THORACIC TRUNCUS SYMPATHICUS 421 
as typical. In one cat the sixth thoracic gray ramus contained 
one myelinated fiber 6.64 in diameter and three others much 
smaller. The seventh was fused with the white ramus, and it 
was difficult to be sure which myelinated fibers belong to it. The 
eighth contained 3, the ninth 20, the tenth 27, the eleventh 14, 
the twelfth 6; and the thirteenth 16, all under 4u in diameter. 
What is the function of these myelinated fibers and from what 
cells do they arise? Many are postganglionic fibers arising 
from the cells of the ganglia of the sympathetic trunk. The 
observations to be found in the literature showing that postgan- 
glionic fibers in some instances acquire myelin sheaths have been 
given qn page 323. As already mentioned, Gaskell showed that 
as a gray ramus reaches its spinal nerve it divides into two fas- 
cicles, one of which is directed peripherally. This peripheral 
branch receives its share of the myelinated fibers. These being 
directed toward the periphery can scarcely be other than post- 
ganglionic fibers. It is reasonable to suppose that many of those 
which turn centrally are of the same nature (Langley, ’92 a). 
In 1896 Langley made a careful study of this problem. After 
a variety of experimental lesions involving degeneration of fibers 
of Various origin in different experiments, he counted and meas- 
ured the normal and degenerated fibers in the gray rami of the 
lumbar and cervical nerves. We quote his conclusions: 
The great majority of these (myelinated) fibers arise from sympa- 
thetic nerve cells in the corresponding sympathetic ganglion. In some 
cases, but not always, a few arise from sympathetic cells in an adjoining 
ganglion. No efferent fibers run from the spinal cord to the sympathetic 
by way of the gray rami. In some cases, but not commonly, a few 
efferent medullated fibers, passing to the sympathetic by the white 
rami, leave the sympathetic by the gray rami. These are to be con- 
sidered as fibers on their way to aberrant sympathetic nerve cells lying 
in the gray rami before they reach the spinal nerves. The afferent 
medullated fibers of the gray rami are of various sizes, 2u, 4u, 6u, and 
in some cases 8 to 12u. These are few in number and rapidly diminish 
(especially those of more than 4u in diameter) in passing from the lower 
lumbar to the coccygeal rami. Most of (these) afferent fibers join the 
sympathetic by white rami (only to leave again by the gray), but there 
is some evidence that a few, especially the larger ones may run to the 
sympathetic by the gray rami. 
