564 TEXT-BOOK OF PHYSIOLOGY. 



After this period the volume remains practically the same for several hours, 

 diminishing as the intensity of sleep diminishes and the waking state is 

 approached. Just previous to the return of consciousness there is a rapid 

 diminution in the volume of the arm. If it be accepted that the enlargement 

 of the cutaneous vessels is followed by a diminution in size of the cerebral 

 vessels, it follows that the former condition stands to the latter in the relation 

 of cause and effect, whereby a portion of the blood is diverted from the brain 

 to the skin. It also naturally follows that the withdrawal of the blood from 

 the brain to the skin and possibly other regions as well, is the fundamental 

 condition for brain repose. 



The Intensity of Sleep. Observations of individuals during sleep 

 show that the intensity or the depth of sleep varies from hour to hour. 

 Attempts have been made to estimate the intensity by measuring the 

 loudness of a sound caused in several ways that is necessary to awaken the 

 sleeper. Accepting this criterion it may be stated from the results of many 

 experiments, that sleep increases in intensity or depth and reaches its max- 

 imum between the first and second hours, after which it rapidly decreases un- 

 til the end of the third hour, when consciousness is so nearly restored, that 

 but a very slight stimulus is required to awaken the sleeper. It is during 

 the latter period when the brain is reviving that dreams arise, the elements 

 of which are formed of previous sensations. 



The Causes of Sleep. Different theories have been proposed to account 

 for the causes of sleep, none of which have been wholly satisfactory. From 

 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. 



