Function

Urination 

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. 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.

Sexual activity

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. Although many erect penises point upwards, 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 (Pic.1).

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. It is rarely 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 phase of the ejaculatory reflex which is under control of the sympathetic nervous system.
  • Ejaculatory phase which is under control of a spinal reflex at the level of the spinal nerves S2 to 4 via the pudendal nerve.

A refractory period succeeds the ejaculation, and sexual stimulation precedes it. 

Development

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 (Pic. 2).

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.

Anatomical structure

The penis is the male organ of copulation (sexual intercourse). It is flaccid for non-sexual actions, such as urination, and turgid and rod-like with sexual arousal. When erect, the stiffness of the organ allows it to penetrate into the vagina and deposit semen into the female reproductive tract. 

The main parts are:

  • Root of the penis (radix) - the attached part, consisting of the bulb of penis in the middle and the crus of penis, one on either side of the bulb. It lies within the superficial perineal pouch.
  • Body of the penis (corpus) - having two surfaces: dorsal (posterosuperior in the erect penis), and ventral or urethral (facing downwards and backwards in the flaccid penis). The ventral surface is marked by a groove in a lateral direction.

Histological structure

Epithelium of the penis consists of the shaft skin, the foreskin, and the preputial mucosa on the inside of the foreskin and covering the glans penis. The epithelium is not attached to the underlying shaft so it is free to glide to and fro (Pic. 3).

The human penis is made up of three columns of tissue (Pic. 4):

  • two corpora cavernosa - lie next to each other on the dorsal side
  • 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.

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Sources

Penis ―sourced from Lumen licensed under CC BY 4.0
Human penis ―sourced from Wikipedia licensed under CC BY-SA 3.0
Erection Development V2 ―by Bebop7 licensed under CC0 1.0
External genitalia ―by Bebop7 licensed under CC0 1.0
Penis with Labels ―by Coastone licensed under CC BY-SA 3.0
Penis lateral cross section ―by CFCF licensed under CC BY-SA 3.0
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