66 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



NERVE FIBERS. 



While the nerve fiber is really to be considered 

 as a process of the nerve cell, it is convenient to de- 

 scribe it separately. Nerve fibers are of two kinds, 

 medullated or white fibers, and non-medullated or 

 gray fibers. 



Medullated Fibers. Each medullated nerve 

 fiber is made up of the following parts: An ex- 

 ternal sheath, called the primitive sheath, neuri- 

 lemma, or nucleated sheath of Schwann; an inter- 

 mediate, known as the medullary or myelin sheath, 

 or white substance of Schwann; and a central 

 thread, the axis-cylinder, or axial fiber. 



The Primitive Sheath. This is a pellucid mem- 

 brane forming the outer investment of the nerve 

 fiber. The sheath is constricted at intervals of a 

 millimeter or less, the nodes of Ranvier. Each in- 

 ternodal segment bears a single nucleus surrounded by a variable amount 

 of protoplasm. This membrane is described as having its origin in the 



FIG. 84. Two Nerve 

 Fibers of the Sciatic 

 Nerve. A, Node of Ran- 

 vier; B, axis-cylinder; 

 C, sheath of Schwann 

 with nuclei. X 300. 

 (Klein and Noble 

 Smith.) 



FIG. 85. A Node of Ranvier in a Medullated Nerve Fiber, viewed from above. The 

 medullary sheath is interrupted and the primitive sheath thickened. Copied from Axel 

 Key and Retzius. X 750. (Klein and Noble Smith.) 



FIG. 86. Gray, Pale, or Gelatinous Nerve Fibers. A, From a branch of the olfactory 

 nerve of the sheep; two dark-bordered or white fibers from the fifth pair are associated 

 with the pale olfactory fibers, B, from the sympathetic nerve. X 450. (Max Schultze.) 



mesoblastic cells, and the nuclei are the indications of the cellular nature 

 of each nodal segment. 



The Medullary or Myelin Sheath. This is the part to which the peculiar 



