Walsh & Hoyt: Circle of Willis

The basal arterial circle, first completely illustrated by Thomas Willis in 1664, forms a system of communications between branches of the ICA and BA. This circle is closed anteriorly by the AComA and the proximal segments of both ACAs. Posteriorly, the circle is completed by the two PComAs and the proximal segments of both PCAs. Under normal conditions, blood flow through the ICA is so rapid that blood from each carotid system supplies each hemisphere without cross-flow from the opposite side. Similarly, the posterior cerebral circulation is normally maintained exclusively by the vertebrobasilar system.