The œsophagus, or gullet, is a tube about nine inches long, reaching from the throat to the stomach. It lies behind the windpipe, pierces the diaphragm between the chest and abdomen, and opens into the stomach. It has in its walls muscular fibers, which, by their worm-like contractions, grasp the successive masses of food swallowed, and pass them along downwards into the stomach. 138. Deglutition, or Swallowing. The food, having been well chewed and mixed with saliva, is now ready to be swallowed as a soft, pasty mass. The tongue gathers it up and forces it backwards between the pillars of the fauces into the pharynx. If we place the fingers on the “Adam’s apple,” and then pretend to swallow something, we can feel the upper part of the windpipe and the closing of its lid (epiglottis), so as to cover the entrance and prevent the passage of food into the trachea. There is only one pathway for the food to travel, and that is down the œsophagus. The slow descent of the food may be seen if a horse or dog be watched while swallowing. Even liquids do not fall or flow down the food passage. Hence, acrobats can drink while standing on their heads, or a horse with its mouth below the level of the œsophagus. 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 raphé. 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