A, The scalp flap has been elevated and the interfascial incision has been completed so that the fat pad containing the branches of the facial nerve to the forehead can be folded downward with the scalp flap. The one-piece orbitozygomatic bone flap is shown in the inset. B, The sylvian fissure has been opened. The M1 bifurcates to form superior and inferior trunks of similar size. The branches forming the M2 begin at the limen insula and cross the insula. The branches forming the M3 loop over the opercular lips, and the M4 branches course on the lateral convexity. C, Enlarged view of the carotid bifurcation. The M1 divides into superior and inferior trunks before reaching the limen insula, which is located at the lateral edge of the anterior perforated substance. A large A1 passes medially above the chiasm. D, The exposure has been directed under the temporal lobe. A large PComA of the fetal type provides the majority of flow to the P2 segment. As the PComA increases in size, it tends to shift laterally. The junction of the posterior communicating and P2 is situated medial to the oculomotor nerve. The tentorial edge has been depressed to expose the superior cerebellar artery. E, Another subtemporal exposure showing a configuration in which the P1 and PComA are of approximately equal size. F, Exposure oriented like C, showing a small PComA with the predominant P2 origin being from the P1. (Images courtesy of AL Rhoton, Jr.)