Segments of the AICA. A, Anterior view. The a1 arises at the pontine level and passes below CN VI. The s2 begins at the level of a rostrocaudal line extending along the most prominent part of the olive and courses along the lateral medulla. The a3 begins where the artery passes above the flocculus to run on the middle cerebellar peduncle, which forms the floor of the cerebellopontine fissure. B, Enlarged view. The cerebellopontine angle, also called the cerebellopontine fissure (Cer. Pon. Fiss.), is formed by the cerebellum, folding around the pons to form an angular cleft with the flocculus and CNs VII and VIII in the center. The fissure has upper and lower limbs that meet at a lateral apex. C, View through a retrosigmoid craniotomy. The a2 is the site of a loop directed toward or into the internal acoustic meatus. The a3 courses above the flocculus (Flocc.) and along the middle cerebellar peduncle toward the apex of the cerebellopontine fissure. The a4 is formed by the cortical branches supplying the petrosal surface of the cerebellum. D, Posterior view with the posterior meatal lip removed to show the a2 looping into the porus of the meatus and the a3 passing above the floccus. E, Right anterolateral view of a1 to a4. F, Anterolateral view of another brainstem and cerebellum showing a1 to a4. (Images courtesy of AL Rhoton, Jr.)