View source for Human anus

{{Short description|External opening of the rectum}}
{{redirect-distinguish|Bumhole|Bunghole}}
{{About|the anuses of humans|the anuses of other animals|Anus|}}
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{{Infobox anatomy
| Name= Human anus
| Latin= anus
| Image= File:Human_anus_ver_1.jpeg
| Caption = A woman's anus, without pubic hair
| Image2= Tractus intestinalis anus.svg
| Caption2 = Scheme of [[Gastrointestinal tract|digestive tract]], with anus marked
| Width = 
| Precursor   = [[Proctodeum]]
| System      = [[Alimentary system|Alimentary]]
| Artery      = [[Inferior rectal artery]]
| Vein        = [[Inferior rectal vein]]
| Nerve       = [[Inferior rectal nerves]], [[pudendal nerve]]
| Lymph       = [[Superficial inguinal lymph nodes]]
}}
{{Gastrointestinal tract sidebar}}
In [[human]]s, the '''anus''' ({{plural form}}: '''anuses''' or '''ani'''; from [[Latin]] ''ānus'', "ring", "circle")<ref name="Albuquerque">{{Cite book |last=Martim de Albuquerque |url=https://books.google.com/books?id=FEn-C_BzRdMC&q=latin+butthole+ring&pg=PA119 |title=Notes and Queries |publisher=Oxford University Press |year=1873 |location=Original from the University of Michigan |page=119}}</ref><ref name="O'Reilly">{{Cite book |last1=O'Reilly |first1=Edward |url=https://books.google.com/books?id=dX8NAAAAQAAJ&q=latin+butthole+ring&pg=PA7 |title=An Irish-English Dictionary |last2=O'Donovan |first2=John |publisher=J. Duffy |year=1864 |location=Original from Oxford University |page=7}}</ref> is the external opening of the [[rectum]] located inside the [[intergluteal cleft]]. Two [[sphincter]]s control the exit of [[Human feces|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]], which are circular [[muscle]]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]], which is also located beneath the [[vulva]] or [[scrotum]].

In part owing to its exposure to feces, a number of medical conditions may affect the anus, such as [[hemorrhoids]].<ref>{{Cite journal |last1=Schubert |first1=MC |last2=Sridhar |first2=S |last3=Schade |first3=RR |last4=Wexner |first4=SD |date=July 2009 |title=What every gastroenterologist needs to know about common anorectal disorders |journal=World J Gastroenterol |volume=15 |issue=26 |pages=3201–09 |doi=10.3748/wjg.15.3201 |issn=1007-9327 |pmc=2710774 |pmid=19598294 |doi-access=free}}</ref> The anus is the site of potential [[infection]]s and other conditions, including [[cancer]] (see [[anal cancer]]).<ref>{{Cite web |title=Anal Cancer |url=https://www.lecturio.com/concepts/anal-cancer/ |access-date=24 July 2021 |website=The Lecturio Medical Concept Library}}</ref>

With [[anal sex]], the anus can play a role in [[Human sexuality|sexuality]]. Attitudes toward anal sex vary, and it is illegal in some countries.<ref name="Discovery.com">{{Cite web |title=Anal Sex, defined |url=http://health.discovery.com/centers/sex/sexpedia/analsex.html |archive-url=https://web.archive.org/web/20020613200634/http://health.discovery.com/centers/sex/sexpedia/analsex.html |archive-date=June 13, 2002 |access-date=July 23, 2013 |publisher=[[Discovery Channel|Discovery.com]]}}</ref> The anus is often considered a [[taboo]] part of the body,<ref name="Discovery.com" /> and is known by many, usually vulgar, [[slang]] terms. Some [[sexually transmitted infection]]s including [[HIV/AIDS]] and [[anal warts]] can be spread via anal sex.

