VAGUS NERVE OF THE TURTLE oly 
explain the increasing proportion of unmyelinated fibers 
found in serial sections as one passed from above downward 
through the region of the two ganglia, it was considered very 
improbable that the unmyelinated fibers of the vagus were to 
any considerable extent postganglionic visceral efferent fibers. 
The work on the turtle shows clearly that none of them can be 
so considered, since there are no ganglion cells in the thoraco- 
abdominal ramus in the neck and yet this branch contains as 
many unmyelinated fibers as the thoracic trunk of the vagus 
in the dog. None of these fibers have been in contact with 
peripheral ganglion cells and they cannot therefore be postgan- 
glionic sympathetic fibers. Since there is no appreciable change 
in the myelination of the vagus above and below the thoraco- 
abdominal ganglion, it is probable that there are no sympa- 
thetic elements in this ganglion. 
CONCLUSIONS 
1. The vagus of the turtle divides high in the neck into a 
cervical and a thoracoabdominal ramus. 
2. The cervical ramus is composed almost entirely of mye- 
linated fibers and the cells of the cervical ganglion of the vagus 
are associated with the fibers of this ramus only. 
3. The thoracoabdominal ramus is composed chiefly of un- 
myelinated fibers among which are scattered myelinated fibers. 
It presents as it passes under the plastron an enlargement, the 
thoracoabdominal ganglion, the cells of which are associated with 
the fibers of this ramus only. 
4. The unmyelinated fibers are present in about the same 
proportion and distributed in the same way in the vagus of the 
turtle as in that of the mammal. 
5. The unmyelinated fibers in the vagus are not postgan- 
glionic visceral efferent fibers arising from sympathetic cells in 
the ganglia of that nerve. 
6. The preganglionic visceral efferent fibers of the vagus do 
not become transformed into unmyelinated fibers in their course 
down the vagus. 
