Posterior View of the Suboccipital Surface of the Cerebellum. Upper, Posterior view of the suboccipital surface of the cerebellum, which is supplied by the PICAs. A wide suboccipital craniotomy and C-1 posterior arch removal have been completed. Both PICAs arise in an extradural location and penetrate the dura along the posterior surface of the VAs. They course behind the medulla and around the cerebellar tonsil and ascend to supply the suboccipital surface of the cerebellum. Lower, Enlarged posterior view. Both PICAs arise from the vertical part of the third segment of the VAs and pursue a similar extradural course parallel and behind the VAs. Intradurally, the PICAs and VAs separate: the PICAs course medially to reach the posterior surface of the medulla, and the VAs pass anterior to reach the ventral brainstem. (Images courtesy of AL Rhoton, Jr.)