The gastric juice is a thin almost colorless fluid with a sour taste and odor. The reaction is distinctly acid, normally due to free hydrochloric acid. Its chief constituents are two ferments called pepsin and rennin, free hydrochloric acid, mineral salts, and 95 per cent of water. Illustration: Fig. 54.—A highly magnified view of a peptic or gastric gland, which is represented as giving off branches. It shows the columnar epithelium of the surface dipping down into the duct D of the gland, from which two tubes branch off. Each tube is lined with columnar epithelial cells, and there is a minute central passage with the “neck” at N. Here and there are seen other special cells called parietal cells, P, which are supposed to produce the acid of the gastric juice. The principal cells are represented at C. Pepsin the important constituent of the gastric juice, has the power, in the presence of an acid, of dissolving the proteid food-stuffs. Some of which is converted into what are called _peptones_, both soluble and capable of filtering through membranes. The gastric juice has no action on starchy foods, neither does it act on fats, except to dissolve the albuminous walls of the fat cells. The fat itself is thus set free in the form of minute globules. The whole contents of the stomach now assume the appearance and the consistency of a thick soup, usually of a grayish color, known as chyme. It is well known that “rennet” prepared from the calf’s stomach has a remarkable effect in rapidly curdling milk, and this property is utilized in the manufacture of cheese. Now, a similar ferment is abundant in the gastric juice, and may be called _rennin_. It causes milk to clot, and does this by so acting on the casein as to make the milk set into a jelly. Mothers are sometimes frightened when their children, seemingly in perfect health, vomit masses of curdled milk. This curdling of the milk is, however, a normal process, and the only noteworthy thing is its rejection, usually due to overfeeding. Experiment 58. _To show that pepsin and acid are necessary for gastric digestion._ Take three beakers, or large test tubes; label them _A_, _B_, _C_. Put into _A_ water and a few grains of powdered pepsin. Fill _B_ two-thirds full of dilute hydrochloric acid (one teaspoonful to a pint), and fill _C_ two-thirds full of hydrochloric acid and a few grains of pepsin. Put into each a small quantity of well-washed fibrin, and place them all in a water bath at 104° Fahrenheit for half an hour. Examine them. In _A_, the fibrin is unchanged; in _B_, the fibrin is clear and swollen up; in _C_, it has disappeared, having first become swollen and clear, and completely dissolved, being finally converted into peptones. Therefore, both acid and ferment are required for gastric digestion.