Gastric Glands.--If we open the stomach of the frog, and remove its contents by carefully washing, its wall is seen to be thrown into folds internally. Between the folds in the stomach of man, as well as in the frog, are located a number of tiny pits. These form the mouths of the _gastric glands_, which pour into the stomach a secretion known as the _gastric juice_. The gastric glands are little tubes, the lining of which secretes the fluid. When we think of or see appetizing food, this secretion is given out in considerable quantity. The stomach, like the mouth, "waters" at the sight of food. Gastric juice is slightly acid in its chemical reaction, containing about .2 per cent free _hydrochloric acid_. It also contains two very important enzymes, one called _pepsin_, and another less important one called _rennin_. Action of Gastric Juice.--If protein is treated with artificial gastric juice at the temperature of the body, it will be found to become swollen and then gradually to change to a substance which is soluble in water. This is like the action of the gastric juice upon proteins in the stomach. The other enzyme of gastric juice, called _rennin_, curdles or coagulates a protein found in milk; after the milk is curdled, the pepsin is able to act upon it. "Junket" tablets, which contain rennin, are used in the kitchen to cause this change. [Illustration: A peptic gland, from the stomach, very much magnified. _A_, central or chief cell, which makes pepsin; _B_, border cells, which make acid. (From Miller's _Histology_.)] The hydrochloric acid found in the gastric juice acts upon lime and some other salts taken into the stomach with food, changing them so that they may pass into the blood and eventually form the mineral part of bone or other tissue. The acid also has a decided antiseptic influence in preventing growth of bacteria which cause decay, and some of which might cause disease.