CHAPTER XXVII 

 SPECIAL SENSES 



Classification of special sense organs. — We have dis- 

 cussed in the previous chapter the mechanism which controls 

 sensory and motor impulses. The peripheral end of each 

 sensory (afferent) nerve fiber is always a specialized structure 

 of some kind and is called a sense organ or end organ. Each 

 kind of sense organ is adapted to respond to a certain kind 

 of stimulus only. The impulse thus originated travels back 

 to the brain or spinal cord through the afferent fiber and the 

 brain then becomes conscious of a sensation, or a reflex 

 action results. 



In former times it was customary to refer to the five 

 senses of man as sight, hearing, touch, taste, and smell. 

 The sense of touch has been shown by physiologists to con- 

 sist of four distinct qualities, pressure, heat, cold and pain. 

 For each quality there is a specialized end organ with its 

 particular fiber passing back to the brain. These four sense 

 organs are grouped together as cutaneous sense organs be- 

 cause their end organs are found in the skin. Certain spots 

 in the skin respond only to pressure, others to heat, others 

 to cold. The pain spots are most numerous and the warmth 

 spots least numerous. These spots may be demonstrated 

 by touching the back of the hand here and there with a 

 metallic point. You will notice, if the point be colder than 

 the hand, that certain spots feel pressure only, others pain, 

 and others cold. These different sensations are due to the 

 microscopic nerve endings, or end organs, that lie in the 



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