238 HANDBOOK OF PHYSIOLOGY. 



or at any rate of both vagi, produces acceleration of the pulse, arid stim- 

 ulation of the distal or peripheral end of the divided nerve produces 

 normally slowing or stopping of the heart's beats. 



It appears that any kind of 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 conse- 

 quence of the fact that within the trunk of the nerve is 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 the nerve on 

 the right side. The effect of the stimulus is not immediately seen; one 

 or more beats may occur before stoppage of the heart takes place, and 

 slight stimulation may produce only slowing and not complete stoppage 



Fig. 194. Tracing showing the actions of the vagus on the heart. Aur., Auricular; vent., ven- 

 tricular tracing. The part between perpendicular lines indicates period of vagus stimulation C 8 

 indicates that the secondary coil was 8 cm. from the primary. The part of tracing to the left 

 shows the regular contractions of moderate height before stimulation. During stimulation and for 

 some time after, the beats of auricle and ventricle are arrested. After they commence again they 

 are single at first, but soon acquire a much greater amplitude than before the apnlication of the 

 stimulus. (From Brunton, after Gaskell.) 



of the heart. The stoppage may be due either to prolongation of the 

 diastole, as is usually the case, or to diminution of the systole. Vagus 

 stimulation inhibits the spontaneous beats of the heart only, it does not 

 do away with the irritability of the heart-muscle, since mechanical stim- 

 ulation may bring out a beat during the still-stand caused by vagus 

 stimulation. The inhibition of the beats varies in duration, but if the 

 stimulation be a prolonged one, the beats may reappear before the cur- 

 rent is shut off. When the beats reappear, the first few are usually 

 feeble, and may be auricular only; after a time the contractions become 

 more and more strong, and very soon exceed both in amplitude and 



