XXXI. SENSATION IN THE LOWER ANIMALS.' 



Sensation may be defined as the perception of stimula- 

 tion. For example, when a hard body presses against our 

 finger it stimulates a nerve impulse which passes to the 

 brain and is there interpreted. As a result we perceive the 

 presence and action of the hard body. In the higher forms 

 of animals the seat of consciousness is the brain, and the 

 various organs where the sensory nerve fibers end are con- 

 nected by these fibers with the brain. It is to be noted 

 here that sensation does not take place necessarily when a 

 sensory fiber is stimulated. It is only when those im- 

 pulses reach the brain and are perceived that we have a 

 true sensation. In short, the seat of sensatibn is the brain, 

 and not the sense organ, and we must distinguish sharply 

 between mere stimulation of sensory impulses and their 

 perception by the brain as sensations. 



Sensation in the protozoans. — It has been known for a long 

 time that the one-celled animals have in their protoplasm 

 a very sensitive substance which reacts to various external 

 stimuli. Whether these reactions indicate true sensation, 

 that is, whether they are accompanied by any consciousness 

 on the part of the animals or not, has caused much specu- 

 lation in the past. In attempting to answer this question 

 two theories have been advanced. One school holds that 

 these tiny organisms do actually possess the rudiments of 

 consciousness, and that their reactions to stimuli indicate 

 the true conscious choice on their part. The other school 



1 See Footnote, p. 146, Chapter X. 

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