M, The A1 branches arise below the medial part of the anterior perforated substance and pass superior. The lateral half of the A1 segment is a richer site of perforating branches than the medial half. Inset: the branches arise from the posterior, superior, or posterosuperior surface of the A1 segment. N, The A1 branches enter the narrow band of anterior perforated substance extending above the optic chiasm. They enter predominantly the posterior and middle zone of the medial territory of the anterior perforated substance. O, As many as four recurrent arteries may arise from the anterior cerebral artery, either proximal to or near the level of the AComA. They pass laterally above the carotid bifurcation and give branches to the full mediolateral extent of the anterior perforated substance. They may wander forward on the posterior part of the orbital surface of the frontal lobe. Inset (lower right): Site of origin of the recurrent arteries. Left inset: Recurrent artery origins near the junction of the A1 and A2 segments. The cross section of the artery at this level is oriented in a transverse plane. These branches arise predominantly from the lateral side of the vessel. Right inset: Site of origin of recurrent arteries arising from the A1 segments. The cross section of the artery at this level has an orientation in the sagittal plane. The branches arise predominantly from the superior or posterosuperior surface. P, The branches of the recurrent artery enter predominantly the anterior half of the anterior perforated substance along its full mediolateral extent, from the interhemispheric fissure to the limen insulae. (Images courtesy of AL Rhoton, Jr.)