnesium sulphate, and removed by filtration, by further saturation with sodium sulphate, sodium nitrate, or iodide of potassium. ft. Paraglobulin can be obtained as a white precipitate from cold serum by adding a considerable excess of water, and passing- through the mixture a current of carbonic acid gas or by the cautious addition of dilute acetic acid. It can also be obtained by saturating scrum with either crystallized magnesium sulphate, or sodium chloride, nitrate, acetate, or carbonate. When obtained in the latter way, precipitation seems to be much more complete than by 90 THE BLOOD. [chap. hi. means of the former method. Paraglobulin belongs to the class of proteids called globulins. c. The salts of sodium predominate in serum as in plasma, and of these the chloride generally forms by far the largest proportion, d. Fats are present partly as fatty acids and partly emulsified. The fats are tri-olein, tri-stearin, and tri-palmitin. The amount of fatty matter varies according to the time after, and the ingredients of, a meal. Of cholesterin and lecithin there are mere traces. e. Grape sugar is found principally in the blood of the hepatic vein, about one part in a thousand. f. The extractives vary from time to time ; sometimes uric and hippuric acids are found in addition to urea, kreatin and kreatinin. Urea exists in proportion from '02 to -04 per cent. g. The yellow pigment of the serum and the odorous matter which gives the blood of each particular animal a peculiar smell, have not yet been exactly differentiated. The former is probably choletelin (MacMunn). (3.) Chemical Composition of the Corpuscles.-a. Coloured. -Analysis of a thousand parts of moist blood corpuscles shows the following result:- Water688 Solids- Organic .... 303-88 Mineral8'12-312 = 1000. Of the solids the most important is Haemoglobin, the substance to which the blood owes its colour. It constitutes, as will be seen from the appended Table, more than 90 per cent, of the organic matter of the corpuscles. Besides haemoglobin there are proteid * and fatty matters, the former chiefly consisting of globulins, and the latter of cholesterin and lecithin. In 1000 parts organic matter are found :- Haemoglobin 905'4 Proteids867 Fats79=1000 Of the inorganic salts of the corpuscles, with the iron omitted- * An account of the proteid bodies, &c., will be found in the Appendix, and should be referred to for explanation of the terms employed in the text. CHAP. HI.] COMPOSITION OF THE CORPUSCLES. 91 In 1000 parts corpuscles (Schmidt) are found :- Potassium Chloride 3'679 Potassium Phosphate ...... 2343 Potassium sulphate '132 Sodium '633 Calcium '094 Magnesium ........ '060 Soda -341 = 7'282 The properties of haemoglobin will be considered in relation to the Gases of the blood (p. 94). b. Colourless.-The corpuscles may be said also to contain fibrinogen, paraglobulin, and the ferment. In consequence of the difficulty of obtaining colourless corpuscles in sufficient number to make an analysis, little is accurately known of their chemical composition; in all probability, however, the stroma of the corpuscles is made up of proteid matter, and the nucleus of nwcZein, a nitrogenous phosphorus-containing body akin to mucin, capable of resisting the action of the gastric juice. The proteid matter, chiefly globulins, soluble in a ten per cent, solution of sodium chloride, the solution being precipitated on the addition of water, by heat and by the mineral acids. The stroma contains fatty granules, and in it also the presence of glycogen has been demonstrated. The salts of the corpuscles are chiefly potassium, and of these the phosphate is in greatest amount. (4.) Chemical Composition of Fibrin.-The part played by fibrin in the formation of a clot has been already described (p. 69), and it is only necessary to consider here its general properties. It is a stringy elastic substance belonging to the proteid class of bodies. It is insoluble in water and in weak saline solutions ; soluble in 10 per cent, solution of sodium chloride, it swells up into a transparent jelly when placed in dilute-hydrochloric acid, but does not dissolve, but in strong acid it dissolves, producing acid-albumin *; it is also soluble in strong saline solutions. Blood contains only "2 per cent, of fibrin. It can be converted by the gastric or pancreatic juice into peptone. It * The use of the two words albumen.and albumin may need explanation. The former is the generic word, which may include several albuminous or proteid bodies, e.g., albumen 'of blood ; the latter which requires to be qualified by another word is the specific form, and is applied to varieties, e.q., egg-albumin, serum-albumin. 92 THE BLOOD. [chap. in. possesses the power of liberating the oxygen from solutions of hydric peroxide H202 or ozonic ether. This may be shown by dipping a few shreds of fibrin in tincture of guaiacum, and then immersing them in a solution of hydric peroxide. The fibrin becomes of a bluish colour, from its having liberated from the solution oxygen, which oxidises the resin of guaiacum contained in the tincture, and thus produces the coloration. The Gases of the Blood. The gases contained in the blood arc Carbonic acid, Oxygen, and Nitrogen, 100 volumes of blood containing from 50 to 60 volumes of these gases collectively. Arterial blood contains relatively more oxygen and less carbonic acid than venous. But the absolute quantity of carbonic acid is in both kinds of blood greater than that of the oxygen. Oxygen. Carbonic Acid. Nitrogen. Arterial Blood 20 vol. per cent. 39 vol. per cent. I to 2 vols. Venous ,, (from muscles at rest) 8 to 12 „ „ „ 46 „ »» „ I to 2 vols. The Extraction of the Gases from the Blood.-As the ordinary air-pumps are not sufficiently powerful for the purpose, the extraction of the gases from the blood is accomplished by means of a mercurial air-pump, of which there are many varieties, those of Ludwig, Alvergnidt, Geissler, and Sprengel being the chief. The principle of action in all is much the same. Ludwig's pump, which may be taken as a type, is represented in fig. 77. It consists of two fixed glass globes, C and F, the upper one communicating by means of the stop-cock B, and a stout india-rubber tube with another glass globe, L, which can be raised or lowered by means of a pulley ; it also communicates by means of a stopcock, B, and a bent glass tube, A, with a gas receiver (not represented in the figure), A, dipping into a bowl of mercury, so that the gas may be received over mercury. The lower globe, F, communicates with C by means of the stopcock, E, with I in which the blood is contained by the stopcock, G, and with a movable glass globe, J/, similar to L, by means of the stopcock, IT, and the stout india-rubber tube, K. In order to work the pump, L and M are filled with mercury, the blood from which the gases are to be extracted is placed in the bulb I, the stop- cocks, IT, E, D, and B, being open, and G closed. M is raised by means of the pulley until F is full of mercury, and the air is driven out. E is then closed, and L is raised so that C becomes full of mercury, and the air driven off. B is then closed. On lowering L the mercury runs into it from C, and a vacuum is established in C. On opening E and lowering JT, a vacuum is similarly established in F; if G be now opened, the blood in I will enter into ebullition, and the gases will pass off into F and C, and on raising M and then L, the stopcock B being opened, the gas is driven through A, and is received into the receiver over mercury. By repeating the experiment CHAP. III.] THE GASES OF THE BLOOD. 93 several times the whole of the gases of the specimen of blood is obtained, and may be estimated. a. The Oxygen of the Blood.-It has been found that a very small proportion of the oxygen which can be obtained, by the aid of the mercurial pump, from the blood, exists in a state of simple solution in the plasma. If the gas were in simple solution, the amount of oxygen in any given quantity of blood exposed to any given atmos- phere ought to vary with the amount of oxygen contained in the atmos- phere. Since, speaking generally, the amount of any gas absorbed by a liquid such as plasma would de- pend upon the proportion of the gas in the atmosphere to which the liquid is exposed-if the proportion is great, the absorption will be great; if small, the absorption will be similarly small. The absorption continues until the proportions of the gas in the liquid and in the atmos- phere are equal. Other things will, of course, influence the absorption, such as the nature of the >jas em- ployed, the nature of the liquid, and the temperature, but coeteris paribus, the amount of a gas which a liquid absorbs depends upon the proportion -the so-called partial pressure-of the gas in the atmosphere to which the liquid is subjected. And con- versely, if a liquid containing a gas in solution be exposed to an atmosphere containing none of the gas, the gas will be given up to the atmosphere until the amount in the liquid and in the atmo- sphere becomes equal. This condition is called a condemn of equal tensions. The condition may be understood by a simple illustration. A large amount of carbonic acid gas is dissolved in a bottle of water by exposing the liquid to extreme pressure of the gas, and a cork is placed in the bottle Fig. 77.-Ludwig's Mercurial Pump. 94 THE BLOOD. [chap. hi. and wired down. The gas exists in the water in a condition of extreme tension, and therefore exhibits a tendency to escape into the atmosphere, in order to relieve the tension ; this produces the violent expulsion of the cork when the wire is removed, and if the aerated water be placed in a glass the gas will continue to be evolved until it has almost entirely passed into the atmosphere, and the tension of the gas in the water approximates to that of the atmosphere in which, it should be remembered, the carbon dioxide is, naturally, in very small amount, viz., '04 per cent. The oxygen of the blood does not obey this law of pressure. For if blood which contains little or no oxygen be exposed to a succession of atmospheres containing more and more of that gas, we find that the absorption is at first very great, but soon becomes relatively very small, not being therefore regularly in proportion to the increased amount (or tension) of the oxygen of the atmospheres, and that conversely, if arterial blood be submitted to regularly diminishing pressures of oxygen, at first very little of the contained oxygen is given off to the atmosphere, then suddenly the gas escapes with great rapidity, and again disobeys the law of pressures. Very little oxygen can be obtained from serum freed from blood corpuscles, even by the strongest mercurial air-pump, neither can scrum be made to absorb a large quantity of that gas ; but the small quantity which is so given up or so absorbed follows the laws of absorption according to pressure. It must be, therefore, evident that the chief part of the oxygen is contained in the corpuscles, and not in a state of simple solu- tion. The chief solid constituent of the coloured corpuscles is haemoglobin, which constitutes more than 90 per cent, of their bulk. This body has a very remarkable affinity for oxygen, absorbing it to a very definite extent under favourable circum- stances, and giving it up when subjected to the action of reducing agents, or to a sufficiently low oxygen pressure. From these facts it is inferred that the oxygen of the blood is combined with haemoglobin, and not simply dissolved; but inasmuch as it is comparatively easy to cause the haemoglobin to give up its oxygen, it is believed that the oxygen is but loosely combined with the substance. Haemoglobin.-Haemoglobin is a crystallizable body which constitutes by far the largest portion of the coloured corpuscles. It is intimately distributed throughout their stroma, and must be dissolved out before it will undergo crystallization. Its percentage composition is C. 53-85 ; H. 7-32 ; N. 16'17 ; 0. 21'84; CHAP. III.] HAEMOGLOBIN CRYSTALS. 95 S. -63 ; Fe. •42 ; and if the molecule be supposed to contain one atom of iron the formula would be CU, H9&, NIS4, Fe S3 O,79. The most interesting of the properties of haemoglobin are its powers of crystallizing and its attraction for oxygen and other gases. Crystals.-The haemoglobin of the blood of various animals possesses the power of crystallizing to very different extents (blood-crystals). In some animals the formation of crystals is almost spontaneous, whereas in others it takes place either with great difficulty or not at all. Among the animals whose blood colouring-matter crys- tallizes most readily are the guinea-pig, rat, squirrel, and dog ; and in these cases to obtain crystals it is gene- rally sufficient to dilute a drop of recently-drawn blood with water and expose it for a few minutes to the air. Light seems to favour the formation of the crystals. In many instances other means must be adopted, t'.r/., the addition of alcohol, ether, or chloroform, rapid freezing, and then thawing, an electric current, a tem- perature of 140° F. (6o° C.), or the addition of sodium sulphate. The haemoglobin of human blood crystallizes with difficulty, as does also that of the ox, the pig, the sheep, and the rabbit. The forms of haemoglobin crystals, as will be seen from the appended figures, differ greatly. Haemoglobin crystals are soluble in water. Both the crystals themselves and also their solutions have the characteristic colour of arterial blood. A dilute solution of haemoglobin gives a characteristic appear- ance with the spectroscope. Two absorption bands are seen between the solar lines I) (which is the sodium band in the yellow) and e (see plate), one in the yellow, with its middle line some little way to the right of d, is very intense, but narrower than the other, which lies in the green near to the left of e. Each band is Fig. 78.-Crystals of oxy-htemoglolin-prismatic from human blood. 96 THE BLOOD. [chap. in. darkest in the middle and fades away at the sides. As the strength of the solution increases the bands become broader and deeper and both the red and the blue ends of the spectrum become encroached upon until the bands coalesce to form one very broad band, and only a slight amount of the green remains unabsorbed, and part of the red; on still further increase of strength the former disappears. If the crystals of oxy- haemoglobin be subjected to a mercurial air-pump they give oft' a definite amount of oxygen (i gramme giving off i'59 c. cm. of oxygen), and they become of a purple colour; and a solution of oxy-haemoglobin may be made to give up oxygen, and to become purple in a similar manner. This change may be also effected by passing through it hydrogen or nitrogen gas, or by the action of reducing agents, of which Stokes's fluid* or ammonium sulphide is the most convenient. With the spectroscope, a solution of deoxidized or re- duced haemoglobin is found to give an entirely different appearance from that of oxi- dized haemoglobin. Instead of the two bands at d and e we Fig. 79.- Oxy-hannoglobln crystals- tetrahedral, from blood of the guinea-pig. Fig. 80.-Hexagonal oxy-ha»moglol>in crystals, from blood of squirrel. On these hexagonal plates prismatic crystals grouped in a stellate manner not unfrequently occur (after Funke). * Stokes's Fluid consists of a solution of ferrous sulphate, to which ammonia has been added and sufficient tartaric acid to prevent precipitation. Another reducing agent is a solution of stannous chloride, treated in away similar to the ferrous sulphate, and a third re-agent of like nature is an aqueous solution of ammonium sulphide. N H4 H S. CHAP. III.] HEMOGLOBIN. 97 find a single broader but fainter band occupying a position mid- way between the two, and at the same time less of the blue end of the spectrum is absorbed. Even in strong solutions this latter appearance is found, thereby differing from the strong solu- tion of oxidised haemoglobin which lets through only the red and orange rays; accordingly to the naked eye the one (reduced haemoglobin solution) appears purple, the other (oxy-haemoglobin solution) red. The deoxidised crystals or their solutions quickly absorb oxygen on exposure to the air, becoming scarlet. If solu- tions of blood be taken instead of solutions of haemoglobin, results similar to the whole of the foregoing can be obtained. Venous blood never, except in the last stages of asphyxia, fails to show the oxy-haemoglobin bands, inasmuch as the greater part of the haemoglobin even in venous blood exists in the more highly oxidised condition. Action, of Gases on Haemoglobin.-Carbonic oxide gas, passed through a solution of haemoglobin, causes it to assume a bluish colour, and its spectrum to be slightly altered ; two bands are still visible, but are slightly nearer the blue end than those of oxy-haemoglobin (see plate). The amount of carbonic oxide taken up is equal to the amount of the oxygen displaced. Although the carbonic oxide gas readily displaces oxygen, the reverse is not the case, and upon this property depends the dangerous effect of coal-gas poisoning. Coal gas contains much carbonic oxide, and when breathed, the gas combines with the haemoglobin of the blood, and produces a compound which cannot easily be reduced. This compound (carboxy-haemoglobin) is by no means an oxygen carrier, and death may result from suffocation due to the want of oxygen notwithstanding the free entry of pure air into the lungs. Crystals of carbonic-oxide haemoglobin closely resemble those of oxy-haemoglobin. Nitric oxide produces a similar compound to the carbonic-oxide haemo- globin, which is even less easily reduced. Nitrous oxide reduces oxy-haemoglobin, and therefore leaves the reduced haemoglobin in a condition to actively take up oxygen. Sulphuretted Hydrogen.-If this gas be passed through a solution of oxy- haemoglobin, the haemoglobin is reduced and an additional band appears in the red. If the solution be then shaken with air, the two bands of oxy- haemoglobin replace that of reduced haemoglobin, but the band in the red persists. Derivatives of Haemoglobin. Methaemoglobin.-If an aqueous solution of oxy-hfemoglobin is exposed to the air for some time, its spectrum undergoes a change ; the two D and E bands become faint, and a new line in the red at c is developed. The solu- tion, too, becomes brown and acid in reaction, and is precipitable by basic lead acetate. This change is due to the decomposition of oxy-hemoglobin, and to the production of metlicemoglobin. On adding ammonium sulphide, reduced haemoglobin is produced, and on shaking this up with air, oxy-haemo- 98 THE BLOOD. [chap. III. globin is reproduced. Methaemoglobin is probably a stage in the deoxidation of oxy-haemoglobin. It appears to contain less oxygen than oxy-htemoglobin, but more than reduced haemoglobin. Its oxygen is in more stable combina- tion, however, than is the case with the former compound. Haematin.-By the action of heat, or of acids or alkalies in the presence of oxygen, haemoglobin can be split up into a substance called Hcematin, which contains all the iron of the haemoglobin from which it was derived, and a proteid residue. Of the latter it is impossible to say more than that it probably consists of one or more bodies of the globulin class. If there be no oxygen present, instead of haematin a body called hsemochromogen is produced, which, however, will speedily undergo oxidation into haematin. Haematin is a dark brownish or black non-crystallizable substance of metallic lustre. Its percentage composition is C. 64'30 ; H. 5'50; N. 9'06 ; Fe. 8'82; 0. 12-32 ; which gives the formula C68, H70, N8, Fe2, 010 (Hoppe- Seyler). It is insoluble in water, alcohol, and ether ; soluble in the caustic alkalies ; soluble with difficulty in hot alcohol to which is added sulphuric acid. The iron may be removed from haematin by heating it with fuming hydrochloric acid to 320° F. (160° C.), and a new body, haematoporphyrin, s produced. Haematoporphyrin (C68, H71, N8, 012, Hoppe-Seyler) may also be obtained by adding blood to strong sulphuric acid, and if necessary fil- tering the fluid through asbestos. It forms a fine crimson solution, which has a distinct spectrum, viz., a dark band just beyond D, and a second all but midway D and E. It may be precipitated from its acid solution by adding water or by neutralisation, and when redissolved in alkalies presents four bands, a pale band between c and D, a second between D and E, nearer D, another nearer E, and a fourth occupying the chief part of the space between b and F. Hcematin in acid solution.-If an excessof acetic acid be added to blood, and the solution is boiled, the colour alters to brown from decomposition of haemoglobin and the setting free of haematin ; by shak- ing this solution with ether, solution of the haematin in acid solution is obtained. The spectrum of the ethereal solution (coloured plate) shows no less than four absorption bands, viz., one in the red between c and D, one faint and narrow close to D, and then two broader bands, one between d and e, and another nearly midway between b and f. The first band is by far the most distinct, and the acid aqueous solution of haematin shows it plainly. Hematin in alkaline solution.-If an alkali be added to blood and the solu- tion is boiled, alkaline haematin is produced, and the solution becomes olive green in colour, the absorption band of which is still in the red, but nearer to D, and the blue end of the spectrum is partially absorbed to a considerable extent. If a reducing agent be added, two bands resembling those of oxy- haemoglobin, but nearer to the blue, appear ; this is the spectrum of reduced hcematin, or haemochromogen. On shaking the reduced haematin with air or oxygen the two bands are replaced by the single band of alkaline haematin. Haematoidin.-This substance is found in the form of yellowish crystals Fig. 81.-Hasmatoidin crystals. (Frey.) chap, nr.] ILEMIN. 99 in old blood extravasations, and is derived from the haemoglobin. Their crystalline form and the reaction they give' with nitric acid seem to show them to be identical with Bilirubin, the chief colouring matter of the Bile. Haemin.-One of the most important derivatives of heematin is haemin. It is usually called Hydrochlorate of Hematin (or hydrochloride), but its exact chemical composition is uncertain. Its formula is H7O, N8, Fe2, Oio, 2 Hcl, and it contains 5'18 per- cent. of chlorine, but by some it is looked upon as simply crys- tallized heematin. Although difficult to obtain in bulk, a speci- men may be easily made for the microscope in the fol- lowing way:- A small drop of dried blood is finely pow- dered with a few- crystals of common salt on a glass slide, and spread out; a cover glass is then placed upon it, and glacial acetic acid added by means of a capillary pipette. The blood at once turns of a brownish colour. The slide is then heated, and the acid mixture evaporated to dryness at a high temperature. The excess of salt is washed away with water from the dried residue, and the specimen may then be mounted. A large number of small, dark, reddish black crystals of a rhombic shape, sometimes arranged in bundles, will be seen if the slide be subjected to microscopic examination. The formation of these haemin crystals is of great interest and importance from a medico-legal point of view, as it constitutes the most certain and delicate test we have for the presence of blood (not of necessity the blood of man) in a stain on clothes, Ac. It exceeds in delicacy even the spectroscopic test. Compounds similar in composition to hsemin, but containing hydrobromic and hydriodic acids, instead of hydrochloric, may be also readily obtained. Estimation of Haemoglobin.-The most exact method is by the estima- tion of the amount of iron in a given specimen of blood, but as this is a somewhat complicated process, a method has been proposed which, though not so exact, has the advantage of simplicity. This consists in comparing the colour of a given small amount of diluted blood with glycerine jelly tinted with carmine and picrocarmine to represent a standard solution of blood diluted one hundred times. The amount of dilution which the given Fig. 82.-Htemin crystals. (Frey.) 100 THE BLOOD. [chap. hi. blood requires will thus approximately represent the quantity of hemoglobin it contains. (Gowers.) Distribution of Haemoglobin.-Hemoglobin occurs not only in the red blood-cells of all Vertebrata (except one fish (leptocephalus) whose blood- cells are all colourless), but also in similar cells in many Worms ; moreover, it is found diffused in the vascular fluid of some other worms and certain Crustacea ; it also occurs in all the striated muscles of Mammals and Birds. It is generally absent from unstriated muscle except that of the rectum. It nas also been found in Mollusca in certain muscles which are specially active, viz., those which work the rasp-like tongue. B. The Carbon Dioxide Gas in the Blood.-Of this gas in the blood, part exists in a state of simple solution in the serum, and the rest in a state of weak chemical combination. It is believed that the latter is combined with the sodium carbonate in a con- dition of bicarbonate. Some observers consider that part of the gas is associated with the corpuscles. C. The Nitrogen in the Blood.-The whole of the small quan- tity of the nitrogen contained in the blood is simply dissolved in the fluid plasma. Chemical Composition of the Blood in Bulk.-Analyses of the blood as a whole differ slightly, but the following table may be taken to represent the average composition : Water784 Solids- Corpuscles130 Proteids (of serum) .... 70 Fibrin (of clot)2-2 Fatty matters (of serum) . . . 1-4 Inorganic salts (of serum) . . . 6 Gases, kreatin, urea and other extractive' matter, glucose and accidental sub- stances 6-4- 2l6 iooo Variations in the Composition of healthy Blood. The conditions which appear most to influence the composition of the blood in health are these: Sex, Pregnancy, Age, and Tem- perament. The composition of the blood is also, of course, much influenced by diet. 1. Sex.-The blood of men differs from that of women, chiefly in being of somewhat higher specific gravity, from its containing a relatively larger quantity of red corpuscles. 2. Pregnancy,-The blood of pregnant women is rather lower than the CHAP. in.] VARIATIONS IN COMPOSITION OF THE BLOOD. 101 average specific gravity, from deficiency of coloured corpuscles. The quantity of the coloured corpuscles, on the other hand, and of fibrin, ip increased. 3. Age.-The blood of the foetus is very rich in solid matter, and especially in coloured corpuscles ; and this condition, gradually diminishing, continues for some weeks after birth. The quantity of solid matter then falls during childhood below the average, rises during adult life, and in old age falls again. 4. Temperament.-There appears to be a relatively larger quantity of solid matter, and particularly of coloured corpuscles, in those of a plethoric or sanguineous temperament. 5. Diet.-Such differences in the composition of the blood as are due to the temporary presence of various matters absorbed with the food and drink, as well as the more lasting changes which must result from generous or poor diet respectively, need be here only referred to. 6. Effects of Bleeding.-The result of bleeding is to diminish the specific gravity of the blood ; and so quickly, that in a single venesection, the portion of blood last drawn has often a less specific gravity than that of the blood that flowed first. This is, of course, due to absorption of fluid from the tissues of the body. [The physiological import of this fact, namely, the instant absorption of liquid from the tissues, is the same as that of the intense thirst which is so common after either loss of blood, or the abstraction from it of watery fluid, as in cholera, diabetes, and the like.] For some little time after bleeding, the want of coloured corpuscles is well marked, but with this exception, no considerable alteration seems to be pro- duced in the composition of the blood for more than a very short time ; the loss of the other constituents, including the coloured corpuscles, being very quickly repaired. Variations in different parts of the Body.-The composi- tion of the blood, as might be expected, is found to vary in different parts of the body. Thus arterial blood differs from venous; and although its composition and general characters are uniform throughout the whole course of the systemic arteries, they are not so throughout the venous system-the blood contained in some veins differing remarkably from that in others. Differences between Arterial and Venous Blood.-The differences between arterial and venous blood are these:- (a.) Arterial blood is bright red, from the fact that almost all its haemoglobin is combined with oxygen (Oxy-haemoglobin, or scarlet haemoglobin), while the purple tint of venous blood is due to the deoxidation of a certain quantity of its oxy-haemoglobin, and its consequent reduction to the purple variety (Deoxidised, or purple haemoglobin). (6.) Arterial blood coagulates somewhat more quickly. (c.) Arterial blood contains more oxygen than venous, and less carbonic acid. 102 THE BLOOD. [chap. III. Some of the veins contain blood which differs from the ordinary standard considerably. These are the Portal, the Hepatic, and the Splenic veins. Portal vein.-The blood which the portal vein conveys to the liver is supplied from two chief sources ; namely, from the gastric and mesenteric veins, which contains the soluble elements of food absorbed from the stomach and intestines during digestion, and from the splenic vein; it must, there- fore, combine the qualities of the blood from each of these sources. The blood in the gastric and mesenteric veins will vary much according to the stage of digestion and the nature of the food taken, and can therefore be seldom exactly the same. Speaking generally, and without considering the sugar, and other soluble matters which may have been absorbed from the alimentary canal, this blood appears to be deficient in solid matters, espe- cially in coloured corpuscles, owing to dilution by the quantity of water absorbed, to contain an excess of proteid matter, and to yield a less tenacious kind of fibrin than that of blood generally. The blood from the splenic vein is generally deficient in coloured cor- puscles, and contains an unusually large proportion of proteids. The fibrin obtainable from the blood seems to vary in relative amount, but to be almost always above the average. The proportion of colourless corpuscles is also unusually large. The whole quantity of solid matter is decreased, the diminution appearing to be of coloured corpuscles. The blood of the portal vein, combining the peculiarities of its two factors, the splenic and mesenteric venous blood, is usually of lower specific gravity than blood generally, is more watery, contains fewer coloured corpuscles, more proteids, and yields a less firm clot than that yielded by other blood, owing to the deficient tenacity of its fibrin. Guarding (by ligature of the portal vein) against the possibility of an error in the analysis from regurgitation of hepatic blood into the portal vein, recent observers have determined that hepatic venous blood contains less water, proteids, and salts than the blood of the portal vein ; but that it yields a much larger amount of extractive matter, in which is one constant element, namely, grape-sugar, which is found, whether saccharine or farina- ceous matter have been present in the food or not. The first formed blood-corpuscles of the human embryo differ much in their general characters from those which belong to the later periods of intra-uterine, and to all periods of extra-uterine life. Their manner of origin is at first very simple. Surrounding the early embryo is a circular area, called the vascular area, in which the first rudiments of the blood-vessels and blood-corpuscles are developed. Here the nucleated embryonal cells of the mesoblast, from which the blood-vessels and corpuscles are to be formed, send out processes in various directions, and these joining together, form an irregular meshwork. The nuclei Development of the Blood-Corpuscles. CHAP. III.] DEVELOPMENT OF THE CORPUSCLES. 103 increase in number, and collect chiefly in the larger masses of protoplasm, but partly also in the processes. These nuclei gather around them a certain amount of the protoplasm, and becoming coloured, form the red blood-corpuscles. The protoplasm of the cells and their branched network in which these corpuscles lie then become hollowed out into a system of canals enclosing fluid, in which the red nucleated corpuscles float. The corpuscles at first are from about aVoo t° 1 >o<j °f an diameter, mostly spherical, and with granular contents, and a well-marked nucleus. Their nuclei, which are about an *n diameter, are Fig. 83.-Part of the network of developing blood-vessels in the vascular area of a guinea-pig. bl, blood-corpuscles becoming free in an enlarged and hollowed out part of the net- work ; a, process of protoplasm. (E. A. Schafer.) central, circular, very little prominent on the surfaces of the corpuscle, and apparently slightly granular or tuberculated. The corpuscles then strongly resemble the colourless corpuscles of the fully developed blood, but are coloured. They are capable of amoeboid movement and multiply by division. When, in the progress of embryonic development, the liver begins to be formed, the multiplication of blood-cells in the whole mass of blood ceases, and new blood-cells are produced by this organ, and also by the lymphatic glands, thymus and spleen. These are at first colourless and nucleated, but afterwards acquire the ordinary blood-tinge, and resemble very much those of the first set. They also multiply by division. In whichever way produced, however, whether from the original formative cells of the embryo, or by the liver and the other organs mentioned above, these coloured nucleated cells begin very early in foetal life to be 104 THE BLOOD. [chap. in. mingled with coloured non-nucleated corpuscles resembling those of the adult, and at about the fourth or fifth month of embryonic existence are completely replaced by them. Origin of the Mature Coloured Corpuscles.-The non- nucleated red corpuscles may possibly be derived from the nucleated, but in all probability are an entirely new formation, and the methods of their origin are the following:-(1.) During foetal life and possibly in some animals, e.g., the rat, which are born in an immature condition, for some little time after birth, the blood discs arise in the connective tissue cells in the following way. Small globules, of varying size, of colouring matter arise Fig. 84.-Development of red corpuscles in connective tissue cells. From the subcutaneous tissue of a new-born rat. A, a cell containing haemoglobin in a diffused form in the proto- plasm ; K, one containing coloured globules of varying size and vacuoles; h", a cell filled with coloured globules of nearly uniform size; /,/'> developing fat cells. (E. A. Schafer.) in the protoplasm of the cells, and the cells themselves become branched, their branches joining the branches of similar cells. The cells next become vacuolated, and the red globules are free in a cavity filled with fluid (fig. 85); by the extension of the cavity of the cells into their processes anastomosing vessels are produced, which ultimately join with the previously existing vessels, and the globules, now having the size and appearance of the ordinary red corpuscles, are passed into the general circulation. This method of formation is called intracellular (Schafer). (2.) From the white corpuscles.-The belief that the red cor- puscles are derived from the white is still very general, although no new evidence has been recently advanced in favour of this view. It is, however, uncertain whether the nucleus of the white corpuscle becomes the red corpuscle, or whether the whole white corpuscle is bodily converted into the red by the gradual clearing up of its contents with a disappearance of the nucleus. Probably the latter view is the correct one. CHAP. III.] ORIGIN OF THE COLOURED CORPUSCLES. 105 (3-) From the medulla of bones.-Coloured corpuscles are to a very large extent derived during adult life from the large pale cells in the red marrow of bones, especially of the ribs (figs. 83, 84). These cells become coloured from the formation of haemoglobin chiefly in one part of then' protoplasm. This coloured part becomes separated from the rest of the cell and forms a red corpuscle, being at first cup-shaped, but soon taking- on the normal appearance of the mature corpuscle. It is supposed that the protoplasm may grow up again and form a number of red corpuscles in a similar way. (4.) From the tissue of the spleen.-It is probable that coloured as well as colourless corpuscles may be produced in the spleen. (5.) From Microcytes. - Hayem describes the small particles (microcytes), pre- viously mentioned as con- tained in the blood (p. 80), and which he calls hannato- blasts, as the precursors of the red corpuscles. They acquire colour, and enlarge to the normal size of red corpuscles. Without doubt, the red corpuscles have, like all other parts of the organism, a tolerably definite term of existence, and in a like manner die and waste away when the portion of work allotted to them has been performed. Neither the length of their life, however, nor the fashion of their decay has been yet clearly made out. It is generally believed that a certain number of the coloured cor- puscles undergo disintegration in the spleen ; and indeed corpuscles in various degrees of degeneration have been observed in that organ. Origin of the Colourless Corpuscles.-The colourless cor- Fig. 85.-Further development of blood-corpuscles in connective tissue cells and transformation of the latter into capillary blood-vessels, a, an elongated cell with a cavity in the proto- plasm occupied by fluid and by blood-cor- puscles which are still globular; b, a hollow' cell, the nucleus of which has multiplied. The new nuclei are arranged around the wall of the cavity, the corpuscles in which have now become discoid; c, shows the mode of union of a " haemapoietic " cell, which, in this instance, contains only one corpuscle, with the prolongation (bl) of a previously existing vessel; a and c, from the new'-bom rat; b, from the fcetal sheep. (E. A. Schafer.) 106 THE BLOOD. [chap. hi. puscles of the blood are derived from the lymph corpuscles, being, indeed, indistinguishable from them ; and these come chiefly from the lymphatic glands. Their number is increased by division. Colourless corpuscles are also in all probability derived from the Fig. 86.-Coloured nucleated corpuscles, from the red marrow of the guinea-pig. (E. A. Schafer.) spleen and thymus, and also from the germinating endothelium of serous membranes, and from connective tissue. The corpuscles are carried into the blood either with the lymph and chyle, or pass directly from the lymphatic tissue in which they have been formed into the neighbouring blood-vessels. 1. Uses of the Blood.-To be a medium for the reception and storing of matter (ordinary food, drink, and oxygen) from the outer world, and for its conveyance to all parts of the body. 2. To be a source whence the various tissues of the body may take the materials necessary for their nutrition and maintenance ; and whence the secreting organs may take the constituents of their various secretions. 3. To be a medium for the absorption of refuse matters from all the tissues, and for their conveyance to those organs whose function it is to separate them and cast them out of the body. 4. To warm and moisten all parts of the body. CHAPTER IV. THE CIRCULATION OF THE BLOOD. The Heart is a hollow muscular organ consisting of four cham- bers, two auricles and two ventricles, arranged in pairs. On the right and left sides of the heart is an auricle joined to and com- municating with a ventricle, but the chambers on the right side do not directly communicate with those on the left side. The circulation of the blood is chiefly carried on by the contraction or systole of the muscular walls of the chambers of the heart: the auricles contracting simultaneously, and their contraction being CHAP. IV.] COURSE OF THE CIRCULATION. 107 followed by the simultaneous contraction of the ventricles. The blood is conveyed away from the left side of the heart (as in the ■diagram, fig. 87) by the arteries, and returned to the right side of the heart by the veins, the arteries and veins being continuous with each other at one end by means of the heart, and at the Fig. 87.-Diagram of the circulation.-The unshaded part of the figure to the right indicates the district of the circulation of arterial blood; the dark part to the left the district of venous blood. other by a fine network of vessels called the capillaries. The blood, therefore, in its passage from the heart passes first into the arteries, then into the capillaries, and lastly into the veins, by which it is conveyed back again to the heart, thus completing a or circulation. As the right side of the heart, however, does not directly com- municate with the left, in order to complete the circulation it is necessary that the blood should pass from the right side to the lungs, through the pulmonary artery, then through the pulmonary 108 CIRCULATION OF TILE BLOOD. [chap. iv. capillary-vessels, and through the pulmonary veins to the left side of the heart (fig. 87). Thus there are two circulations by which the blood must pass; the one, a shorter circuit from the right side of the heart to the lungs and back again to the left side of the heart; the other and larger circuit, from the left side of the heart to all parts of the body and back again to the right side ; but more strictly speaking, there is only one complete circulation, which Larynx. Trachea. Aorta. Light Lung. Pulmonary Artery. Left Lung. .Heart Diaphragm. Fig. 88.- View of heart and lungs in situ. The front portion of the chest-wall, and the outer or parietal layers of the pleuree and pericardium have been removed. The lungs are partly collapsed. may be diagrammatically represented by a double loop, as in the accompanying figure (fig. 87). On reference to this figure, and noticing the direction of the arrows, which represent the course of the stream of blood, it will be observed that while there is a smaller and a larger circle, both of which pass through the heart, yet that these are not distinct, one from the other, but are formed really by one continuous stream, the whole of which must, at one part of its course, pass through the lungs. Subordinate to the two principal circulations, the Pulmonary and Systemic, as they are named, it will be noticed also in the same figure that there is another, by which a portion of the stream of blood having been diverted once into the capil- aries of the intestinal canal, and some other organs, and gathered CHAP. IV.] THE ANATOMY OF THE HEART. 109 up again into a single stream, is a second time divided in its passage through the liver, before it finally reaches the heart and completes a revolution. This subordinate stream through the liver is called the Portal circulation. As a necessary step towards the consideration of the method by which the circulation is maintained, it will be advisable in the first place to devote some time to the description of various important points in the anatomy and minute structure of-I. The Heart; II. The Arteries; III. The Capillaries ; IV. The Veins. We shall then be in a better position to discuss the problems in the physiology of the circulation. (I.) The Heart. The heart is contained in the chest or thorax, and lies between the right and left lungs (fig. 88), enclosed in a membranous sac- the pericardium, which is made up of two distinct parts, an external fibrous membrane, composed of closely interlacing fibres, which has its base attached to the diaphragm or midriff, the great muscle which forms the floor of the chest and divides it from the abdomen -both to the central tendon and to the adjoining muscular fibres, while the smaller and upper end is lost on the large blood-vessels by mingling its fibres with that of their external coats; and an internal serous layer, which not only lines the fibrous sac, but also is reflected on to the heart, which it completely invests. The part which lines the fibrous membrane is called the parietal layer, and that enclosing the heart, the visceral layer, and these being continuous for a short distance along the great vessels of the base of the heart, form a closed sac, the cavity of which in health contains just enough fluid to lubricate the two surfaces, and thus enable them to glide smoothly over each other during the move ments of the heart. Most of the vessels passing in and out of the heart receive more or less investment from this sac. The heart in the chest is situated behind the sternum and costal cartilages, being placed obliquely from right to left, quite two-thirds to the left of the mid-sternal line. It is of pyramidal shape, with the apex pointing downwards, outwards, and towards the left, and the base backwards, inwards, and towards the right. It rests upon the diaphragm, and its pointed apex, formed exclusively of the left side of the heart, is in contact with the chest wall, and during life beats against it at a point called the apex beat, situated in the fifth intercostal space, about two 110 CIRCULATION OF THE BLOOD. [chap. iv. inches below the left nipple, and an inch and a half to the sternal side. The heart is suspended in the chest by the large vessels which proceed from its base, but, excepting the base, the organ itself lies free in the sac of the pericardium. The part which rests upon the diaphragm is flattened, and is known as the posterior surface, whilst the free upper part is called the anterior surface. The margin towards the left is thick and obtuse, whilst the lower margin towards the right is thin and acute. On examination of the external surface the division of the heart into parts which correspond to the chambers inside of it may be traced, for a deep transverse groove called the auriculo-ventricular groove divides the auricles which form the base of the heart from the ventricles which form the remainder, including the apex, the ventricular portion being by far the greater; and, again, the inter-ventricular groove runs between the ventricles both front and back, and separates the one from the other. The anterior groove is nearer the left margin and the posterior nearer the right, as the front surface of the heart is made up chiefly of the right ventricle and the posterior surface of the left ventricle. In the furrows run the coronary vessels, which supply the tissue of the heart with blood, as well as nerves and lymphatics imbedded in more or less fatty material. The Chambers of the Heart.-The interior of the heart is divided by a partition in such a manner as to form two chief chambers or cavities-right and left. Each of these chambers is again sub- divided into an upper and a lower portion, called respectively, as already incidentally mentioned, auricle and ventricle, which freely communicate one with the other; the aperture of communication, however, is guarded by valves, so disposed as to allow blood to pass freely from the auricle into the ventricle, but not in the opposite direction. There are thus four cavities altogether in the heart-the auricle and ventricle of one side being quite separate from those of the other (fig. 89). (1.) Right Auricle.-The right auricle is situated at the right part of the base of the heart as viewed from the front. It is a thin walled cavity of more or less quadrilateral shape, prolonged at one corner into a tongue-shaped portion, the right auricular appendix, which slightly overlaps the exit of the great artery, the aorta, from the heart. The interior is smooth, being lined with the general lining of the heart, the endocardium, and into it open the superior and CHAP. IV.] CHAMBERS OF THE HEART. 111 inferior veme cava), or great veins, which convey the blood from all parts of the body to the heart. The former is directed down- wards and forwards, the latter upwards and inwards ; between the Fig. 89.-The right auricle and ventricle opened, and a part of then- right and anterior walls removed, so as to show their interior. J.-1, superior vena cava; 2, inferior vena cava ; 2', hepatic veins cut short ; 3, right auricle ; 3', placed in the fossa ovalis, below which is the Eustachian valve; 3", is placed close to the aperture of the coronary vein; +, +, placed in the auriculo-ventricular groove, where a narrow portion of the adja- cent walls of the auricle and ventricle has been preserved ; 4, 4, cavity of the right ventricle, the upper figure is immediately below the semilunar valves; 4', large columna camea or musculus papillaris; 5, 5', 5", tricuspid valve; 6, placed in the interior of the pulmonary artery, a part of the anterior wall of that vessel having been removed, and a narrow portion of it preserved at its commencement, where the semilunar valves are attached ; 7, concavity of the aortic arch close to the cord of the ductus arteriosus ; 8, ascending part or sinus of the arch covered at its commencement by the auricular appendix and pulmonary artery; 9, placed between the innominate and left carotid arteries ; 10, appendix of the left auricle ; n, n, the outside of the left ventricle, the lower figure near the apex. (Allen Thomson.) entrances of these vessels is a slight tubercle called tubercle of Lower. The opening of the inferior cava is protected and partly covered by a membrane called the Eustachian valve. In the 112 CIRCULATION OF THE BLOOD. [chap. iv. posterior wall of the auricle is a slight depression called the fossa ovalis, which corresponds to an opening between the right and left auricles which exists in foetal life. The right auricular appendix is of oval form, and admits three fingers. Various veins, including the coronary sinus, or the dilated portion of the right coronary vein, open into this chamber. In the appendix are closely set ■elevations of the muscular tissue covered with endocardium, and ■on the anterior wall of the auricle are similar elevations arranged parallel to one another, called musculi pectinati. (2.) Right Ventricle.-The right ventricle occupies the chief part of the anterior surface of the heart, as well as a small part of the posterior surface : it forms the right margin of the heart. It takes no part in the formation of the apex. On section its cavity, in consequence of the encroachment upon it of the septum ventri- ■culorum, is semilunar or crescentic (fig. 91); into it are two •openings, the auriculo-ventricular at the base, and the opening •of the pulmonary artery also at the base, but more to the left; the part of the ventricle leading to it is called the conns arteriosus ■or infundibulum; both orifices are guarded by valves, the former called tricuspid and the latter sem'Z'wnar or sigmoid. In this ventricle are also the projections of the muscular tissue called columnce carnece (described at length p. 116). (3.) Left Auricle.-The left auricle is situated at the left and posterior part of the base of the heart, and is best seen from behind. It is quadrilateral, and receives on either side two pul- monary veins. The auricular appendix is the only part of the auricle seen from the front, and corresponds with that on the right side, but is thicker, and the interior is more smooth. The left auricle is only slightly thicker than the right, the difference being as i| lines to 1 line. The left auriculo-ventricular orifice is •oval, and a little smaller than that on the right side of the heart. There is a slight vestige of the foramen between the auricles, which exists in foetal life, on the septum between them. (4.) Left Ventricle.-Though taking part to a comparatively .slight extent in the anterior surface, the left ventricle occupies the .chief part of the posterior surface. In it are two openings very close together, viz. the auriculo-ventricular and the aortic, guarded by the valves corresponding to those of the right side of the heart, viz. the bicuspid or mitral and the semilunar or sigmoid. The first ■opening is at the left and back part of the base of the ventricle, .and the aortic in front and towards the right. In this ventricle. CHAI*. IV.] LEFT CHAMBERS OF THE HEART. 113 as in the right, are the columnar cameie, which are smaller but more closely reticulated. They are chiefly found near the apex and along the posterior wall. They will be again referred to in Fig. 90.-The left auricle and ventricle opened and a part of their anterior and left walls removed. J.-The pulmonary artery has been divided at its commencement; the opening into the left ventricle is earned a short distance into the aorta between two of the segments of the semilunar valves ; and the left part of the auricle with its appendix has been removed. The right auricle is out of view. 1, the two right pulmonary veins cut short; their openings are seen within the auricle; 1', placed within the cavity of the auricle on the left side of the septum and on the part which forms the remains of the valve of the foramen ovale, of which the crescentic fold is seen towards the left hand of 1'; 2, a narrow portion of the wall of the auricle and ventricle preserved round the auriculo-ventricular orifice; 3, 3', the cut surface of the walls of the ventricle, seen to become very much thinner towards 3", at the apex ; 4, a small part of the anterior wall of the left ventricle which has been preserved with the principal anterior columna carnea or musculus papillaris attached to it; 5, 5, musculi papillares ; 5', the left side of the septum, between the two ventricles, within the cavity of the left ventricle; 6, 6', the mitral valve; 7, placed in the interior of the aorta near its commencement and above the three segments of its semilunar valve which are hanging loosely together; 7', the exterior of the great aortic sinus ; 8, the root of the pulmonary artery and its semilunar valves ; 8', the separated portfon of the pulmonary artery remaining attached to the aorta by 9, the cord of the ductus arteriosus ; 10, the arteries rising from the summit of the aortic arch (Allen Thomson). 114 CIRCULATION OF THE BLOOD. [chap. IV. the description of the valves. The walls of the left ventricle, which are nearly half an inch in thickness, are, with the exception of the apex, twice or three times as thick as those of the right. Capacity of the Chambers.- The capacity of the two ven- tricles is about four to six ounces of blood, the whole of which is impelled into their respective arteries at each con- traction. The capacity of the auricles is rather less than that of the ventricles: the thickness of their walls is con- siderably less. The latter condition is adapted to the small amount of force which the auricles require in order to empty themselves into their adjoining ventricles; the former to the circumstance of the ventricles being partly filled with blood before the auri- cles contract. Size and Weight of the Heart.- The heart is about 5 inches long, 3I inches greatest width, and 2t> inches in its extreme thickness. The average weight of the heart in the adult is from 9 to 10 ounces; its weight gradually in- creasing throughout life till mid- dle age ; it diminishes in old age. Structure.-The walls of the heart are constructed almost en- tirely of layers of muscular fibres; but a ring of connective tissue, to which some of the muscular fibres are attached, is inserted between each auricle and ventri- cle, and forms the boundary of the auriculo-ventricular opening. Fibrous tissue also exists at the origins of the pulmonary artery and aorta. Fig. 91.-Transverse section of bullock's heart in a state of cadaveric rigidity, a, cavity of left ventricle, b, cavity of right ventricle. (Dalton.) Fig. 92.-Network of muscular fibres (striated) from the heart of a pig. The nuclei of the muscle-corpuscles are well shown, x 450. (Klein and Noble Smith.) STOOD-SPECTRA COMPARED WITH SPECTRUM OF ARCrAND-LAMP 1 Spectrum of Argand-lamp with Fraunhofer's line's in position 2 Spectrum of Oxyhaemoglobin in diluted blood 3 Spectrum of Reduced Haemoglobin. 4- Spectrum of Carbonic oxide Haemoglobin 5 Spectrum of Acid Haematm in ethenal solution. 6 Spectrum of Alkaline Haematm 7 Spectrum of Chloroform extract of acidulated Ox-bile. 8 Spectrum of Methaemoglobin. 9 Spectrum of Haemochromogen 10 Spectrum of Hsematoporphyrin Most of the above Spectra have been dnuvn from observations htf M- WLepraik FCS. Danielsson 4 Ceron Lendon lith CHAP. IV.] STRUCTURE OF THE HEART. 115 The muscular fibres of each auricle are in part continuous with those of the other, and partly separate; and the same remark holds true for the ventricles. The fibres of the auricles are, how- ever, quite separate from those of the ventricles, the bond of connection between them being only the fibrous tissue of the auriculo-ventricular openings. The muscular fibres of the heart, unlike those of most of the involuntary muscles, are striated; but although, in this respect, they resemble the skeletal muscles, they have distinguishing characteris- tics of their own. The fibres which lie side by side are united at frequent intervals by short branches (fig. 92). The fibres are smaller than those of the ordinary striated muscles, and their striation is less marked. No sarcolemma can be discerned. The muscle-corpuscles are situate in the middle of the substance of the fibre; and in correspondence with these the fibres appear under cer- tain conditions subdivided into oblong portions or " cells," the off- sets from which are the means by which the fibres anastomose one with another (fig. 93). Endocardium.-As the heart is clothed on the outside by a thin transparent layer of pericardium, so its cavities are lined by a smooth and shining membrane, or endocardium, which is directly continuous with the internal lining of the arteries and veins. The endocardium is composed of connective tissue with a large ad- mixture of elastic fibres; and on its inner surface is laid down a single tesselated layer of flattened endothelial cells. Here and there unstriped muscular fibres are sometimes found in the tissue of the endocardium. Valves of the Heart.-The arrangement of the heart's valves is such that the blood can pass only in one direction (fig. 94). The tricuspid valve (5, fig. 89) presents three principal cusps or subdivisions, and mitral or bicuspid valve, because it has two such portions (6, fig. 90). But in both valves there is between each two principal portions a smaller one; so that more properly, the rig- 93--Muscular fibre cells from, the heart. (E. A. Schafer.) 116 CIRCULATION OF THE BLOOD. [chap. iv. tricuspid may be described as consisting of six, and the mitral of four, portions. Each portion is of triangular form, its base is continuous with the bases of the neighbouring portions, so as to form an annular membrane around the auriculo-ventricular open- ing, and is fixed to a tendinous ring which encircles the orifice between the auricle and ventricle and receives the insertions of the muscular fibres of both. In each principal cusp may be distinguished a central part, extending from base to apex, and Fig. 94.-Diagram oj the circulation through the heart (Dalton). including about half its width. It is thicker, and much tougher than the border-pieces or edges. While the bases of the cusps of the valves are fixed to the tendinous rings, their ventricular surface and borders are fastened by slender tendinous fibres, the chordae tendineoe, to the internal surface walls of the ventricles, the muscular fibres of which project into the ventricular cavity in the form of bundles or columns- the columns camera,. These columns are not all alike, for while some are attached along their whole length on one side, and by their extremities, others are attached only by their extremities ; and a third set, to which the name musculi papillares has been given, are attached to the wall of the ventricle CHAP. IV.] VALVES OF THE HEART. 117 by one extremity only, the other projecting, papilla-like, into the cavity of the ventricle (5, fig. 89), and having attached to it chorda? tendinese. Of the tendinous cords, besides those which pass from the walls of the ventricle and the musculi papillares to the margins of the valves, there are some of especial strength, which pass from the same parts to the edges of the middle and thicker portions of the cusps before referred to. The ends of these cords arc spread out in the substance of the valve, giving its middle piece its peculiar strength and toughness; and from the sides numerous other more slender and branching cords are given off, which are attached all over the ventricular surface of the adjacent border-pieces of the principal portions of the valves, as well as to those smaller portions which have been mentioned as lying between each two principal ones. Moreover, the musculi papillares are so placed that, from the summit of each, tendinous cords proceed to the adjacent halves of two of the principal divisions, and to one intermediate or smaller division, of the valve. The preceding description applies equally to the mitral and tricuspid valve; but it should be added that the mitral is con- siderably thicker and stronger than the tricuspid, in accordance with the greater force which it is called upon to resist. The semilunar valves, three in number, guard the orifices of the pulmonary artery and of the aorta. They are nearly alike on both sides of the heart; but the aortic valves are altogether thicker and more strongly constructed than the pulmonary valves, in accordance with the greater pressure which they have to withstand. Each valve is of semilunar shape, its convex margin being attached to a fibrous ring at the place of junction of the artery to the ventricle, and the concave or nearly straight border being free, so that each valve forms a little pouch like a watch-pocket (7, fig. 90). In the centre of the free edge of the valve, which contains a fine cord of fibrous tissue, is a small fibrous nodule, the corpus Arantii, and from this and from the attached border fine fibres extend into every part of the mid sub- stance of the valve, except a small lunated space just within the free edge, on each side of the corpus Arantii. Here the valve is thinnest, and composed of little more than the endocardium. Thus constructed and attached, the three semilunar valves are placed side by side around the arterial orifice of each ventricle, so as to form three little pouches, which can be separated by the blood passing out of the ventricle, but which immediately afterwards are 118 CIRCULATION OF THE BLOOD. [chap. iv. pressed together so as to prevent any return (7, fig. 89, and 7, fig. go). This will be again referred to. Opposite each of the semilunar cusps, both in the aorta and pulmonary artery, there is a bulging outwards of the wall of the vessel: these bulgings are called the sinuses of Valsalva. Structure of the Valves.-The valves of the heart are formed essentially of thick layers of closely woven connective and elastic tissue, over which, on every part, is reflected the endocardium. II. The Arteries. Distribution.-The arterial system begins at the left ventricle in a single large trunk, the aorta, which almost immediately after Fig. 95.-Minute artery viewed in longitudinal section, e. Nu- cleated endothelial membrane, with faint nuclei in lumen, looked at from above, i. Thin elastic tunica intima, m. Mus- cular coat or tunica media. a. Tunica adventitia. (Klein and Noble Smith.) x 250. Fig. 96.-Transverse section thi'ough a large branch of the inferior mesenteric artery of a pig. e, endothelial membrane; i, tu- nica elastica interna, no subendothelial layer is seen ; m, muscular tunica media, containing only a few wavy elastic fibres; e, e, tunica elastica externa, dividing the media from the connective tissue adven- titia, a. (Klein and Noble Smith.) x 350. its origin gives off in the thorax three large branches for the supply of the head, neck, and upper extremities; it then traverses the thorax and abdomen, giving off branches, some large and some small, for the supply of the various organs and tissues it passes on its way. In the abdomen it divides into two chief branches, for the supply of the lower extremities. The arterial branches CHAP. IV.] STRUCTURE OF THE ARTERIES. 119 wherever given off divide and subdivide, until the calibre of each subdivision becomes very minute, and these minute vessels pass into capillaries. Arteries are, as a rule, placed in situations protected from pressure and other dangers, and are, with few exceptions, straight in their course, and frequently communicate (anastomose or inosculate) with other arteries. The branches are usually given off at an acute angle, and the area of the branches Fig. 97.-Portion of a fenestrated membrane from the femoral artery. X 200. a, b, c, Perforations. (Henle.) Fig. 98.-Muscular fibre-cells from human arteries, magnified 350 diameters. (Kol- liker.) a. Nucleus. b. A fibre-cell treated with acetic acid. . of an artery generally exceeds that of the parent trunk; and as the distance from the origin is increased, the area of the combined branches is increased also. After death, arteries are usually found dilated (not collapsed as the veins are) and empty, and it was to this fact that their name was given them, as the ancients believed that they conveyed air to the various parts of the body. As regards the arterial system of the lungs (pulmonary system) it begins at the right ventricle in the pulmonary artery, and is distributed much as the arteries belonging to the general systemic circulation. Structure.-The walls of the arteries are composed of three principal coats, termed (a) the external or tunica adventitia, (6) the middle or tunica media, and (c) the internal or tunica intima. (ci) The external coat or tunica adventitia (figs. 95 and 96 a.), the strongest and toughest part of the wall of the artery, is 120 CIRCULATION OF THE BLOOD. [CHAP. IV. Fig. 99--Transverse section of aorta through internal and about half the middle coal. a. Lining endothelium with the nuclei of the cells only shown, b. Subepithelial layer of connec- tive tissue, e, d. Elastic tunica intima proper, with fibrils running circularly or longitudinally, c, f. Middle coat, consisting of elastic fibres arranged longitudinally, with muscle-flbi'es cut obliquely, or longitudinally. (Klein.) formed of areolar tissue, with which is mingled throughout a net- work of elastic fibres. At the inner part of this outer coat the Fig. 100.-Transverse section of small artery from soft palate, e, endothelial lining, the nuclei of the cells are shown; i, elastic tissue of the intima, which is a good deal folded ; c.m. circular muscular coat, showing nuclei of the muscle cells ; t.a. tunica adventitia. X 300. (Schofield.) slastic network forms in most arteries so distinct a layer as to be sometimes called the external elastic coat (fig. 99, e. f.). CHAP. IV.] STRUCTURE OF THE ARTERIES. 121 (6) The middle coat (fig. 95, ?n.) is composed of both muscular and elastic fibres, with a certain proportion of areolar tissue. In the larger arteries (fig. 99) its thickness is comparatively as well as absolutely much greater than in the small, constituting, as it does, the greater part of the arterial wall. The muscular fibres, which are of the unstriped variety (fig. 98), are arranged for the most part transversely to the long axis of the artery (fig. 95, m.); while the elastic element, taking Fig. 101.-Two blood-vessels from a frog's mesentery, injected with nitrate of silver, showing th* outlines of the endothelial cells, a. Artery. The endothelial cells are long and narrow ; the transverse markings indicate the muscular coat. La. Tunica adventitia, v. Vein, showing the shorter and wider endothelial cells with which it is lined, c, c. Twa capillaries entering the vein. (Schofield.) also a transverse direction, is disposed in the form of closely inter- woven and branching fibres, which intersect in all parts the layers of muscular fibre. In arteries of various size there is a difference in the proportion of the muscular and elastic element, elastic tissue preponderating in the largest arteries, while this condition is reversed in those of medium and small size. (c) The internal coat is formed by layers of elastic tissue, con- sisting in part of coarse longitudinal branching fibres, and in part of a very thin and brittle membrane which possesses little elasti- city, and is thrown into folds or wrinkles when the artery contracts. This latter membrane, the striated or fenestrated coat of Henle (fig- 97)5 is peculiar in its tendency to curl up, when peeled off from the artery, and in the perforated and streaked appearance 122 CIRCULATION OF THE BLOOD. [chap. iv. which it presents under the microscope. Its inner surface is lined with a delicate layer of elongated endothelial cells (fig. ioi, a.), which make it smooth and polished, and furnish a nearly imper- meable surface, along which the blood may flow with the smallest possible amount of resistance from friction. Immediately external to the endothelial lining of the artery is fine connective tissue, sub-endothelial layer, with branched cor- Fig. 102.-Blood-vessels from mesocolon of rabbit, a. Artery, with two branches, showing tr. n. nuclei of transverse muscular fibres; l.n. nuclei of endothelial lining; t.a. tunica adventitia, v. Vein. Here the transverse nuclei are more oval than those of the artery. The vein receives a small branch at the lower end of the drawing; it is distinguished from the artery among other things by its straighter course and larger calibre, c. Capil- lary, showing nuclei of endothelial cells, x 300. (Schofield.) puscles. Thus the internal coat consists of three parts, (a) an endothelial lining, (6) the sub-endothelial layer, and (c) elastic layers. Vasa Vasorum.-The walls of the arteries, with the possible exception of the endothelial lining and the layers of the internal coat immediately outside it, are not nourished by the blood which they convey, but are, like other parts of the body, supplied with little arteries, ending in capillaries and veins, which, branching CHAP. IV.] STRUCTURE OF THE CAPILLARIES. 123 throughout the external coat, extend for some distance into the middle, but do not reach the internal coat. These nutrient vessels are called vasa vasorum. Nerves.-Most of the arteries are surrounded by a plexus of Fig. ioj.-Ramification of nerves and termination in the muscular coat of a small artery of the frog (Arnold). sympathetic nerves, which twine around the vessel very much like ivy round a tree ; and ganglia are found at frequent intervals. The smallest arteries and capillaries are also surrounded by a very delicate network of similar nerve-fibres, many of which appear to end in the nuclei of the transverse muscular fibres (fig. 103). III. The Capillaries. Distribution.-In all vascular textures except some parts of the corpora cavernosa of the penis, and of the uterine placenta, and of the spleen, the transmission of the blood from the minute branches of the arteries to the minute veins is effected through a network of capillaries. They may be seen in all minutely injected preparations. The point at which the arteries terminate and the minute veins commence, cannot be exactly defined, for the transition is gradual; but the capillary network has, nevertheless, this peculiarity, that 124 CIRCULATION OF THE BLOOD. [chap. IV. the small vessels which compose it maintain the same diameter throughout: they do not diminish in diametei' in one direction, like arteries and veins; and the meshes of the network that they compose are more uniform in shape and size than those formed by the anastomoses of the minute arteries and veins. Structure.-This is much more simple than that of the arteries or veins. Their walls are composed of a single layer of elongated or radiate, flattened and nucleated cells, so joined and dovetailed together as to form a continuous transparent membrane (fig. 105). Outside these cells, in the larger capillaries, there is a structureless, or very finely fibrillated membrane, on the inner surface of which they are laid down. In some cases this external mem- brane is nucleated, and may then be regarded as a miniature representative of the tunica adventitia of arteries. Here and there, at the junction of two or more of the delicate endothelial cells which compose the capillary wall, pseudo-stomata may be seen (p. 25). The endothelial cells are often con- tinuous at various points with pro- cesses of adjacent connective-tissue cor- puscles. Capillaries are surrounded by a deli- cate nerve-plexus resembling, in miniature, that of the larger blood-vessels. The diameter of the capillary vessels varies somewhat in the different textures of the body, the most common size being about of an inch. Among the smallest may be mentioned those of the brain, and of the follicles of the mucous membrane of the intestines; among the largest, those of the skin, and especially those of the medulla of bones. The size of capillaries varies necessarily in different animals in relation to the size of their blood corpuscles: thus, in the Proteus, the capillary circulation can just be discerned with the naked eye. The form of the capillary network presents considerable variety Fit?. 104.-lilood-cessels of an ■ intestinal villus, representing the arrangement of capillaries between the ultimate venous and arterial branches; a, a, the arteries ; b, the vein. CJJAl'. IV.] STRUCTURE OF THE CAPILLARIES. 125 in the different textures of the body ; the varieties consisting principally of modifications of two chief kinds of mesh, the rounded and the elongated. That kind in which the meshes Pig. 105.- Capillary blood-vessels from the omentum of rabbit, showing the nucleated endo- thelial membrane of which they are composed. (Klein and Noble Smith.) or interspaces have a roundish form is the most common, and prevails in those parts in which the capillary network is most Fig. 106.-Network of capillary vessels of theair-cells of the horse's lung magnified, a, a, capillaries proceeding from b, b, terminal branches of the pulmonary artery. (Frey.) Fig. 107.-Injected capillary vessels of muscle seen with a low mag- nifying power. (Sharpey.) ■dense, such as the lungs (fig. 106), most glands, and mucous membranes, and the cutis. The meshes of this kind of network are not quite circular but more or less angular, sometimes presenting a nearly regular quadrangular or polygonal form, but 126 CIRCULATION OF THE BLOOD. [chap. iv. being more frequently irregular. The capillary network with elongated meshes (fig. 107) is observed in parts in which the vessels are arranged among bundles of fine tubes or fibres, as in muscles and nerves. In such parts, the meshes usually have the form of a parallelogram, the short sides of which may be from three to eight or ten times less than the long ones; the long sides always corresponding to the axis of the fibre or tube, by which it is placed. The appearance of both the rounded and elongated meshes is much varied according as the vessels composing them have a straight or tortuous form. Sometimes the capillaries have a looped arrangement, a single capillary projecting from the common network into some prominent organ, and returning after forming one or more loops, as in the papillae of the tongue and skin. The number of the capillaries and the size of the meshes in different parts determine in general the degree of vascularity of those parts. The parts in which the network of capillaries is closest, that is, in which the meshes or interspaces are the smallest, are the lungs and the choroid membrane of the eye. In the iris and ciliary body, the interspaces are somewhat wider, yet very small. In the human liver the interspaces are of the same size, or even smaller than the capillary vessels themselves. In the human lung they are smaller than the vessels; in the human kidney, and in the kidney of the dog, the diameter of the injected capillaries, compared with that of the interspaces, is in the proportion of one to four, or of one to three. The brain receives a very large quantity of blood ; but the capillaries in which the blood is dis- tributed through its substance are very minute, and less numerous than in some other parts. Their diameter, according to E. H. Weber, compared with the long diameter of the meshes, being in the proportion of one to eight or ten; compared with the trans- verse diameter, in the proportion of one to four or six. In the mucous membranes-for example in the conjunctiva and in the cutis vera, the capillary vessels are much larger than in the brain, and the interspaces narrower,-namely, not more than three or four times wider than the vessels. In the periosteum the meshes are much larger. In the external coat of arteries, the width of the meshes is ten times that of the vessels (Henle). It may be held as a general rule, that the more active the functions of an organ are, the more vascular it is. Hence the narrowness of the interspaces in all glandular organs, in mucous CHAP. IV.] STRUCTURE OF THE VEINS. 127 membranes, and in growing parts ; their much greater width in bones, ligaments, and other very tough and comparatively inactive tissues; and the usually complete absence of vessels in cartilage, and such parts as those in which, probably, very little vital change occurs after they are once formed. IV. The Veins. Distribution.-The venous system begins in small vessels which are slightly larger than the capillaries from which they spring. Fig. 108.- Transverse section through a small artery and vein of the mucous membrane of a child's epiglottis : the contrast between the thick-walled artery and the thin-walled vein is well shown. A. Artery', the letter is placed in the lumen of the vessel, e. Endothelial cells with nuclei clearly visible : these cells appear very thick from the contracted state of the vessel. Outside it a double wavy line marks the elastic tunica intima, m. Tunica media forming the chief part of arterial wall and consisting of unstriped muscular fibres circularly arranged : their nuclei are well seen. u. Part of the tunica adventitia showing bundles of connective-tissue fibre in section, with the circular nuclei of the connective-tissue corpuscles. This coat gradually merges into the surrounding connective-tissue. V. In the lumen of the vein. The other letters indicate the same as in the artery. The muscular coat of the vein (>n) is seen to be much thinner than that of the artery, x 350. (Klein and Noble Smith.) These vessels are gathered up into larger and larger trunks until they terminate (as regards the systemic circulation) in the two venae cavie and the coronary veins, which enter the right auricle, and (as regards the pulmonary circulation) in four pulmonary veins, which enter the left auricle. The total capacity of the veins 128 CIRCULATION OF THE BLOOD. [chap. iv. diminishes as they approach the heart; but, as a rule, their capacity exceeds by twice or three times that of their correspond- ing arteries. The pulmonary veins, however, are an exception to this rule, as they do not exceed in capacity the pulmonary arteries. The veins are found after death as a rule to be more or less collapsed, and often to contain blood. The veins are usually distributed in a superficial and a deep set which communicate frequently in their course. -In structure the coats of veins bear a general re- semblance to those of arteries (fig. 108). Thus, they possess an Fig. 109.-Diagram showing valves of veins. A, part of a vein laid open and spread out, with two pairs of valves, b, longitudinal section of a vein, showing the apposition of the edges of the valves in their closed state, c, portion of a distended vein, exhibiting a swelling in the situation of a pair of valves. ■outer, middle, and interned coat. The outer coat is constructed of nreolar tissue like that of the arteries, but is thicker. In some veins it contains muscular fibre-cells, which are arranged longitu- dinally. The middle coat is considerably thinner than that of the arteries ; and, although it contains circular unstriped muscular fibres or fibre-cells, these arc mingled with a larger proportion of yellow elastic and white fibrous tissue. In the large veins, near the heart, namely the venee cavce and pulmonary veins, the middle coat is replaced, for some distance from the heart, by circularly arranged striped muscular fibres, continuous with those of the auricles. The internal coat of veins is less brittle than the corresponding ■coat of an artery, but in other respects resembles it closely. Valves.-The chief influence which the veins have in the CH Al'. IV.] VALVES OF VEINS. 129 circulation, is effected with the help of the valves, which are placed in all veins subject to local pressure from the muscles between or near w'hich they run. The general construction of these valves is similar to that of the semilunar valves of the aorta and pul- monary artery, already described; but their free margins are turned in the opposite direction, i.e., towards the heart, so as to stop any movement of blood backward in the veins. They are commonly placed in pairs, at various distances in different veins, A B Fig. no.-a, vein with valves open, b, vein with valves closed: stream of blood passing off by lateral channel. (Dalton.) but almost uniformly in each (fig. 109). In the smaller veins, single valves are often met with ; and three or four are sometimes placed together, or near one another, in the largest veins, such as the subclavian, and at their junction with the jugular veins. The valves are semilunar; the unattached edge being in some examples concave, in others straight. They are composed of inextensile fibrous tissue, and are covered with endothelium like that lining the veins. During the period of their inaction, when the venous blood is flowing in its proper direction, they lie by the sides of the veins; but when in action, they close together like the valves of the arteries, and offer a complete barrier to any backward move- ment of the blood (figs. 109 and no). Their situation in the superficial veins of the forearm is readily discovered by pressing along its surface, in a direction opposite to the venous current, 130 CIRCULATION OF THE BLOOD. [chap. iv. i.e., from the elbow towards the wrist ; when little swellings (fig. 109 c) appear in the position of each pair of valves. These swellings at once disappear when the pressure is relaxed. Valves are not equally numerous in all veins, and in many they are absent altogether. They are most numerous in the veins of the extremities, and more so in those of the leg than the arm. They are commonly absent in veins of less than a line in diameter, and, as a general rule, there are few or none in those which are not subject to muscular pressure. Among those veins which have no valves may be mentioned the superior and inferior vena cava, the trunk and branches of the portal vein, the hepatic and renal veins, and the pulmonary veins; those in the interior of the cranium and vertebral column, those of the bones, and the trunk and branches of the umbilical vein are also destitute of valves. Lymphatics of Arteries and Veins. -Lymphatic spaces are present in the coats of both arteries and veins ; but in the tunica adventitia or ex- ternal coat of large vessels they form a distinct plexus of more or less tubular vessels. In smaller ves- sels they appear as sinous spaces lined by endothelium. Sometimes, as in the arteries of the omentum, mesentery, and membranes of the brain, in the pulmonary, hepatic, and splenic arteries, the spaces are continuous with vessels Ashich distinctly ensheath them^-perivascular lymphatic sheaths (fig. in). Lymph channels are said to be present also in the tunica media. Flg.m.-Sar/ace view of an artery from the mesentery of a frog, en- sheathed in a perivascular lym- phatic vessel, a. The artery, with its circular muscular coat (media) indicated by broad trans- verse markings, with an indica- tion of the adventitia outside. I. Lymphatic vessel; its wall is a simple endothelial membrane. (Klein and Noble Smith.) CHAI'. IV.] PHYSIOLOGY OF THE CIRCULATION. 131 The Action of the Heart. Tlie heart's action in propelling the blood consists in the suc- cessive alternate contraction (systole) and relaxation (diastole) of the muscular walls of its two auricles and two ventricles. i. Action of the Auricles.-The description of the action of the heart may be commenced at that period in each action which immediately precedes the beat of the heart against the side of the chest. At this period the -whole heart is in a passive state, the walls of both auricles and ventricles are relaxed, and their cavities are becoming dilated. The auricles are gradually filling with blood flowing into them from the veins; and a portion of this blood passes at once through them into the ventricles, the opening between the cavity of each auricle and that of its corresponding ventricle being, during all the pause, free and patent. The auricles, however, receiving more blood than at once passes through them to the ventricles, become, near the end of the pause, fully distended ; and at the end of the pause, they contract and expel their contents info the ventricles. The contraction of the auricles is sudden and very quick; it commences at the entrance of the great veins into them, and is thence propagated towards the auriculo-ventricular opening ; but the last part which contracts is the auricular appendix. The effect of this contraction of the auricles is to quicken the flow of blood from them into the ventricles ; the force of their contraction not being sufficient under ordinary circumstances to cause any back-flow in the veins. The reflux of blood into the great veins is moreover resisted not only by the mass of blood in the veins and the force with which it streams into the auricles, but also by the simultaneous contraction of the muscular coats with which the large veins are provided near their entrance into the auricles. Any slight regurgitation from the right auricle is limited also by the valves at the junction of the subclavian and internal jugular veins, beyond which the blood cannot move backwards; and the coronary vein is preserved from it by a valve at its mouth. In birds and reptiles regurgitation from the right auricle is prevented by valves placed at the entrance of the great veins. During the auricular contraction the force of the blood pro- pelled into the ventricle is transmitted in all directions, but being 132 CIRCULATION OF THE BLOOD. [chap. iv\ insufficient to separate the semilunar valves, it is expended in distending the ventricle, and, by a reflux of the current, in raising and gradually closing the auriculo-ventricular valves, which, when the ventricle is full, form a complete septum between it and the auricle. 2. Action of the Ventricles.-The blood which is thus driven, by the contraction of the auricles, into the corresponding ventricles, being added to that which had already flowed into them during the heart's pause, is sufficient to complete their diastole. Thus distended, they immediately contract: so imme- diately, indeed, that their systole looks as if it were continuous with that of the auricles. The ventricles contract much more slowly than the auricles, and in their contraction probably always thoroughly empty themselves, differing in this respect from the auricles, in which, even after their complete contraction, a small quantity of blood remains. The shape of both ventricles during systole undergoes an alteration, the left probably not altering in length but to a certain degree in breadth, the diameters in the plane of the base being diminished. The right ventricle does actually shorten to a small extent. The systole has the effect of diminishing the diameter of the base, especially in the plane of the auriculo-ventricular valves; but the length of the heart as a whole is not altered. (Ludwig.) During the systole of the ventiicles, too, the aorta and pulmonary artery, being filled with blood by the force of the ventricular action against considerable resistance, elongate as well as expand, and the whole heart moves slightly towards the right and forwards, twisting on its long axis, and exposing more of the left ventricle anteriorly than is usuallv in front. AV hen the systole ends the heart resumes its former position, rotating to the left again as the aorta and pulmonary artery contract. Functions of the Valves of the Heart.-(i) The Auriculo- Ventrit ulcer. | he distension of the ventricles with blood continues throughout the whole period of their diastole. The auriculo- Tun^KiCUjar va}ves are gradually brought into place by some of the blood getting behind the cusps and floating them up ; and bv the time that the diastole is complete, the valves are no doubt in apposition, the completion of this being brought about by the reflux current caused by the systole of the auricles. This eleva- tion of the auriculo-ventricular valves is materially aided by the action of the elastic tissue which has been shown to exist so chap, iv.] FUNCTIONS OF THE VALVES OF THE HEART. 133 largely in their structure, especially on the auricular surface. At any rate at the commencement of the ventricular systole they are completely closed. It should be recollected that the diminution in the breadth of the base of the heart in its transverse diameters during ventricular systole is especially marked in the neighbour- hood of the auriculo-ventricular rings, and this aids in rendering the auriculo-ventricular valves competent to close the openings, by greatly diminishing their diameter. The margins of the cusps of the valves are still more secured in apposition with another, by the simultaneous contraction of the musculi papillares, whose chorda) tendinea) have a special mode of attachment for this object (p. 117). The cusps of the auriculo-ventricular valves meet not by their edges only, but by the opposed surfaces of their thin outer borders. The form and position of the fleshy columns on the internal walls of the ventricle no doubt help to produce the obliteration of the ventricular cavity during contraction ; and the completeness of the closure may often be observed on making a transverse section of a heart shortly after death, in any case in which rigor mortis is very marked (fig. 91). In such a case only a central fissure may be discernible to the eye in the place of the cavity of each ventricle. If there were only circular fibres forming the ventricular wall, it is evident that on systole the ventricle would elongate ; if there were only longitudinal fibres the ventricle would shorten on systole ; but there are both. The tendency to alter in length is thus counter-balanced, and the whole force of the contraction is expended in diminishing the cavity of the ventricle; or, in other words, in expelling its contents. On the conclusion of the systole the ventricular walls tend to expand by virtue of their elasticity, and a negative pressure is set up, which tends to suck in the blood. This negative or suctional pressure on the left side of the heart is of the highest importance in helping the pulmonary circulation. It has been found to be equal to 23 mm. of mercury, and is quite independent of the aspiration or suction power of the thorax, which will be described in the chapter on Respiration. Function of the Musculi Papillares.-The special function of the musculi papillares is to prevent the auriculo-ventricular valves from being everted into the auricle. For the chordae tendineae might allow the valves to be pressed back into the auricle, were 134 CIRCULATION OF THE BLOOD. [chap. IV. it not that when the wall of the ventricle is brought by its con- traction nearer the auriculo-ventricular orifice, the musculi papillares more than compensate for this by their own contraction -holding the cords tight, and, by pulling down the valves, adding slightly to the force with which the blood is expelled. What has been said applies equally to the auriculo-ventricular valves on both sides of the heart, and of both alike the closure is generally complete every time the ventricles contract. But in some circumstances the closure of the tricuspid valve is not complete, and a certain quantity of blood is forced back into the auricle. This has been called the safety-valve action of this valve. The circumstances in which it usually happens are those in which the vessels of the lung are already full enough when the right ventricle contracts, as e.$r., in certain pulmonary diseases, in very active exertions, and in great efforts. In these cases, the tricuspid valve does not completely close, and the regurgitation of the blood may be indicated by a pulsation in the jugular veins synchronous with that in the carotid arteries. (2) Of the Semilunar Valves.-The arterial or semilunar valves are forced apart by the out-streaming blood, with which the con- tracting ventricle dilates the large arteries. The dilation of the arteries is, in a peculiar manner, adapted to bring the valves into action. The lower borders of the semilunar valves are attached to the inner surface of the tendinous ring, which is, as it were, inlaid at the orifice of the artery, between the muscular fibres of the ventricle and the elastic fibres of the walls of the artery. The tissue of this ring is tough, and does not admit of extension under such pressure as it is commonly exposed to ; the valves are equally inextensile, being, as already mentioned, formed mainly of tough, close-textured, fibrous tissue, with strong interwoven cords. Hence, when the ventricle propels blood through the orifice and into the canal of the artery, the lateral pressure which it exercises is sufficient to dilate the walls of the artery, but not enough to stretch in an equal degree, if at all, the unyielding valves and the ring to which their lower borders are attached. The effect, therefore, of each such propulsion of blood from the ventricle is, that the wall of the first portion of the artery is dilated into three pouches behind the valves, while the free margins of the valves are drawn inward towards its centre (fig. 112 b). Their positions may be explained by the diagrams, in which the continuous lines represent a transverse section of the CHAP. IV.] ACTION OF THE SEMILUNAR VALVES. 135 arterial walls, the dotted ones the edges of the valves, firstly, when the valves are nearest to the walls (a), as in the dead heart A B Fig. 112.-Sections of aorta, to show the action of the semilunar valves, a is intended to show the valves, represented hy the dotted lines, lying near the arterial walls, represented by the continuous outer line, b (after Hunter) shows the arterial wall distended into three pouches (a), and drawn away from the valves, which are straightened into the form of an equilateral triangle, as represented by the dotted lines. and, secondly, when, the walls being dilated, the valves are drawn away from them (b). rig. 113.- View of the base of the ventricular part of the heart, shoving the relative position of the arterial and auriculo-ventricular orifices. -5. The muscular fibres of the ven- tricles are exposed by the removal of the pericardium, fat, blood-vessels, etc.; the pulmonary artery and aorta have been removed by a section made immediately beyond the attachment of the semilunar valves, and the auricles have been removed imme- diately above the auriculo-ventricular orifices. The semilunar and auriculo-ventricular valves are in the nearly closed condition. 1, 1, the base of the right ventricle ; 1', the conus arteriosus; 2, 2, the base of the left ventricle ; 3, 3, the divided wall of the right auricle; 4, that of the left; 5, 5', 5", the tricuspid valve ; 6, 6', the mitral valve. In the angles between these segments are seen the smaller fringes frequently observed; 7, the anterior part of the pulmonary artery ; 8, placed upon the posterior part of the root of the aorta ; 9, the right, 9', the left coronary artery. (Allen Thomson ) This position of the valves and arterial walls is retained so long as the ventricle continues in contraction : but, as soon as it relaxes, and the dilated arterial walls can recoil by their elasticity, 136 CIRCULATION OF THE BLOOD. [chap. IV. the blood is forced backwards towards the ventricles as onwards in the course of the circulation. Part of the blood thus forced back lies in the pouches (sinuses of Valsalva) (a, fig. i t 2, b) between the valves and the arterial walls; and the valves are by it pressed together till their thin lunated margins meet in three lines radiating from the centre to the circumference of the artery (7 and 8, fig. 113). The contact of the valves in this position, and the complete closure of the arterial orifice, are secured by the peculiar con- struction of their borders before mentioned. Among the cords which are interwoven in the substance of the valve, are two of greater strength and prominence than the rest; of which one extends along the free border of each valve, and the other forms a double curve or festoon just below the free border. Each of these cords is attached by its outer extremities to the outer end of the free margin of its valve, and in the middle to the corpus Arantii; they thus en- close a lunated space from a line to a line and a half in width, in which space the substance of the valve is much thinner and more pliant than else- where. When the valves are pressed down, all these parts or spaces of their surfaces come into contact, and the closure of the arterial orifice is thus secured by the apposition not of the mere edges of the valves, but of all those thin lunated parts of each which lie between the free edges and the cords next below them. These parts are firmly pressed together, and the greater the pressure that falls on them the closer and more secure is their apposition. The corpora Arantii meet at the centre of the arterial orifice when the valves are down, and they probably assist in the closure ; but they are not essential to it, for, not they are wanting in the valves of the pul- monary artery, which are then extended in larger, thin, flapping margins. In valves of this form, also, the inlaid cords are less distinct than in those with corpora Arantii; yet the closure by contact of their surfaces is not less secure. Fig'. 114. -Vertical section through the aorta at its junction with the left ventricle, a, Section of aorta. ftA, Section of two valves, c, Section of wall of ventricle, d, Internal surface of ventricle. CHAP. IV. ] CARDIAC CYCLE. 137 It has been clearly shown that this pressure of the blood is not entirely sustained by the valves alone, but in part by the muscular substance of the ventricle (Savory). By making vertical sections (fig. 114) through various parts of the tendinous rings it is possible to show clearly that the aorta and pulmonary artery, expanding towards, their termination, are situated upon the outer edge of the thick upper border of the ventricles, and that consequently the portion of each semilunar valve adjacent to the vessel passes over and rests upon the muscular substance-being thus supported, as it were, on a kind of muscular floor formed by the upper border of the ventricle. The result of this arrangement is that the reflux of the blood is most efficiently sustained by the ventricular wall. As soon as the auricles have completed their contraction they begin again to dilate, and to be refilled with blood, which flows into them in a steady stream through the great venous trunks. Indeed, a chief function of the auricles is to form a receptacle for the on-streaming blood during the ventricular contraction. They are thus filling during all the time in which the ventricles are contracting; and the contraction of the ventricles being ended, these also again dilate, and receive again the blood that flows into them from the auricles. By the time that the ventricles are thus from one-third to two-thirds full, the auricles are distended : these, then suddenly contracting, fill up the ventricles, as already described (p. 131). Cardiac Cycle.-If we suppose a cardiac cycle divided into five parts, one of these will be occupied by the contraction of the auricles, two by that of the ventricles, and two by repose of both auricles and ventricles. Contraction of Auricles . . . i + Repose of Auricles . . -4 = 5 „ Ventricles . . 2 + „ Ventricles . . 3 = 5 Repose (no contraction of either auricles or ventricles) ... 2 + Contraction (of either auri- - cles or ventricles) . -3 = 5 5 If the speed of the heart be quickened, the time occupied by each cardiac revolution is of course diminished, but the diminution affects only the diastole and pause. The systole of the ventricles occupies very much the same time, about sec., whatever the pulse-rate. The periods in which the several valves of the heart are in 138 CIRCULATION OF THE BLOOD. [chap. iv. action may be connected with the foregoing table; for the auriculo-ventricular valves are closed, and the arterial valves are open during the whole time of the ventricular contraction, while, during the dilation and distension of the ventricles, the latter valves are shut, the former open. Thus w'henever the auriculo- ventricular valves are open, the arterial valves are closed and vice versa. The Sounds of the Heart. When the ear is placed over the region of the heart, two sounds may be heard at every beat of the heart, which follow in quick succession, and are succeeded by a pause or period of silence. The first sound is dull and prolonged; its commencement coincides with the movement or impulse of the heart against the chest wall, and just precedes the pulse at the wrist. The second is a shorter and sharper sound, with a somewhat flapping character, and follows close after the arterial pulse. The period of time occu- pied respectively by the two sounds taken together, and by the pause, are almost exactly equal. The relative length of time occupied by each sound, as compared with the other, is a little uncertain. The difference may be best appreciated by considering the different forces concerned in the production of the two sounds. In one case there is a strong, comparatively slow, contraction of a large mass of muscular fibres, urging forward a certain quantity of fluid against considerable resistance; while in the other it is a strong but shorter and sharper recoil of the clastic coat of the large arteries,-shorter because there is no resistance to the flapping back of the semilunar valves, as there was to their opening. The sounds may be expressed by saying the words lubb-dup (C. J. B. Williams). The events which correspond, in point of time, with the first sound, are (i) the contraction of the ventricles, (2) the first part of the dilatation of the auricles, (3) the tension of the auriculo- ventricular valves, (4) the opening of the semilunar valves, and (5) the propulsion of blood into the arteries. The sound is suc- ceeded, in about one-thirtieth of a second, by the pulsation of the facial arteries, and in about one-sixth of a second, by the pulsa- tion of the arteries at the wrist. The second sound, in point of time, immediately follows the cessation of the ventricular con- traction, and corresponds with (a) the tension of the semilunar chap, iv.] CAUSES OF THE SOUNDS OF THE HEART. 139 valves, (6) the continued dilatation of the auricles, (c) the com- mencing dilatation of the ventricles, and (cl) the opening of the auriculo-ventricular valves. The pause immediately follows the second sound, and corresponds in its first part with the completed distension of the auricles, and in its second with their contraction, and the completed distension of the ventricles; the auriculo- ventricular valves being, all the time of the pause, open, and the arterial valves closed. Causes.-The exact causes of the first sound of the heart are not exactly known. Two factors probably enter into it, viz. the vibration of the auriculo-ventricular valves and chordae tendineae, due to their stretching, and also, but to a less extent, of the ventricular walls, and coats of the aorta and pulmonary artery, all of which parts are suddenly put into a state of tension at the moment of ventricular contraction; and secondly the mus- cular sound produced by contraction of the mass of muscular fibres which form the ventricle. The first factor is probably the more important. The cause of the second sound is more simple than that of the first. It is probably due entirely to the vibration consequent on the sudden closure of the semilunar valves when they are pressed down across the orifices of the aorta and pulmonary artery. The influence of the valves in producing the sound is illustrated by the experiment performed on large animals, such as calves, in which the results could be fully appreciated. In these experi- ments two delicate curved needles were inserted, one into the aorta, and another into the pulmonary artery, below the line of attachment of the semilunar valves, and, after being carried upwards about half an inch, were brought out again through the coats of the respective vessels, so that in each vessel one valve was included between the arterial walls and the wire. Upon applying the stethoscope to the vessels, after such an operation, the second sound had ceased to be audible. Disease of these valves, when so extensive as to interfere with their efficient action, also often demonstrates the same fact by modifying or destroying the distinctness of the second sound. One reason for the second sound being a clearer and sharper one than the first may be, that the semilunar valves are not covered in by the thick layer of fibres composing the walls of the heart to such an extent as are the auriculo-ventricular. It might be expected therefore that their vibration would be more 140 CIRCULATION OF THE BLOOD. [GHAF. IV. easily heard through a stethoscope applied to the walls of the chest. The contraction of the auricles which takes place in the end of the pause is inaudible outside the chest, but may be heard, when the heart is exposed and the stethoscope placed on it, as a slight sound preceding and continued into the louder sound of the ven- tricular contraction. The Impulse of the Heart. At the commencement of each ventricular contraction, the heart may be felt to beat with a slight shock or impulse against the walls of the chest. The force of the impulse, and the extent to which it may be perceived beyond this point, vary considerably in different individuals, and in the same individual under different circumstances. It is felt more distinctly, and over a larger extent of surface, in emaciated than in fat and robust persons, and more during a forced expiration than in a deep inspiration ; for, in the one case, the intervention of a thick layer of fat or muscle between the heart and the surface of the chest, and in the other the inflation of the portion of lung which overlaps the heart, prevents the impulse from being fully transmitted to the surface. An excited action of the heart, and especially a hypertrophied condition of the ventricles, will increase the impulse ; while a depressed condition, or an atrophied state of the ventricular walls, will diminish it. Cause of the Impulse.--During the period which precedes the ventricular systole, the apex of the heart is situated upon the diaphragm and against the chest-wall in the fifth intercostal space. When the ventricles contract, their walls become hard and tense, since to expel their contents into the arteries is a distinctly labo- rious action, as it is resisted by the elasticity of the vessels. It is to this sudden hardening that the impulse of the heart against the chest-wall is due, and the shock of the sudden tension may be felt not only externally, but also internally, if the abdomen of an animal be opened and the finger be placed upon the under surface of the diaphragm, at a point corresponding to the under surface of the ventricle. The shock is felt, and possibly seen more dis- tinctly because of the partial rotation of the heart, already spoken of, along its long axis towards the right. The movement pro- duced by the ventricular contraction against the chest-wall may CHAP, IV.] THE CARDIOGRAPH. 141 be registered by means of an instrument called the cardiograph, and it will be found to correspond almost exactly with a tracing obtained by the same instrument applied over the contracting ventricle itself. The Cardiograph (fig. 115) consists of a cup-shaped metal box over the open front of which is stretched an elastic india-rubber membrane, upon which is fixed a small knob of hard wood or ivory. This knob, however, may be attached instead, as in the figure, to the side of the box by means of a spring, and may be made to act upon a metal disc attached to the elastic membrane. The knob (A) is for application to the chest-wall over the place of the greatest im- pulse of the heart. The box or tympanum communicates by means of an air-tight elastic tube (/) with the interior of a second tym- panum (fig. 116, Z*), in connection with which is a long and light lever (a). The shock of the heart's impulse being communi- cated to the ivory knob, and through it to the first tympanum, the effect is, of course, at once transmitted by the column of air in the elastic tube to the interior of the second tympanum, also closed, and through the elastic and movable lid of the latter to the lever, which is placed in connection with a registering apparatus. This generally con- sists of a cylinder or drum covered with smoked paper, revolving according to a definite velocity by clock-work. The point of the lever writes upon the paper, and a tracing of the heart's impulse or cardiogram, is thus obtained. By placing three small india-rubber air-bags or cardiac sounds in the Fig. 115.-Cardiograph. (Sander- son's.) Fig. 116.-Marey's Tambour (&), to which the movement of the column of air in the first tympanum is conducted by the tube, /, and from which it is communicated by the lever a, to a revolving cylinder, so that the tracing of the movement of the impulse beat is obtained. interior respectively of the right auricle, the right ventricle, and in an intercostal space in front of the heart of living animals (horse), and placing These bags, by means of long narrow tubes, in communication with three 142 CIRCULATION OF THE BLOOD. [CHAP. IV. levers, arranged one over the other in connection with a registering apparatus (fig. 117), MM. Chauveau and Marey have been able to record and measure with much accuracy the variations of the endocardial pressure Fig. 117.-Apparatus of MM. Chauveau and Marey for estimating the variations of endo- cardial pressure, and production of impulse of the heart. and the comparative duration of the contractions of the auricles and ventricles. By means of the same apparatus, the synchronism of the impulse with the contraction of the ventricles, is also well shown ; and the causes of the several vibrations of which it is really composed, have been demon- strated. In the tracing (fig. 118), the intervals between the vertical lines represent periods of a tenth of a second. The parts on which any given vertical line falls represent simultaneous events. It will be seen that the contraction of the auricle, indicated by the marked curve at A in first tracing, causes a slight increase of pressure in the ventricle, which is shown at A' in the second tracing, and produces also a slight impulse, which is indicated by A" in the third tracing. The closure of the semilunar valves causes a momen- tarily increased pressure in the ventricle at d', affects the pressure in the auricle D, and is also shown in the tracing of the impulse also, d". The large curve of the ventricular and the impulse tracings, between a' and d', and A" and d", are caused by the ventri- cular contraction, while the smaller undulations, between B and c, b' and c', b" and c", are caused by the vibrations consequent on the tightening and closure of the auriculo-ventricular valves. The method thus described may, as a rule, demonstrate quite correctly the Fig. 118. - Tracings of (i), Intia- auricular, and (2), Intra-ventricular pressures, and (3), of the impulse of the heart, to be read from left to right, obtained by Chauveau and Marey's apparatus. chap, iv.] FREQUENCY OF THE HEART'S ACTION. 143 variations of endocardial pressure, and these variations only, but there is a danger lest the muscular walls should grip the air-bags, even after the complete expulsion of the fluid contents of the chamber, and if so the lever would remain at its highest point for too long a time. The highest curve under such circumstances would represent on the tracing not only, as it ought to do, the endocardiac pressure, but also in addition the muscular pressure exerted upon the cardiac sound itself. (M. Foster.) Frequency and Force of the Heart's Action. The heart of a healthy adult man contracts from seventy to seventy-five times in a minute ; but many circumstances cause this rate, which of course corresponds with that of the arterial pulse., to vary even in health. The chief are age, temperament, sex, food and drink, exercise, time of day, posture, atmospheric pressure, temperature. (1.) Age.-The frequency of the heart's action gradually diminishes from the commencement tonear the end of life, but is said to rise again somewhat in extreme old age, thus:- Before birth the average number of pulsations per minute is 150 Just after birthfrom 140 to 130 During the first year130 to 115 During the second year115 to 100 During the third year100 to 90 About the seventh year90 to 85 About the fourteenth year, the average number of pulses in a minute is from85 to 80 In adult age80 to 70 In old age70 to 60 In decrepitude75 to 65 (2.) Temperament and Sex.-In persons of sanguine temperament, the heart acts somewhat more frequently than in those of the phlegmatic ; and in the female sex more frequently than in the male. (3 and 4.) Food and Drink. Exercise.-After a meal the heart's action is accelerated, and still more so, during bodily exertion or mental excite- ment ; it is slower during sleep. (5.) Diurnal Variation.-In the state of health, the pulse is most frequent in the morning, and becomes gradually slower as the day advances: and that this diminution of frequency is both more regular and more rapid in the evening than in the morning. (6.) Posture.-The pulse, as a general rule, especially in the adult male, is more frequent in the standing than in the sitting posture, and in the latter than in the recumbent position; the difference being greatest between the standing and the sitting postures. The effect of change of posture is greater as the frequency of the pulse is greater, and, accordingly, is more marked in the morning than in the evening. By supporting the body in different positions, without the aid of muscular effort of the individual, it 144 CIRCULATION OF THE BLOOD. [chav. IV. has been proved that the increased frequency of the pulse in the sitting and standing positions is dependent upon the muscular exertion engaged in maintaining them ; the usual effect of these postures on the pulse being almost entirely prevented when the usually attendant muscular exertion was rendered unnecessary. (Guy.) (7.) Atmospheric Pressure.-The frequency of the pulse increases in a ■corresponding ratio with the elevation above the sea. (8.) Temperature.-The rapidity and force of the heart's contractions are largely influenced by variations of temperature. The frog's heart, when excised, ceases to beat if the temperature be reduced to 320 F. (0° C.). When heat is gradually applied to it, both the speed and force of the contractions increase till they reach a maximum. If the temperature is still further raised, the beats become irregular and feeble, and the heart at length stands still in a condition of " heat-rigor." Similar effects are produced in warm-blooded animals. In the rabbit, the number of heart-beats is more than doubled when the temperature of the air was maintained at 105° F. (4Oo-5 C.). At 113°-1140 F. (450 C.), the rabbit's heart ceases to beat. Relative Frequency of the Heart's Contractions to the number of Respirations.-In health there is observed a nearly uniform relation between the frequency of the beats of the heart and of the respirations; the proportion being, on an average, one respiration to three or four beats. The same relation is generally maintained in the cases in which the action of the heart is naturally accele- rated, as after food or exercise ; but in disease this relation usually ■ceases. In many affections accompanied with increased frequency ■of the heart's contraction, the respiration is, indeed, also acce- lerated, yet the degree of its acceleration may bear no definite proportion to the increased number of the heart's actions : and in many other cases, the heart's contraction becomes more frequent without any accompanying increase in the number of respirations ; •or, the respiration alone may be accelerated, the number of pulsations remaining stationary, or even falling below7 the ordinary standard. The Force of the Ventricular Action.-The force of the left ventricular systole is more than double that exerted by the contrac- tion of the right ventricle : this difference results from the walls of the left ventricle being about twice or three times as thick as those of the right. And the difference is adapted to the greater -degree of resistance which the left ventricle has to overcome, -compared with that to be overcome by the right: the former having to propel blood through every part of the body, the latter only through the lungs. The actual amount of the intra- ventricular pressures during systole in the dog has been found to CHAP. IV.] FORCE OF THE CONTRACTIONS. 145 be 2'4 inches (60 mm.) of mercury in the right ventricle, and 6 inches (150 mm.) in the left. During diastole there is in the right ventricle a negative or suction pressure of about g of an inch (-17 to-16 mm.), and in the left ventricle from 2 inches to 4 of an inch ( - 52 to - 20 mm.). Part of this fall in pressure, and possibly the greater part, is to be referred to the influence of respiration; but without this the negative pressure of the left ventricle caused by its active dilatation is about equal to of an inch (23 mm.) of mercury. The right ventricle is undoubtedly aided by this suction power of the left, so that the whole of the work of conducting the pulmonary circulation does not fall upon the right side of the heart, but is assisted by the left side. The Force of the Auricular Contractions.-The maximum pressure within the right auricle is about A of an inch (20 mm.) of mercury, and is probably somewhat less in the left. It has been found that during diastole the pressure within both auricles sinks considerably below that of the atmosphere ; and as some fall in pressure takes place, even when the thorax of the animal operated upon has been opened, a certain proportion of the fall must be due to active auricular dilatation independent of respiration. In the right auricle, this negative pressure is about - 10 mm. Work Done by the Heart.-In estimating the work done by any machine it is usual to express it in terms of the " unit work." In England, the unit of work is the " foot-pound," and is defined to be the energy expended in raising a unit of weight (1 lb.) through a unit of height (1 ft.) : in France, the " kilogram-metre." The work done by the heart at each contraction can be readily found by multiplying the weight of blood expelled by the ventricles by the height to which the blood rises in a tube tied into an artery. This height was found to be about 9 ft. in the horse, and this estimate is nearly correct for a large artery in man. Taking the weight of blood expelled from the left ventricle at each systole at 6 oz., i.e., | lb., we have 9 x j = 3*375 foot-pounds as the work done by the left ventricle at each systole; and adding to this the work done by the right ventricle (about one-third that of the left) we have 3375 x 1-125 = 43 foot-pounds as the work done by the heart at each contraction. Other estimates give 2 kilogram-metre, or about 3I foot-pounds. Haughton estimates the total work of the heart in 24 hours as about 124 foot-tons. 146 CIRCULATION OF THE BLOOD. [chap. iv. Influence of the Nervous System on the Action of the Heart. The hearts of warm-blooded animals cease to beat very soon after removal from the body, and are, therefore, unfavourable for the study of the nervous mechanism which regulates their action. The hearts of cold-blooded animals, therefore, e.y., the frog, tortoise, and snake, which will continue to beat under favourable conditions for many hours after removal from the body, are generally employed, as more convenient for the purpose. Of these animals, the frog is the one most frequently used, and, indeed, until recently, it was from the study of the frog's heart that the chief part of our information on the subject was obtained. If removed from the body entire, the frog's heart will continue to beat for many hours and even days, and the beat has no apparent difference from the beat of the heart before removal from the body ; it will take place without the presence of blood or other fluid within its chambers. If the beats have become infrequent, an additional one may be induced by mechanically stimulating the heart by means of a blunt needle; but the time before the stimulus applied produces its results (the latent period) is very prolonged, and as in this way the cardiac beat is like the con- traction of unstriped muscle, it has been likened to a peristaltic contraction. There is much uncertainty about the nervous mechanism of the beat of the frog's heart, but what has just been said shows, at any rate, two things : firstly, that as the heart will beat when removed from the body in a way differing not at all from the normal, it must contain within itself the mechanism of rhythmical contraction ,• and, secondly, that as it can beat without the presence of fluid Fig. 119A.-The heart o f a Frog (Rana escule.nta) from the front. V, ventricle; Ad, right auricle ; As, left auricle ; R, bulbus arte- riosus, dividing into right and left aortse. (Ecker.) CHAP. IV.] NERVOUS MECHANISM OF THE HEART. 147 within its chambers, the movement cannot depend solely on reflex excitation by the entrance of blood. The nervous apparatus existing in the heart itself has been found to consist of collections of microscopic ganglia, and of nerve- fibres proceeding from them. These ganglia are demonstrable as being collected chiefly into three groups : one is in the wall of the sinus venosus at the junction of the sinus with the auricles {Reniak's) ; a second, near the junction between the auricles and ventricle (Bidder's); and the third in the septum between the auricles. It is generally believed that the rhythmical contractions of the frog's heart are, under ordinary circumstances, closely associated with these ganglia. Thus, (i) if the heart be removed entire from the body, the sequence of the contrac- tion of its several beats will take place with rhythmi- cal regularity, viz., of the sinus venosus, the auri- cles, the ventricle, and bulbus arteriosus, in order. (2) If the heart be re- moved at the junction of the sinus and auricle, the former, remaining in situ, will continue to beat, but the removed portion will for a short variable time stop beating, and when it resumes its beats, it will be with a diffe- rent rhythm to that of the sinus ; and, further, (3) if the ventricle only be removed, it will take a still longer time before recom- mencing its pulsation after its removal than the larger portion consisting of the auricles and ventricle does in experiment (2), and its rhythm is different from that of the unremoved portion, and not so regular. It will not continue to pulsate so long ; but during the period of stoppage a contraction will occur if it be mechanically or otherwise stimulated. (4) If the lower two-thirds or apex of the ventricle be removed, the remainder of the heart will go on beating C..V. S;- A.v.- J.7<- F.p. c.s.cl. -A.d. c. Fig. 119B.- 2'Ae Heart of a Frog {Rana esculenta) from the back. s.v., sinus venosus opened; c.s.s., left vena cava superior; c.s.d., right vena cava supe- rior; c.i., vena cava inferior; v.p., vena pulmo- nales; A.d., right auricle; A.s., left auricle; A .p., opening of communication between the right auricle and the sinus venosus. x -3. (Ecker.) 148 CIRCULATION OF THE BLOOD. [chap. IV. regularly in the body, but the part removed will remain motionless and will not beat spontaneously, although it will respond to stimuli by a single beat for each stimulus. (5) If the heart be divided lengthwise, its parts will continue to pulsate rhythmically, and the auricles may be cut up into pieces, and the pieces will continue their movements of rhythmical con- traction. It will be thus seen that the rhythmical movements appear to be more marked in the parts sup- plied by the ganglia, and that the apical portion of the ventricle, in which the ganglia are not found, does not, under ordinary circum- stances, possess the power of auto- matic movement. It has, however, been shown by Gaskell that the extreme apex of the ventricle of the heart of the tortoise, which contains no ganglia, may under appropriate stimuli be made to con- tract rhythmically. This proves that the muscular tissue of the heart is capable of rhythmical contraction, but it does not prove that in the living animal the muscular rhythm occurs without nervous stimulation, nor indeed is this at all likely. Inhibition of the Heart's Action.-- Although, under ordinary conditions, the apparatus of ganglia and nerve-fibres in the sub- stance of the heart forms the medium through which its action is excited and rhythmically maintained, yet they, and through them, the heart's contractions, are regulated by nerves which pass to them from the higher nerve-centres. These nerves are branches from the pneumogastric or vagus and the sympathetic. The influence of the vagi nerves over the heart beat may be shown by stimulating one (especially the right), or both of the nerves, when a record is being taken of the beats of the frog's, heart. If a single induction shock be sent into the nerve, the heart, cm a rule after a short interval, ceases beating, but after the suppression of several beats resumes its action. As already men- tioned, the effect of the stimulus is not immediately seen, and one beat may occur before the heart stops after the application of the Fig. 120.-Course of the nerves in the auri- cular partition wall of the heart of a frog, d, dorsal branch; v, ventral branch. (Ecker.) 0H\P. IV.] INHIBITION OF THE HEART'S ACTION. 149 electric current. The stoppage of the heart may occur apparently in one of two ways, either by diminishing the strength of the systole or by increasing the length of the diastole (figs. 121, 122). The stoppage of the heart may be brought about by the application of the electrodes to any part of the vagus, but most effectually if they are applied near the posi- tion of Remak's ganglia. It is supposed that the fibres of the vagi, there- fore, terminate there in the ganglia in the heart- walls, and that the inhi- bition of the heart's beats by means of the vagus is not a direct action, but that it is brought about indirectly by stimulating these centres in the heart itself. If this idea be correct, it may be sup- posed that the inhibitory centres are paralyzed by injection of atropin, as after this has been done no amount of stimulation of the vagus, or of the heart itself, will produce any effect upon the cardiac beats. Also that urari in large doses paralyzes the vagus fibres, but as the r inhibitory action can be produced by direct stimu- lation of the heart, it is inferred that this drug does not paralyze the ganglia themselves. Mus- carin and pilocarpin ap- pear to produce effects similar to those obtained by stimulating the vagus fibres. They stimulate the inhibitory ganglia. The remarkable effects of ligaturing the heart at various parts (Stannius' experiments) however, complicate if they do not contradict the above expla- nation. If a ligature be tightly tied round the heart over the situation of the ganglia between the sinus and the auricles, the heart below the ligature stops beating. The ligature might be supposed to stimulate the inhibitory ganglia, but for the remarkable fact that the exhibition of atropin does not interfere with the success of the experiment. Fig. 121.-Tracing showing the actions of the vagus on the heart. Aur., auricular; Vent., ventricular tracing. The part between perpendicular lines indicates period of vagus stimulation. C.8 indicates that the second- ary coil was 8 c.m. from the primary. The part of tracing to the left shows the regular contractions of moderate height before stimulation. During stimu- lation and for some time after the beats of auricle and ventricle are arrested. After they commence again they are single at first, but soon acquire a much greater amplitude' than before the application of the stimulus. (From Brunton, after Gaskell.) Fig. 122.- Tracing showing diminished amplitvd' and slowing of the pulsations of the auricle and ven- tricle without complete stoppage during irritation of the vagus. (From Brunton, after Gaskell. I 150 CIRCULATION OF THE BLOOD. [CHAP. IV. Section of the heart at the same situation we have seen has (experiment 2, p. 147) a similar effect to ligature. Again, if the ventricle be separated from the auricles by ligature or by section, it will recommence its pulsation and continue to beat rhythmically, but the auricles will continue at a standstill. It has been suggested as an alternative explanation of these further experiments that the sinus contains the chief motor ganglia of the heart, and that from it as a rule proceed the impulses which cause the sequence of contraction of the other parts ; that the auricles contain inhibitory ganglia which are not sufficiently powerful to prevent the motor impulses from the sinus ganglia,but that when their influence is removed by section, by ligature, or by excessive stimulation that the inhibitory ganglia are able to prevent the rhythmical contraction of the auricles and ventricle, but that the ventricle contains independent motor ganglia, since when it is removed from the i nfluence of the inhibitory ganglia of the auricles, it recommences rhythmical pulsation. Even if this theory cannot be absolutely maintained, yet it is evident that the power of spontaneous contraction is strongest in the sinus, less- strong in the auricles, and less so still in the ventricle, and that, therefore, the sinus ganglia are important in exciting the rhythmical contraction of the whole heart. So far, the effect of the terminal apparatus of the vagi only has- been considered ; there is, however, no doubt that the vagi nerves are simply the media of an inhibitory or restraining influence over the action of the heart, which is conveyed through them from a centre in the medulla oblongata which is always in operation, and, because of its restraining the heart's action, is called the cardio- inhibitory centre. For, on dividing these nerves, the pulsations of the heart are increased in frequency, an effect opposite to that produced by stimulation of their divided (peripheral) ends. The restraining influence of the centre in the medulla may be reflexly increased, so as to produce slowing or stoppage of the heart, through impulses from it passing down the vagi. As an example of the latter, the well-known effect on the heart of a violent blow on the epigastrium may be referred to. The stoppage of the heart's action in this case, is due to the conveyance of the stimulus, by fibres of the sympathetic (afferent) to the medulla oblongata, and its subsequent reflection through the vagi (afferent) to the inhibitory ganglia of the heart. It is also believed that the power of the medullary inhibitory centre may in a similar manner be reflexly lessened so as to produce accelerated action of the heart. Acceleration of the Heart's Action.-The heart receives an accelerating influence from the medulla oblongata through certain fibres of the sympathetic. These accelerating nerve-fibres, issuing from the spinal cord in the lower cervical and upper dorsal regions, chap, iv.] ACCELERATION OF THE HEART'S ACTION. 151 reach the inferior cervical ganglion of the sympathetic, and pass thence to the cardiac plexus, and so to the heart. Their function is shown in the quickened pulsation which follows stimulation of the spinal cord, when the latter has been cut off from all connec- tion with the heart, excepting by these accelerating filaments. Unlike the inhibitory fibres of the pneumogastric, the accelerating fibres are not continuously in action. The accelerator nerves must not, however, be considered as direct antagonists of the vagus ; for if at the moment of their maximum stimulation, the vagus be stimulated with minimum currents, inhibition is produced with the same readiness as if these were not acting. Nor is there any evidence that these fibres are constantly in action like those of the vagus. The connection of the heart with other organs by means of the nervous system, and the influences to which it is subject through them, are shown in a striking manner by the phenomena of disease. The influence of mental shock in arresting or modifying the action of the heart, the slow pulsation which accompanies com- pression of the brain, the irregularities and palpitations caused by dyspepsia or hysteria, are good evidence of the connection of the heart with othei' organs through the nervous system. Other Influences affecting the Action of the Heart. The healthy action of the heart no doubt very materially depends (i) upon a due supply of healthy blood to its muscular tissue. It is not unlikely that the apparently contradictory effect of poisons may be explained by supposing that the influence of some of them is either partially or entirely directed to the muscular tissue itself, and not to the nervous apparatus alone. As will be explained presently, the heart exercises a consider- able influence upon the condition of the pressure of blood within the arteries, but in its turn (2) the blood pressure within the arteries re-acts upon the heart, and has a distinct effect upon its contractions, increasing by its increase, and vice versa, the force of the cardiac beat, although the frequency is diminished as the blood-pressure rises. (3) The quantity (and quality I') of the blood contained in its chambers, too, has an influence upon its systole, and within normal limits the larger the quantity the stronger the contraction. Rapidity of systole does not of necessity indicate strength, as two weak contractions often do no more work than a strong and pro- 152 CIRCULATION OF THE BLOOD. [chap. IV. longed one. (4) In order that the heart may do its maximum work, it must be allowed free space to act ; for if obstructed in its action by mechanical outside pressure, as by an excess of fluid within the pericardium, such as is produced by inflammation, or by an overloaded stomach, or the like, the pulsations become irregular and feeble. Functions of the Arteries. The External Coat.-The external coat forms a strong and tongh investment, which, though capable of extension, appears principally designed to strengthen the arteries and to guard against their excessive distension by the force of the heart's action. It is this coat which alone prevents the complete sever- ance of an artery when a ligature is tightly applied ; the internal and middle coats being divided. In it, too, the little vasa vasorum (p. 122) find a suitable tissue in which to subdivide for the supply of the arterial coats. The Elastic Tissue.-The purpose of the elastic tissue, which enters so largely into the formation of all the coats of the arteries, is, (a) to guard the arteries from the suddenly exerted pressure to which they are subjected at each contraction of the ventricles. In every such contraction, the contents of the ventricles are forced into the arteries more quickly than they can be discharged into and through the capillaries. The blood therefore, being, for an instant, resisted in its onward course, a part of the force with which it was impelled is directed against the sides of the arteries ; under this force their elastic walls dilate, stretching enough to receive the blood, and as they stretch, becoming more tense and more resisting. Thus, by yielding, they break the shock of the force impelling the blood. On the subsidence of the pressure, when the ventricles cease contracting, the arteries are able, by the same elasticity, to resume their former calibre. (6.) It equalizes the current of the blood by maintaining pressure on it in the arteries during the periods at which the ventricles are at rest or dilating. If the arteries had been rigid tubes, the blood, instead of flowing, as it does, in a constant stream, would have been propelled through the arterial system in a series of jerks corresponding to the ventricular contractions, with intervals of almost complete rest during the inaction of the ventricles. But in the actual condition OHAP. IV.] FUNCTIONS OF THE ARTERIES. 153 of the arteries, the force of the successive contractions of the ventricles is expended partly in the direct propulsion of the blood, and partly in the dilatation of the elastic arteries ; and in the intervals between the contractions of the ventricles, the force of the recoil is employed in continuing the same direct propulsion. Of course the pressure they exercise is equally ■diffused in every direction, and the blood tends to move back- wards as well as onwards, but all movement backwards is pre- vented by the closure of the aortic semi-lunar valves (p. 117), which takes place at the very commencement of the recoil of the •arterial walls. By this exercise of the elasticity of the arteries, all the force of the ventricles is expended upon the circulation ; for that part of their force which is used in dilating the arteries, is restored in full when they recoil. There is thus no loss of force; but neither is there any gain, for the elastic walls of the artery cannot originate any force for the propulsion of the blood-they only restore that which they received from the ventricles. The force with which the arteries are dilated every time the ventricles con- tract, might be said to be received by them in store, to be all given out again in the next succeeding period of dilatation of the ventricles. It is by this equalizing influence of the successive ■branches of every artery that at length the intermittent accele- rations produced in the arterial current by the action of the heart, cease to be observable, and the jetting stream is converted into the continuous and equable movement of the blood which we see in the capillaries and veins. In the production of a continuous stream of blood in the smaller arteries and capillaries, the resist- ance which is offered to the blood-stream in these vessels, is a necessary agent. Were there no greater obstacle to the escape of blood from the larger arteries than exists to its entrance into them from the heart, the stream would be intermittent, notwith- standing the elasticity of walls of the arteries. (c.) By means of the elastic and muscular tissue in their walls the arteries are enabled to dilate and contract readily in correspondence with any temporary increase or diminution of the total quantity of blood in the body; and within a certain range of diminution of the quantity, still to exercise due pressure on their contents ; (<Z.) The elastic tissue assists in restoring the normal state after diminution of its calibre, whether this has been caused by a contraction of the muscular coat, 154 CIRCULATION OF THE BLOOD. [chap. iv. oi* the temporary application of a compressing force from without. This action is well shown in arteries which, having contracted by means of their muscular element, after death, regain their average patency on the cessation of post-mortem rigidity, (c.) By means of their elastic coat the arteries are enabled to adapt themselves to the different movements of the several parts of the body. The natural state of all arteries, in regard at least to their length, is one of tension-they are always more or less stretched, and ever ready to recoil by virtue of their elasticity, whenever the opposing force is removed. The extent to which the divided extremities of arteries retract is a measure of this tension, not of their elasticity. (Savory.) The Muscular Coat.-The most important office of the mus- cular coat is, (i) that of regulating the quantity of blood to be received by each part or organ, and of adjusting it to the require- ments of each, according to various circumstances, but, chiefly, according to the activity with which the functions of each are at different times performed. The amount of work done by each organ of the body varies at different times, and the variations often quickly succeed each other, so that, as in the brain, for example, during sleep and waking, within the same hour a part may be now very active and then inactive. In all its active exercise of function, such a part requires a larger supply of blood than is sufficient for it during the times when it is comparatively inactive, ft is evident that the heart cannot regulate the supply to each part at different periods; neither could this be regulated by any general and uniform contraction of the arteries; but it may be regulated by the power which the arteries of each part have, in their muscular tissue, of contracting so as to diminish, and of passively dilating or yielding so as to permit an increase of the supply of blood, according to the requirements of the part to which they are distributed. And thus, while the ventricles of the heart determine the total quantity of blood, to be sent onwards at each contraction, and the force of its propulsion, and while the large and merely elastic arteries distribute it and equalise its stream, the smaller arteries, in addition, regulate and determine, by means of their muscular tissue, the proportion of the whole quantity of blood which shall be distributed to each part. It must be remembered, however, that this regulating function of the arteries is itself governed and directed by the nervous system (see p. 168). CHAP. IV.] FUNCTIONS OF THE ARTERIES. 155 Another function of the muscular element of the middle coat of arteries is (2), to co-operate with the elastic in adapting the calibre of the vessels to the quantity of blood which they contain. For the amount of fluid in the blood-vessels varies very considerably even from hour to hour, and can never be quite constant; and were the elastic tissue only present, the pressure exercised by the walls of the containing vessels on the contained blood would be sometimes very small, and sometimes inordinately great. The presence of a muscular element, however, provides for a certain uniformity in the amount of pressure exercised; and it is by this adaptive, uniform, gentle, muscular contraction, that the normal tone of the blood-vessels is maintained. Deficiency of this tone is the cause of the soft and yielding pulse, and its unnatural excess, of the hard and tense one. The elastic and muscular contraction of an artery may also be regarded as fidfilling a natural purpose when (3), the artery being cut, it first limits and then, in conjunction with the coagulated fibrin, arrests the escape of blood. It is only in consequence of such contraction and coagulation that we are free from danger through even very slight wounds; for it is only when the artery is closed that the processes for the more permanent and secure prevention of bleeding are established. (4) There appears no reason for supposing that the muscular coat assists, to more than a very small degree, in propelling the onward current of blood. (l.) When a small artery in the living subject is exposed to the air or cold, it gradually but manifestly contracts. Hunter observed that the posterior tibial artery of a dog when laid bare, became in a short time so much contracted as almost to prevent the transmission of blood; and the observation has been often and variously confirmed. Simple elasticity could not effect this. (2.) When an artery is cut across, its divided ends contract, and the orifices may be completely closed. The rapidity and completeness of this contraction vary in different animals ; they are generally greater in young than in old animals; and less, apparently, in man than in the lower animals. This contraction is due in part to elasticity, but in part, also, to muscular action ; for it is generally increased by the application of cold, or of any simple stimulating substances, or by mechanically irritating the cut ends of the artery, as by picking or twisting them. (3.) The contractile property of arteries continues many hours after death, and thus affords an opportunity of distinguishing it from their elas- ticity. When a portion of an artery of a recently killed animal is exposed, it gradually contracts, and its canal may be thus completely closed ; in this contracted state it remains for a time, varying from a few hours to two days ; then it dilates again, and permanently retains the same size. 156 CIRCULATION OF THE BLOOD. [chai*. IV. The Pulse. If we place our fingers upon the radial artery at the wrist, or upon any artery of the body which is sufficiently superficial, we experience a sensation as if our fingers were alternately lifted or raised up from the artery and allowed to fall again, and this action is repeated very frequently in the course of a minute. In other words we feel the pulse of the artery. The pulse is generally described as an expansion of the artery produced by the wave of blood set in motion by the injection of blood into the already full aorta at each ventricular systole. As the force of the left ventricle, however, is not expended in dilating the aorta only, the wave of blood passes on, expanding the arteries as it goes, running as it were on the surface of the more slowly travelling blood already contained in them, and producing the pulse as it proceeds. The distension of each artery increases both its length and its diameter. In their elongation, the arteries change their form, the straight ones becoming slightly curved, and those already curved becoming more so; but they recover their previous form as well as their diameter when the ventricular contraction ceases, and their elastic walls recoil. The increase of their curves which accompanies the distension of arteries, and the succeeding recoil, may be well seen in the prominent temporal artery of an old person. In feeling the pulse, the finger cannot distinguish the sensation produced by the dilatation from that produced by the elongation and curving; that which it perceives most plainly, however, is the dilatation, or return, more or less, to the cylindrical form, of the artery which has been partially flattened by the finger. The pulse-due to any given beat of the heart-is not per- ceptible at the same moment in all the arteries of the body. Thus, it can be felt in the carotid a very short time before it is perceptible in the radial artery, and in this vessel again before the dorsal artery of the foot. The delay in the beat is in pro- portion to the distance of the artery from the heart, but the difference in time between the beat of any two arteries never exceeds probably i to i of a second. A distinction must be carefully made between the passage of the wave along the arteries and the velocity of the stream (p. 180) of blood. Both wave and current are present; but the rates at CHAP. IV.] THE SPHYGMOGRAPH. 157 which they travel are very different, that of the wave 16'5 to 33 feet per second (5 to 10 metres), being twenty or thirty times as great as that of the current. The Sphygmograph.-A great deal of light has been thrown Fig. 123.-Diagram of the mode of action of the Sphygmograph. on what may be called the form of the pulse wave by the sphyg- mograph (figs. 123 and 124). The principle on which it acts is very simple (see fig. 123). The small button replaces the finger in the act of taking the pulse, and is made to rest lightly on the artery, the pulsations of which it is desired to investi- gate. The up-and-down movement of the button is communicated to the Fig. 124.-The Sphygmograph applied to the ana. lever, to the hinder end of which is attached a slight spring, which allows the lever to move up, at the same time that it is just strong enough to resist its making any sudden jerk, and in the interval of the beats also to assist in bringing it back to its original position. For ordinary purposes the instru- ment is bound on the wrist (fig. 124). It is evident that the beating of the pulse with the re-action of the spring will cause an up-and-down movement of the lever, the pen of which will write the effect on a smoked card, which is made to move by clockwork in the direction of the arrow. Thus a tracing of the pulse is obtained, and in this way much more delicate effects can be seen than can be felt on the application of the finger. 158 CIRCULATION OF THE BLOOD. [CHAP. IV. The tracing of the pulse (sphygmogram), obtained by the use of the sphygmograph, differs somewhat according to the artery upon which it is applied, but its general characters are much the same in all cases. It consists of:-A sudden upstroke (fig. 125, a), which is somewhat higher and more abrupt in the pulse of the carotid and of other arteries near the heart than in the radial and other arteries more remote; and a gradual decline (b), less abrupt, and therefore taking a longer time than (a). It is seldom, however, that the decline is an uninterrupted fall; it is usually marked about half-way by a distinct notch (c), called the dicrotic notch, which is caused by a second more or less marked ascent of the lever at that point by a second wave called the dicrotic wave (d) ; not unfrequently (in which case the tracing is said to have a double apex) there is also soon after the com- mencement of the descent a slight ascent previous to the dicrotic notch : this is called the pre-dicrotic wave (c), and in addition there may be one or more slight ascents after the dicrotic, called post-di- crotic (e). The explanation of these tracings presents some difficul- ties, not, however, as regards the two primary factors, viz., the upstroke and downstrokc, because they are universally taken to mean the sudden injection of blood into the already full arteries, and that this passes through the artery as a wave and expands them, the gradual fall of the lever signifying the recovery of the arteries by their recoil. It may be demonstrated on a system of elastic tubes, where a syringe pumps in water at regular intervals, just as well as on the radial artery, or on a more complicated system of tubes in which the heart, the arteries, the capillaries and veins are represented, which is known as an arterial schema. If we place two or more sphygmographs upon such a system of tubes at increasing distances from the pump, we may demonstrate that the rise of the lever commences first in that nearest the pump, and is higher and more sudden, while at a longer distance from the pump the wave is less marked, and a little later. So in the Fig. 125.-Diagram of pulse-tracing. A, upstroke; b, down-stroke; c, pre-di- crotic wave ; d, dicrotic; e, post-dicrotie wave. CHAP. IV.] SPHYGMOGRAMS. 159 arteries of the body the wave of blood gradually gets less and less as we approach the periphery of the arterial system, and is lost in the capillaries. By the sudden injection of blood two distinct waves are produced, which are called the tidal and percussion Fig. 126.-Diagram of the formation of the pulse-tracing, a, percussion wave; b, tidal wave; c, dicrotic wave. (Mahomed.) waves. The tidal wave occurs whenever fluid is injected into an elastic tube (fig. 126, b), and is due to the expansion of the tube and its more gradual collapse. The percussion wave occurs (fig. 126, a) when the impulse imparted to the fluid is more Fig. 127.-Pulse-tracing of radial artery, somewhat deficient in tone. (Sanderson.) sudden; this causes an abrupt upstroke of the lever, which then falls until it is again caught up perhaps by the tidal wave which begins at the same time but is not so quick. In this way, generally speaking, the apex of the upstroke is double, the second upstroke, the so-called pre-dicrotic elevation of the lever, representing the tidal wave. The double apex is most marked in tracings from large arteries, especially when their tone 160 CIRCULATION OF THE BLOOD. [chap. IV. is deficient. In tracings, on the other hand, from arteries of medium size, e.g., the radial, the upstroke is usually single. In this case the percussion-impulse is not sufficiently strong to jerk Fig. 128.-Pulse-tracing of radial artery, with double apex. (Sanderson.) up the lever and produce an effect distinct from that of the- systolic wave which immediately follows it, and which continues, and completes the distension. In cases of feeble arterial tension, however, the percussion-impulse may be traced by the sphygmo- graph, not only in the carotid pulse, but to a less extent in the radial also (fig. 128). The interruptions in the downstroke are called the katacrotic- waves, to distinguish them from an interruption in the upstroke, called the anacrotic wave, which is occasionally met with in cases. Fig. 129.-Anacrotic pulse from a case of aortic aneurism, a, anacrotic wave (or percussion wave), b, tidal or pre-dicrotic wave, continued'rise in tension (or higher tidal wave). in which the pre-dicrotic or tidal wave is higher than the percus- sion wave. There is considerable difference of opinion as to whether the- dicrotic wave is generally present in health, and also as to its cause. The balance of opinion, however, appears to be in favour of the belief that the dicrotic wave is present in health, although it may be very faint; while in certain conditions not necessarily diseased, it becomes so marked as to be quite plain to the unaided finger. Such a pulse is called dicrotic. Sometimes the dicrotic rise exceeds the initial upstroke, and the pulse is then called hyperdici'otic. As to the cause of dicrotism, one opinion (i) is that it is due to- a recovery of pressure during the elastic recoil, in consequence of a rebound from the periphery. It may indeed be produced on a. CHAP. IV.] SPHYGMOGRAMS, 161 schema by obstructing the tube at a little distance beyond the spot where the sphygmograph is placed. Against this view, how- ever, is the fact that the notch appears at about the same point in the downstroke in tracings from the carotid and from the radial, and not first in the radial tracing, as it should do, if this theory was correct, since that artery is nearer the peri- phery than the carotid, and as it does in the correspond- ing experiment with the arterial schema when the tube is obstructed. (2) The generally accepted notion among clinical observers, is that the dicrotic wave is due to the rebound from the aortic valves which causes a second wave ; but the ques- tion cannot be considered settled, and the presence of marked dicrotism in cases of haemorrhage, of anaemia, and of other weakening con- ditions, as well as its pre- sence in cases of diminished pressure within the arteries, would imply that it might, at any rate sometimes, be due to the altered specific gravity of the blood within the vessels, either directly or through the indirect effect of these conditions on the tone of the arterial walls. Waves may be produced in any elastic tube when a fluid is being driven through it with an intermittent force, such waves being called waves of oscillation (M. Foster). Their origin has received various explanations. In an arterial schema they vary with the Fig. 130.-Diagrams of pulse curves with exagge- ration of one or other of the three waves. . A, percussion; B, tidal; C, dicrotic. 1, percussion, wave very marked; 2, tidal wave sudden ; 3, dicrotic pulse curve; 4 and the tida 1 wave very exaggerated, irom high tension. (Mahomed.) 162 CIRCULATION OF THE BLOOD. [chap. iv. specific gravity of the fluid used, and with the kind of tubing, and may be therefore supposed to vary in the body with the condition of the blood and of the arteries. Some consider the secondary waves in the downstroke of a normal tracing to be oscillation waves ; but, as just mentioned, even if this be the case, as is most likely with post-dicrotic waves, the dicrotic wave itself is almost certainly due to the rebound from the aortic valves. The anacrotic notch is usually associated with disease of the arteries, e.g., in atheroma and aneurism. The dicrotic notch is called diastolic or aortic, and in point of time indicates the closure of the aortic valves. Of the three main parts then of a pulse-tracing, viz., the per- cussion wave, the tidal, and the dicrotic, the percussion wave is produced by sudden and forcible contraction of the heart, perhaps exaggerated by an excited action, and may be transmitted much more rapidly than the tidal wave, and so the two may be distinct; frequently, however, they are inseparable. The dicrotic wave may be as great or greater than the other two. According to Mahomed, the distinctness of the three waves depends upon the following conditions :- The percussion leave is increased by :-i. Forcible contraction of the Heart; 2. Sudden contraction of the Heart; 3. Large volume of blood; 4. Fulness of vessel; and diminished by the reversed conditions. The tidal wave is increased by:-1. Slow and prolonged con- traction of the Heart; 2. Large volume of blood ; 3. Comparative emptiness of vessels ; 4. Diminished outflow or slow capillary circulation ; and diminished by the reverse conditions. The dicrotic wave is increased by:-1. Sudden contraction of the Heart ; 2. Low blood pressure; 3. Increased outflow or rapid capillary circulation ; 4. Elasticity of the aorta ; 5. Relaxation of muscular coat; and diminished by the reversed conditions. One very important precaution in the use of the sphygmograph lies in the careful regulation of the pressure. If the pressure be too great, the characters of the pulse may be almost entirely obscured, or the artery may be entirely obstructed, and no tracing is obtained ; and on the other hand, if the pressure be too slight, a very small part of the characters may be represented on the tracing. CHAP. JV.] BLOOD-PRESSURE. 163 The Pressure of the Blood within the Arteries (producing arterial tension). It will be understood from all that has been said about the arteries in a normal condition (a) that they are during life continually " on the stretch," even during the cardiac diastole, and that in consequence of the injection of more blood at each systole of the ventricle into the elastic aorta, that this stretched condition is exaggerated each time the ventricle empties itself. This state of distension of the arteries is due to the pressure of blood within them, and arises in consequence of the resistance presented by the smaller arteries and capillaries (peripheral resist- ance) to the sudden emptying of the arterial system between the contractions of the ventricle. It is called the condition of arterial tension. It will be further understood (6) that, as the blood is forcibly injected into the already full arteries against their elasticity, it must be subjected to the pressure of the arterial walls, so that, when an artery is cut across, the blood is projected forwards by this force for a considerable distance. Thus, although the blood distends the arteries and produces tension, yet the elasticity of the arteries re-acts upon the blood, and subjects it to pressure. We have therefore to remember that we have to do with two things related but not identical, viz., the pressure which the blood exerts upon the arterial walls tending to stretch them, and the pressure to which the blood is subject by the arteries tending to drive it on in the direction of least resistance. The only direction in which it can be driven is onwards towards the capillaries, and so the blood-pressure in the arteries is one of the great agents in maintaining the circulation. The relations which exist between the arteries and their contained blood are thus so obviously of importance to the carrying on of the circulation, that it becomes necessary to be Fig. 131.-Diagram of mer- curial manometer. 164 CIRCULATION OF THE BLOOD. [CHAP. IV. able to gauge the alterations in blood-pressure very accurately. This may be done by means of a mercurial manometer in the following way:-The short horizontal limb of this (fig. 131) is connected, by means of an elastic tube and cannula, with the interior of an artery ; a solution of sodium or potassium carbonate I'ig. 132.-Diagram of mercurial kymograph, a, revolving cylinder, worked by a clockwork arrangement contained in the box (b), the speed being regulated by a fan above the box ; cylinder supported by an upright (S), and capable of being raised or lowered by a screw (a), by a handle attached to it; d, c, e, represent mercurial manometer, a somewhat diiferent form of which is showui in next figure. being previously introduced into this part of the apparatus to prevent coagulation of the blood. The blood-pressure is thus communicated to the upper part of the mercurial column; and the depth to which the latter sinks, added to the height to which it rises in the other, will give the height of the mercurial column which the blood-pressure balances; the weight of the soda solution being subtracted. For the estimation of the arterial tension at any given moment, no further apparatus than this, which is called Poiseuilles's CHAP. IV.] MERCURIAL MANOMETER. 165 is necessary; but for noting the variations of pressure in the arterial system, as well as its absolute amount, the instrument is usually combined with a registering apparatus, and in this form is called a kymo- graph. The kymograph, invented by Ludwig, is composed of a haemady- namometer, the open mercurial column of which supports a float- ing piston and vertical rod, with short horizontal pen (fig. 132). The pen is adjusted in contact with a sheet of paper, which is caused to move at an uniform rate by clock- work ; and thus the up-and-down movements of the mercurial column, which are communicated to the rod and pen, are marked or registered on the moving paper, as in the registering apparatus of the sphygmograph, and minute varia- tions are graphically recorded (fig- 134). For some purposes the spring kymo- graph of Fick (tig. 135) is preferable to the mercurial kymograph. It consists of a hollow C-shaped spring, filled with fluid, the interior of which is brought into connection with the interior of an artery, by means of a flexible metallic tube and cannula. In response to the pressure transmitted to its interior, Fig. 133. -Diagram of mercurial mano- meter. a. Floating rod and pen. b. Tube, which communicates with a bottle containing an alka- line solution, c. Elastic tube and cannula, d, the latter being in- tended for insertion in an artery Fig. 134.-Normal tracing ot arterial pressure in the rabbit obtained with the mercurial kymograph. The smaller undulations correspond with the heart beats; the larger curves with the respiratory movements. (Burdon-Sanderson.) the spring, c, tends to straighten itself, and the movement thus produced is communicated by means of a lever, f>, to a writing-needle and registering apparatus. Fig. 136 exhibits an ordinary arterial pulse-tracing, as obtained by the spring-kymograph. 166 CIRCULATION OF THE BLOOD. [chap. iv. From observations which have been made by means of the mercurial manometer, it has been found that the pressure of blood in the carotid of a rabbit is capable of supporting a column of 2 to 31 inches (50 to 90 mm.), of mercury, in the dog 4 to 7 inches (100 to 175 mm.), in the horse 5 to 8 inches (150 to 200 mm.), and in man the pressure is estimated to be about the same. To measure the absolute amount of this pressure in any artery, it is necessary merely to multiply the area of its transverse section by the height of the column of mercury which is already known to be supported by the blood-pressure in any part of the arterial system. The weight of a column of mer- cury thus found will represent the pres- sure of the blood. Calculated in this way, the blood-pres- sure in the human aorta is equal to 4 lb. 4 oz. avoir- dupois ; that in the aorta of the horse being 11 lb. 9 oz. ; and that in the radial artery at the human wrist only 4 drs. Supposing the muscular power of the right ven- tricle to be only one-half that of the left, the blood-pres- sure in the pulmo- nary artery will be only 2 lb. 2 oz. avoirdupois. The amounts above stated represent the arterial tension at the time of the ventricular contraction. The blood-pressure is greatest in the left ventricle and at the beginning of the aorta, and decreases towards the capillaries. It is greatest in the arteries at the period of the ventricular systole, Fig. 135.-A form of Fick's Spring Kymograph, a, tube to be connected with artery; c, hollow spring, the move- ment of which moves b, the writing lever ; «, screw to regulate height of b; d, outside protective spring; <7, screw to fix on the upright of the support. CHAP. IV.] VARIATIONS OF BLOOD-PRESSURE. 167 and is least in the auricles, during diastole, when the pressure there and in the great veins becomes, as we have seen, negative. The mean arterial pressure equals the average of the pressures in all the arteries. The pressure in the veins is never more than one- tenth of the pressure in the corresponding arteries, and is greatest at the time of auricular systole. There is no periodic variation in venous pressure, as there is in the arterial, except in the great veins. Variations of Blood-Pressure.-Many circumstances cause considerable variations in the amount of the blood-pressure. The following are the chief:-(1) Changes in the beat of the Heart; Fig. 136.-Normal arterial tracing obtained with Fick's kymograph in the dog. (Burdon-Sanderson.) (2) Changes in the Arteries and Capillaries ; (3) Changes due to Nerve Action; (4) Changes in the Blood; (5) Respiratory Changes. 1. Changes in the Beat of the Heart.-The systole and diastole of the muscular chambers. The arterial tension increases during systole and diminishes during diastole. The greater the fre- quency, moreover, of the heart's contractions, the greater is the blood-pressure, cceteris paribus. As a rule, however, when the heart contracts frequently, the beats lose in strength, and the increase in frequency may be compensated for by the delivery into the arteries at each beat of a comparatively small quantity of blood. The greater the quantity of blood expelled from tlie heart at each contraction the greater is the blood-pressure. The quantity and quality of the blood nourishing the heart's substance through the coronary arteries must exercise also a very considerable influence upon its action, and therefore upon the blood-pressure. 2. Changes in the Arteries and Capillaries.-Variations in the degree of contraction of the smaller arteries modify the blood- pressure by favouring or impeding the accumulation of blood in the arterial system which follows every contraction of the heart; the contraction of the arterial walls increasing the blood-pressure, and their relaxation lowering it. 168 CIRCULATION OF THE BLOOD. [chap. iv. 3. Changes due to Nerve Action.-The nervous system has a very important action in regulating the blood-pressure. Its influence is exerted chiefly upon the muscular coat of the arteries and not upon the elastic element, which possesses, as must be obvious, rather physical than vital properties. The muscular tissue in the walls of the vessels increases in amount relatively to the other coats as the arteries grow smaller, so that in the smallest arteries it is developed out of all proportion to the other elements; in fact, in passing from capillary vessels, made up as we have seen of endothelial cells with a ground substance, the Fig. 137.-Plethysmograph. By means of this apparatus, the alteration in volume of the arm, e, which is enclosed in a glass tube, a, filled with fluid, the opening through which it passes being firmly closed by a thick gutta-percha band, f, is communicated to the lever, i>, and registered by a recording apparatus. The fluid in a communicates with that in b, the upper limit of which is above that in a. The chief alterations in volume are due to alteration in the blood contained in the arm. When the volume is increased, fluid passes out of the glass cylinder, and the lever, n, also is raised, and when a decrease takes place the fluid returns again from b to a. It will therefore be evident that the apparatus is capable of recording alterations of blood-pressure in the arm. Apparatus founded upon the same principle have been used for recording alte- rations in the volume of the spleen and kidney. first change which occurs as the vessels become larger (on the side of the arteries) is the appearance of muscular fibres. Thus the nervous system is more powerful in regulating the calibre of the smaller than of the larger arteries. Tt was long ago shown by Claude Bernard that if the cervical sympathetic nerve is divided in a rabbit, the blood-vessels of the corresponding side of the head and neck become dilated. This effect is best seen in the ear, which if held up to the light is seen to become redder, and the arteries are seen to become larger. The whole ear is distinctly warmer than the opposite one. This effect is produced by removing the arteries CHAP. IV.] VASO-MOTOR NERVES. 169 from the influence of the central nervous system, which influence normally passes down the divided nerve; for if the peripheral end of the divided nerve (i.e., that farthest from the brain) be stimulated, the arteries which were before dilated return to their natural size, and the parts regain their primitive condition. And, besides this, if the stimulus which is applied is too strong or too long continued, the point of normal constriction is passed, and the vessels become much more contracted than normal. The natural condition, which is somewhere about midway between extreme contraction and extreme dilatation, is called the natural tone of an artery, and if this is not maintained, the vessel is said to have lost tone, or if it is exaggerated, the tone is said to be too great. The influence of the nervous system upon the vessels consists in maintaining a natural tone. The effects described as having been produced by section of the cervical sympathetic and by subsequent stimulation are not peculiar to that nerve, as it has been found that for every part of the body there exists a nerve the division of which produces the same effects, viz., dilatation of the arteries ; such may be cited as the case with the sciatic, the splanchnic nerves, and the nerves of the brachial plexus: when these are divided, dilatation of the blood-vessels in the parts supplied by them takes place. It appears, therefore, that nerves exist which have a*distinct control over the vascular supply of every part of the body. These nerves are called vaso-motor ; they run now in cerebro- spinal, now in the sympathetic nerve-trunks. Vaso-motor centres.-Experiments by Ludwig and others show that the vaso-motor fibres come primarily from grey matter (vaso-motor centre} in the interior of the medulla oblongata, between the calamus scriptorius and the corpora quadrigemina. Thence the vaso-motor fibres pass down in the interior of the spinal cord, and issuing with the anterior roots of the spinal nerves, traverse the various ganglia on the prse-vertebral cord of the sympathetic, and, accompanied by branches from those ganglia, pass to their destination. Secondary or subordinate centres exist in the spinal cord, and local centres in various regions of the body, and through these, directly, under ordinary circumstances, vaso-motor changes are also effected. The influence exerted by the chief vaso-motor centre is not only in constant moderate action, but may be altered in 170 CIRCULATION OF THE BLOOD. [chap. iv. several ways, but chiefly by afferent (sensory) stimuli. These stimuli may act in two ways, either increasing or diminishing the usual action of the centre, which maintains a medium tone of the arteries. This afferent influence upon the centre may be extremely well shown by the action of a nerve the existence of which was demonstrated by Cyon and Ludwig, and which is called the depressor, because of its characteristic influence on the blood-pressure. Depressor Nerve.-This small nerve arises, in the rabbit, from Fig. 138.-Tracing showing the effect on blood-pressure of stimulating the central end of the Depressor nerve in the rabbit. To be read from, right to left. T, indicates the rate at which the recording-surface was travelling, the intervals correspond to seconds; C, the moment of entrance of current; O, moment at which 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 respiratory nerves; the pulse oscillations are very small. (M. Foster.) the superior laryngeal branch, or from this and the trunk of the pneumogastric nerve, and after communicating with filaments of the inferior cervical ganglion proceeds to the heart. If during an observation of the blood-pressure of a rabbit this nerve be divided, and the central end (i.e., that nearest the brain) be stimulated, a remarkable fall of blood-pressure ensues (fig. 138). The cause of the fall of blood-pressure is found to proceed from the dilatation of the vascular district within the abdomen supplied by the splanchnic nerves, in consequence of which it holds a much larger quantity of blood than usual. The en- gorgement of the splanchnic area very greatly diminishes the blood in the vessels elsewhere, and so materially diminishes the blood-pressure. The function of the depressor nerve is pre- sumed to be that of conveying to the vaso-motor centre indi- cations of such conditions of the heart as require a diminution of CHAI'. IV.] ACTION OF THE DEPRESSOR NERVE. 171 the tension in the blood-vessels; as, for example, that the heart cannot, with sufficient ease, propel blood into the already too full or too tense arteries. The action of the depressor nerve illustrates a somewhat un- usual effect of afferent impulses, as it causes an inhibition of the vaso-motor centre. As a rule, the stimulation of the central end of an afferent nerve produces a reverse effect, or, in other w'ords, increases the tonic influence of the centre, and by causing considerable constriction of certain arterioles, either locally or generally, increases the blood-pressure. Thus the effect of stimu- lating an afferent nerve may be either to dilate or to constrict the arteries. Stimulation of an afferent nerve too may produce a kind of paradoxical effect, causing general vascular constriction and so general increase of blood-pressure but at the same time local dilatation which must evidently have an immense influence in increasing the flow of blood through the part. Not only may the vaso-motor centre be reflexly affected, but it may also be affected by impulses proceeding to it from the cere- brum, as in the case of blushing from mind disturbance, or of pallor from sudden fear. It will be shown, too, in the chapter on Respiration that the circulation of deoxygenated blood may directly stimulate the centre itself. Local Tonic Centres.-Although the tone of the arteries is influenced by the centres in the cerebro-spinal axis, certain experi- ments prove that this is not the only way in which it may be influenced. Thus the dilatation which occurs after section of the cervical sympathetic in the first experiment cited above, only remains for a short time, and is soon followed-although a portion of the nerve may have been removed entirely-by the vessels re- gaining their ordinary calibre; and afterwards local stimulation, e.g., the application of heat or cold, will cause dilatation or con- striction. From this it is probable that there exists a distinct local mechanism for each vascular area, and that the influence exerted by the central nervous system acts through it much in the same way as the cardio-inhibitory centre in the medulla acts upon the heart through the ganglia contained within its muscular substance. Central impulses may inhibit or increase the action of the local centres, which may be considered to be sufficient under ordinary circumstances to maintain the tone of the vessels. The observa- tions upon the functions of the vaso-motor nerves themselves appear to divide them into four classes: (i) those on division of 172 CIRCULATION OF THE BLOOD. [CHAP. IV. which dilatation occurs for some time, and which on stimula- tion of their peripheral ends produce constriction; (2) those on division of which momentary dilatation followed by constric- tion occurs, with dilatation on stimulation; (3) those on division of which dilatation is caused, which lasts for a limited time, with constriction if stimulated at once, but dilatation if some time is allowed to elapse before the stimulation is applied ; (4) a class, division of which produces no effect but which, on stimulation, cause according to their function either dilata- tion or constriction. A good example of this fourth class is afforded by the nerves supplying the submaxillary gland, viz., the chorda tympani and the sympathetic. When either of these nerves is simply divided, no change takes place in the vessels of the gland; but on stimulating the chorda tympani the vessels dilate, and, on the other hand, when the sympathetic is stimulated the vessels contract. The nerves acting like the chorda tympani in this case are called vaso-dilators, and those like the sympathetic vaso-constrictors. The third class, which produce at one time dilatation, at another time constriction, are believed to contain both kinds of vaso-motor nerve-fibres, or to act as dilators or contractors according to the condition of the local apparatus. It is probable that all of these nerves act by inhibit- ing or augmenting the action of the local nervous mechanism already referred to; and as they are in connection with the central nervous system, it is through them that the medullary and spinal centres are capable of altering or of maintaining the normal local tone. It may also be supposed that the local nerve-centres themselves may be directly affected by the condition of blood nourishing them. The following table may serve as a summary of the effect of the nervous system upon the arteries and so upon the blood-pressure :- A. An increase of the blood-pressure may be produced:- (x.) By stimulation of the vaso-motor centre in medulla, either a. Directly, as by carbonated or deoxygenated blood. 0. Indirectly, by impressions descending from the cerebrum, e.g., in sudden pallor. 7. Rejlexly, by stimulation of sensory nerves anywhere. (2.) By stimulation of the centres in spinal cord. Possibly directly or indirectly, certainly reflexly. (3-) stimulation of the local centres for each vascular area, by the vaso-constrictor nerves, or directly by means of altered blood. CHAP. IV.] CHANGES IN THE AMOUNT OF BLOOD. 173 B. A decrease of the blood-pressure may be produced :- (l.) By stimulation of the vaso-motor centre in medulla, either (a.) Directly, as by oxygenated or aerated blood, (S.) Indirectly, by impressions descending from the cere- brum-e.g., in blushing. (7.) Keflexly, by stimulation of the depressor nerve, and consequent dilatation of vessels of splanchnic area, and possibly by stimulation of other sensory nerves, the sensory impulse being interpreted as an indication for diminished blood-pressure. (2.) By stimulation of the centres in spinal cord. Possibly directly, indirectly or reflexly. (3-) By stimulation of local centres for each vascular area by the vaso-dilator nerve, or directly by means of altered blood. 4. Changes in the blood.-a. As regards quantity. At first sight it would appear probable that one of the easiest ways to diminish the blood-pressure would be to remove blood from the vessels by bleed- ing. It has been found by experiment, however, that although the blood-pressure sinks whilst large abstractions of blood are taking place, as soon as the bleeding ceases it rises rapidly, and speedily becomes normal; that is to say, unless so large an amount of blood has been taken as to be positively dangerous to life, ab- straction of blood has little effect upon the blood-pressure. The rapid return to the normal pressure is due not so much to the withdrawal of lymph and other fluids from the body into the blood, as was formerly supposed, as to the regulation of the peripheral resistance by the vaso-motor nerves; in other words, the small arteries contract, and in so doing maintain pressure on the blood and favour its accumulation in the arterial system. This is due to the stimulation of the vaso-motor centre from diminution of the supply of blood, and therefore of oxygen. The failure of the blood-pressure to return to normal in the too great abstraction must be taken to indicate a condition of exhaustion of the centre, and consequently of want of regulation of the peri- pheral resistance. In the same way it might be thought that injection of blood into the already pretty full vessels would be at once followed by rise in the blood-pressure, and this is indeed the case up to a certain point-the pressure does rise, but there is a limit to the rise. Until the amount of blood injected equals about 2 to 3 per cent, of the body weight, the pressure continues to rise gradually; but if the amount exceed this proportion, the rise does not continue. Tn this case therefore, as in the opposite when blood is abstracted, the vaso-motor apparatus must counter- 174 CIRCULATION OF THE BLOOD. [chai-, iv. act the great increase of pressure but now by dilating the small vessels, and so diminishing the peripheral resistance, for after each rise there is a partial fall of pressure; and after the limit is reached the whole of the injected blood displaces, as it were, an equal quantity which passes into the small veins, and remains within them. It should be remembered that the veins are capable of holding the whole of the blood of the body. The amount of blood supplied to the heart, both to its substance and to its chambers, has a marked effect upon the blood-pressure. b. As regards quality. The quality of the blood supplied to the heart has a distinct effect upon its contraction, as too watery or too little oxygenated blood must interfere with its action. Thus it appears that blood containing certain substances affects the peripheral resistance by acting upon the muscular fibres of the arterioles themselves or upon the local centres, and so altering directly, as it were, the calibre of the vessels. 5. Respiratory changes affecting the blood-pressure will be considered in the next Chapter. Circulation in the Capillaries. When the capillary circulation is examined in any transparent part of a full grown living animal by means of the microscope (fig. 139), the blood is seen to flow with a constant equable motion ; the red blood-corpuscles moving along, mostly in single file, and bending in various ways to accommodate them- selves to the tortuous course of the capillary, but instantly recovering their normal outline on reaching a wider vessel. It is in the capillaries that the chief resistance is offered to the progress of the blood ; for in them the friction of the blood is greatly increased by the enormous multi- plication of the surface with which it is brought in contact. At the circumference of the stream in the larger capillaries but chiefly in the small arteries and veins, in contact with the walls of the vessel, and adhering to them, there is a layer of liquor Fig. 139.-Capillaries (C.) in the web of the frog's foot connecting a small artery (A) with a small vein V (after Allen Thompson). CHAP. IV.] CIRCULATION IN THE CAPILLARIES. 175 sanguinis which appears to be motionless. The existence of this still layer, as it is termed, is inferred both from the general fact that such an one exists in all fine tubes traversed by fluid, and from what can be seen in watching the movements of the blood- corpuscles. The red corpuscles occupy the middle of the stream, and move with comparative rapidity; the colourless lymph-cor- puscles run much more slowly by the walls of the vessel; while next to the wall there is often a transparent space in which the fluid appears to be at rest; for if any of the corpuscles happen to be forced within it, they move more slowly than before, rolling lazily along the side of the vessel, and often adhering to its wall. Part of this slow movement of the pale corpuscles and their occasional stoppage may be due to their having a natural tendency to adhere to the walls of the vessels. Sometimes, indeed, when the motion of the blood is not strong, many of the white corpuscles collect in a capillary vessel, and for a time entirely prevent the passage of the red corpuscles. Intermittent flow in the Capillaries.-When the peripheral resistance is greatly diminished by the dilatation of the small arteries and capillaries, so much blood passes on from the arteries into the capillaries at each stroke of the heart, that there is not sufficient remaining in the arteries to distend them. Thus, the intermittent current of the ventricular systole is not converted into a continuous stream by the elasticity of the arteries before the capillaries are reached ; and so intermittency of the flow occurs in capillaries and veins and a pulse is produced. The same phe- nomenon may occur when the arteries become rigid from disease, and when the beat of the heart is so slow or so feeble that the blood at each cardiac systole has time to pass on to the capil- laries before the next stroke occurs; the amount of blood sent at each stroke being insufficient to properly distend the elastic arteries. Diapedesis of Blood-Corpuscles.-It was formerly supposed that the occurrence of any transudation from the interior of the capillaries into the midst of the surrounding tissues was confined, in the absence of injury, strictly to the fluid part of the blood ; in other words, that the corpuscles could not escape from the circulat- ing stream, unless the wall of the containing blood-vessel was ruptured. It is true that an English physiologist, Augustus Waller, affirmed, in 1846, that he had seen blood-corpuscles, both red and white, pass bodily through the wall of the capillary vessel in which 176 CIRCULATION OF THE BLOOD. [chap. iv. they were contained (thus confirming what had been stated a short time previously by Addison); and that, as no opening could be seen before their escape, so none could be observed afterwards- so rapidly was the part healed. But these observations did not attract much notice until the phenomena of escape of the blood- corpuscles from the capillaries and minute veins, part from mechanical injury, were re-discovered by Cohnheim in 1867. Cohnheim's experiment demonstrating the passage of the corpuscles through the wall of the blood-vessel, is performed in the following manner. A frog is urarized, that is to say, paralysis is produced by ejecting under the skin a minute quantity of the poison called urari; and the abdo- men having been opened, a portion of small intestine is drawn out, and its transparent mesentery spread out under a microscope. After a variable time, occu- pied by dilatation, following contraction of the minute vessels and accompanying quickening of the blood-stream, there ensues a retardation of the current, and blood-corpuscles, both red and white, begin to make their way through the capillaries and small veins. " Simultaneously with the retardation of the blood-stream, the leucocytes, instead of loitering here and there at the edge of the axial current, begin to crowd in numbers against the vascular wall. In this way the vein becomes lined with a continuous pavement of these bodies, which remain almost motionless, not- withstanding that the axial current sweeps by them as continuously as before, though with abated velocity. Now is the moment at which the eye must be fixed on the outer contour of the vessel, from which, here and there, minute, colourless, button-shaped elevations spring, just as if they were produced by budding out of the wall of the vessel itself. The buds increase gradually and slowly in size, until each assumes the form of a hemispherical projection, of width corresponding to that of the leucocyte. Eventually the hemisphere is converted into a pear-shaped body, Fig. 140.-H large capillary from the frog's mesentery had been set up, showing '"cSta'tathJ 1 (Frey.) CHAP. IV.] DIAPEDESIS. 177 the small end of which is still attached to the surface of the vein, while the round part projects freely. Gradually the little mass of protoplasm removes itself further and further away, and, as it does so, begins to shoot out delicate prongs of transparent protoplasm from its surface, in nowise differing in their aspect from the slender thread by which it is still moored to the vessel. Finally the thread is severed and the process is complete." (Burdon Sanderson.) The process of diapedesis of the red corpuscles, which occurs under circumstances of impeded venous circulation, and conse- quently increased blood-pressure, resembles closely the migration of the leucocytes, with the exception that they are squeezed through the wall of the vessel, and do not, like them, work their way through by amoeboid movement. Various explanations of these remarkable phenomena have been suggested. Some believe that the pseudo stomata between contiguous endothelial cells (p. 25) provide the means of escape for the blood-corpuscles. But the chief share in the process is to be found in the vital endowments with respect to mobility and contraction of the parts concerned-both of the corpuscles (Bastian) and the capillary wall (Stricker). Burdon-Sanderson remarks, " the capillary is not a dead conduit, but a tube of living protoplasm. There is no difficulty in understanding how the membrane may open to allow the escape of leucocytes, and close again after they have passed out; for it is one of the most striking peculiarities of contractile substance that when two parts of the same mass are separated, and again brought into contact, they melt together as if they had not been severed." Hitherto, the escape of the corpuscles from the interior of the blood-vessels into the surrounding tissues has been studied chiefly in connection -with pathology. But it is impossible to say, at present, to what degree the discovery may not influence all present notions regarding the nutrition of the tissues, even in health. Vital Capillary Force.-The circulation through the capillaries must, of necessity, be largely influenced by that which occurs in the vessels on either side of them-in the arteries or the veins ; their intermediate position causing them to feel at once, so to speak, any alteration in the size or rate of the arterial or venous blood-stream. Thus, the apparent contraction of the capillaries, on the application of certain irritating substances, and during 178 CIRCULATION OF THE BLOOD. [chap. iv. fear, and their dilatation in blushing, may be referred to the action of the small arteries, rather than to that of the capillaries themselves. But largely as the capillaries are influenced by these, and by the conditions of the parts which surround and support them, their own endowments must not be disregarded. They must be looked upon, not as mere passive channels for the passage of blood, but as possessing endowments of their own (vital capil- lary force), in relation to the circulation. The capillary wall is actively living and contractile ; and there is no reason to doubt that, as such, it must have an important influence in connection with the blood-current. Blood-Pressure in the Capillaries. - From observations upon the web of the frog's foot, the tongue and mesentery of the frog, the tails of newts, and small fishes (Roy and Brown), as well as upon the skin of the finger behind the nail (Kries), by careful estimation of the amount of pressure required to empty the vessels of blood under various conditions, it appears that the blood- pressure is subject to variations in the capillaries, apparently following the variations of that of the arteries ; and that up to a certain point, as the extravascular pressure is increased, so does the pulse in the arterioles, capillaries, and venules become more and more evident. The pressure in the first case (web of the frog's foot) has been found to be equal to about 14 to 20 mm. of mercury; in other experiments to be equal to about 1 to | of the ordinary arterial pressure. The blood-current in the veins is maintained by the slight vis a tergo remaining of the contraction of the left ventricle. Very effectual assistance, moreover, to the flow of blood is afforded by the action of the muscles capable of pressing on such veins as have valves, as well as by the suction action of the heart. The effect of such muscular pressure may be thus explained. When pressure is applied to any part of a vein, and the current of blood in it is obstructed, the portion behind the seat of pressure becomes swollen and distended as far back as to the next pair of valves. These, acting like the semilunar valves of the heart, and being, like them, inextensible both in themselves and at their margins of attachment, do not follow the vein in its distension, but are drawn out towards the axis of the canal. Then, if the The Circulation in the Veins. CHAP. IV.] THE VENOUS CIRCULATION. 179 pressure continues on the vein, the compressed blood, tending to move equally in all directions, presses the valves down into contact at their free edges, and they close the vein and prevent regurgitation of the blood. Thus, whatever force is exercised by the pressure of the muscles on the veins, is distributed partly in pressing the blood onwards in the proper course of the circulation, and partly in pressing it backwards and closing the valves behind (fig. 109, A and B). The circulation might lose as much as it gains by such compression of the veins, if it were not for the numerous anasto- moses by which they communicate, one with another ; for through these, the closing up of the venous channel by the backward pressure is prevented from being any serious hindrance to the circxdation, since the blood, of which the onward course is arrested by the closed valves, can at once pass through some anastomosing channel, and proceed on its way by another vein. Thus, therefore, the effect of muscular pressure upon veins which have valves, is turned almost entirely to the advantage of the circulation ; the pressure of the blood onwards is all advantageous, and the pressure of the blood backwards is prevented from being a hindrance by the closure of the valves and the anastomoses of the veins. The effects of such muscular pressure are well shown by the acceleration of the stream of blood when, in venesection, the muscles of the fore arm are put in action, and by the general acceleration of the circulation during active exercise : and the numerous movements which are continually taking place in the body while awake, though their single effects may be less striking, must be an important auxiliary to the venous circulation. Yet they are not essential; for the venous circulation continues un- impaired in parts at rest, in paralysed limbs, and in parts in which the veins are not subject to any muscular pressure. Rhythmical Contraction of Veins.-In the web of the bat's wing, the veins are furnished with valves, and possess the remark- able property of rhythmical contraction and dilatation, whereby the current of blood within them is distinctly accelerated. (Wharton Jones.) The contraction occurs, on an average, about ten times in a minute ; the existence of valves preventing regurgi- tation, the entire effect of the contractions was auxiliary to the onward current of blood. Analogous phenomena have been fre- quently observed in other animals. 180 CIRCULATION OF THE BLOOD. [chap. iv. Blood-Pressure in the Veins.-The blood-pressure gradually lessens as we proceed from arteries near the heart to those more remote, and again from these to the capillaries, and thence along the veins to the right auricle. The blood-pressure in the veins is nowhere very great, but is greatest in the small veins, while in the large veins towards the heart the pressure becomes negative, or, in other words, when a vein is put in connection with a mercurial manometer, the mercury will fall in the arm furthest away from the vein and will rise in the arm nearest the vein, which has a ten- dency to suck in rather than to push forward. In the large veins of the neck this tendency to suck in air is especially marked, and is the cause of death in some surgical operations in that region. The amount of pressure in the brachial vein is said to support 9 mm. of mercury, whereas the pressure in the veins of the neck is about equal to a negative pressure of - 3 to - 8 mm. The variations of venous pressure during systole and diastole of the heart are very slight, and a distinct pulse is seldom seen in veins except under very extraordinary circumstances. The formidable obstacle to the upward current of the blood in the veins of the trunk and extremities in the erect posture sup- posed to be presented by the gravitation of the blood, has no real existence, since the pressure exercised by the column of blood in the arteries, will be always sufficient to support a column of venous blood of the same height as itself: the two columns mutually balancing each other. Indeed, so long as both arteries and veins contain continuous columns of blood, the force of gravi- tation, whatever be the position of the body, can have no power to move or resist the motion of any part of the blood in any direction. The lowest blood-vessels have, of course, to bear the greatest amount of pressure; the pressure on each part being directly proportionate to the height of the column of blood above it: hence their liability to distension. But this pressure bears equally on both arteries and veins, and cannot either move, or resist the motion of, the fluid they contain, so long as the columns of fluid are of equal height in both, and continuous. Velocity of the Blood Current. The velocity of the blood-current at any given point in the various divisions of the circulatory system is inversely propor- tional to their sectional area at that point. If the sectional area CHAP. IV.] VELOCITY OF THE CIRCULATION. 181 of all the branches of a vessel united were always the same as that of the vessel from which they arise, and if the aggregate sectional area of the capillary vessels were equal to that of the aorta, the mean rapidity of the blood's motion in the capillaries would be the same as in the aorta and largest arteries; and if a similar correspondence of capacity existed in the veins and arteries, there would be an equal correspondence in the rapidity of the circulation in them. But the arterial and venous systems may be represented by two truncated cones with their apices directed towards the heart; the area of their united base (the sectional area of the capillaries) being 400-800 times as great as that of the truncated apex representing the aorta. Thus the velocity of blood in the capillaries is not more than of that in the aorta. (a.) In the Arteries.-The velocity of the stream of blood is greater in the arteries than in any other part of the circulatory system, and in them it is greatest in the neighbourhood of the heart, and during the ventricular systole. The rate of movement diminishes during the diastole of the ven- tricles, and in the parts of the arterial system most distant from the heart. Chauveau has estimated the rapidity of the blood-stream in the carotid of the horse at over 20 inches per second during the heart's systole, and nearly 6 inches during the diastole (520-150 mm.). Estimation of the Velocity.-Various instru- ments have been devised for measuring the velocity of the blood-stream in the arteries. Ludwig's " Stromuhr" (fig. 141) consists of a U-shaped glass tube dilated at a and a', the ends of which, It and 2, are of known calibre. The bulbs can be filled by a common opening at k. The instru- ment is so contrived that at b and b', the glass part is firmly fixed into metal cylinders, attached to a circular horizontal table, c o', capable of horizontal movement on a similar table d d' about the vertical axis marked in figure by a dotted line. The opening in c c', when the instrument is in position, as in fig., corresponds exactly with those in d d'; but if c c' be turned at right angles to its present position, there is no communication between h and a, and i and a', but k communicates directly with i; and if turned through two right angles o' communicates with d, and e with d', and there is no direct communication between h and ?. The experiment is performed in the following way :- The artery to be experimented upon is divided and connected with two Fig. 141.-Ludwig's Stromuhr. 182 CIRCULATION OF THE BLOOD. [chap. iv. canhulse and tubes which fit it accurately with h and i-h the central end, and i the peripheral ; the bulb a is filled with olive oil up to a point rather lower than k, and a' and the remainder of a is filled with defibrinated blood ; the tube on k is then carefully clamped ; the tubes d and d' are also filled with defibrinated blood. When everything is ready, the blood is allowed to flow into a through k, and it pushes before it the oil, and that the defibrinated blood into the artery through i, and replaces it in a'; when the blood reaches the former level of the oil in a, the disc c o' is turned rapidly through two right angles, and the blood flowing through d into a' again displaces the oil which is driven into a. This is repeated several Jig. 142.-Diagram of Chauveau's Instrument, a. Brass tube for introduction into the lumen of the artery, and containing an index-needle, which passes through the elastic membrane in its side, and moves by the impulse of the blood-current, c. Graduated scale, for measuring the extent of the oscillations of the needle. times, and the duration of the experiment noted. The capacity of a and a' is known ; the diameter of the artery is also known by its corresponding with the cannulas of known diameter, and as the number of times a has been filled in a given time is known, the velocity of the current can be calculated. Chauveau's instrument (fig. 142) consists of a thin brass tube, a, in one side of which is a small perforation closed by thin vulcanised indiarubber. Passing through the rubber is a fine lever, one end of which, slightly flattened, extends into the lumen of the tube, while the other moves over the face of a dial. The tube is inserted into the interior of an artery, and ligatures applied to fix it, so that the movement of the blood may, in flowing through the tube, be indicated by the movement of the outer extremity of the lever on the face of the dial. The Hcematochometer of Vierordt, and the instrument of Lortet, resemble in principle that of Chauveau. (d.) In the Capillaries.-The observations of Hales, E. H. Weber, and Valentin agree very closely as to the rate of the blood- current in the capillaries of the frog; and the mean of their estimates gives the velocity of the systemic capillary circulation at about one inch (25 mm.) per minute. The velocity in the capil- chap, iv.] VELOCITY IN THE VEINS. 183 laries of warm-blooded animals is greater. In the dog to yuu inch ('5 to '73 mm.) a second. This may seem inconsistent with the facts, which show that the whole circulation is accomplished in about half a minute. But the whole length of capillary vessels, through which any given portion of blood has to pass, probably does not exceed from to of an inch (-5 mm.); and therefore the time required for each quantity of blood to traverse its own appointed portion of the general capillary system will scarcely amount to a second. (c.) In the Veins.-The velocity of the blood is greater in the veins than in the capillaries, but less than in the arteries : this fact depending upon the relative capacities of the arterial and venous systems. If an accurate estimate of the proportionate areas of arteries and the veins corresponding to them could be made, we might, from the velocity of the arterial current, calcu- late that of the venous. A usual estimate is, that the capacity of the veins is about twice or three times as great as that of the arteries, and that the velocity of the blood's motion is, there- fore, about twice or three times as great in the arteries as in the veins, 8 inches (about 200 mm.) a second. The rate at which the blood moves in the veins gradually increases the nearer it ap- proaches the heart, for the sectional area of the venous trunks, compared with that of the branches opening into them, becomes gradually less as the trunks advance towards the heart. (<Z.) Of the Circulation as a whole.-It would appear that a portion of blood can traverse the entire course of the circulation, in the horse, in half a minute. Of course it would require longer to traverse the vessels of the most distant part of the extremities than to go through those of the neck : but taking an average length of vessels to be traversed, and assuming, as we may, that the movement of blood in the human subject is not slower than in the horse, it may be concluded that half a minute represents the average rate. Satisfactory data for these estimates are afforded by the results of experiments to ascertain the rapidity with which poisons in- troduced into the blood are transmitted from one part of the vascular system to another. The time required for the passage of a solution of potassium ferrocyanide, mixed with the blood, from one jugular vein (through the right side of the heart, the pulmonary vessels, the left cavities of the heart, and the general circulation) to the jugular vein of the opposite side, 184 CIRCULATION OF THE BLOOD. [chap. iv. varies from twenty to thirty seconds. I he same substance was transmitted from the jugular vein to the great saphena in twenty seconds; from the jugular vein to the masseteric artery, in between fifteen and thirty seconds ; to the facial artery, in one experiment, in between ten and fifteen seconds ; in another ex- periment in between twenty and twenty-five seconds ; in its transit from the jugular vein to the metatarsal artery, it occupied between twenty and thirty seconds, and in one instance more than forty seconds. The result was nearly the same whatever was the rate of the heart's action. In all these experiments, it is assumed that the substance injected moves with the blood, and at the same rate, and does not move from one part of the organs of circulation to another by diffusing itself through the blood or tissues more quickly that the blood moves. The assumption is sufficiently probable, to be considered nearly certain, that the times above mentioned, as occupied in the passage of the injected substances, arc those in which the portion of blood, into which each was injected, was carried from one part to another of the vascular system. Another mode of estimating the general velocity of the circu- lating blood, is by calculating it from the quantity of blood supposed to be contained in the body, and from the quantity which can pass through the heart in each of its actions. But the conclusions arrived at by this method are less satisfactory. For the total quantity of blood, and the capacity of the cavities of the heart, have as yet been only approximately ascertained. Still the most careful of the estimates thus made accord very nearly with those already mentioned; and it may be assumed that the blood may all pass through the heart in from twenty-five to fifty seconds. Local Peculiarities of the Circulation. The most remarkable peculiarities attending the circulation of blood through different organs are observed in the cases of the brain, the erectile organs, the lungs, the liver, and the kidneys. i. In the Brain.-For the due performance of its functions, the brain requires a large supply of blood. This object is effected through the number and size of its arteries, the two internal carotids, and the two vertebrals. It is further necessary that the CHAP. IV.] THE CIRCULATION IN THE BRAIN. 185 force with which this blood is sent to the brain should be less, or at least should be subject to less variation from external circum- stances than it is in other parts, and so the large arteries are very tortuous and anastomose freely in the circle of Willis, which thus insures that the supply of blood to the brain is uniform, though it may by an accident be diminished, or in some way changed, through one or more of the principal arteries. The transit of the large arteries through bone, especially the carotid canal of the temporal bone, may prevent any undue distension ; and uniformity of supply is further insured by the arrangement of the vessels in the pia mater, in which, previous to their distribution to the sub- stance of the brain, the large arteries break up and divide into innumerable minute branches ending in capillaries, which, after frequent communication with one another, enter the brain, and carry into nearly every part of it uniform and equable streams of blood. The arteries are also enveloped in a special lymphatic sheath. The arrangement of the veins within the cranium is also peculiar. The large venous trunks or sinuses are formed so as to be scarcely capable of change of size ; and composed, as they are, of the tough tissue of the dura mater, and, in some instances, bounded on one side by the bony cranium, they are not compressible by any force which the fulness of the arteries might exercise through the substance of the brain ; nor do they admit of distension when the flow of venous blood from the brain is obstructed. The general uniformity in the supply of blood to the brain, which is thus secured, is well adapted, not only to its functions, but also to its condition as a mass of nearly incompressible sub- stance placed in a cavity with unyielding walls. These conditions of the brain and skull formerly appeared, indeed, enough to justify the opinion that the quantity of blood in the brain must be at all times the same. But it was found that in animals bled to death, without any aperture being made in the cranium, the brain became pale and anaemic like other parts. And in death from strangling or drowning, there was congestion of the cerebral vessels; while in death by prussic acid, the quantity of blood in the cavity of the cranium was determined by the position in which the animal was placed after death, the cerebral vessels being congested when the animal was suspended with its head downwards, and com- paratively empty when the animal was kept suspended by the ears. Thus, it was concluded, although the total volume of the contents of the cranium is probably nearly always the same, yet 186 CIRCULATION OF THE BLOOD. [chap. iv. the quantity of blood in it is liable to variation, its increase or diminution being accompanied by a simultaneous diminution or increase in the quantity of the cerebro-spinal fluid, which, by readily admitting of being removed from one part of the brain and spinal cord to another, and of being rapidly absorbed, and as readily effused, would serve as a kind of supplemental fluid to the other contents of the cranium, to keep it uniformly filled in case of variations in their quantity (Burrows). And there can be no doubt that, although the arrangements of the blood-vessels, to which reference has been made, ensure to the brain an amount of blood which is tolerably uniform, yet, inasmuch as with every beat of the heart and every act of respiration, and under many other circumstances, the quantity of blood in the cavity of the cranium is constantly varying, it is plain that, were there not pro- vision made for the possible displacement of some of the contents of the unyielding bony case in which the brain is contained, there would be often alternations of excessive pressure with insufficient supply of blood. Hence we may consider that the cerebro-spinal fluid in the interior of the skull not only subserves the mechanical functions of fat in other parts as a packing material, but by the readiness with which it can be displaced into the spinal canal, provides the means whereby undue pressure and insufficient supply of blood are equally prevented. Chemical Composition of Cerebro-spinal Fluid.-The cerebro-spinal fluid is transparent, colourless, not viscid, with a saline taste and alkaline reaction, and is not affected by heat or acids. It contains 981-984 parts water, sodium chloride, traces of potassium chloride, of sulphates, carbonates, alkaline and earthy phosphates, minute traces of urea, sugar, sodium lactate, fatty matter, cholesterin, and albumen (Flint). 2. In Erectile Structures.-The instances of greatest variation in the quantity of blood contained, at different times, in the same organs, are found in certain structures which, under ordinary cir- cumstances, are soft and flaccid, but, at certain times, receive an unusually large quantity of blood, become distended and swollen by it, and pass into the state which has been termed erection. Such structures are the corpora cavernosa and corpus spongiosum of the penis in the male, and the clitoris in the female; and, to a less degree, the nipple of the mammary gland in both sexes. The corpus cavernosum penis, which is the best example of an erectile structure, has an external fibrous membrane or sheath ; and from the inner surface of the latter are prolonged numerous CHAP. IV.] ERECTILE TISSUES. 187 fine lamellae which divide its cavity into small compartments looking like cells when they are inflated. Within these is situated the plexus of veins upon which the peculiar erectile property of the organ mainly depends. It consists of short veins which very closely interlace and anastomose with each other in all directions, and admit of great variation of size, collapsing in the passive state of the organ, but, for erection, capable of an amount of dilatation which exceeds beyond comparison that of the arteries and veins which convey the blood to and from them. The strong fibrous tissue lying in the intervals of the venous plexuses, and the external fibrous membrane or sheath with which it is connected, limit the distension of the vessels, and, during the state of erection, give to the penis its condition of tension and firmness. The same general condition of vessels exists in the corpus spongiosum urethrae, but around the urethra the fibrous tissue is much weaker than around the body of the penis, and around the glans there is none. The venous blood is returned from the plexuses by comparatively small veins; those from the glans and the fore part of the urethra empty themselves into the dorsal veins of the penis ; those from the cavernosum pass into deeper veins which issue from the corpora cavernosa at the crura penis; and those from the rest of the urethra and bulb pass more directly into the plexus of the veins about the prostate. For all these veins one condition is the same ; namely, that they are liable to the pressure of muscles when they leave the penis. The muscles chiefly concerned in this action are the erector penis and accelerator urinse. Erection results from the distension of the venous plexuses with blood. The principal exciting cause in the erection of the penis is nervous irritation, originating in the part itself, or derived from the brain and spinal cord. The nervous influence is communicated to the penis by the pudic nerves, which ramify in its vascidar tissue : and after their division in the horse, the penis is no longer capable of erection. This influx of the blood is the first condition necessary for erection, and through it alone much enlargement and turgescencc of the penis may ensue. But the erection is probably not com- plete, nor maintained for any time except when, together with this influx, the muscles already mentioned contract, and by com- pressing the veins, stop the efflux of blood, or prevent it from being as great as the influx. It appears to be only the most perfect kind of erection that 188 CIRCULATION OF THE BLOOD. [chap. iv. needs the help of muscles to compress the veins; and none such can materially assist the erection of the nipples, or that amount of turgescence, just falling short of erection, of which the spleen and many other parts are capable. For such turgescence nothing more seems necessary than a large plexiform arrangement of the veins, and such arteries as may admit, upon occasion, augmented quantities of blood. (3, 4, 5). The circulation in the Lungs, Liver, and Kidneys will be described under their respective heads. Agents concerned in the circulation. Before quitting the subject of the circulation it will be as well to bring together in a tabular form the various agencies concerned in maintaining the circulation. 1. The Systole and Diastole of the Heart, the former pumping into the aorta and so into the arterial system a certain amount of blood, and the latter to some extent sucking in the blood from the veins. 2. The elastic and muscular coats of the arteries, which serve to keep up an equable and continuous stream. 3. The so-called vital capillary force. 4. The pressure of the muscles on veins xvith valves, and the slight rhythmic contraction of the veins. 5. .ds/nrccta'on of the thorax during inspiration, by means of which the blood is drawn from the large veins into the thorax (to be treated of in next Chapter). Proofs of the Circulation of the Blood. The following are the main arguments by which Harvey established the fact of the circulation :- 1. The heart in half an hour propels more blood than the whole mass of blood in the body. 2. The great force and jetting manner with which the blood spurts from an opened artery, such as the carotid, with every beat of the heart. 3. If true, the normal course of the circulation explains why after death the arteries are commonly found empty and the veins full. 4. If the large veins near the heart were tied in a fish or snake, CHAP. IV.] PROOFS OF THE CIRCULATION. 189 the heart became pale, flaccid, and bloodless; on removing the ligature, the blood again flowed into the heart. If the artery were tied, the heart became distended; the distension lasting until the ligature was removed. 5. The evidence to be derived from a ligature round a limb. If it be drawn very tight, no blood can enter the limb, and it be- comes pale and cold. If the ligature be somewhat relaxed, blood can enter but cannot leave the limb; hence it becomes swollen and congested. If the ligature be removed, the limb soon regains its natural appearance. 6. The existence of valves in the veins which only permit the blood to flow towards the heart. 7. The general constitutional disturbance resulting from the introduction of a poison at a single point, e.y., snake poison. To these may now be added many further proofs which have accumulated since the time of Harvey, e.g.:- 8. Wounds of arteries and veins. In the former case haemor- rhage may be almost stopped by pressure between the heart and the wound, in the latter by pressure beyond the seat of injury. 9. The direct observation of the passage of blood corpuscles from small arteries through capillaries into veins in all transparent vascular parts, as the mesentery, tongue or web of the frog, the tail or gills of a tadpole, &c. 10. The results of injecting certain substances into the blood. Further, it is obvious that the mere fact of the existence of a hollow muscular organ (the heart) with valves so arranged as to permit the blood to pass only in one direction, of itself suggests the course of the circulation. The only part of the circulation which Harvey could not follow is that through the capillaries, for the simple reason that he had no lenses sufficiently powerful to enable him to see it. Malpighi (1661) and Leeuwenhoek (1668) demonstrated it in the tail of the tadpole and lung of the frog. 190 RESPIRATION. [chap. V. CHAPTER V. The maintenance of animal life necessitates the continual absorption of oxygen and excretion of carbonic acid ; the blood being, in all animals which possess a well developed blood-vascular system, the medium by which these gases are carried. By the blood, oxygen is absorbed from without and conveyed to all parts of the organism; and, by the blood, carbonic acid, which comes from within, is carried to those parts by which it may escape from the body. The two processes,-absorption of oxygen and excretion of carbonic acid,-are complementary, and their sum is termed the process of Respiration. In all Vertebrata, and in a large number of Invertebrata, certain parts, either lungs or gills, are specially constructed for bringing the blood into proximity with the aerating medium (atmospheric air, or water containing air in solution). In some of the lower Vertebrata (frogs and other naked Amphibia) the skin is important as a respiratory organ, and is capable of supplementing, to some extent, the functions of the proper breathing apparatus ; but in all the higher animals, including man, the respiratory capacity of the skin is so infinitesimal that it may be practically disregarded. Essentially, a lung or gill is constructed of a fine transparent membrane, one surface of which is exposed to the air or water, as the case may be, while, on the other, is a network of blood- vessels,-the only separation between the blood and aerating medium being the thin wall of the blood-vessels, and the fine membrane on one side of which vessels are distributed. The difference between the simplest and the most complicated re- spiratory membrane is one of degree only. The various complexity of the respiratory membrane, and the kind of aerating medium, are not, however, the only conditions which cause a difference in the respiratory capacity of different animals. The number and size of the red blood-corpuscles, the mechanism of the breathing apparatus, the presence or absence of a pulmonary heart, physiologically distinct from the systemic, are, all of them, conditions scarcely second in importance. RESPIRATION. CHAP. V.] THE RESPIRATORY TISSUES. 