human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

s, the anus (from

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

'' meaning "ring", "circle") is the external opening of the

rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the l ...

, located inside the intergluteal cleft and separated from the

genitals A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, ...

by the

perineum The perineum in humans is the space between the anus and scrotum in the male, or between the anus and the vulva in the female. The perineum is the region of the body between the pubic symphysis (pubic arch) and the coccyx (tail bone), inclu ...

. Two

sphincter A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the hu ...

s control the exit of

feces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a rela ...

from the body during an act of

defecation Defecation (or defaecation) follows digestion, and is a necessary process by which organisms eliminate a solid, semisolid, or liquid waste material known as feces from the digestive tract via the anus. The act has a variety of names ranging ...

, which is the primary function of the

. These are the

internal anal sphincter The internal anal sphincter, IAS, (or sphincter ani internus) is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal; its inferior border is in contact with, but quite separate from, the external anal sphincter. It is ...

and the external anal sphincter, which are circular

muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

s 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

which is located behind the

vagina In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hy ...

scrotum The scrotum or scrotal sac is an anatomical male reproductive structure located at the base of the penis that consists of a suspended dual-chambered sac of skin and smooth muscle. It is present in most terrestrial male mammals. The scrotum co ...

. In part owing to its exposure to feces, a number of medical conditions may affect the anus such as

hemorrhoids Hemorrhoids (or haemorrhoids), also known as piles, are vascular structures in the anal canal. In their normal state, they are cushions that help with stool control. They become a disease when swollen or inflamed; the unqualified term '' ...

. The anus is the site of potential

infections An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...

and other conditions, including

cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal bl ...

(see

Anal cancer Anal cancer is a cancer which arises from the anus, the distal opening of the gastrointestinal tract. Symptoms may include bleeding from the anus or a lump near the anus. Other symptoms may include pain, itchiness, or discharge from the anus. A ...

). With

anal sex Anal sex or anal intercourse is generally the insertion and thrusting of the erect penis into a person's anus, or anus and rectum, for sexual pleasure.Sepages 270–271for anal sex information, anpage 118for information about the clitoris. O ...

, the anus can play a role in

sexuality Human sexuality is the way people experience and express themselves sexually. This involves biological, psychological, physical, erotic, emotional, social, or spiritual feelings and behaviors. Because it is a broad term, which has varied wi ...

. Attitudes toward anal sex vary, and it is illegal in some countries. The anus is often considered a

taboo A taboo or tabu is a social group's ban, prohibition, or avoidance of something (usually an utterance or behavior) based on the group's sense that it is excessively repulsive, sacred, or allowed only for certain persons.''Encyclopædia Britannic ...

part of the body, and is known by many usually vulgar

slang Slang is vocabulary (words, phrases, and usage (language), linguistic usages) of an informal register, common in spoken conversation but avoided in formal writing. It also sometimes refers to the language generally exclusive to the members of p ...

terms. Some

sexually transmitted infections Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal diseases, are infections that are spread by sexual activity, especially vaginal intercourse, anal sex, and ora ...

including

HIV/AIDS Human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV), a retrovirus. Following initial infection an individual ...

and

anal warts Genital warts are a sexually transmitted infection caused by certain types of human papillomavirus (HPV). They are generally pink in color and project out from the surface of the skin. Usually they cause few symptoms, but can occasionally be pa ...

can be spread via anal sex.

Structure

The anus is the final part of the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans a ...

, and directly continues from the

. The anus passes through the

pelvic floor The pelvic floor or pelvic diaphragm is composed of muscle fibers of the levator ani, the coccygeus muscle, and associated connective tissue which span the area underneath the pelvis. The pelvic diaphragm is a muscular partition formed by the l ...

. The anus is surrounded by muscles. The top and bottom of the anus are surrounded by the

internal Internal may refer to: * Internality as a concept in behavioural economics *Neijia, internal styles of Chinese martial arts *Neigong Neigong, also spelled ''nei kung'', ''neigung'', or ''nae gong'', refers to any of a set of Chinese breathing, ...

