Regarding the arterial components of the Circle of Willis, all of the following statements are correct EXCEPT:

## Circle of Willis — Arterial Anatomy ### Structure Overview The Circle of Willis is a polygonal arterial anastomosis at the base of the brain that provides collateral circulation between the anterior and posterior circulations. ### Components of the Circle of Willis | Arterial Segment | Origin/Course | Function | |---|---|---| | Anterior communicating artery (AComm) | Connects the two ACAs | Links left and right anterior circulation | | Posterior communicating artery (PComm) | Arises from ICA, joins PCA | Links anterior and posterior circulation | | Anterior cerebral artery (ACA) | Terminal branch of ICA | Part of circle; supplies medial cerebral hemisphere | | Posterior cerebral artery (PCA) | Terminal branch of basilar artery | Part of circle; supplies occipital and temporal lobes | | Middle cerebral artery (MCA) | Terminal branch of ICA | **NOT part of the Circle of Willis** | ### Key Point: **The MCA is NOT a component of the Circle of Willis.** The MCA arises as the larger terminal branch of the internal carotid artery but courses laterally into the lateral sulcus (Sylvian fissure) and does not participate in the polygonal anastomosis at the base of the brain. ### High-Yield: The Circle of Willis is formed by: - **Anteriorly:** ACA + AComm + ACA - **Posteriorly:** PCA + PComm + PCA - **Laterally:** ICA segments proximal to the ACA and PCA origins ### Clinical Pearl: Variations in the Circle of Willis (hypoplasia or absence of PComm or AComm) are common (up to 50% of the population) and may affect collateral flow in stroke scenarios. An intact MCA is essential for lateral cerebral perfusion but operates independently of the circle. ### Warning: ~~The MCA is sometimes incorrectly included as part of the Circle of Willis~~ — it is a major cerebral artery but does NOT form an anastomotic loop with the posterior circulation.