A, The callosomarginal artery arises anterior to the genu of the corpus callosum. The distal part of the ACA, the part beginning at the AComA, is divided into four segments: The A2 extends from the AComA to the lower margin of the corpus callosum; the A3 courses around the anterior part of the corpus callosum; the A4 and A5 course above the anterior and posterior half of the corpus callosum, respectively. The anterior part of the falx cerebri is more widely separated from the corpus callosum than the posterior part. The inner edge of the anterior part of the falx is widely separated from the anterior part of the corpus callosum, but the space between the falx and callosal surface narrows as it proceeds posteriorly so that the posterior falx tightly hugs the splenium. The wide opening anteriorly between the falx and the corpus callosum permits the anterior part of the hemisphere and the more forward branches of the ACA to exhibit greater shift anteriorly than posteriorly. B, The falx has been removed. The distal ACA branches extend around the margins of the hemisphere to reach the orbital surface of the frontal lobe and the anterior two thirds of the lateral convexity. The distal part of the pericallosal artery ascends to course along the cingulate sulcus to reach the paracentral lobule. C, The callosomarginal artery arises just distal to the AComA in the cistern of the lamina terminalis and ascends along the cingulate sulcus. The narrow band of the inner edge of the falx that contains the inferior sagittal sinus has been preserved to show the relationship of the branches of the pericallosal artery. The yellow arrow shows the site at which the ACA would show a sharp angulation when shifted to the opposite side by a mass lesion. A callosal artery arises just below the genu of the corpus callosum and crosses the upper callosal surface toward the splenium. D, The pericallosal artery arises in the subcallosal area several millimeters distal to the AComA and sends branches across the superior margin of the hemisphere to supply the adjacent part of the lateral convexity. E, The pericallosal artery turns anteriorly at the level of the lower margin of the genu of the corpus callosum and courses along the cingulate sulcus, where it gives rise to the callosomarginal artery. The pericallosal artery gives rise to a long callosal artery that courses posteriorly to reach the splenium. The pericallosal artery turns anteriorly at the level of the lower margin of the genu of the corpus callosum and courses along the cingulate sulcus, where it gives rise to the callosomarginal artery. The pericallosal artery gives rise to a long callosal artery that courses posteriorly to reach the splenium. F, The callosomarginal artery arises at the level of the lower margin of the callosal genu. The distal segments (A2 to A5) are shown. The ascending ramus of the cingulate sulcus marks the posterior border of the paracentral lobule formed by the central and precentral sulci overlapping onto the medical surface. (Images courtesy of AL Rhoton, Jr.)