191 In the heart of man and all other Mammalia, the right side from which the blood is propelled into and through the lungs may be termed the " pul- monary " heart; while the left side is " systemic " in function. In many of the lower animals, however, no such distinction can be drawn. Thus, in Fish the heart propels the blood to the respiratory organs (gills) ; but there is no contractile sac corresponding to the left side of the heart, to propel the blood directly into the systemic vessels. It may be well to state here that the lungs arc only the medium for the exchange, on the part of the blood, of carbonic acid for oxygen. They are not the seat, in any. special manner, of those combustion-processes of which the production of carbonic acid is the final result. These occur in all parts of the body- more in one part, less in another : chiefly in the substance of the tissues. The object of respiration being the interchange of gases in the lungs, it is necessary that the atmospheric air shall pass into them and be expelled from them. The lungs are contained in the chest or thorax, which is a closed cavity having no com- munication with the outside, except by means of the respiratory passages. The air enters these passages through the nostrils or through the mouth, thence it passes through the larynx into the trachea or windpipe, which about the middle of the chest divides into two tubes or bronchi, one to each (right and left) lung. The Larynx is the upper part of the passage which leads exclusively to the lung : it is formed by the thyroid, cricoid, and arytenoid cartilages (fig. 144), and contains the vocal' cords, by the vibration of which the voice is chiefly produced. These vocal cords are ligamentous bands attached to certain cartilages capable of movement by muscles. By their approximation the cords can entirely close the entrance into the larynx ; but under ordinary conditions, the entrance of the larynx is formed by a more or less triangular chink between them, called the rima glottidis. Projecting at an acute angle between the base of the tongue and the larynx to which it is attached, is a leaf-shaped cartilage, with its larger extremity free, called the epiglottis (fig. 144, e). The whole of the larynx is lined by mucous membrane, which, however, is extremely thin over the vocal The Respiratory Passages and Tissues. 192 RESPIRATION. [chap. v. cords. At its lower extremity the larynx joins the trachea.* With the exception of the epiglottis and the so-called cornicula laryngis, the cartilages of the larynx are of the hyaline variety. Fig- U3- Structure of the Epiglottis.-The supporting cartilage of the epiglottis is composed of yellow elastic cartilage, enclosed in a fibrous sheath (perichondrium), and covered on both sides with mucous membrane. The anterior surface, which looks towards the back of the tongue, is covered with mucous membrane, the basis of which is fibrous tissue, elevated towards both surfaces in the form of rudimentary papillae, and covered with several layers * A detailed account of the structure and function of the Larynx will be found in Chapter XVI. CHAP. V.] THE TRACHEA AND BRONCHI. 193 of squamous epithelium. In it ramify capillary blood-vessels, and in its meshes are a large number of lymphatic channels. Under the mucous membrane, in the less dense fibrous tissue of which it is composed, are a number of tubular glands. The posterior or laryngeal sur- face of the epiglottis is covered by a mucous membrane, simi- lar in structure to that on the other surface, but its epithelial coat is thinner, the number of strata of cells are less, and the papillae few and less distinct. The fibrous tissue which con- stitutes the mucous membrane is in great part of the ade- noid variety, and is here and there collected into distinct masses or follicles. The glands of the posterior surface are smaller but more numerous than those of the other sur- face. In many places the glands which are situated near- est to the perichondrium are directly continuous through apertures in the cartilage with those on the other side, and often the ducts of the glands from one side of the cartilage pass through and open upon the mucous surface of the other side. Taste goblets have been found in the epithelium of the posterior surface of the epi- glottis, and in several other situations in the laryngeal mucous membrane. The Trachea and Bronchial Tubes.-The trachea or wind- pipe extends from the cricoid cartilage, which is on a level with Fig. 144.-Outline showing the general form of the larynx, trachea, and bronchi, as seen from be- fore. h, the great cornu of the hyoid bone; e, epiglottis; t, superior, and t', inferior cornu of the thyroid cartilage; c, middle of the cricoid cartilage; tr, the trachea, showing sixteen cartilaginous rings ; b, the right, and ft'.theleftbronchus. (AllenThomson.) xj. 194 RESPIRATION. [chap. v. the fifth cervical vertebra, to a point opposite the third dorsal vertebra, where it divides into the two bronchi, one for each lung (fig. 144). It measures, on an average, four or four- and-a-half inches in length, and from three-quarters of an inch to an inch in dia- meter. Structure.-The trachea is essentially a tube of fibro- elastic membrane, within the layers of which are en- closed a series of cartilagi- nous rings, from sixteen to twenty in number. These rings extend only around the front and sides of the trachea (about two-thirds of its circumference), and are deficient behind; the interval between their pos- terior extremities being bridged over by a continua- tion of the fibrous mem- brane in which they are enclosed (fig. 144). The cartilages of the trachea and bronchial tubes are of the hyaline variety. Immediately within this tube, at the back, is a layer of unstriped muscular fibres, which extends, trans- versely, between the ends of the cartilaginous rings to which they are attached, and opposite the intervals between them, also; their evident function being to diminish, when required, the calibre of the trachea by approxi- mating the ends of the cartilages. Outside these are a few' longi- Fig. 145.-Outline showing th- general form of the larynx, trachea, and bronchi as seen from behind, h, great cornu of the hyoid bone ; t, superior, . and t', the inferior cornu of the thyroid carti- lage ; e, epiglottis; a, points to the back of both the arytenoid cartilages, which are sur- mounted by the cornieula; c, the middle ridge on the back of the cricoid cartilage; tr, the posterior membranous part of the trachea; b, b', right and left bronchi. (Allen Thomson.) j. chap, v.] STRUCTURE OF THE TRACHEA AND BRONCHI. 195 tudinal bundles of muscular tissue, which, like the preceding, are attached both to the fibrous and cartilaginous framework. The mucous membrane consists of adenoid tissue, separated from the stratified columnar epithelium which lines it by a homogeneous basement membrane. This is pene- trated here and there by channels which connect the adenoid tissue of the mucosa with the intercellular sub- stance of the epithelium. The stratified columnar epithelium is formed of several layers of cells (fig. 146), of which the most superficial layer is ciliated, and is often branched down- wards to join connective- tissue corpuscles; while between these branched cells are smaller elongated cells prolonged up towards the surface and down to the basement membrane. Beneath these are one or more layers of more irregu- larly shaped cells. In the deeper part of the mucosa are many elastic fibres between which lie con- nective - tissue corpuscles and capillary blood-vessels. Numerous mucous glands are situate on the exterior and in the substance of the fibrous framework of the trachea ; their ducts perforating the various structures which form the wall of the trachea, and opening the mucous membrane into the interior. The two bronchi into which the trachea divides, of which the right is shorter, broader, and more horizontal than the left (fig. Fig. 146.-Section of the trachea, a, columnar ciliated epithelium ; ft and c, proper structure of the mucous membrane, containing clastic fibres cut across transversely ; d, submucous tissue containing mucous glands, e, separated from the hyaline cartilage, <7, by a fine fibrous tissue, /; A, external investment of fine M*r fibroustissue. (S. K. Alcock.) 196 RESPIRATION. [chap. v. 144), resemble the trachea exactly in structure, and in the arrange- ment of their cartilaginous rings. On entering the substance of the lungs, however, the rings, although they still form only larger or smaller segments of a circle, are no longer confined to the front and sides of the tubes, but are distributed impartially to all parts of their circumference. The bronchi divide and sub-divide, in the substance of the lungs, into a number of smaller and smaller branches, which pene- Fig. 147.-Transverse section of a bronchus, about A inch in diameter, e, Epithelium (ciliated) ; immediately beneath it is the mucous membrane or internal fibrous layer, of varying thickness ; m, muscular layer ; s, m, submucous tissue; f, fibrous tissue; cartilage enclosed within the layers of fibrous tissue; <7, mucous gland. (F. E. Schulze.) trate into every part of the organ, until at length they end in the smaller sub-divisions of the lungs, called lobules. All the larger branches still have walls formed of tough mem- brane, containing portions of cartilaginous rings, by which they are held open, and unstriped muscular fibres, as well as longi- tudinal bundles of elastic tissue. They are lined by mucous membrane, the surface of which, like that of the larynx and trachea, is covered with ciliated epithelium (fig. 146). The mucous membrane is abundantly provided with mucous glands. As the bronchi become smaller and smaller, and their walls thinner, the cartilaginous rings become scarcer and more irregular, until, in the smaller bronchial tubes, they are represented only by minute and scattered cartilaginous flakes. And when the bronchi, by successive branches are reduced to about -4\, of an inch in diameter, they lose their cartilaginous element altogether, and their walls are formed only of a tough fibrous elastic membrane, CHAP. V.] THE LUNGS AND PLEURJE. 197 with circular muscular fibres ; they are still lined, however, by a thin mucous membrane, with ciliated epithelium, the length of the cells bearing the cilia having become so far diminished, that the cells are now almost cubical. In the smaller bronchi the circular muscular fibres arc more abundant than in the trachea and larger bronchi, and form a distinct circular coat. The Lungs and Pleurae.-The Lungs occupy the greater por- tion of the thorax. They are of a spongy elastic texture, and on section appear to the naked eye as if they were in great part solid Fig. 148.- Transverse section of the chest. organs, except here and there, at certain points, where branches of the bronchi or air-tubes may have been cut across, and show, on the surface of the section, their tubular structure. Tn fact, however, the lungs are hollow organs, each of which communicates by a separate orifice with a common air-tube, the trachea. The Pleura.-Each lung is enveloped by a serous membrane- the pleura, one layer of which adheres closely to its surface, and provides it with its smooth and slippery covering, while the other adheres to the inner surface of the chest-wall. The continuity of the two layers, which form a closed sac, as in the case of other serous membranes, will be best understood by reference to fig. 148. The appearance of a space, however, between the pleura which covers the lung (visceral layer), and that which lines the inner surface of the chest (parietal layer), is inserted in the drawing only for the sake of distinctness. These layers are, in health, everywhere in contact, one with the other ; and between them is 198 RESPIRATION. [chap. v. only just so much fluid as will ensure the lungs gliding easily, in their expansion and contraction, on the inner surface of the parietal layer, which lines the chest-wall. While considering the subject of normal respiration, we may discard altogether the notion of the existence of any space or cavity between the lungs and the wall of the chest. If, however, an opening be made so as to permit air or fluid to enter the pleural sac, the lung, in virtue of its elasticity, recoils, and a considerable space is left between it and the chest- wall. In other words, the natural elasticity of the lungs would cause them at all times to contract away from the ribs, were it not that the contraction is resisted by atmospheric pressure which bears only on the inner surface of the air-tubes and air-cells. On the admission of air into the pleural sac, atmospheric pressure bears alike on the inner and outer surfaces of the lung, and their elastic recoil is thus no longer prevented. Structure of the Pleura and Dung.-The pulmonary pleura con- sists of an outer or denser layer and an inner looser tissue. The former or pleura proper consists of dense fibrous tissue with elastic fibres, covered by endothelium, the cells of which are large, flat, hya- line, and transparent when the lung is expanded, but become smaller, thicker, and granular when the lung collapses. In the pleura is a lymph-can alien lar system ; and connective tissue cor- puscles are found in the fibrous tissue which forms its groundwork. The inner, looser, or subpleural tissue contains lamellae of fibrous connective tissue and connective tissue cor- puscles between them. Numerous lymphatics arc to be met with, which form a dense plexus of vessels, many of which contain valves. They are simple endothelial tubes, and take origin in the lymph- canalicular system of the pleura proper. Scattered bundles of unstriped muscular fibre occur in the pulmonary pleura. They Fig. i-ig.-Ciliary epithelium of the human trachea, a, Bayer of longitudinally arranged elastic fibres; b, basement membrane; c, deepest cells, circular in form ; d, intermediate elon- gated cells; r, outermost layer of cells fully developed and bearing eilia, x 350. (Kiilliker. CHAP. V.J STRUCTURE OF THE LUNG. 199 are especially strongly developed on the anterior and internal sur- faces of the lungs, the parts which move most freely in respiration : their function is doubtless to aid in expiration. The struc- ture of the parietal portion of the pleura is very similar to that of the visceral layer. Each lung is partially sub- divided into separate portions called lobes; the right lung into three lobes, and the left into two. Each of these lobes, again, is composed of a large number of minute parts, called lobules. Each pulmonary lobule may be considered a lung in miniature, consisting, as it does, of a branch of the bronchial tube, of air-cells, blood vessels, nerves, and lymphatics, with a sparing amount of areolar tissue. On entering a lobule, the small bronchial tube, the structure of which has been just described (a, fig. 151), divides and sub- divides; its walls at the same time becoming thinner and thinner, until at length they are formed only of a thin membrane < >f areolar and elastic tissue, lined by a layer of squamous epithelium, not provided with cilia. At the same time, they are altered in shape ; each of the minute ter- minal branches widening out funnel-wise, and its walls being pouched out irregularly into small saccular dilatations, called air-cells (fig. 151, 6). Such a funnel-shaped terminal branch of the bronchial tube, with its group of pouches or air-cells, has been called an infundibulum (figs. 150, 151), and the irregular oblong space in Fig. 150.-terminal branch of a bronchial tube, with its infundibula and air-cells, from the margin of the lung of a monkey, injected with quicksilver, a, terminal bronchial twig; b b, infundibula and air-cells. X 10. (F. E. Schulze.) Fig. 151.-Two small infundibula or group* of air-cells, a a, with air-cells, b b, and the ultimate bronchial tubes, c c, w ith which the air-cells communicate. From a new-born child. (Kolliker.) 200 RESPIRATION. [chap. v. its centre, with which the air-cells communicate, an intercellular passage. The air-cells, or air-vesicles, may be placed singly, like recesses from the intercellular passage, but more often they arc arranged in groups or even in rows, like minute sacculated tubes; so that a short series of vesicles, all communicating with one another, Fig. 152.-From a section of the lung of a cat, stained with silver nitrate. A. I). Alveolar duct or intercellular passage. S. Alveolar septa. N. Alveoli or air-cells, lined with large flat, nucleated cells, with some smaller polyhedral nucleated cells. U. Unstriped muscular fibres. Circular muscular fibres are seen surrounding the interior of the alveolar duct, and at one part is seen a group of small polyhedral cells continued from the bronchus. (Klein and Noble Smith.), open by a common orifice into the tube. The vesicles are of various forms, according to the mutual pressure to which they are subject; their walls are nearly in contact, and they vary from A. to 7l(j of an inch in diameter. Their walls are formed of fine membrane, similar to that of the intercellular passages, and con- tinuous with it, which membrane is folded on itself so as to form a sharp-edged border at each circular orifice of communication between contiguous air-vesicles, or between the vesicles and the bronchial passages. Numerous fibres of elastic tissue are spread CHAP. V.] CAPILLARIES OF THE LUNGS. 201 out between contiguous air-cells, and many of these are attached to the outer surface of the fine membrane of which each cell is composed, imparting to it additional strength, and the power of recoil after distension. The cells are lined by a layer of epithe- lium (fig. 152), not provided with cilia. Outside the cells, a net- work of pulmonary capillaries is spread out so densely (fig. 153), that the interspaces or meshes are even narrower than the vessels, which are, on an average, of an inch in diameter. Between Fig. 153.-Capilla-ry net-work of the pulmonary Hood-vessels in the human lung1, x 60. (Kiilliker.) the atmospheric air in the cells and the blood in these vessels, nothing intervenes but the thin walls of the cells and capillaries; and the exposure of the blood to the air is the more complete, because the folds of membrane between contiguous cells, and often the spaces between the walls of the same, contain only a single layer of capillaries, both sides of which are thus at once exposed to the air. The air-vesicles situated nearest to the centre of the lung are smaller and their networks of capillaries are closer than those nearer to the circumference. The vesicles of adjacent lobules do not communicate ; and those of the same lobule or proceeding from the same intercellular passage, do so as a general rule only near angles of bifurcation; so that, when any bronchial tube is closed or obstructed, the supply of air is lost for all the cells opening into it or its branches. 202 RESPIRATION. [onAB. v. Blood-supply.-The lungs receive blood from two sources, (a) the pulmonary artery, (/>) the bronchial arteries. The former conveys venous blood to the lungs for its arterialization, and this blood takes no share in the nutrition of the pulmonary tissues through which it passes. (Z») The branches of the bronchial arteries ramify for nutrition's sake in the walls of the bronchi, of the larger pulmonary vessels, in the interlobular connective tissue, <tc. ; the blood of the bronchial vessels being returned chiefly through the bronchial and partly through the pulmonary veins. Lymphatics.-The lymphatics are arranged in three sets :- i. Irregular lacunae in the walls of the alveoli or air-cells. The lymphatic vessels which lead from these accompany the pulmonary vessels towards the root of the lung. 2. Irregular anastomosing spaces in the walls of the bronchi. 3. Lymph-spaces in the pulmonary pleura. The lymphatic vessels from all these irregular sinuses pass in towards the root of the lung to reach the bronchial glands. Nerves.-The nerves of the lung are to be traced from the anterior and posterior pulmonary plexuses, which are formed by branches of the vagus and sympathetic. The nerves follow the course of the vessels and bronchi, and in the walls of the latter many small ganglia are situated. Mechanism of Respiration. Respiration consists of the alternate expansion and contraction of the thorax, by means of which air is drawn into or expelled from the lungs. These acts are called Inspiration and Expiration respectively. For the inspiration of air into the lungs it is evident that all that is necessary is such a movement of the side-walls or floor of the chest, or of both, that the capacity of the interior shall be enlarged. By such increase of capacity there will be of course a diminution of the pressure of the air in the lungs, and a fresh quantity will enter through the larynx and trachea to equalise the pressure on the inside and outside of the chest. For the expiration of air, on the other hand, it is also evident that, by an opposite movement which shall diminish the capacity of the chest, the pressure in the interior will be increased, and air will be expelled, until the pressure within and without the chest are again equal. In both cases the air passes through the trachea < HAP. V.] RESPIRATORY MOVEMENTS. 203 and larynx, whether in entering or leaving the lungs, there being no other communication with the exterior of the body ; and the lung, for the same reason, remains under all the circumstances described closely in contact with the walls and floor of the chest. To speak of expansion of the chest, is to speak also of expansion of the lung. We have now to consider the means by which the respiratory movements arc effected.. Respiratory Movements. A. Inspiration.-The enlargement of the chest in inspiration is a muscular act; the effect of the ' action of the inspiratory Fig. 154. - Ding rant of axes of movement of ribs. muscles being an increase in the size of the chest-cavity (a) in the vertical, and (6) in the lateral and antero-posterior diameters. The muscles engaged in ordinary inspiration are the diaphragm ' the external intercostals ; parts of the internal intercostals ; the. levatores costarum; and serratus posticus superior. (fl.) 'fhe vertical diameter of the chest is increased by the con- traction and consequent descent of the diaphragm,-the sides of 204 RESPIRATION. [chai*, v. the muscle descending most, and the central tendon remaining comparatively unmoved; while the intercostal and other muscles, by acting at the same time, prevent the diaphragm, during its contraction, from drawing in the sides of the chest. (6.) The increase in the lateral and antero-posterior diameters of the chest is effected by the raising of the ribs, the greater number Fig. 155.-Diagram of movement of a rib in inspiration. of which are attached very obliquely to the spine and sternum {see Figure of Skeleton in frontispiece). The elevation of the ribs takes place both in front and at the sides-the hinder ends being prevented from performing any up- ward movement by their attachment to the spine. The movement of the front extremities of the ribs is of necessity accompanied by an upward and forward movement of the sternum to which they are attached, the movement being greater at the lower end than at the upper end of the latter bone. The ares of rotation in these movements are two ; one corre- sponding with a line drawn through the two articulations which the rib forms with the spine (a b, fig. 154); and the other, with a line drawn from one of these (head of rib) to the sternum (A B, fig. 154, and fig. 155); the motion of the rib around the latter axis being somewhat after the fashion of raising the handle of a bucket. The elevation of the ribs is accompanied by a slight opening out of the angle which the bony part forms with its cartilage (fig. CHAP. V.] MOVEMENTS OE THE BIBS. 205 158, A) ; and thus an additional means is provided for increasing the antero-posterior diameter of the chest. The muscles by which the ribs are raised, in ordinary quiet inspiration, arc the external intercostals, and that portion of the- internal intercostals which is situate between the costal cartilages ; and these are assisted by the levatores costarum, and the serratua posticus superior. The action of the levatores and the serratus is. very simple. Their fibres, arising from the spine as a fixed point, pass obliquely downwards and forwards to the ribs, and neces- sarily raise the latter when they contract. The action of tho intercostal muscles is not quite so simple, inasmuch as, passing merely from rib to rib, they seem at first sight to have no fixed point towards which they can pull the bones to which they arc attached. A very simple apparatus will make their action plain. Such an apparatus, is shown in fig. 156. A B is an upright bar, representing the spine, with Fig. 156.-Diagram of apparatus showing the action of the external intercostal muscles. Fig. 157.-Diagram of apparatus showing the action of the internal intercostal muscles. which arc jointed two parallel bars. C and D, which represent two of the ribs, and are connected in front by moveable joints with another upright, representing the sternum. If with such an apparatus elastic bands be connected in imitation of the intercostal muscles, it will be found that when stretched on the bars after the fashion of the external intercostal fibres (fig. 156, G D), ?.r., passing downwards and forwards, they raise them (fig. 156 C' D') ; while on the 206 RESPIRATION. [chap. V. other hand, if placed in imitation of the position of the internal intercostals (fig. 157, E F), i.e., passing downwards and backwards, they depress them (fig. 157, E' F'). The explanation of the foregoing facts is very simple. The intercostal muscles, in contracting, merely do that which all other contracting fibres do, viz., bring nearer together the points to which they are attached ; and in order to do this, the external intercostals must raise the ribs, the points 1' and D (fig. 156) being nearer to each other when the parallel bars are in the position of the dotted lines. The limit of the movement in the apparatus is reached when the elastic band extends at right angles to the two bars which it connects-the points of attachment C' and D' being then at the smallest possible distance one from the other. The internal intercostals (excepting those fibres which are attached to the cartilages of the ribs), have an opposite action to that of the external. Tn contracting they must pull down the ribs, because the points E and F (fig. 157) can only be brought nearer one to another (fig. 157, E' F') by such an alteration in their position. On account of the oblique position of the cartilages of the ribs with reference to the sternum, the action of the inter-cartilaginous fibres of the internal intercostals must, of course, on the foregoing principles, resemble that of the external intercostals. In tranquil breathing, the expansive movements of the lower part of the chest are greater than those of the upper. In forced inspiration, on the other hand, the greatest extent of movement appears to be in the upper antero-posterior diameter. Muscles of Extraordinary Inspiration.-In extraordinary or forced inspiration, as in violent exercise, or in cases in which there is some interference with the due entrance of air into the chest, and in which, therefore, strong efforts are necessary, other muscles than those just enumerated, are pressed into the service. It is very difficult or impossible to separate by a hard and fast line, the so-called muscles of ordinary from those of extraordinary inspiration; but there is no doubt that the following are but little used as respiratory agents, except in cases in which unusual efforts are required-the scaleni muscles, the sternomastoid, the serratns magnus, the perforates, and the trapezius. Types of Respiration.-The expansion of the chest in inspi- ration presents some peculiarities in different persons. In young children, it is effected chiefly by the diaphragm, which being highly arched in expiration, becomes flatter as it contracts, and, descending, presses on the abdominal viscera, and pushes forward the front walls of the abdomen. The movement of the abdominal walls being here more manifest than that of any other part, it is usual to call this the abdominal type of respiration. In men, CHAP. V.] TYPES OF RESPIRATION. 207 together with the descent of the diaphragm, and the pushing forward of the front wall of the abdomen, the chest and the sternum are subject to a wide movement in inspiration (inferior costal type). In women, the movement appears less extensive in Fig. 158.-The changes of the thoracic and abdominal walls of the male during respi- ration. The back is supposed to be fixed, in order to throw forward the respiratory movement as much as pos- sible. The outer black continuous line in front represents the ordinary breathing movement: the anterior margin of it being the boundary of inspiration, the posterior margin the limit of expiration. The line is thicker over the abdomen, since the ordinary respiratory movement is chiefly ab- dominal : thin over the chest, for there is less movement ovei' that region. The dotted line indicates the movement on deep inspiration, during which the sternum advances while the abdomen recedes. Fig. 159.-The respiratory movement in ths female. The lines indicate the same changes as in the last figure. The thickness of the continuous line over the sternum shows the larger extent of the ordinary breathing movement over that region in the female than in the male. (John Hutchinson.) 'Die posterior continuous line represents in both figures the limit of forced expi- ration. the lower, and more so in the upper, part of the chest (superior costal type). (See figs. 158, 159.) B. Expiration.-From the enlargement produced in inspira- tion, the chest and lungs return in ordinary tranquil expiration, by their elasticity; the force employed by the inspiratory muscles in distending the chest and overcoming the elastic resistance of 208 RESPIRATION. [chap. V. the lungs and chest-walls, being returned as an expiratory effort when the muscles are relaxed. This elastic recoil of the chest and lungs is sufficient, in ordinary quiet breathing, to expel air from the lungs in the intervals of inspiration, and no muscular power is required. In all voluntary expiratory efforts, however, as in speaking, singing, blowing, and the like, and in many involuntary actions also, as sneezing, coughing, etc., something more than merely passive elastic power is necessary, and the proper expira- tory muscles are brought into action. By far the chief of these are the abdominal muscles, which, by pressing on the viscera of the abdomen, push up the floor of the chest formed by the dia- phragm, and by thus making pressure on the lungs, expel air from them through the trachea and larynx. All muscles, however, which depress the ribs, must act also as muscles of expiration,, and therefore we must conclude that the abdominal muscles are- assisted in their action by the greater part of the internal inter- costals, the triangularis sterni, the serratus posticus inferior, and quadratus lumborum. When by the efforts of the expiratory muscles, the chest has been squeezed to less than its average diameter, it again, on relaxation of the muscles, returns to the normal dimensions by virtue of its elasticity. The construction of the chest-walls, therefore, admirably adapts them for recoil- ing against and resisting as well undue contraction as undue dilatation. In the natural condition of the parts, the lungs can never con- tract to the utmost, but are always more or less " on the stretch,"- being kept closely in contact with the inner surface of the walls- of the chest by cohesion as well as by atmospheric pressure, and can contract away from these only when, by some means or other,, as by making an opening into the pleural cavity, or by the effusion of fluid there, the pressure on the exterior and interior of the- lungs becomes equal. Thus, under ordinary circumstances, the degree of contraction or dilatation of the lungs is dependent on that of the boundary walls of the chest, the outer surface of the one being in close contact with the inner surface of the other, and obliged to follow it in all its movements. Respiratory Rhythm.-The acts of expansion and contraction of the chest, take up, under ordinary circumstances, a nearly equal time. The act of inspiring air, however, especially in women and children, is a little shorter than that of expelling it, and there is commonly a very slight pause between the end of expiration and oiiap. v.] RESPIRATORY MOVEMENTS OF NOSTRILS. 209 the beginning of the next inspiration. The respiratory rhythm may be thus expressed :- Inspiration ...... 6 Expiration . . . . . . . 7 or 8 A very slight pause. Respiratory Sounds.-If the ear be placed in contact with the wall of the chest, or be separated from it only by a good conductor of sound or stethoscope, a faint respiratory murmur is heard during inspiration. This sound varies somewhat in diffe- rent parts-being loudest or coarsest in the neighbourhood of the trachea and large bronchi (tracheal and bronchial breathing), and fading off into a faint sighing as the ear is placed at a distance from these (vesicular breathing). It is best heard in children, and in them a faint murmur is heard in expiration also. The cause of the vesicular murmur has received various explanations. Most observers hold that the sound is produced by the friction of the air against the walls of the alveoli of the lungs when they are undergoing distension (Laennec, Skoda), others that it is due to an oscillation of the current of air as it enters the alveoli (Chauveau), whilst others believe that the sound is produced in the glottis, but that it is modified in its passage to the pulmonary alveoli (Beau, Gee). Respiratory Movements of the Nostrils and of the Glottis.-During the action of the muscles which directly draw air into the chest, those which guard the opening through which it enters are not passive. In hurried breathing the instinctive dilatation of the nostrils is well seen, although under ordinary conditions it may not be noticeable. The opening at the upper part of the larynx, however, or rima glottidis (fig. 143), is dilated at each inspiration, for the more ready passage of air, and be- comes smaller at each expiration; its condition, therefore, corres- ponding during respiration with that of the walls of the chest. There is a further likeness between the two acts in that, under ordinary circumstances, the dilatation of the rima glottidis is a muscular act, and its contraction chiefly an elastic recoil; although, under various conditions, to be hereafter mentioned,' there may be, in the latter, considerable muscular power exer- cised. Terms used to express Quantity of Air breathed.- a. Breathing or tidal air, is the quantity of air which is habitually 210 RESPIRATION. [chap. v. and almost uniformly changed in each act of breathing. In a healthy adult man it is about 30 cubic inches. b. Complemented air, is the quantity over and above this which can be drawn into the lungs in the deepest inspiration; its amount is various, as will be presently shown. c. Reserve air.-Rttev ordinary expiration, such as that which expels the breathing or tidal air, a certain quantity of air remains in the lungs, which may be expelled by a forcible and deeper expiration. This is termed reserve air. d. Residual air is the quantity which still remains in the lungs after the most violent expiratory effort. Its amount depends in great measure on the absolute size of the chest, but may be esti- mated at about 100 cubic inches. The total quantity of air which passes into and out of the lungs of an adult, at rest, in 24 hours, is about 686,000 cubic inches. This quantity, however, is largely increased by exertion; the average amount for a hard-working labourer in the same time, being 1,568,390 cubic inches. e. Respiratory Capacity.-The greatest respiratory capacity of the chest is indicated by the quantity of air which a person can expel from his lungs by a forcible expiration after the deepest inspiration that he can make; it expresses the power which a person has of breathing in the emergencies of active exercise, violence, and disease. The average capacity of an adult (at 6o° F. or 15'4° C.) is about 225 cubic inches. The respiratory capacity, or as Hutchinson called it, vital capacity, is usually measured by a modified gasometer (spirometer of Hutchinson), into which the experimenter breathes,-making the most prolonged expira- tion possible after the deepest possible inspiration. The quantity of air which is thus expelled from the lungs is indicated by the height to which the air chamber of the spirometer rises ; and by means of a scale placed in connection with this, the number of cubic inches is read off. In healthy men, the respiratory capacity varies chiefly with the stature, weight, and age. It was found by Hutchinson, from whom most of our infor- mation on this subject is derived, that at a temperature of 6o° F., ,225 cubic inches is the average vital or respiratory capacity of a healthy person, five feet seven inches in height. Circumstances affecting the amount of respiratory capacity.-For every inch of height above this standard the capacity is increased, on an average, by eight cubic inches ; and for every inch below, it is diminished by the same amount. CHAP. V.] RELATION BETWEEN RESPIRATION AND PULSE. 211 The influence of weight on the capacity of respiration is less manifest and considerable than that of height : and it is difficult to arrive at any definite conclusions on this point, because the natural average weight of a healthy man in relation to stature has not yet been determined. As a general state- ment, however, it may be said that the capacity of respiration is not affected by weights under 161 pounds, or nJ stones ; but that, above this point, it is diminished at the rate of one cubic inch for every additional pound up to 196 pounds, or 14 stones. By age, the capacity appears to be increased from about the fifteenth to the thirty-fifth year, at the rate of five cubic inches per year ; from thirty, five to sixty-five it diminishes at the rate of about one and a half cubic inch per year; so that the capacity of respiration of a man of sixty years old would be about 30 cubic inches less than that of a man forty years old, of the same height and weight. (John Hutchinson.) Number of Respirations, and Relation to the Pulse.- The number of respirations in a healthy adult person usually ranges from fourteen to eighteen per minute. It is greater in infancy and childhood. It varies also much according to different circumstances, such as exercise or rest, health, or disease, etc. Variations in the number of respirations correspond ordinarily with similar variations in the pulsations of the heart. In health the proportion is-about 1 to 4, or 1 to 5, and when the rapidity of the heart's action is increased, that of the chest movement is commonly increased also; but not in every case in equal pro- portion. It happens occasionally in disease, especially of the lungs or air-passages, that the number of respiratory acts increases in quicker proportion than the beats of the pulse; and, in other affections, much more commonly, that the number of the pulses is greater in proportion than that of the respirations. There can be no doubt that the number of respirations of any given animal is largely affected by its size. Thus, comparing anitnals of the same kind, in a tiger (lying quietly) the number of respirations was 20 per minute, while in a small leopard (lying quietly) the number was 30. In a small monkey 40 per minute ; in a large baboon, 20. The rapid, panting respiration of mice, even when quite still, is familiar, and contrasts strongly with the slow breathing of a large animal such as the elephant (eight or nine times per minute). These facts maybe explained as followsThe heat-producing power of any given animal depends largely on its bulk, while its loss of heat depends to a great extent upon the surface area of its body. If of two animals of similar shape, one be ten times as long as the other, the area of the large animal (representing its loss of heat) is 100 times that of the small one, while its bulk (representing production of heat) is about 1000 times as great. Thus in order to balance its much greater relative loss of heat, the smaller animal must have all its vital functions, circulation, respiration, &c., carried on much more rapidly. 212 RESPIRATION. [chap. v. Force of Inspiratory and Expiratory Muscles.-The force with which the inspiratory muscles are capable of acting is greatest in individuals of the height of from five feet seven inches to five feet eight inches, and will elevate a column of three inches of mercury. Above this height, the force decreases as the stature increases; so that the average of men of six feet can elevate only about two and a half inches of mercury. The force manifested in the strongest expiratory acts is, on the average, one-third greater than that exercised in inspiration. But this difference is in great measure due to the power exerted by the elastic reaction of the walls of the chest; and it is also much influenced by the dispro- portionate strength which the expiratory muscles attain, from their being called into use for other purposes than that of simple expiration. The force of the inspiratory act is, therefore, better adapted than that of the expiratory for testing the muscular strength of the body. (John Hutchinson.) The instrument used by Hutchinson to gauge the inspiratory and expira tory power was a mercurial manometer, to which was attached a tube fitting the nostrils, and through which the inspiratory or expiratory effort was made. The following table represents the results of numerous experiments : Power of Power of Inspiratory Muscles. Expiratory Muscles. i'5 in. . . Weak . 2'0 in. 20 „ . Ordinary . • • 2'5 „ 2'5 „ • . Strong . • 3'5 >, 3'5 » . Very strong • • 4'5 v 4'5 „ • . Remarkable . • 5'8 „ 5'5 „ . Very remarkable • • 7'0 6-o ,, . Extraordinary ■ 8-5 „ 7'° „ . Very extraordinary . . IOO The greater part of the force exerted in deep inspiration is employed in overcoming the resistance offered by the elasticity of the walls of the chest and of the lungs. The amount of this elastic resistance was estimated by observing the elevation of a column of mercury raised by the return of air forced, after death, into the lungs, in quantity equal to the known capacity of respiration during life ; and Hutchinson calculated, according to the well-known hydro- static law of equality of pressures (as shown in the Bramah press), that the total force to be overcome by the muscles in the act of inspiring 200 cubic inches of air is more than 450 lbs. ; The elastic force overcome in ordinary inspiration is, according to the same authority, equal to about 170 lbs. Douglas Powell has shown that within the limits of ordinary CHAP. V.] RESPIRATORY CHANGES IN AIR. 213 tranquil respiration, the elastic resilience of the walls of the chest favours inspiration; and that it is only in deep inspiration that the ribs and rib-cartilages offer an opposing force to their dilata- tion. In other words, the elastic resilience of the lungs, at the end of an act of ordinary breathing, has drawn the chest-walls within the limits of their normal degree of expansion. Under all circumstances, of course, the elastic tissue of the lungs opposes inspiration, and favours expiration. Functions of Muscular Tissue of Lungs.-It is possible that the contractile power which the bronchial tubes and air- vesicles possess, by means of their muscular fibres may (1) assist in expiration; but it is more likely that its chief purpose is (2) to regulate and adapt, in some measure, the quantity of air admitted to the lungs, and to each part of them, according to the supply of blood ; (3) the muscular tissue contracts upon and gradually expels collections of mucus, which may have accumulated within the tubes, and which cannot be ejected by forced expiratory efforts, owing to collapse or other morbid conditions of the portion of lung con- nected with the obstructed tubes (Gairdner). (4) Apart from any of the before-mentioned functions, the presence of muscular fibre in the walls of a hollow viscus, such as a lung, is only what might be expected from analogy with other organs. Subject as the lungs are to such great variation in size it might be antici pated that the elastic tissue, which enters so largely into their composition, would be supplemented by the presence of much muscular fibre also. Respiratory Changes in the Air and. in the Blood. A. In the Air. Composition of the Atmosphere.-The atmosphere we breathe has, in every situation in which it has been examined in its natural state, a nearly uniform composition. It is a mixture of oxygen, nitrogen, carbonic acid, and watery vapour, with, commonly, traces of other gases, as ammonia, sulphuretted hydrogen, &c. Of every 100 volumes of pure atmospheric air, 79 volumes (on an average) consist of nitrogen, the remaining 21 of oxygen. By weight the proportion is N. 75, O. 25. The proportion of carbonic acid is ex- tremely small; 10,000 volumes of atmospheric air contain only about 4 or 5 of carbonic acid. The quantity of watery vapour varies greatly according to the 214 RESPIRATION. [chap. v. temperature and other circumstances, but the atmosphere is never without some. In this country, the average quantity of watery vapour in the atmosphere is 1-40 per cent. Composition of Air which has been breathed.-The changes effected by respiration in the atmospheric air are : 1, an increase of temperature; 2, an increase in the quantity of carbonic acid ; 3, a diminution in the quantity of oxygen; 4, a diminution of volume ; 5, an increase in the amount of watery vapour; 6, the addition of a minute amount of organic matter and of free ammonia. 1. The expired air, heated by its contact with the interior of the lungs, is (at least in most climates) hotter than the inspired air. Its temperature varies between 970 and 99'5° F. (36°- 37'5° C.), the lower temperature being observed when the air has remained but a short time in the lungs. Whatever may be the temperature of the air when inhaled, it nearly acquires that of the blood before it is expelled from the chest. 2. The Carbonic Acid is always increased; but the quantity exhaled in a given time is subject to change from various circumstances. From every volume of air inspired, about 4-8 per cent, of oxygen is abstracted; while a rather smaller quantity, 4*3 of carbonic acid is added in its place : the air will contain, therefore, 434 vols. of carbonic acid in 10,000. Under ordinary circumstances, the quantity of carbonic acid exhaled into the air breathed by a healthy adult man amounts to 1346 cubic inches, or about 636 grains per hour. According to this estimate, the weight of carbon excreted from the lungs is about 173 grains per hour, or rather more than 8 ounces in twenty-four hours. These quantities must be considered approximate only, inasmuch as various circumstances, even in health, influence the amount of carbonic acid excreted, and, correlatively, the amount of oxygen absorbed. Circumstances influencing the amount of carbonic acid excreted.-The following are the chief :-Age and sex. Respiratory movements. External temperature. Season of year. Condition of respired air. Atmospheric conditions. Period of the day. Food and drink. Exercise and sleep. a. Age and Sex.-The quantity of carbonic acid exhaled into the air breathed by males, regularly increases from eight to thirty years of age ; from thirty to fifty the quantity, after remaining stationary for a while, gradually diminishes, and from fifty to extreme age it goes on diminishing, till it scarcely exceeds the quantity exhaled at ten years old. In females (in whom the quantity exhaled is always less than in males of the same age) the same regular increase in quantity goes on from the eighth year to chap, v.] CONDITIONS OF CARBONIC ACID EXCRETION. 215 the age of puberty, when the quantity abruptly ceases to increase, and remains stationary so long as they continue to menstruate. When menstruation has ceased, it soon decreases at the same rate as it does in old men. b. Respiratory Movements.-The more quickly the movements of respira- tion are performed, the smaller is the proportionate quantity of carbonic acid contained in each volume of the expired air. Although, however, the pro- portionate quantity of carbonic acid is thus diminished during frequent respiration, yet the absolute amount exhaled into the air within a given time is increased thereby, owing to the larger quantity of air which is breathed in the time. The last half of a volume of expired air contains more carbonic acid than the half first expired ; a circumstance which is explained by the one portion of air coming from the remote part of the lungs, where it has been in more immediate and prolonged contact with the blood than the other has, which comes chiefly from the larger bronchial tubes. <•. External temperature.-The observation made by Vierordt at various temperatures between 38° F. and 750 F. (3'4°-23'8° C.) show, for warm- blooded animals, that within this range, every rise equal to io° F. causes a diminution of about two cubic inches in the quantity of carbonic acid exhaled per minute. d. Season of the Year.-The season of the year, independently of tempe- rature, materially influences the respiratory phenomena; spring being the season of the greatest, and autumn of the least activity of the respiratory and other functions. (Edward Smith.) e. Purity of the Respired Air.-The average quantity of carbonic acid given out by the lungs constitutes about 4'3 per cent, of the expired air ; but if the air which is breathed be previously impregnated with carbonic acid (as is the case when the same air is frequently respired), then the quantity of carbonic acid exhaled becomes much less. f. Hygrometric State of Atmosphere.-The amount of carbonic acid exhaled is considerably influenced by the degree of moisture of the atmo- sphere, much more being given off when the air is moist than when it is dry. (Lehmann.) g. Period of the Day.-During the day-time more carbonic acid is exhaled than corresponds to the oxygen absorbed ; while, on the other hand, at night very much more oxygen is absorbed than is exhaled in carbonic acid. There is, thus, a reserve fund of oxygen absorbed by night to meet the requirements of the day. If the total quantity of carbonic acid exhaled in 24 hours be represented by 100, 52 parts are exhaled during the day, and 48 at night. While, similarly, 33 parts of the oxygen are absorbed during the day, and the remaining 67 by night. (Pettenkofer and Voit.) h. Food and Drink.-By the use of food the quantity is increased, whilst by fasting it is diminished ; it is greater when animals are fed on farinaceous food than when fed on meat. The effects produced by spirituous drinks depend much on the kind of drink taken. Pure alcohol tends rather to increase than to lessen respiratory changes, and the amount therefore of carbonic acid expired ; rum, ale, and porter, also sherry, have very similar effects. On the other hand, brandy, whisky, and gin, particularly the latter, almost always lessened the respiratory changes, and consequently the amount of carbonic acid exhaled. (Edward Smith.) 2l6 RESPIRATION. [chap. v. i. Exercise.-Bodily exercise, in moderation, increases the quantity to about one-third more than it is during rest: and for about an hour after exercise the volume of the air expired in the minute is increased about 118 cubic inches : and the quantity of carbonic acid about 7'8 cubic inches per minute. Violent exercise, such as full labour on the treadwheel, still further increases the amount of the acid exhaled. (Edward Smith.) A larger quantity is exhaled when the barometer is low than when it is high. 3. The oxygen is diminished, and its diminution is generally proportionate to the increase of the carbonic acid. For every volume of carbonic acid exhaled into the air, 1'17421 volumes of oxygen are absorbed from it, and 1346 cubic inches, or 636 grains being exhaled in the hour the quantity of oxygen absorbed in the same time is 1584 cubic inches or 542 grains. According to this estimate, there is more oxygen absorbed than is exhaled with carbon to form carbonic acid. 4. The volume of air expired in a given time is less than that of the air inspired (allowance being made for the expansion in being heated), and that the loss is due to a portion of oxygen absorbed and not returned in the exhaled carbonic acid, all observers agree, though as to the actual quantity of oxygen so absorbed, they differ even widely. The amount of oxygen absorbed is on an average 4'8 per cent, so that the expired air contains 16'2 volumes per cent, of that gas. The quantity of oxygen that does not combine with the carbon given off in carbonic acid from the lungs is probably disposed of in forming some of the carbonic acid and water given off from the skin, and incombininig with sulphur and phosphorus to form part of the acids of the sulphates and phosphates excreted in the urine, and probably also, with the nitrogen of the decomposing nitrogenous tissues. The quantity of oxygen in the atmosphere surrounding animals, appears to have very little influence on the amount of this gas absorbed by them, for the quantity consumed is not greater even though an excess of oxygen be added to the atmosphere experi- mented with. It has often been discussed whether Nitrogen is absorbed by or exhaled from the lungs during respiration. At present, all that can be said on the subject is that, under most circumstances, animals appear to expire a very small quantity above that which exists in the inspired air. During prolonged fasting, on the con- trary, a small quantity appears to be absorbed. 5. The watery vapour is increased.-The quantity emitted is, as CHAP. V.] CHANGES IN THE AIR. 217 a general rule, sufficient to saturate the expired air, or very nearly so. Its absolute amount is, therefore, influenced by the following circumstances, (i), by the quantity of air respired ; for the greater this is, the greater also will be the quantity of moisture exhaled ; (2), by the quantity of watery vapour contained in the air previous to its being inspired ; because the greater this is, the less will be the amount required to complete the saturation of the air ; (3), by the temperature of the expired air; for the higher this is, the greater will be the quantity of watery vapour required to saturate the air; (4), by the length of time which each volume of inspired air is allowed to remain in the lungs; for although, during ordinary respiration, the expired air is always saturated with watery vapour, yet when respiration is performed very rapidly the air has scarcely time to be raised to the highest temperature, or be fully charged with moisture ere it is expelled. The quantity of water exhaled from the lungs in twenty-four hours ranges (according to the various modifying circumstances already mentioned) from about 6 to 27 ounces, the ordinary quantity being about 9 or 10 ounces. Some of this is probably formed by the chemical combination of oxygen with hydrogen in the system; but the far larger proportion of it is water which has been absorbed, as such, into the blood from the alimentary canal, and which is exhaled from the surface of the air-passages and cells, as it is from the free surfaces of all moist animal membranes, particularly at the high temperature of warm-blooded animals. 6. A small quantity of ammonia is added to the ordinary constituents of expired air. It seems probable, however, both from the fact that this substance cannot be always detected, and from its minute amount when present, that the whole of it may be derived from decomposing particles of food left in the mouth, or from carious teeth or the like; and that it is, therefore, only an accidental constituent of expired air. 7. The quantity of organic matter in the breath is increased and is about 3 grains in about twenty-four hours. (Ransome.) Method of Experiment.-The following represents the kind of experiment by which the foregoing facts regarding the excretion of carbonic acid, water, and organic matter, have been established. A bird or mouse is placed in a large bottle, through the stopper of which two tubes pass, one to supply fresh air, and the other to carry oif that which has been expired. Before entering the bottle, the air is made to bubble through a strong solution of caustic potash, which absorbs the carbonic acid, and then through lime-water, which by remaining limpid, proves the absence 218 RESPIRATION. [chap. v. of carbonic acid. The air which has been breathed by the animal is made to bubble through lime-water, which at once becomes turbid and soon quite milky from the precipitation of calcium carbonate ; and it finally passes through strong sulphuric acid, which, by turning brown, indicates the pre- sence of organic matter. The watery vapour in the expired air will con dense inside the bottle if the surface be kept cool. By means of an apparatus sufficiently large and well-constructed, experi- ments of the kind have been made extensively on man. Methods by which the Respiratory Changes in the Air are effected. The method by which fresh air is inhaled and expelled from the lungs has been explained. It remains to consider how it is that the blood absorbs oxygen from, and gives up carbonic acid to, the air of the alveoli. In the first place, it must be remembered that the tidal air only amounts to about 25-30 cubic inches at each inspiration, and that this is of course insufficient to till the lungs, but it mixes with the stationary air by diffusion, and so supplies to it new oxygen. The amount of oxygen in expired air, which may be taken as the average composition of the mixed air in the lungs, is about 16 to 17 per cent. ; in the pulmonary alveoli it may be rather less than this. From this air the venous blood has to take up oxygen in the proportion of 8 to 12 vols. in every hundred volumes of blood, as the difference between the amount of oxygen in arterial and venous blood is no less than that. It seems therefore somewhat difficult to understand how this can be accomplished at the low oxygen tension of the pulmonary air. But as was pointed out in a previous Chapter (IV.), the oxygen is not simply dissolved in the blood, but is to a great extent chemi- cally combined with the haemoglobin of the red corpuscles; and when a fluid contains a body which enters into loose chemical combination in this way with a gas, the tension of the gas in the fluid is not directly proportional to the total quantity of the gas taken up by the fluid, but to the excess above the total quantity which the substance dissolved in the fluid is capable of taking up (a known quantity in the case of haemoglobin, viz., i'59 cm. for one grm. haemoglobin). On the other hand, if the substance be not saturated, i.e., if it be not combined with as much of the gas as it is capable of taking up, further combination leads to no increase of its tension. However, there is a point at which the haemoglobin gives up its oxygen when it is exposed to a low partial pressure of oxygen, and there is also a point at which CHAP. V.] CHANGES IN THE BLOOD. 219 it neither takes up nor gives out oxygen; in the case of arterial blood of the dog, this is found to be when the oxygen tension of the atmosphere is equal to 3'9 per cent, (or 29'6 mm. of mercury),, which is equivalent to saying that the oxygen tension of arterial blood is 3'9 per cent. ; venous blood, in a similar manner, has been found to have an oxygen tension of 2'8 per cent. At a. higher temperature, the tension is raised, as there is a greater tendency at a high temperature for the chemical compound to. undergo dissociation. It is therefore easy to see that the oxygen tension of the air of the pulmonary alveoli is quite sufficient, even supposing it much less than that of the expired air, to enable the- venous blood to take up oxygen, and what is more, it will take it up until the haemoglobin is very nearly saturated with the gas. As regards the elimination of carbonic acid from the blood* there is evidence to show that it is given up by a process of simple diffusion, the only condition necessary for the process being that the tension of the carbonic acid of the air in the pulmonary alveoli should be less than the tension of the carbonic acid in venous, blood. The carbonic acid tension of the alveolar air probably does not exceed in the dog 3 or 4 pei' cent., while that of the- venous blood is 5'4 per cent., or equal to 41 mm. of mercury. B. In the Blood. Circulation of Blood in the Respiratory Organs.-To be exposed to the air thus alternately moved into and out of the air cells and minute bronchial tubes, the blood is propelled from the right ventricle through the pulmonary capillaries in steady streams, and slowly enough to permit every minute portion of it to be for a few seconds exposed to the air, with only the thin walls of the- capillary vessels and the air-cells intervening. The pulmonary circulation is of the simplest kind : for the pulmonary artery branches regularly; its successive branches run in straight lines, and do not anastomose : the capillary plexus is uniformly spread over the air-cells and intercellular passages ; and the veins derived from it proceed in a course as simple and uniform as that of the arteries, their branches converging but not anastomosing. The veins have no valves, or only small imperfect ones prolonged from their angles of junction, and incapable of closing the orifice of either of the veins between which they are placed. The pul- monary circulation also is unaffected by changes of atmospheric pressure, and is not exposed to the influence of the pressure of 220 RESPIRATION. [chap. v. muscles: the force by which it is accomplished, and the course of the blood are alike simple. Changes in the Blood.-The most obvious change which the blood of the pulmonary artery undergoes in its passage through the lungs is is£, that of colour, the dark crimson of venous blood being exchanged for the bright scarlet of arterial blood; 2nd, and in connection with the preceding change, it gains oxygen; yrd, it loses carbonic acid; Afh, it becomes slightly cooler', $th, it coagulates sooner and more firmly, apparently con- taining more fibrin. The oxygen absorbed into the blood from the atmospheric air in the lungs is combined chemically with the haemoglobin of the red blood-corpuscles. In this condition it is •carried in the arterial blood to the various parts of the body, and brought into near relation or contact with the tissues. In these tissues, and in the blood which circulates in them, a certain portion of the oxygen, which the arterial blood contains, dis- appears, and a proportionate quantity of carbonic acid and water is formed. The venous blood, containing the new-formed carbonic acid returns to the lungs, where a portion of the carbonic acid is exhaled, and a fresh supply of oxygen is taken in. It will be well here, perhaps, to explain some respiratory acts, which appear at first sight somewhat complicated, but cease to be so when the mechanism by which they are performed is clearly understood. The accompanying diagram (fig. 160) shows that the cavity of the chest is separated from that of the abdomen by the diaphragm, which, when acting, will lessen its curve, and thus descending, will push downwards and forwards the abdominal viscera; while the abdominal muscles have the opposite eftect, and in acting will push the viscera upwards and backwards, and with them the diaphragm, supposing its ascent to be not from any cause interfered with. From the same diagram it will be seen that the lungs communicate with the exterior of the body through the glottis, and further on through the mouth and nostrils-through either of them separately, or through both at the same time, according to the position of the soft palate. The stomach communicates with the exterior of the body through the oesophagus, pharynx, and mouth ; while below the rectum opens at the anus, and the bladder through the urethra. All these Mechanism, of Various Respiratory Actions. CHAP. V.] SPECIAL RESPIRATORY ACTS. 221 openings, through which the hollow viscera communicate with the exterior of the body, are guarded by muscles, called which can act independently of each other. The position of the latter is indicated in the diagram. Sighing.-In sighing there is a rather prolonged inspiration ; the ail- almost noiselessly passing in through the glottis, and by Fig. 160. the elastic recoil of the lungs and chest-walls, and probably also of the abdominal walls, being rather suddenly expelled again. Now, in the first, or inspiratory part of this act, the descent of the diaphragm presses the abdominal viscera downwards, and of course this pressure tends to evacuate the contents of such as communicate with the exterior of the body. Inasmuch, however, as their various openings are guarded by sphincter muscles, in a state of constant tonic contraction, there is no escape of their 222 11ESP1KATI0N. [chap. V. contents, and air simply enters the. lungs. In the second, or ■expiratory part of the act of sighing, there is also pressure made on the abdominal viscera in the opposite direction, by the elastic or muscular recoil of the abdominal walls; but the pressure is relieved by the escape of air through the open glottis, and the relaxed diaphragm is pushed up again into its original position. The sphincters of the stomach, rectum, and bladder, act in the same manner as before. Hiccough resembles sighing in that it is an inspiratory act; but the inspiration is sudden instead of gradual, in consequence of the diaphragm acting suddenly and spasmodically ; and the air, therefore suddenly rushing through the unprepared rima glottidis, causes vibration of the vocal cords, and the peculiar sound. Coughing.-In the act of coughing, there is most often first of all a deep inspiration, followed by an expiration ; but the latter, instead of being easy and uninterrupted, as in normal breathing, is obstructed, in consequence of the glottis being momentarily closed by the approximation of the vocal cords. The abdominal muscles, then strongly acting, push up the viscera against the diaphragm, and thus make pressure on the air in the lungs until its tension is sufficient to noisily burst open the vocal cords which oppose its outward passage. In this way considerable force is exercised, and mucus or any other matter that may need expulsion from the air-passages is quickly and sharply expelled by the outstreaming current of air. It will be evident on reference to the diagram (fig. 160), that pressure exercised by the abdominal muscles in the act of cough- ing, acts as forcibly on the abdominal viscera as on the lungs, inasmuch as the viscera form the medium by which the upward pressure on the diaphragm is made, and there is of necessity quite as great a tendency to the expulsion of their contents as of the air in the lungs. The instinctive, and if necessary, voluntarily increased contraction of the sphincters, however, prevents any ■escape at the openings guarded by them, and the pressure is •effective at one part only, at the rima glottidis. Sneezing.-The same remarks that apply to coughing, are almost exactly applicable to the act of sneezing; but in this instance the blast of air, on escaping from the lungs, is directed, by an instinctive contraction of the pillars of the fauces and descent of the soft palate, chiefly through the nose, and any ■offending matter is thence expelled. CHAP. V.] SPECIAL RESPIRATORY ACTS. 223 Speaking.-In speaking, there is a voluntary expulsion of air through the glottis by means of the expiratory muscles. The vocal cords are put, by the muscles of the larynx, in a proper position and state of tension for vibrating as the air passes over them, and thus sound is produced. The sound is moulded into articulate speech by the tongue, teeth, lips, etc.-the vocal cords producing the sound only, and having nothing to do with articulation. Singing.-Singing resembles speaking in the manner of its production; the laryngeal muscles, by variously altering the position and degree of tension of the vocal cords, producing the different notes. Words used in the act of singing are of course framed, as in speaking, by the tongue, teeth, lips, &c. Sniffing.-Sniffing is produced by a rapidly repeated but incomplete action of the diaphragm and other inspiratory muscles. The mouth is closed, and the whole stream of air is made to enter the air-passages through the nostrils. The alee nasi are, commonly, at the same time, instinctively dilated. Sobbing.-Sobbing consists of a series of convulsive inspira- tions, at the moment of which the glottis is usually more or less closed. ■Laughing.-Laughing is made up of a series of short and rapid expirations. Yawning.-Yawning is an act of inspiration, but is unlike most of the preceding actions, as it is always more or less in- voluntary. It is attended by a stretching of various muscles about the palate and lower jaw, which is probably analogous to the stretching of the muscles of the limbs in which a weary man finds relief, as a voluntary act, when they have been some time out of action. The involuntary and reflex character of yawning probably depends on the fact that the muscles concerned are themselves at all times more or less used involuntarily, and require, therefore, something beyond the exercise of the will to set them in action. For the same reason, yawning, like sneezing, cannot be well performed voluntarily. Sucking.-Sucking is not properly a respiratory act, but it may be most conveniently considered in this place. It is caused chiefly by the depressor muscles of the os hyoides. These, by drawing downwards and backwards the tongue and floor of the mouth, produce a partial vacuum in the latter: and the weight of the atmosphere then acting on all sides tends to produce equili- 224 RESPIRATION. [CHAP. V. brium on the inside and outside of the mouth as best it may. The communication between the mouth and pharynx is completely shut off by the contraction of the pillars of the soft palate and descent of the latter so as to touch the back of the tongue ; and the equilibrium, therefore, can be restored only by the entrance of something through the mouth. The action, indeed, of the tongue and floor of the mouth in sucking may be compared to that of the piston in a syringe, and the muscles which pull down the os hyoides and tongue, to the power which draws the handle. Like all other functions of the body, the discharge of which is necessary to life, respiration is essentially an involuntary act. Unless this were the case, life would be in constant danger, and would cease on the loss of consciousness for a few moments, as in sleep. It is, however, also necessary that respiration should be to some extent under the control of the will. For were it not so, it would be impossible to perform those voluntary respiratory acts which have been just discussed, such as speaking, singing, and the like. The respiratory movements and their rhythm, so far as they are involuntary and independent of consciousness, as they are on all ordinary occasions, are under the governance of a nerve-centre in the medulla oblongata which corresponds in position with the origin of the pneumogastric nerves; that is to say, the muscles concerned in the respiratory movements, are excited by stimuli which issue from this part of the nervous system, and which are conveyed by the various motor nerves supplying the muscles. These nerves are the phrenics and intercostals chiefly. On division of one phrenic, for example, the corresponding half of the diaphragm supplied by it ceases to take part in the respiratory movement, and on division of both nerves, the whole muscle ceases to act. Similarly, division of the intercostal nerves one by one produces cessation of action of the muscles supplied by them. To what extent the medullary centre acts automatically, i.e., how far the stimulus originates in it, or how far it is merely a nerve-centre for reflex action, is not certainly known. It is clear, however, that the medullary centre is bilateral or double, since the respiratory movements continue after the medulla at this point is bisected in the middle line. Influence of the Nervous System in Respiration. chap, v.] ACTION OF THE RESPIRATORY CENTRES. 225 There is considerable evidence in favour of its automatic action. Thus it has been shown that if the spinal cord be divided below the medulla, so that no afferent impulses can reach the centre from below, that the nasal and laryngeal respiration continues. The only possible course of the afferent impulses would, under such circumstances, be through the cranial nerves; and when the cord and medulla are intact the division of these nerves pro- duces no effect upon respiration, and indicates that they are not used for the transmission of afferent impulses to the medullary centre. It appears evident, therefore, that afferent stimuli are not absolutely necessary for maintaining the respiratory movements. The respiratory centre, although automatic in its action, may, however, be reflexly excited. The chief channel of this reflex influence is the vagus nerve, for when the nerve of one side is divided, respiration is slowed, and if both vagi are cut it becomes still slower. The influence of the vagus trunk upon the centre may be twofold, for if the nerve is divided below the origin of the superior laryngeal branch and the central end is stimulated, respiratory movements are increased in rapidity, and indeed follow one another so quickly if the stimuli be increased in number, that after a time cessation of respiration in inspiration takes place in consequence of a tetanus of the respiratory muscles (diaphragm). Whereas if the superior' laryngeal branch is divided, although no effect, or scarcely any, follows the mere division, on stimulation of the central end respi- ration is slowed, and after a time, if the stimulus is sufficiently increased, stops, not in inspiration as in the other case, but in expiration. Thus the vagus trunk contains fibres which are capable of slowing and fibres which are capable of accelerating- respiration. The theory that the respiratory centre in the floor of the medulla consists of two parts, one of which tends to produce inspiration and the other to produce expiration, is very plausible. The inspiratory part of the centre is complementary to the expiratory, and the two parts send out impulses alternately. If we adopt this theory, we must look upon the main trunk of the vagus as aiding the inspiratory, and upon the superior laryngeal as aiding the expiratory part of the centre, the first nerve possibly inhibiting the action of the expiratory centre, whilst it aids the in- spiratory, and the latter nerve having the very opposite effect. But inasmuch as the respiration is slowed on division of the vagi, and not quickened or manifestly affected at all on simple division of 226 RESPIRATION. [chap. v. the superior laryngeal, it must be supposed that the vagi fibres are always in action, but that the superior laryngeal fibres are not. It appears that there are, in some animals at all events, subor- dinate centres in the spinal cord which are able, under certain conditions, to discharge the function of the chief respiratory centre in the medulla. The centre in the medulla may be influenced not only by afferent impulses proceeding along the vagus and laryngeal nerves but also by impulses passing downward from the cere- brum ; by impressions made upon the nerves of the skin, or upon part of the fifth nerve distributed to the nasal mucous membrane; or upon other sensory nerves. Such afferent influences are exem- plified in the deep inspiration excited by the application of cold to the surface of the skin, and by the production of sneezing on the slightest irritation of the nasal mucous membrane. At the time of birth, the separation of the placenta, and the consequent non-oxygenation of the foetal blood, are the circumstances which immediately lead to the issue of automatic impulses from the respiratory centre in the medulla oblongata. Methods of Stimulation of Respiratory Centre.-The means by which the respiratory centre or centres are stimulated must now be considered. It is well known that the more venous the blood, the more marked are the inspiratory impulses, and that if the air is pre- vented from entering the chest, that the respiration in a short time becomes very laboured. The obstruction to the entrance of air, whether partial or complete, is followed by an abnormal rapidity of the inspiratory acts, which make up even in depth for the previous stoppage. The condition caused by the obstruction, or by any circumstance in consequence of which the oxygen of the blood is used up in an abnormally quick manner, is known as dyspnoea, and as the aeration of the blood becomes more and more interfered with, not only are the ordinary respiratory muscles employed, but also those extraordinary muscles which have been previously enumerated (p. 206). As the blood becomes more and more venous the action of the medullary centre becomes more and more active. The question arises as to what quality of the venous blood it is which causes this increased activity; whether it is its deficiency , of oxygen or its excess of carbonic acid. This question has been answered by the experiments, which show on the one CHAP. V.] EFFECTS OF VITIATED AIR. 227 hand that dyspnoea occurs when there is no obstruction to the exit of carbonic acid, as when an animal is placed in an atmosphere of nitrogen, and that it cannot therefore be due to the accumu- lation of carbonic acid ; and on the other, that if plenty of oxygen is supplied, true dyspnoea does not occur, although the carbonic acid of the blood is in excess. It is highly probable, therefore, that the respiratory centre is stimulated to action by the absence of sufficient oxygen in the blood circulating in it, and not by the presence of an excess of carbonic acid. The means by which the vagus is excited to increase the activity of the respiratory centre, appears to be that the venous blood circulating in the lungs, or the air in the pulmonary alveoli, stimulates the peripheral fibres of the nerve. If these be the stimuli it will be evident that the vagus action must help to increase the activity of the centre, when the blood in the lungs becomes more and more venous. No doubt the venous condition of the blood affects all the sensory nerves in a similar manner. It has been shown that the circulation of too little blood through the centre, as when its blood supply is cut off, greatly increases its inspiratory action. Effects of Vitiated Air.-Ventilation.-As the air expired from the lungs contains a large proportion of carbonic acid and a minute amount of organic putrescible matter, it is obvious that if the same air be breathed again and again, the proportion of carbonic acid and organic matter will constantly increase till it becomes unfit to be breathed, but long before this point is reached, uneasy sensations occur, such as headache, languor, and a sense of oppression. It is a remarkable fact, however, that the organism after a time adapts itself to such a vitiated atmosphere, and that a person soon comes to breathe, without sensible inconvenience, an atmosphere which, when he first entered it, felt intolerable. Such an adaptation, however, can only take place at the expense of a depression of all the vital functions, which must be injurious if long continued or often repeated. This power of adaptation is well illustrated by the experiments of Claude Bernard. A sparrow is placed under a bell-glass of such a size that it will live for three hours. If now at the end of the second hour (when it could have survived another hour) it be taken out and a fresh healthy sparrow introduced, the latter will perish instantly. It must be evident that provision for a constant and plentiful supply of fresh air, and the removal of that which is vitiated, is of 228 RESPIRATION. [chap. v. far greater importance than the actual cubic space per head of occupants. Not less than 2000 cubic feet per head should be allowed in sleeping apartments (barracks, hospitals, Arc.), and with this allowance the air can only be maintained at the proper standard of purity by such a system of ventilation as provides for the supply of 1500 to 2000 cubic feet of fresh air pei- head per hour. (Parkes.) The Effect of Respiration on the Circulation. The heart and great vessels being situated in the air-tight thorax, are exposed to a certain alteration of pressure when the capacity of the latter is increased; for although the expansion of the lungs during inspiration tends to counter-balance this increase of area, it never does so entirely, since part of the pressure of the air which is drawn into the chest through the trachea is expended in overcoming the elasticity of the lungs themselves. The amount thus used up increases as the lungs become more and more expanded, so that the pressure inside the thorax during inspiration, as far as the heart and great vessels are concerned, never quite equals that outside, and at the conclusion of inspiration is con- siderably less than the atmospheric pressure. It has been ascer- tained that the amount of the pressure used up in the way above described, varies from 5 or 7 mm. of mercury during the pause, and to 30 mm. of mercury when the lungs are expanded at the end of a deep inspiration, so that it will be understood that the pres- sure to which the heart and great vessels are subjected diminishes as inspiration progresses. It will be understood from the accom- panying diagram how, if there were no lungs in the chest, but if its capacity were increased, the effect of the increase would be expended in pumping blood into the heart from the veins, but even with the lungs placed as they are, during inspiration the pressure outside the heart and great vessels is diminished, and they have therefore a tendency to expand and to diminish the intra-vascular pressure. The diminution of pressure within the veins passing to the right auricle and within the right auricle itself, will draw the blood into the thorax, and so assist the circulation. This suction action is independent of the suction power of the diastole of the auricle about which we have previously spoken (p. 145). The effect of sucking more blood into the right auricle will, coeteris paribus, increase the amount passing through the right ventricle, CHAP. V.] EFFECTS OF INSPIRATION. 229 which also exerts a similar suction action, and through the lungs into the left auricle and ventricle and thus into the aorta. This all tends to increase the arterial tension. The effect of the diminished pressure upon the pulmonary vessels will also help towards the same end, t>., an increased flow through the lungs, Fig. i6x.-Diagram of an apparatv illustrating the effect of inspiration upon the heart ami great vessels within the thorax.-I, the thorax at rest; fl, during inspiration ; n, repre- sents the diaphragm when relaxed ; u', when contracted (it must be remembered that this position is a mere diagram), i.e., when the capacity of the thorax is enlarged; ii, the heart; v, the veins entering it, and a, the aorta; rZ, t.Z, the right and left lung ; t, the trachea; m, mercurial manometer in connection with the pleura. The increase in the capacity of the box representing the thorax is seen to dilate the heart as well as the lungs, and so to pump in blood through v, whereas the valve prevents reflex through a . The position of the mercury in m shows also the suction which is taking place. (Landois.) so that, as far as the heart and its veins are concerned, inspiration increases the blood pressure in the arteries. The effect of inspira- tion upon the aorta and its branches within the thorax would be, however, contrary; for as the pressure outside is diminished the vessels would tend to expand, and thus to diminish the tension of the blood within them, but inasmuch as the large arteries are 230 RESPIRATION. [chai*. V. capable of little expansion beyond their natural calibre, the dimi- nution of the arterial tension caused by this means would be in- sufficient to counteract the increase of arterial tension produced by the effect of inspiration upon the veins of the chest, and the balance of the whole action would be in favour of an increase of arterial tension during the inspiratory period. But if a tracing of the variation be taken at the same time that the respiratory movements are being recorded, it will be found that, although s]leaking generally, the arterial tension is increased during inspira- Fig. 162.- Comparison of blood-pressure curve with curve of intra-thoracic pressure. (To be read from left to right.) a is the cun-e of blood-pressure with its respiratory undula- tions, the slower beats on the descent being very marked ; b is the curve of intra- thoracic pressure obtained by connecting one limb of a manometer with the pleural cavity. Inspiration begins at i and expiration at e. The intra-thoracic pressure rises very rapidly after the cessation of the inspiratory effort, and then slowly falls as the air issues from the chest; at the beginning of the inspiratory effort the fall becomes more rapid. (M. Foster.) tion, the maximum of arterial tension does not correspond with the acme of inspiration (fig. 162). As regards the effect of expiration, the capacity of the chest is diminished, and the intra-thoracic pressure returns to the normal, which is not exactly equal to the atmospheric, pressure. The effect of this on the veins is to increase their intra-vascular pres- sure, and so to diminish the flow of blood into the left side of the heart, and with it the arterial tension, but this is almost exactly balanced by the necessary increase of arterial tension caused by the increase of the extra-vascular pressure of the aorta and large arteries, so that the arterial tension is not much affected during expiration either way. Thus, ordinary expiration does not pro- duce a distinct obstruction to the circulation, as even when the CHAI'. V.] TRAUBE-HERING'S CURVES. 231 expiration, is at an end the intra-thoracic pressure is less than the extra-thoracic. The effect of violent expiratory efforts, however, has a distinct Fig. 163.-Traube-Hering's curves. {To be read from left to right.) The curves 1, 2, 3, 4, and 5 are portions selected from one continuous tracing forming the record of a pro- longed observation, so that th„ several curves represent successive stages of the same experiment. Each curve is placed in its proper position relative to the base line, which is omitted; the blood-pressure rises in stages from 1, to 2, 3, and 4, but falls again in stage 5. Curve 1 is taken from a period when artificial respiration was being kept up, but the vagi having been divided, the pulsations on the ascent and descent of the undulations do not differ ; when artificial respiration ceased these undulations for a while disappeared, and the blood-pressure rose steadily while the heart-beats became slower. Soon, as at 2, new undulations appeared; a little later, the blood-pressure was still rising, the heart beats still slower, but the undulations still more obvious (3) ; still later (4), the pressure was still higher, but the heart-beats were quicker, and the undulations flatter, the pressure then began to fall rapidly (5), and continued to fall until some time after artificial respiration was resumed. (M. Foster.) action in preventing the current of blood through the lungs, as seen in the blueness of the face from congestion in straining ; this condition being produced by pressure on the small pulmonary vessels. . . . . . 232 RESPIRATION. [chap. v. We may summarise this mechanical effect of respiration on the blood pressure therefore, and say that inspiration aids the circula- tion and so increases the arterial tension, and that although expi- ration does not materially aid the circulation, yet under ordinary conditions neither does it obstruct it. Under extraordinary con- ditions, however, as in violent expirations, the circulation is decidedly obstructed. But we have seen that there is no exact correspond- ence between the points of extreme arterial tension and the end of inspiration, and we must look to the nervous system for an explanation of this apparently contradictory result. The effect of the nervous system in producing a rhythmical alteration of the blood pressure is two-fold. In the first place the cardio-inhibitory centre is believed to be stimulated during the fall of blood pressure, producing a slower rate of heart-beats during expiration, which will be noticed in the tracing (fig. 162). The undulations during the decline of blood-pressure being longer but fess frequent, this effect disappears when, by section of the vagi, the effect of the centre is cut off from the heart; and in the second place, the vaso-motor centre is also believed to send out rhythmical impulses, by which undulation of blood pressure is produced independently of the mechanical effects of respiration. The action of the vaso-motor centre in taking part in pro- ducing rhythmical changes of blood pressure which are called respiratory, is shown in the following way :-In an animal under the influence of urari, a record of whose blood pressure is being taken, and where artificial respiration has been stopped, and both vagi cut, the blood pressure curve rises at first almost in a straight line, but after a time new rhythmical undulations occur very like the original respiratory undulations, only somewhat larger. These are called Traube's or Traube-IIering's curves. They continue whilst the blood pressure continues to rise and only cease when the vaso-motor centre and the heart are exhausted, when the pressure speedily falls. These curves must be dependent upon the vaso-motor centre, as the mechanical effects of respiration have been eliminated by the poison and by the cessation of artifi- cial respiration, and the effect of the cardio-inhibitory centre by the division of the vagi. It may be presumed therefore that the vaso-motor centre, as well as the cardio-inhibitory, must be con- sidered to take part with the mechanical changes of inspiration and expiration in producing the so-called respiratory undulations of blood-pressure. CHAP. V.] APNCEA, DYSPNCEA, ASPHYXIA. 233 Cheyne-Stokes' breathing .-'This is a rhythmical irregularity in respira- tions which has been observed in various diseases, and is especially connected with fatty degeneration of the heart. Respirations occur in groups, at the beginning of each group the inspirations are very shallow, but each succes- sive breath is deeper than the preceding until a climax is reached, after which the inspirations become less and less deep, until they cease after a slight pause altogether. As blood which contains a normal proportion of oxygen suffi- ciently excites the respiratory centre (p. 227) to produce normal respiration, and, as the excitement and consequent respiratory muscular movements are greater (dyspnoea) in proportion to the deficiency of this gas, so an abnormally large proportion of oxygen in the blood leads to diminished breathing movements, and, if the proportion be large enough, to their temporary cessation. This condition of absence of breathing is termed Apnoea* and it can be demonstrated, in one of the lower animals, by performing artificial respiration to the extent of saturating the blood with oxygen. When, on the other hand, the respiration is stopped, by, e.g., interference with the passage of air to the lungs, or by supplying air devoid of oxygen, a condition ensues, which passes rapidly from Hyperpncea (excessive breathing) to the state of Dyspniea (difficult breathing), and afterwards to Asphyxia ; and the latter quickly ends in death. The ways by which this condition of asphyxia may be produced are very numerous. As, for example, by the prevention of the due entry of oxygen into the blood, either by direct obstruction of the trachea or other pare of the respiratory passages, or by intro- ducing instead of ordinary air a gas devoid of oxygen, or, by inter- ference with the due interchange of gases between the air and the blood. Symptoms.-The symptoms of asphyxia may be divided into three groups, which correspond with the stages of the condition which are usually recognised, these are (1), the stage of exagge- rated breathing; (2), the stage of convulsions; (3), the stage of exhaustion. Apnoea.-Dyspnoea.-Asphyxia. * This term has been, unfortunately, often applied to conditions of dyspnoea or asphyxia; but the modern application of the term, as in the text, is the more convenient. 234 RESPIRATION. [chav. V. In the first stage the patient breathes more rapidly and at the same time more deeply than usual, the inspirations at first being especially exaggerated and prolonged. The muscles of extraordi- nary inspiration are called into action and the effort to respire is laboured and painful. This is soon followed by a similar increase in the expiratory efforts, which become excessively prolonged, being aided by all the muscles of extraordinary expiration. During this stage, which lasts a varying time, from a minute upwards, according as the deprivation of oxygen is sudden or gradual, the patient's face and lips become blue, his eyes are prominent, and his expression intensely anxious. The prolonged respira- tions are accompanied by a distinctly audible sound; the muscles attached to the chest stand out as distinct cords. The stage includes the two conditions hyperpnoea and dyspnoea already spoken of. It is due to the increasingly powerful stimu- lation of the respiratory centres by the increasingly venous blood. In the second stage, which is not marked out by any distinct line of demarcation from the first, the violent expiratory efforts give way to general convulsions (in men and other warm-blooded animals at any rate), which arise from the further stimulation of the centres. The spasms of the muscles are those of the body in general, and not of the respiratory muscles only. The convulsive stage is a short one and soon passes into the third stage, of exhaustion. In it, the respirations all but cease, the spasms give way to flaccidity of the muscles, the patient is insensible, the conjunctivae are insensitive and the pupils are widely dilated. Every now and then a prolonged sighing inspiration takes place, at longer and longer intervals until they cease altogether, and the patient dies. During this stage the pulse is scarcely to be felt, but the heart may beat for some seconds after respirations have quite ceased. The condition is due to the gradual paralysis of the respiratory centre by the prolonged action of the increasingly venous blood. As with the first stage, the duration of the second and third stages depends upon the manner of the deprivation of oxygen, whether sudden or gradual. The convulsive stage is short, lasting, it may be, only one minute. The third stage may last three minutes and upwards. The circulatory conditions which accompany these symptoms are- CHAP. V.] CONDITION IN ASPHYXIA. 235 (1) More or less interference with the passage of the blood through the pulmonary blood-vessels. (2) Accumulation of blood in the right side of the heart and in the systemic veins. (3) Circulation of impure (non-aerated) blood in all parts of the body. Cause of death.-The causes of these conditions and the manner in which they act, so as to be incompatible with life, may be here briefly considered. (1) The obstruction to the passage of blood through the lungs is not very great; and such as there is occurs chiefly in the later stages of asphyxia, when, by the violent and convulsive action of the expiratory muscles, pressure is indirectly made upon the lungs, and the circulation through them is proportionately interfered with. (2) Accumulation of blood, with consequent distension of the right side of the heart and of the systemic veins, is the direct result, at least in part, of the obstruction to the pulmonary circu- lation just referred to. Other causes, however, are in operation. (a) The vaso-motor centres stimulated by blood deficient in oxygen, causes contraction of all the small arteries with increase of arterial tension, and as an immediate consequence the filling of the systemic veins. (6) The increased arterial tension is followed by inhibition of the action of the heart, and, the heart, contracting less frequently, and also gradually enfeebled by deficient supply of oxygen, becomes over-distended with blood which it cannot expel. At this stage the left as well as the right cavities are over- distended. The ill effects of thes° conditions are to be looked for partly in the heart, the muscular fibres of which, like those of the urinary bladder or any other hollow muscular organ, may be paralysed by over-stretching; and partly in the venous congestion, and conse- quent interference with the function of the higher nerve-centres, especially the medulla oblongata. (3) The passage of non-aerated blood through the lungs and its distribution over the body are events incompatible with life in one of the higher animals, for more than a few minutes; the rapidity with which death ensues in asphyxia being due, more particularly, to the effect of non-oxygenised blood on the medulla oblongata, and, through the coronary arteries, on the muscular substance of the heart. The excitability of both nervous and 236 RESPIRATION. [chap. V. muscular tissue is dependent on a constant and large supply of oxygen, and, when this is interfered with, excitability is rapidly lost. The diminution of oxygen has a more direct influence in the production of the usual symptoms of asphyxia than the increased amount of carbonic acid. Indeed, the fatal effect of a gradual accumulation of the latter in the blood, if a due supply of oxygen is maintained, resembles rather that of a narcotic poison, and not of asphyxia. In some experiments performed by a committee appointed by the Medico- Chirurgical Society to investigate the subject of Suspended Animation, it was found that, in the dog, during simple asphyxia, i.e., by simple privation of air, as by plugging the trachea, the average duration of the respiratory movements after the animal had been deprived of air, was 4 minutes 5 seconds ; the extremes being 3 minutes 30 seconds, and 4 minutes 40 seconds. The average duration of the heart's action, on the other hand, was 7 minutes 11 seconds; the extremes being 6 minutes 40 seconds, and 7 minutes 45 seconds. It would seem, therefore, that on an average, the heart's action continues for 3 minutes 15 seconds after the animal has ceased to make respiratory efforts. A very similar relation was observed in the rabbit. Recovery never took place after the heart's action had ceased. The results obtained by the committee on the subject of drowning were very remarkable, especially in this respect, that whereas an animal may recover, after simple deprivation of air for nearly four minutes, yet, after submersion in water for i| minute, recovery seems to be impossible. This remarkable difference was found to be due, not to the mere submersion, nor directly to the struggles of the animal, nor to depression of temperature, but to the two facts, that in drowning, a free passage is allowed to air out of the lungs, and a free entrance of water into them. It is probably to the entrance of water into the lungs that the speedy death in drowning is mainly due. The results of post-mortem examination strongly support this view. On examining the lungs of animals deprived of air by plugging the trachea, they were found simply congested ; but in the animals drowned, not only was the congestion much more intense, accompanied with ecchymosed points on the surface and in the substance of the lung, but the air tubes were com- pletely choked up with a sanious foam, consisting of blood, water, and mucus, churned up with the air in the lungs by the respiratory efforts of the animal. The lung-substance, too, appeared to be saturated and sodden with water, which, stained slightly with blood, poured out at any point where a section was made. The lung thus sodden with water was heavy (though it floated), doughy, pitted on pressure, and was incapable of collapsing. It is not difficult to understand how, by such infarction of the tubes, air is de- barred from reaching the pulmonary cells ; indeed the inability of the lungs to collapse on opening the chest is a proof of the obstruction which the froth occupying the air-tubes offers to the transit of air. We must carefully distinguish the asphyxiating effect of an insufficient supply of oxygen from the directly poisonous action of such gases as carbonic oxide, which is contained to a considerable CHAP. VI.] FOODS AND DIET. 237 amount in common coal-gas. The fatal effects often produced by this gas (as in accidents from burning charcoal stoves in small, close rooms), are due to its entering into combination with the haemoglobin of the blood-corpuscles (p. 97), and thus expelling the oxygen. CHAPTER VI. FOODS AND DIET. Tn order that life of the individual may be maintained it is necessary that his body should be supplied with food in proper quality and quantity. The food taken in by the animal body is used for the purpose of replacing the waste of the tissues. In order to arrive, there- fore, at a reasonable estimation of the proper diet required in the twenty-four hours, it is essential that we should know the amount and composition of the excreta daily eliminated from the body. Careful analysis of the excreta show that they are made up chiefly of the chemical elements, carbon, hydrogen, oxygen, and nitrogen, but that they also contain to a less extent, sulphur, phosphorus, chlorine, potassium, sodium, and certain other of the elements. Since this is the case it must be evident that, to balance this waste, foods must be supplied containing all these elements to a certain degree, and some of them, viz., those which take a principal part in forming the excreta, in large amount. Of the excreta we have seen in the last Chapter that carbonic acid and ammonia, which are made up of the elements, carbon, oxygen, nitrogen, hydrogen, are given off from the lungs. By the excretion of the kidneys-the urine-many elements are elimi- nated from the blood, especially nitrogen, hydrogen, and oxygen. In the sweat, the elements chiefly represented are carbon, hydrogen, and oxygen, and also in the faeces. By all the excre- tions large quantities of water are got rid of daily, but chiefly by the urine. The relations between the amounts of the chief elements con- tained in these various excreta in twenty-four hours may be thus summarised : 238 FOOD. [chap. vr. - Water. C. H. N. 0. By the lungs ... 330 248'8 65115 By the skin ... 660 2'6 7-2 By the urine ... 1700 9'8 3'3 15'8 11 -i By the feces ... 128 20- 3' 3' I2- Grammes ... 2818 28l -2 6'3 18-8 681-41 To this should be added 296' grammes water, which are pro- duced by the union of hydrogen and oxygen in the body during the process of oxydation (t.f., 32'89 hydrogen and 263'ri oxygen). There are twenty-six grammes of salts got rid of by the urine and six by the faeces. The quantity of carbon daily lost from the body amounts to about 281'2 grammes or nearly 4,500 grains, and of nitrogen 18'8 grammes or nearly 300 grains; and if a man could be fed by these elements, as such, the problem would be a very simple one; a corresponding weight of charcoal, and, allowing for the oxygen in it, of atmospheric air, would be all that is necessary. But an animal can live only upon these elements when they are arranged in a particular manner with others, in the form of an organic compound, as albumen, starch, and the like; and the relative proportion of carbon to nitrogen in either of these com- pounds alone, is, by no means, the proportion required in the diet of man. Thus, in albumen, the proportion of carbon to nitrogen is only as 3'5 to 1. If, therefore, a man took into his body, as food, sufficient albumen to supply him with the needful amount of carbon, he would receive more than four times as much nitrogen as he wanted; and if he took only sufficient to supply him with nitrogen, he would be starved for want of carbon. It is plain, therefore, that he should take with the albuminous part of his food, which contains so large a relative amount of nitrogen in proportion to the carbon he needs, substances in which the nitrogen exists in much smaller quantities relatively to the carbon. It is therefore evident that the diet must consist of several substances, not of one alone, and we must therefore turn to the available food-stuffs. For the sake of convenience they may be classified as under: CHAP. VI.] NITROGENOUS FOODS. 239 A. ORGANIC. I. Nitrogenous, consisting of Proteids, e.g., albumen, casein, syntonin, gluten, legumin and their allies; and Gelatins, which include gela- tin, elastin, and chondrin. All of these contain carbon, hydrogen, oxygen, and nitrogen and some in addition, P. and S. II. Non-Nitrogenous, comprising : (i.) Amyloid or saccharine bodies, chemically known as carbo-hydrates, since they contain carbon, hydrogen, and oxygen, with the last two elements in the proportion to form water, i.e., H„n On. To this class belong starch and sugar. (2.) Oils and fats.-These contain carbon, hydrogen, and oxygen, but the oxygen is less in amount than in the amyloids and saccharine bodies. B. INORGANIC. I. Mineral and saline matter. II. Water. To supply the loss of nitrogen and carbon, it is found by expe- rience that it is necessary to combine substances which contain a large amount of nitrogen with others in which carbon is in considerable amount; and although, without doubt, if it were possible to relish and digest one or other of the above-mentioned proteids when combined with a due quantity of an amyloid to supply the carbon, such a diet, together with salt and water, ought to support life; yet we find that for the purposes of ordinary life this system does not answer, and instead of confining our nitro- genous foods to one variety of substance we obtain it in a large number of allied substances, for example, in flesh, of bird, beast, or fish; in eggs; in milk; and in vegetables. And, again, we are not content with one kind of material to supply the carbon neces- sary for maintaining life, but seek more, in bread, in fats, in vege- tables, in fruits. Again, the fluid diet is seldom supplied in the form of pure water, but in beer, in wines, in tea and coffee, as well as in fruits and succulent vegetables. Man requires that his food should be cooked. Very few organic substances can be properly digested without previous exposure to heat and to other manipulations which constitute the process of cooking. A.-Foods containing nitrogenous principles chiefly. I.-Flesh of Animals, of the ox (beef, ■seal), sheep (mutton, lamb), pig (pork, bacon, ham). Of these, beef is richest in nitrogenous matters, containing about 240 FOOD. [chap, vl 20 per cent., whereas mutton contains about 18 per cent., veal, 16'5, and pork, 10; the flesh is also firmer, more satisfying, and is supposed to be more strengthening than mutton, whereas the latter is more digestible. The flesh of young animals, such as lamb and veal, is less digestible and less nutritious. Pork is comparatively indigestible and contains a large amount of fat. Flesh contains:-(1) Nitrogenous bodies: myosin, serum-albu- min, gelatin (from the interstitial fibrous connective tissue) ; elastin (from the elastic tissue), as well as haemoglobin. (2) Fatty matters, including lecithin and cholesterin. (3) Extractive matters, some ot which are agreeable to the palate, e.g., osmazome, and others, which are weakly stimulating, e.g., kreatin. Besides, there are sarcolactic and inositic acids, taurin, xanthin, and others. (4) Salts, chiefly of potassium, calcium, and magnesium. (5) Water, the amount of which varies from 15 per cent, in dried bacon to 39 in pork, 51 to 53 in fat beef and mutton, to 72 per cent, in lean beef and mutton. (6) A certain amount of carbo-hydrate material is found in the flesh of some animals, in the form of inosite, dextrin, grape sugar, and (in young animals) glycogen. Table of Per-centage Composition of Beef, Mutton, Pork, and Veal.-(Letheby.) Water. Albumen. Fats. Salts. Beef.-Lean . 72 193 3-6 5'i „ Fat • • 5i 148 29-8 4'4 Mutton.-Lean 72 18'3 4'9 4'8 „ Fat . • • 53 124 31-1 3'5 Veal 63 15'8 47 Pori'.-Fat • • 39 9'8 48'9 23 Together with the flesh of the above-mentioned animals, that of the deer, hare, rabbit, and birds, constituting venison, game, and poultry, should be added as taking part in the supply of nitro- genous substances, and also fish-salmon, eels, Arc., and shell-fish, e.g., lobster, crab, mussels, oysters, shrimps, scollops, cockles, &c. Table of Per-centage Composition of Poultry and Fish.- (Letheby.) Water. Albumen. Fats. Salts. Poultry. 74 21 3-8 1'2 (Singularly devoid of fat, and is therefore generally eaten with bacon or pork.) CHAP. VI.] COMPOSITION OF MILK. 241 Water. Albumen. Fats. Salts. White Fish • 78 181 2'9 r Salmon . 77 i6'i 5'5 i'4 Eels (very rich in fat) 75 9'9 13-8 i'3 Oysters .... 7574 1172 2'42 273 (7'39 consist of non-nitrogenous matter and loss.) (Payen.) Even now the list of fleshy foods is not complete, as the flesh of nearly all animals has been occasionally eaten, and we may presume that except for difference of flavour, &c., the average composition is nearly the same in every case. II. Milk.- Is intended as the entire food of young animals, and as such contains, when pure, all the elements of a typical diet. (1) Albuminous substances in the form of casein, and serum- albumin. (2) Fats in the cream. (3) Carbohydrates in the form of lactose or milk sugar. (4) Salts, chiefly calcium ))hosphate; and (5) Water. From it we obtain (a) cheese, which is the casein precipitated with more or less of fat according as the cheese is made of skim milk (skim cheese), of fresh milk with its cream (Cheddar and Cheshire), or of fresh milk plus cream (Stilton and double Gloucester). The precipitated casein is allowed to ripen, by which process some of the albumen is split up, with formation of fat. (3) Cream, consists of the fatty globules incased in casein, and which being of low specific gravity float to the surface, (y) Butter, or the fatty matter deprived of its casein envelope by the process of churning. (8) Butter-milk, or the fluid obtained from cream after butter has been formed; very rich therefore in nitrogen, (e) Whey, or the fluid which remains after the precipi- tation of casein; it contains sugar, salt, and a small quantity of albumen. Table of Composition of Milk, Butter-milk, Cream, and Cheese.-(Letheby and Payen.) Nitrogenous matters. Fats. Lactose. > Salts. Water. Milk (Cow) . . . 4'1 3'9 5'2 •8 86 Buttermilk 4'1 7 6'4 •8 88 Cream . . . 27 267 2'8 1*8 66 Cheese.-Sk im . . 44'8 6'3 - 4'9 44 „ Cheddai • . 28'4 311 N on-nitrogenous matter and loss. 4'5 36 „ Neufchatel (Fresh) 8' 4071 36-58 ■5i 36-58 III. Eggs.-The yelk and albumen of eggs are in the same relation as food for the embryoes of oviparous animals that milk 242 FOOD. [chap. VI. is to the young of mammalia, and afford another example of the natural admixture of the various alimentary principles. Table of the Per-centage Composition of Fowls' Eggs. Nitrogenous substances. Fats. Salts. Water White . . . 20'4 - r6 78 Yelk . i6' 307 i'3 52 IV. Leguminous fruits are used by vegetarians, as the chief source of the nitrogen of the food. Those chiefly used arc peas, beans, lentils, Arc., they contain a nitrogenous substance called legumin, allied to albumen. They contain about 25'30 per cent, of this nitrogenous body, and twice as much nitrogen as wheat. B. Foods containing carbohydrate bodies chiefly. I. Bread, made from the ground grain obtained from various so-called cereals, viz., wheat, rye, maize, barley, rice, oats, &c., is the direct form in which the carbohydrate is supplied in an ordinary diet. Flour, however, besides the starch, contains gluten, a nitrogenous body, and a small amount of fat. Table of Per-centage Composition of Bread and Flour. Nitrogenous matters. Carbo- hydrates. Fats. Salts. Water. Bread . . . 8'1 51' 1'6 23 37 Flour 10'8 70-85 2- i'7 15 Various articles of course besides bread are made from flour, «.</., sago, macaroni, biscuits, <fcc. II. Vegetables, especially potatoes. They contain starch and sugar. III. Fruits contain sugar, and organic acids, tartaric, malic, citric, and others. C. Substances supplying fatty bodies principally. The chief are butter, lard (pig's fat), suet (beef and mutton fat). D. Substances supplying the salts of the food. Nearly all the foregoing substances in A, B, and C, contain a greater or less amount of the salts required in food, but green CHAP. VI.] LIQUID FOODS. 243 vegetables and fruit supply certain salts, without which the normal health of the body cannot be maintained. E. Liquid foods. Water is consumed alone, or together with certain other sub- stances used to flavour it, e.//., tea, coffee, &c. Tea in moderation is a stimulant, and contains an aromatic oil to which it owes its peculiar aroma, an astringent of the nature of tannin, and an alkaloid, theine. The composition of coffee is very nearly similar to that of tea. Cocoa, in addition to similar substances contained in tea and coffee, contains fat, albuminous matter, and starch, and must be looked upon more as a food. Beer, in various forms, is an infusion of malt (barley which has sprouted, and in which its starch is converted in great part into sugar), boiled with hops and allowed to ferment. Beer contains from i'2 to 8'8 per cent, of alcohol. Cider and Perry, the fermented juice of the apple and pear. JIT/ic, the fermented juice of the grape, contains from 6 or 7 (Rhine wines, and white and red Bordeaux) to 24-25 (ports and sherries) per cent, of alcohol. Spirits, obtained from the distillation of fermented liquors. They contain upwards of 40-70 per cent, of absolute alcohol. Effects of cooking upon Food. In general terms this may be said to make food more easily digestible; this usually implies two alterations,-food is made more agreeable to the palate and also more pleasing to the eye. Cooking consists in exposing the food to various degrees of heat, cither to the direct heat of the fire, as in roasting, or to the indirect heat of the fire, as in broiling, baking, or frying, or to hot water, as in boiling or stewing. The effect of heat upon (a) flesh is to coagulate the albumen and colouring matter, to solidify fibrin, and to gelatinize tendons and fibrous connective tissue. Previous beating or bruising (as with steaks and chops), or keeping (as in the case of game), renders the meat more tender. Pro- longed exposure to heat also developes on the surface certain empyreumatic bodies, which are agreeable both to the taste and smell. By placing meat in hot water, the external coating of ■albumen is coagidated, and very little, if any, of the constituents of the meat are lost afterwards if boiling be prolonged ; but if the constituents of the meat are to be extracted, it should be exposed 244 FOOD. [CHAP. VI. to prolonged simmering at a much lower temperature, and the " broth " will then contain the gelatin and extractive matters of the meat, as well as a certain amount of albumen. The addition of salt will help to extract myosin. The effect of boiling upon (6) an egg is to coagulate the albu- men, and this helps to render the article of food more suitable for adult dietary. Upon (c) milk, the effect of heat is to produce a scum composed of albumen and a little casein (the greater part of the casein being uncoagulated) with some fat. Upon (<7) vege- tables, the cooking produces the necessary effect of rendering them softer, so that they can be more readily broken up in the mouth; it also causes the starch grains to swell up and burst, and so aids the digestive fluids in penetrating into their substance. The albuminous matters are coagulated, and the gummy, saccharine and saline matters are removed. The conversion of flour into dough is effected by mixing it with water, and adding a little salt and a certain amount of yeast. Yeast consists of the cells of an organised ferment (Torula cerevisi(l'), and it is by the growth of this plant, which lives upon the sugar produced from the starch of the flour, that a quantity of carbonic acid gas and alcohol is formed. By means of the former the dough rises. Another method of making dough consists in mixing the flour with water containing a large quantity of carbonic acid gas in solution. By the action of heat during baking (rf) the dough continues to expand, and the gluten being coagulated, the bread sets as a permanently vesiculated mass. I.-Effects of an insufficient diet. Hunger and Thirst.-The sensation of hunger is manifested in consequence of deficiency of food supplied to the system. The mind refers the sensation to the stomach; yet since the sensation is relieved by the introduction of food either into the stomach itself, or into the blood through other channels than the stomach, it would appear not to depend on the state of the stomach alone. This view is confirmed by the fact, that the division of both pneu- mogastric nerves, which are the principal channels by which the brain is cognisant of the condition of the stomach, does not appear to allay the sensations of hunger. But that the stomach has. some share in this sensation is proved by the relief afforded, though only temporarily, by the introduction of even non-alimen- CHAP. VI.] THIRST AND STARVATION. 245 tary substances into this organ. It may, therefore, be said that the sensation of hunger is caused both by a want in the system generally, and also by the condition of the stomach itself, by which condition, of course, its own nerves are more directly affected. The sensation of thirst, indicating the want of fluid, is referred to the fauces, although, as in hunger, this is, in great part, only the local declaration of a general condition. For thirst is relieved for only a very short time by moistening the dry fauces; but may be relieved completely by the introduction of liquids into the blood, either through the stomach, by injections into the blood- vessels, or by absorption from the surface of the skin or the intes- tines. The sensation of thirst is perceived most naturally when- ever there is a disproportionately small quantity of water in the blood : as well, therefore, when water has been abstracted from the blood, as when saline or any solid matters have been abun- dantly added to it. And the cases of hunger and thirst are not the only ones in which the mind derives, from certain organs, a peculiar predominant sensation of some condition affecting the whole body. Thus, the sensation of the "necessity of breathing," is referred especially to the air-passages ; but, as Volkmann's ex- periments show, it depends on the condition of the blood which circulates everywhere, and is felt even after the lungs of animals are removed; for they continue, even then, to gasp and manifest the sensation of want of breath. Starvation.-The effects of total deprivation of food have been made the subject of experiments on the lower animals, and have been but too frequently illustrated in man. (1.) One of the most notable effects of starvat;on, as might be expected, is loss of weight; the loss being greatest at first, as a rule, but afterwards not varying very much, day by day, until death ensues. Chossat found that the ultimate proportional loss was, in different animals experimented on, almost exactly the same ; death occurring when the body had lost two-fifths (forty per cent.) of its original weight. Different parts of the body lose weight in very different propor- tions. The following results are taken, in round numbers, from the table given by M. Chossat:- Fat loses . . -93 per cent. Blood . . . . 75 „ Spleen . . 71 ,, Pancreas . . . 64 ., Liver loses . -52 per cent. Heart . . . . 44 „ Intestines . . 42 „ Muscles of locomotion 42 . ., 246 FOOD. [chap. vi. Stomach loses . . 39 per cent. Pharynx, (Esophagus 34 ,. Skin . . . . 33 Kidneys lose . . 31 ,, Respiratory apparatus 22 per cent. Bones . . . 16 „ Eyes . . . . 10 „ Nervous System . 2 (nearly). (2.) The effect of starvation on the temperature of the various animals experimented on by Chossat was very marked. For some time the variation in the daily temperature was more marked than its absolute and continuous diminution, the daily fluctuation amounting to 50 or 6° F. (30 C.), instead of r° or 20 F. (*5° to 1° C.), as in health. But a short time before death, the temperature fell very rapidly, and death ensued when the loss had amounted to about 30° F. (16'2° C.). It has been often said, and with truth, although the statement requires some qualification, that death by starvation is really death by cold; for not only has it been found that differences of time with regard to the period of the fatal result are attended by the same ultimate loss of heat, but the effect of the application of external warmth to animals cold and dying from starvation, is more effectual in reviving them than the administration of food. In other words, an animal exhausted by deprivation of nourishment is unable so to digest food as to use it as fuel, and therefore is dependent for heat on its supply from without. (3.) The sywptfoTns produced by starvation in the human sub- ject are hunger, accompanied, or it may be replaced, by pain, referred to the region of the stomach; insatiable thirst; sleep- lessness ; general weakness and emaciation. The exhalations both from the lungs and skin are ftetid, indicating the tendency to decomposition which belongs to badly-nourished tissues; and death occurs, sometimes after the additional exhaustion caused by diarrhoea, often with symptoms of nervous disorder, delirium or convulsions. (4.) In the human subject death commonly occurs within six to ten days after total deprivation of food. But this period may be considerably prolonged by taking a very small quantity of food, or even water only. The cases so frequently related of survival after many days, or even some weeks, of abstinence, have been due either to the last-mentioned circumstances, or to others 110 less effectual, which prevented the loss of heat and moisture. Cases in which life has continued after total abstinence from food and drink for many weeks, or months, exist only in th imagination of the vulgar. chap. vi.J EFFECTS OF IMPROPER DIET. 247 (5.) The appearances presented after death from starvation are those of general wasting and bloodlessness, the latter condition being least noticeable in the brain. The stomach and intestines are empty and contracted, and the walls of the latter appear remarkably thinned and almost transparent. The various secre- tions are scanty or absent, with the exception of the bile, which, somewhat concentrated, usually fills the gall-bladder. All parts of the body readily decompose. II.-Effects of Improper Diet. Experiments on Feeding.-Experiments illustrating the ill-effects produced by feeding animals upon one or t wo alimentary substances only have been often performed. Dogs were fed exclusively on sugar and distilled water. During the first seven or eight days they were brisk and active, and took their food and drink as usual; but in the course of the second week they began to get thin, although their appetite continued good, and they took daily between six and eight ounces of sugar. The emaciation increased during the third week, and they became feeble, and lost their activity and appetite. At the same time an ulcer formed on each cornea, followed by an escape of the humours of the eye : this took place in repeated experiments. The animals still continued to eat three or four ounces of sugar daily; but became at length so feeble as to be incapable of motion, and died on a day varying from the thirty-first to the thirty-fourth. On dissection, their bodies presented all the appearances produced by death from starvation indeed, dogs will live almost the same length of time without any food at all. When dogs were fed exclusively on gum, results almost similar to the above ensued. When they were kept on olive-oil and water, all the phenomena produced were the same, except that no ulcera- tion of the cornea took place ; the effects were also the same with butter. The experiments of Chossat and Letellier prove the same ; and in men, the same is shown by the various diseases to which those who consume but little nitrogenous food are liable, and especially by the affection of the cornea which is observed in Hindus feeding almost exclusively on rice. But it is not only the non-nitrogenous substances, which, taken alone, are insufficient for the maintenance of health. The experiments of the Academies 248 FOOD. [chap. vi. of France and Amsterdam were equally conclusive that gelatin alone soon ceases to be nutritive. III.-Effect of Too Much Food. Sometimes the excess of food is so great that it passes through the alimentary canal, and is at once got rid of by increased peristaltic action of the intestines. In other cases, the unabsorbed portions undergo putrefactive changes in the intestines, which are accompanied by the production of gases, such as carbonic acid, carburetted and sulphuretted hydrogen, and a distended condition of the bowels, together with symptoms of indigestion, is the result. An excess of the substances required as food may undergo absorption. It is a well-known fact that numbers of people habitually eat too much, and especially of nitrogenous food. Dogs can digest an immense amount of meat if fed often, and the amount of meat taken by some men would supply not only the nitrogen, but also the carbon which is requisite for an ordinary natural diet. A method of getting rid of an excess of nitrogen is provided by the digestive processes in the duodenum, to be presently described, whereby the excess of the albuminous food is capable of being changed before absorption into nitrogenous crystalline matters easily converted into urea and so easily excreted by the kidneys, affording one variety of what is called luxus con- sumption; but no doubt after a time the organs, especially the liver upon which the extra amount of the ingested diet throws most of the stress, will yield to the strain of the over-work, and will not reduce the excess of nitrogenous material brought to it into urea, but into other less oxidised products, such as uric acid; general plethora, and gout being the result. This state of things however, is delayed for a long time, if not altogether obviated, when large meat-eaters take a considerable amount of exercise. Excess of carbohydrate food produces an accumulation of fat, which may not only be an inconvenience by causing obesity, but may interfere with the proper nutrition of muscles, causing a feebleness of the action of the heart, and other troubles. The accumulation of fat is due to the excess of carbohydrate being stored up by the protoplasm in the form of fat. Starches when taken in great excess are almost certain to give rise to dyspepsia, with acidity and flatulence. Excess of starch or of sugar in the food may, however, be got rid of by the urine in the form of CHAP. VI.] NORMAL DIETS. 249 glycosuria. There is evidently a limit to the absorption of starch and of fat, as, if taken beyond a certain amount, they appear unchanged in the faeces. Requisites of a Normal Diet. It will have been understood that it is necessary that a normal diet should be made up of various articles, that they should be well cooked, and that they should contain about the same amount of carbon and nitrogen as are got rid of by the excreta. No doubt these desiderata may be satisfied in many ways, and it would be unreasonable to expect that the diet of every adult should be unvarying. The age, sex, strength, and circumstances of each individual must ultimately determine his diet. A dinner of bread and hard cheese with an onion contains all the requisites for a meal, but such diet would be suitable only for those possessing strong digestive powers. It is a well-known fact that the diet of the continental nations differs from that of our own country, and that of cold from that of hot climates, but the same principle underlies them all, viz., the replacement of the loss of the excreta in the most convenient and economical way possible. Without going into detail in the matter, it may be said that anyone in active work requires more nitrogenous matter than one at rest, and that children and women require less than adult men. The quantity of food for a healthy adult man of average height and weight may be stated in the following table :- Table of Food required for a Healthy Adult. (Parkes.) In laborious occupation. . . At rest. Nitrogenous substances, e.g„ flesh . 6 to 7 oz. av. 2-5 oz. Fats 3'5 to 4-5 oz. 1 oz. Carbo-hydrates 16 to 18 oz. 12 oz. Salts 1'2 to 1'5 oz. •5 oz- 267 to 31 oz. 16 oz. The above table contains the weights of dry or solid food required. Such food is found in practice to be nearly always combined with 50 to 60 per cent, of water, and so the above numbers should be correspondingly increased. The amount of liquids required in addition is about three pints per diem. 250 DIGESTION. [CHAP. VII. Full diet scale for an adult male in hospital Bartholomew's Hospital). B reahfast .-1 pint of tea (with milk and sugar), bread and butter. Dinner. of cooked meat, Alb. potatoes, bread and beer. Tea.-1 pint of tea, bread and butter. Supper.-Bread and butter, beer. Dally allowance to each patient.-2 pints of tea, with milk and sugar ; 140Z. bread ; Alb. of cooked meat : |lb. potatoes ; 2 pints of beer; ioz. butter. 3ioz. solid, and 4 pints (80 oz.), liquid. CHAPTER VII. DIGESTION. The object of digestion is to prepare the food to supply the waste of the tissues, which we have seen is its proper function in the economy. Few of the articles of diet are taken in the exact condition in which it is possible for them to be absorbed into the system by the blood vessels and lymphatics, without which they would be useless for the purposes they have to fulfil. Almost the whole of the food, therefore, undergoes various digestive changes before it is fit for absorption. Having been received into the mouth, it is subjected to the action of the teeth and tongue, and is mixed with the first of the digestive juices-the saliva. It is then swailowed, and, passing through the pharynx and oesophagus into the stomach, is subjected to the action of the gastric juice- the second digestive juice. Thence it passes into the intestines, where it meets with the bile, the pancreatic juice and the intestinal juices, all of which exercise an influence upon the portion of the food not absorbed in the stomach. By this time most of the food is capable of absorption, and the residue of undigested matter leaves the body in the form of fences by the anus. The course of the food through the alimentary canal of man will be readily seen from the accompanying diagram (fig. 164). The Mouth is the cavity contained between the jaws and inclosed by the cheeks laterally, the lips anteriorly; behind it opens into the pharynx by the fauces, and is separated from the nasal cavity above, by the hard palate in front, and the soft palate behind, which forms its roof. The tongue forms the lower part or floor. In the jaws are contained the teeth, and when the mouth is CHAP. VII.] THE MOUTH AND TONGUE. 251 shut these form its anterior boundaries. The whole of the mouth is lined with mucous membrane, covered with stratified squamous epithelium, which is continuous in front along the lips with the epithelium and the skin, and posteriorly with that of the pharynx. Fig. 164.- Diagram of the Alimentary Canal. The small intestine of man is from about 3 to 4 times as long as the large intestine. The mucous membrane is provided with numerous glands (small tubular), called mucous glands, and into it open the ducts of the salivary glands, three chief glands on each side. The tongue is not only a prehensile organ, but is also the chief seat of the sense of taste. We shall first of all devote some little space to the consideration 252 DIGESTION. [CHAP. VII. of the structure and development of the teeth, and then shall proceed to discuss, in detail, the process of digestion, as it takes place in each stage of the journey of the food through the alimentary canal. During the course of his life, man in common with other mammals, is provided with two sets of teeth, the first set is ailed the temporary or milk teeth, which makes its appearance The Teeth. Fig. 165.-Part of the lower jaw of a child of three or four years old, showing the relations of the temporary and permanent teeth. The specimen contains all the milk-teeth of the right side, together with the incisors of the left; the inner plate of the jaw has been removed, so as to expose the sacs of all the permanent teeth of the right side, except the eighth or wisdom tooth, which is not yet formed. The large sac near the ascending ramus of the jaw is that of the first permanent molar, and above and behind it is the commencing rudiment of the second molar. (Quain.) in infancy, and is in the course of a few years shed and replaced by the second or permanent set. The temporary or milk teeth have only a very limited term of existence. They are ten in number in each jaw, namely, on either side from the middle line two incisors, one canine, and two deciduous molars, and are replaced by ten permanent teeth. The number of permanent teeth in each jaw is, however, in- creased to sixteen, by the development of three others on each side of the jaw. The following formula shows, at a glance, the comparative arrangement and number of the temporary and permanent teeth:- Temporary Teeth . DEC. MO. CA. IX. CA. MO. Upper 2 i 4 i 2 = io - 20 Lower 2 1 4 1 2 =10 chap, vii.] TEMPORARY AND PERMANENT TEETH. 253 TRUE BIt'USPIDS , .... OR PR.U- CA. IN. CA. BI. MO. MOLARS. M0LABg> Upper 3 2 1 4 1 2 3 = 16 -=32 Lower 3 2 1 4 1 2 3 = 16 Permanent Teeth From this formula it will be seen that the two bicuspid or prse- molar teeth in the adult are the successors of the two deciduous molars in the child. They differ from them, however, in some respects, the temporary molars having a stronger likeness to the permanent than to their immediate descendants, the so-called bicuspids. The temporary incisors and' canines differ from their successors but little except in their smaller size. The following tables show the average times of eruption of the Temporary and Permanent teeth. In both cases, the eruption of any given tooth of the lower jaw precedes, as a rule, that of the corresponding tooth of the upper. Temporary or Milk Teeth. The figures indicate in months the age at which each tooth appears. DECIDUOUS MOLARS. CANINES. INCISORS. CANINES. DECIDUOUS MOLARS. 24 12 18 9 7 7 9 18 12 24 Permanent Teeth. The age at which each tooth is cut is indicated in this table in years. BISCUPID OR MOLARS. PREMOLARS. CANIaES. INCISORS. BISCUPID OR CANINES. PREMOLARS. MOLARS. 17 12 to to 6 10 9 II to 12 877s 11 to 12 .9 IO 12 17 6 to to 25 13 13 25 The times of eruption given in the above tables are only approximate : the limits of variation being tolerably wide. Some children may cut their first teeth before the age of six months, and others not till nearly the twelfth month. In nearly all cases the two central incisors of the lower jaw are cut first; these being suc- ceeded after a short interval by the four incisors of the upper jaw, next follow the lateral incisors of the lower jaw, and so on as indi- cated in the table till the completion of the milk dentition at about the age of two years. 254 DIGESTION. [<JHAl'. VII. The milk teeth usually come through in batches, each period of eruption being succeeded by one of quiescence lasting sometimes several months. The milk-teeth arc in use from the age of two up to five and a half years; at about this age the first permanent molars (four in number) make their appearance behind the milk- molars, and for a short time the child has four permanent and twenty temporary teeth in position at once. It is worthy of note that from the age of five years to the shedding of the first milk-tooth the child has no fewer than forty- eight teeth, twenty milk-teeth and twenty-eight calcified germs of permanent teeth (all in fact except the four wisdom teeth). Structure of a Tooth. A tooth is generally described as possessing a crown, neck, and fang or fangs. The is the portion which projects beyond the level of the gum. The neck is that constricted portion just below the crown Fig. 166.-a. Longitudinal section of a human molar tooth: r, cement; d, dentine ; enamel; v, pulp cavity. (Owen.) b. Transverse section. The letters indicate the same a-s id a. 'which is embraced by the free edges of the gum, and the fang includes all below this. On making a longitudinal section through its centre (figs. 166, 167), a tooth is found to be principally composed of a hard material, dentine or ivory, which is hollowed out into a central cavity which resembles in general shape the outline of the tooth, and is called the pulp cavity, from its containing the very vascular ami sensitive CHAP. VII.] DENTINE. 255 tooth pulp which is composed of connective-tissue, blood-vessels, and nerves. The blood-vessels and nerves enter the pulp through a small opening at the extremity of the fang. A layer of very hard calcareous matter, the enamel, caps that part of the dentine which projects beyond the level of the gum ; while sheathing the portion of dentine which is beneath the level of the gum, is a layer of true bone, called the cement or crusta petrosa. At the neck of the tooth, where the enamel and cement come into contact, each is reduced to an exceedingly thin layer. The covering of enamel becomes thicker to- wards the crown, and the cement towards the lower end or apex of the fang. 1.- Dentine. Chemical composition.-Den- tine closely resembles bone in chemical composition. It con- tains, however, rather less animal matter; the propor- tion in a hundred parts being about twenty-eight animal to seventy-two of earthy. The former, like the animal matter of bone, may be resolved into gelatin by boiling. The earthy matter is made up chiefly of calcium phosphate, with a small portion of the carbonate, and traces of calcium fluoride and magnesium phosphate. Structure.-Under the microscope dentine is seen to be finely channelled by a multitude of delicate tubes, which, by their inner ends, communicate with the pulp-cavity, and by their outer ex- tremities come into contact with the under part of the enamel and Fig. 167.molar tooth of cat in situ. Verti- cal section. 1. Enamel with decussating aud parallel strife. 2. Dentine with Schreger's lines. 3. Cement. 4. Perios- teum of the alveolus. 5. Inferior maxil- lary bone, showing' canal for the inferior dental nerve and vessels, which appears nearly circular in transverse section. 256 DIGESTION. [CHAP. VII. cement, and sometimes even penetrate them for a greater or less distance (fig. 168). In their course from the pulp-cavity to the surface, the minute tubes form gentle and nearly parallel curves and divide and sub- divide dichotomously, but without much lessening of their calibre until they are approaching their peripheral termination. From their sides proceed other exceedingly minute secondary canals, which extend into the dentine between the tubules, and anastomose with each other. The tubules of the dentine, the average diameter of which at their inner and larger extremity is ■jyoo °f an inch, contain fine prolongations from the tooth-pulp, which give the dentine a certain faint sensitiveness under ordi- nary circumstances and, without doubt, have to do also with its C ./ (I C b 9 c a Fig. 168.-Section of a portion of the dentine and cement from the middle of the root of an incisor tooth, a, dental tubuli ramifying and terminating, some of them in the inter- globular spaces b and c, which somewhat resemble bone lacuna?; d, inner layer of the cement with numerous closely set canaliculi; e, outer layer of cement; f, lacunte; </, canaliculi. x 350. (Kdlliker.) nutrition. These prolongations from the tooth-pulp are really processes of the dentine-cells or odontoblasts, which are branched cells lining the pulp-cavity; the relation of these processes to the tubules in which they lie being precisely similar to that of the processes of the bone-corpuscles to the canaliculi of bone. The outer portion of the dentine, underlying both the cement and enamel, forms a more or less distinct layer termed the granular or inter-globular layer. It is characterised by the presence of a number of minute cell-like cavities, much more closely packed than the lacunae in the cement, and communicating with one another and with the ends of the dentine-tubes (fig. 168), and containing cells like bone-corpuscles. CHAP. VII.] STRUCTURE OF A TOOTH. 257 II.-Enamel. Chemical composition.-The enamel, which is by far the hardest portion of a tooth, is composed, chemically, of the same elements that enter into the composition of dentine and bone. Its animal matter, however, amounts only to about 2 or 3 per cent. It contains a larger proportion of inorganic matter and is harder than any other tissue in the body. Structure.-Examined under the mi- croscope, enamel is found composed of fine hexagonal fibres (figs. 169,170) -yoVo of an inch in diameter, which are set on end on the surface of the dentine, and fit into corresponding depressions in the same. They radiate in such a manner from the dentine that at the top of the tooth they are more or less vertical, while towards the sides they tend to the horizontal direction. Like the dentine tubules, they are not straight, but dis- posed in wavy and parallel curves. The fibres are marked by transverse lines, and are mostly solid, but some of them contain a very minute canal. The enamel-prisms are connected together by a very minute quantity of hyaline cement-substance. In the deeper part of the enamel, between the prisms, are small lacunce, which com- municate with the " interglobular spaces " on the surface of the dentine. The enamel itself is coated on the outside by a very thin calcified membrane, sometimes termed the cuticle of the enamel. Fig. 169.- Thin section of the enamel and a part of the den- tine. a, cuticular pellicle of the enamel; b, enamel fibres, or columns with fissures be- tween them and cross striaj; c, larger cavities in the enamel, communicating with the extre- mities of some of the tubuli (d). X 350. (Kolliker.) ill.-Crusta Petrosa. The crusta petrosa, or cement (fig. 168, c, d\ is composed of true bone, and in it are lacunae (/) and canaliculi (</), which some- 258 DIGESTION. [chap. VII. times communicate with the outer finely branched ends of the dentine tubules. Its laminae are as it were bolted together by Fig. 170.-Enamel fibres. A, fragments and single fibres of the enamel, isolated by the action of hydrochloric acid. B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres, x 350. (Kolliker.) perforating fibres like those of ordinary bone, but it differs from ordinary bone in possessing Haversian canals only in the thickest part. Development of the Teeth. Development of the Teeth.-The first step in the development of thekteeth consists in a downward growth (fig. 171, a, t) from the stratified epithelium of the mucous membrane of the mouth, which first becomes thickened in the neighbourhood of the jaws or maxilla) which are in the course of formation. This process passes down- ward into a recess (enamel groove) of the imperfectly developed tissue of the embryonic jaw. The downward epithelial growth forms the primary enamel organ or enamel germ, and its position is indicated by a slight groove in the mucous membrane of the jaw. The next step in the process consists in the elongation downward of the enamel groove and of the enamel germ and the inclination outward of the deeper part (fig. 171, b, /'), which is now inclined at an angle with the upper portion or neck (/), and has become bulbous. After this, there is an increased development at certain points corresponding to the situations .of the future milk-teeth, chap. vii.] DEVELOPMENT OF THE TEETH. 259 and the enamel germ, or common enamel germ, as it may be called, becomes divided at its deeper portion, or extended by further growth, into a number of special enamel germs corre- sponding to each of the above-mentioned milk- teeth, and connected to the common germ by a narrow neck, each tooth being placed in its own special recess in the em- bryonic jaw (fig. 171, B, .//')• As these changes pro- ceed, there grows up from the underlying tis- sue into each enamel germ (fig. 171, c, p), a distinct vascular papilla (dental papilla), and upon it the enamel germ be- comes moulded, and pre- sents the appearance of a cap of two layers of epithelium separated by an interval (fig. 171, €, /'). Whilst part of the sub-epithelial tissue is elevated to form the dental papillae, the part which bounds the em- bryonic teeth forms the dental sacs (fig. 171, c, s); and the rudiment of the jaw, at first a bony gutter in which the teeth germs lie, sends up processes forming partitions between the teeth. In this way small chambers are produced in which the dental sacs are contained, and thus the sockets of the teeth are formed. The papilla, which is really part of the dental sac (if one thinks of this as the whole of A B c Big. 171.-Section of the upper jaw of a foetal she<p A. -1, common enamel-germ dipping down into the mucous membrane; 2, palatine process of jaw. B. -Section similar to A, but passing through one of the special enamel-germs here becoming flask- shaped ; c, c', epithelium of mouth; f, neck; f, body of special enamel-germ. C.-A later stage; e, out- line of epithelium of gum ; f, neck of enamel germ ; fenamel organ; p, papilla; s, dental sac forming; fp, the enamel germ of permanent tooth. (Wal- deyer and Kolliker.) Copied from Quain's Anatomy. 260 DIGESTION. [chai*, vii. the sub-epithelial tissue surrounding the enamel organ and interposed between the enamel germ and the developing bony jaw), is composed of nucleated cells arranged in a meshwork, the outer or peripheral part being covered with a layer of columnar nucleated cells called odontoblasts. The odontoblasts form the dentine, while the remainder of the papilla forms the tooth-pulp. The method of the formation of the dentine from the odontoblasts is as follows :-The cells elongate at their outer part, and these processes are directly converted into the tubules of dentine (fig. 172). The continued formation of dentine proceeds by the elongation of the odontoblasts, and their subsequent conversion by a process of Fig. 172.-Part of section of developing tooth of a young rat, showing the mode of deposition of the dentine. Highly magnified, a, outer layer of fully formed dentine ; b, uncalci- fied matrix -with one or two nodules of calcareous matter near the calcified parts; c, odontoblasts sending processes into the dentine; d, pulp. The section is stained in carmine, which colours the uncalcified matrix but not the calcified part. (E. A. Schafer.) calcification into dentine tubules. The most recently formed tubules are not immediately calcified. The dentine fibres con- tained in the tubules are said to be formed from processes of the deeper layer of odontoblasts, which arc wedged in between the cells of the superficial layer (fig. 172) which form the tubules only. Since the papilla.1 are to form the main portion of each tooth, i.e., the dentine, each of them early takes the shape of the crown of the tooth to which it corresponds. As the dentine increases in thickness, the papilla* diminish, and at last when the tooth is cut, only a small amount of the papilla remains as the dental pulp, and is supplied by vessels and nerves which enter at the end of the fang. The shape of the crown of the tooth is taken by the corresponding papilla, and that of the single or double fang by the subsequent constriction below the crown, or by division of the lower part of the papilla. CHAP. VII.] DEVELOPMENT OF ENAMEL AND DENTINE. 261 The enamel cap is found later on to consist (fig. 17 3) of three parts : (a) an inner membrane, composed of a layer of columnar epithe- lium in contact with the dentine, called enamel cells, and outside of these one or more layers of small polyhedral nucleated cells {stratum intermedium of Hannover) • (6) an outer membrane of several layers of epithelium ; (c) a middle membrane formed of a matrix of non-vascular, gelatinous tissue, contain- ing a hyaline interstitial substance. The enamel is formed by the enamel cells of the inner membrane, by the elongation of their dis- tal extremities, and the direct conversion of these processes into enamel. The calcification of the enamel processes or prisms takes place first at the periphery, the centre remaining for a time transparent. The cells of the stratum inter- medium are used for the regeneration of the enamel cells, but these and the middle membrane after a time disappear. The cells of the outer membrane give origin to the cuticle of the enamel. The cement or crusta pe- trosa is formed from the tissue of the tooth sac, the structure and function of which are identical with those of the osteogenetic layer of the periosteum. In this manner the first set of teeth, or the milk-teeth, are formed ; and each tooth, by degrees developing, presses at length on the wall of the sac enclosing it, and, causing its absorption, is cut, to use a familiar phrase. The temporary or milk-teeth, are speedily replaced by the growth of the permanent teeth, which push their way up from beneath Fig. 173.- Vertical transverse section oj th dental sac, pulp, &c., of a kitten, a, dental papilla or pulp; l>, the cap of dentine formed upon the summit; c, its covering of enamel; d, inner layer of epithelium of the enamel organ; e, gelatinous tissue ; f, outer epithe- lial layer of the enamel organ ; <7, inner layer, and h, outer layer of dental sac. x 14 (Thiersch.) 262 DIGESTION. [chap. vir. them, absorbing in their progress the whole of the fang of each milk-tooth, and leaving at length only the crown as a mere shell, which is shed to make way for the eruption of the permanent teeth (fig. 165). Each temporary tooth is replaced by a corresponding tooth of the permanant set which is developed from a small sac set by, so to speak, from the sac of the temporary tooth which precedes it, and called the cavity of reserve. Mastication. The act of chewing or mastication is performed by the biting and grinding movement of the lower range of teeth against the upper. The simultaneous movements of the tongue and cheeks assist partly by crushing the softer portions of the food against the hard palate and gums, and thus supplementing the action of the teeth, and partly by returning the morsels of food to the action of the teeth, again and again, as they are squeezed out from between them, until they have been sufficiently chewed. Muscles.-The simple up and down, or luting movements of the lower jaw, are performed by the temporal, masseter, and internal pterygoid muscles, the action of which in closing the jaws alter- nates with that of the digastric and other muscles passing from the os hyoides to the lower jaw, which open them. The grinding or side to side movements of the lower jaw are performed mainly by the external pterygoid muscles, the muscle of one side acting alter- nately with the other. When both external pterygoids act together, the lower jaw is pulled directly forwards, so that the lower incisor teeth are brought in front of the level of the upper. Temporo-maxillary Fibro-cartilage.-The function of the inter- articular fibro-cartilage of the temporo-maxillary joint in mastica- tion is to serve : (i) As an elastic pad to distribute the pressure caused by the exceedingly powerful action of the masticatory muscles. (2) As a joint-surface or socket for the condyle of the lower jaw, when the latter has been partially drawn forward out of the glenoid cavity of the temporal bone by the external pterygoid muscle, some of the fibres of the latter being attached to its front surface, and consequently drawing it forward with the condyle which moves on it. Nervous Mechanism.-The act of mastication is partly voluntary and partly reflex and involuntary. The consideration of such CHAP. VII.] THE SALIVARY GLANDS. 263 sensori-motor actions will come hereafter (see Chapter on the Nervous System). It will suffice here to state that the afferent nerves chiefly concerned are the sensory branches of the fifth and the glosso- pharyngeal, and the efferent are the motor branches of the fifth and the ninth (hypoglossal) cerebral nerves. The nerve-centre through which the reflex action occurs, and by which the move- ments of the various muscles are harmonised, is situated in the medulla oblongata. In so far as mastication is voluntary or mentally perceived, it becomes so under the influence, in addition to the medulla oblongata, of the cerebral hemispheres. Insalivation. The act of mastication is much assisted by the saliva which is secreted by the salivary glands in largely increased amount during the process, and the intimate incorporation of which with the food, as it is being chewed, is termed insalivation. The human salivary glands are the parotid, the sub-maxillary, and the sub-lingual, and numerous smaller bodies of similar struc- ture, and with separate ducts, which are scattered thickly beneath the mucous membrane of the lips, cheeks, soft palate, and root of the tongue. Structure.-The salivary glands arc compound tubular glands. They are made up of lobules. Each lobule consists of the branch- ings of a subdivision of the main duct of the gland, which are generally more or less convoluted towards their extremities, and sometimes, according to some observers, sacculated or pouched. The convoluted or pouched portions form the alveoli, or proper secreting parts of the gland. The alveoli are composed of a base- ment membrane of flattened cells joined together by processes to produce a fenestrated membrane, the spaces of which are occupied by a homogeneous ground-substance. Within, upon this membrane, which forms the tube, the nucleated salivary secreting cells, of cubical or columnar form, are arranged parallel to one another enclosing a central canal. The granular appearance frequently seen in the salivary cells is due to the very dense network of fibrils which they contain. When isolated, the cells not unfre- quently are found to be branched. Connecting the alveoli into The Salivary Glands. 264 DIGESTION. [chap. vii. lobules is a considerable amount of fibrous connective tissue, which contains both flattened and granular protoplasmic cells, lymph corpuscles, and in some cases fat cells. The lobules are connected to form larger lobules (lobes), in a similar manner. The alveoli pass into the intralobular ducts by a narrowed portion (inter- calary), lined with flattened epithelium with elongated nuclei. The intercalary ducts pass into the intralobular ducts by a Fig. 174.-Section of sub-maxillary gland of dog. Showing gland cells, b, and a duct, a, in section. (Kolliker.) narrowed neck, lined with cubical cells with small nuclei. The intralobular duct is larger in size, and is lined with large columnar nucleated cells, the parts of which, towards the lumen of the tube, present a fine longitudinal striation, due to the arrangement of the cell network. It is most marked in the submaxillary gland, 'fhe intralobular ducts pass into the larger ducts, and these into the main duct of the gland. As these ducts become larger they acquire an outside coating of connective tissue, and later on some unstriped muscular fibres. The lining of the larger ducts consists of one or more layers of columnar epithelium, the cells of which contain an intracellular network of fibres arranged longitudinally. Varieties.-Certain differences in the structure of salivary glands may be observed according as the glands secrete pure saliva, or saliva mixed with mucus, or pure mucus, and therefore the glands have been classified as :- (i) True salivary glands (called most unfortunately by some serous glands), e.g., the parotid of man and other animals, and the '■HAP. VII.] VARIETIES OF SALIVARY GLANDS. 265 submaxillary of the rabbit and guinea-pig (fig. 175). In this kind the alveolar lumen is small, and the cells lining the tubule are short granular columnar cells, with nuclei presenting the intra- nuclear network. During rest the cells become larger, highly granular, with obscured nuclei, and the lumen becomes smaller. During activity, and after stimu- lation of the sympathetic, the cells become smaller and their contents more opaque ; the gran- ides first of all disappearing from the outer part of the cells, and then being found only at the ex- treme inner part and contiguous border of the cell. The nuclei reappear, as does also the lumen. (2) In the true mucus-secreting glands, as the sublingual of man and other animals, and in the submaxillary of the dog, the tubes are larger, contain a larger lumen and also have larger cells lining them. The cells are of two kinds, (a) mucous or central cells, which are transparent columnar cells with nuclei near the basement mem- brane. The cell substance is made up of a fine net- work, which in the resting state contains a transparent substance called mucigen, during which the cell does not stain well with logwood (fig. 176). When the gland is secreting, mucigen is con- verted into mucin, and the cells swell up, appear more transparent, and stain deeply in logwood (fig. 177). During rest, the cells become smaller and more granular from having discharged their contents- The nuclei appear more distinct. (6) Semilunes of Heidenhain. Fig. 175.-From a section through a true salivary gland, a, the gland alveoli, lined with albuminous " salivary ceils; " l>, intralobular duet cut transversely. (Klein and Noble Smith.) Fig. 176.-From a section through a mucous gland in a quiescent state. The alveoli are lined with transparent mucous cells, and outside these are the semilunes of Heidenhain. The cells should have been represented as more or less granular. (Heidenhain.) 266 DIGESTION. [chav. vii. (fig. 176), which are crescentic masses of granular parietal cells found here and there between the basement membrane and the central cells. The cells composing the mass are small, and have a very dense reticulum, the nuclei are spherical, and increase in size during secretion. In the mucous gland there are some large tubes, lined with large transparent central cells, and having besides a few granular parietal cells ; other small tubes are lined with small granular parietal cells alone; and a third variety are lined equally with each kind of cell. (3) In the muco-salivary or mixed glands, as the human submaxillary gland, part of the gland presents the struc- ture of the mucous gland, whilst the remainder has that of the salivary glands proper. Nerves and blood-vessels.- Nerves of large size are found in the salivary glands, they are principally contained in the con- nective tissue of the alveoli, and in certain glands, especially in the dog, are provided with ganglia. Some nerves have special endings in Pacinian corpuscles, some supply the blood-vessels, and others, according to Pfliiger, penetrate the basement membrane of the alveoli and enter the salivary cells. The blood-vessels form a dense capillary network around the ducts of the alveoli, being carried in by the fibrous trabecula? between the alveoli, in which also begin the lymphatics by lacunar spaces. I77---4 part of a section through a mucous gland after prolonged electrical stimulation. The alveoli are lined with small granular cells. (Lavdovski.) Saliva. Saliva, as it commonly flows from the mouth, is mixed with the secretion of the mucous glands, and often with air bubbles, which, being retained by its viscidity, make it frothy. When obtained from the parotid ducts, and free from mucus, saliva is a trans- parent watery fluid, the specific gravity of which varies from 1004 to 1008, and in which, when examined with the microscope, are found floating a number of minute particles, derived from the CHAP. VII.] MIXED SALIVA. 267 secreting ducts and vesicles of the glands. In the impure or mixed saliva are found, besides these particles, numerous epithe- lial scales separated from the surface of the mucous membrane of the mouth and tongue, and the so-called salivary corpuscles, dis- charged probably from the mucous glands of the mouth and the tonsils, which, when the saliva is collected in a deep vessel, and left at rest, subside in the form of a white opaque matter, leav- ing the supernatant salivary fluid transparent and colourless, or with a pale bluish-grey tint. In reaction, the saliva, when first secreted, appears to be always alkaline. During fasting, the saliva, although secreted alkaline, shortly becomes neutral; especially when it is secreted slowly and is allowed to mix with the acid mucus of the mouth, by which its alkaline reaction is. neutralized. Chemical Composition of Mixed Saliva (Frerichs). Water 994'10 Solids :- Ptyalin . . . . . .141 Fat 0'07 Epithelium and Proteids (including Serum-Albumin, Globulin, Mucin. &c.) 213 Salts :- Potassium Sulpho-Cyanate Sodium Phosphate . . . . Calcium Phosphate Magnesium Phosphate . . . Sodium Chloride . . . . Potassium Chloride . . . . 2'29 5'9 IOOO The presence of potassium sulphocyanate (or thiocyanate) (C N K S) in saliva, may be shown by the blood-red colonration which the fluid gives with a solution of ferric chloride (Fe.2 C1.6), and which is bleached on the addition of a solution of mercuric chloride (Hg CL), but not by hydrochloric acid. Hate of Secretion and Quantity.-The rate at which saliva is secreted is subject to considerable variation. When the tongue and muscles concerned in mastication are at rest, and the nerves of the mouth are subject to no unusual stimulus, the quantity secreted is not more than sufficient, with the mucus, to keep 268 DIGEST] ON. [chap. vn. the mouth moist. During actual secretion the flow is much accelerated. The quantity secreted in twenty-four hours varies: its average amount is probably from i to 3 pints (1 to 2 litres). Uses of Saliva.-The purposes served by saliva are (1) mechanical and (2) chemical. I. Mechanical.-(1) It keeps the mouth in a due condition of moisture, facilitating the movements of the tongue in speaking, and the mastication of food. (2) It serves also in dissolving sapid substances, and rendering them capable of exciting the nerves of taste. But the principal mechanical purpose of the saliva is, (3) that by mixing with the food during mastication, it makes it a soft pulpy mass, such as may be easily swallowed. To this purpose the saliva is adapted both by quantity and quality. For, speaking generally, the quantity secreted during feeding is in direct proportion to the dryness and hardness of the food. The quality of saliva is equally adapted to this end. It is easy to see how much more readily it mixes with most kinds of food than water alone does; and the saliva from the parotid, labial, and other small glands, being more aqueous than the rest, is that which is chiefly braided and mixed with the food in mastication ; while the more viscid mucous secretion of the submaxillary, palatine, and tonsillitic glands is spread over the surface of the softened mass, to enable it to slide more easily through the fauces and oesophagus. II. Chemical.-The chemical action which the saliva exerts upon the food in the mouth is to convert the starchy materials which it contains into some kind of sugar. This power the saliva owes to one of its constituents ptyalin, which is a nitrogenous body of uncertain composition. It is classed among the unorganized ferments, which are substances of uncertain composition capable of producing changes in the composition of other bodies with which they come into contact, without themselves undergoing change or suffering diminution. The conversion of the starch under the influence of the ferment into sugar takes place in several stages, and in order to understand it, a knowledge of the structure and composition of starch granules is necessary. A starch granule consists of two parts: an envelope of cellulose, which does not give a blue colour with iodine except on addition of sulphuric acid, and of granulose, which is contained within, and which gives a blue with iodine alone. Briicke states that a third CHAP. VII.] CH EMICA J, USE OF SALIVA. 269 body is contained in the granule, which gives a red with iodine, viz., erythro-granulose. On boiling, the granulose swells up, bursts the envelope, and the whole granule is more or less completely converted into a paste or gruel, which is called gelatinous starch. When ptyalin or other amylolytic ferment is added to boiled starch, sugar almost at once makes its appearance in small quan- tities, but in addition there is another body, intermediate between starch and sugar, called erythro-dextrin, which gives a reddish- brown colouration with iodine. As the sugar increases in amount, the erythro-dextrin disappears, but its place is taken in part by another dextrin, achroo-dextrin, which gives no colour with iodine. However long the reaction goes on, it is unlikely that all the dextrin becomes sugar. Next with regard to the kind of sugar formed, it is, at first at any rate, not glucose but maltose, the formula for which is. UI2 H2, 0„. Maltose is allied to saccharose or cane-sugar more nearly than to glucose; it is crystalline; its solution has the .property of polarising light to a greater degree than solutions of glucose; is not so sweet, and reduces copper sulphate less easily. It can be converted into glucose by boiling with dilute acids, and by the further action of the ferment. According to Brown and Heron the reactions may be represented thus :- One molecule of gelatinous starch is converted by the action of an amylolytic- ferment into n molecules of soluble starch. One molecule of soluble starch = 10 (C12 H20 Olo) 4- 8 (H2 0), which is. further converted by the ferment into I. Erythro-dextrin (giving red with iodine) + Maltose. 9 (Cia K2O Ow) ' (Cia Haa Ou) then into 2. Erythro-dextrin (giving yellow with iodine) + Maltose. 8 (C« Hao Olo) ' 2 (C18 Haa On) next into 3. Achroo-dextrin + Maltose. 7 (C12 Hao Ol(>) 3 (Cl2 Haa On) And so on ; the resultant being :- IO (Cla Hao 01o) + 8 (Ha 0) = 8 (C12 H22 Ou) + 2 (C12 H20 O10) Soluble starch Water Maltose Achroo-dextrin. Test for Sugar.-In such an experiment the presence of sugar is at once discovered by the application of Trommer's test, which consists in the addition of a drop or two of a solution of copper- sulphate, followed by a larger quantity of caustic potash. When the liquid is boiled, an orange-red precipitate of copper suboxide- indicates the presence of sugar. 270 DIGESTION. [chap. VI1. The action of saliva on starch is facilitated by: ft) Moderate heat, about roo F. (37'8° C.). (6) A slightly alkaline medium. '(c) Removal of the changed material from time to time. Its action is retarded by : (a) Cold ; a temperature of 320 F. (o° C.) stops it for a time, but does not destroy it, whereas a high tem- perature above 140° F. (60° C.) destroys it. (6) Acids or strong ■alkalies either delay or stop the action altogether, (c) Presence •of too much of the changed material. Ptyalin, in that it converts starch into sugar, is an amylolytic ferment. Starch appears to be the only principle of food upon which saliva acts chemically : the secretion has no apparent influence on any of the other ternary principles, such as sugar, gum, cellulose, or on fat, and seems to be equally destitute of power over albu- minous and gelatinous substances. Saliva from the parotid is less viscid, less alkaline, clearer, and more watery than that from the submaxillary. It has moreover a less powerful action on starch. Sublingual saliva is the most viscid, and contains more solids than either of the other two, but does not appear to be so powerful in its action. The salivary glands of children do not become functionally active till the age of 4 to 6 months, and hence the bad effect of feeding them before this age on starchy food, corn-flour, &c., which they are unable to render soluble and capable of absorption. Influence of the Nervous System. The secretion of saliva is under the control of the nervous system. It is a reflex action. Under ordinary conditions it is •excited by the stimulation of the peripheral branches of two nerves, viz., the gustatory or lingual branch of the inferior maxil- lary division of the fifth nerve, and the glosso-pharyngeal part of the eighth pair of nerves, which are distributed to the mucous membrane of the tongue and pharynx conjointly. The stimula- tion occurs on the introduction of sapid substances into the mouth, and the secretion is brought about in the following way. From the terminations of the above-mentioned sensory nerves distributed in the mucous membrane an impression is conveyed upwards (afferent) to the special nerve centre situated in the medulla, which controls the process, and by it is reflected to certain nerves supplied to the salivary glands, which will be presently indicated. In other words, the centre, stimulated to action by the sensory CHAP. VII. J NERVOUS INFLUENCE ON SALIVARY SECRETION. 271 impressions carried to it, sends ont impulses along efferent or secretory nerves supplied to the salivary glands, which cause the saliva to be secreted by and discharged from the gland cells. Other stimuli, however, besides that of the food, and other sensory nerves besides those mentioned, may produce reflexly the same effects. For example, saliva may be caused to flow by irritation of the mucous membrane of the mouth with mechanical, chemical, electrical, or thermal stimuli, also by the irritation of the mucous membrane of the stomach in some way, as in nausea, which pre- cedes vomiting, when some of the peripheral fibres of the vagi are irritated. Stimulation of the olfactory nerves by smell of food, of the optic nerves by the sight of it, and of the auditory nerves by the sounds which are known by experience to accompany the pre- paration of a meal, may also, in the hungry, stimulate the nerve centre to action, hi addition to these, as a secretion of saliva follows the movement of the muscles of mastication, it may be assumed that this movement stimulates the secreting nerve fibres of the gland, directly or reflexly. From the fact that the flow of saliva may be increased or diminished by mental emotions, it is evident that impressions from the cerebrum also are capable of stimulating the centre to action or of inhibiting its action. Salivary secretion may also be excited by direct stimulation of the centre in the medulla. A. On the Submaxillary Gland.-The submaxillary gland has been the gland chiefly employed for the purpose of experimentally demonstrating the influence of the nervous system upon the secre- tion of saliva, because of the comparative facility with which, with its blood-vessels and nerves, it may be exposed to view in the dog, rabbit, and other animals. The chief nerves supplied to the gland are : (i) the chorda tympani, a branch given off from the facial (or portio dura of the seventh pair of nerves), in the canal through which it passes in the temporal bone, in its passage from the interior of the skull to the face; and (2) branches of the sympa- thetic nerve from the plexus around the facial artery and its branches to the gland. The chorda (fig. 178, ch. t.), after quitting the temporal bone, passes downwards and forwards, under cover of the external pterygoid muscle, and joins at an acute angle the lingual or gustatory nerve, proceeds with it for a short distance, and then passes along the submaxillary gland duct (fig. 178, sm. d.), to which it is distributed, giving branches to the submaxillary ganglion (fig. 178, sm. gl.), and sending others to terminate in 272 DIGESTION. [CHAP. VII. the superficial muscles of the tongue. If this nerve be exposed and divided anywhere in its course from its exit from the skull to the gland, the secretion, if the gland be in action, is arrested, and no stimulation either of the lingual or of the glosso-pharyngeal will produce a flow of saliva. But if the peripheral end of the divided nerve be stimulated, an abundant secretion of saliva ensues, and the blood supply is enormously increased, the arteries being dilated. The veins even pulsate, and the blood contained within them is more arterial than venous in character. When, on the other hand, the stimulus is applied to the sympa- thetic filaments (mere division producing no apparent effect), the arteries contract, and the blood stream is in consequence much diminished; and from the veins, when opened, there escapes only a sluggish stream of dark blood. The saliva, instead of being abundant and watery, becomes scanty and tenacious. If both chorda tympani and sympathetic branches be divided, the gland, released from nervous control, secretes continuously and abun- dantly (paralytic secretion). The abundant secretion of saliva, which follows stimulation of the chorda tympani, is not merely the result of a filtration of fluid from the blood-vessels, in consequence of the largely increased cir- culation through them. This is proved by the fact that, when the main duct is obstructed, the pressure within may considerably exceed the blood-pressure in the arteries, and also that when into the veins of the animal experimented upon some atropin has been previously injected, stimulation of the peripheral end of the divided chorda produces all the vascular effects as before, without any secretion of saliva accompanying them. Again, if an animal's head be cut off, and the chorda be rapidly exposed and stimulated with an interrupted current, a secretion of saliva ensues for a short time, although the blood supply is necessarily absent. These experiments serve to prove that the chorda contains two sets of nerve fibres, one set (vaso-di/ator) which, when stimulated, act upon a local vaso-motor centre for regulating the blood supply, inhibiting its action, and causing the vessels to dilate, and so pro- ducing an increased supply of blood to the gland; while another set, which are paralyzed by injection of atropin, directly stimulate the cells themselves to activity, whereby they secrete and dis- charge the constituents of the saliva which they produce. These latter fibres very possibly terminate in the salivary cells them- selves. If, on the other hand, the sympathetic fibres be divided, chap, vii.] INFLUENCE OF THE SYMPATHETIC. 273 stimulation of the tongue by sapid substances, or of the trunk of the lingual, or of the glosso-pharyngeal continues to produce a flow of saliva. From these experiments it is evident that the chorda tympani nerve is the principal nerve through which effe- rent impulses proceed from the centre to excite the secretion of this gland. Fig. 178.-Diagrammatic representation of the submaxillary gland of the dog with its nerves and blood-vessels. (This is not intended to illustrate the exact anatomical relations of the several structures.) sm. gid., the submaxillary gland into the duct (sm. d.), of which a cannula has been tied. The sublingual gland and duct are not shown n. I., n. I'., the lingual or gustatory' nerve; ch. t., ch. t'., the chorda tympani proceeding from the facial nerve, becoming conjoined with the lingual at n. I1., and afterwards diverging and passing to the gland along the duct; sm. gl., submaxillary ganglion with its roots; n. I., the lingual nerve proceeding to the tongue; a. car., the cartoid artery, two branches of which, a. sm. a. ar.I r. sm. p. pass to the anterior and posterior parts of the gland ; v. sm., the anterior and posterior veins from the gland ending in v.j., the jugular vein; v. sym., the conjoined vagus and sympathetic trunks; gl. cer. s., the superior-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 posterior portion of the gland. The arrows indicate the direction taken by the ner-vous impulses; during reflex stimulations of the gland they ascend to the brain by the lingual and descend by the chorda tympani. (M. Foster.) The sympathetic fibres appear to act principally as a vaso- constrictor nerve, and to exalt the action of the local vaso-motor centres. The sympathetic is more powerful in this direction than the chorda. There is not sufficient evidence in favour of the belief that the submaxillary ganglion is ever the nerve centre which controls the secretion of the submaxillary gland. B. On the Parotid Gland.-The nerves which influence secre- tion in the parotid gland are branches of the facial (lesser super- 274 DIGESTION. [chap, vil ficial petrosal) and of the sympathetic. The former nerve, after passing through the otic ganglion, joins the auriculo-temporal branch of the fifth cerebral nerve, and, with it, is distributed to the gland. The nerves by which the stimulus ordinarily exciting secretion is conveyed to the medulla oblongata, are, as in the case of the submaxillary gland, the fifth, and the glosso-pharyn- geal. The pneumogastric nerves convey a further stimulus to the secretion of saliva, when food has entered the stomach ; the nerve centre is the same as in the case of the submaxillary gland. Changes in the Gland Cells.-The method by which the salivary cells produce the secretion of saliva appears to be divided into aRC Fig. 179.-Alveoli of true salivary gland. A, at rest; B. in tlie first stage of secretion ; C, after prolonged secretion. (Langley.) two stages, which differ somewhat according to the class to which the gland belongs, viz., whether to (i) the true salivary, or (2) to the mucous type. In the former case, it has been noticed, as has been already described (p. 264), that during the rest which follows an active secretion the lumen of the alveolus becomes smaller, the gland cells larger, and very granular. During secre- tion the alveoli and their cells become smaller, and the granular appearance in the latter to a considerable extent disappears, and at the end of secretion, the granules are confined to the inner part of the cell nearest to the lumen, which is now quite distinct (fig- 179)- It is supposed from these appearances that the first stage in the act of secretion consists in the protoplasm of the salivary cell taking up from the lymph certain materials from which it manu- factures the elements of its own secretion, and which are stored up in the form of granules in the cell during rest, the second stage consisting of the actual discharge of these granules, with or without previous change. The granules are taken to represent CHAP. VII.] THE PHARYNX. 275 the chief substance of the salivary secretion, i. e., the ferment ptyalin. In the case of the submaxillary gland of the dog, at any rate, the sympathetic nerve-fibres appear to have to do with the first stage of the process, and when stimulated the protoplasm is extremely active in manufacturing the granules, whereas the chorda tympani is concerned in the production of the second act, the actual discharge of the materials of secretion, together with a considerable amount of fluid, the latter being an actual secretion by the protoplasm, as it ceases to occur when atropin has been subcutaneously injected. In the mucous-secreting gland, the changes in the cells during secretion have been already spoken of (p. 265). They consist in the gradual secretion by the protoplasm of the cell of a substance called mucigen, which is converted into mucin, and discharged on secretion into the canal of the alveoli. The mucigen is, for the most part, collected into the inner part of the cells during rest, pressing the nucleus and the small portion of the protoplasm which remains, against the limiting membrane of the alveoli. The process of secretion in the salivary glands is identical with that of glands in general; the cells which line the ultimate branches of the ducts being the agents by which the special con- stituents of the saliva are formed. The materials which they have incorporated with themselves are almost at once given up again, in the form of a fluid (secretion), which escapes from the ducts of the gland ; and the cells, themselves, undergo disintegration,- again to be renewed, in the intervals of the active exercise of their functions. The source whence the cells obtain the materials of their secretion, is the blood, or, to speak more accurately, the plasma, which is filtered off from the circulating blood into the interstices of the glands as of all living textures. The Pharynx. That portion of the alimentary canal which intervenes between the mouth and the oesophagus is termed the Pharynx (fig. 164)] It will suffice here to mention that it is constructed of a series of three muscles with striated fibres (constrictors), which are covered by a thin fascia externally, and are lined internally by a strong fascia (pharyngeal aponeurosis), on the inner aspect of which is areolar (submucous) tissue and mucous membrane, con- 276 DIGESTION. [chap. VII. tinuous with that of the mouth, and, as regards the part concerned in swallowing, is identical with it in general structure. The epithelium of this part of the pharynx, like that of the mouth, is stratified and squamous. The pharynx is well supplied with mucous glands (fig. 182)- The Tonsils. Between the anterior and posterior arches of the soft palate are situated the Tonsils, one on each side. A tonsil con- sists of an elevation of the mucous mem- brane presenting 12 to 15 orifices, which lead into crypts or recesses, in the walls of which are placed nodules of adenoid or lymphoid tissue (fig. 181). These nodules are enveloped in a less dense adenoid tissue Fig. 180.-Lingual follicle or crypt, a, involution of mucous membrane with its papillae ; b, lymphoid tissues, with several lym- phoid sacs. (Frey.) Fig. 181.-Vertical section through a crypt of the human tonsil, a, entrance to the crypt which is divided below by the elevation which does not quite reach the surface •' I, stratified epithelium; c, masses of adenoid tissue; d, mucous glands cut across • e, fibrous capsule. Semidiagrammatic. (V. D. Harris.) which reaches the mucous surface. The surface is covered with stratified squamous epithelium, and the subepithelial or mucous CHAP. VII.] THE (ESOPHAGUS. 277 membrane proper may present rudimentary papillae formed of adenoid tissue. The tonsil is bounded by a fibrous capsule (fig. 181, f). Into the crypts open the ducts of numerous mucous glands. The viscid secretion which exudes from the tonsils serves to lubricate the bolus of food as it passes them in the second part of the act of deglutition. The (Esophagus or Gullet. The (Esophagus or Gullet (fig. 164), the narrowest portion of the alimentary canal, is a muscular and mucous tube, nine or ten inches in length, which extends from the lower end of the pharynx to the cardiac orifice of the stomach. Structure. - The oesophagus is made up of three coats-viz., the outer, muscular; the middle, sub- mucous; and the inner, mucous. The muscular coat (fig. 183, g and t), is covered externally by a varying amount of loose fibrous tissue. It is composed of two layers of fibres, the outer being arranged longitudinally, and the inner circularly. At the upper part of the oesophagus this coat is made up principally of stri- ated muscle fibres, as they are con- tinuous with the constrictor muscles of the pharynx ; but lower down the unstriated fibres become more and more numerous, and towards the end of the tube form the entire coat. The muscular coat is connected with the mucous coat by a more or less developed layer of areolar tissue, which forms the submucous coat (fig. 183, /), in which is contained in the lower half or third of the tube many mucous glands, the ducts of which, passing through the mucous membrane (fig. 183, c) open on its surface. Sepa- Fig. 182.-Section of a mucous gland from tie tongue. A, opening of the duct on the free surface ; C, base- ment membrane with nuclei; B, flattened epithelial cells lining duct. The duct divides into several branches, which are convoluted and end blindly, being lined through- out by columnar epithelium. D, lumen of one of the tubuli of the gland, x 90. (Klein and Noble Smith.) 278 DIGESTION. [chap. vit. rating this coat from the mucous membrane proper is a well- developed layer of longitudinal, unstriatcd muscle (<7), called the muscularis mucosa;. The mucous membrane is composed of a closely felted meshwork of fine connective tissue, which, towards Fig. 183.-Longitudinal section of the oesophagus of a dog towards the lower end. a, stratified epithelium of the mucous membrane; b, mucous membrane proper; c, duct of mucous gland ; d, muscularis ftiucosee ; e, mucous glands; f, submucous coat; g, circular muscular layer; h, intermuscular layer, in which is contained the ganglion cells of Auerbach; i, longitudinal muscular layer; h, outside investment of fibrous tissue. Semidiagrammatic. (V. D. Harris.) the surface, is elevated into rudimentary papilla?. It is covered with a stratified epithelium, of which the most superficial layers are squamous. The epithelium is arranged upon a basement membrane. In newly-born children the mucous membrane exhibits, in many parts, the structure of lymphoid tissue (Klein). Blood- and lymph-vessels, and nerves, are distributed in the walls of the oesophagus. Between the outer and inner layers of the muscular coat, nerve-ganglia of Auerbach arc also found. CHAP. VII.] TIIE STAGES OF DEGLUTITION. 279 Deglutition or Swallowing. When properly masticated, the food is transmitted in successive portions to the stomach by the act of deglutition or swallow- ing. This, for the purpose of description, may be divided into three, acts. In the first, particles of food collected to a morsel are made to glide between the surface of the tongue and the palatine arch, till they have passed the anterior arch of the fauces ; in the second, the morsel is carried through the pharynx ; and in the third, it reaches the stomach through the oesophagus. These three acts follow each other rapidly, (i.) The first act may be voluntary, although it is usually performed unconsciously; the morsel of food, when sufficiently masticated, being pressed between the tongue and palate, by the agency of the muscles of the former, in such a manner as to force it back to the entrance of the pharynx. (2.) The second act is the most complicated, because the food must pass by the posterior orifice of the nose and the upper opening of the larynx without touching them. When it has been brought, by the first act, between the anterior arches of the palate, it is moved onwards by the movement of the tongue backwards, and by the muscles of the anterior arches contracting on it and then behind it. The root of the tongue being retracted, and the larynx being raised with the pharynx and carried for- wards under the base of the tongue, the epiglottis is pressed over the upper opening of the larynx, and the morsel glides past it; the closure of the glottis being additionally secured by the simul- taneous contraction of its own muscles : so that, even when the epiglottis is destroyed, there is little danger of food or drink pass- ing into the larynx so long as its muscles can act freely. At the same time, the raising of the soft palate, so that its posterior edge touches the back part of the pharynx, and the approximation of the sides of the posterior palatine arch, which move quickly in- wards like side curtains, close the passage into the upper part of the pharynx and the posterior nares, and form an inclined plane, along the under surface of which the morsel descends ; then the pharynx, raised up to receive it, in its turn contracts, and forces it onwards into the oesophagus. (3.) In the third act, in which the food passes through the oesophagus, every part of that tube, as it receives the morsel and is dilated by it, is stimulated to con- tract : hence an undulatory contraction of the oesophagus, which is easily observable in horses while drinking, proceeds rapidly 280 DIGESTION. [chap. vii. along the tube. It is only when the morsels swallowed are large, or taken too quickly in succession, that the progressive contrac- tion of the oesophagus is slow, and attended with pain. Division of both pneumogastric nerves paralyses the contractile power of the oesophagus, and food accordingly accumulates in the tube. The second and third parts of the act of deglutition are involuntary. Nerve Mechanism.-The nerves engaged in the reflex act of deglutition are :-sensory, branches of the fifth cerebral supplying the soft palate ; glosso-pharyngeal, supplying the tongue and pharynx ; the superior laryngeal branch of the vagus, supplying the epiglottis and the glottis; while the motor fibres concerned are :-branches of the fifth, supplying part of the digastric and mylo-hyoid muscles, and the muscles of mastication • the facial, supplying the levator palati; the glosso-pharyngeal, supplying the muscles of the pharynx; the vagus, supplying the muscles of the larynx through the inferior laryngeal branch, and the hypoglossal, the muscles of the tongue. The nerve-centre by which the muscles are harmonised in their action, is situate in the medulla oblongata. In the movements of the oesophagus, the ganglia con- tained in its walls, with the pneumo-gastrics, are the nerve-struc- tures chiefly concerned. It is important to note that the swallowing both of food and drink is a muscular act, and can, therefore, take place in opposition to the force of gravity. Thus, horses and many 'other animals habitually drink up-hill, and the same feat can be performed by jugglers. The Stomach. In man and those Mammalia which are provided with a single stomach, it consists of a dilatation of the alimentary canal placed between and continuous with the oesophagus, which enters its larger or cardiac end on the one hand, and the small intes- tine, which commences at its narrowed end or pylorus, on the other. It varies in shape and size according to its state of distension. The Ruminants (ox, sheep, deer, &c.) possess very complex stomachs ; in most of them four distinct cavities are to be distinguished (fig. 184). 1. The Paunch or Rumen, a very large cavity which occupies the cardiac end, and into which large quantities of food are in the first instance swal- lowed with little or no mastication. 2. The Reticulum, or Honeycomb stomach, so called from the fact that its mucous membrane is disposed in a CHAP. VII.] STRUCTURE OF THE STOMACH. 281 number of folds enclosing hexagonal cells. 3. The Psalterium, m Manyplies, in which the mucous membrane is arranged in very prominent longitudinal folds. 4. Abomasum, Reed, or Rennet, narrow and elongated, its mucous membrane being much more highly vascular than that of the other divisions. In the process of rumination small portions of the contents of the rumen and reticulum are successively regurgitated into the mouth, and there thoroughly masticated and insalivated (chewing the cud) : they are then again swal- lowed, being this time directed by a groove (which in the figure is seen running from the lower end of the oesophagus) into the manyplies, and thence into the abomasum. It will thus be seen that the first two stomachs Fig. 184.-Stomach of a sheep, as, oesophagus; Itu, rumen ; Ret, reticulum; Ps, psalterium, or manyplies; A, abomasum; Dti, duodenum; g, groove from oesophagus to psalte- rium. (Huxley.) (paunch and reticulum) have chiefly the mechanical functions of storing and moistening the fodder : the third (manyplies) probably acts as a strainer, only allowing the finely divided portions of food to pass on into the fourth stomach, where the gastric juice is secreted and the process of digestion carried on. The mucous membrane of the first three stomachs is lowly vascular, while that of the fourth is pulpy, glandular, and highly vascular. In some other animals, as the pig, a similar distinction obtains between the mucous membrane in different parts of the stomach. In the pig the glands in the cardiac end are few and small, while towards the pylorus they are abundant and large. A similar division of the stomach into a cardiac (receptive) and a pyloric (digestive) part, foreshadowing the complex stomach of ruminants, is seen in the common rat, in which these two divisions of the stomach are dis- tinguished, not only by the characters of their lining membrane, but also by a well-marked constriction. In birds the function of mastication is performed by the stomach (gizzard) which in granivorous orders, e.g., the common fowl, possesses very powerful muscular walls and a dense horny epithelium. Structure.-The stomach is composed of four coats, called respectively-an external or (i) peritoneal, (2) muscular, (3) sub- mucous, and (4) mucous coat; with blood-vessels, lymphatics, and nerves distributed in and between them. (1) The peritoneal coat has the structure of serous membranes 282 DIGESTION. [chap. vii. in general. (2) The muscular coat consists of three separate layers or sets of fibres, which, according to their several directions, are named the longitudinal, circular, and oblique. The longitu- dinal set are the most superficial: they are continuous with the longitudinal fibres of the oesophagus, and spread out in a diverging manner over the cardiac end and sides of the stomach. They extend as far as the pylorus, being especially distinct at the lesser or upper curvature of the stomach, along which they pass in several strong bands. The next set are the circular or transverse fibres, which more or less completely encircle all parts of the stomach; they are most abundant at the middle and in the pyloric portion of the organ, and form the chief part of the thick projecting ring of the pylorus. These fibres are not simple circles, but form double or figure-of-8 loops, the fibres intersecting very obliquely. The next, and consequently deepest set of fibres, are the oblique, continuous with the circular muscular fibres of the oesophagus, and having the same double-looped arrangement that prevails in the preceding layer: they are comparatively few in number, and are placed only at the cardiac orifice and portion of the stomach, over both surfaces of which they are spread, some passing obliquely from left to right, others from right to left, around the cardiac orifice, to which, by their interlacing, they form a kind of sphincter, continuous with that around the lower end of the oesophagus. The muscular fibres of the stomach and of the intestinal canal arc unstriated, being composed of elongated, spindle-shaped fibre-cells. (3) and (4) The mucous membrane of the stomach, which rests upon a layer of loose cellular membrane, or submucous tissue, is smooth, level, soft, and velvety; of a pale pink colour during life, and in the contracted state thrown into numerous, chiefly longi- tudinal, folds or rugae, which disappear when the organ is distended. The basis of the mucous membrane is a fine connective tissue, which approaches closely in structure to adenoid tissue; this tissue supports the tubular glands of which the superficial and chief part of the mucous membrane is composed, and passing up between them assists in binding them together. Here and there are to be found in this coat, immediately underneath the glands, masses of adenoid tissue sufficiently marked to be termed by some lym- phoid follicles. The glands are separated from the rest of the mucous membrane by a very fine homogeneous basement membrane. At the deepest part of the mucous membrane are two layers chap, vn.] MUCOUS MEMBRANE OF STOMACH. 283 (circular and longitudinal) of unstriped muscular fibres, called the muscularis mucoscv, which separate the mucous membrane from the scanty submucous tissue. When examined with a lens, the internal or free surface of the sto- mach presents a peculiar honeycomb appearance, produced by shallow polygonal depressions, the diameter of which varies generally from rsoth to of an inch ; but near the py- lorus is as much as of an inch. They are separated by slightly elevated ridges, which sometimes, especially in certain morbid states of the stomach, bear mi- nute, narrow vascular processes, which look like villi, and have given rise to the erroneous supposition that the sto- mach has absorbing villi, like those of the small intestines. In the bot- tom of these little pits, and to some extent be- tween them, minute openings are visible, which are the orifices of the ducts of perpendicu- larly arranged tubular glands (fig. 185), im- bedded side by side in sets or bundles, on the surface of the mucous membrane, and composing nearly the whole structure. Fig. 185.-From a vertical section through the. mucous membrane of the cardiac end of stomach. Two pep- tic glands are shown with a duct common to both, one gland only in part. <1, duct with columnar epithelium becoming shorter as the cells are traced downward; n, neck of gland tubes, with central and parietal or so-called peptic cells ; b, fundus with curved ceecal extremity-the parietal cells are not so numerous here. X 400. (Klein and Noble Smith.) 284 DIGESTION. [chap. vii. Gastric Glands.-Of these there are two varieties, (a) Peptic, (6) Pyloric or Mucous. (а) Peptic glands are found throughout the whole of the stomach except at the pylorus. They are arranged in groups of four or five, which are separated by a fine connective tissue. Two or three tubes often open into one duct, which forms about a third of the whole length of the tube and opens on the surface. The ducts are lined with columnar epithelium. Of the gland tube proper, «.e., the part of the gland below the duct, the upper third is the neck and the rest the body. The neck is narrower than the body, and is lined with granular cubical cells which are continuous with the columnar cells of the duct. Between these cells and the membrana propria of the tubes, are large oval or spherical cells, opaque or granular in ap- pearance, with clear oval nuclei, bulging out the membrana pro- pria ; these cells are called peptic or parietal cells. They do not form a continuous layer. The body, which is broader than the neck and terminates in a blind extremity or fundus near the mus- cularis mucosa), is lined by cells continuous with the cubical or central cells of the neck, but longer, more columnar and more transparent. In this part are a few parietal cells of the same kind as in the neck (fig. 185). As the pylorus is approached the gland ducts become longer, and the tube proper becomes shorter, and occasionally branched at the fundus. (б) Pyloric Glands.-These glands (fig. 187) have much longer ducts than the peptic glands. Into each duct two or three tubes open by very short and narrow necks, and the body of each tube is branched, wavy, and convoluted. The lumen is very large. The ducts are lined with columnar epithelium, and the neck and body with shorter and more granular cubical cells, which corre- spond with the central cells of the peptic glands. During secretion the cells become, as in the case of the peptic glands, larger and the granules restricted to the inner zone of the cell. As they approach the duodenum the pyloric glands become larger, more u c- u - C- b b Fig. 186.- Transverse section through lower part of peptic glands of a cat. a, peptic cells; b, small spheroidal or cubical cells ; c, transverse section of capillaries. (Frey.) CHAT. VII.] LYMPHATICS AND BLOOD-VESSELS OF STOMACH. 285 convoluted and more deeply situated. They are directly continu- ous with Brunner's glands in the duodenum. (Watney.) Changes in the gland cells during secretion.-The chief or cubical cells of the peptic glands, and the corresponding cells of the pyloric glands during the early stage of digestion, if hardened in alcohol, appear swollen and granular, and stain readily. At a later stage the cells become smaller, but more granular and stain even more readily. - The parietal cells swell up, but are otherwise not altered during digestion. The granules, however, in the alcohol- hardened specimen, are believed not to exist in the living cells, but to have been precipitated by the hard- ening re-agent; for if examined during life they appear to be con- fined to the inner zone of the cells, and the outer zone is free from granules, whereas during rest the cell is granular throughout. These granules are thought to be pepsin, or the substance from which pepsin is formed, pepsinogen, which is during rest stored chiefly in the inner zone of the cells and discharged into the lumen of the tube during secretion. (Langley.) Lymphatics.-Lymphatic vessels surround the gland tubes to a greater or less extent. Towards the fundus of the peptic glands are found masses of lymphoid tissue, which may ap- pear as distinct follicles, somewhat like the solitary glands of the small intestine. Blood-vessels.-The blood-vessels of the stomach, which first break up in the sub-mucous tissue, send branches upward between the closely packed glandular tubes, anastomosing around them by means of a fine capillary network, with oblong meshes. Con- tinuous with this deeper plexus, or prolonged upwards from it, so to speak, is a more superficial network of larger capillaries, which branch densely around the orifices of the tubes, and form the Fig. 187.-Section showing the pyloric glands, s, free surface ; d, ducts of pyioric glands; n, neck of same; m, the gland alveoli; mm, muscularis mucosae. (Klein and Noble Smith.) 286 DIGESTION. [chap. vii. framework on which are moulded the small elevated ridges of mucous membrane bounding the minute, polygonal pits before referred to. From this superficial network the veins chiefly take their origin. Thence passing down between the tubes, with no very free connection with the deeper inter-tubular capillary plexus, they open finally into the venous network in the submucous tissue. Nerves. - The nerves of the stomach are derived from the pneumogastric and sympathetic, and form a plexus in the sub- mucous and muscular coats, con- taining many ganglia (Remak, Meissner). -c b d Gastric Juice. Gastric Juice. - The func- tions of the stomach are to secrete a digestive fluid (gastric juice), to the action of which the food is subjected after it has entered the cavity of the stomach from the oesophagus; to thoroughly incorporate the fluid with the food by means of its muscular movements; and to absorb such sub- stances as are ready for absorption. While the stomach contains no food, and is inactive, no gastric fluid is secreted ; and mucus, which is either neutral or slightly alkaline, covers its surface. But immediately on the introduction of food or other substance the mucous membrane, previously quite pale, becomes slightly turgid and reddened with the influx of a larger quantity of blood ; the gastric glands commence secreting actively, and an acid fluid is poured out in minute drops, which gradually run together and flow down the walls of the stomach, or soak into the substances within it. Chemical Composition.-The first accurate analysis of gastric juice was made by Prout: but it does not appear to have been collected in any large quantity, or pure and separate from food, Fig. 188.-Plan of the blood-vessels of the stomach, as they would be seen in a vertical section, a, arteries, passing up from the vessels of submucous coat; b, capillaries branching between and around the tubes ; c, superficial plexus of capillaries occupying the ridges of the mucous membrane; <Z, vein formed by the union of veins which, having collected the blood of the superficial capillary plexus, are seen passing down between the tubes. (Brinton.) CHAP. VII.] THE GASTRIC JUICE. 287 until the time when Beaumont was enabled, by a fortunate circum- stance, to obtain it from the stomach of a man named St. Martin, in whom there existed, as the result of a gunshot wound, an open- ing leading directly into the stomach, near the upper extremity of the great curvature, and three inches from the cardiac orifice. The introduction of any mechanical irritant, such as the bulb of a thermometer, into the stomach, through this artificial opening, excited at once the secretion of gastric fluid. This was drawn off, and was often obtained to the extent of nearly an ounce. The introduction of alimentary substances caused a much more rapid and abundant secretion than did other mechanical irritants. No increase of temperature could be detected during the most active secretion; the thermometer introduced into the stomach always stood at too0 F. (37'8° C.) except during muscular exertion, when the temperature of the stomach, like that of other parts of the body, rose one or two degrees higher. The chemical composition of human gastric juice has been also investigated by Schmidt. The fluid in this case was obtained by means of an accidental gastric fistula, which existed for several years below the left mammary region of a patient between the cartilages of the ninth and tenth ribs. The mucous membrane was excited to action by the introduction of some hard matter, such as dry peas, and the secretion was removed by means of an elastic tube. The fluid thus obtained was found to be acid, limpid, odourless, with a mawkish taste-with a specific gravity of 1002, or a little more. It contained a few cells, seen with the microscope, and some fine granular matter. The analysis of the fluid obtained in this way is given below. The gastric juice of dogs and other animals obtained by the introduction into the stomach of a clean sponge through an artificially made gastric fistula, shows a decided difference in composition, but possibly this is due, at least in part, to admixture with food. Chemical Composition of Gastric Juice. Water Solids Dogs. 971'17 28'82 Human. 994'4 5'39 Solids- Ferment-Pepsin 17-5 3'19 Hydrochloric acid (free) 27 '2 Salts- Calcium, sodium, and potassium, chlorides ; and calcium, magnesium, and iron, phos- phates . . 8'57 2'18 288 DIGESTION. [chap. vii. The quantity of gastric juice secreted daily has been variously estimated; but the average for a healthy adult may be assumed to range from ten to twenty pints in the twenty-four hours. The acidity of the fluid is due to free hydrochloric acid, although other acids, e.g., lactic, acetic, butyric, are not unfrequently to be found therein as products of gastric digestion or abnormal fermen- tation. The amount of hydrochloric acid varies from 2 to -2 per 1000 parts. In healthy gastric juice the amount of free acid may be as much as -2 per cent. As regards the formation of pepsin and acid, the former is produced by the central or chief cells of the peptic glands, and also most likely by the similar cells in the pyloric glands ; the acid is chiefly found at the surface of the mucous membrane, but is in all probability formed by the secreting action of the parietal cells of the peptic glands, as no acid is formed by the pyloric glands in which this variety of cell is absent. The ferment Pepsin can be procured by digesting portions of the mucous membrane of the stomach in cold water, after they have been macerated for some time in water at a temperature 8o°-ioo° F.(27>O-37'8° C.) The warm water dissolves various substances as well as some of the pepsin, but the cold water takes up little else than pepsin, which is contained in a greyish-brown viscid fluid, on evaporating the cold solution. The addi- tion of alcohol throws down the pepsin in greyish-white flocculi. Glycerine also has the property of dissolving out the ferment ; and if the mucous membrane be finely minced, and the moisture removed by absolute alcohol, a powerful extract may be obtained by throwing into glycerine. Functions.- The digestive power of the gastric juice depends on the pepsin and acid contained in it, both of which are, under ordinary circumstances, necessary for the process. The general effect of digestion in the stomach is the conversion of the food into chyme, a substance of various composition accord- ing to the nature of the food, yet always presenting a character- istic thick, pultaceous, grumous consistence, with the undigested portions of the food mixed in a more fluid substance, and a strong, disagreeable acid odour and taste. The chief function of the gastric juice is to convert proteids into peptones. This action may be shown by adding a little gastric juice (natural or artificial) to some diluted egg-albumin, and keep- ing the mixture at a temperature of about ioo° F. (37'8° C.); it is soon found that the albumin cannot be precipitated on boiling, but that if the solution be neutralised with an alkali, a preci- CHAP. VII.] PEPTONES. 289 pitate of acid-albumin is thrown down. After a while the propor- tion of acid-albumin gradually diminishes, so that at last scarcely any precipitate results on neutralization, and finally it is found that all the albumin has been changed into another proteid sub- stance which is not precipitated on boiling or on neutralization. This is called peptone. Characteristics of Peptones.- Peptones have certain characteristics which distinguish them from other proteids. i. They are diffu- sible, i.e., they possess the property of passing through animal membranes. 2. They cannot be precipitated by heat, by nitric, or acetic acid, or by potassium ferrocyanide and acetic acid. They are, however, thrown down by tannic acid, by mercuric chloride and by picric acid. 3. They are very soluble in water and in neutral saline solutions. In their diffusibility peptones differ remarkably from egg- albumin, and on this diffusibility depends one of their chief uses. Egg-albumin as such, even in a state of solution, would be of little service as food, inasmuch as its indiffusibility would effec- tually prevent its passing by absorption into the blood-vessels of the stomach and intestinal canal. Changed, however, by the action of the gastric juice into peptones, albuminous matters diffuse readily, and are thus quickly absorbed. After entering the blood the peptones are very soon again modified, so as to re-assume the chemical characters of albumin, a change as necessary for preventing their diffusing out of tbe blood-vessels, as the previous change was for enabling them to pass in. This is effected, probably, in great part by the agency of the liver. Products of Gastric Digestion.-The chief product of gastric digestion is undoubtedly peptone. We have seen, however, in the above experiment that there is a by-product, and this is almost identical with syntonin or acid albumin. This body is probably not exactly identical, however, with syntonin, and its old name of parapeptone had better be retained. The conversion of native albumin into acid-albumin may be effected by the hydrochloric acid alone, but the further action is undoubtedly due to the ferment and the acid together, as although under high pressure any acid solution may, it is said, if strong enough, produce the entire conversion into peptone, under the condition of digestion in the stomach this w'ould be quite impossible; and, on the other hand, pepsin will not act without the presence of acid. The pro- 290 DIGESTION. [chap. vii. duction of two forms of peptone is usually recognised, called respectively onfo'-peptone and Ziewu'-peptone. Their differences in chemical properties have not yet been made out, but they are distinguished by this remarkable fact, that the pancreatic juice, while possessing no action over the former, is able to con- vert the latter into leucin and tyrosin. Pepsin acts the part of a hydrolytic ferment (proteolytic), and appears to cause hydration of albumin, peptone being a highly hydrated form of albumin. Circumstances favouring Gastric Digestion.-i. A temperature of about ioo° F. (37'8° C.); at 32° F. (o° C.) it is delayed, and by boiling is altogether stopped. 2. An acid medium is necessary. Hydrochloric is the best acid for the purpose. Excess of acid or neutralization stops the process. 3. The re- moval of the products of digestion. Excess of peptone delays the action. AcZwn of the Gastric Juice on Bodies other than Proteids. -All proteids are converted by the gastric juice into pep- tones, and, therefore, whether they be taken into the body in meat, eggs, milk, bread, or other foods, the resultant still is peptone. Milk is curdled, the casein being precipitated, and then dissolved. The curdling is due to a special ferment of the gastric juice (curdling or rennet ferment}, and is not due to the action of the free acid only. The effect of rennet, which is a decoction of the fourth stomach of a calf in brine, has long been known, as it is used extensively to cause precipitation of casein in cheese manufacture. The ferment which produces this curdling action is distinct from pepsin. Gelatin is dissolved and changed into peptone, as are also chondrin and elastin; but Mucin, and the Horny tissues, which contain keratin generally are unaffected. On the Amylaceous articles of food, and upon pure Oleaginous principles the gastric juice has no action. In the case of adipose tissue, its effect is to dissolve the areolar tissue, albuminous cell- walls, <tc., which enter into its composition, by which means the fat is able to mingle more uniformly with the other constituents of the chyme. The gastric fluid acts as a general solvent for some of the saline constituents of the food, as, for example, particles of common salt, which may happen to have escaped solution in the CHAP. VII.] GASTRIC DIGESTION. 291 saliva ; while its acid may enable it to dissolve some other salts which are insoluble in the latter or in water. It also dissolves cane sugar, and by the aid of its mucus causes its conversion in part into grape sugar. The action of the gastric juice in preventing and checking putrefaction has been often directly demonstrated. Indeed, that the secretions which the food meets with in the alimentary canal are antiseptic in their action, is what might be antici- pated, not only from the proneness to decomposition of organic matters, such as those used as food, especially under the in- fluence of warmth and moisture, but also from the well-known fact that decomposing flesh (e.g., high game) may be eaten with impunity, while it would certainly cause disease were it allowed to enter the blood by any other route than that formed by the organs of digestion. Time occupied in Gastric Digestion.-Under ordinary condi- tions, from three to four hours may be taken as the average time occupied by the digestion of a meal in the stomach. But many circumstances will modify the rate of gastric digestion. The chief are : the nature of the food taken and its quantity (the stomach should be fairly filled-not distended) ; the time that has elapsed since the last meal, which should be at least enough for the stomach to be quite clear of food; the amount of exercise previous and subsequent to a meal (gentle exercise being favourable, over-exertion injurious to digestion) ; the state of mind (tranquillity of temper being essential, in most cases, to a quick and due digestion) ; the bodily health; and some others. Movements of the Stomach.-The gastric fluid is assisted in accomplishing its share in digestion by the movements of the stomach. In granivorous birds, for example, the contraction of the strong muscular gizzard affords a necessary aid to digestion, by grinding and triturating the hard seeds which constitute part of the food. But in the stomachs of man and other Mammalia the movements of the muscular coat are too feeble to exercise any such mechanical force on the food ; neither are they needed, for mastication has already done the mechanical work of a gizzard ; and experiments have demonstrated that substances are digested even enclosed in perforated tubes, and consequently protected from mechanical influence. The normal actions of the muscular fibres of the human 292 DIGESTION. [chap. vii. stomach appear to have a three-fold purpose: (i) to adapt the stomach to the quantity of food in it, so that its walls may be in contact with the food on all sides, and, at the same time, may exercise a certain amount of compression upon it; (2) to keep the orifices of the stomach closed until the food is digested ; and (3) perform certain peristaltic movements, whereby the food, as it becomes chymified, is gradually propelled towards, and ultimately through, the pylorus. In accomplishing this latter end, the movements without doubt materially contribute towards effecting a thorough intermingling of the food and the gastric fluid. When digestion is not going on, the stomach is uniformly con- tracted, its orifices not more firmly than the rest of its walls ; but, if examined shortly after the introduction of food, it is found closely encircling its contents, and its orifices are firmly closed like sphincters. The cardiac orifice, every time food is swallowed, opens to admit its passage to the stomach, and immediately again closes. The pyloric orifice, during the first part of gastric digestion, is usually so completely closed, that even when the stomach is separated from the intestines, none of its contents escape. But towards the termination of the digestive process, the pylorus seems to offer less resistance to the passage of substances from the stomach ; first it yields to allow the successively digested portions to go through it; and then it allows the transit of even undigested substances. It appears that food, so soon as it enters the stomach, is subjected to a kind of peristaltic action of the muscular coat, whereby the digested portions are gradually moved towards the pylorus. The movements were observed to increase in rapidity as the process of chymification advanced, and were continued until it was completed. The contraction of the fibres situated towards the pyloric end of the stomach seems to be more energetic and more decidedly peristaltic than those of the cardiac portion. Thus, it was found in the case of St. Martin, that when the bulb of the thermo- meter was placed about three inches from the pylorus, through the gastric fistula, it was tightly embraced from time to time, and drawn towards the pyloric orifice for a distance of three or four inches. The object of this movement appears to be, as just said, to carry the food towards the pylorus as fast as it is formed into chyme, and to propel the chyme into the duodenum ; the undigested portions of food being kept back ( hap. vi 1.] INFLUENCE OF THE NERVOUS SYSTEM. 293 until they are also reduced into chyme, or until all that is digestible has passed out. The action of these fibres is often seen in the contracted state of the pyloric portion of the stomach after death, when it alone is contracted and firm, while the cardiac portion forms a dilated sac. Sometimes, by a pre- dominant action of strong circular fibres placed between the cardia and pylorus, the two portions, or ends as they are called, of the stomach, are partially separated from each other by a kind of hour- glass contraction. By means of the peristaltic action of the mus- cular coats of the stomach, not merely is chymified food gradually propelled through the pylorus, but a kind of double current is continually kept up among the contents of the stomach, the circumferential parts of the mass being gradually moved onward towards the pylorus by the contraction of the muscular fibres, while the central portions are propelled in the opposite direction, namely, towards the cardiac orifice; in this way is kept up a constant circulation of the contents of the viscus, highly con- ducive to their free mixture with the gastric fluid and to their ready digestion. Influence of the Nervous System on Gastric Digestion. -The normal movements of the stomach during gastric digestion are directly connected with the plexus of nerves and ganglia con- tained in its walls, the presence of food acting as a stimulus which is conveyed to the ganglia and reflected to the muscular fibres. The stomach is, however, also directly connected with the higher nerve-centres by means of branches of the vagus and solar plexus of the sympathetic. The vaso-motor fibres of the latter are derived, probably, from the splanchnic nerves. The exact function of the vagi in connection with the move- ments of the stomach is not certainly known. Irritation of the vagi produces contraction of the stomach, if digestion is proceed- ing ; while, on the other hand, peristaltic action is retarded or stopped, when these nerves are divided. Bernard, watching the act of gastric digestion in dogs which had fistulous openings into their stomachs, saw that on the instant of dividing their vagic nerves, the process of digestion was stopped, and the mucous membrane of the stomach, pre- viously turgid with blood, became pale, and ceased to secrete. These facts may be explained by the theory that the vagi are the media by which, during digestion, an inhibitory impulse is conducted to the vaso-motor centre in the medulla ; such impulse 294 DIGESTION. [chap. vii. being reflected along the splanchnic nerves to the blood-vessels of the stomach, and cansing their dilatation (Rutherford). From other experiments it may be gathered, that although divi- sion of both vagi always temporarily suspends the secretion of gastric fluid, and so arrests the process of digestion, being occa- sionally followed by death from inanition; yet the digestive powers of the stomach may be completely restored after the opera- tion, and the formation of chyme and the nutrition of the animal may be carried on almost as perfectly as in health. This would indicate the existence of a special local nervous mechanism which controls the secretion. Bernard found that galvanic stimulus of these nerves excited an active secretion of the fluid, while a like stimulus applied to the sympathetic nerves issuing from the semilunar ganglia, caused a diminution and even complete arrest of the secretion. The influence of the higher nerve-centres on gastric digestion, as in the case of mental emotion, is too well known to need more than a reference. Dig-estion of the Stomach after Death.-If an animal die during the process of gastric digestion, and when, therefore, a quantity of gastric juice is present in the interior of the stomach, the walls of this organ itself are frequently themselves acted on by their own secretion, and to such an extent, that a perforation of considerable size may be produced, and the contents of the stomach may in part escape into the cavity of the abdomen. This phenomenon is not unfrequently observed in post-mortem examina- tions of the human body. If a rabbit be killed during a period of digestion, and afterwards exposed to artificial warmth to prevent its temperature from falling, not only the stomach, but many of the surrounding parts will be found to havelbeen dissolved (Pavy). From these facts, it becomes an interesting question why, during life, the stomach is free from liability to injury from a secretion, which, after death, is capable of such destructive effects ? It is only necessary to refer to the idea of Bernard, that the living stomach finds protection from its secretion in the presence of epithelium and mucus, which are constantly renewed in the same degree that they are constantly dissolved, in order to remark that although the gastric mucus is probably protective, this theory, so far as the epithelium is concerned, has been disproved by experiments of Pavy's, in which the mucous mem- brane of the stomachs of dogs was dissected off for a small space, and, on killing the animals some days afterwards, no sign of digestion of the stomach was visible. " Upon one occasion, after removing the mucous membrane, and exposing the muscular fibres over a space of about an inch and a half in diameter, the animal was allowed to live for ten days. It ate food every day, and seemed scarcely affected by the operation. Life was destroyed whilst digestion was being carried on, and the lesion in the stomach was found very nearly repaired ; new matter had been deposited in the place chap, vn.] THE ACT OF VOMITING. 295 of what had been removed, and the. denuded spot had contracted to much less than its original dimensions." Pavy believes that the natural alkalinity of the blood, which circulates so freely during life in the walls of the stomach, is sufficient to neutralize the acidity of the gastric juice ; and as may be gathered from what has been previously said, the neutralization of the acidity of the gastric secre- Fig. 189.