422 MARTIN R. CHASE 



as being post-gaiiglioiiic fibers arising from sympathetic cells 

 in the vagus ganglia. 



Molhant '10 working with the Cajal reduced silver method, 

 saw masses of unmyelinated fibers in the vagus nerve and thought 

 they were of sympathetic origin, being presumably post-gan- 

 glionic fibers arising from cells in the sympathetic ganglia. Nei- 

 ther Gaskell nor Molhant demonstrated unmyelinated fibers in 

 the roots of the vagus. 



Langley has suggested the possibility of j)reganglionic vis- 

 ceral efferent fibers losing their myelin sheaths during their 

 course down the vagus. 



Ranson ('15) has recently studied the sti-ucture of the vagus 

 nerve in the turtle, and has made observations of importance 

 in clearing up the origin of the unmyehnated fibers in the vagus 

 nerve. The vagus nerve in the turtle divides high in the neck 

 into a cervical and a thoraco-abdominal ramus. The cervical 

 ramus is composed almost entirely of myelinated fibers and the 

 cells of the cervical ganglion of the vagus are associated only 

 with the fibers of this ramus. 



The thoraco-abdominal ramus presents the same structure 

 as the thoracic vagus in mammals. It has a ganglion in the 

 upper part of the thoraco-abdominal cavity. It consists largely 

 of unmyelinated fibers, with scattered myelinated fibers, and 

 maintains the same structure from its origin to the origin of 

 the recurrent nerve. Ranson shows that the unmyelinated 

 fibers are not post-ganglionic fibers arising from cells in the cer- 

 vical ganglion, since the thoraco-abdominal trunk is not asso- 

 ciated with this ganglion. Neither do preganglionic visceral 

 efferent fibers lose their myelin sheaths, since the structure of 

 the thoraco-abdominal ramus does not change. There is no 

 association with sympathetic ganglia, and the proportion of 

 unmyelinated fibers does not change below the thoraco-abdomi- 

 nal ganglion, so there are probably no sympathetic elements in 

 this ganglion. 



In the dog there is a very close association of the vagus and 

 sympathetic trunks in the neck, and it seemed possible that some 

 of the unmyehnated fibers of the thoracic vagus in this animal 



