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