CENTRAL NERVOUS SYSTEM. 293 



worthy of attention, and Howell, 1 after carefully reviewing the several 

 theories of sleep, emphasizes the fatigue of the vaso-motor centre in the 

 bulb as the important cause of the diminished blood-supply to the brain, 

 this fatigue in turn being caused by the continuous activity of this centre 

 during the waking - hours." 



Cessation of stimuli, decreased responsiveness of the active tissues, a 

 change in the composition of the blood, and a diminution of the blood-supply 

 to the brain are the preliminaries to sleep. 



A condition superficially resembling sleep can be induced in various ways. 

 Removal of all external stimuli, extreme cold, anaesthetics, hypnotic sugges- 

 tion, compression of the carotids, a blow on the head, loss of blood, all pro- 

 duce a state of unconsciousness which, in so far, has the similitude of sleep. 

 These conditions produce this state, however, by mechanically decreasing the 

 blood-supply or cutting off the peripheral stimuli. 



1 



Mil. 



.i"»i ,1'n 1 " 1 ' 



ii' 



il , i"| l il.l |,| " , 'l, 



"illininiii'lV"' 

 n 



\ 

 ., "' 



Fig. 123.— Plethysmography record taken from the arm of a person sleeping in the laboratory. A tail 

 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 a>leep 

 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). 



Normal sleep is tested by the fact that during its progress the changes 

 that occur in the central system are recuperative, whereas this feature may 

 be almost absent in the states which nearly resemble it. 



Condition of the System During- Sleep. — It appears that during sleep 

 the capacity of the central system to react is never lost. Were such the ease 

 it would not be possible to awaken the sleeper. The reactions most depressed 

 during sleep are those which require the lull activity of the cerebral cortex 

 for their occurrence. Conversely, it is the spinal cord which is least affected. 

 Moreover, the sleeping person is far more responsive to stimuli from without 

 than at first might be thought, The close relations between dreams and ex- 

 ternal stimuli have been recognized, and plethysmography studies show still 

 more clearly how the matter stands. 



It was found that when a subject fell asleep with the arm in a plethys- 

 mograph, various stimuli which did not waken the sleeper still served to 

 cause a diminution in the volume of the arm which was certainly due to the 



1 Howell : Journal of Experimental Medicine, 1897. 



2 De Manaceine: "Sleep: Its Physiology, Pathology, Hygiene, and Psychology," Contem- 

 porary Science Series, London, 1897. 



