THE CAUSATION OF THE HEART-BEAT 1079 



capillaries is the same as in the normal animal. On this account, 

 after a lesion of the aortic valves has been established, the average 

 of the arterial pressure remains the same as before, although the 

 oscillations of pressure with each heart-beat are increased in extent. 

 The augmented output by the ventricles naturally involves increased 

 work on the part of their muscular walls, which react in the same 

 way as skeletal muscle does to increased work, i.e. by hyper- 

 trophy ; the final effect therefore is a heart bigger than normal, with 

 hypertrophied and thickened walls, but capable of maintaining an 

 adequate circulation throughout all parts of the body ; in other words, 

 in the healthy animal complete compensation has taken place. 



THE INFLUENCE OF TEMPERATURE ON THE HEART RATE 



The frequency of the heart varies directly with the temperature. 

 The higher the temperature the greater the frequency. At 40 C. the 

 contraction of the mammalian heart may be four times as frequent as 

 at 15 C. 



FIGF. 441. Tracing of contractions of a frog's heart (by RINGER), showing 

 effect of adding a trace of CaCl 2 to the NaCl solution used previously for 

 perfusion. The arrow marks the point at which the addition was made. 



INFLUENCE OF THE CHEMICAL COMPOSITION OF THE 



SURROUNDING MEDIUM ON THE HEART-MUSCLE 

 The tissues of the heart, like all other cells of the body, require 

 for the normal display of their functions a definite osmotic environ- 

 ment, i.e. a certain molecular concentration of the fluid with which 

 they are bathed. This is equivalent to a 0-65 per cent, sodium 

 chloride solution for the frog's heart, and to a O9 per cent, solu- 

 tion for the mammalian heart. As Ringer first showed, the nature 

 of the neutral salt employed for making up the normal solution 

 is all-important to the heart-muscle. Thus a strip of muscle from 

 the apex of the tortoise's ventricle as a rule does not beat spon- 

 taneously. If, however, it be immersed in a 0*7 per cent, solution 

 of sodium chloride it begins to beat rhythmically after a short latent 

 period. The contractions soon reach a maximum and then gradually 

 die away. Sodium chloride therefore acts as a stimulus to contraction, 

 but is unable to maintain the beats for any considerable length of 

 time. The strip of muscle ceases contracting in a condition of relaxa- 

 tion. On now adding to the solution a trace of calcium chloride or 

 calcium sulphate, the contractions begin again (Fig. 441). Now, 



