200 THE CIRCULATION OF THE BLOOD 



mammalian heart also responds sharply to a change in the salt content of 

 the perfusion solution. For example, addition of potassium chloride to a 

 Locke solution slows or even suppresses the rate, as is shown in figure 177. 



THE REGULATIVE INFLUENCE OF THE CENTRAL NERVOUS 

 SYSTEM ON THE HEART. 



The heart is capable of automatic rhythmic movement, yet while in the 

 body its beats are under the constant control of the central nervous system. 

 The influence which is exerted by the central nervous system is of two kinds: 

 first, in the direction of slowing or inhibiting the beats, and second, in the 

 direction of accelerating or augmenting the beats. The influence of the 

 first kind is brought to bear upon the heart through the fibers of the pneu mo- 

 gastric or vagus nerves, and that of the second kind through the sympathetic 

 nerves. 



The Inhibitory Nerves. It has long been known, indeed since the 

 experiments of the Weber brothers in 1845, that stimulation of one or both 



FIG. 178. Effect on the Heart Rate and on the Arterial Blood Pressure of Stimulating 

 the Right Vagus of the Dog. Stimulus applied at the mark "on" and removed at "off." 

 Pressure in millimeters of mercury shown by the scale to the left. Time in seconds. (New 

 figure by Hill and Chilton.) 



vagi produces slowing of the rhythm of the heart. It has since been shown, 

 in all of the higher vertebrate animals experimented with, that this is the 

 normal reaction to vagus stimulation. Moreover, a section of one vagus, 

 or at any rate of both vagi, produces acceleration of the pulse; and stimu- 

 lation of the distal or peripheral end of the divided nerve normally pro- 

 duces slowing or stopping of the heart's beats. 



