Physiology and Hygiene for Secondary Schools

by Francis M. Walters

The Intestinal Juice is a clear liquid with an alkaline reaction, containing water, mineral salts, and certain proteid substances that may act as enzymes. It assists in bringing about an alkaline condition in the small intestine and aids in the reduction of cane sugar and maltose to the simple sugars, dextrose and levulose. Since it is difficult to obtain this liquid in sufficient quantities for experimenting, its uses have not been fully determined. Recent investigators, however, assign to it an important place in the work of digestion.

Work of the Small Intestine.—The small intestine is the most important division of the alimentary canal. It serves as a receptacle for holding the food while it is being acted upon; it secretes the intestinal juice and mixes the food with the digestive fluids; it propels the food toward the large intestine; and, in addition to all this, serves as an organ of absorption.

Digestion is practically finished in the small intestine, and a large portion of the reduced food is here absorbed. There is always present, however, a variable amount of material that is not digested. This, together with a considerable volume of liquid, is passed into

[pg 158]The Large Intestine.—The large intestine is a tube from five to six feet in length and averaging about one and one half inches in diameter. It begins at the lower right side of the abdominal cavity, forms a coil which almost completely surrounds the coil of small intestine, and finally terminates at the surface of the body (Figs. 2, 71 and 73). It has three divisions, known as the cæcum, the colon, and the rectum.

Fig. 73—Passage from small into large intestine. At the ileo-cæcal valve is the narrowest constriction of the food canal.

The cæcum is the pouch-like dilatation of the large intestine which receives the lower end of the small intestine. It measures about two and one half inches in diameter and has extending from one side a short, slender, and blind tube, called the vermiform appendix. This structure serves no purpose in digestion, but appears to be the rudiment of an organ which may have served a purpose at some remote period in the history of the human race. The cæcum gradually blends into the second division of the large intestine, called the colon.

The colon consists of four parts, described as the ascending colon, the transverse colon, the descending colon, and the sigmoid flexure, or sigmoid colon. The first three divisions are named from the direction of the movement of materials through them and the last from its shape, which is similar to that of the Greek letter sigma (Σ).

The rectum is the last division of the large intestine[pg 159] It is a nearly straight tube, from six to eight inches in length, and connects with the external surface of the body.

The general structure of the large intestine is similar to that of the small intestine, and, like the small intestine, it is held in place by the peritoneum. It differs from the small intestine, however, in its lining of mucous membrane and in the arrangement of the muscular coat. The mucous membrane presents a smooth appearance and has no villi, while the longitudinal layer of the muscular coat is limited to three narrow bands that extend along the greater length of the tube (Fig. 74). These bands are shorter than the coats, and draw the large intestine into a number of shallow pouches, by which it is readily distinguished from the small intestine (Fig. 71).