circle of Willis

[wĭlĭs]
n.
  1. A roughly circular anastomosis that is located at the base of the brain and formed by the anterior communicating artery, the two anterior cerebral, the two internal carotid, the two posterior communicating, and the two posterior cerebral arteries.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.