question: What is the point of your anus having so many nerve endings? Why is it so sensitive?
comment: Some believe that defecating feels good because our bodies often reward us for necessary functions, such as eating, defecating, urinating and procreating. In other words, we have nerves down there so that it feels good when we poop, so that we enjoy pooping, so that we don't hold the poop in and mess up our bodies.
passages: The abundance of nerve endings in the anal region and rectum can make anal sex pleasurable for men or women. The internal and external sphincter muscles control the opening and closing of the anus; these muscles, which are sensitive membranes made up of many nerve endings, facilitate pleasure or pain during anal sex. The "Human Sexuality: An Encyclopedia" states that "the inner third of the anal canal is less sensitive to touch than the outer two-thirds, but is more sensitive to pressure" and that "the rectum is a curved tube about eight or nine inches long and has the capacity, like the anus, to expand".
   There are several reasons that contribute to the jamming of rectal bodies inside the rectum. Many of the objects used for sexual stimulation have a conical tip in order to facilitate penetration, while the base is flat. Extraction by the user may be impossible if the base of the object passed the anus towards the rectum. In order to receive a stronger stimulation, the object may be inserted deeper than intended. In this case, the sphincter prevents, by mechanical means, the extraction of the foreign body.
  
  Section::::Causes.:By mouth.
   With anal sex, the anus can play a role in sexuality. Attitudes towards anal sex vary and it is illegal in some countries. The anus is often considered a taboo part of the body, and it is known by a large number of 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 , though this euphemism is rarely heard now that medical terms are widely acceptable.
  
  Section::::Structure.
   In one study researchers sampled subjects' urethra, scrotum and penis. Samples taken from the urethra added less than 1% to the HPV rate. Studies like this led Giuliano to recommend sampling the glans, shaft and crease between them, along with the scrotum, since sampling the urethra or anus added very little to the diagnosis. Dunne recommends the glans, shaft, their crease, and the foreskin.
   Insertion of foreign objects into the anus is not without dangers. This area is fragile, the intestinal walls do not feel pain and for objects pushed too far, surgery may be necessary for removal (even without injury). Unsafe anal masturbation methods cause harm and a potential trip to the hospital emergency room. However, anal masturbation can be carried out in greater safety by ensuring that the bowel is emptied before beginning, the anus and rectum are sufficiently lubricated and relaxed throughout, and the inserted object is not of too great a size.
  
  Section::::Safety.:Objects.
   Unlike the vagina, which is closed off by the cervix, the rectum leads to the sigmoid colon. Objects that are inserted into the rectum can therefore potentially travel up into the bowel: the flared end on a butt plug exists to prevent this. Some dildos lack a flared end, and thus it is ill-advised to use such dildos anally since they may get stuck; rectal foreign bodies may require medical extraction.
   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 the motor supply to various pelvic muscles, including the male or female external urethral sphincter and the external anal sphincter. If damaged, most commonly by childbirth, lesions may cause sensory loss or fecal incontinence. The nerve may be temporarily blocked as part of an anaesthetic procedure.
   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. 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, 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 are widely regarded as a necessity while performing anal sex as well as a slow and cautious penetration.
   Section::::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.
  
  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".
   Section::::Clinical significance.
  
  BULLET::::- Birth defects, including imperforation, stenosis, Tailgut cyst
  
  BULLET::::- Fistula and anal fissure
  
  BULLET::::- Hemorrhoids
  
  BULLET::::- Anal abscesses
  
  BULLET::::- Sexually transmitted infections
  
  BULLET::::- Anal warts, also called "anal condyloma"
  
  BULLET::::- Anal cancer, also called "anal carcinoma", and Anal intraepithelial neoplasia
  
  BULLET::::- Itchiness, called Pruritus ani
  
  BULLET::::- Trauma; damaged anal sphincter (patulous anus in more severe cases)—caused by careless or sometimes necessarily sacrificial surgery in the perineal region or by rough/abrupt penetration in anal sex—can lead to flatus and/or fecal incontinence, chronic constipation and, ultimately, megacolon.
  
  Section::::Psychology.
  
  In psychology, the Freudian term anal fixation is used.
  
