180 SMELL, TASTE, ALLIED SENSES 



To ascertain into which of these three groups a re- 

 ceptor falls it is necessary to know how it is stimulated 

 after which its classification is simple and immediate. 



Although a grouping of receptors based upon their 

 stimuli will of necessity always be complete, this plan of 

 arrangement is not entirely devoid of difficulties. Chief 

 among these is the fact that the same stimulus may ac- 

 tivate what we know from other standpoints to be differ- 

 ent receptors. Thus, as already stated, parabrombenzoic 

 sulphinide excites sweet receptors as well as bitter ones, 

 and strong material vibrations will stimulate the organs 

 of touch as well as the ear. But such instances appar- 

 ently occur only between closely related receptors, for 

 the organs for sweet and for bitter are so closely related 

 as to be regarded by many as belonging to one category 

 and hearing is certainly very near akin to touch. Herrick 

 (1918) has discussed the definition and classification 

 of receptors and has urged for this purpose the use of 

 four criteria : the sensation, the stimulus, the sensory 

 mechanism, and the type of response. In his opinion, 

 however, none of these affords a wholly satisfactory basis 

 for discrimination and grouping, operations that can be 

 successfully carried out only when sufficient information 

 is at hand. But experience scarcely warrants such a 

 conclusion, for it is much more difficult now to discover 

 the interrelation of the twenty or more human receptors 

 with all that is known about them than it was to make a 

 corresponding statement about the original five. The real 

 difficulty lies in the fact that the numerous receptors that 

 we now recognize have undergone varying degrees of 

 differentiation and hence their mutual affinities are ex- 

 tremely diverse. This brings us at once face to face with 



