48 SMELL, TASTE, ALLIED SENSES 



tory surfaces may be much intensified by the condensa- 

 tion of moisture within the nasal cavity. Zwaardemaker 

 (1917) has called attention to the fact that a fog formed 

 from a vaporized salt solution is very much less stable 

 when it includes odorous substances than when it does 

 not. This condition is believed to depend upon the elec- 

 tric charges carried by the particles concerned, and 

 Durand (1918a, 1918b) recently claimed that olf action 

 is more or less dependent upon an appropriate hygro- 

 metric state in the olfactory atmosphere and that what- 

 ever facilitates the condensation of watery vapor there 

 facilitates olfaction. 



Among the older physiologists Bidder (1844) main- 

 tained that olfaction was possible on inspiration and that 

 expired air could not stimulate the organ of smell. Paul- 

 sen's observations show that this opinion is improbable 

 and the direct test of breathing odorous air in through 

 the mouth and out through the nasal cavity has de- 

 monstrated that it is quite erroneous. The olfactory 

 sensations produced on expiration are noticeably less 

 than on inspiration and this is probably due partly to the 

 lower course maintained in the nasal cavity by the ex- 

 pired air and partly to the previous elimination of much 

 of the odorous material by attachment to the moist sur- 

 faces of the mouth, pharynx, and other parts over which 

 the air passes on its way to the nasal chamber. Never- 

 theless, as Nagel (1904) has pointed out, the odors of our 

 food during mastication are the results of stimulating 

 material that reaches the olfactory surfaces through the 

 choanaB rather than through the external nares. The 

 importance of these odors in promoting the various kinds 



