The heredofamilial or endogenous diseases of the central nervous system are characterized by germ layer selectivity, system selectivity and segment selectivity (Schaffer).1The exogenous diseases of the brain and spinal cord, the etiology of which is unknown or which belong definitely to the group of infectious maladies, are localized in a characteristic way in the gray or white matter of the central nervous system. Both the gray and the white substance of either the brain or the spinal cord may be selectively the seat of pathologic alterations. Undoubtedly, local anatomic, physiologic and biologic factors play an important part in such selectivity. The nature of these local physiologic and biologic properties of the brain or the spinal cord is unknown. In regard to anatomic structure, it is known that the gray matter is more richly supplied with capillaries than the white substance, and this may result in physicochemical differences. In