<prompt> The circle of Willis lies in:Subarachnoid spaceSubdural spaceIntraventricular spaceNone of the above  <response>: The circulus aeriosus circle of Willis is a large aerial anastomosis which unites the internal carotid and veebrobasilar systems.It lies in the subarachnoid space within the deep interpeduncular cistern, and surrounds the optic chiasma, the infundibulum and other structures of the interpeduncular fossa.Ref: Grays anatomy 40th edition, Chapter 17. 