digestive canal, the food on which they feed being already of their own nature, and containing a larger quantity of nourishment in a less bulk; and hence demanding a smaller proportion both of time and space to become fit for use. In this respect man holds a medium between the two: his digestive canal is less complex than that of most animals that feed on grass alone, and more extensive than that of most animals that are confined to a diet of their own kind. Man is hence omnivorous, and is capable of subsisting on an aliment of either sort; and from his digestive organs, as well as from various others, is better qualified for every variety of soil and climate than any other animal.

Man, however, is by no means the only omnivorous animal in the world; for the great Author of nature is perpetually showing us that, though he ope rates by general laws, he is in every instance the lord and not the slave of them. Hence, among quadrupeds, the swine, and among insects the ant, possesses as omnivorous a power as man himself, and feeds equally on the fleshy parts of animals, and on grain, and the sweet juices of vegetables. In consequence of this omnivorous power in the ant, we may often make use of him as a skilful anatomist; for, by putting a dead frog, mouse, or other small animal in a box perforated with holes, and placing it near an ant-hill, we shall find it in a few days reduced to a perfect and exquisite skeleton, every atom of the soft parts being separated and devoured.

The solid materials of the food are first masticated and moistened in the mouth, excepting in a few cases, in which it is swallowed whole. It is then introduced into the stomach, and converted into an homogeneous pulp or paste, which is called chyme; and shortly afterward, by an additional process, into a fluid for the most part of a milky appearance, denominated chyle; in which state it is absorbed or drunk up voraciously by thousands and tens of thousands of little mouths of very minute vessels, which are not often found in the stomach, but line the whole of the interior coating of that part of the intestinal tube into which the stomach immediately empties itself, and which are perpetually waiting to imbibe its liquid contents. These vessels constitute a distinct part of the lymphatic system; they are called lacteals from the usual milky appearance of the liquid they absorb and contain. They progressively anastomose or unite together, and at length terminate in one common trunk, named the thoracic duct, which conveys the different streams thus collected and aggregated to the sanguineous system, to be still farther operated upon, and elaborated by the action of the heart and the lungs.

The means by which the food is broken down and rendered pultaceous after being received into the stomach are various and complicated. In the first place, the muscular tunic of the stomach acts upon it by a slight contraction of its fibres, and so far produces a mechanical resolution: secondly, the high temperature maintained in the stomach by the quantity of blood contained in the neighbouring viscera and sanguiferous vessels, gives it the benefit of accumulated heat, and so far produces a concoctive resolution: and, thirdly, the stomach itself secretes and pours forth from the mouths of its minute arteries a very powerful solvent, which is by far the chief agent in the process, and thus produces a chemical resolution. In this manner the moistened and manducated food becomes converted into the pasty mass we have already called chyme: and, fourthly, there are a variety of juices separated from the mass of the blood by distinct glands situated for this purpose in its vicinity, which are thrown into the duodenum, or that part of the canal into which the stomach immediately opens, by particular conduits, and in some way or other appear to contribute to the common result, and to transform the chyme into chyle, but concerning the immediate powers or modes of action of which we are in a considerable degree of darkness. Of these glands the most remarkable and the most general are the liver and the pancreas or sweet-bread; the first of which secretes the bile, and is always of a considerable size, and appears to produce a very striking effect on the blood itself, by a removal of several of its principles independently of its office as a digestive organ.

From this brief survey of the process of digestion it is obvious that the stomach itself performs by far the principal part; in some animals, indeed, it appears to perform the whole; and it is hence necessary that we examine the general structure and powers of this organ with a little more minuteness. In man the stomach is situated on the left side of the midriff; in its figure it resembles the pouch of a bag-pipe; its left end is most capacious; its upper side is concave, its lower convex; and the two orifices for receiving and discharging the food are both situated in the upper part. In its substance it consists of three distinct coats or layers, the external and internal of which are membranous, and the middle muscular. The internal coat, moreover, is lined with a villous or downy apparatus, and is extremely convoluted or wrinkled; the wrinkles increasing in size as the diameter of the stomach

contracts.

