BIOLOGY LIBRARY G HUMAN PHYSIOLOGY: DESIGNED FOR COLLEGES AND THE HIGHER CLASSES IN SCHOOLS, FOR GENERAL READING. BY WORTHINGTON HOOKER, M. D., PROFESSOR OF THE THEORY AND PRACTICE OF MEDICINE IN YALJC COI AUTHOR OF "PHYSICIAN AND PATIENT." Illustrated by nearly 200 Engravings. NEW Y O E K : PRATT, OAKLEY & COMPANY, 21 MURRAY STREET. R1OLOGY ENTERED, according to Act of Congress, in the year of our Lord. One thousand eight hundred and fifty-four, BY WORTIIIXGTOX HOOKER. M D., In the C'erk's Office of the District Court of Connecticut. CONTENTS. PART I. CHAPTER I. PA3K . ORGANIZED AND UNORGANIZED SUBSTANCES. . . .13 CHAPTER II. THE DISTINCTION BETWEEN ANIMALS AND PLANTS. . 21 CHAPTER III. MAN IN HIS RELATIONS TO THE THREE KINGDOMS OF NA- TURE, 27 PART II. CHAPTER IV. GENERAL VIEWS or PHYSIOLOGY. WITH A BRIEF ACCOUNT OK SOME OF THE STRUCTURES OF THE BODY, . 35 CHAPTER V. DIGESTION, ......... 42 CHAPTER VI. CIRCULATION OF THE BLOOD, . . . . 64 CHAPTER VII. RESPIRATION, 86 CHAPTER VIII. FORMATION AND REPAIR, 109 CHAPTER IX. CELL LIFE, 123 IV CONTENTS. PART III. THE CHAPTER X. PAGE. 139 THE CHAPTER XL BONES, 170 THE CHAPTER XII. MUSCLES, 196 THE CHAPTER XIII. LANGUAGE OF THE MUSCLES, .... 222 THE CHAPTER XIV. VOICE, 243 THE CHAPTER XV. EAR, 271 THE CHAPTER XVI. EYE, 287 CHAPTER XVII. CONNECTION OF THE MIND WITH THE BODY, . . ' 318 CHAPTER XVIII. DIFFERENCES BETWEEN MAN AND THE INFERIOR ANIMALS, 347 CHAPTER XIX. VARIETIES OF THE HUMAN RACE, 367 CHAPTER XX. LIFE AND DEATH, 381 CHAPTER XXL HYGIENE, , 390 APPENDIX, ... 411 PREFACE. I HAVE aimed so to write this book, that it shall be fitted both for gen- eral reading, and for instruction. It is designed for the family as well as for the school. It seemed desirable that these two objects should be ao complished at the same time, and I have not found them to be at all in- compatible. The instruction needed by the family on this subject, differs not from that which is required in the school-room, either in regard to the facts to be communicated, or the manner in which it should be done. No one will question the truth of this, so far as the facts are concerned. But it is true even as to the mode of communicating them. In both cases there need to be clearness in statement, and fullness of illustration. Actual instruction is to be given in both cases, and to minds that are very nearly in the same attitude. I could not, therefore, see the necessity of writing a book on this subject for the people which should differ from one written for the school. Besides, it has seemed to me desirable that there should be a greater community of interest between the school and the family than as yet exists ; and this object books equally interesting to both will tend to promote. It may be proper for me to say a word in relation to the style of the work. I have adopted the style of the lecture-room, because, that while it is not inconsistent with conciseness, it is the more natural mode of in- struction, especially when so much reference is made to illustrative figures. It has enabled me also to keep in view more effectually the attitude of the minds I address. I have had my readers before me continually as an imaginaiy audience. I have avoided technical terms as far as possible. Whenever they are used they are sufficiently explained at the time, so that no glossary is needed. Some points commonly considered hard to be understood are treated of, but I have endeavored to simplify them, by VI PKEFACE. full illustration, and by a presentation of the truth uncomplicated with speculations and hypotheses. And these points are so introduced, that the mind is prepared by the previous investigation to understand them. I have aimed so to arrange the topics, as to have a preparation constantly going on in the mind of the student, fitting him for the proper under- standing of what is to come after. By this natural gradation in the de- velopment of the whole subject some of the deep things in Physiology can be made clear, which it would otherwise be impossible for the student to understand. It is proper to state here, that I intend to prepare a worK for younger scholars, in which some of the simple points in Physiology will be illustrated. This, by familiarizing their minds with the subject, will fit them for a more thorough understanding of the present work. Although Physiology is 1 becoming a prominent study in the schools and colleges in some parts of our country, its importance is no where as yet appreciated as it should be. It should be made a regular branch in our Educational System. This has been already done in France. " A com- petent knowledge," says Carpenter, " of Animal Physiology and Zoology is there required from every candidate for University honors ; and men of the highest scientific reputation do not think it beneath them to write elementary books, for the instruction of the beginner." The importance of Physiology as a study, will appear from various con* siderations. Many of the subjects comprised in Physiology have, in the case of most students, been already studied in a different phase, or mode, in other branches. Thus, if the student has attended to the Mechanical Powers in his Natural Philosophy, he finds in the human body the principles of the pulley and the lever illustrated in great variety and perfection. The principles in relation to strength in the form and arrangement of struc- ture he sees exemplified in the frame-work of the body in the most ad- mirable manner. If he has studied Hydraulics, he sees in the body the most perfect, and at the same time the most complicated hydraulic ma- chinery, working incessantly throughout life in the circulation of the blood. The principles of Pneumatics he finds applied in the respiration those of Optics in the eye those of Acoustics in the ear and those of Musical Sounds in the apparatus of the voice. And then, his chemical PKEFACE. VII knowledge meets with new applications in his observation of the changes and the processes going on in the body. The relations, then, of Physiology to some of the common branches taught in the higher classes in schools, are of the most intimate charac- ter. Physiology, in part, merely extends these branches into a new and interesting field ; and the student who has once entered this field recurs to these same branches with a renewed interest. Hydraulics, Pneuma- tics, Optics, &c., have now a new attraction for him, from this, to him novel, application of their principles. The interest thus awakened in his mind is worth much in itself, aside from the mere addition made to his knowledge. And the interest is enhanced by the consideration, that in the human body he sees the applications of these principles to mechan- ism that exhibits the skill of perfect wisdom and almighty power. But there are relations of Physiology to still other studies which should be noticed. The analogies that exist between the human body and all other living things, in relation to structure and growth, are numerous and striking. Though life is so diverse in its processes and in the forms which we see it evolve in the whole range of animated nature, it in some important re- spects displays a great similarity, which it is interesting to trace through- out its diversified manifestations. Growth, or nutrition, as you will see in the following pages, is essentially the same in the Plant as it is in the Animal. Botany, therefore, taught as it should be, has quite an intimate relation to Animal Physiology. The Science of Life is, in many respects, one Science ; and if, in studying any of its subdivisions, we fail to take this broad view of it, and to trace out the analogies referred to, we lose a large part of the interest of the study. Human Physiology, the subject of study in this book, is but a part of a science which offers to the student wide fields of observation exceedingly diversified and full of interest. This being so, I could not avoid in the following pages making occasional reference to the analogies existing between the phenomena of life as ex- hibited in the human system, and those which we see in the living world around us. So that as the student proceeds with the study, he will find himself interested in the phenomena of life in whatever form they are VU1 PREFACE. This leads me to say that this study of nature, in its broad common re- lations and its beautiful and extensive analogies, should be made very prominent in our systems of education. It is the application of the prin- ciples of abstract science to the forms, and especially the living forms of nature all about us, that gives interest to these principles, and makes us to understand and appreciate them. It is here that we find a very seri- ous defect in the prevalent mode of education, even at the present time, notwithstanding all our improvements. Let us look at it a moment. We live in the midst of a material world, animate and inanimate, and have daily converse, so to speak, with material forms of every variety, present- ing phenomena of the highest interest and of endless diversity. And yet, through almost all the period of childhood, and perhaps we may say youth also, this book of nature is in the school-room very nearly a sealed book. The very process of education shuts in the pupil from this broad contemplation of the world in which he lives. He is drilled through spelling, reading, grammar, &c., but he is left in total ignorance of the beautiful flowers, and the majestic trees outside of the school-room. How very few even of thoroughly educated adults, know the processes by which a plant or a tree grows! And the same can be said of other phenomena of nature. The defect which I have pointed out runs through the whole of educa- tion. We can see it even in the prevalent mode of teaching the natural sciences themselves. One would suppose that here the facts, the phe- nomena, would command the chief attention of the teacher and the stu- dent. But it is very commonly not so. The mere technicalities and the classification are made much too prominent. Botany, really one of the most interesting of all branches of natural science, is thus ordinarily made one of the driest of studies. To teach this aright, the phenomena of vegetation, so varied and so beautiful, should constitute the chief ma- terial of instruction, and the mere classification should be considered, al- though necessary, as wholly a secondary thing. The great facts of the world, both of mind and matter, should furnish really the material for education, and those branches that are ordinarily pursued with such assiduity should be considered as merely subsidiary to the teaching of these facts. The whole order of education must be re- PREFACE. IX versed. Instead of beginning the child's education with learning to spell and read, the object should be to make him an observer of nature, and the spelling and reading should be done in connection with this, and as subsidiary to it. Things and not words, or mere signs, should from the first, constitute the substantial part of instruction. The child should be made, at home, in the school, and everywhere, a naturalist in the largest sense of that word. We should aim to impart to him a spirit in con- sonance with the following precept of Hugh Miller, the famous self- taught geologist. " Learn to make a right use of your eyes ; the com- monest things are worth looking at even stones and weeds, and the most familiar animals." As it is now, no one becomes a naturalist early in life, except in spite of the tendencies of his education. The study of nature is not only not encouraged, but is absolutely discouraged in our educational system. If any one, like Hugh Miller, by the force of a taste that can not be repress- ed by the training of the school-room, undertakes to make a " right use of his eyes," and curiously examines " stones and weeds," he is regard- ed by the world of spellers and readers and grammarians and cipherers, as a strange genius. But he is pursuing from an irresistible internal force, the very course that I would have every student, even from his childhood, encouraged to pursue, in a measure at least, by the external circumstances of his education. The tendencies of his training should be decidedly in this direction. If the general mode of education were changed in the manner indicated, education would have much less of the character of mere drudgery than it now has. Not that there would be any the less labor ; but the labor would be made lighter by the interest imparted to it the interest, which always results from the study of facts and phenomena, and never from the learning of mere words and technicalities and classifications. I would gladly dwell on this subject, and show by varied illustrations how the mode of instruction referred to, should be pursued, and especially with younger scholars $ but the limits of a preface will not allow me to enter BO large a field. The change which I have pointed out can not be effected at once. It will require tune. Confirmed traditional customs are to be done away, PREFACE. the habits of teachers are to be altered, and the proper books are to * great extent to be yet written, especially such as are fitted for the first years of education. If the study of nature should be thus made prominent in education, human physiology would be considered altogether its most interesting and important branch, and for several reasons. First : there is no where to be found so curious a collection of mechanisms, or so interesting and wonderful a series of processes, as in the human body. In nothing else in the wide world are the principles of so many departments of science so extensively and perfectly exemplified. Life works here its most com- plicated set of machinery. Secondly : the singular and mysterious con- nection of the immaterial and immortal soul with the material and perish- able body, gives intense interest to this study. In Physiology we do not study matter alone, or spirit alone, but both matter and spirit united, and often acting together. This circumstance distinguishes this from all other studies. Thirdly : it is our own frames, moved by the spirit within us, that we study. The subject has a personal interest for us, that is not presented by most studies, and by none in so large a degree as in this. And Fourthly : the study is of great importance, because a judicious and efficient Hygiene must be based upon a knowledge of the laws of physi- ology. We cannot know how to keep our functions in the condition of health, without understanding the laws that regulate them. I have said but little in this book in regard to hygiene, and that only incidentally, be- cause that subject would require of itself a whole volume to elucidate it properly. I have not thought it proper to indulge to any great extent in those re- flections, which the contemplation of so perfect and diversified a congeries of mechanisms as are presented in man would naturally suggest, in regard to the skill of the great builder of the universe. Such reflections would extend the book to too great length. Besides, they are so readily sug- gested to the mind of both teacher and scholar, that it is entirely un- necessary for the author to dwell on them. I have treated of some subjects, on which, from the difficulty of un- derstanding them, there has been a disposition in many minds to go be- *-ond what we know, and indulge in unwarranted speculation. On these PKEFACE. XI points I have taken pains to draw the line very distinctly between what is known, and what is supposed. I deem it to bo important to prevent the minds of the young from being led away from the simple truths of science by ingenious speculations and plausible reasonings. Let me not be understood to decry all hypothesis. I only object to the mingling of facts and suppositions together in one indiscriminate mass, as is often done. The disposition to do this, which is more common than is generally supposed, exerts so injurious an influence upon the habile of the mind, and so confuses its views of truth, that we ought to look upon it as one of the most serious evils to be guarded against in education. It is really one of the most prominent obstacles to the progress of truth on all sub- jects, both in individual minds, and in the minds of the community at large. This disposition, so apt to be fostered in the enthusiastic mind of youth, by ingenious but dreamy speculations, should be corrected at tho outset, and the mind should in its forming stage, ba habituated to the dis- crimination between the proved, the true, and that which rests on pre- sumptive, perhaps merely plausible evidence. This discrimination should therefore be exemplified in books designed for instruction, and this I have attempted in the present volume. I have divided the book into Three Parts. The First, which I have made as short as possible, is merely preliminary to the consideration of the particular subject of the book. In the Second Part, I present the human structure, simply as a structure, and show how it is constructed and kept in repair. In the Third Part, I treat of all those subjects which relate to the uses for which the structure is designed. This natural division of the whole subject, not only presents it to the mind of the student in an interesting point of view, but secures that natural grada- tion in its development, which I have spoken of as being necessary to a clear understanding of its deeper and more intricate portions. NOTE. SINCE the book was first published, the Author has, in obedience to the requests of many Teachers, added a chapter on HYGIENE, and also an APPENDIX containing questions. A full INDEX is also subjoined. PHYSIOLOGY PART FIRST, CONTAINING, CHAPTER I ORGANIZED AND UNORGANIZED SUBSTANCES. CHAPTER II Tmt DisnNcnorr BETWEEN ANIMALS AND PLANTS. CHAPTER III. MAJT IN HIS RELATIONS TO TIM Tmir* KINGDOMS or NATURE. CHAPTER I. ORGANIZED AND UNORGANEED SUBSTANCES. 