THE CENTRAL NERVOUS SYSTEM. 



45 



The chemical changes are equally marked. The complex lipoids 

 which compose the myelin are broken down with the formation of 

 simpler substances. Lecithin is split up into a nitrogenous base, 

 choline, a fatty acid, which is usually oleic acid, phosphoric acid, and 

 glycerol. In consequence of these chemical changes it is possible at 



FIG. 8. Degeneration and regeneration of nerve fibres in the rabbit. (Ranvier. ) 

 From Schafer's Essentials of Histology. 



A, part of a nerve fibre in which degeneration has commenced in consequence of the section, 

 fifty hours previously, of the trunk of the nerve higher up ; my, medullary sheath becoming 

 broken up into drops of myelin ; p, granular protoplasmic substance which is replacing the 

 myelin ; n, nucleus ; </, neurolemma. B, another fibre in which degeneration is proceeding, 

 the nerve having been cut four days previously ; cy, axis-cylinder partly broken up. C, more 

 advanced stage of degeneration. D, commencing regeneration of a nerve-fibre. Several 

 small fibres, t', t", have sprouted from the somewhat bulbous cut end, 6, of the original 

 fibre, t ; a, an axis-cylinder which has not yet acquired its medullary sheath ; *, s', neuro- 

 lemma of the original fibre. 



this stage to distinguish degenerating from normal fibres by means 

 of Marchi's fluid, which is a mixture of osmic acid with potassium 

 bichromate. This fluid stains degenerating myelin black, but leaves 

 normal nerve fibres unaffected. When all the myelin has been 

 absorbed, the completely degenerated fibre no longer stains with 

 Marchi's fluid ; hence this method of identifying degenerating nerve 



