ANIMAL SLEEP 15 



which have only been experienced by the sleeper in 

 previous dreams. The experiments by which M. 

 Jouffroy conceived that he had proved that the 

 brain was always dreaming, because persons awakened 

 at various times all said they were then dreaming, 

 are not conclusive. The extraordinary quickness 

 with which the association of ideas follows a sound 

 and produces dreams might account for the dream 

 at the time of awakening, even if the interval be- 

 tween the sound made to rouse the sleeper and that 

 of consciousness were only momentary. But some 

 functions of the brain can be kept alert in sleep ; 

 and the animal which passes all day in the constant 

 apprehension of danger, naturally preserves its vigilant 

 faculties during sleep in a very high degree. Such 

 sleep can hardly be restful, and it is not improbable 

 that this want of complete and secure repose accounts 

 in a measure for the shortness of animal life, even 

 when aided by the healthy influence of their free 

 and open-air existence. 



