Rhythmical Pulsation in Scyphomeduse. 124 
CaCl, + KCl is a stimulant for nerves and muscles, although not so power- 
ful as a pure NaCl solution, and that magnesium is a relaxing, or anesthetic, 
agent, which renders the muscles incapable of contraction. Calciuum-tetanus 
is muscular not nervous in nature. 
We see that magnesium is as essential to recurrent (“ rhythmical”) 
pulsation as is sodium, potassium, or calcium, for it holds the tissue in 
check, and guards it against the too powerful stimulus and tetanus produced 
by NaCl+ KCIl+ CaCl,. It is thus a counterbalancing reagent. 
The importance of magnesium in vital phenomena is at present under- 
estimated, despite the researches of Tullberg, Meltzer and Auer, and others. 
For example, Loeb, 1906,’ lays special stress upon the importance of Na, 
K, and Ca in maintaining pulsation, but regards magnesium as of minor 
importance. 
It is true that a Ringer’s solution, consisting of chlorides of sodium, 
potassium, and calcium, will maintain pulsation longer than will any com- 
bination of any two of these elements with magnesium, but if pulsation 
is to endure indefinitely the pulsating organ must contain or be sur- 
rounded by sodium, potassium, calcium, and magnesium. In this connec- 
tion it is interesting to see that Burnett, 1907,” finds that strips of the ven- 
tricle of the turtle’s heart will live as long in isotonic, diluted sea-water as 
in Ringer’s solution; and indeed my own experiments upon the heart of the 
embryonic loggerhead turtle confirm this observation. 
A pure NaCl solution produces the most rapid initial pulsation possible 
for the tissues to sustain, but in less than one hour the medusa is thor- 
oughly exhausted, and all movement ceases. In NaCl-+KCl, or in 
NaCl + CaCl,, pulsation is slower but endures longer, and in NaCl-+ KCl 
+ CaCl, + MgSO,+ MgCl, in the amounts and proportions found in sea- 
water, pulsation is still slower, and is normal in all respects. It is evident 
that the NaCl of the sea-water is a powerful stimulant; and that the Mg, 
Ca, and K are inhibitors which restrain its affects. 
We can prove that the NaCl of the sea-water is a powerful nervous 
and muscular stimulant. If, as in figure 11, we cut a strip of subumbrella 
tissue leaving a sense-organ (s) at the one end only, then lay this strip 
across three shallow glass dishes, A, B, and C; and place natural sea-water 
in the two end dishes, 4 and C, and a solution of 54m NaCl in the middle 
dish, B, the sense-organ in the dish A gives forth pulsation stimuli in a 
normal manner, but each pulsation-wave is greatly increased as it passes 
through the NaCl in B, and it still maintains some of this increased ampli- 
tude in the dish C, although here it passes through normal sea-water. 
If, on the other hand, we placed pure solutions of Wo. Ca, or KK, er 
any appreciable excess of these salts in sea-water, in the middle dish B the 
* Dynamics of Living Matter, p. 95. 
? Biological Bulletin, vol. 13, No. 4, p. 203-210. 
