IN 1919, Dandy1 showed evidence of formation of the cerebrospinal fluid (CSF) by the choroid plexus and attributed the development of hydrocephalus proximal to a block in the CSF pathways to back pressure. As proof for this conclusion, he presented the following observations: A block in the foramen of Monro results in enlargement of the lateral ventricle on the same side. If the aqueduct of Sylvius is obstructed after unilateral choroid plexectomy and occlusion of the foramen of Monro on the same side, there is enlargement of the opposite ventricle with the intact choroid plexus and collapse of the plexectomized ventricle with the foramen of Monro blocked.
Whether a plexectomized lateral ventricle would enlarge if the foramen of Monro is left open and the aqueduct occluded was never put to a test.
From experiments designed to test Dandy's conclusions, Bering,2 in 1962, concluded that back pressure in