In human anatomy, the left and right posterior communicating arteries are small: 471  arteries at the base of the brain that form part of the circle of Willis. Anteriorly, it unites with the internal carotid artery (ICA) (prior to the terminal bifurcation of the ICA into the anterior cerebral artery and middle cerebral artery); posteriorly, it unites with the posterior cerebral artery. With the anterior communicating artery, the posterior communicating arteries establish a system of collateral circulation in cerebral circulation..