human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s, the anus (: anuses or ani; from

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

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

rectum The rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...

located inside the

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

. Two

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

s control the exit of

feces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) 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 ...

from the body during an act of

defecation Defecation (or defaecation) follows digestion and is the necessary biological process by which organisms eliminate a solid, semisolid, or liquid metabolic waste, waste material known as feces (or faeces) from the digestive tract via the anus o ...

, which is the primary function of the anus. 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. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular musc ...

and the

external anal sphincter The external anal sphincter (or sphincter ani externus) is an oval tube of skeletal muscle fibers. Distally, it is adherent to the skin surrounding the margin of the anus. It exhibits a resting state of tonical contraction and also contracts duri ...

, which are circular

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

s that normally maintain constriction of the orifice and which relax as required by normal physiological functioning. The inner sphincter is involuntary and the outer is voluntary. Above the anus is the

perineum The perineum (: perineums or perinea) in placentalia, placental mammals is the space between the anus and the genitals. The human perineum is between the anus and scrotum in the male or between the anus and vulva in the female. The perineum is ...

, which is also located beneath the

vulva In mammals, the vulva (: vulvas or vulvae) comprises mostly external, visible structures of the female sex organ, genitalia leading into the interior of the female reproductive tract. For humans, it includes the mons pubis, labia majora, lab ...

scrotum In most terrestrial mammals, the scrotum (: scrotums or scrota; possibly from Latin ''scortum'', meaning "hide" or "skin") or scrotal sac is a part of the external male genitalia located at the base of the penis. It consists of a sac of skin ...

. 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

infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...

s and other conditions, including

cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

(see anal cancer). With

anal sex Anal sex or anal intercourse principally means the insertion and pelvic thrusting, thrusting of the Erection, erect human penis, penis into a person's Human anus, anus, or anus and rectum, for sexual pleasure.Sepages 270–271for anal sex inform ...

, 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 ...

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

taboo A taboo 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, offensive, sacred or allowed only for certain people.''Encyclopædia Britannica ...

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

slang A slang is a vocabulary (words, phrases, and linguistic usages) of an informal register, common in everyday conversation but avoided in formal writing and speech. It also often refers to the language exclusively used by the members of pa ...

terms. Some

sexually transmitted infection A sexually transmitted infection (STI), also referred to as a sexually transmitted disease (STD) and the older term venereal disease (VD), is an infection that is Transmission (medicine), spread by Human sexual activity, sexual activity, e ...

s including

HIV/AIDS The HIV, human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a Preventive healthcare, pr ...

and

anal warts Genital warts are a sexually transmitted infection Human papillomavirus infection, caused by certain types of human papillomavirus (HPV). They may be flat or project out from the surface of the skin, and their color may vary; brownish, white, 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 Digestion, digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascula ...

, and directly continues from the

, passing through the

pelvic floor The pelvic floor or pelvic diaphragm is an anatomical location in the human body which has an important role in urinary and anal continence, sexual function, and support of the pelvic organs. The pelvic floor includes muscles, both skeletal and ...

. 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 The pectinate line (dentate line) is a line which divides the upper two-thirds and lower third of the anal canal. Developmentally, this line represents the hindgut- proctodeum junction. It is an important anatomical landmark in humans, and fo ...

. 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 junct ...

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 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, which branch from the

pudendal nerve The pudendal nerve is the main nerve of the perineum. It is a Mixed nerve, mixed (motor and sensory) nerve and also conveys Sympathetic nervous system, sympathetic Autonomic nervous system, autonomic fibers. It carries sensation from the exter ...

Microanatomy

The pseudostratified columnar

epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

of the gastrointestinal tract transitions to stratified squamous epithelium at the pectinate line. The stratified squamous epithelium gradually accumulates

sebaceous A sebaceous gland or oil gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in t ...

and

apocrine glands Apocrine () is a term used to classify the mode of secretion of exocrine glands. In apocrine secretion, secretory cells accumulate material at their apical ends, often forming blebs or "snouts", and this material then buds off from the cells, ...

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 female, the testicles i ...

, as testosterone triggers

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

growth on the body,

pubic hair Pubic hair (or pubes , ) is terminal hair, terminal body hair that is found in the sex organ, genital area and pubic region of adolescent and adult humans. The hair is located on and around the sex organs, and sometimes at the top of the inside ...

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 rectum 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 s ...

. 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 fissures 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 nitroglycerine), also known as trinitroglycerol (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless or pale yellow, oily, explosive liquid most commonly produced by ...

creams, and

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

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 Botulinum toxin, or botulinum neurotoxin (commonly called botox), is a neurotoxic protein produced by the bacterium ''Clostridium botulinum'' and related species. It prevents the release of the neurotransmitter acetylcholine from axon en ...

