A, The anterior incisural space extends forward from the interpeduncular fossa below the floor of the third ventricle and around the optic chiasm to the lamina terminalis. The middle incisural space extends upward into the crural cistern located between the uncus and cerebral peduncle and the ambient cisterns located between the lateral midbrain and the temporal lobe. The posterior incisural space is located behind the midbrain and includes the quadrigeminal cistern and pineal region. The anterior part of the tentorial edge has grooved the uncus. B, The medial edge of the parahippocampal gyrus has been removed to expose the roof of the ambient cistern formed by the lower surface of the thalamus and the geniculate bodies. The optic tract extends posteriorly in the roof of the crural cistern and terminates in the lateral geniculate body located in the anterior part of the roof of the ambient cisterns. The dentate gyrus and the fimbria of the fornix are located in the lateral margin of the ambient cistern above the parahippocampal gyrus. C, The part of the parahippocampal gyrus below the temporal horn has been removed while preserving the fimbria of the fornix. The choroid plexus in the temporal horn is attached along the choroidal fissure located between the fimbria and the thalamus. D, All but the upper part of the left temporal lobe and fimbria has been removed. The optic tract extends posteriorly through the crural cistern to the anterior part of the ambient cistern where it terminates in the lateral geniculate body. The posterior incisural space between the midbrain and the tentorial apex borders the atrium laterally. (Images courtesy of AL Rhoton, Jr.)