288 'Journal of Comparative Neurology afjd Psychology. 



The dorsal roots show distinctly more fibers in the animals 

 operated on at four months of age than in either of the other cases 

 (Table XI); since, as Table IV clearly indicates, these older ani- 

 mals do not possess as much regenerative capacity, and since it 

 is certain that at least one-third more medullated fibers were cut 

 in operating upon them, it seems altogether probable that the 

 larae number of fibers in the dorsal roots in the adult rats is to be 

 explained as in the case of the ventral roots on the basis of a greater 

 resistance of the adult neurones.^ 



The interpretation of the numerical results obtained for the 

 spinal ganglion and dorsal root is exceedmgly difficult. A final 

 statement can only be made when we have the results of the 

 Marchi test and the Nissl stain to assist us in drawing conclu- 

 sions, since in these ways we can tell what proportion of the fibers 

 degenerate and whether the large or the small cells are chiefly 

 concerned in the changes going on in the ganglion. Investigations 

 along these lines are now in progress. 



' At first sight it seems a contradiction to say that the dorsal roots show varying degrees of resistance, 

 but that there is no difference in the degree of degeneration seen in the spinal ganglion. If, however, 

 it were the small cells not directly associated with medullated fibers that had disappeared from the 

 ganglion it would be easy to understand how the dorsal ioot fibers, associated as they are with the large 

 cells, would be quite independent of the decrease in the number of the ganglion cells. These large cells 

 with their medullated processes would then suffer varying degrees of injury, usually not resulting in 

 the destruction of the perikarya, but in a certain proportion of the cases bringing about a degeneration 

 of the associated dorsal root fibers. The more mature the animal, the less seriously would the large 

 neurones be injured, and the fewer would those be that could not maintain their dorsal root fibers intact. 



The possible causes of variation in the dorsal roots may be stated as follows: 



1. While the total number of cells destroyed is constant, the proportion of large and small cells 

 afltected may vary, and accompanying a greater destruction of large cells, there may be a greater 

 degeneration of the dorsal root. This supposition is very improbable. 



2. All the large cells may drop out in every case, the variation in the dorsal roots depending upon 

 the extent of the compensatory development of the small cells. 



3. Most of the large cells may pass through the stages of reaction and repair, while the small cells 

 drop out in large numbers. In this case the medullated nerve fibers associated with a varying number 

 of large neurones might degenerate although the perikarya of these same neurones survive. This is 

 the most probable explanation of the results, as will be shown in another paper. At the present moment 

 it seems probable that the key to the explanation of all these conflicting results is to be found in the 

 existence in the nerve of many non-medullated fibers which are the axons of the small cells. This would 

 furnish an explanation for the degeneration of the small cells after the section of the nerve. 



