280 journal of Comparative Neurology and Psychology. 



The number of medullated afferent nerve fibers cut in the opera- 

 tion on a twelve-day old rat is about 1500. [The average number 

 of medullated fibers in the dorsal roots of twelve-day old rats was 

 found to be 1568 (p. 274); to this must be added a 10 per cent, 

 "distal excess" to find the number of medullated afferent fibers in 

 the nerve (p. 274); and from this result 13 per cent, must be sub- 

 tracted for the afferent fibers running in the uninjured ramus 

 anterior (p. 275). This calculation gives 1500 medullated afferent 

 fibers which would be injured at the operation.] And were all the 

 cells associated with these 1500 fibers to drop out, the loss w^ould 

 only amount to 17 per cent. Or, expressed in other words, nd^rly 

 three times as many cells have disappeared as can be accounted for 

 in terms of medullated axons injured at the time of the operation. 

 Even if we assume that all the axons ever to develop are present 

 (partly as non-meduUated fibers) at the time of the operation on 

 the young rat, and if we let this be represented by the number of 

 medullated afferent axons in the adult nerve, we find that even this 

 number, which does not exceed 2500 (see p. 274) i^s inadequate to 

 account for the number of degenerated cells. For the explanation 

 of these results we are, therefore, forced to fall back upon the 

 existence of some as yet unknown relations within the spinal 

 ganglion. 



There was not sufficient disturbance of the blood supply to ac- 

 count for the degeneration, since the artery and vein accompany- 

 ing each root were not in any way injured. The objection that 

 the degeneration was due to a septic infection has been answered 

 in connection with the discussion of the technique, and against 

 such an objection there also speaks the fact that infection could 

 not produce such uniform results. 



It might be supposed that the fact that only the dorsal branch 

 was cut and the ventral branch left intact explained the occurrence 

 of a partial degeneration, and that if both branches had been cut 

 all the cells would have disappeared. This supposition is, how- 

 ever, manifestly incorrect since the intact ventral branch did not 

 contain more than 13 per cent, of the afferent fibers (see footnote, 

 p. 275), and hence cannot be responsible for the 48 per cent, of the' 

 cells which survive. 



The results obtained by the enumeration of the medullated nerve 

 fibers in the ventral and dorsal roots of young rats surviving two 

 months after the section of the second cervical nerve are much less 



