344 ELEMENTARY PHYSIOLOGY less. 



When we touch a hot or a rough body with the tip 

 of a finger, we are aware not only that we are dealing 

 with a hot or a rough body, but also that the hot or rough 

 body is in contact with the tip of the finger ; we "refer," 

 as is said, the sensation to that part of the tip of the finger 

 which is being acted upon by the body rn question. With 

 the other sensations the case is different. When we smell a 

 bad smell, though we know that we smell by the nose, we 

 do not consider that the smell arises in the nose ; we con- 

 clude that there is some object outside ourselves which is 

 causing the bad smell. We refer the origin of the sensa- 

 tion to some external cause, and that even wlien the sen- 

 sation is after all due to changes taking place in the nose 

 itself independently of external objects, as in the un- 

 pleasant odour.s which accompany certain diseases of the 

 nose. Similarly all our sensations of sight and of hearing 

 are referred to external objects ; and even in the case of 

 taste, when a lump of sugar is taken into the mouth, we 

 are simply aware of a sensation of sweetness and do not 

 associate that sensation of sweetness with any particular 

 part of the mouth, though, by the sense of touch, which the 

 inside of the mouth also possesses, we can tell pretty ex- 

 actly whereabouts in tlie mouth the melting lump is lying. 



4. The General Plan of a Sense-organ. — In these 

 sensations, thus arising in special sense-organs, and hence 

 often spoken of as " special " sensations, each sensation or 

 feeling results from the application of a particular kind of 

 stimulus to its appropriate sense-organ ; and, in each 

 case, the structui-e of the sense-organ is arranged in such a 

 manner as to render that organ peculiarly sensitive to its 

 appropriate stimulus. 



Thus the sensations of siglit are brought about by the 

 action of the vibrations of the luminiferous ether ; and the 

 eye, or sense-organ of sight, is constructed in such a way 

 that rays of light which falling on any other part of the 

 body produce no appreciable effect, give rise to vivid 

 sensations when they fall upon it. 



