Views of the M1 Segment. A, Anterior view of the left M1 segment. The frontal operculum has been retracted to expose the insular courses of the orbitofrontal and prefrontal arteries arising from a common trunk, which is an EFB. The single main trunk of the MCA gives rise to an ETB and an EFB. The temporal branch arises more proximally than the EFB. The EFB arises from the M1 segment, passes upward, forward, and somewhat laterally toward the anterosuperior portion of the insula. The EFB bifurcates into orbitofrontal and prefontal arteries at the level of the transverse gyrus of the insula. The orbitofrontal artery courses along the anterior limiting sulcus of the insula before ascending to supply the inferior portion of the pars orbitalis. The prefrontal artery courses along the anterior short gyri of the insula to reach the junction between the anterior and superior edges of the limiting sulci. B and C, Right pterional exposure of another specimen with a large ETB arising on the proximal half of the prebifurcation M1 segment. The M1 segment bifurcates at the level of the limen insula proximal to the genu. C, Another view of the same specimen. The ETB arising from the proximal half of the main trunk of the MCA gives rise to four cortical arteries: the temporopolar and the anterior, middle, and posterior temporal arteries. After arising from the common trunk, the anterior temporal artery passes laterally across the temporal operculum, but does not reach the limen insulae. The middle temporal artery sends branches to the limen area. The posterior temporal artery courses along the inferior edge of the limiting insular sulcus, parallel to the inferior trunk of the MCA, prior to ascending over the temporal operculum, en route to the cortex. D and E, Anomalous arteries. D, An accMCA arises from the proximal A2 segment. The diameter of the accMCA is smaller than those of the ACA and the MCA. The accMCA courses parallel to the prebifurcation part of the M1 segment, supplying the accessory and the transverse gyri of the insula, and terminates in the territory of the orbitofrontal artery. The prebifurcation M1 segment gives rise to an ETB on its proximal half, and LSAs arise from the M1 segment distal to the origin of the early branch. E, Left pterional exposure. A duplMCA originates from the ICA. The prebifurcation M1 gives rise to an ETB on its distal half. The ETB has been retracted to expose the origin of the duplMCA, which has been marked with an arrow. The duplMCA originates between the anterior choroidal artery and the ICA bifurcation. The anomalous artery courses parallel and lateral to the M1 segment and terminates in the temporopolar territory. (accMCA = accessory MCA, duplMCA = duplicate MCA). (Images courtesy of AL Rhoton, Jr.)