The Cecum and Appendix
The cecum and appendix are part of the gastrointestinal tract. They are the most proximal part of the large intestine, located between the ileum (distal small bowel) and the ascending colon.
Having served as a site for cellulose digestion in our ancestors, the cecum now simply acts as a reservoir for chyme which it receives from the ileum. The appendix contains a large amount of lymphoid tissue, but has no vital functions in the human.
In this article, we shall look at the anatomy of the cecum and appendix – their anatomical structure and relations, neurovascular supply and lymphatic drainage.
Anatomical Structure and Relations
The cecum is the most proximal part of the large intestine and can be found in the right iliac fossa and suprapubic region. It lies slightly inferior to the iliocaecal junction, and can be palpated in the right iliac fossa if enlarged due to faeces, gas or malignancy.
The name is derived from the Latin word ‘caecus’, meaning ‘blind’, due to its blind-end inferiorly. Superiorly, however, the open end of the cecum is continuous with the ascending colon. Unlike the ascending colon above it, the cecum is intraperitoneal.
The appendix, also known as the vermiform (worm-shaped) appendix is a narrow, blind ended tube attached to the posteromedial end of the cecum. The position of the free-end of the appendix is highly variable, and can be categorised into seven main locations. The most common positions are retrocaecal and subileal. A simple way to remember the positions is by imagining the appendix as the hour hand of a clock:
- Pre-ileal – Anterior to the terminal ileum – 1 o’clock.
- Post-ileal – Posterior to the terminal ileum – 2 o’clock.
- Sub-ileal – Parallel with the terminal ileum – 3 o’clock.
- Pelvic – Descending over the pelvic brim – 5 o’clock.
- Sub-caecal – Below the cecum – 6 o’clock.
- Paracaecal – Alongside the lateral border of the cecum – 10 o’clock.
- Retrocaecal – Behind the cecum – 11 o’clock.
The cecum is derived from the embryologic midgut – and therefore blood is supplied by a branch of the superior mesenteric artery. The branch is the ileocolic artery, which splits into anterior and posterior caecal arteries to supply the cecum. Venous drainage is provided by the ileocolic vein, which empties into the superior mesenteric vein.
The appendix receives its blood supply via the appendicular artery (derived from the ileocolic artery), and drains through the appendicular vein. Both are contained, along with lymphatic vessels and nerves, within the mesoappendix, a fold of mesentery which suspends the appendix from the terminal ileum.
The autonomic nervous system innervates the cecum and appendix. It achieves this by means of the ileocolic branch of the superior mesenteric plexus, which follows the same course as the ileocolic artery.
Lymph from the cecum and appendix ultimately drains into the upper and lower ileocolic lymph nodes, which surround the ileocolic artery.
However, lymph from the cecum travels via a number of intermediate mesenteric nodes, whereas that of the appendix travels via a single intermediate node in the mesoappendix.
Clinical Relevance: Appendicitis
Appendicitis is acute inflammation of the appendix, and is the most common cause for acute, severe abdominal pain. The abdomen is most tender at McBurney’s point – one third of the distance from the right anterior superior iliac spine to the umbilicus. This corresponds to the location of the base of the appendix.
The aetiology depends on age. In the young, it is mostly due to an increase in lymphoid tissue size, which occludes the lumen. From 30 years old onwards, it is more likely to be blocked due to faecal matter.
Initially, the appendix cannot drain, and so increases in size, stretching the visceral peritoneum. This causes a vague pain in the periumbilical region. As the appendix swells, it irritates the parietal peritoneum, and causes severe pain in the right lower quadrant.
If the appendix is not removed, it can become necrotic and rupture, resulting in peritonitis (inflammation of the peritoneum).