\eSSSmm ARMY MEDICAL LIBRARY WASHINGTON Founded 1836 IN HEX Section. ffiN i7T?T Number ../..C.Zr:.. nirnnlntinn Tf n pared for seeing the circulation. arrests the circulation, n a kidiTlieouturbark0f the plant. single globule happens to become detached, it immediately whirls round and conducts itself much in the same manner with those minute ani- Stem of the Chara. a, The outside bark. A, ft. The 00 CIRCULATION IN PLANTS. Fig. 14. A portion of the stem of the Chara, highly magnified. mals that are furnished with cilia (84), and we have every reason to believe that the cause of motion is the same in both. The larger rings in the figure represent moats floating with the sap. 88. The polypi not only resemble plants in their ex- ternal appearance, after the manner of the zoanthus or animal flower (81), but they even multiply by buds, like a tree. These buds are called gemmules. They soon fall off, and commence an independent existence. They are provided, from the first, with moving cilia, which carry them off in search of a proper place of perma- nent residence, the moment they are detached from the parent. In fig. 15, you see a figure of the gemmule of a flustra, covered with its cilia. Even those polypi which remain fixed for life in one spot, Fig. 15. have thus the power of transporting their race to a distance, by means of a locomotive power jm which the young lose for ever the moment that V^LK'^J they select their station: but they make this %^Cy selection voluntarily and with judgment, though the motion of the cilia is constant, and seem- ^Fiustll^ ingly involuntary in many of them. 89. By far the majority of the various kinds of polypi live together in extensive societies, of which the num- bers defy calculation. The different members of each group remain connected together, in such a manner that the whole community forms one living mass, and each polypus, instead of being a distinct and separate animal, resembles one of the divisions of the original hydra represented at fig. 4 (62). SOLID SUPPORTS OF THE POLYPI. 61 90. Now such vast communities composed of such soft materials, could not possibly preserve themselves from destruction without some solid support. Provi- dence has, therefore, bestowed upon them the power to form for themselves cells or stems of lime or horny mat- ter, in which their soft flesh may be encased, or over which it may be spread. 91. Sometimes this support is a Fig. 16. jointed tube, branching beautifully like a tree, with openings in the side of each joint, through which the mouth and tentacula of a polypus peep forth, and expand themselves like a flower; as in the sertularia. Fig. 16. 92. Sometimes the support consists of round cells placed side by side, like the barrels of an organ; as you see in the tubipora, a kind of coral. Fig. 17. 93. When the flesh of the commu- nity is spread over the surface, instead of being enclosed within the support, the bodies of the individual polypi are often enclosed in cells formed in the flesh, from Tubipora. Precious Coral. a, A portion of the stem with its poly- pi, of the natural size, ft, A magnified portion of the stem with its fleshy cover- ing and polypi, c, A portion of the fluted solid axis "with the fleshy matter re- moved. which cells they project themselves in search of food. The solid axis often bears the strongest resemblance to a plant with leaves or flowers. Sometimes it is com- 62 SECRETION. posed chiefly of lime; as in the common or precious red coral, fig. 18, where a represents a stem of the natural size, b, a portion with three of the polypi, one contracted, the other two expanded, and the whole highly magnified. In other species the axis is horny; as in the gorgonia— fig. 19, which represents a portion of the gorgonia bri- arius with a section of the flesh, show- ing the axis, the cells, and some of the Fig. 19. individual polypi within them, also great- ly magnified. 94. In many kinds of coral, called madrepores, the solid support of the community of polypi is as massive and almost as firm as a limestone rock, and the hard cells merely indent the surface of the rock, which continues growing with rapidity; the old cells be- Gorgonia magnified. ing obliterated and new ones formed as one generation of these little architects succeeds another. 95. You have heard, no doubt, that in tropical seas the coral rocks grow with such rapidity that vessels are frequently wrecked upon them, where a few years before the soundings were very deep; and that new islands are continually appearing where once the largest vessels might navigate in safety. Yet all this growth of seeming rock is produced by an exudation from the bodies of countless millions of little animals, composed nearly, if not entirely, of simple cellular membrane, without any distinct organs except the cilia, of the true nature of which we as yet know nothing. 96. You may now be able to comprehend what is meant by secretion — a term applied by physiologists to that process by which a living body separates from the fluids which nourish it any substance which is required for a definite use, or which it is desirable to remove from the body. The rocky base or branching stems of corals and gorgonia are secretions from the substance of the polypus, as the outer skin or cuticle of a man — that which we see raised by a blister — is secreted by the surface of the membrane beneath it. If the cuticle be rubbed off a man's hand, a new one is almost imme- NUTRITION. 63 diately formed, and if the hard cell of a polypus be broken, it is rapidly repaired. The power of secreting lime or horny matter is not confined to the surface of the polypus, but is as general as its other functions; for we often find grains of the same material scattered through the substance of the flesh. 97. This powrer is one inherent in animal cellular tissue, the material of which the true skin and much of the solid bulk of all animals is composed. Finding it thus exemplified on the very confines of animal life, we are less surprised at its effects in more complicated beings, where we observe it clothing the shell-fish with their thousand elegant coverings, the insects with hard and jointed shells serving them as a kind of external skeleton, the reptiles with scales that are sometimes used as a house to live in (tortoises), and sometimes in place of feet for crawling (snakes). 98. The hair, claws, horns, nails, teeth, &c, of the more perfect animals and man, are all produced by the action of a similar power, and consist of horny or cal- careous matter, according to the purpose for which they are designed. As if to prove that all parts of the body are capable of forming substances of this nature, the history of disease furnishes us with many examples of the irregular deposit of bony or horny matter, in the substance of all the organs of the human body. The most common affections of this character are called ossifications, and these have been found in the muscles, the blood-vessels, the heart, liver, brain, &c. Some- times large incrustations of bone have been formed on the surface of the skin, where they have grown and fallen off, time after time, without producing any sore, or leaving any mark behind them. 99. As you advance in the study of physiology, you will discover that, as the complexity of the organization of animals increases, the number of secretions, or mat- ters separated from the general mass of the fluids, be- comes greater and greater, until it almost defies calcu- lation. Very many of these substances are deposited in the interior of the animals, to form and support the several different organs; as the bones, the muscles, the 64 CONTRACTILITY. brain, &c. The secretion of such substances is. termed nutrition. 100. Another class of secretions, more commonly so called, which we see in the higher orders of animals, are of a fluid character, and are designed, not to assist in the formation or growth of the frame, but to answer some useful purpose in the performance of the vital functions. The bile, for instance, seems to be a natural purgative secreted by the liver; and the tears, which are secreted by two little organs situated within the orbits of the eyes, are intended to prevent them from being injured by the friction of the eyelids. 101. The functions of assimilation (47, 48), nutrition (99), and secretion (96),—or, in other words, those which are connected with the growth and sustenance of the frame of a living being,—are common to all organized beings, whether plants or animals; and have been called the functions of organic life, to distinguish them from sensation and voluntary motion, which, being peculiar to animals, have been termed the functions of animal life. 102. I will close the present chapter with some defi- nitions and illustrations of a few terms which it is necessary that you should understand before we enter upon the study of the organization of more complex animals; a study that I hope will prove more entertain- ing than these preliminary but indispensable remarks. 103. You have been told that the power by which the cellular tissue that forms the bodies of polypi forces its fluids from cell to cell, so as to change its form and enable it to move its arms, &c, is called by physiolo- gists contractility (85). The same power is employed in pushing the fluids or blood from place to place, in order to nourish all parts of the frame (32). This motion is so gentle and slow, however, in the minute beings of which we have been speaking, that it cannot be perceived by the eye. 104. Now this contractility is a peculiar property of living things, and differs entirely from that power of contraction often observed, in consequence of cohesive attraction, in things which have not life. It has nothing in common with the cause that makes a globule of CONTRACTILITY. 65 quicksilver assume a rounded form when laid upon a china plate, or draws back and shortens a piece of molasses candy, after being stretched. It displays itself in plants as well as animals, — in every thing that is organized (20)—and is therefore a property or function of organic Fig. 20. life (101). 105. In plants, this contracti- lity rarely produces very sud- den and obvious motions, though there can be no doubt that it is interested in moving the sap, as it moves the fluids of a polypus. In the sensitive plant, however, in the hedysarum gyrans,— a shrub that keeps its branches continually rising and falling almost with the regularity of the pendulum, — and in the tube of the chara hispida (87), we see much more striking results of this property. But even these remarkable examples are trifling in comparison with many that we observe in the animal kingdom. In illustration of this fact I will give you a description of the Portuguese man-of-war, a most beautiful marine animal called by naturalists a physalia. Fig. 20. 106. This little creature some- what resembles one or more groups of hydras deprived of their arms, d, d, and suspended from the under surface of a large bladder, a, composed of very transparent cellular mem- brane and distended with air. This bladder is called the body pirysaiia Megaiista. 6 66 CONTRACTILITY. of the animal. At one extremity, it is occasionally curved so elegantly as to resemble the neck of a swan. It floats upon the surface of the sea and is surmounted by a membranous sail, which, as you see in the figure, is full of cavities, ranged side by side, like the fingers of a glove, b. From the middle of each group of jug- shaped appendages, d, d, which seem to be so many sepa- rate stomachs, you may observe a number of slender organs depending, by which the physalia seizes its prey. The sailors call the largest of these, c, c, the cable, and naturalists term them tentacula—a name given to a great variety of organs designed for a similar purpose in the lower orders of animals (81). When fully extended, in a physalia six inches long, this cable may measure five or six yards. The upper part of the sail is of the most splendid carmine colour; the back of the bladder, of ultramarine blue; the intermediate space is shaded ele- gantly through every tint of purple, and the whole sur- face is iridescent in oblique lights. When you recollect that the substance of the animal on which nature has impressed such glorious hues is more transparent than the palest amber, you will be able to form some con- ception of the exquisite beauty of the little being that looks so humble in the figure; — a beauty that I could as readily describe, as a painter could reduce to can- vass the ever-changing features of a sunset sky. 107. The colour of the groups Fig. 21. of stomachs is blue, and the cables, for tentacula, are generally of the same hue; but sometimes they are carmine. In fig. 21 you have a plan of a portion of the cable, very highly magnified, and at a you h observe that the general form of J^£ the organ is cylindrical. It is ^^ studded with numerous little bead- like bodies ranged round it in a spiral line, and each of these beads is covered with minute and hard spines, of which we know not cable of Physalia. the nature. One of them is repre- CONTRACTILITY. 67 sented at b. Their spines are so sharp as to enter the hardest wood; and when the cable accidentally touches the wood work of the vessel, as the naturalist lifts the animal over the rail in the little gauze dip-net used for catching it, the cable is generally broken before it can be detached. The moment that a small fish, crab, or other marine animal comes in contact with the organ, it is dis- abled by the wounds received from the prickles, which are supposed to infuse a poison. The pain induced when the cable touches the skin of a man is very severe, and lasts sometimes for twenty-four hours, though it has been much exaggerated by travellers. The medusa? (fig. 2), and many other soft or gelatinous marine creatures, have similar organs. It is not improbable that the prickles are hollow, and seated upon poison sacs, like the veno- mous teeth of the rattlesnake, and the spines of nettles. 108. Now, when the physalia wishes to spread its sail, it excites the contractility of the air sac, and forces the air into the finger-like cavities already noticed. Then, by using one end of its body as a kind of rudder, it can sail not only before the wind, but obliquely, in the man- ner that seamen term sailing on a wind. t 109. The moment the prickles on the cable have secured any prey, the organ contracts so strongly that it measures scarcely more than as many inches as it pre- viously measured yards. The little beads are brought into contact with each other (fig. 21, a), and the prey lies within reach of the bottle-shaped stomachs, by one or other of which it is swallowed. This is perhaps the most remarkable instance of vital contractility with which nature presents us. 110. It will be evident to you, on a little reflection, that this vital contractility, which produces either per- ceptible or imperceptible motions in various parts of the animal frame, is not necessarily dependent upon con- sciousness and will. For no one dreams that a plant can feel the sap flowing through it, any more than a man can feel the blood circulating through his veins: nor is it more difficult to believe that without sensation, the cable of the Portuguese man-of-war may contract the moment that it strikes its prey, than it is to comprehend 68 CONTRACTILITY—STIMULANTS. how a whole branch of a sensitive plant should shrink the instant that we rudely touch one of its leaves, though it will not do so when shaken by the breeze. If, then, I have said that even the simplest animals seem to give evidence of ivill in many of their motions (17), it is not because their frame or their organs possess such powers of contraction as have been described, but because they all perform occasional motions like those of the hydra in walking, which are obviously voluntary. 111. Nowr almost every part of the most perfect ani- mals, including man, displays contractility of some kind; and yet but few of these parts are gifted with feeling, and very many of their motions are altogether indepen- dent of the will. 112. Contractility, then, is a power resident in all organized bodies; but it produces no motion until it is excited by some internal or external cause. In the case of the cable of the Portuguese man-of-war, we see it excited by the contact of a fish or some other small animal; and here the cause is sufficiently obvious. When the air bladder of this little creature contracts in order to expand the sail (108), the organ is obviously excited by the will; and here the cause is much more^ obscure. The stomach of the polypus, like that of more perfect animals, is excited into action by the food; and the direction of the motion is determined sometimes by the quality of the food, and sometimes by the changes which it has undergone during digestion. Hence, it drives the nourishment from its general cavity into the arms and back again, and also ejects altogether any injurious or indigestible matter that may have been swallowed accidentally; as when its voracity has in- duced it to swallow another polypus (77). 