The penis (pl. penises, penes) is an external male sexual organ. It is the male reproductive organ and, for mammals, also serves as the organ of urination.
The human penis differs from those of some other mammals as it has no baculum or erectile bone; instead relying 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 It cannot be withdrawn into the groin and is larger than average in the animal kingdom in proportion to body mass.
The human penis is made up of three columns of erectile tissue:
- the two corpora cavernosa (sing. corpus cavernosum); and
- one corpus spongiosum
The corpus spongiosum lies on the underside (known also as the ventral side) of the penis; the two corpora cavernosa lie next to each other on the upper side (dorsal side).
The end of the corpus spongiosum is enlarged, 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 both a passage 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 under side of the penis, running from the meatus (opening of the urethra) across the scrotum to the perineum (area between scrotum and anus).
When a male-bodied person enters puberty, their testicles begin to grow the penis begins to enlarge just like the rest of their genitals. The penis grows longer until the age of 18, but growth in width only begins at the age of 13. During the process, pubic hair grows above and around the penis.