The jejunum and ileum measure about 20 ft (6 m) long; the upper two fifths of this length make up the jejunum. Each has distinctive features, but there is a gradual change from one to the other.
The jejunum begins at the duodeno jejunalflexure, and the ileum ends at the ileocecal junction. The coils of jejunum and ileum are freely mobile and are attached to the posterior abdominal wall by a fan-shaped fold of peritoneum known as the mesentery of the small intestine. The long free edge of the fold encloses the mobile intestine. The short root of the fold is continuous with the parietal peritoneum on the posterior abdominal wall along a line that extends downward and to the right from the left side of the 2nd lumbar vertebra to the region of the right sacroiliac joint. The root of the mesentery permits the entrance and exit of the branches of the superior mesenteric artery and vein, lymph vessels, and nerves into the space between the two layers of peritoneum forming the mesentery.