Male External Genitalia Anatomy
The male external genitalia comprise the scrotum and the penis. An account of the scrotum and its contents is given on pp 149–151. The penis consists of a shaft, which is free, and a root, which lies in the superficial perineal pouch, attached to the inferior surface of the perineal membrane.
The perineal membrane is a shelf of dense fascia spanning the triangular interval between the right and left ischial rami and inferior pubic rami (Fig. 5.34). The membrane has a long free posterior margin and a short anterior margin that is separated from the pubic symphysis by a small gap transmitting the deep dorsal vein of the penis. The central part of the membrane is pierced by the urethra and bulbourethral ducts.
At the root of the penis and around the neck of the scrotum, the membranous layer of perineal subcutaneous tissue attaches laterally to the ischial and pubic rami. Posteriorly, the fascia is anchored to the free edge of the perineal membrane. The space contained by the membranous layer of the penis and the scrotum is called the superficial perineal pouch. This pouch communicates with the subcutaneous tissues of the anterior abdominal wall but not with the ischioanal fossae, the thighs or pelvic cavity. If the spongy part of the urethra is ruptured, urine may escape into the superficial pouch (superficial extravasation of urine, p. 140).
On the pelvic aspect of the perineal membrane lie the external urethral sphincter and bulbourethral glands in a space often called the deep perineal pouch. The term urogenital diaphragm is a misnomer sometimes applied to the deep perineal pouch and its boundaries.
The erectile tissue of the shaft of the penis consists of the paired corpora cavernosa, lying in apposition, and the midline corpus spongiosum.
The corpus spongiosum is uniform in diameter except at its extremity, where it expands into the glans (Fig. 5.19), whose prominent margin forms the corona of the penis. Proximally, the corpus spongiosum continues into the root of the penis to form the bulb, which is attached to the inferior surface of the perineal membrane (Fig. 5.38). The urethra pierces the perineal membrane, enters the bulb from above and curves downwards and forwards. It traverses the corpus spongiosum and glans (Fig. 5.19) and terminates at the external urethral meatus near the apex of the glans. Dorsal to the corpus spongiosum are the paired corpora cavernosa (Fig. 5.39), which extend distally as far as the concave proxi- mal surface of the glans. Proximally, the corpora cavernosa continue inferior to the pubic symphysis and diverge as the crura. Each crus tapers posteriorly and is attached to the inferior surface of the perineal membrane and the adjacent rami of the pubis and ischium (Figs 5.34 & 5.38).
The three corpora of the penis are enveloped by a sleeve of deep fascia, which also covers the dorsal vessels and nerves of the organ (Fig. 5.39). Proximally, the deep fascia is anchored to the front of the pubic symphysis by the suspensory ligament of the penis (Fig. 5.37), while distally it terminates at the corona by fusing with the corpora. Covering the deep fascia is the superficial fascia.
The subcutaneous tissue of the penis is membranous, devoid of fat and traversed by superficial nerves and vessels (Fig. 5.39). Proximally, it is continuous with the subcutaneous tissue of the anterior abdominal wall and can be traced around the scrotum, where it contains smooth muscle, the dartos.
The cutaneous covering of the penis is freely mobile except over the glans to which it is adherent. The skin is reflected distally beyond the corona over the glans to form the hood-like prepuce or foreskin. This is attached to the undersurface of the glans by a vascular fold, the frenulum. Removal of the foreskin (circumci- sion) is commonly performed for both medical and religious reasons.
Surrounding the bulb of the penis are the paired bulbospongiosus muscles (Fig. 5.38). Their fibres attach to the perineal membrane and pass downwards and backwards to meet at a midline raphe and blend posteriorly with the external anal sphincter. The bulbospongiosus muscles contract during ejaculation and the terminal stages of micturition to compress the urethra and expel its contents.
Covering each crus is the ischiocavernosus muscle (Fig. 5.38). From the posterior end of each crus, a small superficial transverse perineal muscle runs medially to the penile bulb. All the penile muscles are innervated by the perineal branch of the pudendal nerve.
The erectile tissues of the penis have a rich blood supply. Each internal pudendal artery (p. 241) provides branches to the bulb and to the corresponding crus before terminating as the dorsal and deep arteries. The deep artery traverses the length of the corpus cavernosum, while the artery to the bulb continues along the corpus spongiosum to reach the glans. Diseases affecting these arteries or their autonomic supply may lead to erectile dysfunction (impotence). The principal venous drainage is via the midline deep dorsal vein, which runs beneath the deep fascia (Fig. 5.39). This vein is accompanied by the dorsal arteries and passes inferior to the pubic symphysis into the pelvic cavity, where it terminates in the prostatic venous plexus (Fig. 5.21). From this plexus, blood drains into either the internal iliac veins or the internal pudendal veins.
Erection of the penis is controlled by parasympathetic nerves (the pelvic splanchnic nerves or nervi erigentes) from the sacral segments of the spinal cord. Stimulation of these nerves causes arterial dilatation and simultaneous venous constriction. This results in engorgement of the erectile tissues leading to enlargement and stiffening of the penis. Diseases affecting these arteries or their autonomic supply may lead to erectile dysfunction.
The skin of the penis and the front of the scrotum are supplied by the external pudendal branches of the femoral arteries. The superficial veins of the penis and the anterior part of the scrotum drain via the external pudendal veins, which are tributaries of the great saphenous vein (Fig. 6.11). The posterior part of the scrotum is supplied by posterior scrotal branches from the internal pudendal artery, and venous blood passes into the internal pudendal veins.
The pudendal nerve supplies most of the penis and scrotum via its dorsal and posterior scrotal branches. However, the anterior part of the scrotum and the proximal part of the shaft of the penis are innervated by the ilioinguinal nerve, which descends from the superficial inguinal ring (Fig. 4.21).
Lymph from the superficial tissues of the penis and scrotum passes to the superficial inguinal nodes, while that from the deeper tissues is conveyed via lymphatic vessels accompanying the internal pudendal artery and passes to the internal iliac nodes.