Middle cerebral artery in Sylvian fissure. The left middle cerebral artery, the larger of the two terminal branches of the internal carotid artery, is shown entering the Sylvian (lateral) fissure where it progressively divides into four M segments. The last or M4 segment sends branches into their final cortical territory. The middle cerebral arteries supply most of the lateral surface of the hemisphere, except the superior part of the parietal lobe and inferior part of the temporal and occipital lobes. The anterior cerebral artery, the smaller terminal branch of the ICA, supplies the medial aspect of the frontal lobes and the superior part of the parietal lobes. The optic nerve system is exposed with the optic nerves, chiasm, and tracts revealed. Mobilization of the temporal lobe and subsequent dissection of the petrous apex and roof of the cavernous sinus shows the trigeminal or Gasserian ganglion and the three divisions of this nerve, V1 or ophthalmic nerve; V2, or maxillary nerve; and V3, mandibular nerve. Traction on the cerebral lobes exposes the oculomotor nerves coursing through the subarachnoid space toward the roof of the cavernous sinus. The cavernous sinus surrounds the cavernous segment of ICA and the abducens and trochlear nerves are shown coursing toward the superior orbital fissure and orbit in company with the ophthalmic nerve. (Image courtesy of PA Rubino)