Structure
The colon is recognised by three surface features that the small intestine lacks: taeniae coli, haustra and omental appendices.
- Taeniae coli: three bands of longitudinal muscle. They converge on the base of the appendix and spread out to form a continuous coat at the rectosigmoid junction.
- Haustra: sacculations produced because the taeniae are shorter than the wall.
- Omental appendices (appendices epiploicae): small tags of fat on the serosa.
The rectum has none of these features.
Parts
| Part | Position | Peritoneum |
|---|---|---|
| Caecum | Right iliac fossa, below the ileocaecal junction | Usually intraperitoneal, without a true mesentery |
| Appendix | From the posteromedial caecum, below the ileocaecal junction | Intraperitoneal, with a mesoappendix |
| Ascending colon | Right flank to the right colic (hepatic) flexure under the liver | Retroperitoneal |
| Transverse colon | Hepatic to left colic (splenic) flexure, hanging in a loop | Intraperitoneal, on the transverse mesocolon |
| Descending colon | Splenic flexure to the pelvic brim in the left flank | Retroperitoneal |
| Sigmoid colon | Pelvic brim to S3 | Intraperitoneal, on the sigmoid mesocolon |
| Rectum | S3 to the anorectal junction, following the sacral curve | Upper third covered in front and at the sides, middle third in front only, lower third none |
The splenic flexure lies higher and more posterior than the hepatic flexure and is held by the phrenicocolic ligament. The appendix has a constant base but a variable tip: retrocaecal is the commonest position, followed by pelvic. Its base lies deep to McBurney's point, one-third of the way from the anterior superior iliac spine to the umbilicus.
Relations
The large intestine forms a frame around the small bowel, and its fixed retroperitoneal parts lie directly on the posterior abdominal wall.
- Ascending colon: on iliacus, quadratus lumborum and the lower pole of the right kidney.
- Transverse colon: below the stomach, attached to it by the greater omentum; the transverse mesocolon crosses the second part of the duodenum and the pancreas.
- Descending colon: on the left kidney, quadratus lumborum and iliacus.
- Sigmoid colon: the root of its mesocolon forms an inverted V over the left common iliac vessels, with the left ureter at the apex.
- Rectum: behind lie the sacrum, coccyx and median sacral vessels; in front lie the bladder, seminal glands and prostate in males, or the rectouterine pouch, uterus and vagina in females.
Blood supply and innervation
The large intestine up to about two-thirds of the way along the transverse colon is midgut and supplied by the superior mesenteric artery; the rest is hindgut and supplied by the inferior mesenteric artery.
| Artery | Parent | Territory |
|---|---|---|
| Ileocolic | SMA | Terminal ileum, caecum, appendix (via the appendicular artery), start of ascending colon |
| Right colic | SMA (inconstant) | Ascending colon |
| Middle colic | SMA | Transverse colon |
| Left colic | IMA | Distal transverse and descending colon |
| Sigmoid arteries | IMA | Sigmoid colon |
| Superior rectal | IMA | Upper rectum |
| Middle and inferior rectal | Internal iliac and internal pudendal | Lower rectum and anal canal |
The marginal artery (of Drummond) links these branches along the mesenteric border. Two areas depend on thin connections and are prone to ischaemic colitis: the splenic flexure, between SMA and IMA territories, and the rectosigmoid junction. The appendicular artery is an end artery, so an inflamed appendix can become gangrenous.
Veins follow the arteries; the inferior mesenteric vein joins the splenic vein. Lymph drains through nodes on the colon wall and along the arteries to nodes at the SMA and IMA origins. Parasympathetic supply is from the vagus to the midgut part and from the pelvic splanchnic nerves (S2 to S4) to the hindgut part. Midgut pain is referred to the umbilicus and hindgut pain to the suprapubic region.
Clinical relevance
The arterial territories and lymphatic drainage define the standard colorectal resections, and the peritoneal arrangement explains volvulus and pain patterns.
- Appendicitis: early periumbilical visceral pain moves to the right iliac fossa when parietal peritoneum becomes inflamed; a retrocaecal appendix may give only flank pain and a positive psoas sign.
- Colectomy: a right hemicolectomy divides the ileocolic, right colic and right branch of the middle colic arteries; a left hemicolectomy or anterior resection takes the IMA branches with their nodes.
- Ureter at risk: the left ureter lies under the apex of the sigmoid mesocolon and must be identified in left-sided resections.
- Sigmoid volvulus: a long sigmoid mesocolon with a narrow root lets the loop twist.
- Diverticular disease: commonest in the sigmoid, where mucosa herniates at points where vessels pierce the wall.
- Portosystemic anastomosis: superior rectal (portal) and middle and inferior rectal (systemic) veins meet in the rectal wall.