of the muscular coat of the stomach, that the con- | tents, after having undergone the application of the gastric menstruum, are gradually pressed out. In dogs and cats, this action of the coats of the stomach has been displayed to the eye. It is a slow and gentle undulation, propagated from one orifice of the stomach to the other. For the same reason that I omitted, for the present, offering any observation upon the digestive fluid, I shall say nothing concerning the bile or the pancreatic juice, farther than to observe upon the mechanism, viz. that from the glands in which these secretions are elaborated, pipes are laid into the first of the intestines, through which pipes the product of each gland flows into that bowel, and is there mixed with the aliment, as soon almost as it passes the stomach; adding also as a remark, how grievously this same bile offends the stomach itself, yet cherishes the vessel that lies next to it.

which we call chyle, are, by a series of gentle compressions, squeezed into the narrow orifices of the lacteal veins. Thirdly, it was necessary that these tubes, which we denominate lacteals, or their mouths at least, should be made as narrow as possible, in order to deny admission into the blood to any particle which is of size enough to make a lodgment afterward in the small arteries, and thereby to obstruct the circulation: and it was also necessary that this extreme tenuity should be compensated by multitude; for a large quantity of chyle (in ordinary constitutions, not less, it has been computed, than two or three quarts in a day) is, by some means or other, to be passed through them. Accordingly, we find the number of the lacteals exceeding all powers of computation; and their pipes so fine and slender, as not to be visible, unless filled, to the naked eye; and their orifices, which open into the intestines, so small, as not to be discernible even by the best microscope. Fourthly, the main pipe which carries the chyle from the reservoir to the blood, viz. the thoracic duct, being fixed in an almost upright position, and wanting that advantage of propulsion which the arteries possess, is furnished with a succession of valves to check the ascending fluid, when once it has passed them, from falling back. These valves look upward, so as to leave the ascent free, but to prevent the return of the chyle, if, for want of sufficient force to push it on, its weight should at any time cause it to descend. Fifthly, the chyle enters the blood in an odd place, but perhaps the most commodious place possible, viz. at a large vein in the neck, so situated with respect to the circulation, as speedily to bring the mixture to the heart. And this seems to be a circumstance of great moment; for had the chyle entered the blood at an artery, or at a distant vein, the fluid, composed of the old and the new materials, must have performed a considerable part of the circulation, before it received that churning in the lungs, which is, probably, necessary for the intimate and perfect union of the old blood with the recent chyle. Who could have dreamt of a communication between the cavity of the intestines and the left great vein of the neck? Who could have suspected that this communication should be the medium through which all nourishment is derived to the body; or this the place, where, by a side-inlet, the important junction is formed between the blood and the material which feeds it?

Secondly, We have now the aliment in the intestines, converted into pulp; and, though lately consisting of ten different viands, reduced to nearly a uniform substance, and to a state fitted for yielding its essence, which is called chyle, but which is milk, or more nearly resembling milk than any other liquor with which it can be compared. For the straining of this fluid from the digested aliment in the course of its long progress through the body, myriads of capillary tubes, i. e. pipes as small as hairs, open their orifices into the cavity of every part of the intestines. These tubes, which are so fine and slender as not to be visible unless when distended with chyle, soon unite into larger branches. The pipes, formed by this union, terminate in glands, from which other pipes of a still larger diameter arising, carry the chyle from all parts, nto a common reservoir or receptacle. This receptacle is a bag of size enough to hold about two table-spoons full; and from this vessel a duct or main pipe proceeds, climbing up the back part of the chest, and afterward creeping along the gullet till it reach the neck. Here it meets the river: here it discharges itself into a large vein, which soon conveys the chyle, now flowing along with the old blood, to the heart. This whole route can be exhibited to the eye; nothing is left to be supplied by imagination or conjecture. Now, beside the subserviency of this structure, collectively considered, to a manifest and necessary purpose, we may remark two or three separate particulars in it, which show, not only the contrivance, but the perfection of it. We may remark, first, the length of the intestines, which, in the human subject, is six times that of the body. Simply for a passage, these voluminous bowels, this prolixity of gut, seems in no wise necessary; but in order to allow time and space for the successive extraction of the chyle from the digested aliment, namely, that the chyle which escapes the lacteals of one part of the guts may be taken up by those of some other part, the length of the canal is of evident use and conduciveness. 1. It is not a simple diluent, but a real solvent. Secondly, we must also remark their peristaltic A quarter of an ounce of beef had scarcely touchmotion; which is made up of contractions, follow-ed the stomach of a crow, when the solution being one another like waves upon the surface of a fluid, and not unlike what we observe in the body of an earth-worm crawling along the ground; and which is effected by the joint action of longitudinal and of spiral, or rather perhaps of a great number of separate semicircular fibres. This curious action pushes forward the grosser part of the aliment, at the same time that the more subtile parts,

