178 JOURNAL OF CoMPARATIVE NEUROLOGY. 
I enumerated the total number of the spinal ganglion 
cells and dorsal root fibers at different ages in several spinal 
nerves of the rat, and arrived at the following conclusions( ’02): 
1. The total number of the spinal ganglion cells remains 
approximately constant between 10.3 grams (age 10 days) and 
167 grams of body-weight (age 100 to 200 days). 
2. All of the small cells contained in the spinal ganglia 
are constantly growing, and some of them become large cells. 
So that the number of large cells increases with age, while 
there is a corresponding decrease in the number of small cells. 
3. The number of the cells in the spinal ganglia is 
always more than twice the number of medullated fibers in the 
corresponding dorsal nerve roots. 
From the summary it is evident that the small cells are in 
a developing state and are becoming larger, while the fact that 
there is no numerical decrease in the total number of the cells 
favors my view, and is against that of BUHLER. 
In the present investigation the observation has been 
extended to the ventral root fibers. The method of prepara- 
tion and the technique of counting were the same as used in 
the earlier investigation (’02). The following are the main 
results from this latter study: 
TABLE I. 
The Total Number of the Medullated Ventral Root Fibers in the 
Spinal Nerves Here Named. 
Weight of Rat 10.3 grms. 25.4 68.0 164.9 264.3 
Z2e2 
Gy OME Gers. 558 1007 1302 1474 1522 | 225 
Sele bor. 286 434 561 613 772 \) Cneics 
2 1 II Lumb. 333 698 704 1028 965 De 
| Bee 
SEEN IES 
As we see from Table I, the number of ventral root fibers 
steadily increases from 10.3 to 264.3 grams. The relative 
increase is nearly the same for the different nerves examined. 
Generally speaking, the total number of the medullated fibers 
in the adult animal is approximately 2.7 times that in the 10.3 
gram rat, and the fibers increase most rapidly between the 
