Ventral view of posterior brain vasculature. Paired vertebral arteries ascend through the foramen magnum where they give off posterior inferior cerebellar arteries (PICA). The PICA pass between the roots of the vagus (CN X) and spinal accessory (CN XI) nerves on their way to supply the cerebellum. The vertebral arteries unite to form the basilar artery which runs along the ventral pons. The anterior inferior cerebellar artery (AICA) branches from the basilar artery often between the roots of the facial (CN VII) and vestibulocochlear (CN VIII) nerves. At the distal end of the basilar artery, superior cerebellar arteries (SCA) branch and the basilar artery terminates at the posterior cerebral arteries (PCA). The oculomotor nerve (CN III) exits the midbrain-pontine junction between the SCA and PCA. (Image courtesy of AL Rhoton, Jr.)