A, The left cerebral hemisphere has been removed. The tentorial incisura is located between the tentorial edges and is the only site of communication behind the supra and infratentorial spaces. The tentorial apex is located at the junction of the vein of Galen and the straight sinus. The tentorial edges slope downward from the apex. The free edge passes along the side of the brainstem and anteriorly blends into the dura covering the petrous apex and the anterior and posterior clinoid processes. The incisura, in relation to the midbrain, is divided into anterior, middle, and posterior spaces. The anterior incisural space extends above the optic chiasm to the lamina terminalis and below the chiasm and third ventricular floor to the interpeduncular fossa. The middle incisural space is located between the midbrain and tentorial edge, opens upward into the ambient and crural cisterns, and extends inferiorly into the anterior part of the cerebellomesencephalic fissure. The posterior incisural space, located between the posterior midbrain and the tentorial apex, encompasses the quadrigeminal cistern, which extends into the cerebellomesencephalic fissure and along the outer surface of the upper part of the fourth ventricular roof. The anterior incisural space, located below the frontal horn, contains the basilar bifurcation. The PCA and SCA arise in the anterior and pass around the brainstem to reach the middle and posterior incisural spaces. The branches of the PCA and SCA pass through the lateral part of the posterior incisural space, and the large venous structures converging on the vein of Galen course in the medial part of the posterior incisural space. B, Part of the left central hemisphere and all of the left thalamus have been removed, while preserving the fornix and choroid plexus. The frontal horn and anterior part of the third ventricle is located above the anterior incisural space. The middle incisural space is located medial to the temporal horn, between the temporal lobe and midbrain. The posterior incisural space is located between the tentorial apex and posterior midbrain surface. (Images courtesy of AL Rhoton, Jr.)