Stepwise Dissection of Right Cerebral Hemisphere. A, The inferior frontal gyrus is composed of the partes orbitalis, triangularis, and opercularis. A gyral bridge connects the lower end of the pre- and postcentral gyri so that the central sulcus does not open into the sylvian fissure. The posterior ramus of the sylvian fissure turns upward into the supramarginal gyrus at its posterior end. B, The frontoparietal operculum has been removed to expose the insula and show the relationship between the central sulcus on the convexity and the central insular sulcus. The inferior end of the central sulcus on the convexity is located less than 5 mm posterior to the central insular sulcus. The Heschl gyrus, the most anterior transverse temporal gyrus, extends obliquely backward and medially from the cortical surface and intersects the posterosuperior angle of the insula. The superior limiting sulcus is oriented horizontally and separates the upper border of the insula from the sylvian surface of the frontal and parietal lobes. The central insular sulcus divides the insula into a larger anterior portion and a smaller posterior portion. Typically, there are three anteriorly placed short gyri that form a radiating pattern converging at the anteroinferior end of the insula. In this hemisphere, there is a fourth short gyrus, the accessory insular gyrus, positioned in front of the other three. The lower end of the accessory gyrus blends into the transverse gyrus, which in turn blends into the orbital gyri. The posterior portion of the insula behind the central insular sulcus consists of a single long gyrus in this hemisphere. C, Anterolateral view of the insula in another right cerebral hemisphere. The frontal operculum has been retracted to expose the most anterior portion of the insula. The anterior, middle, and posterior short gyri form a radiating pattern that converges at the anteroinferior end of the insula. The insular apex is the highest or most prominent laterally projecting site on the insular cortex. The insular pole, the anteroinferior edge of the insula below the apex, is located between the anterior limiting sulcus and the central insular sulcus, and blends inferomedially into the gyri on the orbital surface of the frontal lobe. The deep end of the anterior margin of the pole is joined to the posterior orbital gyri by the transverse insular gyrus. The accessory insular gyrus is located superior to, and blends into, the transverse gyrus. The anterior limiting sulcus extends obliquely upward and forward, deep to the frontal operculum. D–F, Photographs obtained during stepwise dissection of right hemisphere display the relationships between the anterior horizontal, ascending, and frontoorbital rami of the sylvian fissure, and the insular sulci and gyri. D, The frontoparietal and temporal opercula have been retracted to expose the insula. The most consistent rami of the sylvian fissure are the anterior horizontal, anterior ascending, and posterior rami. The anterior ascending and anterior horizontal rami divide the inferior frontal gyrus, from anterior to posterior, into the partes orbitalis, triangularis, and opercularis. The MCA bifurcation is located at the level of the limen insulae in this hemisphere. E, The inferior frontal gyrus has been retracted anterosuperiorly to open partially the anterior horizontal and ascending rami of the sylvian fissure. An additional anterior ramus of the sylvian fissure, called the “frontoorbital ramus,” arises below the anterior horizontal ramus, medial to the pars orbitalis, and crosses the basal surface of the frontal lobe. Opercular landmarks can be used to localize deeper structures in the insular area. The apices of the partes triangularis (yellow arrow), orbitalis (blue arrow), and opercularis (red arrow) lie superficial to the anterior short gyrus and the adjacent short sulci. The free surface of the three parts of the inferior frontal gyrus, if followed to the depths, ends at the limiting sulci. F, Inferolateral view directed along the deep side of the frontal operculum. The anterior ascending, anterior horizontal, and frontoorbital rami have been opened to show the relationship between the opercular landmarks and the insula. Opening the anterior horizontal, anterior ascending, and frontoorbital rami plus the adjacent portion of the posterior ramus of the fissure provides access to the upper anterior portion of the insula. Opening the anterior ascending ramus exposes the superior limiting sulcus. Opening the anterior horizontal ramus exposes the anterosuperior angle of the insula formed by the junction of the superior and inferior limiting sulci. Opening the frontoorbital ramus of the sylvian fissure exposes the anterior limiting sulcus. The upper edge of the pars orbitalis lies superficial to the upper portion of the anterior short gyrus and the adjacent portion of the superior limiting sulcus (blue arrow). The upper end of the pars triangularis is positioned superficial to the upper portion of the anterior short gyrus and the adjacent portion of the superior limiting sulcus (yellow arrow). The upper end of the pars opercularis lies superficial to the superior portion of the middle short gyrus and the adjacent portion of the superior limiting sulcus (red arrow). G and H, Photographs obtained during stepwise dissection of another right hemisphere, showing the relationships between the anterior ascending and anterior horizontal rami, and the insula. G, The anterior horizontal ramus separates the partes orbitalis and triangularis; the anterior ascending ramus separates the partes triangularis and opercularis. H, Opening the anterior horizontal ramus exposes the anterosuperior insular angle and the adjacent portion of the middle short gyrus (orange arrow) in this specimen. Opening the anterior ascending ramus exposes the superior limiting sulcus (green arrow) and the adjacent portion of the middle short gyrus. The superior limiting sulcus of the insula lies in the depths of the upper end of the anterior ascending ramus (green arrow). The anterosuperior insular angle lies in the depth of the anterior end of the anterior horizontal ramus (orange arrow). (Images courtesy of AL Rhoton, Jr.)