694 A MANUAL OF PHYSIOLOGY 



Regeneration of Nerve. Degeneration of nerve is followed, 

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

 regeneration, 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 de- 

 generated fibres, ultimately acquire a medullary sheath, and 

 develop into complete nerve-fibres, restoring first sensation, and 

 later on voluntary motion, to the paralyzed part. Or they may 

 possibly unite with imperfect fibres developed in the peripheral 

 stump. 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 complete 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 fibre? 

 are directed in their growth, so that they join their centres to 

 the appropriate 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 voluntary 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 



