70 



THE ESSENTIALS OF HISTOLOGY 



this is not present in all medullated fibres, being absent in tnose which 

 are within the nerve-centres. 



The medullary sheath is composed of a highly refracting fatty 



A B 



FIG. 80. WHITE OR MEDULLATED 

 NERVE -FIBRES, SHOWING THE 

 SINUOUS OUTLINE AND DOUBLE 



CONTOURS. 



FIG. 81. PORTIONS OF TWO NERVE- FIBRES 



STAINED WITH OSMIC ACID (FROM A 



YOUNG RABBIT). (425 diameters.) 



R, R. Nodes of Ranvier, with axis-cylinder 

 passing through. , primitive sheath of 

 the nerve, c, opposite the middle of the 

 segment, indicates the nucleus and proto- 

 plasm lying between the primitive sheath 

 and the medullary sheath. In A the nodes 

 are wider, and the intersegmental substance 

 more apparent than in B. 



material, which gives a characteristic dark contour and tubular appear- 

 ance to the nerve-fibres. It affords a continuous investment to the 

 axis-cylinder, except that it is interrupted at regular intervals in the 



