THE CEREBRUM 615 



all the facts which have been presented it would appear that one cause is a 

 decline in the irritability of the nerve-cells of the brain and associated sense- 

 organs, and the development of fatigue conditions, the result of prolonged 

 activity. 



A second cause is the withdrawal of a large portion of the blood from the 

 brain, on the presence of which, here as elsewhere, normal activity depends. 

 As to whether the diminished activity of the brain is the cause of, or the result 

 of the withdrawal of the blood there has been much difference of opinion. 

 Howell has offered a plausible explanation for the withdrawal of the blood 

 from the brain to the cutaneous vessels, based on the activity of the vaso 

 motor center. He assumes that for a variable number of hours, correspond 

 ing to the usual waking state, this center possesses a certain average tonus, 

 due in all probability to reflex influences, by virtue of which it maintains a 

 certain average contraction of the cutaneous vessels. But at the end of 

 this period it too becomes fatigued, declines in irritability, becomes less 

 responsive to reflex influences, and hence loses its control over the vessels. 

 As a result they dilate and thus reduce the amount of blood flowing to the brain 

 to a level insufficient to maintain its activity, after which sleep supervenes. 

 During sleep the irritability and tonus of the center are restored, when its 

 control of the blood-vessels is regained. Unless the brain in its functional 

 activities differs from all other organs of the body, it may be inferred that 

 cessation of activity or repose is the result partly of fatigue and partly of a 

 diminution of the blood-supply. 



