Penis anatomy and function

THE PENIS

The human penis is the largest of any living primate and, unlike the males of many species (e.g. whale, bear, walrus, cattle, bats, rodents and lower monkeys) has evolved without the need for a strengthening bone. It is an amazing example of bio-engineering, based on three inflatable cylinders of erectile tissue: two larger corpora cavernosa (cavernous bodies) on the upper surface and the thinner corpus spongi-osum (spongy body) running centrally up the underside.
On the upper (dorsal) side of the penis, a dorsal vein drains blood away from the organ; two dorsal arteries, which supply blood to the skin, pulsate where the penis joins the lower abdomen. Several superficial veins are also visible, which drain the skin and glans of the penis, but not the deeper erectile tissues.

The Corpus Spongiosum

The single corpus spongiosum contains the urethra--the tube through which urine flows from the bladder and out. At the tip of the penis the corpus spongiosum expands to form the bulky helmet or glans. At the base, behind the scrotum, the corpus spongiosum thickens again to form the root or bulb of the penis. This is attached to a thick fibrous membrane for stab-ility and is surrounded by a muscle (bulbospongiosus) that contracts rhythmically during ejaculation. The corpus spongiosum also contains erectile tissue that swells in a similar manner to the corpora cavernosa during erection.


The two corpora cavernosa run side by side throughout the penile shaft. Their tips are embedded in the glans penis; at the base they flare apart to form two crura (legs). The crura are covered by muscle and each one attaches to a bone (ischium) on either side of the lower pelvis. This forms an anchorage that allows the penis to stand upright and stable during intercourse. Contraction of these ischiocavernosus muscles are also involved in expelling sperm during ejaculation. Further stab-ility comes from a suspensory ligament stretching from the pubic bone to the base of the penis at the front.
The inside of each corpora cavernosa is divided into several cavernous spaces. A deep artery runs through the centre of each corpora cavernosa and its branches supply blood directly into the spongy tissues. When blood supply is normal, these spaces form the equivalent of tiny puddles. When the arteries dilate and the blood supply increases, the spaces rapidly distend to form the equivalent of giant lakes. This quickly causes rigidity.


Erections

Erections are not under voluntary control but are triggered by emotional, physical and hormonal signals. Testosterone hormone is important but not essential as, albeit rarely, castrated males have experienced erectile activity. Most men, even some who are normally impotent, experience 1¬5 erections while asleep. These last approximately 30 minutes each and are often in evidence on waking.
Erection occurs when small arteries at the base of the penis dilate. This is triggered by activity in a set of nerves (the parasympathetic nervous system) which relaxes the tiny muscles within arterial walls, making the arteries open up. Blood rushes into the penis and is shunted into the expandable tissues of the corpora cavernosa and corpus spongiosum. These fill under high pressure to compress outlet veins so blood cannot drain back out again.
The corpora cavernosa act rather like inflatable bungs to prevent urination during engorgement and maintain erection using the fluid tension of trapped blood. They transform the penis from a low-volume, low-pressure system into a large-volume, high-pressure one by increasing the inflow of arterial blood.
In effect, the penis acquires its own hydrostatic skeleton ¬ a method of support also relied upon by lower life forms such as the garden earthworm.
(See orgasm)

Penile Size

The size of a man's penis varies less than is popularly believed. The average erect penis measures 16 cm (6.3 in) when measured from tip to base on the upper surface (the side with the wiggly vein). Ninety per cent of all men fall between the extremes of 14.5 (5.6 in) and 17.5 cm (7 in), despite any claims to the contrary!
Size when flaccid is not a reliable indication of size when erect. A flaccid penis ranges from 7.5 cm (3 in) to 15 cm (6 in) depending on room temperature, and generally lengthens by around 5 cm (2 in) when erect. Penises that are short when flaccid tend to lengthen proportionately more than longer ones.
Kinsey, one of the earliest sexologists, had a patient whose penis was only 2.5 cm (1 in) when erect. In a survey carried out by Forum in 1970, the smallest penis reported was 12 cm (4.7 in) long. In some medical conditions where the penis fails to develop properly, an erect penis may not exceed 1 cm (0.4 in) in length.
As long as the penis can enter the female partner there is no reason why intercourse and insemination should not occur. There is one argument in favour of a small penis: it is more likely to enter and disengage repeatedly from the vagina during intercourse, which increases clitoral stimulation. Most women say that size bears little relation to satisfactory performance.
An extra long penis is a boast many men make but few can deliver. Forum reported one penis that was 24 cm (9.5 in) long, while Kinsey registered one that was 25 cm (10 in) long. The longest authentically recorded penis measured an impressive 30 cm (12 in) when erect, and was 5.5 cm (2.25 in) in diameter. A penis measuring 35 cm (14 in) when erect was described in Everything You Always Wanted to Know about Sex by Dr David Reuben, but no source was quoted. It would seem that the longest penises average somewhere between 25 cm and 30 cm.
While a penis with an extra-wide base may improve female sexual pleasure, size is not the great attribute many men believe. A large penis can cause physical pain to a female partner during intercourse either by inducing friction sores or by hitting the ovaries ¬ which are just as sensitive as the male testicles. In some cases, a large penis makes intercourse physically very difficult.

Improving on Nature

There is currently a vogue for surgical enlargement of what nature bestowed. Textured silicone (bioplastique) or fat sucked from the abdominal wall can be introduced just under the penile skin using multiple injections. The corpora cavernosa are not affected.
These procedures aim to increase the weight of the penis by around 30 g and add several centimetres to the width of the penis at the base.
If fat cells are used, the procedure is known as CAPE ¬ Circumferential Autologous Penile Engorgement. Trans- planted cells hopefully remain viable and 'take root' within the penile shaft. If the cells die, the fat globules tend to harden leading to an unfortunate side-effect: lumpiness.
An operation perfected in China by (incredibly) a Dr Long, lengthens the penis by up to 50 per cent. The operation takes an hour and is performed under a general anaesthetic. The suspensory ligament attaching the penis to the front of the pubic bone is cut and the root of the penis (40 per cent of which is hidden in the pelvis) is pulled forwards and re-stabilized with stitches. A triangular flap of skin is taken from the pubic hair region and used to cover the newly exposed penile shaft. There are two major after-effects:
1. hair grows on the first 2¬3 cm of the penis (but can be removed by electrolysis)
2. the angle of erection decreases from an upright 45 degrees to a flatter 60 degrees. As the penis has been surgically stabilized, however, this change does not interfere with the man's ability to make love.
Sexual activity is banned for three weeks after the operation and erections prevented by drugs. After this, a normal sex life can restart. At present this technique is only available in China, South Africa and most recently in the U.S.