44 SMELL, TASTE, ALLIED SENSES 



called irritants, true olfaction was accomplished only 

 through the olfactory terminals, which have to do with 

 delicate perfumes, aromas, and the like, many of which 

 were associated with food. 



The recognition in nasal stimulation of the two classes 

 of substances, irritants acting on trigeminal terminals, 

 and true odors affecting the olfactory endings, is of funda- 

 mental importance, and the failure to appreciate this 

 distinction is responsible in part at least for much of the 

 confusion that exists in what has been written on the 

 olfactory stimulus. As early as 1851 Frohlich pointed out 

 this distinction and called attention to the fact that irri- 

 tants or stimuli for the fifth nerve ordinarily induce 

 vigorous reflexes, respiratory and the like, whereas true 

 odors are in nature much milder and seldom call forth 

 strong responses. It is quite possible that some materials 

 are stimuli for both classes of end-organs ; thus tobacco 

 smoke not only carries with it an aroma or true odor but 

 also acts as an irritant. These two actions, however, may 

 depend upon different chemical substances in the smoke. 

 Other stimuli such as oil of mustard or possibly ammonia, 

 that are chemically much more homogeneous than tobacco 

 smoke, may affect, nevertheless, both sets of receptors and 

 thus exhibit the characteristics of both irritants and 

 true odors. A revision of the so-called olfactory stimuli 

 from this standpoint is much to be desired. 



2. Passage of Air through the Nasal Cavity. In ordi- 

 nary respiration in man the passage of air through the 

 nasal cavity does not necessarily excite olfaction at once. 

 Sooner or later, however the odor may be slightly sensed 

 after which a few deep breaths or sniffing movements 

 are usually made, whereupon full stimulation ensues. 