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 ''h ...

s are visible blood vessels from the internal or external venous plexuses of the anus. Haemorrhoids may; cause bleeding after passing a motion; be painful; cause an itch; or 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 gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Conception (biology), Conception usually occurs ...

. 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 In anatomy, a fistula (: fistulas or fistulae ; from Latin ''fistula'', "tube, pipe") is an abnormal connection (i.e. tube) joining two hollow spaces (technically, two epithelialized surfaces), such as blood vessels, intestines, or other h ...

Birth defects A birth defect 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 range from mild to severe. Birth de ...

, including imperforation,

stenosis Stenosis () is the abnormal narrowing of 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'' as a term is usually used when narrowing ...

, 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, fever, abdominal distension, and weight loss. Complications outside of the ...

. 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 infection A sexually transmitted infection (STI), also referred to as a sexually transmitted disease (STD) and the older term venereal disease (VD), is an infection that is Transmission (medicine), spread by Human sexual activity, sexual activity, e ...

s * Anal warts, also called anal condyloma

Cancer

Anal cancer, also called "anal carcinoma", and anal intraepithelial neoplasia.

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 and thrush; or skin conditions such as

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

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 ...

. 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 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—including flatus (gas), liquid stool elements and mucus, or solid feces. FI is a sign or a symptom ...

, chronic constipation and

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

. 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 Sensory processing, sensitivity, the sexual stimulation, stimulation of wh ...

, which 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 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 inf ...

and the

dorsal nerve of the penis The dorsal nerve of the penis is the deepest of three divisions of the pudendal nerve; it accompanies the internal pudendal artery along the Ischium#Structure, ramus of the ischium; it then runs forward along the margin of the inferior ramus of t ...

. 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 an male accessory gland, accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found in all male mammals. It differs between species anatomically, chemica ...

for males, and

vagina In mammals and other animals, the vagina (: vaginas or vaginae) is the elastic, muscular sex organ, reproductive organ of the female genital tract. In humans, it extends from the vulval vestibule to the cervix (neck of the uterus). The #Vag ...

, 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, anal 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 sex, oral–genital or anilingus, oral–anal contact. The sitting partner may face in either directi ...

anilingus Anilingus (also spelled analingus) is an oral sex, oral and anal sex act (anal–oral contact or anal–oral sex) in which one person stimulates the human anus, anus of another by using their tongue or lips. The anus has a relatively high conc ...

, 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 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 device used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection (STI). There are both external condoms, also called male condoms, and internal (female) ...

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 (), also called buggery in British English, principally refers to either anal sex (but occasionally also oral sex) between people, or any Human sexual activity, sexual activity between a human and another animal (Zoophilia, bestiality). I ...

or buggery, and is considered

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

jurisdiction Jurisdiction (from Latin 'law' and 'speech' or 'declaration') is the legal term for the legal authority granted to a legal entity to enact justice. In federations like the United States, the concept of jurisdiction applies at multiple level ...

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 (, ) is a bowl or receptacle designed to be sat upon in order to wash a person's Sex organ, genitalia, perineum, inner buttocks, and human anus, anus. The modern variety has a plumbed-in water supply and a drainage opening, and is thus a ...

or a wipe with

toilet paper Toilet paper (sometimes called toilet/bath/bathroom tissue, or toilet roll) is a tissue paper product primarily used to clean the human anus, anus and surrounding region of Human feces, feces (after defecation), and to clean the external gen ...

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

Cosmetics

Shaving, trimming,

depilatory Hair removal is the deliberate removal of body hair or head hair. This process is also known as epilation or depilation. Hair is a common feature of the human body, exhibiting considerable variation in thickness and length across different pop ...

(hair removal), or Brazilian waxing can clear the

of hair. Anal bleaching is a process in which the anus and perineum is lightened. Perineum sunning is a process in which the anus is sun tanned by deliberate exposure to sunlight, resulting in a darkening of the area. A true anal piercing is rare because it may interfere with the function of the anus and cause infections. Surface piercings of the perineum are easier to care for and much more common. Some people have their anuses

tattoo A tattoo is a form of body modification made by inserting tattoo ink, dyes, or pigments, either indelible or temporary, into the dermis layer of the skin to form a design. Tattoo artists create these designs using several tattooing processes ...

ed.

Slang

The anus has many slang terms including '' asshole'', ''butthole'' (and their respective British equivalents ''arsehole'', ''bumhole''), ''cornhole'', and ''bunghole''.

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 File:Raphe1.png, Raphe perinealis of a human female. The raphe perinealis is a fibrous plate of tissue that extends from the anus to the scrotum or vulva.

References

{{Authority control Digestive system

Structure

Microanatomy

Development

Function

Defecation

Clinical significance

Infections

Cancer

Itching, incontinence and constipation

Society and culture

Sexuality

Hygiene

Cosmetics

Slang

Additional images

See also

References