Circle of willis Health Dictionary

A circle of arteries at the base of the brain, formed by the junction of the basilar, posterior cerebral, internal carotid and anterior cerebral arteries. Congenital defects may occur in these arteries and lead to the formation of aneurysm (see ANEURYSM).

a circle on the undersurface of the brain formed by linked branches of the arteries that supply the brain (see illustration). Most cerebral *aneurysms occur on or near the circle of Willis. [T. Willis (1621–75), English anatomist]