113. Any cause which excites a part to contract is called a stimulant to that part. Thus, in man, the will, through the medium of certain nerves, stimulates the voluntary muscles, one after another, so as to cause him to walk or strike a blow. The flow of blood into the heart stimulates that organ, and causes it to urge forward the circulation. 114. There is a kind of contractility observable in all TONE--TONICITY. 69 living bodies, which is always excited while life remains, though it acts more powerfully at certain times and in certain conditions of the body. I mean that power which causes all parts to compress their contents with a certain degree of firmness. If it were not for this kind of contractility, the polypi and other soft animals could not preserve their forms; for a simple cellular membrane, capable of being greatly stretched by disten- tion, and filled with nothing but fluids, could have no stability if it did not at all times press upon its contents. That it does so in all animals is easily proved, but I will present you with only a few examples drawn from the natural history of man. 115. If you pull your finger with some force, as though you designed to draw it from the hand, you perceive that you can very readily separate the surfaces of the bones at the joints to a certain distance; but the moment you let go your hold, the finger is drawn back, even against your will. This shows that the muscles are always in such a state of active contraction. The same thing is seen in the face; for however it may be distorted by passion, when the mind becomes calm the habitual expression returns without any effort of the will. This kind of contractility is called tonicity, and the force with which it contracts is called its tone. 116. You cannot separate the surfaces of a large joint, like the shoulder, without using considerable exer- tion ; because the powerful tone of the large muscles which surround it draws the bones together with great force. But sometimes an accident, such as a severe blow or an attack of palsy, destroys the tone of these muscles; and then the mere weight of the arm will sometimes draw the head of the shoulder-bone entirely away from its socket. You all know how different is i he expression of the limbs and face of a sleeping or fainting person, and that of the same individual when at rest, but awake. This difference results from the fact that, during the fainting and sleeping conditions the tonicity of all animal bodies is much diminished. 117.* The same evidences of tonicity are observed in the skin, though in a much less marked degree. Cold E ii" 70 TONICITY. weather increases the tone of the skin, while heat diminishes it; hence we see that all parts of the body look comparatively firm in winter and relaxed in summer. 118. In young persons who have been rendered thin by severe illness, the skin seems relaxed, and hangs loosely over certain parts of the body; but when the health improves, the skin contracts so rapidly that long before the patient has " recovered his flesh," as we commonly but not very properly say, it appears as firm and as tight as ever. This is an evidence of tonic con- traction, tone, or tonicity, and you will find, as you ad- vance in this little volume, that tonic contraction plays a very important part in the economy of health. Were it not for this kind of contraction in the half-emptied blood- vessels, a person who had once fainted would never recover; for the heart cannot carry on the circulation of the blood in vessels that no longer contract upon their contents (113). I introduce this illustration to show that the subjects on which we are now conversing are not so unimportant to man and his interests as they may at first appear to you. The power of the heart and the nature of the circulation you will understand much better hereafter. 119. Whether the several forms of contractility which have been described may not all be the result of the same general cause, is, perhaps, doubtful; but by many physiologists they have been regarded as distinct pro- perties. There are also other forms of contractility displayed by the muscles; but these you are not yet prepared to comprehend. 120. Having now given you some idea of the struc- ture of the simplest animals, the manner in which they are supplied with the materials necessary for their growth and support by digestion, the mode in which they often supply themselves with a solid support by secretion, and the nature of the vital forces by means of which they preserve their form, move from place to place, seize their prey, and urge the nutritive fluids throughout their structure so as to nourish all parts of their frame, it is time to close this chapter. 7] CHAPTER IV. ON THE NECESSITY FOR MASTICATORY AND DIGESTIVE APPARATUS IN COMPLEX ANIMALS. 121. Most of the animals of which we have been speaking are so extremely simple, and at the same time so minute, that they require but slender protection, and hardly stand in need of any distinct organs for the per- formance of their proper functions. A single cavity, lined by what seems to be merely a continuation of their skin, suffices to receive and digest their food. All parts of their bodies lie so near this cavity that each portion is nourished by absorbing the digested fluids directly from the stomach. It does not appear that any particular organ of taste or smell is required to enable them to distinguish what food is proper or injurious for them, but they take what the beneficence of Providence sends them, without asking questions. They do not chew their food, and hence require no solid parts like teeth or jaws. We shall soon perceive, however, that much more complex apparatus is employed by animals a little more elevated in the scale of creation. 122. In many of the medusae (52) the thickness and bulk of the body or cap of the animal is so great that it cannot be conveniently nourished by absorption from a simple central cavity; and in these animals we find the cavity which answers the purpose of a stomach divided into four principal sacs in the form of a cross, the corners of which are extended into tubes that pene- trate the substance of the body, ramifying continually as they go, the smaller branches opening into each other, so ; hich you may see in almost any museum, public or private. The jaws of the common caterpillar you maj observe at any time during the summer, while the little animal is engaged in gnawing the edge of a leaf. Its jaws are horny like those of all insects, and not bony or composed of lime. 126. The masticatory organs of animals are not al- ways confined to the neighbourhood of the mouth; for in the lobster we find, in addition to very complex jaws, a set of teeth within the stomach itself, which enables this singular being to chew its food even after it has been swallowed. Many of the sea shell-fish have a long and solid tongue covered with rough ridges and spines that give them some powers of mastication. Not a few of them have horny organs in the interior which are much more powerful. There is a singular set of organs of this character near the stomach of a little shell- fish, lately brought from the coast of California by Mr. Nuttal, the celebrated naturalist. In general form this shell looks, to common eyes, very much like some of our common fresh-water snails, and like them it lives upon the edge of the water, breathing the air. It feeds upon coral, which it swallows in fragments with the animal adhering to it. The masticatory organs are found at a considerable distance from the mouth and near the principal stomach of this little animal. They resemble three rasps bound together by circular fibres, and occupy the whole of the passage for the food, so that nothing can reach the stomach without passing be- tween them. Now, the small portions of coral swallowed by this shell-fish are so ground and broken by these files that not only is the animal matter or food torn off from them, but the very stems of hard lime on which the polypi of coral grow, are formed into little rounded pebbles which fill the intestines below the stomach. 127. The passage through which the food is conveyed, in all animals that have such a passage, (for you see that the Portuguese man-of-war has not,) is called the alimentary canal. 74 ALIMENTARY CAIVAL. 128. The organs which masticate food after it has passed fairly into the alimentary canal, are generally called gizzards, and this name has been given to the appa- ratus just described ; something similar to which is found in many shell-fish. 129. Gizzards, or internal masticatory organs, are found chiefly in those animals which have no jaws ; as in the shell-fish; and in those whose jaws are too weak to crush their proper food; such as birds which live upon hard grains, after the manner of the common fowl, the turkey, &c. In birds, the gizzard is not provided with anything resembling teeth, being composed of a very firm flesh, lined with a hard, horny matter, and posessing so great a degree of contractile power that the gizzard of a turkey has been known to break to pieces a steel needle without being at all injured thereby. The domestic fowls are in the habit of swallowing hard pebbles, and these supply the place of teeth in assisting them to grind their food. When deprived of pebbles, they never con- tinue healthy, and are apt to die of indigestion. 130. You can very readily understand that the more nearly the food of an animal approaches in its nature to the substance of the animal that subsists upon it, the easier it is for the digestive apparatus to act upon it; and you will naturally infer that when the process of assimi- lation is simple, the alimentary canal will be proportion- ably simple. Such is the fact. In those animals that live upon meats, or are carnivorous, the alimentary canal is usually short and straight; its most essential portion, the stomach, is not complicated, and digestion is rapid. But, on the contrary, in those animals that feed on vege- tables,—which, though composed of organized matter, differ very widely in their organization from the animal frame,—the labour of digestion is much greater, and the digestive apparatus more involved. ALIMENTARY CANAL. 75 Alimentary canal of a limpit. 131. To obtain some idea of the Fig. 23. very complex character of the diges- tive apparatus observed even in ani- mals which you may consider insig- nificant, you have only to examine fig. 23, which represents the alimen- tary canal of the common lirnpit, a little shell-fish adhering to rocks on the sea-coast. M represents the mouth of this animal; T is the tongue; S the stomach, and O the intestine wound round and folded upon itself so as to occupy but little space. 132. Now, in many fishes and birds of prey, the alimentary canal passes almost directly through the body, and the stomach is but a slight enlargement of the canal, while many other animals not only have much more complicated intestines, but are provided with other en- largements of the canal besides the stomach, such as the craw or crop in pigeons and fowls. All beasts that chew the cud, or ruminate, such as the ox and the sheep, have four stomachs in the place of one, and each of them has its own peculiar duty to perform in effecting the digestion of the food. The first of them, for instance, receives the food when it is taken, and retains it for some time, until the animal is at leisure to chew it more deliberately. It is then passed into the second stomach to be there moulded and thrown up in small parcels into the mouth again to be fully masticated; after which it descends into the following stomachs, which continue the process of assimilation. Connected with the alimen- tary canal of the camel we find receptacles for contain- ing pure water, which enable this animal to traverse the wide and arid deserts of Africa, where water cannot be had. The traveller in these deserts is often preserved from death, by thirst, in consequence of the supply of water obtained by killing one of his camels. 76 CHAPTER V. ON THE NECESSITY FOR A SPECIAL APPARATUS OF MOTION— THE MUSCULAR AND OSSEOUS SYSTEMS AND THEIR APPEN- DAGES. 133. By this time you must perceive, very plainly, that the motions, both internal and external, performed by animals of much more elevated rank than the polypi, are far more numerous and powerful than theirs. The force required to break down the solid food on which many of them subsist, by means of firm organs, such as teeth, jaws, or gizzards, is much greater than the soft and delicate cellular tissue of an animal could exert. Such beings, therefore, require, and are consequently provided with, a separate system of contractile organs, called the muscular system. The muscles which compose this system display prodigious powers of contractility and, when called into action, they draw together the parts to which they are attached with a strength that is altogether astonishing, when we consider their softness and apparent tenderness. Thus a strong man can rise upon his toes while lifting a weight that requires the muscles of the calf of the leg to exert the force of a ton and a half. Yet so completely is this strength de- pendent upon the principle of life, that, immediately after death, a small portion of the force just mentioned is sufficient to tear the organs to tatters. 134. The muscular system, then, is the apparatus by means of which the more perfect animals perform all motions that are very prompt, and all those that require much force. The limbs and body are provided with muscles to enable them to perform all their mechanical actions; the alimentary canal is also surrrounded with muscles to propel the food from place to place, as the VOLUNTARY AND INVOLUNTARY MUSCLES. 77 progress of digestion requires such changes, &c. But, these motions, being extensive and performed only when occasion requires them, seem to be dependent on a to- tally different kind of stimulation from the tonicity that the muscles display at all times, in common with most other parts of the body, (114, 115). You should, there- fore, avoid confusing the more active muscular contrac- tion, which appears to be the result of the action of pe- culiar stimuli upon these organs, with the tone of the muscles, which is the result of causes producing the same constant contraction of all other parts. 135. As some of the motions of a complex animal, such as those which are designed to carry him about in search of food, or to masticate that food when found, require to be under the government of the will, the mus- cles which perform these motions are called the i/i/scles of voluntary motion. 136. But the motion of the food during digestion and those other operations upon which the growth and nu- trition of the body depend, could not be trusted with safety to the control of the will, lest the passions, the follies, or the indiscretions of the animal should be con- tinually arresting or embarrassing those operations, thus destroying all security for the continued health, and per- haps the life of the individual. Providence has there- fore wisely ordered that the muscles upon which these motions depend shall act under the impression of their proper stimulants, without the control or the conscious- ness of the animal. They are, therefore, called the in- voluntary muscles. 137. The acts which are performed by the involun- tary muscles are such as are necessary to the functions of assimilation and nutrition, the digestion of food, the absorption and circulation of the nutritive fluids, the growth and the support of the organs. Now these vital functions are common to all organized beings; they are functions of organic life (101); and hence the muscles of which we are now speaking are called the muscles of organic life—while the voluntary muscles, which do not 78 APPARATUS OF MOTION. directly contribute to the same processes, but to others which are peculiar to animals, are called also the muscles of animal life. 138. There are certain operations directly connected with organic life that cannot be safely entrusted to the absolute government of the will, on the one hand, nor entirely removed from its control on the other. Thus life cannot be supported for more than a few minutes without breathing, but it would be impossible to carry on the ordinary business of life if man were compelled to breathe at all times, or at perfectly regular intervals. Again: If obliged to attempt an inspiration when under water, or when the head is immersed in a poisonous air or gas, the consequence would be fatal. The muscles that perform the motions required in breathing are, there- fore, partly under the control of the will, but after they have been at rest for a short time, no determination on the part of the animal can prevent them from recom- mencing their functions. Muscles of this character have been termed, rather rudely, the mixed muscles. 