NATURE OF THE NERVE IMPULSE. 127 



suture or grow together in any way, then, under the influence of the 

 central end, the " band fiber " gradually becomes transformed into 

 a normal nerve fiber, with myelin sheath and axis cylinder (Fig. 58). 

 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, but more probably there is an actual down- 

 growth of the axis cylinders from the central ends. In support 

 of this latter view, it may be said that the outgrowth of the 

 new axis cylinders from the old ones present in the fibers of the 

 central stump has been followed more or less successfully by a 

 number of histologists. 



Bethe* has thrown some doubt upon this view, for he has shown appar- 

 ently that in young mammals (eight days to eight weeks) the regeneration of 

 the fibers in the peripheral stump does not stop at the stage of "band fibers," 

 but progresses until perfectly normal nerve fibers are produced, even though 

 no connection is made with the central stump. It should be added, however, 

 that the fibers so formed do not persist indefinitely unless they become con- 

 nected 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, if true, that in the young fiber the regeneration is complete seems to 

 indicate that the axis cylinder may arise independently of the fibers in the 

 central stump. 



Whether or not Bethe's observations upon the autoregeneration of the 

 axis cylinders in the severed nerves of young animals can be accepted is 

 doubtful, the balance of evidence at present seems to indicate that what he 

 took for autoregenerated fibers were really fibers which grew into the de- 

 generated trunk from the surrounding tissue. 



Degenerative Changes in the Neuron on the Central Side 

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

 as originally stated, 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 degeneration and 'regeneration similar to that 

 described for the fibers of the peripheral stump. The immediate 

 degenerative changes in the fibers in the central stump were supposed 

 to extend back only to the first node of Ranvier, to affect, there- 

 fore, only the internodal segment actually injured. Later it was 

 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 was supposed to remain intact as long 

 as its cell of origin was normal. 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 



* Bethe, "Allgemeine Anat. u. Physiologic des Nervensystems," 1903. 



