CHAPTER XXIII 



THE SPECIAL SENSE ORGANS 



Questions. 1. How can some animals get along without special sense 

 organs ? 2. Is it true that if one of the senses is injured, the others be- 

 come more keen to make up for it ? 3. Why is it difficult or impossible 

 to distinguish flavors or food when one has a cold in the nose ? 4. Are 

 all animals equally sensitive to odors ? to sounds ? 5. How can we tell 

 whether other animals perceive the world through the senses (see, hear, 

 smell, taste) just as we do ? 



183. The nervous system and the outside world. The behav- 

 ior of a living thing is always related to the outside world. 

 Indeed, life has been defined as continuous adjustment to ex- 

 ternal changes. This constant interplay between the environ- 

 ment and internal processes can be readily observed in very 

 simple animals and plants. In complex organisms like the 

 human body many things happen on the inside for which ad- 

 justment is necessary. For example, an increase of muscular 

 activity calls for an increase of heart work, of lung work, and of 

 kidney work. But that is another way of saying that the organ- 

 ism acts as a whole, for increase of muscular activity has to do 

 with the outside, or the environment : it brings about changes 

 either in the position of the organism or in the environment itself. 



Many of the reflexes protect from injurious contacts or ex- 

 posures, as the winking reflex, or the pupil reflex, or a with- 

 drawing-from-pain reflex. Others serve to get the animal food. 

 If you ever catch a fish with a hook and line, you depend upon 

 a reflex for your success. You simply have to make sure that 

 you have the right kind of bait ; your "luck" depends upon the 

 fish's seeing the bait, and the reflex does the rest. 



We must not think of reflexes as perfect instruments for get- 

 ting the necessaries or for escaping danger and enemies. From 



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