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The penis is a pendulous organ suspended from the front and sides of the pubic arch and containing the greater part of the urethra. In the flaccid condition it is cylindrical in shape, but when erect assumes the form of a triangular prism with rounded angles, one side of the prism forming the dorsum. It is composed of three cylindrical masses of cavernous tissue bound together by fibrous tissue and covered with skin. Two of the masses are lateral, and are known as the corpora cavernosum penis; the third is median, and is termed the corpus spungiosum.
Corpora cavernosum penis
The Corpora Cavernosum Penis form the greater part of the substance of the penis. For their anterior three-fourths they lie in intimate position with one another, but behind they diverge in the form of two tapering processes, known as the crura, which are firmly connected to the rami of the pubic arch. Traced from behind forward, each crus begins by a blunt-pointed process in front of the tuberosity of the ischium. Just before it meets its fellow it presents a slight enlargement. Beyond this point the crus undergoes a constriction and merges into the corpus cavernosum proper, which retains a uniform diameter to its anterior end. Each corpus cavernosum penis ends abruptly in a rounded extremity some distance from the point of the penis. The corpora cavernosum penis are surrounded by a strong fibrous envelope consisting of superficial and deep fibers. The superficial fibers are longitudinal in direction, and form a single tube which encloses both corpora; the deep fibers are arranged circularly around each corpus, and form by their junction in the median plane the septum of the penis. This is thick and complete behind, but is imperfect in front, where it consists of a series of vertical bands arranged like the teeth of a comb; it is therefore named the septum pectiniforme.
The Corpus corpus spongiosum contains the urethra. Behind, it is expanded to form the bulb, and lies in apposition with the inferior fascia of the urogenital diaphragm, from which it receives a fibrous investment. The urethra enters the bulb nearer to the upper than to the lower surface. On the latter there is a median sulcus, from which a thin fibrous septum projects into the substance of the bulb and divides it imperfectly into two lateral lobes or hemispheres. The portion of the corpus spongiosum in front of the bulb lies in a groove on the under surface of the conjoined corpora cavernosa penis. It is cylindrical in form and tapers slightly from being forward. Its anterior end is expanded in the form of an obtuse cone, flattened from above downward. This expansion, termed the glans penis, is moulded on the rounded ends of the corpora cavernosa penis, extending farther on their upper than on their lower surfaces. At the summit of the glans is the slit-like vertical external urethral orifice. The circumference of the base of the glans forms a rounded projecting border, the corona of the glans, overhanging a deep retroglandular sulcus, behind which is the neck of the penis.
Divisions of the Penis
For descriptive purposes it is convenient to divide the penis into three regions: the root, the body, and the extremity. The root of the penis is triradiate in form, consisting of the diverging crura, one on either side, and the median bulb. Each crus is covered by the Ischiocavernosus, while the bulb is surrounded by the Bulbospongiosus. The root of the penis lies in the perineum between the inferior fascia of the urogenital diaphragm and the fascia of Colles. In addition to being attached to the fasciae and the pubic rami, it is bound to the front of the pubic symphysis by the fundiform and suspensory ligaments. The fundiform ligament springs from the front of the sheath of the Rectus abdominis and the linea alba; it splits into two fasciculi which encircle the root of the penis. The upper fibers of the suspensory ligament pass downward from the lower end of the linea alba, and the lower fibers from the pubic symphysis; together they form a strong fibrous band, which extends to the upper surface of the root, where it blends with the fascial sheath of the organ. The body extends from the root to the ends of the corpora cavernosa penis, and in it these corpora cavernosa are intimately bound to one another. A shallow groove which marks their junction on the upper surface lodges the deep dorsal vein of the penis, while a deeper and wider groove between them on the under surface contains the corpus spongiosum. The body is ensheathed by fascia, which is continuous above with thescarpas fascia, and below with the dartos fascia of the scrotum and the fascia of Colles. The extremity is formed by the glans penis, the expanded anterior end of the corpus spongiosum. The integument covering the penis is remarkable for its thinness, its dark color, its looseness of connectin with the deeper parts of the organ, and its absence of adipose tissue. At the root of the penis it is continuous with that over the pubes, scrotum, and perineum. At the neck it leaves the surface and becomes folded upon itself to form the prepuce or foreskin. The internal layer of the prepuce is immediately behind the external urethral orifice it forms a small secondary reduplication, attached along the bottom of a depressed median raphe, which extends from the meatus to the neck; this fold is termed the frenulum of the prepuce. The integument covering the glans is continuous with the urethral mucous membrane at the orifice; it is devoid of hairs, but projecting from its free surface are a number of small, highly sensitive palpillae. Scattered glands on the corona, neck, glans and inner layer of the prepuce. The prepuce covers a variable amount of the glans, and is separated from it by a potential sac- the preputial sac- which presents two shallow fossae, one on either side of the frenulum.
From the internal surface of the fibrous envelope of the corpora cavernosa penis, as well as from the sides of the septum, numerous bands or cords are given off, which cross the interior of these corpora cavernosa in all directions, subdividing them into a number of separate compartments, and giving the entire structure a spongy appearance. These bands and cords are called trabeculae, and consists of white fibrous tissue, and elastic fibers. In them are contained numerous arteries and nerves. The component fibers which form the trabeculae are larger and stronger around the circumference than at the centers of the corpora cavernosa; they are also thicker behind than in front. The cavernous spaces, on the contrary, are larger at the circumference, their long diameters being directed transversely. They are filled with blood, and are lined by a layer of flattened cells similar to the endothelial lining of veins. The fibrous envelope of the corpus spongiosum is thinner, whiter in color, and more elastic than that of the corpora cavernosa penis. The trabeculae are more delicate, nearly uniform in size, and the meshes between them smaller than in the corpora cavernosa penis. The external envelope or outer coat of the corpus spongiosum is a layer of the same tissue immediately surrounding the canal of the urethra.
Vessels and nerves
The arteries bringing the blood to the cavernous spaces are the deep arteries of the penis and branches from the dorsal arteries of the penis, which perforate the fibrous capsule, along the upper surface, especially near the forepart of the organ. On entering the cavernous structure the arteries divide into branches, which are supported and enclosed by the trabeculae. Some of these arteries end in a capillary network, the branches of which open directly into the cavernous spaces; others assume a tendril-like appearance, and form convoluted and somewhat dilated vessels. They open into the spaces, and from them are also given off small capillary branches to supply the trabecular structure. They are bound down in the spaces by fine fibrous processes, and are most abundant in the back part of the corpora cavernosa. The blood from the cavernous spaces is returned by a series of vessels, some of which emerge in considerable numbers from the base of the glans penis and converge on the dorsum of the organ to form the deep dorsal vein; others pass out on the upper surface of the corpora cavernosa and join the same vein; but the greater number pass out at the root of the penis and join the prostatic plexus.
The nerves are derived from the pudendal nerve and the pelvic plexuses.