What is the rectum? The rectum is located at the end of the colon and is about 5 inches in length. The rectum is normally empty, except when stool is propelled by the upper colon into the rectum just prior to a bowel movement. At that time, stool is ready to be excreted through the anal canal. The anal canal has two muscular "valves", called the internal and external sphincters, through which the stool must pass. The sphincters allow us to retain stool until we are ready to have a bowel movement, at which time the sphincters relax, releasing the stool. What is rectal cancer? Rectal cancer is malignant (or cancerous) tissue that grows in the wall of the rectum. The majority of tumors begin when normal tissue in the rectum wall forms an adenomatous polyp, or pre-cancerous growth, projecting from the rectal wall lining. As this polyp grows larger, the tumor is formed. This process ... read more on Rectal Cancer: The Basics