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特許 権利維持 A sphincter is an anatomical structure, a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals; there are over 50 types in the human body, some microscopically small, in particular the millions of precapillary sphincters.[1] Sphincters relax at death, possibly releasing fluids.