A sphincter (from the Greek sphinkter, meaning that which binds tight) is a ring-like band of muscle fibers that contracts a passage or orifice to close it, or relaxes to open it. Sphincters are important in regulating what passes through and when. Animals have many types of sphincters. In the human body, there are about 60 types. An example is the anal sphincter that opens when expelling feces outside the body. Sphincters may be classified into anatomical sphincters and functional sphincters. Anatomical sphincters are typically circular, localized muscle thickening. Functional sphincters, in contrast, do not have localized muscle thickening but still involves muscle contraction. It may contract around the structure and be referred to as extrinsic sphincter or contract within the structure and be referred to as intrinsic sphincter. Synonym: musculus sphincter.
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