MEDULLATED NERVE FIBRES 



65 



C-- 



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 mem- 

 brane is described as having its origin in the meso- 

 blastic 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 opaque white aspect of 

 medullated nerves is due. The thickness of this 

 layer of a nerve fiber varies considerably. It is a semifluid, fatty substance 

 of high refractive power. It possesses a fine reticulum (Stilling, Klein), in 



FIG. 84. Two Nerve 

 Fibers of the Sciatic Nerve. 

 A, Node of Ranvier, 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 medul- 

 lary 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.) 



the meshes of which is embedded the fatty material. It stains well with 

 osmic acid. 



The Axis-Cylinder. The central thread of a medullated nerve fiber is 

 the axis-cylinder. It is the prolongation of a nerve-cell and extends un- 

 5 



