Anatomy of the Anus
The anus is the last part of the digestive tract, at the end of the rectum. It consists of a muscular ring (called a sphincter) that opens during a bowel movement to allow stool (also called feces) to pass through, as well as flat cells that line the inside of the anus. Most anal cancers start in these flat lining cells, which are also called squamous cells. The lower part of the anus, where it meets the skin, is also known as the anal margin.