STRUCTURE OF NERVE-FIBRES. 



85 





being encompassed 

 only by the primi- 

 tive sheath. Hence 

 the primitive sheath 

 appears at these 

 spots to produce a 

 constriction in the 

 nerve-fibre, and the 

 interruptions of the 

 medullary sheath are 

 accordingly known 

 as the constrictions or 

 mules of Banner (fig. 

 98,R,R; fig. 100, L), 

 the term nodes being 

 .applied from the re- 

 semblance which they FlG. 99. A SMALL PART OF A MEDUL- 

 LATED FIBRE, HIGHLY MAGNIFIED. 



bear to the nodes 



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 outside the medul- 

 lary sheath, and the delicate stria? which 

 ai-e visible in the middle of the fibre pro- 

 bably indicate the fibrillated axis-cylinder. 



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 (figs. 99, 100), 

 which subdivide it 

 into irregular por- 

 tions, which have 

 been termed medul- 



lary segments, but FIG. 101. -Two PORTIONS OF MEDULLATED 

 there is reason to NERVE-FIBRES, AFTER TREATMENT 



WITH OSMIC ACID, SHOWING THE AXIS- 



believe that the clefts CYLINDER, AND THE MEDULLARY 



SHEATHS ' (Key and 



are artificially pro- 



duced. Osmic acid A, node of Ranvier. B, middle of an inter- 

 c;tflinc tViA morliillayTr node with nucleus. c, axis-cylinder, pro- 

 lliary jecting ; p, primitive sheath, within which 

 the medullary sheath, which is stained dark 

 by the osmic acid, is somewhat retracted. 



FIG. 100. NERVE- 

 FIBRE STAINED 

 WITH OSMIC ACID. 



(Key&Retzius.) 



