How it is built
The penis is made of three cylinders of erectile tissue: two corpora cavernosa, side by side on top, like the two barrels of a shotgun, and a corpus spongiosum underneath, which surrounds the urethra — the channel through which urine and semen pass, never at the same time — and ends in the glans. The glans, highly sensitive, is covered by the foreskin, held underneath by a small band, the frenulum, which is fragile and sometimes too short. At the tip of the glans opens the meatus. The corpora cavernosa do not stop at the visible base: they continue inside the pelvis for several centimetres, anchored to the pubic bones.
Below the penis, the scrotum holds the two testicles, which make sperm and testosterone; it is normal for one to hang lower than the other — the left, most often — and for the skin to contract in the cold or with arousal. On each testicle the epididymis stores sperm, which the vas deferens carries to the prostate and the seminal vesicles, which produce most of the fluid of ejaculation. The perineum, between the scrotum and the anus, hides the root of the penis and the muscles involved in erection and ejaculation.
How an erection works
An erection is plumbing controlled by nerves. Under a stimulus — a thought, an image, a touch, or nothing conscious during sleep — the nervous system releases nitric oxide into the corpora cavernosa; their arteries widen, blood floods the thousands of small cavities of the erectile tissue, which swell and press against the outer sheath the veins through which the blood should leave. Trapped inside under pressure, it maintains rigidity until arousal subsides or ejaculation triggers the venous outflow. Night-time erections, three to five a night, maintain the tissue; their disappearance is a signal. The drug that made this mechanism famous, sildenafil, merely prevents the breakdown of the chemical messenger of dilation — so it only works in the presence of desire. The page on erectile dysfunction explains what goes wrong and why a lasting failure is also a warning for the heart.
Size: what the measurements say
The largest published synthesis, in 2015, from 15,500 men measured by clinicians rather than by themselves, gives an average length of 13.1 cm erect (9.2 cm flaccid) and an average circumference of 11.7 cm erect, with 90% of men between 10 and 16 cm. These figures are lower than most men believe, because studies based on self-report exaggerate, and because everyone's angle of view on his own body shortens. One measures on top, from the pubic bone (pressing the ruler against the bone, not the fat) to the tip of the glans, at full erection.
Two facts relieve many worries. Flaccid size predicts nothing about erect size: a penis that is small at rest can more than double, a large one may hardly change. And size has no measurable relationship with the partner's pleasure: the first two thirds of the vagina are the most innervated, and what triggers female orgasm is above all clitoral stimulation — see the female genitals. A true micropenis — under 7 cm erect — is rare and a matter for endocrinology; between 7 and 13 cm there is only the average, and nobody needs it.
Shape, curvature, colour
No two penises are alike: straight, curved upward, downward or sideways when erect; a glans wider or narrower than the shaft; a colour darker than the rest of the skin; visible veins; small whitish bumps around the rim of the glans (pearly papules, entirely harmless) or visible sebaceous glands on the shaft. All of this is normal. A curvature that appears in adulthood, especially with pain or a hard plaque under the skin, is something else — Peyronie's disease — and is best seen early, when it is most treatable.
The foreskin and circumcision
The foreskin protects the glans and slides over it during intercourse. In a small boy it is normally attached and does not retract; it frees itself on its own, usually before adolescence, and is never forced. A foreskin that remains tight in adulthood (phimosis) is treated first with a cream, then if necessary with minor surgery. Circumcision, which removes the foreskin, is performed for religious or cultural reasons in nearly one man in three worldwide, and for medical reasons in cases of phimosis or repeated infections; the WHO also recommends it as an HIV prevention measure in regions of high epidemic, where it reduces by about 60% the risk of a man being infected during heterosexual intercourse. Outside these cases it is neither necessary nor harmful; it does not measurably change pleasure.
Hygiene, self-examination, and when to see a doctor
Daily washing with water, foreskin pulled back, is enough; smegma, the whitish deposit under the foreskin, is a normal mixture of sebum and cells that only causes problems if it builds up. The testicles are checked once a month, in the shower: a hard, painless lump that was not there before is the first sign of testicular cancer, the most common cancer in men aged 15 to 35 and curable in almost every case when caught early. See a doctor also for pain or swelling of a testicle (urgently if sudden), a discharge, a sore or spots on the glans — see the STIs —, blood in the semen, a foreskin stuck behind the glans, or an erection lasting more than four hours.