On the border-land of Lunacy lie the criminal populations. It is a debateable region; and no more vexed problem comes before the Medical Psychologist than this—viz: where badness ends and madness begins in criminals. The inmates of Asylums and of Prisons are so nearly allied that “thin partitions do their bounds divide.” From large experience among criminals I have come to the conclusion, that the principal business of Prison Surgeons must always be with mental diseases; that the number of physical diseases are less than the psychical; that the diseases and causes of death among prisoners are chiefly of the nervous system; and in fine that the treatment of crime is a branch of psychology.