THORACIC TRUNCUS SYMPATHICUS 425 
come from the next higher or next lower ganglion. The ganglia 
and gray rami are therefore more nearly segmental than the white 
rami. As we shall see, few if any postganglionic fibers, arising 
in the ganglia of the sympathetic trunk, pass by way of the 
splanchnic nerves to the abdominal viscera. 
With these facts in mind we are prepared to understand the 
observations which follow and which show that the sympathetic 
trunk is a well myelinated nerve. 
The sympathetic trunk caudad to the sixth thoracic ganglion 
has the structure shown in figure 5. It consists like other por- 
tions of the thoracic trunk of two fascicles which, though not 
sharply separated from each other by connective-tissue septa, 
maintain their identity throughout, and in cross-sections of the 
stained nerve are easily distinguished from each other because 
of their markedly different fiber content. The larger fascicle, 
well myelinated, presents in cross-section a round outline and 
occupies the greater part of the area of the cross-section. The 
other, which makes up but a small part of the area, is composed 
almost exclusively of unmyelinated fibers, and is flattened out like 
a erescent upon the surface of the larger bundle. For conven- 
ience of reference and until its nature is better known we will 
speak of this bundle as the crescent. The larger, more rounded, 
area will be referred to as the oval. 
When followed in serial sections the crescent of unmyelinated 
fibers is seen to enter the ganglion at either end of the internodal 
segment and become lost in the ganglion. The crescent contains 
a very few fine myelinated fibers, and has in fact the structure of a 
gray ramus. ‘The fibers of the larger well myelinated bundle, the 
oval, run in part into the ganglion at either end of the internodal 
segment, but in even larger part pass by along the side of the 
ganglion. ‘Every internodal segment of the thoracic sympathetic 
trunk presents this separation into two fascicles—and in every 
case where serial sections of an internodal segment, including the 
ganglia at both ends, were examined the crescent was found to be 
continuous from ganglion to ganglion. 
The same structure was described as a peripheral fascicle of 
unmyelinated fibers in the cervical sympathetic trunk. As 
