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.