( ( For ( many years ) ) ( , ( scientists ( ( believed ( that ( migraines ( were ( linked ( to ( ( the ( ( dilation and ) constriction ) ) ( of ( ( blood vessels ) ( in ( the head ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN For) (NP (JJ many) (NNS years))) (, ,) (NP (NNS scientists)) (VP (VBD believed) (SBAR (IN that) (S (NP (NNS migraines)) (VP (VBD were) (VP (VBN linked) (PP (TO to) (NP (NP (DT the) (NN dilation) (CC and) (NN constriction)) (PP (IN of) (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head)))))))))))) (. .)))
For many years, scientists believed that migraines were linked to the dilation and constriction of blood vessels in the head.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head)))) (VP (VBP dilate)))) (, ,) (NP (NP (DT a) (NN migraine)) (, ,) (NP (DT a) (JJ severe) (NN headache)) (, ,)) (VP (MD can) (VP (VB occur))) (. .)))