776 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) . Bet he, 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 occur 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 ab- 

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

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

 cerebellum and spinal ganglia of young mammals when cultivated on 

 clotted plasma outside of the body (Fig. 327, p. 829). 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 (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 

 sutured both to its own distal end and to the distal end of an adjacent 

 nerve- trunk, the sum of the nerve-fibres in the two distal trunks after 

 regeneration has occurred is greater than the number of fibres in the 

 central stump (Kilvington). That this is due to splitting of axons is 

 shown by the fact that an axon reflex (p. 885) can be elicited on dividing 

 one of the distal trunks and stimulating its central end after complete 

 separation of the proximal or parent stem from the central nervous 

 system-. Even when the second path offered, to the regenerating motor 

 axon is a sensory path, bifurcation of the axon occurs, one branch 



