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An anatomic study of the intra-and extracranial circulations in the dog was presented by de la Torre, Netsky and Meschan ('59). The cerebral circulation was shown angiographically with radiopaque material injected into the internal carotid artery. Under normal conditions of cerebral circulation, however, contrast medium thus injected did not pass to the posterior communicating arteries. As a consequence, the vessels of the posterior portion of the circle of Willis were not seen in the living animal ( fig. 1).