  Section::::Society and culture.
   As with most forms of sexual activity, anal sex participants risk contracting sexually transmitted infections (STIs/STDs). Anal sex is considered a high-risk sexual practice because of the vulnerability of the anus and rectum. The anal and rectal tissues are delicate and do not provide lubrication like the vagina does, so they can easily tear and permit disease transmission, especially if a personal lubricant is not used. Anal sex without protection of a condom is considered the riskiest form of sexual activity, and therefore health authorities such as the World Health Organization (WHO) recommend safe sex practices for anal sex.
   Imperforate anus is associated with an increased incidence of some other specific anomalies as well, together being called the VACTERL association:
  
  BULLET::::- V – Vertebral anomalies
  
  BULLET::::- A – "Anal atresia"
  
  BULLET::::- C – Cardiovascular anomalies
  
  BULLET::::- T – Tracheoesophageal fistula
  
  BULLET::::- E – Esophageal atresia
  
  BULLET::::- R – Renal (kidney) and/or radial anomalies
  
  BULLET::::- L – Limb defects
   The development of the anus was an important stage in the evolution of multicellular animals. It appears to have happened at least twice, following different paths in protostomes and deuterostomes. This accompanied or facilitated other important evolutionary developments: the bilaterian body plan, the coelom, and metamerism, in which the body was built of repeated "modules" which could later specialize, such as the heads of most arthropods, which are composed of fused, specialized segments.
  
  Section::::Development.
   Section::::Role in continence.
   Section::::History.
   Section::::Signs and symptoms.
  
  Anal fistulae can present with the following symptoms:
  
  BULLET::::- skin maceration
  
  BULLET::::- pus, serous fluid and/or (rarely) feces discharge — can be bloody or purulent
  
  BULLET::::- pruritus ani — itching
  
  BULLET::::- depending on presence and severity of infection:
  
  BULLET::::- Thick discharge, which keeps the area wet
  
  Section::::Diagnosis.
   Since the mid-twentieth century, English speaking sex manuals like "The Joy of Sex" have acknowledged the erogenous sensitivity of the anus, and its role in sexual pleasure. Its author, Alex Comfort, recognised that anal intercourse was part of many heterosexual as well as homosexual relationships (as indeed had Freud at the start of the century). Comfort also stressed the importance (in the French tradition) of postillionage or anal fingering prior to orgasm.
  
  Some decades after Comfort, Margot Anand explored how liberating anal eroticism might contribute to peak experience in sexual activity.
  
  Section::::Developmental.
   An Anatomico-Functional Classification of internal rectal intussusception has been described, with the argument that other factors apart from the height of intussusception above the anal canal appear to be important to predict symptomology. The parameters of this classification are anatomic descent, diameter of intussuscepted bowel, associated rectal hyposensitivity and associated delayed colonic transit:
  
  BULLET::::- Type 1: Internal recto-rectal intussusception
  
  BULLET::::- Type 1W Wide lumen
  
  BULLET::::- Type 1N Narrowed lumen
  
  BULLET::::- Type 2: Internal recto-anal intussusception
  
  BULLET::::- Type 2W Wide Lumen
  
  BULLET::::- Type 2N Narrowed lumen
  
  BULLET::::- Type 2M Narrowed internal lumen with associated rectal hyposensitivity or early megarectum
   In humans the perforation of the anus and mouth happen at 8 weeks and 4 weeks respectively. When both the anus and mouth are perforated, the organism becomes topologically a torus.
  
  Section::::Evolutionary origin.
   Section::::Male to female.
  
  Section::::Male to female.:Behaviors and views.
  
  The anal sphincters are usually tighter than the pelvic muscles of the vagina, which can enhance the sexual pleasure for the inserting male during male-to-female anal intercourse because of the pressure applied to the penis. Men may also enjoy the penetrative role during anal sex because of its association with dominance, because it is made more alluring by a female partner or society in general insisting that it is forbidden, or because it presents an additional option for penetration.
   Anus
  
  The anus (from Latin "anus" meaning "ring", "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, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may include: matter which the animal cannot digest, such as bones; food material after all the nutrients have been extracted, for example cellulose or lignin; ingested matter which would be toxic if it remained in the digestive tract; and dead or excess gut bacteria and other endosymbionts.
   Parts of the vulva, especially the clitoris, are erogenous zones. While the vagina is not especially sensitive as a whole, its lower third (the area close to the entrance) has concentrations of the nerve endings that can provide pleasurable sensations during sexual activity when stimulated; this is also called the anterior wall of the vagina or the outer one-third of the vagina, and it contains the majority of the vaginal nerve endings, making it more sensitive to touch than the inner two-thirds of the vaginal barrel.
   The ureters are richly innervated by nerves that travel alongside the blood vessels, building the ureteric plexus. The primary sensation to the ureters is provided by nerves that come from T12-L2 segments of the spinal cord. Thus pain may be referred to the dermatomes of T12-L2, namely the back and sides of the abdomen, the scrotum (males) or labia majora (females) and upper part of the front of the thigh.
  
  Section::::Structure.:Microanatomy.
   The gastrointestinal tract forms an important part of the immune system. The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court. With such a large exposure (more than three times larger than the exposed surface of the skin), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems. Fundamental components of this protection are provided by the intestinal mucosal barrier which is composed of physical, biochemical, and immune elements elaborated by the intestinal mucosa. Microorganisms also are kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT)
   Section::::Water.:Southeast Asia.
  