Fatty foods undergo little or no chemical alteration until they reach the small intestine. During their stay in the stomach they naturally become liquid from the heat of the body, and there is more or less liberation of fat from the digestive action of gastric juice on cell walls, connective tissues, etc. Most food fat is in the form of so-called neutral fat, which must undergo hydrolysis or saponification before it can be absorbed and thus made available for the body. This is accomplished by the enzyme lipase, or steap-sin, of the pancreatic juice, aided indirectly by the presence of bile. Under the influence of this fat-splitting enzyme all neutral fats, whether animal or vegetable, are broken apart, through hydrolysis, into glycerin and a free fatty acid; the latter reacting: in some measure with the sodium carbonate of the pancreatic juice to form a sodium salt, or soluble soap, while perhaps the larger part of the fatty acid is held in solution by the bile present. Soap, free acid, and glycerin are then absorbed from the intestine and are found again combiued in the lymph as neutral fat. In this way the fats of the food are rendered available for the nourishment of the body.

1 Otto Cohnheim: Zur Spaltung des Nahningseiweisses im Darm. Zeit-schrift f. physiologische Chemie, Band 49, p. 64.

2 Bergell and Lewio:. ZeiUchrift fiir experimentelle Pathologic und Therapie, Band 8, p. 426.

The next important chemical change taking place in the small intestine is that induced by the amylopsin of the pancreatic juice, which, acting in essentially the same manner as the ptyalin of saliva, converts any unaltered starch into dex-trins and sugar. The latter substance, maltose, is exposed to the action of another enzyme contained in the intestinal secretion termed maltase, which transforms it into dextrose, a monosaccharide.

In these ways the proteids, fats, and carbohydrates of the food are gradually digested, so far as conditions will admit, digestion being practically completed by the time the material reaches the ileocæcal valve at the beginning of the large intestine. Throughout the length of the small intestine absorption proceeds rapidly; water, salts, and the products of digestion passing out from the intestine into the circulating blood and lymph. At the ileocecal valve, however, the contents of the intestine are practically as fluid as at the beginning of the small intestine, due to the fact that water is continually being secreted into the intestine. In the large intestine, the contents become less and less fluid through reabsorption of the water, and as the propulsive movements of the circular and longitudinal muscle fibres of the intestinal wall carry the material onward toward the rectum, the last portions of available nutriment are absorbed. Finally, in varying degree, certain putrefactive changes are observed in the large intestine involving a breaking down of some residual proteid matter, through the agency of micro-organisms almost invariably present, with formation of such substances as indol, skatol, phenol, fatty acios, etc. These processes, however, in health are held rigidly in check, and count for little in fitting the food for absorption. Digestion, on the other hand, extending as we have seen from the mouth cavity to the ileocæcal valve, is the handmaiden of nutrition, preparing all three classes of organic foodstuffs for their passage into the circulating blood and lymph, and thus paving the way for their utilization by the hungry tissue cells.