DEGENERATION AND REGENERATION OF NERVE FIBERS 507 
up into segments of varying length. Short, light segments 
alternate with longer, very much darker segments (fig. 5). The 
dark segments are granular, a detail which has been omitted in 
the drawing. For control we have the normal nerve from Dog 
x1, carried through the same solutions at the same time as the 
divided nerves of Dogs xt and x1r. As these changes are not 
seen in the normal nerve, we may be sure we are not dealing 
with artifacts. Three days after the operation, the darker 
segments, still granular in appearance, stain less intensely than 
in the specimen taken a day earlier. A considerable number of 
smooth, black, uniformly stained, non-medullated fibers can be 
seen—but these are not so much in evidence as at a later date 
when the other fibers have undergone more complete degen- 
eration. Again in this experiment we have the normal nerve 
carried through the same solutions at the same time as the 
divided nerve, to serve as a control, and show that the described 
changes are not artifacts due to an irregular deposit of silver. 
Two dogs were killed four days after the operation. Both 
showed only a little advance over the specimen taken on the 
third day; but many of the fibers are already taking a light 
yellow stain with little differentiation into lighter and darker 
segments. 
After eight days many of the non-medullated fibers are no 
longer visible, others appear as light yellow bands along which 
an occasional nucleus can be seen. 
It is at this stage when the other fibers are very lightly stained 
or have disappeared that the resistant undegenerated fibers 
are most evident. ‘These are more numerous and persist longer 
than the resistant medullated fibers. Since the majority of 
the non-medullated fibers degenerate very early and since there 
are few fibers which show an intermediate degree of resistance, 
it is possible that we are dealing here with two different kinds 
of fibers. Those which degenerate early in the first week may 
be afferent spinal fibers; while those which degenerate in the 
second and third weeks may be of sympathetic origin. This 
supposition is the more probable because the susceptible fibers 
represent about the same proportion, which on other grounds 
