Morphological examination of the circulus arteriosus cerebri humani (circle of Willis). I. Anterior and posterior communicating arteries.

The circulus arteriosus cerebri (circle of Willis) was examined to observe morphological variations in 50 normal human brains. Multiformed variations, as well as unruptured intracranial aneurysms, were observed. Among such variations, particular attention was paid to configurations of the two communicating arteries, anterior and posterior, respectively. Variations of the anterior communicating artery were characterized by multiplicity in their forms, on the other hand the posterior communicating arteries exhibited hypoplastic or primitive configurations. We suggest that, possible morphological variations which occur in the course of circulus arteriosus cerebri and their main branches must not be confused with congenital malformations. There has been no critical definition of the normal circle arteriosus cerebri. Differences in morphological variations between the anterior and posterior communicating arteries are discussed from the viewpoint of both embryology and the clinical findings.