268 



THE NERVOUS SYSTEM 



B 



D 



^ 



A 



A 





4- 



from above B (1, 2 and 3) undergo complete degeneration 

 and disappear. Section is then made between B and C, 

 and three weeks later the newly-degenerated fibres (4, 5 

 and 6) arising from B will be visible lower down on 



staining with osmic acid. 



4. Retrograde Degeneration. — 

 When a nerve-fibre is cut, though 

 the proximal part does not un- 

 dergo Wallerian degeneration, the 

 cell-body undergoes a diminution 

 in size and chromatoly sis or failure 

 of formation of the Nissl bodies 



A — changes which can be readily 



{yA made out by staining with methyl- 

 ' ^ ene blue. In this way it is pos- 

 sible, for example, to find out from 

 what cells in the cord a motor 

 nerve arises. 



5. Histological Method. — The 

 tissue is stained in bulk with 

 methylene blue or silver nitrate. 



The main tracts are the follow- 

 ing:— 



Descending Tracts : — 



1. Pyramidal Tracts. — These 

 arise from large cells (Betz cells) 

 situated in the motor or 2)re- 

 Rolandic area of the cerebral cortex. 

 As they pass inwards they form a 

 converging mass of fibres — corona 

 radiata. They then form in turn the posterior hmb of the 

 internal capsule and the middle part of the crus cerebri. 

 In the pons some of the fibres end by arborising around 

 the nuclei pontis, the fibres of which pass transversely to 

 the cerebellum in the middle peduncle. These transverse 

 fibres break up the main tract into a number of bundles, 



I 



A 



^ 



^ 



% 6 



/\ 



Fig. 42.— To show the 

 method of successive 

 degeneration. 



