24 



V. G. DETHIER 



Adrian (1950), recording from olfactory tract fibers in the rabbit, was able 

 to demonstrate that there are large scale differences in the level and evolution 

 of excitation in different parts of the olfactory epithelium. However, odors of 

 any class could stimulate any area provided the concentration was sufficiently 

 high. He thought it was probable that olfactory analysis is brought about by 

 the effect of the general arrangement of the olfactory organ on stimulus accessi- 

 bility and strength rather than by differential sensitivity of receptors in differ- 

 ent parts of the organ. Nevertheless, owing to regional analysis, whether result- 

 ing from differential receptor sensitivity or differences imposed upon the 

 stimulus by the structure of the organ, different smells set up different patterns 

 of excitation. No indication was found that the olfactory epithelium is a uniform 

 mosaic of receptors each with specific affinities for certain kinds of smells. Yet 

 the entire olfactory organ seems to act as an analyzer for molecules of different 

 physical and chemical properties. 



The evidence given above is the best to date that there may be a number of 

 functionally different types of olfactory receptors. It should be mentioned before 

 leaving this subject that the highly specific reactions of certain organisms to 



30 



4.0 



CL 

 O 



5.0 



6.0 



O 



5 



2 70 



80 



• REJECTION THRESHOLDS 

 o ACCEPTANCE THRESHOLD 



LOG NUMBER OF C ATOMS 



Fig. 21. Relation of olfactory thresholds (acceptance and rejection) for aldehydes 

 to chain length. The broken line represents the relation for alcohols based upon the 

 data of Dethier and Yost (1952). The square and triangle represent rejection and 

 acceptance thresholds, respectively, for iso-valderaldehyde. Acceptance thresholds are 

 based upon maximum response. (Dethier, 1954a). 



