Posterior view of contents of the posterior cranial fossa and spinal canal. Removal of occipital bone and laminectomy of the cervical vertebrae provides in situ continuity of the central nervous system (CNS). The periosteal dura mater of the tentorium cerebelli that attached to the transverse groove of the occipital bone can be seen. Internal to this layer, between it and the meningeal dura of this dural fold, is the transverse sinus. At the midline, the occipital sinus, contained within the falx cerebelli (removed) separating the cerebellar hemispheres, is shown draining into the confluens of sinuses. The transverse sinuses continue laterally to become the sigmoid sinuses. Cranial and spinal dura have been cut to demonstrate the underlying arachnoid mater. Pia mater, intimately applied to the surface of the CNS, remains intact. The posterior inferior cerebellar arteries, the largest branches of the vertebral arteries, can be seen proximal to the foramen magnum. Dorsal roots of cervical spinal nerves emerge from the dorsolateral segments of the cervical cord. Distally, the cell bodies of these sensory neurons are contained within dorsal root (spinal) ganglia near intervertebral foramina. Along the lateral border of the spinal cord between dorsal and ventral roots, are longitudinal white glistening structures, the denticulate ligaments. These attach at tooth-like focal adhesions to the spinal dura to stabilize the cord within the dural sac. These ligaments represent consolidations of pia mater. Longitudinally directed posterior spinal arteries supply the posterior one-third of the spinal cord (the anterior spinal artery supplies the anterior two-thirds). At any give cord level (represented here by the C2 spinal nerve), dorsal and ventral roots exit the dural sac and unite to form very short, definitive spinal nerves (mixed nerves). These nerves exit the intervertebral foramen and divide into smaller diameter dorsal primary rami that supply deep back muscles and a limited area of skin posteriorly, and larger diameter ventral primary rami that supply motor innervation to body wall and limb musculature and remaining skin dermatomes. There are seven cervical vertebrae but eight pairs of cervical spinal nerves. The first cervical nerve, the suboccipital nerve, emerges between the skull and the posterior arch of the first cervical vertebra (atlas). The remaining cervical nerves leave the vertebral canal above their corresponding vertebral level (e.g., C2 spinal nerves emerge above the C2 vertebra (axis), between the atlas and the axis). (Image courtesy of AL Rhoton, Jr.)