602 TEXT-BOOK OF PHYSIOLOGY 



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

 activity. 



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

 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 resul 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 diminution of the blood-supply. 



