When approached by the President of the Medico-Psychological Association to give a short address at the Annual Meeting I chose the above title. On reflection, however, I find it is somewhat over- ambitious and cannot be dealt with in the time at our disposal. It would be better, perhaps, if I endeavoured to place before you some broad principles relative to the close association between neuropathology and psychiatry, and indicate the data in support of the view that anatomically, physiologically, and pathologically the entire nervous system must be regarded, along with its endocrinic appendages, as a unit, each of whose component parts reacts upon the others.