ONE ASPECT of the relation of the brain to the mind which may be contributed to by experimental study is concerned with the neural management of wakefulness. The waking state is certainly not essential to mental activity, for in the lighter stages of sleep vivid impressions may be experienced as dreams. In states of deep sleep, unconsciousness, or coma, however, mental activity seems practically in abeyance, and interrelations of the two have been expressed succinctly by Sherrington, in concluding a moving account of arousal from sleep, with the statement, "The brain is awakening and, with it, the mind returning."
The waking state has generally been thought to depend in an important fashion upon the arousing influence at the cerebral cortex of afferent messages initiated by sensory stimulation. It is a commonplace observation that afferent impulses from within or without can arouse a sleeping subject, and reduction of sensory impressions is