962 PHYSIOLOGY 



traction, and the heart at its next systole throws out not only the normal 

 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 

 places-- - _-_ ~ -,-. ;_" 



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 contrac- 

 tion of the mammalian heart may be four times as frequent as at 25 C. 



FIG. 440. 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 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 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, jfodium 

 chloride therefore acts as a stimulus to contraction, but is. unable_tO-main- 

 tain the beats for any considerable length of time. The strip ol nmscle. 

 ceases contracting in a condition of relaxation. On now adding to the 

 solution a trace of calcium chloride or calcium sulphate, the contractions 



