An opening constructed when the bowel has to be brought to the skin surface to convey gastrointestinal contents to the exterior. When the colon is involved, it is called a COLOSTOMY; bringing the stomach to the surface is gastrostomy; doing so for the urethra is urostomy (see URETER). Colostomy may be undertaken because of malignancy of the colon or rectum (see INTESTINE) or as a result of inflammatory bowel diseases such as CROHN'S DISEASE. Urostomies usually take the form of an isolated loop of ileum into which the ureters have been implanted and which in its turn is either brought to the skin's surface or converted into an artificial bladder. This is undertaken either because of bladder cancer or because of neurological diseases of the bladder. A gastrostomy may be used for feeding someone for whom swallowing is impossible or risky. Otherwise, stomas drain into appliances such as disposable plastic bags. Modern appliances collect the effluent of the stoma without any leak or odour.