Alcoholic and Ethereal fluids may be enumerated here as soluble products of the vegetable kingdom. Alcohol mixes with water to any extent; and one part of Ether is soluble in ten parts of water. Nitric Ether and Chloroform are also sufficiently soluble. So is Creosote, 1.25 parts of which dissolve in 100 of water. But this may be absorbed in another way, as will be seen presently. Volatile Oils and Turpentine come also under this head. They are all slightly soluble in water. The former, when given in small doses, are probably absorbed in this way. Turpentine, when given in large doses, may perhaps, by undergoing a change, come under the head of vegetable substances dissolved by alkalies. Camphor may be included here: one part is soluble in 1000 parts of water.
The soluble gummy matters of plants, when added to the substances enumerated above, constitute an aggregate which is called the _watery extractive_ of a vegetable product: _i.e._ that part which is capable of being dissolved out of it by pure water.
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.)