-Auerbach's nerve-plexus in small intestine. The plexus consists of flbrillated substance, and is made up of trabeculse of various thicknesses. Nucleus-like elements and ganglion-cells are imbedded in the plexus, the whole of which is enclosed in a nucleated sheath. (Klein.) tion is quite sufficient to destroy its digestive powers ; but the experiments adduced in favour of this theory are open to many objections, and afford only a negative support to the conclusions they are intended to prove. Again, the pancreatic secretion acts best on proteids in an alkaline medium; but it has no digestive action on the living intestine. It must be confessed that no entirely satisfactory theory has been yet stated. The expulsion of the contents of the stomach in vomiting, like that of mucus or other matter from the lungs in coughing, is preceded by an inspiration ; the glottis is then closed, and imme- diately afterwards the abdominal muscles strongly act; but here occurs the difference in the two actions. Instead of the vocal cords yielding to the action of the abdominal muscles, they remain tightly closed. Thus the diaphragm being unable to go up, forms an unyielding surface against which the stomach can be Vomiting. 296 DIGESTION. [chap, vii. pressed. In this way, as well as by its own contraction, the diaphragm is fixed, to use a technical phrase. At the same time the cardiac sphincter-muscle being relaxed, and the orifice which it naturally guards being actively dilated, while the pylorus is closed, and the stomach itself also contracting, the action of the abdominal muscles, by these means assisted, expels the contents of the organ through the oesophagus, pharynx, and mouth. The reversed peristaltic action of the oesophagus probably increases the effect. It has been frequently stated that the stomach itself is quite passive during vomiting, and that the expulsion of its contents is effected solely by the pressure exerted upon it when the capacity of the abdomen is diminished by the contraction of the diaphragm, and subsequently of the abdominal muscles. The experiments and observations, however, which are supposed to confirm this statement, only show that the contraction of the abdominal muscles alone is sufficient to expel matters from an unresisting bag through/the oesophagus; and that, under very abnormal circumstances, the stomach, by itself, cannot expel its contents. They by no means show that in ordinary vomiting the stomach is passive; and, on the other hand, there are good reasons for believing the contrary. It is true that facts are wanting io demonstrate with certainty this action of the stomach in vomiting; but some of the cases of fistulous opening into the organ appear to support the belief that it does take place; and the analogy of the case of the stomach with that of the other hollow viscera, as the rectum and bladder, may be also cited in confirmation. The muscles concerned in the act of vomiting, are chiefly and primarily those of the abdomen; the diaphragm also acts, but usually not as the muscles of the abdominal walls do. They contract and compress the stomach more and more towards the diaphragm; and the diaphragm (which is usually drawn down in the deep inspiration that precedes each act of vomiting) is fixed, and presents an unyielding surface against which the stomach may be pressed. The diaphragm is, therefore, as a rule passive, during the actual expulsion of the contents of the stomach. But there are grounds for believing that sometimes this muscle actively contracts, so that the stomach is, so to speak, squeezed between the descending diaphragm and the retracting abdominal walls. CHAP. VII.] THE INTESTINES 297 Some persons possess the power of vomiting at ivill, without applying any undue irritation to the stomach, but simply by a voluntary effort. It seems also, that this power may be acquired by those who do not naturally possess it, and by continual prac- tice may become a habit. There are cases also of rare occurrence in which persons habitually swallow their food hastily, and nearly unmasticated, and then at their leisure regurgitate it, piece by piece, into their mouth, remasticate, and again swallow it, like members of the ruminant order of Mammalia. The various nerve-actions concerned in vomiting are governed by a nerve-centre situate in the medulla oblongata. The sensory nerves are the fifth, glosso-pharyngeal and vagus principally ; but, as well, vomiting may occur from stimulation of sensory nerves from many organs, e.g., kidney, testicle, Arc. The centre may also be stimulated by impressions from the cerebrum and cerebellum, so-called central vomiting occurring in disease of those parts. The efferent impulses are carried by the phrenics and other spinal nerves. The Intestines. The Intestinal canal is divided into two chief portions, named from their differences in diameter, the (I.) small and (II.) large intestine (fig. 164). These are continuous with each other, and communicate by means of an opening guarded by a valve, the ileo-caecal valve, which allows the passage of the products of digestion from the small into the large bowel, but not, under ordinary circumstances, in the opposite direction. I. The Small Intestine.-The Small Intestine, the average length of which in an adult is about twenty feet, has been divided, for convenience of description, into three portions, viz., the duo- denum, which extends for eight or ten inches beyond the pylorus; the jejunum, which forms two-fifths, and the ileum, which forms three-fifths of the rest of the canal. Structure.-The small intestine, like the stomach, is constructed of four principal coats, viz., the serous, muscular, sub-mucous, and mucous. (1.) The serous coat, formed by the visceral layer of the peri- toneum, and has the structure of serous membranes in general. (2.) The muscular coats consist of an internal circular and an external longitudinal layer: the former is usually considerably 298 DIGESTION. [chap. vii. the thicker. Both alike consist of bundles of unstriped muscular tissue supported by connective tissue. They are well provided with lymphatic vessels, which form a set distinct from those of the mucous membrane. Between the two muscular coats is a nerve-plexus (Auerbach's plexus, plexus myentericus) (fig. 189), similar in structure to Meissner's (in the submucous tissue), but with more numer- ous ganglia. This plexus regu- lates the peristaltic move- ments of the muscular coats of the intestines. (3.) Between the mucous and muscular coats, is the submucous coat, which consists of connective tissue, in which numerous blood vessels and lymphatics ramify. A fine plexus, consisting mainly of non-medullated nerve-fibres, Meissner's plexus, with gang- lion cells at its nodes, occurs in the submucous tissue from the stomach to the anus. From the position of this plexus and the distribution of its branches, it seems highly probable that it is the local centre for regulating the calibre of the blood-vessels supplying the intestinal mucous membrane, and presiding over the processes of secretion and absorption. (4.) The mucous membrane is the most important coat in rela- tion to the function of digestion. The following structures, which enter into its composition, may now be successively described :- the valvulce conniventes ; the villi ; and the glands. The general structure of the mucous membrane of the intestines resembles that of the stomach (p. 282), and, like it, is lined on its inner surface by columnar epithelium. Adenoid tissue (fig. 190, c and d) enters largely into its construction; and on its deep surface is the muscularis mucosce (mm, fig. 191), the fibres of which are arranged in two layers: the outer longitudinal and the inner circular. Valvulee Conniventes.-The valvula; conniventes (fig. 192) com- mence in the duodenum, about one or two inches beyond the Fig. 190.-Horizontal section of a small fragment of the mucous membrane, including one entire crypt of Lieberkiilin and parts of several others: a, cavity of the tubular glands or crypts; b, one of the lining epithelial cells ; c, the lymphoid or reti- form spaces, of which some are empty, and others occupied by lymph cells, as at d. CHAP. VII.] VALVULE CONNIVENTES. 299 pylorus, and. becoming larger and more numerous immediately beyond the entrance of the bile duct, continue thickly arranged and well developed throughout the jejunum; then, gradually diminishing in size and number, they cease near the middle of the ileum. They are formed by a doubling inwards of the mucous membrane; the crescentic, nearly circular, folds thus formed being arranged trans- versely to the axis of the in- testine, and each individual fold seldom extending around more than | or f of the bowel's circumference. Unlike the rugae in the oesophagus and stomach, they do not disap- pear on distension of the canal. Only an imperfect notion of their natural position and function can be obtained by looking at them after the intes- tine has been laid open in the usual manner. To understand them aright, a piece of gut should be distended either with air or alcohol, and not opened until the tissues have become hardened. On then making a section it will be seen that, instead of disappearing, they stand out at right angles to the general surface of the mucous membrane (fig. 192). Their functions are (1) that they offer a largely increased surface for secretion and absorption, and (2) that they prevent the too rapid passage of the very liquid products of gastric diges- tion, immediately after their escape from the stomach, and (3), by their projection, and consequent interference with an uniform and untroubled current of the intestinal contents, that they assist in the more perfect mingling of the latter with the secretions poured out to act on them. Fig. 191.-Vertical section through portion of small intestine of dog. v, two villi showing e, epithelium; g, goblet cells. The free surface is seen to be formed by the " striated basilar border," while inside the villus the adenoid tissue and un- striped muscle-cells are seen; If, Lieber- kuhn's follicles; m m, muscularis mu- cosre, sending up fibres between the follicles into the villi; sm, submucous tissue; containing {gin), ganglion cells of Meissner's plexus. (Schofield.) 300 DIGESTION. [chap. vn. Glands.-The glands are of three principal kinds :-viz., those of (i) Lieberkuhn, (2) Brunner, and (3) Peyer. (1.) The glands or crypts of Lieberkuhn are simple tubular de- pressions of the intestinal mucous membrane, thickly distributed over the whole surface both of the large and small intestines. In the small intestine they are visible only with the aid of a lens; and their orifices appear as minute dots scattered between the villi. They are larger in the large intestine, and increase in size the nearer they approach the anal end of the intestinal tube; and in the rectum their orifices may be visible to the naked eye. In length they vary from to of a line. Each tubule (fig. 194) is constructed of the same essential parts as the intestinal mucous membrane, viz., of a fine mcwz&rana propria, or basement membrane, a layer of cylindrical epithelium lining it, and capillary blood- vessels covering its exterior, the free surface of the columnar cells presenting an appear- ance precisely similar to the " striated basilar border " which covers the villi. Their con- tents appear to vary, even in health ; the varieties being dependent, probably, on the period of time in relation to digestion at which they are examined. Among the columnar cells of Lieberkuhn's follicles, goblet cells frequently occur (fig. 193). (2.) Brunner's glands (fig. 196) are confined to the duodenum; they are most abundant and thickly set at the commencement of this portion of the intestine, diminishing gradually as the duo- denum advances. They are situated beneath the mucous mem- brane, and imbedded in the submucous tissue, each gland is a branched and convoluted tube, lined with columnar epithelium. As before said, in structure they are very similar to the pyloric glands of the stomach, and their epithelium undergoes a similar change during secretion; but they are more branched and con- voluted and their ducts arc longer. (Watney.) The duct of each gland passes through the muscularis mucosa), and opens on the surface of the mucous membrane. (3.) The glands of Peyer occur chiefly but not exclusively in the Fig. 192.-Piece of small intestine (previously dis- tended and hardened by alcohol) laid open to show the normal posi- tion of the valvulee con- niventes. CHAP. VII.] GLANDS OF THE INTESTINES. 301 small intestine. They are found in greatest abundance in the lower part of the ileum near to the ileo-caecal valve. They are met with in two conditions, viz., either scattered singly, in which case they are termed glandute solitarte, or aggregated in groups varying from one to three inches in length and about half-an-inch Fig. 193. Transverse section through four crypts of Lieberkuhn from the large intestine of the pig. They are lined hy columnar epithelial cells, the nuclei being placed in the outer part of the cells. The divisions between the cells are seen as lines radiating from L, the lumen of the crypt; G, epithelial cells, which have become transformed into goblet cells. X 350. (Klein and Noble Smith.) Fig. 194. J gland of Lieberkuhn in lon- gitudinal section. (Brinton.) in width, chiefly of an oval form, their long axis parallel with that of the intestine. In this state, they are named glandute agmi- nate, the groups being commonly called Peyer's patches (fig. 197), and almost always placed opposite the attachment of the mesen- tery. Tn structure, and in function, there is no essential difference between the solitary glands and the individual bodies of which each group or patch is made up. They are really single or aggre- gated masses of adenoid tissue forming lymph-follicles. In the condition in which they have been most commonly examined, each gland appears as a circular opaque-white rounded body, from to ~ inch in diameter, according to the degree in which it is developed. They are principally contained in the submucous coat, but sometimes project through the muscularis mucosa into the mucous membrane. Tn the agminate glands, each follicle reaches the free surface of the intestine, and is covered with columnar epithelium. Each gland is surrounded by the openings of Liebcrkiihn's follicles. 302 DIGESTION. [chap. vii. The adjacent glands of a Peyer's patch are connected together by adenoid tissue. Sometimes the lymphoid tissue reaches the free surface, replacing the epithelium, as is also the case with some of the lymphoid follicles of the tonsil (p. 276). Peyer's glands are surrounded by lymphatic sinuses which do not penetrate into their interior; the interior is, however, traversed Fig. 195.-Transverse section of injected Peyer's glands (from Kolliker). The drawing was taken from a preparation made by Frey: it represents the tine capillary-looped net- work spreading from the surrounding blood-vessels into the interior of three of Peyer's capsules from the intestine of the rabbit. by a very rich blood capillary plexus. If the vermiform appendix of a rabbit which consists largely of Peyer's glands be injected with blue by pressing the point of a fine syringe into one of the lym- phatic sinuses, the Peyer's glands will appear as greyish white spaces surrounded by blue; if now the arteries of the same be injected with red, the greyish patches will change to red, thus proving that they are surrounded by lymphatic spaces but pene- trated by blood-vessels. The lacteals passing out of the villi com- municate with the lymph sinuses round Peyer's glands. It is to be noted that they are largest and most prominent in children and young persons. Villi.-The Villi (figs. 191, 196, 198, and 199), are confined CHAP. VII.] VILLI. 303 exclusively to the mucous membrane of the small intestine. They are minute vascular processes, from a quarter of a line to a line and two-thirds in length, covering the surface of the mucous membrane, and giving it a peculiar velvety, fleecy appearance. Krause estimates them at fifty to ninety in number in a square line at the upper part of the small intestine, and at forty to seventy in the same area at the lower part. They vary in form even in the same animal, and differ according as the lymphatic vessels they contain are empty or full of chyle; being usually, in the former case, flat and pointed at their summits, in the latter cylindrical or cleavate. Each villus consists of a small projection of mucous membrane, and its interior is therefore supported throughout by fine adenoid tissue, which forms the framework or stroma in which the other constituents are contained. The surface of the villus is clothed by columnar epithelium, which rests on a fine basement membrane; while within this are found, reckon- ing from without inwards, blood- vessels, fibres of the muscularis mu- cosa, and a single lymphatic or lac- teal vessel rarely looped or branched (fig. 200); besides granular matter, fat-globules, etc. The epithelium is of the columnar kind, and continuous with that lining the other parts of the mucous membrane. The cells are arranged with their long axis radiating from the surface of the villus (fig. 199), and their smaller ends resting on the base- ment membrane. The free surface of the epithelial cells of the villi, like that of the cells which cover the general surface of the mucous membrane, is covered by a fine border which exhibits Fig'. 196.- Vertical section of duode- num, showing a, villi; ft, crypts of Lieberkuhn, and c, Brunner's glands in the subniucosa s, with ducts, d; muscularis mucosae, m ; and circular muscular coat f. (Scho- field.) 304 DIGESTION. [CHAP. VII. very delicate striations whence it derives its name, "striated basilar border." Beneath the basement or limiting membrane there is a rich supply of blood-vessels. Two or more minute arteries are distri- buted within each villus; and from their capillaries, which form a dense network, proceed one or two small veins, which pass out at the base of the villus. The layer of the inusciilaris mucosae in the villus forms a kind Fig. 197.-Agminate follicles, or Peye.i's patch, in a state of distension, x 5. (Boehm.) of thin hollow cone immediately around the central lacteal, and is, therefore, situate beneath the blood-vessels. It is with- out doubt instrumental in the propulsion of chyle along the lacteal. The lacteal vessel enters the base of each villus, and passing up in the middle of it, extends nearly to the tip, where it ends commonly by a closed and somewhat dilated extremity. In the larger villi there may be two small lacteal vessels which end by a loop (fig. 200), or the lacteals may form a kind of network in the villus. The last method of ending, however, is rarely or never seen in the human subject, although common in some of the lower animals (a, fig. 201). The office of the villi is the absorption of chyle and other liquids from the intestine. The mode in which they effect this will be considered in the next Chapter. II. The Large Intestine.-The Large Intestine, which in an adult is from about 4 to 6 feet long, is subdivided for descriptive purposes into three portions (fig. 164) viz. :-the ccecum, a short wide pouch, communicating with the lower end of the small CHAP. VII.] STRUCTURE OF THE LARGE INTESTINE. 305 intestine through an opening, guarded by the ileo-ccecal valve ; the colon, continuous with the pseCum, which forms the principal part Fig. 198.-Section of small intestine, showing villi, Lieberkuhn's glands and a Peyer's solitary gland, m, in, muscularis mucosae. (Klein and Noble Smith.) of the large intestine, and is divided into ascending, transverse, and descending portions; and the rectum, which, after dilating at its lower part, again contracts, and immediately afterwards opens Fig. 199.- Vertical section of a villus of the small intestine of a cat,, a, striated basilar border of the epithelium ; Z>, columnar epithelium ; <•, goblet cells ; d, central lymph-vessel: e, smooth muscular fibres ; f, adenoid stroma of the villus in which lymph corpuscles lie. (Klein.) *15 1 externally through the anus. Attached to the is the small append ix vermiform is. Structure.- Like the small intestine, the large intestine is con- structed of four principal coats, viz., the serous, rnuscular, sub- 306 DIGESTION. [CHAP. VII. mucous, and mucous. The serous coat need not be here particu- larly described. Connected with it are the small processes of peritoneum containing fat, called appendices epiploica?. The fibres of the muscular coat, like those of the small intestine, are arranged in two layers-the outer longitudinal, the inner circular. In the caecum and colon, the longitudinal fibres, besides being, as in the Fig. zoo.-A. Villus of shtep. H. Villi of man. (Slightly altered from Teiehmann.) small intestine, thinly disposed in all parts of the wall of the bowel, are collected, for the most part, into three strong bands, which, being shorter, from end to end, than the other coats of the intestine, hold the canal in folds, bounding intermediate sacculi. On the division of these bands, the intestine can be drawn out to its full length, and it then assumes, of course, an uniformly cylindrical form. In the rectum, the fascicidi of these longitu- dinal bands spread out and mingle with the other longitudinal fibres, forming with them a thicker layer of fibres than exists on chap, vii.] MUCOUS MEMBRANE OF LARGE INTESTINE. 307 any other part of the intestinal canal. The circular muscular fibres are spread over the whole surface of the bowel, but are somewhat more marked in the intervals between the sacculi. Towards the lower end of the rectum they become more numerous, Fig. 201.-Diagram of lacteal vessels in small intestine. A, lacteals in villi; p, Peyer's glands ; b and t>, superficial and deep network of lacteals in submucous tissue; l, Lieberkuhn's glands; e, small branch of lacteal vessel on its way to mesenteric gland; n and o, muscular fibres of intestine; s, peritoneum. (Teichmann.) and at the anus they form a strong band called the internal ■sphincter muscle. The mucous membrane of the large, like that of the small intestine, is lined throughout by columnar epithelium, but, unlike it, is quite smooth and destitute of villi, and is not projected in the form of valvula conniventes. Its general microscopic structure resembles that of the small intestine: and it is bounded below by the muscularis mucosa. The general arrangement of ganglia and nerve-fibres in the large intestine resembles that in the small (p. 298). 308 DIGESTION. [chap. VII. Glands.-The glands with which the large intestine is provided arc of two kinds, (i) the tubular and (2) the lymphoid. (1.) The tubular glands, or glands of Lieberkuhn, resemble those of the small intestine, but are somewhat larger and more numerous. They are also more uniformly distributed, (2.) Follicles of adenoid or lymphoid tissue are most numerous in the caecum and vermiform appendix. They resemble in shape Fig'. 202.-Horizontal section through a portion of the mucous membrane of the intestine, shewing Lieberkuhn's glands in transverse section, a, lumen of gland-lining of columnar cells with c, goblet cells, b, supporting connective tissue. Highly magnified. (V. D. Harrii.) and structure, almost exactly, the solitary glands of the small intestine. Peyer's patches are not found in the large intestine. Ileo-csecal Valve.-The ileo-caecal valve is situate at the place of junction of the small with the large intestine, and guards against any reflex of the contents of the latter into the ileum. It is composed of two semilunar folds of mucous membrane. Each fold is formed by a doubling inwards of the mucous mem- brane, and is strengthened on the outside by some of the circular muscular fibres of the intestine, which are contained between the outer surfaces of the two layers of which each fold is composed. While the circular muscular fibres, however, of the bowel at the junction of the ileum with the caecum are contained < IIAP. VII.] THE PANCREAS AND ITS SECRETION. 309 between the outer opposed surfaces of the folds of mucous mem- brane which form the valve, the longitudinal muscular fibres and the peritoneum of the small and large intestine respectively are continuous with each other, without dipping in to follow the cir- cular fibres and the mucous membrane. In this manner, therefore, the folding inwards of these two last-named structures is pre- served, while on the other hand, by dividing the longitudinal muscular fibres and the peritoneum, the valve can be made to disappear, just as the constrictions between the sacculi of the large intestine can be made to disappear by performing a similar operation. The inner surface of the folds is smooth ; the mucous membrane of the ileum being continuous with that of the caecum. That surface of each fold which looks towards the small intestine is covered with villi, while that which looks to the caecum has none. When the caecum is distended, the margin of the folds are stretched, and thus are brought into firm apposition one with the other. Digestion in the Intestines. After the food has been duly acted upon by the stomach, such as has not been absorbed passes into the duodenum, and is there subjected to the action of the secretions of the pancreas and liver which enter that portion of the small intestine. Before consider- ing the changes which the food undergoes in consequence, attention should be directed to the structure and secretion of these glands, and to the secretion (succus entericus) which is poured out into the intestines from the glands lining them. The Pancreas, and its Secretion. The Pancreas is situated within the curve formed by the duodenum; and its main duct opens into that part of the small intestine, through a small opening, or through a duct common to it and to the liver, about two and a half inches from the pylorus. Structure.-In structure the pancreas bears some resemblance to the salivary glands. Its capsule and septa, as well as the blood- vessels and lymphatics, are similarly distributed. It is, however, looser and softer, the lobes and lobules being less compactly arranged. The main duct divides into branches (lobar ducts), one for each lobe, and these branches subdivide into intralobular ducts, 310 DIGESTION. [chap. vn. and these again by their division and branching form the gland tissue proper. The intralobular ducts correspond to a lobule, while between them and the secreting tubes or alveoli are longer or shorter intermediary ducts. The larger ducts possess a very distinct lumen and a membrana propria lined with columnar epithelium, the cells of which are longitudinally striated, but are shorter than those found in the ducts of the salivary glands. In the intralobular ducts the epithelium is short and the lumen is smaller. The intermediary ducts opening into the alveoli possess a dis- tinct lumen, with a membrana propria lined with a single layer of flattened elongated cells. The alveoli are branched and convoluted tubes, with a membrana propria lined with a single layer of columnar cells. They have no distinct lumen, the centre portion of the tube being occupied by fusiform or branched cells. Heidenhain has observed that the alveolar cells in the pancreas of a fasting- dog consist of two zones, an inner or central zone which is finely granular, and which stains feebly, and a smaller parietal zone of finely striated protoplasm which stains easily. The nucleus is partly in one, partly in the other zone. During digestion, it is found that the outer zone increases in size, and the central zone diminishes; the cell itself becoming smaller from the discharge of the secretion. At the end of digestion the first condition again appears, the inner zone enlarging at the expense of the outer. It appears that the granules are formed by the protoplasm of the cells, from material supplied to it by the blood. The granules are thought to be not the ferment itself, but material from which, under certain con- ditions, the ferments of the gland are made, and therefore called Zymogen. The special form of nerve terminations, called Pacinian corpuscles, are often found in the pancreas. Pancreatic Secretion.-The secretion of the pancreas has 1'ig. 203.-Section of the pancreas of a dog during digestion, a, alveoli lined with cells, the outer zone of which is well stained with hrematoxylin ; d, intermediary duct lined with squamous epithelium. X 350. (Klein and Noble Smith.) chap, vii.] CHEMICAL COMPOSITION OF PANCREATIC FLUID. 311 been obtained for purposes of experiment from the lower animals, especially the dog, by opening the abdomen and exposing the duct of the gland, which is then made to communicate with the exterior. A pancreatic fistula is thus established. An extract of pancreas made from the gland which has been removed from an animal killed during digestion possesses the active properties of pancreatic secretion. It is made by first dehydrating the gland, which has been cut up into small pieces, by keeping it for some days in absolute alcohol, and then, after the entire removal of the alcohol, placing it in strong glycerin. A glycerin extract is thus obtained. It is a remarkable fact, however, that the amount of the ferment trypsin greatly increases if the gland be exposed to the air for twenty-four hours before placing in alcohol : indeed, a glycerin extract made from the gland immediately upon removal from the body often appears to contain none of the ferment. This seems to indicate that the conversion of zymogen in the gland into the ferment only takes place during the act of secretion, and that the gland, although it always contains in its cells the materials (trypsinogen) out of which trypsin is formed, yet the conversion of the one into the other only takes place by degrees. Dilute acid appears to assist and accelerate the conversion, and if a recent pancreas be rubbed up with dilute acid before dehydration, a glycerin extract made afterwards, even though the gland may have been only recently removed from the body, is very active. Properties.-Pancreatic juice is colourless, transparent, and slightly viscid, alkaline in reaction. It varies in specific gravity from 1010 to 1015, according as it is obtained from a per- manent fistula-then more watery-or from a newly-opened duct. The solids vary in a temporary fistula from 80 to 100 parts per thousand, and in a permanent one from 16 to 50 per thousand. Chemical Composition of the Pancreatic Secretion. From a permanent fistula. (Bernstein.) Water975 Solids-Ferments (including trypsin, amylopsin, rennet, and ? steapsin) : Proteids, including Serum-Albumin and Casein Leucin and Tyrosin ; Fats and Soaps • i7 Inorganic residue, especially Sodium j Carbonate' 8 25 IOOO Functions.-(i.) By the aid of its proteolytic ferment, trypsin, if converts proteids into peptones, the intermediate product being not 312 DIGESTION. [chap, vi 1. akin to syntonin or acid-albumin as in gastric digestion, but to alkali-albumin. Kiihne calls the intermediate products, both in the peptic and pancreatic digestion of proteids, anti- albumose and hemi-albumose, and states that the peptones formed correspond to these products, which he therefore terms anti-peptone and hemi-peptone. The hemipeptone is capable of being converted by the action of the pancreatic ferment-trypsin-into leucin or amido-caproic acid (C6HI2N02) and tyrosin, (CgH„NO3), but is not so changed by pepsin : the antipeptone cannot be further split up. The products of pancreatic digestion are sometimes further com- plicated by the appearance of certain faecal substances of which indol (C3 H2 N), skatol (C9 H9 N), phenol (C6 H6 0), and napthilamine are the most important. (Kiihne.) When the digestion goes on for a long time the indol is formed in considerable quantities, and emits a most disagreeable faecal odour. These further products are produced by the presence of numerous micro-organisms in the pancreatic digestion fluid. All the albuminous or proteid substances which have not been converted into peptone and absorbed in the stomach, and the par- tially changed substances, i.e., the para-peptones, are converted into peptone by the pancreatic juice, and then in part into leucin and tyrosin. (2.) The action of the pancreatic juice upon the gelatins, or nitrogenous bodies other than proteids, is not so distinct. Mucin can, however, be dissolved, but not keratin in horny tissues. Gelatin itself is formed into peptone (gelatin-peptone}. (3.) Starch is converted into maltose and then into glucose in an exactly similai manner to that which happens with the saliva; erythro- and achroo-dextrine being intermediate products. If the sugar which is at first formed is maltose, the ferment of the pan- creatic juice after a time completes the whole change of starch into glucose. This distinct amylolytic ferment in the pancreatic juice which cannot be distinguished from ptyalin, is called Amylopsin. (4.) Pancreatic juice possesses the property of curdling milk, con- taining a special (rennet) ferment for that purpose. The ferment is distinct from trypsin, and will act in the presence of an acid (W. Roberts). It is best extracted by brine. (5.) Oils and fats are emulsified and saponified by pancreatic secre- tion. The terms emulsification and saponification may need a little explanation. The former is used to signify an important mechanical change in oils or fats, whereby they are made into an emulsion, or CHAP. VII.] EMULSIFICATION AND SAPONIFICATION. 313 in other words arc minutely subdivided into small particles. If a small drop of an emulsion be looked at under the microscope it will be seen to be made up of an immense number of minute rounded particles of oil or fat, of varying sizes. The more complete the emulsion the smaller are these particles. An emulsion is formed at once if oil or fat, which nearly always is slightly acid from the presence of free fatty acid, is mixed with an alkaline solution. Saponification signifies a distinct chemical change in the composi- tion of oils and fats. An oil or a fat is made up chemically of glycerin, a triatomic alcohol (see Appendix), and one or more fatty acid radicles. When an alkali is added to a fat and heat is applied, two changes take place, firstly, the oil or fat is split up into glycerin, and its corresponding fatty acid; secondly, the fatty acid combines with the alkali, to form a soap which is chemically known as stearate, oleate, or palmitate of potassium or sodium. Thus saponification means a chemical splitting up of oils or fats into new compounds, and emulsification means merely a mechanical splitting of them up into minute particles. The pancreatic juice has been for many years credited with the possession of a special ferment, which was called by Claude Bernard steapsin, and which was supposed to aid in one or both of these processes. It appears very doubtful, however, if either the mechanical or the chemical splitting up of fats by the alkaline pancreatic juice is a ferment action at all. Several cases have been recorded in which the pancreatic duct being obstructed, so that its secretion could not be discharged, fatty or oily matter was abundantly discharged from the intestines. In nearly all these cases, indeed, the liver was coincidentally diseased, and the change or absence of the bile might appear to contribute to the result ; yet the frequency of extensive disease of the liver, unaccompanied by fatty dis- charges from the intestines, favours the view that, in these cases, it is to the absence of the pancreatic fluid from the intestines that the excretion or non-absorption of fatty matter should be ascribed. Conditions favourable to the Action.- These are similar to those which are favourable to the action of the saliva, and the reverse (p. 270). The Liver, the largest gland in the body, situated in the abdomen on the right side chiefly, is an extremely vascular organ, and receives its supply of blood from two distinct sources, The Liver. 314 DIGESTION. [chap. VJt. viz., from the portal vein and from the hepatic artery, while the blood is returned from it into the vena cava inferior by the hepatic veins. Its secretion, the bile, is conveyed from it by the hepatic duct, either directly into the intestine, or, when digestion is not going on, into the cystic duct, and thence into the gall- bladder, where it accumulates until required. The portal vein, Fig. 204.-The tinder surface of the liver, a. b., gall-bladder; it. d., common bile-duct; h. a., hepatic artery; v. p., portal vein; l. q., lobulus quadratus; l. s., lobulus spigelii; l. c., lobulus caudatus ; u. v., ductus venosus ; u. v., umbilical vein. (Noble Smith.) hepatic artery, and hepatic duct branch together throughout the liver, while the hepatic veins and their tributaries run by themselves. On the outside, the liver has an incomplete covering of peri- toneum, and beneath this is a very fine coat of areolar tissue, con- tinuous over the whole surface of the organ. It is thickest where the peritoneum is absent, and is continuous on the general surface of the livei' with the fine and, in the human subject, almost imperceptible areolar tissue investing the lobules. At the transverse fissure it is merged in the areolar investment called Glisson's capsule, which, surrounding the portal vein, hepatic artery, and hepatic duct, as they enter at this part, ac- companies them in their branchings through the substance of the liver. Structure.-The liver is made up of small roundish or oval portions called lobules, each of which is about of an inch in diameter, and composed of the minute branches of the portal vein, hepatic artery, hepatic duct, and hepatic vein; while the inter- CHAT. VII.] THE LIVER CELLS. 315 stices of these vessels are filled by the liver cells. The hepatic cells (fig. 205), which form the glandular or secreting part of the liver, are of a spheroidal form, somewhat polygonal from mutual pressure about to -5-oWo inch in diameter, possessing one, some- times two nuclei. The cell-sub- stance contains numerous fatty molecules, and some yellowish- brown granules of bile-pigment. 'fhe cells sometimes exhibit slow amoeboid movements. They are held together by a very delicate sustentacular tissue, continuous with the interlobular connective tissue. To understand the distribution of the blood-vessels in the liver, Fig. 205.-A. Liver-cells. B. Ditto, con- taining various-sized particles of fat. Fig'. 206.-Longitudinal section of a portal canal, containing a portal vein, hepatic artery and hepatic duct, from the pig. p, branch of vena portae, situate in a portal canal formed amongst the lobules of the liver, I, I, and giving off vaginal branches ; there are also seen within the large portal vein numerous orifices of the smallest interlobular veins arising directly from it; a, hepatic artery ; d, hepatic duct, x 5. (Kiernan.) it will be well to trace, first, the two blood-vessels and the duct which enter the organ on the under surface at the transverse fissure, viz., the portal vein, hepatic artery, and hepatic duct. As before remarked, all three run in company, and their appearance 316 DIGESTION. [chap. vn. on longitudinal section is shown in fig. 206. Running together through the substance of the liver, they are contained in small channels called portal canals, their immediate investment being a sheath of areolar tissue (Glisson's capsule). To take the distribution of the portal vein first:-In its course through the liver this vessel gives off small branches which divide and subdivide between, the lobules surrounding them and limiting them, and from this circumstance called inter-lobular veins. From these small vessels a dense capillary 1 ig. 207.- Cross section of a lobule of the human liver, in which the capillary network between the portal and hepatic veins has been fully injected, i, section of the int/'o-lobular vein ; 2, its smaller branches collecting blood from the capillary network ; 3, inter- lobular branches of the vena porta; with their smaller ramifications passing inwards towards the capillary network in the substance of the lobule, -r 60. (Sappey.) network is prolonged into the substance of the lobule, and this network gradually gathering itself up, so to speak, into larger vessels, converges finally to a single small vein, occupying the centre of the lobule, and hence called tWra-lobular. This arrange- ment is well seen in fig. 207, which represents a transverse section of a lobule. 'fhe small ntfm-lobular veins discharge their contents into veins called swMobular (hh h, fig. 208); while these again, by their union, form the main branches of the hepatic veins, which leave the posterior border of the liver to end by two or three principal trunks in the inferior vena cava, just before its passage through chap. vii.] THE VEINS OF THE LIVER. 317 the diaphragm. The swfc-lobular and hepatic veins, unlike the portal vein and its companions, have little or no areolar tissue around them, and their coats being very thin, they form little more than mere channels in the liver substance which closely surrounds them. 'I'he manner in which the lobules are connected with the I'ig. 208.-Section of a portion of liver passing longitudinally through a considerable hepatic vein, from the pig. 11, cepatic venous trunk, against which the sides of the lobules (Z) are applied ; h, h, h, sublobular hepatic veins, on which the bases of the lobules rest, and through the coats of which they are seen as polygonal figures; i, mouth of the intralobular veins, opening into the sublobular veins ; i', intralobular veins shown passing up the centre of some divided lobules ; I, I. cut surface of the liver; c, c, walls of the hepatic venous canal, formed by the polygonal bases of the lobules. X 5. (Kiernan.) sublobular veins by means of the small intralobular veins is well seen in the diagram (fig. 209 and in fig. 208), which represent the parts as seen in a longitudinal section. The appearance has been likened to a twig having leaves without footstalks-the lobules representing the leaves, and the sublobular vein the small branch from which it springs. On a transverse section, the appearance of the intralobular veins is that of 1, fig. 207, while both a transverse and longitudinal section are exhibited in fig. 208. The hepatic artery, the function of which is to distribute blood for nutrition to Glisson's capsule, the walls of the ducts and blood 318 DIGESTION. [chap. vii. vessels, and other parts of the liver, is distributed in a very similar manner to the portal vein, its blood being returned by small branches either into the ramifications of the portal vein, or into the capillary plexus of the lobules which connects the inter- and zWra-lobular veins. The hepatic duct divides and sub- divides in a manner very like that of the portal vein and hepatic artery, the larger branches being lined by cylin- drical, and the smaller by small poly- yonal epithelium. The bile-capillaries commence be- tween the hepatic cells, and are bounded by a delicate membranous wrall of their own. They appear to be alw ays bounded by hepatic cells on all sides, and are thus separated from the nearest blood-capil- lary by at least the breadth of one cell (figs. 2i i and 212). LobuU* Lobules Fig. 209.-Diagram showing the manner in which the lobules of the liver rest on the sublobular veins. (After Kiernan.) The Gall-bladder. The Gall-bladder (g, b, fig. 204) is a pyriform bag, attached to the under surface of the liver, and supported also by the peri- Fig. 2io.-Capillary network of the lobules of the rabbit's liver. The figure is taken from a very successful injection of the hepatic veins, made by' Harting : it shows nearly the whole of two lobules, and parts of three others ; p, portal branches running in the interlobular spaces ; h, hepatic veins penetrating and radiating from the centre of the lobules, x 45. (Kolliker.) CHAP. VII.] THE GALL BLADDER. 319 toneum, which passes below it. The larger end or fundus, projects beyond the front margin of the liver; while the smaller end contracts into the cystic duct. Structure.-The walls of the gall- bladder are constructed of three principal coats, (i) Externally (ex- cepting that part which is in con- tact with the liver), is the serous coat, which has the same structure as the peritoneum with which it is continuous. Within this is (2) the fibrous or areolar coat, constructed of tough fibrous and elastic tissue, with which is mingled a consider- able number of plain muscular fibres, both longitudinal and circular. (3) Internally the gall-bladder is lined by mucous membrane, and a layer of columnar epithelium. The sur- face of the mucous membrane pre- sents to the naked eye a minutely honeycombed appearance from a number of tiny polygonal depressions with intervening ridges, by which its surface is mapped out. In the cystic duct the mucous membrane is raised up in the form of Fig. 211.-Portion of a lobule of liver, a, bile capillaries between liver- cells, the network in which is well seen ; b, blood capillaries. X 350. (Klein and Noble Smith.) Fig. 212.- Hepatic cells and bile capillaries, from the liver of a child three months old. Both figures represent fragments of a section carried through the periphery of a lobule. The red corpuscles of the blood are recognized by their circular contour: vp, corre- sponds to an interlobular vein in immediate proximity with which are the epithe- lial cells of the biliary ducts, to which, at the lower part of the figures, the much larger hepatic cells suddenly succeed. (E. Hering.) 320 DIGESTION. [chap. vii. crescentic folds, which together appeal' like a spiral valve, and which minister to the function of the gall-bladder in retaining the bile during the intervals of digestion. The gall-bladder and all the main biliary ducts are provided with mucous glands, which open on their internal surface. Functions of the Liver.-The functions of the Liver may be classified under the following heads:-1. The Secretion of Bile. 2. The Elaboration of Blood; under this head may be included the Glycogenic Function. I. The Secretion of Bile. The Bile.-Properties.-The bile is a somewhat viscid fluid, of a yellow or reddish-yellow colour, a strongly bitter taste, and, when fresh, with a scarcely perceptible odour : it has a neutral or slightly alkaline reaction, and its specific gravity is about 1020. Its colour and degree of consistence vary much, quite inde- pendent of disease; but, as a rule, it becomes gradually more deeply coloured and thicker as it advances along its ducts, or when it remains long in the gall-bladder, wherein, at the same time, it becomes more viscid and ropy, of a darker colour, and more bitter taste, mainly from its greater degree of concentration, on account of partial absorption of its water, but partly also from being mixed with mucus. Chemical Composition of Human Bile. (Frerichs.) Water 8592 Solids-Bile salts or Bilin 91'5 Fat 92 Cholesterin 2'6 Mucus and colouring matters . . . . 29'8 Salts 77 140'8 10000 («) Bile salts, or Bilin, can be obtained as colourless, exceed- ingly deliquescent crystals, soluble in water, alcohol, and alkaline solutions, giving to the watery solution the taste and general characters of bile. They consist of sodium salts of glycocholic CHAP. VII.] THE BILE SALTS. 321 and taurocholic acids. The former salt is composed of cholic acid combined with glycin (see Appendix), the latter of the same acid combined with taurin. The proportion of these two salts in the bile of different animals varies, e.g., in ox bile the glycocho- latc is in great excess, whereas the bile of the dog, cat, bear, and other carnivora contains taurocholate alone; in human bile both are present in about the same amount (glycocholate in excess?). Preparation of Bile Salts.-Bile salts may be prepared in the follow- ing manner : mix bile which has been evaporated to a quarter of its bulk with animal charcoal, and evaporate to perfect dryness in a water bath. Next extract the mass whilst still warm with absolute alcohol. Separate the alcoholic extract by filtration, and to it add perfectly anhydrous ether as long as a precipitate is thrown down. The solution and precipitate should be set aside in a closely stoppered bottle for some days, when crystals of the bile salts or bilin will have separated out. The glycocholate may be separated from the taurocholate by dissolving bilin in water, and adding to it a solution of neutral lead acetate, and then a little basic lead acetate, when lead glycocholate separates out. Filter and add to the filtrate lead acetate and ammonia, a precipitate of lead taurocholate will be formed, which may be filtered off. In both cases, the lead may be got rid of by suspending or dissolving in hot alcohol, adding hydrogen sulphate, filtering and allowing the acids to separate out by the addition of water. The Test for bile salts is known as Pettenkofer's. If to an aqueous solution of the salts strong sulphuric acid be added, the bile acids are first of all precipitated, but on the further addition of the acid are re-dissolved. If to the solution a drop of solution of cane sugar be added, a fine deep cherry red to purple colour is developed. The re-action will also occur on the addition of grape or fruit sugar instead of cane sugar, slowly with the first, quickly with the last; and a colour similar to the above is produced by the action of sulphuric acid and sugar on albumen, the crystalline lens, nerve tissue, oleic acid, pure ether, cholesterin, morphia, codeia and amylic alcohol. The spectrum of Pettenkofer's reaction, when the fluid is mode- rately diluted, shows four bands-the most marked and largest at E, and a little to the left; another at F; a third between D and E, nearer to D ; and the fourth near D. (6) The yellow colouring matter of the bile of man and the Carnivora is termed Bilirubin or Bilifulvin (ClS Hi8 N2 O3) crystal lizable and insoluble in water, soluble in chloroform or carbon disulphide ; a green colouring matter, Biliverdin (Cl6 N, 0.) 322 DIGESTION. [CHAP. VI1. which always exists in large amount in the bile of Herbivora, being formed from bilirubin on exposure to the air, or by subjecting the bile to any other oxidizing agency, as by adding nitric acid. When the bile has been long in the gall-bladder, a third pigment, Biliprasin, may be also found in small amount. In cases of biliary obstruction, the colouring matter of the bile is re-absorbed, and circulates with the blood, giving to the tissues the yellow tint characteristic of jaundice. The colouring matters of human bile do not appear to give characteristic absorption spectra; but the bile of the guinea pig, rabbit, mouse, sheep, ox, and crow do so, the most constant of which appears to be a band at F. The bile of the sheep and ox give three bands in a thick layer, and four or five bands with a thinner layer, one on each side of D, one near E, and a faint line at F. (McMunn.) There seems to be a close relationship between the colour- matters of the blood and of the bile, and it may be added, between these and that of the urine (urobilin), and of the faeces (ster- cobilin) also ; it is probable they are, all of them, varieties of the same pigment, or derived from the same source. Indeed it is maintained that Urobilin is identical with Hydrobilirubin, a sub- stance which is obtained from bilirubin by the action of sodium amalgam, or by the action of sodium amalgam on alkaline hsematin ; both urobilin and hydrobilirubin giving a characteristic absorption band between b and F. They are also identical with stercobilin, which is formed in the alimentary canal from bile pigments. The Best (Gmelin's) for the presence of bile-pigment consists of the addition of a small quantity of nitric acid, yellow with nitrous acid; if bile be present, a play of colours is produced, beginning with green and passing through blue and violet to red, and lastly to yellow. The spectrum of Gmelin's test gives a black band extending from near b to beyond F. (c) Fatty substances are found in variable proportions in the bile. Besides the ordinary saponifiable fats, there is a small quantity of Uholesterin, a so-called non-saponifiable fat, which is really an alcohol, and, with the free fats, is probably held in solution by the bile salts. It is a body belonging to the class of monatomic alcohols H44 0), and crystallizes in rhombic plates (fig. 213). It is insoluble in water and cold alcohol, but dissolves easily in boiling alcohol or ether. It gives a red colour with strong sul- CHAP. VII.] THE CONSTITUENTS OF THE BILE. 323 phuric acid, and with nitric acid and ammonia; also a play of colours beginning with blood red and ending with green on the addition of sulphuric acid and chlorform. Lecithin (Cw NPO9), a phosphorus-containing body and Neurin (Cs HIS NO,), are also found in bile, the latter probably as a decomposition product of the former. (cZ) The Mucus in bile is de- rived from the mucous membrane and glands of the gall-bladder, and of the hepatic ducts. It con- stitutes the residue, after bile is treated with alcohol. The epithe- lium with which it is mixed may be detected in the bile with the microscope in the form of cylin- drical cells, either scattered or still held together in layers. To the presence of the mucus is probably to be ascribed the rapid decomposition of the bile ; for, according to Berzelius, if the mucus be separated, it will remain unchanged for many days. (e) The Saline or inorganic constituents of the bile are similar to those found in most other secreted fluids. It is possible that the carbonate and neutral phosphate of sodium and potassium, found in the ashes of bile, are formed in the incineration, and do not exist as such in the fluid. Oxide of iron is said to be a common constituent of the ashes of bile, and copper is generally found in healthy bile, and constantly in biliary calculi. (/) Gas.-Small amounts of carbonic acid, oxygen, and nitrogen gases, may be extracted from bile. Mode of Secretion and Discharge.-The secretion of bile is continually going on, but it appears to be retarded during fasting, and accelerated on taking food. This has been shown by tying the common bile-duct of a dog, and establishing a fistulous opening between the skin and gall-bladder, whereby all the bile secreted was discharged at the surface. It was noticed that when the animal was fasting, sometimes not a drop of bile was dis- charged for several hours; but that, in about ten minutes after the introduction of food into the stomach, the bile began to flow abundantly, and continued to do so during the whole period of digestion. Fig. 213.- Crystalline scales 324 DIGESTION. [chap. VII The bile is formed in the hepatic cells ; thence, being discharged into the minute hepatic ducts, it passes into the larger trunks, and from the main hepatic duct may be carried at once into the duodenum. But, probably, this happens only while digestion is going on; during fasting, it regurgitates from the common bile- duct through the cystic duct, into the gall-bladder, where it accu- mulates till, in the next period of digestion, it is discharged into the intestine. The gall-bladder thus fulfils what appears to be its chief or only office, that of a reservoir; for its presence enables bile to be constantly secreted, yet ensures its employment in the service of digestion, although digestion is periodic, and the secretion of bile constant. The mechanism by which the bile passes into the gall-bladder is simple. The orifice through which the common bile-duct com- municates with the duodenum is narrower than the duct, and appears to be closed, except when there is sufficient pressure behind to force the bile through it. The pressure exercised upon the bile secreted during the intervals of digestion appears insuffi- cient to overcome the force with which the orifice of the duct is closed; and the bile in the common duct, finding no exit in the intestine, traverses the cystic duct, and so passes into the gall- bladder, being probably aided in this retrograde course by the peristaltic action of the ducts. The bile is discharged from the gall-bladder and enters the duodenum on the introduction of food into the small intestine: being pressed on by the contrac- tion of the coats of the gall-bladder, and of the common bile- duct also; fo* both these organs contain unstriped muscular fibre-cells. Their contraction is excited by the stimulus of the food in the duodenum acting so as to produce a reflex movement, the force of which is sufficient to open the orifice of the common bile-duct. Bile, as such, is not pre-formed in the blood. As just observed, it is formed or secreted by the hepatic cells, although some of the material may be brought to them almost in the condition for immediate secretion. When it is, however, prevented by an obstruction of some kind, from escaping into the intestine (as by the passage of a gall-stone along the hepatic duct) it is absorbed in great excess into the blood, and, circulating with it, gives rise to the well-known phenomena of jaundice. This is explained by the fact that the pressure of secretion in the ducts is normally very low, and if it exceeds ? inch of mercury (16 mm.) the secretion CHAP, VII.] FUNCTIONS OF THE BILE. 325 ceases to be poured out, and if the opposing force be increased, the bile finds its way into the blood. Quantity.-Various estimates have been made of the quantity of bile discharged into the intestines in twenty-four hours : the quantity doubtless varying, like that of the gastric fluid, in pro- portion to the amount of food taken. A fair average of several computations would give 20 to 40 oz. (600-900 cc.) as the quantity daily secreted by man. Functions.-(r) As an excrementitious substance, the bile may serve especially as a medium for the separation of excess of carbon and hydrogen from the blood; and its adaptation to this purpose is well-illustrated by the peculiarities attending its secretion and disposal in the foetus. During intra-uterine life, the lungs and the intestinal canal are almost inactive; there is no respiration of open air or digestion of food; these are unnecessary, on account of the supply of well elaborated nutriment received by the vessels of the foetus at the placenta. The liver, during the same time, is proportionately larger than it is after birth, and the secretion of bile is active, although there is no food in the intestinal canal upon which it can exercise any digestive property. At birth, the intes- tinal canal is full of thick bile, mixed with intestinal secretion ; the meconium, or faeces of the foetus, containing all the essential principles of bile. Composition of Meconium (Frerichs) : Biliary resin 15 6 Common fat and cholesterin .... 15'4 Epithelium, mucus, pigment, and salts . . 69'0 1000 In the foetus, therefore, the main purpose of the secretion of bile must be the purification of blood by direct excretion, i.e., by sepa- ration from the blood, and ejection from the body without further change. Probably all the bile secreted in foetal life is incorpo- rated in the meconium, and with it discharged, and thus the liver may be said to discharge a function in some sense vicarious of that of the lungs. For, in the foetus, nearly all the blood coming from the placenta passes through the liver, previous to its distri- bution to the several organs of the body ; and the abstraction of carbon, hydrogen, and other elements of bile will purify it, as in extra-uterine life it is purified by the separation of carbonic acid and water at the lungs. 326 DIGESTION. [chap. VII. Disposal of the Bile.-The evident disposal of the foetal bile by excretion, makes it highly probable that the bile in extra-uterine life is also, at least in part, destined to be discharged as excremen- titious. The analysis of the faeces of both children and adults shows, however, that (except wheu rapidly discharged iu purgation) they contain very little of the bile secreted, probably not more thau one-sixteenth part of its weight, aud that this portion includes chiefly its colouring matter in the form of stercobilin, and some of its fatty matters, and to only a rery slight degree, its salts, almost all of which have been re-absorbed from the intestines into the blood. The elementary composition of bile-salts shows such a pre- ponderance of carbon and hydrogen, that probably, after ab- sorption, it combines with oxygen, and is excreted in the form of carbonic acid and water. The change after birth, from the direct to the indirect mode of excretion of the bile may, with much probability, be connected with a purpose in relation to the development of heat. The temperature of the foetus is maintained by that of the parent, and needs no source of heat within itself; but, in extra-uterine life, there is (as one may say) a waste of material for heat when any excretion is discharged unoxidised ; the carbon and hydrogen of the bilin, therefore, instead of being ejected in the faeces, are re-absorbed, in order that they mav be combined with oxygen, and that in the combination heat may be generated. It appears that taurocholic acid may easily be split up in the intestine into taurin and cholalic acid. The former does not appear in the faeces, but the latter has been found there. So that in part it is excreted, but part is re-absorbed in the intes- tine and returned to the liver. It is probable that although part of this may unite to re-form glycocholic or taurocholic acid, the remainder is united with oxygen, and is burnt off in the form of carbonic acid and water. A substance, which has been discovered in the faeces, aud named stercorin is closely allied to cholesterin : and it has been suggested that while one great function of the liver is to excrete cholesterin from the blood, as the kidney excretes urea, the stercorin of faeces is the modified form iu which cholesterin finally leaves the body. Ten grains and a half of stercorin are excreted daily (A. Flint). From the peculiar manner in which the liver is supplied with much of the blood that flows through it, it is probable that this chap, vii.] THE BILE AS A DIGESTIVE FLUID. 327 organ is excretory, not only for such hydro-carbonaceous matters as may need expulsion from any portion of the blood, but that it serves for the direct purification of the stream which, arriving by the portal vein, has just gathered up various substances in its course through the digestive organs-substances which may need to be expelled, almost immediately after their absorption. For it is easily conceivable that many things may be taken up during digestion, which not only are unfit for purposes of nutrition, but which would be positively injurious if allowed to mingle with the general mass of the blood. The liver, therefore, may be supposed placed in the only road by which such matters can pass unchanged into the general current, jealously to guard against their further progress, and turn them back again into an excretory channel, 'fhe frequency with which metallic poisons are either excreted by the liver, or intercepted and retained, often for a considerable time, in its own substance, may be adduced as evidence for the probable truth of this supposition. (2.) As a digestive fluid.-Though one chief purpose of the secretion of bile may thus appear to be the purification of the blood by ultimate excretion, yet there are many reasons for believing that, while it is in the intestines it performs an important part in the process of digestion. In nearly all animals, for example, the bile is discharged, not through an excretory duct communicating with the external surface or with a simple reservoir, as most excretions are, but is made to pass into the intestinal canal, so as to be mingled with the chyme directly after it leaves the stomach; an arrangement, the constancy of which clearly indicates that the bile has some important relations to the food with which it is thus mixed. A similar indication is furnished also by the fact that the secre- tion of bile is most active, and the quantity discharged into the intestines much greater, during digestion than at any other time ; although, without doubt, this activity of secretion during diges- tion may, however, be in part ascribed to the fact that a greater* quantity of blood is sent through the portal vein to the liver at this time, and that this blood contains some of the materials of the food absorbed from the stomach and intestines, which may need to be excreted, either temporarily, (to be afterwards re-absorbed), or permanently. Respecting the functions discharged by the bile in digestion, there is little doubt that it («.) assists in emulsifying the fatty 328 DIGESTION. [chap, vil portions of the food, and thus rendering them capable of being absorbed by the lacteals. For it has appeared in some experiments in which the common bile-duct was tied, that, although the process of digestion in the stomach was unaffected, chyle was no longer well formed ; the contents of the lateals consisting of clear, colourless fluid, instead of being opaque and white, as they ordinarily are, after feeding. (6.) It is probable, also, that the moistening of the mucous mem- brane of the intestines by bile facilitates absorption of fatty matters through it. (c.) The bile, like the gastric fluid, has a considerable anti- septic power, and may serve to prevent the decomposition of food during the time of its sojourn in the intestines. Experiments show that the contents of the intestines,are much more foetid after the common bile-duct has been tied than at other times: more- over, it is found that the mixture of bile with a fermenting fluid stops or spoils the process of fermentation. (cZ.) The bile has also been considered to act as a natural purgative, by promoting an increased secretion of the intestinal glands, and by stimulating the intestines to the propulsion of their contents. This view' receives support from the constipation which ordinarily exists in jaundice, from the diarrhoea which accompanies excessive secretion of bile, and from the purgative properties of ox-gall. (e) The bile appears to have the power of precipitating the gastric parapeptones and peptones, together with the pepsin, which is mixed up witl them, as soon as the contents of the stomach meet it in the duodenum. The purpose of this operation is probably both to delay any change in the parapeptones until the pancreatic juice can act upon them, and also to prevent the pepsin from exercising its solvent action on the ferments of the pancreatic juice. II. Blood-elaboration. The secretion of bile, as already observed, is only one of the purposes fulfilled by the liver. Another very important function appears to be that of so acting upon certain constituents of the blood passing through it, as to render some of them capable of assimilation with the blood generally, and to prepare others for l>eing duly eliminated in the process of respiration. It appears that the peptones, conveyed from the alimentary canal by the chap, vii.] THE GLYCOGENIC FUNCTION OF THE LIVER. 329 blood of the portal vein, require to be submitted to the influence of the liver before they can be assimilated by the blood ; for if such albuminous matter is injected into the jugular vein, it speedily appears in the urine; but if introduced into the portal vein, and thus allowed to traverse the liver, it is no longer ejected as a foreign substance, but is incorporated with the albuminous part of the blood. Glycogenic Function. One of the chief uses of the liver in connection with that elaboration of the blood is known as its glycogenic function. The important fact that the liver normally forms-glucose, or a substance readily convertible into it, was discovered by Claude Bernard in the following way: he fed a dog for seven days with food containing a large quantity of sugar and starch; and, as might be expected, found sugar in both the portal and hepatic veins. And this dog was fed with meat only, and, to his surprise, sugar was still found in the hepatic veins. Repeated experiments gave invariably the Same result; no sugar being found, under a meat diet, in the portal vein, if care were taken, by applying a ligature on it at the transverse fissure, to prevent reflux of blood from the hepatic venous system. Bernard found sugar also in the substance of the liver. It thus seemed certain that the liver formed sugar, even when, from the absence of saccharine and amyloid matters in the food, none could be brought directly to it from the stomach or intestines. Excepting cases in which large quantities of starch and sugar were taken as food, no sugar was found in the blood after it had passed through the lungs; the sugar formed by the liver, having presumably disappeared by combustion, in the course of the pulmonary circulation. Bernard found, subsequently to the before-mentioned experi- ments, that a liver, removed from the body, and from which all sugar had been completely washed away by injecting a stream of water through its blood-vessels, will be found, after the lapse of a few hours, to contain sugar in abundance. This post-mortem production of sugar was a fact which could only be explained in the supposition that the liver contained a substance, readily con- vertible into sugar in the course merely of post-mortem decom- position ; and this theory was proved correct by the discovery of a 330 DIGESTION. [chap. VII. substance in the liver allied to starch, and now generally termed glycogen. We may believe, therefore, that the liver does not form sugar directly from the materials brought to it by the blood, but that glycogen is first formed and stored in its substance : and that the sugar, when present, is the result of the transformation of the latter. Quantity of Glycogen formed.-Although, as before mentioned, glycogen is produced by the liver when neither starch nor sugar is present in the food, its amount is much less under such a diet. Average amount of Glycogen in the Liver of Dogs under various Diets (Pavy). Diet. Amount of Glycogen in Liver. Animal food 7'i9 per cent. Animal food with sugar (about | lb. of sugar daily) 14'5 h Vegetable diet (potatoes, with bread or barley-meal) 17'23 » The dependence of the formation of glycogen on the food taken is also well shown by the following results, obtained by the same experimenter :- Average quantity of Glycogen found in the Liver of Rabbits after Fasting, and after a diet of Starch and Sugar respectively. Average amount of Glycogen in Liver. After fasting for three days .... Practically absent. „ diet of starch and grape-sugar . . . 15'4 per cent. „ „ cane-sugar 16'9 Regarding these facts there is no dispute. All are agreed that glycogen is formed, and laid up in store, temporarily, by the liver- cells ; and that it is not formed exclusively from saccharine and amy- laceous foods, but from albuminous substances also; the albumen, in the latter case, being probably split up into glycogen, which is temporarily stored in the liver, and urea, which is excreted by the kidneys. Destination of Glycogen.-There are two chief theories on the subject of the destination of glycogen, (i.) That the conversion of glycogen into sugar takes place rapidly during life by the agency of a ferment (liver diastase) also formed in the liver: and the sugar is conveyed away by the blood of the hepatic veins, and soon undergoes combustion. (2.) That the conversion into sugar only occurs after death, and that during life no sugar exists in healthy livers; glycogen not undergoing this transformation. The chief arguments advanced in support of this view are, (a) that scarcely a trace of sugar is found in blood drawn during life from the right ventricle, or in blood collected from the right CHAP. VII.] GLYCOSURIA. 331 bide of the heart immediately after an animal has been killed ; while if the examination be delayed for a very short time after death, sugar in abundance may be found in such blood ; (6), that the liver, like the venous blood in the heart, is, at the moment of death, completely free from sugar, although afterwards its tissue speedily becomes saccharine, unless the formation of sugar be pre- vented by freezing, boiling, or other means calculated to interfere with the action of a ferment on the amyloid substance of the organ. Instead of adopting Bernard's view, that normally, during life, glycogen passes as sugar into the hepatic venous blood, and thereby is conveyed to the lungs to be further disposed of, Pavy inclines to the belief that it may represent an intermediate stage in the formation of fat from materials absorbed from the alimentary canal. Liver-Sug-ar.-To demonstrate the presence of sugar in the liver, a portion of this organ, after being cut into small pieces, is bruised in a mortar to a pulp with a small quantity of water, and the pulp is boiled with sodium-sulphate in order to precipitate albuminous and colouring matters. The decoction is then filtered and may be tested for glucose. Glycogen (C8 H10 O5) is an amorphous, starch-like substance, odourless, and tasteless, soluble in water, insoluble in alcohol. It is converted into glucose by boiling with dilute acids, or by contact with any animal ferment. It may be obtained by taking a portion of liver from a recently killed rabbit, and, after cutting it into small pieces, placing it for a short time in boiling water. It is then bruised in a mortar, until it forms a pulpy mass, and subsequently boiled in distilled water for about a quarter of an hour. The glycogen is precipitated from the filtered decoction by the addition of alcohol. Glycogen has been found in many other structures than the liver (See Appendix.) Glycosuria.-The facility with which the glycogen of the liver is transformed into sugar would lead to the expectation that this chemical change, under many circumstances, would occur to such an extent that sugar would be present not only in the hepatic veins, but in the blood generally. Such is frequently the case ; the sugar when in excess in the blood being secreted by the kidneys, and thus appearing in variable quantities in the urine (Glycosuria). Influence of the Nervous System.-Glycosuria may be experi- mentally produced by puncture of the medulla oblongata in the region of the vaso-motor centre. The better fed the animal the larger is the amount of sugar found in the urine ; whereas in the case of a starving animal no sugar appears. It is, therefore, highly probable that the sugar comes from the hepatic glycogen, 332 DIGESTION. [CHAE. VII. since in the one case glycogen is in excess, and in the other it is almost absent. The nature of the influence is uncertain. It may be exercised in dilating the hepatic vessels, or possibly may be exerted on the liver cells themselves. The whole course of the nervous stimulus cannot be traced to the liver, but at first it passes from the lower part of the floor of the fourth ventricle and medulla down the spinal cord as far as-in rabbits-the fourth dorsal vertebra, and thence to the first thoracic ganglion. Many other circumstances will cause glycosuria. It has been observed after the administration of various drugs, after the injec- tion of urari, poisoning with carbonic oxide gas, the inhalation of ether, chloroform, etc., the injection of oxygenated blood into the portal venous system. It has been observed in man after injuries to the head, and in the course of various diseases. The well-known disease, diabetus mellitus, in which a large quantity of sugar is persistently secreted daily with the urine, has, doubtless, some close relation to the normal glycogenic func- tion of the liver; but the nature of the relationship is at present quite unknown. The Intestinal Secretion, or Succus Entericus. On account of the difficulty in isolating the secretion of the glands in the wall of the intestine (Brunner's and Lieberkuhn's) from other secretions poured into the canal (gastric juice, bile, and pancreatic secretion), but little is known regarding the composition of the former fluid fntestinal juice, succus entericus). It is said to be a yellowish alkaline fluid with a specific gravity of ion, and to contain about per cent, of solid matters (Thiry). Functions.-The secretion of Brunner's glands is said to be able to convert proteids into peptones, and that of Lieberkuhn's is be- lieved to convert starch into sugar. To these functions of the succus entericus the powers of converting cane into grape sugar, and of turning grape sugar into lactic, and afterwards into butyric acid, are added by some physiologists. It also probably contains a milk-curdling ferment (W. Roberts). The reaction which represents the conversion of cane sugar into grape sugar may be represented thus :- 2C12H22On + 2 H2 O = C12H24O12 + C12H21012 Saccharose Water Dextrose levulose CHAI'. VII.] DIGESTION IN THE SMALL INTESTINES. 333 The conversion is probably effected by means of a hydrolytic ferment. (Inversive ferment, Bernard.) The length and complexity of the digestive tract seem to be closely con- nected with the character of the food on which an animal lives. Thus in all carnivorous animals, such as the cat and dog, and pre-eminently in car- nivorous birds, as hawks and herons, it is exceedingly short. The seals, which, though carnivorous, possess a very long intestine appear to furnish an exception : but this is doubtless to be explained as an adaptation to. their aquatic habits ; their constant exposure to cold requiring that they should absorb as much as possible from their intestines. Herbivorous animals, on the other hand, and the ruminants especially, have very long intestines (in the sheep 30 times the length of the body) which is no doubt to be connected with their lowly nutritious diet. In others, such as the rabbit, though the intestines are not excessively long, this is compensated by the great length and capacity of the caecum. In man, the length of the intestines is intermediate between the extremes of the carnivora and herbivora. and his diet also is intermediate. Summary of the Digestive Changes in the Small Intestine. hi order to understand the changes in the food which occur during its passage through the small intestine, it will be well to refer briefly to the state in which it leaves the stomach through the pylorus. It has been said before, that the chief office of the stomach is not only to mix into an uniform mass all the varieties of food that reach it through the oesophagus, but especially to dissolve the nitrogenous portion by means of the gastric juice. The fatty matters, during their sojourn in the stomach, become more thoroughly mingled with the other constituents of the food taken, but are not yet in a state fit for absorption. The con- version of starch into sugar, which began in the mouth, has been interfered with, if not altogether stopped. The soluble matters-both those which were so from the first, as sugar and saline matter, and the gastric peptones-have begun to dis- appear by absorption into the blood-vessels, and the same thing has befallen such fluids as may have been swallowed-wine, water, <fcc. The thin pultaceous chyme, therefore, which during the whole period of gastric digestion, is being constantly squeezed or strained through the pyloric orifice into the duodenum, consists of albu- minous matter, broken down, dissolving and half dissolved; fatty matter broken down and melted, but not dissolved at all; starch 334 DIGESTION. [chap. vn. very slowly in process of conversion into sugar, and as it becomes sugar, also dissolving in the fluids with which it is mixed ; while, with these are mingled gastric fluid, and fluid that has been swallowed, together with such portions of the food as are not ■digestible, and will be finally expelled as part of the faeces. On the entrance of the chyme into the duodenum, it is sub- jected to the influence of the bile and pancreatic juice, which are then poured out, and also to that of the succus entericus. All these secretions have a more or less alkaline reaction, and by their admixture with the gastric chyme, its acidity becomes less and less until at length, at about the middle of the small intestine, the reaction becomes alkaline and continues so as far as the ileo-cfecal valve. The special digestive functions of the small intestine may be taken in the following order :- (i.) One important duty of the small intestine is the alteration of the fat in such a manner as to make it fit for absorption; and there is no doubt that this change is chiefly effected in the upper part of the small intestine. What is the exact share of the pro- cess, however, allotted respectively to the bile, to the pancreatic secretion, and to the intestinal juice, is still uncertain. The fat is changed in two ways, (a.) To a slight extent it is chemically decomposed by the alkaline secretions with which it is mingled, and a soap is the result. (6.) It is emulsionised, ?>., its particles are minutely subdivided and diffused, so that the mixture assumes the condition of a milky fluid, or emulsion. As will be seen in the next Chapter, most of the fat is absorbed by the lacteals of the intestine, but a small part, which is saponified, is also absorbed by the blood-vessels. (2.) The albuminous substances which have been partly dis solved in the stomach, and have not been absorbed, are subjected to the action of the pancreatic and intestinal secretions. The pepsin is rendered inert by being precipitated together with the gastric peptones and parapeptones, as soon as the chyme meets with bile. By these means the pancreatic ferment trypsin k enabled to proceed with the further conversion of the parapeptones into peptones, and of part of the peptones (hemipeptone, Kuhne) into leucin and tyrosin. Albuminous substances, which are chemically altered in the process of digestion (peptones) and gelatinous matters similarly changed, are absorbed by the blood- vessels and lymphatics of the intestinal mucous membrane. Albu- chap, vii.] DIGESTION IN THE SMALL INTESTINES. 335 minous matters, in state of solution, which have not undergone the peptonic change, are probably, from the difficulty with which they diffuse, absorbed, if at all, almost solely by the lymphatics. (3.) The starchy, or amyloid portions of the food, the conversion of which into dextrin and sugar was more or less interrupted during its stay in the stomach, is now acted on briskly by the pancreatic juice and the succus entericus; and the sugar as it is formed, is dissolved in the intestinal fluids, and is absorbed chiefly by the blood-vessels. (4.) Saline and saccharine matters, as common salt, or cane sugar, if not in a state of solution beforehand in the saliva or other fluids which may have been swallowed with them, are at once dissolved in the stomach, and if not here absorbed, are soon taken up in the small intestine ; the blood-vessels, as in the last case, being chiefly concerned in the absorption. Cane sugar is in part or wholly converted into grape-sugar before its absorption. This is accomplished partially in the stomach, but also by a ferment in the succus entericus. (5.) The liquids, including in this term the ordinary drinks, as water, wine, ale, tea, etc., which may have escaped absorption in the stomach, are absorbed probably very soon after their entrance into the intestine ; the fluidity of the contents of the latter being preserved more by the constant secretion of fluid by the intestinal glands, pancreas, and liver, than by any given portion of fluid, whether swallowed or secreted, remaining long unabsorbed. From this fact, therefore, it may be gathered that there is a kind of circulation constantly proceeding from the intestines into the blood, and from the blood into the intestines again; for as all the fluid-a very large amount-secreted by the intestinal glands, must come from the blood, the latter would be too much drained, were it not that the same fluid after secre- tion is again re-absorbed into the current of blood-going into the blood charged with nutrient products of digestion-coming out again by secretion through the glands in a comparatively uncharged condition. At the lower end of the small intestine, the chyme, still thin and pultaceous, is of a light yellow colour, and has a distinctly faecal odour. This odour depends upon the formation of indol and its allies. In this state it passes through the ileo-caccal opening into the large intestine. 336 DIGESTION. [CHAP. VII. Summary of the Digestive Changes in the Large Intestine. The changes which take place in the chyme in the large in- testine are probably only the continuation of the same changes that occur in the course of the food's passage through the upper part of the intestinal canal. From the absence of villi, however, we may conclude that absorption, especially of fatty matter, is in great part completed in the small intestine; while, from the still half-liquid, pultaceous consistence of the chyme when it first enters the crccum, there can be no doubt that the absorp- tion of liquid is not by any means concluded. The peculiar odour, moreover, which is acquired after a short time by the contents of the large bowel, would seem to indicate a further chemical change in the alimentary matters or in the digestive fluids, oi' both. The acid reaction, which had disappeared in the small bowel, again becomes very manifest in the csecum- probably from acid fermentation-processes in some of the materials of the food. There seems no reason to conclude that any special ' secondary digestive ' process occurs in the ceocum or in any othei' part of the large intestine. Probably any constituent of the food which has escaped digestion and absorption in the small bowel may be digested in the large intestine; and the power of this part of the intestinal canal to digest fatty, albuminous, or other matters, may be gathered from the good effects of nutrient enemata, so frequently given when from any cause there is difficulty in introducing food into the stomach. In ordinary healthy digestion, however, the changes which ensue in the chyme after its passage into the large intestine, are mainly the absorption of the more liquid parts ; the chief function of the large intestine being to act as a reservoir for the residues of digestion before their expulsion from the body. Movements of the Intestines. It remains only to consider the manner in which the food and the several secretions mingled with it are moved through the intestinal canal, so as to be slowly subjected to the influence of CHAP. VII.] MOVEMENTS OF THE INTESTINES. 337 fresh portions of intestinal secretion, and as slowly exposed to the absorbent power of all the villi and blood vessels of the mucous membrane. The movement of the intestines is peristaltic or vermicular, and is effected by the alternate contractions and dilatations of successive portions of the intestinal coats. The contractions, which may commence at any point of the intestine, extend in a wave-like manner along the tube. In any given portion, the longitudinal muscular fibres contract first, or more than the circular ; they draw a portion of the intestine upwards, or, as it were, backwards, over the substance to be propelled, and then the circular fibres of the same portion contracting in succession from above downwards, or, as it were, from behind forwards, press on the substance into the portion next below, in which at once the same succession of action next ensues. These movements take place slowly and, in health, commonly give rise to no sensation ; but they are perceptible when they are accelerated under the influence of any irritant. The movements of the intestines are sometimes retrograde; and there is no hindrance to the backward movement of the contents of the small intestine. But almost complete security is afforded against the passage of the contents of the large into the small in- testine by the ileo-ceecal valve. Besides,-the orifice of communi- cation between the ileum and ceecum (at the borders of which orifice are the folds of mucous membrane which form the valve) is encircled with muscular fibres, the contraction of which prevents the undue dilatation of the orifice. Proceeding from above downwards, the muscular fibres of the large intestine become, on the whole, stronger in direct propor- tion to the greater strength required for the onward moving of the faeces, which are gradually becoming firmer. The greatest strength is in the rectum, at the termination of which the circular unstriped muscular fibres form a strong band called the internal sphincter; while an external sphincter muscle with striped fibres is placed rather lower down, and more externally, and as we have seen above, holds the orifice close by a constant slight tonic contraction. Experimental irritation of the brain or cord produces no evident or constant effect on the movements of the intestines during life ; yet in consequence of certain mental conditions the movements are accelerated or retarded; and in paraplegia the intestines appear after a time much weakened in their power, and costiveness, with 338 DIGESTION. [CHAP. VII. a tympanitic condition, ensues. Immediately after death, irritation of both the sympathetic and pneumo-gastric nerves, if not too strong, induces genuine peristaltic movements of the intestines. Violent irritation stops the movements. These stimuli act, no doubt, not directly on the muscular tissue of the intestine, but on the ganglionic plexus before referred to. Influence of the Nervous System on Intestinal Digestion. As in the case of the oesophagus and stomach, the peristaltic movements of the intestines are directly due to reflex action through the ganglia and nerve fibres distributed so abundantly in their walls (p. 298); the presence of chyme acting as the stimulus, and few or no movements occurring when the intestines are empty. The intestines are, moreover, connected with the higher nerve-centres by the splanchnic nerves, as well as other branches of the sympathetic which come to them from the coeliac and other abdominal plexuses. The splanchnic nerves are in relation to the intestinal move- ments, inhibitory-these movements being retarded or stopped when the splanchnics are irritated. As the vasomotor nerves of the intestines, the splanchnics are also much concerned in intestinal digestion. Duration of Intestinal Digestive Period.-The time occu- pied by the journey of a given portion of food from the stomach to the anus, var'es considerably even in health, and on this account probably it is that such different opinions have been expressed in regard to the subject. About twelve hours are occupied by the journey of an ordinary meal through the small intestine, and twenty-four to thirty-six hours by the passage through the large bowel. The contents of the large intestine, as they proceed towards the rectum, become more and more solid, and losing their more liquid and nutrient parts, gradually acquire the odour and consistence characteristic of faces. After a sojourn of uncertain duration in the sigmoid flexure of the colon, or in the rectum, they are finally expelled by the act of defalcation. The average quantity of solid faecal matter evacuated by the human adult in twenty-four hours is about six or eight ounces. CHAP. VII.] THE MECHANISM OF DEFECATION. 339 Water 733'00 Solids Composition of Faeces. Special excrementitious constituents :-Excretin, excre- toleic acid (Marcet), and stercorin (Austin Flint). Salts :-Chiefly phosphate of magnesium and phosphate of calcium, with small quantities of iron, soda, lime, and silica. Insoluble residue of the food (chiefly starch grains, woody tissue, particles of cartilage and fibrous tissue, un- digested muscular fibres or fat, and the like, with insoluble substances accidentally introduced with the food. Mucus, epithelium, altered colouring matter of bile, fatty acids, etc. Varying quantities of other constituents of bile, and de- rivatives from them. 267'00 iooo Defsecation.-The act of the expulsion of faeces is in part due to an increased reflex peristaltic action of the lower part of the large intestine, namely of the sigmoid flexure and rectum, and in part to the more or less voluntary action of the abdominal muscles. In the case of active voluntary efforts, there is usually, first an inspiration, as in the case of coughing, sneezing, and vomiting ; the glottis is then closed, and the diaphragm fixed. The abdo- minal muscles are contracted as in expiration; but as the glottis is closed, the whole of their pressure is exercised on the abdo- minal contents. The sphincter of the rectum being relaxed, the evacuation of its contents takes place accordingly; the effect being, of course, increased by the peristaltic action of the intes- tine. As in the other actions just referred to, there is as much tendency to the escape of the contents of the lungs or stomach as of the rectum ; but the pressure is relieved only at the orifice, the sphincter of which instinctively or involuntarily yields. Nervous Mechanism.-The anal sphincter muscle is normally in a state of tonic contraction. The nervous centre which governs this contraction is probably situated in the lumbar region of the spinal cord, inasmuch as in cases of division of the cord above this region the sphincter regains, after a time, to some extent the tonicity which is lost immediately after the operation. By an •effort of the will, acting through the centre, the contraction may 340 DIGESTION. [CHAP. VII. be relaxed or increased. In ordinary cases the apparatus is set in action by the gradual accumulation of faeces in the sigmoid flexure and rectum, pressing by the peristaltic action of these parts of the large intestine against the sphincter, and causing by reflex action its relaxation; this sensory impulse acting through the brain and reflexly through the spinal centre. The Gases contained in the Stomach and Intestines.- Under ordinary circumstances, the alimentary canal contains a considerable quantity of gaseous matter. Any one who has had occasion, in a post-mortem examination, either to lay open the intestines, or to let out the gas which they contain, must have been struck by the small space afterwards occupied by the bowels, and by the large degree, therefore, in which the gas, which naturally distends them, contributes to fill the cavity of the abdomen. Indeed, the presence of air in the intestines is so con- stant, and, within certain limits, the amount in health so uniform, that there can be no doubt that its existence here is not a mere accident, but intended to serve a definite and important purpose, although, probably, a mechanical one. Sources.-The sources of the gas contained in the stomach and bowels may be thus enumerated :- i. Air introduced in the act of swallowing either food or saliva ; 2. Gases developed by the decomposition of alimentary matter, or of the secretions and excretions mingled with it in the stomach and intestines ; 3. It is probable that a certain mutual interchange occurs between the gases con- tained in the alimentary canal, and those present in the blood of these gastric and intestinal blood-vessels ; but the conditions of the exchange are not known, and it is very doubtful whether anything like a true and definite secretion of gas from the blood into the intestines or stomach ever takes place. There can be no doubt, however, that the intestines may be the proper excretory organs for many odorous and other substances, either absorbed from the air taken into the lungs in inspiration, or absorbed in the upper part of the alimentary canal, again to be excreted at a portion of the same tract lower down-in either case assuming rapidly a gaseous form after their excretion, and in this way, perhaps, obtaining a more ready egress from the body. It is probable that, under ordinary circumstances, the gases of the stomach and intestines are derived chiefly from the second of the sources which have been enumerated. It is now very generally admitted that the decompositions of food in the alimentary canal are partially the result of the growth of various kinds of micro-organisms, some of which have been already mentioned, and that these decompositions are independent of as well as distinct from the action of the digestive fluids. It is to these special fermentative changes that the gases in the intestines are chiefly due. CHAP. VIII.] ABSORPTION. 341 Composition of Gases contained in the Alimentary Canal. (Tabulated from various authorities by Brinton.) Whence obtained. Composition by Volume. Oxygen. Nitrog. Carbon. Acid. Hydrog. Carburet. Hydrogen. Sulphuret. Hydrogen. Stomach II 71 14 4 Small Intestines . . - 32 3° 38 - Caecum .... - 66 12 8 13 Colon . . . . - 35 57 6 8 Li ace. Rectum .... - 46 43 - 11 Expelled per anum . . - 22 4i 19 19 i CHAPTER VIII. ABSORPTION. The process of Absorption has, for one of its objects, the intro- duction into the blood of fresh materials from the food and air, and of whatever comes into contact with the external or internal surfaces of the body; and, for another, the gradual removal of parts of the body itself, when they need to be renewed. In absorption from without and absorption from within, the process manifests some variety, and a very wide range of action; and in both two sets of vessels are, or may be, concerned, namely, the Blood-vessels, and the Lymph-vessels or Lymphatics to which the term Absorbents has been specially applied. Lymphatic Vessels. Distribution.-The principal vessels of the lymphatic system are, in structure and general appearance, like very small and thin- walled veins. They are provided with valves. They commence in fine microscopic lymph-capillaries, in the organs and tissues of the body, and they end directly or indirectly in two trunks which open into the large veins near the heart (fig. 214). The lymph 342 ABSORPTION. [chap. viii. and chyle which they contain, unlike the blood, pass only in one direction, namely, from the fine branches to the trunk and so to the large veins, on entering which they are mingled with the stream of blood, and form part of its constituents. Remembering Lymphatics of head ana neck, right. Right internal jugular vein. Right subclavian vein. Lymphatics of right arm. Receptaculum chyli. 4 Lymphatics of lower extremities. Lymphatics of head and neck, left. Thoracic duct. Left subclavian vein. Thoracic duct. Lacteals. Lymphatics of lower extremities. Fig. 214.-Diagram of the principal groups of Lymphatic vessels (from Quain). the course of the fluid in the lymphatic vessels, viz., its passage in the direction only towards the large veins in the neighbourhood of the heart, it will readily be seen from fig. 2 j 4 that the greater part of the contents of the lymphatic system of vessels passes through a comparatively large trunk called the thoracic duct, which finally empties its contents into the blood-stream, at the junction of the internal jugular and subclavian veins of the left side. There is a smaller duct on the right side. The lymphatic chap, viii.] ORIGIN OF LYMPH CAPILLARIES. 343 vessels of the intestinal canal are called lacteals, because during digestion, the fluid contained in them resembles milk in appear- ance ; and the lymph in the lacteals during the period of digestion is called chyle. There is no essential distinction, how- ever, between lacteals and lymphatics. In some parts of their course all lymphatic vessels pass through certain bodies called lymphatic glands. Lymphatic vessels are distributed in nearly all parts of the Fig. 215.-Lymphatics of central tendon of rabbit's diaphragm, stained with silver nitrate. The ground substance has been shaded diagrammatical!}' to bring out the lympha- tics clearly. I. Lymphatics lined by long narrow endothelial cells, and showing v. valves at frequent intervals. (Schofield.) body. Their existence, however, has not yet been determined in the placenta, the umbilical cord, the membranes of the ovum, or in any of the so-called non-vascular parts, as the nails, cuticle, hair, and the like. Origin of Lymph Capillaries.-The lymphatic capillaries com- mence most commonly either (a) in closely-meshed networks, or (6) in irregular lacunar spaces between the various structures of which the different organs are composed. Such irregular spaces, forming what is now termed the lymph-canalicular system, have been shown to exist in many tissues. In serous membranes such as the omentum and mesentery they occur as a connected system 344 ABSORPTION. [chap. vih. of very irregular branched spaces partly occupied by connective tissue-corpuscles, and both in these and in many other tissues are found to communicate freely with regular lymphatic vessels. In many cases, though they are formed mostly by the chinks and Fig. 216.-Lymphatic vessels of the head and neck and the upper part of the trunk (Mascagni). J.-The chest and pericardium, have been opened on the left side, and the left mamma detached and thrown outwards over the left arm, so as to expose a great part of its deep surface. The principal lymphatic vessels and glands are shown on the side of the head and face, and in the neck, axilla, and mediastinum. Between the left internal jugular vein and the common carotid artery, the upper ascending part of the thoracic duct marked 1, and above this, and descending to 2, the arch and last part of the duct. The termination of the upper lymphatics of the diaphragm in the mediastinal glands, as well as the cardiac and the deep mammary lymphatics, is also shown. crannies between the blood-vessels, secreting ducts, and other parts which may happen to form the framework of the organ in which they exist, they are lined by a distinct layer of endo- thelium. The lacteals offer an illustration of another mode of origin, chap, viii.] STRUCTURE OF LYMPH CAPILLARIES. 345 namely, (c) in blind dilated extremities ; but there is no essential difference in structure between these and the lymphatic capillaries of other parts. Structure of Lymph Capillaries. -The structure of lymphatic ca- pillaries is very similar to that of blood-capillaries : their walls consist of a single layer of endothelial cells of an elongated form and sinuous outline, which cohere along their edges to form a delicate membrane. They differ from blood-capillaries mainly in their larger and very variable calibre, and in their numer- ous communications with the spaces of the lymph-canalicular system. Communications of the Lymphatics. -The fluid part of the blood con- stantly exudes from or is strained through the walls of the blood-ca- pillaries, so as to moisten all the surrounding tissues, and occupies the interspaces which exist among their different elements, which form the beginnings of the lymyh-capil- laries; and the latter, therefore, are the means of collecting the exuded blood plasma, and returning that part which is not directly ab- sorbed by the tissues into the blood- stream. It is not necessary to assume the presence of any special channels between the blood and lymphatic vessels, inasmuch as even blood-corpuscles can pass bodily, without much difficulty, through the walls of the blood-capillaries and small veins, and could pass with still less trouble, probably, through the comparatively ill-defined walls of the capillaries which contain lymph. Fig. 217.-Superficial lymphatics of the forearm and palm of the hand, J.- 5. Two small glands at the bend of the arm. 6. Radial lymphatic ves- sels. 7. Ulnar lymphatic vessels. 8, 8. Palmar arch of lymphatics. 9, 9'. Outer and inner sets of ves- sels. b. Cephalic vein. d. Radial vein. e. Median vein. f. Ulnar vein. The lymphatics are repre- sented as lying on the deep fascia. (Mascagni.) 346 ABSORPTION. [chap. viii. It lias been already mentioned that in certain parts of the body, openings or stomata exist, by which lymphatic capillaries directly communicate with parts hitherto supposed to be closed cavities. When absorption into the lym- phatic system takes place in mem- branes covered by epithelium or endothelium through the intersti- tial or intercellular cement-sub- stance, it is said to take place through pseudostomata, already alluded to. Demonstration of Lymphatics of Diaphragm.-The stomata on the peri- toneal surface of the diaphragm are the openings of short vertical canals which lead up into the lymphatics, and are lined by cells like those of germinating endothelium. By in- troducing a solution of Berlin blue into the peritoneal cavity of an animal shortly after death, and suspending it, head downwards, an injection of the lymphatic vessels of the dia- phragm, through the stomata on its peritoneal surface, may readily be obtained, if artificial respiration be carried on for about half an hour. In this way it has been found that in the rabbit the lymphatics are arranged between the tendon bundles of the centrum tendineum; and they are hence termed interfascicular. The centrum tendineum is coated by endo- thelium on its pleural and peritoneal surfaces, and its substance consists of tendon bundles arranged in concentric rings towards the pleural side and in radiating bundles towards the peritoneal side. The lymphatics of the anterior half of the diaphragm open into those of the anterior mediastinum, while those of the posterior half pass into a lymphatic vessel in the posterior mediastinum, which soon enters the thoracic duct. Both these sets of vessels, and the glands into which they pass, are readily injected by the method above described ; and there can be little doubt that during life the flow of lymph along these channels is chiefly caused by the action of the diaphragm during respiration. As it descends in inspiration, the spaces between the radiating tendon bundles dilate, and lymph is sucked from the peritoneal cavity, through the widely open stomata, into the interfascicular lymphatics. During expiration, the spaces between the concentric tendon bundles dilate, and the lymph is squeezed into Fig. 218.-Superficial lymphatics of right groin and upper part of thigh, J. 1. Upper inguinal gla.ids.' 2, 2'. Lower inguinal or femoral glands. 3,3'. Plexus of lymphatics in the course of the long saphenous vein. (Mascagni.) chap, vni.] STRUCTURE OF LYMPHATIC VESSELS. 347 the lymphatics towards the plural surface (Klein). It thus appears probable that during health there is a continued sucking in of lymph from the peri- toneum into the lymphatics by the " pumping " action of the diaphragm ; and there is doubtless an equally continuous exudution of fluid from the general serous surface of the peritoneum. When this balance of transuda- tion and absorption is disturbed either by increased transudation or some impediment to absorption, an accumulation of fluid necessarily takes place (ascites). Stomata have been found in the pleura; and as they may be presumed to exist in other serous membranes, it would seem as if the serous cavities, hitherto supposed closed, form but a large Fig. 219.-Peritoneal surface of septum cisternce lymphatic# magnat of frog. The stomata, some of which are open, some collapsed, are surrounded by germinating endothelium. X 160. (Klein.) lymph-sinus or widening out, so to speak, of the lymph-capillary system with which they directly communicate. Structure of Lymphatic Vessels.-The larger vessels are very like veins, having an external coat of fibro-cellular tissue, with elastic filaments; within this, a thin layer of fibro-cellular tissue, with plain muscular fibres, which have, principally, a circular direction, and are much more abundant in the small than in the larger vessels; and again, within this, an inner elastic layer of longitudinal fibres, and a lining of epithelium; and numerous valves. The valves, constructed like those of veins, and with the free edges turned towards the heart, are usually arranged in pairs, and, in the small vessels, are so closely placed, that when the vessels are full, the valves constricting them where their edges arc attached, give them a peculiar beaded or knotted appearance. 348 ABSORPTION. [chap yxu. Current of the Lymph.-With the help of the valvular mechanism (i) all occasional pressure on the exterior of the lym- phatic and lacteal vessels propels the lymph towards the heart: thus muscular and other external pressure accelerates the flow of the lymph as it does that of the blood in the veins. The actions of (2) the muscular fibres of the small intestine, and probably the layer of unstriped muscle present in each intestinal villus, seem to assist in propelling the chyle : for, in the small intestine of a mouse, the chyle has been seen moving with intermittent propul- sions that appeared to correspond with the peristaltic movements of the intestine. But for the general propulsion of the lymph and chyle, it is probable that, together with (3) the vis a tergo resulting from absorption (as in the ascent of sap in a tree), and from external pressure, some of the force may be derived (4) from the contractility of the vessel's own walls. The respiratory movements, also, (5) favour the current of lymph through the thoracic duct as they do the current of blood in the thoracic veins. Lymph-Hearts.-In reptiles and some birds, an important auxiliary to the move- ment of the lymph and chyle is supplied in certain muscular sacs, named lymph- hearts (fig. 220), and it has been shown that the caudal heart of the eel is a lymph- heart also. The number and position of these organs vary. In frogs and toads there are usually if our, two anterior and two posterior; in the frog, the posterior lymph-heart on each side is situated in the ischiatic region, just beneath the skin ; the anterior lies deeper, just over the transverse process of the third vertebra. Into each of these cavities several lym- phatics open, the orifices of the vessels being guarded by valves, which prevent the retrograde passage of the lymph. From each heart a single vein proceeds, and conveys the lymph directly into the venous system. In the frog, the inferior lymphatic heart, on each side, pours its lymph into a branch of the ischiatic vein ; by the superior, the lymph is forced into a branch of the jugular vein, which issues from its anterior surface, and which becomes turgid each time that the sac contracts. Blood is prevented from passing from the vein into the lymphatic heart by a valve at its orifice. The muscular coat of these hearts is of variable thickness ; in some cases it Fig. 220. - Lymphatic heart (g lines long, 4 lines broad) of a large species of serpent, the Python bivittatus. 4. The external cellular coat. 5. The thick muscular coat. Four muscular columns run across its cavity, which communicates with three lymphatics (1-only one is seen here), and with two veins (2,2). 6. The smooth lining mem- brane of the cavity. 7. A small appendage, or auricle, the cavity of which is continuous with that of the rest of the organ (after E. Weber). CHAP. VIII.] LYMPHATIC GLANDS. 349 can only be discovered by means of the microscope ; but in every case it is composed of striped fibres. The contractions of the hearts are rhythmical, occurring about sixty times in a minute, slowly, and, in comparison with those of the blood-hearts, feebly. The pulsations of the cervical pair are not always synchronous with those of the pair in the ischiatic region, and even the corresponding sacs of opposite sides are not always synchronous in their action. Unlike the contractions of the blood-heart, those of the lymph-heart appear to be directly dependent upon a certain limited portion of the spinal cord. For Volkmann found that so long as the portion of spinal cord corresponding to the third vertebra of the frog was uninjured, the cervical pair of lymphatic hearts continued pulsating after all the rest of the spinal cord and the brain were destroyed ; while destruction of this portion, even though all other parts of the nervous centres were uninjured, instantly arrested the heart's movements. The posterior, or ischiatic, pair of lymph- hearts were found to be governed, in like manner, by the portion of spinal cord corresponding to the eighth vertebra. Division of the posterior spinal roots did not arrest the movements ; but division of the anterior roots caused them to cease at once. Lymphatic Glands. Lymphatic glands are small round or oval compact bodies varying in size from a hempseed to a bean, interposed in the course of the lymphatic vessels, and through which the chief part of the lymph passes in its course to be discharged into the blood vessels. They are found in great numbers in the mesentery, and along the great vessels of the abdomen, thorax, and neck ; in the axilla and groin ; a few in the popliteal space, but not further down the leg, and in the arm as far as the elbow. Some lympha- tics do not, however, pass through glands before entering the thoracic duct. Structure.-A lymphatic gland is covered externally by a capsule of connective tissue, generally containing some unstriped muscle. At the inner side of the gland, which is somewhat concave (hilus), (fig. 221 a), the capsule sends inwards processes called trabeculae in which the blood vessels are contained, and these join with other processes prolonged from the inner surface of the part of the capsule covering the convex or outer part of the gland; they have a structure similar to that of the capsule, and entering the gland from all sides, and freely communicating, form a fibrous supporting stroma. The interior of the gland is seen on section, even when examined with the naked eye, to be made up of two parts, an outer or cortical (fig. 221 c, c), which is light coloured, and an inner of redder' appearance, the medullary 350 ABSORPTION. [chap. viii. portion (fig. 221). In the outer or cortical part of the gland (fig. 223) the intervals between the trabecula? are comparatively large, and form more or less triangular intercommuni- cating spaces termed al- veoli ; whilst in the more central or medullary part is a finer meshwork formed by the more free anasto- mosis of the trabecular pro- cesses. Within the alveoli of the cortex and in the meshwork formed by the trabeculae in the medulla, is contained the proper gland structure. In the former it is arranged as follows: occupying the central and chief part of each alveolus, is a more or less wedge-shaped mass of 22i.-5ecbo» o/u as, slightly magnified, a, Hilus ; b (m the cen- tral part of the figure), medullary substance; c, cortical substance with indistinct alveoli; d, capsule. (Kiilliker.) Fig. 222.-Section of medullary substance of an inyainal gland of an ox; a, a, glandular substance or pulp forming rounded cords joining in a continuous net (dark in the figure); c, c, trabeculfe ; the space, b, b, between these and the glandular substance is the lymph sinus, washed clear of corpuscles and tiaversed by filaments of retiform connective-tissue x 90. (Kiilliker.) adenoid tissue, densely packed with lymph corpuscles ; but at the periphery surrounding the central portion and immediately next the capsule and trabeculae, is a more open meshwork of adenoid tissue constituting the lymph sinus or channel, and containing fewer chap, viii.] STRUCTURE OF LYMPHATIC GLANDS. 351 lymph corpuscles. The central mass is enclosed in endothelium, the cells of which join by their processes, the processes of the adenoid framework of the lymph sinus. The trabecuke are also covered with endothelium. The lining of the central mass does not prevent the passage of fluids and even of corpuscles into the lymph sinus. Fig. 223.-Diagrammatic section of lymphatic gland, a.l., afferent; e.l. efferent lympha- tics ; C, cortical substance; l.h., reticulating eords of medullary substance; Z..?., lymph-sinus; c., fibrous coat sending in trabecula:; t.r., into the substance of the gland. (Sharpey.) The framework of the adenoid tissue of the lymph sinus is nucle- ated, that of the central mass is non-nucleated. At the inner part of the alveolus, the wedge-shaped central mass divides into two or more smaller rounded or cord-like masses which joining with those from the other alveoli, form a much closer arrangement of the gland tissue than in the cortex; spaces (fig. 223 6), are left within those anastomosing cords, in which are found portions of the trabecular meshwork and the continuation of the lymph sinus. The essential structure of lymphatic-gland substance resembles that which was described as existing, in a simple form in the interior of the solitary and agminated intestinal follicles. The lymph enters the gland by several afferent vessels, which 352 ABSORPTION. [chap. viii. open beneath the capsule into the lymph-channel or lymph-path ; at the same time they lay aside all their coats except the endothelial lining, which is continuous with the lining of the lymph-path. The efferent vessels begin in the medullary part of the gland, and are continuous with the lymph-path here as the afferent vessels Fig. 224.-A small portion of medullary substance from a mesenteric gland of the ox, d, d, trabeculae ; a, part of a cord of glandular substances from which all but a few of the lymph-corpuscles have been washed out to show its supporting meshwork of retifonn tissue and its capillary blood-vessels (which have been injected, and are dark in the figure); 6. b, lymph-sinus, of which the retiform tissue is represented only at c, c. X 300. (Kolliker.) were with the cortieal portion; the endothelium of one is con- tinuous with that of the other. The efferent vessels leave the gland at the hilus, the more or less concave inner side of the gland, and generally either at once or very soon after join together to form a single vessel. Blood-vessels which enter and leave the gland at the hilus arc freely distributed to the trabecular tissue and to the gland-pulp. CHAP. VIII.] NATURE OF THE LYMPH AND CHYLE. 353 The Lymph and Chyle. Lymph is, under ordinary circumstances, a clear, transparent, and yellowish fluid. It is devoid of smell, is slightly alkaline, and has a saline taste. As seen with the microscope in the small transparent vessels of the tail of the tadpole, it usually contains no corpuscles or particles of any kind ; and it is only in the larger trunks that any corpuscles are to be found. These corpuscles are similar to colourless blood-corpuscles. The fluid in which the corpuscles float is albuminous, and contains no fatty particles; but is liable to variations according to the general state of the blood, and to that of the organ from which the lymph is derived. As it advances towards the thoracic duct, after passing through the lymphatic glands, it becomes spontaneously coagulable and the number of corpuscles is much increased. Chyle, found in the lacteals after a meal, is an opaque, whitish, milky fluid, neutral or slightly alkaline in reaction. Its whiteness and opacity are due to the presence of innumerable particles of oily or fatty matter, of exceedingly minute though nearly uniform size, measuring on the average about of an inch. These constitute what is termed the molecular base of chyle. Their number, and consequently the opacity of the chyle, are dependent upon the quantity of fatty matter contained in the food. The fatty nature of the molecules is made manifest by their solubility in ether. Each molecule probably consists of a droplet of oil coated over with albumen, in the manner in which minute drops of oil always become covered in an albuminous solution. This is proved when water or dilute acetic acid is added to chyle, many of the molecules are lost sight of, and oil-drops appeal- in their place, as the investments of the molecules have been dissolved, and their oily contents have run together. Except these molecules, the chyle taken from the villi or from lacteals near them, contains no other solid or organised bodies. The fluid in which the molecules float is albuminous, and does not spontaneously coagulate. But as the chyle passes on towards the thoracic duct, and especially whilst traversing one or more of the mesenteric glands, it is elaborated. The quantity of molecules and oily particles gradually diminishes ; cells, to which the name of chyle-corpuscles is given, appear in it; and it acquires the property of coagulating spontaneously. The higher in the thoracic duct the chyle advances, the greater is the number of chyle-cor- 354 ABSORPTION. [chap. vhi. puscles, and the larger and firmer is the clot which forms in it when withdrawn and left at rest. Such a clot is like one of blood without the red corpuscles, having the chyle-corpuscles entangled in it, and the fatty matter forming a white creamy film on the surface of the serum. But the clot of chyle is softer and moister than that of blood. Like blood, also, the chyle often remains for a long time in its vessels without coagulating, but coagulates rapidly on being removed from them. The existence of the materials which, by their union form fibrin, is, therefore, certain; and their increase appears to be commensurate with that of the corpuscles. The structure of the chyle-corpuscles was described when speak- ing of the white corpuscles of the blood, with which they are identical. The lymph, in chemical composition, resembles diluted plasma, and from what has been said, it will appear that }}erfect chyle and lymph are, in essential characters, nearly similar, and scarcely differ, except in the preponderance of fatty and proteid matter in the chyle. Chemical Composition of Lymph and Chyle (Owen Rees). I. Lymph. II. Chyle. Mixed Lvmph & (Donkey). (Donkey). Chyle (Human . Water 96-536 90'237 90-48 Solids 3'454 9763 9'52 Solids- Proteids, including Serum-Albu- "I 3'886 7 08 min, Fibrinogen, and Globulin. ) 1320 Extractives,includingin(iand 11) 1 Sugar,Urea. Leucin & Colesterin. J ' 1'559 i'565 •108 Fatty matter .... a trace 3-601 •92 Salts •585 711 •44 Quantity.-The quantity which would pass into a cat's blood in twenty-four hours has been estimated to be equal to about one- sixth of the weight of the whole body. And, since the estimated weight of the blood in cats is to the weight of their bodies as i to 7, the quantity of lymph daily traversing the thoracic duct would appear to be about equal to the quantity of blood at any time contained in the animals. By another series of experiments, the quantity of lymph traversing the thoracic duct of a dog in twenty- four hours was found to be about equal to two-thirds of the blood in the body. CHAP. VIII.] ABSORPTION BY THE LYMPHATICS. 355 The Process of Absorption (a.) By the Lacteals.-During the passage of the chyme along the intestinal canal, its completely digested parts are absorbed by the blood-vessels and lacteals distributed in the mucous membrane. The lacteals appear to absorb only certain constituents of the digested food, including particularly the fatty portions. The absorption by both sets of vessels is carried on most actively but not exclusively, in the villi of the small intestine; for in these minute processes, both the capillary blood-vessels and the lacteals are brought almost into contact with the intestinal contents. There seems to be no doubt that absorption of fatty matters during digestion, from the contents of the intestines, is effected chiefly between the epithelial cells which line the intestinal tract (Watney), and especially those which clothe the surface of the villi. Thence, the fatty particles are passed on into the interior of the lacteal vessels, but how they pass, and what laws govern their passage, are not at present exactly known. The process of absorption is assisted by the pressure exercised on the contents of the intestines by their contractile walls; and the absorption of fatty particles is also facilitated by the presence of the bile, and the pancreatic and intestinal secretions, which moisten the absorbing surface. For it has been found by experi- ment, that the passage of oil through an animal membrane is made much easier when the latter is impregnated with an alkaline fluid. (6.) By the Lymphatics.-The real source of the lymph, and the mode in which its absorption is effected by the lymphatic vessels, were long matters of discussion. But the problem has been much simplified by more accurate knowledge of the ana- tomical relations of the lymphatic capillaries. The lymph is, as has been pointed out, diluted liquor sanguinis, which is always exuding from the blood-capillaries into the interstices of the tissues in which they lie; and as these interstices form in most parts of the body the beginnings of the lymphatics, the source of the lymph is sufficiently obvious. In connection with this may be mentioned the fact that changes in the character of the lymph correspond very closely with changes in the character of either the whole mass of blood, or of that in the vessels of the part from which the lymph is exuded. Thus it appears that the coagulability 356 ABSORPTION. [chap, viir. of the lymph, although always less than, is directly proportionate to that of the blood; and that when fluids are injected into the blood-vessels in sufficient quantity to distend them, the injected substance may be almost directly afterwards found in the lymphatics. Some other matters than those originally contained in the exuded liquor sanguinis may, however, find their way with it into the lymphatic vessels. Parts which having entered into the composition of a tissue, and, having fulfilled their purpose, require to be removed, may not be altogether excrementitious, but may admit of being re-organised and adapted again for nutrition ; and these may be absorbed by the lymphatics, and elaborated with the other contents of the lymph in passing through the glands. (c.) By Blood-Vessels.-In the absorption by the lymphatic or lacteal vessels just described, there appears something like the exercise of choice in the materials admitted into them. But the absorption by blood-vessels presents no such appearance of selection of materials; rather, it appears, that every substance, whether gaseous, liquid, or a soluble, or minutely divided solid, may be absorbed by the blood-vessels, provided it is capable of permeating their walls, and of mixing with the blood; and that of all such substances, the mode and measure of absorption are determined almost solely by their physical or chemical proper- ties and conditions, and by those of the blood and the walls of the blood-vessels. Method of Absorption. (a.) Osmosis.-The phenomena of absorption of all the materials of the food except the fats arc, to a great extent, comparable to that passage of fluids through membrane, which occurs quite inde- pendently of vital conditions, and the earliest and best scientific investigation of which was made by Dutrochet. The instrument which he employed in his experiments was named an endosmometer. It may consist of a graduated tube expanded into an open- mouthed bell at one end, over which a portion of membrane is tied (fig. 225). If now the bell be filled with a solution of a salt-say sodium chloride, and be immersed in water, the water will pass into the solution, and part of the salt will pass out into the water • the water, however, will pass into the solution much more rapidly than the salt will pass out into the water, and the diluted solution chap, vni.] METHOD OF ABSORPTION. 357 will rise in the tube. To this passage of fluids through membrane the term Osmosis is applied. The nature of the membrane used as a septum, and its affinity for the fluids subjected to experiment have an important influence, as might be anticipated, on the rapidity and dura- tion of the osmotic current. Thus, if a piece of ordinary bladder be used as the septum between water and alcohol, the current is almost solely from the water to the alcohol, on account of the much greater affinity of water for this kind of membrane ; while, on the other hand, in the case of a mem- brane of caoutchouc, the alcohol, from its greater affinity for this substance, would pass freely into the water. Absorption by blood-vessels is the consequence of their walls being, like the membranous septum of the endosmometer, porous and capable of imbib ing fluids, and of the blood being so composed that most fluids will mingle with it. Thus the relation of the chyme in the stomach and intestines to the blood circulating in the vessels of the gastric and intestinal mucous membrane is evidently just that which is required for osmosis. The chyme contains substances which have been so acted upon by the digestive juices as to have become quite able to pass through an animal membrane, or to dialyse as it is called. The thin animal membrane is the coat of the blood-vessels with the intervening mucous membrane. The nature of the fluid within the vessels, the very feeble power of dialyzation which the albuminous blood possesses, determines the direction of the osmotic current, viz., into and not out of the blood-vessels. The current is of course aided by the fact of the constant change in the blood presented to the osmotic surface, as it rapidly circulates within the vessels. As a rule the current is from the stomach or intestine into the blood, but the reversed action may occur, when, for example, a certain salt, e.g., sulphate of magnesia, is taken into the stomach, in which case there is a rapid discharge of water from the blood-vessels into the alimentary canal resulting in purgation. The presence of various substances in the food has the power of diminishing the rate of absorption, their influence is probably exerted upon the membrane, diminishing its power of permitting osmosis. Whereas the pre- , Big. 225.-Ena- osmometer. 358 ABSORPTION. [chap. VIII. sence of a little hydrochloric acid in the contents of the stomach appears to determine the direction of the osmosis, or at any rate to diminish or prevent exosmosis. The conditions for osmosis exist not only in the alimentary mucous membrane, but also in the serous cavities and the tissues elsewhere. The process of absorption, in an instructive, though very imperfect degree' may be observed in any portion of vascular tissue removed from the body. If such a one be placed in a vessel of water, it will shortly swell, and become heavier and moister, through the quantity of water imbibed or soaked into it ; and if now, the blood contained in any of its vessels be let out, it will be found diluted with water, which has been absorbed by the blood-vessels and mingled with the blood. The water round the piece of tissue also will become blood-stained; and if all be kept at perfect rest, the stain derived from the solution of the colouring matter of the blood, together with some of the albumen and other parts of the liquor sanguinis, will spread more widely every day. The same will happen if the piece of tissue be placed in a saline solution instead of water, or in a solution of colouring or odorous matter, either of which will give their tinge or smell to the blood, and receive, in exchange, the colour of the blood. Various substances have been classified according to the degree in which they possess the property of passing, when in a state of solution in water, through membrane; those which pass freely, inasmuch as they are usually capable of crystallization, being termed crystalloids, and those which pass with difficulty, on account of their, physically, glue-like character, colloids. This distinction, however, between colloids and crystalloids which is made the basis of their classification, is by no means the only difference between them. The colloids, besides the absence of power to assume, a crystalline form, are characterised by their inertness as acids or bases, and feebleness in all ordinary chemical relations. Examples of them are found in albumin, gelatin, starch, hydrated alumina, hydrated silicic acid, etc.; while the crystal- loids are characterised by qualities the reverse of those just mentioned as belonging to colloids. Alcohol, sugar, and ordinary saline substances are examples of crystalloids. (6.) Filtration, or transudation. A distinction must be drawn between osmosis and filtration. The latter means the passage of fluids through the pores of a membrane under pressure. The greater the pressure the greater the amount which passes through the membrane. Colloids will filter, although less easily than crystalloids. The nature of the substance to be filtered and the CHAP. VIII.] RATE OF ABSORPTION. 359 nature of the membrane which acts as the filter materially affect the activity of the process. No doubt both osmosis and filtration go on together in the process of absorption. An excellent example of filtration oi' transudation occurs in the pathological condition known as dropsy, in which the connective tissues become infiltrated with serous fluid. The fluid passes out of the vein when the intra- venous pressure passes a certain point, the fluid is, as it were, squeezed through the walls of the vessels by this excess of pressure. Rapidity of Absorption.-The rapidity with which matters may be absorbed from the stomach, probably by the blood-vessels chiefly, and diffused through the textures of the body, has been found by experiment. It appears that lithium chloride may be diffused into all the vascular textures of the body, and into some of the non-vascular, as the cartilage of the hip-joint, as well as into the aqueous humour of the eye, in a quarter of an hour after being- given on an empty stomach. Into the outer part of the crystalline lens it may pass after a time, varying from half an hour to an hour and a half. Lithium carbonate, when taken in five or ten-grain doses on an empty stomach, may be detected in the urine in 5 ol- io minutes; or, if the stomach be full at the time of taking the dose, in 20 minutes. It may sometimes be detected in the urine, moreover, for six, seven, or eight days. Some experiments on the absorption of various mineral and vegetable poisons, have brought to light the singular fact, that, in some cases, absorp- tion takes place more rapidly from the rectum than from the stomach. Strychnia, for example, when in solution, produces its poisonous effects much more speedily when introduced into the rectum than into the stomach. When introduced in the solid form, however, it is absorbed more rapidly from the stomach than from the rectum, doubtless because of the greater solvent property of the secretion of the former than of the latter. With regard to the degree of absorption by living blood-vessels, much depends on the facility with which the substance to be absorbed can penetrate the membrane or tissue which lies between it and the blood-vessels. Thus, absorption will hardly take place through the epidermis, but is quick when the epidermis is removed, and the same vessels are covered with only the surface of the cutis, or with granulations. In general, the absorption through membranes is in an inverse proportion to the thickness of their epithelia ; so that the urinary bladder of a frog is traversed in less than a second ; and the absorption of poisons by the stomach 360 ABSORPTION. [chap. viii. or lungs appears sometimes accomplished in an immeasurably small time. Conditions for Absorption.-i. The substance to be absorbed must, as a general rule, be in the liquid or gaseous state, or, if a solid, must be soluble in the fluids with which it is brought into con- tact. Hence the marks of tattooing, and the discoloration pro- duced by silver nitrate taken internally, remain. Mercury may be absorbed even in the metallic state : and in that state may pass into and remain in the blood-vessels, or be deposited from them; and such substances as exceedingly finely-divided charcoal, when taken into the alimentary canal, have been found in the mesenteric veins; the insoluble materials of ointments may also be rubbed into the blood-vessels; but there are no facts to deter- mine how these various substances effect their passage. Oil, minutely divided, as in an emulsion, will pass slowly into blood-vessels, as it will through a filter moistened with water ; and, without doubt, fatty matters find their way into the blood-vessels as well as the lymph-vessels of the intestinal canal, although the latter seem to be specially intended for their absorption. 2. The less dense the fluid to be absorbed, the more speedy, as a general rule, is its absorption by the living blood-vessels. Hence the rapid absorption of water from the stomach; also of weak saline solutions; but with strong solutions, there appears less absorption into, than effusion from, the blood-vessels. 3. The absorption is the less rapid the fuller and tenser the blood- vessels are; and 1 he tension may be so great as to hinder altoge- ther the entrance of more fluid. Thus, if water is injected into a dog's veins to repletion, poison is absorbed very slowly; but when the tension of the vessels is diminished by bleeding, the poison acts quickly. So, when cupping-glasses are placed over a poisoned wound, they retard the absorption of the poison not only by dimi- nishing the velocity of the circulation in the part, but by filling all its vessels too full to admit more. 4. On the same ground, absorption is the quicker the more rapid the circulation of the blood; not because the fluid to be absorbed is more quickly imbibed into the tissues, or mingled with the blood, but because as fast as it enters the blood, it is carried away from the part, and the blood being constantly renewed, is con- stantly as fit as at the first for the reception of the substance to be absorbed. CHAP. IX.] VARIATIONS IN BODY TEMPERATURE. 361 CHAPTER IX. ANIMAL HEAT. The Average Temperature of the human body in those internal parts which are most easily accessible, as the mouth and rectum,, is from 98'5° to 99'5° F. (36'9°-37'4° C.). In different parts of the external surface of the human body the temperature varies only to the extent of two or three degrees (F.), when all are alike protected from cooling influences; and the difference which under these circumstances exists, depends chiefly upon the different degrees of blood-supply. In the armpit- the most convenient situation, under ordinary circumstances, for examination by the thermometer-the average temperature is 98'6C F. (36'9" C.). In different internal parts, the variation is one or two degrees : those parts and organs being warmest which contain most blood, and in which there occurs the greatest amount of chemical change, e.y., the glands and the muscles ; and the temperature is highest, of course, when they are most actively working : while those tissues which, subserving only a mechanical function, are the seat of least active circulation and chemical change, are the coolest. These differences of temperature, however, are actually but slight, on account of the provisions which exist for maintaining uniformity < >f temperature in different parts. Circumstances causing Variations in Temperature.- The chief circumstances by which the temperature of the healthy body is influenced are the following:-Age; Sex; Period of the day ; Exercise ; Climate and Season ; Food and Drink. Age.-The average temperature of the new-born child is only about i° F. ('54° C.) above that of the adult; and the difference becomes still more trifling during infancy and early childhood. The temperature falls to the extent of about -2°-'5 F. from early infancy to puberty, and by about the same amount from puberty to fifty or sixty years of age. In old age the temperature again rises, and approaches that of infancy; but although this is the case, yet the power of resisting cold is less in them-exposure to a low temperature causing a greater reduction of heat than in young persons. 362 ANIMAL HEAT. [chap. IX. Sex.-The average temperature of the female is very slightly higher than that of the male. Period of the Day.-The temperature undergoes a gradual alteration, to the extent of about i° to 1'5° F. ('54-'8° C.) in the course of the day and night; the minimum being at night or in the early morning, the maximum late in the afternoon. Exercise.-Active exercise raises the temperature of the body from i° to 20 F. ('54°-i'o8° C.). This may be partly ascribed to generally increased combustion-processes, and partly to the fact, that every muscular contraction is attended by the develop- ment of one or two degrees of heat in the acting muscle; and that the heat is increased according to the number and rapidity of these contractions, and is quickly diffused by the blood cir- culating from the heated muscles. Possibly, also, some small amount of heat may be generated in the various movements, stretchings, and recoilings of the other tissues, as the arteries, whose elastic walls, alternately dilated and contracted, may give out some heat, just as caoutchouc alternately stretched and recoiling becomes hot. Climate and Season.-The temperature of the human body is the same in temperate and tropical climates. (Furnell.) In summer the temperature of the body is a little higher than in winter; the difference amounting to about a third of a degree F. Food and Drink.-The effect of a meal upon the temperature of a body is but small. A very slight rise usually occurs. Cold alcoholic drinks depress the temperature somewhat (•5° to 1 F.). Warm alcoholic drinks, as well as warm tea and coffee, raise the temperature (about '5° F.). In disease the temperature of the body deviates from the normal standard to a greater extent than would be anticipated from the slight effect of external conditions during health. Thus, in some diseases, as pneumonia and typhus, it occasionally rises as high as 1060 or 107° F. (41°-4i-6° C.); and considerably higher tempe- ratures have been noted. In Asiatic cholera, on the other hand, a thermometer placed in the mouth may sometimes rise only to 77° or 790 F. (250-26-2 C.). The temperature maintained by Mammalia in an active state of life, according to the tables of Tiedemann and Rudolphi, averages ioi° (38'3° C.). The extremes recorded by them were 96° and 106°, the former in the nar- whal, the latter in a bat (Vespertilio pipistrella). In Birds, the average is as high as 107° (41'2° C.) ; the highest temperature, Iir25° (46'2° C.) : CHAP. IX.] SOURCES OF HEAT. 363 being in the small species, the linnets, &c. Among Reptiles, while the medium they were in was 75° (23'9°C-) their average temperature, was 82-5° (31-2° O). As a general rule, their temperature, though it falls witli that of the surrounding medium, is, in temperate media, two or more degrees higher ; and though it rises also with that of the medium, yet at very high degrees it ceases to do so, and remains even lower than that of the medium. Fish and invertebrata present, as a general rule, the same temperature as the medium in which they live, whether that be high or low ; only among fish, the tunny tribe, with strong hearts and red meat-like muscles, and more blood than the average of fish have, are generally 70 (3'8° C.) warmer than the water around them. The difference, therefore, between what are commonly called the warm and the cold-blooded animals, is not one of absolutely higher or lower tem- perature ; for the animals which to us in a temperate climate, feel cold (being like the air or water, colder than the surface of our bodies), would in an external temperature of ioo° (37'8° C.) have nearly the same tempera- ture and feel hot to us. The real difference is that what we call warm- blooded animals (Birds and Mammalia), have a certain " permanent heat in all atmospheres," while the temperature of the others, which we call cold-blooded, is " variable with every atmosphere." (Hunter.) The power of maintaining a uniform temperature, which Mammalia and Birds possess, is combined with the want of power to endure such changes of body temperature as are harmless to the other classes ; and when their power of resisting change of temperature ceases, they suffer serious dis- turbance or die. Sources and Mode of Production of Heat in the Body.-The heat which is produced in the body arises from combustion, and is due to the fact that the oxygen of the atmosphere taken into the system is ultimately com- bined with carbon and hydrogen, and discharged from the body as carbonic acid and water. Any changes, indeed, which occur in the protoplasm of the tissues, resulting in an exhi- bition of their function, are attended by the evolution of heat and the formation of carbonic acid and water. The more active the changes, the greater is the heat produced and the greater is the amount of the carbonic acid and water formed. But in order that the protoplasm may perform its function, the waste of its own tissue (destructive metabolism), must be repaired by the due supply of food material and therefore for the production of heat food is necessary. In the tissues, therefore, two processes are con- tinually going on : the building up of the protoplasm from the food (constructive metabolism), which is not accompanied by the evolu- tion of heat but possibly by the reverse, and the oxidation of the protoplastic materials, resulting in the production of energy, by which heat is produced and carbonic acid and water are evolved. Some heat also is generated in the combination of sulphur and 364 ANIMAL HEAT. [chap. ix. phosphorus with oxygen, but the amount thus produced is but small. It is not necessary to assume that the combustion processes, which ultimately issue in the production of carbonic acid and water, are as simple as the bare statement of the fact might seem to indicate. But complicated as the various stages may be, the ultimate result is as simple as in ordinary combustion outside the body, and the products are the same. The same amount of heat will be evolved in the union of any given quantities of carbon and oxygen, and of hydrogen and oxygen, whether the combination be rapid and direct, as in ordinary combustion, or slow and almost imperceptible, as in the changes which occur in the living body. And since the heat thus arising will be dis- tributed wherever the blood is carried, every part of the body will be heated equally, or nearly so. This theory, that the maintenance of the temperature of the living body depends on continual chemical change, chiefly by oxidation of combustible materials existing in the tissues, has long been established by the demonstration that the quantity of carbon and hydrogen which, in a given time, unites in the body with oxygen, is sufficient to account for the amount of heat generated in the animal within the same period : an amount capable of maintaining the temperature of the body at from g8°-100" F. (36,8°-37*8° C.), notwithstanding a large loss by radiation and evaporation. It should be remembered that some heat may be introduced into the body by means of warm drinks and foods, and, again, that it is possible for the preliminary digestive changes to be accom- panied by the evolution of heat. Chief Heat-producing Tissues.-The chemical changes which produce the body-heat appear to be especially active in certain tissues -(1), In the Muscles, which form so large a part of the organism. The fact that the manifestation of muscular energy is always attended by the evolution of heat and the production of carbonic acid has been demonstrated by actual experiment ; and when not actually in a condition of active contraction, a metabolism, not so active but still actual, goes on, which is accompanied by the manifestation of heat. The total amount set free by the muscles, therefore, must be very great; and it has been calculated in a way which will be referred to later on, that even neglecting the heat produced by the quiet CHAP. IX.] REGULATION OF THE BODY TEMPERATURE. 365 metabolism of muscular tissue, the amount of heat generated by muscular activity supplies the principal part of the total heat produced within the body. (2), In the Secreting glands, and principally in the liver as being the largest and most active. It has been found by experiment that the blood leaving the glands is considerably warmer than that entering them. The metabolism in the glands is very active and, as we have seen, the more active the metabolism the greater the heat produced. (3), In the Brain; the venous blood having a higher tempe- rature than the arterial. It must be remembered, however, that although the organs above mentioned are the chief heat-producing parts of the body, all living tissues contribute their quota, and this in direct proportion to their activity. The blood itself is also the seat of metabolism, and, therefore, of the production of heat ; but the share which it takes in this respect, apart from the tissues in which it circulates, is very inconsiderable. Regulation of the Temperature of the Human Body. The average temperature of the body is maintained under different conditions of external circumstances by mechanisms which permit of (i) variation in the amount of heat got rid of, and (2) variations in the amount of heat produced or introduced into the body. In healthy warm-blooded animals the loss and gain of heat are so nearly balanced one by the other that, under all ordinary circumstances, an uniform temperature, within two or three degrees, is preserved. I. Methods of Variation in the amount of Heat got rid of.-The loss of heat from the human body is principally regu- lated by the amount lost by radiation and conduction from its surface, and by means of the constant evaporation of water from the same part, and (2) to a much less degree from the air- passages ; in each act of respiration, heat is lost to a greater or less extent according to the temperature of the atmosphere; unless indeed the temperature of the surrounding air exceed that of the blood. We must remember too that all food and drink which enter the body at a lower temperature than itself abstract a small measure of heat: while the urine and faeces which leave the body at about its own temperature are also means by which a small amount is lost. (a.) Loss of Heat from the Surface of the Body ; the Skin.-By 366 ANIMAL HEAT. [chap. IX, far the most important loss of heat from the body,-probably 70 or 80 per cent, of the whole amount, is that which takes place by radiation, conduction, and evaporation from the skin. The means by which the skin is able to act as one of the most important organs for regulating the temperature of the blood are-(1), that it offers a large surface for radiation, conduction, and evaporation; (2), that it contains a large amount of blood ; (3), that the quantity of blood contained in it is the greater under those circumstances which demand a loss of heat from the body, and vice versd. For the circumstance which directly determines the quantity of blood in the skin, is that which governs the supply of blood to all the tissues and organs of the body, namely, the power of the vaso-motor nerves to cause a greater or less tension of the muscular element in the walls of the arteries, and, in correspondence with this, a lessening or increase of the calibre of the vessel, accompanied by a less or greater current of blood. A warm or hot atmosphere so acts on the nerve fibres of the skin, as to lead them to cause in turn a relaxation of the muscular fibre of the blood-vessels ; and, as a result, the skin becomes full-blooded, hot, and sweating; and much heat is lost. With a low temperature, on the other hand, the blood-vessels shrink, and in accordance with the consequently diminished blood-supply, the skin becomes pale, and cold, and dry; and no doubt a similar effect may be produced through the vaso-motor centre in the medulla and spinal cord. Thus, by means of a self-regulating apparatus, the skin becomes the most important of the means by which the temperature of the body is regulated. In connection with loss of heat by the skin, reference has been made to that which occurs both by radiation and conduction, and by evaporation; and the subject of animal heat has been considered almost solely with regard to the ordinary case of man living in a medium colder than his body, and therefore losing heat in all the ways mentioned. The importance of the means however, adopted, so to speak, by the skin for regulating the temperature of the body, will depend on the conditions by which it is surrounded ; an inverse proportion existing in most cases between the loss by radiation and conduction on the one hand, and by evaporation on the other. Indeed, the small loss of heat by evaporation in cold climates may go far to compensate for the greater loss by radiation; as, on the other hand, the great CHAP. IX.] THE REGULATING POWER OF THE SKIN. 367 amount of fluid evaporated in hot air may remove nearly as much heat as is commonly lost by both radiation and evaporation in ordinary temperatures; and thus, it is possible that the quantities of heat required for the maintenance of an uniform proper tempera- ture in various climates and seasons are not so different as they, at first thought, seem. Many examples may be given of Ute power which the body possesses of resisting the effects of a high temperature, in virtue of evaporation from the skin. Blagden and others supported a temperature varying between 198°-2110 F. (920-100° C.) in dry air for several minutes; and in a subsequent experiment he remained eight minutes in a temperature of 260° F. (126'5° C.). " The workmen of Sir F. Chantrey were accustomed to enter a furnace, in which his moulds were dried, whilst the floor was red-hot, and a thermometer in the air stood at 350° F. (177-8° C.), and Chabert, the fire-king, was in the habit of entering an oven, the temperature of which was from 400° to 6oo° F. (205°-315° C.). (Carpenter.) But such heats are not tolerable when the air is moist as well as hot, so as to prevent evaporation from the body. C. James states, that in the vapour baths of Nero he was almost suffocated in a temperature of 112° F. (44'5° C.), while in the caves of Testaccio, in which the air is dry, he was but little incommoded by a temperature of 176° F. (80° C.). In the former, evaporation from the skin was impossible ; in the latter it was abundant, and the layer of vapour which would rise from all the surface of the body would, by its very slowly conducting power, defend it for a time from the full action of the external heat. (The glandular apparatus, by which secretion of fluid from the skin is effected, will be considered in the Section on the Skin.) The ways by which the skin may be rendered more efficient as a cooling-apparatus by exposure, by baths, and by other means which man instinctively adopts for lowering his temperature when necessary, are too well known to need more than to be mentioned. Although under any ordinaiy circumstances, the external application of cold only temporarily depresses the temperature to a slight extent, it is otherwise in cases of high temperature in fever. In these cases a tepid bath may reduce the temperature several degrees, and the effect so produced lasts in some cases for many hours. (6.) Loss of Heat front the Lungs.-As a means for lowering the temperature, the lungs and air-passages are very inferior to the skin; although, by giving heat to the air we breathe, they stand next to the skin in importance. As a regulating power, the inferiority is still more marked. The air which is expelled from the lungs leaves the body at about the temperature of the blood, and is always saturated with moisture. No inverse proportion, 368 ANIMAL HEAT. [chap. ix. therefore, exists, as in the case of the skin, between the loss of heat by radiation and conduction on the one hand, and by evaporation on the other. The colder the air, for example, the greater will be the loss in all ways. Neither is the quantity of blood which is exposed to the cooling influence of the air diminished or increased, so far as is known, in accordance with any ueqd in relation to temperature. It is true that by varying the number and depth of the respirations, the quantity of heat given off by the lungs may be made, to some extent, to vary also. But the respiratory passages, W'hile they must be considered important means by which heat is lost, are altogether subordinate, in the power of regulating the temperature, to the skin. (c.) By Clothing.-The influence of external coverings for the body must not be unnoticed. In warm-blooded animals, they are always adapted, among other purposes, to the maintenance of uniform temperature ; and man adapts for himself such as are, for the same purpose, fitted to the various climates to which he is exposed. By their means, and by his command over food and fire, he maintains his temperature on all accessible parts of the surface of the earth. II. Methods of Variation in the Amount of Heat Pro- duced.-It may seem to have been assumed, in the foregoing pages, that the only regulating apparatus for temperature required by the human body is one that shall, more or less, produce a cooling effect; and as if the amount of heat produced were always, therefore, in excess of that which is required. Such an assumption would be incorrect. We have the power of regu- lating the production of heat, as well as its loss. (a.) By Regulating the Quantity and Quality of the Food taken. -In food we have a means for elevating our temperature. It is the fuel, indeed, on which animal heat ultimately depends -altogether. Thus, when more heat is wanted, we instinctively take more food, and take such kinds of it as are good for com- bustion ; while every day experience shows the different power of resisting cold possessed, respectively, by the well-fed and by the starved. Tn northern regions, again, and in the colder seasons of more southern climes, the quantity of food consumed is (speaking very generally) greater than that consumed by the same men or animals in opposite conditions of climate and season. And the food which appears naturally adapted to the inhabitants of the Coldest climates, such as the several fatty and oily substances, CHAP. IX.] VARIATIONS IN THE AMOUNT OF HEAT PRODUCED. 369 abounds in carbon and hydrogen, and is fitted to combine ulti- mately with the large quantities of oxygen which, breathing cold dense air, they absorb from their lungs. (6.) By Exercise.-In exercise, we have an important means of raising the temperature of our bodies. (c.) By Influence of the Nervous System.--The influence of the nervous system in modifying the production of heat must be very important, as upon nervous influence depends the amount of the metabolism of the tissues. The experiments and observations which best illustrate it are those showing, first, that when the supply of nervous influence to a part is cut off, the temperature of that part after a time falls below its ordinary degree ; and, secondly, that when death is caused by severe injury to, or removal of, the nervous centres, the temperature of the body rapidly falls, even though artificial respiration be performed, the circulation maintained, and to all appearance the ordinary chemical changes of the body be completely effected. It has been repeatedly noticed, that after division of the nerves of a limb its tempe- rature ultimately falls ; and this diminution of heat has been remarked still more plainly in limbs deprived of nervous influence by paralysis. With equal certainty, though less definitely, the influence of the nervous system on the production of heat, is shown in the rapid and momentary increase of temperature, sometimes general, at other times quite local, which is observed in states of nervous excitement; in the general increase of warmth of the body, sometimes amounting to perspiration, which is excited by passions of the mind ; in the sudden rush of heat to the face, which is not a mere sensation; and in the equally rapid diminution of temperature in the depressing passions. But none of these instances suffice to prove that heat is generated by mere nervous action, independent of any chemical change ; all are explicable, on the supposition that the nervous system alters, by its power of controlling the calibre of the blood-vessels (p. 168), the quan- tity of blood supplied to a part; while any influence which the nervous system may have in the production of heat, apart from this influence on the blood-vessels, is an indirect one, and is derived from its power of causing such nutritive change in the tissues as may, by involving the necessity of chemical action, involve the production of heat. The existence of nerve-centres and nerves which regulate animal heat (thermogenic) otherwise 370 ANIMAL HEAT. [chap. ix. than by their influence in trophic (nutritive) or vasomotor changes, although by many considered probable, is not yet proven. Inhibitory heat-centre.-Whether a centre exists which regulates the production of heat in warm-blooded animals, is still unde- cided. Experiments have shown that exposure to cold at once increases the oxygen taken in, and the carbonic acid given out, indicating an increase in the activity of the metabolism of the tissues, but that in animals poisoned by urari, exposure to cold diminishes both the metabolism and the temperature, and warm- blooded animals then re-act to variations of the external tempe- rature just in the same way as cold-blooded. These experiments seem to suggest that there is a centre, to which, under normal circumstances, the impression of cold is conveyed, and from which by efferent nerves impulses pass to the muscles, whereby an increased metabolism is induced, and so an increased amount of heat is generated. The centre is probably situated above the medulla. Thus in urarised animals, as the nerves to the muscles, the metabolism of which is so important in the production of heat, are paralyzed, efferent impulses from the centre cannot induce the necessary metabolism for the production of heat, even though afferent impidses from the skin, stimulated by the altera- tion of temperature, have conveyed to it the necessity of altering the amount of heat to be produced. The same effect is produced when the medulla is cut. Influence of Extreme Heat and. Cold.-In connection with the regulation of animal temperature, and its maintenance in health at the normal height, may be noted the result of circum- stances too powerful, either in raising or lowering the heat of the body, to be controlled by the proper regulating apparatus. Walther found that rabbits and dogs kept exposed to a hot sun, reached a temperature of 114-8° F., and then died. Cases of sunstroke furnish us with several examples in the case of man ; for it would seem that here death ensues chiefly or solely from elevation of the temperature. In many febrile diseases the immediate cause of death appears to be the elevation of the temperature to a point inconsistent with the continuance of life. The effect of mere loss of bodily temperature in man is less we] 1 known than the effect of heat. From experiments by Walther, it appears that rabbits can be cooled down to 48° F. (8'9° C.), before CHAP. X.] THE PROCESS OF SECRETION. 371 they die, if artificial respiration be kept up. Cooled down to 64° F. (17'8° C.), they cannot recover unless external warmth be applied together with the employment of artificial respiration. Rabbits not cooled below 77° F. (250 C.) recover by external warmth alone. CHAPTER X. SECRETION. Secretion is the process by which materials arc separated from the blood by the cells of secreting glands and membranes, and are either elaborated for the purpose of serving some ulterior office in the economy, or are discharged from the body as useless or injurious. In the former case, the separated materials are termed secretions ; in the latter, they are termed excretions. Most of the secretions consist of substances which, probably, do not pre-exist in the same form in the blood, but require special cells and a process of elaboration for their formation, e.g., the liver cells for the formation of bile, the mammary gland-cells for the formation of milk. The excretions, on the other hand, commonly or chiefly consist of substances which exist ready-formed in the blood, and are merely abstracted therefrom. If from any cause, such as extensive disease or extirpation of an excretory organ, the separation of an excretion is prevented, and an accumulation of it in the blood ensues, it frequently escapes through other organs, and may be detected in various fluids of the body. But this is never the case with secretions; at least with those that are most elaborated; for after the removal of the special organ by which each of them is elaborated, the secretion is no longer formed. Cases sometimes occur in which the secretion continues to be formed by the natural organ, but not being able to escape towards the exterior, on account of some obstruction, is re-absorbed into the blood, and afterwards discharged from it by exudation in other ways; but these are not instances of true vicarious secretion, and must not be thus regarded. These circumstances, and their final destination, are, however, 372 SECRETION. [chap. x. the only particulars in which secretions and excretions can be distinguished ; for, in general, the structure of the parts engaged in eliminating excretions is as complex as that of the parts con- cerned in the formation of secretions. And since the differences of the two processes of separation, corresponding with those in the several purposes and destinations of the fluids, are not yet ascer- tained, it will be sufficient to speak in general terms of the process of separation or secretion. Every secreting apparatus possesses, as essential parts of its structure, a simple and almost textureless membrane, named the primary or basement-membrane ; certain cells ; and blood-vessels. These three structural elements arc arranged together in various ways j but all the varieties may be classed under one or other of two principal divisions, namely, membranes and glands. Organs and Tissues oe Secretion. The principal secreting membranes are (1) the Serous and Synovial membranes; (2) the Mucous membranes; (3) the Mam- mary gland ; (4) the Lachrymal gland ; and (5) the Skin. The serous membranes are especially distinguished by the characters of the endothelium covering their free surface: it always consists of a single layer of polygonal cells. The ground substance of most serous membranes consists of connective-tissue corpuscles of various forms lying in the branching spaces which constitute the "jlymph canalicular system" (p. 343), and inter- woven with bundles of white fibrous tissue, and numerous delicate elastic fibrillae, together with blood-vessels, nerves, and lymphatics. In relation to the process of secretion, the layer of connective tissue serves as a ground-work for the ramification of blood-vessels, lymphatics, and nerves. But in its usual form it is absent in some instances, as in the arachnoid covering the dura mater, and in the interior of the ventricles of the brain. The primary membrane and epithelium are always present, and are concerned in the formation of the fluid by which the free surface of the membrane is moistened. Serous membranes are of two principal kinds : 1st Those which jiiie visceral cavities,--the arachnoid, pericardium, pleura*, perito- (i) Serous Membranes. CHAP. X.] SEROUS AND SYNOVIAL MEMBRANE. 373 neum, and tunicas vaginales. 2nd. The synovial membranes lining the joints, and the sheaths of tendons and ligaments, with which, also, are usually included the synovial bursa*, or burses mucosa', whether these be sub- cutaneous, or situated beneath tendons that glide over bones. The serous mem- branes form closed sacs, and exist wherever the free surfaces of viscera come into contact with each other or lie in cavi- ties unattached to sur- rounding parts. The viscera invested by a serous membrane are, as it were, pressed into the shut sac which it forms, carrying before them a portion of the mem- brane, which serves as their investment. To the law that serous mem- branes form shut sacs, there is, in the human subject, one exception, viz. : the opening of the Fallopian tubes into the abdominal cavity,-an arrangement which exists in man and all Vertebrata, with the exception of a few fishes. Functions.-The principal purpose of the serous and synovial membranes is to furnish a smooth, moist surface, to facilitate the movements of the invested organ, and to prevent the injurious effects of friction. This purpose is especially manifested in joints, in which free and extensive movements take place ; and in the stomach and intestines, which, from the varying quantity and movements of their contents, are in almost constant motion upon one another and the walls of the abdomen. Fluid.-The fluid secreted from the free surface of the serous membranes is, in health, rarely more than sufficient to ensure the maintenance of their moisture. The opposed surfaces of each serous sac are at every point in contact with each other. After Fig'. 226.-Section of synovial membrane. a, endothelial covering of the elevations of the membrane ; />, subserous tissue containing fat and blood-vessels ; c, ligament covered by the synovial membrane. (Cadiat.) 374 SECRETION. [chap. x. death, a larger quantity of fluid is usually found in each serous sac ; but this, if not the product of manifest disease, is probably such as has transuded after death, or in the last hours of life. An excess of such fluid in any of the serous sacs constitutes dropsy of the sac. The fluid naturally secreted by the serous membranes appears to be identical, in general and chemical characters, with very dilute liquor sanguinis. It is of a pale yellow or straw-colour, slightly viscid, alkaline, and on account of the presence of albu- men, coagulable by heat. This similarity of the serous fluid to the liquid part of blood, and to the fluid with which most animal tissues are moistened, renders it probable that it is, in great measure, separated by simple transudation, through the walls of the blood-vessels. The probability is increased by the fact that, in jaundice, the fluid in the serous sacs is, equally with the serum of the blood, coloured with the bile. But there is reason for supposing that the fluid of the cerebral ventricles and of the arachnoid sac are exceptions to this rule ; for they differ from the fluids of the other serous sacs not only in being pellucid, colour- less, and of much less specific gravity, but in that they seldom receive the tinge of bile when present in the blood, and are not coloured by madder, or other similar substances introduced abundantly into the blood. It is also probable that the formation of synovial fluid is a process of more genuine and elaborate secretion, by means of the epithelial cells on the surface of the membrane, and especially of those which are accumulated on the edges and processes of the synovial fringes; for, in its peculiar density, viscidity, and abun- dance of albumen, synovia differs alike from the serum of blood and from the fluid of any of the serous cavities. (2) Mucous Membranes. The mucous membranes line all those passages by which internal parts communicate with the exterior, and by which either matters are eliminated from the body or foreign substances taken into it. They are soft and velvety, and extremely vascular. The external surfaces of mucous membranes are attached to various other tissues; in the tongue, for example, to muscle; on cartilaginous parts, to perichondrium; in the cells of the ethmoid bone, in the frontal and sphenoidal sinuses, as well as in the tympanum, to CHAP. X.] MUCOUS MEMBRANES. 375 periosteum ; in the intestinal canal, it is connected with a firm submucous membrane, which on its exterior gives attachment to the fibres of the muscular coat. The mucous membranes line certain principal tracts-Gastro-Pulmonary and Genito-Urinary; the former being subdivided into the Digestive and Respiratory tracts. 1. The Digestive, tract commences in the cavity of the mouth, from which prolongations pass into the ducts of the salivary glands. From the mouth it passes through the fauces, pharynx, and oesophagus, to the stomach, and is thence continued along the whole tract of the intestinal canal to the termination of the rectum, being in its course arranged in the various folds and depressions already described, and prolonged into the ducts of the intestinal glands, the pancreas and liver, and into the gall-bladder. 2. The Respiratory tract includes the mucous membrane lining the cavity of the nose, and the various sinuses communicating with it, the lachrymal canal and sac, the conjunctiva of the eye and eyelids, and the prolongation which passes along the Eusta- chian tubes and lines the tympanum and the inner surface of the membrana tympani. Crossing the pharynx, and lining that part of it which is above the soft palate, the respiratory tract leads into the glottis, whence it is continued, through the larynx and trachea, to the bronchi and their divisions, which it lines as far as the branches of about of an inch in diameter, and continuous with it is a layer of delicate epithelial membrane which extends into the pulmonary cells. 3. The Genito-urinary tract, which lines the whole of the urinary passages, from their external orifice to the termination of the tubuli uriniferi of the kidneys, extends also into the organs of generation in both sexes, and into the ducts of the glands con- nected with them; and in the female becomes continuous with the serous membrane of the abdomen at the fimbriae of the Fallopian tubes. Structure.-These mucous tracts, and different portions of each of them, present certain structural peculiarities of the mucous membrane, adapted to the functions which each part has to discharge; yet in some essential characters the mucous mem- brane is the same, from whatever part it is obtained. In all the principal and larger parts of the several tracts, it presents, as just remarked, an external layer of epithelium, situated upon a basement membrane, and beneath this, a stratum of vascular tissue of vari- 376 SECRETION. [chap. X. able thickness, containing lymphatic vessels and nerves. The vascular stratum or corium, together with the basement membrane and epithelium, in different cases, is elevated into minute papilla? and villi, or depressed into involutions in the form of glands. But in the prolongations of the tracts, where they pass into gland- ducts, these constituents are reduced in the finest branches of the ducts to the epithelium, the primary or basement-membrane, and the capillary blood-vessels spread over the outer surface of the latter in a single layer. The primary or basement membrane is a thin transparent layer, simple, homogeneous, or composed of endothelial cells. In the minuter divisions of the mucous membranes, and in the ducts of glands, it is the layer continuous and correspondent with this basement-membrane that forms the proper walls of the tubes. The cells also which, lining the larger and coarser mucous mem- branes, constitute their epithelium, are continuous with, and often similar to those which, lining the gland-ducts, are called gland- cells. No certain distinction can be drawn between the epithelium- cells of mucous membranes and gland-cells. Mucous Fluid : Mucus.-From all mucous membranes there is secreted either from the surface or from certain special glands, or from both,-a more or less viscid, greyish, or semi-transparent fluid, of alkaline reaction and high specific gravity, named mucus. It mixes imperfectly with water, but, rapidly absorbing liquid, it swells considerably when water is added. Under the micro- scope it is found to contain epithelium and leucocytes. It is found to be made up, chemically, of a nitrogenous principle called mucin, which forms its chief bulk, of a little albumen, of salts chiefly chlorides and phosphates, and water with traces of fats and extractives. Secreting Glands. The structure of the elementary portions of a secreting apparatus, namely epithelium, simple membrane, and blood-vessels having been already described in this and previous chapters, we may proceed to consider the manner in which they are arranged to form the varieties of secreting glands. The secreting glands are the organs to which the function of secretion is more especially ascribed ; for they appear to be occupied with it alone. They present, amid manifold diversities CHAP. X.] VARIETIES OF SECRETING GLANDS. 377 of form and composition, a general plan of structure, by which they are distinguished from all other textures of the body; espe- cially, all contain, and appear constructed with particular regard to, the arrangement of the cells, which, as already expressed, both line their tubes or cavities as an epithelium, and elaborate, as secreting cells, the substances to be discharged from them. (Hands are provided also with lymphatic vessels and nerves. The distri- bution of the former is not peculiar, and need not be here con- sidered. Nerve-fibres are distributed both to the blood-vessels of the gland and to its ducts; and to the secreting cells also in some glands. Varieties.- i. The simple tubule, or tubular gland (a, tig. 227), examples of which are furnished by some mucous glands, the follicles of Lieberkuhn, and the tubular glands of the stomach. These appear to be simple tubular depressions of the mucous membrane, the wall of which is formed of primary membrane, is lined with secreting cells arranged as an epithelium. To the same class may be referred the elongated and tortuous sudoriferous glands. 2. The compound tubular glands (d, fig. 227) form another division. These consist of main gland-tubes, which divide and sub-divide. Each gland may consist of the subdivisions of one or more main tubes. The ultimate sub-divisions of the tubes are generally highly convoluted. They are formed of a basement- membrane, lined by epithelium of various forms. The larger tubes may have an outside coating of fibrous, areolar, or muscular tissue. The Kidney, Testis, Salivary glands, Pancreas, Brunner's glands with the Lachrymal and Mammary glands, and some Mucous glands are examples of this type, but present more or less marked variations among themselves. 3. The aggregate or racemose glands, in which a number of vesicles or acini are arranged in groups or lobules (c, fig. 227). The Meibomian follicles are examples of this kind of gland. These various organs differ from each other only in secondary points of structure; such as, chiefly, the arrangement of their excretory ducts, the grouping of the acini and lobules, their con- nection by areolar tissue, and supply of blood-vessels. The acini commonly appear to be formed by a kind of fusion of the walls of several vesicles, which thus combine to form one cavity lined or tilled with secreting cells which also occupy recesses from the main cavity. The smallest branches of the gland-ducts sometimes open 378 SECRETION. [chap. x. into the centres of these cavities ; sometimes the acini are clustered round the extremities, or by the sides of the ducts : but, whatever secondary arrangement there may be, all have the same essential Fig. 227.-Plans of extension of secreting membrane by inversion or recession in form of cavities. A, simple glands, viz. g, straight tube ; h, sac ; i, coiled tube. B, multilocular crypts ; k, of tubular form ; Z, saccular. C, racemose, or saccular compound gland ; in, entire gland, showing branched duct and lobular structure; n, a lobule, detached with o, branch of duct proceeding from it. D, compound tubular gland (Sharpey). character of rounded groups of vesicles containing gland-cells, and opening by a common central cavity into minute ducts, which ducts in the large glands converge and unite to form larger and larger branches, and at length by one common trunk, open on a free surface of membrane. CHAP. X.] PROCESS OF SECRETION. 379 Among these varieties of structure, all the secreting glands are alike in some essential points, besides those which they have in common with all truly secreting structures. They agree in presenting a large extent of secreting surface within a compara- tively small space; in the circumstance that while one end of the gland-duct opens on a free surface, the opposite end is always closed, having no direct communication with blood-vessels, or any other canal; and in a uniform arrangement of capillary blood- vessels, ramifying and forming a network around the walls and in the interstices of the ducts and acini. Process of Secretion.-In secretion two distinct processes are concerned which may be spoken of as I. Physical, and IL Chemical. i. Physical processes.-These, already discussed in the last chapter, are such as can be closely imitated in the laboratory, inasmuch as they consist in the operation of well-known physical laws : they are-(a) Filtration ; (6) Dialysis. (a) Filtration is, as we have already mentioned, simply the passage of a fluid through a porous membrane under the influence of pressure. If two fluids be separated by a porous membrane, and the pressure on one side is greater than on the other, it is evident that in the absence of counteracting osmotic influences (see below), there will be a filtration through the membrane until the pressure on the two sides is equalized. Of course there may be fluid only on one side of the membrane, as in the ordinary process of filtering through blotting-paper, and then the filtration will continue as long as the pressure (in this case, the weight of the fluid) is sufficient to force it through the pores of the filter. The necessary inequality of pressure may be obtained either by diminishing it on one side, as in the case of cupping ; or increasing it on the other, as in the case of the increased blood-pressure, and consequent increased flow of urine resulting from copious drinking. By filtration, not merely water, but various salts in solution, and even colloids of all kinds, may transude from the blood-vessels. The amount of a liquid which will pass through a filter in a given time depends not only upon the amount of pressure to which it is subjected, but also upon the nature of the fluid filtered, and upon the kind of membrane employed as the filter. It seems probable that some fluids, such as the secretions of serous membranes, are simply exudations or oozings (filtration) from the blood-vessels, whose qualities are determined by those of the 380 SECRETION. [chap. X. liquor sanguinis, while the quantities are liable to variation, and are chiefly dependent upon the blood-pressure. (6) Dialysis is the passage of fluids through a moist animal membrane independent of pressure, and sometimes actually in opposition to it. There must always be in this process two fluids differing in composition, one or both possessing an affinity for the intervening membrane, and the fluids must be capable of mixing one with the other; the osmotic current continuing in each direction (when both fluids have an affinity for the membrane) until the chemical composition of the fluid on each side of the septum becomes the same. 2. Chemical processes.-The chemical processes constitute the process of secretion, properly so called, as distinguished from mere transudation spoken of above. In the chemical process of secre- tion various materials which do not exist as such in the blood are elaborated by the agency of the gland-cells from the blood, or to speak more accurately, from the plasma which exudes from the blood-vessels into the interstices of the gland-textures. The best evidence in favour of this view is: 1st. That cells and nuclei are constituents of all glands, however diverse their outer forms and other characters, and that they are in all glands placed on the surface or in the cavity whence the secretion is poured. 2»<7. That many secretions which are visible with the microscope may be seen in the gland-cells before they arc discharged. Thus, bile may be often discerned by its yellow tinge in the cells of the liver; spermatozoids in the cells of the tubules of the testi- cles ; granules of uric acid in those of the kidneys (of fish) ; fatty particles, like those of milk, in the cells of the mammary gland. Secreting cells, jike the cells or other elements of any other organ, appear to develop, grow, and attain their individual per- fection by appropriating nutriment from the fluid exuded by adjacent blood-vessels and elaborating it, so that it shall form part of their own substance. In this perfected state, the cells subsist for some brief time, and when that period is over they appear to dissolve, wholly or in part, and yield their contents to the peculiar material of the secretion. And this appears to be the case in every part of the gland that contains the appropriate gland-cells ; there- fore not in the extremities of the ducts or in the acini alone, but in great part of their length. We have described elsewhere the changes which have been CHAI'. x.J CIRCUMSTANCES INFLUENCING SECRETION. 381 noticed from actual experiment in the cells of the salivary glands, pancreas, and peptic gland. Discharge of Secretions from glands may either take place as soon as they are. formed; or the secretion may be long retained within the gland or its ducts. The former is the case with the sweat glands. But the secretions of those glands whose activity of function is only occasional are usually retained in the cells in an undeveloped form during the periods of the gland's inaction. And there are glands which are like both these classes, such as the lachrymal, which constantly secrete small portions of fluid, and on occasions of greater excitement discharge it more abundantly. When discharged into the ducts, the further course of secre- tions is affected (i) partly by the pressure from behind ; the fresh quantities of secretion propelling those that were formed before. In the larger ducts, its propulsion is (2) assisted by the contraction of their walls. All the larger ducts, such as the ureter and common bile-duct, possess in their coats plain muscular fibres ; they contract when irritated, and sometimes manifest peristaltic movements. Rhythmic contractions in the pancreatic and bile- ducts have been observed, and also in the ureters and vasa deferentia. It is probable that the contractile power extends along the ducts to a considerable distance within the substance of the glands whose secretions can be rapidly expelled. Saliva and milk, for instance, are sometimes ejected with much force. Circumstances Influencing Secretion.-The principal con- ditions which influence secretion are (1) variations in the quantity of blood, (2) variations in the quantity of the peculiar materials for any secretion that the blood may contain, and (3) variations in the condition of the nerves of the glands. (1.) An increase in the quantity of blood traversing a gland, as in nearly all the instances before quoted, coincides generally with an augmentation of its secretion. Thus, the mucous membrane of the stomach becomes florid when, on the introduction of food, its glands begin to secrete; the mammary gland becomes much more vascular during lactation; and all circumstances which give rise to an increase in the quantity of material secreted by an organ produce, coincidently, an increased supply of blood ; but we have seen that a discharge of saliva may occur under extra- ordinary circumstances, without increase of blood-supply, and so it 382 SECRETION. [chap. x. may be inferred that this condition of increased blood-supply is not absolutely essential. (2.) An increase in the amount of the materials which the glands are designed to separate or elaborate, contained in the blood supplied to them, increases the amount of any secretion. Thus, when an excess of nitrogenous waste is in the blood, whether from excessive exercise, or from destruction of one kidney, a healthy kidney will excreate more urea than it did before. (3.) Influence of the Nervous System on Secretion.-The process of secretion is largely influenced by the condition of the nervous system. The exact mode in which the influence is exhibited must still be regarded as somewhat obscure. In part, it exerts its influence by increasing or diminishing the quantity of blood supplied to the secreting gland, in virtue of the power which it exercises over the contractility of the smaller blood vessels ; while it also has a more direct influence, as was described at length in the case of the submaxillary gland, upon the secreting cells them- selves ; this may be called trophic influence. Its influence over secretion, as well as over other functions of the body, may be excited by causes acting directly upon the nervous centres, upon the nerves going to the secreting organ, or upon the nerves of other parts. In the latter case, a reflex action is produced : thus the impression produced upon the nervous centres by the contact of food in the mouth, is reflected upon the nerves supplying the salivary glands, and produces, through these, a more abundant secretion of the saliva. Through the nerves, various conditions of the brain also influ- ence the secretions. Thus, the thought of food may be sufficient to excite an abundant flow of saliva. And, probably, it is the mental state which excites the abundant secretion of urine in hysterical paroxysms, as well as the perspirations, and, occa- sionally, diarrhoea, which ensue under the influence of terror, and the tears excited by sorrow or excess of joy. The quality of a secretion may also be affected by mental conditions, as in the cases in which, through grief or passion, the secretion of milk is altered, and is sometimes so changed as to produce irritation in the alimentary canal of the child, or even death (Carpenter). Relations between the Secretions.-The secretions of some of the glands seem to bear a certain relation or antagonism to each other, by which an increased activity of one is usually followed by diminished activity of one or more of the others ; chap, x.] STRUCTURE OF THE MAMMARY GLANDS. 383 and a deranged condition of one is apt to entail a disordered state in the others. Such relations appear to exist among the various mucous membranes; and the close relation between the secretion of the kidney and that of the skin is a subject of constant observation. The Mammary Glands. Structure.-The mammary glands are composed of large divi- sions or tabes, and these are again divisible into lobules,-the Fig. 228.-Dissection of the lower half of the female mamma during the period of lactation, j.-In the left-hand side of the dissected part the glandular lobes are exposed and partially unravelled ; and on the right-hand side, the glandular substance has been removed to show the reticular loculi of the connective-tissue in which the glandular lobules are placed: i, upper part of the mamilla or nipple ; 2, areola; 3, subcutaneous masses of fat; 4, reticular loculi of the connective-tissue which support the glandular substance and contain the fatty masses ; 5, one of three lactiferous ducts shown pass- ing towards the mamilla where they open; 6, one of the sinus lactei or reservoirs; 7, some of the glandular lobules which have been unravelled ; 7', others massed together (Luschka). lobules being composed of the convoluted subdivision of the main ducts (alveoli). The lobes and lobules are bound together by areolar tissue; penetrating between the lobes, and covering the general surface of the gland, with the exception of the nipple, is a considerable quantity of yellow fat, itself tabulated by sheaths and processes of tough areolar tissue (fig. 228) connected both with 384 SECRETLOX. [chap. x. the skin in front and the gland behind; the same bond of con- nection extending also from the under surface of the gland to the sheathing connective tissue of the great pectoral muscle on which it lies. The main ducts of the gland, fifteen to twenty in number, called the lactiferous or galactophorous ducts, are formed by the union of the smaller (lobular) ducts, and open by small separate orifices through the nipple. At the points of junction of lobular ducts to form lactiferous ducts, and just before these enter the base of the nipple, the ducts are dilated (fig. 228) ; and, during lactation, the period of active secretion by the gland, the dilata- tions form reservoirs for the milk, which collects in and distends them. The walls of the gland-ducts are formed of areolar with some unstriped muscular tissue, and are lined internally by short columnar and near the nipple by squamous epithelium. The alveoli consist of a membrana propria of flattened endothelial cells lined by low columnar epithelium, and are filled with fat globules. The nipple, which contains the terminations of the lactiferous ducts, is composed also of areolar tissue, and contains unstriped muscular fibres. Blood-vessels are also freely supplied to it, so as to give it a species of erectile structure. On its surface are very sensitive papillae ; and around it is a small area or areola of pink or dark-tinted skin, on which are to be seen small projections formed by minute secreting glands. Blood-vessels, nerves, and lymphatics are plentifully supplied to the mammary glands; the calibre of the blood-vessels, as well as the size of the glands, varying very greatly under certain condi- tions, especially those of pregnancy and lactation. Changes in the Glands at certain Periods.-The minute changes which occur in the mammary gland during its periods of evolution (pregnancy), and involution (when lactation has ceased), are the following :- The most favourable period for observing the epithelium of the mammary gland fully developed is shortly before the end of pregnancy. At this period the acini which form the lobules of the gland, are found to be lined with a mosaic of polyhedral epithelial cells (fig. 229), and supported by a connective tissue stroma. The rapid formation of milk during lactation results from a fatty metamorphosis of the epithelial cells. In the earlier days of lactation, epithelial cells partially trans- chap, x.] THE SECRETION OF THE MAMMARY GLAND. 385 formed are discharged in the secretion : these are termed " colos- trum corpuscles," but later oh the cells are completely transformed into fat before the secretion is discharged. After the end of lactation, the mamma gradually returns to its original size (involution). The acini, in the early stages of invo- lution, are lined with cells in all degrees of vacuolation. As involution proceeds the acini diminish considerably in size, and at length, instead of a mosaic of lining epithelial cells (twenty to thirty in each acinus), we have five or six nuclei (some with no sur- rounding protoplasm) lying in an irregular heap within the acinus. During the later stages of involu- tion, large yellow granular cells are to be seen. As the acini diminish in size, the connective tissue and fatty matter between them increase, and in some animals, when the gland is completely inactive, it is found to consist of a thin film of glandular tissue overlying a thick cushion of fat. Many of the products of waste are carried off by the lymphatics. During pregnancy the mammary glands and mammae undergo changes (evolution) which are readily observable. They enlarge, become harder and more distinctly lobulated: the veins on the surface become more prominent. The areola becomes enlarged and dusky, with projecting papillae; the nipple too becomes more prominent, and milk can be squeezed from the orifices of the ducts. This is a very gradual process, which commences about the time of conception, and progresses steadily during the whole period of gesta- tion. The acini enlarge, and a series of changes occur, exactly the reverse of those just described under the head of Involution. Fig. 229.-Section of mammary ghnni of bitch, showing acini, lined with epithelial cells of a polyhedral or short columnar form. X 200. (V. D. Harris.) The Mammary Secretion : Milk. The secretion of the mammary glands, or milk, is a bluish-white opaque fluid with a pleasant sweet taste. It is a true emulsion. Under the microscope, it is found to contain a number of globules of various sizes (fig. 230), the majority about -fo o 0 0 of an inch in 386 SECRETION. [chap. x. diameter. They are composed of oily matter, probably coated by a fine layer of albuminous material, and arc called milk-globules ; while, accompanying these, are numerous minute particles, both oily and albuminous, which exhibit ordinary molecular movements. The milk which is secreted in the first few days after parturition, and which is called the colostrum, differs from ordinary milk in containing a larger quantity of solid matter; and under the microscope are to be seen certain granular masses called colostrum- corpuscles. These, which appear to be small masses of albuminous and oily matter, are probably secreting cells of the gland, either in a state of fatty degeneration, or old cells which in their at- tempt at secretion under the new circumstances of active need of milk, are filled with oily matter ; which, however, being unable to discharge, they are themselves shed bodily to make room for their successors. Colostrum-corpuscles have been seen to exhibit contractile movements and to squeeze out drops of oil from their interior. Chemical Composition.-In addition to the oil existing in numberless little globules, coated with a thin 1 ayer of albuminous matter, floating in a large quantity of water, milk contains casein, serum-albumin, milk-sugar (lactose), and several salts. Its per- centage composition has been already mentioned, but may be here repeated. Its reaction is alkaline: its specific gravity about 1030. Fig. 230.-Globules and molecules of Cow's milk, x 400. Table of the Chemical Composition of Milk. Water Solids . Human. . 890 . . no . Cow. • • 858 . . 142 IOOO IOOO CHAP. X.] COMPOSITION OF MILK. 387 Human. Cow. Proteids, including Casein . and Serum-Albumin 35 . 68 Fats or Butter .. . . 25 • • ■ • 38 Sugar (with extractives) 48 . . 30 Salts (chiefly potassium, sodium, and calcium, chlo- rides, and phosphates) 2 . . . 6 I IO 142 When milk is allowed to stand, the fat globules, being the lightest portion, rise to the top, forming cream. If a little acetic acid be added to a drop of milk under the microscope, the albu- minous film coating the oil drops is dissolved, and they run together into larger drops. The same result is produced by the process of churning, the effect of which is to break up the albuminous coating of the oil drops: they then coalesce to form butter. Curdling of Milk.-The curdling of milk is due to the coagulation of the casein which is kept in solution under normal conditions by the alkaline calcium phosphate. On the addition of an acid, such as acetic, the casein is precipitated. This occurs, too, if it be allowed to stand for some time, its reaction becomes acid : in popular language it " turns sour." The change appears to be due to the conversion of the milk-sugar into lactic acid, by means of a special micro-organism, Bacterium lactis; this causes the precipitation (curdling) of the casein: the curd contains the fat globules : the remaining fluid (whey) consists of water holding in solution albumen, milk-sugar and certain salts. The same effect is produced in the manufacture of cheese, which is really casein coagulated by the agency of rennet (p. 290). When milk is boiled, the scum which forms consists chiefly of serum-albumin. Curdling Ferments.-The effect of the ferments of the gastric, pancreatic, and intestinal juices in curdling milk (curdling fer- ments) has already been mentioned in the Chapter on Digestion. The salts of milk are chlorides, sulphates, phosphates, and carbonates of potassium, sodium, and calcium. Traces of iron, fluorine, and silica are also found, and the gases, carbonic acid, oxygen and nitrogen. 388 STRUCTURE AND FUNCTIONS OF THE SKIN. [CHAP. XI. CHAPTER XI. THE STRUCTURE AND FUNCTIONS OF THE SKIN. The skin serves-(i), as an external integument for the pro- tection of the deeper tissues, and (2), as a sensitive organ in the exercise of touch; it is also (3), an important secretory and excre- tory, and (4), an ab- sorbing organ; while it plays an important part in (5) the regula- tion of the tempera- ture of the body. Structure. - The skin consists, princi- pally, of a vascular tissue named the co- rium, derma, or cutis vera, and an exter- nal covering of epi- thelium termed the cuticle or epidermis. Within and beneath the corium are im- bedded several organs with special function, namely sudoriferous glands, sebaceous glands, and hair fol- licles ; and on its sur- face are sensitive pa- pillae. The so-called appendages of the skin-the hair and nails- are modifications of the epidermis. A. Epidermis.-The epidermis is composed of several strata of cells of various shapes and sizes; it closely resembles in its structure the epithelium of the mucous membrane that lines the Fig. 231.-Vertical section of the epidermis of the prepuce, a, stratum comeum, of very few layers, the stratum lucidum and stratum granulosum not being distinctly represented; b, c, d, and e, the layers of the stratum Malpighii, a certain number of the cells in layers d and e showing signs of segmentation; layer c consists chiefly of prickle or ridge and furrow cells ; f, basement mem- brane ; g, cells in cutis vera. (Cadiat.) CHAP. XI.] THE EPIDERMIS. 389 month. The following four layers may be distinguished in a more or less developed form. i. Stratum corneum (fig. 231, a), consist- ing of superposed layers of homy scales. The different thickness of the epidermis in different regions of the body is chiefly due to variations in the thickness of this layer; e.g., on the horny parts of the palms of the hands and soles of the feet it is of great thick- ness. The stratum corneum of the buccal epithelium chiefly differs from that of the epidermis in the fact that nuclei are to be distin- guished in some of the cells even of its most superficial layers. 2. Stratum lucidum, a bright homogeneous membrane consist- ing of squamous cells closely arranged, in some of which a nucleus can be seen. 3. Stratum granulosum, consist- ing of one layer of flattened cells which appear fusiform in verti- cal section: they are distinctly nucleated, and a number of gran- ules extend from the nucleus to the margins of the cell. 4. Stratum Malpigliii or Rete mucosum consists of many strata. The deepest cells, placed imme- diately above the cutis vera, are columnar with oval nuclei: this layer of columnar cells is suc- ceeded by a number of layers of more or less polyhedral cells with spherical nuclei; the cells of the more superficial layers are con- siderably flattened. The deeper surface of the rete mucosum is accurately adapted to the papillae of the true skin, being, as it were, moulded on them. It is very constant in thickness in all parts of the skin. The cells of the middle layers of the stratum Malpighii are almost all connected by processes, and thus form 4 ' prickle cells " (fig. 27). The pigment of the skin, the varying quantity of which causes the various tints observed in different individuals and different races, is contained in the deeper cells of rete mucosum; the pigmented cells as they approach the free surface gradually losing their colour. Epidermis maintains its Fig. 232.- Vertical section of skin of the negro, a, a, Cutaneous papilla?, b. Undermost and dark-coloured layer of oblong vertical epidermis-cells, c. Stratum Malpighii. d. Superfi- cial layers, including stratum cor- neum, stratum lucidum, and stratum granulosum, the last two not differen- tiated in fig. X 250. (Sharpey.) 390 STRUCTURE AND FUNCTIONS OF THE SKIN. [chap. xr. thickness in spite of the constant wear and tear to which it is subjected. The columnar cells of the deepest layer of the " rete mucosum" elongate, and their nuclei divide into two (fig. 231, e). Lastly the upper part of the cell divides from the lower; thus from a long columnar cell are produced a polyhedral and a short columnar cell : the latter elongates and the process is repeated. The polyhedral cells thus formed are pushed up towards the free surface by the production of fresh ones beneath them, and become flattened from pressure : they also become gradually horny by evaporation and transformation of their protoplasm into keratin, till at last by rubbing they are detached as dry horny scales at the free surface. There is thus a constant production of fresh cells in the deeper layers, and a constant throwing off of old ones from the free surface. When these two processes are accurately balanced, the epidermis maintains its thickness. When, by inter- mittent pressure a more active cell-growth is stimulated, the production of cells exceeds their waste and the epidermis increases in thickness, as we see in the horny hands of the labourer. The thickness of the epidermis on different portions of the skin is directly proportioned to the friction, pressure, and other sources of injury to which it is exposed ; for it serves as well to protect the sensitive and vascular cutis from injury from without, as to limit the evaporation of fluid from the blood-vessels. The adaptation of the epidermis to the latter purposes may be well shown by exposing to the air two dead hands or feet, of which one has its epidermis perfect, and the other is deprived of it • in a day, the skin of the latter will become brown, dry, and horn- like, while that of the former will almost retain its natural moisture. B. Cutis vera.-The corium or cutis vera, which rests upon a layer of adipose and cellular tissue of varying thickness, is a dense and tough, but yielding and highly elastic structure, composed of fasciculi of areolar tissue, interwoven in all directions, and forming, by their interlacements, numerous spaces or areolae. These areola? are large in the deeper layers of the cutis, and are there usually filled with little masses of fat (fig. 234): but, in the superficial parts, they are small or entirely obliterated. Plain muscular fibres are also abundantly present. Papillae.-The cutis vera presents numerous conical elevations, or papillae, with a single or divided free extremity, which are more prominent and more densely set at some parts than at others (fig. CHAP. XI.] THE GLANDS OF THE CUTIS VERA. 391 233). This is especially the case on the palmar surface of the hands and fingers, and on the soles of the feet-parts, there- fore, in which the sense of touch is most acute. On these parts they are disposed in double rows, in parallel curved lines, separated from each other by depressions. Thus they may be easily seen on Fig. 233.-Compound papilla from the palm of the hand, a, basis of a papilla ; b, b, divi- sions or branches of the same; c, c, branches belonging to papillae, of which the bases are hidden from view, x 60. (Kolliker.) the palm, whereon each raised line is composed of a double row of papillae, and is intersected by short transverse lines or furrows corresponding with the interspaces between the successive pairs of papillae. Over other parts of the skin they are more or less thinly scattered, and are scarcely elevated above the surface. Their average length is about of an inch, and at their base they measure about yyy of an inch in diameter. Each papilla is abundantly supplied with blood, receiving from the vascular plexus in the cutis one or more minute arterial twigs, which divide into capillary loops in its substance, and then reunite into a minute vein, which passes out at its base. This abundant supply of blood explains the turgescence or kind of erection which they undergo when the circulation through the skin is active. The majority, but not all, of the papillae contain also one or more terminal nerve - fibres, from the ultimate ramifications of the cutaneous plexus, on which their exquisite sensibility depends. The nerve-terminations in the skin are described under the Sensory Nerve Terminations. Glands of the Skin.-The skin possesses glands of two kinds ; (a) Sudoriferous, or Sweat Glands; (b) Sebaceous Glands. (a) Sudoriferous, or Sweat Glands.-Each of these glands con- sists of a small lobular mass, formed of a coil of tubular gland- duct, surrounded by blood-vessels and embedded in the sub- cutaneous adipose tissue (fig. 234, C). From this mass, the duct ascends, for a short distance in a spiral manner through the 392 STRUCTURE AND FUNCTIONS OF THE SKIN. [chap. XI. deeper part of the cutis, then passing straight, and then sometimes again becoming spiral, it passes through the cuticle and opens by B C c I) E an oblique valve-like aperture. In the parts where the epi- dermis is thin, the ducts them- selves are thinner and more nearly straight in their course (fig. 234). The duct, which maintains nearly the same diameter through- out, is lined with a layer of columnar epithelium (fig. 235) continuous with the epidermis; while the part which passes through the epidermis is com- posed of the latter structure only; the cells which immediately form the boundary of the canal in this part being somewhat differently arranged from those of the adjacent cuticle. The coils or terminal portions of the gland are lined with at least two layers of short columnar cells with very distinct nuclei (fig. 235), and possess a large lumen distinctly bounded by a special lining or cuticle. Fig. 234.-Vertical section of skin. A. Sebaceous gland opening into hair follicle. B. Muscu- lar fibres. C. Sudoriferous or fot^E^Fundusofhair-fomci™ Noble SnirthP)1118e' <Klemand chap, xi.] SEBACEOUS GLANDS. 393 The sudoriferous glands are abundantly distributed over the whole surface of the body; but are especially numerous, as well as very large, in the skin of the palm of the hand, and of the sole of the foot. The glands by which the peculiar odorous matter of the axillee is secreted form a nearly complete layer under the cutis, and are like the ordinary sudoriferous glands, except in being larger and having very short ducts. The peculiar bitter yellow substance secreted by the skin of the external auditory passage is named cerumen, and the glands themselves ceruminous glands; but they do not much differ in structure from the ordinary sudori- ferous glands. (6) Sebaceous Glands.- The sebaceous glands (fig. 236), like the sudoriferous glands, are abundantly dis- tributed over most parts of the body. They are most numerous in parts largely supplied with hair, as the scalp and face, and are thickly distributed about the entrances of the various passages into the body, as the anus, nose, lips, and external ear. They are entirely absent from the palmar surface of the hand and the plantar surfaces of the feet. They are minutely lobulated glands com- posed of an aggregate of small tubes or sacculi filled with opaque white substances, like soft ointment. Minute capillary vessels over- spread them; and their ducts open either on the surface of the skin, close to a hair, or, which is more usual, directly into the follicle of the hair. In the latter case, there are generally two or more glands to each hair (fig. 234). Hair.-A hair is produced by a peculiar growth and modifica- tion of the epidermis. Externally it is covered by a layer of fine scales closely imbricated, or overlapping like the tiles of a house, but with the free edges turned upwards (fig. 237, a). It is called the cuticle of the hair. Beneath this is a much thicker layer of elongated horny cells, closely packed together so as to resemble a fibrous structure. This, very commonly, in the human Fig. 235.-Terminal hibules of sudoriferous glands, cut in various directions from the skin of the pig's ear. (V. D. Harris.) 394 STRUCTURE AND FUNCTIONS OF THE SKIN. [chap. XI. subject, occupies the whole of the inside of the hair; but in some cases there is left a small central space filled by a substance called the medulla or pith, composed of small collections of irregularly shaped cells, containing some- times pigment granules or fat, but mostly air. The follicle, in which the root of each hair is contained (fig. 238), forms a tubular depres- sion from the surface of the skin, - descending into the subcutaneous fat, generally to a greater depth than the sudo- riferous glands, and at its deepest part enlarging in a bulbous form, and often curv- ing from its previous recti- linear course. It is lined throughout by cells of epithe- lium, continuous with those of the epidermis, and its walls are formed of pellucid mem- brane, which commonly, in the follicles of the largest hairs, has the structure of vascular fibrous tissue. At the bottom of the follicle is a small papilla, or projection of true skin, and it is by the production and outgrowth of epidermal cells from the surface of this papilla Fig. 236.-Sebaceous gland from human skin. (Klein and Noble Smith.) Fig. 237.-Surface of a u-hite hair, magnified 160 diameters. The wave lines mark the upper or free edges of the cortical scales. 11, separated scales, magnified 350 diameters. (Kblliker.) that the hair is formed. The inner wall of the follicle is lined by epidermal cells continuous with those covering the general CHAP. XI.] HAIRY AND HAIR FOLLICLES. 395 surface of the skin; as if indeed the follicle had been formed by a simple thrusting in of the surface of the integument (fig. 238). This epidermal lining of the hair-follicle, or root-sheath of the hair, is composed of two layers, the inner one of which is so moulded on the imbricated scaly cuticle of the hair, that its inner surface becomes imbricated also, but of course in the opposite direction. When a hair is pulled out, the inner layer of the root sheath and part of the outer layer also are commonly pulled out with it. Nails.-A nail, like a hair, is a peculiar arrangement of epidermal cells, the undermost of which, like those of the general surface of the integument, are rounded or elon- gated, while the superficial are flattened, and of more horny con- sistence. That specially modified portion of the corium, or true skin, by which the nail is secreted, is called the matrix. The back edge of the nail, or the roo£ as it is termed, is received into a shallow crescentic groove in the matrix, while the front part is free and projects beyond the extremity of the digit. The intermediate por- tion of the nail rests by its broad under surface on the front part of the matrix, which is here called the bed of the nail. This part of the matrix is not uniformly smooth on the surface, but is raised in the form of longitudinal and nearly parallel ridges or laminae, on which are moulded the epidermal cells of which the nail is made up (fig. 241). The growth of the nail, like that of the hair, or of the epidermis generally, is effected by a constant production of cells from beneath and behind, to take the place of those which are worn or cut Fig. 238.-Medium-sized hair in its follicle, a, stem cut short; Z>, root; c, knob; d, hair cuticle ; e, internal, and f, external root-sheath ; (j, h, dermic coat of follicle; i, papilla ; k, k, ducts of sebaceous glands ; l, corium; m, mucous layer of epi- dermis ; o, upper limit of internal root sheath, x 50. (Kblliker.) 396 STRUCTURE AND FUNCTIONS OF THE SKIN. [chap. XI. away. Inasmuch, however, as the posterior edge of the nail, from its being lodged in a groove of the skin, cannot grow backwards, on additions being made to it, so easily as it can pass in the opposite direction, any growth at its hinder part pushes the whole forwards. At the same time fresh cells are added to its under surface, and thus each portion of the nail becomes gradually thicker as it moves to the front, until, projecting beyond the Fig. 239.-Longitudinal section of a hair follicle, a, Stratum of Malpighi, deep layer forming the external root-sheath, and continued to the surface of the papilla to form the medullary sheath of the hair; b, second external sheath; c, internal root sheath ; d, fibroid sheath of the hair; e, medullary sheath or medulla ; f, hair papilla ; g, blood-vessels of the hair papilla ; A, fibro-vascular sheath. (Cadiat.) surface of the matrix, it can receive no fresh addition from beneath, and is simply moved forwards by the growth at its root, to be at last worn away or cut off. Functions of the Skin. (i.) By means of its toughness, flexibility and elasticity, the skin is eminently qualified to serve as the general integument of the body, for defending the internal parts from external violence, and readily yielding and adapting itself to their various movements and changes of position. (2.) The skin is the chief organ of the sense of touch. Its whole surface is extremely sensitive ; but its tactile properties are due more especially to the abundant papillae with which it is studded. (See Chapter on Special Senses.) CHAP. XI.] FUNCTIONS OF THE SKIN. 397 Although destined especially for the sense of touch, the papillae are not so placed as to come into direct contact with external objects; but like the rest of the surface of the skin, are covered by one or more layers of epithelium, forming the cuticle or epidermis. The papillae adhere very intimately to the cuticle, which is thickest in the spaces between them, but tolerably level on its outer surface : hence, when stripped off from the cutis, as after maceration, its internal surface presents a series of pits and elevations corresponding to the papillae and their interspaces, Fig. 240.-Transverse section of a hair and hair-follicle made below the opening of the seba- ceous gland, a, medulla or pith of the hair; 6, fibrous layer or cortex; c, cuticle ; d, Huxley's layer; e, Henle's layer of internal root-sheath ; / and g, layers of external root sheath, outside of g is a light layer, or "glassy membrane," which is equivalent to the basement membrane ; h, fibrous coat of hair sac ; i, vessels. Cadiat.) of which it thus forms a kind of mould. Besides affording by its impermeability a check to undue evaporation from the skin, and providing the sensitive cutis with a protecting investment, the cuticle is of service in relation to the sense of touch. For bv being thickest in the spaces, between the papillae, and only thinly spread over the summits of these processes, it may serve to sub- divide the sentient surface of the skin into a number of isolated points, each of which is capable of receiving a distinct impression from an external body. By covering the papillae it renders the 398 STRUCTURE AND FUNCTIONS OF THE SKIN. [chap. xi. sensation produced by external bodies more obtuse, and in this manner also is subservient to touch : for unless the very sensitive papilla; were thus defended, the contact of substances would give rise to pain, instead of the ordinary impressions of touch. This is shown in the extreme sensitiveness and loss of tactile power in a part of the skin when deprived of its epidermis. If the cuticle Fig 241.- Vertical transverse section through a small portion oj the nail ami matrix largely magnified. A, eorium of the nail-bed, raised into ridges or laminae a, fitting in between corresponding laminae l>, of the nail. />', Malpighian, and C, horny layer of nail; d, deepest and vertical cells ; e, upper flattened cells of Malpighian layer (Kolliker.) is very thick, however, as on the heel, touch becomes imperfect, or is lost. (3.) The Skin is an organ of Secretion, as it possesses Sebaceous Glands.-The secretion of the sebaceous glands and hair-follicles (for their products cannot be separated) consists of cast-off epithelium cells, with nuclei and granules, together with an oily matter, extractive matter, and stearin ; in certain parts, also, it is mixed with a peculiar odorous principle, which contains caproic, butyric, and rutic acids. It is, perhaps, nearly similar in composition to the unctuous coating, or vernix caseosa, which is formed on the body of the foetus while in the uterus, and which contains large quantities of ordinary fat. Its purpose seems to be that of keeping the skin moist and supple, and, by its oily nature, of both hindering the evaporation from the surface, and guarding CHAP. XI.] COMPOSITION OF SWEAT. 399 the skin from the effects of the long-continued action of moisture. But while it thus serves local purposes, its removal from the body entitles it to be reckoned among the excretions of the skin ; though the share it has in the purifying of the blood cannot be discerned. (4.) The Skin is also an organ of Excretion, as it con- tains Sweat Glands.-The fluid secreted by the sweat-glands is usually formed so gradually that the watery portion of it escapes by evaporation as fast as it reaches the surface. But, during strong exercise, exposure to great external warmth, in some dis- eases, and when evaporation is prevented, the secretion becomes more sensible, and collects on the skin in the form of drops of fluid. The perspiration, as the term is sometimes employed in physiology, includes all that portion of the secretions and exudations from the skin which passes off by evaporation ; the sweat includes that which may be collected only in drops of fluid on the surface of the skin. The two terms are, however, most often used synonymously ; and for distinction, the former is called insensible perspiration ; the lattei' sensible perspiration. The fluids are the same, except that the sweat is commonly mingled with various substances lying on the surface of the skin. The contents of the sweat are, in part, matters capable of assuming the form of vapour, such as carbonic acid and water, and in part, other matters which are deposited on the skin, and mixed with the sebaceous secretion. Table of the Chemical Composition of Sweat. Water 995 Solids Organic Acids (formic, acetic, butyric, propionic, caproic, caprylic) '9 Salts, chiefly sodium chloride . . r8 Neutral fats and cholesterin. . . 7 Extractives (including urea), with epithelium i-6 5 IOOO The sweat is a colourless, slightly turbid fluid, alkaline, neutral acid in reaction, of a saltish taste, and peculiar characteristic odour. Of the several substances it contains, however, only the carbonic acid and water need particular consideration. a. Watery vapour.-The quantity of watery vapour excreted 400 STRUCTURE AND FUNCTIONS OF THE SKIN. [chap. XI. from the skin is on an average between ij and 2 lb. daily. This subject has been very carefully investigated by Lavoisier and Sequin. The latter chemist enclosed his body in an air-tight bag, with a mouth-piece. The bag being closed by a strong band above, and the mouth-piece adjusted and gummed to the skin around the mouth, he was weighed, and then remained quiet for several hours, after which time he was again weighed. The difference in the two weights indicated the amount of loss by pulmonary exhalation. Having taken off the air-tight dress, he was immediately weighed again, and a fourth time after a certain interval. The difference between the two weights last ascertained gave the amount of the cutaneous and pulmonary exhalation together; by subtracting from this the loss by pulmonary exhalation alone, while he was in the air-tight dress, he ascertained the amount of cutaneous tran- spiration. During a state of rest, the average loss by cutaneous and pulmonary exhalation in a minute, is eighteen grains,-the minimum eleven grains, the maximum thirty-two grains; and of the eighteen grains, eleven pass off by the skin, and seven by the lungs. The quantity of watery vapour lost by transpiration is of course influenced by all external circumstances which affect the exhala- tion from other evaporating surfaces, such as the temperature, the hygrometric state, and the stillness of the atmosphere. But, of the variations to which it is subject under the influence of these conditions, no calculation has been exactly made. b. Carbonic Acid.