530 



CIRCULATION OF BLOOD AND LYMPH. 



lation through the coronary arteries.* It was found that the high- 

 est temperature at which the heart will beat is about 44 to 45 C., 

 although a slightly higher temperature may be withstood under 

 special conditions. At the other extreme the mammalian heart 

 ceases to beat when the temperature falls as low as 17 to 18 C. 



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The rate of the heart beat may be influenced also by many sub- 

 stances added to the blood. The influence of atropin and muscarin 

 have already been alluded to, but changes also in the normal con- 

 stituents of the blood may have similar effects. Thus, an increase in 

 the sodium carbonate of the blood affects the heart beat, particu- 

 larly, however, in regard to the amplitude or force of the contraction. 



* Martin and Applegarth, " Studies from the Biological Laboratory, 

 Johns Hopkins University," 4, 275, 1890; also "Collected Physiological 

 Papers," p. 97, 1895. 



