Cerebral arterial circle

The cerebral arterial circle (also called circle of Willis, Willis' circle, loop of Willis or Willis polygon) is a circulatory anastomosis of the internal carotid and vertebral arteries, forming an arterial polygon that supplies blood to the brain.

The cerebral arterial circle is composed of the following arteries:

• Anterior circulation:
  • Horizontal segment of anterior cerebral artery (ACA A1) (left and right)
  • Anterior communicating artery (ACOM)
  • Internal carotid artery (ICA) (left and right)
• Posterior circulation:
  • Horizontal segment of oosterior cerebral artery (PCA P1) (left and right)
  • Posterior communicating artery (PCOM) (left and right)

The middle cerebral arteries are not considered part of the circle of Willis.

Considerable anatomic variation exists in the cerebral arterial circle, the classic anatomy of the circle is only seen in 20 to 34.5% of cases.