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The ileum is narrow; its diameter is about 3.75 centimeters. Its coats are thinner and less vascular than those of the jejunum. It possesses few circular folds and they are small and disappear entirely toward it lower end. Peyer’s patches are larger and more numerous. The ileum chiefly occupies the umbilical, hypogastric, right iliac, and pelvic regions. The terminal part of the ileum usually lies in the pelvis, from which it ascends over the right psoas and right iliac vessels. It ends in the right iliac fossa by opening into the medial side of the commencement of the large intestine. The jejunum and ileum are attaché to the posterior abdominal wall by mesentery. The mesentery allows the freest motion so that each coil can accommodate itself to changes in form and position. Between the two layers of mesentery are blood vessels, nerves, lacteals, lymph glands, and a variable amount of fat.
The jejunum and ileum are supplied by branches of the superior mesenteric artery. The intestinal branches of this artery reach the attached border of the bowel, run between the tunica serosa and muscularis. They anastomose with other branches running around the opposite surface of the gut. From these vessels, numerous branches are given off, which pierce the muscular coat, supplying it and forming an intricate plexus in the submucous tissue. From this plexus, minute vessels pass to the glands and villi of the mucous membrane.
The veins follow a similar path to the arteries.
The nerves of the small intestines are derived from the plexuses of sympathetic nerves around the superior mesenteric artery. From this source, they run to the myenteric plexus situated between the circular and longitudinal muscular fibers. From this the nervous branches are distributed to the muscular coats of the intestine. From this the plexus of the submucosa is derived. It is formed by branches which have perforated the circular muscular fibers. This plexus lies in the tela submucosa of the intestine. The nerve bundles of the submucous plexus are finer than those of the myenteric plexus.