Holding on
Two rings of muscle close the anal canal. The inner one, the thickened end of the gut's smooth muscle, stays contracted without thought. The external anal sphincter round it is skeletal muscle under voluntary control. Above it the puborectalis loops behind the gut and pulls it towards the pubic bone, bending the passage at a sharp angle.
The rectum and anal canal cut lengthwise, with the internal and external sphincters, Gray's Anatomy, 1918, figure 1078Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray1078.png)
When the time is right, the external sphincter and puborectalis relax, the angle straightens and the abdominal muscles push to empty the rectum. When it is not, they tighten, and the urge fades after a few minutes, coming back hours later or when more faeces arrive.
The levator ani, the muscle of the pelvic floor to which the puborectalis belongs, seen from inside the pelvis, Gray's Anatomy, 1918, figure 404Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray404.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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