A, The upper part of the right cerebral hemisphere has been removed to expose the temporal horn, atrium, and the basal cisterns. The part of the left hemisphere anterior to the midportion of the body of the lateral and above the sylvian fissure has been removed. The ICAs ascend on the lateral side of the optic nerves. The MCAs travel laterally in the sylvian fissures. The M1 crosses below the anterior perforated substance. The trunks of the M2 cross the insula and the M3 extends around the opercular lips. The M4 is formed by the cortical branches on the convexity. The PCAs pass posteriorly in the crural and ambient cisterns to reach the quadrigeminal cistern. The ACA passes above the optic chiasm. The floor of the third ventricle and the calcarine and parieto-occipital sulcus have been exposed. The upper lip of the parieto-occipital sulcus formed by the precuneus has been removed. The lower lip of the parieto-occipital sulcus is formed by the cuneus, which also forms the upper lip of the calcarine sulcus. B, Enlarged view. The AChAs enter the choroid plexus in the temporal horn. The sylvian point is located where the most posterior branch of the M2 turns away from the insular surface and toward the lateral convexity. C, The anterior part of the left hemisphere has been removed down to the level of the temporal lobe and the midbrain. The AChAs pass around the upper medial part of the uncus to reach the temporal horn. The P2A courses medial to the uncus in the crural cistern, the P2P courses in the ambient cistern, and the P3 courses in the quadrigeminal cistern. D, Enlarged view. The M2 crosses the insula just above and lateral to the temporal horn. The artery forming the sylvian point often has its apex directed medially toward the atrium. The parieto-occipital branch of the PCA courses along the parieto-occipital sulcus. The calcarine branch is directed backward in the calcarine sulcus. E, The right temporal lobe has been removed while preserving the M1, M2, and M3. The P2 courses along the medial surface of the temporal lobe. The AChA arises from the carotid artery and takes a somewhat tortuous course to reach the choroid plexus and temporal horn. (Images courtesy of AL Rhoton, Jr.)