In mammals, the prepuce or foreskin is the retractable double-layered fold of skin that covers the head of the penis, (the glans penis). It serves a protective function for the ridged band, the frenulum and glans penis, keeping these sexual tissues moist, lubricated and protected from abrasion and injury. The prepuce is a double folded tube of skin and muscosal tissue mucous membrane (like the inside of the mouth) which attaches at the crested end of the glans at the suculas, and to the body above the pubic bone and scrotum. Unlike the skin on the rest of the body which is attached to the underlying tissue, the prepuce and shaft skin are free to glide along the shaft of the penis, which reduces friction, abrasion and loss of lubricating fluid during intercourse.
Some scientists believe the foreskin to be a vector for disease and see circumcision as a preventive measure, but the matter is hotly debated. Opponents of circumcision believe the foreskin to be an essential part of the penis and circumcision to be potentially harmful. See medical analysis of circumcision.
In the prepuce a band of smooth muscle and specialised nerve endings called the "Ridged Band" radiates from the frenulum and surrounds the inner tip of the prepuce. When the prepuce is pulled back along the penis shaft this ridged band lies just behind the crest of the glans. This highly innervated tissue contains about 20,000 Messier corpuscles and Van Der Patter bodies which respond to light touch, stretching, folding, pressure, and temperature. The ridged band contains about 80% of the male erogenous tissue. Like all other specialized neurologic tissue in the human body, the ridged band has a complex and ample blood supply.
The smooth muscle, called Dartos muscle, of the ridged band and frenulum return the prepuce over the glans when the penis is flaccid (not erect), both to protect the highly sensitive nerves of the ridged band and to a lesser extent the glans. The muscle fibres are intertwined like a tiny rope to close off the preputal opening to keep out contamination. Like most other human body parts, the coverage of the glans by the prepuce is a highly-variable characteristic, as some men have abundant overhang when flaccid, while others don't have complete glans coverage. The muscle fibres keep the foreskin close to the glans penis but render the foreskin highly elastic.
There exists a condition called phimosis, in which the prepuce cannot be fully retracted. It is entirely normal for the prepuce to be adherent to the glans in babies, and for the prepuce to not fully retract in young boys. Premature retraction, which is extremely painful, can cause permanent damage to the glans and ridged band, and is also the primary cause of infection in this area. As a boy grows to sexual maturity the prepuce normally becomes retractable. Consult a doctor if phimosis is a problem. Sometimes a prescription steroidal cream can be used to widen the preputial opening.
Where the foreskin is retractable, a conditon called paraphimosis may occur, where the foreskin becomes trapped behind the glans. This is a serious condition which must be treated as a medical emergency.
The inner mucosal tissue of the prepuce contains Langerhans cells which produce anti-bacterial and anti-viral compounds, to protect the owner from infection. Some scientists have speculated that Langerhans cells may have increased receptibilty and "trap" the HIV virus from a partner, and studies are currently in progress regarding a correlation between HIV transfer and foreskin presence (see medical analysis of circumcision). If the man does not wash under the prepuce for a few days the accumulation of secretions, sloughed skin cells and dead bacteria create a white creamy paste between the prepuce and glans penis called smegma. An identical process takes place in the labia folds of a woman's genitals. Smegma is harmless and easily washed away with just water.
Removal of the prepuce, for whatever reason, is called circumcision. Some circumcised men are engaging in foreskin restoration by tensioning the remaining skin to encourage natural skin expansion. Subincision is another method of preputial modification, generally practiced as a fetish practice.
An interesting side note is that, among many tropical South American indians, the closure of the foreskin was regarded as the only point of propriety among the men, who otherwise were essentially naked. In other words, it was considered shameful for a man's glans penis to be seen in public, but completely acceptable for his genitals to otherwise be seen. In some tribes, such as in the Mato Grosso, the foreskin would be held shut by a clamping device made from a palm fronds. In other tribes, such as the Yanomami, the foreskin would be clamped into a string-like belt worn around the waist. In this case, the foreskin would tend to continue to grow as a result of the sustained tension, resulting in foreskins that had an overhang of possibly two inches or more.
Among the Maori of New Zealand (unlike many other Polynesian peoples, who superincised) an exposed glans (tehe) was considered obscene, and a string hanging down from a waistband was tied around the foreskin to prevent it.
In ancient Greece too, only a nude glans was considered obscene, and a thong (kynodesme) was worn to hold the foreskin closed. A long tapering foreskin was considered to be a mark of male beauty.