SMELL, TASTE, AND CHEMICAL SENSE IN VERTEBRATE. 231 



Diagrams illustrating the receptor systems of the following vertebrati sense organs: 



A, the olfactory organ; B, the common chemical sense organ; C, the gustatory 



ORGAN. 



solutions and may, therefore, serve the animal as a distance receptor. Structur- 

 ally and functionally this sense is in strong contrast with the common chemical 

 sense and with taste. In the common chemical sense the nervous receptors are 

 the epidermal free nerve-endings of a fiber whose cell-body lies in some deep-seated 

 ganglion and whose proximal end is imbedded in the central organ thus consti- 

 tuting a sensory neurone of the first order (Diag. E). In the sense of taste a 

 neurone like that described for the common chemical sense is associated distally 

 with certain specialised gustatory cells forming taste buds (Diag. C). In both 

 the common chemical sense and the sense of taste, the receptors are stimulated 

 only by solutions very much more concentrated than in the case of the olfactory 

 organ. Consequently the receptors of these senses are stimulated only when the 

 source of the exciting material is in close contact with the sense organ itself, 

 i. e., they serve the animal chiefly as non-distance or surface receptors. 



These three classes of sense organs have already been recognized by several 

 investigators, notably Herrick (1908) and Sheldon (1909), and in their opinion 

 the sense that I have called the common chemical sense is a primitive one from 

 which the olfactory and gustatory senses have been differentiated. To me, 

 however, it seems more probable that the olfactory sense more nearly represents 

 the primitive chemical sense from which the common chemical sense and taste 

 have been derived. I am led to this conclusion because the olfactory neurone 

 presents a condition almost the exact duplicate of many primitive sensory neu- 

 rones in the invertebrates and, further, because its sensitiveness to very minute 

 amounts of substance and its capacity, therefore, to serve as a distance receptor 



