CIRCLE OF WILLIS

The system of arteries which conduct blood to the brain, the Basilar and the Internal Carotids, are connected in a rather special way. They terminate in the Circle of Willis, a vascular structure located on the floor of the cranial cavity. The Circle of Willis loops around the brainstem, above the Pons, giving off the major vessels supplying the brain: the Anterior, Middle and Posterior Cerebral Arteries. Because this loop is supplied by three vessels, in an arrangement which provides collateral flow, damage to any one will not compromise the blood supply to the brain. Damage to the circle itself is often clinically insignificant, because blood can flow in either direction around it. The cerebral vessels, however, have no anatomic reserves, and any obstruction occuring there is likely to result in tissue damage.