-The quantity of carbonic acid exhaled by the skin on an average is about to of that furnished by the pulmonary respiration. The cutaneous exhalation is most abundant in the lower classes of animals, more particularly the naked Amphibia, as frogs and toads, whose skin is thin and moist, and readily permits an interchange of gases between the blood circulating in it, and the surrounding atmosphere. Bischoff found that, after the lungs of frogs had been tied and cut out, about a quarter of a cubic inch of carbonic acid gas was exhaled by the skin in eight hours. And this quantity is very large, when it is remembered that a full-sized frog will generate only about half a cubic inch of carbonic acid by his lungs and skin together in six hours. The importance of the respiratory function of the skin, which was once thought to be proved by the speedy death of animals whose skins, after removal of the hair, were covered with an impermeable varnish, has been shown by further observations to have no foundation in fact ; the immediate cause of death in such cases being the loss of temperature. A varnished animal is said to have suffered no harm when surrounded by cotton wadding and to have died when the wadding was removed. CHAP. XL] FUNCTIONS OF THE SKIN. 401 Influence of the Nervous System on Sweat-Excretion.-- Any increase in the amount of sweat secreted is usually accom- panied by dilatation of the cutaneous vessels. It is, however, probable that the secretion is like the other secretions, e.g., the saliva, under the direct action of a special nervous apparatus, in that various nerves contain fibres which act directly upon the cells of the sweat-glands in the same way that the chorda tympani contains fibres which act directly upon the salivary cells. The local apparatus is under control of the central nervous system- sweat centres probably existing both in the medulla and spinal cord-and may be reflexly as well as directly excited. The nerve fibres which induce sweating may act independently of the vaso- motor fibres, whether vaso-dilator or vaso-constrictor. This will explain the fact that sweat occurs not only when the skin is red, but also when it is pale, and the cutaneous circulation languid, as in the sweat which accompanies syncope or fainting, or which immediately precedes death. (5.) The Skin has a farther function, that of Absorption. -Absorption by the skin has been already mentioned, as an instance in which that process is most actively accomplished. Metallic preparations rubbed into the skin have the same action as when given internally, only in a less degree. Mercury applied in this manner exerts its specific influence upon syphilis, and excites salivation ; potassio-tartrate of antimony may excite vomit- ing, or an eruption extending over the whole body ; and arsenic may produce poisonous effects. Vegetable matters, also, if soluble, or already in solution, give rise to their peculiar effects, as cathartics, narcotics, and the like, when rubbed into the skin. The effect of rubbing is probably to convey the particles of the matter into the orifices of the glands whence they are more readily absorbed than they would be through the epidermis. When simply left in contact with the skin, substances, unless in a fluid state, are seldom absorbed. It has long been a contested question whether the skin covered with the epidermis has the power of absorbing water; and it is a point the more difficult to determine because the skin loses water by evaporation. But, from the result of many experiments, it may now be regarded as a well-ascertained fact that such absorp- tion really occurs. The absorption of water by the surface of the body may take place in the lower animals very rapidly. Not only frogs, which have a thin skin, but lizards, in which the 402 STRUCTURE AND FUNCTION OF KIDNEYS. [chap. XII. cuticle is thicker than in man, after having lost weight by being kept for some time in a dry atmosphere, are found to recover both their weight and plumpness very rapidly when immersed in water. When merely the tail, posterior extremities, and posterior part of the body of the lizard are immersed, the water absorbed is distributed throughout the system. And a like absorption through the skin, though to a less extent, may take place also in man. In severe cases of dysphagia, when not even fluids can be taken into the stomach, immersion in a bath of warm water or of milk and water may assuage the thirst; and it has been found in such cases that the weight of the body is increased by the immersion. Sailors also, when destitute of fresh water, find their urgent thirst allayed by soaking their clothes in salt water, and wearing them in that state ; but these effects are in part due to the hindrance to the evaporation of water from the skin. (6.) For an account of the important function of the skin in the regulation of temperature, see Chapter on Animal Heat. CHAPTER XII. THE STRUCTURE AND FUNCTION OF THE KIDNEYS. The Kidneys are two in number, and are situated deeply in the lumbar region of the abdomen on either side of the spinal column behind the peritoneum. They correspond in posi- tion to the last two dorsal and two upper lumbar vertebrae; the right being slightly below the left in consequence of the position of the liver on the right side of the abdomen. They are about 4 inches long, 2| inches broad, and i| inch thick. The weight of each kidney is about 4 J oz. Structure.-The kidney is covered by a tough fibrous capsule, which is slightly attached by its inner surface to the proper substance of the organ by means of very fine fibres of areolar t issue CHAP. XII. ] STRUCTURE OF THE KIDNEY. 403 and minute blood-vessels. From the healthy kidney, therefore, it may be easily torn off without injury to the subjacent cortical portion of the organ. At the hilus or notch of the kidney, it becomes continuous with the external coat of the upper and dilated part of the ureter (fig. 242). On dividing the kidney into two equal parts by a section carried through its long convex bor- der (fig. 242), the main part of its substance is seen to be composed of two chief portions, called respectively the cortical and the medullary portion, the latter being also sometimes called the pyramidal portion, from the fact of its being com- posed of about a dozen conical bundles of urine tubes, each bundle being called a pyramid. The upper part of the duct of the organ, or the ureter, is dilated into what is called the pelvis of the kidney ; and this again, after separating into two or three principal divi- sions, is finally subdivided into still smaller portions, vary- ing in number from about 8 to 12, or even more, and called calyces. Each of these little calyces or cups, which are often arranged in a double row, receives the pointed extremity or papilla of a pyramid. Sometimes, however, more than one papilla is received by a calyx. The kidney is a compound tubular gland, and both its cortical and medullary portions are composed essentially of secreting tubes, the tubuli uriniferi, which, by one extremity, in the cortical portion, end commonly in little saccules containing blood-vessels, called Malpighian bodies, and, by the other, open through the papillae into the pelvis of the kidney, and thus discharge the urine which flows through them. Fig. 242.-Plan of a longitudinal section through the pelvis and, substance of the right hidney, |; a, the cortical substance ; b, b, broad part of the pyramids of Malpighi; c, c, the divisions of the pelvis named calyces, laid open ; c', one of those unopened ; d, summit of the pyramids of papilla1 pro- jecting into calyces ; e, e, section of the narrow part of two pyramids near the calyces; p, pelvis or enlarged divisions of the ureter within the kidney; u, the ureter; «, the sinus ; h, the hilus. 404 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xn. In the pyramids the tubes are chiefly straight-dividing and diverging as they ascend through these into the cortical portion ; while in the latter region they spread out more irregularly, and become much branched and convoluted. Tubuli Uriniferi.-The tubuli uriniferi (fig. 243) are composed of a nearly homogeneous membrane, and are lined internally by epithelium. They vary considerably in size in different parts of their course, but are, on an average, about of an inch in diameter, and are found to be made up of several distinct sections which differ from one another very markedly, both in situation and structure. According to Klein, the fol- lowing segments may be made out : (1) The Malpighian corpuscle (figs. 244, 245), composed of a hyaline membrana. propria, thickened by a varying amount of fibrous tissue, and lined by flattened nucleated epithelial plates. This cap- sule is the dilated extremity of the uriniferous tubule, and contains within it a glomerulus of convoluted capil- lary blood-vessels supported by connec- tive tissue, and covered by flattened epithelial plates. The glomerulus is connected with an efferent and an afferent vessel. (2) The constricted neck of the capsule (fig. 244, 2), lined in a similar manner, connects it with (3) The Proximal convoluted tubule, which forms several distinct curves and is lined with short columnar cells, which vary somewhat in size. The tube next passes almost vertically downwards, forming (4) The Spiral tubule, which is of much the same diameter, and is lined in the same way as the convoluted portion. So far the tube has been contained in the cortex of the kidney ; it now passes vertically downward through the most external part (boundary layer) of the Malpighian pyramid into the more internal part (papillary layer), where it curves up sharply, forming altogether the (5 and 6) Loop of Henle, which is a very narrow tube lined with flattened nucleated cells. Passing vertically upwards just as the tube reaches the boundary layer (7), it suddenly enlarges and becomes lined with polyhedral cells. (8) About midway in Fig. 243.-A. Portion of a secreting tubule from the cortical substance of the kidney, b. The epithelial or gland-cells, x 700 times. CHAP. XII.] THE TUBULI URINIFEBT. 405 Fig. 244.-A Diagram of the sections of uriniferous tubes. A, Cortex limited externally by the capsule ; a, subcapsular layer not containing Malpighian corpuscles ; a', inner stratum of cortex, also without Malpighian capsules ; B, boundary layer; C, Papillary part next the boundary layer; 1, Bowman's capsule of Malpighian corpuscle; 2, neck of capsule; 3, proximal convoluted tubule; 4, spiral tubule; 5, descending limb of Henle's loop ; 6, the loop proper ; 7, thick part of the ascending limb ; 8, spiral part of ascending limb ; 9, narrow ascending limb in the medullary ray; 10, the irregular tubule; n, the intercalated section, or the distal convoluted tubule; 12, the curved collecting tubule ; 13, the straight collecting tubule of the medullary ray; 14, the collecting tube of the boundary layer; 15, the large collecting tube of the papillary part which, joining with similar tubes, forms the duct. (Klein and Noble Smith.) 406 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xn. the boundary layer the tube again narrows, forming the ascend- ing spiral of Henle's loop, but is still lined with polyhedral cells. At the point where the tube enters the cortex (9) the ascending limb narrows, but the diameter varies considerably ; here and there the cells are more flattened, but both in this as in (8), the cells are in many places very angular, branched, and im- Fig. 245.-From a vertical section through the kidney of a dog-the capsule of which is sup- posed to be on the right, a. The capillaries of the Malpighian corpuscle-viz., the glomerulus, are arranged in lobules ; n, neck of capsule ; c, convoluted tubes cut in various directions; b, irregular tubule; <Z, e, and /, are straight tubes running towards capsules forming a so-called medullary ray; d, collecting tube; e, spiral tube; f, narrow section of ascending limb, x 380. (Klein and Noble Smith.) bricated. It then joins (io) the "irregular tubulef which has a very irregular and angular outline, and is lined with angular and imbricated cells. The tube next becomes convoluted (n), form- ing the distal convoluted tube or intercalated section of Schweigger- Seidel, which is identical in all respects with the proximal con- voluted tube (12 and 13). The curved and straight collecting tubes, the former entering the latter at right angles, and the latter passing vertically downwards, are lined with polyhedral, CHAP. XII.] BLOOD-SUPPLY OF THE KIDNEY. 407 or spindle-shaped, or flattened, or angular cells. The straight collecting tube now enters the boundary layer (14), and passes on to the papillary layer, and, joining with other collecting tubes, form larger tubes, which finally open at the apex of the papilla. These collecting tubes are lined with transparent nucleated columnar or cubical cells (14, 15). The cells of the tubules with the exception of Henle's loop and Fig. 246.-Transverse section of a renal papilla ; a, larger tubes or papillary ducts ; b, smaller tubes of Henle; c, blood-vessels, distinguished by their flatter epithelium; d, nuclei of the stroma (Kblliker). x 300. all parts of the collecting tubules, are, as a rule, possessed of the intra nuclear as well as of the intra-cellular network of fibres, of which the vertical rods are most conspicuous parts. Heidenhain observed that indigo-sulphate of sodium, and other pigments injected into the jugular vein of an animal, were apparently excreted by the cells which possessed these rods, and therefore concluded that the pigment passes through the cells, rods, and nucleus themselves. Klein, however, believes that the pigment passes through the intercellular substances, and not through the cells. In some places, it is stated that a distinct membrane of flattened cells can be made out lining the lumen of the tubes (centrotubular membrane). Blood-supply.-In connection with the general distribution of blood-vessels to the kidney, the Malpighian Corpuscles may be 408 STRUCTURE AND FUNCTION OF KIDNEYS. [chap. xii. further considered. They (fig. 247) are found only in the cortical part of the kidney, and are confined to the central part, which, however, makes up about seven-eighths of the whole cortex. On a section of the organ, some of them are just visible to the naked eye as minute red points ; others are too small to be thus seen. Their average diameter is about of an inch. Each of them is composed, as we have seen above, of the dilated extremity of an uriniferous tube, or Malpighian capsule, which encloses a tuft of blood-vessels. The renal artery divides into several branches, which, passing in at the hilus of the kidney, and covered by a fine sheath of areolar tissue derived from the capsule, enter the substance of the organ chiefly in the intervals between the papillae, and at the junction between the cortex and the boundary layer. The main branches then pass almost horizontally, giving off branches up- wards to the cortex and downwards to the medulla. The former are for the most part straight; they pass almost vertically to the surface of the kidney, giving off laterally in all directions longer or shorter branches, which ultimately supply the Malpi- ghian bodies. The small afferent artery (figs. 247 and 249) which enters the Malpighian corpuscle, breaks up in the interior as before mentioned into a dense and convoluted and looped capillary plexus, which is ultimately gathered up again into a single small efferent vessel, comparable to a minute vein, which leaves the capsule just by the point at which the afferent artery enters it. On leaving, it does not immediately join other small veins as might have been expected, but again breaking up into a network of capillary vessels, is distributed on the exterior of the tubule, from whose dilated end it had just emerged. After this second breaking up it is finally collected into a small vein, which, by union with others like it, helps to form the radicles of the renal vein. Fig. 247.-Diagram showing the rela- tion of the Malpighian body to the uriniferous ducts and blood-vessels, a, one of the interlobular arteries ; a', afferent artery passing into the glomerulus ; c, capsule of the Mal- pighian body, forming the termi- nation of and continuous with t, the uriniferous tube ; e', e', efferent vessels which subdivide in the plexus; p, surrounding the tube, and finally terminate ii the branch of the renal vein e. (after Bowman). CHAP. XII.] BLOOD SUPPLY OF THE KIDNEY. 409 Thus, in the kidney, the blood entering by the renal artery, traverses two sets of capillaries before emerging by the renal vein, an arrangement which may be compared to the portal system in miniature. The tuft of vessels in the course of development is, as it were, thrust into the dilated extremity of the urinary tubule, which finally completely invests it just as the pleura invests the lungs or the tunica vaginalis the testicle. Thus the Malpighian capsule is lined by a parietal layer of squamous cells and a visceral or reflected layer immediately covering the vascular tuft (fig. 245), and sometimes dipping down into its interstices. This reflected Fig. 248.-Transverse section of a developing Malpighian capsule and tuft (human) X 300. From a foetus at about the fourth month ; a, flattened cells grow- ing to form the capsule; l>, more rounded cells; continuous with the above, reflected round c, and finally enveloping it; c, mass of embiyonic cells which will later become developed into blood-vessels (W. Pye). Fig. 249.-Epithelial elements of a Malpighian capsule and tuft, with the commencement of a urinary tubule showing the afferent and efferent vessel; a, layer of tesselated epithelium forming the capsule; l>, simi- lar, but rather larger epithelial cells, placed in the walls of the tube ; c, cells, covering the vessels of the capillary tuft; d, commencement of the tubule, somewhat narrower than the rest of it (W. Pye). layer of epithelium is readily seen in young subjects, but cannot always be demonstrated in the adult. (See figs. 248 and 249-) The vessels which enter the medullary layer break up into smaller arterioles, which pass through the boundary layer, and proceed in a straight course between the tubules of the papillary 410 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xii. layer, giving off on their way branches, which form a fine arterial meshwork around the tubes, and ending in a similar plexus from which the venous radicles arise. Besides the small afferent arteries of the Malpighian bodies, there are, of course, others which are distributed in the ordinary manner, for the nutrition of the different parts of the organ; and in the pyramids, between the tubes, there are numerous straight vessels, the vasta recta, some of which are branches of vasa efferentia from Malpighian bodies, and therefore comparable to the venous plexus around the tubules in the cortical portion, while others arise directly as small branches of the renal arteries. Between the tubes, vessels, etc., which make up the substance of the kidney, there exists, in small quantity, a fine matrix of areolar tissue. Nerves.--The nerves of the kidney are derived from the renal plexus. The Ureters.-The ducts of each kidney, or ureter, is a tube about the size of a goose-quill, and from twelve to sixteen inches in length, which, continuous above with the pelvis of the kidney, ends below by perforating obliquely the walls of the bladder, and opening on its internal surface. Structure.-It is constructed of three principal coats (a) an outer, tough, fibrous and elastic coat; (6) a middle muscular coat, of which the fibres are unstriped, and arranged in three layers- the fibres of the central layer being circular, and those of the other two longitudinal in direction; and (c) an internal mweows lining continuous with that of the pelvis of the kidney above, and of the urinary bladder below. The epithelium of all these parts (fig. 250) is alike stratified and of a somewhat peculiar form; the cells on the free surface of the mucous membrane being usually spheroidal or polyhedral with one or more spherical or oval nuclei; while beneath these are pear-shaped cells, of which the broad ends are directed towards the free surface, fitting in beneath the cells of the first row, and the apices are prolonged into processes of various lengths, among which, again, the deepest cells of the epithelium are found spheroidal, irregularly oval, spindle-shaped or conical. The Urinary Bladder.-The urinary bladder, which forms a re- ceptacle for the temporary lodgment of the urine in the intervals of its expulsion from the body, is more or less pyriform, its wddest part, which is situate above and behind, being termed the fundus : CHAP. XII.] THE URINARY BLADDER. 411 and the narrow constricted portion in front and below, by which it becomes continuous with the urethra, being called its ctrvix or neck. Structure.-It is constructed of four principal coats,-serous, muscular, areolar or submucous, and (a) The serous coat, which covers only the posterior and upper half of the bladder, has the same structure as that of the peritoneum, with which it is continuous. (6) The fibres of the muscular coat, which are un- striped, are arranged in three principal layers, of which the external and internal have a general longitudinal, and the middle Fig. 250.-Epithelium of the bladder ; a, one of the cells of the first row; b, a cell of the second row; c, cells in situ, of first, second, and deepest layers (Obersteiner). layer a circular direction. The latter are especially developed around the cervix of the organ, and are described as forming a sphincter vesicce. The muscular fibres of the bladder, like those of the stomach, are arranged not in simple circles, but in figure-of-8 loops, (c) The areolar or submucous coat is constructed of con- nective tissue with a large proportion of elastic fibres. (cZ) The mucous membrane, which is rugose in the contracted state of the organ, does not differ in essential structure from mucous membranes in general. Its epithelium is stratified and closely resembles that of the pelvis of the kidney and the ureter (fig. 250). The mucous membrane is provided with mucous glands, which are more numerous near the neck of the bladder. The bladder is well provided with blood- and lymph-vessels, and with nerves. The latter are branches from the sacral plexus (spinal) and hypogastric plexus (sympathetic). A few ganglion-cells are found, here and there, in the course of the nerve-fibres. 412 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xn. Physical Properties.-Healthy urine is a perfectly transparent, amber-coloured liquid, with a peculiar, but not disagreeable odour, a bitterish taste, and slight acid reaction. Its specific gravity varies from 1015 to 1025. On standing for a short time, a little mucus appears in it as a flocculent cloud. Chemical Composition.-The urine consists of water, holding in solution certain organic and saline matters as its ordinary consti- tuents, and occasionally various other matters; some of the latter are indications of diseased states of the system, and others are derived from unusual articles of food or drugs taken into the stomach. The Urine. Table of the Chemical Composition of the Urine. Water 967 Solids- Urea 14'230 Other nitrogenous crystalline bodies- Uric acid, principally in the form of alka- line Urates, a trace only free. Kreatinin, Xanthin, Hypoxanthin. Hippuric acid, Leucin, Tyrosin, Taurin, Cystin, &c., all in small amounts and not constant. Mucus and Pigment. io-635 Salts : - Inorganic- v Principally Sulphates, Phosphates, and \ Chlorides of Sodium, and Potassium, with Phosphates of Magnesium and Calcium, traces of Silicates of and Chlorides. Organic-• Lactates, Hippurates, Acetates and For- mates, which only appear occasionally. 8-135 - 33 Sugara trace sometimes. Gases (nitrogen and carbonic acid principally). 1000 Reaction.-The normal reaction of the urine is slightly acid. This acidity is due to acid phosphate of sodium, and is less marked CHAP. XII.] COMPOSITION OF THE URINE. 413 soon after meals. The urine contains no appreciable amount of free acid, as it gives no precipitate of sulphur with sodium hyposulphite. After standing for some time the acidity increases from a kind of acid fermentation, due in all probability to the presence of mucus and fungi, and acid urates or free uric acid is deposited. After a time, varying in length according to the temperature, the reaction becomes strongly alkaline from the change of urea into ammonium carbonate, due to the presence of one or more specific micro- organisms (micrococcus urem). The urea takes up two molecules of water-a strong ammoniacal and foetid odour appears, and deposits of triple phosphates and alkaline urates take place. This does not occur unless the urine is freely exposed to the air, or, at least, until air has had access to it. Reaction of Urine in different classes of Animals.-In most herbivorous animals the urine is alkaline and turbid. The difference depends, not on any peculiarity in the mode of secretion, but on the differences in the food on which the two classes subsist; for when carnivorous animals, such as dogs, are restricted to a vegetable diet, their urine becomes pale, turbid, and alkaline, like that of an herbivorous animal, but resumes its former acidity on the return to an animal diet; while the urine voided by herbivorous animals, e.g.. rabbits, fed for some time exclusively upon animal substances, presents the acid reaction and other qualities of the urine of Carnivora, its ordinary alkalinity being restored only on the substitution of a vegetable for the animal diet. Human urine is not usually rendered alkaline by vegetable diet, but it becomes so after the free use of alkaline medicines, or of the alkaline salts with carbonic or vegetable acids ; for these latter are changed into alkaline carbonates previous to elimination by the kidneys. Average daily quantity of the chief constituents of the Urine (by healthy male adults). Water 52' fluid ounces. Urea 512'4 grains. Uric acid 8'5 „ Hippuric acid, uncertain probably 10 to 15- „ Sulphuric acid 31'11 „ Phosphoric acid ...... 45' „ Potassium, Sodium, and Ammonium Chlorides and free Chlorine .... . . 323-25 „ Lime 3'5 Magnesia . • 3- „ Mucus 7- „ Extractives Kreatinin I Pigment I Xanthin . Hypoxanthin ■ ■ 154- Resinous matter, &c. 414 STRUCTURE AND FUNCTION OF KIDNEYS. [chap. XII. Variations in the Quantity of the Constituents.-From the proportions given in the above table, most of the constituents are, even in health, liable to variations. The variations of the ivater in different seasons, and according to the quantity of drink and exercise, have already been mentioned. The water of the urine is also liable to be influenced by the condition of the nervous system, being sometimes greatly increased, e.y., in hysteria, and in some other nervous affections; and at other times diminished. In some diseases it is enormously increased ; and its increase may be either attended with an augmented quantity of solid matter, as in ordinary diabetes, or may be nearly the sole change, as in the affection termed diabetes insipidus. In other diseases, e.g., the various forms of albuminuria, the quantity may be considerably diminished. A febrile condition almost always diminishes the quantity of water; and a like diminution is caused by any affec- tion which draws off a large quantity of fluid from the body through any other channel than, that of the kidneys, e.g., the bowels or the skin. Method of estimating the Solids.-A useful rule for approximately esti- mating the total solids in any given specimen of healthy urine is to multiply the last two figures representing the specific gravity by 2'33. Thus, in urine of sp. gr. 1025, 2'33 x 25 = 58'25 grains of solids, are contained in 1000 grains of the urine. In using this method it must be remembered that the limits of error are much wider in diseased than in healthy urine. Variations in the Specific Gravity.-The average specific gravity of the human urine is about 1020. The relative quantity of water and of solid constituents of which it is composed is mate- rially influenced by the condition and occupation of the body during the time at which it is secreted; by the length of time which has elapsed since the last meal; and by several other acci- dental circumstances. The existence of these causes of difference in the composition of the urine has led to the secretion being described under the three heads of Urina sanguinis, Urina potus, and Urina cibi. The first of these names signifies the urine, or that part of it which is secreted from the blood at times in which neither food nor drink has been recently taken, and is applied especially to the urine which is evacuated in the morning before breakfast. The terms urina potus indicates the urine secreted shortly after the introduction of any considerable quantity of fluid into the body: and the urina cibi, the portions secreted during the period immediately succeeding a meal of solid food. The last chap, xn.] THE URINARY SOLIDS. 415 kind contains a larger quantity of solid matter than either of the others; the first or second, being largely diluted with water, pos- sesses a comparatively low specific gravity. Of these three kinds, the morning urine is the best calculated for analysis in health, since it represents the simple secretion unmixed with the elements of food or drink ; if it be not used, the whole of the urine passed during a period of twenty-four hours should be taken. The specific gravity of the urine may thus, consistently with health, range widely on both sides of the usual average. It may vary from 1015 in the winter, to 1025 in the summer ; but varia- tions of diet and exercise, and many other circumstances, may make even greater differences than these. In disease, the varia- tion may be greater; sometimes descending, in albuminuria, to 1004, and frequently ascending in diabetes, when the urine is loaded with sugar, to 1050, or even to 1060. Quantity.-The total quantity of urine passed in twenty-four hours is affected by numerous circumstances. On taking the mean of many observations by several experimenters, the average quantity voided in twenty-four hours by healthy male adults from 20 to 40 years of age has been found to amount to about 52J- fluid ounces (i| to 2 litres). Abnormal Constituents.-In disease, or after the ingestion of special foods, various abnormal substances occur in urine, of which the following maybe mentioned-Serum-albumin, Globulin, Ferments (apparently present in health also), Peptone, Blood, Sugar, Bile acids and pigments, Casts, Fats, various Salts taken as medicine, Micro-organisms of various kinds, and other matters. The Solids of the Urine. (i.) Urea.-(CH4N20.)-Urea is the principal solid consti- tuent of the urine, forming nearly one-half of the whole quantity of solid matter. It is also the most important ingredient, since it is the chief substance by which the nitrogen of decomposed tissue and of any superfluous food is excreted from the body. For its removal, the secretion of urine seems especially provided ; and by its retention in the blood the most pernicious effects are produced. Properties.-Urea, like the other solid constituents of the urine, exists in a state of solution. When in the solid state, it appears in 416 STRUCTURE AND FUNCTION OF KIDNEYS. [chap. xii. the form of delicate silvery acicular crystals, which, under the micro- scope, appear as four-sided prisms (fig. 251). It may be obtained in this state by evaporating urine carefully to the consistence of honey, acting on the inspissated mass with four parts of alcohol, then evaporating the alcoholic so- lution to dryness, and purifying the residue by repeated solution in water or in alcohol, and finally allowing it to crystallize. It readily combines with some acids, like a weak base ; and may thus be conveniently procured in the form of crystals of nitrate or oxa- late of urea (figs. 252 and 253). Urea is colourless when pure ; when impure it may be yellow or brown: it is without smell, and of a cooling nitre-like taste; it has neither an acid nor an alkaline re-action, and deliquesces in a moist and warm atmosphere. At 59° F. (15° C.) it requires for Fig. 251.-Crystals of Urea. Fig. 252.-Crystals of Urea nitrate. Fig. 253.-Crystals of Urea oxalate. its solution less than its own weight of water; it is dissolved in all proportions by boiling water ■ but it requires five times its weight of cold alcohol for its solution. It is insoluble in ether. At 248° F. (120° C.) it melts without undergoing decomposition; at a still higher temperature ebullition takes place, and carbonate of ammonium sublimes ; the melting mass gradually acquires a pulpy consistence ; and if the heat is carefully regulated, leaves a grey- white powder, cyanic acid. Chemical Nature.-Urea is isomeric with ammonium cyanate CNO. It was first of all artificially prepared from that substance. It is usually considered to be a diamide of carbonic acid, in other words, carbonic acid, CO (0H)'2, with two of hydroxyl, CHAP. XII.] THE PROPERTIES OF UREA. 417 (0H)'2 replaced by two of amidogen (NH2)'2. It may also be written as if it were a monamide of carbamic acid (C00HNH2), thus CONH2. NH,; one of amidogen NH2 in the latter replacing one of hydroxyl in the former. On heating, urea is converted into ammonium carbonate and cyanic acid. A similar decomposition of the urea with development of ammonium carbonate ensues spontaneously when urine is kept for some days after being voided, and explains the ammoniacal odour then evolved. The urea is sometimes decomposed before it leaves the bladder, when the mucous membrane is diseased, and the mucus secreted by it is both more abundant, and, probably, more prone to act as a fer- ment ; although the decomposition does not often occur unless atmospheric germs have had access to the urine. Variations in Quantity excreted.-The quantity of urea excreted is, like that of the urine itself, subject to considerable variation. For a healthy adult 500 grains (about 32'5 grms.) per diem may be taken as rather a high average. Its percentage in healthy urine is 1'5 to 2-5. Its amount is materially influenced by diet, being greater when animal food is exclusively used, less when the diet is mixed, and least of all with a vegetable diet. As a rule, men excrete a larger quantity that women, and persons in the middle periods of life a larger quantity than infants or old people. The quantity of urea excreted by children, relatively to their body-weight, is much greater than by adults. Thus the quantity of urea excreted per kilogram of weight was, in a child, 0'8 grm. : in an adult only 0'4 grm. Regarded in this way, the excretion of carbonic acid gives similar results, the proportions in the child and adult being as 82 : 34. The quantity of urea does not necessarily increase and decrease with that of the urine, though on the whole it would seem that whenever the amount of urine is much augmented, the quantity of urea also is usually increased ; and it appears that the quantity of urea, as of urine, may be especially increased by drinking large quantities of water. In various diseases the quantity is reduced considerably below the healthy standard, while in other affections it is above it. Quantitative Estimation.-There are two chief methods of esti- mating the amount of urea in the urine. (1.) By decomposing it by means of an alkaline solution of sodium hypobromite, or hypo- chlorite, and calculating the amount in a measured quantity, by collecting and measuring the amount of nitrogen evolved under such 418 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xn. circumstances. Urea contains nearly half its weight of nitrogen, hence the amount of the gas collected may be taken as a measure of the urea decomposed. The percentage of urea can of course be readily calculated from the volume of nitrogen evolved from a measured quantity of the urine, but this calcula- tion is avoided by graduating the tube in which the nitrogen is collected with numbers which indicate the corresponding per- centage of urea. The reaction is C0N2H4 + 3NaBrO + 2NaH0 = 3NaBr + 3IUO + Na2CO3 + N2. (2.) By precipitating the urea by adding to a given amount of urine, freed from sulphates and phosphates, a standard solution of mercuric nitrate from a burette, until the whole of it has been thrown down in an insoluble form; then reading off the exact amount of the mer- curic nitrate solution, which it was necessary to use. As the amount of urea which each cubic centimetre of the standard solution will precipitate is previously known, it is easy to calcu- late the amount in the sample of urine taken. The precipitate which is formed is generally said to be composed of mercuric oxide and urea. Some, however, consider that it is a mixture of mercuric nitrate itself and urea. (2.) Uric Acid (C5H4N4O3).-Uric or lithic acid is rarely absent from the urine of man or animals, though in the feline tribe it seems to be sometimes entirely replaced by urea. Properties.-Uric acid when pure is colourless, but when deposited from the urine is yellowish-brown. It crystallises in various forms (fig. 254). It is odourless and tasteless. It is slightly soluble in cold water, and a little more so in hot water, quite insoluble in alcohol and ether. It dissolves freely in solution of the alkaline carbonates and other salts. The proportionate quantity of uric acid varies considerably in different animals. In man, and Mammalia generally, especially the Herbivora, it is comparatively small. In the whole tribe of birds, and of serpents, on the other hand, the quantity is very large, greatly exceeding that of the urea. In the urine of granivorous birds, indeed, urea is rarely if ever found, its place being entirely supplied by uric acid. Variations in Quantity.-The quantity of uric acid, like that of urea, in human urine, is increased by the use of animal food, and decreased by the use of food free from nitrogen, or by an exclusively vegetable diet. In most febrile diseases, and in plethora, it is formed in unnaturally large quantities; and in gout it is deposited in and around joints, in the form of urate chap, xii.] PROPERTIES OF URIC ACID. 419 of soda, of which the so-called chalk-stones of this disease are principally composed. The average amount secreted in twenty- four hours is 8'5 grains (rather more than half a gramme). Condition in the Urine.-The condition in which uric acid exists in solution in the urine has formed the subject of some discussion, because of its difficult solubility in water. The uric acid exists as urate of soda, produced by the uric acid as soon as it is formed combining with part of the base of the alkaline sodium phosphate of the blood. Hippuric acid, which exists in human urine also, acts upon the alkaline phosphate in the same way, and increases still more the quantity of acid phosphate, on the presence of which it is probable that a part of the natu- ral acidity of the urine depends. It is scarcely possible to say whether the union of uric acid with the base sodium, and pro- bably ammonium, takes place in the blood, or in the act of secre- tion in the kidney : the latter is more likely; but the quantity of either uric acid or urates in the blood is probably too small to allow of this question being solved. Owing to its existence in combination in healthy urine, uric acid for examination must generally be precipitated from its bases by a stronger acid. Frequently, however, when excreted in excess, it is deposited in a crystalline form (fig. 254), mixed with large quantities of ammonium or sodium urate. In such cases it may be procured for microscopic examination by gently warming the portion of urine containing the sediment; this dissolves urate of ammonium and sodium, while the comparatively insoluble crystals of uric acid subside to the bottom. The most common form in which uric acid is deposited in urine, is that of a brownish or yellowish powdery substance, consisting of granules of ammonium or sodium urate. When deposited in crystals, it is most frequently in rhombic or diamond-shaped laminae, but other forms are not uncommon (fig. 254). When deposited from urine, the crystals are generally more or less Fig. 254.- Various forms of uric acid crystals. 420 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xii. deeply coloured, from being combined with the colouring prin- ciples of the urine. Tests.-There are two chief tests for uric acid besides the micro- scopic evidence of its crystalline structure : (i) The Murexide test, which consists of evaporating to dryness a mixture of strong nitric acid and uric acid in a water bath. This leaves a yellowish-red residue of Alloxan (C4H.,N204) and urea, and on addition of ammo- nium hydrate, a beautiful purple colour (ammonium purpurate, C8H4 (NH4) N5O6), deepened on addition of caustic potash, takes place. (2) Schiff's test consists of dissolving the uric acid in sodium carbonate solution, and of drop- ping some of it on a filter paper moistened with silver nitrate. A black spot appears, which corre- sponds to the reduction of silver by the uric acid. (3) Hippurie Acid (C9H9NO3) has long been known to exist in the urine of herbivorous ani- mals in combination with soda. It also exists naturally in the urine of man, in a quantity equal or rather exceeding that of the uric acid. The quantity of hippurie acid excreted is increased by a vegetable diet. It appears to be formed in the body from benzoic acid or from some allied substance. The benzoic acid unites with glycin, probably in the kidneys, and hippurie acid and water are formed thus, C7H5O2 (Benzoic acid) + C2HSNO2 (Glycin) = C9H9NO3 (Hippurie acid) + H20 (water). It may be decomposed by acids into benzoic acid and glycin. Properties.-It is a colourless and odourless substance of bitter taste, crystallises in semi-transparent rhombic prisms (fig. 255). It is more soluble in cold water than uric acid, and much more soluble in hot water. It is soluble in alcohol. (4) Pigments.-The pigments of the urine are the following :- 1. Urochrome, a yellow colouring matter, giving no absorption band; of which but little is known. Urine owes its yellow colour mainly to the presence of this body. 2. Urobilin, an orange pig- ment, of which traces may be found in nearly all urines, and which is especially abundant in the urines passed by febrile patients. It fig. 255.-Crystals of hippuric acid. CHAP. XII.] THE PIGMENTS OF URINE. 421 is characterised by a well-marked spectroscopic absorption band at the junction of green and blue, best seen in acid solutions ; and by giving a green fluorescence when excess of ammonia with a little chloride of zinc is added to it. The very vexed question of the relation of the pigments of urine to bile pigments turns largely upon the spectroscopic appearances of urobilin; for orange- coloured solutions having the same absorption band as urobilin may be prepared from bile pigments in two different ways-i, by reduction with sodium amalgam-Hydrobilirubin (Maly); ii, by oxidation with nitric acid-Choletelin (Jaffe), and both these bile derivatives give .a fluorescence with ammonia and a drop of chloride of zinc. It is not satisfactorily settled which of these, if either, is the same as urobilin of urine. It is worth noting that choletelin may be oxidised a stage further; it then loses its absorption band, remaining however of a yellow colour. It is very possible that the urochrome of normal urine may be this oxidised choletelin, and that the presence of the absorption band of urobilin in urines may mean that some of the pigment is in the stage of choletelin ; i.e., that its oxidation is not quite completed. Those who believe urobilin to be identical with hydrobilirubin suppose that the bilirubin is reduced by the putrefactive processes in the intestines, and is conveyed in its reduced form by the blood stream to the kidneys. 3. Uro-erythrin is the pigment which is found in the pink deposits of urates which are sometimes seen in urines; it com- municates a rich red-orange colour to urine when in solution, and its solutions have two broad faint absorption bands in the green. 4. Uromelanin. When urine is boiled with strong acids it darkens to a reddish brown colour. This change, once ascribed to the formation of a new pigment uromelanin, is now believed to be due to the presence in urine of pyrocatechin and allied bodies which are capable of taking up oxygen when boiled with acids, yielding C02 and brown or black residual products. 5. Indigo is found rarely in urines, to which it may communi- cate a blue or green colour. Urine frequently contains a com- pound which is either a glucoside, Indican; or more probably a salt of indoxyl-sulphuric acid. It yields indigo blue when treated with strong hydrochloric acid and left to stand for some hours exposed to the air; the indigo may be separated by treatment with boiling chloroform, which takes it up, forming a blue solution. There is a similar compound of skatol and sulphuric acid which 422 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xii. is sometimes i'ecognised in the urine, by the production of a red colour when nitric acid is added to it. Many medicinal substances colour the urine, for instance: Rhubarb, Santonin, Senna, Fuchsine, Carbolic Acid. Bromides and Iodides yield Bromine or Iodine, when nitric acid is added to the urine of patients taking these drugs. In the case of iodides the liberated iodine communicates a strong mahogany colour to the urine thus treated. (5) Mucus.-Mucus in the urine consists principally of the epithelial debris from the mucous surface of the urinary passages. Particles of epithelium, in greater or less abundance, may be de- tected in most samples of urine, especially if it has remained at rest for some time, and the lower strata are then examined (fig. 256). As urine cools, the mucus is sometimes seen suspended in it as a delicate opaque cloud, but generally it falls. In inflamma- tory affections of the urinary pas- sages, especially of the bladder, mucus in large quantities is poured forth, and speedily undergoes decomposition. The presence of the decomposing mucus excites chemical changes in the urea, whereby carbonate of ammonium is formed, which, combining with the excess of acid in the super- phosphates in the urine, produces insoluble neutral or alkaline phosphates of calcium and magnesium, and phosphate of ammo- nium and magnesium. These mixing with the mucus, constitute the peculiar white, viscid, mortar-like substance which collects upon the mucous surface of the bladder, and is often passed with the urine, forming a thick tenacious sediment. (6) Extractives.-In addition to those already considered, urine contains a considerable number of nitrogenous compounds. These are usually described under the generic name of extractives. Of these, the chief are : (1) Kreatinin (C4H7N3O), a substance derived, probably, from the metamorphosis of muscular tissue, crystallizing in colourless oblique rhombic prisms ; a fairly definite amount of this substance, about 15 grains (1 grm.), appears in the urine daily, so that it must be looked upon as a normal con- Fig. 256.-deposited from urine. CHAP. XII.] SALINE MATTERS OF THE URINE. 423 stituent; it is increased on an increase of the nitrogenous consti- tuents of the food ; (2) Xanthin (C5N4H4O2), an amorphous powder soluble in hot water ; (3) Hypo-xanthin, or sarkin (C5N4H4O); (4) Oxaluric acid (C3H4N2O4), in combination with ammonium in the urine of the new-born child : (5) Allantoin (C4H6N4O3). All these extractives are chiefly interesting as being closely connected with urea, and mostly yielding that substance on oxidation. Leucin and tyrosin can scarcely be looked upon as normal con- stituents of urine. (7) Saline Matter.-(a) The Sulphuric acid in the urine is combined chiefly or entirely with sodium or potassium ; forming- salts which are taken in very small quantity with the food, and are scarcely found in other fluids or tissues of the body; for the sulphates commonly enumerated among the constituents of the ashes of the tissues and fluids are for the most part, or entirely, produced by the changes that take place in the burning. Only about one-third of the sulphuric acid found in the urine is derived directly from the food (Parkes). Hence the greater part of the sulphuric acid which the sulphates in the urine contain, must be formed in the blood, or in the act of secretion of urine ; the sulphur of which the acid is formed being probably derived from the decomposing nitrogenous tissues, the other elements of which are resolved into urea and uric acid. It may be in part derived also from the sulphur-holding taurin and cystin, which can be found in the liver, lungs, and other parts of the body, but not generally in the excretions ; and which, therefore, must be broken up. The oxygen is supplied through the lungs, and the heat generated during combination with the sulphur, is one of the sub- ordinate means by which the animal temperature is maintained. Besides the sulphur in these salts, some also appears to be in the urine, uncombined with oxygen; for after all the sulphates have been removed from urine, sulphuric acid may be formed by drying and burning it with nitre. From three to five grains of sulphur are thus daily excreted. The combination in which it exists is uncertain: possibly it is in some compound analogous to cystin or cystic oxide (p. 258). Sulphuric acid also exists normally in the urine in combination with phenol (C6H6O) as phenol sulphuric acid or its corresponding salts, with sodium, Ac. (b) The phosphoric acid in the urine is combined partly with the alkalies, partly with the alkaline earths-about four or five times as much with the former as with the latter. In blood, 424 STRUCTURE AND FUNCTION OF KIDNEYS. [CHAP. XII. saliva, and other alkaline fluids of the body, phosphates exist in the form of alkaline, neutral, or acid salts. In the urine they are acid salts, viz., the sodium, ammonium, calcium, and magnesium phosphates, the excess of acid being (Liebig) due to the appropria- tion of the alkali with which the phosphoric acid in the blood is combined, by the several new acids which are formed or discharged at the kidneys, namely, the uric, hippuric, and sulphuric acids, all of which are neutralised with soda. The phosphates are taken largely in both vegetable and animal food; some thus taken are ex- creted at once; others, after being transformed and incorporated with the tissues. Calcium phos- phate forms the principal earthy constituent of bone, and from the decomposition of the osseous tissue the urine derives a large quantity of this salt. The de- composition of other tissues also, but especially of the brain and nerve-substance, furnishes large supplies of phosphorus to the urine, which phosphorus is sup- posed, like the sulphur, to be united with oxygen, and then com- bined with bases. This quantity is, however, liable to considerable variation. Any undue exercise of the brain, and all circumstances producing nervous exhaustion, increase it. The earthy phosphates are more abundant after meals, whether on animal or vegetable food, and are diminished after long fasting. The alkaline phos- phates are increased after animal food, diminished after vegetable food. Exercise increases the alkaline, but not the earthy phos- phates. Phosphorus uncombined with oxygen appears, like sulphur, to be excreted in the urine. When the urine undergoes alkaline fermentation, phosphates are deposited in the form of a urinary sediment, consisting chiefly of ammonio-magnesium phos- phates (triple phosphate) (fig. 257). This compound does not, as such, exist in healthy urine. The ammonia is chiefly or wholly derived from the decomposition of urea. (c) The Chlorine of the urine occurs chiefly in combination with sodium (next to urea, sodium chloride is the most abundant Fig. 257.- Urinary sediment of triple phos- phates (large prismatic crystals) and urate of ammonium, from urine which had undergone alkaline fermentation. CHAP. XII.] OCCASIONAL CONSTITUENTS OF THE URINE. 425 solid constituent of the urine), but slightly also with ammonium, and, perhaps, potassium. As the chlorides exist largely in food, and in most of the animal fluids, their occurrence in the urine is easily understood. (8) Occasional Constituents.-Cystin (C3H7N SO2) (fig. 258) is an occasional constituent of urine. It resembles taurin in containing a large quantity of sulphur- more than 25 per cent. It does not exist in healthy urine. Another common morbid con- stituent of the urine is Oxalic acid, which is frequently deposited in combination with calcium (fig. 259) as a urinary sediment. Like cystin, but much more commonly, it is the chief constituent of cer- tain calculi. Of the other abnormal consti- tuents of the urine which were mentioned on p. 415, it will be unnecessary to speak at length in this work. (9) Gases.-A small quantity of gas is naturally present in the urine in a state of solution. It consists of carbonic acid (chiefly) and nitrogen and a small quantity of oxygen. Fig. 258.-Crystals of Cystin. The Method of the Excre- tion of Urine. The excretion of the urine by the kidney is believed to consist of two, more or less distinct pro- cesses-viz., (i.) of Filtration, by which the water and the ready- formed salts are eliminated ; and, (2.) of True Secretion, by which certain substances forming the chief and more important part of the urinary solids are removed from the blood. This division of function corresponds more or less to the division in the functions of other glands of which we have already treated. It will be as well to consider them separately. (1.) Of Filtration.-This part of the renal function is per- Fig. 259.- Crystals of Calcium Oxalate. 426 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xii. formed within the Malpighian corpuscles by the renal glomeruli. By it not only the water is strained off' but also certain other constituents of the urine, e.g., sodium chloride, are separated. The amount of the fluid filtered off depends almost entirely upon the blood-pressure in the glomeruli. The greater the blood-pressure in the arterial system generally, and consequently in the renal arteries, the greater, cceteris paribus, will be the blood-pressure in the glomeruli, and the greater the quantity of urine separated; but even without increase of the general blood-pressure, if the renal arteries be locally dilated, the pressure in the glomeruli will be increased and with it the secre- tion of urine. All the numerous causes therefore, which increase the general blood-pressure (p. 172) will, as a rule, secondarily increase the secretion of urine. Of these- (1.) The heart's action is amongst the most important. When the cardiac contractions are increased in force, increased diuresis is the result. (2.) Since the connection between the general blood-pressure and the nervous system is so close it will be evident that the amount of urine secreted depends greatly upon the influence of the latter. This may be demonstrated experimentally. Thus, division of the spinal cord, by producing general vascular dilatation, causes a great diminution of blood-pressure, and so diminishes the amount of water passed; since the local dilatation in the renal arteries is not sufficient to counteract the general diminution of pressure. Stimulation of the cut cord produces, strangely enough, the same results-i.e., a diminution in the amount of the urine passed, but in a different way, viz., by constricting the arteries generally, and, among others, the renal arteries; the diminution of blood-pressure resulting from the local re- sistance in the renal arteries being more potent to diminish blood-pressure in the glomeruli than the general increase of blood-pressure is to increase it. Section of the renal nerves or of any others which produce local dilatation without greatly dimi- nishing the general blood-pressure will cause an increase in the quantity of fluid passed. (3.) The fact that in summer or in hot weather the urine is diminished may be attributed partly to the copious elimination of water by the skin in the form of sweat which occurs in summer, as contrasted with the greatly diminished functional activity of the skin in winter, but also to the dilated condition of the vessels of the skin causing a decrease in the general blood- CHAP. XII.] BLOOD-PRESSURE IN THE KIDNEY. 427 pressure. Thus we see that in regard to the elimination of water from the system, the skin and kidneys perform similar functions, and are capable to some extent of acting vicariously, one for the other. Their relative activities are inversely proportional to each other. The intimate connection between the condition of the kidney and the blood-pres- sure has been ex- ceedingly well shown by means of an instrument called the Oncometer, recently introduced by Roy, which is a modifica- tion of the plethys- mograph (fig. 260). By means of this ap- paratus any alteration in the volume of the kidney is communi- cated to an apparatus (oncograph) capable of recording graphi- cally, with a writing lever, such variations. It has been found that the kidney is ex- tremely sensitive to any alteration in the general blood-pres- sure, every fall in the general blood-pres- sure being accom- panied by a decrease in the volume of the kidney, and every rise, unless produced by considerable constriction of the peripheral vessels, including those of the kidney, being accompanied by a corresponding increase of volume. Increase of volume is followed by an increase in the amount of urine secreted, and decrease of volume by a decrease in the secretion. In addition, however, to the response of the kidney to alterations in the general blood- pressure, it has been further observed that certain substances, when injected into the blood, will also produce an increase in volume of Fig. 260.-Diagram of Jtoy's Oncometer, a, represents the kidney enclosed in a metal box which opens by hinge f; V, the renal vessels and duct. Surrounding the kidney are two chambers formed by membranes, the edges of which are firmly fixed by being clamped between the outside metal capsule, and one (not represented in the figure) inside, the two being firmly screwed together by screws at A, and below. The membranous chamber below is filled with a varying amount of warm oil, according to the size of the kidney experimented with, through the opening then closed with the plug i. After the kidney has been enclosed in the capsule, the mem- branous chamber above is filled with warm oil through the tube e, which is then closed by a tap (not repre- sented in the diagram); the tube d communicates with a recording apparatus, and any alteration in the volume of the kidney is communicated by the oil in the tube to the chamber d of the Oncograph, fig. 261. 428 STRUCTURE AND FUNCTION OF KIDNEYS. [chap. xii. the kidney, and consequent increased flow of urine, without affect- ing the general blood-pressure-such bodies as sodium acetate and other diuretics. These observations appear to prove that local dilatation of the renal vessels may be produced by alterations in the blood acting upon a local nervous mechanism, as this happens when all of the renal nerves have been divided. The alterations are not only produced by the addition of drugs, but also by the introduction of comparatively small quan- tities of water or saline solution. To this alteration of the blood acting upon the renal vessels (either directly or) through a local vaso- motor mechanism, and not to any great alteration in the general blood-pressure, must we attribute theeffects of meals, &c., observed by Roberts. " The renal ex- cretion is increased after meals and diminished during fasting and sleep. The increase began within the first hour after break- fast, and continued during the succeeding two or three hours; then a diminution set in, and continued until an hour or two after dinner. The effect of dinner did not appeal' until two or three hours after the meal; and it reached its maximum about the fourth hour. From this period the excretion steadily decreased until bed-time. During sleep it sank still lower, and reached its minimum-being not more than one-third of the quantity excreted during the hours of digestion." The increased amount of urine passed after drinking large quantities of fluid probably depends upon the diluted condition of the blood thereby induced. The following table * will help to explain the dependence of the filtration function upon the blood-pressure and the nervous system :- Fig. 261.-Hoy's Oncograph, or apparatus for re- cording alterations in the volume of the kid- ney, &c., as shown by the oncometer-a, up- right, supporting recording lever I, which is raised or lowered by needle h, which works through /, and which is attached to the piston e, working in the chamber d, with which the tube from the oncometer communicates. The oil is prevented from being squeezed out as the piston descends by a membrane, which is clamped between the ring-shaped surfaces of cylinder by the screw i working upwards; the tube h is for filling the instrument. * Modified from M. Foster. CHAP. XII.] VARIATIONS IN THE AMOUNT OF URINE. 429 TABLE OF THE RELATION OF THE SECRETION OF URINE TO ARTERIAL PRESSURE. A. Secretion of urine may be increased- a. Uy increasing the general blood-pressure ; by 1. Increase of the force or frequency of heart-beat. 2. Constriction of the small arteries of areas other than that of the kidney. b. By increasing the local blood-pressure, by relaxation of the renal artery, with out compensating relaxation elsewhere ; by 1. Division of the renal nerves (causing polyuria). 2. Division of the renal nerves and stimulation of the cord, below the medulla (causing greater polyuria). 3. Division of the splanchnic nerves; but the polyuria pro- duced is less than in 1 or 2, as these nerves are dis- tributed to a wider area, and the dilatation of the renal artery is accompanied by dilatation of other vessels, and therefore with a somewhat diminished general blood supply. 4. Puncture of the floor of fourth ventricle or mechanical irritation of the superior cervical ganglion of the sym- pathetic, possibly from the production of dilatation of the renal arteries. B. Secretion of urine may be diminished- a. By diminishing the general blood-pressure ; by 1. Diminution of the force or frequency of the heart-beats. 2. Dilatation of capillary areas other than that of the kidney. 3. Division of spinal cord below the medulla, which causes dilatation of general abdominal area, and urine gene- rally ceases being secreted. b. By increasing the blood-pressure, by stimulation of the spinal cord below the medulla, the constriction of the renal artery, which follows not being compensated for by the increase of general blood-pressure. c. By constriction of tlve renal artery, by stimulating the renal or splanchnic nerves, or the spinal cord. Fig. 262.-Curve taken hy renal oncometer compressed-with that of ordinary Hood-pressure. a, Kidney curve ; ft, blood-pressure curve. (Roy.) Although it is convenient to call the processes which go on in the renal glomeruli, filtration, there is reason to believe that they are not absolutely mechanical, as the term might seem to imply, 430 STRUCTURE AND FUNCTION OF KIDNEYS. [chap. xii. since, when the epithelium of the Malpighian capsule has been, as it were, put out of order by ligature of the renal artery, on removal of the ligature, the urine has been found temporarily to contain albumen, indicating that a selective power resides in the healthy epithelium, which allows certain constituent parts of the blood to be filtered off", and not others. (2.) Of True Secretion.-That there is a second part in the process of the excretion of urine, which is true secretion, is suggested by the structure of the tubuli uriniferi, and the idea is supported by various experiments. It will be remembered that the convoluted portions of the tubules are lined with an epithe- lium, which bears a close resemblance to the secretory epithelium of other glands, whereas the Malpighian capsules and portions of the loops of Henle are lined simply by endothelium. The two functions are, then, suggested by the differences of epithelium, and also by the fact that the blood supply is different, since the convoluted tubes are surrounded by capillary vessels derived from the breaking up of the efferent vessels of the Malpighian tufts. The theory first suggested by Bowman (1842), and still generally accepted, of the function of the two parts of the tubules, is that the cells of the convoluted tubes, by a process of true secretion, separate from the blood substances such as urea, whereas from the glomeruli is separated the water and the inorganic salts. Another theory suggested by Ludwig (1844) is that in the glomeruli is filtered off from the blood all the con- stituents of the urine in a very diluted condition. When this passes along the tortuous uriniferous tube, part of the water is re-absorbed into the vessels surrounding them, leaving .the urine in a more concentrated condition-retaining all its proper con- stituents. This osmosis is promoted by the high specific gravity of the blood in the capillaries surrounding the convoluted tubes, but the return of the urea and similar substances is prevented by the secretory epithelium of the tubules. Ludwig's theory, how-ever plausible, must, we think, give way to the first theory, which is more strongly supported by direct experiment. By using the kidney of the newt, which has two distinct vas- cular supplies, one from the renal artery to the glomeruli, and the other from the renal-portal vein to the convoluted tubes, Nuss- baum has shown that certain substances, e.g., peptones and sugar, when injected into the blood, are eliminated by the glomeruli, and so are not got rid of w hen the renal arteries are tied; whereas certain other substances, e.<?., urea, when injected into the blood, CHAP. XII.] RELATION BETWEEN FILTRATION A SECRETION. 431 are eliminated by the convoluted tubes, even when the renal arteries have been tied. This evidence is very direct that urea is excreted by the convoluted tubes. Heidenhain also has shown by experiment that if a substance (sodium sulphindigotate), which ordinarily produces blue urine, be injected into the blood after section of the medulla which causes lowering of the blood-pressure in the renal glomeruli, that when the kidney is examined, the cells of the convoluted tubules (and of these alone) are stained with the substance, which is also found in the lumen of the tubules. This appears to show that under ordinary circumstances the pigment at any rate is elimi- nated by the cells of the convoluted tubules, and that when by diminishing the blood-pressure, the filtration of urine ceases, the pigment remains in the convoluted tubes, and is not, as it is under ordinary circumstances swept away from them by the flushing of them which ordinarily takes place with the watery part of urine derived from the glomeruli. It therefore is probable that the cells, if they excrete the pigment, excrete urea and other substances also. But urea acts somewhat differently to the pig- ment, as when it is injected into the blood of an animal in which the medulla has been divided, and the secretion of urine stopped, a copious secretion of urine results, which is not the case when the pigment is used instead under similar conditions. The flow of urine, independent of the general blood-pressure, might be supposed to be due to the action of the altered blood upon some local vaso-motor mechanism • and, indeed, the local blood- pressure is directly affected in this way, but there is reason for believing that part of the increase of the secretion is due to the direct stimulation of the cells by the urea contained in the blood. To sum up, then, the relation of the tw'O functions : (i.) The process of filtration, by which the chief part, if not the whole, of the fluid is eliminated, together with certain inorganic salts, and possibly other solids, is directly dependent upon blood- pressure, is accomplished by the renal glomeruli, and is accom- panied by a free discharge of solids from the tubules. (2.) The process of secretion proper, by which urea and the principal urinary solids are eliminated, is only indirectly, if at all, depen- dent upon blood-pressure, is accomplished by the cells of the con- voluted tubes, and is sometimes (as in the case of the elimination of urea and similar substances) accompanied by the elimination of copious fluid, produced by the chemical stimulation of the epithelium of the same tubules. 432 STRUCTURE AND FUNCTION OF KIDNEYS, [chap. xii. Sources of the Nitrogenous Urinary Solids. Urea.-In speaking of the method of the secretion of urine, it was assumed that the part played by the cells of the uriniferous tubules was that of mere separation of the constituents of the urine which existed ready-formed in the blood : there is consider- able evidence to favour this assumption. What may be called the specially characteristic solid of the urine, i.e., urea (as well as most of the other solids), may be detected in the blood, and in other parts of the body, e.g., the humours of the eye, even while the functions of the kidneys are unimpaired : but when from any cause, especially extensive disease or extirpation of the kidneys, the separation of urine is imperfect, the urea is found largely in the blood, and in most other fluids of the body. It must, therefore, be clear that the urea is for the most part made somewhere else than in the kidneys, and simply brought to them by the blood for elimination. It is not abso- lutely proved however, that all the urea is formed away from these organs,rand it is possible that a small quantity is actually secreted by the cells of the tubules. The sources of the urea, which is brought to the kidneys for excretion, may be stated to be the two. (i.) From the, splitting up the Elementsof the, Nitrogenous Food.- The origin of urea from this source is shown by the increase which ensues on substituting an animal or highly nitrogenous for a vegetable diet; in the much larger amount-nearly double -excreted by Carnivora than Herbivora, independent of exercise ; and in its diminution to about one-half during starvation, or during the exclusion of nitrogenous principles of food. Part, at any rate, of the increased amount of urea which appears in the urine soon after a full meal of proteid material may be attributed to the production of a considerable amount of leucin and tyrosin by the pancreatic digestion. These substances are carried by the portal vein to the liver, and it is there that the change in all probability takes place, as when the functions of the organ are gravely interfered with, as in the case of acute yellow atrophy, the amount of urea is distinctly diminished, and its place appears to be taken by leucin and tyrosin. It has been found by experi- ment, too, that if these substances be introduced into the alimen- tary canal, the introduction is followed by a corresponding increase CHAP. XII.] SOURCES OF UREA. 433 in the amount of urea, but not by the presence of the bodies themselves in the urine. (2.) From the Nitrogenous metabolism of the tissues.-This second source of urea is shown by the fact that that body continues to be excreted, though in smaller quantity than usual, when all nitrogenous substances are strictly excluded from the food, as, for example, when the diet consists for several days of sugar, starch, gum, oil, and similar non-nitrogenous substances. It is excreted also, even though no food at all is taken for a considerable time; thus it is found in the urine of reptiles which have fasted for months; and in the urine of a madman, who had fasted eighteen days, Lassaigne found both urea and all the components of healthy urine. Turning to the muscles, however, as the most actively meta- bolic tissue, we find as a result of their activity not urea, but Kreatin; and although it maybe supposed that some of this latter body appears naturally in the urine as Kreatinin, or hydrated Kreatin, yet it is not in sufficient quantity to represent the large amount of it formed by the muscles, and, indeed, by others of the tissues. It is assumed that kreatin therefore is the nitrogenous antecedent of urea; where its conversion into urea takes place is doubtful, but very likely the liver, and possibly the spleen, may be the seat of the change. It is possible, however, that part-but if so, a small part-reaches the kidneys without previous change, leaving it to the cells of the renal tubules to complete the action. In speaking of kreatin as the antecedent of urea, it should be recollected that other nitrogenous products, such as xanthin (C5 H4 N4 02), appear in conjunction with it, and that these may also be converted into urea. It was formerly taken for granted that the quantity of urea in the urine is greatly increased by active exercise; but numerous observers have failed to detect more than a slight increase under such circumstances; and our notions concerning the relation of this excretory product to the destruction of muscular fibre, con- sequent on the exercise of the latter, have undergone considerable modification. There is no doubt, of course, that like all parts of the body, the muscles have but a limited term of existence, and are being constantly although very slowly renewed, at the same time that a part of the products of their disintegration appears in the urine in the form of urea. But the waste is not so fast as it was formerly supposed to be; and the theory that the amount of 434 STRUCTURE AND FUNCTION OF KIDNEYS, [chap, xii- work done by the muscles is expressed by the quantity of urea excreted in the urine must without doubt be given up. Uric Acid.-Uric acid probably arises much in the same way as urea, either from the disintegration of albuminous tissues, or from the food. The relation which uric acid and urea bear to each other is, however, still obscure : but uric acid is said to be a less advanced stage of the oxidation of the products of proteid metabolism. The fact that they often exist together in the same urine, makes it seem probable that they have different origins; but the entire replacement of either by the other, as of urea by uric acid in the urine of birds, serpents, and many insects, and of uric acid by urea, in the urine of the feline tribe of Mammalia, shows that either alone may take the place of the two. At any rate, although it is true that one molecule of uric acid is capable of splitting up into two molecules of urea and one of mes-oxalic acid, there is no evidence for believing that uric acid is an ante- cedent of urea in the nitrogenous metabolism of the body. Some experiments seem to show that uric acid is formed, at any rate in part, in the kidney. Hippuric Acid (C9 H9 NO3).-The source of hippuric acid is not satisfactorily determined : in part it is probably derived from some constituents of vegetable diet, though man has no hippuric acid in his food, nor, commonly, any benzoic acid that might be converted into it; in part from the natural disintegration of tissues, independent of vegetable food, for Weismann constantly found an appreciable quantity, even when living on an exclusively animal diet. Hippuric acid arises from the union of benzoic acid with glycin (C2HsN02 + C7H602=C9H9N03 + H20), which union may take place in the kidneys themselves, as well as in the liver. Extractives.-The source of the extractives of the urine is probably in chief part the disintegration of the nitrogenous tissues, but we are unable to say whether these nitrogenous bodies are merely accidental, having resisted further decomposition into urea, oi- whether they are the representatives of the decomposition of special tissues, or of special forms of metabolism of the tissues. There is, however, one exception, and this is in the case of kreatinin; there is great reason for believing that the amount of this body which appears in the urine is derived from the meta- bolism of the nitrogenous food, as when this is diminished, it diminishes, and when stopped, it no longer appears in the urine. CHAP. XII.] MICTURITION. 435 The Passage of Urine into the Bladder. As each portion of urine is secreted it propels that which is already in the uriniferous tubes onwards into the pelvis of the kidney. Thence through the ureter the urine passes into the bladder, into which its rate and mode of entrance has been watched in cases of ectopia vesica., i.e., of such fissures in the ante- rior or lower part of the walls of the abdomen, and of the front wall of the bladder, as expose to view its hinder wall together with the orifices of the ureters. The urine does not enter the bladder at any regular rate, nor is there a synchronism in its movement through the two ureters. During fasting, two or three drops enter the bladder every minute, each drop as it enters first raising up the little papilla on which, in these cases, the ureter opens, and then passing slowly through its orifice, which at once again closes like a sphincter. In the recumbent posture, the urine collects for a little time in the ureters, then flows gently, and, if the body be raised, runs from them in a stream till they are empty. Its flow is increased in deep inspiration, or straining, and in active exercise, and in fifteen or twenty minutes after a meal. The urine collecting is prevented from regurgitation into the ureters by the mode in which these pass through the walls of the bladder, namely, by their lying for between half and three-quarters of an inch between the muscular and mucous coats before they turn rather abruptly forwards, and open through the latter into the interior of the bladder. Micturition.-The contraction of the muscular walls of the bladder may by itself expel the urine with little or no help from other muscles. In so far, however, as it is a voluntary act, it is performed by means of the abdominal and other expiratory muscles, which in their contraction, as before explained, press on the abdominal viscera, the diaphragm being fixed, and cause the expulsion of the contents of those whose sphincter muscles are at the same time relaxed. The muscular coat of the bladder co-operates, in micturition, by reflex involuntary action, with the abdominal muscles; and the act is completed by the accelerator urince, which, as its name implies, quickens the stream, and expels the last drops of urine from the urethra. The act, so far as it is not directed by volition, is under the control of a nervous centre in the lumbar spinal cord, through which, as in the case of 436 THE VASCULAR GLANDS. [chap. xin. the similar centre for defecation, the various muscles concerned are harmonized in their action. It is well known that the act may be reflexly induced, e.g., in children who suffer from intestinal worms, or other such irritation. Generally the afferent impulse which calls into action the desire to micturate is excited by over distension of the bladder, or even by a few drops of urine passing into the urethra. CHAPTER XIII. THE VASCULAR GLANDS. In addition to the various glands the structure and functions of which have been considered in the preceding chapters, and which have been shown either to secrete from the blood materials of use in digestion or to excrete from the blood materials of no further use in the economy, there are others which have not to do with secretion or excretion, at all events directly. These are called Vascular glands, and comprise the Spleen, the Thymus gland, the Tonsils, and the Solitary and Agminated glands of Peyer in the intestine, all of which are made up chiefly of lymphatic tissue, resembling lymphatic glands, and which are evidently closely connected with the lymphatic system; the Supra-renal capsules or Adrenals ; the Thyroid gland ; the Pineal and Pituitary glands and the Parotid and Coccygeal glands. The Spleen. The spleen is the largest of these so-called vascular glands; it is situated to the left of the stomach, between it and the diaphragm. It is of a deep red colour, of a variable shape, generally oval, somewhat concavo-convex. Vessels enter and leave the gland at the inner side or hilus. Structure.-The spleen is covered externally almost completely by a serous coat derived from the peritoneum, while within this is the proper fibrous coat or capsule of the organ. The latter, composed of connective tissue, with a large preponderance of chap, xiii.] STRUCTURE OF THE SPLEEN. 437 elastic fibres, and a certain proportion of unstriated muscular tissue, forms the immediate investment of the spleen. Prolonged from its inner surface are fibrous processes or trabeculce, containing much unstriated muscle, which enter the interior of the organ, and, dividing and anastomosing in all parts, form a kind of sup* Fig'. 263.-Section of dog's spleen injected", e, capsule; tr, trabecula?; m, two Malpighian bodies with numerous small arteries and capillaries ; a, artery; ?, lymphoid tissue, consisting of closely-packed lymphoid cells supported by very delicate retiform tissue; a light space unoccupied by cells is seen all round the trabecula;, which corresponds to the " lymph path " in lymphatic glands. (Schofield.) porting frame-work or s£ro?na, in the interstices of which the proper substance of the spleen (spleen-pulp) is contained (fig. 264). At tl ic hilus of the spleen, the blood-vessels, nerves, and lymphatics enter, and the fibrous coat is prolonged into the spleen-substance in the form of investing sheaths for the arteries and veins, which sheaths again are continuous with the trabecula; before referred to. The spleen-pulp, which is of a dark red or reddish-brown colour, is composed chiefly of cells, imbedded in a matrix of fibres formed 438 THE VASCULAR GLANDS. [CHAP. XIII. of the branchings of large flattened nucleated endotheloid cells. The spaces of the network only partially occupied by cells form a freely communicating system. Of the cells some are granular corpuscles resembling the lymph-corpuscles, more or less connected with the cells of the meshwork, both in general appearance and in being able to perform amoeboid move- ments ; others are red blood-corpuscles of normal appearance or variously changed ; while there are also large cells containing either a pigment allied to the colouring matter of the blood, or rounded corpuscles like red blood-corpuscles. The splenic artery, after entering the spleen by its concave surface, divides and subdivides, with but little anastomosis between its branches ; at the same time its branches are sheathed by the prolonga- tions of the fibrous coat, which they, so to speak, carry into the spleen with them. The arteries send off branches into the spleen-pulp which end in capillaries, and these either communicate, as in other parts of the body, with the radicles of the veins, or end in lacunar spaces in the spleen-pulp, from which veins arise. The walls of the smaller veins are more or less incomplete, and readily allow lymphoid corpuscles to be swept into the blood- current. The blood from the arterial capillaries is emptied into a system of intermediate passages, which are directly bounded by the cells and fibres of the network of the pulp, and from which the smallest venous radicles with their cribriform walls take origin (Frey). The veins are large and very distensible: the whole tissue of the spleen is highly vascular, and becomes readily engorged with blood : the amount of distension is, how- ever, limited by the fibrous and muscular tissue of its capsule and trabeculae, which forms an investment and support for the pulpy mass within. On the face of a section of the spleen can be usually seen readily with the naked eye, minute, scattered rounded or oval whitish spots, mostly from -jL to inch in diameter. These are the Malpighian corpuscles of the spleen, and are situated on the sheaths of the minute splenic arteries, of which, indeed, they may be said to be outgrowths (fig. 263). For while the sheaths of Fig. 264.-Reticulum of the spleen of a Cat, shewn by injection with gela- tine and silver nitrate. (Cadiat.) chap, xirr.] FUNCTIONS OF THE SPLEEN. 439 the larger arteries are constructed of ordinary connective tissue, this has become modified where it forms an investment for the smaller vessels, so as to be composed of adenoid tissue, with abundance of corpuscles, like lymph-corpuscles, contained in its meshes, and the Malpighian corpuscles are but small outgrowths of this cytogenous or cell-bearing connective tissue. They are composed of cylindrical masses of corpuscles, intersected in all parts by a delicate fibrillar tissue, which, though it invests the Malpighian bodies, docs not form a complete capsule. Blood- capillaries traverse the Malpighian corpuscles and form a plexus in their interior. The structure of a Malpighian corpuscle of the spleen is, therefore, very similar to that of lymphatic-gland substance. Functions.-With respect to the office of the spleen, we have the following data, (i.) The large size which it gradually acquires towards the termination of the digestive process, and the great increase observed about this period in the amount of the finely-granular albuminous plasma within its parenchyma, and the subsequent gradual decrease of this material, seem to indicate that this organ is concerned in elaborating the albuminous mate- rials of the food, and for a time storing them up, to be gradually introduced into the blood, according to the demands of the general system. (2.) It seems probable that the spleen, like the lymphatic glands, is engaged in the formation of blood-corpuscles. For it is quite certain, that the blood of the splenic vein contains an unusually large amount of white corpuscles; and in the disease termed leucocythaemia, in which the pale corpuscles of the blood are remarkably increased in number, there is almost always found an hypertrophied state of the spleen or of the lymphatic glands. In Kblliker's opinion, the development of colourless and also coloured corpuscles of the blood is one of the essential functions of the spleen, into the veins of which the new-formed corpuscles pass, and are thus conveyed into the general current of the circulation. (3.) There is reason to believe, that in the spleen many of the red corpuscles of the blood, those probably which have discharged their office and are worn out, undergo disintegration; for in the coloured portions of the spleen-pulp an abundance of such cor- puscles, in various stages of degeneration, are found, while the red corpuscles in the splenic venous blood are said to be relatively 440 THE VASCULAR GLANDS. [chap. XIII. diminished. This process appears to be as follows. The blood- corpuscles, becoming smaller and darker, collect together in roundish heaps, which may remain in this condition, or become each surrounded by a cell-wall. The cells thus produced may contain from one to twenty blood-corpuscles in their interior. These corpuscles become smaller and smaller; exchange their red for a golden yellow, brown, or black colour; and at length, are converted into pigment-granules, which by degrees become paler and paler, until all colour is lost. The corpuscles undergo these changes whether the heaps of them are enveloped by a cell-wall or not. (4.) From the almost constant presence of uric acid, in larger quantities than in other organs, as well as of the nitrogenous bodies, xanthin, hypoxanthin, and leucin, in the spleen, some special nitrogenous metabolism, may be fairly inferred to occur in it. (5.) Besides these, its supposed direct offices, the spleen is believed to fulfil some purpose in regard to the portal circulation, with which it is in close connection. From the readiness with which it admits of being distended, and from the fact that it is generally small while gastric digestion is going on, and enlarges when that act is concluded, it is supposed to act as a kind of vascular reservoir, or diverticulum to the portal system, or more particularly to the vessels of the stomach. That it may serve such a purpose is also made probable by the enlargement which it undergoes in certain affections of the heart and liver, attended with obstruction to the passage of blood through the latter organ, and by its diminution when the congestion of the portal system is relieved by discharges from the bowels, or by the effusion of blood into the stomach. This mechanical influence on the circu- lation, however, can hardly be supposed to be more than a very subordinate function. It is only necessary to mention that Schiff believes that the spleen manu- factures a substance without which the pancreatic secretion cannot act upon proteids, so that when the spleen is removed the digestive action of the pancreatic juice is stopped. Influence of the Nervous System upon the Spleen.-When the spleen is enlarged after digestion, its enlargement is probably due to two causes, (1) a relaxation of the muscular tissue which forms so large a part of its framework; (2) a dilatation of the vessels. Both these phenomena are doubtless under control of the nervous system. It has been found by experiment that when the splenic CHAI', xiir.] STRUCTURE OF THE THYMUS. 441 nerves are cut the spleen enlarges, and that contraction can be brought about (1) by stimulation of the spinal cord (or of the divided nerves); (2) reflexly by stimulation of the central stumps of certain divided nerves, e.g., vagus and sciatic ; (3) by local stimulation by an electric current; (4) the exhibition of quinine and some other drugs. It has been shown by the oncometer of Roy (fig. 260), that the spleen undergoes rhythmical contractions and dilatations, due no doubt to the contraction and relaxation of the muscular tissue in its capsule and trabecula). It also shows the rhythmical alteration of the general blood pressure, but to a less extent than the kidney. The Thymus. This gland must be looked upon as a temporary organ, as it attains its greatest size early after birth, and after the second year gradually diminishes, until in adult life hardly a vestige re- mains. At its greatest develop- ment it is a long narrow body, situated in the front of the chest behind the sternum and partly in the lower part of the neck. It is of a reddish or greyish colour, distinctly lobulated. Structure. -The gland is sur- rounded by a fibrous capsule, which sends in processes, form- ing trabeculae, which divide the glands into lobes, and carry the blood and lymph-vessels. The large trabeculae branch into small ones, which divide the lobes into lobules. The gland is incased in a fold of the pleura. The lobules are further subdivided into follicles by fine connective tissue. A follicle (fig. 266) is seen on section to be more or less polyhedral in shape, and consists of cortical and medullary por- tions, both of which are composed of adenoid tissue, but in the Fig. 265.- Transverse section of a lobule of an injected infantile thymus gland, a. cap- sule of connective-tissue surrounding the lobule ; b, membrane of the glandu- lar vesicles; c, cavity of the lobule, from which the larger blood-vessels are seen to extend towards and ramify in the spheroidal masses of the lobule, x 10. (Kolliker). 442 THE VASCULAR GLANDS. [chap, xij 1. medullary portion the matrix is coarser, and is not so filled up with lymphoid corpuscles as in the cortex. The adenoid tissue of the cortex, and to a less marked extent that of the medulla, con- sists of two elements, one with small meshes formed of fine fibres with thickened nodal points, and the other enclosed within the first, composed of branched connective tissue corpuscles (Watney). Scattered in the adenoid tissue of the medulla are the concentric corpuscles 0/ Hassall, which are protoplasmic masses of various sizes, Fig. 266.-From a horizontal section through superficial part of the thymus of a calf, slightly mag- nified. Showing in the centre a follicle of polygonal shape with similarly shaped follicles round it. (Klein and Noble Smith.) Fig. 267.-The reticulum of the Thymus, a, epithelial elements ; 6. corpuscles of Hassall. (Cadiat.) consisting of a nucleated granular centre, surrounded by flattened nucleated endothelial cells. In the reticulum, especially <>f the medulla, are large transparent giant cells. In the thymus of the dog and of other animals are to be found cysts, probably derived from the concentric corpuscles, some of which are lined with ciliated epithelium, and others with short columnar cells. Haemoglobin is found in the thymus of all animals, either in these cysts, or in cells near to or of the concentric corpuscles. In the lymph issuing from the thymus are cells containing coloured blood corpuscles and haemoglobin granules, and in the lymphatics of the thymus there are more colourless cells than in the lymphatics of the neck. Tn the blood of the thymic vein, there appears some- times to be an increase in the colourless corpuscles, and also masses of granular matter (corpuscles of Zimmermann) (Watney). The arteries radiate from the centre of the gland. Lymph sinuses may be seen occasionally surrounding a greater or smaller portion of the periphery of the follicles (Klein). The nerves are very minute. CHAP. XIII.] THE STRUCTURE OF THE THYROID. 443 Function.-The thymus appears to take part in producing coloured corpuscles, both from the large corpuscles containing haemoglobin, and also indirectly from the colourless corpuscles (Watney). Respecting the thymus gland in the hybernating animals, in which it exists throughout life, as each successive period of hybernation approaches, the thymus greatly enlarges and becomes laden with fat, which accumulates in it and in fat glands con- nected with it, in even larger proportions than it does in the ordinary seats of adipose tissue. Hence it appears to serve for the storing up of materials which, being re-absorbed in inactivity of the hybernating period, may maintain the respiration and the temperature of the body in the reduced state to which they fall during that time. It has been shown also to be a source of the red blood-corpuscles, at any rate in early life. The Thyroid. The thyroid gland is situated in the neck. It consists of two lobes one on each side of the trachea extending upwards to the thyroid cartilage, covering its inferior cornu and part of its body ; these lobes are connected across the middle line by a middle lobe or isthmus. The thyroid is covered by the muscles of the neck. It is highly vascular, and varies in size in different individuals. Structure.-The gland is encased in a thin transparent layer of dense areolar tissue, free from fat, containing elastic fibres. This capsule sends in strong fibrous trabeculae, which enclose the thyroid vesicles-which are rounded or oblong irregular sacs, con- sisting of a wall of thin hyaline membrane lined by a single layer of short cylindrical or cubical cells. These vesicles are filled with a coagulable fliud or transparent colloid material. The colloid substance increases with age, and the cavities appear to coalesce. In the interstitial connective tissue is a round meshed capillary plexus, and a large number of lymphatics. The nerves adhere closely to the vessels. In the vesicles there are in addition to the yellowish glassy colloid material, epithelium cells, colourless blood-corpuscles, and also coloured corpuscles undergoing disintegration. Function.-There is little known definitely about the function of the thyroid body. It, however, produces colloid material of 444 THE VASCULAR GLANDS. [chap. xiii. the vesicle, which is carried off by the lymphatics, and discharged into the blood, and so may contribute its share to the elaboration of that fluid. The destruction of red blood-corpuscles is also supposed to go on in the gland. Tn certain animals its removal Fig. 268.-Part of a section of the human Thyroid, a, fibrous capsule; 6, thyroid vesicles filled with, e, colloid substance; c, supporting fibrous tissue; d, short columnar cells lining vesicles ; f, arteries ; g, veins filled with blood; h. lymphatic vessel filled with colloid substance. X (S. K. Alcock.) appears to produce a peculiar condition in which mucin is deposited in its tissues. A similar condition, known as Myxcedema, and Cretinism are closely associated with disease or removal of the thyroid gland in the human subject. Supra-renal Capsules or Adrenals. These are two flattened, more or less triangular or cocked-hat shaped bodies, resting by their lower border upon the upper border of the kidneys. -The gland is surrounded by an outer sheath of OHAP. XIII.] STRUCTURE OF THE ADRENALS. 445 connective tissue, which sometimes consists of two layers, sending in exceedingly fine prolongations forming the framework of the gland. The gland tissue proper consists of an outside firmer cortical portion, and an inside soft dark medullary portion. (i.) The cortical portion is divided into (fig. 269) an external (L i C > "(1 Fig. 269. -Vertical section through part of the cortical portion of supra-renal of guinea-pig, a, capsule; b, zona glomerulosa; c, zona fasciculata ; d, connective tissue supporting the columns of the cells of the latter, and also indicating the position of the blood- vessels. x (8. K. Alcock.) narrow layer of small rounded or oval spaces, the zona glomerulosa, made by the fibrous trabeculae, containing multinucleated masses of protoplasm, the differentiation of which into distinct cells, cannot be made out. (6) A layer of cells arranged radially, the zona fasciculate (c). The substance of this layer is broken up into cylinders, each of which is surrounded by the connective tissue framework. The cylinders thus produced are of three kinds-one containing an opaque, resistant, highly refracting mass (probably of a fatty nature); frequently a large number of nuclei are present; the individual cells can only be made out with difficulty. The second variety of cylinders is of a brownish colour, and contains finely granular cells, in which are fat globules. The third variety consists of grey cylinders, containing a number of cells whose 446 THE VASCULAR GLANDS. [CHAP. XIII. nuclei are filled with a large number of fat granules. The third layer of the cortical portion is the zona reticularis (not shown in fig. 269). This layer is apparently formed by the breaking up of the cylinders, the elements being dispersed and isolated. The cells tire finely granular, and have no deposit of fat in their interior; but in some specimens fat may be present, as well as certain large yellow granules, which may be called pigment granules. (2.) The medullary substance consists of a coarse rounded or irregular meshwork of fibrous tissue, in the alveoli of which are Fig- 270.-Section through a portion of the medullary part of the supra-renal of guinea-pig. The vessels are very numerous, and the fibrous stroma more distinct than in the cortex, and is moreover reticulated. The cells are irregular and larger, clean, and free from oil globules, x (8. K. Alcock.) masses of multinucleated protoplasm (fig. 270); numerous blood- vessels ; and an abundance of nervous elements. The cells are very irregular in shape and size, poor in fat, and occasionally branched ; the nerves run through the cortical substance, and anastomose over the medullary portion. Function.-Of the function of the supra-renal bodies nothing can be definitely stated, but they are in all probability connected with the lymphatic system. Addison's Disease.-The collection of large numbers of cases in which the supra-renal capsuleshave been diseased, has demonstrated the very close relation subsisting between disease of those organs and brown discoloration of the skin (Addison's disease) ; but the explanation of this relation is still involved in obscurity, and consequently does not aid much in determining the functions of the supra-renal capsules. CHAP. XIII.] PITUITARY AND PINEAL BODIES. 447 Pituitary Body. This body is a small reddish-grey mass, occupying the sella turcica of the sphenoid bone. Structure.-It consists of two lobes-a small posterior one, consisting of nervous tissue ; an anterior larger one, resembling the thyroid in structure. A canal lined with flattened or with ciliated epithelium, passes through the anterior lobe ; it is con- nected with the infundibulum. The gland spaces are oval, nearly round at the periphery, spherical towards the centre of the organ ; they are filled with nucleated cells of various sizes and shapes not unlike ganglion cells, collected together into rounded masses, filling the vesicles, and contained in a semi-fluid granular substance. The vesicles are enclosed by connective tissue, rich in capillaries. Function.-Nothing is known of the function of the pituitary body. Pineal Gland. This gland, which is a small reddish body, is placed beneath the back part of the corpus callosum, and rests upon the corpora quadrigemin a. Structure.-It contains a central cavity lined with ciliated epithelium. The gland substance proper is divisible into-(i.) An outer cortical layer, analogous in structure to the anterior lobe of the pituitary body; and (2.) An inner central layer, wholly nervous. The cortical layer consists of a number of closed follicles, containing (a) cells of variable shape, rounded, elongated, or stellate; (6) fusiform cells. There is also present a gritty matter (acervulus cerebri), consisting of round particles aggregated into small masses. The central substance consists of white and grey matter. The blood-vessels are small, and form a very deli- cate capillary plexus, Function.-Of this there is nothing known. The Coccygeal and Carotid Glands. These so-called glands are situated, the one in front of the tip of the coccyx, and the other at the point of bifurcation of the common carotid artery on each side. They are made up of a plexus of small arteries, are enclosed and supported by a capsule 448 THE VASCULAR SYSTEM. [CHAP. XIIf. of fibrous tissue, which contains connective tissue corpuscles. The blood-vessels are surrounded by one or more layers of cells like secreting-cells, which are said to be modified plasma-cells of the connective tissue. The function of these bodies is unknown. The opinion that the vascular glands serve for the higher organization of the blood, is supported by their being all especially active in the discharge of their functions during foetal life and childhood, when, for the development and growth of the body, the most abundant supply of highly organised blood is necessary. The bulk of the thymus gland, in proportion to that of the body, appears to bear almost a direct proportion to the activity of the body's development and growth, and when, at the period of puberty, the development of the body may be said to be complete, the gland wastes, and finally disappears. The thyroid gland and supra-renal capsules, also, though they probably never cease to discharge some function, yet are proportionally much smaller in childhood than in foetal life and infancy; and with the years advancing to the adult period, they diminish yet more in pro- portionate size and apparent activity of function. The spleen more nearly retains its proportionate size, and enlarges nearly as the whole body does. Although the functions of all the vascular glands may be similar, in so far as they may all alike serve for the elaboration and maintenance of the blood, yet each of them probably dis- charges a peculiar office, in relation either to the whole economy, or to that of some other organ. Respecting any special office of the thyroid gland, nothing reasonable has been hitherto suggested ; nor is there any certain evidence concerning that of the supra- renal, capsules. Bergman believed that they formed part of the sympathetic nervous system from the richness of their nervous supply. Kolliker looked upon the two parts as functionally distinct, the cortical part belonging to the blood vascular system, and the medullary to the nervous system. Functions of the Vascular Glands in General. chap, xiv.] DISTRIBUTION OF UNSTRIPED MUSCLE. 449 CHAPTER XIV. THE MUSCULAR SYSTEM. There are two chief kinds of muscular tissue, differing both in minute structure as well as in mode of action, viz., (1.) the plain or non-striated, and (2.) the striated. The striped form of muscular fibre is sometimes called voluntary muscle, because all muscles under the control of the will are constructed of it. The plain or unstriped variety is often termed involuntary, because it alone is found in the greater number of the muscles over which the will has no power. I. Structure of Muscular Tissue. (1.) Unstriped or Plain Muscle. Distribution.-Unstriped muscle forms the proper muscular coats (i.) of the digestive canal from the middle of the oesophagus to the internal sphincter ani; (2.) of the ureters and urinary bladder; (3.) of the trachea and bronchi; (4.) of the ducts of glands; (5.) of the gall- bladder ; (6.) of the vesicular seminales; (7.) of the pregnant uterus; (8.) of blood-vessels and lymphatics; (9.) of the iris, and some other parts. This form of tissue also enters largely into the composition (10.) of the tunica dartos, the contraction of which is the principal cause of the wrinkling and contraction of the scrotum on exposure to cold. Unstriped muscular tissue occurs largely also in the cutis generally, being especially abundant in the interspaces between the bases of the papillae. Hence when it contracts under the influence of cold, fear, electricity, or any other stimulus, the papillae are made unusually prominent, and give rise to the peculiar roughness of the skin termed cutis anserina, or goose skin. It occurs also in the super- ' a b Fig. 271.-Vertical section through the scalp with two hair sacs; a, epidermis; b, cutis; c, muscles of the hair-follicles. (Kolliker.) 450 THE MUSCULAB SYSTEM. [chap. xiv. ficial portion of the cutis, in all parts where hairs occur, in the form of flattened roundish bundles, which lie alongside the hair- Fig. 272.-A, unstriped muscle cells from the mesentery of a ueu't. The sheath exhibits trans- verse markings, x 180. B, from a similar preparation, showing that each muscle cell consists of a central bundle of fibrils. F, (contractile part), connected with the intra-nuclear network, N, and a sheath with annular thickenings, St. The cells show varicosities due to local contraction and on these the annular thickenings are most marked. X 450. (Klein and Noble Smith.) follicles and sebaceous glands. They pass obliquely from without inwards, embrace the sebaceous glands, and are attached to the hair-follicles near their base (fig- 271). Structure - U nstriated muscles are made up of elongated, spindle-shaped, nucleated cells (fig. 272), which in their perfect form are flat, from about 4-5V0 Woo an *nch broad, and 600 to of an inch in length,-very clear, granu- lar, and brittle, so that when they break they often have abruptly rounded or square extremities. Each cell of these consists of a fine sheath, probably elas- tic ; of a central bundle of fibrils representing the contractile substance ; and of an oblong nucleus, which includes within a membrane a fine network anasto- mosing at the poles of the nucleus with the contractile fibrils. The ends of fibres are usually single, sometimes divided. Between the fibres is an albuminous cementing material or endomysivni in Kg. 273.-Plexus of bundles of unstriped muscle cells from the pulmonary pleura of the guinea- pig. x 180. (Klein and Noble Smith.) A, branching fibres; B, their long central nuclei. CHAP. XIV.] DISTRIBUTION OF STRIPED MUSCLE. 451 which are found connective tissue corpuscles, and a few fibres. The perimysium is continuous with the endomysium in the fibrous connective tissue surrounding and separating the bundles of muscle cells. (2.) Striated or Striped Muscle. Distribution .-The striated muscles include the whole of the voluntary muscles of the body, the heart, and those muscles neither completely voluntary nor in- voluntary, which form part of the walls of the pharynx, and exist in certain other parts of the body, as the internal ear, urethra, &c. Structure.-All these mus- cles are composed of fleshy bundles called fasciculi, en- closed in coverings of fibro- cellular tissue or perimysium, by which each is at once con- nected with and isolated from those adjacent to it (fig. 274). Each fasciculus is made up of several smaller bundles, similarly ensheathed. A bundle is made up of muscle fibres with small processes and connective- tissue cells between them or endomysium. Each muscular fibre is thus constructed :-Externally is a fine, transparent, structureless mem- brane, called the sarcolemma, which in the form of a tubular investing sheath forms the outer wall of the fibre, and is filled up by the contractile material of which the fibre is chiefly made up. Some- times, from its comparative tough- ness, the sarcolemma will remain untorn, when by extension the contained part can be broken (fig. 275), and its presence is in this way best demonstrated. The fibres, which are cylindriform or prismatic, with an average diameter of about of an inch, are of a pale yellow colour, and apparently marked by fine striae, which pass transversely round them, in slightly curved or wholly Fig. 274.-A small portion of muscle natural size, consisting of larger and smaller fasci- culi, seen in a transverse section, and a, the same magnified 5 diameters. (Sharpey.) Fig. 275.-Musculo r fibre torn across; the sarcolemma still connecting the two parts of the fibre. (Todd and Bowman.) 452 THE MUSCULAR SYSTEM. [chap. xiv. parallel lines. Each fibre is found to consist of broad dim bands of highly refractive substance representing the contractile portion of the muscle fibre-the contractile discs-alternating with narrow bright bands of a less refractive substance-the interstitial discs. After hardening, each contractile disc becomes longitudinally striated, the thin oblong rods thus formed being the sarcous elements of Bowman. The sarcous elements are not the optical units, since each consists of minute doubly-refracting elements - the dis- diaclasts of Brucke. When seen in transverse section the contractile discs appear to be subdivided by clear lines into polygonal areas Cohnheim's fields (fig. 278), each corresponding to one sarcous element prism. The clear lines are due to a transparent interstitial fluid substance pressed out of the sarcous elements when they coagulate. The sarcolemma is a transparent struc- tureless elastic sheath of great resistance which surrounds each fibre (fig. 275). There is still some doubt regarding the nature of the fibrils. Each of them appears to be composed of a single row of minute dark quadrangular particles, called sarcous elements, which are se- parated from each other by a bright space formed of a pellucid substance continuous with them. According to Sharpey, even in a fibril so constituted, the ultimate anatomical elements of the fibre are not isolated. His view was that each fibril with quadrangular sarcous elements is composed of a number of other fibrils still finer, so that the sarcous element of an ultimate fibril would be not quadrangular but as a streak. In either case the appearance of striation in the whole fibre would be produced by the arrange- ment, side by side, of the dark and light portions respectively of the fibrils (fig. 277). A fine black streak can usually be discerned passing across the interstitial disc between the sarcous elements: this streak is Fig. 276.-Part of a striped muscle- fibre of a water beetle prepared with absolute alcohol. A, sar- eolemma ; B, Krause's mem- brane. The sarcolemma shows regular bulgings. Above and below Krause's membrane are seen the transparent " lateral discs." The chief mass of a muscular compartment is occu- pied by the contractile disc composed of sarcous elements. The substance of the individual sarcous elements has collected more at the extremity than in , the centre : hence this latter is more transparent. The optical effect of this is that the con- tractile disc appears to possess a " median disc " (Disc of Hen- sen). Several nuclei of muscle corpuscles, C and D, are shown, and in them a minute network. x 300. (Klein and Noble Smith.) CHAP. XIV.] STRUCTURE OF STRIPED MUSCLE. 453 termed Krause's membrane : it is continued at each end with the sarcolemma investing the muscular fibre (fig. 276 B). Thus the space enclosed by the sarcolemma is divided into a series of compartments by the transverse partitions known as Fig. 277.-A. Portion of a medium-sized human muscular fibre. X 800. B. Separated bundles of fibrils equally magnified ; a, a, larger, and b, b, smaller collections ; c, still smaller ; d, d, the smallest which could be detached, possibly representing a single series of sarcous elements. (Sharpey.) Krause's membranes; these compartments being occupied by the true muscle substance. On each side (above and below) of this membrane is a bright border (lateral disc). In the centre of the dark zone of sarcous elements a lighter band can sometimes be dimly discerned : this is termed the middle disc of Hensen (see fig. 276, A). In some fibres, chiefly those from insects, each lateral disc con- tains a row of bright granules forming the granular layer of Flogel. The fibres contain nuclei, which are roundish ovoid, or spindle-shaped in different animals. These nuclei are situated close to the sarcolemma, their long axes being parallel to the fibres which contain them. Each nucleus is composed of a uniform network of fibrils, and is embedded in a thin, more or 454 THE MUSCULAR SYSTEM. [chap. XIV. less branched film of protoplasm. The nucleus and protoplasm together form the muscle cell or muscle corpuscle of Max Schultze. The sarcous elements and Krause's membranes are doubly refracting, the rest of the fibre singly refracting. (Briicke.) According to Schafer, the granules, which have been mentioned on either side of Krause's membrane, are little knobs attached to the ends of " muscle-rodsand these muscle-rods, knobbed at each end, and em- bedded in a homogeneous protoplasmic ground-substance, form the substance of the muscles. This view of the structure of muscle requires further confirmation. Although each muscular fibre may be considered to be formed of a number of longitudinal fibrils, arranged side by side, it is also true that they are not naturally separate from each other, there being lateral cohesion, if not fusion, of each sarcous element with those around and in contact with it j so that it happens that there is a tendency for a fibre to split, not oidy into separate fibrils, but also occasionally into plates or discs, each of which is com- posed of sarcous elements laterally adherent one to another. Muscular Fibres of the Heart (figs. 92 and 93) form the chief, though not the only exception to the rule, that involuntary muscles are constructed of plain fibres ; but although striated and so far resem- bling those of the voluntary muscles, they present these distinctions:- Each muscular fibre is made up of elongated, nucleated, and branched cells, the nuclei or muscle-corpuscles being centrally placed in the fibre. The fibres are finer and less distinctly striated than those of the voluntary muscles ; and no sarcolemma can be usually discerned. Fig. 278.-Three muscular fibres running longitudinally, and two bundles of fibres in transverse section, M, from the tongue. The capillaries, C, are injected, x 150. (Klein and Noble Smith.) Fig. 279- Transverse section through muscular fibres of human tongue. The muscle-corpuscles are indicated by interstitial substance. X 450. (Klein and Noble Smith.) chap, xiv.] BLOOD AND NERVE SUPPLY OF MUSCLE. 455 Blood and Nerve Supply.-The voluntary muscles are freely supplied with blood-vessels; the capillaries form a network with oblong meshes around the fibres on the outside of the sarcolemma. No vessels penetrate tlie sarcolemma to enter the interior of the fibre. Nerves also are supplied freely to muscles ; the voluntary muscles receivingthem from the cerebro-spinal system, and the unstriped muscles from the sym- pathetic or ganglionic system. The nerves terminate in the mus- cular fibre in the following ways :- (i.) In unstriped muscle, the nerves first of all form a plexus, called the ground plexus (Arnold), correspond- ing to each group of muscle bundles; the plexus is made by the anasto- mosis of the primitive fibrils of the axis-cylinders. From the ground plexus, branches pass off, and again anastomosing, form plexuses which correspond to each muscle bundle- intermediary plexuses. From these plexuses branches consisting of primitive fibrils pass in between the individual fibres and anastomose. These fibrils either send off finer branches, or terminate themselves in the nuclei of the muscle cells. (2.) In striped muscle the nerves end in motorial end-plates, having first formed, as in the case of unstriped fibres, ground and intermediary plexuses. The fibres are, however, medullated, and when a branch of the intermediary plexus passes to enter a muscle-fibre, its primitive sheath becomes continuous with the sarcolemma, and the axis-cylinder forms a network of its fibrils on the surface of the fibre. This network lies embedded in a flattened granular mass containing nuclei of several kinds; this is the motorial end-plate (fig. 281). In batrachia, besides end- plates, there is another way in which the nerves end in the muscle-fibres, viz., by rounded extremities, to which oblong nuclei are attached. Development.-(1.) Unstriped. - The cells of unstriped muscle are Fig'. 280.-From a preparation of the nerve-termination in the muscular fibres of a snake, a, End plate seen only broad surfaced. ?>, End plate seen as narrow surface. (Lingard and Klein.) 456 THE MUSCULAR SYSTEM. [chap. xiv. derived directly from embryonic cells, by an elongation of the cell, and its nucleus ; the latter changing from a vesicular to a rod shape. (2.) Striped.-Formerly it was supposed that striated fibres were formed by the coalescence of several cells, but recently it has been proved, that each fibre is formed from a single cell, the process in- volving an enormous in- crease in size, a multiplica- tion of the nucleus by fission, and a differentiation of the cell-contents. This view differs but little from the other, that the muscular fibre is produced, not by multiplication of cells, but by arrangement of nuclei in a growing mass of proto- plasm (answering to the cell in the theory just referred to), which becomes gradu- ally differentiated so as to assume the characters of a fully developed muscular fibre. Growth of Muscle.- The growth of muscles both striated and non-striated, is the result of an increase both in the number and size of the individual ele- ments. In the pregnant uterus the fibre-cells may become enlarged to ten times their original length. In involution of the uterus after parturition the re- verse changes occur, ac- companied generally by some fatty infiltration of the tissue and degeneration of the fibres. Fig'. 281.- Two striped muscle-fibres of the hyoglossus of frog, a, Nerve end-plate ; b, nerve-fibres leaving the end-plate; c, nerve-fibres, terminating after dividing into branches d, a nucleus in which two nerve-fibres anastomose, x 600. (Arndt.) II. The Chemical Composition of Muscle. A. Proteids.-The principal substance which can be extracted from muscle, when examined after death, is a proteid body, called Myosin. This body appears to bear the same relation to the living muscle, as fibrin does to the living blood, since the coagulation of muscle after death is due to the formation of myosin. Thus chap, xiv.] CHEMICAL COMPOSITION OF MUSCLE. 457 if coagulation be delayed in muscles removed immediately from recently killed animals, by subjecting them to a temperature below o° C., it is possible to obtain from them by expression a viscid fluid of slightly alkaline reaction, called muscle plasma Halliburton). And muscle plasma, if exposed to the ordinary temperature of the air (and more quickly at 37-40° C.), un- dergoes coagulation much in the same way as does blood plasma, separated from the blood by the action of a low temperature, under similar circumstances. The appearances presented by the fluid during the process are also very similar to the phenomena of blood-clotting, viz., that first of all an increased viscidity on the surface of the fluid, and at the sides of the containing vessel, appears, which gradually extends throughout the entire mass, until a fine transparent clot is obtained. In the course of some hours the clot begins to contract, and to squeeze out of its meshes a fluid corresponding to blood-serum. In the course of coagula- tion, therefore, muscle plasma separates into muscle clot and muscle serum. The muscle clot is the substance myosin. It differs from fibrin in being easily soluble in a 2 per cent, solution of hydrochloric acid, and in a 10 per cent, solution of sodium chloride. It is insoluble in distilled water, and its solutions coagulate on application of heat. It is a body, therefore, belong- ing to the globulin class of proteids. During the process the reaction of the fluid becomes distinctly acid. The coagulation of muscle plasma can not only be prevented by cold, but also, as Halliburton has shown, by the presence of neutral salts in certain proportions; for example, of sodium chloride, of magnesium sulphate, or of sodium sulphate. It will be remembered that this is also the case with blood plasma. Dilution of the salted muscle plasma will produce its slow coagulation, which is prevented by the presence of the neutral salts if in strong solution. It is highly probable that the formation of muscle-clot is a ferment action {myosin ferment'). The antecedent of myosin in living muscle has received the name of myosinogen, in the same way as the fibrin-forming element in the blood is called fibrinogen. Myosinogen is, however, made up of two globulins, which coagulate at the temperatues 47° C. and 56° C. respectively. Myosin may also be obtained from dead muscle by subjecting it, after all the blood, fat, and fibrous tissue, and substances soluble in water have been removed, to a 10 per cent, solution of sodium 458 THE MUSCULAR SYSTEM. [chap. XIV. chloride, or 5 per cent, solution of magnesium sulphate, or 10 to 15 per cent, solution of ammonium chloride, filtering and allow- ing the filtrate to drop into a large quantity of water, when myosin separates out as a white flocculent precipitate. A very remarkable fact with regard to the properties of myosin has been demonstrated by Halliburton, namely, that a solution of muscle which has undergone rigor mortis, in strong neutral saline solution, possesses very much the same properties as muscle plasma, and that if diluted with twice or three times its bulk of water, myosin will separate out as a clot, which clot can be again dissolved in a strong neutral saline solution, and the solution can be again made to clot on dilution. This process can be often repeated ; but in the fluid which exudes from the clot there is no proteid present. Myosin then when dissolved in neutral saline fluids is converted into myosinogen, but reappears on dilution of the fluid. Muscle clot is almost pure myosin; but it appears to be combined with a certain amount of salts, for if it be freed from salts, especially of those of calcium, by prolonged dialysis, it loses its solubility. If a small amount of calcium salts be added, however, it regains that property. Muscle serum is acid in reaction, and almost colourless. Lt contains three proteid bodies, viz.--(a.) A globulin (myo-globulin), which can be precipitated by saturation with sodium chloride, or magnesium sulphate, and which can be coagulated at 63° C. (6.) Serum-albumin, which coagulates at 730 C., but is not precipitated by saturation with cither of those salts. And (c.) Myo-albumin, which is neither precipitated by heat, nor by saturation with sodium chloride or magnesium sulphate, but may be by saturation with ammonium sulphate. It is closely connected with, even if it is not itself, myosin ferment. Neither casein nor peptone has been found by Halliburton in muscle extracts. In extracts of muscles, especially of red muscles, there is a certain amount of Hcemoglobin, and also of a pigment special to muscle, called by McMunn Myo-haanatin, which has a spectrum quite distinct from haemoglobin, viz., a narrow band just before D, two very narrow between 1) and E, and two other faint bands, near the violet, E b, and between E and F close to F (McMunn). B. Ferments.-In addition to muscle ferments, already men- tioned, muscle extracts contain certain small amounts of pepsin and fibrin ferment, and also of an amylolytic ferment. C. Acids, particularly sarco-lactic, also acetic and formic. chap. xiv.J THE THREE CONDITIONS OF MUSCLE. 459 D. Glycogen and Glucose, also Inosite. E. Nitrogenous crystalline bodies, such as Kreatin, Hypo- xanthin, or camin, Taurin, and Urea, the last in very small amount. F. Salts, the chief of which is potassium phosphate. III. Physiology of Muscle. Muscle may exist in three different conditions: - A. during rest; B. during activity; and C. in rigor. A. Rest. Physical condition.-During rest or inactivity a muscle has a slight but very perfect Elasticity ; it admits of being considerably stretched; but returns readily and completely to its normal con- dition. In the living body the muscles are always stretched some- what beyond their natural length, they are always in a condition of slight tension; an arrangement which enables the whole force of the contraction to be utilised in approximating the points of attachment. It is obvious that if the muscles were lax, the first part of the contraction until the muscle became tight would be wasted. There is no doubt that even in a condition of rest Oxygen is abstracted from the blood, and carbonic acid is given out by a muscle; for the blood becomes venous in the transit, and since the muscles form by far the largest element in the composition of the body, chemical changes must be constantly going on in them as in other tissues and organs, although not necessarily accompanied by contraction. When cut out of the body such muscles retain their contractility longer in an atmosphere of oxygen than in an atmosphere of hydrogen or carbonic acid, and during life, an amount of oxygen is no doubt necessary to the manifestation of energy as well as for the metabolism going on in the resting condition. The reaction of living muscle in a resting or inactive condition is neutral or faintly alkaline. In muscles which have been removed from the body, it has been found that/or sotwc little time electrical currents can be demonstrated passing from point to point on their surface; but as soon as 460 THE MUSCULAR SYSTEM. [chap. XIV. the muscles die or enter into rigor mortis, these currents dis- appear. The Method of Demonstration usually employed is as follows:- The frog's muscles are the most convenient for experiment; and a muscle of regular shape, in which the fibres are parallel, is selected. The ends are cut off by clean vertical cuts, and the resulting piece of muscle is called a regular muscle prism. The muscle prism is insulated, and a pair of non- polarisable electrodes connected with a very delicate galvanometer is applied to various points of the prism, and by a deflection of the needle to a greater or less extent in one direction or another, the strength and direc- tion of the currents in the piece of muscle can be estimated. It is neces- Fig. 282.-Diagram of Du Bois Beymond's non-polarisable electrodes, a, glass tube filled with a saturated solution of zinc sulphate, in the end, c, of which is china clay drawn out to a point; in the solution a well amalgamated zinc rod is immersed and connected, by means of the wire which passes through a, with the galvanometer. The remainder of the apparatus is simply for convenience of application. The muscle and the end of the second electrode are to the right of the figure. sary to use non-polarisable and not metallic electrodes in this experiment, as otherwise there is no certainty that the whole of the current observed is communicated from the muscle itself, and is not derived from the metallic electrodes, in consequence of the action of the saline juices of the tissues upon them. The form of the non-polarisable electrodes is a modification of du Bois Reymond's apparatus (fig. 282), which consists of a somewhat flattened glass cylinder a, drawn abruptly to a point, and fitted to a socket capable of movement, and attached to a stand A, so that it can be raised or lowered as required. The lower portion of the cylinder is filled with china clay moistened with saline solution, part of which projects through its drawn-out point; the rest of the cylinder is fitted with a saturated solution of zinc sulphate into which dips a well amalgamated piece of zinc which is connected by means of a wire with the galvanometer. In this way the zinc sulphate forms a homogeneous and non-polarisable conductor between the zinc and the china clay. A second electrode of the same kind is, of course, necessary. CHAP. XIV.] MUSCLE CURRENTS. 461 In a regular muscle prism the currents are found to be as follows :- If from a point on the surface a line-the equator-be drawn across the muscle prism equally dividing it, currents pass from this point to points away from it, which are weak if the points are near, and increase in strength as the points are further and further aw7ay from the equator; the strongest passing from the equator to Fig. 283.-Diagram of the currents in a muscle prism. (Du Bais Reymond.) a point representing the middle of the ent ends (fig. 283, 2) ; currents also pass from points nearer the equator to those more remote (fig. 283, 1, 3, 4), but not from points equally distant, or iso-electric points (fig. 283, 6, 7, 8). The cut ends are always negative to the equator. These currents are constant for some time after removal of the muscle from the body, and in fact remain as long as the muscle retains its life. They are in all probability due to chemical changes going on in the muscles. The currents are diminished by fatigue and are increased by an increase of temperature within natural limits. If the uninjured tendon be used as the end of the muscle, and the muscle be examined without removal from the body, the currents arc very feeble, but they are at once much increased by injuring the muscle, as by cutting off its tendon. The last observation appears to show that they are right who believe that the currents do not exist in muscles uninjured in situ, but that injury, either mechanical, chemical or thermal, will render the injured part electrically negative to other points on the muscle. In a frog's heart it has been shown, too, that no currents exist during its inactivity, but that as soon as it is injured in any way they 462 THE MUSCULAR SYSTEM. [chap. xiv. are developed ; the injured part being negative to the rest of the muscle. The currents which have been above described are called either natural muscle currents or currents of rest, according as they are looked upon as always existing in muscle or as deve- loped when a part of the muscle is subjected to injury; in either case, up to a certain point, it is agreed that the strength of the currents is in direct proportion to the injury. B. Activity. The property of muscular tissue, by which its peculiar func- tions are exercised, is its Contractility, which is excited by all kinds of stimuli applied either directly to the muscles, or indirectly to them through the medium of their motor nerves. This pro- perty, although commonly brought into action through the nervous system, is inherent in the muscular tissue. For-(i.) it may be manifested in a muscle which is isolated from the influence of tlie nervous system by division of the nerves supplying it, so long as the natural tissue of the muscle is duly nourished ; and (2.) it is manifest in a portion of muscular fibre, in which, under the microscope, no nerve-fibre can be traced. (3.) Substances such as urari, which paralyse the nerve-endings in muscles, do not at all diminish the irritability of the muscle. (4.) When a muscle is fatigued, a local stimulation is followed by a contraction of a small part of the fibre in the immediate vicinity without any regard to the distribution of nerve-fibres. If the removal of nervous influence be long continued, as by division of the nerves supplying a muscle, or in cases of paralysis of long-standing, the irritability, i.e., the power of both per- ceiving and responding to a stimulus, may be lost ; but probably this is chiefly due to the impaired nutrition of the muscular tissue, which ensues through its inaction. The irritability of muscles is also of course soon lost, unless a supply of arterial blood to them is kept up. Thus, after ligature of the main arte- rial trunk of a limb, the power of moving the muscles is partially or wholly lost, until the collateral circulation is established; and when, in animals, the abdominal aorta is tied, the hind legs arc rendered almost powerless. The same fact may be readily shown by compressing the abdo- minal aorta in a rabbit for about 1 o minutes; if the pressure be released and the animal be placed on the ground, it will work itself CHAP. XIV.] MUSCULAR CONTRACTION. 463 along with its front legs, while the hind legs sprawl helplessly behind. Gradually the muscles recover their power and become quite as efficient as before. So, also, it is to the imperfect supply of arterial blood to the muscular tissue of the heart, that the cessation of the action of this organ in asphyxia is in some measure due. Besides the property of contractility, the muscles, especially the striated, possess Sensibility by means of the sensory nerve-fibres distributed to them. The amount of common sensibility in muscles is not great; for they may be cut or pricked without giving rise to severe pain, at least in their healthy condition. But they have a peculiar sensibility, or at least a peculiar modification of common sensibility, which is shown in that their nerves can communicate to the mind an accurate knowledge of their states and positions when in action. By this sensibility, we are not only made conscious of the morbid sensations of fatigue and cramp in muscles, but acquire, through muscular action, a knowledge of the distance of bodies and their relation to each other, and are enabled to estimate and compare their weight and resistance by the effort of which we are conscious in measuring, moving, or raising them. The Phenomena of Muscular Contraction. The power which muscles possess of contraction may then be called forth by stimuli of various kinds, and these stimuli may also be applied directly to the muscle or indirectly to the nerve supplying it. There are distinct advantages, however, in applying the stimulus through the nerves, as it is more convenient, as well as more potent. The stimuli are of four kinds, viz. :- (i.) Mechanical stimuli, as by a blow, pinch, prick of the muscle or its nerve, will produce a contraction, repeated on the repetition of the stimulus; but if applied to the same point for a limited number of times only, as such stimuli will soon destroy the irritability of the preparation. (2.) Thermal stimuli.-If a needle be heated and applied to a muscle or its nerve, the muscle will contract. A temperature of over ioo° F. (37'8° C.) will cause the muscles of a frog to pass into a condition known as heat rigor. (3.) Chemical stimuli.-A great variety of chemical substances will excite the contraction of muscles, some substances being more 464 THE MUSCULAR SYSTEM. [chap. xiv. potent in irritating the muscle itself, and other substances having more effect upon the nerve. Of the former may be mentioned, dilute acids, salts of certain metals, e.y., zinc, copper and iron; to the latter belong strong glycerin, strong acids, ammonia and bile salts in strong solution. (4.) Electrical Stimuli.-For the purpose of experiment elec- trical stimuli are most frequently used, as the strength of the stimulus may be more conveniently regulated. Any form of elec- trical current may be employed for this purpose, but galvanism or the induced current is usually chosen. (1.) Galvanic currents are usually obtained by the employment of a con- tinuous current battery such as that of Daniell, by which an electrical current Fig. 284.-Diagram, of a Daniell's battery. which varies but little in intensity is obtained. The battery (fig. 285) consists of a positive plate of well-amalgamated zinc immersed in a porous cell, con- taining dilute sulphuric acid ; and this cell is again contained within a larger copper vessel (forming the negative plate), containing besides a saturated solution of copper sulphate. The electrical current is made continuous by the use of the two fluids in the following manner. The action of the dilute sulphuric acid upon the zinc plate partly dissolves it, and liberates hydrogen, and this gas passes through the porous vessel, and decomposes the copper sulphate into copper and sulphuric acid. The former is deposited upon the copper plate, and the latter passes through the porous vessel to renew the sulphuric acid which is being used up. The copper sulphate solution is renewed by spare crystals of the salt, which are kept on a little shelf attached to the copper plate, and slightly below the level of the solution in the vessel. The current of electricity supplied by this battery will continue without variation for a considerable time. Other continuous current batteries, such as Grove's, may be used in place of Daniell's. The way in which the apparatus is arranged is to attach wires to the copper and zinc plates, and to bring them to a key, which is a little apparatus for connecting the wires of a battery. One often employed is du Bois Reymond's (fig. 287, d) ; it consists of two pieces of brass about pHAP. XIV.] INDUCTION APPARATUS. 465 an inch long, in each of which are two holes for wires and binding screws to hold them tightly ; these pieces of brass are fixed upon a vulcanite plate, to the under surface of which is a screw clamp by which it can be secured to the table. The interval between the pieces of brass can be bridged over by means of a third thinner piece of similar metal fixed by a screw to one of the brass pieces, and capable of movement by a handle at right angles, so as to touch the other piece of brass. If the wires from the battery are brought to the inner binding screws, and the bridge connects them, the current passes across it and back to the battery. Wires are connected with the outer binding screws, and the other ends are approximated Fig. 285.-Du Bois Reymond? s induction coil. for about two inches, but, being covered except at their points, are insu- lated, the uncovered points are about an eighth of an inch apart. These wires are the electrodes, and the electrical stimulus is applied to the muscle, if they are placed behind its nerve, and the connection between the two brass platesof the key be broken by depressing the handle of the bridge, and so raising the connecting piece of metal. The. key is then said to be opened. (2.) An induced current is developed by means of an apparatus, called an induction coil, and the one employed for physiological purposes is mostly Du Bois Reymond's, the one seen in fig. 285. Wires from a battery are brought to the two binding screws d' and d, a key intervening. These binding screws are the ends of a coil of coarse covered wire c, called the primary coil. The ends of a coil of finer covered wire g, are attached to two binding screws to the left of the figure, one only of which is visible. This is the secondary coil, and is capable of being moved nearer to c along a grooved and graduated scale. To the binding screws to the left of g, the wires of electrodes used to stimulate the muscle are attached. If the key in the circuit of wires from the battery to the primary coil (primary circuit) be closed, the current from the battery passes through the primary coil, and across the key to the battery, and 466 THE MUSCULAR SYSTEM. [chap. XIV. continues to pass as long as the key continues closed. At the moment of closure of the key, at the exact instant of the completion of the primary circuit, an instantaneous current of electricity is induced in the secondary coil, g, if it be sufficiently near; and the nearer it is to c, the stronger is the current induced. The current is only momentary in duration, and does not continue during the whole of the period whilst the primary circuit is complete. When, however, the primary current is broken by opening the key, a second, also momentary, current is induced in g. The former induced current is called the malting, and the latter the breaking shock ; the former is in the opposite direction to, and the latter in the same as, the primary current. The induction coil may be used to produce a rapid series of shocks by means of another and accessory part of the apparatus at the right of the fig., called the magnetic in- terrupter. If the wires from a battery are con- nected with the two pillars by the binding screws, one below c, and the other, a, the course of the current is indicated in fig. 286, the di- rection being indicated by the arrows. The current passes up the pillar from c, and along the springs if the end of d' is close to the spring, the current passes to the primary coil c, and to wires covering two up- right pillars of soft iron, from them to the pillar a, and out by the wires to the battery ; in passing along the wire, b, the soft iron is converted into a magnet, and so attracts the hammer,/, of the spring, breaks the connection of the spring with d', and so cuts off the current from the primary coil, and also from the electro-magnet. As the pillars, b, are no longer magnetized the spring is released, and the current passes in the first direction, and is in like manner interrupted. At each make and break of the primary current, currents corresponding are induced in the secondary coil. These currents are opposite in direction, but are not equal in intensity, the break shock being greater. In order that the shocks should be nearly equal at the make and break, a wire (fig. 286 e') connects e and d', and the screw d' is raised out of reach of the spring, and d is raised (as in fig. 286), so that part of the current always passes through the primary coil and electro- magnet. When the spring touches d, the current in b is diminished, but never entirely withdrawn, and the primary current is altered in intensity at each contact of the spring with <7, but never entirely broken. Record of Muscular Contraction under Stimuli.-The muscles of the frog are most convenient for the purpose of recording contractions. The frog is pithed, that is to say, its central nervous system is entirely destroyed by the insertion of a stout needle into the spinal cord, and the parts above it. One of its lower extremities is used in the following manner. Fig. 286.-Diagram of the course of the. current in the magnetic interrupter of Du Bois Reymond's induction coil. (Helmholz's modification.) Chap, xiv.] RECORDING CONTRACTIONS. 467 The large trunk of the sciatic nerve is dissected out at the back of the thigh, and a pair of electrodes is inserted behind it. The tendo-achillis is divided from its attachment to the os calcis, and a ligature is tightly tied round it. This tendon is part of the broad muscle of the thigh (gastro- cnemius), which arises from above the condyles of the femur. The femur is now fixed to a board covered with cork, and the ligature attached to the tendon is tied to the upright of a piece of metal bent at right angles (fig. 287, b), which is capable of movement about a pivot at its knee, the horizontal portion carrying a writing lever (myograph). When the muscle Fig. 287.-Arrangement of the apparatus necessary for recording muscle contractions with a revolving cylinder carrying smoked paper. A, revolving cylinder ; B, the frog arranged upon a cork-covered board which is capable of being raised or lowered on the upright, which also can be moved along a solid triangular bar of metal attached to the base of the recording apparatus-the tendon of the gastrocnemius is attached to the writing lever, properly weighted, by a ligature. The electrodes from the secondary coil pass to the apparatus-being, for the sake of convenience, first of all brought to a key, D (Du Bois Reymond's) ; C, the induction coil; F, the battery (in this fig. a bichromate one); E, the key (Morse's) in the primary circuit. contracts, the lever is raised. It is necessary to attach a small weight to the lever. In this arrangement the muscle is in situ, and the nerve disturbed from its relations as little as possible. The muscle may, however, be detached from the body with the lower end of the femur from which it arises, and the nerve going to it may be taken away with it. The femur is divided at about the lower third. The bone 468 THE MUSCULAR SYSTEM. [chap. xiv. is held in a firm clamp, the nerve is placed upon two electrodes connected with an induction apparatus, and the lower end of the muscle is connected by means of a ligature attached to its tendon with a lever which can write oh a recording apparatus. To prevent evaporation this so-called nerve-muscle preparation is placed under a glass shade, the air in which is kept moist by means of blotting paper saturated with saline solution. Effects of a Single Induction Shock.-With a nerve-muscle prepara- tion arranged in either of the above ways, on closing or opening the key in the primary circuit, we obtain and can record a contraction, and if we use the clockwork apparatus revolving rapidly, a curve is traced such as is shown in fig. 288. Another way of recording the contraction is by the pendulum myograph Fig. 288.-Muscle-curve obtained by means of the pendulum myograph, s, indicates the exact instant of the induction shock ; c, commencement; and m x, the maximum elevation of lever ; t, the line of a vibrating tuning-fork. (M. Foster.) (fig. 289). Here the movement of the pendulum along a certain arc is substituted for the clockwork movement of the other apparatus. The pendulum carries a smoked glass plate upon which the writing lever of a myograph is made to mark. The opening or breaking shock is sent into the nerve-muscle preparation by the pendulum in its swing opening a key (fig. 289, ('.) in the primary circuit. Single Muscle Contraction.-The tracing obtained of a single muscle contraction {muscle curve') is seen in fig. 288, and may be thus explained. The upper line (wt) represents the curve traced by the end of the lever after stimulation of the muscle by a single induction- shock : the middle line (/) is that described by the marking-lever, and indicates by a sudden drop the exact instant at which the induction-shock was given. The lower wavy line (f) is traced by CHAP. XIV.] A MUSCLE CURVE. 469 a vibrating tuning-fork, and serves to measure precisely the intervals of time occupied in each part of the contraction. It will be observed that after the stimulus has been applied, as indicated by the vertical line s, there is an interval before the con- traction commences, as indicated by the line c. This interval, Fig. 289.-Simple form of pendulum myograph and accessory parts. A, pivot upon which pendulum swings; B, catch on lower end of myograph opening the key, C, in its swing; D, a spring-catch which retains myograph, as indicated by dotted lines, and on pressing down the handle of which the pendulum swings along the arc to D on the left of figure, and is caught by its spring. termed (a) the latent period, when measured by the number of vibrations of the tuning-fork between the lines s and c, is found to l>e about sec. The latent period is longer in some muscles than in others, and differs also according to the condition of the muscle, being longer in fatigued muscles, and the kind of stimulus employed. During the latent period there is no apparent change in the muscle. The second part is the (6) stage of contraction proper. The lever is raised by the sudden contraction of the muscle. The 470 THE MUSCULAR SYSTEM. [chap. xiv. contraction is at first very rapid, but then progresses more slowly to its maximum, indicated by the line m x, drawn through its highest point. It occupies in the figure T sec. (c) The next stage, stage of elongation. After reaching its highest point, the lever begins to descend, in consequence of the elongation of the muscle. At first the fall is rapid, but then becomes more gradual until the lever reaches the abscissa or base line, and the muscle attains its precontraction length, indicated in the figure by the line c'. This stage occupies second. Very Fig. 290.-Tracing of a double muscle-curve. To be read from left to right. While the muscle was engaged in the first contraction (whose complete course, had nothing inter- vened, 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. (M. Foster.) often after the main contraction the lever rises once or twice to a slight degree, producing curves, one of which is seen in fig. 290. These contractions, due to the elasticity of the muscle, are called most properly (<Z) Stage of elastic after-vibration, or contraction remainder. The muscle curve obtained from the heart resembles that of unstriped muscles in the long duration of the effect of stimulation ; the descending curve also is very much prolonged. The greater part of the latent period is taken up by changes in the muscle itself, and the remainder occupied in the propagation of the shock along the nerve. Tetanus.-If we stimulate the nerve-muscle preparation with two induction shocks, one immediately after the other, when the point of stimulation of the second one corresponds to the maximum of the first, a second curve (fig. 290) will occur, which will com- mence at the highest point of the first and will rise nearly as high, so that the sum of the height of the two curves almost exactly CHAP. XIV.] CURVES OF TETANUS. 471 equals twice the height of the first. If a third and a fourth shock be passed, a similar effect will ensue, and curves one above the other will be traced, the third being slightly less than the second, Fig. 291.-Curve of tetanus, obtained from the gastrocnemius of a frog, where the shocks were sent in from an induction coil, about sixteen times a second, by the interruption of the primary cun-ent by means of a vibrating spring, which dipped into a cup of mercury, and broke the primary cunent at each vibration. and the fourth than the third. If a more numerous series of shocks occur, however, the lever after a time ceases to rise any further, and the contraction, which has reached its maximum, is maintained. The condition which ensues is called Tetanus. A tetanus is really a summation of contractions, and unless the stimuli become very rapid indeed, the muscle will be then Fig. 292.-Curve of tetanus, from a series of very rapid shocks from a magnetic interrupter. in a condition of vibratory contraction and not of unvarying contraction. Tf the shocks, however, be repeated at very short intervals, being 15 per second for the frog's muscle, but varying in each animal, the muscle contracts to its utmost suddenly and con- tinues at its maximum contraction for some time and the lever rises almost perpendicularly, and then describes a straight line (fig. 292). If the stimuli are not quite so rapid the line of maximum contraction 472 THE MUSCULAR SYSTEM. [chap. xiv. becomes somewhat wavy, indicating a slight tendency of the muscle to relax during the intervals between the stimuli (fig. 291). Muscular Work.-We have seen that work is estimated by multiplying the weight raised, by the height through which it has been lifted. It has been found that in order to obtain the maximum of work, a muscle must be moderately loaded: if the weight is increased beyond a certain point, the muscle becomes strained and raises the weight through so small a distance that less work is accomplished. If the load is still further increased Fig. 293.-Diagram of fatigue muscle-curves. (Ray Lankester the muscle is completely overtaxed, and cannot raise the weight. No work is then done at all. Practical illustrations of these facts must be familiar to every one. The power of a muscle is usually measured by the maximum weight which it will support without stretching. In man this is readily determined by weighting the body to such an extent, that it can no longer be raised on tiptoe : thus the power of the calf-muscles is determined. The power of a muscle thus estimated depends of course upon its cross- section. The power of a human muscle is from two to three times as great as a frog's muscle of the same sectional area. Fatigue of Muscle.-A muscle becomes rapidly exhausted from repeated stimulation, and the more rapidly, the more quickly the induction-shocks succeed each other. This is indicated by the diminished height of the muscular contractions. CHAP. XIV.] ACCOMPANIMENTS OF MUSCULAR CONTRACTION. 473 It will be seen in fig. 293 that the vertical lines, which indicate the extent of the muscular contraction, decrease in length from left to right. The line A b drawn along the tops of these lines is termed the "fatigue curve." It is' usually a straight line. In the first diagram the effects of a short rest are shown : there is a pause of three minutes, and when the muscle is again stimulated, it con- tracts up to A', but the recovery is only temporary, and the fatigue curve, after a few more contractions, becomes continuous with that before the rest. In the second diagram is represented the effect of a stream of oxygenated blood. Here we have a sudden restoration of energy : the muscle in this case makes an entirely fresh start from A, and the new fatigue curve is parallel to, and never coincides with the old one. A fatigued muscle has a much longer latent period than a fresh one. The slowness with which muscles respond to the will when fatigued must be familiar to every one. In a muscle which is exhausted, stimulation only causes a contraction producing a local bulging near the point irritated. A similar effect may be produced in a fresh muscle by a sharp blow, as in striking the biceps smartly with the edge of the hand, when a hard muscular swelling is instantly formed. Accompaniments of Muscular Contraction. (i.) Heat is developed in the contraction of muscles. Becquerel and Breschet found, with the thermo-multiplier, about x° Fahr, of heat produced by each forcible contraction of a man's biceps ; and when the actions were long continued, the temperature of the muscle increased 2°. This estimate is probably high, as in the frog's muscle a considerable contraction has been found to produce an elevation of temperature equal on an average to less than 1° C. It is not known whether this development of heat is due to chemical changes ensuing in the muscle, or to the friction of its fibres vigorously acting : in either case, we may refer to it a part of the heat developed in active exercise. (2.) Sound is said to be produced when muscles contract forcibly, as mentioned above. Wollaston showed that this sound might be easily heard by placing the tip of the little finger in the ear, and then making some muscles contract, as those of the ball of the thumb, whose sound may be conducted to the ear through the substance of the hand and finger. A low shaking or rumbling sound is heard, the height and loudness of the note being in direct proportion to the force and quickness of the mus- 474 THE MUSCULAR SYSTEM. [chap. xiv. cular action, and to the number of fibres that act together, or, as it were, in time. (3.) Changes in Shape.-The mode of contraction in the trans- vcrsely-striated muscular tissue has been much disputed. The most probable account is, that the contraction is effected by an approximation of the constituent parts of the fibrils, which, at the instant of contraction, without any alteration in their general direction, become closer, flatter, and wider ; a condition which is rendered evident by the approximation of the transverse stria? seen on the surface of the fasciculus, and by its increased breadth h f c <x b d e § Fig. 294.- The microscopic appearances during a muscular contraction in the individual fibrilla after Engelmann. 1. A passive muscle fibre ; c to d = doubly refractive dises, with median disc a b in it; h and g are lateral discs; f and e are secondary discs, only slightly doubly refractive ; fig. on right same fibre in polarised light; bright part is doubly refracted, black ends not so. 2. Transition stage; and 3. Stage of entire contraction ; in each case the right-hand figure represents the effect of polarised light. (Landois after Engelmann.) and thickness. '■■The appearance of the zigzag lines into which it was supposed the fibres are thrown in contraction, is due to the relaxation of a fibre which has been recently contracted, and is not at once stretched again by some antagonist fibre, or whose extremities are kept close together by the contractions of other fibres. The contraction is therefore a simple, and, according to Ed. Weber, a uniform, simultaneous, and steady shortening of each fibre and its contents. What each fibril or fibre loses in length, it gains in thickness: the contraction is a change of form not of size; it is, therefore, not attended with any diminution in bulk, from condensation of the tissue. This has been proved for entire muscles, by making a mass of muscle, or many fibres together, contract in a vessel full of water, with which a fine, perpendicular, graduated tube communicates. Any diminution of the bulk of the contracting muscle would be attended by a fall of fluid in the tube; but when the experiment is carefully performed, the level of the water in the tube remains the same, whether the muscle be contracted or not. CHAP, XIV.] CHANGES IN A CONTRACTING MUSCLE. 475 In thus shortening, muscles appear to swell up, becoming rounder, more prominent, harder, and apparently tougher. But this hardness of muscle in the state of contraction, is not due to increased firmness or condensation of the muscular tissue, but to the increased tension to which the fibres, as well as their tendons and other tissues, are subjected from the resistance ordinarily opposed to their contraction. When no resistance is offered, as when a muscle is cut off from its tendon, not only is no hardness perceived during contraction, but the muscular tissue is even softer, more extensile, and less clastic than in its ordinary uncon- tracted state. During contraction in each fibre it is said that the anisotropous or doubly refractive elements become less refractive and the singly refractive more so (fig. 294). (4.) Chemical changes.-(a) The reaction of the muscle which is normally alkaline or neutral becomes decidedly acid, from the development of sarcolactic acid. (6) The muscle gives out car- bonic acid gas and takes up oxygen, the amount of the C02 given out not appearing to be entirely dependent upon the 0 taken in, and so doubtless in part arising from some other source, (c) Certain imperfectly understood chemical changes occur, in all probability connected with (a) and (6). Glycogen is diminished, and glucose, or muscle sugar (inosite) appears ; the extractives are increased. (5.) Electrical changes.-When a muscle contracts the natural muscle current or currents of rest undergo a distinct diminution, which is due to the appearance in the actively contracting muscle of currents in an opposite direction to those existing in the muscle at rest. This causes a temporary deflection of the needle of a galvanometer in a direction opposite to the original current, and is called by some the negative variation of the muscle current, and by others a current of action. Conditions of Contraction.-(a) The irritability of muscle, Fig. 295.-Muscle-curves from the gastrocnemius of a frog, illustrating effects of alterations in temperature. as indicated by length of latent period, velocity and extent of con- traction, is greatest at a certain mean temperature ; (6) after a number of contractions a muscle gradually becomes exhausted ; 476 THE MUSCULAR SYSTEM. [chap. XIV. (c) the activity of muscles after a time disappears altogether when they are removed from the body or the arteries are tied ; (<7) oxygen is used up in muscular contraction, but a muscle will act for a time in vacuo or in a gas which contains no oxygen: in this case it is of course using up the oxygen already in store ; (e) the contraction is greater if the stimulus is applied to the nerve, than if it be applied to the muscle directly. Response to Stimuli.-The two kinds of fibres, the striped and the unstriped, have characteristic differences in the mode in which they act on the application of the same stimulus; differ- ences which may be ascribed in great part to their respective differences of structure, but to some degree, possibly, to their respective modes of connection with the nervous system. When irritation is applied directly to a muscle with striated fibres, or to the motor nerve supplying it, contraction of the part irritated, and of that only, ensues ; and this contraction is instantaneous, and ceases on the instant of withdrawing the irritation. But when any part with unstriped muscular fibres, e.g., the intestines or bladder, is irritated, the subsequent contraction ensues more slowly, extends beyond the part irritated, and, with alternating relaxation, continues for some time after the withdrawal of the irritation. The difference in the modes of contraction of the two kinds of muscular fibres may be particularly illustrated by the effects of the repeated stimuli with the magnetic interrupter. The rapidly succeeding shocks given by this means to the nerves of muscles excite in all the transversely-striated muscles, except in the case of the heart, a fixed state of tetanic contraction as previously described, which lasts as long as the stimulus is con- tinued, and on its withdrawal instantly ceases; but in the muscles with unstriped fibres they excite a slow vermicular movement, which is comparatively slight and alternates with rest. It con- tinues for a time after the stimulus is withdrawn. In their mode of responding to these stimuli, all the skeletal muscles, or those with transverse striae, are alike ; but among those with unstriped fibres there are many differences-a fact which tends to confirm the opinion that their peculiarity depends as well on their connection with nerves and ganglia as on their own properties. The ureters and gall-bladder are the parts least excited by stimuli ; they do not act at all till the stimulus has been long applied, and then contract feebly, and to a small extent. The contractions of the caecum and stomach are quicker and wider-spread ; still quicker those of the iris, and of the urinary bladder if it be not too full. The actions of the small and large intestines, of the vas deferens, and pregnant uterus, are yet more vivid, more regular, and more sustained ; CHAP. XIV.] PHENOMENA OF RIG OK MORTIS. 477 and they require no more stimulus than that of the air to excite them. The heart, on account, doubtless, of its striated muscle, is the quickest and most vigorous of all the muscles of organic life in contracting upon irri- tation, and appears in this, as in nearly all other respects, to be the connecting member of the two classes of muscles. All the muscles retain their property of contracting under the influence of stimuli applied to them or to their nerves for some time after death, the period being longer in cold-blooded than in warm-blooded Vertebrata, and shorter in Birds than in Mammalia. It would seem as if the more active the respiratory process in the living animal, the shorter is the time of duration of the irritability in the muscles after death : and this is confirmed by the comparison of different species in the same order of Vertebrata. But the period during which this irritability lasts, is not the same in all persons, nor in all the muscles of the same persons. In a man it ceases, according to Nysten, in the following order :-first in the left ventricle, then in the intestines and stomach, the urinary bladder, right ventricle, oeso- phagus, iris ; then in the voluntary muscles of the trunk, lower and upper extremities ; lastly, in the right and left auricle of the heart. C. Rigor Mortis. After the muscles of the dead body have lost their irritability or capability of being excited to contraction by the application of a stimulus, they spontaneously pass into a state of contraction, apparently identical with that which ensues during life. It affects all the muscles of the body; and, where external circum- stances do not prevent it, commonly fixes the limbs in that which is their natural posture of equilibrium or rest. Hence, and from the simultaneous contraction of all the muscles of the trunk, is produced a general stiffening of the body, constituting the rigor mortis or post-mortem rigidity. When this condition has set in, the muscle (a) becomes acid in reaction (due to development of sarco-lactic acid), (6) gives off car- bonic acid in great excess, (c) Its volume is slightly diminished; Iff) the muscular fibres become shortened and opaque, and their substance sets firm. It comes on much more rapidly after muscular activity, and is hastened by warmth. It may be brought on, in muscles exposed for experiment, by the action of distilled water and many acids, also by freezing and thawing again. Cause.-The immediate cause of rigor seems to be a chemical one, namely, the coagulation of the muscle plasma. We may dis- tinguish three main stages-(i.) Gradual coagulation. (2.) Con- traction of coagulated muscle-clot (myosin), and squeezing out of muscle-serum. (3.) Putrefaction. After the first stage, restora- tion is possible through the circulation of arterial blood through 478 THE MUSCULAR SYSTEM. [chap. xiv. the muscles, and even when the second stage has set in, vitality may be restored by dissolving the coagulum of the muscle in salt solution, and passing arterial blood through its vessels. In the third stage recovery is impossible. Order of Occurrence.-The muscles are not affected simul- taneously by rigor mortis. It affects the neck and lower jaw first ; next, the upper extremities, extending from above down- wards ; and lastly, reaches the lower limbs; in some rare in- stances only, it affects the lower extremities before, or simul- taneously with, the upper extremities. It usually ceases in the order in which it began : first at the head, then in the upper extremities, and lastly, in the lower extremities. It never com- mences earlier than ten minutes, and never later than seven hours, after death ; and its duration is greater in proportion to the lateness of its accession. Heat is developed during the passage of a muscular fibre into the condition of rigor mortis. Since rigidity does not ensue until muscles have lost the capacity of being excited by external stimuli, it follows that all circumstances which cause a speedy exhaustion of muscular irri- tability, induce an early occurrence of the rigidity, while con- ditions by which the disappearance of the irritability is delayed, arc succeeded by a tardy onset of this rigidity. Hence its speedy occurrence, and equally speedy departure in the bodies of persons exhausted by chronic diseases ; and its tardy onset and long con- tinuance after sudden death from acute diseases. In some cases of sudden death from lightning, violent injuries, or paroxysms of passion, rigor mortis has been said not to occur at all; but this is not always the case. It may, indeed, be doubted whether there is really a complete absence of the post-mortem rigidity in any such cases; for the experiments of Brown-Sdquard make it pro- bable that the rigidity may supervene immediately after death, and then pass away with such rapidity as to be scarcely observable. Experiments. - Brown-S6quard took five rabbits, and killed them by removing their hearts. In the first, rigidity came on in io hours, and lasted 192 hours ; in the second, which was feebly electrified, it commenced in 7 hours, and lasted 144; in the third, which was more strongly electrified, it came on in two, and lasted 72 hours ; in the fourth, which was still more strongly electrified, it came on in one hour, and lasted 20; while, in the last rabbit, which was submitted to a powerful electro-galvanic current, the rigidity ensued in seven minutes after death, and passed away in 25 minutes. From this it appears that the more powerful the electric CHAP. XIV.] MUSCULAR ACTIONS. 479 current, the sooner does the rigidity ensue, and the shorter is its duration : and as the lightning shock is so much more powerful than any ordinary electric discharge, the rigidity may ensue so early after death, and pass away so rapidly as to escape detection. The influence exercised upon the onset and duration of post-mortem rigidity by causes which exhaust the irritability of the muscles, was well illustrated in further experiments by the same physiologist, in which he found that the rigor mortis ensued far more rapidly, and lasted for a shorter period in those muscles which had been powerfully electrified just before death than those which had not been thus acted upon. The occurrence of rigor mortis is not prevented by the previous existence of paralysis in a part, provided the paralysis has not been attended with very imperfect nutrition of the muscular tissue. The rigidity affects the involuntary as well as the voluntary muscles, whether they be constructed of striped or unstriped fibres. The rigidity of involuntary muscles with striped fibres is shown in the contraction of the heart after death. The con- traction of the muscles with unstriped fibres is shown by an experiment of Valentin, who found that if a graduated tube connected with a portion of intestine taken from a recently-killed animal, be filled with water, and tied at the opposite end, the water will in a few hours rise to a considerable height in the tube, owing to the contraction of the intestinal walls. It is still better shown in the arteries, of which all that have muscular coats con- tract after death, and thus present the roundness and cord-like feel of the arteries of a limb lately removed, or those of a bodv recently dead. Subsequently they relax, as do all the other muscles, and feel lax and flabby, and lie as if flattened, and with their walls nearly in contact. Actions of the Voluntary Muscles. The greater part of the voluntary muscles of the body act as sources of power for removing levers,-the latter consisting of the various bones to which the muscles are attached. Examples of the three orders of levers in the Human Body.-All levers have been divided into three kinds, according to the relative position of the power, the weight to be removed, and the axis of motion or fulcrum. In a lever of the first kind the power is at one extremity of the lever, the weight at the other, and the fulcrum between the two. If the initial letters only of the power, weight, and f ulcrum be used, the arrangement will stand thus:-P.F.W. A poker as ordinarily used, or the bar in fig. 296, may be 480 THE MUSCULAR SYSTEM. [chap. XIV. cited as an example of this variety of lever ; while, as an instance in which the bones of the human skeleton are used as a lever of the same kind, may be mentioned the act of raising the body from the stooping posture by P F W P ]? *V Fig. 206. means of the hamstring muscles attached to the tuberosity of the ischium (fig. 296). In a lever of the second kind, the arrangement is thus :-P.W.F.; and this leverage is employed in the act of raising the handles of a wheelbarrow, or in stretching an elastic band, as in fig. 297. In the human body the act of opening the mouth by depressing the lower jaw is an example of the same kind-the tension of the muscles which close the jaw representing the weight (fig. 297). In a lever of the third kind the arrangement is-F.P.W., and the act of raising a pole, as in fig. 297, is an example. In the human body there arc numerous examples of the employment of this kind of leverage. The act of bending the fore-arm may be mentioned as an instance (fig. 298). The act of biting is another example. Fig. 297. CHAP. XIV.] MUSCULAR ACTIONS. 481 At the ankle we have examples of all three kinds of lever. 1st kind- Extending the foot. 3rd kind-Flexing the foot. In both these cases the foot represents the weight: the ankle joint the fulcrum, the power being the calf muscles in the first case and the tibialis anticus in the second case. F P W F p W Fig'. 298. 2nd kind-When the body is raised bn tip-toe. Here the ground is the fulcrum, the weight of the body acting at the ankle joint the weight, and the calf muscles the power. In the human body, levers are most frequently used at a disadvantage as regards power, the latter being sacrificed for the sake of a greater range of motion. Thus in the diagrams of the first and third kinds it is evident that the power is so close to the fulcrum, that great force must be exercised in order to produce motion. It is also evident, however, from the same diagrams, that by the closeness of the power to the fulcrum a great range of move- ment can be obtained by means of a comparatively slight shortening of the muscular fibres. The greater number of the more important muscular actions of the human body-those, namely, which are arranged harmoniously so as to subserve some definite purpose or other in the animal economy-are described in various parts of this work, in the sec- tions which treat of the physiology of the processes by which these muscular actions are resisted or earned out. There are, however, one or two very important and somewhat complicated muscular acts which may be best described in this place. Walking.-In the act of walking, almost every voluntary muscle in the body is brought into play, either directly for purposes of progression, or indirectly for the proper balancing of the head and trunk. The muscles of the arms are least concerned ; but even these are for the most part instinctively in action also to some extent. Among the chief muscles engaged directly in the act of walking are those of the calf, which, by pulling up the heel, pull up also the astragalus, and with it, of course, the whole body, the weight of which is transmitted through the tibia to this bone (fig. 298). When starting to walk, say with 482 THE MUSCULAR SYSTEM. [CHAP. XIV. the left leg, this raising of the body is not left entirely to the muscles of the left calf, but the trunk is thrown forward in such a way, that it would fall prostrate were it not that the light foot is brought forward and planted on the ground to support it. Thus the muscles of the left calf are assisted in their action by those muscles on the front of the trunk and legs which, by their contraction, pull the body forwards; and, of course, if the trunk form a slanting line, with the inclination forwards, it is plain that when the heel is raised by the calf-muscles, the whole body will be raised, and pushed obliquely forwards and upwards. The successive acts in taking the first step in walking are represented in fig. 299, 1, 2, 3. Now it is evident that by the time the body has assumed the position No. 3, it is time that the right leg should be brought forward to support it 1 2 3 4 5 Fig. 299. and prevent it from falling prostrate. This advance of the other leg (in this case the is effected partly by its mechanically swinging forwards, pendulum-wise, and partly by muscular action ; the muscles used being- 1st, those on the front of the thigh, which bend the thigh forwards on the pelvis, especially the rectus femoris, with the psoas and the iliacus ; zndly, the hamstring muscles, which slightly bend the leg on the thigh ; and, ydly, the muscles on the front of the leg, which raise the front of the foot and toes, and so prevent the latter in swinging forwards from hitching in the ground. The second part of the act of walking, which has been just described, is showm in the diagram (4, fig. 299). When the right foot has reached the ground the action of the left leg has not ceased. The calf-muscles of the latter continue to act, and by pulling up the heel, throw the body still more forwards over the right leg, now bearing nearly the whole weight, until it is time that in its turn the left leg should swing forwards, and the left foot be planted on the ground to prevent the body from falling prostrate. As at first, while the calf muscles of one leg and foot are preparing, so to speak, to push the body forward and upward from behind by raising the heel, the muscles on the front of the trunk and of the same leg (and of the other leg, except when it is swinging forwards) are helping the act by pulling the legs and trunk, so as to make them incline forward, the rotation in the inclining forwards being effected mainly at the ankle joint. Two main kinds of leverage are, therefore, employed in the act of walking, and if this idea be firmly grasped, the details will be under- stood with comparative ease. One kind of leverage employed in walking is essentially the same with that employed in pulling forward the pole, as in fig. 298. And the other, less exactly, is that employed in raising the handle CHAP. XIV.] MUSCULAR ACTIONS, 483 of a wheelbarrow. Now, supposing the lower end of the pole to be placed in the barrow, we should have a very rough and inelegant, but not altogether bad representation of the two main levers employed in the act of walking. The body is pulled forward by the muscles in front, much in the same way that the pole might be by the force applied at P. (fig. 298), while the raising of the heel forwards of the trunk by the calf-muscles is roughly represented on raising the handles of the barrow. The manner in which these actions are performed alternately by each leg, so that one after the other is swung forwards to support the trunk, which is at the same time Fig. 300. pushed and jniUed forwards by the muscles of the other, may be gathered from the previous description. There is one more ithing to be noticed especially in the act of walking. Inasmuch as the body is being constantly supported and balanced on each leg alternately, and therefore on only one at the same moment, it is evident that there must be some provision made for throwing the centre of gravity over the line of support formed by the bones of each leg, as, in its turn, it supports the weight of the body. This may be done in various ways, and the manner in which it is eifected is one element in the differences which exist in the walking of different people. Thus it may be done by an instinctive slight rotation of the pelvis on the head of each femur in turn, in such a manner that the centre of gravity of the body shall fall over the foot of this side. Thus when the body is pushed onwards and upwards by the raising, say, of the right heel, as in fig. 299, 3, the pelvis is instinctively by various muscles, made to rotate on the head of the left femur at the ace- tabulum, to the left side, so that the weight may fall over the line of support formed by the left leg at the time that the right leg is swinging forwards, and leaving all the work of support to fall on its fellow. Such a " rocking " 484 THE MUSCULAR SYSTEM. [chap. xiv. movement of the trunk and pelvis, however, is accompanied by a movement of the whole trunk and leg over the foot which is being planted on the ground (fig. 300) ; the action being accompanied with a compensatory out- ward movement at the hip, more easily appreciated by looking al the figure (in which this movement is shown exaggerated) than described. Thus the body in walking is continually rising and swaying alternately from one side to the other, as its centre of gravity has to be brought alter- nately over one or other leg ; and the curvatures of the spine are altered in correspondence with the varying position of the weight which it has to support. The extent to which the body is raised or swayed differs much in different people. In walking, one foot or the other is always on the ground. The act of leaping1 or jumping-, consists in so sudden a raising of the heels by the sharp and strong contraction of the calf-muscles, that the body is jerked off the ground. At the same time the effect is much increased by first bending the thighs on the pelvis, and the legs on the thighs, and then suddenly straightening out the angles thus formed. The share which this action has in producing the effect may be easily known by attempting to leap in the upright posture, with the legs quite straight. Running- is performed by a series of rapid low jumps with each leg alter- nately ; so that, during each complete muscular act concerned, there is a moment when both feet are off the ground. In all these cases, however, the description of the manner in which any given effect is produced, can give but a very imperfect idea of the infinite number of combined and harmoniously arranged muscular contractions which are necessary for even the simplest acts of locomotion. Action of the Involuntary Muscles.-The involuntary muscles are for the most part not attached to bones arranged to act as levers, but enter into the formation of such hollow parts as require a diminution of their calibre by muscular action, under particular circumstances. Examples of this action are to be found in the intestines, urinary bladder, heart and blood-vessels, gall- bladder, gland-ducts, etc. The difference in the manner of contraction of the striated and non-striated fibres has been already referred to (p. 476); and the peculiar vermicular or peristaltic action of the latter fibres has been described at p. 476. Source of Muscular Action.- It was formerly supposed that each act of contraction on the part of a muscle was accompanied by a correlative waste or destruction of its own substance; and that the quantity of the nitrogenous excreta, especially of urea, presumably the expression of this waste, was in exact proportion to the amount of muscular work performed. It has been found, however, both that the theory itself is erroneous, and that the supposed facts on which it was founded do not exist. It is true that in the action of muscles, as of all other parts, CHAP. XIV.] ELECTRICAL CURRENTS IN NERVES. 485 there is a certain destruction of tissue or, in other words, a certain ' wear and tear ' which may be represented by a slight increase in the quantity of urea excreted; but it is not the correlative expression or only source of the power manifested. The increase in the amount of urea which is excreted after mus- cular exertion is by no means so great as was formerly supposed; indeed, it is very slight. And as there is no reason to believe that the waste of muscle-substance can be expressed, with un- important exceptions, in any other way than by an increased excretion of urea, it is evident that we must look elsewhere than in destruction of muscle, for the source of muscular action. For, it need scarcely be said, all force manifested in the living body must be the correlative expression of force previously latent in the food eaten or the tissue formed; and evidences of force expended in the body must be found in the excreta. If, there- fore, the nitrogenous excreta, represented chiefly by urea, are not in sufficient quantity to account for the work done, we must look to the non-nitrogenous excreta, as carbonic acid and water, which, presumably, cannot be the expression of wasted muscle- substance. The quantity of these non-nitrogenous excreta is undoubtedly increased by active muscular efforts, and to a considerable extent; and whatever may be the source of the water, the carbonic acid, at least, is the result of chemical action in the system, and espe- cially of the combustion of non-nitrogenous food, although, doubt- less, of nitrogenous food also. We are, therefore, driven to the conclusion,-that the substance of muscles is not wasted in pro- portion to the work they perform; and that the non-nitrogenous as well as the nitrogenous foods may, in their combustion, afford the requisite conditions for muscular action. The urgent neces- sity for nitrogenous food, especially after exercise, is probably due more to the need of nutrition by the exhausted muscles and other tissues for which, of course, nitrogen is essential, than to such food being superior to non-nitrogenous substances as a source of muscular power. Electrical Currents in Nerves. The electrical condition of Nerves is so closely connected with the phenomena of muscular contraction, that it will be convenient to consider it in the present chapter. 486 THE MUSCULAR SYSTEM. [chap. xiv. If a piece of nerve be removed from the body and subjected to examination in a way similar to that adopted in the case of muscle which has been described (p. 460), electrical currents are found to exist which correspond exactly to the natural muscle currents, and which are called natural nerve currents or currents of rest, according as one or other theory of their existence be adopted, as in the case with muscle. One point (equator) on the surface being positive to all other points nearer to the cut ends, and the greatest deflection of the needle of the galvanometer taking place when one electrode is applied to the equator and the other to the centre of either cut end. As in the case of muscle, these nerve-currents under- go a negative variation when the nerve is stimulated, the varia- tion being momentary and in the opposite direction to the natural currents; and are similarly known as the currents of action. The currents of action are propagated in both directions from the point of the application of the stimulus, and are of momentarv duration, Rheoscopic Frog:.-This negative variation may be demonstrated by means of the following experiment.-The new current produced by stimu- lating the nerve of one nerve-muscle preparation may be used to stimulate the nerve of a second nerve muscle preparation. The foreleg of a frog with the nerve going to the gastrocnemius cut long is placed upon a glass plate, and arranged in such way that its nerve touches in two places the sciatic nerve, exposed but preserved in situ in the opposite thigh of the frog. The electrodes from an induction coil are placed behind the sciatic nerve of the second preparation, high up. On stimulating it with a single induction shock, the muscles not only of the same leg are found to undergo a twitch, but also those of the first preparation, although this is not near the elec- trodes, and so the stimulation cannot be due to an escape of the current into the first nerve. This experiment is known under the name of the rheoscopic Nerve-stimuli.-Nerve-fibres require to be stimulated before they can manifest any of their properties, since they have no power of themselves of generating force or of originating impulses. The stimuli which are capable of exciting nerves to action, arc, as in the case of muscle, very diverse. They are very similar in each case. The mechanical, chemical, thermal, and electric stimuli which may be used in the one case are also, with certain differences in the methods employed, efficacious in the other. The chemical stimuli are chiefly these: withdrawal of water, as by drying, strong solutions of neutral salts of potassium, sodium, Ac., free inorganic acids, except phosphoric; some organic acids; ether, chloroform, and bile salts. The electrical stimuli employed are CHAP. XIV.] NERVE CURRENTS. 487 the induction and continuous currents concerning which the observations in reference to muscular contraction should be con- sulted, p. 465. Weaker electrical stimuli will excite nerve than will excite muscle; the nerve stimulus appears to gain strength as it descends, and a weaker stimulus applied far from the muscle will have the same effect as a stronger one applied to the nerve near the muscle. It will be only necessary here to add some account of the effect of a constant electrical current, such as that obtained from Daniell's battery, upon a nerve. This effect may be studied with the apparatus described before. A pair of electrodes are placed behind the nerve of the nerve-muscle preparation, with a Du Bois Reymond's key arranged for short circuiting the battery current, in such a way that when the key is opened the current is sent into the nerve, and when closed the current is cut off. It will be found that with a current of moderate strength there will be a contraction of the muscle both at the opening and at the closing of the key (called respectively making and breaking contractions), but that during the interval between these two events the muscle remains flaccid, provided the battery current continues of constant intensity. If the current be a very weak or a very strong one the effect is not quite the same; one or other of the contractions may be absent. Which of these contractions is absent depends upon another circumstance, viz., the direction of the current. The direction of the current may be ascending or descending; if ascending, the anode or positive pole is nearer the muscle than the kathode or negative pole, and the current to return to the battery has to pass up the nerve, if descending, the position of the electrodes is reversed. It will be necessary before considering this question further to return to the apparent want of effect of the constant current during the interval between the make and break contraction: to all appearances no effect is produced at all, but in reality a very important change is brought about in the nerve by the passage of this constant (polarising) current. This may be shown in two ways, first of all by the galvanometer. If a piece of nerve be taken, and if at either end an arrangement be made to test the electrical condition of the nerve by means of a pair of non-polarisable electrodes connected with a galvanometer, while to the central portion a pair of electrodes connected with a Daniell's battery be applied, it will be found that the natural nerve-currents are profoundly altered on the passage of the constant 488 THE MUSCULAR SYSTEM. [chap. XIV. current in the neighbourhood. If the polarising current be in the same direction as the latter the natural current is increased, but if in the direction opposite to it, the natural current is diminished. This change, produced by the continual passage of the battery-current through a portion of the nerve, is to be distin- guished from the negative variation of the natural current to which allusion has been already made, and which is a momentary change Occurring on the sudden application of the stimulus. The condition produced by the passage of a constant current is known by the name of Electrotonus. The other way of showing the effect of the same polarising cur- Fig. 301.-Diagram illustrating the effects of various intensities of the polarising currents, n, n', nerve; a, anode; k, kathode; the curves above indicate increase, and those below decrease of irritability, and when the current is small the increase and decrease are both small, with the neutral point near a, and so on as the current is increased in strength. rent is by taking a nerve-muscle preparation and applying to the nerve a pair of electrodes from an induction coil whilst at a point further removed from the muscle, electrodes from a Daniell's battery are arranged with a key for short circuiting and an ap- paratus (reverser) by which the battery current may be reversed in direction. If the exact point be ascertained to which the secon- dary coil should be moved from the primary coil in order that a minimum contraction be obtained by the induction shock, and the secondary coil be removed slightly further from the primary, the induction current cannot now produce a contraction ; but if the polarising current be sent in a descending direction, that is to say, with the kathode nearest the other electrodes, the induction cur- rent, which was before insufficient, will prove sufficient to cause a contraction; whereby indicating that with a descending current the irritability of the nerve is increased. By means of a somewhat similar experiment it may be shown that an ascending current CHAP. XIV.] LAW OF CONTRACTIONS. 489 will diminish the irritability of a nerve. Similarly, if instead of applying the induction electrodes below the other electrodes they are applied between them, like effects are demonstrated, indicating that in the neighbourhood of the kathode the irritability of the nerve is increased by a constant current, and in the neighbourhood of the anode diminished. This increase in irritability is called kathelectrotonus, and similarly the decrease is called andectrotonus. As there is between the electrodes both an increase and a decrease of irritability on the passage of a polarising current, it must be evident that the increase must shade off into the decrease, and that there must be a neutral point where there is neither increase nor decrease of irritability. The position of this neutral point is found to vary with the intensity of the polarising current-when the current is weak the point is nearer the anode, when strong nearer the kathode (fig. 301); when a constant cun-ent passes into a nerve, therefore, if a contraction result, it may be assumed that it is due to the increased irritability produced in the neighbour- hood of the kathode, but the breaking contraction must be produced by a rise in irritability from a lowered state to the normal in the neighbourhood of the anode. The contractions produced in the muscle of a nerve-muscle preparation by a constant current have been arranged in a table which is known as Pfluger's Law of Contractions. It is really only a statement as to when a contraction may be expected :- Descending Curren r. Ascending Current. Make. Break. Make. Break. Weak - Ye . No. Yes. No. Moderate ... Yes. Yes. Yes. Yes. Strong Yes. No. No. Yes. The difficulty in this table is chiefly in the effect of a weak current, but the following statement will explain it. The increase of irritability at the kathode is more potent to produce a contrac- tion than the rise of irritability at the anode, and so with weak currents the only effect is a contraction at the make of both cur- rents, and the descending current is more potent than the ascend- ing (and with still weaker currents is the only one which produces any effect), since the kathode is near the muscle; whereas in the 490 INCOME AND EXPENDITURE OF BODY. [chap. xv. case of the ascending current the stimulus has to pass through a district of diminished irritability, which with a very strong cur- rent acts as a block, but with a weak only slightly affects the contraction. As the current is stronger recovery from anelectro- tonus is able to produce a contraction as well as kathelectrotonus, a contraction occurs both at the make and the break of the cur- rent. The absence of contraction with a very strong current at the break of the ascending current may be explained by supposing that the region of fall in irritability at the kathode blocks the stimulus of the rise in irritability at the anode. Thus we have seen that two circumstances influence the effect of the constant current upon a nerve, viz., the strength and direc- tion of the current. It is also necessary that the stimulus should be applied suddenly and not gradually, and that the irritability of the nerve be normal, and not increased or diminished. Sometimes (when the nerve is specially irritable ?) instead of a simple con- traction a tetanus occurs at the make or break of the constant current. This is especially liable to occur at the break of a strong ascending current which has been passing for some time into the preparation ; this is called Ritter's tetanus, and may be increased by passing a current in an opposite direction or stopped by passing a current in the same direction. CHAPTER XV. NUTRITION ; THE INCOME AND EXPENDITURE OF THE HUMAN BODY. The various physiological processes which occur in the human body have, with the exception of those in the nervous and gene- rative systems, which will be considered in succeeding chapters, now been dealt with, and it will be as well to give in this chapter a summary of what has been considered more at length before. The subject may be considered under the following heads, (i). The Evidence and Amount of Expenditure. (2). The Sources and Amount of Income. (3). The Sources and Objects of Expen- diture. CHAP, xv.] AMOUNT OF EXPENDITURE. 491 1. Evidence and Amount of Expenditure.-There is com- plete evidence of Expenditure by the living body. From the table (p. 238) it will be seen how the various amounts of the excreta are calculated. a. From the Lungs there is exhaled every 24 hours, Of Carbonic Acid, about .... 15,000 grains ,, Water 5,000 „ Traces of organic matter. b. From the Skin- Water . . . . . . . 11,500 grains Solid and gaseous matter . . . . 250 „ c. From the Kidneys- Water ....... 23,000 grains Organic matter 680 „ Minerals or salines ..... 420 „ d. From the Intestines- Water 2,000 grains Various organic and mineral substances . 800 ., In the account of Expenditure, must be remembered in addition the milk (during the period of suckling), and the products of secretion from the generative organs (ova, menstrual blood, semen); bnt, from their variable and uncertain amounts, these cannot be reckoned with the preceding. Altogether, the expenditure of the body represented by the sum of these various excretory products amounts every 24 hours to- Solid and gaseous matter .... 17.150 grains (1,113 grms.) Water (either fluid or combined with the solids and gaseous matter) . . . . 49.500 „ (2,695 » ) The matter thus lost by the body is matter the chemical at- tractions of which have been in great part satisfied; and which remains quite useless as food, until its elements have been again separated and re-arranged by members of the vegetable world. It is especially instructive to compare the chemical constitution of the products of expenditure, thus separated by the various excretory organs, with that of the sources of income to be imme- diately considered. It is evident from these facts that if the human body is to maintain its size and composition, there must be added to it matter corresponding in amount with that which is lost. The income must equal the expenditure. 492 INCOME AND EXPENDITURE OF BODY. [chap. xv. 2. Sources and Amount of Income.-The Income of the body consists partly of Food and Drink, and partly of Oxygen. Into the stomach there is received daily :- Solid (chemically dry) food .... 8,000 grains (520 grms.) Water (as water, or variously combined with solid food)35,000-40,000 „ (2,444 „ ) By the Lungs there is absorbed daily:- Oxyg°n13-000 „ (844 „ ) The average total daily receipts, in the shape of food, drink and oxygen, correspond, therefore, with the average total daily expenditure, as shown by the following table. Income. Expenditure. Solid food . . . 8,000 grains Lungs 20,000 grains. W ater . • • 37,650 „ Skin . . . . 11,75° ,, Oxygen . 13.000 „ Kidneys 24,100 ,. j, Intestines . . . 2,800 „ 58,650 grains (Generative and mam- (3,808 grins , or about 8 J lb.) mary-gland products are supposed to be included) 58,650 grains (About 3,808 grms.) These quantities are approximate only. But they may be taken as fair averages for a healthy adult. The absolute identity of the two numbers (in grains') in the two tables is of course diagram- matic. No such exactitude in the account occurs in any living body, in the course of any given twenty-four hours. But any difference which exists between the two amounts of income and expenditure at any given period, corresponds merely with the slight variations in the amount of capital (weight of body) to which the healthiest subject is liable. The chemical composition of the food (p. 239) may be profit- ably compared with that of the excreta, as before mentioned. The greater part of our food is composed of matter which contains much potential energy; and in the chemical changes (combustion and other processes) to which it is subject in the body, active energy is manifested. 3. The Sources and Objects of Expenditure.-The sources chap, xv.] OBJECTS OF EXPENDITURE. 493 of the necessary waste and expenditure in the living body are various and extensive. They may be comprehended under the following heads:- (1) Common wear and tear ; such as that to which all structures, living and not living, are subjected by exposure and work; but which must be especially large in the soft and easily decaying structures of an animal body. (2) Manifestations of Force in the form either of Heat or Motion. In the former case (Heat), the combustion must be sufficient to maintain a temperature of about ioo° F. (37'8° C.) throughout the whole substance of the body, in all varieties of external tem- perature, notwithstanding the large amount continually lost in the ways previously enumerated. In the case of Motion, there is the expenditure involved in the (a) Ordinary muscular movements, as in Prehension, Mastication, Locomotion, and numberless other ways: as well as in (6) Various involuntary movements, as in Respiration, Circulation, Digestion, Ac. (3) Manifestation of Nerve-force; as in the general regulation of all physiological processes, c.y., Respiration, Circulation, Digestion; and in Volition and all other manifestations of cerebral activity. (4) The energy expendeel in all physiological processes, e.g., Nutri- tion, Secretion, Growth, and the like. The total expenditure or total manifestation of energy by an animal body can be measured, with fair accuracy; the terms used being such as are employed in connection with other than vital operations. All statements, however, must be considered for the present approximate only, and especially is this the case with respect to the comparative share of expenditure to l>e assigned to the various objects just enumerated. The amount of energy daily manifested by the adult human body in (a) the maintenance of its temperature; (6) in internal mechanical work, as in the movements of the respiratory muscles, the heart, Ac.; and (c) in external mechanical work, as in loco- motion and all other voluntary movements, has been reckoned at about 3,400 foot-tons. Of this amount only one-tenth is directly expended in internal and external mechanical work; the remainder being employed in the maintenance of the body's heat. The latter amount represents the heat which would be required to raise 48'4 lb. of water from the freezing to the boiling point; or if converted into mechanical power, it would suffice to raise 494 INCOME AND EXPENDITURE OF BODY. [chap. xv. the body of a man weighing about 150 lb. through a vertical height of miles. To the foregoing amounts of expenditure must be added the quite unknown quantity expended in the various manifestations of nerve-force, and in the work of nutrition and growth (using these terms in their widest sense). By comparing the amount of energy which should be produced in the body from so much food of a given kind, with that which is actually manifested (as shown by the various products of combustion, in the excretions) attempts have been made, indeed, to estimate, by a process of exclusion, these unknown quantities; but all such calculations must be at present considered only very doubtfully approximate. Sources of Error.-Among the sources of error in any such calculations must be reckoned, as a chief one, the, at present, entirely unknown extent to which forces external to the body (mainly heat) can be utilised by the tissues. We are too apt to think that the heat and light of the sun are directly correlated, as far as living beings are concerned, with the chemico-vital transformations involved in the nutrition and growth of the members of the vegetable world only. But animals, although comparatively independent of external heat and other forces, probably utilise them, to the degree occasion offers. And although the correlative manifestation of energy in the body, due to external heat and light, may still be measured in so far as it may take the form of mechanical work ; yet, in so far as it takes the form of expenditure in nutrition or nerve-force, it is evidently impossible to include it by any method of estimation yet dis- covered ; and all accounts of it must be matters of the purest theory. These considerations may help to explain the apparent discrepancy between the amount of energy which is capable of being produced by the usual daily amount of food, with that which is actually manifested daily by the body; the former leaving but a small margin for anything beyond the maintenance of heat, and mechanical work. In the foregoing sketch we have supposed that the excreta are exactly replaced by the ingesta. Nitrogenous Equilibrium and Formation of Fat.-If an animal, however, which has undergone a starving period, be fed upon a diet of lean meat it is found that instead of the greater part of the nitrogen being stored up, as one would expect, the chief part of it appears in the urine as urea, and continuing with the diet chap, xv.] NITROGENOUS EQUILIBRIUM. 495 the excreted nitrogen approximates more and more closely to the ingested nitrogen until at last the amounts are equal in both cases. This is called nitrogenous equilibrium. There may, how- ever, be an increase of weight which is due to the putting on of fat. If this is the case it must be apparent that the protoplasm of the tissues is able to form fat out of proteid material and to split it up into urea and fat. If fat be given in small quantities with the meat, for a time the carbon of the egesta and ingesta are equal, but if the fat be increased beyond a certain point the body weight increases from a deposition of fat; not, however, by a mere mechanical deposition or filtration from the blood, but by an actual act of secretion by the protoplasm whereby the fat globules are stored up within itself: similarly as regards carbo- hydrates, if they are in small quantity, the carbon appears in the excreta, but beyond a certain amount a considerable portion of it is retained in fat, having been by the protoplasm stored up within itself in that material. The amount of proteid material required to produce nitrogenous equilibrium is considerable, but it may be materially diminished by the addition of carbo-hydrate or fatty food or of gelatine to the exclusively meat diet. It is of much interest to consider how the protoplasm acts in converting food into energy and decomposition products, since the substance itself does not undergo much change in the process except a slight amount of wear and tear. We may assume that it is the property of protoplasm to separate from the blood the materials which it may require to produce secretions, in the case of the protoplasm of secreting glands, or to enable it to evolve heat and energy, as in the case of the protoplasm of muscle. The substances are very possibly different for each process, and the decomposition products, too, may be different in quality or quan- tity. Proteid materials appear to be specially needed, as is shown by the invariable presence of urea in the urine even during starvation ; and as in the latter case, there has been no food from which these materials could have been derived, the urea is con- sidered to be derived from the disintegration of the nitrogenous tissues themselves. The removal of all fat from the body in a starvation period, as the first apparent change, would lead to the supposition that fat is also a specially necessary pabulum for the production of protoplasmic energy; and the fact that, as mentioned above, with a diet of lean meat an enormous amount appears to be required, suggests that in that case protoplasm obtains the fat 496 VOICE AND SPEECH. [chap. xvi. it needs from the proteid food, which process must be evidently a source of much waste of nitrogen. The idea that proteid food has two destinations in the economy, viz., to form organ or tissue proteid which builds up organs and tissues, and circulating proteid, from which the organs and tissues derive the materials of their secretions or for producing their energy, is a convenient one, and it is unlikely that protoplasm would go to the expense of con- struction simply for the sake of immediate destruction. CHAPTER XVI. THE VOICE AND SPEECH. The Larynx.-In nearly all air-breathing vertebrate animals there are arrangements for the production of sound, or voice, in some parts of the respiratory apparatus. In many animals, the Cornu min: Cornu maj: m. Stemo-hyoideus. m. Stemc-hyoideus. Cornu sup: - Lig: crico-thyr. med: w. Stemo-hyoideus. Cart: crieoidea. m, Crico-thyroideus. Lig: crico-tracheae. - Cart: tracheale. Fig-. 302.-The Larynx, as seen from the front, showing the cartilages and ligaments. The muscles, with the exception of one crico-thyroid, are cut oft' short. (Stoerk.) sound admits of being variously modified and altered during and after its production; and, in man, one such modification occurring in obedience to dictates of the cerebrum, is speech. CHAP. XVI.] THE ANATOMY OF THE LARYNX. 497 It has been proved by observations on living subjects, by means of the laryngoscope (p. 500), as well as by experiments on the larynx taken from the dead body, that the sound of the human voice is the result of the vibration of the inferior laryngeal ligaments, or true vocal cords (A, cv, fig. 307) which bound the glottis, caused by currents of expired air impelled over their edges. If a free open- ing exists in the trachea, the sound of the voice ceases, but it returns if the opening is closed. An opening into the air-passages above the glottis, on the contrary, does not prevent the voice being produced. By forcing a current of air through the larynx in the dead subject, clear vocal sounds are elicited, though the epiglottis, the upper ligaments of the larynx or false vocal cords, the ventricles between them and the inferior ligaments or true vocal cords, and the upper part of the arytenoid cartilages, be all removed; provided the true vocal cords remain entire, with their points of attachment, and be kept tense and so approximated that the fissure of the glottis may be narrow. Lig. Ary.-epiglott. Cart. Wrisbergii Cart. Santorini Cart, aryten. Proc, muscul. Lig. cerato-crico. post. sup. lag. crico-aryten. Lig. carat-crieo. post. inf. Cornu infer. Cart, tracheae -Lars membran. Fig. 303.- The larynx as seen from behind after removal of the muscles. The cartilages and ligaments only remain. (Stoerk.) The vocal ligaments or cords, therefore, are regarded as the proper organs for the production of vocal sounds : the modifica- tions of these sounds being effected, as will be presently explained, by other parts-tongue, teeth, lips, etc., as well as by them. The 498 VOICE AND SPEECH. [chap. xvi. structure of the vocal cords is adapted to enable them to vibrate like tense membranes, for they are essentially composed of elastic tissue; and they are so attached to the cartilaginous parts of the larynx that their position and tension can be variously altered by the contraction of the muscles which act on these parts. Thus it will be seen that the larynx is the organ of voice. It may be said to consist essentially of the two vocal cords and the various cartilaginous, muscular, and other apparatus by means of which not only can the aperture of the larynx (rima glottidis), of which they are the lateral boundaries, be closed against the entrance and exit of air to or from the lungs, but also by means of which the cords themselves can be stretched or relaxed, shortened or lengthened, in accordance with the conditions that may be neces- sary for the air in passing over them, to set them vibrating and produce various sounds. Their action in respiration has been already referred to. Anatomy of the Larynx.-The principal parts entering into the formation of the larynx (figs. 302 and 303) are-the thyroid cartilage ; the cricoid cartilage ; the two arytenoid cartilages ; and the two true vocal cords (fig. 307). The epiglottis (fig. 303), has but little to do with the voice, and is chiefly useful in protecting the upper part of the larynx from the entrance of food and drink in deglutition. It also probably guides mucus or other fluids in small amount from the mouth around the sides of the upper opening of the glottis into the pharynx and oesophagus : thus preventing them from entering the larynx. The false vocal cords (cr«, fig. 307), and the ventricle of the larynx, which is a space between the false and the true cord of either side, need be here only referred to. Cartilages.-(a) The thyroid cartilage (fig. 304, 1 to 4) does not form a complete ring around the larynx, but only covers the front portion, (ft) The cricoid cartilage (fig. 304. 5, 6), on the other hand, is a complete ring ; the back part of the ring being much broader than the front. On the top of this broad portion of the cricoid are (c) the arytenoid cartilages (fig. 304, 7), the connection between the cricoid below and arytenoid car- tilages above being a joint with synovial membrane and ligaments, the latter permitting tolerably free motion between them. But although the arytenoid cartilages can move on the cricoid, they of course accompany the latter in all its movements, just as the head may nod or turn on the top of the spinal column, but must accompany it in all its movements as a whole. Joints and Ligaments.-The thyroid cartilage is also connected with the cricoid, not only by ligaments, but also by joints with synovial membranes ; the lower cornua of the thyroid clasping, or nipping, as it were, the cricoid between them, but not so tightly but that the thyroid can revolve, within a certain range, around an axis passing transversely through the two joints at which the cricoid is clasped. The vocal cords are attached (behind) to the front portion of the base of the arytenoid cartilages, and (in front) to the re-entering angle at the back part of the thyroid ; it is evident, chap, xvi.] THE INTRINSIC MUSCLES OF THE LARYNX. 499 therefore, that all movements of either of these cartilages must produce an effect on them of some kind or other. Inasmuch, too, as the arytenoid cartilages rest on the top of the back portion of the cricoid cartilage, and are connected with it by capsular and other ligaments, all movements of the cricoid cartilage must move the arytenoid cartilages, and also produce an effect on the vocal cords. Intrinsic Muscles. - The so-called intrinsic muscles of the larynx, or those which, in their action, have a direct action on the vocal cords, are nine in number-four pairs, and a single muscle; namely, two erico-thy raid muscles, two thy ro-arytenoid, two posterior crico- arytenoid, two lateral crico-arytenoid, and one arytenoid muscle. Their actions are as follows:-When the erico-thyroid muscles (io, fig. 306) contract, they rotate the cricoid on the thyroid cartilage in such a manner, that the upper and back part of the former, and of necessity the arytenoid cartilages on the top of it, are tipped backwards, while the thyroid is inclined forward ; and thus, of course, the vocal cords being attached in front to one, and behind to the other, are " put on the stretch." The thy ro-arytenoid muscles on the other hand, have an opposite action- pulling the thyroid backwards, and the arytenoid and upper and back part of the cricoid cartdages forwards, and thus re- laxing the vocal cords. The • crico-arytenoidei postici muscles (tig. 303) dilate the glottis, and separate the vocal cords, the one from the other, by an action on the arytenoid cartilage. By their contraction they tend to pull together the outer angles of the arytenoid cartilages in such a fashion as to rotate the latter at their joint with the cricoid, and of course to throw asunder their anterior angles to which the vocal cords are attached. These posterior crico-arytenoid muscles are opposed by the erico-ary- tenoidei laterales, which, pulling in the opposite direction from the other side of the axis of rotation, have of course exactly the opposite effect, and close the glottis. The aperture of the glottis can be also contracted by the arytenoid muscle (fig. 305), which, in its contraction, pulls together the upper parts of the arytenoid cartilages between which it extends. Nerve Supply.-In the performance of the functions of the larynx the sensory filaments of the superior laryngeal branch of the vagi supply that acute sensibility by which the glottis is guarded against the ingress of foreign bodies, or of irrespirable gases. The contact of these stimulates the nerve filaments; and the impression conveyed to the medulla oblongata. Fig. 304.- Cartilages of the larynx seen, from the f ront. 1 to 4, thyroid cartilage; 1, vertical ridge or pomum Adami; 2, right ala; 3, superior, and 4, inferior cornu of the right side; 5, 6, cricoid carti- lage ; 5, inside of the posterior part; 6, anterior narrow part of the ring; 7, arytenoid cartilages. X >. 500 VOICE AND SPEECH. [chai*, xvi. whether it produce sensation or not, is reflected to the filaments of the recurrent or inferior laryngeal branch, and excites contraction of the muscles that close the glottis. Both these branches of pneumogastric co-operate also in the production and regulation of the voice ; the inferior Lig. ary epiglott. Cart. Wrisbergii Cart. Santorini mm. Ary ten. obliqu. m. Crieo-arytenoid. post. Lig. cerato-crie. Cornu inferior Pars. post. inf. membrani Pars, cartilag. Fig, 305.-7 he larynx as seen from behind. To show the intiinsic muscles posteriorly. (Stoerk.) laryngeal determining the contraction of the muscles that vary the tension of the vocal cords, and the superior laryngeal conveying to the mind the sensation of the state of these muscles necessary for their continuous guidance. And both the branches co-operate in the actions of the larynx in the ordinary slight dilatation and contraction of the glottis in the acts of expiration and inspiration, and more evidently in those of coughing and other forcible respiratory movements. The laryngoscope is an instrument employed in investigating during life the condition of the pharynx, larynx, and trachea. It consists of a large concave mirror with perforated centre, and of a smaller mirror fixed in a long handle. It is thus used : the patient is placed in a chair, a good light (argand burner, or limp) is arranged on one side of, and a little above his head. The operator fixes the large mirror round his head in such a manner, that he looks through the central aperture with one eye. He then seats himself opposite the patient, and so alters the position of the mirror, which is for this purpose provided with a ball and socket joint, that a beam of light is reflected on the lips of the patient. The patient is now directed to throw his head slightly backwards, and to open his mouth ; the reflection from the mirror lights up the cavity of the mouth, and by a little alteration of the distance between the operator and the patient the point at which the greatest amount of light is reflected by the mirror-in other words its focal length-is readily discovered. The small mirror fixed in the handle is then warmed, either by holding it over CHAP, xvr.] MOVEMENTS OF THE VOCAL CORDS. 501 the lamp, or by putting it into a vessel of warm water ; this is necessary to prevent the condensation of breath upon its surface. The degree of heat is regulated by applying the back of the mirror to the hand or cheek, when it should feel warm without being painful. After these preliminaries the patient is directed to put out his tongue, which is held by the left hand gently but firmly against the lower teeth, by means of a handkerchief. The warm mirror is passed to the back of the mouth, until it rests upon and slightly raises the base of the uvula, and at the same time the light is directed upon it: an inverted image of the larynx and trachea will be seen in the mirror. If the dorsum of the tongue be alone seen, the handle of the mirror must be slightly lowered until the larynx comes into view ; care should be taken, however, not to move the mirror upon the uvula, as it excites retching. The observation should not be prolonged, but should rather be repeated at short in- tervals. The structures seen will vary somewhat according to the condition of the parts as to inspiration, expiration, phonation,&c. ; they are (figs. 307) first, and apparently at the posterior part, the base of the tongue, immediately below which is the arcuate outline of the epiglottis, with its cushion or tubercle. Then are seen in the central line the true vocal cords, white and shining in their normal condition. The cords approximate (in the inverted image) posteriorly ; between them is left a chink, narrow whilst a high note is being sung, wide during a deep inspiration. On each side of the true vocal cords, and on a higher level, are the pink false vocal cords. Still more externally than the false vocal cords is the aryteno-epiglottidean fold, in which are situated upon each side three small elevations ; of these the most external is the cartilage of Wrisberg, the intermediate is the cartilage of Santorini, whilst the summit of the arytenoid cartilage is in front, and somewhat below the preceding, being only seen during deep inspiration. The rings of the trachea, and even the bifurcation of the trachea itself, if the patient be directed to draw a deep breath, may be seen in the interval between the true vocal cords. Movements of the Vocal Cords.-The placing of the vocal cords in a position parallel one with the other, is effected by a combined action of the various intrinsic muscles which act on them -the thyro-arytenoidei having, without much reason, the credit of taking the Largest share in the production of this effect. Fig. Fig. 306.-Lateral view of exterior of the. larynx. 8, thyroid cartilage ; <j, cricoid cartilage ; io, crico-thyroid muscle ; ii, crico-thyroid ligament; 12, first rings of trachea. (Willis.) 502 VOICE AND SPEECH. [chap. XVI. 307 is intended to show the various positions of the vocal cords under different circumstances. Thus, in ordinary tranquil breath- ing, the opening of the glottis is wide and triangular (b), becoming Fig. yrj.-Three laryngoscopic views of the superior aperture of the larynx and surrounding parts. A, the glottis during the emission of a high note in singing ; B. in easy and quiet inhalation of air; C, in the state of widest possible dilatation, as in inhaling a very' deep breath. The diagrams A', B', and O', show in horizontal sections of the glottis the position of the vocal ligaments and arytenoid cartilages in the three several states represented in the other figures. In all the figures, so far as marked, the letters indicate the parts as follows, viz.: I, the base of the tongue ; c, the upper free part of the epiglottis ; the tubercle or cushion of the epiglottis ; ph, part of the anterior wall of the pharynx behind the larynx; in the margin of the aryteno-epiglottidean fold w, the swelling of the membrane caused by the cartilages of Wrisberg; .s, that of the cartilages of Santorini; a, the tip or summit of the arytenoid cartilages ; c v, the true vocal cords or lips of the rima glottidis ; <• v s, the superior or false vocal cords ; between them the ventricle of the larynx ; in (', tr is placed on the anterior wall of the receding trachea, and b indicates the commenc ement of the two bronchi beyond the bifurcation which may be brought into view in this state of extreme dilatation. (Quain after Czermak.) a little wider at each inspiration, and a little narrower at each expiration. On making a rapid and dee]) inspiration the opening of the glottis is widely dilated (as in c), and somewhat lozenge- shaped. At the moment of the emission of sound, it is narrowed, chap, xvi.] MOVEMENTS OF THE VOCAL CORDS. 