Brain vessels, illustration. At the level of the cut brainstem (lower frame) the vertebral arteries form the central basilar artery. It supplies the posterior region of the brain and the cerebellum. Adjacent to the optic chiasm (central cross-shape) the internal carotid arteries will supply most of the cerebral cortex. Branches of the vertebral and internal carotid arteries form an anastomosis around the optic chiasm called the circle of Willis. This equalizes the circulation in different parts of the brain. From Schafer, E.A. And Thane, G.D. 1899 Quain's Elements of Anatomy, Longmans Green, London.