The midbrain is the rostralmost part of the brainstem where it connects to the cerebrum.
The midbrain is traversed by the cerebral aqueduct, part of the ventricular system that connects the third ventricle of the diencephalon to the fourth ventricle above the pons and medulla. The roof or “tectum” of the midbrain lies above the cerebral aqueduct and contains the four elevations of the corpora quadrigemina, that is, the two superior colliculi and two inferior colliculi. The region of the midbrain below the cerebral aqueduct is known as the cerebral peduncle and consists of the midbrain tegmentum and crus cerebri. The midbrain tegmentum contains nuclei such as the oculomotor nucleus, red nucleus, and substantia nigra. The crus cerebri are large bundles on the base of the midbrain that contain major tracts which originate in the cerebral cortex and descend through the internal capsule into the brainstem (corticobulbar tract) and spinal cord (corticospinal...