318 S. W. RANSON AND P. R. BILLINGSLEY 
no bundles of closely packed unmyelinated axons and no indi- 
vidual ones can be made out with certainty. From a study of 
the normal truncus sympathicus we may conclude that it is 
composed almost exclusively of small myelinated fibers. 
The fine peripheral bundles, which represent branches of dis- 
tribution from the superior cervical ganglion, can usually be 
followed in serial sections to the point where they are given off 
as fine branches from the trunk. They do not degenerate after 
section of the nerve more caudally. The structure of these 
peripheral bundles is entirely different from that of the rest of 
the nerve and corresponds to that of the other branches of dis- 
tribution given off from the superior cervical ganglion. They 
contain a few small myelinated fibers, 1.54 to 6u in diameter, 
scattered among the unmyelinated fibers. Such a bundle is seen 
at a in figure 2 where the area occupied by the umyelinated 
fibers is indicated by stippling. In osmie acid preparations 
bundles of unmyelinated fibers are recognized by their being 
somewhat more darkly stained than the rest of the background. 
A fascicle of axons, even though lightly stained, is easily differen- 
tiated from connective tissue. Additional information may be 
obtained by the study of the degenerated nerve. In an osmic 
acid preparation taken from a cat eight days after neurotomy 
of the sympathetic trunk in the neck most of the medullated fibers 
are degenerated, although a few cannot be distinguished from 
normal fibers. But eighteen days after the operation all the 
medullated fibers were degenerated except for a small number in a 
single peripheral fascicle, such as has been described and which 
is not to be regarded as belonging to the nerve. There were 16 
myelinated fibers in this bundle varying in size from 1.8 to 3.6un. 
All the other myelinated fibers in the nerve were degenerated. 
From this we may conclude that all the myelinated fibers in the 
cephalic end of the sympathetic trunk (exclusive of branches 
of the superior cervical ganglion which may be incorporated with 
it for a short distance) are ascending fibers. There are no medul- 
lated fibers arising in the superior cervical ganglion and running 
to the ganglia placed more caudally in the truncus sympathicus. 
