Intestine, Large

the portion of the gut of vertebrate animals and man that extends from the termination of the small intestine to the anus in cyclostomes, man, and most fishes and mammals. In chondrichthians, lungfish, amphibians, reptiles, birds, and monotremes, the large intestine empties into a cloaca. Considerable absorption of water from the residual chyme takes place in the large intestine, with subsequent formation of the feces.

In mammals, including man, the large intestine is divided into the colon, with its ascending, transverse, descending, and sigmoid portions, and the rectum. The cecum, or blind gut, is situated at the place where the small intestine passes into the large intestine. In herbivores, marsupials, ungulates, and some rodents the cecum is of considerable size; it is absent in some insecti-vores, ursids, edentates, cetaceans, and hippopotamuses. Many rodents and some predators, prosimians, and monkeys, in addition to man, have a narrow tube at the end of the cecum, the vermiform appendix.

The length of the large intestine is 1–2 m in man, 6.5–14 m in cattle, 0.28–0.9 m in dogs, 0.2–0.45 m in cats, and 3.5–6 m in swine. In man the diameter of the large intestine is 7 cm in the initial section and 4 cm in the descending colon. The wall of the large intestine is composed of mucous, submucous, and serous membranes and a muscular coat. Bacteria multiply enormously in the large intestine; they help form an immunologic barrier and react against pathogenic microbes. The bacteria complete the hydrolization of the components of digestive secretions and of undigested food residues; they also synthesize vitamins, enzymes, and other physiologically active substances.