== Structure ==
[[File:Human anus-en.svg|thumb|upright= 1.8|Anatomy of the human anus. Frontal section.]]
The anus is the final part of the [[gastrointestinal tract]], and directly continues from the [[rectum]], passing through the [[pelvic floor]]. The top and bottom of the anus are surrounded by the [[internal anal sphincter|internal]] and [[external anal sphincter]]s, two muscular rings which control defecation.<ref name="GRAYS2005">{{Cite book |last1=Drake |first1=Richard L. |title=Gray's anatomy for students |last2=Vogl |first2=Wayne |last3=Tibbitts |first3=Adam W.M. Mitchell |publisher=Elsevier/Churchill Livingstone |others=illustrations by Richard; Richardson, Paul |year=2005 |isbn=978-0-8089-2306-0 |location=Philadelphia}}</ref>{{rp|397}}
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]] in the embryo. Below this point, the mucosa of the internal anus becomes skin.<ref name="GRAYS2005" /> {{rp|397}} 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]].<ref>{{Cite book |last1=Moore |first1=Keith L. |url=https://books.google.com/books?id=-Le5bc5F0sYC |title=Clinically Oriented Anatomy |last2=Dalley |first2=Arthur F. |last3=Agur |first3=A. M. R. |date=2013-02-13 |publisher=Lippincott Williams & Wilkins |isbn=9781451119459 |language=en}}</ref>

=== 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 gland|sebaceous]] and [[Apocrine sweat gland|apocrine glands]].<ref name="WHEATERS2006">{{Cite book |last1=Young |first1=Barbara |last2=Lowe |first2=James S. |last3=Stevens |first3=Alan |last4=Heath |first4=John W. |year=2006 |title=Wheater's functional histology: a text and colour atlas |publisher=Churchill Livingstone, Elsevier |edition=5th}}</ref>{{rp|285}}

=== Development ===
During [[puberty]], as testosterone triggers [[androgenic hair]] 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 ===
{{Main|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 [[peristalsis|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 ==
[[File:Hemorrhoid.png|thumbnail|Image of the anus affected by [[hemorrhoids]]. The [[pectinate line]], which separates the internal from the external anus, can be seen.]]
[[Anal fissure]]s 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.<ref name="Davidsons2018">{{Cite book |last=Davidson |first=Stanley |title=Davidson's principles and practice of medicine |date=2018 |publisher=Elsevier |isbn=978-0-7020-7028-0 |editor-last=Ralston |editor-first=Stuart H. |edition=23rd |pages=835–6 |chapter=Anorectal disorders |author-link=Stanley Davidson |editor-last2=Penman |editor-first2=Ian D. |editor-last3=Strachan |editor-first3=Mark W. |editor-last4=Hobson |editor-first4=Richard P.}}</ref> 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.<ref name="Davidsons2018" /> The external anal sphincter can be relaxed by the application of [[glyceryl trinitrate]] creams, and [[constipation]] is managed with [[laxative]]s and improving hydration.<ref name="Davidsons2018" /> Some fissures may require [[botulinum toxin]] injection; worst cases may require surgical intervention, such as "lateral internal [[anal sphincterotomy]] or advancement anoplasty"{{clarify|date=June 2020}}.<ref name="Davidsons2018" />

[[Hemorrhoid]]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 {{wt|en|prolapse}} out of the anus.<ref name="Davidsons2018" /> Haemorrhoids are often associated with straining due to constipation, and [[pregnancy]].<ref name="Davidsons2018" /> Usually, haemorrhoids are managed with medications to [[stool softener|make motions more soft]] and prevent straining during constipation. Some haemorrhoids require surgery to manage, which may involve [[Rubber band ligation|placing a band around the haemorrhoid]], in order for it to lose blood supply; or surgical excision.<ref name="Davidsons2018" />

Other
* [[Anal fistula|Fistula]]
* [[Birth defects]], including [[Imperforate anus|imperforation]], [[stenosis]], [[Tailgut cyst]]

===Infections===
[[Anal abscess]]es usually result from infection of the normal glands of the anus, or sometimes because of [[Crohn's disease]].<ref name="Davidsons2018" /> 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<ref name="Davidsons2018" /> and antibiotics.<ref>{{Cite web |title=Anorectal Abscess |url=https://www.hopkinsmedicine.org/health/conditions-and-diseases/anorectal-abscess |url-status=live |archive-url=https://web.archive.org/web/20210810081108/https://www.hopkinsmedicine.org/health/conditions-and-diseases/anorectal-abscess |archive-date=10 August 2021 |access-date=10 August 2021 |website=Johns Hopkins Medicine}}</ref><ref>{{Cite web |last1=Ansorge |first1=R |last2=Robinson |first2=J |date=15 September 2019 |title=Anal Abscess |url=https://www.webmd.com/a-to-z-guides/anal-abscess |access-date=10 August 2021 |website=WebMD}}</ref>

