THE PHYSIOLOGY OF SLEEP. 245 



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. 108. Curves made by similar experimental methods are 

 reported by Ho well and by Michelson.* The striking feature about 

 all the curves is the sharp increase in intensity shortly after falling 

 asleep, the maximum being reached at the first or second hour of 

 slumber. Subsequently the curve again falls rapidly and the sleep 

 is light, but may show a greater or less increase in intensity toward 

 the end of the period. 



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 has been able to show 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 

 blood, while a smaller amount flows to the brain. The volume 

 of the foot or hands was measured in these experiments by incasing 

 it in a plethysmograph (see section on circulation). The author || 

 has extended these observations so as to obtain a plethysmographic 

 record of the volume of the hand and part of the forearm during 

 a period of normal sleep. One of the records thus obtained is given 

 in Fig. 109. The amount of dilatation is given by the ordinates 

 below the base line. Granting that the increase in volume of the 

 hand and arm is caused by an increase in the volume of blood 

 contained in their blood-vessels, the curve shows that during and 

 after the onset of sleep the blood-vessels in the arm slowly dilate 

 until between one and two hours after the beginning of sleep. 



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

 Bon, "Dissertation," Dorpat, 1891. 



fTarchanoff, "Archives italiennes de biologic," 21, 318, 1894. 



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

 1901. 



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



II Howell, loc. cit. 



