In the routine study of many brains and cords, great variation in the arachnoid was noted. It was found that in certain cases small masses of cells were scattered irregularly within the arachnoid; the morphology of these cells proved them to be of arachnoid origin. The first description of these cell clusters was made in 1859 by Meyer1 and in 1864 by Cleland,2 although little attention has been paid to their findings. Cushing and Weed,3 in 1915, were probably the first to corroborate Meyer's and Cleland's findings.
It would be well briefly to outline the present status of our knowledge of the normal structure of the arachnoid, with which we are here especially concerned. The arachnoid separates the subdural from the subarachnoid space. Histologically, it is composed of a very fine reticulum covered on both sides by its characteristic cells arranged so as to be only one