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.
[pg 150]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.
In addition to emptying the stomach into the small intestine, these muscles also aid in emptying the organ upward and through the esophagus and mouth, should occasion require. Vomiting in case of poisoning, or if the food for some reason fails to digest, is a necessary though unpleasant operation. It is accomplished by the[pg 151] contraction of all the muscles of the stomach, together with the contraction of the walls of the abdomen. During these contractions the pyloric valve is closed, and the muscles of the esophagus and pharynx are in a relaxed condition.[59]
Fig. 70—Passage from stomach into small intestine. Illustration also shows arrangement of mucous membrane in the two organs. D. Bile duct.
The Small Intestine.—This division of the alimentary canal consists of a coiled tube, about twenty-two feet in length, which occupies the central, lower portion of the abdominal cavity (Fig. 71). At its upper extremity it connects with the pyloric end of the stomach (Fig. 70), and at its lower end it joins the large intestine. It averages a little over an inch in diameter, and gradually diminishes in size from the stomach to the large intestine. The first eight or ten inches form a short curve, known as the duodenum. The upper two fifths of the remainder is called the jejunum, and the lower three fifths is known as the ileum. The ileum joins that part of the large intestine known as the cæcum, and at their place of union is a marked constriction which prevents material from passing from the large into the small intestine (Fig. 73). This is known as the ileo-cæcal valve.