THE SENSES. 625 



invariably producing a sensation of light, of the auditory 

 nerve a sensation of some modification of sound. The 

 phenomenon must, therefore, be ascribed to a peculiar 

 quality belonging to each nerve of special sense. It has 

 been supposed, indeed, that irritation of a nerve of special 

 sense, when excessive, may produce pain ; but experiments 

 seem to have proved that none of these nerves possess the 

 faculty of common sensibility. Thus Magendie observed 

 that when the olfactory nerves laid bare in a dog were 

 pricked, no signs of pain were manifested ; and other ex- 

 periments of his seemed to show that both the retina and 

 optic nerve are insusceptible of pain. 



External impressions on a nerve can give rise to no kind 

 of sensation which cannot also be produced by internal 

 causes, exciting changes in the condition of the same nerve. 

 In the case of the sense of touch, this is at once evident. 

 The sensations of the nerves of touch (or common sensi- 

 bility), excited by causes acting from without, are those of 

 cold and heat, pain and pleasure, and innumerable modifi- 

 cations of these, which have the same kind of sensation as 

 their element. All these sensations are constantly being 

 produced by internal causes, in all parts of our body en- 

 dowed with sensitive nerves. The sensations of the nerves 

 of touch are therefore states or qualities proper to them- 

 selves, and merely rendered manifest by exciting causes, 

 whether external or internal. The sensation of smell, also, 

 may be perceived independently of the application of any 

 odorous substance from without, through the influence 

 of some internal condition of the nerve of smell. The 

 sensations of the sense of vision, namely, colour, light, and 

 darkness, are also often perceived independently of all 

 external exciting causes. So, also, whenever the auditory 

 nerve is in a state of excitement, the sensations peculiar to 

 it, as the sounds of ringing, humming, etc., are perceived. 



The same cause, whether internal or external, excites in 

 the different senses different sensations ; in each sense the 



