Circle of Willis
The major arteries supplying the brain are the right and left ICAs (which distribute blood primarily to the rostral portion of the brain, anterior circulation) and the right and left vertebral arteries (which join to form the basilar artery, posterior circulation). The basilar artery distributes blood primarily to the brainstem and the posterior portion of the brain. These arteries interconnect at the base of the brain at the circle of Willis, also called the cerebral arterial circle (Figs 3-27, 3-28; see also Figs 3-3, 3-16). These interconnections (anastomoses) help distribute blood to all regions of the brain, even when a portion of the system becomes occluded. CN III, in particular, can be affected by vascular lesions within this region.
Figure 3-27 The circle of Willis represents an anastomosis of the anterior, middle, and posterior cerebral arteries. Branches from these vessels supply the distal segment of the intracranial optic nerves, optic chiasm, and optic tract. a. = artery; aa. = arteries; Ant. = anterior; Int. = internal.
(Modified with permission from Liu GT, Volpe NJ, Galetta SL. Neuro-Ophthalmology: Diagnosis and Management. 2nd ed. New York: Elsevier; 2010:295.)
A, Magnetic resonance angiogram showing the circle of Willis in an anteroposterior view. B, An oblique view from the same patient. ACA = anterior cerebral artery; BA = basilar artery; MCA = middle cerebral artery; PCA = posterior cerebral artery; PCoA = posterior communicating artery.
(Courtesy of T. Talli, MD, and W. Yuh, MD.)
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.