A, The tentorial incisura is divided into an anterior incisural space located anterior to the brainstem, a middle incisural space located be-tween the midbrain and tentorial edge, and a posterior incisural space located between the tentorial apex and posterior surface of the midbrain. The anterior incisural space contains the chiasmatic and interpeduncular cisterns. The middle incisural space communicates with the ambient and crural cistern. The posterior incisural space contains the quadrigeminal cistern. B, Superior view with the left cerebrum and left half of the tentorium removed. The frontal horn sits above the anterior incisural space. The thalamus sits directly above the midbrain in the center of the tentorial incisura. The middle incisural space is located between the midbrain and tentorial edge. The atrium faces the posterior incisural space and quadrigeminal cistern. C, Another specimen. The axial section of the right hemisphere extends through the internal capsule. The frontal horn is located above the anterior incisural space. The thalamus is located above the midbrain in the center of the incisura and above the middle incisural spaces. The medial wall of the atrium forms the lateral wall of the quadrigeminal cistern and posterior incisural space. The internal capsule is situated above the lateral edge of the three incisural spaces. D, Comparison of the relationships in the tentorial incisura (D-left) and temporal horn (D-right). The neural structures on the right have been removed except the temporal horn. The temporal lobe on the left was removed to expose the tentorial incisura. The choroidal fissure opens between the fimbria and the thalamus into the middle incisural space located lateral to the midbrain. The temporal horn is positioned lateral to the middle incisural space. The lower part of the medial wall of the atrium faces the posterior incisural space. (Images courtesy of AL Rhoton, Jr.)