Colostomy. Illustration of a colostomy and human digestive system. At the top are illustrations of a stoma (the outlet) and various colostomy bags, including one that aerates the skin while retain- ing odour (top right). A colostomy is an operation in which part of the colon (large intestine) is brought through an incision in the abdominal wall and formed into an artificial anus called a stoma, which is then connected to a lightweight bag that collects faeces. Temporary colostomies are sometimes carried out in order to enable surgery to be carried out on the colon. Permanent colostomies may be needed if the rectum or anus are removed to treat cancer.