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Concept
Human anus
Summary
In humans, the anus (: anuses or ani; from Latin ānus, "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.
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.
The anus is the final part of the gastrointestinal tract, and directly continues from the rectum. The anus passes through the pelvic floor. The anus is surrounded by muscles. The top and bottom of the anus are surrounded by the internal and external anal sphincters, two muscular rings which control defecation.
The anus is surrounded in its length by folds called anal valves, which converge at a line known as the pectinate line. This represents the point of transition between the hindgut and the ectoderm in the embryo. Below this point, the mucosa of the internal anus becomes skin. The pectinate line is also the division between the internal and external anus.
The anus receives blood from the inferior rectal artery and innervation from the inferior rectal nerves, which branch from the pudendal nerve.
Official source
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