Superior view of right cerebral vasculature. The ophthalmic segment of the internal carotid artery is intradural after having passed through the distal dural ring into the subarachnoid space and it runs parallel with the optic nerve. This portion of artery gives to the ophthalmic artery, which passes into the optic canal with the optic nerve. The terminal segment of the ICA is the communicating segment. It gives rise to the anterior choroidal artery (AChA) and posterior communicating artery (PCoA) before terminating into the anterior and middle cerebral arteries. The oculomotor nerve (CN III) passes between the superior cerebellar (SCA) and posterior cerebral (PCA) arteries. SCA arises from the basilar artery as does the PCA, which connects to the ICA via the PCoA. A short anterior communicating artery unites the right and left anterior cerebral arteries to complete the arterial circle of Willis. (Image courtesy of AL Rhoton, Jr.)