Central nervous system disorders

V./1.4.: Arterial Circle of Willis

This well-know circle of arteries of the skull base enables the interconnection between the circulation of the right and left sides as well as the carotid and the vertebrobasilar systems. In the front the two sides are connected by the anterior communicating artery, posteriorly the right and left circulations are joined together in the single basilar artery, while the two posterior communicating arteries link the carotid circulation to the vertebrobasilar system.

The Circle of Willis is composed of:

• 1) the anterior communicating artery,

• 2) the A1 segment of the anterior cerebral arteries,

• 3) the bifurcation of the ICA,

• 4) the posterior communicating arteries,

• 5) the P1 segment of both posterior cerebral arteries and

• 6) the bifurcation of the basilar artery.

Considerable anatomic variation exists in the circle, in the cases of an absent artery (most commonly the posterior communicating artery) we can not speak of an actual circle.