The Central Core at the Center of the Brain. A, An axial cut of the brain at the level of the middle frontal gyrus was made, and the frontal operculum over the insular surface was removed. The central core is exposed and found at the morphological center of the brain, with its medial aspects represented by the caudate nucleus and the thalamus found and with its lateral aspect represented by the insular surface. B, An axial cut was performed through the central core, revealing its composition by basal ganglia and fiber pathways. These fiber pathways are organized inside the central core into capsules and named according to their position in relation to the basal ganglia: the extreme capsule is found between the insular surface and the claustrum, the external capsule between the claustrum and the lentiform nucleus, and the internal capsule (formed by the anterior limb, genu, posterior limb, and retrolenticular and sublenticular portions) between the lentiform nucleus laterally and the caudate nucleus and thalamus medially. C, The insular surface is the most superficial aspect of the central core, has a triangular shape, and is easily seen after the sylvian fissure is split. The insular surface is bounded by the anterior, superior, and inferior limiting sulci, with the limen insula at its anteroinferior vertex. The meeting points of these sulci were used in this study and were defined as follows: anterior insular point (where the anterior and superior limiting sulci meet), posterior insular point (where the superior and inferior limiting sulci meet), frontal limen point (where the limen and the anterior limiting sulcus meet), and temporal limen point (where the limen and the inferior limiting sulcus meet). D, A deep cut made around the insular surface, at the limiting sulci, passes through the cerebral isthmus, reaching the lateral ventricle and detaching the central core from the rest of the cerebral hemisphere. E, Most temporal mesial structures are already separated from the central core by the choroidal fissure, a natural cleft between the fornix and the thalamus. The temporal pole and the amygdala are attached to the central core and can be separated by a line traced from the bifurcation of the internal carotid artery or the proximal segment of M1 to the inferior choroidal point (carotid–choroidal line), just lateral to the optic tract. F, The frontal, parietal, and occipital lobes were removed, and the choroidal fissure is seen inside the lateral ventricle. This circular cleft naturally separates the thalamus from the hippocampal formation (fornix and hippocampus), and its opening will reach the roof of the third ventricle (velum interpositum cistern), the pineal region (quadrigeminal cistern), and the parapeduncular region (ambient cistern). (Images courtesy of E de Oliveira)