than any other liquor with which it can be compared. For the straining off 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. [Pl. XIX.] 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, into a common reservoir or receptacle. This receptacle is a bag large enough to hold about a table-spoon full; and from this vessel a duct or main pipe proceeds, climbing up the back part of the chest, and afterwards creeping along the gullet till it reach the neck. Here it meets the river: here it d scharges 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 whole structure, 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 nowise 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. Secondly, we must also remark their peristaltic motion; which is made up of contractions, following 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, a the same time that the more subtile parts, which we call chyle, are, by a series of gentle compressions, squeezed into the narrow orifices of the lacteal vessels. Thirdly, it was necessary that these tubes, which we denominate lacteals, or their mouths at least, should be as narrow as possible, in order to deny admission into the blood to any particle which is of size enough to make a lodge ment afterwards 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 near 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 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 dreamed of a communication between the cavity of the intestines and the left great vein near 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? II. We postponed the consideration of digestion, lest t should interrupt us in tracing the course of the food to e blood; but, in treating of the alimentary system, so p.incipal 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. I* 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 the 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 chemical pro-` cess with which we are acquainted. And the most wonderful thing about it is its appropriation; its subserviency to the particular economy of each animal. The gastric juice of an owl, falcon, or kite, will not touch grain; no, not even to finish the macerated and half-digested 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 chemistry. 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 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 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. [Pl. XVIII. fig. 1, 2.] 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 ductus 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 gallbladder 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 distension of that vessel: the effect of which compression Phil. Trans. vol. Ixii. p. 447. is, to force out fron 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. It is possible that the gall may be changed, and for some purposes meliorated, by keeping. 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 inte the cavity of the intestine.§ 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. 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, deserves to be reckoned amongst the most intelligible pieces of mechanism with which we are acquainted. [Pl. XX. fig. 1, 2.] The saliva, we all know, is used in the mouth; but much of it is * Keill's Anat. p. 64. Keill's from Malpighius, p. 62. † Anat. p. 164. |