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 medullary sheath, 

 which at these places exhibits a tendency to 

 split into laminae. The primitive sheath is 

 here and there apparent ontside the medul- 

 lary sheath, and the delicate striee which 

 are visible in the middle of the fibre pro- 

 bably indicate the fibrillated axis-cylinder. 



FIG. 84. Two PORTIONS OF MEDULLATED 



NERVE- FIBRES, 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 nucleus, c, axis-cylinder, pro- 

 jecting ; p, primitive sheath, within which 

 the medullary sheath, which is stained dark 

 by the osmic acid, is somewhat retracted. 



FIG. 83. NERVE- 

 FIBKE STAINED 

 WITH OSMIC AC11>. 



course of the peri- 

 pheral nerve-fibres, 

 the axis-cylinder at 

 these places being 

 encompassed only by 

 thejarimitive 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, 

 K, B ; 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 interned e ; 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. 



