TEMPERATURE AND AUTONOMOUS RESPONSES 337 



temperature displaces it upwards or towards relaxation. 

 These facts are of great importance, and should be borne in 

 mind in reference to the explanation of the cause of variation 

 of amplitude and period, which I shall bring forward. 



Explanation of diminution of amplitude of pulsation 

 with rise of temperature. — We have thus seen, as we should 

 theoretically have expected, that with the increased absorp- 

 tion of energy at a higher temperature, the amplitude of 

 vibration is also increased. How is it, then, that with the 

 still further increase of absorption of energy, at still higher 

 temperatures, the amplitude should undergo a diminution ? 

 When approaching the maximum point, where heat-rigor 

 takes place, we can understand that the excitability of the 

 tissue would be very much decreased, with a consequent 

 reduction of amplitude of pulsation. But at temperatures of 

 2 5 C. to 30 C. excitability of the tissue could not be 

 diminished. Indeed, I shall in Chapter XXXIII. adduce 

 considerations to show that it must, at that temperature, be 

 highly excitable, and we should have expected that this 

 would have produced, in addition to the increased energy, an 

 augmentation of vibrational amplitude. But, instead of this, 

 we obtain the curious result which has been described, of 

 a diminution, in the cases both of cardiac muscle and of 

 Desmodium. 



It might be suggested that if increased activity, due to 

 rise of temperature, increased the frequency of vibration, 

 then this fact would be sufficient in itself to account for a 

 diminution of amplitude ; for in this case, less time being 

 allowed for each single vibration, its extent must be cur- 

 tailed. But this consideration alone would not explain all the 

 facts of the case ; for we have seen that on approaching 

 the thermo-tonic minimum, though the frequency of vibra- 

 tion is reduced, and the period very much extended, yet 

 the amplitude is also at the same time decreased (fig. 140). 

 W T e thus see that the question of internal energy is important 

 in this connection. An increase of internal energy may be 

 expressed, as I shall show, in two different ways ; either, that 



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