80 



NEEVE 



[CH. VII. 



when it passes into the white matter of the brain or spinal cord, and 

 a primitive sheath when it leaves the nerve-centre and gets into the 

 nerve. But at first the axis cylinder is not sheathed at all. 



The formation of the sheaths is still a matter of doubt, but the 

 generally accepted opinion is that the primitive sheath is formed by 

 mesoblastic cells which become flattened out and wrapped round the 

 fibre end to end. These are separated at the nodes by intercellular or 

 cement substance stainable by silver nitrate (fig. 93). 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, 



FIG. 101. Mul bipolar nerve-cell from anterior horn of spinal cord ; a, axis cylinder process. 



Sclmltze.) 



(Max 



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 degeneration, to be described later, 

 all tend to confirm the same view. 



