Expansion
No taeniae, no haustra, no appendices epiploicae and no mesentery
Expansion
The rectum begins at S3 and is about 12 cm long. It lacks all three colonic features:
- No taeniae coli - the longitudinal muscle forms a complete layer
- No haustra
- No appendices epiploicae
- No mesentery
Instead it has:
- Valves of Houston - three transverse folds, the middle one at the level of the peritoneal reflection
- The mesorectum - a fatty envelope containing the superior rectal vessels and lymph nodes
Peritoneal covering diminishes as it descends: the upper third is covered anteriorly and laterally, the middle third anteriorly only, and the lower third is entirely extraperitoneal.
The mesorectal fascial envelope is the basis of total mesorectal excision, which dramatically reduced local recurrence in rectal cancer surgery.