A, Anterior view of A1s of nearly equal size. The AComA is hypoplastic and is hidden between the ACAs. Recurrent arteries arise from the A2s at the same level on both sides. B, The A2s have been separated to expose the AComA, which is the site of a perforating branch that enters the brain through the region of the lamina terminalis. C, The A1s are of equal size and give rise to A2s of approximately the same size. The AComA is broad and somewhat dimpled and is expanding behind the right A2 in what may be the beginning of an aneurysm. Both recurrent arteries arise from the proximal A2. D, The left A1 is larger than the right A1. The right recurrent artery arises from a frontopolar artery and passes laterally toward the carotid bifurcation. The AComA is of approximately the same diameter as the left A1 and is the predominant source of flow to both A2s. The floor of the third ventricle has been opened to expose the basilar apex and the P1s. (Images courtesy of AL Rhoton, Jr.)