BODY FLUIDS AND CIRCULATION 121 



Crustacea and Xiphosura. Excess sodium generally has a stimulatory 

 effect on the crustacean heart, increasing rate and amplitude. In a solution 

 of NaCl alone, the heart ceases beating in systole. Low sodium depresses 

 cardiac function and the heart stops in diastole. In the presence of excess 

 potassium, the beat of the lobster heart becomes irregular, and amplitude 

 and tonus increase. In a solution containing only KC1 the heart stops in 

 systole; in low potassium the heart continues beating normally or stops in 

 diastole. These effects are ascribed to stimulation of the cardiac pacemaker. 

 Calcium also acts on the pacemaker of the lobster heart, high calcium 

 depressing the heart and stopping it in diastole, low calcium causing 

 acceleration and reducing amplitude. Magnesium is not essential for con- 

 tinued beating, but high magnesium inhibits and arrests in diastole (28). 



Comparable effects have been observed in Limulus where high concen- 

 trations of ionic sodium and potassium excite the heart and arrest it in 

 systole. The heart beat becomes fast and weak in excess KC1. High 

 potassium increases the frequency of neuronal discharge in the cardiac 

 ganglia; low potassium decreases the number of impulses, and in absence 

 of potassium ganglionic discharge gradually ceases. With high calcium, 

 cardiac frequency decreases and the heart becomes arrested in diastole. 

 Decreased calcium is accompanied by an increase in ganglionic discharge 

 frequency and in the number of active ganglionic units, effects which cause 

 acceleration of the heart (23, 106). 



Selachii. The vertebrate heart is accelerated by excess sodium, with 

 tendency to arrest in systole, whereas high potassium at first stimulates, 

 and then arrests in diastole. The predominant effect of Ca ++ is enhanced 

 cardiac contraction, resulting in systolic arrest, while in the absence of 

 Ca ++ the heart relaxes and stops in diastole. As far as known the fish 

 heart, exemplified by Scyliorhinus and Raja, reacts in a similar manner. 

 Magnesium levels are low in vertebrate blood, and heightened magnesium 

 depresses the heart and brings about diastolic arrest. A peculiarity of 

 selachian blood is the existence of high levels of urea (p. 51). This sub- 

 stance appears to be osmotically neutral so far as internal tissues are 

 concerned, since it freely penetrates blood cells and heart muscle. Removal 

 of urea arrests the heart of Raja; substitution of urea by sucrose is ineffec- 

 tive. It thus appears that urea has become necessary to the functional 

 activity of elasmobranch heart muscle (50, 118). 



Conclusions. Since the heart depends upon a balanced medium for 

 functional integrity, it is difficult to judge an ion in isolation. The effects 

 of any one ion vary with the levels of others and demand a consideration 

 of ionic ratios. Calcium antagonizes the stimulatory effect of sodium and 

 potassium on the pacemaker of hearts, and a proper balance of (Na + + 

 K + )/Ca ++ is essential for optimal functioning. A proper total osmotic 

 concentration is also essential, but within certain limits an increase in one 

 ion is counterbalanced by proportional increases of the others. Since Na + 

 already occurs in high concentrations it is difficult to raise the concentra- 

 tion of this ion without increasing the osmotic effect; reduction of Na + 



