The author describes the anatomical findings among the variants of brain arteries in 360 deceased where 160 of them had a hemorrhagic or ischemic stroke, while 200 had no brain diseases. Those cases where strokes were revealed the author found a separation in the posterior part of the circle of Willis--between the carotid and vertebro-basillar systems. This could happen due to an aplasia of one or both posterior connective arteries, or due to the absence of a connecting vessel between an atypically diverging posterior brain artery (posterior trifurcation) and the basillar artery. It is assumed that this circumstance has a definite significance in the pathogenesis of hemorrhages.