A, Inferior view. The anterior perforated substance forms of the roof of the sphenoidal compartment of the sylvian fissure. It extends from the olfactory striae anteriorly to the optic tract and stem of the temporal lobe posteriorly. On the medial side, it extends to the interhemispheric fissure and laterally to the limen insula. The anterior uncal segment faces the anterior perforated substance. The posterior segment faces the cerebral peduncle. B, The right temporal pole has been removed down to the level of the stem of the temporal lobe that forms the posterolateral margin of the anterior perforated substance. The limen insula is situated at the lateral margin of the anterior perforated substance anterior to the stem of the temporal lobe. On the left side, the lower part of the posterior uncal segment has been removed to expose the upper part formed largely by the hippocampal head. The anterior perforated substance has a salt-and-pepper appearance, created by small openings through which the perforating arteries and veins penetrate the hemisphere. The area where a third ventriculostomy is performed is located in front of the mamillary bodies. C, The view has been directed lateral to the limen to the insula and frontal operculum. The lower ends of the short and long gyri of the insula are exposed lateral to the stem of the temporal lobe and medial to the gyri on the frontal operculum. D, Anterior view with the lips of the opening into the operculoinsular component of the sylvian fissure retracted. The sylvian fissure, lateral to the limen, extends backward and upward, and between the insula medially and the frontal and temporal opercula laterally. The posterior perforated substance is located between the cerebral peduncles. E, Lateral view of the right hemisphere. In this hemisphere, the frontal and parietal opercula do not meet the temporal operculum, thus exposing the inferior part of the long and short gyri of the insula. F, The opercular lips have been retracted to expose the long and short gyri of the insula and the circular sulcus at the outer insular border. (Images courtesy of AL Rhoton, Jr.)