120 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



weeks) the regeneration of the fibers in the peripheral stump does 

 not stop at the stage of " band fibers," but progresses until per- 

 fectly normal nerve fibers are produced, even though no connec- 

 tion is made with the central stump. It should be added, however, 

 that the fibers so formed do not persist indefinitely unless they 

 become connected with the central stump. If this connection fails 

 to take place the newly formed fibers will degenerate after an 

 interval of some months. Still, the fact that in the young fiber 

 the regeneration is complete seems to indicate definitely that the 

 axis cylinder may arise independently of the fibers in the central 

 stump. The power of regeneration in the older animals is more 

 limited and carries the fiber only to the stage of the " band fiber." 

 If under the influence of the central stump an axis cylinder and 

 myelin sheath is now formed in this band fiber it is possible that 

 this result is due to local processes in the band fiber stimulated 

 by nutritive influences of some kind from the central stump, in- 

 stead of being due to an actual downgrowth of the axis cylinder 

 from the central ends. 



Degenerative Changes in the Neuron on the Central Side 

 of the Lesion. According to the Wallerian law of degeneration, 

 the nerve fiber on the central side of the injury and the nerve cell 

 itself do not undergo any change. As a matter of fact, the central 

 stump immediately contiguous to the lesion undergoes typical de- 

 generation and regeneration similar to that described for the fibers 

 of the peripheral stump. The degenerative changes in the fibers 

 in the central stump were supposed to extend back only to the first 

 node of Ranvier, -to affect, therefore, only the internodal segment 

 actually injured. As a matter of observation, however, it is found 

 that the degeneration may extend back over a distance of several 

 internodal segments. This limited degeneration on the central 

 side must be considered as traumatic, that is, it involves only 

 those portions directly injured by the lesion. The central end of 

 the fiber in general remains intact as long as its cell of origin is nor- 

 mal. It was thought at first that after simple section of a nerve 

 trunk, in amputation, for instance, the nerve cells and central stumps 

 remain normal throughout the life of the individual. Dickinson, 

 however, in 1869 * showed that in amputations of long stand- 

 ing the motor cells in the anterior horn of the cord decrease in num- 

 ber and the fibers in the central stump become atrophied. This 

 observation has been corroborated by other observers, and it is 

 now believed that after section of a nerve chronic degenerative 

 changes ensue in the course of time in the central fibers and their 

 cells, resulting in their permanent atrophy. We have, in such 

 cases, what has been called an atrophy from disuse. A fact that 

 * " Journal of Anatomy and Physiology," 3, 176, 1869. 



