106 SMELL, TASTE, ALLIED SENSES 



son of these receptors with those for taste Parker 

 (1908a); Herrick (1908). More recently Coghill (1914) 

 has declared that since tactile and chemical irritability 

 develop simultaneously in certain amphibian larvae, 

 chemical irritability is in reality tactile in nature. It 

 must also be perfectly evident that the receptors under 

 consideration have striking resemblances to those con- 

 cerned with pain. 



The fact that organs of taste always involve special- 

 ized end-organs, such as taste-buds, whereas receptive 

 surfaces for chemical irritants may contain only free- 

 nerve endings, shows that the relation of these two 

 classes of receptors is at best only distant. This con- 

 clusion is supported by an observation by Parker and 

 Stabler (1913) that the minimum concentration of ethyl 

 alcohol necessary for the stimulation of the irritant 

 receptors in man, 5 to 10 molar, is decidedly stronger 

 than that which will stimulate the human gustatory 

 organs, 3 molar. 



The relation of the receptors for irritants to those 

 for touch and for pain seems to be clearly indicated in 

 the results of experiments in which exhaustion and nar- 

 cotics have been used. If the tail of an amphioxus is 

 subjected to about twenty applications of weak nitric 

 acid, 0.025 molar, in fairly rapid succession, the fish will 

 cease to respond to this kind of stimulus. After the 

 exhaustion of the mechanism for this type of reception, 

 the tail of the fish will be found fully sensitive to the 

 touch of a camel's hair brush. If, now, the tail of 

 a fresh individual is vigorously stroked some thirty times 

 in succession, the fish will cease to respond to this form 

 of mechanical stimulation but it will still be found very 



