SECOND CERVICAL NERVE OF THE RAT 155 
The growth of the medullary sheath as compared with that of 
the axis cylinder is emphasized in this series. At seven days the 
area of a cross section of the axis cylinder is noticeably larger 
than that of the medullary sheath. After that age the medullary 
sheath grows more rapidly than the axis cylinder, at fourteen 
days having a greater area than the axis cylinder. At thirty-six 
days it has one and one-fourth times the area of the axis cylinder. 
From thirty-six days the axis cylinder grows more rapidly until 
at nine months there exists the one-to-one relation of the adult 
which has been found in many vertebrates including the albino 
rat. In old age there is a relatively greater loss in the axis than 
in the medullary sheath, so that the area of the medullary sheath 
is the greater. 
While male and female rats may be grouped together in the 
study of the influence of age upon the size of the medullated nerve 
fibers, accuracy demands their separation when the influence of 
weight is to be considered. The growth curve for the two sexes 
appears to be different for the individual elements of the nervous 
system, as it has been shown to be for the central nervous system 
and for the body. 
Usually for both males and females of a fixed age the greater 
average area of the largest medullated nerve fibers of the ventral 
root fibers of the second cervical nerve is found with the greatest 
body weight. But if the less fiber area is found with greater 
body weight the number of fibers is always greater. Greater 
number may be found with greater area and greater body weight. 
This correlation between the body weight and the size of the med- 
ullated nerve fibers is an argument in favor of the theory of a 
certain relation between the amount of tissue to be innervated and 
the caliber of the innervating pathway. 
In the later periods of growth among both males and females the 
size of the medullated nerve fiber runs more closely parallel with 
the body weight than among the more immature individuals, 
that is, growth processes in the individual nerve elements are more 
rapid and also more variable among the immature than among 
more mature individuals. 
