E, The right half of the medulla has been removed. The anterior spinal artery arises predominantly from the left vertebral artery, but has a small contribution from the right vertebral artery. Two bundles of right hypoglossal rootlets penetrate the dura. F, Enlarged view. The medulla has been removed to expose the vertebral and anterior spinal arteries. The C1 nerve roots penetrate the dura with the vertebral artery. G, The intradural segment of the vertebral arteries and the dura lining the anterior margin of the foramen magnum have been removed to expose the tectorial membrane, a rostral extension of the posterior longitudinal ligament, and the vertebral venous plexus, which courses just outside the dura. H, The tectorial membrane has been removed to expose the cruciform and alar ligaments. The horizontal portion of the cruciform ligament, called the transverse ligament of the atlas, extends laterally to be attached to the medial edges of the lateral masses of the atlas, and the vertical portion ascends to attach to the anterior margin of the foramen magnum deep to the tectorial membrane. The alar ligaments pass upward and laterally and attach to the lateral edges of the foramen magnum. Anterior meningeal arteries pass along the dura and ligamentous structures in the anterior spinal canal. I, The vertical portion of the cruciform ligament has been folded downward to expose the synovial joint between the anterior surface of the cruciform ligament and the posterior surface of the dens. There is also another synovial joint between the anterior surface of the dens and the posterior surface of the anterior atlantal arch. The apical ligament of the dens extends upward to be attached to the margin of the foramen magnum. (Images courtesy of AL Rhoton, Jr.)