SECTION IV 



THE SENSES 



IN order that each particular kind of change in our sur- 

 roundings should produce its appropriate reaction, it is 

 essential that the different kinds of changes should act in 

 different ways that the contact of gross matter, changes of 

 temperature, vibrations of the air, vibrations of the ether, 

 and various chemical changes should each produce a special 

 effect. 



To secure this, special peripheral developments of neurons 

 have been evolved which react more particularly to each of 

 these special kinds of change, and with these peripheral 

 neurons particular parts of the brain are connected and 

 associated, so that a reaction to the various stimuli may 

 occur. These reactions may be accompanied by changes in 

 the consciousness by sensations ; and since our consciousness 

 is our instrument of knowledge our Ego these sensations 

 appear to us the chief and most important part of the action 

 of the mechanism. That, in many reactions, it is not an 

 essential part, we have already indicated. 



A. COMMON SENSIBILITY. 



Before dealing with the special senses the phenomena of 

 common sensibility may be considered. By this is meant a 

 series of somewhat indescribable but quite definite sensations 

 by which our attention is directed to the state of the body 

 as a whole or of various parts of the body. The ordinary 

 sensations of thirst and hunger are examples of these, sensa- 

 tions which, although due to changes in the mouth, throat, 

 or stomach, give us information as to the general needs of 

 the body. Such sensations may be considered as normal 

 and physiological. But when abnormal conditions exist in 



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