A, The P2 divides into a P2A, which passes through the crural cistern located between the posterior segment of the uncus and the cerebral peduncle, and a P2P, which courses through the ambient cistern, located below the lateral midbrain and parahippocampal gyrus. The P3 passes through the quadrigeminal cistern where it gives rise to the P4 formed by the cortical branches, including the parieto-occipital and calcarine arteries that course in the parieto-occipital and calcarine sulci where they are commonly hidden between the sulcal lips. B, The lips of the parieto-occipital and calcarine sulci have been retracted to expose the parieto-occipital and calcarine branches coursing along the sulci. A MPChA encircles the brainstem to reach the third ventricular roof. The cuneus forms the upper lip and the lingula forms the lower lip of the calcarine sulcus. The precuneus forms the upper lip and the cuneus forms the lower lip of the parieto-occipital sulcus. C, Another hemisphere. The terminal branches of the PCA pass posteriorly within the parieto-occipital and calcarine sulci. The arrows are on branches that pass around the occipital pole to reach the adjacent lateral surface. D, The lips of the parieto-occipital and calcarine sulci have been retracted. The parieto-occipital artery courses within its sulcus. The calcarine artery courses just below the calcarine sulcus and gives rise to several small branches that course along the depths of the sulcus. E, Posteroinferior view of occipital pole showing the branches (red arrow) of the PCA coursing around the occipital pole to reach the adjacent part of the lateral convexity. F, Posterior view of both occipital lobes. The P4 branches course around the posterior and lower border of the occipital lobe to reach the lateral cortical surface. The P3s course on the quadrigeminal cistern. (Images courtesy of AL Rhoton, Jr.)