K, A suboccipital craniectomy has been completed and the right half of the posterior arch of C1 has been removed. The posterior root of the transverse foramen of the atlas has been removed while preserving the portion of the tip of the transverse process of the atlas to which the rectus capitis lateralis, levator scapulae, and the superior oblique attach. The atlanto-occipital joint and the posterior condylar emissary vein are exposed. The ventral rami of the C1 and C2 nerve roots pass behind the vertebral artery. The dorsal ramus of C2 gives rise to the greater occipital nerve, which passes through the semispinalis capitis to reach the posterior scalp. L, The area above the occipital condyle has been drilled to the depth of the cortical bone surrounding the hypoglossal canal. The change from cancellous to cortical bone indicates that the hypoglossal canal has been reached. M, The hypoglossal canal has been opened to expose the venous plexus, which surrounds the hypoglossal nerve in the canal and connects the basilar venous plexus with the marginal sinus, which encircles the foramen magnum. The dorsal ramus of the C1 nerve root, also termed the suboccipital nerve, passes backward between the posterior arch of the atlas and the vertebral artery, supplies the muscles bordering the suboccipital triangle, and sends fibers to the rectus capitis posterior minor and the semispinalis capitis muscles. N, An upper cervical laminectomy has been completed and the dura opened. The dural incision completely encircles the vertebral artery, leaving a narrow dural cuff on the artery so that the artery can be mobilized. The drilling in the supracondylar area exposes the hypoglossal nerve in the hypoglossal canal, and can be extended extradurally to the level of the jugular tubercle to increase access to the front of the brainstem. O, Enlarged view of the site of dural penetration by the vertebral artery in another specimen. The rostral end of the dentate ligament ascends behind the vertebral artery with the accessory nerve and attaches to the dura along the lateral margin of the foramen magnum. (Images courtesy of AL Rhoton, Jr.)