690. While these processes are going on, another and a very important function is performed by the remaining portion of the alimentary canal. It is the office of this part of the apparatus to carry out of the body that portion of the aliment which is incapable of being converted into chyle. The preparation of the excrementitious part of the aliment for its expulsion constitutes the process of fecation. The organs in which this process is carried on, and by which the excrementitious matter, when duly prepared for its removal, is conveyed from the body, are the large intestines.
691. The large intestines (fig. [CLXXIX].) consist of the cæcum, the colon and the rectum (fig. [CLXXIX]. ). The cæcum varies in length from two inches to six; the colon is about five feet in length, and the rectum is about eight inches.
692. The ilium opens into the cæcum (fig. [CLXXIX]. 8, 10), just as the esophagus opens into the stomach. At this point the ilium is elongated, forming two concentric folds which join at their horns, and between the folds are placed a number of muscular fibres. In this manner is constructed a valve, which is termed the valve of the colon. It is placed in a transverse direction across the intestine, and its action as a valve is very complete. It admits of the free passage of the contents of the small intestines into the large, but it prevents the return of any portion of the contents of the latter into the former.
Fig. CLXXIX.—View of the Abdominal Portion of the Digestive Organs.
1. Esophagus. 2. Stomach. 3. Spleen. 4. Liver. 5. Gall-bladder with its ducts. 6. Pancreas with its duct. 7. Duodenum. 8. Small intestines. 9. Large intestines dividing into—10. Cæcum. 11. Ascending colon. 12. Arch of the colon. 13. Descending colon. 14. Sigmoid flexure here imperfectly represented. 15. Rectum.
Portion of the large intestine, showing the arrangement of the muscular fibres. 1. The longitudinal fibres collected into bands, and forming larger fasciculi. 2. The circular fibres arranged as in the other intestines.
693. The colon is distinguished by its capacious size, its great length, and its longitudinal bands, which consist of strong muscular fasciculi (fig. [CLXXIX]. 11). It is divided into an ascending portion which occupies the right iliac and hypochondriac regions (fig. [CLXXIX]. 11); the transverse portion, called its arch, which is placed directly across the epigastric region (fig. [CLXXIX]. 12), a descending portion which occupies the left hypochondriac region (fig. [CLXXIX]. 13), and a fourth portion, which being curved somewhat like the italic letter S, is called the sigmoid flexure, which occupies the left iliac region (fig. [CLXXIX]. 14). The sigmoid flexure terminates in the last portion of the alimentary canal, called the rectum (fig. [CLXXIX]. 15), which is placed in the hollow of the sacrum, and which follows the curvature of that bone (fig. XLV. 5). The circular fibres of the rectum are accumulated at the termination of the bowel to form the internal sphincter of the anus. External to this is placed another set of fibres, which constitute the external sphincter.