K, The initial cut through the hemisphere exposes the frontal horns and bodies of the lateral ventricles. Three cuts, two coronal cuts and one horizontal, are then completed to expose the atrium and posterior part of the temporal horn. The posterior coronal cut (No. 1) is directed obliquely forward along the medial wall of the atrium. The second coronal cut (No. 2) crosses the hemisphere at the anterior part of the atrium just behind the pulvinar. The horizontal cut (No. 3) is located at the level of the floor of the atrium. The three cuts expose the atrium from the pulvinar back to the medial wall. L, Superolateral view obtained with cuts shown in K. M, The temporal horn is exposed using two cuts. One (No. 1) is directed through the lower margin of the circular sulcus to the temporal horn, and the second is a transverse cut (No. 2) located at the level of the floor of the temporal horn. Removing the block of tissue between the two cuts exposes the temporal horn. The collateral eminence overlying the deep end of the collateral sulcus is well seen, but it is difficult to see the hippocampus because it is located further medially below the insula and lentiform nucleus. N, A sagittal cut medial to the insula exposes the lentiform nucleus. The incision extends through the lentiform nucleus and amygdala. The full length of the choroidal fissure from the foramen of Monro to the inferior choroidal point, located behind the head of the hippocampus, is exposed. The bulb of the corpus callosum overlying the forceps major and the calcar avis overlying the deep end of the calcarine sulcus are exposed in the medial wall of the atrium. O, Enlarged view of the foramen of Monro. The columns of the fornix pass around the superior and anterior margins of the foramen of Monro. The anterior nucleus of the thalamus sits in the posterior margin of the foramen of Monro. The thalamostriate vein passes forward between the caudate nucleus and thalamus and through the posterior margin of the foramen of Monro. The choroidal fissure in the body of the lateral ventricle is located between the body of the fornix and the thalamus. A superior choroidal vein passes along the choroid plexus. P, The opening in the choroidal fissure is begun by dividing the tenia fornix, the delicate membrane that attaches the lateral margin of the fornix to the choroid plexus. Opening the tenia on the thalamic side, by opening the tenia thalami, carries greater risk of damaging the thalamostriate vein than opening the forniceal side of the fissure. The internal cerebral vein and medial posterior choroidal arteries are exposed in the roof of the third ventricle. (Images courtesy of AL Rhoton, Jr.)