MEDICAL SCHOOL Gift of Hans Lisser, M.D. " ' / ' X X- . , / ? ff s ' } HAND-BOOK OF PHYSIOLOGY. /HAND-BOOK PHYSIOLOGY WILLIAM SENIIOUSE KIEKES, M.D. EDITED BY . MOEEANT BAKER, E.K.C.S. LECTURER OX PHYSIOLOGY, AXD WAUDEX OF THE COLLEGE, AT ST. BARTHOLOMEW'S HOS1M Sl-RGEOX TO THE EVELIXA HOSI'ITAL FOR SICK CUILDIJE.V. i V '' ( /x I WITH TWO HUNDRED AND FORTV V -ONK ILLUSTRATIONS. PHILADELPHIA LINDSAY AND BLAKISTON 1869. LONDON: PRINTED BY WERTHEIHER, LEA AND CO. CIRCUS PLACE, FINSBURY CIRCUS. (4 .2 Students are advised to make the following correction before reading the took : Page 163, nth 1. from top, the words "as in fig. 46," to be transferred to the end of the preceding paragraph. PREFACE TO THE SEVENTH EDITION. A NEW Edition of Dr. Kirkes's Handbook of Physio- logy having been so soon called for, a sufficient time has not elapsed for the accumulation of any consider- able number of facts, needing incorporation with the' text. The physiological discoveries of two years, although many, do not of necessity indeed, cannot include a large number of those broader truths, the enunciation of which occupies the chief portion of this work. On the other hand, suggestions, from friendly critics too numerous to thank individually, regarding reconstruction of certain portions of the book, especially in correspondence with the wants of students, have been neither few nor unheeded. Hence, though a record of all important and well- founded recent observations has been inserted in various sections, the chief difference in the present vi PREFACE TO THE SEVENTH EDITION. when compared with the previous edition, will be found to consist mainly of additions and re-arrange- ments of old facts, and, it is hoped, a plainer exposition of them. The number of illustrations has been considerably increased. For the sphygmographic tracings in the chapter on the circulation I am indebted to Dr. Burdon Sanderson. W. MORRANT BAKER. The College, St. Bartholomew's Hospital, October, 1869. PREFACE TO THE SIXTH EDITION. IN the preparation of a new edition of Dr. Kirkes's Hand- book of Physiology, it seemed to me that one of two courses must be pursued either, on the one hand, simply to add to the book such new theories or facts as may have been recorded since the publication of the last edition ; or, on ' the other, without disregarding work in physiology that may have been lately done, thoroughly to revise the whole work as a hand-book for students, and to regard the discussion of all new and crude matter as secondary in importance to a clear enunciation of those grander facts on which alone all true knowledge of the science must be founded. Taking into consideration the position held by the book as one especially for the use of students, there seemed to be no doubt that the latter was the right course ; and I have, therefore, as rigidly as was possible, abstained from inserting all contradictory statements, raw theories, and descriptions of imperfectly or ill-observed facts. A con- stant production of these is of course unavoidable, and indeed necessary for the progress of such a science as physiology, the growth of which can be effected, like that of the beings with whose life it deals, only by gradual assimilation of crude matter, with ceaseless displacement and reconstruction. With the manner of growth, how- ever, of the science, the student has little to do ; he has but to consider the result as it is in his time ; and with this idea in view, I have, although very diffidently, altered such portions of the last edition of this work as appeared to me to fail, either by the unnecessary recital of contra- viii PREFACE. dictory opinions, or, where the enumeration of these was unavoidable, by the absence of any guide to what it is necessary for the student to believe. I have striven to do this without sacrifice of what may be called physiological truth ; and I trust that the difficulty will not be thought an altogether mean excuse for some apparent errors of commission or omission in the performance. In the re- construction of the parts referred to, and in bringing the present edition up to the standard required by recent in- vestigations, it has been necessary to re-write many parts of the work; and where descriptions, especially of the minute structure of organs and tissues, seemed insufficient, the same plan has been adopted. In doing this, however, I have endeavoured to preserve the unity of style and plan so essential in treatises intended for students. A short account of such elementary tissues as are not described in the body of the work has been added in a separate chapter to the present edition, in conformity with the expressed wishes of a large number of students. To many friends my thanks are due for counsel and assistance ; but especially I am indebted to Mr. Savory, not only for] much valuable help during the revision of the whole work, but for his kindness in looking over many of the proof sheets in the passage of the book through the press. Wherever new matter has been taken from other works a reference has of course been given. A special acknowledgment is, however, due to the seventh edition of Quain's Anatomy, for assistance in re-writing many portions of the work devoted to the description of the microscopic anatomy of organs and tissues, and for the use of a considerable number of illustrations. W. MOEKANT BAKER. The College, St. Bartholomew's Hospital. October, 1867. CONTENTS. CHAPTER I. PAGE THE GENERAL AND DISTINCTIVE CHARACTERS OF LIVING BEINGS . . . . . . . . . . . . r CHAPTER II. THE KELATION OF LIFE TO OTHER FORCES . . . . 9 CHAPTER III. CHEMICAL COMPOSITION OF THE HUMAN BODY . . . . 16 CHAPTER IY. STRUCTURAL COMPOSITION OF THE HUMAN BODY . . . . 28 CHAPTER Y. STRUCTURE OF THE ELEMENTARY TISSUES .. .. 38- Epithelium . . . . , . . . . . ib. Areolar, Cellular, or Connective Tissue . . . . 44 Adipose Tissue . . . . . . . . . . 47 Pigment . . . . . . . . . . . . 48 Cartilage . . . . . . . . . . . . 50 Bones and Teeth .. .. .. ... .. 54 CHAPTER VI. THE BLOOD .. .. .. .. .. .. 65 Quantity of Blood .. .. .. .. .. 67 Coagulation of the Blood . . . . . . . . 69 Conditions affecting Coagulation . . . . . . 75 Chemical Composition of the Blood . . . . . . 77 The Blood-Corpuscles, or Blood-Cells . . . . . . 78 Chemical Composition of Red Blood- Cells . . . . 83 The White Corpuscles .. .. .. .. 85 The Serum . . . . . . . . . . 87 Variations in the Principal Constituents of the Liquor Sanguinis . . . . , . . . . . 88- x CONTENTS. PAGE THE BLOOD, continued. Variations in Healthy Blood under Different Circumstances 9 1 Variations in the Composition of the Blood in Different Parts of the Body . . . . . . ... 93 Gases contained in the Blood . . . . . . 98 Blood-Crystals .. .. .. .. .. ib. Development of the Blood . . . . . . . . 100 Uses of the Blood . . . . . . . . . . 106 Uses of the various Constituents of the Blood . . . . ib. CHAPTER VI. (continued.) CIRCULATION OF THE BLOOD . . . . . . . . 109 The Systemic, Pulmonary, and Portal Circulations . . no THE HEART.. .. .. .. .. .. 112 The Action of the Heart . . . . . . . . 119 Function of the Valves of the Heart . . . . . . 122 Sounds of the Heart . . . . . . . . 129 Impulse of the Heart . . . . . . . . 132 Frequency and Force of the Heart's Action . . . . 134 Cause of the Rhythmic Action of the Heart . . . . 138 Effects of the Heart's Action . . . . . . . . 142 THE ARTERIES . . . . . . . . . . 143 The Pulse . . . . . . . . . . . . 155 Force of the Blood in the Arteries . . . . . . 164 Velocity of the Blood in the Arteries . . . . . . 167 THE CAPILLARIES .. .. .. .. .. ib. The Size, Number, and Arrangement of Capillaries . . 169 Circulation in the Capillaries . . . . . . 172 THE VEINS .. .. .. .. .. .. 178 Agents concerned in the Circulation of the Blood . . . 183 Velocity of Blood in the Veins .. .. .. 186 Velocity of the Circulation .. .. .. .. 187 PECULIARITIES OF THE CIRCULATION IN DIFFERENT PARTS 191 Cerebral Circulation . . . . . . . . ib. Erectile Structures . . . . . . . . . . 194 CHAPTER VII. RESPIRATION .. .. .. .. .. .. 197 Position and Structure of the Lungs . . . . . . ib. Mechanism of Respiration . . . . . . . . 204 Respiratory Movements . . . . . . . . 206 Respiratory Rhythm .. .. .. .. 210 CONTENTS. xi PAG INSPIRATION, continued. Quantity of Air respired . . . . . . . . 211 Movements of the Blood in Respiratory Organs . . 219 Changes of the Air in Respiration . . . . . . 220 Changes produced in the Blood by Respiration . . . . 229 Mechanism of various Respiratory Actions .. .. 231 Influence of the Nervous System in Respiration . . 236 Effects of the Suspension and Arrest of Respiration . . 239 CHAPTER VIII. ANIMAL HEAT . . . . . . . . . . 242 Sources and Mode of Production of Heat in the Body . . 246 CHAPTER IX. DIGESTION .. .. .. .. .. .. 255 Food .. .. .. .. .. .. ib. PASSAGE OF FOOD THROUGH THE ALIMENTARY CANAL . . 264 The Salivary Glands and the Saliva . . . . . . ib. Passage of Food into the Stomach .. .. .. 271 DIGESTION OF FOOD IN THE STOMACH . . . . . . 273 Structure of the Stomach . . . . . . . . ib. Secretion and Properties of the Gastric Fluid . . . . 279 Changes of the Food in the Stomach . . . . . . 287 Movements of the Stomach . . . . . . . . 294 Influence of the Nervous System on Gastric Digestion . . . 298 Digestion of the Stomach after Death . . . . . . 301 DIGESTION IN THE INTESTINES . . . . . . 304 Structure and Secretions of the Small Intestine . . . . ib. Valvulse Conniventes . . . . . . . . 305 Glands of the Small Intestine . . . . . . 306 TheVilli 312 Structure of the Large Intestine .. .. .. 315 The Pancreas, and its Secretion .. .. .. 318 Structure of the Liver . . . . . . . . 321 Functions of the Liver . . . . . . . . 328 The Bile . . . . . . . . . . . . ib. Summary of the Changes which take place in the Food during its Passage through the small Intestine . . 342 Summary of the Process of Digestion in the large Intestine 345 Gases contained in the Stomach and Intestines . . . . 348 Movements of the Intestines . . . . .'. . . 349 CHAPTER X. ABSORPTION .. .. .. .. .. .. 353 Structure and Office of the Lacteal and Lymphatic Vessels and Glands . . . . . . . ib. xn CONTENTS. PAGE ABSORPTION, continued. Lymphatic Glands . . . . . . . . . . 358 Properties of Lymph and Chyle . . . . . . 360 Absorption by the Lacteal Vessels . . . . . . 366 Absorption by the Lymphatics . . . . . . 367 Absorption by Blood-vessels . . . . . . . . 370 CHAPTER XL NUTRITION AND GROWTH .. .. .. .... 379 NUTRITION . . . . . . . . . . . . ib. GROWTH . . . . . . . . . . . . 395 CHAPTER XII. SECRETION . . . . . . . . . . . . 399 SECRETING MEMBRANES . . . . . . . . 400 SECRETING GLANDS . . . . . . . . . . 406 PROCESS OF SECRETION . . . . . . . . 409 CHAPTER XIII. VASCULAR GLANDS; OR GLANDS WITHOUT DUCTS .. 415 CHAPTER XIY. THE SKIN AND ITS SECRETIONS.. .. .. .. 421 Structure of the Skin . . . . . . . . ib. Structure of Hair and Nails . . . . . . . . 430 Excretion by the Skin . . . . ' . . . . 434 Absorption by the Skin . . . . . . . . 439 CHAPTER XV. THE KIDNEYS AND THEIR SECRETION . . . . . . 442 Structure of the Kidneys . . . . . . . . ib. Secretion of Urine . . . . . . . . . . 448 The Urine ; its general Properties . . . . . . 450 Chemical Composition of the Urine .. .. .. 452 CHAPTER XVI. THE NERVOUS SYSTEM . . . . . . . . . . 465 Elementary Structures of the Nervous System . . . . 466 Functions of Nerve-Fibres . . . . . . . . 476 Functions of Nerve-Centres . . . . . . . . 485 CEREBRO-SPINAL NERVOUS SYSTEM . . . . . . 490 Spinal Cord and its Nerves . . . .- . . . . ib. Functions of the Spinal Cord .. . . .. 497 THE MEDULLA OBLONGATA .. .. .. .. 510 Its Structure . . . . . . . . . . ib. Distribution of the Fibres of the Medulla Oblongata . . 512 Functions of the Medulla OLlongata .. .. .. 514 CONTENTS. xiii PAGE STRUCTURE AND PHYSIOLOGY OF THE PONS VAROLII, CRURA CEREBRI, CORPORA QUADRIGEMINA, CORPORA GENICU- LATA, OPTIC THALAMI, AND CORPORA STRIATA .. 519 Pons Varolii . . . . . . . . . . ib. Crura Cerebri . . . . . . . . . . 520 Corpora Quadrigemina . . . . . . . . 522 The Sensory Ganglia . . . . . . . . 523 STRUCTURE AND PHYSIOLOGY OF THE CEREBELLUM . . 525 STRUCTURE AND PHYSIOLOGY OF THE CEREBRUM .. 532 PHYSIOLOGY OF THE CEREBRAL AND SPINAL NERVES .. 539 Physiology of the Third, Fourth, and Sixth Cerebral or Cranial Nerves . . . . . . . . . . 540 Physiology of the Fifth or Trigeminal Nerve . . . . 544 Physiology of the Facial Nerve .. .. .. 551 Physiology of the Glosso-Pharyngeal Nerve . . . . 554 Physiology of the Pneumogastric Nerve . . . . 557 Physiology of the Spinal Accessory Nerve . . . . 563 Physiology of the Hypoglossal Nerve . . . . . . 565 Physiology of the Spinal Nerves . . . . . . 567 PHYSIOLOGY OF THE SYMPATHETIC NERVE .. .. ib. CHAPTER XVII. CAUSES AND PHENOMENA OF MOTION .. .. .. 579 CILIARY MOTION . . . . . . . . . . ib. MUSCULAR MOTION . . . . . . . . . . 581 Muscular Tissue . . . . . . . . . . ib. Properties of Muscular Tissue . . . . . . 588 Action of the Voluntary Muscles . . . . . . 596 Action of the Involuntary Muscles . . . . . . 603 Source of Muscular Action . . . . . . . . ib. CHAPTER XVIII. OF VOICE AND SPEECH . . . . . . . . 605 Mode of Production of the Human Voice . . . . ib. The Larynx . . . . . . . . . . 607 Application of the Voice in Singing and Speaking . . 615 SPEECH .. .. .. .. .. .. 620 CHAPTER XIX. THE SENSES .. .. .. .. .. .. 623 THE SENSE OF SMELL .. .. .. .. 631 THE SENSE OF SIGHT . . . . . . . . 637 Structure of the Eye . . . . . . . . ib. Phenomena of Vision . . . . . . . . 646 Reciprocal Action of different parts of the Retina . . 662 Simultaneous Action of the two Eyes . . . . . . 665 xiv CONTENTS. PACK THE SENSE OF HEARING . . . . . . . . 672 Anatomy of the Organ of Hearing . . . . . . ib. Physiology of Hearing . . . . . . . . 680 Functions of the External Ear . . . . . . 68 1 Functions of the Middle Ear ; the Tympanum, Ossicula, and Fenestrse . . . . . . . . . . 684 Functions of the Internal Ear . . . . . . 690 Sensibility of the Auditory Nerve . . . . . . 693 THE SENSE OF TASTE . . . . . . . . 698 THE SENSE OF TOUCH . . . . . . . . 707 CHAPTER, XX. GENERATION AND DEVELOPMENT .. .. .. 714 Generative Organs of the Female .. .. .. 715 Unimpregnated Ovum .. .. .. .. 719 Discharge of the Ovum . . . . . . . . 724 Corpus Luteum . . . . . . . . . . 728 IMPREGNATION OF THE OVUM . . . . . . . . 732 Male Sexual Functions . . . . . . . . ib. DEVELOPMENT . . . . . . . . . . 740 Changes of the Ovum previous to the Formation of the Embryo . . . . . . . . . . ib. Changes of the Ovum within the Uterus . . . . 743 The Umbilical Vesicle .. .. .. .. 746 The Amnion and Allantois . . . . . . . . 748 The Chorion . . . . . . . . . . 752 Changes of the Mucous Membrane of the Uterus and Formation of the Placenta . . . . . . . . 754 DEVELOPMENT OF ORGANS . . . . . . . . 760 Development of the Vertebral Column and Cranium . . ib. Development of the Face and Visceral Arches . . . . 762 Development of the Extremities . . . . . . 764 Development of the Vascular System . . . . . . 765 Circulation of Blood in the Foetus . . . . . . 769 Development of the Nervous System .. .. .. 772 Development of the Organs of Sense . . . . . . ib. Development of the Alimentary Canal .. .. 775 Development of the Respiratory Apparatus . . . . 778 The Wolffian Bodies, Urinary Apparatus, and Sexual Organs 779 THE MAMMARY GLANDS . . . . . . . . 784 INDEX . . . . . . . . . . . . 787 LIST OF WORKS REFERRED TO . . .. .. .. 827 HANDBOOK OF PHYSIOLOGY. CHAPTER I. ON THE GENERAL AND DISTINCTIVE CHARACTERS OF LIVING BEINGS. HUMAN PHYSIOLOGY is the science which treats of the life 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. Having, then, man as the object of its study, it is un- necessary to speak here of the laws of life in general, and the means by which they are carried out, further than is requisite for the more clear understanding of those of the life of man in particular. Yet it would be impossible to understand rightly the working of a complex machine without some knowledge of its motive power in the sim- plest form ; and it will be well to examine briefly the meaning of the term life, as we see it manifested most, simply, before noticing its character in such composite beings as ourselves. If we take one of the most elementary forms in which life exists, say a vegetable cell, we find that those parts of its history which compel us to give it the title of living being, are roughly as follows : First, it is derived from a parent, into whose likeness it will grow ; and its earliest sign of life is inherent power of growth ; that is to say, it is able, under certain influ- ences, of which heat is the most essential, to take up and 2 GROWTH : DEVELOPMENT : DECAY. decompose the simple compounds carbonic acid, water, and ammonia by which it is surrounded, and use them as material wherewith so to build up its own intimate structure as to increase in size and weight, until that boundary is reached which, on living things, is always imposed. But this is not all that is implied in the term growth ; for together with the building up there is always more or less falling down, and the two processes of decay and repair go on together ; the addition, however, is greater than the loss, and so there is growth. With growth, too, there is development. Together with the mere increase in size, to which alone the term growth should be applied, there is a gradual change effected in the structures of which a living being is composed, so that they become more and more highly organized, and Jbetter adapted to subserve those purposes for which they are formed. Thus, by growth and development, a being attains such perfection as is allotted to it, and in this state it remains for a time. Then, having provided for the continuance of its race, by setting aside portions of itself, with power 1 to grow up in the likeness of their parent, it begins to die. Perfection gives place to imperfection ; instead of growth there is shrinking ; instead of development there is dege- neration ; and finally death ensues that state in which a once-living structure decays, without renewal, and the elements of which it is composed return to the inorganic world, whence they were derived. Such is life as we see it in its simplest expressions, and stripped of accessories, and such we find it in the highest animal and in the lowest vegetable. The description may seem to require some alteration here and there in its application to individual examples; but the change re- quired is more apparent than real, and the laws of birth, growth, development, decay, and death, are such as all living beings must obey. ORGANIC AND INORGANIC STRUCTURES. 3 It may be well, here, to state more particularly what are the differences between organic living forms and inorganic matter. Some of them are essential, others only rules with numerous exceptions. In regard to form, a great distinction is noticeable. The shapes which inorganic matter naturally assumes are modelled after a straighter and more severe pattern than those with which life has had to do. The sharp angles and straight lines of a crystal are not matched even by the Jiard outline of those structures, as shells and corals, which, though formed under the guidance of life alone, have never been the seat of living actions ; and they resemble much less the rounded outlines characteristic of living or once-living structures. The difference can scarcely be called essential, but it is too general to be trifling. The intimate arrangement, again, of an inorganic mass, is totally different from that of an organized structure. The term organization is intended, indeed, to express a peculiarity which belongs to those structures only in which life is or has been manifested. The particles of which a lump of chalk is made up, have no mutual relation other than that of simple apposition, and each particle, however small, has like properties to those of the whole mass. In organized matter, on the other hand, there is some kind of mutual dependence of one part on another ; not necessarily to the extent of its being composed of different parts worthy the name of organs; but still with such differences, that one would be able to separate a small part that by itself would be less complete than the whole. This spe- ciality of structure, indicated by the word organization, is not only peculiar as an evidence of present or past life, but no life, so far as we know, is possible without it. Again, growth is not confined to the living, that is, if we make the word mean simply increase of size ; but the manner of growth of an inorganic mass, say a crystal, is very different from the increase of living beings. In one B 2 4 ORGANIC AND INORGANIC STRUCTURES. case, there is simply a laying of particle after particle on the outside, with no change in the interior ; in the other, the increase occurs at every part of the growing structure, the nutrition is interstitial. And wherever, in living forms, the growth is superficial only and there are nu- merous examples of this the growing part performs no vital functions ; and although it is the product of life, it is not itself living. Thus the greater part of the stem of a large tree is to all intents and purposes dead; and its increase, as might be anticipated, takes place after the manner of the increase of dead things in general, the nutrition is superficial. The lignine which is laid down in the interior of the cells, although produced by the agency of life, is not living matter ; and the fashion of its growth, therefore, is no exception to the rule. During the growth of lifeless matter, moreover, there is no decay proceeding at the same time. But in living organisms, as before re- marked, destruction is proceeding always side by side with repair, and must of necessity do so, seeing that the per- sistence of living matter depends not on freedom from decomposition and change, but on the maintenance by repair of that which is being ever destroyed. To the living, death is a necessary condition of life. In a crystal there is no life, and therefore no death. Again, the growth of a crystal has no definite limit, if it be placed under favourable conditions. But it is far otherwise with things living. The term and amount of growth, as well as the duration of life in each species, are well defined. It is true that among the lowest of living forms, life seems to be dependent entirely on such external conditions as heat and moisture, and to be capable of being stayed or even completely stopped for a time on the withdrawal of these, as it can be started or hastened when again placed under their influence. Thus, many infusoria can be dried for a considerable time, so that no vital action can possibly ORGANIC AND INORGANIC STRUCTURES. 5 continue and yet again acquire full possession of all their faculties on the re- application of moisture. But strange as this may seem, and contradictory to the statement that life's term is limited, it forms no real exception. The total duration of real, active life, is the same, whether it be spread over a few days or weeks, or over years. Life implies constant change, and inasmuch as there is no change in these structures while desiccated, there is no life. The lethargic state in which they lie, in the absence of moisture, has been called a condition of dormant vita- lity ; but the term was coined at a time when all life was supposed to be due to a store of vital power laid up and hidden in the structure which was afterwards to manifest life, and, as will be explained hereafter, can only mean capability of living. We may say, therefore, that the period of real life, in all creatures possessing it, is limited and definite. Another great peculiarity by which organised matter is distinguished from inorganic, is difference in chemical com- position. The elements in the two are, indeed, the same ; that is to say, all that is found in organic can be found in inorganic matter ; as must of necessity be the case, since the nutrition of organised structures can only take place by abstraction of matter, directly or indirectly, from the inorganic world. But the arrangement is remarkably different. The manner of this will be considered more par- ticularly hereafter. Suffice it to say now, that in organized matter the number of elements is mostly larger, and their arrangement in an atomic formula requires the use of very high multiples. Its tendency, moreover, on this account, to decomposition, is much greater than that of inorganic matter. It must be remembered, however, that in their chemical constitution there are many more links of con- nection between the two classes of bodies than exist with respect to the other peculiarities just noticed. Thus in shape, in capacity and manner of growth, in 6 ANIMALS CONTRASTED intimate structural and chemical composition, and, it may be added most commonly, in consistence, there is a very definite and marked difference between organic and in- organic structures. Between animals and plants, on the other hand, there is much less distinction. It seems a strange notion at first that it is possible to confound an animal with a vegetable, but it is true with respect to the lowest members of the two kingdoms, many of which have been frequently passed by the naturalist from one domain to the other, and for a long time refused a permanent place in either. And the reason is to be found in the fact that the essentials of life, so to speak, which alone are manifested in these, are the same in both. Both are born of a parent, and beget offspring ; both grow and develop, decay and die. And in the lowest kind of animal there is nothing in external appearance, no evidence of consciousness or volition, suffi- ciently distinct to raise it into a place above vegetables. Nevertheless there are several well-marked characters which prevent any great difficulty for the most part in deciding to which kingdom a living being belongs. Perhaps the most essential of these is the presence or absence of power to live upon inorganic material ; in other words, to act chemically on carbonic acid, ammonia and water, so as to make use of their component elements as food. Indeed one ought probably to say that a question concerning the capability of the lower kinds of animal to live in this way cannot be entertained; and that such a manner of life should decide at once in favour of a vegetable nature, whatever might be the attributes which seemed to point to. an opposite conclusion. The power of living upon organic matter would seem to be less decisive of an animal nature, for some fungi appear to derive support almost entirely from this source. There is, commonly, a marked difference in general chemical composition between vegetables and animals, WITH VEGETABLES. 7 even in their lowest forms ; for while the former consist mainly of a substance containing carbon, hydrogen, and oxygen only, arranged so as to form a compound closely allied to starch, and called cellulose, the latter are com- monly composed almost exclusively of the three elements just named, together with a fourth, nitrogen, the proxi- mate principles formed from these being identical, or nearly so, with albumen. It must not be supposed, how- ever, that either of these typical compounds alone, with its allies, is confined to one kingdom of nature. Nitro- genous or albuminous compounds are freely produced by vegetable structures, although they form an infinitely smaller proportion of the whole organism than cellulose or starch. And while the presence of the latter in animals is much more rare than is that of the former in vegetables, there are many animals in which traces of it may be dis- covered, and some, the Ascidians, in which it is found in considerable quantity. 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 as 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 zoo- spores of many of the Cryptogamia exhibit movements of a like kind to those seen in animalcules ; and even among the higher orders of plants, may 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 character- istic of animal nature, is, when taken by itself, no proof of it. Of course, if the movement were such as to indicate any kind of purpose, whether of getting food or ANIMALS CONTRASTED any other, the case would be different, and we should justly call a being exhibiting such motion, an animal. But low down in the scale of life, where alone there exists any difficulty in distinguishing the two classes, movements, although almost always more lively, are scarcely or not at all more purposive in one than the other and even if we decide on the animal nature of a being, it by no means follows that we are bound to acknowledge the presence of sensation or volition in the slightest degree. There may be at least no evidence of its possessing a trace of those tissues, nervous and muscular, by which, in the higher members of the animal kingdom, these qualities are mani- fested. Probably there is no more of either of them in the lowest animals than in vegetables. In both, movement is effected by the same means ciliary action, and hence the greater value, for purposes of classification, of the power to live on this or that kind of food, on organic or inorganic matter. As the main purpose of the lowest members of the vegetable kingdom is doubtless to bring to organic shape the elements of the inorganic world around, so the function of the lowest animals is, in like manner, to act on degenerating organic matter, "to arrest the fugitive organised particles, and turn them back into the ascending stream of animal life." And, because sensation and voli- tion are accompaniments of life in somewhat higher animal forms, it is needless to suppose that these qualities exist under circumstances in which, as we may believe, they could be of no service. It is as needless as to dogmatise on the opposite side, and say that no feeling or voluntary movement is possible without the presence of those tissues which we call nervous and muscular. The presence of a stomach is a very general mark by which an animal can be distinguished from a vegetable. But the 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 WITH VEGETABLES. 9 of their body being adapted to absorb and digest, they have no need of a special receptacle for nutrient matter, and accordingly have no stomach. 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. It is necessary to compare, side by side, the lowest mem- bers of the two kingdoms, in order to understand rightly how faint are the boundaries between them. CHAPTER II. ON THE RELATION OF LIFE TO OTHER FORCES. IN the foregoing chapter we have seen how life exhibits itself most simply, how many and great differences there are between living forms and those of inorganic matter, how few and small distinctions between life as it is in the lowest animal and the lowest vegetable. It will be well now to examine very briefly what is the nature of life, and how far it can be shown to have any relation to other forces. Before attention had been drawn to the mutual conver- tibility of the various so-called physical forces, heat, light, electricity, etc., and before it had been shown that these are limited and measurable, and no more to be created at will than matter itself, it was not strange that life should be considered, like them, a peculiar power, innate in certain structures, but essentially independent of them, and of the physical forces by which it was surrounded. When, however, our ideas concerning these forces underwent a complete revolution, it was to be expected that our notions of life would suffer a radical change also. We know now . .10 BIRTH : GROWTH. that all force is but the representative of some other force with which it is in exact correlation, and into which it can be re-converted. Thus, in the language of modern science, the motion of a steam-engine is the expression in another form of just so much expansive power in the steam which moves the piston; and this power, again, is the trans- formed expression and exact correlate of the heat produced by the combustion of so much fuel. Coal and wood, again, represent that amount of force, in the shape of the light and heat of the sun, which was expended in their production ; and when they are burnt, give out again just the same amount. Thus, the light and heat of the sun, in this instance, are transformed, so to speak, into chemical affinity, then into heat and light again, then motive power. And, again, were this motive power interfered with as motion, it might be made to re-appear as heat and light. We may use the term transformation of force, or say that it is the same force throughout, variously manifesting itself as heat, light, motion, etc. It matters \ not so that we remember that no force can be exercised I where 1 no force has been previously expended. It cannot \ spring up from nothing, any more than can the matter through which it acts. It is, equally with this, measur- able and indestructible. It has been said in the last chapter, that life manifests itself in those beings that have it, by birth, growth, de- velopment, decay, and death. To understand the nature of life, we must take these events in succession, and con- sider what series of actions each of them represents, and whether there is any reason to suppose that life is the correlative expression of any other force. The term birth, it need scarcely be said, is applied to that period of an animal's life at which, having arrived at a fit state for a more independent existence, it leaves the body of the parent, or, as the phrase goes, is " brought into the world." BIKTH: GROWTH. n It is, however, an indefinite term, and indicates no par- ticular period of development. It is not the beginning of life. To understand this we must go farther back, and see what is the nature of the germ, the development of which will issue in birth. We may take for this purpose, the seed of a plant, or the ovum of the highest animal. Both alike contain something that has the power, under par- ticular circumstances, of growing up into the likeness of the being from which it is derived. It would be beside our purpose to consider here how this portion of organized matter is separated from its parent. The process belongs to the subject of generation. All we have to do with, now, is the fact that it is a part of a living organism that is separated, and has power to develop into the likeness of that from which it is cast off. How is it thus developed ? Formerly it was said that the seed or egg contained a store of vitality laid up in it ; and inasmuch as this might not show itself actively for days, months, or years, and only when placed in appropriate circumstances, of heat, mois- ture, etc., the vitality was said to be dormant. Now, how- ever, we explain the so-called dormant condition of an undeveloping seed or ovum by the fact that as no force *-r can arise by itself, so in this instance there is need of some external force to be transformed by the incipient organized structure into vital power, the latter being only a con- venient term of expression for that power which issues in growth, development, and other actions characteristic of life. The formation of vegetable matter has been just referred to as an example of the employment of external force, namely, the light and heat of the sun, for the pur- pose of decomposing certain chemical compounds, espe- cially carbonic acid and ammonia; for rending asunder, that is to say, the elements of which they are composed, and building up a particular structure with the material in this way obtained. This transformation of heat and light by leaves, into vital power of growth, so to speak, is ana- 12 DOKMANT VITALITY. logons to that which occurs at the very beginning of life, in the germ either of a vegetable or an animal. A seed or ovum contains the germ surrounded by nutrient matter. The germ is the portion of organized structure which can i transform external forces into vital power. But it can of! - course grow only by attracting to itself fresh material;-' and therefore it is surrounded by a store of nutrient matter, which serves until the germ is developed sufficiently to obtain the necessary supplies from without. Heat and moisture seem to be the external conditions most necessary for the beginning of life. Moisture probably is required only as it might be for a simple chemical combination. Heat is the force which, through the medium of the germ, is manifested as growing, vital power. The beginning of life is the beginning of this transformation of force, and the structure raised is the correlative result of its expenditure. Birth is only that period in the process of development at which the new being comes into more immediate relation with the external world. The term "dormant vitality" must be taken to mean that state of an organized being in which there is no evidence of life manifested, simply because there is an absence of those external forces which alone can be trans- formed in the manner before referred to ; or if the force be present, there is absence of the conditions under which it can act. The state of dormant vitality is, speaking roughly, like that of an empty galvanic battery. No electric current passes because no chemical action is going on. Everything is ready for the transformation of chemical into electric power; but no transformation takes place, because there is no chemical force to be transformed. We do not say, in this instance, that there is a store of electricity laid up, in a dormant state, in the battery ; and we have as little reason to say so in regard to an undeveloping seed or ovum. GROWTH AND DEVELOPMENT. 13 How, by means of external force, the embryo can build up its structure with the materials around, we know not in the least. But we are nearly as ignorant of many other transformations of force. The means by which an electric current decomposes a chemical com- pound are as mysterious as those by which vital decom- position and re-construction are effected. When we know the cause in the one case we shall probably in the other also. Growth and development after birth are but continua- tions of the same processes that were active in the embryo. The material wherewith construction is effected is taken from a different source, and elaborated by different organs; but all the vital acts concerned in the further progress of a new being to maturity are dependent for their continuance on external force. This may be applied to them directly, and from without, as when trees absorb nutrient matter, and grow under the light and heat of the sun ; or it may be derived from material taken into the body as food, part of which food is used for constructive purposes, and part as material for the production of force, especially heat, by the transformation of which, construction and other acts charac- teristic of life are alone effected. Capacity of growth, however, is not a property of living beings only. Crystals grow as well as plants or animals, if placed under appropriate conditions. But the manner of growth in the two cases is widely different. A crystal increases in size by fresh additions to its exterior only; while a living form is built up by increase throughout its whole substance. The nutrition is not superficial only but interstitial. There is, however, a greater difference than this. In the case of an inorganic body, there is addition simply, and together with this, there is no waste going on at the same time. Throughout the substance of a living organism, on the contrary, there is constant waste at the same time with repair. In the case of a being that has 14 DECAY. not arrived at maturity, the addition is greater than the loss, and so there is growth. The renewed part, too, is more perfect than that which preceded it, and thus there is development also. Even when growth and development have ceased, there is no cessation of change. Life is not, as was once thought, the power of resisting decay. On the t contrary, the waste which occurs in a living body, is incessant ; and in propor- tion as life is more active, so is decay. But while with the waste of a dead thing, there is no corresponding repair, in the case of the living, destruction and renewal advance side by side. It may be asked what is the reason for this constant waste, which, with corresponding repair, is characteristic of life ? The answer is a simple one, and brings us back to the subject of the correlation of life with other forces. It has been said before that all force must be the represen- tative of the expenditure of force in another form. In the case of the vegetable world, construction is the main object to be attained, and the amount of light and heat expended on the growth of plants is represented by the wood formed. Inasmuch as the latter, when once deposited in the act of growth, is not, for the most part, the seat of any appreciable vital energy, so there is little or no change, either in respect to waste or repair. New matter, more- over, is deposited, chiefly layer by layer, almost as it might be on a crystal. The structures of which animals are composed, on the contrary, are placed under widely different conditions. They are subject to constant wear and tear ; from their construction they are liable to incessant decay ; and many are themselves the means of transforming physical force into some form or other of vital energy. Thus there is constant waste, and if the integrity of the body is to be preserved there must be constant renewal. And as the whole substance of living tissues, and not merely their DEATH. 15 surface, is liable to this incessant change, the nutrition must be not superficial only, but interstitial also. When growth and development cease, there is no real halting at what is called maturity. It is impossible to say when development ends and decline begins ; especially because the two processes often go on in different organs at the same time. Soon, however, all structures alike begin to be less perfectly renewed ; and decline, becoming more and more apparent, is followed by death. Death may be defined simply as that condition of a once- living structure in which no vital transformation of force can be any more effected. It is a state in which waste and decay are not compensated for by repair. The organic matter of which, a dead body is composed follows, unchecked, its natural tendency to return to an inorganic condition. The same would have happened, of course, had life continued. For living structures decay, as dead ones do, although the fashion of the decay is in the two cases different. But now in death, nothing takes the place of that which is lost, and the material body, as an individual, disappears. It may be said that, after all, it is but an alteration of language, to say that life is a correlative expression of physical force ; that we know as little as ever of the means by which the transformation of, say heat into mechanical motion, is effected by a muscle, and still less, if possible, of the transformation by which it is made to happen in obedience to the will ; that the old notion of inherent vital force is almost or quite as rational as the new one, seeing that we know so little of either. But still it is a step in advance to be able to discern not a mere dependence only of life upon other forces, but a distinct correlation the one with the other, and thus, in some degree, at least, to remove life from that isolated position which it has so long assumed. i6 CHAPTER III. CHEMICAL COMPOSITION OF THE HUMAN BODY. THE following Elementary Substances may be obtained by chemical analysis from the human body : Oxygen, Hydro- gen, Nitrogen, Carbon, Sulphur, Phosphorus, Silicon, Chlorine, Fluorine, Potassium, Sodium, Calcium, Magne- sium, Iron, and probably, or sometimes, Manganesium, Aluminium, and Copper. Thus, of the sixty-three or more elements of which all known matter is composed, more than one fourth exist in the human body. A few others have been detected in the bodies of other animals ; but no element has yet been found in any living body which does not also exist in inorganic matter, and for the simple reason that all the structures of a living being are composed of elements which have been withdrawn, directly or indirectly, from the inorganic world, again to return, after a certain time, to their original condition. It is, therefore, in the arrangement of these elements, not in their nature, that we must seek for those distinctions which exist between what is called organic and inorganic matter. The term organic has long ceased to imply a substance that is formed only by organized living tissues, and now signifies only matter with a certain degree of complexity of composition. The term, indeed, for want of a better, is still retained, although its original meaning has been lost. But distinctions founded upon the supposed fact that certain substances can be formed by the agency of life only, are fast disappearing, as the chemist, year by year, adds to the number of his conquests over inorganic matter, and moulds it to organic shape. CHEMICAL COMPOSITION OF HUMAN BODY. 17 Although, however, a large number of so-called organic compounds have long ceased to be peculiar in being formed only by living tissues, it will be well to notice some of the chief characters of the more common kinds of organic matter, inasmuch as they form a large part of all living tissues, and many of them have, up to the present time, been formed by the agency of life only. Among the peculiarities in the chemical characters of animal substances the two following may be specially noticed : 1. They are composed of a larger number of elements than are present in the more common kinds of inorganic matter. Thus the most abundant substances, as albumen, fibrin, and gelatin, in the more highly organized tissues of animals are composed of five elements carbon, hydro- gen, oxygen, nitrogen, and sulphur. 2. Not only are a large number of elements usually combined in an organic compound, but a large number of equivalents or atoms of each of the elements are united to form an equivalent or atom of the compound. In the case of carbonate of ammonia, as an example among inorganic substances, one equivalent of carbonic acid is united with one of ammonia ; the equivalent or atom of carbonic acid consists of one of carbon with two of oxygen ; and that of ammonia of one of nitrogen with three of hydrogen. But in an equivalent or atom of fibrin, or of albumen, there are of the same elements, respectively, 48, 15, 12, and 39 equivalents, according to Dumas, and nearly ten times as many, acording to Mulder. And, together with this union of large numbers of equivalents in the organic compound, it is further observable, that the several numbers stand in no simple arithmetical relation one with another, as the numbers of equivalents combining in an inorganic com- pound do. With these peculiarities in the chemical composition of organic bodies we may connect two other consequent facts ; first, the large number of different compounds that are 1 8 INSTABILITY OF ORGANIC COMPOUNDS. formed out of comparatively few elements ; secondly, their great proneness to decomposition. For it is a general rule, that the greater the number of equivalents or atoms of an element that enter into the formation of an atom of a compound, the less is the stability of that compound. Thus, for example, among the various oxides of lead and other metals, the least stable in composition are those in which each equivalent has the largest number of equiva- lents of oxygen. So, water, composed of one equivalent of oxygen and one of hydrogen, is not changed by any slight force ; but peroxide of hydrogen, which has two equivalents of oxygen to one of hydrogen, is among the substances most easily decomposed. The instability, on this ground, belonging to the more peculiar animal organic compounds, is augmented, 1st, by their containing nitrogen, which, among all the elements, may be called the least decided in its affinities, that which maintains with least tenacity its combinations with other elements ; and, 2ndly, by the quantity of water which, in their natural mode of existence, is combined with them, and the presence of which furnishes a most favourable condition for the decomposition of nitrogenous compounds. Such, indeed, is the instability of animal compounds, arising from these several peculiarities in their constitu- tion, that in dead and moist animal matter, no more is requisite for the occurrence of decomposition than the presence of atmospheric air and a moderate temperature ; conditions so commonly present, that the decomposition of dead animal bodies appears to be, and is generally called, spontaneous. The modes of such decomposition vary ac- cording to the nature of the original compound, the tem- perature, the access of oxygen, the presence of microscopic organisms, and other circumstances, and constitute the several processes of decay and putrefaction ; in the results of which processes the only general rule seems to be, that the several elements of the original compound finally unite NON-AZOTIZED PRINCIPLES. 19 to form those substances, whose composition is, under the circumstances, most stable. The organic compounds existing in the human body may be arranged in two classes, namely, the .azotized, or nitro- genous, and the non-azotized, or non-nitrogenous principles. The non-azotized principles include the several fatty, oily, or oleaginous substances, as margarin, olein, stearin, cholesterin, and others. In the same category of non- nitrogenous substances may be included lactic and formic acids, animal glucose, sugar of milk, etc. The oily or fatty matter which, enclosed in minute cells, forms the essential part of the adipose or fatty tissue of the human body, and which is mingled in minute particles in many other tissues and fluids, consists of a mixture of margarin and olein, with a minute quantity of palmitin ; the proportion of the former being the greater the higher the temperature at which the mixture congeals, and the firmer the mass is when congealed. The animal fats, or suets, that are firmer than human fat, contain also a sub- stance named stearin. Each of these fats is composed of an acid margaric, oleic, stearic, or palmitic, in combina- tion with a base glycerin. Stearin melts under ordinary circumstances at 144 F., margarin at 1 1 6, palmitin at 1 1 8, olein at 25. Human fat is a clear yellow oil, of which different specimens congeal at from 45 to 35. Margarin, when deposited from solution in alcohol, crystal- lizes in pearly scales; and microscopic tufts or balls of fine needle-shaped crystals of margaric acid are often found in fat-cells after death, especially in the fat of diseased parts and of old people. Cholesterin, a fatty matter which melts at 293 F., and is, therefore, always solid at the natural temperature of the body, may be obtained in small quantity from blood, bile, and nervous matter. It occurs abundantly in many biliary calculi ; the pure white crystalline specimens of these con- cretions being formed of it almost exclusively. Minute 20 CHEMICAL COMPOSITION OF HUMAN BODY. rhomboidal scale-like crystals of it are also often found in morbid secretions, as in cysts, the puriform matter of softening and ulcerating tumours, etc. It is soluble in ether and boiling alcohol ; but alkalies do not change it; it is one of those fatty substances which are not saponifiable. The azotized or nitrogenous principles in the human body include what may be called the proper gelatinous and albu- minous substances, besides others of less definite rank and composition, as pepsin and ptyalin, horny matter or keratin, many colouring and extractive matters, etc. The gelatinous substances are contained in several of the tissues, especially those which serve a passive mechanical office in the economy ; as the cellular or fibro- cellular tissue in all parts of the body, the tendons, ligaments, and other fibrous tissues, the cartilages and bones, the skin and serous membranes. These, when boiled in water, yield a material, the solution of which remains liquid while it is hot, but becomes solid and jelly-like on cooling. Two varieties of these substances are described, gelatin and chondrin, the latter being derived from cartilages, the former from all the other tissues enumerated above, and in its purest state, from isinglass, which is the swim- ming bladder of the sturgeon, and which, with the excep- tion of about / per cent, of its weight, is wholly reducible into gelatin. The most characteristic property of gelatin is that already mentioned, of its solution being liquid when warm, and solidifying or setting when it cools. The tem- perature at which it becomes solid, the proportion of gela- tin which must be in solution, and the firmness of the jelly when formed, are various, according to' the source, the quantity, and the quality of the gelatin ; but, as a general rule, one part of dry gelatin dissolved in IOO of water, will become solid when cooled to 60. The solidi- fied jelly may be again made liquid by heating it ; and the ALBUMINOUS SUBSTANCES. 21 transitions from the solid to the liquid state by the alter- nate abstraction and addition of heat, may be repeated several times ; but at length the gelatin is so far altered, and, apparently, oxydized by the process, that it no longer becomes solid on cooling. Gelatin in solutions too weak to solidify when cold, is distinguished by being precipitable with alcohol, ether, tannic acid, and bichloride of mercury, and not precipitable with the ferrocyanide of potassium. The most delicate and striking of these tests is the tannic acid, which is conveniently supplied in an infusion of oak- bark or gall-nuts ; it will detect one part of gelatin in 5,OOO of water; and if the solution of gelatin be strong it forms a singularly dense and heavy precipitate, which has been named tanno-gelatin, and is completely insoluble in water. Chondrin. The kind of gelatin obtained from carti- lages agrees with gelatin in most of its characters, but its solution solidifies on cooling much less firmly, and, unlike gelatin, it is precipitable with acetic and the mineral and other acids, and with alum, persulphate of iron and acetate of lead. The albuminous substances, or protein -compounds, as they are sometimes called, are more different from inorganic bodies than are any of the substances yet considered, or perhaps, any in the body. The chief among them are albumen, fibrin, and casein; the last is found almost exclusively in milk. Principles essentially similar to them all are found also in vegetables, especially in the sap and fruits. And substances much resembling, though not classed with, the albuminous, are horny matter and ex- tractive matter. Albumen exists in some of the tissues of the body, espe- cially the nervous, in the lymph, chyle, and blood, and in many morbid fluids, as the serous secretions of dropsy, pus, and others. In the human body it is most abundant, and most nearly pure, in the serum of the blood. In all 22 CHEMICAL COMPOSITION OF HUMAN BODY. the forms in which it naturally occurs, it is combined with about six per cent, of fatty matter, phosphate of lime, chloride of sodium, and other saline substances. Its most characteristic property is, that both in solution and in the half-solid state in which it exists in white-of-egg, it is coa- gulated by heat, and in thus becoming solid, becomes insoluble in water. The temperature required for the coagulation of albumen is the higher the less the propor- tion of albumen in the solution submitted to heat. Serum and such strong solutions will begin to coagulate at from 150 to 170, and these, when the heat is maintained, be- come almost solid and opaque. But weak solutions require a mush higher temperature, even that of boiling, for their coagulation, and either only become milky or opaline, or produce flocculi which are precipitated. Albumen, in the state in which it naturally occurs, ap- pears to be but little soluble in pure water, but is soluble in water containing a small proportion of alkali. In such solutions it is probably combined chemically with the alkali ; it is precipitated from them by alcohol, nitric, and other mineral acids, by ferrocyanide of potassium (if before or after adding it the alkali combined with the albumen be neutralised), by bichloride of mercury, acetate of lead, and most metallic salts. Coagulated albumen, i.e., albumen made solid with heat, is soluble in solutions of caustic alkali, and in acetic acid if it be long digested or boiled with it. With the aid of heat, also, strong hydrochloric acid dissolves albumen pre- viously coagulated, and the solution has a beautiful purple or blue colour. Fibrin is found most abundantly in the blood and the more perfect portions of the lymph and chyle. It is very doubtful, however, whether fibrin, as such, exist in these fluids, whether, that is to say, it is not itself formed at the moment of coagulation. (See Chapter on the Blood.) If a common clot of blood be pressed in fine linen ALBUMEN: FIBRIN. 23 while a stream of water flows upon it, the whole of the blood-colour is gradually removed, and strings and various pieces remain of a soft, yet tough, elastic, and opaque-white substance, which consist of fibrin, impure, with a mixture of fatty matter, lymph- corpuscles, shreds of the membranes of red blood -corpuscles, and some saline substances. Fibrin somewhat purer than this may be obtained by stirring blood while it coagulates, and collecting the shreds that attach themselves to the instrument, or by retarding the coagulation, and, while the red blood- corpuscles sink, collecting the fibrin un- mixed with them. But in neither of these cases is the fibrin perfectly pure. Chemically, fibrin and albumen can scarcely be distin- guished ; the only difference apparently being that fibrin contains 1-5 more oxygen in every IOO parts than albumen does. Nearly all the changes, produced by various agents, in coagulated albumen, may be repeated with coagulated fibrin, with no greater differences of result than may be reasonably ascribed to the differences in the mechanical properties of the two substances. Of such differences, the principal are, that fibrin immersed in acetic acid swells up and becomes transparent like gelatin, while albumen undergoes no such apparent change; and that deutoxyde of hydrogen is decomposed when in contact with coagulated fibrin, but not with albumen. The difference between albumen and fibrin being thus so small, and due apparently to a slight increase in the amount of oxygen in the latter, it occurred to Mr. Alfred H. Smee that albumen might be converted into fibrin if subjected to the prolonged influence of oxygen. Having exposed completely defibrinated blood to a stream of oxygen for about thirty-six hours, he found at the end of that time that small masses were formed, which possessed the general and microscopic characters of fibrin. A similar formation of fibrin took 24 CHEMICAL COMPOSITION OF HUMAN BODY. place when blood-serum alone, without the red corpuscles, or a solution of egg- albumen, instead of defibrinated blood, was treated in the same way, shewing that the fibrin re- sulted from transformation of the albumen, and not of any other constituent of the blood. The conversion was faci- litated by a temperature of from 98 to I IO Fahr. : was prevented by an alkaline state of the fluid, but readily effected when the fluid was neutral or slightly acid ; and moreover was accompanied by the evolution of sulphur, phosphorus, and carbonic acid. A variety of fibrin, named syntonin, is obtained from muscular tissue, both of the striated and organic kind. It differs from ordinary fibrin in several particulars, especially in being less soluble in nitrate and carbonate of potash, and more soluble in dilute hydrochloric acid. The per-centage composition of albumen, fibrin, casein, gelatin, and chondrin, is thus given by Mulder : Albumen. Fibrin. Casein. Gelatin. Chondrin. Carbon Hydrogen 53'5 7'o 527 6'Q 53^3 7*1 ^ 50-40 6-64 49 '97 6-63 Nitrogen Oxygen Sulphur Phosphorus I5-5 22 -Q 1-6 0'4 I5-4 23 '5 I'2 0"3 15-65 22-52 0-85 i8'34 j 24-62 14-44 ( 28-58 j 0-38 lOO'O lOO'O lOO'OO lOO'OO lOO'OO Horny Matter. The substance of the horny tissues, including the hair and nails (with whale-bone, hoofs, and horns), consists of a protein-compound, with larger pro- portions of sulphur than albumen and fibrin contain. Hair contains 10 per cent, and nails 6 to 8 per cent, of sulphur. The horny substances, to which Simon applied the name of keratin, are insoluble in water, alcohol, or ether; soluble in caustic alkalies, and sulphuric, nitric, and hy- EXTRACTIVE MATTERS. 25 drochloric acids ; and not precipitable from the solution in acids by ferrocyanide of potassium. Mucus, in some of its forms, is related to these horny substances, consisting, in great part, of epithelium detached from the surface of mucous membrane, and floating in a peculiar clear and viscid fluid. But under the name of mucus, several various substances are included, of which some are morbid albuminous secretions containing mucus and pus-corpuscles, and others consist of the fluid secretion variously altered, concentrated, or diluted. The true chemical characters of this fluid are as yet incompletely known. Extractive Matters. Under this name are included sub- stances of mixed and uncertain composition, which form the residue of animal matter when, from almost any of the fluids or solids of the body, the albuminous, gelatinous, and fatty principles have been removed. The remaining animal matter is mixed with various salts, such as lactates, chlorides, and phosphates, and is divisible into two principal portions, of which one is soluble in water alone, the other in alcohol. Doubtless there are in these substances many distinct compounds, of which some exist ready formed in the body, and some are formed in the changes to which the previous chemical examinations have given rise. Many of them, including kreatin and kreatinin, two principles originally discovered among the extractive matters of muscular tissue, but since found in the blood, urine, and elsewhere, are no doubt products of the chemical changes that take place in the natural waste and degeneration of the tissues, and are substances that are to be separated from the tissues for excretion. Such are the chief organic substances of which the human body is composed. It must not be supposed, how- ever, that they exist naturally in a state approaching that 26 CHEMICAL COMPOSITION OF HUMAN BODY. of chemical purity. All the fluids and tissues of the body appear to consist, chemically speaking, of mixtures of several of these principles, together with saline matters. Thus, for example, a piece of muscular flesh would yield fibrin, albumen, gelatin, fatty matters, salts of soda, potash, lime, magnesia, iron, and other substances, such as kreatin, which appear passing from the organic towards the inorganic state. This mixture of substances may be explained in some measure by the existence of many different structures or tissues in the muscles ; the gelatin may be referred principally to the cellular tissue between the fibres, the fatty matter to the adipose tissue in the same position, and part of the albumen to the blood and the fluid by which the tissue is kept moist. But, beyond these general statements, little can be said of the mode in which the chemical compounds are united to form an organized structure ; or of how, in -any organic body, the several incidental substances are combined with those which are essential. The inorganic matters which exist as such in the human body are various. Water forms a large proportion, probably more than two-thirds of the weight of the whole body. Phosphorus occurs in combination, as in the tribasic phosphate of soda in the blood and saliva, the super- phosphates of the muscles and urine, the basic phosphates of lime and magnesia in the bones and teeth. Sulphur is present chiefly in the sulphocyanide of potas- sium of the saliva, and in the sulphates of the urine and sweat. A very small quantity of silica exists, according to Berzelius, in the urine, and, according to others, in the blood. Traces of it have also been found in bones, in hair, and in some other parts of the body. Chlorine is abundant in combination with sodium, potas- sium, and other bases in all parts, fluid as well as solid, of POTASH: SODA: LIME. 27 the body. A minute quantity of fluorine in combination with calcium has been found in the bones, teeth, and urine. Potassium and sodium are constituents of the blood and all the fluids, in various quantities and proportions. They exist in the form of chlorides, sulphates, and phosphates, and probably, also, in combination with albumen, or certain organic acids. Liebig, in his work on the Chemistry of Food, has shown that the juice expressed from muscular flesh always contains a much larger proportion of potash- salts than of soda-salts ; while in the blood and other fluids, except the milk, the latter salts always preponderate over the former ; so that, for example, for every I OO parts of soda-salts in the blood of the chicken, ox, and horse, there are only 40*8, 5*9, and 9*5 parts of potash-salts ; but for every IOO parts of soda-salts in their muscles, there are 381, 279, and 285 parts of potash-salts. The salts of calcium are by far the most abundant of the earthy salts found in the human body. They exist in the lymph, chyle, and blood in combination with phosphoric acid, the phosphate of lime being probably held in solu- tion by the presence of phosphate of soda. Perhaps no tissue is wholly void of phosphate of lime ; ' but its especial seats are the bones and teeth, in which, together with carbonate of lime and fluoride of calcium, it is deposited in minute granules, in a peculiar compound, named bone-earth, containing 51*55 parts of lime, and 48*45 of phosphoric acid. Phosphate of lime, probably the tribasic phosphate, is also found in the saliva, milk, bile, and most other secretions, and superphosphate in the urine, and, according to Blondlot, in the gastric fluid. Magnesium appears to be always associated with calcium, but its proportion is much smaller, except in the juice expressed from muscles, in the ashes of which magnesia preponderates over lime. The especial place of iron is in the cruorin, the colouring- 28 STRUCTUEAL COMPOSITION OF HUMAN BODY. matter of the blood, of which a further account will be given with the chemistry of the blood. Peroxyde of iron is found, in very small quantities, in the ashes of bones, muscles, and many tissues, and of lymph and chyle, albumen of serum, fibrin, bile, and other fluids ; and a salt of iron, probably a phosphate, exists in consider- able quantity in the hair, black pigment, and other deeply coloured epithelial or horny substances. Aluminium, Manganese, and Copper. It seems most likely that in the human body, copper., manganesium, and aluminium 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 and lead, being absorbed, may be deposited in the liver and other parts. CHAPTER IV. STJEITJCTUKAL COMPOSITION OF THE HUMAN BODY. IN the investigation of the structural composition of the human body, it will be well to consider in the first place, what are the simplest, anatomical elements which enter into its formation, and then proceed to examine those more complicated tissues which are produced by their union. It may be premised, that in all the living parts of all living things, animal and vegetable, there is in- variably to be discovered, entering into the formation of their anatomical elements, a greater or less amount of a substance, which, in chemical composition and general character, is indistinguishable from albumen. As it exists in a living tissue or organ, it differs essentially from mere PROTOPLASM. 29 albumen in the fact of its possessing the power of growth, development, and the like ; but in chemical composition it is identical with it. This albuminous substance has received various names according to the structures in which it has been found, and the theory of its nature and uses which may have pre- sented itself most strongly to the mind of its observers. In the bodies of the lowest animals, as the Rhizopoda or Gregarinida, of which it forms the greater portion, it has been called " sarcode," from its chemical resemblance to the flesh of the higher animals. When discovered in vegetable cells, and supposed to be the prime agent in their con- struction, it was termed "protoplasm." As the presumed formative matter in animal tissues it was called "blastema" ; and, with the belief that wherever found, it alone of all matters has to do with generation and nutrition, Dr. Beale has surnamed it " germinal matter." So far as can be discovered, there is no difference in chemical composition between the protoplasm of one part or organism and that of another. The movements which can be seen in certain vegetable cells apparently belong to a sub- stance which is identical in composition with that which constitutes the greater portion of the bodies of the lowest animals, and which is present in greater or less quantity in all the living parts of the highest. So much appears to be a fact ; that in all living parts there exists an albu- minous substance and that in favourable cases for observa- tion in vegetable and the lower animal organisms, there can be noticed in it certain phenomena which are not to be accounted for by physical impressions from without, but are the result of inherent properties we call vital. For example, if a hair of the Tradescantia Virginica, or of many other plants, be examined under the microscope, there is seen in each individual cell a movement of the pro- toplasmic contents in a certain definite direction around the interior of the cell. Each cell is a closed sac or bag, and its 30 STRUCTURAL COMPOSITION OF HUMAN BODY. contents are therefore quite cut off from the direct influence of any motive power from without. The motion of the particles, moreover, in a circuit around the interior of the cell, precludes the notion of its being due to any other than those molecular changes which we call vital. Again, in the lowest animals, whose bodies resemble more than anything else a minute mass of jelly, and w r hich appear to be made up almost solely of this albuminous protoplasm, there are movements in correspondence with the needs of the organism, whether with respect to seizing food or any other purpose, which are unaccountable according to any known physical laws, and can only be called vital. In many, too, there is a kind of molecular current, exactly resembling that which is seen in a vegetable cell. In the higher animals, phenomena such as these are so subordinate to the more complex manifestations of life that they are apt to be overlooked; but they exist nevertheless. The mere nutrition of each part of the body in man or in the higher animals, is performed after a fashion which is strictly analogous to that which holds good in the case of a vegetable cell, or an animalcule ; or, in other words, the life of each anatomical element in a complex structure, as that of the" human body, resembles very closely the life of what in the lowest organisms constitutes the whole being. For example, the thin scaly covering or epidermis, which forms the outer part of a man's skin, is made up of minute cells, which, when living, are composed in part of protoplasm, not to be distinguished from that of a vegetable cell or an animalcule, and which are continually wearing away and being replaced by new similar elements from beneath ; and this process of quick waste and repair could only take place under the very complex conditions of nutrition which exist in man. One w r orking part of the organism of an animal is so inextricably interwoven with that of another, that any want or defect in one, is soon or immediately felt by the whole ; and the epidermis which STRUCTURELESS MEMBRANE. 31 only subserves a mechanical function, would be altered very soon by any defect in the more essential parts concerned in circulation, respiration, etc. But if we take simply the life- history of one of the small cells which constitute the epider- mis, we find that it absorbs nourishment from the parts around, grows, and develops in a manner exactly analogous to that which belongs to one of the cells which constitute the outer covering of a tree, or a cell which by itself forms an independent being. Remembering, however, the invariable presence of a living albuminous matter or protoplasm of apparently identical composition in all living tissues, animal and vegetable, we must not forget that its relations to the parts with which it is incorporated are still very doubtfully known ; and all theories concerning it must be considered only tentative and of uncertain stability. Among the anatomical elements of the human body, some appear, even with the help of the best microscopic ap- paratus, perfectly uniform and simple : they show no trace of structure, i.e., of being composed of definitely arranged dissimilar parts. These are named simple, structureless, or amorphous substances. Such is the simple membrane which forms the walls of most primary cells, of the finest gland- ducts, and of the sarcolemma of muscular fibre ; and such is the membrane enveloping the vitreous humour of the eye. Such also, having a dimly granular appearance, but no really granular structure, is the intercellular sub- stance of the so-called hyaline cartilage. In the parts which present determinate structure, certain primary forms may be distinguished, which, by their various modifications and modes of combination make up the tissues and organs of the body. Such are, I. Gra- nules or molecules, the simplest and minutest of the primary forms. They are particles of various sizes, from immeasur- able minuteness to the !O,OOOth of an inch in diameter; of various and generally uncertain composition, but usually 32 STRUCTURAL COMPOSITION OF HUMAN BODY. so affecting light transmitted through them, that at dif- ferent focal distances their centre, or margin, or whole substance, appears black. From this character, as well as from their low specific gravity (for in microscopic examina- tions they always appear lighter than water), and from their solubility in ether when they can be favourably tested, it is probable that most granules are formed of fatty or oily matter ; or, since they do not coalesce as minute drops of oil would, that they are particles of oil coated over with albumen deposited on them from the fluid in which they float. In any fluid that is not too viscid, they exhibit the phenomenon of molecular motion, shaking and vibrating incessantly, and sometimes moving through the fluid, probably, in great measure, under the influence of external vibration. Granules may be either free, as in milk, chyle, milky serum, yelk- substance, and most tissues containing cells with granules ; or enclosed, as are the granules in nerve-corpuscles, gland-cells, and epithelium-cells, the pigment granules in the pigmentum nigrum and me- dullary substance of the hair; or imbedded, as are the granules of phosphate and carbonate of lime in bones and teeth. 2. Nuclei, or cytollasts (fig. I, 6), appear to be the simplest elementary structures, next to granules. They were thus named in accordance with the hypothesis that they are always connected with cells, or tissues formed from cells, and that in the development of these, each nucleus is the germ or centre around which the cell is formed. The hypothesis is only partially true, but the terms based on it are too familiarly accepted to make it advisable to change them till some more exact and comprehensive theory is formed. Of the corpuscles called nuclei, some are minute cellules or vesicles, with walls formed of simple membrane, enclosing a colourless pellucid fluid, and often one or NUCLEI. 33 more particles, like minute granules, called nucleus- corpuscles, or nudeoli (fig. I, c). Other nuclei, again, appear to be formed simply of a small mass of protoplasm. The composition of the nucleus is uncertain. One of its most general characters, and the most useful in micro- scopic examinations, is, that it is neither dissolved nor made transparent by acetic acid, but acquires, when that fluid is in contact with it, a. darker and more distinct outline. It is commonly, too, the part of the mature cell which is capable of being stained by an ammoniacal solution of carmine the test, it may be remarked, by which, according to Dr. Beale, protoplasm or germinal matter may be always known. Nuclei may be either free or attached. Free nuclei are such as either float in fluid, like those in some of the secretions, which appear to be derived from the secreting cells of the glands, or lie loosely imbedded in solid substance, as in the grey matter of the brain and spinal cord, and most abundantly in some quickly-growing tumours. Attached nuclei are either closely imbedded in homo- geneous pellucid substance, as in rudimental cellular tissue; or are fixed on the surface of fibres, as on those of organic muscle- and organic nerve-fibres ; or are enclosed in cells, or in tissues formed by the extension or junction of cells. Nuclei enclosed in cells appear to be attached to the inner surface of the cell- wall, projecting into the cavity. Their position in relation to the centre or axis of the cell is uncertain ; often when the cell lies on a flat or broad surface, they appear central, as in blood-corpuscles, epithelium-cells, whether tesselated or cylindrical; but, perhaps, more often their position has no regular relation to the centre of the cell. In most instances, each cell contains only a single nucleus ; but in cartilage, especially when it is growing or ossifying, two or more nuclei in each cell are common ; and the development of new cells 34 STRUCTURAL COMPOSITION OF HUMAN BODY. is often effected by a division or multiplication of nuclei in the cavity of a parent cell ; as in the primary blood- cells of the embryo, in the germinal vesicle, and others. When cells extend and coalesce, so that their walls form tubes or sheaths, the nuclei commonly remain attached to the inner surface of the wall. Thus they are seen im- bedded in the walls of the minutest capillary blood-vessels of, for example, the retina and brain ; in the sarcolemma of transversely striated muscular fibres; and in minute gland-tubes. Nuclei are most commonly oval or round, and do not generally conform themselves to the diverse shapes which the cells assume; they are altogether less variable ele- ments, even in regard to size, than the cells are ; of which fact one may see a good example in the uniformity of the nuclei in cells so multiform as those of epithelium. But sometimes they appear to be developed into filaments, elongating themselves and becoming solid, and uniting end to end for greater length, or by lateral branches to form a network. So, according to Henle, are formed the filaments of the striated and fenestrated coats of arteries ; and, according to Beale, the so-called connective tissue corpuscles are to be considered branched nuclei, formed of protoplasm or germinal matter. 3. Cells. The word "cell" of course implies strictly a hollow body, and the term was a sufficiently good one when all so-called cells were considered to be small bags with a membranous envelope, and more or less liquid contents. Many bodies, however, which are still called cells do not answer to this description, and the term there- fore, if taken in its literal signification, is very apt to lead astray, and, indeed, very frequently does so. It is too widely used, however, to be given up, at least for the present, and we must therefore consider the term to indi- cate, 1st, and most commonly, a membranous bag with more or less liquid contents, and almost always a nucleus ; CELLS. 35 2ndly, a small soft semi-solid mass of matter, with no definite boundary wall, but with, most often, a small granular substance in the centre, called, as in the first case, a nucleus. In both cases, the nucleus may contain a nucleolus. Fat-cells (fig. 1 1 ) are examples of the first kind of cells : white blood- corpuscles (fig. 26) of the second. The cell-wall, when there is one, never presents any appearance of structure : it appears sometimes, as in blood- cells, to be a protein substance; sometimes a horny matter, as in thick and dried cuticle. In almost all cases (the dry cells of horny tissue, perhaps, alone excepted) the cell- wall is made transparent by acetic acid, which also pene- trates into the interior and distends it, so that it can hardly be discerned. But in such cases the cell-wall is usually not dissolved ; it may be brought into view again by merely neutralizing the acid with soda or potash. The simplest shape of cells, and that which is probably the normal shape of the primary cell, is oval or spheroidal, as in cartilage-cells and lymph-corpuscles; but in many in- stances they are flattened and discoid, as in the red blood- corpuscles (fig. 26) or scale-like, as in the epidermis and tesselated epithelium (fig. 2). By mutual pressure they may become many-sided, as are most of the pigment cells of the choroidal pigmentum nigrum (fig. 12), and those in close-textured adipose tissue ; they may assume a conical or cylindriform or prismatic shape, as in the varieties of cylinder-epithelium (fig. 4) ; or be caudate, as in certain bodies in the spleen ; they may send out exceedingly fine processes in the form of vibratile cilia (fig. 6), or larger processes, with which they become stellate, or variously caudate, as in some of the ramified pigment-cells of the choroid coat of the eye (fig. 13). The contents of all living cells, including the nucleus, are formed in a greater or less degree of protoplasm, less as the cell grows older. But, besides, cells contain matters D 2 36 STRUCTURAL COMPOSITION OF HUMAN BODY. almost infinitely various, according to the position, office, and age of the cell. In adipose tissue they are the oily matter of the fat; in gland-cells, the contents are the proper substance of the secretion, bile, semen, etc., as the case may be ; in pigment-cells they are the pigment gra- nules that give the colour ; and in the numerous instances in which the cell-contents can be neither seen because they are pellucid, nor tested because of their minute quantity, they are yet, probably, peculiar in each tissue, and con- stitute the greater part of the proper substance of each. Commonly, when the contents are pellucid, they contain granules which float in them ; and when water is added and the contents are diluted, the granules display an active molecular movement within the cavity of the cell. Such a movement may be seen by adding water to mucus-, or granulation-corpuscles, or to those of lymph. In a few cases, the whole cavity of the cell is filled with granules : it is so in yelk-cells and milk-corpuscles, in the large diseased corpuscles often found among the products of inflammation, and in some cells when they are the seat of extreme fatty degeneration. All cells containing abundant granules appear to be either lowly organized, as for nutri- ment, e.g., yelk-cells, or degenerate, e.g., granule-cells of inflammation, or of mucus. The peculiar contents of cells, may be often observed to accumulate first around or directly over the nuclei, as in the cells of black pigment, in those of rnelanotic tumours, and in those of the liver during the retention of bile. Intercellular substance is the material in which, in certain tissues, the cells are imbedded. Its quantity is very variable. In the finer epithelia, especially the columnar epithelium on the mucous membrane of the intestines, it can be just seen filling the interstices of the close-set cells ; here it has no appearance of structure. In cartilage and bone, it forms a large portion of the whole substance of the tissue, and is either homogeneous and finely granular FIBRILS: FIBRES. 37 (fig. 14), or osseous, or, as in fibro-cartilage, resembles fine fibrous tissue (fig. 15). In some cases, the cells are very loosely connected with the intercellular substance, and may be nearly separated from it, as in fibre-cartilage : but in some their walls seem amalgamated with it. The foregoing may be regarded as the simplest, and the nearest to the primary forms assumed in the organization of animal matter ; as the states into which this passes in becoming a solid tissue living or capable of life. By the further development of tissue thus far organized, according to rules which will be hereafter described, higher or secondary forms are produced, which it will be sufficient in this place merely to enumerate. Such are, 4. Filaments, or fibrils, Threads of exceeding fineness, from -g 1 00 th of an inch upwards. Such filaments are cylindriform, as are those of the striated muscular and the fibro-cellular or areolar tissue (fig. 8) ; or flattened, as are those of the organic muscles. Filaments usually lie in parallel fasciculi, as in muscular and tendinous tissues ; but in some instances are matted or reticular with branches and intercommunication, as are the filaments of the middle coat, and of the longitudinally-fibrous coat of arteries ; and in other instances, are spirally wound, or very tortuous, as in the common fibro-cellular tissue (fig. 9). 5 . Fibres, in the instances to which the name is commonly applied are larger than filaments or fibrils, but are by no essential general character distinguished from them. The flattened band-like fibres of the coarser varieties of organic muscle or elastic tissue (fig. 10) are the simplest examples of this form : the toothed fibres of the crystalline lens are more complex ; and more compound, so as hardly to permit of being classed as elementary forms, are the striated mus- cular fibres, which consist of bundles of filaments enclosed in separate membranous sheaths, and the cerebro-spinal nerve-fibres, in which similar sheaths enclose apparently two varieties of nerve substance. 3 8 ELEMENTARY TISSUES. 6. Tubules are formed of simple membrane, such as the minute capillary lymph- and blood-vessels, the investing sheaths of striated muscular and cerebro-spinal nerve- fibres, and the basement membrane or proper wall of the fine ducts of secreting glands. 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 again are variously mixed and interwoven to form more in- tricate combinations. Thus are constructed epithelium and its modifications, connective tissue, fat, cartilage, bone, the fibres of muscle and nerve, etc. ; and these again, with the more simple structures before mentioned, are used as materials wherewith to form arteries, veins and lympha- tics, secreting and vascular glands, lungs, heart, liver and other parts of the body. CHAPTER V.* STRUCTURE OF THE ELEMENTARY TISSUES. Epithelium. of the simplest of the elementary structures of which the human body is made up, is that which has received the name of Epithelium. Composed of nucleated cells which are arranged most commonly in the form of a continuous membrane, it lines the free surfaces both of the inside and outside of the body, and its varieties, with one exception, have been named after the shapes which the individual cells in different parts assume. Classified thus, Epithelium * The following Chapter, containing an outline-description of the elementary tissues, has been inserted for the convenience of students. For a much fuller and better account, the reader may be referred to Dr. Sharpey's admirable descriptions in Quain's Anatomy. EPITHELIUM. 39 presents itself under four principal forms, the characters of each of which are distinct enough in well-marked ex- amples ; but when, as frequently happens, a continuous surface possesses at different parts two or more different epithelia, there is a very gradual transition from one to the other. (i.) The first and most common variety is the squamous or tesselated epithelium (figs. I and 2), which is composed of flat, oval, roundish, or polygonal nucleated cells, of various size, arranged in one, or in many superposed layers. Arranged in several superposed layers this form of Fig. i. Fig. 2.f epithelium covers the skin, where it is called the Epidermis, and is spread over the mouth, pharynx, and oesophagus, the conjunctiva covering the eye, the vagina, and entrance of the urethra in both sexes ; while, as a single layer the same kind of epithelium lines the interior of most of the serous and synovial sacs, and of the heart, blood-vessels and lymph- vessels. (2.) Another variety of epithelium named spheroidal, from * Fig. i. Fragment of epithelium from a serous membrane (peri- toneum); magnified 410 diameters, a. cell; b. nucleus; c. nucleoli (Henle). f Fig. 2. Epithelium scales from the inside of the mouth j magnified 260 diameters (Henle). 40 ELEMENTARY TISSUES. the usually more or less rounded outline of the cells com- posing it (d, fig. 3), is found chiefly lining the interior of the ducts of the compound glands, and more or less completely filling the small sacculations or acini, in which they ter- minate. It commonly indeed occupies the true secreting parts of all glands, and hence is sometimes called glandular epithelium (&, c, and d> fig. 3). Often from mutual pressure, the cells acquire a polygonal outline. From the fact, how- ever, of the term spheroidal epithelium being a generic one for almost all gland-cells, the shapes and sizes of the cells composing this variety of epithelium are, as might be ex- pected, very diverse in different parts of the body. (3.) The third variety is the cylindrical or columnar * Fig. 3. The gastric glands of the human stomach (magnified). a, deep part of a pyloric gastric gland (from Kolliker) ; the cylindrical epithelium is traceable to the csecal extremities, b and c cardiac gastric glands (from Allen Thompson) ; b, vertical section of a small portion of the mucous membrane with the glands magnified 30 diameters ; c, deeper portion of one of the glands, magnified 65 diamaters, showing a slight division of the tubes, and a sacculated appearance produced by the large glandular cells within them ; d, cellular elements of the cardiac glands magnified 250 diameters. EPITHELIUM. 41 epithelium (figs. 4 and 5), which extends from the cardiac orifice of the stomach along the whole of the digestive canal to the anus, and lines the principal gland-ducts which Fig. 4-* open upon the mucous surface of this tract, sometimes throughout their whole extent (a, fig. 3), but in some cases only at the part nearest to the orifice (b and c) . It is also Fig- 5-t \ \ found in the gall-bladder and in the greater portion of the urethra, and in some other parts, as the duct of the parotid gland and of the testicle. It is composed of oblong cells closely packed, and placed perpendicularly to the surface they cover, their deeper or attached extremities being most commonly smaller than those which are free. Each of such cells encloses, at nearly mid-distance between its base and * Fig. 4. Cylindrical epithelium from intestinal villus of a rabbit ; magnified 300 diameters (from Kolliker). t Fig. 5. Cylinders of the intestinal epithelium (after Henle) : B. from the jejunum ; c. cylinders of the intestinal epithelium as seen when looking on their free extremities ; D. ditto, as seen on a transverse section of a villus. 4-2 ELEMENTARY TISSUES. apex, a flat nucleus with, nucleoli (B, fig. 5) ; the nuclei being arranged at such heights in contiguous cells as not to interfere with each other by mutual pressure. (4.) In the fourth variety of epithelium, cells, usually cylindrical, but occasionally of some other shape, are pro- vided at their free extremities with several fine pellucid pliant processes or cilia (figs. 6 and 7) . This form of epi- thelium lines the whole respiratory tract of mucous mem- brane and its prolongations. It occurs also in some parts Fig. 6* Fig. 7-f of the generative apparatus ; in the male, lining the vasa efferentia of the testicle, and their prolongations as far as the lower end of the epididymis ; and, in the female, com- mencing about the middle of the neck of the uterus, and ex- tending to the fimbriated extremities of the Fallopian tubes, and for a short distance along the peritoneal surface of the latter. A tesselated epithelium, with scales partly covered with cilia, lines, in great part, the interior of the cerebral ventricles, and of the minute central canal of the spinal cord. If a portion of ciliary mucous membrane from a living or recently dead animal be moistened and examined with a microscope, the cilia are observed to be in constant motion, moving continually backwards and forwards, and alter- nately rising and falling with a lashing or fanning movement. The appearance is not unlike that of the * Fig. 6. Spheroidal ciliated cells from the mouth of the frog ; magnified 300 diameters (Sharpey). t Fig. 7. Columnar ciliated epithelium cells from the human nasal membrane ; magnified 300 diameters (Sharpey). EPITHELIUM. 43 waves in a field of corn, or swiftly running and rippling water. The general result of their movements is to pro- duce a continuous current in a determinate direction, and this direction is invariably the same on the same surface, being usually in the case of a cavity towards its external orifice. Uses of Epithelium. The various kinds of epithelium serve one general purpose, namely, that of protecting, and at the same time rendering smooth, the surfaces on which, they are placed. But each also, probably, dis- charges a special ofiice in relation to the particular function of the membrane on which it is placed. In mucous and synovial membranes it is highly probable that the epithelium- cells, whatever be their forms and whatever their other functions, are the organs in which by a regular process of elaboration and secretion, such as will be afterwards described, mucus and synovial fluid are formed and discharged. (See chapter on Secretion.) Ciliated epithelium has another superadded function. By means of the current set up by its cilia in the air or fluid in contact with them, it is enabled to propel the fluids or minute particles of solid matter, which come within the range of its influence, and aid in their expulsion from the body. In the respiratory tract of mucous membrane the current set up in the air may also assist in the diffusion and change of gases, on which the due aeration of the blood depends. In the Fallopian tube the direction of the current excited by the cilia is towards the cavity of the uterus, and may thus be of service in aiding the progress of the ovum. Of the purposes served by the cilia which line the ventricles of the brain nothing is known. The nature of ciliary motion and the circumstances by which it is influenced will be considered hereafter. (See chapter on Motion.) Epithelium is devoid of blood-vessels, nerves, and 44 ELEMENTARY TISSUES. lymphatics. The cells composing it are nourished by ab- sorption of nutrient matter from the tissues on which they rest ; and as they grow old they are cast off and replaced by new cells from beneath. The origin of these new cells, whether they are the offspring of pre-existing epithelial cells, or a new generation begotten in some other way, is still a matter of doubt. Areolar, Cellular, or Connective Tissue. This tissue, which has received various names according to the qualities which seemed most important to the authors who have described it, is met with in some form or other in every region of the body ; the areolar tissue of one dis- trict being, directly or indirectly, continuous with that of Fig. 8.* all others. In most parts of the body this structure contains fat, but the quantity of the latter is very variable, * Fig. 8. Filaments of areolar tissue, in larger and smaller bundles, as seen under a magnifying power of 400 diameters. Two or three corpuscles are represented among them (Sharpey). AKEOLAR TISSUE. 45 and in some few regions it is absent altogether (p. 47). Probably no nerves are distributed to areolar tissue itself, although they pass through it to other structures; and although blood-vessels are supplied to it, yet they are sparing in quantity, if we except those destined for the fat which is held in its meshes. Under the microscope areolar tissue seems composed of a mesh-work of fine fibres of two kinds. The first, which makes up the greater part of the tissue, is formed of very fine white structureless fibres, arranged closely in bands and bundles, of wave-like appearance when not stretched out, and crossing and intersecting in all directions (fig. 8). The second kind, or the yellow elastic fibre (fig. 10), has a much Fig. 9.* sharper and darker outline, and is not arranged in bundles, but intimately mingled with the first variety, as more or less separate and well-defined fibres, which twist among and around the bundles of white filaments (fig. 9). Sometimes * Fig. 9. Magnified view of areolar tissues (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 (Sharpey). 4 6 ELEMENTARY TISSUES. Fig. 10* the yellow fibres divide at their ends and anastomose with each other by means of the branches. Among the fibrous parts of areolar or connective tissue are little nuclear bodies of various shapes, called connective-tissue corpuscles, some of which are prolonged at various points of their outline into small processes which meet and join others like them proceeding from their neighbours. The chief functions of areolar tissue seem to consist in the investment and mechanical support of various parts, and as a connecting bond between such structures as may need it. The connective-tissue corpuscles, which, accord- ing to Beale, are small branched particles of germinal matter or protoplasm, probably minister to the nutrition of the texture in which they are seated. In various parts of the body, each of the two constituents of areolar tissue which have been just mentioned, may exist sepa- rately, or nearly so. Thus ten- dons, fasciae, and the like more or less inelastic structures, are formed almost exclusively of the white fibrous tissue, arranged ac- cording to the purpose required, either in parallel bundles or membranous meshes ; while the yellow elastic fibres are found to make up almost alone such elas- tic structures as the vocal cords, the ligamenta subfiava, etc., and to enter largely into the composition of the blood-vessels, the trachea, the lungs, and many other parts of the body. * Fig. 10. Elastic fibres from the ligamenta subflava, magnified about 200 diameters (Sharpey). ADIPOSE TISSUE. 47 Adipose Tissue. In almost all regions of the human body a larger or smaller quantity of adipose or fatty tissue is present ; the chief exceptions being the subcutaneous tissue of the eye- lids, penis and scrotum, the nymphae 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, as the fat of the liver and brain, of the blood and chyle, etc. 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. Under the microscope it is found to Fig. ii.* consist essentially of little vesicles or cells about T -^- th or 3-5-o-th of an inch in diameter, each composed of a struc- tureless and colourless membrane or bag, filled with fatty matter which is liquid during life, but in part solidified after death. A nucleus is always present in some part or other of the cell- wall ; but in the ordinary condition of the * Fig. ii. A small cluster of fat-cells ; magnified 150 diameters ( Sharp ey). 48 ELEMENTAEY TISSUES. cell it is not easily or always visible. The ultimate cells are held together by capillary blood-vessels; 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 and margarin, and there is also a minute proportion of palmi- tin. The harder kinds of fat in some of the lower animals contain stearin also (p. 19). It is doubtful whether lymphatics or nerves are supplied to fat, although both pass through it on their way to other structures. Among the uses of fat, these seem to be the chief: 1. It serves as a store of combustible matter which may be re-absorbed into the blood when occasion requires, and being burnt, may help to preserve the heat of the body. 2. That part of the fat which is situate beneath the skin must, by its want of conducting power, assist in pre- venting undue waste of the heat of the body by escape from the surface. 3. 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, unendangered 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. 4. In the long bones, fatty tissue, in the form known as marrow, serves to fill up the medullary canal, and to sup- port the small blood-vessels which are distributed from it to the inner part of the substance of the bone. Pigment. In various parts of the body there exists a considerable quantity of dark pigmentary matter, e.g., in the choroid PIGMENT-CELLS. 49 coat of the eye, at the back of the iris, in the skin, etc. In all cases the dark colour is due to the presence of so- called pigment- cells. These are for the most part polyhedral (fig. 12) or spheroidal, although sometimes they have irregular processes, as shown in fig. 13. The cell- wall itself is colourless, the dark tint being produced by small dark granules heaped closely together, and more or less con- cealing the nucleus, itself colourless, which each cell contains. The dark tint of the skin, in those of dark com- plexion and in the coloured races, is seated chiefly in the Fig. 12.* Fig. I3.f epidermis, and depends on the presence of pigment-cells, which, except in the presence of the dark granules in their interior, closely resemble the colourless cells with which they are mingled. The pigment-cells are situate chiefly in * Fig. 12. Pigment-cells from the choroid ; magnified 370 diameters (Henle). A, cells still cohering, seen on their surface ; a, nucleus indistinctly seen. In the other cells the nucleus is concealed by the pigment granules. B, two cells seen in profile ; a, the outer or posterior part containing scarcely any pigment. t Fig. 13. Eamified pigment -cells, from the tissue of the choroid coat of the eye ; magnified 350 diameters (after Kolliker). a, cells with pigment ; b, colourless fusiform cells. 50 ELEMENTARY TISSUES. the deeper layer of the epidermis, or the so-called rete mucosum. The pigmentary matter is a very insoluble compound of carbon, hydrogen, nitrogen and oxygen, the carbon largely predominating besides, there is a small quantity of saline matter. The uses of pigment in most parts of the body are not clear. In the eyeball it is evidently intended for the absorption of superfluous rays of light. Cartilage. Cartilage or gristle exists in different forms in the human body, and has been classified under two chief heads, namely, temporary and permanent cartilage ; the former term being applied to that kind of cartilage which, in the foetus and in young subjects, is destined to be con- verted into bone. The varieties of permanent cartilage have been arranged in three classes, namely, the cellular, the hyaline, and the fibrous cartilages, the last-named, being again capable of subdivision into two kinds, namely, elastic or yellow cartilage, and the so-called fibro- cartilage. Elastic cartilage, however, contains fibres, and fibro- cartilage is more or less elastic ; it will be well, therefore, for distinction's sake to term those two kinds white fibro- cartilage and yellow fibro-cartilage respectively. The accompanying table represents the classification of the varieties of cartilage : 1. Temporary. A. Cellular. B. Hyaline. 2. Permanent. , white fibro-cartilage. C. Fibrous. j Yellow nbro . cart iiage. All kinds of cartilage are composed of cells imbedded in a substance called the matrix; and the apparent differences of structure met with in the various kinds of CARTILAGE. cartilage are more due to differences in the character of the matrix than of the cells. Among the latter, however, there is also considerable diversity of form and size. With the exception of the articular variety, cartilage is invested by a thin but tough and firm fibrous membrane called the perichondrium. 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. 1 . Cellular cartilage is not found in the human subject, at least in the adult. It is composed almost entirely of cells (hence its name), with little or no matrix. The latter, when present, consists of very fine fibres, which twine about the cells in various directions and enclose them in a kind of network. The cells are packed very closely together, so much so that it is not easy in all cases to make out the fine fibres often encirling them. 2. Hyaline cartilage is met with largely in the human body, investing the articular ends of bones, and forming the costal cartilages, the nasal cartilages, and those of the larynx, with the exception of the epiglottis and cornicula laryngis. Like other cartilages it is composed of cells imbedded in a matrix (fig. 1(4.). Fig. 14." * Fig. 14. A thin layer peeled off from the surface of the cartilage of the head of the humerus, showing flattened groups of cells. The shrunken cell-bodies are distinctly seen, but the limits of the capsular cavities, where they adjoin one another, are but faintly indicated. Magnified 400 diameters (after Sharpey). 5 2 ELEMENTAEY TISSUES. The cells, which contain a nucleus with nucleoli, are irregular in shape, and generally grouped together in patches. The patches are of various shapes and sizes, and placed 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 in which they are imbedded has a dimly granular appearance, like that of ground glass. 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. The costal cartilages also frequently become ossified in old age, as also do some of those of the larynx. Temporary cartilage closely resembles the ordinary hyaline kind ; the cells, however are not grouped together after the fashion just described, but are more uniformly distributed throughout the matrix. Articular hyaline cartilage is reckoned among the so- called non-vascular structures, no blood-vessels being sup- plied 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 non-vascular parts, is at the best a very artificial one (see chapter on Nutrition) . Nerves are probably not supplied to any variety of cartilage. Fibrous cartilage, as before mentioned, occurs under two chief forms, the yellow and the white fibro-cartilage. Yellow fibro-cartilage is found in the ear, in the epiglottis and cornicula laryngis, and in the eyelid. The cells are rounded or oval, with well-marked nuclei and nucleoli. The matrix in which they are seated is composed almost entirely of fine fibres, which form an intricate interlace- CARTILAGE. 53 ment about the cells, and in their general characters are allied to the yellow variety of fibrous tissue (fig. 15). White fibro-cartilage, which pi _ * is much more widely distri- buted 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-car- tilage it is not unusual to find a great part of its mass composed almost exclusively of fibres, and deserving the name of cartilage only from the fact that in another por- tion, continuous with it, cartilage cells may be pretty freely distributed. The different situations in which white fibro-cartilage is formed have given rise to the following classification: 1. Inter -articular fibro -cartilage, e.g., the semilunar car- tilages of the knee-joint. 2. Circumferential or marginal, as on the edges of the acetabulum and glenoid cavity of the scapula. 3. Connecting, e.g., the inter- vertebral fibro-cartilages. 4. Fibro-cartilage is found in the sheaths of tendons, and sometimes in their substance. The uses of cartilage are the following; in the joints, to form smooth surfaces for easy friction, and to act as a buffer, in shocks; to bind bones together, yet to allow a certain degree of movement, as between the vertebrae ; to form a firm framework and protection, yet without undue stiffness or weight, as in the larynx and chest walls; to deepen joint- cavities, as in the acetabulum, yet not so as * Fig. 15. Section of the epiglottis, magnified 380 diameters (Dr. Baly). 54 ELEMENTARY TISSUES. to restrict the movements of the bones ; to be, where such qualities are required, firm, tough, flexible, elastic, and strong. Structure of Bones and Teeth. 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 phosphate of lime, but besides there is a small quantity, about 1 1 of the 67 per cent., of carbonate of lime, with minute quantities of some other salts. The animal matter is resolved into gelatine 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 general shape of the bone is alike preserved. To the naked eye there appear two kinds of structure in different bones, and in different parts of the same bone, namely, the dense or compact, and the 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 sur- rounds a central canal, the medullary cavity so called from its containing the medulla or marrow (p. 48). 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. The spaces in the cancellous tissue are filled BONE. 55 by a species of marrow, which differs considerably from that of the shaft of the long bones. It is more fluid, and of a reddish colour, and contains very few fat cells. The surfaces of bones, except the parts covered with articular cartilage, are clothed by a tough fibrous mem- brane, the periosteum; and it is from the blood-vessels which are distributed first in this membrane, that the Fig. 16.* 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 * Fig. 1 6. Transverse section of compact tissue (of humerus) mag- nified about 150 diameters. Three of the Haversian canals are seen, with their concentric rings ; also the corpuscles or lacunse, with the canaliculi extending from them across the direction of the lamellae. 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 (after Sharpey). ELEMENTAEY TISSUES. 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. 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 occupied by a multitude of little spaces, called lacuna, with very minute canals or canaliculi, as they are termed, leading from them, and anastomosing with similar little prolongations from other lacunae (fig. 16). In very thin layers of bone, no Fig. 17.* other canals than these may be visible ; but on making a transverse section of the compact tissue, e.g., of a long bone, as the humerus or ulna, the arrangement shewn in fig. 1 6 can be seen. The bone seems mapped out into small circular dis- tricts, at or about the centre of each of which is a hole, and around this an appearance as of concentric layers the lacuna and canaliculi fol- lowing 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 (fig. 17), and * Fig. 17. Haversian canals, seen in a longitudinal section of the compact tissue of the shaft of one of the long bones, a. Arterial canal ; b. Venous canal ; c. Dilatation of another venous canal. BONE. 57 are called Haversian canals, after the name of the phy- sician, Clopton Havers, who first accurately described them. The Haversian canals, the average diameter of which is T l-o 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 canaliculi and lacunae absorbing nutrient matter from the Haversian blood-vessels, and conveying it still more intimately to the very substance of the bone which they traverse. The blood-vessels enter the Haversian canals both from without, by traversing the small holes which exist on the surface of all bones beneath the periosteum, and from within by means of small channels, which extend from the medullary cavity, or from the cancellous tissue. According to Todd and Bowman, the arteries and veins usually occupy separate canals, and the veins which are the larger often present, at irregular intervals, small pouch-like dilatations (fig. 17). The lacuna are occupied by nucleated cells, or, as Dr. Beale expresses it, minute portions of protoplasm or germinal matter ; and there is every reason to believe that the lacunar cells are homologous with the corpuscles of the connective tissue, each little particle of protoplasm ministering to the nutrition of the bone immediately surrounding it, and one lacunar particle 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. Besides the concentric lamella of bone tissue which surround the Haversian canal in the shaft of a long bone, are others, especially near the circumference, which surround the whole bone and are arranged concentrically with regard to the medullary canal. The ultimate structure of the lamella 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, 58 ELEMENTARY TISSUES. and examined with a high power of the microscope, it be found composed, according to Sharpey, of a finely . 8 reticular structure, formed ap- parently of very slender fibres decussating obliquely, but coales- cing at the points of intersection, as if here the fibres were fused rather than woven together (fig. 1 8). In many places these reticular lamellae are perforated by tapering fibres, resembling in character the ordinary white or rarely the elastic fibrous tissue, which bolt the neigh- bouring lamella together, and may be drawn out when the latter are torn asunder (fig. 19). Bone is developed after two different fashions. In one, the tissue in which the earthy matter is laid down is a membrane, composed mainly of fibres and granular cells, like imperfectly developed connective- tissue. Of this kind of ossification in membrane, the flat bones of the skull are examples. In the other, and much more common case, of which a long bone may be cited as an instance, the ossification takes place in cartilage. In most bones ossification begins at more than one point; and, from these centres of ossification, as they are called, the process of deposition of calcareous matter advances in all directions. Bones grow by constant de- velopment of the cartilage or membrane between these centres of ossification, until by the process of calcification advancing at a quicker rate than the development of the softer structures, the bone becomes impregnated through- * Fig. 1 8. Thin layer peeled off from a softened bone, as it appears under a magnifying power of 400. This figure, which is intended to represent the reticular structure of a lamella, gives a better idea of the object Avhen held rather farther off than usual from the eye (from Sharpey). TEETH. 59 out with calcareous matter, and can grow no more. In the long bones the main centres of ossification are seated at the middle of the shaft, and at each of the extremities. Increase of the length of bones, therefore, occurs at the part which intervenes between the ossifying centre in the shaft Fig. 19-* and that at each extremity ; while increase in thickness takes place by the formation of layers of osseous tissue beneath the periosteum. The former is an example of ossification in cartilage ; the latter of ossification in membrane. Teeth. A tooth is generally described as possessing a crown, neck, and. fang or fangs. The crown is the portion which projects beyond the level of the gum. The neck is that constricted portion just below the crown which is * Fig. 19. Lamellse torn off from a decalcified human parietal bone at some depth from the surface, a, a lamella, shewing reticular fibres ; b, b, darker part, where several lamellae are superposed ; c, c, perforating fibres. Apertures through which perforating fibres had passed, are seen especially in the lower part, a, a, of the figure. Magnitude as seen under a power of 200, but not drawn to a scale (from a drawing by Dr. Allen Thompson). 60 ELEMENTARY TISSUES. embraced by the free edges of the gum, and the fang includes'all below this. On making a longitudinal section through the centre of a tooth (figs. 20 and 2 1 ) , it is found to be princi- pally composed of a hard matter, dentine or ivory ; while in the centre this dentine is hollowed out into a cavity resembling in general shape the outline of the tooth, and called the pulp-cavity, from its containing a very vascular and sensi- tive little mass composed of connective tissue, blood-vessels and nerves, which is called the tooth-pulp. The pulp is continuous below, through an opening at the end of the fang, with the mucous membrane of the gum. Capping that part of the dentine which projects beyond the level of the gum, is a layer of very hard calcareous matter, the enamel, 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 the cement is exceedingly thin, but it gradually becomes thicker as it approaches and covers the lower end or apex of the fang. Dentine or ivory in chemical composition closely re- sembles bone. It contains, however, rather less animal matter; the proportion in IOO parts being about 28 of animal matter to 72 of earthy. The former, like the animal matter of bone, may be resolved into gelatin by boiling. The * Fig. 20. Sections of an Incisor and Molar Tooth. The longitudinal sections show the whole of the pulp cavity in the incisor and molar teeth, its extension upwards within the crown, and its prolongation downwards into the fangs, with the small aperture at the point of each ; these and the cross section show the relation of the dentine and enamel (from Quain's Anatomy). TEETH. 61 Fig. 21.* earthy matter is made up chiefly of phosphate of lime, with a small portion of the carbonate, and traces of some other salts. Under the microscope, dentine is seen to be finely channelled by a mul- titude of fine tubes, which, by their ianer ends, communicate with the pulp-cavity, and by their outer extre- mities come into contact with the under part of the enamel and cement, and sometimes even penetrate them for a greater or less distance. In their course from the pulp-cavity to the surface of the dentine, these minute tubes form gentle and nearly parallel curves, and divide and subdivide dichotoniously, but without much lessening of their calibre until they are approaching their peripheral ter- mination. From their sides proceed other exceedingly minute secondary canals, which extend into the dentine between the tubules. The tubules of the dentine, the average diameter of which at their inner and larger extremity is 4-^Vo" f an i ncn > contain fine prolongations from the tooth-pulp which give the dentine a certain faint sensitiveness under ordinary circumstances, and, without doubt, have to do also with its nutrition. * Fig. 21. Magnified Longitudinal Section of a Bicuspid Tooth (after Retzius). I, the ivory or dentine, showing the direction and primary curves of the dental tubuli ; 2, the pulp-cavity, with the small apertures of the tubuli into it ; 3, the cement or crusta petrosa, covering the fang as high as the border of the enamel at the neck, exhibiting lacunae ; 4, the enamel resting on the dentine ; this has been worn away by use from the upper part. 62 ELEMENTARY TISSUES. Fig, 22 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. Examined under the microscope, enamel is found composed of fine hexagonal fibres (figs. 22 and 23), which are set on end on the sur- face 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-tu- bules, they are not straight, but disposed 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 itself is coated on the outside by a very thin calcified membrane, sometimes termed the cuticle of the enamel. The crusta petrosa, or cement, is composed of true bone, and in it are lacunae and canaliculi which sometimes communicate with the outer finely-branched ends of the dentine-tubules. * Fig. 22. Thin section of the enamel and a part of the dentine (from Kolliker) ^p. a, cuticular pellicle of the enamel ; b, enamel fibres, or columns with fissures between them and cross striae ; c, larger cavities in the enamel, communicating with the extremities of some of the tubuli (d). TEETH. 36 Development of Teeth. The teeth are developed after the following manner: Along the free edge of the tooth- less gum in the foetus, there extends a groove, or small Fig. 23.* trench, the primitive dental groove (Goodsir), and from the bottom of this project ten small processes of mucous mem- brane, or papilla, containing blood-vessels and nerves. As these papillce grow up from below, the edges of the small trench begin to grow in towards each other, and over- shadow them, at the same time that each papilla is cut off from its neighbour by the extension of a partition wall from the gum, which grows in from each side to separate the one from the other. Thus closed in above and all around, each dental papilla is at length contained in a separate sac, and gradually assumes the character of a tooth by deposition on its surface of the various hard matters which have been just enumerated as composing the greater part of a tooth's substance. The small vascular * Fig. 23. Enamel fibres (from Kolliker) -. A, fragments and single fibres of the enamel, isolated by the action of h} 7 drochloric acid. B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres. 64 ELEMENTARY TISSUES. papilla is gradually encroached upon and imprisoned by the calcareous deposit, until only a small part of it is left as the tooth-pulp, which remains shut up in the harder substance, with only the before-mentioned small communi- cation with the outside, through the end of the fang. 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, having only a very limited term of existence, gradually decay and are shed, while the permanent teeth push their way from beneath, by gradual increase and development, so as to succeed them. The temporary teeth are ten in each jaw, namely, four incisors, two canines, and four molars, and are replaced by ten permanent teeth, each of 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. The number of the permanent teeth is, however, increased to sixteen, by the development of three others on each side of the jaw after much the same fashion as that by which the milk teeth were themselves formed. The beginning of the development of the permanent teeth of course takes place long before the cutting of those which they are to succeed ; one of the first acts of the newly-formed little dental sac of a milk-tooth being to set aside a portion of itself as the germ of its successor. The following formula shows, at a glance, the com- parative arrangement and number of the temporary and permanent teeth : MO. CA. IN. CA. MO. ( Upper 21412 =10 Temporary Teeth . = 20 ( Lower 21412 =10 MO. BI. CA. IN. CA. BI- MO. (Upper 321412 3=16 Permanent Teeth < - = 32 (Lower 321412 3=16 THE BLOOD. 65 From this formula it will be seen that the two bicuspid teeth in the adult are the successors of the two 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 but little, except in their smaller size, from their successors. CHAPTER VI. THE BLOOD. ALTHOUGH it may seem, in some respects, unadvisable to describe the blood before entering upon the physiology of those subservient processes which have for their end or purpose its formation and development, yet there are many reasons for taking such a course, and we may therefore at once proceed to consider the structural and chemical composition of this fluid. Wherever blood can be seen as it flows in the vessels of a living part, it appears a colourless fluid containing minute particles, the greater part of which are red, and give the blood its colour. The fluid is named liquor sanguinis ; the particles 'are the blood and lymph-corpuscles , or cells. The structural composition of the blood may be thus expressed : f Corpuscles ...... ) Clot (containing also Liquid Blood I Liquor Sanguinis | Fibrin f more or less serum.) When blood flows from the living body, it is a thickish heavy fluid, of a bright scarlet colour when it comes from an artery ; deep purple, or nearly black, when it flows from 66 THE BLOOD. a vein. Its specific gravity at 60 F. is, on an average, 1 05 5 > that of water being reckoned as IOOO; the extremes consistent with health being 1050 and 1059. Its tempera- ture is generally about IOO F. ; but it is not the same in all parts of the body. Thus, while the stream is slightly warmed by passing through the liver and some other parts, it is slightly cooled, according to Bernard, by traversing the capillaries of the skin. The temperature of blood in the left side of the heart is, again i or 2 higher than in the right (Savory). The blood has a slight alkaline reaction ; and emits an odour similar to that which issues from the skin or breath of the animal from which it flows, but fainter. The alka- line reaction appears to be a constant character of blood in all animals and under all circumstances. An exception has been supposed to exist in the case of menstrual blood ; but the acid reaction which this sometimes presents is due to the mixture of an acid mucus from the uterus and vagina. Pure menstrual blood, such as may be obtained with a speculum, or from the uteri of women who die during menstruation, is always alkaline, and resembles ordinary blood. According to Bernard, blood becomes spontaneously acid after removal from the body, owing to conversion of its sugar into lactic acid. The odour of blood is easily perceived in the watery vapour, or halitus as it is called, which rises from blood just drawn : it may also be set free, long afterwards, by adding to the blood a mixture of equal parts of sulphuric acid and water. It is said to be not difficult to tell, by the likeness of the odour to that of the body, the species of domestic animal from which any specimen of blood has been taken : the strong odour of the pig or cat, and the peculiar milky smell of the cow, are especially easy to be thus discerned in their blood (Barruel) . QUANTITY OF BLOOD. 67 Quantity of Blood. Only an imperfect indication of the whole quantity of blood in the body is afforded by measurement of that which escapes, when an animal is rapidly bled to death, inasmuch as a certain amount always remains in the blood vessels. In cases of less rapid bleeding, on the other hand, when life is more prolonged, and when, therefore, sufficient time elapses before death to allow some absorp- tion into the circulating current of the fluids of the body (p. 93), the whole quantity of blood that escapes may be greater than the whole average amount naturally present in the vessels. Various means have been devised, therefore, for obtain- ing a more accurate estimate than that which results from merely bleeding animals to death. Welcker's method is the following. An animal is rapidly bled to death, and the blood which escapes is col- lected and measured. The blood remaining in the smaller vessels is then removed by the injection of water through them, and the mixture of blood and water thus obtained, is also collected. The animal is then finely minced, and infused in water, and the infusion is mixed with the com- bined blood and water previously obtained. Some of this fluid is then brushed on a white ground, and the colour compared with that of mixtures of blood and water whose proportions have been previously determined by measure- ment. In this way the materials are obtained for a fairly exact estimate of the quantity of blood actually existing in the body of the animal experimented on. Another method (that of Vierordt) consists in estimating the amount of blood expelled from the ventricle, at each beat of the heart, and multiplying this quantity by the number of beats necessary for completing the ' round ' of the circulation. This method is ingenious, but open to various objections, the most conclusive being the uncer- F 2 68 THE BLOOD. tainty of all the premises on which the conclusion is founded. Other methods depend on the results of injecting a known quantity of water (Valentin) or of saline matters (Blake) into the blood-vessels; the calculation being founded in the first case, on the diminution of the specific gravity which ensues, and in the other, on the quantity of the salt found diffused in a certain measured amount of the blood abstracted for experiment. A nearly correct estimate was probably made by Weber and Lehmann, 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 represented 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. The most reliable of these various means for estimating the quantity of blood in the body yield as nearly similar results as can be expected, when the sources of error un- avoidably present in all, are taken into consideration ; and it may be stated that in man, the weight of the whole quantity of blood, compared with that of the body, is from about I to 8, to I to IO. It must be remembered, however, that the whole quan- tity of blood varies, even in the same animal, very consider- ably, in correspondence with the different amounts of food and drink, which may have been recently taken in, and the equally varying quantity of matter given out. Bernard found by experiment, that the quantity of blood obtainable from a fasting animal is scarcely more than a half of that which is present soon after a full meal. The estimate above COAGULATION OF BLOOD. '69 given, must therefore be taken to represent only an ap- proximate average. Coagulation of the Blood. "When blood is drawn from the body, and left at rest, certain changes ensue, which constitute a kind of rough analysis of it, and are instructive respecting the nature of some of its constituents. After about ten minutes, taking a general average of many observations, it gradually clots or coagulates, becoming solid like a soft jelly. The clot thus formed has at first the same volume and appearance as the fluid blood had, and, like it, looks quite uniform ; the only change seems to be, that the blood which was fluid is now solid. But presently, drops of transparent yellowish fluid begin to ooze from the surface of the solid clot ; and these gradually collecting, first on its upper surface, and then all around it, the clot or " crassamentum " dimin- ished in size, but firmer than it was before, floats in a quantity of yellowish fluid which is named serum, the quantity of which may continually increase for from twenty-four to forty-eight hours after the clotting of the blood. The changes just described may be thus explained. The liquor sanguinis, or liquid part of the blood, consists of serum, holding fibrin in solution.