PHYSIOLOGY OF THE HEART. 



[CH. XX. 



powerful than is the case with the vagus stimulation ; secondly, 

 a longer time elapses before the effect is manifest ; and thirdly, 

 the augmentation is followed by exhaustion, the beats being after 

 a time feeble and less frequent. 



The fibres of the sympathetic 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 sympathetic ganglion, thence to the second sympathetic 



oots of 

 Vagus 



49. Heart nerves of frog. (Diagrammatic.) 



ganglion, and thence by the annulus of Vieussens (round the sub- 

 clavian arteiy) to the first sympathetic 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. These fibres are indicated by the dark line in fig. 249. 

 The fibres of the sympathetic seen running up into the skull 

 are for the supply of blood-vessels there. It ahould be noted 

 that the frog has no spinal accessory nerve. 



From the fact that the augmentor fibres are joined to the 



