One of the most puzzling and striking features of many of the genetically determined progressive neuromuscular diseases such as the spinal muscular atrophies and the muscular dystrophies is that muscular wasting and weak ness in these cases is curiously selective, at least in the early stages, pick ing out certain skeletal muscles and sparing others. The diagnosis of these conditions has largely depended in the past upon the recognition of specific patterns of involvement of individual muscles and muscle groups, taken along with information derived from the mode of inheritance within the in...