DEGENERATION AND REGENERATION OF NERVE FIBERS 523 
the remains of one old neurilemma sheath. The steps in this 
substitution are not clearly presented in the preparations stud- 
ied. But it seems probable that some of the fibers of the net- 
work atrophy and that others assume a more parallel course. 
It is certain that many branches arise from the axons at a slightly 
higher level and, growing down within the neurilemma sheaths, 
help to take the place of the disappearing network. It is obvi- 
ous that these plexuses, as such, take no part in the final regen- 
eration, although individual fibers, derived from them, may do 
so. Nineteen days after the operation none of the old net- 
works are to be seen; but instead, only parallel coursing fibers, 
branching fibers and twining fibers held together in bundles 
by the old neurilemma sheaths. All of these earliest phenomena 
are confined to the last few tenths of a millimeter of the proxi- 
mal stump. 
d. Formation of lateral branches at some distance above the 
lesion. On the eighth day, when the tubular arrangement just 
described has begun to disappear, one sees that the medullated 
fibers are giving off lateral branches at a higher level. These 
are of good size and can be seen coming off as high as 5 mm. 
above the cut. These side branches become more and more 
abundant in each of the three succeeding stages, fourteen, nine- 
teen, and twenty-five days after the operation. On the nine- 
teenth day, they can be seen in large numbers (fig. 20) chiefly 
in the terminal 5 mm. of the nerve. They end in bulbs and 
usually run within the old sheath, predominantly in a peripheral 
direction, but some run spirally, and others centrally, in the 
old sheath, and still others pierce the sheath and run in the 
endoneurium. In figure 21 one sees a bundle of fibers formed 
in this way within an old sheath. The old axon is thicker, 
lighter and more centrally placed than its branches which are 
accompanying it toward the periphery. Two of these branches 
are seen to end in bulbs directed peripherally. Two fine fibers 
leave the sheath and run in the endoneurium. One branches 
a second time in the connective tissue and the resultant fibers 
run out of the section. The other turns peripherally parallel 
to and just outside the old sheath and ends in a large end bulb. 
