The circle of Willis lies in:
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: Gray's anatomy 40th edition, Chapter 17.
Subarachnoid space
Subdural space
Intraventricular space
None of the above