144 



THE TISSUES 



the distal part of a nerve when that nerve is cut across has long been accepted 

 as one of the fundamental laws of neuropathology (law of Wallerian degenera- 

 tion). In terms of the neurone concept this would mean that an axone cut off 

 from its cell of origin degenerates and disappears and this behavior of the axone 

 would accord with the already stated fact that the cell body is the trophic center 

 of the neurone. The degenerative changes in the axone are best shown in their 

 earlier stages by an osmic acid (p. 7) or by a Marchi (p. 35) stain. Later, 

 when the degenerated fibres have been largely replaced by connective tissue, the 

 Wcigert method is most satisfactory, especially in the central nervous system. 

 In this case, of course, the degeneration is indicated by an absence of stain, while 

 in the Marchi method a positive picture of the degenerating myelin sheaths is 



A 



B 



C 



' Fig. 88. — A, Normal Nerve Fibres from Sciatic Nerve of Rabbit, osmic acid fixa- 

 tion and stain; each fibre shows node of Ranvier. B, Two fibres from distal part of 

 rabbit's sciatic five days after cutting the nerve; shows segmentation of myelin; C, three 

 fibres from distal part of rabbit's sciatic three weeks after cutting nerve; most of the 

 myelin has been absorbed and only traces of the axones remain. 



seen. When the nerve is cut the first changes affect the cut ends and seem to be 

 of a traumatic character such as formation of bulbous enlargements of the ends 

 of the axones, increase and separation of neurofibrils and formation of sprouts. 

 This phase is soon passed over in the distal stump, while in the central it may pass 

 over into a beginning regeneration of the nerve fibres. In the distal portion a 

 degeneration of the nerve fibres throughout their length and including their termi- 

 nal arborization now takes place. All parts of the nerve .fibre are affected. In 

 the medullary sheath the changes consist in segmentation of the sheath, break- 

 ing up of the segments into granules and finally complete absorption (Fig. 88). 

 While undergoing these physical changes chemical changes arc also taking place 

 in the myelin which result in its breaking down into simpler fatty substances 

 which give the fat reaction to the Marchi stain. At the same time the neuro- 

 fibrils become irregular and granular and the axis-cylinders finally disappear. 



