466 Comparative Anhnal Physiology 



cal and physical properties which are directly correlated with the stimulation 

 of the olfactory chemoreceptors, in the hope of arriving at some common 

 denominator which may aid in understanding the mechanism of stimulation. 



The relationship between chemical structure and odor is one that has at- 

 tracted many investigators and is the subject of an enormous quantity of 

 data, which has been adequately reviewed by Moncrieff.^*^ The data relat- 

 ing odor to chemical structure do not submit to any known simple treatment 

 and hence are in a state of chaos. Moncrieff, in an effort to replace chaos 

 by order, has arrived at sixty-two general principles which relate chemical 

 structure to odor. By way of summary it may be stated that whereas sub- 

 stances of similar chemical structure (i.e., a homologous series) may have 

 similar odors, there are numerous examples of isomers and of stereoisomers 

 which have dissimilar odors and, conversely, of substances of quite dissimilar 

 structure which have similar odors. In the final analysis, therefore, the cor- 

 relation of odor with chemical structure is inadequate. 



The relationship between infrared absorption spectra (Raman spectra) 

 and odor has received some attention in the past,--- -'^ and it has been sug- 

 gested that heat waves are reflected to the olfactory epithelium according to 

 the absorption spectra of the compounds. Recently Beck and Miles"*- '^^ have 

 revived interest in this infrared radiation theory as a direct result of some 

 work on olfaction in bees. However attractive this approach may be, it 

 should be emphasized that certain optical isomers, which have identical 

 Raman spectra, have different odors, and that certain isotopic molecules, such 

 as n-butyl alcohol and the deuterated compound, have similar odors but 

 different Raman spectra.''-^ The correlation of odor with infrared absorption 

 must, therefore, be regarded with some caution. 



Examination of odorous substances indicates that they possess in common 

 certain properties which undoubtedly are related to their effectiveness in 

 stimulating the olfactory chemoreceptors. One primary requisite of all odor- 

 ous materials is that they be volatile, i.e., have a high vapor pressure at mod- 

 erate temperatures, so that molecules will readily leave the liquid or the 

 solid state. A second common property of odorous materials is that they are 

 readily adsorbed. Activated carbon will adsorb enormous quantities of odor- 

 ants but very small amounts of substances which are odorless (i.e., hydro- 

 gen, nitrogen, oxygen, carbon monoxide, and carbon dioxide). A third com- 

 mon property of odorants is lipoid solubility. It is true that many odorants 

 are also water-soluble, but some of the most effective olfactory chemorecep- 

 tor stimulants (synthetic musk, vanillin, indole) are relatively insoluble in 

 water. The conjecture may be introduced at this point that the ready ad- 

 sorption of odorants on the olfactory sense cells or their processes, coupled 

 with lipoid solubility, may facilitate the concentration on and penetration 

 of these substances into the olfactory neurones. 



Relatively recent work on the stimulative effectiveness of organic com- 

 pounds on the olfactory sense organs of insects has yielded an interesting re- 

 lation between stimulative ability and boiling point.-" Cook^^ determined 

 the optimal concentration (to flies) of a homologous series of alcohols from 

 methyl to amyl, and of esters from acetates to valerates. The optimal con- 

 centration proved to be a linear function of the boiling point (Fig. 152), or 



