THE RATE OF THE HEART BEAT. 599 



change in the reaction of the blood, which could influence the activ- 

 ity of the medullary cardiac centers, although, as a matter of 

 fact, an adequate explanation of this feature has not been fur- 

 nished by experimental work. The marked immediate effect of 

 the muscular exercise on the heart rate has aroused the most in- 

 terest, and it has been studied with care by a number of observers. 

 The rapidity with which th heart rate begins to increase with 

 muscular work indicates that it is a nervous effect which operates 

 either upon the inhibitory apparatus, decreasing its activity, or 

 upon the accelerator apparatus, increasing its activity. The ex- 

 periments made to determine this latter point have not given 

 concordant results, but the better evidence indicates that the effect 

 in the first place at least is upon the cardio-inhibitory center, 

 decreasing its tone, and, therefore, increasing the heart rate. 

 When, for example, the accelerator nerves are excised in dogs,* 

 moderate exercise causes as prompt and as large an increase in the 

 heart rate as in normal animals. Accepting this view, the further 

 question remains as to whether this effect is an ordinary reflex 

 action. Some authors suppose that it is. They assume that the 

 sensory nerves in the muscles are stimulated, and that through 

 them is brought about a reflex inhibition of the cardio-inhibitory 

 center. Others, however, have suggested that the effect is not a 

 reflex from the periphery, but an example rather of irradiation in 

 the nerve centers, f That is to say, the discharge of voluntary 

 nerve impulses from the brain, in descending to the cord, affects 

 the cardio-inhibitory center in the medulla, presumably, on the 

 neuron hypothesis, by way of collaterals. Between these two 

 points of view it is not possible to decide at present. 



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 O 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 Compo- 

 sition 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 certain optimum temperature. On the 

 heart of the cold-blooded animal this relationship is easily demon- 

 strated by supplying the heart with an artificial circulation of 

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

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



* Gasser and Meek, "American Journal of Physiology," 34, 48, 1914. 

 t Johannson, "Skandinavisches Archiv f. Physiologic," 5, 20, 1895. 



