Percentage of Cortical Branches of the ACA and MCA That Supply the Upper, Middle, and Lower Thirds of the Central Lobe. The watershed area between the ACA and MCA is located approximately 1.5 cm lateral to the superior margin (green shaded area). The branches of the MCA (central, precentral, and anterior parietal arteries) arise near the surface of the sylvian fissure and ascend to anastomose with the descending cortical branches of the ACA (posterior interior frontal, paracentral, and superior parietal arteries), which fold around the superior margin. (Image courtesy of AL Rhoton, Jr.)