Anatomy and function of the human anus

The human anus is situated between the buttocks, posterior to the perineum. It has two anal sphincters, one internal, the other external. These hold the anus closed until the body is ready to defecate.

When the rectum is full the increase in intrarectal pressure forces the walls of the anal canal apart allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincters of the anus allow the feces to be passed by muscles pulling the anus up over the exiting feces.

In some contexts, the anus is considered a sex organ because it contains erotically pleasurable nerve endings. (See: anal sex, rimming.)

For the words "asshole" and "arsehole", and the card game of that name, see Asshole (word)