The characteristic asymmetrical distribution of the posterior cerebral artery of birds

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Abstract
The encephalic arteries of a series of 94 specimens of birds, representing 21 avian orders were studied and compared. Throughout the Class Aves the posterior cerebral arteries normally and consistently exhibit asymmetry in their configuration and distribution to the brain. In only 2 specimens of the series were the major branches of the posterior cerebral artery (interhemispheric and dorsal cerebellar arteries) symmetrical. Four main patterns of asymmetry of these arteries were noted. Major branches of the posterior cerebral system are replaced by branches of the anterior cerebral, ventral cerebellar, or anterior choroidal arteries in certain birds. In some species the posterior cerebral artery is asymmetrically bipartite, consisting of 2 separate vessels on one side. The anterior choroidal artery was a branch of the posterior cerebral system in about 1/3 of the species studied and an independent branch of the cranial ramus of the cerebral carotid in most of the remaining species. Asymmetry of the interhemispheric rami of the anterior cerebral arteries was observed in a number of cases. Sizeable anastomoses between the right and left anterior cerebral arteries, producing a communicans anterior, was only rarely observed. Interspecific variation in the territory of distribution of the posterior cerebral system is discussed.

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