The jejunum and ileum form the second and third part of the small intestine respectively. They collectively measure about 20 feet long. The jejunum is smaller of the two and measures about 8 feet, while the ileum is about 12 feet long. Each of jejunum and ileum has distinctive features, but there is a gradual change from one to the other and that is why it is better to describe both simultaneously.
The jejunum begins at the duodenojejunal flexure. After its origin, it gets coiled and gradually changes its features to become ileum, which ends at the ileocecal junction.
The coils of jejunum and ileum are freely mobile. They are attached to the posterior abdominal wall by the mesentery of small intestine, which is a fan-shaped fold of peritoneum. The root of this fold is continuous with the parietal peritoneum on the posterior abdominal wall along a line that extend from left side of second lumbar vertebra to the region of sacroiliac joint. Thus the root of the mesentery extends downward from left to right. The long free edge of the mesentery encloses the mobile intestine.
The root of the mesentery allows various structures to reach the intestines, from the posterior abdominal wall. The branches of the superior mesenteric artery and vein, lymph vessels and nerves, all reach the intestine through the mesentery.
In living subjects, there are marked differences between the two parts of the intestine, as described below:
The jejunum lies coiled in the upper part of peritoneal cavity below the left of the transverse mesocolon, while the ileum is found in the lower part of the peritoneal cavity and in the pelvis.
The jejunum has wider bore, thicker walls, and is redder in color. The infoldings of the mucous membrane (plicae circulars) are larger, more numerous and more closely set in the jejunum as compared to ileum, where they are smaller and widely spread in the upper part and totally absent in the lower part.
Attachment of mesentery:
The mesentery of jejunum is attached to the left of aorta while the mesentery of ileum is attached to the right.
The mesenteric vessels of jejunum form only one or two arcades, which supply the jejunal wall through long and infrequent branches. On the other hand, the ileum receives numerous short terminal vessels that arise from a series of three, four or even more arcades.
Deposition of fat:
At the jejunal end of mesentery, the fat is deposited near the root and is scanty near the jejunal wall. On the other hand, at the ileal end, the fat is deposited throughout so that it extends from the root to the intestinal wall.
Aggregations of lymphoid tissue:
In the mucous membrane of lower ileum, there are aggregations of lymphoid tissue, known as Peyer's patches. Such aggregations are not found in the jejunum.
Both jejunum and ileum are supplied by the branches of superior mesenteric artery. These branches arise from the left side of the artery (from where no other branches arise) and run through the mesentery to reach the wall of intestine. The branches anastomose with one another to form a series of arcades.
The lowest part of ileum, near the ileocecal junction, is supplied by the ileocolic artery in addition to the usual blood supply.
The veins correspond to the arteries and eventually drain into the superior mesenteric vein.
The lymphatics from the jejunum and ileum drain into the superior mesenteric nodes after passing through a number of intermediate mesenteric nodes.
Both the sympathetic and parasympathetic fibers come from the superior mesenteric plexus.