The anus, in anatomy, is the external opening of the rectum. It is controlled by sphincter muscles. Feces are expelled from the body through the anus during the act of defecation, which is the primary function of the anus.
Anatomy and function of the human anus
The human anus is situated between the buttocks, posterior to the perineum. It has two anal sphincter s, 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.