* The peculiar pro- perty of fibrin, as already said, is its tendency to become solid when at rest, and in some other conditions. When, therefore, a quantity of blood is drawn from the vessels, the fibrin coagulates, and serum and blood corpuscles are held, or, as it were, soaked and entangled in the solid substance which it forms. But after healthy fibrin has thus coagulated, it always * This statement lias "been left unaltered in the text ; but, as will be seoii farther on, it requires some modification. (Eu.) 70 THE BLOOD. contracts ; and what is generally described as one process of coagulation should rather be regarded as consisting of two parts or stages ; namely, first the simple act of clotting, coagulating, or becoming solid ; and, secondly, the con- traction or condensation of the solid clot thus formed. By this second act, much of the serum which was soaked in the clot is gradually pressed out ; and this collects in the vessel around the contracted clot. Thus, by the observation of blood within the vessels, and of the changes which commonly ensue when it is drawn from them, we may distinguish in it three principal constituents, namely, 1st, the fibrin, or coagulating sub- stance ; 2nd, the serum ; 3rd, the corpuscles. That the fibrin is the only spontaneously coagulable material in the blood, may be proved in many ways ; and most simply by employing any means whereby a portion of the liquor sanguinis, i.e., the serum and fibrin, can be separated from the red corpuscles before coagulation. Under ordinary circumstances coagulation occurs 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 dark red colour throughout, somewhat darker, it may be, at the most dependent part, from accu- mulation of red cells, but not to any very marked degree. If, however, from any cause, the red cells sink more quickly than usual, or the fibrin contracts more slowly, then, in either of these cases, the red corpuscles may be observed, while the blood is yet fluid, to sink below its surface ; and the layer beneath which they have sunk, and which has usually an opaline or greyish white tint, will coagulate without them, and form a white clot consisting of fibrin alone, or of fibrin with entangled white cor- puscles ; for the white corpuscles, being very light, tend upwards towards the surface of the fluid. The layer of white clot which is thus formed rests on the top of a coloured clot of ordinary character, i.e., of one in which BUFFY COAT. 71 the coagulating fibrin has entangled the red corpuscles while they were sinking : and, thus placed, it constitutes what has been called a huffy coat. When a buffy coat is formed in the manner just de- scribed, it commonly contracts more than the rest of the clot does, and, drawing in at its sides, produces a cupped appearance on the top of the clot. In certain conditions of the system, and especially when there exists some local inflammation, this buffed and cupped condition of the clot is well marked, and there has been much discussion concerning its origin under these circumstances. It is now generally agreed that two causes combine to produce it. In the first place, the tendency of the red corpuscles to form rouleaux (see p. 82) is much exaggerated in inflam- matory blood ; and as their rate of sinking increases with their aggregation, there is a ready explanation, at least in part, of the colourless condition of the top of the clot. And in the next place, inflammatory blood coagulates less rapidly than usual, and thus there is more time for the already rapidly sinking corpuscles to subside. The colour- less or buffed condition of the upper part of the clot is, there- fore, readily accounted for ; while the cupped appearance is easily explained by the greater power of contraction pos- sessed by the fibrin of inflammatory blood, and by its contraction being now not interfered with by the presence of red corpuscles in its meshes. Although the appearance just described is commonly the result of a condition of the blood in which there is an increase in the quantity of fibrin, it need not of necessity be so. For a very different state of the blood, such as that which exists in chlorosis, may give rise to the same appearance ; but in this case the pale layer is due to a relatively smaller amount of red corpuscles, not to any increase in the quantity of fibrin. It is thus evident that the coagulation of the blood is due 72 THE BLOOD. to its fibrin. The cause of the coagulation of the fibrin, however, is still a mystery. The theory of Prof. Lister, that fibrin 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, is insufficient ; because even if it be true, it still leaves unexplained the manner in which the fibrin, fluid in the living blood-vessels, can, by foreign matter, be thus made solid. If it be a fact, it is a very important one, but it is not an explanation. The same remark may be applied also to another theory which differs from the last, in that while it admits a natural tendency on the part of the blood to coagulation, it supposes that this tendency in the living body is re- strained by some inhibitory power resident in the walls of the containing vessels. This also may, or may not, be true ; but it is only a statement of a possible fact, and leaves unexplained the manner in which living tissue can thus restrain coagulation. Dr. Draper believes that coagulation takes place in the living body, as out of it, or as in the dead ; but in the one case the fibrin is picked out in the course of the circu- lation by tissues which this particular constituent of the blood is destined to nourish ; in the others, it re- mains and becomes evident as a clot. This explanation is ingenious, but requires some kind of proof before it can be adopted. Concerning other theories, as for instance, that coagula- tion is due to the escape of carbonic acid, or of ammonia, it need only be said that they have been completely disproved. We must therefore, for the present, believe that the cause of the coagulation of the blood has yet to be dis- FORMATION OF FIBRIN. 73 covered; but some very interesting observations in con- nexion with the subject have been recently made, and seem not unlikely to lead in time to a solution of this difficult and most vexed question. The observations referred to have been made independently by Alexander Schmidt, although he was forestalled in regard to some of his ex- periments by Dr. Andrew Buchanan of Glasgow, many years ago. "When blood-serum, or washed blood- clot, is added to the fluid of hydrocele, or any other serous effusion, it speedily causes coagulation, and the production of true fibrin. And this phenomenon occurs also on the ad- mixture of serous effusions from different parts of the body, as that of hydrocele with that of ascites, or of either with fluid from the cavity of the pleura. Other sub- stances also, as muscular or nervous tissue, skin, etc., have been found also able to excite coagulation in serous fluids. Thus, fluids which have little or no tendency to coagulate when left to themselves, can be made to produce a clot, apparently identical with the fibrin of blood, by the addition to them of matter which, on its part, was not known to have any special relation to fibrin. As may be supposed, the coagulation is not alike in extent under all these circumstances. Thus, although it occurs when ap- parently few or no blood-cells exist in either constituent of the mixture, yet the addition of these very much increases the effect, and their presence evidently has a very close connexion with the process. From the action of the buffy coat of a clot, in causing the appearance of fibrin in serous effusions, it may be inferred that the pale as well as the red corpuscles are influential in coagulation under these circumstances. Blood-crystals are also found to be effec- tive in producing a clot in serous fluids. The true explanation of these very curious phenomena is, probably, not fully known ; but Schmidt supposes that in the act of formation of fibrin there occurs the union of 74 THE BLOOD. two substances, the qualities of which he proposes to express by the terms fibrino-plastic and fibrinogenous. The substance which he terms fibrino-plastic, and which he has obtained, not only from blood, but from many other liquids and solids, as the crystalline lens, chyle and lymph, connective tissue, etc., which are found capable of exciting coagulation in serous fluids, is probably identical with the globulin of the red corpuscles. The fibrinogenous matter obtained from serous effusions differs but little, chemically, from the fibrino-plastic. Thus in the experiment before mentioned, the globulin or fibrino-plastic matter of the blood-cells in the clot, causes coagulation by uniting with the fibrinogen present in the hydrocele-fluid. And whenever there occurs coagu- lation with the production of fibrin, whether in ordinary blood-clotting, or in the admixture of serous effusions, or in any other way, a like union of these two substances may be supposed to occur. The main result, therefore, of these very interesting experiments and observations has been to make it probable that the idea of fibrin existing in a liquid state in the blood is founded on a mistaken notion of its real nature, and that, probably, it does not exist at all in solution as fibrin, but is formed at the moment of coagulation by the union of two substances which, in fluid blood, exist separately. . The theories before referred to, concerning the coagu- lation of the blood, will therefore, if this be true, resolve themselves into theories concerning the causes of the union of globulin and fibrinogen ; and whether, on the one hand, it is an inhibitory action of the living blood-vessels that naturally restrains, or a catalytic action of foreign matter that excites, the union of these two substances. CONDITIONS AFFECTING COAGULATION. 75 Conditions affecting Coagulation. Although the coagulation of fibrin appears to be spon- taneous, yet it is liable to be modified by the conditions in which it is placed ; such as temperature, motion, the access of air, the substances with which it is in contact, the mode of death, etc. All these conditions need to be considered in the study of the coagulation of the blood. The coagulation of the blood is hastened by the follow- ing means : 1. Moderate warmth, from about TOO F. to I2O F. 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. But rest is not essential to coagulation ; for the coagulated fibrin may be quickly obtained from blood by stirring it with a bundle of small twigs ; and whenever any rough points of earthy matter or foreign bodies are introduced into the blood-vessels, the blood soon coagulates upon them. 3. Contact with foreign matter, and especially multi- plication of the points of contact. Thus, when all other conditions are unfavourable, the blood will coagulate upon rough bodies projecting into the vessels ; as, for example, upon threads passed through arteries or aneurismal sacs, on the heart's valves roughened by inflammatory deposits or calcareous accumulations. And, perhaps, this may explain the quicker coagulation of blood after death in the heart with walls made irregular by the fleshy columns, than in the simple smooth walled arteries and veins. 4. The free access of air. 5. Coagulation is quicker in shallow, than in tall and narrow vessels. 6. The addition of less than twice the bulk of water. The blood last drawn is said to coagulate more quickly than that which is first let out. 76 THE BLOOD. The coagulation of the blood is retarded by the following means : 1 . Cold retards the coagulation of blood ; and it is said that, so long as blood is kept at a temperature below 40 F., 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, even after it has been frozen for several months. Coagulation is accelerated, but the subsequent contraction of the clot is hindered, by a temperature between 100 and 1 2O : a higher temperature retards coagulation, or, by coagulating the albumen of the serum, prevents it altogether. 2. The addition of water in greater proportion than twice the bulk of the blood. 3. Contact with living tissues, and especially with the interior of a living blood-vessel, retards coagulation, although if the blood be at rest it does not prevent it. 4. The addition of the alkaline and earthy salts in the proportion of 2 or 3 per cent, and upwards. When added in large proportion most of these saline substances pre- vent coagulation altogether. Coagulation, however, ensues on dilution with water. The time that 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 coa- gulates 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 coagulates quickly ; but Prof. 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 con- taining vessel. COMPOSITION OF BLOOD. 77 The coagulation of the blood is prevented altogether by the addition of strong acids and caustic alkalies. It has been believed, and chiefly on the authority of Mr. Hunter, that, after certain modes of death, the blood does not coagulate; he enumerates the deaths 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 Mr. 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. Chemical Composition of the Blood. Among the many analyses of the blood that have been published, some, in which all the constituents are enume- rated, are inaccurate in their statements of the proportions of those constituents ; others, admirably accurate in some particulars, are incomplete. The two following tables, constructed chiefly from the analyses of Denis, Lecanu, Simon, Nasse, Lehmann, Becquerel, Rodier, and Gavarret, are designed to combine, as far as possible, the advan- tage of accuracy in numbers with the convenience of presenting at one view, a list of all the constituents of the blood. Average proportions of the principal constituents of the blood in I,OOO parts : Water 784- Eed corpuscles (solid residue) . . . . ,130* Albumen of serum .70* Saline matters . . . . . . . 6*03 Extractive, fatty, and other matters . . . . 777 Fibrin 2 '2 looo- 78 THE BLOOD. Average proportions of all the constituents of the blood in 1,000 parts: Water Albumen . . Fibrin Eed corpuscles (dried) Fatty Matters : Cholesterin . . . . 0*08 Cerebrin . . . . .0*4 Serolin . . . . . o - O2 Oleic and margaric acids Volatile and odorous fatty acids Fat containing phosphorus Inorganic Salts : Chloride of sodium .... Chloride of potassium Tribasic phosphate of soda Carbonate of soda .... Sulphate of soda .... Phosphates of lime and magnesia Oxide and phosphate of iron Extractive matters, biliary colouring matter, gases, and accidental substances , 784- 70- 2 '2 ISO' 3-6 0'2 0-82 0-28 0-25 6*40 Elementary composition of the dried blood of the ox : Carbon .... .... 57^9 Hydrogen. . . . . . . . 7'i Nitrogen . . . . . . . .17*4 Oxygen . 19*2 Ashes . 4-4 These results of the ultimate analysis of ox's blood afford a remarkable illustration of its general purpose, as supply- ing the materials for the renovation of all the tissues. For the analysts (Play fair and Boeckmann) have found that the flesh of the ox yields the same elements in so nearly the same proportions, that the elementary composition of the organic constituents of the blood and flesh may be con- sidered identical, and may be represented for both by the formula C 4S H 39 N 6 O IS . The Blood-Corpuscles or Blood-Cells. It has been already said, that the clot of blood contains, BLOOD-CORPUSCLES. 79 with the fibrin and the portion of the serum that is soaked in it, the blood-corpuscles, or blood-cells. Of these there are Fig. 24.* Mammals. Birds. Reptiles. Amphibia. Fish. * The above illustration is somewhat altered from a drawing, by Mr. 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 8o THE BLOOD. two principal forms, the red and the white corpuscles. When coagulation has taken place quickly, both kinds of corpuscles may be uniformly diffused through the clot; but, when it has been slow, the red corpuscles, being the heaviest constituent of the blood, tend by gravitation to accumulate at the bottom of the clot ; and the white cor- puscles, being among the lightest constituents, collect in the upper part, and contribute to the formation of the buffy coat. The humanredblood-corpuscles (fig. 26) are circular flattened cells of different sizes, the majority varying in diameter from Wo o- to TOW of an inch > and about rznhro of an inch in thickness. Their borders are rounded ; their surfaces, in the most perfect and usual state, slightly concave ; but they readily acquire flat, or convex surfaces when, the liquor sanguinis being diluted, they are swollen by absorb- ing more fluid into their cavity. They are composed of a colourless, structureless, and transparent envelope, enclos- ing a peculiar matter named cruor, or, as it may be termed, cruoro-globulin. The cell-wall is tough and elastic, so that, as the cells circulate, they admit of elongation and other changes of form, in adaptation to the vessels, yet recover their natural shape as soon as they escape from compression. They have no nuclei, and their contents are probably homogeneous ; at least they appear so, when their surfaces are flat or slightly convex ; it is only when 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. It may be here remarked, that the appearance of a nucleus in the red blood-cells of birds, reptiles, amphibia and fish, has been lately shown by Mr. Savory to be the result of post-mortem change ; no nucleus being visible in the cells as they circulate in the living body, or in those which have just escaped from the blood-vessels. RED BLOOD-CORPUSCLES. 81 they are concave that the unequal refraction of transmitted light gives the appearance of a central spot, which is brighter or darker than the border, according as it is viewed in or out of focus.* Their specific gravity is about 1 088. In examining a number of red corpuscles with the micro- scope, it is easy to observe certain natural diversities among them, though they may have been all taken from the same part. The great majority, indeed, are very uniform ; but some are rather larger, and the larger ones generally appear paler and less exactly circular than the rest ; their surfaces also are, usually, flat or slightly convex, they often contain a minute shining particle like a nucleolus, and they are lighter than the rest, floating higher in the fluid in which they are placed. Other deviations from the general charac- ters assigned to the corpuscles, depend on changes that occur after they are taken from the body. Very commonly they assume a granulated or mulberry-like form, in" conse- quence, apparently, of a peculiar corrugation of their cell- walls. Sometimes, from the same cause, they present a very irregular, jagged, indented, or star-like appearance. The larger cells are much less liable to this change than the smaller, and the natural shape may be restored by diluting the fluid in which the corpuscles float ; by such dilution the corpuscles, as already said, may be made to swell up, by absorbing the fluid ; and, if much water be * Although the view above stated with, regard to the structure of the red corpuscles, is the one generally entertained, yet it should be observed, that Dr. Dalton in America, Professor Beale in this country, and Dr. Rollet in Germany, with some others, are of opinion that the corpuscles have no distinct cell-wall, but are homogeneous in structure throughout. Professor Beale believes that each corpuscle has different densities at different parts, being firm externally, but gradually becoming softer, so as to approach to a state of fluidity towards the centre. On the other hand, however, Dr. W. Roberts has recently offered reasons for believing that the blood-cells have a double envelope, the outer covering enclosing an anterior vesicle, within which are the coloured contents. 82 THE BLOOD. added, they will become spherical and pellucid, their colouring matter being dissolved, and, as it were, washed out of them. Some of them may thus be burst; the others are made obscure ; but many of these latter may be brought into view again by evaporating, or adding saline matter to, the fluid, so as to restore it to its previous density. The changes thus produced by water are more quickly effected by weak acetic acid, which immediately makes the cor- puscles pellucid, but dissolves few or none of them, for the addition of an alkali, so as to neutralize the acid, will restore their form though not their colour. A peculiar property of the red corpuscles, which is exag- gerated in inflammatory blood, and which appears to exist in a marked degree in the blood of horses, may be here noticed. It gives them a great tendency to adhere together in rolls or columns, like piles of coins, and then, very quickly, these rolls fasten together 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. 25). Hence, the clot formed in such a thin layer of blood looks mottled with blotches of pink upon a white ground: in a larger quan- tity of such blood, as soon as the corpuscles have clustered and collected in rolls (that is, generally in two or three minutes after the blood is drawn), they begin to sink very quickly ; for in the aggregate they pre- sent less surface to the resistance of the liquor sanguinis than they would if sinking separately. Thus quickly sink- ing, they leave above them a layer of liquor sanguinis, * Fig. 25. Red corpuscles collected into rolls (after Henle). BED BLOOD-COKPUSCLES. 83 and this coagulating, forms a buffy coat, as before des- cribed, the volume of which is augmented by the white corpuscles, which have no tendency to adhere to the red ones, and by their lightness float up clear of them. Chemical Composition of Red Blood-cells. The cell-wall is composed, doubtless, of a protein material, but from the difficulty in isolating it from the cell-contents, its composition cannot be accurately deter- mined. The cruor, which is soluble in water, is com- posed of two substances, intimately and perhaps chemically combined, which have been named globulin and cru- orin. The former, which preponderates in quantity over the cruorin in about the proportion of 90 to IO, is colour- less, and, in its nature and composition closely allied to albumen and casein. It has been found in various structures besides the red blood-cells, and exists in espe- cially large quantity in the crystalline lens ; the globulin from this source, however, differs from blood-globulin in not being crystallizable. When separated from cruorin it is almost insoluble in water, but can be dissolved by the addition of a small quantity either of an acid or alkali. It is precipitated from either solution, acid or alkaline, on neutralization, and in either case can be again dissolved by a slight excess. Heat throws it down from neither of these solutions, although the same means will precipitate it from a solution in neutral salts, not as a curd, but in the form of fine molecules and granules. Carbonic acid passed through its alkaline solution will precipitate it, while it is re-dissolved by the passage of a stream of atmospheric air or oxygen. The other constituent of the cruor, namely, the sub- stance which gives the blood its red colour, has probably not been separated in a pure and unaltered state. The experiments of Prof. Stokes seem to indicate that the heematin of chemists is really a product of chemical 84 THE BLOOD. decomposition, and not the pure and unaltered colouring matter, for which he proposes the name of cruorin. As, however, hsomatin must of necessity be very closely allied to cruorin, even if it be not identical with it, it may be well to enumerate its principal characters. In the purest state in which it can be obtained, it is so far changed as to be insoluble in water, of a deep blackish brown colour, and not liable to change of colour on exposure to gases. Boiling alcohol will dissolve small quantities of it, and it is freely soluble in alcohol acidulated with sulphuric, hydrochloric, or nitric acid, and in weak solutions of potash, soda, or ammonia. According to Mulder, pure heematin consists of carbon, 65*84 per cent. ; hydrogen, 5-37; nitrogen, 10-4; oxygen, 1175,- iron, 6 '64. The presence of so large a proportion of iron con- stitutes a peculiar feature in hsematin. The mode in which the metal exists in it has been much discussed. By some it is supposed to be in the form of an oxide, or a salt, or in the form of peroxide in arterial blood, and carbonate of the protoxide of iron in venous blood (Liebig). But the greater probability is that the iron is combined as an element with the four essential elements, carbon, hydro- gen, nitrogen, and oxygen, in the same manner as, it is believed, sulphur is combined with them in albumen, fibrin, cystic oxide, etc., or as iron in ferrocyanogen. The principal evidence for this view, which is especially sup- ported by Scherer and Mulder, is, I, that when chlorine, which would not decompose an oxide of iron, is passed through a solution of hsematin, chloride of iron is formed, and the iron, thus removed from the other elements of the haematin, is replaced by chlorous acid; 2. that all the iron may be removed from heematin by sulphuric acid, without abstracting from it any of its oxygen, which would not be possible if the iron were more intimately united with the oxygen than with the other elements of the hsematin ; 3, that pure hsernatin may be exposed for several days to WHITE BLOOD-CORPUSCLES. 85 the action of dilute hydrochloric or sulphuric acid, without any loss of its iron ; though these acids would dissolve an oxide of iron or decompose a carbonate. The peculiar colour of haematin depends less on the iron than on its other constituents, for, as Scherer and Mulder have shown, hsematin may retain its colour after all the iron is extracted from it. The blood-corpuscles also possess a small quantity "of a solid phosphuretted fat, in proportion of about 2 parts in IOO of dried corpuscles. By calcination of dried cruor 1*3 per cent, of brown alkaline ashes are obtained, and these consist of carbonate of alkali with traces of phosphate, 0-3 ; phosphate of lime, O'l ; lime, o*2 ; sub-phosphate of iron ? O'l ; peroxide of iron, 0*5 ; carbonic acid and loss O'l. The White Corpuscles. The white corpuscles are much less numerous than the red. On an average, in health, there may be one white to 400 or 500 red corpuscles; but in disease, the propor- tion is often as high as one to ten, and sometimes even much higher. In health, the proportion 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 white cells being very considerably increased by a meal, and diminished again on fasting. They present greater diversities of form than the red ones do ; but the gradations between the extreme forms are so regular, that no sufficient reason can be found for supposing that there is in healthy blood more than one species of white corpuscles. In their most general appearance, they are circular and nearly spherical, about __i__ of an inch in diameter (fig. 26). They have a grey- ish, pearly look, appearing variously shaded or nebulous, the shading being much darker in some than in others. 86 THE BLOOD. Fig. 26. They seem to be formed of some white substance, variously refracting the light, and containing granules which are in some specimens few and very distinct, in others (though rarely) so nu- merous that the whole corpuscle looks like a mass of granules. It is doubtful whe- ther these corpuscles have any true cell-wall. In a few instances an apparent cell -mem- brane can be traced around them ; but, much more commonly, even this is not discernible till after the addition of water or dilute acetic acid, which penetrates the corpuscle, and lifts up and distends what looks like a cell-wall, to the interior of which the material, that before appeared to form the whole corpuscle, remains attached as the nucleus of the ceU (fig. 26). A remarkable property of the white corpuscles, first observed by Mr. Wharton Jones, consists in their capa- bility of assuming different forms, apparently irrespective of any external influence. On watching them while fresh, with a high microscope-power, they can be seen alternately contracting and dilating, at various parts of their cir- cumference, shooting out irregular processes, and again withdrawing them partially or completely, and thus in succession assuming various irregular forms. Besides the red and white corpuscles, the microscope reveals numerous minute molecules or granules in the blood, * Fig. 26. Red and white blood-corpuscles, a, "White corpuscle of natural aspect : b, Three white corpuscles acted on by weak acetic acid. c, Red blood-corpuscles. SERUM OF BLOOD. 87 circular or spherical, and varying in size from the most minute visible speck to the --oVo- ^ an ^ ncn (Gulliver). These molecules are very similar to those found in the lymph and chyle, and are, some of them, fatty, being soluble in ether, others probably albuminous, being soluble in acetic acid. Generally, also, there may be detected in the blood, especially during the height of digestion, very minute equal-sized fatty particles, similar to those of which the molecular base of chyle is constituted (Gulliver). The Serum. The serum is the liquid part of the blood remaining after the coagulation of the fibrine. In the usual mode of coagulation, part of the serum remains soaked in the clot, and the rest, squeezed from the clot by its contraction, lies around and over it. The quantity of serum that appears around the clot depends partly on the total quantity in the blood, but partly also on the degree to which the clot con- tracts. This is affected by many circumstances : generally, the faster the coagulation the less is the amount of con- traction ; and, therefore, when blood coagulates quickly, it will appear to contain a small proportion of serum. Hence, the serum always appears deficient in blood drawn slowly into a shallow vessel, abundant in inflammatory blood drawn into a tall vessel. In all cases, too, it should be remembered, that, 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. The serum is an alkaline, slimy or viscid, yellowish fluid, often presenting a slight greenish, or greyish hue, and with a specific gravity of from 1025 to 1030. It is composed of a mixture of various substances dissolved in about nine times their weight of water. It contains, indeed, the greater part of all the substances enumerated as existing in the blood, with the exceptions of the fibrin and the red 88 THE BLOOD. corpuscles. Its principal constituent is albumen, of which it contains about 8 per cent., and the coagulation of which, when heated, converts nearly the whole of the serum into a solid mass. The liquid which remains uncoagulated, and which is often enclosed in little cavities in the coagu- lated serum, is called serosity : it contains, dissolved in water, fatty, extractive, and saline matters. r Variations in the principal Constituents of the Liquor Sanguinis. The water of the blood is subject to hourly variations in its quantity, according to the period since the taking of food, the amount of bodily exercise, the state of the atmosphere, and all the other events that may affect either the ingestion or the excretion of fluids. According to these conditions, it may vary from 700 to 790 parts in the thousand. Yet uniformity is on the whole maintained; because nearly all those things which tend to lower the proportion of water in the blood, such as active exercise, or the addition of saline and other solid matter, excite thirst ; w r liile, on the other hand, the addition of an excess of water to the blood is quickly followed by its more copious excretion in sweat and urine. And these means for adjusting the proportion of the water find their purpose in maintaining certain im- portant physical conditions in the blood ; such as its proper viscidity, and the degree of its adhesion to the vessels through which it ought to flow with the least possible resistance from friction. On this also depends, in great .measure, the activity of absorption by the blood-vessels, into which no fluids will quickly penetrate, but such as are of less density than the blood. Again, the quantity of water in the blood determines chiefly its volume, and thereby the fulness and tension of the vessels and the quantity of fluid that will exude from them to keep the tissues moist. Finally, the water is the general solvent of all the other materials of the liquor sanguinis. ALBUMEN OF BLOOD. 89 It is remarkable, that the proportion of water in the blood may be sometimes increased even during its abstrac- tion from an artery or vein. Thus Dr. Zimmerman in bleeding dogs, found the last drawn portion of blood contain 12 or 13 parts more of water in IOOO than the blood first drawn; and Polli noticed a corresponding diminution in the specific gravity of the human blood during venesection, and suggested the only probable ex- planation of the fact, namely, that during bleeding, the blood-vessels absorb very quickly a part of the serous fluid with which all the tissues are moistened. The albumen may vary, consistently with health, from 60 to 70 parts in the IOOO of blood. The form in which it exists in the blood is not yet certain. It may be that of simple solution as pure albumen : but it is, more probably, in combination with soda, as an albuminate of soda ; for, if serum be much diluted with water, and then neutralized with acetic acid, pure albumen is deposited. Another view entertained by Enderlin is that the albumen is dis- solved in the solution of the tribasic phosphate of soda, to which he considers the alkaline reaction of the blood to be due, and solutions of which can dissolve large quantities of albumen and phosphate of lime. The proportion of fibrin in healthy blood may vary be- tween 2 and 3 parts in IOOO. In some diseases, such as typhus, and others of low type, it may be as little as 1*034; in other diseases, it is said, it may be increased to as much as 7*528 parts in IOOO. But, in estimating the quantity of fibrin, chemists have not taken account of the white corpuscles of the blood. These cannot, by any mode of analysis yet invented, be separated from the fibrin of mammalian blood : their composition is unknown, but their weight is always included in the estimate of the fibrin. In health, they may, perhaps, add too little to its weight to merit consideration : but in many diseases, espe- cially in inflammatory and other blood diseases in which 90 THE BLOOD. the fibrin is said to be increased, these corpuscles become so numerous that a large proportion of the supposed increase of the fibrin must be due to their being weighed with it. On this account all the statements respecting the increase of fibrin in certain diseases need revision. The enumeration of the fatty matters of the blood makes it probable that most of those which are found in the tissues or secretions exist also ready formed in the blood ; for it contains the cholesterin of the bile, the cerebrin and phosphorised fat of the brain, and the margaric and oleic acids of common fat. The fat named serolin appears to be peculiar to the blood. The volatile fatty acid is that on which the odour of the blood mainly depends ; and it is supposed, that when sulphuric acid is added (see p. 66) , it evolves the odour by combining with the base with which, naturally, this fat is neutralized. According to Lehmann, much of the fatty matter of the blood is accumulated in the red corpuscles. These fatty matters are subject to much variation in quantity, being commonly increased after every meal in which fat, or starch, or saccharine substances have been taken. At such times, the fatty particles of the chyle, added quickly to the blood, are only gradually assimilated ; and their quantity may be sufficient to make the serum of the blood opaque, or even milk-like. As regards the inorganic constituents of the blood, the substances whichTremain as ashes after its complete burning one may observe in general their small quantity in pro- portion to that of the animal matter contained in it. Those among them of peculiar interest are the phosphate and carbonate of soda, and the phosphate of lime. It appears most probable, that the blood owes its alkaline reaction to both these salts of soda. The existence of the tribasic phosphate, a salt consisting of one equivalent of phosphoric acid with two of soda and one of basic water (P0 5 + 2N a O + HO) was proved byEnderlin: the pre- VAKIATIONS OF BLOOD. 91 sence of carbonate of soda has been proved by Lehmann and others. In illustration of the characters which the blood may derive from the phosphate of soda, Liebig points out the large capacity which solutions of that salt have of absorb- ing carbonic acid gas, and then very readily giving it off again when agitated in atmospheric air, and when the atmospheric pressure is diminished. It is probably, also, by means of this salt, that the phosphate of lime is held in solution in the blood in a form in which it is not soluble- in water, or in a solution of albumen. Of the remaining inorganic constituents of the blood, the oxide and phos- phate of iron referred to, exist in the liquor sanguinis, independently of the iron in the corpuscles. Schmidt's investigations have shown that the inorganic constituents of the blood-cells somewhat differ from those contained in the serum ; the former possessing a consider- able preponderance of phosphates and of the salts of potash, while the chlorides, especially of sodium, with phosphate of soda, are particularly abundant in the latter. Among the extractive matters of the blood, the most noteworthy are Kreatin and Kreatinin. Besides these, other organic principles have been found either constantly or generally in the blood, including casein, especially in women during lactation : glucose, or grape-sugar, found in the blood of the hepatic vein, but disappearing during its transit through the lungs (Bernard) ; urea, and in very minute quantities, uric acid (Garrod) ; hippuric and lactic acids ; ammonia (Richardson) : and lastly, certain colouring and odoriferous matters. Variations in healthy Blood under different Circumstances. As the general condition of the body depends so much on the condition of the blood, and as, on the other hand, anything that affects the body must sooner or later, and 92 THE BLOOD. to a greater or less degree, affect the blood also, it might be expected that considerable variations in the qualities of this fluid would be found under different circumstances of disease ; and such is found to be the case. Even in health, however, the general composition of the blood varies con- siderably. The conditions which appear most to influence the com- position of the blood in health, are these : sex, pregnancy, age, and temperament. 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 has a rather lower specific gravity than the average, from de- ficiency of red corpuscles. The quantity of white corpuscles, on the other hand, and of fibrin are increased. 3. Age. From the analysis of Denis it appears that the blood of the footus is very rich in solid matter, and especially in red corpuscles; and this condition, gradually diminishing, continues for some weeks after birth. The quantity of solid matter then falls during childhood below the average, again rises during adult life, and in old age falls again. 4. Temperament. But little more is known concerning the connection of this with the condition of the blood, than that there appears to be a relatively larger quantity of solid matter, and particularly of red 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. Effects of Bleeding. The result of bleeding'is to diminish the specific gravity of the blood ; and so quickly, that in a VARIATIONS OF BLOOD. , 93 single venesection, the portion of blood last drawn has often a less specific gravity that that of the blood that flowed first (J. Davy and Polli). This is, of course/ due to absorption of fluid from the tissues of the body. The physiological im- port 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. It indicates that urgent need which always exists for a certain quantity of blood, irrespective, within certain bounds, of its quality. The benefit which, at least temporarily, results from the injection of warm water into the veins of those dying from haemorrhage or cholera, illustrates the same thing very well. For some little time after bleeding, the want of red blood-cells is well marked ; but with this exception, no considerable alteration seems to be produced in the com- position of the blood for more than a very short time, the loss of the other constituents, including the pale corpuscles, being very quickly repaired. Variations in the Composition of the Blood, in different Parts of the Body. The composition 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 com- position 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. I. Differences between arterial and venous blood. These may be arranged under two heads, differences in colour, and in general composition. a. Colour. Concerning the cause of the difference in colour between arterial and venous blood, there has been 94 THE BLOOD. much doubt, not to say confusion. For while the scarlet colour of the arterial blood has been supposed by some observers, and for some reasons, to be due to the chemical action of oxygen, and the purple tint of that in the veins to the action of carbonic acid, there are facts which made it seem probable that the cause was a mechanical one rather than a chemical, and that it depended on a difference in the shape of the red corpuscles, by which their power of transmitting and reflecting light was altered. Thus, car- bonic acid was thought to make the blood dark by causing the red cells to assume a bi- convex outline, and oxygen was supposed to reverse the effect by contracting them and rendering them bi-concave. We may believe, how- ever, that Prof. Stokes has, at least for the present, set this vexed question at rest. From the results of spectrum analysis, he has been led to the conclusion that the colouring matter of the blood, or cruorin, is capable of existing in two different states of oxidation, and that the respective colours of arterial and venous blood are caused by differences in tint be- tween these two varieties scarlet cruorin and purple cruorin. The change of colour produced by passage of the blood through the lungs, and its consequent exposure to oxygen, is due, probably, to the oxidation of purple cruorin, and its conversion into scarlet cruorin ; while the readiness with which the latter is de-oxidized offers a reasonable explanation of the change, in regard to tint, of arterial into venous blood, the transformation being effected, probably, by the delivering up of oxygen to the tissues, by the scarlet cruorin, during the blood's passage through the capillaries. The changes of colour are more probably due to this cause, namely, a varying quantity of oxygen chemically combined with the cruorin, than to any mechanical effect of this gas, or to the influence of carbonic acid, either chemically, on the colouring matter, or mechanically, on the corpuscles which contain it. We ARTERIAL AND VENOUS BLOOD. 95 are not, perhaps, in a position to deny altogether the possible influence of mechanical conditions of the red corpuscles on the colour of arterial and venous blood respectively ; but it is probable that this cause alone would be quite insufficient to explain the differences in the colour of the two kinds of blood, and therefore if it be an element at all in the change, it must be allowed to take only a subordinate position. The greater part of the cruorin in both arterial and venous blood probably exists in the scarlet or more highly oxidized condition, and only a small part is de-oxidized and made purple in its passage from the arteries into the veins. The differences in regard to colour between arterial and venous blood are sometimes not to be observed. If blood runs very slowly from an artery, as from the bottom of a deep and devious wound, it is often as dark as venous blood. In persons nearly asphyxiated also, and some- times, under the influence of chloroform or ether, the arterial blood becomes like the venous. In the foetus also both kinds of blood are dark. But, in all these cases, the dark blood becomes bright on exposure to the air. Bernard has shown that venous blood returning from a gland in active secretion is almost as bright as arterial blood. b. General Composition. The chief differences between arterial and ordinary venous blood are these. Arterial blood contains rather more fibrin, and rather less albumen and fat. It coagulates somewhat more quickly. Also, it contains more oxygen, and less carbonic acid. According to Denis, the fibrin of venous blood differs from arterial, in that when it is fresh and has not been much exposed to the air, it may be dissolved in a slightly heated solution of nitrate of potash. Some of the veins, however, contain blood which differs from the ordinary standard considerably. These are the portal, the hepatic, and the splenic veins. 96 THE BLOOD. Portal vein. The blood which the portal vein conveys to the liver is supplied from two chief sources ; namely, that in the gastric and niesenteric veins, which contains the soluble elements of food absorbed from the stomach and intestines during digestion, and that in the splenic vein ; it must, therefore, 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, dextrine, and other soluble matters which may have been absorbed from the alimentary canal, this blood appears to be deficient in solid matters, especially in red corpuscles, owing to dilution by the quantity of water absorbed, to contain an excess of albumen, though chiefly of a lower kind than usual, resulting from the digestion of nitrogenized substances, and termed albuminose, and to yield a less tenacious kind of fibrin than that of blood generally. The blood from the splenic vein is probably more definite in composition, though also liable to alterations according to the stage of the digestive process, and other circum- stances. It seems generally to be deficient in red cor- puscles, and to contain an unusually large proportion of albumen. The fibrin seems to vary in relative amount, but to be almost always above the average. The propor- tion of colourless corpuscles appears also to be unusually large. The whole quantity of solid matter is decreased, the diminution appearing to be chiefly in the proportion of red 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 red corpuscles, more albumen, chiefly in the form of albuminose, and BLOOD OF PORTAL VEIN. 97 yields a less firm clot than that yielded by other blood, owing to the deficient tenacity of its fibrin. These characteristics of portal blood refer to the composition of the blood itself, and have no reference to the extraneous substances, such as the absorbed materials of the food, which it may contain ; neither, indeed, has any complete analysis of these been given. Comparative analyses of blood in the portal vein and blood in the hepatic veins have also been frequently made, with the view of determining the changes which this fluid undergoes in its transit through the liver. Great diversity, however, is observable in the analyses of these two kinds of blood by different chemists. Part of this diversity is no doubt attributable to the fact pointed out by Bernard, that unless the portal vein is tied before the liver is removed from the body, hepatic venous blood is very liable to regurgitate into the portal vein, and thus vitiate the result of the analysis. Guarding against this source of error, recent observers seem to have determined that hepatic venous blood contains less water, albumen, and salts, than the blood of the portal vein; but that it yields a much larger amount of extractive matter, in which, according to Bernard and others, is one constant element, namely, grape- sugar, which is found equally the same, whether saccharine or farinaceous matter have been present in the food or not. Besides the rather wide difference between the composi- tion of the blood of these veins and of others, it must not be forgotten that in its passage through every organ and tissue of the body, the blood's composition must be varying con- stantly, as each part takes from it or adds to it such matter as it, roughly speaking, wishes either to have or to throw away. Thus the blood of the renal vein has been proved by experiment to contain less water than does the blood of the artery, and doubtless its salts are diminished also. The blood in the renal vein is said, moreover, by Bernard and Brown-Sequard not to coagulate. 98 GASES OF THE BLOOD. This then is an example of the change produced in the blood by its passage through a special excretory organ. But all parts of the body, bones, muscles, nerves, etc., must act on the blood as it passes through them, and leave in it some mark of their action, too slight though it may be, at any given moment, for analysis by means now at our disposal. On the Gases contained in the Blood. The gases contained in the blood are carbonic acid, oxygen, and nitrogen, IOO volumes of blood containing from 40 to 50 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. The proportion of nitrogen is in both very small. Concerning the manner in which carbonic acid and oxygen exist in the blood, considerable uncertainty still prevails. It is most probable that they are partly free, and partly in a state of weak chemical combination. That portion of the carbonic acid which is chemically combined, is contained partly in a bicarbonate of soda, and partly is united with phosphate of the same base. The oxygen is carried in the red corpuscles. That the oxygen is absorbed chiefly by the red corpuscles is proved by the fact that the serum alone has little or no more power of absorbing this gas than pure water ; and the observations of Professor Stokes, before referred to, make it likely that the colouring matter, or cruorin, is the constituent of the cells which thus holds the oxygen in weak chemical union with itself, to deliver it up as it is wanted in the course of the circulation. Blood Crystals. When blood has been at rest for some time, either within or without the body, and especially if diluted with water, crystals of various kinds not unfrequently form in it. BLOOD-CRYSTALS. 99 Fig. 27.* They are very common in the coagula within aneurismal sacs, in apoplectic clots, and in other masses of extrava- sated blood. Such crystals may also often be formed artificially, by ex- posing a drop of re- cently drawn blood, diluted with water, to the air for a few minutes, and then breathing upon it. The addition of alco- hol, ether, and espe- cially chloroform, is said to facilitate the process. In some cases, however, the blood crystallizes at once, on* simple ex- posure to the air : in others, more com- plex processes have to be resorted to for their production, such as rapid freez- ing and then thaw- ing, electricity, and exhaustion in the air- pump (Rollet). Whe- ther formed natural- ly or artificially, the blood crystals (figs. Fig. 28. t * Figs. 27, 28, and 29, illustrate some of the principal forms of blood crystals : Fig. 27, Prismatic, from human blood, t Fig. 28, Tetrahedral, from blood of the guinea-pig. H 2 TOO DEVELOPMENT OF BLOOD. 27, 28 and 29) have nearly always a more or less red colour, vary much in size and shape, not only in the blood Fig. 29.* of different animals, but in the same blood at different stages of its decom- position, and also present diversities in chemical composi- tion, some being soluble in one re- agent, some in another. The sub- stance of which they are composed has been especially stu- died by Lehmann; it appears to be of an albuminous nature, and probably results from a retrograde trans- formation of the contents of the red corpuscles, namely, the JicBinato-glolulin, or, as it may be now termed, cruoro- globulin. The globulin, however, seems to be essentially the substance of which most of these crystals are composed; for Lehmann has obtained them free from colour, but in other respects, apparently unaltered. Cruorin, or a modi- fication of it, is, however, also crystallizable. This interesting subject is still involved in some obscurity. Development of the Blood. In the development of the blood little more can be traced than the processes by which the corpuscles are formed. The first formed blood-cells of the human embryo differ * Fig. 29. Hexagonal crystals, from blood of squirrel. On these six-sided plates, prismatic crystals, grouped in a stellate manner, not uiifrequently occur (after Funke). DEVELOPMENT OF BLOOD. 101 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 differs also, and it will be well perhaps to consider this first. In the process of development of the embryo, the plan, so to speak, of the heart and chief blood-vessels is first laid out in cells. Thus the heart is at first but a solid mass of cells, resembling those which constitute all other parts of the embryo ; and continuous with this are tracts of similar cells the rudiments of the chief blood-vessels. The formation of the first blood corpuscles is very simple. While the outermost of the embryonic cells, of which the rudimentary heart and its attendant vessels are composed, gradually develop into the muscular and other tissues which form the walls of the heart and blood-vessels, the inner cells simply separate from each other, and form blood-cells ; some fluid plasma being at the same time secreted. Thus, by the same process, blood is formed, and the originally solid heart and blood-vessels are hollowed out. The blood-cells produced in this way, are from about 3 5*00 to l J^ of an inch in diameter, mostly spherical, but some of them oval, pellucid and colourless, with granular contents, and a well-marked nucleus. Gradually, they acquire a red colour, at the same time that the nucleus becomes more defined, and the granular matter clears away. Mr. Paget describes them as, at this period, cir- cular, thickly disc- shaped, full-coloured, and, on an average, about YsVo" ^ an i nc -' 1 i n diameter ; their nuclei, which are about 5-oV Q- of an inch in diameter, are central, circular, very little prominent on the surfaces of the cell, and appa- rently slightly granular or tuberculated. Before the occurrence, however, of this change from the colourless to the coloured state in many instances, probably, during it, and in many afterwards, a process of multiplication takes place by division of the nucleus and subsequently of the cell, into two, and much more rarely, 102 DEVELOPMENT OF BLOOD. three or four new cells, which, gradually acquire the characters of the original cell from, which they sprang. Fig. 30 (B, c, D, E.) Fig. 30.* 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, according to Kolliker, and new blood-cells are produced by this organ. Like those just described, they are at first colourless and nucleated, but afterwards acquire the ordinary blood- tinge, and resemble very much those of the first set. Like them they may also multiply by division. In whichever way produced, however, whether from the original for- mative cells of the embryo, or by the liver, these coloured nucleated cells begin very early in foetal life to be mingled with coloured non-nucleated corpuscles resembling those of the adult, and about the second or third month of embryonic existence are completely replaced by them. * Fig. 30. Development of the first set of blood-corpuscles in the mammalian embryo. A. A dotted, nucleated embryo-cell in process of conversion into a blood- corpuscle : the nucleus provided with a nucle- olus. B. A similar cell with a dividing nucleus ; at c, the division of the nucleus is complete ; at D, the cell also is dividing. E. A blood- corpuscle almost complete, but still containing a few granules. F. Perfect blood-corpuscle. DEVELOPMENT OF BLOOD. 103 The manner of origin of these perfect non-nucleated corpuscles must be now considered. I. Concerning the Cells from which tliey arise. a. Before Birth. It is uncertain whether they are derived only from the cells of the lymph, which, at about the period of their appearance, begins to be poured into the blood ; or whether they are derived also from the nucleated red cells, which they replace, or also from similar nucleated cells, which Kolliker thinks are produced by the liver during the whole time of foetal existence. b. After Birth. It is generally agreed that after birth the red corpuscles are derived from the smaller nucleated lymph or chyle -corpuscles, the white corpuscles of the blood. II. Concerning the Manner of their Development. There is not perfect agreement among physiologists concerning the process by which lymph-globules or white corpuscles (and in the foetus, perhaps the red nucleated cells) are transformed into red non-nucleated blood-cells. For while some maintain that the whole cell is changed into a red one by the gradual clearing up of the con- tents, including the nucleus, it is believed by Mr. Wharton Jones and many others, that only the nucleus becomes the red blood-cell, by escaping from its envelope and acquiring the ordinary blood-tint. We are not in a position at present, perhaps, to say certainly which of these two theories is the true one, but the last-mentioned that which supposes the nucleus of the lymph or chyle globule to be the germ of the future red blood-cell, is the theory now almost universally adopted, at least in this country. The development of red blood-cells from the corpuscles of the lymph and chyle continues throughout life, and there is no reason for supposing that after birth they have any other origin. 104 ASSIMILATION OF BLOOD. Without doubt these little bodies 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, and we can only surmise that in these things they resemble more or less closely those parts of the body which lie more plainly within our observation. From what has been said, it will have appeared that when the blood is once formed, its growth and maintenance are effected by the constant repetition of the development of new portions. In the same proportion that the blood yields its materials for the maintenance and repair of the several solid tissues, and for secretions, so are new materials sup- plied to it in the lymph and chyle, and by development made like it. The part of the process which relates to the formation of new corpuscles has been described, but it is probably only a small portion of the whole process ; for the assimilation of the new materials to the blood must be perfect, in regard to all those immeasurable minute par- ticulars by which the blood is adapted for the nutrition of every tissue, and the maintenance of every peculiarity of each. How precise the assimilation must be for such an adaptation, may be conceived from some of the cases in which the blood is altered by disease, and by assimilation is maintained in its altered state. For example, by the insertion of vaccine matter, the blood is for a short time manifestly diseased ; however minute the portion of virus, it affects and alters, in some way, the whole of the blood. And the alteration thus produced, inconceivably slight as it must be, is long maintained ; for even very long after a successful vaccination, a second insertion of the virus may have no effect, the blood being no longer amenable to its influence, because the new blood, formed after the vaccina- tion, is made like the blood as altered by the vaccine ASSIMILATION OF BLOOD. 105 virus ; in other words, the blood exactly assimilates to its altered self the materials derived from the lymph and chyle. In health we cannot see the precision of the adjustment of the blood to the tissues ; but we may imagine it from the small influences by which, as in vaccination, it is disturbed ; and we may be sure that the new blood is as perfectly assimilated to the healthy standard as in disease it is assimilated to the most minutely altered standard.* How far the assimilation of the blood is effected by any formative power which it may possess in common with the solid tissues, we know not. That this possible formative power is, however, if present, greatly ministered to and assisted by the actions of other parts there can be no doubt ; as, ist, by the digestive and absorbent systems, and pro- bably by the liver, and all of the so-called vascular glands ; and, 2ndly, by the excretory organs, which separate from the blood refuse materials, including in this term not only the waste substance of the tissues, but also such matters as, having been taken with food and drink, may have been absorbed from the digestive canal, and have been sub- sequently found unfit to remain in the circulating current. And, 3rc%, the precise constitution of the blood is adjusted by the balance of the nutritive processes for maintaining the several tissues, so that none of the materials appro- priate for the maintenance of any part may remain in excess in the blood. Each part, by taking from the blood the materials it requires for its maintenance, is, as has been observed, in the relation of an excretory organ to all the rest; inasmuch as by abstracting the matters proper for its nutrition, it prevents excess of such matters as effectually as if they were separated from the blood and cast out altogether by the excreting organs specially present for such a purpose. * Corresponding facts in relation to the maintenance of the tissues b} T assimilation will be mentioned in the chapter on NUTRITION. io6 USES OF BLOOD. Uses of the Blood. The purposes of the blood, thus developed and main- tained, appear, in the perfect state, to be these ; 1st, to be a source whence the various parts of the body may abstract the materials necessary for their nutrition and maintenance ; and whence the secreting organs may take the materials for their various secretions; 2nd, to be an ever constantly replenished store-house of latent chemical force, which in its expenditure will maintain the heat of the body, or be transformed by the living tissues, and manifested by them in various forms as vital power ; ^rd, to convey oxygen to the several tissues which may need it, either for the discharge of their functions, or for combination with their refuse matter ; ^.th, to bring from all parts refuse matters, and convey them to places whence they may be dis- charged; 5*/i, to warm and moisten all parts of the body. Uses of the various Constituents of the Blood. Regarding the uses of the various constituents of the blood it may be said that the matter almost resolves itself into an analysis of the different parts of the body, and of the food and drink which are taken for their nutrition, with a subsequent consideration of how far any given con- stituent of the blood may be supposed to be on its way to the living tissues, to be incorporated with and nourish them, or, having fulfilled its purpose, to be on its way in a more or less changed condition to the excretory organs to be cast out. It must be remembered, however, that the blood contains also matters which serve by their combustion to produce heat, and, again, others which possibly sub- serve only a mechanical, although most important, purpose ; as for instance the preservation of the due specific gravity of the blood, or some other quality by which it is enabled to maintain its proper relation to the vessels containing it and to the tissues through which it passes. Lastly, among the constituents of the blood, are the gases, as oxygen and carbonic acid, and the substances specially adapted to carry USES OF BLOOD. 107 them, which can scarcely be said to take part in the nutri- tion of the body, but are rather the means and evidence of the combustion before referred to, on which to a great extent, directly or indirectly, all vitality depends. Albumen. The albumen which exists in so large a pro- portion among the chief constituents of the blood, is with- out doubt mainly for the nourishment of those textures which contain it or one of the compounds, as gelatin and syntonin, so nearly allied to it (see p. 24). What relationship may exist between the albumen of the liquor sanguinis and that modification of it, the globulin, contained in the red corpuscles, or how each is related to the parts to be nourished, is still a matter of uncertainty. Besides its purpose in nutrition, the albumen of the liquor sanguinis is doubtless of importance also in the maintenance of those essential physical properties of the blood to which refer- ence has been already made. Fibrin. It has been mentioned in a previous part of this chapter that the idea of fibrin existing in the blood, as fibrin, is probably founded in error ; and that it is formed in the act of coagulation by the union of two substances, which before existed separately (p. 71). In considering, therefore, the functions of fibrin, we may exclude the notion of its existence, as such, in the blood in a fluid state, and of its use in the nutrition of certain special textures, and look for the explanation of its functions to those circumstances, whether of health or disease, under which it is produced. In haemorrhage, for example, the formation of fibrin in the clotting of blood, is the means by which, at least for a time, the bleeding is restrained or stopped ; and the material which is produced for the permanent healing of the injured part, contains a coagulable material probably identical, or very nearly so, with the fibrin of clotted blood. Fatty Matters. The fatty matters of the blood subserve more than one purpose. For while they are the means, at least in part, by which the fat of the body, so widely dis- tributed in the proper adipose and other textures, is io8 USES OF BLOOD. replenished, they also, by their union with oxygen, assist in maintaining the temperature of the body. In certain secretions also, notably the milk and bile, fat is an impor- tant constituent. Saline Matter. The uses of the saline constituents of the blood are, first, to enter into the composition of such textures and secretions as naturally contain them, and, secondly, to assist in preserving the due specific gravity and alkalinity of the blood and, perhaps, also in preventing its decomposition. The phosphate and carbonate of soda, besides maintaining the alkalinity of the blood, are said especially to preserve the liquidity of its albumen, and to favour its circulation through the capillaries, at the same time that they increase the absorptive power of the serum for gases. But although, from the constant presence of a certain quantity of saline matter in the blood, we may believe that it has these last-mentioned important functions in connection with the blood itself, apart from the nutri- tion of the body, yet, from the amount which is daily separated by the different excretory organs, and especially by the kidneys, we must also believe that a considerable quantity simply passes through the blood, both from the food and from the tissues, as a temporary and useless constituent, to be excreted when opportunity offers. Corpuscles. The uses of the red corpuscles are probably not yet fully known, but they may be inferred, at least in part, from the composition and properties of their contents. The affinity of cruorin for oxygen has been already men- tioned ; and the main function of the red corpuscles seems to be the absorption of oxygen in the lungs by means of this constituent, and its conveyance to all parts of the body, especially to those tissues, the nervous and muscular, the discharge of whose functions depends in so great a degree upon a rapid and full supply of this element. The readi- ness with which cruorin absorbs oxygen, and delivers it up again to a reducing agent, so well shewn by the experi- ments of Prof. Stokes, admirably adapts it for this purpose- THE CIRCULATION. 109 How far the globulin of the red corpuscles is concerned in nutrition is quite unknown. It has been inferred that the phosphuretted fat contained in them may be concerned particularly in the nutrition of nervous matter, which contains a somewhat similar compound; and that the potash, which is in larger proportion in the red corpuscles than soda, indicates an especial relation to the nutrition of muscle, in which also potash preponderates. The relation of the red to the white corpuscles of the blood has been already considered; of the functions of the latter, other than are concerned in this relation- ship, nothing whatever is known. CHAPTER VI. (Continued). CIRCULATION OF THE BLOOD. THE body is divided into two chief cavities, the chest or thorax and abdomen, by a curved muscular partition, called the diaphragm (fig. 31). The chest is almost entirely filled by the lungs and heart ; the latter being fitted in, so to speak, between the two lungs, nearer the front than the back of the chest, and partly overlapped by them (fig. 31). Each of these organs is contained in a distinct bag, called respec- tively the right and left pleura and the pericardium, which is fibrous in the main, but lined on the inner aspect by a smooth shining epithelial covering, on which can glide, with but little friction, the equally smooth surface of the organ enveloped by it. In fig. 3 1 the containing bags of pleura and pericardium are supposed to have been removed. Entering the chest from above is a large and long air-tube, called the trachea, which divides into two branches, one for each lung, and tlirough which air passes and repasses in respiration. Springing from the upper part or base of the heart may be seen the large vessels, arteries, and veins, which convey blood either to or from this organ. no THE CIRCULATION. Larynx Trachea - Pulm y Artery In the living body the heart and lungs are in constant rhythmic movement, the result of which is an unceasing stream of air through the trachea alternately into and out of Fig. 31.* the lungs, and an unceasing stream of blood into and out of the heart. It is with this last event that we are concerned es- pecially in this chapter, with the means, that is to say, by which the blood which at one moment is forced out of the heart, is in a few moments more re- turned to it, again to depart, and again pass through the body in course of what is technically called the circulation. The purposes for which this unceasing current is maintained, are indicated in the uses of the blood enumerated in the preceding chapter. The blood is conveyed away from, the heart by the arteries, and returned to it by the veins ; the arteries and veins being continuous with each other, at one end by means of the heart, and at the 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 revolution or circulation. As generally described there are two circulations by * Fig. 31. View of heart and lungs in situ. The front portion of the chest-wall, and the outer or parietal layers of the pleurse and pericardium have been removed. The lungs are partly collapsed. THE CIRCULATION. ni which all the blood must pass ; the one a shorter circuit from the heart to the lungs and back again ; the other and larger circuit, from the heart to all parts of the body and back again ; but more strictly speaking, there is only one complete circulation, which may be diagrammatically repre- sented by a double loop, as in the accompanying figure. On reference to pig. 32.* this figure and noticing the di- rection of the ar- rows which repre- sent 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 con- tinuous stream, the whole of which must, at one part of its course, pass through the lungs. Subor- dinate 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 capillaries of the intestinal canal, and some other organs, and gathered up again into a single stream, is a second time divided in its passage through the liver, * Fig. 32. Diagram of the circulation* ii2 THE CIRCULATION. before it finally reaches the heart and completes a revolu- tion. This subordinate stream through the liver is called the portal circulation. The principal force provided for constantly moving the blood through this course is that of the muscular substance of the heart; other assistant forces are (2) those of the elastic walls of the arteries, (3) the pressure of the muscles among which some of the veins run, (4) the movements of the walls of the chest in respiration, and probably, to some extent, (5) the interchange of relations between the blood and the tissues which ensues in the capillary system during the nutritive processes. The right direction of the blood's course is determined and maintained by the valves of the heart to be immediately described; which valves open to permit the movement of the blood in the course described, but close when any force tends to move it in the contrary direction. We shall consider separately each member of the system of organs for the circulation : and first The Heart. The heart is a hollow muscular organ, the interior of which 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 subdivided into an upper and a lower portion, called respectively the auricle and ventricle, which freely communicate one with the other ; the aperture of communication, however, being guarded by valvular curtains, so disposed as to allow blood to pass freely from the auricle into the ventricle, but not in the opposite direc- tion. There are thus four cavities altogether in the heart two auricles and two ventricles ; the auricle and ventricle of one side being quite separate from those of the other. The riglit auricle communicates, on the one hand, with the veins of the general system, and, on the other, with the right ventricle, while the latter leads directly into the pul- monary artery, the orifice of which is guarded by valves. THE HEART. 113 The left auricle again communicates, on the one hand, with the pulmonary veins, and, on the other, with the left ventricle, while the latter leads directly into the aorta a large artery which conveys blood to the general system, the orifice of which, like that of the pulmonary artery, is guarded by valves. The arrangement of the heart's valves is such that the blood can pass only in one definite direction, and this is as follows (fig. 33) : From the right auricle the blood passes into the right ventricle, and thence into the pulmo- nary artery, by which it is con- veyed to the ca- pillaries of the lungs. From the lungs the blood, which is now puri- fied and altered in colour, is ga- thered by the pul- monary veins and taken to the left auricle. From the left auricle it passes into the left ventricle, and thence into the aorta, by which it is distributed to the capillaries of every portion of the body. The branches of the aorta, from being distributed to the general system, are called systemic arteries ; and from thence the blood passes into the sys- temic capillaries, where it again becomes dark and impure, and thence into the branches of the systemic veins, which, * Fig- 33- Diagram of the circulation through the heart (after Dalton), I ii4 THE CIRCULATION. forming by their union two large trunks, called the superior and inferior vena cava, discharge their contents into the right auricle, whence we supposed the blood to start (fig. 3 3). Structure of the Valves of the Heart. It will be well now to consider the structure of the Fig. 34-* * Fig. 34. The right auricle and ventricle opened, and a part of their right and anterior walls removed, so as to show their interior. . I, superior vena cava ; 2, inferior vena cava ; 2', hepatic veins cut short ; 3, right au icle ; 3', placed in the fossa ovalis, below which is the Eustachian valve ; 3", is placed close to the aperture of the coronary STRUCTURE OF HEART'S VALVES. 115 valves of the heart, and the manner in which they perform their function of directing the stream of blood in the course which has been just described. The valve between the right auricle and ventricle is named tricuspid (fig. 34), because it presents three principal cusps or pointed portions, and that between the left auricle and ventricle bicuspid or mitral, because it has two such portions (fig. 35). But in both valves there is between each two principal portions a smaller one ; so that more properly, the tricuspid may be described as consisting of six, and the mitral of four, por- tions. Each portion is of triangular form, its apex and sides lying free in the cavity of the ventricle, and its base, which is continuous with the bases of the neighbouring portions, so as to form an annular membrane around the auriculo -ventricular opening, being 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 portion of the valve may be distinguished a middle-piece, extending from its base to its apex, and including about half its width; this piece is thicker, and much tougher and tighter than the border- pieces which are attached loose and flapping at its sides. While the bases of the several portions of the valves are fixed to the tendinous rings, their ventricular surfaces vein ; + , + , placed in the auriculo -ventricular groove, where a narrow portion of. the adjacent walls of the auricle and ventricle has been pre- served ; 4, 4, cavity of the right ventricle, the upper figure is imme- diately below the semilunar valves ; 4', large columna carnea 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. (From Quoin's Anatomy.) I 2 n6 THE CIRCULATION. and borders are fastened by slender tendinous fibres, the chorda tendinea, to the walls of the ventricles, the muscular fibres of which project into the ventricular cavity in the ^ 35-* * Fig. 35. The left auricle and ventricle opened and a part of their anterior and left walls removed so as to show their interior. 4- Tne pulmonary artery has been divided at its commencement so as to show the aorta : the opening into the left ventricle has been carried a short distance into the aorta between two of the segments of the semilunar valves ; the left part of the auricle with its appendix has been removed. STRUCTURE OF HEART'S VALVES. 117 form of bundles or columns the column carnece. These columns are not all of them alike, for while some of them 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 by one extremity only, the other projecting, papilla-like, into the cavity of the ventricle (5, fig. 35), and having attached to it chorda tendinea. Of the tendinous cords, besides those which pass from the walls of the ven- tricle and the musculi papillares, to the margins of the valves both free and attached, there are some of especial strength, which pass from the same parts to the edges of the middle pieces of the several chief portions of the valve. The ends of these cords are 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 The right auricle has been thrown out of view. I, the two right pul- monary veins cut short : their openings are seen within the auricle ; i', 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 i' ; 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 portion 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. (From Quain's Anatomy.) u8 THE CIRCULATION. attached all over tlie 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 may proceed to the adjacent halves of two of the principal divisions, and to one intermediate or smaller division, of the valve. It has been already said that while the ventricles com- municate, on the one hand, with the auricles, they commu- nicate, on the other, with the large arteries which convey the blood away from the heart ; the right ventricle with the pulmonary artery (6, fig. 34), which conveys blood to the lungs, and the left ventricle with the aorta, which distributes it to the general system (7, fig. 35). And as the auriculo- ventricular orifice is guarded by valves, so are also the mouths of the pulmonary artery and aorta (%s. 34, 35). The valves, three in number, which guard the orifice of each of these two arteries, are called the semilunar valves. They are nearly alike on both sides of the heart; but those of the aorta are altogether thicker and more strongly con- structed than those of the pulmonary artery. Like the tricuspid and mitral valves, they are formed by a dupli- cature of the lining membrane of the heart, strengthened by fibrous tissue. 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 (fig. 35). 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 substance of the valve, excepting 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 ACTION OF THE HEART. 119 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 thrown back and flattened by the blood passing out of the ventricle, but which belly out imme- diately so as to prevent any return (6, fig. 34). This will be again referred to immediately. THE ACTION OF THE HEAHT. The heart's action in propelling the blood consists in the successive alternate contractions and dilatations of the muscular walls of its two auricles and two ventricles. The auricles contract simultaneously ; so do the ventricles ; their dilatations also are severally simultaneous ; and the contractions of the one pair of cavities are synchronous with the dilatations of the other. The description of the action of the heart may best be commenced at that period in each action which immediately precedes the beat of the heart against the side of the chest, and, by a very small interval more, precedes the pulse at the wrist. For at this time the whole heart is in a passive state ; the walls of both auricles and ventricles are relaxed, and their cavities are being 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 ; then, in the end of the pause, they con- tract and empty their contents into 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 120 THE CIKCULATION. appendix. The effect of this contraction of the auricles is to propel nearly the whole of their blood into the ventricles. The reflux of blood into the great veins is 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 for some distance before their entrance into the auricles; a resistance which, however, is not so complete but that a small quantity of blood does regurgi- tate, i.e., flow backwards into the veins, at each auricular contraction. The effect of this regurgitation from the right auricle is limited by the valves at the junction of the subclavian and internal jugular veins, beyond which the blood cannot move backwards ; and the coronary vein, or vein which brings back to the right auricle the blood which has circulated in the substance of the heart, is preserved from it by a valve at its mouth. 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 the dilatation or diastole of the ventricles. Thus distended, they immediately contract ; so immediately, indeed, that their contraction, or systole, looks as if it were continuous with that of the auricles. This has been graphically described by Harvey in the following passage : " These two motions, one of the ven- tricles, another of the auricles, take place consecutively, but in such a manner that there is a kind of harmony, or rhythm, present between them, the two concurring in such wise that but one motion is apparent ; especially in the warmer blooded animals, in which the movements in ques- tion are rapid. Nor is this for any other reason than it is in a piece of machinery, in which, though one wheel gives motion to another, yet all the wheels seem to move simul- taneously ; or in that mechanical contrivance which is adapted to fire-arms, where the trigger being touched, ACTION OF THE HEART. 121 down comes the flint, strikes against the steel, elicits a spark, which, falling among the powder, it is ignited, upon which the flame extends, enters the barrel, causes the explosion, propels the ball, and the mark is attained all of which incidents by reason of the celerity with which they happen, seem to take place in the twinkling of an eye." 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 contrac- tion, a small quantity of blood remains. The form and position of the fleshy columns on the internal walls of the ventricle appear, indeed, especially adapted to produce this obliteration of their cavities during their 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 the contraction of the rigor 'mortis is very marked. In such a case, only a central fissure may be discernible to the eye in the place of the cavity of each ventricle. At the same time that the walls of the ventricles con- tract, the fleshy columns, and especially those of them called the musculi papillares, contract also, and assist in bringing the margins of the auriculo-ventricular valves into apposition, so that they close the auriculo-ventricular openings, and prevent the backward passage of the blood into the auricles (p. 123). The whole force of the ven- tricular contraction is thus directed to the propulsion of the blood through their- arterial orifices. During the time which elapses between the end of one contraction of the ventricles, and the commencement of another, the com- munication between them and the great arteries the aorta on the left side, the pulmonary artery on the right is closed by the three semilunar valves situated at the orifice of each vessel. But the force with which the current of blood is propelled by the contraction of the ventricle 122 THE CIKCULATION. separates these valves from contact with each other, and presses them back against the sides of the artery, making a free passage for the stream of blood. Then, as soon as the ventricular contraction ceases, the elastic walls of the distended artery recoil, and by pressing the blood behind the valves, force them down towards the centre of the vessel, and spread them out so as to close the orifice and prevent any of the blood flowing back into the ven- tricles (p. 127). 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. They are thus filling during all the time in which the ventricles are contracting ; and the con- traction 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. If we suppose a cardiac revolution, which includes the contraction of the auricles, the contraction of the ventricles, and their repose, to occupy rather more than a second, the following table will represent, in tenths of a second, the time occupied by the various events we have considered. Contraction of Auricle . . . I Eepose of Auricles . . 10 ,, ,, Ventricles . 4 ,, ,, Ventricles . . 7 Repose ....... 6 Contraction .... 5 ii Action of the Valves of the Heart. The periods in which the several valves of the heart are in 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 dilatation and distension of the ventricles the latter valves are shut, the former open. FUNCTION OF THE VALVES. 123 Fig. 36.* Each half or side of the heart, through the action of its valves, may be compared with a kind of forcing pump, like the common enema- syringe with two valves, of which one admits the fluid on raising the piston, but is closed again when the piston is forced down; while the other opens for the es- cape of the fluid, but closes when the piston is raised, so as to prevent the re- gurgitation of the fluid already forced through it. The ventricular dilatation is here represented by the raising-up of the piston; the valve thus admit- ting fluid repre- sents the auriculo- ventricular valve, which is closed * Fig. 36. Diagrams of valves of the heart (after Dalton). 124 THE CIRCULATION. again when the piston is forced down, i.e., when the ventricle contracts, and the other, i.e., the arterial, valve opens. The diagrams on the preceding page illustrate this very well. During auricular contraction, the force of the blood pro- pelled into the ventricle is transmitted in all directions, but being insufficient to raise the semilunar valves, it is ex- pended in distending the ventricle, and in raising and gradually closing the auriculo- ventricular valves, which, when the ventricle is full, form a complete septum between it and the auricle. This elevation of the auriculo-ventricular valves is, no doubt, materially aided by the action of the elastic tissue which Dr. Markham has shown to exist so largely in their structure, especially on the auricular sur- face. When the ventricle contracts, the edges of the valves are maintained in apposition by the simultaneous contrac- tion of the columns earner, which are enabled thus to act by the arrangement of their tendinous cords just men- tioned. In this position the segments of the valves are held secure, even though the form and size of the orifice and the ventricle may change during the continued con- traction; for the border-pieces are held by their mutual apposition and the equal pressure of the blood on their ventricular surfaces; and the middle pieces are secure by their great strength, and by the attachment of the ten- dinous cords along their margins, these cords being always held tight by the contraction of the musculi papillares. A peculiar advantage, derived from the projection of these columns into the cavity of the ventricle, seems to be, that they prevent the valve from being everted into the auricle; for, when the ventricle contracts, and the parts of its walls to which, through the medium of the columns, the ten- dinous cords are affixed, approach the auriculo-ventricular orifices, there would be a tendency to slackness of the cords, and the valves might be everted, if it were not that while the wall of the ventricle is drawn towards the orifice, FUNCTION OF THE VALVES. 125 the end of the simultaneously contracting fleshy column is drawn away from it, and the cords are held tight. 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: and, since this may be advantageous, by preventing the over- filling of the vessels of the lungs, it has been called the safety-valve action of this valve (Hunter, Wilkinson King). 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.g., in certain pulmonary diseases, in very active exertion, and in great efforts. In these cases, perhaps, because the right ventricle cannot contract quickly or completely enough, the tricuspid valve does not completely close, and the regurgitation of blood may be indicated by a pulsation in the jugular veins syn- chronous with that in the carotid arteries. The arterial or semilunar valves are, as already said, brought into action by the pressure of the arterial blood forced back towards the ventricles, when the elastic walls of the arteries recoil after being dilated by the blood pro- pelled into them in the previous contraction of the ventricle. The dilatation 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 a 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, does not admit of extension under such pressure as it is commonly exposed to; the valves are equally inextensile, being, as already mentioned, formed of tough, close-textured, fibrous tissue, with strong inter- woven cords, and covered with endocardium. Hence, when 126 THE CIECULATION. 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 unyield- ing 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, which had previously lain in contact with the inner surface of the artery (as at A, fig. 37), are drawn inward towards its Fig. 37.* centre (fig. 37, B). Their positions may be explained by the foregoing diagrams, in which the continuous lines represent a transverse section of the arterial walls, the dotted ones the edges of the valves, firstly, when the valves are in contact with the walls (A), and, secondly, when the walls being dilated, the valves are drawn away from them (B). This position of the valves and arterial walls is retained so long as the ventricle continues in contraction: but, so * Fig. 37. Sections of aorta, to show the action of the semilunar ralves. A. is intended to show the valves, represented by the dotted lines, in contact with 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 straight- ened into the form of an equilateral triangle, as represented by the dotted lines. FUNCTION OF THE VALVES. 127 soon as it relaxes, and the dilated arterial walls can recoil by their elasticity, they press the blood as well towards the ventricles as onwards in the course of the circulation. Part of the blood thus pressed back lies in the pouches (a, fig. 37, 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. 38). * Fig. 38. View of the base of the ventricular part of the heart, showing the relative position of the arterial and auriculo- ventricular orifices. f . The muscular fibres of the ventricles 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 immediately above the auriculo-ventricular orifices. The semilunar and auriculo-ventricular valves are in the nearly closed con- dition. I, I, the base of the right ventricle ; i', 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 pul- monary artery ; 8, placed upon the posterior part of the root of the aorta ; 9, the right, 9', the left coronary artery. (From Quain's Anatomy). 128 THE CIRCULATION. Fig. 39.* Mr. Savory has 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. Availing himself of a method of dissection hitherto apparently overlooked, namely, that of making vertical sections (fig. 39) through various parts of the tendinous rings, he has been enabled 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 conse- quently the portion of each semi- lunar 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 free border of the ventricle. The result of this arrangement will be that the reflux of the blood will be most efficiently sustained by the ventricular wall, which, at the moment of its occurrence, is probably in a state of contraction.! The effect of the blood's pressure on the valves is, as said, to cause their margins to meet in three lines radiating from the centre to the circumference (7 and 8, fig. 38). The con- tact of the valves in this position, and the complete closure of the arterial orifice, are secured by the peculiar construc- tion of their borders before mentioned. Among the cords which are interwoven in the substance of the valves, are * Fig. 39. Vertical section through the aorta at its junction with the left ventricle, i. Section of arterial coat. 2. Section of valve. 3. Section of ventricle. t Mr. Savory's preparations, illustrating this and other points in relation to the structure and functions of the valves of the heart, are in the museum of St. Bartholomew's Hospital. SOUNDS OF THE HEART. 129 ' 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 enclose 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 elsewhere. 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 unfrequently, they are wanting in the valves of the pulmonary 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. 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 pro- longed ; its commencement coincides with the impulse of the heart, 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 occupied respectively by the two sounds 130 THE CIRCULATION. 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 elastic 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 difference may be also expressed, as Dr. C. J. B. Williams has remarked, by saying the words lubb dup. The events which correspond, in point of time, with the first sound, are the contraction of the ventricles, the first part of the dilatation of the auricles, the closure of the auriculo -ventricular valves, the opening of the semilunar valves, and the propulsion of blood into the arteries. The sound is succeeded, in about one-thirtieth of a second, by the pulsation of the facial artery, and in about one-sixth of a second, by the pulsation of the arteries at the wrist. The second sound, in point of time, immediately follows the cessation of the ventricular contraction, and corresponds with the closure of the semilunar valves, the continued dilatation of the auricles, the commencing dilatation of the ventricles, and the opening of the auriculo -ventricular valves. The pause immediately follows the second sound, and corresponds in its first part with the completed disten- sion of the auricles, and in its second with their contraction, and the distension of the ventricles, the auriculo-ventricular valves being all the time open, and the arterial valves closed. Scarcely any subject has been investigated with greater care than that relating to the cause of the sounds of the heart ; yet, although nearly all observers agree about the SOUNDS OF THE HEART. 131 cause of the second of these sounds, there is still great dis- crepancy of opinion respecting the real agent concerned in the production of the first of them. The most rational theory, however, to be at present adopted, with regard to the cause of the first sound, is one which admits the co-operation of several coincident agencies in its production. The chief of these, probably by far the chief, appears to be the vibration of the auriculo-ventricular valves, and also, but to a less extent, of the ventricular walls, and coats of the aorta and pul- monary artery, all of which parts are suddenly put into a state of tension and stretching at the moment of ventricular contraction. This view, long ago advanced by Dr. Billing, is sup- ported by the fact observed by Valentin, that if a portion of a horse's intestine, tied at one end, be moderately filled with water, without any admixture of air, and have a syringe containing water fitted to the other end, the first sound of the heart is exactly imitated by forcing in more water, and thus suddenly rendering the walls of the intes- tine more tense. Although, however, it is most likely that the vibration first referred to, is the chief element in the production of the first sound of the heart, it must be allowed that there are others which may assist, if only in a small degree, in producing the effect. Of these, (l) the noise or bruit resulting from the sudden and forcible contraction of the large mass of muscular fibres composing the walls of the ventricles, and (2), the vibration of the blood itself in its rapid compression and ejection from the heart into the arteries (Leared) with the coincident throwing back of the semilunar valves, are probably the chief. The share which these severally take in the production of the first sound, is, however, very uncertain, and their effect alto- gether is probably but very slight. The cause of the second sound is more simple than that 132 THE CIRCULATION. of the first. It is probably due almost entirely to the sudden closure and consequent vibration of the semilunar valves when they are pressed down across the orifice of the aorta and pulmonary artery ; for, of the other events which take place during the second sound, none is cal- culated to produce sound. The influence of the valves in producing the sound, is illustrated by the experiment already quoted from Valentin, and by others performed on large animals, such as calves, in which the results could be fully appreciated. In these experiments 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 auricula-ventricular. It might be expected therefore that their vibration would be more 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 ventricular contraction. The Impulse of the Heart. At the commencement of each ventricular contraction, the heart may be felt to beat with IMPULSE OF THE HEART. 133 a slight shock or impulse against the walls" of the chest. This impulse is most evident in the space between the fifth and sixth ribs, between one and two inches to the left of the sternum. The force of the impulse, and the extent to which it may be perceived beyond this point, vary con- siderably in different individuals, and in the same indi- viduals 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. The impulse of the heart is probably the result, in part, of a tilting forwards of the apex, so that it is made to strike against the walls of the chest. This tilting move- ment is thought to be effected by the contraction of the spiral muscular fibres of the ventricles, and especially of certain of these fibres which, according to Dr. Reid, arise from the base of the ventricular septum, pass downwards and forwards, forming part of the septum, then emerge and curve spirally around the apex and adjacent portion of the heart. The whole extent of the movement thus produced is, however, but slight. The condition, which, no doubt, contributes most to the occurrence and character of the impulse of the heart, is its change of shape; for, during the contraction of the ventricles, and the consequent approximation of the base towards the apex, the heart becomes more globular, and bulges so much, that a distinct impulse is felt when the finger is placed over the bulging portion, either at the front of the chest, or under the 134 THE CIRCULATION. diaphragm. The production of the impulse is, perhaps, further assisted by the tendency of the aorta to straighten itself and diminish its curvature when distended with the blood impelled by the ventricle ; and, by the elastic recoil of all the parts about the base of the heart, which, accord- ing to the experiments of Kurschner, are stretched down- ward and backward by the blood flowing into the auricles and ventricles during the dilatation of the latter, but re- cover themselves when, at the beginning of the contraction of the ventricles, the flow through the auriculo-ventricular orifices is stopped. But these can only be accessory con- ditions in the perfect state of things ; for the same tilting movement of the heart ensues when its apex is cut off, and no tension or change of form can be produced by the blood. Frequency and Force of the Heart's Action. The frequency with which the heart performs the actions we have described, may be counted by the pulses at the wrist, or in any other artery; for these correspond with the contractions of the ventricles. The heart of a healthy adult man in the middle period of life, acts from seventy to seventy-five times in a minute. The frequency of the heart's action gradually diminishes from the commencement to near the end of life, but is said to rise again somewhat in extreme old age, thus : In the embryo the average number of pulses in a minute is 150 Just after birth from 140 to 130 During the first year 130 to 115 During the second year 115 to 100 During the third year . . . . . 100 to 90 About the seventh year 90 to 85 About the fourteenth year, the average number of pulses in a minute is from . . . 85 to 80 In adult age . . . . . . . 80 to 70 In old age . . . . . . 7 to 60 In decrepitude . . . . . . 75 to 65 In persons of sanguine temperament, the heart acts ACTION OF THE HEART. 135 somewhat more frequently than in those of the phleg- matic ; and in the female sex more frequently than in the male. After a meal its action is accelerated, and sjill more so during bodily exertion or mental excitement ; it is slower during sleep. The effect of disease in producing tem- porary increase or diminution of the heart's action is well known. From the observation of several experimenters, it appears that, 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. It is found, also, that as a general rule, the pulse, 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 posture. The t effect of change of posture is greater as the fre- quency of the pulse is greater, and accordingly, is more marked in the morning than in the evening. Dr. Guy, by supporting the body in different postures, without the aid of muscular effort of the individual, has proved that the increased frequency of the pulse in the sitting and stand- ing 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 un-, necessary. The effect of food, like that of change of posture, is greater in the morning than in the evening. According to Parrot, the frequency of the pulse increases in a corresponding ratio with the elevation above the sea ; and Dr. Frankland, who lately passed a night on the summit of Mont Blanc, informed the author, that his pulse was about double the ordinary standard all the time he was there. After six hours' perfect rest and sleep at the top, it was 1 20, on descending to the corridor it fell to 108, 136 THE CIRCULATION. at the Grands Mulets it was 88, at Chamounix 56 ; normally, his pulse is 60. In health there is observed a nearly uniform relation between the frequency of the pulse and of the respirations ; the proportion being, on an average, one of the latter to three or four of the former. The same relation is generally maintained in the cases in which the pulse is naturally accelerated, as after food or exercise : but in disease this relation usually ceases to exist. In many affections accom- panied with increased frequency of the pulse, the respira- tion, is, indeed, also accelerated, yet the degree of its acceleration bears no definite proportion to the increased number of the heart's actions : and in many other cases, the pulse becomes more frequent without any accompany- ing increase in the number of respirations; or, the respiration alone may be accelerated, the number of pulsations remaining stationary, or even falling below the ordinary standard. (On the whole of this subject the article Pulse, by Dr. Guy, in the Cyclopaedia of Anatomy and Physiology, may be advantageously con- sulted.) The/orce with which the left ventricle of the heart con- tracts is about double that exerted by the contraction of the right : being equal (according to Valentin) to about ^L-th of the weight of the whole body, that of the right being equal only to y^o-th of the same. This difference in the amount of force exerted by the contraction of the two ventricles, results from the walls of the left ventricle being about twice as thick as those of the right. And the dif- ference 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 force exercised by the auricles in their contraction has not been determined. Neither is it known with what CAPACITY OF THE HEART. 137 amount of force either the auricles or the ventricles dilate ; but there is no evidence for the opinion, that in their dilata- tion they can materially assist the circulation by any such action as that of a sucking-pump, or a caoutchouc bag, in drawing blood into their cavities. That the force which the ventricles exercise in dilatation is very slight, has been proved by Oesterreicher. He removed the heart of a frog from, the body, and laid upon it a substance suffi- ciently heavy to press it flat, and yet so small as not to conceal the heart from view ; he then observed that during the contraction of the heart, the weight was raised; but that during its dilatation, the heart remained flat. And the same was shown by Dr. Clendinning, who, applying the points of a pair of spring callipers to the heart of a live ass, found that their points were separated as often as the heart swelled up in the contraction of the ventricles, but approached each other by the force of the spring when the ventricles dilated. Seeing how slight the force exerted -in the dilatation of the ventricles is, it has been supposed that they are only dilated by the pressure of the blood impelled from the auricles; but that both ventricles and auricles dilate spontaneously is proved by their continuing their successive contractions and dilatations when the heart is removed, or even when they are separated from one another, and when therefore no such force as the pressure of blood can be exercised to dilate them. By such spon- taneous dilatation they at least offer no resistance to the influx of blood, and save the force which would otherwise be required to dilate them. The capacity of the two ventricles is probably exactly the same. It is difficult to determine with certainty how much this may be; but, taking the mean of various estimates, it may be inferred that each ventricle is able to contain on an average, about three ounces of blood, the whole of which is impelled into their respective arteries at each contraction. The capacity of the auricles is rather less 138 THE CIRCULATION. than that of the ventricles ; the thickness of their walls is considerably 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 auricles contract. Cause of the Rhythmic Action of the Heart. It has been attempted in various ways to account for the existence and continuance of those peculiar rhythmic move- ments by which the action of the heart is distinguished from that of all the other muscles. By some it has been supposed that the contact of arterial blood with the lining membrane of the left cavities of the heart, and of venous blood with that of the right cavities, furnishes a stimulus, in answer to which the walls of these cavities contract. And they explain the rhythmic order in which these con- tractions ensue, by supposing that the same act, the systole, which expe]s the stimulating fluid from the ven- tricles, causes the auricles to be filled from the veins ; and that the contraction of the auricles thereupon induced, gives rise, in its turn, to the filling and consequent contraction of the ventricles. But the fact that the heart, especially in Amphibia and fishes, will continue to contract and dilate regularly and in rhythmic order after it is removed from the body, completely emptied of blood, and even placed in a vacuum where it cannot receive the stimulus of the atmo- spheric air, is a proof that even if the contact of blood be the ordinary stimulus to the heart's contraction, it cannot alone be an explanation of its rhythmic motion. The influence of the mind, and of some affections of the brain and spinal cord upon the action of the heart, proves that it is not altogether, or at all times, independent of the cerebro- spinal nervous system. Yet the numerous experiments instituted for the purpose of determining the exact relation in which the heart stands towards this KHYTHM OF THE HEART. 139 system, have failed to prove that the action is directly governed by the power of any portion of the brain or spinal cord. The results of the experiments are, in many instances contradictory; but they lead to the general conclusion, that no uniform and decided alteration in the movements of the heart is produced by irritation of any part of either of those nervous centres. Sudden destruc- tion of either the brain or spinal cord alone, or of both together, produces, immediately, a temporary interruption or cessation of the heart's action : but this appears to be only an effect of the shock of so severe an injury ; for, in some such cases, the movements of the heart are subse- quently resumed, and if artificial respiration be kept up, may continue for a considerable time ; and may then again be arrested by a violent shock applied through an injury of the stomach. While, therefore, we must admit an indirect or occasional influence exercised by, or through, the brain and spinal cord upon the movements of the heart, and may believe this influence to be the greater the more highly the several organs are developed, yet it is clear that we cannot ascribe the regular determination and direction of the movements to these nervous centres. The persistence of the movements of the heart in their regular rhythmic order, after its removal from the body, and their capability of being then re-excited by an ordinary stimulus after they have ceased, prove that the cause of these movements must be resident within the heart itself. And it seems probable, from the experiments and observa- tions of various observers, that it may be connected with the existence of numerous minute ganglia of the sympathetic nervous system, which, with connecting nerve-fibres, are distributed through the substance of the heart. These ganglia appear to act as so many centres or organs for the production of motor impulses ; while the connecting nerve- fibres unite them into one system, and enable them to act in concert and direct their impulses so as to excite in 140 THE CIECULATIOJS T . regular series the successive contractions of the several muscles of the heart. The mode in which ganglia thus act as centres and co-ordinators of nervous power will be described in the chapter on the NEHVOTTS SYSTEM ; and it will appear probable that the chief peculiarity of the heart, in this respect, is due to the number of its ganglia, and the apparently equal power which they all exercise ; so that there is no one part of the heart whose action, more than another's, determines the actions of the rest. Thus, if the heart of a reptile be bisected, the rhythmic, suc- cessive actions of auricle and ventricle w r ill go on in both halves: we therefore cannot say that the action of the right side determines or regulates that of the left, or vice versa ; and we must suppose that when they act together in the perfect heart, it is because they are both, as it were, set to the same time. Neither can we say that the auricles determine the action of the ventricles; for, if they are separated, they will both contract and dilate in regular, though not necessarily similar, succession. A fact pointed out by Mr. Maiden shows how the several portions of each cavity are similarly adjusted to act alike, yet independently of each other. If a point of the surface of the ventricle of a turtle's or frog's heart be irritated, it will immediately contract, and very quickly afterwards all the rest of the ventricle will contract; but, at the close of this general contraction, the part that was irritated and contracted first, is slightly distended or pouched out, showing that it was adjusted to contract in, and for only, a certain time, and that therefore as it began to contract first, so it began to dilate first. Mr. Paget, however, has shown that the cause of the rhythmic motion does not exist equally in all parts of the heart. If, for example, the cut-out heart of a tortoise be divided into two pieces, one comprising the auricles and the base of the ventricle, the other comprising the rest of the ventricle, the former will continue to act rhythmically, the KHYTHM OF THE HEART. 141 latter will cease to do so, and no rhythmic action can be, by any means, excited in it. Other sections of the heart, and experiments of other kinds, seem to show that the cause of the rhythmic action of the ventricle, and probably also of the auricles so long as they are associated with it, is situated about the boundary-ring between the auricles and ventricle ; for that which remains connected with this part retains its rhythm, while that which is disconnected from it loses rhythm. They seem to prove, also, that the rhythm does not depend on the properties of the muscular tissue alone or independently, but is derived from the nervous ganglia, as so many centres of rhythmic action, which are chiefly situated in the region named. Why these nervous centres should issue impulses for rhythmic rather than for continuous action, is still a debateable point. The most philosophical interpretation yet given of it, and of rhythmic processes in general, is that by Mr. Paget, who regards them as dependent on rhythmic nutrition, i.e., on a method of nutrition in which the acting parts are gradually raised, with time-regulated progress, to a certain state of instability of composition, which then issues in the discharge of their functions, e.g., of nerve-force in the case of the cardiac ganglia, by which force the muscular walls are excited to contraction. According to this view, there is in the nervous ganglia of the heart, and in all parts originating rhythmic processes, the same alternation of periods of action with periods of repose, during which the waste in the structure is repaired, as is observed in most of, if not all, the organic phenomena of life. All organic processes seem to be regulated with exact observance of time ; and rhythmic nutrition and action, as exhibited in the action of the heart, are but well-marked examples of such chronometric arrangement. We may conclude, then, that the nervous ganglia in the heart's substance are the immediate regulators of the heart's action, but that they are themselves liable to in- 142 THE CIRCULATION. fluences conveyed from without, through branches of the pneumogastric and sympathetic nerves. It is generally believed that the pneumogastric nerves are the media of an inhibitory influence over the action of the heart, from the fact that when by section their influence is withdrawn, the pulsations of the organ are increased in frequency and strength ; and, again, that an opposite effect is produced by stimulating them, the transmission of an electric current, of even moderate strength, diminishing the pulsations, or stopping them altogether. Stimulation of the sympathetic nerves, on the other hand, accelerates and strengthens the heart's action. Various theories have been proposed to account for these peculiar results, but none of them are very satis- factory, and it is probable that many more facts must be discovered before any theory on the subject can be per- manently maintained. The connection of the action of the heart with the other organs, and the influences to which it is subject through them, are explicable from the connection of its nervous system with the other ganglia of the sympathetic, and with the brain and spinal cord through, chiefly, the pneumo- gastric nerves. But this influence is proved in a much more striking manner by the phenomena of disease than by any experimental or other physiological observations. The influence of a shock in arresting or modifying the action of the heart, its very slow action after compression of the brain, or injury to the cervical portion of the spinal cord, its irregularities and palpitations in dyspepsia and hysteria, are better evidence for the connection of the heart with the other organs through the nervous system, than are any results obtained by experiments. Effects of the Heart's Action. That the contractions of the heart supply alone a suffi- cient force for the circulation of the blood, appears to be STKUCTUKE OF ARTEKIES. 143 established by the results of several experiments, of which the following is one of the most conclusive : Dr. Sharpey injected bullock's blood into the thoracic aorta of a dog recently killed, after tying the abdominal aorta above the renal arteries, and found that, with a force just equal to that by which the ventricle commonly impels the blood in the dog, the blood which he injected into the aorta passed in a free stream out of the trunk of the vena cava inferior. It thus traversed both the systemic and hepatic capillaries ; and when the aorta was not tied above the renals, blood injected under the same pressure flowed freely through the vessels of the lower extremities. A pressure equal to that of one and a-half or two inches of mercury was, in the same way, found sufficient to propel blood through the vessels of the lungs. But although it is probably true that the heart's action alone is sufficient to ensure the circulation, yet there is reason to believe in the existence of several other forces which are, as it were, supplementary to the action of the heart, and assist it in maintaining the circulation. The principal of these supplemental forces have been already alluded to, and will now be more fully pointed out. THE AUTERIES. For the purpose of explaining the influence of the arteries in the circulation it will be sufficient to consider the walls of an artery as containing three principal coats : an external, & middle, and an internal coat. The external coat is constructed of ordinary areolar tissue, the fibres of which are chiefly of the white, inelastic kind, especially towards the outer portion of the artery. They are arranged, for the most part, in a longitudinal or oblique direction. The following are the uses of the external coat : (i.) It forms a strong, tough investment, which, though capable of extension, appears principally designed to strengthen the walls of the artery, and to guard against their excessive 144 THE CIRCULATION. distension from the force of the heart's action. (2.) It serves another purpose also in affording a suitable tissue for the ramifications of the vasa vasorum, or nutritive vessels for the supply of the arterial walls. The internal arterial coat is formed by layers of elastic tissue, the outer portion consisting of coarse longitudinal branching fibres, and the inner of a very thin and brit- tle membrane which possesses little elasticity, and is thrown into folds or wrinkles when the artery contracts. This latter membrane, the striated or fenestrated coat of Henle, is peculiar in its tendency to curl up when peeled off in thin films from the artery, and in the perforated and streaked appearance which it presents under the microscope. Its inner surface is lined with a delicate layer of epithelium, composed of thin squamous elongated cells, which make it smooth and polished, and furnish a nearly impermea- ble surface, along which the blood may flow with the smallest possible amount of resistance from friction. The middle coat is the seat of those properties by which arteries chiefly influence the circulation. The outer portion of this coat is made up, chiefly, of fibres of yellow elastic tissue, disposed for the most part circularly, and con- stituting, as Hunter named it, the elastic coat. The inner consists also of circular fibres of yellow elastic with a sparing amount of white fibrous tissue, but mingled with these, sometimes in alternate layers, and having the same transverse direction, are pale, flat fibres, or "fibre-cells" * Fig. 40. Portion of fenestrated membrane from the crural artery, magnified 200 diameters, a, b, c, perforations (from Henle). STRUCTURE OF ARTERIES. 145 Fig. 41.* of Kolliker, which differ in no essential respect from the fibres of organic muscle, such as those which compose the muscular coat of the stomach and intestines. To this part of the middle coat the name of muscular was applied by Hunter. These two elements of the middle coat exist in different rela- tive amounts in different arteries ; and, in general, are in an in- verse ratio to each other, for the arteries which possess most elastic tissue have least mus- cular tissue, while those whose walls are most muscular, are in general least elastic. In the large arteries, such as the aorta and its main branches, scarcely a trace of the muscular element can be found, nearly the whole thickness of their walls consisting of elastic tissue. But in the arteries farther removed from the heart, and of smaller size, the pro- portionate thickness of the elastic element gradually diminishes, while, as a general rule, that of the mus- cular element progressively increases. Moreover, in the arteries of certain organs, probably of those in which the supply of blood is subject to greater than usual variations, in adaptation to fluctuations in the amount of function they discharge, there is proportionately greater development of the muscular tissue. Of the properties which the arteries possess in these two tissues, the muscularity has its seat of course exclu- sively in the muscular tissue, and no artery without this element would present any contraction similar to that of * Fig. 41. Muscular fibre-cells from human arteries, magnified 350 diameters (Kolliker). a, natural state ; 6, treated with acetic acid. L 146 , THE CIRCULATION. muscles. But elasticity is a property not exclusively, though especially, seated in the elastic portion of the middle coat; indeed, all the coats are in some measure elastic, and will recoil after being distended; and the effect their elasticity produces is yet further assisted by the elasticity of the tissues around them. The purposes of the elasticity of arteries are chiefly these ; 1st, To guard them from the suddenly exerted pressure to which they are subjected at each contraction of the ven- tricles. In every such contraction, the contents of the ven- tricles 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, which might burst a brittle tube, their elastic walls dilate, stretching enough to receive the blood, and as they stretch, becoming more tense and more resisting. Thus, by yield- ing, they, as it were, break the shock of the force impelling the blood, and exhaust it before they are in danger of bursting, through being overstretched. Elasticity is thus advantageous in all arteries, but chiefly so in the aorta and its large branches, which are provided, as already said, with a large quantity of elastic tissue, in adaptation to the great force of the left ventricle, which falls first on them, and to the increased pressure of the arterial blood in violent expiratory efforts. On the subsidence of the pressure, when the ventricles cease contracting, the arteries are able, by the same elas- ticity, to resume their former calibre ; and in thus doing, they manifest the 2nd chief purpose of their elasticity, that, namely, of equalising the current of the blood by main- taining pressure on the blood in the arteries during the periods at which the ventricles are at rest or dilating. If some such method as this had not been adopted if for example the arteries had been rigid tubes, the blood, ELASTICITY OF ARTERIES. 14? 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 of the arteries, the force of the successive contractions of the ventricles is ex- pended 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 recoiling and contracting arteries is employed in continuing the same direct propulsion. Of course, the pressure exercised by the recoiling arteries is equally diffused in every direction through the blood, and the blood would tend to move backwards as well as onwards, but that all movement backwards is prevented by the closure of the arterial valves, 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 made advantageous to the circula- tion; for that part of their force which is expended in dilating the arteries, is restored in full, according to that law of action of elastic bodies, by which they return to the state of rest with a force equal to that by which they were disturbed therefrom. 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 ventri- cles ; they would not contract had they not first been dilated, any more than a spiral spring would shorten itself unless it were first elongated. The advantage of elasticity in this respect is, therefore, not that it increases, but that it equalizes or diffuses the force derived from the periodic contractions of the ventricles. The force with which the arteries are dilated every time the ventricles contract, might be said to be received by them in store, to be all given out again in the next succeeding period of dilatation L 2 148 THE CIRCULATION. of the ventricles. It is by this equalizing influence of the successive branches of every artery that, at length, the intermittent accelerations 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. (3.) By means of the elastic tissue in their walls (and of the muscular tissue also), 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. The elastic coat, however, not only assists in restoring the normal calibre of an artery after temporary dilatation, but also, (4.) may assist in restoring it after diminution of the calibre, whether this be caused by a temporary con- traction of the muscular coat, or the application of a com- pressing force from without. This action of the elastic tissue in arteries, is well shown in arteries which contract after death, but regain their average patency on the cessa- tion of post-mortem rigidity (p. 149). (5.) By means of their elastic coat the arteries are enabled to adapt them- selves to the different movements of the several parts of the body. The evidence for the muscularity of arteries may be given at some length. We have already referred to the mus- cular structure of the inner layer of the middle coat of all but the largest arteries, and to the fact, first observed by Henle, that this layer is composed of fibres in all respects similar to those of organic muscle, though mingled with fine elastic filaments. The observation of the action of arteries will show, 1st, the operation of a contractile' power in arteries, essentially distinct from their elasticity ; and, 2ndly, the identity of this power with muscular'' contractility. MUSCULARITY OF ARTERIES. 149 (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; for after death, when the vital muscular power has ceased, and the mechanical elastic one alone operates, the contracted artery dilates again. (2.) When an artery is cut across, its divided ends con- tract, 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 animals. In part this contraction is due to elasticity, but in part, no doubt, 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 pricking or twisting them. Such irritation would not be followed by these effects, if the arteries had no other power of contracting than that depending upon elasticity. (3.) The contractile property of arteries continues many hours after death, and thus affords an opportunity of distin- guishing it from elasticity. When a portion of an artery, the splenic, for example, 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. If, while contracted, the artery be forcibly distended, its con- tractility is destroyed, and it holds a middle or natural size. This persistence of the contractile property after death was well shown in an observation of Hunter, which may be mentioned as proving, also, the greater degree of 150 THE CIRCULATION. contractility possessed by the smaller than by the larger arteries. Having injected the uterus of a cow, which had been removed from the animal upwards of twenty-four hours, he found, after the lapse of another day, that the larger vessels had become much more turgid than when he injected them, and that the smaller arteries had contracted so as to force the injection back into the larger ones. The results of an experiment which Hunter made with the vessels of an umbilical cord prove still more strikingly the long continuance of the contractile power of arteries after death. In a woman delivered on a Thursday after- noon, the umbilical cord was separated from the foetus, having been first tied in two places, and then cut between, so that the blood contained in the cord and placenta was confined in them. On the following morning, Hunter tied a string round the cord, about an inch below the other ligature, that the blood might still be confined in the placenta and remaining cord. Having cut off this piece, and allowed all the blood to escape from its vessels, he attentively observed to what size the ends of the cut arte- ries were brought by the elasticity of their coats, and then laid aside the piece of cord to see the influence of the contractile power of its vessels. On Saturday morning, the day after, the mouths of the arteries were completely closed up. He repeated the experiment the same day with another portion of the same cord, and on the following morning found the results to be precisely similar. On the Sunday, he performed the experiment the third time, but the artery then seemed to have lost its contractility, for on the Monday morning, the mouths of the cut arteries were found open. In each of these experiments there was but little alteration perceived in the orifices of the veins. (4.) The influence of cold in increasing the contraction of a divided artery has been referred to : it has been shown, also, by Schwann, in an experiment on the mesentery of a living toad. Having extended the mesentery under the MUSCULARITY OF ARTERIES. 