ACTION OF HEAT ON THE EXCISED HEART 113 



The rate before cooling was 24 per minute ; during cooling, 7. After 

 removing the cold blood the heart very rapidly increased in rate, and 

 for the first few beats in force. The rate was increased until the beat 

 became faster than the original rate. The force gradually decreased 

 until at last both rate and force after about five minutes returned to 

 their original state. 



At a rather higher temperature 11° C. the same effects are observed, 

 though to a much less marked degree (fig. 94 a). 



At 25° C. it is seen that the rate is greatly increased— from 51 

 to 93 per minute. The force of the ventricular and auricular con- 

 tractions remains practically unaltered, but the time of each is markedly 

 diminished, so that the auricular relaxation is finished before the ven- 

 tricular contraction begins. On removing the warmed blood these 

 changes gradually disappear. 



If a heart be immersed in blood at about 35° C, fig. 94 b, a very 

 interesting result is obtained. The ventricular contraction at once 

 ceases, but the auricular persists — at first very weak, but gradually 

 becoming stronger. If immersion be prolonged one beat is dropped at 

 irregular intervals, at times two successive ones. On cooling, the heart, 

 after some few seconds, once more begins to beat, and at first with 

 greatly increased frequency. Sometimes, as in the tracing reproduced, 

 the ventricular contractions after a time are once more dropped, and 

 the auricular become much weaker. After a varying interval, rhythm 

 for a second time returns, and the heart apparently recovers com- 

 pletely. During both periods, when the ventricular contractions cease, 

 the ventricle will respond with a single beat on mechanical or electrical 

 stimulation, though the stimulus required is greater than normal. 



Another change which this tracing shows is in the amount of ' tone ' 

 of the heart. Directly after immersion the heart begins to elongate, at 

 first fairly rapidly, and then more slowly, i.e. there is a diminution of 

 tone. As soon, however, as the heart recommences beating this tone is 

 recovered, and even becomes greater than before. When the ventricle 

 once more ceases beating the tone immediately falls again. 



In some hearts a different result is produced at this temperature. 

 The heart may at first beat very rapidly before the ventricle finally stops. 

 This is because the heart takes a little time before it reaches the tem- 

 perature at which the ventricle stops, and if the fluid be only just above 

 this temperature the time required to reach this point is much longer 

 than if the temperature be distinctly higher. 



In the last tracing the immersion fluid was at 38° O, and it is seen 

 that after one short contraction the ventricle ceased, and then some 

 five or six beats later the auricle also stopped. The heart then relaxed 

 a little, i.e. there was a diminution of tone. About 30 sees, after im- 



i 