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 The hindgut (or epigaster) is the posterior (caudal) part of the alimentary canal. In mammals, it includes the distal one third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and up to the ano-rectal junc ...

and the

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...

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 The pudendal nerve is the main nerve of the perineum. It carries sensation from the external genitalia of both sexes and the skin around the anus and perineum, as well as the motor supply to various pelvic muscles, including the male or ...

Microanatomy

The pseudostratified columnar epithelium of the gastrointestinal tract transitions to stratified squamous epithelium at the pectinate line. The stratified squamous epithelium gradually accumulates sebaceous and apocrine glands.

Development

During

puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a b ...

, as testosterone triggers

androgenic hair Body hair, or androgenic hair, is the terminal hair that develops on the human body during and after puberty. It is differentiated from the head hair and less visible vellus hair, which is much finer and lighter in color. The growth of and ...

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. In some genetic populations, androgenic hair is less common.

Function

Defecation

Intra-rectal pressure builds as the

fills with feces, pushing the feces against the walls of the

anal canal The anal canal is the part that connects the rectum to the anus, located below the level of the pelvic diaphragm. It is located within the anal triangle of the perineum, between the right and left ischioanal fossa. As the final functional segm ...

. Contractions of abdominal and pelvic floor muscles can create intra-abdominal pressure which further increases intra-rectal pressure. The

(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.

Clinical significance

Anal fissure An anal fissure is a break or tear in the skin of the anal canal. Anal fissures may be noticed by bright red anal bleeding on toilet paper and undergarments, or sometimes in the toilet. If acute they are painful after defecation, but with chroni ...

s, which are tears in the external lining of the lining (mucosa) of the anus. These are exquisitely painful, with pain occurring after a motion is passed; other symptoms may include minor bleeding, discharge, or itch. Generally, fissures are due to injury to the mucosa, or because of a poor local blood supply that prevents proper healing, with spasm of the external anal sphincter contributing. The external anal sphincter can be relaxed by the application of

glyceryl trinitrate Nitroglycerin (NG), (alternative spelling of nitroglycerine) also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive liquid most commonly produced by nitrating g ...

creams, and constipation is managed with

laxative Laxatives, purgatives, or aperients are substances that loosen stools and increase bowel movements. They are used to treat and prevent constipation. Laxatives vary as to how they work and the side effects they may have. Certain stimulant, lub ...

s and improving hydration. Some fissures may require

botulinum toxin Botulinum toxin, or botulinum neurotoxin (BoNT), is a neurotoxic protein produced by the bacterium ''Clostridium botulinum'' and related species. It prevents the release of the neurotransmitter acetylcholine from axon endings at the neuromu ...

injection; worst cases may require surgical intervention such as "lateral internal anal sphincterotomy or advancement anoplasty".

Hemorrhoid Hemorrhoids (or haemorrhoids), also known as piles, are vascular structures in the anal canal. In their normal state, they are cushions that help with stool control. They become a disease when swollen or inflamed; the unqualified term '' ...

s, which are visible blood vessels from the internal or external venous plexuses of the anus. Haemorrhoids may cause bleeding after passing a motion; may be painful; may cause an itch; may out of the anus. Haemorrhoids are often associated with straining due to constipation, and

pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ...

. Usually, haemorrhoids are managed with medications to make motions more soft and prevent straining during constipation. Some haemorrhoids require surgery to manage, which may involve placing a band around the haemorrhoid, in order for it to lose blood supply; or surgical excision. Other *

Fistula A fistula (plural: fistulas or fistulae ; from Latin ''fistula'', "tube, pipe") in anatomy is an abnormal connection between two hollow spaces (technically, two epithelialized surfaces), such as blood vessels, intestines, or other hollow or ...

Birth defects A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities ca ...

, including imperforation,

stenosis A stenosis (from Ancient Greek στενός, "narrow") is an abnormal narrowing in a blood vessel or other tubular organ or structure such as foramina and canals. It is also sometimes called a stricture (as in urethral stricture). ''Stricture' ...

