In approaching a study of certain lesions that may develop at the base of the skull in the vicinity of the chiasm, it seems appropriate to refer to certain anatomic conditions that many influence, to some extent, certain signs or symptoms that are prone to occur.
The circle of Willis, situated in the interpeduncular cistern, forms a complete circular anastomosis in 93 per cent of the cases ; in Stopford's1 series of one hundred and five specimens, the remaining 7 per cent concerned the posterior communicating artery. In early intrauterine life this artery represents the origin of the posterior cerebral from the internal carotid artery, but later, after the posterior cerebral artery has formed its anastomosis with the basilar artery, the posterior communicating artery becomes of less importance. Its ultimate size and importance vary with the efficiency of the anastomosis referred to. In one of Stopford's cases it