A ring of arterial supply at the base of the brain, around the optic chiasm. Named for Thomas Willis, an English doctor with an interest in anatomy and physiology. (more on Thomas Willis at http://es.rice.edu/ES/humsoc/Galileo/Catalog/Files/willis.html)
The blood supply to the circle of Willis:
Arteries supplied by the Circle of Willis:
There are communicating arteries, namely the anterior communicating artery and the posterior communicating arteries which allow for anastomosis of blood supply between the two internal carotid arteries and the basilar artery.
It is due to the anastomoses between the various blood supplies that the brain can continue to function even if, say, one carotid artery is completely occluded by an over-enthusiastic medical student feeling for a pulse. | 
