Retracted view of entrance to left Sylvian fissure. Retraction of the left frontoparietal and temporal lobes has opened the Sylvian (lateral) fissure to reveal the middle cerebral artery and its branches. The M1 (horizontal) segment gives rise to some early branches (temporopolar and anterior temporal arteries) before giving rise to its M2 segment.  The M2 segment in about 80% of cases divides into a superior and inferior trunk, but as shown here (~10% of cases) splits three ways into superior, middle, and inferior divisions. M3 segmental branches arise from these divisions and course laterally along the frontoparietal and temporal operculum and then terminate at the external surface of the Sylvian fissure where they give rise to M4 branches that supply cortical areas along the lateral convexity of the cerebrum. The frontal poles have been elevated to expose the olfactory bulbs and tracts bilaterally, and the left optic nerve. (Image courtesy of PA Rubino)