Circle of Willis

The circle of Willis is a ring of vessels connecting the anterior and posterior circulations of the brain. The ring is bounded anteriorly by a single anterior communicating artery (ACom), which connects the bilateral anterior cerebral arteries (ACA). The ACAs course posterolaterally until reaching their lateral-most connection to the internal carotid artery (ICA), which runs cephalically through the neck and into the brain. As each ICA runs its course, they individually give off an ophthalmic artery. At the point of connection between the ACA and the ICA, the lateral continuation of the ICA becomes the middle cerebral artery (MCA). Coursing posteromedially from each ACA-ICA junction is the posterior communicating artery (PCom). The PCom connects the MCA with the posterior cerebral arteries (PCA), which form the posterior-most aspect of the CoW. The bilateral PCAs fuse to become the basilar artery (BA). The BA courses caudally along the anterior pons, giving off many branches, including the superior cerebellar arteries and pontine arteries, and the anterior inferior cerebellar artery. The BA then divides into the bilateral Vertebral arteries (VAs), which each gives off a posterior inferior cerebellar artery (PICA) and contributes to the formation of a single anterior spinal artery.