We postponed the consideration of digestion, lest it should interrupt us in tracing the course of the food to the blood; but in treating of the alimentary system, so principal a part of the process cannot be omitted.

Of the gastric juice, the immediate agent by which that change which food undergoes in our stomachs is effected, we shall take our account from the numerous, careful, and varied experiments of the Abbé Spallanzani.

gun.

2. It has not the nature of saliva; it has not the nature of the bile; but is distinct from both. By experiments out of the body it appears, that neither of these secretions acts upon alimentary substances, in the same manner as the gastric juice acts.

3. Digestion is not putrefaction: for the digest

mg fluid resists putrefaction most pertinaciously; nay, not only checks its farther progress, but restores putrid substances.

4. It is not a fermentative process: for the solution begins at the surface, and proceeds towards the centre, contrary to the order in which fermentation acts and spreads.

5. It is not the digestion of heat: for the cold maw of a cod or sturgeon will dissolve the shells of crabs or lobsters, harder than the sides of the stomach which contains them.

the gastric juice, not having been weakened by disease, retains its activity,) it has been known to eat a hole through the bowel which contains it.* How nice is this discrimination of action, yet how necessary!

But to return to our hydraulics.

III. The gall-bladder is a very remarkable contrivance. It is the reservoir of a canal. It does not form the channel itself, i. e. the direct communication between the liver and the intestine, which is by another passage, riz, the ductus hepaticus, continued under the name of the ductus communis; but it lies adjacent to this channel, join

In a word, animal digestion carries about it the marks of being a power and a process completely sui generis; distinct from every other; at leasting it by a duct of its own, the ductus cysticus: from every chymical process with which we are by which structure it is enabled, as occasion may acquainted. And the most wonderful thing about require, to add its contents to, and increase the it is its appropriation; its subserviency to the par- flow of bile into the duodenum. And the positicular economy of each animal. The gastric tion of the gall-bladder is such as to apply this juice of an owl, falcon, or kite, will not touch grain; structure to the best advantage. In its natural no, not even to finish the macerated and half-di- situation, it touches the exterior surface of the gested pulse which is left in the crops of the spar-stomach, and consequently is compressed by the rows that the bird devours. In poultry, the tritu- distention of that vessel: the effect of which comration of the gizzard, and the gastric juice, con- pression is to force out from the bag, and send inspire in the work of digestion. The gastric juice to the duodenum, an extraordinary quantity of will not dissolve the grain whilst it is whole. En- bile, to meet the extraordinary demand which the tire grains of barley, enclosed in tubes or sphe- repletion of the stomach by food is about to occarules, are not affected by it. But if the same sion.+ Cheselden describes the gall-bladder as grain be by any means broken or ground, the gas-seated against the duodenum, and thereby liable tric juice immediately lays hold of it. Here then is wanted, and here we find, a combination of mechanism and chymistry. For the preparatory grinding, the gizzard lends its mill. And as all mill-works should be strong, its structure is so, beyond that of any other muscle belonging to the animal. The internal coat also, or lining of the gizzard, is, for the same purpose, hard and cartilaginous. But, forasmuch as this is not the sort of animal substance, suited for the reception of glands or for secretion, the gastric juice, in this family, is not supplied, as in membranous sto machs, by the stomach itself, but by the gullet, in which the feeding glands are placed, and from which it trickles down into the stomach.

to have its fluid pressed out, by the passage of the aliment through that cavity; which likewise will have the effect of causing it to be received into the intestine, at a right time, and in a due proportion.

