The midbrain or mesencephalon (from the Greek mesos - middle, and enkephalos - brain) is a portion of the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 associated with vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation.

Anatomically, it comprises the tectum (or corpora quadrigemina), tegmentum

Tegmentum

The tegmentum is a general area within the brainstem. It is located between the ventricular system and distinctive basal or ventral structures at each level...

, the ventricular mesocoelia (or "iter"), and the cerebral peduncle

Cerebral peduncle

Mainly, the three common areas that give rise to the cerebral peduncles are the cortex, the spinal cord and the cerebellum. The cerebral peduncle, by most classifications, is everything in the mesencephalon except the tectum. The region includes the midbrain tegmentum, crus cerebri and pretectum...

s, as well as several nuclei and fasciculi.