OF THE great variety of neurologic disorders associated with atrophy of muscle, most fit into well delineated clinical and pathologic syndromes; yet the problem of the etiologic factors in the degeneration of muscle are among the least understood in neurology. Many generalized diseases may produce sharply localized areas of muscular atrophy, raising the question of why one group of muscles should be singled out to become atrophic while an analogous group is spared by a process acting equally on the two. For example, in hyperthyroidism, in which increased or altered thyroid hormone circulates through the entire body, all the muscles of the body may weaken, and perhaps waste somewhat; but in certain cases the tiny extraocular muscles may show degenerative changes far beyond those seen in other muscles. Lead and other metallic poisons, various hydrocarbons, diphtheria toxin and the poliomyelitis virus are but a few of the pathogenic agents which