78 Journal of Comparative Neurology. 



VII. The Relations of the Ventral and Dorsal Roots 

 TO each other. 



The absolute numbers of the fibers enumerated in the ven- 

 tral and in the dorsal roots and in the trunks and dorsal branch- 

 es are represented in the curves of Chart II. This chart is con- 

 structed directly from Table I, and is but a charted expression of 

 the numbers given there. In order to complete the chart, the 

 data for the Ilnd and Xth nerves are computed from the results 

 obtained by Birge in the work above cited. 



This interpolation in the curves was made on the basis of the 

 following calculation. The total number of fibers found by Birge 

 in the spinal nerves of a 63 gram frog was taken and the pro- 

 portional values of the numbers for the nerves in question was 

 determined. Then the same proportional values were found of 

 the numbers recorded in Table I. The amounts thus deter- 

 mined for the ventral and dorsal roots and for the trunk of the 

 Ilnd and Xth nerves were inserted to complete the chart. The 

 number of fibers Mn the dorsal branches of these two nerves 

 could only be roughly estimated, since Birge did not deal with 

 with them at all. 



Birge used a different species of frog {esadetita) from the 

 one used in this work and, that the proportion of fibers con- 

 tained in a given nerve is even approximately the same for the 

 two species, can only be assumed. From observations made 

 by Professor Donaldson in this laboratory, it appears that the 

 European Rana esailenta has a central nervous system of pro- 

 portionally smaller weight than the American Rana viresccns. 

 This relation is also indicated by comparing Birge's results with 

 those mentioned in this paper. 



Birge counted in a frog of 63 grams, a much smaller num- 

 ber of fibers than were here found for one of 48 grams, as re- 

 corded in Table I. For the ten spinal nerves of one side of a 

 63 gram frog, Birge found in the two roots a total of 9,618 

 fibers. Of the species here used, a frog weighing 48 grams was 

 found to possess 14,783 fibers in the roots of the ten spinal 

 nerves of one side. While the two sides of the same specimen 



