A, A suboccipital craniectomy and upper cervical laminectomy exposes the dura. The vertebral arteries pass medially across the upper surface of the atlas where they give off the posterior meningeal arteries that ascend to supply the dura on the posterior aspect of the foramen magnum and posterior fossa. Insert, upper right. The upper margin of the left half of the arch of the atlas forms an osseous ring around the vertebral artery just proximal to where it enters the dura. B, Enlarged view of another foramen magnum after opening the dura. The right PICA arises outside the dura and penetrates the dura with the vertebral artery. The rostral end of the dentate ligament passes between the vertebral artery and the PICA to insert into the dura along the lateral margin of the foramen mag-num. The accessory nerve ascends posterior to both the PICA and the vertebral artery. The vertebral artery gives rise to a posterior spinal artery that passes along the posterolateral aspect of the spinal cord and medulla. The hypoglossal rootlets are stretched over the posterior aspect of the vertebral artery. C, The right tonsil has been retracted to expose the caudal end of the fourth ventricle, which is located above the foramen magnum. The right PICA ascends through the foramen magnum and along the posterior margin of the medulla to reach the cerebellomedullary fissure. D, Another specimen. The rostral end of the dentate ligament passes between the posterior spinal artery and vertebral artery and attaches to the dura at the level of the foramen magnum. The accessory nerve ascends behind the posterior spinal artery. The C1 nerve root receives a contribution from the accessory nerve and passes through the dura with the vertebral artery and courses along the lower margin of the artery. The posterior spinal artery arises inside the dura and passes between the dentate ligament and accessory nerve and gives rise to ascending branches to the medulla and descending branches to the spinal cord. E, The anterior skull base has been removed. The vertebral arteries ascend in front of the brainstem and give rise to the anterior spinal artery. F, Enlarged view. The C1 ventral roots penetrate the dura with the vertebral artery. The hypoglossal rootlets pass behind the vertebral arteries. (Images courtesy of AL Rhoton, Jr.)