Natural sleep is that condition of physiological repose in which the molecular movements of the brain are no longer fully and clearly projected upon the field of consciousness. This condition is universally observed in all healthy animals; and its recurrence is intimately associated with the diurnal revolution of the earth, and the succession of day and night. The disappearance of daylight is, for the majority of living creatures, the signal for cessation of active life. Though its onset may be for a time delayed by an effort of the will, the need of rest at length overcomes all opposition, and the most untoward circumstances cannot then prevent the access of unconsciousness. The story of the sailor-boy, sleeping on “the high and giddy mast,” is familiar to every one. An officer in the United States Navy has assured me of more than one instance in which men had fallen asleep under his own eyes, oppressed by exhaustion, during the roar of a long continued bombardment. Thus produced, the relation of cause and effect between weariness and sleep becomes very apparent. The refreshing influence of such repose points clearly to the restorative character of the physiological processes which persist during the suspension of consciousness. It also renders evident the final cause of that periodical interruption of activity which the brain experiences in common with every other living structure. Sleep is usually preceded for some time by a feeling of sleepiness. This sensation, like the analogous sensations of hunger and thirst, represents in some measure the progressive diminution of energy throughout the entire body; but it is chiefly expressive of the failure of cerebral energy. It produces a sense of general heaviness and intellectual dullness; the special senses become less alert, the eyelids droop, numerous groups of muscles experience the spasmodic contraction of yawning, the head drops forward and is recovered with a jerk, the limbs relax, and the whole body tends to assume a position convenient for repose. Every school-boy who has been compelled to pass an evening hour at a dull lecture, under the eye of a martinet monitor, will testify to the suffering which attends any unusual prolongation of this period. But, if the natural course of events be not obstructed, the stage of mere sleepiness is soon passed, and the introductory stage of sleep is entered. This is a state in which the individual is neither awake nor fully asleep. It is known as the hypnagogic state. During this period the phenomena of simple sleepiness become exaggerated to such a degree that the attitude of repose is assumed without effort if the body be permitted to follow the natural inclination of its different members. The eyes close, the other senses become inactive, though the sense of hearing is the most persistent. Released in considerable measure from the control of the brain, the reflex energy of the spinal cord is at first somewhat exalted. Witness the fibrillary