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The human penis is an external male intromittent organ that additionally serves as the urinal duct. The main parts are the root (radix); the body (corpus); and the epithelium of the penis including the shaft skin and the foreskin (prepuce) covering the glans penis. The body of the penis is made up of three columns of tissue: two corpora cavernosa on the dorsal side and corpus spongiosum between them on the ventral side. The human male urethra passes through the prostate gland, where it is joined by the ejaculatory duct, and then through the penis. The urethra traverses the corpus spongiosum, and its opening, the meatus (/miːˈeɪtəs/), lies on the tip of the glans penis. It is a passage both for urination and ejaculation of semen (see male reproductive system.)
A flaccid penis, with surrounding pubic hair removed to show anatomical detail
Most of the penis develops from the same embryonic tissue as does the clitoris in females. The skin around the penis and the urethra come from the same embryonic tissue from which develops the labia minora in females.[1][2] An erection is the stiffening and rising of the penis, which occurs during sexual arousal. Erections can also occur in non-sexual situations; spontaneous non-sexual erections frequently occur during adolescence and during sleep. In its flaccid (unerect) state, the shaft of the penis has the feel of a dense sponge encased in very smooth eyelid-type skin. The glans of the penis, in uncircumcised males, is covered by the foreskin. In its fully erect (hard) state, the shaft of the penis is rigid, with the skin tightly stretched. The glans of the erect penis has the feel of a raw mushroom. An erect penis may be straight or curved and may point at an upward angle, a downward angle, or straight ahead. As of 2015, the best research on penis size concluded that the average erect human penis is 13.12 cm (5.17 inches) long and has a circumference of 11.66 cm (4.59 inches).[3][4] Neither age nor size of the flaccid penis accurately predicts erectile length.
The most common form of alterations of the penis are circumcision and piercings. Circumcision is the removal of part or all of the foreskin for various cultural, religious, and, more rarely, medical reasons, and there is controversy surrounding the practice.
Efforts by scientists to partially or fully regenerate the structures of the human penis are currently underway. Patients who can benefit most from this field are those who have congenital defects, cancer, injuries that have excised parts or all of their penis, and men wishing to reverse forms of involuntary genital modification.
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.[6]
The enlarged and bulbous-shaped end of the corpus spongiosum forms the glans penis with two specific types of sinusoids, which supports the foreskin, or prepuce, a loose fold of skin that in adults can retract to expose the glans.[7] 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.
The urethra, which is the last part of the urinary tract, traverses the corpus spongiosum, and its opening, known as the meatus /miːˈeɪtəs/, 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).[8]
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. A distal ligament buttresses the glans penis and plays an integral role to the penile fibroskeleton, and the structure is called "os analog," a term coined by Geng Long Hsu in the Encyclopedia of Reproduction.[9] It is a remnant of baculum evolved likely due to change in mating practice.[10]
The human penis cannot be withdrawn into the groin, and it is larger than average in the animal kingdom in proportion to body mass. The human penis is reciprocating from a cotton soft to a bony rigidity resulting from penile arterial flow varied between 2-3 to 60-80 mL/Min implies the most ideal milieu to apply Pascal's law in the entire human body; the overall structure is unique.[9]
Penile measurements vary, with studies that rely on self-measurement reporting a significantly higher average size than those which rely on measurements taken by health professionals. As of 2015, a systematic review of 15,521 men (and the best research to date on the topic, as the subjects were measured by health professionals) concluded that the average length of an erect human penis is 13.12 cm (5.17 inches) long, while the average circumference of an erect human penis is 11.66 cm (4.59 inches).[3][4]
Among all primates, the human penis is the largest in girth, but is comparable to the chimpanzee penis and the penises of certain other primates in length.[11] Penis size is affected by genetics, but also by environmental factors such as fertility medications[12] and chemical/pollution exposure.[13][14][15] The longest officially documented human penis was found by physician Robert Latou Dickinson. It was 34.3 cm (13.5 in) long and 15.9 cm (6.26 in) around.[16]
In the developing fetus, the genital tubercle develops into the glans of the penis in males and into the clitoral glans in females; they are homologous. The urogenital fold develops into the skin around the shaft of the penis and the urethra in males and into the labia minora in females.[1] 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 majora; and the foreskin, homologous to the clitoral hood.[1][19] The raphe does not exist in females, because there, the two halves are not connected.
On entering puberty, the penis, scrotum and testicles will enlarge toward maturity. During the process, pubic hair grows above and around the penis. A large-scale study assessing penis size in thousands of 17- to 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.[20]
In males the expulsion of urine from the body is done through the 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 properly 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.[21] 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 preventing urine from entering the upper portion of the urethra during ejaculation.[22]
The distal section of the urethra allows a human male to direct the stream of urine by holding the penis. This flexibility allows the male to choose the posture in which to urinate. 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. It is customary for some boys and men to urinate in seated or crouched positions. The preferred position may be influenced by cultural or religious beliefs.[23] Research on the medical superiority of either position exists, but the data are heterogenic. A meta-analysis[24] summarizing the evidence found no superior position for young, healthy males. For elderly males with LUTS, however, the sitting position when compared to the standing position is differentiated by the following:
This urodynamic profile is related to a lower risk of urologic complications, such as cystitis and bladder stones.
An erection is the stiffening and rising of the penis, which occurs during sexual arousal, though it can also happen in non-sexual situations. Spontaneous erections frequently occur during adolescence due to friction with clothing, a full bladder or large intestine, hormone fluctuations, nervousness, and undressing in a nonsexual situation. It is also normal for erections to occur during sleep and upon waking. (See nocturnal penile tumescence.) 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. The scrotum will usually tighten during erection.
