CHAPTER XX. 



THE ORGANS OF SPECIAL SENSE. 



Whenever the sensory nerves in any part of the body are 

 properly affected, a nervous impulse arises which is then 

 conveyed to the inner centers and may there give rise, and 

 usually does give rise, to what we ordinarily call a sensa- 

 tion. Very few tissues, indeed, in the body do not have 

 this property of sensation. These are the hairs, the nails, 

 portions of cartilage and bone, and other forms of con- 

 nective tissue, but with these obvious exceptions every 

 tissue in the body is able to produce by its proper nerves, 

 changes and sensations in the brain. 



Sensations, however, differ at once fundamentally and 

 fall into two groups. In one group we have the sensations 

 which give states of feeling of our own body usually with- 

 out any relation at all to the outer world. The other sen- 

 sations seem to be projected beyond the body into the 

 external world and produce in us ideas concerning the 

 phenomena of our own environment. Thus, we have the 

 common sensations of the body and the special sensations. 



The common sensations are the general feelings of pain, 

 of hunger and thirst, fatigue and buoyancy and possibly all 

 the varied feelings accompanying disease. In no instance 

 probably are these feelings ever projected into the world of 

 things. From these we gain absolutely no notion of our 

 environment. 



In the second class belong the sensations which are 

 produced by the organs of special sense and those general 

 sensations of touch in so far as they give to us knowledge 

 of external things. It is with these latter sensations that 

 this chapter concerns itself. 



There is possibly not a hard and fast line dividing 

 special sensations from the common sensations. Thus, for 



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