151 microscope, lie placed upon it a few drops of water, the temperature of which was some degrees lower than that of the atmosphere. The contraction of the vessels soon com- menced, and gradually increased until, at the expiration of ten or fifteen minutes, the diameter of the canal of an artery, which at first was 0*0724 of an English line, was reduced to 0*0276. The arteries then dilated again, and at the expiration of half an hour had acquired nearly their original size. By renewing the application of the water, the contraction was reproduced : in this way the experi- ment could be performed several times on the same artery. The veins did not contract. It is thus proved, that cold will excite contraction in the walls of very small, as well as of comparatively large arteries : it could not produce such contraction in a merely elastic substance; but it is a stimulus to the organic muscular fibres in many other parts, as well as in the arterial coat; as, e.g., in the skin, the dartos, and the walls of the bronchi. (5.) Lastly, satisfactory evidence of the muscularity of the arterial coats is furnished by the experiments of Ed. and E. H. Weber, and of Professor Kolliker, in which they applied the stimulus of electro-magnetism to small arteries. One principal circumstance which induced Miiller to deny the muscularity of arteries, was the seeming impossibility of producing contraction in arteries by galvanic and electric stimuli, which excite all true muscular tissues to manifest contraction. An explanation of the failure may be found in the circumstance that, in nearly all the experiments, the arteries examined were of large size, such as the aorta and the carotid, in which there is little or no muscular tissue. The experiments of the Webers were performed on the small mesenteric arteries of frogs ; and the most striking results were obtained when the diameter of the vessels examined did not exceed from i to j'y of a Paris line. When a vessel of this size was exposed to the electric current, its diameter in from. 152 THE CIRCULATION. five to ten seconds, became one-third less, and the area of its section about one-half. On continuing the stimulus, the narrowing gradually increased, until the calibre of the tube became from three to six times smaller than it was at first, so that only a single row of blood-corpuscles could pass along it at once ; and eventually the vessel was closed and the current of blood arrested. With regard to the purpose served by the muscular coat of the arteries, there appears no sufficient reason for supposing that it assists, to more than a very small degree, in pro- pelling the onward current of blood. Its most important office is that of regulating the quantity of blood to be received by each part, and of adjusting it to the require- ments of each, according to various circumstances, but chiefly and most naturally, according to the activity with which the functions of each part are at different times per- formed. 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. It 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 as the requirements of the part may demand. 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, FUNCTIONS OF MUSCULAR COAT. 153 the smaller arteries with muscular tissue add to these two purposes, that of regulating and determining, according to its requirements, 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. The muscular tissue of arteries is supplied with nerves chiefly, if not entirely, by branches from the sympathetic system. These so-called vaso-motor nerves are again con- nected, through the medium of ganglia, with the fibres from the sympathetic system supplied to the organs nourished by these same arteries. Thus, any condition in these organs which causes them to need a different amount of blood, whether more or less, produces a certain im- pression on their nerves, and by these the impression is carried to the ganglia, and thence reflected along the nerves which supply the arteries. The muscular element of these vessels responds in obedience to the impression con- veyed to it by the nerves ; and, according to its contraction or dilatation, is a larger or smaller quantity of blood allowed to pass. Another function of the muscular element of the middle coat of arteries is, doubtless, 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 exer- cised; and it is by this adaptive, uniform, gentle, muscular contraction, that the tone of the blood-vessels is maintained. Deficiency of this tone is the cause of the soft and yield- 154 THE CIRCULATION. ing pulse, and its unnatural excess of the hard and tense one. The elastic and muscular contraction of an artery may also be regarded as fulfilling a natural purpose when, 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. From what has been said in the preceding pages, it appears that the office of the arteries in the circulation is, 1st, the conveyance and distribution of blood to the several parts of the body; 2nd, the equalization of the current, and the conversion of the pulsatile jetting movement given to the blood by the ventricles, into an uniform flow ; ^rd, the regulation of the supply of blood to each part, in accord- ance with its demands. In explanation of the mode in which, by the combination of the elastic and muscular coats of arteries, this three-fold office is accomplished, we may use, as a summary of what has been already said, the words of Mr. Hunter, who observes that, " there are three states in which an artery is found, viz., 1st, the natural pervious state; 2nd, the stretched; and ^rd, the contracted state, which may or may not be pervious. The natural pervious state is that to which the elastic power naturally brings a vessel which has been stretched beyond or contracted within the extent which it held in a state of rest. The stretched is that state produced by the impulse of the blood in consequence of the contraction of the heart ; from which it is again brought back to the natural state by the elastic power, perhaps assisted by the mus- cular. The contracted state of an artery arises from the action of the muscular power, and it is again restored to the natural state by the elastic." THE PULSE. 155 It must be observed, however, that the natural pervious state of an artery here spoken of, is not one of absolute rest ; for Mr. Savory has shown that the natural state of all arteries, in regard at least to their length, is one of tension that 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. The Pulse. The jetting movement of the blood, which, as just stated, it is one of the offices of the arteries to change into an uniform motion, is the cause of the pulse, and therefore needs a separate consideration. We have already said, that as the blood is not able to pass through the arteries so quickly as it is forced into them by the ventricle, on account of the resistance it experiences in the capillaries, a part of the force with which the heart impels the blood is exercised upon the walls of the vessels which it distends. The distension of each artery increases both its length and its diameter. In their elongation, the arteries change their form, the straight ones becoming curved, or having such a tendency, and those already curved becoming more so;* but they recover their previous form as well as their dia- meter 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. The elongation of the artery is in such a case quite manifest. * There is, perhaps, an exception to this in the case of the aorta, of which the curve is by some supposed to be diminished when it is elon- gated ; but if this be so, it is because only one end of the arch is im- moveable ; the other end, with the heart, may move forward slightly when the ventricles contract. 156 THE CIRCULATION. The dilatation or increase of the diameter of the artery is less evident. In several reptiles, it may be seen without aid, in the immediate vicinity of the heart, and it may be watched, with a simple magnifying glass, in the aorta of the tadpole. Its slight amount in the smaller arteries, the difficulty of observing it in opaque 'parts, and the rapidity with which it takes place, are sufficient to account for its being, in Mammalia, imperceptible to the eye. But in these also experiment has proved its occurrence. Flourens, in evidence of such dilatation, says he encircled a large artery with a thin elastic metallic ring cleft at one point, and that at the moment of pulsation the cleft part became perceptibly widened. This dilatation of an artery, and the elongation producing curvature, or increasing the natural curves, are sensible to the finger placed over the vessel, and produce the pulse. The mind 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.* * For this fact, which is contrary to the commonly accepted doctrine, I am indebted to my friend, Dr. Hensley, who has kindly furnished me with the following note on the subject : By determining the conditions of equilibrium of a portion of artery supposed cylindrical and filled with blood at a given pressure, it is easily shown that the transverse tension is double the longitudinal. Also it may be shown experimentally that, if strips of equal breadth, cut in the two directions from one of the larger arteries, be stretched by equal weights, the stretching of the transverse strip is somewhat greater than that of the longitudinal one. (By the word stretching is to be understood amount of stretching, and not increase of length : it may be measured by the ratio which the increase of length bears to the original length : Thus things whose natural lengths are 5 and 10 inches are equally stretched when their lengths are made 6 and 12 inches respectively.) Such experiments also show that, within certain limits, the stretch- ing of each strip varies directly as its tension. Hence it will be seen that the transverse stretching of an artery, when THE SPHYGMOGRAPH. 157 The pulse due to any given beat of the heart is not perceptible 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 proportion to the distance of the artery from the heart, but the difference in time] between the beat of any two arteries never exceeds probably J to J of a second. The pulse, moreover, is but the maximum distension and elongation of the vessel; and it has been shown by M. Marey that the commencement of the act occurs at the same moment in all the arterial system, but that the highest point of distension and lengthening, which alone we recognise as the pulse, is reached more rapidly in proportion to the proximity of the artery to the heart. This observation has been made by means of a special modification of the sphygrnograph, an instrument invented by Vierordt and much improved by M. Marey and others, which has thrown a great deal of light on what may be called the structure of the pulse. The principle on which the sphygmograph acts is very simple (see fig. 42). The small button replaces the finger in the ordinary act of taking the pulse, and is made to rest lightly on the artery, the pulsations of which it is desired to investigate. The up-and-down movement of the button is communicated to the lever, to the hinder end of which is attached a slight filled with blood, must be somewhat more than double its longitudinal stretching. This being true for different blood pressures, the difference between the transverse stretchings for different pressures must be somewhat more than double the difference between the corresponding longitudinal stretchings ; and thus we can hardly be justified in saying that the increase of longitudinal stretching which takes place with the pulse is greater than the increase of transverse stretching. It must also be remembered that the arteries are, under all circum- stances, naturally in a state of tension longitudinally, and that their length, therefore, cannot be increased at all until the blood pressure is increased beyond a certain point. (ED.) THE CIRCULATION. 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 instrument is bound on the wrist (fig. 43). Fig. 42. It is evident that the beating of the pulse with the reaction of the spring will cause an up-and-down move- ment of the lever, and if the extremity of the latter be inked, it will write the effect on the 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. Fig. 44 represents a healthy pulse-tracing of the radial artery, but somewhat deficient in tone. On examination, we see that the upstroke which represents the beat of the pulse is a nearly vertical line, while the down-stroke is very slanting, and interrupted by a slight re-ascent. The more vigorous the pulse, if it be healthy, the less is this PULSE-TRACINGS. '59 re-ascent, and vice versa. Fig. 45 represents the tracing of a healthy pulse in which the tone of the vessel is better than in the last instance, and the down-stroke is there- fore less interrupted. Sometimes the up-stroke' has a double apex, as in fig. 46. This will be explained hereafter. Ft' ft. Ad.* Fift. 46. Before proceeding to consider the formation of the pulse, as shown by these tracings, it is necessary to consider what are the elements combined to produce it. In the first place Ihere is a propelling organ, the heart, which at regular intervals discharges a certain quantity of fluid into a tube with elastic walls, filled, although not distended to the utmost, with fluid. This fresh quantity of blood therefore obtains entrance by the yielding of the artery's elastic walls, and, on the cessation of the propelling force, the blood is prevented from returning into the ventricle whence it issued, by the shutting of the semilunar valves in the manner before described. * Fig. 44. Pulse-tracing of radial artery, somewhat deficient in tone. t Fig. 45. Firm and long pulse of vigorous health. J Fig. 46. Pulse-tracing of radial artery, with double apex. The above tracings are taken from Dr. Sanderson's work "On the Fphygmograph. " 160 THE CIRCULATION. It was formerly supposed that the pulse was caused not by the direct action of the ventricle, but by the propaga- tion of a wave in consequence of the elastic recoil of the large arteries, after their distension ; and successive acts of dilatation and recoil, extending along the arteries in the direction of the circulation, were supposed to account for the later appearance of the pulse in the vessels most dis- tant from the heart. The observation of Mr. Colt, however, that the pulse is perceptible in every part of the arterial system previous to the occurrence of the second sound of the heart, that is, previous to the closure of the aortic valves, is a very forcible objection to this theory. For, if the pulse were the effect of a wave propagated by the alternate dilatation and contraction of successive portions of the arterial tube, it ought, in all the arteries except those nearest to the heart, to follow or coincide with, but could never precede, the second sound of the heart ; for the first effect of the elastic recoil of the arteries first dilated is the closure of the aortic valves ; and their closure produces the second sound. The theory proposed by Mr. Colt, which seems to recon- cile all the facts of the case, and especially those two which appear most opposed, namely, that the pulse always pre- cedes the second sound of the heart, and yet is later in the arteries far from the heart than in those near it, may be thus stated : It supposes that the blood which is impelled onwards by the left ventricle does not so impart its pressure to that which the arteries already contain, as to dilate the whole arterial system at once; but that it enters the arteries, it displaces and propels that which they before contained, and flows on with what may be called a head- wave, like that which is formed when a rapid stream of water overtakes another moving more slowly. The slower stream offers resistance to the more rapid one, till their velocities are equalized : and, because of such resistance, some of the force of the more rapid stream of blood just expelled from the ventricle, is diverted laterally, and with THE PULSE. 161 the rising of the wave the arteries nearest the heart are dilated and elongated. They do not at once recoil, but continue to be distended so long as blood is entering them from the ventricle. The wave at the head of the more rapid stream of blood runs on, propelled and maintained in its velocity by the continuous contraction of the ventricle : and it thus dilates in succession every portion of the arterial system, and produces the pulse in all. At length, the whole arterial system (wherein a pulse can be felt) is dilated; and at this time, when the wave we have sup- posed has reached all the smaller arteries, the entire system may be said to be simultaneously dilated ; then it begins to contract, and the contractions of its several parts ensue in the same succession as the dilatations, commencing at the heart. The contraction of the first portion produces the closure of the valves and the second sound of the heart ; and both it and the progressive contractions of all the more distant parts maintain, as already said, that pressure on the blood during the inaction of the ventricle, by which the stream of the arterial blood is sustained between the jets, and is finally equalized by the time it reaches the capillaries. It may seem an objection to this theory, that it would probably require a larger quantity of blood to dilate all the arteries than can be discharged by the ventricle at each contraction. But the quantity necessary for such a pur- pose is less than might be supposed. Injections of the arteries prove that, including all down to those of about one-eighth of a line in diameter, they do not contain on an average more than one and a half pints of fluid, even when distended. There can be no doubt, therefore, that the three or four ounces which the ventricle is supposed to dis- charge at each contraction, being added to that which already fills the arteries, would be sufficient to distend them all. Besides this wave-like movement, noVever, it must be remembered that, as before stated, the pulse begins in 1 62 THE CIRCULATION. every artery at the same moment, although the maximum effect of distension, which alone we feel with the finger, is attained more slowly in proportion to the distance of the vessel from the heart. There can be no doubt, therefore, that the contractile force of the ventricle is almost instantaneously propagated through the whole arterial system ; and if we consider that the fluid blood, practically incompressible, contained in the arteries, may be compared to a quasi-solid trunk beginning at the heart, and branching into all parts of the body, it is evident that a force applied at the beginning of the trunk will be immediately propa- gated through all the branches, and, if sufficiently strong, will produce movement at all parts. And this force is applied when fresh blood is forced into the aorta by the left ventricle, and the effect is without doubt instantaneously propagated through the continuous and branching column of fluid in the arteries. These vessels being elastic, how- ever, the force of the ventricle is employed, not only in the instantaneous propagation of an impulse, but in part also in distending the vessels along which the blood, wave-like, rolls on to complete the pulse which has already com- menced in all the arteries at the same moment. Returning now to the consideration of the pulse-tracings (p. 159), it may be remarked that, in each, the up-stroke corresponds with the period during which the ventricle is contracting ; the down-stroke, with the interval between its contractions, or in other words with the recoil, after distension, of the elastic arteries. In the large arteries, when at least there is much loss of tone, the up-stroke is double, the instantaneous propagation of the force of con- traction of the left ventricle, or percussion impulse, as it is termed by Dr. Sanderson, being sufficiently strong to jerk up the lever for an instant, while the wave of blood, rather more slowly propagated from the ventricle, catches it, so to speak, as it begins to fall, and again slightly raises it PULSE-TRACING. 1 63 In the radial artery tracings, on the other hand, we see that the up-stroke is single. In this case the percussion- impulse although it causes the pulse to begin at the same moment with that in the carotid, is not sufficiently strong to jerk 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 sphygmograph, not only in the carotid pulse, but to a less extent in the radial also, G~Z #Y\ In looking now at the down-stroke (fig. 44), ao in fig-. 46pin the tracings, we see that it is interrupted by a well- marked notch, or in other words, that the descent is inter- rupted by a slight up-rising. In some cases of disease this re-ascent is so considerable as to be perceptible to the finger, and this double-beat has received the technical name of "dicrotous" pulse. As a diseased condition this has been long recognized, but it is only since the invention of the sphygmograph that it has been found to belong in a certain degree to the normal pulse also. Various theories have been framed to account for this dicrotism of the normal pulse, but the most probable, and that which is generally adopted, supposes it to be due to the aortic valves, the sudden closure of which stops the incipient regurgitation of blood into the ventricle, and causes a momentary rebound throughout the arterial sys- tem. As before remarked, when the tone of the artery is good, the dicrotism is less marked ; and it is often replaced by a series of very slight vibrations. The beginning of the down-stroke, the part, namely, between the end of the up-stroke and the beginning of the slight re-ascent, represents, probably, not as has been supposed, some regurgitation into the ventricle before the final closure of the semilunar valves, but simply the com- mencing subsidence of the artery's tension, after the passage of the systolic wave before referred to. M 2 164 THE CIRCULATION. Fig. 47. Force of the Blood in the Arteries. The force with which the ventricles act in their con- traction, and the reasons for believing it sufficient for the circulation of the blood, have been already mentioned. Both calculation and experiment have proved, that very little of this force is consumed in the arteries. Dr. Thomas Young calculated that the loss of force in overcoming friction and other hindrances in the arteries would be so slight, that if one tube were introduced into the aorta, and another into any other artery, even into one as fine as hair, the blood would rise in the tube from the small vessel to within two inches of the height to which it would rise from the large vessel. The cor- rectness of the calculation is estab- lished by the experiments of Poiseu- ille, who invented an instrument named a haBmadynamometer, for es- timating the statical pressure exer- cised by the blood upon the walls of the arteries. It consists of a lon- ^ glass tube, bent so as to have a short horizontal portion (fig. 47), a branch (2) descending at right angles from it, and a long ascending branch (3). Mercury poured into the ascending and descending portions, will necessarily have the same level in both branches, and in a vertical position^ the height of its column must be the same in both. If, "how, the blood is made to flow from an artery, through the horizontal portion of the tube (which should contain a solution of carbonate of potash to prevent coagulation) into the descending branch, it will exert on the mercury a f-iTOCQl FOKCE OF BLOOD IN ARTERIES. 165 pressure equal to the force by which it is moved in the arteries ; and the mercury will, in consequence, descend in this branch, and ascend in the other. The depth to which it sinks in the one branch, added to the height to which it rises in the other, will give the whole height of the column of mercury which balances the pressure exerted by the blood ; the weight of the blood, which takes the place of the mercury in the descending branch, and which is more than ten times less than the same quantity of quicksilver, being subtracted. Poiseuille thus calculated the force with which the blood moves in an artery, according to the laws of hydrostatics, from the diameter of the artery, and the height of the column of quicksilver ; that is to say, from the weight of a column of mercury, whose base is a circle of the same diameter as the artery, and whose height is equal to the difference in the levels of the mercury in the two branches of the instrument. He found the blood's pressure equal in all the arteries examined ; difference in size, and distance from the heart being unattended by any corresponding difference of force in the circulation. The height of the column of mercury displaced by the blood was the same in all the arteries of the same animal. The correctness of these views having been questioned, Poi- seuille has recently repeated his observations, and obtained the same results. From the mean result of several observations on horses and dogs, he calculated that the force with which the blood is moved in any large artery, is capable of support- ing a column of mercury six inches and one and a half lines in height, or a column of water seven feet one line in height. With these results, the more recent observations of other experimenters closely accord. Poiseuille's experi- ments having thus shown to him that the force of the blood's motion is the same in the most different arteries, he concluded that, to measure the amount of the blood's pressure in any artery of which the calibre is known, it is 1 66 THE CIRCULATION. necessary merely to multiply the area of a transverse sec- tion of a vessel by the height of the column of mercury which is already known to be supported by the force of the blood in any part of the arterial system. The weight of a column of mercury of the dimensions thus found, will represent the pressure exerted by the column of blood. And assuming that the mean of the greatest and least height of the column of mercury found, by experiments on different animals, to be supported by the force of the blood in them, is equivalent to the height of the column which the force of the blood in the human aorta would support, he calculated that about 4 Ibs. 4 oz. avoirdupois would indicate the static force with which the blood is impelled into the human aorta. By the same calculation, he estimated the force of the circulation in the aorta of the mare to be about 1 1 Ibs. 9 oz. avoirdupois : and that in the radial artery at the human wrist only 4 drs. We have already seen that the muscular force of the right ventricle is equal to only half that of the left, consequently, if Poiseuille's estimate of the latter be correct, the force with which the blood is propelled into the lungs will only be equal to 2 Ibs. 2 oz. avoirdupois. The amounts above stated indicate the pressure exerted by the blood at the several parts of the arterial system at the time of the ventricular contraction. During the dila- tation, this pressure is somewhat diminished. Hales observed, that the column of blood in the tube inserted into an artery, falls an inch, or rather more, after each pulse ; Ludwig has observed the same, and recorded it more minutely. The pressure is also influenced by the various circumstances which affect the action of the heart ; the diminution or increase of the pressure being pro- portioned to the weaker or stronger action of this organ. Valentin observed that, on increasing the amount of blood by the injection of a fresh quantity into it, the pressure in the vessels was also increased, while a contrary effect ensued on diminishing the quantity of blood. VELOCITY OF BLOOD IN ARTERIES. 167 Poiseuille, Ludwig, and others have confirmed what Haller and Magendie observed, namely, that the strength of the blood's impulse in the arteries is increased during expiration ; in which act the chest is contracted, and the large vessels in consequence compressed. This point will be again referred to in speaking of the movement of the venous blood. Velocity of the Blood 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 diminishing during the diastole of the ven- tricles, and in the parts of the arterial system most distant from the heart. From Volkmann's experiments with the hsemodromometer, it may be concluded that the blood moves in the large arteries near the heart at the rate of about ten or twelve inches per second. Vierordt calculated the rapidity of the stream at about the same rate in the arteries near the heart, and at two and a quarter inches per second in the arteries of the foot. THE CAPILLABIES. In all organic 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 microscopic vessels, in the meshes of which the proper substance of the tissue lies (fig. 49). This may be seen in all minutely injected preparations ; and during life, by the aid of the microscope, in any trans- parent vascular parts, such as the web of the frog's foot, the tail or external branchiae of the tadpole, or the wing of the bat. The structure of the capillaries is much more simple than that of the arteries or veins. Their walls are composed of a single layer of elongated or radiate, flat- i68 THE CIRCULATION. tened and nucleated cells, so joined and dovetailed together as to form a continuous transparent membrane (fig. 48). It is not quite certain whether outside these cells there is Fig, 48.* or is not a fine structureless membrane, on the inner sur- face of which they are laid down. The ramifications of the minute arteries form repeated anastomoses with each other and give off the capillaries which, by their anasto- moses, compose a continuous and uniform network, from which the venous radicles, on the other hand, take their rise. The reticulated vessels connecting the arteries and * Fig. 48. Magnified view of capillary vessels from the bladder of the cat. A, V, an artery and a vein ; i, transitional vessel between them and c c, the capillaries. The muscular coat of the larger vessels is left out in the figure to allow the epithelium to be seen : at c', a radiate epithelium scale with four pointed processes, running out upon the four adjoining capillaries (after Chrzonszczewesky, Virch. Arch., 1866). THE CAPILLARIES. 169 veins are called capillary, on account of their minute size ; and intermediate vessels, on account of their position. The point at which the arteries terminate and the minute veins commence, cannot be exactly de- fined, for the transition is gradual ; but the intermediate network has, nevertheless, this peculiarity, that the small vessels which compose it maintain the same diameter throughout; they do not diminish in diameter 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 anas- tomoses of the minute arteries and veins. The diameter of the capillary vessels varies somewhat in the different textures of the body, the most common size being about 3 1 O th 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 form of the capillary network presents considerable variety 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 or interspaces have a roundish form is the most common, and prevails in those parts in which the * Fig. 49. Blood-vessels of an intestinal villus, representing the arrangement of capillaries between the ultimate venous and arterial branches ; a, a, the arteries ; b, the vein. 170 THE CIRCULATION. capillary network is most dense, such as the lungs (fig. 50), 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 being more frequently irregular. The capillary network with elon- gated meshes (fig. 51) is observed in parts in which the Fig. so-* Fig. 51.+ 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 * Fig. 50. Net-work of capillary vessels of the air cells of the horse's lung, magnified, a, a, capillaries proceeding from b, 5, terminal branches of the pulmonary artery (after Frey). t Fig. 51. Injected capillary vessels of muscle, seen with a low magnifying power (Sharpey). THE CAPILLARIES. 171 arrangement, a single capillary projecting from the com- mon network into some prominent organ, and returning after forming one or more loops, as in the papillae of the tongue and skin. Whatever be the form of the capillary network in any tissue or organ, it is, as a rule, found to prevail in the corresponding parts of all animals. 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 net- work 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 distributed 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 transverse 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 cellular 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 ; that is, the closer is its capillary network and the larger its 172 THE CIRCULATION. supply of blood. Hence the narrowness of the interspaces in all glandular organs, in mucous membranes, and in growing parts ; their much greater width in bones, liga- ments, and other very tough and comparatively inactive tissues ; and the complete absence of vessels in cartilage, the dense tendons of adults, and such parts as those in which, probably, very little organic change occurs after they are once formed. But the general rule must be modified by the consideration, that some organs, such as the brain, though they have small and not very closely arranged capillaries, may receive large supplies of blood by reason of its more rapid movement. When an organ has large arterial trunks and a comparatively small supply of capillaries, the movement of the blood through it will be so quick, that it may, in a given time, receive as much fresh blood as a more vascular part with smaller trunks, though at any given instant the less vascular part will have in it a smaller quantity of blood. In the Circulation in the Capillaries, as seen in any trans- parent part of a living adult animal by means of the mi- croscope (fig. 52), the blood flows with a constant equable motion. In very young ani- mals, the motion, though continuous, is accelerated at intervals corresponding to the pulse in the larger arteries, and a similar motion of the blood is also seen in the capillaries of adult animals when they are feeble : if their exhaustion is so great that the power of the heart is still more diminished, the red corpuscles are observed to have merely the periodic motion, and to remain stationary in the * Fig. 52. Capillaries in the web of the frog's foot magnified. THE CAPILLARIES. 173 intervals ; while, if the debility of the animal is extreme, they even recede somewhat after each impulse, apparently because of the elasticity of the capillaries and the tissues around them. These observations may be added to those already advanced (p. 142) to prove that, even in the state of great debility, the action of the heart is sufficient to impel the blood through the capillary vessels. Moreover, Dr. Marshall Hall having placed the pectoral fin of an eel in the field of the microscope, and compressed it by the weight of a heavy probe, observed that the movement of the blood in the capillaries became obviously pulsatory, the pulsations being synchronous with the contractions of the ventricle. The pulsatory motion of the blood in the capillaries cannot be attributed to an action in these ves- sels ; for, when the animal is tranquil, they present not the slightest change in their diameter. 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 multipli- cation of the surface with which it is brought in contact. The velocity of the blood is also in them reduced to its minimum, because of the widening of the stream. If, as Professor Miiller says, the sectional area 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 sec- tional 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 cor- respondence in the rapidity of the circulation in them. It is quite true, that the force with which the blood is pro- pelled in the arteries, as shown by the quantity of blood which escapes from them in a certain space of time, is much greater than that with which it moves in the veins ; but this force has to overcome all the resistance offered in 174 THE CIRCULATION. the arterial and capillary system the heart itself, indeed, must overcome this resistance ; so that the excess of the force of the blood's motion in the arteries is expended in overcoming this resistance, and the rapidity of the circu- lation in the arteries, even from the commencement of the aorta, would be the same as in the veins and capillaries, if the aggregate capacity of each of the three systems of vessels were the same. But since the aggregate sectional area of the branches is greater than that of the trunk from which they arise, the rapidity of the blood's motion will necessarily be greater in the trunk, and will diminish in proportion as the aggregate capacity of the vessels increases during their ramification : in the same manner as, other things being equal, the velocity of a stream diminishes as it widens. The observations of Hales, E. H. Weber, and Valentin, agree very closely as to the rate of the blood in the capil- laries of the frog : and the mean of their estimates gives the velocity of the systemic capillary circulation at about one inch per minute. Through the pulmonic capillaries, the rate of motion, according to Hales, is about five times that through the systemic ones. The velocity in the capillaries of warm-blooded animals is greater, but has not yet been accurately estimated. If it be assumed to be three times as great as in the frog, still the estimate may seem too low, and inconsistent with the facts, which show that the whole circulation is accomplished in about a minute. But the whole length of capillary vessels, through which any given portion of blood has to pass, probably does not exceed -^th of an inch ; 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 : while in the pulmonic capil- lary system the length of time required will be much less even than this. The estimates given above are drawn from observations THE CAPILLARIES. i?5 of the movements of the red blood-corpuscles, which move in the centre of the stream. At the circumference of the stream, in contact with the walls of the vessel, and adhering to them, there is a layer of liquor 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-corpuscles run much more slowly by the walls of the vessel ; while next to the wall there is often a trans- parent 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, as E. H. Weber has suggested, 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. But there is no doubt that such a still layer of liquor sanguinis exists next the walls of the vessels, and it is between this and the tissues around the vessels that those interchanges of particles take place which ensue in nutrition, secretion and absorption by the ^blood vessels ; interchanges which are probably facilitated by the tranquillity of the fluids between which they are effected. There is no reason for supposing that under ordinary circumstances, in health, either the pale or red corpuscles ever remain permanently fixed to the wall of the vessels and become united with them, or that they pass through the walls to enter into the structure of the tissues. Their office appears to be fulfilled exclusively within the vessels. 1 76 THE CIRCULATION. Some remarkable observations of Cohnheim, however, to which attention has been lately drawn by Dr. Charlton Bastian, apparently prove without doubt, that under cer- tain circumstances of congestion or inflammation, the red blood corpuscles may pass bodily through the walls of the capillaries ; the process consisting of a gradual protrusion of the capillary-wall by the corpuscle, until at length the latter slowly advances through it and out of it into the tissues around. Various theories have been advanced in explanation of these remarkable phenomena ; but Dr. Bastian' s idea appears the most probable, that the perforation of the capillary wall is caused primarily by certain changes in the shape of the red corpuscles, attended with protrusion of their walls, analogous to the amoeboid movements of the white corpuscles before referred to (p. 86). By means of such movements it appears that the white corpuscles also may, under certain inflammatory conditions, pass through the walls of the minute veins. Much diversity of opinion has long prevailed respecting the possession by capillaries of any power to aid the pro- gressive motion of the blood. It may be stated, with tolerable certainty, that the capillaries themselves possess no such power, and that the influence which they seem to exercise on the movement of their contained blood, is referable in part to the action of the smal,! arteries, and in part to the results of the relation which exists between the tissues without, and the blood within, the capillaries. Thus, the capillaries contract on the application of cold : but this is due not to any contraction similar to that of muscular tissue, but to their elasticity, and to that of the surrounding tissues, which close in, when by the contraction of the small arteries (which, as already stated, can be made to contract by cold), the flow of blood into the capillaries is diminished. THE CAPILLARIES. 177 The apparent contraction of the capillaries too, on the application of certain irritating substances, and during fear, and their dilatation in blushing, may be similarly referred to the action of the small arteries, rather than to that of the capillaries themselves. Still it is very probable that some influence in aid of the general circulation takes place in the capillary system. The results of morbid action, as well as the phenomena of health, strongly support such a view. For example, when the access of oxygen to the lungs is prevented, the circula- tion through the pulmonic capillaries is gradually retarded, the blood- corpuscles cluster together, and their movement is eventually almost arrested, even while the action of the heart continues. In inflammation, also, the capillaries of an inflamed part are enlarged and distended with blood, which either moves very slowly or is completely at rest. In both these cases the phenomena are local, and independent of the action of the heart, and appear to result from some alteration in the blood, which increases the adhesion of its particles to one another, and to the walls of the capil- laries, to an amount which the propelling action of the heart is not able to overcome. The temporary increase in the size of the capillaries, and in the quantity of blood moving through them in any part during an unusually active discharge of its functions, has been cited as evidence of their exercising some power to determine the amount of blood that shall traverse them. Instances of such enlargement are seen in the turgescence of an actively secreting or quickly growing part. But the control here displayed is exercised, not by the capillaries themselves, but by that relation, whatever be its nature, which exists between every tissue and the blood, and by which the condition of the tissue determines the quantity of blood to be supplied to it. It may be concluded then, that the capillaries, which are formed of a simple membrane, destitute of all con- N 178 THE CIRCULATION. tractile power apart from elasticity, can of themselves exercise no direct influence on the movement of their contents : yet that the constant interchange of relations between the blood and the tissues outside the vessels does in some measure facilitate the movement of blood through the capillary system, and thus constitute one of the assist- ant forces of the circulation. THE VEINS. In structure the coats of veins bear a general resem- blance to those of arteries. Thus, they possess an outer, middle, and internal coat. The outer coat is constructed of areolar tissue like that of the arteries, but is thinner. In some veins it contains muscular fibre-cells. The middle coat is considerably thinner than that of the arteries; and, although it contains circular unstriped muscular fibres or fibre-cells, these are mingled with a larger proportion of yellow elastic and white fibrous tissue. In the large veins entering the heart, namely, the vena cavce and pulmonary veins, the middle coat is replaced by circularly arranged striped muscular fibres, continuous with those of the auricles. The internal coat of veins is less brittle than the cor- responding coat of an artery, but in other respects, resembles it closely. The chief influence which the veins have in the circula- tion, is effected with the help of the valves, which are placed in all veins subject to local pressure from the muscles between or near which they run. The general construction of these valves is similar to that of the semi- lunar valves of the aorta and pulmonary artery, already described (p. 1 1 8); 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, but almost uniformly in each (fig. 53). In the smaller VALVES OF VEINS. 179 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 be- ing in some examples concave, in others straight. They are composed of inexten- sile fibrous tissue, and are covered with epithelium 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 movement of the blood (figs. 54 and 55). 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 Fig. 53. Diagrams 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. N 2 i8o THE CIRCULATION. of the bones, and the trunk and branches of the umbilical vein are also destitute of valves. The principal obstacle to the circulation is already over- come when the blood has traversed the capillaries; and the force of the heart which is not yet consumed, is suffi- cient to complete its passage through the veins, in which the obstructions to its movement are very slight. For the formidable obstacle supposed to be presented by the gravi- tation of the blood, has no real existence, since the pres- sure 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 gravitation, 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 ; as if one had a circular tube full of fluid at every part, the fluid might be made to circulate with equal facility in either direction, or in any position of the tube. 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. Their condition may, in this respect, be compared with that of a double bent tube full of fluid held vertically ; whatever be the height and gravitation of the columns of fluid, neither of them can move of its own weight, each being supported by the other ; yet the least pressure on the top of either column will lift up the other : so, when the body is erect, the least pressure on the column of arterial blood may lift up the venous blood, and, were it not for the valves, the least pressure on the venous might lift up the arterial column. PRESSURE IN VEINS. 181 In experiments to determine what proportion of the force of the left ventricle remains to propel the blood in the veins, Valentin found that the pressure of the blood in the jugular vein of a dog as estimated by the hsemadyna- mometer, did not amount to more than -^ or T ^- of that in the carotid artery of the same animal ; and this esti- mate is confirmed, in the instances of several other arteries and their corresponding veins, by Mogk. In the upper part of the inferior vena cava, Valentin could scarcely detect the existence of any pressure, nearly the whole force received from the heart having been, apparently, consumed during the passage of the blood through the capillaries. But slight as this remanent force might be (and the experiment in which it was estimated would reduce the force of the heart below its natural standard), it would be enough to complete the circulation of the blood; for, as already stated, the spontaneous dilatation of the auricles and ventricles, though it may not be forcible enough to assist the movement of blood into them, is adapted to offer to that movement no obstacle. Very effectual assistance to the flow of blood in the veins is afforded by the action of the muscles capable of pressing on such veins as have valves. The effect of muscular pressure on such veins 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 arterial valves, and being, like them, inex- tensile both in themselves and at their margins of attach- ment, do not follow the vein in its distension, but are drawn out towards the axis of the canal. Then, if the 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, what 182 THE CIRCULATION. ever 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. The circulation might lose as much as it gains by such compression of the veins, if it were not for the numerous anastomoses 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 circulation, 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 (figs. 54 and 55). Thus, therefore, the Fig. 54.* Fig. 55- 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 * Fig. 54. t Fig. 55- Vein with valves open (Dalton). Vein with valves closed ; stream of blood passing off by a lateral channel (Dalton). EFFECTS OF PRESSURE ON VEINS. 183 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 venesec- tion, 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 con- tinually taking place in the body while awake, though their single effects may be less striking, must be an im- portant auxiliary to the venous circulation. Yet they are not essential; for the venous circulation continues unimpaired in parts at rest, in paralysed limbs, and in parts in which the veins are not subject to any muscular pressure. Besides the assistance thus afforded by muscular pressure to the movement of blood along veins possessed of valves, it has been discovered by Mr. Wharton Jones that, in the web of the bat's wing, the veins are furnished with valves, and possess the remarkable property of rhythmical con- traction and dilatation, whereby the current of blood within them is distinctly accelerated. Mr. Jones found that the contraction occurred, on an average, about ten times in a minute ; the existence of valves preventing regurgitation, the entire effect of the contractions was auxiliary to the onward current of blood. It is reasonable to infer that veins in other parts may, when furnished with valves, possess a like power. Agents concerned in the Circulation of the Blood. The agents concerned in the circulation of the blood, which have been now described, may be thus enume- rated : 1. The action of the heart and of the arteries. 2. The vital capillary force exercised in the capillaries. 3. The possible slight action of the muscular coat of 1 84 THE CIRCULATION. veins ; and, much more, the contraction of muscles capable of acting on veins provided with valves. It remains only to consider (4) the influence of the respiratory movements on the circulation. Although the continuance of the respiratory movements is essential to the circulation of the blood, and although their cessation is followed, within a very few minutes, by that of the heart's action also, yet their direct mechanical influence on the movement of the current of blood is pro- bably, under ordinary circumstances, but slight. The effect of expiration in increasing the pressure of the blood in the arteries has been already mentioned (page 167), and is minutely illustrated by the experiments of Ludwig. It acts as the pressure of contracting muscles does upon the veins, and is advantageous to the movement of arterial blood, inasmuch as all regurgitation into the heart is prevented by the force of the onward stream of blood from the contracting ventricle, and in the intervals of this con- traction by the closure of the semilunar valves. Under ordinary circumstances, and with a free passage through the capillaries of the lungs, the effect of expiration on the stream of blood in the veins is also probably to assist, rather than retard its movement in the proper direction. For, with no obstruction in front, there is the force of the blood streaming into the heart from behind, to prevent any tendency to a backward flow, even apart from what may be effected by the presence of the valves of the venous system. It is true that in violent expiratory efforts there is a certain retardation of the circulation in the veins. The effect of such retardation is shown in the swelling-up of the veins of the head and neck, and the lividity of the face, during coughing, straining, and similar violent expiratory efforts ; the effects shown in these instances being due both to some actual regurgitation of the blood in the great veins, and to the accumulation of blood in all the veins, from EFFECTS OF RESPIRATION. 185 their being constantly more and more filled by the influx from the arteries. But strong expiratory efforts, as in straining and the like, are not fairly comparable to ordinary expiration, inas- much as they are instances of more or less interference with expiration, and involve probably circumstances lead- ing to obstruction of the circulation in the pulmonary capillaries, such as are not present in the ordinary rhyth- mical exit of air from the lungs. The act of inspiration is favourable to the venous circu- lation, and its effect is not counterbalanced by its tendency to draw the arterial, as well as the venous, blood towards the cavity of the chest. When the chest is enlarged in inspiration, the additional space within it is filled chiefly by the fresh quantity of air which passes through the trachea and bronchial passages to the vesicular structure of the lungs. But the blood being, like the air, subject to the atmospheric pressure, some of it also is at the same time pressed towards the expanding cavity of the chest, and therein towards the heart. The effect of this on the arterial current is hindered by the aortic valves, while they are closed, and by the forcible outward stream of blood from the ventricles when they are open ; while, on the other hand, there is nothing to prevent an increased afflux of blood to the auricles through the large veins. Sir David Barry was the first who showed plainly this effect of inspiration on the venous circulation; and he mentions the following experiment in proof of it. He introduced one end of a bent glass tube into the jugular vein of an animal, the vein being tied above the point where the tube was inserted ; the inferior end of the tube was immersed in some coloured fluid. He then observed that at the time of each inspiration the fluid ascended in the tube, while during expiration it either remained sta- tionary, or even sank. Poiseuille confirmed the truth of this observation in a more accurate manner, by means of J 86 THE CIRCULATION. his heemadynamometer. And a like confirmation has been since furnished by Valentin, and in minute details by Ludwig. The effect of inspiration on the veins is observable only in the large ones near the thorax. Poiseuille could not detect it by means of his instrument in veins more distant from the heart, for example, in the veins of the extremi- ties. And its beneficial effect would be neutralized were it not for the valves ; for he found that, when he repeated Sir D. Barry's experiments, and passed the tube so far along the veins that it went beyond the valves nearest to the heart, as much fluid was forced back into the tube in every expiration as was drawn in through it in every inspiration. Some recent experiments, by Dr. Burdon Sanderson, have proved more directly that inspiration is favourable to the circulation, inasmuch as, during it, the tension of the arterial system is increased. And it is only when the respiratory orifice is closed, as by plugging the trachea, that inspiratory efforts are sufficient to produce an opposite effect to diminish the tension in the arteries. On the whole, therefore, the respiratory movements of the chest are advantageous to the circulation. Velocity of Blood in the Veins. The velocity of the blood is greater in the veins than in the capillaries, but less than in the arteries : and with this fact may be remembered the relative capacities of the arterial and venous systems ; for since the veins return to the heart all the blood that they receive from it in a given time through the arteries, their larger size and propor- tionally greater number must compensate for the slower movement of the blood through them. 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, calculate that of the venous. VELOCITY OF THE CIRCULATION. 187 Perhaps a fair approximation to such, an 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, therefore, about twice or three times as great in the arteries as in the veins. And this is not a slow move- ment ; for if we stop the circulation at the beginning of any superficial vein, and empty the upper part of the vein, immediately upon removing the finger the blood will move along the vein faster than the eye can follow it. The rate at which the blood moves in the veins gradually increases the nearer it approaches 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. Velocity of the Circulation. Having now considered the share which each of the cir- culatory organs has in the propulsion and direction of the blood, we may speak of their combined effects, especially in regard to the velocity with which the movement of the blood through the whole round of the circulation is accom- plished. As Miiller says, the rate of the blood's motion in the vessels must not be judged of by the rapidity with which it flows from a vessel when divided. In the latter case, the rate of motion is the result of the entire pressure to which the whole mass of blood is subjected in the vas- cular system, and which at the point of the incision in the vessel meets with no resistance. In the closed vessels, on the contrary, no portion of blood can be moved forwards except by impelling on the whole mass, and by overcoming the resistance arising from friction in the smaller vessels. From the rate at which the blood escapes from opened vessels we can only judge, in general, that its velocity is, as already said, greater in arteries than in veins, and in both these greater than in the capillaries. More satisfactory data for the estimates are afforded by the results of experiments 1 88 THE CIRCULATION. to ascertain the rapidity with which poisons introduced into the blood are transmitted from one part of the vascular system to another. From eighteen such experiments on horses, Hering deduced that the time required for the passage of a solution of ferrocyanide of potassium, mixed with the blood, from one jugular vein (through the right side of the heart, the pulmonary circulation, the left cavities of the heart, and the general circulation) to the jugular vein of the opposite side, varies from twenty to thirty seconds. The 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 experiment, 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. Poiseuille's observations accord completely with the above ; and show, moreover, that when the ferrocyanide is injected into the blood with other substances, such as acetate of ammonia, or nitrate of potash (solutions of which, as other experiments have shown, pass quickly through capillary tubes), the passage from one jugular vein to the other is effected in from eighteen to twenty- four seconds ; while, if instead of these, alcohol is added, the passage is not completed until from forty to forty-five seconds after injection. Still greater rapidity of transit has been observed by Mr. J. Blake, who found that nitrate of baryta injected into the jugular vein of a horse could be detected in blood drawn from the carotid artery of the opposite side in from fifteen to twenty seconds after the injection. In sixteen seconds a solution of nitrate of potash, injected into the jugular vein of a horse, caused complete arrest of the heart's action, by entering and VELOCITY OF THE CIRCULATION. 189 diffusing itself through the coronary arteries. In a dog, the poisonous effects of strychnia on the nervous system were manifested in twelve seconds after injection into the jugular vein ; in a fowl, in six and a half seconds, and in a rabbit in four and a half seconds. In all these experiments, it is assumed that the sub- . stance injected moves with the blood, and at the same rate as it, and does not move from one part of the organs of circulation to another by diffusing itself through the blood or tissues more quickly than 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, are those in which the portion of blood, into which each was injected, was carried from one part to another of the vascular system. It would, therefore, 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 ex- tremities 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 con- cluded that one minute, which is the estimate usually adopted of the average time in which the blood completes its entire circuit in man, is rather above than below the actual rate. Another mode of estimating the general velocity of the circulating 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 estimates both of the total quantity of blood, and of the capacity of the cavities of the heart, have as yet only approximated to the truth. Still, the most careful of the estimates thus made accord 190 THE CIKCULATION. with those already mentioned; for Valentin has, from these data, calculated that the blood may all pass through the heart in from 43! to 62 1 seconds. The estimate from the speed at which the blood may be seen moving in transparent parts, is not opposed to this. For, as already stated (p. 174), though the movement through the capillaries may be very slow, yet the length of capillary vessel through which any portion of blood has to pass is very small. Even if we estimate that length at the tenth of an inch, and suppose the velocity of the blood therein to be only one inch per minute, then each portion of blood may traverse its own distance of the capillary system in about six seconds. There would thus be plenty of time left for the blood to travel through its circuit in the larger vessels, in which the greatest length of tube that it can have to traverse in the human subject does not exceed ten feet. All the estimates here given are averages ; but of course the time in which a given portion of blood passes from one side of the heart to the other, varies much according to. the organ it has to traverse. The blood which circulates from the left ventricle, through the coronary vessels, to the right side of the heart, requires a far shorter time for the completion of its course than the blood which flows from the left side of the heart to the feet, and back again to the right side of the heart ; for the circulation from the left to the right cavities of the heart may be represented as form- ing a number of arches, varying in size, and requiring proportionately various times for the blood to traverse them; the smallest of these arches being formed by the circulation through the coronary vessels of the heart itself. The course of the blood from the right side of the heart, through the lungs to the left, is shorter than most of the arches described by the systemic circulation, and in it the blood flows, cceteris paribus, much quicker than in most of the vessels which belong to the aortic circulation. For PECULIARITIES OF THE CIRCULATION. 191 although the quantity of blood contained, at any instant, in the greater circulation of the body, is far greater than the quantity within the lesser circulation; yet, in any given space of time, as much blood must pass through the lungs as passes in the same time through the systemic circulation. If the systemic vessels contain five times as much blood as the pulmonary, the blood in them must move five times as slow as in these ; else, the right side of the heart would be either overfilled or not filled enough. Peculiarities of the Circulation in different Parts. The most remarkable peculiarities attending the circula- tion of blood through different organs are observed in the cases of the lungs, the liver, the brain, and the erectile organs. The pulmonary and portal circulations have been already alluded to (pp. 1 1 1, 112), and will be again noticed when considering the functions of the lungs and liver. The chief circumstances requiring notice, in relation to the cerebral circulation, are observed in the arrangement and distribution of the vessels of the brain, and in the con- ditions attending the amount of blood usually contained within the cranium. The functions of the brain seem to require that it should receive a large supply of blood. This is accomplished through the number and size of its arteries, the two in- ternal carotids, and the two vertebrals. But it appears to be further necessary that the force with which this blood is sent to the brain should be less, or at least, subject to less variations from external circumstances than it is in other parts. This object is effected by several provisions ; such as the tortuosity of the large arteries, and their wide anas- tomoses in the formation of the circle of Willis, which will insure that the supply of blood to the brain may be uni- form, 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, 192 THE CIRCULATION. 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 substance of the brain, the large arteries break up and divide into innumerable minute branches ending in capillaries, which, after frequent communications with one another, enter the brain, and carry into nearly every part of it uniform and equable streams of blood. 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 com- posed, 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 substance placed in a cavity with unyielding walls. These conditions of the brain and skull have ap- peared, indeed, to some, enough to justify the opinion that the quantity of blood in the brain must be at all times the same ; and that the quantity of blood received within any given time through the arteries must be always, and at the same time, exactly equal to that removed by the veins. In accordance with this supposition, the symptoms commonly referred to either excess or deficiency of blood in the brain, were ascribed to a disturbance in the balance between the quantity of arterial and that of venous blood. Some experiments performed by Dr. Kellie appeared to establish the correctness of this view. But Dr. Burrows having repeated these experiments, and performed ad- ditional ones, obtained different results. He found that in CIRCULATION IN THE BRAIN. 193 animals bled to death, without any aperture being made in the cranium, the brain became pale and anaemic like other parts. And in proof that, during life, the cerebral circulation is influenced by the same general circumstances that influence the circulation elsewhere, he found con- gestion of the cerebral vessels in rabbits killed by strang- ling or drowning ; while in others, killed by prussic acid, he observed that 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 comparatively empty when the animal was kept suspended by the ears. He concluded, therefore, that although the total volume of the contents of the cranium is probably nearly always the same, yet the quantity of blood in it is liable to variation, its increase or diminution being accompanied by a simultaneous diminu- tion 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. 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 provision made for the possible displace- ment 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 not only subserves the mechanical functions of fat in 194 THE CIRCULATION. 4 other parts as a,' packing material, but by the readiness with which it can be displaced, provides the means whereby undue pressure and insufficient supply of blood are equally prevented. Circulation 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 circumstances, 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 cl^oris 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 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, col- lapsing 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 urethrse, 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 CIRCULATION IN ERECTILE STRUCTURES. 195 venous blood is returned from the plexuses by compara- tively small veins ; those from the glans and the fore part of the urethra empty themselves into the dorsal vein of the penis ; those from the corpus 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 plex- uses with blood. The principal exciting cause in the erec- tion 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 vascular tissue : and Guenther has observed, that, after their division in the horse, the penis is no longer capable of erection. It affords a good example of the subjection of the circulation in an indi- vidual organ to the influence of the nerves ; but the mode in which, they excite a greater influx of blood is not with certainty known. The most probable explanation is that offered by Pro- fessor Kolliker, who ascribes the distension of the venous plexuses to the influence of organic muscular fibres, which are found in abundance in the corpora cavernosa of the penis, from the bulb to the glans, also in the clitoris and other parts capable of erection. While erectile organs are flaccid and at rest, these contractile fibres exercise an amount of pressure on the plexuses of vessels distributed amongst them, sufficient to prevent their distension with blood. But when through the influence of their nerves, these parts are stimulated to erection, the action of these fibres is suspended, and the plexuses thus liberated from pressure, yield to the distending force of the blood, which, o 2 196 THE CIRCULATION. probably, at the same time arrives in greater quantity, owing to a simultaneous dilatation of the arteries of the parts, and thus the plexuses become filled, and remain so until the stimulus to erection subsides, when the organic muscular fibres again contract, and so gradually expel the excess of blood from the previously distended vessels. The influence of cold in producing extreme contraction and shrinking of erectile organs, and the opposite effect of warmth in inducing fulness and distension of these parts, are among the arguments used by Kolliker in support of this opinion. The accurate dissections and experiments of Kobelt, extending and confirming those of Le Gros Clark and Krause, have shown, that this influx of the blood, however explained, is the first condition necessary for erection, and that through it alone much enlargement and turgescence of the penis may ensue. But the erection is probably not complete, nor maintained for any time except when, together with this influx, the muscles already mentioned contract, and by compressing the veins, stop the eiflux of blood, or prevent it from being as great as the influx. It appears to be only the most perfect kind of erection that 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 erec- tion, 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 local occasions, augmented quantities of blood. i 9 7 CHAPTER VII. RESPIRATION. As the blood circulates through the various parts of the body, and fulfils its office by nourishing the several tissues, by supplying to secreting organs the materials necessary for their secretions, and by the performance of other duties with which it is charged, it is deprived of part of its nutritive constituents, and receives impurities which need removal from the body. It is, therefore, necessary that fresh supplies of nutriment should be con- tinually added to the blood, and that provision should be made for the removal of the impurities. The first of these objects is accomplished by the processes of digestion and absorption. The second is principally effected by the agency of the various excretory organs, through which are removed the several impurities with which the blood is charged, whether these impurities are derived altogether from the degeneration of tissues, or in part also from the elements of unassimilated food. One of the most important and abundant of the impurities is carbonic acid, the re- moval of which and the introduction of fresh quantities of oxygen, constitute the chief purpose of respiration a process which, because of its intimate relation to the cir- culation, may be considered here, rather than with the other excretory functions. Position and /Structure of the Lungs. The lungs occupy the greater portion of the chest, or uppermost of the two cavities into which the body is divided by the diaphragm (fig. 31). They are of a spongy elastic texture, and on section appear to the naked eye as if they were in great part solid organs, except here and there, at certain points, where branches of the bronchi or 198 RESPIRATION. air-tubes may have been cut across, and show, on their surface of the section, their tubular structure. In fact, however, the lungs are hollow organs, and we may consider them as really two bags containing air, each of which communicates by a separate orifice with a common air-tube (fig. 31), through the upper portion of which, the larynx, they freely communicate with the external atmosphere. The orifice of the larynx is guarded by muscles, and can be opened or closed at will. It has been said, in the preceding chapter, that each lung is enveloped in a distinct fibrous bag, with a smooth, slippery lining, and that the outer smooth surface of the lung glides easily on the inner smooth surface of the lag which envelopes it. This enveloping bag, which is Fig. 56.* called the pleura, is easily seen in the dead subject ; and when it is opened, as in an ordinary post-mortem examina- tion, there is a considerable space left, by the elastic recoil of the lung, between the outer surface of the lung and the inner surface of the pleura, which is left sticking, so to Fig. 56. Transverse section of the chest (after Gray). STRUCTURE OF THE LUNGS. 199 speak, to the inner surface of the walls and floor of the chest. This space, however, between the lung and the pleura does not exist (except in some cases of disease) so long as the chest is not opened ; and, while considering the subject of normal healthy respiration, we may discard altogether the notion of any space or cavity between the lung and the wall of the chest. So far as the movement of the lung is concerned they might be adherent completely, one to the other, inasmuch as they accompany each other in all their movements ; only there is a slight gliding of the smooth surface of the lung on the smooth inner surface of the pleura, but no separation, in the slightest degree, of one from the other. * The trachea, or tube through which air passes to the lungs, divides into two branches one for each lung ; and these primary branches, or bronchi, after entering the substance of the organ, divide and subdivide into a number of smaller and smaller branches, which penetrate to every part of the organ, until at length they end in the smaller subdivisions of the lung called lobules. All the larger branches have walls formed of tough membrane, contain- ing 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 vibratile ciliary epi- thelium (fig. 58). * It may be here mentioned, that the smooth covering of the lung is really continuous with the inner smooth lining of the walls and floor of the chest, as will be readily seen in fig. 56. Hence the membrane which covers the lung is called the visceral layer of the pleura and that which lines the walls and floor of the chest the parietal layer. The appearance of a cavity or space (fig. 56) between the visceral layer of pleura (covering the lungs) and the parietal layer (covering the inner surface of the wall of the chest and upper part of the diaphragm) is only inserted for the sake of distinctness. 200 RESPIRATION. As the bronchi divide they become smaller and smaller, and their walls thinner ; the cartilaginous rings especially Fig. 57.* becoming 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 -jL- of an incli in diameter, they lose their cartilaginous element alto- gether, and their walls are formed only of a tough, fibrous, elastic membrane, with traces of circular muscular fibres ; they are still lined, however, by a thin mucous membrane, with ciliated epithelium. * Fig. 57- A diagrammatic representation of the heart and great vessels in connection with the lungs. . The pericardium has been removed, and the lungs are turned aside. I, right auricle ; 2, vena cava superior ; 3, vena cava inferior ; 4, right ventricle ; 5, stem of the pul- monary artery ; a a, its right and left branches ; 6, left auricular appendage ; 7, left ventricle ; 8, aorta ; 9, 10, the two lobes of the left lung ; ii, 12, 13, the three lobes of the right lung ; b b, right and left bronchi ; v v, right and left upper pulmonary veins. STRUCTURE OF THE LUNGS. 201 Each lung is partially subdivided into separate portions, called lobes ; the right lung into three lobes, and the left lung into two (fig. 57). Each of these lobes, again, is Fig. 58.* 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, bloodvessels, nerves, and lymphatics, with a sparing amount of areolar tissue. On entering a lobule, the small bronchial tube divides and subdivides; its walls, at the same time, becoming thinner and thinner, until at length they are formed only of a thin membrane of 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 terminal branches widening out funnel-wise, and its walls being pouched out irregularly into small saccular dilatations, called air-cells (fig. 59). Such a funnel-shaped terminal branch of the bronchial tube, with its group of pouches or air-cells, has been called an infundibulum * Fig. 58. Ciliary epithelium of the human trachea magnified 350 diameters, a. Layer of longitudinally-arranged elastic fibres ; b. Base- ment membrane ; c. Deepest cells, circular in form ; d. Intermediate elongated cells ; e. Outermost layer of cells fully developed and bearing cilia (after Kolliker.) 2O2 RESPIEATION. (% 59)> and the irregular oblong space in its centre, with which the air-cells communicate, an intercellular passage. ^59-* The air-ceUs may be placed singly, like recesses from the intercellular pas- sage, but more often they are arranged in groups or even in rows, like minute sacculated tubes ; so that a short series of cells, all communicating with one another, open by a common orifice into the tube. The cells are of various forms, according to the mutual pressure to which they are subject ; their walls are nearly in contact, and they vary from -j-i-o to T L of an inch in diameter. Their walls are formed of fine membrane, similar to that of the intercellular passages, and continuous 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-cells, or between the cells and the bronchial passages. Numerous fibres of elastic tissue are spread 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 (fig. 60, b and c). The cells are lined by a layer of squamous or tesselated epithe- lium, not provided with cilia. Outside the cells, a net- * Fig. 59. Two small groups of air-cells, or infundibula, a a, with air-cells, b b, and the ultimate bronchial tubes, c c, with which the air- cells communicate. From a new-born child (after Kolliker). STRUCTURE OF THE LUNGS. 203 work of pulmonary capillaries is spread out so densely (fig. 6l), that the interspaces or meshes are even narrower than the vessels, which are, on an average, -^-oVo" ^ an inch in diameter. Between the atmospheric air in the cells and the blood in these vessels, nothing intervenes Fig. 60.* but the thin membranes of the cells and capillaries and the delicate epithelial lining of the former ; 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, con- tain only a single layer of capillaries, both sides of which are thus at once exposed to the air. * Fig. 60. Air-cells of lung, magnified 350 diameters, a. Epithelial lining of the cells ; b. Fibres of elastic tissue ; c. Delicate membrane of which the cell-wall is constructed with elastic fibres attached to it (after KoUiker). 204 RESPIRATION. The cells situated nearest to the centre of the lung are smaller, and their networks of capillaries are closer than those nearer to the circumference, in adaptation to the more ready supply of fresh air to the central than the peripheral portion of the lungs. The cells of adjacent Fig. 6 1.* 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. Mechanism of Respiration. For the proper understanding of the mechanism by which air enters and is expelled from the lungs, the follow- ing facts must be borne in mind : The lungs form two distinct hollow bags (communicating with the exterior through the trachea and larynx), and are always closely in contact with the inner surface of the * Fig.. 61. Capillary net-work of the pulmonary blood-vessels in the human lung (from Kblliker) . MECHANISM OF BESPIRATION. 205 chest- walls, while their lower portions are closely in con- tact with the diaphragm, or muscular partition which separates the chest from the abdomen (figs. 3 1 and 65). The lungs follow all movements of the parts in contact with them; and for the evident reason that the outer surface of the lung-bag not being exposed directly to atmospheric pres- sure, while the inner surface is so exposed, the pressure from within preserves the lungs in close contact with the parts surrounding them, and obliterates, practically, the pleural space, and must continue to do so, until from some cause or other say from an opening for the admission of air through the chest-walls, the pressure on the outside of the lung equals or exceeds that on the interior. Any such artificial condition of things, however, need not here be considered. For the inspiration of air into the lungs it will be evi- dent from the foregoing facts, 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 contract the capacity of the chest, the pressure in the interior will be increased, and air will be expelled, until the pressures within and without the chest are again equal. In both cases the air passes through the trachea and larynx, w r hether in entering or leaving the lungs, there being no other communication with the exterior, and the lung, for the reason before mentioned, 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 chest- 206 RESPIRATION. cavity is alternately enlarged and contracted for the entrance and expulsion of atmospheric air ; or, in technical terms, for inspiration and expiration. Respiratory Movements. The chest is a cavity filled by the lungs, heart, and large blood-vessels, etc., and closed everywhere against the en- trance of air except by the way of the larynx and trachea. It is bounded behind and at the sides by the spine and ribs, and in front by the sternum and cartilages of the ribs. Its floor is formed mainly by the diaphragm. The immediate inner lining of all these parts is the outer or polished layer of the pleura ; and this membrane also is stretched continuously across the top of the chest- cavity, and mainly forms its roof. The enlargement of the capacity of the chest in inspira- tion is a muscular act ; the muscles concerned in producing the effect being chiefly the diaphragm and the external intercostal muscles, with that part of the internal inter- costal which is between the cartilages of the ribs. These are assisted by the levatores costarum, the serratus posticus superior, and some others. The vertical diameter of the chest is increased by the contraction and consequent descent of the diaphragm, the sides of the muscle descending most, and the central tendon remaining comparatively unmoved; while the intercostal, and other muscles just mentioned, by acting at the same time, not only prevent the diaphragm during its contraction from drawing in the sides of the chest, but increase the diameter of the chest in the lateral direction, by elevating the ribs ; that is to say, by rotating them, to speak roughly, around an axis passing through their sternal and spinal attachments, somewhat after the fashion of raising the handle o*f a bucket (fig. 62) . This is not all, however. Another effect of the contraction of the intercostal muscles is to increase RESPIRATORY MOVEMENTS. 207 the antero -posterior diameter of the chest, by partially straightening out the angle between the rib and its carti- lage, and thus lengthening the distance between its spinal and sternal attachments (fig. 62, A). In this way, at the same time that the ribs are raised, the sternum is pushed forward. The differences in shape of the upper and lower Fig. 62. true ribs, and the more acute angles formed by the junction of the latter with their cartilages, make the effect much greater at the lower than at the upper part of the chest. The expansion of the chest in inspiration presents some peculiarities in different persons and circumstances. In young children, it is effected almost entirely by the dia- phragm, 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 mode or type of respiration. In adult men, together with the descent of the diaphragm, and the pushing forward of the front wall of the abdomen, the lower part of the chest and the sternum are subject to a wide movement in inspiration. In women, the move- 208 RESPIRATION. merit appears less extensive in the lower, and more so in the upper, part of the chest ; a mode of breathing to which a greater mobility of the first rib is adapted, and which Fig. 64. t may have for its object the provision of sufficient space for * Fig. 63 (after Hutchinson). The changes of the thoracic and abdominal walls of the male during respiration. The back is supposed to be fixed in order to throw forward the respiratory movement as much as possible. 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 abdominal : thin over the chest, for there is less movement over that region. The dotted line indicates the movement on deep inspiration, during which the sternum advances while the abdomen recedes. f Fig. 64 (after Hutchinson). The respiratory movement in the 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. ELASTIC KECOIL OF LUNGS AND CHEST. 209 respiration when the lower part of the chest is encroached upon by the pregnant uterus. MM. Beau and Maissiat call the former the inferior costal, and the latter the superior costal, type of respiration ; but the annexed diagrams will explain the difference better than the names will, for these imply a greater diversity than naturally exists in the modes of inspiration. From the enlargement produced in inspiration, 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 the lungs and chest-walls, being returned as an expira- tory effort when the muscles are relaxed. This elastic recoil, chiefly of the rib -cartilages, but also of the lungs themselves, in consequence of the elastic tissue which they contain in considerable quantity, is sufficient, in ordinary quiet breathing, to expel air from the chest .in the inter- vals 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 of course necessary, and the proper expiratory 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 diaphragm, 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 intercostals and triangularis sterni, the ser- ratus posticus inferior, etc. 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 210 RESPIRATION. elasticity. The construction of the chest-walls, therefore, admirably adapts them for recoiling against and resisting as well undue contraction as undue dilatation. As before mentioned, the lungs, after distension in the act of inspiration, contract by virtue of the elastic tissue which is present in the bronchial tubes, on and between the air-cells, and in the investing pleura. But in the natural condition of the parts, they can never contract 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 atmospheric pressure, and able to 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 dila- tation 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, and can scarcely be said to be separated from each other by an intervening pause. 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 the beginning of the next inspira- tion. The respiratory rhythm may be thus expressed : Inspiration ..... 6 Expiration . . . . 7 or 8 A very slight pause. Respiratory Movements of the Glottis. During the action of the muscles which directly draw QUANTITY OF AIR RESPIRED. 211 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. 65), is dilated at each inspiration, for the more ready passage of air, and collapses somewhat at each expiration, its condition, therefore, corresponding 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 riina glottidis is a muscular act, and its contraction chiefly an elastic recoil; although, under various conditions, to be hereafter mentioned, there may be, as in expiration, con- siderable muscular power exercised. Quantity of Air Respired. The quantity of air that is changed in the lungs in each act of ordinary tranquil breathing is variable, and is very difficult to estimate, because it is hardly possible to breathe naturally while, as in an experiment, one is attending to the process. Probably 30 to 35 cubic inches are a fair average in the case of healthy young and middle-aged men; but Bourgery is perhaps right in saying, that old people, even in health, habitually breathe more deeply, and change in each respiration a larger quantity of air than younger persons do. The total quantity of air which passes into and out of the lungs of an adult, at rest, in 24 hours, has been esti- mated by Dr. E. Smith at about 686, OOO cubic inches. This quantity, however, is largely increased by exertion; and the same observer has computed the average amount for a hard-working labourer in the same time, at 1,568,390 cubic inches. The quantity which is habitually and almost uniformly changed in each act of breathing, is called by Mr. Hutchin- p 2- 212 RESPIRATION. son breathing air. The quantity over and above this which a man can draw into the lungs in the deepest inspiration, he names complemented air: its amount is various, as will be presently shown. After ordinary expiration, such as that which expels the breathing air, a certain quantity of air remains in the lungs, which may be expelled by a forcible and deeper expiration: this he terms reserve air> But, even after the most violent expiratory effort, the lungs are not completely emptied; a certain quantity always remains in them, over which there is no voluntary control, and which may be called residual air. Its amount depends in great measure on the absolute size of the chest, and has been variously estimated at from forty to two hundred and sixty cubic inches. 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. Mr. Hutchinson names this the vital capacity: it expresses the power which a person has of breathing in the emergencies of active exercise, violence, and disease; and in healthy men it varies according to stature, weight, and age. It is found by Mr. Hutchinson, from whom most of our information on this subject is derived, that at a temperature of 60 F., 225 cubic inches is the average vital capacity of a healthy person, five feet seven inches in height. 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. This relation of capacity to height is quite independent of the absolute capacity of the cavity of the chest ; for the cubic contents of the chest do not always, or even generally, increase with the stature of the body ; and a person of small absolute capacity of chest may have a large capacity of respiration, and vice versa. The capacity of respiration is determined only by the mobility of the walls of the chest ; but why VITAL CAPACITY. 213 this mobility should increase in a definite ratio with the height of the body is yet unexplained, and must be difficult of solution, seeing that the height of the body is chiefly determined by that of the legs, and not by the height of the trunk or the depth of the chest. But the vast number of observations made by Mr. Hutchinson seem to leave no doubt of the fact as stated above. The influence of iveiglit 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 statement, however, it may be said, that the capacity of respiration is not affected by weights under l6l pounds, or Ilj 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; so that, for example, while a man of five feet six inches, and weighing less than Il|- stones, should be able to expire 217 cubic inches, one of the same height, weighing \2\ stones, might expire only 203 cubic inches. 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. . Mr. Hutchinson's observations were made almost exclu- sively on men ; and his conclusions are, perhaps, true of them alone ; for women, according to Bourgery, have only half the capacity of breathing that men of the same age have. The number of respirations in a healthy adult person usually ranges from fourteen to eighteen per minute. It is greater in infancy and childhood; and of course 214 RESPIRATION. varies 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 I to 4, or I to 5, and when the rapidity of the heart's action is increased, that of the chest move- ment is commonly increased also ; but not in every case in equal proportion. 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 pro- portion than that of the respirations. According to Mr. Hutchinson, 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 mer- cury. 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 dispropor- tionate 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. The following table expresses the result of numerous experiments by Mr. Hutchinson on this subject, the instru- ment used to gauge the inspiratory and expiratory power being a hsemadynamometer (see p. 164), to which was attached a tube fitting the nostrils, and through which the inspiratory or expiratory effort was made: POWER OF RESPIRATORY MUSCLES. 215 Power of Power of Inspiratory Muscles. Expiratory Muscles. i. 5 in. . . . Weak 2.0 in. 2.0 ... Ordinary 2.5 2-5 3-5 4-5 5-5 6.0 7.0 Strong 3.5 Very strong 4.5 Remarkable 5-^ Very remarkable . . . 7> >> Extraordinary . . . . 8.5 ,, Very extraordinary . . 10.0 ,, Mr. Hutchinson remarks : " Suppose a man to lift by his inspiratory muscles three inches of mercury, what muscular effort has he used ? The mere quantity of fluid lifted may be very inconsiderable (and as such I have found men wonder they could not elevate more), but not so the power exerted, when we recollect that hydrostatic law, which Mr. Bramah adopted to the construction of a very convenient press. To apply this law here, the diaphragm alone must act under such an effort, with a force equal to the weight of a column of mercury 3 inches in height, and whose base is commensurate to the area of the diaphragm. The area of the base of one of the chests now before the Society, is 57 square inches; therefore, had this man raised 3 inches of mercury by his inspiratory muscles, his diaphragm alone in this act must have opposed a resistance equal to more than 23 oz. on every inch of that muscle, and a total weight of more than 83 Ibs. Moreover, the sides of his chest would resist a pressure from the atmosphere equal to the weight of a covering of mer- cury three inches in thickness, or more than 23 oz. on every inch surface, which, if we take at 318 square inches, the chest will be found resisting a pressure of 731 Ibs. ; and allowing the elastic resistance of the ribs as ij inch of mercury, this will bring the weight resisted by the chest as follows : Diaphragm 83 Ibs. Walls of the chest 731 Elastic force. 232 Total 1046 216 RESPIRATION. " In round numbers it may be said, that the parietes of the thorax resisted IOOO Ibs. of atmospheric pressure, and that not counterbalanced, to say nothing of the elastic power of the lungs, which co-operated with this pressure. " I would not venture at present to state exactly the distribution of muscular fibre over the thorax, which is called into action when resisting this 1046 Ibs., but I think I am safe in stating that nine-tenths of the thoracic sur- face conspire to this act. 11 What is here said of the muscular part of the chest re- sisting such a force, must not be confounded with a former statement of 'two-thirds being lifted by the inspiratory muscles, and one-third left dormant,' under a force equal to 301 Ibs. In this case the 301 Ibs. are lifted ; in the other, nine- tenths of 1046 Ibs. are said to be resisted. " The glass receiver of an air-pump may resist 15 Ibs. on the square inch, yet it may be said to lift nothing. This question of the thoracic muscular force and resistance, and muscular distribution, is rendered complicate by the pres- ence of so much osseous matter entering into the composi- tion of the chest, which can scarcely be considered to act the same as muscle." The great force of the inspiratory efforts during apncea was well shown in some of the experiments performed by the Medico-chirurgical Society's Committee on Suspended Animation. On inserting a glass tube into the trachea of a dog, and immersing the other end of the tube in a vessel of mercury, the respiratory efforts during apncea were so great as to draw the mercury four inches up the tube. The influence of the same force was shown in other expe- riments, in which the heads of animals were immersed both in mercury and in liquid plaster of Paris. In both cases the material was found, after death, to have been drawn up into all the bronchial tubes, filling the tissue of the lungs. CONTRACTION OF BRONCHI. 217 Much of the force exerted in inspiration is employed in overcoming the resistance offered by the elasticity of the walls of the chest and of the lungs. Mr. Hutchinson esti- mated the amount of this elastic resistance, 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 he calculated that, in a man capable of breathing 2OO cubic inches of air, the muscular power expended upon the elas- ticity of the walls of the chest, in making the deepest inspiration, would be equal to the raising of at least 301 pounds avoirdupois. To this must be added about 150 Ibs. for the elastic resistance of the lungs themselves, so that the total force to be overcome by the muscles in the act of inspiring 2OO cubic inches of air is more than 450 Ibs. In tranquil respiration, supposing the amount of breathing air to be twenty cubic inches, the resistance of the walls of the chest would be equal to lifting more than I OO pounds; and to this must be added about 70 pounds for the elasticity of the lungs. The elastic force overcome in ordinary inspiration must, therefore, be equal to about I/O pounds. It is probable, that in the ordinary quiet respiration, which is performed without consciousness or effort of the will, the only forces engaged are those of the inspiratory muscles, and the elasticity of the walls of the chest and the lungs. It is not known under what circumstances the contractile power which the bronchial tubes possess, by means of their organic muscular fibres, is brought into action. It is possible, as Dr. R. Hall maintained, that it may exist in expiration ; but it is more likely that its chief purpose is 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. Another pur- pose probably served by the muscular fibres of the bron- 218 EESPIKATION. chial tubes is that of contracting upon and gradually ex- pelling collections of mucus, which may have accumulated within the tubes, and cannot be ejected by forced expiratory efforts, owing to collapse or other morbid conditions of the portion of lung proceeding from the obstructed tubes (Gairdner). The muscular action in the lungs, morbidly excited, is probably the chief cause of the phenomena of spasmodic asthma. It may be demonstrated by galvanizing the lungs shortly after taking them from the body. Under such a stimulus, they contract so as to lift up water placed in a tube introduced into the trachea (C. J. B. Williams) ; and Volkmann has shown that they may be made to contract by stimulating their nerves. He tied a glass tube, drawn fine at one end, into the trachea of a beheaded animal; and when the small end was turned to the flame of a candle, he galvanized the pneumogastric trunk. Each time he did so the flame was blown, and once it was blown out. The changes of the air in the lungs effected by these respiratory movements are assisted by the various con- ditions of the air itself. According to the law observed in the diffusion of gases, the carbonic acid evolved in the air- cells will, independently of any respiratory movement, tend to leave the lungs, by diffusing itself into the external air, where it exists in less proportion; and according to the same law, the oxygen of the atmospheric air will tend of itself towards the air-cells in which its proportion is less than it is in the air in the bronchial tubes or in that external to the body. But for this tendency in the oxygen and carbonic acid to mix uniformly, within and without the lungs, the reserve and residual air would, probably, be very injuriously charged with carbonic acid; for the respiratory movements alone are not enough to empty the air-cells, and perhaps expel only the air which lies in the larger bronchial tubes. Probably also the change PULMONARY CIRCULATION. 219 is assisted by the different temperature of the air within and without the lungs ; and by the action of the cilia on the mucous membrane of the bronchial tubes, the continual vibrations of which may serve to prevent the adhesion of the air to the moist surface of the mem- brane. Movement 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 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 junc- tion, and incapable of closing the orifice of either of the veins between which they are placed. The pulmonary cir- culation also is unaffected by changes of atmospheric pressure, and is not exposed to the influence of the pressure of muscles : the force by which it is accomplished, and the course of the blood are alike simple. The blood which is conveyed to the lungs by the pul- monary arteries is distributed to these organs to be purified and made fit for the nutrition of all other parts of the body. The capillaries of the pulmonary vessels are ar- ranged solely with reference to this object, and therefore can have but little to do with the nutrition of the lungs ; or at least, only of those portions of the lungs with which 220 BESPIRATION. they are in intimate connection for another purpose. For the nutrition of the rest of the lungs, including the pleura, interlobular tissue, bronchial tubes and glands, and the walls of the larger blood-vessels, a special supply of arterial blood is furnished through one or two bronchial arteries, the branches of which ramify in all these parts. The blood of the bronchial artery, when, having served for the nutri- tion of these parts, it has become venous, is carried partly into the branches of the bronchial vein, distributed in the parts about the root of the lung, and partly into the small branches of the pulmonary artery, or, more directly, into the pulmonary capillaries, whence, being with the rest of the blood arterialized, it is carried to the pulmonary veins and left side of the heart. Changes of the Air in Respiration. By their contact in the lungs the composition of both air and blood is changed. The alterations of the former being manifest, simpler than those of the latter, and in some degree illustrative of them, may be considered first. 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, etc. Of every IOO volumes of pure atmospheric air, 79 volumes (on an average) consist of nitrogen, the remaining 21 of oxygen. The proportion of carbonic acid is extremely small; IO,ODO volumes of atmospheric air contain only about 4 or 5 of carbonic acid. The quantity of watery vapour varies greatly, according to the temperature and other circumstances, but the at- mosphere is never without some. In this country, the average quantity of watery vapour in the atmosphere is I '40 per cent. CHANGES OF AIR IN RESPIRATION. 221 The changes produced by respiration on the atmospheric air are, that, I, it is warmed; 2, its carbonic acid is in- creased ; 3, its oxygen is diminished ; 4, its watery vapour is increased. 1. The expired air, heated by its contact with the in- terior of the lungs, is (at least in most climates) hotter than the inspired air. Its temperature varies between 97 and 99i, the lower temperature being 'observed when the air has remained but a short time in the lungs, rather than when it is inhaled at a very low temperature ; for whatever the temperature when inhaled may be, the air nearly acquires that of the blood before it is expelled from the chest. 2. The carbonic acid in respired air 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^ per cent, of oxygen are abstracted ; while a rather smaller quantity of carbonic acid is added in its place. It may be stated, as a general average de- duced from the results of experiments by Valentin and Brunner, that, 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, which cor- responds very closely with the one furnished by Sir H. Davy, and does not widely differ from those obtained by Allen and Pepys, Lavoisier, and Dr. Ed. Smith, the weight of carbon excreted from the lungs is about 173 grains per hour, or rather more than 8 ounces in the course of twenty- four hours. Discrepancies in the results obtained by dif- ferent experimenters may be due to the variations to which the exhalation of carbonic acid is liable in different circumstances ; for even in health the quantity varies accord- ing to age, sex, diversities in the respiratory movements, external temperature, the degree of purity of the respired air, and other circumstances. Each of these deserves a 222 RESPIRATION. brief notice, because it affords evidence concerning either the sources of carbonic acid exhaled, or the mode in which it is separated from the blood. a. Influence of Age and Sex. According to Andral and Gavarret the quantity of carbonic acid exhaled into the air breathed -by males, regularly increases from eight to thirty years of age ; from thirty to forty it is stationary or diminishes a little ; from forty to fifty the diminution is greater ; and from fifty to extreme age it goes on diminish- ing, 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 the age of puberty, when the quantity abruptly ceases to increase, and remains stationary so long as they con- tinue to menstruate. When, however, menstruation has ceased, either in advancing years or in pregnancy, or morbid amenorrhcea, the exhalation of carbonic acid again augments; but when menstruation ceases naturally, it soon decreases again at the same rate that it does in old men. b. Influence of Respiratory Movements. According to Vierordt, the more quickly the movements of respiration are performed, the smaller is the proportionate quantity of carbonic acid contained in each volume of the expired air. Thus he found that, with six respirations per minute, the quantity of expired carbonic acid was 5*528 per cent. ; with twelve respirations, 4*262 per cent. ; with twenty- four, 3'355 ; with forty-eight, 2-984; and with ninety-six, 2 '662. Although, however, the proportionate quantity of carbonic acid is thus diminished during frequent respira- tion, 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. This is the case, whether the respiration be voluntarily accelerated, or naturally increased in frequency, as it is after feeding, INFLUENCE OF TEMPERATURE AND SEASON. 223 active exercise, etc. By diminishing the frequency, and increasing the depth of respiration, the per-centage pro- portion of carbonic acid in the expired air is diminished ; being in the deepest respiration as much as I '97 per cent, less than in ordinary breathing. But for this proportionate diminution also, there is a full compensation in the greater total volume of air which is thus breathed. Finally, 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 im- mediate and prolonged contact with the blood than the other has, which comes chiefly from the larger bronchial tubes. c. Influence of external Temperature. The observations made by Vierordt at various temperatures between 38 F. and 75 F. 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. Letellier, from experiments performed on animals at much higher and lower tempera- tures than the above, also found that the higher the tem- perature of the respired air (as far as 104 F.), the less is the amount of carbonic acid exhaled into it, whilst the nearer it approaches zero the more does the carbonic acid increase. The greatest quantity exhaled at the lower tem- peratures he found to be about twice as much as the smallest exhaled at the higher temperatures. d. Season of the Year. Dr. Edward Smith has shown that the season of the year, independently of temperature, also materially influences the respiratory phenomena ; for with the same temperature, at different seasons, there is a great diversity in the amount of carbonic acid expired. According to his observations, spring is the season of the greatest, and autumn of the least activity of the respira- tory and other functions. 224 RESPIRATION. e. Purity of the Respired Air. The average quantity of carbonic acid given out by the lungs constitutes about 4*48 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 quan- tity of carbonic acid exhaled becomes much less. This is shown by the results of two experiments performed by Allen and Pepys. In one, in which fresh air was taken in at each respiration, thirty-two cubic inches of carbonic acid were exhaled in a minute ; whilst in the other, in which the same air was respired repeatedly, the quantity of carbonic acid emitted in the same time was only 9*5 cubic inches. Thej r found also that, however often the same air may be respired, even if until it will no longer sustain life, it does not become charged with more than 10 per cent, of carbonic acid. The necessity of a constant supply of fresh air, by means of ventilation, through rooms in which many persons are breathing together, or in which, from any other source, much carbonic acid is evolved, is thus ren- dered obvious ; for even when the air is not completely irrespirable, yet in the same proportion as it is already charged with carbonic acid, does the further extrication of that gas from the lungs suffer hindrance. /. Hygrometric State of Atmosphere. Lehmann's obser- vations have shown that the amount of carbonic acid exhaled is considerably influenced by the degree of mois- ture of the atmosphere, much more being given off when the air is moist than when it is dry. g. Period of the Day. The period of day seems to exercise a slight influence on the amount of carbonic acid exhaled in a given time, though beyond the fact that the quantity exhaled is much less by night, we are scarcely yet in a position to state that variations in the amount exhaled occur at uniform periods of the day, independently of the influence of other circumstances. h. Food. By the use of food the quantity is increased, EFFECTS OF EXERCISE AND SLEEP. 225 whilst by fasting it is diminished : and, according to Regnault and Reiset, it is greater when animals are fed on farinaceous food than when fed on meat. Spirituous drinks, especially when taken on an empty stomach, are generally believed to produce an immediate and marked diminution in the quantity of this gas exhaled. Recent observations by Dr. Edward Smith, however, furnish some singular results on this subject. Dr. Smith found, for example, that the effects produced by spirituous drinks depend much on the kind of drink taken. Pure alcohol tended rather to increase than to lessen respiratory changes, and the amount, therefore, of carbonic acid expired : rum, ale and porter, also sherry, had 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. i. Exercise and Sleep. 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 Il8 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, according to Dr. E. Smith, the amount of the acid exhaled. During sleep, on the other hand, there is a considerable diminution in the quantity of this gas evolved ; a result probably in great measure dependent on the tranquillity of breathing : directly after walking, there is a great, though quickly transitory, increase in the amount exhaled. A larger quantity is exhaled when the barometer is -low than when it is high. 3. The Oxygen in respired air is always less than in the same air before respiration, and its diminution is generally proportionate to the increase of the carbonic acid. The experiments of Valentin and Brunner seem to show, that, for every volume of carbonic acid exhaled into the air, Q 226 RESPIRATION. 1*17421 volumes of oxygen are absorbed from it: and that when the average quantity of carbonic acid, i.e., 1 346 cubic inches, or 636 grains, is 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 without change of volume ; and to this general conclusion, namely, that 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 ab- sorbed, they differ even widely. The quantity of oxygen that does not combine with the carbon given off in carbonic acid from the lungs, is pro- bably disposed of in forming some of the carbonic acid and water given off from the skin, and in combining with sulphur and phosphorus to form part of the acids of the sulphates and phosphates excreted in the urine, and pro- bably also, from the experiments of Dr. Bence Jones, with the nitrogen of the decomposing nitrogenous tissues. The quantity of oxygen consumed seems to vary much, not only in different individuals, but in the same individual at different periods ; thus it is considerably influenced by food, being greater in dogs when fed on farinaceous than on animal food, and much diminished during fasting, while it varies at different stages of digestion. Animals of small size consume a relatively much greater amount of oxygen fhan larger ones. 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 experimented with (Regnault and Reiset). The Nitrogen of the Atmosphere, in relation to the respira- EXHALATION OF WATERY VAPOUR. 227 tory process, is supposed to serve only mechanically, by diluting the oxygen, and moderating its action upon the system. This purpose, or the mode of expressing it, has been denied by Liebig, on the ground that if we suppose the nitrogen removed, the amount of oxygen in a given space would not be altered. But, although it be true that, if all the nitrogen of the atmosphere were removed and not replaced by any other gas, the oxygen might still extend over the whole space at present occupied by the mixture of which the atmosphere is composed ; yet since, under ordinary circumstances, oxygen and nitrogen, when mixed together in the ratio of one volume to four, produce a mixture which occupies precisely five volumes, with all the properties of atmospheric air, it must result that a given volume of atmosphere drawn into the lungs con- tains four-fifths less weight of oxygen than an equal volume composed entirely of oxygen. The greater rapidity and brilliancy with which combustion goes on in an atmo- sphere of oxygen than in one of common air, and the increased rapidity with which the ordinary effects of respiration are produced when oxygen instead of atmo- spheric air is breathed, seem to leave no doubt that the nitrogen with which the oxygen of the atmosphere is mixed has the effect of diluting this gas, in the same sense and degree as one part of alcohol is diluted when mixed with four parts of water. It has been often discussed whether nitrogen is ever 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 contrary, a small quan- tity appears to be absorbed. 4. Watery Vapour is, under ordinary circumstances, always exhaled from the lungs in breathing. The quan- tity emitted is, as a general rule, sufficient to saturate the Q 2 228 RESPIRATION. expired air or very nearly so. Its absolute amount is, therefore, influenced by the following circumstances. First, by the volume of air expired ; for the greater this is, the greater also will be the quantity of moisture exhaled. Secondly, 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. Thirdly, 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. Fourthly, by the length of time which each volume of inspired air is allowed to remain in the lungs ; for it seems probable that, 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. For ordinary cases, however, it may be held that the expired air is saturated with watery vapour, and hence is derivable a means of estimating the quantity exhaled in any given time : namely, by subtracting the quantity con- tained in the air inspired from the quantity which (at the barometric pressure) would saturate the same air at the temperature of expiration, which is ordinarily about 99. And, on the other hand, if the quantity of watery vapour in the expired air be estimated, the quantity of air itself may from it be determined, being as much as that quantity of watery vapour would saturate at the ascertained tem- perature and barometric pressure. The quantity of water exhaled from the lungs in twenty- four hours ranges (according to the various modifying cir- cumstances already mentioned) from about 6 to 27 ounces, the ordinary quantity being about 9 or IO ounces. Some of this is probably formed by the combination of the excess of oxygen absorbed in the lungs with the hydrogen of the blood ; but the far larger proportion of it must be the mere CHANGES IN BLOOD. 229 exhalation of the water of the blood, taking place from the surfaces of the air-passages and cells, as it does from the free surfaces of all moist animal membranes, particularly at the high temperature of warm-blooded animals. It is exhaled from the lungs whatever be the gas respired, con- tinuing to be expelled even in hydrogen gas. Carbonic acid and water are, however, not the only prin- ciples given off from the lungs. The Rev. J. B. Reade showed, some years ago, and Dr. Richardson's experiments confirm the fact, that ammonia is among the ordinary constituents of expired air. And Wiederhold has stated, that not only ammonia, but chloride of sodium, and even uric acid and urate of soda and ammonia, may be readily detected in the condensed vapour of expired air. His experiments, therefore, seem to prove that the lungs may furnish a channel for the excretion of some of the same kind of solid principles that are met with in the secretions of the skin and of the kidneys. Changes produced in the Blood by Respiration. The most obvious change which the blood undergoes in its passage through the lungs is that of colour, the dark crimson of venous blood being exchanged for the bright scarlet of arterial blood. The circumstances which have been supposed to give rise to this change, the conditions capable of effecting it independent of respiration, and some other differences between arterial and venous blood, were discussed in the chapter on BLOOD (p. 93). The change in colour is indeed the most striking, and may appear the most important, which the blood undergoes in its passage through the lungs; yet, perhaps, its importance is very little, except so far as it is an indication of other and essential alterations effected in the composition of the blood. Of these alterations the principal are, 1st, that the blood, after passing through the lungs, is l or 2 warmer than it was before ; 2nd, that it coagulates sooner and 23 RESPIRATION. more firmly, and contains, apparently, more fibrin; ^rd, that it contains more oxygen, less carbonic acid, and perhaps less nitrogen. The third difference is, probably, the most important. It might be assumed from what has been said of the changes of the inspired air ; and it is proved, by examination of the blood itself. As before remarked however (p. 98), the absolute quantity of carbonic acid is in both arterial and venous blood greater than that of the oxygen, although the amount is relatively less after the blood's passage through the lungs. The oxygen absorbed into the blood from the atmospheric air in the lungs is in part simply dissolved, but probably for the most part, combined chemically with the cruorin of the red blood corpuscles. In this condition, it is carried in the arterial blood to the various parts of the body, and with it is, in the capillary system of vessels, brought into near relation or contact with the elementary parts of the tissues. Herein, co-operating probably in the process of nutrition, or in the removal of disintegrated parts of the tissues, a certain portion of the oxygen which the arterial blood contains disappears, and a proportionate quantity of carbonic acid and water is formed. But it is not alone in the disintegrating processes to which all parts of the body are liable, that oxygen is con- sumed and carbonic acid and water are formed in its consumption. A like process occurs in the blood itself, independently of the decay of the tissues ; for on the continuance of such chemical processes depend, directly or indirectly, not only the temperature of the body, but all the forces, the nervous, the muscular, and others, manifested by the living organism. 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 again taken in. MECHANISM OF RESPIRATORY ACTIONS. 231 Mechanism of Various Respiratory Actions. 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 un- derstood. The accompanying diagram (fig. 65) shows that the cavity of the chest is separated from that of the abdomen by the diaphragm, which, when acting, will les- sen its curve, and thus des- cending, will "pushdownwards and forwards the abdominal viscera ; while the abdominal muscles have the opposite effect, 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 232 RESPIRATION. separately, or through both at the same time, according to the position of the soft palate. The stomach commu- nicates with the exterior of the body through the oesopha- gus, pharynx, and mouth; while below, the rectum opens at the anus, and the bladder through the urethra. All these openings, through which the hollow viscera commu- nicate with the exterior of the body, are guarded by muscles, called sphincters, which can act independently of each other. The position of the latter is indicated in the diagram. Let us take first the simple act of sighing. In this case there is a rather prolonged inspiratory effort by the dia- phragm and other muscles concerned in inspiration; the air almost noiselessly passing in through the glottis, and by the elastic recoil of the lungs and chest- walls, and probably also of the abdominal w r alls, being rather sud- denly 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 contents, and air simply enters the lungs. In the second, or expira- tory 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 as before. Hiccough resembles sighing in that it is an inspiratory act, but the inspiration is sudden instead of gradual, from the diaphragm acting suddenly and spasmodically ; and the air, therefore, suddenly rushing through the unprepared COUGHING; SNEEZING; VOMITING. 233 \ rima glottidis, causes vibration of the vocal cords, and the peculiar sound. In the act of coughing, there is most often first an in- spiration, and this is followed by an expiration ; but when the lungs have been filled by the preliminary inspiration, instead of the air being easily let out again through the glottis, the latter is momentarily closed by the approximation of the vocal cords ; and then the abdo- minal muscles, 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 burst open noisily the vocal cords which oppose its outward pas- sage. In this way a considerable force is exercised, and mucus or any other matter that may need expulsion from the lungs or trachea is quickly and sharply expelled by the out- streaming current of air. Now it is evident on reference to the diagram (fig. 65), that pressure exercised by the abdominal muscles in the act of coughing, 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 of necessity there is 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 contrac- tion of the sphincters, however, prevents any escape at the openings guarded by them, and the pressure is effective at one part only, namely, the rima glottidis. 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. In the act of vomiting, as in coughing, there is first an inspiration; the glottis is then closed, and immediately afterwards the abdominal muscles strongly act ; but here 234 EESPIRATION. occurs the difference in the two actions. Instead of the vocal cords yielding to the action of the abdominal mus- cles, they remain tightly closed. Thus the diaphragm being unable to go up, forms an unyielding surface against which the stomach can be pressed. It is fixed, to use a technical phrase. At the same time the cardiac sphincter being relaxed w r hile the pylorus is closed (see fig. 65), and the stomach itself also contracting, the action of the abdo- minal 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. In the act of voluntary expulsion of urine or fseces, there is first an inspiration, as in coughing, sneezing, and vomiting; the glottis is then closed, and the diaphragm fixed as in vomiting. Now, however, both the rima glottidis and the cardiac opening of the stomach remain closed, and the sphincter of the bladder or rectum, or of both, being re- laxed, the evacuation of the contents of these viscera takes place accordingly ; the effect being, of course, increased by the muscular and elastic contraction of their own walls. As before, there is as much tendency to the escape of the contents of the lungs or stomach as of the rectum or bladder; but the pressure is relieved only at the orifice, the sphincter of which instinctively or involuntarily yields. In all these expulsive actions the diaphragm is quite passive ; and when it is fixed, it is in consequence of the closure of the glottis (which by preventing the exit of air from the lungs prevents its upward movement), not from any exertion on its own part. In females, during parturition, almost an exactly similar action occurs, so far as the diaphragm and abdominal walls are concerned, to that which takes place in a strain- ing effort at expulsion of urine or fa3ces. The contraction of the uterus, however, is both relatively and absolutely SPEAKING; SINGING; SNIFFING. 235 more powerful than that of the bladder or rectum, although it is greatly assisted by the inspiratory effort, by the fixing of the diaphragm, and by the action of the abdominal muscles, as in the other acts just described. In parturi- tion, as in vomiting, the action of the abdominal muscles is, to a great extent, involuntary more so than it com- monly is in the expulsion of faeces or urine ; but in these latter instances also, in cases of great pain and difficulty, it may cease to be a voluntary act, and be quite beyond the control of the will. In speaking, there is a voluntary expulsion of air through the glottis by means of the abdominal muscles ; and 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 producing sound. The sound is moulded into words by the tongue, teeth, lips, etc. the vocal cords producing the sound only, and having nothing to do with articulation. Singing resembles speaking in the manner of its pro- duction ; 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, etc. Sniffing is produced by a somewhat quick action of the diaphragm and other inspiratory muscles. The mouth is, however, closed, and by these means the whole stream of air is made to enter by the nostrils. The alge nasi are, commonly, at the same time, instinctively dilated. Sucking is not properly a respiratory act, but it may be most conveniently considered in this place. It is caused by the depressor muscles of the os hyoides. These, by drawing down 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 equilibrium on the inside and outside of the mouth as best 236 RESPIRATION. it may. The communication between the mouth and pharynx is shut off, probably by the contraction of the pillars of the soft palate and descent of the latter so as to touch the back of the tongue, so that the air cannot find entrance by this way ; and the equilibrium, therefore, can be restored only by 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, to the power which draws the handle. In the preceding account of respiratory actions, the diaphragm and abdominal muscles have been, as the chief muscles engaged and for the sake of clearness, almost alone referred to. But, of course, in all inspiratory actions, the other muscles of inspiration (p. 206) are also more or less engaged ; and in expiration, the abdominal muscles are assisted by others, previously enumerated (p. 209) as grouped in action with them. Influence of the Nervous System in Respiration. Like all other functions of the body, the discharge of which is necessary to life, respiration must be essentially an involuntary act. Else, life would be in constant danger, and would cease on the loss of consciousness for a few moments, even in sleep. But it is 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 enumerated and explained, as speaking, singing, straining, and the like. The respiratory movements and their regular rhythm, so far as they are involuntary and independent of con- sciousness (as on all ordinary occasions they are), seem to be under the absolute governance of the medulla oblon- gata, which, as a nervous centre, receives the impression of the " necessity of breathing," and reflects it to the INFLUENCE OF NERVOUS SYSTEM. 