248 PHYSIOLOGY OF THE HEART. [CH. xx. 



heart, lungs, oesophagus, and stomach, the remainder joining the 

 cceliac plexus, and contributing to the nerve supply of various 

 abdominal organs. We have, however, in this place only to deal 

 with the cardiac fibres. 



It has been known since the experiments of the Bros. Weber 

 in 1845, that stimulation of one or both vagi produces slowing 

 or stoppage of the beats of the heart. It has since been shown 

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

 the normal result of vagus stimulation. Moreover, section of 

 one vagus produces slight acceleration of the heart ; this result 

 is better marked when both vagi are divided. 



Weak stimulation of the peripheral end of the divided nerve 

 produces slowing of the heart (fig. 248) ; a strong stimulus 

 produces stoppage (fig. 247). It appears that any kind of 



Fig. 247. Tracing showing the actions of the vagus on the heart. Aur., auricular ; Vent., 

 ventricular tracing. The part between the perpendicular lines indicates the period 

 of vagus stimulation. (7.8 indicates that the secondary coil was 8 a.m. from the 

 primary. The part of the tracing to the left shows the regular contractions of mode- 

 rate height before stimulation. During stimulation, and for some time after, the 

 beats of auricle and ventricle are arrested. After they commence again they are 

 small at first, but soon acquire a much greater amplitude than before the application 

 of the stimulus. (From Brunton, after Gaskell.) 



stimulus produces the same effect, either chemical, mechanical, 

 electrical, or thermal, but that of these the most potent is 

 a rapidly interrupted induction current. A certain amount 

 of confusion has arisen as to the effect of vagus stimulation 

 in consequence of the fact that within the trunk of the 

 nerve are contained, in some animals, fibres of the sympathetic, 

 and it depends to some extent upon the exact position of the 

 application of the stimulus, as to the exact effect produced. 

 Speaking generally, however, excitation of any part of the trunk 

 of the vagus produces inhibition, the stimulus being particularly 

 potent if applied to the termination of the vagi in the heart 

 itself, where they enter the substance of the organ at the situation 

 of the sinus ganglia. The stimulus may be applied to either vagus 

 with effect, although it is frequently more potent if applied to 



