146 ELIZABETH HOPKINS DUNN 
der is considerably decreased. The axis cylinder is the more 
affected so that the ratio approximates that of a fourteen day rat 
or a hundred and eighty day rat, in which the medullary sheath 
has a slightly greater area than has the axis cylinder. 
The curves for the averages have been plotted in fig. 1, Curves 
A and B. At seven days Curve A begins at 12 square micra and 
Curve B at 17sq remicra. As age increases the curves diverge 
until at two hundred and seventy days the fiber is twice the size 
of the axis cylinder. In old age, six hundred and forty days, the 
axis cylinder has an average area of 78 square micra, while that of 
the entire fiber is 171 square micra, showing a distinct shrinkage 
in both axis cylinder and medullary sheath. ‘ 
The comparison of the curves for the nerve fiber and for its 
axis cylinder with the curve for the body weight of the same ani- 
mals is full of interest. Curve C has been plotted in the same 
figure to show the relation of the body weights of the groups to 
their age. 
From this comparison we see that the size of the largest nerve 
fibers increases more rapidly than the weight of the body until the 
thirty-sixth day. From the thirty-sixth to the seventy-fifth day 
or until puberty the body growth is very rapid, overshadowing 
that of the nerve fiber. This disporportional growth of the body 
continues until about the one hundred and thirty-fifth day. At 
that age the body has acquired two-thirds of its ultimate size, 
while the largest nerve fibers have acquired only a little more than 
one-half of their ultimate size. The growth of the nerve fibers 
from the one hundred and thirty-fifth day is more rapid than the 
body growth, so that the curves approach one another at the two 
hundred and seventieth day. At six hundred and forty days, or 
old age, the curves are widely separated, due both to the increased 
weight of the body and to the decreased size of the nerve fibers. 
It may be well to discuss more fully the question of size as 
related to all the nerve fibers. 
Measurements have been made upon the largest nerve fibers 
for convenience and for accuracy. ‘To those who have made the 
attempt, the difficulty of making such measurements with the 
ocular micrometer need not be emphasized. The difficulty of this 
