Muscle · Pelvis

Anal sphincter

Musculus sphincter ani

Two rings of muscle around the anal canal that do the same job under completely different control. The INNER one is smooth muscle — a thickening of the bowel's own circular layer, run by the autonomic nervous system, contracted by default and not available to conscious command. The OUTER one is skeletal muscle in three parts, under voluntary control through a branch of the pudendal nerve, and its uppermost part blends with the sling of levator ani above. That layering is the whole design: the inner ring holds the canal closed all day without being thought about, and the outer ring is what can be added deliberately and briefly on top of it.