438 S. W. RANSON AND P. R. BILLINGSLEY 
from which they run through the sympathetic trunk and the 
splanchnic nerves to the viscera. 
The sympathetic trunk is to be looked upon as a series of more 
or less segmentally arranged ganglia bound together by fibers 
frcm the white rami. Above the sixth thoracic ganglion these 
fibers are chiefly ascending, below the tenth descending, but 
between the sixth and tenth both ascending and descending 
fibers are present. In addition to these fibers from the white 
rami, which make up the larger part of the cross-section of the 
trunk in the form of a large well myelinated oval field, there is 
also present throughout the thoracic sympathetic trunk a small 
well-defined bundle consisting chiefly of unmyelinated fibers. 
This in cross-sections appears flattened out like a crescent against 
the larger oval well myelinated field. Some of the fibers in the 
crescentic field are postganglionic, ascending or descending to 
reach adjacent gray rami. Others may be preganglionic fibers 
that have passed through one or more ganglia, giving off collaterals 
end losing their myelin sheaths. 
In the upper part of the thoracic sympathetic trunk the oval 
well myelinated field is composed almost exclusively of fine myelin- 
ated preganglionic fibers, with very few large myelinated and 
unmyelinated afferent fibers. In the lower thoracic segments 
there are in addition to the preganglionic components also affer- 
ent fibers, both myelinated and unmyelinated, which increase 
steadily in number from the sixth internodal segment toward the 
origin of the greater splanchnic nerve. 
The greater splanchnic nerve of the cat usually leaves the trunk 
at or just below the level of the thirteenth thoracic ganglion. 
It is formed by the separation of a large part of the oval well 
myelinated fascicle from the rest of the sympathetic trunk and 
is composed of fine myelinated preganglionic fibers, destined to 
end in the coeliac ganglion, and of both myelinated and unmyelin- 
ated afferent fibers. Occasionally it also contains a bundle of 
postganglionic fibers arising from cells in ganglia of the sympa- 
thetic trunk. 
