circle of willis circle of willis the circle of willis (also called willis' circle, loop of willis, cerebral arterial circle, and willis polygon) is a circulatory anastomosis that supplies blood to the brain and surrounding structures in reptiles, birds and mammals, including humans. it is named after thomas willis (1621–1675), an english physician. circle of willisdiagram of the arterial circulation at the base of the brain (inferior view), the circle of willis is drawn in the upper half. blood flows up to the brain through the vertebral arteries and through the internal carotid arteries.the arteries of the base of the brain. basilar artery labeled below center. the temporal pole of the cerebrum and the cerebellar hemisphere have been removed on the right side. inferior aspect (viewed from below).detailsidentifierslatincirculus arteriosus cerebri circulus willisiimeshd002941ta98a12.2.07.080ta24516fma50454anatomical terminology structure the circle of willis is a part of the cerebral circulation and is composed of the following arteries: anterior cerebral artery (left and right) at their a1 segments anterior communicating artery internal carotid artery (left and right) at its distal tip (carotid terminus) posterior cerebral artery (left and right) at their p1 segments posterior communicating artery (left and right) the middle cerebral arteries, supplying the brain, are not considered part of the circle of willis. origin of arteries the left and right internal carotid arteries arise from the left and right common carotid arteries. the posterior communicating artery is given off as a branch of the internal carotid artery just before it divides into its terminal branches - the anterior and middle cerebral arteries. the anterior cerebral artery forms the anterolateral portion of the circle of willis, while the middle cerebral artery does not contribute to the circle. the right and left posterior cerebral arteries arise from the basilar artery, which is formed by the left and right vertebral arteries. the vertebral arteries arise from the subclavian arteries. the anterior communicating artery connects the two anterior cerebral arteries and could be said to arise from either the left or right side. all arteries involved give off cortical and central branches. the central branches supply the interior of the circle of willis, more specifically, the interpeduncular fossa. the cortical branches are named for the area they supply and do not directly affect the circle of willis. variation