Fatigue of any sort is one of the most energetic causes of sleep. The impossibility of long sustained exertion is a fact almost too familiar to attract attention. Every muscle must be suffered to rest for a time after contraction before it can be again contracted. Even the heart and the muscles of respiration must be allowed to enjoy regular periods of repose many times each minute. These are examples of local rest, not involving the entire body. But if the whole body participate in any violent action, every part will manifest a consequent disposition to rest. Witness the effects of the venereal act. Every muscle is relaxed; the brain, which has officiated as the supreme source of energy, experiences exhaustion, and sleep frequently terminates the voluptuous paroxysm. In like manner, sensations of severe pain, if sufficiently prolonged, become a cause of sleep. Prisoners upon the rack have slept through sheer exhaustion while undergoing the horrors of torture. Little children frequently fall into a deep sleep immediately after painful, though comparatively bloodless, surgical operations performed without anæsthetics. The depressing emotions, even, may so fatigue the brain as to induce sound sleep through reaction from previous excitement. Every wearied portion of the body must rest; and when the brain thus rests, sleep is the consequence. Impressed by the force of such considerations, certain physiologists[18] have reasoned from the analogies suggested by a study of the results of muscular fatigue, and have suggested an hypothesis accounting for the occurrence of sleep by a supposed loading of the cerebral tissues with the acid products of their own disassimilation during wakeful activity. The acid reaction of the brain and of the nerves after exertion, corresponding with the development of acids in the muscular tissues during contraction, suggested the probability that an excessive presence of lactic acid and its sodic compounds might be the real cause of cerebral torpor and sleep. Could this hypothesis be proved, ordinary sleep would take its place along with the states of unconsciousness induced by anæsthetics and hypnotics, and the lactate of sodium should be found the very best of medicines for the relief of wakefulness. Its administration for this purpose, however, has yielded only the most discordant and unsatisfactory results. The fatigue theory, moreover, is insufficient, since it furnishes no explanation of the invincible stupefaction produced by cold, nor does it render intelligible the unbroken sleep of the unborn child.