Relationship of the M1 (blue), M2 (green), M3 (yellow), and M4 (red) segments of the middle cerebral arteries to the insula and sylvian fissure. Upper left and right, superolateral views of the right cerebral hemisphere with the anterior half of the frontal lobe and part of the frontoparietal and temporal opercula removed. Upper left, the removal exposes the anterior quarter of the insula. Upper right, the removal exposes the whole surface of the insula. The sylvian fissure is divided into sphenoidal and operculoinsular compartments. The sphenoidal compartment, in which the M1 segment courses, is located posterior to the sphenoid ridge. The M2 and M3 segments course in the operculoinsular compartment of the sylvian fissure. The operculoinsular compartment is divided into an insular and an opercular cleft. The opercular cleft is located between the frontoparietal and the temporal opercula. The insular cleft is located between the insula and the opercula. The insular cleft is divided into a superior limb, located medial to the frontoparietal operculum, and an inferior limb, located medial to the temporal operculum. The circular sulcus is located at the periphery of the insula. The short gyri of the insula are located above the central sulcus of the insula and the long gyri are located below. The carotid arteries and anterior perforated substance are at the medial end of the sylvian fissure. The lateral ventricles are above the optic nerves. A–D, anterior views of coronal sections of the right cerebral hemisphere. The central diagram shows the level of the sections. A, coronal section at the level of the M1 segment. The M1 segment courses in the sphenoidal compartment, the M2 segment courses on the insulae, the M3 segment passes over the deep surface of the opercula, and the M4 segment courses on the cortical surface. At this anterior level, the frontal operculum covers more of the insula than the temporal operculum. B, coronal section at the midportion of the sylvian fissure where the frontal and temporal opercula are of nearly equal height. C, coronal section at a more posterior level where the temporal operculum covers more of the insula than does the frontoparietal operculum. D, coronal section from the posterior end of the sylvian fissure. Only the opercular cleft remains; the insular cleft has disappeared. (Images courtesy of AL Rhoton, Jr.)