1. THE crystal and the plant are both wonderful growths. As you look at them, you think -of the crystal as having been formed, and of the plant as having grown. But in one sense they have both grown to be what they are. The crystal was once a minute nucleus, and the plant was once a little germ. 2. In one respect they are alike in their growth both have increased from particles taken from things around them. But the processes by which this is done are different in the two cases. The crystal has increased or grown by layer after layer of particles. There are no spaces or passages by which parti- cles of matter can be introduced inside of it. Any part of it, when once formed, is not altered. It can receive additions upon the outside alone. But it is not so with the plant. This enlarges by particles which are introduced into passages and interstices. It grows, as it is expressed, by absorption or by in- tussusception. 3. How, now, is this absorption effected? It is done by cer- tain vessels or organs, constructed in the root of the plant for this purpose. These take up or absorb fluid matter from the earth. There are other organs which circulate this fluid through all the plant ; and others still which use it for the purpose of growth or formation. There are no such organs in the crystal, for it has no inner growth. The plant is therefore said to be an organized substance or being, and the crystal is an unor- ganized substance. And so we speak of the organic structure, or the organization of plants. 14 HUMAN PHYSIOLOGY. Organized beings. Mechanical, chemical, and vital principles. 4. These organs, which thus absorb, and circulate, and con- struct, do not act simply on mechanical principles. The plant is not merely soaked with fluid, which the heat of the sun may expel, as it does water from a porous mineral substance. These organs are active agents, and they perform their duty with a force, and after a manner, for which no mechanical principles can account. No mechanical powers could alone supply the leaves of the mighty tree of the forest with sap from its deep roots ; much less could they form these leaves. 5. Neither do these organs act simply on chemical princi- ples. While man, through the agency of chemistry, can form some of the crystals which are found in nature, he can not by any arrangement of constituents make a plant, a flower, or a leaf. And the plant, left alone to the action of chemical prin- ciples, wilts ; and at length ceases to be a plant, and becomes common unorganized matter. 6. Mechanical and chemical principles, it is true, are both employed to some extent in the growth of plants ; but they are under the control of other principles, which we term vital. And so we speak of the plant not only as an organized substance, but as a living being. 7. What I have said of plants, in distinction from minerals, may also be said of animals. They are also organized living beings, and they have generally a more complex organization than plants, as you will see as I proceed. 8. The whole material world, then, that we see around us, we divide into two parts the unorganized and lifeless, and the organized and living. The distinctions thus pointed out be- tween organized and unorganized matter are essential and fundamental. But let us look at some other distinctions, which either arise from these or accompany them. 9. One distinction is this. All the parts of the mineral are independent of each other, while it is otherwise with the plant or the animal. Accordingly, we examine the properties of min- erals in a different way from those of plants and animals. The chemist can ascertain all the properties of a crystal or a rock, if you give him but a small piece of it. But the botanist can not ascertain all the properties of a plant by looking at some one part of it. If he examine the flower, this gives him no knowledge of the root. In order to know all about the plant, he must examine every part by itself, and then look at it in its relations to the other parts. The same can be said of tha physiologist, in his investigation of the properties of animals. ORGANIZED AND UNORGANIZED SUBSTANCES. 15 Assimilation in organized substances. 10. As the crystal is forming by layer after layer of particles, no change is effected in these particles as they are becoming arranged in the layers. But in the case of the living organ- ized being, a change is produced in the particles which are taken up by the absorbents. And the change, ordinarily, is both a gradual and a complex one. In the plant, a change is produced in the particles in the very act of absorption; but this change is only the beginning of a process which is after- wards perfected. The sap is not thoroughly fitted for nutrition when it first begins to circulate. It is carried up through the vessels of the trunk or stalk to the leaves. There the last step of the process is taken, and the sap is now ready to be used in the growth of the plant or tree. So, also, in the animal, the nutritious part of the food, taken up by the absorbents in the digestive organs, is first acted upon by certain little glands, through which it passes, is then poured into the circulation, to be mingled with the blood, and is carried with the blood to the lungs, to be exposed to the air ; and thus it is fitted for the nu- trition or growth of the body. This process, which is thus car- ried on in the plant and in the animal, is very properly called assimilation. For the particles that are taken up by the ab- sorbents in the root of the plant are, by this process, made like to the plant ; and the particles taken up by the absorbents in the stomach * are made like to the animal. So obvious is this, in the case of the animal, that some French physiologist speaks of the blood as chair coulante, or running flesh. 11. Another prominent distinction between organized and unorganized substances is in relation to permanency. Constant change appears in all organized bodies ; while permanency is written upon all substances which are unorganized. In organ- ized beings, continual change is going on at every point. It is a condition of their being. This is true, not only of the de- cline of a plant or animal, but even of its growth. For, in its growth, as the parts enlarge internally as well as externally, they change not only the arrangement of the particles, but, to a great extent, they change the particles themselves. It is * The word stomach requires some little explanation, as it is used in physiology in two senses in a limited sense, and also in an extended one. It is used in its limited sense, as referring to the cavity at the beginning of the alimentary cann/, as it is termed; this lat- ter term being applied to the series of cavities, the stomach and the small and large, intes- tines, which are fuuod in the digestive apparatus in the higher orders of animals. In comparisons, however, between these animals and those which have a more simple digest- ive apparatus, the word stomach is used in a more extended sense, as being synonymou* with the term alimentary canal. It is used in this sense, also, when, as in the present oase, it is referred to in a comparison between animals and vegetables. 16 HUMAN PHYSIOLOGY. Organized substances changing. Unorganized permanent. true, as well of the towering tree as of the tiny plant, that these changes have been going on during all its growth ; so that, at its maturity, it is, both in relation to the arrangement of its particles, and in relation to the particles themselves, a very different thing from what it was when it pushed its germ up through the ground, or even when it was but a small tree. Not only has it received into its interstices and passages new particles, but it has thrown off from the pores of its leaves, those outlets for the refuse of plants, vast quantities of parti- cles which are no longer of use in its structure. So, in all animals, the same internal changes are going on, and to a much greater extent ; because, from the activity of their na- ture, there is more of wear and tear, and, therefore, more of refuse matter to be disposed of. As you will see in another part of this book, the human body, that most complicated of organized beings, undergoes these changes very largely. 12. It is not thus with unorganized substances. The crys- tal, so fast as it is formed, becomes permanent. No changes occur within it. In itself, it is unchangeable. It can not change its own particles, as the plant or the animal does. It can be changed only by external addition, or by external dimi- nution, through the influence of agents acting upon its surface. 13. With the constant changes going on in organic nature, there is constant succession. Plants and animals produce other plants and animals, and themselves die, making room for their successors. But the crystal does not form other crystals, and then crumble into dust. In itself, it is both unchangeable and unproductive. 14. This distinction between organized and unorganized sub- stances, in relation to change and succession, meets the eye everywhere. The mountains, the rocks, and even the stones under our feet, remain the same year after year, while all vege- table and animal life is ever changing its forms and manifesta- tions. There are the changes of growth, and the changes of decay and death, all around and within us; and they are strangely mingled together. There is death even in the changes of life, as the waste particles are taken away, and are replaced by the new ; and life springs out of the very bosom of death, as from decayed nature new forms of vigor and beauty arise. The mountains stand as they have stood, as the passing generations have looked upon them, while the continual changes of vegetation have been going on upon and around them. The seasons crown their battlements with the emblemi ORGANIZED AND UNORGANIZED SUBSTANCES. 17 Different forms of organized and unorganized substances. of their ever-returning mutations of life, decay, and death; and even the mighty trees, that have shed their leaves from year to year, in obedience to the great law of change, but have themselves stood, at length bow their heads to the same law, and give place to other lords of the forest. From the " ever- lasting hills," which thus remain the same, though change is ever about and upon them, man gets the unchangeable and imperishable rock to construct his habitation, while he himself is changeable and perishable the creature of a day, whose life is as a vapor. He wears the precious stones, and traffics in the golden ores, which have existed from the creation of the world, through all the changing and dying generations, and passes away, leaving them to others as changeable and perish- able as himself. 15. Another distinction between organized and unorganized substances relates to the forms which they assume. There is regularity in both, but it is different in each. Unorganized matter is disposed to arrange its particles in straight lines, and with angles mathematically exact. You see this in the beautiful crystal ; and you also see it, less definitely, but magnificently, displayed in the regular battlements and columns of rocks and mountains. The tendency is to regularity ; and irregularity is the result of interfering circumstances. A similar disposition to regularity is manifest in organized substances, but in a different manner. It is disposed to curved, rather than straight lines, and seldom makes lines or angles with mathematical ex- actness. We see this .law of regularity exemplified both in animal and vegetable life. The leaf, for example, has the same general shape, that is, the same general arrangement of par- ticles, when it attains its full size, that it had when it was small ; and the same can be said of the arm of the man, com- pared with his arm when a child. Illustrations might be cited to any extent, but these are sufficient. 16. While the law of regularity is not commonly as exact in organized substances as it is in the unorganized, it is quite as authoritative. While it does not ordinarily observe the per- fectly straight lines and the unvarying angles which we always find in the crystal, the general plan and contour are very strictly preserved amid all the changes of animal and vegetable life. And, in some cases, the same mathematical exactness that we find in the mineral world is found in organized beings. I know not that this is ever true of straight lines and angles ; but it is often true of curved lines. There are many very 2* 18 HUMAN PHYSIOLOGY. Regularity in form in some cases wonderful. beautiful examples in the vegetable world. I will give but a single one. If you look at the common 'white daisy, before tho hundreds of little buds in its bosom have opened into tiny flowers, you will see them arranged with great exactness in crossing curved lines, such as you often see on the back of a watch case. A similar arrangement you will find in many flowers. 17. This regularity is more wonderful in organized sub- stances than in the unorganized, because it rules in them in the midst of constant change. In the case of the crystal, as there are no internal changes in it, and as each layer of it, when formed, is permanent, regularity is comparatively, so to speak, easily secured. But in the case of the leaf, as it is growing to its full size, and of the arm, as it grows from infancy to be the stalwart arm of manhood, continual change is going on at every point ; and regularity here is obviously a more difficult achievement. 18. This regularity appears still more wonderful, when we look at the infinite variety of forms in organized matter, in both the vegetable and the animal world. In all these forms, each part of every animal and of every plant maintains its own peculiar plan and contour. Take, for example, the leaf in its endless varieties. How definitely does each variety preserve its individual character, and how easily is it distinguished from every other variety ! The same can be said of every part of every organized being. 19. Another circumstance still must be mentioned, as adding to the wonderfulness of this regularity. It has been scrupu- lously maintained, through all the changes of the world from its creation, when God pronounced the works of his hands to be "very good." The leaf of every tree, for example, is like the leaf of its ancestral trees back to that time ; and so it will be in all its successors to the end of the world. " The trees of the garden," which delighted the eyes of our first parents, and refreshed them with their shade in their innocence, and amid which they hid themselves after their sin from the presence of their Maker, undoubtedly had the same characteristic shapes, and the same leaves and flowers which their successors present to our eyes. 20. Again, it is interesting to notice that, in the midst of this regularity, so strictly maintained in each specific form from age to age, there is a measure of irregularity allowed. While each kind of tree, for example, has specific characteristics in ORGANIZED AND UNORGANIZED SUBSTANCES. 19 Variety of form ; yet regularity preserved. Size. the arrangements of branches and other parts, and in the shapes of "its leaves, no two trees of the same kind are exactly alike, and there is always much variety in the leaves of the same kind. The wonder is, that so much latitude is allowed in this respect, and yet the specific characteristics of each kind are so thoroughly preserved. We can readily see that if a pattern, definite in all its details, were to be copied exactly in each kind of vegetable and animal form, the distinctions between them could be more easily preserved. But Omnipotence is able to combine a wide latitude and variety of form in each kind, with a strict and uniform preservation of its characteristic contour and arrangement. We have a striking exemplification of the above remarks in the variety of the human countenance. While the face of man is so entirely different from the face of every other animal, at the same time, among the hundreds of millions of the human family, how uncommon it is to find two faces that are very nearly alike. 