503 th e margins of the arytenoid cartilages being brought into contact and the edges of the vocal cords approximated and made parallel, at the same time that their tension is much increased. The higher the note produced, the tenser do the cords become (fig. 307, a) ; and the range of a voice depends, of course, in the main, on the extent to which the degree of tension of the vocal cords can be thus altered. In the production of a high note, the vocal cords are brought well within sight, so as to be plainly visible with the help of the laryngoscope. In the utterance of grave tones, on the other hand, the epiglottis is depressed and brought over them, and the arytenoid cartilages look as if they were trying to hide themselves under it (fig. 308). The epiglottis, by being somewhat pressed down so as to cover the superior cavity of the larynx, serves to render the notes deeper in tone, and at the same time somewhat duller, just as covering the end of a short tube placed in front of caoutchouc tongues lowers the tone. In no other respect does the epiglottis appear to have any effect in modifying the vocal sounds. The degree of approximation of the vocal cords also usually corresponds with the height of the note produced ; but probably not always, for the width of the aperture has no essential influence on the height of the note, as long as the vocal cords have the same tension: only with a wide aperture, the tone is more difficult to produce, and is less perfect, the rushing of the air through the aperture being heard at the same time. No true vocal sound is pro- duced at the posterior part of the aperture of the glottis, that, viz., which is formed by the space between the arytenoid cartilages. For if the arytenoid cartilages be approximated in such a man- ner that their anterior processes touch each other, but yet leave an opening behind them as well as in front, no second vocal tone is produced by the passage of the air through the posterior opening, but merely a rustling or bubbling sound ; and the height or pitch of the note produced is the same whether the posterior part of the glottis be open or Fig. 308.- View of the upper part of the larynx as seen by means of the laryn- goscope during the utterance of a grave note. c. ep'glottis; s, tuber- cles of the cartilages of Santorini; a, arytenoid cartilages; z, base of the tongue; ph. the posterior wall of the pharynx (Czermak). 504 VOICE AND SPEECH. [chap. XVI. not, provided the vocal cords maintain the same degree of tension. The Voice in Singing and Speaking. Varieties of Vocal Sounds.-The laryngeal notes may observe three different kinds of sequence. The first is the monotonous, in which the notes have nearly all the same pitch as in ordinary speaking ; the variety of the sounds of speech being due to articu- lation in the mouth. In speaking, however, occasional syllables generally receive a higher intonation for the sake of accent. The second mode of sequence is the successive transition from high to low notes, and vice versd, without intervals ; such as is heard in the sounds, which, as expressions of passion, accompany crying in men, and in the howling and whining of dogs. The third mode of sequence of the vocal sounds is the musical, in which each sound has a determinate number of vibrations, and the numbers of the vibrations in the successive sounds have the same relative propor- tions that characterise the notes of the musical scale. In different individuals this comprehends one, two, or three octaves. In singers-that is, in persons apt for singing-it extends to two or three octaves. But the male and female voices com- mence and end at different points of the musical scale. The lowest note of the female voice is about an octave higher than the lowest of the male voice ; the highest note of the female voice about an octave higher than the highest of the male. The com- pass of the male and female voices taken together, or the entire scale of the human voice, includes about four octaves. The prin- cipal difference between the male and female voice is, therefore, in their pitch ; but they are also distinguished by their tone,-the male voice is not so soft. The voice presents other varieties besides that of male and female ; there are two kinds of male voice, technically called the bass and tenor, and two kinds of female voice, the contralto and soprano, all differing from each other in tone. The bass voice usually reaches lower than the tenor, and its strength lies in the low notes ; while the tenor voice extends higher than the bass. The contralto voice has generally lower notes than the soprano, and is strongest in the lower notes of the female voice ; while the soprano voice reaches higher in the scale. But the difference of compass, and of power in different parts of the scale, is not the essential distinction CHAP. XVI.] VARIETIES OF VOCAL SOUNDS. 505 between the different voices ; for bass singers can sometimes go very high, and the contralto frequently sings the high notes like soprano singers. The essential difference between the bass and tenor voices, and between the contralto and soprano, consists in their tone or " timbre," which distinguishes them even when they are singing the same note. The qualities of the barytone and mezzo-soprano voices are less marked ; the barytone being inter- mediate between the bass and tenor, the mezzo-soprano between the contralto and soprano. They have also a middle position as to pitch in the scale of the male and female voices. The different pitch of the male and the female voices depends on the different length of the vocal cords in the two sexes; their relative length in men and women being as three to two. The difference of the two voices in tone or " timbre," is owing to the different nature and form of the resounding walls, which in the male larynx are much more extensive, and form a more acute angle anteriorly. The different qualities of the tenor and bass, and of the alto and soprano voices, probably depend on some peculiarities of the ligaments, and the membranous and cartila- ginous parietes of the laryngeal cavity, which are not at present understood, but of which we may form some idea, by recollecting that musical instruments made of different materials, e.g., metallic and gut-strings, may be tuned to the same note, but that each will give it with a peculiar tone or " timbre." The larynx of boys resembles the female larynx ; their vocal cords before puberty are not two-thirds the length of the adult cords; and the angle of their thyroid cartilage is as little promi- nent as in the female larynx. Boys' voices are alto and soprano, resembling in pitch those of women, but louder, and differing somewhat from them in tone. But, after the larynx has under- gone the change produced during the period of development at puberty, the boy's voice becomes bass or tenor. While the change of form is taking place, the voice is said to " crack • " it becomes imperfect, frequently hoarse and crowing, and is unfitted for sing- ing until the new' tones are brought under command by practice. In eunuchs, who have been deprived of the testes before puberty, the voice does not undergo this change. The voice of most old people is deficient in tone, unsteady, and more restricted in extent: the first defect is owing to the ossification of the cartilages of the larynx and the altered condition of the vocal cords; the want of steadiness arises from the loss of nervous power and command 506 VOICE AND SPEECH. [chap. XV I. over the muscles; the result of which is here, as in other parts, a tremulous movement. These two causes combined render the voices of old people void of tone, unsteady, bleating, and weak. In any class of persons arranged, as in an orchestra, according to the character of voices, each would possess, with the general characteristics of a bass, or tenor, or any other kind of voice, some peculiar character by which his voice would be recognised from all the rest. The conditions that determine these distinc- tions are, however, quite unknown. They are probably inherent in the tissues of the larynx, and are as indiscernible as the minute differences that characterize men's features ; one often observes, in like manner, hereditary and family peculiarities of voice, as well marked as those of the limbs or face. Most persons, particularly men, have the power, if at all capable of singing, of modulating their voices through a double series of notes of different character : namely, the notes of the natural voice, or chest-notes, and the falsetto notes. The natural voice, which alone has been hitherto considered, is fuller, and excites a distinct sensation of much stronger vibration and resonance than the falsetto voice, which has more a flute-like character. The deeper notes of the male voice can be produced only with the natural voice, the highest with the falsetto only ; the notes of middle pitch can be produced either with the natural or falsetto voice ; the two registers of the voice are therefore not limited in such a manner as that one ends when the other begins, but they run in part side by side. Method of the Production of Notes.-The natural or chest-notes, are produced by the ordinary vibrations of the vocal cords. The mode of production of the falsetto notes is still obscure. By Muller the falsetto notes were thought to be due to vibrations of only the inner borders of the vocal cords. In the opinion of Petrequin and Diday, they do not result from vibrations of the vocal cords at all, but from vibrations of the air passing through the aperture of the glottis, which they believe assumes, at such times, the contour of the embouchure of a flute. Others (considering some degree of similarity which exists between the falsetto notes and the peculiar tones called harmonic, which are produced when, by touching or stopping a harp-string at a particular point, only a portion of its length is allowed to vibrate) have supposed that, in the falsetto notes, portions of the vocal ligaments are thus isolated, and made to vibrate while the rest are held still. The question cannot yet be settled; but any one in the habit of singing may assure himself, both by the difficulty of passing smoothly from one set of notes to the other, and by the necessity CHAP. XVI.] VARIETIES OF VOICES. 507 of exercising himself in both registers, lest he should become very deficient in one, that there must be some great difference in the modes in which their respective notes are produced. The strength of the voice depends partly (a) on the degree to which the vocal cords can be made to vibrate; and partly (6) on the fitness for resonance of the membranes and cartilages of the larynx, of the parietes of the thorax, lungs, and cavities of the mouth, nostrils, and communicating sinuses. It is diminished by anything which interferes with such capability of vibration. The intensity or loudness of a given note with maintenance of the same " pitch," cannot be rendered greater by merely increas- ing the force of the current of air through the glottis ; for increase of the force of the current of air, cceteris paribus, raises the pitch both of the natural and the falsetto notes. Yet, since a singer possesses the power of increasing the loudness of a note from the faintest " piano " to " fortissimo " without its pitch being altered, there must be some means of compensating the tendency of the vocal cords to emit a higher note when the force of the current of air is increased. This means evidently consists in modifying the tension of the vocal cords. When a note is rendered louder and more intense, the vocal cords must be relaxed by remission of the muscular action, in proportion as the force of the current of the breath through the glottis is increased. When a note is rendered fainter, the reverse of this must occur. The arches of the palate and the uvula become contracted during the formation of the higher notes; but their contraction is the same for a note of given height, whether it be falsetto or not; and in either case the arches of the palate may be touched with the finger, without the note being altered. Their action, there- fore, in the production of the higher notes seems to be merely the result of involuntary associate nervous action, excited by the voluntarily increased exertion of the muscles of the larynx. If the palatine arches contribute at all to the production of the higher notes of the natural voice and the falsetto, it can only be by their increased tension strengthening the resonance. The office of the ventricles of the larynx is evidently to afford a free space for the vibrations of the lips of the glottis; they may be compared with the cavity at the commencement of the mouth- piece of trumpets, which allows the free vibration of the lips. Speech.-Besides the musical tones formed in the larynx, a great number of other sounds can be produced in the vocal tubes, 508 VOICE AND SPEECH. [chap. xvt. between the glottis and the external apertures of the air-passages, the combination of which sounds by the agency of the cerebrum into different groups to designate objects, properties, actions, etc., constitutes language. The languages do not employ all the sounds which can be produced in this manner, the combination of some with others being often difficult. Those sounds which are easy of combination enter, for the most part, into the formation of the greater number of languages. Each language contains a certain number of such sounds, but in no one are all brought together. On the contrary, different languages are characterised by the prevalence in them of certain classes of these sounds, while others are less frequent or altogether absent. Articulate Sounds.-The sounds produced in speech, or the articulate sounds, are commonly divided into vowels and consonants: the distinction between which is, that the sounds for the former are generated by the larynx, while those for the latter are pro- duced by interruption of the current of air in some part of the air-passages above the larynx. The term consonant has been given to these because several of them are not properly sounded, except consonantly with a vowel. Thus, if it be attempted to pronounce aloud the consonants b, d, and g, or their modifica- tions, p, t, k, the intonation only follows them in their combination with a vowel. To recognize the essential properties of the arti- culate sounds, it is necessary first to examine them as they are produced in whispering, and then investigate which of them can also be uttered in a modified character conjoined with vocal tone. By this procedure we find two series of sounds : in one the sounds are mute, and cannot be uttered with a vocal tone; the sounds of the other series can be formed independently of voice, but are also capable of being uttered in conjunction with it. All the vowels can be expressed in a whisper without vocal tone, that is, mutely. These mute vowel-sounds differ, however, in some measure, as to their mode of production, from the consonants. All the mute consonants are formed in the vocal tube above the glottis, or in the cavity of the mouth or nose, by the mere rushing of the air between the surfaces differently modified in disposition. But the sound of the vowels, even when mute, has its source in the glottis, though its vocal cords are not thrown into the vibra- tions necessary for the production of voice; and the sound seems to be produced by the passage of the current of air between the relaxed vocal cords. The same vowel-sound can be produced in CHAP. XVI.] ARTICULATION. 509 the larynx when the mouth is closed, the nostrils being open, and the utterance of all vocal tone avoided. The sound, when the mouth is open, is so modified by varied forms of the oral cavity, as to assume the characters of the vowels a, e, i, o, u, in all their modifications. The cavity of the mouth assumes the same form for the articu- lation of each of the mute vowels as for the corresponding vowel when vocalized; the only difference in the two cases lies in the kind of sound emitted by the larynx. It has been pointed out that the conditions necessary for changing one and the same sound into the different vowels, are differences in the size of two parts- the oral canal and the oral opening; and the same is the case with regard to the mute vowels. By oral canal, is meant here the space between the tongue and palate : for the pronunciation of certain vowels both the opening of the mouth and the space just mentioned arc widened; for the pronunciation of other vowels both are contracted; and for others one is wide, the other con- tracted. Admitting five degrees of size, both of the opening of the mouth and of the space between the tongue and palate, Kempelen thus states the dimensions of these parts for the following vowel-sounds:- Vowel. Sound. Size of oral opening. Size of oral canal. a as in " far " 5 3 a ,, " name " 4 2 e ., " theme " 3 I ° " "g°"., 2 4 oo ,, "cool" I 5 Another important distinction in articulate sounds is, that the utterance of some is only of momentary duration, taking place during a sudden change in the conformation of the mouth, and being incapable of prolongation by a continued expiration. To this class belong b, p, d, and the hard g. In the utterance of other consonants the sounds may be continuous; they may be prolonged, ad libitum, as long as a particular disposition of the mouth and a constant expiration are maintained. Among these consonants are h, m, n, f, s, r, 1. Corresponding differences in respect to the time that may be occupied in their utterance exist in the vowel sounds, and principally constitute the differences of long and short syllables. Thus the a as in " far " and " fate," the o as in "go" and "fort," may be indefinitely prolonged; but the same vowels (or more properly different vowels expressed 510 VOICE AND SPEECH. [chap. xvi. by the same letters), as in "can" and "fact," in "dog" and " rotten," cannot be prolonged. All sounds of the first or explosive kind are insusceptible of combination with vocal tone (" intonation "), and are absolutely mute ; nearly all the consonants of the second or continuous kind may be attended with " intonation." Ventriloquism.-The peculiarity of speaking, to which the term ven- triloquism is applied, appears to consist merely in the varied modification of the sounds produced in the larynx, in imitation of the modifications which voice ordinarily suffers from distance, &c. From the observations of Midler and Colombat, it seems that the essential mechanical parts of the process of ventriloquism consist in taking a full inspiration, then keeping the muscles of the chest and neck fixed, and speaking with the mouth almost closed, and the lips and lower jaw as motionless as possible, while air is very slowly expired through a very narrow glottis ; care being taken also, that none of the expired air passes through the nose. But, as observed by Muller, much of the ventriloquist's skill in imitating the voices coming from particular directions, consists in deceiving other senses than hearing. We never dis- tinguish very readily the direction in which sounds reach our ear; and, when our attention is directed to a particular point, our imagination is very apt to refer to that point whatever sounds we may hear. Action of the Tongue in Speech.-The tongue, which is usually credited with the power of speech-language and speech being often employed as synonymous terms-plays only a sub- ordinate, although very important part. This is well shown by cases in which nearly the whole organ has been removed on account of disease. Patients who recover from this operation talk imperfectly, and their voice is considerably modified; but the loss of speech is confined to those letters, in the pronunciation of which the tongue is concerned. Stammering- depends on a want of harmony between the action of the muscles (chiefly abdominal) which expel air through the larynx, and that of the muscles which guard the orifice (rima glottidis) by which it escapes, and of those (of tongue, palate, &c.) which modulate the sound to the form of speech Over either of the groups of muscles, by itself, a stammerer may have as much power as other people. But he cannot harmoniously arrange their conjoint actions. CHAP. XVII.] NERVE FIBRES. 511 CHAPTER XVII. THE NERVOUS SYSTEM. I. The Structure of the Nervous Elements. Nervous tissue is found under the microscope to consist essentially of two main elements, namely, of nerve fibres and nerve cells. When the nerve fibres are collected together into bundles they form nerve trunks or nerves. When nerve cells are collected together they form nerve ganglia, but in such ganglia nerve - fibres are also invariably found. A. Nerve Fibres. Varieties.-In most nerve-trunks two kinds of fibres are mingled, called (a) medullated or white fibres, and (b) non-medullated or grey fibres. (a.) Medullated Fibres.--Each medullated nerve-fibre is made up of the following parts:-(i.) An ex- ternal sheath called the primitive nerve sheath, or nucleated sheath of Schwann ; (2.) An intermediate or packing substance known as the med'ullary sheath, or white substance of Schwann ; and (3.) internally the axis-cylinder, primitive band, axis band, or axial fibre. Although these parts can be made out in nerves examined some time after death, in a recent specimen the contents of the nerve - sheath appear to be homogeneous. But by degrees they undergo changes which show them to be composed of two different materials. The internal or central part, occupying the axis of Fig. 309.-Primitive nerve-fibres. A. A perfectly fresh tubule with a single dark outline, b. A tubule or fibre with a double contour from com- mencing post-mortem change, c. the changes further advanced, pro- ducing a varicose or beaded appear- ance. n. A tubule or fibre, the central part of which, in conse- quence of still further changes, has accumulated in separate portions within the sheath (Wagner). 512 THE NERVOUS SYSTEM. [chap. xvn. the tube (axis-cylinder), becomes greyish, while the outer, or cortical portion (white substance of Schwann), becomes opaque and dimly granular or grumous, as if from a kind of coagulation. At the same time, the fine outline of the previously transparent cylindrical tube is exchanged for a dark double contour (fig. 309, b), the outer line being formed by the sheath of the fibre, the inner by the margin of curdled or coagulated medullary substance. The granular material shortly collects into little masses, which distend portions of the tubular membrane; while the intermediate spaces collapse, giving the fibres a varicose, or beaded appearance (fig. 309, c and d), instead of the previous cylindrical form. The whole contents of the nerve-tubules are extremely soft, for when subjected to pressure they readily pass from one part of the tubular sheath to another, and often cause a bulging at the side of the mem- brane. They also readily escape, on pres- sure, from the extremities of the tubule, in the form of a grumous or granular material. (1.) The external nucleated sheath of Schwann is a pellucid membrane, forming the outer investment of the nerve-fibre. Within this delicate structureless mem- brane nuclei are seen at intervals, sur- rounded by a variable amount of protoplasm. The sheath is structureless, like the sarcolemma, and the nuclei appear to be within it: together with the protoplasm which surrounds them, they are the relics of embryonic cells, and from their resem- blance to the muscle corpuscles of striated muscle, may be termed nerve-corpuscles. They are easily stained with logwood and other dyes. (2.) The medullary sheath or white substance of Schwann is the part to which the peculiar white aspect of some nerves is principally due. It is a thick, fatty, semi-fluid substance, as we have seen, possessing a double contour. It is said to be made up of a fine reticulum (Stilling, Klein), in the meshes of which is embedded the bright fatty material. It stains well with osmic acid. Fig. 310.-Two nerve-fibres of sciatic nerve. A. Node of Ranvier, b. Axis-cylinder, c. Sheath of Schwann, with nuclei, x 300. (Klein and Noble Smith.) CHAP. XVII.] STRUCTURE OF NERVE FIBRES. 513 According to McCarthy, the medullary sheath is composed of small rods radiating from the axis-cylinder to the sheath of Schwann. Sometimes the whole space is occupied by them, whilst at other times the rods appear shortened, and compressed laterally into bundles embedded in some homogeneous substance. (3.) The axis-cylinder consists of a large number of primitive fibrillae. This is well shown in the cornea;, where the axis-cylinders of nerves break up into minute fibrils which go to form terminal networks, and also in the spinal cord, where these fibrillae form a large part of the grey matter. From various considerations, such as its invariable presence and unbroken con- tinuity in all nerves, though the primitive sheath or the medullary sheath maybe absent, there can be little doubt that the axis cylinder is the essential part of the fibre, the other parts having the subsidiary function of sup- port and possibly of insulation. At regular intervals in most medullated nerves, the nucleated sheath of Schwann possesses annular constrictions (nodes of Ranvier). At these points (figs. 310, 311), the continuity of the medullary white sub- stance is interrupted, and the primitive sheath comes into immediate contact with the axis-cylinder. Size.-The size of the nerve-fibres varies ; it is said that the same fibres may not preserve the same diameter through their whole length. The largest fibres are found within the trunks and branches of the spinal nerves, in which the majority measure from 14-4/1* to 19/x of an inch in diameter. In the so-called visceral nerves of the brain and spinal cord medullated nerves are found, the diameter of which varies from r8/* to 3-6/1. In the hypoglossal nerve they are intermediate in size, and generally measure 7-2/1 to 10~8p.. (b.) Non-medullated Fibres.-The fibres of the second kind (fig. 312), which constitute the whole of the branches of the olfactory and auditory nerves, the principal part of the trunk and branches Fig.3ti.-A nodeof Ran- vier in a medullated nerve-fibre, viewed from above. The medullary sheath is interrupted, and the primitive sheath thickened. Co- pied from Axel Key and Retzius, x 750. (Klein and Noble Smith.) * a = -ooi mm. 514 THE NERVOUS SYSTEM. [chap. xvn. of the sympathetic nerves, and are mingled in various proportions in the cerebro-spinal nerves, differ from the preceding, chiefly in Fig. 312.- Grey, pale, or gelatinous nerve-fibres. A. From a branch of the olfactory nerve of the sheep: a, a, two dark-bordered or white fibres from the fifth pair, associated with the pale olfactory fibres. B. From the sympathetic nerve. x 450 (Max Schultze.) their fineness, being only about | to i as large in their course within the trunks and branches of the nerves; in the absence of Fig. 313.- Transverse section of the sciatic nerve of a cat about X joo.-It consists of bundles (Funiculi') of nerve-fibres ensheathed in a fibrous supporting capsule, epineurium, A; each bundle has a special sheath (not sufficiently marked out from the epineurium in the figure) or perineurium ; the nerve-fibres N/are separated from one another by endoneurium; L, lymph spaces ; Ar, artery ; V, vein ; F, fat. Somewhat diagrammatic. (V. D. Harris.) the double contour ; in their contents being apparently uniform; and in their having, when in bundles, a yellowish grey hue instead chap, xvii.] STRUCTURE OF NERVE TRUNKS. 515 of the whiteness of the cerebro-spinal nerves. These peculiarities depend on their not possessing the outer layer of medullary substance; their contents being composed exclusively of the axis- cylinder. Yet, since many nerve-fibres may be found which appear intermediate in character between these two kinds, and since the large fibres, as they approach both their central and their peripheral end, lose their medullary sheath, and assume many of the other characters of the fine fibres of the sympathetic system, it is not necessary to suppose that there is any material difference in the twro kinds of fibres. It is worthy of note, that in the foetus, at an early period of development, all nerve-fibres are non-medullated. Nerve. - trunks. - Each nerve- trunk is composed of a variable number of different-sized bundles (funiculi) of nerve-fibres which have a special sheath (peri- neurium or neurilemma). The funiculi are enclosed in a firm fibrous sheath (epineurium); this sheath also sends in processes of connective tissue which connect the bundles together. In the funi- culi between the fibres is a deli- cate supporting tissue (the encloneurium). There are numerous lymph-spaces both beneath the connective tissue investing individual nerve-fibres, and also beneath that which surrounds the funiculi. Course.-Every nerve-fibre in its course proceeds uninter- ruptedly from its origin in a nerve-centre to near its destination, whether this be the periphery of the body, another nervous centre, or the same centre whence it issued. Bundles of fibres run together in the nerve-trunk, but merely lie in apposition with each other; they do not unite : even when they anastomose, there is no union of fibres, but only an inter- change of fibres between the anastomosing funiculi. Although each nerve-fibre is thus single and undivided through pearly its Fig. 314.-Several fibres oj a bundle of medullated nerve-fibres acted upon by silver nitrate to show peculiar beha- viour of nodes of Ranvier, N, towards this reagent. The silver has penetrated at the nodes, and has stained the axis- cylinder, M, for a short distance. S, the white substance. (Klein and Noble Smith.) 516 THE NERVOUS SYSTEM. [CHAP. XVII. whole course, yet as it approaches the region in which it terminates, individual fibres break up into several subdivisions (fig. 315) before their final ending. Plexuses.-At certain parts of their course, nerves form plexuses, Fig'- 3I5--Small branch of a muscular nerve of the frog, near its termination, showing divisions of the fibres, a, into two ; b, into tliree. X 350. (KOlliker.) in which they anastomose with each other, as in the case of the brachial and lumbar plexuses. The objects of such interchange of fibres are, (a), to give to each nerve passing off from the plexus, a wider connection with the spinal cord than it would have if it proceeded to its destination without such communication with other nerves. Thus, each nerve by the wideness of its connec- tions, is less dependent on the integrity of any single portion, whether of nerve-centre or of nerve-trunk, from which it may spring, (b) Each part supplied from a plexus has wider relations with the nerve-centres, and more extensive sympathies; and, by means of the same arrangement, groups of muscles may be co- ordinated, every member of the group receiving motor filaments from the same parts of the nerve-centre, (c) Any given part, say a limb, is less dependent upon the integrity of any one nerve. Nerve terminations.-As medullated nerve-fibres approach their terminations they lose their medullary sheath, and consist then merely of axis-cylinder and primitive sheath. They then lose also the latter, and only the axis-cylinder is left with here and there a chap, xvii.] NERVE CELLS. 517 nerve-corpuscle partly rolled around it. Finally, even this invest- ment ceases, and the axis-cylinder breaks up into its elementary fibrillae. B. Nerve-Cells or Corpuscles. Nerve-cells comprise the second principal element of nervous tissue. They are not generally present in nerve-trunks, but are found in all collections of nervous tissue called ganglia. They vary considerably in shape, size, and structure in different ganglia. a. Some of them are small, generally spherical or ovoid, and have a regular uninterrupted out- line. These simple nerve-cells are most numerous in the sym- pathetic ganglia; each is en- closed in a nucleated sheath. b. Others, which are called caudate or stellate nerve-cells (fig. 317), are larger, and have one, two, or more long processes issu- ing from them, the cells being called respectively unipolar, bipolar, or multipolar: which processes often divide and sub- divide, and appear tubular, and filled with the same kind of granular material that is con- tained within the cell. Of these processes some appear to taper to a point and terminate at a greater or less distance from the cell; some appear to anastomose with similar offsets from other cells; while others are continuous with nerve-fibres, the prolongation from the cell by degrees as- suming the characters of the nerve-fibre with which it is continuous. Ganglion-cells are generally enclosed in a transparent mem- branous capsule similar in appearance to the nucleated sheath of Schwann in nerve-fibres: within this capsule is a layer of small flattened cells. Fig. 316.-Ganglion nerve-corpuscles of different shapes. (Klein and Noble Smith.) 518 THE NERVOUS SYSTEM [chap. xvii. That process of a nerve-cell which becomes continuous with a nerve-fibre is always unbranched as it leaves the cell. It at first has all the characters of an axis-cylinder, but soon acquires a Fig. 317.-An isolated sympathetic ganglion cell of man, showing sheath with nueleated-cell lining, B. A. Ganglion-cell, with nucleus and nucleolus. C. Branched process. D. Unbranched process. (Key and Retzius.) X 750. medullary sheath, and then may be termed a nerve-fibre. This continuity of nerve-cells and fibres may be readily traced out in the anterior cornua of the grey matter of the spinal cord. In many large branched nerve-cells a distinctly fibrillated appearance is observable; the fibrillae are probably continuous with those of the axis-cylinder of a nerve. Any other points in the structure of nerve-cells will be mentioned under the account of the different ganglia. CHAP. XVII.] FUNCTIONS OF NERVE FIBRES. 519 II. Function of Nerve-fibres. From the account of nervous action previously given it will be readily understood (p. 486), that nerve-fibres are stimulated to act by anything which, with sufficient suddenness, increases their irritability, but they are incapable of originating of themselves the condition necessary for the manifestation of their own energy. The stimulus produces its effect upon the termination of the nerve stimulated, being conducted to it by the nerve-fibre. The effect of the stimulus upon a nerve therefore depends upon the nature of its end-organ. A length of a nerve-trunk when freshly removed from the body, if stimulated midway between its extremi- ties, will, as shown by the deflection of the needle of a galvanometer at either end, conduct the electrical impressions in either direction, and it may be considered therefore only an accidental circumstance as it were, whether when in situ it has conducted impressions to the central nervous system from the periphery, or from the central nervous system to the muscles or other tissues. The same fibre cannot be used for the one purpose at one time, and for the other at another, simply because of the nature of its terminal organs. Thus, when a cerebro-spinal nerve-fibre is irritated in the living body as by pinching, or by heat, or by electrifying it, there is, under ordinary circumstances, one of two effects,-either there is pain, or there is twitching of one or more muscles to which the nerve distributes its fibres. From various considerations it is cer- tain that pain is always the result of a change in the nerve-cells of the brain. Therefore, in such an experiment as that referred to, the irritation of the nerve-fibre is conducted in one of two direc- tions, i.e., either to the brain, which is the central termination of the fibre, when there is pain, or to a muscle, which is the peripheral termination, when there is movement. That this is the true explanation is made highly probable, not merely by the absence of any essential structural differences in the two kinds of nerve- fibre, but also by the fact, proved by direct experiment, that if a centripetal nerve (gustatory) be divided, and its central portion be made to unite with the distal portion of a divided motor nerve (hypoglossal) the effect of irritating the former after the parts have healed, is to excite contraction in the muscles supplied by the latter. (Philippeaux and Vulpian.) The effect of this simple experiment is a type of what always occurs when nerve-fibres are engaged in the performance of their 520 THE NERVOUS SYSTEM. [chap. XVII. functions. The result of stimulating them, which roughly imitates what happens naturally in the body, is found to occur at one or other of their extremities, central or peripheral, never at both ; and in accordance with this fact, and because, for any given nerve- fibre, the result is always the same, nerves are commonly classed as sensory or motor. Classification.-The classification of nerve-fibres into sensory and motor is not altogether accurate, and the terms Centripetal or afferent, and Centrifugal or efferent are more properly used in connection with nerve-fibres in lieu of the corresponding terms, because the result of stimulating a nerve of the former kind is not alaays the production of pain or other form of sensation, nor is motion the invariable result of stimulating the latter. The term intercentral is applied to those nerve-fibres which connect more or less distinct nerve-centres, and may, there- fore, be said to have no peripheral distribution, in the ordi- nary sense of the term. Nerve-fibres then are either (a) Centri- petal or afferent, (6) Centrifugal or efferent, or (c) Inter- central. Conduction in centripetal nerves may cause (a) pain, or some other kind of sensation ; (6) special sensation ; or (c) reflex action of some kind; or (cl) inhibition, or restraint of action. Conduction in centrifugal nerves may cause (a) contraction of muscle, (motor nerves); (6) it may influence nutrition (trophic nerves); or (c) may influence secretion (secretory nerves); or (cl) inhibit, augment, or stop any other efferent action. It is a law of action in all nerve-fibres, and corresponds with the continuity and simplicity of their course, that an impression made on any fibre, is simply and uninterruptedly transmitted along it, without being imparted or diffused to any of the fibres lying near it. In other words, all nerve-fibres are mere conductors of impres- sions. Their adaptation to this purpose is, perhaps, due to the contents of each fibre being completely isolated from those of adjacent fibres by the membrane or sheath in which each is enclosed, and which acts, it may be supposed, just as silk, or other non-conductors of electricity do, which, when covering a wire, prevent the electric condition of the wire from being k conducted into the surrounding medium. Velocity of Nerve-force.-The change which a stimulus sets up in a nerve, of the exact nature of which we are unacquainted, appears to travel along a nerve-fibre in both directions in the form of a CHAP. XVII.] CONDUCTION BY NERVES. 521 wave with considerable velocity. Helmholtz and Baxt have esti- mated the average rate of conduction in human motor nerves at in feet (nearly 29 metres) per second; this result agreeing very closely with that previously obtained. It is probably rather under than over the average velocity. Rutherford's observations agree with those of Von Wittich, that the rate of transmission in sensory nerves is about 140 feet per second. Various con- ditions modify the rate of transmission, of which temperature is one of the most important, a very low or a very high temperature diminishing it ; fatigue of the nerve acting in the same direction, but increase of the stimulus up to a certain point increasing it, as does also the kathelectrotonic condition of the nerve. Conduction in Sensory Nerves.-Centripetal nerves appear able to convey impressions only from the parts in which they are distributed, towards the nerve-centre from which they arise, or to which they tend. Thus, when a sensitive nerve is divided, and irritation is applied to the end of the proximal portion, i.e., of the portion still connected with the nervous centre, sensation is perceived, or a reflex action ensues ; but, when the end of the distal portion of the divided nerve is irritated, no effect appears. When an impression is made upon any part of the course of a sensory nerve, the mind may perceive it as if it were made not only upon the point to which the stimulus is applied, but also upon all the points in which the fibres of the irritated nerve are distributed : in other words, the effect is the same as if the irritation were applied to the parts supplied by the branches of the nerve. When the whole trunk of the nerve is irritated, the sensation is felt at all parts which receive branches from it; but when only individual portions of the trunk are irritated, the sensation is perceived at those parts only which are supplied by the several portions. Thus, if we compress the ulnar nerve where it lies at the inner side of the elbow-joint, behind the internal condyle, we have the sensation of " pins and needles," or of a shock, in the parts to which its fibres are distributed, namely, in the palm and back of the hand, and in the fifth and ulna half of the fourth finger. When stronger pressure is made, the sensations are felt in the fore-arm also ; and if the mode and direction of the pressure be varied, the sensation is felt by turns in the fourth finger, in the fifth, and in the palm of the hand, or in the back of the hand, according as different fibres or fasciculi of fibres are more pressed upon than others. Illustrations.-It is in accordance with this law, that when parts are deprived of sensibility by compression or division of the nerves supplying them, irritation of the portion of the nerve connected with the brain still excites sensations which are felt as if derived from the parts to which the peripheral extremities of the nerve-fibres are distributed. Thus, there are cases of paralysis in which the limbs are totally insensible to external stimuli, yet are the seat of most violent pain, resulting apparently from irritation of the sound part of the trunk of the nerve still in connection with the brain, or from irritation of those parts of the nervous centre from which 522 THE NERVOUS SYSTEM. [chap. xvn. the sensory nerve or nerves which supply the paralysed limbs originate. An illustration of the same law is also afforded by the cases in which division of a nerve for the cure of neuralgic pain is found useless, and in which the pain continues or returns, though portions of the nerves are removed. In such cases, the disease is probably seated nearer the nervous centre than the part at which the division of the nerve is made, or it may be in the nervous centre itself. In the same way may be explained the fact, that when part of a limb has been removed by amputation, the remaining portions of the nerves may give rise to sensations which the mind refers to the lost part. When the stump is healed, the sensations which we are accustomed to have in a sound limb are still felt; and tingling and pains arc referred to the parts that are lost, or to particular portions of them, as to single toes, to the sole of the foot, to the dorsum of the foot, etc. It must not be assumed, as it often has been, that the mind has no power of discriminating the very point in the length of any nerve-fibre to which an irritation is applied. Even in the instances referred to, the mind perceives the pressure of a nerve at the point of pressure, as well as in the seeming sensations derived from the extremities of the fibres : and in stumps, pain is felt in the stump, as well as, seemingly, in the parts removed. It is not quite certain whether these sensations are due to conduction through the nerve-fibres which are on their way to be distributed elsewhere, or through the sentient extremities of nerves which are themselves distributed to the many trunks of the nerves, the nervi nervorum. The latter is the more probable supposition. Conduction in the Nerves of Special Sense.-The laws of conduction in the olfactory, optic, auditory, gustatory, resemble in many aspects those of conduction in the nerves of commom sensation, just described. Thus the effect is always central; stimulation of the trunk of the nerve produces the same effect as that of its extremities, and if the nerve be severed, it is the central and not the peripheral extremity which responds to irritation, although the sensation is referred to the periphery. There are, however, certain peculiarities in the effects. Thus the various stimuli, which might cause, through an ordinary sensitive nerve, the sense of pain, would, if applied to the optic nerve, cause a sensation as of flashes of light; if applied to the olfactory, there would be a sense as of something smelt. And so with the other two. Hence the explanation of so-called subjective sensations. Irritation in the optic nerve, or the part of the brain from which it arises, may cause a patient to believe he sees flashes of light, and among the commonest troubles of the nerves of special sense, is the distressing noise in the head (tinnitus auriumj, which depends on some unknown stimu- lation of the auditory nerve or centre quite unconnected with external sounds. Conduction in Motor Nerves.-Conduction in motor nerves presents a remarkable contrast with the foregoing. Thus, the effect of applying a stimulus to the motor nerve is always noticeable, at the peripheral ex- tremity, in the contraction of muscles supplied by it. If a motor nerve be severed, irritation of the distal portion causes contraction of muscle, but no effect whatever is produced by stimulating that part of the nerve which is still in direct connection with the nerve-centre. Contractions are excite ! in all the muscles supplied by the branches given off by the nerve below the point irritated, and in those muscles alone : the CHAP. XVII.] NERVE TERMINATIONS. 523 muscles supplied by the branches which come off from the nerve at a higher point than that irritated, are not directly excited to contraction. And it is from the same fact that, when a motor nerve enters a plexus and contributes with other nerves to the formation of a nervous trunk proceeding from the plexus, it does not impart motor power to the whole of that trunk, but only retains it isolated in the fibres which form its continuation in the branches of that trunk. Nerve Terminations. A. Of Sensory Nerves.-(i.) Paetni'an Corpuscles.-The Pacinian bodies or corpuscles (figs. 318 and 319), named after their discoverer Pacini, also called Corpuscles of Vater, are little elon- gated oval bodies, situated on some of the cerebro-spinal and sympathetic nerves, especially the cutaneous nerves of the hands and feet; and on branches of the large sympathetic plexus about the abdominal aorta (Kolliker). They often occur also on the nerves of the mesentery, and are especially well seen even by the naked eye in the mesen- tery of the cat. They have been observed also in the pancreas, lym- phatic glands and thyroid glands, as well as in the penis of the cat. Each corpuscle is attached by a narrow pedicle to the nerve on which it is situated, and is formed of several concentric layers of fine membrane, consisting of a hyaline ground- membrane with connective tissue fibres, each layer being lined by endothelium (fig. 320) ; through its pedicle passes a single nerve-fibre, which, after traversing the several concentric layers and their immediate spaces, enters a central cavity, and, gradually losing its dark border, and becoming smaller, terminates at or near the distal end of the cavity, in a knob-like enlargement, or in a bifurcation. The enlargement commonly found at the end of the fibre, is said by Pacini to resemble a ganglion corpuscle ; but this observation has not been Fig. 318.-Extremities of a nerve of the finger with Pacinian cor- puscles attached, about the natural size (adapted from Henle and Kolliker). 524 THE NERVOUS SYSTEM. [chap. xvn. confirmed. In some cases two nerves have been seen entering one Pacinian body, and in others a nerve after passing unaltered through one, has been observed to terminate in a second Paci- nian corpuscle. The physio- logical import of these bodies is still obscure. (2.) The corp uscles of Herbst (fig. 321) are closely allied to Pacinian corpuscles, except that they are smaller and longer, with a row of nuclei around the central termina- tion of the nerve in the core. They have been found chiefly in the tongues of ducks. The capsules are nearer together, and towards the centre the endothelial sheath appears to be absent. (3.) End-bulbs are found in the conjunctiva, in the glans penis and clitoris, in the skin, in the lips, and in tendon; each is about gio inch in diameter, oval or spheroidal, and is com- posed of a medullated nerve- fibre, which terminates in cor- puscles of various shapes, with a capsule containing a trans- parent or striated mass, in the centre of which terminates the axis-cylinder of the nerve-fibre, the ending of which is some- what clubbed (fig. 322). (4.) Touch-corpuscles (fig. 323) are found in the papilla? of the skin of the fingers and toes, or among its epithelium ; they may be simple or compound; when simple, they are large and slightly flattened transparent nucleated ganglion cells, enclosed in a Fig. 319.-Pacinian corpuscle of the cat's mesen- tery. The stalk consists of a nerve-fibre (N) with its thick outer sheath. The peri- pheral capsules of the Pacinian corpuscle are continuous with the outer sheath of the stalk. The intermediary part becomes much narrower near the entrance of the axis-cylinder into the clear central mass. A hook-shaped termination with the end- bulb (T) is seen in the upper part. A blood- vessel (V) enters the Pacinian corpuscle, and approaches the end-bulb; it possesses a sheath which is the continuation of the peripheral capsules of the Pacinian corpus- cle. X 100. (Klein and Noble Smith.) CHAP. XVII.] NERVE TERMINATIONS. 525 capsule; when compound the capsule contains several small cells. They are small oblong masses, about inch long, and inch Fig. 320.-Summit of a Pacinian corpuscle of the human finger, showing the endothelial membranes lining the capsules, x 220. (Kleinand Noble Smith.) broad. Some regard touch-corpuscles as little else than masses of fibrous or connective tissue, surrounded by elastic fibres, and Fig. 321.-A corpuscle of Herbst, from the tongue of a duck, a, medulla ted nerve cut away. (Klein.) Fig. 322.-End-bulb of Krause, a, me- dullated nerve-fibre; b, capsule of corpuscle. formed, according to Huxley, by an increased development of the primitive sheaths of the nerve-fibres, entering the papillae. Others, 526 THE NERVOUS SYSTEM [chap. xvii. however, believe that, instead of thus consisting of a homogeneous mass of connective tissue, they are special and peculiar bodies of B A Fig. 323.-Papilla from the skin of the hand, freed from the cuticle and exhibiting tactile corpuscles, a. Simple papilla with four nerve-fibres: a, tactile corpuscles; ft, nerves. b. Papilla treated with acetic acid ; a, corticle layer with cells and fine elastic fila- ments ; ft, tactile corpuscle with transverse nuclei; c, entering nerve with neurilemma or perineurium; d, nerve-fibres winding round the corpuscle, x 350. (Kdlliker.) laminated structure, directly concerned in the sense of touch. They do not occur in all the papillae of the parts where they are found, and, as a rule, in the papillae in which they are present Fig. 324.-A corpuscle of Grandry, from the tongue of a duck. Fig. 325.-A touch-corpuscle of Meissner, from the skin of the human hand. there are no blood-vessels. Since these bodies in which the nerve-fibres end are only met with in the papillae of highly sensitive parts, it is inferred that they are specially concerned in the sense of touch, yet their absence from the papillae of other tactile parts shows that they are not essential to this sense. The peculiar way in which the medullated nerve winds round and round the corpuscle before it enters it is shown in fig. 325. CHAP. XVII.] NERVE TERMINATIONS. 527 It loses its sheath before it enters into the interior, and then its axis-cylinder branches, and the branches coil round the Fig. 326.-Termination of medullated nerve-fibres in tendon near the muscular insertion (Golgf). Fig. 327.-One of the reticulated end- plates of tig. 326, more highly magnified. a, medullated nerve-fibre ; b, reticulated end-plates (Golgi). corpuscle (fig. 325), anastomosing with one another, and ending in pear-shaped enlargements. (5.) The corpuscles of Grandry (fig. 324) form another variety, and have been noticed in the beaks and tongues of birds. They consist of corpuscles oval or spherical, contained within a delicate nucleated sheath, and containing several cells, two or more compressed vertically. The cells are granular and transparent, with a nucleus. The nerve enters on one side, and laying aside its medullary sheath, terminates in or between the cells. (6.) Nerve terminations, probably sensory in function, are found in inter- muscular tissue (figs. 326, 327), and also in tendon. The former are reticulated end plates, and the latter are something like small Pacinian corpuscles (fig. 328). (7.) In addition to the special end organs, sensory fibres may terminate in plexuses, as in the sub-epithelial and the intra-epithelial plexus of the cornea. B. Of Nerves of Special Sense.-The terminations of the nerves of special sense will be considered in the Chapter on the Special Senses. Fig. 328.-A termination of a medullated nerve-flbre in tendon, lower half with convoluted medullated nerve-fibre (Golgi). 528 THE NERVOUS SYSTEM. [chap. xvn. C. Of Motor Nerves.-The terminations of nerves in muscle, both striped and unstriped, have been already described, p. 455. D. Of Secretory Nerves.-The ending of nerves in the cells of the salivary glands has been described by Pfliiger, and has been already alluded to. III. General Plan of the Construction of the Nervous System. The Nervous System is made up of two portions or systems, the (I.) Cerebrospinal, and the (II.) Sympathetic. (I.) The Cerebro-spinal System includes the Brain-includ- ing the Cerebrum, Cerebellum, the Crura cerebri, the Pons Varolii, and the so-called Basic ganglia; the Medulla Oblongata; and the Spinal cord, with the nerves proceeding from them. Its fibres are chiefly, but not exclusively, distributed to the skin and other organs of the senses, and to the voluntary muscles. (II.) The Sympathetic System consists of:-(i) A double chain of ganglia and fibres, which extends from the cranium to the pelvis, along each side of the vertebral column, and from which branches are distributed both to the cerebro-spinal system, and to other parts of the sympathetic system. With these may be included the small ganglia in connection with those branches of the fifth cerebral nerve which are distributed in the neighbour- hood of the organs of special sense : namely, the Ophthalmic, Otic Spheno-palatine, and Submaxillary ganglia. (2) Various ganglia and plexuses of nerve-fibres which give off branches to the thoracic and abdominal viscera, the chief of such plexuses being the Cardiac, Solar, and Hypogastric ; but in intimate connection with these are many secondary plexuses, as the Aortic, Spermatic, and Renal. To these plexuses, fibres pass from the praevertebral chain of ganglia, as well as from cerebro-spinal nerves. (3) Various ganglia and plexuses in the substance of many of the viscera, as in the Stomach, Intestines, and Urinary bladder. These, which are, for the most part, microscopic, also freely communicate with other parts of the sympathetic system, as well as, to some extent, with the cerebro-spinal. (4) By many, the ganglia on the Posterior roots of the spinal nerves, on the Glossopharyngeal and Vagus, and on the Sensory root of the Fifth cerebral nerve (Gasserian ganglion), are also included as sympathetic-nerve structures. We have already considered the functions of nerve-fibres; we must now turn to those of the Nerve-centres, which are made up not only of nerve-fibres but also of nerve-cells. CHAP, xvn.] NERVE CENTRES. 529 IV. Functions of Nerve-Centres. The functions of nerve-centres maybe classified as follows:- i. Conduction. 2. Transference. 3. Reflection. 4. Automatism. 5. Augmentation. 6. Inhibition. Conduction in or through nerve-centres may be thus simply illustrated. The food in a given portion of the intestines, acting as a stimulus, produces a certain impression on the nerves in the mucous membrane, which impression is conveyed through them to the adjacent ganglia of the sympathetic. In ordinary cases, the consequence of such an impression on the ganglia is the movement by reflex action of the muscular coat of that and the adjacent part of the canal. But if irritant substances be mingled with the food, the sharper stimulus produces a stronger impression, and this is conducted through the nearest ganglia to others more and more distant; and, from all these, reflex motor impulses issuing, excite a wide-extended and more forcible action of the intestines. Or even through the sympathetic ganglia, the impression may be further conducted to the spinal cord, whence may issue motor impulses to the abdominal and other muscles, pro- ducing cramp. And yet further, the same morbid impression may be conducted through the spinal cord to the brain, where it may be felt. In the opposite direction, mental influence may be conducted from the brain through a succession of nervous centres-the spinal cord and ganglia, and one or more ganglia of the sympathetic-to produce the influence of the mind on the digestive and other organs ; altering both the quantity and quality of their secretions. i. Conduction. It has been previously stated that impressions conveyed by any centripetal nerve-fibre travel uninterruptedly throughout its whole length, and are not communicated to adjacent fibres. When such an impression, however, reaches a nerve-centre, it may seem to be communicated to another fibre or fibres; as pain or some other kind of sensation may be felt in a part different altogether from that from which, so to speak, the stimulus started. Thus, in disease of the hip, there may be pain in the knee. This apparent change of place of a sensation to a part to which it would not seem properly to belong is termed transference. The transference of impressions may be illustrated by the fact 2. Transference. 530 THE NERVOUS SYSTEM. [chap. xvn. just referred to,-the pain in the knee, which is a common symptom of disease of the hip. In this case the impression made by the disease on the nerves of the hip-joint is conveyed to the spinal cord; there it is transferred to the central ends or connections of the nerve-fibres which are distributed about the knee. Through these the transferred impression is conducted to the brain, which, referring the sensation to the part from which it usually through these fibres receives impressions, feels as if the disease and the source of pain were in the knee. At the same time that it is transferred, the primary impression may be also conducted to the brain; and in this case the pain is felt in both the hip and the knee. And so, in whatever part of the respiratory organs an irritation may be seated, the impression it produces, being con- ducted to the medulla oblongata, is transferred to the central connections of the nerves of the larynx; and thence, being con- ducted as in the last case to the brain, the latter perceives the peculiar sensation of tickling in the glottis, which excites the act of coughing. Or, again, when the sun's light falls strongly on the eye, a tickling may be felt in the nose, exciting sneezing. A variety of transference, which may be termed radiation of impressions, is shown when an impression received by a nervous centre is diffused to many other parts in the same centre, and produces sensations extending far beyond the part from which the primary impression was derived. Hence, as in the former cases, result various kinds of what have been denominated sympathetic sensations. Sometimes such sensations are referred to almost every part of the body: as in the shock and tinkling of the skin produced by some startling noise. Sometimes only the parts immediately surrounding the point first irritated participate in the effects of the irritation; thus, the aching of a tooth may be accompanied by pain in the adjoining teeth, and in all the sur- rounding parts of the face; the explanation of such a case being, that the irritation conveyed to the brain by the nerve-fibres of the diseased tooth is radiated to the central ends of adjoining fibres, and that the mind perceives this secondary impression as if it were derived from the peripheral ends of the fibres. Tn the cases of transference of nerve-force just described, it has been said that all that need be assumed is a communication of the excited condition of an afferent nerve to other parts of its nerve- 3. Reflection. CHAP. XVII.] REFLECTION. 531 centre than that from which it takes its origin. In the case of reflection, on the other hand, the stimulus having been conveyed to a nerve-centre by a centripetal nerve, is conducted away again by a centrifugal nerve, and effects some change-motor, secretory, or nutritive, at the peripheral extremity of the latter-the diffe- rence in effect depending on the variety of centrifugal nerve secondarily affected. As in transference, the reflection may take place from a certain limited set of centripetal nerves to a corre- sponding and related set of centrifugal nerves ; as when in conse- quence of the impression of light on the retina, the iris contracts, but no other muscle moves. Or the reflection may extend to widely different parts: as when an irritation in the larynx brings all the muscles engaged in expiration into coincident movement. Reflex movements, occurring quite independently of sensation, are generally called excito-motor; those which are guided or accom- panied by sensation, but not to the extent of a distinct perception or intellectual process, are termed sensori-motor. (а) For the manifestation of every reflex action, these things are necessary : (i), one or more perfect centripetal nerve-fibres, to convey an impression; (2), a nervous centre for its reception, and by which it may be reflected; (3), one or more centrifugal nerve- fibres, along which the impression may be conducted to (4), the muscular or other tissue by which the effect is manifested. In the absence of any one of these conditions, a proper reflex action cannot take place; and whenever, for example, impres- sions made by external stimuli on sensory nerves give rise to movements, these are never the result of the direct reaction of the sensory and motor fibres of the nerves on each other ; in all such cases the impression is conveyed by the afferent fibres to a nerve- centre, and is therein communicated to the motor fibres. (б) All reflex actions are essentially involuntary, though most of them admit of being modified, controlled, or prevented by a voluntary effort. (c) Reflex actions performed in health have, for the most part, a distinct purpose, and are adapted to secure some end desirable for the well-being of the body; but, in disease, many of them are irregular and purposeless. As an illustration of the first point, may be mentioned movements of the digestive canal, the respira- tory movements, and the contraction of the eyelids and the pupil to exclude many rays of light, when the retina is exposed to a bright glare. These and all other normal reflex acts afford also 532 THE NERVOUS SYSTEM. [chap. XVII. examples of the mode in which the nervous centres combine and arrange co-ordinately the actions of the nerve-fibres, so that many muscles may act together for the common end. Another instance of the same kind is furnished by the spasmodic contractions of the glottis on the contact of carbonic acid gas, or any foreign substance, with the surface of the epiglottis or larynx. Examples of the purposeless irregular nature of morbid reflex action are seen in the convulsive movements of epilepsy, and in the spasms of tetanus and hydrophobia. (<Z) Reflex muscular acts are often more sustained than those produced by the direct stimulus of muscular nerves. The irrita- tion of a muscular organ, or its motor nerve, produces contraction lasting only so long as the irritation continues ; but irritation applied to a nervous centre through one of its centripetal nerves, may excite reflex and harmonious contractions, which last some time after the withdrawal of the stimulus. Classification of Reflex Actions.-Reflex actions may be classified as follows:-i. Those in which both the centripetal and centrifugal nerves concerned are cerebrospinal; e.g., deglutition, sneezing, coughing, and, in pathological conditions, tetanus, epilepsy. 2. Those in which the centripetal nerve is cerebro- spinal, and the centrifugal is sympathetic, most often vaso-motor ; e.g., secretion of saliva, or gastric juice; blushing or pallor of the skin. 3. Those in which the centripetal nerve is of the sympathetic system, and the centrifugal is cerebro-spinal. The majority of these are pathological, as in the case of convulsion, produced by intestinal worms, or hysterical convulsions. 4. Those in which both centripetal and centrifugal nerves are of the sympa- thetic system : as, for example, in the nervous mechanism concerned in the secretion of the intestinal fluids, those which unite the various generative functions, and many pathological phenomena. Relations between the Stimulus and the Resulting Reflex Action.-Certain rules showing the relation between the result- ing reflex action and the stimulus have been drawn up by Pfliiger, as follows:- 1. Law of unilateral reflection.-A slight irritation of sensory nerves is reflected along the motor nerves of the same region. Thus, if the skin of a frog's foot be tickled on the right side, the right leg is drawn up. 2. Law of symmetrical reflection.-A stronger irritation is reflected, not only on one side, but also along the corresponding motor nerves chap, xvii.] VARIETIES OF REFLEX ACTION. 533 of the opposite side. Thus, if the spinal cord of a man has been severed by a stab in the back, when one foot is tickled both legs will be drawn up. 3. Law of intensity.-In the above case, the contractions will be more violent on the side irritated. 4. Law of radiation.-If the irritation (afferent impulse) in- creases, it is reflected along the motor nerves which spring from points higher up the spinal cord, till at length all the muscles of the body are thrown into action. Varieties of Reflex Actions. Simple and Co-ordinated Reflex Actions.-In the simplest form of reflex action a single nerve cell with an afferent and an efferent fibre is concerned, but in the majority of actual actions a number of cells are probably concerned, and the impression is as it were distributed among them, and they act in concert or co-ordination. This co-ordinating power belongs to nerve-centres. Primary and Secondary or acquired Reflex Actions.-We must carefully distinguish between such reflex actions which may be termed primary, and those which are secondary or acquired. As examples of the former class we may cite sucking, contraction of the pupil, drawing up the legs when the toes are tickled, and many others, which are performed as perfectly by the infant as by the adult. The large class of secondary reflex actions consists of acts which require for their first performance, and many subsequent repetitions, an effort of will, but which by constant repetition are habitually though not necessarily performed, mechanically, i.e., without the intervention of consciousness and volition. As instances we may take reading, writing, walking, &c. In endeavouring to conceive how such complicated actions can be performed without consciousness and will, we must suppose that in the first instance the will directs the nerve-force along certain channels causing the performance of certain acts, e.g., the various movements of flexion and extension involved in walking. After a time by constant repetition, these routes become, to use a metaphor, well worn : there is, as it were, a beaten track along which the nerve-force travels with much greater ease than formerly: so much so that a slight stimulus, such as the pressure of the foot on the ground, is sufficient to start and keep going indefinitely the complex reflex actions of walking during entire mental abstraction, or even during sleep. In such acts as reading, writing, and the. 534 THE NERVOUS SYSTEM. [chap. xvii. like, it would appear as if the will set the necessary reflex machinery going, and that the reflex actions go on uninterruptedly until again interfered with by the will. Without this capacity possessed by the nervous system of " organising conscious actions into more or less unconscious ones," education or training would be impossible. A most important part of the process by which these acquired reflex actions come to be performed automatically consists in what is termed associa- tion. If two acts be at first performed voluntarily in succession, and this succession is often repeated, the performance of the first is at once followed mechanically by the second. Instances of this " force of habit " must be within the daily experience of every one. Of course it is only such actions as have become entirely reflex that can be performed during complete unconsciousness, as in sleep. Cases of somnambulism are of course familiar to every one, and authentic instances are on record of persons writing and even playing the piano during sleep. 4. Automatism. To nerve centres, it is said, belongs the property of originating nerve-impulses, as well as of receiving them, and conducting and reflecting them. The term automatism is employed to indicate the origination of nervous impulses in nerve-centres, and their conduction therefrom, independently of previous reception of a stimulus from another part. It is impossible, in the present state of our knowledge, to say definitely what actions in the body are really in this sense automatic. An example of automatic nerve-action has been already referred to, /.<?., that of the respiratory centre, but the apparently best examples of automatism are found, however, in the case of the cerebrum, which will be presently considered. 5 and 6. Augmentation and Inhibition. Nerve-cells not only receive and reflect nerve impulses, and also in some cases even originate such impulses, but they are also capable of increasing the impulse, and the result is what is called augmentation; and when a nerve-centre is in action, its action is also capable of being increased or diminished {inhibition) by afferent impulses. This is the case in whatever way the centre has caused the action, whether of itself, or by means of previous afferent impulses. The action, by which a centre is capable of chap, xvin.] THE CEREBRO-SPINAL NERVOUS SYSTEM. 535 being inhibited or exalted, has been well shown in the case of the vaso-motor centre, before described. This power, which can be exerted from the periphery, is very important in regulating the action even of partially automatic centres such as the respiratory centre. CHAPTER XVIII. THE CEREBRO-SPINAL NERVOUS SYSTEM. The physiology of the cerebro-spinal nervous system includes that of the Spinal Cord, Medulla Oblongata, and Brain, of the several Nerves given off from each, and of the Ganglia on those nerves. Membranes of the Brain and. Spinal Cord.-The Brain and Spinal Cord are enveloped in three membranes-(i) the Dura Mater, (2) the Arachnoid, (3) the Pia Mater. (l.) The Dura Mater, or external covering, is a tough membrane com- posed of bundles of connective tissue which cross at various angles, and in whose interstices branched connective-tissue corpuscles lie : it is lined by a thin elastic membrane, and on the inner surface, and, where it is not adherent to the bone, on the outer surface also is a layer of endothelial cells very similar to those found in serous membranes. (2.) The Arachnoid is a much more delicate membrane, very similar in structure to the dura mater, and lined on its outer or free surface by an endothelial membrane. (3.) The Pia Mater consists of two chief layers, between which numerous blood-vessels ramify. Between the arachnoid and pia mater is a network of fibrous-tissue trabeculae sheathed with endothelial cells : these sub-arachnoid trabeculae divide up the sub-arachnoid space into a number of irregular sinuses. There are some similar trabeculae, but much fewer in number, traversing the sub-dural space, i.e., the space between the dura mater and arachnoid. Pacchionian bodies are growths from the sub-arachnoid network of connective-tissue trabeculae which project through small holes in the inner layers of the dura mater into the venous sinuses of that membrane. The venous sinuses of the dura mater have been injected from the sub-arach- noidal space through the intermediation of these villous outgrowths. A. The Spinal Cord and its Nerves. The Spinal cord is a cylindriform column of nerve-substance connected above with the brain through the medium of the me- dulla oblongata, and terminating below, about the lower border of the first lumbar vertebra, in a slender filament of grey substance, the filum terminale, which lies in the midst of the roots of many nerves forming the cauda equina. 536 THE NERVOUS SYSTEM. [chap, xviii. Structure.-The cord is composed of white and grey nervous sub- stance, of which the former is situated externally, and constitutes its chief portion, while the latter occupies its central or axial portion, and is so arranged, that on the surface of a transverse section of the cord it appears like two somewhat crescentic masses con- nected together by a narrower portion or isthmus (fig. 330). Passing through the centre of this isth- mus in a longitudinal direction is a minute canal (central canal), which is continued through the whole length of the cord, and opens above into the space at the back of medulla oblongata and pons Varolii, called the fourth ven- tricle. It is lined by a layer of columnar ciliated epithelium. The spinal cord con- sists of two exactly symmetrical halves, Fig. 329.- View o f the cerebro- spinal axis of the nervous system. The right half of the cranium and trunk of the body has been removed by a vertical section; the membranes of the brain and spinal cord have also been removed, .and the roots and first part of the fifth and ninth cranial, and of all the spinal nerves of the right side, have been - , ; , ' . K> . _■» dissected out and laid sepa- rately on the wall of the skull and on the several vertebra; opposite to the place of their natural exit from the cranio-spinal cavity. (After Bourgery.) CHAI*. XVIII.] THE SPINAL CORI). 537 separated anteriorly and posteriorly by vertical fissures (the posterior fissure being deeper, but less wide and distinct than the anterior), and united in the middle by nervous matter which is usually described as forming two commissures-an anterior com- missure, in front of the central canal, consisting of medullated nerve-fibres, and a posterior commissure behind the central canal consisting also of medullated nerve-fibres, but with more neuroglia, Fig. 330.-Different views of a portion of the spinal cord from the cervical region, with the roots of the nerves (slightly enlarged). In a, the anterior surface of the specimen is shown; the anterior nerve-root of its right side being divided ; in n, a view of the right side is given ; in c, the upper surface is shown; in d, the nerve-roots and ganglion are shown from below, i. The anterior median fissure ; 2, posterior median fissure ; 3, anterior lateral depression, over which the anterior nerve-roots are seen to spread; 4, posterior lateral groove, into which the posterior roots are seen to sink; 5, anterior roots passing the ganglion; 5', in a, the anterior root divided ; 6, the posterior roots, the fibres of which pass into the ganglion 6'; 7, the united or com- pound nerve; 7',the posterior primary branch, seen in a and i> to be derived in part from the anterior and in part from the posterior root. (Allen Thomson.) which gives the grey aspect to this commissure (fig. 330, b). Each half of the spinal cord is marked on the sides (obscurely at the lower part, but distinctly above) by two longitudinal furrows, which divide it into three portions, columns, or tracts, an anterior, lateral, and posterior. From the groove between the anterior and lateral columns spring the anterior roots of the spinal nerves (b and c, 5) ; and just in front of the groove between the lateral and posterior column arise the posterior roots of the same (b, 6) : a pair of roots on each side corresponding to each vertebra (fig. 329). 538 THE NERVOUS SYSTEM. [chap. xvm. White matter.-The white matter of the cord is made up of medullated nerve-fibres, of various sizes, arranged longitudinally around the cord under the pia mater and passing in to support the individual fibres in the delicate connective tissue or neuroglia made up of a very fine reticulum, with both small cells almost filled up by nuclei and stellate branching corpuscles. The general rule respecting the size of different parts of the cord appears to be, that the size of each part bears a direct proportion to the size and number of nerve-roots given off from Fig. 331.-Section of grey matter of anterior cornu of a calf's spinal cord ; n f, nerve-fibres of white matter in transverse section, showing axis-cylinder in centre of each; a r, anterior roots of spinal nerve passing out through white matter; g c, large stellate nerve-cells with nuclei; they are seen imbedded in neuroglia. (Schofield.) itself, and has but little relation to the size or number of those given off below it. Thus the cord is very large in the middle and lower part of its cervical portion, whence arise the large nerve-roots for the formation of the brachial plexuses and the supply of the upper extremities, and again enlarges at the lowest part of its dorsal portion and the upper part of its lumbar, at the origins of the large nerves which, after forming the lumbar and sacral plexuses, are distributed to the lower extremities. The chief cause of the greater size at these parts of the spinal cord is increase in the quantity of grey matter; for there seems reason to believe that the white or fibrous part of the cord becomes gradually and progressively larger from below upwards, doubtless CHAP. XVIII.] SIZE OF THE SPINAL CORD. 539 from the addition of a certain number of upward passing fibres from each pair of nerves. From careful estimates of the number of nerve-fibres in a trans- verse section of the cord towards its upper end, and the number entering it by the anterior and posterior roots of each pair of nerves, it has been shown that in the human spinal cord not more than half of the total number of nerve- fibres entering the cord through all the spinal nerves are contained in a transverse section near its upper end. It is obvious, therefore, that at least half of the nerve-fibres entering it must terminate in the cord itself. Grey matter. - The grey matter of the cord consists essentially of an extremely delicate network of the primitive fibrillae of axis-cylinders, and which are derived from the rami- fication of multipolar gang- lion cells of very large size, containing large round nuclei with nucleoli. This fine plexus is called Ger- lach's network, and is mingled with the meshes of neuro- glia, which in some parts is chiefly fibrillated, in others mainly granular and punctiform. The neuroglia is pro- longed from the surface into the tip of the posterior cornu of grey matter and forms a jelly-like transparent substance, which when hardened is found to be reticular, and is called the sub- stantia gelatinosa of Rolando. The multipolar cells are either scattered singly or arranged in groups, of which the following are to be distinguished on either Fig. 332.-Transverse section of half the spinal cord in the lumbar enlargement (semi-diagrammatic). 1. Anterior median fissure ; 2, posterior median fissure ; 3, central canal lined with epithelium ; 4, posterior commissure; 5, anterior commis- sure ; 6, posterior column; 7, lateral column; 8, anterior column. The white substance is traversed by radiating trabecula1 of pia mater. 9, Fasiculus of posterior nerve-root entering in one bundle; 10, fasciculi of anterior roots en- tering in four spreading bundles of fibres; b, in the cervix cornu, decussating fibres from the nerve-roots and posterior commissure; c, pos- terior vesicular columns. About half way be- tween the central canal and 7 are seen the group of nerve-cells forming the tractus intermedio- lateralis ; e,e, fibres of anterior roots ; e*, fibres of anterior roots which decussate in anterior commissure. (Allen Thomson.) x 6. 540 THE NERVOUS SYSTEM. [chap. XVIII. side :-(a) In the anterior cornu. The groups found in the anterior cornu are generally two-one at the lateral part near the lateral column, and the other at the tip of the cornu in the middle line- sometimes, as in the lumbar enlargement, there is a third group more posterior. The cells of the anterior group are the largest. Into many of these cells the fibres of the anterior motor nerve-roots can be distinctly traced, (6.) In the tractus intermedio-lateralis. A group of nerve-cells midway between the anterior and posterior cornua, near the external surface of the grey matter. It is espe- cially developed in the dorsal and also in the upper cervical region, (c.) In the posterior-vesicular columns of Lockhart Clarke. These are found in the posterior cornua of grey matter towards the inner surface, extending from the cervical enlargement to the third lumbar nerves (fig. 332, c). (d.) Smaller cells are scattered throughout the grey matter, but are found chiefly at the tip (caput cornu) of posterior cornu, in a finely granular basis, and also among the posterior root fibres (substantia gelatinosa cinerea of Rolando). The anterior nerve-cells are connected by their processes im- mediately with the axis-cylinders of the fibres of the anterior or motor nerve-roots: whereas the nerve-cells of the posterior roots are connected with nerve-fibres, not directly, but only through the intermediation of Gerlach's nerve-network, in which their branching processes lose themselves. Spinal Nerves.-The spinal nerves consist of thirty-one pairs, issuing from the sides of the whole length of the cord, their number corresponding with the intervertebral foramina through which they pass. Each nerve arises by two roots, an anterior and posterior, the latter being the larger. The roots emerge through separate apertures of the sheath of dura mater surrounding the cord ; and directly after their emergence, where the roots lie in the inter- vertebral foramen, a ganglion is found on the posterior root. The anterior root lies in contact with the anterior surface of the gang- lion, but none of its fibres intermingle with those in the ganglion (5, fig- 33°)- But immediately beyond the ganglion the two roots coalesce, and by the mingling of their fibres form a compound or mixed spinal nerve, which, after issuing from the intervertebral canal, gives off anterior and posterior or ventral and dorsal branches, each containing fibres from both the roots (fig.330), as well as a third or visceral branch, ramus communicans, to the sympathetic. The anterior root of each spinal nerve arises by numerous CHAP. XVIII.] TIIE SPINAL NERVE-ROOTS. 541 separate and. converging bundles from the anterior column of the cord; the posterior root by more numerous parallel bundles, from the posterior column, or, rather, from the posterior part of the lateral column (fig. 330), for if a fissure be directed inwards from the groove between the middle and posterior columns, the posterior roots will remain attached to the former. The anterior roots of each spinal nerve consist of centrif ugal fibres; the pos- terior as exclusively of centripetal fibres. Course of the Fibres of the Spinal Nerve-Roots.-(a) The interior roots enter the cord in several bundles, which may be called:-(1) Internal; (2) Middle; (3) External; all being more or less connected with the groups of multipolar cells in the anterior cornua. 1. The internal fibres are partly connected with internal group of nerve-cells of anterior cornu of the same side; but some fibres pass over, through anterior commissure to end in the anterior cornu of opposite side, probably in internal group of cells. 2. The middle fibres are partly in connection with the lateral group of cells in anterior cornu, and in part pass backwards to posterior cornu, having no connection with cells. 3. The external fibres are partly in connection with the lateral group of cells in the anterior cornu, but some fibres proceed direct into the lateral column without connection with cells, and pass upwards in it. (5) The Posterior roots enter the posterior cornua in two chief bundles, either at the tip, through or round the substantia gela- tinosa, or by the inner side. The former enter the grey matter at once, and as a rule, turn upwards or downwards for a certain dis- tance and then pass horizontally; some fibres reach the anterior cornua, passing at once horizontally; and the others, the opposite side, through the posterior grey commissure. Of those which enter by the inner side of the cornua the majority pass up (or down) in the white substance of the posterior columns, and enter the grey matter at various heights at the base of the posterior cornu; perhaps some pass directly upwards and inwards in the posterior median column without entering the grey matter. Those that enter the grey matter pass in various directions, some to join the lateral cells in the anterior cornu, some join the cells in the posterior vesicular column, and some pass across to the other side of the cord in the anterior commissure, whilst others become again longitudinal in the grey matter. It should be here mentioned that the cells in the posterior vesicular column are connected with medullated fibres which 542 THE NERVOUS SYSTEM. [chap, xviii. pass horizontally to the white matter of the lateral columns, and there become longitudinal. Course of the fibres in the cord. The nerve-fibres which form the white matter of the cord arc nearly all longitudinal fibres. It is, however, a matter of great difficulty to trace them by mere dissection, and so other methods have been resorted to. One of these is based upon the fact that nerve- fibres undergo degene- ration when they are cut off from the centre with which they are normally connected, or when the parts to which they are distributed are removed, as in ampu- tation of a limb; and information as to the course of the fibres has been obtained by tracing such degenerated tracts. The second method consists in observing the development of the fibres of the various tracts; some tracts of fibres receive their medullary substance later than others, and are to be traced by their grey appearance. The chief tracts which have been made out are the following:-(i) The direct pyramidal tract (fig. 333, d.p.t.\ a comparatively small portion of the inner part of the anterior columns, which is traceable from the anterior pyramids of the medulla, as far as the mid-dorsal region of the spinal cord. It consists of the fibres of the pyramids which do not undergo decussation in the medulla. They are probably fibres chiefly for the arm and constitute about one-fourth or one-fifth of the whole motor tract. There is reason for believing, however, that the fibres of this tract undergo decussation throughout their course,and also that fibres pass over from it through the anterior commissure to join the lateral pyramidal tract; (2) the Crossed or lateral pyramidal tract (fig. 333, L. p. t.) can be traced from the anterior pyramids of the medulla, and consists of motor fibres which decussate in the anterior fissure and pass downwards in the lateral columns near Ant-Toot. d.jj.l P.rdot P.M.C. Fig. 333.-Diagram of the spinal cord at the lower cervi- cal region to show the track of fibres; d.p. t., direct pyramidal tract; l. p. t., crossed pyramidal tract; n. c. t., direct cerebellar tract; p. m. c., posterior median column. A. G. F., anterior ground fibres ; A. c., anterior commissure; P. c., post, commissure; a. l. a. t., antero-lateral ascending tract ; Ant. C., anterior cornu ; P. Cor., posterior cornu ; c. c. p., intermediate grey substance; l. l. l., lateral limiting layer. (After Gowers.) CHAP. XVIII.] THE PYRAMIDAL TRACTS. 543 the posterior cornu of the grey matter. They may be traced down- wards as far as the lower end of the cord. The number of fibres which decussate in the medulla, and consequently the size of this tract, varies. The fibres which most constantly cross over are those for the leg. The pyramidal tracts end in the grey matter of the anterior cornua; (3) Direct cerebellar tract, D. c. t., which cor- responds to the peripheral portion of the posterior lateral column between the crossed pyramidal tract and the edge of the cord, can be traced upwards directly to the cerebellum and downwards as far as the mid-lumbar region ; (4) Posterior median column, or Fas- ciculus of Goll, is found on either side of the posterior commissure, and is traceable upwards and terminates as the fasciculus gracilis of the medulla. It is traceable downwards as far as the mid-dorsal region. The portion of the posterior column between the posterior median column and the posterior roots of the spinal nerves, known as (5) The Fasciculus cuneatus, Burdock's,or Postero-external column, is composed of fibres of the posterior roots on their way to enter the grey substance and the posterior median column at different heights. The antero-lateral column contains fibres from the anterior cornua of the same as well as of the opposite side; (6) Lateral limiting layer (l. l. l.) consists of fine fibres which pass into the grey matter at different levels ; it probably consists of con- necting fibres to connect the grey matter of different levels. These fibres have not a long course ; (7) .dntfmor ground fibres (a. g. f.) are vertical fibres which probably connect the anterior cornua at different levels. Some fibres pass to the anterior commissure and connect with the anterior cornu of the opposite side; (8) Antero- lateral ascending tract is a tract which degenerates upwards. It is a sensory tract, and is connected with the posterior nerve-roots of the opposite side. Functions of the Spinal Nerve-Roots.-The anterior spinal nerve-roots are efferent or motor: the posterior are afferent or sensory. The fact is proved in various ways. Division of the anterior roots of one or more nerves is followed by complete loss of motion in the parts supplied by the fibres of such roots ; but the sensation of the same parts remains perfect. Division of the posterior roots destroys the sensibility of the parts supplied by their fibres, while the power of motion continues unimpaired. Moreover, irritation of the ends of the distal portions of the divided anterior roots of a nerve excites muscular movements; irritation of the ends of the proximal portions, which are still in 544 THE NERVOUS SYSTEM. [chap. XVIII. connection with the cord, is followed by no appreciable effect. Irri- tation of the distal portions of the divided posterior roots, on the other hand, produces no muscular movements and no manifesta- tions of pain; for, as already stated, sensory nerves convey im- pressions only towards the nervous centres : but irritation of the proximal portions of these roots elicits signs of intense suffering. Occasionally, under this last irritation, muscular movements also ensue; but these are either voluntary, or the result of the irrita- tion being reflected from the sensory to the motor fibres. Occa- sionally, too, irritation of the distal ends of divided anterior roots elicits signs of pain, as well as producing muscular movements : the pain thus excited is probably the result either of cramp or of so-called recurrent sensibility. Recurrent Sensibility.-If the anterior root of a spinal nerve be divided, and the peripheral end be irritated, not only move- ments of the muscles supplied by the nerve take place, but also of other muscles, indicative of pain. If the main trunk of the nerve (after the coalescence of the roots beyond the ganglion) be divided, and the anterior root be irritated as before, the general signs of pain still remain, although the contraction of the muscles does not occur. The signs of pain disappear when the posterior root is divided. From these experiments it is believed that the stimulus passes down the anterior root to the mixed nerve, and returns to the central nervous system through the posterior root by means of certain sensory fibres from the posterior root, which loop back into the anterior root before continuing their course into the mixed nerve-trunk. Functions of the Ganglia on Posterior Roots.--The ganglia act as centres for the nutrition of the nerves, since when the nerves are severed from connection with the ganglia, the parts of the nerves so severed degenerate, whilst the parts which remain in connection with them do not. Functions of the Spinal Cord. The power of the spinal cord, as a nerve-centre, may be arranged under the heads of (i) Conduction; (2) Transference; (3) Reflex action. (1) Conduction.-The functions of the spinal cord in relation to conduction, may be best remembered by considering its anatomical connections with other parts of the body. From these it is evident that, with the exception of some few filaments of the sympathetic, chap, xviii.] CONDUCTION IN THE SPINAL CORD. 545 there is no way by which nerve-impulses can be conveyed from the trunk and extremities to the brain, or vice versa, other than that formed by the spinal cord. Through it, the impressions made upon the peripheral extremities or other parts of the spinal sensory nerves are conducted to the brain, where alone they can be per- ceived. Through it, also, the stimulus of the will, conducted from the brain, is capable of exciting the action of the muscles supplied from it with motor nerves. And for all these conductions of impressions to and fro between the brain and the spinal nerves, the perfect state of the cord is necessary ; for when any part of it is destroyed, and its communication with the brain is interrupted, impressions on the sensory nerves given off from it below the seat of injury, cease to be propagated to the brain, and the brain loses the power of voluntarily exciting the motor nerves proceeding from the portion of cord isolated from it. Illustrations of this are furnished by various examples of paralysis, but by none better than by the common paraplegia, or loss of sensation and voluntary motion in the lower part of the body, in consequence of destruc- tive disease or injury of a portion, including the whole thickness, of the spinal cord. Such lesions destroy the communication between the brain and all parts of the spinal cord below the seat of injury, and consequently cut off from their connection with the brain the various organs supplied with nerves issuing from those parts of the cord. It is not probable that the conduction of impressions along the cord is effected (to any great extent), as was formerly supposed, through the grey substance, i.e., through the nerve-corpuscles and filaments connecting them. All parts of the cord arc not alike able to conduct all impressions • and as there are separate nerve- fibres for motor and for sensory impressions, so in the cord, sepa- rate and determinate tracts serve to conduct always the samekind of impression. Experimental and other observations point to the following conclusions regarding the conduction of sensory and motor im- pressions through the spinal cord. It is important to bear in mind that the grey matter of the cord, even if it conduct some impressions giving rise to sensation, appears not to be sensitive when it is directly stimulated. The explana- tion probably is, that it possesses no apparatus such as exists at the peripheral terminations of sensory nerves, for the reception of sensory impressions. 546 THE NERVOUS SYSTEM. [chap. XVIII. The conducting Paths in the Spinal Cord. a. Sensory Impressions are conveyed to the spinal cord by the posterior nerve-roots, and generally speaking cross over to the opposite side, and are conveyed upwards in two or three paths, according to the nature of the sensory impulse. (r.) Sensibility to Pain is almost certainly conveyed upwards in that part of the lateral column which is called by Gowers the antero-lateral ascending tract (a l a t, fig. 333). It is a tract of vertical fibres immediately in front of the crossed pyramidal and direct cerebellar tracts. The zone extends across the lateral column as a band which is largest in area near the periphery of the cord, where it fills up the angle between the crossed pyramidal and cerebellar tracts, and it reaches the surface of the cord in front of the latter tract ; it then extends forwards in the periphery of the anterior column, almost to the anterior median fissure (Gowers). (2.) Sensibility to Touch (tactile sensibility) is probably con- veyed upwards, after decussating almost as soon as it enters the cord, in the posterior median column. (3.) Sensibility of the Muscles (muscular sensibility).-The path of muscular sensation does not decussate, but passes upwards probably in the posterior median column of the same side, passing up to it from the hinder part of the postero-external column, and according to Flechsig in the direct cerebellar tract. (4.) Sensibility to Temperature.-The path for sensations of temperature is probably near to that of sensibility to pain, in the lateral column. (5.) Sensory Impressions subserving Reflex Actions.-There is considerable probability that all the paths for cutaneous sensibility undergo interruption in the spinal cord, and do not pass straight up, as no ascending tract of degeneration has been demonstrated so far when a lesion has been confined to the nerve-roots. If this be the case, it is probable that the same fibres which convey sensation have also to do with the cutaneous reflexes. In the case of muscular reflexes, however, as the fibres pass upwards without interruption, the reverse is in all probability the case, and special afferent fibres, even if few in number, exist, which are employed in the chain of such reflexes. b. Motor Impressions. - Motor impressions are conveyed downwards from the brain along the pyramidal tracts, viz., the direct or anterior, and the crossed or lateral, chiefly in the latter. CHAP. XVIII.] MOTOR TRACTS IX THE CORD. 547 Generally speaking, the impressions pass down on the side opposite to which they originate, having undergone decussation in the medulla; but some impressions do not cross in the medulla, but lower down, in the cord, being conveyed by the anterior or uncrossed pyramidal fibres, and decussate in the anterior com- missure. The motor fibres for the legs partially pass downwards in the lateral columns of the same side. This is also probably Fig. 334.-Diagram of the decussation of the conductors for voluntary movements, and those for sensation: ar, anterior roots and their continuations in the spinal cord, and decussa- tion at the lower part of the medulla oblongata, m o ; p r, the posterior roots and their continuation and decussation in the spinal cord ; g, g, the ganglions of the roots. The arrows indicate the direction of the nervous action ; r, the right side; Z, the left side, i, 2, 3, indicate places of alteration in a lateral half of the spino-cerebral axis, to show the influence on the two kinds of conductors, resulting from section of the cord at any one of these three places. (After Brown-S&juard.) the case with the bilateral muscles, i.e., muscles of the two sides acting together, such as the intercostal muscles and other muscles of the trunk, as well as the costo-humeral muscles. It is quite certain, as was just now pointed out, that the fibres of the anterior nerve-roots are more numerous than the fibres pro- ceeding downwards from the brain in the pyramidal tracts, or the so-called pyramidal fibres. It is therefore probable that each pyramidal fibre, or set of fibres, corresponds with an apparatus of 548 THE NERVOUS SYSTEM. [chap. XVIII. ganglion cells in the anterior cornu either on the same level, or even above or below, that when this fibre, or set of fibres, is stimulated, very complex co-ordinated movements occur-such co- ordinated movements having been set up by impressions from a connected system of ganglion cells, sent out into the motor nerve fibres which arise from them. In other words, it appears to be probable that in the grey matter of the anterior cornua of various sections of the cord are contained the apparatus for various com- plicated co-ordinated movements. The apparatus of each co-ordi- nated movement may be set in motion either by sensory impressions passing to the cord, when the result of movement would be a reflex action, or by an impression travelling downwards from the brain, and conveyed by one or more pyramidal fibres. Division of the anterior pyramids of the medulla at the point of decussation (2, fig. 334), is followed by paralysis of motion, never quite absolute, in all parts below. Disease or division of any part of the cerebro-spinal axis above the seat of decussation (1, fig. 334) is followed by impaired or lost power of motion on the opposite side of the body ; while a like injury inflicted below this part (3, fig. 334) induces similar, never quite absolute no doubt, on the corresponding side. When one half of the spinal cord is cut through, complete anaesthesia of the other side of the body below the point of section results, but there is often greatly increased sensibility (hyper- esthesia) on the same side ; so much so that the least touch appears to be agonising. This condition may persist for several days. Similar effects may, in man, be the result of injury. In addition to the transmission of ordinary sensory and motor impulses, the spinal cord is the medium of conduction also of im- pulses to and from the Vaso-motor centre in the medulla oblongata, although it probably contains special vaso-motor centres of its own. It will be seen in Chapter XXL that Gaskell considers that the white visceral branches from the spinal cord to the sympathetic system are connected with or arise from the posterior vesicular column of Clarke, and from the anterior lateral cells. Others think that the direct cerebellar tract arises from Clarke's column. Transference.-Examples of the transference of impressions in the cord have been given (p. 558) ; and that the transference takes place in the cord, and not in the brain, is nearly proved by the frequent cases of pain felt in the knee and not in the hip, in diseases of the hip ; of pain felt in the urethra or glans penis, and not in the bladder, in calculus ; for, if both the primary and the secondary or transferred impression were in the brain, both should be felt. chap, xviii.] REFLEX ACTIONS OF THE CORD. 549 Reflex Action or Reflection. In man the spinal cord is so much under the control of the higher nerve-centres, that its own individual functions in relation to reflex action are apt to be overlooked ; so that the result of injury, by which the cord is cut off completely from the influence of the encephalon, is apt to lessen rather than increase our estimate of its importance and individual endowments. Thus, when the human spinal cord is divided, the lower extremities fall into any position that their weight and the resistance of sur- rounding objects combine to give them; if the body is irritated, they do not move towards the irritation; and if they are touched, the consequent reflex movements are disorderly and purposeless ; all power of voluntary movement is absolutely abolished. In other mammals, however, e.g., in the rabbit or dog, after recovery from the shock of the operation, which takes some time, reflex actions in the parts below will occur after the spinal cord has been divided, a very feeble irritation being followed by extensive and co-ordinate movements. In the case of the frog, and many other cold-blooded animals, in which experimental and other injuries of the nerve-tissues are better borne, and in which the lower nerve-centres are less subordinate in their action to the higher, the reflex functions of the cord are still more clearly shown. When, for example, a frog's head is cut off, its limbs remain in, or assume a natural position ; they resume it when disturbed ; and when the abdomen or back is irritated, the feet are moved with the manifest purpose of pushing away the irrita- tion. The main difference in the cold-blooded animals being that 'the reflex movements are more definite, complicated, and effective, although less energetic than in the case of mammals. It might indeed be thought, on superficial examination, that the mind of the animal was engaged in the acts ; and yet all analogy would lead us to the belief that the spinal cord of the frog has no different endowment, in kind, from those which belong to the cord of the higher vertebrata: the difference is only in degree. And if this be granted, it may be assumed that, in man and the higher animals, many actions are performed as reflex movements occurring through and by means of the spinal cord, although the latter cannot by itself initiate or even direct them independently. Cutaneous and. Muscle Reflexes.-In the human subject two kinds of reflex actions dependent upon the spinal cord are usually distinguished, the alterations of which, either in the direction of increase or of diminution, are 550 THE NERVOUS SYSTEM. [chap, xviii. indications of some abnormality, and are used as a means of diagnosis in nervous and other disorders. They are termed respectively (a.) Cutaneous reflexes, and (J.) Muscle reflexes, (a.') Cutaneous reflexes are set up by a gentle stimulus applied to the skin. The subjacent muscle or muscles contract in response. Although these cutaneous reflex actions may be demonstrated almost anywhere, yet certain of such actions as being most characteristic are distinguished, e.g., plantar reflex ; gluteal reflex, i.e., a con- traction of the gluteus maximus when the skin over it is stimulated ; cremaster reflex, retraction of the testicle when the skin of the inside of the thigh is stimulated, and the like. The ocular reflexes, too, are important. They are contraction of the iris on exposure to light, and its dilatation on stimulating the skin of the cervical region. All of these cutaneous reflexes are true reflex actions, but they differ in different individuals, and are more easily elicited in the young. (J.) Muscle reflexes, or as they are often termed, tendon-reflexes, consist of a contraction of a muscle under con- ditions of more or less tension, when its tendon is sharply tapped. The so-called patella-tendon-reflex is the most well-known of this variety of reflexes. If one knee be slightly flexed, as by crossing it over the other, so that the quadriceps femoris is extended to a moderate degree, and the patella tendon be tapped with the fingers or the earpiece of a stethoscope, the muscle contracts and the knee is jerked forwards. Another variety of the same phenomenon is seen if the foot is flexed so as to stretch the calf muscles and the tendo Achillis is tapped ; the foot is extended by the contraction of the stretched muscles. It appears, however, that the tendon reflexes are not exactly what their name implies. The interval between the tap and the contraction is too short for the production of a true reflex action. It is suggested that the contraction is caused by local stimulation of the muscle, but that this would not occur unless the muscle had been reflexly stimulated previously by the tension applied, and placed in a condition of excessive irritability. It is further probable that the condition on which it depends is a reflex spinal irritability of the muscle or (exaggerated) muscular tone, which is admitted to be a reflex phenomenon. Inhibition of Reflex Actions.-The fact that such movements as are produced by irritating the skin of the lower extremities in the human subject, after division or disorganisation of a part of the spinal cord, do not follow the same irritation when the mind is active and connected with the cord through the brain, is, probably, due to the mind ordinarily perceiving the irritation and instantly controlling the muscles of the irritated and other parts ; for, even when the cord is perfect, such involuntary movements will often follow irritation, if it be applied when the mind is wholly occupied. When, for example, one is anxiously thinking, even slight stimuli will produce involuntary and reflex movements. So, also, during sleep, such reflex movements may be observed, when the skin is touched or tickled ; for example, when one touches with the finger the palm of the hand of a sleeping child, the finger is grasped-the impression on the skin of the palm producing a CHAP. XVIII.] INHIBITION OF REFLEX ACTIONS. 551 reflex movement of the muscles which close the hand. But when the child is awake, no such effect is produced by a similar touch. Further, many reflex actions are capable of being more or less controlled or even altogether prevented by the will : thus an inhibitory action may be exercised by the brain over reflex func- tions of the cord and the other nerve centres. The following may be quoted as familiar examples of this inhibitory action :- To prevent the reflex action of crying out when in pain, it is often sufficient firmly to clench the teeth or to grasp some object and hold it tight. When the feet are tickled we can, by an effort of will, prevent the reflex action of jerking them up. So, too, the involuntary closing of the eyes and starting, when a blow is aimed at the head, can be similarly restrained. Darwin has mentioned an interesting example of the way in which, on the other hand, such an instinctive reflex act may over- ride the strongest effort of the will. He placed his face close against the glass of the cobra's cage in the Reptile House at the Zoological Gardens, and though, of course, thoroughly convinced of his perfect security, could not by any effort of the will prevent him- self from starting back when the snake struck with fury at the glass. It has been found by experiment that in a frog the optic lobes and optic thalami have a distinct action in inhibiting or delaying reflex action, and also that more generally any afferent stimulus, if sufficiently strong, may inhibit or modify any reflex action even in the absence of these centres. On the whole, therefore, it may, from these and like facts, be concluded that reflex acts, performed under the influence of the reflecting power of the spinal cord, are essentially independent of the brain and may be performed perfectly when the brain is separated from the cord : that these include a much larger number of the natural and purposive movements of the lower animals than of the warm-blooded animals and man : and that over nearly all of them the mind may exercise, through the higher nerve centres, some control; determining, directing, hindering, or modifying, them, either by direct action, or by its power over associated muscles. To these instances of spinal reflex action, some add yet many more, including nearly all the acts which seem to be performed unconsciously, such as those of walking, running, writing, and the like : for these are really involuntary acts. It is true that at their first performances they are voluntary, that they require education for their perfection, and are at all times so constantly 552 THE NERVOUS SYSTEM. [chap, xviii. performed in obedience to a mandate of the will, that it is difficnlt to believe in their essentially involuntary nature. But the will really has only a controlling power over their performance ; it can hasten or stay them, but it has little or nothing to do with the actual carrying out of the effect. And this is proved by the circumstance that these acts can be performed with complete mental abstraction : and, more than this, that the endeavour to carry them out entirely by the exercise of the will is not only not beneficial, but positively interferes with their harmonious and perfect performance. Anyone may convince himself of this fact by trying to take each step as a voluntary act in walking down stairs, or to form each letter or word in writing by a distinct exercise of the will. These actions, however, will be again referred to, when treating of their possible connection with the functions of the Sensory Ganglia. Morbid reflex actions.--TMe relation of the reflex action to the strength of the stimulus is the same as was shown generally in the action of ganglia, a slight stimulus producing a slight movement, and a greater, a greater movement, and so on ; but in instances in which we must assume that the cord is morbidly more irritable, i.e., apt to issue more nervous force than is proportionate to the stimulus applied to it, a slight impression on a sensory nerve produces extensive reflex movements. This appears to be the condition in tetanus, in which a slight touch on the skin may throw the whole body into convulsion. A similar state is induced by the introduction of strychnia, and, in frogs, of opium, into the blood ; and numerous experiments on frogs thus made tetanic, have shown that the tetanus is wholly unconnected with the brain, and depends on the state induced in the spinal cord. Special Centres in Spinal Cord. It may seem to have been implied that the spinal cord as a single nerve-centre, reflects alike from all parts all the impressions conducted to it. This, however, is not the case, and it should be regarded as we have indicated, as a collection of nervous centres united in a continuous column. This is well illustrated by the fact that segments of the cord may act as distinct nerve-centres, and excite muscular action in the parts supplied with nerves given off from them; as well as by the analogy of certain cases in which the muscular movements of single organs are under the control of certain circumscribed portions of the cord. The special centres are the following :- CHAP. XVIII.] SPECIAL SPINAL CENTRES. 553 (a.) Centre fur Defalcation, or Ano-Spinal centre.-The mode of action of the ano-spinal centre appears to be this. The mucous membrane of the rectum is stimulated by the presence of faeces or gases in the bowel. The stimulus passes up by the afferent nerves of the haemorrhoidal and inferior mesenteric plexus to the centre in the cord, situated in the lumbar enlargement, and is reflected through the pudendal plexus to the anal sphincter on the one hand, and on the other to the muscular tissue in the wall of the lower bowel. In this way is produced a relaxation of the first and a contraction of the second, and expulsion of the contents of the bowel follows. The centre in the spinal cord is partially under the control of the will, so that its action may be either inhibited, or augmented or helped. The action may be helped by the abdominal muscles which are under the control of the will, although under a strong stimulus they may also be compelled to contract by reflex action. (6.) Centre for Micturition, or the Vesico-Spinal centre.-The vesico-spinal centre acts in a very similar way to that of the ano- spinal. The centre is also in the lumbar enlargement of the cord. It may be stimulated to action by impulses descending from the brain, or reflexly by the presence of urine in the bladder. The action of the brain may be voluntary, or it may be excited to action by the sensation of distension of the bladder by the urine. The sensory fibres concerned are the posterior roots of the lower sacral nerves. The action of the centre thus stimulated is double, or it may be supposed that the centre consists of two parts, one which is usually in action and maintains the tone of the sphincter, and the other which causes contraction of the bladder and other muscles. When evacuation of the bladder is to occur, impulses are sent on the one hand to its muscles and to certain other muscles, which cause their contraction, and on the other to the sphincter urethrae which procures its relaxation. The way having been opened by the relaxation of the sphincter, the urine is expelled by the combined action of the bladder and accessory muscles. The cerebrum may act not only in the way of stimulating the centre to action, but also in the way of inhibiting its action. The abdominal muscles may be called into action as in defaecation. (c.) Centre for Emission of Semen, or Genito-Spinal centre.-The centre situated in the lumbar enlargement of the spinal cord is stimulated to action by sensory impressions from the glans penis. Efferent impulses from the centre excite the successive and co- 554 THE NERVOUS SYSTEM. [chap. XVIII. ordinate contractions of the muscular fibres of the vasa deferentia and vesiculse seminales, and of the accelerator urime and other muscles of the urethra; and a forcible expulsion of semen takes place, over which the mind has little or no control, and which, in cases of paraplegia, may be unfelt. (<7.) Centre for the Erection of the Penis.-This centre is also situated in the lumbar region. It is excited to action by the sensory nerves of the penis. Efferent impulses produce dilatation of the vessels of the penis, which also appears to be in part the result of a reflex contraction of the muscles by which the veins returning the blood from the penis are compressed. Centre for Parturition.-The centre for the expulsion of the contents of the uterus in parturition is situated in the lumbar spinal cord rather higher up than the other centres already enumerated. The stimulation of the interior of the uterus by its contents may, under certain conditions, excite the centre to send out impulses which produce a contraction of the uterine walls and expulsion of the contents of the cavity. The centre is independent of the will, since delivery can take place in paraplegic women, and also whilst a patient is under the influence of chloroform. Again, as in the cases of defsecation and micturition, the abdominal muscles assist; their action being for the most part reflex and involuntary. (/.) Centre for Movements of Lymphatic Hearts of Frog.- Volkmann has shown that the rhythmical movements of the anterior pair of lymphatic hearts in the frog depend upon nervous influence derived from the portion of spinal cord corresponding to the third vertebra, and those of the posterior pair on influence supplied by the portion of cord opposite the eighth vertebra. The movements of the heart continue, though the whole of the cord, except the above portions, be destroyed; but on the instant of destroying either of these portions, though all the rest of the cord be untouched, the movements of the corresponding hearts cease. What appears to be thus proved in regard to two portions of the cord, may be inferred to prevail in other portions also; and the inference is reconcilable with most of the facts known concerning the physiology and comparative anatomy of the cord. (<?.) The Centre for the Tone of Muscles.-The influence of the spinal cord on the sphincter ani and sphincter urethra; has been already mentioned (see above). It maintains these muscles in permanent contraction. The condition of these sphincters, how- ever, is not altogether exceptional. It is the same in kind, though CHAP. XVIII.] SPECIAL SPINAL CENTRES. 555 it exceeds in degree that condition of muscles which has been called tone, or passive contraction; a state in which they always when not active appear to be during health, and in which, though called inactive, they are in slight contraction, and certainly are not relaxed, as they are long after death, or when the spinal cord is destroyed. This tone of all the muscles of the trunk and limbs depends on the spinal cord, as the contraction of the sphincters does. If an animal be killed by injury or removal of the brain, the tone of the muscles may be felt and the limbs feel firm as during sleep; but if the spinal cord be destroyed, the sphincter ani relaxes, and all the muscles feel loose, and flabby, and atonic, and remain so till rigor mortis commences. This kind of tone must be distinguished from that mere firmness and tension which it is customary to ascribe, under the name of tone, to all tissues that feel robust and not flabby, as well as to muscles. The tone peculiar to muscles has in it a degree of vital contraction: that of other tissues is only due to their being well nourished, and therefore compact and tense. All the foregoing examples illustrate the fact that the spinal cord is a collection of reflex centres, upon which the higher centres act by sending down impulses to set in motion, modify or control them. The movements or other phenomena of reflex action being as it were the function of the ganglion cells to which an afferent impression is conveyed by the posterior nerve-trunks in connection with them, and that the extent of the movement depends upon the strength of the stimulus, the position in which it is applied as well as the condition of the nerve cells, the connection between the cells being so intimate that a series of co-ordinated movements may result from a single stimulation. Whether the cells possess as well the power of originating impulses (automatism) is doubtful, but this is possible in the case of (7t.) Vaso-motor centres which are situated in the cord (p. 192), and of (7.) Sweating centres which must be closely related to them, and possibly in the case of (/.) The centres for maintaining #7<e tone of muscles. The Nutrition (a) of the muscles, appears to be under the control of the spinal cord. When the anterior motor nerve cells are diseased the muscles atrophy. In the same way (6) the bones and (c) joints are seriously affected when the cord is diseased. The former where the anterior nerve cells are implicated do not 556 THE NERVOUS SYSTEM. [chap, xvi i r. grow, and the latter are disorganised in some cases when the posterior columns are affected. (tZ) The skin too evidently is only maintained in a healthy condition as long as the cord and its nerves are intact. No doubt part of this influence which the cord exercises over nutrition is due to the relationship which it bears to the vaso-motor nerves. Within the cord are contained, for some distance, fibres (a) which regulate the dilation of the pupil, (6) which have to do with the glycogenic function of the liver, (c) which control the nerve-supply of the vessels of the face and head, (<Z) which produce acceleration of the heart's action, and, in fact, all the other so-called sympathetic functions (see Chapter XXL). B. The Medulla Oblongata. The medulla oblongata (figs. 335, 336), is a column of grey and white nervous substance formed by the prolongation upwards of the spinal cord and connecting it with the brain. Structure.-The grey substance which it contains is situated in the interior and variously divided into masses and laminae by the white or fibrous substance which is arranged partly in external columns, and partly in fasciculi traversing the central grey mat- ter. The medulla oblongata is larger than any part of the spinal cord. Its columns are pyriform, enlarging as they proceed towards the brain, and are continuous with those of the spinal cord. Each half of the medulla, there- fore, may be divided into three columns or tracts of fibres, con- tinuous with the three tracts of which each half of the spinal cord is made up. The columns are more prominent than those of the spinal cord, and separated from each other by deeper grooves. The anterior, continuous with the Fig- 335;-Anterior surface of the pons Varolii, and medulla oblongata. a, a, anterior pyramids ; b, their decussation; c, c, olivary bodies; d, d, restifonn bodies; e, arciform fibres; f, fibres passing from the anteri ;■ column of the cord to the cerebellum; g, anterior column of the spinal cord; h, lateral column; p, pons Varolii; i, its upper fibres; 5, 5, roots of the fifth pair of nerves. chap. xvni. ] THE STRUCTURE OF THE MEDULLA. 557 anterior columns of the cord, are called the anterior pyramids ; the posterior, continuous with the posterior columns of the cord, with the addition of the funiculus of Rolando on each side (fig. 337, f.Rf, are called the restiform bodies. On the outer side of the anterior pyramids of each side, near its upper part, is a small oval mass contain- ing grey matter, and named the olivary body ; and at the posterior part of the restiform column, immediately on each side of the posterior median groove, con- tinuous with the posterior me- dian column of the cord, a small tract is marked off by a slight groove from the remainder of the restiform body, and called the posterior pyramid or fasciculus gracilis. The restiform columns, instead of remaining parallel with each other throughout the whole length of the medulla oblongata, diverge near its upper part, and by thus diverging, lay open, so to speak, a space called the fourth ventricle, the floor of which is formed by the grey matter of the interior of the medulla, exposed by this divergence. On separating the anterior pyramids, and looking into the groove between them, some de- cussating fibres of the lateral columns of the cord can be plainly seen. Fin- 336.-Posterior surface of the pons Varolii, corpora quadrigemina, and me- dulla oblongata. The peduncles of the cerebellum are cut short at the side. a, a, the upper pair of corpora quadri- gemina ; b, b, the lover; f,f, superior peduncles of the cerebellum ; c, emin- ence connected with the nucleus of the hypoglossal nerve; e, that of the glosso-pharyngeal nerve ; i, that of the vagus nerve; d, d, restiform bodies; p, posterior pyramids; v, v, groove in the middle of the fourth ventricle, ending below in the calamus scrip- torius; 7, 7, roots of the auditory nerves. Distribution of the Fibres of the Medulla Oblong-ata. a. The anterior pyramid of each side, although mainly composed of con- tinuations of the fibres of the anterior columns of the spinal cord, receives fibres from the lateral columns, both of its own and the opposite side ; the latter fibres forming almost entirely the decussating strands which are seen in the groove between the anterior pyramids. Thus composed, the anterior pyramidal fibres proceeding onwards to the brain are distributed in the following manner :- i. The greater part pass on through the Pons to the Cerebrum. A por- 558 THE NERVOUS SYSTEM. [chap. xvm. tion of the fibres, however, running apart from the others, joins some fibres from the olivary body, and unites with them to form what is called the olivary fasciculus or fillet. 2. A small tract of fibres proceeds to the cerebellum. b. The lateral column of the cord on each side of the medulla, in pro- ceeding upwards, divides into three parts, outer, inner, and middle, which are thusdisposed of :-i. The outer fibres (direct cerebel- lar tract) go with the resti- form tract to the cerebellum. 2. The middle (crossed py- ramidal tract) decussate across the middle line with their fellows, and form a pait of the anterior pyramid of the opposite side. 3. The inner pass on to the cere- brum, at first superficially but afterwards beneath the olivary body and the arcuate fibres, and then proceed along the floor of the fourth ventricle, on each side, un- der the name of the fasci- culus teres. c. The posterior column of the cord is represented in the medulla by i. thej>c«- terior pyramid, or fascicu- lus gracilis, which is a con- tinuation of the posterior median column,and by ii. the rest if or m body, comprising the funiculus cuneatus and the funiculus of Rolando. The fasciculus gracilis (fig. 337, f.g'), diverges above as the broader clava to form one on either side the lower late- ral boundary of the fourth ventricle, then tapers off, and becomes no longer trace- able. The funiculus cune- atus, or the rest of the pos- terior column of the cord, is continued up in the me- dulla as such (fig. 337, f.el) ; but soon, in addition, between this and the continuation of the posterior nerve roots, appears another tract called the funiculus of Rolando (fig. 337:High up, the funiculus cuneatus is covered by a set of fibres (arcuate fibres), which issue from the anterior median fissure, turn upwards Fig. 337--Posterior view of the medulla, fourth ventri- cle, and mesencephalon (natural size), p.n, line of the posterior roots of the spinal nerves ; р. posterior median fissure ; f.g., funiculus gracilis; cl., its clava; /.c., funiculus cuneatus; f.R., funiculus of Rolando ; r.b., restiform body ; с. calamus scriptorius; I, section of ligula or tenia ; part of choroid plexus is seen beneath it ; l.r., lateral recess of the ventricle ; str., striae acusticse; if., infeidor fossa ; s.f., poste- rior fossa ; between it and the median sulcus is the fasciculus teres; cbl., cut surface of the cere- bellar hemisphere ; n.d., central or grey matter; s.m.v., superior medullary velum; ligula; s.c.p., superior cerebellar peduncle cut longitudi- nally; cr., combined section of the three cere- bellar peduncles ; c.a.s., c.q.i., corpora quadri- gemina (superior and inferior) ; fr., fraenulum; f., fibres of the fillet seen on the surface of the tegmentum ; c., crusti ; l.g., lateral groove ; e.g.i., corpus geniculum internus ; th., posterior part of thalamus ; p., pineal body. The roman numbers indicate the corresponding cranial nerves. (E. A. Schafer.) chap, xvni.] GREY MATTER OF THE MEDULLA. 559 over the anterior pyramids to pass directly into the corresponding hemi- sphere of the cerebellum, being joined by the fibres of the direct cerebellar tract ; the funiculus of Rolando, and the funiculus cuneatus, although appearing to join them, do not actually do so, except to a partial extent. Grey Matter of the Medulla.-To a considerable extent the grey matter of the medulla is a continuation of that in the spinal cord, but the arrangement is somewhat different. The displacement of the anterior cornu takes place because of the decussation of a large part of the fibres of the lateral columns in the anterior pyramids passing through the grey matter of the anterior cornu, so that the caput cornu is cut off from the rest of the grey matter, and is, moreover, pushed backwards by the olivary body, to be men- tioned below. It lies in the lateral portion of the me- dulla, and exists for a time as the nucleus lateralis (fig. 338, nV) ; it consists of a reticulum of grey matter, containing ganglion cells intersected by white nerve- fibres. The base of the anterior cornu is pushed more from the anterior sur- face, and when the central canal opens out into the fourth ventricle, forms a collection of ganglion cells, producing the eminence of the fasciculus teres ; from certain large cells in it arise the hypoglossal nerve (fig. 338, XII.'), which passes through the medulla, and appears between the olivary body and the anterior pyramids. In the fasciculus teres, nearer to the middle line as well as to the surface, is a collection of nerve cells called the nucleus of that funiculus (fig. 339, nt). The grey matter of the posterior cornu is displaced somewhat by bands of fibres passing through it. The caput cornu appears at the surface as the funiculus of Rolando, whilst the cervix cornu is broken up into a reticulated structure which is displaced laterally, similar in structure to the nucleus lateralis. From the increase of the base of the posterior cornu, the nuclei of Fig. 338.-Section of the medulla oblongata in the region of the superior pyramidal decussation, a. m./., ante- rior median fissure ; f.a., superficial arciform fibres emerging from the fissure; py., pyramid; n.a.r., nuclei of arciform fibres ; f.a., deep arci- form becoming superficial; o., lower end of olivary nucleus; n.l., nucleus lateralis ; f.r., formatio reticularis; f.a.2, arciform fibres proceeding from the formatio reticularis; g., substantia gelatinosa of Rolando; a.V., ascending root of fifth nerve; n.c., nucleus cuneatus ; n.c'., external cuneate nucleus ; n.g., nucleus gracilis ; f.g. funiculus gra- cilis; p.m.f., posterior median fissure ; c.c., central canal surrounded by grey matter, in which are n.XL, nucleus of the spinal accessory, and n.XII., nucleus of the hypoglossal; s.d., superior pyramidal decussation. (Modified from Schwalbe.) 560 THE NERVOUS SYSTEM. [chap. xvin. the funiculus gracilis and funiculus cuncatus are derived (fig. 339, n.g, n.c), and outside of the latter is an accessory nucleus formed (fig. 339, n.c). Internally to these latter, and also derived from the cells of the base of the posterior cornu and appearing in the floor of the fourth ventricle, when the central canal opens are the nuclei of the spinal accessory, vagus, and glosso- pharyngeal nerves. In the upper part of the medulla also, to the out- side of these three nuclei, is found the principal auditory nucleus. All the above nuclei appear to be derived from a con- tinuation of the grey matter of the spinal cord, but a fresh collection of grey matter not repre- sented is interpolated between the anterior pyramids and the lateral column, contained with- in the olivary promi- nence, the wavy line of which (corpus deuta- tum) is doubled upon itself at an angle with the extremities directed upwards and inwards (fig. 339, o'). There may also be a smaller collec- tion of grey matter on the outer and inner side of the olivary nucleus known as accessory oli- vary nuclei. Functions. - As in case of the spinal cord, the functions of the medulla oblon- gata, like those of the spinal cord, may be considered under the heads of:-1. Conduction ; 2. Reflex action, or Reflection ; and, in addition, 3. Automatism. 1. In conducting impressions the medulla oblongata has a wider extent of function than any other part of the nervous system, since it is obvious that all impressions passing to and fro between the brain and the spinal cord and all nerves arising below the pons, must be transmitted through it. Fig. 339.-Section of the medulla oblongata at about the middle of the olivary body, f.l.a., anterior median tissure ; n.ar., nucleus arciformis; p., pyramid ; XII., bundle of hypoglossal nerve emerging from the surface; at ft, it is seen coursing between the pyramid and the olivary nucleus, o.; f.a.e., external arciform fibres; n./., nucleus lateralis; a., arciform fibres passing towards restiform body, partly through the substantia gelatinosa, g., partly superficial to the ascending root of the fifth nerve, a. F.; X., bundle of vagus root emerging; /.»•., formatio reti- cularis; c.r., corpus restiform, beginning to be formed, chiefly by arciform fibres, superficial and deep ; n.c., nucleus cuneatus ; n.g., nucleus gracilis ; t., attachment of the ligula; funiculus solitarius; n. X., n.X.', two parts of the vagus nucleus; n.XII., hypoglossal nucleus ; n.t., nucleus of the funiculus teres ; n.am., nucleus am- biguus ; r., raphe ; -I., continuation of the anterior column of cord; o', 0", accessory olivary nucleus; p.o., pedunculus olivee. (Modified from Schwalbe.) CHAP. XVIII.] FUNCTIONS OF THE MEDULLA. 561 2. As a nerve-centre by which impressions are reflected, the medulla oblongata also resembles the spinal cord; the only difference between them consisting of the fact that many of the reflex actions performed by the former are much more important to life than any performed by the spinal cord. Demonstration of Functions.-It has been proved by repeated experiments on the lower animals that the entire brain may be gradually cut away in successive portions, and yet life may con- tinue for a considerable time, and the respiratory movements be uninterrupted. Life may also continue when the spinal cord is cut away in successive portions from below upwards as high as the point of origin of the phrenic nerve. In Amphibia, the brain has been all removed from above, and the cord, as far as the medulla oblongata, from below ; and so long as the medulla oblongata was intact, respiration and life were maintained. But if, in any animal, the medulla oblongata is wounded, particularly if it is wounded in its central part, opposite the origin of the pneu- mogastric nerves, the respiratory movements cease, and the animal dies asphyxiated. And this effect ensues even when all parts of the nervous system, except the medulla oblongata, are left intact. Injury and disease in men prove the same as these experiments on animals. Numerous instances are recorded in which injury to the medulla oblongata has produced instantaneous death ; and, indeed, it is through injury of it, or of the part of the cord connecting it with the origin of the phrenic nerve, that death is commonly produced in fractures attended by sudden displacement of the upper cervical vertebra). Special Centres. In the medulla are contained a considerable number of centres which preside over many important and complicated co-ordinated movements of muscles. The majority of these centres are (u.) reflex centres simply, which are stimulated by afferent or by volun- tary impressions. Some of them are (6.) automatic centres, being capable of sending out efferent impulses, generally rhythmical, with- out previous stimulation by afferent or by voluntary impressions. The automatic centres are, however, generally influenced by reflex or by voluntary impulses. Some again of the centres, whether reflex or automatic, are (c.) control centres, by which subsidiary spinal centres are governed. Finally the action of some of the centres 562 THE NERVOUS SYSTEM. [chap, xviii. is (cZ.) tonic, i.e., they exercise their influence, either directly or through another apparatus, continuously and uninterruptedly in maintaining a regular action. (a.) Simple Reflex centres. (i.) A centre for the co-ordinated movements of Mastication, the afferent and efferent nerves of which have been already enumerated (p. 263). (2.) A centre for the movements of Deglutition. The medulla oblongata appears to contain the centre whence are derived the motor impulses enabling the muscles of the palate, pharynx, and oesophagus to produce the successive co-ordinate and adapted movements necessary to the act of deglutition (p. 280). This is proved by the persistence of swallowing in some of the lower animals after destruction of the cerebral hemispheres and cere- bellum ; its existence in anencephalous monsters ; the power of swallowing possessed by the marsupial embryo before the brain is developed; and by the complete arrest of the power of swallowing when the medulla oblongata is injured in experiments. (3 ) A centre for the combined muscular movements of Sucking, the motor nerves concerned being the facial for the lips and mouth, the hypoglossal for the tongue, and the inferior maxillary division of the 5th for the muscles of the jaw. (4.) A centre for the Secretion of Saliva, which has been already mentioned (p. 271). (5.) A centre for Vomiting (p. 295). (6.) A centre for Coughing, which is said to be independent of the respiratory centre, being situated above the inspiratory part of that centre. (7.) A centre for Sneezing, connected no doubt with the respiratory centre. (8.) A centre for the Dilatation of the pupil, the fibres from which pass out partly in the third nerve and partly through the spinal cord (through the last two cervical and two upper dorsal nerves?) into the cervical sympathetic. (6.) Automatic centres. (1.) Respiratory centre.-The action of the respiratory centre has been already discussed. It is only necessary to repeat here that although it can be influenced by afferent impulses, it is also automatic in its action, being capable of direct stimulation, as by the condition of the blood circulating within it. It is also bilateral. It probably consists of an inspiratory part and of an CHAP. XVIII.] CENTRES IN THE MEDULLA. 563 expiratory part. The centre is capable of being influenced both reflexly and to a certain extent also by voluntary impulses. The vagus influence is probably constant in the direction of stimulating the inspiratory portion of the centre, whereas the influence of the superior laryngeal is not always in action, and is inhibitory. (2.) The Cardio-Inhibitory centre. The action of this centre in maintaining the proper rhythm of the heart through the vagus fibres, which terminate in a local intrinsic mechanism, has been already discussed. The centre can be directly stimulated, as by the condition of the blood circulating within it, and also indirectly by afferent stimuli, especially by stimulating the abdominal sympathetic nerves, but also by stimulating any sensory nerve, including the vagus itself. (3.) The Accelerator centre for the heart. A centre from which arise the accelerator fibres of the heart, probably exists in the medulla. It is automatic but not tonic in action. (4.) The Vaso-motor centre, which controls the unstriped muscle of the arteries, is also situated in the medulla. Like the respiratory centre, it is bilateral. As has already been pointed out, this centre may be directly or reflexly stimulated, as well as by impressions conveyed downwards from the cerebrum to the medulla. The condition of the blood circulating in it is the direct stimulus. Its influence is no doubt a tonic or else a rhythmic one. It is also supposed that there is in the medulla a special vaso-dilator centre, not acting tonically, stimulation of which produces vascular dilatation. The diabetic centre is probably a part of the vaso-motor centre, at any rate stimulation of it causes dilatation of the vessels of the liver. (5.) A chief centre for the secretion of Sweat exists in the medulla. It controls the subsidiary spinal sweat centres. It is double, and the two sides may be excited unequally so as to produce unilateral sweating. It is probably automatic and reflex. (6.) A Spasm centre is said to be present in the medulla, on the stimulation of which, as by suddenly produced excessive venosity of the blood, general spasms of the muscles of the body are produced. (c.) Control centres. These are centres whose influence may be directed to controlling the action of subsidiary centres. They are- (r.) The Respiratory centre, which probably controls the action of other subordinate centres in the spinal cord. 564 THE NERVOUS SYSTEM. [CHAP. XVIII. (2.) The Cardio-Inhibitory, which acts upon a local ganglionic mechanism in the heart. (3.) The Accelerator centre, if it exists, probably acts through a local mechanism in the heart. (4.). The Vaso-motor centre controls spinal as well as local tonic centres. (5.) The medullary Sweat centre controls spinal sweat centres. (c?.) Tonic centres. Of the centres whose action is tonic or continuous up to a certain degree, may be cited the vaso-motor and the cardio-inhibitory. It should not be forgotten that in the medulla are the centres for the special senses, Hearing and Taste, and that other special centres are supposed to be localised there, of which may be mentioned one, the hypothetical Inhibitory heat centre, which controls the production of heat by the tissues, independently of the vaso-motor centre. Though respiration and life continue while the medulla oblongata is perfect and in connection with the respiratory nerves, yet, when all the brain above it is removed, there is no more appearance of sensation, or will, or of any mental act in the animal, the subject of the experiment, than there is when only the spinal cord is left. The movements are all involuntary and unfelt; and the medulla oblongata has, therefore, no claim to be considered as an organ of the mind, or as the seat of sensation or voluntary power. These are connected with parts to be afterwards described. C. The Pons Varolii. Structure.-The meso-cephalon, or pons Varolii (vi, fig. 341), is composed principally of transverse fibres connecting the two hemispheres of the cerebellum, and forming its principal transverse commissure. But it includes, interlacing with these, numerous longitudinal fibres which connect the medulla oblongata with the cerebrum, and transverse fibres which connect it with the cere- bellum. Among the fasciculi of nerve-fibres by which these several parts are connected, the pons also contains abundant grey or vesicular substance, which appears irregularly placed among the fibres, and fills up all the interstices. The fibres of the facial nerve probably decussate in the pons. Functions.-The anatomical distribution of the fibres, both transverse and longitudinal, of which the pons is composed, is chap, xvni.] THE PONS VAROLII AND CRURA CEREBRI. 565 sufficient evidence of its functions as a conductor of impressions from one part of the cerebro-spinal axis to another. Concerning its functions as a nerve-centre, little or nothing is certainly known. An important point in the diagnosis of lesions of the pons varolii is the occurrence of a variety of what is called crossed paralysis. If the lesion be in the lower half of the pons, there is paralysis, motor and sensory, more or less complete of the opposite side of the body, with paralysis of the facial nerve of the same side. If the lesion be in the upper half of the pons, the facial nerve is paralysed on the same side as the other paralysis, but other nerves are involved. Hyperpyrexia follows after some lesions of the pons. D. The Crura Cerebri. Structure.-The Crura Cerebri (ill, fig. 341) are principally formed of nerve-fibres, of which the inferior or more superficial Fig. 340.-Diagram of the motor tract as shown in a diagrammatic horizontal section through the Cerebral hemispheres Crura, Pons, and Medulla. Fr., frontal lobe ; Oc., occipital lobe ; AF., ascending frontal, AP., ascending parietal, convolutions; PCF., pre-central fissure in front of the ascending frontal convolution; FR., fissure of Rolando ; IPF., inter-parietal fissure, a section of crus is lettered on the leftside. SN., substantia nigra ; Py., pyramidal motor fibre, which on the right is shown as continuous lines converging to pass through the posterior limb of IC. internal capsule (the knee or elbow of which is shown thus *) upwards into the hemisphere and downwards through the pons to cross the medulla in the anterior pyramids. (Gowers.) (crusta) are in part continuous with those of the anterior pyra- midal tracts of the medulla oblongata, and the superior or deeper 566 THE NERVOUS SYSTEM. [chap. XVIII. fibres (tegmentum) with the lateral and posterior pyramidal tracts, and with the olivary fasciculus. The middle third only of the crusta contains the fibres of the pyramidal tracts. The outer third transmits fibres which connect the cerebellum with the temporal I? g. 341.-Base of the brain. 1, superior longitudinal fissure; 2, 2', 2", anterior cerebri 1 lobe ; 3, fissure of Sylvius, between anterior and 4, 4', 4", middle cerebral lobe ; 5,5', posterior lobe ; 6, medulla oblongata; the figure is in the right anterior pyramid ; 7. 8, 9, 10, the cerebellum ; -j-, the inferior vermiform process. The figures from I. to IX. are placed against the corresponding cerebral nerves ; HI. is placed on the right crus cerebri. VI. and VII. on the pons Varolii; X. the first cervical or suboccipital nerve. (Allen Thomson.) J. and occipital lobes, and the inner third fibres which connect the frontal lobes with the cerebellum through its superior peduncles. Each crus cerebri contains among its fibres a mass of grey sub- stance, the locus niger. Functions.-With regard to their functions, the crura cerebri may be regarded as, principally, conducting organs : the crusta conduct- ing chiefly motor and the tegmentum sensory impressions. As nerve- centres they are probably connected with the functions of the third cerebral nerve, which arises from the locus niger, and through CHAP. XVIII.] FUNCTIONS OF THE TONS. 567 which are directed the chief of the numerous and complicated movements of the eyeball. The crura cerebri are also in all pro bability connected with the co-ordination of other movements Fig, 342.-Dissection of brain, from above, exposing the lateral fourth and fifth ventricles with the surrounding parts, j.-a, anterior part, or genu of corpus callosum ; b, corpus striatum ; V, the corpus striatum of left side, dissected so as to expose its grey sub- stance ; c, points by a line to the taenia semicircularis ; d, optic thalamus ; e, anterior pillars of fornix divided; below they are seen descending in front of the third ventricle, and between them is seen part of the anterior commissure ; in front of the letter e is seen the slit-like fifth ventricle, between the two laminae of the septum lucidum ; f, soft or middle commissure ; g is placed in the posterior part of the third ventricle; immediately behind the latter are the posterior commissure (just visible) and the pineal gland, the two crura of which extend forwards along the inner and upper margins of the optic thalami; h and i, the corpora quadrigemina ; k, superior crus of cerebellum ; close to k is the valve of Vieussens, which has been divided so as to ex- pose the fourth ventricle; I, hippocampus major and corpus fimbriatum, or taenia hippocampi; m, hippocampus minor; n, eminentia collateralis ; o, fourth ventricle ; p, posterior surface of medulla oblongata ; r, section of cerebellum ; s, upper part of left hemisphere of cerebellum exposed by the removal of part of the posterior cerebral lobe. (Hirschfeld and Leveill<5.) besides those of the eye, as either rotatory or disorderly movements result after section of either of them. Lesion of one crus is followed by motor and sensory, partial or complete paralysis of the opposite side of the body and paralysis of the third nerve of the same side as the lesion. 568 THE NERVOUS SYSTEM. [chap. XVIII. E. The Corpora Quadrigemina. The corpora quadrigemina (from which, in function, the corpora geniculata are not distinguishable) are the homologues of the optic lobes in Birds, Amphibia, and Fishes, and may be regarded as the principal nerve-centres for visual sensations. Functions.-(1) The experiments of Flourens, Longet, and Hertwig, show that removal of the corpora quadrigemina wholly destroys the power of seeing ; and diseases in which they are disorganised are usually accompanied by blindness. Atrophy of them is also often a consequence of atrophy of the eyes. Destruc- tion of one of the corpora quadrigemina (or of one optic lobe in birds) produces blindness of the opposite eye. This loss of sight is the only apparent injury of sensibility sustained by the removal of the corpora quadrigemina. The (2) removal of one of them affects the movements of the body, so that animals rotate, as after division of the crus cerebri, only more slowly : but this may be due to giddiness and partial loss of sight. (3) The more evident and direct influence is that produced on the iris. It contracts when the corpora quadrigemina are irritated: it is always dilated when they are removed: so that they may be regarded, in some measure at least, as the nervous centres governing its movements, and adapting them to the impressions derived from the retina through the optic nerves and tracts. (4) The centre for the co-ordination of the movements of the eyes is also contained in them. This centre is closely associated with that for contraction of the pupil, and so it follows that contraction or dilatation follows upon certain definite ocular movements. F. The Cerebrum. Relation to other parts.-The relation of the cerebrum to the other parts of the central nervous system will be understood on reference to figs. 343, 344. It is composed of two so-called halves or hemispheres, and is placed in connection with the Pons and Medulla oblongata by its two Crura or peduncles (III. fig. 341) : it is connected with the cerebellum by the processes called superior crura of the cerebellum, or processus a cerebello ad testes, and by a layer of grey matter, called the valve of Vieussens, which lies between these processes, and extends rom the inferior vermiform process of the cerebellum to the CHAP. XV111.] LOBES OF THE CEREBRUM. 569 corpora quadrigemina of the cerebrum. These parts, which thus connect the cerebrum with the other principal divisions of the cerebro-spinal system, may, therefore, be regarded as the con- tinuation of the cerebro-spinal axis or column; on which, as a kind of offset from the main nerve-path, the cerebellum is placed ; Fig. 343.-Plan in outline of the encephalon, as seen from the right side. J. The parts are represented as separated from one another somewhat more than natural, so as to show their connections. A, cerebrum; f, g, h, its anterior, middle, and posterior lobes; «, fissure of Sylvius ; B, cerebellum ; C, pons Varolii; D, medulla oblongata ; a, ped- uncles of the cerebrum ; Z>, c, d, superior, middle, and inferior peduncles of the cere- bellum. (From Quain.) and on the further continuation of which in the direct line, is placed the cerebrum (fig. 343). When the two hemispheres are separated and turned to either side, a broad connecting band or commissure, the corpus callosum, is seen. Convolutions of the Cerebrum.-For convenience of description, the surface of the brain has been divided lobes (Gratiolet). 1. Frontal (F. figs. 343, 344), limited behind by the fissure of Rolando (central fissure), and beneath by the fissure of Sylvius. Its surface consists of three main convolutions, which are approximately horizontal in direction, and are broken up into numerous secondary gyri. They are termed the superior, middle, and inferior frontal convolutions. In addition, the frontal lobe contains, at its posterior part, a convolution which runs upwards almost vertically (" ascending frontal "), and is bounded in front by a fissure termed the prrecentral, behind by that of Rolando. 2. Parietal (P.). This lobe is bounded in front by the fissure of Rolando, behind by external perpendicular fissure (parieto-occipital), and below 570 TIIE NERVOUS SYSTEM. [CHAP. XVIII. by the fissure of Sylvius. Behind the fissure of Rolando is the " ascending parietal " convolution, which swells out at its upper end into what is termed the superior parietal lobule. The superior parietal lobule is separated from the inferior parietal lobule by the intra-parietal sulcus. The inferior parietal lobule (pli courbe) is situated at the posterior and upper end of the fissure of Sylvius ; it consists of (a) an anterior part (supra-marginal con- Fig. 344.-Lateral view of the brain (semi-diagrammatic). F, Frontal lobe ; P, Parietal lobe ; O, Occipital lobe ; T, Temporo-sphenoidal lobe ; S, fissure of Sylvius ; S', hori- zontal, S", ascending ramus of the same ; c, sulcus centralis (fissure of Kolando) ; A, ascending frontal; B, ascending parietal convolution; Ft, superior; Fz, middle ; F3, inferior frontal convolutions; fi, superior, fz, inferior frontal sulcus; f3, pre- central sulcus ; Pi, superior parietal lobule ; P2, inferior parietal lobule consisting of P2, supramarginal gyrus, and P2', angular gyrus ; ip, interparietal sulcus ; cm, ter- mination of calloso-marginal fissure ; Oi, first, O2, second, O3, third occipital convo- lutions ; po, parieto-occipital fissure ; o, transverse occipital fissure ; 02, sulcus occipitalis inferior ; Ti, first, T2, second, T3, third temporo-sphenoidal convolutions ; ti, first, t2, second temporo-sphenoidal fissures. (Ecker.) volution) which hooks round the end of the fissure of Sylvius, and joins the superior temporal convolution, and a posterior part (V) (angular gyrus) which hooks round into the middle temporal convolution. 3. Temporo-sphenoidal (T.), contains three well-marked convolutions, parallel to each other, termed the superior, middle, and inferior temporal. The superior and middle are separated by the parallel fissure. 4. Occipital (0.). This lobe lies behind the external perpendicular or parieto-occipital fissure, and contains three convolutions, termed the supe- CHAP, xvm.] LOBES OF THE CEREBRUM. 571 rior, middle, and inferior occipital. They are often not well marked. In man, the external parieto-occipital fissure is only to be distinguished as a notch in the inner edge of the hemisphere ; below this it is quite obliterated by the four annectent gyri (plis de passage) which run nearly horizontally Fig. 345.-View of the Corpus Callosum from above. J.-The upper surface of the corpus callosum has been fully exposed by separating the cerebral hemispheres and throwing them to the side ; the gyrus fornicatus has been detached, and the transverse fibres of the corpus callosum traced for some distance into the cerebral medullary substance. 1, the upper surface of the corpus callosum ; 2, median furrow or raphe ; 3, longitu- dinal striae bounding the furrow; 4, swelling formed by the transverse bands as they pass into the cerebrum; 5, anterior extremity or knee of the corpus callosum; 6, posterior extremity; 7, anterior, and 8, posterior part of the mass of fibres proceed- ing from the corpus callosum ; 9, margin of the swelling ; 10, anterior part of the convolution of the corpus callosum ; 11, hem or band of union of this convolution ; 12, internal convolutions of the parietal lobe ; 13, upper surface of the cerebellum. (Sappey after Foville.) The upper two connect the parietal, and the lower two the temporal with the occipital lobe. 5. Central lobe, or island of Keil, which contains a number of radiating convolutions (gyri operti). When the cerebral hemispheres have been removed by trans- verse cuts at a level of the corpus callosum, and that body has been cut through on either side about half an inch from the middle line by an antero-posterior vertical incision, a considerable space on either side of the middle line in the interior of the brain is laid open, called the lateral ventricles. They are separated by 572 THE NERVOUS SYSTEM. [CHAP. XVIII. a thin double partition, the septum lucidum, between the laminse of which is an interval containing fluid, the fifth ventricle; they communicate by an aperture below. They are lined with ciliated epithelium. Each ventricle consists of a narrow interval extending into the anterior and posterior regions from the middle region of the corresponding hemisphere. Its middle portion or body is straight, but each horn is more or less curved. In the floor of the cavity project portions of the chief basal ganglia, the Fig. 346.- View of the brain from above (semi-diagrammatic). Si, end of horizontal ramus of fissure of Sylvius. The other letters refer to the same parts as in Fig. 344. (Ecker.) corpus striatum in front, and the optic thalamus behind, and a. white band of fibres, the taenia semicircularis, between them. For the relation of the other portions of the interior of the brain- including those of the arched white commissure or fornix which extends backwards from the septum lucidum, and consists of two lateral halves joined only in the middle, with two anterior pillars and two posterior crura-and of the third ventricle, refer to fig. 342 and the description. The Internal Surface (Fig. 347) contains the following gyri and sulci : < hap. xviii.] CONVOLUTIONS OF THE CEREBRUM. 573 Gyrus fornicatus, a long curved convolution, parallel to and curving round the corpus callosum, and swelling out at its hinder and upper end into the quadrate lobule (preecuneus), which is continuous with the superior parietal lobule on the external surface. Marginal convolution runs parallel to the preceding, and occupies the space between it and the edge of the longitudinal fissure. The two convolutions are separated by the calloso-marginal fissure. The internal perpendicular fissure is well marked, and runs downwards to its junction with the calcarine fissure : the wedge-shaped mass intervening Fig. 347.-View oj the right hemisphere in the median aspect (semi-diagrammatic). CC, cor- pus callosum longitudinally divided ; Gf, gyrus fornicatus; H, gyrus hippocampi; h, sulcus liippocampi; U, uncinate gyrus; cm, calloso-marginal fissure; Fi, median aspect of first frontal convolution ; c, terminal portion of sulcus centralis (fissure of Rolando); A, ascending frontal; B, ascending parietal convolution ; Pi', priecuneus; Oz, cuneus; po, parieto-occipital fissure; o, sulcus occipitalis transversus; oc, calcarine fissure; oc', superior; oc", inferior ramus of the same ; D, gyrus descendens; T4, gyrus occipito-temporalis lateralis (lobulus fusiformis); T5, gyrus occipito-temporalis medialis (lobulus lingualis). (Ecker.) between these two is termed the cuneus. The calcarine fissure corresponds to the projection into the posterior cornu of the lateral ventricle, termed the Hippocampus minor. The temporo-tphenoidal lobe on its internal aspect is seen to end in a hook (uncinate gyrus). The notch round which it curves is continued up and back as the dentate or hippocampal sulcus : this fissure underlies the projection of the hippocampus major within the brain. There are three internal temporo-occipital convolutions, of which the superior and inferior ones are usually well marked, the middle one generally less so. The collateral fissure (corresponding to the eminentia collateralis) forms the lower boundary of the superior temporo-occipital convolution. All the above details will be found indicated in the diagrams (figs. 346, 347)- Structure.-The cerebrum is constructed, like the other chief 574 THE NERVOUS SYSTEM. [chap, xviii. divisions of the cerebro-spinal system, of grey and white matter; and, as in the case of the Cerebellum (and unlike the spinal cord and medulla oblongata), the grey matter (cortex) is external, and forms a capsule or covering for the white substance. For the evident purpose of increasing its amount without undue occu- pation of space, the grey matter is variously infolded so as to form the cerebral convolutions. The cortical grey matter of the brain consists of five layers (Meynert) (fig. 348). 1. Superficial layer with abun- dance of neuroglia and a few small multipolar ganglion-cells. 2. A large number of closely packed small ganglion-cells of pyramidal shape. 3. The most important layer, and the thickest of all: it contains many large pyramidal ganglion-cells, each with a process running off from the apex vertically towards the free surface, and lateral pro- cesses at the base which are always branched. Also a median process from the base of each cell which is unbranched and becomes continuous with the axis- cylinder of a nerve-fibre. 4. Numerous ganglion-cells : termed the " granular formation " by Meynert. 5. Spindle-shaped and branched ganglion-cells of moder- ate size arranged chiefly parallel to the free surface (vide fig. 348). According to recent observations by Bousfield, the fibres of the medullary centre become connected with the multipolar ganglion cells of the fourth layer, and, from these latter, branches pass to the angles at the bases of the 348--The layers of the cortical grey matter of the cerebrum. (Meynert.) chap. xvni.] STRUCTURE OF THE CEREBRAL CORTEX. 575 pyramidal cells of the third layer of the cortex (fig. 350, a). From the apices of the pyramidal cells, the axis-cylinder processes pass upwards for a considerable distance, and finally terminate in ovoid cor- puscles (fig. 349) closely resembling, and homologous with, the corpuscles in which the ultimate ramifications of the branched cells of Purkinje in the cerebellum terminate. Thus it would seem that the large pyramidal cells of the third layer are themselves homologous with the cells of Purkinje in the cerebellum. The white matter of the brain, as of the spinal cord, consists of bundles of mednllated, and, in the neighbourhood of the grey matter, of non-medul- lated nerve-fibres, which, however, as is the case in the central nervous system generally, have no ex- ternal nucleated nerve-sheath, which are held together by delicate connective tissue. The size of the fibres of the brain is usually less than that of the fibres of the spinal cord : the average diameter of the former being about tu.w0 °f an inch. Chemical Composition.-The chemistry of nerves and nerve cells has been chiefly studied in the brain and spinal cord. Nerve matter contains several albuminous and fatty bodies (cerebrin, lecithin, and some others), also fatty matter which can be extracted by ether (including cholesterin) and various salts, especially Potassium and Magnesium phosphates, which exist in larger quantity than those of Sodium and Calcium. The great relative and absolute size of the Cerebral hemispheres in the adult man, masks to a great extent the real arrangement of the several parts of the brain, which is illustrated in the two accompanying diagrams. From these it is apparent that the parts of the brain are disposed in a linear series, as follows (from before backwards) : olfactory lobes, cerebral hemispheres, optic thalami, and third ventricle, corpora quadrigemina, or optic lobes, cerebellum medulla oblon- gata. This linear arrangement of parts actually occurs in the human foetus ; and it is permanent in some of the lower Vertebrata, Fishes, in which the cerebral hemispheres are represented by a pair of ganglia intervening Fig. 349. Fig. 350- [Drawn by G. Munro Smith from ammonium bichro- mate preparations by E. C. Bous- field]. 576 THE NERVOUS SYSTEM. [chap. xyin. between the olfactory and the optic lobes, and considerably smaller than the latter. In Amphibia the cerebral lobes are further developed, and are larger than any of the other ganglia. In Reptiles and Birds the cerebral ganglia attain a still further develop- ment, and in Mammalia the cerebral hemi- spheres exceed in weight all the rest of the brain. As we ascend the scale, the relative size of the cerebrum increases, till in the higher apes and man the hemispheres, which commenced as two little lateral buds from the anterior cerebral vesicle, have grown upwards and backwards, completely covering in and hiding from view all the rest of the brain. At the same time the smooth surface of the brain, in many lower Mammalia, such as the rabbit, is replaced by the labyrinth of convolutions of the human brain. Weight of the Brain.-The brain of an adult man weighs from 48 to 50 oz.-or about 3 lbs. It exceeds in absolute weight that of all the lower animals except the elephant and whale. Its weight, relatively to that of the body, is only exceeded by that of a few small birds, and some of the smaller monkeys. In the adult man it ranges from i of the body weight. Variations. -In a new-born child the brain (weighing 10-14 oz.) is of the body weight. At the age of 7 years the weight of the brain already averages 40 oz., and about 14 years the brain not unfrequently reaches the weight of 48 oz. Beyond the age of forty years the weight slowly but steadily declines at the rate of about 1 oz. in 10 years. Sex.-The average weight of the female brain is less than the male : and this difference per- sists from birth throughout life. In the adult it amounts to about 5 oz. Thus the average weight of an adult woman's brain is about 44 oz. Intelligence.-The brains of idiots are gene- rally much below the average, some weighing less than 16 oz. Still the facts at present col- lected do not warrant more than a very general statement, to which there are numerous excep- tions, that the brain weight corresponds to some extent with the degree of intelligence. There can be little doubt that the complexity and depth of the convolutions, which indicate the area of the grey matter of the cortex, correspond with the degree of intelligence. Weight of the Spinal Cord.-The spinal cord of man weighs from 1- •oz.; its weight relatively to the brain is about 1 : 36. As we descend the scale, this ratio constantly increases till in the mouse it is 1 : 4. In cold- Fig. 351.-.Diagrammatic hori- zontal section of a Vertebrate brain. The figures serve both for this and the next diagram. Mb, mid brain: what lies in front of this is the fore-, and what lies behind, the hind-brain; it,lamina termi- nalis ; Olf, olfactory lobes ; Hmp, hemispheres; Th. E, thalamencephalon ; Pn, pi- neal gland; Pg, pituitary body; F. M, foramen of Munro; cs, corpus striatum; Th, optic thalamus; CC, •crura cerebri: the mass lying above the canal represents "the corpora quadrigcmina ; Cb, cerebellum; I-IX., the nine pairs of cranial nerves ; 1, olfactory ventricle; 2, late- ral ventricle; 3, third ven- tricle ; 4, fourth ventricle; +, iter a tertio ad quartum ventriculum. (Huxley.) 1 hap. xviii.] CHARACTERISTICS OF THE HUMAN BRAIN, 577 blooded animals the relation is reversed, the spinal cord is the heavier and the more important organ. In the newt, 2:1; and in the lamprey. 75 : 1. Fig. 352..-Longitudinal and vertical Diagrammatic section of a vertebrate brain. Letters as before. Lamina terminalis is represented by the strong black line joining Pn and Pg. (Huxley.) Distinctive Characters of the Human Brain.-The following cha- racters distinguish brain of man and apex from those of all other animals. Fig. 353.-Brain of the Orang, | natural size, showing the arrangement of the convolutions. Sy, fissure of Sylvius; B, fissure of Rolando; E P, external perpendicular fissure ; (Jlf, olfactory lobe ; Cb,. cerebellum; PV, pons Varolii; JZ 0, medulla oblongata. As contrasted with the human brain, the frontal lobe is short and small relatively, the fissure of Sylvius is oblique, the temporo-sphenoidal lobe very prominent, and the external perpendicular fissure very well marked. (Gratiolet.) (a.) The rudimentary condition of the olfactory lobes, (b.) A perfectly defined fissure of Sylvius, (e.) A posterior lobe completely covering the cerebellum, (d.') The presence of posterior cornua in the lateral ven- tricles. 578 THE NERVOUS SYSTEM. [chap, xviii. The most distinctive points in the human brain, as contrasted with that of apes, are :-(l.) The much greater size and weight of the whole brain. The brain of a full-grown gorilla weighs only about 15 oz., which is less than 1 the weight of the human adult male brain, and barely exceeds that of the human infant at birth. (2.) The much greater complexity of the convolu- tions, especially the existence in the human brain of tertiary convolutions in the sides of the fissures. (3.) The greater relative size and complexity, and the blunted quadrangular contour of the frontal lobes in man, which are relatively both broader, longer, and higher, than in apes. In apes the frontal lobes project keel-like (rostrum) between the olfactory bulbs. (4.) The much greater prominence of the temporo-sphenoidal lobes in apes. (5.) The fissure of Sylvius is nearly horizontal in man, while in apes it slants considerably upwards. (6.) The distinctness of the external perpen- dicular fissure, which in apes is a well-defined almost vertical " slash," while in man it is almost obscured by the annectent gyri. Most of the above points are shown in the accompanying figure of the brain of the Orang. Functions of the Cerebrum. Speaking in the most general way, and for the present omitting the accumulating evidence in favour of the direct representation of the various co-ordinated movements of the muscles of the body in ganglia situated in different parts of the cerebral cortex, it may be said that:-(i.) The Cerebral hemispheres are the organs by which are perceived those clear and more impressive sensations which can be retained, and regarding which we can judge. (2.) The Cerebrum is the organ of the will, in so far at least as each act of the will requires a deliberate, however quick determination. (3.) It is the means of retaining impressions of sensible things, and reproducing them in subjective sensations and ideas. (4.) It is the medium of all the higher emotions and feelings, and of the faculties of judgment, understanding, memory, reflection, induction, imagination and the like. Evidence regarding the physiology of the cerebral hemispheres, has been obtained, as in the case of other parts of the nervous, system, from the study of Comparative Anatomy, from Pathology, and from Experiments on the lower animals. The chief evidences, regarding the functions of the Cerebral hemispheres derived from these various sources, are briefly these:-1. Any severe injury of them, such as a general concussion, or sudden pressure by apoplexy, may instantly deprive a man of all power of manifesting externally any mental faculty. 2. In the same general proportion as the higher mental faculties are developed in the Vertebrate- chap, xvin.] EFFECTS OF REMOVAL OF THE CEREBRUM. 579 animals, and in man at different ages and in different individuals, the more is the size of the cerebral hemispheres developed in com- parison with the rest of the cerebro-spinal system. 3. No other part of the nervous system bears a corresponding proportion to the development of the mental faculties. 4. Congenital and other morbid defects of the cerebral hemisphere are, in general, accom- panied by corresponding deficiency in the range or power of the intellectual faculties and the higher instincts. 5. Removal of the cerebral hemispheres in one of the lower animals produces effects corresponding with what might be anticipated from the foregoing facts. Effects of the Removal oj the Cerebrum.-The removal of the cerebrum in the lower animals appears to reduce them to the condition of a mechanism without spontaneity. A pigeon from which the cerebrum has been removed will remain motionless and apparently unconscious unless disturbed. When disturbed in any way it soon recovers its former position; when thrown into the air it flies. In the case of the frog, when the cerebral lobes have been removed, the animal appears similarly deprived of all power of spontaneous movement. But it sits up in a natural attitude, breathing quietly; when pricked it jumps away; when thrown into the water it swims ; when placed upon the palm of the hand it remains motionless, although, if the hand be gradually tilted over till the frog is on the point of losing his balance, he will crawl up till he regains his equilibrium, and comes to be perched quite on the edge of the hand. This condition contrasts with that resulting from the removal of the entire brain, leaving only the spinal cord ; in this case only the simpler reflex actions can take place. The frog does not breathe, he lies flat on the table instead of sitting up ; when thrown into a vessel of water he sinks to the bottom; when his legs are pinched he kicks out, but does not leap away. Unilateral Action.-Respecting the mode in which the brain discharges its functions, there is no evidence whatever. But it appears that, for all but its highest intellectual acts, one of the cerebral hemispheres is sufficient. For numerous cases are recorded in which no mental defect was observed, although one cerebral hemisphere was so disorganised or atrophied that it could not be supposed capable of discharging its functions. The remain- ing hemisphere was, in these cases, adequate to the functions 580 THE NERVOUS SYSTEM. [chap. XV11I. generally discharged by both; but the mind does not seem in any of these cases to have been tested in very high intellectual exer- cises ; so that it is not certain that one hemisphere will suffice for these. In general, the brain combines, as one sensation, the im- pressions which it derives from one object through both hemi- spheres, and the ideas to which the two such impressions give rise are single. In relation to common sensation and the effort of the will, the impressions to and from the hemispheres of the brain are carried across the middle line ; so that in destruction or com- pression of either hemisphere, whatever effects are produced in loss of sensation or voluntary motion, are observed on the side of the body opposite to that on which the brain is injured. Localisation of Functions.-In speaking of the cerebral hemi- spheres as the so-called organs of the mind, they have been regarded as if they were single organs, of which all parts are equally appropriate for the exercise of each of the mental faculties. But it is possible that each faculty has a special portion of the brain appropriated to it as its proper organ. For this theory the principal evidences are as follows :-i. That it is in accordance with the physiology of the compound organs or systems in the body, in which each part has its special function ; as, for example, of the digestive system, in which the stomach, liver, and other organs perform each their separate share in the general process of the digestion of the food. 2. That in different individuals the several mental functions are manifested in very different degrees. Even in early childhood, before education can be imagined to have exercised any influence on the mind, children exhibit various dispositions - each presents some predominant propensity, or evinces a singular aptness in some study or pursuit; and it is a matter of daily observation that every one has his peculiar talent or propensity. But it is difficult to imagine how this could be the case, if the manifestation of each faculty depended on the whole of the brain; different conditions of the whole mass might affect the mind generally, depressing or exalting all its functions in an equal degree, but could not permit one faculty to be strongly and another weakly manifested. 3. The plurality of organs in the brain is supported by the. phenomena of some forms of mental derangement. It is not usual for all the mental faculties in an insane person to be equally disordered; it often happens that the strength of some is increased, while that of others is diminished ; and in many cases one function only of the brain is deranged, CHAP. XVIII.] UNCONSCIOUS CEREBRATION. 581 while all the rest are performed in a natural manner. 4. The same opinion is supported by the fact that the several mental faculties are developed to their greatest strength at different periods of life, some being exercised with great energy in child- hood, others only in adult age ; and that, as their energy decreases in old age, there is not a gradual and equal diminution of power in all of them at once, but, on the contrary, a diminution in one or more, while others retain their full strength, or even increase in power. 5. The plurality of cerebral organs appears to be indi- cated by the phenomena of dreams, in which only a part of the mental faculties are at rest or asleep, while the others are awake, and, it is presumed, are exercised through the medium of the parts of the brain appropriated to them. Unconscious Cerebration.-In connection with the above, some re- markable phenomena should be mentioned which have been described as depending on an unconscious action of the brain. It must be within the experience of every one to have tried to recollect some particular name or occurrence ; and after trying in vain for some time the attempt is given up and quite forgotten amid other occupations, when suddenly, hours or even a day or two afterwards, the desired name or occurrence unexpectedly flashes across the mind. Such occurrences are supposed by many to be due to the requisite cerebral processes going on unconsciously, and, when the result is reached, to our all at once becoming conscious of it. That unconscious cerebration may sometimes occur, is likely enough ; and it is paralleled by the unconscious walking of a somnambulist. But many cases of so-called unconscious cerebration are better explained by the supposition that some missing link in the chain of reasoning cannot at the moment be found ; but is afterwards, by some chance combination of events, suggested, and thus the mental process is at once, with the memory of what has gone before, completed. Again, in the vain endeavour to solve a difficult, or it may be an easy problem, the reasoner is frequently in the condition of a man whose wearied muscles could never, before they have rested, overcome some obstacles. In both cases,-of brain and muscle, after renewal of their textures by rest, the task is performed so rapidly as to seem instantaneous., Sleep.-All parts of the body which are the seat of active change require periods of rest. The alternation of work and rest is a necessary condition of their maintenance, and of the healthy performance of their functions. These alternating periods, however, differ much in duration in different cases ; but, for any individual instance, they preserve a general and rather close uniformity. Thus, as before mentioned, the periods of rest and work, in the case of the heart, occupy, each of them, about half a second ; in the case of the ordinary respiratory muscles the periods are about four or five times as long. In many cases, again (as of the voluntary muscles during violent exercise) while the periods during active exertion alternate very frequently, yet the expenditure goes far ahead of the repair, and, to com- 582 THE NERVOUS SYSTEM. [chap. XVIII. pensate for this, an after repose of some hours becomes necessary ; the rhythm being less perfect as to time, than in the case of the muscles concerned in circulation and respiration. Obviously, it would be impossible that, in the case of the Brain, there should be short periods of activity and repose, or in other words, of con- sciousness and unconsciousness. The repose must occur at long intervals ; and it must therefore be proportionately long. Hence the necessity for that condition which we call Sleep; a condition which seeming at first sight exceptional, is only an unusually perfect example of what occurs, at varying intervals, in every actively working portion of our bodies. A temporary abrogation of the funtions of the cerebrum imitating sleep, may occur, in the case of injury or disease, as the consequence of two apparently widely different conditions. Insensibility is equally produced by a deficient and an excessive quantity of blood within the cranium (coma) ; but it was once supposed that the latter offered the truest analogy to the normal condition of the brain in sleep, and in the absence of any proof to the contrary, the brain was said to be during sleep congested. Direct experimental enquiry has led, however, to the opposite conclusion. By exposing, at a circumscribed spot, the surface of the brain of living animals, and protecting the exposed part by a watch-glass, Durham was able to prove that the brain becomes visibly paler (amemic) during sleep ; and the anaemia of the optic disc during sleep, observed by Hughlings Jackson, may be taken as a strong confirmation, by analogy, of the same fact. A very little consideration will show that these experimental results correspond exactly with what might have been foretold from the analogy of other physiological conditions. Blood is supplied to the brain for two partly distinct purposes, (i.) It is supplied for mere nutrition's sake. (2.) It is necessary for bringing supplies of potential or active energy (Z.e., combustiblc matter or heat) which may be transformed by the cerebral corpuscles into the various manifestations of nerve-force. During sleep, blood is requisite for only the first of these purposes; and its supply in greater quantity would be not only useless, but, by supplying an excitement to work, when rest is needed, would be positively harmful. In this respect the varying circulation of blood in the brain exactly resembles that which occurs in all other energy transforming parts of the body ; e.g., glands or muscles. At the same time, it is necessary to remember that the normal anasmia of the brain which accompanies sleep is probably a result, and not a cause of the quiescence of the cerebral functions. What the immediate cause of this periodical partial abrogation of function is, however, we do not know. Somnambulism and Dreams.-What we term sleep occurs often in very different degrees in different parts of the nervous system ; and in some parts the expression cannot be used in the ordinary sense. The phenomena of dreams and somnambulism are examples of differing degrees of sleep in different parts of the cerebro-spinal nervous system. In the former case the cerebrum is still partially active ; but the mind-pro- ducts of its action are no longer corrected by the reception, on the part of the sleeping sensorium, of impressions of objects belonging to the outer world ; neither can the cerebrum, in this half-awake condition, act on the CHAP. XVIII.] THE MOTORIAL ABEAS. 583 centres of reflex action of the voluntary muscles, so as to cause the latter to contract-a fact within the painful experience of all who have suffered from nigntmare. In somnambulism the cerebrum is capable of exciting that train of reflex nervous action which is necessary for progression, while the nerve-centre of muscular sense (in the cerebellum ?) is, presumably, fully awake ; but the sensorium is still asleep, and impressions made on it are not sufficiently felt to rouse the cerebrum to a comparison of the difference between mere ideas or memories and sensations derived from external objects. The Motor Centres of the Cerebral Cortex. The experiments upon the brains of various animals by means of electrical stimulation have demonstrated that there are definite regions of the cerebral cortex the stimulation of which produces definite movements of co-ordinated groups of muscle of the opposite side of the body. It had long been well-known that the cerebral hemispheres could not be excited by mechanical, chemical, or thermal stimuli, but Fritsch and Hitzig were the first to show that they are amenable to electric irritation. They employed a weak constant current in their experiments, applying a pair of fine electrodes not more than in. apart to different parts of the cerebral cortex. The results thus obtained have been confirmed and extended by Ferrier and many others. The fundamental phenomena observed in all these cases may be thus epitomised :- (i). Excitation of the same spot is always followed by the same movement in the same animal. (2). The area of excitability for any given movement is extremely small, and admits of very accurate definition. (3). In different animals excitations of anatomically corresponding spots produce similar or corresponding results. The various definite movements resulting from the electric stimulation of circumscribed areas of the cerebral cortex, are enumerated in the description of the accompanying figures of the dog and monkey's brain. In the case of the dog, the results obtained are summed up as follows, by Hitzig (a). One portion (anterior) of the convexity of the cerebrum is motor: another portion (posterior) is non-motor. (6). Electric stimulation of the motor portion produces co-ordinated muscular contraction on the opposite side of the body. (c). With very weak currents, the contractions produced are distinctly limited to particular groups of muscles ; with stronger currents the stimulus 584 THE NERVOUS SYSTEM. [CHAP. II. is communicated to other muscles of the same or neighbo ing parts. (cZ). The portions of the brain intervening between these motor centres are inexcitable by similar means. With regard to facts above mentioned, ahi ex- perimenters are agre< 3d, but there is still considerable diversity of opinion as to their explanation. In applying the facts) ascer- tained by these experim ents to elucidate the physiology of the human brain, we must remem- ber that the method of < electric stimulation is an artificial one. differing widely from th<3 ordi- nary stimuli to which th<e brain is subject during life. Effects of Stimulation of Various Regions of Mon- key's Brain.-According to the observations of Ferrier, confirmed and extei ided by later experimenters, stimu- lation of various j 'arts of the monkey's brain, tes indi- Fig- 354- Fig- 355- lJg8-,354 an(l 355--Brain of do'j, viewed from above and in profile. !•', frontal tissur0>, some- times termed crucial sulcus, corresponding to the fissure of Rolando in man ; -fissure of Sylvius, around which the four longitudinal convolutions are concentrfca'J,y a1'- langed ; i, flexion of head on the neck, in the median line ; 2, flexion of hcd-(i 0®i the neck, with rotation towards the side of the stimulus; 3, 4, flexion and ext£ns}°'n of anterior limb; 5, 6, flexion and extension of posterior limb ; 7, 8, 9, cont™0^101'-1 of orbicularis oculi, and the facial muscles in general. The unshaded part is thi expos 1 by opening the skull. (Dalton.) CHAP. XVIII.] FERRIER'S EXPERIMENTS. 585 cated by the numbers in figs. 356, 357, produces movements of definite muscles, thus :- Stimulation of the districts marked 1, causes movement of hind foot; of 2, chiefly adduction of the foot; of 3, movements of hind foot and tail; of 4, of latissimus dorsi; of 5, exten- sion forward of arm; o, 6, c, <7, movements of hand and wrist ; of 6, supination and flexion of fore- arm ; of 7, eleva- tion of the upper lip; of 8, conjoint action of elevation of upper lip and depression <f lower; of 9, opening of mouth and protrusion of tongue; of 10, retraction of tongue; of 11, action of platysma; of 12, elevation of eyebrows and eyelids, dilatation of pupils, and turning head to opposite side ; of 13, eyes directed to opposite side and up- wards, with usually contrac- tion of the pupils ; of ] 3', similar action, but eyes usually directed down- wards ; of 14, retraction of opposite ear, head turns to the opposite side, the eyes widely opened, and pupils dilated; of 15, stimulation of this region, which corresponds to- the tip of the uncinate convolution, causes torsion of the lip and nostril of the same side. Fig. 356. Figs. 356 and 357.-Diagrams of monkey's brain t<- show the effects of electric stimulation of certain- spots. (According to Ferrier.) Fig. 357- 586 THE NERVOUS SYSTEM. [chap, xvn 1. It is thus seen that the motor areas chiefly correspond with the ascending frontal and ascending parietal convolutions, and that the movements of the leg are represented at the upper part of these convolutions, then follow from above downwards the centres for the arms, the face, the lips, and the tongue. According to the further researches of Schafer and Horsley, rig. 358. Motorial areas of the bruin. A.F., ascending'frontal convolution; A.P., ascend- ing parietal; F.R., fissure of Rolando ; /■'. Sy., sylvian fissure. (After Gowers.) ■electrical stimulation, of the marginal convolution internally at the parts corresponding with the ascending frontal and parietal ■convolutions, from before backwards, produces movements of the arm, of the trunk, and of the leg. A good deal of doubt was thrown upon the experiments of Ferrier by Goltz and other observers, from the results of excising the so-called motor •areas, of the dog's brain. It was found that the part might be sliced away or washed away with a stream of water, but that no permanent paralysis ■ensued. Burdon-Sanderson, too, shewed that stimulation of different points in a horizontal section, through the deeper parts of the hemispheres, produces the same effects as stimulation of the so-called " centres." More extensive observations however, have confirmed Ferrier's original statement, at any rate with regard to the monkey's brain. Destruction of the motor areas for the arm produces permanent paralysis of the arm of the opposite side, and similarly of that for the leg, paralysis of the opposite leg. If both areas are destroyed permanent hemiplegia ensues. Paralysis of so extensive and permanent character does not, however, appear the rule when the brain of a dog is used instead of that of the monkey. It is suggested that in the animal lower in the scale, the functions which in the monkey are discharged by the cortical centres may be subserved by the basal ganglia. chap, xviii.] MOTORIAL AREAS OF THE HUMAN BRAIN. 587 Motorial Areas of the Human Brain. - It is naturally of great importance to discover how far the result of experiments upon the dog and monkey hold good with regard to the human brain. Evidence furnished by diseased conditions is not wanting to support the general idea of the existence of cortical motorial centres in the human brain (fig. 358)- So far, however, it has been possible to localize motor functions in the frontal and ascend- ing parietal convolu- tions, only to the convo- lutions which bound the fissure of Rolando, and to those on the inner side of the hemi- spheres which corre- spond thereto. The position of the centres is probably much the same as in the monkey's brain - those for the leg above, those for the arm, face, lips, and tongue from above downwards. Destruc- tion of these parts causes paralysis, corresponding to the district affected, and irritation causes con- vulsions of the muscles of the same part. Again, a number of cases are on record in which aphasia, or the loss of power of expressing ideas in words, has been associated with disease of the posterior part of the lower or third frontal convolution on the left side. This condition is usually associated with paralysis of the right side (right hemiplegia). This district of the brain is now generally known as the wiotor area; and there seems no doubt whatever that from this area Fig. to show the connecting of the Front'd Occipital Lobes with the Cerebellum, <tc. The dotted lines passing in the crusta (toc), outside the motor fibres, indicate the connection between the tem- poro-occipital lobe and the cerebellum, r.c., the fronto-cerebellar fibres, which pass internally to the motor tract in the crusta ; i. f., fibres from the caudate nucleus to the pons, fb., frontal lobe; Oc., occipital lobe; af., ascending frontal; Ar., ascending parietal convolutions ; per., pre-centrai fissure in front of the ascending frontal convolution; fr., fissure of Rolando ; iff., inter-parietal fissure, a section of crus is lettered on the left side, bn., substantia nigra: py. , pyramidal motor fibre, which on the right is shown as continuous lines converging to pass through the posterior limb of ic. internal capsule (the knee or elbow of which is shown thus •) upwards into the hemisphere and downwards through the pons to cross at the medulla in the anterior pyramids. (Gowers.) 588 THE NEItVOUS SYSTEM. [chap, xviii. pass the nerve-fibres which proceed to the spinal cord, and are there represented as the pyramidal tracts. This is the reason, no doubt, that movements are produced on stimulation of the white matter after the superficial grey matter of the animal's brain has been sliced off. Motor tracts in the brain.-These motor fibres are connected with the pyramidal cells of the cortex, and arc indeed their continuations. It will be necessary, therefore, to trace them from the cortex downwards. From the motor area of the cortex they converge to the internal capsule, a comparatively narrow7 band of fibres passing- first of all between the twro parts of the corpus striatum, namely, the intra-ventricular por- tion, or caudate nucleus, and the extra-ventricu- lar portion, or lenticular nucleus, and then be- tween the optic thala- mus internally and the lenticular nucleus exter- nally (fig. 360). The relations of the internal capsule are most im- portant. Corpora Striata. - (I.) The corpora striata are situ- ated in front of the optic thalami, partly within and partly without the lateral ventricle. Each corpus striatum consists of two parts. (</.) An intraventricular portion (caudate nucleus') which is conical in shape, with the base of the cone forwards ; it consists of grey matter, with white substance in its centre. (J.) An extraventricular portion (lenticular nucleus), which is separated from the other portion by a layer of white material, which forms a portion of the internal capsule,-the anterior limb. 1 he lenticular nucleus is seen, on a horizontal section of the hemisphere, to consist of three parts, separated from one another by white matter, of which the smallest is inside, each part somewhat resembling in shape a wedge. Ihe upper and internal surface is in relation with the caudate nucleus, being separated from it by the anterior limb of the internal capsule. Ihe remainder of the internal surface is in relation to the optic thalamus, 360. Diagram to show the relative positions of the several motor tracts in their course from the cortex to the crus. The section through the convolutions is vertical; that through the internal capsule, I, C, horizontal; that through the crus again vertical. <',N, caudate nucleus; O, TH, optic thalamus; L2 and L3, middle and outer part of lenticular nucleus ; f, a, I, face, arm, and leg fibres. The words in italic indicate corresponding cortical centres. (Gowers.) CHAP, xvm.] INTERNAL CAPSULE AND ITS RELATIONS. 589 being separated from it by the posterior limb of the internal capsule. The anterior and posterior limbs of the internal capsule meet at an acute angle, which is known as the knee of the internal capsule. The horizontal section is wider in the centre than at the end. On the outside is the grey lamina (claustrum) separated by a thin white layer-external capsule-from the lenticular nucleus. Optic Thalami. -(2.) The Optic Thalami are ovalin shape, and rest upon the crura cerebri. The upper surface of each thalamus is free, and of white substance, it projects into the lateral ventricle. The posterior surface is also white. The inner sides of the two optic thalami are in partial contact, and are composed of grey material uncovered by white, and are, as a rule, connected to- gether by a tran sverse portion. In the internal capsule the fibres which pass on- wards and down- wards to the pyra- midal tracts of the spinal cord do not occupy more than a small section, namely, that part knownas the knee, and the anterior two-thirds of the posterior segment (fig. 360). In this district the fibres for the face, arm, and leg, are in this relation : those for the face and tongue are just at the knee, and below or behind them come first the fibres for the arm and then those for the leg. The posterior third of the posterior segment is occupied by the sensory fibres. Following the fibres downwards from the internal capsule it is found that those which are motor in function descend in the crusta of the crus on either side, where they are collected into the upper part of the middle third, and that they then pass through the pons to form the anterior pyramids of the medulla. The fibres then either decussate in the middle line, passing over to the Fig. 361.-FerticoZ section through the cerebrum and basic ganglia to show the relations of the latter, co, cerebral convolutions ; c.c., corpus callosum; v.l., lateral ventricle; f, fornix; vIII., third ventricle; n.c., caudate nucleus; th, optic thalamus; nd., lenticular nucleus; c.i., internal capsule; c.l., claustrum; c.e., external capsule; m, corpus mam- millare; t.o., optic tract; s.t.t, stria terminalis; n.a., nucleus amygdala;; cm, soft commissure. (Schwalbe.) 590 THE NERVOUS SYSTEM. [chap, xvni. opposite side to become the lateral or crossed pyramidal tract of the lateral column of the cord, or remain as the direct pyramidal tract of the anterior column on either side of the anterior fissure. The direct pyramidal tracts, it will be remembered, decussate by degrees in the cord. This pathway of the pyramidal fibres is demonstrated by their degeneration when any lesion separates the fibres from their corresponding cortical cells, as, for example, a haemorrhage into the corpus striatum of sufficient extent-but the interruption may take place anywhere in the whole course of the tract. If the whole of these fibres on one side are destroyed transversely, above the decussation, hemiplegia of the opposite side, more or less complete, results. The idea which was formerly held, that some of these fibres pass through the corpus striatum does not appear to be supported by sufficient evidence. They have an interrupted course. The reason why a haemorrhage into the corpus striatum produces hemiplegia appears to be because of the almost certain pressure which such a lesion exerts upon the fibres of the internal capsule. Sensory paths in the brain.-The knowledge which we possess of the distribution of the sensory fibres in the brain is not nearly so definite as that which has been obtained of the motor tracts. As we have seen, the course of the sensory fibres even in the cord is not by any means completely understood. Supposing such fibres to be contained chiefly in the anterior part of the lateral columns and in the posterior columns of the cord, having previously crossed over to the opposite side of the cord to that from whence they came, they probably proceed in the posterior half of the medulla, chiefly in the formatio reticularis, and in the corresponding part of the pons, beneath the corpora quadrigemina to the tegmentum of the crus. In this they pass above the locus niger, and enter the posterior third of the posterior limb of the internal capsule (sensory crossway). From this district the fibres pass on into the white matter of the brain and probably extend into the so-called motorial areas already spoken of situated in the posterior frontal and anterior parietal regions. Some of the fibres pass into the optic thalamus. The fibres of the fifth nerve join the tegmentum, and so in the internal capsule arc included with the other sensory fibres. This is also probably the case with the other nerves of special sense,-smell, vision, and hearing. Cerebro-eerebellar fibres.-The tracts of fibres connecting the cerebellum with the cerebrum are in all probability at least three chap, xvni.] FUNCTIONS OF THE CORPORA STRIATA. 591 in number, (a.) Fibres situated in the crusta to the inside of the pyramidal fibres (fig. 360). These pass upwards in the anterior limb of the internal capsule and proceed into the anterior frontal lobes. In the other direction they descend to the pons, and appear to end in the grey matter within it. But it is very likely that from this grey matter fibres proceed, to the lateral and posterioi' parts of the opposite side of the cerebellum. As the fibres degenerate down- wards they conduct in the same direction, but are arrested at the pons, where they are interrupted by grey matter. (6.) Fibres which in the crusta are situated outside the pyramidal tract do not enter the internal capsule, but at once proceed to the occipital and temporo-sphenoidal lobes. These fibres proceed downwards to the cerebellum, being interrupted in the pons, and from thence proceed to the upper surface of the opposite side of the cerebellum near the middle lobe, (e.) The third tract is situated (fig. 359,1, f) beneath the pyramidal fibres and above the locus niger. The fibres pass from the corpus striatum chiefly from the caudate nucleus to the pons and thence to the cerebellum. Functions of the Corpora Striata.-The idea that the corpora striata are concerned in the transmission of motor impulses, or that they are the great motor ganglia at the base of the brain, rests upon insufficient evidence. It has been already incidentally mentioned that lesions of the corpora striata produce hemiplegia only because of the pressure effects they exercise upon the internal capsule close by. The caudate nucleus is connected with the opposite side of the cerebellum by fibres which conduct downwards, and the lenticular nucleus is connected with the cerebellum by fibres from the teg- mentum and superior cerebellar peduncles which conduct upwards. It is suggested that the corpora striata are central organs analogous to the cerebral cortex itself. " The analogy to those parts of the cortex that are connected with the cerebellum is rendered still greater by the fact that a lesion, even an extensive lesion, may exist in either the caudate or lenticular nucleus, and so long as it does not interfere with the functions of the motor or sensory parts of the internal capsules it causes no persistent symptoms." (Gowers.) Functions of the optic thalami.-That the optic thalami are the great sensory centres at the base of the brain-which was a view- held by many until recently-does not seem to be based upon sufficiently accurate observations. Some fibres from the tegmentum enter it no doubt, but the main body skirts the ganglion on either 592 THE NERVOUS SYSTEM. [chap. xvhi. side, and does not enter it. Fibres connect the optic thalamus with the superior peduncle of the cerebellum of the opposite side. Fibres connect it with the optic nerves. From the optic thalamus of either side fibres pass to the lenticular nucleus as well as to all parts of the cerebral cortex. Lesions of the optic thalamus do not of themselves produce loss of sensation. If such a symptom follows, it is due to pressure upon, or injury to the posterior limb of the internal capsule. The optic thalamus is connected with visual sensations, and may be a reflex-centre for some of the higher reflex actions. Of the functions of the external capsule and of the claustrum nothing definite is known. The Cerebellum. The Cerebellum (7, 8, 9, 10, fig. 341), is composed of an elon- gated central or lobe portion, called the vermiform processes, and two Fig. 362. Cerebellum in section, and fourth ventricle, with the neighbouring parts. 1, median groove of fourth ventricle, ending below in the calamus scriptorius, with the longitudinal eminences formed by the fasciculi teretes, one on each side ; 2, the same groove, at the place where the white streaks of the auditory nerve emerge from it to cross the floor of the ventricle ; 3, inferior crus or peduncle of the cerebellum, formed by the restiform body ; 4, posterior pyramid ; above this is the calamus scriptorius ; 5' AU«nF12 * CivS cerebellum, or processus e cerebello ad cerebrum (or ad testes) ; o, o. hllet to the side of the crura cerebri; 7, 7, lateral grooves of the crura cerebri ; 8, corpora quadngemina. (From Sappey after Hirschfeld and LeveilM.) hemispheres. Each hemisphere is connected with its fellow, not only by means of the vermiform processes, but also by a bundle of fibres called the middle crus or peduncle (the latter forming the chap, xviii.] STRUCTURE OF THE CEREBELLUM. 593 greater part of the pons Varolii), while the superior crura with the valve of Vieussens connect it with the cerebrum (5, fig. 361), and the inferior crura (formed by the prolonged restiform bodies) connect it with the medulla oblongata (3, fig. 361). Structure.- The cerebellum is composed of white and grey matter, the latter being external, like that of the cerebrum, and like it, infolded, so that a larger area may be contained in a given space. The convolutions of the grey matter, however, are arranged after a dif- ferent pattern as shown in fig. 362. Besides the grey substance on the surface, there is, near the centre of the white substance of each hemisphere, a small capsule of grey matter called the corpus dentatum (fig. 363, cd), resembling very closely the corpus dentatum of the olivary body of the medulla oblongata (fig. 363, 0). If a section be taken through the cortical portion of the cerebellum, the fol- lowing distinct layers can be seen (fig. 364) by micro- scopic examination. (1.) Immediately beneath the pia mater (/> m) is a layer of con- siderable thickness, which consists of a delicate connective tissue, in which are scattered several spherical corpuscles like those of the granular layer of the retina, and also an immense number of delicate fibres passing up towards the free surface and branching as they go. These fibres are the processes of the cells of Purkinje. (2.) The Cells of Purkinje (p). These are a single layer of branched nerve-cells, which give off a single unbranched process downwards, and numerous processes up into the external layer, some of which become continuous with the scattered corpuscles. (3.) The granular layer (<7), consisting of immense numbers of corpuscles closely resembling those of the nuclear layers of the retina. (4.) Nerve-fibre layer (/). Bundles of nerve - fibres forming Fig. 363.- Outline sketch of a section of the cere- bellum, showing the corpus dentatum. The section has been carried through the left lateral part of the pons, so as to divide the superior peduncle and pass nearly through the middle of the left cerebellar hemisphere. The olivary body has also been divided longi- tudinally so as to expose in section its corpus dentatum. c r, crus cerebri; f, fillet; q, corpora quadrigemina ; s p, superior peduncle of the cerebellum divided ; m p, middle peduncle or lateral part of the pons Varolii, with fibres passing from it into the white stem ; a v, continuation of the white stem radiating towards the arbor vitae of the folia; e d, corpus dentatum ; o, olivary body with its corpus dentatum ; p, anterior pyramid. (Allen Thomson.) |. 594 THE NERVOUS SYSTEM. [chap. xvm. the white matter of the cerebellum, which, from its branched, appearance has been named the "arbor vitae." Functions.-The physiology of the Cerebellum may be considered in its relation to- sensation, volun- tary motion, and the instincts or higher faculties of the mind. Its- supposed func- tions, like those of every other part of the ner- vous system, have been determined by physiological experiment, by pathological ob- servation, and by its comparative anatomy. (i.) With the exception of its. middle lobe, it is itself insensible to- il ritation, and may be all cut away without eliciting s:gns of pain (Longet). Its re- moval or dis- organization by disease is also ge- nerally unaccom- panied by loss or disorder of sensi- bility ; animals from which it is re- 364. 1 ertical section of dog's cerebellum ; p m, pia mater ; p, corpuscles of Purkinje, which are branched nerve-cells lying in a single layer and sending single processes down- wards and more numerous ones upwards, which branch continuously and extend through the deep " molecular layer towards the free surface ; <7, dense layer of gangrli- omc corpuscles, closely resembling nuclear layers of retina ; /, layer of nerve-fibres, with a few scattered llT\CnC01'PYSeles- This Ust Ia5er (//) constitutes part of the white matter of the cerebellum, while the <xrielS P tlle free surface are grey matter. (Klein and Noble Smith.) chap, xvm.j FUNCTIONS OF THE CEREBELLUM. 595 moved can smell, see, hear, and feel pain, to all appearance, as per- fectly as, before (Flourens ; Magendie). Yet, if any of its crura be touched, pain is indicated ; and, if the restiform tracts of the medulla oblongata be irritated, the most acute suffering appears to be produced. It cannot, therefore, be regarded as a principal organ of sensation. (2.) Co-ordination of Movements.-In reference to motion, the experiments of Longet and many others agree that no irritation of the cerebellum produces movement of any kind. Remarkable results, however, are produced by removing parts of its substance. Flourens (whose experiments have been confirmed by those of Bouillaud, Longet, and others) extirpated the cerebellum in birds by successive layers. Feebleness and want of harmony of mus- cular movements were the consequence of removing the superficial layers. When he reached the middle layers, the animals became restless without being convulsed; their movements were violent and irregular, but their sight and hearing were perfect. By the time that the last portion of the organ was cut away, the animals had entirely lost the powers of springing, flying, walking, standing, and preserving their equilibrium. When an animal in this state was laid upon its back, it could not recover its former posture, but it fluttered its wings, and did not lie in a state of stupor ; it saw the blow that threatened it, and endeavoured to avoid it. Volition and sensation, therefore, were not lost, but merely the faculty of combining the actions of the muscles ; and the endeavours of the animal to maintain its balance were like those of a drunken man. The experiments afforded the same results when repeated on all classes of animals ; and from them and the others before referred to, Flourens inferred that the cerebellum belongs neither to the sen- sory nor the intellectual apparatus; and that it is not the source of voluntary movements, although it belongs to the motor appa- ratus ; but is the organ for the co-ordination of the voluntary movements, or for the excitement of the combined action of muscles. Such evidence as can be obtained from cases of disease of this organ confirms the view taken by Flourens; and, on the whole, it gains support from comparative anatomy; animals whose natural movements require most frequent and exact combinations of muscular actions being those whose cerebella are most developed in proportion to the spinal cord. We must remember, too, that the cerebellum is connected with the posterior columns of the cord as well as with the direct cere- bellar tract, both of which probably convey to the middle lobe muscular sensations. It is also connected with the auditory 596 THE NERVOUS SYSTEM. [chap. xviii. nerves. Movements of the eyes also occur on direct stimulation of the middle lobe. It seems, therefore, to be connected in some way with all of the chief sensory impulses which have to do with the maintenance of the equilibrium. Foville supposed that the cerebellum is the organ of muscular sense, i.e., the organ by which the mind acquires that knowledge of the actual state and position of the muscles which is essential to the exercise of the will upon them ; and it must be admitted that all the facts just referred to are as well explained on this hypothesis as on that of the cerebellum being the organ for combining movements. A harmonious combination of muscular actions must depend as much on the capability of appreciating the con- dition of the muscles with regard to their tension, and to the force with which they are contracting, as on the power which any special- nerve-centre may possess of exciting them to contraction. And it is because the power of such harmonious movement would be equally lost, whether the injury to the cerebellum involved injury to the seat of muscular sense, or to the centre for combining muscular actions, that experiments on the subject afford no proof in one direction more than the other. Forced Movements.-The influence of each half of the cere- bellum is directed to muscles on the opposite side of the body; and it would appear that for the right ordering of movements, the actions of its two halves must be always mutually balanced and adjusted. For if one of its crura, or if the pons on either side of the middle line, be divided, so as to cut off from the medulla oblongata and spinal cord the influence of one of the hemispheres of the cerebellum, strangely disordered movements ensue (forced movements). The animals fall down on the side opposite to that on which the crus cerebelli has been divided, and then roll over continuously and repeatedly ; the rotation being always round the long axis of their bodies, and generally from the side on which the injury has been inflicted. The rotations sometimes take place with much rapidity; as often, according to Magendie, as sixty times in a minute, and may last for several days. Similar move- ments have been observed in men; as by Serres in a man in whom there was apoplectic effusion in the right crus cerebelli; and by Belhomme in a woman, in whom an exostosis pressed on the left crus. They may, perhaps, be explained by assuming that the division or injury of the crus cerebelli produces paralysis or imperfect and disorderly movements of the opposite side of the body; the animal falls, and then, struggling with the disordered side on the ground, and striving to rise with the other, pushes itself over; and so again and again, with the same act, rotates itself. Such movements cease when the other crus cerebelli is divided; but probably only because the paralysis of the body is CHAP. XVIII.] SENSORY CENTRES IN CEREBRAL CORTEX. 597 thus made almost complete. Other varieties of forced movements have been observed, especially those named " circus movements," when the animal operated upon moves round and round in a circle; and again those in which the animal turns over and over in a series of somersaults. Nearly all these movements may result on section of one or other of the following parts ; viz. crura cerebri, medulla, pons, cerebellum, corpora quadrigemina, corpora striata, optic thalami, and even, it is said, of the cerebral hemispheres. Sensory Centres in the Cerebral Cortex. Experimental lesions of various portions of the cerebral cortex and stimulation of such parts appears to show that the special senses are in some way represented at definite spots in the convolutions. Thus (a) the visual or optic centre is localised in the occipital lobe on either side on the outer convex part (fig. 358). This has been demonstrated in the dog's brain by Munk. In the human brain there seems to be a very complex mechanism about this centre. The optic nerve-fibres having partially decussated in the chiasma pass in the optic tract to the optic thalami, and thence to the cortical substance of the occipital lobe. Hemianopia, restric- tion of the field of vision of opposite sides of the two eyes, may be produced, either by a lesion of one optic tract, in which are (chiefly) the crossed fibres from the nasal portion of the retina of the oppo- site eye and the uncrossed fibres of the external portion of the retina of the corresponding eye; or of the occipital centre. Part of the fibres of the optic tract pass to the corpora geniculata and to the corpora quadrigemina. Each of these so-called half-vision cen- tres of opposite sides, situated in the occipital lobes, appears to be in connection with a higher centre in which the retina) of both eyes are represented, but especially that of the opposite eye. If both occipital lobes be extensively diseased total blindness results. (6) The Olfactory centre, is said to be localized in the anterior extremity of the uncinate gyrus. The fibres, however, appear to be connected with a centre on the same side ; others cross over to a centre on the opposite side. (c) The Auditory centre, is situated (according to Ferrier and Munk) in the monkey's brain in the first temporo-sphenoidal con- volution. The auditory fibres pass up the pons in which they cross, and then in the superior portion of the tegmentum through the hinder portion of the internal capsule to this centre. Destruction of the entire region causes deafness of the opposite ear. 598 PHYSIOLOGY OF THE CRANIAL NERVES. [chap. xix. (cZ) The centre for Taste has not yet been localised. According to Gowers, it is quite probable that the whole of the taste-fibres belong to the fifth nerve. Those which are distributed to the an- terior parts of the tongue in the chorda tympani, coming from that nerve through the Vidian, which passes from the spheno-palatine ganglion to the facial, and those which are distributed to the back of the tongue through the glosso-pharyngeal, being derived from the otic ganglion of the fifth nerve through the small petrosal nerve and the tympanic plexus. CHAPTER XIX. PHYSIOLOGY OF THE CRANIAL NERVES. The Cranial nerves are commonly enumerated as nine pairs; but the number is in reality twelve pairs, the seventh nerve con- sisting as it does, of two nerves, and the eighth of three. All arise Fig. 365. 7ourth ventricle, with the medulla oblongata and the corpora quadrigemina. The roman numbers indicate superficial origins of the cranial nerves, while the other num- bers indicate their deep origins, or the position of their central nuclei. 8, 8', 8 ', 8"', auditory nuclei nerves; t, funiculus teres; A, B, corpora quadrigemina; c, g, corpus geni- •uiatum; p c, peaunculus cerebri; m, c, p, middle cerebellar peduncle; s, c, p, supe- JT cere cellar peduncle ; 1, c, p, inferior cerebellar peduncle ; I, c, locus coiruleus ; e, t, ![nentla teres ; a, c, alacinerea; a, n, accessory nucleus ; o, obex : c, clava ; /, c, funi- culus cuneatus ;g, funiculus gracilis. ■CHAP. XIX.] CLASSIFICATION OF THE CRANIAL NERVES. 599 (superficial origin) from the base of the encephalon, in a double series which extends from the under surface of the anterior cerebral lobes to the lower end of the medulla oblongata. Traced into the substance of the brain and medulla, the roots of the nerves are found to take origin from various masses of grey matter, which are all •connected one with another, and with the cerebral hemispheres. The roots of the olfactory and of the optic nerves have been already mentioned. The third and fourth nerves arise from grey matter beneath the corpora quadrigemina ; and the roots of origin of the remainder of the cranial nerves can be traced to grey matter in the medulla oblongata in the floor of the fourth ventricle, and in the more central part of the medulla, around its central canal, as low down as the decussation of the pyramids. According to their several functions, the cranial nerves may be thus arranged:- A. Nerves of special sense . . . Olfactory, Optic, Auditory, part of the Glosso-pharyngeal, and part of the Fifth. B. Nerves of common sensation . The greater portion of the Fifth. C. Nerves of motionThird, Fourth, lesser division of the Fifth,Sixth,Facial,and Hypoglossal. D. Mixed nervesGlosso - pharyngeal, Vagus, and Spinal accessory. The physiology of the First, Second, and Eighth will be considered with the organs of Special sense. The Third. Nerve, or Motor Oeuli. Functions.-The Third nerve, or motor oculi, which arises in three distinct bands of fibres from the grey matter beneath the aqueduct of Sylvius near the middle line in conjunction with the fourth nerve. It supplies the levator palpebrse superioris muscle, and all of the muscles of the eye-ball, but the superior oblique, to which the fourth nerve is appropriated, and the rectus externus which receives the sixth nerve. Through the medium of the ophthalmic or lenticular ganglion, of which it forms what is called the short root, it also supplies motor filaments to the iris and ciliary muscle. The fibres which subserve the three functions, accommodation, contraction of the pupil, and nerve-supply to the external ocular muscles, arise from three distinct groups of cells. When the third nerve is irritated within the skull, all those muscles to which it is distributed are convulsed. When it is paralysed or divided the following effects ensue :-(i) the upper eyelid can be no longer raised by the levator palpebne, but 600 PHYSIOLOGY OF THE CRANIAL NERVES. [chap. XIX. droops (ptosis') and remains gently closed over the eye, under the unbalanced influence of the orbicularis palpebrarum, which is supplied by the facial nerve : (2) the eye is turned outwards (external strabismus') by the unbalanced action of the rectus externus, to which the sixth nerve is appropriated : and hence, from the irregularity of the axes of the eyes, double-sight, diplopia, is often experienced when a single object is within view of both the eyes 1(3) the eye cannot be moved either upwards, downwards, or inwards: (4) the pupil becomes dilated (mydriasis), and in- sensible to light: (5) the eye cannot accom- modate for short dis- tances. Contraction and Di- latation of the Pupil. -The relation of the third nerve to the muscles of the iris is of peculiar interest. Under ordinary circumstances the contraction of the iris is a reflex action, which is produced by the stimulus of light on the retina which is conveyed by the optic nerve to the brain (probably to the corpora quadrigemina or medulla), and thence reflected through the third nerve to the iris. Hence the iris ceases to act when either the optic or the third nerve is divided or destroyed, or when the centre is destroyed or much com- pressed. Hut when the optic nerve is divided, the contraction of the iris may be excited by irritating that portion of the nerve which is connected with the brain ; and when the third nerve is divided, the irritation of its distal portion will still excite the contraction of the iris. The contraction of the iris thus shows all the characters of a reflex act, and in ordinary cases requires the concurrent action of the optic nerve, its centre, and the third nerve; and, probably also, considering the peculiarities of its perfect mode of action, of the ophthalmic ganglion. But, besides, both irides will con- tract under the reflected stimulus of light falling upon one retina or under irritation of one optic nerve only. Thus in amaurosis of one eye, its pupil may contract when the other eye Fig. 366.