762 STUDIES IN GENERAL PHYSIOLOGY 



saturate the foot with the sufficient number of ions to induce 

 the hypersensitiveness. 



It goes without saying that the hypersensitiveness which 

 can be produced by A1C1 3 and sodium citrate does not make 

 itself felt toward water alone, but to salt solutions also. 

 One can find a minimal concentration for each solution of 

 an electrolyte at which a pithed frog almost instantly with- 

 draws its feet when they come in contact with the solution. 

 This minimal concentration is considerably lowered after a 

 treatment of the foot with an A1C1 3 or sodium-citrate solution. 



The production of hypersensitiveness is only one side of 

 the problem. The mitigation of the hypersensitiveness is 

 the other side. The violent reactions of a frog when its 

 feet are dipped in tap- water after a treatment with A1C1 3 

 can be stopped instantly when the feet are put into a normal 

 solution of cane-sugar. When weaker solutions of cane- 

 sugar are used the feet are withdrawn, and the attempts at 

 withdrawing become more noticeable and violent the weaker 

 the sugar solution is. Very concentrated solutions of urea, 

 e. g., 2n solutions, act similarly, but not so powerfully as 

 cane-sugar. Glycerin solutions gave no such results ; 

 neither have I been able to find as yet any solution of an 

 electrolyte which acted this way. The fact that only very 

 concentrated solutions of cane-sugar or urea inhibited the 

 hypersensitiveness gave rise to the idea that the diffusion of 

 water out of the foot might be the inhibiting factor, and 

 that a stream of diffusion in the opposite direction, namely, 

 from the outside into -the skin, might give rise to a with- 

 drawal of the foot. The latter idea could be tested. When 

 the feet of a pithed frog are dipped into a normal solution 

 of cane-sugar, they are not withdrawn, no matter how long 

 they remain in the solution. But if subsequently (after 

 several minutes) the feet are put into pure water, after a 

 few (five to ten) seconds the feet are energetically withdrawn. 



