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; chymification, 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

* See Vol. I. Ser. 1. Lect. x.

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 afterwards clearly detected, and its power incontrovertibly established.

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 yaorp, which is Greek for stomach. Mr. Cruickshank supposes about a pound of it to be poured forth every twentyfour 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 considerably, according to the demand of the system generally, or the state of the stomach itself. In carnivorous birds, whose stomachs are

* Anat. of the Absorbing Vessels, p. 106.

+ Physiol. Institut. xxvii. § 410.

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

* Voyage, p. 181. See also Hunter's Animal Economy, p. 196.

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 neutral; and in graminivorous ruminating animals with four stomachs, and particularly 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 especially 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.