250 PHYSIOLOGY OF THE HEART [CH. XXI. 



A certain amount of confusion has arisen as to the effect of vagus 

 stimulation, because so many experiments have been made on the 

 frog. In this animal the sympathetic fibres join the vagus after it 

 leaves the skull, and so what is usually called the vagus in this 

 animal should more properly be termed the vago-sympathetic. It will 

 readily be understood that by stimulating a mixed nerve, one obtains 



FIG. 229. 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. C.8 

 indicates that the secondary coil was 8 c.m. from the primary. The part of the 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 small at first, but soon acquire a much greater amplitude than before the application of 

 the stimulus. (From Brunton, after Gaskell.) 



an intermixture of effects. If, however, one stimulates the intra- 

 cranial vagus before the sympathetic blends with it, a pure inhibitory 

 effect is obtained. Figs. 229 and 230 show the common effect of 

 stimulating the mixed trunk ; the inhibitory effect is usually mani- 

 fested first, and this is followed by the augmentor effect due to 



Fro. 230. Tracing showing diminished amplitude and slowing of the pulsations of the auricle and 

 ventricle without complete stoppage during stimulation of the vagus. (From Brunton, after 

 Gaskell.) 



sympathetic action. But it is by no means infrequent to obtain the 

 phenomena in the reverse order. It is often stated that the right 

 nerve contains more inhibitory fibres than the left, but this is by no 

 means a constant rule. One can always obtain good inhibition if the 

 stimulus is applied to the wall of the sinus; here one stimulates 

 the post-ganglionic fibres which originate from the nerve-cells in 

 the sinus ganglion around which the vagi terminate. 