139. It is now time to give you a clearer idea of the nature of these highly important organs. You have been told that when you have removed the skin of a quadruped you find beneath it a layer of simple cellular tissue, perhaps containing a portion of fat (71). If you remove this by dissecting it off, you will find, in most parts of the body, a broad smooth expansion of a pearly hue, covering a red substance beneath. It is sometimes thinner than the finest paper, and almost perfectly trans- parent; in other places it is thick, white, and nearly opaque; while in many situations it is altogether want- ing. This membrane is composed of condensed cellu- lar tissue, strengthened by numerous fibres which are generally disposed very irregularly over and through its substance. It is called a fascia. 140. In reading works on physiology or medicine, you would find mention made of many fasciae in differ- ent parts of the body; but in reality these are all con- nected together in various ways throughout the whole FASCIA--MUSCLE. 79 frame, so as to constitute something like a distinct system. 141. The principal uses of fasciae are to separate parts from each other by interposing between them something more resisting than the loose and soft com- mon cellular tissue, and to bind down various muscles or sets of muscles, so as to give them proper and grace- ful form, and prevent them from starting out of their position when they contract. They also arrest or retard the passage#)f the fluids from cell to cell through the cellular tissue*in some forms of dropsy, and exert a powerful influence in limiting the progress of inflamma- tion or other local diseases, which pass through the fasciae with great difficulty. 142. These fasciae are found, not only near the surface of the body, beneath the skin, but are met with between the deeper seated organs, which they surround, cover, or envelope more or less completely, in many places. Were it possible to remove from the body all its harder portions, all its special organs, and all the loose or com- mon cellular tissue, there would remain nothing but a series of large cells or cavities, of various sizes and shapes, composed of the fasciae. Many of these cells would be found imperfect, communicating freely with each other in consequence of the deficiency of their walls. If you now recall to mind the fact that these fasciae are really composed of common cellular tissue strengthened by fibres (139), and that they are embedded in, and continuous with that tissue on all sides, you will have an idea of these parts sufficiently clear for our present purpose. 143. When you cut through the superficial fascia, in a quadruped (139), you find, in most parts of the body, the bulky red substance which we call flesh. To a casual observer, this flesh appears like a rude mass of matter designed to give form to the body, and to supply food for man and other animals. Such is indeed the popular idea of its nature, but the physiologist informs you that it is composed of a great number of distinct organs designed for the production of active and exten- 80 APPARATUS OF MOTION. sive motions. Each of these organs is a muscle, and the whole mass of flesh taken together constitutes the muscular system. 144. Each muscle (except the hollow involuntary muscles, of which I shall speak hereafter) is attached at either end, to the parts which it is intended to draw together, but is generally disconnected from all other organs every where between its extremities. It is found enveloped in a delicate sheath of cellular membrane, and is surrounded by loose cellular tissue*sufficient to allow it to move freely; but the layer o£ this substance in which it is embedded, is often so thin that the eye cannot very readily distinguish the separation of the side of one muscle from that of its neighbour; and this is the reason why the flesh of a limb is_ taken for an undi- vided mass by the ignorant. The skilful anatomist, how- ever, readily dissects around the entire circumference of a muscle by cutting through the loose cellular tissue only, without wounding the flesh in the least. 145. In fig. 24, you see part of a muscle thus dis- sected, to show its form when every thing, including the Fig. 24. Biceps muscle. e, t, Fleshy portions of the muscle, e, the tendon. STRUCTURE OF MUSCLE. 81 bones to which in this case its extremities are attached, has been removed from around it. This is part of the double muscle of the arm, whose function it is to bend the fore-arm. 146. When we examine a muscle more closely, we find it apparently composed of a great multitude of fibres, each surrounded by its own envelope of cellular tissue. These fibres are generally collected together in small bundles, which are again associated into larger groups forming the whole substance of the organ. Each bundle, and, indeed, each particular fibre, enjoys its own parti- cular power of contraction; so that some parts of a large muscle maybe called into action while other parts remain at rest; and thus the same organ may produce various motions, according to the direction of the fibres that happen to contract. Irregular motions of one or more fibres often occur from diseases, such as convulsions or cramp. 147. The muscles of man and the more complex animals are of a bright or deep red colour: but those of animals whose blood is white, such as the insects and many other minute beings, are pearly, colourless, or sometimes even transparent. 148. The attempt has been often made to determine the actual structure of the muscular fibre by means of powerful microscopes, and some writers tell us that it consists of a row of red globular. bodies connected together by transparent matter. Others inform us that no globules really exist in it, but that it resembles a cord or riband, crimpled on the surface, as if thrown into zig-zag folds by its own contraction. One celebrated physiologist of the present day declares that each seem- ing fibre is nothing else than a very long and narrow ceTl. containing a fluid. Now the fact is, that such examinations, made with very powerful lenses, require a degree of knowledge, practice, and judgment which few men in the world "possess; and so numerous are the sources of error, that very little dependence can be placed on the results. This investigation is highly 82 APPARATUS OF MOTION. important to the profound physiologist, but it would only tend to confuse the mind of the elementary student. 149. Whatever the true structure of the muscular fibre may be, it is well known that the common cellular tissue, which seems to form the entire body of the simpler animals, penetrates the muscle in every part, so that when every thing peculiar to the organ has been removed by art or disease, there still remains a mass of that tissue occupying the same place. 150. Sometimes, when bones are broken, a piece of muscle is caught between the broken extremities: the fragments cannot then be knit or reunited until the vital powers have caused the absorption of all the muscular matter that intervenes; and it is found that the part is then reduced to simple cellular tissue, which does not interfere with the knitting; for, new bone is soon de- posited in the tissue, and speedily joins the fragments together. 151. You can readily understand that the muscles would perform their office very awkwardly, (at least in the more complex animals,) unless attached at their extremities to parts more firm than mere cellular tissue; for how could the body be moved to any useful pur- pose, if there were nothing about it to prevent it from bending with equal facility in every direction 1 Now the necessity for such firmer parts is answered in widely different ways in different portions of the animal king- dom. You have been told that even the hydra has an external surface composed of a cellular tissue more dense, and consequently somewhat harder than the other portion of its body (65): and when we examine animals of somewhat more complex structure, we find that nature employs the true skin, — which is mainly composed of the same tissue, very much condensed and strengthened with innumerable harder fibres — as an attachment for the voluntary muscles. She also em- ploys, for the same purpose, the fasciae (139) or internal membranes, which are rendered strong by means of the fibres contained in their structure. The common snail MUSCULAR ATTACHMENTS. 83 found in the damp vaults in which we often keep Our meat and butter, will furnish you with an excellent idea of an animal that performs many and curious motions, and is provided with a multitude of muscles, the greater part of which are connected with the skin. The pro- gression in all such animals is very slow, and is effected with seeming difficulty, because the parts to which the muscles are attached are so soft and flexible that they cannot be made to perform sudden and violent motions. 152. Many animals analogous in some respects to the snail, and classed by naturalists under the general name of mollusca or soft animals, have the power of secreting, upon the external surface of their mantle—a membrane formed by an expansion of their skin, that covers their bodies loosely, like a cloak — a solid shell, composed chiefly of carbonate of lime; from whence this portion of the mollusca are often called testacea. This shell answers the purpose of a house to live in; and although the animal can never leave it, it can thrust the body out or draw it back at pleasure, by means of certain large and strong muscles attached to the shell within its cavity. But even in such animals, all the muscles which enable them to crawl and carry their shell about are connected with the skin, which, in many places, is very thick and hard. You can often find small snail-shells beneath damp boards in the garden, in the moist earth about the lower part of fences, or under the bark of decaying logs or stumps in the woods. By the side of almost any large brook or river you may gather quantities of shells inhabited by animals of the same class, and if you keep a few of these for some hours in a tumbler of water, choosing such as have no hard covering over the mouth of the shell when the animal retires within it, you may now and then enjoy the opportunity of seeing them swim upon the surface, displaying in the most beautiful man- ner the slow motions produced by muscles which arise from one portion of the skin and are inserted into an- other. 153. There is a large class of marine animals known by the very hard name of echinodermata or spiny 84 APPARATUS OF MOTION. skinned animals, in which we find the true skin not covered by a simple cuticle alone (96,) but also by a solid incrustation of lime, enveloping nearly the whole body. Some of these animals are formed like a star; and in these, the rays, which are often divided into many branches, are employed as limbs to walk with. The hard incrustations of these rays and their branches are divided transversely into very numerous segments or rings bound together by a more flexible horny matter; and the muscles of locomotion, passing from one ring or segment to another, serve so to bend them as to enable the animal to move along the sand at the bottom of the ocean. You may find animals of this character dried, and preserved in cabinets by the improper name of star- fish. They are very common on the shores of inlets from the sea. 154. In some of the members of this class, known by the popular name of sea-eggs, there are no rays, the body being of a form approaching to the globular; but the external surface of the incrustation is studded with raised balls of the same substance, perfectly smooth and pol- ished. To each of these balls a strong spine of solid carbonate of lime, sometimes very thick and long, is attached by means of a regular socket exactly fitting the round surface of the ball; and these spines are moved by muscles attached to them, so as to enable the animal to push or roll itself along. The sea-eggs are common on sandy coasts in hot countries, and among rocks in northern climates. You will find their shells or crusts, (sometimes with spines attached, but more frequently without them) in almost every collection of shells. 155. I need not explain to you the manner in which insects and also crabs (crustacea) employ the jointed horny or calcareous plates which are formed in their cuticle and bound together by it. To make yourselves acquainted with the motions of insects, you have only to examine a fly or beetle; and if you live so far from the sea that you cannot procure a common crab or a lob- ster, you can find a crawfish at any time by turning over a few flat stones in the nearest rivulet where the THE OSSEOUS SYSTEM. 85 water runs rapidly. The muscles of locomotion pass from one segment to another in these animals, as they do in the star-fish. 156. The external hard coverings, or, as they may be termed, the external skeletons, of the testacea, the echinodermata, insects, and Crustacea, may all be re- garded as appendages of the skin, being secreted by that membrane, as the solid stems of coral are secreted by the bodies of the polypi (95, 96.) They resemble more or less the nails, horns, scales, and beaks, of man, quadrupeds, fishes, and birds. Like the outer bark of plants, these parts possess no life, and are subject to being worn away by friction and injuries, and afterwards reproduced. Insects and Crustacea cast off their hard covering at certain seasons, and form new ones adapted to their changes of shape and dimensions. 157. But you will readily conceive that external skeletons like those of the Crustacea and insects would be very ill adapted to the necessities of the larger and more important animals. The accuracy of the sense of feeling would be destroyed over nearly the whole body by such an arrangement, while the freedom of motions would be greatly impeded by the rigidity of the envelope. The bulk, weight, and rapid and power- ful motions characterizing the members of the higher orders of the animate creation seem also to require a solid internal frame-work, to give strength to their several parts. Accordingly, we find the reptiles, fishes, birds, quadrupeds, and man provided with another sys- tem of solid organs, situated within the body, and con- nected together by numerous joints. This is called the osseous system, and the individual organs which compose it are the bones. 158. All the voluntary, and most of the mixed muscles are either directly or indirectly connected at each ex- tremity with the bones; and it is by the motions of the osseous system, produced by these muscles, that all the voluntary actions of the animal are effected. Nothing analogous to true bone is found in animals of less dignity than the reptiles and fishes. F 86 APPARATUS OF MOTION. 159. Even the bones of the most perfect animals are soft and flexible at a very early age; and, at a somewhat later period of existence, a portion of almost every bone is still found in the same condition. It is not very un- common to see the arm of a child two or three years old bent and deformed by a fall, without being actually broken; and it may be then restored to its proper shape by the surgeon without producing a fracture. Bones are originally formed of soft cellular tissue, filled with a kind of glutinous fluid. After a time, this fluid is gra- dually absorbed, and a white, elastic substance, resem- bling what anatomists call cartilage, commonly known by the name of gristle, is deposited in its place. The bones then become firm enough to be useful to small or very young animals, and also to some beings of much larger size, that, living entirely in the water, have their weight supported by the fluid in which they float, and are there- fore less liable to falls and heavy blows. A very large family of fishes are found to possess an entire skeleton composed of gristle alone. Even the jaws of that terri- ble animal the shark, are composed of this material, and a portion of the ribs, in man, remains in the same con- dition during life. 160. But the necessities of most of the more perfect animals, when full grown, demand a skeleton or bony frame-work for the body, that is very hard and inflexible. The bones are brought to this condition by the deposition of earthy matter within the substance of the gristle; and this deposition becomes at last so considerable that these organs appear to be entirely composed of it. Two salts of lime, the phosphate in great abundance, and the car- bonate, or common chalk, in small proportion, constitute nearly the whole of this earthy matter. 161. If we heat a perfect bone for a long time in a furnace, all the gristle will be burned out, and the whole will crumble easily under the fingers like a piece of chalk, because the animal matter that bound the earthy particles together has disappeared. By long boiling in water, much of the animal matter may be removed, and the bone reduced to nearly the same condition. STRUCTURE OF BONE. 87 162. On the other hand, if we place one of these organs in a large quantity of dilute acid, the earthy matter will be gradually dissolved, as in the case of the eye-stone surrounded by vinegar (13); and then the gristle will remain, preserving the form of the bone most perfectly, yet becoming so flexible that it may be tied in a knot without breaking, if the specimen be long enough for the purpose. One of the bones of the fore-arm reduced to this condition and thus tied, may be seen in fig. 25. 