6 W. J. CROZIER 



active by killing tlie superficial cells. In the case of picric acid 

 it is probably the II ion which is mainly concerned in stimulation, 

 since 50 annnonium picrate is entirely ineffective, neither does 

 it easily stain the frog's foot. This does not, however, signify 

 that other acids behave as does picric, since as many as 20 suc- 

 cessive reactions may be obtained from w HCl. Moreover, the 

 staining is not directly correlated with the stimulating effect, 

 since the skin of the frog's foot was stained by immersion for 

 3 minutes in g^o picric acid without producing any reaction. 

 The skin of Necturus and the catfish may be stimulated with 

 fQ picric acid without producing any visible stain. 



V. I have repeated on the frog's foot the experiment of Shel- 

 don ('09) and Cole ('10) regarding the separation of tactile 

 irritability and responsiveness to irritating solution, by treat- 

 ment with cocaine. A 0.5 per cent solution of cocaine hydro- 

 chloride was used, and the tests were made by comparing at 

 intervals the reactions of the cocainized foot with those of the 

 untreated one. After about 20 minutes' immersion, the reaction 

 time of the cocainized leg to ^ formic acid (chosen as a powerful 

 stimulant) was usually twice that of the normal foot; after about 

 an hour, varying in some tests to an hour and a half, the cocainized 

 foot no longer reacted to pinching, but gave good responses to 

 the acid with reaction times of 10-15 seconds, still about twice 

 the reaction time of the non-narcotized leg. 



It is therefore possible, I believe, to effect experimentally a 

 separation of sensitivity to mechanical and to chemical stimu- 

 lation in the frog's foot. 



There can be no question of the distinctness of the human 

 sensation attributed to the common chemical sense (Parker, '12; 

 Parker and Stabler, '13) as compared with any tactile sensation; 

 and from tests made upon cocainized areas of my own mouth, ^ I 

 am certain that the two sets of receptors are not only qualita- 

 tively distinct as regards the sensations with which they are 

 connected, but also may be separated by the use of cocaine. 



• These tests concerned mostly the inner surface of the cheek. 



