THE NUTRITION OF THE HEART 167 



proper osmotic pressure (see page 4) ; and (2) to perform the special 

 role of the sodium ion in the origination and maintenance of the auto- 

 matic beat. The latter function of Na can be shown by observing the behav- 

 ior of strips cut out from the ventricle of the turtle heart and placed 

 in solutions of correct osmotic pressure but containing no sodium chlo- 

 ride isotonic solutions of cane sugar, for example. They soon cease 

 to beat, but if a small amount of sodium chloride is added to the cane 

 sugar solution, rhythmic contractions return. The role of the calcium 

 ions is almost entirely a pharmacological one. If a strip of turtle ven- 

 tricle which has been made to cease beating by immersion in isotonic 

 sugar solution is placed in a weak solution of calcium chloride before 

 it is transferred to sodium chloride solution, the spontaneous contrac- 

 tions will return earlier and continue for a longer time. On the other 

 hand, if more than the correct amount of calcium salt is present in the 

 solution, the beats will soon be found to become smaller and smaller 

 in amplitude, because relaxation does not properly occur between them, 

 and ultimately they will cease altogether with the ventricle in a condition 

 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 