From what I have already observed, it must appear that the process of digestion in man consists of three distinct acts: mastication, which is the office of the mouth, and by which the food is first broken down; chymifica tion, or its reduction into pulp, which is the office of the stomach; and chylification, or its dilution into a fluid state, which is the office of that part of the intestinal canal which immediately communicates with the stomach. The whole of this process is completed in about three hours, and under certain states of the stomach, to which I shall advert presently, almost as quickly as the food is swallowed. The most important of these three actions is that of chymification; and, while it takes place, both orifices of the stomach are closed, and a degree of chilliness is often produced in the system generally, from the demand which the stomach makes upon it for an auxiliary supply of heat, without an augmentation of which it appears incapable of performing this important function.

Considering the comparatively slender texture of the chief digesting organ, and the toughness and the solidity of the substances it digests, it cannot appear surprising that mankind should have run into a variety of mistaken theories in accounting for its mode of action. Empedocles and Hippocrates supposed the food to be softened by a kind of putrefaction. Galen, whose doctrine descended to recent times, and was zealously supported by Grew and Santarelli, ascribed the effect to concoction, produced, like the ripening and softening of fruits beneath a summer sun, by the high temperature of the stomach from causes just pointed out. Pringle and Macbride advocated the doctrine of fermentation, thus uniting the two causes of heat and putrefaction assigned by the Greek writers; while Borelli, Keil, and Pitcairn resolved the entire process into mechanical action, or trituration; thus making the muscular coating of the stomach an enormous mill-stone, which Dr. Pitcairn was extravagant enough to conceive ground down the food with a pressure equal to a weight of not less than a hundred and seventeen thousand and eighty pounds, assisted, at the same time, in its gigantic labour, by an equal pressure derived from the surrounding muscles.*

Each of these hypotheses, however, was encumbered with insuperable objections; and it is difficult to say which of them was most incompetent to explain the fact for which they were invented.

Boerhaave endeavoured to give them force by interunion, and hence combined the mechanical theory of pressure with the chemical theory of concoction; while Haller contended for the process of maceration. But still a something else was found wanting, and continued to be so till Cheselden in lucky hour threw out the hint, for at first it was nothing more than a hint, of a menstruum secreted into some part of the digestive system; a hint which was soon eagerly laid hold of, and successfully followed up by Haller, Reaumur, Spallanzani, and other celebrated physiologists. And though Cheselden was mistaken in the peculiar fluid to which he ascribed the solvent energy, namely, the saliva, still he led forward to the important fact, and the GASTRIC JUICE Was soon afterward clearly detected, and its power incontrovertibly established.

See Series 1. Lecture x

This wonderful menstruum, the most active we are acquainted with in nature, is secreted by a distinct set of vessels that exist in the texture of the stomach, and empty themselves into its cavity by innumerable orifices invisible to the naked eye; and it is hence called gastric juice, from yaar?, which is Greek for stomach. Mr. Cruickshank supposes about a pound of it to be poured forth every twenty-four hours. "The drink," says he, "taken into the stomach may be two pounds in twenty-four hours; the saliva swallowed may be one pound in the same period, the gastric juice another, the pancreatic juice another. The bile poured into the intestines Haller supposes about twenty ounces, besides the fluid secreted through the whole of the internal surfaces of the intestines ;"* which Haller calculates at not less than eight pounds in twenty-four hours,―a calculation, nevertheless, that Blumenbach regards as extravagant.

The quantity of the gastric juice, however, seems to vary very considerably, according to the demand of the system generally, or the state of the stomach itself. In carnivorous birds, whose stomachs are membranous alone, and, consequently, whose food is chymified by the sole action of the gastric juice, without any collateral assistance or previous mastication, this fluid is secreted in much larger abundance; as it is also in those who labour under that morbid state of the stomach which is called canine appetite; or when, on recovery from fevers, or in consequence of long abstinence, the system is reduced to a state of great exhaustion, and a keen sense of hunger induces a desire to devour food voraciously and almost perpetually.

