Rhythmical Pulsation in Scyphomedusa. 1^7 



lo -+- KC1 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. Calcium-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 -(- KC1 -\- 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, l 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 -f- KG, or in 

 NaCl -\- CaCL, pulsation is slower but endures longer, and in NaCl -f- KC1 

 -f- CaCL -f- MgSO 4 -|- 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 n, 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, A and C, and a solution of ^m 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 Mg, Ca, or K, or 

 any appreciable excess of these salts in sea-water, in the middle dish B the 



1 Dynamics of Living Matter, p. 95. 



2 Biological Bulletin, vol. 13, No. 4, p. 203-210. 



