The pattern of the circulus arteriosus cerebri was studied on the sagittal plane in 41 brains taking off on children from birth to 14 years. Internal diameters were measured on the segments of the circulus, on the cortical efferent vessels and on the internal carotid afferent arteries. In only 17/41 cases, a symetry of the segment of the cortical efferent branches and of the carotid arteries. We found different factors associated with the assymetry of the circulus arteriosus, namely cerebral conditions as demonstrated in our pathological cases and embryological dispositions. Three grades are proposed according anatomic criteria. They are related to a greater complexity and interaction of the conditions mentionned above:-grade 1 "foetal" : symetry is complete;-grade 2 : there is a symetry of the cortical branches, but an assymetry of the circulus arteriosus, which is obtained by compensatory modelling of the circulus arteriosus, working to equalize the cerebral flow; -grade 3 : there is an assymetry of the circulus and of the cortical efferent branches suggesting a failure of adaptation. It is assumed that this modelling of the circulus has consequences on adult anatomic and functional conditions. Indeed, the other modelating factors which affect the adult circulus refer mainly to the grade obtained in childhood.