Anatomy of the Appendix

The vermiform appendix is a blind-ended true diverticulum of the colon — its wall contains all three layers (mucosa, submucosa, muscularis, serosa). It extends from the posteromedial aspect of the caecum, approximately 2.5 cm inferior to the ileocaecal valve.

The Appendix — Overview Diagram

The appendix arises from the posteromedial caecum at the confluence of the three taeniae coli. The mesoappendix is a peritoneal fold containing the appendiceal artery (an end artery, branch of the ileocolic artery). The appendix has a complete outer longitudinal muscle layer — unlike the colon which has taeniae. Rich in submucosal lymphoid tissue (MALT).

McBurney’s Point

Located at the junction of the lateral 1/3 and medial 2/3 of a line drawn from the right ASIS to the umbilicus. This is the surface marking for the appendiceal base, the site of maximal tenderness in appendicitis, and the centre of both the Gridiron and Lanz surgical incisions.

Appendix & Ileocaecal Valve

The appendiceal orifice opens into the caecal lumen approximately 2.5 cm inferior to the ileocaecal valve. The ileocaecal valve controls the passage of intestinal contents from the terminal ileum into the caecum and prevents reflux. The appendiceal base position is constant — only the tip position is variable.

McBurney’s Point — The Key Surface Landmark

The base of the appendix projects to McBurney’s point — at the junction of the lateral 1/3 and medial 2/3 of a line drawn from the right ASIS to the umbilicus. This is:

• The site of maximal tenderness in acute appendicitis (McBurney’s sign)
• The centre of both the Gridiron (McBurney’s) and Lanz open incisions
• The landmark for LIF laparoscopic port placement

Variable Positions of the Appendix Tip

The base is fixed at the posteromedial caecum. The tip can lie in multiple positions — retrocaecal (65%, most common), subcaecal/pelvic (30%), preileal, postileal, or paracaecal. Each position produces a different clinical presentation, which explains the diverse and atypical presentations of appendicitis.

Mesoappendix & Bloodless Fold of Treves

The mesoappendix is a fold of peritoneum wrapping around the appendix, containing the appendiceal artery and vein. Its proximal attachment is the ileocaecal fold — the bloodless fold of Treves (named after Sir Frederick Treves who operated on King Edward VII, 1902). Despite the name, this fold frequently contains a small vessel and must be handled carefully.

Appendiceal Arterial Supply

The appendiceal artery is a branch of the ileocolic artery (from the SMA). It runs in the free edge of the mesoappendix as an end artery with no collateral supply. In appendicitis, rising intraluminal pressure eventually occludes this single end artery → ischaemia → full-thickness necrosis → perforation. This explains why untreated appendicitis reliably progresses to gangrene and perforation.

• Artery

  Appendiceal artery — branch of the ileocolic artery (SMA → aorta)

  An end artery in the free edge of the mesoappendix. Sole supply with no collateral — rising intraluminal pressure in appendicitis → arterial occlusion → ischaemia → gangrene → perforation. Contrast: the gallbladder has a dual supply (cystic artery + liver bed) → perforation is less common.

• Vein

  Appendiceal vein → ileocolic vein → superior mesenteric vein → portal vein → liver

  Portal venous drainage — infective thrombus from appendicitis can track to the portal vein → pylephlebitis (septic portal vein thrombosis). Rare but life-threatening. Presents with spiking fevers, jaundice, and hepatic abscesses.

• Lymph

  Nodes within the mesoappendix → ileocolic lymph nodes

  Follows the vascular pedicle. Relevant in appendiceal tumour staging — ileocolic nodal involvement determines whether right hemicolectomy is required after appendicectomy.

• Nerve

  Sympathetic afferents → T10 → periumbilical referred pain

  T10 = umbilicus dermatome. Initial visceral pain in appendicitis is a dull periumbilical ache. Pain then migrates to the RIF as parietal peritoneum is irritated (somatic A-delta fibres → sharp, localised). This migration is the hallmark of appendicitis.

• True diverticulum — all three bowel wall layers present (mucosa, submucosa, muscularis, serosa)
• Rich in lymphoid tissue (MALT) — submucosal lymphoid follicles peak in adolescence, explaining peak incidence of appendicitis in young adults. Lymphoid hyperplasia is the most common cause of luminal obstruction in children/young adults.
• M cells (microfold cells) — sample and present antigens from the intestinal lumen to subepithelial lymphoid tissue
• Complete outer longitudinal muscle — unlike the colon, which has taeniae. The appendix has a regular, complete outer muscle layer.
• Lumen may be obliterated with age — progressive fibrosis obliterates the lumen → appendicitis becomes less common with advancing age as lumenal obstruction becomes impossible
• Taeniae coli — three 8 mm-wide longitudinal muscle bands on the colon wall. They converge at the appendiceal base (the basis for the intraoperative landmark). The appendix itself lacks taeniae.