SENSATION AND THE SENSIFEROUS ORGANS. 261 



sations are caused by odorous bodies ; and we may pass 

 on to the next step of the inquiry namely, how the 

 odorous body produces the effect attributed to it. 



The first point to be noted here is another fact re- 

 vealed by experience ; that the appearance of the sensa- 

 tion is governed, not only by the presence of the odorous 

 substance, but by the condition of a certain part of our 

 corporeal structure, the nose. If the nostrils are closed, 

 the presence of the odorous substance does not give rise 

 to the sensation ; while, when they are open, the sensa- 

 tion is intensified by the approximation of the odorous 

 substance to them, and by snuffing up the adjacent air in 

 such a manner as to draw it into the nose. On the other 

 hand, looking at an odorous substance, or rubbing it on 

 the skin, or holding it to the ear, does not awaken the 

 sensation. Thus, it can be readily established by experi- 

 ment that the perviousness of the nasal passages is, in 

 some way, essential to the sensory function ; in fact, that 

 the organ of that function is lodged somewhere in the 

 nasal passages. And, since odorous bodies give rise to 

 their effects at considerable distances, the suggestion is 

 obvious that something must pass from them into the 

 sense organ. "What is this " something," which plays 

 the part of an intermediary between the odorous body 

 and the sensory organ ? 



The oldest speculation about the matter dates back 

 to Democritus and the Epicurean School, and it is to be 

 found fully stated in the fourth book of Lucretius. It 

 comes to this : that the surfaces of bodies are constantly 



