The small intestine is the longest section of the digestive tube and it is arbitrarily divided into three parts. The first 20 to 30 cm is termed the duodenum, the second 2.5 metres the jejunum and the final 3.5 metres the ileum. These regions are not anatomically distinct, although there are differences in absorptive capability and secretion. There is no definite sphincter between the stomach and duodenum although in some studies a zone of elevated pressure between the two regions has been reported to exist. The duodenum has a thick wall with a deeply folded mucous membrane and contains duodenal digestive glands and Brunner's glands. Brunner's glands are found only in the submucosa of the duodenum and produce a protective alkaline secretion which does not contain any enzymes, but serves to neutralize gastric acid. The jejunum is thicker walled and more vascular than the duodenum and has larger and more numerous villi than the ileum. In the ileum, the lymphatic follicles (Peyer's patches) are larger and more numerous than elsewhere in the intestine.
Most of the small intestine is suspended from the body wall by an extension of the peritoneum called the mesentery. The blood vessels which supply the small intestine lie between the two sheets of the mesentery.
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