Large Intestine

Taeniae coli: Three longitudinal bands of thickened outer (longitudinal) smooth muscle, running the length of the colon from the caecum to the rectosigmoid junction (where they merge into a complete circumferential layer around the rectum). They are named the free, mesocolic, and omental taeniae by their relationship to the mesentery and appendices epiploicae attachments.

• Haustra: Sacculations of the colonic wall between the taeniae, produced by the taeniae being shorter than the underlying bowel length, causing the wall to pucker into pouches, a key radiological and endoscopic distinguishing feature of the colon from the small intestine.
• Appendices epiploicae (omental appendices): Small, fat-filled peritoneal pouches projecting from the colonic serosal surface, most numerous along the transverse and sigmoid colon, of relatively minor functional significance but can undergo torsion (epiploic appendagitis), causing localised abdominal pain mimicking other acute abdominal pathology.

• The blind-ending pouch at the beginning of the large intestine, lying in the right iliac fossa, below the level of the ileocaecal junction.
• Usually intraperitoneal (though relatively immobile in most individuals, lacking a true mesentery in the majority, the peritoneum is continuous but does not typically form a free mesentery), though a variable minority have a more mobile caecum with a small mesocaecum.
• Receives the terminal ileum via the ileocaecal valve (a somewhat rudimentary sphincter-like structure, functionally limiting reflux of colonic contents into the ileum, though its physiological competence is variable and debated).

Base:

• Arises from the posteromedial wall of the caecum, at the confluence of the three taeniae coli.
• It is a reliable anatomical landmark for locating the appendiceal base even when the appendix itself is difficult to visualise, since the taeniae can be traced to converge precisely at its origin.
• The base position is relatively constant even though the tip's position varies considerably.

Positions (of the tip: highly variable):

• Retrocaecal (most common, ~60-65%), behind the caecum, often extraperitoneal in its course.
• Pelvic (descending): hanging down into the pelvis, may lie close to the bladder, rectum, or (in females) the ovary/uterine tube, relevant to symptom overlap with gynaecological or urinary pathology.
• Subcaecal: below the caecum.
• Pre-ileal and post-ileal: anterior or posterior to the terminal ileum respectively.
• This variability in tip position explains the correspondingly variable clinical presentation of appendicitis (e.g., a retrocaecal appendix may produce less classic anterior peritoneal signs, given its relatively protected/retroperitoneal-like position, while a pelvic appendix may cause urinary frequency or diarrhoea from irritation of adjacent pelvic structures).

Mesoappendix:

• A small, triangular mesentery suspending the appendix from the terminal ileal mesentery, carrying the appendicular vessels; typically does not reach all the way to the appendix tip, leaving the distal appendix relatively free.

Appendicular artery:

• A single, end-artery (typically the only significant blood supply, with minimal collateral circulation) arising from the ileocolic artery (a branch of the SMA), running within the mesoappendix.
• This end-artery nature is clinically critical. Occlusion (from luminal obstruction and consequent inflammation/pressure, or from the appendix twisting) rapidly compromises the entire appendiceal blood supply. This explains the relatively rapid progression from inflammation to ischaemic necrosis and perforation characteristic of appendicitis, compared to organs with richer collateral supply.

Surface marking:

• McBurney's point: one-third of the distance along a line drawn from the ASIS to the umbilicus, the classic surface landmark for the appendiceal base, and the site of maximal tenderness in classic (anterior, non-retrocaecal) appendicitis.

Ascending Colon

• Position: Right side of the abdomen, ascending from the caecum to the hepatic (right colic) flexure, below the liver.
• Peritoneal relationship: Secondarily retroperitoneal in most individuals (fused to the posterior abdominal wall during development), covered by peritoneum only on its anterior and lateral surfaces.
• Important relations: Right kidney (posteriorly), right lobe of liver and gallbladder (near the hepatic flexure).
• Blood supply: Ileocolic and right colic arteries, branches of the SMA.
• Venous drainage: Ileocolic and right colic veins, to the SMV.
• Lymphatics: Epicolic/paracolic nodes → intermediate (ileocolic/right colic) nodes → superior mesenteric nodes.
• Innervation: SMA plexus (sympathetic, lesser/least splanchnic nerves; parasympathetic (vagus)) midgut-pattern innervation, periumbilical referred pain for proximal pathology.

Transverse Colon

• Position: Crosses the abdomen from the hepatic flexure (right) to the splenic (left colic) flexure (left, typically slightly higher/more superior than the hepatic flexure, held up by the phrenicocolic ligament).
• Peritoneal relationship: Intraperitoneal, suspended by the transverse mesocolon from the posterior abdominal wall (anterior to the second part of the duodenum and pancreas).
• Important relations: Stomach (superiorly, related via the greater omentum), small intestine loops (inferiorly), pancreas and duodenum (posteriorly, via the root of the mesocolon).
• Blood supply: Proximal two-thirds, middle colic artery (SMA branch); distal one-third, left colic artery (IMA branch), reflecting the midgut/hindgut embryological boundary at this point.
• Venous drainage: Middle colic vein to SMV (proximal two-thirds); left colic vein to IMV (distal one-third).
• Lymphatics: Follow the corresponding arterial territory, middle colic nodes to superior mesenteric nodes (proximal); left colic nodes to inferior mesenteric nodes (distal).
• Innervation: Mixed midgut (proximal two-thirds, periumbilical pain pattern) and hindgut (distal one-third, suprapubic/lower pain pattern) innervation, a key transition zone.