21. In the animal world, we see remarkable examples of the preservation of regularity of form in the exact correspondence which exists so commonly between the two halves of the body. For example, the brain has two halves, which are precisely alike, and the same is true of the nerves which are distributed from it. And so of other parts. But, mingled with this symmetri- cal arrangement of parts, there are other parts which are irreg- ular in their shape. This is the case with the stomach, the heart, the liver, ' ORIGINS OP LACTEALS. SECTION OF INTESTINE SHOWING THE LACTEALS. thoracic duct. This duct, which is about the size of a common quill, running up on the left side of the aorta, the great artery of the heart, pours its contents into the junction of two veins at the top of the chest. As the circulation of the chyle in the DIGESTION. 59 Mechanical contrivance of the thoracic duct. Chyle makes blood. thoracic duct needs all the mechanical help that it can have, the mode of the joining of this duct with these veins is calculated to facilitate the freeness of the discharge of the chyle. As the two large currents in the veins, v and v, v, in Fig. 18, FIG. 18. JUNCTION OF THE THORACIC DUCT WITH THE VEINS. unite, there is created, by the forward motion of these cur- rents, a tendency to a vacuum at the angle at which they meet, the point where the thoracic duct, T, D, opens. There is, therefore, a suction power, as it is termed, exerted upon the fluid in this duct. The chyle, thus mingled with the blood, becomes a part of it. Or rather, I should say, that the blood is made from the chyle, and, as it is constantly used for formation and repair in all parts of the system, it is thus as constantly replenished. The material by which all the textures of the body are made and are kept in repair, is furnished to the system through this small duct, in the form of a milky fluid. You observe in Fig. 17, certain lymphatic vessels. These are trunks of absorbents, hereafter to be spoken of particularly, which bring a fluid called lymph, to be mingled with the chyle, and to be poured with it into the circulation. 92. The extent of surface on which the absorbent lacteals open can not be appreciated, if you look merely at the outside of the small intestines. It can be done only by looking at the inner mucous coat. This coat is really much more extensive than the outer coat, or the middle one, the muscular, arid it is full of folds, as represented in Fig. 14, on page 52. The ob- 60 HUMAN PHYSIOLOGY. Extent of absorbing surface in intestines. Alimentary canal in different animals. ject of this is to offer a very large absorbing surface to the chyme as it passes, and also to prevent its passing along as rapidly as it would if the mucous surface were perfectly smooth, instead of having folds. Before leaving this subject, I would again call your attention to the analogy which exists between absorption in animals and in plants. The lacteals do for the animal in its stomach, what the absorbents do for the plant in the extremities of its roots. Both absorb and assimilate nutri- ment. The function is the same. It differs in the two cases only in the circumstances under which it is performed. 93. The digestive apparatus varies much in different animals, according to the kinds of food on which they live. As a gene- ral rule, the more the food differs in character from the animal itself, the more complicated and extensive is the apparatus. Thus, the herbivorous animals have a very long alimentary canal, and the beginning of it, the stomach, is a complicated organ. While, on the other hand, in the carnivorous, the flesh which they eat being very much like their own flesh, and, there- fore, not requiring very much of a process of assimilation, the stomach is a simple organ, and the alimentary canal is very short. In the sheep, for example, the alimentary canal is about, twenty-eight times the length of the body, but in the lion it is only three times its length. In man, who lives on a mixed diet, the alimentary canal is about six times the length of the body. 94. The stomach is more complicated in animals that chew the cud than in any other animals. It has four distinct cavities, and, as you will see, a singular mechanism is called into opera- tion in managing the food as it passes through them. In Fig. 19, you have a representation of the stomachs of the sheep, as they appear exteriorly. The course which the food pursues is this. As the animal crops the food, it passes into the first sto- mach, which is little else than a great reservoir to hold it and to soak it. Then it passes into the second stomach, from which it is returned into the mouth. On being swallowed again, it passes from the oesophagus into the third, and thence into the fourth stomach. In Fig. 20, you see the interior of these four stomachs ; and by the aid of this I will describe the process of digestion in the sheep more particularly. You see the very large first stomach, or paunch, in which the food is accumu- lated. It is not yet masticated thoroughly, for the animal has swallowed it as fast as he could, and packed it away in this reservoir. From this it is passed, in small quantities at a time, DIGESTION". Digestion in the sheep. OESOPHAGUS. ORIFICE OF STOMACH. 3D STOMACH. FIG. 19. STOMACHS OF THE SHEEP. FIG. 20. INTERIOR OF THE STOMACHS OF THE SHEEP. into the second stomach, the honey-comb, so called from the peculiar network of folds in it. Here the food is rolled up into balls by the action of the muscular fibres in this network. 6 62 HUMAN PHYSIOLOGY. Digestive apparatus in birds. Different in the grain-eating and the flesh-eating. Each ball of food is passed up through the oesophagus into the mouth, where it is chewed and thoroughly mixed with the saliva, in doing which the animal seems to have great enjoyment. Then it is swallowed, and, as it passes from the oesophagus, in- stead of going into the paunch, as it did when swallowed the fhst time, it is directed through the groove seen in the Figure into the third stomach, the manyplies. This has many folds, like the leaves of a book, so that the food is exposed to a large surface in this cavity. It passes from this to the fourth sto- mach, the reed. Here, and here only, it is acted upon by the gastric juice. This, therefore, is the true stomach, all the other cavities furnishing only preparatory steps to the true process of digestion. It is from this fourth stomach that what is called the rennet is taken. When fluid matter is swallowed, it goes directly into the second stomach, and not into the first, the paunch ; so that, in the case of the sheep, the drink goes one way, and the solid food another. And, what is still more singu- lar, while the animal is a suckling, the milk passes directly into the fourth stomach through the third, which has its folds so closed together as to form a mere tube to conduct it to its des- tination. And the great paunch and the honey-comb are wholly useless until the animal begins to crop its food for itself. 95. In birds, the digestive apparatus is necessarily very peculiar, from the fact that they do not masticate their food. They have, on this account, an arrangement in the stomach itself for grinding the food. In the cavity called the gizzard are two opposing surfaces, made very hard, so that by rub- bing together they bruise the grains ; and while they are thus ground, as between two millstones, the gastric juice is poured down upon them from above. This arrangement is seen in Fig. 21, which represents the digestive apparatus in the turkey laid open. At b is the gizzard, showing the two hard surfaces, which are rubbed together by the stout muscles that make the great bulk of the organ. Above, at a, are the glands which pour forth the gastric juice. And above this part of the stomach there is, in all grain-eating birds, a large sac bulging out from the oesophagus, called the crop, which is a reservoir for the food, just as the paunch is in the ruminating animals. In those birds that live on flesh or fish there is no such grinding apparatus; and the walls of the stomach are quite thin, and it presents no hard surfaces. 96. It would be interesting, were it consistent with the plau DIGESTION. 63 Pigestion in the turkey. Digestive apparatus in different animals. FIG. 21. STOMACH OF THE TURKEY. of this book, to go into a further examination of the varieties in the digestive apparatus in different animals. They have a very wide range, being according to the wants of the animal in each case. The kind of food, the mode of life, and the pur- pose which the animal is designed to fulfill, are the circumstances which govern these variations. The proportion which the di- gestive apparatus bears to other parts varies very much ; and in some of the lower orders of animals, the body seems to be all stomach. In such cases, the only appendages are those which seize the food and direct it into the orifice of this organ. This 64 HUMAN PHYSIOLOGY. Apparatus of the circulation. Heart, arteries, veins, capillaries. is the case with the hydra, represented in Fig. 1. And, what is very singular, the outside of the body of this animal is just as capable of acting as a stomach as its inside. For you may turn it inside out, as you can a stocking, and yet it will go on to catch and digest its food as usual. But, wide as the varia- tions are in the digestive apparatus of animals, the same com- mon object is aimed at in all the assimilation ( 10) of nu- trient substances to the animal, to produce a material from which its structure can be built and kept in repair. There is, therefore, much that is common to them all in the modes in which this object is accomplished. And even the analogy which exists between the animal and plant, in regard to assimi- lation, does not relate to the fact alone, but in some measure to the modes in which the process is effected. CHAPTER VI. CIRCULATION OF THE BLOOD. 97. IN the last chapter I described the manner in which the blood is made from the food. The blood, thus prepared, is circulated in every part of the body, that it may be used for the purposes of construction and repair. The apparatus by which this is done acts, as I have before said, as the common carrier of the material which is used everywhere in the body by the laborers, the builders, to whom it is thus brought. 98. This apparatus consists of several parts a great central organ, the heart, situated in the chest; the arteries, the tubes by which the blood is conducted to all parts of the body ; the veins, other tubes, which bring the blood back to the heart ; and capillaries, a network of exceedingly minute vessels, through which the blood passes as it goes from the extreme arteries into the beginnings of the veins. The blood goes from the heart through a large artery, called the aorta, which sends forth branches ; and these divide and subdivide, so that the extreme arteries, through which the blood flows into the capillary net- work, are very minute. And the veins which receive the blood from this network to carry it back to the heart, are equally minute ; but joining together more and more, as they proceed THE CIRCULATION. b'5 Heart n forcing and suction pump. Arteries firm tubes. Why. toward the heart, they are at length all united into two great venous trunks, one from above and the other from below, which pour their contents into this organ. The capillaries, taking their namev from the Latin word, capilla, a hair, are so small that the) 7 " can not be seen by the naked eye. In any small cut, the blood which oozes out comes from multitudes of these vessels. They serve to hold the blood, while the formative ves- sels, that construct and repair the body, may select from it such materials as they need for their purposes. 99. The heart is a great central forcing and suction pump, in the midst of this circulating apparatus. When it contracts, it forces the blood out through the aorta and its branching ar- teries into all parts of the system. And when it enlarges or dilates itself, it, by suction, as it is termed, receives the blood returning from the system through the veins. The blood never ceases to go these rounds. The necessity for this continual motion you will perceive as I proceed with the development of the subject. 100. The arteries differ from the veins in their structure and arrangement. The arteries are firm tubes, while the veins are lax in their structure. The object of the difference is obvious. As the blood is forced into the arteries by the powerful action of the heart, it is necessary that they should be strong and firm, else, they would be liable to dilatation and rupture, and death would frequently result. As it is, it is not a common event to have an artery dilate and burst, though it does occa- sionally happen. When dilatation does occur in an artery, it is called an aneurism. But the arteries need to be firm, not only for the sake of security against rupture, but also that the force of the heart may propel the blood to the extremities of the arterial system. If the arteries were lax tubes, like the veins, the impulse would soon be lost in the yielding tubes, and the blood would move very sluggishly in the small arteries at a distance from the heart. What we call the pulse, is caused by this impulse. If the arteries were lax tubes, the pulse would not be felt at any great distance from the heart. Instead of being distinct, as it now is, w^h every beat of the heart almost to the very extremities of the arterial system, it would be ren- dered confused by the yielding of the tubes, even quite near the heart, and at a distance from that organ it would be en- tirely lost. 101. Besides the firmness of the arteries, there is another circumstance which favors the freeness of the flow of blood 6* 66 HUMAN PHYSIOLOGY. Different arrangement of arteries and veins. through them. It is their mode of division. The branch of an artery leaves the main trunk at a sharp angle, making thus only a slight deviation from the direction of the current ; while, on the other hand, in the veins where the current flows in an opposite direction, the branch unites with the trunk at nearly a right angle. This difference is represented in Fig. 22 ; 1 being the artery, and 2 the vein. FIG. 22. ARTERY AND VEIN. 102. The venous system has a much greater capacity than the arterial. That is, all the veins of the body are together ca- pable of holding more blood than all the arteries are. And the blood moves very rapidly and directly from the heart through the arteries, but it conies back to the heart quite slowly through the veins. Every thing is arranged to promote this rapid cir- culation through the arteries, while the venous system is calcu- lated for a slow but sure progress of the blood back to the heart. To secure this, valves, made of folds of the inner lining of the veins are so arranged as to prevent the blood from flow- ing in the wrong direction. Fig. 23 represents a vein cut open so as to show these valves. A shows the valves as they appear when the vein is laid open and spread out ; B, as they appear when the vein is simply laid open ; and C represents the ap- pearance of the outside of the vein where there are valves. THE CIKCULATION. 67 Valves in veins. Dangerous to wound an artery. Therefore well guarded VALVES IN THE VEINS. The need which there is of this help to the circulation through the veins is obvious. The suction power of the heart is not competent, unaided, to move the blood throughout all the lax venous system. These pocket-like valves, therefore, are made in the veins to assist the circulation there. They do so in this way. Every motion of the muscles or other parts about the veins tends to keep the blood in motion, and the valves serve to prevent this motion from being in the wrong direction. The difference in force and velocity with which the blood moves in the arteries and in the veins, is made manifest when they are wounded. The blood flows from a wounded vein in a slow and steady stream. From an artery it flows rapidly, showing the impulse of the heart in its jets, which correspond exactly with the pulse. Hence comes the danger in wounding an ar- tery, while the wound of a vein is ordinarily attended with no danger. Accordingly, we find that the "Maker of our bodies" has so placed the arteries that they cannot easily be wounded, while many of the veins are quite freely exposed. The arteries are deeply seated, except in some few cases where this is im- possible ; but the veins are often superficially situated. You can see this, for example, in the bend of the arm. Some large veins appear there just under the skin, while the artery which supplies the arm is imbedded among the muscles and tendons. In every part of the body, the most secure spot is chosen for an artery. Thus, at the knee joint, the artery, instead of run- ning over the surface of bone, where it would be liable to be 68 HUMAN" PHYSIOLOGY. Few arteries superficial. Mode of stopping the bleeding of an artery. wounded, lies deep in the ham at the rear of the joint The same is true of the elbow joint, just alluded to, and of other parts of the body. Although there are arteries everywhere, they are so uniformly deeply seated, that it is only in a few lo- calities that you can readily find one. You can feel one pul- sating at the wrist, and also on the temple. Here the arteries are superficial, only because it is impossible that it should be otherwise. 