THE COMMON CHEMICAL SENSE 107 



sensitive in the exhausted part of the skin to weak acid. 

 Thus mechanical stimulation and chemical stimulation 

 seem to apply to different sets of terminals and the 

 exhaustion of one set does not involve that of the other. 



On treating a portion of the surface of a dogfish with 

 2 per cent cocaine, Sheldon found that tactile stimulation 

 ceased in from ten to twenty minutes whereas chemical 

 stimulation was effective for a somewhat longer period. 

 By continuing the treatment with cocaine receptivity for 

 chemical irritants was also eventually abolished. In a 

 similar way Cole (1910) found that if the hind foot of a 

 spinal frog was treated with 1 per cent cocaine till the 

 animal no longer responded to pricking or scratching 

 with a needle or to pinching with forceps, it would never- 

 theless respond vigorously to a salt solution. The'se 

 results were confirmed by Crozier in 1916 who used a 

 half per cent solution of cocaine hydrochloride on a 

 frog's foot. After about 20 minutes' immersion in this 

 solution, the reaction time of the cocained foot to formic 

 acid 0.05 molar, was about twice that of the normal foot. 

 After about an hour to an hour and a half of this treat- 

 ment the cocained foot no longer reacted to pinching but 

 gave good responses to acid with reaction times of from 

 ten to fifteen seconds, about twice that of the non-cocained 

 foot. These observations show beyond a doubt that the 

 effect of chemical irritants on the naturally moist skin 

 of vertebrates is not to be ascribed to the stimulation 

 of organs of touch or of pain but to some other form of 

 receptor, the terminals of what has been called the com- 

 mon chemical sense. 



As Crozier has pointed out, there can be no question 

 of the distinctness of the human sensations attributed 



