Lateral view of the brainstem and cerebellum. The three divisions of the brainstem are, from cranio-to-caudal, the midbrain, pons, and medulla.  Attached to the dorsal surface of the brainstem is the cerebellum, divided into anterior, posterior, and flocculonodular lobes by the primary and posterolateral fissures, respectively.  The oculomotor nerve (CN III) exits the anterior aspect of the midbrain in the interpeduncular fossa. The dorsal aspect of the midbrain, posterior to the Sylvian aqueduct, is called the tectum and contains the paired superior and inferior colliculi. The lateral mesencephalic sulcus, extending from the medial geniculate body to the pontomesencephalic sulcus, divides the midbrain on its external surface into tegmentum and tectum. The trochlear nerve (CN IV) exits the dorsal aspect of the midbrain below the inferior colliculi. The trigeminal nerve enters the pons through the middle cerebellar peduncle. The abducens nerve (CN VI) exits the pontomedullary junction cranial to the medullary pyramid. The accessory nerve (CN XI) is formed by a large spinal component and a smaller cranial component that join prior to entering the jugular foramen. The so-called "cranial component" of CN XI is considered by most authors to actually be part of CN X (vagus) and contains motor fibers to pharyngeal and laryngeal muscles. (Image courtesy of AL Rhoton, Jr.)