Hatai, Spenal Nerves of the Rat. 181 
nerves in the adult cat, was not able to observe in that animal 
the numerical differences recorded by Harpesty for the frog. 
I have pointed out already, that the total number of the 
medullated fibers in the two roots increases from 10.3 to 264.3 
grams, although the increase is not so marked between 65 and 
264.3 grams as at the younger ages. From these facts, one 
might expect at the earlier ages an excess of fibers near the 
ganglia in the case of the dorsal roots, and near the cord in the 
case of the ventral roots, since the fibers are outgrowths of the 
cell-body located in spinal ganglion and ventral horn, respec- 
tively. In order to test this point, the fibers were enumerated 
at two levels in the ventral roots, one near the cord and the 
other farther from the cord and at the point where the ventral 
root approaches close to the ganglion, as had been done by 
Haropesty in the frog, and the following results were obtained : 
TABLE IV. 
Numerical Difference of the Medullated Ventral Root Fibers of all 
three nerves, VI Cerv., IV Thor., and IJ Lumb., combined at 
the Two Levels, Proximal (near cord) and Distal (near ganglion). 
Weight of 
Rat 10.3 ZEA: 68.0 164.9 264.3 
Proximal 1299 2244 2696 3169 3279 ) Number of the 
Distal 7 2139 2567 3095 3259 | medallted 
Ditference 122 105 129 74 20 
or 9-4% 4.6% 47% 2.3% 0.6% 
As we see from the table, the ventral root in the proximal 
section, i. e., near the cord, contains more medullated fibers 
than appear in amore distal section. This difference at the 
two levels is greatest in the youngest animal (122 fibers or 
9-4% in excess at 10.3 grams) and diminishes to 0.6% in the 
oldest rat examined. Since the difference decreases as the 
animal becomes older, we most readily explain the decrease of 
the difference by the growth of the fibers which at an earlier 
age are immature, and therefore stop short of the more distal 
section. We infer that even when the growth process stops, 
there are some fibers which are still incomplete and unfinished ; 
this explains the fact that in the oldest rats differences are found 
