190 VETEEINARY PHYSIOLOGY 



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. 53.) 



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. In man it is usually 

 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 

 dpeams may ensue. Or, on the other hand, without con- 

 sciousness being necessarily restored, stimuli may lead to 

 muscular responses of a perfectly definite and purposive 

 character, and to such phenomena as sleep-walking. 



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 respirations and pulse become 

 quickened, the pupils expand, and the sensitiveness of the 

 neuro-muscular mechanism is so increased that merely 

 stroking a group of muscles may throw them into firm con- 

 traction. The individual becomes a reflex machine even as 

 regards the cerebral arcs, and each stimulus is followed 



