The human penis is the largest of any living primate and, unlike the males of many species 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 large corpora cavernosa on the upper surface and the thinner corpus spongiosum running centrally up the underside.

On the upper 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 stability and is surrounded by a muscle 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 Corpora Cavernosa 

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. The crura are covered by muscle and each one attaches to a bone 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 stability comes from a suspensory ligament stretching from the public 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 increase, the spaces rapidly distend to form the equivalent of giant lakes. This quickly causes rigidity.


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 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.

Penile Size 

The size of a man penis varies less than is popularly believed. The average erect penis measures 16 cm when measured from tip to base on the upper surface. Ninety per cent of all men fall between the extremes of 14.5 and 17.5 cm, 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 to 15 cm depending on room temperature, and generally lengthens by around 5 cm 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 when erect. In a survey carried out by Forum in 1970, the smallest penis reported was 12 cm long. In some medical conditions where the penis fails to develop properly, an erect penis may not exceed 1 cm 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 increase 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 long, while kinsey registered one that was 25 cm long. The longest authentically recorded penis measured an impressive 30 cm when erect, and was 5.5 cm in diameter. A penis measuring 35 cm 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 silicome 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. Transplanted 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 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 public bone is cut and the root of the penis is pulled forwards and re-stabilized with stitches. A triangular flap of skin is taken from the public hair region and used to cover the newly exposed penile shaft. There are two major after-effect:

  1. Hair grows on the first 2-3 cm of the penis
  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 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.


The penis is enclosed by a loose sleeve of thin, hairless skin rich in muscles fibres. This has expansile and contractile properties that allow it to respond to changing penile length during erection.

In uncircumcised males, this sleeve of skin folds over on itself to form the foreskin. Only 4 per cent are retractile at birth. Most foreskins remain firmly stuck to the glans during the first few years of life and should never be forcibly retracted. The adhesions between the glans and foreskin slowly break down and by the age of three years, 90 per cent of boys foreskins can slide to and fro over the helmet to some extent. Remnants of the cells attaching the foreskin to the glans may prevent full retraction in up to 60 per cent of nine years old, but have usually disappeared by the age of 17 years.

After full separation, the mature foreskin remains tethered to the glans on the underside to form a right of skin, the frenulum which contains a small artery. The frenulum and glands especially the corona, or ridge are usually the most sensitive parts of the penis.

The mature foreskin acts as a cover to protect the glans while flaccid and to keep it in a moist, sensitive state. During erection, the foreskin slips back to clothe the elongating penile shaft.


Bacteria, yeasts, stale urine and sloughed skin cells rapidly accumulate under the foreskin to form a white, smelly, cheesy substances known as smegma.

It starts to develop at an early age and is seen in 1 per cent of seven year old, and 8 per cent of 17 year old. Smegma that is allowed to build up can cause irritation and soreness. Smegma has also been linked with the development of cancer of the penis, although this is unproven.

Males over the age of seven years who have an intact foreskin should be taught how to retract it and wash underneath regularly, at least once a day preferably after every urination. This procedure should be done gently and carefully, as forceful retraction, especially if adhesions are still present, can cause injury, scarring and even a phimosis

After washing under the foreskin it is important to draw it back over the glans so a paraphimosis does not form


Phimosis is a tightness of the foreskin so that it cannot be drawn back over the widest part of the glans. This may be normal up until the age of two or three years, but in 10 per cent of boys it is still present beyond the age of three. Phimosis is often associated with an excessively long foreskin that contains an abnormal amount of fibrous scar tissue. Sometimes it develops at a late age following a foreskin tear that heals to leave a contraction.

A tight phimosis may cause difficulty in urinatiny, so that the foreskin balloons when passing water. This can be distressing, especially if urine trapped under the foreskin continues to leak after urination has finished. Even if the foreskin cannot be retracted, easing it back gently while passing water usually helps.

Phimosis in older males cause pain and difficulty with erection, masturbation and love-making. It also predisposes towards tearing of the foreskin, balanitis and cancer of the penis. Surgical correction by circumcision is the usual treatment.


Paraphimosis is a constriction of the penis behind the glans, due to an extremely tight foreskin. This usually occurs when the tight foreskin is drawn back to expose the glans and is then not subsequently pulled back over. The tight foreskin constricts circulation at the end of the penis and gross swelling of the glans and foreskin results. If this is not treated, the tissues will eventually become gangrenous.

In most cases the foreskin can be massaged back over the glans using in ice-pack to reduce swelling, squeezing the glans to expel excess fluid and applying a lubricating gel. This should not be done as a DIY procedure, but left to the skill of a surgeon. It is painful and often requires an anaesthetic. If manual reduction proves impossible, circumcision is essential. Some surgeons cut the constricting band first and let swelling subside before circumcising. Others perform circumcision straightaway.

Foreskin Redevelopment

A Dr Bigelow in the US has perfected a technique that redevelops a circumcised foreskin. This is referred to as uncircumcising. The skin covering the end of the penile shaft is gently stretched and encouraged over the glans using a specially shaped plaster. This is worm continuously and cut to allow urination.

The skin of the glans quickly becomes less tough, more moist and increasingly sensitive as the new sleeve of skin starts to develop. As the stretched skin covering the penile shaft expands, further tension is required. Either small lead weight or a funnel-shaped expansion device is used during the end stages of foreskin redevelopment. A full foreskin will reform over 2_6 years, depending on how tightly the original operation was performed. Surgical restoration of the foreskin using skin grafts is also possible.



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