A large portion of the oxygen needed by the brain is supplied via a set of vessels located at the base of the brain that, together, form a structure generally known as the arterial circle of Willis. Principally formed by junctures and communicating vessels (anastomosis) among the basilar artery--formed by the fusion of the left and right vertebral arteries--and the left and right internal carotid arteries, the circle of Willis aids in assuring a continuous supply of oxygenated blood to all portions of the brain.

Although physiological studies utilizing radioisotopes and other traceable markers establish that the majority of the blood originally passing through the left vertebral and left internal carotid arteries normally supply the left side of the brain (with a similar situation found on the right with the right vertebral and right internal carotid arteries), the circle of Willis allows a communication...