THE NERVOUS TISSUES. 



181 



black. Occasionally, a peculiar transverse striation is seen in the longi- 

 tudinal portions of the crosses. These are known as Frommann's lines. 

 Their origin and significance have not as yet been satisfactorily ex- 

 plained. 



To demonstrate the fibrils of the axial cord a piece of a small 

 nerve is stretched on a match or toothpick and fixed for four hours in a 

 0.5% osmic acid solution, after which it is washed in water for the same 

 length of time and immersed in 90% alcohol for twenty-four hours. The 

 preparation is now stained for another twenty-four hours in a saturated 

 aqueous solution of fuchsin S and 

 then placed for three days in abso- 

 lute alcohol. Finally, the nerve is 

 passed as rapidly as possible through 

 toluol, toluol- paraffin, and then im- 

 bedded in paraffin. The proper 

 orientation of the specimen is of the 

 greatest importance, as is also the 

 cutting of thin sections. In a lon- 

 gitudinal section red fibrils of almost 

 uniform thickness and evenly dis- 

 tributed throughout the axial space Axis-cylin- d 



are seen lying in the colorless neuro- 



Medullary 

 sheath. 



Fig. 148. Ranvier's crosses from sci- 

 atic nerve of rabbit treated with silver ni- 

 trate solution ; X * 2O - Frommann' s lines 

 can be seen in a few fibers. 



Fig. 149. Medullated nerve-fiber 

 from sciatic nerve of frog. In two places 

 the medullary sheath has been pulled 

 away by teasing, showing the "naked 

 axis-cylinder" ; X 2I2 - 



plasm, and parallel to the long axis of the nerve-fiber. In cross-section 

 the axial fibrils appear as evenly distributed dots. Attention must be 

 called to the fact that the fibrils are not equally well stained in all cases 

 (Kupffer, 83, II ; compare also Jacobi and Joseph). 



When the fiber is less carefully treated, the fibrils fuse with the 

 neuroplasm to form the ' ' axis-cylinder ' ' of authors. As the appearance 

 of the latter is due to a shrinkage of the contents of the axial space, it is 

 easy to understand that one reagent may have a greater effect in this re- 

 spect than another. The thinnest axis-cylinders are produced by chromic 

 acid and its salts, while thicker ones are seen in nerve-fibers fixed in 

 alcohol. These variations are best seen in cross-sections, in which the 



