450 THE SPECIAL SENSES 



Stimulation and sensitiveness. — The explanation of the 

 stimulation of an olfactory nerve fiber is as follows : 



It is supposed that all odoriferous bodies give off par- 

 ticles in the form of vapor, which latter when dissolved in 

 the liquids of the mucous membrane stimulate the sensi- 

 tive hairs of the end organs. The vapor may be given 

 off by a near or distant body, but the question of whether 

 it be smelled or not is determined by the currents of air 

 which carry this vapor. If the air is inhaled it carries 

 this vapor through the nasal passages, where the dis- 

 solved particles produce the sensation in the manner 

 described. Similarly, swallowed bodies may send their 

 vapors up into the nasal passages through the posterior 

 openings, and stimulate in the same manner. 



The sensitiveness of the olfactory nerves to stimuli 

 varies greatly in different persons and between persons 

 and animals. While in man the sensitiveness is' not so 

 great as in dogs and many of the lower vertebrates, it is 

 still very acute. By taking odoriferous substances and 

 diluting them to known amounts it has been shoM'n that 

 the sense is so keen in. the average man that one part of 

 camphor in four hundred thousand parts of air can still be 

 readily detected, while odors like musk and vanilla can 

 be diluted to eight and ten million parts of air and still be 

 detected. These olfactory cells, however, soon become 

 exhausted by continuous stimulation and cease to react. 

 Their sensitiveness is also affected by lack of secretion on 

 the part of the membranes. 



Kinds of olfactory sensations. — Smell sensations are usually 

 classified as agreeable and disagreeable, according as their effect 

 is pleasant or unpleasant, and these effects undoubtedly serve to 

 guard the person from evil atmospheres. This is hardly a true 



