THE RATE OF THE HEART BEAT. 



591 



Ringer's solution, which can be heated or cooled at pleasure. The 

 rate and force of the beat increase to a maximum, which is 

 reached at about 30° C. (see Fig. 247). Beyond this optimum 

 temperature the beats decrease in force and also in rate, becoming 

 irregular or fibrillar before the heart finally comes to rest. Newell 

 Martin* has shown the same relationship in a very conclusive 

 way upon the isolated heart of the dog. Within physiological 

 limits the rate of beat rises and 

 falls substantially parallel to the 

 variations in temperature as is 

 shown by the chart reproduced in 

 Fig. 248. The accelerated heart 

 rate in fevers is therefore due 

 probably to the direct influence of 

 the high temperature upon the 

 heart itself. The same observer de- 

 termined experimentally the upper 

 and lower lethal limits of tempera- 

 ture for the mammalian heart. 

 The experiments were made upon 

 cats' hearts kept alive by artificial 

 circulation through the coronary 

 arteries, t It was found that the 

 highest 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. An increased out- 

 put of epinephrin from the adrenal glands, such as may result from 

 emotional excitement (p. 612), will modify the heart beat either 



* Martin, "Croonian Lecture, Philosophical Transactions, Royal Society," 

 London, 174, 663, 1883; also "Collected Physiological Papers," p. 40, 1895. 



t Martin and Applegarth, "Studies from the Biological Laboratory, Johns 

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

 p. 97, 1895. 



Fig. 247.— To show 



the effect of 

 temperature on the rate and force oi 

 the heart beat. Contractions of the 

 terrapin's ventricle at different tem- 

 peratures. Kymograph moving st 

 the same speed. At 30° the rate ia 

 still increasing, but the extent of con- 

 traction has pa.ssed its optimum. 