237 phrenic and such other motor nerves as will bring into co-ordinate and adapted action the muscles necessary to inspiration. In the cases of voluntary respiratory acts, we may believe that the brain, as well as the medulla oblongata, is engaged in the process ; for we have no evidence of the mind exercising either perception or will through any other organ than the brain. But even when the brain is thus in action, it appears to be the medulla oblongata which combines the several respiratory muscles to act together. In such acts, for example, as those of coughing and sneezing, the mind first perceives the irritation at the larynx or nose, and may exercise a certain degree of will in determining the actions, as e.g., in the taking of the deep inspiration which always precedes them. But the mode in which the acts are performed, and the combi- nation of muscles to effect them, are determined by the medulla oblongata, independently of the will, and have the peculiar character of reflex involuntary movements, in being always, and without practice or experience, precisely adapted to the end or purpose. In these, and in all the other extraordinary respiratory actions, such as are seen in dyspnoea, or in straining, yawning, hiccough, and others, the medulla oblongata brings into adapted combination of action many other muscles besides those commonly exerted in respiration. Almost all the muscles of the body, in violent efforts of dyspnoea, coughing, and the like, may be brought into action at once, or in quick succession ; but more particu- larly the muscles of the larynx, face, scapula, spine, and abdomen co-operate in these efforts with the muscles of the chest. These, therefore, are often classed as secondary muscles of respiration ; and the nerves supplying them, including especially the facial, pneumogastric, spinal accessory, and external respiratory nerves, were classed by Sir Charles Bell with the phrenic, as the respiratory 238 RESPIRATION. system of nerves. There appears, however, no propriety in making a separate system of these nerves, since their mode of action is not peculiar, and many besides them co- operate in the respiratory acts. That which is peculiar in the nervous influence, directing- the extraordinary move- ments of respiration, is, that so many nerves are com- bined towards one purpose by the power of a distinct nervous centre, the medulla oblongata. In other than respiratory movements, these nerves may act singly or together, without the medulla oblongata; but after it is destroyed, no movement adapted to respiration can be per- formed by any of the muscles, even though the part of the spinal cord from which they arise be perfect. The phrenic nerves, for example, are unable to excite respiratory move- ments of the diaphragm when their connection with the medulla oblongata is cut off, though their connection with the spinal cord may be uninjured.* Effects of the Suspension and Arrest of Respiration. These deserve some consideration, because of the illus- tration which they afford of the nature of the normal processes of respiration and circulation. When the process of respiration is stopped, either by arresting the respiratory movements, or permitting them to continue in an atmo- sphere deprived of uncombined oxygen, the circulation of blood through the lungs is retarded, and at length stopped. The immediate effect of such retarded circulation is an obstruction to the exit of blood from the right ventricle : this is followed by delay in the return of venous blood to the heart ; and to this succeeds venous congestion of the nervous centres and all the other organs of the body. In such retardation, also, an unusually small supply of blood is transmitted through the lungs to the left side of the heart ; and this small quantity is venous. * The influence of the nervous system in respiration will be again and more particularly considered in the section treating of the medulla oblongata and pneumogastric nerves. DEATH BY SUFFOCATION. 239 The condition, then, in which a suffocated, or asphyxi- ated animal dies is, commonly, that the left side of the heart is nearly empty, while the lungs, right side of the heart, and other organs, are gorged with venous blood. To this condition many things contribute. 1st. The ob- structed passage of blood through the lungs, which appears to be the first of the events leading to suffocation, seems to depend on the cessation of the interchange of gases, as if blood charged with carbonic acid could not pass freely through the pulmonary capillaries. But the stagnation of blood in the pulmonary capillaries would not, perhaps, be enough to stop entirely the circulation, unless the action of the heart were also weakened. Therefore, 2ndly, the fatal result is probably due, in some measure, to the enfeebled action of the right side of the heart, in con- sequence of its over-distension by blood continually flowing into it ; this flow, probably, being much increased by the powerful but fruitless efforts continually made at inspira- tion (Eccles). And $rdly, because of the obstruction at the right side of the heart, there must be venous congestion in the medulla oblongata and nervous centres : and this evil is augmented by the left ventricle receiving and propelling none but venous blood. Hence, slowness and disorder of the respiratory movements and of the movements of the heart may be added. Under all these conditions combined, the heart at length ceases to act ; the cessation of its action being also in great measure, probably, brought about, 4thly, by the imperfect supply of oxygenated blood to its muscular tissue. In some experiments recently 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 apncea, i.e., 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 240 RESPIRATION. 3 minutes 30 seconds, and 4 minutes 40 seconds. The average duration of the heart's action, on the other hand, was 7 minutes 1 1 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 depriva- tion of air for nearly 4 minutes, yet, after submersion in water for ij minutes, recovery appears 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. In proof of the correctness of this explanation, it was found that when two dogs of the same size, one, however, having his windpipe plugged, the other not, were submerged at the same moment, and taken out after being under water for 2 minutes, the former recovered on removal of the plug, the latter did not. 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, accom- panied with ecchymosed points on the surface and in the substance of the lung, but the air-tubes were completely 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, DEATH BY DROWNING. 241 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 was 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 debarred 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. The entire depend- ence of the early fatal issue, in apncea by drowning, upon the open condition of the windpipe, and its results, was also strikingly shown by the following experiment. A strong dog had its windpipe plugged, and was then sub- merged in water for four minutes ; in three quarters of a minute after its release it began to breathe, and in four minutes had fully recovered. This experiment was re- peated with similar results on other dogs. When the entrance of water into the lungs, and its drawing up with the air into the bronchial tubes by means of the respira- tory efforts, were diminished, as by rendering the animal insensible by chloroform previously to immersion, and thus depriving it of the power of making violent respiratory efforts, it was found that it could bear immersion for a longer period without dying than when not thus rendered insensible. Probably to a like diminution in the respira- tory efforts, may also be ascribed the greater length of time persons have been found to bear submersion without being killed, when in a state of intoxication, poisoning by narcotics, or during insensibility from syncope. It is to the accumulation of carbonic acid in the blood, and its conveyance into the organs, that we must, in the first place, ascribe the phenomena of asphyxia. For when this does not happen, all the other conditions may exist without injury; as they do, for example, in hybernating warm-blooded animals. In these, life is supported for 242 ANIMAL HEAT. many months in atmospheres in which the same animals, in their full activity, would be speedily suffocated. During the periods of complete torpor, their respiration almost entirely ceases ; the heart acts very slowly and feebly ; the processes of organic life are all but suspended, and the animal may be with impunity completely deprived of atmospheric air for a considerable period. Spallanzani kept a marmot, in this torpid state, immersed for four hours in carbonic acid gas, without its suffering any apparent inconvenience. Dr. Marshall Hall kept a lethar- gic bat under water for sixteen minutes, and a lethargic hedgehog for 22^ minutes; and neither of the animals appeared injured by the experiment. CHAPTER VIII. ANIMAL HEAT. INTIMATELY associated with the process of respiration are the production of animal heat and the maintenance of a uniform temperature of the body ; conditions as essential to the continuance of life in warm-blooded animals, as the extrication of carbonic acid and the absorption of oxygen are. The average temperature of the human body, in those in- ternal parts which are most easily accessible, such as the mouth and rectum, may be estimated at from 98 to 103 F. In children the temperature is commonly as high as I O2 F. In old persons it is about the same as in adults. Of the external parts of the body, the temperature becomes lower the further they are removed from the centre of the body ; thus, in the human subject, a thermometer placed TEMPERATURE OF HUMAN BODY. 243 in the axilla was found by Dr. John Davy to stand at 98 F., at the loins it indicated a temperature of 96^-, on the thigh 94, on the leg 9 3 or 91, on the sole of the foot 90. In disease, the temperature of the body may deviate, by several degrees above and below, from the average of health. In some diseases, as scarlatina and typhus, it rises as high as 106 or 107 F.; and in children, M. Roger has observed the temperature of the skin to be raised to 108*5 Fah. In the morbus camlem, in which there is defective arterialization of the blood from malformation of the heart, the temperature of the body is often as low as 79 or 77^ ; in Asiatic cholera a thermometer placed in the mouth sometimes rises only to 77 or 79. M. Roger observed the temperature of the body in children to be sometimes reduced in disease to 74' 3. The temperature of the body in health is about lj F. lower during sleep than while awake. According to Dr. Davy it is highest in the morning after rising from sleep, continues high but fluctuating till evening, and is lowest about midnight. Sustained mental exertion elevates it slightly; continued bodily exercise does so to a certain extent ; after feeding, also, it is somewhat raised. All these facts are important, both as showing variations in the temperature of the body correspondent with those in the production of carbonic acid in the same circum- stances, and as proving that the influence which slight changes in the organic economy of warm-blooded animals have, is as great or greater than that exercised by even extreme variation in the external temperature to which they are exposed. For in warm climates, Dr. Davy found the temperature of the interior of the body only from 27 to 3-6 F. higher than in temperate climates ; and during the voyage of the " Bonite," the French naturalists, who had an opportunity of observing the influence of various climates on the same persons, found that the temperature of the human body rises and falls in R 2 244 ANIMAL HEAT. only a slight degree, even in extremes of external tem- perature ; that it falls slowly in passing from, hot to cold climates, and rises more rapidly in returning towards the torrid zone : but that these changes in the temperature of the body, are more considerable in some individuals than in others. The temperature maintained by Mammalia in an active state of life, according to the tables of Tiedemann and Rudolphi, averages 101. The extremes recorded by them were 96 and 106, the former in the narwhal, the latter in a bat (Vespertilio Pipistrella). In birds, the average is as high as 107; the highest temperature, 111*25, being in the small species, the linnets, etc. Among reptiles, Dr. John Davy found, that while the medium they were in was 75> their average temperature was 82 '5. As a general rule, their temperature, though it falls with 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, insects, and other 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 gene- rally 7 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 temperature ; 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 I OO, have nearly the same temperature and feel hot to us. The real difference is, as Mr. Hunter expressed it, that what we call warm- blooded animals (birds and Mammalia), have a certain " permanent heat in all atmospheres," while the tempera- MAINTENANCE OF TEMPERATURE. 245 ture of the others, which we call cold-blooded, is "variable with every atmosphere." The power of maintaining- a uniform temperature, which Mammalia and birds possess, is combined with the want of power to endure such changes of temperature as are harmless to the other classes; and when their power of resisting change of temperature ceases, they suffer serious disturbances or die. M. Magendie has shown that birds and rabbits die when, being exposed to great external heat, their temperature is raised as much as 9 above the natural standard: but that they bear a reduction of the temperature of the interior of the body to a much greater amount before very dangerous or fatal consequences ensue. In all the ordinary circumstances of life, the maintenance of uniform temperature is effected by the production of heat sufficient to compensate for that which is constantly lost in radiation into the medium in which we live, or in combination with the fluids evaporating from the exposed surfaces of the body. The losses thus sustained are extremely various in different circumstances; and the degrees of power which animals possess of adapting themselves to such differences are equally various. Some live best in cold regions, where they produce abundant heat for radiation, and cannot endure the heat of warm climates, where the heat that they habitually produce would, probably, be excessive, and by its continual, though perhaps small excess, would generate disease; others, naturally inhabiting warm cli- mates, die if removed to cold ones, as if because their power of producing heat were not quite sufficient to com- pensate for the constantly larger abstraction of it by radiation. Man, with the aid of intellect for the provision of artificial clothing, and with command over food, is, in these respects, superior to all other creatures; possessing the greatest power of adaptation to external temperature, and being capable of enduring extreme degrees of heat, 246 ANIMAL HEAT. as well as of cold, without injury to health. His power of adaptation is sufficient for the maintenance of a uniform temperature in a range of upwards of 2OO Fahrenheit ; a power which is only shared by some of the domestic animals who are his companions in his various abodes. Sources and Mode of Production of Heat in the Body. To explain the production of heat in the body, several theories have been advanced ; but it now appears certain that the correct one is that which refers the generation of heat, primarily and in general, to certain chemical pro- cesses going on in the system ; but admits, at the same time, that as these chemical changes are carried on in parts whose functions are, to a certain extent, under the influence of the nervous system, therefore the production of heat is liable to be modified, either locally or in every part, by the operation of that system. In explaining the chemical changes effected in the process of respiration (p. 230), it was stated that the oxygen of the atmosphere taken into the blood is, most probably, combined, in the course of the circulation, and mainly in the systemic capillary vessels, with the carbon and the hydrogen of disintegrated and absorbed tissues, and of certain elements of food which have not been con- verted into tissues. That such a combination between the oxygen of the atmosphere and the carbon and hydro- gen in the blood, is continually taking place, is made nearly certain by the fact, that a larger amount of carbon and hydrogen is constantly being added to the blood from the food than is required for the ordinary purposes of nutrition, and that a^ quantity of oxygen is also constantly being absorbed from the air in the lungs, of the disposal of which no account can be given except by regarding it as combining, for the most part, with the excess of carbon and hydrogen, and being evaporated in the form of car- bonic acid and water. In other words, the blood of warm- PRODUCTION OF HEAT. 247 blooded animals appears to be always receiving from, the digestive canal and the lungs more carbon, hydrogen, and oxygen than are consumed in the repair of the tissues, and to be always emitting carbonic acid and water, for which there is no other known source than the combination of these elements. By such combination, heat must be continually produced in the animal body. 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 evident, as in ordinary combustion, or slow and imperceptible, as in the changes which are believed to occur in the living body. And since the heat thus arising will be generated wher- ever the blood is carried, every part of the body will be heated equally, or nearly so. To establish this theory, it needs to be shown 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 time : an amount capable of maintaining the tem- perature of the body at from 98 to IOO, notwithstanding a large loss by radiation and evaporation.* An attempt to determine this point was made by Dulong and Despretz. Dulong introduced different mammiferous animals, carnivorous as well as herbivorous, into a receiver, in which the changes produced in the air by respiration, and the volume of the different products, could be deter- mined at the same time that the amount of heat lost by the animal could be ascertained. His experiments led him to conclude, among other points, that supposing all the * Some heat will also be generated in the combination of sulphur and phosphorus with oxygen, to which reference has been made (p. 226) ; but the amount thus produced has not been estimated, and need not be considered in the exposition of a theory which can, at present, be stated in only the most general terms. 248 ANIMAL HEAT. oxygen absorbed into the blood from the air in the lungs were combined with carbon and hydrogen in the system, and that as much heat was thus generated as would be developed during the quick combustion of equal quantities of oxygen and carbon, and of oxygen and hydrogen, still, the whole quantity of heat produced would amount to only from J to of that which is developed during the same space of time by carnivorous as well as herbivorous animals. Despretz placed animals in a vessel surrounded with water ; an uninterrupted current of air to and from the vessel was maintained, and the volume and composition of the air employed were ascertained both before and after the experiment (which was continued I J or 2 hours), as well as the increase in the temperature of the surrounding water during its progress; by this means it was found that the heat which should have been generated, accord- ing to the chemical theory of respiration, would account for from 0*76 to O'QI only of that which the animals really gave out during the same time. The failing of these experiments to account for all the heat produced threw doubts on the chemical theory of animal heat (as the pro- posed explanation has been called), till Liebig showed that Dulong and Despretz were in error in their conclusions, from having formed too low an estimate of the heat pro- duced in the combustion of carbon and hydrogen. On repeating their experiments, and using the more accurate numbers to represent these combustion-heats, Liebig found reason to believe that the quantity of heat which would be generated, by the union of the oxygen absorbed into the blood from the atmosphere with the carbon and hydro- gen taken into the system as food, would be sufficient to account for the whole of the caloric formed in the animal body. Many things observed in the economy and habits of animals are explicable by this theory, and are, therefore, evidence for its truth. Thus, as a general rule, in the RELATION TO ACTIVITY OF RESPIRATION. 249 various classes of animals, as well as in individual ex- amples of each class, the quantity of heat generated in the body is in direct proportion to the activity of the respiratory process. The highest animal temperature, for example, is found in birds, in whom the function of respiration is most actively performed. In Mammalia, the process of respiration is less active, and the average tem- perature of the body less, than in birds. In reptiles, both the respiration and the heat are at a much lower standard; while in animals below them, in which the function of respiration is at the lowest point, a power of producing heat is, in ordinary circumstances, hardly dis- cernible. Among these lower animals, however, the observations of Mr. Newport supply confirmatory evidence. He shows that the larva, in which the respiratory organs are smaller in comparison with the size of the body, has a lower temperature than the perfect insect. Volant insects have the highest temperature, and they have always the largest respiratory organs and breathe the greatest quan- tity of air; while among terrestrial insects, those also produce the most heat which have the largest respiratory organs and breathe the most air. During sleep, hyber- nation, and other states of inaction, respiration is slower or suspended, and the temperature is proportionately diminished ; while, on the other hand, when the insect is most active and respiring most voluminously, its amount of temperature is at its maximum, and corresponds with the quantity of respiration. Neither the rapidity of the circulation, nor the size of the nervous system, according to Mr. Newport, presents such a constant relation to the evolution of heat. Similar evidence in favour of this theory of animal heat is furnished by the fact that heat is sometimes evolved by plants, in a quantity which appears to be in direct propor- tion to the amount of oxygen they at the same time absorb and convert into carbonic acid. For example, their evo- 250 ANIMAL HEAT. lution of heat is most evident during flowering and the germination of seeds, the times at which the largest amount of carbonic acid is exhaled. The quantity and quality of food consumed by man and animals in the different climates and seasons, also appear to be adapted to the production of various amounts of heat by the combination of carbon and hydrogen with oxygen. In northern regions, for example, 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 seasons. And the food which appears naturally adapted to the inhabitants of the coldest climates, such as the several fatty and oily substances, abounds in carbon and hydrogen, and is fitted to combine with the large quantities of oxygen which, breathing cold dense air, they absorb from their lungs. The influence of the nervous system in modifying the pro- duction of heat has been already referred to. The experi- ments 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 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 temperature falls; and this diminution of heat has been remarked still more plainly in limbs deprived of nervous influence by paralysis. For example, Mr. Earle found the temperature of the hand of a paralysed arm to be 70, while the hand of the sound side had a tempera- ture of 92 F. On electrifying the paralysed limb, the temperature rose to 77. In another case, the temperature INFLUENCE OF NERVOUS SYSTEM. 251 of the paralysed finger was 56 F., while that of the un- affected hand was 62. With equal certainty, though less definitely, the in- fluence 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 tem- perature in the depressing passions. But none of these instances suffices to prove that heat is generated by mere nervous action, independent of any chemical change ; all are explicable, on the supposition that the influence of the nervous system alters, indirectly, the chemical processes from which the heat is commonly generated. There are ample proofs that the nervous system, especially in the most highly organized animals, does so modify all the functions of organic life ; and it appears more reasonable to suppose that it thus influences the production of heat, than to ascribe to it any more direct agency. The temporary increase of heat in a part, as in the instances mentioned above, is no doubt in great measure due to a larger influx of blood to the part, in consequence of temporary relaxation of the walls of the small arteries through nervous agency. M. Bernard, for example, found that when he divided, on one side of the neck, the trunk which unites the sympathetic ganglia, or when he removed the superior cervical ganglion, an increase of temperature at once took place on the corresponding side of the face, and continued for m'any months. This observation has since been abundantly confirmed. It will be more fully considered when speaking of the physiology of the sympa- thetic nerve. In the foregoing pages, the illustrations of the power of 252 ANIMAL HEAT. maintaining an uniform temperature have had reference to the ordinary case of man living in a medium colder than his body, and therefore losing heat both by radiation and evaporation. The losses in these two ways will bear, in general, an inverse proportion to one another ; 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 amount of fluid evaporated in hot air may remove nearly as much heat as is commonly lost by both radiation and evaporation in ordinary tem- peratures. Thus, it is possible, that the quantities of heat required for the maintenance of an uniform proper temperature in various climates and seasons are not so different as they may at first thought seem : but on these points no accurate information has yet been obtained. Neither, as to the maintenance of the temperature of the body in hot air is much more known, than that great heat can for a time be borne with little change in the proper temperature of the body, provided the air be dry. Sir Charles Blagden and others supported a temperature varying between 198 and 211 F. in dry air for several minutes; and in a subsequent experiment he remained eight minutes in a temperature of 260. Delaroche and Berger observed that the temperature of rabbits was raised only a few degrees when they were exposed to heat varying from 122 to 194. But such heats are not tolerable when the air is moist as well as hot, so as to pre- vent evaporation from the body. M. C. James states, that in the vapour baths of Nero he was almost suffocated in a temperature of 1 1 2, while in the caves of Testaccio, in which the air is dry, he was but little incommoded by a temperature of 176. In the former, evaporation from the skin was impossible ; in the latter, it was, probably, abundant, and the layer of vapour which would rise from all the surface of the body would, by its very slowly con- EFFECTS OF AGE. 253 ducting power, defend it for a time from the full action of the external heat. It remains to notice certain conditions by which the pro- duction of heat is modified. The effects of age are noticeable. M. Edwards found the power of generating heat to be less in old people ; and the same was observed by Dr. Davy, who, in eight people, between eighty-seven and ninety-five years old, found that, although the average temperature of the body was not lower than that of younger persons, yet the power of resisting cold was less in them exposure to a low tem- perature causing a greater reduction of heat than in young persons. The same rapid diminution of temperature was observed by M. Edwards in the new-born young of most carnivorous and rodent animals when they were removed from the parent, the temperature of the atmosphere being between 50 and 533- F.; whereas, while lying close to the body of the mother, their temperature was only 2 or 3 degrees lower than' hers. The same law applies to the young of birds. Young sparrows, a week after they were hatched had a temperature of 95 to 97, while in the nest ; but when taken from it, their temperature fell in one hour to 66 J, the temperature of the atmosphere being at the time 62^-. It appears from his investigations, that in respect of the power of generating heat, some Mammalia are born in a less developed condition than others ; and that the young of dogs, cats, and rabbits, for example, are inferior to the young of those animals which are not born blind. The need of external warmth to keep up the temperature of new-born children is well known ; the researches of M. Edwards show, that the want of it is, as Hunter suggested, a much more frequent cause of death in new-born children than is generally supposed, and furnish a strong argument against the idea, that children, by early exposure to cold, can soon be hardened into resisting its injurious influence. 254 ANIMAL HEAT. Active exercise, as already stated, raises the temperature of the body. This may be partly ascribed to the fact, that every muscular contraction is attended by the development 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 may be quickly diffused by the blood circulating from the heated muscles. Possibly also, some heat may be generated in the various move- ments, 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. But the heat thus developed cannot be great. Moreover, the increase of temperature throughout the whole body, produced by active exercise, is but small ; the great apparent increase of heat depending, in a great measure, on the increased circulation and quantity of blood ? and, therefore, greater heat, in parts of the body (as the skin, and especially the skin of the extremities), which, at the same time that they feel more acutely than others any changes of temperature, are commonly by some degrees colder than organs more centrally situated. That the increased temperature of the skin dicing exercise is not accompanied by corresponding increase of the heat of other parts, which are naturally much warmer, is well shown by some observations of Dr. J. Davy. 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 com- mand over food and fire, perhaps as much as by his capacity for developing appropriate amounts of heat, he maintains his temperature on all accessible parts of the surface of the earth. 255 CHAPTER IX. DIGESTION. DIGESTION is the process by which those parts of our food which may be employed in the formation and repair of the tissues, or in the production of heat, are made fit to be absorbed and added to the blood. Food. Food may be considered in its relation to these two pur- poses the nutrition of the tissues, and the production of heat. But, under the first of these heads will be included many other allied functions, as, for example, secretion and generation : and under the second, not the production of heat only as such, but of all the other forces correlated with it, which are manifested by the living body. The various articles of food may be artificially classified according as they are chiefly subservient to one or the other of these purposes. All articles of food that are to be employed in the production of heat, contain a large amount of carbon and hydrogen ; and of those which are appro- priate for the maintenance of the several tissues (except the adipose) nearly all are characterised by the possession of nitrogen, and are capable of ready conversion into the nitrogenous principles of the blood. The name of nutritive or plastic is given to those principles of food which admit of ready conversion into the albumen of the blood, and of being subsequently assimilated, through the medium of the blood, by the tissues. And those prin- ciples, comprising the greater part of the non-nitrogenous materials of food, in the form of fat, starch, sugar, gum, and other similar substances, which are believed to be employed in the production of heat, are named calorifacient, or, sometimes, respiratory food. It must be borne in mind, however, that the nitrogenous articles of food are in part 256 DIGESTION. devoted to the maintenance of the heat of the body ; and it is equally true that the non-nitrogenous or so-called calorifacient food has to do in part with nutrition ; so that care must be taken not to rely implicitly on the name which has been given to these two classes of nutriment. For purposes of accurate classification, the terms may be best abandoned altogether. The following is a convenient tabular classification of the usual and more necessary kinds of food : NITROGENOUS : Albumen, Casein, Gluten, Gelatin, and their allies (containing Carbon, Hydrogen, Oxygen, and Nitrogen; some of them, also Sulphur and Phosphorus). NoN-NlTKOGENOUS ! (i). Starch, Sugar, Alcohol, and their allies (containing Carbon, Hydrogen and Oxygen). (2). Oils and Fats (containing Carbon, Hydrogen, and Oxygen ; the oxygen in much smaller proportion than in starch or sugar). (3). Mineral or Saline Matters ; as Chloride of Sodium, Phosphate of Lime, etc. (4). Water. Animals cannot subsist on any but organic substances, and these must contain the several elements and com- pounds which are naturally combined with them : in other words, not even organic compounds are nutritive unless they are supplied in their natural state. Pure fibrin, pure gelatin, and other principles purified from the substances naturally mingled with them, are incapable of supporting life for more than a brief time. Moreover, health cannot be maintained by any number of substances derived exclusively from one of the three groups of alimentary principles. A mixture of nitrogenous and non-nitrogenous substances, together with the inorganic principles which are severally contained in them, is essen- tial to the well-being, and, generally, even to the existence of an animal. The truth of this is demonstrated by experi- COMPOSITION OF MILK AND EGGS. 257 merits performed for the purpose r and is illustrated by the composition of the food prepared by nature, as the exclu- sive source of nourishment to the young of Mammalia, namely, milk. COMPOSITION OF MILK. Human. Cows. Water 890 / . 858 Solids no ........ 142 1,000' 1,000 Casein 35 68 Butter 25 38 Sugar (with extractives) 48 30 Salts 2. . 6 no 142 In milk, as will be seen from the preceding table, the albuminous group of aliments is represented by the casein, the oleaginous by the butter, the aqueous by the water, the saccharine by the sugar of milk. Among the salts of milk are likewise phosphate of lime, alkaline and other salts, and a trace of iron ; so that it may be briefly said to include all the substances which the tissues of the growing animal need for their nutrition, and which are required for the production of animal heat. The yelk and albumen of eggs are in the same relation as food for the embryoes of oviparous animals, that milk is to the young of Mammalia, and afford another example of mixed food being provided as the most perfect nutrition. COMPOSITION OF HENS' EGGS. White. Yelk. "Water 80*0 5373 Albumen 15-5 17-47 Mucus 4-5 Yellow Oil . . 2875 Salts 4'o 6'o The experiments illustrating the same principle have been chiefly performed by Magendie. Dogs were fed exclusively on sugar and distilled water. During the first seven or eight days they were brisk and active, and took 8 258 DIGESTION. their food and drink as usual ; but in the course of the second week, they began to get thin, although their appe- tite 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 with- out 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 ulceration of the cornea took place : the effects were also the same with butter. Tiedemann and Gmelin obtained very similar results. They fed different geese, one with sugar and water, another with gum and water, and a third with starch and water. All gradually lost weight. The one fed with gum died on the sixteenth day ; that fed with sugar, on the twenty-second ; the third, which was fed with starch, on the twenty-fourth ; and another on the twenty- seventh day ; having lost, during these periods, from one-sixth to one-half of their weight. The experiments of Chossat and Latellier prove the same ; and in men, the same is shown by the various diseases to which they who consume but little nitrogenous food are liable, and especially, as Dr. Budd has shown, 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 insuffi- cient for the maintenance of health. The experiments of EXPERIMENTS WITH DIFFERENT FOODS. 259 the Academies of France and Amsterdam were equally conclusive that gelatin alone soon ceases to be nutritive. Mr. Savory's observations on food confirm and extend the results obtained by Magendie, Chossat, and others. They show that animals fed exclusively on non-nitrogenous diet speedily emaciate and die, as if from starvation ; that a much larger amount of urine is voided by those fed with nitrogenous than by those with non -nitrogenous food ; and that animal heat is maintained as well by the former as by the latter a fact which seems to prove that nitrogenous elements of food, as well as non-nitrogenous, may be regarded as calorifacient. The non-nitrogenous principles, however, he believes to be calorifacient essentially, not being first converted into tissue : but of the nitrogenous, he believes that only a part is thus directly calorifacient, the rest being employed in the formation of tissue. Con- trary to the views of Liebig and Lehmann, Savory has shown that, while animals speedily die when confined to non-nitrogenous diet, they may live long when fed exclu- sively with nitrogenous food. Man is supported as well by food constituted wholly of animal substances, as by that which is formed entirely of vegetable matters, on the condition, of course, that it contain a mixture of the various nitrogenous and non- nitrogenous substances just shown to be essential for healthy nutrition. In the case of carnivorous animals, the food upon which they exist, consisting as it does of the flesh and blood of other animals, not only contains all the elements of which their own blood and tissues are composed, but contains them combined, probably, in the same forms. Therefore, little more may seem requisite, in the prepara- tion of this kind of food for the nutrition of the body, than that it should be dissolved and conveyed into the blood in a condition capable of being re -organised. But in the case of the herbivorous animals, which feed exclusively upon vegetable substances, it might seem as if there would s 2 260 DIGESTION. be greater difficulty in procuring food capable of assimila- tion into their blood and tissues. But the chief ordinary articles of vegetable food contain substances identical in composition, with the albumen, fibrin, and casein, which constitute the principal nutritive materials in animal food. Albumen is abundant in the juices and seeds of nearly all vegetables ; the gluten which exists, especially in corn and other seeds of grasses as well as in their juices, is identical in composition with fibrin, and is often named vegetable fibrin ; and the substance named legumen, which is obtained especially from peas, beans, and other seeds of leguminous plants, and from the potato, is identical with the casein of milk. All these vegetable substances are, equally with the corresponding animal principles, and in the same manner, capable of conversion into blood and tissue ; and as the blood and tissues in both classes of animals are alike, so also the nitrogenous food of both may be regarded as, in essential respects, similar. It is in the relative quantities of the nitrogenous and non-nitrogenous compounds in these different foods that the difference lies, rather than in the presence of substances in one of them which do not exist in the other. The only non-nitrogenous compounds in ordinary animal food are the fat, the saline matters, and water, and, in some instances, the vegetable matters which may chance to be in the digestive canals of such animals as are eaten whole. The amount of these, however, is altogether much less than that of the non-nitrogenous substances represented by the starch, sugar, gum, oil, etc., in the vegetable food of herbivorous animals. It has been just remarked that man can live upon animal matters alone, or upon vegetables. The structure of his teeth, however, as well as experience, seems to declare that he is best fitted for a mixed diet ; and the same inference may be readily gathered from other DAILY LOSS OF CARBON AND NITROGEN. 261 facts and considerations. Thus, the food a man takes into his body daily, represents, or ought to represent, the quantity and kind of matter necessary for replacing that which is daily cast out by way of the lungs, skin, kidneys, and other organs. To find out, therefore, the quantity and kind of food necessary for a healthy man, it will evidently be the best plan to consider in the first place what he loses by excretion. For the sake of example, we may now take only two elements, carbon and nitrogen, and, if we discover what amount of these is respectively discharged in a given time from the body, we shall be in a position to judge what kind of food will most readily and economically replace their loss. The quantity of carbon daily lost from the body, amounts to about 4,500 grains, and of nitrogen 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, as before remarked, an animal can live only upon these elements when they are arranged in a particular man- ner with others, in the form of an organic compound, as albumen, starch, and the like ; and the relative propor- tion of carbon to nitrogen in either of these compounds alone, is, by no means, the proportion required in the diet of man. The amount, 4,500 grains of carbon, repre- sents about fifteen times the quantity of nitrogen required in the same period; and, in albumen, the proportion of carbon to nitrogen is only as 3 5 to I . 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 262 DIGESTION. of his food, which contains so large a relative amount of nitrogen in proportion to the carbon he needs, sub- stances in which the nitrogen exists in much smaller quantities. Food of this kind is provided in such compounds as starch and fat. The latter indeed as it exists for the most part in considerable amount mingled with the flesh of animals, removes to a great extent, in a diet of animal food, the difficulty which would otherwise arise from a deficiency of carbon fat containing a large relative pro- portion of this element, and no nitrogen. To take another example ; the proportion of carbon to nitrogen in bread is about 30 to I. If a man's diet were confined to bread, he would eat, therefore, in order to obtain the requisite quantity of nitrogen, twice as much carbon as is necessary; and it is evident that, in this instance, a certain quantity of a substance with a large relative amount of nitrogen is the kind of food necessary for redressing the balance. To place the preceding facts in a tabular form, and taking meat as an example instead of pure albumen : meat contains about 10 per cent, of carbon, and rather more than 3 per cent, of nitrogen. Supposing a man to take meat for the supply of the needful carbon, he would require 45 ,OOO grains, or nearly 61-lbs., containing : Carbon ........ 4, 500 grains Nitrogen ....... 1,350 ,, Excess of Nitrogen above the amount required. 1,050 ,, Bread contains about 30 per cent, of carbon and I per cent, of nitrogen. If bread alone, therefore, were taken as food, a man would require in order to obtain the requisite nitrogen, 3O ; OOO grains, containing: Carbon . . . . . . . . 9,000 grains Nitrogen ....... 300 ,, Excess of Carbon above the amount required . 4,500 ,, NECESSITY FOR CHANGES OF DIET. 263 But a combination of bread and meat would supply much more economically what was necessary. Thus : Carbon. Nitrogen. I5,ooogrs. of bread (or rather more than 2 Ib.) contain 4,5oogrs. i5ogrs. 5, ocx) ,, of meat (or about fib.) contain . . 500 ,, 150 ,, 5,000 300 So that | Ib. of meat, and less than 2 Ibs. of bread would supply all the needful carbon and nitrogen with but little waste. From these facts it will be plain that a mixed diet is the best and most economical food for man ; and the result of experience entirely coincides with what might have been anticipated on theoretical grounds only. It must not be forgotten, however, that the value of certain foods may depend quite as much on their digesti- bility, as on the relative quantities of the necessary elements which they contain. In actual practice, moreover, the quantity and kind of food to be taken with most economy and advantage cannot be settled for each individual, only by considerations of the exact quantities of certain elements that are required. Much will of necessity depend on the habits and digestive powers of the individual, on the state of his excretory organs, and on many other circumstances. Food which to one person is appropriate enough, may be quite unfit for another : and the changes of diet so instinctively prac- tised by all to whom they are possible, have much more reliable grounds of justification than any which could be framed on theoretical considerations only. In many of the experiments on the digestibility of various articles of food, disgust at the sameness of the diet may have had as much to do with inability to consume and digest it, as the want of nutritious properties in the substances which were experimented on. And that disease may occur from the want of particular food, is well shown by the occurrence of scurvy when fresh vegetables are deficient, and its rapid cure when they are again eaten: 264 DIGESTION. and the disease which is here so remarkably evident in its symptoms, causes, and cure, is matched by numberless other ailments, the causes of which, however, although analagous, are less exactly known, and therefore less easily combated. With regard to the quantity, too, as well as the kind of food necessary, there will be much diversity in different individuals. Dr. Dalton believed, from some experiments which he performed, that the quantity of food necessary for a healthy man, taking free exercise in the open air, is as follows i- Meat 16 ounces, or i -oo Ib. avoird. Bread 19 ,, 1-19 ,, Butter or Fat . . 3^ ,, ,, 0-22 ,, ,, Water. -. . . , . 52 fluid ozs. ,, 3-38 ,, ,, The quantity of meat, however, here given is probably more in proportion to the other articles of diet enumerated than is needful for the majority of individuals under the circumstances stated. PASSAGE OF JFOOD THROUGH THE ALIMENTARY CANAL. The course of the food through the alimentary canal of man will be readily seen from the accompanying diagram (fig. 66). The food taken into the mouth passes thence through the oesophagus into the stomach, and from this into the small and large intestine successively ; gradually losing, by absorption, the greater portion of its nutritive constituents. The residue, together with such matters as may have been added to it in its passage, is discharged from the rectum through the anus. We shall now consider, in detail, the process of diges- tion, as it takes place in each stage of this journey of the food through the alimentary canal. The Salivary Glands and the Saliva. The first of a series of changes to which the food is sub- jected in the digestive canal, takes place in the cavity of SALIVARY GLANDS AND SALIVA. 265 the mouth ; the solid articles of food are here submitted to the action of the teeth (p. 59), whereby they are divided and crushed, and by being at the same time mixed with Fig. 66.* the fluids of the mouth, are reduced to a soft pulp, capable of being easily swallowed. The fluids with which the food * Fig. 66. Diagram of the alimentary canal. The small intestine of man is from about 3 to 4 times as long as the large intestine. 266 DIGESTION. is mixed in the mouth consist of the secretion of the salivary glands, and the mucus secreted by the lining membrane of the whole buccal cavity. The glands concerned in the production of saliva, are very extensive, and in man and Mammalia generally, are presented in the form of four pairs of large glands, the parotid, submaxillary, sublingual, and numerous smaller bodies, of similar structure and with separate ducts, which are scattered thickly beneath the mucous membrane of the lips, cheeks, soft palate, and root of the tongue. The structure of all these glands is essentially the same. Each is composed of several parts, called lobes, which are joined together by areolar tissue ; and each of these lobes, again, is made up of a number of smaller parts called lobules, bound together as before by areolar tissue. Each of these small divisions, called lobules, is a miniature representation of the whole gland. It contains a small branch of the duct, which, subdividing, ends in small vesicular pouches, called acini, a group of which may be considered the Fig. 67*. dilated end of one of the smaller ducts (fig. 67). Each of the acini is about -^-^ of an inch in diameter, and is formed of a fine structureless membrane, lined on the inner surface * Fig. 67. Diagram of a racemose or saccular compound gland ; m, entire gland, showing branched duct and tabular structure; n, a lobule detached, with o t branch of duct proceeding from it (after Sharpey). . SALIVA. 267 and often filled by spheroidal or glandular epithelium; while on the outside there is a plexus of capillary blood- vessels. The accompanying diagram is intended to show the typical structure of such glands as the salivary (fig. 67). Saliva, as it commonly flows from the mouth, is mixed with the secretion of the mucous membrane, 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 transparent watery fluid, the specific gravity of which varies from I -004 to I -oo8, and in which, when examined with the microscope, are found floating a number of minute particles, derived from the secreting ducts and vesicles of the glands. In the impure or mixed saliva are found, besides these particles, numerous epithelial scales separated from the surface of the mucous membrane of the mouth and tongue, and mucus- corpuscles, discharged for the most part from 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, leaving 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 ; and that from the parotid gland is said to be more strongly alkaline than that from the other salivary glands. This alkaline condition is most evident when digestion is going on, and according to Dr. Wright, the degree of alkalinity of the saliva bears a direct proportion to the acidity of the gastric fluid secreted at the same time. During fasting, the saliva, although secreted alkaline, shortly becomes neutral ; and it does so especially when secreted slowly and allowed to mix with the acid mucus of the mouth, by which its alkaline reaction is neutralized. The following analysis of the saliva is by Frerichs : 268 DIGESTION. Composition of Saliva. "Water 994*io Solids 5-90 Ptyalin !- 4 i Fat ....... 0*07 Epithelium and Mucus . . . 2 '13 Salts : Sulpho-Cyanide of Potassium . Phosphate of Soda .... ,, ,, Lime .... / 2*29 ,, ,, Magnesia . Chloride of Sodium .... ,, Potassium 5-90 The rate at which saliva is secreted is subject to consider- able 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 the mouth moist. But the flow is much accelerated when the move- ments of mastication take place, and especially when they are combined with the presence of food in the mouth. It may be excited also, even when the mouth is at rest, by the mental impressions produced by the sight or thought of food ; also by the introduction of food into the stomach. The influence of the latter circumstance was well shown in a case mentioned by Dr. Gairdner, of a man whose pharynx had been divided : the injection of a meal of broth into the stomach was followed by the secretion of from six to eight ounces of saliva. Under these varying circumstances, the quantity of saliva secreted in twenty-four hours varies also ; its average amount is probably from two to three pints in twenty-four hours. In a man who had a fistulous opening of the parotid duct, Mitscherlich found that the quantity of saliva discharged from it during twenty-four hours, was from two USES OF SALIVA. 269 to three ounces ; and the saliva collected from the mouth during the same period, and derived from the other sali- vary glands, amounted to six times more than that from the one parotid. The purposes served by saliva are of several kinds. In the first place, acting mechanically, 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 ren- dering 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 : as M. Lassaigne has shown by a table of the quantity produced in the mastication of a hundred parts of each of several kinds of food, thirty parts suffice for a hundred parts of crumb of bread, but not less than I2O for the crusts; 42*5 parts of saliva are produced for the hundred of roast meat; 3*7 for as much of apples ; and so on, according to the general rule above- stated. 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 M. Bernard has shown that 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 mucoid 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 ossophagus. This view obtains confirmation from the interesting fact pointed out by Professor Owen, that in the great ant-eater, whose enormously elongated tongue is kept moist by a large quantity of viscid saliva, the sub- 270 DIGESTION. maxillary glands are remarkably developed, while the parotids are not of unusual size. Beyond these, its mechanical purposes, saliva performs (4) some chemical part in the digestion of the food. When saliva, or a portion of a salivary gland, or even a portion of dried ptyalin, is added to starch-paste, the starch is very rapidly transformed into dextrin and grape-sugar; and when common raw starch is masticated and mingled with saliva, and kept with it at a temperature of 90 or I OO, the starch-grains are cracked or eroded, and their contents are transformed in the same manner as the starch-paste. Changes similar to these are effected on the starch of farinaceous food (especially after cooking) in the stomach; and it is reasonable to refer them to the action of the saliva, because the acid of the gastric fluid tends to retard or prevent, rather than favour the trans- formation of the starch. It may therefore be held, that one purpose served by the saliva in the digestive process is that of assisting in the transformation of the starch, which enters so largely into the composition of most articles of vegetable food, and which (being naturally insoluble) is converted into soluble dextrin and grape- sugar, and made fit for absorption. Besides saliva, many azotized substances, especially if in a state of incipient decomposition, may excite the trans- formation of starch, such as pieces of the mucous mem- brane of the mouth, bladder, rectum, and other parts, various animal and vegetable tissues, and even morbid products ; but the gastric fluid will not produce the same effect. The transformation in question is effected much more rapidly by saliva, however, than by any of the other fluids or substances experimented with, except the pan- creatic secretion, which, as will be presently shown, is very analogous to saliva. The actual process by which these changes are effected is still obscure. Probably the azotized substance, ptyalin, acts as a kind of ferment, like diastase in the process of malting, and excites molecular changes DEGLUTITION. 271 in the starch which result in its transformation, first into dextrin and then into sugar. The majority of observers agree that the transformation of starch into sugar ceases on the entrance of the food into the stomach, or on the addition of gastric fluid to it in a test-tube : while others maintain that it still goes on. Probably all are right : for, although gastric fluid added to saliva appears to arrest the action of the latter on starch, yet portions of saliva mingled with food in mas- tication may, for some time after their entrance into the stomach, remain unneutralized by the gastric secretion, and continue their influence upon the starchy principles in contact with them. Starch appears to be the only principle of food upon which saliva acts chemically : it has no apparent influence on any of the other ternary principles, such as sugar, gum, mucous, cellulose, or (according to Bernard) on fat, and seems to be equally destitute of power over albuminous and gelatinous substances, so that we have as yet no infor- mation respecting any purpose it can serve in the digestion of Carnivora, beyond that of softening or macerating the food; though, since such animals masticate their food very little, usually " bolting" it, the saliva has probably but little use even in this respect, in the process of digestion. Passage of Food into the Stomach. When properly masticated, the food is transmitted in successive portions to the stomach by the act of deglutition or swallowing. This act, for the purpose of description, may be divided into three parts. In the first, particles of food collected to a morsel 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. The first is performed volun- 272 DIGESTION. tarily by the muscles of the tongue and cheeks. The second also is effected with the aid of muscles which are in part endued with voluntary motion, such as the muscles of the soft palate and pharynx ; but it is, nevertheless, an invo- luntary act, and takes place without our being able to prevent it, as soon as a morsel of food, drink, or saliva is carried backwards to a certain point of the tongue's sur- face. When we appear to swallow voluntarily, we only convey, through the first act of deglutition, a portion of food or saliva beyond the anterior arch of the palate ; then the substance acts as a stimulus, which, in accordance with the laws of reflex movements hereafter to be described, is carried by the sensitive nerves to the medulla oblongata, where it is reflected to the motor nerves, and an involun- tary adapted action of the muscles of the palate and pharynx ensues. The third act of deglutition takes place in the oesophagus, the muscular fibres of which are entirely beyond the influence of the will. The second act of deglutition 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 tongue being carried 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 forwards under 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 simultaneous contraction of its own muscles : so that, even when the epi- glottis is destroyed, there is little danger of food or drink passing 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 inwards like side-curtains, close STRUCTURE OF THE STOMACH. 273 the passage into the upper part of the pharynx and the posterior nares, and form an incline 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. 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 contract: hence an undulatory contraction of the oesophagus, which is easily observable in horses while drinking, proceeds rapidly along the tube. It is only when the morsels swallowed are large, or taken too quickly in succession, that the progres- sive contraction of the oesophagus is slow, and attended with pain. Division of both pneumogastric nerves para- lyzes the contractile power of the oesophagus, and food accordingly accumulates in the tube (Bernard). DIGESTION OF FOOD IN THE STOMACH. Structure of the Stomach. It appears to be an almost universal character, of ani- mals, that they have an internal cavity for the production of a chemical change in the aliment a cavity for diges- tion ; and when this cavity is compound, the part in which the food undergoes its principal and most important changes is the stomach. In man and those Mammalia which are provided with a single stomach, its walls consist of three distinct layers or coats, viz., an external peritoneal, an internal mucous, and an intermediate muscular coat, with blood-vessels, lym- phatics, and nerves distributed in and between them. The muscular coat of the stomach consists of three sepa- rate layers or sets of fibres, which, according to their several directions, are named the longitudinal, circular and oblique. The longitudinal set are the most superficial: they are continuous with the longitudinal fibres of the 274 DIGESTION. oesophagus, and spread out in a diverging manner over the great 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. According to Pettigrew, 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 mus- cular fibres of the oesophagus, and, according to Pettigrew, with 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 fibres of which the several muscular layers of the stomach, and of the intestinal canal generally, are composed, belong to the class of organic muscle, being composed of smooth or unstriped, elongated, spindle-shaped fibre-cells; a fuller description of which will be given under the head of Muscular Tissue. 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 is thrown into numerous, chiefly longitudinal, folds or ruga3, which dis- appear when the organ is distended. When examined with a lens, the internal or free surface presents a peculiar honeycomb appearance, produced by shallow polygonal depressions or cells (fig. 68), the diameter of which varies GLANDS OF THE STOMACH. 275 Fig. 68.* generally from Y^o-th to -g^th of an i ncn '> ^ ut near ^ e pylorus is as much as y^-g-th of an inch. They are separated by slightly elevated ridges, which sometimes, especially in certain morbid states of the stomach, bear minute, nar- row, vascular processes, which look like villi, and have given rise to the erroneous sup- position that the stomach has ab- sorbing villi, like those of the small intestines. In the bottom of the cells minute openings are visible (fig. 68), which are the orifices of perpendicularly-arranged tubular glands (fig. 69), imbedded side by side in sets or bundles, in the substance of the mucous membrane, and composing nearly the whole structure. The glands which are found in the human stomach may be divided into two classes, the tu- bular and lenticular. Tabular glands. The tubular glands may be described as a collection of cylinders with blind extremities, about -th of an inch in Fig. 69. f * Fig. 68. Small portion of the surface of the mucous membrane of the stomach (from Ecker) 3 T . The specimen shows the shallow de- pressions, in each of which the smaller dark spots indicate the orifices of a variable number of the gastric tubular glands. t Fig. 69. Portion of human stomach (magnified 30 diameters) cut vertically, both in a direction parallel to its long axis, and across it (altered from Brinton). T "2 2 7 6 DIGESTION. length, and ^^ in diameter, packed closely together, with their long axis at right angles to the surface of the mucous membrane on which they open, their blind ends resting on the submucous tissue. (See fig. 69.) They are all composed of basement membrane, and lined by epithe- lial cells, but they are not all of exactly similar shape; for while some are simple straight tubes, open at one end and closed at the other (fig. 69), others present at their deeper extremities a varicose, pouched, or in some cases, even a branched appearance (fig. 70, b and c). The epithelium lining them is not the same throughout. In the upper third or fourth of their length it is cylindrical and continuous with that which covers the free mucous surface of the rest of the stomach. In their lower part, on * Fig. 70. The gastric glands of the human stomach (magnified). a, deep part of a pyloric gastric gland (from Kolliker) ; the cylindrical epithelium is traceable to the csecal extremities. 5, and c, cardiac gastric glands (from Allen Thompson) ; b, vertical section of a small portion of the mucous membrane with the glands magnified 30 diameters ; c, deeper portion of one of the glands, magnified 65 diameters, showing a slight division of the tubes, and a sacculated appearance, produced by the large glandular cells within them ; d, cellular elements of the cardiac glands magnified 250 diameters. GLANDS OF THE STOMACH. 277 the other hand, it is of the variety called glandular or spheroidal, the cells being oval or somewhat Angular, and th of an inch in diameter. The cells, however, about 1200 do not completely fill up the cavity of the gland which they line, but leave a slight, central, thread-like space, the immediate lining of which is a layer of small angular cells, Fig. 71.* continuous with the cylindrical epithe- lium in the upper portion of the tube. This description will become plain on reference to fig. 71, which represents on a larger scale a longitudinal section of one of the glands depicted in fig. 69. In the greater number of the glands which are branched at their deeper extremities, the spheroidal epithelium exists in the divisions, while the main duct and the upper part of the branches are lined by the cylindrical variety (fig. 70, c). In the human stomach, according to Dr. Brinton, the simple undivided tubes are the rule, and the branched the exception. The varieties in the epithelial cells lining the different parts of the tubes, correspond probably with differences in the fluid secreted by their agency the cylinder-epithe- lium, like that on the free surface of the stomach, being probably engaged in separating the thin alkaline mucus which is always present in greater or less quantity, while the larger granular cells probably secrete the proper gas- tric juice. Near the pylorus there exist glands branched at their * Fig. 71. Part of one. of the gastric glands, highly magnified, to show the arrangement of the epithelium in its interior ; a, columnar cells lining the upper part of the tube ; b, small angular cells, into which these merge below to form a central or axial layer within ; c, the proper gastric or glandular cells (after Brinton). 278 DIGESTION. deep extremities, which are lined throughout by cylinder- epithelium (^g. 70, a.) } and probably serve only for the secretion of mucus. All the tubular glands, while they open by one end into the cavity of the stomach, rest by their blind extremities on a bed or matrix of dense areolar tissue (fig. 69), which is prolonged upwards between them, so as to invest and support them. The matrix contains a variable quantity of unstriped muscular fibres. Lenticular glands. Besides the cylindrical glands, there are also small closed sacs beneath the surface of the mucous membrane, resembling exactly the solitary glands of the intestine, to be described hereafter. Their num- ber is very variable, and they are found chiefly along the lesser curvature of the stomach, and in the pyloric region, but they may be present in any part of the organ. According to Dr. Brinton they are rarely absent in children. Their function probably resembles that of the intestinal solitary glands, but nothing is certainly known regarding it. The blood-vessels of the stomach, which first break up in the submucous tissue, send branches upward between the closely packed glandular tubes, anastomosing around them by means of a fine capillary network with oblong meshes. Continuous 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 framework on which are moulded the small elevated ridges of mucous membrane bounding the minute, polygonal pits above 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. The nerves of the stomach are derived from the pneumo- gastric and sympathetic. THE GASTRIC FLUID. 279 Secretion and Properties of the Gastric Fluid. 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 imme- diately on the introduction of food or other foreign sub- stance into the stomach, 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 introduced. The quantity of this fluid secreted daily has been variously estimated ; but the average for a healthy adult has been assumed to range from ten to twenty pints in the twenty-four hours (Brinton). The first accurate analysis of the gastric fluid was made by Dr. Prout ; but it does not appear that it was collected in any large quantity, or pure and separate from food, until the time when Dr. Beaumont was enabled, by a fortunate circumstance, to obtain it from the stomach of a man named St. Martin, in whom there existed, as the result of a gunshot wound, an opening leading directly into the stomach, near the upper extremity of the great curvature, and three inches from the cardiac orifice. The external opening was situate two inches below the left mamma, in a line drawn from that part to the spine of the left ilium. The borders of the opening into the stomach, which was of considerable size, had united, in healing, with the margins of the external wound ; but the cavity of the stomach was at last sepa- rated from the exterior by a fold of mucous membrane, which projected from the upper and back part of the opening, and closed it like a valve, but could be pushed back with the finger. The introduction of any mechanical 280 DIGESTION. irritant, such as the bulb of a thermometer, into the stomach, excited at once the secretion of gastric fluid. This could be drawn off with a caoutchouc tube, and could often be obtained to the extent of nearly an ounce. The introduction of alimentary substances caused a much more rapid and abundant secretion of pure gastric fluid than the presence of other mechanical irritants did. No in- crease of temperature could be detected during the most active secretion; the thermometer introduced into the stomach always stood at IOO Fahr., except during mus- cular exertion, when the temperature of the stomach, like that of other parts of the body, rose one or two degrees higher. M. Blondlot and subsequently M. Bernard, and since then, several others, by maintaining fistulous openings into the stomachs of dogs, have confirmed most of the facts discovered by Dr. Beaumont. And the man St. Martin has frequently submitted to renewed experiments on his stomach, by various physiologists. From all these obser- vations it appears, that pepper, salt, and other soluble stimulants, excite a more rapid discharge of gastric fluid than mechanical irritation does ; so do alkalies generally, but acids have a contrary effect. When mechanical irri- tation is carried beyond certain limits so as to produce pain, the secretion, instead of being more abundant, diminishes or ceases entirely, and a ropy mucus is poured out instead. Very cold water, or small pieces of ice, at first render the mucous membrane pallid, but soon a kind of reaction ensues, the membrane becomes turgid with blood, and a larger quantity of gastric fluid is poured out. The application of too much ice is attended by diminution in the quantity of fluid secreted, and by consequent re- tardation of the progress of digestion. The quantity of the secretion seems to be influenced also by impressions made on the mouth ; for Blondlot found that when sugar was introduced into the dog's stomach, either alone, or mixed V COMPOSITION OF GASTRIC FLUID. 281 with human saliva, a very small secretion ensued ; but when the dog had himself masticated and swallowed it, the secretion was abundant. Dr. Beaumont described the secretion of the human stomach as " a clear transparent fluid, inodorous, a little saltish, and very perceptibly acid. Its taste is similar to that of thin mucilaginous water, slightly acidulated with muriatic acid. It is readily diffusible in water, wine, or spirits ; slightly effervesces with alkalies ; and is an effec- tual solvent of the materia alimentaria. It possesses the property of coagulating albumen in an eminent degree; is powerfully antiseptic, checking the putrefaction of meat ; and effectually restorative of healthy action, when applied to old foetid sores and foul ulcerating surfaces." The chemical composition of the gastric juice in the human subject has been particularly investigated by Schmidt, a favourable case for his doing so occurring in the person of a peasant named Catherine Kiitt, aged 35, who for three years had had a gastric fistula under the left mammary gland, between the cartilages of the ninth and tenth ribs. The fluid was obtained by putting into the stomach some hard indigestible matter, as dry peas, and a little water, by which means the stomach was excited to secre- tion, at the same time that the matter introduced did not complicate the analysis by being digested in the fluid secreted. The gastric juice was drawn off through an elastic tube inserted into the fistula. The fluid thus obtained was acid, limpid, and odourless, with a mawkish taste. Its density varied from roO22 to I '0024. Under the microscope a few cells from the gastric glands and some fine granular matter were observable. The following table gives the mean of two analyses of the above-mentioned fluid ; and arranged by the side of it, for purposes of comparison, is an analysis of gastric juice from the sheep and dog. 282 DIGESTION. Composition of Gastric Juice. Human Sheep's Dog's Gastric Juice. Gastric Juice. Gastric Juice. Water .... 994-40 986-14 97i'i7 Solid Constituents . . 5-59 13-85 28-82 /Ferment, Pepsin (with a trace of Ammonia) 3 '19 4*20 17*5 Hydrochloric Acid . 0-20 . 1*55 2-70 , Chloride of Calcium . 0-06 o'li i'66 **\ Sodium . 1-46 4-36 3-14 ,, Potassium 0-55 1-51 1-07 Phosphate of Lime, v Magnesia, and Iron 0-12 2-09 2*73 In all the above analyses the amount of water given must be reckoned as rather too much, inasmuch as a cer- tain quantity of saliva was mixed with the gastric fluid. The allowance, however, to be made on this account is only very small. Considerable difference of Opinion has existed concern- ing the nature of the free acid contained in the gastric juice, chiefly whether it is hydrochloric or lactic. The weight of evidence, however, is in favour of free hydro- chloric acid, being that to which, in the human subject, the acidity of the gastric fluid is mainly due ; although there is no doubt that others, as lactic, acetic, butyric, are not unfrequently to be found therein. The animal matter mentioned in the analysis of the gas- tric fluid is named pepsin, from its power in the process of digestion. It is an azotised substance, and is best pro- cured 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 between 80 and I OO F. The warm water dissolves various substances as well as some of the pepsin, but the cold water takes up little else than pepsin, which, on evaporating the cold DIGESTIVE POWER OF GASTRIC FLUID. 283 solution, is obtained in a greyish-brown viscid fluid. The addition of alcohol throws down the pepsin in greyish- white flocculi ; and one part of the principle thus prepared, if dissolved in even 6o,OOO parts of water, will digest meat and other alimentary substances. The digestive power of the gastric fluid is manifested in its softening, reducing into pulp, and partially or completely dissolving various articles of food placed in it at a tempe- rature of from 90 to I OO. This, its peculiar property, requires the presence of both the pepsin and the acid; neither of them can digest alone, and when they are mixed, either the decomposition of the pepsin, or the neutralization of the acid, at once destroys the digestive property of the fluid. For the perfection of the process also, certain conditions are required, which are all found in the stomach; namely (l), a temperature of about I OO F. ; (2), such movements as the food is subjected to by the muscular actions of the stomach, which bring in succession every part of it in contact with the mucous membrane, whence the fresh gastric fluid is being secreted ; (3), the constant removal of those portions of food which are already digested, so that what remains undigested may be brought more completely into contact with the solvent fluid; and (4), a state of softness and minute division, such as that to which the food is reduced by mastication previous to its introduction into the stomach. The chief circumstances connected with the mode in which the gastric fluid acts upon food during natural diges- tion, have been determined by watching its operations when removed from the stomach and placed in conditions as nearly as possible like those under which it acts while within that viscus. The fact that solid food, immersed in gastric fluid out of the body, and kept at a temperature of about I OO, is gradually converted into a thick fluid similar to chyme, was shown by Spallanzani, Dr. Stevens, Tiede- mann and Gmelin and others. They used the gastric fluid 284 DIGESTION. of dogs obtained by causing the animals to swallow small pieces of sponge, which, were subsequently withdrawn, soaked with the fluid and proved nearly as much as the latter experiments of the same kind of gastric fluid by Blondlot, Bernard, and others. But these need not be particularly referred to, while we have the more satisfac- tory and instructive observations which Dr. Beaumont made with the fluid obtained from the stomach of St. Martin. After the man had fasted seventeen hours, Dr. Beaumont took one ounce of gastric fluid, put into it a solid piece of boiled recently salted beef weighing three drachms, and placed the vessel which contained them in a water-bath heated to IOO. " In forty minutes digestion had distinctly commenced over the surface of the meat ; in fifty minutes, the fluid had become quite opaque and cloudy, the external texture began to separate and become loose; and in sixty minutes chyme began to form. At I p.m." (two hours after the commencement of the expe- riment) "the cellular texture seemed to be entirely destroyed, leaving the muscular fibres loose and uncon- nected, floating about in small fine shreds, very tender and soft." In six hours, they were nearly all digested a few fibres only remaining. After the lapse of ten hours, every part of the meat was completely digested. The gastric juice, which was at first transparent, was now about the colour of whey, and deposited a fine sediment of the colour of meat. A similar piece of beef was, at the time of the commencement of this experiment, suspended in the stomach by means of a thread : at the expiration of the first hour it was changed in about the same degree as the meat digested artificially; but at the end of the second hour, it was completely digested and gone. In other experiments, Dr. Beaumont withdrew through the opening of the stomach some of the food which had been taken twenty minutes previously, and which was completely mixed with the gastric juice. He continued ARTIFICIAL DIGESTION. 285 the digestion, which had already commenced, by means of artificial heat in a water-bath. In a few hours the food thus treated was completely chymified; and the artificial seemed in this, as in several other experiments, to be exactly similar to, though a little slower than, the natural digestion. The apparent identity of the process in and outside of the stomach thus manifested, while it shows that we may regard digestion as essentially a chemical process, when once the gastric fluid is formed, justifies the belief that Dr. Beaumont's other experiments with the digestive fluid may exactly represent the modifications to which, under similar conditions, its action in the stomach would be liable. He found that, if the mixture of food and gastric fluid were exposed to a temperature of 34 F., the process of digestion was completely arrested. In another experi- ment, a piece of meat which had been macerated in water at the temperature of IOO for several days, till it acquired a strong putrid odour, lost, on the addition of some fresh gastric juice, all signs of putrefaction, and soon began to be digested. From other experiments he obtained the data for estimates of the degrees of digestibility of various articles of food, and of the ways in which the digestion is liable to be affected, to which reference will again be made. When natural gastric juice cannot be obtained, many of these experiments may be performed with an artificial digestive fluid, the action of which, probably, very closely resembles that of the fluid secreted by the stomach. It is made by macerating in water portions of fresh or recently- dried mucous membrane of the stomach of a pig^ or other omnivorous animal, or of the fourth stomach of the calf, * The best portion of the stomach of the pig for this purpose is that between the cardiac and pyloric orifices ; the cardiac portion appears to furnish the least active digestive fluid: 286 DIGESTION. and adding to the infusion a few drops of hydrochloric acid about 3*3 grains to half an ounce of the mixture, according to Sehwann. Portions of food placed in such fluid, and maintained with it at a temperature of about I OO, are, in an hour or more, according to the toughness of the substance, softened and changed in just the same manner as they would be in the stomach. The nature of the action by which the mucous mem- brane of the stomach and its secretion work these changes in organic matter is exceedingly obscure. The action of the pepsin may be compared with that of a ferment, which at the same time that it undergoes change itself, induces certain changes also in the organic matters with which it is in contact. Or its mode of action may belong to that class of chemical processes termed " catalytic," in which a substance excites, by its mere presence, and without itself undergoing change as ordinary ferments do, some chemical action in the substances with which it is in contact. So, for example, spongy platinum, or charcoal, placed in a mixture, however voluminous, of oxygen and hydrogen, makes them combine to form water; and diastase makes the starch in grains undergo transforma- tion, and sugar is produced. And that pepsin acts in some such manner appears probable from the very minute quantity capable of exerting the peculiar digestive action on a large quantity of food, and apparently with little diminution in its active power. The process differs from ordinary fermentation, in being unattended with the for- mation of carbonic acid, in not requiring the presence of oxygen, and in being unaccompanied by the production of new quantities of the active principle, or ferment. It agrees with the processes of both fermentation and organic catalysis, in that whatever alters the composition of the pepsin (such as heat above IOO, strong alcohol, or strong acids), destroys the digestive power of the fluid. DIGESTION OF FOOD IN THE STOMACH. 287 Changes of the Food in the Stomach. The general effect of digestion in the stomach is the conversion of the food into chyme, a substance of various composition according to the nature of the food, yet always presenting a characteristic thick, pultaceous, grumous con- sistence, with the undigested portions of the food mixed in a more fluid substance, and a strong, disagreeable acid odour and taste. Its colour depends on the nature of the food, or on the admixture of yellow or green bile which may, apparently even in health, pass into the stomach. Reduced into such a substance, all the various materials of a meal may be mingled together, and near the end of the digestive process hardly admit of recognition ; but the experiments of artificial digestion, and the examination of stomachs with fistulee, have illustrated many of the changes through which the chief alimentary principles pass, and the times and modes in which they are severally disposed of. These must now be traced. The readiness with which the gastric fluid acts on the several articles of food is, in some measure, determined by the state of division, and the tenderness and moisture of the substance presented to it. By minute division of the food, the extent of surface with which the digestive fluid can come in contact is increased, and its action proportion- ably accelerated. Tender and moist substances offer less resistance to the action of the gastric juice than tough, hard, and dry ones do, because they may be thoroughly penetrated by it, and thus be attacked not only at the surface, but at every part at once. The readiness with which a substance is acted upon by the gastric fluid does not, however, necessarily imply the degree of its nutritive property; for a substance may be nutritious, yet, on account of its toughness and other qualities, hard to digest; and many soft, easily-digested substances contain comparatively a small amount of nutriment. But for a 288 DIGESTION. substance to be nutritive, it must be capable of being assimilated to the blood; and to find its way into the blood, it must, if insoluble, be digestible by the gastric fluid or some other secretion in the intestinal canal. There is, therefore, thus far, a necessary connection between the digestibility of a substance and its power of affording nutriment. Those portions of food which are liquid when taken into the stomach, or which are easily soluble in the fluids therein, are probably at once absorbed by the blood- vessels in the mucous membrane of the stomach. Magen- die's experiments, and better still, those of Dr. Beaumont, have proved this quick absorption of water, wine, weak saline solutions, and the like; that they are absorbed without manifest change by the digestive fluid, and that, generally, the water of such liquid food as soups is absorbed at once, so that the substances suspended in it are concentrated into a thicker material, like the chyme from solid food, before the digestive fluid acts upon them. The action of the gastric fluid on the several kinds of solid food has been studied in various ways. In the earliest experiments, perforated metallic and glass tubes, filled with the alimentary substances, were introduced into the stomachs of animals, and after the lapse of a certain time withdrawn, to observe the condition of the contained sub- stances; but such experiments are fallacious, because gastric fluid has not ready access to the food. A better method was practised in a series of experiments by Tiede- mann and Gmelin, who fed dogs with different substances, and killed them in a certain number of hours afterwards. But the results they obtained are 'of less interest than those of the experiments of Dr. Beaumont on his patient, St. Martin, and of Dr. Gosse, who had the power of vomiting at will. Dr. Beaumont's observations show, that the process of digestion in the stomach, during health, takes place so DIGESTION IN THE STOMACH. 289 rapidly, that a full meal, consisting of animal and vege- table substances, may nearly all be converted into chyme in about an hour, and the stomach left empty in two hours and a half. The details of two days' experiments will be sufficient examples : Exp. 42. April ^th, 8 A.M. St. Martin breakfasted on three hard-boiled eggs, pancakes, and coffee. At half-past eight o'clock, Dr. Beaumont examined the stomach, and found a heterogeneous mixture of the several articles slightly digested At a quarter past ten, no part of the breakfast remained in the stomach. Exp. 43. At eleven o'clock the same day, he ate two roasted eggs and three ripe apples. In half an hour they were in an incipient state of digestion ; and a quarter past twelve no vestige of them remained. Exp. /]/]. At two o'clock P.M. the same day, he dined on roasted pig and vegetables. At three o'clock they were half chymified, and at half-past four nothing remained but a very little gastric juice. Again, Exp. 46. April Qth. At three o'clock P.M. he dined on boiled dried codfish, potatoes, parsnips, bread, and drawn butter. At half-past three o'clock examined, and took out a portion about half digested ; the potatoes the least so. The fish was broken down into small filaments; the bread and parsnips were not to be dis- tinguished. At four o'clock examined another portion. Very few particles of fish remained entire. Some of the few potatoes were distinctly to be seen. At half-past four o'clock, he took out and examined another portion; all completely chymified. At five o'clock, stomach empty. Many circumstances besides the nature of the food are apt to influence the process of chymification. Among them are, the quantity of food taken ; 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 IT 2QO DIGESTION. previous and subsequent to the meal, gentle exercise being favourable, over-exertion injurious to digestion ; the state of mind tranquillity of temper being apparently essential to a quick and due digestion : the bodily health : the state of the weather. But under ordinary circumstances, from three to four hours may be taken as the average time occupied by the digestion of a meal in the stomach. Dr. Beaumont constructed a table showing the times required for the digestion of all usual articles of food in St. Martin's stomach, and in his gastric fluid taken from the stomach. Among the substances most quickly digested were rice and tripe, both of which were chymified in an hour; eggs, salmon, trout, apples, and venison, were digested in an hour and a half; tapioca, barley, milk, liver, fish, in two hours ; turkey, lamb, potatoes, pig, in two hours and a half; beef and mutton required from three hours to three and a half, and both were more digestible than veal ; fowls were like mutton in their degree of digestibility. Animal substances were, in general, con- verted into chyme more rapidly than vegetables. Dr, Beaumont's experiments were all made on ordinary articles of food. A minuter examination of the changes produced by gastric digestion on various tissues has been made by Dr. Rawitz, who examined microscopically the product of the artificial digestion of different kinds of food, and the contents of the faeces after eating the same kinds of food. The general results of his examinations, as regards animal food, show that muscular tissue breaks up into its constituent fasciculi, and that these again are divided transversely; gradually the transverse striae become indistinct, and then disappear ; and finally, the sarcolemma seems to be dissolved, and no trace of the tissue can be found in the chyme, except a few fragments of fibres. These changes ensue most rapidly in the flesh of fish and hares, less rapidly in that of poultry and other animals. The cells of cartilage and fibre-cartilage, except those of fish, DIGESTION IN THE STOMACH. 291 pass unchanged through the stomach and intestines, and may be found in the fseces. The interstitial tissues of these structures are converted into pulpy textureless substances in the artificial digestive fluid, and are not discoverable in the fseces. Elastic fibres are unchanged in the digestive fluid. Fat-cells are sometimes found quite unaltered in the fseces : and crystals of cholesterin may usually be obtained from fseces, especially after the use of pork fat. As regards vegetable substances, Dr. Rawitz states that he frequently found large quantities of cell-membranes un- changed in the fseces ; also starch-cells, commonly deprived of only part of their contents. The green colouring prin- ciple, chlorophyll, was usually unchanged. The walls of the sap-vessels and spiral vessels were quite unaltered by the digestive fluid, and were usually found in large quan- tities in the fseces ; their contents, probably, were removed. From these experiments, we may understand the structural changes which the chief alimentary substances undergo in their conversion into chyme ; and the proportions of each which are not reducible to chyme, nor capable of any further act of digestion. The chemical changes undergone in and by the proximate principles are less easily traced. Of the albuminous principles, the casein of milk, and, according to Dr. Beaumont, fluid albumen, are coagulated by the acid of the gastric fluid ; and thus, before they are digested, come into the condition of the other solid prin- ciples of the food. These, including solid albumen and fibrin, in the same proportion that they are broken up and anatomically disorganized by the gastric fluid, appear to be reduced or lowered in their chemical composition. This chemical change is probably produced, as suggested by Dr. Prout, by the principles entering into combination with water. It is sufficient to conceal nearly all their characteristic properties ; the albumen is rendered scarcely coagulable by heat ; the gelatin, even when its solution is evaporated, does not congeal in cooling j the fibrin and TJ 2 292 DIGESTION. casein cannot be found by their characteristic tests. It would seem, indeed, that all these various substances are converted into one and the same principle, a low form of albumen, not precipitable by nitric acid or heat, and now generally termed albuminose or peptone, from which, after being absorbed, they are again raised, in the elaboration of the blood, to which they are ultimately assimilated. The change of molecular constitution suffered by the albuminous parts of the food, in consequence of the action of the gastric juice, has an important relation to their absorption by the blood-vessels of the stomach. From the condition of ' colloids,' or substances, so named by Pro- fessor Graham, which are absorbed with extreme difficulty, they appear, from experiments of Funke, to assume to a great degree the character of ' crystalloids,' which can pass through animal membranes with ease.* Whatever be the mode in which the gastric secretion affects these principles, it, or something like it, appears essential, in order that they may be assimilated to the blood and tissues. For, when Bernard and Barreswil in- jected albumen dissolved in water into the jugular veins of dogs, they always, in about three hours after, found it in the urine. But if, previous to injection, it was mixed with gastric fluid, no trace of it could be detected in the urine. The influence of the liver seems to be almost as efficacious as that of the gastric fluid, in rendering albu- men assimilable ; for Bernard found that, if diluted egg- albumen, unmixed with gastric fluid, is injected into the portal vein, it no longer makes its appearance in the urine, and is, therefore, no doubt, assimilated by the blood. Probably, most of the albuminose, with other soluble and fluid materials, is absorbed directly from the stomach by the minute blood-vessels with which the mucous mem- brane is so abundantly supplied. * These terms will be further explained and illustrated in the Chapter on Absorption. MOVEMENTS OF THE STOMACH. 293 The saccharine including the amylaceous principles are at first, probably, only mechanically separated from the vege- table substances within which they are contained, by the action of the gastric fluid. The soluble portions, viz., dextrin and sugar, are probably at once absorbed. The insoluble ones, viz., starch and lignin, (or some parts of them) are rendered soluble and capable of absorption, by being converted into dextrin or grape-sugar. It is pro- bable that this change is carried on to some extent in the stomach ; but this conversion of starch into sugar is pro- bably effected, not by the gastric fluid, but by the saliva introduced with the food, or subsequently swallowed. The transformation of starch is continued in the intestinal canal, as will be shown, by the secretion of the pancreas, and perhaps by that of the intestinal glands and mucous membrane. The power of digesting uncooked starch is, however, very -limited in man and Garni vora, for when starch has been taken raw, as in corn and rice, large quantities of the granules are passed unaltered with the excrements. Cooking, by expanding or bursting the envelopes of the granules, renders their interior more amenable to the action of the digestive organs; and the abundant nutriment furnished by bread, and the large proportion that is absorbed of the weight consumed, afford proof of the completeness of their power to make its starch soluble and prepare it for absorption. Of the oleaginous principles, as to their changes in the stomach, no more can be said than that they appear to be reduced to minute particles, and pass into the intestines mingled with the other constituents of the chyme. In the case of the solid fats, this effect is probably produced by the solvent action of the gastric juice on the areolar tissue, albuminous cell-walls, etc., which enter into their com- position, and by the solution of which the true fat is able to mingle more uniformly with the other constituents of the chyme. Being further changed in the intestinal canal, fat is rendered capable of absorption by the lacteals. 294 DIGESTION. Movements of the Stomach. ' It has been already said, that 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 neces- sary aid to digestion, by grinding and triturating the hard seeds which constitute part of the food. But in the stomachs of man and Mammalia the motions of the mus- cular 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 the experiments of Reaumur and Spallanzani have demon- strated that substances enclosed in perforated tubes, and consequently protected from mechanical influence, are yet digested. The normal actions of the muscular fibres of the human stomach appear to have a three-fold purpose ; first, 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 com- pression upon it ; secondly, to keep the orifices of the stomach closed until the food is digested ; and, thirdly, to 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 contracted, 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 ori- fices 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 MOVEMENTS OF THE STOMACH. 295 so completely closed, that even when the stomach is sepa- rated 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. From the observations of Dr. Beaumont on the man St. Martin, it appears that food, so soon as it enters the stomach, is subject to a kind of peristaltic action of the muscular coat, whereby the digested portions are gradually approximated towards the pylorus. The movements were observed to increase in rapidity as the process of chymifica- tion 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, Dr. Beaumont found that when the bulb of the thermometer was placed about three inches from the pylorus, 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 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 con- tracted and firm, while the cardiac portion forms a dilated sac. Sometimes, by a predominant action of strong circular fibres placed between the cardia and pylorus, the two por- tions, or ends as they are called, of the stomach, are sepa- rated from each other by a kind of hour-glass contraction. The interesting researches of Dr. Brinton have clearly established that, by means of this peristaltic action of the muscular coats of the stomach, not merely is chymified 296 DIGESTION. food gradually propelled through the pylorus, but a kind of double current is continually kept up among the con- tents of the stomach, the circumferential parts of the mass being gradually moved onward towards the pylorus by the peristaltic 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 conducive to their free mixture with the gastric fluid and to* their ready digestion. These actions of the stomach are peculiar to it and inde- pendent. But it is, also, adapted to act in concert with the abdominal muscles, in certain circumstances which can hardly be called abnormal, as in vomiting and eructation. It has indeed been frequently stated that the stomach itself is quite passive during vomiting, and that the ex- pulsion 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 sub- sequently of the abdominal muscles. The experiments and observations, however, which are supposed to confirm this statement, only show that the contraction of the abdo- minal 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 or rather does not 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 wanted to 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 * A collection of cases of fistulous communication with the stomach, through the abdominal pai-ietes, has been given by Dr. Murchison in vol. xli. of the Medico-Chirurgical Transactions. VOMITING. 297 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. Besides the influence which it may thus have by its con- traction, the stomach also essentially contributes to the act of vomiting, by the contraction of its pyloric orifice at the same time that the oblique fibres around the cardiac orifice are relaxed. For, until the relaxation of these fibres, no vomiting can ensue ; when contracted, they can as well resist all the force of the contracting abdominal and other muscles, as the muscles by which the glottis is closed can resist the same force in the act of straining. Doubtless we may refer many of the acts of retching and ineffectual attempts to vomit, to the w T ant of concord between the re- laxation of these muscles and the contraction of the others. The muscles with which the stomach co-operates in con- traction during vomiting, are chiefly and primarily those of the abdomen ; the diaphragm also acts, but not as the muscles of the abdominal walls do. They contract and compress the stomach more and more towards the back and upper parts of the diaphragm : and the diaphragm (which is usually drawn down in the deep inspiration that precedes each act of vomiting) holds itself fixed in contrac- tion, and presents an unyielding surface against which the stomach may be pressed. It is enabled to act thus, and pro- bably only thus, because the inspiration which precedes the act of vomiting is terminated by the closure of the glottis ; after which the diaphragm can neither descend further, except by expanding the air in the lungs, nor, except by compressing the air, ascend again until, the act of vomiting having ceased, the glottis is opened again (see diagram, p. 231 ; see also p. 233). Some persons possess the power of vomiting at will, 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 pos- 298 DIGESTION. sess it, and by continual practice may become a habit. There are cases also of rare occurrence in which persons habitually swallow their food hastily, and nearly unmas- ticated, and then at their leisure regurgitate it, piece by piece, into their mouth, remasticate, and again swallow it, exactly as is done by the ruminant order of Mammalia. Influence of the Nervous System on Gastric Digestion. This influence is manifold; and is evidenced, 1st, in the sensations which induce to the taking of food : 2nd, in the secretion of the gastric fluid ; ^rd, in the movements of the food in and from, the stomach. The sensation of hunger is manifested in consequence of deficiency of food in 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 divi- sion of both pneumogastric nerves, which are the principal channels by which the mind 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-alimentary substances into this organ. It may, therefore, be said that the sensa- tion of hunger is derived from the system generally, but chiefly from the condition of the stomach, the nerves of which, we may suppose, are more affected by the state of the insufficiently replenished blood than those of other organs are. The sensation of thirst, indicating the want of fluid, is referred to the fauces, although, as in hunger, this is merely^ the local declaration of a general condition existing in the system. For thirst is relieved for only a very short HUNGEE AND THIRST. 299 time by moistening the dry fauces ; but may be relieved completely by the introduction of liquids into the blood, either through the stomach, or by injections into the blood-vessels, or by absorption from the surface of the skin, or the intestines. The sensation of thirst is per- ceived most naturally whenever there is a disproportion- ately 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 abundantly added to it. We can express the fact (even if it be not an explanation of it), by saying that the nerves of the mouth and fauces, through which the sense of thirst is chiefly derived, are more sensitive to this condition of the blood than other nerves are. 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 sensa- tion of the " necessity of breathing," is referred especially to the lungs; but, as Volkmann's experiments 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. And, as with respiration when the lungs are removed, the mind may still feel the body's want of breath, so in hunger and thirst, even when the stomach has been filled with innu- tritious substances, or the pneumogastric nerves have been divided, and the mouth and fauces are kept moist, the mind is still aware, by the more obscure sensations in other parts, of the whole body's need of food and water. The influence of the nervous system on the secretion of gastric fluid, is shown plainly enough in the influence of the mind upon digestion in the stomach ; and is, in this regard, well illustrated by several of Dr. Beaumont's observations. M. Bernard also, watching the act of gastric digestion in dogs which had fistulous openings into their stomachs, 300 DIGESTION. saw that on the instant of dividing their pneumogastric nerves, the process of digestion was stopped, and the mucous membrane of the stomach, previously turgid with blood, became pale, and ceased to secrete. These, how- ever, and the like experiments showing the instant effect of division of the pneumogastric nerves, may prove no more than the effect of a severe shock, and the effect that influences affecting digestion may be conveyed to the stomach through those nerves. From other experiments it may be gathered, that although, as in M. Bernard's, the division of both pneumogastric nerves always tem- porarily suspends the secretion of gastric fluid, and so arrests the progress of digestion, and is occasionally followed by death from inanition ; yet the digestive powers of the stomach may be completely restored after the operation, and the formation of chyme and the nutrition of the animal may be carried on almost as perfectly as in health. In thirty experiments on Mammalia, which M. Wern- scheidt performed under Miiller's direction, not the least difference could be perceived in the action of narcotic poisons introduced into the stomach, whether the pneu- mogastric had been divided on both sides or not, provided the animals were of the same species and size. It appears, however, that such poisons as are capable of being rendered inert by the action of the gastric fluid, may, if taken into the stomach shortly after division of both pneu- mogastric nerves, produce their poisonous effects; in consequence, apparently, of the temporary suspension of the secretion of gastric fluid. Thus, in one of his experiments, M. Bernard gave to each of two dogs, in one of which he had divided the pneumogastric nerves, a dose of emulsine, and half an hour afterwards a dose of amygdaline, substances which are innocent alone, but when mixed produce hydrocyanic acid. The dog whose nerves were cut, died in a quarter of an hour, the sub- stances being absorbed unaltered and mixing in the blood ; POST-MORTEM DIGESTION. 301 in the other, the emulsine was decomposed by the gastric fluid before the amygdaline was administered ; therefore, hydrocyanic acid was not formed in the blood, and the dog survived. The influence of the pneumogastric nerves over the secretion of gastric fluid has been pf late even more de- cidedly shown by M. Bernard, who 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 dimi- nution and even complete arrest of the secretion. The influence of the nervous system on the movements of the stomach has been often seen in the retardation or arrest of these movements after division of the pneumogastric nerves. The results of irritating the same nerves were ambiguous ; but the experiments of Longet, and Bischoff have shown that the different results depended on whether the stomach were digesting or not at the time of the experi- ment. In the act of digestion, the nervous system of the stomach appears to participate in the excitement which prevails through the rest of its organization, and a stimulus applied to the pneumogastric nerves is felt intensely, and active movements of the muscular fibres of the stomach follow ; but in the inaction of fasting, the same stimulus produces no effect. So, while the stomach is digesting, the pylorus is too irritable to allow anything but chyme to pass; but when digestion is ended, the undigested parts of the food, and even large bodies, coins, and the like, may pass through it. Digestion 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 302 DIGESTION. 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 examinations of the human body; but, as Dr. Pavy observes, the effect may be rendered, by experiment, more strikingly manifest. " If, for instance," he remarks, " an animal, as a rabbit, be killed at 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 have been dissolved. With a rabbit killed in the evening, and placed in a warm situation to prevent its temperature from falling, not only the stomach but many of the surrounding parts will be found to have been dissolved. With a rabbit killed in the evening, and placed in a warm situation (100 to 110 Fahr.) during the night, I have seen in the morning, the stomach, dia- phragm, part of the liver and lungs, and the intercostal muscles of the side on which the animal was laid all digested away, with the muscles and skin of the neck and upper extremity on the same side also in a semi-digested state." 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 ? John Hunter, who particularly drew attention to the phenomena of post-mortem digestion, ex- plained the immunity from injury of the living stomach, by referring it to the protective influence of the " vital principle." But this dictum has been called in question by subsequent observers. It is, indeed, rather a statement of a fact, than an explanation of its cause. It must be confessed, however, that no entirely satisfactory theory has been yet stated as a substitute. It is only necessary to refer to the idea of Bernard, that the living stomach finds protection from its secretion in the POST-MORTEM DIGESTION. 303 presence of epithelium and mucus, which are constantly renewed in the same degree that they are so constantly dis- solved, in order to remark that this theory has been disproved by experiments of Pavy's, in which the mucous membrane of the stomachs of dogs was dissected off for a small space, and, on killing the animals some days after- wards, no sign of digestion of the stomach was visible. " Upon one occasion, after removing the mucous mem- brane 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 des- troyed 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 of what had been removed, and the denuded spot had contracted to much less than its original dimensions." Dr. 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, were it, so to speak, to make an attempt at digesting parts with which it has no business ; and as may be gathered from what has been previously said, (p. 283) the neutralization of the acidity of the gastric secretion is quite sufficient to destroy its digestive powers. He also very ingeniously argues that this very alkalinity must, from the conditions of the circulation naturally existing in the walls of the stomach, be increased in proportion to the need of its protective influence. " In the arrangement of the vascular supply," he remarks, " a doubly effective barrier is, as it were, provided. The vessels pass from below upwards towards the surface : capillaries having this direction ramify between the tubules by which the acid of the gastric juice is secreted ; and being separated by secretion below, must leave the blood that is proceeding upwards correspondingly increased in alkalinity ; and thus 304 DIGESTION. at the period when the largest amount of acid is flowing into the stomach, and the greatest protection is required, then is the provision afforded in its highest state of efficiency." Dr. Pavy's theory is the best and most ingenious hitherto framed in connection with this subject; but the experiments adduced in its favour are open to many objections, and afford only a negative support to the conclusions they are intended to prove. The matter, therefore, can scarcely be considered finally settled. DIGESTION IN THE INTESTINES. The intestinal canal is divided into two chief portions, named, from their differences in diameter, the small and large intestine. These are continuous with each other, and communicate by means of an opening guarded by a valve, the ileo-ccBcal valve, which allows the passage of the pro- ducts of digestion from the small into the large bowel, but not, under ordinary circumstances, in the opposite direction. The structure and functions of each organ or tissue con- cerned in intestinal digestion will be first described in detail, and afterwards a summary will be given of the changes which the food undergoes in its passage through the intestines, 1st, from the pylorus to the ileo-ca3cal valve; and, 2nd, from the ileo-ca3cal valve to the anus. Structure and Secretions of the Small Intestine. The small intestine, the average length of which in an adult is about twenty feet, has been divided, for conve- nience of description, into three portions, viz., the duo- denum, which extends for eight or ten inches beyond the pylorus; the jejunum, which occupies two-fifths, and the Hewn, which occupies three-fifths of the rest of the canal. The small intestine, like the stomach, is constructed of three principal coats, viz., the serous, muscular, and mucous. The serous coat, formed by the visceral layer of the peritoneum, need not be here specially described. The THE VALVUL^E CONNIVENTES. 305 fibres of the muscular coat of the small intestine are arranged in two layers; those of the outer layer being disposed longitudinally; those of the inner layer trans- versely, or in portions of circles encompassing the canal. They are composed of the unstriped kind of muscular fibre. Between the mucous and muscular coats, there is a layer of submucous tissue, in which numerous blood-vessels and a rich plexus of nerves and ganglia are imbedded (Meissner). The mucous membrane is the most important coat iu relation to the function of digestion, and the following structures which enter into its composition may be now successively described ; the valvula conniventes ; the villi ; and the glands. The mucous membrane throughout is lined on its inner surface by columnar epithelium. Valvulce Conniventes. The valvulse conniventes commence in the duodenum, about one or two inches beyond the pylorus, and becoming larger and more numerous immediately beyond the en- trance 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. In structure they are formed by a doubling inwards of the mucous membrane, the crescentic, nearly circular, folds thus formed being arranged trans- versely with regard to the axis of the intestine, and each individual fold seldom extending around more than J or |^ of the bowel's circumference. Unlike the rugas in the stomach, they do not disappear on distension. Only an imperfect notion of their natural position and function can be obtained by looking at them after the intestine 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 may be seen that x 306 DIGESTION. instead of disappearing, as the rugre in the stomach would under similar circumstances, they stand out at right angles . * to the general surface of the mucous membrane (fig. 72). Their functions are probably these Besides ( I ) offer- ing a largely increased surface for secretion and absorption, they proba- bly (2) prevent the too rapid passage of the very liquid products of gastric digestion, immediately after their es- cape from the stomach, and (3), by their projection, and consequent inter- ference with an uniform and untrou- bled current of the intestinal contents, probably assist in the more perfect mingling of the latter with the secre- tions poured out to act on them. Glands of tlie Small Intestine. The glands are of three principal kinds, named after their describers, the glands of pi Lieberkiihn, of Peyer, and of Brunn. The glands or follicles 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 3 ens; 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 yL- of a line. Each tubule (fig. 73) is constructed * Fig. 72. Piece of small intestine (previously distended and hardened by alcohol) laid open to show the normal position of the valvulse con- niventes. + Fig. 73. A gland of Lieberkiihn. PETER'S GLANDS. 37 of the same essential parts as the intestinal mucous mem- brane, viz., a fine structureless membrana propria, or base- ment membrane, a layer of cylindrical epithelium lining it and capillary blood-vessels covering its exterior. Their contents appear to vary, even in health; the varieties being dependent, probably, on the period of time in rela- tion to digestion at which they are examined. At the bottom of the follicle, the contents usually consist of a granular material, in which a few cytoblasts or nuclei are imbedded; these cytoblasts, as they ascend towards the surface, are supposed to be gradually developed into nucleated cells, some of which are discharged into the intestinal cavity. The purpose served by the material secreted by these glands is still doubtful. Their large number and the extent of surface occupied by them, seem, however, to indicate that they are concerned in other and higher offices than the mere production of fluid to moisten the surface of the mucous membrane, although, doubtless, this is one of their functions. The glands of Peyer occur exclusively in the small intes- Fig. 74* tine. They are found in greatest abundance in the lower part of the ileum near to the ileo-csecal valve. They are * Fig. 74. Agminate follicles, or Peter's prtrh, in a state of dis- tension : magnified about 5 diameters (after Boelim). x 2 308 DIGESTION. met with in two conditions, viz., either scattered singly, in which case they are termed glandula solitaricB, or aggre- gated in groups varying from one to three inches in length and about half-an-inch in width, chiefly of an oval form, their long axis parallel with that of the intestine. In this state, they are named glandulce agminatce, the groups being commonly called Peyers patches (fig. 74). The latter are placed almost always opposite the attachment of the mesentery. In structure, and probably in function, there Fig. 76. t is no essential difference between the solitary glands and the individual bodies of which each group or patch is made up ; but the surface of the solitary glands (fig. 75) is beset with villi, from which those forming the agminate patches (fig. 76) are usually free. In the condition in which they have been most commonly examined, each gland appears as a circular opaque-white sacculus, from half a line to a line in diameter, and, according to the * Fig. 75. Solitary gland of small intestine, after Boelim. *t* Fig. 76. Part of a patch of the so-called Peyer's glands magnified, showing the various forms of the sacculi, with their zone of foramina. The rest of the membrane marked with Lieberkiihn's follicles, and sprinkled with villi (after Boehm). PEYER'S GLANDS. 39 degree in which, it is developed, either sunk beneath, or more or less prominently raised on, the surface of a depression or fossa in the mucous membrane. Each gland is surrounded by openings like those of Lieberkiihn's follicles (see fig. 76) except that they are more elongated; and the direction of the long diameter of each opening is such that the whole produce a radiated appearance around the white sacculus. These openings appear to belong to tubules identical with Lieberkiihn's follicles : they have no commu- nication with the sacculus, and none of its contents escape through them on pressure. fi Neither can any perma- nent opening be detected in the sacculus or Peyer's gland itself (see fig. 77). Each gland is a closed sac or follicle formed of a tolerably firm membranous capsule of imperfectly de- veloped connective tissue, imbedded in a rich plexus of minute blood-vessels, many fine branches from which pass through the capsule and enter, chiefly loopwise, the interior of the follicle (fig. 78). The contents of each sac amid which these minute vessels are distributed, con- sist of a pale greyish opalescent pulp, formed of albu- minous and fatty matter, and a multitude of nucleated cells of various sizes, many of which, according to Kolliker, exhibit well-marked endogenous cell-multiplication. * Fig. 77. Side-view of a portion of intestinal mucous membrane of a cat, showing a Peyer's gland (a) : it is embedded in the submucous tissue (/), the line of separation between which and the mucous mem- brane passes across the gland : b, one of the tubular follicles, the orifices of which form the zone of openings around the gland : c, the fossa in the mucous membrane : d, villi : e, follicles of Lieberkiihn (after Bendz). 310 DIGESTION. The real office of these Peyerian glands or follicles is still unknown. It was formerly believed that each follicle was a kind of secreting cell, which, when its contents were fully matured, forming a communication with the cavity of the intestine by the absorption or bursting of its own cell-wall, and of the portion of mucous membrane over it, and thus discharged its secretion into the intes- tinal canal. A small shallow cavity or space was thought to remain, for a time, after this absorption or dehiscence, Fig. 78.* but shortly to disappear, together with all trace of the previous gland. More recent acquaintance with the -real structure of * Fig. 78. Transverse section of injected Peyer's glands (from Kb'l- liker). The drawing was taken from a preparation made by Frey : it represents the fine capillary looped network spreading from the sur- rounding blood-vessels into the interior of three of Peyer's capsules from the intestine of the rabbit. BRUXX'S GLANDS. 3 11 these bodies seems, however, to prove that they are not mere temporary gland-cells which thus discharge their elaborated contents into the intestine and then disappear, but that they are rather to be regarded as structures analogous to lymphatic or absorbent glands, and that their office is to take up certain materials from the chyle, elaborate and subsequently discharge them into the lacteals, with which vessels they appear to be closely connected, although no direct communication has been proved to exist between them. Moreover, it has been lately suggested that since the molecular and cellular contents of the glands are so abundantly traversed by minute blood-vessels, important changes may mutually take place between these contents and the blood in the vessels, material being abstracted from the latter, elaborated by the cells, and then restored to the blood, much in the same manner as is believed to be the case in the so-called vascular glands, such as the spleen, thymus, and others ; and that thus Peyer's glands should also be regarded as closely analogous to these vas- cular glands. Possibly they may combine the functions both of lymphatic and vascular glands, absorbing and elaborating material both from the chyle and from the blood within their minute vessels, and transmitting part to the lacteal system and part direct to the blood. Brunn's glands (fig. 79) are confined to the duodenum ; they are most abundant and thickly set at the commence- ment of this portion of the intestine, diminishing gradually as the duodenum advances. Situated beneath the mucous membrane, and imbedded in the submucous tissue, they are minutely lobulated bodies, visible to the naked eye, like detached small portions of pancreas, and provided with permanent gland-ducts, which pass through the mucous membrane and open on the internal surface of the intestine. As in structure, so probably in function, they resemble the pancreas j or at least stand to it in a similar relation to 312 DIGESTION. that which, the small labial and buccal glands occupy in relation to the larger salivary glands, the parotid and subm axillary. The Villi (figs. 80, 81) are confined 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 mem- brane, and giving it a peculiar velvety, fleecy appearance. Fig. 79-* Krauss 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 clavate. Each villus consists of a small projection of mucous membrane, and is composed from without inwards- of the following parts : Epithelium, basement membrane, blood- * Fig. 79. Enlarged view of one of Brunn's glands from the human duodenum (from Frey). The main duct is seen superiorly ; its branches are elsewhere hidden by the bunches of opaque glandular vesicles. THE VILLI. vessels, unstriped muscular fibres, and a single lymphatic or lacteal vessel rarely looped or branched (fig. 80) ; besides granular matter, fat-globules, &c. Fig. 80.* The epithelium is of eht columnar kind, and continuous with that lining the parts of the other mucous membrane. The cells are arranged with their long axis radiatiDg from the surface of the villus (fig. 80), and their smaller ends resting on the basement membrane. Some doubt exists concerning the minute structure of these cells and their relation to the deeper parts of the villus. But the opi- nions given by different observers are so contradictory * Fig. 80. (Slightly altered from Teichmann.) A. Villus of sheep. B. Villi of man. DIGESTION. that we may for the present consider them as like simple columnar epithelium in other parts. (See chapter on Absorption). Fia. 8 1.* Beneath the basement or limiting membrane tlure is a * Fig. 81. (From Teichmann.) A, lacteals in villi. P, Payer's glands. B and D, superficial and deep network of lacteals in submucous tissue. L, Lieberkiilm's glands. E, small branch of lacteal vessel on its way to ineseriteric gland. H and o, muscular nbres of intestine, s, peritoneum. THE LARGE INTESTINE. 315 rich supply of blood-vessels. Two or more minute arteries are distributed within each villus ; and from their capil- laries, which form a dense network, proceed one or two small veins, which pass out at the base of the villus. The layer of organic muscular fibres forms a kind of thin hollow cone immediately around the central lacteal, and is, therefore, situate beneath the blood-vessels and much of the granular basis of the villus. The addition of acetic acid to the villus brings out the characteristic nuclei of the muscular fibres and shows the size and position of the layer most distinctly. Its use is still un- known, although it is impossible to resist the belief, that it is instrumental in the propulsion of chyle along the lacteal. Kolliker has lately shown that this layer is continuous with a layer of organic muscular fibres situated within the mu- cous membrane of the intestine. The lacteal vessel enters the base of each villus, and pass- ing up in the middle of it, extends nearly to the tip, where it ends commonly by a closed and somewhat dilated ex- tremity. In the larger villi there may be two small lacteal vessels which end by a loop (fig 80), 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. 80). The office of the villi is the absorption of chyle fromNthe completely digested food in the intestine. The mode in which they effect this will be considered in the chapter on ABSORPTION. Structure of 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, viz. : The cacum, a short wide pouch, commu- nicating with the lower end of the small intestine through an opening, guarded by the ileo-cacal valvej the colon, 316 DIGESTION. continuous with the caecum, which forms the principal part of the large intestine, and is divided into an ascend- ing, transverse and descending portion ; and the rectum, which, after dilating at its lower part, again contracts, and immediately afterwards opens externally through the anus. Attached to the caecum is the small appendix vermiformis. Like the small intestine the large is constructed of three principal coats, viz., the serous, muscular, and mucous. The serous coat need not be here particularly described. Connected with it are the small processes of peritoneum containing fat, called appendices epeploicce. The fibres of the muscular coat, like those of the small intestine, are arranged in two layers the outer longitudinally, the inner circularly. In the caecum and colon the longi- tudinal fibres, besides being, as in the small intestine, thinly disposed in all parts of the wall of the bowels, 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 inter- mediate sacculi. On the division of these bands, the intes- tine can be drawn out to its full length, and it then assumes, of course, an uniformly cylindrical form. In the rectum, the fasciculi of these longitudinal bands spread out and mingle with the other longitudinal fibres, forming with them a thicker layer of fibres than exists on 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, 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 epithe- lium, but, unlike it, is quite smooth and destitute of villi, and is not projected in the form of valvulse conniventes. THE ILEO-CLECAL VALVE. 