PHYSIOLOGICAL CHANGES IN THE CENTRAL SYSTEM. 289 



and must be regarded rather as mutually influencing 

 than as excluding one another. Such being the case 

 in respect to the diffusion of impulses, it becomes 

 important to examine the physiological status thus 

 created by all those impulses entering the central system 



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11 ,,,, 



FIG. 66. Plethysmographic record taken from the arm of a person 

 sleeping in the laboratory. A fall in the curve indicates a 

 decrease in the volume of the arm. The curve is to He read 

 in the direction of the arrow, i, the night watchman enter- 

 ing the laboratory ; 2, the watchman spoke ; 3, watchman 

 went out. These changes occurred without awakening the 

 subject. Permission to publish these records was granted by 

 Prof. W. H. Howell. The experiments were made by Messrs. 

 Bardeen and Nichols, students at John Hopkins Medical 

 School. 



FlG. 67. Record similar to that in Fig. 66. Change in the 

 volume of the arm of the sleeping subject caused by the sound 

 of a music box which was started at *. 



at any moment, since this status forms the background 

 upon which reactions following fresh stimuli must be 

 developed. 



The evident stimuli can be measured or estimated, 



19 



