Rectum and Anal Canal
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Direct answer
Twelve centimetres long, curving with the sacrum from the rectosigmoid junction at S3 to the anorectal ring, the rectum loses every colonic feature — no taeniae coli (they spread into a complete coat), no sacculations, no appendices epiploicae, no mesentery — and shows three lateral flexures whose internal folds are the valves of Houston. Below it, the 4 cm anal canal runs to the anal verge, and everything of clinical consequence in it — pain, epithelium, blood supply, lymph drainage, embryology — switches at the dentate line, 2 cm from the verge.
What you must remember
- Rectum: about 12 cm, with a dilated ampulla above the pelvic floor; upper third covered by peritoneum in front and at the sides, middle third in front only, lower third extraperitoneal.
- Valves of Houston: three transverse folds — left, right, left from below upwards — negotiated during sigmoidoscopy.
- Pouch of Douglas or rectovesical pouch: the lowest peritoneal point, about 7.5 cm from the anal verge in the male, slightly less in the female — within reach of the examining finger.
- Anorectal ring: puborectalis with the deep external sphincter and upper internal sphincter; division of the ring sacrifices continence.
- Sphincters: internal — involuntary smooth muscle, the resting tone; external — voluntary striated muscle in subcutaneous, superficial and deep parts, supplied by the inferior rectal branch of the pudendal nerve (S2, S3, S4 — "S2, 3, 4 keep the gut off the floor").
- Dentate line, the great divide: above — columnar epithelium (endoderm), visceral afferents so painless, superior rectal vessels, lymph to inferior mesenteric and internal iliac nodes; below — stratified squamous (ectoderm), somatic inferior rectal nerve so painful, inferior rectal vessels, lymph to superficial inguinal nodes.
- Anal glands: opening into the anal sinuses at the dentate line, they cause most perianal abscesses and fistulas when infected.
- Haemorrhoids: internal plexus above the line — painless bleeding, superior rectal vein draining to the portal system; external plexus below — painful, inferior rectal vein to the systemic circulation.
One line of examination, two worlds of anatomy
A patient reports painless bright bleeding; proctoscopy shows purple swellings at three, seven and eleven o'clock — the classic positions of internal haemorrhoids. The line explains them: above it the superior rectal vein lies in unsupported submucosa of the anal columns; straining engorges the cushions and they slide down, and because the mucosa carries visceral afferents the bleeding is painless. Below the line the picture inverts: thrombosed external haemorrhoids under the inferior rectal nerve are exquisitely painful.
The same line stages a cancer. A tumour above it drains to the inferior mesenteric and para-aortic nodes, growing silently; a tumour below it drains to the superficial inguinal nodes, so both groins are examined. Sphincter preservation hangs on the ring: dividing the puborectalis sling commits the patient to a permanent stoma. Infection follows the map too — a blocked anal gland at the line seeds a perianal abscess tracking through the ischioanal fossa, and the fistula's internal opening lies at the anal crypts.
Where students slip
The anal verge and the dentate line are treated as one landmark; they are about 2 cm apart, and the surgical anal canal runs from the dentate line to the verge while the anatomical canal is measured from the anorectal ring — examiners ask for both definitions. The sphincters are blurred: internal is involuntary smooth muscle holding resting tone, external is voluntary striated muscle providing the squeeze, so a pudendal block abolishes the squeeze but not the resting pressure. Lymphatics below the line are wrongly sent to pelvic nodes — they go to the superficial inguinal nodes, which is why anal tumours below the line present with groin nodes. And the peritoneal story is muddled: only the upper third is clothed in front and at the sides, the middle third in front alone, the lower third bare — which is why a low rectal biopsy does not enter the peritoneal cavity.
Frequently asked questions
Why is the dentate line the key landmark of the anal canal?
Across it change the epithelium (columnar to squamous), nerve supply (painless visceral to painful somatic), blood supply and lymph drainage — endoderm meeting ectoderm.
Why do internal haemorrhoids bleed painlessly?
They arise from the superior rectal venous plexus above the dentate line, where visceral afferents carry no pain; external haemorrhoids below the line under the inferior rectal nerve are painful.
What is the anorectal ring, and why must the surgeon preserve it?
The puborectalis sling with the deep external sphincter and upper internal sphincter at the anorectal junction, palpable on digital examination; dividing it destroys the angulation and squeeze that maintain continence.
What is the nerve supply of the external anal sphincter?
The inferior rectal branch of the pudendal nerve from S2, S3 and S4; the internal sphincter, smooth muscle, holds resting tone without somatic supply. The pouch of Douglas, 7.5 cm from the verge, is within reach of the examining finger.