A, Superior view. The central core is the portion located between the insular surface laterally and the lateral and third ventricles medially. We refer to the narrow strip of white matter deep to the circular sulcus and connecting the central core to the remaining hemisphere as the cerebral isthmus. The isthmus, at the margin of the core, conveys all the fibers related to all of the motor and sensory pathways, including those that form the internal capsule and optic radiations. The anterior margin of the circular sulcus is separated from the frontal horn by the relatively thin anterior part of the isthmus, and the posterior margin of the circular sulcus is separated from the atrium by the narrow posterior part of the isthmus. The upper margin of the isthmus separating the upper margin of the circular sulcus and the lateral ventricle is somewhat thicker than the anterior or posterior margin of the isthmus. The transverse temporal gyri, the most anterior of which is Heschl’s gyrus, are located lateral to the posterior margin of the insula on the planum temporale. An area without gyri anterior to the planum temporale on the anterior part of the upper surface of the temporal lobe, called the planum polare, faces the anterior part of the lateral surface of the insula. The transverse temporal gyri radiate forward and laterally from the posterior isthmus located lateral to the atrium. The sylvian point, the site at which the last insular branch of the middle cerebral artery turns laterally from the insula, is located lateral to the posterior isthmus and the posterior part of the circular sulcus. B, Superior view of the central core and the anterior and posterior parts of the cerebral isthmus. The transverse temporal gyri seem to radiate laterally and forward from an apex situated lateral to the atrium, the posterior part of the circular sulcus, and the posterior isthmus. The section extends through the anterior and posterior limb and genu of the internal capsule, thalamus, and lentiform and caudate nuclei. The thalamus is located directly above the midbrain in the center of the tentorial incisura. C, Superolateral view of the insula, circular sulcus, and the anterior and posterior isthmi. The circular sulcus extends completely around the margin of the insula and is located superficial to the white matter forming the cerebral isthmus. The isthmus is the thinnest area between the insular and ventricular surfaces. D, The upper surface of the left temporal lobe has been retracted to expose the lower part of the circular sulcus and isthmus located deep to the circular sulcus. An incision extending through the thin isthmus at the lower margin of the circular sulcus will expose the temporal horn, but will also cut across the optic and auditory radiations and the sublenticular part of the internal capsule, unless only a short segment of the anterior part of the lower isthmus is opened. E, Lateral view of the central core. The cerebral hemisphere has been removed by dividing the isthmus, located deep to the circular sulcus and extending around the margin of the insula. The corpus callosum and fibers crossing the midline were also divided. Middle cerebral branches course along the insular surface. The lower margin of the circular sulcus is located deep to the superior temporal sulcus. F, All of the central core has been removed. It includes the caudate and lentiform nuclei, thalamus, and some of the corona radiata and internal capsule. The medial part of the core has been separated from the ventricular surface by opening the choroidal fissure, the natural cleft and cleavage plane between the thalamus and fornix. The body, crus, and fimbria of the fornix form the outer border of the choroidal fissure. The transverse temporal gyri, forming the planum temporale, radiate forward from the posterior edge of the circular sulcus located lateral to the atrium. The quadrigeminal cistern is located medial to the crus of the fornix. (Images courtesy of AL Rhoton, Jr.)