144 ELIZABETH HOPKINS DUNN 
nerve and its branches in various adult rabbits but, unfortunately 
for comparison with other records, do not state the age or sex of 
the animal. There is shown however in his table 4 (p. 143) a 
variation in the total number of medullated nerve fibers in the 
motor portion of the fifth cerebral nerve which might indicate the 
influence of an age factor. 
It would seem from the present more extended series from albino 
rats that the increase in the number of medullated nerve fibers 
practically ceases with the attainment of the adult condition and 
that the presence of an individually greater number after that 
period is dependent upon some factor determining the number to 
be supplied to the adult body and is not likely to be due to the 
maturing of immature elements in the nervous system as it seems 
to be at the earlier ages. 
This increase in number of medullated nerve fibers with the 
increase in weight and age may have several interpretations. 
When it appears as a condition most marked during the period of 
rapid growth, it may well seem to be associated with the increase 
of the number of body elements which must be innervated. Dur- 
ing the same period, or more naturally later, it may be dependent 
upon an increased innervation of already innervated material. 
Our data as regards the number of innervated peripheral ele- 
ments either sensory or motor are extremely limited. 
Intimately bound up with this question is that of the splitting 
of peripheral nerve fibers, and no absolute correlation between the 
number of nerve fibers in the spinal roots and the number of 
peripheral innervated elements can be made with our existing 
knowledge. 
It would appear in general that the increase in size of the imma- 
ture body is accomplished by the increase in both number and 
size of the individual elements, while that of the mature body is 
rather due to increase in size of the already formed elements. 
Boughton’s (’06) findings lead him to state that after the initial 
laying down of nerve fibers the added fibers never attain the size 
of the earlier formed fibers, and the added fibers must be small 
fibers. Under this interpretation size is determined by age and 
