It is the major vascular anastomatic trunk of the brain. It is at the base of the brain. Three major arteries go into the circle and three major cerebral leave the circle. Three communicating interconnect the circle. They are the anterior communicating artery which is continuous with each internal carotid, and the two posterior communicating which connect the anterior and middle cerebral arteries to the posterior cerebral arteries. The communication arteries provide a backup system when one internal carotid or vertebral artery are occluded.