THE CIRCULATION OF THE BLOOD. 309 



that some fibers of the vagus at least diminish the tonicity of the heart- 

 muscle. 



After cessation of the stimulation, the heart resumes its activity. At 

 first the beat usually is slow and feeble, but with each succeeding beat both the 

 rate and force increase, until they attain or exceed that observed prior to the 

 stimulation. In some cases, however, the heart begins to beat with as much 

 and even more vigor than it did prior to the stimulation. The duration of 

 the inhibitor effect varies with the duration of the stimulation. Thus during 

 and after a stimulation of thirty-eight seconds the heart of the toad remained 



FIG. 143. TRACING SHOWING THE DIMINUTION IN THE RATE OF THE HEART-BEAT 

 FOLLOWING WEAK TETANIZATION OF THE VAGUS TRUNK. 



at rest for 292 seconds (Gaskell); the heart of a snake for from one-half to 

 one hour (Meyer); the heart of a turtle for four and a half hours (Mills). 

 The period of inhibition will depend on the strength of the electric current 

 employed, the nerve stimulated, the season of the year, etc. 



The effects on the heart-beat which wilt follow stimulation of the vago- 

 sympathetic in its course vary, however, because of the antagonistic action 

 of the inhibitor and accelerator nerve impulses. Thus stimulation of the 

 peripheral end of the divided trunk of the vagus in the frog or the toad with 

 weak tetanizing induced electric currents is followed by an increase in the 



FIG. 144. TRACING SHOWING COMPLETE INHIBITION FOLLOWING STRONG TETAN- 

 IZATION OF THE VAGUS TRUNK. 



rate of the heart-beat because of the stimulation of the accelerator fibers, 

 which apparently respond before the inhibitor fibers; stimulation with some- 

 what stronger currents is followed by a diminution in the rate of the beat 

 because of the greater effect on the inhibitor nerve-fibers (Fig. 143). Stim- 

 ulation with strong tetanizing currents is followed by complete inhibition 

 (Fig. 144). 



The foregoing facts lead to the inference that the cardio-accelerator 

 and the cardie-inhibitor centers have as their function the discharge of nerve 

 impulses which are conducted by their related nerves, the efferent sympathetic 



