628 THE SENSES. 



cate to the mind the image of a vibrating body, and can 

 distinguish the vibrations when they are very slow ; it may 

 be also that the vibrations are communicated to the optic 

 as to the auditory nerve in such a manner that it repeats 

 them, or receives their impulses. 



We are made acquainted with chemical actions principally 

 by taste, smell, and touch, and by each of these senses in 

 the mode proper to it. Volatile bodies disturbing the 

 conditions of the nerves by a chemical action, exert the 

 greatest influence upon the organ of smell; and many 

 matters act on that sense which produce no impression 

 upon the organs of taste and touch, for example, many 

 odorous substances, as the vapour of metals, such as lead, 

 and the vapour of many minerals. Some volatile sub- 

 stances, however, are perceived not only by the sense of 

 smell, but also by the senses of touch and taste, provided 

 they are of a nature adapted to disturb chemically the 

 condition of those organs, and in case of the organ of 

 taste, to be dissolved by the fluids covering it. Thus, the 

 vapours of horse-radish and mustard, and acrid suffo- 

 cating gases, act upon the conjunctiva and the mucous 

 membrane of the lungs, exciting through the common 

 sensitive nerves, merely modifications of common feeling ; 

 and at the same time they excite the sensations of smell 

 and of taste. 



Sensations are referred from their proper seat towards 

 the exterior ; but this is owing, not to anything in the 

 nature of the nerves themselves, but to the accompanying 

 idea derived from experience. For in the perception of sen- 

 sations, there is a combined action both of the mind and of 

 the nerves of sense ; and the mind by education or experi- 

 ence, has learned to refer the impressions it receives to> 

 objects external to the body. Even when it derives impres- 

 sions from internal causes, it commonly refers them to ex- 

 ternal objects. The light perceived in congestion of the 

 retina seems external to the body : the ringing of the ears 



