THORACIC TRUNCUS SYMPATHICUS 431 
At the point where the first and largest splanchnic nerve 
is given off, which is usually near the thirteenth thoracic ganglion, 
the size of the trunk becomes abruptly reduced, and it is ob- 
vious, when serial sections are studied, that a large part of its 
myelinated fibers run into this nerve. 
The sympathetic trunk cephalad to the sixth thoracic ganglion 
is characterized by the small number of large myelinated fibers 
which it contains. The peripheral crescent-like bundle of un- 
myelinated fibers is seen here as well as in lower segments and 
can be traced continuously from one ganglion into another. 
Above the fourth thoracic ganglion the trunk sometimes breaks 
up into two or three fascicles which run parallel to one another to 
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Fig.7 One of three fascicles composing the sympathetic trunk above the level 
of the fourth thoracic ganglion in the cat. Osmic acid. X 425. 
reach the stellate ganglion. A cross-section of such a fascicle 
just above the level of the fourth thoracic ganglion is seen in 
figure 7. The crescent of unmyelinated fibers was joined with 
one of the other two fascicles of which the trunk was composed. 
The fascicle which is seen in cross-section in figure 7 contained 
no bundle of unmyelinated fibers large enough to be recognized 
in osmic acid preparations. With the exception of two larger 
fibers, it was composed of myelinated fibers of uniformly small 
size. The other fascicles of the trunk were also characterized by 
the paucity of large and medium-sized myelinated fibers. 
