PHYSIOLOGY OF OLFACTION 79 



to the aliphatic series. He abandons the idea that spe- 

 cial odors are to be associated with particular osmopho- 

 ric groups. In odors these groups are significant, not 

 because of the structure they themselves possess, but 

 because of the positions they may occupy on the benzene 

 ring. Osmophoric groups are such as the hydroxyl, al- 

 dehyde, keton, ester, nitro, and nitril groups. None of 

 these, however, is associated with a particular odor, but 

 any one may be the occasion of odor, if it occupies an 

 appropriate place on a benzene ring. The position on the 

 ring not the particular radical, according to Henning, is 

 the determining factor so far as odor is concerned. 



Henning is further convinced that in a general way 

 types of chemical constitution can be indicated for the 

 six groups of odors that he was able to distinguish 

 (Fig. 18). Thus the class of spicy odors is represented 

 by compounds in which the osmophoric groups are in 

 para-position (Fig. 18a), as in anisaldehyde. In the 

 flowery odors the osmophoric groups are in the meta- or 

 the ortho-positions (Fig. 18b), as in tuberon. In the 

 fruity odors the groups are forked (Fig. 18c) as in cit- 

 ral. In the resinous odors the groups are within the 

 ring (Fig. 18d) as in pinene. In the burnt odors the 

 ring is smooth (Fig. 18e) as in pyridin, and in the foul 

 odors the ring is fragmentary (Fig. 18f ) as in cacodyl. 

 In this way each class of odors is associated with a spe- 

 cial feature in the constitution of the molecule though 

 not necessarily with a particular osmophoric group. In- 

 termediate odors are due to combinations of groupings 

 which partake of the nature of the two classes between 

 which the intermediate lies. Thus vanillin has an odor 

 between spicy and flowery and its three osmophoric 



