The midbrain or mesencephalon (from the Greek mesos - middle, and enkephalos - brain) is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation.
Anatomically, it comprises the tectum (or corpora quadrigemina), tegmentum, the ventricular mesocoelia (or "iter"), and the cerebral peduncles, as well as several nuclei and fasciculi. Caudally the mesencephalon adjoins the pons (metencephalon) and rostrally it adjoins the diencephalon (Thalamus, hypothalamus, etc.). The midbrain is located below the cerebral cortex, and above the hindbrain placing it near the center of the brain.
Read more about Midbrain: Development, Functional Role, Corpora Quadrigemina, Cerebral Peduncle, Anatomical Features of Cross-sections Through The Midbrain, Organization