The anus

The anus is part of the digestive system. Stool leaves the body through the anus. It is the opening at the end of the large intestine after the rectum.

The anal canal is a tube about 4 cm long. It connects the anus to the rectum, which is the holding area for stool. The junction between the rectum and anal canal is called the anorectal line.

The anal canal is surrounded by ring-like muscles called the anal sphincter (made up of the internal anal sphincter and the external anal sphincter). These muscles relax to let stool pass out of the body.

The anal canal is lined by a mucous membrane, which is a thin layer of moist tissue. It contains gland cells that make mucus (a thick, slippery fluid). This mucus helps stool move easily out of the body. The upper part of the anal canal contains folds of epithelium called anal columns. The lower part of the anal canal contains squamous cells. The dentate line marks the place where the anal columns change to squamous cells. At the anus, the tissue starts to change from moist to dry and more like skin on the rest of the body.

The perianal skin is the skin around the anus. The perianal skin is made up of squamous cells and is similar to skin found anywhere else on the body.