802 NERVE 



mechanically and electrically, and of conducting impulses towards 

 the centre. In about eight weeks they become medullated, but at 

 first are of small calibre (Head and Ham) . Bethe, the most strenuous 

 defender of the inherent regenerative power of the isolated peri- 

 pheral stump (autogenetic theory), has even stated that complete 

 regeneration occurs in young animals in nerves entirely separated 

 from their centres. There is no doubt that this result is due to 

 some error of technique or of interpretation. The controversy 

 turns largely upon the precautions judged necessary to prevent the 

 ingrowth of central fibres. And while it is comparatively easy to 

 make sure, by removing a large part of it, that the central end of 

 the nerve under observation shall remain completely unconnected 

 with the peripheral end, it is often a matter of the greatest difficulty 

 to prevent the union of the distal stump with central fibres from 

 other sources e.g., from the nerves cut in the wound. Many of the 

 results which seemed to favour the autogenetic theory were cer- 

 tainly due to this cause. 



The most conclusive evidence in favour of central and against 

 autogenetic regeneration, because the most direct and uncompli- 

 cated, has been afforded by the demonstration that the development 

 of axis-cylinders occurs in vitro in a suitable plasmatic medium, in 

 the absence of any other elements than the nerve-cells from which 

 they arise (Harrison). This observer, working with the medullary 

 plates of tadpoles, in which the nerve-cells originate in the embryo, 

 showed further that peripheral nerves do not develop when the 

 nerve-centres are removed, and that the sheath-cells of Schwann are 

 not essential to the growth of axis-cylinders, since in their absence 

 the latter continue to grow and reach their normal length. It has 

 also been proved that nerve-fibres grow out from pieces of the cere- 

 bellum and spinal ganglia (Fig. 281) of young mammals when cul- 

 tivated on clotted plasma outside of the body (Fig. 337, p. 857). 

 Many fibres sprouting out from the spinal ganglia attain a length of 

 more than half a millimetre in forty-eight hours, and their growth 

 need not be accompanied by either neuroglia or connective tissue. 

 When portions of normal peripheral nerve are incubated in plasma 

 no growth of axis cylinders -is ever observed, nor does the Wallerian 

 degeneration occurring in peripheral nerves incubated in Ringer's 

 solution take place (F ; g. 282). On the other hand, if nerves in which 

 Wallerian degeneration has been produced by section are placed in 

 plasma on the fifth day after section or later, growth of the elements 

 of the sheath of Schwann may be seen, again unaccompanied by any 

 growth of axis cylinders (Ingebrigtsen). 



A fact of great physiological interest, and also of practical impor- 

 tance, in connection with the anastomosis of nerves for the relief of 

 certain forms of paralysis, is the bifurcation of axons in regeneration, 

 when the conditions are such that the axons of the central stump are 

 offered more than one path along which to regenerate. If, for instance, 

 a limb nerve-trunk containing motor fibres is cut, and its central end 



