A, Superior view. The left PComA is hypoplastic and the right is larger than its corresponding P1. The left PComA is straight and short and the right is long and convex medially. The right P2 segment is a direct continuation of the PComA. An MPChA courses medial to the left P2. Thalamoperforating branches arise at the basilar bifurcation. B, Both P1s arise predominantly from the basilar artery. The hypoplastic PComAs course above and medial to the oculomotor nerves. C, The right PComA and P1 are of approximately equal size, and the junction of the PComA and the P2 is sharply angulated. The left P1 is directed anterior before joining the junction of the P2 and the PComA. The right PComA is much longer than the left. D, The right P1 arises predominantly from the PComA. The right P1 segment is small and short, being only long enough to reach above the oculomotor nerve. The left PComA and P1 are of approximately equal size, but the left P1 is short. The junction of the PComAs and the P2s are sharply angulated on both sides. E, Inferior view. The left P1 is hypoplastic and the left P2 arises mainly from the PComA. The right PCA arises predominantly from the basilar artery. F, Large tortuous PComAs almost touch in the midline. The P2s arise predominantly from the large PComAs, which are larger than the P1 segments. Premamillary perforating branches of the PComA arise on both sides. (Images courtesy of AL Rhoton, Jr.)