The jejunum is the middle section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms middle intestine or mid-gut may be used instead of jejunum. The jejunum lies between the duodenum and the ileum. The change from the duodenum to the jejunum is usually defined as the ligament of Treitz. In adult humans, the small intestine is usually between 5.5-6m long, 2.5m of which is the jej… (More on Jejunum)