Anus
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. The anus also plays an important role in human and animal sexuality.
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 until time to defecate or time for sexual activity. 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.
Role in defecation
main article: defecation
When the rectum is full the increase in intrarectal pressure forces the walls of the anal canal apart allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincters of the anus allow the feces to be passed by muscles pulling the anus up over the exiting feces.
To prevent diseases of the anus and to promote general hygiene, humans often follow the action of the sphincter with a cleansing activity after emptying the bowels. A wash with a bidet or a wiping with toilet paper are often used to clear the anus of fecal matter.
Role in sexuality
main article: anal sex
As the anus has a relatively high concentration of nerve endings, it is an erogenous zone, and plays a role in sexuality. Sigmund Freud's theory of psychosexual development described an anal stage, hypothesizing that toddlers derive pleasure from retaining and expelling feces. This is the source of the derogatory term anal-retentive.
Anal intercourse can be pleasurable for both the insertive partner and the receptive partner. The anus contains many of the same kinds of nerves as the penis or clitoris. For females, pleasure derived from anal intercourse is also thought to be related to the shared wall between the rectum and the vagina as well as the G-spot. For males, the tightness of the anus is often said to be a source of pleasure in penetrative anal sex, while the presence of the prostate gland near the rectal wall is generally seen as a source of pleasure during receptive anal intercourse. Animals have also been observed practicing anal sex.
Anal intercourse is generally considered taboo in most societies, and it has at times been a crime carrying severe punishment.
Anal sex need not include penetration. The anus also plays an important role in face sitting, coprophilia and anilingus.
Anal stretching can stimulate the nerves around the anus and can be considered pleasurable. Care must be taken to maintain elasticity.
Maintenance
Hygiene is important for good anal health. Washing daily 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.
Penetration with a penis or sex toy can irritate or tear the inside of the anus. Lubrication is often recommended to ease penetration.
Kegel exercises can improve the tone of the outer sphincter muscle.
Cosmetics
Shaving, trimming, depilatories or Brazilian waxing can clear the perineum of hair.
Anal bleaching is a relatively new West coast phenomenon where the perineum, which darkens over the years, is lightened for a more youthful appearance. Actress Lara Flynn Boyle has expressed enthusiasm over this process.
Anal piercing is among the more extreme piercings and usually interferes with the function of the anus.
Pathology
Anal cancer, abcess, warts, fistula, fissure, itching and hemorrhoids are among the diseases of the anus that benefit from medical intervention. Birth defects of the anus include stenosis and imperforation. The anus is also a frequent site of sexually transmitted infections.
See also
