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mid·brain 'mid-.brān n the middle division of the three primary divisions of the developing vertebrate brain or the corresponding part of the adult brain that includes a ventral part containing the cerebral peduncles and a dorsal tectum containing the corpora quadrigemina and that surrounds the aqueduct of Sylvius connecting the third and fourth ventricles called also mesencephalon
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n.
the small portion of the brainstem, excluding the pons and the medulla, that joins the hindbrain to the forebrain.
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mid·brain (midґbrān″) mesencephalon.Medical dictionary. 2011.