DEGENERATION AND REGENERATION OF NERVE FIBERS 9503 
which occur at a distance of at least 5 mm. from the end of the 
stump. At that distance from the cut surface no changes can 
be detected in the medullated fibers in Dog x1, killed one day 
after the operations The medullated axons still show their 
characteristic, smooth contour and uniform light yellow stain 
characteristic of them in their normal state. In Dog x11, killed 
two days after the operation, many axons have an irregular 
surface and are stained more intensely but less uniformly than 
normal axons (fig. 1). The same changes are seen in Dog 1, 
which died about twenty-four hours after the operation. On 
the third day, although these changes are somewhat more 
advanced, there is as yet no fragmentation of the axons. This 
begins on the fourth day and is shown in figure 2. One of the 
fibers, a, is broken up into large granules, staining dark brown 
and grouped into clumps of irregular size and shape. ‘These 
masses are for the most part still connected with each other. 
There is a very marked difference in susceptibility of the axons 
to degenerative changes. Many of the fibers at this stage are 
normal in appearance or are in the stage described as character- 
istic for the second day. Such a fiber, showing only very slight 
alterations, is seen in figure 2, b. 
Changes in the myelin sheaths cannot be clearly seen in these 
specimens. Howell and Huber (’92) found the first evidences 
of segmentation of the myelin sheath on the fourth day. It 
is therefore plain that granular degeneration of the axons is well 
advanced before the segmentation of the myelin sheath begins. 
This agrees with the observations of Bethe (’03). 
After eight days the first 3 mm. of the distal stump are almost 
completely degenerated; the degeneration becomes less and less 
marked during the next 2 mm.; and at a distance of 5 mm. 
from the cut surface we have a condition which is characteristic 
for the remainder of the distal stretch included in the section 
(an additional 5 mm.). It is probable that this stage of degen- 
eration would be found throughout all the rest of the distal 
portion of the nerve. It is also probable that those who have 
described the degeneration as progressing from the point of 
injury downward along the nerve have been misled by this rapid 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 22, NO. 6 
