Dorsal view of the midbrain and quadrigeminal (tectal) plate. Visualized are the superior and inferior colliculi of the midbrain. The trochlear nerve arises from the dorsal midbrain at the level of the cerebellomesencephalic fissure and courses laterally around the midbrain. The pineal gland and third ventricle are visualized after removal of the velum interpositum. (Image courtesy of AL Rhoton, Jr.)