THE RATE OF THE HEART BEAT. 



549 



plied in the first factor given above, the effect of the products of 

 muscular metabolism. 



Variations with the Gaseous Conditions of the Blood. In con- 

 ditions of asphyxia the altered gaseous contents of the blood 

 increase in CO 2 and decrease in 2 , act upon the medullary centers 

 of the cardiac nerves, causing, first, 

 an increase and then a decrease in 

 heart rate. 



The Variations in Pulse Rate 

 Due to Changes in the Composi- 

 tion or Properties of the Blood. 

 The condition under this head that 

 has the most marked influence upon 

 the heart rate is the temperature of 

 the blood. Speaking generally, the 

 rate of beat increases regularly with 

 the temperature of the blood or 

 other circulating liquid up to a cer- 

 tain optimum temperature. On the 

 heart of the cold-blooded animal this 

 relationship is easily demonstrated 

 by supplying the heart with an arti- 

 ficial circulation of Ringer's solu- 

 tion, 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. 226). Beyond this opti- 

 mum temperature the beats decrease 

 in force and also in rate, becoming 

 irregular or fibrillar before the heart 

 finally comes to rest. Newell Mar- 

 tin* has shown the same relation- 

 ship 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. 227. 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 determined experimentally the upper and lower 

 lethal limits of temperature for the mammalian heart. The experi- 

 ments were made upon cats' hearts kept alive by an artificial circu- 



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

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



Fig. 226. To show the effect of 

 temperature on the rate and force of 

 the heart beat. Contractions of the 

 terrapin's ventricle at different tem- 

 peratures. Kymograph moving at 

 the same speed. At 30 the rate is 

 still increasing, but the extent of con- 

 traction has passed its optimum. 



