A, Superior view of the thalamoperforating arteries arising from the P1 segment. The left thalamoperforating artery is larger than the right one. The medial part of the A1s was removed to provide this view of the basilar apex. B, Inferior view of another basilar bifurcation. Both P1s contribute to the tuft of thalamoperforating arteries entering the interpeduncular fossa. The right AChA courses above and lateral to the PComA and turns laterally above the uncus. An MPChA arises from the right P1. C, Superior view of the thalamoperforating arteries arising from P1. The basilar artery, below the P1 origins, also send perforating branches in the same area. D, Inferior view. The right P1 is much smaller than the left P1, but the right P1 gives rise to a tuft of thalamoperforating arteries that pass upward to enter the interpeduncular fossa. A nerve hook holds up a premamillary branch of the PComA. (Images courtesy of AL Rhoton, Jr.)