532 STUDIES IN GENERAL PHYSIOLOGY 



in the serum. In the former solution the muscle begins to 

 twitch after about eighty minutes, and continues to contract 

 rhythmically from twelve to forty hours. In the second 

 solution either no contractions whatsoever occur, or the con- 

 tractions begin later and cease earlier, that is, are decreased 

 in number. This is only a repetition of what has been said. 

 If now, after the contractions have ceased in the pure NaCl 

 solution, the muscle which has been in the NaCl solution 

 containing Ca, but which has not contracted, is put into the 

 pure NaCl solution, it begins to contract in this solution, 

 while the other muscle will contract in neither of the two 

 solutions. 



The two facts can perhaps be explained upon the following 

 basis: From my observations on the absorption of fluids by 

 muscle, it follows that Na, Ca, and K ions must be present 

 in the muscle in combinations in which these three ions can 

 be exchanged one for the other. If the muscle is contained 

 in a solution of one or more of these ions, the relative com- 

 bination of three kinds of ions is governed by the laws of 

 chemical equilibrium. If only Na ions are present, but no 

 K or Ca ions, a number of Na ions will take the place of Ca 

 ions and K ions until the laws of equilibrium have been 

 fulfilled. In this process the muscle becomes poorer in Ca 

 compounds, or at least in such Ca compounds in which Ca 

 exists in the ionic state and in which it can be replaced by 

 Na ions. (Ca may yet exist in still another form in the 

 muscle substance, and be able to enter from this form into 

 the ionic form through metabolic changes.) If, on the other 

 hand, Ca ions are in the solution then at a low (very low) 

 concentration of the Ca ions, a substitution of Na ions for 

 the Ca ions is at once compensated by an equal substitution 

 of Ca ions for Na ions. If the concentration of the Ca ions 

 is greater, the Ca ions must crowd out a part of the Na ions 

 from these combinations. Thus far this is no hypothesis, as 



