The Anterior Circulation. A, Photograph obtained during a left pterional exposure. Inset features the position of the head and the scalp incision. The M1 segment begins at the origin of the MCA, extends laterally within the depths of the sylvian fissure, crossing below the anterior perforated substance, and ends at the site of a 90-degree turn, the genu, located at the level of the limen insulae. The postbifurcation trunks of the M1 segment ran nearly parallel to each other, diverging only minimally prior to reaching the genu. B and C, Photographs featuring stepwise dissections of left cerebral hemisphere. B, Anterior view. The coronal cross-section extends through the foramen of Monro. The ICA ascends lateral to the optic nerve. The M1 segment begins at the origin of the MCA, extends laterally below the anterior perforated substance, and ends at the genu, located at the level of the limen insulae. It gives rise to the LSAs. The M2 segment begins at the genu, where the MCA trunks pass the limen insulae, and ends at the limiting sulci along the periphery of the insula. The M3 segment courses around the opercular lips. The sylvian point is located at the point at which the most posterior branch of the M2 segment turns away from the insular surface toward the lateral convexity near or at the junction of the superior and inferior limiting sulci. C, Anterosuperior view of the same hemisphere. The coronal cut has been extended to the level of the lateral geniculate body, and the temporal horn has been exposed through an additional axial cut. The M2 segment crosses the insula above and lateral to the temporal horn. The M3 branches turn laterally away from the insula and cross the transverse temporal gyri to reach the cortical surface. The most anterior transverse temporal gyrus forms the Heschl gyrus. D, Photograph of another specimen of the left cerebral hemisphere, left pterional exposure. The sphenoidal compartment of the sylvian fissure sits behind the sphenoidal ridge in the area proximal to the limen insulae. The M1 segment gives rise to early frontal and temporal branches and bifurcates at the level of the limen insulae. The LSAs arise from both of the early branches. (Images courtesy of AL Rhoton, Jr.)