764 STUDIES IN GENERAL PHYSIOLOGY 
In this case, as in the case of rhythmical contractions, 
the oxalates and citrates are the most powerful anions of 
this series. It is clear that, in the determination of the 
lowest concentration of a salt which is still able to cause the 
immediate withdrawal of the foot, one must remember that 
a number of solutions (e. g., AgNO,, AICI,, FeCl, , HCl, 
NaOH, Na, citrate, etc.) have an after-effect which makes 
itself felt in an increase of irritability. Other solutions 
(e. g., those of calcium salts) may possibly have the opposite 
effect, namely, to raise the threshold of stimulation for sub- 
sequent tests. 
It was of some interest to ascertain whether the results in 
these experiments were produced through an action of the 
electrolytes upon the nerve-endings, or upon the nerves 
themselves. In experiments upon frogs whose skin had been 
removed from the feet, the results described in this paper 
could not be produced. The experiment of putting the nerves 
themselves into the above-mentioned solutions remained prac- 
tically without effect. It is possible that with solutions of 
much greater concentration results may be obtained. It is, 
therefore, certain that the results observed in our experiments 
are due to an action of the electrolytes upon the nerve-endings 
in the skin, and not to an action upon the sensory nerves.! 
IV. CONCLUSIONS 
The experiments mentioned in this paper were undertaken 
with two aims in view, a practical and a theoretical one. As 
far as the former is concerned, it follows from our investiga- 
tions that abnormal muscular twitchings and contractions 
may be brought about in an organism by a reduction in the 
proportion of calcium (or magnesium) in the muscles or the 
blood, or an increase in the proportion of Na and other 
1The chemical irritability of muscles is, as far as electrolytes are concerned, 
also greater than that of motor nerves. The reverse is true for electrical stimula- 
tion. 
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