380 THE HUMAN BODY 



motor changes which have been looked upon as very instructive; 

 and as affording us, indeed, our only satisfactory method of study- 

 ing sleep experimentally. Observations upon sleeping individuals 

 have shown that normal sleep is frequently accompanied by a 

 considerable fall in general blood-pressure, resulting from exten- 

 sive vasodilation. This is itself sufficient to account for the dimin- 

 ished cerebral activity with its accompanying loss of conscious- 

 ness which constitutes sleep, and many physiologists are inclined 

 to believe, therefore, that the vasomotor changes may form the 

 underlying basis for the phenomenon. A theory which expresses 

 this view looks upon the vasoconstrictor center as the controlling 

 mechanism of sleep. When this center is in good condition the 

 constant stream of afferent impulses playing upon it maintains it 

 in strong activity, and vasomotor tone is kept high. With the 

 passage of hours of such ceaseless activity the center becomes 

 fatigued and tends to respond less strongly to the afferent impulses 

 coming to it. The result will be a falling off of vasomotor tone, 

 unless by an effort of the will or an increase in the stream of af- 

 ferent impulses, such as follows muscular exercise, for example, 

 the center is whipped up to renewed activity. "Keeping awake" 

 when one is sleepy is, according to this view, a matter of stimu- 

 lating the tired vasoconstrictor center to continued effort. The 

 effect may be produced by an artificial stimulant, such as coffee, 

 or by an act of the will. The usual preparations for sleep are such 

 as favor diminished activity of the vasoconstrictor center by les- 

 sening the afferent impulses coming to it. Lying in a comfort- 

 able position removes most of the impulses of muscle sense; by 

 closing the eyes visual stimuli are gotten rid of. Thus unless the 

 center is so irritable that the small stream of inevitable afferent 

 impulses keeps it up to the mark the essential condition for sleep, 

 loss of vasomotor tone, is fulfilled. The act of waking, according 

 to this theory, results either from an undue stimulation of the 

 vasoconstrictor center, as when one is waked by being violently 

 shaken, or from a gradual restoration of the irritability of the 

 center during its period of rest, to a point where the minimal 

 stream of afferent impulses, inseparable from the living Body, is 

 sufficient to stimulate it to the maintenance of waking vasomotor 

 tone. 

 It must be admitted that not all experiments upon sleep have 



