218 



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 cylinder. 

 It is wanting at the origin of the 

 fibers in the centers and at their 

 peripheral distribution. It is prob- 

 ably not necessary to conductivity. 

 In fresh nerves this substance is 

 'strongly refractive, 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 

 myeline sheath acts as an insulator 

 lacks supporting evidence. The 

 theory that it is nutritional is 

 plausible; but no sufficient differ- 

 ence in the medullated and non- 

 medullated fibers in this respect 

 has been found to establish the the- 

 ory as a fact. At certain points in 

 in the course of medullated fibers 

 there are seen constrictions called 

 the nodes of Ranvier. At these 

 points the medullary substance is 

 wanting and the sheath of 

 Schwann is in contact with the 

 axis cylinder. It is not improbable 

 that these nodes furnish a mode of 

 access for the nutrient plasma. 

 Certain it is that they are most 