, Tailgut cyst

Infections

Anal abscesses usually result from infection of the normal glands of the anus, or sometimes because of

Crohn's disease Crohn's disease is a type of inflammatory bowel disease (IBD) that may affect any segment of the gastrointestinal tract. Symptoms often include abdominal pain, diarrhea (which may be bloody if inflammation is severe), fever, abdominal distension, ...

. They usually occur to the sides of the sphincters, and between the internal and external sphincters, either on the surface, or deeper. They may get bigger, enlarging in the direction of the rectum, and resulting in an abnormal connection called an

anorectal fistula Anal fistula is a chronic abnormal communication between the epithelialised surface of the anal canal and usually the perianal skin. An anal fistula can be described as a narrow tunnel with its internal opening in the anal canal and its externa ...

. They are usually managed with surgical drainage and antibiotics. Additional * Sexually transmitted infections * Anal warts, also called "anal condyloma"

Cancer

, also called "anal carcinoma", and

Anal intraepithelial neoplasia Intraepithelial neoplasia (IEN) is the development of a benign neoplasia A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm ...

Itching, incontinence and constipation

Itchiness called Pruritus ani, can affect the anus area. It is most often due to long-term exposure of the anus to faeces, with reasons including diseases of the anus such as haemorrhoids, fistulas and fissures; poor hygiene or chronic diarrhoea; local infections such as

tapeworm Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass is Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Cestod ...

and

thrush ''The Man from U.N.C.L.E.'' is an American spy fiction television series produced by Metro-Goldwyn-Mayer Television and first broadcast on NBC. The series follows secret agents, played by Robert Vaughn and David McCallum, who work for a secret ...

, skin conditions such as

psoriasis Psoriasis is a long-lasting, noncontagious autoimmune disease characterized by raised areas of abnormal skin. These areas are red, pink, or purple, dry, itchy, and scaly. Psoriasis varies in severity from small, localized patches to complet ...

and

contact dermatitis Contact dermatitis is a type of acute or chronic inflammation of the skin caused by exposure to chemical or physical agents. Symptoms of contact dermatitis can include itchy or dry skin, a red rash, bumps, blisters, or swelling. These rashes ar ...

. If there is a specific cause identified, the cause may be treated to relieve the itch. Otherwise, treatment includes keeping the area clean and dry, ceasing topical creams and ointments, and potentially bulk-forming laxatives to reduce the chance of faecal contamination. Damage or injury to the anal sphincter ( patulous anus in more severe cases) as a result of damage during surgery, such as to the perineal region, or resulting from

; can lead to

flatus Flatulence, in humans, is the expulsion of gas from the intestines via the anus, commonly referred to as farting. "Flatus" is the medical word for gas generated in the stomach or bowels. A proportion of intestinal gas may be swallowed environ ...

and/or

fecal incontinence Fecal incontinence (FI), or in some forms encopresis, is a lack of control over defecation, leading to involuntary loss of bowel contents, both liquid stool elements and mucus, or solid feces. When this loss includes flatus (gas), it is referre ...

, chronic

constipation Constipation is a bowel dysfunction that makes bowel movements infrequent or hard to pass. The stool is often hard and dry. Other symptoms may include abdominal pain, bloating, and feeling as if one has not completely passed the bowel moveme ...

and

megacolon Megacolon is an abnormal dilation of the colon (also called the large intestine). This leads to hypertrophy of the colon. The dilation is often accompanied by a paralysis of the peristaltic movements of the bowel. In more extreme cases, the f ...

. File:Piles Grade 4.svg, A Grade IV hemorrhoid protrudes out of the anus.

Society and culture

Sexuality

The anus has a relatively high concentration of nerve endings and can be an

erogenous zone An erogenous zone (from Greek , ''érōs'' "love"; and English ''-genous'' "producing", from Greek , ''-genḗs'' "born") is an area of the human body that has heightened sensitivity, the stimulation of which may generate a sexual response, suc ...

