From Encyclopedia of Sex and Sexuality
The male’s sexual and reproduction organ. In sexual intercourse, the erect penis (see erection) is inserted in the female partner’s vagina and remains there until orgasm and ejaculation of seminal fluid, which may or may not impregnate the female.
The human penis has a soft bulge at the tip called the “glans,” a body, or “shaft,” and a root, called the “crus” of the penis, buried in the pubic area. In uncircumcised males the glans is covered by loose skin called the foreskin. Many, if not most, males in the industrialized West are circumcised; their foreskins have been surgically removed shortly after birth for hygienic or religious reasons (see circumcision).
Three elongated structures, the two corpora cavernosa and the urethra, constitute the body of the penis. Within the penis, but located closer to the top or the back of the penis’s body, are the corpora cavernosa, two cylindrical tubes, usually flattened, placed side by side. Each is composed of inner sponge-like tissue surrounded by a very dense fascia, a band-like lining, called the tunica albuginea. (This is the densest fascia in humans, much thicker than the dura mater, the fascia that surrounds the brain.) This lining does not totally separate the two corpora cavernosa, and several passages allow blood to circulate from one to the other. The sponge-like tissue inside them is best described as a mesh of spaces and cavities separated and surrounded by thick layers of muscle tissue. In the center of each corpus cavernosum is an artery with corkscrew-shaped branches that supply blood to each of the tiny cavities.
Underneath the two corpora cavernosa, on the penis’s midline, lies the third tubular structure, the urethra. The urethra serves as the conduit for urine or semen to the opening at the tip of the glans, called the meatus. The urethra is surrounded by the corpus spongiosum, sponge-like tissue that is also capable of engorgement and erection. The corpus spongiosum also forms the glans. Unlike the corpora cavernosa, the erectile tissue of the corpus spongiosum and glans is not contained in a thick fascia like the tunica albuginea. No vascular communication exists between the corpus cavernosum and the corpus spongiosum.
The corpora cavernosa separate near the root, or base, of the penis like two branches of the letter ”Y.” At the point of bifurcation, a ligament links the corpora to the pubic bone. Furthermore, each branch of the “Y” is attached to a region of the pelvic bone known as the ischium. This buttressing of the corpora to the bone accounts for the change of angulation that occurs when a flaccid penis becomes erect. As a man ages, the suspensory ligament of the penis stretches, explaining why in a man of seventy the erect penis may be pointing downward. The urethra curves upward, passes between the separated corpora, and connects to the bladder.
The head of the penis—known medically as the glans—has a conical shape, with the meatus (penis’s opening) at its tip and a corona (crown or cornice) at its base. The glans is the external extension of the corpus spongiosum, and as noted, in the uncircumcised male it is covered by the foreskin. During erection, blood pressure in the glans is about half the pressure in the corpus cavernosum, enabling blood to drain freely into the penis’s shaft. One function of the glans is to act as a shock absorber and protect the woman’s cervix from the man’s thrusting during intercourse. Vaginal pressure keeps the glans full and improves the seal with the corona; this increases friction and promotes ejaculation and the retention of semen near the cervix, where sperm enter to impregnate the woman.
The foreskin covers the glans almost completely at birth. Its opening is tight and the infant’s foreskin usually cannot be retracted from the glans. Unless it is removed by circumcision shortly after birth, the foreskin usually loosens and retraction becomes possible as the infant matures. (It is an unresolved question whether the presence of a foreskin increases the sexual pleasure experienced by both partners). The foreskin’s function is to protect the sensitive skin of the glans, and some have postulated that it may play a role in the development of the glans during gestation. Removal of the foreskin after birth is a religious practice of Jews and Muslims and has become common even among the general population in some Western societies because it makes it easier to clean the area around the glans. In recent years objections have been raised by by some parents and physicians that male circumcision is unnecessary, and some parents refuse it for newborn sons. Male circumcision is totally unlike female circumcision in its objectives and effects (see clitoridectomy).
There is a great variation in penis size and in the change in penis size from the flaccid, or soft, state to partial or full erection. This is true both when different persons are compared and for any single individual over his lifetime and even in the course of a single day. Sex therapists and researchers have established that size variations of the penis have little to do with the ability to please a partner or to father children. Variations in penis size are due, in part, to differences in the proportion of the penis that is buried (the crus) and exposed (the body) Individuals with a short crus have larger penises, but during erection the penis may point downward. Those with a larger crus may have penises with shorter bodies and erections that point outward or upward. Variation in the change of size of the penis from the flaccid to the erect state means that men with penises of similar size when flaccid will have erect penises of very different sizes. Individuals will also have erections of different sizes and firmness depending on their physical and emotional state and their state of arousal.
There is no correlation between body size and penis length. Evidence exists that penis size and length decrease with age, but this occurs gradually and most often is not noticed. Penis length can be altered by weight loss: this diminishes the thickness of fat in the pubic area. A good rule of thumb is, “one inch gained in penis length for every thirty pounds of weight loss.”