Anatomy of the Anus
The anus is the last part of the digestive tract. It’s at the end of the rectum. It's where stool comes out of the body. It consists of a muscular ring (called a sphincter) that opens during a bowel movement to allow stool (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. These are also called squamous cells. The lower part of the anus, where it meets the skin, is called the anal margin.