The jejunum and ileum is slung from the posterior abdominal wall by the mesentery of the small intestines and, therefore, is extremely mobile. The mesentery of the small intestine arises from the root of the mesentery which extends from the duodenojejunal flexure to the ileocecal junction.
The jejunum is about 2.5m (8ft) long and passes imperceptibly into the ileum, which is about 4m (12ft) long. this part of the small intestine occupies a central position in the abdominal cavity, below the liver and the stomach, and behind the transverse mesocolon, the transverse colon and the greater omentum. The lowest coils of the intestine lie in the pelvic cavity. The purple dotted line in the lower image is an arbitrary line that can be used to separate the jejunum which is to the upward left of the line and the ileum which is to the downward and right of the line.