A, Superior view. The part of the left hemisphere above the upper edge of the insula and circular sulcus has been removed. The central sulcus ascends on the right hemisphere and intersects the superior margin of the hemisphere above the posterior part of the body of the lateral ventricle. The upper part of the body of the caudate that extends above the level of the upper margin of the circular sulcus has been removed. Anteriorly, the circular sulcus is located superficial to the anterior edge of the caudate head. The posterior edge of the circular sulcus is situated lateral to the anterior wall of the atrium. B, The section of the right hemisphere has been extended through the upper part of the lentiform nucleus, thalamus, and caudate head. On the left side, the axial section remains at the level of the upper edge of the circular sulcus. The anterior part of the cerebral isthmus is located between the frontal horn and anterior part of the circular sulcus and the posterior part is located between the posterior part of the circular sulcus and the anterior part of the atrium. C, The external and extreme capsule and the claustrum fill the interval between the insula and the lentiform nucleus. The axial section in the left hemisphere extends through the internal capsule just above and lateral to the foramen of Monro where the genu of the capsule reaches the ventricular surface. The anterior limb of the internal capsule is separated from the frontal horn by the caudate nucleus and the posterior limb is separated from the body of the ventricle by the thalamus, but the genu reaches the ventricular surface lateral to the foramen of Monro. D, The section through the left hemisphere has been extended downward below the frontal horn to the level of the anterior commissure. The anterior part of the section extends through the deep gray matter below the frontal horn anterior limb of the internal capsule. At higher levels, the lentiform and caudate nuclei are separated by the anterior limb of the internal capsule, but at this level below the anterior limb of the internal capsule, the two nuclei blend into a mass of gray matter located above the anterior perforated substance and adjacent part of the orbital surface of the frontal lobe. The caudate and lentiform nuclei blend into the nucleus basalis located below the anterior commissure and the nucleus accumbens situated anterior to the nucleus basalis to form a massive collection of gray matter in the basal part of the hemisphere. (Images courtesy of AL Rhoton, Jr.)