( ( A sphincter ) ( ( ( is ( ( a ring ) ( of muscle ) ) ) ( that ( contracts ( to ( ( close off ) ( ( a ( ( passage or ) opening ) ) ( in ( the body ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN sphincter)) (VP (VBZ is) (NP (NP (DT a) (NN ring)) (PP (IN of) (NP (NN muscle)))) (SBAR (IN that) (S (NP (NNS contracts)) (VP (TO to) (VP (VB close) (PRT (RP off)) (NP (NP (DT a) (NN passage) (CC or) (NN opening)) (PP (IN in) (NP (DT the) (NN body))))))))) (. .)))
A sphincter is a ring of muscle that contracts to close off a passage or opening in the body.
(ROOT (S (S (VP (VBG Acting) (PP (IN like) (NP (NNS drawstrings))) (S (VP (TO to) (VP (VB close) (PRT (RP off)) (NP (DT the) (JJ alimentary) (NN canal))))))) (, ,) (NP (DT the) (NN sphincter)) (VP (VBZ regulates) (NP (NP (DT the) (NN passage)) (PP (IN of) (NP (NP (NN material)) (PP (IN between) (NP (NNS compartments))))))) (. .)))