Periclinoidal Exposure via Transsylvian Approach. A and B, Right pterional exposure. An ETB is shown arising at a right angle from the proximal half of the prebifurcation part of the M1 segment. The MCA bifurcation is located at the level of the limen insulae. The early branch gives rise to multiple LSAs on its common trunk. B, Enlarged view. The MCA bifurcation has been retracted to expose LSAs arising from the M1 segment distal to the origin of the ETB. The LSAs arise from the posterior and posterosuperior aspects of the ETB, ascend, and cross the superior surface of the M1 segment to reach the anterior perforated substance. C, Right pterional exposure of another specimen. The superior trunk of the MCA has been elevated to expose the LSAs. Approximately 17% of LSAs arise distal to the MCA bifurcation, as demonstrated in this specimen. These perforating arteries have a longer course than those arising from the main trunk of the MCA. The M1 segment divides into a bifurcation at the limen insulae. The postbifurcation trunks of the M1 segment run in nearly parallel courses, diverging only minimally before reaching the genu. In the majority of the specimens, the bifurcation occurs at a variable distance proximal to the genu. The junction of the M1 and M2 segments is at the genu. The M2 segment is composed of trunks coursing over the insula. D and E, Another specimen, left pterional exposure. The main trunk of the M1 segment ends in a trifurcation at the limen insulae. The prebifurcation M1 segment gives rise to a large ETB. E, The M1 segment has been retracted to expose the LSAs arising from the main trunk. The large ETB, arising on the proximal half of the main trunk of the MCA, could be confused with the postbifurcation trunk of the M1 segment in this specimen; however, LSAs are identified on the main trunk distal to the origin of the ETB prior to the trifurcation. The ETB arises at a right angle with respect to the main trunk of the MCA. F, Left pterional exposure of another specimen giving rise to three ETBs. The ETBs are numbered based on whether they are the first, second, or third branch distal to the MCA origin. The first ETB gives rise to a number of LSAs. The course of the LSAs arising from the first ETB is longer than that of the LSAs arising from the prebifurcation M1 segment. The left recurrent artery of Heubner arises from the distal A1 segment, passes laterally below the anterior perforated substance, and intermingles with the LSAs arising from the M1 segment. A small aneurysm is identified at the origin of the third ETB. A. = arteries or artery; Ant. = anterior; A1 = A1 segment of the ACA; A2 = A2 segment of the ACA; Bif. = bifurcation; Br. = branch; Car.A. = carotid artery; Cer. = cerebral; Clin. = clinoid process; CN = cranial nerve; E.T.Br.1 = first ETB; E.T.Br.2 = second ETB; E.T.Br3 = third ETB; Inf. = inferior; Len.Str. = lenticulostriate; Mid. = middle; M1 = M1 segment of the MCA; M2 = M2 segment of the MCA; P.C.A. = posterior cerebral artery; Pit. = pituitary; Post. = posterior; Rec.A. = recurrent artery of Heubner; S.C.A. = superior cerebellar artery; Sup. = superficial or superior; Temp. = temporal; Temp.Br. = temporal branch; Tent. = tentorium; Tr. = trunk; Trif. = trifurcation; V. = vein. (Images courtesy of AL Rhoton, Jr.)