317 Glands of the Large Intestine. The glands with, which the large intestine is provided are of two kinds, the tubular and lenticular. The tubular glands, or glands of Lieberkiihn, resemble those of the small intestine, but are somewhat larger and more numerous. They are also more uniformly distributed. The lenticular glands are most numerous in the caecum and vermiform appendix. They resemble in shape and structure, almost exactly, the solitary glands of the small intestine, and, like them, have no opening. Just over them, however, there is commonly a small depression in the mucous membrane, which has led to the erroneous belief that some of them open on the surface. The functions discharged by the glands found in the large intestine are not known with any certainty, but there is no reason to doubt that they resemble very nearly those discharged by the glands of like structure in the small intestine. The difficulty of determining the function of any single set of the intestinal glands seems indeed almost insuper- able, so many fluids being discharged together into the intestine ; for all acting, probably, at once, produce a com- bined effect upon the food, so that it is almost impossible to discern the share of any one of them in digestion. lleo-cacal valve. The ileo-csecal valve is situate at the place of junction of the small with the large intestine, and guards against any reflux 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 membrane, 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. The inner surface of the folds is smooth; the mucous membrane of the ileum being continuous with that of the caecum. That surface DIGESTION. 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 margins of the folds are stretched and thus are brought into firm apposition with each other. While the circular muscular fibres of the bowel at the junction of the ileum with the caecum are contained between the outer opposed surfaces of the folds of mucous membrane which form the valve, the longitudinal mus- cular fibres and the peritoneum of the small and large intestine respectively, are continuous with each other, without dipping in to follow the circular fibres and the mucous membrane. In this manner, therefore, the folding inwards of these two last named structures is preserved, 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 per- forming a similar operation. 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 intestine, either through a small opening or through a duct common to itself and to the liver. The pancreas, in its minute anatomy, closely resembles the salivary glands; and the fluid elaborated by it appears almost identical with saliva. When obtained pure, in all the different animals in which it has been hitherto examined, it has been found colourless, transparent, and slightly viscid. It is alkaline when fresh, and contains a peculiar animal matter named pancreatin and certain salts, both of which are very similar to those found in saliva. In pancreatic secretion, however, there is no sulpho-cyanogen. Pancreatin is a substance coagulable by heat, and in many other respects very THE PANCREATIC SECRETION. 319 like albumen : to it the peculiar digestive power of the pancreatic secretion is probably due. Like saliva, the pancreatic fluid, shortly after its escape, becomes neutral and then acid.' The following is the mean of three analyses by Schmidt : Composition of Pancreatic Secretion. Water . 980*45 Solids 19-55 Pancreatin. ...... 1271 Inorganic bases and salts . . . . 6 '84 The functions of the pancreas are probably as follows : I. Numerous experiments have shown, that starch is acted upon by the pancreatic secretion, or by portions of pancreas put in starch-paste, in the same manner that it is by saliva and portions of the salivary glands. And although, as before stated (p. 270), many substances be- sides those glands can excite the transformation of starch into dextrin and grape-sugar, yet it appears probable that the pancreatic fluid, exercising this power of trans- formation, is largely subservient to the purpose of digesting starch. MM. Bouchardat and Sandras have shown that the raw starch-granules which have passed unchanged through the crops and gizzards of granivorous birds, or through the stomachs of herbivorous Mammalia, are, in the small intestine, disorganized, eroded, and finally dis- solved, as they are when exposed, in experiment, to the action of the pancreatic fluid. The bile cannot effect such a change in starch ; and it is most probable that the pan- creatic secretion is the principal agent in the transfor- mation, though it is by no means clear that the office may not be shared by the secretion of the intestinal mucous 320 DIGESTION. membrane, which also seems to possess the power of con- verting starch into sugar. 2. The existence of a pancreas in Garni vora, which have little or no starch in their food, and the results of various observations and experiments, leave very little doubt that the pancreatic secretion also assists largely in the digestion of fatty matters, by transforming them into a kind of emulsion, and thus rendering them capable of absorption by the lacteals. Several cases have been recorded in which the pancreatic duct being obstructed, so that the secretion could not be discharged, fatty or oily matter was abun- dantly discharged from the intestines. In nearly all these cases, indeed, the liver was coincidently 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 discharges 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. In Bernard's experiments too, fat always ap- peared in the evacuations when the pancreas was destroyed or its duct tied. Bernard, indeed, is of opinion that to emulsify fat is the express office of the pancreas, and the evidence that he and others have brought forward in sup- port of this view is very weighty. The power of emulsi- fying fat, however, although perhaps mainly exercised by the secretion of the pancreas, is evidently possessed to some extent by other secretions poured into the intestines, and especially by the bile. 3. The pancreatic secretion has been supposed to dis- charge a third function, namely, that of dissolving albu- minous substances. It is very doubtful, however, whether its effect in this way is more than a slight one ; and, at any rate, the function is quite subordinate to the other two that have been mentioned. STRUCTURE OF THE LIVER. 321 Structure of the. Liver. The liver is an extremely vascular organ, and receives its supply of blood from two distinct vessels, the portal vein and hepatic artery, while the blood is returned from it into the vena cava inferior by the hepatic vein. Its secre- tion, the Hie, 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, hepatic artery and hepatic duct branch together throughout the liver, while the hepatic vein and its tributaries run by themselves. On the outside the liver has an incomplete covering of peritoneum, and beneath this is a very fine coat of areolar tissue, continuous over the whole surface of the organ. It is thickest where the peritoneum is absent, and is con- tinuous on the general surface of the liver 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 ^ > g 2 this part, accompanies them B in their branchings through the substance of the liver. The liver is made up of small roundish or oval por- tions called lobules, each of which is about ^L- of an inch in diameter, and composed of the minute branches of the portal vein, hepatic artery, hepatic duct, and hepatic vein; while the interstices of these vessels are filled by the liver cells. These cells (fig. 82) which make up a great portion of the substance of the organ, are rounded or polygonal from about Tr ^ ir to Y 3 22 DIGESTION. Fig. 83-' 1 ' 00 of an inch in diameter, containing well-marked nuclei and granules, and having sometimes a yellowish tinge, especially about their nuclei; frequently, they contain also various sized particles of fat (fig. 82 B). Each lobule is very sparingly invested by areolar tissue. To understand the distribution of the blood-vessels in the liver, it will be well to trace, first, the two blood-vessels and the duct which enter the organ on the under surface at the trans- verse fissure, viz., the portal vein, hepatic artery, and hepatic duct. As before re- marked, all three run in company, and their appearance on longitudinal section is shown in fig. 83. 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, called 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 * Fig. 83. Longitudinal section of a portal canal, containing a portal vein, hepatic artery and hepatic duct, from the pig (after Kieruan), f. p, branch of vena portse, situated in a portal canal, formed amongst the lobules of the liver, and giving off vaginal branches ; there are also seen within the large portal vein numerous orifices of the smallest inter- lobular veins arising directly from it ; a, hepatic artery ; d, hepatic duct. STRUCTURE OF THE LIVER. 323 surrounding them and limiting them, and from this cir- cumstance called inter-lobula? veins. From these small vessels a dense capillary network is prolonged into the substance of the lobule, and this network gradually gather- ing itself up, so to speak, into larger vessels, converges finally to a single small vein, occupying the centre of the lobule, and hence called wra-lobular. This arrangement is well seen in fig. 84, which represents a transverse sec- tion of a lobule. The smaller branches of the portal vein being closely surrounded by the -lobules, give off directly Fig. 84.* w : -" ' ^Mf^M^^M^fM wter-lobular veins, (see fig. 83); but here and there, espe- cially where the hepatic artery and duct intervene, branches called vaginal first arise, and breaking up in the sheath are * Fig. 84. Cross section of a lobule of the human liver, in which the capillary network between the portal and hepatic veins has been fully injected (from Sappey), . I. Section of the wra-lobular vein ; 2, its smaller branches collecting blood from the capillary network ; 3, iuter- lobular branches of the vena portte with their smaller ramifications passing inwards towards the capillary network in the substance of the lobule. Y 2 324 DIGESTION. subsequently distributed like the others around the lobules and become wter-lobular. The larger trunks of the portal vein being more separated from the lobules by a thicker sheath of Glisson's capsule, give off vaginal branches alone, which, however, after breaking up in the sheath, are distributed like the others between the lobules, and become iter-lobular veins. The small mra-lobular veins discharge their contents into veins called sw&-lobular (fig. 86), while these again, by their Fin. 8s.* * Fig. 85. Section of a portion of liver passing longitudinally through a considerable hepatic vein, from the pig (after Kiernan), T . H, hepatic venous trunk, against which the sides of the lobules (I) 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. STRUCTURE OF THE LIVER. 325 union, form the main branches of the hepatic vein, which Fig. 86.* Lobule* Lobules leaves the posterior border of the liver to end by two or three principal trunks in the inferior vena cava, just before its passage through the dia- phragm. The sw&-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 chan- nels in the liver substance which closely surrounds them. The manner in which the lobules are connected with the sublobular veins by means of the small intralobular veins is well seen in the diagram fig. 86, and in fig. 85, which represent the parts as seen in a longitudinal section. The appearance has been likened to a twig having leaves with- out 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 intra- lobular veins, is that of I, fig. 84, while both a transverse and longitudinal section highly magnified are exhibited in %. 87. The hepatic artery, the function of which is to distribute blood for nutrition to Glisson's capsule, the walls of the ducts and blood-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 witra-lobular veins. * Fig. 86. Diagram showing the manner in which the lobules of the liver rest on the sublobular veins (after Kiernan). DIGESTION. The hepatic duct divides and subdivides in a manner very like that of the portal vein and hepatic artery, the larger branches being lined by cylindrical, and the smaller Fig. 87.* by small polygonal epithelium. The exact arrangement of its terminal branches, however, and their relation to the liver- cells have not been clearly made out, or at least, have not been agreed upon by different observers. The chief theories on the subject are three in number : 1. That the terminal branches of the hepatic duct form an interlobular network, which abuts on the outermost cells of a lobule, but does not enter the inside of the lobule, or only for a little way. 2. That minute branches begin in the lobules between the cells, not enclosing them. 3. That the ultimate branches begin in the lobules and enclose hepatic cells. * Fig. 87. Capillary network of the lobules of the rabbit's liver (from Kolliker), ^. 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. STKUCTUKE OF THE LIVER. 327 The illustrations below will show the conflicting theories at a glance. Fig. 88.* * Fig. 88. Diagrams showing the arrangement of the radicles of the hepatic duct, according to the different observers. 1. d, d, are two branches of the hepatic duct, which is supposed to commence in a plexus situated towards the circumference of the lobule marked b, b, called by Kiernan the biliary plexus. Within this is seen the central part of the lobule, containing branches of the intra- lobular vein. 2. A small fragment of an hepatic lobule, of which the smallest intercellular biliary ducts were filled with colouring matter during life, highly magnified (from Chrzonszezewsky). 3. View of some of the smallest biliary ducts illustrating Beale's view of their relation to the biliary cells (from Kb'lliker after Beale), ^p. The drawing is taken from an injected preparation of the pig's liver ; 328 DIGESTION. Functions of the Liver. The Secretion of Bile is the most obvious, and one of the chief functions which the liver has to perform ; but, as will be presently shown, it is not the only one ; for recent dis- coveries have shown that important changes are effected in certain constituents of the blood in its transit through this gland, whereby they are rendered more fit for their subsequent purposes in the animal economy. The Bile. Composition of the Bile. The bile is a somewhat viscid fluid, of a yellow, or greenish-yellow colour, a strongly bitter taste, and when fresh with a scarcely perceptible odour ; it has a neutral or slightly alkaline re-action, and its specific gravity is about IO2O. Its colour and degree of consistence vary much, apparently independent 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. The following analysis is by Frerichs : Composition of Human Bile. Water 859-2 Solids . .... 140-8 a, small branch of an interlobular hepatic duct ; b, smallest biliary ducts ; c, portions of the cellular part of the lobule in which the cells are seen within tubes which communicate with the finest ducts. THE BILE. 329 Biliary acids combined ) with alkalies ( Fat Cholesterin . Mucus and colouring matters . Salts 9i '5 9-2 2-6 29-8 77 140-8 The Ellin or biliary matter described by Berzelius, when freed by ether from the fat with which it is combined, is a resinoid substance, soluble in water, alcohol, and alkaline solutions, and giving to the watery solution the taste and general character of bile. It appears to be not the single substance it was once supposed to be, but a compound of soda combined with one or both of two resinous acids, named by Lehmann, glycocholic and taurocholic, because the former consists, he believes, of cholic acid conjugated with glycin (or sugar of gelatin), the latter of the same acid conjugated with taurin. Fatty substances are found in variable proportions, such Fig. 89.* as olein and margarin, or their acids, oleic and mar- garic, and stearic acids, com- bined with potash and soda. Besides, there is a small quan- tity of cholesterin (see p. 19), which, with the other free fats, is probably held in solution by the tauro-chlorate of soda. The colouring matter of the bile has not yet been obtained pure, owing to the facility with which it is decomposed. It occasionally deposits itself in the gall-bladder as a yel- low substance mixed with mucus, and in this state has been frequently examined. It is composed of two colour- Fig. 89. Crystalline scales of cholesterin. 33 DIGESTION. ing matters, called biliverdin and bilifulvin. By oxidising agencies, as exposure to the air, or the addition of nitric acid, it assumes a dark green colour. In cases of biliary obstruction, it is often re-absorbed, circulates with the blood, and gives to the tissues the yellow tint characteristic of jaundice. There seems to be some relationship between the colour- ing matters of the blood and bile, and it may be added, between these and that of the urine also, so that it is possible they may be, all of them, varieties of the same pigment, or derived from the same source. Nothing, however, is at present certainly known regarding the relation in which one of them stands to the other. The mucus in bile is derived chiefly from the mucous membrane of the gall-bladder, but in part also from the hepatic ducts and their branches. It constitutes the residue after bile is treated with alcohol. The epithelium with which it is mixed may be detected in the bile with the microscope in the form of cylindrical cells, either scattered or still held together in layers. To the presence of this mucus is probably to be ascribed the rapid decomposition undergone by the bilin ; for, according to Berzelius, if the mucus be separated, bile will remain unchanged for many days. 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 tribasic phosphate of soda and potash, 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. Such are the principal chemical constituents of bile ; but its physiology is, perhaps, better illustrated by its ultimate elementary composition. According to Liebig's analysis, THE BILE. 33i the biliary matter, consisting of bilin and the products of its spontaneous decomposition yields, on analysis, 76 atoms of carbon, 66 of hydrogen, 22 of oxygen, 2 of nitrogen, and a certain quantity of sulphur.* Comparing this with the ultimate composition of the organic parts of blood which may be stated at C 48 H 36 N 6 O I4 with sulphur and phosphorus it is evident that bile contains a large preponderance of carbon and hydrogen, and a deficiency of nitrogen. The import of this will presently appear. The process of secreting bile is probably continually going on, but appears to be retarded during fasting, and ac- celerated on taking food. This was shown by Blondlot who, having tied the common bile-duct of a dog, and established a fistulous opening between the skin and gall- bladder, whereby all the bile secreted was discharged at the surface, noticed that when the animal was fasting, sometimes not a drop of bile was discharged for several hours ; but that, in about ten minutes after the introduc- tion of food into the stomach, the bile began to flow abundantly, and continued to do so during the whole period of digestion. Bidder and Schmidt's observations are quite in accordance with this. The bile is probably formed 'first in the hepatic cells ; then, 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 flows from the common bile-duct into the cystic duct, and thence into the gall-bladder, where it accumulates till, in the next period of digestion, it is dis- charged into the intestine. The gall-bladder thus fulfils * The sulphur is combined with the taurin^ one of the substances yielded by the decomposition of bilin. According to Dr. Kemp, the sulphur in the bile of the ox, dried and freed from mucus, colouring matter, and salts, constitutes about 3 per cent. 332 DIGESTION. what appears to be its chief or only office, that of a reser- voir ; for its presence enables bile to be constantly secreted for the purification of the blood, yet insures that it shall all be employed in the service of digestion, although diges- tion 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 communicates 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 insufficient to overcome the force with which the orifice of the duct is closed ; and the bile in the common duct, finding no exit into 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 intro- duction of food into the small intestine : being pressed on by the contraction of the coats of the gall-bladder, and pro- bably of the common bile-duct also ; for both these organs contain organic 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. 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 proportion to the amount of food taken. A fair average of several computations would give thirty to forty ounces as the quantity daily secreted by man. The purposes served by the secretion of bile may be con- sidered to be of two principal kinds, viz., excrementitious and digestive. As an excrementitious substance, the bile serves espe- cially as a medium for the separation of excess of carbon THE BILE: MECONIUM. 333 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, because of the supply of well-elaborated nutriment received by the vessels of the foetus at the placenta. The liver, during the same time, is proportionally larger than it is after birth, and the se- cretion of bile is active, although there is no food in the intestinal canal upon which it can exercise any digestive property. At birth, the intestinal canal is full of thick bile, mixed with intestinal secretion ; for the meconium, or faeces of the foetus, are shown by the analyses of Simon and of Frerichs to contain all the essential principles of bile. Composition of Meconium (Frerichs) : Biliary resin . . . . . . 15-6 Cholesterin, olein, and margarin . . 15*4 Epithelium, mucus, pigment, and salts . 69' loo- In the foetus, therefore, the main purpose of the secretion of bile must be the purification of the blood by direct excretion, i.e., by separation from the blood, and ejection from the body without further change. Probably all the bile secreted in foetal life is incorporated 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 dis- tribution 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 the separation of carbonic acid and water at the lungs does. The evident disposal of the fcetal bile by excretion, makes it highly probable that the bile in extra-uterine life is 334 DIGESTION. also, at least in part, destined to be discharged as excrementitious. But the analysis of the faeces of both children and adults shows that (except when rapidly dis- charged in purgation) they contain very little of the bile secreted, probably not more than one-sixteenth part of its weight, and that this portion includes only its colouring, and some of its fatty matters, but none of its essential principle, the bilin. All the bilin is again absorbed from the intestines into the blood. But the elementary compo- sition of bilin (see p. 331) shows such a preponderance of carbon and hydrogen, that it cannot be appropriated to the nutrition of the tissues ; therefore, it may be presumed that after absorption, the carbon and hydrogen of the bilin combining with oxygen, are excreted as carbonic acid and water. The destination of the bile is, on this theory, essentially the same in both foetal and extra- uterine life ; only, in the former, it is directly excreted, in the latter indirectly, being, before final ejection, modified in its absorption from the intestines, and mingled with the blood. The change from the direct to the indirect mode of excretion of the bile may, with much probability, be con- nected 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 the body of the foetus itself; but, in extra-uterine life, there is (as one may say) a waste of material for heat when any excretion is discharged unoxidized ; the carbon and hydro- gen of the bilin, therefore, instead of being ejected in the faeces, are re-absorbed, in order that they may be com- bined with oxygen, and that in the combination, heat may be generated. From the peculiar manner in which the liver is supplied with much of the blood that flows through it, it is probable, as Dr. Budd suggests, that this organ is excretory, not only for such hydro-carbonaceous matters as may need THE BILE. 335 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 into the general current, jealously to guard against their further progress, and turn them back again into an excretory channel. The 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. Though the 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 secretions 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 secretion of bile is more 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 digestion may, however, be in part ascribed to the fact that a greater quantity of blood is sent 336 . DIGESTION. 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 re-absorbed, or permanently. Respecting the functions discharged by the bile in digestion, there is little doubt that it assists in emulsifying the fatty 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 lacteals consisting of clear, colourless fluid, instead of being opaque and white, as they ordinarily are, after feeding. (2.) It is probable, also, from the re- sult of some experiments by Wistinghausen and Hoffmann, that the moistening of the mucous membrane of the in- testines by bile may facilitate absorption of fatty matters through it. (3.) The bile, like the gastric fluid, has a strongly antiseptic power, and may serve to prevent the decompo- sition of food during the time of its sojourn in the intes- tines. The experiments of Tiedemann and Gmelin show that the contents of the intestines are much more foetid after the common bile-duct has been tied than at other times; and the experiments of Bidder and Schmidt on animals with an artificial biliary fistula, confirm this ob- servation ; moreover, it is found that the mixture of bile with a fermenting fluid stops or spoils the process of fer- mentation. (4.) The bile has also been considered to act as a kind of 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 FUNCTIONS OF THE LIVER. 337 secretion of bile, and from the purgative properties of ox- gaU. Nothing is known with certainty respecting the changes which the re- absorbed portions of the bile undergo, either in the intestines or in the absorbent vessels. That they are much changed appears from the impossibility of de- tecting them in the blood ; and that part of this change is effected in the liver is probable from an experiment of Magendie, who found that when he injected bile into the portal vein a dog was unharmed, but was killed when he injected the bile into one of the systemic vessels. The secretion of bile, as already observed, is only one of the purposes fulfilled by the liver. Another very im- portant 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 being duly eliminated in the process of respiration. From the labours of M. Bernard, to whom we owe most of what we know on this subject, it appears that the low form of albuminous matter, or albuminose, conveyed from the alimentary canal by the blood of the portal vein, requires to be sub- mitted to the influence of the liver before it can be assi- milated 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 probably incorporated with the albuminous part of the blood. An important influence seems also to be exerted by the liver upon the saccharine matters derived from the alimen- tary canal. The chief purpose of the saccharine and amylaceous principles of food is, probably, in relation to respiration and the production of animal heat ; but in order that they may fulfil this, their main office, it seems to 338 DIGESTION. be essential that they should undergo some intermediate change, which is effected in the liver, and which consists in their conversion into a peculiar form of saccharine matter, very similar to glucose, or diabetic sugar. That such influence is exerted by the liver seems proved by the fact that when cane sugar is injected into the jugular vein it is speedily thrown out of the system, and appears in the urine ; but when injected into the portal vein, and thus enabled to traverse the liver, it ceases to be excreted at the kidneys ; and, what is still more to the point, a very large quantity of glucose may be injected into the venous system without any trace of it appearing in the* urine. So that it may be concluded, that the saccharine principles of the food undergo, in their passage through the liver, some transformation necessary to the subsequent purpose they have to fulfil in relation to the respiratory process, and without which, such purpose probably could not be pro- perly accomplished, and the substances themselves would be eliminated as foreign matters by the kidneys. Then, again, it has been discovered by Bernard, and the discovery has been amply confirmed by Lehmann and other distinguished animal chemists, that the liver possesses the remarkable property of forming sugar, or a substance readily convertible into sugar, even out of principles in the blood which contain no trace of saccharine or amylaceous matter. In Herbivora and in animals living on mixed diet, a large part of the sugar is derived from the saccha- rine and amylaceous principles introduced in their food. But in animals fed exclusively on flesh, and deprived therefore of this source of sugar, the liver furnishes the means whereby it may be obtained. Not only in Carni- vora, however, but apparently in all classes of animals, the liver is continually engaged, during health, in forming sugar, or a substance closely allied to it, in large amount. This substance may always be found in the liver, even when absent from all other parts of the body. FORMATION OF SUGAR IN THE LIVER. 339 The researches of Bernard and others, however, have shown that the sugar is not formed at once at the liver, but that this organ has the power of producing a peculiar ternary substance, which is readily convertible into sugar when in contact with any animal ferment. This substance has received from various chemists the different names of glycose, glycogen, glycogenic substance, glycocene, animal starch, amylon, amyloid substance, hepatin. There are two chief theories concerning the immediate destination of this substance, (i.) According to Bernard and most other physiologists, its conversion into sugar takes place rapidly during life, and the sugar is conveyed away by the blood of the hepatic veins to be consumed in respiration at the lungs. (2.) Pavy and others believe that the conversion into sugar only occurs after death, and that during life no sugar exists in healthy livers, the amyloid substance or hepatin being prevented by some force from undergoing the transformation. The chief arguments advanced by Pavy in support of this view are, first, that scarcely a trace of sugar is found in blood drawn during life from the right ventricle, or in blood collected from the right side of the heart immediately after an animal has been suddenly deprived of life, while if the examination be delayed for a little while after death, sugar in abundance may be found in such blood; secondly, that the liver, like the venous blood in the heart, is, at the moment of death, almost completely free from sugar, although after- wards its tissue speedily becomes saccharine, unless the formation of sugar be prevented 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, it passes as sugar into the hepatic venous blood, and thereby is conveyed to the lungs to be further disposed of, Pavy inclines to believe that it is destined to enter the in- testinal canal through the biliary ducts, and then possibly Z 2 340 DIGESTION. to subserve some purpose in relation to the formation of fat. For the present we must remain uncertain as to which of these theories contains most truth in it. It is right, however, to remark that experiments made by Dr. Thudi- chum, and by Dr, Harley, with Dr. Sharpey's co-operation, tend to invalidate the conclusions to which Dr. Pavy has been led. Whatever be the destination of this peculiar amyloid substance formed at the liver, most recent observers agree tli at it is formed at, and exists within, the hepatic cells, from which it may be readily extracted by boiling, and then separated from the decoction by the addition of alcohol, which at once precipitates it. When isolated and purified it is said to be whitish, tasteless, soluble in water, and to possess the chemical qualities intermediate between those of hydrated starch and dextrin. Its composition is C I2 H IO O I0 . It manifests, as already observed, a remark- able tendency to pass into sugar in presence of any animal ferment, such, for example, as may be found in the blood, urine, saliva, or pancreatic secretion. Whether this be its natural destination is, however, still an open question. Much doubt exists also respecting the mode in which this glycogenic or amyloid substance is formed in the liver, and the materials which furnish its source. Since its quantity is increased after feeding, especially on substances containing much sugar or starch, it is probable that part of it is derived from saccharine principles absorbed from 'the digestive canal ; but since its formation continues even when there is no starch or sugar in the food, the albumin- ous or fatty principles also have been thought capable of furnishing part of it. Numerous experiments, however, having proved that the liver continues to form sugar in animals after prolonged starvation, and during hybernation, and even after death, its production is clearly independent of the elements of food. Oiie of Bernard's experiments FORMATION OF SUGAE IN THE LIVER. 341 may be quoted in proof of this : Having- fed a healthy dog for many days exclusively on flesh, he killed it, removed the liver at once, and before the contained blood could have coagulated, he thoroughly washed out its tissue by passing a stream .of cold water through the portal vein. He continued the injection until the liver was completely exsanguined, until the issuing water contained not a trace of sugar or albumen, and until no sugar was yielded by portions of the organ cut into slices and boiled in water. Having thus deprived the liver of all saccharine matter, he left it for twenty-four hours, and on then examining it, found in its tissue a lajge quantity of soluble sugar, which must clearly have been formed subsequently to the organ being washed, and out of some previously insoluble and non-saccharine substance. This and other experiments led him and others to the conclusion that the formation of the amyloid substance by the liver is the result of a kind of secretion or elaboration out of materials in the solid tissues of the gland such secretion being probably effected by the hepatic cells, in which, indeed, as already observed, the substance has been detected. According to this view, then, the liver may be regarded as an organ engaged in forming two kinds of secretion, namely, bile and sugar, or rather, the glycogenic substance readily convertible into sugar. The former, chiefly excre- mentitious, passes along the bile-ducts into the intestines, where it may subserve some purposes in relation to diges- tion, and is then for the most part re-absorbed, and ultimately eliminated during the processes concerned in the production of animal heat. The latter, namely sugar, being soluble, is, unless Pavy's view be correct, taken up by the blood in the hepatic vein, conveyed through the right side of the heart to the lungs, where it is probably consumed in the respiratory process, and thus contributes to the production of animal heat. The experiments of Lehmann led him to believe that the liver sugar is con- 342 DIGESTION. verted into lactic fluid before it is finally disposed of in respiration. The rapidity with which the sugar-forming process at the liver goes on seems to be influenced by various cir- cumstances; but, on the whole, Bernard's experiments appear to prove that it is directly proportioned to the rapidity of the portal circulation. Whatever expedites this, increases the quantity formed, and vice versa. Hence it is, probably, that irritation of various parts of the nervous system, especially of the sympathetic, causes an increased formation of sugar, by stimulating the portal circulation. Summary of the Changes which take place in tJie Food during its Passage through the Small Intestine. In 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 consti- tuents of the food taken, but are not yet in a state fit for absorption. The conversion of starch into sugar, which began in the mouth, has been interfered with, although not stopped altogether. The soluble matters both those which were so from the first, as sugar -and saline matter, and those which have been made so by the action of the saliva and gastric juice have begun to disappear by ab- sorption into the blood-vessels, and the same thing has befallen such fluids as may have been swallowed, wine, water, etc. The thin pultaceous chyme, therefore, which, during the DIGESTION IN SMALL INTESTINE. 343 whole period of gastric digestion, is being constantly squeezed or strained through the pyloric orifice into the duodenum, consists of albuminous matter, broken down, dissolving and half dissolved, fatty matter, broken down, but not dissolved at all, starch very slowly becoming 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 subjected to the influence of the fluid secreted by Lieber- kiihn's and Brunn's glands, before described, and to that of the bile and pancreatic juice, which are poured into this part of the intestine. Without doubt, that part of digestion which it is the chief duty of the small intestine to perform, is the altera- tion 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 process, however, allotted respectively to the bile, pancreatic secretion, and the secre- tion of the intestinal glands, is still uncertain. It is most probable, however, that the pancreatic secretion, and the bile are the main agents in emulsifying the fat, and that they do this by direct admixture with it. They also pro- mote its absorption by moistening the surface of the villi (p- 336). During digestion in the small intestine, the villi become turgid with blood, their epithelial cells become filled, by absorption, with fat globules, which, after minute division, transude into the granular basis of the villus, and thence into the lacteal vessel in the centre, by which they are conveyed along the mesentery to the lymphatic glands, and thence into the thoracic duct. The term chyle is sometimes applied to the emulsified contents of the intes- 344 DIGESTION. tine after their admixture with the bile and pancreatic juice ; but more strictly to the fluid contained in the lacteal vessels during digestion, which differs from ordinary lymph contained in the same vessels at other times, chiefly in the greatly increased quantity of fat particles which have been absorbed from the small intestine. Although the most evident function of the small intes- tine is the digestion of fat, it must not be forgotten that a great part of the other constituents of the food is by no means completely digested when it leaves the stomach. Indeed, its leaving it unabsorbed would, alone, be proof of this fact. The albuminous substances which have been partly dis- solved in the stomach continue to be acted on by the gastric juice which passes into the duodenum with them, and the effect of the last-named secretion is assisted or comple- mented by that of the pancreas and intestinal glands. As the albuminous matters are dissolved, they are absorbed chiefly by the blood-vessels, and only to a small extent, probably, by the lacteals. The starchy, or amylaceous portion of the food, the con- version of which into dextrin and sugar was more or less interrupted during its stay in the stomach, is now acted on briskly by the secretion of the pancreas, and of Brunn's glands, and perhaps of Lieberkiihn's glands also, and the sugar as it is formed dissolves in the intestinal fluids, and afterwards, like the albumen, is absorbed chiefly by the blood-vessels. The liquids, swallowed as such, 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 con- stant 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 DIGESTION IN LARGE INTESTINE. 345 of circulation constantly proceeding from the intestines into the blood, and from the blood into the intestines again ; for, as all the fluid, probably 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 secretion is again re- absorbed into the cur- rent of blood going into the blood charged with nutrient products of digestion, coming out again by secretion through the glands in a comparatively uncharged con- dition. It has been said before that the contents of the stomach during gastric digestion have a strongly acid reaction. On the entrance of the chyme into the small intestine, this is gradually neutralized to a greater or less degree by admixture with the bile and other secretions with which it is mixed, and the acid reaction becomes less and less strongly marked as the chyme passes along the canal towards the ileo-caecal valve. Thus, all the materials of the food are acted on in the small intestine, and a great portion of the nutrient matter is absorbed the fat chiefly by the lacteals, the other principles, when in a state of solution, chiefly by the blood- vessels, but neither, probably, exclusively by one set of vessels. 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. In this state it passes through the ileo-caacal opening into the large intestine. Summary of the Process of Digestion in the Large Intestine. The changes which take place in the chyme after its passage from the small into the large intestine 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 346 DIGESTION. 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 csecum, there can be no doubt that the absorption 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 the addition to them, in this region, of some special matter, probably excretory. The acid reaction, which had become less and less distinct in the small bowel, again becomes very manifest in the caecum probably from acid fermentation processes in some of the materials of the food. There seems no reason, however, to conclude that any special, ' secondary,' digestive process occurs in the caecum or in any other 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, and the addition of the special excretory products which give it the charac- teristic odour. At the same time, as before said, it is probable that a certain quantity of nutrient matter always escapes digestion in the small intestine, and that this happens more especially when food has been taken in excess, or when it is of such a kind as to be difficult of digestion. Under these circumstances there is no doubt that such changes as were proceeding in it at the lower part of the ileum may go on unchecked in the large bowel, the process being assisted by the secretion of the nume- rous tubular glands therein present. COMPOSITION OF FJECES. 347 By these means 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 faeces. After a sojourn of uncertain duration in the rectum, they are finally expelled by the contraction of its muscular coat, aided, under ordinary circumstances, by the contraction of the abdominal muscles. For a description of the mechanism by which the act of defsecation is accomplished, see p. 234. The average quantity of solid fsecal matter evacuated by the human adult in twenty-four hours is about five ounces ; an uncertain proportion of which consists simply of the undigested or chemically modified residue of the food and the remainder of certain matters, which are excreted in the intestinal canal. Composition of Fceces. Water Solids 733-00 267 'oo Special excrementitious constituents : Excretin, \ excretoleic acid (Marcet), and stercorin (Austin Flint). Salts : Chiefly phosphate of magnesia and phos- phate of lime, with small quantities of iron, soda, lime, and silica. Insoluble residue of the food (chiefly starch, ) 267 'oo grains, woody tissue, particles of cartilage, and fibrous tissue, undigested 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. / The time occupied by the journey of a given portion of food from the stomach to the anus, varies considerably even in health, and on this account, probably, it is that such 348 DIGESTION. different opinions have been expressed in regard to the subject. Dr. Brinton supposes twelve hours to be occupied by the journey of an ordinary meal through the small intes- tine, and twenty-four to thirty-six hours by the passage through the large bowel. On the Gases contained in the Stomach and Intestines. It need scarcely be remarked that, under ordinary cir- cumstances, the alimentary canal contains a considerable quantity of gaseous matter. Any one who has had occa- sion, 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 constant, 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, pro- bably, a mechanical one. The sources of the gas contained in the stomach and bowels may be thus enumerated : 1 . 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 contained in the alimentary canal, and those present in the blood of the gastric and in- testinal 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 GASES OF INTESTINAL CANAL. 349 be no doubt, however, that the intestines may be the proper excretory organs for many odorous and other sub- stances, 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. Tabular Analysis of Gases contained in the Alimentary Canal. Whence obtained. Composition by Volume. Oxygen. Nitrog. Carbon. Acid. Hydrog. Carburet. Hydrogen. Sulphuret. Hydrogen. Stomach . . . Small Intestine Caecum .... Colon .... Rectum . . . .. Expelled per .anum II 71 3 2 66 9 22 H 30 12 57 43 4* 1 6 19 II 19 | trace. k The above tabular analysis of the gases contained in the alimentary canal has been quoted from the analyses of Jurine, Magendie, Marchand, and Chevreul by Dr. Brinton, from whose work the above enumeration of the sources of the gas has been also taken. 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 fresh portions of intestinal 35 DIGESTION. secretion, and as slowly exposed to the absorbent power of all the villi and blood-vessels of the mucous mem- brane. The movement of the intestines is peristaltic or vermicular, and is effected by the alternate con- tractions 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 longi- tudinal 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 actions next ensues. These movements take place slowly, and, in health, are commonly unperceived by the mind ; but they are perceptible when they are accelerated under the in- fluence of any irritant. The movements of the intestines are sometimes retro- grade ; and there is no hindrance to the backward move- ment 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 intestine by the ileo- ceecal valve. Besides, the orifice of communication between the ileum and caecum (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 proportion 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 MOVEMENTS OF THE INTESTINES. 351 external sphincter muscle with striped fibres is placed rather lower down, and more externally, and holds the orifice close by a constant slight contraction under the influence of the spinal cord. The peculiar condition of the sphincter, in relation to the nervous system, will be again referred to. The remaining portion of the intestinal canal is under the direct influence of the sympathetic or ganglionic system, and, indirectly, or more distantly, is subject to the influence of the brain and spinal cord, which influence appears to be, in some degree, transmitted through the vagus nerve. Experimental irri- tation of the brain or cord produces no evident or constant effect on the movements of the intestines during life ; yet in consequence of certain conditions of the mind, the movements are accelerated or retarded ; and in paraplegia the intestines appear after a time much weakened in their power, and costiveness, with 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 rich ganglionic structure shown by Meissner to exist in the sub -mucous tissue. This regulates and controls the movements and gives to them their peculiar slow, orderly, rhythmic, and peristaltic character, both naturally, and when artificially excited. 35 2 i CHAPTER X. ABSORPTION". THE process of absorption has, for one of its objects, the introduction 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 taking away of parts of the body itself, when, having fulfilled their office, or otherwise requiring removal, they need to be renewed. In both these offices, i.e., in both absorption from without and absorption from within, the process manifests some variety, and a very wide range of action ; and in both it is probable that two sets of vessels are, or may be, concerned, namely, the blood-vessels, and the lacteals or lymphatics, to which the term absorbents has been especially applied. Structure and Office of the Lacteal and Lymphatic Vessels and Glands. Besides the system of arteries and veins, with their inter- mediate vessels, the capillaries, there is another system of canals in man and other vertebrata, called the lymphatic system. Both these systems of vessels are concerned in absorption. The vessels of the lymphatic system are, in structure and general appearance, like very small and thin-walled veins, and like them are provided with valves. By one extremity they commence by fine microscopic branches in the organs and tissues of nearly every part of the body, and by their other extremities they end directly or indirectly in two trunks which open into the large veins near the heart (fig. 90). Their contents, the lymph and chyle, unlike the blood, pass only in one direction, namely, from the fine branches to COURSE OF THE LYMPHATICS. 353 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 the course of the fluid in the Fig. 90.* Lvmphatics of head and neck, right. Right internal jug- ular vein. Right subclaviau vein. Lymphatics of right arm. Receptaculuin chyli. Lymphatics of lower 'extremities. Lymphatics of head and neck, left. Tioracic duct. Left subclavian vein Thoracic duct. Lymphatics of lower extremities. lymphatic vessels, viz., its passage in the direction only towards the large veins in the neighbourhood of the heart, it will be readily seen from fig. go that the greater part of the contents of the lymphatic system of vessels passes through a comparatively large trunk called the thoracic * Fig. 90. Diagram of the piincipal groups of lymphatic vessse (from Quain). A A 354 ABSORPTION. duct, which finally empties its contents into the blood- stream at the junction of the internal jugular and sub- clavian veins of the left side. There is a smaller duct on the right side. In some part of their course all lymphatic vessels pass through certain bodies called lymphatic glands. The lymphatic vessels of the intestinal canal are called lacteals, because, during digestion, the fluid contained in them resembles milk in appearance ; and the lymph in the lacteals during the period of digestion is called chyle. There is no essential distinction, however, between lacteals and lymphatics.. Lymphatic vessels are distributed in nearly all parts of the body. Their existence, however, has not yet been determined in the placenta, the umbilical cord, the mem- branes of the ovum, or in any of the non-vascular parts, as the nails, cuticle, hair, and the like. The lymphatics and lacteals commence most commonly either (i), in closely-meshed networks, or (2), in irregular lacunar spaces between the various structures of which the different organs are composed. The former is the rule of origin with those lymphatics which are placed most super- ficially, as, for instance, immediately beneath the skin, or under the mucous and serous membranes ; while the latter is most common with those which arise in the substance of organs. In the latter instance the small irregular channels and spaces from which the lymphatics take their origin, although they are formed merely by the chinks and cran- nies between the blood-vessels, secreting ducts, and other parts which may happen to form the framework of the organ in which they exist, yet have a layer of epithelial cells to define and bound them, the cells resembling those lining the lymphatic vessels.* By some, the lymphatics and lacteals are believed to * See Dr. Sharpey's Article, " Lymphatic System," in Quain's Anatomy, 7th edition. OEIGIN OF LYMPHATICS. 355 arise (3), by communications with connective-tissue cor- puscles (p. 46) ; while the lacteals offer an illustration Fig. 91.* * Fig. 91. Lymphatic vessels of the head and neck of the upper part of the trunk (from Mascagni). ^. 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 i, and ahove 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, are also shown. A A 2 356 ABSOEPTION. of another mode of origin, namely (4), in blind dilated ex- tremities (figs. 80, 8l), unless, indeed, the views to which reference has been made, p. 367, be correct, and in that Fig. 92.* Fig. 93 t. * Fig. 92. Superficial lymphatics of the forearm and palm of the hand, 1 (after Mascagni). 5. Two small glands at the bend of the arm. 6. Eadial lymphatic vessels. 7. Ulnar lymphatic vessels. 8, 8. Palmar arch of lymphatics. 9, 9'. Outer and inner sets of vessels, b. Cephalic STRUCTURE OF LYMPHATIC VESSELS. 357 case, yet unproved, the latter mode of origin does not occur. In structure, tlie lymphatic and lacteal vessels are very like veins ; having, according to Kolliker, an external coat of fibre-cellular tissue, with elastic filaments ; within this, a thin layer of fibro-cellular tissue, with organic muscular fibres, which have, principally, a circular direc- tion, 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 are attached, give them a peculiar braided or knotted ap- pearance (fig. 95). "With the help of the valvular mechanism, all occasional pressure on the exterior of the lymphatic and lacteal ves- sels 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 (seep. I Si). The actions of the muscular fibres of the small intestine, and probably the layer of organic muscle present in each intestinal villus (p. 315), seem to assist in propelling the chyle : for, in the small intestine of a mouse, Poiseuille saw the chyle moving with intermittent propulsions 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 the vis a tergo vein. d. Radial vein. e. Median vein. /. Ulnar vein. The lymph- atics are represented as lying on the deep fascia. t Fig. 93. Superficial lymphatics of right groin and upper part of thigh, (after Mascagni). I. Upper inguinal glands. 2'. Lower inguinal or femoral glands. 3, 3. Plexus of lymphatics in the course of the long saphenous vein. 358 ABSORPTION. resulting from absorption (as shown in the ascent of sap in a tree), and from external pressure, some of the force may be derived from the contractility of the vessel's own walls. Kolliker, after watching the lymphatic in the transparent tail of the tadpole, states that no distinct movements of their walls can ever be seen, but as they are emptied after death they gradually contract, and then, after some time, again dilate to their former size, exactly as the small arteries do under the like circumstances. Thus, also, the larger vessels in the human subject commonly empty themselves after death; so that, although absorption is probably usually going on just before the time of death, it is not common to see the lymphatic or lacteal vessels full. Their power of contraction under the influence of stimuli has been demonstrated by Kolliker, who applied the wire of an electro-magnetic apparatus to some well-filled lymphatics on the skin of a boy's foot, just after the re- moval of his leg by amputation, and noticed that the calibre of the vessels diminished at least one half. It is most probable that this contraction of the vessels occurs during life, and that it consists, not in peristaltic or undu- latory movements, but in an uniform contraction of the successive portions of \he vessels, by which pressure is steadily exercised upon their contents, and which alternates with their relaxation. Lymphatic Glands. Almost all lymphatic and lacteal vessels in some part of their course pass through one or more small bodies called lymphatic glands (fig. 94). These are something more than mere plexuses of the vessels. Each gland has an investing capsule of connective tissue, from which prolongations dip into its substance forming partitions. Immediately within the investing capsule occurs what is named the cortical portion of the gland (fig. 94, c), and immediately within this the medullary LYMPHATIC GLANDS. 359 substance (fig. 94, 6). The cortical portion consists of a spongy alveolar tissue formed of septa of fibrous tissue traversed by minute blood-vessels. The la- cunse or spaces in this tissue freely communi- cate with each other, and are filled with fluid containing nuclei and cells. They constitute, indeed, the direct con- tinuations of the afferent lymphatic vessels which enter the gland, and, after pene- trating the capsule, lose their proper coats, and open out into these spaces in the cortical part of the gland. After- wards they acquire fresh coverings, pass into the central or medullary part of the gland, where they form a dense plexus of vessels embedded in a stroma of connective tissue ; and then unite into one or more efferent vessels (fig. 95), which, on issuing from the gland, receive an external coat, and proceed on their way towards the main lymphatic duct, in which they end. By the peculiar arrangement of the lymphatic vessels at the cortex of the glands, their contents are freely brought into relation, in the lacunae, with the capillary blood-vessels spread over the septa of these spaces, and a mutual interchange of materials and influence is thus allowed. The main result of this interchange appears to be the development of lymph-corpuscles, for it is well known that the amount of cell-contents of lymph and chyle beyond the gland is far greater than it was previously. An increase of fibrin also occurs as a consequence of the transit through the glands. * Fig. 94 (after Kolliker). Section of a mesenteric gland from the ox, slightly magnified, a, hilus ; b (in the central part of the figure), medullary substance ; c, cortical substance with indistinct alveoli ; d, capsule. 360 ABSORPTION. It has been supposed, that the lymphatics, at their origin # and in the substance of absorbent glands, communicate directly with the blood- vessels ; but there is not sufficient evi- dence for believing that this is ever the case in Mammalia and birds, although it may be so in Amphibia and fish. In man and Mammalia, the lymphatics and blood-vessels are directly connected only by the principal lymphatic trunk, the thoracic duct, which opens into the junction of the left internal jugular and subclavian veins, and by a corresponding but smaller trunk, which pours its con- tents into the corresponding part on the right side. Properties of Lymph and Chyle. The fluid contained in the lymphatic vessels or lymph, is, under ordinary circumstances, clear, transparent, and colourless, or of a pale yellow tint. 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, the lymph usually contains no corpuscles or particles of any kind ; and it is probably only in the larger trunks in which, by a process similar to that to be described in the chyle, the lymph is more elaborated, that any corpuscles are formed. These corpuscles are similar to those in the chyle, but less numerous. The fluid in which the cor- puscles float is commonly and in health albuminous, and * Fig. 95. A lymphatic gland from the axilla, with its afferent and efferent vessels, injected with mercury (after Bendz). CHYLE. 361 contains no fatty particles or molecular base ; but it is liable to variations according to the general state of the blood, and that of the organ from which the lymph is derived. As it advances towards the] thoracic duct, and passes through the lymphatic glands, it becomes, like chyle, spontaneously coagulable from the formation of fibrin. The fluid contained in the lacleals, or lymphatic vessels of the intestines, is clear and transparent during fasting, and differs in no respect from ordinary lymph; but during digestion, it becomes milky, and acquires the other charac- ters of chyle. Chyle is an opaque, whitish fluid, resembling milk in appearance, and having a neutral or slightly alkaline re- action. Its whiteness and opacity are due to the presence of innumerable particles of oily or fatty matter, of exceed- ingly minute though nearly uniform size, measuring on the average about 7T -L_ ff . of an inch (Gulliver). These con- stitute what Mr. Gulliver appropriately terms 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. Hence, as a rule, the chyle is whitest and most turbid in carnivorous animals ; less so in Herbivora; while in birds it is usually transparent. The fatty nature of the molecules is made manifest by their solubility in ether, and, when the ether evaporates, by their being deposited in various-sized drops of oil.* Yet, since they do not run together and form a larger drop, as particles of oil would, it appears very probable that each molecule consists of oil coated over with albu- men, in the manner in which, as Ascherson observed, oil always becomes covered when set free in minute drops in an albuminous solution. And this view is supported by the fact, that when water or dilute acetic acid is added to * Some of the molecules may remain undissolved by the ether : but tliis appears to be due to their being defended from the action of the ether by being entangled within the albumen which it coagulates. 3^2 ABSORPTION. chyle, many of the molecules are lost sight of, and oil- drops appear in their place, as if the investments of the molecules had been dissolved, and their oily contents had run together. Except these molecules, the chyle taken from the villi or from lacteals near them, contains no other solid or organized bodies. The fluid in which the molecules float is albuminous, and does not spontaneously coagulate, though coagulable by the addition of ether. But as the chyle passes on towards the thoracic duct, and especially while it traverses one or more of the mesenteric glands (propelled by forces which have been described with the structure of the vessels), it is elaborated. The quantity of molecules and oily particles gradually diminishes ; cells, to which the name of chyle-corpuscles is given, are developed in it ; and by the formation of fibrin, it acquires the pro- perty of coagulating spontaneously. The higher in the thoracic duct the chyle advances, the more is it, in all these respects, developed; the greater is the number of chyle -corpuscles, 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 (Bouisson). The existence of fibrin, or of the materials which, by their union, form it (p. 73 et seq.), is, therefore, certain; its increase appears to be commensurate with that of the corpuscles; and, like them, it is not absorbed as such from the chyme (for no fibrin exists in the chyle in the villi), but is gradually elaborated out of the albumen which chyle, in its earliest condition, contains. The structure of the chyle-corpuscles was described COMPOSITION OF LYMPH AND CHYLE. 363 when speaking of the white or rudimentary corpuscles of the blood, with which they are identical (see pp. 85 and 103). Their mode of origin, probably in the lympjiatic glands, is obscure. From what has been said, it will appear that perfect chyle and lymph are, in essential characters, nearly similar, and scarcely differ except in the preponderance of fatty matter in the chyle. The comparative analysis of the two fluids obtained from the lacteals and the lymphatics of a donkey, is thus given by Dr. Owen Rees. Water Albumen. . Fibrin .. Animal extractive Fatty matter . Salts lOO'OOO Lymph. 96-536 1-200 0-120 1-559 a trace. 0-585 lOO'OOO The analyses of Nasse afford an estimate of the rela- tive compositions of the lymph, chyle, and blood of the horse.* "Water .... Corpuscles ... Albumen. ... Fibrin Extractive matter . . Fatty matter ... Alkaline salts ... Phosphate of lime and magne- sia, oxide of iron, etc. Lymph. Chyle. Blood. 950- 935" 810- 4* 92*8 39-11 31' 80- 075 2-8 4-88 6-25 5'2 0-09 15- i-55 5-61 r 6-7 0-31 i* '95 1000' * The analysis of the blood differs rather widely from that given at page 78 ; but if it be erroneous, it is probable that corresponding errors exist in the analysis of the lymph and chyle ; and that therefore the tables in the text may represent accurately enough the relation in which the three fluids stand to each other. ABSOKPTIOK The contents of the thoracic duct, including both the lymph and chyle mixed, in an executed criminal, were examined by Dr. Rees, who foimd them to consist of: Water 90-48 Albumen and fibrin 7-08 Extractive matter . . . . . . o - io8 Fatty 0-92 Saline ,, 0-44 From all the analyses of lymph and chyle, it appears that they contain essentially the same organic constituents that are found in the blood, viz., albumen, fibrin, and fatty matter, the same saline substances, and iron. Their com- position differs from that of the blood in degree rather than in kind; they contain a less proportion of all the substances dissolved in the water (see Nasse's analyses, just quoted), and much less fibrin. The fibrin* of lymph, besides being less in quantity, appears to be in a less elaborated state than that of the blood, coagulating less rapidly and less firmly. According to Virchow, it never coagulates, under ordinary circumstances, within the lymphatic vessels, either during life or after death. These differences gradually diminish, while the lymph and chyle, passing towards and through the thoracic duct, gradually approach the place at which they are to be mingled with the blood. For, in the thoracic duct, besides the higher and more abundant development of the fibrin, the lymph and chyle-corpuscles are found more advanced towards their development into red blood-corpuscles; sometimes even that development is completed, and the lymph has a pinkish tinge from the number of red blood-corpuscles that it contains. The general result, therefore, of both the microscopic and the chemical examination of the lymph and chyle, demonstrate that they are rudimental blood; their fluid * For observations on the nature of fibrin, see p. 73. QUANTITY OF LYMPH AND CHYLE. 365 part being, like the liquor sanguinis, diluted, but gradually becoming more concentrated; and their corpuscles being in process of development into red blood-corpuscles. Thus, in quality, the lymph and chyle are adapted to replenish the blood ; and their quantity, so far as it can be estimated, appears ample for this purpose. In one of Magendie's experiments, half an ounce of chyle was collected in five minutes, from the thoracic duct of a middle-sized dog; Collard de Martigny obtained nine grains of lymph, in ten minutes, from the thoracic duct of a rabbit which had taken no food for twenty -four hours; and Gieger, from three to five pounds of lymph daily from the foot of a horse, from whom the same quantity had been flowing several years without injury to health. Bidder found, on opening the thoracic duct in cats, immediately after death, that the mingled lymph and chyle continued to flow from one to six minutes ; and, from the quantity thus obtained, he estimated that if the contents of the thoracic duct con- tinued to move at the same rate, the quantity which would pass into a cat's blood in twenty-four hours would 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 : 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. Schmidt's observations on foals have yielded very similar results. By another series of experiments, Bidder estimated that the quantity of lymph traversing the thoracic duct of a dog in twenty-four hours, is about equal to two-thirds of the blood in the body. If we take these estimates, it will not follow from them that the whole of an animal's blood is daily replaced by the development of lymph and chyle; for even if the quantity of lymph and chyle daily formed be equal to that of the blood, the solid contents of the blood will be much too great to be replaced by those of the lymph and chyle. According to 366 ABSOEPTION. Nasse's analyses, the solid matter of a given quantity of blood could not be replaced out of less than three or four times the quantity of lymph and chyle. Absorption by the Lacteal Vessels. During the passage of the chyme along the whole tract of the intestinal canal, its completely digested parts are absorbed by the blood-vessels and lacteals distributed in the mucous membrane. The blood-vessels appear to absorb none but the dissolved portions of the food, and these, including especially the albuminous and saccharine, they imbibe without choice; whatever can mix with the blood passes into the vessels, as will be presently described. But the lacteals appear to absorb only certain constituents of the 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 intes- tine; for in these minute processes, both the capillary blood-vessels and the lacteals are brought almost into contact with the intestinal contents. It has been already stated that the villi of the small intestine (figs. 80 and 81) are minute vascular processes of mucous membrane, each containing a delicate net- work of blood-vessels and one or more lacteals, and are invested by a sheath of cylindrical epithelium. In the interspaces of the mucous membrane between the villi, as well as over all the rest of the intestinal canal, the lacteals and blood-vessels are also densely distributed in a close net- work, the lacteals, however, being more sparingly supplied to the large than to the small intestine. It has long been, and is still, very difficult to explain how absorption by the lacteals is effected. Various theories on the subject have been promulgated, but there is still much doubt whether any correct interpretation has yet been suggested. Probably the cause of the confusion in ABSORPTION BY LYMPHATICS. 367 which this subject is at present involved is due rather to imperfect knowledge of the laws which regulate absorp- tion as it occurs in the living body, than to ignorance of the exact microscopic structure of the tissues concerned ; although, even in the latter respect, there is much differ- ence of opinion, and therefore error. There seems to be no doubt that absorption of fatty matters during digestion, from the contents of the intestines, is effected by the epithelial cells which line the intestinal tract, and mainly by those which clothe the surface of the villi, (fig. 80). From these epithelial cells, again, the fatty particles are passed on into the interior of the lacteal vessels (figs. 80 and Si), but how they pass, and what laws govern their so doing,' we know not at all. Heidenhain believes that the deeper ends of the epithelial cells are continuous by fine processes with prolongations from con- nective-tissue corpuscles in the interior of the villus ; while he supposes other prolongations from the same corpuscles to be directly continuous with the fine terminations or rather beginnings of the lacteal vessels. This view deserves consideration if only on account of its ingenuity, but requires confirmation. The opinion before referred to, that the epithelial cells exercise a certain selective power over the materials which they absorb, is supported by various circumstances, which will be considered under the head of absorption by the blood-vessels. Absorption by the Lymphatic Vessels. The real source of the lymph, and the mode in which its absorption is effected by the lymphatic vessels, are still among the enigmas of physiology. It may, however, be held with tolerable certainty, that the lymph, like the chyle, is chiefly of a nutritive nature, capable of a higher organization, and of contributing to the nutrition of the body. Whether it is derived exclusively from the liquor 368 ABSORPTION. sanguinis effused for the. nutrition of the tissues, or from the fluid with which the tissues are kept moist, or, in part also, from degenerated or used portions of the tissues, cannot yet with certainty be determined. 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- organized and adapted to the nutrition of the same or some lower tissue; and these may be absorbed by the lymphatics. On the whole, however, it is most probable that the lymph is derived in great part, from the liquor sanguinis; since changes in the character of the former usually correspond very closely with changes in the cha- racter of either the whole mass of blood, or of that in the vessels of the part from which the lymph is examined. Thus Herbst found that the coagulability of the lymph is directly proportionate to that of the blood ; and that when fluids are injected into the blood-vessels in sufficient quan- tity to distend them, the injected substance may be almost directly afterwards found in the lymphatics. Lymph- Hearts. In reptiles and some birds, an important auxiliary to the movement of the lymph and chyle is sup- plied in certain muscular sacs, named lymph-hearts (fig. 96), and Mr. Wharton Jones has lately 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 four, 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 lymphatics 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 LYMPHATIC HEARTS. 369 branch of the ischiatic vein ; by the superior, the lymph is forced into a branch of the pi jugular vein, which issues from its anterior surface, and which becomes turgid each time that the sac contracts. Blood is pre- vented 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 can only be dis- covered by means of the micro- scope ; but in every case it is composed of transversely- fcnated fibres. The contractions of the hearts are rhyth- mical, 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 cer- imited portion of the spinal cord. For Volkmann 3und that so long as the portion of spinal cord correspond- ing 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 was destroyed ; while destruction of this portion, even though aU other * Fig. 96. Lymphatic heart (9 lines long, 4 lines broad), of a L-n-e species of serpent, the Python bivittatus ; (after E. Weber). 4 The external cellular coat. 5. The thick muscular coat. Four muscular columns run across its cavity, which communicates with three lympha- cs (i-only one is seen here), with two veins (2, 2). 6 The smooth 6 f the "^ 7 " A SmaU W&* -luriclT h is continuous with that of the rest of the organ. B B 370 ABSORPTION. 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. Absorption by Blood-vessels. The process thus named is that which has been com- monly called absorption by the veins; but the term here employed seems preferable, since, though the materials absorbed are commonly found in the veins, this is only because they are carried into them with the circulating blood, after being absorbed by all the blood-vessels (but chiefly by the capillaries) with which they were placed in contact. There is nothing in the mode of absorption by blood-vessels, or in the structure of veins, which can make the latter more active than arteries of the same size, or so active as the capillaries, in the process. In the absorption by the lymphatics or lacteal vessels just described, there appears something like the exercise of choice in the materials admitted into them; for the chyle and lymph have a nearly constant composition, and we must admit, as a hypothesis, either that these vessels are so constructed that only certain materials, capable of being assimilated to their proper contents, can traverse the walls, or else that the materials from which the perfect chyle and lymph are to be developed, are secreted into the lacteals and lymphatics from the adjacent blood-vessels. In either hypothesis, we assume something which brings the absorption by lacteals and lymphatics into the category of vital processes. But the absorption by blood-vessels presents no such appearance of selection of materials; rather, it appears, that every substance, whether gaseous, OSMOSIS. 37i 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 solely by their physical or chemical properties and conditions, and by those of the blood and the walls of the blood-vessels. The phenomena are, indeed, exactly comparable to that passage of fluids through membrane, which occurs quite independently 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 f{g t 97. a graduated tube expanded into an open- mouthed bell at one end, over which a por- tion of membrane is tied (fig. 97) . If now the bell be filled with the solution of a salt say chloride of sodium, 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 will pass into the solution much more rapidly than the salt will pass out into the water, and the diluted solution 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 sub- jected to experiment, have an important influence, as might be anticipated, on the rapidity and duration 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 membrane of caoutchouc, the alcohol, from B B 2 372 ABSORPTION. its greater affinity for this substance, would pass freely into the water. Various opinions have been advanced in regard to the nature of the force by which fluids of different chemical composition thus tend to mix through an intervening membrane. According to some, this power is the result of the different degrees of capillary attraction exerted by the pores of the membrane upon the two fluids. Prof. Graham, however, believes that the passage or osmose of water through membrane may be explained by supposing that it combines with the membranous septum, which thus becomes hydrated, and that on reaching the other side it partly leaves the membrane, which thus becomes to a certain degree de-hydrated. For example, a membrane such as that used in the endosmometer, is hydrated to a higher degree if placed in pure water than in a neutral saline solution. Hence, in the case of the endosmometer filled with the saline solution and placed in water, the equilibrium of hydration is different on the two sides ; the outer surface being in contact with pure water tends to hydrate itself in a higher degree than the inner surface does. " When the full hydration of the outer surface ex- tends through the thickness of the membrane, and reaches the inner surface, it there receives a check. The degree of hydration is lowered, and water must be given up by the inner layer of the membrane." Thus the osmose or current of water through the membrane is caused. The passage outwards of the saline solution, on the other hand, is not due, probably, to any actual fluid current ; but to a solution of the salt in successive layers of the water con- tained in the pores of the membrane, until it reaches the outer surface and diffuses in the water there situate. Thus, " the water movement in osmose is an affair of hydration and of de-hydration in the substance of the membrane or other colloid septum, and the diffusion of the saline solution placed within the osmometer has little or COLLOIDS AND CRYSTALLOIDS. 373 nothing to do with the osmotic result, otherwise than as it affects the state of hydration of the septum." Prof. Graham has classed various substances according to the degree in which they possess this property of passing when in a state of solution in water, through membrane ; those which pass freely being termed crystalloids, and those which pass with difficulty, colloids. This distinction, however, between colloids and crystal- loids 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 crystal- line form, are characterised by their inertness as acids or bases, and feebleness in all ordinary chemical relations. Examples of them are found in albumen, gelatin, starch, hydrated alumina, and hydrated silicic acid, etc. ; while the crystalloids are characterised by qualities the reverse of those just mentioned as belonging to colloids. Alcohol, sugar, and ordinary saline substances are examples of crystalloids. Absorption by blood-vessels is the consequence of their walls being, like the membranous septum of the endos- mometer, porous and capable of imbibing fluids, and of the blood being so composed that most fluids will mingle with it. 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 an 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 which chemistry would detect some of the albumen and other parts of the liquor sanguinis) will spread more widely 374 ABSORPTION. 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. Even so simple an experiment will illustrate the ab- sorption by blood-vessels during life ; the process it shows is imitated, but with these differences : that, during life, as soon as water or any other substance is admitted into the blood, it is carried from the place at which it was absorbed into the general current of the circulation, and that the colouring matter of the blood is not dissolved so as to ooze out of the blood-vessels into the fluid which they are absorbing. The absorption of gases by the blood may be thus simply imitated. If venous blood be suspended in a moist bladder in the air, its surface will be reddened by the contact of oxygen, which is first dissolved in the fluid that moistens the bladder, and is then carried in the fluid to the surface of the blood : while, on the other hand, watery vapour and carbonic acid will pass through the membrane, and be exhaled into the air. In all these cases alike there is a mutual interchange between the substances ; while the blood is receiving water, it is giving out its colouring matter and other constituents : or, while it is receiving oxygen, it is giving out carbonic acid and water ; so that, at the end of the experiment, the two substances employed in it are mixed : and if, instead of a piece of tissue, one had taken a single blood-vessel full of blood, and placed it in the water, both blood and water would, after a time, have been found both inside and outside the vessel. In such a case, moreover, if one were to determine accurately the quantity of water that passed to the blood, and of blood that passed to the water, it would be found that the former was always greater than the latter. And so with other substances ; it almost alway ABSORPTION BY BLOOD-VESSELS. ' 375 happens, that if the two fluids placed on opposite sides of a membrane be of different densities or specific gravities, a larger quantity of the lees dense fluid passes into the more dense, than of the latter into the former. While the question was being discussed, whether ab- sorption (using the term generally) were effected by the lymphatics or the veins, many experiments were formed to demonstrate the fact of absorption by the blood-vessels, which may be quoted, not only as evidence for that fact, but in illustration of the difference between the absorption by lymphatics and that by blood-vessels, in regard to the materials they severally receive and convey into the cir- culation. Various odorous and saline matters taken with the food, or injected into the intestines of an animal, are soon found in the blood of the vena portae, or other blood-vessels, or in the urine, but are not found in the chyle ; or, if found there, not till they may have passed into the lacteals from their blood-vessels. This is shown by numerous experi- ments, especially by those of Tiedemann and Gmelin, and Panizza. The substances used in the experiments were ferrocyanide of potassium, sulphate of potash, several salts of lead, iron, and other metals, indigo, madder, rhubarb, camphor, musk, alcohol, turpentine, etc. Mayer also, when he injected ferrocyanide of potassium into the lungs, found it in the left side of the heart sooner than in the right ; showing that it had taken the course of the blood, not of the lymph, which would have carried it to the right side of the heart first. All these substances, therefore, appear to be absorbed by blood-vessels exclusively. Again, if any of these substances be included within a portion of an animal's intestine tied at both ends, and if all the vessels of that portion of the intestine be cut away, except its artery and vein, the substances being absorbed will be found in the blood of the vein ; but, if the main arteries and veins be tied, and the lacteals left entire, the 376 ABSORPTION. same substances will not be found in them. So with, poisons, such as opium and strychnia, in the experiments of Magendie and Segalas. When one of these poisons was put into a piece of intestine, of which the lacteals were tied, but the blood-vessels were free, poisoning took place within six minutes after returning the intestine into the abdomen ; but if the vein or veins of the piece of intestine were tied, so as to stop the circulation of blood, the effects of the poison were delayed for an hour or more, though the lacteals were free to absorb and carry it to the blood. The numerous experiments, proving that poisons are not absorbed, or only very slowly, after insertion into the hinder extremities of animals in which the aorta or vena cava inferior is tied, tend to the same conclusion, that these are among the substances not absorbed by^ the lymphatic or lacteal vessels, but absorbed without choice by the blood-vessels. 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, may be gathered from the history of some experiments by Dr. Bence Jones. From these it appears that even in a quarter of an hour after being given on an empty stomach, chloride of lithium 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. 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. Carbonate of lithia, when taken in five or ten grain doses on an empty stomach, may be detected in the urine in 5 or IO minutes ; or, if the stomach be full at the time of taking the dose, in 2O minutes. It may sometimes be detected in the urine, moreover, for six, seven, or even eight days. Some experiments on the absorption of various mineral and vegetable poisons, by Mr. Savory, have brought to light the singular fact that, in some cases, absorption ABSORPTION BY BLOOD-VESSELS. 377 takes place more rapidly from the rectum than from the stomach. Strychnia, for example, when in solution, pro- duces its poisonous effects much more speedily when intro- duced 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 that of the latter. It is probably a general truth, that in parts which are supplied with both blood-vessels and lymphatics, the lym- phatics (or lacteals for the intestines) absorb only certain materials for the replenishing of the blood, while the blood-vessels absorb not only nutrient matters, but all other soluble materials that are accidentally brought into contact with them. But in parts which receive only blood- vessels, these alone must perform the whole function of absorption, as they do in invertebrate animals. 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; for, naturally, the blood-vessels are not bare to absorb. 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 Miiller found the urinary bladder of a frog traversed in less than a second; and the absorption of poisons by the stomach or lungs appears sometimes accomplished in an immeasurably small time. 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 in contact. Hence the marks of tattooing, and the discoloration produced by nitrate of silver taken internally, remain. Mercury may 378 ABSORPTION. 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 (Oesterlen) ; and such substances as exceedingly finely-divided charcoal, when taken into the alimentary canal, have been found in the mesenteric veins (Oesterlen) ; the insoluble materials of ointments may also be rubbed into the blood-vessels; but there are no facts to determine how these various substances effect their pas- sage. Oil, minutely divided, as in an emulsion, will pass slowly into blood vessels, as it will through a filter moistened with water (Vogel). As in the experiments before referred to, 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. The absorption is the less rapid the fuller and tenser the blood-vessels are; and the tension may be so great as to hinder altogether the entrance of more fluid. Thus, Magendie found that when he injected water into a dog's veins to repletion, poison was absorbed very slowly; but when he diminished the tension of the vessels by bleeding, the poison acted quickly. So when cupping glasses are placed over a poisoned wound, they retard the absorption of the poison, not only by diminishing the velocity of the circulation in the part, but by filling all its vessels too full to admit more. 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 constantly as fit as at the first for the reception of the substance to be absorbed. 379 CHAPTER XI. NUTRITION AND GROWTH. NUTRITION or nutritive assimilation is that modification of the formative process peculiar to living bodies by which tissues and organs already formed maintain their integrity. By the incorporation of fresh nutritive principles into their substance, the loss consequent on the waste and natural decay of the component particles of the tissues is repaired ; and each elementary particle seems to have the power not only of attracting materials from the blood, but of causing them to assume its structure, and participate in its vital properties. The relations between development, growth, and simple nutrition or maintenance, have been already stated (Chap. I. and IL); under the head of NUTRITION will be now considered the process by which parts are maintained in the same general conditions of form, size, and composition, which they have already, by development and growth, attained ; and this, notwithstanding, but rather by means of, continual changes in their component particles. It is by this process that an adult person, in health, is main- tained, through a series of some years, with the same general outline of features, the same size and foi*m, and perhaps even the same weight ; although, during all this time, the several portions of his body are continually changing : their particles decaying and being removed, and then replaced by the formation of new ones, which, in their turn, also die and pass away. Neither is it only a general similarity of the whole body which is thus main- tained. Every organ or part of the body, as much as the whole, exactly maintains its form and composition, as 380 NUTRITION. the issue of the changes continually taking place among its particles. The change of component particles, in which the nutri- tion of organs consists, is most evidently shown when, in growth, they maintain their form and other general charac- ters, but increase in size. When, for example, a long bone increases in circumference, and in the thickness of its walls, while, at the same time, its medullary cavity enlarges, it can only be by the addition of materials to its exterior, and a coincident removal of them from the interior of its wall ; and so it must be with the growth of even the minutest portions of a tissue. And that a similar change of particles takes place, even while parts retain a perfect uniformity, may be proved, if it can be shown that all the parts of the body are subject to waste and impair- ment. In many parts, the removal of particles is evident. Thus, as will be shown when speaking of secretion, the elementary structures composing glands are the parts of which the secretions are composed : each gland is con- stantly casting off its cells, or their contents, in the secretion which it forms : yet, each gland maintains its size and proper composition, because for every cell cast off a new one is produced. So also the epidermis and all such tissues are maintained. In the muscles, it seems nearly certain, that each act of contraction is accompanied with a change in the composition of the contracting tissue, although the change from this cause is less rapid and extensive than was once supposed. Thence, the develop- ment of heat in acting muscles, and thence the discharge of urea, carbonic acid and water the ordinary products of the decomposition of the animal tissues which fol- lows all active muscular exercise. Indeed, the researches of Helmholtz almost demonstrate the chemical change that muscles undergo after long-repeated contractions; yet the muscles retain their structure and composition, NUTRITION. 381 because the particles thus changed are replaced by new ones resembling those which preceded them. So again, the increase of alkaline phosphates discharged with the urine after great mental exertion, seems to prove that the various acts of the nervous system are attended with change in the composition of the nervous tissue ; yet the condition of that tissue is maintained. In short, for every tissue there is sufficient evidence of impairment in the discharge of its functions : without such change, the pro- duction or resistance of physical force is hardly conceivable : and the proof as well as the purpose of the nutritive pro- cess appears in the repair or replacement of the changed particles ; so that, notwithstanding its losses, each tissue is maintained unchanged. But besides the impairment and change of composition to which all parts are subject in the discharge of their natural functions, an amount of impairment which will be in direct proportion to their activity, they are all liable to decay and degeneration of their particles, even while their natural actions are not called forth. It may be proved, as Dr. Carpenter first clearly showed, that every particle of the body is formed for a certain period of existence in the ordinary condition of active life ; at the end of which period, if not previously destroyed by outward force or exercise, it degenerates and is absorbed, or dies and is cast out. The simplest examples that can be adduced of this are in the hair and teeth; and it may be observed, that, in the process which will now be described, all the great features of the process of nutrition seem to be represented.* An eyelash which naturally falls, or which can be drawn * These and other instances are related more in detail in the first six of Mr. Paget's Lectures on Surgical Pathology, of which the principal part of this chapter is an abstract. In connection with this subject, Mr. Paget's subsequent Lectures on Repair and Inflammation, in the same work, may be consulted with advantage. 382 NUTRITION. out without pain, is one that has lived its natural time, and has died, and been separated from the living parts. In its bulb such an one will be found different from those that are still living in any period of their age. In the early period of the growth of a dark eyelash, the medullary substance appears like an interior cylinder of darker granular sub- stance, continued down to the deepest part, where the hair enlarges to form the bulb. This enlargement, which is of nearly cup - like form, appears to depend on the accumulation of nucleated cells, whose nuclei, according to their position, are either, by narrowing and elongation, to form the fibrous substance of the outer part of the growing and further protruding hair, or are to be transformed into the granular matter of its medullary portion. At the time of early and most active growth, all the cells and nuclei contain abundant pigment-matter, and * Fig. 98. Intended to represent the changes undergone by a hair towards the close of its period of existence. At A, its activity of growth is diminishing, as shown by the small quantity of pigment contained in the cells of the pulp, and by the interrupted line of dark medullary sub- stance. At B, provision is being made for the formation of a new hair, by the growth of a new pulp connected with the pulp or capsule of the old hair. c. A hair at the end of its period of life, deprived of its sheath and of the mass of cells composing the pulp of a living hair. NUTRITION OF HAIR. 383 the whole bulb looks nearly black. The sources of the material out of which the cells form themselves are at least two ; the inner surface of the sheath or capsule, which dips into the skin, enveloping the hair, and the surface of a vascular pulp which fits in a conical cavity in the bottom of the hair-bulb. Such is the state of parts so long as the growing hair is all dark. But as the hair approaches the end of its exist- ence, instead of the almost sudden enlargement at its bulb, it only swells a little, and then tapers nearly to a point ; the conical cavity in its base is contracted ; and the cells produced on the inner surface of the capsule contain no pigment. Still, for some time, it continues thus to live and grow ; and the vigour of the pulp lasts rather longer than that of the sheath or capsule, for it continues to pro- duce pigment-matter for the medullary substance of the hair after the cortical substance has become white. Thus the column of dark medullary substance appears paler and more slender, and perhaps interrupted, down to the point of the conical pulp which, though smaller, is still distinct, because of the pigment-cells covering its surface. At length the pulp can be no longer discerned, and un- coloured cells are alone produced, and maintain the latest growth of the hair. With these it appears to grow yet some further distance ; for traces of the elongation of their nuclei into fibres appear in lines running from the inner surface of the capsule inwards and along the surface of the hair ; and the column of dark medullary substance ceases at some distance above the lower end of the contracted hair-bulb. The end of all is the complete closure of the conical cavity in which the hair-pulp was lodged, the cessa- tion of the production of new cells from the inner surface of the capsule, and the detachment of the hair which, as a dead part, is separated and falls. Such is the life of a hair, and such its death ; which death is spontaneous, independent of exercise, or of any 384 NUTRITION. mechanical external force the natural termination of a certain period of life. Yet, before the hair dies, provision is made for its successor ; for when its growth is failing, there appears below its base a dark spot, the germ or young pulp of the new hair covered with cells containing pigment, and often connected by a series of pigment cells with the old pulp or capsule (fig. 98, B). Probably there is an intimate analogy between the pro- cess of successive life and death, and life communicated to a successor, which is here shown, and that which constitutes the ordinary nutrition of a part. It may be objected, that the death and casting out of the hair cannot be imitated in internal parts ; therefore, for an example in which the assumed absorption of the worn-out or degenerate internal particles is imitated in larger organs at the end of their appointed period of life, the instance of the deciduous or milk-teeth may be adduced. Each milk-tooth is develop- ed from its germ ; and in the course of its own development, separates a portion of itself to be the germ of its successor ; and each, having reached its perfection, retains for a time its perfect state, and still lives, though it does not grow. But at length, as the new tooth comes, the deciduous tooth dies ; or rather its crown dies, and is cast out like the dead hair, while its fang, with its bony sheathing, and vascular and nervous pulp, de- generates and is absorbed (fig. 99). The degeneration is * Fig. 99. Section of a portion of the upper jaw of a child, showing a new tooth in process of formation, the fang of the corresponding deciduous tooth being absorbed. MAINTENANCE BY NUTRITION. 3^5 accompanied by some unknown spontaneous decomposition of the fang ; for it could not be absorbed unless it was first so changed as to be soluble. And it is degeneration, not death, which precedes its removal ; for when a tooth- fang dies, as that of the second tooth does in old age, then it is not absorbed, but is cast out entire, as a dead part. Such, or generally such, it seems almost certain, is the process of maintenance by nutrition ; the hair and teeth may be fairly taken as types of what occurs in other parts, for they are parts of complex organic structure and com- position, and the teeth-pulps, which are absorbed as well as the fangs, are very vascular and sensitive. Nor are they the only instances that might be adduced. The like development, persistence for a time in the perfect state, death, and discharge, appear in all the varieties of cuticles and gland- cells ; and in the epidermis, as in the teeth, there is evidence of decomposition of the old cells, in the fact of the different influence which acetic acid and potash exercise on them and on the young cells. Seeing, then, that the process of nutrition, as thus displayed, both in active organs and in elementary cells, appears in these respects similar, the general conclusion may be that, in nutrition, the ordinary course of each complete elementary organ in the body, after the attainment of its perfect state by development and growth, is to remain in that state for a time ; then, independently of the death or decay of the whole body, and in some measure, independently of its own exercise, or exposure to external violence, to die or to degenerate ; and then, being cast out or absorbed, to make way for its successor. It appears, moreover, that the length of life which each part is to enjoy is fixed and determinate, though, in some degree subject to accidents and to the expenditure of life in exercise. It is not likely that all parts are made to last a certain and equal time, and then all need to be changed. The bones, for instance, when once completely formed, c c 386 NUTRITION. must last longer than the muscles and other softer tissues. But when we see that the life of certain parts is of determined length, whether they be used or not, we may assume, from analogy, the same of nearly all. Now, the deciduous human teeth have an appointed average duration of life. So have the deciduous teeth of all other animals ; and in all the numerous instances of moulting, shedding of antlers, of desquamation, change of plumage in birds, and of hair in Mammalia, the only ex- planation is that these organs have their severally appointed times^of living, at the ends of which they degenerate, die, are 'cast away, and in due time are replaced by others which, in their turn, are to be developed to perfection, to live their life in the mature state, and in their turn to be cast off. So also, in some elementary structures we may discern the same laws of determinate period of life, death, or degeneration, and replacement. They are evident in the history of the blood-corpuscles, both in the superseding of the first set of them by the second at a definite period in the life of the embryo, and in the replacement of those that degenerate by others new-formed from lymph-cor- puscles (see p. 103). And if we could suppose the blood- corpuscles grouped together in a tissue instead of floating, we might have in the changes they present an image of the nutrition of the elements of the tissues. The duration of life in each particle is, however, liable to be modified ; especially by the exercise of the function of the part. The less a part is exercised the longer do its component particles appear to live : the more active its functions are, the less prolonged is the existence of its individual particles. So in the case of single cells ; if the general development of the tadpole be retarded by keeping ' it in a cold, dark place, and if hereby the function of the , blood-corpuscles be slowly and imperfectly discharged, / they will maintain their embryonic state for even several t weeks later than usual, the development of the second set NUTRITIVE REPRODUCTION AND REPETITION. 387 of corpuscles will be proportionately postponed, and the individual life of the corpuscles of the first set will be, by the same time, prolonged. Such being the mode in which the necessity for the pro- cess of nutritive maintenance is created, such the sources of impairment and waste of the tissues, the next conside- ration may be the manner in which the perfect state of a part is maintained by the insertion of new particles in the place of those that are absorbed or cast off. The process by wliick a new particle is formed in the place of the old one is probably always a process of develop- ment ; that is, the cell or fibre, or other element of tissue, passes in its formation through the same stages of develop- ment as those elements of the same tissue did which were first formed in the embryo. This is probable from the analogy of the hair, the teeth, the epidermis, and all the tissues that can be observed : in all, the process of repair or replacement is effected through development of the new parts. The existence of nuclei or cytoblasts in nearly all parts that are the seats of active nutrition makes the same probable. For these nuclei, such as are seen so abundant in strong, active muscles, are not remnants of the embryonic tissue, but germs or organs of power for new formation, and their abundance often appears directly proportionate to the activity of growth. Thus, they are always abundant in the fcetal tissues, and those of the young animal ; and they are peculiarly numerous in the muscles and the brain, and their disappearance from a part in which they usually exist is a sure accompaniment and sign of degeneration. A difference may be drawn between what may be called nutritive reproduction and nutritive repetition. The former is shown in the case of the human teeth. As the deciduous tooth is being developed, a part of its productive capsule is detached, and serves as a germ for the formation of the second tooth ; in which eecond tooth, therefore, the first may be said to be reproduced, in the same sense as that in c c 2 388 NUTRITION. which we speak of the organs by which new individuals are formed, as the reproductive organs. But in the shark's jaws, and others, in which we see row after row of teeth succeeding each other, the row behind is not formed of germs derived from the row before : the front row is simply repeated in the second one, the second in the third, and so on. So, in cuticle, the deepest layer of epidermis- cells derives no germs from the layer above them ; their development is not like a reproduction of the cells that have gone on towards the surface before them : it is only a repetition. It is not improbable that much of the difference in the degree of repair, of which the several tissues are capable after injuries or diseases, may be con- nected with these differences in their ordinary mode of nutrition. In order that the process of nutrition may be perfectly accomplished, certain conditions are necessary. Of these, the most important are ; I . A right state and composition of the blood, from which the materials for nutrition are derived. 2. A regular and not far distant supply of such blood. 3. A certain influence of the nervous system. 4. A natural state of the part to be nourished, I . This right condition of the blood does not necessarily imply its accordance with any known standard of com- position, common to all kinds of healthy blood, but rather the existence of a certain adaptation between the blood and the tissues, and even the several portions of each tissue. Such an adaptation, peculiar to each individual, is determined in its first formation, and is maintained in the concurrent development and increase of both blood and tissues; and upon its maintenance in adult life, appears to depend the continuance of a healthy process of nutrition, or at least, the preservation of that exact sameness of the whole body and its parts, which constitutes the perfection of nutrition. Some notice of the maintenance of this sameness in the blood has been given already (p. 104), in CONDITIONS NECESSARY FOR NUTRITION. 389 speaking of the power of assimilation which the blood exercises, a power exactly comparable with this of main- tenance by nutrition in the tissues. And evidence of the adaptation between the blood and the tissues, and of the exceeding fineness of the adjustment by which it is main- tained, is afforded by the phenomena of diseases, in which, after the introduction of certain animal poisons, even in very minute quantities, the whole mass of the blood is altered in composition, and the solid tissues are perverted in their nutrition. It is necessary to refer only to such diseases as syphilis, small-pox, and other eruptive fevers, in illustration. And when the absolute dependence of all the tissues on the blood for their very existence is remembered, on the one hand, and, on the other, the rapidity with which substances introduced into the blood are diffused into all, even non- vascular textures (p. 376), it need be no source of wonder that any, even the slightest alteration, from the normal constitution of the blood, should be immediately reflected, so to speak, as a change in the nutrition of the solid tissues and organs which it is destined to nourish. 2. The necessity of an adequate supply of appropriate blood in or near the part to be nourished, in order that its nutrition may be perfect, is shown in the frequent examples of atrophy of parts to which too little blood is sent, of morti- fication or arrested nutrition when the supply of blood is entirely cut off, and of defective nutrition when the blood is stagnant in a part. That the nutrition of a part may be perfect, it is also necessary that the blood should be brought sufficiently near to it for the elements of the tissue to imbibe, through the walls of the blood-vessels, the nutritive materials which they require. The blood-vessels themselves take no share in the process of nutrition, except as carriers of the nutritive matter. Therefore, provided they come so near that this nutritive matter may pass by imbibition into the part to be nourished, it is compara- tively immaterial whether they ramify within the substance 390 NUTRITION. of the tissue, or are distributed only on its surface or border. The blood-vessels serve alike for the nutrition of the vascular and the non-vascular parts, the difference between which, in regard to nutrition, is less than it may seem. For the vascular, the nutritive fluid is carried in streams into the interior ; for the non-vascular, it flows on the sur- face ; but in both alike, the parts themselves imbibe the fluid ; and although the passage through the walls of the blood-vessels may effect some change in the materials, yet all the process of formation is, in both alike, outside the vessels. Thus, in muscular tissue, the fibrils in the very centre of the fibre, nourish themselves : yet these are distant from all blood-vessels, and can only by imbibition receive their nutriment. So, in bones, the spaces between the blood-vessels are wider than in muscle; yet the parts in the meshes nourish themselves imbibing materials from the nearest source. The non-vascular epidermis, though no vessels pass into its substance, yet imbibes nutritive matter from the vessels of the immediately subjacent cutis, and maintains itself, and grows. The instances of the cornea and vitreous humour are stronger, yet similar ; and sometimes even the same tissue is in one case vascular, in the other not, as the osseous tissue, which, when it is in masses or thick layers, has blood-vessels running into it ; but when it is in thin layers, as in the lachrymal and tur- binated bones, has not. These bones subsist on the blood flowing in the minute vessels of the mucous membrane, from which the epithelium derives nutriment on one side, the bone on the other, and the tissue of the membrane itself on every side : a striking instance how, from the same source, many tissues maintain themselves, each exer- cising its peculiar assimilative and self-formative power. 3. The third condition said to be essential to a healthy nutrition, is a certain influence of the nervous system. It has been held that the nervous system cannot be INFLUENCE OF NERVOUS' SYSTEM. 391 essential to a healthy course of nutrition, because in plants and the early embryo, 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, than 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 inserted and blended in a more and more inti- mate 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 nutri- tion, and by the most striking of these facts being observed in the higher animals, and especially in man. The influ- ence 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 por- tions 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 followed by mortification of portions of the paralysed parts ; and this may take place very quickly, as in a case by Sir B. C. Brodie, in which the ancle sloughed within twenty-four hours after an injury to the spine. After such lesions also, the repair of injuries in the paralysed parts may take place less completely than in others ; so, Mr. Travers men- tions a case in which paraplegia was produced by fracture of the lumbar vertebrae, 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 illus- 39 2 NUTRITION. trated by some experiments of Dr. Baly, 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, he found that the end of the tail was reproduced more slowly than in other salamanders in whom Vthe spinal cord was left uninjured above the point at which the tail was amputated. Illustrations of the same kind are furnished by the several cases in which division or destruction of the trunk of the trigeminal nerve has been followed by incomplete and morbid nutrition of the cor- responding 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 degene- ration 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. Further evidence of the influence of the nervous system upon nutrition is furnished by those cases in which, from mental anguish, or in severe neuralgic headaches, the hair becomes grey very quickly, or even in a few hours. So many and various facts leave little doubt that the nervous system exercises an influence over nutrition as over other organic processes ; and they cannot be explained by supposing that the changes in the nutritive processes are only due to the variations in the size of the blood- vessels supplying the affected parts. The question remains, through what class of nerves is the influence exerted ? When defective nutrition occurs in parts rendered inactive by injury of the motor nerve alone, INFLUENCE OF NERVOUS SYSTEM. 393 as in the muscles and other tissues of a paralysed face or limb, it may appear as if the atrophy were the direct con- sequence of the loss of power in the motor nerves; but it is more probable that the atrophy is the consequence of the want of exercise of the parts; for if the muscles be exercised by artificial irritation of their nerves, their nutri- tion will be less defective (J. Reid). The defect of the nutritive process which ensues in the face and other parts, in consequence of destruction of the trigeminal nerve, must be referred directly or indirectly to the loss of influence exercised through the sensitive or sympathetic nerves ; for the motor nerves of the face and eye, as well as the olfactory and optic, have no share in the defective nutrition which follows injury of the trigeminal nerve; and one or all of them may be destroyed without any direct disturbance of the nutrition of the parts they severally supply. The influence exercised by the sensitive and sympathetic nerves over the process of nutrition, thus proved in the case of the trigeminal nerve, is probably only an example of what is generally true. A similar influence is shown in the cases in which sloughing of parts from injury or disease of the spinal cord has ensued earlier and more extensively when sensation than when motion alone was lost, and in other cases in which the wasting of a paralysed limb is, after a certain time, more marked when both sensation and motion are impaired than when the power of motion alone is interfered with. It is not at present possible to say whether the influence on nutrition is exercised through the sensitive or through the sympathetic nerves, which, in the parts on which the observation has been made, are generally combined in the same sheath. The truth perhaps is, that it may be exerted through either or both of these nerves. The defect of nutrition which ensues after lesion of the spinal cord alone, the sympathetic nerves being uninjured, and the 394 NUTRITION. general atrophy which, sometimes occurs in consequence of diseases of the brain, seem to prove the influence of the cerebro-spinal system: while the observation of Magendie and Mayer, that inflammation of the eye is a constant result of ligature of the sympathetic nerve in the neck, and many other observations of a similar kind exhibit very well the influence of the latter nerve in nutrition. 4. The fourth condition necessary to healthy nutrition is a healthy state of the part to be nourished. This seems proved by the very nature of the process, which consists in the formation of new parts like those already existing ; for, unless the latter are healthy the former cannot be so. Whatever be the condition of a part, it is apt to be per- petuated by assimilating exactly to itself, and endowing with all its peculiarities, the new particles which it forms to replace those that degenerate. So long as a part is healthy, and the other conditions of healthy nutrition exist, it maintains its healthy condition. But, according to the same law, if the structure of a part be diseased or in any way altered from its natural condition, the alteration is maintained; the altered, like the healthy structure, is perpetuated. The same exactness of the assimilation of the new parts to the old, which is seen in the nutrition of the healthy tissues, may be observed also in those that are formed in disease. By it, the exact form and relative size of a cicatrix are preserved from year to year ; by it, the thickening and induration to which inflammation gives rise are kept up, and the various morbid states of the blood in struma, syphilis, and other chronic diseases are maintained, notwithstanding all diversities of diet. By this precision of the assimilating process, may be explained the law that certain diseases occur only once in the same person, and that certain others are apt to recur frequently; because in both cases alike, the alteration produced by the first attack of the disease is maintained by the exact like- ness which the new parts bear to the old ones. GROWTH. 395 The period, however, during which an alteration of structure may be exactly maintained by nutrition, is not unlimited ; for in nearly all altered parts there appears to exist a tendency to recover the perfect state, and, in many cases, this state is, in time, attained. To this, we may attribute the possibility of re-vaccination after the lapse of some years ; the occasional recurrence of small-pox, scarlet- fever, and the like diseases, in the same person ; the wearing out of scars, and the complete restoration of tissues that have been alter ated by injury or disease. Such are some of the more important conditions which appear to be essential to healthy nutrition. Absence or defect of any one of them is liable to be followed by dis- arrangement of the process ; and the various diseases resulting from defective nutrition appear to be due to the failure of these conditions, more often than to imperfection of the process itself. GROWTH. Growth, as has been already observed, consists in the increase of a part in bulk and weight by the addition to its substance of particles similar to its own, but more than sufficient to replace those which it loses by the waste or natural decay of its tissue. The structure and composi- tion of the part remain the same; but the increase of healthy tissue which it receives is attended with the capa- bility of discharging a larger amount of its ordinary function. While development is in progress, growth frequently proceeds with it in the same part, as in the formation of the various organs and tissues of the embryo, in which parts, while they grow larger, are also gradually more developed until they attain their perfect state. But, commonly, growth continues after development is com- pleted, and in some parts, continues even after the full stature of the body is attained, and after nearly every 39^ NUTRITION. portion of it has gained its perfect state in both size and composition. In certain conditions, this continuance or a renewal of growth may be observed in nearly every part of the body. When parts have attained the full size which in the ordi- nary process of growth they reach, and are then kept in a moderate exercise of their functions, they commonly (as already stated), retain almost exactly the same dimensions through the adult period of life. But when, from any cause, a part already full-grown in proportion to the rest of the body, is called upon to discharge an unusual amount of its ordinary function, the demand is met by a corresponding increase or growth of the part. Illustrations of this are afforded by the increased thickening of cuticle at parts where it is subjected to an unusual degree of occasional pressure or friction, as in the palms of the hands of persons employed in rough manual labour ; by the enlargement and increased hardness of muscles that are largely exer- cised ; and by many other facts of a like kind. The increased power of nutrition put forth in such growth is greater than might be supposed ; for the immediate effect of increased exercise of a part must be a greater using of its tissue, and might be expected to entail a permanent thinning or diminution of the substance of the part. But the energy with which fresh particles are formed is suffi- cient not only to replace completely those that are worn away, but to cause an increase in the substance of the part the amount of this increase being proportioned to the more than usual degree in which its functions are exercised. The growth of a part from undue exercise of its functions is always, in itself, a healthy process ; and the increased size which results from it must be distinguished from the various kinds of enlargement to which the same part may be subject from disease. In the former case, the enlarge- ment is due to an increased quantity of healthy tissue, HYPERTROPHY. 397 providing more than the previous power to meet a par- ticular emergency; the other may be the result of a deposit of morbid material within the natural structure of the part diminishing, instead of augmenting, its fitness for its office. Such a healthy process of growth in a part, attended with increased power and activity of its functions, may, however, occur as the consequence of disease in some other part ; in which case it is commonly called Hypertrophy, i.e., excess of nutrition. The most familiar examples of this are in the increased thickness and robustness of the muscular walls of the cavities of the heart in cases of continued obstruction to the circulation; and in the increased development of the muscular coat of the urinary bladder when, from any cause, the free discharge of urine from it is interfered with. In both these cases, though the origin of the growth is the consequence of disease, yet the growth itself is natural, and its end is the benefit of the economy; it is only common growth renewed or exercised in a part which had attained its size in due proportion to the rest of the body. It may be further mentioned, in relation to the phy- siology of this subject, that when the increase of function, which is requisite in the cases from which hypertrophy results, cannot be efficiently discharged by mere increase of the ordinary tissue of the part, the development of a new and higher kind of tissue is frequently combined with this growth. An example of this is furnished by the uterus, in the walls of which, when it becomes enlarged by pregnancy, or by the growth of fibrous tumours, organic muscular fibres, found in a very ill-developed condition in its quiescent state, are then enormously developed, and provide for the expulsion of the foetus or the foreign body. Other examples of the same kind are furnished by cases in which, from obstruction to the discharge of their contents and a consequently increased necessity for propulsive power, the coats of reservoirs and of ducts become the seat 39^ NUTRITION. of development of organic muscular fibres, which could be said only just to exist in them before, or were present in a very imperfectly developed condition. Respecting the mode and conditions of the process of growth, it need only be said, that its mode seems to differ only in degree from that of common maintenance of a part ; more particles are removed from, and many more added to a growing tissue, than to one which only maintains itself. But so far as can be ascertained, the mode of removal, the disposition of the removed parts, and the insertion of the new particles, are as in simple maintenance. The conditions also of growth are the same as those of common nutrition, and are equally or more necessary to its occurrence. When they are very favourable or in excess, growth may occur in the place of common nutrition. Thus hair may grow profusely in the neighbourhood of old ulcers, in consequence, apparently, of the excessive supply of blood to the hair-bulbs and pulps ; bones may increase in length when disease brings much blood to them ; and, cocks' spurs transplanted from their legs into their combs grow to an unnatural length ; the conditions common to all these cases being both an increased supply of blood, and the capability, on the part of the growing tissue, of avail- ing itself of the opportunity of increased absorption and nutrition thus afforded to it. In the absence of the last- named condition, increased supply of blood will not lead to increased nutrition. 399 CHAPTER XII. SECRETION. SECRETION is the process by which materials are separated from the blood and from the organs in which they are formed, for the purpose either of serving some ulterior office in the economy, or being discharged from the body as excrement. In the former case, both the separated materials and the processes for their separation are termed secretions ; in the latter, they are named excretions. Most of the secretions consist of substances which, pro- bably, do not pre-exist in the same form in the blood, but require special organs and a process of elaboration for their formation, e.g., the liver for the formation of bile, the mammary gland for the formation of milk. The ex- cretions, on the other hand, commonly or chiefly consist of substances which, as urea, carbonic acid, and probably uric acid, exist ready-formed in the blood, and are merely abstracted therefrom. If from any cause, such as exten- sive disease or extirpation of an excretory organ, the sepa- ration 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 organs by which any of them is elaborated, it 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 ac- count 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 secre- tion, and must not be thus regarded. These circumstances, and their final destination are, 400 SECRETION. however, the only particulars in which secretions and excretions can be distinguished; for in general, the struc- ture of the parts engaged in eliminating excretions, e.g., the kidneys, is as complex as that of the parts concerned 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 ascertained, 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 apparently 'textureless mem- brane, named the primary or basement-membrane; certain cells; and blood-vessels. These three structural elements are arranged together in various ways; but all the varieties may be classed under one or other of two principal divi- sions, namely, membranes and glands. SECRETING MEMBRANES. The principal secreting membranes are the serous and synovial membranes, the mucous membranes, and the skin.* Fig. loo.f A The serous membranes are formed of fibro- cellular tissue, interwoven so as to constitute a membrane, the free surface of which is covered with a single layer of flattened cells, forming, in most instances, a simple tesselated epithelium. Between the epithelium and the subjacent layer of fibro - cellular tissue, is situated the primary or basement mem- brane (Bowman). * The skin will be described in a subsequent chapter. f Fig. 100. Plan of a secreting membrane ; a, mcmbrana propria, or, basement membrane ; b, epithelium composed of secreting nucleated cells ; c, layer of capillary blood-vessels (after Sharpey). SEROUS MEMBEANES. 4i In relation to the process of secretion, the layer of fibro- cellular 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 ven- tricles of the brain. The primary membrane and epithe- lium are probably always present, and are concerned in the formation of the fluid by which the free surface of the membrane is moistened. The serous membranes are of two principal kinds : 1st. Those which line visceral cavities, the arachnoid, pericardium, pleurae, peritoneum, and tunicse vaginales. 