Shortly after food begins to enter the stomach churning movements are set up. These consist of regular contractions of the smooth muscle beginning at about the middle of the stomach in the form of a ring and so causing a deep depression to form right around the stomach, which then travels along toward the sphincter at the outlet. This, of course, crowds the food up toward that end; but if the sphincter does not relax, the food instead of escaping simply squirts back through the space left in the middle of the ringlike construction and so is actively churned. A regular procession of these waves travels over this part of the stomach during the whole time that gastric digestion is going on. The outer half of the stomach, the side toward the esophagus, does not join in this churning motion. The walls here remain quiet, merely pressing upon the food to crowd it up into the part where the churning is going on as fast as room is made for it there. At intervals during this process the sphincter relaxes and a small mass of food is crowded through into the small intestine; then the sphincter closes again, preventing more from passing until the former mass has had time to be acted upon by the digestive juices in the intestine. The way in which this sphincter is controlled is one of the very interesting facts of physiology, but before describing it we shall have to say something about actual gastric digestion. The juice which the glands in the stomach wall secrete is called gastric juice. This contains three important constituents, first of which is hydrochloric acid. From the standpoint of chemistry it is a very interesting thing that the gastric glands should he able to manufacture a mineral acid like hydrochloric acid. That they do so, however, is proven by the facts of digestion in everybody. In addition to hydrochloric acid gastric juice contains two enzymes; the first is pepsin, which begins the digestion of protein foods, although it does not carry it through to completion. The other constituent is the enzyme rennin, which clots milk; this property appears to be useful in that clotted milk will not pass the sphincter into the small intestine as quickly as it would if it were liquid, and so the digestive processes can go on in milk after clotting just as it goes on in any other of our foods. Since milk is the chief, and in many cases the only, food of the young it is of course very important that its digestion should be very efficiently carried on. As the food is churned by the stomach muscles, it becomes mixed with gastric juice. Any milk that is present will be clotted and any protein will begin to be digested. The food will also be mixed with hydrochloric acid. It happens that the salivary enzyme, ptyalin, will not act in the presence of an acid, so that as soon as any of the food in the stomach comes in contact with hydrochloric acid of the gastric juice, the digestion of starch by ptyalin stops. Since