Circle of Willis

Circulus arteriosus cerebri; ring of arterial blood vessels within the cranium at the base of the brain; formed by the paired internal carotid arteries and vertebral arteries; supplies blood to the mid- and forebrain. 2 The appearance of the circle and the pattern of its major branches is quite constant among mammals, although the sources from which the circle is supplied and the directions in which blood flows in certain vessels vary.