THE NERVES OF THE FROG'S HEART 



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leaving it as the nerve comes towards the anterior mid line. The main 

 trunk of the nerve branches near the heart, the larger half running 

 backwards towards the lung ; the other is the cardiac branch. The 

 vagus must be isolated completely from the surrounding tissues. It is 

 important to remember that the nerve thus isolated is not the vagus 

 only, but has already been joined, immediately after its exit from the 

 skull, by a large branch from the sympathetic. The cardiac branch 

 therefore contains both vagal and sympathetic fibres. 



To expose the sympathetic before it joins the vagus, cut away the 

 whole of the lower jaw by a single transverse incision. Pick up the 

 mucous membrane covering the roof of the mouth and cut it away, 

 removing it well down to the oesophagus. This brings into view a 

 triangular-shaped muscle, the levator anguli scapulae, m, fig. 99. 

 Very carefully cut this through near to its attachment to the base of 

 the skull and turn it down. This 

 brings the sympathetic into view. 

 It is usually accompanied by a blood- 

 vessel which lies over it, and being 

 pigmented serves as a guide to the 

 nerve. 



The nerve is very readily seen as 

 it crosses the large root of the second 

 nerve, above which it bifurcates to 

 pass round the subclavian artery, 

 forming the annulus of Vieussens. 

 Carefully isolate the sympathetic 

 between the annulus and its junction 

 with the vagal trunk. Pass a fine 

 ligature round it and tie it as far down 

 as possible. 



Cut the nerve below the ligature. 

 The cardiac fibres of the sympathetic 

 leave the cord mainly in the ramus 

 communicans from the third nerve, 

 and to a less extent in that of the 

 fourth nerve. In the same dissection 



it is easy to expose the petro-hyoid muscles and remove them, and in 

 this way the vago-sympathetic in its course from the ganglion of the 

 trunk of the vagus to the heart can be readily traced. 



Having exposed the two vagi and passed threads under them their 

 action should be studied as follows : — 



Fig. 99. — The Course of the 



SYMPATHETIC IN THE FkOC. 



