Large intestine and anal canal
Large Intestine, Appendix, Rectum and Anal Canal: Anatomy for FCPS
Colon, appendix, rectum and anal canal for FCPS Part 1: identifying features, blood supply, pectinate line, sphincters and fascial planes. Links the anatomy to appendicitis, volvulus, haemorrhoids, Hirschsprung disease and anorectal sepsis.
Identifying the colon
- Taeniae coli: three bands of outer longitudinal muscle. Haustra: sacculations because the taeniae are shorter than the bowel. Appendices epiploicae: fat-filled peritoneal tags.
- The taeniae converge on the base of the appendix (a guide at surgery) and spread into a complete longitudinal coat at the rectosigmoid junction (about S3). That change, with loss of appendices epiploicae and mesentery, is how a surgeon knows the sigmoid has become the rectum.
- On X-ray, colonic haustra cross only part of the bowel width; small bowel plicae (valvulae conniventes) cross the full width.
- Histology: colon has straight crypts full of goblet cells, no villi; the gallbladder has no goblet cells, no muscularis mucosae and no submucosa.
- The caecum has the widest diameter, so it perforates first in large bowel obstruction with a competent ileocaecal valve (closed loop).
Peritoneal attachments and the plane of Toldt
- Transverse colon hangs on the transverse mesocolon, attached along the anterior border of the pancreas; sigmoid on the sigmoid mesocolon.
- Ascending and descending colon are secondarily retroperitoneal: their embryonic mesentery fused with the posterior wall, leaving the fusion fascia of Toldt. Incising the peritoneum lateral to the colon (white line of Toldt) and lifting the colon medially opens a nearly bloodless plane. Going too deep enters Gerota's fascia and endangers the ureter and gonadal vessels.
- Sigmoid volvulus: long sigmoid mesocolon on a narrow base; elderly, constipated, institutionalised patients; massive distension; coffee-bean loop arising from the left iliac fossa/pelvis. First treatment is endoscopic (sigmoidoscopic) decompression with a flatus tube, then elective resection. Caecal volvulus affects younger patients with a mobile caecum.
Appendix
- Arises from the posteromedial caecum about 2 cm below the ileocaecal valve. Positions: retrocaecal commonest (about 65%), then pelvic, subcaecal, pre- and post-ileal.
- Base at McBurney's point: one-third of the way from the ASIS to the umbilicus.
- Appendicular artery: branch of the ileocolic artery (SMA) in the mesoappendix; an end artery, so obstruction quickly causes gangrene and perforation.
- Lymph drains to ileocolic nodes, then superior mesenteric nodes.
- Pain: early periumbilical pain carried by visceral afferents with sympathetic fibres to T10 (midgut); later right iliac fossa pain when parietal peritoneum is inflamed. Retrocaecal: psoas sign; pelvic: diarrhoea, frequency, rectal tenderness.
- Mucinous neoplasm of the appendix can rupture and fill the peritoneum with mucin (pseudomyxoma peritonei).
Blood supply and lymph
| Segment | Artery | Lymph nodes |
|---|---|---|
| Caecum, ascending, proximal two-thirds transverse | SMA: ileocolic, right colic, middle colic | Superior mesenteric |
| Distal third transverse, descending, sigmoid | IMA: left colic, sigmoid | Inferior mesenteric |
| Upper rectum | Superior rectal (IMA), the main supply of the rectum | Inferior mesenteric |
| Middle and lower rectum | Middle rectal (internal iliac) | Internal iliac |
| Anal canal below pectinate line | Inferior rectal (internal pudendal) | Superficial inguinal |
- Marginal artery (of Drummond) links SMA and IMA along the colon, so IMA ligation in a healthy patient is usually tolerated.
- Ischaemia from hepatic flexure to mid-transverse colon: middle colic artery. SMA occlusion spares the descending and sigmoid colon (IMA territory).
- Watershed areas: splenic flexure (Griffiths' point) and rectosigmoid (Sudeck's point).
Rectum
- 12–15 cm, from S3 to the anorectal junction, following the sacral curve; no taeniae, appendices epiploicae or true mesentery, but a fatty mesorectum holding nodes and vessels.
- Peritoneum: upper third front and sides, middle third front only, lower third none.
| Fascia | Position | Importance |
|---|---|---|
| Denonvilliers fascia (rectovesical septum) | Anterior, between rectum and prostate, seminal vesicles, bladder | Anterior plane in total mesorectal excision; barrier between rectum and bladder |
| Waldeyer fascia (rectosacral) | Posterior, between mesorectum and sacrum | Tearing it causes presacral venous bleeding |
| Mesorectal fascia | Envelope of the mesorectum | Removed intact in total mesorectal excision |
- Anterior relations: men, bladder, seminal vesicles, prostate; women, uterus, vagina, rectouterine pouch.
