The orifice by which the food enters is called the cardiac opening, because it is near the heart. The other opening, by which the food leaves the stomach, and where the small intestine begins, is the pyloric orifice, and is guarded by a kind of valve, known as the pylorus, or gatekeeper. The concave border between the two orifices is called the _small curvature_, and the convex as the _great curvature_, of the stomach. 140. Coats of Stomach. The walls of the stomach are formed by four coats, known successively from without as serous, muscular, sub-mucous, and mucous. The outer coat is the serous membrane which lines the abdomen,—the peritoneum (note, p. 135). The second coat is muscular, having three sets of involuntary muscular fibers. The outer set runs lengthwise from the cardiac orifice to the pylorus. The middle set encircles all parts of the stomach, while the inner set consists of oblique fibers. The third coat is the sub-mucous, made up of loose connective tissues, and binds the mucous to the muscular coat. Lastly there is the mucous coat, a moist, pink, inelastic membrane, which completely lines the stomach. When the stomach is not distended, the mucous layer is thrown into folds presenting a corrugated appearance. Illustration: Fig. 53.—Pits in the Mucous Membrane of the Stomach, and Openings of the Gastric Glands. (Magnified 20 diameters.) 141. The Gastric Glands. If we were to examine with a hand lens the inner surface of the stomach, we would find it covered with little pits, or depressions, at the bottom of which would be seen dark dots. These dots are the openings of the gastric glands. In the form of fine, wavy tubes, the gastric glands are buried in the mucous membrane, their mouths opening on the surface. When the stomach is empty the mucous membrane is pale, but when food enters, it at once takes on a rosy tint. This is due to the influx of blood from the large number of very minute blood-vessels which are in the tissue between the rows of glands. The cells of the gastric glands are thrown into a state of greater activity by the increased quantity of blood supply. As a result, soon after food enters the stomach, drops of fluid collect at the mouths of the glands and trickle down its walls to mix with the food. Thus these glands produce a large quantity of gastric juice, to aid in the digestion of food. 142. Digestion in the Stomach. When the food, thoroughly mixed with saliva, reaches the stomach, the cardiac end of that organ is closed as well as the pyloric valve, and the muscular walls contract on the contents. A spiral wave of motion begins, becoming more rapid as digestion goes on. Every particle of food is thus constantly churned about in the stomach and thoroughly mixed with the gastric juice. The action of the juice is aided by the heat of the parts, a temperature of about 99° Fahrenheit.