THE CERVICAL SYMPATHETIC TRUNK oL7 
STRUCTURE OF THE CEPHALIC END OF THE SYMPATHETIC TRUNK 
As has been said, the cervical portion of the sympathetic trunk 
serves to convey preganglionic fibers from the upper white rami 
to the superior cervical ganglion. Whether it also contains other 
than preganglionic fibers is a question which we will consider in 
this paper. In the cat this nerve, a short distance below the 
superior cervical ganglion, has the structure shown in figure 2. 
In cross-section it is uniform throughout except for one or two 
small well-defined bundles at the periphery. These bundles are 
not constant and, as we shall see, represent fine branches of dis- 
tribution from the ganglion which have been incorporated for 
a short distance in the trunk. 
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Fig. 2 From a section of the truncus sympathicus a short distance below the 
ganglion cervicale superius in the cat. a, area occupied by a bundle of unmye- 
linated fibers. Osmic acid. X 425. 
Exclusive of these peripheral bundles which really do not 
belong to it, the sympathetic trunk below the superior cervical 
ganglion in the cat consists almost exclusively of myelinated 
fibers as shown in figure 2. These are uniformly distributed and 
closely packed. It is as well myelinated a nerve as there is any- 
where in the body. The fibers are all rather fine. The majority 
vary in diameter from 1.5u to 3.5u. Between these two extremes 
there are fibers of all sizes and about as many of one size as 
another. Fibers larger than 4.54 are few in number but there 
may be two or three as large as 6.5 or 7u. Pyridine silver prep- 
arations show rather small axons, each surrounded by an un- 
stained halo representing a myelin sheath; these are uniformly 
distributed, each well separated from its neighbor. ‘There are 
