has become a mere memory of childhood. Wordsworth understood the full significance of this change when he wrote: There was a time when meadow, grove, and stream, The earth, and every common sight, To me did seem Apparelled in celestial light, The glory and the freshness of a dream. It is not now as it has been of yore; Turn whereso’er I may, By night or day, The things which I have seen I now can see no more! ... I know, where’er I go, That there has passed away a glory from the earth. If, however, despite the loss of much that was beautiful and attractive in the myths of antiquity, we take advantage of the Years that bring the philosophic mind, we shall surely find in the scientific investigation of sleep enough to awaken “thoughts too deep for” words. CONTENTS. CHAPTER I. THE NATURE AND CAUSE OF SLEEP. Definition of sleep--The invasion of sleep--The hypnagogic state--Depth and duration of sleep--Diagrammatic illustration of the phases of sleep--Modifications of physiological functions produced by sleep--Effect of sleep upon the processes of respiration, circulation, calorification, secretion, and nutrition--Consequences of the progressive invasion of the nervous system by sleep-- Effect upon the organs of special sense--Effects observed in the muscular apparatus of the body--Condition of intellectual functions during the invasion of sleep--Does the mind ever sleep?--Arguments adduced by Sir William Hamilton and others to prove the continued activity of the mind during the sleep of the brain--Reasons for supposing that the mind may sleep--Variability of the depth of sleep--Experiments of Kohlshüter to estimate the degree of variation--Alternation of day and night considered as a cause of sleep--Diminution of sensation a cause of sleep-- Illustrative observation by Strümpell--Fatigue a cause of sleep--Hypothesis of Obersteiner regarding the cause of sleep--Hypothesis of Pflüger--Production of artificial sleep by impregnation of the brain with narcotic substances--Analogous production of natural sleep by accumulation of cerebral waste-products--Observations regarding the duration of sensory impressions requisite for the excitement of conscious perception--Difference between syncope and sleep--Observations of Mosso regarding the state of the cerebral circulation during sleep--Cause of the change in the cerebral circulation during sleep-- Molecular conditions necessary for the production of sleep--Somnolence--Sleeping Dropsy, or Maladie du Sommeil-- Coma--Lethargy--Apparent death--Lucid lethargy. 1 CHAPTER II. INSOMNIA, OR WAKEFULNESS.