SMELL, TASTE, AND ALLIED 

 SENSES IN THE VERTEBRATES 



CHAPTER L 



NATURE OF SENSE ORGANS. 



Contents. — 1. Older Conception of Sense Organs. 2. 

 Modified View due to Theory of Reflex Action. 3. The 

 Genesis of Receptors. 4. Bibliography. 



1. Older Conception of Sense Organs. In the con- 

 ventional text-book, sense organs are commonly looked 

 upon as structures that supply the brain with those nerv- 

 ous impressions from which the mental life of the indi- 

 vidual is built. During normal activity these organs are 

 incessantly in operation and flood the central apparatus 

 with a stream of impulses by which are carried to us evi- 

 dences of the multitudinous alterations of the environ- 

 ment. Through the ear and the eye pass continuous 

 streams of change by which we adjust ourselves not only 

 to the immediate material world about us but to the 

 world of ideas whose elements are spoken and writ- 

 ten words. 



Sense organs from a structural standpoint are organs 

 whose cells are so specialized that they are subject to stim- 

 ulation by only a particular category of external changes. 

 As Keith Lucas has expressed it, sense cells approximate 

 a unif unctional state. The changes by which they are 

 brought into action form rather homogeneous groups of 



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