THORACIC TRUNCUS SYMPATHICUS 437 
evidence on some of these points will be presented in the paper 
which follows. 
CONCLUSIONS 
We shall make no attempt to summarize the detailed observa- 
tions presented in the preceding pages, but will merely state a 
few general conclusions that seem warranted by the facts already 
given. 
The segmental character of the sympathetic trunk is evident 
in its ganglia and gray rami. The fibers of each gray ramus arise 
chiefly, and sometimes exclusively, from the cells of its own seg- 
mental ganglion and are distributed through its associated spinal 
nerve. When two or more segmental ganglia are fused together 
the resulting compound ganglion gives rise to gray rami running 
to the corresponding spinal nerves. Except for compound 
ganglia like the cervical and stellate, the ganglia of the trunk 
are best designated by the number of the spinal nerve with 
which their gray rami are associated. The white rami have 
a restricted origin from the first thoracic to the fourth lumbar 
spinal nerves, inclusive, and their fibers are distributed through 
the sympathetic trunk to ganglia at higher and lower levels. 
A white ramus is in no special sense associated with its own 
segmental ganglion. 
The gray rami contain in addition to the unmyelinated also a 
few, mostly fine, myelinated fibers. The latter are for the 
most part postganglionic. No preganglionic fibers run from the 
spinal cord to the sympathetic ganglia by way of these rami. 
The white rami consist of myelinated fibers ranging from 
1.5 to 13u, though fibers of more than 10u are rare except in the 
first two white rami and most of the large fibers have a diameter 
of about 6 or 7u. The majority of the myelinated fibers are small, 
measuring 1.5 to 3.54. Most of these are preganglionic. The 
afferent components include myelinated fibers of all sizes, some 
of the very smallest as well as those of medium and large size, 
and also unmyelinated axons. The number of the large myelin- 
ated fibers varies greatly in the different white rami. They 
were found to be most numerous in the seventh to the tenth, 
