INTERRELATION OF THE CHEMICAL SENSES 181 



one of the problems of this inquiry, namely, the genetic 

 relations of receptors. 



5. Genetic Relations of Chemoreceptors. The three 

 sets of receptors mentioned in the last section, the 

 chemoreceptors, the mechanicoreceptors, and the radiore- 

 ceptors, are more than mere convenient assemblages ; they 

 represent natural groups of organs whose relations with- 

 in each group have a certain genetic character. This can 

 be illustrated by the chemoreceptors. 



FIG. 37. Diagrams illustrating the receptor systems of the following verte- 

 brate chemoreceptors: a, olfactory organ and vomero-nasal organ; b, organ of the 

 common chemical sense; c, gustatory organ. After Parker, 1912. 



If the structure of the several vertebrate chemore- 

 ceptors is compared, it will be found that they present 

 three types of organization (Fig.37). These types can 

 be best appreciated from the standpoint of their constit- 

 uent neurones. In the olfactory and vomero-nasal 

 organs the neurones have cell bodies in the receptive 

 epithelium and their axons extend as nerve-fibers from 

 these bodies into the central organ. In the common chem- 

 ical organs the receptors are free-nerve terminations in 

 the mucous epithelium of the mouth, the nose, the eye and 

 other such apertures, from which axons provided with 



