Our conception of the ætiological significance of myxœdema in reference to those forms of mental disorder generally known under the name of “insanity of myxœdema,” has undergone considerable modification since the time when hypothyroidism was regarded as the principal causal factor. The almost dramatic effect of thyroid treatment gave rise at first to a justifiable optimism. This for a while tended to obscure the possibility that a deeper-lying disorder of mind might exist, which would be naturally brought into prominence by the peculiar disturbance in the functioning of the brain associated with this state of disordered metabolism.