THE NEURON AND ITS CONDUCTING PATHS 123 



of the second view, it will be shown later on that the former concep- 

 tion of regeneration is the more correct. 



Assuming, therefore, that the regeneration of the fibers results 

 in consequence of a central proliferation of neuroplasmic material,^ 

 the question may be asked whether this outgrowth takes place from 

 the axis-cylinder or from its investments. Briefly stated, it appears 



Fig. 69. — Histology op a Degenerating Nerve Fiber. (Howell.) 



that this process begins with a hyperplasia of the neurolemma at the 

 site of the section of the nerve, and while the central as well as the 

 peripheral stumps participate in this reaction, the principal part is 

 played by the former. ^ The cytoplasm surrounding the nuclei of 

 this locality, is rapidly increased in amount, as is also the number of 



Fig. 70. — Embryonic Fibers in a Regenerating Nerve. (Howell.) 



the nuclei themselves. In this way numerous protoplasmic streamers 

 are developed which become well differentiated in the course of from 

 foiu" to six days and progress into the distal stump, where they form 

 thickened bands of cytoplasm within the neurolemmal sheaths. The 

 axis-cylinders of the central stump follow -along these protoplasmic 

 bridges and thus close the defect. In many cases they may even be 



Fig. 71. — A Newly Developed Fiber in a Regenerating Nerve Fiber. 



(Howell.) 



seen to penetrate the cicatricial tissue at the site of the injury. To 

 begin with, these axons are non-medullated but acquire a myelin sheath 

 in the course of from five to six weeks (dog) — provided, of course, that 

 they were meduUated previously. This medullation begins proximally 

 and progresses toward the periphery. 



A continuity having been established in this way, the correspond- 

 ing cell-bodies and their collaborating neurons gradually regain their 



1 Purpura, Archivio ed atti della Soc. ital. di, Chirurgia, 1909 and 1911; also 

 see: Perroncito, Mem. del R. 1st. Lombardo di Sc. et Lett., xx, 1908. 



2 Kirk and Lewis, Johns Hopkins Hosp. Bull., xxviii, 1917. 



