It is not surprising that the treatment of mental disorder should have brought in its train a clearer insight into the ordinary course of events in the human mind. In this as in every other department of medical experience the hidden processes of nature are revealed to advantage when they become manifest in some abnormal form. One has only to think of the importance of the part played by the recognition of heart-block and of muscular paralysis in elucidating the meaning of the successive phases of the cardiac cycle. In a wider field a greater claim can be made for clinical pathology in its contributions to knowledge of the structure and function of the nervous system. In the work of Hughlings Jackson (1) alone may be recognized the foundations of a new era not only of neurological medicine and surgery, but also in the more scientific spheres of anatomy and physiology. The subtle complexities that underlie the apparently even tenor of nature's average path may first come to light when disturbed by disease.