Dorsal cross-sectional view of the posterior fossa at the level of the medulla. The brainstem has been transected at the rostral medulla, and removal of the cerebellum reveals the caudal portion of the floor of the 4^th ventricle. The spinal accessory nerve (CNXI) consists of a number of small rootlets which course rostral and lateral from the medulla to enter the jugular foramen along with the glossopharyngeal (CNIX) and vagus (CNX) nerves. The hypoglossal nerve (CNXII) courses in the lateral medullary cistern to the hypoglossal canal. Both vertebral arteries are visible with their respective posterior inferior cerebellar arteries (PICA); in this case, the lateral medullary segment of the left PICA courses between the vagus and spinal accessory nerves before proceeding as the tonsillar medullary segment. The facial (CNVII) and vestibulocochlear (CNVIII) nerves have been transected and are visible only in the internal auditory canal. (Image courtesy of AL Rhoton, Jr.)