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.  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.)]