In anatomy, the anus (from Latin ''ānus'' "ring, anus") 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.

The anus plays a role in sexuality, though attitudes towards anal sex vary and it is even illegal in some countries. The anus is also the site of potential infections and other conditions including cancer. The subject is often considered a taboo part of the body, and is known by a large number of usually vulgar slang terms.

Structure

The human anus is situated between the buttocks, posterior to the perineum. It has two anal sphincters, one internal, the other external. These hold the anus closed until the need arises to defecate. One sphincter consists of smooth muscle and its action is involuntary; the other consists of striated muscle and its action is voluntary. In many animals, the anus is surrounded by anal sacs.

Role in defecation

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.

To prevent diseases of the anusand to promote general hygiene, humans often clean the exterior of the anus after emptying the bowels. A rinse with water from a bidet or a wipe with toilet paper are often used for this purpose.

Role in sexuality

The anus has a relatively high concentration of nerve endings and is an erogenous zone. Sigmund Freud's theory of psychosexual development, for example, described an anal stage, hypothesizing that toddlers derive pleasure from retaining and expelling feces. This is the source of the term "anal" and the derived, derogatory vulgarism "anal-retentive."

Anal intercourse can be pleasurable for both the insertive partner and the receptive partner. For the receptive partner, pleasure from anal intercourse is also thought to be related to the shared wall between the rectum and the vagina (for females) as well as the G-spot or prostate (for males). For the insertive partner, the tightness of the anus is often said to be a source of pleasure in penetrative anal sex.
Anal intercourse, sometimes referred to as sodomy or buggery, is a human sexual activity, but is considered taboo in a number of moral systems, and it has been, and in some jurisdictions continues to be, a crime carrying severe punishment.

Anal sexual activity need not include penetration. The anus also plays an important role in facesitting, coprophilia and anilingus.

Anal stretching can stimulate the nerves around the anus and can be considered pleasurable. Care must be taken to maintain elasticity.

Lubricant is widely regarded as a necessity while performing anal sex.

Health

Hygiene is important for good anal health and anal sex. Washing with a mild soap and water will keep the anus clean. Harsh soaps or wiping vigorously with toilet paper can irritate the skin around the anus, making it itchy or sore. Pinworms are sometimes the source of anal itching.

Care should be taken not to strip the anus of natural oils that keep the skin around the opening supple and elastic.

Penetration with a penis or sex toy can irritate or tear the inside of the anus. Lubrication is often recommended to ease penetration. The risk of injury to the anal sphincter should be a concern, which can lead on to lack of control of defecation and fecal incontinence. Similarly if the anus is torn, this can occasionally cause a fistula formation which can not only cause fecal leaking, but also can be very difficult to treat.

Kegel exercises can improve the tone of the outer sphincter muscle.

Cosmetics

Shaving, trimming, depilatory (hair removal), or Brazilian waxing can clear the perineum of hair.

Anal bleaching is a process where the perineum, which darkens over the years, is lightened for a more youthful appearance.

Anal piercing is among the more extreme piercings and usually interferes with the function of the anus.

Pathology

Diseases of the anus include anal cancer, abscess, warts, fistula, anal fissure, itching and hemorrhoid. The anus is also a frequent site of sexually transmitted infections. These benefit from medical intervention.

Birth defects of the anus include stenosis and imperforation. These benefit from surgical intervention.

Damaged anal sphincter (patulous anus in more severe cases) — caused by careless or sometimes necessarily sacrificial surgery in the perineal region or by rough/abrupt penetration in anal sex — can lead to flatus and/or fecal incontinence, chronic constipation and, ultimately, megacolon.

In psychology the Freudian term anal fixation is used.