The Penis

The penis, the male organ of copulation, is composed essentially of erectile tissue, and incloses the extrapelvic part of the urethra. It extends from the ischial arch forward between the thighs on to the umbilical region of the abdominal wall. It is supported by the fascia penis and the skin, and its prescrotal portion is situated in a cutaneous pouch, the prepuce or sheath. It is cylindrical in form, but much compressed laterally in the greater part of its extent.

In the quiescent state it is about 20 inches (ca. 50 cm.) long; of this, about 6 to 8 inches (ca. 15 to 20 cm.) is free in the prepuce. In erection it increases 50 per cent. or more in length.

It may be divided into a root, a body, and a terminal enlargement, the glans.

The root (Radix penis) is attached to the lateral parts of the ischial arch by two crura, which converge and unite below the arch (Fig. ). The urethra passes over the ischial arch between the crura, and curves forward to become incorporated with the penis.

The body (Corpus penis) begins at the junction of the crura and constitutes the bulk of the organ. At its origin it is attached to the symphysis ischii by two strong flat bands, the suspensory ligaments of the penis (Ligamenta suspensoria penis), which blend with the tendon of origin of the graciles muscles (Figs. , 456). This part of the penis is flattened laterally for the most part, but becomes rounded and smaller anteriorly. It presents four surfaces. The dorsal surface or dorsum penis, is narrow and rounded; on it are the dorsal arteries and nerves of the penis and a rich venous plexus. The ventral or urethral surface (Facies urethralis) is rounded, and along it runs the urethra, embedded in the deep urethral groove of the corpus cavernosum. The lateral surfaces are high and flattened, except anteriorly, where they are lower and rounded.

The glans penis is the enlarged free end of the organ. It is marked off from the body by a faintly marked neck (Collum glandis). In front of this is a prominent circular ridge, the corona glandis, which is notched below. The base of the glans is rounded and extends further backward dorsally than ventrally; it is marked in its lower part by a deep depression, the fossa glandis, in which the urethra protrudes for about an inch (ca. 2.5 cm.) as a free tube, the urethral process (Processus urethræ), covered by a thin integument. The urethra is thus surrounded by a circular fossa, which opens superiorly into the urethral sinus, a bilocular diverticulum lined by thin black skin. This diverticulum is filled sometimes with a caseous mass of sebaceous matter and epithelial débris.

Structure.—The penis consists essentially of two erectile bodies, the corpus cavernosum penis and the corpus spongiosum.

The corpus cavernosum penis forms the greater part of the bulk of the penis except at its free extremity. It arises from each side of the ischial arch by a crus, which is embedded in the ischio-cavernosus muscle. Below the ischial arch the crura fuse completely to form the laterally compressed body of the corpus cavernosum; this presents ventrally the urethral groove (Sulcus urethralis), which contains the urethra and corpus spongiosum. Anteriorly, the corpus cavernosum divides into three processes, a long central one, which is capped by the glans penis, and two short blunt lateral ones. The corpus cavernosum is inclosed by the tunica albuginea, a thick capsule of fibrous tissue which contains many elastic fibers and some unstriped muscle. Externally the fibers are chiefly longitudinal; internally they are mainly circular and are looser in arrangement. Numerous trabeculæ pass inward from the tunic and form a sponge-like framework in the interior of the corpus cavernosum, which is thus divided into numerous spaces (Cavernæ). These spaces may be regarded as greatly enlarged capillaries; they contain blood, are lined with flat endothelial cells, and are directly continuous with the veins of the penis. Their walls are composed very largely of unstriped muscle. Erection is produced by distention of these spaces with blood; at other times the spaces are mere slits.

In man there are two distinct corpora cavernosa, separated by a complete septum penis, except in the middle part of the organ, where the septum is composed of vertical trabeculæ, between which are slit-like intervals; through the latter the blood-spaces of the two corpora cavernosa communicate. In the horse no distinct septum exists except near the root, but in the proximal and distal parts of the corpus cavernosum there are vertical trabeculæ.

The corpus spongiosum or corpus cavernosum urethræ forms a tube around the urethra, and is much enlarged at its free end to form the bulk of the glans penis; throughout the remainder of its course it is practically uniform in size, and does not form any very distinct bulb (Bulbus urethræ) at its origin, as in man and many animals. The glans has been described in part; it should be noted that the corpus spongiosum extends backward dorsally a distance of about four inches (ca. 10 cm.) on the middle process of the corpus cavernosum penis, forming the processus dorsalis glandis. The urethral process is covered by a thin layer of corpus spongiosum.

In structure the corpus spongiosum has a general resemblance to the corpus cavernosum, but the trabeculæ are finer and more elastic. In the glans the trabeculæ are highly elastic, and the spaces are large and very extensible; the latter are continuous at the extremity of the processus dorsalis with large veins on the dorsum penis. There is a partial septum glandis. The skin covering the glans is thin, destitute of glands, and richly supplied with nerves and special nerve-endings.

Vessels and Nerves.—The penis is supplied with blood by three arteries, viz., the internal pudic, obturator, and external pudic. The veins form a rich plexus on the dorsum and sides of the penis, which is drained by the external pudic and obturator veins; from the root the blood is carried by the internal pudic veins. The lymphatics run with the veins and go to the inguinal and lumbar glands. The nerves are derived chiefly from the pudic nerves and the pelvic plexus of the sympathetic.