After the internal carotid arteries enter the skull from each side, they will each trifurcate into the anterior cerebral artery, middle cerebral artery, and posterior communicating artery.
The vertebral arteries, entering the skull posteriorly, will join to form the basilar artery. The basilar artery bifurcates into the right and left posterior cerebral arteries.
The two anterior cerebral arteries from both sides are joined together anteriorly by the anterior communicating artery. The posterior commicating arteries join the posterior cerebral arteries, completing the circle of Willis.
As the arteries in the circle of Willis form a complete ring, they effectively act as anastomoses for each other. This means that if any one of the communicating arteries becomes blocked, blood can flow from another part of the circle to ensure that blood flow is not compromised.