XXVIII. THE SPECIAL SENSES. 

 Touch, Taste, and Smell. 



In the preceding chapters we have discussed, mainly, 

 the mechanism by which sensory and motor impulses are 

 conveyed and controlled; and the areas of the central 

 system where these sensory impulses are received and 

 translated into conscious sensation. The manner in which 

 impulses originate and their character requires a study of 

 the arrangements at the surface ends of the afferent fibers. 

 Such arrangements are grouped together mider the name 

 of end or sense organs. They are aU specialized struc- 

 tures adapted for response to certain kinds of stimuli only, 

 and are named according to the effect of the impulses 

 upon the consciousness. Thus, organs wliich produce 

 sensations of sight are called sight organs, those which 

 produce sensations of touch, tactile organs, etc. It must 

 be distinctly borne in mind, however, that all of these 

 organs merely originate impulses, and it is -only when these 

 impulses have traveled over the afferent fibers and stim- 

 ulated certain tracts of the brain cortex that we become 

 conscious of the sensations which they produce. In short, 

 the sense organ is simply a structure adapted for response 

 to a special kind of stimulus; and the impulse sent out by 

 this stimulus, the brain interprets as sight, pain, heat, 

 etc., according to the quality of the impulses. 



Laws of sensory impulses. — There are three laws of 

 sensory impulses which are applicable to aU kinds of sen- 

 sory fibers. 



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