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 penis is made up of three separate cylinders. 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.
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.