SECOND CERVICAL NERVE OF THE RAT 133 
the purpose of uniformity the fixed point of section was opposite 
the spinal ganglion and central to the fusion of the two roots. 
The methods of preparation of the material were those used for 
the enumerations and measurements in the leopard frog, (Dunn, 
00, ’02, 09) and differed but slightly from those of Boughton, 
(06) Donaldson and Hoke (’05) and those of other investigators 
with whose findings comparison will be made. The nerve roots, 
ganglion and a portion of the nerve were fixed and stained unsep- 
arated in a one per cent solution of osmic acid, imbedded in par- 
affin, cut to a thickness of 4 micra and mounted serially. 
The counts were made by the aid of an ocular net, and the 
measurements with an ocular micrometer, using an oil immersion 
lense at a magnification such that ten of the subdivisions of the 
ocular micrometer equalled one of the stage micrometer, or one 
one-hundredth of a millimeter. One division equalled 1 micron. 
This magnification facilitated greatly the final computations which 
were at best very tedious and required the closest control to elim- 
inate the chances for error. 
In selecting the nerve fibers for measurement, the entire sec- 
tion was surveyed and the largest fibers methodically selected. 
Each fiber was measured in two diameters, and its axis cylinder 
immediately measured in the same diameters. 
The counting was done in daylight and the measurements taken 
under an oil immersion lense by the aid of an electric microscopic 
bulb of thirty-two candle power. 
It was at first intended to select a variable number of nerve 
fibers for measurement, making the number proportional to the 
number of medullated nerve fibers in the section. But this plan 
was abandoned in favor of that of a fixed number which showed 
more accurately the increase in size at successive ages. The 
number fixed upon was ten. The results of this examination are 
found in tables 1 and 2. 
Table 1 shows individual by individual, the age, the sex, the 
weight in grams, and the number of medullated nerve fibers in 
the ventral root of the second cervical nerve. Then for the ten 
largest medullated nerve fibers, the average diameter in micra, 
and the average area ‘in square micra for the fibers, and the 
