182 JOURNAL OF COMPARATIVE NEUROLOGY. 
between the proximal and distal sections, although at this phase 
of the life cycle, the difference does not necessarily mean that 
crowth in these rats is still taking place. Thus Harbesty’s 
observations on the frog are confirmed in the growing white rat. 
Summary. 
1. The total number of the medullated fibers in the 
ventral roots of the spinal nerves increases as the animal be- 
comes older (Table I). The rate of increase of these fibers is 
not the same for the different ages. It is most rapid between 
the weights of 10.3 and 25.4 grams (10-30 days), after which 
it becomes slower. The number at maturity is about 2.7 
times that found in the 10.3 gram rat. 
2. The total number of the medullated fibers in the 
ventral roots in the 10.3 gram rat is approximately one-third of 
that in the dorsal root fibers (1:2.9), while in the adult the 
ratio is 1:2.3 (Tables IJ and III). Thus the increase of 
medullated fibers in the ventral roots is more rapid than in the 
dorsal roots. 
3. At all ages, the ventral root near the cord contains 
more medullated fibers than appear more distally, the second 
section being taken near the ganglion of the corresponding 
dorsal root; in other words, the total number of the medul- 
lated fibers in the ventral root decreases from the spinal cord 
towards the periphery (Table IV). 
BIBLIOGRAPHY. 
BIRGE, E. A. 
82. Die Zahl der Nervenfasern und der motorischen Ganglienzellen im 
Riickenmark des Frosches. Arch. f. Anat. u. Phystol., Physiol. 
Abtheil. Hefte 5 u. 6. 
BUHLER, A. 
’98. Untersuchungen iiber den Bau der Nervenzellen. Verhandlungen 
der Physik. med. Gesellschaft zu Wiirzburg. N.F., Bd. 31. 
DALE, H. H. 
oo. On some Numerical Comparisons of the Centripetal and Centri- 
fugal Medullated Nerve Fibers Arising in the Spinal Ganglia of the 
Mammal. /ourn. of Physiol. (Foster), Vol. XXV, No. 3. 
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