THE CIRCULATION OF THE BLOOD. 



293 



the respective actions of the two sets of fibers they must be stimu- 

 lated or divided before their union at the vagus ganglion. 



Stimulation of the intra-cranial roots of the vagus with very weak 

 induced currents is followed by a gradual diminution in the rate or 

 rhythm and a diminution in the force of the heart-beat. If the in- 

 duced currents are moderate in strength, the heart will at once come 

 to a standstill in diastole. Since stimulation of the nerve, which in 

 all probability exaggerates its normal function, is followed by a period 



FIG. 129. TRACING SHOWING THE DIMINUTION IN THE RATE or THE HEART-BEAT 

 FOLLOWING WEAK TETANIZATION OF THE VAGUS TRUNK. 



of rest or inactivity, the vagus is said to have a retarding or an inhibi- 

 tory influence on the automatic pulsations of the heart. 



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 rate and force increase, until they attain or exceed that 

 observed prior to the stimulation. The duration of the inhibitory 

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

 after a stimulation of thirty-eight seconds the heart of the toad re- 

 mained at rest for 292 seconds (Gaskell). Stimulation of the sym- 



FIG. 130. TRACING SHOWING COMPLETE INHIBITION FOLLOWING STR^G TETANI- 

 ZATION OF THE VAGUS TRUNK. 



pathetic fibers prior to their union with the vagus is followed by an 

 increase in the rate or an augmentation in the force of the heart, or 

 both at the same time. With the cessation of the stimulus the heart 

 returns to its normal condition. From the foregoing facts the vagus 

 is said to be the inhibitor, the sympathetic, the accelerator or aug- 

 mentor nerve of the heart, according as it increases the rate or force. 

 Stimulation of the trunk of the vagus in the frog or the toad with 

 weak tetanizing induced electric currents is followed merely by a 



