Structure of penis

The penis is made up of three separate cylinders (see Figure 1.1). The two paired cylinders called the corpora cavernosa make up the majority of the bulk and the erectile functioning of the penis. Both these cylinders actually communicate with each other for approximately three-quarters of their length through small holes between the cylinders. (This is why penile injections are applied into only one shaft or cylinder of the penis.) As the penis approaches the body, these two cylinders split and are anchored to the pelvic bone by a tough membrane. Each of these cylinders is encased in a very tough thick sheath called the tunica albuginea. A tough thick membrane surrounds the penis so that when it is filled with blood under pressure it creates a firm structure that allows penetration.

Figure 1.1. Frontal view of the penis. (Vivus, Inc.)

The third cylinder of the penis is called the corpus spongiosum, and it contains the urethra. The tissue around this erectile body is much thinner, and the cylinder actually sits in a groove created by the other two cylinders. As this structure approaches the end of the penis, it becomes swollen and is known as the glans, or the head of the penis. As this layer gets closer to the body, it expands to form the bulb. Covering all three of these cylinders is a thick tough membrane called Buck's fascia. Finally, a final layer covers this area called Colles fascia, or the superficial layer. This is actually continuous with the abdominal wall and makes this whole supporting structure of the penis very tough, allowing it to take quite a bit of force and trauma.

The skin covering the penis is extremely mobile and expandable. This is necessary to allow an erection to take place. The skin of the penis is unique in this property, and it is controlled by the hormonal system. The head of the penis, or the glans, is an anatomically distinct structure covered by a foreskin. This is a double layer of penile skin that is very freely moveable. Because of its movability and expand ability, it is very sensitive to any degree of swelling or trauma. It is for this reason that the skin of the penis can become massively swollen even with minimal trauma in a short period of time. For instance, a bee sting or spider bite to the penis can produce massive swelling and actually distort the penis to the point where the head of the penis is no longer visible.

The body of the penis is anchored to the pubic bone, and a thickening of the rectus muscle anchors the top of the penis. The rectus muscles, or "abs," are the muscles in the middle of the abdominal wall. This thickened layer, called the fundiform ligament, extends off the rectus muscle to anchor the penis. When this ligament is cut, as in so-called penis-lengthening operations, the penis may appear longer although it simply hangs lower from the body because it is disattached.

The blood supply of the penis comes from a main blood vessel that goes down the back of the body called the aorta. The aorta then branches to an internal and external iliac artery, and finally a pudendal artery passes underneath the pelvic bone and terminates in the common penile artery. When sitting and especially when riding a bicycle, a man can cut off blood circulation to this common penile artery. When this artery is damaged, arterial insufficiency and subsequent erectile dysfunction occur. A cavernosal artery supplies blood into each of the erectile bodies of the penis.

The blood supply to the glans, or head, of the penis is part of a separate system. It is for this reason that men can achieve an erection without swollen glans, such as in conditions known as priapism. This is also true for men who have penile implants; the glans or head of the penis will not become enlarged.

The underlying mechanism of an erection is the corporo-veno-occlusive mechanism. When the veins cannot become compressed or blocked, an erection cannot be maintained. Without this very sensitive mechanism, blood leaks prematurely from the penis and produces the loss of an erection. This type of erectile dysfunction is called a venous leak.

The nervous system of the penis is involved with both the creation and maintenance of an erection as well as an ejaculation. The most numerous sensory nerves are located on the head, or glans, of the penis. To achieve an erection it takes multiple input from numerous areas of both the brain and the spinal cord. The first part of a penile erection is controlled by the brain, known as a psychogenic erection. This occurs under any type of mental or erotic stimulation. Penile erections can also be caused by friction of the skin of the penis alone. This is known as a reflexogenic erection, which commonly occurs among men who have had damage to the spinal cord and who are unable to get an erection unless physically stimulated.

The scrotum is a unique structure with very thin, loose skin that is slightly hair bearing. It is also controlled by the endocrine system and functions as a receptacle for the testes. Keeping the testicles outside the body produces a cooler environment and thus provides the best area for spermatogenesis, or the creation of sperm, to take place. Warmth tends to prevent sperm from developing properly. Underlying the skin of the scrotum is the cremasteric muscle. This muscle is incorporated into the scrotum and by contracting, it elevates the testicles. This is done in response to cool and warm weather and noxious or painful stimuli.

The testicles are roughly the size of a small egg. They are responsible for the development of sperm as well as the manufacture of the hormone testosterone. Behind the testicles sits the epididymis, a single coiled tube that is the site of sperm maturation and storage. The end of the epididymis results in a thick muscular tube, called the vas deferens, which carries sperm from the epididymis to the prostate to be ejaculated. The vas deferens is the most common site of sterility operations, or vasectomy.

The sperm is carried in the vas deferens to two structures that sit behind the prostate. These structures are called the seminal vesicles, glands roughly 2 inches in length that form a secretion which nourishes the sperm and which attach to the prostate as well. The prostate sits at the base of the bladder and creates a fluid that allows the nourishment and activation of sperm. The primary purpose of both the seminal vesicles and the prostate is to provide nourishment and a place for sperm to live before ejaculation. In fact, the majority of the ejaculate is composed of fluid from both these glands. A very small component, usually less than 5 percent, is actually spermatozoa, which is why the amount of ejaculate remains relatively unchanged after a vasectomy.

The prostate secretes the majority of the fluid. Two small glands sit just outside the diaphragm of the urogenital diaphragm, or the thick area that anchors the penis to the pubic bone. These structures, called the bulbourethral glands, produce a very small amount of clear fluid. This is the clear fluid that we see just prior to ejaculation. It may also contain very small amounts of spermatozoa as well.

"Copyright © 1998 NTC/Contemporary Publishing Group. From The Impotence Sourcebook, by arrangement with The RGA Publishing Group."

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