The involuntary rhythmic oscillations of the eyeballs, the nystagmic movements, are in reality defensive reactions of the ocular muscles in the interest of ocular fixation and orientation in space. Normal function of these muscles, that is, the ability to fix on a given object, depends on calculated correct movements and on the ability, after bringing the object into the line of the fovea centralis of both eyes, to keep it there for the necessary period, an act accomplished by normal tonus. The production of these movements and the necessary tonus depends, on the one hand, on voluntary innervation and, on the other, on certain reflex phenomena, the afferent arm of which is initiated principally in the retina and in the labyrinth (Stellungapparat and Blickapparat1). Any factors clinicopathologically or experimentally induced by interference with visual fixation or labyrinthine function as well as with the posterior longitudinal bundle may produce