MYELINATED FIBERS—PERONEAL NERVE OF RAT 409 
Examination of the number of fibers on both right and left 
sides reveals the fact that the older the animals in both the ‘pneu- 
monia’ and control groups the less is the number of myelinated 
fibers. Thus members of litter 7, aged 251 days, have an aver- 
age of 2251 fibers when right and left nerves of both animals are 
considered. Members of litter 29, aged 335 days, show in like 
manner, an average of 2093 fibers, while members of litter 35, 
aged 454 days show an average of only 1971 fibers. 
If, for greater accuracy, the females are omitted, a similar re- 
sult is obtained; the 251 day rats average 2251 fibers, the 335 
day rats 2109 fibers, and the 454 day rats 1997 fibers. 
Dunn (12) observed that old rats (640 days of age) showed a 
decrease in size of medullated fibers in the ventral roots of the 
second cervical, and states that her observations were made on 
albino rats of widely varying weights but not in good health. 
Table 3 shows that in the eight animals examined the pero- 
neal fibers of the greatest sectional areas occur in animals 335 
days of age, or at the end of the first third of the entire span 
of the rat’s life. In this series animals both younger and older 
than 335 days have fibers of less sectional areas. 
EFFECTS OF ELECTRICAL STIMULATION 
In further pursuit of the problem of changes in the myelinated 
fibers in animals in good health, the experiment was modified and 
instead of subjecting animals to involuntary exercise the nerve 
under examination was electrically stimulated in the following 
manner: 
Kach animal was etherized, both right and left sciatic nerves 
were exposed and cut in quick succession. The wound in the 
left leg was closed, while a small portion of the right peroneal 
(a branch of the sciatic) was exposed, kept moist with normal 
salt solution and stimulated for 25 to 30 minutes by being touched 
at one second intervals by platinum electrodes carrying a weak 
interrupted current. At the end of this period little or no mus- 
cular contraction, following each contact of the electrode, was 
apparent. The stimulated (right peroneal) nerve and the 
control (left peroneal) nerve were then removed, fixed in | per 
