Organs and Processes of Digestion : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 12 of 31)

The Stomach. - The stomach is the largest dilatation of the alimentary canal. It is situated in the abdominal cavity, immediately below the diaphragm, with the larger portion toward the left side. Its connection with the esophagus is known as the cardiac orifice and its opening into the small intestine is called the pyloric orifice. It varies greatly in size in different individuals, being on the average from ten to twelve inches at its greatest length, from four to five inches at its greatest width, and holding from three to five pints. It has the coats common to the canal, but these are modified somewhat to adapt them to its work.

The mucous membrane of the stomach is thick and highly developed. It contains great numbers of minute tube-shaped bodies, known as the gastric glands. These are of two general kinds and secrete large quantities of a liquid called the gastric juice. When the stomach is empty, the mucous membrane is thrown into folds which run lengthwise over the inner surface. These disappear, however, when the walls of the stomach are distended with food.

The muscular coat consists of three separate layers which are named, from the direction of the fibers, the circular layer, the longitudinal layer, and the oblique layer. The circular layer becomes quite thick at the pyloric orifice, forming a distinct band which serves as a valve.

The outer coat of the stomach, called the serous coat, is a continuation of the peritoneum, the membrane lining the abdominal cavity.

Stomach Digestion. - In the stomach begins the definite work of dissolving those foods which are insoluble in water. This, as already stated, is a double process. There is first a chemical action in which the insoluble are changed into soluble substances, and this is followed immediately by the dissolving action of water. The chief substances digested in the stomach are the proteins. These, in dissolving, are changed into two soluble substances, known as peptones and proteoses. The digestion of the proteins is, of course, due to the

Gastric Juice. - The gastric juice is a thin, colorless liquid composed of about 99 percent of water and about 1 percent 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 proteins, 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 proteins into peptones and proteoses.

Other Effects of the Gastric Juice. - In addition to digesting proteins, 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. 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 protein layers that enclose the fat particles are dissolved away, 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 proteins.

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 laevulose (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 chime. 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 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.

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. At its upper extremity it connects with the pyloric end of the stomach, 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 cecum, and at their place of union is a marked constriction which prevents material from passing from the large into the small intestine. This is known as the ileo-cecal valve.

D.C. Heath and Co. - Publishers
Original copyright 1909