There may be other purposes answered by this contrivance; and it is probable that there are. The contents of the gall-bladder are not exactly of the same kind as what passes from the liver through a direct passage.§ It is possible that the gall may be changed, and for some purposes me→ liorated, by keeping.

coat, it runs near two fingers' breadth between the coats, before it opens into the cavity of the intes tine. The same contrivance is used in another part, where there is exactly the same occasion for it, viz. in the insertion of the ureters in the blad der. These enter the bladder near its neck, run

The entrance of the gall-duct into the duodenum furnishes another observation. Whenever either smaller tubes are inserted into larger tubes, or tubes into vessels and cavities, such receiving In sheep, the gastric fluid has no effect in di- tubes, vessels, or cavities, being subject to muscugesting plants, unless they have been previously lar constriction, we always find a contrivance to masticated. It only produces a slight maceration, prevent regurgitation. In some cases, valves are nearly such as common water would produce, in used; in other cases, amongst which is that now a degree of heat somewhat exceeding the medium before us, a different expedient is resorted to, which temperature of the atmosphere. But provided may be thus described: The gall-duct enters the that the plant has been reduced to pieces by chew-duodenum obliquely after it has pierced the first ing, the gastric juice then proceeds with it, first by softening its substance; next by destroying its natural consistency; and lastly, by dissolving it so completely, as not even to spare the toughest and most stringy parts, such as the nerves of the leaves. So far our accurate and indefatigable Abbé.-ning obliquely for the space of an inch between Dr. Stevens, of Edinburgh, in 1777, found, by experiments tried with perforated balls, that the gastric juice of the sheep and the ox speedily dissolved vegetables, but made no impression upon beef, mutton, and other animal bodies. Dr. Hunter discovered a property of this fluid, of a most curious kind; viz. that in the stomachs of animals which feed upon flesh, irresistibly as this fluid acts upon animal substances, it is only upon the dead substance that it operates at all. The living fibre suffers no injury from lying in contact with it. Worms and insects are found alive in the stomachs of such animals. The coats of the human stomach, in a healthy state, are insensible to its presence; yet in cases of sudden death, (wherein |

its coats. It is, in both cases, sufficiently evident, that this structure has a necessary mechanical tendency to resist regurgitation: for whatever force acts in such a direction as to urge the fluid back into the orifices of the tubes, must, at the same time, stretch the coats of the vessels, and thereby compress that part of the tube which is included between them.

IV. Amongst the vessels of the human body the pipe which conveys the saliva from the place where it is made, to the place where it is wanted,

Phil. Trans. vol. Ixii. p. 447. Keill's Anat. p. 64. ‡ Anat. p. 164. § Keill. (from Malpighius,) p. 63. Keill's Anat. p. 62. T Cheselden's Anat. p. 260

deserves to be reckoned amongst the most intelli- | gible pieces of mechanism with which we are acquainted. The saliva, we all know, is used in the mouth but much of it is produced on the outside of the cheek, by the parotid gland, which lies between the ear and the angle of the lower jaw. In order to carry the secreted juice to its destination, there is laid from the gland, on the outside, a pipe, about the thickness of a wheat straw, and about three fingers' breadth in length; which, after riding over the masseter muscle, bores for itself a hole through the very middle of the cheek; enters by that hole, which is a complete perforation of the buccinator muscle, into the mouth; and there discharges its fluid very copiously.

glottis: I do not mean in the same individual, but in a succession of generations. Not only the action of the parts has no such tendency, but the animal could not live, nor consequently the parts act, either without it, or with it in a half-formed state. The species was not to wait for the gradual formation or expansion of a part which was, from the first, necessary to the life of the in| dividual.

Not only is the larynx curious, but the whole wind-pipe possesses a structure adapted to its peculiar office. It is made up (as any one may perceive by putting his fingers to his throat) of stout cartilaginous ringlets, placed at small and equal distances from one another. Now this is not the case with any other of the numerous conduits of the body. The use of these cartilages is to keep the passage for the air constantly open; which they do mechanically. A pipe with soft membranous coats, liable to collapse and close when empty, would not have answered here; although this be the general vascular structure, and a structure which serves very well for those tubes which are kept in a state of perpetual distension by the fluid they enclose, or which afford a passage to solid and protruding substances.

the descent of solid food. The same rings are also bevelled off at the upper and lower edges, the better to close upon one another, when the trachea is compressed or shortened.

