Illustration of Sylvian Dissection. A, View through a left frontotemporal craniotomy (inset). The outer arachnoid membrane and the sylvian fissure have been opened to expose 10 of the 12 arachnoid membranes seen in this study. These membranes are: the lateral, intermediate, medial, and proximal sylvian; olfactory; lateral lamina terminalis; medial lamina terminalis; medial carotid; intracarotid; and intracrural membranes. The proximal sylvian membrane extends from the posterior orbital gyri to the anterior part of the uncus, and it separates the sylvian and the carotid cisterns. The lateral sylvian membrane spans the outer margin of the fissure deep to the outer arachnoid membrane. The superficial sylvian veins are sandwiched into the narrow space between the outer arachnoid and lateral sylvian veins. The intermediate sylvian membrane extends downward from the frontoparietal operculum, at a deeper level than the lateral membrane, and attaches to the upper surface of the temporal lobe along the medial edge of the superior and transverse temporal gyri. The M2 trunks course deep to the intermediate sylvian membrane. The medial sylvian membrane extends inferomedially from the medial edge of the frontoparietal operculum and attaches to the insula. The M2 trunks course on the inferolateral side of the medial sylvian membrane. The medial carotid membrane extends downward from the lower surface of the optic chiasm and attaches to the outer arachnoidal membrane covering the posterior clinoid process and lateral sellar area, and it separates the carotid and the chiasmatic cisterns. The intracarotid membrane extends downward within the carotid cistern from the inferolateral surface of the optic tract and attaches to the anterior segment of the uncus. The intracrural membrane extends obliquely across the interval between the posterior part of the uncus and the cerebral peduncle at the entrance into the crural cistern, and it separates the cistern into upper and lower parts. The lateral lamina terminalis membrane extends from the gyrus rectus to the optic chiasm, and it separates the lamina terminalis and the carotid cisterns. The medial lamina terminalis membrane extends from the adjacent posteromedial edges of the paired rectus gyri to the outer arachnoid membrane, and it separates the lamina terminalis and pericallosal cisterns. The olfactory membrane extends from the posterior orbital gyri and below the olfactory tract to the posterior part of the gyrus rectus. B, Enlarged view to show Liliequist’s membrane. Liliequist’s membrane arises from the outer arachnoid membrane covering the posterior clinoid processes and dorsum sellae. As this membrane spreads upward from the dorsum and across the interval between the oculomotor nerves, it splits into 2 separate arachnoid sheets. One sheet, the diencephalic membrane, extends upward and attaches to the floor of the third ventricle at the mamillary bodies and separates the chiasmatic and interpeduncular cisterns. The second sheet, the mesencephalic membrane, extends backward around the basilar artery just below the origin of the posterior cerebral arteries and attaches along the junction of the midbrain and pons to separate the interpeduncular and prepontine cisterns. (Images courtesy of AL Rhoton, Jr.)