Such was the situation of Admiral Byron and his two friends, Captains Cheap and Hamilton, after they had been shipwrecked on the western coast of South America, and had been emaciated, as he tells us, to skin and bone, by having suffered with hunger and fatigue for some months. "The governor," says Admiral Byron," ordered a table to be spread for us with cold ham and fowls, which only we three sat down to, and in a short time despatched more than ten men with common appetites would have done. It is amazing that our eating to that excess we had done from the time we first got among these kind Indians had not killed us; we were never satisfied, and used to take all opportunities, for some months after, of filling our pockets when we were not seen, that we might get up two or three times in the night to cram ourselves."‡

When pure and in a healthy state, the gastric juice is a thin, transparent, and uninflammable fluid, of a weak saline taste, and destitute of smell. Generally speaking, it is neither acid nor alkaline; but it appears to vary more or less in these properties, not only in animals whose organs of digestion are of a different structure, but even in the very same animal under different circumstances. It may, however, be laid down as an established rule, that in carnivorous and graminivorous animals possessing only a single stomach, this fluid is acid, and colours blue vegetable juices red; in omnivorous animals as man, whose food is composed both of vegetable and animal diet, it is neu tral; and in graminivorous ruminating animals with four stomachs, and par ticularly in the adults of these tribes, it has an alkaline tendency, and colours blue vegetable juices green.

There are two grand characteristics by which this fluid is pre-eminently distinguished; a most astonishing faculty of counteracting and even correcting putrefaction; and a faculty, equally astonishing, of dissolving the toughest and most rigid substances in nature.

Of its ANTISEPTIC POWER abundant proofs may be adduced from every class of animals. Among mankind, and especially in civilized life, the food is usually eaten in a state of sweetness and freshness; but fashion, and the luxurious desire of having it softened and mellowed to our hands, tempt us to keep several kinds as long as we can endure the smell. The wandering hordes of gypsies, however, and the inhabitants of various savage countries, and espe

† Physiol. Institut. xxvii. § 410.

* Arat. of the Absorbing Vessels, p. 100.
I

t Voyage, p. 181. See also Hunter's Animal Economy, p. 196.

cially those about the mouth of the Orange river in Africa, carry this sort of luxury to a much higher pitch, for they have no objection to an offensive smell, and appear to value their food in proportion to its approach towards putrefaction. Now all these foods, whatever be the degree of their putridity, are equally restored to a state of sweetness by the action of this juice, a short time after they have been introduced into the stomach.

Dr. Fordyce made a variety of experiments in reference to this subject upon the dog, and found uniformly that the most putrid meat he could be made to swallow, was in a very short time deprived of its putrescency. We cannot, therefore, be surprised that crows, vultures, and hyenas, who find a pleasure in tainted flesh, should fatten upon so impure a diet; nor that the dunghill should have its courtiers among insects as well as the flowergarden.

The gastric juice has hence been employed as an antiseptic in a variety of cases out of the body.

Spallanzani has ascertained that the gastric juice of the crow and the dog will preserve veal and mutton perfectly sweet, and without consumption, thirty-seven days in winter; while the same meats immersed in water emit a fetid smell as early as the seventh day, and by the thirtieth are resolved into a state of most offensive liquidity.

Physicians and surgeons have equally availed themselves of this corrective quality, and have occasionally employed the gastric juice, internally in cases of indigestion from a debilitated stomach, and externally as a check to gangrenes, and a stimulus to impotent and indolent ulcers. I do not know that this practice has hitherto taken place very largely in our own country, but it has been extensively resorted to on the Continent, and especially in Switzerland and Italy; and in many cases with great success.

But the gastric juice is as remarkable for its solvent as for its antiputrescent property. Of this any industrious observer may satisfy himself by attending to the process of digestion in many of our most common animals; but it has been most strikingly exemplified in the experiments of Reaumur and Spallanzani. Pieces of the toughest meats, and of the most solid bones, enclosed in small perforated tin cases to guard against all muscular action, have been repeatedly thrust into the stomach of a buzzard: the meats were uniformly found diminished to three-fourths of their bulk in the space of twentyfour hours, and reduced to slender threads; and the bones were wholly digested, either upon the first trial or a few repetitions of it. Dr. Stevens repeated the experiment on the human stomach by means of a perforated ivory ball, which he hired a person at Edinburgh alternately to swallow and disgorge, when a like effect was observed.

The gastric juice of the dog dissolves ivory itself and the enamel of the teeth; that of the hen has dissolved an onyx and diminished a louis-d'or ;* even among insects we find some tribes that fatten upon the fibrous parts of the roots of trees, and others upon metallic oxides. And it is not long since that, upon examining the stomach and intestinal tube of a man who died in one of the public hospitals of this metropolis, and who had some years before swallowed a number of clasp-knives out of hardihood, their handles were found digested, and their blades blunted, though he had not been able to discharge them from his body.

