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  .
 
 
 
Published under license with Merriam-Webster, Incorporated.  © 1997-2024.