Male Pelvis Showing Flaccid Penis: Medical visualization of the male pelvic region with an erect penis, also visible are the testicles, pelvis, femurs, and surrounding musculature. There are three columns of erectile tissue in the penis: at the top (or dorsal side) of the penis are the two corpora cavernosa, and at the bottom (or ventral side) is the corpus spongiosum. Upon arousal, the arteries that supply the penis dilate and allow blood to fill the three spongy erectile tissue columns, causing it to lengthen and stiffen. The engorged erectile tissue presses against penile veins, preventing blood from flowing back out of the penis.
A penis (plural penises or penes /-niːz/) is the primary sexual organ that male and hermaphrodite animals use to inseminate sexually receptive mates (usually females and hermaphrodites respectively) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males do not bear a penis in every animal species, and in those species in which the male does bear a so-called penis, the penes in the various species are not necessarily homologous. For example, the penis of a mammal is at most analogous to the penis of a male insect or barnacle.
The term penis applies to many reproductive intromittent organs, but not to all; for example the intromittent organ of most cephalopoda is the hectocotylus, a specialised arm, and male spiders use their pedipalps. Even within the Vertebrata there are morphological variants with specific terminology, such as hemipenes.
In most species of animals in which there is an organ that might reasonably be described as a penis, it has no major function other than intromission, or at least conveying the sperm to the female, but in the placental mammals the penis bears the distal part of the urethra, which discharges both urine during urination and semen during copulation as the occasion requires.
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