THE CIRCULATION" OP THE BLOOD. 239 



frequency those which occurred before the application of the stimulus 

 (fig. 194). 



Influence of the Sympathetic. The influence of the sympathetic 

 may be considered, to a certain extent, as the reverse of that of the 

 vagus. Stimulation of the sympathetic, even of one side, produces ac- 

 celeration of the heart-beats, and according to certain observers, section- 

 of the same nerve produces slowing. The acceleration produced by stim- 

 ulation of the sympathetic 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 par- 

 ticulars besides the augmentation which is produced : firstly, the stimulus 

 required to produce any effect must be more powerful than is the case 

 with the vagus stimulation; secondly, a longer time lapses before the 

 effect is manifest; and thirdly, the augmentation is followed by exhaus- 

 tion, the beats being after a time feeble and less frequent. 



Origin of the cardiac nerve-fibres. 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 spinal nerve, and pass 

 thence by the ramus communicans to the third spinal ganglion, thence 

 to the second spinal ganglion, and thence by the annulus of Vieussens 



Fig. 195. Tracing showing diminished amplitude and slowing of the pulsations of the auricle 

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

 Gaskell.) 



(round ttie subclavian artery) to the first spinal ganglion, and thence in 

 the main trunk of the sympathetic, to near the exit of the vagus from 

 the cranium, where it joins that nerve and runs down to the heart within 

 its sheath, forming the joint vago-sympathetic trunk. 



In the dog, the augmentor fibres leave the cord by the second and 

 third dorsal nerves, and possibly by anterior roots of two or more lower 

 nerves, passing by the rami communicantes to the ganglion stellatum, 

 or first thoracic ganglion, thence by the annulus of Vieussens to the 

 inferior cervical ganglion of the sympathetic fibres from the annulus, or 

 from the inferior cervical ganglion proceed to the heart. 



From the fact that the augmentor fibres are joined to the vagus 