2nd. The synovial membranes lining the joints, and the sheaths of tendons and ligaments, with which, also, are usually included the synovial bursse, or bursa mucos, whether these be subcutaneous, or situated beneath ten- dons that glide over bones. The serous membranes form closed sacs, and exist wherever the free surfaces of viscera come into contact with each other, or lie in cavities unattached to surround- ing parts. The viscera which are invested by a serous membrane, are, as it were, pressed into the shut sac which it forms, carrying before them a portion of the membrane, which serves as their investment. To the law that serous membranes 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. The principal purpose of the serous and synovial mem- branes 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 move- ments take place ; and in the stomach and intestines which from the varying quantity and movements of their contents, D D 402 SECRETION. are in almost constant motion upon one another and the walls of the abdomen. The fluid secreted from the free surface of the serous membranes is, in health, rarely more than suflicient to ensure the maintenance of their moisture. The opposed surfaces of each serous sac, are at every point in contact with each other, and leave no space in which fluid can collect. After 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 the serum of the blood, or with very dilute liquor san- guinis. It is of a pale yellow or straw-colour, slightly viscid, alkaline, and, because of the presence of albumen, coagulable by heat. The presence of a minute quantity of fibrin, at least in the dropsical fluids effused into the serous cavities, is shown by their partial coagulation into a jelly-like mass, on the addition of certain animal substances, or on mixture with certain fluids, especially such as contain cells (p. 73 et seq.). 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 in the blood, and are not coloured by madder, or other similar substances introduced abundantly into the blood. MUCOUS MEMBRANES. 403 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 mem- brane, and especially of those which are accumulated on the edges and processes of the synovial fringes ; for, in its peculiar density, viscidity, and abundance of albumen, synovia differs alike from the serum of blood and from the fluid of any of the serous cavities. 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. Their general structure resembles that of serous membranes. It consists of epithelium, basement membrane, and fibro-cellular or areolar tissue containing blood-vessels, lymphatics, and nerves. The structure of mucous membranes is less uniform, especially as regards their epithelium, than that of serous membranes ; but the varieties of structure in different parts are described in connection with the organs in which mucous membranes are present, and need not be here noticed in detail. 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 sphenoid sinuses, as well as in the tympanum, to 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 are described as lining certain principal tracts. I. 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 D D 2 404 SECRETION. 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 pancreas and liver and into the gall-bladder. 2. The respiratory tract includes the mucous membrane lining the cavity of the nose, and tjie various sinuses communicating with it, the lachrymal canal and sac, the conjunctiva of the eye and eyelids, and the prolongation which passes along the Eustachian 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 con- tinuous with it is a layer of delicate epithelial mem- brane 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 into and through the organs of generation in both sexes, into the ducts of the glands connected with them ; and in the female becomes continuous with the serous membrane of the abdo- men at the fimbrise of the Fallopian tubes. Along each of the above tracts, and in different portions of each of them, the mucous membrane presents certain struc- tural peculiarities adapted to the functions which each part has to discharge ; yet in some essential characters mucous membrane 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 basement-membrane, and beneath this, a stratum of vascular tissue of variable thickness, which in different cases presents either out-growths in the form of papillse and villi, or depressions or involutions in the form of glands. But in the prolongations of the tracts, where SECRETING GLANDS. 45 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 trans- parent layer, simple, homogeneous, and with no discernible structure, which on the larger mucous membranes that Lave a layer of vascular fibro-cellular tissue, may appear to be only the blastema or formative substance, out of which successive layers of epithelium cells are formed. But 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 membranes, constitute their epithelium, are continuous with, and often similar to, those which, lining the gland-ducts, are called gland-cells, rather than epithelium. Indeed, no certain distinction can be drawn between the epithelium-cells of mucous membranes and gland-cells. In reference to their position, as covering surfaces, they might all be called epithelium- cells, whether they lie on open mucous membranes or in gland-ducts ; and in reference to the process of secretion, they might all be called gland-cells, or at least secreting- cells, since they probably all fulfil a secretory office by separating certain definite materials from the blood and from the part on which they are seated. It is only an artificial distinction which makes them epithelial cells in one place, and gland-cells in another. It thus appears, that the tissues essential to the pro- duction of a secretion are, in their simplest form, a simple membrane, having on one surface blood-vessels, and on the other a layer of cells, which may be called either epithelium-cells, or gland-cells. The structure of these elementary portions of a 406 SECRETION. 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. SECRETING GLANDS. The secreting glands are. the organs to which the office of secreting is more especially ascribed : for they appear to be occupied with it alone. They all present, amid manifold diversities of form and composition, a general plan of structure, by which they are distinguished from all other textures of the body; especially, 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. For convenience of description, they may be divided into three principal groups, the characters of each of which are determined by the different modes in which the sacculi or tubes containing the secreting cells are grouped : I. The simple tubule, or tubular gland (A, fig. 10 1 ), examples of which are furnished by the several tubular follicles in mucous membranes : especially by the follicles of Lieber- kiihn in the mucous membrane of the intestinal canal (p. 306), and the tubular or gastric glands of the stomach (p. 2/5). These appear to be simple tubular depressions of the mucous membrane on which they open, each consisting of an elongated gland-vesicle, the wall of which is formed of primary membrane, and is lined with secreting cells arranged as an epithelium. To the same class may be referred the elongated and tortuous sudoriparous glands of the skin (p. 428), and the Meibomian follicles beneath the palpebral conjectiva; though the latter are made more complex by the presence of small pouches along their sides (B, fig. lOl), and form a connecting link between the SECRETING GLANDS. 407 members of this division and the next, as the former by their length and tortuosity do between the first division and the third (D, fig. 101). * Fig. 101. Plans of extension of secreting membrane by inversion or recession in form of cavities. A, simple glands, viz., g, straight tube ; h, sac ; i t coiled tube. B, multilocular crypts ; k, of tubular form ; 7, 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, (after Sharpey). 408 SECRETION. 2. The aggregated glands, including those that used to be called conglomerate, in which a number of vesicles or acini are arranged in groups or lobules (c, fig. I o I ). Such are all those commonly called mucous glands, as those of the trachea, vagina, and the minute salivary glands. Such, also, are the lachrymal, the large salivary and mammary glands, Brunn's, Cowper's, and Duverney's glands, the pancreas and prostate. 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 connection by fibro- cellular 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 filled with secreting cells which also occupy recesses from the main cavity. The smallest branches of the gland-ducts sometimes open into the centres of these cavities; some- times the acini are clustered round the extremities, or by the sides of the ducts : but, whatever secondary arrange- ment there may be, all have the same essential character of rounded groups of vesicles containing gland- cells, and opening, either occasionally or permanently, 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. 3. The convoluted tubular glands (D, fig. lOl), such as the kidney and testis, form another division. These consist of tubules of membrane, lined with secreting cells arranged like an epithelium. Through nearly the whole of their long course, the tubules present an almost uniform size and structure ; ultimately they terminate either in a cul-de-sac, or by dilating, as in the Malpighian capsules of the kidney, or by forming a simple loop and returning, as in the testicle. Among these varieties of structure, all the permanent PROCESS OF SECRETION. 409 glands are alike in some essential points, besides those which they have in common with all truly secreting struc- tures. They agree in presenting a large extent of secreting surface within a comparatively small space ; in the circum- stance 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 canals ; and in an uniform arrangement of capillary blood- vessels, ramifying and forming a network around the walls and in the interstices of the ducts and acini. PROCESS OP SECRETION. From what has been said, it will have already appeared that the modes in which secretions are produced are at least two. Some fluids, such as the secretions of serous mem- branes, appear to be simply exudations or oozings from the blood-vessels, whose qualities are determined by those of the liquor sanguinis, while the quantities are liable to variation, or are chiefly dependent on the pressure of the blood on the interior of the blood-vessels. But, in the production of the other secretions, such as those of mucous membranes and all glands, other besides these mechanical forces are in operation. Most of the secretions are indeed liable to be modified by the circumstances which affect the simple exudation from the blood-vessels, and the pro- ducts of such exudations, when excessive, are apt to be mixed with the more proper products of .all the secreting organs. But the act of secretion in all glands is the result of the vital processes of cells or nuclei, which, as they develop themselves and grow, form in their interior the proper materials of the secretion, and then discharge them. The best evidence for this view is : 1st. That cells and nuclei are constituents of all glands, however diverse their outer forms and other characters, and are in all glands 4io SECEETION. placed on the surface or in the cavity whence the secretion is poured. 2nd. That many secretions which are visible with the microscope may be seen in the cells of their glands before they are discharged. Thus, bile may be often discerned by its yellow tinge in the gland-cells of the liver; spermatozoids in the cells of the tubules of the testicles ; granules of uric acid in those of the kidneys of fish ; fatty particles, like those of milk, in the cells of the mammary gland. The process of secretion might, therefore, be said to be accomplished in, and by the life of, these gland-cells. They appear, like the cells or other elements of any other organ, to develop themselves, grow, and attain their indi- vidual perfection by appropriating the nutriment from the adjacent blood-vessels and elaborating it into the materials of their walls and the contents of their cavities. In this perfected state, they subsist for some brief time, and when that period is over they appear to dissolve or burst and yield themselves and their contents as 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 ; therefore not in the extremities of the ducts or in the acini alone, but in great part of their length. In these things there is the closest resemblance between secretion and nutrition ; for, if the purpose which the secreting glands are to serve in the economy be disre- garded, their formation might be considered as only the process of nutrition of organs, whose size and other con- ditions are maintained in, and by means of, the continual succession of cells developing themselves and passing away. In other words, glands are maintained by the development of the cells, and their continuance in the perfect state : and the secretions are discharged as the constituent gland-cells degenerate and are set free. The processes of nutrition and secretion are similar, also, in their obscurity : there is the same difficulty in saying why, DISCHAEGE OF SECRETIONS. 411 out of apparently the same materials, the cells of one gland elaborate the components of bile, while those of another form the components of milk, and of a third those of saliva, as there is in determining why one tissue forms cartilage, another bone, a third muscle, or any other tissue. In nutrition, also, as in secretion, some elements of tissues, such as the gelatinous tissues, are different in their chemical properties from any of the constituents ready- formed in the blood. Of these differences, also, no account can be rendered; but, obscure as the cause of these diversities may be, they are not objections to the explanation of secretion as a process similar to nutrition ; an explanation with which all the facts of the case are reconcilable. It may be observed that the diversities presented by the other constituents of glands afford no explanation of the differences or peculiarities of their several products. There are many differences in the arrangements of the blood- vessels in different glands and mucous membranes ; and, in accordance with these, much diversity in the rapidity with which the blood traverses them. But there is no reason for believing that these things do more than in- fluence the rate of the process and the quantity of the material secreted. Cceteris paribus, the greater the vascu- larity of a secreting organ, and the larger the supply of blood traversing its vessels in a given time, the larger is the amount of secretion ; but there is no evidence that the quantity or mode of movement of the blood can directly determine the quality of the secretion. The Discharge of Secretions from glands may take place as soon as they are formed ; or the secretion may be long retained within the gland or its ducts. The secretions of glands which are continually in active function for the purification of the blood, such as the kidneys, are generally discharged from the gland as rapidly as they are formed. But the secretions of those whose activity of function is 412 SECRETION. only occasional, such, as the testicle, are usually retained in the ducts during the periods of the gland's inaction. And there are glands which are like both these classes, such as the lachrymal and salivary, 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 secretions is effected partly by the pressure from behind ; the fresh quantities of secretion propelling those that were formed before. In the larger ducts, its propulsion is assisted by the contraction of their walls. All the larger ducts, such as the ureter and common bile-duct, possess in their coats organic muscular fibres; they contract when irritated, and sometimes manifest peristaltic movements. Bernard and Brown-Sequard, indeed, have observed rhyth- mic contractions in the pancreatic and bile ducts, 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; doubtless by the energetic and simultaneous contraction of many of the ducts of their respective glands. The contrac- tion of the ducts can only expel the fluid they contain through their main trunk ; for at their opposite ends all the ducts are closed. Circumstances influencing Secretion. The influence of ex- ternal conditions on the functions of glands is manifested chiefly in alterations of the quantity of secretion ; and among the principal of these conditions are variations in the quantity of blood, in the quantity of the peculiar ma- terials for any secretion that it may contain, and in the conditions of the nerves of the glands. In general, an increase in the quantity of blood traversing a gland, coincides with an augmentation of its secretion. Thus the mucous membrane of the stomach becomes florid INFLUENCE OF NERVOUS SYSTEM. 413 when, on the introduction of food, its glands begin to secrete ; the mammary gland becomes much more vascular during lactation ; and it appears that all circumstances which give rise to an increase in the quantity of material secreted by an organ, produce, coincidently, an increased supply of blood. In most cases, the increased supply of blood rather follows than precedes the increase of secre- tion ; as, in the nutritive processes, the increased nutrition of a part just precedes and determines the increased supply of blood; but, as also in the nutritive process, an increased supply of blood may have, for a consequence, an increased secretion from the glands to which it is sent. Glands also secrete with increased activity when the blood contains more than usual of the materials they are designed to separate. Thus, when an excess of urea is in the blood, whether from excessive exercise, or from destruction of one kidney, a healthy kidney will excrete more than it did before. It will, at the same time, grow larger ; an interesting fact, as proving both that secretion and nutrition in glands are identical, and that the presence of certain materials in the blood may lead to the formation of structures in which they may be incorporated. The production of secretion is, also, largely influenced by the condition of the nervous system. It is not possible to say, with certainty, whether the secretion of a gland would be arrested by the division or destruction of all the nerves distributed to it, for the branches of the nerves are largely spread over the blood-vessels, so that their destruction cannot be effected without serious injury to these, and to other structures entering into the formation of the gland. It is probable, however, that the influence of the nerves in secretion is due mainly, if not entirely, to their controlling influence on the calibre of the blood- vessels. It appears immaterial to the perfection of secretion, from which nervous centres a secreting organ receives its 414 SECRETION. supply of nervous influence ; for some glands are supplied with sympathetic, others with cerebro-spinal nerves, and others with both kinds ; yet the mode of secretion appears to be in all alike. The experiments of Bernard, Brown- Sequard, . and others, however, seem to show that all secreting glands are provided with two distinct orders of nerves, namely, motor and ganglionic : the latter appear to act by causing contraction of the blood-vessels supplying the gland, and thus diminishing the secretion, the former lead to dilatation of the vessels, and an abundant supply of blood, whereby the secretion is augmented. This was well shown in Bernard's experiments on the secretion of gastric fluid, irritation of the pneumo-gastric increasing it, irritation of the sympathetic branches of the semilunar ganglia diminishing or arresting it. The exact mode, however, in which the nervous system influences secretion, must be still regarded as obscure. In many cases, it probably exerts its influence, as just said, 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. 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 supposed to be reflected upon the nerves supplying the salivary glands, and to produce, through these, a more abundant secretion of saliva. Through the nerves, various conditions of the mind also influence 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, occasionally, diarrhoea which ensue under THE DUCTLESS GLANDS. 415 the influence of terror, and the tears excited by sorrow or excess of joy. The quality of a secretion may also be affected by the mind ; 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). 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; 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. CHAPTER XIII. THE VASCULAR GLANDS } OR GLANDS WITHOUT DUCTS. THE materials separated from the blood by the ordinary process of secretion by glands, are always discharged from the organ in which they are formed, and either straight- way expelled from the body, or if they are again received into the blood, it is only after they have been altered from their original condition, as in the cases of the saliva and bile. There appears, however, to be a modification of the process of secretion, in which certain materials are ab- stracted from the blood, undergo some change, and are added to the lymph or restored to the blood, without being previously discharged from the secreting organ, or made use of for any secondary purpose. The bodies in which 4i 6 THE DUCTLESS GLANDS. this modified form of secretion takes place, are usually described as vascular glands, or glands without ducts, and include the spleen, the thymus and thyroid glands, the supra-renal capsules, and, according to (Esterlin and Ecker and Gull, the pineal gland and pituitary body; possibly, also the tonsils. Peyer's glands of the intestine, and lymph- glands in general, also closely resemble them ; indeed, both in structure and function, the vascular glands bear a close relation, on the one hand, to the true secreting glands, and on the other, to the lymphatic glands. The evidence in favour of the view that these organs exercise a function analogous to that of secreting glands, has been chiefly obtained from investigations into their structure, which have shown that all the glands without ducts contain the same essential structures as the secreting glands, except the ducts. They are mainly com- posed of vesicles, or sacculi, either simple and closed, as in the thyroid (Simon), spleen, and supra-renal capsules (Ecker), or variously branched, and with the cavities of the several branches communicating in and by common canals, as in the thymus (Simon). These vesicles, like the acini of secreting glands, are formed of a delicate homo- geneous membrane, are surrounded with and often traversed by a vascular plexus, and are filled with finely molecular albuminous fluid, suspended in which are either granules of fat, or cytoblasts or nuclei, or nucleated cell, or a mix- ture of all these. These general resemblances in structure between the vascular glands and the true glands lead to the supposi- tion that both sets of organs pursue, up to a certain point, a similar course in the discharge of their functions. It is assumed that certain principles in an inferior state of organization are effused from the vessels into the sacculi, and gradually develop into nuclei or cytoblasts, which may be further developed into cells; that in the growth of these THE DUCTLESS GLANDS. 417 nuclei and cells, the materials derived from the Hood are elaborated into a higher condition of organization; and that, when liberated by the dissolution of these cells, they pass into the lymphatics, or are again received into the blood, whose aptness for nutrition they contribute to maintain. The opinion that the vascular glands thus serve for the higher organization of the blood, is supported by their being all especially active in the discharge of their functions during festal life and childhood, when, for the development and growth of the body, the most abundant supply of highly-organized 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 dis- appears. The thyroid gland and supra-renal capsules, also, though they probably never cease to discharge some amount of 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 proportionate size and apparent activity of func- tion. The spleen more nearly retains its proportionate size, and enlarges nearly as the whole body does. The function of the vascular glands seems not essential to life, at least not in the adult. The thymus wastes and disappears ; no signs of illness attend some of the diseases which wholly destroy the structure of the thyroid gland ; and the spleen has been often removed in animals, and in a few instances in men, without any evident ill-consequence. It is possible that, in such cases, some compensation for the loss of one of the organs may be afforded by an in- creased activity of function in those that remain. The experiment, to be complete, should include the removal of all these organs, an operation of course not possible 4i 8 THE DUCTLESS GLANDS. without immediate danger to life. Nor, indeed, would this be certainly sufficient, since there is reason to suppose that the duties of the spleen, after its removal, might be performed by lymphatic glands, between whose structure and that of the vascular glands there is much resemblance, and which, it is said, have been found peculiarly enlarged when the spleen has been removed (Meyer). 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 discharges a peculiar office, in relation either to the whole economy, or to that of some other organ. Respecting the special office of the thyroid gland, nothing reasonable can be suggested; nor is there any certain evidence concerning that of the supra-renal capsules.* Respecting the thymus gland, the observations of Mr. Simon, confirmed by those of Friedleben and others, have shown that 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 connected 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 the 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. With respect to the office of the spleen, recent investi- * Mr. J. Hutchinson, and, more recently, Dr. "Wilks, following out Dr. Addison's discovery, have, by the collection of a large and valuable series of cases in which the supra-renal capsules were diseased, de- monstrated most satisfactorily the very close relation subsisting between disease of these organs and brown discoloration of the skin ; but the explanation of this relation is still involved in obscurity, and conse- quently does not aid much in determining the functions of the supra- renal capsules. THE DUCTLESS GLANDS. 4*9 gations seem to have furnished us with more definite infor- mation. In the first place, 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 or formative materials of food, and for a time storing them up, to be gradually introduced into the blood, according to the demands of the general system. The small amount of fatty matter in such plasma, leads to the inference that the gland has little to do in regard to the preparation of material for the respira- tory process. Then again, it seems not improbable that, as Hewson originally suggested, the spleen, and perhaps to soma extent the other vascular glands, are, like the lymphatic glands, engaged in the formation of the germs of subse- quent blood-corpuscles. For it seems quite certain, that the blood of the splenic vein contains an unusually large amount of white corpuscles ; and in the disease termed leucocythsemia, 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 thy- roid body, or some of the lymphatic glands. Accordingly there seems to be a close analogy in function between the so-called vascular and the lymphatic glands : the former elaborating albuminous principles, and forming the germs of new blood- corpuscles out of alimentary materials ab- sorbed by the blood-vessels; the latter discharging the like office on nutritive materials taken up by the general absorbent system. In Kolliker'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. E E 2 420 THE DUCTLESS GLANDS. There is reason to believe, too, 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 portion of the spleen- pulp an abundance of such corpuscles, in various stages of degeneration, are found, while the red corpuscles in the splenic venous blood are said to be relatively diminished. According to Kolliker's description of this process of disin- tegration, 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 cor- puscles 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. 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 con- cluded, 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 circulation, however, can hardly be supposed to be more than a very subordinate THE SKIN. 421 part of the office of an organ of so great complexity as the spleen, and containing so many other structures besides blood-vessels. The same may also be said with regard to the opinion that the thyroid gland is important as a diver ticulum for the cerebral circulation, or the thymus for the pulmonary in childhood. These, like the spleen, must have peculiar and higher, though as yet ill under- stood, offices. CHAPTER XIV. THE SKIN AND ITS SECRETION. To complete the consideration of the processes of organic life, and especially of those which, by separating materials from the blood, maintain it in the state necessary for the nutrition of the body, the structure and functions of the skin must be now considered : for besides the purposes which it serves (i) as an external integument for the protection of the deeper tissues, and (2) as a sensitive organ in the exercise of touch, it is also (3) an important excretory, and (4) an absorbing organ. Structure of the Skin. The skin consists, principally, of a layer of vascular tissue, named the corium, derma or cutis vera, and an external covering of epithelium termed the cuticle or epidermis. Within and beneath the corium are imbedded several organs with special functions, namely, sudoriparous glands, sebaceous glands, and hair-follicles ; and on its sur- face are sensitive papilla. The so-called appendages of the skin the hair and nails are modifications of the epidermis. 4 22 THE SKIN. Epidermis. The epidermis is composed of several layers of epithelial cells of the squamous kind (p. 39), the deeper cells, however, being rounded or elongated, and in the latter instance having their long axis arranged vertically as regards the general surface of the skin, while the more superficial cells are flattened and scaly (fig. 102). The deeper part of the epider- Fig. 102.* mis, which is softer and more opaque than the su- perficial, is called the rete mucosum. Many of the epidermal cells contain pigment, and the varying quantity of this is the source of the different shades of tint in the skin, both of individuals and races. The colouring mat- ter is contained chiefly in the deeper cells composing the rete mucosum, and be- comes less evident in them as they are gradually pushed up by those under them, and become, like their predecessors, flattened and scale-like (fig. IO2). It is by this process of production from beneath, to make up for the waste at the surface, that the growth of the cuticle is effected. 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 ; and the more it is subjected to such injury, within certain limits, * Fig. 102. Skin of the negro, in a vertical section, magnified 250 diameters, a, a, cutaneous papillae ; 6, undermost and dark coloured layer of oblong vertical epidermis-cells; c, mucous or Malpighian layer; d, horny layer (from Sharpey). THE COBIUM OR CUTIS YERA. 423 the more does it grow, and the thicker and more horny does it become ; for it serves as well to protect the sen- sitive 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. Cutis vera. The corium or cutis, 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 fibro-cellular tissue, interwoven in all directions, and forming, by their interlacements, numerous spaces or areolse. These areola3 are large in the deeper layers of the cutis, and are there usually filled with little masses of fat (fig. 105) : but, in the more super- ficial parts, they are exceedingly small or entirely oblite- rated. By means of its toughness, flexibility, and elasticity, the skin is eminently qualified to serve as the general integu- ment 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. But, from the abundant supply of sensitive nerve-fibres which it receives, it is enabled to fulfil a not less important pur- pose in serving as the principal organ of the sense of touch. The entire surface of the skin is extremely sen- sitive, but its tactile properties are due chiefly to the abundant papillae with which it is studded. These papilla are conical elevations of the corium, with a single or divided free extremity, more prominent and more densely set at some parts than at others (figs. 103 and 104). The parts on which they are most abundant and most prominent are the palmar surface of the hands and fingers, and the 424 THE SKIN. soles of the feet parts, therefore, 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 (fig. 105). Thus they may be seen easily on the palm, whereon each raised line is composed of a double row of papillae, and is intersected by Fig. 103.* 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 -i-g-th of an inch, and at their base they measure about -s^-o-th 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. The abundant supply of blood which the papillae thus receive 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, * Fig. 103. Papillae, as seen with a microscope, on a portion of the true skin, from which the cuticle has been removed (after Breschet). t Fig. 104. Compound papillae from the palm of the hand, magnified 60 diameters : a, basis of a papilla ; 6, 6, divisions or branches of the same ; c, c, branches belonging to papillae, of which the bases are hidden from view (after Kolliker). STRUCTURE OF THE PAPILLA. 425 from the ultimate ramifications of the cutaneous plexus, on which their exquisite sensibility depends. The exact mode in which these nerve-fibres terminate is not yet Fig. 105.* satisfactorily determined. In some parts, especially those in which the sense of touch is highly developed, as for example, the palm of the hand and the lips, the fibres appear to terminate, in many of the papillae, by one or more free ends in the substance of a dilated oval-shaped body, not unlike a Pacinian corpuscle (figs. 129, 130,) * Fig. 105. Vertical section of the skin and subcutaneous tissue, from end of the thumb, across the ridges and furrows, magnified 20 diameters (from Kolliker) : a, horny, and b, mucous layer of the epidermis ; c, corium ; d, panniculus adiposus ; e, papillse on the ridges ; /, fat- clusters ; y, sweat-glands ; h, sweat-ducts ; i, their openings on the surface. 426 THE SKIN. occupying the principal part of the interior of the papilla, and termed a touch-corpuscle (fig. 1 06). The nature of this body is obscure. Kolliker, Huxley, and others, regard it as little else than a mass of fibrous, or connective tissue, surrounded by elastic fibres, and formed, according to Huxley, by an increased development of the neurilemma of the nerve-fibres entering the papilla. Wagner, how- ever, to whom seems to belong the merit of first fully describing these bodies, believes that, instead of thus con- sisting of a homogeneous mass of connective tissue, they are special and peculiar bodies of 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. 106. Papillae from the skin of the hand, freed from the cuticle and exhibiting the tactile corpuscles. Magnified 350 diameters. A. Simple papilla with four nerve-fibres : a, tactile corpuscle ; b, nerves. B. Papilla treated with acetic acid : a, cortical layer with cells and fine elastic filaments; b, tactile corpuscle with transverse nuclei; c, entering nerve with neurilemma or perineuriurn ; d, nerve-fibres winding round the corpuscle, c. Papilla viewed from above so as to appear as a cross section : a, cortical layer ; &, nerve-fibre ; c, sheath of the tactile cor- puscle containing nuclei ; d } core (after Kolliker). TOUCH-CORPUSCLES ; END-BULBS. 427 there are no blood-vessels. Since these peculiar bodies in which the nerve-fibres end are only met with in the papillae of Highly sensitive parts, it may be 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. Closely allied in structure to the Pacinian corpuscles and touch-corpuscles are some little bodies about -B^th of an inch in diameter, first particularly described by Krause, and somewhat awkwardly named by him " end-bulbs." They are generally oval or spheroidal, and composed externally of a coat of connective tissue enclosing a softer matter, in which the extremity of a nerve terminates. These bodies have been found chiefly in the lips, tongue, palate, and the skin of the glans penis (fig. 107). Although destined especially for the sense of touch, the papillae are not so placed as to come into direct contact Fig. 107.* * Fig. 107. End-bulbs in papillae (magnified) treated with acetic acid. A, from the lips ; the white loops in one of them are capillaries. B, from the tongue. Two end-bulbs seen in the midst of the simple papillee : a, a, nerves (from Kolliker). 428 THE SKIN. 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, 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 ta the sense of touch. For, by 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, ea.ch of which is capable of receiving a distinct impression from an external body. By covering the papillae it renders the sensation produced by external bodies more obtuse, and in this manner also is subservient to touch : for unless the very sensitive papillae 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 is very thick, however, as on the heel, touch becomes imperfect, or is lost, through the inability of the tactile papillae to receive impressions through the dense and horny layer covering them. Sudoriparous Glands. In the middle of each of the transverse furrows between the papillae, and irregularly scattered between the bases of the papillae in those parts of the surface of the body in which there are no furrows between them, are the orifices of ducts of the sudoriparous or sweat glands, by which it is probable that a large portion of the aqueous and gaseous materials excreted by the skin SUDORIPAROUS OR SWEAT GLANDS. 429 are separated. Each of these glands consists of a small lobular mass, which appears formed of a coil of tubular gland-duct, surrounded by blood-vessels and embedded in the subcutaneous adipose tissue (fig. 105). From this mass, the duct ascends, for a short distance, in a spiral manner through the deeper part of the cutis, then passing straight, and then sometimes again becoming spiral, it passes through the cuticle and opens by an oblique valve- like aperture. In the parts where the epidermis is thin, the ducts themselves are thinner and more nearly straight in their course (fig. 108). The canal of the duct, which maintains nearly the same diameter throughout, is lined with a layer of epithelium continuous with the epidermis ; and its walls are formed of pellucid membrane continuous with the surface of the cutis. The sudoriparous 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, where, according to Krause, they amount to 2736 in each superficial square inch, and according to Mr. Erasmus Wilson, to as many as 3528. They are almost equally abundant and large in the skin of the sole. The glands by which the peculiar odorous matter of the axillae is secreted form a nearly complete layer under the cutis, and are like the ordinary sudoriparous glands, except in being larger and having very short ducts. In the neck and back, where they are least numerous, the glands amount to 417 on the square inch (Krause). Their total number Krause estimates at 2,381,248; and, supposing the orifice of each gland to present a surface of -3^^ ^ a line in diameter (and regarding a line as equal to y^-th of an inch), he reckons that the whole of the glands would present an evaporating surface of about eight square inches.* * The peculiar bitter yellow substance secreted by the skin of the external auditory passage is named cerumen, and the glands themselves 43 THE SKIN. Sebaceous Glands. Besides the perspiration, the skin Fig. 108.* secretes a peculiar fatty matter, and Ifor this purpose is provided with another set of special organs, termed sebaceous glands (fig. 108), which, like the sudoriparous glands, are abun- dantly distributed 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 surfaces of the hands and the plantar surfaces of the feet. They are minutely lobulated glands, com- posed of an aggregate of small vesi- cles or sacculi filled with opaque white substances, like soft ointment. Minute capillary vessels overspread them; and their ducts, which have a beaded appearance, as if formed of rows of cells, 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 glands to each hair (fig. 1 08). Structure of Hair and Nails. Hair. A hair is produced by a peculiar growth and ceruminous glands ; but they do not much differ in structure from the ordinary sudoriparous glands. * Fig. 108. Sebaceous and sudoriparous glands of the skin (after Gurlt) : i, the thin cuticle ; 2, the cutis ; 3, adipose tissue ; 4, a hair,' in its follicle (5) ; 6, Sebaceous gland, opening into the follicle of the hair by an efferent duct ; 7, the sudoriparous gland. STRUCTURE OF HAIR. 43 1 modification 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, 109, 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, at least in the human subject, occupies the whole of the inside of the hair ; but Fig. iog* in some cases there is left a small central space filled by v a substance called the medulla or pith, composed of small collections of irregularly shaped cells, containing fat- and pigment-granules. The follicle, in which the root of each hair is contained (fig. no), forms a tubular depression from the surface of the skin, descending into the subcutaneous fat, generally to a greater depth than the sudoriparous glands, and at its deepest part enlarging in a bulbous form, and often curving from its previous rectilinear course. It is lined throughout by cells of epithelium, continuous with those of the epidermis, and its walls are formed of pellucid membrane, which commonly, in the follicles of the largest hairs, has the structure of vascular fibro-cellular tissue. At the bottom of the follicle is a small papilla, or projec- tion of true skin, and it is by the production and out- * Fig. 109. A, surface of a white hair, magnified 1 60 diameters. The waved lines mark the upper or free edges of the cortical scales. B, separated scales, magnified 350 diameters (after Kb'lliker). 43 2 THE SKIN. growth of epidermal cells from the surface of this papilla that the hair is formed. The inner wall of the follicle is Fig. 1 10.* Fig. iii.f * Fig. 1 10. Medium-sized hair in its follicle, magnified 50 diameters (from Kolliker). a, stem cut short ; b, root ; c, knob ; d, hair cuticle ; e, internal, and /, external root-sheath ; g, h, dermic coat of follicle ; i, papilla ; k, k, ducts of sebaceous glands ; I, corium ; m, mucous layer of epidermis ; o, upper limit of internal root-sheath (from Kolliker). t Fig. i ii. Magnified view of the root of a hair (after Kohlrausch). a, stem or shaft. of hair cut across ; b, inner, and c, outer layer of the epidermal lining of the hair-follicle, called also the inner and outer root- sheath ; d, dermal or external coat of the hair-follicle, shown in part ; STRUCTURE OF NAILS. 433 lined by epidermal cells continuous with those covering the general surface of the skin ; as if indeed the follicle had been formed by a simple thrusting in of the surface of the integument (figs. 110, 111). 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 com- monly 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 elongated, while the superficial are flattened, and of more horny consistence. 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 root 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 portion 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 laminse, on which are moulded the epidermal cells of which the nail is made up (fig. 112). The growth of the nail, like that of a 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 away. Inasmuch, however, as the e, imbricated scales about to form a cortical layer on the surface of the hair. The adjacent cuticle of the root- sheath is not represented, and the papilla is hidden in the lower part of the knob where that is represented lighter. F F 434 THE SKIN. posterior edge of the nail, from its being lodged in a groove of the skin, cannot grow backwards, on ad- ditions being made to it, so easily as it can pass in the op- posite direction, any growth at its hinder part sim- ply pushes the whole forwards. At the same time fresh cells are added to its un- ^ . der surface, and thus each portion of the nail becomes gradually thicker as it moves to the front, until, projecting beyond the 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. Excretion by the Skin. The skin, as already stated, is the seat of a two-fold excretion; of that formed by the sebaceous glands and hair-follicles, and of the more watery fluid, the sweat or perspiration, eliminated by the sudoriparous glands. The secretion of the sebaceous glands and hair-follicles * Fig. 112. Vertical transverse section through a small portion of the nail and matrix largely magnified (after Kolliker). A, corium of the nail-bed, raised into ridges or laminae a, fitting in between corresponding laminae &, of the nail. B, Malpighian, and O, horny layer of mil : d, deepest and vertical cells ; e, upper flattened cells of Malpighian layer. THE SWEAT. 435 (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 is said by Dr. Fischer to contain caproic, butyric, and rutic acids. It is, perhaps, nearly similar in composi- tion 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 both of olein and margarin (J. Davy). 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 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. The fluid secreted by the sudoriparous 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 diseases, and when evaporation is prevented by the application of oiled silk or plaster, the secretion becomes more sensible and collects on the skin in the form of drops of fluid. A good analysis of the secretion of these glands, unmixed with other fluids secreted from the skin, can scarcely be made ; for the quantity that can be collected pure is very small. Krause, in a few drops from the palm of the hand, found an acid reaction, oily matter, and margarin, with water. The perspiration of the skin, 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 F F 2 436 THE SKIN. for distinction, the former is called insensible perspiration ; the latter, 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. Thenard collected the perspiration in a flannel shirt which had been washed in distilled water, and found in it chloride of sodium, acetic acid, some phosphate of soda, traces of phosphate of lime, and oxide of iron, together with an animal substance. In sweat which had run from the forehead in drops, Berze- lius found lactic acid, chloride of sodium, and chloride of ammonium. Anselmino placed his arm in a glass cylinder, and closed the opening around it with oiled silk, taking care that the arm touched the glass at no point. The cutaneous exhalation collected on the interior of the glass, and ran down as a fluid : on analysing this, he found water, acetate of ammonia, and carbonic acid ; and in the ashes of the dried residue of sweat he found carbonate, sulphate, and phosphate of soda, and some potash, with chloride of sodium, phosphate and carbonate of lime, and traces of oxide of iron. Urea has also been shown to be an ordinary constituent of the fluid of perspiration. The ordinary constituents of perspiration may, there- fore, according to Gorup-Besanez, be thus summed up : water, fat, acetic, butyric and formic acids, urea, and salts. The principal salts are the chlorides of sodium and potas- sium, together with, in small quantity, alkaline and earthy phosphates and sulphates ; and, lastly, some oxide of iron. Of these several substances, none, however, need particular consideration, except the carbonic acid and water. The quantity of watery vapour excreted from the skin, was estimated very carefully by Lavoisier and Sequin. The latter chemist enclosed his body in an air-tight bag, EXHALATION FROM THE SKIN. 437 with a mouthpiece. The bag being closed by a strong band above, and the mouthpiece adjusted and gummed to the skin around the mouth, he was weighed, and then re- mained quiet for several hours, after which time he was again weighed. The difference in the two weights indi- cated 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 transpiration. The repetition of these experiments during a long period, showed that during a state of rest, the average loss by cutaneous and pulmonary exhalation in a minute, is from seventeen to eighteen grains, the minimum eleven grains, the maximum thirty-two grains; and that of the eighteen grains, eleven pass off by the skin, and seven by the lungs. The maximum loss by exhalation, cutaneous and pulmo- nary, in twenty-four hours, is about 3 J Ib. ; the minimum about l^- Ib. Valentin found the whole quantity lost by exhalation from the cutaneous and respiratory surfaces of a healthy man who consumed daily 40,000 grains of food and drink, to be 19,000 grains, or 2|- Ib. Subtracting from this, for the pulmonary exhalation, 5,OOO grains, and, for the excess of the weight of the exhaled carbonic acid over that of the equal volume of the inspired oxygen, 2,256 grains, the remainder, 11,744 grains, or nearly i|- Ib., may represent an average amount of cutaneous exhalation in the day. The large quantity of watery vapour thus exhaled from the skin, will prove that the amount excreted by simple transudation through the cuticle must be very large, if we may take Krause's estimate of about eight square inches for the total evaporating surface of the sudoriparous 43 8 THE SKIN. glands; for not more than about 3,365 grains could be evaporated from such, a surface in twenty-four hours, under the ordinary circumstances in which the surface of the skin is placed. This estimate is not an improbable one, for it agrees very closely with that of Milne Edwards, who calculated that when the temperature of the atmo- sphere is not above 68 F., the glandular secretion of the skin contributes only J to the total sum of cutaneous exhalation. The quantity of watery vapour lost by transpiration, is of course influenced by all external circumstances which affect the exhalation from other evaporating surfaces, such as the temperature, the hydrometric state, and the stillness of the atmosphere. But, of the variations to which it is subject under the influence of these conditions, no calcula- tion has been exactly made. Neither, until recently, has there been any estimate of the quantity of carbonic acid exhaled by the skin on an average, or in various circumstances. Regnault and Reiset attempted to supply this defect, and concluded, from some careful experiments, that the quantity of carbonic acid exhaled from the skin of a warm-blooded animal is about g-L of that furnished by the pulmonary respiration. Dr. Edward Smith's calculation is somewhat less than this. The cutaneous exhalation is most abundant in the lower classes of animals, more particularly the naked Am- phibia, 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 continued to be exhaled by the skin. 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 (Milne Edwards and Miiller). That the respiratory func- ABSORPTION BY THE SKIN. 439 tion of the skin is, perhaps, even more considerable in the higher animals than appears to be the case from the ex- periments of Regnault and Reiset just alluded to, is made probable by the fact observed by Magendie and others, that if the skin of animals is covered with an impermeable varnish, or the body enclosed, all but the head, in a caoutchouc dress, animals soon die, as if asphyxiated; their heart and lungs being gorged with blood, and their temperatures, during life, gradually falling many degrees, and sometimes as much as 36 F. below the ordinary standard (Magendie). Some recent experiments of Lashke- witzch appear, indeed, to confirm the opinion of Valentin, that loss of temperature is the immediate cause of death in these cases, although it is not easy to understand the cause of the loss of temperature. A varnished animal is said to have suffered no harm when surrounded by cotton wadding, but it died when the wadding was removed. Absorption by the skin has been already mentioned, as an instance in which that process is most actively accom- plished. 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- tai?,trate of antimony may excite vomiting, 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 440 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 absorption really occurs. M. Edwards has proved that 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 cuticle is thicker than in man, after having lost weight by being kept for some time in a dry atmosphere, were 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 were immersed, the water absorbed was distributed throughout the system. And a like absorption through the skin, though to a less extent, may take place also in man. Dr. Madden, having ascertained the loss of weight, by cutaneous and pulmonary transpiration, that occurred during half an hour in the air, entered the bath, and remained immersed during the same period of time, breath- ing through a tube which communicated with the air exterior to the room. He was then carefully dried and again weighed. Twelve experiments were performed in this manner; and in ten there was a gain of weight, varying from 2 scruples to 5 drachms and 4 scruples, or a mean gain of l drachm 2 scruples and 1 3 grains. The loss in the air during the same length of time (half an hour) varied in ten experiments from 2\ drachms to I ounce 2\ scruples, or in the mean was about 6\ drachms. So that, admitting the supposition that the cutaneous transpiration was entirely suspended, and estimating the loss by pulmonary exhalation at 3 drachms, there was, in these ten experiments of Dr. Madden, an average absorp- tion of 4 drachms I scruple and 3 grains, by the surface of the body, during half an hour. In four experiments ABSORPTION BY THE SKIN. 44 1 performed by M. Berthold, the gain in weight was greater than in those of Dr. Madden. 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 may be in part due to the hindrance to the evaporation of water from the skin. The absorption, also, of different kinds of gas by the skin is proved by the experiments of Abernethy, Cruik- shank, Beddoes, and others. In these cases, of course, the absorbed gases combine with the fluids, and lose the gaseous form. Several physiologists have observed an absorption of nitrogen by the skin. Beddoes says, that he saw the arm of a negro become pale for a short time when immersed in chlorine ; and Abernethy observed that when he held his hands in oxygen, nitrogen, carbonic acid, and other gases contained in jars, over mercury, the volume of the gases became considerably diminished. The share which the evaporation from the skin has in the maintenance of the uniform temperature of the body, and the necessary adaptation thereto of the production of heat, have been already mentioned (p. 252), 442 CHAPTER XV. THE KIDNEYS AND THEIR SECRETION. Structure of the Kidneys. THE kidney is covered on the outside by a rather tough fibrous capsule, which is slightly attached by its inner sur- face to the proper substance of the organ by means of very fine fibres of areolar tissue and minute blood-vessels. From the healthy kidney, therefore, it may be easily torn off without injury to the subjacent cortical portion of the Fig. 113-* organ. At the kilns or notch of the kidney, it becomes continuous with the external coat of the upper and dilated part of the ureter. On making a section length-wise through the kidney (fig. 113) 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 some- times called the pyramidal portion, from the fact of its being composed of about a * Fig. 113. Plan of a longitudinal section through the pelvis and substance of the right kidney, \ : 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 these unopened ; d, summit of the pyramids or papillae projecting 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 j u, the ureter ; s t the sinus ; A, the hilus. STRUCTURE OF THE KIDNEY. 443 Fig. 114-* 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 divisions, is finally subdivided into still smaller portions, varying in number from about 8 to 12, or even more, and called calyces. Each of these little calyces or cups, again, receives the pointed extremity or papilla of a pyramid. Sometimes, however, more than one papilla is received by a calyx. The kidney is a gland of the class called tubular, and both its cortical and medullary portions are composed essentially of secreting tubes, the tubuli uriniferi, which by one extremity, in the cortical portion, and commonly in little saccules containing blood-ves- sels, called Malpighian bodies, and by the other open through the papilla into the pelvis of the kidney, and thus discharge the urine which flows through them. In the pyramids they 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. The tubuli uriniferi (fig. 114) are composed of a nearly homogeneous membrane, lined internally by spheroidal epithelium, and for the greater part of their extent are about -- of an inch in diameter, becoming somewhat * Fig. 114. A. Portion of a secreting canal from the cortical substance of the kidney. B. The epithelium or gland-cells, more highly magnified (700 times). 444 THE KIDNEYS AND THEIR SECRETION, larger than this immediately before they open through the Fig. 115 .* Papilla. On tracing these tu- bules upwards from the papillae, they are found to divide dicho- tomously as they ascend through the pyramids, and on reaching the bases of the latter, they begin to branch and diverge more widely, and to form by their branches and convolutions the essential part of the cortical portion of the organ. At their extremities they become dilated into the Malpighian capsules. Until very recently, it has been believed that the straight tu- bules in the pyramids branch out and become convoluted im- mediately on reaching the bases of the pyramids ; but it is now commonly believed that between the straight tubes in the pyra- mids and the convoluted tubes in the cortical portion, there is a system of tubules of smaller diameter than either, which form inter-communications between the two varieties for- merly recognised. These intervening tubules, called the looped tubes of Henle, arising from the straight tubes in * Fig. 115. Diagram of the looped uriniferotis tubes and their con- nection with the capsules of the glomeruli (from Southey, after Ludwig). In the lower part of the figure one of the large branching tubes is shown opening on a papilla ; in the middle part two of the looped small tubes are seen descending to form their loops, and re-ascending in the medul- lary substance ; while in the upper or cortical part, these tubes, after some enlargement, are represented as becoming convoluted and dilated in the capsules of glomeruli. STRUCTURE OF THE KIDNEY. 445 some part of their course, or being continued from their extremities at the bases of the pyramids, pass down loop- wise in the pyramids for a longer or shorter distance, and then, again turning up, end in the convoluted tubes whose extremities are dilated into the Malpighian capsules before referred to (fig. 115). On a transverse section of a pyramid (fig. 1 1 6), these looped tubes are seen to be of much smaller calibre than the straight ones, which are passing down to open through the papillae. The Malpighian bodies are found only in the cortical part of the kidney. 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 -y-i^- of an inch. Each of them is com- posed of the dilated extremity of an urinary tube, or Malpighian capsule, enclosing a tuft of blood-vessels. In connection Fig. 116.* with these little bodies the general distribution of blood - vessels to the kidney may be here considered. The renal ar- tery divides into several branches, which, passing in at the hilus of the kidney, and ^ps/^*' j ^ ; " covered by a fine ^^^^Hl TK3^ sheath of areolar tissue derived from the capsule, enter the substance of the * Fig. 1 1 6. Transverse section of a renal papilla (from Kolliker), ^~. a, larger tubes or papillary ducts ; b, smaller tubes of Henle ; c, blood- vessels, distinguished by their flatter epithelium ; d, nuclei of the stronia. 446 THE KIDNEYS AND THEIE SECRETION. organ chiefly in the intervals between the papillae, and penetrate the cortical substance, where this dips down be- tween the bases of the pyramids. Here they form a tolerably dense plexus of an arched form, and from this are given off smaller arteries which ultimately supply the Malpighian bodies. ^ II7 * The small, afferent artery (fig. 117), which enters the Malpi- ghian body by perforating the capsule, breaks up in the interior 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 Malpighian 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 capil- lary vessels, is distributed on the exterior of the tubule, from whose dilated end it has 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 immediate radicles of the renal vein. The Malpighian capsule is lined by a layer of fine squa- mous epithelial cells ; but whether the small glomerulus or tuft of capillaries in the interior is covered by a similar layer is uncertain. Kolliker believes that such a covering, * Fig. 117. Diagram showing the relation of the Malpighian body to the uriniferous ducts and blood-vessels (after Bowman) : , one of the interlobular arteries ; a', afferent artery passing into the glomerulus ; c, capsule of the Malpighian body, forming the termination of and con- tinuous with t, the uriniferous tube ; e, e, efferent vessels which sub- divide in the plexus p, surrounding the tube, and finally terminate in the branch of the renal vein e. STRUCTURE OF THE KIDNEY. 447 although exceedingly thin, is present, and has delineated the appearance in the accompanying diagram (fig. 118). Besides the small afferent arteries of the Malpighian Fig. 1 1 8. bodies, there are, of course, others which are distri- buted in the ordinary manner, for nutrition's sake, to the different parts of the organ; and in the pyramids, between the tubes, there are numerous straight vessels, the vasa recta, the origin of which is somewhat uncertain, some observers supposing them to be branches of vasa efferentia from Mal- pighian bodies, and therefore comparable to the venous plexus around the tubules in the cortical portion, while others think that they arise directly from small branches of the renal arteries. Between the tubes, vessels, etc., which make up the main substance of the kidney, there exists in small quantity a fine matrix of areolar tissue. The nerves of the kidney are derived from the renal plexus, f J * Fig. 1 1 8. Semidiagrammatic representation of a Malpighian body in its relation to the uriniferous tube (from Kolliker), ^. a, capsule of the Malpighian body ; d, epithelium of the uriniferous tube ; ^de- tached epithelium ; /, afferent vessel ; g, efferent vessel ; h, convoluted vessels of the glomerulus. f For a more detailed accoune of the structure of the kidney and a summary of the various opinions on the subject, the student may be referred especially to Quain's Anatomy, 7th ed., and to a paper by Dr. Reginald Southey in vol. i. of the St. Bartholomew's Hospital Eeports. 448 THE KIDNEYS AND THEIR SECRETION. Secretion of Urine. The separation from the blood of the solids in a state of solution in the urine is probably effected, like other secretions, by the agency of the gland cells, and equally in all parts of the urine-tubes. The urea and uric acid, and perhaps some of the other constituents existing ready formed in the blood, may need only separation, that is they may pass from the blood to the urine without further elaboration; but this is not the case with some of the other principles of the urine, such as the acid phosphates and the sulphates, for these salts do not exist as such in the blood, and must be formed by the chemical agency of the cells. The watery part of the urine is probably in part sepa- rated by the same structures that secrete the solids, but the ingenious suggestion of Mr. Bowman that the water of the urine is mainly strained off, so to speak, by the Malpighian bodies, from the blood which circulates in their capillary tufts, is exceedingly probable ; although if, as Kolliker and others maintain, there is an epithelial cover- ing to these tufts or glomeruli, it is very likely that the solids of the urine may be in part secreted here also. We may, therefore, conclude that all parts of the tubular system of the kidney take part in the secretion of the urine as a whole, but that there is a provision also in the arrangement of the vessels in the Malpighian bodies for a more simple draining off of water from, the blood when required. The large size of the renal arteries and veins permits so rapid a transit of the blood through the kidneys, that the whole of the blood is purified by them. The secretion of urine is rapid in comparison with other secretions, and as each portion is secreted, it propels that which is already in the tubes onwards into the pelvis of the kidney. Thence through the ureter the urine passes into the bladder, into PASSAGE OF URINE INTO THE BLADDER. 449 which its rate and mode of entrance has been watched in cases of ectopia vesicse, i.e., of such fissures in the anterior and 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. Some good observations on such cases were made by Mr. Erichsen. 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 same observations, also, showed how fast some substances pass from the stomach through the circulation, and through the vessels of the kidneys. Ferrocyanide of potassium so passed on one occasion in a minute : vegetable substances, such as rhubarb, occupied from sixteen to thirty-five minutes ; neutral alkaline salts with vegetable acids, which were generally decomposed in transits, made the urine alkaline in from twenty-eight to forty-seven minutes. But the times of passage varied much ; and the transit was always slow when the substances were taken during digestion. The urine collecting in the urinary bladder 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, and then turning rather abruptly forwards, and opening through the latter, it collects till the distension of the bladder is felt either G G 450 THE URINE. by direct sensation, or in ordinary cases, by a transferred sensation at and near the orifice of the urethra. Then, the effort of the will being directed primarily to the muscles of the abdomen, and through them (by reason of its tendency to act with them) to the urinary bladder, the latter, though its muscular walls are really composed of invo- luntary muscle, contracts, and expels the urine. (See also P- 234-) The Urine : its general Properties. Healthy urine is a clear limpid fluid, of a pale yellow or amber colour, with a peculiar faint aromatic odour, which becomes pungent and ammoniacal when decomposition takes place. The urine, though usually clear and trans- parent at first, often becomes as it cools opaque and turbid from the deposition of part of its constituents pre- viously held in solution ; and this may be consistent with health, though it is only in disease that, in the tempera- ture of 98 or 1 00, at which it is voided, the urine is turbid even when first expelled. Although ordinarily of pale amber colour, yet, consistently with health, the urine may be nearly colourless, or of a brownish or deep orange tint, and between these extremes, it may present every shade of colour. When secreted, and most commonly when first voided, the urine has a distinctly acid reaction in man and all car- nivorous animals, and it thus remains till it is neutralized or made alkaline by the ammonia developed in it by decomposition. In most herbivorous animals, on the con- trary, 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 sub- SPECIFIC GRAVITY OF URINE. 45 T stances, presents the acid reaction and other qualities of the urine of Carnivora, its ordinary alkalinity being re- stored only on the substitution of a vegetable for the animal diet (Bernard). 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 car- bonic or vegetable acids ; for these latter are changed into alkaline carbonates previous to elimination by the kidneys. Except in these cases, it is very rarely alkaline, unless ammonia has been developed in it by decomposition com- mencing before it is evacuated from the bladder. The average specific gravity of the human urine is about IO2O. Probably no other animal fluid presents so many varieties in density within twenty-four hours as the urine does; for the relative quantity of water and of solid constituents of which it is composed is materially influ- enced 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 accidental 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 urina potus indicates the urine secreted shortly after the introduction of any considerable quantity of fluid into the body : and the urina cibi the por- tions secreted during the period immediately succeeding a meal of solid food. The last kind contains a larger quantity of solid matter than either of the others; the first or second, being largely diluted with water, possesses a com- paratively low specific gravity. Of these three kinds, the morning-urine is the best calculated for analysis, since it represents the simple secretion unmixed with the elements G G 2 452 THE URINE. 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. In accordance with the various circumstances above-mentioned, the specific gravity of the urine may, consistently with health, range widely on both sides of the usual average. The average healthy range may be stated at from 1015 in the winter to 1025 in the summer (Prout), and variations of diet and exercise may make as great a difference. In disease, the variation 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 (Watson). The whole quantity of urine secreted in twenty-four hours is subject to variation according to the amount of fluid drunk, and the proportion of the latter passing off from the skin, lungs, and alimentary canal. It is because the secretion of the skin is more active in summer than in winter, that the quantity of urine is smaller, and its specific gravity proportionately higher. On taking the mean of numerous observations by several experimenters, Dr. Parkes found that the average quantity voided in twenty-four hours by healthy male adults from twenty to forty years of age, amounted to 52 J fluid ounces. Chemical Composition of the Urine. The urine consists of water, holding in solution certain animal and saline matters as its ordinary constituents, and occasionally various matters taken into the stomach as food salts, colouring matter, and the like. The quan- tities of the several natural and constant ingredients of the urine are stated somewhat differently by the different chemists who have analysed it ; but many of the differences are not important, and the well-known accuracy of the several chemists renders it almost immaterial which of the analyses is adopted. The analysis by A. Becquerel being adopted by Dr. Prout, and by Dr. Golding Bird, will be here employed. (Table I.) COMPOSITION OF URINE. 453 Table II. has been compiled from the observations of Dr. Parkes, and of numerous other authors quoted in his admirable work on the urine. TABLE I. Average quantity of each constituent of the Urine in 1000 parts. Water 967- Urea 14-230 Uric acid "468 Colouring matter. .... ) inseparable from ) if the observed. When, for example, an animal's right cms cerebelli is divided, he rolls from his own right to his own left, but from the left to the rigLt of one who is standing in front of him. M M 2 532 THE NERVOUS SYSTEM. much rapidity; as often, according to M. Magendie, as sixty times in a minute, and may last for several days. Similar movements have been observed in men ; as by M. Serres in a man in whom there was apoplectic effusion in the right crus cerebelli ; and by M. 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 im- perfect 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 thus made almost complete. STRUCTURE AND PHYSIOLOGY OF THE CEREBRUM. The cerebrum is placed in connection with the pons and medulla oblongata by its-two crura or peduncles (fig. 142) : 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 from the inferior vermiform process of the cerebellum to the corpora quadrigemina of the cerebrum. These parts, which thus connect the cerebrum with the other princi- pal divisions of the cerebro- spinal nervous centre, form parts of the walls of a cavity (the fourth ventricle) and a canal (the iter a tertio ad quartum ventriculum) , which are the continuation of the canal that in the foetus extended through the whole length of the spinal cord and brain. * See such cases collected and recorded by Dr. Paget in the Ed. Med. and Surg. Journal for 1847. THE CEREBRUM. 533 They may, therefore, be regarded as the continuation of the cerebro- spinal axis or column ; on which, as a develop- ment from the simple type, the cerebellum is placed ; and, on the further continuation of which, structures both larger and more numerous are raised, to form the cerebrum (%. 142). Fig. 142.* The cerebral convolutions appear to be formed of nearly parallel plates of fibres, the ends of which are turned towards the surface of the brain, and are overlaid and mingled with successive layers of grey nerve- substance. The external grey matter is so arranged in layers, that a * Fig. 142. Plan in outline of the encephalon, as seen from the right side. \. (From Quain). The parts are represented as separated from one another somewhat more .than natural, so as to show their connections. A, cerebrum ; /, g, h, its anterior, middle, and posterior lobes ; e, fissure of Sylvius ; B, cerebellum ; C, pons Yarolii ; D, medulla oblongata ; a, peduncles of the cerebrum ; b, c, d, superior, middle, and inferior pedun- cles of the cerebellum. 534 THE NERVOUS SYSTEM. vertical section of a convolution generally presents the appearance of three layers of grey with two intervening layers of white substance, a grey layer being most ex- ternal. In these grey layers, the outer is formed prin- cipally of granular matter and nuclei, like those of nerve corpuscles ; in the deeper layer are more perfectly formed cells. It is nearly certain that the cerebral hemispheres are the organ by which, 1st, we perceive those clear and more impressive sensations which we can retain, and according to which we can judge; 2ndly, by which are performed those acts of will, each of which requires a deliberate, however quick, determination ; ^rdly, they are the means of retaining impressions of sensible things, and reproducing them in subjective sensations and ideas ; 4thly, they are the medium of the higher emotions and feelings, and of the faculties of judgment, understanding, memory, reflection, induction, and imagination, and others of the like class. The evidences that the cerebral hemispheres have the functions indicated above, are chiefly these: I. That 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. That in the same general proportion as the higher sensuous mental faculties are developed in the ver- tebrate animals, and in man at different ages, the more is the size of the cerebral hemispheres developed in com- parison with the rest of the cerebro- spinal system. 3. That no other part of the nervous system bears a corresponding proportion to the development of the mental faculties. 4. That congenital and other morbid defects of the cerebral hemispheres are, in general, accompanied with correspond- ing deficiency in the range or power of the intellectual faculties and the higher instincts. Respecting the mode in which the brain discharges its FUNCTIONS OF THE CEREBRUM. 535 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 disorganized or atrophied that it could not be supposed capable of dis- charging its functions. The remaining hemisphere was, in these cases, adequate to the functions generally dis- charged by both ; but the mind does not seem in any of these cases to have been tested in very high intellectual exercises; so that it is not certain that one hemisphere will suffice for these. In general, the mind combines, as one sensation, the impressions which it derives from one object through both hemispheres, 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 destruc- tion or compression 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. In speaking of the cerebral hemispheres as the so-called organs of the mind, they have been regarded as if they were single organs, of which all parts are equally appro- priate 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 among those collected by Drs. Gall and Spurzheim are as follows : I. That it is in accordance with the physiology of the other 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 536 THE NERVOUS SYSTEM. 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 predomi- nant 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 mind is deranged, 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 childhood, 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 indicated 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. These facts have been so illustrated and adapted by phrenologists, that the theory of the plurality of organs in the cerebrum, thus made probable, has been commonly THE CORPUS CALLOSUM. 537 regarded as peculiar to phrenology, and as so essentially connected with it, that if the system of Gall and Spurzheim be untrue, this theory cannot be maintained. But it is plain that all the system of phrenology built upon the theory may be false, and the theory itself true ; for the school of Gall and Spurzheim assume, not only this theory, but also that they have determined all the primitive facul- ties of which the mind consists, i.e., all the faculties to which special organs must be assigned, and the places of all those organs in the cerebral hemispheres and the cere- bellum. That this is a system of error there need be no doubt, but it is possibly founded on a true theory : the cerebrum may have many organs, and the mind as many faculties ; but what are the faculties that require separate organs, and where those organs are situate, are subjects of which only the most general and rudimentary knowledge has been yet attained. Of the physiology of the other parts of the brain, little or nothing can be said. Of the offices of the corpus callosum, or great transverse and oblique commissure of the brain, nothing positive is known. But instances in which it w^as absent, or very deficient, either without any evident mental defect, or with only such as might be ascribed to coincident affections of other parts, make it probable that the office which is com- monly assigned to it, of enabling the two sides of the brain to act in concord, is exercised only in the highest acts of which the mind is capable. And this view is confirmed by the very late period of its development, and by its absence in all but the placenta! Mammalia.* To the fornix and other commissures no special function * See cases of congenital deficiency of the corpus callosum, by Mr. Paget and Mr. -Henry in the twenty-ninth and thirty-first volumes of the Medico- Chirurgical Transactions. 538 THE NERVOUS SYSTEM. can be assigned; but it is a reasonable hypothesis that Fig. 143.* * Fig. 143. Yiew of the corpus callosum from above (from Sappey after Foville). \. 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. I, the upper surface of the corpus callosum; 2, median furrow or raphe ; 3, longitudinal strise bounding the furrow ; 4, swelling formed by the transverse bands as they pass into the cere- brum ; 5, anterior extremity or knee of the corpus callosum ; 6, posterior extremity ; 7, anterior, and 8, posterior part of the mass of fibres pro- ceeding from the corpus callosum ; 9, margin of the swelling ; 10, ante- rior part of the convolution of the corpus callosum ; 1 1, hem or band of union of this convolution ; 12, internal convolutions of the parietal lobe ; 13, upper surface of the cerebellum. THE CEEEBEAL NERVES. 539 they connect the action of the parts between which they are severally placed, As little is known of the functions of the pineal and pituitary glands. Indeed, Oesterlen and others raise the question whether either their structure or functions are those of nervous organs, and class them among the glands without ducts. PHYSIOLOGY OF THE CEREBRAL AND SPINAL NERVES. The cerebral nerves are commonly enumerated as nine pairs ; but the number is in reality twelve, the seventh nerve consisting, as it does, of two nerves, and the eighth of three. These and the spinal nerves, of which there are thirty-one pairs, symmetrically arranged on each side of what, reduced to its simplest form, may be regarded as a column or axis of nervous matter, extending from the olfactory bulbs on the ethmoid bone to ihefilum terminate of the spinal cord in the lumbar and sacral portion of the vertebral canal. The spinal nerves all present certain characters in common, such as their double roots ; the isolation of the fibres of sensation in the posterior roots, and those of motion in the anterior roots ; the formation of the ganglia on the posterior root ; and the subsequent mingling of the fibres in trunks and branches of mixed functions. Similar characters pro- bably belong essentially to the cerebral nerves ; but even when one includes the nerves of special sense, it is not possible to discern a conformity of arrangement in any besides the fifth, or trifacial, which, from its many analogies to the spinal nerves, Sir Charles Bell designed as a spinal nerve of the head. According to their several functions, the cerebral or cranial nerves may be thus arranged : Nerves of special sense . Olfactory, optic, auditory, part of the glosso- pharyngeal, and the lingual branch of the fifth. ,, of common sensation The greater portion of the fifth, and part of the glosso-pharyngeal. 540 THE NERVOUS SYSTEM. Nerves of motion . . . Third, fourth, lesser division of the fifth, sixth, facial, and hypoglossal. Mixed nerves .... Pneumogastric, and accessory. The physiology of the several nerves of the special senses will be considered with the organs of those senses. of the Third, Fourth, and Sixth Cerebral or Cranial Nerves. The physiology of these nerves may be in some degree combined, because of their intimate connection with each other in the actions of the muscles of the eyeball, which they supply. They are probably all formed exclusively of motor fibres : some pain is indicated when the trunk of the third nerve is irritated near its origin ; but this may be because of some filaments of the fifth nerve running back- wards to the brain in the trunk of the third, or because adjacent sensitive parts are involved in the irritation. The third nerve, or motor oculi, supplies the levator palpebrse superioris muscle, and, of the muscles of the eye- ball, all but the superior oblique or trochlearis, 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 the motor filaments to the iris. When the third nerve is irritated within the skull, all those muscles to which it is distributed are convulsed. When it is paralyzed or divided, the following effects ensue : first, the upper eyelid can be no longer raised by the levator palpebrse, but drops and remains gently closed over the eye, under the unbalanced influence of the orbi- cularis palpebrarum, which is supplied by the facial nerve : secondly, the eye is turned outwards by the unbalanced action of the rectus externus, to which the sixth nerve is appropriated : and hence, from the irregularity of the axes THE THIED CEREBRAL NERVE. 541 of the eyes, double- sight is often experienced when a single object is within view of both the eyes : thirdly, the eye cannot be moved either upwards, downwards, or inwards ; fourthly, the pupil is dilated. The relation of the third nerve to the iris is of peculiar interest. In ordinary circumstances the contraction of the iris is a reflex action, which may be explained as produced by the stimulus of light on the retina being conveyed by the optic nerve to the brain (probably to the corpora quadrigemina), 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 corpora quadrigemina are destroyed or much compressed. But 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 contraction of the iris in which its fibres are distributed. The contraction of the iris thus shows all the character of a reflex act, and in ordinary cases requires the concurrent action of the optic nerve, corpora quadrigemina, and third nerve ; and, probably also, considering the peculiarities of its perfect mode of action, the ophthalmic ganglion. But, besides, both irides will contract their pupils under the reflected stimulus of light falling only on one retina or under irritation of one optic nerve. Thus, in amaurosis of one eye, its pupil may contract when the other eye is ex- posed to a stronger light : and generally the contraction of each of the pupils appears to be in direct proportion to the total quantity of light 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 inwards, or upwards and inwards, by the action of the third nerve distributed in the rectus internus and 542 THE NERVOUS SYSTEM. rectus superior, the iris contracts, as if under direct volun- tary 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. The fourth nerve, or Nervus trochlearis or patlieticm, is exclusively motor, and supplies only the trochlearis or obliquus superior muscle of the eyeball. The sixth nerve, Nervus dbducens or ocularis externus, is also, like the fourth, exclusively motor, and supplies only the rectus externus muscle.* The rectus externus is, therefore, convulsed, and the eye is turned outwards, when the sixth nerve is irritated; and the muscle paralyzed when the nerve is disorganized, compressed, or 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 within the cavity of the skull does. But the import of these communications with the sympathetic, and the subsequent distribution of its fila- * In several animals it sends filaments to the iris (Radclyffe Hall) ; and it has probably done so in man, in some instances in which the iris has not been paralyzed, while all the other parts supplied by the third nerve were (Grant). MOTOR NERVES OF THE EYE. 534 merits after joining the sixth, nerve, are quite unknown; and there is no reason to believe that the sixth nerve is, in function, more closely connected with the sympathetic than any other cerebral nerve is. The question has often suggested itself why the six muscles of the eyeball should be supplied by three motor nerves when all of them are within reach of the branches of one nerve ; and the true explanation would have more interest than attaches to the movements of the eye alone ; since it is probable, that we have, in this instance, within a small space, an example of some general rule according to which associate or antagonist muscles are supplied with motor nerves. Now, in the several movements of the eyes, we sometimes have to act with symmetrically-placed muscles, as when both eyes are turned upwards or downwards, inwards or outwards.* All the symmetrically-placed muscles are sup- plied with symmetrical nerves, i.e., with corresponding branches of the same nerves on the two sides; and the action of these symmetrical muscles is easy and natural, as we have a natural tendency to symmetrical movement in most parts. But because of this tendency to symme- trical movements of muscles supplied by symmetrical nerves, it would appear as if, when the two eyes are to be moved otherwise than symmetrically, the muscles to effect such a movement must be supplied with different nerves. So, when the two eyes are to be turned towards one side, say the right, by the action of the rectus externus of the right eye and the rectus internus of the left, it appears as if the tendency to action through the similar branches of corresponding nerves (which would move both eyes in- wards or outwards) were corrected by one of these muscles * It is sometimes said, that the external recti cannot be put in action simultaneously : yet they are so when the eyes, having been both directed inwards, are restored to the position which they have in looking straight forwards. 544 THE NERVOUS SYSTEM. being supplied by the sixth, and the other by the third nerve. So with the oblique muscles : the simplest and easiest actions would be through branches of the corre- sponding nerves, acting similarly as symmetrical muscles ; but the necessary movements of the two eyes require the contraction of the superior oblique of one side, to be asso- ciated with the contraction of the inferior oblique and the relaxation of the superior oblique, of the opposite side. For this, the fourth nerve of one side is made to act with a branch of the third nerve of the other; as if thus the tendency to simultaneous action through the similar nerves of the two sides were prevented. At any rate, the rule of distribution of nerves here seems to be, that when in frequent and necessary movements any muscle has to act with the antagonist of its fellow on the opposite side, it and its fellow's antagonist are supplied from different nerves. Physiology of the Fifth or Trigeminal Nerve. 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 sensitive, 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 sensitive root to join the third branch or division which issues from it. The first and second divisions of the nerve, which arise wholly from the larger root, are purely sensitive. The third division being joined, as before said, by the motor root of the nerve, is of course both motor and sensitive. Through the branches of the greater or ganglionic por- tion 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 THE FIFTH CEREBRAL NERVE. 545 sense, from which, common sensations are conveyed through the fifth nerve, and their peculiar sensation through their several nerves of special sense. The muscles, also, of the face and lower jaw acquire muscular sensibility through the filaments of the ganglionic portion of the fifth nerve distributed to them with their proper motor nerves. Through branches of the lesser or non-ganglionic por- tion of the fifth, the muscles of mastication, namely, the temporal, masseter, two pterygoid, anterior part of the digastric, and mylo-hyoid, derive their motor nerves. The motor function of these branches is proved by the violent contraction of all the muscles of mastication in experi- mental irritation of the third, or inferior maxillary, division of the nerve ; by paralysis of the same muscles, when it is divided or disorganized, 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 paralyzed together with the other muscles that are supplied by the facial, but retains its power when the other muscles of mastication are paralyzed. Whether, however, the branch of the fifth nerve which is supplied to the buccinator muscle is entirely sensitive, 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, sensitive ; although it is not quite certain that it may not give a few motor filaments to the buccinator muscle. N N 546 THE NERVOUS SYSTEM. The sensitive function of the branches of the greater division of the fifth nerve is proved by all the usual evi- dences, such as their distribution in 'parts that are sensitive and not capable of muscular contraction, the exceeding sensibility of some of these parts, their loss of sensation when the nerve is paralyzed or divided, the pain without convulsions produced by morbid or experimental irritation 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. But although formed of sensitive filaments exclusively, the branches of the greater or ganglionic portion of the fifth nerve exercise a manifold influence on the movements of the muscles of the head and face, and other parts in which they are distributed. They do so, in the first place, by providing the muscles themselves with that sensibility without which the mind, being unconscious of their position and state, cannot voluntarily exercise them. It is, pro- bably, 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 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 feeding may cease, or be im- perfect ; a fact which led Sir Charles Bell into one of the very few errors of his physiology of the nerves. He sup- posed that the motion of the upper lip, in grasping food, THE FIFTH CEREBRAL NERVE. 547 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. Mr. 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 Sir C. 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. Lastly, the fifth nerve has an intimate connection with muscular 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, that of the tongue no flowing of saliva ; and although tears and saliva may flow naturally, their afflux is not increased by the me- chanical or chemical or other stimuli, to the indirect or reflected influence of which it is liable in the perfect state of this nerve. The fifth nerve, through its ciliary branches and the branch which forms the long root of the ciliary or ophthal- mic ganglion, exercises also some influence on the move- ments of the iris. When the trunk of the ophthalmic portion is divided, the pupil becomes, according to Valentin, contracted in men and rabbits, 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 N N 2 548 THE NERVOUS SYSTEM. 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 divi- sion to the iris ; or, as Dr. R. Hall ingeniously suggests, the influence of the fifth nerve on the movements of the iris may be ascribed to the affection of vision in con- sequence 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, in- creased circulation making the retina more irritable might induce extreme contraction of the iris ; or, under moderate stimulus of light, producing partial blindness, might induce dilatation : but it does not appear why, if this be the true explanation, the iris should in either case be immovable and unaffected by the various degrees of light. Furthermore, the complete paralysis on division of the fifth nerve, by the morbid effects which it produces in the organs of special sense, makes it probable that, in the nor- mal state, the fifth nerve exercises some indirect influence on all these organs or their functions. Thus, after such complete paralysis, 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 inflam- matory process ensues in the conjunctiva, sclerotica, and interior parts of the eye ; and within one of a few weeks, the whole eye may be quite disorganized, 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 im- paired ; so may the hearing ; and commonly, whenever the fifth nerve is paralysed, the tongue loses the sense of taste in its anterior and lateral parts, i.e., in the portion in which the lingual or gustatory branch of the inferior maxillary division of the fifth is distributed.* 4 * That complete paralysis of the fifth nerve may, however, be unac- companied, at least, for a considerable period, by injury to the organs THE FIFTH CEREBRAL NERVE. 549 The loss of the sense of taste is no doubt chiefly due to the lingual branch of the fifth nerve being a nerve of special sense; partly, also, perhaps, it is due 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. 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 the 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. According to Magendie and Longet, destruction of the eye ensues more quickly after division of the trunk of the fifth beyond the Gasserian ganglion, or after divi- sion of the ophthalmic branch, than after division of the roots of the fifth between the brain and the ganglion. of special sense, with the exception of that portion of the tongue which is supplied by its gustatory branch, is well illustrated by a valuable case lately recorded by Dr. Althaus. * Two of the best cases are published, with analyses of others, by Mr. Dixon, in the Medico-Chirurgical Transactions, vol. xxviii. 550 THE NERVOUS SYSTEM. Hence it would appear as if the influence on nutrition were conveyed through the filaments of the sympathetic, Which join the branches of the fifth nerve at and beyond the Gasserian ganglion, rather than through the filaments of the fifth itself; and this is confirmed by experiments in which extirpation of the superior cervical ganglion of the sympathetic produced the same destructive disease of the eye that commonly follows the division of the fifth nerve. And yet, that the filaments of the fifth nerve, as well as those of the sympathetic, may conduct such influence, ap- pears certain from the cases, including that by Mr. Stanley, in which the source of the paralysis of the fifth nerve was near the brain, or at its very origin, before it receives any communication from the sympathetic nerve. The existence of ganglia of the sympathetic in connection with all the principal divisions of the fifth nerve where it gives off those branches which supply the organs of special sense for example, the connection of the ophthalmic ganglion with the ophthalmic nerve at the origin of the ciliary nerves ; of the spheno-palatine ganglion with the superior maxillary division, where it gives its branches to the nose and the palate; of the otic ganglion with the inferior maxillary near the giving off of filaments to the internal ear ; and of the sub-maxillary ganglion with the lingual branch of the fifth all these connections suggest that a peculiar and probably conjoint influence of the sympa- thetic and fifth nerves is exercised in the nutrition of the organs of the special senses; and the results of experi- ment and disease confirm this, by showing that the nutrition of the organs may be impaired in consequence of impair- ment of the power of either of the nerves. A possible connection between the fifth nerve and the sense of sight, is shown in cases of no unfrequent occur- rence, in which blows or other injuries implicating the frontal nerve as it passes over the brow, are followed by total blindness in the corresponding eye. The blindness THE FACIAL NERVE. 551 appears to be the consequence of defective nutrition of the retina; for although, in some cases, it has ensued imme- diately, as if from concussion of the retina, yet in some it has come on gradually like slowly progressive arnaurosis, and in some with inflammatory disorganisation, followed by atrophy of the whole eye.* Physiology of ike Facial Nerve. The facial, or portio dura of the seventh pair of nerves, 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 (p. 545) ; it supplies, also, through the connection of its trunk with the Vidian nerve, by the petrosal nerves, some of the muscles of the soft palate, most probably the levator palati and azygos uvulse ; 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 lingualis and some other muscular fibres of the tongue ; 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. To the greater number of these muscles it is the sole motor nerve, and it is probably exclusively motor in its power ; no pain is produced by irritating it near its origin (Valentin) and the indications of pain which are elicited when any of its branches are irritated, may be explained by the abundant communications which, in all parts of its course, it forms with sensitive nerves, whose filaments being mingled with its own are the true source of the pain. Such communications are effected with the fifth nerve through the petrosal nerves, and probably also through the chorda tympani, and with the pneumogastric nerve through its auricular branch, even before the facial leaves the cranium. * Such a case is recorded by Snabilie in the Nederlandsch Lancet, August, 1846, 55 2 THE NERVOUS SYSTEM. When the facial nerve is divided, or in any other way paralyzed, the loss of power in the muscles which it sup- plies, 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 un- balanced action of the levator palpebrse ; and the conjunc- tiva, 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. In paralysis of the facial nerve, also, tears are apt to flow constantly over the face, apparently because of the paralysis of the tensor tarsi muscle, and the loss of the proper direction and form of the orifices of the puncta lacrymalia. From these circumstances, the sense of sight is impaired. The sense of hearing, also, is impaired in many cases of paralysis of the facial nerve ; not only in such as are instances of simultaneous disease in the auditory nerves, but in such as may be 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 distri- buted ; 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 only by the influence which, through the chorda tympani, it THE FACIAL NERVE. 553 exercises on the movements of the lingualis and the adjacent muscular fibres of the tongue ; and, according to some, or probably in some animals, on the movements of the stylo-glossus. We may therefore suppose that the accurate movement of these muscles in the tongue is in some way connected with the proper exercise of taste. Together with these effects of paralysis of the facial nerve, the muscles of the face being all powerless, the countenance acquires on the paralyzed side a characteristic, vacant look, from the absence of all expression : the angle of the mouth is lower, and the paralyzed half of the mouth looks longer than that on the other side; the eye has an unmeaning stare. All these peculiarities increase, the longer the paralysis lasts ; and their appearance is exag- gerated 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. As a nerve of expression, the seventh nerve must not be considered independent of the fifth nerve, with which it forms so many communications ; for, although it is through the facial nerve alone that all the muscles of the face are put into their naturally expressive actions, yet the power which the mind has of suppressing or controlling all these expressions can only be exercised by voluntary and well- educated actions directed through the facial nerve with the guidance of the knowledge of the state and position of every muscle, and this knowledge is acquired only through the fifth nerve, which confers sensibility on the muscles, and appears, for this purpose, to be more abundantly sup- plied to the muscles of the face than any other sensitive nerve is to those of other parts. 554 THE NERVOUS SYSTEM. Physiology of the Glosso-Pharyngeal Nerve. The glosso-pharyngeal nerves (16, fig. 144), in the enume- ration 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 pneumogastric and accessory nerves. But the union of the nerves under one term is inconvenient, although in some parts the glosso-pharyngeal and pneu- mogastric are so combined in their distribution that it is impossible to separate them in either anatomy or phy- siology. The glosso-pharyngeal nerve appears to give 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 communi- cating, 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 mem- brane, 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. Some experiments make it probable that the glosso- pharyngeal nerve contains, even at its origin, some motor fibres, together with those of common sensation and the sense of taste. Whatever motor influence, however, is conveyed directly through the branches of the glosso- pharyngeal, may be ascribed to the filaments of the pneu- mogastric or accessory that are mingled with it. The experiments of Dr. John Reid, confirming those of THE GLOSSO-PHARYNGEAL NERVE. 555 Panizza and Longet, tend to the same conclusions; and their results probably express nearly all the truth regard- ing the part of the glosso-pharyngeal nerve which is distributed to the pharynx. These results were that, I. Pain was produced when the nerve, particularly its pharyngeal branch, was irritated. 2. Irritation of the nerve before the origin of its pharyngeal, or of any of these branches, gave rise to extensive muscular motions of the throat and lower part of the face : but when the nerve was divided, these motions were excited by irritating the upper or cranial portion, while irritation of the lower end, or that in connection with the muscles, was followed by no movement ; so that these motions must have depended on a reflex influence transmitted to the muscles through other nerves by the intervention of the nervous centres. 3. When the functions of the brain and medulla oblon- gata were arrested by poisoning the animal with prussic acid, irritation of the glosso-pharyngeal nerve, before it was joined by any branches of the pneumogastric, gave rise to no movements of the muscles of the pharynx or other parts to which it was distributed ; while, on irrita- ting the pharyngeal branch of the pneumogastric, or the glosso-pharyngeal nerve, after it had received the commu- nicating branches just alluded to, vigorous movements of all the pharyngeal muscles and of the upper part of the oesophagus followed. The most probable conclusion, therefore, may be that what motor influence the glosso-pharyngeal nerve may seem to exercise, is due either to the filaments of the pneumogastric or accessory that are mingled with it, or to impressions conveyed through it to the medulla oblongata, and thence reflected to muscles through motor nerves, especially the pneumogastric, accessory, and facial. Thus, the glosso-pharyngeal nerve excites, through the medium of the medulla oblongata, the actions of the muscles of deglutition. It is the chief centripetal nerve engaged in 556 THE NERVOUS SYSTEM. these actions ; yet not the only one, for, as Dr. John Reid has shown, the acts are scarcely disturbed or retarded when both the glosso-pharyngeal nerves are divided. But besides being thus a nerve of common sensation in the parts which it supplies, and a centripetal nerve through which impressions are conveyed to be reflected to the adjacent muscles, the glosso-pharyngeal is also a nerve of special sensation ; being the gustatory nerve, or nerve of taste, in all the parts of the tongue to which it is distri- buted. 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 both nerves of this special function. For very numerous experiments and cases have shown that when the trunk of the fifth nerve or its lingual branch is paralysed or divided, the sense of taste is completely lost in the superior surface of the anterior and lateral parts of the tongue. The loss is instantaneous after division of the nerve ; and, therefore, cannot be ascribed to the defective nutrition of the part, though to this, per- haps, may be ascribed the more complete and general loss of the sense of taste when the whole of the fifth nerve has been paralysed. But, on the other hand, while the loss of taste in the part of the tongue to which the lingual branch of the fifth nerve is distributed proves that to be a gustatory nerve, the fact that the sense of taste is at the same time retained in the posterior and postero-lateral parts of the tongue, and in the soft palate and its anterior arch, to which (and to some parts of which exclusively) the glosso- pharyngeal is distributed, proves that this also must be a gustatory nerve. In a female patient at St. Bartholomew's Hospital, the left lingual branch of the fifth nerve was divided in removing a portion of the lower jaw : she lost both common sensation and the sensation of taste in the tip and the anterior parts of the left half of the tongue, THE PNEUMOGASTRIC NERVE. 557 but retained both, in all the rest of the tongue. M. Lisfranc and others have noted similar cases ; and the phenomena in them are so simple and clear, that there can scarcely be any fallacy in the conclusion that the lingual branches of both the fifth and the glosso-pharyngeal nerves are gus- tatory nerves in the parts of the tongue which they severally supply. This conclusion is confirmed by some experiments on animals, and, perhaps, more satisfactorily as concerns the sense of taste in man, by observation of the parts of the tongue and fauces, in which the sense is most acute. According to Valentin's experiments made on thirty students, the parts of the tongue from which the clearest sensations of taste are derived, are the base, as far as the foramen caecum and lines diverging forwards on each side from it ; the posterior palatine arches down to the epi- glottis ; the tonsils and upper part of the pharynx over the root of the tongue. These are the seats of the distribu- tion of the glosso-pharyngeal nerve. The anterior dorsal surface, and a portion of the anterior and inferior surface of the tongue, in which the lingual branch of the fifth is alone distributed, conveyed no sense of taste in the ma- jority of the subjects of Valentin's experiments ; but even if this were generally the case, it would not invalidate the conclusion that, in those who have the sense of taste in the anterior and upper part of the tongue, the lingual branch of the fifth is the nerve by which it is exercised. Physiology of the Pneumogastric Nerve. The pneumogastric nerve, nervus vagus, or par vagum (l, fig. 144), has, of all the cranial and spinal nerves, the most various distribution, and influences the most various func- tions, either through its own filaments, or those which, derived from other nerves, are mingled in its branches. The parts supplied by the branches of the pneumogastric 55^ THE NERVOUS SYSTEM. nerve are as follows : by its pharyngeal branches, which enter the pharyngeal plexus, a large portion of the mucous membrane, and, probably, all the muscles of the pharynx ; 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 ; 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; by cesophageal branches, the mucous membrane and muscular coats of the oesophagus. Moreover, the branches of the pneumogastric nerve form a large portion of the supply of nerves to the heart and the great arteries through the cardiac nerves, derived from both the trunk and the recurrent nerve; to the lungs, through both the anterior and the posterior pulmonary plexuses; and to the stomach, by its terminal branches passing over the walls of that organ. From the parts thus enumerated as receiving nerves from the pneumogastric, it might be assumed that this latter is a nerve of mixed function, both sensitive and motor. Expe- riments prove that it is so from its origin, for the irritation of its roots, even within the cranial cavity, produces both pain and convulsive movements of the larynx and pharynx ; and when it is divided within the skull, the same move- ments follow the irritation of the distal portion, showing that they are not due to reflex action. Similar experiments prove that, through its whole course, it contains both sensitive 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 the branches of the pneumogastric. The likeness THE PNEUMOGASTBIC NERVE. , 559 to the sympathetic which it thus acquires, is further in- creased by its containing 1 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 importantly communicates with the pneu- mogastric is the accessory nerve, whose internal branch joins its trunk, and is lost in it. Properly, therefore, the pneumogastric might be re- garded as a triple-mixed nerve, having out of its own sources, motor, sensitive, and sympathetic or ganglionic nerve-fibres ; and to this natural complexity it adds that which it derives from the reception of filaments from the sympathetic, accessory, and cervical nerves, and, probably, the glosso-pharyngeal and facial. The most probable account of the particular functions which the branches of the pneumogastric nerve discharge in the several parts to which they are distributed, may be drawn from Dr. John Reid's experiments on dogs. They show that, I. The pharyngeal branch is the principal, if not the sole motor nerve of the pharynx and soft palate, and is most probably wholly motor ; a part of its motor fibres being derived from the internal branch of the acces- sory nerve. 2. The inferior laryngeal nerve is the motor nerve of the larynx, irritation of it producing vigorous movements of the arytenoid cartilages ; while irritation of the superior laryngeal nerve gives rise to no action in any of the muscles attached to the arytenoid cartilages, but merely to contractions of the crico-thyroid muscle. 3. The superior laryngeal nerve is chiefly sensitive ; the inferior, for the most part, motor; for division of the recurrent nerves puts an end to the motions of the glottis, but without lessening the sensibility of the mucous membrane ; and division of the superior laryngeal nerves leaves the 560 THE NERVOUS SYSTEM. movements of the glottis unaffected, but deprives it of its sensibility. 4. The motions of the oesophagus are depen- dent on motor fibres of the pneumogastric, and are pro- bably excited by impressions made upon sensitive fibres of the same ; for irritation of its trunk excites motions of the oesophagus, which extend over the cardiac portion of the stomach; and division of the trunk paralyzes the oeso- phagus, which then becomes distended with the food. 5. The cardiac branches of the pneumogastric nerve are one, but not the sole channel through which the influence of the central organs and of mental emotions is transmitted to the heart. 6. The pulmonary branches form the prin- cipal, but not the sole channel by which the impressions on the mucous surface of the lungs that excite respiration, are transmitted to the medulla oblongata. Dr. Reid was unable to determine whether they contain motor fibres. From these results, and by referring to what has been said in former chapters, the share which the pneumogastric nerve takes in the functions of the several parts to which it sends branches, may be understood : 1 . In deglutition, the motions of the pharynx are of the reflex kind. The stimulus of the food, or other substance to be swallowed, acting on the filaments of the glosso- pharyngeal nerve as well as the filaments of the superior laryngeal given to the pharynx, and of some other nerves, perhaps, with which these communicate, is conducted to the medulla oblongata, whence it is reflected, chiefly through the pneumogastric, to the muscles of the pharynx. 2. In the functions of the larynx, the sensitive filaments of the pneumogastric 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 filaments of the superior laryngeal branch of the pneumogastric ; and the impression conveyed to the medulla oblongata, whether it produce sensation or not, is reflected to the filaments of the recurrent or inferior THE PNEUMOGASTRIC NERVE. 5 6 * laryngeal branch, and excites contraction of the muscles that close the glottis. Both these branches of the pneumo- gastric co-operate also in the production and regulation of the voice ; the inferior laryngeal determining the con- traction of the muscles that vary the tension of the vocal cords, and the superior laryngeal conveying to the mind the sensations 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 (P- 233)- 3. It is partly through their influence on the sensibility and muscular movements in the larynx, that the pneumo- gastric nerves exercise so great an influence on the respira- tory process, and that the division of both the nerves is commonly fatal. To determine how death is in these cases produced, has been the object of innumerable, and often contradictory, experiments. It is probably produced differently in different cases, and in many is the result of several co-operating causes. Thus, after division of both the nerves, the respiration at once becomes slower, the number of respirations in a given time being commonly diminished to one half, probably because the pneumo- gastric nerves are the principal conductors of the impres- sion of the necessity of breathing to the medulla oblongata. Respiration does not cease; for it is probable that the impression may be conveyed to the medulla oblongata through the sensitive nerves of all parts in which the im- perfectly aerated blood flows (see p. 517); yet the respira- tion being retarded, adds to the other injurious effects of division of the nerves. Again, division of both pneumogastric trunks, or of both their recurrent branches, is often very quickly fatal in young animals; but in old animals, the division of the o o 562 THE NERVOUS SYSTEM. recurrent nerve is not generally fatal, and that of both the pneumogastric trunks is not always fatal (J. Reid), and, when it is so, the 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 (Legallois). In old animals, the rigidity and prominence of the arytenoid cartilages prevent the glottis from being completely closed by the atmospheric pressure ; even when all the muscles are paralyzed, a portion at its posterior part remains open, and through this the animal continues to breathe. Yet the diminution of the orifice for respira- tion may add to the difficulty of maintaining life. In the case of slower death, after division of both the pneumogastric nerves, the lungs are commonly found gorged with blood, oedematous, or nearly solid, or 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 are due, perhaps in part, to the influence which the pneumogastric nerves exercise on the movements of the air-cells and bronchi ; yet, since they are not always produced in one lung when its pneumogastric nerve is divided, they cannot be ascribed wholly to the suspension of organic nervous influence (J. Reid). Rather, they may be ascribed to the hindrance to the passage of blood through the lungs, in consequence of the diminished supply of air and the excess of carbonic acid in the air- cells and in the pulmonary capillaries (see p. 239) ; in part, perhaps, to paralysis of the blood-vessels leading to congestion; and in part, also, as the experiments of Traube especially show, 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 stimulated or closed in con- THE SPINAL ACCESSOKY NERVE. 563 sequence of their contact. He says, that if the trachea be divided and separated from the oesophagus, or if only the oesophagus be tied, so that no food or secretion from above can pass down the trachea, no degeneration of the tissue of the lungs will follow the division of the pneumogastric nerves. So that, on the whole, death after division of the pneumogastric nerves may be ascribed, when it occurs quickly in young animals, to suffocation through me- chanical closure of the paralyzed glottis: and, when it occurs more slowly, to the congestion and pneumonia pro- duced by the diminished supply of air, by paralysis of the blood-vessels, and by the passage of foreign fluids into the bronchi ; and aggravated by the diminished frequency of respiration, the insensibility to the diseased state of the lungs, the diminished aperture of the glottis, and the loss of the due nervous influence upon the process of respiration. 4. Respecting the influence of the pneumogastric nerves on the movements of the oesophagus and stomach, the secretion of gastric fluid, the sensation of hunger, absorp- tion by the stomach, and the action of the heart, former pages may be referred to. Physiology of the Spinal Accessory Nerve. In the preceding pages it is implied that all the motor influence which the pneumogastric nerves exercise, is con- veyed through filaments which, from their origin, belong to them : and this is, perhaps, true. Yet a question, which has been often discussed, may still be entertained, whether all or a great part of the motor filaments that appear to belong to the pneumogastric nerves are not given to them from the accessory nerves. The principal branch of the accessory nerve, its external branch, supplies the sterno-mastoid and trapezius muscles ; and, though pain is produced by irritating it, is composed almost exclusively of motor fibres. It might appear very 002 564 THE NERVOUS SYSTEM. probable, therefore, that the internal branch, which is added to the trunk of the pneumogastric just before the giving off of the pharyngeal branch, is also motor ; and that through it the pneumogastric nerve derives part of the motor fibres which it supplies to the muscles enumer- ated above. And further, since the pneumogastric nerve has a ganglion just above the part at which the internal branch of the accessory nerve joins its trunk, a close analogy may seem to exist between these two nerves and the spinal nerves with their anterior and posterior roots. In this view, Arnold and several later physiologists have regarded the accessory nerve as constituting a motor root of the vagus nerve ; and although this view cannot now be maintained, yet it is very probable that the accessory nerve gives some motor filaments to the pneumogastric. For, among the experiments made on this point, many have shown that when the accessory nerve is irritated within the skull, convulsive movements ensue in some of the muscles of the larynx ; all of which, as already stated, are supplied, apparently, by branches of the pneumo- gastric ; and (which is a very significant fact) Vrolik states that in the chimpanzee the internal branch of the accessory does not join the pneumogastric at all, but goes direct to the larynx. On the whole, therefore, although in some of the experiments no movements in the larynx followed irritation of the accessory nerve, yet it may be concluded that this nerve gives to the pneumogastric some of the motor filaments which pass, with the laryngeal branches, to the muscles of the larynx, especially to the crico-thyroid (Bernard). It is not certain, whether, besides these, the accessory gives to the pneumogastric any other motor filaments ; for the experiments undertaken to determine whether, on irritating the accessory within the skull, the muscles of the pharynx, oesophagus, or other parts besides the larynx are convulsed, are completely contradictory, and there appears THE HYPOGLOSSAL NEUVE. 565 no other means than that of experiment by which the diffi- culty may be solved. It is, however, certain that the accessory nerve does not supply all the motor filaments which the branches of the pneumogastric contain; for division of the pneumogastric produces a much more ex- tensive paralysis of motion in all the parts that it supplies, than division of the accessory or its internal branch does, especially in regard to the larynx, and other respiratory organs : almost the only effects of destruction of the acces- sory are loss of voice, and panting in great efforts (Ber- nard). Among the roots of the accessory nerve, the lower, arising from the spinal cord, appear to be composed ex- clusively of motor fibres, and to be destined entirely to the trapezius and sterno-mastoid muscles ; the upper fibres, arising from the medulla oblongata, contain many sensitive as well as motor fibres. Physiology of the Hypoglossal Nerve. 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 noni), 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. The function of the hypoglossal, is, probably, exclu- sively motor. 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 566 THE NEEVOUS SYSTEM. deviation in the direction of the protruded tongue ; pro- bably because the tongue is so compact and firm that the Fig. 144.* * Fig. 144. View of the nerves of the eighth pair, their distribution and connections on the left side (from Sappey after Hirschfeld and Leveille"). |. i, pneumogastric nerve in the neck ; 2, ganglion of its trunk ; 3, its union with the spinal accessory ; 4, its union with the THE SYMPATHETIC NERVE. 567 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. Physiology of the Spinal Nerves. Little need be added to what has been already said of these nerves (pp. 495 to 497). The anterior roots of the spinal nerves are formed exclusively of motor fibres ; the posterior roots exclusively of sensitive fibres. Beyond the ganglia all the spinal nerves appear to be mixed nerves, and to contain as well sympathetic fila- ments. Of the functions of the ganglia of the spinal nerves nothing very definite is known. That they are not the reflectors of any of the ascertained reflex actions through the spinal nerves, is shown by the reflex movements ceasing when the posterior roots are divided between the ganglia and the spinal cord. PHYSIOLOGY OF THE SYMPATHETIC NERVE. The sympathetic nerve, or sympathetic system of nerves, obtained its name from the opinion that it is the means through which are effected the several sympathies in hyppglossal ; 5, pharyngeal branch ; 6, superior laryngeal nerve ; 7, external laryngeal ; 8, laryngeal plexus ; 9, inferior or recurrent laryn- geal ; 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 pneumo- gastric, and by its outer, passing into the sterno-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. 568 THE NERVOUS SYSTEM. morbid action which distant organs manifest. It has also been called the nervous system of organic life, upon the sup- position, now proved erroneous, that it alone, as a nervous system, influences the organic processes. Both terms are defective; but, since the title sympathetic nerve has the advantage of long and most general custom in its favour, and is not more inaccurate than the other, it will be here employed. The general differences between the fibres of the cere- bro-spinal and sympathetic nerves have been already stated (p. 470) ; and it has been said, that although such general differences exist, and are sufficiently discernible in selected filaments of each system of nerves, yet they are neither so constant, nor of such a kind, as to warrant the supposition, that the different modes of action of the two systems can be referred to the different structures of their fibres. Rather, it is probable, that the laws of conduction by the fibres are in both systems the same, and that the differences manifest in the modes of action of the systems are due to the multiplication and separation of the nervous centres of the sympathetic : ganglia, or nerve-centres, being placed in connection with the fibres of the sympathetic in nearly all parts of their course. According to the most general view, the sympathetic system may be described as arranged in two principal divisions, each of which consists of ganglia and connecting fibres. The first division may include those ganglia which are seated on and involve the main trunks or branches of cerebral and spinal nerves. This division will include the large Gasserian ganglion on the sensitive trunk of the fifth cerebral nerve (fig. 145), the ganglia on the glosso-pharyn- geal and pneumogastric nerves, and the ganglia on the posterior or sensitive branches of the spinal nerves (fig. 1 34). To the second division belong the double chain of pra> vertebral ganglia (24 to 30, fig. 144) and their branches, extending from the interior and base of the skull to the THE SYMPATHETIC NERVE. 5 6 9 coccyx ; the various sympathetic visceral plexuses and Fig. 145-* * Fig. 145. General plan of the branches of the fifth pair (after a sketch by Charles Bell). ^. i, lesser root of the fifth pair; 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 connected with the ophthalmic ganglion ; 4, placed on the bone close to the foramen rotun- dum, 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 submaxillary 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 submax- illary ganglion placed above it in connection with the gustatory nerve ; 6, the chorda tympanij 7, the facial nerve issuing from the stylo- mastoid foramen. 57 THE NEKVOUS SYSTEM. their ganglia, as the cardiac, the solar, the renal and hypo- gastric plexuses ; and in the same division may be included the ganglia in the neighbourhood of the head and neck, namely, the ophthalmic or lenticular, the spheno-palatine, the otic, and the submaxillary ganglia (fig. 145). The structure of all these ganglia appears to be essen- tially similar, all containing 1st, nerve-fibres traversing them ; 2ndly, nerve-fibres originating in them ; ^rdly, nerve or ganglion-corpuscles, giving origin to these fibres ; and 4-thly, other corpuscles that appear free. And in the trunk, and thence proceeding branches of the sympathetic, there appear to be always 1st, fibres which arise in its own ganglia; 2ndfy, fibres derived from the ganglia of the cerebral and spinal nerves ; ^rdly, fibres derived from the brain and spinal cord and transmitted through the roots of their nerves. The spinal cord, indeed, appears to furnish a large source of the fibres of the sympathetic nerve. Respecting the course of the filaments belonging to the sympathetic, the following appears to have been deter- mined. Of the filaments derived from the ganglia on the cerebral nerves, some may pass towards the brain ; for, in the trunks of the nerves, between the ganglia and the brain, fine filaments like those of the sympathetic are found. But these may be proceeding from the brain to the ganglia ; and, on the whole, it is probable that nearly all the filaments originating in the ganglia on cerebral nerves, go out towards the tissues and organs to be sup- plied, some of them being centrifugal, some centripetal ; so that each ganglion with its outgoing filaments may form a kind of special nervous system appropriated to the part in which its filaments are placed. Such, for example, may be the ophthalmic ganglion with the ciliary nerves, con- nected with the brain and the rest of the sympathetic system by the branches of the third, fifth, and sympa- thetic nerves that form its roots, yet, by filaments of its THE SYMPATHETIC NERVE. 