Anus

The human anus (from Latin anūs meaning "ring", "circle") 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. With anal sex, the anus can play a role in sexuality. Attitudes towards anal sex vary and it is illegal in some countries. The anus 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.

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