530 STUDIES IN GENERAL PHYSIOLOGY 
that found in the serum of the turtle (according to Greene). 
One can scarcely conceive of a more convincing experiment 
than the foregoing. In the pure NaBr solution rhythmical 
contractions occurred after a minute, and lasted uninter- 
ruptedly for twelve hours, when the experiment was brought 
toaclose. In the second solution, which contained the least 
amount of Ca, occasional very weak contractions occurred 
which were noticeable during the first thirty minutes, but 
for the remainder of the day the muscle remained absolutely 
quiet. In the other solutions containing greater amounts of 
CaCl, no contractions whatever occurred. 
So slight an amount of calcium as 0.026 per cent.—that 
is to say, the amount of calcium contained in blood serum— 
is sufficient to render almost entirely impossible the periodi- 
cal contractions brought about through Na and Br ions, and 
a little more, namely, 0.038 per cent. of CaCl, , suffices to do 
away with the contractions entirely.’ 
It might be doubted that in these experiments the Ca 
acted as the specific inhibiting substance for the muscular 
contractions. It might be considered possible that the 
CaCl, had decreased the irritability of the muscle to such an 
extent that the liberation of contraction through Na and Br 
ions had been rendered impossible; but that is not the case. 
I tested after three hours the faradic irritability of all the 
muscles. That contained in the NaBr gave the first con- 
tractions when the distance of the secondary coil was 310 
mm.; that in the NaBr plus 0.026 CaCl, solution had the 
same threshold of stimulation; that in the other solutions 
was only slightly lower. On the following morning the 
muscles contained in the CaCl, solution were more irritable 
than that in the pure NaBr solution, which had contracted 
continually. We can, therefore, only be dealing with the 
fact that the entrance of Ca ions into the muscle inhibits the 
1 We are therefore indebted to the calcium contained in our blood for the fact 
that our muscles do not twitch continually. 
Digitized by Microsoft® 
