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By experiments out of the body it appears, that neither of these fecretions acts upon alimentary substances, in the fame manner as the gastric juice acts.

3. Digestion is not putrefaction; for it refifts putrefaction moft pertinaciously; nay, not only checks its further progress, but reftores putrid substances.

4. It is not a fermentative process; for the folution begins at the furface, and proceeds towards the centre, contrary to the order in which fermentation acts and fpreads.

5. It is not the digeftion of beat; for the cold maw of a cod or fturgeon will diffolve the fhells of crabs and lobsters, harder than the fides of the ftomach which contains them.

In a word, animal digestion carries about it the marks of being a power and a process, completely fui generis; diftinct from every other; at least from every chymical process with which we are acquainted. And the moft wonderful thing about it is its appropriation; its fubferviency 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 half digested pulse, which is left in the crops

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of the fparrows that the bird devours. poultry, the trituration of the gizzard, and the gaftric juice, confpire in the work of digeftion. The gaftric juice will not diffolve the grain whilt it is whole. Grains of barley inclosed in tubes or spherules are not effected by it. But if the fame 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 ftrong, its ftructure is fo, beyond that of any other muscle belonging to the animal. The internal coat also, or lining of the gizzard, is, for the fame purpose, hard and cartilaginous. But, forafmuch as this is not the fort of animal fubftance fuited for the reception of glands, or for fecretion, the gaftric juice, in this family, is not fupplied, as in membranous ftomachs, by the ftomach itself, but by the gullet, in which the feeding glands are placed, and from which it trickles down into the ftomach.

In theep, the gaftric fluid has no effect in digefting plants, unless they have been previously mafticated. It only produces a flight macera

tion; nearly fuch as common water would produce, in a degree of heat fomewhat exceeding the medium temperature of the atmofphere. But provided that the plant has been reduced to pieces by chewing, the gastric juice then proceeds with it, firft by softening its fubftance; next by destroying its natural confiftency; and, laftly, by diffolving it fo completely, as not even to spare the toughest and most stringy parts, fuch 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 diffolved vegetables, but made no impreffion upon beef, mutton, and other animal bodies. Dr. Hunter difcovered a property of this fluid, of a most curious kind; viz. that, in the ftomachs of animals which feed upon flesh, irrefiftibly as this fluid acts upon animal fubftances, it is only upon the dead fubftance, that it operates at all. The living fibre fuffers no injury from lying in contact with it. Worms and infects are found alive in the ftomachs of fuch animals. coats of the human stomach, in a healthy ftate,

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are infenfible to its prefence: yet, in cafes of fudden death, (wherein the gastric juice, not having been weakened by difeafe, retains its activity,) it has been known to eat a hole through the bowel which contains it*. How nice is this dilcrimination of action, yet how neceffary?

But to return to our hydraulics.

III. The gall bladder is a very remarkable contrivance. It is the refervoir of a canal. It does not form the channel itself, i. e. the direct communication between the liver and the inteftine, which is by another paffage, 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 cyfticus: by which fructure it is enabled, as occafions may require, to add its contents to, and increafe, the flow of, bile into the duodenum. And the pofition of the gall bladder is fuch as to apply this ftructure to the beft advantage. In its natural fituation it touches the exterior surface, of the ftomach, and confequently is compreffed by the diftenfion of that veflel: the effect of which compreffion is, to force cut from the

*Phil. Tranf. vol. lxii. p. 447•

bag,

bag, and send into the duodenum, an extraordinary quantity of bile, to meet the extraor dinary demand which the repletion of the ftomach by food is about to occafion*. Chefelden describes † the gall bladder as seated against the duodenum, and thereby liable to have its fluid preffed out, by the paffage of the aliment through that cavity; which likewife will have the effect of causing it to be received into the inteftine, 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 fame kind as what paffes from the liver through the direct paffage ‡. It is poffible that the gall may be changed, and for fome purposes meliorated, by keeping.

The entrance of the gall duct into the duodenum furnishes another obfervation. Whenever either smaller tubes are inferted into larger tubes, or tubes into veffels and cavities, fuch receiving tubes, veffels, or cavities, being fubject to muscular conftriction, we always

*Keill's Anat. p. 64.

+ Anat. p. 164.

+ Keill from Malpighius, p. 63.

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