60 SMELL, TASTE, ALLIED SENSES 



of gaseous or vaporous particles, they probably enter 

 quickly into solution in the watery mucous and in this state 

 come in contact with the olfactory hairs. From the nature 

 of the surroundings, then, it would seem extremely im- 

 probable that the stimulating material for the olfactory 

 terminals should be in any other state than that of a solu- 

 tion. This opinion seems to be gaining ground rapidly 

 among the more recent workers, for it has found clear 

 expression within the last few years in papers by Back- 

 man (1917a), by Durand (1918b), and in a qualified way 

 by Henning (1916). 



As already indicated, the difficulty met with in at- 

 tempting to stimulate adequately the human olfactory 

 epithelium with solutions of odorous material is due in 

 all probability to the effects of the solvent on the olfac- 

 tory hairs and not to the incapacity of these terminals to 

 be stimulated by solutions. These hairs are apparently 

 very delicately attuned to a mucous environment that 

 would be very difficult to duplicate experimentally and yet 

 this environment seems to be essential to a wholly suc- 

 cessful test. Care as to temperature and salt contents of 

 the solvent, as emphasized by Aronsohn, Vaschide, and 

 Veress, are probably only the first steps in this direction. 



The relation of the solubility of a substance to its 

 efficiency as an olfactory stimulus has been discussed 

 recently by Backman (1917a). This investigator has 

 expressed the opinion that not only the aqueous environ- 

 ment of the olfactory hairs must be considered but also 

 the substance of the hairs themselves. This he believes 

 to be lipoid in character, an opinion that is supported by 

 the well known fact that these hairs are best demon- 

 strated by osmic acid. If the embedding mucous layer is 



