468 



THE NERVOUS SYSTEM. 



Fig. 126.* 

 B c 



undergo changes which make it probable that their con- 

 tents are composed of two different materials. The internal 

 or central part, occupying the axis of the tube, becomes 

 greyish, while the outer, or cortical portion, becomes 

 opaque and dimly granular or grumous, as if from a kind 

 of coagulation. At the same time, the fine outline of the 

 previously transparent cylindrical tube is exchanged for a 

 dark double contour (fig. 126, B), the outer line being 

 formed by the sheath of the fibre, the inner by the margin 

 of curdled or coagulated medullary substance. The gra- 

 nular material shortly collects 

 into little masses, which distend 

 portions of the tubular mem- 

 brane, while the intermediate 

 spaces collapse, giving the fibres 

 a varicose, or beaded appearance 

 (fig. 126, c ando), instead of the 

 previous cylindrical form. 



The difference produced in the 

 contents of the nerve-fibres when 

 exposed to the same conditions, 

 has, with other facts, led to the 

 opinion now generally adopted, 

 that the central part or axis- 

 cylinder of each nerve-fibre differs 

 from the outer portion. The 

 outer portion is usually called the medullary or white 

 substance of Schwann, being that to which the peculiar 

 white aspect of cerebro- spinal nerves is principally due. 

 The whole contents of the nerve-tubules appear to be ex- 

 tremely soft, for when subjected to pressure they readily 



* Fig. 126. Primitive nerve-tubules. A. A perfectly fresh tubule with 

 a single dark outline. B. A tubule or fibre with a double contour from 

 commencing post-mortem change, c. The changes further advanced, 

 producing a varicose or beaded appearance. D. A tubule or fibre, the 

 central part of which, in consequence of still further changes, has 

 accumulated in separate portions within the sheath (after "Wagner). 



