NERVOUS TISSUE. 



105 



The contour of the fibre becomes irregular and the myelin 

 sheath shows interruptions in its course. With perosmic acid 

 a characteristic picture is produced. This acid colors only the 

 fat-containing medullary sheath. The interruptions are thus 

 left unstained. These are called Schmidt- Lantcrmann lines or 

 funnels, and Ranvier' s nodes (Fig. 75). 



FIG. 75. 



FIG. 76. 



Piece of a niedu Hated nerve fibre from the tiervus ischiadicus of the frog. A node of 

 Ranvier (b) and the lines of Schmidt-Lantermann (a) are shown, x 370. 



The former appear on optical section as oblique lines run- 

 ning down to the axis cylinder, and are therefore funnel- 

 shaped. They thus divide the myelin sheath into cylindro- 

 conical segments. The apices of these segments may be 

 directed either toward the cell or away from it. Some authors 

 regard these lines as artifacts produced by fixing reagents. 

 Others are of the opinion that they occur 

 normally, and that they are composed of 

 a different substance from the myelin. 



The Ranvier's nodes (Fig. 75) are 

 large annular interruptions which divide 

 the fibres into what are known as inter- 

 annular or Ranvier 's segments. The ab- 

 sence of the myelin at these nodes is so 

 distinct that the axis cylinder becomes 

 joined with the neurilemma by a cement 

 substance. These nodes are probably of 

 use in the nourishment of the axis cylin- 

 der, for at these places nutritive fluids 

 could pass more easily into the centre of 

 the fibre. By treating the nerve with 

 silver nitrate and reducing this in the sunlight, we often obtain 

 a brown striation on the axis cylinder (Fromman's silver line], 

 which is usually considered as an artifact. This striation is 

 especially marked at the nodes of Rauvier, and becomes less 



Medullated nerve fibres of 

 a rabbit, treated with silver 

 nitrate. The crosses of Ran- 

 vier are shown. X 300. 



