Lateral view of the midbrain with deep structures dissected.  The corticospinal tract has been isolated in the anterior aspect of the cerebral peduncle as it descends through the brainstem toward the spinal cord.  The lateral mesencephalic sulcus separates the cerebral peduncle from the midbrain tegmentum. The red nucleus is located within the midbrain tegmentum at the level of the superior colliculus. The fibers of the superior cerebellar peduncles decussate in the caudal midbrain at the level of the inferior colliculus. The oculomotor nucleus is located anterior to the cerebral aqueduct within the midbrain tegmentum. Fibers from this nucleus emerge on the anterior aspect of the midbrain as the oculomotor nerve.  The cerebral aqueduct courses through the midbrain and opens inferiorly into the fourth ventricle between the pons and cerebellum.  Posterior to the cerebral aqueduct, the midbrain tectum consists of the superior and inferior colliculi.  The trochlear nerves emerge from the dorsal aspect of the midbrain below the inferior colliculi and course around the lateral aspect of the brainstem at the pontomesencephalic junction. (Image courtesy of AL Rhoton, Jr.)