. 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 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 giv