For a long time it was supposed that progressive muscular atrophy, as first systematically described by Aran in 1850, was a disease of uniform pathology, resulting from degeneration of the ganglion-cells in the anterior horns of the spinal cord. Later studies, however, have shown that such atrophy may result also in the absence of appreciable lesion of the spinal cord or of the motor nerves representing the peripheral continuation of the anterior ganglion-cells of the spinal gray matter. In explanation of the cases included in the second group it is assumed that the disease originates in the muscles themselves, constituting the so-called myopathies, in contradistinction to the cases of spinal origin, or myelopathies. In some cases, further, it had been observed that with motor weakness there was associated increase rather than diminution, in the size of the muscles functionally concerned. To Erb,1in especial, belongs the credit of having pointed out that in both the cases with wasting and those with apparent hypertrophy the essential demonstrable lesion is the same, viz., increase in size of some muscular fibres, with diminution in size of others, and degenerative changes; and more or less increase in the interstitial connective and fatty tissues. Erb is, however, not entirely willing to admit that the changes in the muscles are purely myopathic, contending that they may depend upon impalpable nutritional alterations in the ganglion-cells of the anterior horns of the spinal cord not capable of demonstration with our present resources. He, therefore, proposes to designate this group of cases as muscular dystrophies, thus not committing one definitely to a theory of their pathology. Several types of the disease have been described, as, for instance, the idiopathic, the pseudo-hypertrophic, the juvenile or scapulo-humeral of Erb, the infantile of Duchenne, or the facio-scapulo-humeral of Landouzy-Déjérine, and the hereditary of Leyden; but it is doubtful if any useful purpose is subserved by this classification, inasmuch as the boundaries of the several so-called types are ill-defined, and many if not most cases present features of two or more varieties. It is probable that these several forms represent rather differences in degree and distribution than variations in essential character.