562 STUDIES IN GENERAL PHYSIOLOGY 
contracts much more slowly, but the contractions last longer. 
In a solution of 98 c.c. $n NaCl+2 cc. $n KCl it does 
not beat at all, with the exception of a few contractions at 
the beginning. In a solution of 96 c.c. $m NaCl+2 cc. 
$n KCl+2 cc. 12 CaCl, it beats very slowly, but much 
longer than in any other of the solutions mentioned. In 
pure $n KCl or 42 CaCl, solutions no contractions 
occur. 
The explanation of all these facts seems to me to be as 
follows: If a Gonionemus be put into a pure NaCl solution, 
Na ions begin to enter the tissues. As soon as they contain 
a certain number of Na ions, any further increase of the Na 
ions raises the rate of contractions. On the other hand, 
the substitution of Ca and K ions for Na ions has the 
opposite effect (as long as not too many Na proteids are 
formed). If too many Na ions have entered into combina- 
tion with the proteids, the irritability ceases. We shall see. 
later that in this case the substitution of Ca or K ions for 
Na ions restores the irritability. 
Thus the Na ions play an important réle in the rhythmical 
contractions. Itis just as necessary that a certain number 
of Na proteids exist in the tissues of the Gonionemus as 
that a certain number of Ca and K proteids be present. 
The proportion of these three proteids is, however, apparently 
different in the margin and in the center. In both kinds of 
tissue the relative number of Na proteids is greater than 
that of the other proteids. 
The view differs from the one generally held in connec- 
tion with the heart-beat, that the NaCl in the blood serves 
mainly the purpose of preventing the tissues from losing or 
taking up water,’ while the Ca salts are considered as the 
cause of the systole and the KCl is said to favor the diastole 
of the heart. The fact that a Medusa not only contracts 
1 HOWELL, American Journal of Physiology, Vol. IT (1898), p. 47. 
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