The jejunum (/dʒɨˈdʒuːnəm/) is the second part of the small intestine in humans and most higher vertebrates, including mammals, reptiles, and birds.The jejunum lies between the duodenum and the ileum.The jejunum is considered to begin at the attachment of the suspensory muscle of the duodenum to the duodenum, a location called the duodenojejunal flexure. The division between the jejunum and ileum is not anatomically distinct.he jejunum lies between the duodenum and the ileum. The jejunum is considered to begin at the attachment of the suspensory muscle of the duodenum to the duodenum, a location called the duodenojejunal flexure.