234 PHYSIOLOGY CHAP. 



morphological alteration there is a chemical metamorphosis of 

 the myelin which, probably owing to the formation of fat, now 

 stains black with osmic acid, after the nerve has been mordanted 

 in a chrome solution. Marchi's method of distinguishing between 

 the healthy and the degenerated nerve-fibres is based on this 

 chemical change of the myelin sheath (Fig. 148, A-J). 



Many hold, on the strength of Banvier's studies, that the 

 fragmentation and fatty degeneration of the myelin is accom- 

 panied by a multiplication of the nuclei of the neurolemma, 

 and increase of its protoplasm, which interrupts the continuity 

 of the medullary sheath. The axis-cylinder, too, is broken up 

 by the same process as the myelin, i.e. by increase of the 

 protoplasm at the level of the nuclei of the interannular segments. 

 But according to the recent and more accurate work of Bethe 

 and Monckeberg the degenerative alteration of the axis-cylinder 

 precedes the other changes, and takes place pari passu with the 

 diminution and loss of excitability in the nerve. First, the 

 fibrils of the axis-cylinder stain less readily ; next, they fuse 

 into a compact cord, which looks knotted in places, and also 

 shows large fusiform nodules; lastly, they break up and then 

 dissolve into a detritus of colourless granules. The acute period 

 of the degenerative process is followed by a slow stage, in which 

 the products of disintegration are absorbed (by phagocytosis ?) 

 so that they entirely disappear after three to four weeks. When 

 clear of the degeneration products the fibres of the nerve are 

 seen as strands filled with large fusiform elements, which are 

 derived from the cells of the neurolemma. What part do these 

 spindle-shaped elements play in the regeneration of the nerve ? 



The regenerative process in the divided nerve proceeds to a 

 large extent along with the degenerative, to which it is the active 

 reaction, directed to the morphological and functional recovery 

 of the injured nerve. 



There are two principal theories to explain the process of 

 nerve regeneration, which are related to the two fundamental 

 conceptions of the morphological structure of the nervous 

 system discussed earlier in this chapter. 



In correspondence with the neurone theory, many authors 

 hold that the regeneration of the axis-cylinders in the peripheral 

 end of the cut nerve is due exclusively to an outgrowth of the 

 axis-cylinders of the central end. These increase in size, become 

 bulbous at their extremities, and send out fibrils in a centrifugal 

 direction, which pierce the cicatricial tissue that has united 

 the two stumps, and then penetrate the old neurolemmal sheaths, 

 or grow along them until they reach their peripheral termination. 

 Ranvier, Vanlair, Strobe are the chief promoters of this theory 

 (Fig. 148, K, L). 



The other conception of the regenerative process in nerve 



