The rectum
The rectum, the last 12 cm or so of the large intestine, follows the curve of the sacrum into the pelvis and ends at the anal canal, about 4 cm long. It is usually empty. A mass movement from the descending colon fills it, and the stretch of its wall is felt as the urge to empty the bowel.
The sigmoid colon (labelled pelvic colon), the rectum and the anal canal from the front, Gray's Anatomy, 1918, figure 1076Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray1076.png)
The stretch also starts a reflex. Waves of contraction run down the sigmoid colon and rectum, and the inner ring of muscle round the anal canal relaxes. Nerve endings in the lining of the canal can tell gas from liquid or solid, so a person can let out wind and hold the rest.
The lining of the lower rectum and anal canal, with the internal sphincter in its wall, Gray's Anatomy, 1918, figure 1080Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray1080.png)
Sources
- Susan Standring (ed.), Gray's Anatomy, 42nd ed. (2020), chapter on the large intestine.
- Arthur Guyton and John Hall, Textbook of Medical Physiology, 13th ed. (2016), ch. 64.
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