103. When the physician bleeds a patient, he commonly does it at the bend of the arm, as being the most convenient place for the operation. A ligature of some sort, as a ribbon, is tied around the arm above the elbow, with sufficient tightness to interrupt the flow of blood toward the heart in the super- ficial veins, but not so tightly as to prevent the free supply of blood to the arm by the artery. It is commonly tied as tightly as it can be without stopping the pulse at the wrist. An open- ing is then made in one of the veins ; and, as the blood flows freely into the arm from the heart through the artery, on its return, so much of it as passes through the opened vein is dis- charged at that point. 104. It will be proper here to give some practical instruc- tion, in regard to stopping the flow of blood from a wounded arterv, as many lives have been lost from the ignorance of by- standers when such accidents have happened. Enveloping the part in cloths, which is so commonly done at such times, does no good, but only serves to catch and conceal the blood as it flows. Pressure upon the artery, on that side of the wound which is toward the heart, will of course interrupt the supply of blood from this organ to the wound. Firm pressure with the thumb will do it. But the pressure must be made at the right point, that is, directly upon the artery. You may not, in all cases, press upon the right spot at once. If you do not, the blood will continue to flow. In this case, press at different points, until you find the point at which you see that pressure stops the flow of blood from the wound. But you may not be able to find the right spot. If you can not, you can *ie a slip of strong cloth or a handkerchief around the limb, above the wound, and twist a stick in it until the bleeding stops. In one or the other of these ways, you can prevent the loss of blood until the surgeon arrives to take charge of the case. 105. Although there is no such free communication between arteries as exists between the capillaries, there is some amount of communication, and particularly in certain parts of the body. THE CIRCULATION. 69 Aneurism. Communication between arteries. And it is well that it is so, for it sometimes helps the surgeon to save a lirnb, when he could not do it if there were no com- munication. I have already alluded to a disease of the arteries called aneurism. An artery has three coats, one of which is a strong fibrous one. When this is thinned or ruptured, the other two coats bulge out, forming a pulsating tumour. And, as the blood is constantly pumped into this by the force of the heart, it enlarges, and at length it may burst, and the life of the patient will be destroyed by the loss of blood. When an aneurism formed in a limb, as for example in the ham, the sur- geon, in former times, used to save the life of the patient by amputating the limb above the aneurism. Putting a ligature round the artery above the aneurism would of course stop the flow of blood into it ; but it was supposed that the limb would die, in that case, from the want of a proper supply of blood. But it was found, at length, that this was not so ; and surgeons now, in such cases, cure the disease, and save the limb too, by tying the artery. Immediately after the operation the limb is cold, and there is plainly very little circulation in it. But in a few hours the circulation becomes free, and in a little time it is as well established as ever. This is effected by the communi- cations which exist between the branches which go off from the artery above the aneurism, and those which go off below it. It is obvious, however, that this would not be thoroughly effected if no change took place in the size of the communicat- ing arteries. But this change does occur. Some of them be- come enlarged to meet the necessity of the case. This is a most interesting fact ; and so is also the fact, that these commu- nications between branches of arteries are very common in the neighborhood of those places in the body, where aneurism, from strains produced by violent and sudden motion, is peculi- arly apt to appear. This same provision avails, of course, when aneurism is cured by pressure made upon the artery above it, a measure which modern surgery has found in many cases to be as effectual as tying the artery. 106. There have been great differences of opinion among physiologists, in regard to the proportionate amounts of agency that the different parts of the apparatus have in carrying on the circulation. The heart manifestly exerts the chief agency, both by its forcing and its suction power. You can get a clear idea of the manner in which it exerts these two forces in this way. Fill a ball of India rubber, to which a tube is attached, with water, and immerse the tube in water in a vessel. If you 70 HUMAN PHYSIOLOGY. Action of the heart illustrated. Agency of the capillaries in the circulation. press the sides of the ball together, some of the water is forced out into the vessel. This represents the contraction of the heart. If, now, you allow the ball by its elasticity to resume its round shape, the water rushes into it from the vessel. Tnis represents the dilatation of the heart. The dilatation of the ball results from its elasticity ; and so it is supposed by some that the dilatation of the heart results from the same cause, its contraction alone being produced by muscular action. Whether this be so or not, the dilatation is an active one, and the blood rushes into the heart from the veins by suction, as it is termed. The dilatation is so active that, as has been shown by experi- ments on animals, even a great amount of pressure is not able to prevent its taking place. 107. But, great as the agency of the heart is, it is not true that it is the only moving power, and that the arteries and veins are mere passive conducting tubes. There are various phenomena which show that the arteries, the capillaries, and even the lax veins, exert a considerable agency in circulating the blood. I will merely allude to some of these phenomena. Determina- tions of blood to particular parts show that the blood-vessels have an active agency in the circulation. In inflammation of any part, there is an increased activity of the particular portion of the circulating apparatus supplying that part. In the act of blushing, there is a local activity of the circulation somewhat independent of the heart. This is also true of the circumscribed flush of hectic. 108. There is one portion of the circulation in which the active agency of the capillaries is especially manifest. The veins, as I have told you, receiving the blood from all parts of the body, at length are all united into two veins, which empty their contents into the heart. But there is a very remarkable exception to this. The veins which collect the blood from the viscera in the abdomen unite in one large trunk, called the vena portae ; and this, instead of pouring its contents into the large vein that goes up to the heart, divides, like an artery, into branches, which take all this blood to the liver for the manufac- ture of bile. Fig. 24 represents this circulation of the vena porta3. 1, 1, are the veins coming from the intestines ; 2 is the trunk of the vena portse ; and 3, 3, are the branches of it dis- tributed in the liver. Now, it can not be pretended that the suction power of the heart extends its influence through the veins that bring the blood from the liver, then through the capillaries of this organ, and then through all the veins that bring the THE CIRCULATION. 71 Circulation in the liver. Why the veins are full and the arteries empty after death. 1 CIRCULATION OF VENOUS BLOOD IN THE LIVER. blood to the liver, even to the capillaries of the abdominal vis- cera. There must be, in this case, some propelling power in the capillaries, and some, too, also in the veins. If there were not, another subordinate heart would obviously be needed in the vena portae, to pump up the blood from all the veins of the abdominal viscera, and then to send it through all its branches into the capillaries of the liver. 109. The veins have a less active agency in the circulation than any of the other parts of the apparatus. It is for this reason that commonly after death the veins are found quite full of blood, while the arteries are nearly empty. The appa- ratus of the circulation may be regarded as forming a circle of organs in this order the heart, the arteries, the capillaries, and the veins. The blood is constantly going the rounds of this circle. It is plain that, as the apparatus is about to stop, there must be an accumulation in the weakest, least active, and most relaxed of this circle of organs. The arteries and capillaries force the blood into the veins to the last moment of life. This effec* 72 HUMAN PHYSIOLOGY. The blood changed in the capillaries from red to dark. probably extends no further than the smaller veins ; but the heart, by its active dilatation, draws the blood from them into the larger veins. And as these two forces, at the two ends of the venous system, are at work up to the last moment, the whole of this system is filled with blood. 110. The fact, that the larger arteries are commonly found nearly empty of blood after death, gave the ancients the idea that air circulated in arteries, while blood circulated in veins. Hence, the name, artery, is derived from two Greek words, sig- nifying to hold air. And hence, also, by long established cus- tom, in common language, the blood is spoken of as running in our veins; and it would sound strangely, if, in common, and especially in poetical language, we should speak of it as running in our arteries also. Although there were from time to time some glimpses of the true idea of the circulation, it was not really developed and demonstrated till about two hundred and thirty years ago. Harvey spent eight years in maturing his ideas on the subject. When he published them, they en- countered much opposition ; but he lived long enough to see them almost universally received by the medical world, although the profession was in a much less enlightened state than it is at the present day. 111. I will now take you a step farther in the development of the plan of the circulation. I have said that the office of the arteries is to conduct the blood to the network of capil- laries, and that in the capillaries the blood has reached its place of destination where it is to be used. The formative ves- sels, appended to the capillaries, take from the blood what they need for their various purposes, and at the same time there is added to the blood refuse matter from the waste of the tissues. The blood, then, is changed while it is in the capillaries. You see the change in its color. In the arteries it was red ; but, after passing through the capillaries, it appears in the veins of a purple color. It is also as much changed in other properties. It is no longer fitted to nourish the body. It would even prove a poison to any organ if it should flow into its capillaries. If it should, for example, be sent to the brain, instead of bright ar- terial blood, that organ would cease to do its office ; insensibility would ensue, and life would soon be destroyed, if the flow of red blood could not be established. 112. This purple blood, which comes back to the heart from the capillaries by the veins, must, therefore, be in some way changed to red blood, before it is again sent all over the system THE CIRCULATION. 73 Change in the blood in the lungs. Course of the circulation through the arteries. This change is effected in the lungs. As the purple blood returns to the heart, it is sent by the heart to the lungs, in order to be exposed to the air before it is sent again over the system. For this purpose there are two circula- tions, and the heart is a double organ ; or rather, there are in effect two hearts for the two circulations, for the two sides of the heart have no communication with each other. The appa- ratus for all this is very complicated, but I think it can be made clear to you. 113. I present, first, a diagram, which is intended to repre- sent merely the course of the circulation, without regard to proportionate size, or to minutiae in the arrangement of the ap- paratus. Let a represent the right side of the heart, c the left side, b the lungs, and d the general system of the body. The arrows show the direction in which the blood flows. In all the shaded part the blood is venous or purple, and in the part not shaded it is arterial or red. We will now take some point of beginning, and trace on the Figure the course of the circulation. PIG. 25. DIAGRAM SHOWING THE COURSE OF THE CIRCULATION". We will start at a, the right side of the heart. The blood re- ceived here, of a purple color, from the whole body by the veins, is sent by the heart to ft, the lungs. Here it changes to red blood, and passes by veins back to the heart but, observe, it is to the left side of the heart, c. It is now sent by this left half of the heart to all parts of the system, represented by d. Here, in the capillaries, it is changed to purple blood, and goes back by veins to the right side of the heart, a, the place where we started. 7 74 HUMAN PHYSIOLOGY. Two circulations and two hearts. Arrangement of valves. 114. You see, then, that there are two separate circulations, one through the general system, and the other through the lungs alone. In both circulations the blood is sent from the heart by arteries, and is brought back to it by veins. But notice that, while in the general circulation the red blood is in the arteries, and the purple in the veins, in the circulation through the lungs it is reversed the red blood is in the veins, and the purple is in the arteries. So, also, while the change of the blood in the capillaries of the general system is from red to purple, in the capillaries of the lungs it is from purple to red. 115. There are not only two sides or halves of the heart, separated entirely from each other, but each of these sides has two apartments, with valves or folding doors between them, so arranged that the blood can pass one way through them, but not the other. There are also valves at the beginning of the great artery of the heart, the aorta. These are so arranged that the blood can go freely out of the heart into the artery, but not a drop can get back from the artery into the heart. There are similar valves, also, at the beginning of the great ar- tery, by which the purple blood is sent from the heart to the lungs. 116. In Fig. 26, is represented a section of the right side of the heart, for the purpose of giving you an idea of the arrange- ment and the relative size of the two apartments. The auricle, a, so called because a part of it has some resemblance to an ear, receives the blood from the whole system by two large veins, 6, b, called the venae cavte. From the auricle it passes into the ven- FIG. 26. tricle, v, which by its contractions sends a it to the lungs through the pulmonary artery, /. The valve between the au- ricle and ventricle is composed of three membraneous sheets, which are held at their edges by small tendinous cords, o?, just as a sail is held by the ropes at its corners. This valve permits the blood to pass from the auricle into the ventri- cle ; but when it attempts to pass back from the ventricle to the auricle, it SECTION OF THE RIGHT pushes back the sheets of the valve, they SIDE OF THE HEART. being prevented from going too far back by the tendinous cords. There are also valves at e, the beginning of the pulmonary artery, which allow the blood to pass through THE CIRCULATION. 75 Relation between the auricles and the ventricles. them into the artery, but no blood can pass through them from the artery back into the ventricle. I shall soon call your atten- tion again to these different valves, that you may see more par- ticularly their structure and arrangement. 117. The auricle and ventricle act in this way in propelling the blood. When the auricle contracts, the ventricle dilates * to receive the blood from the auricle. The valves between them are open while this is taking place. But the next moment the ventricle contracts and the auricle dilates. You at once see, that if now the valves between them should be open, the blood would be forced back into the auricle. But the membranous sheets of these valves shut upon each other as the ventricle contracts, and thus prevent the blood from going back. It therefore is discharged through the pulmonary artery, /, the valves there being open. And when the ventricle dilates, you can see that the blood would, from suction, enter it from the artery as well as from the auricle, if the valves at the orifice of the artery should remain open. They are accordingly shut when the ventricle dilates. You see, then, that when the auricle dilates and the ventricle contracts, the valves between the auricle and ventricle are closed, and the valves at the mouth of the pulmonary artery are open ; and, on the other hand, when the ventricle, dilates and the auricle contracts, the valves between them are open, and the valves of the pulmonary artery are closed. 