582 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 
Naions. (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 
Digitized by Microsoft® 
