THE VAGUS NERVE 427 



ent in the entire cross section only an occasional myelin ring, 

 and most of these clearly represent degenerating fibers. 



Figure 4 is a composite drawing. Figure 4a, the upper half 

 of the figure, is taken from a cross section of a normal right 

 vagus nerve just below the origin of the bronchial rami (fig. 1, 

 4a). There are present in this field nine small myelinated axones. 

 The remainder of the field is packed with unmyelinated axones. 

 Figiu'e 4a may be taken to represent the condition in the normal 

 thoracic vagus, it being remembered that above the bronchial 

 rami there are more myelinated axones, while near the dia- 

 phragm there are even fewer than in the figure. Figure 4b 

 is a drawing from a cross section of the same nerve as figures 

 2 and 3, taken at a level just proximal to the union with the 

 branch from the left vagus nerve (fig. 1, 4^). There are in the 

 field no normal unmyelinated or myelinated axones, and the 

 entire cross section shows the same structure, there being only 

 an occasional naked axone. 



In both pyridine silver and osmic acid preparations of the 

 nerve below the level described sections, both of the right nerve 

 after its union with the branch from the left vagus, and of the 

 left nerve after \mion with the brancli from the right, show a 

 mingling of the degenerated nerve with the normal, but the 

 degeneration can be clearly traced to the diaphragm. 



In the second case, wliere both tlie \agus and sym])athetic 

 were cut in the mid-cervical region, no bundles of fibers of sym- 

 pathetic origin coukl be found, the entire nerve being degenerated. 



We conclude, then, that following section of the cervical 

 vagus ner\-e in the dog, with or without section of the sympathetic 

 trunk, all fibers of the vagus, myelinated and unmyelinated 

 alike, imdergo complete degeneration. There are to be found 

 in th(^ degenerated thoracic tiimk, above the level of the pul- 

 monary plexuses, in .some individuals bundles of unmyelinated 

 nerve fibers having their origin in the cells of the .symi)athetic, 

 presumably in the inferior cervical ganglion. In some individuals 

 no bundles of this sort are found. In no case does the total 

 number of fibers of sympathetic origin present amount to more 

 than an insignificant fraction of the total mnuber of unmyelinated 



