356 RECENT LITERATURE OF NERVES. 



along the course of those which have been destroyed as a result of the section. Of 

 the numerous fibres in the groups above described, no doubt a few only eventually 

 assume the function of the fibres which they replace, but the later steps of the 

 process of regeneration have not yet been fully followed out. 



Except close to the actual place of section, where they are somewhat hypertrophied, 

 the connective-tissue sheaths of the nerves remain unaltered. In the cicatrix the 

 new nerve-fibres do not at first run in definite sheaths, but these become subsequently 

 developed from the connective-tissue around, so that at length the restoration of 

 continuity of all the structures in the nerve becomes complete. Vanlair states 

 that the outbudding of the axis-cylinders of the central end may occur as much as 

 one or two centimeters from the point of section, and may involve at first only the 

 peripheral fibres of a funiculus. 



Ranvier looks upon the regeneration of a nerve by growth from the intact central 

 ends of the fibres as illustrating the tendency which, he believes, all nerve-fibres 

 exhibit, to grow continuously until a hindrance is met with, and he compares the 

 result of cutting a nerve-fibre in causing the growth of a number of new fibres in 

 place of the original one, to that produced when the leading shoot of a plant is 

 removed, in causing the production of a number of lateral buds. 



Some have thought that under favourable circumstances an immediate union 

 between the ends of the nerve-fibres may happen after section ; but considering the 

 impossibility of procuring exact apposition of the individual fibres, end to end, as 

 well as the inevitable extension of the effects of the mechanical injury caused by the 

 section along the soft contents of the primitive sheath, it seems improbable that 

 such direct union should ever occur. 



The degeneration does not affect, as we have seen, the part of the nerve 

 remaining in connection with the nervous centre, which seems to exert an influence 

 in maintaining the nutrition of the nerve. The ganglia, as well as the grey matter 

 of the brain and spinal cord, are centres of this influence. It is found that, in the 

 central portion of a divided spinal nerve, while the fibres belonging to the anterior 

 root owe their integrity to their connection with the spinal cord (and especially with 

 the large cells of the anterior cornu), those of the posterior root are similarly depen- 

 dent on the ganglion ; and that if the posterior root be cut between the ganglion 

 and the spinal cord, not only will the fibres which pass from it into the trunk of the 

 nerve beyond the ganglion remain unchanged, but also those above the ganglion, in 

 the portion of the root left in connection with it ; whereas the fibres of the same root 

 which remain connected with the cord but severed from the ganglion degenerate. 



The degeneration of the peripheral end of a cut nerve and the breaking up of the substance 

 of the medullary sheath were first noticed by Nasse in 1839. But the discovery by Augustus 

 Waller in 1852 of the dependence of the process upon isolation of the nerve-fibre from its 

 nutritive centre, and his application of this discovery to the tracing the course of nerve-fibres 

 in peripheral parts (now known as the Wallerian method) first gave full interest and import- 

 ance to the observation of Nasse. Stated briefly, the law may be formulated as follows : 

 " Degeneration occurs along the whole extent of any nerve-fibre which is cut off from the cell 

 which governs its nutrition," and this, as the observations of His have shown, is in every case 

 the cell from which the nerve-fibre has originally grown. 1 



RECENT LITERATURE. 



Arnold, Julius, Bemerkunyen uber Spiralfasern und pericellulare Fadcnnetze an den Ganylien- 

 zellen des Sympathicus, Anatomischer Anzeiger, 1890. 



Aronson, H., Beitraye zur Kenntniss der centralen und periphcren A'ervenendigungen, Dissert., 

 Berlin, 1886. 



Boveri, Th., Beitraye zur Kenntniss der Nervenfasern, Abhandl. d. k. bayeriscb. Akad. d. 

 "Wissenschaften zu Miinchen, 1885. 



1 The addendum which is sometimes made to the effect that the course of the degeneration follows 

 that of the nervous impulses which are ordinarily conducted along the fibre is manifestly erroneous. 