-Diagram of a longitudinal section through the pons, showing the relation of the nuclei for the ocular muscles, co, corpora quadrigemina; 3, third nene; in., its nu- cleus ; 4, fourth nerve; iv., its nucleus, the posterior part of the third ; 6, sixth nerve. The probable posi- tion of the centre and nerve fibres for accommodation is shown at a and a'; for the reflex action of iris, at b, and 7/; for the external rectus muscles, at c, c'. The lines beneath the floor of the fourth ventricle indicate fibres, which connect the nuclei. (Gowers.) ( HAP, XIX.] THE FOURTH AND FIFTH NERVE. 601 is exposed to a stronger light: and generally the contraction of each of the pupils appears to be in direct proportion to the total quantity of fight which stimulates either one or both retinae, according as one or both eyes are open. The iris acts also in association with certain other muscles supplied by the third nerve : thus, when the eye is directed in- wards, or upwards and inwards, by the action of the third nerve distributed in the rectus internus and rectus superior, the iris con- tracts, as if under direct voluntary influence. The will cannot, however, act on the iris alone through the third nerve; but this aptness to contract in association with the other muscles supplied by the third, may be sufficient to make it act even in total blindness and insensibility of the retina, whenever these muscles are contracted. The contraction of the pupils, when the eyes are moved inwards, as in looking at a near object, has probably the purpose of excluding those outermost rays of light which would be too far divergent to be refracted to a clear image on the retina ; and the dilatation in looking straight forwards as in looking at a distant object, permits the admission of the largest number of rays, of which none are too divergent to be so refracted. Functions.-The Fourth nerve, Nervus trochlearis, or Patheticus, is exclusively motor, and supplies only the trochlearis or obliquus superior muscle of the eyeball. It arises from above the fourth ventricle from the valve of Vieussens, but its fibres can be traced to the lower part of the nucleus of the third (fig. 366) nerve. It decussates with its fellow between its deep and superficial origins. The Fourth Nerve, or Trochlearis. Functions.-The Fifth or Trigeminal nerve resembles, as already stated, the spinal nerves, in that its branches are derived through two roots ; namely, the larger or sensory, in connection with which is the Gasserian ganglion, and the smaller or motor root which has no ganglion, and which passes under the ganglion of the sensory root to join the third branch or division which ensues from it. The fibres of origin of the fifth nerve appear to come from under the floor of the fourth ventricle. The motor root to the inside of the sensory, about the middle of each lateral half. The sensory fibres, however, can be traced down in the medulla as far as the upper part of the cord, these latter fibres bringing sensory impres- sions from the tongue. In addition to these sensory fibres, coming The Fifth Nerve, or Trigeminus. 602 PHYSIOLOGY OF THE CRANIAL NERVES, [chap. xix. from the nucleus and the spinal cord, there are it is said others coming from the cerebellum. The motor centre is connected with the cerebral cortex of the opposite side. Fibres for the motor root also come from the corpora quadrigemina along the aqueduct of Sylvius. The first and second divisions of the nerve, which arise wholly from the larger root, are purely sensory. The third division being joined, as before said, by the motor root of the nerve, is of course both motor and sensory. (a.) Motor Functions.-Through branches of the lesser or non-ganglionic portion of the fifth, the muscles of mastication, namely, the temporal, masseter, twro pterygoid, anterior part of the digastric, and mylo-hyoid, derive their motor nerves. Filaments are also supplied to the tensor tympani and tensor palati. The motor function of these branches is proved by the violent con- traction of all the muscles of mastication in experimental irrita- tion of the third or inferior maxillary division of the nerve ; by paralysis of the same muscles, when it is divided or disorganised, or from any reason deprived of power; and by the retention of the power of these muscles, when all those supplied by the facial nerve lose their power through paralysis of that nerve. The last instance proves best, that though the buccinator muscle gives passage to, and receives some filaments from, a buccal branch of the inferior division of the fifth nerve, yet it derives its motor power from the facial, for it is paralysed together with the other muscles that are supplied by the facial, but retains its power when the other muscles of mastication are paralysed. Whether, how- ever, the branch of the fifth nerve which is supplied to the buccinator muscle is entirely sensory, or in part motor also, must remain for the present doubtful. From the fact that this muscle, besides its other functions, acts in concert or harmony with the muscles of mastication, in keeping the food between the teeth, it might be supposed from analogy, that it would have a motor branch from the same nerve that supplies them. There can be no doubt, however, that the so-called buccal branch of the fifth is, in the main, sensory; although it is not quite certain that it docs not give a few motor filaments to the buccinator muscle. (b.) Sensory Functions.-Through the branches of the greater or ganglionic portion of the fifth nerve, all the anterior and antero- lateral parts of the face and head, with the exception of the skin of the parotid region (which derives branches from the cervical spinal nerves), acquire common sensibility; and among these parts may be included the organs of special sense, from which CHAP. XIX.] FUNCTIONS OF THE FIFTH NERVE. 603 common sensations are conveyed through the fifth nerve, and their special sensations through their several nerves of special sense. The muscles, also, of the face and lower jaw acquire mus- cular sensibility, through the filaments of the ganglionic portion of the fifth nerve distributed to them with their proper motor nerves. The sensory function of the branches of the greater division of the fifth nerve is proved, by all the usual evidences, such as their dis- tribution in parts that are sensitive and not capable of muscular con- traction, the ex- ceeding sensi- bility of some of these parts, their loss of sensation when the nerve is paralyzed or divided, the pain without convul- sions produced by morbid or ex- perimental irrita- tion of the trunk or branches of the nerve, and the analogy of this portion of the fifth to the posterior root of the spinal nerve. Other Func- tions.-In rela- tion to muscular movements, the branches of the greater or ganglionic portion of the fifth nerve exercise a manifold influence on the movements of the muscles of Fig. 367.- General plan of the branches oj the fifth pair, k- 1, lesser root of the fifth pail- ; 2, greater root passing forwards into the Gasserian ganglion; 3, placed on the bone above the ophthalmic nerve, which is seen dividing into the supra-orbital, lachrymal, and nasal branches, the latter con- nected with the ophthalmic ganglion ; 4, placed on the bone close to the foramen rotundum, marks the superior maxillary division, which is connected below with the spheno-palatine ganglion, and passes forwards to the infra-orbital foramen; 5, placed on the bone over the foramen ovale, marks the inferior maxillary nerve, giving off the anterior auricular and muscular branches, and continued by the inferior dental to the lower jaw, and by the gustatory to the tongue ; a. the submaxillary gland, the submaxillary ganglion placed above it in connection with the gustatory nerve ; 6, the chorda tympani; 7, the facial nerve issuing from the stylomastoid foramen. (Charles Bell.) 604 PHYSIOLOGY OF THE CRANIAL NERVES, [chai*, xix. the head and face, and other parts in which they are distributed. They do so, in the first place (<i), by providing the muscles themselves with that sensibility without which the mind, being unconscious of their position and state, cannot voluntarily exer- cise them. It is, probably, for conferring this sensibility on the muscles, that the branches of the fifth nerve communicate so frequently with those of the facial and hypoglossal, and the nerves of the muscles of the eye ; and it is because of the loss of this sensibility that when the fifth nerve is divided, animals are always slow and awkward in the movement of the muscles of the face and head, or hold them still, or guide their movements by the sight of the objects towards which they wish to move. Again, the fifth nerve has an indirect influence on the muscular movements, by (6) conveying sensations of the state and position of the skin and other parts : which the mind perceiving, is enabled to determine appropriate acts. Thus, when the fifth nerve or its infra-orbital branch is divided, the movements of the lips in feed- ing may cease, or be imperfect. Bell supposed that the motion of the upper Up in grasping food depended directly on the infra- orbital nerve; for he found that, after he had divided that nerve on both sides in an ass, it no longer seized the food with its lips, but merely pressed them against the ground, and used the tongue for the prehension of the food. Mayo corrected this error. He found, indeed, that after the infra-orbital nerve had been divided, the animal did not seize its food with the lip, and could not use it well during mastication, but that it could open the lips. He, therefore, justly attributed the phenomena in Bell's experiments to the loss of sensation in the lips; the animal not being able to feel the food, and, therefore, although it had the power to seize it, not knowing how or where to use that power. The fifth nerve has also (c), an intimate connection with mus- cular movements through the many reflex acts of muscles of which it is the necessary excitant. Hence, when it is divided and can no longer convey impressions to the nervous centres to be thence reflected, the irritation of the conjunctiva produces no closure of the eye, the mechanical irritation of the nose excites no sneezing. Through its ciliary branches and the branch which forms the long root of the ciliary or ophthalmic ganglion, it exercises also (d), some influence on the movements of the iris. M hen the trunk of the ophthalmic portion is divided, the pupil becomes, according to \ alentin, contracted in men and rabbits, chap, xix.] FUNCTIONS OF THE FIFTH NERVE. 605 and dilated in cats and dogs ; but in all cases, becomes immovable even under all the varieties of the stimulus of light. How the fifth nerve thus affects the iris is unexplained; the same effects are produced by destruction of the superior cervical ganglion of the sympathetic, so that, possibly, they are due to the injury of those filaments of the sympathetic which, after joining the trunk of the fifth, at and beyond the Gasserian ganglion, proceed with the branches of its ophthalmic division to the iris; or, as has been ingeniously suggested, the influence of the fifth nerve on the move- ments of the iris may be ascribed to the affection of vision in consequence of the disturbed circulation or nutrition in the retina, when the normal influence of the fifth nerve and ciliary ganglion is disturbed. In such disturbance, increased circulation making the retina more irritable might induce extreme contraction of the iris ; or under moderate stimulus of light, producing partial blind- ness, might induce dilatation: but it does not appear why, if this be the true explanation, the iris should in either case be immov- able and unaffected by the various degrees of light. Trophic influence.-Furthermore, the morbid effects which division of the fifth nerve produces in the organs of special sense, make it probable that, in the normal state, the fifth nerve exer- cises some special or trophic influence on the nutrition of all these organs; although, in part, the effect of the section of the nerve is only indirectly destructive by abolishing sensation, and therefore the natural safeguard which leads to the protection of parts from external injury. Thus, after such division, within a period varying from twenty-four hours to a week, the cornea begins to be opaque ; then it grows completely white; a low destructive inflammatory process ensues in the conjunctiva, sclerotica, and interior parts of the eye; and within one or a few weeks, the whole eye may be quite disorganised, and the cornea may slough or be penetrated by a large ulcer. The sense of smell (and not merely that of mechanical irritation of the nose), may be at the same time lost or gravely impaired; so may the hearing, and commonly, when- ever the fifth nerve is paralysed, the tongue loses the sense of taste in its anterior and lateral parts, and according to Gowers in the posterior part as well. In relation to Taste.-The loss of tactile sensibility as well as the sense of taste, is no doubt due (a) to the lingual branch of the fifth nerve being a nerve of tactile sense, and also because with it runs the chorda tympani, which is one of the nerves of taste; 606 PHYSIOLOGY OF THE CRANIAL NERVES, [chap. xix. partly, also, it is due (6), to the fact that this branch supplies, in the anterior and lateral parts of the tongue, a necessary condition for the proper nutrition of that part; while (c), it forms also one chief link in the nervous circle for reflex action, in the secretion of saliva. But, deferring this question until the glosso-pharyngeal nerve is to be considered, it may be observed that in some brief time after complete paralysis or division of the fifth nerve, the power of all the organs of the special senses may be lost; they may lose not merely their sensibility to common impressions, for which they all depend directly on the fifth nerve, but also their sensibility to their several peculiar impressions for the reception and conduction of which they are purposely constructed and supplied with special nerves besides the fifth. The facts observed in these cases can, perhaps, be only explained by the influence which the fifth nerve exercises on the nutritive processes in the organs of the special senses. It is not unreasonable to believe, that, in paralysis of the fifth nerve, their tissues may be the seats of such changes as are seen in the laxity, the vascular congestion, oedema, and other affections of the skin of the face and other tegumentary parts which also accompany the paralysis ; and that these changes, which may appear unimportant when they affect external parts, are sufficient to destroy that refinement of structure by which the organs of the special senses are adapted to their functions. The Sixth Nerve, or Abducens. Functions.-The sixth nerve, Nervus abducens or ocularis ex- ternus, is also, like the fourth, exclusively motor, and supplies only the rectus externus muscle. It arises from the floor of the fourth ventricle from the anterior region in the deeper part. It is connected (fig. 367) with the nuclei of the third, fourth, and seventh nerves. It is nearer the middle line than the nuclei of the fifth. The rectus externus is convulsed, and the eye is turned out- wards, when the sixth nerve is irritated • and the muscle is paralysed when the nerve is divided. In all such cases of paralysis, the eye squints inwards, and cannot be moved outwards. In its course through the cavernous sinus, the sixth nerve forms larger communications with the sympathetic nerve than any other nerve w ithin the cavity of the skull does. But the import of these communications with the sympathetic, and the subsequent CHAP. XIX.] THE FACIAL NERVE. 607 distribution of its filaments after joining the sixth nerve, are quite unknown. The Seventh or Facial Nerve. Functions.-The facial, or portio dura of the seventh pair of nerves, arises from the floor of the central part of the fourth ventricle to the outside of and deeper down than the sixth nucleus. It may be connected with the hypoglossal nucleus. There are two roots, the lower and smaller is called the portio inter- media, is the motor nerve of all the muscles of the face, including the platysma, but not including any of the muscles of mastication already enumerated ; it supplies, also, the parotid gland, and through the connection of its trunk with the Vidian nerve, by the petrosal nerves, some of the muscles of the soft palate, probably the levator palati and azygos uvulae; by its tympanic branches it supplies the stapedius and laxator tympani, and, through the otic ganglion, the tensor tympani ■ through the chorda tympani it sends branches to the submaxillary gland and to the lingualis and some other muscular fibres of the tongue, and to the mucous mem- brane of its anterior two-thirds; and by branches given off before it comes upon the face, it supplies the muscles of the external ear, the posterior part of the digastricus, and the stylo-hyoideus. Besides its motor influence, the facial is also, by means of the fibres which are supplied to the submaxillary and parotid glands, a secretory nerve. For, through the last-named branches, impres- sions may be conveyed which excite increased secretion of saliva. Symptoms of Paralysis of Facial Nerve.-When the facial nerve is divided, or in any other way paralysed, the loss of power in the muscles which it supplies, while proving the nature and extent of its functions, displays also the necessity of its perfection for the perfect exercise of all the organs of the special senses. Thus, in paralysis of the facial nerve, the orbicularis palpebrarum being powerless, the eye remains open through the unbalanced action of the levator palpebree; and the conjunctiva, thus continually exposed to the air and the contact of dust, is liable to repeated inflammation, which may end in thickening and opacity of both its own tissue and that of the cornea. These changes, however, ensue much more slowly than those which follow paralysis of the fifth nerve, and never bear the same destructive character. The sense of hearing, also, is impaired in many cases of paralysis, of the facial nerve ; not only in such as are instances of simul- taneous disease in the auditory nerves, but in such as may be 608 PHYSIOLOGY OF THE CRANIAL NERVES. [CHAP. XIX. explained by the loss of power in the muscles of the internal ear. The sense of smell is commonly at the same time impaired through the inability to draw air briskly towards the upper part of the nasal cavities in which part alone the olfactory nerve is distributed ; because, to draw the air perfectly in this direction, the action of the dilators and compressors of the nostrils should be perfect. Lastly, the sense of taste is impaired, or may be wholly lost in paralysis of the facial nerve, provided the source of the paralysis be in some part of the nerve between its origin and the giving off of the chorda tympani. This result, which has been observed in many instances of disease of the facial nerve in man, appears explicable on the supposition that the chorda tympani is the nerve of taste to the anterior two-thirds of the tongue, its fibres being distributed with the so-called gustatory or lingual branch of the fifth. Some look upon the chorda as partly or entirely made up of fibres from the fifth nerve, and not strictly speaking as a branch of the facial; others consider that it receives its taste fibres from communications with the glosso-pharyngeal. Together 'with these effects of paralysis of the facial nerve, the muscles of the face being all powerless, the countenance acquires on the paralysed side a characteristic, vacant look, from the absence of all expression: the angle of the mouth is lower, and the paralysed half of the mouth looks longer than that on the other side ; the eye has an unmeaning stare. All these pecu- liarities increase, the longer the paralysis lasts; and their appearance is exaggerated when at any time the muscles of the opposite side of the face are made active in any expression, or in any of their ordinary functions. In an attempt to blow or whistle, one side of the mouth and cheek acts properly, but the other side is motionless, or flaps loosely at the impulse of the expired air; so in trying to suck, one side only of the mouth acts ; in feeding, the lips and cheek are powerless, and food lodges between the cheek and gum. The Ninth, or G-losso-Pharyngeal Nerve. The glosso-pharyngeal nerves (ix., fig. 341), in the enumeration •of the cerebral nerves by numbers according to the position in which they leave the cranium, are considered as divisions of the eighth pair of nerves, in which term are included with them the pneumo-gastric and accessory nerves. But the union of the CHAP. XIX.] THE NINTH NERVE. 609 nerves under one term is inconvenient, although in some parts the glosso-pharyngeal and pneumogastric arc so combined in their distribution that it is impossible to separate them in either their anatomy or physiology. Distribution. - The glosso-pharyngeal nerve gives filaments through its tympanic branch (Jacobson's nerve), to the fenestra ovalis, and fenestra rotunda, and the Eustachian tube; also, to the carotid plexus, and, through the petrosal nerve, to the spheno- palatine ganglion. After communicating, either within or without the cranium, with the pneumogastric, and soon after it leaves the cranium, with the sympathetic, digastric branch of the facial, and the accessory nerve, the glosso-pharyngeal nerve parts into the two principal divisions indicated by its name, and supplies the mucous membrane of the posterior and lateral walls of the upper part of the pharynx, the Eustachian tube, the arches of the palate, the tonsils and their mucous membrane, and the tongue as far forwards as the foramen caecum in the middle line, and to near the tip at the sides and inferior part. Functions.-The glosso-pharyngeal nerve contains some motor fibres, together with those of common sensation and the sense of taste. i. Its motor influences are distributed to the glosso-pharyn- geal, the stylo-pharyngei, palato-glossi, and constrictors of the pharynx. Besides being (2) a nerve of common sensation in the parts which it supplies, and a centripetal nerve through which impres- sions are conveyed to be reflected to the adjacent muscles, the glosso-pharyngeal is also a nerve of special sensation ; being the nerve of taste (from its fibres derived from the fifth, Gowers), in all the parts of the tongue and palate to which it is distributed. After many discussions, the question, Which is the nerve of taste ?-the lingual branch of the fifth, or the glosso-pharyngeal ? -may be most probably answered by stating that they are not themselves, strictly speaking, nerves of this special function, but through their connection with the fifth nerve. For very numerous experiments and cases have shown that when the trunk of the fifth nerve is paralysed or divided, the sense of taste is com- pletely lost in the superior surface of the anterior and lateral parts of the tongue, at the back of the tongue, on the soft palate and palatine arches. The loss is instantaneous after division of the nerve; and, therefore, cannot be ascribed wholly to the 610 PHYSIOLOGY OF THE CRANIAL NERVES, [chap. xix. defective nutrition of the part, though to this, perhaps, may be ascribed the more complete and general loss of the sense of taste when the whole of the fifth nerve has been paralysed. The Tenth or Pneumogastric Nerve. The Vagus or Par Vagum. The origin of the Vagus nerve is in the lower half of the calamus scriptori us in the ala cinerea (fig. 365). Its nucleus very probably represents the cells of Clarke's posterior vesicular column of the spinal cord. In origin it is closely connected with the glosso-pharyngeal, spinal accessory, and the hypoglossal. It supplies sensory branches, which accompany the sympathetic on the middle meningeal artery, and others which supply the back part of the meatus and the adjoining part of the external ear. It is connected with the petrous ganglion of the glosso- pharyngeal, by means of fibres to its jugular ganglion ; with the spinal accessory which supplies it with its motor fibres for the larger and upper portion of the oesophagus, and with its inhibitory fibres for the heart; also with the hypoglossal, with the superior cervical ganglion of the sympathetic and with the cervical plexus. Distribution.-The Pneumogastric nerve, Nervus Vagus, or Par Vagum (1, fig. 368), has, of all the cranial and spinal nerves, the most various distribution, and influences the most various functions, either through its own filaments, or through those which, derived from other nerves, are mingled in its branches. The parts supplied by the branches of the vagus nerve are as follows:- (1.) By its pharyngeal branches, which enter the pharyngeal plexus, a large portion of the mucous membrane, and, probably, all the muscles of the pharynx. (2.) By the superior laryngeal nerve, the mucous membrane of the under surface of the epiglottis, the glottis, and the greater part of the larynx, and, the crico-thyroid muscle. (3-) By the inferior laryngeal nerve, the mucous membrane and muscular fibres of the trachea, the lower part of the pharynx and larynx, and all the muscles of the larynx except the crico-thyroid. (4-) By its oesophageal branches, the mucous membrane and muscular coats of the (Esophagus. (5-) Through the cardiac nerves, moreover, the branches of the vagus form a large portion of the supply of nerves to the heart CHAP. XIX.] THE TENTH NERVE. 611 Fig. 368 - View of the nerves of the eighth pair, their distribution and connections on the leftside, j.-1, pneumogastric nerve in the neck; 2, ganglion of its trunk; 3, its union with the spinal accessory; 4, its union with the hypoglossal; 5, pharyngeal branch; 6, superior laryngeal nerve ; 7, external laryngeal; 8, laryngeal plexus ; 9. inferior or recurrent laryngeal; 10, superior cardiac branch; n, middle cardiac; 12, plexiform part of the nerve in the thorax; 13, posterior pulmonary plexus ; 14, lingual or gustatory nerve of the inferior maxillary ; 15, hypoglossal, passing into the muscles of the tongue, giving its thyro-hyoid branch, and uniting with twigs of the lingual; 16, glosso-pharyngeal nerve; 17, spinal accessory nerve, uniting by its inner branch with the pneumogastric, and by its outer, passing into the stemo-mastoid muscle; 18, second cervical nerve; 19, third; 20, fourth; 21, origin of the phrenic nerve, 22, 23, fifth, sixth, seventh, and eighth cervical nerves, forming with the first dorsal the brachial plexus; 24, superior cervical ganglion of the sympathetic; 25, middle cervical ganglion; 26, inferior cervical ganglion united with the first dorsal ganglion; 27, 28, 29, 30, second, third, fourth, and fifth dorsal ganglia. (From Sappey after Hirschfeld and Leveillee) 612 PHYSIOLOGY OF THE CRANIAL NERVES. [chap. xix. and the great Arteries derived from both the trunk and the recurrent nerve. (6.) Through both the anterior and the posterior pulmonary plexuses to the Lungs. (7.) Through its gastric branches and to the Stomach, by its terminal branches passing over the walls of that organ. (8.) Through hepatic and splenic branches the Liver and the Spleen are partly supplied with nerves. Communications.-Throughout its whole course, the vagus contains both sensory and motor fibres ; but after it has emerged from the skull, and, in some instances even sooner, it enters into so many anastomoses that it is hard to say whether the filaments it contains are, from their origin, its own, or whether they are derived from other nerves combining with it. This is particularly the case with the filaments of the sympathetic nerve, which are abundantly added to nearly all its branches. The likeness to the sympathetic which it thus acquires is further increased by its con- taining many filaments derived, not from the brain, but from its own petrosal ganglia, in which filaments originate, in the same manner as in the ganglia of the sympathetic, so abundantly that the trunk of the nerve is visibly larger below the ganglia than above them (Bidder and Volkmann). Next to the sympathetic nerve, that which most communicates with the vagus is the acces- sory nerve, whose internal branch joins its trunk, and is lost in it. Functions.-The particular functions which the branches of the pneumogastric nerve discharge in the several parts to which they are distributed, may be thus summarised. They show that,-i. The pharyngeal branch is the principal motor nerve of the pharynx and soft palate, and is most probably wholly motor ; the chief part of its motor fibres being derived from the internal branch of the accessory nerve. 2. The inferior or recurrent laryngeal nerve is the motor nerve of the larynx. 3. The superior laryngeal nerve is chiefly sensory : the muscles supplied by it being the crico-thyroid, the arytenoid in part (?), and the inferior con- strictor of the pharynx. 4. The motions of the oesophagus, the stomach and part of the small intestines are dependent on motor fibres of the vagus, and are probably excited by impressions made upon sensitive fibres of the same. 5. The cardiac branches com- municate, from the centre in the medullary channel, impulses (inhibitory) regulating the action of the heart. 6. The pulmonary branches form the principal channel by which the sensory impres- CHAP, XIX.] THE FUNCTIONS OF THE VAGUS. 613 sions on the mucous surface of the trachea, bronchi and lungs that influence respiration, are transmitted to the medulla oblongata; and some fibres also supply motor influence to the muscular portions of the fibres of the trachea and bronchi. 7. Branches to the stomach and intestine not only convey motor but also vcrso-ynotfor impulses to those organs. 8. The action of the so-called depressor branch (p. 170) in inhibiting the action of the vaso-motor centre has already been treated of, and also the influence of the vagus in stimulating the secretion of the salivary glands, as in the nausea which pre- cedes vomiting. To summarise, therefore, the many functions of this nerve, it may be said that it supplies (1) motor influence to the pharynx and oesophagus, stomach and small intestine, the larynx, trachea, bronchi and lung; (2) sensory and in part (3) vaso-motor influence to the same regions; (4) inhibitory influence to the heart; (5) inhibitory afferent impulses to the vaso-motor centre ; (6) excito-secretory to the salivary glands; (7)excito- motor in coughing, vomiting, Ac. Effects of Section.-Division of both vagi, or of both their recur- rent branches, is often very quickly fatal in young animals; but in old animals the division of the recurrent nerve is not generally fatal, and that of both the vagi is not always fatal, and, when it is so, death ensues slowly. This difference is, probably, because, the yielding of the cartilages of the larynx in young animals permits the glottis to be closed by the atmospheric pressure in inspiration, and they are thus quickly suffocated unless tracheotomy be performed. In old animals, the rigidity and promi- nence of the arytenoid cartilages prevent the glottis from being completely closed by the atmospheric pressure ; even when all the muscles are paralysed, a portion at its posterior part remains open, and through this the animal continues to breathe. In the case of slower death, after division of both the vagi, the lungs are commonly found gorged with blood, oedematous, or nearly solid, with a kind of low pneumonia, and with their bronchial tubes full of frothy bloody fluid and mucus, changes to which, in general, the death may be proximately ascribed. These changes arc due, perhaps in part, to the influence which the nerves exercise on the movements of the air-cells and bronchi ; yet, since they are not always produced in one lung when its nerve is divided, they cannot be ascribed wholly to the suspension of organic nervous influence. Rather, they may be ascribed to 614 PHYSIOLOGY OF THE CRANIAL NERVES, [chap. xix. the hindrance to the passage of blood through the lungs, in con- sequence of the diminished supply of air and the excess of carbonic acid in the air-cells and in the pulmonary capillaries ; in part, perhaps, to paralysis of the blood-vessels, leading to congestion; and in part, also, they appear due to the passage of food and of the various secretions of the mouth and fauces through the glottis, which, being deprived of its sensibility, is no longer stimu- lated or closed in consequence of their contact. References to other functions of Vag-i.-Regarding the influence of the vagus, see also Heart (p. 149), Arteries (p. 170), Salivary Gland (p. 271), Glottis and Larynx (p. 499), Respiration (p. 224), Pharynx and Oesophagus (p. 280), Stomach (p. 293). Origin and Connections.-The nerve arises by two distinct origins -one from a centre in the floor of the 4th ventricle, partly but chiefly in the medulla, and connected with the vagus nucleus; the other, from the outer side of the anterior corner of the spinal cord as low down as the 5th or 6th cervical vertebra. The fibres from the two origins come together at the jugular foramen, but separate again into two branches, the inner of which, arising from the medulla, joins the vagus, to which it supplies its motor fibres, consisting of small medullated or visceral nerve-fibres, whilst the outer consisting of large medullated fibres, supplies the trapezius and sterno-mastoid muscles. The small-fibred branch probably arises from a nucleus which corresponds to the posterior vesicular column of Clarke. The principal branch of the accessory nerve, its external branch, then supplies the sterno-mastoid and trapezius muscles; and, though pain is produced by irritating it, is composed almost ex- clusively of motor fibres. The internal branch accessory nerve supplies chiefly viscero-motor filaments to the vagus. The muscles of the larynx, all of which, as already stated, are supplied, apparently, by branches of the vagus, are said to derive their motor nerves from the accessory ; and (which is a very significant fact) A rolik states that in the chimpanzee the internal branch of the ac- cessory does not join the vagus at all, but goes direct to the larynx. Among the roots of the accessory nerve, the lower or external, arising from the spinal cord, appears to be composed exclusively of motor fibres, and to be destined entirely to the trapezius and The Eleventh or Spinal Accessory Nerve. chap, xix.] THE TWELFTH NERVE. 615 sterno-mastoid muscles ; the upper fibres, arising from the medulla oblongata, contain many sensory as well as motor fibres. The Twelfth or Hypoglossal Nerve. Origin and Connections.-The hypoglossal nerve arises from two large celled and one small celled, nuclei in the lowest part of the floor of the 4th ventricle near the middle line. The fibres of origin are continuous with the anterior roots of the spinal nerves. It is connected with the vagus, the superior cervical ganglion of the sympathetic and with the upper cervical nerves. Distribution.-The hypoglossal or ninth nerve, or motor lingua}, has a peculiar relation to the muscles connected with the hyoid bone, including those of the tongue. It supplies through its descending branch (descendens nonV), the sterno-hyoid, sterno- thyroid, and omo-hyoid; through a special branch, the thyro- hyoid, and through its lingual branches the genio-hyoid, stylo- glossus, hyo-glossus, and genio-hyo-glossus and linguales. It contributes, also, to the supply of the submaxillary gland. Functions.-The function of the hypoglossal is exclusively motor, except in so far as its descending branch may receive a few sensory filaments from the first cervical nerve. As a motor nerve, its influence on all the muscles enumerated above is shown by their convulsions when it is irritated, and by their loss of power when it is paralysed. The effects of the paralysis of one hypoglossal nerve are, however, not very striking in the tongue. Often, in cases of hemiplegia involving the functions of the hypoglossal nerve, it is not possible to observe any deviation in the direction of the protruded tongue ; probably because the tongue is so compact and firm that the muscles on either side, their insertion being nearly parallel to the median line, can push it straight forwards or turn it for some distance towards either side. Spinal Nerves. Functions.-Little need be added to what has been already said of these nerves (pp. 543, 544). The anterior roots of the spinal nerves are formed exclusively of motor fibres; the posterior roots exclusively of sensory fibres. Beyond the ganglia, all the spinal nerves are mixed nerves, and contain as well sympathetic filaments. 616 THE SENSES. [CHAP. XX. CHAPTER XX. THE SENSES. General Considerations.-Through the medium of the Nervous system the mind obtains a knowledge of the existence both of the various parts of the body, and of the external world. This know- ledge is based upon sensations resulting from the stimulation of certain centres in the brain, by irritations conveyed to them by afferent (sensory) nerves. Under normal circumstances, the following structures are necessary for sensation : (a) A peripheral organ for the reception of the impression ; (6) a nerve for con- ducting it; (c) a nerve-centre for feeling or perceiving it. Classification of Sensations.-Sensations may be conveniently classed as (i) common and (2) special. (r.) Common Sensations.-Under this head fall all those general sensations which cannot be distinctly localized in any particular part of the body, such as Fatigue, Discomfort, Faintness, Satiety, together with Hunger and Thirst, in which, in addition to a general discomfort, there is in many persons a distinct sensation referred to the stomach or fauces. In this class must also be placed the various irritations of the mucous membrane of the bronchi, which give rise to coughing, and also the sensations derived from various viscera indicating the necessity of expelling their contents; e.</., the desire to defalcate, to urinate, and, in the female, the sensations which precede the expulsion of the foetus. We must also include such sensations as itching, creeping, tick- ling, tingling, burning, aching, etc., some of which come under the head of pain : they will be again referred to in describing the sense of Touch. It is impossible to draw a very clear line of demarca- tion between many of the common sensations above mentioned, and the sense of touch, which forms the connecting link between the general and special sensations. Touch is, indeed, usually classed with the special senses, and will be considered in the same group with them; yet it differs from them in being common to many nerves; e.g., all the sensory spinal nerves, the vagus, glosso-pharyngeal, and fifth cerebral nerves, and in its impressions being communicable through many organs. Among common CHAP. XX.] SPECIAL SENSATIONS. 617 sensations must also be ranked the muscular sense, which has been already alluded to. It is by means of this sense that we become aware of the condition of contraction or relaxation of the various muscles and groups of muscles, and thus obtain the information necessary for their adjustment to various purposes-standing, walking, grasping, etc. This muscular sensibility is shown in our power to estimate the differences between weights by the different muscular efforts necessary to raise them. Consider- able delicacy may be attained by practice, and the difference between ig| oz. in one hand and 20 oz. in the other is readily appreciated. This sensibility with which the muscles are endowed must be carefully distinguished from the sense of contact and of pressure, of which the skin is the organ. When standing erect, we can feel the ground (contact), and further there is a sense of jsressMre, due to our feet being pressed against the ground by the weight of the body. Both these are derived from the skin of the sole of the foot. If now we raise the body on the toes, we are conscious (muscular sense) of a muscular effort made by the muscles of the calf, which overcomes a certain resistance. (2.) Special Sensations.-Including the sense of touch, the special senses are five in number-Touch, Taste, Smell, Hearing, Sight. Difference between Common and Special Sensations.-The most important distinction between common and special sensations is that by the former we are made aware of certain conditions of various parts of our bodies, while from the latter we gain our knowledge of the external world also. This difference will be clear if we compare the sensations of pain and touch, the former cf which is a common, the latter a special sensation. " If we place the edge of a sharp knife on the skin, we feel the edge by means of our sense of touch; we perceive a sensation, and refer it to the object which has caused it. But as soon as we cut the skin with the knife, we feel pain, a feeling which we no longer refer to the cutting knife, but which we feel within ourselves, and which communicates to us the fact of a change of condition in cui' own body. By the sensation of pain we are neither able to recognise the object which caused it, nor its nature." General Characteristics: Seat.-In studying the phenomena of sensation, it is important clearly to understand that the Sensorium, cr seat of sensation, is in the Brain, and not in the particular 618 THE SENSES. [chap. xx. organ (eye, ear, etc.) through which the sensory impression is received. In common parlance we arc said to see with the eye, hear with the ear, etc., but in reality these organs are only adapted to receive impressions which are conducted to the sensorium, through the optic and auditory nerves respectively, and there give rise to sensation. Hence, if the optic nerve is severed (although the eye itself is perfectly uninjured), vision is no longer possible ; since, although the image falls on the retina as before, the sensory impression can no longer be conveyed to the sensorium. When any given sen- sation is felt, all that we can with certainty affirm is that the sensorium in the brain is excited. The exciting cause may be (in the vast majority of cases is), some object of the external world {objective sensation); or the condition of the sensorium may be due to some excitement within the brain, in which case the sensation is termed subjective. The mind habitually refers sen- sations to external causes; and hence, whenever they are sub- jective (due to causes within the brain), we can hardly divest ourselves of the idea of an external cause, and an illusion is the result. Illusions.-Numberless examples of such illusions might be quoted. As familiar cases may be mentioned, humming and buzzing in the ears caused by some irritation of the auditory nerve or centre, and even musical sounds and voices (sometimes termed auditory spectra) ; also so-called optical illusions : persons and other objects are described as being seen, although not present. Such illusions are most strikingly exemplified in cases of delirium tremens or other forms of delirium, in which cats, rats, creeping loathsome forms, etc., are described by the patient as seen with great vividness. Cases of Illusions.-One uniform internal cause, which may act on all the nerves of the senses in the same manner, is the accumulation of blood in their capillary vessels, as in congestion and inflammation. This one cause excites in the retina, while the eyes are closed, the sensations of light and luminous flashes ; in the auditory nerve, the sensation of humming and ringing sounds; in the olfactory nerve, the sense of odours ; and in the nerves of feeling, the sensation of pain. In the same way, also, a narcotic substance introduced into the blood, excites in the nerves of each sense peculiar symptoms : in the optic nerves, the appearance of luminous sparks before the eyes ; in the auditory nerves, "tinnitus auriumand in the common sensory nerves, the sensation of creeping over the surface. So, also, among external causes, the chap, xx.] SENSATIONS, PERCEPTIONS, AND JUDGMENTS. 619 stimulus of electricity, or the mechanical influence of a blow, concussion, or pressure, excites in the eye the sensation of light and colours ; in the ear, a sense of a loud sound or of ringing; in the tongue, a saline or acid taste ; and in the other parts of the body, a perception of peculiar jarring or of the mechanical impression, or a shock like it. Experiments seem to have proved, however, that none of the nerves of special sense possess the faculty of common sensibility. Thus, Magendie observed that when the olfactory nerves, laid bare in a dog, were pricked, no signs of pain were manifested ; and other experiments of his seem to show that both the retina and optic nerve are insusceptible of pain. Further, the optic nerve is insusceptible to the stimulus of light when severed from its connection with the retina which alone is adapted to receive luminous impressions. Sensations and Perceptions.- The habit of constantly re- ferring our sensations to external causes, leads us to interpret the various modifications which external objects produce in our sen- sations, as properties of the external bodies themselves. Thus we speak of certain substances as possessing a disagreeable taste and smell; whereas, the fact is, their taste and smell are only dis- agreeable to us. It is evident, however, that on this habit of referring our sensations to causes outside ourselves (perception), depends the reality of the external world to us; and more especially is this the case with the senses of touch and sight. By the co-operation of these two senses, aided by the others, we are enabled gradually to attain a knowledge of external objects which daily experience confirms, until we come to place unbounded confidence in what is termed the " evidence of the senses." Judgments.-We must draw a distinction between mere sensations, and the judgments based, often unconsciously, upon them. Thus, in looking at a near object, we unconsciously estimate its distance, and say it seems to be ten or twelve feet off: but the estimate of its distance is in reality a judg- ment based on many things besides the appearance of the object itself; among which may be mentioned the number of inter- vening objects, the number of steps which from past experience we know we must take before we could touch it, and many others. Sensation of Motion is, like motion itself, of two kinds,-pro- 620 THE SENSES. [chap. xx. gressive and vibratory. The faculty of the perception of pro- gressive motion is possessed chiefly by the senses of vision, touch, and taste. Thus an impression is perceived travelling from one part of the retina to another, and the movement of the image is interpreted by the mind as the motion of the object. The same is the case in the sense of touch ; so also the movement of a sensation of taste over the surface of the organ of taste, can be recognised. The motion of tremors, or vibrations, is perceived by several senses, but especially by those of hearing and touch. Sensations of Chemical Actions.-We are made acquainted with chemical actions principally by taste, smell, and touch, and by each of these senses in the mode proper to it. Volatile bodies, disturbing the conditions of the nerves by a chemical action, exert the greatest influence upon the organ of smell; and many matters act on that sense which produce no impression upon the organs of taste and touch,-for example, many odorous substances, as the vapour of metals, such as lead, and the vapour of many minerals. Some volatile substances, however, are perceived not only by the sense of smell, but also by the senses of touch and taste. Thus, the vapours of horse-radish and mustard, and acrid suffocating gases, act upon the conjunctiva and the mucous membrane of the lungs, exciting through the common sensory nerves, merely modi- fications of common feeling ; and at the same time they excite the sensations of smell and of taste. The Special Senses. I. Touch. Seat.-The sense of touch is not confined to particular parts of the body of small extent, like the other senses ; on the contrary, all parts capable of perceiving the presence of a stimulus by ordi- nary sensation are, in certain degrees, the seat of this sense ; for touch is simply a modification or exaltation of common sensation or sensibility. The nerves on which the sense of touch depends are, therefore, the same as those which confer ordinary sensation on the different parts of the body, viz., those derived from the posterior roots of the nerves of the spinal cord, and the sensory cerebral nerves. But, although all parts of the body supplied with sensory nerves CHAP. XX.] VARIETIES OF TOUCH SENSATIONS. 621 are thus, in some degree, organs of touch, yet the sense is exer- cised in perfection only in those parts the sensibility of which is extremely delicate, e.g., the skin, the tongue, and the lips, which are provided with abundant papilla). A peculiar and, of its own kind in each case, a very acute sense of touch is exercised through the medium of the nails and teeth. To a less extent the hair may be reckoned an organ of touch ; as in the case of the eyelashes. The sense of touch renders us conscious of the presence of a stimulus, from the slightest to the most intense degree of its action, by that indescribable something which we call feeling, or common sensation. The modifications of this sense often depend on the extent of the parts affected. The sensation of pricking, for example, informs us that the sensitive particles are intensely affected in a small extent; the sensation of pressure indicates a slighter affection of the parts in the greater extent, and to a greater depth. It is by the depth to which the parts are affected that the feeling of pressure is distinguished from that of mere contact. Varieties.-(a) The sense of Touch, strictly so-called (tactile sensibility), (6) the sense of Pressure, (c) the sense of Tempera- ture. These when carried beyond a certain degree are merged in (d) the sensation of Pain. Various peculiar sensations, such as tickling, must be classed with pain under the head of common sensations, since they give us no information as to external objects. Such sensations, whether pleasurable or painful, are in all cases referred by the mind to the part affected, and not to the cause which stimulates the sensory nerves of the part. The sensation of tickling may be produced in many parts of the body, but with especial intensity in the soles of the feet. Among other sensations belonging to this class, and confined to particular parts of the body, may be mentioned those of the genital organs and nipples. (a) Touch, proper.-In almost all parts of the body which have delicate tactile sensibility the epidermis, immediately over the papillae, is moderately thin. When its thickness is much in- creased, as over the heel, the sense of touch is very much dulled. On the other hand, when it is altogether removed, and the cutis laid bare, the sensation of contact is replaced by one of pain. Further, in all highly sensitive parts, the papillae are numerous and highly vascular, and usually the sensory nerves are connected with special End-organs. The acuteness of the sense of touch depends very largely on 622 THE SENSES. [chap. xx. the cutaneous circulation, which is of course largely influenced by external temperature. Hence the numbness, familiar to everyone, produced by the application of cold to the skin. Special organs of touch are present in most animals, among which may be mentioned the antennae of insects, the •" whiskers " (vibrissae) of cats and other carnivora, the wings of bats, the trunk of the elephant, and the hand of man. Judgment of the Form and Size of Bodies.-By the sense of touch the mind is made acquainted with the size, form, and other external characters of bodies. And in order that these characters may be easily ascertained, the sense of touch is especially developed in those parts which can be readily moved over the surface of bodies. Touch, in its more limited sense, or the act of examining a body by the touch, consists merely in a voluntary employment of this sense combined with movement, and stands in the same relation to the sense of touch, or common sensibility, generally, as the act of seeking, following, or examining odours, does to the sense of smell. The hand is best adapted for it, by reason of its peculiarities of structure,-namely, its capability of pronation and supination, which enables it, by the movement of rotation, to examine the whole circumference of the body ; the power it pos- sesses of opposing the thumb to the rest of the hand, and the relative mobility of the fingers ; and lastly from the abundance of the sensory terminal organs which it possesses. In forming a conception of the figure and extent of a surface, the mind multi- plies the size of the hand or fingers used in the inquiry by the number of times which it is contained in the surface traversed ; and by repeating this process with regard to the different dimen- sions of a solid body, acquires a notion of its cuoical extent, but, of course, only an imperfect notion, as other senses, e.g., the sight, are required to make it complete. Acuteness of Touch.-The perfection of the sense of touch on different parts of the surface is proportioned to the power which such parts possess of distinguishing and isolating the sensations produced by two points placed close together. This power de- pends, at least in part, on the number of primitive nerve-fibres distributed to the part; for the fewer the primitive fibres which an organ receives, the more likely is it that several impressions on different contiguous points will act on only one nervous fibre, and hence be confounded, and perhaps produce but one sensation. .CHAP. XX.] VARIATIONS IN TACTILE SENSIBILITY. 623 Experiments have been made to determine the tactile properties of different parts of the skin, as measured by this power of dis- tinguishing distances. These consist in touching the skin, while the eyes are closed, with the points of a pair of compasses sheathed with cork, and in ascertaining how close the points of compasses might be brought to each other, and still be felt as two bodies. Table of variations in the tactile sensibility of different parts.- The measurement indicates the least distance at which the two Hunted points of a pair of compasses could he separately distinguished. (E. H. Weber.) Tip of tonguea inch. Palmar surface of third phalanx of forefinger . . . . A „ Palmar surface of second phalanges of fingers ... j „ Red surface of under-lip j „ Tip of the nosej „ Middle of dorsum of tongue A „ Palm of hand£ ,. Centre of hard palatein Dorsal surface of first phalanges of fingers . . . . ., Back of hand i(i „ Dorsum of foot near toesi j ., Gluteal region„ Sacral regioni| „ Upper and lower parts of forearmiA ,, Back of neck near occiput 2 ,, Upper dorsal and mid-lumbar regions2 ,, Middle part of forearm . . . . . . . 2| „ Middle of thigh „ Mid-cervical region ,, Mid-dorsal region2A „ Moreover, in the case of the limbs, it was found that before they were recognised as two, the points of the compasses had to be further separated when the line joining them was in the long axis of the limb, than when in the transverse direction. According to Weber the mind estimates the distance between two points by the number of unexcited nerve-endings which in- tervene between the two points touched. It would appear that a certain number of intervening unexcited nerve-endings are necessary before two points touched can be recognised as separate, and the greater this number the more clearly are the points of contact distinguished as separate. By practice the delicacy of a sense of touch may be very much increased. A familiar illustra- 624 THE SENSES. [chai-, xx. tion occurs in the case of the blind, who, by constant practice, can acquire the power of reading raised letters the forms of which are almost if not quite undistinguishable, by the sense of touch to an ordinary person. The power of correctly localising sensations of touch is gradually derived from experience. Thus infants when in pain simply cry, but make no effort to remove the cause of irritation, as an older child or adult would, doubtless on account of their imperfect knowledge of its exact situation. By long experience this power of localisation becomes perfected, till at length the brain pos- sesses a complete "picture'' as it were of the surface of the body, and is able with marvellous exactness to localise each sensation of touch. Illusions of Touch.-The different degrees of sensitiveness pos- sessed by different parts may give rise to errors of judgment in estimating the distance between two points where the skin is touched. Thus, if blunted points of a pair of compasses (main- tained at a constant distance apart) be slowly drawn over the skin of the cheek towards the lips, it is almost impossible to resist the conclusion that the distance between the points is gradually increasing. When they reach the lips they seem to be consider- ably further apart than on the cheek. Thus, too, our estimate of the size of a cavity in a tooth is usually exaggerated when based upon sensation derived from the tongue alone. Another curious illusion may here be mentioned. If we close the eyes, and place a small marble or pea between the crossed fore and middle fingers, we seem to be touching two marbles. This illusion is due to an error of judgment. The marble is touched by two surfaces which, under ordinary circumstances, could only be touched by two separate marbles, hence, the mind taking no cognizance of the fact that the fingers are crossed, forms the conclusion that two sensations arc due to two marbles. (6) Pressure.-It is extremely difficult to separate touch proper from sensations of pressure, and, indeed, the former may be said to depend upon the latter. If the hand be rested on the table and a very light body such as a small card placed on it, the only sensation produced is one of contact; if, however, an ounce weight be laid on the card an additional sensation (that of pressure) is experienced, and this becomes more intense as the weight is increased. If now the weight be raised by the hand, we are con- scious of overcoming a certain resistance ; this consciousness is CHAP. XX.] SENSATIONS OF TEMPERATURE. 625 due to what is termed the " muscular sense." The estimate of a weight is, therefore, usually based on two sensations, (i) of pres- sure on the skin, and (2) the muscular sense. The estimate of weight derived from a combination of these two sensa- tions (as in lifting a weight) is more accurate than that derived from the former alone (as when a weight is laid on the hand) ; thus Weber found that by the former method he could generally distinguish oz. from 20 oz., but not 19I oz. from 20, while by the latter he could at most only distinguish 14A oz. from 15 oz. It is not the absolute, but the relative, amount of the difference of weight which we have thus the faculty of perceiving. It is not. however, certain, that our idea of the amount of muscular force used is derived solely from sensation in the muscles. We have the power of estimating very accurately beforehand, and of regulating, the amount of nervous influence necessary for the production of a certain degree of move- ment. When we raise a vessel, with the contents of which we are not acquainted, the force we employ is determined by the idea we have con- ceived of its weight. If it should happen to contain some very heavy sub- stance, as quicksilver, we shall probably let it fall; the amount of muscular action, or of nervous energy, which we had exerted being insufficient. The same thing occurs sometimes to a person descending stairs in the dark ; he makes the movement for the descent of a step which does not exist. It is possible that in the same way the idea of weight and pressure in raising bodies, or in resisting forces, may in part arise from a consciousness of the amount of nervous energy transmitted from the brain rather than from a sensation in the muscles themselves. The mental conviction of the inability longer to support a weight must also be distinguished from the actual sensa- tion of fatigue in the muscles. So, with regard to the ideas derived from sensations of touch combined with movements, it is doubtful how far the consciousness of the extent of muscular movement is obtained from sensations in the muscles themselves. The sensation of movement attending the motions of the hand is very slight; and persons who do not know that the action of particular muscles is necessary for the production of given movements, do not suspect that the movement of the fingers, for example, depends on an action in the forearm. The mind has, nevertheless, a very definite knowledge of the changes of position produced by movements ; and it is on this that the ideas which it conceives of the extension and form of a body are in great measure founded. (c) Temperature.-The whole surface of the body is more or less sensitive to differences of temperature. The sensation of heat is distinct from that of touch; and it would seem reasonable to suppose that there are special nerves and nerve-endings for tem- perature. At any rate the power of discriminating temperature may remain unimpaired when the sense of touch is temporarily in 626 THE SENSES. [chap. xx. abeyance. Thus if the ulnar nerve be compressed at the elbow till the sense of touch is very much dulled in the fingers which it supplies, the sense of temperature remains quite unaffected. The sensations of heat and cold are often exceedingly fallacious, and in many cases are no guide at all to the absolute temperature as indicated by a thermometer. All that we can with safety infer from our sensations of temperature, is that a given object is warmer or cooler than the skin. Thus the temperature of our skin is the standard j and as this varies from hour to hour accord- ing to the activity of the cutaneous circulation, our estimate of the absolute temperature of any body must necessarily vary too. If we put the left hand into water at 40° F. and the right into water at 110° F., and then immerse both in water at 80 F., it will feel warm to the left hand but cool to the right. Again, a piece of metal which has really the same temperature as a given piece of wood will feel much colder, since it conducts away the heat much more rapidly. For the same reason air in motion feels very much cooler than air of the same temperature at rest. Perhaps the most striking example of the fallaciousness of our sensations as a measure of temperature is afforded by fever. In a shivering fit of ague the patient feels excessively cold, whereas his actual temperature is several degrees above the normal, while in the sweating stage which succeeds it he feels very warm, whereas really his temperature has fallen several degees. In the former case the cutaneous circulation is much diminished, in the latter much increased; hence the sensations of cold and heat respectively. In some cases we are able to form a fairly accurate estimate of absolute temperature. Thus, by plunging the elbow into a bath, a practised bath-attendant can tell the temperature sometimes within i° F. The temperatures which can be readily discriminated are between 50°-115° F. (io°-450 C.); very low and very high temperatures alike produce a burning sensation. A temperature appears higher according to the extent of cutaneous surface ex- posed to it. Thus, water of a temperature which can be readily borne by the hand, is quite intolerable if the whole body be immersed. So, too, w'ater appears much hotter to the hand than to a single finger. The delicacy of the sense of temperature coincides in the main with that of touch, and appears to depend largely on the thick- CHAP. XX. ] THE SENSE OF TASTE. 627 ness of the skin; hence, in the elbow, where the skin is thin, the sense of temperature is delicate, though that of touch is not re- markably so. Weber has further ascertained the following facts : two compass points so near together on the skin that they pro- duce but a single impression, at once give rise to two sensations, when one is hotter than the other. Moreover, of two bodies of equal weight, that which is the colder feels heavier than the other. As every sensation is attended with an idea, and leaves behind it an idea in the mind which can be reproduced at will, we are enabled to compare the idea of a past sensation with another sensation really present. Thus we can compare the weight of one body with another which we had previously felt, of which the idea is retained in our mind. Weber was indeed able to distinguish in this manner between temperatues, experienced one after the other, better than between temperatures to which the two hands were simultaneously subjected. This power of comparing present with past sensations diminishes, however, in proportion to the time which has elapsed between them. After-sensations left by impres- sions on nerves of common sensibility or touch are very vivid and durable. As long as the condition into which the stimulus has thrown the organ endures, the sensation also remains, though the exciting cause should have long ceased to act. Both painful and pleasurable sensations afford many examples of this fact. Subjective sensations, or sensations dependent on internal causes, are in no sense more frequent than in the sense of touch. All the sensations of pleasure and pain, of heat and cold, of light- ness and weight, of fatigue, Ac., may be produced by internal causes. Neuralgic pains, the sensation of rigor, formication or the creeping of ants, and the states of the sexual organs occurring during sleep, afford striking examples of subjective sensations. The mind has a remarkable power of exciting sensations in the nerves of common sensibility; just as the thought of the nauseous excites sometimes the sensation of nausea, so the idea of pain gives rise to the actual sensation of pain in a part predisposed to it; numerous examples of this influence might be quoted. II.-Taste. Conditions necessary.-The conditions for the perceptions of taste arc:-r, the presence of a nerve and nerve-centre with 628 THE SENSES. [chap. xx. special endowments ; 2, the excitation of the nerve by the sapid matters, which for this purpose must be in a state of solution. The nerves concerned in the production of the sense of taste have been already considered (pp. 605 and 609). The mode of action of the substances which excite taste consists in the production of a change in the condition of the gustatory nerves, and the conduc- tion of the stimulus thus produced to the nerve-centre; and, according to the difference of the substances, an infinite variety of changes of condition of the nerves, and consequently of stimu- lations of the gustatory centre, may be induced. The matters to be tasted must either be in solution or be soluble in the moisture covering the tongue ; hence insoluble substances are usually taste- less, and produce merely sensations of touch. Moreover, for the perfect action of a sapid, as of an odorous substance, it is necessary that the sentient surface should be moist. Hence, when the tongue and fauces are dry, sapid substances, even in solution, are with difficulty tasted. The nerves of taste, like the nerves of other special senses, may have their peculiar properties excited by various other kinds of irritation, such as electricity and mechanical impressions. Thus, a small current of air directed upon the tongue gives rise to a cool saline taste, like that of saltpetre ; and a distinct sensation of taste similar to that caused by electricity, may be produced by a smart tap applied to the papillae of the tongue. Moreover, the mechanical irritation of the fauces and palate pro- duces the sensation of nausea, which is probably only a modification of taste. Seat.-The principal seat (apparent seat, that is, to our senses) of the sense of taste is the tongue. But the results of experi- ments as well as ordinary experience show that the soft palate and its arches, the uvula, tonsils, and probably the upper part of the pharynx, are also endowed with taste. These parts, together with the base and posterior parts of the tongue, are supplied with branches of the glosso-pharyngeal nerve, and evidence has been already adduced that the sense of taste is conferred upon them by this nerve. In most, though not in all persons, the anterior parts of the tongue, especially the edges and tip, are endowed with the sense of taste. The middle of the dorsum is only feebly endowed with this sense, probably because of the density and thickness of the epithelium covering the filiform papillae of this part of the tongue, which will prevent the sapid substances from penetrating to their sensitive parts. CHAP. XX.] STRUCTURE OF THE TONGUE. 629 The Tongue. Structure.-The tongue is a muscular organ covered by mucous membrane. The muscles, which form the greater part of the Fig. 369. -PapUlar surface of the tongue, with the fauces and tonsils. 1, 1, circumvallate papilbe, in front of 2, the foramen crecum; 3, fungiform papillse; 4, filiform and conical papillae ; 5, transverse and oblique rugte ; 6, mucous glands at the base of the tongue and in the fauces ; 7, tonsils ; 8, part of the epiglottis ; 9, median glosso-epiglot- tidean fold (frienum epiglottidis). (From Sappey.) substance of the tongue (intrinsic muscles) are termed linguales; and by these, which are attached to the mucous membrane chiefly, its smaller and more delicate movements are chiefly performed. 630 THE SENSES. [chap. xx. By other muscles (extrinsic muscles), as the genio-hyoglossus, the styloglossus, &c., the tongue is fixed to surrounding parts; and by this group of muscles its larger movements are per- formed. The mucous membrane of the tongue resembles other mucous membranes in essential points of structure, but contains papilla, more or less peculiar to itself; peculiar, however, in details of structure and arrangement, not in their nature. The tongue is beset with numerous mucous follicles and glands. The use of the tongue in relation to mastication and deglutition has already been considered. The larger ptapillce of the tongue are thickly set over the anterior two-thirds of its upper surface, or dorsum (fig. 369), and give to it its characteristic roughness. In carnivorous animals, especially those of the cat tribe, the papillae attain a large size, and are developed into sharp recurved horny spines. Such papillae cannot be regarded as sensitive, but they enable the tongue to play the part of a most efficient rasp, as in scraping bones, or of a comb in cleaning fur. Their greater prominence than those of the skin is due to their interspaces not being filled up with epithelium, as the interspaces of the papillae of the skin are. The papillae of the tongue present several diversities of form; but three principal varieties, differing both in seat and general characters, may usually be distinguished, namely, the (1) circum- vallate, the (2) fungiform, and the (3) filiform papillae. Essentially these have all of them the same structure, that is to say, they are all formed by a projection of the mucous membrane, and contain special branches of blood-vessels and nerves. In details of struc- ture, however, they differ considerably one from another. The surface of each kind is studded by minute conical processes of mucous membrane, which thus form secondary papillae. Simple papillae also occur over most other parts of the tongue not occupied by the compound papillae, and extend for some distance behind the papillae circumvallatae. They are commonly buried beneath the epi- thelium ; hence they are often overlooked. The mucous membrane imme- diately in front of the epiglottis is, however, free from them. (1.) Circumvallate.-Ihese papillae (fig. 370), eight or ten in number, are situate in two V-shaped lines at the base of the tongue (1, 1, fig. 369). They are circular elevations from to of an inch wide, each with a central depression, and sur- chap, xx.] VARIETIES OF PAPILLjE. 631 rounded by a circular fissure, at the outside of which again is a slightly elevated ring, both the central elevation and the ring being formed of close-set simple papillae (fig. 370). (2.) Fungiform.-The fungiform papillae (3, fig. 369) are scattered chiefly over the sides and tip, and sparingly over the middle of the dorsum, of the tongue ; their name is derived from their being usually narrower at their base than at their summit. They also consist of groups of simple papillae (A. fig. 371), each of which con- tains in its interior a loop of capillary blood- vessels (B.), and a nerve-fibre. (3.) Conical or Filiform.-These, which are the most abundant Fig. 370.- Ferticad section of a circumvallate papilla of the calf. 1 and 3, epithelial layers covering it; 2, taste goblets; ; and f, duct of serous gland open- ing out into the pit in which papilla is situated ; 5 and 6, nerves ramifying within the papilla. (Engelmann.) Fig. 371.-Surface and section of the fungiform papilla!. A, the surface of a fungiform papilla, partially denuded of its epithelium ; p, secondary papillte ; e, epithelium. B, section of a fungiform papilla with the blood-vessels injected; a, artery; v, vein; c, capillary loops of similar papilla? in the neighbouring structure of the tongue; rf, capillary loops of the secondary papilla*; e, epithelium. (From Kiilliker, after Todd and Bowman.) papillae, are scattered over the whole surface of the tongue, but especially over the middle of the dorsum. They vary in shape somewhat, but for the most part are conical or filiform, and covered by a thick layer of epidermis, which is arranged over them, either in an imbricated manner, or is prolonged from their 632 THE SENSES. [chap. XX. surface in the form of fine stiff projections, hair-like in appearance, and in some instances in structure also (fig. 371). From their peculiar structure, it seems likely that these papillae have a mechani- cal function, or one allied to that of touch rather than of taste ; the latter sense being pro- bably seated especially in the other two varie- ties of papillae, the cir- cumvallate and the fungi- form. The epithelium of the tongue is stratified with the upper layers of the squamous kind. It covers every part of the surface ; but over the fungiform papillae forms a thinner layer than elsewhere. The epithe- lium covering the fili- form papillae is extremely dense and thick, and, as before mentioned, pro- jects from their sides and summits in the form of long, stiff, hair-like processes (fig. 372). Many of these processes bear a close resemblance to hairs. Blood-vessels and nerves are supplied freely to the papillae. The nerves in the fungiform and circumvallate papillae form a kind of plexus, spreading out brush-wise (fig. 370), but the exact mode of termination of the nerve-filaments is not cer- tainly known. Taste Goblets.-In the circumvallate papillae of the tongue of man peculiar structures known as gustatory buds or taste goblets, have been discovered. They are of an oval shape, and consist Fig. 372.-Two filiform papilla;, one with epithelium, the other without, y.--p, the substance of the papilla? dividing at their upper extremities into secondary papillie ; a, artery, and v, vein, dividing into capillary loops ; e, epithelial covering, lami- nated between the papilla1, but extended into hair-like processes, f, from the extremities of the secondary papillae. (From Kolliker, after Todd and Bowman.) CHAP. XX.] TASTE GOBLETS. 633 of a number of closely packed, very narrow and fusiform, cells (gustatory cells'). This central core of gustatory cells is enclosed in a single layer of broader fusiform cells (encasing cells). The gustatory cells terminate in fine spikes not unlike cilia, which project on the free surface (fig. a, 373). Fig'. 373;-Taste-goblet from dog's epiglottis (laryngeal surface near the base), precisely similar in structure to those found in the tongue, a, depression in epithelium over goblet; below the letter are seen the fine hair-like processes in which the cells termi- nate ; c, two nuclei of the axial (gustatory) cells. The more superficial nuclei belong to the superficial (encasing) cells; the converging lines indicate the fusiform shape of the encasing cells, x 400. (Schofield.) These bodies also occur side by side in considerable numbers in the epithelium of the papilla foliata, which is situated near the root of the tongue in the rabbit, and also in man. Similar taste- goblets have been observed on the posterior (laryngeal) surface of the epiglottis. It seems probable, from their distribution, that these taste goblets are gustatory in function, though no nerves have been distinctly traced into them. Other Functions. - Besides the sense of taste, the tongue, by means also of its papillae, is endued (2) especially at its side and tip, with a very delicate and accurate sense of touch which renders it sensible of the impressions of heat and cold, pain and mechanical pressure, and consequently of the form of surfaces. The tongue may lose its common sensibility, and still retain the sense of taste, and vice versd. This fact renders it probable that, although the senses of taste and of touch may be exercised by the same papillae supplied by the same nerves, yet the nervous conductors.for these two different sensa- tions are distinct, just as the nerves for smell and common sensi- bility in the nostrils are distinct; and it is quite conceivable that the same nervous trunk may contain fibres differing essentially in 634 THE SENSES. [chap. XX. their specific properties. Facts already detailed seem to prove that the lingual branch of the fifth nerve is the conductor of sen- sations of taste in the anterior part of the tongue ; and it is also certain, from the marked manifestations of pain to which its division in animals gives rise, that it is likewise a nerve of common sensibility. The glosso-pharyngeal also seems to contain fibres both of common sensation and of the special sense of taste. The functions of the tongue in connection with (3) speech, (4) mastication, (5) deglutition, (6) suction, have been referred to in other chapters. Taste and Smell: Perceptions.-The concurrence of common and special sensibility in the same part makes it sometimes diffi- cult to determine whether the impression produced by a substance is perceived through the ordinary sensitive fibres, or through those of the sense of taste. In many cases, indeed, it is probable that both sets of nerve-fibres are concerned, as when irritating acrid substances are introduced into the mouth. Much of the perfection of the sense of taste is often due to the sapid substances being also odorous, and exciting the simultaneous action of the sense of smell. This is shown by the imperfection of the taste of such substances when their action on the olfactory nerves is prevented by closing the nostrils. Many fine wines lose much of their apparent excellence if the nostrils are held close while they are drunk. Varieties of Tastes.-Among the most clearly defined tastes are the sweet and bitter (which are more or less opposed to each other), the acid, alkaline, and saline tastes. Acid and alkaline taste may be excited by electricity. If a piece of zinc be placed beneath and a piece of copper above the tongue, and their ends brought into contact, an acid taste (due to the feeble galvanic current) is pro- duced. The delicacy of the sense of taste is sufficient to discern 1 part of sulphuric acid in 1000 of water; but it is far surpassed in acuteness by the sense of smell. After-taste.-Nery distinct sensations of taste are frequently left after the substances which excited them have ceased to act on the nerve ; and such sensations often endure for a long time, and modify the taste of other substances applied to the tongue after- wards. Thus, the taste of sweet substances spoils the flavour of wine, the taste of cheese improves it. There appears, therefore, to exist the same relation between tastes as between colours, of which those that are opposed or complementary render each other CHAP. XX.] THE SENSE OF SMELL. 635 more vivid, though no general principles governing this relation have been discovered in the case of tastes. In the art of cooking, however, attention has at all times been paid to the consonance or harmony of flavours in their combination or order of succession, just as in painting and music the fundamental principles of harmony have been employed empirically while the theoretical laws were unknown. Frequent and continued repetitions of the same taste render the perception of it less and less distinct, in the same way that a colour becomes more and more dull and indistinct the longer the eye is fixed upon it. Thus, after frequently tasting first one and then the other of two kinds of wine, it becomes impossible to dis- criminate between them. The simple contact of a sapid substance with the surface of the gustatory organ seldom gives rise to a distinct sensation of taste ; it needs to be diffused over the surface, and brought into intimate contact with the sensitive parts by compression, friction, and motion between the tongue and palate. Subjective Sensations of Taste.- The sense of taste seems capable of being excited only by external causes, such as changes in the conditions of the nerves or nerve-centres, produced by con- gestion or other causes, which excite subjective sensations in the other organs of sense. But little is known of the subjective sensations of taste ; for it is difficult to distinguish the phenomena from the effects of external causes, such as changes in the nature of the secretions of the mouth. III.-Smell. Conditions necessary.-(i.) The first conditions essential to the sense of smell are a special nerve and nerve-centre, the changes in whose condition are perceived in sensations of odour, for no other nervous structure is capable of these sensations, even though acted on by the same causes. The same substance which excites the sensation of smell in the olfactory centre may cause another peculiar sensation through the nerves of taste, and may produce an irritating and burning sensation on the nerves of touch; but the sensation of odour is yet separate and distinct from these, though it may be simultaneously perceived. (2.) The second condition of smell is a peculiar change produced in the olfactory nerve and its centre by the stimulus or odorous substance. (3.) 636 THE SENSES. [chap. xx. The material causes of odours are, usually, in the case of animals living in the air, either solids suspended in a state of extremely fine division in the atmosphere ; or gaseous exhalations often of so subtile a nature that they can be detected by no other re-agent than the sense of smell itself. The matters of odour must, in all cases, be dissolved in the mucus of the mucous membrane before they can be immediately applied to, or affect the olfactory nerves; therefore a further condition necessary for the perception of odours is, that the mucous membrane of the nasal cavity be moist. When the Schneiderian membrane is dry, the sense of smell is impaired or lost; in the first stage of catarrh, when the secretion of mucus within the nostrils is lessened, the faculty of perceiving odour is either lost, or rendered very imperfect. (4.) In animals living in the air, it is also requisite that the odorous matter should be trans- mitted in a current through the nostrils. This is effected by an inspiratory movement, the mouth being closed ; hence we have voluntary influence over the sense of smell; for by interrupting respiration we prevent the perception of odours, and by repeated quick inspiration, assisted, as in the act of sniffing, by the action of the nostrils, we render the impression more intense. An odorous substance in a liquid form injected into tlie nostrils appears incapable of giving rise to the sensation of smell ; thus Weber could not smell the slightest odour when his nostrils were completely filled with water containing a large quantity of eau-de-Cologne. Seat.-The human organ of smell is formed by the filaments of the olfactory nerves, distributed in the mucous membrane cover- ing the upper third of the septum of the nose, the superior turbinated or spongy bone, the upper part of the middle turbinated bone, and the upper wall of the nasal cavities beneath the cribri- form plates of the ethmoid bones (figs. 374 and 376). The olfactory region is covered by cells of cylindrical epithelium, prolonged at their deep extremities into fine branched processes, but not ciliated ; and interspersed with these are fusiform (olfactory) cells, with both superficial and deep processes (fig. 377), the latter being probably connected with the terminal filaments of the olfactory nerve. The lower, or respiratory part, as it is called, of the nasal fossae is lined by cylindrical ciliated epithelium, except in the region of the nostrils, where it is squamous. The branches of the olfactory nerves retain much of the same soft and greyish texture which distinguishes those of the olfactory tracts within the cranium. Their filaments, also, are peculiar, chap. xx.] STRUCTURE OF OLFACTORY MUCOUS-MEMBRANE. 637 more resembling those of the sympathetic nerve than the filaments of the other cerebral nerves do, containing no outer white sub- Fie - 374--Nerves of the septum nasi, seen from the right side. |.-I, the olfactory bulb; i, the olfactory nerves passing through the foramina of the cribriform plate, and de- scending to be distributed on the septum ; 2, the internal or septal twig of the nasal branch of the ophthalmic nerve; 3, naso-palatine nerves. (From Sappey, after Hirsch- feld and LeveillG.) stance, and being finely granular and nucleated. The sense of smell is derived exclusively through those parts of the nasal cavities in which the olfactory nerves are distri- buted ; the accessory cavities or sinuses communicating with the nostrils seem to have no relation to it. Air im- pregnated with the vapour of cam- phor was injected into the frontal sinus through a fistulous opening and odorous substances have been injected into the antrum of Highmore; but in neither case was any odour perceived by the patient. The purposes of these sinuses appear to be, that the bones, necessarily large for the action of the muscles and other parts connected with them, may be as light as possible, and that there may be more room for the resonance of the air in vocalising. The former purpose, which is in other bones obtained by filling their Fig. 375--Section through the olfactory mucous membrane of the new-born child, a, non- nuclear ; and b, nucleated portions of the epithelium; c, nerves; dd, glands, marked out by Schultze as Bow- mans. (M. Schultze.) 638 THE SENSES. [chap. xx. cavities with fat, is here attained, as it is in many bones of birds, by their being filled with air. Other Functions of the Olfactory Region.-All parts of the nasal cavities, whether or not they can be the seats of the sense of smell, are endowed with common sensibility by the nasal branches of the first and second divisions of the fifth nerve. Hence the sen- sations of cold, heat, itching, tickling, and pain; and the sensation of tension or pres- sure in the nostrils. That these nerves cannot perform the function of the olfactory nerves is proved by cases in which the sense of smell is lost, while the mucous mem- brane of the nose remains susceptible of the various modifications of common sen- sation or touch. But it is often difficult to distinguish the sensation of smell from that of mere feeling, and to ascertain what belongs to each separately. This is the case particularly with the sensa- tions excited in the nose by acrid vapours, as of ammonia, horse-radish, mustard, etc., which resemble much the sensations of the nerves of touch; and the difficulty is the greater, when it is remembered that these acrid vapours have nearly the same action upon the mucous membrane of the eyelids. It was because the common sensibility of the nose to these irritating substances remained after the destruction of the olfactory nerves, that Magendie was led to the erroneous belief that the fifth nerve might exercise this special sense. Varieties oj Odorous Sensations.-Animals do not all equally perceive the same odours ; the odours most plainly perceived by an herbivorous animal and by a carnivorous animal are different. Fig. 376.-Nerves of the outer walls of the nasal fossa:. |.-1, network of the brunches of the olfactory nerve, descending upon the region of the superior and middle tur binated bones; 2, external twig of the ethmoidal branch of the nasal nerves ; 3, spheno- palatine ganglion ; 4, ramification of the anterior palatine nerves ; 5, posterior, and 6, middle divisions of the palatine nerves ; 7, branch to the region of the inferior- turbinated bone ; 8, branch to the region of the superior and middle turbinated bones; 9, naso - palatine branch to the septum cut short. (From Sappev, after Hirschfeld and Leveille.) CHAP. XX.] VARIETIES OF ODOROUS SENSATIONS. 639 The Carnivora have the power of detecting most accurately by the smell the special peculiarities of animal matters, and of tracking other animals by the scent ; but have apparently very little sensibility to the odours of plants and flowers. Herbivorous animals are peculiarly sensitive to the latter, and have a narrower sensibility to animal odours, espe- cially to such as proceed from other individuals than their own species. Man is far inferior to many animals of both classes in respect of the acuteness of smell; but his sphere of susceptibility to various odours is more uniform and extended. The cause of this difference lies probably in the endowments of the cerebral parts of the olfactory apparatus. The delicacy of the sense of smell is most re- markable ; it can discern the presence of bodies in quantities so minute as to be undiscoverable even by spectrum analysis ; j^(000 of a grain of musk can be distinctly smelt (Valentin). Opposed to the sensation of an agreeable odour is that of a disagreeable or disgusting odour, which corre- sponds to the sensations of pain, dazzling and disharmony of colours, and dissonance in the other senses. The cause of this difference in the effect of different odours is unknown ; but this much is certain, that odours are pleasant or offensive in a relative sense only, for many animals pass their existence in the midst of odours which to us are highly disagreeable. A great difference in this respect is, indeed, observed amongst men : many odours, generally thought agreeable, are to some persons intolerable; and different persons describe differently the sensations that they severally derive from the same odorous substances. There seems also to be in some persons an insensibility to certain odours, com- parable with that of the eye to certain colours; and among different persons, as great a difference in the acuteness of the sense of smell as among others in the acuteness of sight. We have no exact proof that a relation of harmony and disharmony exists between odours as between colours and sounds ; though it is pro- bable that such is the case, since it certainly is so with regard to the sense of taste; and since such a relation would account in some measure for the different degrees of perceptive power in E E Olf Fig. 377.-Epithelial and olfactory cells of man. The let- ters are placed on the free surface. E, E, epithelial cells; Olf., olfac- tory cells. (Max Schultze.) 640 THE SENSES. [chap. xx. different persons ; for as some have no ear for music (as it is said), so others have no clear appreciation of the relation of odours, and therefore little pleasure in them. Subjective Sensations.-The sensations of the olfactory nerves, independent of the external application of odorous substances, have hitherto been little studied. The friction of the electric machine produces a smell like that of phosphorus. Ritter, too, has observed, that when galvanism is applied to the organ of smell, besides the impulse to sneeze, and the tickling sensation excited in the filaments of the fifth nerve, a smell like that of ammonia was excited by the negative pole, and an acid odour by the positive pole ; whichever of these sensations were produced, it remained constant as long as the circle was closed, and changed to the other at the moment of the circle being opened. Subjective sensations occur frequently in connection with the sense of smell. Frequently a person smells something which is not present, and which other persons cannot smell; this is very frequent with nervous people, but it occasionally happens to every one. In a man who was constantly conscious of a bad odour, the arachnoid was found after death to be beset with deposits of bone; and in the middle of the cerebral hemispheres were scrofulous cysts in a state of suppuration. Dubois was acquainted with a man who, ever after a fall from his horse, which occurred several years before his death, believed that he smelt a bad odour. IV.-Hearing. Anatomy of the Ear.-For descriptive purposes, the Ear, or Organ of Hearing, is divided into three parts, (i) the external, (2) the middle, and (3) the internal ear. The two first are only acces- sory to the third or internal ear, which contains the essential parts of an organ of hearing. The accompanying figure shows very well the relation of these divisions,-one to the other (fig. 378). (1.) External Ear.-The external ear consists of the pinna or auricle, and the external aWz'fory canal or meatus. The principal parts of the pinna (fig. 379) are two prominent rims enclosed one within the other (helix and antihelix'), and enclosing a central hollow named the concha ; in front of the concha, a prominence directed backwards, the tragus, and opposite to this one directed forwards, the antitragus, from the concha, the auditory canal, with a slight arch directed upwards, passes inwards and a little forwards to the membrana tympani, to which it thus serves to convey the vibrating air. Its outer part consists of CHAP. XX.] ANATOMY OF THE EAR. 641 fibro-cartilage continued from the concha ; its inner part of bone. Both are lined by skin continuous with that of the pinna, and extending over the outer part of the membrana tympani. Fig. 378.-Diagrammatic view from before of the parts composing the organ of hearing of the left side. The temporal bone of the left side, with the accompanying soft parts, has been detached from the head, and a section has been carried through it transversely, so as to remove the front of the meatus externus, half the tympanic membrane, the upper and anterior wall of the tympanum and Eustachian tube. The meatus internus has also been opened, and the bony labyrinth exposed by the removal of the surrounding parts of the petrous bone, i, the pinna and lobe; 2, 2', meatus externus; 2', membrana tympani; 3, cavity of the tympanum; 3', its opening backwards into the mastoid cells ; between 3 and 3', the chain of small bones; 4, Eustachian tube ; 5, meatus in- terims, containing the facial (uppermost) and the auditory nerves; 6, placed on the vestibule of the labyrinth above the fenestra ovalis; a, apex of the petrous bone; 6, internal carotid artery; c, styloid process; d, facial nerve issuing from the stylo- mastoid foramen; e, mastoid process ; /, squamous part of the bone covered by integu- ment, &c. (Arnold.) Towards the outer part of the canal are fine hairs and sebaceous glands, while deeper in the canal are small glands, resembling the sweat-glands in structure which secrete a peculiar yellow substance called cerumen, or ear-wax. (2.) Middle Ear or Tympanum.-The middle ear, or tym- panum (3, fig. 378), is separated by the membrana tympani from the external auditory canal. It is a cavity in the temporal bone, opening through its anterior and inner wall into the Eustachian tube, a cylindriform flattened canal, dilated at both ends, com- posed partly of bone and partly of cartilage, and lined with mucous membrane, which forms a communication between the tympanum and the pharynx. It opens into the cavity of the pharynx just behind the posterior aperture of the nostrils. The cavity of the 642 THE SENSES. [chap. xx. tympanum communicates posteriorly with air-cavities, the mastoid cells in the mastoid process of the temporal bone; but its only opening to the external air is through the Eustachian tube (4, fig. 378). The walls of the tympanum are osseous, except where apertures in them are closed with membrane, as at the fenestra rotunda, and fenestra ovalis, and at the outer part where the bone is replaced by the membrana tympani. The cavity of the tympanum is lined with mucous membrane, the epi- thelium of which is ciliated and contin- uous with that of the pharynx. It con- tains a chain of small bones (ossicula auditus) which extends from the membrana tympani to the fenestra ovalis. The Membrana Tympani is placed in a slant- ing direction at the bottom of the external audi- tory canal, its plane being at an angle of about 45° with the lower wall of the canal. It is formed chiefly of a tough and tense fibrous membrane, the edges of which are set in a bony groove ; its outer surface is covered with a continuation of the Fig. 379.- Outer surface of the pinna of the right auri- cle. 1, helix ; 2, fossa of the helix; 3, antihelix ; 4, fossa of the antihelix ; 5, antitragus; 6, tragus; 7, concha; 8, lobule. J. Fig. 382.-The stapes, or stirrup-bone. 1, base; 2 and 3, arch ; 4, head of bone, which articu- lates with orbicular process of the incus; 5, constricted part of neck; 6, one of the crura. (Schwalbe.) Fig. 381.- The incus, or anvil-\>OT&. 1, body ; 2, ridged articulation for the malleus; 4, processus brevis, with 5, rough articular surface for ligament of incus; 6, processus magnus, with articu- lating surface for stapes; 7, nu- trient foramen (Schwalbe.) Fig. 380.-The hammer- bone or malleus, seen from the front. 1, The head; 2, neck; 3, short process; 4, long process. (Schwalbe). cutaneous lining of the auditory canal, its inner surface with part of the ciliated mucous membrane of the tympanum. The ossicles or small bones of the ear are three; named Malleus, ant Napes. The Malleus, or hammer-bone, is attached by a long sig y-cuned piocess, called its handle, to the membrana tympani ; the me of attachment being vertical, including the whole length of the handle, and extending from the upper border to the centre of the membrane. The CHAP. XX.] THE OSSICULA OF THE EAR. 643 head of the malleus is irregularly rounded ; its neck, or the line of boundary between it and the handle, supports two processes; a short conical one, which receives the insertion of the tensor tympani, and a slender one, processus gracilis, which extends forwards, and to which the laxator tym- pani muscle is attached. The incus, or anvil-bone, shaped like a bicuspid molar tooth, is articulated by its broader part, corresponding with the surface of the crown of a tooth, to the malleus. Of its two fang- like processes, one, directed back- wards, has a free end lodged in a depression in the mastoid bone ; the other, curved downwards and more pointed, articulates by means of a roundish tubercle, formerly called os orbiculare, with the stapes, a little bone shaped exactly like a stirrup, of which the base or bar fits into the fenestra ovalis. To the neck of the stapes, a short pro- cess, corresponding with the loop of the stirrup, is attached the stape- dius muscle. " The ossicula of aquatic mam- malia are very bulky and relatively large, especially in the true seals and the sirenia (Manatee and Dugong). In the cetacea the stapes is generally ankylosed to the fenestra ovalis, the malleus is always ankylosed to the tym- panic bone, yet the membrana tympani is well formed and there is a manu- brium, often ill-developed, but always attached to the membrane by a long process. In the Otariae or Sea-lions, where the ossicula are far smaller rela- tively, and less solid than in whales, manatees, and the earless true seals, there are well-formed, moveable external ears. The ossicula seem to be vestigial relics utilized for the auditory function. In land animals they vary in shape according to the type of the animal rather than in relation to its acuteness of hearing. I have never found a muscular laxator tympani in any animal, but the tensor exists as a ligament in whales where the malleus is fixed." (Alban Doran.) The bones of the ear are covered with mucous membrane re- flected over them from the wall of the tympanum; and are moveable both altogether and one upon the other. The malleus moves and vibrates with every movement and vibration of the membrana tympani, and its movements are communicated through the incus to the stapes, and through it to the membrane closing the fenestra ovalis. The malleus, also, is moveable in its articu- lation with the incus ; and the membrana tympani moving with it is altered in its degree of tension by the laxator and tensor tympani muscles. The stapes is moveable on the process of the incus, when the stapedius muscle acting, draws it backwards. Fig. 383.-Interior view of the tympanum, with membrana tympani and bones in natural posi- tion. 1, Membrana tympani; 2, Eustachian tube ; 3, tensor tympani muscle; 4, lig. mallei superior; 5, lig. mallei super. ; 6, corda- tympanic nerve; a, b, and c, sinuses about ossicula. (Schwalbe.) 644 THE SENSES. [CHAP. XX. The axis round which the malleus and incus rotate is the line joining the processus gracilis of the malleus and the posterior (short) process of the incus. (3.) The Internal Ear.-The proper organ of hearing is formed by the distribution of the auditory nerve within the internal ear, or labyrinth, a set of cavities within the petrous portion of the temporal bone. The bone which forms the walls of these cavities is denser than that around it, and forms the osseous labyrinth; the membrane within the cavities forms the membranous labyrinth. The membranous labyrinth contains a fluid called endo- lymph; while outside it, be- tween it and the osseous labyrinth, is a fluid called perilymph. The osseous labyrinth con- sists of three principal parts, namely the vestibule, the coch- lea, and the semicircular canals. The Anatomy of the Internal Ear.-The vestibule is the middle cavity of the labyrinth, and the central organ of the whole audi- tory apparatus. It presents, in its inner wall, several openings for the entrance of the divisions of the auditory nerve ; in its outer wall, the fenestra oralis (2, fig. 384), an opening filled by the base of the stapes, one of the small bones of the ear ; in its posterior and supe- rior walls, five openings by which the semicircular canals communi- cate with it: in its anterior wall, an opening leading into the cochlea. The hinder part of the inner wall of the vestibule also presents an opening, the orifice of the aquasductus vestibuli, a canal leading to the posterior margin of the petrous bone, with uncertain contents and unknown purpose. The semicircular canals (figs. 384, 385), are three arched cylindriform bony canals, set in the substance of the petrous bone. They all open at both ends into the vestibule (two of them first coalescing). The ends of each are dilated just before opening into the vestibule; and one end of each being more dilated than the other is called an ampulla. Two of the canals form nearly vertical arches ; of these the superior is also anterior ; the posterior is inferior; the third canal is horizontal, and lower and shorter than the others. The cochlea (6, 7, 8, figs. 384 and 385), a small organ, shaped like a common snail-shell, is seated in front of the vestibule, its base resting on Fig. 384.-Right long labyrinth, viewed from the outer side. The specimen here repre- sented is prepared by separating piecemeal the looser substance of the petrous bone from the dense walls which immediately enclose the labyrinth. 1, the vestibule ; 2, fenestra ovalis; 3, superior semicircular canal; 4, horizontal or external canal; 5, posterior canal; *, ampulla; of the semi- circular canals ; 6, first turn of the cochlea; 7, second turn; 8, apex; 9, fenestra rotunda. The smaller figure in outline below shows the natural size, (Summering.) CHAP. XX.] THE COCHLEA. 645 the bottom of the internal meatus, where some apertures transmit to it the cochlear filaments of the auditory nerve. In its axis, the cochlea is traversed by a conical column, the modiolus, around which, a spiral canal winds with about two turns and a half from the base to the apex. At the apex of the cochlea the canal is closed; at the base it presents three openings, of which one, already mentioned, communicates with the vestibule; another called fenestra rotunda, is sepa- rated by a membrane from the cavity of the tympanum ; the third is the orifice of the aquee ductus cochlea;, a canal leading to the jugular fossa of the petrous bone, and corresponding, at least in obscurity of purpose and origin, to the aquaaductus vestibuli. The spiral canal is divided into two passages, or scalae, by a partition of bone and membrane, the lamina spiralis. The osseous part or zone of this lamina is connected with the modiolus; the membranous part, with a muscular zone, forming its outer margin, is attached to the outer wall of the canal. Com- mencing at the base of the cochlea, between its vestibular and tym- panic openings, they form a parti- tion between these apertures ; the two scalae are, therefore, in corre- spondence with this arrangement, named scala vestibuli and scala tympani (fig. 386). At the apex of the cochlea, the lamina spiralis ends in a small hamulus, the inner and concave part of which, being detached from the summit of the modiolus, leaves a small aperture named heli- cotrema, by which the two scalae, separated in all the rest of their length, communicate. Besides the scala vestibuli and scala tympani, there is a third space between them, called scala media or canalis membranaceus (CC. fig. 387). In section it is trian- gular, its external wall being formed by the wall of the cochlea, its upper wall (separating it from the scala vestibuli) by the mem- brane of Reissner, and its lower wall (separating it from the scala tympani) by the basilar membrane, these two meeting at the outer edge of the bony lamina spiralis. Following the turns of the cochlea to its apex, the scala media there terminates blindly ; Fig. 385.- View of the interior of the left labyrinth. The bony wall of the labyrinth is removed superiorly and externally. 1, fovea hemielliptica; 2, fovea hemispherica; 3, common opening of the superior and posterior semicircular canals ; 4, opening of the aqueduct of the vestibule; 5, the superior, 6, the posterior, and 7, the ex- ternal semicircular canals ; 8, spiral tube of the cochlea (scala tympani); 9, opening of the aqueduct of the cochlea ; 10, placed on the lamina spiralis in the scala vestibuli. (Sbmmering.) Fig. 386.- View of the osseous cochlea divided through the middle. 1, central canal of the modiolus; 2, lamina spi- ralis ossea; 3, scala tympani; 4, scala vestibuli; 5, porous substance of the modiolus near one of the sections of the canalis spiralis modioli, f. (Arnold.) 646 THE SENSES. [chap. xx. while towards the base of the cochlea it is also closed with the exception of a very narrow passage (canalis reunions) uniting it with the sacculus. The scala media (like the rest of the membranous labyrinth) contains endo- lymph. Organ of Corti.-Upon the basilar membrane are arranged cells of various shapes. About midway between the outer edge of the lamina spiralis and the outer wall of the cochlea are situated the rods of Corti. Viewed sideways, the rods of Corti are seen to consist of an ex- ternal and internal pil- lar, each using from an expanded foot or base on the basilar membrane (a, n, fig. 388). They slant inwards towards each other, and each ends in a swellingtermed the head ; the head of the inner pillar over- lying that of the outer (fig. 388). Each pair of pillars forms, as it were, a pointed roof arching over a space, and by a succession of them, a little tunnel is formed. It has been estimated that there are about 3000 of these pairs of pillars, in proceeding from the base of the cochlea towards its apex. They are found progressively to increase in length, and become more oblique ; in other words the tunnel becomes wider, but diminishes in height as we approach the apex of the cochlea. Leaning, as it were, against these external and internal pillars are certain other cells, of which the external ones, Ztair cells, terminate in small hair-like processes. Most of the above details are shown in the accompanying figure (fig. 388). This complicated structure rests, as we have seen, upon the basilar membrane; it is roofed in by a remarkable fenestrated membrane or lamina reticularis into the fenestrse of which the tops of the various rods and cells are received. When viewed from above, the organ of Corti show's a remarkable resemblance to the key-board of a piano. In close relation with the rods of Corti and the cells inside and outside them, and probably projecting by free ends into the little tunnel containing fluid (roofed in by them), are filaments of the auditory nerve. Membranous Labyrinth.-This corresponds generally with the form of the osseous labyrinth, so far as regards the vestibule and semicircular canals, but is separated from the walls of these parts by fluid (endolymph), except where the nerves enter into Tiff. 387.-Section through one of the coils of the cochlea (diagrammatic). S T, scala tympani ; ,$' V, scala vesti- buli; C C, canalis cochleae or canalis membranaceus; It, membrane of Reissner ; I s o, lamina spiralis ossea ; I I s, limbus laminte spiralis ; s s, sulcus spiralis ; n c, cochlear nerve; g s, ganglion spirale; t, membrana tectoria ; (below the membrana tectoria is the lamina reticularis ;) 6, membrana basilaris ; Co, rods of Corti; I sp, ligamentum spirale. (Quain.) chap, xx.] THE ORGAN OF CORTI. 647 connection within it. Between its outer surface and the inner surface of the walls of the vestibule and semicircular canals is another collection of similar fluid, called perilymph; so that all the sonorous vibrations impressing the auditory nerves on these parts of the internal ear, are conducted through fluid to a membrane suspended in and containing fluid. In the cochlea, Fig. 388.- Vertical section of the organ of Corti from the dog. i to 2, homogeneous layer of the so-called membrana basilaris ; u, vestibular layer ; v, tympanal layer, with nuclei and protoplasm; a, prolongation of tympanal periosteum of lamina spiralis ossea; c, thickened commencement of the membrana basilaris near the point of perforation of the nerves h; d, blood-vessel (vas spirale); e, blood-vessel; f, nerves; g, the epithe- lium of the sulcus spiralis internus ; i, internal or tufted cell, ■with basil process k, sur- rounded with nuclei and protoplasm (of the granular layer), into which the nerve- fibres radiate; I, hairs of the internal hair-cell; n, base or foot of inner pillar of organ of Corti; m, head of the same uniting with the corresponding part of an external pillar, whose under half is missing, while the next pillar beyond, 0, presents both middle portion and base ; r, s, d, three external hair cells ; t, bases of two neighbour- ing hair or tufted cells; x, so-called supporting eell of Hensen; w, nerve-fibre terminating in the first of the external hair-cells ; 11 to I, lamina reticularis. X 800. (Waldeyer.) the membranous labyrinth completes the septum between the two scales, and encloses a spiral canal, previously mentioned, called canalis membranaceous or canalis cochlea? (fig. 387). The fluid in the scales of the cochlea is continuous with the perilymph in the vestibule and semicircular canals, and there is no fluid external to its lining membrane. The vestibular portion of the membranous labyrinth comprises two, probably communicating cavities, of which the larger and upper is named the ntriculus; the lower, the sacculus. They are lodged in depressions in the bony laby- rinth termed respectively " fovea hemielliptica " and " fovea hemispherica." Into the former open the orifices of the mem- branous semicircular canals ; into the latter the canalis cochlea?. The membranous labyrinth of all these parts is laminated, trans- parent, very vascular, and covered on the inner surface with 648 THE SENSES. [chap. xx. nucleated cells, of which those that line the ampullae are pro- longed into stiff hair-like processes; the same appearance, but to a much less degree, being visible in the utricule and saccule. In the cavities of the utriculus and sacculus are small masses of calcareous particles, otoconia or otoliths; and the same, although in more minute quantities, are to be found in the interior of some other parts of the membranous labyrinth. Auditory Nerve.-For the appropriate exposure of the fila- ments of the auditory nerve to sonorous vibrations all the organs now described are provided. It is characterised as a nerve of special sense by its softness (whence it derived its name of portio mollis of the seventh pair), and by the fineness of its component fibres. It enters the labyrinth of the ear in two divisions ; one for the vestibule and semicircular canals, and the other for the cochlea. The branches for the vestibule spread out and radiate on the inner surface of the membranous labyrinth : their exact termina- tion is unknown. Those for the semicircular canals pass into the ampullte, and form, within each of them, a forked projection which corresponds with a septum in the interior of the ampulla. The branches for the cochlea enter it through orifices at the base of the modiolus, which they ascend, and thence successively pass into canals in the osseous part of the lamina spiralis. In the canals of this osseous part or zone, the nerves are arranged in a plexus, containing ganglion cells. Their ultimate termination is not known with certainty ; but some of them, without doubt, end in the organ of Corti, probably in cells. Physiology of Hearing. All the acoustic contrivances of the organ of hearing are means for conducting sound, just as the optical apparatus of the eye are media for conducting light. Since all matter is capable of propagating sonorous vibrations, the simplest conditions must be sufficient for mere hearing; for all substances surrounding the auditory nerve would communicate sound to it. The whole development of the organ of hearing, therefore, can have for its object merely the rendering more perfect the propagation of the sonorous vibrations, and their multiplication by resonance; and, in fact, all the acoustic apparatus of the organ may be shown to have reference to these two principles. CHAP. XX.] FUNCTIONS OF THE EAR. 649 Functions of the External Ear.-The external auditory passage influences the propagation of sound to the tympanum in three ways :-i, by causing the sonorous undulations, entering directly from the atmosphere, to be transmitted by the air in the passage immediately to the membrana tympani, and thus pre- venting them from being dispersed • 2, by the walls of the passage conducting the sonorous undulations imparted to the external ear itself, by the shortest path to the attachment of the membrana tympani, and so to this membrane; 3, by the resonance of the column of air contained within the passage; 4, the external ear, especially when the tragus is provided with hairs, is also, doubtless, of service in protecting the meatus and membrana tympani against dust, insects, and the like. 1. As a conductor of undulations of air, the external auditory passage receives the direct undulations of the atmosphere, of which those that enter in the direction of its axis produce the strongest impressions. The undula- tions which enter the passage obliquely are reflected by its parietes, and thus by reflexion reach the membrana tympani. 2. The walls of the meatus are also solid conductors of sound ; for those vibrations which are communicated to the cartilage of the external ear, and not reflected from it, are propagated by the shortest path through the parietes of the passage to the membrana tympani. Hence, both ears being close stopped, the sound of a pipe is heard more distinctly when its lower extremity, covered with a membrane, is applied to the cartilage of the external ear itself, than when it is placed in contact with the surface of the head. 3. The external auditory passage is important, inasmuch as the air which it contains, like all insulated masses of air, increases the intensity of sounds by resonance. Regarding the cartilage of the external ear, therefore, as a con- ductor of sonorous vibrations, all its inequalities, elevations, and depressions, which are useless with regard to reflexion, become of evident importance; for those elevations and depressions upon which the undulations fall perpendicularly, will be affected by them in the most intense degree; and, in consequence of the various form and position of these inequalities, sonorous undula- tions, in whatever direction they may come, must fall perpendicu- larly upon the tangent of some one of them. This affords an explanation of the extraordinary form given to this part. Functions of the Middle Ear.-In animals living in the atmosphere, the sonorous vibrations are conveyed to the auditory nerve by three different media in succession ; namely, the air, the 650 THE SENSES. [chap. xx. solid parts of the body of the animal and of the auditory apparatus, and the fluid of the labyrinth. Sonorous vibrations are imparted too imperfectly from air to solid bodies, for the propagation of sound to the internal ear to be adequately effected by that means alone; yet already an instance of its being thus propagated has been mentioned. In passing from air directly into water, sonorous vibrations suffer also a considerable diminution of their strength ; but if a tense membrane exists between the air and the water, the sonorous vibrations are communicated from the former to the latter medium with very great intensity. This fact, of which Muller gives experimental proof, furnishes at once an explanation of the use of the fenestra rotunda, and of the membrane closing it. They are the means of communicating, in full intensity, the vibrations of the air in the tympanum to the fluid of the labyrinth. This peculiar property of membranes is the residt, not of their tenuity alone, but of the elasticity and capability of displacement of their particles; and it is not impaired when, like the mem- brane of the fenestra rotunda, they are not impregnated with moisture. Sonorous vibrations are also communicated without any per- ceptible loss of intensity from the air to the water, when to the membrane forming the medium of communication, there is attached a short, solid body, which occupies the greater part of its surface, and is alone in contact with the water. This fact elucidates the action of the fenestra ovalis, and of the plate of the stapes which occupies it, and, with the preceding fact, shows that both fenestrse-that closed by membrane only, and that with which the moveable stapes is connected-transmit very freely the sonorous vibrations from the air to the fluid of the labyrinth. A small, solid body, fixed in an opening by means of a border of membrane, so as to be moveable, communicates sonorous vibra- tions from air on the one side, to water, or the fluid of the labyrinth, on the other side, much better than solid media not so constructed. But the propagation of sound to the fluid is ren- dered much more perfect if the solid conductor thus occupying the opening, or fenestra ovalis, is by its other end fixed to the middle of a tense membrane, which has atmospheric air on both sides. A tense membrane is a much better conductor of the vibrations of air than any other solid body bounded by definite surfaces : and the vibrations are also communicated very readily CHAP. XX.] FUNCTIONS OF THE OSSICULA. 651 by tense membranes to solid bodies in contact with them. Thus, then, the membrana tympani serves for the transmission of sound from the air to the chain of auditory bones. Stretched tightly in its osseous ring, it vibrates with the air in the auditory passage, as any thin tense membrane will, when the air near it is thrown into vibrations by the sounding of a tuning fork or a musical string. And, from such a tense vibrating membrane, the vibra- tions are communicated with great intensity to solid bodies which touch it at any point. If, for example, one end of a flat piece of wood be applied to the membrane of a drum, while the other end is held in the hand, vibrations are felt distinctly when the vibrating tuning-fork is held over the membrane without touching it ; but the wood alone, isolated from the membrane, will only very feebly propagate the vibrations of the air to the hand. In comparing the membrana tympani to the membrane of a drum, it is necessary to point out certain important differences. When a drum is struck, a certain definite tone is elicited (fundamental tone) ; similarly a drum is thrown into vibration when certain tones are sounded in its neighbourhood, while it is quite unaffected by others. In ■other words it can only take up and vibrate in response to those tones whose vibrations nearly correspond in number with those of its own fundamental tone. The tympanic membrane can take up an immense range of tones pro- duced by vibrations ranging from 30 to 4000 or 5000 per second. This would be clearly impossible if it were an evenly stretched membrane. The fact is, that the tympanic membrane is by no means evenly stretched, and this is due partly to its slightly funnel-like form, and partly to its-being connected with the chain of auditory ossicles. Further, if the membrane were quite free in its centre, it would go on vibrating as a drum does some time after it is struck, and each sound would be prolonged, leading to con- siderable confusion. This evil is obviated by the ear-bones, which check the continuance of the vibrations like the " dampers " in a pianoforte. The ossicula of the ear are the better conductors of the sonorous vibrations communicated to them, on account of being isolated by an atmosphere of air, and not continuous with the bones of the cranium ; for every solid body thus isolated by a different medium, propagates vibrations with more intensity through its own sub- stance than it communicates them to the surrounding medium, which thus prevents a dispersion of the sound; just as the vibra- tions of the air in the tubes used for conducting the voice from one apartment to another are prevented from being dispersed by the solid walls of the tube. The vibrations of the membrana tympani are transmitted, therefore, by the chain of ossicula to the fenestra 652 THE SENSES. [chap. XX. ovalis and fluid of the labyrinth, their dispersion in the tympanum being prevented by the difficulty of the transition of vibrations from solid to gaseous bodies. The necessity of the presence of air on the inner side of the membrana tympani, in order to enable it and the ossicula auditus to fulfil the objects just described, is obvious. Without this provision, neither would the vibrations of the membrane be free, nor the chain of bones isolated, so as to propagate the sonorous undulations with concentration of their intensity. But while the oscillations of the membrana tympani are readily communicated to the air in the cavity of the tympanum, those of the solid ossicula will not be conducted away by the air, but will be propa- gated to the labyrinth without being dispersed in the tympanum. The propagation of sound through the ossicula of the tympanum to the labyrinth, must be effected either by oscillations of the bones, or by a kind of molecular vibration of their particles, or, most probably, by both these kinds of motion. Movements of the ossicula.-E. Weber has shown that the existence of the membrane over the fenestra rotunda will permit approximation and removal of the stapes to and from the labyrinth. When by the stapes the membrane of the fenestra ovalis is pressed towards the labyrinth, the membrane of the fenestra rotunda may, by the pressure communi- cated through the fluid of the labyrinth, be pressed towards the cavity of the tympanum. The long process of the malleus receives the un- dulations of the membrana tympani (fig. 389, a, a) and of the air in a direction indicated by the arrows, nearly perpendicular to itself. From the long process of the malleus they are propagated to its head (Z») : thence into the incus (c), the long pro- cess of which is parallel with the long process of the malleus. From the long process of the incus the undulations are communicated to the stapes (<Z) which is united to the incus at right angles. The several changes in the direction of the chain of bones have, however, no influence on that of the undulations, which remain the same as it was in the meatus externus and long process of the mal- leus, so that the undulations are communicated by the stapes to the fenestra ovalis in a perpendicular direction. Increasing tension of the membrana tympani diminishes the facility of transmission of sonorous undulations from the air to it. Savart observed that the dry membrana tympani, on the approach of a body emitting a loud sound, rejected particles of sand strewn upon it more Fig. 389. CHAP. XX.] ACTION OF THE TENSOR TYMPANI. 653 strongly when lax than when very tense ; and inferred, therefore, that hear- ing is rendered less acute by increasing the tension of the membrana tym- pani. Muller has confirmed this by experiments with small membranes arranged so as to imitate the membrana tympani ; and it may be confirmed also by observations on one's self. The pharyngeal orifice of the Eustachian tube is usually shut; during swallowing, however, it is opened ; this may be shown as follows :-If the nose and mouth be closed and the cheeks blown out, a sense of pressure is produced in both ears the moment we swallow ; this is due, doubtless, to the bulging out of the tympanic membrane by the compressed air, which at that moment enters the Eustachian tube. Similarly the tympanic membrane may be pressed in by rarefying the air in the tympanum. This can be readily accomplished by closing the mouth and nose, and making an inspiratory effort and at the same time swallowing (Valsalva). In both cases the sense of hearing is temporarily dulled ; proving that equality of pressure on both sides of the tympanic membrane is necessary for its full efficiency. Functions of Eustachian Tube.-The principal office of the Eustachian tube, in Muller's opinion, has relation to the prevention of these effects of increased tension of the membrana tympani. Its existence and openness will provide for the maintenance of the equilibrium between the air within the tympanum and the external air, so as to prevent the inordinate tension of the membrana tympani which would be produced by too great or too little pressure on either side. 'While discharging this office, however, it will serve to render sounds clearer, as (Henle suggests) the aper- tures in violins do; to supply the tympanum with air; and to be an outlet for mucus. If the Eustachian tube were permanently open, the sound of one's own voice would probably be greatly intensified, a condition which would of course interfere with the perception of other sounds. At any rate, it is certain that sonorous vibrations can be propagated up the Eustachian tube to the tym- panum by means of a tube inserted into the pharyngeal orifice of the Eustachian tube. Action of the Tensor Tympani.-The influence of the tensor tympani muscle in modifying hearing may also be probably ex- plained in connection with the regulation of the tension of the membrana tympani. If, through reflex nervous action, it can be excited to contraction by a very loud sound, just as the iris and orbicularis palpebrarum muscle are by a very intense light, then it is manifest that a very intense sound would, through the action of this muscle, induce a deafening or muffling of the ears. In favour of this supposition we have the fact that a loud sound 654 THE SENSES. [chap. XX. excites, by reflection, nervous action, winking of the eyelids, and, in persons of irritable nervous system, a sudden contraction of many muscles. Action of the Stapedius.-The influence of the stapedius muscle in hearing is unknown. It acts upon the stapes in such a manner as to make it rest obliquely in the fenestra ovalis depressing that side of it on which it acts, and elevating the other side to the same extent. It prevents too great a movement of the bone. Functions of the Fluid of the Labyrinth.-The fluid of the labyrinth is the most general and constant of the acoustic pro- visions of the labyrinth. In all forms of organs of hearing, the sonorous vibrations affect the auditory nerve through the medium of liquid-the most convenient medium, on many accounts, for such a purpose. The crystalline pulverulent masses (otoliths) in the labyrinth would reinforce the sonorous vibrations by their resonance, even if they did not actually touch the membranes upon which the nerves are expanded ; but, inasmuch as these bodies lie in contact with the membranous parts of the labyrinth, and the vestibular nerve- fibres are imbedded in them, they communicate to these membranes and the nerves, vibratory impulses of greater intensity than the fluid of the labyrinth can impart. This appears to be their office. Sonorous undulations in water are not perceived by the hand itself immersed in the water, but are felt distinctly through the medium of a rod held in the hand. The fine hair-like prolongations from the epithelial cells of the ampullae have, probably, the same function. Functions of the Semicircular canals.-Besides the function of collecting in their fluid contents sonorous undulations from the bones of the cranium, the semicircular canals appear to have another function less directly connected with the sense of hearing. Experiments show that when the horizontal canal is divided in a pigeon a constant movement of the head from side to side occurs, and similarly, when one of the vertical canals is operated upon, up and down movements of the head are observed. These movements are associated, also, with loss of co-ordination, as after the opera- tion the bird is unable to fly in an orderly manner, but flutters and falls when thrown into the air, and, moreover, is able to feed with difficulty. Hearing remains unimpaired. It has been sug- gested, therefore, that as loss of co-ordination results from section CHAP. XX.] FUNCTIONS OF THE COCHLEA. 655 of these canals, and as co-ordinate muscular movements appear to depend to a considerable extent for their due performance upon a correct notion of our equilibrium, that the semicircular canals are connected in some way with this sense, possibly by the constant alterations of the pressure of the fluid within them: the change in the pressure of the fluid in each canal which takes place on any movement of the head, producing sensations which aid in forming an exact judgment of the alteration of position which has occurred. Functions of the Cochlea.-The cochlea seems to be con- structed for the spreading out of the nerve-fibres over a wide extent of surface, upon a solid lamina which communicates with the solid walls of the labyrinth and cranium, at the same time that it is in contact with the fluid of the labyrinth, and which, besides exposing the nerve-fibres to the influence of sonorous undulations, by two media, is itself insulated by fluid on either side. The connection of the lamina spiralis with the solid walls of the labyrinth, adapts the cochlea for the perception of the sonorous undulations propagated by the solid parts of the head and the walls of the labyrinth. The membranous labyrinth of the vesti- bule and semicircular canals is suspended free in the perilymph, and is destined more particularly for the perception of sounds through the medium of that fluid, whether the sonorous undula- tions be imparted to the fluid through the fenestrse, or by the intervention of the cranial bones, as when sounding bodies are brought into communication with the head or teeth. The spiral lamina on which the nervous fibres are expanded in the cochlea, is, on the contrary, continuous with the solid walls of the laby- rinth, and receives directly from them the impulses which they transmit. This is an important advantage; for the impulses im- parted by solid bodies, have, cateris paribus, a greater absolute intensity than those communicated by water. And, even when a sound is excited in the water, the sonorous undulations are more intense in the water near the surface of the vessel containing it, than in other parts of the water equally distant from the point of origin of the sound; thus we may conclude that, cateris paribus, the sonorous undulations of solid bodies act with greater intensity than those of water. Hence, we perceive at once an important use of the cochlea. This is not, however, the sole office of the cochlea; the spiral 656 THE SENSES. [CHAP. XX. lamina, as well as the membranous labyrinth, receives sonorous impulses through the medium of the fluid of the labyrinth from the cavity of the vestibule, and from the fenestra rotunda. The lamina spiralis is, indeed, much better calculated to render the action of these undulations upon the auditory nerve efficient, than the membranous labyrinth is; for as a solid body insulated by a different medium, it is capable of resonance. The rotZs of Corti are probably arranged so that each set to vibrate in unison with a particular tone, and thus strike a par- ticular note, the sensation of which is carried to the brain by those filaments of the auditory nerve with which the little vibrating rod is connected. The distinctive function, therefore, of these minute bodies is, probably, to render sensible to the brain the various musical notes and tones, one of them answering to one tone, and one to another ; while perhaps the other parts of the organ of hearing discriminate between the intensities of different sounds, rather than their qualities. " In the cochlea we have to do with a scries of apparatus adapted for performing sympathetic vibrations with wonderful exactness. We have here before us a musical instrument which is designed, not to create musical sounds, but to render them perceptible, and which is similar in construction to artificial musical instruments, but which far surpasses them in the delicacy as well as the simplicity of its execution. For, while in a piano every string must have a separate hammer by means of which it is sounded, the ear possesses a single hammer of an ingenious form in its ear-bones, which can make every string of the organ of Corti sound separately." (Bernstein.) About 3000 rods of Corti are present in the human ear ; this would give about 400 to each of the seven octaves which are within the compass of the ear. Thus about 32 would go to each semi-ftme. Weber asserts that accom- plished musicians can appreciate differences in pitch as small as of a tone. Thus on the theory above advanced, the delicacy of discrimination would, in this case, appear to have reached its limits. Sensibility of the Auditory Nerve.--Any elastic body, e.g., air, a membrane, or a string performing a certain number of regular vibrations in the second, gives rise to what is termed a musical sound or tone. We must, however, distinguish between a musical sound and a mere noise; the latter being due to irregular vibrations. CHAP. XX.J QUALITIES OF MUSICAL SOUNDS. 657 Qualities of Musical Sounds.-Musical sounds are distinguished from each other by three qualities. i. Strength or intensity, which is due to the amplitude or length of the vibrations. 2. Pitch, which depends upon the number of vibrations in a second. 3. Quality, Colour, or Timbre. It is by this property that we distinguish the same note sounded on two instruments, e.g., a piano and a flute. It has been proved by Helmholtz to depend on the number of secondary notes, termed harmonics, which are present with the predominating or fundamental tone. It would appear that two impulses, which are equivalent to four single or half vibrations, are sufficient to produce a definite note, audible as such through the auditory nerve. The note produced by the shocks of the teeth of a revolving wheel, at regular intervals upon a solid body, is still heard when the teeth of the wheel are removed in succession, until two only are left; the second produced by the impulse of these two teeth has still the same definite value in the scale of music. The maximum and minimum of the intervals of successive impulses still appreciable through the auditory nerve as determinate sounds, have been determined by M. Savart. If their intensity is sufficiently great, sounds are still audible which result from the succession of 48,000 half vibrations, or 24.000 impulses in a second; and this, probably, is not the extreme limit in acuteness of sounds perceptible by the ear. For the opposite extreme, he has succeeded in rendering sounds audible which were produced by only fourteen or eighteen half vibrations, or seven or eight impulses in a second ; and sounds still deeper might probably be heard, if the individual impulses could be sufficiently prolonged. By removing one or several teeth from the toothed wheel the fact has been demonstrated that in the case of the auditory nerve, as in that of the optic nerve, the sensation continues longer than the impression which causes it; for a removal of a tooth from the wheel produced no interruption of the sound. The gradual cessation of the sensation of sound renders it difficult, however, to determine its exact duration beyond that of the impression of the sonorous impulses. Direction.-The power of perceiving the direction of sounds is not a faculty of the sense of hearing itself, but is an act of the mind judging on experience previously acquired. From the modifications which the sensation of sound undergoes according to the direction in which the sound reaches us, the mind infers the position of the sounding body. The only true guide for this inference is the more intense action of the sound upon one than Sounds. 658 THE SENSES. [chap. XX. upon the other ear. But even here there is room for much deception, by the influence of reflexion or resonance, and by the propagation of sound from a distance, without loss of intensity, through curved conducting tubes filled with air. By means of such tubes, or of solid conductors, which convey the sonorous vibrations from their source to a distant resonant body, sounds may be made to appear to originate in a new situation. The direction of sound may also be judged of by means of one ear only ; the position of the ear and head being varied, so that the sonorous undulations at one moment fall upon the ear in a perpendicular direction, at another moment obliquely. But when neither of these circumstances can guide us in distinguishing the direction of sound, as when it falls equally upon both ears, its source being, for example, either directly in front or behind us, it becomes impossible to determine whence the sound comes. Distance.-The distance of the source of sounds is not recog- nised by the sense itself, but is inferred from their intensity. The sound itself is always seated but in one place, namely, in our ear; but it is interpreted as coming from an exterior soniferous body. When the intensity of the voice is modified in imitation of the effect of distance, it excites the idea of its originating at a distance. Ventriloquists take advantage of the difficulty with which the direction of sound is recognised, and also the influence of the imagination over our judgment, when they direct their voice in a certain direction, and at the same time pretend, them- selves, to hear the sounds as coming from thence. The effect of the action of sonorous undulations upon the nerve of hearing, endures somewhat longer than the period during which the undulations are passing through the ear. If, however, the impressions of the same sound be very long con- tinued, or constantly repeated for a long time, then the sensation produced may continue for a very long time, more than twelve or twenty-four hours even, after the original cause of the sound has ceased. Binaural Sensations.-Corresponding to the double vision of the same object with the two eyes, is the double hearing with the two ears; and analogous to the double vision with one eye, dependent on unequal refraction, is the double hearing of a single sound with one ear, owing to the sound coming to the ear through media of unequal conducting power. The first kind of double hearing is very rare ; instances of it, however, have been recorded. chap, xx.] SUBJECTIVE SENSATIONS OF SOUND. 659 The second kind, which depends on the unequal conducting power of two media through which the same sound is transmitted to the ear, may easily be experienced. If a small bell be'sounded in water, while the ears are closed by plugs, and a solid conductor be interposed between the water and the ear, two sounds will be heard differing in intensity and tone ; one being conveyed to the ear through the medium of the atmosphere, the other through the conducting-rod. Subjective Sensations.-Subjective sounds are the result of a state of irritation or excitement of the auditory nerve produced by other causes than sonorous impulses. A state of excitement of this nerve, however induced, gives rise to the sensation of sound. Hence the ringing and buzzing in the ears heard by persons of irritable and exhausted nervous system, and by patients with cerebral disease, or disease of the auditory nerve itself; hence also the noise in the ears heard for some time after a long journey in a rattling noisy vehicle. Ritter found that electricity also excites a sound in the ears. From the above truly subjective sound we must distinguish those dependent, not on a state of the auditory nerve itself merely, but on sonorous vibra- tions excited in the auditory apparatus. Such are the buzzing- sounds attendant on vascular congestion of the head and ear, or on aneurismal dilatation of the vessels. Frequently even the simple pulsatory circulation of the blood in the ear is heard. To the sounds of this class belong also the buzz or hum, heard during the contraction of the palatine muscles in the act of yawning, during the forcing of air into the tympanum so as to make tense the membrana tympani, and in the act of blowing the nose, as well as during the forcible depression of the lower jaw. Irritation or excitement of the auditory nerve is capable of giving rise to movements in the body, and to sensations in other organs of sense. In both cases it is probable that the laws of reflex action, through the medium of the brain, come into play. An intense and sudden noise excites, in every person, closure of the eyelids, and, in nervous individuals, a start of the whole body or an unpleasant sensation, like that produced by an electric shock, throughout the body, and sometimes a particular feeling in the external ear. Various sounds cause in many people a disagreeable feeling in the teeth, or a sensation of cold tickling through the body, and, in some people, intense sounds are said to make the saliva collect. 660 THE SENSES. [CHAP. XX. V. Sight. Anatomy of the Optical Apparatus.-The eyelids consist of two moveable folds of skin, each of which is kept in shape by a thin plate of yellow elastic tissue. Along their free edges are inserted a number of curved hairs (eyelashes), which, when the lids are half closed, serve to protect the eye from dust and other foreign bodies : their tactile sensibility is also very delicate. On the inner surface of the elastic tissue are disposed a number of small racemose glands (Meibomian), whose ducts open near the free edge of the lid. The orbital surface of each lid is lined by a delicate, highly sensitive mucous membrane (conjunctiva), which is continuous with the skin at the free edge of each lid, and after lining the inner surface of the eyelid is reflected on to the eyeball, being somewhat loosely adherent to the sclerotic coat. The epithelial layer is continued over the cornea at its anterior epithelium. At the inner edge of the eye the conjunctiva becomes con- tinuous with the mucous lining of the lachrymal sac and duct, which again is continuous with the mucous membrane of the inferior meatus of the nose. The lachrymal gland is lodged in the upper and outer angle of the orbit. Its secretion, which issues from several ducts on the inner surface of the upper lid, under ordinary circumstances just suffices to keep the conjunctiva moist. It passes out through two small openings (puncta lachrymalia) near the inner angle of the eye, one in each lid, into the lachrymal sac, and thence along the nasal duct into the inferior meatus of the nose. The excessive secretion poured out under the influence of any irritating vapour or painful emotion overflows the lower lid in the form of tears. The eyelids are closed by the contraction of a sphincter muscle (orbicularis), supplied by the Facial nerve ; the upper lid is raised by the Levator palpebrce superioris, which is supplied by the Third nerve. The eyeball or the organ of vision (fig. 390) consists of a variety of structures which may be thus enumerated :- The sclerotic, or outermost coat, envelops about five-sixths of the eyeball : continuous with it, in front, and occupying the re- maining sixth, is the cornea. Immediately within the sclerotic is the choroid coat, and within the choroid is the retina. The interior of the eyeball is well-nigh filled by the aqueous and vitreous humours and the crystalline lens; but, also, there is suspended in the interior a contractile and perforated curtain,-the iris, for regu- lating the admission of light, and behind the junction of the sclerotic and cornea is the ciliary muscle, the function of which is to adapt the eye for seeing objects at various distances. Structure of the Sclerotic Coat.-The sclerotic coat is composed The Eyeball. CHAP. XX.] THE EYE. 661 of connective tissue, arranged in variously disposed and inter- communicating layers. It is strong, tough, and opaque, and not very elastic. Structure of the Cornea.-The cornea is a transparent membrane which forms a segment of a smaller sphere than the rest of the eyeball, and is let in, as it were, into the sclerotic with which it is continuous all round. It is coated with a laminated anterior epithelium (a, fig. 393), consisting of seven or eight layers of cells, -Sclerotic coat Choroid coat -Retina -Vitreous humour Ciliary muscle- Ciliary process- Canal of Petit- Cornea- Anterior chamber- Lens- Iris- Ciliary process- Ciliary muscle Fig. 39°. of which the superficial ones are flattened and scaly, and the deeper ones more or less columnar. Immediately beneath this is the anterior elastic lamina (Bowman). The cornea tissue proper as well as its epithelium is, in the adult, completely destitute of blood-vessels ; it consists of an intercellular ground-substance of rather obscurely fibrillated flattened bundles of connective tissue, arranged parallel to the free surface, and forming the boundaries of branched anastomos- ing spaces in which the cornea-corpuscles lie. These branched cornea-corpuscles have been seen to creep by amoeboid movement from one branched space into another. At its posterior surface the cornea is limited by the posterior elastic lamina, or membrane of Descemet, the inner layer of which consists of a single stratum of epithelial cells (fig. 391, cZ). 662 THE SENSES. [chap. xx. Nerves.-The nerves of the cornea are both large and numerous : they are derived from the ciliary nerves. They traverse the sub- stance of the cornea, in which some of them terminate, in the direction of its anterior surface, near which the axis cylinders break up into bundles of very delicate beaded fibrillae (fig. 391): these form a plexus immediately beneath the epithelium, from which delicate fibrils pass up between the cells anastomosing with horizontal branches, and forming 1- 9 J 4 5 Fig. 392.-Section through the choroid coat of the human eye. 1, elastic membrane, struc- tureless or finely fibrillated; 2, chorio- capillaris or tunica Ruyschiana; 3, Proper substance of the choroid with large vessels cut through ; 4, suprachoroidea ; 5, scle- rotic. (Schwalbe.) Fig. 391.-Vertical section of rab- bit's cornea, stained with gold chloride, e, Laminated ante- rior epithelium. Immediately beneath this is the anterior elastic lamina of Bowman, n, Nerves forming a delicate sub- epithelial plexus, and sending up fine twigs between the epi- thelial cells to end in a second plexus on the free surface ; d, Descemet's membrane, consist- ing of a fine elastic layer, and a single layer of epithelial cells ; the substance of the cornea, f, is seen to be fibrillated, and contains many layers of branched corpuscles, arranged parallel to the free surface, and here seen edgewise. (Schofield.) a deep intra-epithelial plexus, from which fibres ascend, till near the sur- face they form a superficial intra-epithe- lial net-work. Structure of the Choroid Coat (tunica, vasculosa).-This coat of the eyeball is formed by a very rich network of ca- pillaries (chorio-capillaris) outside which again are connective-tissue layers of stellate pigmented cells, suprachoroidea, (fig. 39 2) with numerous arteries and veins. It is separated from the retina by a fine elastic membrane, which is either structureless or finely fibrillated. chap, xx.] STRUCTURE OF THE RETINA. 663 The choroid coat ends in front in what are called the ciliary processes (fig. 399). Structure of the Retina.-The retina (fig. 396) is a delicate mem- Fig. 393.--Vertical section of rabbit's cornea., a, Anterior epithelium, showing the different shapes of the cells at various depths from the free surface ; b, portion of the substance of cornea. (Klein.) brane, concave, with the concavity directed forwards and ending in front, near the outer part of the ciliary processes, in a finely Fig. .594.-Horizontal preparation of cornea of frog; showing the network of branched cornea corpuscles. The ground substance is completely colourless, x 400. (Klein.) notched edge,-the ora serrata. Semitransparent when fresh, it soon becomes clouded and opaque, with a pinkish tint from the blood in its minute vessels. It results from the sudden spreading out or expansion of the optic nerve, of whose terminal fibres, ap- parently deprived of their external white substance, together with nerve cells, it is essentially composed. Exactly in the centre of the retina, and at a point thus corre- 664 THE SENSES. [chap. xx. spending to the axis of the eye in which the sense of vision is most perfect, is a round yellowish elevated spot, about of an inch in diameter, having a minute aperture at its summit, and called after its discoverer the yellow spot of Soemmering. In its centre is a minute depression called/owa centralis. About Jq of an inch to the inner side of the yellow spot, and consequently of the axis of the eye, is the point at which the optic nerve begins to spread Fig. 395.-Surface view of part of lamella of'kitten's cornea, prepared first with caustic potash and then with nitrate of silver. (By thia method the branched cornea-corpuscles with their granular protoplasm and large oval nuclei are brought out.) X 450. (Klein and Noble Smith.) out its fibres to form the retina. This is the only point of the surface of the retina from which the power of vision is absent. The retina consists of certain nervous elements arranged in several layers, and supported by a very delicate connective tissue. From the nature of the case there is still considerable uncer- tainty as to the character (nervous or connective tissue) of some of the layers of the retina. The following ten layers, from within outwards, are usually to be distinguished in a vertical section (figs- 396, 399)- [. Membrana limitans interna: a delicate membrane in contact with the vitreous humour. 2. Fibres of optic nerve. This layer is of very varying thickness in different parts of the retina : it consists chiefly of non-medullated fibres which interlace, and some of which are continuous with pro- cesses of the large nerve-cells forming the next layer. 3. Layer of ganglionic corpuscles, consisting of large multipolar nerve-cells, sometimes forming a single layer. In some parts of the retina, especially near the macula lutea, this layer is very thick, consisting of several distinct strata of nerve-cells. These cells lie in the spaces of a connective-tissue framework. chap, xx.] RODS AND CONES OF THE RETINA. 665 4. Molecular layer. This presents a finely granulated appear- ance. It consists of a punctiform connective tissue traversed by numberless very fine fibrillar processes of the nerve-cells. 5. Internal granular layer. This consists chiefly of numerous small round cells with a very small quantity of protoplasm sur- rounding a large nucleus; they are generally bipolar, giving off one process outwards and another inwards. They greatly resemble the ganglionic corpuscles of the cerebellum. Besides these there are large oval nuclei (e', fig. 396 A) belonging to the sustentacular connective tissue fibres. 6. Intergranular layer; which closely resembles the molecular layer but is much thinner. It consists of finely-dotted connec- tive tissue with nerve fibrils. 7. External granular layer; which consists of several strata of small cells resembling those of the internal granular layer; they have been classed as rod and cone granules, according as they are connected by very delicate fibrils with the rods and cones respec- tively. They are lodged in the meshes of a connective tissue framework. Both the internal and external granular layer stain very rapidly and deeply with haema- toxylin, while the rod and cone layer remains quite unstained. 8. Membrana limitans externa; a delicate, well-defined mem- brane, clearly marking the internal limit of the rod and cone layer. 9. Rod and cone layer, bacillar layer, or membrane of Jacob, con- sisting of two kinds of elements : the "rods," which are cylindrical and of uniform diameter throughout, and the " cones," whose internal portion is distinctly conical, and surmounted externally Fig. 396.-Diagram of the retina. A, con- nective tissue portion; B, nervous por- tion ; {the two must he combined to form the complete retina;) a a, membrana limitans externa; b, rods; c, cones; b', rod-granule; c', cone-granule; both belonging to the external granule layer; e, Muller's sustentacular fibres, with their nuclei e'; d, intergranular layer ; f, internal granule layer; g, molecular layer, connective-tissue portion; g', molecular layer, nerve-fibril portion ; A, ganglion cells ; A', their axis-cylinder process; i, nerve-fibre layer. (Max Schultze.) 666 THE SENSES. [chap. XX. by a thin rod-like body. According to the researches of Max Schultze, the rods show traces of longitudinal fibrillation, and, moreover, have a great tendency to break up into a number of transverse discs like a pile of coins. In the rod and cone layer of birds, the cones usually predomi- nate largely in number, whereas in man the rods are by far the Fig. 398.-The posterior half of the retina of the eft eye, viewed from before; s, the cut edge of the sclerotic coat; ch, the choroid; r, the retina ; n the interior at the middle, the macula lutea with the depression of the fovea centralis is represented by a slight oval shade ; towards the left side the light spot indicates the colli- culus or eminence at the entrance of the optic nerve, from the centre of which the arteria centralis is seen spreading its branches into the retina, leaving the part occupied by the macula comparatively free. (After Henle.) Fig. 397.-Ciliary processes, as seen from behind. 8, posterior surface of the iris, with the sphincter muscle of the pupil; 2, anterior part of the cho- roid coat; 3, one of the ciliary pro- cesses, of which about seventy are represented. -J-. more numerous. In nocturnal birds, however, such as the owl, only rods are present, and the same appears to be the case in many nocturnal and burrowing mammalia, e.g., bat, hedge-hog, mouse, and mole. io. Pigment cell layer, which was formerly considered part of the choroid. It consists of hexagonal and unbranched cells with a light nucleus. In the centre of the yellow spot (macula lutea) all the layers of the retina become greatly thinned out and almost disappear, except the rod and cone layer, which considerably increases in thickness, and comes to consist almost entirely of long slender cones, the rods being very few in number, or entirely absent. There are capillaries here, but none of the larger branches of the retinal arteries. CHAP. XX.] BLOOD VESSELS OF THE EYE. 667 With regard to the connection of the various layers there is still some uncertainty. Fig. 396 represents the view of Max Schultze. According to this there are certain sustentacular fibres of connective tissue (radiating fibres of Muller) which spring from the membrana limitans interna almost vertically, and traverse the retina to the limitans externa, whence very delicate connective tissue processes pass up between the rods and cones. The framework which they form is represented in fig. 396, A. The nervous Fig. 399.-Section through the eye carried through the ciliary processes. 1, Cornea; 2, mem- brane of Descemet; 3, sclerotic; 3', comeo-scleral junction; 4, canal of Schlemm; 5, vein ; 6, nucleated network on inner wall of canal of Schlemm; 7, lig. pectinatum iridis, abc ; 8, iris stroma ; 9, pigment of iris; to, ciliary processes; 11, ciliary muscle; 12, choroid tissue ; 13, meridional and 14, radiating fibres of ciliary muscle ; 15, ring- muscle of Midler; 16, circular or angular bundles of ciliary muscle. (Schwalbe.) elements of the retina are represented in fig. 396, B, and consist of delicate fibres passing up from the nerve-fibre layer to the rods and cones, and con- nected with the ganglionic corpuscles and granules of the internal and external layer. Blood-vessels of the Eyeball.-The eye is very richly sup- plied with blood-vessels. In addition to the conjunctival vessels which are derived from the palpebral and lachrymal arteries, there are at least two other distinct sets of vessels supplying the tunics of the eyeball. (1) The vessels of the sclerotic, choroid, and iris, and (2) The vessels of the retina. (l.) These are the short and long posterior ciliary arteries which pierce the sclerotic in the posterior half of the eyeball, and the anterior ciliary which enter near the insertions of the recti. These vessels anastomose and form a very rich choroidal plexus ; they also supply the iris and ciliary pro- cesses, forming a very highly vascular circle round the outer margin of the iris and adjoining portion of the sclerotic. The distinctness of these vessels from those of the conjunctiva is well seen in the difference between the bright red of blood-shot eyes (conjunc- 668 THE SENSES. [chap. xx. tival congestion), and the pink zone surrounding the cornea which indicates deep-seated ciliary congestion. (2.) /The retinal vessels are derived from the arteria centralis retina', which enters the eyeball along the centre of the optic nerve. They ramify all over the retina, chiefly in its inner layers. They can be seen by direct ophthalmoscopic examina- tion. The Crystalline Lens. Structure.-The lens is made up of a series of concentric lamina; (fig. 403), which when it has been har- dened, can be peeled off like the leaves of an onion. The lamina; consist of long ribbon- shaped fibres, which in the course of develop- ment are derived from cells. The fibres, there- fore, when young contain oval nuclei, but these disappear in the fully developed lens except at the outside. The super- ficial fibres are softer. The fibres are really six- sided prisms when seen in section, and fit exactly together with little con- necting material. The capsule is a homogeneous transparent elastic membrane. The hardest portion of the lens is that which is most internal. It forms the so-called nucleus of the lens (fig. 403, i)« Fig. 400.-A. Section of the retina, choroid, and part of the sclerotic, moderately magnified. «, membrana limitans interna ; b, nerve-fibre layer- traversed by Muller's sustentacular fibres (of the connec- tive tissue system) ; c, ganglion-cell layer; d, molecular layer; e, internal granular layer; f, intergranular layer ; g, external granular layer ; h, membrana limitans externa, running along the lower part of i, the layer of rods and cones ; k, pigment cell layer formerly described as part of the choroid ; I, m, internal and external vas- cular portions of the choroid, the first contain- ing capillaries, the second larger blood-vessels, cut in tranverse section; n, sclerotic. (W. Pye.) CHAI'. XX.] THE REFRACTION OF MEDIA. 669 Optical Apparatus. The eye may be compared to the camera used by photographers, and the transparent media correspond to the lens which is screwed into the front part. In the photographic camera images of external 2- 3 4 f> tr io' Fig. 401.-Section through the macula lutea and fovea centralis of human retina, a, fovea; b, descent of the macula towards fovea. The numbers indicate the layers of the retina. (Kuhnt.) objects are thrown upon a ground-glass screen at the back of a box, the interior of which is painted black. In the eye the camera proper is represented by the eye-ball with its choroidal Fig. 402.-Meridional section through the lens of a rabbit. 1, Lens capsule; 2, epithelium of lens; 3, transition of the epithelium into the fibres; 4, lens fibres. (Bubuchin.) pigment, and the screen by the retina. In the case of the camera the screen is enabled to receive clear images of objects at different distances, by an apparatus for focussing, and the convex lens too can be screwed in and out. The corresponding contrivance in the eye will be described under the head of Accommodation. The essential constituents of the optical apparatus of the eye may be thus enumerated: (i) A nervous structure (the retina) to be stimulated by light and to transmit by means of the optic nerve, of which it is the terminal expansion, the impression of the stimulation to the brain, in which it excites the sensation of 670 THE SENSES. [chap. XX. vision; (2) An apparatus consisting of certain refracting media, cornea, crystalline lens, aqueous and vitreous humour, the function of which is to collect together into one point, the different divergent rays emitted by each point of every external body and of giving them such directions that they are exactly focussed upon the retina, and thus produce an exact image of the object from which they proceed. For as light radiates from a luminous body in all directions, when the media offer no impediment to its transmission, a luminous point will necessarily illuminate all parts of a surface, such as the retina opposed to it, and not merely one single point. A retina, therefore, without any optical apparatus placed in front of it to separate the light of diffe- rent objects, would not allow of distinct vision, but would merely transmit such a general impression of daylight as would distinguish it from the night; (3) A contractile diaphragm (iris) with a central aper- ture for regulating the quantity of light admitted into the eye; and (4) an arrangement by which the chief refracting medium shall be so controlled as to enable objects to be seen at various distances, causing convergence of the rays of light that fall upon and traverse it (accommodation). Of the refracting media the cornea is in a twofold manner capable of refracting and causing con- vergence of the rays of light that fall upon and traverse it. It thus affects them first, by its density; for it is a law in optics that when rays of light pass from a rarer into a denser medium, if they impinge upon the surface in a direction removed from the perpendicular, they are bent out of their former direction towards that of a line perpendicular to the surface of the denser medium , and, secondly, by its convexity; since rays of light impinging upon a convex transparent surface, are refracted towards the centre, those being most refracted which are farthest from the centre of the convex surface. Behind the cornea is a space containing a thin watery fluid, the aqueous humour, holding in solution a small quantity of sodium chloride and extractive matter. The space containing the Fig. 403.-Laminated structure of the crystalline lens. The laminae are split up after hardening in alcohol. 1, the denser central part or nu- cleus ; 2, the successive external layers, (Arnold.) CHAP. XX.] ACTION OF THE IRIS. 671 aqueous humour is divided into an anterior and posterior chamber by a membranous partition, the iris, to be presently again men- tioned. The effect produced by the aqueous humour on the rays of light traversing it, is not yet fully ascertained. Its chief use, probably, is to assist in filling the eyeball, so as to maintain its proper convexity, and at the same time to furnish a medium in which the movements of the iris can take place. Behind the aqueous humour and the iris, and embedded in the anterior part of the medium next to be described, viz., the vitreous humour, is seated a doubly-convex body, the crystalline lens, which is the most important refracting structure of the eye. The struc- ture of the lens is very complex. It consists essentially of fibres united side by side to each other, and arranged together in very numerous laminae, which are so placed upon one another, that when hardened in spirit the lens splits into three portions in the form of sectors, each of which is composed of superimposed con- centric laminae. The lens increases in density and, consequently, in power of refraction, from without inwards; the central part, usually termed the nucleus, being the most dense. The vitreous humour constitutes nearly four-fifths of the whole globe of the eye. It fills up the space between the retina and the lens, and its soft jelly-like substance consists essentially of nume- rous layers, formed of delicate, simple membrane, the spaces between which are filled with a watery, pellucid fluid. Its prin- cipal use appears to be that of giving the proper distension to the globe of the eye, and of keeping the surface of the retina at a proper distance from the lens. Action of the Iris.-The iris is a vertically-placed mem- branous diaphragm, provided with a central aperture, the pupil, for the transmission of light. It is composed of plain muscular fibres imbedded in ordinary fibro-cellular or connective tissue. The muscular fibres have a direction, for the most part, radiating from the circumference towards the pupil; but as they approach the pupillary margin, they assume a circular direction, and at the very edge form a complete ring. By the contraction of the radiating fibres (dilator pupilla?) the size of the pupil is enlarged : by the contraction of the circular ones (sphincter pupillae), it is diminished. The object effected by the movements of the iris, is the regulation of the quantity of light transmitted to the retina. The posterior surface of the iris is coated with a layer of dark pigment, so that no rays of light can pass to the 672 THE SENSES. [chap. xx. retina, except such as are admitted through the aperture of the pupil. This iris is very richly supplied with nerves and blood-vessels. Its circular muscular fibres are supplied by the third (by the short ciliary branches of the ophthalmic ganglion), and its radiating fibres, by the sympathetic and fifth cranial nerve (by the long ciliary branches of the nasal nerve). Contraction of the pupil occurs under the following circum- stances : (i) On exposure of the eye to a bright light; (2) when the eye is focussed for near objects; (3) when the eyes converge to look at a near object; (4) on the local application of eserine (active principle of Calabar bean); (5) on the administration in- ternally of opium, aconite, and in the early stages of chloroform and alcohol poisoning ; (6) on division of the cervical sympathetic or stimulation of the third nerve. Dilatation of the pupil occurs (1) in a dim light; (2) when the eye is focussed for distant objects; (3) on the local application of atropine and its allied alkaloids; (4) on the internal administra- tion of atropine and its allies ; (5) in the latei' stages of poisoning by chloroform, opium, and other drugs; (6) on paralysis of the third nerve ; (7) on stimulation of the cervical sympathetic, or of its centre in the floor of the front of the aqueduct of Sylvius. The contraction of the pupil appears to be under the control of a centre in the medulla or on the corpora quadrigemina, and this is reflexly stimulated by a bright light, and the dilatation when the reflex centre is not in action is due to the more powerful sympathetic action; but in addition, it appears that both con- traction and dilatation may be produced by a local mechanism, upon which certain drugs can act, which is independent of and probably often antagonistic to the action of the central apparatus of the third and sympathetic nerve. The action of the fifth nerve upon the pupil is not well understood, but its apparent effect in producing dilatation is due to the mixture of sympathetic fibres with its nasal branch. The sympathetic influence upon the radiating fibres is believed to be conveyed not by the long ciliary branches of that nerve, but by the short ciliary branches from the ophthalmic ganglion. 1 he close sympathy subsisting between the two eyes is nowhere better shown than by the condition of the pupil. If one eye be shaded by the hand its pupil will of course dilate; but the pupil of the other eye will also dilate, though it is unshaded. CHAP. XX.] THE MECHANISM OF ACCOMMODATION. 673 Ciliary Muscle.-The ciliary muscle is composed of plain muscular fibres, which form a narrow zone around the interior of the eyeball, near the line of junction of the cornea with the sclerotic, and just behind the outer border of the iris. The outermost fibres of this muscle are attached in front to the inner part of the sclerotic and cornea at their line of junction, and diverging somewhat, are fixed to the ciliary processes, and a small portion of the choroid immediately behind them. The inner fibres immediately within the preceding, form a circular zone around the interior of the eyeball, outside the ciliary processes. They compose the ring formerly called the ciliary ligament. Accom modation. The distinctness of the image formed upon the retina, is mainly dependent on the rays emitted by each luminous point of the object being brought to a perfect focus upon the retina. If this focus occur at a point either in front of, or behind the retina, indistinctness of vision ensues, with the production of a halo. The focal distance, i.e., the distance of the point at which the luminous rays from a lens are collected, besides being regulated by the degree of convexity and density of the lens, varies with the distance of the object from the lens, being greater as this is shorter, and vice versd. Hence, since objects placed at various distances from the eye can, within a certain range, different in different persons, be seen with almost equal distinctness, there must be some provision by which the eye is enabled to adapt itself, so that whatever length the focal distance may be, the focal point may always fall exactly upon the retina. This power of adaptation of the eye to vision at different distances has received the most varied explanations. It is obvious that the effect might be produced in either of two ways, viz., by altering the convexity, and thus the refracting power, either of the cornea or lens; or by changing the position either of the retina or of the lens, so that whether the object viewed be near or distant, and the focal distance thus increased or diminished, the focal points to which the rays are converged by the lens may always be at the place occupied by the retina. The amount of either of these changes required in even the widest range of vision, is extremely small. For, from the refractive powers of the media of the eye, 674 THE SENSES. [chap. xx. the difference between the focal distances of the images of an object at such a distance that the rays are parallel, and of one at the distance of four inches, is only about 0'143 °f an On this calculation, the change in the distance of the retina from the lens required for vision at all distances, supposing the cornea and lens to maintain the same form, would not be more than about one line. It is now almost universally believed that Helmholtz is right in his statement that the immediate cause of the adaptation of the eye for objects at different distances is a varying shape of the lens, its front surface becoming more or less con- vex, according to the distance of the object looked at. The nearer the object, the more convex does the front surface of the lens become, and vice versd; the back surface taking little or no share in the pro- duction of the effect required. The following simple experiment illus- trates this point. If a small flame be held a little to one side of a person's eye, an observer looking at the eye from the other side sees three distinct images of the flame (fig- 404). The first and brightest is (1) a small erec image foimed by the anterior convex surface of the cornea : the second (2) is also erect, but larger and less distinct than the preceding, and is formed at the anterior convex surface of the lens: the third (3) is smaller and reversed, it is formed at the posterior surface of the lens, which is concave forwards, and therefore, like all concave mirrors, gives a reversed image. If now the eye under obseivation be made to look at a near object, the second image becomes smaller, clearer, and approaches the first. If the eye be now adjusted for a far point, the second image enlarges again, becomes less distinct, and recedes from the first. In both cases alike the first and third images remain unaltered in size and rela- the position. 1 his proves that during accommodation for near objects the curvature of the cornea, and of the posterior of the ens, remains unaltered, while the anterior surface of the lens becomes more convex and approaches the cornea. Fig. 404.-Diagram showing three re- flections of a candle. 1, From the anterior surface of cornea ; 2, from the anterior surface of lens; 3, from the posterior surface of lens'. For fui-ther explanation, see text. The experiment is best performed by employing an instrument in- vented by Helmholtz, termed a Phakoscope, CHAP. XX.] THE MECHANISM OF ACCOMMODATION. 675 Mechanism.-Of course the lens has no inherent power of contraction, and therefore its changes of outline must be pro- duced by some power from without; and there seems no reason to doubt that this power is supplied by the ciliary muscle. It is .sometimes termed the tensor choroidece. As this name implies, Fig. 405.-Phakoscope of Helmholtz. At B ff are two prisms, by which the light of a candle is concentrated on the eye of the person experimented with at O'; A is the aperture for the eye of the observer. The observer notices three double images, as in fig. 404, reflected from the eye under examination when the eye is fixed upon a distant object; the position of the images having been noticed the eye is then made to focus a near object, such as a reed pushed up by C; the images from the anterior surface of the lens will be observed to move towards each other, in consequence of the lens becoming more convex. from its attachment, it is able to draw forwards the choroid and therefore slackens the tension of the suspensory ligament of the lens which arises from it. The lens is usually partly flattened by the action of the suspensory ligament; and the ciliary muscle by diminishing the tension of this ligament diminishes, to a proportional degree, the flattening of which it is the cause. On diminution or cessation of the action of the ciliary muscle, the lens returns, in a corresponding degree, to its former shape, by virtue of the elasticity of its suspensory ligament (fig. 406). From this it will appear that the eye is usually focussed for distant objects. In viewing near objects 676 THE SENSES. [chap. xx. the pupil contracts, the opposite effect taking place on with- drawal of the attention from near objects, and fixing it on those distant. Range of Distinct Vision. Near-point.-In every eye there is a limit to the power of accommodation. If a book be brought nearer and nearer to the eye, the type at last becomes indistinct and cannot be brought into focus by any effort of accommodation, Fig. 406. Diagram representing by dotted lines the alteration tn the shape of the lens on accom- modation for near objects. (E. Eandolt.) however strong. This, which is termed the near-point, can be determined by the following experiment (Scheiner). Two small holes are pricked in a card with a pin not more than a line apart, at any rate their distance from each other must not exceed the diameter of the pupil. The card is held close in front of the c} e, and a small needle viewed through the pin-holes. At a moderate distance it can be clearly focussed, but when brought nearer, beyond a certain point, the image appears double or at any rate blurred. This point where the needle ceases to appear single is the near-point. Its distance from the eye can of course be readily measured. It is usually about 5 or 6 inches. In the accompanying figure (fig. 407) the lens b represents the eye ; ef the two pin-holes in the card, nn the retina; a represents the position of the needle. When the needle is at a moderate dis- tance, the two pencils of light coming from e and /, are focussed at a single point on the retina nn. If the needle be brought nearer than the near-point, the strongest effort of accommodation is not sufficient to focus the two pencils, they meet at a point CHAI'. XX.] SCilEINER'S EXPERIMENT. 677 behind the retina. The effect is the same as if the retina were shifted forward to mm. Two images h.g. arc formed, one from each hole. It is interesting to note that when two images are produced, the lower one g really appears in the position q, while the upper one appears in the position p. This may be readily verified by covering the holes in succession. Course of a Ray of Light.-With the help of the diagram, representing a vertical section of the eye from before back- wards, the mode in which, by means of the refracting media Fig. 407.-Diagram of experiment to ascertain the minimum distance of distinct vision. of the eye, an image of an object of sight is thrown on the retina, may be rendered intelligible. The rays of the cones of light emitted by the points a b, and every other point of an object placed before the eye, are first refracted, that is, are bent towards the axis of the cone, by the cornea c c, and the aqueous humour contained between it and the lens. The rays of each cone are again refracted and bent still more towards its central ray or axis by the anterior surface of the lens e e ; and again as they pass out through its posterior surface into the less dense medium of the vitreous humour. For a lens has the power of refracting and causing the convergence of the rays of a cone of light, not only on their entrance from a rarer medium into its anterior convex surface, but also at their exit from its posterior convex surface into the rarer medium. In this manner the rays of the cones of light issuing from the points a and b are again collected to points a and b; and, if the retina F be situated at a and b, perfect, though reversed, images of the points a and B will be formed upon it: but if the retina be not at a and b, but either before or behind that situation,-for instance at H or G,-circular luminous spots c and f or e and 0, instead of points, will be seen ; for at n the rays have not yet 678 THE SENSES. [chap. XX. met, and at G they have already intersected each other, and are again diverging. The retina must therefore be situated at the proper focal dis- tance from the lens, otherwise a defined image will not be formed, Fig. 408.-Diagram of the course of a ray of light, to show how a blurred or indistinct image is formed if the object be not exactly focussed upon retina. or, in other words, the rays emitted by a given point of the object will not be collected into a corresponding point of focus upon the retina. Defects in the Optical Apparatus. A. Defects in th.e Refracting Media.-Under this head we may consider the defects known as (i) Myopia, (2) Hypermetropia, (3) Astigmatism, (4) Spherical Aberration, (5) Chromatic Aberra- tion. The normal (emmetropic) eye is so adjusted that parallel rays arc brought exactly to a focus on the retina without any effort of accommodation (1, fig. 409). Hence all objects except near ones (practically all objects more than twenty feet off) are seen without any effort of accommodation; in other words, the far-point of the normal eye is at an infinite distance. In viewing near objects we are conscious of an effort (the contraction of the ciliary muscle) by which the anterior surface of the lens is rendered more convex, and rays which would otherwise be focussed behind the retina are converged upon the retina (see dotted lines 2, fig- 408). CHAP. XX.] DEFECTS IN THE REFRACTING MEDIA. 679 1. Myopia (short-sight) (4, fig. 409).-This defect is due to an abnormal elongation of the eye-ball. The eye is usually larger than normal and is always longer than normal; the lens is also probably too convex. The retina is too far from the lens and consequently parallel rays are focussed in front of the retina, and, crossing, form little circles on the retina ; thus the images of distant objects are blurred and indistinct. The eye is, as it were, permanently adjusted for a near-point. Rays from a point near the eye are exactly focussed in the retina. But those which issue from any object beyond a certain distance (/ar-joowtf) cannot be distinctly focussed. This defect is corrected by concave glasses which cause the rays entering the eye to diverge; hence they do not come to a focus so soon. Such glasses of course arc only needed to give a clear vision of distant objects. For near objects, except in extreme cases, they are not required. 2. Hypermetropia (long-sight) (3, fig. 409).-This is the reverse defect. The eye is too short and the lens too flat. Parallel rays are focussed behind the retina : an effort of accom- modation is required to focus even parallel rays on the retina; and when they are divergent, as in viewing a near object, the accommodation is insufficient to focus them. Thus in well- marked cases distant objects require an effort of accommodation and near ones a very powerful effort. Thus the ciliary muscle is constantly acting. This defect is obviated by the use of convex glasses, which render the pencils of light more convergent. Such glasses are of course especially needed for near objects, as in reading, Arc. They rest the eye by relieving the ciliary muscle from excessive work. 3. Astigmatism.-This defect, which was first discovered by Airy, is due to a greater curvature of the eye in one meridian than in others. The eye may be even myopic in one plane and hypermetropic in others. Thus vertical and horizontal lines crossing each other cannot both be focussed at once ; one set stand out clearly and the others are blurred and indistinct. This defect, which is present in a slight degree in all eyes, is generally seated in the cornea, but occasionally in the lens as well ; it may be corrected by the use of cylindrical glasses (i.e., curved only in one direction). 4. Spherical Aberration.-The rays of a cone of light from an object situated at the side of the field of vision do not meet all in the same point, owing to their unequal refraction ; for the 680 THE SENSES. [CHAP. XX. refraction of the rays which pass through the circumference of a lens is greater than that of those traversing its central portion. Fig. 409.-Diagrams showing-1, normal (emmetropic) eye bringing parallel rays exactly to a focus on the retina ; 2, normal eye adapted to a near point; without accommodation the rays would be focussed behind the retina, but by increasing the curvature of the an- terior surface of the lens (shown by a dotted line) the rays are focussed on the retina (as indicated by the meeting of the two dotted lines) ; y,'hypermetropic eye, in this case the axis of the eye is shorter, and the lens flatter, than normal; parallel rays are focussed behind the retina; 4, myopic eye; in this case the axis of the eye is abnormally long, and the lens too convex ; parallel rays are focussed in front of the retina. I his defect is known as spherical aberration, and in the camera, telescope, microscope, and other optical instruments, it is remedied by the interposition of a screen with a circular aperture in the path of the rays of light, cutting off all the marginal rays and only allowing the passage of those near the centre. Such correc- CHAP. XX.] CHROMATIC ABERRATION. 681 tion is effected in the eye by the iris, which forms an annular diaphragm to cover the circumference of the lens, and to prevent the rays from passing through any part of the lens but its centre which corresponds to the pupil. The posterior surface of the iris is coated with pigment, to prevent the passage of rays of light through its substance. The image of an object will be most defined and distinct when the pupil is narrow, the object at the proper distance for vision, and the light abundant ; so that, while a sufficient number of rays are admitted, the narrowness of the pupil may prevent the production of indistinctness of the image by spherical aberration. But even the image formed by the rays passing through the circumference of the lens, when the pupil is much dilated, as in the dark, or in a feeble light, may, under certain circumstances, be well defined. Distinctness of vision is further secured by the outer surface of the retina as well as the posterior surface of the iris and the ciliary processes, being coated with black pigment, which absorbs any rays of light that may be reflected within the eye, and prevents their being thrown again upon the retina so as to interfere with the images there formed. The pigment of the retina is especially important in this respect; for with the exception of its outer layer the retina is very transparent, and if the surface behind it were not of a dark colour, but capable of reflecting the light, the luminous rays which had already acted on the retina would be reflected again through it, and would fall upon other parts of the same membrane, producing both dazzling from excessive light, and indistinctness of the images. 5. Chromatic Aberration.-In the passage of light through an ordinary convex lens, decomposition of each ray into its ele- mentary coloured parts commonly ensues, and a coloured margin appears around the image, owing to the unequal refraction which the elementary colours undergo. In optical instruments this, which is termed chromatic aberration, is corrected by the use of two or more lenses, differing in shape and density, the second of which continues or increases the refraction of the rays produced by the first, but by recombining the individual parts of each ray into its original white light, corrects any chromatic aberration which may have resulted from the first. It is probable that the unequal refractive power of the transparent media in front of the retina may be the means by which the eye is enabled to guard against the effect of chromatic aberration. The human eye is 682 THE SENSES. [chap. xx. achromatic, however, only so long as the image is received at its- focal distance upon the retina, or so long as the eye adapts itself to the different distances of sight. If either of these conditions be interfered with, a more or less distinct appearance of colours is- produced. An ordinary ray of white light in passing through a prism, is- refracted, i.e., bent out of its course, but the different coloured rays which go to make up white light are refracted in different degrees, and therefore appear as coloured bands fading off into- each other : thus a coloured band known as the " spectrum " is produced, the colours of which are arranged as follows - red, orange, yellow, green, blue, indigo, violet; of these the red ray is the least, and the violet the most refracted. Hence, as Helmholtz has shown, a small white object cannot be accurately focussed on the retina, for if wre focus for the red rays, the violet are out of focus, and vice versd: such objects, if not exactly focussed, are often seen surrounded by a pale yellowish or bluish fringe. For similar reasons a red surface looks nearer than a blue one- at an equal distance, because, the red rays being less refrangible, a stronger effort of accommodation is necessary to focus them, and the eye is adjusted as if for a nearer object, and therefore the- red surface appears nearer. From the insufficient adjustment of the image of a small white object, it appears surrounded by a sort of halo or fringe. This phenomenon is termed Irradiation. It is from this reason that a white square on a black ground appears larger than a black square of the same size on white ground. As an optical instrument, the eye is superior to the camera in the following, among many other particulars, which may be enumerated in detail. I. The correctness of images even in a large field of view. 2. The simplicity and efficiency of the means by which chromatic aberration is avoided. 3. The perfect efficiency of its adaptation to different distances. In the photo- graphic camera, it is well known that only a comparatively small object can be accurately focussed. In the photograph of a large object near at land, the upper and lower limits are always more or less hazy, and vertical ines appear curved. This is due to the fact that the image produced >y a convex lens is really slightly curved and can only be received without 77""°? °n a curved concave screen, hence the distortion on a . a surface of ground glass. It is different with the eye, since it possesses a concave background, upon which the field of vision is depicted, and with " ich the curved form of the image coincides exactly. Thus, the defect of the camera obscura is entirely avoided ; for the eye is able to embrace CHAP. XX.] VISUAL SENSATIONS. 683 a large field of vision, the margins of which are depicted distinctly and without distortion. If the retina had a plane surface like the ground glass plate in a camera, it must necessarily be much larger than is really the case if we were to see as much ; moreover, the central portion of the field of vision alone would give a good clear picture. (Bernstein.) B. Defective Accommodation - Presbyopia.-This con- dition is due to the gradual loss of the power of accommodation which is part of the general decay of old age. In consequence the patient would be obliged in reading to hold his book further and further away in order to focus the letters, till at last the letters are held too far for distinct vision. The defect is remedied by weak convex glasses, which are very commonly worn by old people. It is due chiefly to the gradual increase in density of the lens, which is unable to swell out and become convex when near objects are looked at, and also to a weakening of the ciliary muscle, and a general loss of elasticity in the parts concerned in the mechanism. Excitation of the Retina.-Light is the normal agent in the excitation of the retina. The only layer of the retina capable of reacting to the stimulus is the rods and cones. The proofs of this statement may be summed up thus :- (i.) The point of entrance of the optic nerve into the retina, where the rods and cones are absent, is insensitive to light and is called the blind spot. The phenomenon itself is very readily demon- strated. If we direct one eye, the other being closed, upon a point at such a distance to the side of any object, that the image of the latter must fall upon the retina at the point of entrance of the optic nerve, this image is lost eithei' instantaneously, or very soon. If, for example, we close the left eye, and direct the axis of the right Visual Sensations. eye steadily towards the circular spot here represented, while the page is held at a distance of about six inches from the eye, both dot and cross are visible. On gradually increasing the distance between the eye and the object, by removing the book farther and farther from the face, and still keeping the right eye steadily on 684 THE SENSES. [CHAE. XX. the dot, it will be found that suddenly the cross disappears from view, while on removing the book still farther, it suddenly comes in sight again. The cause of this phenomenon is simply that the portion of retina which is occupied by the entrance of the optic nerve, is quite blind ; and therefore that when it alone occupies the field of vision, objects cease to be visible. (2.) In the fovea centralis and macula lutea -which contain rods and cones but no optic nerve-fibres, light produces the greatest effect. In the latter, cones occur in larger numbers, and in the former cones without rods are found, whereas in the rest of the retina which is not so sensitive to light, there are fewer cones than rods. We may conclude, therefore, that cones are even more important to vision than rods. (3.) If a small lighted candle be moved to and fro at the side of and close to one eye in a dark room while the eyes look steadily forward into the darkness, a remarkable branch- ing figure (Purkinje's figures') is seen floating before the eye, con- sisting of dark lines on a reddish ground. As the candle moves, the figure moves in the opposite direction, and from its whole appear- ance there can be no doubt that it is a reversed picture of the retinal vessels projected before the eye. The two large branching arteries passing up and down from the optic disc are clearly visible together with their minutest branches. A little to one side of the disc, in a part free from vessels, is seen the yellow spot in the form of a slight depression. This remarkable appearance is ■doubtless duo to shadows of the retinal vessels cast by the candle. T he branches of these vessels are chiefly distributed in the nerve-fibre and ganglionic layers ; and since the light of the candle falls on the retinal vessels from in front, the shadow is cast behind them, and hence those elements of the retina which perceive the shadows must also lie behind the vessels. Here, then, we have a clear proof that the light-perceiving elements of the retina are not the fibres of the optic nerve forming the innermost layer of the retina, but the external layers of the retina, almost certainly the rods and cones, which indeed appear to be the special terminations of the optic nerve-fibres. Duration of Visual Sensations.-The duration of the sensa- tion produced by a luminous impression on the retina is always greater than that of the impression which produces it. However brief the luminous impression, the effect on the retina always lasts for about one-eighth of a second. Thus, supposing an object in motion, say a horse, to be revealed on a dark night by a flash of CHAP. XX.] FECHNER'S LAW. 685 lightning. The object would be seen apparently for an eighth of a second, but it would not appear in motion; because, although the image remained on the retina for this time, it was really revealed for such an extremely short period (a flash of lightning being almost instantaneous) that no appreciable movement on the part of the object coidd have taken place in the period during which it was revealed to the retina of the observer. And the same fact is proved in a reverse way. The spokes of a rapidly revolving wheel are not seen as distinct objects, because at every point of the field of vision over which the revolving spokes pass, a given impression has not faded before another comes to replace it. Thus every part of the interior of the wheel appears occupied. The duration of the after-sensation, produced by an object, is greater in a direct ratio with the duration of the impression which caused it. Hence the image of a bright object, as of the panes of a window through which the light is shining, may be perceived in the retina for a considerable period, if we have previously kept our eyes fixed for some time on it. But the image in this case is negative. If, however, after shutting the eyes for some time, we open them and look at an object for an instant, and again close them, the after-image is positive. Intensity of Visual Sensations.-It is quite evident that the more luminous a body the more intense is the sensation it pro- duces. But the intensity of the sensation is not directly propor- tional to the intensity of the luminosity of the object. It is neces- sary for light to have a certain intensity before it can excite the retina, but it is impossible to fix an arbitrary limit to the power of excitability. As in other sensations, so also in visual sensations, a stimulus may be too feeble to produce a sensation. If it be increased in amount sufficiently it begins to produce an effect which is increased on the increase of the stimulation; this increase in the effect is not directly proportional to the increase in the excitation, but, according to Fechner's law, "as the logarithm of the stimulus," i.e., in each sensation, there is a constant ratio between the increase in the stimulus and the increase in the sensation, this constant ratio for each sensation expresses the least perceptible increase in the sensation or minimal increment of excitation. This law, which is true only within certain limits, may be best understood by an example. When the retina has been stimulated 686 THE SENSES. [chap. xx. by the light of one candle, the light of two candles will produce a difference in sensation which can be distinctly felt. If, however, the first stimulus had been that of an electric light, the addition of the light of a candle would make no difference in the sensation. So, generally, for an additional stimulus to be felt, it may be proportionately small if the original stimulus have been small, and must be greater if the original stimulus have been great. The stimulus increases as the ordinary numbers, while the sensation increases as the logarithm. The Ophthalmoscope.- Part of the light which enters the eye is absorbed, and produces some change in the retina, of which we shall treat further on; the rest is reflected. Every one is perfectly familiar with the fact, that it is quite impossible to see the fundus or back of another person's eye by simply looking into it. The interior of the eye forms a perfectly black background to the pupil. The same remark applies to an ordinary photographic camera, and may be illustrated by the difficulty we experience in seeing into a room from the street through the window7, unless the room be lighted within. In the case of the eye this fact is partly due to the feebleness of the light reflected from the retina, most of it being absorbed by the choroid, as mentioned above; but far more to the fact that every such ray is reflected straight back to the source of light (e.g., candle), and cannot, therefore, be seen by the unaided eye without intercepting the incident light from the candle, as well as the reflected rays from the retina. This difficulty is surmounted by the use of the vphthal/nioscope. The ophthalmoscope, brought into use by Helmholtz, consists in its simplest form of a, a slightly concave mirror of metal or silvered glass perforated in the centre, and fixed into a handle; and b, a biconvex lens of about 2J-3 inches focal length. Two methods of examining the eye with this instrument are in common use-the direct and the indirect: both methods of investigation should be employed. A normal eye should be examined , a drop of a solution of atropia (two grains to the ounce) or of hom-atropia hydrobromate, should be instilled about twenty minutes before ie examination is commenced ; the ciliary muscle is thereby paralysed, the power of accommodation is abolished, and the pupil is dilated. This will nm eria 5 acilitate the examination ; but it is quite possible to observe all the details to be presently described without the use of this drug. The 1JClng, nCJ''' darkened, the observer seats himself in front of the person w ose eye e is about to examine, placing himself upon a somewhat higher ' rulliant aucl steady light is placed close to the left ear of the pa ion . e atropia having been put into the right eye only of the patient, CHAP. XX.] THE OPHTHALMOSCOPE. 687 this eye is examined. Taking the mirror in his right hand, and looking through the central hole, the operator directs a beam of light into the eye of the patient. A red glare, known as the reflex, is seen ; it is due to the illumination of the retina. The patient is then told to look at the little finger of the observer's right hand as he holds the mirror ; to effect this the eye is rotated somewhat inwards, and at the same time the reflex changes from red to a lighter colour, owing to the reflection from the optic disc. The ob- server now approximates the mirror, and with it his eye to the eye of the patient, taking care to keep the light fixed upon the pupil, so as not to lose the reflex. At a certain point, which varies with different eyes, but is usually when there is an in- terval of about two or three inches between the observed and the observing eye, the vessels of the retina will become visible as lines running in different directions. Distinguish the smaller and brighter red arteries from the larger and darker coloured veins. Examine carefully the fundus of the eye, i.e. the red surface- until the optic disc is seen ; trace its cir- cular outline, and observe the small cen- tral white spot, the porus opticus, or physiological pit: near the centre is the central artery of the retina breaking up upon the disc into branches ; veins also are present, and correspond roughly to the course of the arteries. Trace the vessels over the disc on to the retina. The optic disc is bounded by two delicate rings, the more external being the choroidal, whilst the more internal is the sclerotic opening. Somewhat to the outer side, and only visible after some practice, is the yellow spot, with the small lighter-coloured fovea centralis in its centre. This constitutes the direct method of examination; by it the various details of the fundus are seen as they really exist, and it is this method which should be adopted for ordinary use. If the observer is ametropic, i.e., is myopic or hypermetropic, he will be unable to employ the direct method of examination until he has remedied his defective vision by the use of proper glasses. In the indirect method the patient is placed as before, and the operator holds the mirror in his right hand at a distance of twelve to eighteen inches from the patient's right eye. At the same time he rests his little finger lightly upon the temple, and holding the lens between his thumb and fore- finger, two or three inches in front of the patient's eye, directs the light through the lens into the eye. The red reflex, and subsequently the white one, having been gainei, the operator slowly moves his mirror, and with it Fig. 410. - The ophthalmoscope. The small upper mirror is for direct, the larger for indirect illumination. 688 THE SENSES. [CHAI*. XX. his eye, towards or away from the face of the patient, until the outline of one of the retinal vessels becomes visible, when very slight movements on the part of the operator will suffice to bring into view the details of the fundus above described, but the image will be an inverted one. The lens should be kept fixed at a distance of two or three inches, the mirror being alone moved until the disc becomes visible : should the image of the mirror, however, obscure the disc, the lens may be slightly tilted. Visual Purple.-The method by which a ray of light is able to stimulate the endings of the optic nerve in the retina in such a manner that a visual sensation is perceived by the cerebrum is not yet understood. It is supposed that the change effected by the agency of the light which falls upon the retina is in fact a chemical alteration in the protoplasm, and that this change stimulates the optic nerve-endings. The discovery of a certain temporary reddish-purple pigmentation of the outer limbs of the retinal rods in certain animals (e.g., frogs) which have been killed in the dark, forming the so-called visual purple, appeared likely to offer some explanation of the matter, especially as it was also found that the pigmentation disappeared when the retina was exposed to light, and re-appeared when the light was removed, and also that it underwent distinct changes of colour when other than white light was used. The visual purple cannot however be absolutely essential to the due production of visual sensations, as it is absent from the retinal cones, and from the macula lutea and fovea centralis of the human retina, and does not appear to exist at all in the retinas of some animals, e.g., bat, dove, and hen, which are, nevertheless, possessed of good vision. If the operation be performed quickly enough, the image of an object may be fixed in the pigment on the retina by soaking the retina of an animal, which has been killed in the dark, in alum solution. Electrical Currents.-According to the careful researches of Dewar and McKendrick, and of Holmgren, it appears that the stimulus of light is able to produce a variation of the natural electrical current of the retina. The current is at first increased and then diminished. McKendrick believes that this is the electrical expression of those chemical changes in the retina of which we have already spoken. Visual Perceptions and Judgments. Reversion of the Image.-The direction given to the rays by their refraction is regulated by that of the central ray, or axis of the cone, towards which the rays are bent. The image of any point of an object is, therefore, as a rule (the exceptions to which chap, xx.] VISUAL PERCEPTIONS. 689 need not here be stated), always formed in a line identical with the axis of the cone of light, as in the line of b a, or a 6 (fig. 411), so that the spot where the image of any point will be formed upon the retina may be determined by prolonging the central ray of the cone of light, or that ray which traverses the centre of the pupil. Thus Ab is the axis or central ray of the cone of light issuing from a ; b a the central ray of the cone of light issuing from b ; the image of a is formed at b, the image of b at a, in the inverted position ; therefore what in the object was above is in the image below, and vice versd,-the right-hand part of the object is in the image to the left, the left-hand to the right. If an opening A B Fig. 411.-Diagram of the formation of the image on the retina. be made in an eye at its superior surface, so that the retina can be seen through the vitreous humour, this reversed image of any bright object, such as the windows of the room, may be perceived at the bottom of the eye. Or still better, if the eye of any albino animal, such as a white rabbit, in which the coats, from the absence of pigment, are transparent, is dissected clean, and held with the cornea towards the window, a very distinct image of the window completely inverted is seen depicted on the posterior translucent wall of the eye. Volkmann has also shown that a similar experiment may be successfully performed in a living person possessed of large prominent eyes, and an unusually trans- parent sclerotic. An image formed at any point on the retina is referred to a point outside the eye, lying on a straight line drawn from the point on the retina outwards through the centre of the pupil. Idins an image on the left side of the retina is referred by the mind to an object on the right side of the eye, and vice versd. Thus all images on the retina are mentally, as it were, projected in front of the eye, and the objects are seen erect though the image on the retina is reversed. Much needless confusion and difficulty have 690 THE SENSES. [chap. xx. been raised on this subject for want of remembering that when we are said to see an object, the mind is merely conscious of the picture on the retina, and when it refers it to the external object, or "projects" it outside the eye, it necessarily reverses it and secs the object as erect, though the retinal image is inverted. This is further corroborated by the sense of touch. Thus an object whose picture falls on the left half of the retina is reached by the right hand, and hence is said to lie to the right. Or, again, an object whose image is formed on the upper part of the retina is readily touched by the feet, and is therefore said to be in the leaver part of the field, and so on. Hence it is, also, that no discordance arises between the sensa- tions of inverted vision and those of touch, which perceives every- thing in its erect position; for the images of all objects, even of our own limbs, in the retina, are equally inverted, and therefore maintain the same relative position. Even the image of our hand, while used in touch, is seen in- verted. The position in which we see objects, we call, therefore, the erect position. A mere lateral inversion of our body in a mirror, where the right hand occupies the left of the image, is indeed scarcely remarked : and there is but little discordance between the sensations acquired by touch in regulating our move- ments by the image in the mirror, and those of sight, as, for example, in tying a knot in the cravat. There is some want of harmony here, on account of the inversion being only lateral, and not complete in all directions. The perception of the erect position of objects appears, there- fore, to be the result of an act of the mind. And this leads us to a consideration of the several other properties of the retina, and of the co-operation of the mind in the several other parts of the act of vision. To these belong not merely the act of sensation itself and the perception of the changes produced in the retina, as light and colours, but also the conversion of the mere images depicted in the retina into ideas of an extended field of vision, of proximity and distance, of the form and size of objects, of the reciprocal influence of different parts of the retina upon each other, the simultaneous action of the two eyes, and some other phenomena. Field, of Vision.-The actual size of the field of vision depends on the extent of the retina, for only so many images can be seen at any one time as can occupy the retina at the same time; and thus chap. XX.] FIELD OF VISION. 691 considered, the retina, the conditions of which are perceived by the brain, is itself the field of vision. But to the mind of the individual the size of the field of vision has no determinate limits; sometimes it appears very small, at another time very large; for the mind has the power of projecting images on the retina towards the exterior. Hence the mental field of vision is very small when the sphere of the action of the mind is limited to impediments near the eye : on the contrary, it is very extensive when the projection of the images on the retina towards the exterior, by the influence of the mind, is not impeded. It is very small when we look into a hollow body of small capacity held before the eyes; large when Fig. 412.-Diagram of the optical angle. we look out upon the landscape through a small opening ; more extensive when we look at the landscape through a window ■ and most so when our view is not confined by any near object. In all these cases the idea which we receive of the size of the field of vision is very different, although its absolute size is in all the same, being dependent on the extent of the retina. Hence it follows, that the mind is constantly co-operating in the acts of vision, so that at last it becomes difficult to say what belongs to mere sensation, and what to the influence of the mind. By a mental operation of this kind, we obtain a correct idea of the size of individual objects, as well as of the extent of the field of vision. To illustrate this, it will be well to refer to fig. 412. The angle x, included between the decussating central rays of two cones of light issuing from different points of an object, is called the optical angle-angulus opticus seu visorius. This angle becomes larger, the greater the distance between the points a and B; and since the angles x and y are equal, the distance between the points a and b in the image on the retina increases as the angle becomes larger. Objects at different distances from the eye, but having the same optical angle x-for example, the objects, e, 692 THE SENSES. [chap. xx. d, and e,-must also throw images of equal size upon the retina; and, if they occupy the same angle of the field of vision, their image must occupy the same spot in the retina. Nevertheless, these images appear to the mind to be of very unequal size when the ideas of distance and proximity come into play; for, from the image a b, the mind forms the conception of a visual space extending to e, d, or c, and of an object of the size which that represented by the image on the retina appears to have when vi( wed close to the eye, or under the most usual circumstances. Estimation of Size.-Our estimate of the size of various objects is based partly on the visual angle under which they are seen, but much more on the estimate we form of their distance. Thus a lofty mountain many miles off may be seen under the same visual angle as a small hill near at hand, but we infer that the former is much the larger object because we know it is much further off than the hill. Our estimate of distance is often erroneous, and consequently the estimate of size also. Thus persons seen walking on the top of a small hill against a clear twylight sky appear unusually large, because we over-estimate their distance, and for similar reasons most objects in a fog appear immensely magnified. The same mental process gives rise to the idea of depth in the field of vision; this idea being fixed in our mind principally by the circumstance that, as we ourselves move forwards, different images in succession become depicted on our retina, so that we seem to pass between these images, which to the mind is the same thing as passing between the objects themselves. The action of the sense of vision in relation to external objects is, therefore, quite different from that of the sense of touch. The objects of the latter sense are immediately present to it; and our own body, with which they come into contact, is the measure of theii size. 1 he part of a table touched by the hand appears as huge as the part of the hand receiving an impression from it, for a part of our body in which a sensation is excited, is here the measure by which we judge of the magnitude of the object. In the sense of vision, on the contrary, the images of objects are mere fiactions of the objects themselves realised upon the retina, the extent of which remains constantly the same. But the imagina- tion, which analyses the sensations of vision, invests the images of objects, together with the whole field of vision in the retina, with CHAP. XX.] PERCEPTION AND DIRECTION OF FORM. 693 very varying dimensions; the relative size of the image in proportion to the whole field of vision, or of the affected parts of the retina to the whole retina, alone remaining unaltered. Estimation of Direction.-The direction in which an object is seen, depends on the part of the retina which receives the image, and on the distance of this part from, and its relation to, the central point of the retina. Thus, objects of which the images fall upon the same parts of the retina lie in the same visual direction; and when, by the action of the mind, the images or affections of the retina are projected into the exterior world, the relation of the images to each other remains the same. Estimation of Form.-The estimation of the form of bodies by sight is the result partly of the mere sensation, and partly of the association of ideas. Since the form of the images perceived by the retina depends wholly on the outline of the part of the retina affected, the sensation alone is adequate to the distinction of only superficial forms of each other, as of a square from a circle. But the idea of a solid body as a sphere, or a body of three or more dimensions, e.g., a cube, can only be attained by the action of the mind constructing it from the different superficial images seen in different positions of the eye with regard to the object, and, as shown by Wheatstone and illustrated in the stereoscope, from two different perspective projections of the body being presented simultaneously to the mind by the two eyes. Hence, when, in adult age, sight is suddenly restored to persons blind from infancy, all objects in the field of vision appear at first as if painted flat on one surface; and no idea of solidity is formed until after long exercise of the sense of vision combined with that of touch. The clearness with which an object is perceived irrespective of accommodation, would appear to depend largely on the number of rods and cones which its retinal image covers. Hence the nearer an object is to the eye (within moderate limits) the more clearly are all its details seen. Moreover, if we want carefully to examine any object, we always direct the eyes straight to it, so that its image shall fall on the yellow spot where an image of a given area will cover a larger number of cones than anywhere else in the retina. It has been found that the images of two points must be at least 12 *00 in. apart on the yellow spot in order to be distinguished separately; if the images are nearer together, the 694 THE SENSES. [chap. xx. points appear as one. The diameter of each cone in this part of the retina is about 12 in- Estimation of Movement.-We judge of the motion of an object, partly from the motion of its image over the surface of the retina, and partly from the motion of our eyes following it. If the image upon the retina moves while our eyes and our body are at rest, we conclude that the object is changing its relative position with regard to ourselves. In such a case the movement of the object may be apparent only, as when we are standing upon a body which is in motion, such as a ship. If, on the other hand, the image does not move with regard to the retina, but remains fixed upon the same spot of that membrane, while our eyes follow the moving body, we judge of the motion of the object by the sensa- tion of the muscles in action to move the eye. If the image moves over the surface of the retina while the muscles of the eye are acting at the same time in a manner corresponding to this motion, as in reading, we infer that the object is stationary, and we know that we are merely altering the relations of our eyes to the object. Sometimes the object appears to move when both object and eye are fixed, as in vertigo. The mind can, by the faculty of attention, concentrate its activity more or less exclusively upon the sense of sight, hearing, and touch alternately. When exclusively occupied with the action of one sense, it is scarcely conscious of the sensations of the others. The mind, when deeply immersed in contemplations of another nature, is indifterent to the actions of the sense of sight, as of every other sense. We often, when deep in thought, have our eyes open and fixed, but see nothing, because of the stimulus of ordinary light being unable to excite the brain to perception, when otherwise engaged. The attention which is thus necessary for vision, is necessary also to analyse what the field of vision presents. The mind does not perceive all the objects presented by the field of vision at the same time with equal acuteness, but directs itself first to one and then to another. The sensation becomes more intense, according as the particular object is at the time the prin- cipal object of mental contemplation. Any compound mathe- matical figure produces a different impression according as the attention is directed exclusively to one or the other part of it. Thus in fig. 413, we may in succession have a vivid perception of the whole, or of distinct parts only; of the six triangles near the CHAP. XX.] COLOUR SENSATIONS. 695 outer circle, of the hexagon in the middle, or of the three large triangles. The more numerous and varied the parts of which a figure is composed the more scope does it afford for the play of the attention. Hence it is that architectural ornaments have an enlivening effect on the sense of vision, since they afford constantly fresh subject for the action of the mind. Colour Sensations.-If a ray of sunlight be allowed to pass through a prism, it is decomposed by its passage into rays of different colours, which are called the colours of the spectrum; they are red, orange, yellow, green, blue, indigo, and violet. The red rays are the least turned out of their course by the prism, and the violet the most, whilst the other colours occupy in order places between these two extremes. The differences in the colour of the rays, depend upon the number of vibrations producing each, the red rays being the least rapid and the violet the most. In addition to the coloured rays of the spectrum, there arc others which are invisible, but which have definite properties, those to the left of the red, and less refrangible, being the calorific rays which act upon the thermometer, and those to the right of the violet which are called the actinic or chemical rays, which have a powerful chemical action. The rays which can be perceived by the brain, i.e., the coloured rays, must stimulate the retina in some special manner in order that coloured vision may result, and two chief explanations of the method of stimulation have been suggested. (i.) The one, originated by Young and elaborated by Helmholtz, holds that there are three primary colours, viz., red, green, and violet, and that in the retina are contained rods or cones A\hich answer to each of these primary colours, whereas the innumerable intermediate shades of colour are produced by stimulation of the three primary and colour terminals in different degrees, the sen- sation of white is produced at the same time when the three elements arc equally excited. Thus if the retina be stimulated by rays of certain wave length, at the red end of the spectrum, the terminals of the other colours, green and violet, are hardly stimulated at all, but the red terminals are strongly stimulated, the resulting sensation being red. The orange rays excite the red terminals considerably, the green rather more, and the violet slightly, the resulting sensation being that of orange, and so on. Fig- 4i3- 696 THE SENSES. [chap. xx. (2.) The second theory of colour (Hering's) supposes that there are six primary colour sensations, of three pair of antagonistic or complemental colours, black and white, red and green, and yellow and blue, and that these are produced by the changes either of disintegration or of assimilation taking place in certain substances, somewhat it may be supposed of the nature of the visual purple, which (the theory supposes to) exist in the retina. Each of the substances corresponding to a pair of colours, being capable of undergoing two changes, one of construction and the other of disintegration, with the result of producing one or other colour. Fig. 414.-Diagram of the three primary colour sensations. (Young-Helmholtz theory.) 