163. By a careful and-diffi- cult process even the gristle Fig. 25. maybe removed, so as to leave nothing but the soft cellular tissue in and by which the bone was originally formed. By preparing a bone thus re- duced with spirit of turpentine, it may be rendered so tran- sparent that you can read a book through its thickness. 164. The changes thus ef- fected by art, are often accom- plished in the living body by disease. There is a very ter- rible affection sometimes seen in Europe, but scarcely ever in this country, which reduces all the human bones nearly to the Condition of gristle, SO that Bone deprived of earthy matter. they will bend with the weight of the body or the limbs, until the unfortunate patient becomes horribly deformed and finally dies. In scro- fulous or cancerous complaints, a part or the whole of a particular bone may be reduced nearly to simple cel- lular tissue;—and in consequence of this change, I have known a person to break an arm by simply turning in bed. In a fewr rare instances, the gristle and earthy matter have been restored by the vital powers after such an alteration. 165. From what has been already said of the struc- 88 APPARATUS OF MOTION. ture of muscles and bones, you are now prepared for the statement of a general truth, which I introduce in this place in order to avoid the necessity of frequent repetition. Every part of the body of an animal, and consequently every organ that it contains, is composed, in part, of cellular tissue : after death, it may be reduced by art to the condition of simple cellular membrane. Any organ not essential to life may undergo this change in consequence of disease, and may be restored by the vital powers to its former condition. 166. This membrane, which, as you have been told, seems to form the entire body of the simplest animals, such as the hydra (65), is really the instrument by which all the organs are created. There is a time in the history of every animal before birth, when the body is composed entirely of cellular membrane, and is as simple in its structure as the hydra. The younger an animal is, the more nearly all its organs approach to this simple state. 167. When an earth-worm is cut in half (43, 44), it is the cellular tissue that grows, so as to form a new head or a new tail. And when the leg of a salamander (a little water lizard) is bitten off by a bird, or a fish, the same tissue buds out, like the branch of a tree, and forms a new limb, gradually constructing within itself the bones, the muscles, and all the other organs belonging to the perfect member. So, in man, when he is wounded, it matters not whether the injury occurs in a bone, in a muscle, or any other particular organ, it is always the cellular membrane that first unites or heals, and the matter peculiar to the organ is afterwards deposited within it. 168. Why it is that cellular tissue should form a bone in one part of the body, a muscle in another, &c, we know not, because the principle of life—the power that regulates the vital functions — is a mystery beyond the reach of human learning. 169. But let us return from this digression. It is scarcely necessary to tell you that the skeleton is com- posed of a great many bones, most of which are con- nected together by movable joints. If the extremities CARTILAGE--SYNOVIAL MEMBRANE. 89 of the bones at the joints were permitted to come in contact with each other, without the interposition of any softer matter, there would be great danger that the edges of the bones would be broken off in consequence of slight falls, blows, or violent motions; for bone is very brittle, and cannot be compressed. To guard against this danger, the extremities of the bones, where they form movable joints, are covered with a thick cap of white elastic matter called cartilage. 170. Cartilage bears a strong resemblance to the gristle of which the entire skeletons of many full-grown, and those of all very young animals are formed (159), and hence anatomists have termed the elastic covering of the joints the articular cartilages, to distinguish them from all other organs of somewhat similar appearance. 171. You may examine for yourselves the structure of the articular cartilages in the joints of any of the larger animals when cooked for the table; for, although the process of roasting or boiling alters them considera- bly, they will still serve your purpose very well unless they have been overdone. A knuckle of veal or a pig's foot will furnish you with the best example, and may be examined in the kitchen before it is dressed. One such examination will give you clearer ideas than a volume of description. 172. To prevent friction between the articular carti- lages when the body is in motion, every movable joint is provided with a delicate sac of very thin and perfectly smooth membrane, called a synovial membrane. This lies between the articular cartilages, covering them so closely wherever it touches them that it can scarcely be separated from them; but at the sides of the joints the membrane is much less closely connected with the sur- rounding parts; so that it maybe more readily seen. The synovial membrane or sac always contains a small quantity of a peculiar unctuous fluid called synovia, which "answers the same purpose with the oil that we pour upon the axle or pivot of a wheel to make it turn more easily; and this fluid is secreted by the mem- brane which contains it. 8 90 APPARATUS OF MOTION. 173. When we were speaking of that form of con- tractility which is called tonicity (115, 116), you were informed that the habitual tone of the muscles keeps the bones, or rather the articular cartilages; alw7ays pressed against each other with a certain degree of force. But, in extensive and sudden motions of the members, the bones would be continually liable to be put out of joint, or dislocated, were they not bound together by some firmer material than muscle, and one less capable of being stretched or contracted. To secure the animal against such accidents, the joints are provided with an- other set of organs called ligaments. 174. The ligaments are composed of cellular tissue very much condensed, and strengthened by strong and numerous fibres. They are white like the fasciae, in- elastic, and cannot be suddenly stretched to any consi- derable extent except by most violent forces. Though flexible like membranes, and soft to the touch, they are much stronger in proportion to their size than the bones w7hich they bind together. Their principal function ap- pears to be the prevention of too extensive motions in the joints; for many of them remain perfectly loose while the bones are in an easy or common attitude, but when they are bent as far as they are intended to go, some of the ligaments are drawn tight, like cords, and thus prevent either the muscles or slight accidents from moving the joints any further. Let me give you an illustration. The leg, in man, is intended to bend back- ward in walking, and to remain straight in standing. It can be bent backward until the heel touches the thigh, without straining any of the ligaments, because the thigh itself prevents it from being carried further in this direc- tion than is suitable to the wants of the animal. But if you endeavour to bend the leg forward or to either side, you soon find it impossible, because there are very powerful ligaments on the sides and in the interior of the knee joint, which are put on the stretch whenever you attempt to cause such a motion. Tremendous forces sometimes dislocate the strongest joints; but, whenever this occurs, either some of the ligaments are LIGAMENTS--PERIOSTEUM. 91 broken, or the parts of the bones to which they are attached are torn off. The latter accident is even more frequent than the former. In fig. 26 you have a repre- sentation of the ligaments of the elbow joint. 175. You have been told that each of the muscles is inclosed in a kind of sheath or covering of cellular mem- brane or fascia (144). Each of the bones is inclosed in a similar manner by a mem- brane composed of cellular tissue strengthened by very numerous and irregular fibres, so that its structure bears considerable resem- blance to that of the liga- ments. As we may have occasion to mention this kind of membrane again it is well to name it at once. It is called the periosteum. 176. The periosteum ad- heres very firmly to the bone, and covers all parts of it, except those which give ori- gin or insertion to the liga- ments and muscles, and those which are coated with car- tilage. In some places, the Ligaments of the Elbow Joint. " . • ,, j„j „..„^. c, The bone of the arm. ft, c. Bones periosteum IS extended OVerof t'ne forearm, d, A lateral ligament ttio ciTrfrar-c* r\f n nnrtilfirrp • "f the elbow joint, e, The capsular me SUIIdCe Ol d. oai mage , ,igament of the elbow joint. /, A liga- and the membrane then takes ment connecting the bones of the fore- another hard name. It isann- not essentially changed in its nature, and it is hardly right to task your memory with its title. It is called the perichondrium. The periosteum covering the outside of the bones of the skull has received the name of peri- cranium. 177. Having now enumerated the principal classes of 92 APPARATUS OF MOTION. organs, &c, that belong or are appended to the osseous or bony system, namely; the bones, the cartilages, the ligaments and the periosteum; let us return for a few moments to the muscles. 178. Most of the voluntary muscles are large, for they are designed to exert great force. Now, if they were so formed as to preserve the same fbshy and bulky character throughout their whole extent, the joints which they surround or cover in their passage from one bone to another would be buried as deeply as • • any other parts of the bones. The elbow would be at least as thick as the arm, and the knee would rival the calf of the leg. Moreover, the bones would not present sufficient surface for the attachment of such a multi- tude of fleshy fibres. All symmetry of form would be destroyed, and the strength would be exceedingly diminished. But, to prevent these inconveniences, the muscular fibres of many of the principal voluntary muscles are made to terminate in much finer fibres of a pearly hue, possessing far greater strength than those of the red, fleshy-portion of the muscle. These smaller fibres are crowded together so as to occupy very little space in comparison with the more bulky part of the organ. Any bundle of such fibres which may be con- nected with a single muscle is called a tendon. A drawing of one of these accessories belonging to a double muscle is seen at fig. 24, where a and b repre- sent portions of the two fleshy bellies of this muscle, both terminating in the single tendon c. 179. Some of the tendons are round, like a cord, and others are flattened until they resemble a very thick fascia, from which, indeed, they do not differ very widely in composition. One of the former kind you may examine in your own person by grasping the back of your ankle an inch above the heel. The thick hard cord that you feel there is a tendon connected with the muscles that make the foot point downwards, or lift the whole body when we stand on the toes. Small as it is, every fibre of the flesh composing the bulk of the calf terminates in it. ARRANGEMENT OF TENDONS. 93 180. The tendons do not contract like the fleshy fibres, nor can they be stretched any more than the ligaments (174). They act like simple ropes or bands to connect the ends of certain muscles with the parts that those muscles are intended to move. The me- chanical arrangements of the tendons in the larger animals and man are often exceedingly curious. Some of them run over pulleys formed by grooves in the bones near the joints, which pulleys are covered with cartilage and synovial sacs to prevent friction (172). Sometimes Fig. 27. Section of the Orbit. The Human Eye and its Muscles. a The outer straight muscle of the eye, cut off" from its attachment at the bottom of the orbit, and turned up to display the other parts. 6, c, d. The other straight muscles e The superior oblique muscle, with its tendon running through a car- tilaginous pulley near the edge of the orbit, and turning back to be inserted on the outside of the globe of the eye. /, The optic nerve. they are bound down in their places by ligaments which stretch across the grooves. Some of the tendons are perforated by smooth openings, resembling button-holes, through which other tendons pass to reach their destina- tion &But one of the most curious of these arrangements 8* 94 APPARATUS OF MOTION. is seen in an oblique muscle of the eye, of which the tendon runs through a pulley within the orbit, and then doubles itself backward, so as to move the eye in a direc- tion opposite to that of the motion of the muscle. (See fig. 27.) If you wish to examine the action of a tendon for yourself, take the leg of a dead bird; cut off the skin with a sharp knife, and draw with your fingers any of the white cords that surround the bone. You will immediately see a motion produced in the foot or claws, and the kind of motion will depend upon the tendon which you happen to have seized. In birds, some of the tendons are often partly composed of bone; and the cap of the knee in man, though a bone, appears to belong rather to the great tendon of the muscles on the front of the thigh than to the skeleton, with which it is not directly connected. 181. The involuntary muscles are rarely provided with tendons. They are scarcely ever formed into dis- tinct masses, like the voluntary muscles; but, are nearly always composed of fibres interlacing or overlapping each other in various directions; and, instead of being connected with the bones or hard parts of the animal, they are usually found spread out, like a membrane, around some hollow organ, such as the stomach for instance, to which they furnish a distinct coat called the muscular coat. When called into action, the fibres of the muscular coat contract in such a manner as to expel the contents of the organ that they envelope. All parts of the alimentary canal of the more complex animals are provided with a muscular coat, designed to drive forward the food and its products, as the process of digestion advances. • 95 CHAPTER VI. ON THE GENERAL DIVISIONS OF THE VASCULAR SYSTEM. 182. Many of the organs that have been mentioned are large and solid. Their structure, and, consequently, the materials of which they are composed, are very various. In some of them we observe many different kinds of matter combined to form a single organ. Thus: in each of the bones, when perfect, there are found the cellular tissue, the cartilaginous matter, and the earthy substance or lime (160). Now these various and often very complex organs, must be provided with the mate- rials necessary for their growth and support from the same nutritive fluid; and you will naturally conclude that it would be scarcely possible to convey this fluid throughout all parts of a machine so complicated, by suffering it to pass from cell to cell through the whole body, as in the polypi (59, 60.) Nor could it be more conveniently distributed by means of a stomach branch- ing and sending canals to every part, as occurs in the medusa (122). Accordingly, we find that in all the more important animals, the nutritive fluid formed by the process of digestion in the alimentary canal, instead of being absorbed into the general cellular tissue, as in the hydra, &c. finds its way, by a process that will be ex- plained hereafter, into a great number of minute ves- sels, canals, or tubes, that all tend toward some common centre or receptacle in the substance of the body, en- tirely distinct and removed from the alimentary canal. These tubes or canals are known by the general name of the blood-vessels, and the nutritive fluid having been sufficiently prepared to enter them by the first steps in the process of assimilation (47, 48), is then properly called the blood. 96 VASCULAR SYSTEM--VEINS. 183. The blood-ves- sels through which the blood flows toward the common centre just mentioned, are called the veins; and in ani- mals placed high in the order of nature, the mi- nute veins are found in every part of the body in countless numbers. To obtain some idea of their number and ar- rangement, you may glance at the figure of the venous system in man, as represented in fig. 28. 184. The blood in the veins is constantly flow- ing toward the common centre or receptacle (182); for these vessels are generally provided, internally, with nume- rous valves or flood- gates, which will not allow any thing to pass in the opposite direction. The structure of these valves you will be better prepared tO Understand The General Venous System. hereafter, but fig. 29 will convey some idea of their ap- pearance in a vein that has been laid open. 185. The common centre or receptacle is very dif- ferently constructed in different animals. In insects and worms it is merely a single very large blood-vessel, running lengthwise along the back, and provided with a muscular coat or some such contrivance to force the blood forward towards the organs that it is intended to VALVES OF THE VEINS. 