148 



PHYSIOLOGY OF NERVE 



[CH. XIV. 



A great amount of attention has been directed to this process of 

 degeneration, because it has formed a valuable method of research in 

 tracing nervous tracts, and ascertaining the 

 nerve-cells from which they originate. It 

 must not, however, be regarded as an isolated 

 phenomenon in physiology ; it is only an illus- 

 tration of the universal truth that any portion 

 of a cell (in this case the axis-cylinder process) 

 cut off from the nucleus of the cell degenerates 

 and dies. 



Regeneration of Nerve-Fibres. 



If a nerve is cut and allowed to heal, 

 restoration of function occurs after the lapse 

 of a variable time, which can be shortened if 

 the cut ends of the nerve are sutured together. 

 This surgical assistance is of special import- 

 ance when the nerve is a large one, and the 

 formation of dense cicatricial tissue between 

 the ends is thus minimised. The restoration 

 of function is due to regeneration of nerve- 

 fibres, which sprout out from the central end 

 of the cut nerve and grow distalwards, replacing 

 those which have degenerated. The new fibres, 

 which are the earliest to appear, are of a much 

 narrower diameter than those they replace ; 

 this is illustrated in fig. 153, D. Later the 

 It is obvious that a mass of dense scar tissue 



will hinder the successful growth of the nerve-fibres. When regene- 

 ration does not take place, the central ends of the cut fibres and the 

 cells from which they originate undergo slow atropic changes (disuse 

 atrophy). 



The view that in the embryo each nerve-fibre develops as an 

 outgrowth from a nerve-cell, and grows in a distal direction, finally 

 becoming united to other tissues in the periphery of the body, is 

 associated especially with the name of His, and has been accepted 

 by the majority of embryologists. There have been other views held, 

 but it will be sufficient to mention only one of these, for it is the one 

 which, next to that of His, has been favoured by investigators. 

 Briefly it is as follows : the nerve-fibre is not a secondarily formed 

 bridge between the central nervous system and the peripheral 

 organs, but exists from the very first, and in subsequent develop- 

 ment it merely undergoes elaboration, and increases in bulk and 

 in length as the distance from the central nervous system and 



1. 154. Degenerated nerve- 

 fibres at an early stage, 

 where the fragmentation 

 of the medullary sheath 

 into myelin droplets is 

 well shown. Stained by 

 osmic acid. (S. Martin.) 



new fibres are larger. 



