THE SENSES. 491 



and the olfactory only by odorous particles yet each of these 

 nerves may have its peculiar properties called forth by other 

 conditions. In fact, in whatever way and to whatever degree 

 a nerve of special sense is stimulated, the sensation produced 

 is essentially of the same kind ; irritation of the optic nerve 

 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 experiments 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 sensa- 

 tions of the nerves of touch (or common sensibility), excited 

 by causes acting from without, are those of cold and heat, 

 pain and pleasure, and innumerable modifications 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 endowed with sensitive nerves. 

 The sensations of the nerves of touch are therefore states or 

 qualities proper to themselves, and merely rendered manifest 

 by exciting causes, whether external or internal. The sensa- 

 tion of smell, also, may be perceived independently of the ap- 

 plication 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, color, 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, &c., are perceived. 



The same cause, whether internal or external, excites in the 

 different senses different sensations ; in each sense the sensa- 

 tions peculiar to it. For instance, one uniform internal cause, 

 which may act on all the nerves of the senses in the same 

 manner, is the accumulation of blood in their capillary vessels, 

 as in congestion and inflammation. This one cause excites in 

 the retina, while the eyes are closed, the sensations of light 

 and luminous flashes ; in the auditory nerve, the sensation of 

 humming and ringing sounds ; in the olfactory nerve, the 



