Demyelination 103 



stroyed. Schwann cells proliferate and the nerve sheaths thicken 

 to become Schwann tubes along which grow the new axon tips 

 from the intact central stump. 



Myelination spreads down the nerve in a peripheral direction 

 and eventually the myelin sheaths of the regenerated nerve fibres 

 are closely similar to those of a normal nerve. 



After section the water content of the nerve increases, reaching 

 a maximum, in the cat, in thirty-two days and returning to normal 

 by 144 days. The total lipid content of the nerve decreases during 

 the first sixteen days, remains constant up to the forty-eighth day 

 and then increases gradually. Even after 144 days the concentra- 

 tion of total lipid is less than that of corresponding normal nerves. 

 Neutral fat decreases rapidly but is normal once again by forty- 

 eight days. The myelin lipids— cerebroside, free cholesterol and 

 sphingomyelin— decrease during the first thirty-two days, remain 

 constant until the ninety-sixth day after which they increase gradu- 

 ally. By 144 days myelin-lipid concentration in the cat's sciatic 

 nerve is 44 per cent of the normal value. Both total and free choles- 

 terol decrease during the first thirty-two days, free cholesterol more 

 rapidly than total, for cholesterol ester, absent in normal control 

 nerves, appears during this period. Free cholesterol behaviour 

 parallels that of the myelin lipid but cholesterol disappears by 

 144 days. Total phospholipin decreases during the first thirty-two 

 days, then remains fairly constant until after ninety-six days when 

 it increases gradually. Sphingomyelin follows a similar course, but 

 cephalin decreases more rapidly and lecithin more slowly. The 

 concentration of the latter is similar at 144 days to that at sixty- 

 four days (Burt et ah, 1950) . These changes are very like those 

 that occur during autolysis. They suggest that a phospholipid- 

 splitting mechanism is concerned in both cases. 



DEMYELINATION 



Demyelinating diseases are common disorders of the central 

 nervous system in which the myelin sheath of the white matter is 

 lost. They have many causes that range from vascular occlusion 

 leading to cerebral softening, neoplastic compression, Wallerian 

 degeneration due to loss or damage of the neurones, to ascending 