V. Another exquisite structure, differing indeed from the four preceding instances, in that it does not relate to the conveyance of fluids, but still belonging, like these, to the class of pipes or conduits of the body, is seen in the larnyr. We all know that there go down the throat two pipes, one leading to the stomach, the other to the lungs; the one being the passage for the food, the other for the breath and voice: we know also that both these passages open into the bottom of the mouth; the gullet, necessarily, for the conveyance of food; Nevertheless (which is another particularity and the wind-pipe, for speech and the modulation well worthy of notice,) these rings are not comof sound, not much less so: therefore the difficulty plete, that is, are not cartilaginous and stiff all was, the passages being so contiguous, to prevent round; but their hinder part, which is contiguous the food, especially the liquids, which we swal- to the gullet, is membranous and soft, easily yieldlow into the stomach, from entering the wind-ing to the distensions of that organ occasioned by pipe, i. e. the road to the lungs; the consequence of which error, when it does happen, is perceived by the convulsive throes that are instantly produced. This business, which is very nice, is managed in this manner. The gullet (the passage for food) opens into the mouth like the cone or upper part of a funnel, the capacity of which forms indeed the bottom of the mouth. Into the side of this funnel, at the part which lies the lowest, enters the wind-pipe, by a chink or slit, with a lid or flap, like a little tongue, accurately fitted to the orifice. The solids or liquids which we swallow, pass over this lid or flap, as they descend by the funnel into the gullet. Both the weight of the food, and the action of the muscles concerned in swallowing, contribute to keep the lid close down upon the aperture, whilst any thing is passing; whereas, by means of its natural cartilaginous spring, it raises itself a little, as soon as the food is passed, thereby allowing a free inlet and outlet for the respiration of air by the lungs. Such is its structure: and we may here remark the almost complete success of the expedient, viz. how seldom it fails of its purpose, compared with the number of instances in which it fulfils it. Reflect how frequently we swallow, how constantly we breathe. In a city feast, for example, what deglutition, what anhelation! yet does this little cartilage, the epiglottis, so effectually interpose its office, so securely guard the entrance of the wind-pipe, that whilst morsel after morsel, draught after draught, are coursing one another over it, an accident of a crumb or a drop slipping into this passage (which nevertheless must be opened for the breath every second of time,) excites in the whole company, not only alarm by its danger, but surprise by its novelty. Not two guests are choked in a century.

There is no room for pretending that the action of the parts may have gradually formed the epi

The constitution of the trachea may suggest likewise another reflection. The membrane which lines its inside, is, perhaps, the most sensible, irritable membrane of the body. It rejects the touch of a crumb of bread, or a drop of water, with a spasm which convulses the whole frame; yet, left to itself, and its proper office, the intromission of air alone, nothing can be so quiet. It does not even make itself felt; a man does not know that he has a trachea. This capacity of perceiving with such acuteness, this impatience of offence, yet perfect rest and ease when let alone, are properties, one would have thought, not likely to reside in the same subject. It is to the junction, however, of these almost inconsistent qualities, in this, as well as in some other delicate parts of the body, that we owe our safety and our comfort;-our safety to their sensibility, our comfort to their repose.

The larynx, or rather the whole wind-pipe taken together, (for the larynx is only the upper part of the wind-pipe,) besides its other uses, is also a musical instrument, that is to say, it is mechanism expressly adapted to the modulation of sound; for it has been found upon trial, that, by relaxing or tightening the tendinous bands at the extremity of the wind-pipe, and blowing in at the other end, all the cries and notes might be produced of which the living animal was capable. It can be sounded, just as a pipe or flute is sounded.

Birds, says Bonnet, have, at the lower end of the wind-pipe, a conformation like the reed of a hautboy, for the modulation of their notes. A tuneful bird is a ventriloquist. The seat of the song is in the breast.

The use of the lungs in the system has been said to be obscure; one use however is plain, though in some sense external to the system, and that is, the formation, in conjunction with the larynx, of voice and speech. They are, to animal utterance, what the bellows are to the organ.

often within how small a compass. It is a cluster of contrivances. In a canary-bird, for instance, and in the single ounce of matter which composes his body, (but which seems to be all employed,) we have instruments for eating, for digesting, for nourishment, for breathing, for generation, for running, for flying, for seeing, for hearing, for smelling; each appropriate,-each entirely different from all the rest.

I. The first is, the exact correspondency of the two sides of the same animal: the right hand answering to the left, leg to leg, eye to eye, one side of the countenance to the other; and with a precision, to imitate which in any tolerable degree forms one of the difficulties of statuary, and requires on the part of the artist, a constant attention to this property of his work, distinct from every other.

