=The gullet= (or esophagus) is a muscular tube about nine inches long. (See Fig. 89.) Like the rest of the food tube, it is lined with mucous membrane. It has two layers of muscles in its walls, the fibers of one layer running lengthwise, and the fibers of the other layer being circular. In _swallowing_, the food does not fall down the gullet of its own weight, but _the circular bands of muscle in front of the food relax_, and _those behind it contract and push it on into the stomach_. This wavelike motion is called _peristalsis_. =The stomach=, the greatest enlargement of the food tube, is like _a large bag lying sideways_. It lies to the left side of the abdomen. The walls of the stomach consist chiefly of _muscular fibers which run lengthwise, crosswise, and slantwise_, making three coats (Exp. 7, also Fig. 95). As soon as the food reaches the stomach, the layers of muscles begin to contract, changing the size of the stomach, first in length, then in breadth, thus churning the food to and fro, and mixing it with the gastric juice, a fluid more active than the saliva. For as the food enters the stomach, the mucous membrane lining it turns a bright red, and many little gastric glands in the lining begin to secrete gastric juice. [Illustration: FIG. 95.--MUSCULAR AND OTHER LAYERS IN WALL OF STOMACH. 1, mucous lining; 2, layer of blood vessels and connective tissue; 3, muscular layers (involuntary muscles); 4, connective-tissue fibers. (Peabody.)] =Digestion in the Stomach.=--The stomach churns the food from two to four hours after the meal, according to the kind of food eaten, the way it has been cooked, and the thoroughness with which it has been chewed. The _gastric juice_ is chiefly water, and contains two ferments called _pepsin_ and _rennin_, and a small quantity of _hydrochloric acid_. Rennin acts upon the curd of milk, and is abundant only during infancy. Hydrochloric acid kills germs that may enter the stomach, and changes the food which has been made alkaline by the saliva into an acid condition (Exp. 1). This enables the _pepsin to act upon the proteid part of the food_, for pepsin will not act while the food is alkaline. Gastric juice _digests lean meat_, which is a proteid food, by first dissolving the connective tissue that holds the fibers in place, and they fall apart; it then acts upon the fibers separately and makes them soluble. Like human fatty tissue (Fig. 14), _fat meat_ consists of cells filled with fat and held together by threads of connective tissue. The cell walls and the threads, both being proteid, are soon dissolved by the gastric juice, and the free fat is melted into oil, but still undigested. The food is reduced in the stomach to a creamy, half-fluid mass called _chyme_. Where the stomach opens into the small intestine, there is a folding in or narrowing of the tube so as to form a kind of valve called the _pylorus_. After the food has been changed