In attempts (by Dr. Oldberg) to obstruct the sylvian aqueduct according to the method of Dandy1 (by way of the fourth ventricle), the pledgets of cotton used for this purpose were occasionally pushed sideways into the nerve parenchyma (fig. 1). Figure 1 shows such a pledget below a transverse slit (above, at IV) representing a continuation of the fourth ventricle, which was also found to be obstructed by pledgets of cotton (in five specimens). Where the pledget of cotton obstructed the fourth ventricle, a part of the caudal part of the sylvian aqueduct was invaded. In both locations the presence of the foreign body gave rise to the formation of several varieties of reacting cells. The reacting cells were fundamentally different from those that arise around plugs of necrosed, coagulated tissue produced by plunging into the brain a hot needle, a sharp knife, a spicule of bone or a