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.
Muscles
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.
Vessels
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.
Cutaneous innervation
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).
Lymphatic drainage
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.
