CHAPTER XXL 



THE SENSES. 



Common Sensation and Special Senses. Changes in 

 many parts of our bodies are accompanied or followed by 

 states of consciousness which we call sensations. All 

 such parts (sensitive parts) are in connection, direct or in- 

 direct, with the brain by sensory nerve-fibres. Since all feel- 

 ing is lost in any region of the body when this connecting 

 path is severed, it is clear that all sensations, whatever their 

 primary exciting cause, are finally dependent on conditions 

 of the brain. Since all nerves lie within the body as circum- 

 scribed by the skin, one might be inclined to suppose that the 

 cause of all sensations would appear to be within our bodies 

 themselves; that the thing felt would be recognized as a 

 modification of some portion of the person feeling. This is 

 the case with regard to many sensations: a headache, tooth- 

 ache, or earache gives us no idea of any external object ; it 

 merely suggests to each one a particular state of a sensitive 

 portion of himself. As regards many sensations this is not 

 so; they suggest to us external causes, to properties of which, 

 and not to states of our bodies, we ascribe them; and so they 

 lead us to the conception of an external universe in which we 

 live. A knife laid on the skin produces changes in it which 



With what are all sensitive parts of the body in connection ? By 

 what nerve-fibres? How do we know that all sensations finally de- 

 pend on the brain ? 



Why might we suppose that the causes of all sensations would 

 seem to lie within the body? Name sensalions merely suggesting 

 to us a state of the body itself. What do some other sensations sug- 

 gest to us? Illustrate. 



