Chemoreception 



461 



It has been suggested that there are nine types of sense cells, each sensi- 

 tive primarily to one of these classes of odors. Proof of die existence of these 

 nine types would constitute a great step forward in the study of the theory 

 of olfaction. There is some evidence that during recovery from anesthesia 

 the abihty to smell odors of class 6 (empyreumatic) recovers first and that 

 for odors of class 8 (repulsive) recovers last. It has also been found that 

 fatigue of the olfactory organ to an odor of one class (or subclass) also causes 

 reduced sensitivity to other members of the same class (or subclass). The 

 best evidence of this nature was obtained by Ohma for the five subclasses 

 of class 2. For instance, he found that camphor, eucalyptol, and eugenol, 

 members of the same subclass, produced perfect fatigue for each other, but 

 only partial fatigue for citrol and safrol which are in the same class but in 

 a different subclass. 



The olfactory capabilities of the insect chemoreceptors have not been ex- 

 haustively explored. Von Frisch •" trained bees to find food in one con- 



Y 



\ 



W > 



SOUR 



m 



SALT 



BITTER 



SWEET 



m 



(/ 





Fig. 150. Diagrams of the right half of the tongue of the human (above) and of the 

 cat (below), indicating the areas of lowest threshold for the four primary tastes, deter- 

 mined by punctiform exploration (human) and electrical recording (cat). After Hanig 

 and Pfaffmann,"' from Fulton."" 



tainer from which emanated the odor of one essential oil and related com- 

 pounds. This odor was then presented simultancousK with other odors (in 

 different containers) and the frequency of the visits to each was recorded. 

 Of the forty-seven substances tested (all crude preparations of essential oils), 

 three in addition to the training odor were attractive. The four essential 

 oils were all distilled from citrus fruit and contained one common substance, 

 limolene, and had similar odors as judged b\ man. Comparison of pure vola- 

 tile organic compounds showed that bees react in a manner similar to man. 

 Nitrobenzene and benzaldehxde, although of widely di\erse chemical struc- 

 ture, are confused as one by man, as well as by the bee; methyl ester of 

 anthranilic acid and beta naphthol methyl ether, which are nearly identical 