97 nourish. In the higher orders of animals, it is a very strong hollow muscle, designed to re- ceive a small quantity of blood at a time, and then, by contracting, to urge that quantity on- ward. The receptacle, when constructed in this manner, is called a heart; and the beat- ing of the heart, as it is called, is produced by the motion of this most important organ in pumping its con- tents. 186. When the blood from the veins has filled the heart or the great vessel that answers the same purpose, it is ne- cessary that it should be conducted through another set of channels to all parts of the body, and into the substance of every organ, in order to nourish it. For this purpose another set of blood-vessels, called the arte- ries, is provided. One or more great arteries originate from the heart, and pursue their course toward the ex- tremities. Each artery soon branches into two or more trunks, and each trunk'is again and again divided, until at length the number of branches exceeds all calculation; and there are few parts of the body into which a pin can penetrate without wounding one. For a general idea of the distribution of the arteries in man, you may refer to the view of the arterial system as represented in fig. 30. 187. The current of blood produced by the action of the heart is very rapid; and you are not to suppose that any part of the body employs all the blood which is sent A Vein laid open to show the Valves. o, The trunk of the vein; 6, 6, the valves; c, a branch of the vein entering it. 98 THE VASCULAR SYSTEM. to jt for its growth or Fig. 30. sustenance. In fact a very small proportion of the whole amount is actually converted into cellular tissue, muscle, or other solid matter, in the course of a single day. But the heart drives for- ward so much, at every beat or pulsa- tion, that, in a full grown, healthy man, all the blood in the body must pass through that organ several times in an hour. You perceive, therefore, the necessi- ty of some connexion between the arteries and the veins, in order that the blood driven by the heart through the arteries into the organs, may be re- turned through the veins to the heart. This communication is effected by the con- tinuation of the ex- tremely minute bran- ches of the arterial system into the equally minute roots of the venous system: so that if you inject a large quan- tity of coloured water into the principal artery of an animal, soon after death, the water will pass into the veins, and return through them to the heart. The small- est divisions or ramifications of both sets of blood-vessels are scarcely, if at all, visible to the naked eye; and as The General Arterial System. SIMPLE FORMS OF CIRCULATION. 99 they are as fine or finer than a hair, they are called by physiologists the capillaries or capillary blood-vessels. 188. What you have now been told will give you some idea of the nature of the circulation, which is that process by which the blood, in those animals that are provided with blood-vessels, is kept continually in motion toward and from every part of the body. 189. The more closely you study physiology and na- tural history, the more you will be surprised at the gra- dual and beautiful manner in which one organ is added after another, as you proceed from the observation of the more simple to that of the more complex animals; you will observe that each of the principal vital func- tions, which, in the hydra, is performed seemingly by the skin or the common cellular tissue, requires, in the higher classes of animals, a peculiar system of organs; you will see this system rendered more and more complex as animals rise in what has been termed the scale of nature ; and the performance of the function will be found more and more perfect in proportion to this com- plexity. Common cellular tissue may digest well enough to support the frame of a hydra (72,) but it is not suffi- ciently active to nourish an insect: and insects have, therefore, a very complex alimentary canal for digestion. Again : The contractility of the cellular tissue alone may be sufficient to drive the nutritious fluid to all parts of the body of a polypus, but it would fail to answer the same purpose even in an earth-worm; and an earth-worm is, therefore, provided with blood-vessels and a proper circulation. 190. In most perfect insects, (that is, in most of those that have reached their full development, like the cat- terpillar that has become a butterfly,) the circulation does not appear to be complete. They have a large blood- vessel running along their back, often terminating at either end in some branches which have been supposed to open into the general cellular tissue of the animal. In this blood-vessel, which is highly contractile, the fluid is driven onward in waves, sometimes in one direction and some- times in the other, but generally from behind forwards. 100 THE VASCULAR SYSTEM. This blood-vessel may be regarded as an artery; but as veins have been detected in very few insects, it is still believed by many that the blood is merely agitated or mixed in this vessel, which is supposed to receive it by suction or absorption from the cellular tissue at one of its extremities and at certain other places, and to drive it out into the same tissue at the other extremity. In several of the imperfectly developed insects, such as those larvae* which come from the water and afterwards form the dragon-fly, we find a complete circulation, and these animals furnish us with the simplest example of a circulatory apparatus. It consists of the dorsal vessel just described, and another which may be considered as a vein, running along near the under surface of the body. These two vessels communicate with each other at either end by means of numerous branches; they both send out several lateral ramifications toy various parts of the body and limbs, and there can be no doubt that these branches communicate with each other in the substance of the various organs. The blood in these larvae is seen to flow from the tail toward the head, through the princi- pal artery or dorsal vessel; and, as it passes, it sends its divided current into the smaller arteries till these become too minute for examination. We can then detect it flow- ing through all the little veins toward the principal vein or inferior vessel, through which it is constantly moving from the head toward the tail; whence it is forced to return again into the dorsal vessel. 191. In the leech, and most marine worms, we find several other large vessels running longitudinally, and receiving a portion of the blood, for purposes that you are not yet prepared to understand; but, like the inferior vessel, they return this blood to the great artery. Even in animals apparently so insignificant as the oyster and other shell-fish, but which take higher rank than the * Most insects pass through at least four forms or conditions during their lifetime. —1. The egg. 2. The larva. 3. The pupa. 4. The imago. In the silkworm, you may easily make yourselves acquainted with these changes. The larva is the worm, the pupa is found wrapped up in the cocoon, and the imago is the perfect fly. CHYME AND CHYLE. 101 insects from their organization, we find the circulation much more complex, for they have no longer a single dorsal artery but a regular heart, sending the blood into many sets of vessels. You will not be surprised, then, to hear that the circulatory system in man is explained with difficulty to those who have never considered that system in those animals which are very simple in their structure. After these remarks, however, I trust you will find it an easier study when we reach the subject. 192. You have been promised an explanation of the process by which the nutritive fluid makes its way from the alimentary canal into the blood-vessels (182,) and it is right to say a few words upon that subject here. That peculiar kind of absorption seemingly resident in cellular tissue, by which it takes into the body the nour- ishment derived from the food in the hydra (59, 60) and the medusa (122, 123,) is commonly called imbibition. We know nothing of its nature, it is true, but we know that it takes place, and it is therefore convenient to give it a name, as we do when we call the power which makes a stone fall to the ground attraction, though we only know the simple fact that stones will fall to the ground when unsupported. 193, By imbibition, then, it is probable that the nour- ishment extracted from the food by digestion, (which crude nourishment we call the chyme,) is taken into the cellular tissue of insects, worms, and other animals with a very simple circulation ; for we cannot trace any inter- mediate passage^ between the circulatory and the diges- tive apparatus in these animals. Now no openings are known to exist in the sides of the blood-vessels; and these vessels, like all the other organs of the body, how- ever complex, are formed originally of the cellular tis- sue. It is, therefore, reasonable to conclude that the nutritive fluid, after entering the substance of the ani- mals just mentioned, is carried thence into the blood- vessels by imbibition. 194. But here I must pause to explain some other facts. The chyme, while it remains in the alimentary canal, is as yet imperfectly assimilated (47, 48,) and requires 102 THE VASCULAR SYSTEM. further changes to fit it for entering the circulatory apparatus. These changes probably commence at the moment of its first imbibition; and when it has once entered the substance of the body, it is called the chyle. Even the chyle is not exactly similar to the blood in animals that have a circulatory apparatus, but requires to be mingled with that fluid, and to circulate for a time before it becomes fitted to nourish the several organs of the body. These facts are ascertained by examinations made upon the larger animals, and you will comprehend them better hereafter. 195. In the higher orders of animals, the chyle is never found wandering in the cellular tissue, as it may be, per- haps, in insects, (190,) but is conveyed to the blood-vessels through another set of vessels, called the lacteals. These, though they supply the blood by carrying into it the nourishment extracted from the food, are not a part of the circulatory apparatus, but constitute a separate sys- tem of canals passing from the bowels to the blood-ves- sels—a system unknown in the simpler animals. 196. The chyle in the lacteals is always white or milky, even in those creatures whose blood is red. Yet it is an organized fluid, and contains globules, like the blood and the sap of some plants (49), though of smaller size than those observed in the arteries and veins. 197. The lacteals originate in countless numbers from the internal surface of the alimentary canal below the stomach. There is no reason to suppose that their mouths stand open, so as to drink in the nourishment from the chyme (193) as it passes; but they imbibe this nourishment through the cellular tissue, of which their sides are formed; so that there is no direct communi- cation between the lacteals and the bowels. These vessels, like the fine branches of the roots of a plant— which seem to answer the same purpose in the vegeta- ble kingdom—continually join with each other so as to form larger trunks as they pursue their course toward the centre of the circulatory system (182, 183,) until at length they are all collected into one great canal, called the thoracic duct, which opens and pours its contents THE LACTEAL VESSELS AND GLANDS. 103 into one of the largest veins of the body, just before it enters the heart. 198. The lacteals are furnished with valves, like the veins, to pre- vent the chyle from flowing in any other direction than towards the blood-vessels; but these valves are much more numerous, occurring so frequently as to give the vessels a peculiar knotty appearance; as you see in fig. 31. 199. You have been already in- formed (194) that the chyle is but imperfectly assimilated when it first enters the body, and requires fur- ther changes after it enters the blood, in the route of circulation. Now it appears that in those ani- mals which are furnished with lacteals the chyle is continually changing, and becoming more and more assimilated, from the moment of its first imbibition until it reaches the thoracic duct (197). In order that time may be allowed for this mysterious change, the lacteals pur- sue a very winding course, and every here and there they are studded with little rounded bodies — fig. 31, /;, — into which several branches are seen to enter, and from which a smaller number of larger trunks usually make their exit. These bodies are called glands, and in their interior, the little lacteal tubes are rolled and tangled together, like a bundle of fishing-worms, so that they very con- siderably increase the length of the route by which the chyle has to travel toward the blood-vessels. 200. By this time you must be much less surprised Lacteals. a, Branches of the lacteals. b, A gland. 104 THE VASCULAR SYSTEM. than formerly at learning that many of the simpler animals may be cut to pieces without being killed. For, if a polypus be divided, each piece is capable of digest- ing its food, and may grow:—if a worm be cut in half, each end has part of all its great blood-vessels and some of their connecting branches left; and it can still carry on a circulation, provided the ends of the vessels con- tract so as to keep it from bleeding to death:—but, in a quadruped or bird, if the main trunk of the lacteals be injured, the creature must starve, even though he may continue to digest his food and his circulatory system be in perfect order. His nourishment cannot then reach the blood-vessels, and of course his organs cannot be supported for any great length of time. 201. The lacteals, however, are not the only vessels that convey substances into the circulation, though there is every reason to believe that they furnish the only very important route through which nourishment can be introduced in the larger animals. Let me ex- plain. If you put a blister upon any part of the body, you can easily cut away the cuticle or scarf-skin (28), so as to lay bare the true skin beneath; but you do not produce a wound, or lay open any blood-vessel by so doing. Yet if you then dust the blistered surface with certain medicinal powders, these will be found to act on the patient precisely^ if they had been taken into the stomach: and, in most cases, these effects can be rationally explained on no other supposition than that part of the medicine is absorbed and carried into the circulation. There is every reason to suppose that water, mercury, and some other substances are even imbibed through the cuticle so as to enter the blood. When a poisonous snake has bitten any part of the body, the poison very soon circulates and produces the most serious consequences; and I could recount a thousand facts of a similar nature. Some substances artificially introduced in the extremity of an animal, through a wound, have been afterwards found in the blood, and have been actually collected from it. Now, here there are no lacteals to convey these matters into THE LYMPHATIC VESSELS AND GLANDS. 105 the circulatory system. How, then, do they arrive at their destination? 202. It is thought by many, that the veins of the part may sometimes imbibe these substances directly. And, indeed, we have reason to believe that, even in man, the cellular tissue and the blood-vessels retain the power of displaying all the functions that they perform in the hydra, the medusa, or the earth-worm; imbibition among the number (58, 59, 189); though these functions are far too feebly exercised to supply the wants of so noble a creature as the lord of creation. 203. But anatomy displays for us another route through which these substances may and actually do reach the circulation. We find in the more complex animals a countless multitude of little vessels originating from almost every part of the body, even from the interior of almost every organ. These vessels are very much like the lacteals ; but they are constantly filled with a colour- less or slightly bluish fluid, called the lymph, and the vessels themselves are called the lymphatics. The lymph is always flowing towards the centre of the circulatory system, and the vessels that convey it are continually uniting into larger trunks, a great majority of which empty into the general receptacle of the lacteals (197), where their contents are mingled with the chyle before it mixes with the blood. The other lymphatics empty directly into some of the larger veins. 