404 M. J. GREENMAN 
PRELIMINARY TESTS 
A preliminary test was made upon two female gray rats (Mus 
norvegicus) of practically the same body weight, 236.5 and 226.5 
grams, respectively. One rat, No. 323, was used as a control 
while the other, No. 322, was placed in a revolving cage operated 
by a motor and exercised continuously for four hours. At the 
end of this period the animal was given water but no food and 
left in the cage until the following day when the cage was again 
started and the animal exercised for a further period of fifteen 
minutes. At the end of this brief period the animal died. 
The unexpected death of the animal complicated the test to 
some extent, since from previous experience with rats in revolvy- 
ing cages the amount of exercise given this animal should not 
have resulted in death. It is possible that the animal was un- 
healthy, though no external signs of disease were noted. No 
autopsy was made. However, as the object of the test was 
to detect changes in nerve fibers due to fatigue, I proceeded to 
remove the right peroneal nerve for comparison with that of the 
control animal. 
Following the method of previous work on this nerve, 10 mm. 
of the right peroneal nerve just distal to the sciatic bifurcation 
were excised, in both the exercised (No. 322) and the control 
animal (No. 323), fixed in 1 per cent osmic acid, embedded in 
paraffine and cut into sections 7 micra thick. The sections from 
the middle zone of each nerve were examined. The number of 
myelinated fibers was determined and the sectional areas of the 
twenty largest fibers were measured. 
In making these determinations Hardesty’s (99) method for 
counting fibers was employed; after making a photograph of 
each section to be counted, each fiber was automatically recorded 
and counted by pricking a hole in each fiber image of a photo- 
graphic print, while the original section was observed under a 
Zeiss 2 mm. apochromatie objective with a No. 4 compensating 
ocular and a tube length of 160 mm. 
The largest fibers of each section were selected by carefully 
going over each successive zone of the section. The sectional 
