1) Midbrain 2) Neural structure 1) Midbrain 
• (n.) The middle segment of the brain; the midbrain. Sometimes abbreviated to mesen. See Brain.
(mez″en-sef´ә-lon) the short part of the brainstem just above the pons; it contains the nerve pathways between the cerebral hemispheres and the medulla oblongata, as well as nuclei (relay stations or centers) of the third and fourth cranial nerves. The center for visual reflexes, such as moving the head and ...
Region of the brain below the thalamus and above the pons developed from the middle of the three cerebral vesicles of the embryonic nervous system. Includes the superior and inferior colliculi and cerebral peduncles.
Mes`en·ceph'a·lon noun [ New Latin See Meso- and Encephalon .] (Anat.) The middle segment of the brain; the midbrain. Sometimes abbreviated to mesen . See Brain . 
Type: Term Pronunciation: mes-en-sef′ă-lon Definitions: 1. That part of the brainstem developing from the middle of the three primary cerebral vesicles of the embryo (the caudal of these being the rhombencephalon or hindbrain, the rostral the prosencephalon or forebrain). In the adult, the mesencephalon is characterized by the unique con...
(mes;en-sef;ua-lon) The midbrain, which contains the corpora quadrigemina and the cerebral peduncles.
mesencephalon The part of the brain developed from the middle cerebral vesicle and consisting of the tectum and cerebral peduncles, and traversed by the cerebral aqueduct; midbrain.
Greek mesos = middle, and enkephalos = brain; adjective - mesencephalic.
the middle portion of the brain