698 THE SENSES 



as of ammonia, horse-radish, mustard, etc., and the difficulty is the greater 

 when it is remembered that these acrid vapors have nearly the same action 

 upon the mucous membrane of the eyelids. 



The true olfactory membrane is limited to the small area on either side 

 of the superior meatus and supplied by the olfactory nerve. It is stimulated 

 by odorous substances when they penetrate the upper chamber of the nose. 

 Currents of air can be drawn over this membrane more certainly and effect- 

 ively by sniffing the air, as noticed in the acts of a dog following the trail. 

 The odorous particles must come into contact with the olfactory cells when 

 in solution in the moisture over the surface and produce the stimulus by chem- 

 ical change. Mere presence in solution is not always adequate to a stimula- 

 tion. It seems that movement over the surface is necessary, at least to effect- 

 ive stimulation. Haycraft has repeated some of the older experiments and 

 finds that eau de Cologne can be introduced into the nasal cavity in warm 

 saline solutions without producing a sensation of smell even when 10 per cent, 

 solutions are used. He also showed that Cologne, bergamot, etc., can be 

 slowly diffused into the nasal cavity without producing a stimulus. If, 

 while the vapor is thus in the nasal cavity, the nostril be closed and the 

 person goes into pure air and breathes, then an odorous sensation is at once 

 experienced. This shows that even odorous gases "must be moved over the 

 olfactory surface" in order to produce a stimulus. 



The presence of bodies in quantities so minute as to be undiscernible 

 even by spectrum analysis, 0.000,000,03 of a grain of musk, can be distinctly 

 smelt (Valentin). Opposed to the sensation of an agreeable odor is that of 

 a disagreeable or disgusting odor, which corresponds to the sensations of 

 pain, dazzling and disharmony of colors, and dissonance in the other senses. 

 Thecause 01 this difference in the effect of different odors is unknown; but this 

 much is certain, that odors are pleasant or offensive in a relative sense only, 

 for many animals pass their existence in the midst of odors which to us are 

 highly disagreeable. A great difference in this respect is, indeed, observed 

 among men. Many odors, generally thought agreeable, are to some per- 

 sons intolerable; and different persons describe differently the sensations 

 that they severally derive from the same odorous substances. There seems 

 also to be in some persons an insensibility to certain odors, comparable with 

 that of the eye to certain colors; and among different persons, as great a 

 difference in the acuteness of the sense of smell as among others in the acute- 

 ness of sight. We have no exact proof that a relation of harmony exists 

 between odors as between colors and sounds, though it is probable that such 

 is the case, since it certainly is so with regard to the sense of taste. Such a 

 relation would account in some measure for the different degrees of perceptive 

 power in different persons; for as some have no ear for music, so others have 

 no clear appreciation of the relation of odors, and therefore little pleasure 

 in them. 



