The collateral circulation to the brain exists in 3 main areas: (1) through extracranial arterial connections; (2) through developmental extracranial to intracranial persisting arteries; and (3) through intracranial cerebral arterial anastomoses. The intracranial cerebral arterial collateral occurs in 3 main areas: (1) through connections of the large base vessels (circle of Willis); (2) through connections between the perforating vessels to the basal cell masses; and (3) through interarterial connections on the suprasegmental areas of the brain. All collateral occurs on the surface of the brain.