Anus
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The anus, in anatomy, is the external opening of the rectum. Closure 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. Most animals — from simple worms to elephants and humans — have a tubular gut, with a mouth at one end and an anus at the other.
Structure
The human anus is situated between the buttocks, posterior to the perineum. It has two anal sphincters, one internal, the other external. One sphincter consists of smooth muscle and its action is voluntary; the other, striated muscle and involuntary. In many animals, the anus is surrounded by anal sacs.