176 SMELL, TASTE, ALLIED SENSES 



with Oehrwall's opinion (1901), must be regarded as 

 generic and to consist of at least three and probably four 

 senses, namely the sense of sour, of saline, of bitter, and 

 of sweet. These senses are really distinct and separate. 

 They have independent receptors and give rise to sensa- 

 tions that do not intergrade. Their association under 

 one head as members of the sense of taste is in a way a 

 misconception due doubtless to the fact that in ordinary 

 activity all four senses are commonly in operation at 

 once, and hence acquire a certain degree of functional as- 

 sociation. Taste then is not the name for a single sense 

 but for a group of senses and it is likely that smell is of 

 the same nature, but until olfaction is better understood, 

 it is impossible to indicate the elements of which it is com- 

 posed. Thus the chemical senses, like the others already 

 briefly enumerated, show the same tendency to increase 

 in number as they become better known. 



4. Classification of Receptors in General. A detailed 

 investigation of the chemoreceptors leads to an increas- 

 ing multiplicity of elements as in the other receptor sys- 

 tems, and raises the question of what constitutes a unitary 

 sense and how such units are related. When one or 

 more similarly organized receptors are excited to activity 

 by a single category of stimuli and give rise to the same 

 kind of sensation we think of the aggregate as a sense. 

 Thus when a deforming pressure impinges upon any part 

 of the skin, touch receptors are stimulated and we re- 

 ceive a uniform impression characteristic of the sense of 

 touch. Or when one of a variety of sounds falls upon 

 the ear, we experience hearing. In the second instance 

 the stimulus, different sounds, is open to much greater 

 variety than in the first where the stimulus is, a deform- 



