## Distinguishing Features of ACA vs MCA ### Origin and Caliber Comparison **Key Point:** The MCA is the direct continuation of the internal carotid artery (ICA) and represents the largest and most direct branch, whereas the ACA is a smaller terminal branch of the ICA formed after the MCA bifurcates. ### Anatomical Relationships | Feature | MCA | ACA | |---------|-----|-----| | **Origin** | Direct continuation of ICA | Terminal branch after ICA bifurcation | | **Caliber** | Larger (60–80% of ICA flow) | Smaller (20–40% of ICA flow) | | **Course** | Lateral, within lateral sulcus | Medial, crosses corpus callosum | | **Dominance** | Usually dominant on one side | Often hypoplastic on one side | **High-Yield:** The MCA carries approximately 80% of the cerebral blood flow from the ICA, making it the dominant cerebral artery. This explains why MCA occlusion causes the most extensive middle cerebral territory infarction. ### Clinical Correlation **Clinical Pearl:** In circle of Willis anatomy, the anterior communicating artery (AComm) connects the two ACAs, allowing cross-flow compensation. However, the ACA itself is NOT a direct branch of the AComm — it arises from the ICA bifurcation. The AComm is a connecting vessel, not an origin point. **Mnemonic:** **ICA → MCA + ACA** — The internal carotid artery bifurcates into the larger MCA (lateral) and smaller ACA (medial), with the anterior communicating artery bridging the two ACAs. [cite:Standring Anatomy 41e Ch 27] 
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