With the ever-increasing number of processes proved to be due to the action of micro-organisms, bacterial and protozoal, it is natural that a similar line of research should be suggested in the study of the psychoses. It has long been realised that the classification and differentiation of the various conditions by mental signs alone is confusing and unscientific. Gradually more importance has come to be placed on general clinical observations as indicating broadly the primary disturbances in the economy of the body, secondary consideration being given to mental symptoms as showing the change effected in the individual nervous system by the general disturbance.