168 SMELL, TASTE, ALLIED SENSES 



which this material must make its way before it becomes 

 effective. In olfaction, moreover, it is probably not 

 simply a question of aqueous solution but, as already ex- 

 plained, one of solution in oil as well, for the olfactory 

 stimulus seems to be a material that must reach its recep- 

 tors through an aqueous medium that covers them and 

 then enter them through their lipoid components. What 

 has been said of the stimulation of the olfactory organ 

 is probably true of the vomero-nasal organ also. Thus 

 in one way or another all appropriate stimuli of the so- 

 called chemoreceptors are materials in solution. 



But not all soluble materials stimulate the chemorecep- 

 tors. Thus such elementary gases as hydrogen, oxygen, 

 and nitrogen are odorless and tasteless, and a number of 

 organic substances have no stimulating capacity for 

 these organs. Those substances that do stimulate, as 

 was pointed out especially in the case of taste, fall into 

 groups whose characteristics are chemical and not phys- 

 ical and, though such an analysis cannot at present be 

 made with certainty for smell, it has already been pointed 

 out that the variety of smells can be explained only on a 

 chemical basis. Thus chemoreceptors are stimulated not 

 simply by material in solution, but by the chemical activ- 

 ity of dissolved material. On this assumption it is nat- 

 ural to expect that there would be a certain number of 

 substances, chemically inert toward the given receptors, 

 that would, therefore, be incapable of acting as stimuli 

 for them. Such substances as the gases already men- 

 tioned probably represent this group. 



The stimulus for the chemoreceptor, however, is not 

 only a solution of a chemically active material, but it is 

 such a solution applied directly to the terminal organ. 



