LARGE INTESTINE

posterior section of the vertebrate intestine, consisting typically of four regions: the cecum, colon, rectum, and anal canal (qq.v.). The term colon is sometimes used to refer to the entire large intestine. The large intestine is wider and shorter than the small (in humans, approximately 5 feet, or 1.5 metres, in length as compared with 22 to 25 feet, or 6.7 to 7.6 metres, for the small intestine) and has a smooth inner wall. In the proximal, or upper, half of the large intestine, enzymes from the small intestine complete the digestive process, and bacteria produce the B vitamins (B12, thiamin, and riboflavin) as well as vitamin K. The large intestine's primary function, however, is absorption of water and electrolytes from digestive residues (which in humans usually takes about 24 to 30 hours) and storage of fecal matter until it can be expelled. Churning movements of the intestine gradually expose digestive residue to the absorbing walls. A progressive and more vigorous type of movement known as mass movement (gastrocolic reflex), which occurs only two or three times daily, propels the material toward the anus. In primarily vegetarian animals the large intestine is usually longer. The immature frog (or tadpole), for example, eats mainly plant matter and has a long, highly coiled large intestine. As the frog matures and begins to eat mostly insects, its intestine becomes considerably shorter. High-protein food, such as meat, can readily be digested by the small intestine; much more chemical action and agitation are required, however, to reduce the tough cellulose fibres of plant cells. The large intestine performs this function with its slow digestive process. In newborn humans, the large intestine does not contain the bacteria essential for production of vitamin K, lack of which may cause excessive bleeding. Infant diets should be supplemented with this vitamin for a few weeks until the infant is capable of producing its own supply. Common afflictions of the large intestine include inflammation, such as colitis; diverticulosis; and abnormal growths, such as benign or malignant tumours.