254 SENSATION AND THE SENSIFEROUS ORGANS. [LECT. 



or into that of the notion of causation. Assuming 

 the existence of an external world, there is no diffi- 

 culty in obtaining experimental proof that, as a 

 general rule, olfactory sensations 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 

 revealed by experience; that the appearance of the 

 sensation 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 sub- 

 stance does not give rise to the sensation ; while, 

 when they are open, the sensation 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, look- 

 ing 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 

 experiment 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 odor- 

 ous 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 in- 

 termediary between the odorous body and the sensory 

 organ ? 



