756 STUDIES IN GENERAL PHYSIOLOGY 
water in the same way as the presence of Ca in the blood 
seems to prevent our muscles from beating. In order to 
- test this idea, I added to the sea-water various salts which 
precipitate Ca, e. g., NaF and Na,HPO,. I found that 
when a little more of these salts had been added than re- 
quired to precipitate all the Ca in the sea-water, the center 
behaved indeed in the same way as if it had been put into a 
pure NaCl solution. When a little less Na,HPO, was 
added, the beats began after a latent period, which varied 
according to the amount of Na, HPO, added. Rapid con- 
tractions began at once when 32 c.c. of a ® Na,HPO, 
solution was added to 68 ¢.c of sea-water. The same result 
was obtained when 16 c.c. of a normal NaF solution was 
added to 100 c.c. of sea-water. 
The addition of about 13 c.c. of m sodium-citrate solution 
to 100 c.c. of sea-water also brought about immediate con- 
traction of the isolated center. This salt does not bring 
about a precipitation of Ca in the sea-water or the tissues, 
but excludes the action of Ca ions in another way. 
I did not succeed in bringing about such results with 
the addition of Na,SO, to sea-water. Even the addition of 
32 c.c of m Na,SO, to 100 cc. of sea-water did not give 
rise to contractions, although the irritability of the center 
was increased. Experiments with the addition of NaHCO, 
remained also negative. But as only a few experiments 
were made with Na,SO, and NaCHO,, it is possible that a 
continuation of the work might lead to positive results. 
It is, therefore, obvious that the centers can be caused to 
beat through a diminution of the amount of Ca they contain, 
and it may be further argued that the presence of Ca in the 
sea-water is the cause, or at least one of the causes, that pre- 
vent the centers from beating in sea-water. 
It should, however, be added that, while a certain diminu- 
tion of Ca in the center is necessary for the development of 
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