In an ileoanal anastomosis, a pouch is used to create a large section of bowel whose function replaces that of the large intestine, or colon. In this operation, the ileum (part of the small intestine) is shaped into a W-shaped pouch (A). An incision is made (B) to open up the shape and create the larger pouch, which is left open at one end and brought through the rectal area (C). The bottom of the pouch acts as a new rectum, and a new anus is fashioned (D). (Illustration by GGS Inc.)