CHAPTER V 

 THE RECEPTORS AND EFFECTORS 



IN the further study of the nervous system as the apparatus 

 of adjustment between the activities of the body and those of 

 environing nature, our first task is the analysis of the receptors 

 (that is, the sense organs) ; for these are the only places through 

 which the forces of the world outside can reach the nervous sys- 

 tem in order to excite its activity. 



"The world is so full of a number of things 

 I'm sure we should all be as happy as kings." 



But in order to attain this fortunate result it is necessary that we 

 should be able to discriminate the essential from the unimportant 

 elements of this environing complex, and to adjust our own be- 

 havior in relation thereto. 



Protoplasm in its simplest form is sensitive to some sorts of 

 mechanical and chemical stimulation. In fact, as we have seen, 

 all of the so-called nervous functions are implicit in undifferen- 

 tiated protoplasm. But the bodies of all but a few of the lowest 

 organisms are protected by some sort of a shell or cuticle from 

 excessive stimulation from the outside, and individual parts of 

 the surface are then differentiated in such a way as to be sensi- 

 tive to only one group of excitations while remaining insensitive 

 to all other forms. Thus arose the sense organs, each of which 

 consists essentially of specialized protoplasm which is highly 

 sensitive to some particular form of energy manifestation, but 

 relatively insensitive to other forms of stimulation. Each sense 

 organ possesses, in addition, certain accessory parts, adapted to 

 concentrate the stimuli upon the essential sensitive protoplasm, 

 to intensify the force of the stimulus, or to so transform the 



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