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The posterior communicating artery (a. communicans posterior) runs backward from the internal carotid, and anastomoses with the posterior cerebral, a branch of the basilar.
It varies in size, being sometimes small, and occasionally so large that the posterior cerebral may be considered as arising from the internal carotid rather than from the basilar.
It is frequently larger on one side than on the other. From its posterior half are given off a number of small branches, the postero-medial ganglionic branches, which, with similar vessels from the posterior cerebral, pierce the posterior perforated substance and supply the medial surface of the thalami and the walls of the third ventricle.