57 I own, controlling in some mode and degree, the processes in the interior of the eye. Of the fibres that arise in the spinal ganglia, some appear to pass into the posterior branches of the spinal nerves, and to be distributed with them; the rest pass through the branches by which the spinal nerves commu- nicate with the trunks of the sympathetic, and then entering the sympathetic are distributed with its branches to the viscera. With these, also, a certain number of the large ordinary cerebro-spinal nerve-fibres, after traversing the ganglia, pass into the sympathetic. Of the fibres derived from the ganglia of the sympa- thetic itself, some go straightway towards the viscera, the rest pass through the branches of communication between the sympathetic and the branches of the spinal nerves, and joining these spinal nerves, proceed with them to their respective seats of distribution, especially to the more sensitive parts. Thus, through these communicating branches, which have been generally called roots or origins of the sympa- thetic nerve, an interchange is effected between all the spinal nerves and the sympathetic trunks ; all the ganglia, also, which are seated on the cerebral nerves, have roots (as they are called) through which filaments of the cere- bral nerves are added to their own. So that, probably, all sympathetic nerves contain some intermingled cerebral or spinal nerve-fibres ; and all cerebral and spinal nerves some filaments derived from the sympathetic system or from ganglia. But the proportions in which these fila- ments are mingled are not uniform. The nerves which arise from the brain and spinal cord retain throughout their course and distribution a preponderance of cerebro- spinal fibres, while the nerves immediately arising from the so-called sympathetic ganglia probably contain a majority of sympathetic fibres. But inasmuch as there is no certainty that in structure the branches of cerebral 572 THE NERVOUS SYSTEM. or spinal nerves differ always from those of the sympathetic system, it is impossible in the present state of our know- ledge to be sure of the source of fibres which from their structure might lead the observer to believe that they arose from the brain or spinal cord on the one hand, or from the sympathetic ganglia on the other. In other words, although the large white tubular fibres are especially cha- racteristic of cerebro-spinal nerves, and the pale or gelatinous fibres of a sympathetic nerve, in which they largely preponderate, there is no certainty to be obtained in a doubtful case, of whether the nerve-fibre is derived from one or the other, from mere examination of its structure. It may be derived from either source. The physiology of the sympathetic nerve is still very obscure ; there are, however, certain statements which may be made in regard to it. And first, it may be stated generally as nearly certain that the sympathetic nerve-fibres are simple conductors of impressions, as those of the cerebro-spinal system are, and that the ganglionic centres have (each in its appropriate sphere) the like powers both of conducting and of commu- nicating impressions. Their power of conducting impres- sions is sufficiently proved in ordinary diseases, as when any of the viscera, usually unfelt, give rise to sensations of pain, or when a part not commonly subject to mental influence is excited or retarded in its actions by the various conditions of the mind ; for in all these cases impressions must be conducted to and fro through the whole distance between the part and the spinal cord and brain. So, also, in experiments, now more than sufficiently numerous, irritations of the semilunar ganglia, the splanchnic nerves, the thoracic, hepatic, and other ganglia and nerves, have elicited expressions of pain, and have excited movements in the muscular organs supplied from the irritated part. In the case of pain excited, or movements affected by the mind, it may be supposed that the conduction of im- THE SYMPATHETIC NERVE. 573 pressions is effected through the cerebro-spinal fibres which are mingled in all, or nearly all, parts of the sym- pathetic nerves. There are no means of deciding this; but if it be admitted that the conduction is effected through the cerebro-spinal nerve-fibres, then, whether or not they pass uninterruptedly between the brain or spinal cord and the part affected, it must be assumed that their mode of conduction is modified by the ganglia. For, if such cerebro-spinal fibres are conducted in the ordinary manner, the parts should be always sensible and liable to the in- fluence of the will, and impressions should be conveyed to and fro instantaneously. But this is not the case ; on the contrary, through the branches of the sympathetic nerve and its ganglia none but intense impressions, or impressions exaggerated by the morbid excitability of the nerves or ganglia, can be conveyed. Either, therefore, the nerve-fibres conduct differently in the sympathetic nerves (which is improbable), or else the ganglia have a power of modifying the method of conduc- tion of impressions. It is as if the facility with which an impression maybe communicated from one fibre to another in the ganglia were such, that the whole force of ordinary impressions on the nerve-fibres is lost in diffusion among the rest of their contents. This seems not improbable; for some cases show that when fibres certainly belonging to cerebro-spinal nerves pass through ganglia of, or con- nected with, the sympathetic, they do not so rapidly or so surely transmit impressions as when they have no such relation to the ganglia. Thus, the iris is not under the direct or perfect influence of the will, though the passage thereto of filaments of the third nerve, is shown by its acting with the muscles supplied by the same nerve. Neither does it always contract when the third nerve is irritated, and when all the other muscles supplied by the same nerve are put in action. So, also, when all the other muscles supplied by the facial nerve contract on irritating 574 THE NERVOUS SYSTEM. its trunk, the levator palati and azygos uvulse, to which its filaments probably pass through the spheno-palatine ganglion, do not contract. We may account for these facts, by believing that the impression, whether of the mind or of artificial irritation, which would be conveyed at once through nerve-fibres, unconnected with ganglia, is, in the ganglia of the sym- pathetic, communicated and diffused among the corpuscles and the other fibres ; and thus, as one may say, is ex- hausted without reaching the muscles, or, in the case of a centripetal nerve, the spinal cord or brain. Whether, then, the conduction be effected through proper sympathetic nerve-fibres, or through cerebro-spinal fibres mingled with them and traversing their ganglia, there is this peculiarity to be ascribed either to the fibres or, more probably, to the ganglia that the conduction is effected more slowly ; so that when, for example, a gan- glion on a sympathetic nerve is irritated, the movements in the parts supplied from it do not immediately ensue, and pain is not indicated till after repeated irritations, or till, by exposure or otherwise, the fibres and ganglia have become morbidly irritable. But, with this exception, it is probable that the laws of conduction of impressions are the same in both cerebro-spinal and sympathetic systems. Respecting the general action of the ganglia of the sympathetic nerve little need be said here, since they may be taken as examples by which to illustrate the common modes of action of all nerve-centres (see p. 485). 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 mind is unconnected. But it will THE SYMPATHETIC NERVE. 575 be more convenient to consider the ganglia now in connec- tion with the functions that they may be supposed to control, in the several organs supplied by the sympathetic system alone, or in conjunction with the cerebro-spinal. The general processes which the sympathetic appears to influence, are those of involuntary motion, secretion, and nutrition. Many movements take place involuntarily in parts sup- plied with cerebro-spinal nerves, as the respiratory and other spinal reflex motions ; but the parts principally supplied with sympathetic nerves are usually capable of none but involuntary movements, and when the mind acts on them at all, it is only through the strong excitement or depressing influence of some passion, or through some voluntary movement with which the actions of the involun- tary part are commonly associated. The heart, stomach, and intestines 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 have arisen from their ganglia, and are therefore of the nature of sympathetic fibres. The parts which are supplied with motor power by the sympathetic nerve 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. Thus, the heart, after it is taken from the body, continues 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 motion of the parts supplied with nerves from the sympathetic are shown to be, in a measure, independent of the brain and spinal cord. It seems to be a general rule, at least in animals that 576 THE NERVOUS SYSTEM. have both cerebro-spinal and sympathetic nerves much de- veloped, 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 disorderly ; and the probability is that, in the natural state, it is through the same ganglia that natural stimuli, impressing centripetal 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, probably, are those of the heart, stomach, and intestines, by their several ganglia. And as with the ganglia of the sympa- thetic and their nerves, so with the medulla oblongata and its nerves distributed to respiratory muscles, if these nerves or the medulla oblongata itself be directly stimu- lated, 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 con- vulsive, are yet combined in the plan of the proper res- piratory acts. Among the ganglia of the sympathetic nerves to which this co-ordination of movements is to be ascribed, must be reckoned, not those alone which are on the principal trunks and branches of the sympathetic external to any organ, but those also which lie in the very substance of the organs ; such as those discovered in the heart by Remak. Those also may be included which have been found in the mesentery close by the intestines, as well as in the sub- mucous tissue of the stomach and intestinal canal (Meissner), and in other parts. The extension of discoveries of such ganglia will probably diminish yet further the number of instances in which the involuntary movements appear to be effected independently of central nervous influence. THE SYMPATHETIC NEKVE. 577 Respecting the influence of the sympathetic nerve in nutrition and secretion, we may refer to the chapters on those processes. The mode in which this influence is exer- cised is still obscure, though probably it is in a great measure exercised through the vaso-motor nerves (p. I53)> and is, therefore, connected with the supply of blood to the parts. The experiments of Bernard, Waller, Brown- Sequard, and others, render it certain that the sympathetic nerve possesses great influence over the contractile power of the blood-vessels, division of the trunk or branch of such nerve being followed by paralysis of the muscular coat of the vessels supplied by the ramifications of the divided nerve, and by consequent congestion and increased temperature of the parts in which such vessels are distri- buted; while galvanic or other stimulus to the nerve is followed by contrary effects, namely, by contraction of the vessels, and by diminution in the quantity of blood and in temperature. So constant and important are these results, that the sympathetic nerve may be regarded as having, for its principal office, the power of regulating and con- trolling the supply of blood to parts, its fibres constituting the true vasomotor nerves. Besides acting directly upon the muscular coat of the vessels, and thus determining the supply of blood, the sympathetic may possibly also influence the nutritive and secretory changes in a part by direct action on its tissue, whereby it is stimulated to increased activity of function ; for the changes in the mode of nutri- tion and secretion in a part cannot be altogether explained by mere variations in the diameter of its blood- vessels) or in the quantity of blood supplied to it. Daily observation shows multiform results in secretion and nutrition in cases of disease, of which all have, as a common condition, the enlargement of the blood-vessels of the diseased part; something, therefore, besides the enlargement of the ves- sels, must in these cases determine the different events : and so when the various exercise of nervous influence in a p P 578 THE NERVOUS SYSTEM. part affects the size of its vessels and the supply of blood, this change cannot be considered as the only source of the change in its mode of secretion or nutrition. It is, of course, very difficult to determine the relative share exercised by the true sympathetic and the ordinary cerebro-spinal fibres in the contraction of blood-vessels, and in the general processes of nutrition and secretion, since both kinds of fibres appear to be distributed to most parts, and there seems to be no possibility of isolating them. The difficulty of determining this point is much greater in the higher than in the lower Vertebrata ; for it would appear that, in the same proportion as the centres of the cerebro-spinal system are developed, so is its connection with the processes of organic life more inti- mate. In frogs, for instance, all the organic functions may be carried on for several days after the removal of the brain and spinal cord, if only the medulla oblongata has been spared for the maintenance of respiration ; but in Mammalia, and, most of all, in man, even a slight injury of either brain or spinal cord, may disturb all the organic functions. The regularity of the movements of the stomach and intestines, the heart and urinary bladder, independently of the spinal cord or brain, is manifested by numerous experiments in reptiles and Amphibia ; but in Mammalia, the separation of these organs from the influence of the spinal cord or brain, is sufficient to render their actions feeble and irregular, or, after a short time, to stop them altogether. Probably, therefore, the safest view of the question at present is, still to regard all the processes of organic life, in man, as liable to the combined influences of the cere- bro-spinal and the sympathetic systems; to consider that those influences may be so combined as that the sympa- thetic nerves and ganglia may be in man, as in the lower animals, the parts through which the ordinary and constant influence of nervous force is exercised on the organic MOTION. 579 processes; while the cerebro-spinal nervous centres and their ganglia are so closely connected with the proper sympathetic ganglia, that neither of them can be said to be independent of the other ; each, as a rule, and under ordinary circumstances, governing its own domain, but always liable to be influenced by the other. CHAPTER XVII. CAUSES AND PHENOMENA OP MOTION. THE so-called vital motions observable in the bodies of animals, are performed almost exclusively in one or other of the following ways : first, by means of the oscillatory motion or vibration of microscopic cilia, with which the surfaces of certain membranes are beset; and secondly, by the contraction of fibres which either have a longitu- dinal direction and are fixed at both extremities, or form circular bands : the contraction or shortening of the fibres bringing the parts to which they are fixed nearer to each other. There are, besides, various molecular movements allied to these which need not here be considered. CILIARY MOTION. As just remarked, ciliary motion consists in the incessant vibration of fine, pellucid, blunt processes, about -^-^-Q of an inch long, termed cilia (figs. 146, 147), situated on the free extremities of the cells of epithelium covering certain surfaces of the body. The distribution and structure of ciliary epithelium and the microscopic appearances of cilia in motion have been already described (pp. 42, 43). Ciliary motion seems to be alike independent of the will, of the direct influence of the nervous system, and of p p 2 580 MOTION. muscular contraction, for it is involuntary; there is no nervous or muscular tissue in the immediate neighbour- hood of the cilia, and it continues for several hours after death or removal from the body, provided the portion of tissue under examination be kept moist. Its independence of the nervous system is shown also in its occurrence in Fig. 146.* Fig. 147. f the lowest invertebrate animals apparently unprovided with anything analogous to a nervous system, in its per- sistence in animals killed by prussic acid, by narcotic or other poisons, and after the direct application of nar- cotics to the ciliary surface, or the discharge of a Leyden jar, or of a galvanic shock through it. The vapour of chloroform arrests the motion; but it is renewed on the discontinuance of the application (Lister). According to Kuhne, the movement ceases in an atmosphere deprived of oxygen, but is revived on the admission of this gas. Carbonic acid stops the movement. The contact of various substances will stop the motion altogether ; but this seems to depend chiefly on destruction of the delicate substance of which the cilia are composed. Little or nothing is known with certainty regarding the nature of ciliary action. As Dr. Sharpey observes, however, it is a special manifestation of a similar pro- perty to that by which the other motions of animals are * Fig. 146. Spheroidal ciliated cells from the mouth of the frog ; magnified 300 diameters (Sharpey). t Fig. 147. Columnar ciliated epithelium cells from the human nasal membrane ; magnified 300 diameters (Sharpey). CILIARY AND MUSCULAR MOTION. 581 effected, namely, by what we term vital contractility. The fact of the more evident movements of the larger animals being effected by a structure apparently different from that of cilia, is no argument against such a supposition. For, if we consider the matter, it will be plain that our prejudices against admitting a relationship to exist between the two structures, muscles and cilia, rests on no definite ground ; and for the simple reason, that we know so little of the manner of production of movement in either case. The mere difference of structure is not an argument in point ; neither is the presence or absence of nerves. The movements of both muscles and cilia are manifestations of force, by certain special structures, which we call respec- tively muscles and cilia. We know nothing more about the means by which the manifestation is effected by one of these structures than by the other ; and the mere fact that one has nerves and the other has not, is no more argument against cilia having what we call a vital power of contrac- tion, than the presence or absence of stripes from voluntary or involuntary muscles respectively, is an argument for or against the contraction of one of them being vital and the other not so. Inasmuch then as cilia are found in living structures only, and inasmuch as they are a means whereby force is transformed (see chap. II.), their peculiar properties have as much right to be invested with the term vital as have those of muscular fibres. The term may be in both instances a bad one, it certainly is an unsatisfactory one, but it is as good for one case as the other. MUSCULAR MOTION. There are two chief kinds of muscular tissue, and they are distinguished by structural peculiarities and mode of action. The first kind includes, with the exception of the heart, the involuntary muscles, which consist of simple smooth filaments ; the second, comprising the volun- 582 MOTION. tary muscles and the heart, consists of compound and apparently striped fibres, or tubes including fibres. The difference, however, between these two kinds of muscular tissue is less marked than was formerly supposed; for both in structure and mode' of action they often closely resemble each other. The involuntary or unstriped muscles are made up, according to Kolliker, of elongated, spindle-shaped, nu cleated fibre-cells (fig. 148), which in their most perfect form are flat, from about -j^Vo" to 33^0" ^ an ^ ncn broad, and about -^^ to -g-J-g- of an inch in length, very clear, granular, and brittle, so that when they break, they often have abruptly rounded or square extremities. Each fibre- cell possesses an elongated nucleus, and many are marked along the middle, or, more rarely, along one of the edges, either by a fine continuous dark streak, or by short isola- ted dark lines, or by dark points arranged in a row, or scattered. These fibre-cells, by their union, form fibres and bundles of fibres (fig. 149). The fibres have no distinct sheath. The fibres of involuntary muscle, such as are here described, form the proper muscular coats of the digestive canal from the middle of the oesophagus to the internal sphincter ani, of the ureters and urinary bladder, the trachea and bronchi, the ducts of glands, the gall-bladder, the vesiculse seminales, the pregnant uterus, of blood-vessels and lymphatics, the iris, and some other parts. This form of tissue also enters largely into the compo- sition of the tunica dartos, and is the principal cause of the wrinkling and contraction of the scrotum on exposure to cold. The fibres of the crem aster assist in some measure in producing this effect, but they are chiefly concerned in drawing up the testis and its coverings towards the inguinal opening. Unstriped muscular tissue occurs largely also in the cutis (p. 423), being especially abundant at the STEUCTUEE OF UNSTEIPED MUSCLE. 583 interspaces between the bases of the papillae. Hence when it contracts under the influence of cold, fear, elec- tricity, or any other stimulus, the papillae are made Fig. I49.t unusually prominent, and give rise to the peculiar rough- ness of the skin termed cutis anserina, or gooseskin. It occurs also in the superficial portion of the cutis, in all parts where hairs occur, in the form of flattened roundish bundles, which lie alongside the hair-follicles and sebaceous glands. They pass obliquely from without inwards, em- brace the sebaceous glands, and are attached to the hair follicles near their base (fig. 150). Fibres of this tissue, also, constitute part of the walls of most gland-ducts, and are concerned in the propulsion of the contents of these canals. * Fig. 148. Muscular fibre-cells from human arteries, magnified 350 diameters (Kolliker). a, natural state ; J, treated with acetic acid. f Fig. 149. Plain muscular fibres from the human bladder, mag- nified 250 diameters, a, in their natural state; b, treated with acetic acid to show the nuclei. 584 MOTION. To this kind of muscular fibre the term organic is often but loosely applied, from the fact that Pig. 150.* it enters especially into the con- struction of such parts as are concerned in what has been called organic life (see note, p. 466). The muscles of animal life, or striped muscles, iclude the whole class of voluntary muscles, the heart, and those muscles neither completely voluntary nor involuntary, which form part of the walls of the pharynx, and exist in many other parts of the body, as the internal ear, urethra, etc. All these muscles are com- posed of fleshy bundles called fasciculi, enclosed in coverings of fibro- cellular tissue, by which each is at once connect- ed with, and isolated from, those adjacent to it (fig. 151). Each bundle is again divided into smaller ones, similarly ensheathed and similarly divisible ; and so on, through an uncertain number of gradations, till, just beyond the reach of the unaided eye, one arrives at thepri- mitive fasciculi, or the muscular fibres peculiarly so called. Fig. 15 1. 1 \ * Fig. 150. Perpendicular section through the scalp, with two hair- sacs ; a, epidermis ) ; b, cutis ; c, muscles of the hair-follicles (after Kolliker). + Fig. 151. A small portion of muscle, natural size, consisting of larger and smaller fasciculi, seen in a transverse section, and the same magnified 5 diameters (after Sharpey). STRUCTURE OF STRIPED MUSCLE. 585 Muscular fibres consist, each, of them, of a tube or sheath of delicate structureless membrane, called the sarcolemma, enclosing a number of filaments or fibrils. They are Fig. 152.* Fig. i53-t cjlindriform or prismatic, with five or more sides, according to the manner in which they are compressed by adjacent fibres. Their average diameter is about 3-^0. of an inch, and their length never exceeds an inch and a half. Each muscular fibre is thus constructed: Externally is a fine, transparent, struc- tureless membrane, called the sarcolemma, which in the form of a tubular in- vesting sheath forms the outer wall of the fibre, and is filled by the contractile material of which the fibre is chiefly made up. Some- times, from its comparative toughness, the sarcolemma will remain untorn, when by extension the contained part can be broken (fig. 152), and its presence is in this way best demonstrated. The fibres, which are cylindriform or prismatic, with an average diameter of about -g-1^- of an inch, are of a pale yellow colour, and apparently marked by fine strise, which pass transversely * Fig. 152. Muscular fibre torn across ; the sarcolemma still con- necting the two parts of the fibre (after Todd and Bowman). f Fig. 153. A few muscular fibres, being part of a small fasciculus, highly magnified, showing the transverse strise. a, end view of b, b, fibres ; c, a fibre split into its fibrils (after Sharpey). 586 MOTION. round them, in slightly curved or wholly parallel lines. Other, but generally more obscure striee, also pass longitudinally over the tubes, and indicate the direction of the filaments or primitive y^n'Zs of which the substance of each fibre is composed (fig. 153). The whole substance of the fibre contained within the sarcolemma may be thus supposed to be constructed of longitudinal fibrils a bundle of fibrils surrounded by the sarcolemma constituting a, fibre. Pig. 154.* If **/ fgt L ' / : ' / ? / t &! 9 // a- L B? M m i rf tar 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 * Fig. 154. A portion of a medium-sized human muscular fibre, magnified nearly 800 diameters. B. Separated bundles of fibrils equally magnified ; a, a, larger, and 6, 6, smaller collections ; c, still smaller ; d, d, the smallest which could be detached, possibly representing a single series of sarcous elements (after Sharpey). MUSCULAR FIBRES OF THE HEART. 5 8 7 elements, which are separated from each other by a bright space formed of a pellucid substance continuous with them. A fine streak can be sometimes discerned passing across the bright interval between the sarcous elements. Dr. Sharpey believes that, even in a fibril so constituted, the ultimate anatomical element of the fibre has not been isolated. He believes 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, and the dark transverse streak on the bright space but a row of dots. In either case the appearance of striation in the whole fibre would be produced by the arrangement, side by side, of the dark and light portions respectively of the fibrils (fig. 1 54). 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 of each sarcous element with those around and in contact with it ; so that it happens that there is a tendency for a fibre to split, not only into separate fibrils, but also fig. 155. occasionally into plates or disks, each of which is composed of sarcous elements laterally adherent one to another. The muscular fibres of the heart, although striped and resembling closely those of the voluntary muscles in their general structure, present these distinctions : They are finer and more faintly striated, they branch and anastomose one with another, and no sar- colemma can be usually discerned (fig. 155). The voluntary muscles are freely supplied with blood- vessels ; the capillaries form a network with oblong meshes * Fig. 155. Muscular fibres from the heart, magnified, showing their cross-striee, divisions and junctions (from Kblliker). 588 MOTION. around the fibres on the outside of the sarcolemma. No vessels penetrate the sarcolemma to enter the interior of the fibre. Nerves also are supplied freely to muscles ; the volun- tary muscles receiving chiefly nerves from the cerebro- spinal system, and the unstriped muscles from the sym- pathetic or ganglionic system. Properties of Muscular Tissue. The property of muscular tissue, by which its peculiar functions are exercised, is its contractility, which, in the contraction or shortening of muscle, 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 property, although commonly brought into action through the nervous system, is inherent in the mus- cular tissue. For 1st, it may be manifested in a muscle which is isolated from the influence of the nervous system by division of the nerves supplying it, so long as the natural tissue of the muscle is duly nourished; and 2ndly, it is manifest in a portion of muscular fibre, in which, under the microscope, no nerve-fibre can be traced. If the removal of nervous influence be long continued, as by division of the nerve supplying a muscle, or in cases of paralysis of long standing, the irritability, i.e., the power of both perceiving 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 (J. Reid). 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 arterial 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 are rendered almost powerless (Segalas) . So, also, it is to the imperfect supply of arterial blood to CONTRACTION IN STRIATED MUSCLES. 59 the muscular tissue of the heart, that the cessation of the action of this organ in asphyxia is in some measure due (P- 239)- Besides the property of contractility, the muscles, espe- cially the striated or those of animal life, possess sensibility by means of the sensitive 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 modifi- cation 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 sensi- bility, we are not only made conscious of the morbid sensa- tion 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. Except with such knowledge of the position and state of each muscle, we could not tell how or when to move it for any required action ; nor without such a sensation of effort could we maintain the muscles in contraction for any pro- longed exertion. The mode of contraction in the transversely-striated mus- cular tissue, has been much disputed. The most probable account, which has been especially illustrated by Mr. Bowman, is that the contraction is effected by an approxi- mation 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 con- dition which is rendered evident by the approximation of the transverse striae seen on the surface of the fasciculus, and by its increased breadth and thickness. The appear- ance of the zigzag lines into which it was supposed the fibres are thrown in contraction, is due to the relaxation of 590 MOTION. 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, an 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, there- fore, not attended with any diminution in bulk, from con- densation of the tissue. This has been proved for entire muscles, by making a mass of muscle, or many fibres to- gether, contract in a vessel full of water, with which a fine, perpendicular, graduated tube communicates. Any dimi- nution of the bulk of the contracting muscle would be attended by a fall of fluid in the tube ; but when the ex- periment is carefully performed, the level of the water in the tube remains the same, whether the muscle be con- tracted or not.* In thus shortening, muscles appear to swell up, becom- ing rounder, more prominent, harder, and apparently tougher. But this hardness of muscle in the state of con- traction, 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 elastic than in its ordinary uncontracted state (Ed. Weber). Heat is developed in the contraction of muscles. Bec- querel and Breschet found, with the thermo -multiplier, * Edward Weber, however, states that a very slight diminution does take place in the bulk of a contracting muscle ; but it is so slight as to be practically of no moment. SOUND OF MUSCULAR CONTRACTION. 591 about 1 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. It is not known whether this development of heat is due to chemi- cal 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 (p. 254). And Nasse suspects that to it is due the higher tempera- ture of the blood in the left ventricle; for he says that this fluid is always warmer in the left ventricle than in the left auricle, and that the blood in the latter is but little warmer than that on the right side of the heart. But these experiments need confirmation. Sound is said to be produced when muscles contract for- cibly. Dr. 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 muscular action, and to the number of fibres that act together, or, as it were, in time. The two kinds of fibres, the striped and unstriped, have characteristic differences in the mode in which they act on the application of the same stimulus ; differences which may be ascribed in great part to their respective differences of structure, but to some degree possibly, to their respec- tive 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 592 MOTION. 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. Ed. Weber particularly illustrated the difference in the modes of contraction of the two kinds of muscular fibres by the effects of the electro-magnetic stimulus. The rapidly succeeding shocks given by this means to the nerves of muscles excite in all the trans- versely-striated muscles a fixed state of tetanic contraction, which lasts as long as the stimulus is continued, and on its withdrawal instantly ceases : but in the muscles with smooth fibres they excite, if any movement, only one that ensues slowly, is comparatively slight, alternates with rest, and continues for a time after the stimulus is withdrawn. In their mode of responding to these stimuli, all the voluntary muscles, or those with transverse strise, are alike; but among those with plain or 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. According to Weber, 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 ; and they require no more stimulus than that of the air to excite them. The heart is the quickest and most vigorous of all the muscles of organic life in contracting upon irritation, 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 un- der the influence of stimuli applied to them or to their RIGOR MORTIS. 593 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 ani- mal, 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 Ver- tebrata. 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 man it ceases, according to Xysten, in the following order : first in the left ventricle, then in the intestines and stomach, the urinary bladder, right ventricle, oesophagus, iris ; then in the voluntary muscles of the trunk, lower and upper extremities ; lastly in the left and right auricle of the heart. After the muscles of the dead body have lost their irri- tability 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 circumstances do not prevent it, com- monly 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. f * If, however, arterial blood be made to circulate through the body or through a limb, the post mortem contraction of the muscles thus sup- plied with blood, may, as Dr. Brown- Sequard has shown, be suspended, and the muscles again admit of contracting on the application of a stimulus. t It should be stated here, however, that the generally accepted ex- planation of the state of the muscles during rigor mortis, namely, that it is due to contraction of the fibres, as in strong action during life, is denied by some physiologists, who maintain that the condition of the muscles is not due to contraction at all, but is caused by a kind of coagulation of the inter-fibrillar juices. This idea has been of late especially supported by Dr. Norris (see Camb. J. of Anat. and Phys., Part I.). Q Q 594 MOTION. The muscles are not affected exactly simultaneously by the post-mortem contraction, but rather in succession. It affects the neck and lower jaw first; next, the upper ex- tremities, extending from above downwards ; and lastly, reaches the lower limbs; in some rare instances only, it affects the lower extremities before, or simultaneously with, the upper extremities. It usually ceases in the order in which it began ; first at the head, then in the upper ex- tremities, and lastly in the lower extremities. According to Sommer, it never ccfmmences earlier than ten minutes, and n-ever later than seven hours, after death; and its duration is greater in proportion to the lateness of its accession. According to Schiffer, and others have con- firmed the truth of his observation, 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 irritability, induce an early occurrence of the rigidity, while conditions by which the disappearance of the irritability is delayed, are 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 continuance 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 M. Brown-Sequard with electro-magnetism make it pro- bable that the rigidity may supervene immediately after death, and then pass away with such rapidity as to be scarcely observable. Thus, he took five rabbits, and killed them by removing their hearts. In the first, rigidity RIGOR MORTIS. 595 came on in 10 hours, and lasted 192 hours; in the second, which was feebly electrified, it commenced in seven 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 cur- rent, the rigidity ensued in seven minutes after death, and passed away in 25 minutes. From this it appears that the more powerful the electric 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 elec- trified just /before death than in 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 nutri- tion of the muscular tissue. The rigidity affects the involuntary as well as the volun- tary 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 contraction of the muscles with un- striped 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-slain 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, Q Q 2 59& MOTION. owing to the contraction of the intestinal walls. It is still better shown in the arteries, of which all tnat have mus- cular coats contract after death, and thus present the roundness and cord-like feel of the arteries of a limb lately removed, or those of a body 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 Musdes. The greater part of the voluntary muscles of the body act as sources of power for moving levers, the latter consisting of the various bones to which the muscles are attached. All levers have been divided into three kinds, according to the relative position of the power, the weight to be moved, 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 fulcrum be used, the arrangement will stand thus : P.F.W. A * Although the preceding remarks represent the views generally enter- tained in regard to muscular action, yet it must be observed that a new and very different theory on the subject has been lately advanced by several writers, and especially developed by Dr. Kadcliffe, who* has also made it the basis of new views on the pathology of various convulsive affections. According to this doctrine, the ordinary relaxed or elongated state of a muscle is due to a certain "state of polarity " in which the muscle is maintained, and contraction is brought about by anything (such as an effort of the will) which liberates the muscle from this influence, and thus leaves it to the operation of the attractive force inherent in the muscular molecules. According to this doctrine, also, the stage of rigor mortis is readily explicable : death depriving the muscles of the "state of polarity " whereby they had hitherto been kept relaxed, and thus allowing the attractive force of the muscular particles to come into play. For facts and arguments in support of this view, and for references and confirmatory opinions, Dr. Radcliffe's work On Epileptic and other Convulsive Affections may be consulted. VARIETIES OF LEVERS. 597 poker, as ordinarily used, or the bar in fig. 156, may be 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 means of the hamstring muscles attached to the tuberosity of the ischium (fig. 156). Fig. 156. 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 Fig. 157- ELASTICBBAND an elastic band as in fig. 157. 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. 157). 59* MOTION. In a lever of the third kind the arrangement is F.P.\V., and the act of raising a pole, as in fig. 158, is an example. In the human body there are numerous examples of the employment of this kind of leverage. The act of bending the fore-arm may be mentioned as an instance (fig. 158). Fig. 158. 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 movement 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 sections which treat of the physiology of the processes by which these muscular actions are assisted or carried out. The combined action of the respiratory muscles, for instance, will be found described in the chapter on " Respiration" ; the action of the heart and blood-vessels, under the head of " Circula- WALKING. 599 are too intimately associated with the function of " Diges- tion," to be described apart from it. There are, however, one or two very important and somewhat complicated mus- cular 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. 159). When starting to walk, say with 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 right 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 con- traction, 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 for- wards and upwards. The successive acts in taking the first step in walking are represented in fig. 159, I, 2, 3. 600 MOTION. 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 and prevent it from falling prostrate. This advance of the other leg (in this case the right) is effected partly by its mechanically swinging for- wards, 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, espe- cially the rectus femoris, with the psoas and the iliacus ; 2ndly, 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 pre- vent the latter in swinging forwards from, hitching in the ground. Anybody who has attentively watched the help- less flapping action of the foot and leg in cases of partial paralysis affecting the muscles of the leg, or who will, in his own case, note the act of bringing the leg forward in walking, will be convinced of the large share which the muscles take in the act in question ; although, of course, their work is rendered much easier by the pendulum -like swinging forward of the leg by its own weight. The second part of the act of walking, which has been just described, is shown in the diagram (4, fig. 159). 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 WALKING. 60 1 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 detail will be understood with comparative ease. One kind of leverage employed in walking is essentially the same with that em- ployed in pulling forward the pole, as in fig. 158. And the other, less exactly, is that employed in raising the handles 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. 158) while the raising of the heel and pushing forwards of the trunk by the calf-muscles is roughly repre- sented 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 sup- port the trunk, which is at the same time pushed and pulled forwards by the muscles of the other, may be gathered from the previous description. There is one more thing to be noticed especially in the act of walking. Inasmuch as the body is being con- stantly 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 effected is one element in the dif- ferences 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 602 MOTION. onwards and upwards by the raising, say, of the right heel, as in fig. 159, 3, the pelvis is instinctively, by various muscles, made to rotate on the head of the left femur at the acetabulum, to the left side, so that the weight may fall over the line of sup- port 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 ' movement of the trunk and pelvis, however, is but an awkward manner of doing what can be done more gracefully by combining a slight ' rocking ' with, a movement of the whole trunk and leg over the foot which is being planted on the ground (fig. 160) ; the action being accompanied with a compensatory outward movement at the hip, more easily appreciated by looking at the figure (160) than described. Thus the body in walking is continually rising and swaying alternately from one side to the other, as its cen- tre of gravity has to be brought alternately 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 leaping, or jumping, consists in so sudden a raising of the heels by the sharp and strong contraction of SOURCE OF MUSCULAR ACTION. 603 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 pro- ducing 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 alternately; 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 man- ner 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. Actions 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. Ex- amples 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. 591) ; and the peculiar vermicular or peristaltic action of the latter fibres in some regions of the body has been described at p. 350. 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 pro- MOTION. portion 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, there is a certain destruction of tissue or, in other words, a certain ' wear and tear/ which may be repre- sented by a slight increase in the quantity of urea excreted : but it is not the correlative expression (p. 9) or only source of the power manifested. The increase in the amount of urea which is excreted after muscular 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 unim- portant 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, therefore, 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 especially of the combustion of non-nitrogenous food, although, doubtless, of the carbon element in nitrogenous food also. "We are, therefore, driven to the conclusion, that the substance of muscles is not wasted in proportion to the VOICE AND SPEECH. 605 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 (see Chap. II) . The urgent necessity for nitrogenous food, especially after exer- cise, 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. CHAPTER XVIII. OF VOICE AND SPEECH. IN nearly all air-breathing vertebrate animals there are arrangements for the production of sound, or voice, in some part of the respiratory apparatus. In many animals, the sound admits of being variously modified and altered during and after its production ; and, in man, one of the results of such modification is speech. Mode of Production of the Human Voice. It has been proved by observations on living subjects, by means of the laryngoscope, 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 inferior laryngeal ligaments, or true vocal cords (A, cv f fig. 165) which bound the glottis, being thrown into vibration by currents of expired air impelled over their edges. Thus, if a free open- ing exists in the trachea, the sound of the voice ceases, but returns on the opening being closed. An opening into the air-passages above the glottis, on the contrary, does not prevent the voice being formed. Injury of the laryngeal nerves supplying the muscles which move the vocal cords 6o6 VOICE AND SPEECH. Fig. 161.* puts an end to the formation of vocal sounds ; and when these nerves are divided on both sides, the loss of voice is complete. Moreover, by forcing a current of air through the larynx in the dead subject, clear vocal sounds are produced, though the epiglottis, the upper ligaments of the larynx or false vocal cords, the ventricles be- tween them, and the in- ferior ligaments or true vocal cords, and the up- per part of the aryte- noid cartilages, be all removed; provided the true vocal cords remain entire, with their points of attachment, and be kept tense and so ap- proximated that the fis- sure of the glottis may be narrow. The vocal ligaments or cord, therefore, may be regarded as the pro- per organs of the mere voice : the modifications of the voice are effected by other * Fig. 161. Outline showing the general form of the larynx, trachea, and bronchi, as seen from before. \. h, the great cornu of the hyoid bone ; c, epiglottis ; t, superior, and t', inferior cornu of the thyroid cartilage ; c, middle of the cricoid cartilage ; tr, the trachea, showing sixteen cartilaginous rings ; I, the right, and &', the left bronchus. THE LARYNX. 607 parts as well as by them. Their struc- ture is adapted to enable them to vi- brate like tense mem- branes, for they are essentially composed of elastic tissue ; and they are so attached to the cartilaginous parts of the laiynx that their position and tension can be variously altered by the contraction of the muscles which act on these parts. The Larynx. The larynx, or or- gan of voice, consists essentially of two elastic lips called the vocal cords, which are so attached to certain cartilages, and so under the control of certain muscles, that they Fig. 162. * Fig. 162. Outline showing the general form of the larynx, trachea, and bronchi, as seen from behind. 4- ^> great cornu of the hyoidbone ; t, superior, and t', the inferior cornu of the thyroid cartilage : c, the epiglottis ; a, points to the back of both the arytenoid cartilages, which are surmounted by the cornicula ; c, the middle ridge on the back of the cricoid cartilage ; tr, the posterior membranous part of the trachea ; I, &', right and left bronchi. 6oS VOICE AND SPEECH. can be made the means not only of closing the larynx against the entrance and exit of air to or from the lungs, but also can be stretched or relaxed, shortened or length- ened, in accordance with the conditions that may be necessary for the air in passing over them, to set them vibrating and produce various sounds. Their action in respiration has been already referred to (p. 2IO), in con- nection with ordinary tranquil respiration, and also (p. 233, et seq.) with other respiratory acts, in which the opening or closing of the glottis, or, in other words, the close apposition or separation of the vocal cords, is an essential part of the performance. In these respiratory acts, how- ever, any sound that may be produced, as in coughing, is, so to speak, an accident, and not performed with purpose. In the present chapter the sound produced by the vibration of the vocal cords is the only part of their function with which we have to deal. It will be well, perhaps, to refer to a few points in the anatomy of the larynx, before considering its physiology in connection with voice and speech. The principal parts entering into the formation of the larynx (figs. 161 and 162) are (t) the thyroid cartilage; (c) the cricoid cartilage ; (a) the two arytenoid cartilages ; and the two true vocal cords (A, cv, fig. 165). The epiglottis (fig. 162, e) has but little to do with the voice, and is chiefly useful in falling down as a ' lid ' over the upper part of the larynx, to prevent the entrance of food and drink in deglutition. The false vocal cords (cvs. fig. 165), 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. The thyroid cartilage (fig. 163, I to 4) does not form a complete ring around the larynx, but only covers the front portion. The cricoid cartilage (fig. 163, 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 por- tion of the cricoid are the arytenoid cartilages (fig. 162, a) THE LAEYNX. 609 the connection between the cricoid below and arytenoid car- tilages above being a joint with synovial membrane and liga- ments, the latter permitting tolerably free motion between them. But, although the aryte- pig. 163.* noid cartilages can move on the cricoid, they of course accompany the latter in all their 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. The thyroid cartilage is also con- nected with the cricoid, not only by ligaments, but by two joints with synovial membrane (t' } figs. 1 6 1 and 162); 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-enter- ing angle at the back part of the thyroid ; it is evident, 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 (a, fig. 162), and are connected with it by capsular and other ligaments, all movements of the cricoid cartilage must move the * Fig. 163. Cartilages of the larynx seen from before, f I to 4, thyroid cartilage ; I, vertical ridge or pomum Adarni ; 2, right ala ; 3, superior, and 4, inferior cormi of the right side ; 5, 6, cricoid cartilage ; 5, inside of the posterior part ; 6, anterior narrow part of the ring ; 7, adenoid cartilages. R R 6io VOICE AND SPEECH. Fig. 164.* arytenoid cartilages, and also produce an effect on the vocal cords. 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 crico-thyroid muscles, two thyro- arytenoid, two posterior crico-arytenoid, two lateral crico- arytenoid, and one arytenoid muscle. Their actions are as follows : When the crico-thyroid muscles (10, fig. 164) 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 thyro-arytenoid mus- cles (7, fig. 167), on the other hand, have an opposite action,' pulling the thyroid backwards, and the arytenoid and upper and back part of the cricoid car- tilages forwards, and thus relaxing the vocal cords. The crico-arytenoidei pos- tici muscles (fig. i64,b)dilate the glottis, and separate the vocal cords, the one from the other, by an action on the arytenoid cartilage, which will be plain on reference to B' and c', (fig. 165). By their * Fig. 164. Lateral view of exterior of the larynx, after Mr. Willis. 8, thyroid cartilage ; 9, Cricoid cartilage ; 10, Crico-thyroid muscle ; II, Crico-thyroid ligament ; 12, first rings of trachea. ACTIONS OF THE LAKYNGEAL MUSCLES. 611 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 crico-arytenoidei 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 (fig. 167, 4 and 5). The aperture of the glottis can be also contracted by the arytenoid muscle (s, fig. 1 66, and 6, fig. 167), which, in its contraction, pulls together the upper parts of the arytenoid cartilages between which it extends. The placing of the vocal cords in a position parallel one with the other, is effected by a combined action of the various little muscles which act on them the thyro-aryte- noidei having, without much reason, the credit of taking the largest share in the production of this effect. Fig. 165 is intended to show the various positions of the vocal cords under different circumstances. Thus, in ordinary tranquil breathing, the opening of the glottis is wide and triangular, becoming a little wider at each inspiration, and a little narrower at each expiration (fig. 165, see also p. 210). On making a rapid and deep inspiration the opening of the glottis is widely dilated, as in c, fig. 165, and somewhat lozenge-shaped. At the moment of the emission of sound, it is more narrowed, the 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. 165, 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 E 2 6l2 VOICE AND SPEECH. utterance of grave tones, on the other hand, the epiglottis is depressed and brought over them, and the arytenoid Fig. 165. * Fig. 165. Three laryngoscopic views of the superior aperture of the larynx and surrounding parts and different states of the glottis during life (from Czermak). 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 dila- tation, as in inhaling a very deep breath. The diagrams A', B', and C', have been added to Czermak's figures, to 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 ; e, the upper free part of the epiglottis ; 6, 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-epiglot- tidean fold w, the swelling of the membrane caused by the cartilages of PRODUCTION OF VOCAL SOUNDS. 613 Fig. 1 66.* cartilages look as if they were trying to hide themselves under it (fig. 168). 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 modify- ing the vocal sounds. The degree of approx- imation of the vocal cords also usually corresponds with the height of the note produced; but pro- bably not always, for the width of the aperture has no essential influence on the height of the note, as long as the vocal cords Wrisberg ; s, that of the cartilages of Santorini ; a, the tip or summit of the arytenoid cartilages ; cv, the true vocal cords or lips of the rima glottidis ; cvs, the superior or false vocal cords ; between them the ventricle of the larynx ; in C, t r is placed on the anterior wall of the receding trachea, and b indicates the commencement of the two bronchi beyond the bifurcation which may be brought into view in this state of extreme dilatation (from Quain's Anatomy). * Fig. 1 66. View of the larynx and part of the trachea from behind, with the muscles dissected : h, the body of the hyoid bone ; e, epiglottis ; t, the posterior borders of the thyroid cartilage ; c, the median ridge of the cricoid ; a, upper part of the arytenoid ; s, placed on one of the oblique fasciculi of the arytenoid muscle ; b, left posterior crico-arytenoid muscle ; ends of the incomplete cartilaginous rings of the trachea ; I, fibrous membrane crossing the back of the trachea ; n, muscular fibres exposed in a part (from Quain's Anatomy). 614 VOICE AND SPEECH. Fig. 167.* 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 ary- tenoid cartilages. For, as Miiller's experiments showed, if the arytenoid cartilages be approximated in such a manner that their anterior processes touch each other, but yet leave an opening behind them as well as in front, no Fig. i68.f second vocal tone is pro- duced by the passage of the air through the pos- terior 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 not, provided the vocal cords maintain the same degree of tension. * Fig. 167. Yiew of the interior of larynx from above. I, aperture of glottis ; 2, arytenoid cartilages ; 3, vocal cords ; 4, posterior crico- arytenoid muscles ; 5, lateral crico-arytenoid muscle of right side, that of left side removed ; 6, arytenoid muscle ; 7, thyro-arytenoid muscle of left side, that of right side removed ; 8, thyroid cartilage ; 9, cricoid cartilage ; 13, posterior crico-arytenoid ligament. "With the exception of the arytenoid muscle, this diagram is a copy from Mr. "Willis's figure. t Fig. 1 68. View of the upper part of the larynx as seen by means of the laryngoscope during the utterance of a grave note, c, epiglottis ; s, tubercles of the cartilages of Santorini ; a, arytenoid cartilages ; z, base of the tongue ; ph, the posterior wall of the pharynx. COMPASS OF THE VOICE. 615 Application of the Voice in Singing and SpeaTdng. The notes of the voice thus produced may observe three different kinds of sequence. The first is the monotonous, iu which the notes have nearly all the same pitch as in ordinary speaking ; the variety of the sounds of speech being due to articulation in the mouth. In speaking, how- ever, occasional syllables generally receive a higher intona- tion for the sake of accent. The second mode of sequence is the successive transition from high to low notes, and vice versa, 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 proportions that characterise the notes of the musical scale. The compass of the voice in different individuals, compre- hends 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 commence and end at dif- ferent 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 principal 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 616 VOICE AND SPEECH. 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 between the different voices ; for bass singers can sometimes go very high, and the con- tralto 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 intermediate 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 voice 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 pecu- liarities of the ligaments, and the membranous and cartilaginous 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 dif- ferent 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 YAKIETIES OF VOCAL TONES. 617 vocal cords before puberty have not two-thirds the length which they acquire at that period ; and the angle of their thyroid cartilage is as little prominent 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 undergone 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 singing 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 re- stricted in extent : the first defect is owing to the ossifi- cation of the cartilages of the larynx and the altered condition of the vocal cord ; the want of steadiness arises from the loss of nervous power and command over the muscles ; the result of which is here, as in other parts, a tremulous motion. 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 characters 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 con- ditions that determine these distinctions are, however, quite unknown. They are probably inherent in the tissues of the larynx, and are as indiscernible as the minute differences that characterise 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 6 1 8 VOICE AND SPEECH. 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 fal- setto notes. The natural voice, which alone has been hitherto considered, is fuller, and excites a distinct sensa- tion 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. 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 Miiller they are 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 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 respec- tive notes are produced. VARIETIES OF YOCAL TONES. 619 The strength of the voice depends partly on the degree to which the vocal cords can be made to vibrate; and partly 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 communi- cating 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 increasing 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 * fortis- simo ' 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 con- traction 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, therefore, 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. 620 VOICE AND SPEECH. 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 com- mencement 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 tube, between the glottis and the external apertures of the air- passages, the combination of which sounds into different groups to designate objects, properties, actions, etc., con- stitutes language. The languages do not employ all the sounds which can be produced in this manner, the com- bination 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 lan- guages. 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. The sounds produced in speech, or articulate sounds, are commonly divided into vowels and consonants; the distinc- tion 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 modifications, p, t, k, the intonation only follows them in their combination with a vowel. To recognize the essential properties of the articulate sounds, we must, according to Muller, first examine them as they are produced in whispering, and then investigate SPEECH. 621 which of them can also be uttered in a modified character conjoined with local 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 the vocal cords are not thrown into the vibrations 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 the larynx when the mouth is closed, the nostrils being open, and the utterance' of all vocal tone avoided. This sound, when the mouth is open, is so modified by varied forms of the oral cavity, as to assume the characters of the vowels , e, i, o, u, in all their modifications. The cavity of the mouth assumes the same form for the articulation of each of the mute vowels as for the cor- responding vowel when vocalized; the only difference in the two cases lies in the kind of sound emitted by the larynx. Kratzenstein and Kempelen have 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, Kempelen means here the space between the tongue and palate : for the pronunciation of certain vowels both the opening of the mouth and the space just Yowel. Sound. Size of oral opening. a as in 'far' 5 a, ' name ' 4 . ' theme ' 3 . o ,, 'go' 2 00 ,, ' cool ' I . 622 VOICE AND SPEECH. mentioned are widened; for the pronunciation of other vowels both are contracted; and for others one is wide, the other contracted. 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 : Size of oral canal. 3 2 I 4 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 7i, m, n,f, s, r, I. 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 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." * The minuter physiology of speech may be best studied in Miiller ; or in the remarkable work by Ammann (from which even Miiller has been instructed), entitled "Dissertatio de Loquela," 1700. THE SENSES. 623 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, etc. From the observations of Miil- ler 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 Miiller, much of the ventriloquist's skill in imitating the voices coming from particular directions, consists in deceiving other senses than hearing. We never distinguish 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. The tongue, which is usually credited with the power of speech, language and speech being often employed as synonymous terms plays only a subordinate, 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 im- perfectly, and their voice is considerably modified ; but the loss of speech is confined to those letters, in the pronuncia- tion of which the tongue is concerned. CHAPTER XIX. THE SENSES. SENSATION consists in the mind receiving, through the medium of the nervous system, and, usually as the result of the action of an external cause, a knowledge of certain 624 THE SENSES. qualities or conditions, not of external bodies but of the nerves of sense themselves; and these qualities of the nerves of sense are in all different, the nerve of each sense having its own peculiar quality. There are two principal kinds of sensation, named common and special. The first is the consequence of the ordinary sensibility or feeling possessed by most parts of the body, and is manifested when a part is touched, or in any ordinary manner is stimulated. According- to the stimulus, the mind perceives a sensation of heat, or cold, of pain, of the contact of hard, soft, smooth, or rough objects, etc. From this, also, in morbid states, the mind perceives itching, tingling, burning, aching and the like sensations. In its greatest perfection, common sensibility constitutes touch or tact. 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 a pro- perty common to many nerves, e.g., all the sensitive spinal nerves, the pneumogastric, glosso-pharyngeal, and fifth cerebral nerves, and in its impressions being communicable through many organs. Including the sense of touch, the special senses are five in number, the senses of sight, hearing, smell, taste, and touch. The manifestation of each of the first three depends on the existence of a special nerve ; the optic for the sense of sight, the auditory for that of hearing, and the olfactory for that of smell. The sense of taste appears to be a property common to branches of the fifth and of the glosso-pharyngeal nerves. The senses, by virtue of the peculiar properties of their several nerves, make us acquainted with the states of our own body ; and thus indirectly inform us of such qualities and changes of external matter as can give rise to changes in the condition of the nerves. That which through the medium of our senses is actually perceived by the mind is, indeed, merely a property or change of condition of our THE SENSES. 625 nerves ; but the mind is accustomed to interpret these modifications in the state of the nerves produced by external influences as properties of the external bodies themselves. This mode of regarding sensations is so habitual in the case of the senses which are more rarely affected by internal causes, that it is only on reflection that we perceive it to be erroneous. In the case of the sense of feeling, on the con- trary, where many of the peculiar sensations of the nerves perceived by the sensorium are excited as frequently by internal as by external causes, we more readily apprehend the truth. For it is easily conceived that the feeling of pain or pleasure, for example, is due to a condition of the nerves, and is not a property of the things which excite it. What is true of these is true of all other sensations ; the mind perceives conditions of the optic, olfactory, and other nerves specifically different from that of their state of rest ; these conditions may be excited by the contact of external objects, but they may also be the consequence of internal changes : in the former case the mind, having knowledge of the object through either instinct or instruction, re- cognises it by the appropriate changes which it produces in the state of the nerves. The special susceptibility of the different nerves of sense for certain influences, as of the optic nerve, or rather its centre, for light ; of the auditory nerve, or centre, for vibrations of the air, etc., and so on, is not due entirely to those nerves having each a specific irritability for such influences exclusively. For although, in the ordinary events of life, the optic nerve is excited only by the undu- lations or emanations of which light may consist, the audi- tory only by vibrations of the air, and the olfactory only by odorous particles yet each of these nerves may have its peculiar properties called forth by other conditions. In fact, in whatever way and to whatever degree a nerve of special sense is stimulated, the sensation produced is essentially of the same kind ; irritation of the optic nerve s s 626 THE SENSES. invariably producing a sensation of light, of the auditory nerve a sensation of some modification of sound. The phenomenon must, therefore, be ascribed to a peculiar quality belonging to each nerve of special sense. It has been supposed, indeed, that irritation of a nerve of special sense, when excessive, may produce pain ; but experiments seem to have proved that none of these nerves 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 ex- periments of his seemed to show that both the retina and optic nerve are insusceptible of pain. External impressions on a nerve can give rise to no kind of sensation which cannot also be produced by internal causes, exciting changes in the condition of the same nerve. In the case of the sense of touch, this is at once evident. The sensations of the nerves of touch (or common sensi- bility) excited by causes acting from without, are those of cold and heat, pain and pleasure, and innumerable modifi- cations of these, which have the same kind of sensation as their element. All these sensations are constantly being produced by internal causes, in all parts of our body en- dowed with sensitive nerves. The sensations of the nerves of touch are therefore states or qualities proper to them- selves, and merely rendered manifest by exciting causes, whether external or internal. The sensation of smell, also, may be perceived independently of the application of any odorous substance from without, through the influence of some internal condition of the nerve of smell. The sensations of the sense of vision, namely, colour, light, and darkness, are also often perceived independently of all external exciting causes. So, also, whenever the auditory nerve is in a state of excitement, the sensations peculiar to it, as the sounds of ringing, humming, etc., are perceived. The same cause, whether internal or external, excites in the different senses different sensations ; in each sense the THE, SENSES. 627 sensations peculiar to it. For instance, 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 capil- lary 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 audi- tory 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 aurium " ; and in the common sensitive nerves, the sensation of creeping over the surface. So, also, among external causes, the stimulus of electricity, or the mechanical influence of a blow, concussion, or pressure, excites in the eye the sensa- tion 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 at the other parts of the body, a perception of peculiar jarring or of the mechanical impression, or a shock like it. Although, in the cases just referred to, and in all ordi- nary conditions, sensations are derived from peculiar con- ditions of the nerves of sense, whether excited by external or by internal causes, yet the mind may have the same sensations independently of changes in the conditions of at least the peripheral portions of the several nerves, and even independently of any connection with the external organs of the senses. The causes of such sensations are seated in the parts of the brain in which the several nerves of sense terminate. Thus pressure on the brain has been observed to cause the sensation of light : luminous spectra may be excited by internal causes after complete amaurosis of the retina : and Humboldt states, that, in a man who had lost one eye, he produced by means of galvanism, luminous appearances on the blind side. Many of the 832 628 THE SENSES. various morbid sensations attending diseases of the brain, the vision of spectra, and the like, are of the same kind. Again, although the immediate objects of the perception of our senses are merely particular states induced in the nerves, and felt as sensations, yet, inasmuch as the nerves of the senses are material bodies, and therefore participate in the properties of matter generally, occupying space, being susceptible of vibratory motion, and capable of being variously changed chemically, as well as by the action of heat and electricity, they make known to the mind, by virtue of the different changes thus produced in them by external causes, not merely their own condition, but also some of the different properties and changes of condition of external bodies; as, e.g., progressive and tremulous motion, chemical change, etc. The information concerning external nature thus obtained by the senses, varies in each sense, having a relation to the peculiar qualities or ener- gies of the nerve. The sensation of motion is, like motion itself, of two kinds, progressive and vibratory. The faculty of the percep- tion of progressive 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. For the sense of hearing, vibrations constitute the ordinary stimulus, and so give rise to the perception of sound. By the sense of touch, vibrations are perceived as tremors, occasionally attended with the general impression of tickling ; for instance, when a vibrating body, such as a tuning fork, is approximated to a very sensible part of the surface, the eye can communi- THE SENSES. 629 cate to the mind the image of a vibrating body, and can distinguish the vibrations when they are very slow; it may be also that the vibrations are communicated to the optic as to the auditory nerve in such a manner that it repeats them, or receives their impulses. 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 sub- stances, however, are perceived not only by the sense of smell, but also by the senses of touch and taste, provided they are of a nature adapted to disturb chemically the condition of those organs, and in the case of the organ of taste, to be dissolved by the fluids covering it. Thus, the vapours of horse-radish and mustard, and acrid suffo- cating gases, 'act upon the conjunctiva and the mucous membrane of the lungs, exciting through the common sensitive nerves, merely modifications of common feeling ; and at the same time they excite the sensations of smell and of taste. Sensations are referred from their proper seat towards the exterior; but this is owing, not to anything in the nature of the nerves themselves, but to the accompanying idea derived from experience. For in the perception of sensations, there is a combined action both of the mind and of the nerves of sense ; and the mind, by education or experience, has learned to refer the impressions it receives to objects external to the body. Even when it derives impressions from internal causes, it commonly refers them to external objects. The light perceived in congestion of 630 THE SENSES. the retina seems external to the body : the ringing of the ears in disease is felt as if the sound came from some distance : the mind referring it to the outer world from which it is in the habit of receiving the like impression. Moreover, the mind not only perceives the sensations, and interprets them according to ideas previously obtained, but it has a direct influence upon them, imparting to them intensity by its faculty of attention. Without simultaneous attention, all sensations are only obscurely, if at all, per- ceived. If the mind be torpid in indolence, or if the attention be withdrawn from the nerves of sense in intel- lectual contemplation, deep speculations, or an intense passion, the sensations of the nerves make no impression upon the mind ; they are not perceived, that is to say, they are not communicated to the conscious " self," or with so little intensity, that the mind is unable to retain the impression, or only recollects it some time after, when it is freed from the preponderating influence of the idea which had occupied it. This power of attention to the sensations derived from a single organ, may also be exercised in a single portion of a sentient organ, and thus enable one to discern the detail of what would otherwise be a single sensation. For example, by well-directed attention, one can distinguish each of the many tones simultaneously emitted by an orchestra, and can even follow the weaker tones of one instrument apart from the other sounds, of which the im- pressions being not attended to are less vividly perceived. So, also, if one endeavours to direct attention to the whole field of vision at the same time, nothing is seen distinctly ; but when the attention is directed first to this, then to that part, and analyses the detail of the sensation, the part to which the mind is directed is perceived with more distinct- ness than the rest of the same sensation. THE SENSE OF SMELL. 631 THE SENSE OF SMELL. The sense of smell ordinarily requires, for its excitement to a state of activity, the action of external matters, which, action produces certain changes in the olfactory nerve; and this nerve is susceptible of an infinite variety of states dependent on the nature of the external stimulus. The first condition essential to the sense of smell is the existence of a special nerve, the changes in whose condition are perceived as sensations of odour ; for no other nerve is capable of these sensations, even though acted on by the same causes. The same substance which excites the sen- sation of smell in the olfactory nerves 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 per- ceived. The second condition of smell is a peculiar state of the olfactory nerve, or a peculiar change produced in it by the stimulus or odorous substance. 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 dis- solved 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 mem- brane 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. In animals living in the air, it is also requisite that the 632 THE SENSE OF SMELL. odorous matter should be transmitted 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 inspirations, assisted, as in the act of sniffing, by the action of the nostrils, we render the impression more intense (see P- 2 35). The human organ of smell is essentially formed by the filaments of the olfactory nerves, distributed in minute Fig. 169.* arrangement, in the mucous membrane covering 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 bone (figs. 169 and I/O). This olfactory region is covered by cells of cylindrical epi- * Fig. 169. Nerves of the septum nasi, seen from the right side (from Sappey after Hirschfeld and Leveilld). |. I, the olfactory bulb ; i, the olfactory nerves passing through the foramina of the cribriform plate, and descending to be distributed on the septum ; 2, the internal or septal twig of the nasal branch of the ophthalmic nerve ; 3, naso-palatine nerves. THE SENSE OF SMELL. 633 thelium not provided with cilia ; and interspersed with, these are peculiar fusiform cells with fine processes, called olfactory cells. They are supposed to have some connection 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 nos- trils, where it is squa- mous. In all the distribu- tion, the branches of the olfactory nerves re- tain much of the same soft and greyish tex- ture which distinguishes their trunks (as the olfactory lobes of the brain are called) within the cranium. Their individual filaments, also, are peculiar, more resembling those of the sympathetic nerve than the filaments of the other cerebral nerves do, con- taining no outer white substance, and being finely granular and nucleated. The branches are distributed principally in close plexuses ; but the mode of termination of the fila- ments is not yet satisfactorily determined. * Fig. 170. Nerves of the outer walls of the nasal fossae (from Sappey after Hirschfeld and Leveille). j}. I, network of the branches of the olfactory nerve, descending upon the region of the superior and middle turbiuated bones ; 2, external twig of the ethmoidal branch of the nasal nerve ; 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 (after Sharpey). 634 THE SENSE OF SMELL. The sense of smell is derived exclusively through those parts of the nasal cavities in which the olfactory nerves are distributed ; the accessory cavities or sinuses com- municating with the nostrils seem to have no relation to it. Air impregnated with the vapour of camphor was injected by Deschamps into the frontal sinus through a fistulous opening, and Richer and injected odorous substances 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 cavities with fat, is here attained, as it is in many bones of birds, by their being filled with air. All parts of the nasal cavities, whether or not they can be the seats of the sense of smell, are endowed with com- mon sensibility by the nasal branches of the first and second divisions of the fifth nerve. Hence the sensations of cold, heat, itching, tickling, and pain; and the sensa- tion of tension or pressure 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 membrane of the nose remains susceptible of the various modifications of common sensation 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 sensations excited in the nose by acrid vapours, as of am- monia, 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 01 the nose to these irritating substances remained after the THE SENSE OF SMELL. 635 destruction of the olfactory nerves, that Magendie was led to believe the fifth nerve might exercise the special sense. 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. The Carnivora have the power of detecting most accurately by the smell the special peculiarities of animal matters, and of track- ing 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, especially 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. Opposed to the sensation of an agreeable odour is that of a disagreeable or disgusting odour, which corresponds 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 un- known ; 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, comparable 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 636 THE SENSE OF SMELL. relation of harmony and disharmony exists between odours as between colours and sounds ; though it is probable 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 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. The sensations of the olfactory nerves, independent of the external application of odorous substances, have hitherto been little studied. It has been found that solutions of inodorous substances, such as salts, excite t no sensation of odour when injected into the nostrils. The friction of the electric machine is, however, known to produce a smell like that of phosphorus. Hitter, 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 was produced, it remained constant as long as the circle was closed, and changed to the other at the moment of the circle being opened. 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, by MM. Cullerier and Maignault, 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. THE SENSE OF SIGHT. 637 THE SENSE OF SIGHT. The eyeball or the organ of vision (fig. 171) consists of a variety of structures which may be thus enumerated : Fig. 171. Cornea Anterior chamber Iris Ciliary process Ciliary muscle The sclerotic or outermost coat, envelopes about iive- sixths of the eyeball: continuous with it, in front, and occupying the remaining sixth, is the cornea. The cornea and front portion of the sclerotic are covered by mucous membrane, the conjunctiva ; that which covers the front of the cornea being little more than squamous epithelium. Immediately within the sclerotic is the choroid coat, and within the choroid is the retina. The interior of the eye- ball 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 regulating 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. 638 THE SENSE OF SIGHT. Fig. 172.* A These structures may be now examined rather more in detail. The sclerotic coat is composed of connective tissue, arranged in variously disposed and intercommunicating layers. It is strong, tough, and opaque, and not very elastic. The cornea (fig. 172) is, like the sclerotic, with which it is continuous, chiefly of a fibrous structure, but the fibres are so modified and arranged as to form a transparent membrane for the passage of light. Both in front of and behind the fibrous tissue of the cornea is a structureless elastic membrane with epithelium. The choroid, which is the next tunic of the eye within the sclerotic and immediately outside the retina, consists of a thin and highly vascu- lar membrane, of which the inter- nal surface is covered by a layer of black pigment-cells. The prin- cipal use of the choroid is to absorb, by means of its pigment, those rays of light which pass through the transparent retina, and thus to prevent their being thrown again upon the retina, so as to interfere :^5 C * Fig. 1 72. Structures of the cornea (after Bowman). A s , B & C, 220. A, Small portion of a vertical section of the cornea in the adult ; a, con- junctival epithelium ; 6, anterior elastic lamina ; etod, fibrous laminae with nuclear bodies interspersed between them ; c, fibres shooting through some of these layers from the external elastic lamina ; d, pos- terior elastic lamina ; e, internal epithelium. B, epithelium of the mem- brane of Demours, as seen looking towards its surface. C, the same seen in section. THE SENSE OF SIGHT. 639 with the distinctness of the images there formed. Hence animals in which the choroid is destitute of pigment, and human Albi- noes, are dazzled by day- lj|j light and see best in the twilight. The choroid coat ends in front in what are called the ciliary pro- cesses (fig. 173). The ? water with considerable intensity. The sound being conducted to the labyrinth by two paths, will, of course, produce so much the stronger impression; for undulations will be thus excited in the fluid of the labyrinth from two different though contiguous points; and by the crossing of these undulations stationary waves of increased intensity will be produced in the fluid. Miiller's experiments show that the same vibrations of the air act upon the fluid of the labyrinth with much greater intensity through the medium of the chain of auditory bones and the fenestra ovalis than through the medium of the air of the tympanum and the membrane closing the fenestra rotunda : but the cases of disease in which the ossicula have been lost with- out loss of hearing, prove that sound may also be well conducted through the air of the tympanum and the mem- brane of the fenestra rotunda. Functions of the Labyrinth. The fluid of the labyrinth is the most general and constant of the acoustic provisions 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 function usually ascribed to the semicircular canals is the collecting in their fluid contents, the sonorous undula- tions from the bones of the cranium. They have probably, FUNCTIONS OF THE LABYEINTH. 691 also, in some degree, the power of conducting sounds in the direction of their curved cavities more easily than the sounds are carried off by the surrounding hard parts in the original direction of the undulations, though this con- ducting power is in them much less perfect than in tubes containing air. Admitting that they have these powers, the increased intensity of the sonorous vibrations thus attained will be of advantage in acting on the auditory nerve where it is expanded in the ampullce of the canals, and in the utri- culus. Where the membranous canals are in contact with the solid parietes of the tubes, this action must be much more intense. But the membranous semicircular canals must have a function independent of the surrounding hard parts; for in the Petromyzon they are not separately enclosed in solid substance, but lie in one common cavity with the utriculus. The crystalline pulverulent masses 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 the office of the otoconia. 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. The cochlea seems to be constructed 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 Y Y 2 692 THE SENSE OF HEARING. 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 percep- tion of the sonorous undulations propagated by the solid parts of the head and the walls of the labyrinth. The membranous labyrinth of the vestibule 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 undulations be imparted to the fluid through the fenestrae, 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 ex- panded in the cochlea, is, on the contrary, continuous with the solid walls of the labyrinth, and receives directly from them the impulses which they transmit. This is an important advantage ; for the impulses imparted 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, cceteris paribus, the sonorous undula- tions 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 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 undu- lations upon the auditory nerve efficient, than the mem- FUNCTION OF RODS OF CORTI. 693 branous labyrinth is ; for, as a solid body insulated by a different medium, it is capable of resonance. The rods of Corti are probably arranged so that each is set to vibrate in unison with a particular tone, and thus strike a particular 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 pick out and 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. Sensibility of the Auditory Nerve* Most frequently, several undulations or impulses on the auditory nerve concur in the production of the impressions of sound. By the rapid succession of several impulses at unequal intervals, a noise or rattle is produced ; from a rapid suc- cession of several impulses at equal intervals, a musical sound results, the height or acuteness of which increases with the number of the impulses communicated to the ear within a given time. A sound of definite musical value is also produced when each one of the impulses, succeeding another thus at regular intervals, is itself compounded of several undulations, in such a way that, heard alone, it would give the impression of an unmusical sound ; that is to say, by a sufficiently rapid succession of short unmusical sounds at regular intervals, a musical sound is generated. 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, 694 THE SENSE OF HEAEING. until two only are left ; the sound 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 suc- cessive 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 pro- duced 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 im- pulses could be sufficiently prolonged. By removing one or several teeth from the toothed wheel before mentioned, M. Savart was also enabled to satisfy himself of the fact that in the case of the auditory nerve, as in that of the optic nerve, the sensation continues louger than the impression which causes it; for the re- moval 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. 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 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 DIKECTION AND DISTANCE OF SOUNDS. 695 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 circum- stances 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. 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 themselves to hear the sounds as coming from thence. The distance of the source of sounds is not recognised by the sense itself, but is inferred from their intensity. The sound itself is always seated but in one place, namely, ill 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 ; and this is also taken advantage of by ventriloquists. The experiments of Savart, already referred to, prove that 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 impression of the same sound be very long continued, or constantly repeated for a long time, when 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. This 696 THE SENSE OF HEARING. must have been experienced by every one who has travelled several days continuously; for some time after the journey,' the rattling noises are heard when the ear is not acted on by other sounds. We have here a proof that the perception of sound, as sound, is not essentially connected with the existence of undulatory pulses; and that the sensation of sound is a state of the auditory nerve, which, though it may be excited by a succession of impulses, may also be produced by other causes. Even if it be supposed that undulations excited by the impulse are kept up in the auditory nerve for a certain time, they must be undulations of the nervous principle itself, which, being excited, continue until the equilibrium is restored. 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 are recorded, however, by Sauvages and Itard. 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 tensity and tone ; one being conveyed to the ear through the medium of the atmosphere, the other through the conducting-rod. The sense of vision may vary in its degree of perfection as regards either the faculty of adjustment to different distances, the power of distinguishing accurately the par- ticles of the retina affected, sensibility to light and dark- ness, or the perception of the different shades of colour. In the sense of hearing, there is no parallel to the faculty SUBJECTIVE SOUNDS. 697 by which the" eye is accommodated to distance, nor to the perception of the particular part of the nerve affected ; but just as one person sees distinctly only in a bright light, and another only in a moderate light, so in different individuals the sense of hearing is more perfect for sounds of different pitch ; and just as a person whose vision for the forms of objects, etc., is acute, nevertheless dis- tinguishes colours with difficulty, and has no perception of the harmony and disharmony of colours, so one, whose hearing is good as far as regards the sensibility to feeble sounds, is sometimes deficient in the power of recognising the musical relation of sounds, and in the sense of harmony and discord; while another individual, whose hearing is in other respects imperfect, has these endowments. The causes of these differences are unknown. 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 audi- tory 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 sounds we must distinguish those dependent, not on a state of the auditory nerve itself merely, but on sonorous vibrations 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 snapping sound in the ear produced by a voluntary effort, and the buzz or hum heard during the contraction of the palatine muscles in the act of yawning"; during the forcing 698 THE SENSE OF TASTE. of air into the tympanum, so as to make tense the mem- brana 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 ner- vous individuals, a start of the whole body or an unpleasant sensation, like that produced by an electric shock, through- out 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. The sense of hearing may in its turn be affected by im- pressions on many other parts of the body ; especially in diseases of the abdominal viscera, and in febrile affections. Here, also, it is probable that the central organs of the ner- vous system are the media through which the impression, is transmitted. SENSE OF TASTE. The conditions for the perception of taste are: I, the presence of a nerve with special endowments ; 2, the excitation of the nerves by the sapid matters ; 3, the solu- tion of these matters in the secretions of the organ of taste. The nerves concerned in the production of the sense of taste have been already considered (pp. 548 and 556). The mode of action of the substances which excite taste probably consists in the production of a change in the internal condition of the gustatory nerves ; and, according to the difference of the substances, an infinite variety of changes of condition, and consequently of tastes, may be induced. It is not, however, necessary for the manifesto- STRUCTURE OF THE TONGUE. 699 tion of taste that sapid substances in solution should be brought into contact with its nerves. For 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 Henle observed that a small current of air directed upon the tongue gives rise to a cool saline taste, like that of saltpetre ; and Dr. Baly has shown that a distinct sensa- tion 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 produces the sensation of nausea, which is probably only a modification of taste. The matters to be tasted must either be in solution or be soluble in the moisture covering the tongue ; hence in- soluble substances are usually tasteless, 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 principal, but not exclusive seat of the sense of taste is the fauces and tongue. The tongue is a muscular organ covered by mucous membrane ; the latter resembling other mucous mem- branes (p. 403) in essential points of structure, but con- taining certain parts, the 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. Its use in relation to mastication and deglutition has already been considered (p. 271). Besides other functions, the mucous membrane of the tongue serves as a ground -work for the ramification of the abundant blood-vessels and nerves which the tongue receives, and affords insertion to the extremities of the 7oo THE SENSE OF TASTE. muscular fibres of which the chief substance of the organ is composed. Fig. 195.* * Fig. 195. Papillar surface of the tongue, with the fauces and tonsils (from Sappey). i, i, circumvallate papillae, in front of 2, the foramen caecum ; 3, fungiform papillae ; 4, filiform and conical papillae ; 5, trans- verse and oblique rugae ; 6, mucous glands at the base of the tongue and in the fauces ; 7, tonsils ; 8, part of the epiglottis ; 9, median glosso-epiglottidean fold, or freemun epiglottidis. PAPILLAE OF THE TONGUE. 701 The larger papilla of the tongue are thickly set over the anterior two-thirds of its upper surface, or dorsum (fig. 195), and give to it its characteristic roughness. 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 circumvallate or ctdyciform, the fungiform, and the filiform papillae. Essen- tially 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 structure, however, they differ considerably one from another. All the three varieties of papillae just described have been commonly regarded as simple processes, like the papillae of the skin ; but Todd and Bowman have shown that the surface of each kind is studded by minute conical processes of mucous membrane, which thus form secondary papillae. These secondary 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. The mucous membrane immediately in front Fig. 196.* * Fig. 196. Vertical section of the circiimvallate papillae (from Kolliker). \. A, the papillae ; B, the surrounding wall ; a, the epithe- lial covering ; 5, the nerves of the papilla and wall spreading towards the surface ; c, the secondary papillae. 702 THE SENSE OF TASTE. of the epiglottis is, however, free from them. They are commonly buried beneath the epithelium ; hence they had been previously overlooked. Circumvallate or Calyciform Papilla. These papillae (fig. 196), eight or ten in number, are situate in two V-shaped lines at the base of the tongue (l, I, fig. 195). They are circular elevations from -^th to y'j-th of an inch wide, each with a central depression, and surrounded by a circular fis- sure, at the outside of which again is a slightly elevated ring, both the central elevation and the ring being formed of close-set simple papilla) (fig. 196). Fungiform Papilla. The fun- giform papillae (fig. 197) are scattered chiefly over the sides and tip, and sparingly over the middle of the dorsum, of the tongue ; their name is de- rived from their being usually narrower at their base than at their summit. They also con- sist of groups of simple papilla^ each of which contains in its interior a loop of capillary blood-vessels, and a nerve-fibre. Conical or Filiform Papilla. These, which are the most abundant papilla?, are scattered over the whole surface of the tongue, but especially over the middle of the dorsum. * Fig. 197. Surface and section of the fuugiform papillae (from Kolliker, after Todd and Bowman). A, the surface of a fungiform papilla, partially denuded of its epithelium, \ 5 ; p, secondary papillae ; c, epithelium. B, section of a fungiform papilla with the blood-vessels injected ; a, artery ; v, vein ; c, capillary loops of simple papillae in the neighbouring structure of the tongue ; d, capillary loops of the secondary papillos ; e, epithelium. C-" PAPILLA OF THE TONGUE. 703 Fig. 198.* 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 im- bricated manner, or is prolonged from their surface in the form of fine stiff pro- jections, hair-like in appearance, and in some instances in structure also (fig. 198). From their peculiar structure, it seems likely that these papillae have a mechanical func- tion, or one allied to that of touch, rather than of taste ; /<" the latter sense be- ing probably seated especially in the other two varieties of papillae, the circum- vallate and the fungi- form. The epithelium of the tongue is of the squamous or tesselated kind, like the epidermis (p. 422). It covers every part of the surface ; but over the fungiform * Fig. 198. Two filiform papillae, one with epithelium, the other without (from Kolliker, after Todd and Bowman). 3 T 5 . p, the substance of the papillae dividing at their upper extremities into secondary papillae ; a, artery, and v, vein, dividing into capillary loops ; e, epithelial cover- ing, laminated "between the papillae, but extended into hair- like pro- cesses/, from the extremities of the secondary papillse. 704 THE SENSE OF TASTE. papilla) forms a thinner layer than elsewhere, so that these papillae stand out more prominently than the rest. The epithelium covering the filiform papillae has been shown by Todd and Bowman, to have a singular arrangement; being extremely dense and thick, and, as before men- tioned, projecting from their sides and summits in the form of long, stiff, hair-like processes. Many of these processes bear a close resemblance to hairs, and some actually contain minute hair-tubes. 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. 196), but the exact mode of termination of the nerve-filaments is not certainly known. Such, in outline, is the structure of the sensitive surface of the tongue. But the tongue is not the only seat of the sense of taste ; for the results of experiments as well as ordinary experience shows that the soft palate and its arches, the uvula, tonsils, and probably the upper part of the pharynx, are endowed with taste. These parts, together with the base and posterior parts of the tongue, are sup- plied with branches of the glosso-pharyngeal nerve, and evidence has been already adduced (p. 556 et seq.) that the sense of taste is conferred upon them by this nerve. In most, though not in all persons, the anterior part of the tongue, especially the edges and tip, are supplied with taste. The middle of the dorsum is only feebly endowed with this sense, probably because of the density and thick- ness 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. The gustatory- property of the anterior part of the tongue is due, as already said, p. 548), to the lingual branches of the fifth nerve. Besides the sense of taste, the tongue, by means also of its papillae, is endued, especially at its sides and tip, with a very delicate and accurate sense of touch, which renders THE SENSE OF TASTE. 75 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 versa. 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 sensations are distinct, just as the nerves for smell and common sensibility in the nostrils are distinct ; and it is quite conceivable that the same nervous trunk may contain fibres differing essentially in their specific properties. Facts already detailed (p. 548) seem to prove that the lingual branch of the fifth nerve is the seat of sensations 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 like- wise 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 concurrence of common and special sensibility in the same part makes it sometimes difficult to determine whether the impression produced by a substance is per- ceived 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. The impressions on the mind leading to the perception of taste seem to result, as already said, from certain changes in the internal condition of the nerves produced by the contact of sapid substances with the papillae in which the fibres of these nerves are distributed. This ex- planation, obscure though it be, may account generelly for the sense; but the variations of taste produced by different substances are as yet inexplicable. In the case z z 7o6 THE SENSE OF TASTE. of hearing, we know that sounds differ from one another according to the differences in the number of undulations producing them ; and in the case of vision, it is reasonably inferred that different colours result from differences in the number of undulations, or in the rate of transit, of the principle of light. But, in the cases of taste and smell, no such probable explanation has yet been offered. It would appear, indeed, from the experiments of Horn, that while some substances taste alike in all regions of the tongue's surface, others excite different tastes, according as they are applied to different papillae of the tongue. This observa- tion, if confirmed, would seem to show that, in some cases at least, different fibres are capable of receiving different impressions from the same sapid substance. 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. Very 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 afterwards. 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 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 con- sonance or harmony of flavours in their combination or order of succession, just as in painting and music the THE SENSE OF TOUCH. 77 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 discriminate between them. The simple contact of a sapid substance with the sur- face 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. The sense of taste seems capable of being excited also by internal causes, such as changes in the conditions of the nerves or nerve-centres, produced by congestion or other causes, which excite subjective sensations in the other organs of sense. But little is known of the sub- jective sensations of taste ; for it is difficult to distin- guish the phenomena from the effects of external causes, such as changes in the nature of the secretions of the mouth. SENSE OF TOUCH. The sense of touch is not confined io 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 ordinary 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 sensitive cerebral nerves. z z 2 7o8 THE SENSE OF TOUCH. But, although all parts of the body supplied with sensi- tive nerves are thus, in some degree, organs of touch, yet the sense is exercised 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 abun- dant papillae. (See chapter on SKIN, and section on TASTE.) The sensations of the common sensitive nerves have as peculiar a character as those of any other organ of sense. The sense of touch renders us conscious of the presence of a stimulus, from the slightest to the most intense degree of its action, neither by sound, nor by light, nor by colour, but 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 a 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. Schiff and Brown-Sequard are of opinion that common sensibility and tactile sensibility manifest them- selves to the individual by the aid of different sets of fibres. Dr. Sieveking has arrived at the same conclusion from pathological observation. 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 ascer- tained, 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 move- ment, and stands in the same relation to the sense of touch, or common sensibility, generally, as the act of seek- THE SENSE OF TOUCH. 709 ing, following, or examining odours, does to the sense of smell. Every sensitive part of the body which can, by means of movement, be brought into different relations of contact with external bodies, is an organ of " touch." No one part, consequently, has exclusively this function. The hand, however, is best adapted for it, by reason of its peculiarities of structure, namely, its capability of pro- nation and supination, which enables it, by the movement of rotation, to examine the whole circumference of a body ; the power it possesses of opposing the thumb to the rest of the hand ; and the relative mobility of the fingers. Besides the hand, and especially the fingers, are abun- dantly endowed with papilla and touch-corpuscles (pp. 424-6) which are specially necessary for the perfect employment of this sense. In forming a conception of the figure and extent of a surface, the mind multiplies 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 dimensions of a solid body, acquires a notion of its cubical extent. 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 sensa- tions produced by two points placed close together. This power depends, 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 con- founded, and perhaps produce but one sensation. Expe- riments to determine the tactile properties of different parts of the skin, as measured by this power of distin- guishing distances, were made by E. H. Weber. One experiment consisted in touching the skin, while the eyes were closed, with the points of a pair of compasses sheathed 710 THE SENSE OF TOUCH. with, cork, and in ascertaining how close the points of the compasses might be brought to each other, and still be felt as two bodies. He examined in this manner nearly every part of the surface of the body, and has given tables showing the relative degrees of sensibility of different parts. Experiments of a similar kind have been per- formed also by Valentin: and, among the numerous results obtained by both these investigators, it appears that the extremity of the third finger, and the point of the tongue are the parts most sensitive : a distance of as little as half a line being here distinguished. Next in sensitive- ness to these is the mucous surface of the lips, which can perceive the two points of the compass when separated to the distance of about a line and a half : on the dorsum of the tongue they require to be separated two lines. The parts in which the sense of touch is least acute are the neck, the middle of the back, the middle of the arm, and the middle of the thigh, on which the points of the com- pass have to be separated to the distance of thirty lines to be perceived as distinct points (Weber). Other parts of the body possess various degrees of sensibility intermediate between the above extremes. A sensation in a part endowed with touch appears to the mind to be, cateris paribiLs, more intense when it is excited in a large extent of surface than when it is con- fined to a small space. The temperature of water into which he dipped his whole hand, appeared to Weber to be higher than that of water of really higher temperature, in which he immersed only one finger of the other hand. Similar observations may be made by persons bathing in warm or cold water. Part of the ideas which we obtain of the conditions of external bodies is derived through the peculiar sensibility with which muscles are endowed the sensibility by which we are made acquainted with their position, and the degree of their contraction. By this sensation, we are enabled to THE SENSE OF TOUCH. 711 estimate the degree of force exerted in resisting pressure or in raising weights. The estimate of weight by mus- cular effort is more accurate than that by pressure on the skin, according to Weber, who states that by the former a difference between two weights may be detected when one is only one-twentieth or one fifteenth less than the other. 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 movement. 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 conceived of its weight. If it should happen to contain some very heavy substance, 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 sensation 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 712 THE SENSE OF TOUCH. 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 fore- arm. The mind has, nevertheless, a very definite know- ledge 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.. In order that an impression made on a sensitive surface may be perceived, it is necessary that there should exist a reciprocal influence between the mind and the sense of touch; for, if the mind does not thus co-operate, the organic conditions for the sensation may be fulfilled, but it remains unperceived. Moreover, the distinctness and intensity of a sensation in the nerves of touch depend, in great measure, on the degree in which the mind co- operates for its perception. A painful sensation becomes more intolerable the more the attention is directed to it : thus, a sensation in itself inconsiderable, as an itching in a very small spot of the skin, may be rendered very troublesome and enduring. 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 com- pare 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 temperatures, 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 pro- portion to the time which has elapsed between them. The after-sensations left by impressions on nerves of com- mon sensibility or touch are very vivid and durable. As SUBJECTIVE SENSATIONS. 713 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 pain- ful and pleasurable sensations afford many examples of this fact. The law of contrast, which we have shown modifies the sensations of vision, prevails here also. After the body has been exposed to a warm atmosphere, a degree of temperature a very little lower, which would under other circumstances appear warm, produces the sensation of cold ; and a sudden change to the extent of a few degrees from a cold temperature to one less severe, will produce the sensation of warmth. Heat and cold are, therefore, rela- tive terms; for a particular state of the sentient organs causes what would otherwise be warmth to appear cold. So, also, a diminution in the intensity of a long -continued pain gives pleasure, even though the degree of pain that remains would in the healthy state have seemed intoler- able. 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 lightness and weight, of fatigue, etc., may be produced by internal causes. Neuralgic pains, the sensa- tion 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, also, 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 sensa- tion of pain in a part predisposed to it. The thought of anything horrid excites the sensation of shuddering ; the feelings of eager expectation, of pathetic emotion, of enthusiasm, excite in some persons a sensation of ". con- centration " at the top of the head, and of cold trickling 714 GENERATION AND DEVELOPMENT. through the body ; fright causes sensations to be felt in many parts of the body ; and even the thought of tickling excites that sensation in individuals very susceptible of it, when they are threatened with it by the movements of another person. These sensations from internal causes are most frequent in persons of excitable nervous systems, such as the hypochondriacal and the hysterical, of whom it is usual to say that their pains are imaginary. If by this is meant that their pains exist in their imagination merely, it is certainly quite incorrect. Pain is never imaginary in this sense ; but is as truly pain when arising from internal as from external causes; the idea of pain only can be unattended with sensation, but of the mere idea no one will complain. Still, it is quite certain that the imagination can render pain that already exists more intense and can excite it when there is a disposition to it. CHAPTER XX. GENERATION AND DEVELOPMENT. THE several organs and functions of the human body which have been considered in the previous chapters, have relation to the individual being. We have now to con- sider those organs and functions which are destined for the propagation of the species. These comprise the several provisions made for the formation, impregnation, and development of the ovum, from which the embryo or foetus is produced and gradually perfected into a fully- formed human being. The organs concerned in effecting these objects are named the generative organs, or sexual apparatus, since part belong to the male and part to the female sex. FEMALE ORGANS OF GENERATION. 715 Generative Organs of the Female. The female organs of generation consist of two Ovaries, whose function is the formation of ova ; of a Fallopian tube, or oviduct, connected with each ovary, for the pur- pose of conducting the ovum from the ovary to the uterus or cavity in 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 append- ages, for the reception of the male generative organ in the Fig. 199-* * Fig. 199. Diagrammatic view of the uterus and its appendages, as seen from behind (from Quain). ^. The uterus and upper part of the vagina have been laid open by removing the posterior wall ; the Fallopian tube, round ligament, and ovarian ligament have been cut short, and the broad ligament removed on the left side ; u, the upper part of the uterus ; c, the cervix opposite the os internum ; the triangular shape of the uterine cavity is shown, and the dilatation of the cervical cavity with the rugse termed arbor vitse ; v, upper part of the vagina ; od, Fallopian tube or oviduct ; the narrow communication of its cavity with that of the cornu of the uterus on each side is seen ; I, round ligament ; lo, ligament of the ovary ; o, ovary ; i, wide outer part of the right Fallopian tube ; fi, its fimbriated extremity ; po, parovarium ; h, one of the hydatids frequently found connected with the broad ligament. 716 GENEKATION AND DEVELOPMENT. act of copulation, and for the subsequent discharge of the foetus. 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 is attached to the uterus by a narrow fibrous cord (the ligament of the ovary), and, more slightly, to the Fallopian tube by one of the fimbrire, into which the walls of the extremity of the tube expand. The ovary is enveloped by a capsule of dense fibro- cellular tissue, which again is surrounded by peritoneum. The internal structure of the organ consists of a peculiar soft fibrous tissue, or stroma, abundantly supplied with blood-vessels, and having imbedded in it, in various stages of development, numerous minute follicles or vesicles, the Graafian vesicles, or sacculi, containing the ova (fig. 2OO). A further account of the Graafian vesicles and of their contained ova will be presently given. 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 Fallopian 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 offimbria, one of which, longer than the rest, is attached to the ovary. The canal by which each Fallopian 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 cavity of the abdomen, surrounded by the zone of fimbrise. Externally, the Fallopian tube is invested with perito- neum ; internally, its canal is lined with mucous mem- brane, covered with ciliary epithelium (p. 42) : between the peritoneal and mucous coats, the walls are composed, like those of the uterus, of fibrous tissue and organic mus- cular fibres (pp. 582-4), THE UTERUS. 717 The uterus (u, c, fig. 199) is a somewhat pyriform, fibrous organ, with a central cavity lined with mucous membrane. In the unimpregnated state it is about three inches in length, two in breadth at its upper part, orfundus, but at its Fig. 200.* 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. The walls of the organ are composed of dense fibro-cellular tissue, with which are intermingled fibres of organic muscle : in the impregnated state the latter are much developed and increased in number. The cavity of the uterus corresponds * Fig. 200. VieAv of a section of the prepared ovary of the cat (from Schrb'n) f. I, outer covering and free border of the ovary ; i', attached border ; 2, the ovarian stroma, presenting a fibrous and vascular 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 surrounded by the fibrous stroma ; 8, more advanced Graafian follicle with the ovum imbedded in the layer of cells constituting the proligerous disc ; 9, the most advanced follicle containing the ovum, etc. ; 9', a follicle from which the ovum has accidentally escaped ; 10, corpus luteum. 7i8 GENERATION AND DEVELOPMENT. 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, and probably only separated from each other by mucus. 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 distinguished into two lips, an interior and posterior. In the mucous membrane of the cervix are found several mucous follicles, termed Ovula or glanduke Nabothi : they probably form the jelly-like substance by which the os uteri is usually found closed. The vagina is a membranous canal, six or eight inches long, extending obliquely downwards and forwards from the neck of the uterus, which it embraces, to the external organs of generation. It is lined with mucous membrane, 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 fibro-cellular tissue, within which, especially around the lower part of the tube, is a layer of erectile tissue. The lower extremity of the vagina is embraced by an orbicular muscle, the constrictor-vaginae ; 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, like the male penis, of two erectile corpora cavernosa, but unlike it, without a corpus spongiosum, and not perforated by the urethra ; of two folds of mucous membrane, termed labia interna, or nymphcB ; and, in front of these, of two other folds, the labia externa, or pudenda, formed of the external integument, and lined internally by mucous mem- brane. Between the nymphao and beneath the clitoris is an angular space, termed the vestibule, at the centre of whose base is the orifice of the rneatus urinarius. Numerous THE tJN IMPREGNATED OVUM. 719 mucous follicles are scattered beneath the mucous mem- brane composing these parts of the external organs of generation; and at the side of the fore part of the vagina, are two larger lobulated glands, named vulva-vaginal, or Duverney's glands, which are analogous to Cowper's glands in the male. Having given this general outline of the several parts which, in the female, contribute to the reproduction of the species, it will now be necessary to examine successively the formation, discharge, impregnation, and development of the ovum, to which these several parts are subservient. Unimpregnated Ovum. If the structure and formation of the human 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 contain, on an average, from fifteen to twenty small vesicles or mem- branous sacs of various sizes; these have been already alluded to as the follicles or vesicles of De Graaf } the anatomist who first accurately described them; they are also sometimes called ovisacs. At their first formation, the Graafian vesicles, according to Schron, are near the surface of the stroma of the ovary, but subsequently become more deeply placed; and again, as they increase in size, make their way towards the surface. When mature, they form little prominences on the exterior of the ovary, covered only by the peritoneum. Each follicle has an external membranous envelope, composed of fine fibro-cellular tissue, and connected with the surrounding stroma of the ovary by networks of blood-vessels (fig. 201). This enve- lope or tunic is lined with a layer of nucleated cells, forming a kind of epithelium or internal tunic, and named membrana granulosa. The cavity of the follicle is filled with an albu- minous fluid in which microscopic granules float ; and it contains also the ovum or ovule. 720 GENERATION AND DEVELOPMENT. The ovum is a minute spherical body situated, in im- mature follicles, near the centre : but in those nearer Fig. 201.* maturity, in contact with the membrana granulosa at that part of the follicle which forms a prominence on the surface of the ovary. The cells of the membrana granu- losa are at that point more numerous than elsewhere, and are heaped around the ovum, forming a kind of granular zone, the discus proligerus (fig. 2Ol). In order to examine an ovum, one of the Graafian vesicles, it matters not whether it be of small size or arrived at maturity, should be pricked, and the contained fluid received upon a piece of glass. The ovum then, being found in the midst of the fluid by means of a simple lens, may be further examined with higher microscopic powers. Owing to its globular form, however, its structure cannot be seen until it is subjected to gentle pressure. The human ovum is extremely small, measuring, accord- ing to Bischoff, from -^-^ to y-i^- of an inch. Its external investment is a transparent membrane, about -^Vo ^ an inch in thickness, which under the microscope, appears as a bright ring (fig. 202), bounded externally and internally by a dark outline : it is called the zona pellucida or vitelline membrane. It adheres externally to the heap of cells con- stituting the discus proligerus. Within this transparent investment or zona pellucida, * Fig. 20 1. Section of the Graafian vesicle of a Mammal, after Von Baer. i. Stroma of the ovary with blood-vessels. 2. Peritoneum. 3 and 4. Layers of the external coat of the Graafian vesicle. 5. Mem- brana granulosa. 6. Fluid of the Graafian vesicle. 7. Granular zone or discus proligerus, containing the ovum (8). THE GERMINAL VESICLE. 7 2 i 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 more or less fluid substance. The smaller granules, which are the more numerous, resemble in their appearance, as well as p . 2Q2 * their constant motion, pigment granules. The larger granules or globules which have the aspect of fat globules, are in greatest num- ber at the periphery of the yelk. The number of the granules is, according to Bischoff, greatest in the ova of carnivorous animals. In the human ovum their quantity is comparatively small. In the substance of the yelk is imbedded the germinal vesicle, or vesicula germinativa (figs. 2O2, 203) . This vesicle is of greatest relative size in the smallest ova, and is in them surrounded closely by the yelk, 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 yelk, and comes to be placed near to its surface. Its size in the human ovum has not yet been ascertained, owing to the difficulty of isolating it ; but it is probably about y^- of an inch in diameter. Ifc 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 yelk is situated the germinal spot (macula germinativa), a finely granulated sub- stance, of a yellowish colour, strongly refracting the rays of light, and measuring, in the Mammalia generally, from TeVo to Woo- of an inch (Wagner). * Fig. 202. Ovum of the sow, after Barry. I. Germinal spot. 2. Germinal vesicle. 3. Yelk. 4. Zona pellucida. 5. Discus proligerus. 6. Adherent granules or cells. 3 A 722 GENERATION AND DEVELOPMENT. Sucli are the parts of which the Graafian follicle and its contents, including the ovum, are composed. The diagram (fig. 203) represents them in their relative posi- tions when mature. With regard to the mode and order of development of these parts there is considerable un- certainty; but it seems most likely that the ovum is formed before the Graafian vesicle or ovisac. Fig. 203.* 3 4- 57 With regard to the parts of the ovum first formed, it appears certain that the formation of the germinal vesicle precedes that of the yelk and zona pellucida, or vitelline membrane. Whether the germinal spot is formed first, and the germinal vesicle afterwards developed around it, cannot be decided in the case of vertebrate animals ; but the observations of Kolliker and Bagge on the develop- ment of the ova of intestinal worms show that in these animals, the first step in the process is the production of round bodies resembling the germinal spots of ova, the * Fig. 203. Diagram of a Graafian vesicle, containing an ovum. i. Stroma or tissue of the ovary. 2 and 3. External and internal tunics of the Graafian vesicle. 4. Cavity of the vesicle. 5. Thick tunic of the ovum or yelk sac. 6. The yelk. 7. The germinal vesicle. 