A, The coronal section of the right hemisphere crosses vertically through the thalamus and lateral geniculate body and the transverse section crosses the cerebral peduncle. The right temporal horn has been opened to expose the hippocampus and amygdaloid nucleus. The floor of the third ventricle is exposed in the midline. The coronal section of the left hemisphere crosses anterior to the thalamus near the foramen of Monro and genu of the internal capsule. The anterior incisural space extends from the interpeduncular fossa, around the chiasm, and into the suprachiasmatic area. B, The right thalamus has been removed while preserving the fornix, which wraps around the thalamus to form the outer edge of the choroidal fissure situated between the thalamus and fornix. The middle incisural space extends upward into the ambient and crural cisterns. The crural cistern is located between the uncus and the cerebral peduncle. The ambient cistern in located between the parahippocampal and dentate gyri and the fimbria of the fornix laterally and the midbrain medially. The posterior part of the tentorial edge is exposed. The quadrigeminal cistern is located in the posterior incisural space between the tentorial apex and the pineal. The atrium is located lateral to the quadrigeminal cistern and posterior incisural space. C, Enlarged view. The coronal section through the left hemisphere has been extended backward to the level of the thalamus and posterior limb of the internal capsule. The left temporal horn is exposed below the basal ganglia. The optic nerves, chiasm, and tracts, and the oculomotor nerves cross the anterior incisural space. The middle incisural space extends into the ambient and crural cisterns, and the posterior incisural space, located in front of the tentorial apex, contains the quadrigeminal cistern. D, The upper parts of the anterior and middle incisural spaces have been exposed by removing the thalami on both sides. The tentorial edges extend forward from the apex, located at the posterior margin of the pineal region, along the side of the midbrain to attach to the petrous ridge and clinoid processes. E, The temporal lobe has been sectioned in a coronal plane and the third ventricular floor has been removed. The lateral wall of the ambient cistern is formed by the parahippocampal and dentate gyri and the fimbria of the fornix. F, Enlarged view. The rounded medial edge of the parahippocampal gyrus, the site of the subiculum, which blends into the hippocampus, joins the dentate gyri and fimbria to form the lateral wall of the ambient cistern. The fimbria arises on the hippocampal surface. (Images courtesy of AL Rhoton, Jr.)