In order to obtain information about the relationship between variations in the circle of Willis and aneurysms, 44 complete circles of Willis with aneurysm were studied macroscopically. The incidence of variations was significantly higher in the aneurysm series than in the control circles without aneurysm. There was a definite correlation between asymmetric proximal segments of the anterior cerebral artery and aneurysms of the anterior communicating artery, and a tendency to correlation was found in the case of asymmetric posterior communicating arteries and aneurysms on the internal carotid artery-posterior communicating artery junction. In the light of these findings it seems likely that through hemodynamic changes variation in the circle of Willis plays some role in the development of cerebral aneurysms.

1.A colony of stroke-prone spontaneously hypertensive rats has been developed by selective breeding.2. These animals developed severe hypertension early in life, the magnitude of the hypertension being closely related to the incidence of stroke.3. No evidence was obtained of any humoral factor responsible for strokes.4. Local factors predisposing to stroke were a scanty arterial supply with characteristic recurrent branching of long and large arteries, together with increased vascular permeability, angio-necrosis, and formation of microaneurysms. 5. Strokes could be prevented by adequate antihypertensive therapy from an early age.

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The present findings indicate that there is an association between apoptosis of medial SMCs and the formation of saccular cerebral aneurysms.