go NERVOUS SYSTEM OF VERTEBRATES. 



the slender branches would appear to be composed of a neuro- 

 fibrilla alone. Finally, in nerve-muscle endings it is conceivable 

 that the neuroplasm and sarcoplasm should be fused together 

 and that the neurofibrillae should continue into the sarcoplasm. 

 This is at present the most probable interpretation of the appear- 

 ances presented by the neurofibrillae. Whether the fibrallae con- 

 stitute a special conducting substance in the nerve cell is doubtful. 

 It is scarcely conceivable that any one substance in the nerve cell 

 can originate and conduct a nerve impulse without interaction 

 with other substances in the cell. However, if the nerve impulse 

 and its transmission are phenomena in the production of which 

 the various substances in the cell cooperate, then the neurofibrillae 

 by reason of their density and their staining properties may 

 reasonably claim our attention as indicators of the course taken 

 by impulses. 



The process of regeneration of injured nerves, although it may 

 not coincide with the processes of normal growth of nerve elements, 

 is instructive as to the nature of the neurone. The course of 

 regeneration of a cut nerve seems to be as follows. The part 

 distal to the cut, i.e. away from the cell-body, degenerates in agree- 

 ment with the observations of Waller. The fibers proximal to 

 the cut remain in a healthy condition and grow along the hne of 

 the degenerated distal part. Whether the growth of the fibers 

 is sufficient to replace the whole length of the part cut away has 

 not been positively demonstrated but is presumed in the absence 

 of evidence to the contrary. When the proximal ends are not 

 prevented from growing down along the degenerated nerve the 

 naked extensions of the axis cyHnders may be traced for a con- 

 siderable distance. The degeneration of the distal part of the 

 axis cylinders leads to changes in the sheaths. The myelin is 

 resorbed and the sheath cells, or some of them, form strands of 

 plasm which are to be regarded as in the nature of embryonic 

 tissue. When the proximal end of the nerve is tied to one side 

 so that its fibers cannot grow out along the nerve bed, it is said 

 (Bethe) that the embryonic strands develop neurofibrillae and 

 become able to conduct impulses. In adult animals the strands 

 do not acquire this power and in any case connection with the 



