( This ( ( is ( part ( of ( the ( brain stem ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (NP (NP (NN part)) (PP (IN of) (NP (DT the) (NN brain) (NN stem))))) (. .)))
This is part of the brain stem.
(ROOT (S (NP (DT The) (NN brain) (NN stem)) (VP (VBZ regulates) (NP (NP (DT the) (NN rate)) (PP (IN of) (NP (NN breathing))))) (. .)))