Liliequist Membrane Dissection. A, Left frontotemporal craniotomy and exposure of the basal cisterns (inset). The outer arachnoid membrane covering the sylvian fissure has been opened, the frontal and temporal operculae retracted, and the membranes in the distal part of the sylvian fissure removed to expose the proximal part of the sylvian fissure and carotid artery. The middle cerebral artery has been elevated to expose the proximal sylvian membrane. B, A forceps holds the part of the outer arachnoid membrane that has been preserved. The outer arachnoidal membrane covering the optic nerve has been opened. The proximal sylvian membrane has been preserved. C, Enlarged view. The proximal sylvian membrane arches across the proximal part of the sylvian fissure from the posterior part of the orbital gyri to the medial part of the uncus, surrounds the middle cerebral artery just distal to its origin, and separates the sylvian and the carotid cisterns. D, The outer arachnoidal membrane has been elevated to expose the proximal sylvian and olfactory membranes. (Images courtesy of AL Rhoton, Jr.)