( Migraines ( ( are ( caused ( by ( ( excessive dilation ) ( of ( ( blood vessels ) ( in ( the head ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Migraines)) (VP (VBP are) (VP (VBN caused) (PP (IN by) (NP (NP (JJ excessive) (NN dilation)) (PP (IN of) (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head))))))))) (. .)))
Migraines are caused by excessive dilation 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))) (. .)))