Additional
* [[Sexually transmitted infection]]s
* [[Anal wart]]s, also called anal condyloma

===Cancer===
{{expand section|date=June 2020}}
[[Anal cancer]], also called "anal carcinoma", and [[anal intraepithelial neoplasia]].<ref>{{Cite web |title=Anal Cancer |url=https://www.lecturio.com/concepts/anal-cancer/ |access-date=10 August 2021 |website=The Lecturio Medical Concept Library}}</ref>

===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 [[candidiasis|thrush]]; or skin conditions such as [[psoriasis]] and [[contact dermatitis]]. 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.<ref name="Davidsons2018" />

Damage or injury to the anal sphincter ([[patulous anus]] in more severe cases) as a result of damage during surgery, such as to the [[Perineum|perineal]] region, or resulting from [[anal sex]]; can lead to [[flatus]] and/or [[fecal incontinence]], chronic constipation and [[megacolon]].<ref>{{Cite web |title=Megacolon |url=https://www.lecturio.com/concepts/megacolon/ |access-date=10 August 2021 |website=The Lecturio Medical Concept Library}}</ref>

<gallery class="center">
File:Piles Grade 4.svg|A [[hemorrhoid|grade IV hemorrhoid]] protrudes out of the anus.
</gallery>

== Society and culture ==

=== Sexuality ===
{{Main|Anal sex}}
[[File:Anogenital_distance_female_and_male.png|thumb|right|The [[anogenital distance]] is a measurement of the distance from the anus to the genitals. In women, this distance tends to be much smaller than in men.]]

The anus has a relatively high concentration of [[Nerve|nerve endings]] and can be an [[erogenous zone]], 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]] and the [[dorsal nerve of the penis]].<ref name="Answer">{{Cite book |last1=Komisaruk |first1=Barry R. |url=https://books.google.com/books?id=Kkts3AX9QVAC&pg=PA108 |title=The Orgasm Answer Guide |last2=Whipple |first2=Beverly |last3=Nasserzadeh |first3=Sara |last4=Beyer-Flores |first4=Carlos |publisher=JHU Press |year=2009 |isbn=978-0-8018-9396-4 |pages=108–109 |author-link=Barry Komisaruk |author-link2=Beverly Whipple |author-link3=Sara Nasserzadeh |access-date=January 20, 2014}}</ref>

