MYELINATED FIBERS—PERONEAL NERVE OF RAT 419 
CONCLUSIONS 
In the five series of albino rats here examined the axis-sheath 
relation varies but slightly. Its range is from 38 per cent 
axis: 62 per cent sheath to 43 per cent axis: 57 per cent sheath; 
an average of 40 per cent axis: 60 per cent sheath. 
The symmetry as to number of fibers in the right and left 
peroneal nerves is almost exact, the difference shown in one 
group of twelve and another of fifteen animals being less than 
0.3 per cent. 
Pathological ccnditions like the so-called ‘pneumonia’ which 
is more or less acute and occurs during the later period of growth 
in albino rats, appears to lessen the number of myelinated pero- 
neal fibers, but produces no measurable change upon the sec- 
tional area of the largest fibers. 
In the examination of fifteen rats measurements show that 
symmetry as to sectional area of the largest peroneal fibers 
exists between the fibers of the right and left peroneal nerves. 
Electrical stimulation of a peroneal nerve for thirty minutes 
appears to have no measurable effect upon the sectional area of 
nerve fibers. 
In the data presented there is confirmation of the work of 
Dunn (712) that sectional area of myelinated fibers decreases 
slightly in old age; of the work of Greenman (’13) that the in- 
tact nerve of an operated animal loses in both number and sec- 
tional area of fibers. 
The number of fibers in the peroneal nerve increases with 
age until age 250 days is reached and begins to decrease at or 
before 335 days of age. After the first year of life the sectional 
area of peroneal fibers decreases with advancing age; at 335 days 
of age this process of reduction has already begun. 
