Human penis

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Human penis
Penis with Labels.jpg
A flaccid penis with labels showing the locations of the shaft, foreskin, glans and meatus. The model has removed body hair.
Latin'penis, penes'
Gray'ssubject #262 1247
ArteryDorsal artery of the penis, deep artery of the penis, artery of the urethral bulb
VeinDorsal veins of the penis
NerveDorsal nerve of the penis
LymphSuperficial inguinal lymph nodes
PrecursorGenital tubercle, Urogenital folds
MeSHPenis
 
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Human penis
Penis with Labels.jpg
A flaccid penis with labels showing the locations of the shaft, foreskin, glans and meatus. The model has removed body hair.
Latin'penis, penes'
Gray'ssubject #262 1247
ArteryDorsal artery of the penis, deep artery of the penis, artery of the urethral bulb
VeinDorsal veins of the penis
NerveDorsal nerve of the penis
LymphSuperficial inguinal lymph nodes
PrecursorGenital tubercle, Urogenital folds
MeSHPenis

The human penis is an external sexual organ of male humans. It is a reproductive, intromittent organ that additionally serves as the urinal duct.

Contents

Human anatomy

Parts

Structure

The human penis is made up of three columns of tissue: two corpora cavernosa lie next to each other on the dorsal side and one corpus spongiosum lies between them on the ventral side.

The enlarged and bulbous-shaped end of the corpus spongiosum forms the glans penis, which supports the foreskin, or prepuce, a loose fold of skin that in adults can retract to expose the glans. The area on the underside of the penis, where the foreskin is attached, is called the frenum, or frenulum. The rounded base of the glans is called the corona. The perineal raphe is the noticeable line along the underside of the penis.

Anatomical diagram of a human penis

The urethra, which is the last part of the urinary tract, traverses the corpus spongiosum, and its opening, known as the meatus (play /mˈtəs/), lies on the tip of the glans penis. It is a passage both for urine and for the ejaculation of semen. Sperm are produced in the testes and stored in the attached epididymis. During ejaculation, sperm are propelled up the vas deferens, two ducts that pass over and behind the bladder. Fluids are added by the seminal vesicles and the vas deferens turns into the ejaculatory ducts, which join the urethra inside the prostate gland. The prostate as well as the bulbourethral glands add further secretions, and the semen is expelled through the penis.

The raphe is the visible ridge between the lateral halves of the penis, found on the ventral or underside of the penis, running from the meatus (opening of the urethra) across the scrotum to the perineum (area between scrotum and anus).

The human penis differs from those of most other mammals, as it has no baculum, or erectile bone, and instead relies entirely on engorgement with blood to reach its erect state. It cannot be withdrawn into the groin, and it is larger than average in the animal kingdom in proportion to body mass.

Physiological functions of the penis

Genital homology between sexes

In short, this is a known list of sex organs that evolve from the same tissue in females and males.

The glans of the penis is homologous to the clitoral glans; the corpora cavernosa are homologous to the body of the clitoris; the corpus spongiosum is homologous to the vestibular bulbs beneath the labia minora; the scrotum, homologous to the labia minora and labia majora; and the foreskin, homologous to the clitoral hood. The raphe does not exist in females, because there, the two halves are not connected.

Penile growth and puberty

On entering puberty, the penis, scrotum and testicles will begin to develop. During the process, pubic hair grows above and around the penis. A large-scale study assessing penis size in thousands of 17–19 year old males found no difference in average penis size between 17 year olds and 19 year olds. From this, it can be concluded that penile growth is typically complete not later than age 17, and possibly earlier.[1]

Urination

The penis allows males, contrary to females, to urinate while standing.Martin van Maële, 1905

In males, the expulsion of urine from the body is done through the male penis. The urethra drains the bladder through the prostate gland where it is joined by the ejaculatory duct, and then onward to the penis. At the root of the penis (the proximal end of the corpus spongiosum) lies the external sphincter muscle. This is a small sphincter of striated muscle tissue and is in healthy males under voluntary control. Relaxing the urethra sphincter allows the urine in the upper urethra to enter the penis proper and thus empty the urinary bladder.

