The
human Anus (from
Latin anus meaning "ring", "circle")
[1][2] is the external opening of the
rectum. Like other animals, its closure is controlled by
sphincter muscles.
Feces are expelled from the body through the anus during the act of
defecation, the primary function of the anus.
The anus plays a role in sexuality, though attitudes towards anal sex vary; it is illegal in some countries.[citation needed] The anus is also the site of potential infections and other conditions including cancer. The subject is often considered a taboo part of the body, and it is known by a large number of usually vulgar slang terms. The traditional polite synonym for anus was fundament, though this euphemism is rarely heard now that medical terms are widely acceptable.
Intra-rectal pressure builds as the rectum fills with feces, pushing the feces against the walls of the anal canal. Contractions of abdominal and pelvic floor muscles can create intra-abdominal pressure which further increases intra-rectal pressure. The internal anal sphincter (an involuntary muscle) responds to the pressure by relaxing, thus allowing the feces to enter the canal. The rectum shortens as feces are pushed into the anal canal and peristaltic waves push the feces out of the rectum. Relaxation of the internal and external anal sphincters allows the feces to exit from the anus, finally, as the levator ani muscles pull the anus up over the exiting feces.
To prevent diseases of the anus[citation needed] and to promote general hygiene, humans often clean the exterior of the anus after emptying the bowels. A rinse with water from a bidet or a wipe with toilet paper are often used for this purpose.
