A, The left cerebrum, above the level of the cerebral peduncle, has been removed to expose the anterior, middle, and posterior incisural spaces. The thalamus, which forms the floor of the body of the lateral ventricle, sits directly above the central part of the tentorial incisura. The right lateral ventricle and the lower wall of the sylvian fissure have been preserved. The left half of the tentorium, except the edge, has been removed to expose the tentorial cerebellar surface. The frontal horn is located above the anterior incisural space. Structures located in the anterior incisural space below the frontal horn include the optic nerves and chiasm, internal arotid arteries, and the upper part of the basilar artery and its branches. The middle incisural space, located between the midbrain and tentorial edge, opens upward into the crural and ambient cisterns and downward into the anterior part of the cerebellomesencephalic fissure. The posterior incisural space, located between the midbrain and the tentorial apex, includes the area of the quadrigeminal cistern and opens into the central part of the cerebellomesencephalic fissure. The atrium of the lateral ventricle is situated lateral to the posterior incisural space. B, View of the tentorial incisura after removing the cerebrum. The tentorial edges sweep along the lateral margin of the cerebral peduncle. The oculomotor nerve passes medial to the anterior edge of the tentorium and enters the cavernous sinus by passing through a triangular patch of dura called the oculomotor trigone. C, Superior view of the tentorial incisura before removing the left temporal lobe. The crural cistern is located between the cerebral peduncle and uncus. The ambient cistern opens upward between the midbrain and the medial surface of the temporal lobe formed by the parahippocampal and dentate gyri. The thalamus and the genu of the internal capsule are located above the central part of the tentorial incisura. D, Enlarged view after removing the temporal lobe. The internal capsule and the lentiform nucleus are located above the middle incisural space. The genu of the internal capsule abuts on the lateral ventricular wall at the level of the foramen of Monro. (Images courtesy of AL Rhoton, Jr.)