A, The upper part of the left hemisphere has been removed to expose the atrium and temporal horn. Part of the optic tract and cerebral peduncle has been preserved. The ACA crosses above the chiasm and along the medial surface of the hemisphere. The MCA passes laterally below the anterior perforated substance and turns posteriorly in the depths of the sylvian fissure on the medial side of the opercular lips. The M1 segment courses below the anterior perforated substance and ends at the limen insula, the M2 segment crosses the insular, the M3 crosses the opercular lips, and the M4 branches course on the lateral convexity. B, Enlarged view. The initial segment of the optic tract has been preserved. The MCA courses laterally in the area above and anterior to the temporal pole and turns posteriorly in the sylvian fissure. The sylvian point, the site at which the last MCA turns away from the insula, coincides with the point where the most posterior of the transverse temporal gyri intersect the insula. The PCA is hidden below the optic tract and cerebral peduncle. C, The anterior part of the right hemisphere has been removed to show the symmetry of the MCAs. Lenticulostriate arteries are exposed below the lentiform nucleus. The upper part of the left cerebral peduncle and optic tract has been removed to expose the PCA and basal veins in the crural and ambient cisterns. D, Enlarged view. The P2 arises at the level of the PComA and passes around the brainstem. The anterior part of the P2, the part that passes through the crural cistern, is designated the P2A, or crural segment, and the posterior part that courses in the ambient cistern is designated the P2P, or ambient segment. The P3 is located in the quadrigeminal cistern and the P4 segment consists of the cortical branches. The calcarine branch courses deeply within the calcarine sulcus, roofed above by the cuneus, which has been removed to expose the floor of the calcarine sulcus formed by the lingula. The calcarine branch courses adjacent to the calcar avis, which is the prominence in the medial wall of the atrium formed by the deep end of the calcarine sulcus. E, Enlarged view. The AChA courses around the anterior and posterior uncal segments and the uncal apex to reach the temporal horn just behind the posterior uncal segment. The PComA courses below and medial to the AChA and joins the P1 at the anterior edge of the crural cistern. (Images courtesy of AL RHoton, Jr.)