A, Stepwise dissection of the left cerebral hemisphere. The inferior frontal gyrus is composed of the pars orbitalis, pars opercularis, and pars triangularis. The precentral gyrus borders the sylvian fissure behind the pars opercularis. The sylvian fissure extends backward and turns up into the supramarginal gyrus at its posterior end. The lower part of the postcentral gyrus is positioned in front of the anterior bank of the supramarginal gyrus. The posterior bank of the supramarginal gyrus is continuous with the superior temporal gyrus. The central sulcus ascends between the pre-and postcentral gyri. There is commonly a gyral bridge (red arrow) connecting the pre- and postcentral gyri below the lower end of the central sulcus, so that the central sulcus does not open directly into the sylvian fissure. Often, with the limited craniotomy opening, the whole sylvian fissure is not exposed to aid in identification of the pre- and postcentral gyri and the central sulcus. The position of the lower end of the pre- and postcentral gyri can be approximated by identifying the pars opercularis just in front of the precentral gyrus and the anterior bank of the supramarginal gyrus just in back of the postcentral sulcus. The angular gyrus wraps around the upturned posterior end of the superior temporal sulcus. B, The part of the frontal lobe above the inferior frontal gyrus and in front of the precentral sulcus has been removed while preserving a thin layer of the medial part of the hemisphere. The inferior frontal sulcus is located on the convexity at the deep level of the lower margin of the corpus callosum and roof of the frontal horn. The gray matter of the cingulate sulcus is exposed above the corpus callosum. C, The opercular lips have been retracted to expose the insula, which is defined at its outer margin by the circular or limiting sulcus. The short gyri are located anteriorly and the long gyri posteriorly. D, Enlarged view with the brain in front of the precentral gyrus removed. The insular gyri radiate upward and backward from the anteroinferior angle situated just lateral to the limen insulae. The short gyri are located deep to the pars triangularis and opercularis. Heschl’s gyrus, the most anterior of the transverse temporal gyri, faces the lower end of the postcentral gyrus across the sylvian fissure. E, Anterosuperior view of the central core of the hemisphere located deep to the insulae. The lentiform nucleus is exposed deep to the insula and is separated from the caudate nucleus by the anterior limb of the internal capsule. The circular sulcus surrounds the insula. F, The supramarginal gyrus has been removed to show its location superficial to the atrium. The posterior margins of the insula and circular sulcus are positioned superficial to the anterior edge of the atrium. The pre- and postcentral gyri are located lateral to the body of the ventricle and the splenium of the corpus callosum. The foramen of Monro is located deep to a point on the pars opercularis approximately 1 cm above the sylvian fissure and deep to the midlevel of the short gyri of the insula. (Images courtesy of AL Rhoton, Jr.)