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 lodgement 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 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 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 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 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. It does not form the channel itself, i. e. the direct communication between the liver and the intestine, which is by another

*Phil. Trans. vol. lxii. p. 447.

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 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 describest the gallbladder 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 passaget. 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

* Keill's Anat. p. 64

† Anat. p. 164.

Keill (from Malpighius), p. 63.