INTERRELATION OF THE CHEMICAL SENSES 179 



the sensation does. The two groups of chemoreceptors 

 and of mechanicoreceptors, already frequently alluded 

 to, show how fundamental this method of classification 

 is, for these two groups represent the two well-recognized 

 activities of our material surroundings and together may 

 be put in strong contrast with radioreceptors such as the 

 organs for heat and for cold and the eye, all of which are 

 stimulated by radiant energy. 



These three classes constitute the fundamental groups 

 of receptors and under some one of these heads every 

 such organ should find its place. To the chemoreceptors 

 discussed in this volume may possibly be added those 

 on the wall of the stomach that according to Carlson 

 (1916) have to do with appetite. The receptors for pain 

 are possibly stimulated by the chemical action of ab- 

 normal tissue juices and the endings for thirst may also 

 depend upon some such form of activation (Cannon, 

 1918), though both of these organs may belong to the 

 group of the mechanicoreceptors (Muller, 1920). To the 

 mechanicoreceptors belong; unquestionably those termi- 

 nals that are excited by a deforming pressure such as 

 the receptors for touch, for pressure, including the or- 

 gans for equilibrium, and for hearing. Very probably 

 pressure is the stimulus for muscle, tendon, and joint re- 

 ceptivity and the sense of fullness in cavities. Pressure 

 produced by the contraction of the muscular walls of 

 the stomach appears to be the stimulus for the hunger 

 pang (Cannon and Washburn, 1912). The lateral-line 

 organs of fishes and amphibians give every evidence of 

 being* mechanicoreceptors. Finally radiorecepto-rs are 

 those organs that are stimulated by radiant energy such 

 as the heat organs, the cold organs, and the eye. 



