ROOTS, TRUNK, BRANCHES OF VAGUS NERVE 57 

 THE SOURCE OF THE NON-MEDULLATED FIBERS OF THE VAGUS 



This great predominance of non-medullated over medullated 

 fibers in the vagus as it passes through the diaphragm seems to us 

 of considerable importance in connection with the interpretation 

 of these fibers in the vagus. It seems ahnost certain that the 

 vagus at this level contains more efferent pre-ganglionic fibers 

 than would be accounted for by the few medullated fibers which 

 it contains; and it does not seem at all probable that all of the 

 non-medullated fibers are afferent in function, otherwise the 

 abdominal vagus would be almost wholly an afferent nerve. It 

 would therefore seem probable that there are efferent non-med- 

 ullated fibers in the thoracic and abdominal portions of the vagus 

 nerve. 



It will be remembered that the proportion of non-medullated 

 fibers increased steadily in following the vagus downward in the 

 neck and thorax. This would fit in well with the theory ad- 

 vanced by Langley and summarized in a preceding paragraph of 

 this paper that some of the pre-ganglionic fibers of the vagus lost 

 theii- sheaths within the vagus trunk. Whether any of the non- 

 medullated fibers in the roots of the vagus are pre-ganglionic 

 fibers or not remains an open question. 



Reasoning from the analogy of the spinal nerves, where the 

 non-medullated fibers of the dorsal roots are probably all affer- 

 ent in function and arise from the small cells of the spinal ganglia 

 (Ranson '12), it seems probable that many, if not all, of the non- 

 medullated fibers in the roots of the vagus arise from cells in 

 the jugular and nodose ganglia and are afferent in function. It 

 seems probable, therefore, that both afferent and efferent non- 

 medullated fibers are found in the vagus nerve. 



SUMMARY 



1. The various rootlets of the vagus and accessory nerves differ 

 markedly in structure. The spinal root of the accessory is com- 

 posed almost entirely of large medullated fibers with a very few 

 small ones. The bulbar rootlets of the accessory are composed of 

 large and small medullated fibers, with the small fibers predomi- 