Physiologically, urination involves coordination between the central, autonomic, and somatic nervous systems. In infants, some elderly individuals, and those with neurological injury, urination may occur as an involuntary reflex. Brain centers that regulate urination include the pontine micturition center, periaqueductal gray, and the cerebral cortex.[2] During erection, these centers block the relaxation of the sphincter muscles, so as to act as a physiological separation of the excretory and reproductive function of the penis, and stopping sperm from entering the upper portion of the urethra during ejaculation.[3]

The part of the urethra in the penis has no muscles, and this serves no physiological function beyond that of a duct. Small amounts of urine usually remaining in the distal portion of the urethra, seeping out after the voluntary expulsion of urine is over. The distal section of the urethra does however allow a human male to direct the stream of urine by holding the penis. In cultures where more than a minimum of clothing is worn, the penis allows the male to urinate while standing without removing much of the clothing, a fact highly appreciated in these circumstances.[4] Females usually sit or squat to urinate and often have to remove some garments in the process.

Erection

The development of a penile erection, also showing the foreskin gradually retracting over the glans.
See also: Commons image gallery
A ventral view of a penis flaccid (left) and erect (middle); a dorsal view of a penis erect (right).

An erection is the stiffening and rising of the penis, which occurs during sexual arousal, though it can also happen in non-sexual situations. The primary physiological mechanism that brings about erection is the autonomic dilation of arteries supplying blood to the penis, which allows more blood to fill the three spongy erectile tissue chambers in the penis, causing it to lengthen and stiffen. The now-engorged erectile tissue presses against and constricts the veins that carry blood away from the penis. More blood enters than leaves the penis until an equilibrium is reached where an equal volume of blood flows into the dilated arteries and out of the constricted veins; a constant erectile size is achieved at this equilibrium.

Erection facilitates sexual intercourse though it is not essential for various other sexual activities.

Erection angle

Although many erect penises point upwards (see illustration), it is common and normal for the erect penis to point nearly vertically upwards or nearly vertically downwards or even horizontally straight forward, all depending on the tension of the suspensory ligament that holds it in position.

The following table shows how common various erection angles are for a standing male, out of a sample of 1,564 males aged 20 through 69. In the table, zero degrees is pointing straight up against the abdomen, 90 degrees is horizontal and pointing straight forward, while 180 degrees would be pointing straight down to the feet. An upward pointing angle is most common.[5]

Occurrence of erection angles
angle (º)
from vertically upwards
Percent
of males
0–305
30–6030
60–8531
85–9510
95–12020
120–1805

Ejaculation

Ejaculation is the ejecting of semen from the penis, and is usually accompanied by orgasm. A series of muscular contractions delivers semen, containing male gametes known as sperm cells or spermatozoa, from the penis. It is usually the result of sexual stimulation, which may include prostate stimulation. Rarely, it is due to prostatic disease. Ejaculation may occur spontaneously during sleep (known as a nocturnal emission or wet dream). Anejaculation is the condition of being unable to ejaculate.

Ejaculation has two phases: emission and ejaculation proper. The emission phase of the ejaculatory reflex is under control of the sympathetic nervous system, while the ejaculatory phase is under control of a spinal reflex at the level of the spinal nerves S2–4 via the pudendal nerve. A refractory period succeeds the ejaculation, and sexual stimulation precedes it.

Normal variations

Disorders

Developmental disorders

Alleged and observed psychological disorders

Altering the genitalia

The penis is sometimes pierced or decorated by other body art. Other than circumcision, genital alterations are almost universally elective and usually for the purpose of aesthetics or increased sensitivity. Piercings of the penis include the Prince Albert, the apadravya, the ampallang, the dydoe, and the frenum piercing. Foreskin restoration or stretching is a further form of body modification, as well as implants under the shaft of the penis.

Male to female transsexuals who undergo sex reassignment surgery, have their penis surgically modified into a neovagina. Female to male transsexuals may have a phalloplasty.

Other practices that alter the penis are also performed, although they are rare in Western societies without a diagnosed medical condition. Apart from a penectomy, perhaps the most radical of these is subincision, in which the urethra is split along the underside of the penis. Subincision originated among Australian Aborigines, although it is now done by some in the U.S. and Europe.

Penis removal is another form of alteration done to the penis.

Circumcision

A labelled dorsal view of a circumcised penis: (1)Shaft, (2)Circumcision scar, (3)Corona, (4)Glans, (5)Meatus.

