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The precommunicating segment of the PCA (P1) extends from the basilar bifurcation to the origin of the posterior communicating artery (PCommA). Its course lies within the interpeduncular cistern, which is demarcated by the
clivus and the two cerebral peduncles. The oculomotor nerve, after its emergence from the brain stem, runs between the PCA and the superior
cerebellar artery. The postcommunicating segment (P2) curves laterally and backward around the crus cerebri and reaches the posterior surface of the midbrain at an intercollicular level.


The precommunicating and postcommunicating segments are together referred to as the pars circularis of the PCA. (Alternatively, the pars circularis may be divided into three segments— interpeduncular, ambient, and quadrigeminal—
named after the cisterns they traverse.) Distal to the pars circularis of the PCA is the pars terminalis, which divides above the tentorium and caudal to the lateral geniculate body to form its terminal branches, the medial and lateral occipital arteries. Pars circularis.
The precommunicating segment gives off fine branches (posteromedial central
arteries) that pierce the interpeduncular perforated substance to supply the anterior thalamus, the wall of the third ventricle, and the globus pallidus. The postcommunicating segment gives off fine branches (posterolateral central
arteries) to the cerebral peduncles, the posterior portion of the thalamus, the colliculi of the mid brain, the medial geniculate body, and the pineal body. Further branches supply the posterior portion of the thalamus (thalamic branches), the cerebral peduncle (peduncular branches), and the lateral geniculate body and
choroid plexus of the third and lateral ventricles (posterior choroidal branches).
Pars terminalis. Of the two terminal branches of this terminal portion of the PCA, the lateral occipital artery (together with its temporal branches) supplies the uncus, the hippocampalgyrus, and the undersurface of the occipital
lobe. The medial occipital artery passes under the splenium of the corpus allosum, giving off branches that supply it (dorsal branch to the corpus callosum) as well as the cuneus and precuneus (parieto-occipital branch), the striate
cortex (calcarine branch), and the medial surfaces of the occipital and temporal lobes (occipitotemporal and temporal banches), including the parasagittal portion of the occipital lobe. Posterior Circulation of the Brain Cerebral