*Gastric Juice.*—The gastric juice is a thin, colorless liquid composed of about 99 per cent of water and about 1 per cent of other substances. The latter are dissolved in the water and include, besides several salts, three active chemical agents—hydrochloric acid, pepsin, and rennin. _Pepsin_ is the enzyme which acts upon proteids, but it is able to act only in an acid medium—a condition which is supplied by the _hydrochloric acid_. Mixed with the hydrochloric acid it converts the proteids into peptones and proteoses. *Other Effects of the Gastric Juice.*—In addition to digesting proteids, the gastric juice brings about several minor effects, as follows: 1. It checks, after a time, the digestion of the starch which was begun in the mouth by the saliva.(58) This is due to the presence of the hydrochloric acid, the ptyalin being unable to act in an acid medium. 2. While there is no appreciable action on the fat itself, the proteid layers that inclose the fat particles are dissolved away (Fig. 79), and the fat is set free. By this means the fat is broken up and prepared for a special digestive action in the small intestine. 3. Dissolved albumin, like that in milk, is curded, or coagulated, in the stomach. This action is due to the _rennin_. The curded mass is then acted upon by the pepsin and hydrochloric acid in the same manner as the other proteids. 4. The hydrochloric acid acts on certain of the insoluble mineral salts found in the foods and reduces them to a soluble condition. 5. It is also the opinion of certain physiologists that cane sugar and maltose (double sugars) are converted by the hydrochloric acid into dextrose and levulose (single sugars). After a variable length of time, the contents of the stomach is reduced to a rather uniform and pulpy mass which is called _chyme_. Portions of this are now passed at intervals into the small intestine. *Muscular Action of the Stomach.*—The muscles in the walls of the stomach have for one of their functions the mixing of the food with the gastric juice. By _alternately_ contracting and relaxing, the different layers of muscle keep the form of the stomach changing—a result which agitates and mixes its contents. This action varies in different parts of the organ, being slight or entirely absent at the cardiac end, but quite marked at the pyloric end. Another purpose of the muscular coat is to empty the stomach into the small intestine. During the greater part of the digestive period the muscular band at the pyloric orifice is contracted. At intervals, however, this band relaxes, permitting a part of the contents of the stomach to be forced into the small intestine. After the discharge the pyloric muscle again contracts, and so remains until the time arrives for another discharge.