The food is
under the control of the will until it has entered the pharynx; all the
later movements are involuntary.

[Illustration: Fig. 51.--A View into the Back Part of the Adult Mouth.
(The head is represented as having been thrown back, and the tongue drawn
forward.)

A, B, incisors;
C, canine;
D, E, bicuspids;
F, H, K, molars;
M, anterior pillar of the fauces;
N, tonsil;
L, uvula;
O, upper part of the pharynx;
P, tongue drawn forward;
R, linear ridge, or raphe.
]

139. The Stomach. The stomach is the most dilated portion of the
alimentary canal and the principal organ of digestion. Its form is not
easily described. It has been compared to a bagpipe, which it resembles
somewhat, when moderately distended. When empty it is flattened, and in
some parts its opposite walls are in contact.

We may describe the stomach as a pear-shaped bag, with the large end to
the left and the small end to the right. It lies chiefly on the left side
of the abdomen, under the diaphragm, and protected by the lower ribs. The
fact that the large end of the stomach lies just beneath the diaphragm and
the heart, and is sometimes greatly distended on account of indigestion or
gas, may cause feelings of heaviness in the chest or palpitation of the
heart. The stomach is subject to greater variations in size than any other
organ of the body, depending on its contents. Just after a moderate meal
it averages about twelve inches in length and four in diameter, with a
capacity of about four pints.

[Illustration: Fig. 52.--The Stomach. A, cardiac end; B, pyloric end, C,
lesser curvature, D, greater curvature]

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