1, is the red ; 2, green, and 3 violet, primary colour sensations. The lettering indicates the colours of the spectrum. The diagram indicates by the height of the curve to what extent the several primary sensations of colour are excited bv vibrations of different wave lengths. lor instance, in the white-black substance, when disintegration is in excess of construction or assimilation, the sensation is white, and when assimilation is in excess of disintegration the reverse is the case ; and similarly with the red-green substance, and with the yellow-blue substance. When the repair and disintegration are equal with the first substance, the visual sensation is grey; but in the other pairs when this is the case, no sensation occurs. The rays of the spectrum to the left produce changes in the red-green substance only, with a resulting sensation of red, whilst the (orange) rays further to the right affect both the red -green and the yellow-blue substances ; blue rays cause constructive changes in the yellow-blue substance, but none in the red- green, and so on. These changes produced in the visual sub- stances in the retina are perceived by the brain as sensations of colour. 1 he spectra left by the images or white or luminous objects, are chap, xx.] COLOUR BLINDNESS. 697 ordinarily white or luminous; those left by dark objects arc dark. Sometimes, however, the relation of the light and dark parts in the image may, under certain circumstances, be reversed in the spectrum ; what was bright may be dark, and what was dark may appear light. This occurs whenever the eye, which is the seat of the spectrum of a luminous object, is not closed, but fixed upon another bright or white surface, as a white wall, or a sheet of wdiite paper. Hence the spectrum of the sun, which, while light is excluded from the eye is luminous, appears black or grey when the eye is directed upon a white surface. The explanation of this is, that the part of the retina which has received the luminous image remains for a certain period afterwards in an exhausted or less sensitive state, while that which has received a dark image is in an unexhausted, and therefore much more excitable condition. The ocular spectra which remain after the impression of coloured objects upon the retina are always coloured ; and their colour is not that of the object, or of the image produced directly by the object, but the opposite, or complemented colour. The spectrum of a red object is, therefore, green ; that of a green object, red; that of violet, yellow'; that of yellow, violet, and so on. The reason of this is obvious. The part of the retina which receives, say, a red image, is w earied by that particular colour, but remains sensi- tive to the other rays which w ith red make up white light; and, therefore, these by themselves reflected from a white object produce a green hue. If, on the other hand, the first object looked at be green, the retina being tired of green rays, receives a red image when the eye is turned to a white object. And so with the other colours ; the retina while fatigued by yellow' rays will suppose an object to be violet, and vice versd; the size and shape of the spectrum corresponding with the size and shape of the original object looked at. The colours which thus reciprocally excite each other in the retina are those placed at opposite points of the circle in fig. 415. The peripheral parts of the retina have no perception of red. The area of the retina which is capable of receiving impressions of coloui' is slightly different for each colour. Colour Blindness or Daltonism.-Daltonism or colour-blind- ness is a by no means uncommon visual defect. One of the com- monest forms is the inability to distinguish between red and green. The simplest explanation of such a condition is, that the elements 698 THE SENSES. [chap. xx. of the retina which receive the impression of red, etc., are absent, or very imperfectly developed, or, according to the other theory, that the red-green substance is absent from the retina. Other red orange. violet yellow' 'Llue green lij?" 4I5- -Diagram of the various simple and compound colours of light, and those which are complemental of each other, i.e., which, when mixed, produce a neutral grey tint. The three simple colours, red, yellow, and blue, are placed at the angles of an equilateral triangle; which are connected together by means of a circle; the mixed colours, green, orange, and violet, are placed intermediate between the corresponding simple or homo- geneous colours; and the complemental colours, of which the pigments, when mixed, would constitute a grey, and of which the prismatic spectra would together produce a white light, will be found to be placed in each case opposite to each other, but con- nected by a line passing through the centre of the circle. The figure is also useful in showing the further shades of colour which are complementary of each other. If the circle be supposed to contain every transition of colour between the six marked down, those which, when united, yield a white or grey colour, will always be found directly opposite to each other ; thus, for example, the intermediate tint between orange and red is complemental of the middle tint between green and blue. varieties of colour blindness in which the other colour-perceiving elements are absent have been shown to exist occasionally. Of the Reciprocal Action of Different Parts of the Retina on each other. Although each elementary part of the retina represents a distinct portion of the field of vision, yet the different elementary parts, or sensitive points of that membrane, have a certain influence on each other; the particular condition of one influencing that of another, so that the image perceived by one part is modified by the image depicted in the other. The phenomena which result fiom this relation between the different parts of the retina, may be arranged in two classes : the one including those where the condition existing in the greater extent of the retina is imparted to the remainder of that membrane ; the other, consisting of those in which the condition of the larger portion of the retina excites, in the less extensive portion, the opposite condition. chap, xx.] . CONTRASTS, 699 1. \\ lien two opposite impressions occur in contiguous parts of an image on the retina, the one impression is, under certain circumstances, modified by the other. If the impressions occupy each one-half of the image, this does not take place ; for in that case, their actions are equally balanced. But if one of the impres- sions occupies only a small part of the retina, and the other the greater part of its surface, the latter may, if long continued, extend its influence over the whole retina, so that the opposite less exten- sive impression is no longer perceived, and its place becomes occupied by the same sensation as the rest of the field of vision. Thus, if we fix the eye for some time upon a strip of coloured paper lying upon a white surface, the image of the coloured object, especially when it falls on the lateral parts of the retina, will gradually disappear, and the white surface be seen in its place. 2. In the second class of phenomena, the affection of one part of the retina influences that of another part, not in such a manner as to obliterate it, but so as to cause it to become the contrast or opposite of itself. Thus a grey spot upon a 'white ground appears darker than the same tint of grey would do if it alone occupied the whole field of vision, and a shadow is always rendered deeper when the light which gives rise to it becomes more intense, owing to the greater contrast. The former phenomena ensue gradually, and only after the images have been long fixed on the retina; the latter are instan- taneous in their production, and are permanent. In the same way, also, colours may be produced by contrast. Thus, a very small dull grey strip of paper, lying upon an extensive surface of any bright colour, does not appear grey, but has a faint tint of the colour which is the complement of that of the surround- ing surface. A strip of grey paper upon a green field, for example, often appears to have a tint of red, and when lying upon a red surface, a greenish tint; it has an orange-coloured tint upon a bright blue surface, and a bluish tint upon an orange-coloured surface; a yellowish colour upon a bright violet, and a violet tint upon a bright yellow surface. The colour excited thus, as a con- trast to the exciting colour, being wholly independent of any rays of the corresponding colour acting from without upon the retina, must arise as an opposite or antagonistic condition of that mem- brane ; and the opposite conditions of which the retina thus be- comes the subject would seem to balance each other by their reciprocal reaction. A necessary condition for the production of 700 THE SENSES. [chap. xx. the contrasted colours is, that the part of the retina in which the new colour is to be excited, shall be in a state of comparative repose ; hence the small object itself must be grey. A second Fig. 416. Diagram of the axes of rotation to the eye. The thin lines indicate axes of rota- tion, the thick the position of muscular attachment. condition is, that the colour of the surrounding surface shall be very bright, that is, it shall contain much white light. Movements of the Eye.-The eyeball possesses movement around three axes indicated in fig. 416, viz., an antero-posterior, a vertical, and a transverse, passing through a centre of rotation a little behind the centre of the optic axis. The movements arc accomplished by pairs of muscles. Direction of Movement. Inwards .... Outwards By what muscles accomplished. • Internal rectus. . External rectus. Upwards . 1 Superior rectus. 1 Inferior oblique. Downwards . ( Inferior rectus. 1 Superior oblique. Inwards and upwards i Internal and superior rectus. (Inferior oblique. CHAP. XX.J DIPLOPIA. 701 Direction of Movement. ] By what muscles accomplished. Inwards and downwards . . . j Internal and inferior rectus. I Superior oblique. Outwards and upwards . . . ] i External and superior rectus. ' Inferior oblique. Outwards and downwards . . . ( External and inferior rectus. ( Superior oblique. Of the Simultaneous Action of the two Eyes. Although the sense of sight is exercised by two organs, yet the impression of an object conveyed to the mind is single. Various theories have been advanced to account for this phenomenon. By Gall it was supposed that we do not really employ both eyes simultaneously in vision, but always see with only one at a time. This especial employment of one eye in vision certainly occurs in persons whose eyes are of very unequal focal distance, but in the majority of individuals both eyes are simultaneously in action, in the perception of the same object; this is shown by the double images seen under certain conditions. If two fingers be held up before the eyes, one in front of the other, and vision be directed to the more distant, so that it is seen singly, the nearer will appear double; while, if the nearer one be regarded, the most distant will be seen double ; and one of the double images in each case will be found to belong to one eye, the other to the other eye. Diplopia.-Single vision results only when certain parts of the two retinae are affected simul- taneously ; if different parts of the retina? receive the image of the object, it is seen double. This may be readily illustrated as follows :-The eyes are fixed upon some near object, and one of them is pressed by the thumb so as to be turned slightly in or out; two images of the object (Diplopia or Double Vision) are at once perceived, just as is frequently the case in persons who squint. This diplopia is due to the fact that the images of the object do not fall on corresponding points in the two retinae. The parts of the retina? in the two eyes which thus correspond Fig. 417. 702 THE SENSES. [chap. xx. to each other in the property of referring the images which affect them simultaneously to the same spot in the field of vision, are, in man, just those parts which would correspond to each other, if one retina were placed exactly in front of, and over the other (as in fig. 417, c). Thus, the outer lateral portion of one eye corresponds to, or, to use a better term, is identi- cal with the inner portion of the other eye; or a of the eye a (fig. 418), with a' of the eye b. The upper part of one retina is also identical with the upper part of the other ; and the lower parts of the two eyes are identical with each other. 1 his is proved by a simple experiment. Pressure upon any part of the ball of the eye, so as to affect the retina, produces a luminous circle, seen at the opposite side of the field of vision to that on which the pressure is made. If, now, in a dark room, we press with the finger at the upper part of one eye, and at the lower part of the other, two luminous circles are seen, one above the other: so, also, two figures are seen when pressure is made simultaneously on the two outer or the two inner sides of both eyes. , It is certain, therefore, that neither the upper part of one retina and the lower part e ot icr are identical, nor the outer lateral parts of the Fig- 418. Fig. 419. CHAP. XX.] CORRESPONDING PARTS OF RETINA. 703 two retinae, nor their inner lateral portions. But if pressure be made with the fingers upon both eyes simultaneously at their lower part, one luminous ring is seen at the middle of the upper part of the field of vision ; if the pressure be applied to the upper part of both eyes a single luminous circle is seen in the middle of the field of vision below. So, also, if we press upon the outer side a of the eye a, and upon the inner side a' of the eye b, a single spectrum is produced, and is apparent at the extreme right of the field of vision; if upon the point b of one eye, and the point b' of the other, a single spectrum is seen to the extreme left. The spheres of the two retinae may, therefore, be regarded as lying one over the other, as in c, fig. 417; so that the left portion of one eye lies over the identical left portion of the other eye, the right portion of one eye over the identical right portion of the other eye; and with the upper and lower portions of the two eyes, a lies over a', b over b', and c over c'. The points of the one retina intermediate between a and c are again identical with the corresponding points of the other retina between a' and c'; those between b and c of the one retina, with those between o' and c' of the other. If the axes of the eyes, a and b (fig. 419), be so directed that they meet at a, an object at a will be seen singly, for the point a of the one retina, and a' of the other are identical. So, also, if the object 3 be so situated that its image falls in both eyes at the same distance from the central point of the retina,- namely, at b in the one eye, and at b' in the other,-/3 will be seen single, for it affects identical parts of the two retina). The same will apply to the object y. In quadrupeds, the relation between the identical and non-identical parts of the retina cannot be the same as in man ; for the axes of their eyes gene- rally diverge, and can never be made to meet in one point of an object. When an animal regards an object situated directly in front of it, the image of the object must fall, in both eyes, on the outer portion of the retina). Thus, the image of the object a (fig. 419) will fall at a,' in one, and at a" in the other : and these points a' and a" must be identical So, also, for distinct and single vision of objects, b or c, the points V and b" or c' e", in the two retinas, on which the images of these objects fall, must be identical. All points of the retina in each eye which receive rays of light from lateral objects only, can have no corresponding identical points in the retina of the other eye ; for otherwise two objects, one situated to the right Fig. 420. 704 THE SENSES. [chap. xx. and the other to the left, would appear to lie in the same spot of the field of vision. It is probable, therefore, that there are in the eyes of animals, parts of the retinas which are identical, and parts which are not identical, i.e., parts in one which have no corresponding parts in the other eye. And the relation of the two retinae to each other in the field of vision may be repre- sented as in fig. 420. Binocular Vision.-The cause of the impressions on the identical points of the two retinas giving rise to but one sensation, and the perception of a single image, must either lie in the structural organization of the deeper or cerebral portion of the A B 0 rig. 421. visual apparatus, or be the result of a mental operation; for in no other case is it the property of the corresponding nerves of the two sides of the body to refer their sensations as one to one spot. Many attempts have been made to explain this remarkable relation between the eyes, by referring it to anatomical relation between the optic nerves. The circumstance of the inner portion of the fibres of the two optic nerves decussating at the commis- sure, and passing to the eye of the opposite side, while the outer portion of the fibres continue their course to the eye of the same side, so that the left side of both retinae is formed from one root of the nerves, and the right side of both retinae from the outer root, naturally led to an attempt to explain the phenomenon by this distribution of the fibres of the nerves. And this explanation is favoured by cases in which the entire of one side of the retina, as far as the central point in both eyes, sometimes be- comes insensible. But Muller shows the inadequateness of this theory to explain the phenomenon, unless it be supposed that chap, xx.] JUDGMENT OF SOLIDITY. 705 each fibre in each cerebral portion of the optic nerves divides in the optic commissure into two branches for the identical points of the two retinae, as is shown in A, fig. 421. But there is no foun- dation for such supposition. By another theory it is assumed that each optic nerve contains exactly the same number of fibres as the other, and that the corresponding fibres of the two nerves are united in the Sen- sorium (as in fig. 421, B). But in this theory no account is taken of the partial decussation of the fibres of the nerves in the optic commissure. According to a third theory, the fibres a and a', fig. 421, C, coming from identical points of the two retinae, are in the optic commissure brought into one optic nerve, and in the brain either are united by a loop, or spring from the same point. The same disposition prevails in the case of the identical fibres b and b'. According to this theory, the left half of each retina would be represented in the left hemisphere of the brain, and the right half of each retina in the right hemisphere. Another explanation is founded on the fact, that at the anterior part of the commissure of the optic nerve, certain fibres pass across from the distal portion of one nerve to the corresponding portion of the other nerves, as if they were commissural fibres forming a connection between the retinge of the two eyes. It is supposed, indeed, that these fibres may connect the corresponding parts of the two retinae, and may thus explain their unity of action ; in the same way that corresponding parts of the cerebral hemispheres are believed to be connected together by the commis- sural fibres of the corpus callosum, and so enabled to exercise unity ■of function. Judgment of Solidity.-On the whole, it is probable, that the power of forming a single idea of an object from a double impression conveyed by it to the eyes is the result of a mental act. This view is supported by the same facts as those employed by Wheatstone to show that this power is subservient to the purpose of obtaining a right perception of bodies raised in relief. When an object is placed so near the eyes that to view it the optic axes must converge, a different perspective projection of it is seen by each eye, these perspectives being more dissimilar as the ■convergence of the optic axes becomes greater. Thus, if any figure of three dimensions, an outline cube, for example, be held at a moderate distance before the eyes, and viewed with each 706 THE SENSES. [chap. xx. eye successively while the head is kept perfectly steady, a (fig. 420) will be the picture presented to the right eye, and b that seen by the left eye. Wheatstone has shown that on this circumstance depends in a great measure our conviction of the solidity of an object, or of its projection in relief. If different perspective drawings of a solid body, one representing the image seen by the right eye, the other that seen by the left (for A B Fig. 422. example, the drawing of a cube, a, b, fig. 422 be presented to corresponding parts of the two retinae, as may be readily done by means of the stereoscope, the mind will perceive not merely a single representation of the object, but a body projecting in relief, the exact counterpart of that from which the drawings were made. By transposing two stereoscopic pictures a reverse effect is produced; the elevated parts appear to be depressed, and vice versa. An instrument contrived with this purpose is termed a pseudoscope. A iewed with this instrument a bust appears as a hollow mask, and as may readily be imagined the effect is most bewildering. CHAP. XXI.] THE SYMPATHETIC SYSTEM. 707 CHAPTER XXI. Having in the preceding chapters completed the description of the Cerebro-spinal nervous system, there remains to be considered the structure and functions of the so-called Sympathetic nervous system, and to this it is now necessary to direct attention. It should, however, be borne in mind that the cerebro-spinal and sympathetic systems are so very intimately connected that the separation of the one from the other may be considered to be purely for the sake of convenience. Distribution.-The various ganglia and nerves of which the sympathetic system is generally said to consist have been already enumerated. Gaskell's researches have suggested a convenient classification of the former into : (i.) The main sympathetic chain, extending from above downwards, in the form of connected ganglia lying upon the bodies of the vertebra?, which may be called, lateral or vertebral ganglia. (2.) A more or less distinct chain, prsevertebral in position, consisting of the semi-lunar, inferior mesenteric and similar plexuses, which may be called, collateral ganglia. (3.) Ganglia situated in the organs and tissues them- selves, called terminal ganglia. (4.) The ganglia of the posterior roots of the spinal nerves. The connection between these parts is as follows: the visceral branch or ramus communicans of each spinal nerve, which is one of the divisions of a typical spinal nerve-the others being the dorsal and ventral-passes first of all into the lateral chain ; from this chain branches, rami efferentes, pass into the collateral ganglia, and from these again other branches pass off into the organs to end in the terminal ganglia. In the thoracic region the rami communi- cantes are composed of two parts, white and grey. The former can be traced backwards into both spinal nerve roots of their correspond- ing spinal nerve ; and in the other direction partly into the lateral sympathetic chain, and partly into the great splanchnic nerves and so into the collateral ganglia without entering the lateral chain at all. The upper white rami (from the 2nd to 5th), however, proceed upwards and join the superior cervical ganglion, instead of passing THE SYMPATHETIC NERVOUS SYSTEM, Fig, 423.-Diagrammatic view of the Sympathetic cord of the right side, showing its connections with the principal cei ebro- spinal nerves and the main prreaortic plexuses. J. (From Quain's Anatomy.) Cerelro-spinal nerves.-VI, a portion of the sixth cranial as it passes through the cavernous sinus, receiving two twigs from the carotid plexus of the sympathetic nerve ; O, ophthalmic ganglion connected by a twig with the carotid plexus ; M, connection of the spheno-palatine gang- lion by the Vidian nerve with the carotid plexus ; C, cervical plexus ; Br, brachial plexus; I) 6, sixth intercostal nerve ; D12, twelfth; L 3, third lumbar nerve; S first sacral nerve ; S 3, third; S 5, fifth; Cr, anterior crural nerve ; Cr', great scia- tic ; pn, vagus in the lower part of the neck; »■, recurrent nerve winding round the subclavian artery. Sympathetic Cord.-c, superior cervical ganglion; c', second or middle ; c", in- ferior : from each of these ganglia cardiac nerves (all deep on this side) are seen descending to the cardiac plexus; d 1, placed immediately below the first dorsal sympathetic ganglion; d 6, is opposite the sixth ; I 1, first lumbar ganglion ; c g, the terminal or coccygeal ganglion. Prceaortic and Visceral Plexuses.-p p, pharyngeal, and, lower down, laryngeal plexus ; pl, post, pulmonary plexus spreading from the vagus on the back of the right bronchus ; c a, on the aorta, the cardiac plexus, towards w'hich, in addition to the cardiac nerves from the three cervi- cal sympathetic ganglia, other branches are seen descending from the vagus and recurrent nbrves; co, right or posterior and co', left or ant. coronary plexus; o, oesophageal plexus in long meshes on the gullet; sp, great splanchnic nen e formed by branches from the fifth, sixth, seventh, eighth, and ninth dorsal ganglia; + , small splanchnic from the ninth and tenth ; -i- +, smallest or third splanchnic from the eleventh : the firstand second of these are shown joining the solar plexus, so; the third descending to the renal plexus, re; connecting branches between the solar plexus and the vagi are also represented; pn', above the place where the right vagus passes to the lower or posterior surface of the stomach; pn", the left dis- tributed on the anterior or upper surface of the cardiac portion of the organ : from the solar plexus large branches are seen surrounding the arteries of the coeliac axis, and descending to m s, the sup. me- senteric plexus; opposite to this is an in- dication of the suprarenal plexus ; below r e (the renal plexus), the spermatic plexus is also indicated ; a 0, on the front of the aorta, marks the aortic plexus, formed by nerves descending from the solar and sup. mesenteric plexuses and from the lumbar ganglia; mi, the inf. mesenteric plexus surrounding the corresponding artery ; hy, hypogastric plexus placed be- tween the common iliac vessels, connected above with the aortic plexus, receiving nerves from the lower lumbar ganglia, and dividing below into the right and left pel- vic or inf : hypogastric plexuses ; pl, the right pelvic plexus ; from this the nerves descending are joined by those from the plexus on the sup. hemorrhoidal vessels, mi' by nerves from the sacral ganglia, and by visceral nerves from the third and fourth sacral spinal nerves, and there are thus formed the rectal, vesical, and other plexuses, which ramify upon the viscera, as towards ir, and v, the rectum and bladder. chap, xxi.] FUNCTIONS OF THE VISCERAL NERVES. 709 downwards into the splanchnics. Other branches go downwards into the lumbar and sacral plexuses. The grey rami of all the spinal nerves are the only apparent representatives of the visceral branches in the regions above the 2nd thoracic nerve root, and below the 2nd lumbar nerve root, with the exception of the roots of the 2nd and 3rd sacral nerves, which have also white rami, and consist of non niedullated fibres, and pass from the ganglia to be distributed chiefly to the spinal column, to the spinal membranes and to the spinal nerve roots themselves. We must look upon the white rami then as the visceral branches proper. A peculiarity in the structure of these white niedullated visceral nerves is the fineness of their fibres. They are a third or a fourth of the diameter of ordinary niedullated fibres, measuring i'8/x to 2"jp. instead of 14'4/1 to 19/1. They are a peculiarity of the spinal nerve roots chiefly in the thoracic region, but are also to be found in the 2nd and 3rd sacral nerves, and constitute there the nervi erigentes which pass directly to the hypogastric plexus, and not first of all into the lateral chain. From this plexus branches pass upwards into the inferior mesenteric ganglia and downwards to the bladder, rectum and generative organs. These nerves, called by Gaskell pelvic splanchnic nerves, differ from the rami viscerales of the thoracic region only in not communicating with the lateral ganglia; the branches which pass upwards from the thoracic region to the neck, he calls cervical splanchnics, and the splanch- nics proper abdominal splanchnics. The white rami viscerales of the upper cervical and cervico-cranial regions do not run with their corresponding grey rami, but form, Gaskell thinks, the internal branch of the spinal accessory nerve, which contains small medul- lated fibres similar to those of the visceral branches in the thoracic region. This branch passes into the ganglion of the trunk of the vagus. Small visceral fibres exist too in the roots of the vagus, and in those of the glosso-pharyngeal in connection with the ganglion of the trunk and ganglion petrosum, as well as in the chorda tympani, in the small petrosal and in other cranial visceral nerves. Functions.-The functions of the sympathetic system are not by any means completely understood. Indeed, until within the last few years, what could be said about them was of a very vague kind. The remarkable researches of Gaskell have, however, done much to clear up the former confusion ; and in the following account the description of the functions of the sympathetic as given by that observer, will be to a great extent followed. 710 THE SYMPATHETIC NERVOUS SYSTEM, [chap. xxi. A. Functions of the nerve fibres.-The efferent nerve fibres of the sympathetic system supply (a) the muscles of the vascular system, to which they send vaso-motor fibres, i.e., vaso- constrictor and cardiac augmentor or accelerator, and vaso-inhibitory fibres, i.e., vaso-dilator and cardiac inhibitory ; (6) the muscles of the viscera, to which they send both vi>cero-mo£or and viscero- inhibitory fibres, (c) The secretory gland-cells. (a) i. Faso-motfor or Vaso-constrictor and Cardio-augmentor Fibres. The vaso-motor nerves for all parts of the body come from the central nervous system, and pass out from the spinal cord in the white rami viscerales of the thoracic region from the 2nd thoracic to the 2nd lum- bar nerve roots inclusive, as fine medullated fibres; they then pass to the lateral or main sympathetic chain, become non-medullated, and are distributed to their muscles either directly or through terminal ganglia. Thus the augmentor nerves of the heart arise in the thoracic rami, pass upwards, and are distributed to the heart through the ganglion stellatum or inferior cervical ganglion; the vaso-motor nerves for the arm pass out of the cord below the origin of the roots of the brachial plexus, in the anterior roots of the 2nd and lower thoracic nerves, and reach that plexus by the same gan- glion ; the vaso-motor nerves of the foot leave the spinal cord high up, and reach the sympathetic lateral ganglia above the origin of the sciatic nerve, into which they pass through the abdo- minal sympathetic. In all cases the nerves lose their medulla in the ganglia. Similarly the vaso-motor nerve supply for the blood vessels of the head and neck and of the abdomen is derived from the cervical and abdominal splanchnics respectively, or from the corresponding rami efferentes of the upper lumbar ganglia. The lateral sympathetic chain Gaskell proposes to call the chain of vaso-motor ganglia. ii. Vaso-inhibitory or Vaso-dilator, and Cardio-inhibitory Fibres. ■-Of these, which are doubtless as widely distributed as the vaso- motor fibres, we have distinct proof in the existence of fibres separate from vaso-motor, e.g., in the inhibitory nerve of the heart, the cardio-vagus ; in the chorda tympani; in the small petrosal, and in the nervi erigentes. these nerve-fibres, as far as we know' at present, leave the central nervous system among the fine medullated nerves of the cervico-cranial and sacral rami communicantes, do not enter the lateral ganglia, but pass without losing their medulla into the collateral or terminal ganglia. chap. xxj.J STRUCTURE AND FUNCTIONS OF THE GANGLIA, 711 (&.) i. Viscero-motor Fibres.-These fibres, upon which depend the peristaltic movements of the thoracic portion of the oesophagus, and of the stomach, and intestines, arise from the central nervous system, as the fine medullated fibres of the upper portion of the cervical region, not in the spinal nerve roots of that region, but as the bundles of fibres which may be called the rami viscerales of the vagus and accessory nerves. They pass to the. ganglion of the trunk of the vagus, where they lose their medulla. ii. Viscero-Inhibitory Fibres.-It appears that the nerve supply to the circular muscles of the alimentary canal and its appen- dages, is contained in the abdominal splanchnics, and consists of those fibres which have not passed through the lateral chain, and which therefore retain their medulla until they reach the proximal or collateral chain. c. Glandular Nerve Fibres.-A double nerve supply, in all pro- bability coinciding with the supply to the visceral muscles, has been demonstrated in the cases of the submaxillary, parotid, and lachrymal glands, and in these cases the course of the fibres is very similar to that of the corresponding fibres for the vaso- muscular supply. Thus the sympathetic supply for these glands passes along with the vaso-motor fibres from the cervical splanchnic (or sympathetic trunk), and superior cervical ganglion; whilst the cerebro-spinal supply conies from the rami viscerales of the cranial nerves in conjunction with the vaso-dilator fibres. Central Origin of the Rami Viscerales.-There appears to be the strongest presumption that the white rami of the thoracic region arise in the spinal cord in, or are connected with, the cells of the posterior vesicular column of Clarke. This conclusion is based upon the fact that these special cells are found in the three regions already mentioned, and in those only where the white rami of fine medullated fibres exist, viz., in the cervico-cranial regions, in the spinal accessory, in the thoracic region, and in the sacral region. But it is probable that the fibres are also connected with the cells of the lateral horn of the grey matter of the spinal cord, and its representative in the medulla, the antero-lateral nucleus of Clarke. B. Structure and Functions of the Ganglia.- The sym- pathetic ganglia all contain-(i.) nerve-fibres traversing them; (2.) nerve-fibres originating in them; (3.) nerve- or ganglion- corpuscles, giving origin to these fibres ; and (4.) other corpuscles that appear free. In the sympathetic ganglia of the frog, ganglion- 712 THE SYMPATHETIC NERVOUS SYSTEM, [chap. xxr. cells of a very complicated structure have been described by Beale, and subsequently by Arnold. The cells are enclosed each in a nucleated capsule: they are pyriform in shape, and from the pointed end two fibres are given off, which gradually acquire the characters of nerve-fibres: one of them is straight, and the other (which sometimes arises from the cell by two roots) is spirally coiled around it. According to Gaskell the functions of the main sympathetic ganglia are the following :-(i.) They effect the conversion of medullated into non-medullated fibres ; (2.) They possess a nutritive influence over the nerves which pass from them to the periphery; (3.) They increase the number of fibres at the same time as they cause the removal of the medulla. As regards their possession of the usual properties of nerve-centres little or nothing is certainly known. It appears unlikely that they possess the reflex functions of the spinal centres. Respecting the general action of the peripheral ganglia of the sympathetic, in reflex or other actions, little need be said, since they may be taken as examples by which to illustrate the common modes of action of all nerve-centres. Indeed, complex as the sympathetic system, taken as a whole, is, it presents in each of its parts a simplicity not to be found in the cerebro-spinal system : for each ganglion with afferent and efferent nerves forms a simple nervous system, and might serve for the illustration of all the nervous actions with which the cerebrum is unconnected. The parts principally supplied with sympathetic nerves are usually capable of none but involuntary movements, and when the cerebrum acts on them at all, it is only through the strong excite- ment or depressing influence of some passion, or through some voluntary movement with which the actions of the involuntary part are commonly associated. The heart, stomach, and intes- tines are examples of these statements ; for the heart and stomach, though supplied in large measure from the pneumogastric nerves, yet probably derive through them few filaments except such as haAe arisen from their ganglia, and are therefore of the nature of sympathetic fibres. The parts which are supplied with motor power by the sym- pathetic nerv e continue to move, though more feebly than before, when they are separated from their natural connections with the rest of the sympathetic system, and wholly removed from the body. 1 bus, the heart, after it is taken from the body, continues CHAP. XXI.] FUNCTIONS OF THE GANGLIA. 713 to beat in Mammalia for one or two minutes, in reptiles and Amphibia for hours; and the peristaltic motions of the intestine continue under the same circumstances. Hence the motions of the parts supplied with nerves from the sympathetic are shown to be, in a measure, independent of the brain and spinal cord; this independent maintenance of their action being, without doubt, due to the fact that they contain, in their own substance, the apparatus of ganglia and nerve-fibres by which their motions are immediately governed. It seems to be a general rule, at least in animals that have both cerebro-spinal and sympathetic nerves much developed, that the involuntary movements excited by stimuli conveyed through ganglia are orderly and like natural movements, while those excited through nerves without ganglia are convulsive and dis- orderly ; and the probability is that, in the natural state, it is through the same ganglia that natural stimuli, impressing cen- tripetal nerves, are reflected through centrifugal nerves to the involuntary muscles. As the muscles of respiration are maintained in uniform rhythmic action chiefly by the reflecting and combining power of the medulla oblongata, so are those of the heart, stomach, and intestines, by their several ganglia. And as with the ganglia of the sympathetic and their nerves, so with the medulla oblon- gata and its nerves distributed to the respiratory muscles-if these nerves of the medulla oblongata itself be directly stimulated, the movements that follow are convulsive and disorderly ; but if the medulla be stimulated through a centripetal nerve, as when cold is applied to the skin, then the impressions are reflected so as to produce movements which, though they may be very quick and almost convulsive, are yet combined in the plan of the proper respiratory acts. Among the ganglia of the sympathetic nerves to which this co-ordination of movements is to be ascribed, must be reckoned those which lie in the very substance of the organs ; such as those of the heart. Those also may be included which have been found in the mesentery close by the intestines, as well as in the muscular and sub-mucous tissue of the stomach and intestinal canal, and in other parts. Respecting the influence of the sympathetic system on the various physiological processes, the sections on the Heart, Arteries, Animal Heat, Salivary Glands, Stomach and Intestines should be referred to. Influence of the Nervous System in general on Nutrition. 714 THE SYMPATHETIC NERVOUS SYSTEM. [chap. xxi. -It has been held that the nervous system cannot be essential to a healthy course of nutrition, because in plants, in the early embrj o, and in the lowest animals, in which no nervous system is developed, nutrition goes on without it. But this is no proof that in animals which have a nervous system, nutrition may be independent of it; rather, it may be assumed, that in ascending development, as one system after another is added or increased, so the highest (and, highest of all, the nervous system) will always be present and blended in a more and more intimate relation with all the rest: according to the general law, that the interdependence of parts augments with their development. The reasonableness of this assumption is proved by many facts showing the influence of the nervous system on nutrition, and by the most striking of these facts being observed in the higher animals, and especially in man. The influence of the mind in the production, aggravation, and cure of organic diseases is matter of daily observation, and a sufficient proof of influence exercised on nutrition through the nervous system. Independently of mental influence, injuries either to portions of the nervous centres, or to individual nerves, are frequently followed by defective nutrition of the parts supplied by the injured nerves, or deriving their nervous influence from the damaged portions of the nervous centres. Thus, lesions of the spinal cord are sometimes quickly followed by gangrene of portions of the paralysed parts. After such lesions also, the repair of injuries in the paralysed parts may take place less completely than in others ; as, in a case in which paraplegia was produced by fracture of the lumbar vertebra), and, in the same accident, the humerus and tibia were fractured. The former in due time united : the latter did not. The same fact was illustrated by some experiments, in which having, in salamanders, cut off the end of the tail, and then thrust a thin wire some distance up the spinal canal, so as to destroy the cord, it was found that the end of the tail was repro- duced more slowly than in other salamanders in whom the spinal cord was left uninjured above the point at which the tail was amputated. Illustrations of the same kind are furnished by the seveial cases in which division or destruction of the trunk of the trigeminal nerve has been followed by incomplete and morbid nutrition of the corresponding side of the face • ulceration of the cornea being often directly or indirectly one of the consequences of such imperfect nutrition. Part of the wasting and slow dege- CHAT. XXI.] ANABOLIC AND KATABOLIC NERVES. 715 lieration of tissue in paralysed limbs is probably referable also to the withdrawal of nervous influence from them; though, perhaps, more is due to the want of use of the tissues. Undue irritation of the trunks of nerves, as well as their division or destruction, is sometimes followed by defective or morbid nutrition. To this may be referred the cases in which ulceration of the parts supplied by the irritated nerves occurs frequently, and continues so long as the irritation lasts. So many and varied facts leave little doubt that the nervous system exercises an influence over nutrition as over other organic processes ; and they cannot be easily explained by supposing that the changes in the nutritive processes are only due to the varia- tions in the size of the blood-vessels supplying the affected parts, although this is, doubtless, one important element in producing the result. As a contribution towards the explanation of the nervous mechanism of nutrition comes in Gaskell's theory of katabolic and anabolic nerves. He supposes that every tissue is supplied with two sets of nerves, the former of which corresponds with the motor nerve, the viscero-motor and the cardio-augmentor, by the stimulation of which an increase of the metabolism takes place, and which is followed by exhaustion. It may be accom- panied either by contraction of a muscle or by an increase of con- traction. Such a nerve is excellently illustrated by the sympa- thetic augmentor or accelerator nerve of the heart, on stimulation of which an increase in the force and frequency of the heart takes place, followed after a time by exhaustion. A katabolic nerve stimulates the destructive metabolism which is always going on in a tissue. The anabolic nerve is the exact opposite of the katabolic nerve in function. It subserves constructive metabolism. Stimu- lation of the nerve produces diminished activity, repair of tissue and building up. An example of this kind of nerve is seen in the cardiac vagus, stimulation of which produces inhibition. Inhibition must generally be looked upon as an anabolic process. It will be seen that the results of stimulation of the nerves to the salivary glands, discussed in a former chapter, appear to support the theory, that the processes of constructive and destruc- tive metabolism are under the control of separate nerve fibres. In the case of the submaxillary gland for example, if the sympa- thetic fibres be stimulated, a katabolic effect is produced, and the materials of secretion are formed at the expense of the protoplasm 716 THE REPRODUCTIVE ORGANS. [chap. xxit. (this action in the case of the gland Heidenhain calls trophic}; if on the other hand the chorda tympani or the secretory nerve be stimulated, two things happen, one being the discharge of water and the materials of secretion from the gland cells, and the other the building up or reconstruction of the protoplasm of the cells. A part of this action at any rate is anabolic, and similar to the action of inhibitory nerves. CHAPTER XXII. THE REPRODUCTIVE ORGANS. Before describing the method of Reproduction or the way in which the species is propagated, it will be advisable to describe Fig. 424.-Diagrammatic view of the uterus and its appendages, as seen from behind. The S+KodT??riPer-t le v have been lai(1 °Pen by removing the posterior A 1 fallopian tube, round ligament, and ovarian ligament have been cut short, X ±removed on the left side; u, the Spper part of the uterus ; c, '.mi fK?0,?', 6?!16 os internum : the triangular shape of the uterine cavity is unnev hurt nf ?.ddatati°n cervical cavity with the ruga? termed arbor vitae ; v, _.Ae F-u ln£l Fallopian tube or oviduct; the narrow communication ment • li .J, 1 In the cornu of the uterus on each side is seen; I, round liga- tiX-' J h«fl^i5tF£thetOVa7; °> ovary; i, wide outer part of the right Fallopian found ■o<mn<<tI1i,niVk<lFXi5emiitV.' /"b Parovarium ; A. one of the hydatids frequently found connected with the broad ligament. J. (Allen Thomson.) ' ' the structure of those organs which in either sex are concerned in reproduction, and which are called the genital or generative organs or the sexual apparatus. CHAP. XXII.] THE FEMALE GENITAL ORGANS. 717 A. The Genital Organs of the Female. The female organs of generation (fig. 424) consist of two Ovaries, whose function is the formation of ova; of a Fallopian tube, or oviduct, connected with each ovary, for the purpose of conducting the ovum from the ovary to the Uterus or cavity in Fig. 425.-Fiew of a section of the ovary of the cat. 1, outer covering and free border of the ovary ; 1', attached border; 2, the ovarian stroma, presenting a fibrous and vascu- lar structure; 3, granular substance lying external to the fibrous stroma; 4, blood- vessels ; 5, ovigerms in their earliest stages occupying a part of the granular layer near the surface; 6, ovigerms which have begun to enlarge and to pass more deeply into the ovary; 7, ovigerms round which the Graafian follicle and tunica granulosa are now formed, and which have passed somewhat deeper into the ovary and are sur- rounded by the fibrous stroma ; 8, more advanced Graafian follicle with the ovum im- bedded in the layer of cells constituting the proligerous disc ; 9, the most advanced follicle containing the ovum, &c.; 9', a follicle from which the ovum has accidentally escaped; 10, corpus lutem. y. (Schriin.) which, if impregnated, it is retained until the embryo is fully developed, and fitted to maintain its existence independently of internal connection with the parent; and, lastly, of a canal, or vagina, with its appendages, for the reception of the male organ in the act of copulation, and for the subsequent discharge of the foetus. a. The Ovaries.-The ovaries are two oval compressed bodies, situated in the cavity of the pelvis, one on each side, enclosed in the folds of the broad ligament. Each ovary measures about an inch and a half in length, three-quarters of an inch in width, and nearly half an inch in thickness, and is attached to the uterus by a narrow fibrous cord (the ligament of the ovary), and, more slightly, to the Fallopian tubes by one of the fimbrife into which the walls of the extremity of the tube expand. 718 THE REPRODUCTIVE ORGANS. [CHAP. XXII. Structure.-The ovary is enveloped by a capsule of dense fibro- cellular tissue, called the Zwnica albuginea, covered on the outside by epithelium (germ-epithelium), the cells of which, although con- tinuous with, and originally derived from, the squamous epithe- lium of the peritoneum, are short columnar (A, fig. 426). The internal structure of the organ consists of a peculiar soft Fig. 426.-Section of the ovary of a cat. A, germinal epithelium; B, immature Graafian follicle ; C, stroma of ovary ; D, vitelline membrane containing the ovum ; E, Graafian follicle showing lining cells ; F, follicle from which the ovum has fallen out. (V. D. Harris.) fibrous tissue-a kind of undeveloped connective tissue, with long nuclei closely resembling unstriped muscle (C, fig. 426)-or stroma, abundantly supplied with blood-vessels, and having embedded in it, in various stages of development, numerous minute follicles or vesicles, the Graafian vesicles, or sacculi, containing the ova (fig. 426). If the ovary be examined at any period between early infancy and advanced age, but especially during that period of life in which the power of conception exists, it will be found to con- tain a number of these vesicles. Immediately under the tunica albuginea (fig. 426) they are small and numerous, either arranged as a continuous layer, as in the cat or rabbit, or in groups, as in the human ovary. These small follicles embedded in the soft stroma of fine connective tissue and unstriped muscle form here the cortical layer ; they are sometimes called owsart. CHAP. XXII.] STRUCTURE OF THE OVUM. 719 Each of the small follicles of this layer has an external mem- branous envelope, or membrana propria. This envelope or tunic is lined with a layer of nucleated cells, forming a kind of epi- thelium or internal tunic, and named the membrana granulosa. The cavity of the follicle is filled up by a nucleated mass of pro- toplasm enclosed in a very delicate membrane, which is the Ovum. The nucleus contains one or more nucleoli. The nucleus is known as the germinal vesicle, and the nucleolus as the germinal spot. The central portion of the stroma of the ovary extends from the cortical layer to the hilum of the organ, at which enter the numerous arteries, fibrous tissue, and unstriped muscle, forming a highly vascular zona vasculosa. Within this central zone are contained the fully-developed Graafian follicles, varying in size, however, but considerably larger than those of the cortical layer. In these follicles the cavity is not nearly filled by the ovum, which is attached at one side to the zona granulosa by a collec- tion of small cells, the discus proligerus, the remainder of the cavity being filled with fluid. The envelope of the ovum, or zona pellucida, is much thicker. The zona granulosa is formed of several layers of cells, instead of one only. Its membrana propria is much thicker, so as to form a distinct fibrous invest- ment the membrana fibrosa and the blood-vessels surrounding it are numerous, and may be said to form a membrana vasculosa about it. The human ovum measures about of an inch. Its external investment, or the zona joelluvida, or vitelline membrane, is a trans- parent membrane, about °f an i in thickness, which under the microscope appears as a bright ring (4, fig. 427), bounded externally and internally by a dark outline. \\ ithin this transparent investment or zona pellucida, and usually in close contact with it, lies the yolk or vitellus, which is composed of granules and globules of various sizes, imbedded in a moie 01 less fluid substance. The smaller granules, which are the more numerous, resemble in their appearance, as v ell as theii constant motion, pigment-granules. The larger granules or globules, which have the aspect of fat-globules, are in greatest number at the periphery of the yolk. The number of the granules is greatest in the ova of carnivorous animals. In the human ovum their quan- tity is comparatively small. In the substance of the yolk is imbedded the germinal vesicle, 720 THE REPRODUCTIVE ORGANS. [chap. xxii. or vesicula germinativa (2, fig. 427). The vesicle is of greatest relative size in the smallest ova, and is in them surrounded closely by the yolk, nearly in the centre of which it lies. During the development of the ovum, the germinal vesicle increases in size much less rapidly than the yolk, and comes to be placed near to its surface. It consists of a fine, transparent, structureless membrane, containing a clear, watery fluid, in which are sometimes a few granules; and at that part of the periphery of the germinal vesicle which is nearest to the periphery of the yolk is situated the germinal spot, or macula germinativa, of a finely granulated appearance and of a yel- lowish colour, strongly refracting the rays of light. Such are the parts of which the Graafian follicle and its contents, including the ovum, are composed. With regard to the mode and order of development of these parts there is considerable un- certainty. It appears that the Graafian follicles arc formed in the fol- lowing manner:-The embryonic ovary is covered with short columnar cells, or the so-called germinal epithelium. The cells of this layer undergo proliferation, so as to form several strata. These cells grow into the ovarian stroma as longer or shorter columns or tubes. By degrees these tubes become cut off from the surface epithelium, and form cell nests, small if near the surface, larger if in the depth of the stroma. The nests increase in size from multiplication of their cells, and may even give off new nests laterally by constriction of them in various directions. Certain of the cells of the germinal epithelium enlarge, and form ■ova; and the formation of ova also takes place in the nests within the stroma. The ova of a nest may multiply by division. The small cells of a nest surround the ova, and form their membrana granulosa, and the stroma growing up separates the surrounded ova into so many Graafian follicles. The other layers, namely, the membrana fibrosa and the membrana vasculosa, are derived from the stroma. 1 he smallest follicles are formed at the surface, and form the cortical layer. It is said by some that the superficial follicles as they ripen become more deeply placed in the ovarian stroma • Fig. 427.- Ovum of the sow. 1, germinal spot; 2, germinal vesicle; 3, yolk; 4, zona pellu- cida; 5, discus proli- gerus; 6, adherent granules or cells. (Barry.) CHAP. XXII.] THE FORMATION OF OVA. 721 and, again, that as they increase in size, they make their way towards the surface (fig. 425). When mature, they form little prominences on the exterior of the ovary, covered only by a thin layer of condensed fibrous tissue and epithelium. Only a few follicles ever reach maturity. From the earliest infancy, and through the whole fruitful period of life, there appears to be a constant formation, develop- ment, and maturation of Graafian vesicles, with their contained Fig. 428.- Germinal epithelium of the surface of the ovary of five, days' chick, a, small ovo- blasts; &, larger ovoblasts. (Cadiat.) ova. Until the period of puberty, however, the process is com- paratively inactive; for, previous to this period, the ovaries are small and pale, the Graafian vesicles in them are very minute, and probably never attain full development, but soon shrivel and disappear, instead of bursting, as matured follicles do; the con- tained ova are also incapable of being impregnated. But, coinci- dent with the other changes which occur in the body at the time of puberty, the ovaries enlarge, and become very vascular, the formation of Graafian vesicles is more abundant, the size and degree of development attained by them are greater, and the ova are capable of being fecundated. b. The Fallopian Tubes or Oviducts.-The Fallopian tubes are about four inches in length, and extend between the ovaries and the upper angles of the uterus. At the point of attachment to the uterus, the tube is very narrow; but in its course to the ovary it increases to about a line and a half in thickness; at its distal extremity, which is free and floating, it bears a number of fimbriae, one of which, longer than the rest, is attached to the ovary. The canal by which each tube is traversed is narrow, especially at its point of entrance into the uterus, at which it will scarcely admit a bristle ; its other extremity is wider, and opens into the 722 THE REPRODUCTIVE ORGANS. [chap. xxii. cavity of the abdomen, surrounded by the zone of fimbria,'. Externally, the fallopian tube is invested with peritoneum ; in- ternally, its canal is lined with mucus membrane, which is apt to be thrown into numerous folds, covered w ith ciliated epithelium : between the peritoneal and mucous coats, the walls are composed, like those of the uterus, of fibrous tissue and unstriped muscular fibres, chiefly circular in arrangement. c. The Uterus.-The Uterus («, c, fig. 424) is somewhat pyriform, and in the unimpregnated state is about three inches in length, two in breadth at its upper part or fundus, but at its lower pointed part or neck, only about half an inch. The part between the fundus and neck is termed the body of the uterus : it is about an inch in thickness. Structure.-The uterus is constructed of three principal layers, or coats-serous, fibrous and muscular, and mucous. (1.) The serous coat, which has the same general structure as the perito- neum, covers the organ before and behind, but is absent from the front surface of the neck. (2.) The middle coat is composed of unstriped muscle, arranged in the human uterus in three layers from without inwards, longitudinal, circular, oblique and cir- cular. They become enormously developed during pregnancy. The arteries and veins are found in large numbers in the outer part of this coat, so as to form almost a special vascular covering. (3.) The mucous membrane of the uterus is lined by columnar ciliated epithelium, which extends also into the interior of the tubular glands, of which the mucous membrane is largely made up. In the neck of the uterus (cervix) the mucous membrane is arranged in permanent longitudinal folds, palmse plicata?, and between these fold open the ducts of the tubular glands. In the fundus the proper tissue is a spongy tissue of interlacing fibrous bundles, forming a system of lymph channels. Here the lining is a single layer of flattened cells. The tubular glands are usually simple and unbranched, and seldom wavy or convoluted. The cavity of the uterus corresponds in form to that of the organ itself: it is very small in the unimpregnated state ; the sides of its mucous surface being almost in contact. Into its upper part, at each side, opens the canal of the corresponding Fallopian tube : below, it communicates with the vagina by a fissure-like opening in its neck, the os uteri, the margins of which are dis- tinguished into two lips, an anterior and posterior. In the chap, xxn.] THE MALE GENITAL ORGANS. 723 mucous membrane of the cervix are found several mucous follicles, termed ovula or glandulce Nabothi: they probably form the jelly-like substance by which the os uteri is usually found closed. The vagina is a membranous canal, five or six inches long, extending obliquely downwards and forwards from the neck of the uterus, which it embraces, to the external organs of genera- tion. It is lined with mucous membrane, covered with stratified squamous epithelium, which in the ordinary contracted state of the canal is thrown into transverse folds. External to the mucous membrane the walls of the vagina are constructed of unstriped muscle and fibrous tissue, within which in the submucosa, especially around the lower part of the tube, is a layer of erectile tissue. This exists also in the mucosa. The lower extremity of the vagina is embraced by an orbicular muscle, the constrictor vagina?; its external orifice, in the virgin, is partially closed by a fold or ring of mucous membrane, termed the hymen. The external organs of generation consist of the clitoris, a small elongated body, situated above and in the middle line, and constructed of two erectile masses or corpora cavernosa. They are not perforated by the urethra; of two folds of mucous membrane, termed labia interna, or nymphce ; and, in front of these, of two other folds, the labia externa, or pudenda, formed of the external integument, and lined internally by mucous membrane. Between the nymphce and beneath the clitoris is an angular space, termed the vesti- bule, at the centre of whose base is the orifice of the meatus urinarius. Numerous mucous follicles are scattered beneath the mucous membrane composing these parts of the external organs of generation; and at the side of the lower part of the vagina are two larger lobulated glands, vulvo-vaginal or Duver ney's glands, which are analogous to Cowper's glands in the male. B. The Genital Organs of the Male. The male organs of generation comprise the two Testes, in which the semen is formed; each with its duct, the Vas Deferens, with the accessory Vesicula Seminalis; and the Penis, an erec- tile organ, through which the semen as well as the urine is dis- charged. The Prostate gland, the exact function of which is not understood, is generally included in the same class. a. The Testes.-The secreting structure of the testicle and its 724 THE REPRODUCTIVE ORGANS. [chap. XXII, duct are disposed of in two contiguous parts, (i) the body of the testicle proper, enclosed within a thick and tough white fibrous membrane, the tunica albuginea, on the outer surface of which is the serous covering formed by the tunica vaginalis, and (2) the epididymis and vas deferens. The Vas deferens, or duct of the testicle, which is about two feet in length, is constructed externally of connective tissue, and internally is lined by a mucous membrane, covered witli columnar epithelium; while between these two coats is a middle coat, very firm and tough, made up of unstriped muscle, chiefly arranged longitudinally, but also containing some circular fibres. When followed back to its origin, the vas deferens is found to pass to the lower part of the epididymis, with which it is directly continuous (fig. 429), and assumes there a much smaller diameter with an exceedingly tortuous course. The Epididymis, which is lined, except at its lowest part, by columnar ciliated epithelium (fig. 429), is com- monly described as consisting (fig. 429) of a globus minor (y), the body (e), and the globus major (f). When unravelled, it is found to be constructed of a single tube, measuring about twenty feet in length. At the globus major this duct divides into ten or twelve small branches, the convolutions of which form coniform masses, named Coni vasculosi; and the ducts continued from these, the Vasa efferentia, after anastomosing, one with another in what is called the Rete testis, lead finally as the Tubuli recti or Vasa recta to the seminal tubules, or form the proper substance of the testicle. The epithelium lining the coni vasculosi and vasa efferentia is columnar and ciliated ■ that of the rete testis is squamous. 1 he seminal tubules are arranged in lobules, separated from one another by incomplete fibrous septa or cords, which pass from the front of the tunica albuginea internally to a firm incomplete vertical septum of thick extending fibrous tissue at the posterior Fig. 429.-Plan of a vertical section of the testicle, showing the ar- rangement of the ducts. The true length and diameter of the ducts have been disregarded. a, a, tubuli seminiferi coiled up in the separate lobes ; b, tubuli recti or vasa recta; c, rete testis; il, vasa efferentia ending in the coni vasculosi; I, e, g, convo- luted canal of the epididymis; h, vas deferens; f, section of the back part of the tunica albuginea; i, i, fibrous pro- cesses running between the lobes ; s, mediastinum. chap, xxn.] THE STRUCTURE OF THE TESTES. 725 border, from the upper to near the lower part, called the corpus Higlvmori, or mediastinum testis. Through this very firm fibrous tissue pass the seminal tubes from the vasa recta. The tunica albuginea is covered by a very fine plexus of blood-vessels in- ternally, derived from the sper- matic vessels. The fibrous cords which may contain unstriped muscle are also covered with a similar capillary plexus. The Seminal Tubes.- The seminal tubes, or tubuli seminiferi, which compose the parenchyma of the testicle, are loosely arranged in lobules between the connective tissue septa. They are relatively large, very wavy, and much convoluted; and they possess a few lateral branches, by which they become connected into a network. They form terminal loops, and in the peripheral portion of the testis the tubules are possessed of minute lateral csecal branchlets. Each seminal tubule in the adult testis is limited by a mem- brana propria, which appears as a hyaline elastic membrane, but which is really made up of several incomplete layers of flattened cells, containing oval flattened nuclei at regular intervals. Inside this membrana propria are several layers of epithelial cells, the seminal cells. These consist of two or more layers, the outermost being situated next the mem- brana propria. These cells are of two kinds, those that are in a resting state, which generally form a complete layer, and those that are in a state of division, of which there may be two layers. The latter arc called mother cells, and the smaller cells resulting from their division are called daughter cells or spermatoblasts. From these the spermatozoa are formed, their head corresponding with the nuclei of the daughter cells; and during their develop- ment they lie in groups (fig. 432), and are supported by irregular Fig. 430.-Section of the epididymis of a dog.-The tube is cut in several places, both transversely and obliquely; it is seen to be lined by a ciliated epithe- lium, the nuclei of which are well shown, c, connective tissue. Scho- field.) 726 THE REPRODUCTIVE ORGANS. [chap. xxii. masses of so-called nutritive cells; but when fully formed, they become detached, and fill the lumen of the seminiferous tubule (fig- 43i). Big- 431* -d section oj dog's testicle, highly mag- nified, showing three "tubuli seminiferi," lined and largely occupied by a spheroidal epithelium, the numerous nuclei of which are well seen; c, connective tissue surrounding and supporting the tubuli; sp, masses of spermatozoa occupying the centre of tubuli: the small black bodies scattered about are the heads of the spermatozoa. (Schofield.) Fig. 432.-Section of a tubule of the testicle of a rat, to show the formation of the spermatozoa. a, spermatozoa ; b, seminal cells ; c, spermato- blasts, to which the spermatozoa are still adherent; d, membrana propria ; e, fibro- plastic elements of the connective tissue. (Cadiat). In the fine connective tissue which supports the tubules of the testis, are to be found flattened and nucleated epithelial cells, A Be g'enithelial ™ a th" teSt'S °^dos,-,\ showing portions of seminal tubes. A, seminal atoblasts converted ?n^el'°US S?aU celJ? loosely urranf?ed i B, the small cells or sperm- ment. (Klein ) !Sl>crma^ozoa 5 C, groups of these in a further stage of develop- probably the remains of the Wolffian body. The lymphatics of the testes are numerous, and may be injected by inserting the needle CHAr. xxi r.] THE MALE GENITAL ORGANS. 727 of an injecting syringe into the tunica albuginea, and pressing in the injection with slight effort. Vesiculae Seminales.-The vesiculce seminales haxe the appear- ance of outgrowths from the vasa deferentia. Each vas deferens, just before it enters the prostate gland, through part of which it passes to terminate in the urethra, gives off a side branch, which bends back from it at an acute angle : and this branch dilating, variously branching, and pursuing in both itself and its branches a tortuous course, forms the vesicula seminalis. Structure.-Each of the vesicuhe may be unravelled into a single branching tube, sacculated, convoluted, and folded up. The structure of the vesicular resembles closely that of the vasa deferentia. The mucous membrane lining the vesicuhe seminales, like that of the gall-bladder, is minutely wrinkled and set with folds and ridges arranged so as to give it a finely reticulated appearance. The Penis.-The penis is composed of three long more or less cylindrical masses, enclosed in remarkably firm fibrous sheaths, of which two the corpora cavernosa are alike, and are firmly joined together, and receive below and between them the third part, oi' corpus spongiosum. The urethra passes through the corpus spongio- sum. The penis is attached to the symphysis pubis by its root. The enlarged extremity or glans penis is continuous with the corpus spongiosum. The integuments covering the penis forms a loose fold from the junction of the glans with the body, called the prepuce or foreskin. Structure. - (a.) The urethra is lined by stratified pavement epithelium in the prostatic portion ; in front of the bulb the epithe- lium becomes columnar, whilst at the fossa navicularis it is again lined with stratified pavement epithelium. The mucous membrane consists chiefly of fibrous connective-tissue, intermixed with which are many elastic fibres. It is surrounded by unstriped muscular tissue. In the intermediate portion many large veins run amongst the bundles of muscular tissue. Many mucous glands, glands of Littre, are present. (6.) The corpora cavernosa, a true erectile structure, consist of a matrix, formed of trabeculae cerva, made up chiefly of unstriped muscle-fibres, which run in all directions from the fibrous sheath, and from the septum, which separates the two corpora cavernosa, intermixed with connective-tissue, and a few elastic fibres. The matrix is arranged in bundles, and thus form a spongy tissue, 728 THE REPRODUCTIVE ORGANS. [chap. XXII. lined everywhere with endothelium, into the interstices of which, the venous sinuses, the venous blood passes. The trabeculee thus constitute the greater part of the substance of each corpus cavernosum. The venous sinuses anastomose with each other to form plexuses. The arteries run in the muscular trabeculee. (c.) The corpus spongio- sum urethra consists of an inner portion or plexus of longitudinal veins, and of an outer or really cavern- ous portion identical in structure with that which has just been described. The lymphatws of the penis are very numerous, both superficially and also around the urethra. They join the inguinal glands. The nerves, derived from the pudic nerves and hypogastric plexus, are distributed to the skin and mucous membrane and to the corpora cavernosa and spongiosum respectively. The nerves are provided with end bulbs and Pacinian corpuscles in the glans penis, and form also a dense subepithelial plexus. Cowper's glands, are two small glands, the ducts of which open into the bulbous part of the urethra. They are small round bodies, of the size of a pea, yellow in colour, resembling the sublingual gland; in structure they are compound tubular mucous glands. The Prostate Gland.-The prostate is situated (fig. 435) at the neck of the urinary bladder, and encloses the commence- ment of the urethra. It is somewhat chestnut-shaped. It measures an inch and a half in breadth, and an inch and a quarter long, and half an inch in thickness. Structure.-The prostate is made up of small compound tubular glands imbedded in an abundance of muscular fibres and connective tissue. The glandular substance, which is nearly absent from the front part of the organ, consists of numerous small saccules, opening into elongated ducts, which unite into a smaller number of Fig. 434.-Erectile tissue of the human penis, a, fibrous trabecula; with their ordinary capil- laries ; &, section of the venous sinuses; c, muscular tissue. (Cadiat.) CHAP. XXII.] THE PROSTATE GLAND. 729 excretory ducts. The acini of the upper part of the prostate, are small and hemispherical; whilst in the middle and lower parts the tubes are longer and more convoluted. The acini are of two kinds, namely, those (a) lined with a single layer of thin and long co- lumnar cells, each with an oval nucleus in outer part of wall; and those (6) acini resembling the fore- going, but with a second layer of small cortical, polyhedral, or fusiform cells be- tween the membrana propria and the co- lumnar cells. The ducts,twelve to twenty in number, open into the urethra. They are lined by a layer of columnar cells, be- neath which is a layer of small poly- hedral cells. The tunica adven- titia consists of dense fibrous tissue of two layers, between which is situated a plexus of veins. Large ves- sels pass into the interior of the organ, to form a broad, meshed, capillary system. Nerves and numerous large ganglion cells sur- round the cortex. Pacinian bodies are sometimes found in the substance of the organ. The muscular tissue of the prostate not only forms the chief part of the stroma of the gland, but also forms a continuous layer inside the fibrous sheath, as well as a layer surrounding the urethra, which is continuous with the sphincter vesica). Fig. 435.-Dissection of the base of the bladder and prostate gland, showing the vesicula seminales and vasa de- ferentia. a, lower surface of the bladder at the place of reflexion of the peritoneum; b, the part above covered by the peritoneum; », left vas deferens, ending in e, the ejaculatory duct; the vas deferens has been divided near i, and all except the vesicle portion has been taken away; s, left vesicula semi- nalis joining the same duct; s, s, the right vas deferens and right vesicula seminalis, which has been unravelled; p, under side of the prostate gland ; m, part of the urethra ; a, it, the ureters (cut short), the right one turned aside. (Haller.) 730 THE REPRODUCTIVE ORGANS. [chap. xxii. A. Of the Female.-In the process of development in the ovary of individual Graafian vesicles, it has been already observed, that as each increases in size, it gradually approaches the surface of the ovary, and when fully ripe or mature, forms a little projec- tion on the exterior. Coincident with the increase of size, caused by the augmentation of its liquid contents, the external envelope of the distended vesicle becomes very thin and eventually bursts. By these means, the ovum and fluid contents of the vesicle are liberated, and escape on the exterior of the ovary, whence they pass into the Fallopian tube or oviduct, the fimbriated processes of the extremity of which are supposed coincidentally to grasp the ovary, while the aperture of the tube is applied to the part corres- ponding to the matured and bursting vesicle. In animals whose capability of being impregnated occurs at regular periods, as in the human subject, and most Mammalia, the Graafian vesicles and their contained ova appeal' to arrive at maturity, and the latter to be discharged at such periods only. But in other animals, e.g., the common fowl, the formation, maturation, and discharge of ova appear to take place almost constantly. It has long been known, that in the so-called oviparous animals, the separation of ova from the ovary may take place independently of impregnation by the male, or even of sexual union. And it is now established that a like maturation and discharge of ova, independently of coition, occurs in Mammalia, the periods at which the matured ova are separated from the ovaries and received into the Fallopian tubes being indicated in the lower Mammalia by the phenomena of heat or rut : in the human female, although not always with exact coincidence, by the phenomena of menstruation. If the union of the sexes take place, the ovum may be fecundated, and if no union occur it perishes. That this maturation and discharge occur periodically, and only during the phenomena of heat in the lower Mammalia, is made probable by the facts that, in all instances in which Graafian vesicles have been found presenting the appearance of recent rupture, the animals were at the time, or had recently been, in heat ; that on the other hand, there is no authentic and detailed account of Graafian vesicles being found ruptured in the Physiology of the Sexual Organs. chap, xxii.] MENSTRUATION. 731 intervals of the period of heat ; and that female animals do not admit the males, and never become impregnated, except at those periods. Relation of Menstruation to the Discharge of Ova.-The human female is subject to the same law as the females of other mammi- ferous animals; namely, in her as in them, ova are matured and discharged from the ovary independent of sexual union. This maturation and discharge occur, moreover, periodically at or about the epochs of menstruation. The evidence of the periodical discharge of ova at the menstrual periods is that in most cases in which signs of menstruation have been found in the uterus, follicles in a state of maturity or of rupture have been seen in the ovary; and although conception is not confined to the periods of menstruation, yet it is more likely to occur about a menstrual epoch than at other times. The exact relation between the discharge of ova and men- struation is not very clear. It was formerly believed that the monthly flux was the result of a congestion of the uterus arising from the enlargement and rupture of a Graafian follicle ; but though a Graafian follicle is, as a rule, ruptured at each menstrual epoch, yet several instances are recorded in which menstruation has occurred where no Graafian follicle can have been ruptured, and on the other hand cases are known where ova have been dis- charged in amenorrhceic women. It must therefore be admitted that menstruation is not dependent on the maturation and dis- charge of ova. It was, moreover, formerly understood that ova were discharged towards the close or soon after the cessation of a menstrual flow. Observations made after death, and facts obtained by clinical investigation, however, do not support this view. Rupture of a Graafian follicle does not happen on the same day of the monthly period in all women. It may occur towards the close or soon after the cessation of a flow ; but only in a small minority of the subjects examined after death was this the case. On the other hand, in almost all such subjects of which there is record, rupture of the follicle appears to have taken place before the commencement of the catamenial flow. Moreover, the custom of the Jews-a prolific race, to whom by the Levitical law sexual intercourse during the week following menstruation was forbidden-militates strongly in favour of the view that conception usually occurs before and not soon after a 732 THE REPRODUCTIVE ORGANS. [chap, xxii menstrual epoch, and. necessarily, therefore, for the view that ova are usually discharged before the catamenial flow. This, together with the anatomical condition of the uterus just before the cata- menia, seem to indicate that the ovum fertilized is that which is discharged in connection with the first absent, and not that with the last present menstruation. Though menstruation does not appear to depend upon the J'ust beS?re menstruation; the shaded portion represents the S of uterus when menstruation has fust ceased, showing the cavity of the uterus deprived of mucous membrane. Fig. 418 - Jhurjram. of uterus a week after the menstrual flux has ceased: the shaded portion repre- sents renewed mucous membrane. (J. Williams.) * P discharge of ova, yet the presence of the ovaries seems necessary for the performance of the function ; for women do not menstruate when both ovaries have been removed by operation. Some in- stances have been recently recorded, indeed, of a sanguineous discharge, occurring periodically from the vagina after both ovaries have been previously removed for disease ; and it has been infened from this that menstruation is a function independent of the o'vary: but this evidence is not conclusive, inasmuch as it is possible that portions of ovarian tissue were left after the operation. chap, xxii.] THE PHENOMENA OF PUBERTY. 733 Source and Characters of Menstrual Discharge.-The menstrual discharge is a thin sanguineous fluid, having a peculiar odour. It is of a dark colour, and consists of blood, epithelium, and mucus from the uterus and vagina, serum, and the debris of a membrane called the decidua menstrualis. This membrane is the developed mucous membrane of the body of the uterus. It does not extend into the Fallopian tube or into the cavity of the cervix. It attains its highest state of development in the unimpregnated organ just before the commencement of a catamenial flow (fig. 436). If impregnation take place, it becomes the decidua vera; if impreg- nation fail, the membrane undergoes rapid disintegration; its vessels are laid open and haemorrhage follows. The blood poured out does not coagulate in consequence partly of the admixture already mentioned; or, very possibly, coagulation occurs, but the process is more or less spoiled, and what clot is .formed is broken down again, so as to imitate liquid blood. Menstruation, therefore, is not the result of congestion, or of a species of erection, but of a destructive process by which the decidua or nidus prepared for an impregnated ovum is carried away. It is not a sign of the capability of being impregnated as much as of disappointed impregnation. Menstrual Life.-The occurrence of a menstrual discharge is one of the most prominent indications of the commencement of puberty in the female sex ; though its absence even for several years is not neces- sarily attended with arrest of the other characters of this period of life, or with inaptness for sexual union, or incapability of impregnation. The average time of its first appearance in females of this country and others of about the same latitude, is from fourteen to fifteen ; but it is much influenced by the kind of life to which girls are subjected, being accelerated by habits of luxury and indolence, and retarded by contrary conditions. On the whole, its appearance is earlier in persons dwelling in warm climes than in those inhabiting colder latitudes ; though the extensive investigations of Robertson show that the influence of temperature on the development of puberty has been exaggerated. Much of the influence attributed to climate appears due to the custom prevalent in many hot countries, as in Hindostan, of giving girls in marriage at a very early age, and inducing sexual excitement previous to the proper menstrual time. The menstrual functions continue through the whole fruitful period of a woman's life, and usually cease between the forty-fifth and fiftieth years. 734 THE REPRODUCTIVE ORGANS. [chap. xxii. The several menstrual periods usually occur at intervals of a lunar month, the duration of each being from three to six days. In some women the intervals are as short as three weeks or even less ; while in others they are longer than a month. The periodical return is usually attended by pain in the loins, a sense of fatigue in the lower limbs, and other symptoms, which are different in different individuals. Menstruation does not usually occur in pregnant women, or in those who are suckling ; but instances of its occurrence in both these conditions are by no means rare. Corpus Luteum.-Immediately before, as well as subsequent to, the rupture of a Graafian vesicle, and the escape of its ovum, certain changes ensue in the interior of the vesicle, which result in the production of a yellowish mass, termed a Corpus luteum. When fully formed the corpus luteum of mammiferous animals is a roundish solid body, of a yellowish or orange colour, and composed of a number of lobules, which surround, sometimes a small cavity, but more frequently a small stelliform mass of white substance, from which delicate processes pass as septa between the several lobules. Very often, in the cow and sheep, there is no white substance in the centre ; and the lobules projecting from the opposite walls of the Graafian vesicle appear in a section to be separated by the thinnest possible lamina of semi-transparent tissue. When a Graafian vesicle is about to burst and expel the ovum, it becomes highly vascular and opaque; and, immediately before the rupture takes place, its walls appear thickened on the interior by a reddish glutinous or fleshy-looking substance. Immediately after the rupture, the inner layer of the wall of the vesicle appears pulpy and flocculent. It is thrown into wrinkles by the contrac- tion of the outer layer, and, soon, red fleshy mammillary processes grow from it, and gradually enlarge till they nearly fill the vesicle, and even protrude from the orifice in the external covering of the ovary. Subsequently this orifice closes, but the fleshy growth within still increases during the earlier period of pregnancy, the colour of the substance gradually changing from red to yellow, and its consistence becoming firmer. The corpus luteum of the human female (fig. 439) differs from that of the domestic quadruped in being of a firmer texture, and having more frequently a persistent cavity at its centre, and in the stelliform cicatrix, which remains in the cases where the jCHAP. XXII.] CORPUS LUTEUM. 735 cavity is obliterated, being proportionately of much larger bulk. The quantity of yellow substance formed is also much less : and although the deposit increases after the vesicle has burst, yet it does not usually form mammillary growths projecting into the cavity of the vesicle, and never protrudes from the orifice, as is the case in other Mammalia. It maintains the character of a uniform, or nearly uniform, layer, which is thrown into wrinkles, in conse- quence of the contraction of the external tunic of the vesicle. After the orifice of the vesicle has closed, the growth of the yellow substance continues during the first half of pregnancy, till the cavity is reduced to a comparatively small size, or is obliterated ; Fig. 439.-Corpora Intea of different periods. B. Corpus luteum of about the sixth week after impregnation, showing its plicated form at that period. 1, substance of the ovary; 2, substance of the corpus luteum; 3, a greyish coagulum in its cavity. (Paterson.) A, corpus luteum two days after delivery; I), in the twelfth week after delivery. (Montgomery.) in the latter case, merely a white stelliform cicatrix remains in the centre of the corpus luteum. An effusion of blood generally takes place into the cavity of the Graafian vesicle at the time of its rupture, especially in the human subject, but it has no share in forming the yellow body ; it gradually loses its colouring matter, and acquires the character of a mass of fibrin. The serum of the blood sometimes remains included, within a cavity in the centre of the coagulum, and then the decolorised fibrin forms a membraniform sac, lining the corpus luteum. At other times the serum is removed, and the fibrin constitutes a solid stelliform mass. The yellow substance of which the corpus luteum consists, both in the human subject and in the domestic animals, is a growth from the inner surface of the Graafian vesicle, the result of an 736 THE REPRODUCTIVE ORGANS. [chap. xxii. increased development of the cells forming the membrana granu- losa, which naturally lines the internal tunic of the vesicle. The first changes of the internal coat of the Graafian vesicle in the process of formation of a corpus luteum seem to occur in every case in which an ovum escapes ; as well in the human subject as in the domestic quadrupeds. If the ovum is impreg- nated, the growth of the yellow substance continues during nearly the whole period of gestation and forms the large corpus luteum commonly described as a characteristic mark of impreg- nation. If the ovum is not impregnated, the growth of yellow substance on the internal surface of the vesicle proceeds, in the human ovary, no further than the formation of a thin layer, which shortly disappears; but in the domestic animals it con- tinues for some time after the ovum has perished, and forms a corpus luteum of considerable size. The fact that a structure, in its essential characters similar to, though smaller than, a corpus luteum observed during pregnancy, is formed in the human subject, independent of impregnation or of sexual union, coupled with the varieties in size of corpora lutea formed during pregnancy, necessarily renders unsafe all evidence of previous impregnation founded on the existence of a corpus luteum in the ovary. The following table by Dalton, expresses well the differences between the corpus luteum of the pregnant and unimpregnated condition respec- tively :- Corpus Luteum of Men- struation. Corpus Luteum of Preg- nancy. At the end of three weeks One month , Two months . Six months , Nine months . Three-quarters of an inch in diameter ; central clot reddish ; convoluted wall pale. Smaller; convoluted wall bright yellow ; clot still reddish. Reduced to the condi- tion of an insignifi- cant cicatrix. Absent, Absent. Larger; convoluted wall bright yellow ; clot still reddish. Seven-eighths of an inch in diame- ter ; convoluted wall bright yel- low ; clot perfectly decolorised. Still as large as at end of second month; clot fibrinous ; convo- luted wall paler. One half an inch in diameter; central clot converted into a radiating cicatrix ; the external wall tolerably thick and con- voluted, but without any bright yellow colour. CHAP. XXII.] SPERMATOZOA. 737 B. Of the Male.-In order that the ovum should be fecun- dated or impregnated, it is necessary that it should meet with the seminal fluid of the male. This is accomplished by the junction of the sexes in the act of coition, whereby the seminal fluid is discharged into the neighbourhood of, if not within, the cervex uteri. Before considering the changes which are produced in the ovum by impregnation, it will be as well to describe the nature of the seminal fluid. This consists essen- tially of the semen secreted by the tes- ticles: and to this are added, a mate- rial secreted by the vesiculae seminales, as well as the secretion of the prostate gland, and of Cowper's glands. Por- tions of these several fluids are dis- charged, together with the proper secre- tion of the testicles. The semen is a viscid, whitish, albu- minous fluid of a peculiar odour. It contains epithelium, granules or colour- less particles, and large numbers of spermatozoa, which are the character- istic and essential element. The sperma- tozoa are minute bodies each consisting of a flattened oval head and attached to it a long slender tapering mobile flagellum or tail. In some forms of spermatozoa there is a small middle piece interposed between the head and the tail. The head is about g oooth inch l°no and ToicToth inch broad. The tail is about ToVoth to 4,booth inch long. The spermatozoa possess the power of active movement, and it is by this sinuous, cilia-like movement that they are propelled in the female and so helped in their pro- gress to meet the ovum. Spermatozoa.-On examining the spermatozoon of Triton cristatus, one of the Amphibia which possess the largest spermatozoa of all Vertebrate animals, Gibbes found that the organism consisted of («) a long pointed head, at the base of which is (&), an elliptical structure joining the head to (c), a long filiform body ; (rZ), a fine filament, much longer than the body, is connected with this latter by (c), a homogeneous membrane. The head, as it appears in the fresh specimen, has a different refractive power from that of the rest of the organism, and with a high power appears to be a light green colour ; there is also a central line running up it, from which it appears to be hollow. Fig. 440. - Spermatic filaments from the human vas deferens. 1, magnified 300 diameters; 2, magnified 800 diameters; a, from the side; b, from above. (From Kiilliker.) 738 THE REPRODUCTIVE ORGANS. [CHAP. XXII. The elliptical structure at the base of the head connects it with the long threadlike body, and the filament springs from it. Whilst the spermatozoon is living, this filament is in constant motion; at first this is so quick that it is difficult to see it, but as its vitality becomes impaired the motion gets slower, and it is then easily perceived to be a continuous waving from side to side. The spermatozoa of all Mammalia ex- amined, consisting of Man, Bull, Dog, Horse, Cat, Pig, Mouse, Hat, Guinea-pig. had, instead of the long-pointed head of the Amphibian, a blunt thick process of different shapes in the different animals : and from the root or neck of this pro- ceeded the long filament, just as in the Amphibia, only so delicate as to be in- visible except with very high powers. In Man the head (fig. 441) is club- shaped, and from its base springs the very delicate filament, which is three or four times as long as the body; and the mem- brane which attaches it to the body is much broader, and allows it to lie at a greater distance from the body than in the spermatozoa of any other Mammal examined. From his investigations, Gibbes con- cluded:-1st. That the head of the spermatozoon is enclosed in a sheath, which is a continuation of the mem- brane which surrounds the filament, and connects it to the body, acting in fact the part of a mesentery. 2ndly. That the substance of the head is quite distinct in its composition from the elliptical structure, the filament and the long body, and that it is readily acted on by alkalies ; these re-agents have no effect, however, on the other part, excepting the membranous sheath. 3rdly. That this elliptical structure has its analogue in the Mammalian spermatozoon ; in the one case the head is drawn out as a long pointed process, in the other it is of a globular form, and surrounds the elliptical structure. 4thly. 1 hat the motive power lies, in a great measure, in the filament and the membrane attaching it to the body. rhe spermatozoa are derived from the breaking up of the seminal cells or daughter cells. They must be looked upon as modified cells. 1 he occurrence of spermatozoa in the impregnating fluid of nearly all classes of animals, proves that they are essential to the process of impregnation, and their actual contact with the ovum is necessary for its development. lhe seminal fluid is, probably, after the period of puberty 1'ig. <141. - Spermatozoa, i, Of salamander; 2, human. (If. Gibbs.' chap, xxn.] FUNCTIONS OF THE VESICULJE SEMINALIS. 739 secreted constantly, though, except under excitement, very slowly, in the tubules of the testicles. From these it passes along the vasa deferentia into the vesiculae seminales, whence, if not ex- pelled in emission, it may be discharged, as slowly as it enters them, either with the urine, which may remove minute quantities, mingled with the mucus of the bladder and the secretion of the prostate, or from the urethra in the act of defalcation. To the vesiculae seminales a double function may be assigned ; for they both secrete some fluid to be added to that of the testicles, and serve as reservoirs for the seminal fluid. The former is their most constant and probably most important office; for in the horse, bear, guinea-pig, and several other animals, in whom the vesiculae seminales are large and of apparently active function, they do not communicate with the vasa deferentia, but pour their secretions, separately, though it may be simultaneously, into the urethra. In man, also, when one testicle is lost, the correspond- ing vesicula seminalis suffers no atrophy, though its function as a reservoir is abrogated. But how the vesiculae seminales act as secreting organs is unknown; the peculiai' brownish fluid which they contain after death does not properly represent their secre- tion, for it is different in appearance from anything discharged during life, and is mixed with semen. It is nearly certain, how- ever, that their secretion contributes to the proper composition of the impregnating fluid j for in all the animals in whom they exist, and in whom the generative functions are exercised at only one season of the year, the vesiculae seminales, whether they commu- nicate with the vasa deferentia or not, enlarge commensurately with the testicles at the approach of that season. That the vesiculae are also reservoirs in which the seminal fluid may lie for a time previous to its discharge, is shown by their commonly containing the seminal filaments in larger abun- dance than any portion of the seminal ducts themselves do. The fluid-like mucus, also, which is often discharged from the vesiculae in straining during defalcation, commonly contains seminal fila- ments. But no reason can be given why this office of the vesiculae should not be equally necessary to all the animals whose testicles are organised like those of man, or why in many animals the vesiculae are wholly absent. There is an equally complete want of information respecting the secretions of the prostate and Cowper's glands, their nature and purposes. That they contribute to the right composition of the 740 DEVELOPMENT. [chap. XXIII. impregnating fluid, is shown both by the position of the glands and by their enlarging with the testicles at the approach of an animal's breeding time. But that they contribute only a sub- ordinate part is shown by the fact, that, when the testicles are lost, though these other organs be perfect, all procreative power ceases. The fluid part of the semen or liquor seminis has not been satisfactorily analysed : but Henle says it contains fibrin, because shortly after being discharged, flocculi form in it by spontaneous coagulation, and leave the rest of it thinner and more liquid, so that the filaments move in it more actively. Nothing has shown what it is that makes this fluid with its corpuscles capable of impregnating the ovum, or (what is yet more remarkable) of giving to the developing offspring all the characters, in features, size, mental disposition, and liability to disease, which belong to the father. This is a fact wholly inex- plicable : and is, perhaps, only exceeded in strangeness by those facts which show that the seminal fluid may exert such an influ- ence, not only on the ovum which it impregnates, but, through the medium of the mother, on many which are subsequently im- pregnated by the seminal fluid of another male. CHAPTER XX11I. Changes which occur in the Ovum. DEVELOPMENT. Of the changes which take place in the ovum, some occur before and are as it were preparatory to impregnation, and others ensue after impregnation. It will be as well to consider the respective changes separately. (i.) Changes prior to Impregnation.-These changes espe- cially concern the germinal vesicle, and have been observed chiefly in the ova of low types. The ovum when ripe and detached from the ovary consists, it will be remembered, of a granular yolk enclosed within the protoplasmic zona pellucida, and containing the germinal vesicle and germinal spot situated eccentrically. The yelk granules are of different sizes, from the minutest molecules up to a diameter of vsooth to of an inch. The germinal vesicle consists of reticulated protoplasm enclosed in a distinct CHAP. XXIII.] CHANGES IN THE OVUM. 741 membrane, and containing one or more nucleoli or germinal spots. The primary change observed in the ovum consists in alterations in the shape of the vesicle, the disappearance of its protoplasmic reticulum, and of its enclosing membrane, with a consequent inden- tation and indistinctness of its outline. Its protoplasm becomes to a considerable extent confounded with the yelk substance, and its germinal spot disappears. The next step in the process is the appearance in the yelk of two stars in a clear space near the poles of the vesicle elongated to a certain extent, and from this results a nuclear spindle, corresponding to a nucleus in the process of division, with the stars at either end lying near the surface of the yelk. This spindle next becomes vertical, and the star nearer the surface protrudes from the ovum enveloped in a protoplasmic mass, which by constriction form the first polar cell. A second polar cell arises in the same way. From the remainder of the spindle within the yelk two or three vesicles arise, and by the junction of these a single nucleus is formed, which is called the female pro-nucleus. This is clearly derived from the original germinal vesicle. It must be remembered that these changes have been so far observed only in a certain number of instances. It is very possible, not to say probable, that such changes are universal in the animal kingdom (Balfour). Balfour's view as to the formation of the polar bodies may be given in his own words:-" My view amounts to the following, viz., that after the formation of the polar-cells, the remainder of the germinal vesicle within the ovum (the female pro-nucleus) is incapable of further deve- lopment without the addition of the nuclear part of the male element (spermatozoon), and that if polar-cells were not formed, parthenogenesis might normally occur." (2.) Changes following Impregnation.-The process of impregnation of the ovum has been observed most accurately in the lower types. In mammalia, although spermatozoa pass in numbers through the yelk envelope, yet their further progress is only inferred from observations on the lower animals. The process in asterias glacialis, according to Balfour, is as follows:- The head of a single spermatozoon joins with an elevation of the yelk substance, the tail remaining motionless, and then disappear- ing. The head enveloped in the protoplasm then sinks into the yelk and becomes a nucleus, from which the yelk substance is arranged in radiating lines. This is the male pro-nucleus. At first, at some distance from the female pro-nucleus, it after a while ap- proaches nearer, and the female pro-nucleus, which was before inac- DEVELOPMENT. [chai*, xxi 11. 742 tive, becomes active. The nuclei at last meet ami unite. The result of their union is the first segmentation sphere, or Blasto-sphere. It is a nucleated protoplasmic cell. The changes which have re- sulted in the formation of the Blasto-sphere or primitive segmenta- tion germ are followed by the process known as segmentation of the yelk. This process and the earlier stages in development are so funda- mentally similar in all vertebrate animals, from Fishes up to Man, that the gaps existing in our knowledge of the process in the higher Mammalia, such as man, may be, in part, at any rate, filled up by the more accurate knowledge which we possess of the development of the ovum in such animals as the trout, frog, and fowl. One important distinction betwen the ova of various Vertebrata should be remembered. In the hen's egg, besides the shell and the white or albumen, two other structures are to be distinguished-the germ, often called the cicatricula or "tread," and the yellt, enclosed in its vitelline membrane. The germ is (as was mentioned in the description already given) essen- tially a cell, consisting of protoplasm enclosing a nucleus and nucleolus. It alone participates in the process of segmentation, the great mass of the yelk (food-yelk) remaining quite unaffected by it. Since only the germ, which forms but a small portion of the yelk, undergoes segmentation, the ovum is called merollastic. In the Mammalia, on the other hand, there is no large unsegmented mass corresponding to the food-yelk of birds; the entire ovum undergoes segmentation, and is hence termed holoblastie. The eggs of Fishes, Reptiles, and Birds, are meroblastic, while those of Amphibia and Mammalia are holoVlaxtic. Of the changes which the mammalian ovum undergoes previous to the formation of the embryo, those which occur while it is still in the ovary are independent of impregnation: others take place after it has reached the Fallopian tube. The knowledge we possess of these changes is derived almost exclusively from observations on the ova of the bitch and rabbit: but it may be inferred that analogous changes ensue in the human ovum. As the ovum approaches the middle of the Fallopian tube, it begins to receive a new investment, consisting of a layer of trans- parent albuminous or glutinous substance, which forms upon the exterior of the zona pellucida. It is at first exceedingly fine, and, owing to this, and to its transparency, is not easily recognised, but at the lower part of the Fallopian tube it acquires considerable thickness. chap, xxiii.] SEGMENTATION OF THE YELK. 743 Segmentation.-The first visible result of fertilisation is a slight amoeboid movement in the protoplasm of the ovum : this has been observed in some fish, in the frog, and in some mammals. Immediately succeeding to this the process of segmentation com- mences, and is completed during the passage of the ovum through the Fallopian tube. In mammals, in which the process is an example of complete seg- mentation, the yelk becomes constricted in the middle, and surrounded by a furrow which gradually deepening, at length cuts it in half, while the same process begins almost immediately in each half of the yelk, and cuts it also in two. The same process is repeated in each of the quarters, and so on, until at last by continual cleav- ings, the whole yelk is changed into a mulberry-like mass of small and more or less rounded bodies, sometimes called vitelline spheres, the whole still enclosed by the zona pellucida or vitelline membrane (fig. 442). Each of these little spherules contains a transparent vesicle, like an oil- globule, which is seen with difficulty, on account of its being enveloped by the yelk- granules which adhere closely to its sur- face. The cause of this singular subdivision of the yelk is quite obscure : though the immediate agent in its production seems to be the central vesicle contained in each division of the yelk. Originally there was probably but one vesicle, situated in the centre of the entire granular mass of the yelk, and probably derived in the manner already described from the germinal vesicle. This divides and sub- divides : each successive division and subdivision of the vesicle being accompanied by a corresponding division of the yelk. About the time at which the Mammalian ovum reaches the uterus, the process of division and subdivision of the yelk appears to have ceased, its substance having been resolved into its ulti- mate and smallest divisions, while its surface presents a uniform Fig. 442.-Diagrams of the various stages of cleavage of the gelk (Dalton.) 744 DEVELOPMENT. [cuai*. xxiii. finely-granular-aspect, instead of its late mulberry-like appearance. The ovum, indeed, appears at first sight to have lost all trace of the cleavage process, and, with the exception of being paler and more translucent, almost exactly resembles the ovarian ovum, its yelk consisting apparently of a confused mass of finely granular substance. But on a more careful examination, it is found that these granules are aggregated into numerous minute spheroidal masses, each of which contains a clear vesicle or nucleus in its centre, and is, in fact, an embryonal cell. The zona pellucida, and the layer of albuminous matter surrounding it, have at this time the same character as when at the lower part of the Fal- lopian tube. The passage of the ovum, from the ovary to the uterus, occupies probably eight or ten days in the human female. When the peripheral cells, which are formed first, are fully developed, they arrange themselves at the surface of the yelk into a kind of membrane, and at the same time assume a polyhedral shape from mutual pressure, so as to resemble pavement epithe- lium. The deeper cells of the interior pass gradually to the surface and accumulate there, thus increasing the thickness of the membrane already formed by the more superficial layer of cells, while the central part of the yelk remains filled only with a clear fluid. By this means the yelk is shortly converted into a kind of secondary vesicle, the walls of which are composed exter- nally of the original vitelline membrane, and within by the newly formed cellular layer, the blastodermic or germinal membrane, as it is called. Segmentation in the Chick.-The embryo chick affords an illustration of what is known as incomplete or partial segmenta- tion, or meroblastic segmentation. In the youngest ova the germinal vesicle is situated subcentrally, but as development proceeds it passes to the periphery, and the protoplasm surround- ing it remaining free from yelk granules, the germinal disc is formed. This germinal disc is not marked out by any sharp line from the remaining protoplasm, but passes insensibly into it. The first change consists in the appearance of a furrow running across the disc dividing it into two; it does not extend across the whole breadth. A second furrow, at right angles, cutting the first a little eccentrically, next appears, and the disc is thus cut into four quadrants. The furrows do not extend through the whole thickness of the disc, and the segments are not separated out CHAP. XXIII.] SEGMENTATION IN THE CHICK. 745 on the lower aspect. The quadrants are next bisected by radiating furrows, and the disc is thus divided into eight parts. The central portion of each segment is now cut off from the peri- pheral furrow, so that a number of smaller central and larger peripheral portions result. As the primary division was eccentric and the succeeding followed the same plan, there results a bi- lateral symmetry ; but the relation of the axis of symmetry and the long axis of the embryo is not known. Rapid division of the segments by furrows in various directions now ensues, and the small central portions are more rapidly broken up than the larger, Fig. 443.- Vertical notion of area pellucida and area opaca (left extremity of figure) of blastoderm of a fresh-laid egg (unincubated). S, superficial layer corresponding to epiblast; D, deeper layer, corresponding to hypoblast, and probably in part to meso- blast ; Jf, large " formative cells," filled with yelk granules, and lying on the floor of the segmentation cavity; J, the white yelk immediately underlying the segmenta- tation cavity (Strieker'. and therefore become more numerous. During this superficial segmentation a similiar process goes on throughout the whole mass, and division goes on not only by vertical but also by hori- zontal furrows. The result of this process of segmentation is that the original germinal disc is cut up into a large number of small rounded protoplasmic cells, small in the centre, larger to the periphery, and that the superficial cells are smaller than those below: the two original layers of the blastoderm are thus early represented. The process of segmentation proceeds at the periphery of the germinal disc, and at the same time further division of the cells at the centre proceeds. The nucleus of the original cell divides coincidently with the protoplasm, and so it comes that the proto- plasmic masses are nucleated ; and besides this, nuclei derived from the original nucleus are found in the ovum below the area of segmentation, and from these, by the protoplasm which surrounds them being constricted off with them, supplementary segmentation masses come to be formed. The blastoderm is thus formed as the result of segmentation, and between it and the subjacent white 746 DEVELOPMENT. [chap. xxm. velk is a cavity containing fluid. The segmentation having been completed towards the centre, although it still proceeds at the periphery, the superficial layer of the blastoderm becomes a layer of columnar nucleated cells, and the lower layer consists of larger masses indistinctly nucleated, still granular and rounded, irregu- larly disposed. In the segmentation cavity are the supplementary segmentation masses or formative cells. When the egg is incubated, rapid changes take place in the blastoderm, resulting in the formation first of all of two, then of the three layers, which have been already mentioned in the first chapter. The superficial layer, or Epiblast, does not at first enter into these changes, but continues to be a layer of nucleated columnar cells. But in the lower layer of larger rounded cells, certain of the cells become flattened horizontally, their granules disappear, and the nuclei become distinct. A membrane of flat- tened nucleated cells is then formed, first of all towards the centre of the area, afterwards peripherally also : this is the Hypoblast. Between the two layers some cells, not belonging to either layer, remain. These cells are almost entirely at the back part of the area. The formations of the intermediate layer of mesoblast is more complicated, and will now be described. At this period it is necessary to return to the surface view of the blastoderm. Before incubation it is seen to consist of a more or less circular transparent area, the area pellucida, surrounded by an opaque rim, which is called the area opaca. The area opaca rests upon the white yelk : beneath the area pellucida is a cavity containing fluid. In the centre of the area pellucida is a white shining spot, or nucleus of Pander, shining through. The nucleus of Pander is the upper dilated extremity of the flask- shaped accumulation of white yelk upon which the blastoderm rests. The yellow yelk consists of spheres 25 p. to 100 p. in diameter, filled with highly refractive granules of an albuminous nature, and the white yelk being distinguished from the yellow not only by its lighter colour, bul also because its vesicles are smaller than those of the yellow. Each contains a highly refractive body. Some large spheres contain a number of spherules. Some of these are vacuolated. The white yelk not only envelopes the yellow yelk in a thin layer, and merges with the central flask-shaped mass, already mentioned, but also is found in the yellow yelk, forming with it alternate layers. Except that the central shining opacity of the pellucid area has disappeared, that the size of the area has increased, and that the • hap. xxiii.] FORMATION OF THE MESOBLAST. 747 opaque area has also increased, no other change can be remarked up to the formation of the two complete layers. There is, however, a slight ill-defined opacity at the posterior part of the area pel- lucida, known as the embryonic shield. This opacity'is probably due to the intermediate cells already mentioned as existing between the epiblast and hypoblast. In the posterior part of the area pellucida now appears an opaque streak which extends about a third of the diameter of the area towards the middle line. This is the Primitive streak. It is found on transverse section of the blastoderm in this neighbourhood to be due to a proliferation downwards of cells, two or more deep, from the epiblast. The area pellucida now becomes oval. As the primitive streak becomes more defined the area pellucida changes its oval for a pear shape, but the streak increases in size faster than the area, and so after a time is about two-thirds of its length. In the axis of the primitive streak agroove, the primitive groove runs. From the primitive streak the cells from the under-surface of the epiblast now extend as lateral Fig. 444.-Impregnated egg, with com- mencement of formation of embryo ; showing the area germinativa or embryonic spot, the area pellucida, and the primitive groove or trace (Dalton). Fig. 445.- Transverse section through embryo chick (26 hours), a, epiblast; 6, mesoblast; c, hypoblast; d, central portion of mesoblast, which is here fused with epiblast; r, primi- tive groove; /, dorsal ridge (Klein). wings to the edge of the pellucid area; they are not joined with the hypoblast. The intermediate layer of cells in this position producing 748 DEVELOPMENT. [chap, xxiii. the primitive streak is a portion of the intermediate layer or mesoblast. It is formed chiefly from the epiblast, but laterally, Fig. 446.-Diagram of transverse section through an embryo before the closing-in of the medullary groove. m, cells of epiblast lining the medullary groove which will form the spinal cord ; 7i, epiblast; d, hypoblast; ch, noto-chord ; u, protovertebra ; sp, mesoblast : w, edge of lamina dorsalis, folding over medullary groove (Kolliker). especially in the front part of the primitive streak, it appears to be derived at any rate in part from the cells of the primitive lower layer. At the most anterior part of the primitive streak, Fig. 447.-Portion of thegerminal membrane, uith rudiments of the embryo; from the ovum of a bitch. The primitive groove, a, is not yet closed, and at its upper or cephalic end presents three dilatations, b, which correspond to the three divisions or vesicles of the brain. At its lower extremity the groove presents a lancet-shaped dilatation (sinus rhomboidalis) c. The margins of the groove consist of clear pellucid nerve-substance. Along the bottom of the groove is observed a faint-streak, which is probably the chorda dorsalis, d. Vertebral plates (Bischoff J. at the point which corresponds to the future posterior end of the embryo, the three layers are all joined together. The next important change which occurs is found in the hypo- blast in front of the primitive streak. The irregular layer of 'HAP. XXIII.] THE NOTOCHOBB. 749 primitive cells of which it is composed, split into two layers, the lower of flattened cells which forms the hypoblast proper, and an upper of several layers of stellate cells, the mesoblast. In the preceding account of the formation of the blastodermic layers, Balfour's description has been chiefly followed. It differs somewhat from that which has been given in previous editions of this book. The mesoblast Fig. 448.- Vertical section of blastoderm of chick (1st day of incubation). S, epiblast, con- sisting of short columnar cells ; D, hypoblast, consisting of a single layer of flattene 1 cells ; JZ, "formative cells." They are seen on the right of the figure, passing in between the epiblast and hypoblast to form the mesoblast; J, white yelk granules. Many of the large " formative cells " are seen containing these granules (Stricker). was described as arising from the hypoblast, together with some of the large formative cells, which migrate by amoeboid movement round the edge of the hypoblast (fig. 448, 3/), and no difference was made in the formation of the mesoblast in the primitive streak and elsewhere. Now appears in the middle line extending forwards from the primitive streak an opaque line, which proceeds almost to the anterior edge of the area pellucida, stopping short at a transverse crescent-shaped line, the future headfold. This line is the com- mencing notochord. It is a collection of mesoblastic cells from the hypoblast in the middle line, and remains connected with the latter after the lateral portions of the mesoblast have become quite detached from it. The notochord and the hypoblast from which it arises are continued posteriorly into the primitive streak. Thus the mesoblast of the area on either side of the middle line in which the embryo is formed arises from the hypoblast, as does also the notochord. In the formation of the medullary plate which now appears, the epiblast is concerned. In the middle line above the collection of cells that will become the notochord that layer becomes thickened. The sides of the central thickened portion are elevated somewhat to form the medullary folds enclosing between them the medullary groove. From this medullary plate is formed the central nervous system. Although behind the groove is a shallow one, if it be traced forwards it becomes deeper and narrower, and at the headfold the folds curve 750 DEVELOPMENT. [chap. XXU1. round, and meet in the middle line. Anterior to the headfold is a second fold parallel to it, which is the commencing amnion. The medullary canal is bounded by its two folds or longitudinal elevations, laminse dorsales, which are folds consisting entirely of cells of the epiblast: these grow up and arch over the medullary groove (fig. 446) till after some time they coa- lesce in the middle line, converting it from an open furrow into a closed tube-the neural canal or the primitive cerebro-spinal axis. Over this closed tube, the walls of which consist of more or less cylindrical cells, the superficial layer of the epiblast is now continued as a distinct membrane. The union of the medullary folds or laminae dorsales takes place first about the neck of the future embryo; they soon after unite over the region of the head, while the closing in of the groove progresses much more slowly towards the hinder extremity of the embryo. The medullary groove is by no means of uniform diameter throughout, but even be- fore the dorsal laminae have united over it, is seen to be dilated at the anterior extremity and obscurely divided by constrictions into the three primary vesicles of the brain. The part from which the spinal cord is formed is of nearly uniform calibre, while towards the posterior extremity is a lozenge-shaped dilata- tion, sinus rhomboidalis, which is the last part to close in (fig. 447). Whilst the changes which have been described are taking place in the area pellucida, which has eir Fig. 449.-Embryo chick (30 hours), viewed from beneath as a transparent object (magnified), pl, outline of pellucid area ; Eli, fore-brain, or first cerebral vesicle : from its sides project op, the optic vesicles; SO, backward limit of somatopleure fold, " tucked in " under head ; a, head-fold of true amnion; o', re- flected layer of amnion, sometimes termed "false amnion ; " sp, back- ward limit of splanchnopleure folds, along which run the omphalomesa- raic veins uniting to form h, the heart, which is continued forwards into ba, the bulbus arteriosus; d, the fore-gut, lying behind the heart, and having a wide crescentic open- ing between the splanchnopleure folds ; HB, hind-brain ; Mil, mid- brain ; pv, protovertebrie lying be- hind the fore-gut; me, line of junc- tion of medullary folds and of notochord; ch, front end of noto- chord ; vpl, vertebral plates; pr, the primitive groove at its caudal end (Foster and Balfour), CHAP. XXIII.] THE SPLITTING OF THE MESOBLAST. 751 larged to a certain extent, the area opaca has considerably extended. The hypoblast and mesoblast have also been pro- longed laterally, not by mere extension, but also from the ger- minal wall, which is the thickened edge of the blastoderm, together with formative cells of the yelk; on each side of the notochord and medullary canal, the mesoblast remains as a longitudinal thickening. It now however splits horizontally into two layers or lamina) {parietal and visceral) : of these the former, when traced out from Fig. 450.-Transverse section through dorsal region or embryo chick (45 hrs.). One half of the section is represented : if completed it would extend as far to the left as to the right of the line of the medullary canal (Jfc). A, epiblast; C, hypoblast, consisting of a single layer of flattened cells ; Jfc, medullary canal; l'c, protovertebra ; Urd, Wolffian duct; So, somatopleure ; Sp, splanchnopleure ; pp, pleuro-peritoneal cavity ; ch, noto- chord ; <10, dorsal aorta, containing blood cells ; v, blood-vessels of the yolk-sac (Foster and Balfour). the central axis, is seen to be in close apposition with the epiblast and gives origin to the parietes of the trunk, while the latter adheres more or less closely to the hypoblast, and gives rise to the serous and muscular walls of the alimentary canal and several other* parts (fig. 450). The united parietal layer of the mesoblast with the epiblast is termed Somatopleure, the united visceral layer and hypoblast, Splanchnopleure. The space between them is the pleuro- peritoneal cavity, which becomes subdivided by subsequent partitions into pericardium, pleura, and peritoneum. The splitting of the mesoblast extends almost to the medullary canal, but a portion on either side (p. v. fig. 450) remains un- divided, the vertebral plate. The divided portion is known as the lateral plate. The longitudinal thickening of the vertebral plate is seen after awhile to be divided, at right angles to the 752 DEVELOPMENT. [chap. XX111. medullary canal by bright transverse lines into a number of square segments. These segments, which are the surface ap- pearance of cubes of mesoblast, are the mesoblastic somites or proto vertebrae. The first three or four of these protovertebrac make their appearance in the cervical region, while one or two more are formed in front of this point: and the series is continued backward till the whole medullary canal is flanked by them (fig. 449). That which is first formed corresponds to the second cervical vertebrae. From these somites the vertebra; and the trunk muscles are derived. Head and Tail Folds. Body Cavity.-Every vertebrate Fig. 451,-Diagrammatic longitudinal section through the axis of an embryo. The head-fold has commenced, but the tail-fold has not yet appeared. FSo, fold of the somato- pleure ; F8i>, fold of the splanehnopleure ; the line of reference, FSo, lies outside the embryo in the "moat," which marks off the overhanging head from the amnion ; />. inside the embryo, is that part which is to become the fore-gut; FSo and Fsp, are both parts of the head-fold, and travel to the left of the figure as development proceeds ; pp, space between somatopleure and splanehnopleure, pleuro-peritoneal cavity; Am, commencing head-fold of amnion; .VC, neural canal; <'A, notochord ; 2Zi, heart; A, II, C, epiblast, mesoblast, hypoblast (Foster and Balfour.) animal consists essentially of a longitudinal axis (vertebral column) with a neural canal above it, and a body-cavity (containing the alimentary canal) beneath. We have seen how the earliest rudiments of the central axis and the neural canal are formed; we must now consider how the general body-cavity is developed. In the earliest stages the embryo lies flat on the surface of the yelk, and is not clearly marked off from the rest of the blastoderm: but gradually the head fold or crescentic depression (with its concavity backwards) is formed in the blastoderm, limiting the head of the embryo ; the blastoderm is, as it were, tucked in under the head, which thus comes to project above the general surface of the membrane : a similar tucking in of blastoderm takes place at the caudal extremity, and thus the head and tail folds are formed (fig. 452). Similar depressions mark off the embryo laterally, until it is chap, xx 111.] THE HEAD FOLD. 753 completely surrounded by a sort of moat which it overhangs on all sides, and which clearly defines it from the yelk. This moat runs in further and further all round beneath the overhanging embryo, till the latter comes to resemble a canoe turned upside-down, the ends and middle being, as it were, Fig. 452.-Diagrammatic section showing the. relation in a mammal between the primitive alimen- targ canal and the. membranes of the ovum. The stage represented in this diagram cor- responds to that of the fifteenth or seventeenth day in the human embryo, previous to the expansion of the allantois ; c, the villous chorion; a, the amnion ; the place of convergence of the amnion and reflexion of the false amnion a" a", or outer or corneous layer ; e, the head and trunk of the embryo, comprising the primitive vertebra) and cerebro-spinal axis ; i, t, the simple alimentary canal in its upper and lower portions. Immediately beneath the right hand r is se'en the fu tai heart, lying in the anterior part of the pleuro-peritoneal cavity; », the yolk-sac or umbilical vesicle ; v i, the vrtello-in- testinal opening; u, the allantois connected by a pedicle with the anal portion of the alimentary canal. (Quain.) decked in by the folding or tucking in of the blastoderm, while on the ventral surface there is still a large communication with the yelk, corresponding to the well or undecked portion of the canoe. This communication between the embryo and the yelk is gra- dually contracted by the further tucking in of the blastoderm from all sides, till it becomes narrowed down, as by an invisible 754 DEVELOPMENT. [chap, xxiii. constricting band, to a mere pedicle which passes out of the body, of the embryo at the point of the future umbilicus. The downwardly folded portions of blastoderm are termed the visceral plates. Thus we see that the body-cavity is formed by the downward folding of the visceral plates, just as the neural cavity is pro- duced by the upward growth of the dorsal laminae, the difference being that, in the visceral or ventral laminae, all three layers of the blastoderm are concerned. The folding in of the splanchnopleure, lined by hypoblast, pinches off, as it were, a portion of the yelk-sac, enclosing it in the body-cavity. This forms the rudiment of the alimentary canal, which at this period ends blindly towards the head and tail, while in the centre it communicates freely with the cavity of the yelk-sac through the canal termed vitelline or omphalomes- enteric duct. The yelk-sac thus becomes divided into two portions which communicate through the vitelline duct, that portion within the body giving rise, as above stated, to the digestive canal, and that outside the body remaining for some time as the umbilical vesicle (fig. 453, ys). The hypoblast forming the epithelium of the intestine is of course continuous with the lining membrane of the umbilical vesicle, while the visceral plate of the mesoblast is continuous with the outer layer of the umbilical vesicle. All the above details will be clear on reference to the accom- panying diagrams. At the posterior end of the embryo chick, when the amniotic fold iscommencingto be formed, and the hind fold of the splanchnopleure has commenced, there remains for a time a communication between the neural canal and the hind gut, which is called the neurenteric canal. It passes in at the point where the notochord falls into the primitive streak. The anterior part of the primitive streak becomes the tail swelling, the posterior part atrophies, and the correspond- ing lateral part of the blastoderm forms part of the body-wall of the embryo. The anterior part of the medullary canal having been completely roofed in; the foremost portion undergoes dilatation, and a bulb, or first cerebral vesicle results. From either side of this dilatation a process, the cavity of which is in communi- cation with it, is separated off; these processes are the optic vesicles. Behind the first cerebral vesicle two other vesicles now arise, and at the posterior part of the head two small pits, the chap, xxin.] THE FCETAL MEMBRANES. 755 auditory pits, are to be seen. The folding of the head, it should be recollected, is the cause of the enclosure below the neural canal (fig- 45a canal ending blindly, which has in front the splanch- nopleure, and which is just as long as the involution of that mem- brane. This canal is the fore-gut. In the interior of the splanch- nopleure fold below it (as seen in fig. 451) in the pleuro-peritoneal cavity the heart is formed, at the point where the splanchnopleure makes its turn forwards. It arises as a thickening of the meso- blast on either side as the two splanchnopleure folds diverge, and of a thickening of the mesoblast at the point of divergence. So that at first the rudiment of the heart is like an inverted V, which by the gradual coming together of the diverging cords is converted into an inverted Y. The cylinders become hollowed out, and are thus converted into tubes, which then coalesce. Layers are separated off towards the interior, which become the epithelial lining, and the mass of the mesoblast surrounding this afterwards forms the muscle and serous covering, whilst at first the rudimentary organ is attached to the gut by a mesoblastic mesentery, the mesocardium. Fcetal Membranes. Umbilical Vesicle or Yelk-sac.-The splanchnopleure, lined by hypoblast, forms the yelk-sac in Reptiles, Birds, and Mammals ; but in Amphibia and Fishes, since there is neither amnion nor allantois, the wall of the yelk-sac consists of all three layers of the blastoderm, enclosed, of course, by the original vitelline membrane. The body of the embryo becomes in great measure detached from the yelk sac or umbilical vesicle, which contains, however, the greater part of the substance of the yelk, and furnishes a source whence nutriment is derived for the embryo. This nutri- ment is absorbed by the numerous vessels (omphalo-mesenteric) which ramify in the walls of the yelk-sac, forming what in birds is termed the area vasculosa. In Birds, the contents of the yelk-sac afford nourishment until the end of incubation, and the .omphalo-mesenteric vessels are developed to a corresponding degree; but in Mammalia the office of the umbilical vesicle •ceases at a very early period, the quantity of the yelk is small, and the embryo soon becomes independent of it by the connections it forms with the parent. Moreover, in Birds, as the sac is emptied, 756 DEVELOPMENT. [chap, xxiii. it is gradually drawn into the abdomen through the umbilical opening, which then closes over it: but in Mammalia it always Fig. 453/-Diagrams showing three successive stages of development. Transverse vertical sections. The yelk-sac, ys, is seen progr essively diminishing in size. In the embryo itself the medullary canal and notochord are seen in section, a', in middle figure, the alimentary canal, becoming pinched off, as it were, from the yelk-sac ; a', in right hand figure, alimentary canal completely closed ; a, in last two figures, amnion; ac, cavity of amnion filled with amniotic fluid ; pp, space between amnion and chorion con- tinuous with the pleuro-peritoneal cavity inside the body ; t>(. vitelline membrane; ys, yelk-sac, or umbilical vesicle. (Foster and Balfour.) remains on the outside ; and as it is emptied it contracts (fig. 455), shrivels up, and together with the part of its duct external to the abdomen, is detached and disappears cither before or at the termi 1'ig. 455.- Human embryo of fifth week with umbilical vesicle; about natural size (Dalton). The human umbilical vesicle never exceeds the size of a small pea. Fig. 454-Diagram showing vascular area in the chick, a, area pellucida ; b, area vasculosa; c. area vitellina. nation of intra-uterine life, the period of its disappearance varying in different orders of Mammalia. When blood'Vessels begin to be developed, they ramify largely over the walls of the umbilical vesicle, and are actively concerned < HAP. XXIII.] FORMATION OF THE AMNION. 757 iii absorbing its contents and conveying them away for the nutrition of the embryo. At an early stage of development of the foetus, and some time before the completion of the changes which have been just described, two important structures, called respectively the amnion and the allantois, begin to be formed. Amnion.-The amnion is produced as follows :-Beyond the head- and tail-folds before described (p. 752), the somatopleure coated by epi blast, is raised into folds, which grow up, arching over the embryo, not only anteriorly and posteriorly but also laterally, and all converging towards one point over its dorsal surface (fig. 453). The growing up of these folds from all sides and their convergence towards one point very closely resembles the folding inwards of the visceral plates already described, and hence, by some, the point at which the amniotic folds meet over the back has been termed the amniotic umbilicus. The folds not only come into contact but coalesce. The inner of the two layers forms the true amnion, while the outer or reflected layer, sometimes termed the false amnion, coalesces with the inner surface of the original vitelline membrane to form the subzonal membrane or false chorion. This growth of the amniotic folds must of course be clearly distinguished from the very similar process, already described, by which the walls of the neural canal are formed at a much earlier stage. The cavity between the true amnion and the external surface of the embryo becomes a closed space, termed the amniotic cavity (ac, fig. 453). At first, the amnion closely invests the embryo, but it becomes gradually distended with fluid (liquor amnii), which, as preg- nancy advances, reaches a considerable quantity. This fluid consists of water containing small quantities of albumen and urea. Its chief function during gestation appears to be the mechanical one of affording equal support to the embryo on all sides, and of protecting it as far as possible from the effects of blows and other injuries to the abdomen of the mother. The embryo up to the end of pregnancy is thus immersed in fluid, which during parturition serves the important purpose of gradually and evenly dilating the neck of the uterus to allow of the passage of the foetus : when this is accomplished the amniotic sac bursts, and the " waters " escape. On referring to the diagrams (fig. 453), it will be obvious that the cavity outside the amnion (between it and the false amnion) 758 DEVELOPMENT. [chap. xxm. is continuous with the pleuro-peritoneal cavity at the umbilicus. This cavity is not entirely obliterated even at birth, and contains a small quantity of fluid ("false waters"), which is discharged during parturition either before, or at the same time as the amniotic fluid. Allantois.-Into the pleuro-peritoneal space the allantois sprouts out, its formation commencing during the development of the amnion. Growing out from or near the binder portion of the intestinal canal (c, fig. 456), with which it communi- cates, the allantois is at first a solid pear- shaped mass of splanchnopleure; but be- coming vesicular by the projection into it of a hollow out-growth of hypoblast, and very soon simply membranous and vascular, it insinuates itself between the amniotic folds, just described, and comes into close contact and union with the outer of the two folds, which has itself, as before said, become one with the external investing membrane of the egg. As it grows, the allantois develops muscular tissue in its external wall and be- comes exceedingly vascular; in birds (fig. 457) it envelops the whole embryo-taking up vessels, so to speak, to the outer investing membrane of the egg, and lining the inner surface of the shell with a vascular membrane, by these means affording an extensive surface in which the blood may be aerated. In the human sub- ject and in other Mammalia, the vessels carried out by the allantois arc distributed only to a special part of the outer membrane or false chorion, where, by interlacement with the vascular system of the mother, a structure called the placenta is developed. In Mammalia, as the visceral laminse close in the abdominal cavity, the allantois is thereby divided at the umbilicus into two portions; the outer part, extending from the umbilicus to the chorion, soon shrivelling; while the inner part, remaining in the abdomen, is in part converted into the urinary bladder; the portion of the inner part not so converted, extending from the bladder to the umbilicus, under the name of the urachus. After birth the um- bilical cord, and with it the external and shrivelled portion of the allantois, are cast off at the umbilicus, while the urachus remains as an impervious cord stretched from the top of the urinary bladder Fiff. 456. - Diagram of fecundated egg. a, um- bilical vesicle; b, am- niotic cavity ; c, allan- tois. (Dalton.) CHAP. XXIII.] THE CHORION. 759 to the umbilicus, in the middle line of the body, immediately beneath the parietal layer of the peritoneum. It is sometimes enumerated among the ligaments of the bladder. It must not be supposed that the phenomena which have been successively described, occur in any regu- lar order one after another. On the con- trary, the development of one part is going- on side by side with that of another. The Chorion.-It has been already re- marked that the allantois is a structure which extends from the body of the foetus to the outer investing membrane of the ovum, that it insinuates itself between the two layers of the amniotic fold, and becomes fused with the outer layer, which has itself become previously fused with the vitelline membrane. By these means the external investing membrane of the ovum, or the true chorion, as it is now called, re- presents three layers, namely, the original vitelline membrane, the outer layer of the amniotic fold, and the allantois. Very soon after the entrance of the ovum into the uterus, in the human subject, the outer surface of the chorion is found beset with fine processes, the so-called villi of the chorion (a, figs. 458, 459), which give it a rough and shaggy ap- pearance. At first only cellular in structure, these little outgrowths subsequently become vascular by the development in them of loops of capillaries (fig. 460) ; and the latter at length form the minute extremities of the blood-vessels which are, so to speak, conducted from the foetus to the chorion by the allantois. The function of the villi of the chorion is evidently the absorption of nutrient matter for the foetusj and this is probably supplied to them at first from the Huid matter, secreted by the follicular glands of the uterus, in which they are soaked. Soon, however, the foetal vessels of the villi come into more intimate relation with the vessels of the uterus. The part at which this relation between the vessels of the foetus and those of the parent ensues, is not, however, over the whole surface of the chorion: for, although all the villi become vascular, yet they become indistinct or disappear except Fig. 457. - Fecundated egg with allantois nearly com- plete. a, inner layer of amniotic fold; b, outer layer of ditto; c. point where the amniotic folds come in contact. The allantois is seen penetrat- ing between the outer and inner layers of the amni- otic folds. This figure, which represents only the amniotic folds and the parts within them, should be compared with figs. 453> 459> in which will be found the structures ex- ternal to these folds. (Dalton.) 760 DEVELOPMENT. [chap. XX11I. at one part where they tire greatly developed, and by their branching give rise, with the vessels of the uterus, to the forma- tion of the placenta. I'igs. 458 and 459. «, chorion with villi. The villi are shovn to be best developed in the part of the chorion to which the allantois is extending ; this portion ultimately In- comes the placenta; b, space between the two layers of the amnion ; c, amniotic cavity; rf, situation of the intestine, showing its connection with the umbilical vesicle ; e, um- bilical vesicle; f, situation of heart and vessels; g, allantois. To understand the manner in which the foetal and maternal blood-vessels come into relation with each other in the placenta, it is necessary briefly to notice the changes which the uterus undergoes after impregnation. These changes consist especially of alterations in struc- ture of the superficial part of the mucous membrane which lines the interior of the uterus, and which forms, after a kind of development to be immediately described, the membrana decidua, so called on account of its being discharged from the uterus at birth. Fig. 4 >o. Formation of the Placenta. The mucous membrane of the hu- man uterus, which consists of a matrix of connective tissue containing numerous corpuscles (adenoid tissue), and is lined internally by columnar ciliated epithelium, is abundantly beset with tubular glands, arranged perpendicularly to the surface (fig. 459'). These follicles are very CHAP. XXIII.] UTERINE SINUSES. 761 small in the unimpregnated uterus ; but when examined shortly after impregnation, they are found elongated, enlarged, and much Fig. 461.-Section of the lining membrane of a human uterus at the period of commencing pregnaifcg showing the arrangement and other peculiarities of the glands, d, d, d, with their orifices, a, a, a, on the internal surface of the organ. Twice the natural size. waved and contorted towards their deep and closed extremity, which is implanted at some depth in the tissue of the uterus, and may dilate into two or three closed sacculi (fig. 461). The glands are lined by columnar ciliated epithelium and they open on the inner surface of the mucous mem- brane by small round orifices set closely together (a, a, fig. 461). On the internal surface of the mucous membrane may be seen the circular orifices of the glands, many of which are, in the early period of pregnancy, surrounded by a whitish ring, formed of the epithelium which lines the follicles (fig. 462). Coincidently with the occurrence of pregnancy, important changes occur in the structure of the mucous membrane of the uterus. The epithe- lium and sub-epithelial connective tissue, together with the tubular glands, increase rapidly, and there is a greatly increased vascularity of the whole mucous membrane, the vessels of the mucous membrane becoming larger and more nume- rous ; while a substance composed chiefly of nucleated cells fills up the interfollicular spaces in which the blood-vessels are con- Fig. 462.-Tivo thin segments of human decidua after recent impregnation, viewed on a dark ground: they show the openings on the surface of the membrane, a is magnified six dia- meters, and b twelve diameters. At 1, the lining of epithelium is seen within the orifices, at 2 it has escaped. (Sharpey.) 762 DEVELOPMENT. [chap. XXI11. tained. The effect of these changes is an increased thickness, softness, and vascularity of the mucous membrane, the superficial part of which itself forms the membrana decidua. The object of this increased development seems to be the pro- duction of nutritive mate- rials for the ovum ; for the cavity of the uterus shortly becomes filled with secreted fluid, consisting almost en- tirely of nucleated cells in which the villi of the cho- rion are imbedded. When the ovum first en- ters the uterus it becomes imbedded in the structure of the decidua, which is yet quite soft, and in which soon afterwards three portions are distinguishable. These have been named the de- cidua vera, the decidua rejlexa, and the decidua serofona. The first of these, the decidua vera, lines the cavity of the uterus ; the second, or decidua rejlexa, is a part of the decidua vera which grows up around the ovum, and, wrapping it closely, forms its immediate investment. The third, or decidua serotina, is the part of the decidua vera which becomes especially developed in connection with those villi of the chorion, which, instead of disappearing, remain to form the foetal part of the placenta. In connection with these villous processes of the chorion, there are developed depressions or crypts in the decidual mucous mem- brane, which correspond in shape with the villi they are to lodge ; and thus the chorionic villi become more or less imbedded in the maternal structures. These uterine crypts, it is important to note, are not, as was once supposed, merely the open mouths of the uterine follicles. As the ovum increases in size, the decidua vera and the decidua i eflexa gradually come into contact, and in the third month of preg- nancy the cavity between them has quite disappeared. Henceforth it is very difficult, or even impossible, to distinguish the two layers. Fig. 463.-Diagram of an early stage of the forma- tion of the human placenta, a, embryo ; 6, am- nion ; c, placental vessels ; d, decidua reflexa ; e, allantois; f, placental villi; <7, mucous membrane. (Cadiat.) chap, xxn[.] FORMATION OF THE PLACENTA. 763 The Placenta.-During these changes the deeper part of the mucous membrane of the uterus, at and near the region where the placenta is placed, becomes hollowed out by sinuses, or cavernous spaces, which communicate on the one hand with Fig. 464.-Diagrammatic view of a vertical transverse section of the uterus at the seventh or eighth week of pregnancy, c, c, c', cavity of uterus, which becomes the cavity of the decidua, opening at c, c, the cornua, into the Fallopian tubes, aud at c' into the cavity of the cervix, which is closed by a plug of inucus ; d v, decidua vera; d r, decidua reflexa, with the sparser villi imbedded in its substance; d s, decidua serotina, in- volving the more developed chorionic villi of the commencing placenta. The foetus is seen lying in the amniotic sac; passing up from the umbilicus is seen the umbilical cord and its vessels, passing to their distribution in the villi of the chorion ; also the pedicle of the yelk sac, which lies in the cavity between the amnion and chorion. (Allen Thomson.) arteries and on the other with veins of the uterus. Into these sinuses the villi of the chorion protrude, pushing the thin wall of the sinus before them, and so come into intimate relation with the blood contained in them. There is no direct communication between the blood-vessels of the mother and those of the foetus ; but the layer or layers of membrane intervening between the blood of the one and of the other offer no obstacle to a free inter- chano,e of matters between them. Thus the villi of the chorion 764 DEVELOPMENT. [chap. XXIII. containing foetal blood, are bathed or soaked in maternal blood contained in the uterine sinuses. The arrangement may be roughly compared to filling a glove with foetal blood, and dipping its fingers into a vessel containing maternal blood. But in the foetal villi there is a constant stream of blood into and out of the loop of capillary blood-vessels contained in it, as there is also into and out of the maternal sinuses. It would seem that, at the villi of the placental tufts, where the foetal and maternal portions of the placenta are brought into close relation with each other, the blood in the vessels of the mother is separated from that in the vessels of the foetus by the interven- tion of two distinct sets of nucleated cells (fig. 465). One of these (6) be- longs to the maternal portion of the placenta, is placed between the mem- brane of the villus and that of the vas- cular system of the mother, and is probably designed to separate from the blood of the parent the materials destined for the blood of the foetus; the other (/) belongs to the foetal portion of the placenta, is situated between, the membrane of the villus and the loop of vessels contained within, and probably serves for the absorption of the material secreted by the other sets of cells, and for its conveyance into the blood-vessels of the foetus. Between the two sets of cells with their investing membrane there exists a space (<?), into which it is probable that the materials secreted by the one set of cells of the villus are poured in order that they may be absorbed by the other set, and thus conveyed into a foetal vessel. Not only, however, is there a passage of materials from the blood of the mother into that of the foetus, but there is a mutual interchange of materials between the blood both of foetus and of parent; the latter supplying the former with nutriment, and in turn abstracting from it materials which require to be removed. Alexander Harvey's experiments were very decisive on this point. The a iew has also received abundant support from Hutchinson's important •observations on the communication of syphilis from the father to the mother. I'>g- 465--Extremity of a placen- tal villus, a, lining membrane of the vascular system of the mother; b, cells immediately lining a; d, space between the maternal and foetal portions of the villus ; e, internal mem- brane of the villus, or external membrane of the chorion; f, internal cells of the villus, or cells of the chorion ; <7, loop of umbilical vessels. (Goodsir.) CHAP. XXIII.] THE TWO PARTS OF THE PLACENTA. 765 through the instrumentality of the foetus ; and still more from Savory's experimental researches, which prove quite clearly that the female parent may be directly inoculated through the foetus. Having opened the abdomen and uterus of a pregnant bitch, Savory injected a solution of strychnia into the abdominal cavity of one foetus, and into the thoracic cavity of another, and then replaced all the parts, every precaution being taken to prevent escape of the poison. In less than half an honr the bitch died from tetanic spasms: the foetuses operated on were also found dead, while the others were alive and active. The experiments, repeated on other animals with like results, leave no doubt of the rapid and direct transmission of matter from the foetus to the mother, through the blood of the placenta. The placenta, therefore, of the human subject is composed of a foetal part and a maternal part,-the term placenta properly in- cluding all that entanglement of foetal villi and maternal sinuses, by means of which the blood of the foetus is enriched and purified after the fashion necessary for the proper growth and develop- ment of those parts which it is designed to nourish. The importance of the placenta is at once apparent if we remember that during the greater portion of intra-uterine life the maternal blood circu- lating in its vessels supplies the foetus with both food and oxygen. It thus performs the functions which in later life are discharged by the alimentary canal and lungs. The whole of this structure is not, as might be imagined, thrown off immediately after birth. The greater part, indeed, comes away at that time, as the after-birth ; and the separation of this portion takes place by a rending or crushing through of that part at which its cohesion is least strong, namely, where it is most bur- rowed and undermined by the cavernous spaces before referred to. In this way it is cast off with the foetal membrane and the decidua eera and ref era, together with a part of the decidua serotina. The remaining portion withers, and disappears by being gradually either absorbed, or thrown off in the uterine discharges or the lochia, which occur at this period. A new mucous membrane is of course gradually developed, as the old one, by its transformation into the decidua, ceases to perform its original functions. The umbilical cord, which in the latter part of foetal life is almost solely composed of the two arteries and the single vein which respec- tively convey foetal blood to and from the placenta, contains the remnants of other structures which in the early stages of the development of the embryo were, as already related, of great com- parative importance. Thus, in early foetal life, it is composed of the following parts:-(r.) Externally, a layer of the amnion, 766 DEVELOPMENT. [chap, xx 111. reflected over it from the umbilicus. (2.) The umbilical vesicle with its duct and appertaining omphalo-mesenteric blood-vessels. (3.) The remains of the allantois, and continuous with it the urachus. (4.) The umbilical vessels, which, as just remarked, ultimately form the greater part of the cord. It remains now to consider in succession the development of the several organs and systems of organs in the further progress of The Development of the Organs. Fig. 466.-Embryo chick (4th day), viewed as a transparent object, lying on its left side (mag- nified) . C H, cerebral hemispheres ; F B, fore-brain or vesicle of third ventricle, with Pn, pineal gland projecting from its summit; J/ B, mid-brain; C b, cerebellum ; IV F, fourth ventricle ; L, lens; c h s, choroidal slit; Gen V, auditory vesicle; s m, superior maxillary process; iF, 2F, &c., first, second, third, and fourth visceral folds; r, fifth nerve, sending one branch (ophthalmic) to the eye, and another to the first visceral arch ; VII, seventh nerve, passing to the second visceral arch ; G Ph, glosso- pharyngeal nerve, passing to the third visceral arch; P g, pneumogastric nerve, pass- ing towards the fourth visceral arch ; i v, investing mass ; c h, notochord ; its front end cannot be seen in the living embryo, and it does not end as shown in the figure, but takes a sudden bend downwards, and then terminates in a point; H t, heart seen through the walls of the chest; M P, muscle-plates ; II', wing, showing commencing differentiation of segments, corresponding to arm, forearm, and hand; HL, hind-limb, as yet a shapeless bud, showing no differentiation. Beneath it is seen the curved tail. (Poster and Balfour.) the embryo. The accompanying figure (fig. 466) shows the chief organs of the body in a moderately early stage of development. The Vertebral Column and Cranium.-The primitive part of the vertebral column in all the Vertebrata is the chorda dorsalis (notochord), which consists entirely of soft cellular cartilage. This cord tapers to a point at the cranial and caudal extremities of the animal. In the progress of its development, it is found to become CHAP. XXIII.] THE CRANIUM AND VERTEBRAE. 767 enclosed in a membranous sheath, which at length acquires a fibrous structure, composed of transverse annular fibres. The chorda dorsalis is to be regarded as the azygos axis of the spinal column, and, in particular, of the future bodies of the vertebra;-, although it never itself passes into the state of hyaline cartilage or bone, but remains enclosed as in a case within the persistent parts of the vertebra] column which are developed around it. It is permanent, how'ever, only in a few animals : in the majority only traces of it persist in the adult animal. In many Fish no true vertebrae are developed, and there is every gradation from the amphioxus, in which the notochord per- sists through life and there are no vertebrae, through the lampreys in which there are a few scattered cartilaginous vertebrae, and the sharks, in which many of the vertebra) are partly ossified, to the bony fishes, such as the cod and herring, in which the vertebral column consists of a number of distinct ossified vertebrae, with remnants of the notochord between them. In Amphibia, Reptiles, Birds, and Mammals, there are distinct vertebrae, which are formed as follows :- The mesoblastic somites, which have been already mentioned (p. 752)j send processes downwards and inwards to surround the notochord, and also upwards between the medullary canal and the epiblast covering it. In the former situation, the cartilaginous bodies of the vertebrae make their appearance, in the latter their arches, which enclose the neural canal. The vertebrae do not exactly correspond in their position with the protovertebrae : but each permanent vertebrae is developed from the contiguous halves of two protovertebrae. The original segmentation of the protovertebrae disappears and a fresh subdi- vision occurs in such a way that a permanent invertebral disc is developed opposite the centre of each protovertebra. Meanwhile the protovertebrae split into a dorsal and ventral portion. The former is termed the musculo-cutaneous plate, and from it are developed all the muscles of the back together with the cutis of the dorsal region (the epidermis being derived from the epiblast). The ventral portions of the protovertebrae, as we have already seen, give rise to the vertebrae and heads of the ribs. The chorda is now enclosed in a case, formed by the bodies of the vertebrae, but it gradually wastes and disappears. Before the disappearance of the chorda, the ossification of the bodies and arches of the vertebrae begins at distinct points. 768 DEVELOPMENT. [chap, xxiii. The ossification of the body of a vertebra is first observed at the point where the two primitive elements of the vertebra) have united inferiorly. Those vertebra) which do not bear ribs, such as the cervical vertebra), have generally an additional centre of ossification in the transverse process, which is to be regarded as an abortive rudiment of a rib. In the foetal bird, these additional ossified portions exist in all the cervical vertebrae, and gradually become so much developed in the lower part of the cervical region as to form the upper false ribs of this class of animals. The same parts exist in mammalia and man ; those of the last cervical vertebra) are the most developed, and in children may, for a con- siderable period be distinguished as a separate part on eachside like the root or head of a rib. The true cranium is a prolongation of the vertebral column, and is developed at a much earlier period than the facial bones. Originally, it is formed of but one mass, a cerebral capsule, the chorda dorsalis being continued into its base, and ending there with a tapering point. At an early period the head is bent down- wards and forwards round the end of the chorda dorsalis in such a way that the middle cerebral vesicle, and not the anterior, comes to occupy the highest position in the head. Pituitary Body.-Tn connection with this must be mentioned the development of the pituitary body. It is formed by the meeting of two out-growths, one from the foetal brain, which grows downwards, and the other from the epiblast of the buccal cavity, which grows up towards it. The surrounding mesoblast also takes part in its formation. The connection of the first process with the brain becomes narrowed, and persists as the infundibulum, while that of the other process with the buccal cavity disappears com- pletely at a spot corresponding with the future position of the body of the sphenoid. Cranium. The first appearance of a solid support at the base of the cranium observed by Muller in fish, consists of two elongated bands of cartilage (trabecula) cranii), one on the right and the other on the left side, which are connected with the cartila- ginous capsule of the auditory apparatus, and which diverge to enclose the pituitary body, uniting in front to form the septum nasi beneath the anterior end of the cerebral capsule. Hence, in CHAP. XXIII.] THE VISCERAL CLEFTS AND ARCHES. 769 the cranium, as in the spinal column, there are at first developed at the sides of the chorda dorsalis two symmetrical elements, which subsequently coalesce, and may wholly enclose the chorda. The brain-case consists of three segments: occipital, parietal, and frontal, corresponding in their relative position to the three primitive cerebral vesicles; it may also be noted that in front of each segment is developed a sense-organ (auditory, ocular, and olfactory, from behind forwards). The basis cranii consists at an early period of an unsegmented cartilaginous rod, developed round the notochord, and continued forward beyond its termination into the trabeculae cranii, which bound the pituitary fossa on either side. In this cartilaginous rod three centres of ossification appear: basi-occipital, basi-sphenoid, and pre sphenoid, one corresponding to each segment. The bones forming the vault of the skull, viz., the frontal, parietal, squamous portion of temporal and the squamo-occipital, are ossified in membrane. As the embryo enlarges, the heart, which at first occupied a position close to the cranial flexure, is carried further and further backwards until a considerable intervening part exists between it and the head, in which the mcsoblast is undivided. This becomes the neck. On a section it is seen that in it the whole three layers are represented in order, and that there is no interval between them. In the neck thus formed soon appear the visceral or branchial clefts on either side, in series, across the axis of the gut not quite at right angles. They are four in number, the most anterior being first found. At their edges the hypoblast and the epiblast are continuous. The anterior border of each cleft forms a fold or lip, the branchial or visceral fold. The posterior border of the last cleft is also formed into a fold, so that there are four clefts and five folds, but the three most anterior are far more prominent than the others, and of these the second is the most conspicuous. The first fold nearly meets its fellow in the middle line, the second less nearly, and the others in order still less so. Thus in the neck there is a triangular interval, into which by the splitting of the mesoblast at that part the pleuro- peritoneal cavity extends. The branchial clefts and arches are not all permanent. The first arch gives off a branch from its front edge, which passes forwards to meet its fellow, but these offshoots do not quite meet, being separated by a process which grows downwards The Visceral Clefts and Arches. 770 DEVELOPMENT. [chap. XXIII. from the head. Between the branches and the main first fold is the cavity of the mouth. The branches represent the superior maxilla, and the main folds the mandible or lower jaw. The central process, which grows down, is the fronto-nasal process. A B 1'ig. 467.-A. Magnified view from before of the head and neck of a human embryo of about three weeks (from Ecker).-i, anterior cerebral vesicle or cerebrum; 2, middle ditto; 3, middle or fronto-nasal process ; 4, superior maxillary process; 5, eye ; 6, inferior maxillary process, or first visceral arch, and below it the first cleft; 7, 8, 9, second, third, and four th arches and clefts, b. Anterior view of the head of a human feetus of about the fifth week (from Ecker, as before, fig. IV.). 1, 2, 3, 5, the same parts as in a ; 4, the external nasal or lateral frontal process; 6, the superior maxillary process : 7, the lower jaw ; x, the tongue; 8, first branchial cleft becoming the meatus audi- torius externus. In this way the so-called visceral arches and clefts are formed, four on each side (fig. 467, a). From or in connection with these arches the following parts are de- veloped :- The first arch (mandibular) contains a cartilaginous rod (Meckel's carti- lage), around the distal end of which the lower jaw is developed, while the malleus is ossified from the proximal end. When the maxillary processes on the two sides fail partially or completely to unite in the middle line, the well-known condition termed cleft palate results. V hen the integument of the face presents a similar deficiency, we have the deformity known as hare-lip. Though these two deformities- frequently co-exist, they are by no means always necessarily associated. lhe upper part of the face in the middle line is developed from the so-called frontal-nasal process (A, 3, fig. 467). From the second arch are developed the incus, stapes, and stapedius muscle, the styloid process of the temporal bone, the stylo-hyoid ligament, and the smaller cornu of the hyoid' bone. From the third visceral arch, the greater cornu and ho ly of the hyoid bone.. In man and other mammalia the fourth visceral arch is indistinct. It occupies the position where the neck is afterwards developed. A distinct connection is traceable between these visceral arches- and certain cranial nerves: the trigeminal, the facial, the glosso- pharyngeal, and the pneumogastric. The ophthalmic division of the trigeminal supplies the trabecular arch • the superior and CHAP. XXIII.] FORMATION OF THE EXTREMITIES. 771 inferior maxillary divisions supply the maxillary and mandibular arches respectively. The facial nerve distributes one branch (chorda tympani) to the Fig. 468.-Embryo chick (4th day), viewed a» a transparent object, lying on its left side (mag- nified). C H, cerebral hemispheres; F B, fore-brain or vesicle of third ventricle, with Pn, pineal gland projecting from its summit; .V B, mid-brain; C b, cerebellum; Zr. F, fourth ventricle; L, lens; c h », choroidal slit; Gen. V, auditory vesicle; «»«' superior maxillary process ; iF, zF, &c., first, second, third, and fourth visceral folds ; I', fifth nerve, sending one branch (ophthalmic) to the eye, and another to the first visceral arch; VII, seventh nerve, passing to the second visceral arch ; G. Ph, glosso- pharyngeal nerve, passing to the third visceral arch ; P g, pneumogastric nerve, pass- ing towards the fourth visceral arch; i v, investing mass; c h, notochord ; its front end cannot be seen in the living embryo, and it does not end as shown in the figure, but takes a sudden bend downwards, and then terminates in a point; Ut, heart seen through the walls of the chest; M P, muscle-plates; JI', wing, showing commencing differentiation of segments, corresponding to arm, forearm, and hand; S 8, somatic stalk ; Al, allantois ; II I., hind-limb, as yet a shapeless bud, showing no differentia- tion. Beneath it is seen the curved tail. (Foster and Balfour.) first visceral arch, and others to the second visceral arch. Thus it divides, enclosing the first visceral cleft. Similarly, the glosso-pharyngeal divides to enclose the second visceral cleft, its lingual branch being distributed to the second, and its pharyngeal branch to the third arch. The vagus, too, sends a branch (pharyngeal) along the third arch, and in fishes it gives off paired branches, which divide to enclose several successive branchial clefts. The extremities are developed in an uniform manner in all vertebrate animals. They appear in the form of leaf-like elevations The Extremities. 772 DEVELOPMENT. [chap. XXIII. from the parieties of the trunk (see fig. 468), at points where more or less of an arch will be produced for them within. The primitive form of the extremity is nearly the same in all Vertebrata, whether it be destined for swimming, crawling, walking, or flying. In the Fig. 469.-A human embryo of the fourth week, 3J lines in length.-1, the chorion ; 3, part of the amnion ; 4, umbilical vesicle with its long pedicle passing into the abdomen ; 7, the heart; 8, the liver ; 9, the visceral arch destined to form the lower jaw, beneath which are two other visceral arches separated by the branchial clefts ; 10, rudiment of the upper extremity ; 11, that of the lower extremity ; 12, the umbilical cord ; 15, the eye; 16, the ear; 17, cerebral hemispheres ; 18, optic lobes, corpora quadrigemina. (Muller.) human foetus the fingers are at first united, as if webbed for swim- ming ; but this is to be regarded not so much as an approximation to the form of aquatic animals, as the primitive form of the hand, the individual parts of which subsequently become more completely isolated. The fore-limb always appears before the hind-limb, and for some time continues in a more advanced state of development. In both limbs alike, the distal segment (hand or foot) is separated by a slight notch from the proximal part of the limb, and this part is sub- sequently divided again by a second notch (knee or elbow-joint). The Vascular System.-At an early stage in the develop- ment of the embryo-chick, the so-called " area vasculosa " begins to make its appearance. A number of branched cells in the mesoblast send out processes which unite so as to form a network of protoplasm with nuclei at the nodal points. A large number of the nuclei acquire a red colour • these form the red blood-cells. The proto-plasmic processes become hollowed out in the centre so as to form a closed system of branching canals, in the walls of CHAI*. XX11I.] FORMATION OF BLOOD-VESSELS. 773 which the rest of the nuclei remain imbedded. In the blood-vessels thus formed, the circulation of the embryonic blood commences. According to Klein's researches, the first blood-vessels in the chick are developed from embryonic cells of the mesoblast, which swell up and become vacuolated, while their nuclei undergo segmentation. These cells send out protoplasmic processes, which unite with corre- sponding ones from other cells, and become hollowed, give rise to the capillary wall composed of endo- thelial cells ; the blood corpuscles being budded off from the en- dothelial wall by a process of gemmation. Heart.-About the same early period the heart makes its appearance as a solid mass of cells of the splanchno-pleure in the manner before indi- cated. At this period the anterior part of the alimentary tube ends blindly beneath the notochord. It is beneath the posterior end of this fore-gut that the heart begins to be developed. The heart when first formed is made up of two not quite com- plete tubes which coalesce to form one, and so when the cavity is hollowed out in the mass of cells, the central cells float freely in the fluid, which soon begins to circulate by means of the rhythmic pulsations of the embryonic heart. These pulsations take place even before the appearance of a cavity, and immediately after the first " laying down " of the cells from which the heart is formed, and long before muscular fibres or ganglia have been formed in the cardiac walls. At first they Fig. 470.-Capillarn blood-vessels of the tail of a young larval frog, a, capillaries perme- able to blood; b, fat granules attached to the walls of the vessels, and concealing the nuclei; c, hollow prolongation of a capillary, ending in a point; d, a branch- ing cell with nucleus and fat-granules ; it communicates by three branches with prolongation of capillaries already formed; e, e, blood corpuscles still con- taining granules of fat. x 350 times. (Kolliker.) 774 DEVELOPMENT. [chap. xxui. seldom exceed from fifteen to eighteen in the minute. The fluid within the cavity of the heart shortly assumes the characters of blood. At the same time the cavity itself forms a communication with the great vessels in contact with it, and the cells of which its walls are com- posed are transformed into fibrous and muscu- lar tissues, and into epithelium. In the de- veloping chick it can be observed with the naked eye as a minute red pul- sating point before the end of the second day of incubation. Blood-vessels. - Blood- vessels appear to be developed in two ways, according to the size of the vessels. In the formation of large blood-vessels, masses of embryonic cells similar to those from which the heart and other struc- tures of the embryo are developed, arrange them- selves in the position, form, and thickness of the developing vessel. Shortly afterwards th cells in the interior of a column of this kind seem to be developed into blood-corpuscles, while the external layer of cells is converted into the walls of the vessel. In the development of capillaries another plan is pursued. This has been well illus- trated by Kblliker, as observed in the tails of tadpoles. The first lateral vessels of the tail have the form of simple arches, passing 471.-Development of capillaries in the regenerating tail of a tadpole, a, b, c, d, sprouts and cords of protoplasm. (Arnold.) aThe !<yn'e 'Won after the lapse of 24 hours. lhe sprouts and cords of protoplasm" have become channelled out into capillaries. (Arnold.) CHAP. XXH1.] FORMATION OF CAPILLARIES. 775 between the main artery and vein, and are produced by the junction of prolongations, sent from both the artery and vein, with certain elongated or star-shaped cells, in the substance of the tail. When these arches are formed and are permeable to blood, new prolongations pass from them, join other radiated cells, and thus form secondary arches. In this manner, the capil- lary network extends in proportion as the tail increases in length and breadth, and it, at the same time, becomes more dense bv the formation, according to the same plan, of fresh vessels within its meshes. The prolongations by which the vessels communicate with the star-shaped cells, consist at first of narrow pointed pro- Fig. 47J?-Capillaries from the vitreous humour oj a fetal calf. Two vessels are seen connected by a "cord" of protoplasm, and clothed with an adventitia, containing numerous nuclei, a, insertion of this "cord" into the primary walls of the vessels. (Frey.) jections from the side of the vessels, which gradually elongate until they come in contact with the radiated processes of the cells. The thickness of such a prolongation often does not exceed that of a fibril of fibrous tissue, and at first it is perfectly solid; but, by degrees, especially after its junction with a cell, or with another prolongation, or with a vessel already permeable to blood, it enlarges, and a cavity then forms in its interior (see figs. 470, 472). This tissue is well calculated to illustrate the various steps in the development of blood-vessels from elongating and branching cells. In many cases a whole network of capillaries is developed from a network of branched, embryonic connective-tissue corpuscles by the joining of their processes, the multiplication of their nuclei, and the vacuolation of the cell-substance. The vacuoles gradually coalesce till all the partitions are broken down, and the originally 776 DEVELOPMENT. [chap. xxin. solid protoplasmic cell-substance is, so to speak, tunnelled out into a number of tubes. Capillaries may also be developed from cells which are originally spheroidal, vacuoles form in the interior of the cells gradually becoming united by fine pro- toplasmic processes: by the extension of the vacuoles into them, capillary tubes are gra- dually formed. Morphology. Heart.-When it first appears, the heart is ap- proximately tubular in form, being at first a double tube then a single one. It receives at its two posterior angles the two omphalo-mesenteric or vitelline veins, and gives oft anteriorly the primitive aorta (fig. 474). The junction of the two veins which pass into the auricle becomes removed farther and farther away from the heart, and the vessel thus formed is called sinus venosus near to the auricle, and ductus venosus farther away, or if it be called by one name that of meatus venosus may be used. It soon, however, becomes curved somewhat in the shape of a horse-shoe, with the convexity towards the right, the venous end being at the same time drawn up towards the head, so that it finally lies behind and somewhat to the right of the arterial. It also becomes partly divided by constrictions into three cavities. Of these three cavities which are developed in all Vertebrata, that at the venous end is the simple auricle, with the sinus venosus, that at the arterial end the bulbus arteriosus, and the middle one is the simple ventricle. These three parts of the heart contract in succession. The auricle and the bulbus arteriosus at this period lie at the ex- tremities of the horse-shoe. The bulging out of the middle portion inferiorly gives the first indication of the future form of the ventricle (fig. 475). The great curvature of the horse-shoe by the same means becomes much more developed than the smaller curva- ture between the auricle and bulbus ; and the two extremities, l'ig. 474--Foetal heart in successive stages of development, i, venous extremity ; 2, arte- rial extremity 53,3, pulmonary'branches ; 4, ductus arteriosus. (Dalton.) CHAP. XXIII.] FORMATION OF THE HEART. 777 the auricle and bulb, approach each other superiorly, so as to pro- duce a greater resemblance to the later form of the heart, whilst the ventricle becomes more and more developed inferiorly. The heart of Fishes retains these four cavities, no further division bv internal septa into right and left chambers taking place. In Amphibia, also, the heart throughout life consists of the three muscular divisions which are so early formed in the embryo and the sinus venosus; but the auricle is divided internally by a septum into a pulmonary and systemic auricle. In Reptiles, not merely the auricle is thus divided into two cavities, but a similar Fig* 475*-Heart of the chick at the 65th, and 85th hours of incubation. 1. the venous trunks ; 2, the auricle; 3, the ventricle ; 4, the bulbus arteriosus. (Allen Thomson.) septum but incomplete is more or less developed in the ventricle. In Birds and Mammals, both auricle and ventricle undergo com- plete division by septa; whilst in these animals as well as in reptiles, the bulbus aortse is not permanent, but becomes lost in the ventricles. The septum dividing the ventricle commences at the apex and extends upwards. The sub-division of the auricles is very early foreshadowed by the outgrowth of the two auricular appendages, which occurs before any septum is formed externally. The septum of the auricles is developed from a semilunar fold, which extends from above downwards. In man, the septum between the ventricles, according to Meckel, begins to be formed about the fourth week, and at the end of eight wreeks is complete. The septum of the auricles, in man and all animals which possess it, remains imperfect throughout foetal life. When the partition of the auricles is first commencing, the two venae cavse have different relations to the tAvo cavities. The superior cava enters, as in the adult, into the right auricle ; but the inferior cava is so placed that it appears to enter the left auricle, and the posterior part of the septum of the auricles is formed by the Eustachian valve, which extends from the point of entrance of the inferior cava. Subsequently, however, the septum, growing from the anterior wall close to the upper end of the ventricular septum, becomes directed more and more to the left of the vena cava 778 DEVELOPMENT. [chap. xxni. inferior. During the entire period of foetal life, there remains an opening in the septum, which the valve of the foramen ovale, developed in the third month, imperfectly closes. The bulbus arteriosus, which is originally a single tube, becomes gradually divided into two by the growth of an internal septum, which springs from the posterior w'all, and extends forwards towards the front wall and downwards towards the ventricles. This partition takes a somewhat spiral direction, so that the two tubes (aorta and pulmonary artery) which result from its completion, do not run side by side, but are twisted round each other. As the septum grows down towards the ventricles, it meets and coalesces with the upwardly growing ventricular septum, and thus from the right and left ventricles, which are now completely separate, arise respectively the pulmonary artery and aorta, which are also quite distinct. The auriculo-ventricular and semilunar valves are formed by the grow'th of folds of the endocardium. At its first appearance, as we have seen, the heart is placed just beneath the head of the foetus, and is very large relatively to the whole body; but with the growth of the neck it becomes further and further removed from the head, and is lodged in the cavity of the thorax. Up to a certain period the auricular is larger than the ventri- cular division of the heart; but this relation is gradually reversed as development proceeds. Moreover, all through foetal life, the walls of the right ventricle are of very much the same thickness as those of the left, which may probably be explained by the fact that in the foetus the right ventricle has to propel the blood from the pulmonary artery into the aorta, and thence into the placenta, while in the adult it only drives the blood through the lungs. Arteries.-The primitive aorta arises from the bulbus arteriosus and divides into two branches which arch backwards, one on each side of the foregut and unite again behind it, and in front of the notochord into a single vessel. This gives off the two omphalo-mesenteric arteries, which distribute branches all over the yolk-sac ; this area vasculosa in the chick attaining a large development, and being limited all round by a vessel known as the sinus terminalis. The blood is collected by the venous channels, and returned through the omphalo-mesenteric veins to the heart. Behind this pair of primitive aortic arches, four more pairs make CHAP. XXIII.] AORTIC ARCHES. 779 their appearance successively, so that there are five pairs in all, each one running along one of the visceral arches. These five are never all to be seen at once in the embryo of higher animals, for the two anterior pairs gradually disappear, Fig. 476.-Diagram of the aortic arches in a mammnf, showing transformations which give nse to the permanent arterial vessels. A, primitive arterial stem or aortic bulb, now divided into A, the ascending part of the aortic arch, and the pulmonary; a a', right and left aortic roots ; A', descending aorta; 1, 2, 3, 4, 5, the five primitive aortic or branchial arches; Z, ZZ, III, IV, the four branchial clefts which, for the sake of clearness, have been omitted on the right side. The permanent systemic vessels are deeply, the pulmonary arteries lightly, shaded ; the parts of the primitive arches which are transitory are simply outlined ; c, placed between the permanent common carotid arteries ; c e, external carotid arteries; c i, internal carotid arteries; s, light sub- clavian, rising from the right aortic root beyond the fifth arch ; v, right vertebral from the same, opposite the fourth arch ; v' s', left vertebral and subclavian arteries rising together from the left, or permanent aortic root, opposite the fourth arch; p, pul- monary arteries rising together from the left fifth arch ; rf. outer or back part of left fifth arch, forming ductus arteriosus; p n, p u', right and left pneumogastric nerves descending in front of aortic arch, with their recurrent branches represented dia- grammatically as passing behind, to illustrate the relations of these nerves respec- tively to the right subclavian artery (4), and the arch of the aorta and ductus arteriosus (d). (Allen Thomson, after Bathkc.) while the posterior ones are making their appearance, so that at length only three remain. In Fishes, however, they all persist throughout life as the branchial arteries supplying the gills, while in Amphibia three pairs persist throughout life. In Reptiles, Birds, and Mammals, further transformations occur. In Reptiles the fourth pair remains throughout life as the 780 1 EVELOl'MENT. [chap, xxiii. permanent right and left aorta; in Birds the right one remains as the permanent aorta, curving over the right bronchus instead of the left as in Mammals. In Mammals the left fourth aortic arch develops into the permanent aorta, the right one remaining as the subclavian artery of that side. Thus the subclavian artery on the right side corresponds to the aortic arch on the left, and this homology is further confirmed by the fact that the recurrent laryngeal nerve hooks under the subclavian on the right side, and the aortic arch on the left. The third aortic arch remains as the internal carotid artery, while the fifth disappears on the right side, but on the left forms the pulmonary artery. The distal end of this arch originally opens into the descending aorta, and this communication (which is permanent throughout life in many reptiles on both sides of the body) remains throughout foetal life under the name of ductus arterio- sus : the branches of the pulmonary artery, to the right and left lung, are very small, and most of the blood which is forced into the pulmonary artery passes through the wide ductus arteriosus into the descend- ing aorta. All these points will become clear on refer- ence to the accompanying diagram (fig. 476). As the umbilical vesicle dwindles in size, the por- tion of the omphalo-mesen- teric arteries outside the Fig. 477.-Diagram of young embryo and its vessels, showing course of circulation in the umbilical vesicle; and also that of the allantois (near the caudal extremity), which is just commencing. (Dalton.) 478. Diagram of embryo and its vessels at a later stagey showing the second circulation. The pharynx, oesophagus, and intestinal canal have become further developed, and the mesenteric arteries have enlarged, while the umbilical vesicle and its vascular branches are very much reduced in size. The large umbilical arteries are seen passing out in the placenta. (Dalton.) CHAP. XXIII.] FORMATION OF VEINS. 781 body gradually disappears, the part inside the body remaining as the mesenteric arteries. Meanwhile with the growth of the allantois two new arteries (umbilical) appear, and rapidly increase in size till they are the largest branches of the aorta: they are given off from the internal iliac arteries, and for a long time are considerably larger than the external iliacs which supply the comparatively small hind-limbs. Ketns.-The chief veins in the early embryo may be divided Fig. 479.-Diagrams illustrating the. development of veins about the liver. B, il c, duets of Cuvier, right and left; c a, right and left cardinal veins ; o, left omphalo-mesenteric vein ; o', right omphalo-mesenteric vein, almost shrivelled up ; u u' umbilical veins, of which u', the right one, has almost disappeared. Between the venae cardinales is seen the outline of the rudimentary liver with its venae hepaticae advehentes, and revehentes. D, ductus venosus ; hepatic veins ; c i, vena cava inferior ; P, portal vein; P P, veme advehentes ; m, mesenteric veins. (Kiilliker.) into two groups, visceral ami parietal : the former includes the omphalo-mesenteric and umbilical, the latter the jugular and cardinal veins. The former may be first considered. The earliest veins to appear in the foetus are the omphalo- mesenteric or vitelline, which return the blood from the yolk-sac to the developing auricle. As soon as the placenta with its umbilical veins is developed, these unite with the omphalo-mesenteric, and thus the blood which reaches the auricle comes partly from the yolk - sac and partly from the placenta. The right omphalo- mesenteric and the right umbilical vein soon disappear, and the united left omphalo-mesenteric and umbilical veins pass through the developing liver on the way to the auricle. Two sets of vessels make their appearance in connection with the liver (veme hepaticae advehentes, and revehentes), both opening into the united omphalo-mesenteric and umbilical veins, in such a way that a portion of the venous blood traversing the latter is diverted into the developing liver, and, having passed through its capillaries, 782 DEVELOPMENT. [chap. xxnr. returns to the umbilical vein through the venae hepaticse revehentes at a point nearer the heart (see fig. 479). The portion of vein between the afterent and efferent veins of the liver becomes the ductus venosus. The veme hepaticae advehentes become the right and left branches of the portal vein, the vena? hepaticae revehentes become the hepatic veins, which open just at the junction of the ductus venosus with another large vein (vena cava inferior), which is now being developed. The mesenteric portion of the omphalo-mesenteric vein returning blood from the developing intestines remains as the mesenteric vein, which, by its union with the splenic vein, forms the portal. Thus the foetal liver is supplied with venous blood from two sources, through the umbilical and portal vein respectively. At birth the circulation through the umbilical vein of course com- pletely ceases and the vessel begins at once to dwindle, so that now the only venous supply of the liver is through the portal vein. The earliest appearance of the panetai system of veins is the for- mation of two short transverse veins (ducts of Cuvier) opening into the auricle on either side, which result from the union of an an- terior cardinal, afterwards forming a jugular, vein, collecting blood from the head and neck, and a posterior cardinal vein which returns the blood from the Wolffian bodies, the vertebral column, and the parieties of the trunk. This arrangement persists throughout life in Fishes, but in Mammals the following transformations occur. As the kidneys are developing a new vein appears (vena cava inferior), formed by the junction of their efferent veins. It receives branches from the legs (iliac) and increases rapidly in size as they grow: further up it receives the hepatic veins, which by now have lost their original opening into the ductus venosus. The heart gradually descends into the thorax, causing the ducts of Cuvier to become oblique instead of transverse. As the fore-limbs develop, the subclavian veins are formed. A transverse communicating trunk now unites the two ducts of Cuvier, and gradually increases, while the left duct of Cuvier becomes almost entirely obliterated (all its blood passing by the communicating trunk to the right side) (fig. 480, c. n). The right duct of Cuvier remains as the right innominate vein, while the communicating branch forms the left innominate. The remnant of the left duct of Cuvier generally remains as a fibrous band, running obliquely down to the coronary vein, which is really the proximal part of the left duct of Cuvier. In front of the root of chap, xxni.] THE FOETAL CIRCULATION. 783 the left lung, another relic may be found in the form of the so- called vestigial fold of Marshall, which is a fold of pericardium running in the same direction. I n many of the lower mammals, such as the rat, the left ductus Cuvieri remains as a left superior cava. Meanwhile, a transverse branch carries across most of the blood ABC I) Fig. 480.-Diagrams illustrating th- development of the great veins, d c, ducts of Cuvier ; j, jugular veins ; h, hepatic veins ; e, cardinal veins; s, subclavian vein; j i, internal jugular vein ; j e, external jugular vein; a z, azygos vein ; c i, inferior vena cava ; r, renal veins; i I, iliac veins; h ij, hypogastric veins. (Gegenbaur.) of the left posterior cardinal vein into the right ; and by this union the great azygos vein is formed. The upper portions of the left posterior cardinal vein remain as the left superior intercostal and vena azygos minor (fig. 480). The circulation of blood in the foetus differs considerably from that of the adult. It will be well, perhaps, to begin its descrip- tion bv tracing the course of the blood, which, after being carried out to the placenta by the two umbilical arteries, has returned, cleansed and replenished, to the foetus by the umbilical vein. It is at first conveyed to the under surface of the liver, and Circulation of Blood in the Fcetus. 784 DEVELOPMENT. [chap. XXII I. there the stream is divided,-a part of the blood passing straight on to the inferior vena cava, through a venous canal called the ductus venosus, while the remainder passes into the portal vein, Fig. 481.-Diagram of the Circulation. and reaches the inferior vena cava only after circulating through the liver. Whether, however, by the direct route through the ductus venosus or by the roundabout way through the liver,-all the blood which is returned from the placenta by the umbilical vein reaches the inferior vena cava at last, and is carried by it chap, xxrn.] THE NERVOUS SYSTEM. 785 to the right auricle of the heart, into which cavity is also pouring the blood that has circulated in the head and neck and arms, and has been brought to the auricle by the superior vena cava. It might be naturally expected that the two streams of blood would be mingled in the right auricle, but such is not the case, or only to a slight extent. The blood from the superior vena cava,-the less pure fluid of the two-passes almost exclusively into the right ventricle, through the auriculo-ventricular opening, just as it does in the adult; while the blood of the inferior vena cava is directed by a fold of the lining membrane of the heart, called the Eusta- chian valve, through the foramen ovale into the left auricle, whence it passes into the left ventricle, and out of this into the aorta, and thence to all the body, but chiefly to the head and neck. The blood of the superior vena cava, which, as before said, passes into the right ventricle, is sent out thence in small amount through the pulmonary artery to the lungs, and thence to the left auricle, as in the adult. The greater part, however, by far, does not go to the lungs, but instead, passes through a canal, the ductus arteriosus, leading from the pulmonary artery into the aorta just below the origin of the three great vessels which supply the uppci* parts of the body ; and there meeting that part of the blood of the inferior vena cava which has not gone into these large vessels, it is distributed with it to the trunk and lower parts,-a portion passing out by way of the two umbilical arteries to the placenta. From the placenta it is returned by the umbilical vein to the under surface of the liver, from which the description started. Changes after Birth.-After birth the foramen ovale closes and so do the ductus arteriosus and ductus venosus, as well as the umbilical vessels ; so that the two streams of blood which arrive at the right auricle by the superior and inferior vena cava respectively, thenceforth mingle in this cavity of the heart, and passing into the right ventricle, go by way of the pulmonary artery to the lungs, and through these, after purification, to the left auricle and ventricle, to be distributed over the body. (See Chapter on Circulation.) The Nervous System. The Cranial and Spinal Nerves.-The cranial nerves are derived from a continuous band, called the neural band. They are formed before the neural canal is complete. The neural band is made up 786 DEVELOPMENT. [chap, xxiii. of two laminae going from the dorsal edges of the neural groove to the external epiblast. It becomes separated from the epiblast, and then forms a crest attached to the upper surface of the brain. The posterior roots of the spinal nerves arise as outgrowths of median processes of cells from the dorsal side of the spinal cord, which become attached laterally to the spinal cord as their original point of attachment disappears. The anterior roots probably arise from the ventral part of the cord as a number of strands for each nerve. They appear later than the posterior roots. The rudiment of the posterior root is differentiated into a proximal round nerve con- nected to the cord, a ganglionic portion and a distal portion. To the last the anterior nerve-root becomes attached. The Spinal Cord.-The spinal cord consists at first of the un- fig. 482.-Diagram of development of spinal cord: c c, central canal; af, anterior fissure; pf, posterior fissure ; g, grey matter ; w, white matter. For further explanation see text. differentiated epiblast of the walls of the neural canal, the cavity of which is large, with almost parallel sides. The walls are at first composed of elongated irregular nucleated columnar cells, arranged in a radiate manner. The cavity then becomes narrow in the middle and of an hour-glass shape (fig. 482). When the spinal nerves make their first appearance, about the fourth day in the chick, the epiblastic walls become differentiated into three parts : (a) the epithelium lining the central canal; (6) the grey matter ; (<•) the external white matter. The last is derived from the outer most part of the epiblastic walls by the conversion of the cells into longitudinal nerve-fibres. The fibres being without any myelin sheath, are for a time grey in appearance. The white matter corresponds in position to the anterior and posterior nerve-roots, and are the anterior and posterior white columns. It is at first a very thin layer, but increases in thickness until it covers the whole cord. The grey matter too arises from the cells by their being prolonged into fibres. The change in the central cells is sufficiently obvious. The anterior and posterior cornua of grey matter and the anterior grey commissure then CHAP. XXIII.] FORMATION OF THE BRAIN. 787 appear. The anterior fissure is formed on the fifth day by the growth downwards of the anterior cornua of grey matter towards the middle line. The posterior fissure is formed later. The whole cord now becomes circular. The posterior grey commissure is then formed. When it first appears, the spinal cord occupies the whole length Fig. 483.-Early stayes in development of human brain (magnified). 1, 2, 3, are from an embryo about seven weeks old ; 4, about three months' old. m, middle cerebral vesicle (mesencephalon) ; c, cerebellum; m o, medulla oblongata; i, thalamencephalon ; h, hemispheres; £' infundibulum; Fig. 3 shows the several curves which occur in the course of development; Fig. 4 is a lateral view, showing the great enlargement of the cerebral hemispheres which have covered in the thalami, leaving the optic lobes, m, uncovered. (Kdlliker.) N.B.-In fig. 2 the line t terminates in the right hemisphere; it ought to be continued into the thalamencephalon. of the medullary canal, but as development proceeds, the spinal column grows more rapidly than the contained cord, so that the latter appears as if drawn up till, at birth, it is opposite the third lumbar vertebra, and in the adult opposite the first lumbar. In the same way the increasing obliquity of the spinal nerves in the neural canal, as we approach the lumbar region, and the "cauda equina " at the lower end of the cord, are accounted for. Brain.-We have seen that the front portion of the medul- lary7 canal is almost from the first widened out and divided 788 DEVELOPMENT. [chap. XXIII. into three vesicles. From the anterior vesicle (thalamencephalon) the two primary optic vesicles are budded off laterally: their further history will be traced in the next section. Somewhat later, from the same vesicle the rudiments of the hemispheres appear in the form of two outgrowths at a higher level, which grow upwards and backwards. These form the prosencephalon. In the walls of the posterior (third) cerebral vesicle, a thicken- ing appears (rudimentary cerebellum) which becomes separated from the rest of the vesicle by a deep inflection. At this time there are two chief curvatures of the brain (fig. 483, 3)- (x-) sharp bend of the whole cerebral mass down- wards round the end of the notochord, by which the anterior vesicle, which was the highest of the three, is bent downwards, and the middle one comes to occupy the highest position. (2.) A sharp bend, with the convexity forwards, which runs in from behind beneath the rudimentary cerebellum separating it from the medulla. Thus, five fundamental parts of the foetal brain may be distin- guished, which, together with the parts developed from them, may be presented in the following tabular view :- Table of Parts Developed from Fundamental Parts of Brain. I. Anterior Primary Vesicle. Cerebral hemispheres, corpora striata, corpus callosum, fornix, lateral ventricles,olfactory bulb. 1. Prosencephalon. 2. Thalamencephalon (Diencephalon) ■ Thalami optici, pineal gland ) (part of), pituitary body, third I ventricle, optic nerve (prima- . rily), infundibulum. II. Middle ' Primary Vesicle. , 3. Mesencephalon. ' Corpora quadrigemina, crura cere- bri, aqueduct of Sylvius, optic „ nerve (secondarily). III. Posterior Primary Vesicle. 4. Epencepbalon. 5. Metencephalon. Cerebellum, pons Varolii, anterior part of fourth ventricle. Medulla oblongata, fourth ventri- cle, auditory nerve. (Quain.) The cerebral hemispheres grow rapidly upwards and back- wards, while from their inferior surface the olfactory bulbs are budded off, and the prosencephalon, from which they spring, chap, xxiii.] THE PARTS OF THE BRAIN. 789 remains to form the third ventricle and optic thalami. The middle cerebral vesicle (mesencephalon) for some time is the most prominent part of the foetal brain, and in Fishes, Amphibia, and Reptiles, it remains uncovered through life as the optic lobes. But in Birds the growth of the cerebral hemispheres thrusts the optic lobes down laterally, and in Mammalia completely overlaps them. Tn the lower Mammalia the backward growth of the hemi- spheres ceases as it were, but in the higher groups, such as the monkeys and man, they grow still further back, until they completely cover in the cerebellum, so that on looking down on the brain from above, the cerebellum is quite concealed from view. The surface of the hemi- spheres is at first quite smooth, but as early as the third month the great Syl- vian fissure begins to be formed (fig. 483, 4). The next to appear is the parieto-occipital or per- pendicular fissure; these two great fissures, unlike the rest of the sulci, are formed by a curving round of the whole cerebral mass. In the sixth month the fissure of Rolando appears: from this time till the end of foetal life the brain grows rapidly in size, and the convolutions appear in quick succession; first the great primary ones are sketched out, then the secondary, and lastly the tertiary ones in the sides of the fissures. The commissures of the brain (anterior, middle, and posterior), and the corpus callosum, are developed by the growth of fibres across the middle line. The Hippocampus major is formed by the folding in of the grey matter from the exterior into the lateral ventricles. The essential points in the structure and arrangement of the various parts of the brain, are diagrammatically shown in the two accompanying figures (figs. 483, 484). Fig. 484.-Side view of fie lai brain at six months, ■showing commencement of formation of the principal fissures and convolutions. F, frontal lobe; F, parietal; 0, occipital; T, temporal; a a a, commencing frontal convo- lutions; s, Sylvian fissure; its anterior division ; c, within it the central lobe or island of Reil; r, fissure of Rolando; p, perpen- dicular fissure. (R. Wagner.) 790 DEVELOPMENT. [chap. XXIII. The Special Sense Organs. The Eye.-Soon after the first three cerebral vesicles have become distinct from each other, the anterior one sends out a lateral vesicle from each side, (primary optic vesicle), which grows out towards the free surface, its cavity of course communi- cating with that of the cerebral vesicle through the canal in its pedicle. It is soon met and invaginated by an in-growing process from the epiblast (fig. 450), very much as the growing tooth is met by the process of epithelium which produces the enamel organ. This process of the epiblast is at first a depression Fig. 485.--section of the primary optic vesicle, in the chick magnified (from Remak). A, from an embryo of sixty-five hours; B, a few hours later; C, of the lourth day; c, the corneous layer or epidermis, presenting in A the open depression tor the lens, which is closed in B and C; 7, the lens follicle and lens; the primary optic vesicle ; in A and B, the pedicle is shown ; in C, the section being to the side of the pedicle, the latter is not shown; r, the secondary ocular vesicle and vitreous humour. which ultimately becomes closed in at the edges so as to produce a hollow ball, which is thus completely severed from the epithelium with which it was originally continuous. From this hollow ball the crystalline lens is developed. By the in-growth of the lens the anterior wall of the primary optic vesicle is forced back nearly into contact with the posterior, and thus the primary optic vesicle is almost obliterated. The cells in the anterior wall are much longer than those of the posterior wall; from the former the retina proper is developed, from the latter the retinal pigment. The cup-shaped hollow in which the lens is now lodged is termed the secondary optic vesicle : its walls grow up all round, leaving, however, a slit at the lower part. Choroidal hissure.-Through this slit (fig. 487), often termed the choroidal fissure, a process of mesoblast containing numerous blood-vessels projects, and occupies the cavity of the secondary CHAP. XXIII.] THE EYE. 791 optic vesicle behind the lens, filling it with vitreous humour and furnishing the lens capsule and the capsulo-pupillary membrane. This process in Mammals projects, not only into the secondary optic vesicle, but also into the pedicle of the primary optic vesicle inva- ginating it for some dis- tance from beneath, and thus carrying up the arteria centralis retince into its per- manent position in the centre of the optic nerve. This invagination of the optic nerve does not occur in birds, and consequent!}' no arteria centralis retina? exists in them. But they possess an important per- manent relic of the original protrusion of the mesoblast through the choroidal fis- sure, forming the pecten, while a remnant of the same fissure some- times occurs in man under the name coloboma iridis. The cavity of the primary optic vesicle becomes com- pletely obliterated, and the rods and cones come into apposition with the pigment layer of the retina. The cavity of its pedicle disappears and the solid optic nerveisformed. Mean- while the cavity which existed in the centre of the primitive lens becomes filled up. by the growth of fibres from its posterior wall. The epithe- lium of the cornea is developed from the epiblast, while the corneal tissue proper is derived from the mesoblast which intervenes between the epi- blast and the primitive lens which Fig. 486.-Diagrammatic sketch of a vertical lon- gitudinal section through the eyeball of a human foetus of four weeks. The section is a little to the side, so as to avoid passing through the ocular cleft; c, the cuticle where it becomes later the corneal epithelium ; I, the lens; op. optic nerve formed by the pedicle of the primary optic vesicle; vp, primary medullary cavity or optic vesicle ; p, the pigment layer of the retina; r, the inner wall forming the retina proper; vs, secondary optic vesicle containing the rudiment of the vitreous humour, x 100. (Kolliker ) Fig. 487.- Transverse vertical section of the eyeball of a human embryo of four weeks. The anterior half of the sec- tion is represented : pr, the remains of the cavity of the primary optie vesicle; p, the inner part of the outer layer forming the retinal pig- ment ; r, the thickened inner part giving rise to the columnar and other structures of the retina ; v, the com- mencing vitreous humour within the secondary optic vesicle; v', the ocular cleft through which the loop of the central blood-vessel, a, projects from below; I, the lens with a central cavity, x 100. (Kiilliker.) 792 DEVELOPMENT. [chap, xxiii. was originally continuous with it. The sclerotic coat is developed round the eye-ball from the general mesoblast in which it is embedded. The iris is formed rather late, as a circular septum projecting inwards, from the fore part of the choroid, between the lens and the cornea. In the eye of the foetus of Mammalia, the pupil is closed by a delicate mem- brane, the membrana pupil- laris, which forms the front portion of a highly vascu- lar membrane that, in the foetus, surrounds the lens, and is named the membrana capsulo-pupillaris (fig. 488). It is supplied with blood by a branch of the arteria cen- tralis retinae, which, passing forwards to the back of the lens, there subdivides. The membrana capsulo-pupil- laris withers and disappears in the human subject a short time before birth. The eyelids of the human subject and mammiferous animals like those of birds, are first developed in the form of a ring. They then extend over the globe of the eye until they meet and become firmly agglutinated to each other. But before birth, or in the Carnivora after birth, they again separate. The Ear.-Very early in the development of the embryo a depression or ingrowth of the epiblast occurs on each side of the head which deepens and soon becomes a closed follicle. This primary otic vesicle, which closely corresponds in its formation to the lens follicle in the eye, sinks down to some distance from the free surface; from it are developed the epithelial lining of the 'membranous labyrinth of the internal ear, consisting of the vesti- bule and its semicircular canals and the scala media of the cochlea. I he surrounding mesoblast gives rise to the various fibrous bony and cartilaginous parts which complete and enclose this mem- branous labyrinth, the bony semicircular canals, the walls of the cochlea with its scala vestibuli and scala tympani. In the meso- 1'ig. 488.-Blood-vessels of the capsulo-pupillary mem- brane of a new-born kitten, magnified. The drawing is taken from a preparation injected by Tiersch, and shows in the central part the convergence of the net-work of vessels in the pupillary membrane. (Kolliker.) CHAP. XXIII.] THE ALIMENTARY CANAL. 793 blast, between the primary otic vesicle and the brain, the auditory nerve is gradually differentiated and forms its central and peripheral attachments to the brain and internal ear respectively. According to some authorities, however, it is said to take its origin from and grow out of the hind brain. The Eustachian tube, the cavity of the tympanum, and the external auditory passage, are remains of the first branchial cleft. The membrana tympani divides the cavity of this cleft into an internal space, the tympanum, and the external meatus. The mucous membrane of the mouth, which is prolonged in the form of a diverticulum through the Eustachian tube into the tympanum, and the external cutaneous system, come into relation with each other at this point ; the two membranes being separated only by the proper membrane of the tympanum. The pinna or external ear is developed from a process of integument in the neighbourhood of the first and second visceral arches, and probably corresponds to the gill-cover (operculum) in fishes. The Nose.-The nose originates like the eye and car in a de- pression of the superficial cpiblast at each side of the fronto-nasal process (primary olfactory groove), which is at first completely separated from the cavity of the mouth, and gradually extends backwards and downwards till it opens into the mouth. The outer angles of the fronto-nasal process, uniting with the maxillary process on each side, convert what was at first a groove into a closed canal. The Alimentary Canal. The alimentary canal in the earliest stages of its development consists of three distinct parts-the fore and hind gut ending blindly at each end of the body, and a middle segment which communicates freely on its ventral surface with the cavity of the yelk-sac through the vitelline or omphalo-mesenteric duct. From the fore-gut are formed the pharynx, oesophagus, and stomachfrom the hind-gut, the lower end of the colon and the rectum. The mouth is developed by an involution of the epiblast between the maxillary and mandibular processes, which becomes deeper and deeper till it reaches the blind end of the fore-gut, and at length communicates freely with the pharynx by the absorption of the partition between the two. 794 DEVELOPMENT. [chap, xxi 1 r. At the other end of the alimentary canal the anus is formed in a precisely similar way by an involution from the free surface, which at length opens into the hind-gut. When the depression from the free surface does not reach the intestine, the condition known as imperforate anus results. A similar condition may exist at the other end of the alimentary canal from the failure of the AB C D Fi"- 489.- Outlines of the form and position of the alimentary canal in successive stages of its development. A, alimentary canal, &c., in an embryo of four weeks ; B, at six weeks ; C, at eight weeks ; D, at ten weeks; I, the primitive lungs connected with the pharynx ; s, the stomach; d, the duodenum ; i, the small intestine ; i', the large ; c, the ceecuiu and vermiform appendage ; r, the rectum ; cl, in A, the cloaca ; a, in B, the anus dis- tinct from s i, the sinus uro-genitalis ; v, the yelk-sac ; v i, the vitello-intestinal duet; v, the urinary bladder and urachus leading to the allantois; g, genital ducts. (Allen Thomson.) involution which forms the mouth, to meet the fore-gut. The middle portion of the digestive canal becomes more and more closed in till its originally wide communication with the yelk-sac becomes narrowed down to a small duct (vitelline). This duct usually completely disappears in the adult, but occasionally the proximal portion remains as a diverticulum from the intestine. Sometimes a fibrous cord attaching some part of the intestine to the umbilicus, remains to represent the vitelline duct. Such a cord has been known to cause in after-life strangulation of the bowel and death. The alimentary canal lies in the form of a straight tube close beneath the vertebral column, but it gradually becomes divided into its special parts, stomach, small intestine, and large intestine chap xxin.] PANCREAS AND SALIVARY GLANDS. 795 (fig. 489), and at the same time comes to be suspended in the abdominal cavity by means of a lengthening mesentery formed from the splanchno-pleure which attaches it to the vertebral co- lumn. The stomach originally has the same direction as the rest of the canal; its cardiac extremity being superior, its pylorus in- ferior. The changes of position which the alimentary canal un- dergoes may be readily gathered from the accompanying figures (fig. 489). Pancreas and Salivary Glands.-The principal glands in connection with the intestinal canal are the salivary, pancreas, and the liver. In Mammalia, each salivary gland first appears as a simple canal with bud-like processes (fig. 490), lying in a gelatinous nidus or blas- tema, and communicating with the cavity of the mouth. As the develop- ment of the gland advances, the canal becomes more and more ramified, increasing at the expense of the blas- tema in which it is still enclosed. The branches or salivary ducts constitute an independent system of closed tubes (fig. 491). The pancreas is developed ex- actly as the salivary glands, but is developed from the hypoblast lining the intes- tine, while the salivary glands are formed from the epiblast lining the mouth. Liver.-The liver is developed by the protrusion, as it were, of a part of the walls of the fore-gut, in the form of two conical Fig. 490.-First appearance of the parotid gland in the embryo of a sheep. Fig. 491.-Lobule* of the parotid, with the salivary ducts, in the embryo of the sheep, at a more advanced stage. 796 DEVELOPMENT. [chap, xxii 1. hollow branches which embrace the common venous stem (figs. 492, 493). The outer part of these cones involves the omphalo- mesenteric vein, which breaks up in its interior into a plexus of capillaries, ending in venous trunks for the conveyance of the blood to the heart. The inner portion of the cones consists of a number of solid cylindrical masses of cells, derived probably from the hypoblast, which become gradually hollowed by the formation of the hepatic ducts, and among which blood - vessels are rapidly de- veloped. The gland-cells of the organs are derived from the hypoblast, the connective tissue and vessels without doubt from the mesoblast. The gall-bladder is developed as a diverticulum from the hepatic duct. Thespleen, lymphatic, and thymus glands are developed from the mesoblast: the thyroid partly also from the hypoblast, which grows into it as a diverticulum from the fore-gut. Fig. 492.-Diagram of part of digestive tract of a chick (4th day). The black line represents hypoblast, the outer shading mesoblast; 1 g, lung diverti- culum, with expanded end forming primary lung-vesicle; S t, stomach ; I, two hepatic diverticula, with their terminations united by solid rows of hypoblast cells; p, diverticulum of the pancreas with the vesicular diver- ticula coming from it. (Gotte.) Fig. 49.;.-Rudiments of the liver on the intestine of a chick at the fifth day of incubation. I, heart; 2, intestine ; 3, diverticulum of the intestine in which the liver (4) is developed ; 5, part of the mucous layer of the germinal membrane. (Miiller.) The Respiratory Apparatus. The lungs, at their first development, appear as small tubercles, or diverticula from the abdominal surface of the oesophagus. chap. xxiii.] THE GENITO-URINARY APPARATUS. 797 The two diverticula at first open directly into the oesophagus, but as they grow, a separate tube (the future trachea) is formed at their point of fusion, opening into the oesophagus on its anterior surface. These primary diverticula of the hypoblast of the ali- mentary canal send off secondary branches into the surrounding mesoblast, and these again give off tertiary branches, forming the air-cells. Thus we have the lungs formed : the epithelium lining their air-cells, bronchi, and trachea being derived from the hypoblast, and all the rest of the lung-tissue, nerves, lymphatics, and blood- vessels, cartilaginous rings, and muscular fibres of the bronchi from the mesoblast. The dia- phragm is early developed. A. B C Fig. 494 illustrates the development of the respiratory organs. A, is the oesophagus of a chick on the fourth day of incuba- tion, with the rudiments of 'the trachea on the lung of the left side, viewed late- rally ; 1, the inferior wall of the oesopha- gus ; 2, the upper tube of the same tube; 3, the rudimentary lung; 4, the stomach. b, is the same object seen from below, so that both lungs are visible, c, shows the tongue and respiratory organs of the embryo of a horse ; 1, the tongue; 2, the larynx; 3, the trachea ; 4, the lungs, viewed from the upper side. (After Rathke.) The Genito-Urinary Apparatus. The Wolffian bodies are organs peculiar to the embryonic state, and may be regarded as temporary, rather than rudiniental, kidneys; for although they seem to discharge the functions of these latter organs, they are not developed into them. The Wolffian duct makes its appearance at an early stage in the history of the embryo, as a cord running longitudinally on each side in the mass of mesoblast, which lies just external to the intermediate cell-mass (ung, fig. 495). This cord, at first solid, becomes gradually hollowed out to form a tube (Wolffian duct) which sinks down till it projects beneath the lining membrane into the pleuro-peritoneal cavity. The primitive tube thus formed sends off secondary diverticula at frequent intervals which grow into the surrounding mesoblast : tufts of vessels grow into the blind ends of these tubes, invagi nating them and producing " Malpighian bodies " very similar in appearance to those of the permanent kidney, which constitute the substance of the Wolffian body. Meanwhile another portion of mesoblast between the Wolffian body and the mesentery projects in the form of a ridge, covered on its free surface with 798 DEVELOPMENT. [chap. xxm. epithelium termed "germ epithelium/' from this projection is developed the reproductive gland (ovary or testis as the case may be). Simultaneously, on the outer wall of the Wolffian body, between it and the body-wall on each side, an involution is formed from the pleuro-peritoneal cavity in the form of a longitudinal Fig. 495.-Transverse of embryo chick (third day). rudimentary spinal cord; the primitive central canal has become constricted in the middle; c h, notochord; u w h, primordial vertebral mass; mf muscle-plate; dr,df, hypoblast and visceral layer of mesoblast lining groove, which is not yet closed in to form the intestines; a, o, one of the primitive aortse ; u n, Wolffian body ; un y, Wolffian duct: v c, vena car Jinalis ; h, epiblast; h p, somatopleure and its reflection to form af, amniotic fold ; P, pleuroperitoneal cavity. (Kulliker.) furrow, whose edges soon close over to form a duct (Muller's duct). All the above points are shown in the accompanying figures, 495, 496> 497- The Wolffian bodies, or temporary kidneys, as they may be termed, give place at an early period in the human foetus to their successors, the permanent kidneys, which are developed behind them. They diminish rapidly in size, and by the end of the third month have almost entirely disappeared. In connection, however, with their upper part, in the male, there are developed from a new mass of blastema, the vasa efferentia, coni vasculosi, and globus major of the epididymis ; and thus is brought about a direct con- nection between the secreting part of the testicle and its duct (Cleland, Banks). The Wolffian ducts persist in the male, and are developed to form the body and globus minor of the epididymis, CHAP. XXI11.] THE KIDNEYS. 799 the vas deferens, and ejaculatory duct on each side, the vesicuhe seminales forming diverticula from their lower part. In the female a small relic of the Wolffian body persists as the " parovarium ; " in the male a similar relic is termed the " organ of Giraldes." The lower end of the Wolffian duct remains in the female as the Fig. 496.