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A MANUAL OF PHYSIOLOGY 



region supplied by the nerve ; but for a time the nerve remains 

 excitable to direct stimulation. The excitability gradually 

 diminishes, and in a few days is completely gone. If portions of 

 the nerve distal to the lesion are examined at different periods 

 after section, a remarkable process of degeneration (commonly 



spoken of as Wallerian de- 

 generation) is seen to be going 

 on. In the medullated fibres 

 this begins on the second or 

 third day with a swelling of 

 the axis-cylinder, which breaks 

 up into detached pieces (frag- 

 mentation), and assumes a 

 granular appearance. The 

 medullary sheath also under- 

 goes fragmentation at the lines 

 of Lantermann, and a little 

 later separates into clumps 

 and droplets of myelin. The 

 nuclei under the neurilemma 

 increase in size, proliferate by 

 mitosis, and insinuate them- 

 selves between the fragments 

 of the medullary sheath and 

 axis-cylinder, which ultimately 

 disappear, leaving the nerve- 

 fibre represented only by a 

 kind of mummy of connective 

 tissue, in which the neuri- 

 lemma with its abnormally 

 numerous nuclei can still be 



FIG. 256. DEGENERATION OF NERVE- 

 FIBRES AFTER SECTION (BARKER, AFTER 

 THOMA). 

 I, normal fibre ; II, degenerating fibre; 



III, further stage of degeneration ; 



A, axis-cylinder ; L, Lantermann's line 

 or cleft ; R, node ; mt, drops of myelin ; 

 a, remains of axis-cylinder ; w, prolifera- 

 ting cells of neurilemma. 



recognised. The protoplasm 

 around the nuclei of the 

 neurilemma also increases in 

 amount, and undergoes other 

 changes, which will be more 

 particularly referred to in 

 describing the regeneration 



of nerve. The degenerative 

 process begins near the cut end, and extends gradually to 

 the periphery, and more' rapidly in warm than in cold- 

 blooded animals. At any rate, that is the interpretation generally 

 given to the fact that at a given period after section the 

 changes especially the breaking-up of the myelin are more 

 pronounced near the proximal end of the peripheral stump. In 

 a mammal degeneration is far advanced in a fortnight, although 