It is in consequence of this wonderful power that the stomach is sometimes found in the extraordinary condition of digesting itself; and of exhibiting, when examined on dissection, various erosions in different parts of it, and especially towards the upper half, into which the gastric juice is supposed to flow most freely. It is the opinion of Mr. John Hunter,+ however, whose opinions are always entitled to respect, that such a fact can never take place except in cases of sudden death, when the stomach is in full health, and the gastric juice, now just poured forth, is surrounded by a dead organ. For he plausibly argues, that the moment the stomach begins to be diseased, it

@wammerdam. Biblia Naturæ, p. 168.

↑ Phil Trans. 1772.

ceases to secrete this fluid, at least in a state of perfect activity; and that so long as it is itself alive, it is capable, by its living principle, of counteracting the effect of this solvent power. Yet a case has lately been published by Mr. Burns of Glasgow, in which the stomach appears to have been eroded, although the death, instead of being sudden, did not take place till after a long illness and great emaciation of the body. It is possible, however, that even here the stomach did not participate in the disease. That the living principle of the stomach is capable, so long as it continues in the stomach, of resisting the action of the gastric juice, can hardly be questioned. And it is to the superior power of this principle of life, that worms and the ova of insects are so often capable of existing in the stomach uninjured, and even of thriving in the midst of so destructible an agency.

But though the solvent juice of the stomach is the chief agent in the process of digestion, its muscular power contributes always something, and in many animals a considerable proportion, towards the general result; and hence, the shape and structure of this organ, instead of being uniformly alike, is varied with the most skilful attention to the nature of the mechanism by which it is to operate.

In its general construction the stomach of different animals may be divided into three kinds; membranous, muscular, and bony. The first is common to graminivorous quadrupeds, and to carnivorous animals of most kinds; to sheep, oxen, horses, dogs, and cats; eagles, falcons, snakes, frogs, newts, and the greater number of fishes, as well as to man himself. The second is common to graminivorous birds; and to granivorous animals of most kinds; to fowls, ducks, turkeys, geese, and pigeons. The third, to a few apterous insects, a few soft-bodied worms, and a few zoophytes; to the cancer-genus, the cuttle-fish, the sea-hedgehog; tubipores and madrepores.

Of the membranous stomach we have already taken notice in describing that of man; and at the bony stomach we took a glance in a late lecture on the teeth and other masticatory organs. It only remains, therefore, that we make a few remarks on that singular variety of the membranous stomach which belongs to ruminant animals, and on the muscular stomach of granivorous and graminivorous birds.

All animals which ruminate must have more stomachs or ventricles than one; some have two, some three; and the sheep and ox not less than four. The food is carried down directly into the first, which lies upon the left side, and is the largest of all; the vulgar name for this is the paunch. There are no wrinkles on its internal surface; but the food is considerably macerated in it by the force of its muscular coat, and the digestive secretions which are poured into it. Yet, in consequence of the vegetable and unanalogous nature of the food, it requires a much farther comminution; and is hence forced up by the esophagus into the mouth, and a second time masticated; and this constitutes the act called rumination, or chewing the cud. After this process, it is sent down into the second ventricle, for the esophagus opens equally into both, and the animal has a power of directing it to whichsoever it pleases. This ventricle is called the bonnet or king's-hood; its internal surface contains a number of cells, and resembles a honey-comb; it macerates the food still farther; which is then protruded into the third ventricle, that, on account of its very numerous folds or wrinkles, is called manyplies, and vulgarly many-plus. It is here still farther elaborated, and is then sent into the fourth ventricle, which, on account of its colour, is called the red, and by the French le caillé, or the curdle, since it is here that the milk sucked by calves first assumes a curdled appearance. It is thus that the process of digestion is completed, and it is this compartment that constitutes the true stomach, to which the others are only vestibules.

There are some animals, however, which do not ruminate, that have more than one stomach; thus the hampster has two, the kangaroo three, and the sloth not less than four. Nor does the conformation terminate even with

• Wiedemann Archív. b. i