The human anus from Latin anus meaning "ring", "circle"   is the external opening of the rectum. Two sphincters control the exit of feces from the body during an act of defecationwhich is the primary function of the anus. These are the internal anal sphincter and the external anal sphincterwhich 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 in females and behind the scrotum in males. With anal sexthe anus can play a role in sexuality. Attitudes towards anal sex vary and it is illegal in some countries.
The anus is also the site of potential infections and other conditions, including cancer. The anus is the final component of the gastrointestinal tractand 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 sphincterstwo 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 anus receives blood from the inferior rectal artery and innervation from the inferior rectal nerveswhich branch from the pudendal nerve. The pseudostratified columnar epithelium of the gastrointestinal tract transitions to stratified squamous epithelium at the pectinate line. The stratified squamous epithelium gradually accumulates sebaceous Average depth of anal penetration apocrine glands.
During pubertyas testosterone triggers androgenic hair growth on the body, pubic hair begins to appear around the anus. Although initially sparse, it fills out by the end of puberty, if not earlier. However, in some genetic populations androgenic hair is less common.
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 Average depth of anal penetration pull the anus up over the exiting feces. Example of a Grade IV hemorrhoidone that protrudes out of the anus.
In psychologythe Freudian term anal fixation is used. The anus has a relatively high concentration of nerve endings and can be an erogenous zonewhich can make anal intercourse pleasurable if performed properly. The pudendal nerve that branches to supply the external anal sphincter also branches to the dorsal nerve of the clitoris and the dorsal nerve of the penis.
Sigmund Freud 's theory of psychosexual developmentfor example, described an anal stagehypothesizing that toddlers derive pleasure from retaining and expelling feces. This is the source of the term " anal-retentive " and the derived informal term "anal".
In addition to nerve endings, pleasure from anal intercourse may be aided by the close proximity between the anus and the prostate for males, and vaginaclitoral legs and anal area for females. This is because of indirect stimulation of the prostate and vagina or clitoral legs. Average depth of anal penetration stretching or fisting is pleasurable for some, but it poses a more serious threat of damage due to the deliberate stretching of the anal and rectal tissues; its injuries include anal sphincter lacerations and rectal and sigmoid colon rectosigmoid perforation, which might result in death.