THE PHYSIOLOGY OF SLEEP. 257 



ball from varying heights upon a lead plate. Only two experi- 

 ments were made each night, and the curves constructed repre- 

 sent, therefore, composites from several periods of sleep. One 

 of the curves obtained is represented in Fig. 114. According 

 to this curve the maximum intensity is reached between the 

 first and second hours, and between the fourth and the fifth 

 hour there is a second slight increase in intensity, giving a 

 second maximum in the curve. This latter feature of a second 

 increase in intensity toward morning is very apparent also in 

 some interesting curves obtained by Czerny from children of 

 different ages. His method of awakening the sleeper was to use 

 induction shocks of varying intensities. In children of four 

 years with a normal period of sleep of about twelve hours the 

 curve shows a very marked increase in intensity toward morning, 

 as shown in Fig. 115. Curves made by similar experimental 

 methods are reported by Howell and by Michelson.* The 

 striking feature about all the curves is the sharp increase in 

 intensity shortly after falling asleep; in most cases the maximum 

 is reached at the first or second hour of slumber, but Michelson 

 believes that there are two classes of individuals in this respect, 

 those with morning dispositions in whom the maximum of 

 mental efficiency occurs early in the day and who upon going to 

 sleep show a maximum of intensity within an hour, and those 

 with evening dispositions whose maximum efficiency comes 

 later in the day and whose curve of sleep reaches its maximum 

 of intensity with relative slowness (If to 3^ hrs.). 



Changes in the Circulation during Sleep. — That the circula- 

 tion undergoes distinct and characteristic changes during sleep 

 has been shown upon man by phlethysmographic observations and 

 upon the lower animals by direct kymographic experiments. 

 Using very young dogs, Tarchanofff has been able to measure 

 their blood-pressure while sleeping. He finds that the pressure 

 in the aorta falls by an amount equal to twenty to fifty millimeters 

 of mercury during sleep, and that the same general fact is true 

 for man is shown by the sphygmomanometric observations reported 

 by Brush and Fayerweather.J Making use of patients with a 

 trephine hole in the skull, Mosso§ found that during sleep the 

 volume of the brain diminishes, while that of the arm or foot 

 increases. The apparent explanation of this fact is that the 

 blood-vessels in the body dilate, and receive, therefore, more 



* Howell, "Journal of Experimental Medicine," 2, 313, 1897. Michel- 

 son, "Dissertation," Dorpat, 1891. 



t Tarchanoff, "Archives italiennes de biologie," 21, 318, 1894. 



t Brush and Fayerweather, "American Journal of Physiology," 5, 199, 

 1901. 



§ Mosso, "Ueber den Kreislauf des Blutes im menschlichen Gehirn," 1881. 



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