Illustration showing arteries at the base of the brain called the "Circle of Willis". In this image, there is an absence of one of the anterior cerebral arteries (upper left). In its place, slender branches form a communication between the single anterior cerebral artery and the internal carotid of the opposite side. It is an example of a very rare variety. 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. It is named after Thomas Willis (1621-1675), an English physician. Color lithograph, c. 1844, by Joseph Maclise.