550 



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. 



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

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

 has 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. 



