THORACIC TRUNCUS SYMPATHICUS 413 
having all the characteristics of small gray rami. Such fascicles 
are, however, always sharply defined and do not form a part of 
the white ramus proper. These composite rami are especially 
frequent in the upper thoracic region where the gray and white 
rami are regularly fused. The first two or three thoracic nerves 
usually have each a single mixed ramus. Mixed rami are also 
not uncommon in the lower thoracic and upper lumbar regions 
where the white ramus runs downward. 
Osmic acid preparations of the white rami of the cat show that 
these contain great numbers of small myelinated fibers with which 
are mingled some larger fibers varying in number in the different 
rami. In the dog Gaskell found that the small fibers measured 
1.8 to 3.6u. He believed that all visceral efferent fibers were of 
this size. Langley (’96 a) states that: 
In the cat, the medullated fibers of the sympathetic appear to be 
somewhat smaller than in the dog. In the dog, I found that most of 
the nerve fibers could be classed as belonging to one of three types, viz., 
large fibers about 8 uw in diameter, medium fibers about 5 yu in diameter, 
and small fibers about 3 uw in diameter, though all sizes from 2 to 12 u 
were present. Inthe cat the corresponding fibers are about 7 uw, 4.5 wand 
2.5 u; fibers from 2 to 10 u and occasionally of greater diameter than 
10 uw being present. 
Our measurements of the medullated fibers in the white rami 
of the cat show that the small fibers vary in diameter from 1.5 
to 3.5u. Between these two extremes there are fibers of all sizes 
and in about equal proportion. There are also fibers of larger 
size, but in much smaller proportion. The second thoracic white 
ramus contains, however, a great many fibers measuring 4.5 or 
5u, as will be seen in figure 2. The larger fibers, which vary in 
size from 5 to 13u are not evenly distributed in the white rami 
and will be considered more in detail when some of the individual 
rami are taken up. 
There does not seem to be any reason for grouping the fibers 
into three classes as Langley has done. There would be more 
reason for making two groups: one less than 4.5y, the other 
larger. It is probable that nearly all of the preganglionic efferent 
fibers would fall in the first group, but this group would also 
contain, as we shall see, some of the smaller sensory fibers. The 
