Jejunum
The jejunum lies between the duodenum and the ileum and is considered to start at the suspensory muscle of the duodenum, a location called the duodenojejunal flexure. The division between the jejunum and ileum is not anatomically distinct due to the fact that in adult humans, the small intestine is usually 6-7m long, about two-fifths of which (2.5 m) is the jejunum.
Gallstone ileus
First described by Thomas Bartholin in 1654, gallstone ileus is a rare form of small bowel obstruction caused by an impaction of a gallstone within the lumen (small opening) of the small intestine. Such a gallstone enters the bowel via a cholecysto-enteric fistula. The presence of large stones, >2.5 cm in diameter, within the gallbladder are thought to predispose to fistula formation by gradual erosion through the gallbladder fundus. Once a fistula has formed, a stone may travel from the gallbladder into the bowel and become lodged almost anywhere along the GI tract. Obstruction occurs most commonly at the near the distal ileum, within 60 cm proximally to the ileocecal valve. Rarely, gallstone ileus may recur if the underlying fistula is not treated.