The midbrain (mesencephalon) lies between the pons and the diencephalon. Ventral view. The ventral view reveals two prominent bundles of fibers converging onto the pons. These are the cerebral peduncles, or, as they are alternatively called, the crura cerebri (singular: crus cerebri). The groove between the peduncles, known as the interpeduncular fossa, is the site of emergence of the two oculomotor nerves (CN III) from the brainstem. The cerebral peduncles disappear caudally as they enter the pons; rostrally, they are encircled by the optic tracts before entering the cerebral hemispheres.

Dorsal view. The dorsal aspect of the midbrain (the midbrain tectum, i.e., “roof”) contains four protrusions collectively termed the quadrigeminal plate. Visual information is processed in the upper two protrusions (the superior colliculi), while auditory information is processed in the lower two protrusions (the inferior colliculi), which are somewhat smaller. The trochlear nerve (CN IV) emerges from the brainstem just below the inferior colliculus on either side and then courses ventrally around the cerebral peduncle. It is the only cranial nerve that emerges from the dorsal aspect of the brainstem.

Lateral view. The two small protrusions lying lateral to the quadrigeminal plate are the medial geniculate body (an auditory relay area) and the lateral geniculate body (a visual relay area). The geniculate bodies are components of the thalamus and thus belong not to the brainstem but to the diencephalon.