Drawing and Anatomic Dissections of the Intrasylvian Membranes. A, Anterior view of a drawing of a coronal section extending through the left sylvian fissure showing the lateral, intermediate, and medial sylvian membranes. The outer arachnoidal membrane forms the outer wall of the sylvian cistern and fissure. The lateral sylvian membrane spans the lateral part of the sylvian fissure and extends from the frontoparietal to the temporal operculum deep to the superficial sylvian veins. The intermediate sylvian membrane spans the interval between the medial part of the frontoparietal operculum above and the medial side of the superior temporal and Heschl’s gyrus below. The medial sylvian membrane extends downward from the medial edge of the frontoparietal operculum and attaches to the insula on the medial side of the M2 trunks. B, Lateral view of a left sylvian fissure. The outer arachnoid membrane covering the sylvian fissure is intact. C and D, The outer arachnoid membrane has been removed to expose the lateral sylvian membrane. The lateral sylvian membrane is positioned deep to the superficial sylvian veins and blends into the outer arachnoid membrane along the margins of the sylvian fissure. E, The inset shows the area of view. The intermediate sylvian membrane stretches from the medial part of the frontoparietal operculum above to the temporal operculum below and separates the medial and lateral parts of the posterior compartment of the sylvian fissure. This membrane is positioned superficial the M2 trunks and is penetrated by the M3 branches as they pass laterally to reach the cortical surface. F, The medial sylvian membrane extends downward from the frontoparietal operculum and attaches to the insula on the medial side of the M2 trunks. (Images courtesy of AL Rhoton, Jr.)