Penis

The penis is the external male reproductive organ in humans and many animals. It has two primary functions: the delivery of semen during sexual intercourse, and the passage of urine from the body, since the urethra runs through the penis.

The English term 'penis' is also used broadly to refer to the male reproductive organ in various animal species, though their structure varies: snakes, reptiles, and some fish possess analogous structures that function similarly but differ in form.

Structure (in humans)

The human penis is composed of a shaft and a head, called the glans. Inside are three columns of spongy erectile tissue that carry blood: two of these (the corpora cavernosa) are the largest, while a third (the corpus spongiosum) surrounds the urethra and forms the glans at the tip. The glans can be covered by a fold of skin called the foreskin (prepuce), which is removed during male circumcision.

How it functions

Erection occurs when blood vessels in the spongy tissue dilate and fill with blood, while outgoing blood flow is restricted. This process is regulated by the autonomous nervous system and chemical signals, particularly nitric oxide. Ejaculation involves contractions of muscles at the base of the penis and combines fluids from the testes, seminal vesicles, and the prostate gland.

Variation across animals

Some mammals, such as dogs, bears, and certain primates, possess a small bone in the penis called the baculum, which aids in rigidity. Humans lack this bone. Reptiles such as snakes and lizards have two structures called hemipenes, of which only one is used at a time. Most birds lack an external reproductive organ and reproduce through contact between their cloacae.

Health

Health conditions include erectile dysfunction, phimosis (tightness of the foreskin preventing retraction over the glans), sexually transmitted infections, and penile cancer, which is rare. Routine hygiene, medical screening, and HPV vaccination are among standard preventive measures.