166 THE CIRCULATION OF THE BLOOD 



Observations on Cold-Blooded Heart 



The earlier experiments for the investigation of the chemical condi- 

 tions necessary for the maintenance of the heartbeat were performed 

 on the heart of the frog or turtle. By perfusing either of these hearts 

 with physiological sodium-chloride solution, it was observed that though 

 the beat might continue for some time, yet it gradually grew feebler 

 and feebler, until at last it ceased altogether with the heart muscle 

 in a condition of extreme relaxation or diastole. If small proportions 

 of potassium and calcium salts (as chloride) were added to the sodium- 

 chloride solution, the beat was much better maintained. Doctor Sidney 

 Ringer proved that the optimum concentration to produce efficient and 

 prolonged contraction for the heart of the frog or terrapin is as follows: 

 potassium chloride, 0.03 per cent; calcium chloride, 0.025 per cent. 

 The effectiveness of the solution was also found to be increased by the 

 addition of 0.003 per cent of sodium bicarbonate. This acts as a buf- 

 fer substance (page 36), holding the hydrogen-ion concentration at a 

 constant level. More recent work has shown that the hydrogen-ion con- 

 centration of the perfusion solutions is of considerable importance in 

 determining the efficiency of the beat, but the optimum is not the same 

 for the hearts of different kinds of animal, and indeed it may differ 

 for different parts of the same heart. 



The question naturally arises as to the relative importance of each 

 of the above salts; or rather, we should say, cations, since the anion, 

 chlorine, is the same for all of them. The function of the sodium chlo- 

 ride in the solutions is twofold: (1) to endow the solution with the 

 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 



