GENERAL HISTOLOGY OF THE NERVOUS SYSTEM. 113 



black (Frommanrfs lines). It appears probable, as Ranvier 

 explains, that, owing to the break in the myeline, at the * c an- 

 nular constriction," the particles of silver gain an entrance to 

 the axis-cylinder at this the only unprotected spot. If the 

 action of the salt is long con- 

 tinued, the axis-cylinder is col- 

 ored for a somewhat longer 

 distance. The transverse bar 

 seems to be formed of two 

 conical segments set base to 

 base. The position of this bi- 

 conical segment usually cor- 

 responds in position with the 

 "annular constriction," but 



i t won 1 rl n rmpa v fh a f fh PV m n v FlG ' 45< ~ a ' Ranvier ' s disk 5 & ' Frommann's lines ; 

 11 WOUld clppedT tlldt tliey nitty c , nucleus of interannular segment. 



be separated, for, when the 



tissue of the nerve has been put upon the stretch, the biconical 

 segment may be drawn away from the annular constriction. 

 (See Fig. 45.) 



Now, as Schwann's sheath is understood to end at the an- 

 nular constriction, where it is cemented to the next adjoining 

 segment just as epithelial cells are joined together, the biconi- 

 cal disk may belong to the axis-cylinder exclusively, and 

 merely constitute a dividing line between its segments. Ac- 

 cording to Engelmann, the axis-cylinder is divided up into 

 portions corresponding with the interannular segments. 



According to Bawitz, Schwann's sheath does not end at the nodes, but is 

 continuous with the sheath of the adjacent interannular segment. 



Staining of the nerve in osmic acid semi-desiccation. 

 Osmic acid is one of the most valuable reagents for histological 

 work, and the method now to be described (a modification of 

 KanvierV) succeeds well. Take the frog's sciatic, or any other 

 peripheral nerve, carefully remove a portion with the surround- 

 ing tissue, keep the whole extended with pins, upon a flat bit 

 of cork, and then dip it into a vessel containing a 1 per cent, 

 watery solution of osmic acid. 2 The vessel is then to be exposed 

 to the light. The whole nerve will be more or less thoroughly 



1 Lemons sur 1'Histologie du Systeme Nerveux, Paris, 1878. 



a The solution should, of course, have been kept in a dark bottle away from the 

 light. 



