516 STUDIES IN GENERAL PHYSIOLOGY 
that one of the changes which occur in the muscle is the 
formation of acid. That such a formation of acid, if it 
occurs, must increase the absorption of water by the muscle 
has been clearly shown in my previous publications. I do 
not, of course, believe that the formation of an acid is the 
only factor which comes into consideration here. The 
observations on the effect of K and Ca on absorption point 
to the possibility that the Na ions which enter the muscle 
alter its substance chemically. 
That this assumption is correct is shown by the following 
facts: If some acid is added to a 0.7 per cent. NaCl solu- 
tion, it compels a muscle immersed in it to take up much 
more water than when the acid is not added. If this same 
amount of acid is added to a 2.5-5 per cent. NaCl solution, 
it has exactly the opposite effect—the muscle finally loses 
more weight in the hypertonic sodium-chloride solution 
which contains acid than in the hypertonic solution without 
acid. I have not yet determined the exact turning-point. 
It lies below 2.4 per cent. NaCl and above 1.3 per cent. 
NaCl. These facts show most strikingly that when a 
sufficient number of NaCl molecules or Na ions—I suspect 
that we are dealing only with the ions—enter a muscle, the 
absorptive power of the muscle for water is altered in a 
similar way as through the introduction of K ions into the 
muscle. Table IV shows the difference in the amount of 
water absorbed by muscles in neutral and in ;+, normal acid 
sodium-chloride solutions in eighteen hours. 
TABLE IV 
NEUTRAL NaCl SoLuTIon iio NORMAL ACID NaCl SoLuTIon 
é Increase in Increase in 
Concentration] Weight in Eight-| Concentration Weight in Eight- 
een Hours een Hours 
4.90% +67 4.907 — 36.0% 
1.22 —2 1.22 +22.2 
0.70 +7 0.70 +40.0 
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