DEGENERATION AND REGENERATION OF NERVE FIBERS 901 
preparation, as each requires a different amount of ammonia 
in the fixing solution (Cajal ’08). Cajal advises the use of a 
mixture of medium strength in order to stain both, but says 
that in practice it 1s very difficult to get just the right concen- 
tration. All this difficulty is avoided by the use of pyridine, 
which causes the old medullated axons to stain yellow and the 
old non-medullated axons, as well as all newly formed fibers, a 
dark brown or black. The new technique is also much more 
certain in its results. With pure chemicals and clean glass- 
ware no failures need be anticipated. It also has the advan- 
tage of giving a uniform stain throughout a large piece of tissue. 
Against the use of the Cajal method in the study of regen- 
erating nerves, Bethe (07) has raised the following objections: 
(1) The myelin sheaths are not stained. This is of course true 
and, since these sheaths are represented only by colorless spaces, 
the method is of little or no value in the study of their degen- 
eration and regeneration. If, however, we wish to study the 
changes in the axons, this very transparency of the myelin 
sheaths renders the preparations more serviceable. The very 
faint yellow of the connective tissue and the transparency of 
the myelin sheath make it possible to use relatively thick prep- 
arations (12 to 15u) in which the axons can be followed for 
a relatively long distance. (2) It shrinks the axons. This is 
also true and is a defect which it has not been possible to over- 
come. (3) It is uncertain in its action. Bethe excuses himself 
for not having used Cajal’s method by saying that his material 
was too valuable to jeopardize by the use of such an uncertain 
technique. This objection certainly cannot be applied to the 
pyridine-silver method. (4) Only very small pieces of tissue 
2 or 3 mm. thick can be used. On the contrary, either with the 
old Cajal method or the pyridine-silver modification, the best 
results are obtained with larger pieces. It is, of course, essential 
that one be able to study large sections in order to secure a 
correct idea of what is going on at different levels. The prepa- 
rations which form the basis of this paper are, for the most part, 
longitudinal serial sections of pieces of nerve from 5 to 8 mm. 
thick and from 15 to 20 mm. long. They represent sometimes 
