Human anus

The human anus (from Latin anus meaning "ring", "circle")[1][2] is the external opening of the rectum. Two sphincters 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, which are circular muscles 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 perineum which is located behind the vagina or scrotum.

Anus
The anus of a female (left, with prominent perineal raphe) and a male (right).
Identifiers
TAA05.7.05.013
FMA15711
Anatomical terminology

With anal sex, the anus can play a role in sexuality. Attitudes towards anal sex vary and it is illegal in some countries.[3] The anus is often considered a taboo part of the body,[3] and is known by many usually vulgar slang terms. The anus is also the site of potential infections and other conditions, including cancer. The traditional polite synonym for anus was fundament, though this euphemism is rarely heard now that medical terms are widely acceptable.

Structure
Image of the anus affected by hemorrhoids. The pectinate line, which separates the internal from the external anus, can be seen.

The anus is the final component of the gastrointestinal tract, and directly continues from the rectum. The anus passes through the pelvic floor. The anus is surrounded by muscles. The top and bottom of the anus are surrounded by the internal and external anal sphincters, two muscular rings which control defecation.[4]: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.[4] :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.[5]

Histology

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.[6] :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. However, in some genetic populations androgenic hair is less common.

Function

Defecation
Main article: 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 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

Psychology

In psychology, the Freudian term anal fixation is used.

Society and culture

Sexuality

The anus has a relatively high concentration of 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.[7]

Sigmund Freud's theory of psychosexual development, for example, described an anal stage, hypothesizing that toddlers derive pleasure from retaining and expelling feces. This is the source of the term "anal-retentive" and the derived informal term "anal".

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, clitoral legs and anal area for females. This is because of indirect stimulation of the prostate and vagina or clitoral legs.[7][8][9] For a male insertive partner, the tightness of the anus can be a source of pleasure via the tactile pressure on the penis.[10][11] Pleasure from the anus can also be achieved through anal masturbation, fingering,[3] facesitting, 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.[12] Lubricant and condoms are widely regarded as a necessity while performing anal sex as well as a slow and cautious penetration.[13]

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 jurisdictions continues to be, a crime carrying severe punishment.[3]

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 or a wipe with toilet paper 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 perineum 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 is rare because it may interfere with the function of the anus. However, surface piercings of the perineum are easier to care for and much more common.

Additional images
  • Muscles of the male perineum
  • Muscles of the female perineum
  • The posterior aspect of the rectum and anus exposed by removing the lower part of the sacrum and the coccyx

See also

References
  1. Martim de Albuquerque (1873). Notes and Queries. Original from the University of Michigan: Oxford University Press. p. 119.
  2. Edward O'Reilly; John O'Donovan (1864). An Irish-English Dictionary. Original from Oxford University: J. Duffy. p. 7.
  3. "Anal Sex, defined". Discovery.com. Archived from the original on June 13, 2002. Retrieved July 23, 2013.
  4. Drake, Richard L.; Vogl, Wayne; Tibbitts, Adam W.M. Mitchell (2005). Gray's anatomy for students. illustrations by Richard; Richardson, Paul. Philadelphia: Elsevier/Churchill Livingstone. ISBN 978-0-8089-2306-0.
  5. Moore, Keith L.; Dalley, Arthur F.; Agur, A. M. R. (2013-02-13). Clinically Oriented Anatomy. Lippincott Williams & Wilkins. ISBN 9781451119459.
  6. Deakin, Barbara Young ... [et al.]; (2006). Wheater's functional histology : a text and colour atlas. drawings by Philip J. (5th ed.). [Edinburgh?]: Churchill Livingstone/Elsevier. ISBN 978-0-443-06850-8.CS1 maint: extra punctuation (link)
  7. Komisaruk, Barry R.; Whipple, Beverly; Nasserzadeh, Sara; Beyer-Flores, Carlos (2009). The Orgasm Answer Guide. JHU Press. pp. 108–109. ISBN 978-0-8018-9396-4. Retrieved January 20, 2014.
  8. Rosenthal, Martha (2012). Human Sexuality: From Cells to Society. Cengage Learning. pp. 133–135. ISBN 978-0-618-75571-4. Retrieved September 17, 2012.
  9. Natasha Janina, Valdez (2011). Vitamin O: Why Orgasms Are Vital to a Woman's Health and Happiness, and How to Have Them Every Time!. Skyhorse Publishing Inc. p. 79. ISBN 978-1-61608-311-3. Retrieved November 6, 2011.
  10. "Anal Sex Safety and Health Concerns". WebMD. Retrieved August 19, 2013.
  11. Joann S. DeLora; Carol A. B. Warren; Carol Rinkleib Ellison (2008) [1981]. Understanding Sexual Interaction. Houghton Mifflin (Original from the University of Virginia). p. 123. ISBN 978-0-395-29724-7. Retrieved November 6, 2011. 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.
  12. John J. Miletich; Tia Laura Lindstrom (2010). An Introduction to the Work of a Medical Examiner: From Death Scene to Autopsy Suite. ABC-CLIO. p. 29. ISBN 0275995089. Retrieved September 15, 2014.
  13. Carroll, Janell L. (2009). Sexuality Now: Embracing Diversity. Cengage Learning. p. 271. ISBN 978-0-495-60274-3. Retrieved 2010-12-19.
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