INTERRELATION OF THE CHEMICAL SENSES 169 



This peculiarity of the chemoreceptors is in strong con- 

 trast with that which occurs in the so-called mechanicore- 

 ceptors, the organs of touch, pressure, and hearing. In 

 these organs the appropriate stimulus is a deforming 

 pressure which may be exerted by an impinging or vi- 

 brating material that does not necessarily touch the 

 terminal organ itself, but may act through a considerable 

 amount of intervening tissue. Hence the mechanicore- 

 ceptors are not necessarily exposed directly to what is 

 ordinarily called the stimulus as chemoreceptors are, but 

 they may be excited more or less indirectly. Our organs 

 of touch and of hearing, therefore, may be lodged in the 

 deeper part of the skin or the head without interfering 

 in any serious way with their efficiency. All chemorecep- 

 tors on the other hand are necessarily either upon the 

 exposed surfaces of the body or are provided with pores 

 that lead from these surfaces directly to the receptors 

 themselves. This condition is in a way merely a corollary 

 of what has already been stated about chemical stimula- 

 tion, for if the organs of smell, taste and the like are 

 acted on chemically by their appropriate stimuli, these 

 stimuli must of necessity come into direct contact with 

 the given terminals. 



2. Differences among Chemoreceptors. The chemore- 

 ceptors agree then in the general character of their stim- 

 uli. Such stimuli are certain chemically active materials 

 in solution applied directly to the receptors themselves. 

 The variety that these organs exhibit ought, therefore, 

 to turn more or less on the extent of their differentiation 

 in relation to the chemical diversity of the environment. 

 The degree of this organic differentiation, however, has 

 been very inadequately worked out. Almost nothing is 



