darkness, the absence of any decided impression on the skin, and the nonexistence of odors and flavors, accomplish this end. In these respects, however, habit exercises great influence, and thus individuals, for instance, who are accustomed to continual loud noises, cannot sleep when the sound is interrupted. As we have already seen, however, the predisposition to sleep is, in healthy persons, generally so great that when it has been long resisted, no sensation, however strong it may be, can withstand its power.