Jejunum and Ileum Anatomy
The jejunum and ileum constitute the major part of the small intestine. The jejunum commences at the duodenojejunal flexure and the ileum terminates at the ileocaecal junction. Between these two sites, the small intestine is about 5 or 6 m long and forms numerous loops that fill most of the infracolic compartment of the abdomen (Figs 4.41 & 4.63). By convention, the proximal two-fifths are called the jejunum and the remainder the ileum. However, no precise anatomical feature marks the junction between the two, there being a gradual morphological transition along the whole length of the small intestine.
As the small intestine is traced distally, there is a gradual reduction in the size of the lumen. The terminal ileum is the narrowest region and it is here that gallstones and foreign bodies may lodge. Mucosal folds (plicae circulares) are numerous in the proximal jejunum (Fig. 4.64) but diminish in both size and number so that in the distal ileum they are often absent. This difference in mucosal structure can be detected by palpation and may also be apparent on radiographs. The distribution of lymphoid tissue in the jejunum is diffuse, whereas in the mucosa of the ileum it is arranged in discrete clumps (Peyer’s patches).
Within a metre of the ileocaecal junction, the ileum occasionally possesses a diverticulum on its antimesenteric border. This diverticulum (Meckel’s diverticulum) is the embryological remnant of the vitellointestinal duct and may be connected to the umbilicus. Inflammation of the diverticulum can give rise to clinical features similar to those of appendicitis. Sometimes the tip of the diverticulum remains connected to the umbilicus by a fibrous cord around which loops of intestine may become twisted giving rise to intestinal obstruction.
The jejunum and ileum are contained within the free border of the mesentery of the small intestine. This fan-shaped structure has a root about 15 cm long attached to the posterior abdominal wall between the duodenojejunal flexure and the ileocaecal junction (Fig. 4.65). The mesentery divides the infracolic compartment of the peritoneal cavity into right and left infracolic spaces. Between its two peritoneal layers, the mesentery contains a quantity of fat, which is particularly abundant in the ileal portion. Embedded in this fat are numerous jejunal and ileal blood vessels (see below), lymphatic vessels and nodes, and autonomic nerves.
Location and relations
Because they are suspended from the mesentery, the jejunum and ileum possess considerable mobility and their coils can change position relative to adjacent organs. The jejunum usually occupies the central part of the abdomen, especially the umbilical region, while the ileum lies at a lower level, mostly in the hypogastrium and the pelvic cavity (Fig. 4.63). The terminal ileum usually ascends from the pelvis into the right iliac fossa to reach the medial aspect of the caecum (Fig. 4.69). The principal anterior relations of the jejunum and ileum are the greater omentum, the transverse colon and its mesocolon, and the anterior abdominal wall. Posteriorly, the coils of small intestine overlie retroperito- neal structures on the posterior wall of the abdomen and pelvis and may also overlap the ascending, descending and sigmoid parts of the colon. Within the pelvis, loops of ileum may lie in contact with pelvic organs, such as the rectum and urinary bladder and, in the female, with the uterus and its appendages.
The superior mesenteric artery supplies the intestine from the descending duodenum to the splenic flexure of the colon. The artery is an anterior branch of the abdominal aorta (Fig. 4.89) at the level of the first lumbar vertebra, just below the coeliac trunk. It descends in front of the left renal vein (Fig. 4.91) and behind the neck of the pancreas. Inclining to the right, the artery continues downwards in front of the uncinate process of the pancreas and across the horizontal duodenum to enter the root of the mesentery (Fig. 4.66).
The superior mesenteric artery gives rise to middle colic and inferior pancreaticoduodenal branches before gaining the mesentery. As it descends in the root of the mesentery, it furnishes the right colic artery, which passes behind the peritoneum to supply the ascending colon. A further branch, also retroperitoneal, is the ileocolic artery, which inclines downwards and to the right towards the caecum. The superior mesenteric artery also gives numerous branches to the jejunum and ileum. Within the mesentery these jejunal and ileal arteries anastomose, producing a series of arterial arcades which are more profuse in the ileal part of the mesentery (Fig. 4.67). The superior mesenteric artery terminates at the upper border of the terminal ileum, where it anastomoses with the lower branch of the ileocolic artery and the distal ileal arteries. If the superior mesenteric artery becomes narrowed by disease, most of the small intestine and part of the large intestine may become ischaemic or necrotic. The superior mesenteric vein begins above the terminal ileum and ascends in the root of the mesentery. It lies on the right of the superior mesenteric artery and its tributaries correspond to the branches of the artery. The vein terminates behind the neck of the pancreas by joining the splenic vein to form the portal vein. Close to its termination, it may be joined by the inferior mesenteric vein (Fig. 4.78), but this more commonly enters the splenic vein.