mesencephalon /mes′ensef″əlon/ [Gk, mesos, middle, enkephalos, brain] , one of the three parts of the brainstem, lying just below the cerebrum and just above the pons. It consists primarily of white matter with some gray matter around the cerebral aqueduct. Deep within the mesencephalon are nuclei of the third and the fourth cranial nerves and the anterior part of the fifth cranial nerve. The mesencephalon also contains nuclei for certain auditory and visual reflexes. Also called midbrain. −mesencephalic, adj.
