The human penis is an external male intromittent organ that additionally serves as the urinal duct. The main parts are the root (radix); the body (corpus); and the epithelium of the penis including the shaft skin and the foreskin (prepuce) covering the glans penis. The body of the penis is made up of three columns of tissue: two corpora cavernosa on the dorsal side and corpus spongiosum between them on the ventral side. The human male urethra passes through the prostate gland, where it is joined by the ejaculatory duct, and then through the penis. The urethra traverses the corpus spongiosum, and its opening, the meatus (), lies on the tip of the glans penis. It is a passage both for urination and ejaculation of semen. (See: male reproductive system.)
Most of the penis develops from the same tissue in the embryo as does the clitoris in females; the skin around the penis and the urethra come from the same embryonic tissue from which develops the labia minora in females. An erection is the stiffening and rising of the penis, which occurs during sexual arousal, though it can also happen in non-sexual situations. Spontaneous non-sexual erections frequently occur during adolescence and during sleep.
In its relaxed (flaccid, i.e. soft/limp) state, the shaft of the penis has the feel of a dense sponge encased in very smooth eyelid-type skin. The tip, or glans of the penis is darker in color, and covered by the foreskin, if present. In its fully erect state, the shaft of the penis is rigid, with the skin tightly stretched. The glans of the erect penis has the feel of a raw mushroom. The erect penis may be straight or curved and may point at an upward or downward angle, or straight ahead. It may also have a tendency to the left or right.
Measurements vary, with studies that rely on self-measurement reporting a significantly higher average than those with a health professional measuring. As of 2015, a systematic review of 15,521 men, and the best research to date on the topic, as the subjects were measured by health professionals, rather than self-measured, has concluded that the average length of an erect human penis is 13.12 cm (5.17 inches) long, while the average circumference of an erect human penis is 11.66 cm (4.59 inches). Neither age nor size of the flaccid penis accurately predicts erectile length.
The most common form of genital alteration is circumcision, removal of part or all of the foreskin for various cultural, religious and, more rarely, medical reasons. There is controversy surrounding circumcision.