THE PHYSIOLOGY OF SLEEP. 



243 



After this maximum is reached the arm remains more or less of 



the same volume for a certain period or else diminishes in volume 



very gradually. Shortly before waking, 



however, the arm begins to diminish ,, 



more rapidly in size, owing doubt- 



less to the contraction of its blood- g 



vessels; so that at the time of awak- ') 



ing it has practically the same volume sr 



as at the beginning of sleep. If, on 



the basis of Mosso's experiments, | 



quoted above, we assume that the | 



blood-flow in the brain stands in a | 



reciprocal relation to that in the arm, g 



this curve may be taken to indicate < 



that before and after the onset of sleep 2, 



the blood-flow through the brain di- | 



minishes rapidly to a certain point 



and that before awaking the blood-flow | 



begins to increase again until it reaches * 



normal proportions. | 



Effect of Sensory Stimulation. 



That sensory stimuli of various kinds gf 



affect a sleeping individual without g- 



entirely awaking him is shown by the | 



movements that may be caused in this 



way, and also by the nature of the ^ 

 dreams which may be provoked. It is 



very interesting to find from plethys- | 



mographic observations that all kinds | 



of sensory stimulations from with- 



out and from within are liable to J 



affect the circulation of the blood 5, 



during sleep. As shown by the plethys- | 



mograph, the volume of the arm dimin- r 

 ishes more or less in proportion to the 



intensity of the stimulus, and the ' g 



probable interpretation of this fact is g- 



that the sensory stimulus acts reflexly g 



upon the vasomotor center in the J? 



medulla and causes through it a con- g 

 traction of the blood-vessels. In the 

 curve shown in Fig. 109 most of the 



irregularities were traceable to causes of this kind, noises 



in the building or street or other sensory stimuli. The 

 same fact is exhibited in a striking way by the curves given in 



