( ( Migraine headaches ) ( ( are ( believed ( to ( result ( from ( dilatation ( of ( ( the ( blood vessels ) ) ( in ( the head ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Migraine) (NNS headaches)) (VP (VBP are) (VP (VBN believed) (S (VP (TO to) (VP (VB result) (PP (IN from) (NP (NP (NN dilatation)) (PP (IN of) (NP (NP (DT the) (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head)))))))))))) (. .)))
Migraine headaches are believed to result from dilatation of the 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))) (. .)))