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 

 to a close. 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 

 CaClg 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 2 , suffices to do 

 away with the contractions entirely. 1 



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 

 Ca01 2 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 2 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 2 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 



i We are therefore indebted to the calcium contained in our blood for the fact 

 that our muscles do not twitch continually. 