204. To prove that the lymphatics do actually convey to the circulation some of the substances mentioned at paragraph 201, it is only necessary to state, that poison- ed wounds not unfrequently produce most terrible effects in consequence of the poison finding its way along the lymphatics running from the part, which it inflames as it goes, so that you can trace by the swelling, redness, and pain, the extent to which the poison has travelled. 205. The lymphatics, like the lacteals, are provided with glands, which are generally found larger and more numerous about the principal joints than in other parts of the body. The glands, in addition to the uses already pointed out at paragraph 30, seem to act as guardians 106 FUNCTIONS TRIBUTARY TO NUTRITION. against the introduction of noxious substances into the circulation; for when a poison has reached one of these organs, in following the route of a lymphatic or lacteal vessel, the cellular membrane between the worm-like folds of the canal (199), inflames and swells. The glands being each inclosed, like many muscles and other organs, in a firm covering of cellular tissue strengthened by fibres not very easily stretched, this inflammation frequently causes the vessel to close by the pressure of the swelling, and cuts off the route of the poison towards the veins and heart. 206. The lymphatics are not discovered in the simpler animals; but, in those of the higher orders, they fulfil most important purposes, which will be explained in the next chapter. When spoken of collectively, they are often called the absorbent system, and the individual ves- sels are not unfrequently styled absorbents. These terms are unfortunately employed by physiologists, for they are calculated to deceive the student and to lead to the be- lief that the lymphatics are the only organs capable of carrying on absorption; which is very far from the truth. Should I use the term absorbent system in the after part of this volume, you will understand me to allude to both the lacteals and the lymphatics, and when the absorbents are mentioned I do not wish to exclude even the veins, for reasons given in paragraph 202. CHAPTER VII. ON THE FUNCTIONS OF SECRETION, RESPIRATION, AND NUTRITION. 207. You have received in the preceding chapter some idea of the complexity of structure observed in the more perfect animals. You have seen that this complexity requires an extension and a corresponding complication NECESSITY OF PERPETUAL SECRETION. 107 of the masticatory apparatus (124), and the digestive system (130), in order to supply proper support to the frame. The number of separate bones, muscles, and other organs demanded to enable the animal to seek and prepare food and to move it along the alimentary canal as the process of digestion advances, requires that the nourishing fluid in these animals should be confined in blood-vessels (182), and conveyed to and from all parts of the body by means of a circulatory apparatus (188), which, in its simplest form, is composed entirely of blood- vessels, but, in creatures a little more complicated, de- mands a heart (185) as a principal moving power to carry on the circulation. You have also learned that, at first, the admission of the nourishment into the circu- lation appears to be effected by simple imbibition (192, 193), but that as animals advance in the scale of nature other assistance is required to convey it from the alimen- tary canal into the blood-vessels. Hence the necessity for the lacteals (195). You have been told, moreover, that in the higher orders of animals certain substances are carried into the blood from the surface of the body, or from the interior of the various organs, and that for this purpose the lymphatics are provided (202, 203). Yet the circulatory system in all the more important animals is much more complicated than you would sup- pose, even from what you have learned heretofore; and in the present chapter I propose to introduce you to an acquaintance with certain deeper mysteries connected with it. In order to do this properly, I must quit for a time the regular course of my narrative to communi- cate some preliminary information. 208. It is easy to understand that, while an animal is growing and forming its various organs, it must con- stantly require food to supply the materials necessary for its growth; and the circulation of the blood must be continued regularly and perpetually. But why should food be demanded, or why should the blood circulate, after the animal has reached its full dimensions, when its organization is complete and perfect? You may reply that the wearing of the cuticle, nails, horns, or 108 FUNCTIONS TRIBUTARY TO NUTRITION. other external parts, demands a supply of food and blood to make up for these losses; for such parts are contin- ually growing as fast as they are worn away, even at a late period in life. But a very small amount of food and blood would be sufficient for this purpose; and yet the full-grown animal requires nearly as much food, and has nearly as much blood in its vessels, as the young one: why is this 1 209. If you place a vase of flowers, or a living plant, under a bell-glass, you will find, in a few hours, that the inside of the glass is obscured by moisture collected in little drops all over the surface: and this experiment proves that vegetables, which absorb water by their roots (33, 34), actually give out water from their leaves and branches. In like manner, the surface of animals is continually pouring forth a fluid which we call the perspiration. You do not see this fluid upon the surface of organized beings at all times, because it is usually thrown off in the form of a gas that is invisible, and combines immediately with the common air. It is only when heat, exercise, or disease has increased very greatly the flow of perspiration, that we see it collected on the surface in the liquid form of sweat. But, to con- vince you that the fluid is at all times escaping, during health, you have only to bind closely upon your arm, or any other part of the body, a piece of India-rubber cloth or oiled silk, and, in a few hours, you will find the surface beneath it completely wet, because the fluid discharged from the skin cannot pass through the covering, and is therefore compelled to collect in such quantities as to arrest attention. If the experiment be long continued, the sweat will generally ooze out round the edges of the cloth and flow down the limb. The escape of gaseous moisture from the skin is called insensible perspiration; but when the discharge is condensed so as to assume the liquid form, it is called the sensible perspiration. 210. When you breathe upon a looking-glass for a short time, you observe the glass to become obscured by the moisture from the breath, which soon accumulates so as to gather itself into large drops that run down the glass. This proves that the same process is going on at INTERSTITIAL ABSORPTION. 109 all times and very actively, within the cavities of the body. 211. Now this constant discharge of perspiration amounts, in twenty-four hours, to a very considerable quantity. It is a secretion (96, 97;) and like all the other secretions, is furnished from the blood. You can now comprehend one of the reasons why full-grown animals require regular supplies of food. This is necessary in order to replenish the blood continually drained by the secretions. 212. The number and quantity of the various secre- tions poured out from the body, and therefore taken from the circulation, is much greater than you might at first suppose. The tears, the mucus lining all the ali- mentary canal and many other passages, as well as the various fluids, such as the saliva, the bile, &c, that are required to assist in the digestion of food, may be men- tioned as important secretions; and their formation demands no inconsiderable supply of nourishment at all ages to maintain the proper amount of blood. 213. In many fevers, the insensible perspiration is checked, and all the secretions are very much dimi- nished in quantity: and this is one reason why the sick often have no desire for food, and why undue nourish- ment so frequently renders them worse by forming too much blood. 214. I must now proceed to explain another much more wonderful vital operation. If an animal in health be deprived of its necessary food, the secretions still continue until the circulation is so far exhausted that it can no longer supply the wants of life, and the animal becomes diseased or dies. In fevers, life may be some- times preserved without food for a greater length of time than in health, because the quantity of the secre- tions is then diminished. The loss of-the circulating fluids during partial starvation renders the animal thin- ner, but it will not account for the extent to which that thinness is often carried. A person who is fat at the commencement of an attack of illness, or a stout man who is compelled to submit to short allowance at sea, soon loses his unnecessary fat; and after a time even 110 FUNCTIONS TRIBUTARY TO NUTRITION. his muscles, (particularly those of animal life) are gra- dually diminished in size until they can no longer per- form their office, and he may become so weak as to be unable to turn in bed. 215. If deprived of all food, an animal generally dies before the solid organs of its body are so very much diminished; because the exhaustion of the fluids by the secretion stops the circulation too suddenly. But when placed in circumstances that enable it to obtain some food, but not enough, the changes which take place in the frame-work of the body are very curious. All the organs are gradually diminished in bulk, but those which are least important to life are diminished most rapidly. The heart, for instance, or the alimentary canal, is rendered feeble, but the muscles of voluntary motion may almost disappear, and the fat is only to be seen in a few places where its presence happens to be essential to the organs in or about which it is formed. If the slow starvation be carried still further, some of the less important parts of the body may be entirely removed. Ulcers break out on the extremities, and some of the organs that can be spared without the sacrifice of life are totally destroyed. I have seen most of these effects produced, in a young man, by a tumour that pressed upon and finally closed the great canal through which the chyle flows into the blood (197:) so that, although he continued to eat, and for many months par- tially digested his food, he was as effectually starved as if he had been inclosed in a dungeon with an allowance of food diminished every day until nothing was left. 216. Now a moment of consideration will convince you that the substances that disappear from the body, wholly or in part, during starvation, must be taken up by absorption from the organs or parts where they had been previously placed, and carried out of the body by some means. There is no route by which they can thus be carried out from those animals* that have a circula- tion except through the blood-vessels; and the blood- vessels have no other efficient means of discharging them but by the secretions. Hence you see that the ALTERNATE LIFE AND DEATH OF PARTICLES. Ill exhaustion of the blood by the secretions, when an animal is deprived of food, is compensated as long as possible by the absorption of the less important particles of the body, which are carried into the circulation by the lymphatics ; and, perhaps, byimbibition into the veins themselves (202.) In other words, we may say the starving animal lives for a time upon itself, eating up by internal absorption such parts of the body as can be spared under urgent necessity, to feed those organs and to continue those functions that are absolutely essential to life. 217. But starvation is not necessary to cause this constant absorption of particles from the interior of the body. I have merely selected this very striking example because you may all observe it for yourselves in the sick-room, or in persons who are ordered to subsist on low diet for a long time. The same operation is going on at all times, even during the highest health. If the organs of an adult animal in health do not diminish, it is only because the blood-vessels nourish them with new particles as fast as the absorbents carry off'the old ones. If all the organs of a young animal grow stronger with time, or if the same effect is produced in any particular muscle by exercise, it is because the blood-vessels, during youth, deposit more particles in a given time than the absorbents can take up. 218. It is one of the most curious laws of life, that there is not a particle in any organized body that caii( fulfil its proper functions beyond a certain length of time. It must then be removed from the body and an- other deposited in its place by the blood-vessels: so that in a few years there will not remain in your own person one atom that now assists in forming your bones, muscles, brain, or any other portion of your frame! You will be the same "if you live, and yet another! for you will be composed of new materials. It is the im- mortal part of man alone that preserves the identity of the individual! You can be no longer surprised that an animal whose organization is perfected requires nearly as much food to support that organization as a younger 112 FUNCTIONS TRIBUTARY TO NUTRITION. one in which many of the organs are still in the act of growing. 219. As the blood-vessels are the reservoirs into which all the worn-out particles of the body that are no longer fitted to fulfil the functions of life are continually poured by the absorbents, it follows that the blood would be- come more and more impure by these additions of ex- hausted matter, until no longer fitted to support the frame, were not some arrangement made for the ejection of such materials from the body. This necessary duty is performed by the secretions. 220. The secretions in animals that have an organi- zation somewhat complex are very numerous and of widely different appearance. Thus; the tears, the bile, the perspiration, the saliva, &c, are all secretions, and all contribute to purify the blood; but they bear little resemblance to each other. 221. Why the blood-vessels should secrete tears in one place, bile in another, and perspiration in a third, we know not. This is one of the mysteries of life that so often lead weak-minded philosophers to travel beyond the bounds of human reason in search of first causes, a journey that always results in the accumulation of a cargo of words instead of things, to be brought home for no other purpose but to confuse the minds of others, and deceive ourselves into the belief that we are acquir- ing a store of facts, while we are really endeavouring to hoard up empty sounds. All that we can reasonably ex- pect to ascertain in relation to the different secretions is the anatomical structure of the parts by which they are constructed. 222. So far as the blood-vessels alone are concerned, there is one point of resemblance between all parts of the body which secrete or separate the secretions from the blood. The capillaries of such parts are divided, branched, or multiplied to such an extent that, when filled with coloured glue, the whole mass often seems at first sight to be composed altogether of blood-vessels; for it will be generally found of a colour almost uniformly red throughout. Such is the structure of the true skin, and SECRETORY GLANDS. 113 of the internal lining of the alimentary and all other canals that open on the surface of the body; called the mucous membranes. The true skin secretes perspira- tion, and the mucous membranes throw out the mucus that lines all such passages, and gives name to these membranes. 223. Many of the secretions are the work of particular organs, expressly designed to construct them. They are called glands, but to distinguish them from another very curious class of organs belonging to the lymphatic and lacteal systems, and known by the same general name, the glands that produce secretions are termed the secre- tory glands. 224. The secretory glands are as various in structure as the secretions which it is their function to produce. In some of them the capillaries are wound or bundled together like a group of earth-worms in a cup ready for a fishing excursion: in some, the minutest branches are arranged in sets more like the teeth of a fine-tooth comb; while in others, they form beautiful brushes like the rays of light flowing from a sharp point placed on the prime conductor of an electrical machine, or the groups of bristles that form a tooth-brush: but these vessels are too small to be distinguished by the naked eye, and it requires the aid of the microscope to render them visible. 225. The secretions of the secretory glands are gene- rally poured out by the capillary blood-vessels into a mul- titude of membranous tubes within the substance of the glands, often as minute as the vessels themselves; and these tubes run together continually, forming trunks larger and larger until they are collected into one or more tubes or passages called ducts, which lead the secretion to the surface of the body or to that of the alimentary canal. And all these ducts are lined with mucous membrane, like the other internal passages that communicate with the surface (212). 