RHYTHMICAL ConTRACTIONS IN MUSCLE 521 
contractions are due to a specific effect of sodium salts. 
Without entering into many details, I will only emphasize 
the fact that lithium, cesium, and rubidium also bring about 
such periodic contractions. So far as the anions are con- 
cerned, rhythmical contractions occur not only in the chlo- 
rides of the metals mentioned above, but also in the Br, I, 
and F salts. In the last-named varieties of salts contrac- 
tions begin even earlier than in the corresponding chlorides. 
In a0.7 per cent. NaCl solution it usually takes more than an 
hour before the regular rhythmical contractions begin. 
These show themselves at first in a slight tremor at the 
femoral end of the gastrocnemius, become stronger, and 
finally affect the whole muscle so that the tendon of Achilles 
executes pendulum-like movements. These periodic move- 
ments can still be observed on the following day even at 
room temperature. In a NaF solution, equimolecular with 
a 0.7 per cent. NaCl solution, the most vigorous contractions 
set in immediately, but last only about half an hour and then 
stop. In LiBr or NaBr the contractions are stronger, and 
occur earlier than in NaCl. But whether this can be 
attributed to the fact that Br ions are more effective than Cl 
ions is questionable. As the muscle is always surrounded 
by a NaCl solution and contains Cl ions, it is clear that F 
or Br or I ions must at first pass in greater number from 
the solution into the muscle than the Cl ions. I cannot 
say definitely that I ions are more effective than Br ions. 
The Rb and Cs ions have a poisonous effect like the F ions; 
that is to say, the rhythmical contractions which are pro- 
duced at first soon cease. In NaBr, LiBr, NaI, Lil, and 
LiCl solutions isosmotic with a 0.7 per cent. NaCl solution 
the contractions may continue at room temperature for one 
or two days, in which, of course, periods of rest are often 
noticed. 
4. If the number of ions entering the muscle determines 
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