THORACIC TRUNCUS SYMPATHICUS AD 
in gray branches. In the thoracic region one cannot often see 
this fascicle of the trunk give off branches. It appears rather 
to serve as a commissural cord joining two successive ganglia 
together. 
There are three possibilities concerning the nature of the fibers 
of this fascicle: 1. It may consist of commissural fibers arising 
from the cells of one ganglion and running to another. Against 
this assumption is all the evidence presented by Langley to show 
that such commissural neurones do not exist. The most im- 
portant evidence in this connection is that based on degeneration 
experiments. Langley has shown (’03 b) that after degeneration 
of the lower thoracic and lumbar spinal roots, the lower part 
of the sympathetic trunk is in the same condition as after injec- 
tion of nicotine. Stimulation between the ganglia has either no 
effect or only such effect as could be interpreted as due to post- 
ganglionic fibers. The literature on this question was con- 
sidered at some length in the second paper of this series on page 320. 
In the paper by Dr. Johnson this question is again considered, 
and what seems to be conclusive evidence is presented that no 
commissural fibers exist in the sympathetic trunk of the frog. 
In view of these facts, it does not seem probable that the crescent 
is composed of commissural fibers. 
2. The crescent may consist of fibers belonging to gray rami 
which ascend or descend in the trunk for a short distance. From 
what has been said about the crescent in the cervical region and ~ 
from the fact that, as we have already stated, the fibers of a given 
gray ramus may come in part from the ganglion next above or 
next below, it seems obvious that some at least of the fibers of the 
crescent must be of this nature. Since, however, gray ramus 
fibers do not ascend or descend in the trunk for more than one 
segment and since such fibers are not numerous nor constantly 
present, it seems doubtful if they can account for the large number 
of fibers constantly present in the crescent. 
3. A third possibility is that the crescent is formed by unmyelin- 
ated terminal branches of preganglionic fibers. There is some 
evidence that these fibers may lose their sheaths before termi- 
nating. This evidence has been summarized by Langley (00) as 
follows: 
