308 S. WALTER RANSON 
greatly from each other in structure. The cervical ramus 
contains myelinated fibers of all sizes about equally mixed to- 
gether, scattered among which there are found a relatively 
small number of unmyelinated fibers. The thoracoabdominal 
ramus is composed chiefly of unmyelinated fibers among which 
are scattered myelinated fibers of all sizes. Of the myelinated 
fibers the majority are small. It is the preponderance of un- 
myelinated fibers that gives this fascicle its light appearance in 
osmic acid preparations. 
These points concerning the structure of the various fascicles 
have been determined from a study of sections stained with 
osmic acid, showing the myelin sheaths, and others stained by 
the pyridine silver method, showing the axons. A comparison 
of the two kinds of sections shows that the thoracoabdominal 
ramus of the vagus is composed of great numbers of axons, 
most of which are very slender, while it contains only scattering 
myelin sheaths. Some very instructive preparations were 
obtained with osmic acid. In these preparations the myelin 
* sheaths are deep black and readily recognized as such on the 
smallest myelinated axons. Some of these myelinated axons 
are even smaller than the average unmyelinated ones and yet 
their myelin sheaths are perfectly distinct. Connective tissue 
and neurilemma stain a light yellow, while all the axons are color- 
less. In such preparations one can readily recognize the un- 
myelinated fibers (fig. 8). The colorless axons are encircled by 
a light yellow neurilemma ring. This yellow ring contains no 
trace of black myelin. The unmyelinated fibers in the sympa- 
thetic trunk of the turtle have an exactly similar appearance in 
good osmic acid preparations. Somewhat similar appearances 
were noted in the mammalian vagus; but in osmic acid prepara- 
ations of the dog’s vagus the individual unmyelinated fibers 
do not stand out with anything like the same clearness. In the 
dog’s vagus the tissue separating the myelinated fibers has 
when highly magnified a ‘reticulated’ appearance (Chase and 
Ranson 714). 
Figure 4 shows the same bundles in slightly different arrange- 
ment at the level of the highest part of the cervical ganglion. 
