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. 694. The mucous membrane of the large intestines is disposed differently from that of the small intestines, and the mucous membrane of the colon still differently from that of the rectum. In the colon the mucous membrane, instead of being disposed in the form of valvulæ conniventes, is so arranged as to divide its whole surface into minute apartments or cells by which the descent of the fecal matter is retarded still more than the descent of the chyle by the valvulæ conniventes. Some particles of chyle do, however, continue to be separated from the fecal matter, even in the large intestines; and in order that nothing may be lost, a few valvulæ conniventes, with their lacteals, appear here also, while the cells of the colon, by retarding the descent of the fecal matter, allow time for the more complete separation and absorption of the chylous particles. 695. In the rectum the mucous membrane is plaited into large transverse folds, which disappear as the fecal matter descends into the bowel, accumulates in it, and distends it; an arrangement which gives to this portion of the intestine its power of distension, so closely connected with our convenience and comfort.