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 AlCl, 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 AIC1, 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 AICI, 
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sugar are used the feet are withdrawn, and the attempts at 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of cane-sugar, they are not withdrawn, no matter how long 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 solation. 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few (five to ten) seconds the feet are energetically withdrawn. 
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