CH. XX.] THE CARDIAC \ 7 AGUS. 249 



the nerve on the right side. The effect of the stimulus is not imme- 

 diately 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 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 spon- 

 taneous beats of the heart only ; it does not do away with the 

 irritability of the heart-muscle, since mechanical stimulation may 

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

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

 stimulation is a prolonged one, the beats may reappear before 

 the current is shut off. When the beats reappear on the 

 cessation of stimulation, the first few are usually feeble, and 

 may be auricular only ; after a time the contractions become 

 stronger, and very soon exceed both in amplitude and frequency 



Fig. 248. Tracing show ing diminished amplitude and slowing of the pulsations of the 

 auricle and ventricle without complete stoppage during stimulation of the vagus. 

 (From Brunton, after Oaskell.) 



those which occurred before the application of the stimulus 

 (figs. 247, 248). 



One branch of the vagus is called the depressor; it is a separate 

 nerve in only a few animals. Unlike the inhibitory branches, 

 it is afferent, not efferent ; it carries impulses to the vaso-motor 

 centre in the bulb from the heart. We shall study its use in 

 connection with blood pressure. 



Tho Sympathetic. The influence of the sympathetic is the 

 reverse of that of the vagus. Stimulation of the sympathetic, 

 even of one side, produces acceleration of the heart-beats, and 

 according to certain observers, section of the same nerve produces 

 slowing. The acceleration produced by stimulation of the sym- 

 pathetic fibres is accompanied by increased force, and so the 

 action of the nerve is more properly termed augmentor. The 

 action of the sympathetic differs from that of the vagus in 

 several particulars besides the augmentation which is produced : 

 first, the stimulus required to produce any effect must be more 