118. Dr. Carpenter has a very good illustration of the rela- tion of the actions of the auricle and ventricle, in a representa- tion given in Fig. 27. The apparatus which is represented consists of two pumps, a and 6, the pistons of which move up and down alternately. These are connected with a pipe, c, /, in which there are two valves, d and e, opening in the direction of the arrows. The portion c of the pipe represents the venous trunk discharging its blood into the heart, and the portion /, the artery which is the outlet for the blood. The pump, a, represents the auricle, and the pump, 6, the ventricle. When the piston in a is raised, the fluid enters through c to fill it by suction, as it is termed. When, now, its piston is lowered, the fluid is forced through the valve d into the pump 6, (which re- presents the ventricle,) whose piston is at the same time raised to receive it. And when the piston in b is lowered in its turn, * This dilatation is an active one, as was stated in 106, when speaking of the heart as a whole. The ventricle does not dilate because the blood is forced into it, but the blood rushes into it because it dilates. 76 HUMAN PHYSIOLOGY Ventricles larger and stronger than the auricles. Valves of the aorta. FIG. 27. a db the fluid being prevented from returning into a, by the closure of the valve d, is forced through the valve e into /, representing the discharging tube, the artery. At the same time, a fresh supply of fluid is received into a by the raising of its piston. 119. I have described the auricle and ventricle of one side of the heart, the right side. The left side is constructed very much in the same way. You will observe, in Fig. 26, that the ventricle is much more capacious than the auricle. The auricle is indeed the antechamber to the ventricle. The ventricle, too, you see, is much thicker in its walls. It is made very strong, because it does by far the principal part of the work. I remark here, in passing, that the size of the whole heart is about that of the closed hand of the individual. 120. I will now call your attention to a more particular view of the valves of the heart. We will take, first, the valves which are at the beginning of the aorta, the great artery of the body, going out from the left ventricle. These are very much like the valves of the veins seen in Fig. 23. There are three of them. They are like little pockets arranged around the ori- fice of the artery, and looking toward the tube of the artery. Of course, when the ventricle contracts, and forces the blood into the artery, these pockets are pressed by the blood flat against the sides of the artery. But when the ventricle dilates, and the blood attempts to go back from the artery into the ventricle, it gets into these pockets, and bulges them out toward the heart, and thus the mouth of the artery is closed. But you can see that if these pocket-like valves had plain curved edges, they would not effect a perfect closure. There would be a THE CIRCULATION. 77 Peculiar provision in the valves of the aorta. FIG. 28. little space in the very middle of the orifice of the artery which would be left open. This is made plain by Fig. 28, which pre- sents the orifice of the artery with its closed valves, as it would appear seen from the interior of the heart, if the three valves had plain curved edges. There would be a space left between them. But this difficulty is remedied by a very simple contriv- ance. A little fleshy projection is placed upon the middle point of the edge of each valve, of such a size that the three projections together just fill the space A. When, there- fore, the valves are closed, no blood can go back from the artery into the ventricle. This arrangement is shown in Fig. 29, in which the aorta, a, is laid open and spread out, so as to show the three valves with their projections on the edges. At 6 and c, are the openings of the two arteries that supply the walls of the heart FIG. 29. VALVES OF THE AORTA. with blood for their growth and repair, tor the heart is con- structed and repaired from its own blood. The valves at the orifice of the pulmonary artery are arranged in the same man- ner as those which are at the orifice of the aorta. 121. The valves which are between the auricles and the 7* 78 HUMAN PHYSIOLOGY. Arrangement of the valves between the auricles and ventricles. ventricles I have already partially described. They are folds of strong white membrane, their edges being held by numerous small tendinous cords. And these cords are manned, as we may express it, by muscles attached to the walls of the heart. The office of these muscles is to hold on to the cords that are fast- ened to the edges of the valves, and prevent these sheets of membrane from flapping back too far when the powerful ven- tricle contracts. It is by a nice adjustment of forces that these valves act with such exactness. They are of greater extent than the valves which are at the mouth of the aorta and the pulmonary artery, and, therefore, it would not do to leave them to act alone, as those valves do, upon simple mechanical princi- ples. The living muscular fibre must be introduced as the agent to control and regulate these principles in their applica- tion here. If it were not done, the consequence would be, that when the ventricle contracts with prodigious force, as it some- times does when the circulation is in a great state of excitement, the light tendinous fastenings would be ruptured by the pres- sure of the blood upon the valves. As it is now, the strong but yielding muscular bundles, to which these tendons are attached, regulate with great exactness the closing of the valves. Even if there were no need of any regulation, by muscular action, of the movement of these valves if the tendons would, in all cases, let the valves go back to just the right point as they are not extensible, and have no elasticity, it is manifest that there would be more danger of rupture than there is with the present arrangement. The tendons cannot be stretched, and, therefore, under great pressure they might break. In Fig. 30 is a representation of a portion of this valvular apparatus. The engraving was made from a drawing of the part taken from the heart, and pinned upon a board for the purpose. At m, you see the sheet of membrane ; o, o, are two of the muscles attached to the inside of the ventricle, to hold on to the ten- dons, A, that are fastened to the edge of the membrane. This membrane is now in the position that it is when the valves are open, that is, lying in a line with the little tendons and their muscles. But when the ventricle contracts, the blood, pushing against the membrane m, carries up the free edge to which the tendons are fastened, which, meeting the free edges of the other valves, closes with them the communication between the auricle and ventricle. 122. In looking at Fig. 26, you observe that, while there are valves between the auricle and ventricle, and at the mouth of THE CIRCULATION. 79 No valves at the openings of the venae cavse. Why this. FIG. 30. PART OF THE VALVULAR APPARATUS BETWEEN THE AURICLE AND THE VENTRICLE. the artery going out from the ventricle, there are none at the openings of the two ven<# cava, the veins that pour their con- tents into the auricle. Why is this ? Why is there no need of valves here to prevent a regurgitation into these veins when the auricle contracts ? It is because that, as the auricle con- tracts, there is at the same time the dilatation of the strong ventricle, making, of course, a suction in that direction so powerful as to counteract most fully any tendency to regurgita- tion into the veins. You readily see, that if the arrangement were reversed, and the auricle were stronger than the ventricle, then, when the auricle contracted, there would be regurgitatior into the vena? cavge, if there were no valves there to prevent it. The same remarks could be made in regard to the pulmonary veins, that pour their contents into the left auricle. 123. Having thus examined the heart in detail, you are now prepared to look at it as a whole. For this purpose, I present to you, in Fig. 31, a front view of the heart, in which a is the right auricle, receiving the purple blood from the whole body by the two large veins, h and i, called the ve nee cav and at the same time the body is made very light. And the heat gener- ated by the effort of flying must expand the air in the air-sacs and swell them out, and thus make the body lighter. In Fig. 47 is seen this arrangement of air-sacs in the ostrich. The lungs, /, , are quite small, but the air-sacs, c, c, c, are very large. The orifices by which they communicate with the lungs vou see FIG. 47. LUNGS OF THE OSTRICH. KESPIRATION. 101 Changes produced in the air in the lungs. in the Figure. In birds of great powers of flight, the air-saca are much more extensive. This arrangement of air-sacs in different parts of the body of the bird bears some analogy to the tracheae distributed in the bodies of insects. 148. You have seen that the object of the apparatus of re- spiration is to change venous blood into arterial, and you have also seen how the air is introduced to the blood in order to effect this change. And now the interesting inquiry arises, what are the actual changes which occur, both in the blood and in the air, in the lungs. If you take a tumbler filled with lime-water, and breathe into it through a tube, the lime- water will become turbid, and will soon deposit a sediment. This is chalk, or carbonate of lime, formed by the union of the carbonic acid gas exhaled from the lungs with the lime in the lime-water. Whence comes this carbonic acid gas, and how is it formed ? In order to answer this question satis- factorily, we must look at the chemical constitution of the air which we breathe. It is composed of two gases, oxygen and nitrogen. In every 100 parts of common air, there are 79 parts of nitrogen and 21 of oxygen. It is found that the oxygen is that constituent of the air which is necessary to life. If an animal be placed in a closed jar filled with com- mon air, he will soon die, and the oxygen will be found to have disappeared, while the nitrogen remains very nearly the same in amount. If, now, you place an animal in a jar of nitrogen, and another in a jar of oxygen, the one in the nitrogen will die immediately, while the other will be very lively until the oxy- gen is mostly used up by his lungs. The animal in the pure oxygen will breathe at first more rapidly than the animal in the jar of common air ; and it is thought that oxygen is too stimulating for the lungs, and therefore needs to be diluted with the nitrogen, as it is in the air that we breathe. 149. In the case of both the animal in the jar of air, and that in the jar of oxygen, carbonic acid is found to have taken the place of the oxygen which has disappeared. This gas is made by a union of oxygen with carbon or charcoal. It was formerly supposed that this union is effected in the lungs that carbon is thrown off from the venous blood in the lungs, and that the oxygen of the air there unites with it, and so car- bonic acid appears in the air expired from the chest. But it has been discovered that the exchange is made in a different man- ner. It is not made in the lungs. The oxygen is absorbed by the blood, and goes with it to the heart to be sent all over the 9* 102 HUMAN PHYSIOLOGY. Changes produced in the blood by the air. system. And it is in the capillaries that the oxygen unites with carbon to form carbonic acid. The union takes placa while the blood is changing from arterial to venous, and is an essential part of the change. The carbonic acid thus formed in the capillaries, is brought back to the heart in the venous blood, and is discharged from the system in the lungs. That the change takes place as stated has been abundantly proved in various ways. It has been found by experiments which I will not detail, that carbonic acid exists in considerable amount in venous blood ; while, on the other hand, there is much oxy- gen in arterial blood. The plain inference from this is, that oxygen unites with the blood as it passes through the lungs, goes with it to the capillaries, and there unites with the carbon, giving us the carbonic acid which we find in the blood in the veins, after it has passed into them from the capillaries. It has been found, also, that if frogs or other cold-blooded animals be placed in hydrogen or nitrogen, (gases which produce no in- jurious effect on them,) they will give off for some time nearly as much carbonic acid as they would have done in common air. In this case, as no oxygen is introduced into the lungs, the carbonic acid can not come from any union effected in these organs between carbon and oxygen, but it must be dis- charged by exhalation from the blood as it is passing through the lungs. Of course the discharge of the carbonic acid ceases after a little time ; for, there being no new supply of oxygen by way of the lungs, as there is when the animal is breathing com- mon air, there can be no new formation of carbonic acid. But even cold-blooded animals can not live in these gases for any great length of time, although they are not positively deleterious to them, for oxygen is needed for the continuance of their func- tions. And in the warm-blooded animals, a constant supply of it is necessary they will die if cut off from this supply even for a short time. 150. The change which takes place in the blood, as it passes through the lungs, occurs to some extent when the blood is ex- posed to the air in any way. Thus, if blood be drawn from a vein into a bowl, the surface of it becomes red by the action of the air upon it. Carbonic acid is discharged from it, and the oxygen of the air takes its place, uniting with the blood, just as the process occurs in the lungs. A larger part of the blood will be thus changed, if it be shaken so as to expose more of it to the air. The change takes place to some extent even if a membrane be interposed between, as when the blood KESPIKATION. 103 Quantity of carbonic acid given out by the lungs. Necessity ol ventilation. is inclosed in a bladder. The oxygen of the air, in this case, is introduced through the minute pores of the bladder, and the carbonic acid gas escapes through them. Precisely in this way is the change effected in the lungs, as already stated in 140. The blood is separated from the air by being con- fined in blood-vessels, and the air in the vesicles acts upon it through the minute pores of these vessels. Arid, as the blood is divided into innumerable little streams, every part of it is acted upon by the air in the vesicles. Though the texture of the lungs is exceedingly delicate, and the separation between the air and the blood is almost as nothing, yet the blood is confined to its limits, even though it courses through these organs with great, rapidity, and it never mingles with the air except as 9 consequence of actual disease. 151. The quantity of carbonic acid gas discharged from the lungs in the course of twenty-four hours is very great. Many experiments have been tried and calculations made to ascertain its -amount, and I am within bounds when I state, that there is at least three-quarters of a pound of charcoal in the carbonic acid thrown off from the lungs of a common-sized adult in the course of twenty-four hours. This gas is a deadly poison. When accumulated in a considerable amount, as when char- coal is burned in an open furnace in a close room, it may prove immediately destructive to life. And in the very prevalent neglect of ventilation, the frequent accumulation of this gas from the respiration must prove more or less injurious to the health. Whenever the proper amount of oxygen gas is with- held from the lungs, and carbonic acid takes its place, the quality of the blood is impaired from incompleteness in the change effected in the lungs, and the vigor of the body must in this way be lessened, to say nothing of the deleterious influ- ence of this gas upon the nervous system. Though the results are not immediate and palpable, great injury is continually done to the health of multitudes by the accumulation of this gas, in small close apartments, and in crowded assemblies. A congregation of twelve hundred people in two hours throw off from their lungs an amount of carbonic acid that contains seventy -five pounds of charcoal. And yet little pains is com- monly taken to carry off this vast quantity of poisonous gas, and replace it with pure air. 152. As so much oxygen is absorbed in the lungs of all ani- mals, and so much carbonic acid is thrown out from them, the inquiry arises how the air is replenished with oxygen, and is 104 HUMAN PHYSIOLOGY. Carbonic acid exhaled from the lungs of animals absorbed by plants. cleared of the carbonic acid which is thus so largely mixed with it. It is found that this is done, to a great extent at least, by the leaves of plants. The process which goes on in these lungs, as they may be called, of the plants, is quite the reverse of that which is going on in the lungs of animals. The carbon of the car- bonic acid which is thrown off from the lungs of animals is ab- sorbed by the leaves of plants, and the leaves replenish the air with the oxygen, which is so constantly and abundantly ab- sorbed in the lungs of the animal creation. Thus, the animal and vegetable kingdoms are sources of supply to each other. But it may be thought that there would be apt to be a surplus of oxygen in the atmosphere in warm climates, where the vege- tation is so luxuriant ; while, on the other hand, there would be an accumulation of carbonic acid gas in the colder regions. This would be so, if the air were not so movable that the equi- librium is readily secured in either case. 153. It is an interesting fact, that the presence of light is necessary to the process which I have described as going on in the leaves of plants. Each leaf may be considered as a labor- atory, and the light as the chief agent in effecting the chemical changes that occur in it. And it is found that no artificial light can do the work. It is only the light of the sun that is competent to this chemistry. And as these innumerable labor- atories are everywhere at work, absorbing the carbon and ex- haling the oxygen, to purify the air rendered noxious by the laboratories of the animal creation, we must confess it to be a mystery as to how the chemistry of the lungs of animals, and that of the leaves of plants should be kept so nicely balanced. The balance is so strictly maintained, that the chemical composition of the air is always found to be almost exactly the same. 154. The heat of the body is maintained by the union which takes place in the capillaries between the carbon and hydrogen of the system, and the oxygen which is introduced into the blood through the lungs. It is a process analogous to combustion. When carbon or charcoal is burned in a ves- sel containing air, the oxygen disappears, for it unites with the carbon, and carbonic acid gas, therefore, appears in its place. The same union occurs in this case between carbon and oxygen, as we find occuring in the capillaries. A sort of com- bustion, then, is going on in every part of our bodies. And, as heat is evolved in the one case, so it is in the other. The same can be said of the burning of hydrogen and oxygen together. Heat is caused by the union thus produced between them, and RESPIRATION. 