ANATOMY OF THE HUMAN PENIS

Knowing how a penis functions can provide useful insight into sex and the human body — whether you want to understand the penis and male anatomy better or learning about it for the first time.
Basically, a penis is composed of three structures, which are made of a spongelike material that can fill with blood:

• The two corpus cavernosa contain the central arteries and lie on the top half of the penis. They are cylindrical tubes and are larger than the other spongy structure.
• The corpus spongiosum, which is under the two corpus cavernosa and surrounds the urethra, is the pipeline for both urine and sperm.
  

When a man becomes sexually excited , the nerves surrounding his penis become active, causing the muscles around the arteries to relax and more blood to flow into the penis. The spongelike material then absorbs the additional blood, making the penis stiff and hard, or erect. This erection tightens the veins so the blood can’t leave the penis, enabling the penis to remain erect. After a man ejaculates or if his arousal fades, detumescence occurs, in which the brain sends a signal to allow the blood to leave the erect penis, and it returns to its flaccid state.

Penises become erect at all different angles — and the angle doesn’t have any effect on the way the penis performs.

The head of the penis, called the glans, is shaped like a cone. The opening of the glans is called the meatus (me-ate-us), and at the base of the glans is a crownlike structure called the corona. The scrotum, a sac of skin located at the base of the penis, holds the testicles. The testicles produce sperm and manufacture hormones (most notably, testosterone).
The glans serves several purposes: increasing the chances for fertilization of an egg, creating extra friction during sex, and acting as a shock absorber within the vagina during intercourse.
At birth, the glans is covered by the foreskin, a sheath of skin that opens at the top. In an infant, this opening is very tight and usually can’t be pulled back (or retracted, to use the medical term). Usually, the foreskin loosens up as the baby grows older. When a male has an erection, the foreskin pulls back entirely to fully reveal the glans. The skin of the glans is very sensitive, and the purpose of the foreskin is to protect it.
In the Jewish and Muslim cultures, the foreskin is always surgically removed in a procedure called circumcision. Circumcision has also become popular in many Western societies because the penis is easier to keep clean without the foreskin. Because of today’s better hygiene, some parents and physicians believe that circumcision is no longer necessary, although the debate isn’t entirely over.

A circumcised penis (l.) with the foreskin removed; an uncircumcised penis (r.) with the foreskin intact.