Anal canal and the pectinate line
About 4 cm. The pectinate line marks the junction of hindgut endoderm and ectodermal proctodeum. The urorectal septum divides the cloaca into the rectum and anal canal behind and the urogenital sinus in front.
| Feature | Above the pectinate line | Below |
|---|---|---|
| Epithelium | Columnar | Stratified squamous |
| Artery | Superior rectal (IMA) | Inferior rectal (internal pudendal) |
| Vein | Superior rectal to portal | Inferior rectal to systemic |
| Lymph | Internal iliac, inferior mesenteric | Superficial inguinal |
| Nerve | Autonomic: painless | Somatic inferior rectal (pudendal): painful |
| Lesions | Internal haemorrhoids, adenocarcinoma | External haemorrhoids, fissure, squamous carcinoma |
Haemorrhoids and fissures
- Haemorrhoids are enlarged anal cushions (submucosal vascular tissue with smooth muscle) at 3, 7 and 11 o'clock in lithotomy.
- The blood is bright red because the cushions contain direct arteriovenous communications from the superior rectal arteries; it is arterial-like, not portal venous blood.
- Haemorrhoids are not varices: anorectal varices in portal hypertension are a separate entity, and portal hypertension does not make haemorrhoids more common.
- Anal fissure: below the pectinate line, posterior midline, painful, with internal sphincter spasm.
Sphincters and continence
- Internal anal sphincter: thickened circular smooth muscle; involuntary; maintains resting tone.
- External anal sphincter: skeletal, voluntary; inferior rectal nerve (pudendal, S2–S4).
- Puborectalis slings behind the anorectal junction and pulls it forwards, creating the anorectal angle. During defecation it relaxes, straightening the angle; with straining and IAS relaxation, stool passes.
- Anorectal ring (puborectalis plus upper EAS and IAS) must be preserved in fistula surgery.
- Sphincter injury after childbirth is assessed by anorectal manometry and endoanal ultrasound.
Anorectal sepsis
- Spaces: perianal, ischioanal, intersphincteric, supralevator, and the deep postanal space behind the anal canal, which connects the two ischioanal fossae. Spread through it gives a horseshoe abscess.
- Goodsall's rule: external openings behind a transverse line through the anus track curvilinearly to the posterior midline; those in front run radially.
Paediatric conditions
- Hirschsprung disease: failure of neural crest migration leaves the distal segment aganglionic and narrow; the dilated bowel above is normal. No meconium in 48 h; suction biopsy shows no submucosal ganglion cells and increased acetylcholinesterase-positive nerve fibres. Chagas disease destroys the myenteric plexus, causing acquired megacolon.
- Intussusception (6–18 months): ileocolic; lead point at this age is hypertrophied Peyer patches after a viral infection; colicky screaming, sausage-shaped mass, red-currant jelly stool. In older children seek a pathological lead point (Meckel's, polyp, lymphoma).
- Lynch syndrome (mismatch repair genes, e.g. MLH1, MSH2): colorectal plus endometrial cancer in the family.
Remember Taeniae meet at the appendix and spread out at the rectum; the pectinate line splits everything into two halves.
Exam trap Denonvilliers fascia is anterior to the rectum; Waldeyer fascia is posterior.
Exam trap In Hirschsprung disease the narrow distal segment is diseased; the dilated part is normal.
How FCPS asks about this
- How the surgeon recognises the rectum: taeniae spread into a complete coat; no appendices epiploicae.
- Commonest appendix position and artery: retrocaecal; appendicular branch of ileocolic (SMA).
- Lymph below the pectinate line: superficial inguinal nodes; lining is squamous and pain-sensitive.
- Bloodless plane for mobilising the colon: fusion fascia of Toldt; ureter at risk if too deep.
- Layer between rectum and prostate: Denonvilliers fascia.
- Coffee-bean sign in an elderly man: sigmoid volvulus; endoscopic decompression first.
- Horseshoe abscess: spread through the deep postanal space.
Test yourself on this topic
5 of the 79 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
With a patient in the lithotomy position, at which clock positions are the primary internal haemorrhoids usually found?
Answer: 3, 7 and 11 o'clock
Internal haemorrhoids are cushions of the anal canal above the dentate line, containing branches of the superior rectal vessels. The main cushions lie at the left lateral, right anterior and right posterior positions, which are 3, 7 and 11 o'clock in the lithotomy position. Secondary haemorrhoids may form between them.
Haemorrhoid positions
- Lithotomy: 3, 7, 11 o'clock
- Left lateral, right anterior, right posterior
- Above the dentate line
Remember 3, 7, 11: the odd haemorrhoid clock.
Exam trap An anal fissure is usually at 6 o'clock (posterior midline).
Why the other options are wrong
- 12, 4 and 8 o'clock Not the classic positions.
- 1, 5 and 9 o'clock Not the classic positions.
- 6 o'clock only Typical site of an anal fissure.
- 2, 6 and 10 o'clock Not the classic positions.