In addition to nerve endings, pleasure from anal intercourse may be aided by the close proximity between the anus and the [[prostate]] for males, and [[vagina]], [[Clitoris|clitoral]] [[Crus of clitoris|legs]] and anal area for females. This is because of indirect stimulation of the prostate and vagina or clitoral legs.<ref name="Answer" /><ref name="Rosenthal">{{Cite book |last=Rosenthal |first=Martha |url=https://books.google.com/books?id=d58z5hgQ2gsC&pg=PT153 |title=Human Sexuality: From Cells to Society |publisher=[[Cengage Learning]] |year=2012 |isbn=978-0-618-75571-4 |pages=133–135 |access-date=September 17, 2012}}</ref><ref name="Valdez">{{Cite book |last=Janina |first=Valdez Natasha |url=https://books.google.com/books?id=5PsNKTB87isC&pg=PT79 |title=Vitamin O: Why Orgasms Are Vital to a Woman's Health and Happiness, and How to Have Them Every Time! |publisher=[[Skyhorse Publishing]] Inc. |year=2011 |isbn=978-1-61608-311-3 |page=79 |access-date=November 6, 2011}}</ref> For a male insertive partner, the tightness of the anus can be a source of pleasure via the tactile pressure on the penis.<ref name="WebMD">{{Cite web |last=Dunkin |first=Mary Anne |title=Anal Sex Safety: What to Know |url=http://www.webmd.com/sex/anal-sex-health-concerns |access-date=August 19, 2013 |publisher=[[WebMD]]}}</ref><ref name="Understanding">{{Cite book |last1=DeLora |first1=Joann S. |url=https://books.google.com/books?id=lP4cAAAAYAAJ |title=Understanding Sexual Interaction |last2=Warren |first2=Carol A. B. |last3=Ellison |first3=Carol Rinkleib |publisher=[[Houghton Mifflin]] (Original from the [[University of Virginia]]) |year=2008 |isbn=978-0-395-29724-7 |page=123 |quote=Many men find anal intercourse more exciting than penile-vaginal intercourse because the anal opening is usually smaller and tighter than the vagina. Probably the forbidden aspect of anal intercourse also makes it more exciting for some people. |access-date=November 6, 2011 |orig-year=1981}}</ref> Pleasure from the anus can also be achieved through [[anal masturbation]], [[Fingering (sexual act)#Anus|anal fingering]],<ref name="Discovery.com" /> [[facesitting]], [[anilingus]], and other penetrative and [[Non-penetrative sex|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.<ref name=" Miletich">{{Cite book |last1=Miletich |first1=John J. |url=https://books.google.com/books?id=_l_9peGFRz0C&pg=PA29 |title=An Introduction to the Work of a Medical Examiner: From Death Scene to Autopsy Suite |last2=Tia Laura Lindstrom |publisher=[[ABC-CLIO]] |year=2010 |isbn=978-0275995089 |page=29 |access-date=September 15, 2014}}</ref> Lubricant and [[condoms]] are widely regarded as a necessity while performing anal sex, as well as a slow and cautious penetration.<ref name="Carroll">{{Cite book |last=Carroll |first=Janell L. |url=https://archive.org/details/sexualitynowembr0000carr_x0m8 |title=Sexuality Now: Embracing Diversity |publisher=Cengage Learning |year=2009 |isbn=978-0-495-60274-3 |page=271 |access-date=2010-12-19 |url-access=registration}}</ref>

Anal intercourse is sometimes referred to as [[sodomy]] or buggery, and is considered [[taboo]] in a number of legal systems. It has been, and in some [[jurisdiction]]s continues to be, a crime carrying severe punishment.<ref name="Discovery.com" />

=== Hygiene ===
{{main|Anal hygiene}}
To prevent diseases of the anus{{Citation needed|date=June 2007}} and 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]] is often used for this purpose, though anal cleansing practices vary greatly between cultures.

=== Cosmetics ===
Shaving, trimming, [[Hair removal|depilatory]] (hair removal), or [[Brazilian waxing]] can clear the [[perineum]] 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.<ref name="u014">{{cite journal | last1=Ottwell | first1=Ryan | last2=Hartwell | first2=Micah | last3=Beswick | first3=Tracy | last4=Rogers | first4=Taylor Calli | last5=Ivy | first5=Heather | last6=Goodman | first6=Marcus | last7=Vassar | first7=Matt | title=Public Interest in a Potentially Harmful, Non–Evidence-Based "Wellness" Practice: Cross-Sectional Analysis of Perineum Sunning | journal=JMIR Dermatology | publisher=JMIR Publications Inc. | volume=4 | issue=1 | date=2021-01-26 | issn=2562-0959 | doi=10.2196/24124 | doi-access=free | page=e24124| pmid=37632796 | pmc=10501511 }}</ref>

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]]ed.

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

== Additional images ==
<gallery class="center">
 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.
</gallery>

== See also ==
{{Wiktionary|anus}}
{{Commons category|Human anus}}
* [[Anal bleaching]]
* [[Anal stage]] (Freudian psychosexual stage)
* [[Anococcygeal nerve]]
* [[Anorectal disorder]]
* [[Buttocks]]
* [[Cloaca]]
* [[Coccydynia]]
* [[Coccyx]]
* [[Flatulence]]

== References ==
{{Reflist}}

{{Human systems and organs}}
{{digestive system}}
{{sex}}
{{Human regional anatomy}}
{{Authority control}}

[[Category:Digestive system]]
[[Category:Anus| ]]