circle of Willis
a ring of blood vessels on the underside of the brain, formed by links between branches of several major arteries—the basilar artery, internal carotid artery, anterior cerebral artery, anterior communicating artery, middle cerebral artery, posterior cerebral artery, and posterior communicating artery. By providing multiple pathways for blood circulation, this arrangement ensures the continuous flow of oxygenated blood to the brain in the event a supplying artery becomes blocked or constricted as a result of an injury, disease, or other condition. Also called arterial circle. [Thomas Willis (1621–1675), English anatomist and physician who was the first to illustrate this area definitively and provide a description of its function]