intestine

Cross section of the small intestine showing the villi and their blood supply. There are hundreds of villi for every square millimetre of the small intestine (itself 5 m/16 ft in length).

In vertebrates, the digestive tract from the stomach outlet to the anus. The human small intestine is 6 m/20 ft long, 4 cm/1.5 in in diameter, and consists of the duodenum, jejunum, and ileum; the large intestine is 1.5 m/5 ft long, 6 cm/2.5 in in diameter, and includes the caecum, colon, and rectum. The contents of the intestine are passed along slowly by peristalsis (waves of involuntary muscular action). The term intestine is also applied to the lower digestive tract of invertebrates.

Both the small and large intestines are muscular tubes comprising an inner lining that secretes alkaline digestive juice, a submucous coat containing fine blood vessels and nerves, a muscular coat, and a serous coat covering all. The intestines are supported by a strong peritoneum, which carries the blood vessels, the lymph vessels, and the nerves.