Describe the rectum and anal canal. Explain the importance of the pectinate line and add a note on the ischiorectal fossa.

The Rectum

• It has NO sacculations, appendices epiploicae, mesentery or taeniae coli — the taeniae spread out to form a continuous longitudinal coat
• Three lateral curves, with corresponding internal folds — the transverse rectal folds (valves of Houston): two on the left and one on the right
• The dilated lower part is the ampulla
• Upper third — peritoneum on the front and sides
• Middle third — peritoneum on the front only
• Lower third — NO peritoneum; entirely extraperitoneal

The Anal Canal

• The pectinate (dentate) line lies at the level of the anal valves, about the middle of the canal
• Anal columns of Morgagni above it, joined below by the anal valves, behind which are the anal sinuses receiving the anal glands

Sphincters

Blood Supply and the Anal Cushions

• Superior rectal artery — the continuation of the inferior mesenteric; the chief supply of the rectum and upper anal canal
• Middle rectal artery — from the internal iliac
• Inferior rectal artery — from the internal pudendal
• The superior rectal artery divides into a left branch and two right branches, so the anal cushions lie at 3, 7 and 11 o'clock in the lithotomy position — the classical sites of primary internal haemorrhoids

The Ischiorectal Fossa

• Contents — ischiorectal fat (which allows the anal canal to distend), the inferior rectal vessels and nerve, and the perineal branch of S4
• Pudendal canal (of ALCOCK) — a fascial tunnel on the lateral wall containing the internal pudendal vessels and the pudendal nerve
• The two fossae communicate behind the anal canal, so infection can pass from one to the other — a horseshoe abscess

Defaecation

• If defaecation is deferred, the external sphincter and puborectalis contract voluntarily, the rectum accommodates, and the urge passes

Applied Aspects

• Internal haemorrhoids are painless (above the pectinate line, autonomic supply) and bleed bright red; external haemorrhoids and fissures are painful (below the line, somatic supply). This single distinction guides diagnosis and treatment
• Anal fissure is usually in the posterior midline, where the sphincter support is weakest and the blood supply poorest; it causes intense pain and sphincter spasm
• Ischiorectal abscess — the fat is poorly vascularised and readily infected, usually from an anal gland; it must be drained early, and may leave a fistula-in-ano
• Goodsall's rule — a fistula with an external opening in front of the transverse anal line opens into the canal radially; one behind it curves to open in the posterior midline
• Carcinoma of the rectum spreads to the internal iliac and para-aortic nodes; that of the anal margin below the pectinate line spreads to the superficial inguinal nodes — which must be examined
• Digital rectal examination palpates the prostate or cervix, the pouch of Douglas, and any rectal growth; about 60% of rectal carcinomas are within reach of the finger
• Rectal prolapse in children is associated with malnutrition and parasitic infestation, and in adults with a weak pelvic floor
• Pilonidal sinus occurs in the natal cleft above the coccyx, not in the anal canal, and differs from a fistula in having no internal opening
• Rectal biopsy is taken from the posterior wall above the pectinate line, where the mucosa is insensitive