Quantitative eye movement recordings in a patient with multiple sclerosis who had both macro square wave jerks and an internuclear ophthalmoparesis supported the concept of an internal brain-stem monitor that mediates corrective eye movements. A brain-stem model of this monitor illustrates the patient's motility disorder and the normal function of providing synergistic version and vergence corrections for dysmetric eye movements. Thus, the careful study of pathologic disturbances provides information relevant to the normal operation of the ocular motor system.