When you did me the very great honour of calling on me to deliver the Maudsley Lecture, I was told that it had been decided that these lectures should be devoted alternately to scientific psychiatry and to subjects of general popular interest, and that the subject of my lecture was to be scientific. I decided that with a year's notice the active preparation of the lecture might well be postponed for eleven months, and that during this gestation period I might hope to fix on one of the many aspects of pathological or physiological science of sufficient interest to justify a not too technical exposition to a mainly clinical audience.