SECOND CERVICAL NERVE OF THE RAT 145 
these additional fibers whatever their function are determined 
by the time of their appearance to be small fibers. 
This phase of the significance of the added fibers is so bound up 
with the discussion of size that we may consider it more readily 
under that topic. 
SIZE OF THE MEDULLATED NERVE FIBERS 
If the period preceding puberty marks the time of the chief 
increase in the number of medullated nerve fibers, it has no such 
notable relation to the increase in size of the medullated nerve 
fibers, which continues at least to the ninth month. This is 
clearly indicated in table 2, which gives the average areas in 
square micra for the ten largest fibers of four individuals in each 
group. In this table, and in fig. 1, curve B, an unbroken increase 
occurs for our averages from the seven day group to the two hun- 
dred and seventy day group. A somewhat similar increase 
occurs in the areas of the axis cylinders of the same fibers, showing 
that the integral portion of the nerve cell increases in size during 
health with increasing age at least to the ninth month of life. 
The ratio of the average for the axis cylinder and for the entire 
fiber (table 2) gives some interesting information as to the growth 
of the medullary sheath. Using the average area of the axis 
cylinder as the unit, at seven days the area of the entire fiber is 
about one and one-half times that of the axis cylinder, that is the 
medullary sheath has half the area of the axis cylinder. At the 
succeeding stages of body growth to the two hundred and seventy 
day period the medullary sheath is thicker than the axis cylinder. 
The greatest relative thickness of the medullary sheath appears 
in our series at the thirty-sixth day when, contrasted with the 
unit area of the axis cylinder, the area of the medullary sheath 
is 1.47. From this age there appears a readjustment in the growth 
of the two, until at nine months the ratio is 1:0.99 or the one to 
one relation of the normal adult (Donaldson and Hoke, ’05). 
Among the members of our group of aged and infirm male rats 
the nerve fiber is affected in both axis cylinder and medullary 
sheath. The average area of the entire fiber and of the axis cylin- 
