420 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



measurements by Clausen of the rate of production of Carbon Dioxide by 

 germinating seeds showed that this process is approximately doubled 

 in velocity by an elevation of 10 in temperature until an upper limit 

 somewhat exceeding 40 is attained, when the rate of the tissue-respira- 

 tion falls off owing to heat-injury. This method of inquiry was extended 

 to animal tissues by C. D. Snyder, who investigated the influence of 

 temperature upon the Rate of the Heart-beat in the isolated heart of 

 the Pacific terrapin, Clemmys marmorata. The following are illus- 

 trative results : 



It is evident that the rate of the beat is approximately doubled for 

 each 10 rise in temperature. From the data quoted and others 

 obtained by Snyder the following average coefficients may be com- 

 puted: 



HEART-BEAT OF CLEMMYS MARMORATA 



Temperature-coefficient 

 Temperature-interval. for 10 intervals. 



10 to 20 2.3 



20 to 30 22 



30 to 37 '. ... 1.6 



at temperatures exceeding 37 the rate of the beat, instead of increas- 

 ing, diminished until the heart came to a standstill owing to irreparable 

 heat-injury. These experiments were subsequently repeated upon 

 the isolated heart of another species of terrapin, Emys europea, by 

 Galeotti and Piccinini, and by Snyder upon the isolated heart of the 

 frog, and by Kanitz upon the isolated mammalian heart. 



The heart in situ is, however, considerably modified in its behavior 

 and particularly in the rate of beat by the nervous control to which it 

 is subjected. The study of the heart-beat in the intact animal there- 

 fore involves more numerous and more complex factors than that of the 

 beat of the excised heart. Nevertheless in this case also the rate of 

 the beat is primarily determined by the velocity of underlying chemical 

 changes. Thus in the minute transparent fresh-water crustacean, 

 Ceriodaphnia, the heart can be viewed through the body-wall of the 