The most common form of genital alteration is circumcision: removal of part or all of the foreskin for various cultural, religious, and more rarely medical reasons. For infant circumcision, modern devices such as the Gomco clamp, Plastibell, and Mogen clamp are available.[11]

With all modern devices the same basic procedure is followed. First, the amount of foreskin to be removed is estimated. The foreskin is then opened via the preputial orifice to reveal the glans underneath and ensured that it is normal. The inner lining of the foreskin (preputial epithelium) is then separated from its attachment to the glans. The device is then placed (this sometimes requires a dorsal slit) and remains there until blood flow has stopped. Finally, part, or all, of the foreskin is then removed.

Adult circumcisions are often performed without clamps and require 4 to 6 weeks of abstinence from masturbation or intercourse after the operation to allow the wound to heal.[12] In some African countries, male circumcision is often performed by non-medical personnel under unsterile conditions.[13] After hospital circumcision, the foreskin may be used in biomedical research,[14] consumer skin-care products,[15] skin grafts,[16][17][18] or β-interferon-based drugs.[19] In parts of Africa, the foreskin may be dipped in brandy and eaten by the patient, eaten by the circumciser, or fed to animals.[20] According to Jewish law, after a Brit milah, the foreskin should be buried.[21]

There is controversy surrounding circumcision. Advocates of circumcision argue, for example, that it provides important health advantages that outweigh the risks, has no substantial effects on sexual function, has a low complication rate when carried out by an experienced physician, and is best performed during the neonatal period.[22] Opponents of circumcision argue, for example, that the practice has been and is still defended through the use of various myths; that it interferes with normal sexual function; that it is extremely painful; and that when performed on infants and children, it violates the individual's human rights.[23]

The American Medical Association stated in 1999: "Virtually all current policy statements from specialty societies and medical organizations do not recommend routine neonatal circumcision, and support the provision of accurate and unbiased information to parents to inform their choice."[24]

The World Health Organization (WHO; 2007), the Joint United Nations Programme on HIV/AIDS (UNAIDS; 2007), and the Centers for Disease Control and Prevention (CDC; 2008) state that evidence indicates male circumcision significantly reduces the risk of HIV acquisition by men during penile-vaginal sex, but also state that circumcision only provides partial protection and should not replace other interventions to prevent transmission of HIV.[25][26] In addition, some doctors have expressed concern over the policy and the data that supports it.[27][28]

Surgical replacement

The first successful penis allotransplant surgery was done in September 2005 in a military hospital in Guangzhou, China.[29] A man at 44 sustained an injury after an accident and his penis was severed; urination became difficult as his urethra was partly blocked. A recently brain-dead man, aged 23, was selected for the transplant. Despite atrophy of blood vessels and nerves, the arteries, veins, nerves and the corpora spongiosa were successfully matched. But, on 19 September (after two weeks), the surgery was reversed because of a severe psychological problem (rejection) by the recipient and his wife.[30]

In 2009, researchers Chen, Eberli, Yoo and Atala have produced bioengineered penises and implanted them on rabbits.[31] The animals were able to obtain erection and copulate, with 10 of 12 rabbits achieving ejaculation. This study shows that in the future it could be possible to produce artificial penises for replacement surgeries or phalloplasties.

Size

While results vary across studies, the consensus is that the average erect human penis is approximately 12.9–15 cm (5.1–5.9 in) in length with 95% of adult males falling within the interval (10.7 cm, 19.1 cm) or (4.23 in, 7.53 in). Neither patient age nor size of the flaccid penis accurately predicted erectile length. Stretched length most closely correlated with erect length.[32][33][34] The average penis size is slightly larger than the median size (i.e., most penises are below average in size).

Length of the flaccid penis does not necessarily correspond to length of the erect penis; some smaller flaccid penises grow much longer, while some larger flaccid penises grow comparatively less.[35] Among all apes, the human penis is the largest, both in length and girth.[36]

A research project, summarizing dozens of published studies conducted by physicians of different nationalities, shows that, worldwide, erect-penis size averages vary between 9.6 and 16 cm (3.8 and 6.3 in). It has been suggested that this difference is caused not only by genetics but also by environmental factors such as fertility medications,[37] culture, diet, and chemical/pollution exposure.[38][39][40] Endocrine disruption resulting from chemical exposure has been linked to genital deformation in both sexes (among many other problems).

The longest officially documented human penis was found by Doctor Robert Latou Dickinson. It was 34.3 cm (13.5 in) long and 15.9 cm (6.26 in) around.[41]

Cultural aspects

References

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  29. ^ Guangzhou Daily
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