The cerebral arterial circle (of Willis) is formed at the base of the brain by the interconnecting vertebrobasilar and internal carotid systems of vessels (Fig. 8.38). This anastomotic interconnection is accomplished by: an anterior communicating artery connecting the left and right anterior cerebral arteries to each other, and two posterior communicating arteries, one on each side, connecting the internal carotid artery with the posterior cerebral artery (Figs. 8.38 and 8.39).