CHAPTER XVIII 

 SENSATION-TOUCH-TASTESMELL 



WE have seen that afferent impulses reaching the central 

 nervous system may by reflex action give rise in the central 

 nervous system to motor impulses or to other efferent im- 

 pulses, such as secretory. Many reflex actions take place 

 \\ithout our being aware of any afferent impulses. When 

 afferent impulses give rise in us to a feeling by which we are 

 aware something is happening, we speak of it as a sensation. 

 Some sensations arise altogether in ourselves and are in- 

 definite, so that we cannot say where in our bodies they 

 arise ; such are sensations of fatigue, restlessness, and the 

 like. Other sensations are very definite ; we judge them at 

 once to be caused by something around us, or by some in- 

 fluence re;iching us from without ; such are the five sensations 

 touch, taste, smell, hearing, and sight. Besides these 

 five special senses, as they are called, we have as distinct from 

 the sensations of touch proper, that is. of pressure, sensations 

 of temperature, and in addition the somewhat peculiar 

 sensations which we call sensations of pain and those which 

 are called muscular sensations. 



The five special sensations arise in special parts of the 

 body only ; sight and hearing are confined to the eye and the 

 ear ; and smell and taste to certain parts of the mucous 

 membrane lining the nose and mouth ; and touch, though it is 

 not so restricted as these, arises only in the skin and certain 

 parts of the mucous membrane lining the alimentary and other 

 passages. The particular parts of the body where these 

 sensations arise are called the special sense organs. The 

 skin is tlic sense organ for touch just as the ear is the sense 



