314 S. WALTER RANSON 
through its thoracoabdominal branches. This would indicate 
that the unmyelinated fibers do not represent a recently 
acquired group of fibers in the mammalian nervous system, 
but that they are present in lower vertebrates in about the same 
proportion and distributed in the same way as in the mammals. 
It would seem that the distinction between the two kinds of 
fibers is a fundamental one. The unmyelinated fibers of the 
cerebrospinal nerves do not form a phylogenetically young 
system and the incompleteness of their ontogenetic development 
(i.e., their failure to develop a myelin sheath) must find some 
other explanation. 
2. In the mammalian vagus it was found that the relative 
proportion of unmyelinated fibers increases rapidly from above 
downward, at least in the region of the jugular and nodose 
ganglia where the cervical branches are given off. Most of this 
change in the relative proportion of the two kinds of fibers was 
considered due to the cervical branches taking out almost noth- 
ing but myelinated fibers, leaving behind an increasing pro- 
portion of unmyelinated fibers. But it was considered possible 
that the preganglionic visceral efferent fibers of the vagus might 
lose their sheaths in their course down the vagus, myelinated 
being thus transformed into unmyelinated fibers. This pos- 
sibility has been definitely ruled out by the study of the turtle’s 
vagus. The fibers destined for the thoracoabdominal viscera 
separate all at once into a well defined nerve trunk, just below 
the base of the cranium. This trunk has the same structure as 
the thoracic vagus of the mammal and it retains this same 
structure throughout the neck. There is no increase in the pro- 
portion of unmyelinated fibers from the place where the cervical 
ramus splits off to the point of origin of the recurrent nerve in 
the thoracoabdominal cavity. The preganglionic myelinated 
fibers of the turtle’s vagus do not to any appreciable extent 
lose their myelin sheaths during their course down the vagus. 
3. It was also possible that some of the unmyelinated fibers 
of the mammalian vagus were post ganglionic visceral efferent 
fibers arising from sympathetic cells in the jugular and nodose 
ganglia. Although such an assumption would have helped 
