THE CAUSATION OF THE HEART BEAT 1005 



quantity of blood but also that which it has received back from the aorta. 

 The arterial system thus receives at each beat the normal quantity of blood 

 plus the amount which leaks back into the ventricle after each systole; 

 so that the amount of blood remaining in the aorta and available for passage 

 on to the 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 hypertrophy. 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. 



FIG. 449. 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 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 25 C. 



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 tl^eir functions a definite osmotic environment, 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 0-9 per cent, solution for the mammalian heart. As Kinger 

 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 

 spontaneously. If 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 relaxation. On now adding 



