CHAPTER VIII. 



INTEEBELATION OF THE CHEMICAL SENSES. 



Contents. 1. Common Features in the Stimulation of 

 Chemoreceptors. 2. Differences among Chemoreceptors. 

 3. Groups of Chemoreceptors. 4. Classification of Recep- 

 tors in General. 5. Genetic Relations of Chemoreceptors. 

 6. Bibliography. 



1. COMMON Features in the Stimulation of Chemo- 

 receptors. The sense organs that have been discussed in 

 this volume, the olfactory organs, the vomero-nasal or- 

 gans, the common chemical receptors, and the organs of 

 taste, form a more or less natural group of organs under 

 the general title of chemical receptors or Chemoreceptors. 

 This designation is justified by the fact that in stimula- 

 tion these several types of receptors present certain 

 important features in common. In all instances they 

 are activated by solutions. This is most obvious in the 

 sense of taste whose stimuli from fishes to mammals con- 

 sist of materials in solution either in the water that enters 

 the mouth or in the saliva that is mingled with the crushed 

 food. An aqueous solution is also the stimulus for the 

 common chemical receptors. The nasal cavities of fishes 

 are likewise bathed by a continuous stream of water that 

 carries the stimulating substances to the olfactory sur- 

 faces. And in the air-inhabiting vertebrates, as already 

 pointed out, the olfactory terminals are probably not 

 exposed in any direct way to the air that carries the stim- 

 ulating material but are immersed in mucous through 



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