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?

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.

1. It is not a simple diluent, but a real solvent. A quarter of an ounce of beef had scarcely touched the stomach of a crow, when the solution began.

2. It has not the nature of saliva; it has not the nature of 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 digesting 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.

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 least from every chymical process with which we are acquainted. And the most wonderful thing about it is its appropriation; its subserviency to the particular œconomy of each animal. The gastric juice of an owl, falcon, or kite, will not touch grain; no, not even to finish the macerated and halfdigested pulse which is left in the crops of the sparrows that the bird devours. In poultry, the trituration of the gizzard, and the gastric juice, conspire in the work of digestion. The gastric juice will not dissolve the grain whilst it is whole. Entire grains of barley, enclosed in tubes or spherules, are not affected by it. But if the same grain be by any means broken or ground, the gastric 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-work 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 stomachs, by the stomach itself, but by the gullet, in which the feeding-glands are placed, and from which it trickles down into the stomach.

In sheep, the gastric fluid has no effect in digesting plants, unless they have been previously masticated. It only produces a slight maceration; nearly such as common water would produce, in a degree of heat somewhat exceeding the medium temperature of the atmosphere. But provided that the plant has been reduced to pieces by chewing, 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.-Dr. Stevens, of Edinburgh, in 1777, found, by experiments tried with perforated balls, that the gastric juice of the sheep and the ox speedily dis

solved 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 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.

IV. 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, viz. the ductus hepaticus, continued under the name of the ductus communis; but it lies adjacent to this channel, joining it by a duct of its own, the duct cysticus; by which structure it is enabled, as occasion may require, to add its contents to, and increase, the flow of bile into the duodenum. And the position of the gall-bladder is such as to apply this structure to the best advantage. In its natural situation, it touches the exterior surface of the stomach, and consequently is compressed by the distention of that vessel: the effect of which compression is to force out from the bag, and send into the duodenum, an extraordinary quantity of bile, to meet the extraordinary demand which the repletion of the stomach by food is about to occasion". Cheselden describes the gall-bladder as seated against the duodenum, and thereby liable 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 the direct passage d. It is possible that the gall may be changed, and for some purposes meliorated, by keeping.

a Phil. Trans. vol. lxii.

с

Anat. p. 164.

p. 447.

b Keill's Anat. p. 64. Keill, (from Malpighius,) p. 63.

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 tubes, vessels, or cavities, being subject to muscular constriction, we always find a contrivance to prevent regurgitation. In some cases, valves are used; in other cases, amongst which is that now before us, a different expedient is resorted to, which may be thus described: The gall-duct enters the duodenum obliquely; after it has pierced the first coat, it runs near two fingers' breadth between the coats before it opens into the cavity of the intestine 2. 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 bladder. These enter the bladder near its neck, running obliquely for the space of an inch between its coats b. 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.

V. 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, deserves to be reckoned amongst the most intelligible 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 beteen 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.

VI. 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 larynx. 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 a Keill's Anat. p. 62. b Ches. Anat. p. 260.

that both these passages open into the bottom of the mouth; the gullet, necessarily, for the conveyance of food; and the windpipe, for speech and the modulation of sound, not much less so therefore the difficulty was, the passages being so contiguous, to prevent the food, especially the liquids, which we swallow into the stomach, from entering the wind-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 windpipe, 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 crum 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 epiglottis: 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