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.
Retroperistalsis
In the small intestine, local stimulation of the intestinal smooth muscle by the presence of food particles causes contractions that tend to travel from the stimulated point in both directions. Under normal circumstances, the progress of the contractions in an oral direction is quickly inhibited, while the contractions travelling away from the mouth tend to persist. If the intestine is paralyzed by applying such drugs as nicotine or cocaine to the intestinal wall, the contractions initiated by local stimulations travel equally well in both directions. Ordinarily, peristaltic waves appear in the small intestine at irregular intervals and travel for varying distances; some travel only a few inches, others a few feet. They serve to expose food to the intestinal wall for absorption and to move it forward.