The Circle of Willis provides multiple paths for oxygenated blood to supply the brain if any of the principal suppliers of oxygenated blood (i.e., the vertebral and internal carotid arteries) are constricted by physical pressure, occluded by disease, or interrupted by injury. This redundancy of blood supply is generally termed collateral circulation.
Found on the inferior brain surface surrounding the pituitary gland which was cut off to show the circle. The stalk of the pituitary remains highlighted in aqua. The optic nerves are also cut (yellow ).
The arteries on the right side are easily viewed. The left side of the circle is folded but the middle cerebral artery (f) is seen disappearing into the temporal lobe. The vertebral arteries that lead into the basilar artery (e) have been cut off.