There is but one important preliminary to my paper. It is that I wish to take this opportunity of expressing my most sincere thanks to the medical officers of Bethlem Hospital (Dr. Percy Smith, Dr. Hyslop, and Dr. Craig) for their great kindness and courtesy in putting every facility in my way while I was making the following observations, for the interest which they took in my work, and for the valuable suggestions which they so willingly afforded me from time to time.