sphincter muscle

A sphincter is a structure, usually a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. There are over 40 different sphincters in the human body; some of these sphincters are microscopic in size.
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Muscle /mus·cle/ an organ which by contraction produces movement of an animal organism. abductor muscle of great toe origin, medial tubercle of [more]Sphincter muscle - Community and Resources

Noun
1. a ring of muscle that contracts to close an opening
(synonym) sphincter, anatomical sphincter
(hypernym) muscle, musculus
(hyponym) physiological sphincter