MINUTE STRUCTURE OF THE NERVOUS TISSUES 35 



belongs, its most distant portion is yet dependent upon 

 the influence of the perikaryon for its preservation in good 

 condition. Cut off from its perikaryon it inevitably 

 degenerates. This is a fact which has proved most help- 

 ful to the investigators who have sought to unravel the 

 tangled fabric of the nervous system. 



Regeneration. When a nerve has been cut and de- 

 generation has taken place, regeneration remains a possi- 

 bility. This requires months for its accomplishment. 

 New axons from the old perikarya grow along the track 

 of the former set. It seems marvellous that they should 

 push through to make appropriate connections, but they 

 often do. Of course, this may be prevented by obstacles 

 in the path; if the two ends of the cut nerve were dis- 

 placed laterally instead of remaining in end-to-end con- 

 tact the outgrowth of the fresh fibers would not occur. 

 Unfortunately for the human race, the reconstruction of 

 nerve-fibers which we observe outside the cord and brain 

 does nol take place in the central axis. Injuries which 

 result in degeneration within the confines of the central 

 nervous system are anatomically irreparable. Physio- 

 logically, the accompanying losses are sometimes made 

 good by the substitution of other paths for those which 

 have suffered interruption. 



