But there are other active parts of vegetables, such as oily and resinous matters, and some neutral acrid principles, which, before they can be dissolved by water, require the aid of an alkali. These will be considered separately. We now arrive at a fifth class of matters which are taken up by absorption. 5. Animal and vegetable products dissolved by the gastric juice.--The most important of these are the nitrogenous and nutritive constituents of the flesh of animals and of the parts of vegetables. Albumen, Glutine, Fibrine, and Caseine, are connected together as compounds of Proteine. Animal Fibrine, and the analogous Glutine of vegetables, are quite insoluble in water. Albumen and Caseine, though soluble, are immediately precipitated by acids. This is known to be at first effected by the gastric juice on their entry into the stomach. But the action of the gastric juice which contains an acid, and a peculiar nitrogenous material called Pepsin,--together with the temperature of the body, which is about 100°,--causes at length the gradual solution of these previously insoluble matters. This is found to take place out of the body when the above conditions are imitated with an artificial gastric fluid. The result of the process is a viscid fluid, which is then absorbed. The hard Gelatine of gristle and bone is not soluble in water at this temperature, but is readily soluble in the acid gastric juice. The Pepsine seems to be an important agent in this process, for an acid by itself is found to produce an imperfect solution. The nitrogenous matters thus digested and absorbed constitute that portion of the food which is of most use in the nutriment of the system; for the starchy compounds cannot be appropriated to the more solid tissues, although in some cases they may be converted into fat, as in herbivorous animals. (Liebig's _Animal Chemistry_, p. 113.) Starch itself is one of this class. Tiedemann and Gmelin found that by the action of the gastric juice it was slowly converted into Dextrine, which afterwards changed into grape sugar. Both of these are very soluble. From the researches of Bouchardat and others, it appears that other fluids, as the Saliva, the secretion of Brunner's glands, and the Pancreatic juice, possess also this power of converting starch into a more soluble compound. So that if any of it escape the action of the stomach-secretion, it is probably reduced to solution and absorbed in the small intestine. This change of starch is the first of a series of transformations, now ascertained, the ultimate result of which is its combustion and resolution into carbonic acid. Thus the nitrogenous compounds are called the _nutritive_, and the starchy materials the _calorifacient_ elements of the food. Considered as medicines, these substances belong to the division of Aliments.