702 STUDIES IN GENERAL PHYSIOLOGY 
left column of the above list to those of the right column. 
But it is obvious too that as soon as this change occurs the 
surface layer gradually undergoes an alteration, for example, 
in air, sugar solution, NaCl solution, etc. This change, in 
which the contact-irritability is lost, occurs most rapidly in 
a CaCl, solution. This suggests the following possibility. 
The loss of contact-irritability of the muscle in air or oil, 
etc., is due to the migration of Ca ions from the interior of 
the fiber or the muscle to the surface, thus re-establishing 
approximately the original normal surface condition. If we 
then put the muscle back for a short time into a sodium- 
citrate or sodium-fluoride, etc., solution, a diminution of 
Ca ions will again occur in the surface layers and the con- 
tact-irritability will be re-established, As is to be expected 
the time the muscle remains in the solution is as important 
as the concentration of the solution. If we dip a muscle for 
a few seconds only into a sodium-citrate solution (1 gram- 
molecule in 10 liters) the contact-irritability cannot be pro- 
duced, as there is not time for a large enough number of 
anions to diffuse into the muscle. 
Still another fact harmonizes with our assumption. If we 
lift only a piece of the muscle out of the sodium-citrate 
solution, not the whole muscle contracts, but only the in- 
dividual fibers that come in contact with the air. Similarly 
amore powerful contraction occurs when we lift the thick 
femur end of the gastrocnemius out of the solution than if 
we expose the thin tendon-Achilles end to the air. 
Finally it should be mentioned that the latent period is 
somewhat long in these experiments. I have not measured 
it yet exactly; but it may be a considerable fraction of a 
second, especially when the contact-irritability is about to 
disappear. This somewhat long latent period would harmo- 
nize well with the assumption of contact-phenomena. 
Although I have spoken chiefly of the diminution of 
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