Effect of Salts Upon the Invertebrate Heart. 243 



Coincident with this decHne is the characteristic increase of 

 muscular tone generally associated with the action of the sugar 

 solutions. A particular feature of the action of solutions con- 

 taining potassium is that a single muscle twitch occupies much 

 less time than when the muscle is immersed in a pure sugar 

 solution, or even in other solutions in which the amount of potas- 

 sium is much less. 



E. The Effect of the Addition of Magnesium Chloride to a Sugar 



Solution. 



When small amounts of a -f\ rn- solution of magnesium 

 chloride are added to a solution of cane sugar in which the hearts 

 are immersed it is found that the quality of the contraction 

 becomes modified although the length of time during which the 

 heart will continue to beat is not altered. The first contractions 

 are usually stronger than normal. After a short series of these 

 powerful beats the beats lost strength and became somewhat 

 irregular, and finally were weak and rapid, with occasional strong 

 contractions scattered among the much w^eaker ones. 



F. The Effects of the Addition of Calcium and Magnesium to a 



Sugar Solution. 



Small amounts of | m. CaCl, and a | m. MgCL solution 

 added to a solution of cane sugar have a marked influence upon 

 the action of a heart immersed in such a solution. The beats 

 become more regular as to time and intensity and the average 

 length of time during which the heart will continue to beat is 

 greater than in the solution with either one salt alone. 



G. The Effect of Sodium Chloride and Calcium Chloride. 



While in a pure NaCl solution the heart tracings are very 

 similar to the fatigue curves of voluntary muscle, as has been 

 noted by other observers upon other hearts, the addition of a 

 slight amount of a | or y% m. solution of calcium chloride 

 exerts a profound influence upon the heart action. The cardiac 



