Penis

The term penis applies to many intromittent organs, but not to all. As an example, the intromittent organ of most Cephalopoda is the hectocotylus, a specialized arm, and male spiders use their pedipalps. Even within the Vertebrata, there are morphological variants with specific terminology, such as hemipenes.

Etymology

The word "penis" is taken from the Latin word for "tail". Some derive that from Indo-European *pesnis, and the Greek word πέος = "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,^[3] 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."^[4] According to Wiktionary, this term meant (among other senses) "rod" or "bar".


As with nearly any aspect of the body involved in sexual or excretory functions, the penis is the subject of many slang words and euphemisms for it, a particularly common and enduring one being "cock". See WikiSaurus:penis for a list of alternative words for penis.


The Latin word "phallus" (from Greek φαλλος) is sometimes used to describe the penis, although "phallus" originally was used to describe representations, pictorial or carved, of the penis.^[5]

Evolution and function

The external genital organs appeared in the Devonian, about 410 million years ago, when tetrapods began to abandon the aquatic environment.^[6] In fact, it was necessary to overcome the absence of a liquid phase in which to release the gametes was achieved through the transition to internal fertilization.


Among amniotes, the development of an erectile penis occurred independently for mammals, reptiles and archosaurs (crocodiles and birds).


Over time, birds have lost this organ, with the exception of Paleognathae and Anseriformes.^[7]


The penis is an intromittent organ used to transfer sperm into the female genital tract for potential fertilization and, in the case of placentals, also for the excretion of urine.^[8] The penises of different animal groups are not homologous with each other, but were created several times independently of each other in the course of evolution.


An erection is the stiffening and rising of the penis, which occurs during sexual arousal, though it can also happen in non-sexual situations. 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.


The last common ancestor of all living amniotes (mammals, birds and reptiles) likely possessed a penis.^[9]