ROOTS, TRUNK, BRANCHES OF VAGUS NERVE 55 



remaining large medullated fibers. The pharyngeal, superior 

 laryngeal, and recurrent nerves take out from the vagus large 

 numbers of medullated fibers, while of these only the superior 

 laryngeal contains appreciable numbers of non-medullated fibers, 

 and even this nerve contains relatively few of them. It will be 

 seen, therefore, that practically all of the non-medullated fibers 

 in the cervical vagus are carried down with the thoracic vagus, 

 where they are more compactly grouped, due to the fact that the 

 larger part of the medullated fibers have left the vagus in its cer- 

 vical branches. This explains in part, at least, why the thoracic 

 vagus contains a much greater proportion of non-medullated fibers 

 than does the upper cervical trunk. 



It is doubtful if this accounts for the entire increase in the pro- 

 portion of non-medullated fibers in the thoracic \'agus. It may 

 be that there has been some admixture of sympathetic fibers. 

 It seems probable that an important factor in this increase is a 

 change in the character of the pre-ganglionic fibers. It seems 

 probable that some of these lose theii* sheaths as they pass down 

 the vagus. Langley's argument in fa\'or of this has been given 

 in a preceding paragraph. 



Figure 17 is a drawing of an osmic acid }Dreparation of the tho- 

 racic vagus in the cat above the level of the puhnonary rami. It 

 shows that there are a few large medullated fibers, but that the 

 majority of the medullated fibers are either small or of medium 

 size. The most noticeable feature is the wide spacing of these 

 fibers and the large amount of intervening unstained substance. 

 In pyi'idine-silver preparations these unstained spaces are seen to 

 be filled by myriads of non-medullated axons. 



THE VAGUS BELOW THE BRONCHIAL RAMI 



As the vagus passes behind the root of the lung it gives off 

 several branches to the pulmonary plexuses. These bronchial 

 rami contain large numbers of medullated fibers and take from 

 the vagus a considerable proportion of those medullated fibers 

 which have continued down the vagus to this level. We have 

 no data as to the presence or absence of non-medullated fibers in 



