The waste of the nerve-centers having become such that the stimuli received from the external world no longer suffice to call forth from them adequate discharges, there results a diminished impulse to those internal organs which subserve nervous activity, including more especially the heart. Consequently, the nerve-centers, already working feebly, are supplied with less blood and begin to work more feebly, responding still less to impressions, and discharge still less to the heart. And so the two act and react until there is reached a state of profound unimpressibility and inactivity. Between this state and the waking state the essential distinction is great reduction of waste, which falls so low that the rate of repair exceeds it.... During the day the loss is greater than the gain, whereas during the night the gain is diminished by scarcely any loss. Hence results accumulation; there is restoration of nerve-tissue to its state of integrity. According to Mr. Spencer, that rhythmical variation in nervous activity which we see in sleep and waking is the result of adaptation, due to survival of the fittest. "An animal so constituted that waste and repair were balanced from moment to moment throughout the twenty-four hours would, other things equal, be overcome by an enemy or competitor that could evolve greater energy during the hours when light facilitates action, at the expense of being less energetic during the hours of darkness and concealment."[13] With some qualification, the foregoing statement is about as satisfactory as any that has yet been offered as to the proximate cause of sleep. During the waking hours the vaso-motor center in the medulla is doubtless under inhibition by the superior centers, and there is relative relaxation of the cerebral arterioles, with dilatation of the capillaries; when the cells of the hemispheres are exhausted, they are no longer able to exercise this inhibition--in common parlance, they no longer powerfully extract the blood--and the vaso-motor center "puts on the brakes"; the blood supply is then no longer sufficient for function, though enough for nutrition. An ingenious theory has lately been proposed by Preyer, of Jena,[14] according to which, to use a homely illustration, the fire ceases to burn because the flues are clogged with cinders.