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 of the ...

'' anus'' 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 Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...

, located inside the

intergluteal cleft The intergluteal cleft or just gluteal cleft, also known by a number of synonyms, including natal cleft, butt crack, and cluneal cleft, is the groove between the buttocks that runs from just below the sacrum to the perineum, so named because it f ...

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, a ...

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), includi ...

. 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 h ...

s 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 The external anal sphincter (or sphincter ani externus ) is a flat plane of skeletal muscle fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus. Anatomy The external anal sphincter measures about 8 ...

, which are circular

muscle Skeletal muscles (commonly referred to as muscles) are 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 types of muscl ...

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 hymen ...

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. 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 di ...

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 b ...

(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, 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 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 Britannica ...

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

slang Slang is vocabulary (words, phrases, and 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 particular in-gro ...

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 oral se ...

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 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 organ (biology), organs of the digestive syste ...

, 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 lev ...

. 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 or "internal skills", a type of exercise in meditation associated with Daoism *''Internal (album)'' by Safia, 2016 ...

and

s, 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 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 t ...

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 The inferior rectal artery (inferior hemorrhoidal artery) is an artery that supplies blood to the lower third of the anal canal below the pectinate line. Structure The inferior rectal artery arises from the internal pudendal artery as it passes ab ...

and innervation from the

inferior rectal nerves The Inferior rectal nerves (inferior anal nerves, inferior hemorrhoidal nerve) usually branch from the pudendal nerve but occasionally arises directly from the sacral plexus; they cross the ischiorectal fossa along with the inferior rectal artery a ...

, 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 fem ...

Microanatomy

The pseudostratified columnar

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

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 boy. ...

, 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 andr ...

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 seg ...

. 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 The levator ani is a broad, thin muscle group, situated on either side of the pelvis. It is formed from three muscle components: the pubococcygeus, the iliococcygeus, and the puborectalis. It is attached to the inner surface of each side of the ...

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 ...

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, lubri ...

s and improving hydration. Some fissures may require botulinum toxin 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 ''he ...

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 ca ...

. 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 can ...

, including imperforation, stenosis, Tailgut cyst

Infections

Anal abscess Anorectal abscess (also known as an anal/rectal abscess, or perianal/perirectal abscess) is an abscess adjacent to the anus. Most cases of perianal abscesses are sporadic, though there are certain situations which elevate the risk for developing ...

es usually result from infection of the normal glands of the anus, or sometimes because of Crohn's disease. 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. 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

Itching, incontinence and constipation

Itchiness called

Pruritus ani Pruritus ani is the irritation of the skin at the exit of the rectum, known as the anus, causing the desire to scratch. The intensity of anal itching increases from moisture, pressure, and rubbing caused by clothing and sitting. At worst, anal ...

, 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 Cestodar ...

and thrush, 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 complete ...

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 are ...

. 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 anal sex; 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 environm ...

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 movement ...

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 feces ...

. 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 A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

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, s ...

, 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 The dorsal nerve of the clitoris is a nerve in females that branches off the pudendal nerve to innervate the clitoris. The nerve is important for female sexual pleasure, and it may play a role in clitoral erections. It travels from below the infe ...

and the

dorsal nerve of the penis The dorsal nerve of the penis is the deepest division of the pudendal nerve; it accompanies the internal pudendal artery along the ramus of the ischium; it then runs forward along the margin of the inferior ramus of the pubis, between the superio ...

. 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 Male accessory gland, 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, ...

for males, and

clitoral The clitoris ( or ) is a female sex organ present in mammals, ostriches and a limited number of other animals. In humans, the visible portion – the glans – is at the front junction of the labia minora (inner lips), above the open ...

legs A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element ...

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 Anal masturbation is the erotic self-stimulation of the anus and rectum. For humans, common methods of anal masturbation include manual stimulation of the anal opening, and the insertion of an object or objects. Items inserted may be a body p ...

, 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 a ...

, anilingus, and other penetrative and non-penetrative acts. Anal 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. Lubricant and

condoms A condom is a sheath-shaped barrier device used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection (STI). There are both male and female condoms. With proper use—and use at every act of inte ...

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 ''sodo ...

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. Jur ...

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 subjec ...

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 u ...

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

Cosmetics

Shaving, trimming,

depilatory Hair removal, also known as epilation or depilation, is the deliberate removal of body hair or head hair. Hair typically grows all over the human body and can vary in thickness and length across human populations. Hair can become more visible d ...

(hair removal), or

Brazilian waxing Bikini waxing is the removal of pubic hair using a special wax, which can be hot or cold, that adheres to hairs and pulls them out when the wax is removed quickly from the skin, usually with a cloth strip. While the practice is mainly associ ...

can clear the

of hair.

Anal bleaching Anal bleaching is the process of lightening the color of the skin around the anus. It is done for cosmetic purposes, to make the color of the anus more uniform with the surrounding area. Some treatments are applied in an office or salon by a cosm ...

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 in the surrounding area of the human anus, usually the perineum. Despite the name, piercing of the anal sphincter itself is rather rare, because even once the wound canal has healed, piercings through a functi ...

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