-Section of intermediate cell-mass on the fourth day. m, mesentery; L, somato- pleure ; «', germinal epithelium, from which z, the duct of Muller, becomes involuted ; a, thickened part of germinal epithelium in which the primitive ova 0 and o, are lying; i', modified mesoblast, which will form the stroma of the ovary; IF A', Wolffian body; y, Wolffian duct; x 160. (Waldeyer.) "duct of Gaertner " which descends towards, and is lost upon, the anterior wall of the vagina. From the lower end of the Wolffian duct a diverticulum grows back along the body of the embryo towards its anterior extremity, and ultimately forms the ureter. Secondary diverticula are given oft' from it and grow into the surrounding blastema of blood- vessels and cells. Malpighian bodies are formed just as in the Wolffian body, by the invagination of the blind knobbed end of these diverticula by a tuft of vessels. This process is precisely similar to the invagination of the primary optic vesicle by the rudimentary lens. 800 DEVELOPMENT. [chap, xxi 11. Tims the kidney is developed, consisting at first of a number of separate lobules; this condition remaining throughout life in many of the lower animals, e.g., seals and whales, and traces of this Fig. 497.-Diagram showing the relations of the female {the left-hand figure ? ) and of the male {the right-hand figure S ) reproductive organs to the general plan {the middle figure) of these organs in the higher vertebrata (including man). C I, cloaca; It, rectum; B I, urinary bladder; 17, ureter; K, kidney ; U h, urethra ; G, genital gland, ovary, or testis; W, 'Wolffian body: W d, Wolffian duct; Jf, Mullerian duct; P s t, prostrate gland ; C p, Cowper's gland ; C sp, corpus spongiosum; C c, corpus cavemosum. In the female.-V, vagina ; U t, uterus ; F p, Fallopian tube ; G t, Gaertner's duct; P v, parovarium ; A, anus ; C c, C s p, clitoris. /« rte male. - C s p, C c, penis ; U t, uterus masculinis ; V s, vesicula seminalis ; V </, vas deferens. (Huxley.) lobulation being visible in the human foetus at birth. In the adult all the lobules are fused into a compact solid organ. The supra-renal capsules originate in a mass of mesoblast just above the kidneys ; soon after their first appearance they are very much larger than the kidneys (see fig. 497), but by the more rapid growth of the latter this relation is soon reversed. The first appearance of the generative gland has been already described : for some time it is impossible to determine whether an ovary or testis will be developed from it; gradually however the special characters belonging to one of them appear, and in either CHAP. XXIII.] DESCENT OF THE TESTICLES. 801 case the organ soon begins to assume a relatively lower position in the body; the ovaries being ultimately placed in the pelvis ; while towards the end of foetal existence the testicles descend into the scrotum, the testicle entering the internal inguinal ring in the seventh month of foetal life, and completing its descent through the inguinal canal and external ring into the scrotum by the end of the eighth month. A pouch of peritoneum, the processus vagi- nalis, precedes it in its descent, and ultimately forms the tunica vaginalis or serous covering of the organ ; the communication between the tunica vaginalis and the cavity of the peritoneum being closed only a short time before birth. In its descent, the testicle or ovary of course retains the blood-vessels, nerves, and lymphatics, which were supplied to it while in the lumbar region, and which are compelled to accompany it, so to speak, as it assumes a lower position in the body. Hence the explanation of the other- wise strange fact of the origin of these parts at so considerable a distance from the organ to which they are distributed. Descent of the Testicles into the Scrotum.-The means by which the descent of the testicles into the scrotum is effected are not fully and exactly known. It was formerly believed that a mem- branous and partly muscular cord, called the gubernaculum testis, which extends while the testicle is yet high in the abdomen, from its lower part, through the abdominal wall (in the situation of the inguinal canal) to the front of the pubes and lower part of the scrotum, was the agent by the contraction of which the descent was effected. It is now generally thought, however, that such is not the case; and that the descent of the testicle and ovary is rather the result of a general process of development in these and neighbouring parts, the tendency of which is to produce this change in the relative position of these organs. In other words, the descent is not the result of a mere mechanical action, by which the organ is dragged down to a lower position, but rather one change out of many which attend the gradual development and re-arrangement of these organs. It may be repeated, how- ever, that the details of the process by which the descent of the testicle into the scrotum is effected are not accurately known. The homologue, in the female, of the gubernaculum testis is a structure called the round ligament of the uterus, which extends through the inguinal canal, from the outer and upper part of the uterus to the subcutaneous tissue in front of the symphysis pubis. 802 DEVELOPMENT. [chap, xxiii. At a very early stage of foetal life, the Wolffian ducts, ureters, and Mullerian ducts, open into a receptacle formed by the lower end of the allantois, or rudimentary bladder; and as this com- municates with the lower extremity of the intestine, there is for the time, a common receptacle or cloaca for all these parts, which opens to the exterior of the body through a part corresponding with the future anus, an arrangement which is permanent in Reptiles, Birds, and some of the lower Mammalia. In the human foetus, however, the intestinal portion of the cloaca is cut oft' from that which belongs to the urinary and genera- tive organs; a separate passage or canal to the exterior of the body, belonging to these parts, being called thesinusuro- genitalis. Subsequently, this canal is divided, by a process of division ex- tending from before back- wards or from above downwards, into a ' pars urinaria ' and a ' pars genitalis.' The former, continuous with the ura- chus, is converted into the urinary bladder. The Fallopian tubes, the uterus, and the vagina are developed from the Mullerian ducts (fig. 498, m and fig. 501) whose first appear- ance has been already described. The two Miil- lerian ducts are united below into a single cord, called the genital cord, and, from this are developed the vagina, as well as the cervix and the lower portion of the body of the uterus; while the ununited por- tion of the duct on each side forms the upper part of the uterus, and the Fallopian tube. In certain cases of arrested or abnormal big. 198, - Diagram of the Wolffian bodies, Mullerian ducts and adjacent parts previous to sexual distinction, as seen from before, sr, the supra-renal bodies; £ef£}oney& 0V?^mon1 bvstema °U>v,®ies or testicles; W, Wolffian bodies; w, Wolffian ducts; m m, Mullerian ducts ; y c, genital cord ; w<7, sinus urogenitalis; i, intestine; cl, cloaca. Alien Thompson.) chap, xxnr.] FORMATION OF THE GENITAL ORGANS. 803 development, these portions of the Miillerian ducts may not become fused together at their lower extremities, and there is left a cleft or horned condition of the upper part of the uterus re- sembling a condition which is permanent in certain of the lower animals. Fig. 409. Fig. 500. Fig. 501. Fig. 502. Urinary and generative organs of a human female embryo, measuring inches in length. Fig. 499.-General view of these parts ; 1, supra-renal capsules; 2, kidneys; 3, ovary; 4, Fallopian tube ; 5, uterus ; 6, intestine; 7, the bladder. Fig. 500.-Bladder and Generative organs of the same embryo viewed from the side ; a, the urinary bladder (at the upper part is a portion of the urachus) ; 2, urethra ; 3, uterus (with two cornea); 4, vagina; 5, part as yet common to the vagina and urethra; 6, common orifice of the urinary and generative organs ; 7, the clitoris. Fig. 501.-Internal generative organs of the same embryo; i, the uterus; 2, the round ligaments; 3, the Fallopian tubes (formed by the Miillerian ducts) ; 4, the ovaries ; 5, the remains of the Wolffian bodies. Fig. 502.-External generative oigans of the same embryo; 1, the labia majora; 2, the riymphee; 3, clitoris; 4, anus. (Muller.) In the male, the Mullerian ducts have no special function, and are but slightly developed. The hydatid of Morgagni is the remnant of the upper part of the Mullerian duct. The small prostatic pouch, uterus masculinus, or sinus pocularis, forms the atrophied remnant of the distal end of the genital cord, and is, of 804 THE RELATION OF LIFE TO OTHER FORCES. [CHAP. XXIV. course, therefore, the homologue, in the male, of the vagina and uterus in the female. The external parts of generation are at first the same in both sexes. The opening of the genito-urinary apparatus is, in both sexes, bounded by two folds of skin, whilst in front of it there is formed a penis-like body surmounted by a glans, and cleft or furrowed along its under surface. The borders of the furrows diverge posteriorly, running at the sides of the genito-urinary orifice internally to the cutaneous folds just mentioned (see figs. 499-5O2)- I11 the female, this body becoming retracted, forms the clitoris, and the margins of the furrow on its under surface are converted into the nymphae, or labia minora, the labia majora pudendae being constituted by the great cutaneous folds. In the male foetus, the margins of the furrow at the under surface of the penis unite at about the fourteenth week, and form that part of the urethra which is included in the penis. The large cutaneous folds form the scrotum, and later (in the eighth month of development), receive the testicles, which descend into them from the abdominal cavity. Sometimes the urethra is not closed, and the deformity called hypospadias then results. The appearance of hermaphroditism may, in these cases, be increased by the retention of the testes within the abdomen. CHAPTER XXIV." ON THE RELATION OF LIFE TO OTHER FORCES. An enumeration of theories concerning the nature of life would be beside the purpose of the present chapter. They are interest- ing as marks of the way in which various minds have been influenced by the mystery which has always hung about vitality ; their destruction is but another warning that any theory we can * This chapter is a reprint, with some verbal alterations, of an essay con- tributed to St. Bartholomew's Hospital Reports, 1867, 1869, bv W. Morrant Baker. ■ hap. xxiv.] THE RELATION OF LIFE TO OTHER FORCES. 805 frame must be considered only a tie for connecting present facts, and one that must yield or break on any addition to the number which it is to bind together. Before attention had been drawn to the mutual convertibility of the various so-called physical forces-heat, light, electricity, and others-and until it had been shown that these, like the matter through which they act, are limited in amount, and strictly measurable; that a given quantity of one force can produce a certain quantity of another and no more; that a given quantity of combustible material can produce only a given quantity of steam, and this again only so much motive power ; it was natural that men's minds should be satisfied with the thought that vital force was some peculiar innate power, un- limited by matter, and altogether independent of structure and organisation. The comparison of life to a flame is probably as early as any thought about life at all. And so long as light and heat were thought to be inherent qualities of certain material which perished utterly in their production, it is not strange that life also should have been reckoned some strange spirit, pent up in the germ, expending itself in growth and development, and finally declining and perishing with the body which it had inhabited. With the recognition, however, of a distinct correlation between the physical forces, came as a natural consequence a revolution of the commonly accepted theories concerning life also. The dictum, so long accepted, that life was essentially independent of physical force began to be questioned. As it is well-nigh impossible to give a definition of life that shall be short, comprehensive, and intelligible, it will be best, perhaps, to take its chief manifestations, and see how far these seem to be dependent on other forces in nature, and how connected with them. Life manifests itself by Birth, Growth, Development, Decline, and Death ; and an idea of life will most naturally arise by taking these events in succession, and studying them individually, and in relation to each other. When the embryo in a seed awakes from that state, neither life nor death, which is called dormant vitality, and, bursting its envelopes, begins to grow up and develope, it may be said that there is a birth. And so, when the chick escapes from the egg, and when any living form is, as the phrase goes, brought into the 806 THE RELATION OF LIFE TO OTHER FORCES. [( HAP. xxiv. world. In each case, however, birth is not the beginning of life, bnt only the continuation of it under different conditions. To understand the beginning of life in any individual, whether plant or animal, existence must be traced somewhat further back, and in this way an idea gained concerning the nature of the germ, the development of which is to issue in birth. The germ may be defined as that portion of the parent which is set apart with power to grow up into the likeness of the being from which it has been derived. The manner in which the germ is separated from the parent does not here concern us. It belongs to the special subject of generation. Neither need we consider apart from others those modes of propagation, as fission and gemmation, which differ more apparently than really from the ordinary process typified in the formation of the seed or ovum. In every case alike, a new individual plant or animal is a portion of its parent: it may be a mere outgrowth or bud, which, if separated, can maintain an independent existence; it may be not an outgrowth, but simply a portion of the parent's structure, which has been naturally or artificially cut off, as in the spontaneous or artificial cleaving of a polype ; it may be the embryo of a seed or ovum, as in those cases in which the process of multiplication of different organs has reached the point of separation of the individual more or less completely into two sexes, the mutual conjugation of a portion of each of which, the sperm-cell and the germ-cell, is necessary for the production of a new being. We arc so accustomed to regard the conjugation of the two sexes as necessary for what is called generation, that wc are apt to forget that it is only gradually in the upward progress of development of the vege- table and animal kingdoms, that those portions of organised matter which are to produce new beings are allotted to two separate individuals. In the least developed forms of life, almost any part of the body is capable of assuming the characters of a separate individual; and propagation, therefore, occurs by fission or gemmation in some form or other. Then, in beings a little higher in rank, only a special part of the body can become a separate being, and only by conjugation with another special part. Still there is but one parent; and this hermaphrodite-form of generation is the rule in the vegetable and least developed portion of the animal kingdom. At last, in all animals but the lowest, and in some plants, the portions of organised structure specialised chai-, xxiv.] THE RELATION OF LIFE TO OTHER FORCES. 807 for development after their mutual union into a new individual, are found on two distinct beings, which we call respectively male and female. The old idea concerning the power of growth resident in the germ of the new being, thus formed in various ways, was expressed by saying that a store of dormant vitality was laid up in it, and that so long as no decomposition ensued, this was capable of manifesting itself and becoming active under the influence of certain external conditions. Thus, the dormant force supposed to be present in the seed or the egg was assumed to be the primary agent in effecting development and growth, and to continue in action during the whole term of life of the living being, animal or vegetable, in which it was said to reside. The influence of external forces-heat, light, and others-was noticed and appreciated ; but these were thought to have no other connec- tion with vital force than that in some way or other they called it into action, and that to some extent it was dependent on them for its continuance. They were not supposed to be correlated with it in any other sense than this. Now, however, we are obliged to modify considerably our notions and with them our terms of expression, when describing the origin and birth of a new being. To take, as before, the simplest case-a seed or egg. We must suppose that the heat, which in conjunction with moisture is necessary for the development of those changes which issue in the growth of a new plant or animal, is not simply an agent which so stimulates the dormant vitality in the seed or egg as to make it cause growth, but it is a force, which is itself transformed into chemical and vital power. The embryo in the seed or egg is a part which can transform heat into vital force, this term being a convenient one wherewith to express the power which particular structures possess of growing, developing, and performing other actions which we call vital.* Of course the embryo can grow only by taking up fresh material, and incorporating it with its own structure, and therefore it is surrounded in the seed or ovum with matter sufficient for nutri- * The term " vital force " is here employed for the sake of brevity. Whether it is strictly admissible will be discussed hereafter. The general term force is used as synonymous with what is now often termed energy. 808 THE RELATION OF LIFE TO OTHER FORCES. [< hap. xxiv. tion until it can obtain fresh supplies from without. The absorption of this nutrient matter involves an expenditure of force of some kind or other, inasmuch as it implies the raising of simple to more complicated forms. Hence the necessity for heat or some other power before the embryo can exhibit any sign of life. It would be quite as impossible for the germ to begin life without external force as without a supply of nutrient matter. Without the force wherewith to take it, the matter would be useless. The heat, therefore, which in conjunction with moisture is necessary for the beginning of life, is partly expended as chemical power, which causes certain modifications in the nutrient material surrounding the embryo, c.y., the transforma- tion of starch into sugar in the act of germination ; partly, it is transformed by the germ itself into vital force, whereby the germ is enabled to take up the nutrient material presented to it, and arrange it in forms characteristic of life. Thus the force is expended, and thus life begins-when a particle of organised matter, which has itself been produced by the agency of life, begins to transform external force into vital force, or in other words into a power by which it is enabled to grow and develop. 'Phis is the true beginning of life. The time of birth is but a particular period in the process of development at which the germ, having arrived at a fit state for a more independent existence, steps forth into the outer world. The term " dormant vitality," must be taken to mean simply the existence of organised matter with the capacity of transform- ing heat or other force into vital or growing power, when this force is applied to it under proper conditions. The state of dormant vitality is like that of an empty voltaic battery, or a steam-engine in which the fuel is not yet lighted. In the former case no electric current passes, because no chemical action is going on. There is no transformation into electric force, because there is no chemical force to be transformed. Yet, we do not say, in this instance, that there is a store of electricity laid up in a dormant state in the battery ; neither do we say that a store of motion is laid up in the steam-engine. And there is as little reason for saying there is a store of vitality in a dormant seed or ovum. Next to the beginning of life, we have to consider how far its continuance by growth and development is dependent on external force, and to what extent correlated with it. CHAP. XXIV.] THE RELATION OE LIFE TO OTHER FORCES. 809 Mere growth is not a special peculiarity of living beings. A crystal, if placed in a proper solution, will increase in size and preserve its own characteristic outline ; and even if it be injured, the flaw can be in part or wholly repaired. The manner of its growth, however, is very different from that of a living being, and the process as it occurs in the latter will be made more evident by a comparison of the two cases. The increase of a crystal takes place simply by the laying of material on the sur- face only, and is unaccompanied by any interstitial change. This is, however, but an accidental difference. A much greater one is to be found in the fact that with the growth of a crystal there is no decay at the same time, and proceeding with it side by side. Since there is no life there is no need of death-the one being a condition consequent on the other. During the whole life of a living being, on the other hand, there is unceasing change. .At different periods of existence the relation between waste and repair is of course different. In early life the addition is greater than the loss, and so there is growth ; the reconstructed part is better than it was before, and so there is development. In the decline of life, on the contrary, the renewal is less than the destruction, and instead of development there is degeneration. But at no time is there perfect rest or stability. It must not be supposed, therefore, that life consists in the capability of resisting decay. Formerly, when but little or nothing was known about the laws which regulate the existence of living beings, it was reasonable enough to entertain such an idea; and, indeed, life was thought to be, essentially, a myste- rious power counteracting that tendency to decay which is so evident when life has departed. Now, we know that so far from life preventing decomposition, it is absolutely dependent upon it for all its manifestations. The reason of this is very evident. Apart from the doctrine of correlation of force, it is of course plain that tissues which do work must sooner or later wear out if not constantly supplied with nourishment; and the need of a continual supply of food, on the one hand, and on the other, the constant excretion of matter, which, having evidently discharged what was required of it, was fit only to be cast out, taught this fact very plainly. But although, to a certain extent, the dependence of vital power on supplies of matter from without was recognized and appreciated, the true relation between the demand and supply was not until recently 810 THE RELATION OF LIFE TO OTHER FORCES. [chap. XXIV. thoroughly grasped. The doctrine of the correlation of vital with other forces was not understood. To make this more plain, it will be well to take an instance of transformation of force more commonly known and appreciated. In the steam-engine a certain amount of force is exhibited as motion, and the immediate agent in the production of this is steam, which again is the result of a certain expenditure of heat. Thus, heat is in this instance said to be transformed into motion, or, in other language, one-molecular-mode of motion, heat, is made to express itself by another-mechanical-mode, ordinary movement. But the heat which produced the vapour is itself the product of the combustion of fuel, or, in other words, it is the correlated expression of another force- chemical, namely, that affinity of carbon and hydrogen for oxygen which is satisfied in the act of combustion. Again, the production of light and heat by the burning of coal and wood is only the giving out again of that heat and light of the sun which were used in their pro- duction. For, as it need scarcely be said, it is only by means of these solar forces that the leaves of plants can decompose carbonic acid, Ac., and thereby provide material for the construction of woody tissue. Thus, coal and wood being products of the expen- diture of force, must be taken to represent a certain amount of power; and, according to the law of the correlation of forces, must be capable of yielding, in some shape or other, just so much as was exercised in their formation. The amount of force requisite for rending asunder the elements of carbonic acid is exactly that amount which will again be manifested when they clash together again. The sun, then, really, is the prime agent in the movement of the steam-engine, as it is indeed in the production of nearly all the power manifested on this globe. In this particular instance, speaking roughly, its light and heat arc manifested successively as vital and chemical force in the growth of plants, as heat and light again in the burning fuel, and lastly by the piston and wheels of the engine as motive power. We may use the term transformation of force if we will, or say that throughout the cycle of changes there is but one force variously manifesting itself. It matters not, so that we keep clearly in view the notion that all force, so far at least as our present knowledge extends, is but a representative, it may be in the same form or another, of some previous force, and incapable like matter, of being CHAP. XXIV.] THE RELATION OF LIFE TO OTHER FORCES. 811 created afresh, except by the Creator. Much of our knowledge on this subject is of course confined to ideas, and governed by the words with which we are compelled to express them, rather than to actual things or facts; and probably the term force will soon lose the signification which we now attach to it. What is now known, however, about the relation of one force to another, is not sufficient for the complete destruction of old ideas; and, therefore, in applying the examples of transformation of physical force to the explanation of vital phenomena, we are compelled still to use a vocabulary which was framed for expressing many notions now obsolete. The dependence of the lowest kind of vital existence on external force, and the manner in which this is used as a means whereby life is manifested, have been incidentally referred to more than once when describing the origin of vegetable tissues. The main functions of the vegetable kingdom are construction, and the perpetuation of the race; and the use which is made of external physical force is more simple than in animals. The transformation indeed which is effected, while much less mysterious than in the latter instance, forms an interesting link between animal and crystalline growth, The decomposition of carbonic acid or ammonia by the leaves of plants may be compared to that of water by a galvanic current. In both cases a force is applied through a special material medium, and the result is a separation of the elements of which each compound is formed. On the return of the elements to their original state of union, there will be the return also in some form oi' other of the force which was used to separate them. Vegetable growth, moreover, with which we are now specially concerned, resembles somewhat the increase of unorganised matter. The accidental difference of its being in one case superficial, and in the other interstitial, is but little marked in the process as it occurs in the more permanent parts of vegetable tissues. The layers of lignine are in their arrangement nearly as simple as those of a crystal, and almost or quite as lifeless. After their deposition, moreover, they undergo no further change than that caused by the addition of fresh matter, and hence they are not instances of that ceaseless waste and repair which have been referred to as so characteristic of the higher forms of living tissue. There is, how- ever, no contradiction here of the axiom, that where there is life there is constant change. Those parts of a vegetable organism in 812 the RELATION OF LIFE TO OTHER FORCES, [chap. xxiv. which active life is going on are subject, like the tissues of animals, to constant destruction and renewal. But, in the more permanent parts, life ceases with deposition and construction. Addition of fresh matter may occur, and so may decay also of that which is already laid down, but the two processes are not related to each other, and not, as in living parts inter-dependent. Hence the change is not a vital one. The acquirement in growth, moreover, of a definite shape in the case of a tree, is no more admirable or mysterious than the pro- duction of a crystal. That chloride of sodium should naturally assume the form of a cube is as inexplicable as that an acorn should grow into an oak, or an ovum into a man. When we learn the cause in the one case we shall probably in the other also. There is nothing, therefore, in the products of life's more simple forms that need make us start at the notion of their being the products of only a special transformation of ordinary physical force, and we cannot doubt that the growth and de- velopment of animals obey the same general laws that govern the formation of plants. The connecting links between them are too numerous for the acceptance of any other supposition. Both kingdoms alike are expressions of vital force, which is itself but a term for a special transformation of ordinary physical force. The mode of the transformation is, indeed, mysterious, but so is that of heat into light, or of either into mechanical motion or chemical affinity. All forms of life are as absolutely dependent on external physical force as a fire is dependent for its continuance on a supply of fuel; and there is as much reason to be certain that vital force is an expression or representation of the physical forces, especially heat and light, as that these are the correlates of some force or other which has acted or is acting on the substances which, as we say, produce them. In the tissues of plants, as just said, there is but little change, except such as is produced by additions of fresh matter. That which is once deposited alters but little; or, if the part be tran- sient and easily perishable, the alteration is only or chiefly one produced by the ordinary process of decay. Little or no force is manifested ; or, if it be, it is only the heat of the slow oxidation whereby the structure again returns to inorganic shape. There is no special transformation of force to which the term vital can be applied. \\ ith construction the chief end of vegetable existence CHAP. XXIV.] THE RELATION OF LIFE TO OTHER FORCES. 813 lias been attained, and the tissue formed represents a store of force to be used, but not by the being which laid it up. The labours of the vegetable world are not for itself but for animals. The power laid up by the one is spent by the other. Hence the reason that the constant change, which is so great a character of life, is comparatively but little marked in plants. It is present, but only in living portions of the organism, and in these it is but limited. In a tree the greater part of the tissues may be con- sidered dead ; the only change they suffer is that fresh matter is piled on to them. They are not the seat of any transformation of force, and therefore, although their existence is the result of living action, they do not themselves live. Force is, so to speak, laid up in them, but they do not themselves spend it. Those portions of a vegetable organism which are doing active vital work-which are using the sun's light and heat, as a means whereby to prepare building material, are, however, the seat of unceasing change. Their existence as living tissue depends upon this fact-upon their capability of perishing and being renewed. And this leads to the answer to the question, What is the cause of the constant change which occurs in the living parts of animals and vegetables, which is so invariable an accompani- ment of life, that we refuse the title of "living" to parts not attended by it ? It is because all manifestations of life are exhibi- tions of power, and as no power can be originated by us: as, according to the doctrine of correlation of force, all power is but the representative of some previous force in the same or another form, so, for its production, there must be expenditure and change somewhere or other. For the vital actions of plants the light and heat of the sun are nearly or quite sufficient, and there is no need of expenditure of that store of force which is laid up in them- selves ; but with animals the case is different. They cannot directly transform the solar forces into vital power; they must seek it elsewhere. The great use of the vegetable kingdom is therefore to store up power in such a form that it can be used by animals; that so, when in the bodies of the latter, vegetable organised material returns to an inorganic condition, it may give out force in such a manner that it can be transformed by animal tissues, and manifested variously by them as vital power. Hence, then, we must consider the waste and repair attendant on living growth, and development as something more than these words, taken by themselves, imply. The waste is the return to a 814 THE RELATION OF LIFE TO OTHER FORCES. [chap. XXIV. lower from a higher form of matter; and, in the fall, force is manifested. This force, when specially transformed by organised tissues, we call vital. In the repair, force is laid up. The analogy with ordinary transmutations of physical force is perfect. By the expenditure of heat in a particular manner a weight can be raised. By its fall heat is returned. The molecular motion is but the expression in another form of the mechanical. So with life. There is constant renewal and decay, because it is only so that vital activity can take place. The renewal must be something more than replacement, however, as the decay must be more than simple mechanical loss. The idea of life must include both storing up of force, and its transformation in the expenditure. Hence we must be careful not to confound the mere preservation of individual form under the circumstances of concurrent waste and repair, with the essential nature of vitality. Life, in its simplest form, has been happily expressed by Savory as a state of dynamical equilibrium, since one of its most characteristic features is continual decay, yet with maintenance for the individual by equally constant repair. Since, then, in the preservation of the equilibrium there is ceaseless change, it is not static equilibrium but dynamical. Care must be taken, however, not to accept the term in too strict a sense, and not to confound that which is but a necessary attendant on life with life itself. For, indeed, strictly, there is no preservation of equilibrium during life. Each vital act is an advance towards death. We are accustomed to make use of the terms growth and development in the sense of progress in one direction, and the words decline and decay with an opposite signi- fication, as if, like the ebb of the tide, there were after maturity a reversal of life's current. But, to use an equally old comparison, life is really a journey always in one direction. It is an ascent, more and more gradual as the summit is approached, so gradual that it is impossible to say when development ends and decline begins. But the descent is on the other side. There is no perfect equilibrium, no halting, no turning back. I he term, therefore, must be used with only a limited significa- tion. 1 here is preservation of the individual, yet, although it may seem a paradox, not of the same individual. A man at one period of his life may retain not a particle of the matter of which formerly he was composed. The preservation of a living being during growth and development is more comparable, indeed, to that of a chap, xxiv.] THE RELATION OF LIFE TO OTHER FORCES. 815 nation, than of an individual as the term is popularly understood. The elements of which it is made up fulfil a certain work the traditions of which were handed down from their predecessors, and then pass away, leaving the same legacy to those that follow them. The individuality is preserved, but, like all things handed down by tradition, its fashion changes, until at last, perhaps, scarce any likeness to the original can be discovered. Or, as it sometimes happens, the alterations by time arc so small that we wonder, not at the change, but the want of it. Yet, in both cases alike, the individuality is preserved, not by the same individual elements throughout, but by a succession of them. Again, concurrent waste and repair do not imply of necessity the existence of life. It is true that living beings are the chief instances of the simultaneous occurrence of these things. But this happens only because the conditions under which the func- tions of life are discharged are the principal examples of the necessity for this unceasing and mingled destruction and renewal. They are the chief, but not the only instances of this curious conjunction. A theoretical case will make this plain. Suppose an instance of some permanent structure, say a marble statue. If we imagine it to be placed under some external conditions by which each particle of its substance should waste and be replaced, yet with maintenance of its original size and shape, we obtain no idea of life. There is waste and renewal, with preservation of the individual form, but no vitality. And the reason is plain. With the waste of a substance like carbonate of calcium whose attractions are satisfied, there would be no evolution of force; and even if there were, no structure is present with the power to transform or manifest anew' any power which might be evolved. With the repair, likewise, there would be no storing of force. The part used to make good the loss is not different from that which disappeared. There is therefore neither storing of force, nor its transformation, nor its expenditure; and therefore there is no life. But real examples of the preservation of an individual substance under the circumstances of constant loss and renewal, may be found, yet without any semblance in them of life. Chemistry, perhaps, affords some of the neatest and best examples of this. One, suggested by Shepard, seems particularly apposite. It is the ease of trioxide of nitrogen N2O3 in the preparation of sulphuric acid. The gas from which this acid is 8l6 THE RELATION OF LIFE TO OTHER FORCES. [CHAV. XXIV. obtained is sulphur dioxide, and the addition of an equivalent of oxygen and the combination of the resulting sulphur trioxide (SO.) with water (H.O) is all that is required. Thus: SO4 + 0 + H'O = H.,S04 Sulph. dioxide : Oxygen : Water - Sulphuric Acid. Sulphur dioxide, however, cannot take the necessary oxygen directly from the atmosphere, but it can abstract it from trioxide of nitrogen (N2O3), when the two gases are mingled. The trioxide, accordingly, by continually giving up an equivalent of oxygen to an equivalent of sulphur dioxide, causes the formation of sulphuric acid, at the same time that it retains its composition by continually absorbing a fresh quantity of oxygen from the atmosphere. In this instance, then, there is constant waste and repair, yet without life. And here an objection cannot be raised, as it might be to the preceding example, that both the destruction and repair come from without, and are not dependent on any inherent qualities of the substance with which they have to do. The waste and renewal in the last-named example are strictly dependent on the qualities of the chemical compound which is subject to them. It has but to be placed in appropriate conditions, and destruction and repair will continue indefinitely. Force, too, is manifested, but there is nothing present which can transform it into vital shape, and so there is no life. Hence, our notion of the constant decay which, together with repair, takes place throughout life, must be not confined to any simply mechanical act. It must include the idea, as before said, of laying up of force, and its expenditure-its transformation too, in the act of being expended. The growth, then, of an animal or vegetable, implies the ex- penditure of physical force by organized tissue, as a means whereby fresh matter is added to and incorporated with that already existing. In the case of the plant the force used, trans- formed, and stored up, is almost entirely derived from external sources; the material used is inorganic. The result is a tissue which is not intended for expenditure by the individual which has accumulated it. The force expended in growth by animals, on the other hand, cannot be obtained directly from without. For them a supply of force is necessary in the shape of food derived directly or indirectly from the vegetable kingdom. Part of this force- chap, xxiv.] TIIE RELATION OF LIFE TO OTHER FORCES. 817 containing food is expended as fuel for the production of power; and the latter is used as a means wherewith to elaborate another* portion of the food, and incorporate it as animal structure. Un- like vegetable structure, however, animal tissues are the seat of constant change, because their object is not the storing up of power, but its expenditure; so there must be constant waste; and if this happen, then for the continuance of life there must be equally constant repair. But, as before said, in early life the repair surpasses the loss, and so there is growth. The part repaired is better than before the loss, and thus there is development. The definite limit which has been imposed on the duration of life has been already incidentally referred to. Like birth, growth, and development, it belongs essentially to living beings only. Dead structures and those which have never lived are subject to change and destruction, but decay in them is uncertain in its beginning and continuance. It depends almost entirely on ex- ternal conditions, and differs altogether from the decline of life. The decline and death of living beings are as definite in their occurrence as growth and development. Like these they may be hastened or stayed, especially in the lower forms of life, by various influences from without; but the putting off of decline must be the putting off also of so much life ; and, apart from disease, the reverse is true also. A living being starts on its career with a certain amount of work to do-varying infinitely in different individuals, but for each well-defined. In the lowest members of both the animal and vegetable creation the progress of life in any given time seems to depend almost entirely on external circum- stances ; and at first sight it seems almost as if these lowly-formed organisms were but the sport of the surrounding elements. But it is only so in appearance, not in reality. Each act of their life is so much expended of the time and work allotted to them; and if, from absence of those surrounding conditions under which alone life is possible, their vitality is stayed for a time, it again proceeds on the renewal of the necessary conditions, from that point which it had already attained. The amount of life to be manifested by any given individual is the same, whether it takes a day or a year for its expenditure. Life may be of course at any moment interrupted altogether by disease and death. But sup- posing it, in any individual organism, to run its natural course, it will attain but the same goal, whatever be its rate of move- 818 THE RELATION OF LIFE TO OTHER FORCES. [CHAP. XXIV. ment. Decline and death, therefore, are bnt the natural termina- tions of life ; they form part of the conditions on which vital action begins; they are the end towards which it naturally tends. Death, not by disease or injury, is not so much a violent interrup- tion of the course of life, as the attainment of a distant object which was in view from the commencement. In the period of decline, as during growth, life consists in continued manifestations of transformed physical force ; and there is of necessity the same series of changes by which the individual, though bit by bit perishing, yet by constant renewal retains its entity. The difference, as has been more than once said, is in the comparative extent of the loss and reproduction. In decline there is not perfect replacement of that which is lost. Repair becomes less and less perfect. It does not of necessity happen that there is any decrease of the quantity of material added in the place of that which disappears. But although the quantity may not be lessened, and may indeed absolutely increase, it is not perfect as material for repair, and although there may be no wasting, there is degeneration. No definite period can be assigned as existing between the end of development and the beginning of decline, and chiefly because the two processes go on side by side in different parts of the same organism. The transition as a whole is therefore too gradual for appreciation. But, after some time, all parts alike share in the tendency to degeneration ; until at length, being no longer able to subdue external force to vital shape, they die; and the elements of which they are composed simply employ what remnant of power, in the shape of chemical affinity, is still left in them, as a means whereby they may go back to the inorganic world. Of course the same process happens constantly during life; but in death the place of the departing elements is not taken by others. Here, then, a sharp boundary line is drawn where one kind of action stops and the other begins; where physical force ceases to be manifested except as physical force, and where no further vital transformation takes place, or can in the body ever do so. I or the notion of death must include the idea of impossibility of revival, as a distinction from that state of what is called " dor- mant vitality," in which, although there is no life, there is capabi- lity of living. Hence the explanation of the difference between the eflect of appliance of external force in the two cases. Take, chap, xxiv.] THE RELATION OF LIFE TO OTHER FORCES. 819 for examples, the fertile but not yet living egg, and the barren or dead one. Every application of force to the one must excite movement in the direction of development; the force, if used at all, is transformed by the germ into vital energy, or the power by which it can gather up and elaborate the materials for nutrition by which it is surrounded. Hence its freedom throughout the brooding time from putrefaction. In the other instance, the appli- ance of force excites only degeneration; if transformed at all, it is only into chemical force, whereby the progress of destruction is hastened ; hence it soon rots. To the one, heat is the signal for development, to the other for decay. By one it is taken up and manifested anew, and in a higher form ; to the other it gives the impetus for a still quicker fall. Life, then, does not stand alone. It is but a special manifesta- tion of transformed force. " But if this be so," it may be said- " if the resemblance of life to other forces be great, are not the differences still greater?" At the first glance, the distinctions between living organised tissue and inorganic matter seem so great that the difficulty is in finding a likeness. And there is no doubt that these wide differ- ences in both out ward configuration and intimate composition have been mainly the causes of the delay in the recognition of the claims of life to a place among other forces. And reasonably enough. For the notion that a plant or an animal can have any kind of relationship in the discharge of its functions to a galvanic battery or a steam engine is sufficiently startling to the most credulous. But so it has been proved to be. Among the distinctions between living and unorganised matter, that which includes differences in structure and proximate chemical composition has been always reckoned a great one. The very terms organic and inorganic were, until quite recently, almost synonymous w ith those which implied the influence of life and the want of it. The science of chemistry, however, is a great leveller of artificial distinctions, and many complex substances which, it was supposed, could not be formed without the agency of life can be now made directly from their elements or from very simple combinations of these. The number of complex substances so formed artificially is constantly increasing ; and there seems to be no reason for doubting that even such as albumen, gelatin, and the like, will be ultimately produced without the intermediation of living structure. 820 THE RELATION OF LIFE TO OTHER FORCES. [CHAP. XXIV. The formation of the latter, such an organised structure for instance as a cell or a muscular fibre, is a different thing alto- gether. There is at present no reason for believing that such will ever be formed by artificial means ; and, therefore, among the peculiarities of living force-transforming agents, must be reckoned as a great and essential one, a special intimate structure, apart from mere idtimate or proximate chemical composition, to which there is no close likeness in any artificial apparatus, even the most complicated. This is the real distinction, as regards com- position, between a living tissue and an inorganic machine; namely, the difference between the structural arrangement by which force is transformed and manifested anew. The fact that one agent for transforming force is made of albumen or the like, and another of zinc or iron, is a great distinction, but not so essential or fundamental an one as the difference in mechanical structure and arrangement. In proceeding to consider the difference between what may be called the transformation-products of living tissue, and of an arti- ficial machine, it will be well to take one of the simple cases first -the production of mechanical motion; and especially because it is so common in both. In one we can trace the transformation. We know, as a fact, that heat produces expansion (steam), and by constructing an apparatus which provides for the application of the expansive power in opposite directions alternately, or by alternating con- traction with expansion, we are able to produce motion so as to subserve an infinite variety of purposes. For the continuance of the motion there must be a constant supply of heat, and therefore of fuel. In the production of mechanical motion by the alternate con- tractions of muscular fibres we cannot trace the transformation of force at all. We know that the constant supply of force is as necessary in this instance as in the other ; and that the food which an animal absorbs is as necessary as the fuel in the former case, and is analogous with it in function. In what exact rela- tion,. however, the latent force in the food stands to the movement in the fibre, we are at present quite ignorant. That in some way or other, however, the transformation occurs, we may feel quite certain. There is another distinction between the two exhibitions of force which must be noticed. It has been universally believed, CHAP. XXIV.] THE RELATION OF LIFE TO OTHER FORCES. 821 almost up to the present time, that in the production of living force the result is obtained by an exactly corresponding waste of the tissue which produces it; that, for instance, the power of each contraction of a muscle is the exact equivalent of the force produced by the more or less complete descent of so much mus- cular substance to inorganic, or less complex organic shape; in other words,-that the immediate fuel which an animal requires for the production of force is derived from its own substance; and that the food taken must first be appropriated by, and enter into the very formation of living tissue before its latent force can be transformed and manifested as vital power. And here, it might be said, is a great distinction between a living structure and a simply mechanical arrangement such as that which has been used for comparison ; the fuel which is analogous to the food of a plant or animal does not, as in the case of the latter, first form part of the machine which transforms its latent energy into another variety of power. We are not, at present, in a position to deny that this is a real and great distinction between the two cases; but modern investigations in more than one direction lead to the belief that we must hesitate before allowing such a difference to be an universal or essential one. The experiments referred to seem conclusive in regard to the production of muscular power in greater amount than can be accounted for by the products of muscular waste excreted ; and it may be said with justice, that there is no intrinsic improbability in the supposed occurrence of transformation of force, apart from equivalent nutrition and sub- sequent destruction of the transforming agent. Argument from analogy, indeed, would be in favour of the more recent theory as the likelier of the two. Whatever may be the result of investigations concerning the relation of waste of living tissue to the production of power, there can be no doubt, of course, that the changes in any part which is the seat of vital action must be considerable, not only from what may be called " wear and tear," but, also, on account of the great instability of all organised structures. Between such waste as this, however, and that of an inorganic machine there is only the difference in degree, arising necessarily from diversity of structure, of elemental arrangement, and so forth. But the repair in the two cases is different. The capability of reconstruction in a living body is an inherent quality like that 822 THE RELATION OF LIFE TO OTHER FORCES. [CHAP. XXIV. which causes growth in a special shape or to a certain degree. At present we know nothing really of its nature, and we are therefore compelled to express the fact of its existence by such terms as " inherent power," " individual endowment," and the like, and wait for more facts which may ultimately explain it. This special quality is not indeed one of living things alone. The repair of a crystal in definite shape is equally an " indi- vidual endowment," or " inherent peculiarity," of the nature of which we are equally ignorant. In the case, however, of an inorganic machine there is nothing of the sort, not even as in a crystal. Faults of structure must be repaired by some means entirely from without. And as our notion of a living being, say a horse, would be entirely altered if flaws in his composition were repaired by external means only; so, in like manner, would our idea of the nature of a steam-engine be completely changed had it the power of absorbing and using part of its fuel as matter wherewith to repair any ordinary injury it might sustain. It is this ignorance of the nature of such an act as reconstruc- tion which causes it to be said, with apparent reason, that so long as the term " vital force " is used, so long do we beg the question at issue-What is the nature of life *? A little consideration, how- ever, will show that the justice of this criticism depends on the manner in which the word " vital " is used. If by it we intend to express an idea of something which arises in a totally different manner from other forces-something which, we know not how, depends on a special innate quality of living beings, and owns no dependence on ordinary physical force, but is simply stimulated by it, and has no correlation with it-then, indeed, it would be just to say that the whole matter is merely shelved if we retain the term "vital force." But if a distinct correlation be recognised between ordinary physical force and that which in various shapes is manifested by living beings ; if it be granted that every act-say, for example, of a brain or muscle-is the exactly correlated expression of a certain quantity of force latent in the food with which an animal is nourished; and that the force produced either in the shape of thought or movement is but the transformed expression of external foice, and can no more originate in a living organ without sup- plies of force from without, than can that organ itself be formed 01 nourished without supplies of matter •-if these facts be recog- nised, then the term used in speaking of the powers exercised by CHAP. XXIV.] THE RELATION OF LIFE TO OTHER FORCES. 823 a living being is not of very much consequence. We have as much right to use the term " vital " as the words galvanic and chemical. All alike are but the expressions of our ignorance concerning the nature of that power of which all that we call "forces" are various manifestations. The difference is in the apparatus by which the force is transformed. It is with this meaning that, for the present, the term " vital force " may still be retained when we wish shortly to name that combination of energies which we call life. For, exult as we may at the discovery of the transformation of physical force into vital action, we must acknowledge not only that, with the exception of some slight details, we are utterly ignorant of the process by which the transformation is effected; but, as well, that the result is in many ways altogether different from that of any other force with which we are acquainted. It is impossible to define in what respects, exactly, vital force differs from any other. For while some of its manifestations are identical with ordinary physical force, others have no parallel whatsoever. And it is this mixed nature which has hitherto baffled all attempts to define life, and, like a Will-o'-the-wisp, has led us floundering on through one definition after another only to escape our grasp and show our impotence to seize it. In examining, therefore, the distinctions between the products of transformations by a living and by an inorganic machine, we have first to recognise the fact, that while in some cases the difference is so faint as to be nearly or quite imperceptible, in others there seems not a trace of resemblance to be discovered. In discussing the nature of life's manifestations-birth, growth, development, and decline-the differences which exist between them and other processes more or less resembling them, but not dependent on life, have been already briefly considered and need not be here repeated. It may be well, however, to sum up very shortly the particulars in which life as a manifestation of force differs from all others. The mere acquirement of a certain shape by growth is not a peculiarity of life. But the power of developing into so composite a mass even as a vegetable cell is a property possessed by an organised being only. In the increase of inorganic matter there is no development. The minutest crystal of any given salt has exactly the same shape and intimate structure as the largest. With the growth there is no development. There is increase of 824 THE RELATION OF LIFE TO OTHER FORCES. [chap. XXIVo size with retention of the original shape, but nothing more. And if we consider the matter a little we shall see a reason for this. In all force-transformers, whether living or inorganic, with but few exceptions-and these are, probably, apparent only-some- thing more is required than homogeneity of structure. There seems to be a need for some mutual dependence of one part on another, some distinction of qualities, which cannot happen when all portions are exactly alike. And here lies the resemblance between a living being and an artificial machine. Both are developments, and depend for their power of transforming force on that mutual relation of the several parts of their structure which we call organisation. But here, also, lies a great difference. The development of a living being is due to an inherent tendency to assume a certain form ; about which tendency we know abso- lutely nothing. We recognise the fact, and that is all. The development of an inorganic machine-say an electrical apparatus -is not due to any inherent or individual property. It is the result of a power entirely from without; and we know exactly how to construct it. Here, then, again, we recognise the compound nature of a living being. In structure it is altogether different from a crystal-in inherent capacity of growth into definite shape it resembles it. Again, in the fact of its organisation it resembles a machine made by man: in capacity of growth it entirely differs from it. In regard, therefore, to structure, growth, and development, it has combined in itself qualities which in all other things are more or less completely separated. That modification of ordinary growth and development called generation, which consists in the natural production and separa- tion of a portion of organised structure, with power itself to trans- form force so as therewith to build up an organism like the being from which it was thrown off, is another distinctive peculiarity of a living being. We know of nothing like it in the inorganic world. And the distinction is the greater because it is the ful- filment of a purpose, towards which life is evidently, from its very beginning, constantly tending. It is as natural a destiny to separate parts which shall form independent beings as it is to develop a limb. Hence it is another instance of that carrying out of certain projects, from the very beginning in view, which is so characteristic of things living and of no other. It is especially in the discharge of what are called the animal CHAP. XXIV.] THE RELATION OF LIFE TO OTHER FORCES. 825 functions that we see vital force most strangely manifested. It is true that one of the actions included in this term-namely mecha- nical movement-although one of the most striking, is by no means a distinctive one. For it must be remembered that one of the commonest transformations of physical force with which we are acquainted is that of heat into mechanical motion, and that this may be effected by an apparatus having itself nothing whatever to do with life. The peculiarity of the manifestation in an animal or vegetable is that of the organ by which it is effected, and the manner in which the transformation takes place, not in the ulti- mate result. The mere fact of an animal's possessing capability of movement is not more wonderful than the possession of a similar property by a steam engine. In both cases alike, the motion is the correlative expression of force latent in the food and fuel respectively ; but in one case we can trace the transformation in the arrangement of parts, in the other we cannot. The consideration of the products of the transformation of force effected by the nervous system would lead far beyond the limits of the present chapter. But although the relation of mind to matter is so little known that it is impossible to speak with any freedom concerning such correlative expressions of physical force as thought and nerve-products, still it cannot be doubted that they are as much the results of transformation of force as the mechanical motion caused by the contraction of a muscle. But here the mystery reaches its climax. We neither know how the change is effected, nor the nature of the product, nor its analogies with other forces. It is therefore better, for the present, to con- fess our ignorance, than, with the knowledge which we have lately gained to build up rash theories, serving only to cause that confusion which is worse than error. It may be said, with perfect justice, that even if the foregoing conclusions be accepted, namely, that all manifestations of force by living beings are correlative expressions of ordinary physical force, still the argument is based on the assumption of the existence of the apparatus which we call living organised matter, with power not only to use external force for its own use in growth, development, and other vital manifestations, but for that modi- fication of these powers which consists in the separation of a part that shall grow up into the likeness of its parent, and thus con- tinue the race. We are therefore, it may be added, as far as ever from any explanation of the origin of life. This is of course quite 826 THE RELATION OF LIFE TO OTHER FORCES, [chap. xxiv. true. The object of the present chapter, however, is only to deal with the relations of life, as it now exists, to other forces. The manner of creation of the various kinds of organised matter, and the source of those qualities, belonging to it, which from our ignorance we call inherent, are different questions altogether. To say that of necessity the power to form living organised matter will never be vouchsafed to us, that it is only a mere materialist who would believe in such a possibility, seems almost as absurd as the statement that such inquiries lead of necessity to the denial of any higher power than that which in various forms is manifested as " force," on this small portion of the universe. It is almost as absurd, but not quite. For, surely, he who recog- nises the doctrine of the mutual convertibility of all forces, vital and physical, who believes in their unity and imperishableness, should be the last to doubt the existence of an all-powerful Being, of whose will they are but the various correlative expressions from whom they all come ; to whom they return. APPENDIX. The Chemical Basis of the Human Body. Of the sixty-seven known chemical elements no less than seventeen combine, in larger or smaller quantities, to form the chemical basis of the animal body. The substances which contribute the largest share are the non- metallic elements, Oxygen, Carbon, Hydrogen, and Nitrogen -oxygen and carbon making up altogether about 85 per cent, of the whole. The most abundant of the metallic elements are Calcium, Sodium, and Potassium. The following table represents the relative proportion of the various elements.-(Marshall.) Oxygen . . .72'0 Carbon . 13-5 Hydrogen 91 Nitrogen . 2'5 Calcium . .13 Phosphorus . . . . I'15 Sulphur . . T476 Sodium . 'I Chlorine .... "085 Ffuorine . . . . -c8 Potassium . . . '026 Iron . . . . . "01 Magnesium . . . 'coi2 Silicon . . ... '0002 (Traces of copper, lead, and aluminium) . I OCT Compounds.-Few of these elementary substances occur free or un- combined in the animal body. They are generally united in various numbers, and in variable proportions to form compounds. Traces of uncombined Oxygen and Nitrogen, however, have been found in the blood, and of Hydrogen as well as of Oxygen and Nitrogen in the intestinal canal. It was formerly thought that the more complex compounds built up by the animal or vegetable organism were peculiar, and could not be made artificially by chemists from their elements, and under this idea they were formed into a distinct class, termed organic. This idea has been given up, but the name is still in use, with a different significa- tion. The term is now applied simply to the compounds of the element Carbon, irrespective of their origin. 828 APPENDIX. Characteristics of Organic Compounds.-A large number of the animal organic compounds are characterized by their complexity. Many elements may enter into their composition, thereby distinguishing them from bodies as simple as water (H2 0), hydrochloric acid (HC1), and ammonia (N H3), which may be taken as types of inorganic compounds. Many atoms of the same element also may occur in each molecule. This latter fact no doubt explains also the reason of the instability of these compounds. Another great cause of the instability is the frequent presence of Nitrogen, which may be called negative or undecided in its affinities, and may be easily separated from combination with other elements. Animal tissues, containing as they do these organic nitrogenous compounds, are extremely prone to undergo chemical decomposition. They also contain a large quantity of water, a condition most favourable for the breaking up of such substances. It is due to this tendency to decomposition that we meet with so large a number of decomposition products among the chemical substances forming the basis of the animal body. The various substances found in the animal organism may be con- veniently considered according to the following classification : - i. Organic a. Nitrogenous. b. Non-Nitrogenous. 2. Inorganic. 1. Organic. (a.) Nitrogenous bodies take the chief part in forming the solid tissues of the body, and are found also to a considerable extent in the circulating fluids (blood, lymph, chyle), the secretions and excretions. They often contain in addition to Carbon, Hydrogen, Nitrogen, and Oxygen, the elements Sulphur and Phosphorus ; but although the composition of most of them is approximately known, no general rational formula can at present be given. Several classes of organic nitrogenous bodies may be distinguished, and it is convenient to consider them under the following heads:- (l.) Proteids or albuminoids. (2.) Gelatinous substances. (3.) Decomposition nitrogenous bodies. (4.) Certain nitrogenous bodies, the exact composition of which has not been made out. (1.) Proteids or Albuminoids are the most important of the nitrogenous animal compounds, one or more of them entering as essential parts into the formation of all living tissue. In the lymph, chyle, and blood, they also exist abundantly. Their atomic formula APPENDIX. 829 is uncertain. Their composition, according to Hoppe-Seyler, may be taken to be :- Carbon, from 51'5 to 54'5 ; Hydrog-en, from 6'9 to 73 ; Nitrog-en, from 15'2 to I7' ; Oxygen, from 20'9 to 233 ; Sulphur, from '3 to 2'. Physical Properties.-Proteids are all amorphous and non-crystallisable, so that they possess as a rule no power (or scarcely any) of passing through animal membranes. They are soluble, but undergo alteration in composition in strong acids and alkalies ; some are soluble in water, others in neutral saline solutions, some in dilute acids and alkalies, few in alcohol or ether. Their solutions exercise a left-handed action on polarised light. Chemical Properties.-Certain general reactions are given for proteids. They are a little varied in each particular case :- i. Xantho-Proteic Reaction.-A solution boiled with strong nitric acid, becomes yellow, and the colour is darkened on addition of ammonia. ii. Biuret Reaction.-With a trace of copper sulphate and an excess of potassium or sodium hydrate they give a purple colouration. iii. Millon's Reaction.-With Millon's reagent (a solution of metallic mercury in strong nitric acid), they give a white or pinkish clotted precipitate, becoming more pink on boiling. iv. They are, with the exception of peptone, entirely precipitated from their solutions by saturation with ammonium sulphate. Many of the proteids give, in addition, the following tests : v. With excess of acetic acid, and potassium ferrocyanide, a white precipitate. vi. With excess of acetic acid and a saturated solution of sodium sulphate, on boiling, a white precipitate. This test is often used to get rid of all traces of proteids, except peptones, from solutions. vii. Boiled with strong hydrochloric acid, they give a violet red coloura- tion. viii. With cane sugar and strong sulphuric acid, on heating, they give a purplish colouration. ix. They are precipitated on addition of- Citric or acetic acid, and picric acid ; or, Citric or acetic acid, and sodium tungstate ; or, Citric or acetic acid, and potassio-mercuric iodide. Varieties.-Proteids are divided into seven classes, chiefly on the basis of their solubilities in various reagents. Each class, however, if it contains more than one substance, may often be distinguished by other properties common to its members. (1.) Native-Albumins.-These substances are soluble in water and in saline solutions, and are coagulated, i.e., turned into coagulated proteid, on heating. (2.) Derived-Albumins.-These are soluble in acids or alkalies, but insoluble in saline solutions and in water, and are not coagulated on heating. (3.) Globulins.-These are soluble in strong or in weak saline solu- tions, in dilute acids and alkalies, and insoluble in water. They are coagulated on heating. 830 APPENDIX. (4.) Fibrin.-It is insoluble in water, in dilute saline solutions, or in dilute acids or alkalies ; soluble in strong saline solutions (partly) and in strong acids ; soluble to a certain extent in strong saline solu- tions and in gastric or pancreatic fluids. (5.) Peptones.-These are soluble in water, saline solutions, acids, or alkalies ; they are not coagulated on heating. (6.) Coagulated Proteids.-These are soluble only in gastric or pan- creatic fluids, forming peptones. (7.) Amyloid substance, or Lardacein.-This body is generally in- soluble, even in gastric or pancreatic fluids at ordinary temperatures. It gives a brown colouration with iodine. Class I.-Native-Albumins. (a) Egg-Albumin, is contained in the white of the egg. Properties.-When in solution in water it is a transparent, frothy, yellowish fluid, neutral or slightly alkaline in reaction. It gives all of the general proteid reactions. At a temperature not exceeding 40° C. it is dried up into a yellowish, transparent, glassy mass, soluble in water. At a temperature of 70° C. it is coagulated, i.e., changed into a new substance, coagulated proteid, which is quite insoluble in water. It is coagulated also by the prolonged action of alcohol ; by strong mineral acids, especially by nitric acid, also by tannic acid, or carbolic acid ; by ether the coagulum is soluble in caustic soda. It is precipitated without coagulation, i.e., forms an insoluble com- pound with the reagent, soluble on removal of the salt by dialysis, with either mercuric chloride, lead acetate, copper sulphate or silver nitrate, the precipitate being soluble in slight excess of the reagent. With strong nitric acid the albumin is precipitated at the point of contact with the acid in the form of a fine white or yellow ring. (b) Serum-Albumin is contained in blood serum, lymph, serous and synovial fluids, and the tissues generally; it appears in the urine in the condition known as albuminuria. Two varieties, metalbumin and paraZ&um-in, have been described as existing in dropsical fluids and ovarian cysts respectively. It gives similar reactions to egg-albumin, but differs from it in not being coagulated by ether. It also differs from egg-albumin in not being easily precipitated by hydrochloric acid, and in the precipi- tate being easily soluble in excess of that acid. Serum-albumin, either in the coagulated or precipitated form, is more soluble in excess of strong acid than egg-albumin. The compound nature of what is usually called serum-albumin, or serine, and its differentiation into three substances, coagulable at different temperatures, viz., a. at 73° C., /3. at 77° C., and y. at 85° C., as demonstrated by Halliburton, as well as its other properties, are mentioned at p. 89. APPENDIX. 831 Class II.-Derived-Albumins. (a) Acid-Albumin.-Acid-albumin is made by adding small quantities of dilute acid (of which, the best is hydrochloric, '4 per cent, to 1 per cent.), to either egg- or serum-albumin diluted with five to ten times its bulk of water, and keeping the solution at a temperature not higher than 50° C. for not less than half an hour. It may also be made by dissolving coagulated native-albumin in strong acid, or by dissolving any of the globulins in acids. It is not coagulated on heating, but on exactly neutralising the solution, a flocculent precipitate is produced. This may be shown by adding to the acid-albumin solution a little aqueous solution of litmus, and then adding, drop by drop, a weak solution of caustic potash from a burette, until the red colour disappears. The precipi- tate is the derived-albumin. It is soluble in dilute acid, dilute alkalies and dilute solutions of alkaline carbonates. The solution of acid-albumin gives the proteid tests. The substance itself is coagulated by strong acids, e.</., nitric acid, and by strong alcohol ; it is insoluble in distilled water, and in neutral saline solu- tions ; it is precipitated from its solutions by saturation with sodium chloride. On boiling in lime-water it is partially coagulated, and a further precipitation takes place on addition to the boiled solution of calcium chloride, magnesium sulphate, or sodium chloride. (b) Alkali-Albumin.-If solutions of native-albumin, or coagu- lated or other proteid, be treated with dilute or strong fixed alkali, alkali-albumin is produced. Solid alkali-albumin may also be pre- pared by adding caustic soda or potash, drop by drop, to undiluted egg-albumin, until the whole forms a jelly. This jelly is soluble in dilute alkalies on boiling. A solution of alkali-albumin gives the tests corresponding to those of acid-albumin. It is not coagulated on heating. It is thrown down on neutralising its solution, except in the presence of alkaline phosphates, in which case the solution must be distinctly acid before a precipitate falls. To differentiate between Acid- and Alkali-Albumin, the following method may be adopted. Alkali-albumin is not precipitated on exact neutralisation, if sodium phosphate has been previously added. Acid- albumin is precipitated on exact neutralisation, whether or not sodium phosphate has been previously added. (c) Casein.-Casein is the chief proteid of milk, from which it may be prepared by the following process: The milk should be diluted with three to four times its volume of water, sufficient dilute acetic acid should then be added to render the solution distinctly acid, and the casein which is thrown down may be separated by filtration. It may then be washed with alcohol and afterwards with ether, to free it from fat. Casein may also be prepared by adding to milk an excess of crystal- 832 APPENDIX. lized magnesium sulphate or sodium chloride, either of which salt causes it to separate out. Casein gives much the same tests as alkali-albumin. It is soluble in dilute acid or alkalies ; it is reprecipitated on neutralisation, but if potassium phosphate be present the solution must be distinctly acid before the casein is deposited. Class III.-Globulins. General Properties of Globulins.-They give the general proteid tests; are insoluble in water ; are soluble in dilute saline solutions ; are soluble in acids and alkalies forming the corresponding derived-albumin. Most of them are precipitated from their solutions by saturation with solid sodium chloride, magnesium sulphate, and other neutral salts. They are coagulated, but at different temperatures, on heating. (a) Globulin or Crystallin.-It is obtained from the crystalline lens by rubbing it up with powdered glass, extracting with water or with dilute saline solution, and by passing through the extract a stream of carbon dioxide. It differs from other globulins, except vitellin, in not being precipi- tated by saturation with sodium chloride. (b) Myosin.-Myosin may be prepared, as was before described, from dead muscle by removing all fat, tendon, etc., and washing repeatedly in water, until the washing contains no trace of proteids, mincing it and then treating with io per cent, solution of sodium chloride, or similar solution of ammonium chloride, magnesium sulphate, which will dissolve a large portion into a viscid fluid, which filters with difficulty. If the viscid filtrate be dropped little by little into a large quantity of distilled water, a white flocculent precipitate of myosin will occur. It is soluble in io per cent, saline solution; it is coagulated at 6o° C. into coagulated proteid ; it is soluble without change in very dilute acids ; it is precipitated by picric acid, the precipitate being re- dissolved on boiling ; it may give a blue colour with ozonic ether and tincture of guaiacuni. The formation of a clot of myosin on dilution of the strong saline solution in which it is contained, has been already commented upon. (c) Paraglobulin.-Paraglobulin is contained in serum and in serous and synovial fluids, and may be precipitated by saturating serum with solid sodium chloride or magnesium sulphate, as a bulky floccu- lent substance, which can be removed by filtration after standing for some time. It may also be prepared by diluting blood serum with ten volumes of water, and passing carbonic acid gas rapidly through it. The fine precipitate may be collected on filter, and washed with water contain- ing carbonic acid gas. APPENDIX. 833 It is very soluble in dilute saline solutions, from which it is pre- cipitated by carbonic acid gas or by dilute acids ; its solution is coagu- lated at 70° C. ; even dilute acids and alkalies convert it into acid- or alkali-albumin. (d) Fibrinogen.-Fibrinogen is prepared from hydrocele fluid or other serous transudation by methods similar to those employed in pre- paring paraglobulin from serum. Its general reactions are similar to those of paraglobulin ; its solu- tion is coagulated at 52°_55° C. Its characteristic property is that, under certain conditions, it forms fibrin. (e) Vitellin.-Vitellin is prepared from yolk of egg by washing with ether until all the yellow matter has been removed. The residue is dissolved in 10 per cent, saline solution, filtered, and poured into a large quantity of distilled water. The precipitate which falls is impure vitellin. It gives the same tests as myosin, but is not precipitated on satura- tion with sodium chloride ; it coagulates between 70° and 83° C. (f) G-lobin.-Is the proteid residue of haemoglobin. Class IV. Fibrin.-Fibrin can be obtained as a soft, white, fibrous, and very- elastic substance by whipping blood with a bundle of twigs, and wash- ing the adhering mass in a stream of water until all the blood-colouring matter is removed. Tests.-It differs from all other proteids, in having a filamentous structure. It is insoluble in water and in dilute saline solutions; slightly soluble in concentrated saline solutions, soluble on boiling in strong acids and alkalies. On boiling it is converted into coagulated proteid. When dissolved in strong saline solution it gives many of the same reactions as myosin. When dissolved in acids or alkalies, it is converted into the corresponding derived-albumin. It gives a blue colour with tincture of guaiacum and ozonic ether. Class V. Peptone.-Peptone is formed by the action of the digestive fer- ments, pepsin, or trypsin, on other proteids, and on gelatine. The properties and tests for peptone are at present very unsatis- factory, owing to the fact that the substance can be obtained in a pure condition with extreme difficulty. Many of the following tests therefore which are usually given for the substance are very likely due to impurities, intermediate digestion products, or albumose. A solution of commercial peptone in water gives the following tests : It is not coagulated on heating ; it is not precipitated by saturation with NaCl, or MgSO4, or by C03. It is not precipitated by boiling 834 APPENDIX. with sodium sulphate and acetic acid. It is not precipitated by addi- tion of dilute acid or alkali. It is precipitated from neutral or slightly acid solutions by; Mercuric chloride, the precipitate being only partly soluble in excess ; argentic nitrate ; lead acetate ; potassio-mercuric iodide; bile salts ; phosphoro-molybdic acid ; tannin, the precipitate being soluble in dilute acid, but not in excess of the reagent. Picric acid (saturated solution), the precipitate disappears on heating and partly returns on cooling. It is precipitated but not coagulated by absolute alcohol, and by ether. The solution of impure peptone gives The Xanthoproteic reaction easily, but there is very slight, if any previous precipitation with the nitric acid. The Biuret reaction-but the colour is pink instead of violet. With Millon's test-not so easily as do native albumins. With Ferrocyanide and acetic acid-only in cases where the peptone is very impure, is there any precipitate. The only substance which appears to separate the whole of the other proteids from peptone is ammonium sulphate. It dialyses freely. Class VI. Coagulated proteids are formed by the action of heat upon other proteids; the temperature necessary in each case varying in the manner previously indicated. They may also be produced by the prolonged action of alcohol upon proteids. They are soluble in strong acids or alkalies ; slightly so in dilute ; are soluble in digestive fluids (gastric and pancreatic). Are insoluble in saline solutions. Class VII. Lardacein is found in organs which are the seat of amyloid de- generation. It is insoluble in dilute acids and in gastric juice at the temperature of the body.-It is coloured brown by iodine and bluish-purple by methyl violet. (2.) The Gelatins or Nitrogenous Bodies other than Proteids. (a) Gelatin.-Gelatin is contained in bone, teeth, fibrous connec- tive-tissues, tendons, ligaments, etc. It may be obtained by prolonged action of boiling water in a Papin's digester or of dilute acetic acid at a low temperature (150 C.). Properties.-The percentage composition is O, 23'21, H, 7'15 APPENDIX, 835 N, 18'32, C, 5076, S, 0'56. It contains more nitrogen and less carbon than proteids. It is amorphous, and transparent when dried. It does not dialyse; it is insoluble in cold water, but swells up to about six times its volume : it dissolves readily on the addition of very dilute acids or alkalies. It is soluble in hot water, and forms a jelly on cooling, even when only 1 per cent, of gelatin is present. Pro- longed boiling in dilute acids, or in water alone, destroys this power of forming a jelly on cooling. A fairly strong solution of gelatin-2 per cent, to 4 per cent.- gives the following reactions ; (a) With proteid tests: (i.) Xanthoproteic test.-A yellow colour with no previous precipitate with nitric acid, becoming darker on the addition of ammonia, (ii.) Biuret test.-A violet colour, (iii.) Millon's test.-A pink precipitate, (iv.) Potassium ferro- cyanide and acetic acid.-No reaction. (v.) Boiling with sodium sulphate and acetic acid.-No reaction. (b) Special reactions: (i.) No precipitate with acetic acid, (ii.) No precipitate with hydrochloric acid, (iii.) A white precipitate with tannic acid, not soluble in excess or in dilute acetic acid, (iv.) A white precipitate with mercuric chloride, unaltered by excess of the reagent, (v.) A white precipitate with alcohol, (vi.) A yellowish-white precipitate with picric acid, dissolved on heating and reappearing on cooling. Bone consists of an organised matrix of connective-tissue which yields gelatin and inorganic salts. Inorganic salts can be removed by digesting it in hydrochloric acid. The gelatinous matter left retains the form of the bone. By long boiling in water it is converted into a solution of a gelatin. When bone is heated, the first action is to decompose the organic matter, leaving a deposit of carbon. On further ignition in air this carbon burns away, and only inorganic salts (principally calcic phosphate) are left. (b) Mucin.-Mucin is the characteristic component of mucus; it is contained in foetal connective-tissue, tendons, and salivary glands. It may be prepared from ox-gall, by acidulation with acetic acid and subsequent filtration, or from ox-gall by precipitation with alcohol, afterwards dissolving in water, and again precipitating by means of acetic acid. It can also be obtained from mucus by diluting it with water, filtering, treating the insoluble portion with weak caustic alkali, and precipitating the mucus with acetic acid. Properties.-Mucin has a ropy consistency. It is precipitated by alcohol and by mineral acids, but dissolved by excess of the latter. It is dissolved by alkalies and in lime water. It gives the proteid re- action with Millon's reagent and nitric acid, but not with copper sulphate. Neither mercuric chloride nor tannic acid gives a precipitate with it (?). It does not dialyse. 836 •APPENDIX. (c) Elastin is found in elastic tissue, in the ligamenta subflava, ligamentum nuchse, etc. Take the fresh ligamentum nuchse of an ox, cut in pieces, and boil in alcohol and ether to remove the fat. Remove the gelatin by boiling for some hours in water. Boil the residue with acetic acid for some time, and remove the acid by boiling in water, then boil with caustic soda until it begins to swell. Remove the alkali, and leave it in cold hydrochloric acid for twenty-four hours, and afterwards wash with water. Properties.-It is insoluble, but swells up both in cold and hot water. Is soluble in strong caustic soda. It is precipitated by tannic acid; does not gelatinize. Gives the proteid reactions with strong nitric acid and ammonia, and imperfectly with Millon's reagent.. Yields leucin on boiling with strong sulphuric acid. (d) Chondrin is found in cartilage. It is prepared by boiling small pieces of cartilage for several hours,, and filtering. The opalescent filtrate will form a jelly on cooling. Chondrin is precipitated from the warm filtrate on addition of acetic acid.. Properties.-It is soluble in hot water, and in solutions of neutral salts, e.g., sulphate of sodium, in dilute mineral acids, caustic potash,, and soda. Insoluble in cold water, alcohol, and ether. It is precipi- tated from its solutions by dilute mineral acids (excess re-dissolves it), by alum, by lead acetate, by silver nitrate, and by chlorine water. On boiling with strong hydrochloric acid, it yields grape-sugar and certain nitrogenous substances. Prolonged boiling in dilute acids, or in water,, destroys its power of forming a jelly on cooling. (e) Keratin is obtained from hair, nails, and dried skin. It contains sulphur evidently only loosely combined. (3.) Decomposition Nitrogenous products.-These are formed by the chemical actions which go on in digestion, secretion, and nutrition. Amido-Acids. c2h.nos Glycin, Glycocol, Glyco- cin, or Amido-acetic acid This substance occurs in the body in combination as in the biliary acids, but is never free. Glycocholic acid, when treated with weak acids, with alkalies, or with baryta water, splits up into cholic acid and. glycin, or aniido-acetic acid. Thus: C26H43NO(; + H2O = C26 O5. + C2 H5 NO2. Glycocholic acid + water = cholic acid + glycin, and under similar circumstances Taurocolic acid splits up into cholic acid and taurin :-C26 H45 O3 NSO2 + H2O = C2ti H40 O5 + C2 H7 NSO3, or amido-isethionic. Taurocholic acid + water = cholic acid and taurin. Glycin occurs also in hippuric acid. It can be prepared from gelatin by the action of acids or alkalies, it can also be obtained from hippuric acid. APPENDIX. 837 j-C3H7NO2( = CH/^CH3-y I(. .g a constituent of kreatin, and also of caffeine, but has never been found free in the human body. It may be obtained from these bodies by boiling with baryta water. Sarcosin or Methyl Glycin, j- c6 h13 no2 (= ch3. ch2 ch2 ch2. ch (NH2) CO OH occurs normally in many of the organs of the body and is a product of the pancreatic digestion of proteids. It is pre- sent in the urine in certain diseases of the liver in which there is loss of substance, especially in acute yellow atrophy. It occurs in circular oily discs or crystallises in plates, and can be prepared either by boiling horn shavings, or any of the gelatins with sulphuric acid, or out of the products of pancreatic digestion. • Leucin or Amido- caproic Acid, C2 H7 NSO3 (=C2 H4 is a consti- tuent of the bile acid, taurocholic acid, and is found also in traces in the muscles and lungs. It has been prepared synthetically from isethionic acid. It is a crystalline substance, very stable. Taurin, or Amido- isethionic Acid, Amido-Sulphonic Acids. Benzoyl Amido-Acids. C9 H9 NO3=(C6 H5 CONH CHa COOH), a normal constituent of human urine, the quantity excreted being in- creased by a vegetable diet, and therefore it is present in greater amount in the urine of herbivora. It may be decomposed by acids into glycin and benzoic acid. It crystallises in semi-transparent rhombic prisms, almost insoluble in cold water, soluble in boiling water. (See also p. 420). Tyrosin, C9 Hn NO3, is found generally together with leucin, in certain glands, e.g. pancreas and spleen; and chiefly in the products of pancreatic digestion or of the putrefaction of proteids. It is found in the urine in some diseases of the liver, especially acute yellow atrophy. It crystallises in fine needles, which collect into feathery masses. It gives the proteid test with Millon's reagent, and heated with strong sulphuric acid, on the addition of ferric chloride gives a violet colour. Lecithin, C42 Hg4 P N09, is a complex nitrogenous fatty body, containing phos- phorus, which has been found mixed with cerebrin and oleophosphoric acid in the brain. It is also found in blood, bile and serous fluids, and in larger quantities in nerves, pus, yelk of egg, semen, and white blood-corpuscles. On boiling with acids it yields cholin, glycero-phosphoric acid, palmic and oleic acids, Cerebrin, C17 II33 N03, is found in nerves, pus corpuscles, and in the brain. Its chemical constitution is not known. It is a light amorphous powder, tasteless and odourless. Swells up like starch when boiled with water, and is converted by acids into a saccharine substance and other bodies. The so-called Protagon is a mixture of lecithin and cerebrin. Hippuric Acid, or Benzolglycin, 838 APPENDIX. Urea and its Allies. Urea or Carbamide, C'0N2 H4, is the last product of the oxidation of the albuminous tissues of the body and of the albuminous foods. It occurs as the chief nitrogenous constituent of the urine of man, about 2 to 3 per cent., and of some other animals. It has been found in the blood and serous fluids, in lymph, and in the liver. Properties.-Crystallises in thin glittering needles, or in prisms with pyramidal ends. Easily soluble in water and alcohol, insoluble in ether. It may be produced artificially by treating carbonyl chloride (CO Cl2) with ammonia; or by heating ethyl carbonate with ammonia /m1 TT CO <^q(<2 jj5 + 2NH3 = CO N2 H4 2C0 H6 0 ; by heating carbonate of /NH ammonium «C0 =C0 N2 H4 -|- H2 O; by adding water to cyan- amide CN. NH2, or by evaporating ammonium cyanate in aqueous solution. When heated with water in a sealed tube to too0 C. or on allowing urine to stand, urea splits up into carbonic acid and ammonia ; when heated to a high temperature urea loses ammonia and first yields Inuret C2 Hg N3 02, and after cyanuric acid, C3 H3 03 N3. It is de- composed by sodium hypochlorite or hypobromite, or by nitrous acid with evolution of N. It forms compounds with acids, of wdiich the chief are urea hydrochloride CH4 N20. HCL ; urea nitrate, CH4 N„ 0 HNO3; and urea phosphate CH4 N2 O. H3PO4. It forms compounds with metals such as HgO. CH4 N20 ; with silver CH2 N20 Ag2; and with salts such as Ilg Cl2. Urea is isomeric with ammonium cyanate C from which it was first artificially prepared. Kreatin, C4 H9 N3 02 is one of the primary products of muscular disintegration. It is always found in the juice of muscle. It is gene- rally decomposed in the blood into urea and kreatinin, and seldom, unless under abnormal circumstances, appears as such in the urine. Treated with either sulphuric or hydrochloric acid, it is converted into kreatinin ; thus- C4 H9 N3 02 = C4 H7 N3 O + H2 0. It lias been made synthetically by bringing together cyanimide and sarcosine. Kreatinin, C4 H7 N3 0, is present in human urine, derived from oxidation of kreatin. It does not appear to be present in muscle. Uric Acid, C5 H4 N4 03, occurs in the urine, sparingly in human urine, abundantly in that of birds and reptiles, where it represents the chief nitrogenous decomposition product. It occurs also in the blood, spleen, liver, and sometimes is the only constituent of urinary calculi. It is probably converted in the blood into urea and carbonic acid. It generally occurs in urine in combination with bases, forming ■urates, and never free unless under abnormal conditions. A deposit of urates APPENDIX. 839 may occur when the urine is concentrated or extremely acid, or when, as during febrile disorders, the conversion of uric acid into urea is incompletely performed. Properties.-Crystallises in many forms, of which the most common are smooth, transparent, rhomboid plates, diamond-shaped plates, hexagonal tables, &c. Very insoluble in water, and absolutely so in alcohol and ether. Dried with strong nitric acid in a water bath, a compound is formed called alloxan, which gives a beautiful violet red with ammonium hydrate (murexide'), and a blue colour with potassium hydrate. It is easily precipitated from its solutions by the addition of a free acid. It forms both acid and neutral salts with bases. The most soluble urate is lithium urate. Composition.-Very uncertain ; has been however recently produced artificially, but it is not easily decomposed; it may be regarded as di- ureide of tartronic acid. The chief product of its decomposition is urea. Guanin, C5 H, N5 0, has been found in the human liver, spleen, and faeces, but does not occur as a constant product. Xanthin, C8 H4 N4 02, has been obtained from the liver, spleen, thymus, muscle, and the blood. It is found in normal urine, and is a constituent of certain rare urinary calculi. Hypoxanthin, C5 H4 N4 O, or sarkin, is found in juice of flesh, in the spleen, thymus, and thyroid. Allantoin, C4 H6 N4 03, found in the allantoic fluid of the foetus, and in the urine of animals for a short period after their birth. It is i >ne of the oxidation products of uric acid, which on oxidation gives urea. In addition to the above compounds and probably related to them, are certain colouring and excrementitious matters, which are also most likely distinct decomposition compounds. Pigments, &c. Bilirubin, C9 H9 N02, is the best known of the bile pigments. It is best made by extracting inspissated bile or gall stones with water (which dissolves the salts, &c.), then with alcohol, which takes out cholesterin, fatty, and biliary acids. Hydrochloric acid is then added, which decomposes the lime salt of bilirubin and removes the lime. After extracting with alcohol and ether, the residue is dried and finally extracted with chloroform. It crystallises of a bluish-red colour. It is allied in composition to haematin. Biliverdin, C8 H9 NO2, is made by passing a current of air through an alkaline solution of bilirubin, and by precipitation with hydrochloric acid. It is a green pigment. Bilifuscin, C9 Hu NO3, is made by treating gall stones with ether, then with dilute acid, and extracting with absolute alcohol. It is a non-crystallizable brown pigment. Biliprasin is a pigment of a green colour, which can be obtained from gall stones. 840 APPENDIX. Bilihumin (Staedeler) is a dark brown earthy-looking substance, of which the formula is unknown. Urochrome (see p. 420). Urobilin occurs in bile and in urine, and is probably identical with stercobilin, which is found in the faeces. Uroerythrin is one of the colouring matters of the urine. It is orange red and contains iron. Melanin is a dark brown or black material containing iron, occurring in the lungs, bronchial glands, the skin, hair, and choroid. Choletelin (p. 421). Haematin has been fully treated of, p. 98, et seq. Indican is supposed to exist in the sweat and urine. It has not however been satisfactorily isolated. Indigo, C8 Hg N9 0, is formed from indican, and gives rise to the bluish colour which is occasionally met with in the sweat and urine (also 421). Indol, C8 H2 N, is found in the feces, and is formed either by decomposition of indigo, or of the proteid food materials. It gives the characteristic disagreeable smell to feces (see p. 312). (4.) Nitrogenous Bodies of Uncertain Nature. Ferments are bodies which possess the property of exciting chemical changes in matter with which they come in contact. They are at present divided into two classes, called (1) organised, and (2) un- organised or soluble. (1.) Of the organised, yeast may be taken as an example. Its activity depends upon the vitality of the yeast cell, and disappears as soon as the cell dies, neither can any substance be obtained from the yeast by means of precipitation with alcohol or in any other way which has the power of exciting the ordinary change produced by yeast. The action of micro-organisms in the alimentary canal and elsewhere is also an example of the same nature. (2.) Unorganised or soluble ferments are those which are found in secretions of glands, or are produced by chemical changes in animal or vegetable cells in general; when isolated they are colourless, tasteless, amorphous solids soluble in water and glycerin, and precipitated from the aqueous solutions by alcohol and acetate of lead. Chemically many of these are said to contain nitrogen. Mode of action.-Without going into the theories of how these un- organised ferments act, it will suffice to mention that: (1.) Their activity does not depend upon the actual amount of the ferment present. (2.) That the activity is destroyed by high tempera- ture, and various concentrated chemical reagents, but increased by moderate heat, about 40° C., and by weak solutions of either an acid or alkaline fluid. (3.) The ferments themselves appear to undergo no change in their own composition, and waste very slightly during the process. APPENDIX. 841 Varieties.-The chief classes of unorganised ferments are :- (i.) Amylolytic, which possess the property of converting starch into glucose. They add a molecule of water, and may be called hydro- lytic. The probable reaction is given, p. 267. The principal amylolytic ferments are Ptyalin, found in the saliva, and a ferment, probably distinct, in the pancreatic juice, called Amy- lopsin. These both act in an alkaline medium. Amylolytic ferments have been found in the blood and elsewhere. (2.) Proteolytic convert proteids into peptones. The nature of their action is probably hydrolytic. The proteolytic ferments of the body are called Pepsin, acting in an acid medium from the gastric juice. Trypsin, acting in an alkaline medium from the pancreatic juice. The Succus entericus is said to contain a third such ferment. (3.) Inversive, which convert cane sugar or saccharose into grape sugar or glucose. Such a ferment was found by Claude Bernard in the Succus entericus ; and probably exists also in the stomach mucus. (4.) Ferments which act upon fats.-Such a body, called Steapsin, has been found in pancreatic juice. (5.) Milk-curdling ferments.-It has been long known that rennet, a decoction of the fourth stomach of a calf, in brine, possessed the power of curdling milk. This power does not depend upon the acidity of the gastric juice, since the curdling will take place in a neutral or alkaline medium ; neither does it depend upon the pepsin, as pure pepsin scarcely curdles milk at all, and the rennet which rapidly curdles milk has a very feeble proteolytic action. From this and other evidence it is believed that a distinct milk-curdling ferment exists in the stomach. W. Roberts lias shown that a similar but distinct ferment exists in pancreatic extract, which acts best in an alkaline medium, next best in an acid medium, and worst in a neutral medium. The ferment of rennet acts best in an acid medium, and worst in an alkaline, the reaction ceasing if the alkalinity be more than slight. In addition to the above ferments, many others most likely exist in the body, of which the following are the most important: (6.) Fibrin-forming ferment (Schmidt), (see p. 73, et seq.), found in the blood, lymph and chyle. (7.) A ferment which converts glycogen into glucose in the liver ; being therefore an amylolytic ferment. (8.) Myosin ferment. (6.) Organic non-nitrogenous bodies consist of (1.) Oils and Fats. Most oils and fats are mixtures of palmitin C51 Hso O6, stearin Hn0 O6, and olein C57 H104 O6, in different proportions. They are formed by the union of fatty acid radicals with the triatomic alcohol, Glycerin C3 H5 (0H)3, and are etherial salts of that alcohol. The radicals 842 APPENDIX. are C18 H 35 0, C16 H31 0, and C']8 H33 0, respectively. Human fat con- sists of a mixture of palmitin, stearin, and olein, of which the two former contribute three-quarters of the whole. Olein is the only liquid con- stituent. General characteristics.-Insoluble in water and in cold alcohol ; soluble in hot alcohol, ether, and chloroform. Colourless and tasteless ; easily decomposed or saponified by alkalies or super-heated steam into glycerin and the fatty acids. Cholesterin, C.26 H44 O, is the only alcohol which has been found in the body in a free state. It has been called a non-saponifial>le fat. It occurs in small quantities in the blood and various tissues, and forms the principal constituent of gall-stones. It is found in dropsical fluids, especially in the contents of cysts, in disorganised eyes, and in plants (especially peas and beans). It is soluble in ether, chloroform, or benzol. It crystallises in white feathery needles. See also under the head of the constituents of the bile. Excretin (Marcet), and Stercorin (Flint), are crystalline fatty bodies which have been isolated from the faeces. (2.) Carbo-hydrates or Amyloids. Carbo-hydrates are bodies composed of six or twelve atoms of carbon with hydrogen and oxygen, the two latter elements being in the proportion to form water. Amyloses, C6 H]0 05, Starch, Dextrin, Glycogen, Znuh'n, Cellu- lose, Gum. Saccharoses, C12 On, Saccharose, or Cane sugar, Lactose, Mal- tose, Melitose. Glucoses, C6 H12 0(i, Dextrose or Grape sugar, Ltevulose or Fruit-sugar, Inosite, Afanm'tose. Of these the most important are : (a) Starch (C6 H10 05) which is contained in nearly plants, and in many seeds, roots, stems, and some fruits. Characters.-It is a soft white powder composed of granules having an organised structure, consisting of granulose (soluble in water) con- tained in a coat of cellulose (insoluble in water) ; the shape and size of the granules varying according to the source whence the starch has been obtained. Tests.--It is insoluble in cold water, in alcohol, and in ether; it is soluble after boiling for some time, and may be filtered, in consequence of the swelling up of the granulose, which bursts the cellulose coat, and becoming free, is entirely dissolved in water. This solution is a solution of soluble starch or amydin. It gives a blue coloration with iodine, which disappears on heating and returns on cooling. It is converted into dextrine and grape-sugar by diastase or by boiling with dilute acids. APPENDIX. 843 (b) Glycogen. Glycogen, usually obtained from the livers, is also present to a considerable extent in the muscles of very young animals. In order to prepare it in con- siderable amount, it is best to use the liver of a rabbit. The animal should be large, and should have been well fed on a diet of grain and sugar for some days, or even weeks, previously. It should have had a full meal of grain, carrots, and sugar, about two hours before it is killed, in order that it may be in full digestion. The rabbit is killed either by decapitation, or by a blow on the head, and the abdomen is then rapidly opened, and the liver is torn out, is chopped up as quickly as possible with the knife, and is thrown into boiling water. It is important that this operation should be performed within half a minute of the death of the animal, and that the water should not be allowed to fall below the boiling point. The liver is to remain in the hot water for five minutes ; it is then poured into a mortar, and reduced to a pulp, and is again boiled for ten minutes. The liquid is filtered, and the filtrate is rapidly cooled. The albuminous substances in the cold filtrate are precipitated by adding potassio- mercuric iodine and dilute hydrogen chloride alternately as long as any precipitate is produced. The mixture is then stirred, is allowed to stand for five minutes, and is filtered. Alcohol is added to this second filtrate until glycogen is pre- cipitated, which occurs after about 60 per cent, of absolute alcohol has been added. The precipitate is then filtered off, and is washed with weak spirit, strong spirit, absolute alcohol (two or three times), and finally with ether. It is then dried on a glass plate at a moderate heat, and, if pure, should remain as a white amorphous powder. If the water has not been completely removed, the glycogen will form a gummy mass ; in this case it must again be treated with absolute alcohol. Properties.-It is freely soluble in water, and its solution looks opalescent ; it gives a port-wine coloration with iodine, which dis- appears on heating and returns on cooling. It is insoluble in absolute alcohol and in ether. It exists in the liver during life, but very soon after death is changed into sugar. It is converted into sugar by diastase ferments, or by boiling with dilute acids. (c) Dextrine.-This substance is made in commerce by heating dry potato-starch to a temperature of 400°. It is also produced in the process of the conversion of starch into sugar by diastase, and by the salivary and pancreatic ferments. Properties.-A yellowish amorphous powder, soluble in water, but insoluble in absolute alcohol and in ether. It corresponds almost exactly in tests with, glycogen ; but one variety (achroo-dextrine) does not give the port-wine coloration with iodine. (d) Glucose occurs widely diffused in the vegetable kingdom, in diabetic urine, in the blood, etc.; it is usually obtained from grape- juice, honey, beetroot or carrots. It really is a mixture of two isomeric bodies, Dextrose or grape-sugar, which turns a ray of polarised light to the right, and Lcevulose or fruit-sugar, which turns the ray to the left. Properties.-It is easily soluble in water ; not so sweet as cane-sugar; the relation of its sweetness to that of cane-sugar is as 3 to 5. It is not so easily charred by strong sulphuric acid as cane-sugar. It is not entirely soluble in alcohol. Tests.-(i.) Trommels.-This test depends upon the power sugar 844 APPENDIX. possesses of reducing copper salts to their sub-oxide. It is done in the following way :-An excess of caustic potash and then a solution of ■copper sulphate, drop by drop, is added to the solution, containing the sugar in a test-tube, as long as the blue precipitate which forms re- dissolves on shaking the tube. The upper portion of the fluid is then heated, and a yellowish-brown precipitate of copper suboxide appears. (ii.) Moore's.- If a solution of sugar in a test-tube is boiled with caustic potash, a brown coloration appears. (iii.) Fermentation.-If a solution of sugar be kept in the warm plate for a time after the addition of yeast, the sugar is converted into alcohol and carbon dioxide. (C6H]2O6= 2C2H5OH-f-2CO,2.) (iv.) Bottcher's test.-A little bismuth oxide or subnitrate and an excess of caustic potash are added to the solution in a test-tube, and the mixture is heated ; the solution becomes at first grey and then black. (v.) Picric acid test.-To the solution about a fourth of its bulk of picric acid (saturated solution) and an equal quantity of caustic potash are added, and the solution is boiled ; the liquid becomes of a very deep coffee-brown. (e) Lactose is contained in milk (p. 386). Properties.-It is less soluble in water than glucose ; not sweet, and is gritty to the taste ; but it is insoluble in absolute alcohol. Under- goes alcoholic fermentation with extreme difficulty; gives the tests similar to glucose, but less readily. (f) Inosite. - Inosite is a non-fermentible variety of glucose occurring in the heart and voluntary muscles, as well as in beans and other plants. It crystallises in the form of large colourless monoclinic tables, which are soluble in water, but insoluble in alcohol or ether. Inosite may be detected by evaporating the solution containing it nearly to dryness, and by then adding a small drop of a solution of mercuric nitrate, and afterwards evaporating carefully to dryness, a yellowish-white residue is obtained ; on further cautiously heating, the yellow changes to a deep rose-colour, which disappears on cooling, but reappears on heating. If the inosite be almost pure, its solution may be evaporated nearly to dryness. After the addition of nitric acid, the residue mixed with a little ammonia and calcium chloride, and again evaporated, yields a rose-red coloration. (g) Maltose is formed in the conversion of starch into glucose by the saliva and pancreatic fluids. It is also formed by the action of malt upon starch by the ferment diastase, and in the formation of glucose from starch. It is converted into dextrine by dilute sulphuric acid. It is dextro-rotatory ; ferments with yeast; reduces copper salts, and crystallises in fine needles. Monatomic Fatty Acids. Formic CH2 O2, acetic C2 H4 O2, and propionic C3 Hs O2, acids art- present in sweat, but normally in no other human secretion. They APPENDIX. 845 have been found elsewhere in diseased conditions. Butyric acid, C4 H8 O.,, is found in sweat. Various others of these acids have been obtained from blood, muscular juice, faeces, and urine. Diatomic Fatty Acids. Lactic acid, C3 H6 O3, exists in a free state in muscle plasma, and is increased in quantity by muscular contraction, is never contained in healthy blood, and when present in abnormal amount seems to produce rheumatism. Oxalates are present in the urine in certain diseases, and after drink- ing certain carbonated "beverages, and after eating rhubarb, &c. Aromatic Series, Benzoic . . . . . . C, H6 02 PhenolCo Ha O Benzoic acid, C3 H6 O2, is always found in the urine of herbivora, and can be obtained from stale human urine. It does not exist free elsewhere. Phenol.-Phenyl alcohol or carbolic acid exists in minute quantity in human urine. It is an alcohol of the aromatic series. 2. Inorganic Principles. The inorganic proximate principles of the human body are numerous. They are derived, for the most part, directly from food and drink, and pass through the system unaltered. Some are, however,, decomposed on their way, as chloride of sodium, of which only four-fifths of the quantity ingested are excreted in the same form ; and some are newly formed within the body,-as for example, a part of the sulphates and carbonates, and some of the water. Much of the inorganic saline matter found in the body is a necessary constituent of its structure,-as necessary in its way as albumin or any other organic principle ; another part is important in regulating or modifying various physical processes, as absorption, solution, and the like ; while a part must be reckoned only as matter, which is, so to speak, accidentally present, whether derived from the food or the tissues, and which will, at the first opportunity, be excreted from the body. Gases.-The gaseous matters found in the body are Oxygen, Hy- drogen, Nitrogen, Carburetted and Sulphuretted hydrogen, and Carbonic acid. The first three have been referred to (p. 827). Carburetted and sulphuretted hydrogen are found in the intestinal canal. Carbonic acid is present in the blood and other fluids, and is excreted in large quantities by the lungs, and in very minute amount by the skin. It has been specially considered in the chapters on Respiration and elsewhere. 846 APPENDIX. Water, the most abundant of the proximate principles, forms a large proportion,-more than two-thirds of the weight of the whole body. Its relative amount in some of the principal solids and fluids of the body is shown in the following table (quoted by Dalton, from Robin and Verdeil's table, compiled from various authors) :- Teeth 100 Bones ...... 130 Cartilage . . . . .550 Muscles . . . . . 750 Ligament .... 768 Brain . . . . . . 789 Blood ..... 795 Synovia ..... 805 Quantity of Water in 1000 Parts. Bile . . . . 880 Milk 887 Pancreatic juice . . . 900 Urine. ..... 936 Lymph . . . . . 960 Gastric juice . . . . 975 Perspiration .... 986 Saliva...... 995 Uses of the Water of the Body. - The importance of water as a constituent of the animal hody may be assumed from the preceding table, and is shown in a still more striking manner by its withdrawal. If any tissue, as muscle, cartilage, or tendon, be subjected to heat sufficient to drive off the greater part of its water, all its characteristic physical properties are destroyed; and what was previously soft, elastic, and flexible, becomes hard and brittle, and horny, so as to be scarcely recognisable. In all the fluids of the body-blood, lymph, &c.,-water acts the part of a general solvent, and by its means alone circulation of nutrient matter is possible. It is the medium also in which all fluid and solid aliments are dissolved before absorption, as well as the means by which all, except gaseous, excretory products are removed. All the various processes of secretion, transudation, and nutrition, depend of necessity on its presence for their performance. Source.-The greater part, by far, of the water present in the body is taken into it as such from without, in the food and drink. A small amount, however, is the result of the chemical union of hydrogen with oxygen in the blood and tissues. The total amount taken into the body every day is about lbs. ; while an uncertain quantity (perhaps | to j lb.) is formed by chemical action within it.- (Dalton.) Loss.-The loss of water from the body is intimately connected with excretion from the lungs, skin, and kidneys, and, to a less extent, from the alimentary canal. The loss from these various organs may be thus apportioned (quoted by Dalton from various observers). From the Alimentary canal (faeces) .... 4 per cent. ,, Lungs . . . . . . . . 20 ,, ,, Skin (perspiration) . . . . . 30 ,, Kidneys (urine) . . . . . . . . 46 „ 100 APPENDIX. 847 Sodium, and Potassium Chlorides are present in nearly all parts of the body. The former seems to be especially necessary, judging from the instinctive craving for it on the part of animals in whose food it is deficient, and from the diseased condition which is consequent on its withdrawal. In the blood, the quantity of sodium chloride is greater than that of all its other saline ingredients taken together. In the muscles, on the other hand, the quantity of sodium chloride is less than that of the chloride of potassium. Calcium Fluoride, in minute amount, is present in the bones and teeth, and traces have been found in the blood and some other fluids. Calcium, Potassium, Sodium, and Magnesium Phosphates •are found in nearly every tissue and fluid. In some tissues-the bones and teeth-the phosphate of calcium exists in very large amount and is the principal source of that hardness of texture, on which the proper performance of their functions so much depends. The phosphate of calcium is intimately incorporated with the organic basis or matrix, but it can be removed by acids without destroying the general shape of tlie bone ; and, after the removal of its inorganic salts, a bone is left soft, tough,, and .flexible. Potassium and sodium phosphates with the carbonates, maintain the alkalinity of the blood. Calcium Carbonate occurs in bones and teeth, but in much smaller quantity than the phosphate. It is found also in some other parts. The small concretions of the internal ear (otoliths) are com- posed of crystalline calcium carbonate, and form the only example of inorganic crystalline matter existing as such in the body. Potassium and Sodium Carbonates are found in the blood, and some other fluids and tissues. Potassium, Sodium, and Calcium Sulphates are met with in small amount in most of the solids and fluids. Silicon.-A very minute quantity of silica exists in the urine, and in the blood. Traces of it have been found also in bones, hair, and some other parts. Iron.-The especial place of iron is in haemoglobin, the colouring- matter of the blood, of which a full account has been given with the chemistry of the'blood. Peroxide of iron is found, in very small quantities, in the ashes of bones, muscles, and many tissues, and in lymph and chyle, albumin of serum, fibrin, bile, milk, and other fluids ; and a salt of iron, probably a phosphate, exists in the hair, black pigment, and other deeply coloured epithelial or horny substances. Aluminium, Manganese, Copper, and Lead.-It seems most likely that in the human body, copper, manganesiupi, aluminium, and lead are merely accidental elements, which, being taken in minute quantities with the food, and not excreted at once with the faeces, are absorbed and deposited in some tissue or organ, of which, however, they form no necessary part. In the same manner, arsenic, being absorbed, may be deposited in the liver and other parts. APPENDIX B. MEASURES OF WEIGHT (Avoirdupois), (Averages.) lbs. ozs. Recent Skeleton . .21 8 Muscles and Tendons . . 77 8 Skin and Subcutaneous tissue . . . . 16 5 Blood . . .. ti to 14 - lbs. ozs. Liver 38 Lungs (both) . . . . 2 10 (Esophagus. . . . - if Ovaries (both) . | to - | Pancreas . . . . - 3 Salivary Glands (both sides), ijto - 2 Stomach . . . . - 7 Spinal Cord, divested of its nerves and membranes . - 11 Spleen - 7 Suprarenal Capsules (both), Mo - i Testicles (both) . to - 2 Thyroid body and remains of Thymus gland . . - | Tongue and Hyoid bone . - 3 Uterus (virgin) , . i to - f Cerebrum . . 212 Cerebellum . . - 5} Pons and Medulla oblongata . . - 1 Brain Encephalon . . 3 2J Eyes 4 Heart - 10 Intestines, small . . 1 u| „ large . . . 1 1 Kidneys (both) . . - ioJ Larynx, Trachea,and larger Bronchi . . . . - 2| MEASURES OF LENGTH (Average). ft. in. Appendix vermiformis 3 to - 6 Bronchus right . . . - i| left . . . . - Caecum . . . . - 2| Duct, common bile . . . - 3 ., „ ejaculatory, | to - 1 ,, of Cowper's gland . - i| „ hepatic , . . . - 2 ,, nasal . . . . - | „ parotid . . . . - „ sub-maxillary . . - 2 Epididymis . . . . - i| „ unravelled . 20 - Eustachian tube , . . - x| Fallopian tube . . • - 3i Intestine, large . . 5 to 6 - „ small . . . 20 - Ligament, round, of uterus . - ft. in. Ligament of ovary . . - i| Meatus auditorius externus . - Medulla oblongata . . - (Esophagus . . . . - io Pancreas . . . . - 7 Pharynx ~ 4i Rectum . . . . - 8 Spinal cord . . ..15 Tubulus seminiferus . .23 Urethra, male . . . . - 8 „ female . . . - xj Ureter 14 Vagina . . . 4 to - 6 Vas deferens . . . . 2 - Vesicula seminalis . . - 2 „ „ unravelled, 4 to - 6 Vocal cord , . . . - APPENDIX. 849 SIZES OF VARIOUS HISTOLOGICAL ELEMENTS AND TISSUES. Average size infractions of .an inch. Air-cells, Jg to JL. P>lood-cells (red), (breadth). „ „ lofco (thickness). „ (colourless), Canaliculus of bone, jAg (width). Capillary blood-vessels, gJgg (lung) to 1250 (bone). Cartilage-cells (nuclei of), ggig. Chyle-molecules, 30igg. cilia> sho to gig (length). Cones of retina (at yellow spot), jigg to igfeg (width). Connective-tissue fibrils, igg (width). Dentine-tubules, -Jgg (width). Enamel-fibres, (width). End-bulbs, gig. Epithelium columnar (intestine), (length), spheroidal (hepatic), TtJjg to i.. squamous (peritoneum)jgigg (width). „ (mouth), (i „ » (skin), 30o Elastic (yel.) fibres, gJgg to 4ig (wide). Fat-cells, i, to i,. ' (Terminal vescicle, yig. ., Spot, g J,g. Glands gastric i to i (length). » 303 to jL (width). Lieberkuhn's (small intestines), 5Jg to ifo (length). Lieberkuhn's (small intestine), (width). Peyer's (follicles), JL to Sweat, (width). ,, in axilla, to | (width). Haversian canals, 1{ig to (width). Lacunae (bone), igLg (length). saw (width). Macula lutea, a. Malpighian bodies (kidney), i;l0. . ,, corpuscles (spleen), to Muscle (striated), to Jg (width). „ -cell (plain), gL to (length). >. „ ; to (width). Xerve-corpuscles (brain), aJ-xl to ., -fibres (medullated) I5igg to J-*, (width). (non-medullated) gJLg to 3<feo (width). Ovum, Pacinian to l. (length). „ „ Js to i (width). Papillae of skin (palm), to Tgg (length). >' !> (faCe), jgg tO ggg ,, „ tongue (circumvallate), ?g t° is (width). „ ., (fungiform), £ to (width). „ „ (filiform),a (length). Pigment-cells of choroid (hexagonal). loco' „ granules, Spennatazoon, gjgto (length). ,, head, jggg ,, „ „ w&s5 (width). Touch-eorpuscles, gjg (length). Tubuli seminiferi, 5(ig to yi_ (width). „ uriniferi, Villi, to | (length). „ sfe t0 to (width). METRICAL SYSTEM OF WEIGHTS AND MEASURES COMPARED WITH THE COMMON MEASURES. Metre . . = 39? inches (about). Centimetre = i inch (nearly). Millimetre = 5'5 „ „ Gramme . - 15.} grains (nearly). Centigramme = grain (about). Milligramme = „ Litre = about i| pint (351 oz.). 850 APPENDIX. CLASSIFICATION OF THE ANIMAL KINGDOM. Mammalia Typical Examples. Monodelphia PrimatesMan, ape. Cheiroptera .... . . Bat. Insectivora. ..'... Hedgehog. CarnivoraCat, dog, bear. Proboscidea Elephant. HyracoideaHyrax. UngulataHorse, sheep, pig. SireniaDugong. Cetacea Whale. Rodentia .... . . Rabbit, rat. Edentata . . .... Armadillo. DidelphiaKangaroo. OrnithodelphiaDuck-billed platypus. Aves. CarinateeFowl, duck. RatitesOstrich. Reptilia CrocodiliaCrocodile. Ophidia . Snake. CheloniaTortoise. Lacertilia . Lizard. AmphibiA . Anura Frog. .Urodela Newt. PiscesLamprey, shark, cod. A.-VERTEBRATA. B.-INVERTEBRATA. Mollusca Odontophora Whelk, snail. Lamellibranchiata . . . . Mussel, oyster. Brachiopoda Terebratula. Polyzoa . . . . . . . Sea mat. Arthropoda Crustacea Lobster. ArachnidaScorpion, spider. Insecta Bee, fly. Myriapoda . Centipede. Echinodermata Sea stars. APPENDIX. 851 INVERTEBRATA (continued). Typical Examples. Vermes AnnelidaEarthworm. Platyhelminth.es Tapeworm, fluke. Nemathelminthes Round-worm, thread-worm. ('(ELENTERATA Actinozoa Sea anemone. Hydrozoa Hydra. ProtozoaAmoeba, Vorlicella. INDEX. A. Abdominal muscles, action of in respira- tion, 208 Aberration, chromatic, 681 spherical, 680 Abomasum, 281 Absorbents. See Lymphatics. Absorption, 341 by blood-vessels, 356 by lacteal vessels, 355 by lymphatics, ib. conditions for, 360 by the skin, 401 process of osmosis, ib. rapidity of, 359 See Chyle, Lymph, Lymphatics, Lac- teals. Accelerator centre, 563 Accidental elements in human body, 847 Accommodation of eye, 669 Acids, organic, 844 acetic, ib. in gastric j uice, 288 Acid-albumin, 289, 831 Acini of secreting glands, 337 Actinic rays, 695 Addison's disease, 446 Adenoid tissue, 40 Adipose tissue, 42. See Fat. development, 43 situations of, 42 structure of, 42 Adrenals, 444 After-birth, 765 After-sensations, taste, 634 touch, G27 vision, 685 Aggregate glands, 377 Agminate glands, 300 Air, atmospheric, composition of, 213 breathing, 209 complemental, ib. reserve, ib. Ammonia. Air, continued. residual, 209 tidal, ib. changes by breathing, 214 quantity breathed, 210 transmission of sonorous vibrations through, 649 in tympanum, for hearing, 652 et seq. undulations of, conducted by external ear, 649 Air-cells, 199 Air-tubes, 193. See Bronchi. Albumin, 830 acid, 289, 831 action of gastric fluid on, ib. alkali, 831 characters of, ib. chemical composition of, 829 derived, 831 egg, 830 native, ib. serum, 89 tissues and secretions in which it exists, 830 of blood, 89 Albuminoids, 826 Albuminous substances, 828 absorption of, 334 action of gastric fluid on, 288 of liver on, 329 of pancreas on, 311 Alcoholic drinks, effect on respiratory changes, 215 Alimentary canal, 275, 340 development of, 793 Alkali-albumin, 831 Allantoin, 423, 839 Allantois, 758 Alloxan, 420 Aluminium, 847 Ammonia, cyanate of, isomeric with urea, 417, 838 exhaled from lungs, 217 urate of, 41 854 INDEX. Amnion. Amnion, 755, 757 fluid of, 757 Amoeba, 4 Amoeboid movements, 84 cells, 4 colourless corpuscles, 84 cornea-cells, 340 protoplasm, 4 'fradescantia, 5 Amphioxus, 767 Ampulla, 644 Amyloids or Starches, 842 action of pancreas and intestinal glands, 312, 332 of saliva on, 269 Amylopsin, 312, 841 Amyloses, 842 Anabolic nerves, 715 Anacrotic wave, iCx) Anastomoses of muscular fibres of heart, 114 of nerves, 516 of veins, 179 in erectile tissues, 186 Anelectrofonus, 489 Angle, optical, 689 Angulus opticus sen visorius, ib. Animal heat, 361. Heat and Tem- perature. Animals, distinctive characters, 11 Antialbumose, 312 Antihelix, 640 Anti peptone, 312 Antitragus, 640 Anus, 339 Aorta, 118 development, 678 pressure of blood in, 166 valves of, 117 action of, 134 Aphasia, 587 Apnoea, 233 Appendices epiploica1, 306 Appendix vermiformis, 305 Aqmeduetus, cochlea?, 645 vestibuli, 644 Aqueous humour, 670 Arachnoid, 535 Arches, visceral, 769 Area germin ativa, 746 opaca, ib. pellucida, ib. vasculosa, 755 Areolar tissue, 38 Arsenic, 847 Arterial tension, 163 Arteries, 118 circulation in, 152 velocity of, 181 distribution, 118 muscular contraction of, 15; effect of cold on, 135 Bass. Arteries-continued. effect of division, ift. elasticity, 152 purposes of, 153 muscularity, 154 governed by nervous system, 168 purposes of, 155 nerves of, 168 nervous system, influence of, iZ>. office of, ib. pressure of blood in, 163 pulse, 156. See Pulse, rhythmic contraction, 154 et seq. structure, 117 et seq. distinctions in large and small ar- teries, ib. systemic, 118 tone of, 169 umbilical, 785 velocity of blood in, 181 Articulate sounds, classification of, 508 See Vowels and Consonants. Arytenoid cartilages, 498 effect of approximation, 499 movements of, ib. muscle, 499 Asphyxia, 233 causes of death in, 235 experiments on, 236 symptoms, 233 Astigmatism, 679 Atmospheric air, 213 See Air. pressure in relation to respiration, 20; Auditory canal, 648 et seq. function, ib. Auditory centre, 597 Auditory nerve, 643 distribution, ib. effects of irritation of, 656 fits, 733 Auerbach's plexus, 298 Auricles of heart, 1 IO, 112 action, 131 capacity, 114 development, 777 dilatation, 145 force of contraction, ib. Automatic action, 534 cerebrum, 378 medulla oblongata, 561 et seq. respiratory, 562 Axis-cylinder of neive-fibre, 513 B. Bacterium lactis, 387 Barytone voice, 505 Basement-membrane, of mucous membranes, 376 of secreting membranes, 372 Bass voice, 504 INDEX. 855 Battery. Battery, Daniell's, 464 Benzoic acid, 420, 845 Bicuspid valve, 112 Bidder's ganglia, 147 Bile, 320 antiseptic power, 328 colouring matter, 321 composition of, 320 digestive properties, 327 excrementitious, 325 fat made capable of absorption by, 327 functions in digestion, 325 mixture with chyme, 328 mucus in, 323 natural purgative, 328 process of secretion, 323 quantity, 325 re-absorption, 329 salts, 321 secretion and flow, 323 secretion in foetus, 323 tests for, 321, 322 uses, 325 Bilifulvin, Biliprasin, Bilirubin, Bili- verdin, 321, 839 Bilin, 320 preparation of, 321 re-absorption of, 326 Bioplasm, 3. See Protoplasm. Bladder, urinary, 410 Sec Urinary Bladder. Blastema. See Protoplasm. Blastodermic membrane, 744 Bleeding, effects of, on blood, 101 Blind spot, 713 Blood, 65 albumin, 88 arterial and venous, 101 buffy coat, 68 chemical composition, 87 coagulation, 6s et seq. colour, 65 changed by respiration, 220 colouring matter, 99 et seq. colouring matter, relation to that of bile, 322 composition, chemical, 87 variations in, 100 corpuscles or cells of, 79. See Blood corpuscles. red, 80 white, 83 crystals, 95 cupped clot, 68 development, 102 extractive matters, 90 fatty matters, iZ>. fibrine, 68 separation of, 68 formation in liver, 103 in spleen, 439 gases of, 92 Bone. Blood-con tin tied. haemoglobin, 94 hepatic, 102 menstrual, 733 odour dr halitus of, 65 portal, characters of, 102 purification of by liver, 325 quantity, 15 -C '' reaction, 65 relation of, to lymph, 354 saline constituents, 91 serum of, 88 compared with secretion of serous membrane, 354 specific gravity, 65 splenic, 102 structural composition, 79 temperature, 65 uses, 106 of various constituents, ib. variations of, in different circum- stances, 100 in different parts of body, 101 Blood-corpuscles, red, 79 action of reagents on, 81 chemical composition, 90 development, 104, 774 disintegration and removal, 439 method of counting, 86 rouleaux, 81 sinking of, 68 specific gravity, 80 stroma, ib. tendency to adhere, 81 varieties, 80 vertebrate, various, 82 Blood-corpuscles, white, 83 amoeboid movements of, 84 derivation of, 105 formation of, in spleen, 106, 439 locomotion, 84 Blood-crystals, 95 Blood-pressure, 163 influence of vaso-motor system of, 168 variations, 167 Blood-vessels, absorption by, 356 circumstances influencing, 360 difference from lymphatic absorp- tion, 355 et seq. osmotic character of, 356 rapidity of, 359 development, 774 influence of nervous system on, 170 Bone, 49 canaliculi, 52 cancellous, 50 chemical composition, 49 compact, 50 development, 55 et seq. functions, 64 Haversian canals, 52 lacunae, 52 856 INDEX. Bone. Bone, continued. lamellae, 54 marrow, 50 medullary canal, ii. periosteum, 51 structure, 50 growth, 64 Branchial clefts, 769 Brain. See Cerebellum, Cerebrum, Pons, etc. adult, 576 amphibia, ib. apes, 577 birds, 576 capillaries of, 185 child, 576 circulation of blood in, 184 convolutions, 569 development, 787 female, 576 fish, 575 gorilla, 577 idiots, 576 lobes, 569 male, 576 mammalia, ib. orang, 578 proportion of water in, 846 quantity of blood in, 186 rabbit, 576 reptiles, ib. weight, ib. relative, 575 Breathing, 190. See Respiration. Breathing-air, 209 Bronchi, arrangement and structure of, Bronchial arteries and veins, 202 Brunner's glands, 300 Buffy coat, formation of, 68 Bulbus arteriosus, 778 Burdach's column, 543 Bursae mucosae, 373 Butyric acid, 288 C. Caecum, 304 Calcification compared with ossification, 63 Calcium, 827 fluoride, 841 phosphate, ib. carbonate, ib. Calculi, biliarv, containing cholesterin, 842 Calyces of the kidney, 403 Canal, alimentary. See Stomach, Intes- tine, etc. external auditory, 640 function of, 649 spiral, of cochlea, 645 Cartilage. Canaliculi of bone, 52 Canalis menibranaeeus, 645 Canals, Haversian, 52 portal, 316 semicircular, 644 function of, 654 Cancellous texture of bone, 50 Capacity of chest, vital, 209 of heart, 114 Capillaries, 123 circulation in, 174 rate of, 182 contraction of, 177 development, 774 diameter of, 120 influence of on circulation, 178 lymphatic, 345 network of, 124 number, 126 passage of corpuscles through walls of, 175 pressure in, 198 resistance to flow of blood in, 176 still layer in, 175 structure of, 123 of lungs, 123 of stomach, 285 external, 589 internal, 588 Capsule of Glisson, 316 Capsules, Malpighian, 408 Carbon, 827 Carbonic acid in atmosphere, 213 in blood, 100 effect of, 227 exhaled from skin, 400 increase of in breathed air, 214 in lungs, 218 in relation to heat of body, 364 Carbonates, 847 Cardiac orifice of stomach, action of, 396 sphincter of, ib. relaxation in vomiting, ib. Cardiac revolution, 137 Cardiograph, 141 Cardio-inhibitory centre, 563 Carnivorous animals, food of, 281 sense of smell in, 639 Carotid gland, 447 Cartilage, 45 articular, 46 cellular, 47 chondrin obtained from, 49 classification, 45 development, 49 elastic, 47 fibrous, Sec Fibro-cartilage, hyaline, 45 matrix, 45 ossification, 57 perichrondrium of, 55 structure, 45 INDEX. 857 Cartilage. Cartilage-contin ued. temporary, 47 uses, 49 varieties, 45 Cartilage of external ear, used in hear- ing, 649 Cartilages of larynx, 498 Casein, 831. See Milk. Cauda equina, 535 Caudate ganglion corpuscles, 517 nucleus, 588 Cause of fluidity of living blood, 77 Cells, 2 abrasion, 11 amoeboid, 4 blood, 79. See Blood-eorpuscles. cartilage, 45 chemical transformation, 11 ciliated, 28 classification, 16 decay and death, 11 'definition of, 2 epithelium, 21. See Epithelium, fission, 8, 10 formative, 746 functions, 4 et seq. gemmation, 8, 10 gustatory, 633 lacunar of bone, 53 modes of connection, 19 nutrition, 6 olfactory, 636 pigment, 22 reproduction, 8 segmentation, 14 structure, 18 et seq. transformation, 11 varieties, 16 vegetable, 13 distinctions from animal cells, 13 Cellular cartilage, 47 Cement of teeth, 261 Centres, nervous, &c. See Nerve- centres. of ossification, 63 Centrifugal nerve-fibres, 520 Centripetal nerve-fibres, ib. Cerebellum, 592 co-ordinating function of, 595 cross-action of, 595 effects ofinjury of crura, ib. of removal of, 594 functions of, ib. in relation to sensation, 595 to motion, ib. to muscular sense, 596 structure of, 593 Cerebral circulation, 194 hemispheres, 568. See Cerebrum. Cerebral nerves, 568 third, ib. effects of irritation andinjury of, iZ>. relation of to iris, 600 Chondrin. Cerebral nerves-eon tinued. fourth, 6oi fifth, (x)i distribution of, ib. effect of division of, 602 influence of, on muscles of mastication, 602 on organs of special sense, 605 et seq. relation of, to nutrition, 605 resemblance to spinal nerves, 601 sensory function of greater division of fifth, 602 sixth, 606 communication of,with sympathetic, ib. seventh, 607. See Facial Nerve, ninth, 608 tenth, 610 eleventh, 614, twelfth, 615 Cerebration, unconscious, 581 Cerebrin, 837 Cerebro-cerebellar fibres, 590 Cerebro-spinal fluid, relation to circula- tion, 186 Cerebro-spinal nervous system, 528 et seq. See Brain, Spinal Cord, etc. Cerebrum, its structure, 574 chemical composition, 575 convolutions of, 569 et seq. crura of, 565 development, 787 distinctive character in man, 578 effects of injury, 578 removal, 579 electrical stimulation, 584 functions of, 578 grey matter, 574 in relation to speech, 586 other parts, 568 localization of functions, 580 structure, 573 et seq. unilateral action of, 579 white matter, 575 Cerumen, or ear-wax, 393 Characteristics of organic compounds, 828 Chemical composition of the human body, 827 Chest, its capacity, 202 contraction of in expiration, 203 enlargement of in inspiration, 202 Chest-notes, 506 Cheyne-Stokes' breathing, 233 Chlorine, 827 in human body, 827 in urine, 424 Cholesteriu, 842 in bile, 322 Choletelin, 421, 840 Chondrin, 836 858 INDEX. Chorda. Chorda dorsalis, 749 Chorda tympani, 271 et seq. Chord® tendine®, 116 action of, 133 Chorion, false, 759 true, 754 villi of, 759 Choroid coat of eye, 662 blood-vessels, ib. Choroidal fissure, 740 Chromatic aberration, 681 Chyle, 353 absorption of, 355 analysis of, 354 coagulation of, ib. compared with lymph, 353 corpuscles of, 354. See Chyle-cor- puscles. fibrin of, 354 forces propelling, 348 molecular base of, 353 quantity found, 354 relation of, to blood, ib. Chyle-corpuscles, 374 Chyme, 288 absorption of digested parts of, 334 changes of in intestines, 334 et seq. Cilia, 28 Ciliary epithelium, 28 function of, 29 Ciliary motion, 29 nature of, 29 Ciliary-muscles, 673 action of in adaptation to distances, 675 Ciliary processes, 667 Circulation of blood, 106 action of heart, 131 agents concerned in, 188 arteries, 152 brain, 184 capillaries, 174 course of, 106 et seq. discovery, 189 erectile structures, 186 foetal, 783 forces acting in, 188 influence of respiration on, 228 peculiarities of, in different parts, 184 portal, 316 proofs, 188 pulmonary, 219 systemic, 106 in veins, 178 velocity of, 180 Circumvallate papillae, 632 Claustrum, 589 Claviculi of Gagliardi, 55 Clefts, visceral, 769 Clitoris, 723 development of, 804 Cords. Cloaca, 802 Clot or coagulum of blood, 67 See Coagulation. of chyle, 354 Coagulation of blood, 67 absent or retarded, 75 conditions affecting, 75 theories of, 78 of chyle, 354 of lymph, tb. Coat, ouffy, 68 Coats of arteries, 101 Coccygeal gland, 447 Cochlea of the ear, 644 office of, 655 Cohnheim's fields, 452 Cold-blooded animals, 363 extent of reflex movements in, 549 retention of muscular irritability in, 466 Colloids, 360 Colon, 304 Colostrum, 386 Colour-blindness, 697 Colouring matter, of bile, 321 of blood, 94 of urine, 420 Colours, optical phenomena of, 695 et seq. Column® came®, 116 Columnar epithelium, 26 Complemental air, 2C9 colours, 695 et seq. Compounds, 828 inorganic, 846 organic, 828 Concha, 672 use of, 679 Conducting paths in cord, 546 Cones of retina, 603 Coni vasculosi, 724 Conjunctiva, 660 Connective tissues, 33 classification, 33 corpuscles of, ib. fibrous, 36 gelatinous, 39 retiform, 40 varieties, 33 Consonants, 308 varieties of, ib. Contralto voice, 504 Control centres, 563 Convolutions, cerebral, 569 et seq. Co-ordination of movements, office of cerebellum in, 595 Copper, an accidental element in the body, 847 Cord, spinal. See Spinal Cord, umbilical, 765 Cords, tendinous, in heart, 116 vocal. See Vocal Cords. INDEX. 859 COKIUM. Corium, 390 Cornea, 601 corpuscles, 662 nerves, ib. structure, ib. Corpora Arantii, 117 geniculata, 568 quadrigemina, ti. their function, ib. striata, 588 their function, 591 Corpus callosum, 569 cavernosum penis, 186 dentatum of cerebellum, 593 of olivary body, 560 luteum, 734 of human female, ib. of mammalian animals, ib. of menstruation and pregnancy compared, 736 spongiosum urethra), 186 Corpuscles of blood, 79. See Blood- corpuscles. of connective tissue, 34 Zimmerman, 442 Correlation of life with other forces, 804 Cortical substance of kidney, 402 of lymphatic glands, 345 Corti's rods, 646 office of, 656 Costal types of respiration, 206 Coughing, influence on circulation in veins, 231 mechanism of, 223 Cowper's glands, 728 Cranial nerves. See Cerebral nerves. Cranium, development of, 766 Crassamentum, 67 Crescents of Gianuzzi. See Semilunes of Heidenhain. Crico-arytenoid muscles, 499 Cricoid cartilages, 498 Crossed pyramidal tract, 542 paralysis, 565 Crura cerebelli, effect of dividing, 595 et seq. of irritating, ib. cerebri, 565 their office, 566 Crusta, 565 Crusta petrosa, 257 Crystallin, 832 Crystalline lens, 668 in relation to vision at different distances, 671 Crystalloids, 360 blood, 94 et seq. Cubic feet of air for rooms, 228 Cupped appearance of blood-clot, 68 Curdling ferments, 312 Currents of action, 486 ascending, 487 Development. Currents of action-continued. continuous, 464 descending, 487 induced, 465 muscle, 459 natural, 460 negative variation, 486 nerve, ib. polarising, 487 rest, 486 Curves, Traube-Hering's, 232 Cuticle. See Epidermis, Epithelium. of hair, 394 Cutis vera, 390 Cystic duct, 319 Cystin in urine, 425 D. Daltonism, 697 Daniell's battery, 464 Decidua, menstrualis, 733 reflex a, 762 serotina, ib. vera, ib. Decomposition, tendency of animal com- pounds to, 827 Decomposition-products, 836 Decussation of fibres in medulla ob- longata, 557, 558 in spinal cord, 541 of optic nerves, 597 Defalcation, mechanism of, 339 influence of spinal cord on, 553 Deglutition, 279. See Swallowing. Dentine, 255 Depressor nerve, 170 Derived albumins, 835 Derma, 390 Descendens noni nerve, 615 Descemet's membrane, C61 Development, 740 of organs, 766 alimentary canal, 793 arteries, 778 blood, 102 et seq. blood-vessels, 778 bone, 55 brain, 787 capillaries, 774 cranium, 766 ear, 792 embryo, 740 extremities, 771 eye, 790 face and visceral arches, 769 heart, 773 liver, 809 lungs, 796 medulla oblongata, 787 860 INDEX. Development. Development-continued. muscle, 453, 455 nerves, 785 nervous system, nose, 793 organs of sense, 790 pancreas, 796 pituitary body, 798 respiratory apparatus, 796 salivary glands, 795 spinal cord, 786 teeth, 258 vascular system, 772 veins, 781 vertebral column and cranium, 766 visceral arches and clefts, 769 of Wolftian bodies, urinary apparatus and sexual organs, 797 Dextrin, 843 Diabetes, 331 Diapedesis of blood-corpuscles, 175 Diaphragm, action of, on abdominal viscera, 208 in inspiration, 203 in various respiratory acts, 208 in vomiting, 295 Diastase of liver, 330 Diastole of heart, 131 Dicrotous pulse, 160 Diet- daily, 249 mixed, necessity of, 239 et seq. Diffusion of gases in respiration, 218 Digestion, 250 in the intestines, 333 et seq. in the stomach, 286 influence of nervous system on, 338 of stomach after death, 294 See Gastric fluid. Food, Stomach. Dilatation of pupil, 562 Diplopia, 701 Direct cerebellar tract, 543 pyramidal tract, 542 Direction of sounds, perception of, 657 Discus proligerus, 719 Disdiaclasts, 452 Distance, adaptation of eye to, 763 of sounds, how judged of, 658 Distinctness of vision, how secured, 680 et seq. Divisions of functions, 14 Dorsal laminae, 750 Double hearing, 659 vision, 600 Dreams, 582 Drowning, cause of death in, 236 Ductless glands, 436 Ducts of Cuvier, 782 Ductus arteriosus, 780 venosus, 789 closure of, 780 Duodenum, 297 Epidermis. Duration of impressions on retina, 687 intestinal digestion, 338 Duverney's glands, 723 Dyspnoea, 233 E Ear, 640 bones or ossicles of, 642 function of, 652 development of, 792 external, 640 function of, 649 internal, 642 function of, 654 middle, 641 function of, 649 Ectopia vesicae, 435 Efferent nerve fibres, 52 vessels of kidney, 412 Egg-albumin, 830' Elastic cartilage, 47 fibres, 35 tissue, 37 Elastin, 36, 836 Elastic after-vibration, 470 Electricity, in muscle, 457 nerve, 486 retina, 688 Electrodes, 460 Electrotonus, 488 Elementary substances in the human body, 827 accidental, 847 Embryo, 740 et seq. Sec Development. Embryonic shield, 747 Emmetropic eye, 678 Emotions, connection of with cerebral hemispheres, 578 Emulsification, 312 Enamel of teeth, 257 Enamel organ, 258 End-bulbs, 524 End-plates, motorial, 455 Endocardium, 115 Endolymph, 644 function of, 654 Endomysium, 451 Endoneurium, 516 Endosmometer, 357 Endothelium, 23 distinctive characters, ib. germinating, 25 Energy, relations of vital to physical, chap, xxiv. daily amount expended in body, 492 Epencephalon, 788 Epiblast, 14, 746 Epidermis, 388 INDEX. 861 Epidermis. Epidermis-continued. functions of, 397 pigment of, 389 structure of, 388 Epididymis, 724 Epiglottis, 192 structure, 192 Epineurium, 516 Epithelium, 21 air-cells, 199 arteries, 122 bronchi, 195 bronchial tubes, ib. ciliated, 28 cogged, 31 columnar, 26 cylindrical, ib. functions, 32 glandular, 26 goblet-shaped, 27 mucous membranes, 375 olfactory region, 636 secreting glands, 399 serous membranes, 394 spheroidal, 26 squamous or tesselated, 22 stratified, 30 transitional, 29 Erect position of objects, perception of, 688 Erectile structures, circulation in, 186 Erection, <6. cause of, ib. influence of muscular tissue in, ib. a reflex act, 384 Erythro-granulose, 269 Erythro-dextrin, ib. Eunuchs, voice of, 505 Eustachian tube, 641 development, 793 function of, 653 Eustachian valve, 129 Excreta in relation to muscular action, 491 et seq. Excretin, 842 Excretion, 371 Exercise, effects of, on production of carbonic acid, 216 on temperature of body, 362 Expenditure of body, 491 et seq Expiration, 207 influence of, on circulation, 230 mechanism of, 208 muscles concerned in, ib. relative duration of, 209 Expired air, properties of, 214 et seq. Extractive matters, in blood, 90 in urine, 434 Extremities, development of, 771 Eye, 660 Fibro-Cartilage. Eye-continued. adaptation of vision at different dis- tances, 673 et seq. blood-vessels, 607 development of, 790 optical apparatus of, 667 refracting media of, ib. resemblance to camera, ib. Eyelids, 660 development of, 792 Eyes, simultaneous action of in vision, 704 F. Face, development of, 769 Facial nerve, 607 effects of paralysis of, ib. relation of, to expression, 607 Fsects, composition of, 339 quantity of, 338 Fallopian tubes, 74 opening into abdomen, Falsetto notes, 506 Fasciculus, cuneatus, 558 muscle, 457 olivary, 558 teres, ib. Fasting, influence on secretion of bile, 323 Fat. See Adipose tissue. action of bile on, 327 of pancreatic secretion, 13 of small intestine on, 334 absorbed by lacteals, 353 formation of, 494 in blood, 90 in relation to heat of body, 368 of bile, 322 of chyle, 354 situations where found, 42 uses of, 44 Fechner's law, 685 Female generative organs, 717 Fenestra ovalis, 644 rotunda, 645 Ferments, 73, 841 Fibres, 20 of Muller, 667 Fibrils or filaments, 20 Fibrin, 833, in blood, 68 in chyle, 353, 354 formation of, 69 in lymph, 354, 355 sources and properties of, 833 vegetable, 242 Fibrinogen, 71 et seq., 833 Fibrinoplastin, ib. Fibro-cartilage, 47 classification, ib. 862 INDEX. Fibro-Cartilagf.. Fibro-cartilage-continued. development, 49 ■white, 48 yellow, 47 Fibrous tissue, 36 white, 36 yellow, 37 development, 41 Fick's kymograph, 165 Field of vision, actual and ideal size of, 690 Fifth nerve. See Cerebral Nerves. Fillet, 558 Filtration, 358, 425 Filum terminate, 535 Fimbria? of Fallopian tube, 721 Fingers, development of, 772 Fish, temperature of, 363 Fissures, of brain, 569 et seq. of spinal cord, 337 Fistula, gastric, experiments in cases of, 281 Flesh, of animals, 240 Fluids, passage of, through membranes, 356 Fluoride of calcium, 847 Focal distance, 673 Foetus, blood of, 102 circulation in, 783 communication with mother, 763 faeces of, 325 membranes, 753 et seq. office of bile in, ib. pulse in, 143 Folds, head and tail, 752 Follicles, Graafian. See Graafian Vesi- cles. Food, 237 albuminous, changes of, 288, 312 amyloid, changes of, 269, 312, 332 classification of, 239 digestibility of articles of, 239 value dependent on, ib. digestion of, in intestines, 333 et seq. in stomach, 288 et seq. improper, 247 of man, 239 mixed, the best for man, 239 mixture of necessary, ib. too little, 244 too much, 248 vegetable, contains nitrogenous prin- ciples, 242 Foot-pound, 145 Foot-ton, ib. Foramen ovale, 112 Forced movements, 596 Form of bodies, how estimated, 693 Formation of fat, 495 Gland. Formic acid, 844 Fornix, 572 Fourth cranial nerve, 601 ventricle, 557 Fovea centralis, 664 Fundus of bladder, 410 Fundus of uterus, 722 Fungiform papillae of tongue, 631 Funiculus of Rolando, 559 G. Galactophorous ducts, 384 Gall-bladder, 318 structure, 319 Ganglia. See Nerve centres. Ganglion, Gasserian, 601 corpuscles, 517 See Nerve-corpuscles. Gases, 827 in bile, 323 in blood, 92 extraction from blood, ib. in stomach and intestines, 340 in urine, 425 Gastric glands, 284 Gastric juice, 288 acid in, ib. action of, on nitrogenous food, 288 on non-nitrogenous food, 290 on saccharine and amyloid prin- ciples, ib. characters of, 281 composition of, 286 digestive power of, 288 experiments with, iZ>. pepsin of, ib. quantity of, 288 secretion of, 287 how excited, ib. influence of nervous system on. Gelatin, 834 as food, 248 action of gastric juice on, 286 action of pancreatic juice on, 312 Gelatinous substances, 834 Generation and development, 716 Generative organs of the female, 717 of the male, 723 Gerlach's network, 539 Germinal area, 746 epithelium, 718 matter, 3. See Protoplasm. Germinal membrane, 744 spot, 719 vesicle, ib. Gill, 190 Gizzard, action of, 281 Gland, pineal, 447 pituitary, ib. INDEX. 863 Gland-Cells. Gland, prostate, 728 Gland-cells, agents of secretion, 371 changes in during secretion, 285, 310, 384 Gland-ducts, arrangement of, 381 contractions of, ib. Glands, aggregate, 377 Brunner's, 300 ceruminous, 393 Cowper's, 728 ductless, 436. See Vascular. Duverney's, 723 of large intestine, 308 of Lieberkuhn, 300 lymphatic, 349. See Lymphatic Glands. mammary, 383 of Peyer, 360 salivary, 263 sebaceous, 393 secreting, 376. See Secreting Glands, of small intestines, 300 of stomach, 284 sudoriferous, 391 tubular, 377 vascular, 436. See Vascular Glands, vulvo-vaginal, 723 Glandula Nabothi, 723 Glisson's capsule, 314 Globulin, 89, 832 distinctions from albumin, ib. Globus major and minor, 724 development, 798 Glosso-pharyngeal nerve, 608 communications of, 609 motor filaments, ib. a nerve of common sensation and of taste, ib. Glottis, action of laryngeal muscles on, 499 . . dosed in vomiting, 295 et seq. forms assumed by, 501 narrowing of, proportioned to height of note, 503 respiratory movements of, 209 Glucose, 269, 842 in liver, 331 test for, 269 Gluten in vegetables, 242 Glycerin, 313, 842 Glycin, 836 Glycocholic acid, 320 Glycogen, 329, 843 characters, 331 destination, 330 preparation, 331 quantity formed, 330 variation with diet, Glycosuria, 331 artificial production of, Gmelin's test, 322 Goll's column, 543 Graafian vesicles, 718 Heaht. Graafian vesicles-continued. formation and development of, 720 et seq. relation of ovum to, ib. rupture of, changes following, 734 Granular layers of retina, 665 Fibgel, 453 Grape-sugar. See Glucose. Grey matter of cerebellum, 593 of cerebrum, 574 of crura cerebri, 566 of medulla oblongata, 559 of pons Varolii, 564 of spinal cord, 539 Groove, primitive, 747 Growth, 7 coincident with development, 7 of bone, 64 not peculiar to living beings, 7 Guanin, 839 Gubernaculum testis, 801 II. Habitual movements, 533 H rematin, 98 hydrochlorate of, 99 Hremadynamometer, 165 Hrematochometer, 182 Hrematoidin, 98 Hrematoporphyrin, 98 Hremin, 99 Haemochromogen, 98 Hremocytometer, 86 Haemoglobin, 94 et seq. action of gases on, 97 derivatives of, 97 distribution, 100 estimation of, 99 spectrum, 95 Hair-follicles, 394 their secretion, 398 Hairs, 393 chemical composition of, 836 structure of, 393 Half vision centre, 597 Hamulus, 645 Hare-lip, 770 Hassall, concentric corpuscles of, 442 Haversian canals, 52 Head-folds, 752 Hearing, anatomy of organ of, 640 double, 658 impaired by lesion of facial nerve, 607 influence of external ear on, 649 of labyrinth, 654 of middle ear, 649 physiology of, 648 See Sound, Vibrations, etc. Heart, 109 et seq. action of, 131 864 INDEX. Heart. Heart-continued. accelerated, 150 force of, 143 frequency of, ib. inhibited, 148 after removal, ib. rhythmic, 146 et seq. work of, 145 auricles of, no, 112 See Auricles. capacity, 114 chambers, no chordae tendinese of, 116 column® earn® of, ib. course of blood in, 131 development, 776 endocardium, 115 force, 143 frog's, 146 ganglia of, 147 impulse of, 140 tracing by cardiograph, 141 et seq. influence of pneumogastric nerve, 148 of s\ mpathetic nerve, 150 investing sac, 109 muscular fibres of, 114 musculi papillares, 112 nervous connections with other organs, ISO rhythm, 137 nervous system, influence on, 147 revolution of, 146 situation, 109 sounds of, 138 causes, 139 structure of, 114 tendinous cords of, 116 tubercle of Lower in, 111 valves, 115 arterial or semilunar, 117 function of, 134 auriculo-ventricular, 115 function of, 132 ventricles, their action, 132, 144 capacity, 114 weignt of, ib. work of, 144 Heat, animal. See Temperature, influence of nervous system, 369 of various circumstances on, 361 et seq. losses by radiation, etc., 366 sources and modes of production, 363 developed m contraction of muscles, 364 Heat centres, 369 Heat-producing tissues, 363 Heat or rut, 730 analogous to menstruation, ib. Height, relation to respiratory capacity, 211 Helicotrema, 645 Inorganic Matter. Helix of ear, 640 Helmholz's modification, 466 Hemipeptone, 312 Hemispheres, Cerebral, 574. <S'e? Cere- brum. Henson's disc, 453 Hepatic cells, 315 ducts, 318 veins, 316 vessels, arrangement of, 315 1/ wy. Herbivorous animals, perception of odours by, 639 Hering's theory, 696 Hiccough, mechanism of, 222 Hippuric acid, 420, 434, 837 Horse's blood, peculiar coagulation of, 68 Howship's lacunae, 52 Hunger, sensation of, 244 Hyaline cartilage, 45 Hybernation, state of thymus in, 468 Hydrobilirubin, 421 Hydrochloric acid, 288 Hydrogen, 827 Hydrolytic ferments, 841 Hymen, 723 Hyperesthesia, result of injury to spinal cord, 548 Hypermetropia, 679 Hyperpneea, 233 Hyperpyrexia, 565 Hypoblast, 746 Hypoglossal nerve, 615 Hypoxanthin, 422, 839 I. Ideas, connection of, with cerebrum, 578 Ileum, 297 Ileo-caecal valve, 305, 308 Image, formation of, on retina, 610 Impulse of heart, 140 Income of body, 490 et seq. compared with expenditure, jj. Incus, 642 function of, 651 Indican, 44, 840 Indigo, 421, 840 Indol, 312, 840 Induction coil, 465 current, ib. Infundibulum, 199 Inhibitory influence of pneumogastric nerve, 148 Inhibitory action of brain, 578 on blood-vessels, 121 Inhibitory heat-centre, 370 Inorganic matter, distinction from or- ganised, 827 principles, 845 INDEX. 865 Inosite. Inosite, 844 Insalivation, 263 Inspiration, 203 elastic resistance overcome by, ib. extraordinary, 206 force employed in, 212 influence of, on circulation, 228 mechanism of, 205 Intercellular substance, 19 Intercostal muscles, action in inspira tion, 206 et seq. in expiration, 208 Interlobular veins, 316 Internal capsule, 588 Intestinal juice, 332 Intestines, digestion in, 297 et seq. development, 794 gases, 341 large, digestion in, 336 structure, 304 movements, 336 small, changes of food in, 333 structure of, 297 Intralobular veins, 317 Inversive ferments, 332 Involuntary muscles, actions of, 484 structure of, 449 Iris, 671 action of, ib. et seq. in adaptation to distances, 675 blood-vessels, 671 development of, 797 influence of fifth nerve on, 662 of third nerve, ib. relation of, to optic nerve, Iron, 847 Irradiation, 682 J. Jacobson's nerve, 609 Jaw, interarticular cartilage, 262 Jejunum, 297 Juice, gastric, 286 pancreatic, 310 Jumping, 484 K. Karyokinesis, 10 Katabolic nerves, 715 Katacrotic wave, 160 Katelectrotonus, 489 Keratin, 836 Key, 465 Kidneys, their structure, 402 Leucin. Kidneys-con tin tied. blood-vessels of, how distributed, 408 capillaries of, 407 development of, 812 function of, 412. See Urine. Malpighian corpuscles of, 404 nerves, 410 tubules of, 403 et seq. Knee, pain of, in diseased hip, 530 Krause's membrane, 453 Kreatin, 433, 838 Kreatinin, 433, 434, 838 Kymograph, 165 tracings, ib. spring-, 166 L. Labia externa and interna, 723 Labyrinth of the ear. See Ear. Lachrymal apparatus, 660 gland, ib. Lactation, 385 Lacteals, 304 absorption by, 355 contain lymph in fasting, 343 origin of, 344 structure of, 345 in villi, 304 Lactic acid, 845 in gastric fluid, 288 Lactiferous ducts, 384 Lactose, 842 Lacunae of bone, 52 Lamellae of bone, 54 Laminae dorsales, 750 viscerales or ventrales, 754 Large intestine. See Intestii.e. Larynx, construction of, 497 muscles of, 499 nerves of, ib. variations in according to sex and age, 5°5 ventricles of, 530 vocal cords of, 498 Latent period, 469 Lateral plate, 751 Lateral ventricles, 571 Laughing, 223 Laxator tympani muscle, 685 Lead an accidental element, 847 Leaping, 484 Lecithin, 323, 337 Legumen identical with casein, 242 Lens, crystalline, 668 Lenticular ganglion, relation of third nerve to, 559 nucleus, 589 Leucic acid, 857 Leucin, 312, 837 866 INDEX. Leucocytes. Leucocytes. See Blood Corpuscles (White). Leucocythaemia, state of vascular glands T in, 439 Levers, different kinds of, 479 Lieberkuhn's glands, 300 in large intestines, 308 in small intestines, 301 Life, 804 relation to other forces, ib. simplest manifestations of, 4 Ligamentum nuchae, 37 Lightning, condition of blood after death , . h-v' Z6 Lime, salts of, in human body, 847 Lingual branch of fifth nerve, 605 Lips, influence of fifth nerve on move- ments of, 604 Liquor amnii, 757 Liquor sanguinis, or plasma, 8 lymph derived from, 355 Liver, 313 action of, on albuminous matters, 327 on saccharine matters, 330 blood-elaborating organ, 329 blood-vessels of, 314 capillaries of, ib. cells of, 315 circulation in, 314 development of, 795 ducts of, 318 functions of, 320 et seq. glycogenic function of, 329 secretion of, 320. See Bile, structure of, 314 sugar formed by, 566 et seq. Locus niger, 331 Loss of water, 846 Ludwig's air pump, 92 Lungs, 197 blood-supply, 202 capillaries of, 201 cells of, 199 changes of air in, 214 •changes of blood in, 219 circulation in, 202 contraction of, 207 coverings of, 197 •development of, 796 ■elasticity of, 207 lobes of, 196 lobules of, ib. lymphatics, 198 muscular tissue of, 213 nerves, 202 nutrition of, ib. position of, 191 structure of, 198 Luxus consumption, 248 Lymph, 353 compared with chyle, ib. with blood, 354 current of, 348 Meatus of Ear. Lymph-continued. quantity formed, 354 Lymph-corpuscles, 353 in blood, 106 development of into red blood-cor- puscles, 105 origin of, ib.' Lymph-hearts, structure and action of, 34? relation of to spinal cord, ib. Lymphatic glands, 348 Lymphatic vessels, 341 absorption by, 355 communication with serous cavities, 345 communication with blood-vessels, ib. course of fluid in, 348 distribution of, 361 origin of, 343 propulsion of lymph by, 368 structure of, 347 et seq. valves of, ib. Lymphoid or retiform tissue, 40. See Adenoid Tissue. M. Macula germinativa, 719 Magnesium, 847 Male sexual functions, 737 Malleus, 642 function of, 651 Malpighian bodies or corpuscles of kid- ney, 404 capsules, ib. corpuscles of spleen, 438 et seq. Maltose, 269, 844 Mammalia, blood-corpuscles of, 82 brain of, 576 Mammary glands, 383 evolution, 384 involution, 385 lactation, ib. structure, 383 Mandibular arch, 710 Manganese, 847 Manometer, 165 experiments on respiratory power with, 202 Marrow of bone, 50 Mastication, 262 centre, 562 fifth nerve supplies muscles of, 263 muscles of, 262 Mastoid cells, 673 Matrix of cartilage, 45 Mature corpuscles, origin of, 104 Meatus of ear, 640 venosus, 777 urinarius, opening of in female, 723 INDEX. 867 Meckel's Cartilage. Meckel's cartilage, 770 Meconium, 325 Medulla of bone, 50 Medulla oblongata, 556 et seq. columns of, 556 conduction of impressions, 560 decussation of fibres, 557 effects of injury and disease of, 560 fibres of, how distributed, 557 functions of, 560 et seq. important to life, 560 nerve-centres in, 560 pyramids of, anterior, 556, 557 posterior, 557 structure of, 356 Medullary portion of kidney. See Kid- ney. folds, 749 groove, 749 plate, 749 substance of lymphatic glands, 349 substance of nerve fibre, 512 Meissner's plexus, 298 Melanin, 840 Membrana decidua, 760 granulosa, 719 development of into corpus luteum, • 734 limitans externa, 665 interna, 669 Membrana propria or basement mem- brane. See Basement Membrane. pupillaris, 792 capsulo-pupillari<, ib. tympani, 642 office of, 651 Membrane, blastodermic. 744 of the brain and spinal cord, 535 ossification in, 55 primary or basement. See Basement membrane. vitelline, 719 Membranes of brain, 535 Membranes, mucous, 374. See Mucous membranes. Membranes, passage of fluids through, 366. See Osmosis. secreting, 372 Membranes, serous, 372. See Serous membranes. Membranous labyrinth. See Ear. Memory, relation to cerebral hemi- spheres, 578 et seq. Menstrual discharge, composition of, ,r 733 . Menstruation, 730 coincident with discharge of ova, 731 corpus luteum of, 736 time of appearance and cessation, 733 Mercurial air-pump, 92 Mercurial manometer, 165 Mercury, absorption of, 401 Mesencephalon, ;88 Muller's Fibres. Mesoblast, 746 Mesocephalon, 788 Metallic substances, absorption of by skin, 426 Metencephalon, 788 Methaemoglobin, 97 Mezzo-soprano voice, 505 Micro-organisms, 312 Micturition, 435 Milk, as food, 312 chemical composition, 386 properties of, 387 secretion of, 386 Milk-curdling ferments, 389 Milk-globules, 386 Milk-teeth, 252 et seq. Millon's re-agent, 829 Mind, cerebral hemisphere the org ms of, 578 Mitral valve, 117 Modiolus, 645 Molecules, or granules, 5 in blood, 80 in milk, 386 movement of in cells, 5 Molars. See Teeth. Molecular base of chyle, 353 Morphological development, 14 Motor centres, 583 Motion, ciliary, 29 sensation of, 650 Motor impulses, transmission of in cord, 546 n. nerve-fibres, 520 laws of action of, 522 Motor linguae nerve, 615 oculi, or third nerve, 599 Motor paths, 546 Motorial areas, 581 Motorial end-plates, 455 Mouth, changes of food in, 250 et seq. Movements, of eyes, 700 of intestines, 336 of voluntary muscles, 439 Mucigen, 265 Mucin, 835 Mucous membrane, 374 basement membrane of, 375 epithelium-cells of, ib. See Epithe- lium. digestive tract, 375 gastro-pulmouary tract, ib. genito-urinary tract, ii. gland-cells of, 376 of intestines, 298, 307 of stomach, 282 of uterus, changes of in pregnancy, 760 respiratory tract, 375 Muco-salivary glands, 266 Mucus, 376 Muller's fibres, 667 868 INDEX. Murexide. Murexide, 420 Muscle, 449 activity, 462 chemical constitution, 456 clot, 457 contractility, 462 contraction, mode of, 468 corpuscles, 454 curves, 468 development, 455 disc of Hensen, 453 effect of pressure of, on veins, 179 elasticity, 459 electric currents in, 460, 475 fatigue, 472 curves, ib. growth, 456 heart, 454 heat developed in contraction of, ' • involuntary, 449 actions of, 484 Krause's membrane, 453 muscle rods, 454 natural currents, 460 nerves of, 455 non-striated, 449 nutrition of, 459 physiology of, 459 plain, 449 plasma, 457 reaction, 459 response to stimuli, 462, 476 rest of, 459 rigor, 477 sarcolemma, 451 sensibility of, 463 serum, 457 shape, changes in, 474 sound, developed in contraction of, 473 source of action of, 484 stimuli, 463 striated, 450 structure, ib. et seq. tetanus, 470 twitch, 468 unstriped, 449 voluntary, 451 actions of, 479 blood-vessels and nerves of, 455 work of, 472 Muscular action, 462 conditions of, 475 force, 472 Muscular irritability, 462 duration of, after death, 477 Muscular motion, 462 et seq. sense, 596 cerebellum the organ of, 596 tone, 554 Musculaiis mucosse, 282, 298, 316 M it sculi p a pillar es, 133 Nerves. Musical sounds, 657 Myo-albumin, 458 Myograph, 467 pendulum, 469 Myo-heematin, 458 Myopia, or short-sight, 678 Myosin, 457, 832 ferment, ib. Myosinogen, ib. N. Nabothi glandulee, 723 Nails, 395 growth of, ib. structure of, ib. Napthilamine, 312 Nasal cavities in relation to smell, 63? et seq. Native albumins, 830 Natural organic compounds, 828 classification of, ib. Nerve-centre, 529. See Cerebellum, Cerebrum, &c. ano-spinal, 553 automatic action, 534 cardio-inhibitory, 563 ci Lio-spinal, 562 conduction in, 529 deglutition, 562 diabetic, 393 diffusion in, 530 erection, 554 functions of, 529 genito-urinary, 553 mastication, 263 micturition, 435 motor, 583 radiation in, 530 reflexion in, ib. laws and conditions of, 531 respiratory, 224, 562 secretion of saliva, 270 sweat, 555 transference of impressions, 531 vaso-motor, 164, 555 vesico-spinal, 553 Nerve-corpuscles, 517 caudate or stellate, ib. polar, ib. Nerves, 511 accelerator, 150 action of stimuli on, 486 currents of, 485 afferent, 520 axis-cylinder of, 513 cells, 517 centrifugal, 520 centripetal, ib. cerebro-spinal, 511 INDEX. 869 Nerves. Nerves-continued. classification, 520 conduction by, 520 et seq. rate of, 520 continuity, 515 course of, 515 cranial. See Cerebral Nerves, depressor, 170 efferent, 520 electrical currents of, 483 functions of, 519 effect of chemical stimuli on, 486 of mechanical irritation, ib. of temperature, ib. funiculi of, 515 grey, 513 impressions on,referred to periphery, . 519 inhibitory, 520. See Inhibitory Action, intercentral, 520 laws of conduction, 319 et seq. of motor nerves, 522 of sensory nerves, 521 medullary sheath, 511 medullated, 511 motor, 522 laws of action in, ib. natural currents, 486 neurilemma, 515 nodes of Ranvier, 513 non-medullated, 513 nuclei, 512 of special sense, 522 plexuses of, 516 primitive nerve sheath, 512 sensory, 520 laws of action in, 521 size of, 513 spinal, 543. See Spinal Nerves, stimuli, 486 structure, 513 sympathetic. See Sympathetic Nerve. tei minations of, 523 central, 5 in cells, 528 in corpuscles of Golgi, 527 in corpuscles of Graudry, 527 in "corpuscles of Herbst, 525 in end-bulbs, 524 in motorial end-plates, 453 in networks or plexuses, 527 in Pacinian corpuscles, 525 in touch-corpuscles, 524 trophic, 716 ulnar, effect of compression of, 521 varieties of, 511 vaso-constrictor, 172 vaso-dilator, 710 vaso-inhibitory, ib. vaso-motor, ib. velocity of nerve-force, 520 white, 514 Odours. Nervi nervorum, 556 Nervi vasorum, 123 Nervous force, velocity of, 510 Nervous'system, 540 cerebro-spinal, 535 development, 785 elementary structure of, 511 influence of on animal heat, 369 on arteries, 168 on contraction of blood-vessels, 171 on erection, 187 on gastric digestion, 293 on the heart's action, 146 on movements of intestines, 335 of stomach, 293 on respiration, 224 on secretion, 369 on sphincter ani, 583 sympathetic, 707 Network, intracellular, 18 nuclear, 19 Neural canal, 750 Neurenteric canal, 754 Neurilemma, 516 Neuroglia, 41 Nipple, an erectile organ, 384 structure of, ib. Nitrogen, 827 in blood, 100 influence of in decomposition, 828 in relation to food, 240 et seq. in respiration, 213 Nitrogenous compounds, 239 non-nitrogenous compounds, 739 Nitrogenous equilibrium, 494 Nitrogenous food, 239 in relation to muscular work, 432 et seq. in relation to urea, to uric acid, 434 Nodes of Ranvier, 513 Non-azotized or Non-nitrogenous food, 239 organic principles, 842 Nose. See Smell, development of, 793 Notochord, 749 Nucleus, 18 lateralis, 539 position, 18 staining of, 18 Nutrition, 490 general nature, 490 of cells, 6 Nymphae, 742 0. Odontoblasts, 260 Odours, causes of, 635 et seq. 870 INDEX. Odours. 0 lours-continued. different kinds of, 638 perception of, ib. varies in different classes, 639 relation to taste, 634 (Esophagus, 277 Oil, absorption of, 355 Oleaginous principles, digestion of, 312 Oleic acid, 841 Olfactory cells, 638 centre, 397 nerve, 636 subjective sensations of, 640 Olivary body, 360 fasciculus, io. Oa phalo-mesenteric arteries, 781 duct, 734 veins, 776 Oncograph, 427 Oncometer, ib. Ophthalmic ganglion, relation of third nerve, 599 Ophthalmoscope, 686 Optic lobes, corpora quadrigemina, homo- logues of, 568 functions of, ib. nerve, decussation of, 597 point of entrance insensible to light, 683 thalamus, function of, 591 vesicle, primary, 790 secondary, ib. Optical angle, 689 apparatus of eye, 660 Ora serrata of retina, 663 Orang, brain of, 577 Organ of Corti, 646 Organic compounds in body, 828 instability of, ib. Organs of sense, development of, 790 Osmosis, 356 Os uteri, 722 Osseous labyrinth, 644 Ossicles of the ear, 642 Ossicula auditus, 642 Ossification, 57 et seq. Osteoblasts, 56 Osteoclasts, 59 Otoconia or Otoliths, 648 use of, 654 Ovaries, 717 enlargement of, at puberty, 733 Graafian vesicles in, 718 Ovisacs, ib. Ovoblasts, 720 Ovum, 719 action of seminal fluid on, 741 et seq. changes of, in ovary, 720 previous to formation of embryo, 741 Peduncles. Ovum-con t in tied. subsequent to cleavage, 742 et seq. in uterus, 742 et seq. cleaving of yelk, 743 connexion of with uterus, 733 discharge of from ovary, io. formation of, 720 germinal vesicle and spot of, 719 e! seq. impregnation of, 741 structure of, 719 unimpregnated, 719 Oviduct, or fallopian tube, 721 Oxalic acid, 425 Oxalic acid in urine, ib. Oxaluric acid, 423 Oxygen, 827 in blood, 93 consumed in breathing, 216 effects of on colour of blood, 94 proportion of to carbonic acid, 214 et seq. Oxyhaemoglobin, 94 spectrum, 96 P. Pacchionian bodies, 535 Pacinian bodies or corpuscles, 523 Pain, 621 Palmitin, 841 Pancreas, 309 development of, 795 functions of, 316 et seq. structure, 309 Pancreatic fluid, 310 Papilla foliata, 664 Papillae of the kidney, 403 of skin, distribution of, 390 epithelium of, 391 of teeth, 259 of tongue, 630 Paraglobulin, 99, 832 et seq. Par vagum. See Pneumogastric nerve. Paraglobulin, 89 Paralysis, cross, 565 Parapeptone, 289 Paraplegia, delivery in, 554 reflex movements in, ib. Parotid gland, saliva from, 270 nerves influencing secretion by, 273 Patellar reflex, 550 Paunch, 280 Pause in heart's action, 137 respiratory, 209 Pecten of birds, 791 Peduncles, of the cerebellum, 592 of the cerebrum, or Crura Cerebri, 565 INDEX. 871 Pelvis of the Kidney. Pelvis of the kidney, 408 Penis, corpus cavernosum of, 186, 727 development of, 804 erection of, explained, 186 structure, 727 Pepsin, 288 Pepsinogen, 285 Peptic cells, 284 Peptones, 289, 833 et seq. Pericardium, 104 Perichondrium, 45 Perilymph, or fluid of labyrinth of ear, 044 use of, 654 Perimysium, 451 Perineurium, 516 Periosteum, 51 Peristaltic movements of intestines, 336 of stomach, 292 Perivascular lymphatic sheaths, 130 Permanent teeth, 252. See Teeth. Perspiration, cutaneous, 399 insensible and sensible, ib. ordinary constituents of, ib. Pettenkofer's test, 321 Peyer's glands, 300 patches, ib. structure of, 300 Pfliiger's law, 489 Phakoscope, 676 Pharynx, 275 action of in swallowing, 229 influence of glosso-pharyngeal nerve on, 280 of pneumogastric nerve on, 280 Phenol, 312, 845 Phenomena of life, 1 Phosphates, 847 Phosphates in tissues, 847 Phosphorus in- the human body, ib. Pia mater, 535 Pigment, 22 Pigment cells, forms of, 22 movements of granules in, 22 Pineal gland, 447 Pinna of ear, 641 Pituitary body, 447 development, 768 Placenta, 758 et seq. foetal and maternal, ib. Plants, distinctions from animals, 11 Plasma of blood, 87 salts of, 88 Plasmine, 70 composition, 71 nature of, ib. Plethysmograph, 168 1'leura, 197 Pleuro-peritoneal cavity, 751 Plexus, 516 Pseudoscope. Pie x us-con t i n tied. terminal, 549 Pneumogastric nerve, 6IO distribution of, ib. influence on action of heart, 148 deglutition, 610 gastric digestion, 293 larynx, 610 lungs, 225 oesophagus, 610 pharynx, 6lO respiration, 225 secretion of gastric fluid, 293 sensation of hunger, 244 stomach, 293 mixed function of, 612 origin from medulla oblongata, 6icr Poisoned wounds, absorption from, 381 Polar cell, 741 Pons Varolii, its structure, 564 functions, ib. Portal blood, characters of, 102 canals, 317 circulation, 317 function of spleen with regard to,. . 440 veins, arrangement of, 317 et seq. Portio dura, of seventh nerve, 607 mollis, of seventh nerve, ib. Potassium, 847 sulphocyanate, 283 Pregnancy, absence of menstruation during, 732 et seq. corpus luteum of, 736 influence on blood, 101 Presbyopia, or long-sight, 683 Primitive groove, 747 streak, ib. Primitive nerve-sheath, or Schwann's sheath, 543 Pro-nucleus, female, 741 male, 741 Propionic acid, 844 Prosencephalon, 788 Prostate gland, 728 Proteids, 828 chemical properties, 829 et seq. physical properties, ib. tests for, ib. varieties of, ib. Proteolytic ferments, 307 Protoplasm, 2 chemical characters, 4 movement, 3 physical characters, 4 et sen. physiological characters, ib. reproduction, 8 transformation of, 11 Proto-vertebrae, 752 Psalterium, 281 Pseudoscope, 735 872 INDEX. Pseudo-stomach. Pseudo-stomach, 52 Ptyalin, 267 action of, 268 Puberty, changes at period of, 739 indicated by menstruation, ib. Pulmonary artery, valves of, 117 capillaries, 202 circulation, ib. Pulse, arterial, 156 cause of, ib. dicrotous, 160 frequency of, 143 influence of age on, ib. of food, posture, etc., ib. relation of to respiration, 144 sphygmographic tracings, 158 et seq. variations, 158 et seq. Purkinje's figures, 684 Pylorus, structure of, 285 Pyramidal portion of kidney, 428 Pyramidal tracts, 542 et seq. sounds, 209 Pyramids of medulla oblongata, 584 Q. Quantity of air breathed, 209 blood, 65 et seq. saliva, 268 R. Racemose glands, 377 Radiation of impressions, 530 Rami viscerales, 709 707 communicantes, 707 Rectum, 305 Recurrent sensibility, 544 Reflex actions, acquired, 533 augmentation, 535 classification, 532 compound, 533 conditions necessary to, 531 cutaneous, 549 in disease, 532 examples of, 631 excito-motor and sensori motor, 531 inhibition of, 556 irregular in disease, 532 after separation of cord from brain, , 579 laws of, 532 morbid, 552 muscle, 549 Retina. Reflex actions-continued. of medulla oblongata, 560 et seq. of spinal cord, 540 purposive in health, 531 relation between a stimulus and, 532 secondary, 533 simple, ib. varieties, ib. Refracting media of eye, 670 Refraction, laws of, ib. Regions of body. See Frontispiece. Registering apparatus, cardiograph, 141 kymograph, 165 sphygmograph, 157 Relations between secretions, 382 Remak's ganglia, 149 Reptiles, blood-corpuscles, 82 Requisites of diet, 249 Reserve air, 209 Residual air, «i. Respiration, 140 abdominal type, 206 changes of air, 213 of blood, 219 costal type, 207 force, 212 frequency, 211 influence of nervous system, 227 mechanism, 205 et seq. movements, 203 nitrogen in relation thereto, 213 organic matter excreted, 217 quantity of air changed, 216 relation to the pulse, 211 suspension and arrest, 230 et seq. types of, 206 Respiratory capacity of chest, 209 cells, 199 functions of skin, 400 movements, 203 axes of rotation, 204 et seq. of glottis, 209 influence on amount of carbonic acid, 214 on arterial tension, 228 rate, 211 relation to pulse rate, ib. size of animal, ib. relation to will, 224 et seq. various mechanism, 220 muscles, 205 et seq. daily work, 212 power of, ib. nerve-centre, 224, 562 rhythm, 208 sounds, 209 Restiform bodies, 558 et seq. Retiform or adenoid, or lymphoid tissue, 40 Reticulum, 280 Retina, (>63 INDEX. 873 Retina. Retina-continued. blind spot, 664 blood-vessels, 667 duration of impression on, 685 of after-sensations, ib. excitation of, 683 focal distance of, 703 fovea centralis, 664 . functions of, 677 et seq. image on, how formed distinctly, 677 inversion of, how corrected, 717 insensible at entrance of optic nerve, 664 layers, ib. in quadrupeds, 666 reciprocal action of parts of, 698 in relation to direction of vision, 693 to motion of bodies, 694 to single vision, 704 to size of field of vision, 690 structure of, 663 vessels, 667 visual purple, 688 Rheoscopic frog, 486 Rhinencephalon, 788 Ribs, axis of rotation, 204 et seq. Rigor mortis, 477 affects all classes of muscles, 479 phenomena and causes of, 477 Rima glottidis, movements of in respi- ration, 209 Ritter's tetanus, 490 Rods of Corti, 646 use of, 656 Rotatory movements, 567 Rouleaux, formation of in blood, 81 Ruminants, stomach of, 280 Rumination, ii. Running, mechanism of, 484 Rut or heat, 730 S. Saccharine principles of food, digestion of, 335 Saccharoses, 842 Sacculus, 648 Saliva, 266 composition, 261 process of secretion, 275 quantity, 267 rate of secretion, ib. uses, 268 Salivary glands, 263 development of, 795 influence of nervous system, 266, 562 mixed, 266 nerves, ib. Sensation. Salivary glands-continued. secretion, 266 structure, 263 true, 264 varieties, 264 Saponification, 313 Sarcode, 2. See Protoplasm. Sarcolemma, 451 Sarcosin, 839 Sarcous elements, 452 Scala media, 645 tympani, ib. . • vestibuli, ib. Scheiner's experiment, 676 Schiff's test, 420 Schwann's sheath, 513 Sclerotic, 660 Scurvy from want of vegetables, 243 Sebaceous glands, 393 their secretion, 398 Secreting glands, 376 aggregated, 377 convoluted tubular, ib. tubular or simple, ib. Secreting membranes, 372. See Mucous and Serous membranes. Secretion, 371 apparatus necessary for, 372 et seq. changes in gland-cells during, 381 sensory paths, 546 circumstances influencing, 381 discharge of, 381 influence of nervous system, 382 of urine, 430 process of physical and chemical, 379 serous, 372 synovial, 374 Segmentation of cells, 743 in chick, 744 ovum, ib. Semen, 731 composition of, ib. emission of, a reflex act, 553 filaments or spermatozoa, 737 tubes, 725 Semicircular canals of ear, 644 development of, 792 et seq. use of, 654 Semilunar valves, 117 functions of, 134 Semilunes of Heidenhain, 265 Sensation, 695 colour, 695 common, 616 conditions necessary to, 616 excited by mind, ib. by internal causes, ib. of motion, 619 nerves of, 520 et seq. of pain, 621 of pressure, 624 special, 617 nerves of, 520 874 INDEX. Sensation. Sen sation-eon tinned. stimuli of, 521 of special, ib. subjective, 618. See also Special Senses, 648 tactile, 620 temperature, 621 tickling, 621 touch, 620 transference and radiation of, 524 et seq. of weight, 625 Senses, special, 616 organs o*', development of, 790 Sensory centres in cerebral cortex, 597 Sensory impressions, conduction of, 54 by spinal cord, 546 in brain, 590 nerves, 520 Septum between auricles, formation of, 777 between ventricles, formation of, ib. lucidum, 572 Serine, 89, 830 Serous fluid, 373 Serous membranes, 372 arrangement of, ib. communication of lymphatics with, epithelium, 23 fluid secreted by, 373 functions, ib. lining joints, etc., 372 et seq. visceral cavities, ib. structure of, 372 Serum, of blood, 88 albumin, 831 separation of, 88 Seventh cerebral nerve, 607 Sex, intluence on blood, 100 influence on production of carbonic acid, 214 relation of to respiratory movements, 206 Sexual organs and functions in the female, 717 in the male, 720 Sighing, mechanism of, 221 Sight, 660. See Vision. Silica, parts in which found, 847 Silicon, ib. Singing, mechanism of, 223 et seq. Single vision, conditions 01, 704 Sinus pocularis, 803 rhomboidalis, 79b urogenitalis, ib. Sinuses of dura mater, 185 et seq. of Valsalva, 118 Sixth cerebral nerve, 606 Size of field of vision, 690 Skatol, 312 Speaking. Skeleton. See Frontispiece. Skin, 388 absorption by, 401 of metallic substances, ib. of water, ib. cutis vera of, 390 epidermis of, 388 evaporation from, 399 excretion by, 399 exhalation of carbonic acid from, 400 of watery vapour from, 400 functions of, 396 respiratory, 480 glands, 391 papillae of, 390 perspiration of, 399 rete mucosum of, 388 sebaceous glands of, 393 structure of, 388 sudoriferous glands of, 391 Sleep, 580 Smell, sense of, 635 conditions of, ib. delicacy, 639 different kinds of odours, 638 impaired by lesion of facial nerve, 608 impaired by lesion of fifth nerve, 606 internal excitants of, 640 limited to olfactory region, 636 structure of organ of, 636 subjective sensations, 640 varies in different animals, 639 Sneezing, centre, 562 mechanism of, 222 Sniffing, mechanism of, 223 smell, aided by, 636 Sobbing, 223 Sodium, 847 in human body, ib. sulphindigotate, 431 Solitary glands. See Peyer's. Soluble ferments, 852 Somatopleure, 751 Somnambulism, 582 Sonorous vibrations, how communicated in ear, 648 et seq. in air and in water, ib. See Sound. Soprano voice, ?O4 Sound, binaural sensations, 658 conduction of by ear, 648 heart, 138 movements and sensations produced by, 659 perception, of direction of, 657 of distance of, 658 permanence of sensation of, 658 production of, 657 subjective, 659 Source of water, 846 Spasms, reflex acts, 563 Speaking, 507 INDEX 875 Speaking. Speaking, mechanism of, 223, 507 Special senses, 617 Spectrum-analysis of blood, 95 Speech, 507 function of tongue in, 510 Spermatozoa, development of, 726 form and structure of, 737 function of, 741 motion of, 737 Spherical aberration, 680 correction of, 681 Spheroidal epithelium, 26 Sphincter ani. See Defalcation. Sphygmograph, 133 tracings, 158 et seq. Spinal accessory nerve, 614 Spinal cord, 535 automatism, 333 canal of, 536 centres in, 580 a collection of nervous centres, 552 columns of, 537 commissures of, ib. conduction of impressions by, 544 et seq. course of fibres in, 542 decussation of sensory impressions in, 546 development of, 786 effect of injuries of, on conduction of impressions, 548 et seq. fissures and furrows of, 537 functions of, 544 of columns, 546 influence on lymph-hearts, 554 on sphincter ani, 333 on tone, 355 morbid irritability of, 552 nerves of, 540 reflex action of, 549 in disease, 580 inhibition of, 550 special centres in, 532 structure of, 536 et seq. transference, 548 weight, 576 relative, ib. white matter, 538 grey matter, 539 Spinal nerves, 540 origin of, 542 et seq. physiology of, 543 Spirometer, 210 Splanchnic nerves, 210, 709 Splanchnopleure, 765 Spleen, 430 functions, 439 hilus of, 430 influence of nervous system, 440 Malpighian corpuscles of, 438 pulp, 436 et seq. stroma of, ib. structure of, ib. Sulphates. Spleen, trabeculae of, 436 et seq. Splenic vein, blood of, 102 Spot, germinal, 719 Squamous epithelium, 23 Stammering, 510 Stannius' experiments, 149 Stapedius muscle, 674 function of, 685 Stapes, 641 Starch, 269, 842 digestion of in mouth, 267 Starvation, 245 appearances after death, 246 effect on temperature, ib. loss of weight in, ib. period of death in, ib. symptoms, ib. Steapsin, 313 Stearic acid, 842 Stearin, 824 Stercorin, 326 allied to cholesterin, 326 Stereoscope, 706 Stimuli, protoplasmic, 6 St. Martin, Alexis, case of, 287 Stomach, 280 blood-vessels, 285 development, 793 et seq. digestion in, 286 circumstances favouring, 290 products of, 289 digestion after death, 294 glands, 284 lymphatics, 285 movements, 291 influence of nervous system, 293 mucous membrane, 282 muscular coat, 282 nerves, 243 ruminant, 280 secretion of, 286. See Gastric fluid. structure, 281 temperature, 287 Stomata, 25, 346 Stratum intermedium (Hannover), 261 Striated muscle, 451 Stroma fibrin, 69 Stromiihr, 181 Structural basis of human body, 16 Submaxillary gland, 271 Succus entericus, 332 functions of, ib. Sucking, mechanism of, 223 centre, 562 Sudoriferous glands, 391 their distribution, 393 number of, ib. their secretion, 399 Suffocation, 233 et seq. Sugar. See Glucose. tests, 269 Sulphates, 847 876 INDEX. Sulphates. S ulphates-continued. in tissues, 847 in urine, 423 Sulphuretted hydrogen, 827 Suprarenal capsules, 444 development of, 800 disease of, relation to discoloration of skin, 446 structure, 444 Sun, a source of energy, Chap. XXIV. Swallowing, 279 centre, 562 nerves engaged, 280 Sweat, 399 Sympathetic nervous system, 528, 707 conduction of impressions by, 712 distribution, 528 divisions of, ib. fibres, differences of from cerebro- spinal fibres, 513 functions, 710 et seq. ganglia of, 711 action of, 711 et seq. co-ordination of movements by, 712 structure, 707 in substance of organs, ib. influence on blood-vessels, 168 et seq. heart, 150 involuntary motion, 710 et seq. salivary glands, 273 et seq. secretion, 710 structure of, 707 Synovial fluid, secretion of, 374 membranes, ib. Syntonin, 289 Systemic circulation. See Circula- tion. vessels, ib. Systole of heart, 131 T. Table of diet, 249 Taste, 658 after-tastes, 666 centre, 598 conditions for perception of, 627 connection with smell, 634 impaired by injury of facial nerve, 608 of fifth nerve, 606 nerves of, 609 seat of, 628 subjective sensations, 635 varieties, 634 Taste-goblets, 632 Taurin, 837 Tissue. Taurocholic acid, 321 Teeth, 252 development, 258 eruption, times of, 253 structure of, 254 et seq. temporary and permanent, 252 et seq. Tegmentum, 566 Temperament, influence on blood, 108 Temperature, 361 average of body, ib. changes of, effects of, 366 et seq. circumstances modifying, 386 of cold-blooded and warm-blooded animals, 363 in disease, 362 loss of, 365 maintenance of, 365 of Mammalia, Birds, etc., 363 of paralysed parts, 369 regulation of, 365 of respired air, 214 sensation of variation of, 656. See Heat. Temporo-inaxillary fibro-cartilage, 262 Tendon-reflex, 550 Tendons, structure of, 36 cells of, 37 Tenor voice, 505 Tension, arterial, 163 Tension of gases in lungs, 218 Tensor tympani muscle, 653 office of, ib. Tesselated epithelium, 22 Testicle, 724 development, 801 descent of, ib. structure of, 724 ctseq. Tetanus, 470 Thalamencephalon, 788 Thalami optici, function of, 591 Thermogenic nerves and nerve-centres, 1 hirst, 244 Thoracic duct, 342 contents, 334 Thymus gland, 441 function of, 443 structure, 441 Thyro-arytenoid muscles, 499 Thyroid cartilage, structure and con- nections of, 498 Thyroid-gland, 443 function of, ib. structure, ib. Timbre of voice, 50 Tissue, adipose, 42 areolar, cellular, or connective, 38 elastic, 37 fatty, 42 fibrous, 36 gelatinous, 39 retiform, 40 INDEX. 877 Tissues. T issues, connective, 33 elementary structure of, 34 et seq. erectile, 188 Tone of blood-vessels, 169 of muscles, 555 of voice, 526 Tongue, 629 action of in deglutition, 279 in sucking, 223 action of in speech, 510 epithelium of, 630 influence of facial nerve on, 608 motor nerve of, 615 an organ of touch, 633 papillae of, 630 parts most sensitive to taste, 632 structure of, 629 Tonic centres, 564 Tonsils, 226 Tooth-ache, radiation of, sensation in, 620 after sensation, 629 conditions for perfection of, 621 connection of with muscular sense, 624 co-operation of mind with, 627 hand an organ of, 621 illusions, 624 modifications of, 621 a modification of common sensation, 620 special organs, ib. subjective sensations, 627 the tongue an organ of, 621 various degrees of in different parts, 623 Touch-corpuscles, 524 Trachea, 193 Tracts in the spinal cord, 542 Tradescantial Virginica, movements in cells of, 5 Tragus, 672 Transference of impressions, 529 Traube-Hering's curves, 258 Tricuspid valve, 115 safety-valve action of, 134 Trigeminal or fifth nerve, 601 effects of injury of, 603 Trophic nerves, 706 Trypsin, 312 Trypsinogen, 311 Tubercle of Lower, ill Tubes, Fallopian, 721. See Fallopian tubes. Tubular glands, 377 Tubules, 20 Tubuli seminiferi, 725 uniferi, 404 et seq. Tunica albuginea of testicle, 724 Tympanum or middle ear, 641 development of, 805 Urine. Tympanum or middle ear-continued. functions of, 649 membrane of, 651 structure of, ib. use of air in, 650 Types of respiration, 206 Tyrosin, 312, 837 U. Ulceration of parts attending injuries of nerves, 382 Ulnar nerve, effects of compression of, 521 Umbilical arteries, 785 cord, 765 vesicle, 755 Unconscious cerebration, 581 Unorganised ferments, 840 Unstriped muscular fibre, 449 development, 455 distribution, 449 structure, 450 Urachus, 758 Urate of ammonium, 419 of sodium, ib. Urea, 415, 838 apparatus for estimating quantity, 4*7 chemical composition of, 415 identical with cyanate of ammonium, id., 417 properties, 415 quantity, 417 in relation to muscular exertion, 433 sources, 432 Ureter, 410 Uric acid, 418, 828 condition in which it exists in urine, 4T9 . . forms in which it is deposited, 419 proportionate quantity of, 410 source of, 434 tests, 420 variations in quantity, 418 Urina sanguinis, potfls, et cibi, 414 Urinary bladder, 410 development, 815 nerves, 411 structure, 410 Urinary ferments, 413 Urine, 412 abnormal, 415 analysis of, 412 chemical composition, ib, colouring matter of, 420 cystin in, 425 decomposition by mucus, 413 effect of blood-pressure on, 427 expulsion, 435 878 INDEX. Urine. U rine-continued. extractives, 422, 434 flow of into bladder, 435 gases, 425 hippuric acid in, 420 mucus in, 422 oxalic acid in, 425 physical characters, 412 pigments, 420 quantity 01 chief constituents, 414 reaction of, 420 in different animals, 421 made alkaline by diet, 421, 423 saline matter, 423 secretion, 425 effects of posture, etc., on, 435 rate of, ib. solids, 415 variations of, 414 specific gravity of, ib. variations of, ib. urates, 419 urea, 415 uric acid in, 418 variations of specific gravity, 414 of water, 417 Urobilin, 322 Urochrome, 420, 840 Uroerythrin, 421 Uromelanin, 421 Uses of blood, 106 Uterus, 722 change of mucous membrane of, 760 et seq. development of in pregnancy, ib. follicular glands of, ib. masculinus, 803 structure, 722 Utriculus of labyrinth, 648 Uvula in relation to voice, 507 V. Vagina, structure of, 723 Vagus nerve. See Pneumogastric. Valve, ileo-csecal, structure of, 303 Valves of heart, 117 action of, 132 bicuspid or mitral, 117 semilunar, 117 tricuspid, 115 of lymphatic vessels, 347 of veins, 128 Valvula; conniventes, 307 Vas deferens, 724 Vasa eff'erentia of testicle, 724 recta of testicle, 724 vasorum, 122 Vascular area, 755 Vascular glands, 436 Vesicula Gebminativa. Vascular glands-continued. in relation to blood, 448 several offices of, ib. Vascular system, development of, 772 Vaso-constrictor nerves, 172 Vaso-dilator nerves, 172 Vaso-motor influence on blood-pressure, 168 et seq. Vaso-motor nerves, 168 effect of section, 168 et seq. influence upon blood-pressure, 169 Vaso-motor nerve-centres, ib., 563 reflection by, 170 Vegetables and animals, distinctions between, 11 Veins, 127 blood-pressure in, 180 circulation in, 178 et seq. rate of, 182 cardinal, 781 collateral circulation in, 129 cranium, 186 development, 781 distribution, 127 effects of respiration on, 228 influence of expiration, 230 inspiration, 228 influence of gravitation in, 180 parietal system of, 782 et seq. pressure in, 180 rhythmical action in, 179 structure of, 128 systemic, 129 umbilical, 785 valves of, 128 velocity of blood in, 183 visceral system of, 781 et seq. Velocity of blood in arteries, 181 in capillaries, 182 in veins, ib. of circulation, 180 of nervous force, 520 conditions modifying, 521 Vena portae, 314 Venae hepaticae advehentes, 781 revenentes, ib. Ventilation, 229 Ventricles of heart, 112 capacity of, 132, 144 contraction of, 132 dilatation of, ib., 144 force of, ib. of larynx, office of, 507 lateral, 571 Ventriloquism, 5 IO Vermicular movement of intestines, TT 336 V ermiform process, 305 Vertebrae, development of, 766 Vertebral plate, 751 Vesicle, germinal, 719 Graafian, 718 Vesicula germinativa, 719 INDEX. 879 Vesiculje Seminales. Vesiculae seminales, 727 functions of, 739 structure, 727 Vestibule of the ear, 644 Vibrations, conveyance of to auditory nerve, 649 etseq. Vidian nerve, 607 Villi in chorion, 759 in placenta, 764 Villi of intestines, 302 action in digestion, 304 Visceral arches, development of, 769 connection with cranial nerves, 770 laminae or plates, 734 Visceral plates, 754 Viscero-inhibitory nerves, 711 motor, ib. Vision, 660 angle of, 691 at different distances, adaptation of eye to, 673 et seq. centre, 597 corpora quadrigemina, the principal nerve-centres of, 568 correction of aberration, 681 et seq. of inversion of image, 688 defects of, 678 et seq. distinctness of, how secured, 677 et seq. duration of sensation in, 684 estimation of the form of objects, of their direction, 693 of their motion, 694 of their size, 692 field of, size of, 690 focal distance of, 677 impaired by lesion of fifth nerve, 606 influence of attention on, 694 modified by different parts of the retina, 726 purple, 688 single, with two eyes, 701 Visual direction, 693 Vital or respiratory capacity of chest, 209 Vital capillary force, 177 Vitellin, 833 Vitelline duct, 794 membrane, 743 spheres, ib. Vitiated air, effects of, 227 Vitreous humour, 671 Vocal cords, 497 et seq. action of in respiratory actions, 209 et seq. approximation of, effect on height of note, 507 longer in males than in females, 505 position of, how modified, 501 vibrations of, cause voice, 497 Voice, 504 of boys, 505 Work of Heart. Voice-continued. compass of, 506 conditions on which strength depends, 5°7 human, produced by vibration of vocal cords, 505 in eunuchs, 505 influence of age on, ib. of arches of palate and uvula, 507 of epiglottis, 503 of sex, 504 of ventricles of larynx, 507 of vocal cords, 304 in male and female, ib. cause of different pitch, ib. modulations of, ib. natural and falsetto, 506 peculiar characters of, 505 varieties of, 504 et seq. Vomiting, 295 action of stomach in, 296 centre, 562 nerve-actions in, 297 voluntary and acquired, Vowels and consonants, 508 Vulvo-vaginal or Duvemey's glands, 723 W. Walking, 481 Water, 840 absorbed by skin, 401 by stomach, 289 amount, in blood, variations in, IOO, 107 exhaled from lungs, 46 from skin, 399 forms large part of human body, 846 influence of on decomposition, 828 in urine, excretion of, 415 variations in, 414 loss of from body, 846 uses, ib. quantity in various tissues, ib. source, ib. vapour of in atmosphere, 216 Wave of blood causing the pulse, 155 velocity of, 156 White corpuscles, 156. See Blood-cor- puscles, white; and Lymph-Cor- puscles. White fioro-cartilage, 48 fibrous tissue, 36 Willis, circle of, 184 Wolffian bodies, 797 et seq. Wooldridge, 78 Work of heart, 145 880 INDEX. Xanthin. Xanthin, 423, 839 Xantho-proteic reaction, 829 , Y. Yawning, 223 Yelk, or vitellus, 719 changes of, in Fallopian tube, 742 cleaving of, 743 constriction of, by ventral laminae, 756 Zona Pellucida. Yelk-sac, 755 et seq. Yellow elastic fibre, 37 fibro-cartilage, 47 spot of Sommering, 664 Young-Helmholtz theory, 695 Z. Zimmermann, corpuscles of, 442 Zona pellucida, 743 THE END. BRADBURY, AGNEW <fc CO., PRINTERS, WHITEFRIARS. CATALOGUE No. 7. OCTOBER, 1888. A CATALOGUE OF Books for Students. PAGES PAGES New Series of Manuals, 2,3,4,$ Obstetrics. . . . .10 Anatomy, . . . . 6 Pathology and Histology, . 11 Chemistry, . . . .6 Pharmacy, , . . .13 Children's Diseases, . . 7 Physical Diagnosis, . .11 Dentistry, . . . ,8 Physiology, . . . .11 Dictionaries, . . .8 Practice of Medicine, . . 12 Eye Diseases, . . .8 Prescription Books, . . 12 Electricity, . . . . 9 ? Quiz-Compends? . . 16 Gynsecology, . . .10 Skin Diseases, . . .13 Hygiene, .... 9 Surgery, . . . .13 Materia Medica, . . .9 Therapeutics, . . .9 Medical Briefs, . . .15 Throat, . . . .14 Medical Jurisprudence, . 9 Urine and Urinary Organs, 14 Miscellaneous, . . .10 Venereal Diseases, . . 14 CONTENTS. PUBLISHED BY P. BLAKISTON, SON & CO., Medical Booksellers, Importers and Publishers. LARGE STOCK OF ALL STUDENTS' BOOKS, AT THE LOWEST PRICES. 1012 Walnut Street, Philadelphia. For sale by all Booksellers, or any book will be sent by mail, postpaid, upon receipt of price. Catalogues of books on all branches of Medicine, Dentistry, Pharmacy, etc., supplied upon application. " An excellent Series of Manuals."-Archives of Gynaecology. A NEW SERIES OF STUDENTS' MANUALS On the various Branches of Medicine and Surgery. Can be used by Students of any College. Price of each, Handsome Cloth, $3.00. Full Leather, $3.50. The object of this series is to furnish good manuals for the medical student, that will strike the medium between the compend on one hand and the prolix text- book on the other-to contain all that is necessary for the student, without embarrassing him with a flood of theory and involved statements. They have been pre- pared by well-known men, who have had large experience as teachers and writers, and who are, therefore, well informed as to the needs of the student. Their mechanical execution is of the best-good type and paper, handsomely illustrated whenever illustrations are of use, and strongly bound in uniform style. Each book is sold separately at a remarkably low price, and the immediate success of several of the volumes shows that the series has met with popular favor. No. 1. SURGERY. 236 Illustrations. A Manual of the Practice of Surgery. By Wm. J. Walsham, m.d., Asst. Surg. to, and Demonstrator of Surg. in, St. Bartholomew's Hospital, London, etc. 228 Illustrations. Presents the introductory facts in Surgery in clear, precise language, and contains all the latest advances in Pathology, Antiseptics, etc. " It aims to occupy a position midway between the pretentious manual and the cumbersome System of Surgery, and its general character may be summed up in one word-practical."-The Medi- cal Bulletin. " Walsham, beside being an excellent surgeon, is a teacher in its best sense, and having had very great experience in the preparation of candidates for examination, and their subsequent professional career, may be relied upon to have carried out his work successfully. Without following out in detail his arrange- ment, which is excellent, we can at once say that his book is an embodiment of modern ideas neatly strung together, with an amount of careful organization well suited to the candidate, and, indeed, to the practitioner."-British Medical Journal. Price of each Book, Cloth, $3.00. Leather, $3.50. THE NEW SERIES OF MANUALS. 3 No. 2. DISEASES OF WOMEN. 130 Ulus. The Diseases of Women. By Dr. F. Winckel, Professor of Gynsecology and Director of the Royal University Clinic for Women, in Munich. Translated from the German by Dr. J. H. Williamson, Resident Physician Allegheny General Hospital, Allegheny, Penn'a, under the supervision of, and with an intro- duction by, Theophilus Parvin, m.d., Professor of Obstetrics and Diseases of Women and Children in Jefferson Medical College. Illustrated by 132 fine Engravings on Wood, most of which are new. " The book will be a valuable one to physicians, and a safe and satisfactory one to put into the hands of students. It is issued in a neat and attractive form, and at a very reasonable price."-Boston Medical and Surgl. Journal. No. 3. OBSTETRICS. 227 Illustrations. A Manual of Midwifery. By Alfred Lewis Galabin, M.A., M.D., Obstetric Physician and Lecturer on Mid- wifery and the Diseases of Women at Guy's Hospital, London; Examiner in Midwifery to the Conjoint Examining Board of England, etc. With 227 Ulus. " This manual is one we can strongly recommend to all who desire to study the science as well as the practice of midwifery. Students at the present time not only are expected to know the principles of diagnosis, and the treatment of the various emergen- cies and complications that occur in the practice of midwifery, but find that the tendency is for examiners to ask more questions relating to the science of the subject than was the custom a few years ago. * * * The general standard of the manual is high ; and wherever the science and practice of midwifery are well taught, it will be regarded as one of the most important text-books on the subject."-London Practitioner. No. 4. PHYSIOLOGY. Third Edition. 331 ILLUSTRATIONS AND A GLOSSARY. A Manual of Physiology. By Gerald F. Yeo, m.d., f.r.c.s., Professor of Physiology in King's College, London. 321 Illustrations and a Glossary of Terms. Third American from second English Edition, revised and improved. 758 Pages. This volume was specially prepared to furnish students with a new text-book of Physiology, elementary so far as to avoid theories which ha not borne the test of time and such details of methods as are unnecessary for students in our medical colleges. While endeavoring to save the student from doubtful and erroneous doctrines, great care has been taken not to omit any important facts that are necessary to an acquirement of a clear idea of the Price of each Book, Cloth, $3.00. Leather, $3.50. 4 THE NEW SERIES OF MANUALS. principles of Physiology. Such subjects as are useful in the practice of medicine and surgery are treated more fully than those which are essential only to an abstract physiological knowledge. "The brief examination I have given it was so favorable that I placed it in the list of text-books recommended in the circular of the University Medical College.''-Prof. Lewis A. Stimson, m.d., 57 East 33d Street, New York. No. 5. POTTER'S MATERIA MEDIC A, PHARMACY AND THERAPEUTICS. OVER 600 PRESCRIPTIONS, FORMULA, ETC. A Handbook of Materia Medica, Pharmacy and Therapeutics-including the Physiological Action of Drugs, Special Therapeutics of Diseases, Official and Extemporaneous Pharmacy, etc., etc. By Sam'l O. L. Potter, m.a., m.d., Professor of the Practice of Medicine in Cooper Medical College, San Francisco, Late A. A. Surg., U. S. A., Author of the " Quiz- Compends" of Anatomy and Materia Medica, etc. This book contains many unique features of style and arrange- ment ; no time or trouble has been spared to make it most complete and yet concise in all its parts. It contains many prescriptions of practical worth, a great mass of facts conveniently and concisely put together, also many tables, dose lists, diagnostic hints, etc., all rendering it the most complete manual ever published. " Dr. Potter's handbook will find a place, and a very important one, in our colleges and the libraries of our practitioners.''-N. Y. Medical Journal. No. 6. DISEASES OF CHILDREN. A Manual. By J. F. Goodhart, m.d., Phys, to the Evelina Hospital for Children; Asst. Phys, to Guy's Hospital, London. American Edition. Edited by Louis Starr, m.d., Clinical Prof, of Dis. of Children in the Hospital of the Univ, of Pennsylvania, and Physician to the Children's Hospital, Phila. Containing many new Prescriptions, a list of over 50 Formulae, conforming to the U. S. Pharmacopoeia, and Directions for making Artificial Human Milk, for the Artificial Digestion of Milk, etc. "As it is said of some men, so it might be said of some books, that they are ' born to greatness.' This new volume has, we believe, a mission, particularly in the hands of the younger members of the profession. In these days of prolixity in medical literature, it is refreshing to meet with an author who knows both what to say and when he has said it. The work of Dr. Goodhart Price of each Book, Cloth, $3.00. Leather, $3.50. THE NEW SERIES OF MANUALS. 5 (admirably conformed, by Dr. Starr, to meet American require- ments) is the nearest approach to clinical teaching without the actual presence of clinical material that we have yet seen."-New York Medical Record. No. 7. PRACTICAL THERAPEUTICS. FOURTH EDITION, WITH AN INDEX OF DISEASES. Practical Therapeutics, considered with reference to Articles of the Materia Medica. Containing, also, an Index of Diseases, with a list of the Medicines applicable as Remedies. By Edward John Waring, m.d., f.r.c.p. Fourth Edition. Rewritten and Re- vised. By Dudley W. Buxton, m.d., Asst, to the Prof, of Medicine at University College Hospital. " We wish a copy could be put in the hands of every Student or Practitioner in the country. In our estimation, it is the best book of the kind ever written."-N. Y. Medical Journal. No. 8. MEDICAL JURISPRUDENCE AND TOXICOLOGY. By John J. Reese, m.d., Professor of Medical Jurispru- dence and Toxicology in the University of Pennsyl- vania; Vice-President of the Medical Jurisprudence Society of Phila.; Physician to St. Joseph's Hospital. "This admirable text-book."-Amer. Jour, of Med. Sciences. " We lay this volume aside, after a careful perusal of its pages, with the profound impression that it should be in the hands of every doctor and lawyer, it fully meets the wants of all students He has succeeded in admirably condensing into a handy volume all the essential points."-Cincinnati Lancet and Clinic. No. 9. ORGANIC CHEMISTRY. Or the Chemistry of the Carbon Compounds. By Prof. Victor von Richter, University of Breslau. Au- thorized translation, from the Fourth German Edition. By Edgar F. Smith, m.a., ph.d. ; Prof, of Chemistry in Wittenberg College, Springfield, Ohio ; formerly in die Laboratories of the University of Pennsylvania; Member of the Chem. Socs. of Berlin and Paris. " I must say that this standard treatise is here presented in a remarkably compendious shape."-J. IY. Holland, m.d., Professor of Chemistry, Jefferson Medical College, Philadelphia. " This work brings the whole matter, in simple, plain language, to the student in a clear, comprehensive manner. The whole method of the work is one that is more readily grasped than that of jjlder and more famed text-books, and we look forward to the time when, to a great extent, this work will supersede others, on the score of its better adaptation to the wants of both teacher and student."-Pharmaceutical Record. Price of each Book, Cloth, $3-00. Leather, $3.50. 6 STUDENTS' TEXT-BOOKS AND MANUALS. ANATOMY. Holden's Anatomy. A manual of Dissection of the Human Body. Fifth Edition. Enlarged, with Marginal References and over 200 Illustrations. Octavo. Cloth, 5.00; Leather, 6.00 Bound in Oilcloth, for the Dissecting Room, $4.50. " No student of Anatomy can take up this book without being pleased and instructed. Its Diagrams are original, striking and suggestive, giving more at a glance than pages of text description. * * * The text matches the illustrations in directness of prac- tical application and clearness of detail."-New York Medical Record. Holden's Human Osteology. Comprising a Description of the Bones, with Colored Delineations of the Attachments of the Muscles. The General and Microscopical Structure of Bone and its Development. With Lithographic Plates and Numerous Illus- trations. Seventh Edition. 8vo. Cloth, 6.00 Heath's Practical Anatomy. Sixth London Edition. 24 Col- ored Plates, and nearly 300 other Illustrations. Cloth, 5.00 Potter's Compend of Anatomy. Fourth Edition. 117 Illus- trations. Cloth, 1.00; Interleaved for Notes, 1.25 CHEMISTRY. 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Cloth, 3.00; Leather, 3.50 fS~See pages 2 to 5 for list of Students' Manuals. STUDENTS' TEXT-BOOKS AND MANUALS. 7 Chemistry ;-Continued. Trimble. Practical and Analytical Chemistry. A Course in Chemical Analysis, by Henry Trimble, Prof, of Analytical Chem- istry in the Phila. College of Pharmacy. Illustrated. Second Edition. 8vo. Cloth, 1.50 Tidy. Modern Chemistry, 2d Ed. Cloth, 5.50 Leffmann's Compend of Chemistry. Inorganic and Organic. Cloth, 1.00; Interleaved for Notes, 1.25 Muter. Practical and Analytical Chemistry. Second Edi- tion, Revised and Illustrated. Cloth, 2.00 Holland. The Urine, Chemical and Microscopical. For Laboratory Use. Illustrated. Cloth, .50 Van Nuys. Urine Analysis. Illus. Cloth, 2.00 Wolff's Applied Medical Chemistry. By Lawrence Wolff, m.d., Demonstrator of Chemistry in Jefferson Medical College, Philadelphia. Cloth, 1.50 CHILDREN. Goodhart and Starr. The Diseases of Children. A Manual for Students and Physicians. By J. F. Goodhart, m.d., Physi- cian to the Evelina Hospital for Children; Assistant Physician to Guy's Hospital, London. American Edition, Revised and Edited by Louis Starr, m.d., Clinical Professor of Diseases of Children in the Hospital of the University of Pennsylvania; Physician to the Children's Hospital, Philadelphia. Containing many new Prescriptions, a List of over 50 Formulae, conforming to the U. S. Pharmacopoeia, and Directions for making Arti- ficial Human Milk, for the Artificial Digestion of Milk, etc. Cloth, 3.00; Leather, 3.50 Day. On Children. A Practical and Systematic Treatise. Second Edition. 8vo. 752 pages. Cloth, 3.00; Leather, 4.00 Meigs and Pepper. The Diseases of Children. Seventh Edition. 8vo. Cloth, 5.00; Leather, 6.00 Starr. Diseases of the Digestive Organs in Infancy and Childhood. With chapters on the Investigation of Disease, and on the General Management of Children. Illus. Cloth, 2.50 Keating and Edwards. 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Cloth, 3.00; Leather, 3.50 Roberts' Compend of Materia Medica and Pharmacy. By the author of " Roberts' Practice." Cloth, 2.00 Starr, Walker and Powell. Synopsis of Physiological Action of Medicines,based upon Prof. H. C. Wood's " Materia Medica and Therapeutics." 3d Ed. Enlarged. Cloth, .75 Waring. Therapeutics. With an Index of Diseases and an Index of Remedies. A Practical Manual. Fourth Edition. Revised and Enlarged. Cloth, 3.00; Leather, 3.50 MEDICAL JURISPRUDENCE. Reese. A Text-book of Medical Jurisprudence and Toxi- cology. By John J. Reese, m.d., Professor of Medical Juris- prudence and Toxicology in the Medical and Law Departments of the University of Pennsylvania; Vice-President of the Med- ical Jurisprudence Society of Philadelphia; Physician to St. &S~See page ib for list of f Quiz-Contpends ? 10 STUDENTS' TEXT-BOOKS AND MANUALS. Medical Jurisprudence :- Continued. Joseph's Hospital; Corresponding Member of The New York Medico-legal Society. 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The Practice of Medicine and Surgery, as applied to the Diseases and Accidents Incident to Women. By W. H. Byford, a.m., m.d., Professor of Gynaecology in Rush Medical College and of Obstetrics in the Woman's Med- ical College, etc., and Henry T. Byford, m.d., Surgeon to the Woman's Hospital of Chicago; Gynaecologist to St. Luke's Hospital, etc. Fourth Edition. Revised, Rewritten and En- larged. With 306 Illustrations, over 100 of which are original. Octavo. 832 pages. Cloth, 5.00; Leather, 6.00 Parvin's Winckel's Diseases of Women. Edited by Prof. Theophilus Parvin, Jefferson Medical College, Philadelphia. 117 Illustrations. See page 3. Cloth, 3.00; Leather, 3.50 Morris. Compend of Gynaecology. Illustrated. In Press. SEf' See pages 2 to 5for list of New Manuals. STUDENTS' TEXT-BOOKS AND MANUALS 11 Obstetrics and Gynaecology :-Continued. Landis' Compend of Obstetrics. Illustrated. 3d edition. Cloth, 1.00; Interleaved for Notes, 1.25 Galabin's Midwifery. A New Manual for Students. By A. Lewis Galabin, m.d., f.r.c.p., Obstetric Physician to Guy's Hospital, London, and Professor of Obstetrics in the same Insti- tution. 227 Illustrations. Cloth, 3.00; Leather, 3.50 Glisan's Modern Midwifery. 2d Edition. Cloth, 3.00 Rigby's Obstetric Memoranda. By Alfred Meadows, m.d. 4th Edition. Cloth, .50 Meadows' Manual of Midwifery. Including the Signs and Symptoms of Pregnancy, Obstetric Operations, Diseases of the Puerperal State, etc. 145 Illustrations. 494 pages. Cloth, 2.00 Swayne's Obstetric Aphorisms. For the use of Students commencing Midwifery Practice. 8th Ed. tamo, Cloth, 1.25 PATHOLOGY AND HISTOLOGY. Bowlby. Surgical Pathology and Morbid Anatomy, for Students. 135 Illustrations, izmo. Cloth, 2.00 Rindfleisch's General Pathology. By Tyson. For Students and Physicians. By Prof. Edward Rindfleisch, of Wurzburg. Translated by Wm. H. Mercur, m.d., of Pittsburgh, Pa. Edited by James Tyson, m.d., Professor of Pathology and Morbid Anatomy in the University of Pennsylvania, izmo. Cloth, 2.00 Gilliam's Essentials of Pathology. A Handbook for Students. 47 Illustrations. I2mo. Cloth, 2.00 *** The object of this book is to unfold to the beginner the funda- mentals of pathology in a plain, practical way, and by bringing them within easy comprehension to increase his interest in the study of the subject. Gibbes' Practical Histology and Pathology. Third Edition. Enlarged, izrno. Cloth, 1.75 PHYSICAL DIAGNOSIS. Bruen's Physical Diagnosis of the Heart and Lungs. By Dr. Edward T. Bruen, Assistant Professor of Clinical Medicine in the University of Pennsylvania. Second Edition, revised. With new Illustrations. i2mo. Cloth, 1.50 ***The subject is treated in a plain, practical manner, avoiding questions of historical or theoretical interest, and without laying special claim to originality of matter, the author has made a book that presents to the student the somewhat difficult points of Physi- cal Diagnosis clearly and distinctly . PHYSIOLOGY. Yeo's Physiology. Third Edition. The most Popular Stu- dents' Book. By Gerald F. Yeo, m.d., f.r.c.s., Professor of See page ib for list of ? Quiz- Compends f 12 STUDENTS' TEXT-BOOKS AND MANUALS. Physiology :- Continued. Physiology in King's College, London. Small Octavo. 758 pages. 321 carefully printed Illustrations. With a Full Glossary and Index. See Page3. Cloth, 3.00; Leather, 3.50 Brubaker's Compend of Physiology. Illustrated. Fourth Edition. Cloth, 1.00; Interleaved for Notes, 1.25 Stirling. Practical Physiology, including Chemical and Ex- perimental Physiology. 142 Illustrations. Cloth, 2.25 Kirke's Physiology. nthEd. Ulus. Cloth,4.00; Leather, 5.00 Landois' Human Physiology. Including Histology and Micro- scopical Anatomy, and with special reference to Practical Medi- cine. Second Edition. Translated and Edited by Prof. Stirling. 583 Illustrations. Cloth, 6.50; Leather, 7.50 " So great are the advantages offered by Prof. Landois' Text- book, from the exhaustive and eminently practical manner in which the subject is treated, that, notwithstanding it is one of the largest works on Physiology, it has yet passed through four large editions in the same number of years. Dr. Stirling's annotations have materially added to the value of the work. . . . Admirably adapted for the practitioner. . . . With this Text-book at his command, no student could fail in his examination."-Lancet. Sanderson's Physiological Laboratory. Being Practical Ex- ercises for the Student. 350 Illustrations. 8vo. Cloth, 5.00 Tyson's Cell Doctrine. Its History and Present State. Illus- trated. Second Edition. Cloth, 2.00 PRACTICE. Roberts' Practice. New Revised Edition. A Handbook of the Theory and Practice of Medicine. By Frederick T. Roberts, m.d. ; m.r.c.p., Professor of Clinical Medicine and Therapeutics in University College Hospital, London. Seventh Edition. Octavo. Cloth, 5.50; Sheep, 6.50 Hughes. Compend of the Practice of Medicine. 3d Ed. Two parts, each, Cloth, 1.00; Interleaved for Notes, 1.25 Part t.-Continued, Eruptive and Periodical Fevers, Diseases of the Stomach, Intestines, Peritoneum, Biliary Passages, Liver, Kidneys, etc., and General Diseases, etc. Part ii.-Diseases of the Respiratory System, Circulatory System and Nervous System; Diseases of the Blood, etc. Tanner's Index of Diseases, and Their Treatment. Cloth, 3.00 "This work has won for itself a reputation. ... It is, in truth, what its Title indicates."-N. Y. Medical Record. PRESCRIPTION BOOKS. Wythe's Dose and Symptom Book. Containing the Doses and Uses of all the principal Articles of the Materia Medica, etc. Seventeenth Edition. Completely Revised and Rewritten. Just Ready. 32010. Cloth, 1.00; Pocket-book style, 1.25 Pereira's Physician's Prescription Book. Containing Lists of Terms, Phrases, Contractions and Abbreviations used in Prescriptions, Explanatory Notes, Grammatical Construction of Prescriptions, etc., etc. By Professor Jonathan Pereira, m.d. Sixteenth Edition. 32010. Cloth, 1.00; Pocket-book style, 1.25 hn See pages 2 to 5 for list of New Manuals. STUDENTS' TEXT-BOOKS AND MANUALS. 13 PHARMACY. Stewart's Compend of Pharmacy. Based upon Remington's Text-Book of Pharmacy. Second Edition, Revised. Cloth, i.oo ; Interleaved for Notes, 1.25 SKIN DISEASES. Anderson, (McCall) Skin Diseases. A complete Text-Book, with Colored Plates and numerous Wood Engravings. 8vo. Just Ready. Cloth, 4.50 ; Leather, 5.50 " We welcome Dr. Anderson's work not only as a friend, but as a benefactor to the profession, because the author has stricken off mediaeval shackles of insuperable nomenclature and made crooked ways straight in the diagnosis and treatment of this hitherto but little understood class of diseases. The chapter on Eczema is alone worth the price of the book."-Nashville Medical News. " Worthy its distinguished author in every respect; a work whose practical value commends it not only to the practitioner and stu- dent of medicine, but also to the dermatologist."-Janies Nevens Hyde, m.d., Prof, of Skin and Venereal Diseases, Rush Medical College, Chicago. Van Harlingen on Skin Diseases. A Handbook of the Dis- eases of the Skin, their Diagnosis and Treatment. By Arthur Van Harlingen, m.d., Prof, of Diseases of the Skin in the Phila- delphia Polyclinic; Consulting Physician to the Dispensary for Skin Diseases, etc. With colored plates, izmo. Cloth, 1.75 Bulkley. The Skin in Health and Disease. By L. Duncan Bulkley, Physician to the N. Y. Hospital. Ulus. Cloth, .50 SURGERY. Heath's Minor Surgery, and Bandaging. Eighth Edition. 142 Illustrations. 60 Formulae and Diet Lists. Cloth, 2.00 Horwitz's Compend of Surgery, including Minor Surgery, Amputations, Fractures, Dislocations, Surgical Diseases, and the Latest Antiseptic Rules, etc., with Differential Diagnosis and Treatment. By Orville Horwitz, b.s., m.d., Demonstrator of Anatomy, Jefferson Medical College ; Chief, Out-Patient Surgi- cal Department, Jefferson Medical College Hospital. 3d edition. Very much Enlarged and Rearranged. 91 Illustrations and 77 Formulae, izmo. No. 9 ?Quiz-Compend f Series. Cloth, 1.00; Interleaved for the addition of Notes, 1.25. Pye's Surgical Handicraft. A Manual of Surgical Manipula- tions, Minor Surgery, Bandaging, Dressing, etc., etc. With special chapters on Aural Surgery, Extraction of Teeth, Anaes- thetics, etc. 208 Illustrations. 8vo. Cloth, 5.00 Swain's Surgical Emergencies. New Edition. Ulus. Clo.,1.50 Walsham. Manual of Practical Surgery. For Students and Physicians. By Wm. J. Walsham, m.d., f.r c.s., Asst. Surg. to, and Dem. of Practical Surg. in, St. Bartholomew's Hospital, Surgeon to Metropolitan Free Hospital, London. With 236 Engravings. See Page 2. Cloth, 3.00; Leather, 3.50 Watson on Amputation of the Extremities, and their Compli- cations. 2 colored plates and 250 wood cuts. 8vo. Cloth, 5.50 thg- See page ib for list of ? Quiz-Compends f 14 STUDENTS' TEXT-BOOKS AND MANUALS. THROAT. Mackenzie on the Throat and Nose. New Edition. By Morell Mackenzie, m.d., Senior Physician to the Hospital for Diseases of the Chest and Throat; Lecturer on Diseases of the Throat at the London Hospital, etc. Revised and Edited by D. Bryson Delavan, m.d., Prof, of Laryngology and Rhinology in the N. Y. Polyclinic; Chief of Clinic, Department of Diseases of the Throat, College of Physicians and Surgeons, N. Y.; Sec'y of the Amer. Laryngological Assoc., etc. Complete in one vol- ume, over 200 Illustrations, and many formulae. Preparing. Diseases of the CEsophagus, Nose and Naso-Pharynx, with Formulae and 93 Illustrations. Cloth, 3.00; Leather, 4.00 " It is both practical and learned ; abundantly and well illustrated; its descriptions of disease are graphic and the diagnosis the best we have anywhere seen."-Philadelphia Medical Times. Cohen. The Throat and Voice. Illustrated. Cloth, .50 James. Sore Throat. Its Nature, Varieties and Treatment. izmo. Illustrated. Paper cover, .75; Cloth, 1.25 URINE, URINARY ORGANS, ETC. Acton. The Reproductive Organs. In Childhood, Youth, Adult Life and Old Age. Sixth Edition. Cloth, 2.00 Beale. Urinary and Renal Diseases and Calculous Disorders. Hints on Diagnosis and Treatment, izmo. Cloth, 1.75 Holland. The Urine. Chemical and Microscopical, for Labo- ratory Use. Illustrated. Cloth, .50 Ralfe. Kidney Diseases and Urinary Derangements. 42 Illus- trations. izmo. 572 pages. Cloth, 2.75 Legg. On the Urine. A Practical Guide. 6th Ed. Cloth, .75 Marshall and Smith. On the Urine. The Chemical Analysis of theUrine. By John Marshall, m.d., Chemical Laboratory, Univ, of Penna; and Prof. E. F. Smith, ph.d. Col. Plates. Cloth, 1.00 Thompson. Diseases of the Urinary Organs. Seventh Edition. Illustrated. Cloth, 1.25 Tyson. On the Urine. A Practical Guide to the Examination of Urine. With Colored Plates and Wood Engravings. 5th Ed. Enlarged, izmo. Cloth, 1.50 Van Nuys, Urine Analysis. Ulus. Cloth, 2.00 VENEREAL DISEASES. Hill and Cooper. Student's Manual of Venereal Diseases, with Formulae. Fourth Edition, izmo. Cloth, 1.00 Durkee. On Gonorrhoea and Syphilis. Ulus. Cloth, 3.50 See page lb for list of f Quiz-Compends ? MEDICAL BRIEFS. A new series of short, concise compends for the Med- ical Student and Practitioner. i2mo. Cloth. Price of Each Book, $1.00. No. i. POST-MORTEM EXAMINATIONS. With Especial Reference to Medico-Legal Practice. By Prof. Rudolph Virchow, of Berlin Charite Hos- pital, author of Cellular Pathology; Translated by T. P. Smith, m.d., Member of the Royal College of Sur- geons of England. 2d American, from the 4th German Edition. With new Plates. Illustrated by Four Lith- ographs. " We are informed in precise and exact terms how a post-mortem examination should be made, both with regard to the plan to be pursued, and the manner of making the several cuts into the various organs and tissues. The method of recording the results of the investigation is clearly indicated by the addition of the detailed account of the examination of four cases; and the value of the ob- jective evidence is accurately stated in the form of the inferences drawn concerning the manner and cause of death."-American Journal of Medical Sciences. No. 2. MANUAL OF VENEREAL DISEASES. A Concise Description of those Affections and of their Treatment, including a list of Sixty-seven Prescrip- tions for Vapor Bath, Gargles, Injections, Lotions, Mixtures, Ointments, Paste, Pills, Powders, Solutions and Suppositories. By Berkeley Hill, m.d., Pro fessor of Clinical Surgery in University College; Sur- geon to University College and Lock Hospitals; and Arthur Cooper, m.d., formerly House Surgeon, Lock Hospital, London. 4th Edition, Revised and Enlarged. " I have examined it with care, and find it to be a practical and useful compendium of knowledge on the subjects discussed, well adapted to the use of medical students and those physicians in general practice who have occasional need to consult a work of this kind."-James Neven Hyde, m.d., Professor of Skin and Venereal Diseases, Kush Medical College, Chicago. No. 3. MEDICAL ELECTRICITY. A Com- pend of Electricity and its Medical and Surgical Uses. By Chas. F. Mason, m.d., Ass't Surg. U. S. Army; with an introduction by Charles H. May, m.d., Instructor in Ophthalmology, New York Polyclinic. Illustrated. OTHER VOLUMES IN PREPARATION. Price of Each Book, bound in Cloth, $1.00. ? QUIZ-COMPENDS? A new series of books for Students' use in the Quiz Class and when preparing for Examinations. Price of each Book, Cloth, $1.00; Interleaved, 1.25. I. ANATOMY. By Prof. S. O. L. Potter, Late A. A. Surg. U. S. A. Fourth Edition. 117 Illustrations. II. PRACTICE. Part 1. By Dan'l E. Hughes, m.d., late Demonstrator of Clinical Medicine in Jefferson College. Third Edition, Enlarged. III. PRACTICE. Part 11. Same author as above. Third Edition, Enlarged. IV. PHYSIOLOGY. By A. P. Brubaker, Demon- strator of Physiology in Jeff. Med. College, Philadel- phia. Fourth Edition, Improved. New Illustrations. V. OBSTETRICS. By Prof. Henry G. Landis. Third Edition. Illustrated. VI. MATERIA MEDICA, THERAPEUTICS AND PRESCRIPTION WRITING. By Prof. Sam'l O. L. Potter. 5th Ed., Enlarged and Imp. VII. GYNAECOLOGY. By Henry Morris, m.d., Demonstrator of Obstetrics and Diseases of Women and Children, Jeff. Medical College. Ulus. In Press. VIII. DISEASES OF THE EYE AND RE- FRACTION. By L. Webster Fox, m.d., Chief Clinical Assistant Ophthalmological Dept., Jefferson Medical College, and Geo. M. Gould. 60 Illus. IX. SURGERY. Including Fractures, Wounds, Dislocations, Sprains, Amputations, etc., Inflammation, Suppuration, Ulcers, Syphilis Tumors, Shock, etc., Diseases of Bladder, Testicles, Anus and other Surgi- cal Diseases, Antisepsis, etc. By Orville Horwitz, Demonstrator of Anatomy, Jefferson Medical College, etc. Third Edition. 91 Illustrations. 77 Formulae. X. INORGANIC AND ORGANIC CHEM- ISTRY. Including Medical Chemistry, Urine and Water Analysis, etc. By Henry Leffmann, m.d. XL PHARMACY. By F. E. Stewart, m.d., ph.g., Quiz Master at Philadelphia College of Pharmacy. Based, by permission, upon " Remington's Text-Book of Pharmacy." Second Edition. O"The ? Quiz-Compends ? can be used by students of any college. They contain the latest and best information in such shape that it may be easily memorized. PBIOE OF EACH BOOH, CLOTH, $1.00; INTE2LEAVED, $1.25.