Anatomy of the Penis...

Your penis is basically a roll of flesh with a hollow tube down the middle. This tube, the urethra, is the passage to the outside world for urine and semen. It normally opens as either a small round hole or a long narrow slit on the tip of the glans, although some men have a condition called hypospadias, where the urethra opens somewhere on the underneath of the penile shaft. Often there are two little lips surrounding the end of the urethra on the glans. These lips are full of nerve endings which can provide immense excitement during sexual play. Their function - apart from sexual pleasure - may be to alert a man to any intrusion into his sensitive urethra.

Below is a diagram of an uncircumcised penis:


The glans is the most sensitive part of the penis, as you'd expect, since it literally leads the way into the vagina (or anus) during sexual intercourse. Both the coronal ridge of the glans and the frenulum - which is a band of skin between the shaft and the glans, often missing on circumcised penises - seem to be crucial in stimulating ejaculation during intercourse. The foreskin may actually be more pleasure-sensitive than the glans, which sometimes feels painful when it is touched; this is especially true after orgasm, when many men find their glans is almost unpleasantly hypersensitive. Many men say that the frenulum is the most sensitive part of their penis.

Inside the penile shaft, the urethra is surrounded by spongy tissue called the corpus spongiosum. During sexual arousal, a network of small arteries fills the corpus spongiosum with blood; this blood cannot escape because the veins near the base of the penis are constricted by the swelling of the spongy tissue of the corpus spongiosum. As the amount and pressure of blood in the penis increases, an erection begins to develop. If sexual arousal continues, the continuing constriction of the veins traps more blood inside the penis and the erection proceeds to full firmness.

There are two common causes of impotence or erectile dysfunction - which means you can't get an erection or stay hard. First, narrowing of the penile arteries by fatty deposits and, second, venous leakage. In the diagram above you can see that there are two other channels of spongy tissue in the penile shaft, each known as a corpus cavernosa; these also fill with blood during an erection.

Unfortunately, as a man ages, the blood supply to his penis can decrease because of the increased amounts of fat which are deposited in his penile arteries - and it doesn't take much to block them, for they are very small in diameter. Such reduced blood flow slowly causes the muscle fibers inside the penis, whose job is to regulate blood flowing through the blood vessels, to degenerate into connective tissue. The greater the amount of connective tissue, the greater the difficulty getting an erection - eventually it will be impossible to get hard at all. Even in a penis with clear arteries, it seems that normal blood flow may not be sufficient to keep the internal tissues healthy, and some researchers have suggested that the function of spontaneous erections during sleep is to ensure good oxygenation of the penis through increased blood flow; in any event, it seems that the more erections you have, the healthier your penis will be. Make of that what you will!

The penis goes deep back inside the body, with its "root" reaching back underneath the prostate gland towards the anus. This hidden penile tissue is very sensitive, and stimulating it during sex with fingertip pressure applied either through the scrotum or on the perineum (the area between scrotum and anus) can be very pleasurable. In the significant number of men who have a penis which shows little or no shaft outside their body when they are not sexually aroused, or men whose penis looks just like a glans sticking out of their body when they are flaccid, it is this hidden part of the penis which can generate a sizable erection when they become sexually aroused.

As a baby boy develops in his mother's uterus, his penis develops as a tube of flesh which gradually seals along its length. Normally, the only remaining sign of this process after birth is a thin ridge of skin along the underside of the penis known as the raphe.

The skin of the penis is infiltrated with muscle fibers that extend from the muscles of the scrotum. The strength or activity of this muscle in any individual male largely determines whether he has a scrotum which hangs loosely, or one which is held tighter against his body: the more active the muscle, the tighter and more wrinkly his scrotum appears, although it is of course also stimulated into contraction by fear or cold. These muscle fibers are also largely responsible for the contraction of the penis in length and for the variation in size of the preputial sphincter - which is just a fancy name for the opening of the foreskin at the end of the penis. This opening is not always seen in its classic textbook form, located just beyond the end of the glans, because like every other aspect of the penis, the length of the foreskin is highly variable, so that in some men there is only partial coverage of the glans, even when the penis is flaccid.

The outer surface of the foreskin is skin, but the inner surface is a sensitive mucous membrane that contains sebaceous glands which lubricate and protect the glans from friction. Because circumcision removes these lubricating glands, many circumcised men complain that their glans tissue is too sensitive as it rubs against their clothing. Generally, the foreskin is much larger than you might think - it can be up to ninety square centimeters in area in an adult man. Considering how many more nerve cells the foreskin has compared to the glans, one might guess that circumcision destroys a great deal of a man's sexual sensitivity. The foreskin is tethered to the shaft of the penis by the frenulum, which stops it retracting too far and causes it to move forward again.

Finally, the ridged bands shown in the diagram above are located on the inner surface of the foreskin near the tip; they merge with the frenulum, and have the highest number of nerve endings of any part of the foreskin. These nerve endings are of a type sensitive to changes in pressure and tension, and are thought to play a part in stimulating and controlling orgasm and ejaculation during intercourse.