8. The germinal spot. CHANGE IN POSITION OF OVUM. 723 germinal vesicles being subsequently developed around these in the form of transparent membranous cells. The more important changes that take place in the ovum next to the formation of these its essential com- ponent parts, consist in alterations of the size and position of these parts with relation to each other, and of the ovum itself with relation to the Graafian vesicle, and in the more complete elaboration of the yelk. The earlier the stage of development the larger is the germinal vesicle in relation to the whole ovum, and of the ovum in relation to the Graafian vesicle. For, as the ovum becomes mature, although all these parts increase in size, the Graafian vesicle enlarges most, and the germinal vesicle least. Changes take place also in the position of the parts. The ovum at first occupies the centre of the Graafian vesicle, but subsequently is removed to its periphery. The germinal vesicle, too, which in young ova is in the centre of the yelk, is in mature ova found at the peri- phery. The change of position of the ovum, from the centre to the periphery of the Graafian vesicle, is possibly connected with the formation of the membrana granulosa which lines the vesicle. For, according to Valentin, at a very early period, the contents of the vesicle between its wall and the ovum are almost wholly formed of granules : but in the process of growth a clear fluid collects in the centre of the vesicle, and the granules, which from the first have a regular arrangement, are pushed outwards, and form the membrana granulosa. Now, as the mature ovum lies embedded in a thickened portion of the membrana granu- losa, it is possible that when the elementary parts of this membrane are pushed outwards, in the way just described, the ovum is carried with them from the centre to the peri- phery of the follicle. While the changes here described take place, the zona pellucida increases in thickness. 3 A 2 724 GENERATION AND DEVELOPMENT. According to Bischoff, the number of the granules of the yelk is greater the more mature the ovum, conse- quently the yelk is more opaque in the mature, and more transparent in the immature ova. The matter in which the granules are contained is fluid in the immature ova of all animals ; in some it remains so ; but in others, as the human ovum, it subsequently becomes a consistent gela- tinous substance. From the earliest infancy, and through the whole fruit- ful period of life, there appears to be a constant formation, development, and maturation of Graafian vesicles, with their contained ova. Until the period of puberty, however, the process is comparatively inactive ; for, previous to this period, the ovaries are small and pale, the Graafian vesicles in them are very minute, few in number, and probably never attain full development, but soon shrivel and dis- appear, instead of bursting, as matured follicles do ; the contained ova are also incapable of being impregnated. But, coincident 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. Discharge of the Ovum. In the process of development of individual 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 projection on the exterior. Coincident with the increase of size, caused by the augmen- tation of its liquid contents, the external envelope of the distended vesicle becomes very thin and eventually bursts. By this means, the ovum and fluid contents of the Graafian PERIODICAL DISCHARGE OF OVA. 725 vesicle are liberated, and escape on the exterior of the ovary, whence they pass into the Fallopian tube, the tim- briated processes of the extremity of which are supposed coincidently to grasp the ovary, while the aperture of the tube is applied to the part corresponding 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 appear to arrive at maturity, and the latter to be dis- charged 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, by the phenomena of menstruation. Sexual desire manifests itself in the human female to a greater degree at these periods, and in the female of mammiferous animals at no other time. 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 Mam- malia, 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 726 GENERATION AND DEVELOPMENT. Graafian vesicles being found ruptured in the intervals of the periods of heat ; and that female animals do not admit the males, and never become impregnated, except at those periods. Many circumstances make it probable that the human female is subject, in these respects, to the same law as the females of other mammiferous animals; namely, that in her as in them, ova are matured and discharged from the ovary independent of sexual union, and that this matura- tion and discharge occur periodically at the epochs of menstruation. Thus Graafian vesicles recently ruptured have been frequently seen in ovaries of virgins or women who could not have been recently impregnated, and although it is true that the ova discharged under these circumstances have rarely been discovered in the Fallopian tube,* 4 partly on account of their minute size, and partly because the search has seldom been prosecuted with much care, yet analogy forbids us to doubt that in the human female, as in the domestic quadrupeds, the result and purpose of the rupture of the follicles is the discharge of the ova. The evidence of the periodical discharge of ova at the epochs of menstruation is first, that nearly all authors who have touched on the point, agree that no traces of follicles having burst are ever seen in the ovaries before puberty or the first menstruation ; secondly, that in all cases in which ovarian follicles have been found burst, indepen- dently of sexual intercourse, the women were at the time menstruating, or had very recently passed through the menstrual state ; thirdly, that although conception is not confined to the periods of menstruation, yet it is more likely to occur within a few days after the cessation of the * See, however, the record of two such cases by Dr. Letheby, in the Philosophical Transactions, 1851. MENSTRUATION. 727 menstrual flux than at other times; and, lastly, that the ovaries of the human female become turgid and vas- cular at the menstrual periods, as those of animals do at the time of heat. From what has been said, it may, therefore, be concluded that the two states, heat and menstruation, are analogous, and that the essential accompaniment of both, is the matu- ration and extrusion of ova. In both there is a state of active congestion of the sexual organs, sympathising with the ovaries at the time of the highest degree of develop- ment of the Graafian vesicles; and in both, the crisis of this state of congestion is attended by a discharge of blood or mucus, or both, from the external organs of generation. 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 necessarily 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 subject, 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 Mr. Roberton 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 in- ducing sexual excitement previous to the proper menstrual time. The menstrual functions continue through the 728 GENERATION AND DEVELOPMENT. whole fruitful period of a woman's life, and usually cease between the forty-fifth and fiftieth years. The several menstrual periods occur usually 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 indi- viduals. 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. The menstrual discharge consists of blood effused from the inner surface of the uterus, and mixed with mucus from the uterus, vagina, and external parts of the gene- rative apparatus. Being diluted by this admixture, the menstrual blood coagulates less perfectly than ordinary blood; and the frequent acidity of the vaginal mucus tends still further to diminish its coagulability. This has led to the erroneous supposition that the menstrual blood contains an unusually small quantity of fibrin, or none at all. The blood-corpuscles exist in it in their natural state : mixed with them may also be found numerous scales of epithelium derived from the mucous passages along which the discharge flows. Corpus Luteum. Immediately before, as well as subsequent to, the rup- ture 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 CORPUS LUTEUM. 729 animals is a roundish solid body, of a yellow or orange colour, and composed of a number of lobules, which, sur- round, sometimes a small cavity, but more frequently a small stelliform mass of white substance, from which deli- cate processes pass as septa between the several lobules. Very often, in the cow and sheep, there is no white sub- stance in the centre of the corpus luteum ; 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, im- mediately before the rupture takes place, its walls appear thickened on their 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 contraction of the outer layer, and, soon, red fleshy mammillary pro- cesses 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 closer, 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. 204) 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 cavity is obliterated, being propor- tionately 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 73 GENERATION AND DEVELOPMENT. the case in other Mammalia. It maintains the character of a uniform, or nearly uniform, layer, which is thrown into wrinkles, in consequence 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; 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 decolorized fibrin forms a membraniform sac, lining the corpus luteum. At * Fig. 204. Corpora lutea of different periods. B. Corpus luteum of about the sixth week after impregnation, showing its plicated form at that period, i. Substance of the ovary. 2. Substance of the corpus luteum. 3. A greyish coagulum in its cavity. After Dr. Paterson. A. Corpus luteum, two days after delivery. D. In the twelfth week after delivery. After Dr. Montgomery. CORPUS LUTEUM. 731 other times the serum is removed, and the fibrin consti- tutes a solid stelliform mass. The yellow substance of which the corpus luteum con- sists, 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 increased development of the cells forming the membrana granulosa, 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 impregnated, the growth of the yellow sub- stance goes on during nearly the whole period of gestation, and forms the large corpus luteum commonly described as a characteristic mark of impregnation. 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 continues 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 dif- ferences between the corpus luteum of the pregnant and unimpregnated condition respectively. 732 GENERATION AND DEVELOPMENT. At the end of three weeks One month Two months Six months Nine months CORPUS LUTEUM OF MEN- CORPUS LUTEUM OF PREG- STRUATION. NANCY. Three-quarters of an inch in diameter ; central clot red- dish ; convoluted Avail pale. Smaller ; convoluted wall bright yellow ; clot still reddish. Reduced to the condition of an insignificant cicatrix. Absent. Absent. Larger ; convoluted wall bright yellow ; clot still reddish. Seven-eighths of an inch in diameter ; convoluted wall bright yellow; clot per- fectly decolorised. Still as large as at end of second month ; clot fibri- nous ; convoluted wall paler. One-half an inch in diame- ter ; central clot converted into a radiating cicatrix; the external wall tolerably thick and convoluted, but without any bright yellow colour. IMPREGNATION OF THE OVTJM. Male Sexual Functions. The fluid of the male, by which the ovum is impregnated, consists essentially of the semen secreted by the testicles ; and to this are added, as necessary, perhaps, to its perfec- tion, a material secreted by the vesiculse seminales, in which, as in reservoirs, the semen lies before its discharge, as well as the secretion of the prostate gland, and of Cowper's glands. Portions of these several fluids are, probably, all discharged, together with the proper secre- tion of the testicles. The secreting structure of the testicle is disposed in two contiguous parts, (l) the body of the testicle enclosed within a tough fibrous membrane, the tunica albuginea, on the outer surface of which is the serous covering formed by the tunica vaginalis, and (2.) the epididymis. The vas deferens, THE STRUCTURE OF THE TESTICLE. 733 the main trunk of the secreting tube, when followed back to its origin, is found to pass to the lower part of the epi- didymis, and assumes there a much less diameter with a very tortuous course : with its various convolutions it forms first the mass named globus minor, then the body, and then the globus major of the epididymis. At the last-named part, the duct divides into ten or twelve small branches, the convolutions of which form coniform masses, named coni vasculosi ; and the vessels continued from these, the vasa efferentia, after anastomosing, one with another, in what is called the rete testis, lead finally through the tubuli recti or vasa recta to the tubules which form the proper substance of the testicle, wherein they are arranged in lobules, closely packed, and all attached to the tough fibrous tissue at the back of the testicle. The seminal tubes, or tubuli seminiferi, which compose the proper substance of the testicle, are fine thread-like tubules, formed of simple homogene- ous membrane, measuring on an average j^-th to ^w^ ^ an mc ^ in diameter, and lined with epithe- lium or gland-cells. Rarely branching, they extend as simple tubes through a great length, with the same uniform structure, and probably terminate either in free closed extremities or in loops. Their walls are covered with fine capillary blood- vessels, through which, reckoning their great extent in Fig. 205.* * Fig. 205. Plan of a vertical section of the testicle, showing the arrangement 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 ; d, vasa efferentia ending in the coni vasculosi ; I, e, g, convoluted canal of the epididymis ; h, vas deferens ; /, section of the back part of the tunica albuginea ; i t i, fibrous processes running between the lobes ; s, mediastinum. 734 GENEKATION AND DEVELOPMENT. A, spermatic fila- ments from the human vas deferens (from Kolliker). i, magnified 350 diameters ; 2, magnified 800 dia- meters ; a, from the side ; b, from above. B, spermatic cells and spermatozoa of the bull undergoing development (from Kolliker) i|2. i, spermatic cells, with one or two nuclei, one of them clear ; 2, 3, free nuclei, with spermatic filaments forming ; 4, the filaments elongated and the body widened ; 5, filaments nearly fully developed. C, escape of the spermatozoa from their cells in the same animal, i, spermatic cell containing the spermatozoon coiled up within it ; 2, the cells elon- gated by the partial uncoiling of the spermatic filament ; 3, a cell from which the filament has in part be- come free ; 4, the same with the body also partially free ; 5, sperma- tozoon from the epididymis with vestiges of the cell adherent ; 6, sper- matozoon from the vas deferens, showing the small enlargement, b, on the filament. THE SEMINAL "FLUID. 735 comparison with, the size of the spermatic artery, the blood must move very slowly. The seminal fluid secreted by the testicle is one of those secretions in which a process of development is continued after its formation by the secreting cells, and its discharge from them into the tubes. The principal part of this de- velopment consists in the formation of the peculiar bodies named seminal filaments, spermatozoa or spermatozoids (fig. 206) the complete development of which, in their full propor- tion of number, is not achieved till the semen has reached, or has for some time lain in, the vesiculae seminales. Earlier, after its first secretion, the semen contains none of these bodies, but granules and round corpuscles (seminal corpuscles), like large nuclei, enclosed within parent-cells (fig. 206). Within each of these corpuscles, or nuclei, a seminal filament is developed, by a similar process in nearly all animals. Each corpuscle, or nucleus, is filled with granular matter ; this is gradually converted into a spermatozoid, which is at first coiled up, and in contact with the inner surface of the wall of the corpuscle (fig. 206, 0,1). Thus developed, the human seminal filaments consist of a long, slender, tapering portion, called the body or tail, to distinguish it from the head, an oval or pyriform por- tion of larger diameter, flattened, and sometimes pointed. They are from J-^th to -^ of an inch in length, the length of the head alone being from -oVo^ 1 to ^^th ^ an inch, and its width about half as much. They present no trace of structure, or dissimilar organs; a dark spot often observed in the head, is probably due to its being concave, like a blood corpuscle. They move about in the fluid like so many minute corpuscles, with each a ciliary process, lashing their tails, and propelling their heads forwards in various lines. Their movement, which is pro- bably essentially, as well as apparently, similar to that of ciliary processes, appears nearly independent of external 736 GENERATION AND DEVELOPMENT. conditions, provided the natural density of the fluid is pre- served ; disturbing this condition, by either evaporating the semen or diluting it, will stop the movement. It may continue within the body of the female for seven or eight days, and out of the body for at least nearly twenty- four hours. The direction of the movement is quite un- certain : but in general, the current that each excites keeps it from the contact of others. The rate of motion, according to Valentin, is about one inch in thirteen minutes. Respecting the purpose served by these seminal fila- ments, or concerning their exact nature, little that is certain can be said. Their occurrence in the impregnating fluid of nearly all classes of animals, proves that they are essential to the process of impregnation ; but beyond this, and that their contact with the ovum is necessary for its development, nothing is known. The seminal fluid is, probably, after the period of puberty, secreted constantly, though, except under excitement, very slowly, in the tubules of the testicles. From these it passes along the vasa deferentia into the vesiculse seminales, whence, if not expelled 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 defecation. The vesicula seminales have the appearance of out-growths 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, constructs the vesicula seminalis. Each of the vesiculse, therefore, might be unravelled into a single branching tube, sacculated, convoluted, and folded up. THE VESICUL^E SEMINALES. 737 The mucous membrane lining the vesiculee 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 rest of their walls is formed, chiefly of a layer of organic muscular fibres, from which Fig. 207.* they derive contractile power for the expulsion of their contents. To the vesiculae seminales a double function may be assigned ; for they both secrete some fluid to be added to * Fig. 207. Dissection of the base of the bladder and prostate gland, showing the vesiculse seminales and vasa deferentia (from Haller). a', lower surface of the bladder at the place of reflexion of the peritoneum ; ft, the part above covered by the peritoneum ; i, left vas deferens, ending in e, the ejaculatory duct ; the vas deferens has been divided near i, and all except the vesical portion has been taken away ; s, left vesicula seminalis joining the same duct ; s, s, the right vas deferens and right vesi- cula seminalis, which has been unravelled ; p, under side of the prostate gland ; m, part of the urethra ; u, u, the ureters (cut short near u), the right one turned aside. 3 B 738 GENERATION AND DEVELOPMENT. 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 vesiculse 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 corresponding vesicula seminalis suffers no atrophy, though its function as a reservoir is abrogated. But how the vesiculse seminales act as secreting organs is unknown; the peculiar brownish fluid which they contain after death does not properly represent their secretion, for it is different in appearance from anything discharged during life, and is mixed with semen. It is nearly certain, however, that their secretion contributes to the proper composition of the impregnating fluid ; 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 vesiculse seminales, whether they communicate with the vasa deferentia or not, enlarge commensurately with the testicles at the approach of that season. That the vesiculee are also reservoirs in which the semi- nal fluid may lie for a time previous to its discharge, is shown by their commonly containing the seminal filaments in larger abundance than any portion of the seminal ducts themselves do. The fluid-like mucus, also, which is often discharged from the vesiculae in straining during defeca- tion, commonly contains seminal filaments. But no reason can be. given why this office of the vesiculse 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 re- specting the secretions of the prostate and Cowper's glands, their nature and purposes. That they contribute to INFLUENCE OF FCETUS ON MOTHER. 739 the right composition of the 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 subordinate part is shown by the fact, that, when the testicles are lost, though these other organs be perfect, all procreative power ceases. The mingled secretions of all the organs just described, form the semen or seminal fluid. Its corpuscles have been already described (p. 735) : its fluid part 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 off- spring all the characters, in features, size, mental disposi- tion, and liability to disease, which belong to the father. This is a fact wholly inexplicable : and is, perhaps, only exceeded in strangeness by those facts which show that the seminal fluid may exert such an influence, not only on the ovum which it impregnates, but, through the medium of the mother, on many which are subsequently impreg- nated by the seminal fluid of another male. It has been often observed for example, that a well-bred bitch, if she have been once impregnated by a mongrel dog, will not bear thorough-bred puppies in the next two. or three litters after that succeeding the copulation with the mongrel. But the best instance of the kind was in the case of a mare belonging to Lord Morton, who, while he was in India, wished to obtain a cross-breed between the horse and quagga, and caused this mare to be covered by a male quagga. The foal that she next bore had distinct marks of the quagga, in the shape of its head, black bars on the legs and shoulders, and other characters. After 740 GENERATION AND DEVELOPMENT. this time she was thrice covered by horses, and every time the foal she bore had still distinct, though decreasing, marks of the quagga; the peculiar characters of the quagga being thus impressed not only on the ovum then impregnated, but on the three following ova impregnated by horses. It would appear, therefore, that the constitu- tion of an impregnated female may become so altered and tainted with the peculiarities of the impregnating male, through the medium of the foetus, that she necessarily imparts such peculiarities to any offspring she may sub- sequently bear by other males. Of the direct means by which a peculiarity of structure on the part of a male is thus transmitted, nothing whatever is known. DEVELOPMENT. Changes in the Ovum previous to the Formation of the Embryo. Of the changes which the ovum undergoes previous to the formation of the embryo, some occur while it is still in the ovary, and are apparently independent of impreg- nation : others take place after it has reached the Fallo- pian tube. The knowledge we possess of these changes is derived almost exclusively from observations on the ova of mammiferous animals, especially the bitch and rabbit : but it may be inferred that analogous changes ensue in the human ovum. Bischoff describes the yelk of an ovarian ovum after coitus as being unchanged in its characters, with the single exception of being fuller and more dense ; it is still granular, as before, and does not possess any of the cells subsequently found in it. The germinal vesicle always disappears, sometimes before the ovum leaves the ovary, at other times not until it has entered the Fallopian tube ; but always before the commencement of the metamorphosis of the yelk. As the ovum approaches the middle of the Fallopian tube, it begins to receive a new investment, consisting of CLEAVAGE OF THE YELK. 741 a layer of transparent 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. About this time, that it is to say, during its passage through the Fal- lopian tube, a very remarkable change takes place in the interior of the ovum. The whole yelk be- comes constricted in the middle, and surrounded by a furrow, which, gradually deepening, at length cuts the yelk 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 cleavings 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. 208). 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 surface. The cause of this singular subdivision of the yelk is * Fig. 208. Diagrams of the various stages o? cleavage of the yelk (after Dalton). 742 GENERATION AND DEVELOPMENT. quite obscure : though, the immediate agent in its produc- tion 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 from the germinal vesicle. This, by some process of multiplication, divides and subdivides : then each division and subdivision attracts around itself, as a centre, a certain portion of the sub- stance of the yelk. About the time at which the mammiferous ovum reaches the uterus, the process of division and subdivision of the yelk appears to have ceased, its substance having been resolved into its ultimate and smallest divisions, while its surface presents a uniform finely-granular aspect, instead of its late mulberry-like appearance. The ovum, indeed, appears at first sight to have lost all trace of the cleaving 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 examina- tion, it is found that these granules are aggregated into numerous minute spherical masses, each of which contains a clear vesicle in its centre, but is not, at this period, provided with an enveloping membrane, and possesses none of the other characters of a 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 Fallopian tube. The time occupied in the passage of the ovum, from the ovary to the uterus, occupies, probably, eight or ten days in the human female. Shortly after this, important changes ensue. Each of the several globular segments of the yelk becomes sur- rounded by a membrane, and is thus converted into a cell, the nucleus of which is formed by the central vesicle, the contents by the granular matter originally composing the SEROUS AND MUCOUS LAYERS. 743 globule : these granules usually arrange themselves con- centrically around the nucleus. When the peripheral cells, which are formed first, are fully developed, they arrange themselves at the surface of the yelk into a kind of mem- brane, and at the same time assume a pentagonal or hexagonal shape from mutual pressure, so as to resemble pavement -epithelium. As the globular masses of the interior are gradually converted into cells, they also pass 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 externally of the original vitelline membrane, and within by the newly formed cellular layer, the blastodermic or germinal membrane, as it is called. Very soon, however, the latter, by the develop- ment of new cells, increases in thickness, and splits into two layers, so that now the ovum has three coats. The vitelline membrane on the outside, and, within this, the outer and the inner layers of the blastodermic membrane. Of the last-named layers, the superior or outer, which lies next to the zona pellucida, is called the serous layer ; from it are developed the organs of the animal system of the body, e.g., the bones, muscles, and integuments. The inferior or inner layer, in contact with the yelk itself, is named the mucous layer, and serves for the formation of the internal or visceral system of organs. Changes of the Ovum within the Uterus. Very soon after its formation, and division into two layers, the blastodermic vesicle or membrane presents at one point on its surface an opaque roundish spot, which is produced by an accumulation of cells and nuclei of cells, of less transparency than elsewhere. This space, the 744 GENERATION AND DEVELOPMENT. " area germinativa" or germinal area, is the part at which the embryo first appears. At first the area germinativa has a rounded form, but Fig. 209.* ^ soons loses this and becomes oval, then pear-shaped, and while this change in form is taking place, there gradually appears in its centre a clear space or area pellucida (fig. 209), bounded externally by a more opaque circle, the obscurity being due to the greater accumulation of nu- cleated cells and nuclei at that part than in the area pellucida. The first trace of the embryo in the centre of the area pellucida consists of a shallow groove or channel, the primitive groove (fig. 209), formed of the external or serous fold of the germinal membrane, the groove being wider at its anterior or cephalic extremity, and tapering towards the opposite extremity. Coincidently with the formation of the primitive groove, two oval masses of cells, the lamina dorsales, appear, one on each side of the groove. At first scarcely elevated above the plane of the germinal membrane, they soon rise into two prominent masses, the upper borders of which gradually tend towards each other, turning inwards over the primi- tive groove. The parts from opposite sides then unite, and convert the primitive groove into a tube, large and rounded in front, narrow and lancet-shaped behind, which is the central canal of the cerebro-spinal axis, and contains the rudimental spinal cord and brain, which are developed in its interior (fig. 210). * Fig. 209. (After Dalton.) Impregnated egg, with commencement of formation of embryo ; showing the area germinativa or embryonic spot, the area pellucida, and the primitive groove or trace. THE CHORDA DORSALIS. 745 Immediately beneath, and in a line parallel with, the primitive groove, may be seen, about the same time, a narrow linear mass of cells, the chorda dorsalis, which forms the basis around which the bodies of the vertebrae are developed. The development of this column is early Fig. 210.* indicated by the appearance of a few square, at first in- distinct, plates, the rudiments of vertebne (fig. 2IO, D), which begin to appear at about the middle of each dorsal lamina. * Fig. 210. Portion of the germinal membrane, with 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. After Bischoff. 746 GENERATION AND DEVELOPMENT. Fig. 211* While the dorsal laminoo are closing over the primitive groove, thickened prolonga- tions of the same serous layer are given off from the lower margin of each of them, and are named lamina viscerates sen ventrales. These visceral laminae by degrees bend downwards and inwards, and at length, enclosing a part of the yelk, unite and form the anterior walls of the trunk enclosing the abdominal cavity below, as the dorsal plates enclose the cerebro-spinal canal above. Umbilical Vesicle. The ventral laminae, as they extend downwards and in- wards, at first proceed on the same plane with the inner layer of the germinal membrane, which immediately lines them. Soon, however, they show a tendency to turn in- wards, so as to constrict the yelk, and enclose only a part of it ; and soon afterwards the yelk and the inner layer of the germinal membrane that contains it, are separated into two portions, one of which is retained within the body of the embryo, while the other remains outside, and receives the name of the umbilical vesicle (v, fig. 212). The cavity of the latter communicates for some time with that of the abdomen, through what is called the umbilicus, by means of a gradually narrowing canal, called the vitelline duct ; the interior of the abdomen and that of the umbilical vesicle being lined by a continuous layer of the inner stratum, or mucous layer of the germinal membrane ; while around both of them is a continuation of the outer, or serous layer * Fig. 211. Diagram showing vascular area in the chick, a, Area pellucida. I. Area A-asculosa. c. Area vitellma. THE UMBILICAL VESICLE. 747 (fig. 212). From that portion of the mucous layer which is now enclosed within the body of the embryo, the intes- tinal canal is developed. Fig. 212.* fk /* Thus, by the constriction which the fold of germinal membrane, in which the abdominal walls are formed, pro- duces at the umbilicus, the body of the embryo becomes * Fig. 212. Diagrammatic section showing the relation in a mammal and in man between the primitive alimentary canal and the membranes of the ovum. The stage represented in this diagram corresponds 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 arnnion ; a', 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 vertebrae and cerebro-spinal axis ; i, i, the simple alimentary canal in its upper and lower portions ; v, the yolk- sac or umbilical vesicle ; v i, the vitello-intestinal opening ; u, the allantois connected by a pedicle with the anal portion of the alimentary canal. 748 GENEEATION AND DEVELOPMENT. in great measure detached from, the yelk- sac or umbilical vesicle, though the cavity of the rudimentary intestine still communicates with it through the vitelline or omphalo- mesenteric duct, and contains part of the yelk substance with which the vesicle was filled. The yelk-sac contains, however, the greater part of the substance of the yelk, and furnishes a source whence nutriment is derived for the embryo. In birds, the contents of the yelk-sac afford nourishment until the end of incubation: but in Mam- malia, the office of the corresponding umbilical vesicle ceases at a very early period, the quantity of 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, it is gradually drawn into the abdo- men through the umbilical opening, which then closes over it : but in Mammalia it always remains on the outside ; Fig. 213.* and as it is emptied it contracts (fig. 213), shrivels up, and together with the part of its duct external to the abdomen, is de- tached and disappears either before, or at the termination of intra-uterine life, the period of its disappearance varying in different orders of Mammalia. When blood-vessels begin to be de- veloped, they ramify largely over the walls of the umbilical vesicle, and are actively concerned in absorbing its contents and conveying them away for the nutrition of the embryo. The Amnion and Allantois. 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 * Fig. 213. Human embryo with umbilical vesicle ; about the fifth week (after Dalton). THE AMNION AND ALLANTOIS. 749 the amnion and the allantois, begin to be formed the am- nion being developed by the external, and the allantois by the internal layer of the blastodermic membrane. The amnion is produced in the fol- Fig. 214.* lowing manner : The external layer of the blastodermic membrane is raised up in the form of a fold around the body of the embryo, so that the latter appears as if sunk in a kind of de- pression, with the outer layer of the membrane raised up wall-like around it. On section, the appearance is that represented in fig. 214. Soon the edges of the fold rising higher and higher above and around the embryo, coalesce over it; and the double layer of membrane at their place of junction being absorbed, the two layers of which the fold was originally made up are separated from each other (figs. 2 1 6, 2 17). The inner of the two forms the amnion, and remains continuous with the integument of the foetus at the umbilicus ; while the outer layer, receding farther and farther, is fused and forms one with the inner surface of the original vitelline membrane, which in the mean time has undergone various alterations, to be immediately described. As the term of pregnancy advances, the amnion becomes more and more separated from the body of the foetus by a considerable quantity of fluid, the so-called liquor amnii. During the process of development of the amnion, the allantois (c, fig. 214) begins to be formed. Growing out from, or near the hinder portion of the intestinal canal, with which it communicates, it is at first a pear-shaped mass of cells ; but becoming vesicular and very soon simply membra- nous and vascular, it insinuates itself between the amniotic folds, just described, and comes into close contact and * Fig. 214. Diagram of fecundated egg (after Dalton). a, um- bilical vesicle ; b, amniotic cavity ; c, allantois. 750 GENERATION AND DEVELOPMENT. union with the outer of the two folds, which has itself, as before said, become one with the external investing mem- Fig. 215.* brane of the egg. As it grows, the allantois becomes exceedingly vascu- lar, and in birds (fig. 215) envelopes the whole embryo taking out vessels, so to speak, to the outer investing membrane of the egg, and lining the inner surface of the shell with a vas- cular membrane; by these means afford- ing an extensive surface in which the blood may be aerated. In the human subject and in other mammalia, the vessels carried out by the allantois are distributed only to a special part of the outer membrane, at which a structure called the placenta is developed. In Mammalia, as the visceral laminae close in the abdo- minal cavity, the allantois is thereby divided at the umbi- licus 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 urachiis. After birth the umbilical 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 to the umbilicus, in the middle line of the body, immediately beneath the parietal layer of the peritoneum. It is sometimes enumerated among the liga- ments of the bladder. * Fig. 215. Fecundated egg with allantois nearly complete, a, inner layer of amniotic fold ; &, outer layer of ditto ; c, point where the amniotic folds come in contact. The allantois is seen penetrating between the outer and inner layers of the amniotic folds. This figure, which represents only the amniotic folds and the parts within them, should be compared with figs. 216, 217, in which Avill be found the structures external to these folds. THE AREA VASCULOSA. 75 1 It must not be supposed that the phenomena which have been successively described, occur in any regular order one after another. On the contrary, the development of one part is going on side by side with that of another. Fig. 217.* Development of Blood-vessels. At an early period of development, and during the changes just described, an accumulation of cells ensues between the mucous and serous laminae at a part of the germinal mem- brane named the area vasculosa (&, fig. 2Il). Within this mass, which constitutes a third or middle layer of the blasto- dermic membrane, is laid the foundation for the develop- ment of the vascular system. At the circumference of the vascular area, insulated red spots and lines make their appearance, and these soon unite, so as to form a network of vessels filled with blood. The margin of the vascular layer is at first limited and quite circular, being bounded * Figs. 216 and 217 (after Todd and Bowman), a, chorion with villi. The villi are shown to be best developed in the part of the chorion to which the allantois is extending ; this portion ultimately becomes the placenta. &, space between the two layers of the amnion. c, amniotic cavity, d, situation of the intestine, showing its connexion with the umbilical vesicle, e, umbilical vesicle. /, situation of heart and vessels, g, allantois. 752 GENERATION AND DEVELOPMENT. by vessels united in a circulus venosus, or sinus terminalis, but it soon extends over the whole surface of the germinal membrane. At about the same time, the rudimentary heart is formed in the same layer of the germinal membrane. As shown by Schwann, the blood-vessels are developed originally from nucleated cells. These cells send out processes ; the processes from, different cells unite ; and in this way rami- fications and a network are produced vessels extending from this network in the area vasculosa into the area pellucida, and joining the rudimentary heart (see p. 765). The Chorion. It has been already remarked 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 Fi 2i8 ovum, or the chorion, as it is now called, represents three layers, namely, the original vi- telline 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 tlieclwrion(a, figs. 2 1 6, 2 1 7), which give it a rough and shaggy ap- pearance. At first only cellular in structure, these little outgrowths subsequently become vascular by the develop- ment in them of loops of capillaries (fig. 2 1 8) ; and the latter THE VILLI OF THE CHORION. 755 at length form the minute extremities of the blood-vessels which are, so to speak, conducted from the fcetus to the chorion by the allantois (fig. 219). The function of the Fig. 219.* villi of the chorion is evidently the absorption of nutrient * Fig. 2 19 represents a perpendicular section of a uterus, with a fully formed ovum. A plug of lymph (i) occupies the cervix uteri ; 2, indi- cates the opening of the Fallopian tube on one side ; 3, the decidua vera; 4, the cavity of the uterus nearly filled by the ovum ; 5, the decidua reflexa ; 6, the chorion ; 7, the decidua serotina (see p. 756) ; 8, the allantois and the situation of the future placenta ; 9, the amnion ; 10, the umbilical vesicle ; 1 1, the umbilical cord. 3 c 754 GENERATION AND DEVELOPMENT. matter for the foetus; and this is probably supplied to them at first from the fluid 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 at one part where they are greatly developed, and by their branching give rise, with the vessels of the uterus, to the formation of the placenta (fig. 219). 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 structure 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 the period of parturition. Changes of the Mucous Membrane of the Uterus, and Formation of the Placenta. The mucous membrane of the human uterus is abun- dantly beset with tubular follicles, arranged perpen- dicularly to the surface. These follicles are very small in the unimpregnated uterus; but when examined shortly after impregnation, they are found elongated, enlarged, and much waved and contorted towards their deep and closed extremity, which is implanted at some depth in the tissue of the uterus, and commonly dilates into two or three closed sacculi(fig. 220). According to Dr. Sharpey, tire glands of the mucous THE GLANDS OF THE UTERUS. 755 membrane of the bitch's uterus (and, according to H. Miiller, that of the human female also) are of two kinds, Fig. 220.* simple and compound. The former, which are the more numerous, are merely very short unbranched tubes closed at one end (fig. 221, y); the latter ( 2 , 2 ) have a long duct dividing into convoluted branches; both open on the inner surface of the membrane by small round orifices, lined with epithelium and set closely together. On the internal surface Fiq> 22i.f 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 epi- thelium which lines the follicles (fig. 222). Coincidently with the increasing size of the follicles, the quantity of their secretion is augmented, the vessels of * Fig. 220. Section of the lining membrane of a human uterus at the period of .commencing pregnancy, 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. t Fig. 221. A vertical section of the mucous membrane, showing uterine glands of the bitch, magnified twelve diameters ; I, i, simple glands ; 2, 2, compound ditto (from Sharpey). 3 2 756 GENERATION AND DEVELOPMENT. the mucous membrane become larger and more numerous, while a substance composed chiefly of nucleated cells fills up the interfollicular spaces in which the blood-vessels are Fig. 222.* contained. The effect of these changes is an in- creased thickness, softness, and vascularity of the mu- cous membrane, the super- ficial part of which itself forms the memlrana de- cidua. The object of this in- creased development seems to be the production of nutritive materials for the ovum ; for the cavity of the uterus shortly becomes filled with secreted fluid, consisting almost entirely of nucleated cells, in which the villi of the chorion are embedded. When the ovum first enters the uterus it becomes im- bedded in the structure of the decidua, which is yet quite soft, and in which soon afterwards three portions are distinguish- able. These have been named the decidua vera, the decidua refl,exa, and the decidua serotina. The first of these, the decidua vera (3, fig. 219), lines the cavity of the uterus; the second, or decidua reflexa (5), 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 (7), 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. * Fig. 222. Two thin segments of human decidua after recent im- pregnation, viewed on a dark ground : they show the openings on the surface of the membrane. A is magnified six diameters, and B twelve diameters. At I, the lining of epithelium is seen within the orifices, at 2 it has escaped (from Sharpey). THE PLACENTA. 757 As the ovum increases in size, the decidua vera and the decidua reflexa gradually come into contact, and in the third month of pregnancy the cavity between them has quite disappeared. Henceforth it is very difficult, or even impossible to distinguish the two layers. 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 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- change of matters between them. Thus the villi of the chorion, 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 con- taining maternal blood. But in the foetal villi there is a constant stream of blood into and out of the loop of capil- lary blood-vessel contained in it, as there is also into and out of the maternal sinuses. It would seem from the observations of Professor Goodsir, that, at the villi of the placental tufts, \vhere 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 intervention of two distinct sets of nucleated cells (fig. 223). One of these (6), belongs to the maternal portion of the placenta, is placed between the membrane of the villus and that of the vascular system of the mother, and is probably designed to separate from the blood of the 758 GENERATION AND DEVELOPMENT. 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 (d), Fig. 223.* 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 the fcetal vessels. Not only, however, is there a pas- sage of materials from the blood of the mother into that of the foetus, but there can be no doubt of the existence of 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. Dr. Alexander Harvey's experiments were very decisive on this point. The view has also received abundant support of late from Mr. Hutchinson's important observations on the communi- cation of syphilis from the father to the mother, through the instrumentality of the foetus; and still more from Mr. 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, Mr. Savory injected a solution * Fig. 223. Extremity of a placental 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 ; c, internal membrane of the villus, or external membrane of the chorion ; /, internal cells of the villus, or cells of the chorion ; g, loop of umbilical vessels (after Goodsir). THE PLACENTA. 759 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 hour, 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 com- posed of a fatal part and a maternal part, the term, placenta, properly including 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 development of those parts which it is destined to nourish. 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 crush- ing through of that part at which its cohesion is least strong, namely, where it is most burrowed and undermined by the cavernous spaces before referred to. In this way it is cast off with the fcetal membranes and the decidua vera and reflexa, together with a part of the decidua serotina. The remaining portion withers, and disappears by being gra- dually either absorbed, or thrown off in the uterine discharges or the loclda, which occur at this period. A new mucous membrane is of course gradually de- veloped, as the old one, by its peculiar transformation into what is called the decidua, ceases to perform its original functions. The umbilical cord ( 1 1, fig. 2 19), which in the later period of foetal life is almost solely composed of the two arteries and the single vein which respectively convey fcetal blood to 760 GENERATION AND DEVELOPMENT. 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 comparative importance. Thus, in early fcetal life, it is composed of the following parts: (l). Externally, a layer of the amnion, reflected over it from the umbilicus (fig. 219). (2). The umbilical vesicle (10, fig. 219), 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, ulti- mately form the greater part of the cord. DEVELOPMENT OF ORGANS. It remains now to consider in succession the development of the several organs and systems of organs in the further progress of the embryo. i Development of the Vertebral Column and Cranium. The primitive part of the vertebral column in all the Vertebrata is the gelatinous chorda dorsalis, which con- sists entirely of cells. 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 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 vertebrae, although it never itself passes into the cartilaginous or osseous state, but remains enclosed as in a case within the persistent parts of the vertebral column which are developed around it. It is permanent, however, only in a few animals : in the majority it disappears at an early period. The cartilaginous or osseous vertebrae are always first developed in pairs of lateral elements at the sides of the DEVELOPMENT OF VERTEBRAE. 761 chorda dorsalis. From these lateral elements are formed the bodies and the arches of the vertebrae. In some animals, as the sturgeon, however, the lateral elements of the vertebrae undergo no further development, and it is here that the chorda dorsalis is persistent through life. In the myxinoid fishes the spinal column presents no ver- tebral segments, and there exists merely the chorda dorsalis with the fibrous layer surrounding its sheath, which is the layer in which the skeleton originates. This fibrous layer also forms superiorly the membranous cover- ing of the vertebral canal. In reptiles, birds, and mammals, the mode in which the vertebrae are formed around the chorda dorsalis seems to be different. When the formation of these parts from the blastema commences, there appears at each side of the chorda dorsalis a series of quadrangular figures, the rudiments of the future vertebrae. These gra- dually increase in number and size, so as to surround the chorda both above and below, sending out, at the same time, superiorly, processes to form the arches des- tined to enclose the spinal cord. In this primitive con- dition the body and arches of each vertebra are formed by one piece on each side. At a certain period these two primary elements, which have become cartilaginous, unite inferiorly by a suture. The chorda is now enclosed in a case, formed by the bodies of the vertebrae, but it gra- dually wastes and disappears. Before the disappearance of the chorda, the ossification of the bodies and arches of the vertebrae begins at distinct points. The ossification of the body of a vertebra is first ob- served at the point where the two primitive elements of the vertebrae have united inferiorly. Those vertebrae which do not bear ribs, such as the cervical vertebras, 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 762 GENEKATION AND DEVELOPMENT. ossified portions exist in all the cervical vertebra), 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 vertebreo are the most developed, and in children may, for a considerable period, be distinguished as a separate part on each side, 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. This relation of the chorda dorsalis to the basis of the cranium is persistent through life in some fish, e.g., the sturgeon. The first appearance of a solid support at the base of the cranium observed by Miiller in fish, consists of two elongated bands of cartilage, one on the right and the other on the left side, which are connected with the car- tilaginous capsule of the auditory apparatus, and united with each other in an arched manner anteriorly beneath the anterior end of the cerebral capsule. Hence, in the cranium, as in the spinal column, there are at first de- veloped at the sides of the chorda dorsalis two symmetrical elements, which subsequently coalesce, and may wholly enclose the chorda.* Development of the Face and Visceral Arclies. It has been said before that at an early period of development of the embryo, there grow up on the sides of the primitive groove the so-called dorsal lamina;, which at * For much new and original matter relating to the development of the cranium, the reader is referred to the important lectures on Com- parative Anatomy, delivered at the College of Surgeons by Professor Huxley. THE VISCERAL ARCHES AND CLEFTS. 763 Fig. 224.* length coalesce, and complete by their union the spinal canal. The same process essentially takes place in the head, so as to enclose the cranial cavity. The so-called visceral lamina have been also described as passing forwards, and gradually coalescing in front, as the dorsal laminae do behind, and thus enclosing the thoracic and abdominal cavity. An analogous process occurs in the facial and cervical regions, but the enclosing laminae, instead of being simple, as in the former instances, are cleft. In this way the so-called visceral arches and clefts are formed, four on each side (fig. 224, A), and from or in connection with these arches the following parts are developed : From the first arch, and its maxillary process, the superior maxillary, the palate bone, and the internal pterygoid plate of the sphenoid bone, the incus and malleus and the lower jaw. The upper part of the face in the middle line is developed from the so-called fronto-nasal process (A, 3, fig. 224). From the second arch are de- veloped the stapes, the stapedius muscle, the styloid process of the temporal bone, * Fig. 224 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 fourth arches and clefts. B, anterior view of the head of a human foetus of about the fifth week (from Ecker, as before, fig. IV. ). i, 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 auditorius externus. 7 ^ote. of sympathetic, 573. of third nerve, 541, 573. relation of to optic nerve, 541. structure and function of, 645. Iron in hsematin of blood-cells, 84. parts of body in which found, 27. Irritability of muscular tissue, 588. Isolation of nerve-fibres, 479. Iter a tertio ad quartum ventricu- lum, 532. Ivory of teeth, 60. J. Jacob's membrane, 641. Jacobson's nerve, 554. Jejunum, 304. Jetting flow of blood in arteries, 148. Jumping, 602. K. Keratin, 24. Kidney, increased function of one, 413. Kidneys, their structure, 442. blood-vessels of, how distributed, 455- capillaries of, 171, 446. development of, 779. function of, 448. See Urine. Malpighian bodies of, 445. tubules of, 443. Knee, pain of, in diseased hip, 487, 501. Kreatin and Kreatinin, 25. in blood, 91. in urine, 462. Labia externa and interna, 718. Labyrinth of the ear, 676. membranous, 679. osseous, 676. function of, 690. Lacteals, 354. absorption by, 366. contain lymph in fasting, 361. origin of, 354. structure of, 357. in villi, 315, 366. Lactic acid in blood, 91. in gastric fluid, 282. Lactiferous ducts, 785. Lacunae of bone, 56, 57. Lamellae of bone, 57. Lamina spiralis, 678. use of, 692. Laminae dorsales, 744> 7^2. viscerales or ventrales, 746, 763. Language, how produced, 620. Large intestine. Sec Intestine. Laryngeal nerves, 559. INDEX. 807 Larynx, construction of, 607. influence of pneumogastric nerve on, 560-63. irritation referred to, 487. muscles of, 610. variations in according to sex and age, 6 1 6. ventricles of, 620. vocal cords of, 608, 611. Laws of functions of nerves, 490. Laxator tympani muscle, 675. Leaping, 602. Legumen identical with casein, 260. Lens, crystalline, 644. Lenticular ganglion, relation of third nerve to, 540. Lenticular glands of stomach, 278. of large intestine, 317. Leucocythsemia, state of vascular glands in, 419. Levator palpebrse superioris, nerve supplying, 540. Levers, different kinds of, 596. Lieberkiihn's glands, in large intestine, 317. in stomach, 306. Life, animal, 466. dependence of on medulla oblon- gata, 514. natural term of, for each particle, 386. organic, 466. relation of to other forces, 9-15. simplest manifestation of, i. Lightning, condition of blood after death by, 77. Lime, salts of, in human body, 27. phosphate of, in albumen, 22. in blood, 90. in bones and teeth, 27, 54. in tissues, 27. Lingual branch of fifth nerve, 548, 549, 55 6 > 70S- Lips, influence of fifth nerve on movements of, 546. Liquid part of food, absorption of, 288, 344. Liquor amnii, 749. Liquor sanguinis, 65, 69. lymph derived from, 368. still layer of in capillaries, 175. Lithium, absorption of salts of, 376. Liver, 321. action of on albuminous matters, 337- on saccharine matters, ib. a blood-making organ, 102. blood-vessels of, 322-5. capillaries of, 171. cells of, 321. circulation in, in. development of, 777. ducts of, 327. functions of, 328. in foetus, 332. secretion of, 328. Sec Bile, structure of, 321. sugar formed by, 338-42. Living bodies, properties of, I. tissues, contact with retards co- agulation, 76. Lobes of lungs, 201. Lobules of liver, 321. of lungs, 201. Locus niger, 522. Love, physical, cerebellum in rela- tion to, 529. Luminous circles produced by pres- sure on eyeball, 666 impressions, duration of, 660. Lungs, 197-204. capillaries of, 171. cells of, 202-4. changes of air in, 218. circulation in, in, 219. congestion of, in asphyxia, 239. after division of pneumogastric nerve, 562. contraction of, 209. coverings of, 198. development of, 778. elasticity of, 209. intercellular passages in, 202. lobes of, 201. lobules of, 20 r. movements of in respiration, 204- 10. nutrition of, 219. position of, 197. structure of, 197, 201. supplied by pneumogastric nerve, 558, 560. Lymph, analysis of, 363. compared with chyle, ib. with blood, 364. general characters of, 360. 8o8 INDEX. Lymph, continued. quantity formed, 365. relation to blood, 364, 368. Lymph- corpuscles, structure of, 361. in blood, 65. development into blood- corpus- cles, 103. Lymph-hearts, structure and action of, 368. , relation of to spinal cord, 369, 509. Lymphatic glands, structure of, 354. function of, 355. Lymphatic vessels, absorption by, 367. communication with blood- vessels, 360. contraction of, 358. course of fluid in, 352. distribution of, 354. origin of, 354. propulsion of lymph by, 357. structure of, 352, 357. valves of, 357. M. Macula germinativa, 721. Magnesium in human body, 28. Maintenance or assimilation, nature of the process. See Growth. nutritive, 379. of blood, 104. Male sexual functions, 732. voice, 615. Malleus, 675. function of, 687. Malpighian bodies, 443, 445. capsules, 444, 445, 446. Mammary glands, 784. Manganese,an accidental element,28. Margarin, 19. in bile, 329. Marginal fibro- cartilages, 53. Marrow of bone, 54. Mastication, 265. fifth nerve supplies muscles of, id c< Mastoid cells, 674. Matrix of cartilage, 50. of nails, 433. Meatus of ear, 672. urinarius, opening of in female, 718. Mechanical irritation, violent, effect on nerves, 478. Meconium, biliary principles in, 333. Medulla of bone, 54. of hair, 431. Medulla oblongata, 510-19. analogy to spinal cord, 513. columns of, 511. distribution of fibres of, 513. conduction of impressions in, 514. congested in asphyxia, 239. decussation of fibres in, 513, 514. development of, 772. effects of injury and disease of, 5I5- fibres of, how distributed, 512. functions of, 514. important to maintenance of life, ib. influence on deglutition, 517. on respiration, 237, 515, 519. on speech, 519. maintenance of power in, $19. as a nerve centre, 514. pyramids of, anterior, 512. posterior, 513. reflecting power of, 516. sensation and voluntary power not seated in, 518. structure of, 511. Medullary portion of kidney, 442. substance of lymphatic glands, 358. substance of nerve-fibre, 468. Membrana decidua, 756. granulosa, 719. development of into corpus lu- teum, 731. limitans, 641. propria. See Basement Membrane, pupillaris, 774. capsulo-pupillaris, ib. tympani, 674. office of, 685, 688. Membrane, blastodermic, 743. Jacob's, 641. ossification in, 58. primary or basement. See Base- ment Membrane, vitelline, 720, 741. Membranes, mucous. See Mucous membranes. Membranes, serous. See Serous membranes. Membranes,passagc of fluids through, 371. secreting, 400. INDEX. 809 Membranous labyrinth, 677, 679. Memory, relation to cerebral hemi- spheres, 534. Menstruation, 725. analogous with heat, 727. coincident with discharge of ova, 726. corpus luteum of, 731. phenomena of, 728. time of appearance and cessation, 727. Menstrual discharge, composition of, 66, 728. Mental derangement, 536. exertion, effect onheat of body, 243 . on phosphates in urine, 464. faculties, development of in pro- portion to brain, 534. theory of special localisation of, field of vision, 655. Mercury, absorption of, 377, 439. Mesenteric arteries, contraction of, IS 1 - veins, blood of, 96.. Meshes of capillary network, 171. Mesocephalon, 519. Metallic substances, absorption of by skin, 439. Mezzo-soprano voice, 616. Micturition, action of spinal cord in, 506. Milk, as food, 257. properties of, 786. secretion of, 785. Milk-globules, 786. Milk-teeth, 64. Mind, cerebral hemispheres the organs of, 534"537- influence on action of heart, 138. on animal heat, 251. on digestion, 290, 299. on hearing, 694. on movements of intestines, 35 1 . on nutrition, 391. on respiratory acts, 237, 299. on secretion, 414. on secretion of saliva, 268. in taste, 705. in touch, 708. in vision, 654-60. perception of special sensations by, 624. independently of organs, 626. Mind, continued. perception of transferred impres- sions, 487. power of concentration on the senses, 659. of exciting sensations, 713. reflex movements independent of, 502. sensitive impressions referred to parts by, 483. Mitral valve, 115. Mixed food, for man, 260. necessity of, 256. Modiolus, 677. Molecules, or granules, 31. in blood, 86. in milk, 786. movement of in cells, 36. Molecular base of chyle, 361. motion, 32. Monotonous voice, 615. Mortification from deficient blood, 389- Motion, causes and phenomena of,5 79 ciliary, 599. See Cilia, molecular, 32. muscular, 581. action on bones as levers, 596. nerves of, 477. of objects, how judged, 658. power of, not essentially distinc- tive of animals, 7. sensation of, 628. Motor impulses, transmission of in cord, 500. nerve-fibres, 477. nerves, cranial, 540. laws of action of, 484. roots of spinal nerves, 496, 567. Motor linguae nerve, 565. oculi, or third nerve, 540. Motorial end-plates, 473. Mouth, changes of food in, 265. moistened with saliva, 269. Movements of intestines, 349. of muscles, 596. habitual, 507, 523. reflex. See Keflex actions, sensation of, 711. symmetrical, 543. produced by irritation of auditory nerve, 698. of respiration, 206. of stomach, 294. 8io INDEX. Mucous layer of blastodermic mem- brane, 743. Mucous membranes, 403. basement membrane of, 404, 405. capillaries of, 171. component structures of, 404. epithelium - cells of, 405. See Epithelium. gland-cells of, 405. tracts of, 403. of intestines, 305, 316. of stomach, 274. of tongue, 699. of uterus, changes of in'pregnancy, 754- Mucus, nature of, 25. acid, of vagina, 66. in bile, 330. of mouth, mixed with saliva, 267. in urine, 461. Multipolar nerve-cells, 476. Muscles of animal life, 584. assisting erection, 195. assisting vomiting, 297.' changes in, by exercise, 380. contraction of, 589. eifect of pressure of, on veins, 181. expiratory, 209. heat developed in contraction of, 590. involuntary, 581. moving chest, 206. eyeball, 540-44. larynx, 610. nerves of, 588. nutrition of, 390. of organic life, 584. sensibility of, 589. 3d in contraction sound developec of, 591. source of action of, 603. striated, 584, 589. voluntary, 584. action of, 596. blood-vessels and nerves of, 587. work of, how estimated, 604. Muscular coat of arteries, 145. of large intestine, 316. of small intestine, 305. of stomach, 273. fibres, involuntary, 582. voluntary, 585. of stomach, action of, 294. of villi in intestines, 315. Muscular, continued. force, idea of, how derived, 711. motion, 581. movements. See Movements, sense, 589, 710. cerebellum the organ of, 529. strength tested by respiratory efforts, 214. tissue, of animal life, 584. in arteries, 145. contractility of, 588. contraction of, 589. heat developed in, 590. sound in, 591. effect of stimuli on, 478, 591. of heart, 587. involuntary, 581. irritability of, 588. duration of, after death, 593. of organic life, 582. properties of, 588. rigidity of, after death, 593. sensibility of, 589. striped, 584. structure of, 584-87. unstriped, 582-84. situations where found, 582. structure of, ib. in veins, 1 78. tone, 510. Muscularity, of arteries, 145. evidence of, 148. purposes of, 152. of lymphatics, 357. of lymph-hearts, 369. Musical sounds, 615. Myopia or short-sightedness, 652. K. Nabothi glandulse, 718. Nails, chemical composition of, 24. structure of, 433. growth of, ib. Narcotic poisons in stomach, experi- ments on, 300. Nasal cavities in relation to smell,634. Nates (brain), 521. Natural organic compounds, 17. classification of, 19. Nerve-centres, 467. See Cerebellum, Cerebrum, etc. conduction in or through, 486. INDEX. 811 Nerve-centres, continued. congestion of in asphyxia, 239. diffusion or radiation in, 488. functions of, 485. perception in, 486. reflexion in, 488. conditions of, 489. transference of impressions in, 48 7. Nerve- corpuscles, 475. caudate or stellate, 476. of retina, 642. simple, 476. Nerve-fibres, 467-85. axis-cylinder of, 468. cerebro-spinal, 467. conduction of impressions by, 479. of one kind only, 480. rate of, 480. continuity of, 471. course of, ib. difference in function not attended by difference of structure, 47 7. effects of injury and division, 481, 482, 484. fasciculi of, 471. force not generated by, 477. functions of, 476. effect of chemical stimuli on, 479- of mechanical irritation, 478, 481. of temperature, 478. impressions on, referred to peri- phery, 483. kinds of, 467. laws of action, 479. of motor nerves, 484. of sensitive nerves, 480. medullary or white substance of, 468. plexuses of, 472. of retina, 642. size of. 469. structure of, 467. sympathetic, 469. terminations of, 472. in cells, 473. in free ends, ib. in motorial end-plates, ib. in networks or plexuses, ib. in special terminal organs, ib. Nerves, action of stimuli on, 477'79- afferent, 477. Nerves, continued. centrifugal, 477. centripetal, ib. cerebral, physiology of, 539. Sec Cerebral Nerves, efferent, 477. of motion, or motor, 477. laws of action in, 484. respiratory, 237. of sensation, or sensitive, 477. laws of action in, 480. of special sense, 539. spinal, 495. See Spinal Nerves, stimuli of, 478. structure of, 467. sympathetic. See Sympathetic Nerve. ulnar, effect of compression of, 481. of division of, 484. vaso-motor, 153, 577. Nervi nervorum, 483. Nervous force, velocity of, 490. layer of retina, 642. Nervous substance, changes in from mental exertion, 381. fibrous, 467. phosphorus in urine from, 463. vesicular, 466, 475. Nervous system, 465. cerebro-spinal, 465, 490. development of, 744, 772. elementary structure of, 466. See Nerve-corpuscles and Nerve- fibres. influence of on animal heat, 250. on arteries, 153. on contractility, 588. on contraction of blood-vessels, 153, 578. on erection, 195. on gastric digestion, 298. on the heart's action, 138. on movements of intestines, 351. of stomach, 301. on nutrition, 390. on respiration, 236. on secretion, 413. on sphincter ani, 351. of organic life, 466, 568. sympathetic, 466. Nervus abducens sen ocularis ex- ternus, 542. patheticus sen trochlearis, 542. vagus, 557. See Pneumogastric. 8l2 INDEX. Networks, capillary, 171. See Capil- laries. Neuralgia, division of nerves for, 482. New-born animals, heat of, 253. Ninth cerebral nerve, 565. Nipple, an erectile organ, 194. structure of, 785. Nitrogen, in blood, 98. influence of in decomposition, 18. in relation to food, 261-63. in respiration, 226. Nitrogenous food, 256. in relation to muscular work, 604. to urea, 456. to uric acid, 458. principles, 20. Noise, how produced, 693. Noises in ears, 697. Nose. See Smell. irritation referred to, 487. restoration of, sensitive pheno- mena in, 483. Non-azotized or Non-nitrogenous food, 256. organic principles, 19. Non- vascular parts, nutrition of, 390. Nuclei, description of, 31-3. in developing and growing parts, 387- Nucleoli or nucleus-corpuscles, 32. Nutrition compared with secretion, 410. conditions necessary to, 388. examples of, 381. general nature of, 14, 379. influence of conditions of blood on, 388. of nervous system on, 390, 550. of state of part on, 394. of supply of blood on, 389. of sympathetic nerves on, 577. in paralysed parts, 391. in vascular and non-vascular parts, 390. Nutritive food, 255. repetition, 387. reproduction, ib. Nymphse, 718. Oblique muscles of the eye, action of, 544. Ocular cleft, 774. spectrum, 660. Odoriferous matters in blood, 91. Odour of blood, 66. Odours, causes of, 631. different kinds of, 635. perception of, 631. varies in different classes, 635. relation to taste, 706. (Esophagus, action of in deglutition, 273- reflex movements of, 502. Oil, absorption of, 378. Oily matter, 19. coated with albumen, 361. Oleaginous principles, digestion of, 293- Olein, 19. in bile, 329. Olfactory cells, 633. lobes, functions of, 523. nerve, 632. subjective sensations of, 636. Olivary body, 512. fasciculus, 513. Ophthalmic ganglion, relation of third nerve to, 540. Optic lobes, corpora quadrigemina, homologues of, 522. functions of, 523. nerve, decussation of, 669. fibres of, 477. point of entrance insensible to sight, 664. thalamus, function of, 523. vesicle, primary, 772. secondary, 774. Optical angle, 656. Ora serrata of retina, 639. Oral canal and oral opening, 621. Organic and inorganic bodies, 3. compounds, instability of, 18. peculiarities of some, 17. life, its phenomena, i. muscles of, 584. nervous influences regulating, 578. nervous system of, 466, 568. processes, influence of sympathetic nerve upon, 575. Organisation, definition of, 3. Organs, plurality of cerebral, 535. Organs of sense, development of, 772. INDEX. 813 Os orbieulare, 675. Os uteri, 718. Osmosis, 371. Osseous labyrinth, 677. Ossicles of the ear, 675. office of, 686. Ossification, 58. Ossicula auditus, 674. Otoconia or Otolithes, 680. use of, 691. Ovaries, 716. enlargement of at puberty, 724. Graafian vesicles in, 719. Ovisacs, 719. Ovula Nabothi, 718. Ovum, 720. action of seminal fluid on, 739. changes of in ovary, 723. previous to formation of em- bryo, 740. subsequent to cleaving, 742. in uterus, 743. cleaving of yelk, 741. connexion of with uterus, 752. discharge of from ovary, 724-28. formation of, 722. germinal membrane of, 743. germinal vesicle and spot of, 721. impregnation of, 732. nature of the, n. structure of, 720. Oviduct, or Fallopian tube, 715, 716. Oxalic acid in urine, 465. Oxygen, in blood, 98, 230. consumed in breathing, 221, 225. effects of on colour of blood, 94. on pulmonary circulation, 177. proportion of to carbonic acid, 225. union with carbon and hydrogen, producing heat, 246. P. Pacinian bodies or corpuscles, 473. Pain excited by the mind, 713. in paralysed parts, 482. Palate in relation to deglutition, 271, 272. nerves of, 559. Palate and uvula in relation to voice, 619. Palmitin, 19. Pancreas, 318. Pancreas, continued. development of, 776. functions of, 319. Pancreatic fluid, 319. Pancreatin, 318. Papillae, of the kidney, 443. of skin, distribution of, 423. end-bulbs in, 427. epithelium of, 428. nerve-fibres in, 424. supply of blood to, 424. touch-corpuscles in, 426. of teeth, 63. of tongue, 699, 701. circumvallate or calyciform, 702. conical or filiform, ib. fungiform, ib. use of, 705. Par vagum, 557. See Pneumogas- tric nerve. Paralysed parts, pain in, 482. nutrition of, 391. limbs, temperature of, 250. preservation of sensibility in, 484 Paralysis, cross, 514. seat of, according to part of cere- bro-spinal axis injured, 500. Paraplegia, delivery in, 507. influence of spinal cord shown in, 498. reflex movements in, 503. state of intestines in, 351. Parotid gland, saliva from, 267. Particles, changes of in nutrition, 379. duration of life in each, 386. . natural decay and death, 381. process of forming new, 387. removal when impaired or effete, 380. Parturition, mechanism of, 234. Patheticus, or fourth nerve, 542. Pause in heart's action, 129, 130. respiratory, 210. Peduncles, of the cerebellum, 526. of the cerebrum, 532. Pelvis of the kidney, 443. Penis, corpus cavernosum of, 194. 8i 4 INDEX. Penis, continued. development of, 784. erection of, explained, 195. reflex action in, 196, 507. Pepsin, 282. Peptone, 292. Perception of sensations by cerebral hemispheres, 486, 534. Perichondrium, 51. Perilymph, or fluid of labyrinth of ear, 679. use of, 690. Periosteum, 55. Peristaltic movements of intestines, 350. of stomach, 295. Permanent cartilage, 50. glands, 408. teeth, 64. Perspiration, cutaneous, 435. insensible and sensible, 436. ordinary constituents of, ib. Peyer's glands, 307. functions of, 310. patches, 308. resemblance to vascular glands, 311, 416. structure of, 309. Pharynx, action of in swallowing, 272, 502, 560. influence of glosso-pharyngeal nerve on, 554. of pneumogastric nerve on, 559, 56o. Phosphates in tissues, 26, 27. present in albumen, 22. in blood, 90, 91. in urine, 463. Phosphorus in human bocty, 26. union of with oxygen producing heat, 246, note. in urine, source of, 463. Phosphuretted fat in blood-corpus- cles, 85, 90. Phrenology, 535-37. Physical forces, relation of life to, 9-15- Physiology, definition of, I. Pia mater, circulation in, 192. Pigment, 48. of choroid coat of eye, 638. composition of, 50. of hair, 382, 431. Pigment, continued. of skin, 422. uses of, 50. Pigment -cells, form of, 35, 49. Pineal gland, 539. Pinna of ear, 672. " Pins and needles," sensation of, 481. Pitch of voice, 619. Pith of hair, 431. Pituitary gland, 539. Placenta, 750. formation of, 754. fretal and maternal, 759. relation of to the liver, 333. structures composing, 759. Plants, distinctions from animals, 6. heat evolved by, 249. Plastic food, 255. Plexuses of nerves, 472. terminal, ib. conduction through, 585. of spinal nerves, relation to cord, 494. Pneumogastric nerve, 557. distribution of, 558. mixed function of, ib. influence on action of heart, 142. on deglutition, 273. on digestion, 300. on functions of larynx, 559, 560. of oesophagus, 560. of lungs, ib. of pharynx, 559, 560. on movements of stomach, 301. on respiration, 515, 561. on secretion of gastric fluid, 300, 301. on sensation of hunger, 298. of thirst, 299. origin of from medulla oblongata, 5 J 5- Poisoned wounds, absorption from, 378. Poisons, absorption of from intes- tines, 376. narcotic, introduced in stomach, 300. Polarity of muscles, 596, note. Potygamous birds, their cerebella, 530- Pons Varolii, its structure, 519. INDEX. Portal blood, characters of, 96. canals, 322. circulation, 112. function of spleen with, regard to, 420. veins, arrangement of, 322. Portio dura, of seventh nerve, 551. mollis, of seventh nerve, 680. Post mortem rigidity. See Eigor Mortis. Posture, effects of on the heart's action, 135. Potassium, salts of, in fluids and tissues, 27. Pregnancy, absence of menstruation during, 728. corpus luteum of, 732. influence on blood, 92. Presbyopia, or long-sightedness, 653. Pressure on eye, effects of, 666. Primary membrane, 400, 405. Primitive, dental groove, 63. fasciculi and fibrils of muscle, 584-86. groove in embryo, 744. Principles, nitrogenous, 20. non-nitrogenous, 19. Process, vermiform, 525. Processus gracilis, 675. a cerebello ad testes, 532. Prostate gland, 732. functions of secretion unknown, 738. Protein-compounds, 21. Ptyalin, action of, 270. Puberty, changes at period of, 724. indicated by menstruation, 727. Pudenda, 718. Pulmonary artery, valves of, 118, 129. capillaries, 203. circulation, in, 219. influence of carbonic acid 011,239. of pneumogastric nerve, on, 562. velocity of, 190. Pulp of hair, 383. of teeth, 60. Pulse, arterial, 155. cause of, ib. dicrotous, 163. difference of time in, 157. explanation of, 160. Pulse, arterial, continued. frequency of, 134. influence of age in, ib. of food, posture, etc., 135. observations on with sphygmo- graph, 157-159. relation of to respiration, 136. tracings of, 159, 162. in large arteries, 162. in radial artery, 163. variations in, 134-36. in capillaries, 172, 173. Pupil of eye, office of, 645. relation of third nerve to, 541. Purgative action of bile, 336. Pus, contains albumen, 21. Putrefaction. See Decomposition. arrested by gastric fluid, 285. Pylorus, structure of, 274. action of, 295, 297. Pyramidal portion of kidney, 442. Pyramids of medulla oblongata, 512, SIS- Q. Quadrupeds, retinse of, 668. E. Eadiation of impressions, 488, 501. Eectum, 316. evacuation of, a reflex act, 506. mechanism of, 234. Eeflexion of impressions, 488. by medulla oblongata, 516. by spinal cord, 501. Eeflex actions, 488, 501. in accidents, 507. conditions necessary to, 489. in disease, 508. examples of, 502, 506. excito-motor and sensori-motor, 505, note. general rules of, 489. independent of mind, 489, 502-6. influence of cord on, 501, 509. irregular in disease, 489. after separation of cord from brain, 502, 506. purposive in health, 489. relation of fifth nerve to, 547. relation to volition, 506. to walking, running, etc., 507. sustained, 490. in tetanus, &c., 508. 3x6 INDEX. Reflex functions of medulla ob- longata, 516. of spinal cord, 501. Refraction, laws of, 643. Refracting media of eye, 643. Renal arteries, arrangement of, 445. veins, blood of, 97. Repair. See Nutrition. retarded in paralysed parts, 39 1 . Repetition, nutritive, 387. Reproduction, nutritive, ib. Reserve air, 212. Residual air, ib. Respiration, 197. abdominal type of, 207. ammonia and other products ex- haled by, 229. carbonic acid increased by, 221. changes of air in, 219, 220. of blood in, 229. costal types of, 209. force of, 215-18. frequency of, 213. influence of brain on, 505. of medulla oblongata, 236-38, 5H-I7. of pneumogastric nerve, 515, 561. mechanism of, 205. movements of, 206-210. See Re- spiratory Movements. of air in, 218. of blood in, 219. nitrogen in relation to, 226. oxygen diminished by, 225. quantity of air changed in, 211. relation of to the pulse, 136. structure of organs of, 197-204. suspension and arrest of, 238. temperature of air increased by, 221. types of, 207, watery vapour exhaled in, 227. Respiratory capacity of chest, 212. food, 255. function of skin, 438. movements, 206-10. of air- tubes, 217. centre of, the medulla oblongata, SIS- effect of on circulation, 184. excited through nerves, 516. by various stimuli, 517. of expiration, 209. Respiratory movements, continued. of glottis, 210. influence on amount of carbonic acid, 222. of inspiration, 206. relation to will, 505. various, mechanism of, 231. muscles, 206, 209. power of, 214. secondary, 237. nerves, 237. rhythm, 210. tract of mucous membrane, 404. Rest, favourable to coagulation, 75. Restiform bodies, 512, 513. Retching, explanation of, 297. Rete mucosum, 422. testis, 733. Retina, 639. duration of impressions on, 660. of after-sensations, 660. effect of pressure on, 666. focal distance of, 650. function of, 642. image on, how formed distinctly, 648-50. inversion of, how corrected, 654. insensible at entrance of optic nerve, 664. insufficient alone for distinct vision, 643. in quadrupeds, 668. reciprocal action of parts of, 662. in relation to direction of vision, 657. to motion of bodies, 658. to single vision, 665-72. to size of field of vision, 655-57. structure of, 639. Rhythm of Heart, cause of, 138. See Heart. respiratory, 210. Rigor mortis, 593. affects all classes of muscles, 595. phenomena and causes of, 594. Rima glottidis, movements of in re- spiration, 211. Rods of Corti, 679. use of, 693. Root of nail, 433. Root-sheath of hair, 433. Roots of spinal nerves, 493, 495. anterior and posterior, special pro- perties of, 496. INDEX. 817 Rotation, folloAving injury of crura cerebelli, 531- produced by dividing the crura cerebri, 522. Rouleaux, formation of in blood, 71, 82. Rubbing, influence on cutaneous absorption, 439. Rugae or folds of stomach, 274. Rumination, 298. Running, mechanism of, 603. Rut or heat, 725. S. Saccharine principles of food, diges- tion of, 293. action of bile on, 337. absorption of, 344. Sacculus, 679. Safety-valve action of tricuspid valve, 125. Saline constituents of bile, 330. Saline constituents of blood, 90. use of, 1 08. of urine, 462. matters, absorption of, 375. Saliva, action of on food, 269. on starch, 270. composition of, 268. digestive properties of, 270. mechanical purposes of, 269. organs for production of, 264. physical properties of, 267. purposes of, 269. quantity secreted, 268. rate of secretion, ib. reaction of, 267. relation to gastric fluid, 271. Salivary glands, 264. development of, 776. Salts, alkaline and earthy, influence on coagulation, 76. Sarcode, 29. Sarcolemma, 585. Sarcous elements, 586. Scala media, 679. tympani, 678. vestibuli, ib. Sclerotic, 638, 646. Scurvy from want of vegetables, 263. Season, influence on carbonic acid expired, 223. Sebaceous glands, 430. their secretion, 434. Secreting glands, 406. aggregated, 408. convoluted tubular, 408. tubular or simple, 406. Secreting membranes, 400. See Mucous and Serous Mem- branes. Secretion, 399. action of cells and nuclei in, 409. apparatus necessary for, 400. circumstances influencing, 412. discharge of, 411. general nature of, 399. influence of nervous system on, 4 1 3 . of sympathetic nerve, 577. of quantity of blood, 412. process of, 399. relation or antagonism of, 415. resemblance to nutrition, 410. by membranes, 400. mucous, 405. serous, 400. synovial, 403. in vascular glands, 415. Selection of materials for absorption 370- Semicircular canals of ear, 677. development of, 775. use of, 690. Semilunar valves, 118. action of, 125. Seminal fluid, 735. composition of, 739. corpuscles and granules of, 735. emission of, a reflex act, 506. influence on ovum and embryo, 739- filaments, 735. purpose of, 736. tubes, 733. vesicles, 736. Sensation attended by ideas, 712. cerebral nerves of, 539. common, 624. conditions necessary to, 712. conduction of in spinal cord, 498. contrasts in, 713. definition of, 623. excited by mind, 713. by internal causes, 626. of hunger, 298. influence of attention on, 630, 659. 3 G 8i8 INDEX. Sensation, continued. influence of mind necessary to, 712. of motion, how perceived, 628. muscular, 589, 710-12. of necessity of breathing, 299. nerves of, 477. convey impressions to centres only, 480. impressions on referred to peri- phery, 481-3. laws of action of, 490. perceived in cerebrum, 534. preservation of in paralysed nerves, 483. referred to exterior, 483. special, 624. stimuli of, 477-79. of special, 625-27. in stumps, 482. subjective, 713. of thirst, 298. sympathetic, 488. touch a modification of, 707. transference and radiation of, 488, 501. two kinds of, 624. of volatile bodies, 629. of weight, 711. Sense, of hearing, 672. See Hearing, Sound. of sight, 631. (See Vision, of smell, ib. See Smell, of taste, 698. See Taste. of touch, 707. See Touch, muscular, 529, 710-12. special, nerves of, 539. organs of, development of, 772. Senses, special, general properties of, 623. action of external and internal stimuli on, 626. impairment of from division of the facial nerve, 552. from division of the fifth nerve, 548. influence of attention on, 630. of internalimpressions on nerves of, 626. qualities of external matter per- ceived by, 624, 628. special nerves of, 624. stimulus excites in each nerve its own sensation, 625. Sensitive impressions, conduction of, 480. by spinal cord, 497, 498. reference of, 481-83. nerves, 477. Sensory ganglia, 523. Septum between auricles, formation of, 769. between ventricles, formation of, 768. Serolin, 90. Serosity of blood, 88. Serous layer of blastodermic mem- brane, 743, 746. Serous membranes, 400. arrangement of, 401. epithelium of, 400. fluid secreted by, 402. lining joints, etc., 401. visceral cavities, ib. purpose of, 401. structure of, 400. Serum, of blood, 87. chief source of albumen, 21. separation of, 69, 87. Seventh cerebral nerve, auditory portion, 680, 693. facial portion, 551. Sex, influence on blood, 92. influence on production of car- bonic acid, 222. relation of to capacity of chest, 213. to respiratory movements, 207. Sexual organs and functions in the female, 715-32. in the male, 732-40. Sexual passion, connection of with cerebellum, 529. Sighing, mechanism of, 232. Sight. See Vision. Silica, parts in which found, 26. Singing, mechanism of, 235, 615. Single vision, conditions of, 665. Sinus terminalis, 752. urogenitalis, 781. Sinuses of dura mater, 192. Sixth cerebral nerve, 542. Size of field of vision, 655-58. Skin, 421. absorption by, 439. of gases, 441. of metallic substances, 440. of water, ib. INDEX. 819 Skin, continued. capillaries of, 171. cutis vera of, 425. epidermis of, 424. evaporation from, 440. excretion by, 434-39- exhalation of carbonic acid from, 438. of watery vapour from, 437. functions of, 425. respiratoiy, 438. papillse of, 425-28. perspiration of, 435. rete mucosum of, 422. sebaceous glands of, 430. structure of, 421. sudoriparous glands of, 428. Sleep, influence of on production of carbonic acid, 224. in relation to heat of body, 243. Smell, sense of, 631. conditions of, 631. different kinds of odours, 635. impaired by lesion of facial nerve, 552. impaired by lesion of fifth nerve, 548. internal excitants of, 636. limited to olfactory region, 634. relation to common sensibility, 634- structure of organ of, 632. subjective sensations of, 636. varies in different animals, 635. Sneezing, caused by sun's light, 488. mechanism of, 233. Sniffing, mechanism of, 235. smell aided by, 632. Soda, salts of in blood, 90. in solids and fluids, 27. Sodium in human body, 27. chloride of, in albumen, 22. Solid food, action of gastric fluid on, 288. Solitary glands, 308. Sonorous vibrations, how communi- cated in ear, 684, e. s. in air and water, 684. See Sound. Soprano voice, 615. Sound, conduction of by ear, 680. by external ear, 681-84. by internal ear, 690-93. by middle ear, 684-90. Sound, continued. movements and sensations pro- duced by, 698. perception of, 693. of direction of, 694. of distance of, 695. a state of the auditory nerve, 696. permanence of sensation of, 695. produced by contraction of muscle, 591. production of, 693. subjective, 697. Sounds as expressions of passion, 615. classified, 615. of heart, 129. causes of, 130. Sources of nervous force, 485. Spasms, reflex acts, 508. Speaking, 615. mechanism of, 235. Special sense. See Senses. Spectrum-analysis of blood, 94. Spectrum, ocular, 660-62. Speech, 620. function of tongue in, 623. influence of medulla oblongata on, SW' Spermatozoids, development of, 735. form and structure of, ib. function of, 736. motion of, 735. Spherical aberration, how corrected in the eye, 649. Spheroidal epithelium, 39. Sphincter ani, external, 351. internal, 316, 350. influence of spinal cord on, 504, 509- Sphygmograph, 157. Spinal accessory nerve, 563. See Accessory nerve. Spinal cord, 490. canal of, 493. a collection of nervous centres, 509. columns of, 493. commissure of, ib. conduction of impressions by, 497- 501. course of fibres in, 493. decussation of sensitive impres- sions in, 500. 3 o 2 820 INDEX. Spinal cord, effect of injuries of, on conduction of impressions, 498. on nutrition, 391-93. enlargement of parts of, 494. fissures and furrows of, 493. functions of, 497-510. of columns, 498. influence of on heart's action, 138. on lymph-hearts, 369, 509. on sphincter ani, 502, 509. on tone, 510. morbid irritability of, 508. nerves of, 495-97. reflex function of, 501. See Re- flex Action, size of parts of, 494. structure of, 492. transference and radiation in, 487, 501. Spinal nerves, origin of, 495. physiology of, 567. Spiral canal of cochlea, 677. lamina of cochlea, 678. function of, 692. Spleen, as a blood-forming organ, 419. in relation to digestion, ib. to portal circulation, 420. Splenic vein, blood of, 96. Spontaneous decomposition, 18. Spot, germinal, 721. Squamous epithelium, 39. Stapedius muscle, 675. function of, 689. Stapes, 675, 677. Starch, action of cooking on, 293. of pancreatic secretion, 319. of saliva, 270. of various substances, ib. - animal, 339. digestion of in small intestine, 344- in stomach, 293. Statical pressure of blood, 164. Stature, relation to capacity of chest, 212. Stearin, 19. Stellate nerve-corpuscles, 476. Stercorin, 347. Stereoscope, 672. Still layer in capillaries, 175. Stimuli, as excitants of contractility, 591-93- of sensation, 477, 479. of special senses, 625-27. St. Martin, Alexis, case of, 238, 248. Stomach, blood-vessels of, 278. development of, 776. digestion in, 287-93. influence of nervous system on, 298. digestion of after death, 30. examined through fistula?, 279, 288. glands of, 275. lenticular, 278. tubular, 275. movements of, 294. influence of nervous system on, . 30i. . . in vomiting, 296. mucous membrane of, 274. muscular coat of, 273. passage of substances from to urine, 449. presence of not absolutely distinc- tive of animals, 8. in relation to hunger, 298. secretion of, 279. See Gastric fluid. structure of, 273. temperature of, 280. Striped muscular fibre, 584-88. Stroma of ovary, 716. Structural changes of food in sto- mach, 291. Structural composition of human body, 28. m Stumps, sensations in, 482. Subjective sensations, 713. of sound, 697. of taste, 707. Sublobular veins, 324. Sucking, mechanism of, 235. Sudoriparous glands, 428. their distribution, 429. number of, ib. their secretion, 435. Suet, or animal fat, 19. Suffocation, 238-42. Sugar, digestion of, 293, 337. as food, experiments with, 257. formation of in liver, 337-342. Sulphates in urine, 462. INDEX. 821 Sulphur in bile, 331. in human body, 26. union of with oxygen producing heat, 226, 247, note. in urine, 462. Suprarenal capsules, 416. development of, 779. disease of, relation to discolora- tion of skin, 418, note. Swallowing, 271. an example of reflex acts, 779. Sweat, 435. Sympathetic nerve, 567. character of movements executed through, 576. communication of with fifth nerve, 550, 568. with glosso-pharyngeal nerve, 568. with pneuinogastrie nerve, 559, 568. with sixth nerve, 542. with spinal nerves, 569. conduction of impressions by, 5 72. divisions of, 568. fibres of, course of, 570. differences of from cerebro- spinal fibres, 470, 568. mixture with cerebro-spinal fibres, 571. relation to cerebro-spinal sys- tem, 578. ganglia of, 568. action of, 575. co-ordination of movements by, 576. in substance of organs, 576. influence of on blood-vessels, 153, 578. on heart's action, 139. on involuntary motion, 575. on nutrition, 393, 577. on secretion, 414, 577. physiology of, 572-79. structure of, 567-72. Synovial fluid, secretion of, 403. membranes, 401. Syntonin, 24. Systemic circulation, in. &'&; Cir- culation, vessels, 113. T. Tact, 624. See Touch. Taimic acid, test for gelatin, 21. Tanno-gelatin, ib. Taste, 698. conditions for perception of, 698. connection with smell, 706. impaired by injury of facial nerve, offiftl th nerve, 548. nerves on which the sense depends, 556, 704, 705. permanence of impressions, 706. seat of, 699. subjective sensations, 707. variations of, 705. Taurin, sulphur combined with, 331, note. Taurocholic acid, 329. Teeth, 59. development and casting of, 63, 384. parts of, 59. structure of, 60. temporary and permanent, 64. Temperament, influence on blood, 92. Temperature, average of body, 242. changes of, effects of, 245. circumstances modifying, 243. of cold-blooded and warm-blooded* animals, 244. in diseases, 243. increased, power of supporting, 252. influence on amount of carbonic acid produced, 223. on nerves, 478. maintenance of, 245. of Mammalia, birds, etc., 244. modified by age, etc., 253. of paralysed parts, 250. relation of to combustion of car- bon and hydrogen, 246-48. of respired air, 221. sensation of variations of, 713. variations of, in sleep, etc., 243. See Heat. Temporary cartilage, 50, 52. teeth, 64. Tendinous cords, 117. Tendons, structure of, 46. Tenor voice, 615. Tensor tyrnpani muscle, 675. office of, 689. Tesselated epithelium, 39, 42. 822 INDEX. Testicle, 732. development of, 779- structure of, 732. Tetanus, reflex movements in, 508. Thalami optici, function'of, 523. Third cerebral nerve, 540. Thirst, allayed by cutaneous absorption, 441. cause of, 93. sensation of, 298. Thoracic duct, 353. its contents, 365. Thorax, 109. Thymus gland, 416. function of, 418. Thyro-arytenoid muscles, 610. Thyroid gland, 416, 417. function of, 418. Thyroid cartilage, structure and connections of, 608, 609. Timbre of voice, 616. Tissue, adipose, 47. areolar, cellular, or connective, 44. fatty, 47. muscular, 581. Tissues, absorption of, 368. elementary, structure of, 38. decay and removal of, 379-81. erectile, 194. gelatin from, 20. mutually excretory, 105. nitrogenous, urea derived from, 457- nutrition of. See Nutrition, relation to blood, 177. vascular and non-vascular, 390. Tone of blood-vessels, 153. of muscles, 510. of voice, 6 1 6. Tpngue, 699. action of in deglutition, 271. in sucking, 235. epithelium of, 703. function of in speech, 623. influence of facial nerve on muscles of, 553. motor nerve of, 565. an organ of touch, 704. papillae of, 699, 701. parts most sensitive to taste, 557, 704. structure of, 699. Tooth-ache, radiation of sensation in, 488. Tooth-fang, 64. absorption of, 384. Tooth-pulp, 60, 64. Touch, 707. after- sensation of, 712. characters of external bodies as- certained by, 708. conditions for perfection of, 709. connexion of with muscular sense, 710. co-operation of mind with, 712. function of cuticle with regard to, 428. of papillae of skin with regard to, 426. the hand an organ of, 709. modifications of, 708. a modification of common sensa- tion, 624, 707. special organs of, 708. subjective sensations of, 713. the tongue an organ of, 704. Touch-corpuscles, 426, 473. Trachea, 109. structure of, 199. Tracts of medulla oblongata, 511. of mucous membrane, 403. of spinal cord, 493. Tradescantia Virginica, movements in cells of, 29. Tragus, 672. Transference of impressions, 487,501 . Transplanted' skin, sensation in, 483. Tricuspid valve, 115. safety-valve action of, 125. Trigeminal or fifth nerve, 544. effects of injury of, 392, 548. Trochlearis nerve, 542. Tube, Eustachian, 689. Tubes, Fallopian, 716. See Fallopian tubes. looped, of Henle, 444. Tubular glands, 406. convoluted, 408. simple, 406. of intestines, 317. of stomach, 275. Tubules, general structure of, 38. Tubuli seminiferi, 733. uriniferi, 443. Tunica albuginea of testicle, 732. Tympanum or middle ear, 674. INDEX. 823 Tympanum, continued. development of, 775. functions of, 684. membrane of, 635. structure of, ib. use of air in, 686. Types of respiration, 207, 209. U. Ulceration of parts attending injuries of nerves, 392, 548, 549. Ulnar nerve, effects of compression of, 481. of division of, 484. Umbilical arteries, contraction of, 150. vesicle, 746. Understanding, relation of to cere- brum, 534. Unstriped muscular fibre, 582. Urachus, 750. Urate of ammonia, 459. of soda, 458, 459. Urea, 454. in blood, 91, 457. chemical composition of, 455. identical with cyanate of ammonia, 455- properties of, 455. quantity of, 456. in relation to muscular exertion, 457, 604. sources of, 456. Ureter, 443. arrangement of, 449. Urethra, development of, 784. Uric acid, 459. in blood, 91. condition in which it exists in urine, 459. in expired air, 229. forms in which it is deposited, 460. proportionate quantity of, 458. source of, 459. Urina sanguinis, potus, et cibi, 451. Urinary bladder, action of, 449. development of, 781. evacuation of, a reflex act, 506. hypertrophy of, 397. regurgitationfrom prevented, 649. Urine, 448-65. analyses of, 453. animal extractive in, 461. Urine, continued. chemical composition of, 452. chlorine in, 464. colour of, 450. colouring matter of, 461. cystin in, 464. decomposition of by mucus, 461. expulsion of, 450. flow of into bladder, 449. gases in, 465. general properties of, 450. hippuric acid in, 460. kreatin and kreatinin in, 462. mucus in, 461. oxalic acid in, 464. phosphorus in, 463. quantity secreted, 452. reaction of, 450. made alkaline by diet, 451. saline matters in, 462. secretion of, 448. effects of posture, etc., on, 449. rate of, ib. specific gravity of, 451. sulphur in, 462. urea in, 454. uric acid in, 458. variations of, 451. of water in, 454. Urohsematin, 461. Uterus, 717. changes of mucous membrane of, 754- contractions of its arteries, 150. development of in pregnancy, 397. follicular glands of, 754. reflex action of, 507. simple and compound glands of, 755- structure of, 717. Utriculus of labyrinth, 679. Uvula in relation to voice, 619. y. Vagina, structure of, 718. Vaginal veins of liver, 323. Vagus nerve. See Pneumogastric. Valve, ileo-csecal, structure of, 317. of Vieussens, 532. Valves of heart, 114. action of, 122-29. bicuspid or mitral, 115. semilunar, 118. tricuspid, 115. 824 INDEX. Valves, continued. of lymphatic vessels, 357. of veins, 178. Valvulse conniventes, 305. Vas deferens, 732. Vasa efferentia, 447, 733. recta, 447, 733. vasorum, 144. Vascular area, 751. Vascular glands, 415. analogous to secreting glands, 416. in relation to blood, 417. several offices of, 418-21. Vascular parts, nutrition of, 390. system, development of, 765. Vaso-motor nerves, 153, 577. Vascularity, degrees of, 171. Vegetable life, its phenomena, I. Vegetable matters, absorption of, 439. Vegetable substances, digestion of, 291. Vegetables and animals, distinctions between, 6. Veins, no, 178. absorption by, 370. anastomoses of, 182. circulation in, 180. coats of, 178. of cranium, 192. effects of muscular pressure on,i 81. of respiration on, 1 84. in erectile tissues, 194. force of heart's action remaining in, 1 80. influence of gravitation in, ib. muscular tissue in large, 183. rhythmical action in, ib. structure of, 178. systemic, 113. valves of, 178. velocity of blood in, 186. Velocity of blood in arteries, 167. in capillaries, 174. in veins, 186. of circulation, 187. of nervous force, 480. Vena portse, its arrangement, in, 322. Venous blood, characters of, 93-8. Ventilation, necessity of, 224. Ventricle, fourth, of brain, 512, 532. Ventricles of heart, 112. capacity of, 137. contraction of, 120. Ventricles of heart, continued. contraction of, effect on arteries, 155, 160. oh circulation, 142. on veins, 181. force of, 136. development of, 767. dilatation of, 122. of larynx, office of, 620. Ventriloquism, 695. mechanism of, ib. Vermicular movement of intestines, 3.50. Vermiform process, 525. Vertebra, development of, 745, 760. Vesicle, germinal, 721. Graafian, 716, 719. bursting of, 723-26. umbilical, 746. Vesicles df vascular glands, 416. Vesicula germinativa, 721. Vesiculse seminales, 736. functions of, 737. reflex movements of, 507. Vesicular nervous substance, 475. Vestibule of the ear, 677. of vagina, 718. Vibrations, conveyance of to auditory nerve, 684. perception of, 628. of vocal cords, 618. Vidian nerve, 551. Villi of intestines, 312. action in digestion, 343. on intestinal glands, 308. Villi of chorion, 752. in placenta, 757. Visceral arches, development of, 762. cavities, serous membranes of, 401 . laminae, 746, 762. layer of pleura, 199. Vision, 637. angle of, 656. at different distances, adaptation of eye to, 650-52. contrasted with touch, 657. corpora quadrigemina the princi- pal nerve-centres of, 522. correction of aberration, 649, 650. of inversion, 654. direction of, 657. direction of rays in, howregulated, 653. distinctness of, how secured, 648. INDEX. 825 Vision, continued. double, 667. duration of sensation in, 660. estimation of the form of objects, 657. of their motion, 658. of their size, 655. field of, size of, ib. focal distance of, 650. impaired by lesion of fifth nerve, 547, 548. influence of attention on, 659. modified by different parts of the retina, 663. organ of, 637. See Eye. phenomena of, 646. in quadrupeds, 668. single, with two eyes, 665. its cause, 668-72. structures essential for, 646. Visual direction, 657. Vital capacity of chest, 212. motions, 579. Vitality. See Life. dormant, 4, 12. Vitelline duct, 746. membrane, 720, 741. spheres, 741. Vitellus, or yelk, 721. See Yelk. Vitreous humour, 645. Vocal cords, action of in respiration, 210, 233. approximation of, effect on height of note, 613. attachment of, 609. elastic tissue in, 46. longer in males than infemales,6i6 position of, how modified, 611. vibrations of, cause voice, 605, 619. Voice, 605. of boys, 6 1 6. compass of, 615. conditions on which strength de- pends, 619. human, produced by vibration of vocal cords, 605, 619. impaired by destruction of ac- cessory nerve, 565. in eunuchs, 617. influence of age on, 616. of arches of palate and uvula, 619. of epiglottis, 613. of sex, 615. Voice, continued. influence of ventricles of larynx, 620. of vocal cords, 605, 6n. in male and female, 615. cause of different pitch, 6 1 6. modulations of, 618. natural and falsetto, ib. peculiar characters of, 617. varieties of, 615. Volatile bodies, influence of on sense of smell, 634. Voluntary muscles. See Muscles. Vomiting, action of stomach in, 296. mechanism of, 233. influence of spinal cord in, 506. voluntary and acquired, 297. Vowels and consonants, 620. Vulvo- vaginal glands, 719. W. Walking,muscularactionin,599,6o2 Warm-blooded animals, 244. Water, absorbed by skin, 439. by stomach, 288. in blood, variations in, 88. conduction of sound through, 684. deficient in thirst, 93. exhaled from lungs, 227, 437. from skin, 436. forms large part of human body, 26. influence of on coagulation of blood, 75, 76. on decomposition, 18. in urine, excretion of, 448. variations in, 454. vapour of in atmosphere, 220. Wave of blood in the pulse, 160. Weight, relation to capacity of chest, 213. sensation of, 711. White corpuscles. See Blood-cor- puscles, white ; and Lymph-cor- puscles. fibro-cartilage, 50, 53. White substance of nerve-fibre, 468. Will, reflex actions amenable to, 506-8. transmission of through cord, 500. Willis, circle of, 191. Wolffian bodies, 779. 826 INDEX. Y. Yelk-sac, 748. Yelk, or vitellus, 721. Yellow elastic fibre, 45. changes of, in Fallopian tube, 741. fibro- cartilage, 50, 52. in uterus, 743. spot of Sb'mniering, 640, 642. cleaving of, 741. constriction of by ventral lamina, Z. 746. Zona pellucida, 720, 741. LIST OF WORKS REFERRED TO. PAGE 23. SMEE. Proc. R. Soc. 1863. 27. PREPONDERANCE OF MAGNESIA OVER LIME IN JUICE-OF MUSCLES. Liebig. Chemistry of Food. 1847. 66, Jth line from t. BERNARD. Lemons de Pbys. Experimentale. Paris, 1859. SAVORY. On the Relative Temperature of Arterial and Yenous Blood. Pamphlet. ,, i^th line from b. BERNARD. The Medical Times and Gazette. April 21, 1860. ,, BARRUEL. Annales d'Hygiene Publique et de Medecine Ldgale. 68. VALENTIN. Repert. f. Anat. und Phys. Bd. iii., p. 281. ,, BLAKE. Prof. Dunglison : Physiol. 7th Ed., vol. ii., p. 102. LEHMANN. Physiolog. Chem. Cavend. Soc. Edit., vol.ii., p. 269. ,, BERNARD. Legons. 1859. T. i., p. 419. 73. ALEXANDER SCHMIDT. Archiv. fur Anatomic, Physiologie, etc. By Reichert and Du Bois-Raymond, being a continuation of Neil's, Meckel's, and Job. Miiller's Archiv. Leipzig, 1861, p. 545 ; and 1862, pp. 428 and 583. DR. ANDREW BUCHANAN. Proceedings of the Glasgow Philo- sophical Society. Feb., 1845. 77. MR. GULLIVER. London Medical Gazette. Vol. xli., p. 1087. 8 1, note. DR. JOHN C. DALTON. A Treatise on Human Physiology. P. 1 8 1. Philadelphia, 1859. ,, PROFESSOR BEALE. Dr. Thudiclium : Treatise on the Pathology of the Urine. Vol. ii., p. 236. London, 1858. ,, DR. ROLLET. Canstatt : Jahresbericht iiber die Fortschritte in der Biologic. 1863, p. 212. Erlangen. ,, DR. W. ROBERTS. Proceedings of the Royal Society. No.lv., 1863. 85. SCHERER & MULDER. Proceedings of the Royal Society. 1863-4, p. 360. 86. MR. WHARTON JONES. Philosophical Transactions. 1846. 90. LEHMANN. Dr. Geo. E. Day : Chemistry, in Relation to Physio- logy and Medicine. P. 216. 1860. Bailliere. ,, ENDERLIN. Annalen der Chemie und Pharmacie. Von Liebig und Wb'hler. 1844. Sz8 LIST OF AUTHORS. PAGE 91. SCHMIDT. Dr. Geo. E. Day : Chemistry, in Relation to Physiology and Medicine, p. 217. 1860. Bailliere. 92. DENIS. F. Simon : Animal Chemistry. Translated by Dr. Day for the Sydenham Soc. 93. JOHN DAVY. Anat. and Phys. Researches. Vol. ii., p. 28. PO.LLI. Researches and Experiments upon the Human Blood. Noticed in the Medico-Ckirurgical Review, Oct., 1847. 94. PROF. STOKES. Proceedings of the Royal Society. 1863-4. 99. ROLLET. Canstatt : Jahresbericht iiber die Fortschritte in der Biologie. 1863, p. 212. Erlangen. 100. FUNKE. Atlas der Physiologischen Chemie. Leipzig, 1853. 121. HARVEY'S Works. Syd. Soc. Edition. 1847, p. 31. 128. "W. S. SAVORY. Observations on the Structure and Connection of the Valves of the Human Heart. Pamphlet, 1851. 131. VALENTIN. De Functionibus Nervorum Cerebralium et Nervi Sjonpathetici. Berne, 1839. Bd. L, p. 427. 133. DR. REID. The Cyclopaedia of Anatomy and Physiology. Edited by Dr. Todd. Vol. ii., p. 606. 134. KiiRSCHNER. R. Wagner : Handworterbuch der Physiologic. Braunschweig. Art. Herzthatigkeit. 135. DR. GUY. Guy's Hospital Reports. Nos. vi. and vii. 136. VALENTIN. Lehrbuch der Physiologic des Menschen. Bd. i., p. 415, etc. Braunschweig, 1844. 137. OESTERREICHER. Lehre vom Kreislauf des Blutes. P. 33. Nuremberg, 1826. 139. GANGLIA IN THE SUBSTANCE OF THE HEART. Remak : Medi- cinische Zeitung des Vereins fur Heilkunde in Preussen. No. ii., 1840. VOLKMANN. Miiller: Archiv. fiir Anatomic, Physiologic, mid wissenschaftliche Medecin. 1844, p. 424. Berlin. ,, DR. R. LEE. Proceedings of the Royal Society. 1847. ,, London Medical Gazette. Vol. xlv., p. 224. ,, PAGET. Reports on the Use of the Microscope, and On the Pro- gress of Human Anatomy and Physiology, in the British and Foreign Medical Review. 1844-5, p. 13. 141. MR. PAGET. Proceedings of the Royal Society. May, 1857. 143. DR. SHARPEY. Edinburgh Medical and Surgical Journal. Vol. Ixiii., p. 20. 149. JOHN HUNTER. Works of, Edited by Mr. Palmer. Vol. iii., p. 157. London, Longmans, 1835. 150. JOHN HUNTER. Works of, etc. Vol. iii., p. 158. 151. E. H. WEBER. Miiller : Archiv. fiir Anatomic, Physiologic, und wissenschaftliche Medecin. 1847, p. 232. Berlin. LIST OF AUTHOKS. 829 PAGE 151. PROF. KOLLIKEK. British and Foreign Medico-Chirurgical Re- view. July, 1850, p. 241. 154. JOHN HUNTER. Works of, Edited by Mr. Palmer. Vol. iii., p. 159. 1835. 155. W. S. SAVORY. On the Shape of Transverse Wounds of Blood- vessels. Pamphlet, 1859. 156. JOHN HUNTER. Works of, Edited by Mr. Palmer. Vol. iii., p. 216, note. 1835. 157. nth line from t. MAREY. Journ. de la Phys. Brown- Se'quard. 1859. 1 60. MR. COLT. London Med. Gazette. Vol. xxxvi., p. 456. 164. DR. THOMAS YOUNG. Philosophical Transactions. Vol. xcix. ,, POISEUILLE. Magendie : Journ. de Phys. T. viii., p. 272. 165. loth line from t. POISEUILLE. Comptes Rendus des Stances de- 1' Academic Royale des Sciences de Paris. 1860, p. 238. 1 66. LUDWIG. Miiller : Archiv., etc. 1847, p. 242. 169. For instances of occasional direct communications between arteries and veins see Suchet : Bulletin de lAcad. de Med. T. xxvi., p. 825. 173. DR. MARSHALL HALL. Edinburgh Med. and Surg. Journ. 1843. 174. HALES. Statist. Essays. Vol. ii. London, 1740. ,, WEBER. Miiller : Archiv. fur Anat., Phys., und wissenschaftliche Medeciu. Berlin, 1838, p. 450. ,, VALENTIN. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Vol. i., p. 468. 181. MOGK. Henle and Pfeufer : Zeitschrift fur Rationelle Medizin. Heidelberg, 1845, p. 33. VALENTIN. Lehrbuch der Phys. des Menschen. Braunschweig, 1844, p. 477. 183. WHARTON JONES. Philosoph. Trans. 1852. 184. LUDWIG. Miiller : Archiv. fur Anat., Phys., und wissenschaft- liche Medecin. Berlin, 1847, p. 242. 1 86. VALENTIN. Lehrbuch der Phys. des Menschen. Braunschweig, 1844, p. 478. LUDWIG. Miiller : Archiv. fur Anat., Phys., und wissenschaft- liche Medecin. Berlin, 1847, p. 242. ,, DR. BURDON SANDERSON. Proc. of the Royal Soc. 1867. 1 88. POISEUILLE. Annales des Sciences Naturelles : Zoologie. 1843. MR. J. BLAKE. Edinburgh Medical and Surgical Journal. Oct., 1841. 192. DR. KELLIE. London Med. Gaz. May, 1843. ,, DR. BURROWS. Disorders of the Cerebral Circulation. 1846. 195. GUENTHER. Meckel : Archiv. fur Anat. und Phys. 1828, p. 364. 830 LIST OF AUTHORS. PAGE 196. KOLLIKER. Das anatomische und physiologische Yerlialten der cavernb'sen Korper der Sexualorgane. ,, KOBELT. Spallanzani : Versuch iiber das Verdauungsgeschaft. Leipzig, 1785. ,, LE GROS CLARK. Lond. Med. Gaz. Vol. xviii., p. 437. ,, KRAUSE. Miiller : Archiv. fur Anat., Phys., und wissenschaftliche Medecin. Berlin, 1837. 209. MM. BEAU & MAISSIAT. Archives Gene'rales de Me'decine. 1842-3. ,, note. Mr. HUTCHINSON. Trans, of the Koyal Med.-Chir. Soc. Vol. xxix. 211. BOURGERY. Archives Ge'ne'rales de Me'decine. 1843. ,, DR. E. SMITH. Carpenter : Princ. of Human Phys. Edited by H. Power. 1864. 212. top line. MR. HUTCHINSON. Trans, of the Royal Med.-Chir. Soc. Vol. xxix. 216. Rep. of Med.-Chir. Committee. Trans, of Royal Med.-Chir. Soc. 1862. 217. DR. RADCLYFFE HALL. On the Action of the Muscular Coat of the Bronchial Tubes. Pamphlet, 1851. 218. GAIRDNER. Monthly Journ. of Med. Science. Edinburgh. May, 1851. ,, C. J. B. WILLIAMS. Carpenter : Princ. of Human Phys. 3rd Ed., p. 588. ,, VOLKMANN. R. "Wagner : Handworterbuch der Phys. Braun- schweig. Art. Nervenphysiologie, p. 586. 221. VALENTIN & BRUNNER. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Vol. i., p. 547. ,, DR. ED. SMITH. Philosoph. Trans. 1859. 222. ANDRAL ET GAVARRET. Recherches sur la quantite* d'Acide Car- bonique exhale' par le Poumon. Paris, 1843. ,, VIERORDT. Phys. des Athmens. 1845. 223. LETTELLIER. Annales de Chimie et de Physique. 1845. ,, DR. ED. SMITH. Philosoph. Trans. 1859. 224. ALLEN & PEPYS. Philosoph. Trans. 1808-9. ,, LEHMANN. Dr. Geo. E. Day : Chemistry in Relation to Phys. and Med. 1860. Bailliere, p. 469. 225. DR. ED. SMITH. Philosoph. Trans. 1859. ,, VALENTIN & BRUNNER. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Bd. i. 226. DR. BENCE JONES. Chem. Gaz. April, 1851. ,, DR. BENCE JONES. The Med. Times. August 3oth, 1851. ,, REGNATTLT & REISET. Brit, and For. Med.-Chir. Rev. July, 1850, p. 252. LIST OF AUTHOES. 831 PAGE 227. LIEBIG. Animal Chem. Trans, by Dr. Gregory. 3rd Ed., p. 184. ,, $rd par. Comptes Rendus des Seances de 1'Acad. Royale des Sciences de Paris. 1848. ,, Annales de Chimie et de Pharmacie. 1849. 229. WIEDERHOLD. Brit, and For. Med.-CMr. Rev. 1859, p. 155. 239. ECCLES. London Med. Gaz. Vol. xliv., p. 657. , , Report of Med. -Chir. Committee. Trans, of the Royal Med. -Chirur. Soc. 1862. 242. DE. MARSHALL HALL. The Cyclopaedia of Anat. and Phys. Ed. by Dr. Todd. Vol. ii., p. 771. 243. DR. J. DAYY. Phil. Trans. 1844. ,, M.ROGER. Archives Generates de Me'decine. 1844.' ,, DR. DAVY, ijth line from t. Proceedings of the Royal Soc. June, 1845. 244. VOYAGE OF THE "BONITE." Comptes Rendus des Stances de TAcad. Roy ale des Sciences de Paris. 1838, p. 456. ,, TIEDEMANN AND RuDOLPHi. Tiedemann : Phys. translated by Gully and Lane, p. 234. JOHN HUNTER. Works of, Ed. by Mr. Palmer. 1835. Vol. iii., p. 1 6, and vol. iv. p. 131 et seq. 245. M. Magendie. L' Union Medicale. 1850. 249. MR. NEWPORT. Philosoph. Transac. 1837. 250. MR. EARLE. Trans, of the Royal Med. -Chir. Soc. Vol. vii., p. 173. 251. M. BERNARD. Dr. Carpenter: Princ. of Human Phys. 5th edt. 1855, p. 418. 252. DELAROCHE ET BERGER. Exp. sur les Effets qu'une forte Cha- leur produit dans 1'Economie Animale. Paris, 1806. ,, MR. C. JAMES. Gazette Mgdicale de Paris, April, 1844. 253. DR. DAVY. Philosoph. Transac. 1844. 257. MAGENDIE. Phys. transl., by Milligan, 4th ed. 258. CHOSSAT. Gaz. Md. de Paris, Oct., 1843. ,, LETTELLIER. Annales de Chimie et de Physique. 1844. 259. top line. PAGET. Reports on the use of the Microscope, and on the Progress of Human Anat. and Phys. in the Brit, and For. Med. Rev. 1843-4, p. 35. 267. DR. WRIGHT. The Lancet. 1842-3. 269. PROF. OWEN. Dr. Carpenter : Princ. of Human Phys. 5th ed. 1855, p. 76, note. 273. BERNARD. The Med. Times and Gaz. July 7th, 1860. 279. DR. BRINTON. On Food and its Digestion. Churchill, 1861. ,, BEAUMONT. Experiments and Observations on the Gastric Juice, and the Phys. of Digestion, by W. Beaumont. U.S., 1834. Reprinted with notes by Dr. A. Combe. Edinburgh, 1838. 832 LIST OF AUTHOKS. PAGE 280. BLONDLOT. Traite Analytique de la Digestion. 8vo. Paris, 1844. ,, BERNARD. Gaz. M^d. de Paris. June, 1844. 281. DR. GEO. E. DAY. Chem. in relation to Phys. and Med. 1860. Bailliere. p. 158. ,, Dr. GEO. E. DAY. Chem., etc., p. 159. 284. BEAUMONT. Experiments and Observations on the Gastric Juice, and the Phys. of Digestion, by "W. Beaumont. U.S., 1834. Reprinted with notes by Dr. A. Combe. Edinburgh, 1838, p. 120. 288. Dr. GORSE. Spallanzani : Versuch tiber das Verdauungsgescha'ft. Leipzig, 1785. 290. RAWITZ. Weber : die Einfachen Nahrungsmittel. Breslau, 1846. 292. 8th line from b. BERNARD. Gaz. Med. de Paris. 1850, p. 889. 295. DR. BRINTON. London Med. Gaz. 1849. ,, DR. BRINTON. On Food and its Digestion. Churchill, 1 86 1, p. 100. 301. BERNARD. The Med. Times and Gaz. Aug. 1860. LONGET. Anat. et Phys. du Systeme Nerveux, etc. Paris, 1842. Vol. i. p. 323. ,, BISCHOFF. Muller: Archiv. fur Anat., Phys. und wissenchaft- liche Medecin. " Berlin, 1848. Jahresbericht, p. 140. 305. MEISSNER. Henle and Pfeufer : Zeitschrift fur Kationelle Medi- zin. Heidelberg, 2nd Ser., vol. viii. ~>6 LIEBERKUHN, J. N. Diss. de Fabric^ et Actione Villorum Intesti- no rum tenuium. 1782. ,, Peyer. De Glandulis Intestinorum. 1682. BRUNN, J. C. Glandulse Duodeni seu Pancreas secundariunu 4to. 1715- 315. KOLLIKER. Manual of Human Microscopic Anat. Parker, 1860, P- 327. 319. MM. BOUCHARDAT & SANDRAS. Gaz. Med. ^e Paris. Jan. 1845. 320. DISCHARGE OF FATTY MATTERS FROM INTESTINE. Trans, of the Eoyal Med. -Chir. Soc. Vol. xviii., p. 57. ,, BERNARD. Quarterly Jour, of Microscop. Science. Churchill. 328. TA BULAR COMPOSITION OF BILE, by Frerichs. V. Gorup-Besanez. Physiologic Chemie. 1862, p. 469. 330. PRESENCE OF COPPER IN BILE AND BILIARY CALCULI. Gorup- Besanez. Untersuchungen iiber Galli. Erlangen, 1846. 331. BLONDLOT. Essai sur lesFonctions du Foie. Paris, 1846, p. 62. DR. KEMP. Chemical Gaz. No. 99, 1846, note. 333. SIMON. Animal Chem. Trans, by Dr. Day for the Sydenham Soc. Vol. ii. p. 367. ,, FRERICHS. Kanking : Half-yearly Abst. of the Med. Sciences. Churchill. Vol. iii. p. 314. LIST OF AUTHORS. 833 PAGE 339. 6th line from b. PAVY. Phil. Trans. 1860, p. 595. ,, yd line from b. PAVY. On the Nature and Treatment of Dia- betes. Churchill. 1862. 340. THUDICHUM. The Brit. Med. Jour. 1860. ,, HARLEY. Proceed, of the Royal Soc. 1861. 349. BRINTON. On Food and its Digestion. Churchill. 1861. ,, MEISSNER. Henle and Pfeufer : Zeitschrift fur Kationelle Medi- zin. Heidelberg, 2nd Ser., Vol. viii. 358. KOLLIKER. Annales des Sciences Naturales : Geologic. 1846, p. 99. ,, KOLLIKER. Brit, and For. Med.-Chir. Rev. July, 1850. 360. COMMUNICATION BETWEEN LYMPHATICS AND BLOOD-VESSELS. Paget. Reports on the use of the Microscope, and on the Progress of Human Anat. and Phys., in the Brit, and For. Med. Rev. 1842, p. 45. 361. GULLIVER. Hewson : Works, Ed. for the Syd. Soc. by Mr. Gulli- ver. 1846-7, p. 82, note. ,, ASCHERSON. Miiller : Archiv. fur Anat., Phys. und wissenschaft- liche Medecin. Berlin, 1840. 362. BOUISSON. Gaz. Med. de Paris. 1844. 363. DR. OWEN REES. London Med. Gaz. Jan., 1841. 364. R. VIRCHOW. Die Cellular Pathologie (since translated by I Chance). Berlin, 1858. s. 143. 365. BIDDER. Miiller : Archiv. fur Anat, Phys. und wissenschaftlicLe Medecin. Berlin, 1845. ,, SCHMIDT. New Syd. Soc.'s Year-Book of Med., etc. London, 1863, p. 24. 368. HERBST. Das Lymphagefass system und seine Verrichtungen. Gb'ttingen, 1844. 369. LYMPH-HEARTS. I J. Miiller. Elements of Phys. transl. by Dr. Baly. 2nd ed., 1840, p. 293. ,, VOLKMANN. Miiller : Archiv. fur Anat. , Phys. und wissenschaft- liche Medecin. Berlin, 1844. ,, MAYER. Meckel : Archiv. fur Anat. und Phys. T. iii. p. 485. 376. MAGENDIE. Phys. transl. byMilligan. 4th ed., p. 314. ,, SEGALAS. Magendie : Journ. de Phys. T. ii., p. 117. BENCE JONES. Proceedings of the Royal Soc. Vol. xiv. 377. SAVORY. The Lancet. 1863, May 9 & 16. 378. OESTERLEN. Oesterreichische Mediciuische Wochenschrift. Wien, Feb., 1844. ,, OESTERLEN. Archiv. fiir Phys. und Pathol. Chemie und Mikro- scopie. Von J. F. Heller. Wien, 1847, p. 56. 3 H 834 LIST OF AUTHORS. PAGE 380. HELMHOLTZ. Miiller : Archiv. fur Anat. Pliys. und wissenschaft- liche Medecin. Berlin, 1845. 381. CARPENTER. Princ. of Human Phys. 3rd ed., p. 623. ,, NUTRITION, see Paget : Lectures on Snrg. Pathol. 1853. 391. BRODIE. Lectures on Pathol. and Surg. 1846, p. 309. ,, TRAVERS. Further Inquiry concerning Constitutional Irritation. p. 436. 392. BALY. J. Miiller : Elements of Phys. Trans, by Dr. Baly. 2nd ed., 1840, p. 396. STANLEY. London Med. Gaz. Vol. i., p. 531. ,, ERASMUS WILSON. Diseases of the Skin: A Practical and Theo- retical Treatise on the Diagnosis, Pathology, and Treatment of Cutaneous Diseases. 1842, p. 28. ,, Defective Nutrition from Irritation of Nerves : London Med. Gaz., Vol. xxxix., p. 1022. 393. 2nd par. Transac. of the Royal Med.-Chir. Soc. Vol. xx. 400. BOWMAN. The Cyclopsedia of Anat. and Phys., Edited by Dr. Todd. Art. Mucous Membrane. ,, GOODSIR. Anat. and Pathol. Observations, by John andH. D. S. Goodsir. 1845, p. 41. 403. GOODSIR. Anat. and Pathol., etc. 1845. ,, RAINEY. Proceedings of the Royal Society. 1847. 404. RAINEY. Transac. of the Royal Med.-Chir. Soc. Vol. xxviii. 412. PERISTALTIC MOVEMENTS OF LARGE GLAND-DUCTS. T. Miiller.: Elements of Phys. Trans, by Dr. Baly. 2nd Ed., 1840, p. 521. ,, DR. BR'OWN-SQUARD. Journ. de Phys. 1858. 414. BERNARD. Quart. Journ. of Microscop. Science. ,, BROWN-SEQUARD. The Lancet. 1858. 415. CARPENTER. Princ. of Human Phys. 3rd Edition, p. 476. 416. SIMON. A Physiological Essay on the Thymus Gland. London, 1845. 4to. ,, ECKER. Der feinere Bau der Neben-nieren beim Menschen und den Vier Wirbelthierclassen. Braunschweig, 1846. SIMON. A Physiological Essay on the Thymus Gland. London, 1845. 4to. 418. MEYER. Boehm, L. De Glandularum intestinalium Structura peni- tiori. Berol., 1835. March, 1845. ,, A Physiological Essay on the Thymns Gland. London, 1845. 4to. ,, FRIEDLEBEN. Die Phys. der Thynmsdriise. Frankfort, 1858. ,, HUTCHTNSON. Fuuke : Atlas der Phys. Chemie. Leipzig, 1853-6. ,, WILKES. Guy's Hosp. Rep. 1862. 419. KOLLIKER. Manual of Human Miscroscop. Anat. Parker, 1860, P- 374- LIST OF AUTHORS. 835 PJLGE 420. Kb'LLiKER. Manual, etc. p. 365. 429. KRAUSE. R. Wagner: Handworterbuch der Phys. Braunschweig. Article Haut. ,, ERASMUS WILSON. A Practical Treatise on Healthy Skin. Churchill, 1846. ,, ROHIN. Gaz. He'd, de Paris. Sept., 1845. ,, HORNER. American Journ. of Med. Sciences. Jan., 1846. 435. J. DAVY. Trans, of the Royal Med.- Chir. Soc. Vol. xxvii.,p. 189. ,, KRAITSE. Bulletin de I'Acad^mie Royale de Medecine. 436. BERZELIUS. Traite" de Chimie, traduit par Esslinger. 8 vols., 8vo. Paris. Vol. vii. contains the Chemistry of Animal Structures. ,, ANSELMINO. Zoochemie, by D.Lehmanri. Heidelberg, 1858, p. 301. ,, GORUP-BESANEZ. Lehrbuch der Phys. Chemie. 1862, p. 504. ,, LAVOISIER & SEQUIN. Memoires de 1'Acad. des Sciences do Paris. 1790. ,, KRAUSE. Paget: Reports on the Use of the Microscope, and on the Progress of Human Anat. and Phys. ; in the Brit, and For. Med. Rev. 1843-4, p. 40. 438. MILNE-EDWARDS. Influence des Agens Physiques sur la Vie. Trans, by Dr. Hodgkin. ,, REGNAULT & REISET. Annales de Chimie et de Pharmacie. 1849. ,, EDWARD SMITH. Dr. Carpenter: Princ. of Human Phys. 5th Ed., 1855. 6th Ed., p. 293. ,, MILNE-EDWARDS & MULLER. J. Miiller: Elements of Phys. ; Trans, by Dr. Baly. 2nd Ed., 1840, p. 328. 439. MAGENDIE. Gaz. Med. de Paris. Dec., 1843. 440. MILNE-EDWARDS. Influence des Agens Physiques sur la Vie. Translated by Dr. Hodgkin. ,, MADDEN. Experimental Inquiry into the Phys. of Cutaneous Absorption. Edinburgh, 1835. 441. BERTHOLD. Miiller: Archiv. fiir Anat, Phys., und wissen- schaftliche Medecin. Berlin, 1838, p. 177. 449. ERICHSEN. London Med. Gaz. 1845. 451. BERNARD. Comptes Rendus des Seances de I'Academie Royale des Sciences de Paris. 1846. ,, PROUT. On the Nature and Treatment of Stomach and Renal Diseases. 4th Ed., 1843, p. 518. 452. PROUT. On the Nature, etc. ,, WATSON. Lectures on the Princ. and Prac. of Physic. 1843. Vol. ii., p. 557. ,, PROUT. On the Nature and Treatment of Stomach and Renal Diseases. 4th Ed., 1843, p. 519. 836 LIST OF AUTHORS. PAOE 452. GOLDING BIRD. Urinary Deposits. 1844,^31. ,, PAEKES. On the Composition of the Urine. 1866. 455. WOHLEB. Annales de Chimie et de Pharmacie. xxvii., 196. 456. LECANU. Bulletin de 1'Acad. Koyale de Med. T. xxv., p. 261. 457. LEHMANN. F. Simon : Animal Chem. Trans, by Dr. Davy for the Syd. Soc. Vol. ii., p. 163. ,, LASSAIGNE. Journal de Chimie Medicale. p. 272. ,, UREA IN THE BLOOD. Archiv. fiir Phys. und Pathol. Chemie und Mikroscopie. Yon J. F. Heller. Wien, 1848. 458. MILLON. Comptes Rendus des Seances de 1'Acad. Eoyale des Sciences de Paris. 1843. G. BIRD. London Med. Gaz. Vol. xli., p. 1106. 459. LIEBIG. The Lancet. June, 1844. 460. LIEBIG. The Lancet. June, 1844. WEISMANN. Henle and Pfeufer : Zeitschrift fiir Rationelle Medizin. Heidelberg. 3 ser., p. 337. ,, URE. Transac. of the Royal Med. -Chir. Soc. Yol. xxiv, 461. LIEBIG. Chem. of Food. Walton and Maberly, 1847. ,, HEINTZ. Canstatt : Jahresbericht iiber die Fortschritte in der Biologie. Eiiangen, 1847, p. 105. 463. RONALDS. Philosophical Mag. 1846. 464. RONALDS. Philosophical Mag. 1846. 469. LISTER & TURNER. Quar. Jour, of Microscop. Science. 1859. ,, LOCKHART CLARKE. Philosoph. Transac. 1859. ,, STILLING. Ueber den Bau der Nerven-primitivfaser und der Nerven-zelle. 1856. ,, G. TREVIRANUS. Beitrage zur Auf klarung der Erscheinungen des Lebens. Heft. ii. 472. GULL. The Med. Times. 1849. 473. KOLLIKER. Manual of Human Microscopic Anat. Parker, 1860. p. 248. 474. PACINI. Annali Universali di Medicini. Luglio. 1845, p. 208. 480. HELMHOLTZ & BAXT. Camb. Journ. of Anat. and Phys. P. i., new series, p. 190. "484. SAVORY. The Lancet. Aug. ist, 1868. 493. LOCKHART CLARKE. Philosoph. Transac. 1851 to 1859. 495. KOLLIKER. Manual of Human Microscopic Anat. Parker. 1860, p. 244. 496. KOLLIKER. Manual, etc. p. 247. 498. BROWN-SEQUARD. On the Phys. and Pathol. of the Cerebral Nervous System. Philadelphia, 1860. 502. GRAINGER. Obs. on the Spinal Cord. London, 1837. 509. YOLKMANN. Miiller : Archiv. fiir Anat., Phys., und wissen- schaftliche Medecin. Berlin, 1844. LIST OF AUTHORS. 837 PAGE 515. LEGALLOIS. (Euvres Completes, Edited by M. Pariset. Paris, 1830. T. i., p. 64. ,, FLOURENS. Kecherches Expe"rimentales sur les Fonc. du Systeme Nerveux, etc. Paris. ,, LONGET. Anat et Phys. du Systeme Nerveux, etc. Paris, 1842. 516. BROWN-SEQTJARD. The Med. Times and Gaz. 1858, p. 600. 517. J. REID. Edinburgh Med. and Surg. Journ. 1838. 518. J. REID. Edinburgh Med. and Surg. Journ. 1838. 519. SCHROEDER VAN DER KOLK. On the Structure of the Spinal Cord and Medulla Oblongata. New Syd. Soc., 1859. 527. LONGET. Anat. et Phys. du Systeme Nerveux, etc. Paris, 1842. T. i., p. 733, and others. 528. FLOURENS. Recherches Experimentales sur les Fonc. du Systeme Nerveux, etc. Paris. ,, MAGENDIE. Legons sur les Fonctions du Systeme Nerveux. ,, BOUILLAUD. Recherches Clim'ques et Experimentales sur le Cervelet. Referred to by ,, LONGET. Anat. et Phys. du Systeme Nerveux, etc. Paris, 1842. T. i., p. 740. 530. LONGET. Anat. et Phys. du Systeme Nerveux, etc. Paris, 1842. T. i, p. 762. ,, COMBIETTE. Revue Mddicale. 539. OESTERLEN. Beitrage zur Physiologic. 1843. 542. GRANT. See Longet, 1. c. T. ii., p. 388. 547. VALENTIN. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Vol. ii. p. 666. 554. REID. Edinburgh Med. and Surg. Journ. 1838. 559. BIDDER & VOLKMANN. R. Wagner : Handworterbuch der Phys. Braunschweig. Art. Nervenphysiologie. ,, REID. Edinburgh Med. and Surg. Journ. Vols. xlix. and Ii. 562. REID. Edinburgh Med. and Surg. Journ. Vols. xlix. and Ii. ,, LEGALLOIS. (Euvres Completes; Edited by M. Pariset. Paris, 1830. ,, TRATJBE. Beitrage zur Experimentellen Pathologic und Phys. Berlin, 1846. 564. BERNARD. Archives Ge'ne' rales de Medecine. 1844. 565. BERNARD. Archives Generales de Medecine. 1844. 580. KUHNE. Camb. Journ. of Anat. and Phys. Part ii. 582. ^th to Sth line from t. ELLIS. Phil. Trans. 1859. ,, KOLLIKER. Manual of Human Microscopic Anat. Parker, 1860, p. 63. 587. SHARPEY. Quain : Anat. 7th Ed. 588. SEGALAS. J. Miiller : Elements of Phys. Trans, by Dr. Baly. 2nd Ed., 1840, p. 895. 838 LIST OF AUTHORS. PAGE 589. BOWMAN. Phil. Trans. 1840, 1841. 590. No DIMINUTION IN BULK OF CONTRACTING MUSCLE. Mayo. J. Miiller : Elements of Phys. ; Trans, by Dr. Baly. 2nd Ed. , 1840, p. 886. Valentin : Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Matteucci : Erdmann's Journ. ,, ED. WEBER. R. Wagner : Handwb'rterbuch der Phys. Braun- schweig. Art. Muskelbewegung. 592. ED. WEBER. R. Wagner : Handwb'rterbuch der Phys. Braun- schweig. Art. Muskelbewegung. 594. SCHIFFER : Camb. Journ. Part ii., new series, p. 416 ; Part iii., new series, p. 236. ,, BROWN-SEQUARD. Proc. of the Royal Soc. 1862, p. 204. BROWN-SEQUARD. London Med. Gaz. May 16, 1851. 595. VALENTIN. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Bd. ii., p. 36. 618. PETREQUIN & DIDAY. Gaz. Med. de Paris. 623 . MULLER & COLOMBAT. Froriep : Neue Notizen aus dem Gebiete der Natur. Weimar, 1840. 626. MAGENDIE. Journ. de Phys. T. iv., p. 180. 640. TODD & BOWMAN. The Phys. Anat. and Phys. of Man. Vol. ii., p. 31. 653. VOLKMANN. R. Wagner : Handworterbuch der Phys. Braun- schweig. Art> Sehen, p. 286. 687, note. ED. WEBER. Archives d'Anat. Ge'nerale et de Phys. 1846. 708. SCHIFF & BROWN-SEQUARD. The Lancet. 1858. ,, SIEVEKING. Brit, and For. Med. -Chir. Rev. 1858, Vol. ii., p. 501. 710. VALENTIN. Lehrbuch der Phys. des Menschen. Braunschweig, 1844. Vol. ii., p. 566. 723. VALENTIN. Miiller : Archiv. fur Anat., Phys., und wissenschaft- liche Medecin. Berlin, 1838. 731. D ALTON. Phys. 1864, p. 585. 740. BISCHOFF. Entwickelungs-Geschichte der Saugethiere und des- Menschen. 1842. 755. H. MULLER. Brit, and For. Med. -Chir. Rev. Vol. xiii., p. 546; 758. HARVEY. On a remarkable Effect of Cross- Breeding. By Dr. Alex. Harvey. Blackwood and Sons, 1851. ,, HUTCHINSON. Med. Times and Gaz. Dec., 1856. ,, SAVORY. On Effects upon the Mother of Poisoning the Foetus. Pamphlet, 1858. 767. KOLLIKER. Annales des Sciences Naturelles : Zoologie. Aug.,, 1846. Srimtifit PRINTED FOR JAMES WALTON, 137, GOWER STREET, LONDON. QUAIN'S ANATOMY. Seventh Edition. 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" This is an almost perfect guide to the art of bandaging and the application of surgical apparatus a subject with which all students are now required to be thoroughly acquainted, and on which they are specially examined at the College of Surgeons. To dressers and to students about to present themselves for examination, Mr. Hill's handbook (which is admirably illustrated) will be henceforth indispensable." British Medical Journal. ON SYPHILIS & LOCAL CONTAGIOUS DISORDERS. By BERKELEY HILL, M.B., F.k.C.S., Surgeon to Out-Patients at the Lock Hospital, Assistant Surgeon in University College Hospital. 8vo., 1 6s. "1>his book is marvellously complete The description of the eruptions is most Excellent, at once the simplest and most complete we have read in any language. . ' . . Mr. Hill's practical experience at the Lock Hospital enables him to give some very valu- able hints as to treatment. " Edinburgh Medical Journal. ELLIS'S DEMONSTRATIONS OF ANATOMY. A Guide to the Dissection of the Human Body. New Edition (Sixth), with 146 Illustrations on Wood. Small 8vo., I2s. 6d. ILLUSTRATIONS OF DISSECTIONS, in a Series of Original Coloured Plates, the size of life, representing the Dissection of the Hitman body. By G. V. ELLIS, Professor of Anatomy in University College, London, and G. H. FORD, Esq. Complete in 29 Parts, Imperial folio, price ^5 3$., or half-bound in Morocco, price /66s. Parts I. to XXVIII., each 35. 6d. Part XXIX. 53. With these plates, and such as these, by his side, the learner will be well guided in his dissection ; and under their guidance he may safely continue his study when out of the dissecting-room. With such plates as these, the surgeon will be fully reminded of all that is needful in anatomy when engaged in planning an operation." Medical Times. DISEASES OF CHILDREN, TREATED CLINICALLY. Founded upon Lectures delivered at the Hospital for Sick Children. By THOMAS HILLIER, M.D., Physician to the Hospital for Sick Children. Small 8vo., 8s. 6d. " It is a thoroughly sound piece of observation and practical application of experience/ It is so thoroughly clinical that it is impossible to review it. But from the therapeutical point of view, which chiefly interests us, we may recommend it with great confidence ; and it is certainly a very much needed work in this respect, for the text-books, which have hitherto been standards on the subject, ha.ve been extraordinarily conservative in their tendencies, and have tended to perpetuate not a little of the old routine drugging of children. " The Practitioner. ON THE WASTING DISEASES OF CHILDREN. By EUSTACE SMITH, M.D., Physician Extraordinary to His Majesty the King of the Belgians. Physician to the North-west London Free Dispensary for Sick Children, and to the Metropolitan Dispensary. Small 8vo., 7s. 6d. " We can most honestly recommend the volume as one full of valuable practical infor- mation, not only concerning the diseases of children of which it treats, but also as to their rood and general hygienic management." British Medical Journal. DR. GARROD ON GOUT AND RHEUMATIC GOUT. Second Edition, with extensive alterations. Coloured and other Illus- trations. Small 8vo., 155. " Dr. Garrod has in this edition incorporated the results of his increased experience of the nature and treatment of gout ; and has added a chapter on the diseases to which gouty persons are peculiarly liable.'' British Medical Journal, 5716 University of California Medical School Library