560 STUDIES IN GENERAL PHYSIOLOGY 
and its ganglia, and the central region which has no ganglia, 
but is said to possess scattered ganglion cells. The case is 
similar to that of the heart, which has ganglia in the auricles 
and sinus vinosus, whose ventricle, however, is free from 
ganglia, but contains scattered ganglion cells. 
Romanes first stated that if we cut a Hydromedusa in 
two, the marginal part with the ganglia will continue to beat 
rhythmically very much like the whole Medusa, while the 
center ceases to beat. These results have been confirmed by 
several authors, but I have found that the statement of 
Romanes is only correct for sea-water. If the center of a 
Hydro-medusa be put into a pure $n NaCl or 3” NaBr solu- 
tion, it begins to beat rhythmically for an hour immediately 
after the operation. Hence the center as well as the margin 
is capable of spontaneous contractions. But why does the 
center not beat rhythmically in sea-water? If it be put in- 
to a solution of 98 c.c. $n NaCl+ 2c.c.142n CaCl,, it no 
longer beats rhythmically. The same is true for a solution 
of 98 cc. 3n NaCl+2c.c. §n KCl, or a solution of 96 
NaCl+2 CaCl, +2 KCl. Hence the Ca and K ions of the 
sea-water prevent the center from beating rhythmically. 
This harmonizes with my previous experiments on the 
muscles of the skeleton.’ The latter are able to beat rhyth- 
mically in a pure NaCl or a NaBr solution or any solution 
with Naions. But a small addition of Ca ions or K ions, or 
both, prevents rhythmical contractions. We owe it to the 
presence of these ions in our blood that our muscles do not 
contract rhythmically like the heart. 
Thus we see that there is a typical difference between the 
effects of ions on rhythmical contractions originating in the 
muscles directly and those originating in parts which con- 
tain ganglia or which originate in the latter themselves. 
Inasmuch as the whole Gonionemus beats in the rhythm of 
1 Part II, p. 518. 
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