Below the pelvic floor lies the perineum, a superficial region traversed by the anal canal and the lower parts of the genital and urinary tracts. The perineum is diamond-shaped, extending anteriorly to the pubic symphysis, posteriorly to the coccyx and laterally to the ischial tuberosities (Figs 5.34 & 5.3). On each side, the region is bounded by the conjoined rami of the ischium and pubis and by the sacrotuberous ligament, which is overlapped by the inferior border of gluteus maximus. Inferiorly, the perineum is bounded by skin. By convention, the perineum is divided into two triangular areas by a line joining the ischial tuberosities. Posteriorly is the anal triangle, containing the anal canal and the ischioanal (ischiorectal) fossae, and anteriorly lies the urogenital triangle, containing the external genitalia.
The anal canal is the terminal part of the intestine and is approximately 4 cm long. Beginning at the anorectal junction, it passes downwards and backwards as far as the anus (Fig. 5.5). Its upper part is lined by mucous membrane bearing several longitudinal ridges, the anal columns; the lower part is lined by skin. The smooth muscle coat of the rectum continues into the wall of the anal canal and thickens to form the internal anal sphincter. Striated fibres from the levator ani muscles (Fig. 5.35) blend with the outer layers of the wall and continue as far as the perianal skin.
In addition, the lower two-thirds of the anal canal are encircled by the external anal sphincter (Fig. 5.36), composed of striated muscle fibres. This sphincter comprises three parts, of which the uppermost, the deep part, blends with the levator ani muscles. Inferior to this lies the superficial part, attached posteriorly to the coccyx and anococcygeal raphe and anteriorly to the posterior border of the perineal membrane. The most inferior component of the sphincter, the subcutaneous part, encircles the anal opening. Continence of faeces is not dependent on the external sphincter alone; the fibres of the levator ani muscles that maintain the anorectal angulation play a major role.
The anal canal is supplied by inferior rectal branches (Fig. 5.36) from the pudendal nerve, which innervate the external sphincter and the cutaneous lining (p. 243). The internal sphincter and the mucous membrane lining the upper part are innervated, like the rectum, by autonomic nerves. The blood supply is provided by inferior rectal branches of the internal pudendal artery. The anal canal is a site of portacaval anastomosis because venous blood passes not only via inferior rectal veins to the internal iliac veins but also into the superior rectal vein (p. 185). These anastomoses dilate in the presence of portal hypertension. The lining of the lower part of the anal canal has a rich nerve supply. Tears of the mucosa of the lower part of the anal canal may result in painful fissures. Lymph drains from the upper part of the canal to the internal iliac nodes, but from the lower part, it passes to the superficial inguinal nodes. Therefore, a patient with an anal tumour may present with inguinal lymphadenopathy. Lymph drains from the upper part of the canal to the internal iliac nodes, but from the lower part, it passes to the superficial inguinal nodes.
On each side of the anal canal is a fat-filled space extending laterally as far as obturator internus, upwards to the levator ani muscle and downwards to the perianal skin (Fig. 5.35). The fossae communicate behind the anal canal. Anteriorly, each fossa tapers and continues above the perineal membrane and external urethral sphincter almost to the pubic symphysis. Entering each fossa from the gluteal region via the lesser sciatic foramen are the nerve to obturator internus, the pudendal nerve (Fig. 5.37) and the internal pudendal artery with its venae comitantes. Although infection in this fossa (ischiorectal abscess) often requires surgical incision, the vessels and nerves supplying the anal canal are protected by their superior location within the fossa and are rarely damaged.
The nerve to obturator internus arises from the sacral plexus and supplies the muscle from within the ischioanal fossa (Fig. 5.28).
The pudendal nerve and the internal pudendal vessels pass along the lateral wall of the fossa in a fascial tunnel, the pudendal canal. Branches from the nerve and artery traverse the upper part of the fossa to supply the anal canal and the posterior part of the scrotum or labium majus (Fig. 5.36).
Internal pudendal artery
This vessel provides most of the arterial blood to the perineum. Arising from the internal iliac artery within the pelvis (Fig. 5.32), it enters the buttock through the greater sciatic foramen and then traverses the lesser sciatic foramen to enter the ischioanal fossa. The artery runs forwards along the lateral wall of the fossa in the pudendal canal and gives off inferior rectal branches and poste- rior scrotal or labial branches. The artery continues above the perineal membrane, where it provides branches to the bulb of the penis or vestibule and crus of the penis or clitoris, and terminates by dividing into deep and dorsal arteries of the penis or clitoris. In the male, the dorsal artery passes below the pubic symphysis and continues along the dorsum of the penile shaft (Fig. 5.37), lateral to the deep dorsal vein, and gives branches to the erectile tissue of the corpus cavernosum. The deep artery pierces the perineal membrane and supplies the erectile tissue of the corpus cavernosum. In the female, there are comparable but smaller branches to the clitoris. The internal pudendal artery and its branches are accompanied by venae comitantes, which drain into the internal iliac vein.
The pudendal nerve provides the principal innervation to the perineum. It arises from the sacral plexus (S2, S3 & S4) and accompanies the internal pudendal artery into the perineum. In the ischioanal fossa, the nerve divides into the dorsal nerve of the penis (or clitoris) and the perineal nerve (Fig. 5.37).
The perineal nerve supplies an inferior rectal branch to the anal canal and posterior scrotal or labial branches to the skin of the genitalia (Fig. 5.36). On reaching the anterior part of the perineum, the perineal nerve gives branches to all muscles in the superficial perineal pouch, as well as sensory branches to the urethra.
The dorsal nerve of the penis continues forwards with the internal pudendal artery above the perineal membrane and below the pubic symphysis. It runs along the dorsum of the penis, lateral to the dorsal artery (Fig. 5.39), and innervates the skin of the distal two-thirds of the organ. The dorsal nerve of the clitoris has a similar course and distribution.