In colorectal surgery, which of the following sites is the classical watershed between the lowest sigmoid artery and the superior rectal artery, where perfusion may be poor if the communicating branch is divided?
Answer: Rectosigmoid junction (Sudeck point)
Sudeck critical point lies at the rectosigmoid junction, between the last sigmoid branch and the superior rectal artery. If the superior rectal artery is tied below the last sigmoid branch, the communication may be lost and the rectosigmoid can become ischaemic. Griffith point is the other colonic watershed, at the splenic flexure where the SMA and IMA territories meet through a weak marginal artery. Knowing both helps surgeons choose levels of ligation and anastomosis.
Colonic watershed points
- Griffith: splenic flexure (SMA–IMA)
- Sudeck: rectosigmoid (sigmoid–superior rectal)
- Ischaemia risk in low flow or ligation
- Plan ligation levels around them
Remember Griffith at the bend, Sudeck at the bottom.
Exam trap Do not confuse the two watersheds; the splenic flexure is the commoner site of ischaemic colitis.
Why the other options are wrong
- Splenic flexure (Griffith point) The SMA–IMA watershed, a different site.
- Hepatic flexure Well supplied by right and middle colic.
- Ileocaecal junction Well supplied by the ileocolic artery.
- Anorectal junction Well supplied by middle and inferior rectal arteries.
In adhesive obstruction, X-ray shows central dilated loops crossed completely by thin, closely spaced folds. Which of the following are these folds?
Answer: Valvulae conniventes
Valvulae conniventes (plicae circulares) are permanent circular folds of mucosa and submucosa in the small intestine, most prominent in the jejunum. On X-ray, dilated small bowel shows them as thin lines crossing the whole width of the loop, giving a stacked-coin appearance, and the loops lie centrally. Colonic haustra are thicker, more widely spaced and cross only part of the width, and the colon lies peripherally.
Small vs large bowel X-ray
- Small bowel: valvulae conniventes cross fully
- Central loops, smaller diameter
- Colon: haustra cross partly
- Colon peripheral, may contain faeces
Remember Lines all the way across: small bowel.
Exam trap The terminal ileum has few folds and can look featureless.
Why the other options are wrong
- Haustra Colonic; thick and cross only part of the width.
- Taeniae coli Longitudinal muscle bands of the colon.
- Appendices epiploicae Fat tags on the colon; not seen as folds.
- Peyer patches Lymphoid tissue; not seen on plain X-ray.
After shock, colonoscopy shows ischaemia from the hepatic flexure to the mid-transverse colon. Which of the following arteries supplies this segment?
Answer: Middle colic artery
The middle colic artery arises from the superior mesenteric artery as it enters the small bowel mesentery, runs in the transverse mesocolon and supplies the proximal two-thirds of the transverse colon from the hepatic flexure. The right colic supplies the ascending colon, the ileocolic the caecum and terminal ileum, and the left colic (from the IMA) the distal transverse and descending colon. Its branches join the marginal artery, which may be poor at the splenic flexure.
Colic arteries
- Ileocolic: caecum, terminal ileum
- Right colic: ascending colon
- Middle colic: proximal two-thirds of transverse
- Left colic (IMA): distal third, descending
Remember Middle colic, middle of the colon.
Exam trap The left colic artery comes from the IMA, not the SMA.
Why the other options are wrong
- Right colic artery Supplies the ascending colon.
- Left colic artery From the IMA; supplies the distal transverse and descending colon.
- Ileocolic artery Supplies the caecum, appendix and terminal ileum.
- Sigmoid arteries Supply the sigmoid colon.
A 22-year-old man has right iliac fossa and flank pain with fever but little anterior tenderness. Pain increases when the right hip is passively extended while he lies on his left side. Where is the appendix most likely to lie?
Answer: Retrocaecal
A retrocaecal appendix lies behind the caecum on the posterior abdominal wall, in contact with the iliopsoas. Inflammation irritates psoas, so stretching it by extending the hip causes pain (psoas sign), and the patient may hold the hip slightly flexed. Because the caecum lies in front, anterior tenderness and guarding can be mild, and pain may be felt in the flank. A pelvic appendix instead irritates obturator internus (obturator sign) and the rectum or bladder.
Psoas sign
- Hip extension stretches psoas
- Positive in retrocaecal appendix
- Little anterior tenderness
- Pelvic appendix: obturator sign
Remember Pain on hip extension: appendix behind caecum.
Exam trap A retrocaecal appendix can give few anterior signs and be missed.
Why the other options are wrong
- Pelvic Gives an obturator sign and pelvic symptoms.
- Pre-ileal Lies in front of the ileum, away from psoas.
- Subcaecal Lies below the caecum, in the iliac fossa.
- Paracaecal (anterior) Gives marked anterior tenderness.
Where this sits in the CPSP syllabus
- Paper 1: Abdomen, Large intestine, appendix, rectum and anal canal
- Surgery & Allied, Paper 2: Abdomen, Large intestine, appendix, rectum and anal canal