FOR the sake of method, we have considered animal bodies under three divisions; their bones, their muscles, and their vessels: and we have The human, or indeed the animal frame, constated our observations upon these parts separately. sidered as a mass or assemblage, exhibits in its But this is to diminish the strength of the argu- composition three properties, which have long ment. The wisdom of the Creator is seen, not in struck my mind as indubitable evidences, not only their separate but their collective action; in their of design, but of a great deal of attention and acmutual subserviency and dependance; in their con-curacy in prosecuting the design. tributing together to one effect, and one use. It has been said, that a man cannot lift his hand to his head, without finding enough to convince him of the existence of a God. And it is well said; for he has only to reflect, familiar as this action is, and simple as it seems to be, how many things are requisite for the performing of it: how many things which we understand, to say nothing of many more, probably, which we do not; viz. first, a long, hard, strong cylinder, in order to give to the arm its firmness and tension; but which, being rigid, and, in its substance, inflexible, can only turn upon joints: secondly, therefore, joints for this purpose; one at the shoulder to raise the arm, another at the elbow to bend it; these joints continually fed with a soft mucilage to make the parts slip easily upon one another, and holden together by strong braces, to keep them in their position: then, thirdly, strings and wires, i. e. muscles and tendons, artificially inserted for the purpose of drawing the bones in the directions in which the joints allow them to move. Hitherto we seem to understand the mechanism pretty well; and, understanding this, we possess enough for our conclusion: nevertheless, we have hitherto only a machine standing still; a dead organization, -an apparatus. To put the system in a state of activity; to set it at work; a farther provision is necessary, viz. a communication with the brain by means of nerves. We know the existence of this communication, because we can see the communicating threads, and can trace them to the brain: its necessity we also know, because if the thread be cut, if the communication be intercepted, the muscle becomes paralytic: but beyond this we know little; the organization being too minute and subtile for our inspection.

To what has been enumerated, as officiating in the single act of a man's raising his hand to his head, must be added likewise, all that is necessary, and all that contributes to the growth, nourishment, and sustentation, of the limb, the repair of its waste, the preservation of its health: such as the circulation of the blood through every part of it; its lymphatics, exhalants, absorbents; its excretions and integuments. All these share in the result; join in the effect: and how all these, or any of them, come together without a designing, disposing intelligence, it is impossible to conceive.

CHAPTER XI.

Of the Animal Structure regarded as a Mass. CONTEMPLATING an animal body in its collective capacity, we .cannot forget to notice what a number of instruments are brought together, and

It is the most difficult thing that can be to get a wig made even; yet how seldom is the face awry? And what care is taken that it should not be so, the anatomy of its bones demonstrates. The upper part of the face is composed of thirteen bones, six on each side, answering each to each, and the thirteenth, without a fellow, in the middle; the lower part of the face is in like manner composed of six bones, three on each side respectively corresponding, and the lower jaw in the centre. In building an arch, could more be done in order to make the curve true, i. e. the parts equi-distant from the middle, alike in figure and position?

The exact resemblance of the eyes, considering how compounded this organ is in its structure, how various and how delicate are the shades of colour with which its iris is tinged; how differently, as to effect upon appearance, the eye may be mounted in its socket, and how differently in different heads eyes actually are set,—is a property of animal bodies much to be admired. Of ten thousand eyes, I do not know that it would be possible to match one, except with its own fellow; or to distribute them into suitable pairs by any other selection than that which obtains.

This regularity of the animal structure is rendered more remarkable by the three following considerations. First, the limbs, separately taken, have not this correlation of parts, but the contrary of it. A knife drawn down the chine, cuts the human body into two parts, externally equal and alike; you cannot draw a straight line which will divide a hand, a foot, the leg, the thigh, the cheek, the eye, the ear, into two parts equal and alike. Those parts which are placed upon the middle or partition line of the body, or which traverse that line, as the nose, the tongue, the lips, may be so divided, or, more properly speaking, are double organs: but other parts cannot. This shows that the correspondency which we have been describing, does not arise by any necessity in the nature of the subject: for, if necessary, it would be universal; whereas it is observed only in the system or assemblage: it is not true of the separate parts; that is to say, it is found where it conduces to beauty or utility; it is not found, where it would subsist at the expense of both. The two wings of a bird always correspond: the two sides of a feather frequently do not. In centi