STRUCTURE OF NERVE-FIBRES 



71 





FlG. 82. A SMALL PART OF A MEDUL- 

 LATED FIBRE, HIGHLY MAGNIFIED. 



The fibre looks in optical section like a tube 

 hence the term tubular, formerly applied to 

 these fibres. Two partial breaches of con- 

 tinuity are seen in the medu'lary sheath, 

 which at these places exhibits a tendency to 

 split into laminae. The primitive sheath is 

 here and there apparent outside the medul- 

 lary sheath, and the delicate striae which, 

 are visible in the middle of the fibre pro- 

 bably indicate the fibrillated axis-cylinder. 



FIG. 84. Two PORTIONS OF MEDULLATED 



NERVE- FIBRE*, AFTER TREATMENT 

 WITH OSMIC ACID, SHOWING THE AXIS- 

 CYLINDER, AND THE MEDULLARY AND 

 PRIMITIVE SHEATHS. 



A. Node of Ranvier. B. Middle of an inter- 

 node with nuc eus. c. axis-cylinder, pro- *? C o VT^ 

 jecting ; p, primitive sheath, within which K IG ' ed ' ~~ 



the medullary sheath, which is stained dark 

 by the osmic acid, is somewhat retracted. 



FIBRE STAINED 

 WITH OSMIC ACID. 



course of the peri- 

 pheral nerve -fibres, 

 the axis-cylinder at 

 these places being 

 encompassed only by 

 the primitive sheath. 

 Hence the primitive 

 sheath appears at 

 these spots to pro- 

 duce a constriction 

 in the nerve-fibre, 

 and the interruptions 

 of the medullary 

 sheath are accord- 

 ingly known as the 

 constrictions or nodes 

 of Ranvier (fig. 81, 

 B, R ; fig. 83, L), the 

 term nodes being ap- 

 plied from the resem- 

 blance which they 

 bear to the nodes 

 of a bamboo. The 

 length of nerve be- 

 tween two successive 

 nodes may be termed 

 an internode ; in the 

 middle of each inter- 

 node is one of the 

 nuclei of Schwann's 

 sheath. Besides 



these interruptions 

 the medullary sheath 

 shows a variable 

 number of oblique 

 clefts (fig. 83) which 

 subdivide it into 

 irregular portions, 

 which have been 

 termed medullary 

 segments, but there 

 is reason to believe 

 that the clefts are 

 artificially produced. 

 Osmic acid stains the 

 medullary sheath 



black. 



