In humans, the anus (from Latin anus meaning "ring", "circle") is the external opening of the rectum, located inside the intergluteal cleft and separated from the genitals by the perineum. Two sphincters control the exit of feces from the body during an act of defecation, which is the primary function of the anus. These are the internal anal sphincter and the external anal sphincter, which are circular muscles that normally maintain constriction of the orifice and which relaxes as required by normal physiological functioning. The inner sphincter is involuntary and the outer is voluntary. It is located behind the perineum which is located behind the vagina or scrotum.
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In part owing to its exposure to feces, a number of medical conditions may affect the anus such as hemorrhoids. The anus is the site of potential infections and other conditions, including cancer (see Anal cancer).
With anal sex, the anus can play a role in sexuality. Attitudes toward anal sex vary, and it is illegal in some countries. The anus is often considered a taboo part of the body, and is known by many usually vulgar slang terms. Some sexually transmitted infections including HIV/AIDS and anal warts can be spread via anal sex.