105 Animal heat. Produced by a sort of combustion. Three sources of fuel. so it is when they unite in the body. The water which is ex- haled from the lungs comes from this union of oxygen and hydrogen. It was formerly supposed that the union between the oxygen and the carbon and hydrogen takes place in the lungs, and that the heat is made there, and then is distributed over the whole system. But it was objected to this supposi- tion, that it made the lungs a sort of furnace for the rest of the body, and that, if the supposition were correct, there ought to be a much higher degree of heat in these organs than any- where else, which is not the case. Ingenious theories were broached to get over this difficulty ; but it was at length dis- covered that the union between the oxygen and the carbon and hydrogen occurs in the capillaries of the body, instead of the lungs, and that the combustion, therefore, that produces the heat is everywhere, instead of being in one locality. 155. The fuel for this combustion comes from three sources. One of these is the waste of the tissues. This furnishes a con- siderable amount of the carbon and hydrogen for the union with the oxygen, in all animals that are subjected, from their activity, to much wear and tear of the system. I barely al- lude to this now, and shall enlarge upon it soon. Another source of the fuel for combustion is food. The oily, sugary, and starchy kinds of food are devoted in a great measure to this particular purpose. These furnish a sort of floating fuel, as we may express it, which is carried about in the blood. Hence, we see, that our diet must necessarily be varied accord- ing to the weather and the climate. In cold weather, the heat of the body is more rapidly abstracted than in warm weather, and, therefore, we need then more of that food which affords a supply of carbon and hydrogen. And so as to climate. The enormous quantity of oily food often consumed by inhabitants of very cold climates is used up by being burned, as we may say, in the capillaries to keep up the animal heat. Of course, keeping the body warm by fire and clothing relieves from the necessity of taking any large quantities of fuel-making food. Still, under the most favorable circumstances in this respect, there is a need of variation in diet to suit the weather and the climate, and we make this variation for the most part instinct- ively. Indeed there is a marked provision in nature for it. I will mention but a single example of this provision. While there is a large amount of fat in the bears and seals and whales which afford food for the Esquimaux and Greenlander, there is very little in the animals which furnish a part of the diet of the 106 HUMAN PHYSIOLOGY. Animal heat differs in cold and warm-blooded animals. Why. inhabitants of tropical climates. Another source, still, of ani- mal heat is the store of fat which is laid up in the body. Ona design of this accumulation of fat in different parts of the body seems to be to provide for the heat when other sources fail. Thus, when disease destroys the appetite, and thus cuts off the supply of food, the fat wastes away, or rather is burned up, to keep up the temperature of the body. The fat is the great means of maintaining the requisite temperature when hi- bernating animals become torpid for the winter. They become very fat in the autumn, before crawling into their winter quar- ters, and in the spring they come out very lean, their fat having been consumed in keeping up the low degree of temperature re- quired during this time. 156. As the amount of heat produced, when charcoal is burned in air, or when oxygen and hydrogen are burned together, depends upon the quantities of carbon and hydrogen that unite with the oxygen, so, also, the degree of animal heat depends upon the quantities of carbon and hydrogen that unite with the oxygen in the capillaries. This may be illustrated by referring to the effects of exercise on the heat of the body. When the circulation is quickened by exercise, the blood passes more rapidly than usual through the lungs, the respiration is consequently quickened, more air is introduced into the lungs, and therefore oxygen is more rapidly absorbed by the blood. At the same time, the action of the muscles effects a waste in their structure by the wear and tear, so that more carbon and hydrogen are ready to be released to be united with the in- creased oxygen. Hence comes the heat produced by exercise. So, too, those animals which are the most active, ordinarily have the most animal heat, and have the most extensive respi- ratory apparatus, so that there may be a free supply of absorbed oxygen to unite with the carbon and hydrogen of the changing tissues. It is in birds and insects that this union takes place most largely, and in them, therefore, the respiratory apparatus is very largely developed. This is to be attributed to their mus- cular activity, which produces so much waste matter that must be removed from the system. Cold-blooded animals, on the other hand, are very inactive. There is not, therefore, much wear and tear of the tissues. There is comparatively little waste, therefore, to be thrown off. And so but little oxygen needs to be introduced into the lungs, and consequently little heat is generated. To realize fully the contrast between the warm and the cold-blooded animals in these respects, observe, a* KESPIRATION. 107 Uniformity of animal heat in the warm-blooded. Interesting experiments. the representative of the one class, a canary bird, and a frog ai the representative of the other. The frog is generally quiet, and only now and then takes a leap or croaks ; but the bird is ever in restless motion, and sings much of the time with all his might. The bird is warm with the heat generated by the constant union of oxygen with carbon and hydrogen in its capillaries ; but the frog is nearly as cold as the water in which he is immersed. The bird breathes rapidly, to let the oxygen of the air largely into his lungs ; but the frog scarcely seems to breathe at all, so scanty is the supply of oxygen which he needs. 157. Cold-blooded animals are very nearly of the same tem- perature with the substances that are around them ; but warm blooded animals have a certain degree of temperature, which they maintain with considerable uniformity under all variations of temperature in the atmosphere. This in man is about ninety- eight degrees of Fahrenheit. This, you observe, is above the temperature of the surrounding air, except in exceedingly hot weather. The human body is therefore always giving off heat. Indeed it is essential to comfort that it should part with con- siderable heat, for any near approach of the atmosphere to ninety-eight degrees produces an uncomfortable sensation of heat. But the amount of heat which the human body can bear for a short time is much greater than the facts above alluded to would lead us to suppose. It was long taken for granted, that it could not safely bear, even for a short time, a heat much higher than that which is endured in hot climates. The truth on this subject was at length discovered by accident. Two Frenchmen were employed by government, in 1760, to devise some method of destroying an insect which infested the grain at that time. The result of their experiments was the discovery, that by subjecting the grain to a certain degree of heat in an oven the insect was destroyed, and the grain not injured. While they were trying their experiments, a girl offered to go into the oven and mark the height of the mer- cury in the thermometer. It stood at 260 ; and, after remain- ing there for ten minutes, which she found that she could do without any great inconvenience, she marked it at 288, that is, 76 above the boiling point of water. These facts led to the famous experiments of Dr. Fordyce and Sir Charles Blagden, in England. With wooden shoes, tied on with list, they went into a room in which the thermometer showed the air to be at 260. Their watch chains were so hot that they could scarcely 108 HUMAN PHYSIOLOGY. Different degrees of torpor in hybernating animals. touch them, and eggs were roasted hard in twenty minutes, and beefsteak was cooked in thirty-three minutes. And yet tha same air that produced these results was breathed by them with impunity, and it raised the heat of the body but very little. The air which was breathed out from the lungs was so much cooler than the air of the room, that it was refreshingly cool to the nostrils, and to the fingers as they blowed upon them. In such cases, the evil effects of the heat are prevented chiefly by the great amount of perspiration that occurs, the vaporization of this abstracting the heat, which would otherwise accumulate in the body and produce disastrous results. The exhalation from the lungs, also, has some influence. 158. In the state of hibernation, to which I have several times referred, the torpidity varies in degree in different ani- mals. In cold-blooded animals, respiration and circulation may cease altogether in this state. In them the movements of life are often, perhaps we may say generally, as fully suspended as they are in the seed that is kept from heat and moisture. They may be preserved in this state for a long time and yet revive. Serpents and frogs have been kept in an ice-house for three years, and then have been revived on being brought out into a warm atmosphere. In the warm-blooded animals that hiber- nate the torpidity is less deep than in those which are cold blooded. In them the respiration and the circulation become very slow, but never entirely cease. Indeed some species take food with them into their winter quarters, and occasionally wake up sufficiently to eat. But most of them are in a quiet, deep sleep, from which they do not arouse at all till the winter is past. In this state, as life is nearly, sometimes quite at a stand, there is no wear and tear, and therefore no change in the tissues, and so there is no need of the introduction of oxygen by the respiration. Dr. M. Hall, in his experiments and observations, found that the bat, when completely torpid, consumed no oxy- gan, and discharged no carbonic acid from the lungs, although its circulation was not entirely suspended. 159. The more active is the respiration of animals, the less able are they to bear a deprivation of air. A warm-blooded animal will die if it be under the water only a few minutes ; but a cold-blooded animal can live under the water for some time, because it is not in so urgent need of oxygen. And, for the same reason, a warm-blooded animal, in a state of hiberna- tion, may be kept under water for a long time without destroy- ing life, although when in its active state it would die on being FORMATION AND REPAIR. 109 Formative vessels appended to the capillaries. kept under water for only a few minutes. And this suggests a probable explanation of those cases, in which individuals hava been restored, after having been under the water longer than the usual time that suffices to destroy life in drowning. In such cases, the condition is not simply that of a drowned person. A blow, or the shock of body or mind, or both, may have in- duced a suspension of active vitality, like that which we see in the animal in a state of hybernation. The bare fact of immer- sion in the water may have but little or even nothing to do with the actual condition. Such a state of things is especially to be suspected in those cases in which the countenance does not exhibit the usual dark and full appearance of drowned per- sons. 160. I have thus shown the extensive play which the respi- ration has in the vital operations of the system. I have shown what the chemical changes are, which it effects directly in the lungs, and indirectly in the system. And you have seen how the animal heat is produced by these changes, and how unac- countably it is so regulated, that it seldom varies to any ex- tent from ite fixed standard. But it is to be remembered that, while the lungs, and even the capillaries, everywhere are thus chemical laboratories, the nervous system exerts a constant influence upon this chemistry of the body. This is especially seen in regard to the production of heat, but it is true of the whole range of the chemical operations. The laboratories would all cease their work if their nervous connections were destroyed. CHAPTER VIII. FORMATION AND REPAIR. 161. THE building and the repairing of the various struc- tures of the body are done by vessels appended to the capil- laries. The capillaries having received from the arteries the blood, the building material, the formative vessels select from it, while it is in these capillaries, whatever they need for their purposes. The selection is made according to the tissue or structure to be formed. Those vessels which, for example, form bone, select from the blood very different constituents from those which make nerve or muscle. 10 110 HUMAN PHYSIOLOGY. Selecting power of the formative vessels. Their concert of action. 162. It is wonderful that the blood can be formed from such a variety of food as is often taken into the stomach. But it is far more wonderful that from the blood can be made so many and such different structures. How different are the teeth from the gums which surround them ; and yet both are made from the blood. Observe, in some particular part of the body, how many different structures there are which are all made from the same common material. Take, for example, those which are in and around the eye. There are, the skin of the eyelids ; the eyelashes ; the vascular lining on the inside of the lids ; the cartilages of the lids ; the firm, white coat of the eye, giving to the eyeball its firmness ; the thin, transparent window in front, setting into the firm, white coat, like a watch-glass into the case ; the beautiful iris, a round moving curtain with a cen- tral opening ; the lens behind this opening ; the optic nerve ex- panded on the inside of the cavity of the eye ; the muscles that move the eye, with their tendons; the tear-gland; the cushion of fat on which the eye reposes ; the bone which forms the socket, &c. All these various textures are formed from the blood ; and the different workmen are as unerring in their se- lections from this common material, as if they were intelligent beings. Indeed, no ordinary intelligence could accomplish such a selection. It is effected, inscrutably to us, under the direction of an all-wise Intelligence, and by Almighty power. 