Penis
The penis (plural penises, penes) is an external sexual organ of certain biologically male organisms. The penis is a reproductive organ and, for mammals, additionally serves as the external organ of urination.

# 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 end of the corpus spongiosum is enlarged and bulbous-shaped and forms the glans penis. The glans 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, lies on the tip of the glans penis. It is a passage both for urine and for the ejaculation of semen. Sperm is 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. It has no baculum, or erectile bone; instead it relies entirely on engorgement with blood to reach its erect state. It cannot be withdrawn into the groin, and is larger than average in the animal kingdom in proportion to body mass.

# Linguistics

## Etymology
The word "penis" was taken from Latin and originally meant "tail." Some derive that from Indo-European *pesnis, and the Greek word  = "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 examples of the word yard used in this sense from 1379, 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."
The Latin word "phallus" (from Greek ) is sometimes used to describe the penis, although "phallus" originally was used to describe images, pictorial or carved, of the penis.

## Slang
As with nearly any aspect of the human body that is involved in sexual or excretory functions, the word penis is considered inherently funny from a juvenile perspective and there are many slang words for the penis, including "dick", "wang" or "cock". Many of these are noted in the bathroom humor article.
"Penii" is sometimes facetiously or mistakenly used as a plural form of "penis" instead of "penes" or "penises," its correct forms.

# Puberty
When a boy enters puberty, after the testicles begin to develop, the penis begins to enlarge, alongside the rest of the genitals. The penis grows longer until about the age of 16, and growth in width begins at roughly the age of 11. 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 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. 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 straightforward, all depending on the tension of the suspensory ligament that holds it in position. Stiffness or erectile angle can vary.

# Size
As a general rule, an animal's penis is proportional to its body size, but this varies greatly between species  even between closely related species. For example, an adult gorilla's erect penis is about 4 cm (1.5 in) in length; an adult chimpanzee, significantly smaller (in body size) than a gorilla, has a penis size about double that of the gorilla. In comparison, the human penis is larger than that of any other primate, both in proportion to body size and in absolute terms.
While results vary across studies, the consensus is that the average human penis is approximately 12.7-15 cm (5-5.9 in) in length and 12.3 cm (4.85 in) in circumference when fully erect. The average penis size is slightly larger than the median size. Most of these studies were performed on subjects of primarily European descent; worldwide averages may vary.
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 cm (3.7 in) and 16 cm (6.2 in). It has been suggested that this difference is caused not only by genetics, but also by environmental factors such as culture, diet, chemical/pollution exposure, etc.
As with any other bodily attribute, the length and girth of the penis can be highly variable between individuals of the same species. In many animals, especially mammals, the size of a flaccid penis is much smaller than its erect size. In humans and some other species, flaccid vs. erect penis size varies greatly between individuals, such that penis size when flaccid is not a reliable predictor of size when erect.
Except for extreme cases at either end of the size spectrum, penis size does not correspond strongly to reproductive ability in almost any species.

# Normal variations
Depending on temperature, a flaccid (not erect) penis of average size can withdraw almost completely within the body. During erection the penis will return to its normal (erect) size.
- Other 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 affecting the penis
Edema (swelling) of the foreskin can result from sexual activity, including masturbation.
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.
Lymphangiosclerosis is a hardened lymph vessel, although it can feel like a hardened, almost calcified or fibrous, vein. It tend to not 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 tend to go away if given rest and more gentle care, for example by use of lubricants.

### Developmental disorders of the penis
Hypospadias is a developmental disorder where the meatus is positioned wrongly at birth. Hypospadias can also occur iatrogenically by the downward pressure of an indwelling urethral catheter. It is usually corrected by surgery. The Intersex Society of North America classifies hypospadias as an intersex condition. They believe in halting all medically unnecessary surgeries, including many of those done on people with hypospadias.
A micropenis is a very small penis caused by developmental or congenital problems. Diphallia, or penile duplication (PD), is the condition of having two penises. However, this disorder is exceedingly rare.

### Alleged and observed psychological disorders
- penis panic (koro in Malaysian/Indonesian) - delusion of shrinkage of the penis and retraction into the body. This appears to be culturally conditioned and largely limited to Sudan, China, Japan, and Southeast Asia.
- penis envy - the contested Freudian belief of a woman envying men for having a penis.
- small penis syndrome - disorder when men believe that their penis is smaller than average

## Altering the genitalia
The most prevalent form of genital alteration in some countries is circumcision: removal of part or all of the foreskin for various cultural, religious, and more rarely medical reasons. In many cases, such as in some United States hospitals, the frenulum and part of the shaft skin is also removed. 
Less commonly, 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 piercing, the apadravya piercing, the ampallang piercing, the dydoe piercing, and the frenum piercing. Foreskin restoration or stretching is a further form of body modification.
Other practices which 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 replacement
The first successful penis allotransplant surgery was done on September 2005 in a military hospital in Guangzhou, China. 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 newly brain-dead man, at 23, was tracked down and his penis selected for the transplant. Despite atrophy of blood vessels and nerves, the arteries, veins, nerves and the corpora spongiosa were successfully matched. On September 19th, the surgery was reversed because of a severe psychological problem of the recipient and his wife.

# Non-human penises
Most marsupials, except for the two largest species of kangaroos, have a bifurcated penis. That is, it separates into two columns, and so the penis has two ends. Urban legend alleges that the dolphin has prehensile control over his penis (it is true, however, that whales and dolphins can move and to a certain degree bend their penis tips to facilitate mating). In the realm of absolute size, the smallest vertebrate penis belongs to the common shrew (5 mm or 0.2 inches). The largest penis belongs to the blue whale estimated at over 2 m (more than 612 feet). Accurate measurements are difficult to take because the whale's erect length can only be observed during mating. Gorillas have relatively small penises, so it is an often used subtle insult in some countries to insinuate or directly state that one is 'hung like a gorilla'.
The Icelandic Phallological Museum is devoted entirely to collecting penis specimens from all sorts of land and sea mammals. The museum has received a legally-certified gift token for a future specimen belonging to Homo sapiens.
Among birds, only paleognathes (tinamous and ratites) and Anatidae (ducks, geese and swans) possess a penis. It is different in structure from mammal penises, being an erectile expansion of the cloacal wall and being erected by lymph, not blood. It is usually partially feathered and in some species features spines and brush-like filaments, and in flaccid state curled up inside the cloaca. The Argentine Blue-bill has the largest penis in relation to body size of all vertebrates; while usually about half the body size (20 cm), a specimen with a remarkable 42.5 cm-long penis is documented.
Male specimens of the Squamata order of reptiles have two paired organs called hemipenes. In fish, the gonopodium, andropodium, and claspers are various organs developed from modified fins. In male insects, the structure homologous to a penis is known as aedeagus. The male copulatory organ of various lower invertebrate animals is often called the cirrus.

# Cultural aspects involving penises

## Uses of animal penises
- Culinary, e.g., in Chinese gastronomy
- Magical and therapeutic, in medicine and/or superstition, especially as an alleged aphrodisiac or even cure for impotence
- Also used for punitive implements and dog toys, such as the bull pizzle

## Uses of human penises in cultural traditions
- Aesthetical, e.g., Body modification
- For the symbolic and artistic use, see under phallus; in heraldry, the term is pizzle
- In humor, considered indecent or completely taboo in various cultures