is the gastric juice, the presence of which determines the location in the food-tract of the stomach proper. This is because the fluid is produced by the lining of the stomach, and not by a distinct organ. That even the microscopic animals have some digestive fluid, like gastric juice, is regarded as proved by the fact, already noticed, that solid food is dissolved by them without mechanical aid. This fluid is well shown in the radiated animals. Its active principle is a ferment called pepsin, which acts only in the presence of an acid. The acidity of the fluid is given by free hydrochloric acid. Gastric juice dissolves only nitrogenous substances, as meat, albumen, and gelatine, having little or no effect on oil or starch.
Next to the gastric juice in importance, if we may judge by its early appearance in the animal kingdom, is the bile. This alkaline fluid is found in all animals having a distinct digestive cavity. The earliest biliary organs are minute cells upon the stomach-lining, as in the anemone. A higher form is found in the small tubes surrounding
Fig. 11.—Water-cells of Camel's Stomach.
the intestines of the insect, from which there are slow gradations to the superior liver of the higher mollusks and fishes. In the articulates, mollusks, and all higher animals, the bile is poured into the intestine and so separated from the gastric juice. The action of the bile is not fully known; but it appears to dissolve fats slightly, and helps to subdivide them into minute particles which are "diffused through the liquid like atoms of butter in milk." It probably aids also in the process of absorption.
The pancreatic juice, another alkaline fluid, is found below the vertebrates only in the higher mollusks. As a gland the pancreas is rudimentary in the cephalopods, but appears better developed in the fishes, and proportionally largest in birds. The function of this fluid is a general one, as it acts on nearly all aliments, and seems to be the principal means of digesting oils and starch, or carbonaceous foods. It is poured into the small intestine near the stomach.
By the mucous lining of the intestines there is produced an alkaline
