CENTRAL NERVOUS SYSTEM. 



741 



drawal of blood from it, the blood-supply to the brain being probably at the 

 same time increased (see Fig. 211). 



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FIG. 211. 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 be read in the direction of 

 the arrow. 1, the night watchman entering the laboratory, waking the subject, who shortly fell asleep 

 again ; 2, the watchman spoke ; 3, watchman went out ; these changes (2 and 3) occurred without awak- 

 ening the subject (from experiments made by Messrs. Bardeen and Nichols, Johns Hopkins Medical 

 School). 



This experiment shows that during sleep the nervous system is capable of 

 reactions which are not remembered in any way, but which naturally form a 

 feature of the condition intermediate between waking and deep slumber. The 

 depth of sleep as determined by the strength of the stimulus necessary to elicit 

 an efficient response has been measured. The stimulus in these experiments 

 was the sound caused by the fall of a ball upon a plate, and the measure was 



Strength of stimulus 



Hours QJ5 LO 1.5 .0 2.5 3.0 3.5 4.0 45 5.0 5.5 &0 65 7.0 7.5 7.6 



FIG. 212. Curve illustrating the strength of an auditory stimulus (a ball falling from a height) neces- 

 sary to waken a sleeping person. The hours marked below. The tests were made at half-hour intervals. 

 The curve indicates that the distance through which the ball required to be dropped increased during the 

 first hour, and then diminished, at first very rapidly, then slowly (Kohlschutter). 



the height from which the ball must fall in order to produce a sound loud 

 enough to awaken a sleeping person. The results of the observations are shown 

 in Figure 212. 