226. When we throw a very fine coloured fluid with some force into the blood-vessels of a dead young ani- mal properly prepared, the fluid can be made to flow 114 FUNCTIONS TRIBUTARY TO NUTRITION. into the ducts of the secretory glands, and into all the passages lined with mucous membrane; but the most careful examination does not detect the slightest com- munication between the capillaries and the ducts or the other passages. It appears that the blood in the vessels is brought extremely near to the ducts or the surface designed to be bathed by the secretions, but there is every reason to believe that there is always an astonish- ingly thin layer of cellular membrane between the blood and the ducts or the surface. Through this layer the secreted fluids must pass in order to escape from the circulation; and the process by which this passage is effected is called transpiration ;* a process closely re- sembling perspiration. This is one of the proofs that the cellular tissue in the more complex animals exercises all the functions that distinguish it in the hydra and the polypi, where it effects all the secretions without the aid of blood-vessels. 227. It is observed that all the phenomena of nature give evidence of a beautiful economy; and this is clearly exemplified in the history of most of the secretions. Though these fluids are composed in part, and perhaps principally, of the worn-out particles of the body (216), yet nearly all of them are made useful in some way be- fore they leave the frame entirely. Thus the tears in man, which are secreted by a small gland within the bony orbit of the eye, are poured out through six or more little ducts running down near the outer corner of the upper eyelid, where they may sometimes be seen by reverting the eyelid. Here the tears spread themselves over the eye to prevent friction between the ball and the lids, which would be extremely irritating to an organ so delicate. They are then taken up or absorbed by two other ducts that run from near the inner corner of each eyelid to a canal leading into the nose, where they assist in preserving the moisture necessary to the perfection * Transpiration is a term often used generically, to signify the passage of fluids or gases through membranes, internally or externally; but per spiration is a specific term signifying transpiration on to the external sur face. RESPIRATORY APPARATUS. 115 of the sense of smell, and prevent the extreme dryness of the mucus, that would otherwise result from the almost continual rush of air through the nose in breath- ing. Around the mouth there are found several glands called salivary glands, that secrete the saliva, pouring it through as many ducts into the mouth. The saliva as- sists in preventing too much friction from the food in the act of swallowing, or deglutition. It also assists in preparing the food for digestion, and probably aids in producing healthy chyme (193), for we find another gland, called the pancreas or sweet-bread, in the inte- rior of all large animals, which secretes a similar fluid, and empties it through a duct into the alimentary canal just below the stomach, where it is mingled with the chyme as it passes from the latter organ, and before it is absorbed by the lacteals. The bile is the secretion of the largest gland in the body, called the liver, of which we shall have occasion to speak in another part of this volume. The bile passes through thousands of little ducts in the interior of the gland until these are col- lected into one great duct that passes into the alimentary canal at the same place with the duct of the pancreas. What part the bile plays in perfecting the chyme we know not, but there is strong reason to believe that it acts as the natural purgative, and accelerates the pas- sage of the food along the alimentary canal. 228. But the most important, and the most universal of the secretions, is that which is carried on by the or- gans employed in breathing, or respiration. The func- tion of respiration is performed by all organized beings. In plants, the leaves are the breathing organs, and their office is so important that if all the leaves be plucked or prevented from growing during the summer while the vital functions are carried on actively in the stem and branches of a plant, it will die as certainly as a man when strangled or confined under water. 229. The principal object of breathing, in animals, is to free the body from the worn-out particles of one of the principal substances that compose the animal frame; and it may be well to enumerate these substances, in 116 FUNCTIONS TRIBUTARY TO SECRETION. order that you may better comprehend the nature of this most interesting function. 230. Besides several metals, sulphur, and phosphorus, which contribute in small quantities to the formation of the animal frame, there are four different kinds of matter which, combined in various proportions, compose nearly the whole mass of every animal. These are, 1st, carbon, which we see nearly pure in the diamond, and mixed with but little other matter in common charcoal: 2d, oxygen, the gas or air that supports the flame of com- bustible bodies, and gives to common air the power of maintaining the life of animals and plants: 3d, nitrogen, a kind of air that will not support life, and extinguishes a candle when immersed in it, but which forms, when mixed with a proper portion of oxygen, a considerable part of the air we breathe; and, 4th, hydrogen, a gas that combines with oxygen to form water, and with carbon to give us the gas that is burned in our streets in the place of oil. Oil itself owes its inflammable pro- perties to the presence of this gas. 231. Now, as the four substances above mentioned (230), combined in different proportions, and rendered liquid or solid according to circumstances, compose nearly the whole animal, and as all the particles of all parts of the animal require to be taken up by absorption from time to time, to be carried into the circulation and rejected from the body (216), it follows that the blood, as it travels through the capillaries in the substance of the different organs, must become loaded with these four substances to such an extent as to require to be con- tinually purified from them. And as the arteries are the organs that convey the blood to all parts of the body in its purer condition, to nourish the frame (186), while the lymphatics, which empty into the veins, and the capillary veins themselves (206) receive all the worn- out particles, it is in the veins that you would expect to find the blood most in need of purification. The oxygen and hydrogen are easily discharged from all parts of the body in the form of water or watery vapour, in the sensible and insensible perspiration and other secretions. RESPIRATORY APPARATUS. 117 The nitrogen escapes in many ways without the neces- sity of any particular organ for separating it from the blood, but the carbon is not so easily dismissed. 232. It is the presence of an excess of this substance in the veins of the red-blooded animals that gives to the blood in the veins its dark purple or bluish tint; and it is the removal of the same substance that restores the bright crimson of the blood always seen in the arteries. Now a part of the surpliis carbon is got rid of in the liver by the secretion of bile; but a far greater amount of purification is demanded for maintaining the vital functions in health, and special organs are required for the purpose. These organs, taken collectively, are called the respiratory apparatus, and the process by which they perform their functions is called respiration. 233. In order to purify the blood of its excess of car- bon, it is necessary to bring the circulating fluid to the external air, that its carbon may unite with the oxygen contained in the atmosphere; for it is found that wher- ever the living blood is thus placed, the substances just mentioned will unite and form that gas which is known among chemists by the name of carbonic acid; the same that escapes from beer, cider, or mineral water. Wherever a portion of air has been breathed, or sub- mitted to the action of the respiratory apparatus of an animal, it is found that a portion of its oxygen has disappeared, and that a proportional quantity of carbonic acid gas has taken its place. 234. As many animals live altogether in the water, and as this fluid contains oxygen as well as air, it is very commonly supposed that such animals breathe the water itself. But all water, in its natural state, contains a large quantity of atmospheric air, which, though we can- not perceive it, may be extracted by art, as you will learn when you see it placed upon an air-pump. While the air-pump is being exhausted, you will observe bubbles of air continually rising through the water. Now, it is generally believed by physiologists, that fish and other animals that live altogether in the water, breathe only the air that it contains, and not the water itself; and it h 10* 118 FUNCTIONS TRIBUTARY TO SECRETION. is certain that all the experiments yet tried tend to prove that when water has been artificially deprived of its air it can no longer maintain animal life ; so that a fish may then be drowned in its own element. 235. You all know that a fish, when taken from the water, will soon die; proving that too much air will kill as effectually as too little. Thus; although the birds, quadrupeds, and man, in breathing, use little else than the oxygen contained in the air, yet if we enclose an animal of either of these classes in a vessel of pure oxygen, he will soon die. You will now readily under- stand why changes of air, such as those which occur in moving from the mountains to the sea, from a swampy to a dry situation, or the reverse, may seriously affect the health of man and beast, particularly when in a feeble condition. But this is wandering from the direct course of our studies. 236. It is not necessary that the blood should actually touch the external air in order to part with its carbon; for this operation takes place through the sides of the blood-vessels, by imbibition and exhalation or transpira- tion, like all the organic functions of the polypi and the hydra. 237. The function of respiration in the simplest ani- mals is performed by or through the skin; and even in many of those which are much more complex in their organization, some portions of the surface preserve the same power of action ; but, even in these latter animals, life cannot be prolonged beyond a definite period without the aid of a special respiratory apparatus. Thus; we know beyond dispute that the toad can breathe through the skin of the back, and this power no doubt assists in preserving its life for a long time when shut up in the hol- lows of trees, or buried in fissures of rock where it can make no use of its special respiratory organs, and must depend exclusively upon the air contained in the crevices of its living tomb, or in the fluids that accidentally trickle around it. Anecdotes of toads living for months or years in such situations are not uncommon. 238. There is reason to believe that even man may RESPIRATORY APPARATUS. 119 breathe, to a certain extent, by his skin; and different substances are known to find their way into and out of the body by this route. Although this kind of respira- tion is altogether insufficient for the purposes of an ani- mal so noble and complex in his organization, the effect of cleanliness in promoting health and a ruddy com- plexion is in part due to the removal of all obstacles to the proper exercise of this function by the human skin. Many things in the history of wounds and inflammation tend to establish this fact. 239. But in all animals, except those of the very simplest character, some definite apparatus is devoted to the particular purpose of respiration; and in nearly all those whose organization in this respect is under- stood, the most essential part of this apparatus is formed on one general principle. One or more blood-vessels are provided, to convey a portion or the whole of the blood to some organ where it may be acted upon by the atmosphere, or by the air contained in water (234.) These blood-vessels, though they convey the impure or venous blood to the purified, appear to be constructed like an artery. Another vessel, or set of vessels, re-con- veys the blood, after purification, back to the circula- tion; and although these vessels are thus filled with arterial blood fitted to supply nourishment to the frame, they are constructed like the veins. It is in the capil- laries of these vessels and through their sides (236) that the function of respiration is performed, and the blood loses its surplus carbon. 240. The capillaries which are expressly devoted to carrying on the function of respiration are always found collected together, in such a manner as to form one or more somewhat irregular organs bearing more or less resemblance to glands (223,) and generally situated on opposite sides of the body. In a few animals these pairs of organs are fixed so near the middle line of the body that they seem to be united into one. 241. The only important exception to the general prin- ciple on which is regulated the formation of the respiratory 120 FUNCTIONS TRIBUTARY TO NUTRITION. apparatus (239,) is found in the insects, certain spiders, and some kindred tribes that seem not to possess a per- fect circulation. In the insects, the air is admitted into the substance of the body through numerous openings ranged along the side or lower surface of the animal. These openings are the mouths of as many tubes, which divide themselves in the interior into many branches communicating with each other, and bringing the air almost into contact with the nutritive fluid or blood in the cellular tissue around their organs. These tubes are called trachece, and the kind of respiration per- formed by them is called tracheal respiration. Many of the worms have also numerous openings to admit air into small sacs beneath their skin, for the purpose of respiration; but I will not saddle your memory with a description of the endless varieties of the respiratory ap- paratus of the lower orders of animals. 242. As a general rule, those animals that live en- tirely in the water have their breathing organs at or near the surface of the body. These are sometimes in the form of tufts of hair or prickles that may be useful in crawling; as in the long red worm so often seen creep- ing about the hinges of salt oysters. Sometimes they resemble little paddles or limbs that assist the animal in swimming; as in a few of the molluscous tribes that float near the surface of the ocean. But more generally they are composed of cartilaginous rays, with branches ranged much like the teeth of a fine-tooth comb, and covered with a delicate tissue as in the fishes. 243. All respiratory organs designed for breathing under water, and formed on the models mentioned in the last paragraph, are termed branchice or gills, how- ever various their number and shape may be, and whether they are placed altogether externally, or en- closed in superficial cavities. The kind of respiration performed by them is called branchial respiration. 244. The different forms of branchias observed in aquatic animals are indefinite in number; but all of them are furnished with innumerable capillary vessels that RESPIRATORY APPARATUS. 121 approach so nearly to the surface that they bring the blood almost into contact with the air contained in the water, in order to be purified of its carbon. 245. In many of the lower orders of animals, the branchia hang suspended in the water without any very apparent apparatus to produce a current towards them, so that they would seem, at first sight, to depend for their supplies of air entirely upon the water that chances to come in contact with them. The common fresh-water muscle of our brooks and mill-dams will furnish you with a beautiful example of this kind of respiratory apparatus. If you open one of these shells very carefully, you find it lined internally with a soft membrane called the mantle. Between this mantle and the tough, muscular, tongue-like organ lying next the opening, (by means of which the animal pushes himself along through the mud, and which is therefore termed the foot,) you see two delicate membranes on each side, resembling the leaves of a book. These membranes are the branchiae, and the delicate misty lines which you may detect ranged like the teeth of a comb along their margin, are the principal blood-vessels of respiration, •which the transparency of the animal permits you to distinguish. As no motion in these branchiae is visible by the naked eye, you would naturally suppose that the supply of air that they obtain in still water is very small and precarious; but if you long observe one of these shell-fish in a vessel of water, when undisturbed, you will see the shell slightly open, and if there be a few motes in the water, you will soon perceive that there is a constant current running in at one end of the shell and out at the other; thus the branchiae are supplied with fresh fluid at every moment. The microscope ex- poses the cause of this mysterious motion; for it dis- plays the branchiae covered with innumerable cilia like those of the polypi, which, by their motion, produce the current just mentioned (81, 82). When a portion of one of these membranes is carefully cut off, it is seen to move about like an independent animal by the powers of the cilia, and hence many naturalists 122 FUNCTIONS TRIBUTARY TO NUTRITION. conclude that the latter class of organs are employed as a respiratory apparatus even by the simplest animals. 