150 ELIZABETH HOPKINS DUNN 
enced by the richness of the fiber branching, by the size of the 
innervated elements, by the frequency of their use, or possibly, 
if neurofibrils are the conducting elements, by the number or size 
of the neurofibrils. However this may be, we must be grateful 
for every attempt to delimit the problem, while admitting that 
much further information is desirable. 
Increase in size is indeed a matter of growth, and it is extremely 
confusing to the problem that age plays so large a part in deter- 
mining the actual and possibly the relative size of the medullated 
nerve fibers. It would appear that the next step in the interpre- 
tation involves the accumulation of a mass of data regarding the 
direct relation of the size of the nerve fibers to the tissues inner- 
vated and much further information as to the relation of these 
successively appearing fibers to the peripheral structures. The 
latter involves a study of the axonic relations to the periphery and 
the time of medullation of these axones. 
My own findings (Dunn, ’02 and ’09) on the distribution of the 
medullated nerve fibers to the segments of the leg of the leopard 
frog suggest that difference in time of outgrowth from the central 
nervous system is not sufficient to account for the appearance of 
the largest medullated nerve fibers in each instance in the segment 
nearest to the body. A peripheral factor seems to be present 
here. 
At the moment it is sufficient to inquire as to the information 
regarding the factors determining the size of the medullated 
nerve fibers which may be drawn from the present investigation. 
It has been pointed out that, unlike the oculomotor nerve, the 
number of medullated viscero-motor fibers is small, and that the 
number of very small medullated nerve fibers is much less in the 
adult than in a rat of seven days. 
Among the possible influences upon the size of the medullated 
nerve fibers has been mentioned the size of the animal. Dhéré 
(03) has shown that the extraction of myelin is greater among 
mammals of greater size than among those of less size. It will 
be interesting to inquire as to the influence of weight among the 
individual groups of white rats. Comparing females with females 
and males with males, table 1, in each group of fixed age, the 
