With the recognition of a certain set of factors responsible for the formation of the several known forms of hydrocephalus, the term idiopathic hydrocephalus is gradually being abandoned. The work of Dandy and Blackfan,1 Thomas,2 Dandy3 and others has established beyond doubt that one form of ventricular hypertrophy with coexisting stasis of the cerebrospinal fluid is due mainly to an obstructive lesion which retards or completely blocks the normal outflow of that fluid from the ventricular cavities. Such an obstruction may be caused by a tumor which, by its location, occludes partly or completely some part of the ventricular or aqueductal channel; it may be in the nature of an inflammatory process in the ventricular lining, producing local (Spiller,4 Tillgren5), or widespread ependymitis (Globus and Strauss6), or it may finally take the character of a congenital malformation in the form of an atresia, such