246. All animals that live in air are provided with internal respiratory organs, which are called lungs or respiratory cavities, and the kind of respiration effected by these organs is called pulmonary respiration. 247. The pulmonary cavities are sometimes single, and formed of a simple sac with an external opening to admit the air. This is the case with those snails that breathe in the air only. Many even of those snail- shells called lymnaeae by naturalists that we find along the margin of our rivers and streams, living in the water, are provided with organs of this kind. They would drown if kept continually immersed ; and if you observe their habits when preserved in a tumbler of fresh water, you may see them crawling up the glass at intervals until they reach the surface and take in a fresh supply of air. This they do by opening a small round orifice leading to their pulmonary cavity. When the air therein has been sufficiently changed, they close the orifice again, and carry their fresh supply with them, wherever they travel, until its oxygen is exhausted. (233). These pulmonary cavities render the animals much lighter, and assist them in floating upon the surface in the manner-already described (152). 248. The respiratory capillaries in these animals, instead of being spread over the outside of solid organs, as in the branchiae, (243) are distributed over the mem- brane forming the pulmonary cavity, where they bring the blood nearly into contact with the contained air,— nothing being placed between the sides of the blood- vessels and the cavity except an exceedingly thin layer of the membrane. 249. The pulmonary cavities of the larger animals, such as the quadrupeds, are constructed upon the same model; but instead of a single cavity,these are composed of a large mass of little cells, collected together like a bunch of grapes, but clustered in incalculable numbers, and formed into two large organs, one placed on each side of the chest, and called the right and left lungs. RESPIRATORY APPARATUS. 123 You see a Every one of these cells contains air, and the respiratory capillaries are distributed over their thin walls to purify the blood. 250. In order to admit the air to the lungs in these animals, a canal passes from the back part of ihe mouth, just behind the tongue, down the neck of the animal into its chest, where it divides into two great branches, one of which passes into the left and the other into the right lung. As soon as these branches have entered the lungs they are again divided, and continue to ramify, like the blood-vessels, until they become exceedingly small, and each of the minute branches terminates in a group of air-cells. rude picture of this arrangement in fig. 32. In fig. 32 you have the left lung of a man remaining entire, 5, but the right lung has had its substance and its air-cells cut away, so as to show you the large branches of the canal as they divide within its substance, 7, 7, &c, and a few of the smaller branches also, 8. Fig. 33 will give you some little idea of the manner in which the smaller ramifications, 1, termi- nate in the air-cells, 2, 2, 2, &c. The parts are highly magnified; the air-cells being but barely visible in the human lungs when fully distended. 251. The great canal that passes from the root of the chest, — fig. 32, 2, — is called the trachea.* The principal branches passing to the right and left lungs, are called * It is perhaps unfortunate that this organ should bear the same name with the air-passages of insects, although it performs an analogous func- tion. It would be well for the preceptor to guard the pupil against the confusion likely to result from this identity of terms. Trachea and its branches. 124 FUNCTIONS TRIBUTARY TO NUTRITION. the bronchia;, 3, 4, and the title of bronchial tubes is given to the various ramifications of the bronchiae in the sub- stance of the lungs, 7, 7, 8. Air-cells of the Lungs magnified. 1, A minute bronchial tube ; 2, 2, 2, groups of air-cells; 3, the same parts laid open. 252. If we compare the lungs to a gland intended to secrete the carbon of the blood, the bronchial tubes, bronchiae, and trachea may be compared to the ducts of a secretory gland. Like all such ducts, they are lined throughout with mucous membrane, but, unlike them, are never closed or collapsed when emptied of every thing but air; for the whole length of the main canal and its branches, is surrounded by a series of cartila- ginous arches or rings external to the lining membrane, which hold it open at all times. 253. The pulmonary respiration of certain shell-fish (247), requires no machinery for drawing the air into the respiratory organs and thrusting it out again; but the larger bodies of animals whose lungs are placed deep in the body, and who consume a large quantity of air very rapidly, stand in need of such an apparatus. They are therefore provided with movable bones in the chest, called ribs, and numerous muscles for moving those ribs, which will be more fully noticed hereafter. These muscles, when in action, alternately raise and depress the ribs; so as to increase and diminish the size of the chest and cause the air to rush in and out through the RESPIRATORY APPARATUS. 125 trachea, to supply the lungs with fresh oxygen, and to remove the carbonic acid formed in them. The act of drawing in the air is called inspiration; and the act of forcing it out again is callea expiration. These things you can study on your own person. 254. In birds, it'is necessary that the bones should be very light, in order that they may not embarrass these animals in flying; and as the laws of Providence are such that every accidental circumstance connected with the organization of living things is rendered as useful as possible, most of the bones of birds are made hollow, and the air in breathing is admitted into their cavities, where a great number of capillary blood-vessels are brought nearly into contact with the air. Thus these cavities in the bones become a part of the respiratory apparatus. 255. You know that when the eggs of a frog are hatched, the young animal appears at first as a tadpole, residing altogether in the water, and leading the life of a fish. It is then provided with gills, and has a regular branchial respiration (243). But after a while its legs beo-in to grow, and its tail is diminished in length by absorption. At this time a pair of true lungs begin to be found in its chest, and the animal comes often to the surface to take in air. For a period, it retains both forms of respiratory organs; but as the lungs grow larger, the gills are gradually absorbed, until its respira- tion becomes entirely pulmonary, if we except its power of breathing by the'skin of the back (237). When the animal becomes perfectly developed, it maybe drowned by being kept too long under water. 256 In the great majority of the lower orders of those animals that have any respiratory organs whatever, only a small portion and not the whole of the blood is sent through the branchiae or the lungs; so that the arteries are always filled with a mixed blood, partly pure and partly impure. The pure blood is that portion which is carried from the principal blood-vessels, through the respiratory arteries (239), into the branchiae or lungs; where it loses its carbon, and is then carried back by the respira 11 126 FUNCTIONS TRIBUTARY TO NUTRITION. tory veins into the principal blood-vessels again. The impure blood is that which passes directly along the principal blood-vessels from the arteries to the veins, without passing through the respiratory organs at all. 257. Now, it is found that all the vital functions are performed most vigorously in those animals whose arte- ries circulate the purest blood; and hence those beings alluded to in the last paragraph are remarkable for the sluggishness of their motions and functions, and for their power of retaining life for some time without air. Snakes, tortoises, and lizards, which are amphibious, are of this class; and so are a multitude of still less complex animals. 258. But in man, quadrupeds, and birds, all the blood in the veins is made to pass through the lungs before it recommences its route through the circulation; so that the various parts of the body are supplied exclusively with pure blood from the arteries. It is this circum- stance that renders these animals so rapid and powerful in their motions, and enables them to display so much activity of all the vital functions, while, at the same time, it makes them more dependent upon the good quality and ample supply of air for breathing. 259. I shall not attempt to describe, in this work, the forces that compel the blood to flow through the vessels, or the various forms that the heart assumes in different animals; for you will be much better prepared to read understandingly on these matters hereafter. But it is necessary that I should give you some definitions of terms connected with circulation and respiration that we may shortly have occasion to employ. As, in the most perfect animals, the respiratory arteries carry only impure blood in order that it may be purified in their capillaries, they cannot properly support the growth and nutrition of the respiratory organs themselves. These organs are therefore supplied with another and much smaller set of arteries springing from some of the principal arterial trunks carrying pure blood. The arteries of this small set nourish the respiratory organs, but have nothing to do directly with the function of respiration. They are STRUCTURE OF THE HEART. 127 called the nutritive arteries of the lungs or branchice. Both the respiratory and the nutritive arteries have their corresponding veins, to carry back the blood that they have conveyed into the respiratory organs. Those attached to the former system deliver their pure contents into the great arteries that nourish the whole frame, but those of the latter system deliver their impure contents into the principal veins that bring back the- blood from all parts of the body to be purified. Thus you see that the nutritive system of vessels is completely distinct from the respiratory system, even in the respiratory organs themselves. The respiratory system of blood- vessels is called branchial when the animal breathes by gills, and pulmonary when it is furnished with lungs: bu* these terms are not applied to nutritive vessels. Fig. 34. The Heart in the Pericardium. 128 FUNCTIONS TRIBUTARY TO NUTRITION. 260. To distinguish the respiratory system of vessels from that which conveys nourishment to all the organs, it has been customary to call the latter the systematic circulatory apparatus; but having objected to the term system, as applied to the whole body (25), because it is likely to confuse the mind when thus employed, I prefer the term general or nutritive system to designate this class of vessels. 261. It is now time togive you some idea of the func tions of the heart in carrying on the circulation of blood in all the vessels of the larger animals and man. Al fig. 34 you see a representation of the human heart in closed in a thin membrane that covers it like a bag, and surrounded by the large blood-vessels that spring from Fiv. 35. it. At fig. 35 you see the human heart divided from side to side, so as to show that it contains the „ four different cavities marked with the numbers 3,4,10, and 11. You see a solid division running down g the middle of the organ, marked 6, separating the two cavities on the right from those on the left; and it is necessary for you to re- member that you are looking at the organ as it would appear if the individual to whom it belonged were facing you, so that the left side of the heart is next your right hand. This division between the two sides of the heart in the larger animals and man is always complete after birth, except in some rare cases of disease; so that no blood can pass from the cavities marked 3, 4, to those marked 10, 11. But between the cavities marked 10 and 11 there is a division, 5, that is not complete. It is composed partly of thick muscular and tendinous matter, like 6, but there is a large opening in its centre which is furnished with a valve composed of a thin membrane that lines not only the heart, but also the whole length of the arteries. This valve is scolloped so as to form three festoons, each oc- STRUCTURE OF THE HEART. 129 cupying about one-third of the circumference of the opening, with their loose edges hanging down a little toward the cavity marked 11. When the cavity 10 is full of blood, this fluid can pass easily into cavity 11 by pushing open these festoons; but when it attempts to re- turn it arrests itself at once by forcing the festoons against each other so as to close the passage. To guard against the valve being driven upward through the opening by a sudden rush of blood, the loose edges of the festoons are secured by a number of little tendons arising from columns of muscular fibres springing from the sides of cavity 11. These tendons prevent the festoons from rising so high as to be inverted upward, which would destroy their usefulness. Between cavities 3 and 4 there is a valve, also marked 5, similar in all respects, except that it is scolloped into only two festoons. 262. The cavities marked 10 and 3 are called the right and left auricles. They receive all the blood brought to the heart by the veins of the two systems, the gene- ral and the respiratory (259, 260); and, when full, they contract and force it through the two valves, 5, 5, into the cavities 11 and 4. These latter cavities are called the right and left ventricles. All the arteries in the body, both general and respiratory, spring from these ventri- cles by two great trunks, each of which continues di- viding again and again until its ramifications form the capillaries in the manner already described (186, 187). Now, when the ventricles contract, the blood that they have received from the auricles endeavours to flow back into those cavities, but it is immediately stopped by the closure of the valves (261); and it is therefore forced into the arteries, which furnish the only outlet. The two great arteries are also provided with valves at their origin where they leave the heart; so that the blood that has once entered them cannot flow back into the ventri- cles, but must flow forward into the capillaries, and thus into the veins, before it can return to the heart. These are the only valves seen in the arterial system. Although the great veins near the heart are not provided with valves, the smaller ones which unite to form them have 11* 130 FUNCTIONS TRIBUTARY TO NUTRITION. very numerous valves; as you have been informed alrea- dy (see fig. 29, page 97); and this will explain why the auricles, when they contract, do not force their contents back into those vessels. Thus you perceive that the blood is compelled to move regularly in one direction, or to fol- low one fixed route of circulation. Let us trace that route. 263. All the veins from the head, neck, and upper ex- tremities, before they reach the heart, form one great venous trunk called the superior or descending vena cava, fig. 35, 1; and all the veins coming from the body and lowrer extremities form a similar trunk called the inferior or ascending vena cava, 2. These two great vessels, filled with the dark-coloured or impure blood (232), meet together just behind the heart, so as to resemble but one continued vein. (See fig. 28, page 96.) At this point they communicate directly by means of a large opening in their side, with the right auricle of the heart, 10, fig. 35; into which they empty their contents. 264. At every beat of the heart the right auricle con- tracts and forces its contents into the right ventricle, 11. This ventricle then immediately contracts and drives the blood into the great arterial trunk that arises from it (262), which is called the pulmonary artery, 7. This artery soon divides, as you see at 8, into a right branch going to the right lung, and a left branch going to the left lung. The two branches of the pulmonary artery convey the impure blood into the lungs, and there distri- bute it to the pulmonary capillaries, which separate its carbon in the manner already described (239), and ren- der it fit to support and nourish the frame. The pure or bright red blood thus formed then passes from the pulmonary capillaries into the minute brar