CH. XXI.] 



CAEDIAC SYMPATHETIC NERVES 



251 



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 of 

 the heart (fig. 230). The stoppage may be due either to prolongation 

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

 Vagus stimulation lessens the conductivity of the cardiac tissue, but 

 it does not abolish the irritability of the heart-muscle, since direct 

 mechanical stimulation may bring out a beat during the standstill 

 caused by vagus stimulation. The inhibition of the beats varies in 

 duration, but if the stimulation is a prolonged one, the beats reappear 

 before the current is shut off. This is known as " vagus escape," and 

 is probably due to fatigue of the vagal endings. 



The Sympathetic. The influence of the sympathetic is the 

 reverse of that of the vagus. Stimulation of the sympathetic 

 produces acceleration of the 

 heart-beats, and as a rule, sec- 

 tion of the nerve produces 

 slowing. Hence the nerve is 

 also in constant action like 

 the vagus. The acceleration 

 produced by stimulation of the 

 sympathetic fibres is accom- 

 panied by increased force, and 

 so the action of the nerve is 

 also termed augmentor. It is 

 probable that the augmentor 

 fibres are distinct from the 

 accelerator fibres, because in 

 mammals one or two of the 

 small nerves leaving the stel- 

 late ganglion on stimulation 

 produce augmentation without 

 acceleration. 



The fibres of the sympa- 

 thetic system which influence 

 the heart-beat in the frog, 

 leave the spinal cord by the 

 anterior root of the third 



Post, root 



spinal nerve, and pass by the 



ran ins communicans to the 



third sympathetic ganglion, 



then to the second sympathetic ganglion, then by the annulus of 



Vieussens (round the subclavian artery) to the first sympathetic 



ganglion, and finally in the main trunk of the sympathetic, to near 



the exit of the vagus from the cranium, where it joins that nerve 



FIG. 231. Heart nerves of frog. (Diagrammatic.) 



