Posterior view into the posterior cranial fossa and occipito-atlantal area. The vertebro-basilar arterial system is exposed to show the vertebral arteries (V3 segments) passing around the lateral masses of the atlas and piercing the atlanto-occipital membrane (to become the V4 segments) to enter the posterior fossa through the foramen magnum where they join to form the basilar artery. The origins of the three cerebellar arteries (posterior, anterior, and superior) are in view as are multiple cranial nerves coursing from the brainstem (removed) to their respective foramina. (Image courtesy of AL Rhoton, Jr.)