798 NERVE 



Regeneration of Nerve. Degeneration of nerve is followed, if 

 its divided ends are not kept artificially apart, by a process of re- 

 generation, already distinct under favourable conditions in from 

 three to four weeks after the section, and indeed in some cases 

 commencing as early as the second week. This consists in the 

 outgrowth of new axis-cylinders, in the form of fine fibres, from 

 the ends of the divided axis-cylinders of the central stump of the 

 nerve. These push their way into and along the degenerated 

 fibres, ultimately acquire a medullary sheath, and develop into 

 complete nerve-fibres, restoring first sensation, and later 'on volun- 

 tary motion, to the paralyzed part. The process needs several 

 months for its completion, even in warm-blooded animals. It 

 takes place under the influence of the nucleated portion of the 

 neuron (the cell-body), and is never completed if the peripheral 

 and central portions of the nerve are permanently separated by a 

 substance through which the new axis-cylinders cannot grow or 

 by a gap too wide for them to bridge over. When the 'cut ends of 

 the nerve are carefully sutured together, the conditions for com- 

 plete and speedy regeneration are rendered more favourable a 

 fact which finds its application in the surgical treatment of injured 

 nerves. The cycle of chemical changes described in the degenerating 

 nerve is retraced in the reverse order. In the cat's sciatic the 

 first sign of the return of the phosphorus was seen with the beginning 

 of the normal myelin reaction about the sixtieth day after section. 

 At the one-hundredth day the phosphorus content was almost as 

 great as that of the normal nerve (a little under i per cent, of the 

 solids for the regenerated, as compared with a little over i per cent, 

 for the normal nerve). 



It is not as yet well understood how the regenerating fibres are 

 directed in their growth, so that they join their centres to the appro- 

 priate end-organs without mistake. That they have a high capacity 

 for finding their way is indicated by the results of cross-suturing 

 such nerves as the median and ulnar i.e., of uniting the central end 

 of the one with the peripheral end of the other. Howell and Huber 

 found that after this operation in the dog, both co-ordinated volun- 

 tary motion and sensation returned in large measure in the parts 

 supplied by the nerves. Here the motor fibres of the median nerve 

 must, of course, have made connection with muscles previously 

 supplied by the ulnar, being guided to them along the nerve-sheaths 

 of the latter. Doubtless the old nerve-sheaths serve to some 

 extent as mechanical guides by offering to the new axons a path of 

 least resistance. And when a nerve-trunk containing motor and 

 sensory fibres is simply crushed so as to destroy all physiological 

 continuity, but is not cut, no distortion of the motor and sensory 

 ' patterns ' of the nerve in other words, no ' straying ' of the fibres 

 from their old paths can be detected on regeneration. When the 



