This research was undertaken to establish, as far as possible, Drs. Ford Robertson's and McRae's ætiological hypothesis for general paralysis of the insane. It is hardly necessary to emphasise the importance of giving the fullest consideration to this theory, being, as it is, the first serious attempt of its kind to bring mental disturbance into the category of physical disease, to establish the wealth of histological observation in its place as an effect, and to break from the bugbear of metaphysics and perversion of psychic function. I should like to make it clear that though this research may controvert the ætiological significance of the bacilli of Ford Robertson, it in no wise attacks the fundamental theory of a toxic cause acting through the very definite channels of infection established by Drs. Orr and Rows.