(From digero, to dissolve). Digestion-. In surgery it is the disposing an ulcer or wound to suppuration, by the application of proper remedies.

In pharmacy it is the subjecting of bodies, included in proper vessels, to the action of a gentle heat. The term digestion is often used for maceration; and, in this case, the process is without heat: where this circumstance is not expressed, digestion always implies the operation of heat. In some cases, digestion is used to produce a change in a single body, as in hydrargyrus nitratus ruber; in others, to promote solutions, or differ-ent combinations. Circulation is a mode of digesting: the vessels generally used are matrasses, or Florence wine flasks, either of which may be converted into cir-ry vessels; or the neck of one may be inverted into the neck of the other. The operation is generally performed in a sand bath, by which the degrees of heat may be regulated according to the intention of the chemist. This heat is never so great as to make it boil. Digestion is used for making tinctures, wines, and elixirs.

In the animal economy, it is the conversion of aliment into chyle, and then into blood. By digestion, the specific differences of all substances are abolished; the blood, formed from different kinds of aliment, whether used singly or together, does not sensibly differ in its properties, provided that the organ of digestion be sufficiently powerful to convert them into blood. Digestion, in the stomach alone, is capable of converting our food into chyme, and the continuation of the process can alone assimilate it to our own nature.

This function of the animal economy is of most difficult explanation. The ingenuity of physiologists has been exhausted in the solution of the problem; and we are still at a distance from any theory that will explain all the different appearances satisfactorily.

When, with Hippocrates, we attribute digestion to putrefaction; with Pringle and Macbride, to fermentation; or, with Mailer, to the joint action of solution and fermentation; we alter the language only, not the opinions. Van Helmont attributed it to the energy of his archaeus, which resided in the stomach; and, though fanciful in language, we shall probably find this process very intimately connected with the vital principle. We must, however, premise some peculiar circumstances in the structure of the parts concerned in the operation, omitted in the general descriptions; because they would appear more advantageously, when their application could be at the same time perceived.

The alimentary canal extends from the mouth to thc anus, enlarged at different parts, to detain the food, and assist the changes it is destined to undergo. When the oesophagus passes down on the left side, the canal crosses the body; and we here find the first dilatation, which we style the stomach. In its empty state it appears a bag, into which a substance falls, and from which it must rise to pass out at the other aperture, the pylorus. When, however, the stomach is full, the fundus of this sac is raised against the integuments, and it assumes the form of a crescent. The angle to which the substance passing out must rise, in the empty state, is obliterated; but the food is retained by the contraction of the strait fibres, which draw the pylorus towards the cardia. The fundus of the stomach enlarges between the folds of the epiploon, which is its mesentery; and the large vessels of this membrane are thus emptied, discharging their contents into the stomach, which receives also, at the same time, blood, from its pressure on the spleen, which determines a larger proportion of the vital fluid through the vasa brevia.

The next dilatation is a little below the stomach, at the part styled the duodenum. The intestine, in this part, is not confined by the peritonaeum so closely as to form a mesentery; but is loosely connected to the back bone, and admits of considerable distention. Here the chyme receives the bile and the pancreatic juice; and, in this part, its animalisation begins, and chyle is formed. The intestines then proceed, of an uniform diameter, till the end of the ilium is inserted, in the manner before described, into the large intestine (see Colon), where it again stagnates; apparently to admit of the absorption of the remaining chyle.

The food of animals is very various; yet it consists only of a few principles, and these may be reduced to oil, gelatine, and sugar, with the animal matter already prepared. In proportion as the food is of the last kind its remora in the stomach and duodenum is shorter; but the digestion of matter already animalised, is a problem too simple to detain us. We must remark, however, that the action of the stomach differs from almost every known power. It has no effect on living bodies: its

4C effects are in proportion to the vital energy; and when this is greatly diminished from any sudden cause, the digestion suffers at the same moment, and in the same ratio. Digestion sweetens also, in a short period, the most putrid substances which the stomach can retain. So different is this process, as well as its instruments, from every other, that we must turn with contempt from the philosophers attempting to imitate it in their phials, did not their exalted character in other pursuits change our contempt to surprise.

The changes produced by digestion are very considerable. The principles of vegetable substances) as we have already seen, consist of oxygen, carbone, and hydrogen; animal substances contain the same principles, with a less proportion of carbone, and with the addition of azote. The production of the azote has occasioned the chief difficulty; for we know that some proportion of carbone is separated with oxygen, by respiration, in the form of carbonic acid gas. We must, however, pursue the subject in its proper order.

The changes in the stomach are, by the most modern physiologists, attributed either to fermentation or solution. Each, however, gives a due share of the cre-tlit to the division by mastication, combination with the saliva, the effects of heat, and gentle agitation in the stomach by the action of its fibres, producing a regular progressive and retrograde motion. Fermentation, it is contended, does not probably take place, because we do not find its productions, an ardent spirit, and an acid. Wc have more than once had occasion to remark that we often find an acid, which we know to be the product of fermentation, though we cannot detect the previous form. We discover, in digestion, an extrication of air, which, when the process is not interrupted, is again combined with the mass, and an acid is perhaps always formed; because, when by accident digestion is disturbed, or stopped, In an early period, it is obvious. In better circumstances this acid disappears, in conse-cpaence of a subsequent union. In short, as our vegetable food is susceptible of fermentation, and in circumstances which would most probably favour it, we see no reason for denying that this change takes place.