A, The cerebellar tonsils, the foramen of Magendie, and lower part of the fourth ventricle are situated above the foramen magnum. The vertebral artery penetrates the dura below the foramen magnum and ascends through the foramen in front of the dentate ligament and accessory nerves. The glossopharyngeal, vagus, and accessory nerves pass through the jugular foramen, which is located lateral to the anterior half of the foramen magnum. B, The cerebellum has been removed. The vertebral arteries pass through the foramen magnum to reach the front of the medulla. C, Enlarged view of the left half of the foramen magnum. The vertebral artery passes behind and below the atlanto-occipital joint, penetrates the dura, and passes in front of the dentate ligament and accessory nerve. The rostral end of the dentate ligament attaches to the dura at the level of the foramen magnum. The C1 nerve penetrates the dura with the vertebral artery. The hypoglossal nerve passes behind the vertebral artery and enters the hypoglossal canal. The hypoglossal nerve is separated into several bundles as it penetrates the dura. The posterior spinal artery arises as the vertebral artery enters the dura and gives rise to ascending and descending branches. D, A longitudinal strip of the medulla and floor of the fourth ventricle has been removed to expose the vertebrobasilar junction, the origin of the anterior spinal artery, and the median anterior medullary and median anterior spinal veins. (Images courtesy of AL Rhoton, Jr.)