264 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



we assume that the blood-flow in the brain stands in a reciprocal re- 

 lation to that in the arm, this curve may be taken to indicate that 

 before and after the onset of sleep the blood-flow through the 

 brain diminishes 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 vari- 

 ous kinds affect a sleeping individual without 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 pro- 

 voked. It is very interesting to find from plethysmographic 

 observations that all kinds of sensory stimulations from without 

 and from within are liable to affect the circulation of the blood 

 during sleep. As shown by the plethysmograph, the volume of 



Fig. 117. Sleep: A, effect of external impression (music box), insufficient to awaken 

 sleeper, a marked diminution in volume of the arm ; B, effect of external impression 

 (music box) sufficient to awaken sleeper; a stronger diminution in volume followed by 

 dilatation as the subject again fell asleep. 



the arm diminishes more or less in proportion to the intensity 

 of the stimulus, and the probable interpretation of this fact is 

 that the sensory stimulus acts reflexly upon the vasomotor 

 center in the medulla and causes through it a contraction of 

 the blood-vessels. In the curve shown in Fig. 116 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 Fig. 

 117. In these experiments the recorder attached to the plethys- 



