activity. Almost any vigorous sensory stimulation may have the same effect. During sleep the vasoconstrictor center is usually not very active; the cause of sleep is not completely understood, but one of the most satisfactory theories regarding it is that during the period of waking the vasoconstrictor center becomes gradually more and more fatigued, and so requires more and more stimulation to keep it active. This stimulation may come in part through the ordinary channels of the sense organs and in part from the higher brain centers, as when one keeps awake by an effort of the will. Upon going to bed sensory stimulation is cut off to a very large extent, also the will to remain awake is no longer present. Under these circumstances the fatigued vasoconstrictor center is under a minimum of stimulation and tends, therefore, to lessen its activity. The result is that the blood pressure falls and presently the circulation through the brain drops below the level of consciousness and the individual is asleep. During the period of sleep the fatigued center recuperates, so that it becomes more susceptible to sensory stimulations and, in course of time, is aroused by such stimuli as accompany the returning day to sufficient activity to restore the brain to consciousness. Of course it will be perceived that there are many things about sleep which are not satisfactorily explained on this theory; in fact no one at the present time pretends that we can account for it completely on this basis of changes of circulation through the brain. It is believed, however, that they have a good deal to do with it and it is certainly true that in healthy individuals the course of sleep follows very closely the activity of the vasoconstrictor center. Undue wakefulness in persons not suffering from disease can nearly always be explained on the basis of excessive activity on the part of this center. The activity may be the result of chemical stimulation, as when persons are kept awake by drinking coffee or strong tea in the evening, or by the persistence of adrenalin in the blood following a period of great excitement. Mental activity, itself, tends to keep the vasoconstrictor center whipped up, so that one who allows his mind to work actively during the time when he should be asleep is very apt to find sleep refusing to come when it is desired. The center is often stimulated from the digestive tract; gastric irritation, even though not acute enough to be recognized as indigestion, may cause wakefulness by arousing nervous disturbances which play upon the vasoconstrictor center. Flatulence, namely the presence of large volumes of gas in the digestive tract, frequently acts as a mechanical source of irritation by which wakefulness is induced. Evidently the factors favoring healthy sleep are the inducing of fatigue, preferably by physical exercise, the avoidance of chemical irritation or of excitement in the hours just