A, Coronal section at the level of the rostrum of the corpus callosum and anterior part of the frontal horn. The anterior wall and adjacent part of the roof of the frontal horn are formed by the genu of the corpus callosum, the floor by the rostrum, and the lateral wall by the caudate nucleus. The insular surface is small at this level. The gyrus rectus is located medial to the olfactory tracts. B, The section has been extended to the midportion of the frontal horn. The roof is formed by the body of the corpus callosum, the lateral wall by the caudate nucleus, the floor by the rostrum, and the medial wall by the septum pellucidum. The anterior limb of the internal capsule passes between the caudate and the lentiform nuclei. The caudate nucleus blends into the lentiform nucleus in the area below the anterior limb of the internal capsule. The planum polare on the upper surface of the anterior part of the temporal lobe is devoid of gyri and has a shallow trough along which the middle cerebral artery courses. C, The cross section has been extended posteriorly to the level of the lamina terminalis and the anterior commissure. The columns of the fornix pass around the anterior and superior margin of the foramen of Monro and turn downward behind the lamina terminalis toward the mamillary bodies. At this level, the lentiform nucleus has taken on its characteristic triangular or lens shape in cross section. D, Enlarged view. The lamina terminalis has been opened. The anterior limb of the internal capsule separates the caudate and lentiform nuclei. The lentiform nucleus is formed by the putamen and globus pallidus. The anteroinferior part of the caudate and lentiform nuclei blends without clear demarcation into the large mass of gray matter above the anterior perforated substance and adjacent part of the orbital surface of the frontal lobe that also includes the nucleus basalis and accumbens. The nucleus basalis is located below the anterior commissure and the accumbens is located anterior to the basalis without clear demarcation between these two nuclei or the adjacent part of the lentiform and caudate nuclei. The anterior segment of the uncus is exposed lateral to the carotid artery. (Images courtesy of AL Rhoton, Jr.)