163. But these builders of the body not only have the power of selecting their building materials from the blood, but they work in concert. Each company of builders work together in harmony, as if they were under intelligent leaders. And though different companies may be in close proximity, there is no disagreement nor interference. For example, the builders of a tooth and the builders of the gum around it, do not en- croach on each other ; but each do their appropriate work with- in their assigned limits. Even when different structures are intermingled, as when tendon and muscle mingle together at their place of union, there is no confusion- in the work of the two sets of laborers. In Fig. 48 you see the difference in struc- ture between the transparent cornea in the front part of the eye and the white coat, the sclerotic coat, into which the cornea is set like the crystal of a watch. It is represented as seen magnified 320 times. The dotted lines mark the place of union. The cornea, a, is a much more open structure, you ob- serve, than the sclerotic coat, b. The builders of these two structures, though some of them are in such near neighborhood FORMATION AND REPAIR. Concert of action shown in producing different shapes. FIG. 48. never encroach on each other, but each set adheres strictly to its own kind of work. The sclerotica-makers never go to mak- ing the open work which you see in the cornea. If they should do so at any point there would be a little transparent window at that point in the white of the eye ; and if the cornea-makers should at any point make close work like that in the sclerotica, here would be a white spot in the cornea. 164. The concert of action which we observe in the different sets of formative vessels is to be looked at in another point of view. It is such that they give a definite and peculiar shape to the structure which they make. Each bone differs in shape from every other bone, each muscle from every other muscle ; and so of other parts. There is very great variety of shape in the structures of the body ; and each shape can be determined only by a certain concert among the builders. That you may realize in some measure the extent of this variety, observe again the numerous different textures which I have mentioned as making up the eye. Each of these has its own peculiar shape, and its definite limits. Its builders work after a fixed plan, and within fixed bounds. 165. This concert of action may be looked at in still another point of view. If the different structures in the body were made, as a crystal is, by layer after layer of particles deposited upon the outside, wonderful as the concert among the little builders would be in that case, it would not be any thing like as wonderful as it is now. In the growth, that is the construc- tion of any part, the addition is made by the formative vessels at every point of the part, and not upon the outside merely. As these builders are at work enlarging the part in the growth from infancy to childhood, they must so act in concert, as to 112 HUMAN PHYSIOLOGY. Change of action. The teeth. Tadpole and frog. preserve the same general form in the part during all the suc- cessive stages of growth. And, as all the different stiuctures of the body enlarge together, there must be agreement between different sets ; else there would be encroachment and confusion. Thus in the growth of the tiny arm of infancy to the sturdy arm of manhood, each set of builders must during all this time keep within its proper limits, so that there may be just the right proportion, and the right position of bone, and muscle, and tendon, and ligament, and cellular membrane, and skin, and nail, &c., that make up the arm. 166. But this concert of action appears the most wonderful when a new action, or change of action is called for. In the transition from childhood to youth, for example, the builders of the apparatus of the voice, the larynx, all at once become unusually active in their work, and a great enlargement of this musical instrument, for such it is, takes place, so that it may now utter the grave notes of manhood. Soon, too, the beard- builders begin their new work upon the face. And during the period of childhood new operations have been continually insti- tuted among the builders of the teeth, as one tooth after another has made its appearance, and as the new set have re- placed the old. To produce in the enlarging jaw a new set of teeth to take the place of the smaller and less numerous first set, and to bring them out in a symmetrical arrangement, re- quire a very complicated series of operations. To effect each one of these, there must be concert of action among the forma- tive vessels ; aiid there must be a most wonderful concert among the different successive sets of builders, to make all tliese series of operations work out at length the general result. 167. This change of action in the formative vessels is strikingly exemplified in some animals. I refer to those that so entirely change their forms during the period of their exist- ence. I will give two examples. The first is the common frog. He is at first what is termed a tadpole, and goes through many successive changes to become a complete frog. Tliese changes are represented in the following figures. The relative sizes are not preserved, the tadpole state being represented re- latively much too large, for the purpose of showing more clearly the development of the legs. The young tadpole is represented in Fig. 49. It has a large head and body, and a long flat tail by which it swims easily. There are no promi- nences to indicate the putting forth of any thing like limbs. It FORMATION AND REPAIR. 113 Change of action in the silk-worm. Concert preserved in these cases. FIG. 50. has gills, which are loose fringes on each side of the head. These gills after a time disappear, and it has another set of gills arranged under a fold of skin very much like the gills of a fish. The form is then as in Fig. 50. The next change is this. The hind legs begin to grow out as seen in Fig. 51. Next, the fore legs appear as seen in Fig. 52. The tail is still very large. This now gradually disappears while the legs grow as represented in Fig. 53. In Fig. 54, representing the perfect frog, the tail has entirely disappeared. With these exterior changes interior ones have been going on also. The animal, which was at the first a real fish, breathing with gills and swimming in water, has lost its gills, and has now a pair of lungs ; and it is no longer able to remain long under water, without coming to the surface to breathe the air. 168. The other example is the silk- worm. It is represented in Fig. 55. When it has attained its full growth, it passes into what is termed its chrysalis state, Fig. 56, it having previously woven for itself from its silken thread a case or cocoon. While it is in this state of inactivity great changes are going on in its structure, and it at length becomes a perfect winged insect, as represented in Fig. 57. In the two cases which I have described, in each successive change, the concert of action in the formative vessels is pre- 10* 114 HUMAN PHYSIOLOGY. Change of action to meet new exigencies. served, but it is after a new plan. This change of plan makei the concert of action exceedingly wonderful. FIG. 55. FIG. 56. FIG. 57. 169. The change of action in the formative vessels, which is sometimes called for by accident and disease, exhibits in an in- teresting manner the concert between these vessels as in- fluenced by circumstances. When a bone is broken, these formative vessels set themselves to work to repair the injury, by forming new bone between and around the two ends of bone, which new bone we call callus. In this case, the bone- builders extend their range of operations to meet the new necessity ; and in doing so they maintain the same concert which marked their usual operations before the bone was broken. I stated in 105, that when an artery is tied, to cure an aneurism, the circulation in the limb is kept up by the small arteries that go off from it above the ligature, communi- cating with those that branch off below ; and that, in order to make the circulation perfect, some of these communicating arteries gradually enlarge, to meet the necessities of the case Now, this enlargement is not a mere dilatation produced tr- ibe distending blood. The arteries grow in thickness as wel. as in capacity. The artery-builders are awakened to a new ac- tivity, and make the arteries in this quarter after a larger pat- tern than the one originally designed for them. 170. Concert of action under successive changes is strikingly exhibited in the processes of inflammation. The following ac- count of these processes is from a work published by the author, entitled " Physician and Patient." " You see a swelling. FORMATION AND REPAIR. 115 Illustration from processes of inflammation. It after a while begins to soften. There is matter in it, but it is not yet very near the surface. But soon, at some point, it comes nearer and nearer to the surface, the wall of the abscess thus becoming constantly more thin, till, at length, it opens and discharges. The discharge continues till the swelling is nearly all gone, and the remainder is absorbed, and the part is restored to its natural state. Just look for a moment at the complicated character of this apparently simple operation. Here is quite a large deposition of substance which is to be re- moved; and this is the object to be effected. Observe how it is done. The softening of the swelling is not a mere change of solid substance into a fluid, as if by decay, but it is the re- sult of an active process, which we call suppuration. When this process is properly performed good pus is made, or as the old writers in medicine rather quaintly expressed it, laud- able pus. This process of suppuration, when it is well done, does not go on here and there in the swelling, making it like a honeycomb with a multitude of little abscesses ; but there is a consent, an agreement of action by the vessels of the part, as really as if they worked intelligently. It is this consent of action which not only makes the line of movement in the abscess, but points it towards the surface, instead of giving it some other direction, laterally or inward, upon some of the internal organs. But it is further to be observed, that in this agreement of action, the vessels of the part do not all do one thing. Three different offices are performed by them in the different quarters of the abscess. While some of these little workmen are forming the pus, there are others thinning the wall of the abscess in the direction of the surface, by absorbing or taking up the substance there ; while there are others still, in the rear, and at the sides of the abscess, deposit- ing substance, in order to make a barrier to prevent the pus from being diffused in the surrounding parts. Each class of these workmen perform their particular work with even more exactness and harmony, than would be expected of any com- pany of intelligent laborers under the direction of a leader. The absorbents absorb together, the wall-builders build together, and the makers of pus make pus together, and deposit it in a common reservoir. 171. But observe farther, and you will soon see an entire change come over the whole scene of operations. When the absorbents have completed their passage for the pus through the skin, the pus is gradually discharged from its reservoir, and 116 HUMAN PHYSIOLOGY. Formative vessels and absorbents act in concert. the " occupation " of the pus-makers is soon " gone." The wall builders also cease their work, and while the vacancy becomes filled up by contraction and deposition, the wall of defense, so carefully maintained so long as was needed, is now taken up by the absorbents, workmen which seem to know just when, as well as how, to do their duty." 172. Here you have concert of action exemplified in a complicated set of associated actions, to accomplish a tempo- rary purpose. These actions, as you see, change in the different stages of the process, each one being performed just at the time, and during the period that it is wanted. And when the temporary purpose aimed at is accomplished, the vessels of the part resume at once their ordinary duties. It is to be ob- served also, that the concert of action is not confined to the formative vessels ; but it appears also in those vessels called absorbents, of which I shall speak soon more particularly. And these two sets of vessels do not interfere with each other, but have a sort of agreement together in accomplishing the general result. This concert of action is plainly seen among the absorbents, not only in this case, but in all the cases that I have cited as exhibiting it among the formative vessels. For example, in the case of the frog ( 167) while the formative vessels are constructing the legs, the absorbents are removing the tail. So in the case of the teeth ( 166) while the formative vessels are constructing the second set, the absorbents remove the ends of the fangs of the teeth in the first set, so that they are loosened in their sockets, and are thus taken out of the way of the coming teeth. And indeed, wherever there is formation, there is absorption ; and the same concert of action always appears. 173. I have spoken of the great variety of structures, which are made out of the same material, the blood. Besides this, all the different secretions are also formed from the same material. This appears wonderful when we look at the differ- ence between such secretions as the tears, the ear-wax, the gastric juice, the bile, lor to the part. FIG. 60. CAPILLARIES IN THE WEB OF A FROG'S FOOT. 1 90. Cells may be seen in most of the fluids besides the blood, and also in the solids. The solid parts of animal bodies, CELL-LIFE. 125 Character and color of tissues dependent on the contents of cells. are composed either of cells, or of structures produced by cells, or of a mixture of these structures with cells. The same can be said also of plants. Cells, therefore, are the real formative vessels in both classes of organized beings. 191. We have very striking exhibitions of the FIG. 61. cells in the lower orders of animals. The Hydra, a representation of which is given in Fig. 1, seems to be made up of little else than cells. If you observe under the microscope one of its arms, as it moves about, the motion appears to be a motion of the cells upon each other. There are no fibres to be seen, to which the mo- tion can be attributed. Fig. 61 represents one of these arms highly magnified. The cells, as you see, have somewhat of a spiral arrangement. 192. The character of many of the tissues in the body depends on the contents of the cells. The cell itself, or the cell -wall, as it is termed, is considered to be always the same. But the contents vary, and this variation makes generally the variation in the character, and in the color also, of the various textures. For ex- ample, all the glands are constructed essentially on the same plan ; and their difference depends upon the contents of the cells in them. Thus the liver differs from the tear-gland, chiefly be- cause the former has cells which fill themselves from the blood with the components of bile, while the other has cells which fill themselves with the components of the tears. The color of various parts, as the iris of the eye, the skin of the dark-colored, the hair, &c., depends upon a coloring matter, which constitutes either a part or the whole of the contents of particular cells. So in plants the various colors displayed result from the various coloring matters which cer- tain cells contain. Some contain yellow coloring matter, others red, &c. When various colors appear together in any flower, there are, where the colors bound upon each other, cells lying side by side which contain different coloring matters. And in the shading off of the colors, the effect is produced wholly by the variation in the quantities of the coloring matter in the cells. 193. It is clear from the facts which have been stated, that the cells have a selecting power. In the body they take from the common pabulum or material, the blood, such constituents CELLS In the nrm of ths Hydra. 126 HUMAN PHYSIOLOGY. Cells absorb and select. Cells real laboratories. or substances as they need for their particular purposes. I have already given illustrations of this, in speaking of the difference in the glands. This selecting power is seen in the cells everywhere. Every cell contains its own peculiar consti- tuents, which it has taken from the blood. For example, there are fat-cells which receive fatty matter from the blood, rejecting every thing else ; pigmentary cells receiving nothing but color- ing matter from the blood, &c. The same thing appears too in plants. There are cells which receive from the sap volatile oil ; others, fixed oil ; others, starch ; others, coloring matter, &c. 194. Fluids, and sometimes gases enter the cells continually. The pores through which they enter are not visible even through the microscope, but of course such pores must exist. Their entrance is controlled by the selecting power to which I have alluded. 195. This selecting absorption thus performed by cells, as revealed by the microscope, is one of the most wonderful and mysterious phenomena in the material world. There is here a power in these cells which is unaccountable. The selection is made by the little cell as unerringly, as if its pores were con- trolled by an intelligence residing there. It has been said that this selection is a mere result of affinity ; that a certain affinity exists between the contents of the cell, or the cell itself, for the constituents which are absorbed. But if it be so, the mystery comes no nearer to being solved than before. For how are these affinities, so numerous and various, established, and what are the principles by which they are governed ? In either case the wisdom and power of the Creator may be considered as making, in this minute interior life of all organized sub- stances, some of their most wonderful manifestations. 