MuscunarR TWITCHINGS T57 
rhythmical contractions, the diminution has its limit. It 
appears that, if too much Ca is removed from the tissues, 
the beats will also cease. This is demonstrated by the fol- 
lowing facts: When we put the center of a Medusa into 
sea-water to which enough sodium citrate has been added, 
beats begin at once, last for a certain time, and then cease. 
If at this time the centers are put back into sea-water with 
less or no sodium citrate, beats will begin again. The ex- 
planation of this phenomenon seems to be as follows: The 
normal center of a Medusa contains too much Ca for sponta- 
neous rhythmical contractions. If we put a center into sea- 
water to which a large amount of NaF, Na, HPO,, or sodium 
citrate has been added, so much of the salt will diffuse at 
once into the organism that at least in the superficial cells 
enough Ca will be eliminated from the field of action to 
allow the spontaneous contractions to begin. Subsequently 
the same will happen in the deeper cells. The process of 
elimination of calcium in the cell proceeds, and very soon 
a period comes when the loss of Ca in all the cells will be too 
great for the contractions to go on. If, as soon as this 
occurs, the center is thrown into normal sea-water, or sea- 
water with only a little sodium citrate or phosphate, citrate 
and phosphate anions will diffuse back from the tissues into 
the sea-water, or Ca ions will diffuse into the cells, or both 
phenomena will occur, and beats will again begin. The same 
reasoning applies probably to the rhythmical contractions of 
muscles and the apex of the heart. 
2. When we put the margin containing the central 
nervous system into a pure NaCl solution, it behaves very 
much like the center, e. g., it begins to beat very rapidly, 
and the rapidity of the beats increases, at first steadily, until 
the poisonous effects of the pure NaCl solutions make them- 
selves felt. But even the addition of large quantities of Ca 
does not inhibit these contractions. For instance, when we 
Digitized by Microsoft® 