Descending Colon

• Position: Left side of the abdomen, descending from the splenic flexure to the pelvic brim, where it becomes the sigmoid colon.
• Peritoneal relationship: Secondarily retroperitoneal, similar to the ascending colon, covered by peritoneum only anteriorly/laterally.
• Important relations: Left kidney (posteriorly), spleen (near the splenic flexure).
• Blood supply: Left colic artery, branch of the IMA.
• Venous drainage: Left colic vein to the IMV (which typically drains into the splenic vein).
• Lymphatics: Epicolic/paracolic nodes → intermediate (left colic) nodes → inferior mesenteric nodes.
• Innervation: IMA plexus (sympathetic, lumbar splanchnic nerves; parasympathetic (pelvic splanchnic nerves, S2-S4, NOT the vagus) a key distinguishing feature of hindgut innervation compared to foregut/midgut), hindgut-pattern innervation, suprapubic/lower abdominal referred pain.

Sigmoid Colon

• Position: S-shaped loop in the left iliac fossa/pelvis, continuing from the descending colon to the rectum (at approximately the S3 vertebral level).
• Peritoneal relationship: Intraperitoneal, suspended by the sigmoid mesocolon: a relatively mobile segment, predisposing it to volvulus.
• Important relations: Bladder (males) or uterus/posterior fornix (females) anteriorly, given its position within the pelvis; left ureter crosses close to the sigmoid mesocolon's root (relevant to surgical injury risk during sigmoid colectomy).
• Blood supply: Sigmoid arteries, branches of the IMA.
• Venous drainage: Sigmoid veins to the IMV.
• Lymphatics: Follow the sigmoid vessels to inferior mesenteric nodes.
• Innervation: Hindgut pattern, as for the descending colon.

• A continuous anastomotic arterial arcade running along the inner border of the colon, formed by the union of terminal branches from the ileocolic, right colic, middle colic (SMA territory), left colic, and sigmoid arteries (IMA territory).
• Provides collateral circulation along essentially the entire length of the colon, from the ileocaecal junction to the rectosigmoid junction.
• Clinically vital in providing collateral flow if one of the main contributing arteries is occluded, though this protective function is least reliable at the watershed points.

• SMA territory: Caecum, appendix, ascending colon, and the proximal two-thirds of the transverse colon (midgut).
• IMA territory: The distal one-third of the transverse colon, splenic flexure, descending colon, sigmoid colon, and the upper rectum (hindgut).

• Splenic flexure (Griffith's point): The junction between SMA (middle colic artery) and IMA (left colic artery) territories. The marginal artery's collateral connection here is often relatively poor/attenuated compared to elsewhere along its length. This makes the splenic flexure particularly vulnerable to ischaemic colitis, especially in states of reduced perfusion (hypotension, shock, low cardiac output).
• Rectosigmoid junction (Sudeck's point): A second, less consistently described watershed point, between the sigmoid arteries (IMA) and the superior rectal artery's territory, also considered a relative watershed zone, though anatomically Griffith's point at the splenic flexure is the more classically and consistently taught watershed.

Appendicitis:

• Luminal obstruction (typically by a faecolith, less commonly lymphoid hyperplasia or other cause) leads to increased intraluminal pressure, venous congestion, bacterial overgrowth, and progressive ischaemia, given the appendix's single end-artery blood supply, this progresses relatively rapidly toward necrosis and perforation if untreated.
• Classic pain pattern: initial poorly localised periumbilical pain (visceral, midgut-pattern referred pain, since the appendix is a midgut derivative supplied by the SMA). This migrates over hours to localised right iliac fossa pain (McBurney's point) as the inflammation progresses to involve the overlying parietal peritoneum (somatic, well-localised pain).
• Presentation varies considerably with the appendix's anatomical position.

Volvulus (colonic):

• Sigmoid volvulus (most common site of colonic volvulus), related to the sigmoid colon's relatively long, mobile mesocolon and its redundant looping in the pelvis, predisposing to twisting around the axis of the mesocolon's root. It presents with large bowel obstruction, classically with a characteristic "coffee bean" sign on imaging.
• Caecal volvulus: less common, requires an abnormally mobile caecum (a minority anatomical variant, given the caecum's typically fixed retroperitoneal-like position in most individuals) to twist around its own mesentery/blood supply.

Colonic ischaemia:

• Most classically affects the watershed regions (splenic flexure, and to a lesser extent the rectosigmoid junction), particularly in the setting of systemic hypoperfusion (shock, cardiac failure, major vascular surgery compromising IMA flow).
• Presents with abdominal pain, bloody diarrhoea, and variable severity depending on the depth and extent of ischaemic injury (mucosal ischaemia being reversible, transmural ischaemia risking perforation).

Referred pain:

• Colonic pain follows the embryological/vascular pattern established throughout this section.
• Midgut-derived colon (caecum to proximal two-thirds transverse colon) refers pain periumbilically.
• Hindgut-derived colon (distal one-third transverse colon onward) refers pain to the suprapubic/lower abdominal region, a direct continuation of the same organising principle covered for the small intestine and stomach.