THE NUTRITION OF THE HEART 167 



of extreme contraction, called calcium rigor. The importance of calcium 

 may also be shown by attempting to perfuse a turtle heart with blood 

 serum from which the calcium has been removed by the addition of 

 sodium oxalate (which precipitates it as insoluble calcium oxalate). The 

 heart soon ceases to beat, but can readily be made to do so again by 

 adding a slight excess of calcium chloride. 



The potassium ions do not appear, like those of calcium and sodium, to 

 be absolutely essential for the maintenance of the heartbeat; at least the 

 heart of the turtle will beat for a long time when perfused with a solu- 

 tion containing only sodium and calcium salts. The explanation of this 

 result need not, however, necessarily be that potassium is an unessential 

 constituent of the perfusion fluid, for it may well depend on the fact that 

 there is a sufficient store of potassium locked away in the muscle fiber 

 to supply the requirements of the heart muscle for this ion for at least 

 as long as the beat would continue under any circumstances. In any 

 case, we know that potassium has a profound influence on the heart- 

 beat, for when the proportion of it in the perfusion fluid is increased, the 

 beat becomes very slow and the tone of the heart is greatly diminished 

 that is, it becomes extremely relaxed between the beats; and if the 

 amount is further increased, will very soon come to a standstill in a 

 greatly dilated or diastolic position. 



The striking antagonism displayed by these inorganic cations upon 

 the heartbeat has led some investigators to suggest that the stimulus re- 

 sponsible for the rhythmic activity of the heart depends on some sort 

 of chemical union occurring between the inorganic cations and the con- 

 tractile substance of the heart. Union of calcium with the contractile 

 substance will lead to systole or contraction, whereas union of sodium 

 or potassium will lead to relaxation or diastole. 



Observations on Mammalian Heart 



Investigation of the efficiency of various saline solutions on the iso- 

 lated mammalian heart has shown that the proportion of the above salts 

 must be somewhat different from that used for the cold-blooded heart. 

 As might be expected, the most efficient proportions are those present 

 in the blood serum of the particular animal whose heart is being per- 

 fused. Basing his proportions upon the results of analyses of the inor- 

 ganic constituents of mammalian blood serum, Locke found that an 

 inorganic solution of the following composition is most efficient: so- 

 dium chloride, 0.9 per cent; calcium chloride, 0.024 per cent; potassium 

 chloride, 0.042 per cent; and sodium bicarbonate, 0.01 to 0.03 per cent. 

 When "Locke's solution," as it is called, is perfused, with oxygen in it, 

 under pressure through the isolated mammalian heart at body tempera- 



