The basilar communicating artery (or mesencephalic artery) gives off more often one than two paramedian thalamic arteries. The paramedian territory, infero-medial, has a variable extent. It can include the polar territory when the polar artery does not exist. The paramedian artery arises as often from the controlateral basilar communicating artery than from the ipsilateral one. One basilar communicating artery may so have a very extensive bilateral territory.