, which can make anal intercourse pleasurable if performed properly. The

that branches to supply the external anal sphincter also branches to the dorsal nerve of the clitoris and the dorsal nerve of the penis. In addition to nerve endings, pleasure from anal intercourse may be aided by the close proximity between the anus and the

prostate The prostate is both an accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found only in some mammals. It differs between species anatomically, chemically, and phys ...

for males, and

, clitoral legs and anal area for females. This is because of indirect stimulation of the prostate and vagina or clitoral legs. For a male insertive partner, the tightness of the anus can be a source of pleasure via the tactile pressure on the penis. Pleasure from the anus can also be achieved through anal masturbation, fingering,

facesitting Facesitting, also known as queening or kinging, is a sexual practice with one partner sitting over the other's face, sometimes allowing for oral–genital or oral–anal contact. The sitting partner may face in either direction. The position ...

, anilingus, and other penetrative and non-penetrative acts. Anal stretching or

fisting Fisting, handballing, fist-fucking, brachiovaginal, or brachioproctic insertion is a sexual activity that involves inserting a hand into the vagina or rectum. Once insertion is complete, the fingers are either clenched into a fist or kept straig ...

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 The sigmoid colon (or pelvic colon) is the part of the large intestine that is closest to the rectum and anus. It forms a loop that averages about in length. The loop is typically shaped like a Greek letter sigma (ς) or Latin letter S (thus ''s ...

(rectosigmoid) perforation, which might result in death. Lubricant and

condoms A condom is a sheath-shaped Barrier contraception, barrier device used during sexual intercourse to reduce the probability of pregnancy or a Sexually transmitted disease, sexually transmitted infection (STI). There are both male and female con ...

are widely regarded as a necessity while performing anal sex as well as a slow and cautious penetration. Anal intercourse is sometimes referred to as

sodomy Sodomy () or buggery (British English) is generally anal or oral sex between people, or sexual activity between a person and a non-human animal ( bestiality), but it may also mean any non-procreative sexual activity. Originally, the term ''so ...

or buggery, and is considered

in a number of legal systems. It has been, and in some

jurisdiction Jurisdiction (from Latin 'law' + 'declaration') is the legal term for the legal authority granted to a legal entity to enact justice. In federations like the United States, areas of jurisdiction apply to local, state, and federal levels. Ju ...

s continues to be, a crime carrying severe punishment.

Hygiene

To prevent diseases of the anus and to promote general hygiene, humans often clean the exterior of the anus after emptying the bowels. A rinse with water from a

bidet A bidet ( or ) is a bowl or receptacle designed to be sat on in order to wash one's genitalia, perineum, inner buttocks, and anus. The modern variety has a plumbed-in water supply and a drainage opening, and is thus a plumbing fixture subject to ...

or a wipe with

toilet paper Toilet paper (sometimes called toilet tissue or bathroom tissue) is a tissue paper product primarily used to clean the anus and surrounding anal region of feces after defecation, and to clean the perineal area and external genitalia of ...

is often used for this purpose, though anal cleansing practices vary greatly between cultures.

Cosmetics

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

of hair. Anal bleaching is a process in which the anus and perineum, which may darken after puberty depending on individual genetics, is lightened for a more youthful appearance. True

anal piercing An anal piercing is a body piercing Body piercing, which is a form of body modification, is the practice of puncturing or cutting a part of the human body, creating an opening in which jewelry may be worn, or where an implant could be inser ...

is rare because it may interfere with the function of the anus. Surface piercings of the perineum are easier to care for and much more common.

Additional images

File:1116 Muscle of the Male Perineum.png, Muscles of the male perineum File:1116 Muscle of the Female Perineum.png, Muscles of the female perineum File:Gray1077.png, The posterior aspect of the rectum and anus exposed by removing the lower part of the sacrum and the coccyx

References

{{Authority control Digestive system Anus