NERVE INJURIES 133 



nerves. If it is allowed that in the embryo the nerves 

 grow out from the central nervous system, the theory 

 of ^central regeneration is placed upon a strong basis, 

 and indeed it is now almost universally accepted, 

 whereas ten years ago it was losing favour. 



Two questions of great interest have recently 

 received answers. First, Why does the medullary 

 sheath of a nerve fibre break up into fatty droplets 

 when it is cut off from its trophic centre, that is, 

 from its cell of origin in the central nervous system ? 

 Second, How does the budding axis cylinder of the 

 central end of a divided nerve manage to find its 

 way so accurately along the old path ? 



The questions are intimately related. Each fur- 

 nishes the answer to the other. The medullary sheath 

 breaks up that it may liberate the chemical substance 

 which attracts the sprouting axis cylinder. The new 

 fibre follows the old path, because of the chemical 

 attraction along that path. 



Nature is full of analogies to this process of chemical 

 attraction. Chemical particles, though infinitely 

 diluted with air or soil, attract the vulture to the 

 corpse in the desert, or the bloodhound to the hunted 

 criminal. Smell is only a chemical analysis. Simi- 

 larly, the leucocytes crowd out of the vessels to an 

 inflamed area, in obedience to a law of chemical 

 attraction. 



If two celloidin tubes are presented to the central 

 end of a divided nerve, the one containing emulsion 

 of liver, and the other emulsion of brain, all the 

 sprouting fibres pass into the brain emulsion, none 

 into the tube containing liver (Forssman). The 



