26 V. G. DETHIER 



LOG F, (25° C) 



Fig. 23. Comparison in terms of thermodynamic activity of the stimulating effec- 

 tiveness of homologous aliphatic aldehydes on the olfactory receptors of the blowfly. 

 The open circles represent rejection thresholds and the solid circles, acceptance thresh- 

 olds. 



should be able to work with single receptor units. But, as with taste, prelim- 

 inary studies of populations of receptors are of introductory value. On the basis 

 of measurements made with insects by Dethier and Yost (1952) and Dethier 

 (1954a) and with man by Mullins (1955a) it is clear that the olfactory thresholds 

 stated as thermodynamic activities (pt/ps) for members of certain homologous 

 series, notably alcohols and aldehydes, are nearly constant over the middle range 

 of chain length (Figs. 10, 21, 22, 23). On the other hand, recent measurements of 

 fatty and olfactory thresholds in the dog (Neuhaus, 1953) and in the honeybee 

 (Schwarz, 1955) do not exhibit such a clean-cut relationship when expressed in 

 activity units. When expressed as numbers of molecules per cc. of air at thresh- 

 old both series show an increase in sensitivity for C2 to C 4 (although the differ- 

 ence is very small in the bee) and a decrease from C 4 to C 8 . Dethier (1954b) 

 pointed out that the relationship observed with the alcohols suggests that the 

 phenomenon under investigation may involve the establishment of an equilib- 

 rium and as such represent a physical rather than a chemical process involving 

 specific receptor substances. Mullins (1954) has analyzed the data of Dethier 

 and Yost (1952) by plotting the product of the thermodynamic activity at 

 threshold and the molal volume of the compounds against chain length and 

 assigning a calculated membrane solubility parameter of 11.5. The analysis 

 suggests that the vapor of the first three alcohols may not be in equilibrium 

 with the organism. 2 



2 In a recent paper Mullins (1955b) has analyzed existing data and presented 

 additional data which indicate that olfaction is not at all similar to an equilibrium 



