A ring-shaped muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning.
From Greek 'sphinktēr' meaning 'that which binds tight', from 'sphingein' (to bind tight). The term was adopted into medical Latin and then English in the 16th century to describe these important muscular valves throughout the body.
Sphincters are among the most essential yet underappreciated muscles in our bodies - we have over 50 of them controlling everything from our pupils to our digestive system, working tirelessly without conscious thought to maintain our bodily functions.
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