iv GENEKAL PHYSIOLOGY OF NEKVOUS SYSTEM 233 



days with an opposite regenerative process. This finally leads to 

 the gradual recovery of function in the nerve, and so of sensibility 

 and motility in the region which it innervates. 



These morphological studies of the degeneration and re- 

 generation of nerves severed 

 from their centres were 

 begun by Steinbriick (1838), 

 Nasse (1839), Giinther and 

 Schon (1840) ; but they only 

 acquired significance after 

 the discovery of the so-called 

 trophic centres of the spinal 

 roots by Augustus Waller in 

 1852. 



We must here confine 

 ourselves to a summary of 

 the changes produced by 

 severing the fibres of a mixed 

 peripheral nerve from their 

 trophic centres. Two to four 

 days after section the whole 

 peripheral part of the nerve 

 and a short length of its 

 central portion (according to 

 Engelmann to the nearest 

 node of Kanvier, but accord- 

 ing to other authors as far 

 as the second or third node) 

 begins to undergo a process 

 of degeneration, which is 

 easily traced under the micro- 

 scope, and which leads to the 

 disintegration of the fibres. 

 It is usually held that the 

 degenerative process does not 

 advance progressively from 

 the seat of the lesion towards 

 the periphery, but that it 

 appears simultaneously 

 throughout the whole distal 

 portion. The rapidity of the 

 degenerative process is greater in young than in old animals, 

 in strong than in weak, in warm-blooded than in the cold- 

 blooded. 



The most apparent change occurs in the myelin sheath of 

 the fibres, which undergoes progressive fragmentation till it 

 is reduced to small irregular lumps or drops. Along with this 



FIG. 148. Degeneration and regeneration of nerve- 

 fibres. (Banvier.) A, rabbit's sciatic four days 

 after section; B, C, the same fifty hours after 

 section ; D, fibre stained with carmine only, to 

 show axis -cylinder; F, G, pigeon's fibres three 

 days after section ; H, two fibres of rabbit's vagus 

 six days after section ; J, lymph cell from inter- 

 fibrillary connective tissue, containing ingested 

 globules of myelin. Throughout the figure, n, n, 

 are nuclei ; x, x, myelin broken up by increase 

 of the protoplasm ; ac, axis - cylinder ; K, L, re- 

 generation of nerve-fibres ; H, of rabbit's vagus 

 seventy-two days after section ; L, of rabbit's 

 sciatic ninety days after section ; e, conical ending 

 of white matter of central end of the nerve ; s, 

 sheath ; ria, new axis - cylinder. L shows two 

 globules of myelin left over from the degeneration 

 of the old fibre. 



