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Penis

The penis (plural penises, penes) is an external sexual organ of certain biologically male organisms, in both vertebrates and invertebrates.

The penis is a reproductive organ, technically an intromittent organ, and for placental mammals, additionally serves as the external organ of urination. The penis is generally found on mammals and reptiles.

Linguistics 

Etymology 

The word "penis" is taken from the Latin word for "tail." Some derive that from Indo-European *pesnis, and the Greek word πεος = "penis" from Indo-European *pesos. Prior to the adoption of the Latin word in English the penis was referred to as a "yard". The Oxford English Dictionary cites an example of the word yard used in this sense from 1379,[1] and notes that in his Physical Dictionary of 1684, Steven Blankaart defined the word penis as "the Yard, made up of two nervous Bodies, the Channel, Nut, Skin, and Fore-skin, etc."[2]

The Latin word "phallus" (from Greek φαλλος) is sometimes used to describe the penis, although "phallus" originally was used to describe images, pictorial or carved, of the penis.[3]

The adjectival form of the word penis is penile. This adjective is commonly used in describing the male copulatory organ's various accessory structures which are commonly found in many kinds of invertebrate animals.

Slang 

As with nearly any aspect of the human body that is involved in sexual or excretory functions, the word penis is considered funny from a juvenile perspective, and there are many slang words and euphemisms for the penis, such as "cock", "dick", "schlong", "johnson", "willy", or "pecker".

"Penii" is sometimes facetiously or mistakenly used as a plural form of "penis" instead of "penes" or "penises," its correct forms.

Humans

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 urethra, which is the last part of the urinary tract, traverses the corpus spongiosum, and its opening, known as the meatus (pronounced /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).

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.

Puberty 

On entering puberty, the testicles will begin to develop and the genitalia will grow. The penis begins to grow between the ages of as early as 10 or as late as age 15. Growth is usually complete by age 18-21. During the process, pubic hair grows above and around the penis. 

Sexual homology 

In short, this is a known list of sex organs that evolve from the same tissue in a human life.

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.

Erection 

An erection is the stiffening and rising (see Erection Angle) 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. 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. [4] 

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 (and into the vagina, if for reproductive intention via sexual intercourse). 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 (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 

* Pearly penile papules are raised bumps of somewhat paler color around the base of the glans and are normal.
* Fordyce's spots are small, raised, yellowish-white spots 1-2 mm in diameter that may appear on the penis.
* Sebaceous prominences are raised bumps similar to Fordyce's spots on the shaft of the penis, located at the sebaceous glands and are normal.
* Phimosis is an inability to retract the foreskin fully, is harmless in infancy and pre-pubescence, occurring in about 8% of boys at age 10. According to the British Medical Association, treatment (steroid cream, manual stretching) does not need to be considered until age 19.

* Curvature: few penises are completely straight, with curves commonly seen in all directions (up, down, left, right). Sometimes the curve is very prominent but it rarely inhibits sexual intercourse. Curvature as great as 30° is considered normal and medical treatment is rarely considered unless the angle exceeds 45°. Changes to the curvature of a penis may be caused by Peyronie's disease.

Disorders 

Paraphimosis is an inability to move the foreskin forward, over the glans. It can result from fluid trapped in a foreskin which is left retracted, perhaps following a medical procedure, or accumulation of fluid in the foreskin because of friction during vigorous sexual activity.

In Peyronie's disease, anomalous scar tissue grows in the soft tissue of the penis, causing curvature. Severe cases can benefit from surgical correction.

A thrombosis can occur during periods of frequent and prolonged sexual activity, especially fellatio. It is usually harmless and self-corrects within a few weeks.

Infection with the herpes virus can occur after sexual contact with an infected carrier; this may lead to the development of herpes sores.

Pudendal nerve entrapment is a condition characterized by pain on sitting and loss of penile (or clitoral) sensation and orgasm. Occasionally there is a total loss of sensation and orgasm. The pudendal nerve can be damaged by narrow, hard bicycle seats and accidents.

Penile fracture can occur if the erect penis is bent excessively. A popping or cracking sound and pain is normally associated with this event. Emergency medical assistance should be obtained. Prompt medical attention lowers likelihood of permanent penile curvature.

In diabetes, peripheral neuropathy can cause tingling in the penile skin and possibly reduced or completely absent sensation. The reduced sensations can lead to injuries for either partner and their absence can make it impossible to have sexual pleasure through stimulation of the penis. Since the problems are caused by permanent nerve damage, preventive treatment through good control of the diabetes is the primary treatment. Some limited recovery may be possible through improved diabetes control.

Erectile dysfunction or impotence is the inability to have and maintain an erection sufficiently firm for satisfactory sexual performance. Diabetes is a leading cause, as is natural aging. A variety of treatments exist, including drugs, such as sildenafil citrate (marketed as Viagra) which works by vasodilation.

Priapism is a painful and potentially harmful medical condition in which the erect penis does not return to its flaccid state. The causative mechanisms are poorly understood but involve complex neurological and vascular factors. Potential complications include ischaemia, thrombosis, and impotence. In serious cases the condition may result in gangrene, which may necessitate amputation. The condition has been associated with a variety of drugs including prostaglandin but not sildenafil (Viagra).[5]

Lymphangiosclerosis is a hardened lymph vessel, although it can feel like a hardened, almost calcified or fibrous, vein. It tends not to share the common blue tint with a vein however. It can be felt as a hardened lump or "vein" even when the penis is flaccid, and is even more prominent during an erection. It is considered a benign physical condition. It is fairly common and can follow a particularly vigorous sexual activity for men, and tends to go away if given rest and more gentle care, for example by use of lubricants.

Carcinoma of the penis is rare with a reported rate of 1 person in 100,000 in developed countries. Circumcision is said to protect against this disease but this notion remains controversial.[6]