196. There is not only a selecting power in the cell, but there is often a converting power, by which new compounds are formed from the constituents introduced into it. The cell in this case, though so small as to be seen only by a microscope of considerable power, is a real laboratory, effecting chemical changes in its contents. There can often be seen quite a brisk movement in the molecules in the cell while these changes are going on. 197. Some cells produce other cells. This is the sole office of some of them. In some cases new cells are made by a separation of a cell into two or more. A sort of hourglass contraction takes place at the middle, by an inflection or fold- CELL-LIFE. 127 Different offices of cells. Office of red cells in the blood. ing in of the inner cell-wall, for the cell has two walls. At the tame time the cell becomes elongated. An entire separation into two cells is thus, after a little time, effected ; and then each of these cells becomes two more, and so on. In other cases cells are formed within cells. When this takes place, the nucleus, that is an aggregation or mass* of solid matter in the cell, separates into two different parts, each of which has a cell formed around it. 198. Cells, as you have already seen, do not all perform the same office, but there are cells for a great variety of purposes. A consideration of these will develope to you still greater won- ders in the cell-life, and show you in the most interesting man- ner how great the Creator is in the minute operations of nature, as well as in those which are large and obvious to the naked eye. 199. There are different kinds of cells in the blood. There are colored and colorless ones. The office of the colorless ones has not yet been satisfactorily determined. But we know more about the colored ones. These give the red color to the blood. They are not red when looked at singly, but are of a yellow cast ; and the red color appears only when several are together. One office of these colored cells is to carry oxygen to all parts of the system, and return the carbonic acid to the lungs to be thrown off. By carrying these cargoes back and forth in the circulation, these little cells perform a very important office. A very valuable part of the cargo of these cells is iron. In low states of the system, when the red cells are deficient, the ad- ministration of iron in some form is often found to be very effectual, in connection with a good diet, in remedying the defi- ciency. The proportion of these red cells varies much in different animals. It is largest in those which are the most active, and which, therefore, as you saw in the chapter on Respiration, consume the largest quantity of oxygen. The proportion is greater generally in birds than in the mammalia, and it is much greater in the latter than in reptiles or fishes. In man it varies much in different individuals. These cells are abundant in the ruddy, strong, and active ; while it is other- wise in the inactive, pale, and feeble. 200. There are cells for absorption, and cells for secretion * To the common ear the word mass, which is ordinarily used in relation to aggregates of some size, sec:ns out of place when applied to a collection of molecules which is so small, that it c .. only be seen by a microscope of high power ; but though fco small, it is to the little . jntaining it a mass. 128 HUMAN PHYSIOLOGY. Manner in which absorption is performed by cells. and excretion. Of these I will give some examples. I have said in the chapter on Digestion, that the vessels called lacteala absorb chyle from the contents of the intestine. It was formerly supposed that they did this through their open mouths on the surface of the mucous membrane. But the microscope has shown that this is not so. The absorption is accomplished by cells, which are developed for this purpose at the extremities of the lacteals. They take up the chyle and discharge it into the lacteals, and they are dissolved away in the very act of emptying themselves. A new crop therefore of cells appears eve Y time the process of absorption is to be performed. And, wha: is still more curious, every time that absorption is to take place, ihere is cast off, as a preparatory step, a sort of pavement of cells f rom over every point in the mucous membrane where there is n extremity of a lacteal. The absorbing cells are thus uncovered, so that they can perform their duty. All this can be made clear by the following diagram. I must premise that the surfac of the mucous membrane of the intestine is not a perfectly smo( h surface, but examined by a microscope it is seen to be covered with eminences and depressions. Absorp- tion takes place on the eminences, while the depressions are the seats of secretion. In the diagram. Fig. 62, you have a FIG. 62. DIAGRAM SHOWING ABSORPTION IN A MUCOUS MEMBRANE. representation of the arrangement of one of the eminences highly magnified. A, represents it as it is in the intervals of digestion when absorption is not going on, and B as i-t is during absorption ; a a are the absorbent vessels or lacteals ; b b basement membrane, as it is termed, an exceedingly thin membrane acting as a basement to the pavement cells c c ; d d, CELL-LIFE. 129 Manner in which secretion is effected by them. the absorbing cells. When absorption is not going on, the prominence is somewhat shrunken, and the pavement cells cover it. There are some granules or small grains, c?, in A, which are, it is supposed, the germs of the absorbing cells, which you see developed in B. When absorption is taking place, the prominence is swelled out as represented, the lacteal vessels are full, and the absorbing cells appear at their ex- tremities, while the pavement cells have been thrown off, so that the chyle may have free access to the absorbing cells through the pores or interstices of the basement membrane. 201. While absorption thus goes on in the eminences, secretion takes place in the depressions. The diagram, Fig. 63, FIG. 63. DIAGRAM SHOWING SECRETION IN A MUCOUS MEMBRANE. represents one of these depressions, or follicles, as they are termed, in two opposite states, when secreting, and when not secreting. In A, secretion is not going on, and the cells e, in the follicle remain quiet. In B, on the other hand, secretion is taking place, and it is done by the casting off of cells, as represented. These cells discharge their fluid contents into the cavity of the intestine, and disappear, while other cells take their places. These follicles are really little glands. And the various glands, the salivary glands, the liver, the pancreas, &c., are made up essentially of such follicles arranged in different ways. You see, therefore, in this diagram, the manner in which secretion is effected everywhere. The secreted matter is received by the absorbing cells, through the interstices of the basement membrane, from the blood in the capillaries which lie under this membrane. 202. The pavement cells, of which I have spoken, cover every part of the mucous coat or membrane, and answer as a protection to it. There is a similar arrangement over the whole outer surface of the body. Next to the true skin is a 130 HUMAN PHYSIOLOGY. Muscles made up of cells. Their tontraction nnd relaxation. basement membrane, and upon these, as in the case of the mucous coat of the alimentary canal, lie pavement cells. These cells, constituting the cuticle or scarf-skin, are much more numerous than in the alimentary canal. There are many layers of them. The outer cells dry by exposure to the air, and become scales. As these are rubbed off. the cells below take their places ; and there is a constant supply of fresh cells from the basement membrane. 203. There are some cells which are devoted entirely to the production of motion, for an ordinary muscle is composed of great numbers of chains of cells included in sheaths bound to- gether. A muscle appears to the naked eye to be made up of fibres. Each one of these fibres is found by the miscroscope to be composed of from 500 to 800 fibrillce, or minute fibres. And each of these fibrillse is a series or chain of cells. In Fig. 64, a, is represented a fibre as seen under the microscope, FIG. 64. a FIG. 65. FIBRE OF A MUSCLE. showing the fibrillse of which it is com- posed. They are separated at the broken end by the violence in tearing the fibre. In 6, you see one of the fibrillae very highly magnified, showing that it is a chain of cells. In the diagram, Fig. 65, is repre- sented the condition of a fibrilla in the two states of contraction and relaxation. In a it is relaxed. In 6 it is contracted, the cells being shortened, and at the same time widened. And as all the cells in the muscle are thus widened when the muscle contracts, we see the cause of the well known swelling out of muscles when they are in action. That you may form some idea of the size of these cells in muscles, I will state that in the space of the square of a tenth part of an inch, thus, there are over 100,000 of these cells. M When MUSCULAR FIBRIL; a relaxed ; 6 contracted. CELL-LIFE. 131 Hoofs, horns, nails, and teeth made by cells. a large muscle contracts what an innumerable multitude of these cells are set in action ! 204. There are cells whose office it is to make certain solid deposits. Hoofs, horns, nails, and teeth are made in this way. Even the hard enamel of the teeth is constructed by cells. They deposit it in the form of prisms of hexagonal shape as seen in Fig. 66, which represents a vertical section of enamel as seen under the microscope. Their shape is more plainly seen in A, Fig. 67, which represents a transverse section of enamel. The line of these prisms is generally wavy, but they are for the most part parallel to each other. At B are some of these prisms separated. They are more magnified here than in Fig. 66. FIG. 66. VERTICAL SECTION OF ENAMEL. FIG. 67. ENAMEL. A, Transverse section. B, Separated prisms of it 205. Perhaps the most wonderful exhibitions of the functions of the cell are presented to us in the nervous system. The nerves are bundles of tubes of exceeding fineness. They vary from TsVoth to i o.oouth of an inch in diameter. Now, each of these little tubes, or tubuli, as they are called, was once a chain of cells. The cells in each chain or row, as the micros- 132 HUMAN PHYSIOLOGY. Nerves composed of tubes made from cells. Cells in the gray substance of the brain. cope has shown, gradually became incorporated together to be- come a tube, and in this tube is contained the true nervous matter. And it is supposed that each of these tubuli preserves itself separate and distinct, from its origin in the brain, or some other of the central organs of the nervous system, to its ter- mination in some fibre, or on some surface. For no communi- cations between the tubuli have ever been foun,d by any micros- copist. The manner in which these tubuli are made from cells may be illustrated by the diagram in Fig. 68, in which the steps by FIG. 68. which the row of cells A becomes A B the tube B are represented. 206. It is these tubuli, thus formed from cells, that constitute the means of nervous communica- tion between all parts of the sys- tem. Thus, when a muscle con- tracts in obedience to the will, an impression is conveyed through those tubuli that connect the brain with the fibres of the muscle, or rather with the cells of which these fibres are composed. These tubuli exist in all the nerves, and in the white parts of the brain and spinal marrow. They transmit, but they have nothing to do with originating what is transmitted. This is done by another part of the nervous system, the reddish gray substance, which is seen in the brain and spinal marrow, as entirely distinct from the white portion. This gray substance, in which all nerve force, as it is termed, is produced, is made up chiefly of cells. These cells, which have a nucleus or central particle, are originally globular, but many of them assume various shapes, and often shoot out branches. Some of the shapes are very fantastic as represented in Fig. 69. These are magnified 200 diameters. 207. In the views which I have given of cell-life, I have not attempted to describe all the phenomena which have been dis- covered, but only enough of them to give the student a general view of this interior unseen life, that is at work so busily at every point of every living substance. The cell, you have seen, performs a great variety of functions. It is the agent by which all vital operations are carried on. The very beginning of life, so far as we can see, is in the cell which the microscope reveals to us. Its first manifestation is here. We can suppose a germ as the origin of a cell, but we do not see it if it exist. 208. All animated nature is built up by cells. The first CELL-LIFE. 133 All organized substances built up by cells. FIG. 69. NERVE CELLS IN THE GRAY SUBSTANCE. thing which comes from the supposed germ is a cell. And this single cell is the parent of all the cells which build up the whole structure, whatever it be. It is by these cells thus pro- duced, that all plants and animals are constructed. "A globu- lar mass," says Carpenter, " containing a large number of cells is formed before any diversity of parts shows itself; and it is by the subsequent development, from this mass, of different sets of cells, of which some are changed into cartilage, others into nerve, others into muscle, others into vessels, and so on, that the several parts of the body are ultimately formed. Of the cause of these transformations, and of the regularity with which they take place in the different parts, according to the type or plan upon which the animal is constructed, we are en- tirely in the dark ; and we may probably never know much more than we do at present." 209. A beautiful exemplification of what has just been stated is seen in the development of the animal in the interior of an egg, and, particularly- in the egg of the bird tribe. By an ex 12 134 HUMAN PHYSIOLOGY. Arrangement of the parts of the egg. animation of different eggs at different stages of the process of hatching, the various steps in the development of the animal have been observed and noted. It is a series of most wonderful processes, that go on concealed from our view by that sym- metrical inclosure of lime. Of these I will present the general outlines. In the middle of the egg is the yellow yolk, com- posed of albumen and oil globules. It is surrounded by an ex- ceedingly thin sac, which keeps it separate from the albumen, the white of the egg that envelopes it. The yolk, 6, Fig. 70, is SECTION OF A BIRDS' EGG. lighter than the white, and it therefore always seeks the highest point in the egg. But there is a particular contrivance which prevents it from actually touching the shell. It is held down by two very delicate ligaments e,e, connecting it with the white lining of the shell. And you will observe, too, that the cica- tricula, or germ-spot, a, which is a collection of cells beginning the process which is to form the animal, being lighter than the yolk is always at the top of it, in order to receive the warmth from the body of the bird as it sets upon its eggs. Besides all this, there is at the blunt end of the egg, /, a bubble of air which is intended as an invigorating draught for the lungs of the young bird, preparatory to its bursting its shell. 210. When the processes preparatory to the formation of the animal commence, the yolk itself is composed in part of cells, as represented in Fig. 71, A. In the midst of it there is a germinal spot, a, with a vesicle in it, b. This vesicle produces CELL-LIFE. 135 Succession of cells in the yolk before the animal is formed. FIG. 71. B ? DEVELOPMENT OF CELLS IN THE YOLK DURING INCUBATION. a cluster of cells. But these cells, and those which in part compose the yolk are temporary, and all disappear. Before, however, the cluster of cells in the germinal spot disappear, there are seen in the midst of them two twin cells. These multiply ; and what is singular, they do it by doubling, so that there are successively 4, 8, 16, 32, &c. At length there is a mass of them, like a mulberry, as at e, in B. This mass then sends off cells at its edges which makes a layer,/, all round the yolk as represented in C. A second layer, ^, is formed inside of the first as seen in D. In the case of the higher animals a third layer is added. 211. There is no formation of the animal yet. But now a single large cell appears in the centre of the mulberry-shaped mass of cells, and from this begins the formation of the animal. All the other parts of the egg the cells, the yolk, the white are tributary to the action which proceeds from this cell. Within its wall is a ring-like nucleus. This takes the shape of a pear, and then it is afterward very much like a violin. From this nucleus are produced cells which form all the various parts of the animal, the heart, lungs, stomach, brain, limbs,