INNERVATION OF HEART AND BLOOD-VESSELS 555 



THE AUGMENTOR NERVES OF THE HEART 



In the frog both the augmentor and the inhibi- 

 tory nerves reach the heart through the splanch- 

 nic, branch of the vagus. The augmentor fibres 

 leave the spinal cord in the third spinal nerve, and 

 pass through the ramus communicans of this 

 nerve into the third sympathetic ganglion, where 

 they probably end in contact with the body or 

 processes of sympathetic cells. The axis-cylin- 

 ders of these sympathetic cells pass up the cer- 

 vical sympathetic chain to the ganglion of the 

 vagus (Fig. 72), and thence down the vagus trunk 

 to the heart. Thus in the greater part of its 

 course the vagus cannot be stimulated without 

 exciting both the augmentor and the inhibitory 

 cardiac fibres. To excite either alone it is neces- 

 sary to stimulate the respective nerves above 

 their junction. 



Preparation of the Sympathetic. Cut away the 

 lower jaw of a large frog, the brain of which has 

 been destroyed by pithing, and continue the slit 

 from the angle of the mouth downwards for a 

 short distance. Avoid cutting the vagus nerve 

 (Fig. 73). Turn the parts well aside, and expose 

 the vertebral column where it joins the skull. 

 Remove the mucous membrane covering the 

 roof of the mouth. The sympathetic is situated 



