98 NERVE [CH. VII. 



probably mesoblastic. The medullary sheath is formed, according to 

 some, by a fatty change occurring in the parts of these same cells 

 which are nearest to the axis cylinder, but it is much more probable 

 that it is formed from the peripheral layer of the axis cylinder ; the 

 presence of neurokeratin in it distinctly points to an epiblastic origin. 

 The fact also that, in the nerve centres, the medullated nerve-fibres 

 have no primitive sheath, and the phenomena of Wallerian degenera- 

 tion, to be described later, all tend to confirm the same view. 



