Basal Arachnoid Membranes. E, The outer arachnoidal membrane has been opened to expose the lateral lamina terminalis membrane, which extends from the posterolateral edge of the gyrus rectus to the lateral edge of the optic chiasm and separates the lamina terminalis and the carotid cisterns. The anterior cerebral artery passes above the optic tract and through the lateral terminalis membrane to enter the lamina terminalis cistern. The proximal sylvian, olfactory, and lateral lamina terminalis membranes blend into each other near their attachment to the frontal lobe. The olfactory membrane extends from the posterior orbital gyrus and below the olfactory tract to the gyrus rectus. F, The frontal lobes have been retracted to expose the lamina terminalis and pericallosal cistern. The medial lamina terminalis membrane extends across the adjacent posteromedial edges of the paired rectus gyri and separates the lamina terminalis from the pericallosal cistern. The proximal A2s penetrate this membrane to exit the lamina terminalis cistern and enter the pericallosal cistern. G, Inferior view of the proximal part of the left sylvian fissure. The proximal sylvian membrane separates the sylvian and carotid cisterns. The olfactory membrane extends from the posterior orbital gyrus and below the olfactory tract to the gyrus rectus. The proximal sylvian, olfactory, and lateral lamina terminalis membranes blend into each other along their margins. H, The optic nerve has been retracted downward. The lateral lamina terminalis membrane extends from the posterolateral part of the gyrus rectus to the optic chiasm, separates the lamina terminalis and carotid cisterns, and blends laterally into the olfactory membrane, which extends below the olfactory tract. (Images courtesy of AL Rhoton, Jr.)