Perspective of the dorsal midbrain and fourth ventricle. The superior medullary velum arching between the superior cerebellar peduncles has been largely resected, revealing the fourth ventricle. Prominent structures of the dorsal mesencephalon include the midline pineal gland and paired superior and inferior colliculi. The trochlear nerves emerge from the dorsal midbrain just inferior to the inferior colliculi and superior to the superior medullary velum. Within the fourth ventricle, the nodule of the cerebellar vermis and the cranial poles of the cerebellar tonsils are visible beneath the inferior medullary velum, as are branches of the posterior inferior cerebellar arteries. Laterally, the anterior inferior cerebellar artery courses near the facial and vestibulocochlear nerves toward the infernal acoustic meatus. (Image courtesy of AL Rhoton, Jr.)