2l8 



THE NERVOUS SYSTEM 



Node of Ranvier' 



Primitive sheath. 



Nerve corpuscles. 



Axis cylinder. 



White substance, 

 of Schwann. 



Node of Ranvier. 



FIG. 62. Scheme of a 

 medullated nerve fiber of a 

 rabbit acted on by osmic 



acid. 



The incisures are omitted. 

 X 400. (Landois.) 



It is the white substance of Schwann 

 which gives to the nerve its peculiar 

 whitish appearance. This is a fatty 

 substance of a semi-fluid consistence. 

 It fills the tube made by the sheath of 

 Schwann and surrounds the axis cylin- 

 der. It is wanting at the origin of the 

 fibers in the centers and at their periph- 

 eral distribution. It is probably not 

 necessary to conductivity. In fresh 

 nerves this substance is strongly refrac- 

 tive, and the optical effect produced by 

 its varying thickness in the center and 

 at the edges is the appearance of dark 

 borders. It easily coagulates into an 

 opaque mass. The idea that the mye- 

 line sheath acts as an insulator lacks 

 supporting evidence. The theory that 

 it is nutritional is plausible; but no suf- 

 ficient difference in the medullated and 

 non-medullated fibers in this respect 

 has been found to establish the theory 

 as a fact. At certain points in the 

 course of medullated fibers there are 

 seen constrictions called the nodes of 

 Ranvier. At these points the medul- 

 lary substance is wanting and the 

 sheath of Schwann is in contact with 

 the axis cyclinder. It is not improb- 

 able that these nodes furnish a mode 

 of access for the nutrient plasma. 

 Certain it is that they are most numer- 

 ous where physiological activity is 

 supposed to be most active. 



