twitching of the muscles, and the convulsive state, which may often be observed during the stage of somnolence after severe fatigue. The uneasy sleeper may even be roused to complete wakefulness by such involuntary movements. But, as sleep becomes more profound, the reflex functions of the cord are also weakened.[1] As the sensory organs retire from action, the intellectual faculties lose their equilibrium. First, the power of volition ceases. Then the logical association of ideas comes to an end. The reasoning faculty disappears, and judgment is suspended. We become, therefore, no longer capable of surprise or astonishment at the vagaries of memory and of imagination, the only faculties that remain in action. To their more or less unfettered activity we owe the presentation in consciousness of those disorderly pictures which, occurring in this stage of imperfect sleep, have been termed hypnagogic hallucinations.[2] During the early moments of this period an observant person may often retain a power of reasoning sufficient to remark the fact of dreaming, and this effort of attention may produce a partial awakening; but, usually, the subsidence of cerebral function is progressive and rapid. The fire of imagination fades, the field of consciousness becomes less and less vividly illuminated, the entire nervous apparatus yields to the advancing tide, and, finally, the dominion of sleep is fully confirmed. The sleeper knows nothing of the external world, and has lost all consciousness of his own existence. But the duration of profound repose is brief. From the end of the first hour the depth of sleep, at first, rapidly, then, more gradually, subsides. Dreams disturb its tranquility, mental activity increases, the power of volition revives, and, at the end of six or eight hours, the individual is once more awake. The subjoined diagram, borrowed from the Dictionaire Encyclopédique des Sciences Médicales, will facilitate the apprehension of these successive phases in the course of sleep: