418 M. J. GREENMAN 
weight, but were not known to be of the same litter or of the 
same age. 
Comparing male ‘pneumonia’ animals with male controls the 
total number of right and left peroneal fibers of the diseased 
animal is in every case less than the total number of right and 
left peroneal fibers of the control; this difference is 5.8 per cent. 
No significant differences were observed between the sec- 
tional areas of fibers from ‘pneumonia’ rats and those from the 
controls. The axis-sheath relation in both ‘pneumonia’ rats 
and their controls is 40 per cent axis to 60 per cent sheath. 
In this group of rats from 251 to 454 days of age, the older the 
animal the fewer the myelinated fibers found in their peroneal 
nerves. This applies to both ‘pneumonia’ and control rats. The 
fibers of the greatest sectional area occur in those animals 335 
days of age, the younger and the older animals present fibers of 
less sectional area. 
Data are presented to show that from age 147 days to age 
251 days the peroneal nerve gradually acquires more fibers at 
the rate of about two fibers per day. In two groups of three 
animals each, aged 335 days and 454 days, respectively, the 
number of fibers is shown to decrease with advancing age. Be- 
tween 251 and 335 days of age the rat acquires its largest and 
its greatest number of peroneal fibers. 
In a series of twelve animals in which the right peroneal nerve 
of each rat was electrically stimulated for thirty minutes the sec- 
tional areas of the stimulated nerve fibers were slightly less when 
compared with those of the left or intact nerve, but this dif- 
ference may be regarded as insignificant. 
Fifteen albino rats were examined to determine the normal 
size of peroneal fibers of the right and left sides. This exami- 
nation of the largest fibers showed that there is practical sym- 
metry between the right and left peroneal fibers. 
Twelve normal inbred albino rats of 153 days average age have 
an average of 2070 fibers in their peroneal nerves and the aver- 
age sectional area of the ten largest fibers of these peroneal nerves 
is 108.6 square micra. 
