From the time of Parkinson to within the past decade, paralysis agitans has been attributed to a variety of lesions, from minor cytologic changes to gross anatomic destruction, in numerous and widely separated portions of the central nervous system. Examination of the peripheral nerves had failed to add material knowledge of a positive nature. The internal secretory glands and the striated musculature were also studied without furnishing any convincing pathologic evidence. In 1911, Mendel1 published a comprehensive account of the disease with a critical review of the pathologic studies recorded up to that period. Evidence was accumulating which suggested that the lesion might be situated in one or more of the central nuclear masses. Brissaud, Maillard and Jelgersma had observed changes in the substantia nigra, in the red nucleus, and in the lenticulostriate and hypothalamic structures; but the lesions were often diffuse, wide-spread, and of uncertain meaning. Similar observations