Erection facilitates sexual intercourse though it is not essential for various other sexual activities.
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.[25]
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. 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.[26]
The human penis has been argued to have several evolutionary adaptations. The purpose of these adaptations is to maximise reproductive success and minimise sperm competition. Sperm competition is where the sperm of two males simultaneously resides within the reproductive tract of a female and they compete to fertilise the egg.[27] If sperm competition results in the rival male's sperm fertilising the egg, cuckoldry could occur. This is the process whereby males unwittingly invest their resources into offspring of another male and, evolutionarily speaking, should be avoided.[28]
The most researched human penis adaptations are testis and penis size, ejaculate adjustment and semen displacement.[29]
Evolution has caused sexually selected adaptations to occur in penis and testis size in order to maximise reproductive success and minimise sperm competition.[30][31]
Sperm competition has caused the human penis to evolve in length and size for sperm retention and displacement.[31] To achieve this, the penis must be of sufficient length to reach any rival sperm and to maximally fill the vagina.[31] In order to ensure that the female retains the male's sperm, the adaptations in length of the human penis have occurred so that the ejaculate is placed close to the female cervix.[32] This is achieved when complete penetration occurs and the penis pushes against the cervix.[33] These adaptations have occurred in order to release and retain sperm to the highest point of the vaginal tract. As a result, this adaptation also leaves the sperm less vulnerable to sperm displacement and semen loss. Another reason for this adaptation is that, due to the nature of the human posture, gravity creates vulnerability for semen loss. Therefore, a long penis, which places the ejaculate deep in the vaginal tract, could reduce the loss of semen.[34]
Another evolutionary theory of penis size is female mate choice and its associations with social judgements in modern-day society.[31][35] A study which illustrates female mate choice as an influence on penis size presented females with life-size, rotatable, computer generated males. These varied in height, body shape and flaccid penis size, with these aspects being examples of masculinity.[31] Female ratings of attractiveness for each male revealed that larger penises were associated with higher attractiveness ratings.[31] These relations between penis size and attractiveness have therefore led to frequently emphasized associations between masculinity and penis size in popular media.[35] This has led to a social bias existing around penis size with larger penises being preferred and having higher social status. This is reflected in the association between believed sexual prowess and penis size and the social judgement of penis size in relation to 'manhood'.[35]
Like the penis, sperm competition has caused the human testicles to evolve in size through sexual selection.[30] This means that large testicles are an example of a sexually selected adaptation. The human testicles are moderately sized when compared to other animals such as gorillas and chimpanzees, placing somewhere midway.[36] Large testicles are advantageous in sperm competition due to their ability to produce a bigger ejaculation.[37] Research has shown that a positive correlation exists between the number of sperm ejaculated and testis size.[37] Larger testes have also been shown to predict higher sperm quality, including a larger number of motile sperm and higher sperm motility.[30]
Research has also demonstrated that evolutionary adaptations of testis size are dependent on the breeding system in which the species resides.[38] Single-male breeding systems—or monogamous societies—tend to show smaller testis size than do multi-male breeding systems or extra-pair copulation (EPC) societies. Human males live largely in monogamous societies like gorillas, and therefore testis size is smaller in comparison to primates in multi-male breeding systems, such as chimpanzees. The reason for the differentiation in testis size is that in order to succeed reproductively in a multi-male breeding system, males must possess the ability to produce several fully fertilising ejaculations one after another.[30] This, however, is not the case in monogamous societies, where a reduction in fertilising ejaculations has no effect on reproductive success.[30] This is reflected in humans, as the sperm count in ejaculations is decreased if copulation occurs more than three to five times in a week.[39]
One of the primary ways in which a male's ejaculate has evolved to overcome sperm competition is through the speed at which it travels. Ejaculates can travel up to 30–60 centimetres at a time which, when combined with its placement at the highest point of the vaginal tract, acts to increase a male's chances that an egg will be fertilised by his sperm (as opposed to a potential rival male's sperm), thus maximising his paternal certainty.[34]
In addition, males can—and do—adjust their ejaculates in response to sperm competition and according to the likely cost-benefits of mating with a particular female.[40] Research has focused primarily on two fundamental ways in which males go about achieving this: adjusting ejaculate size and adjusting ejaculate quality.
The number of sperm in any given ejaculate varies from one ejaculate to another.[41] This variation is hypothesised to be a male's attempt to eliminate, if not reduce, his sperm competition. A male will alter the number of sperm he inseminates into a female according to his perceived level of sperm competition,[29] inseminating a higher number of sperm if he suspects a greater level of competition from other males.
In support of ejaculate adjustment, research has shown that a male typically increases the amount he inseminates sperm into his partner after they have been separated for a period of time.[42] This is largely due to the fact that the less time a couple is able to spend together, the chances the female will be inseminated by another male increases,[43] hence greater sperm competition. Increasing the number of sperm a male inseminates into a female acts to get rid of any rival male's sperm that may be stored within the female, as a result of her potential extra-pair copulations (EPCs) during this separation. Through increasing the amount he inseminates his partner following separation, a male increases his chances of paternal certainty. This increase in the number of sperm a male produces in response to sperm competition is not observed for masturbatory ejaculates.[29]
Males also adjust their ejaculates in response to sperm competition in terms of quality. Research has demonstrated, for example, that simply viewing a sexually explicit image of a female and two males (i.e. high sperm competition) can cause males to produce a greater amount of motile sperm than when viewing a sexually e
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