Histologic studies of the pathology had shown that the obstruction was most commontly either (1) within the ventricular system itself, that is proximal to the outlets of the fourth ventricle as in, for example, aqueductal stenosis; or (2) outside the ventricular system where inadequate circulation over the convexities of the brain adequate reabsorption, for example, after meningitis or subarachnoid hemorrhage.