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).
Ileal diverticulum
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.
