RHYTHMICAL CONTRACTIONS IN MuscLE 531 
liberation of rhythmical contractions through Na and Br 
ions. Because of the importance of this fact, I will briefly 
report another experiment. I repeated the same experiment 
given above, with this difference, that a 0.7 per cent. NaCl 
solution was used instead of NaBr. In this case the periodic 
contractions began in the pure NaCl solution in eighty 
minutes; in the remaining solutions, which contained a trace 
of CaCl,, contractions did not occur at all, not even in the 
solution which contained 0.026 per cent. CaCl,. 
It could be shown in these experiments also that the 
faradic threshold of stimulation of the muscle contained in 
the solution containing Ca was no lower than that of the 
muscle which beat rhythmically in the NaCl solution free 
from Ca. 
In a more concentrated CaCl, solution—for example, in 
a solution isosmotic with a 0.7 per cent. NaCl solution—not 
only do no contractions occur, but the faradic irritability is also 
rapidly destroyed. If, however, only a small amount of Ca 
is present in the NaCl solution—for example, as much as 
there is contained in an equal volume of serum—only the 
specific inhibiting effect of the Ca ions upon the liberation 
of the rhythmical contractions through Na and other ions 
appears, while the irritability does not suffer. 
We therefore come to this necessary conclusion, that there 
are ions the entrance of which into the muscle has a specific 
inhibiting effect upon the liberation of rhythmic contractions. 
For practical purposes the most important of these ions are 
Ca and K. 
11. An apparent contradiction to what has been said is 
found when the following experiments are made. Two 
solutions are prepared, one a 0.7 per cent. NaCl solution, 
and a second similar solution to which 2 c.c. of an equimo- 
lecular CaCl, solution have been added to 100 c.c. of NaCl. 
The latter solution then contains the amount of CaCl, found 
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