The circle of Willis (circulus arteriosus cerebri) is an collection of arteries that are located at the base of the brain. The “circle” was named after Thomas Willis (1621-1675) by his student, Richard Lower. Willis authored Cerebri Anatome which described and depicted this vascular ring of arteries. The circle of Willis encircles the stalk of the pituitary gland and provides a connection between the internal carotid (the internal carotid is cut in this image) and the vertebrobasilar arterial brain supply systems. The circle of Willis is formed when each internal carotid artery divides into the anterior cerebral artery and middle cerebral artery. The anterior cerebral arteries are then united by an anterior communicating artery. Posteriorly, the basilar artery branches into a left and right posterior cerebral artery, forming the posterior circulation. The circle of Willis is completed by the posterior communicating arteries which join the posterior and internal carotid arteries. This arrangement potentially allows blood supply to the brain when there is occlusion of one of the internal carotid arteries, or vertebral arteries. However, the collateral circulation provided by this circle is typically insufficient to fully compensate for such an occlusion.
Image courtesy of Drs. Suárez-Quian and Vilensky.
Suárez-Quian, C.a. and Vilensky, J.A. 2016 All-in-One Anatomy Exam Review. Image-Based Questions and Answers. Volume 6: The Head. Apple iBooks.