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 degrees Fahrenheit.

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.