( ( The ( ( mesencephalon or ) midbrain ) ) ( ( is ( ( a part ) ( of ( the ( brain stem ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN mesencephalon) (CC or) (NN midbrain)) (VP (VBZ is) (NP (NP (DT a) (NN part)) (PP (IN of) (NP (DT the) (NN brain) (NN stem))))) (. .)))
The mesencephalon or midbrain is a 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))))) (. .)))