THE NERVOUS SYSTEM 177 



Fatigue of the central nervous system is manifest both 

 upon the receiving and reacting mechanism ; upon the 

 receiving, on the physical side by prolongation of the 

 reaction period, and on the metaphysical side by diminished 

 power of attention. Upon the reacting side it is shown by 

 lessened power of muscular contraction. (See also p. 94.) 



4. Sleep. Fatigue of the cerebral mechanism is closely 

 connected with sleep. As the result of fatigue, external 

 stimuli produce less and less effect, and thus the changes 

 which are the physical basis of consciousness become less 

 and less marked. At the same time, by artificial means 

 stimuli are usually excluded as far as possible. Absence of 

 light, of noise, and of tactile and thermal stimuli all conduce 

 to sleep. Consciousness fades away, and, as the cerebral 

 activity diminishes, the arterioles throughout the body 

 dilate, and the arterial blood pressure falls, and thus less 

 blood is sent to the brain, and the organ becomes more 

 bloodless. The eyelids close, the eyeballs turn upwards, 

 the pupils contract, and the voluntary muscles relax. 



The depth of sleep may be measured by the strength of 

 the stimuli required to overcome it, and it has been found 

 that usually it is deepest at about the end of an hour, 

 and that it then rapidly becomes more and more shallow 

 until, as the result of some stimulus, or when the brain has 

 regained its normal condition, it terminates. In the later 

 hours of sleep the consciousness may be temporarily aroused 

 without the other conditions of sleep disappearing, and as a 

 result of this dreams may ensue. Or, on the other hand, 

 without consciousness being necessarily restored, stimuli may 

 lead to muscular response of a perfectly definite and pur- 

 posive character, and sleep-walking may occur. 



Hypnosis is a condition in some respects allied to sleep. 

 It may be produced in many individuals by powerfully 

 arresting the attention, and is probably due to a removal 

 of the influence of the higher centres over the lower. When 

 the condition is produced the respiration and pulse become 

 quickened, the pupil expands, the sensitiveness of the neuro- 

 muscular mechanism is so increased that merely stroking 

 a group of muscles may throw them into firm contraction. 



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