Colon, rectum, and anus

The colon is one structural unit with two embryological origins. The cecum and right and midtransverse colons are of midgut origin and as such are supplied by the superior mesenteric artery (SMA). The distal transverse, splenic flexure, and descending and sigmoid colon are of hindgut origin and receive blood from the inferior mesenteric artery (IMA). The entire colon starts as a midline structure that rotates during development and attaches laterally to the right and left posterior peritoneum. The right and left colonic mesenteries are obliterated, fusing to the posterior peritoneum in these regions, leaving these portions of the colon covered by peritoneum on the lateral, anterior, and medial surfaces. The transverse and sigmoid colons, in contrast, are completely covered with peritoneum and are attached by long mesenteries, allowing for great variation in the location of these structures (Fig. 51.1).