THE STRUCTURE OF NERVE FIBRES 2 5] 



as the posterior root ganglion, or may be one of the mass of cells and ir 

 lacing processes making up a central nervous system. All nerves are -il.k > 

 in possessing as their conducting part the continuous strand of protoplas 

 produced from the nerve-cell and known as the axon or axis cylinder By 

 special methods the axon may be shown to be made up of fibrillse or 

 neuro-fibrils, embedded in a more fluid material (Fig. 101). These neuro- 



FIG. 101. 



Medullated nerve fibres, showing continuity of the neuro-fibrils across 



the node of Ranvier. (BETHE.) 

 a, longitudinal ; &, transverse section. 



fibrils are supposed to be continuous throughout the cell and the axis 

 cylinder and to represent the essential conducting constituents of the nerve. 

 In the course of growth the nerves develop certain histological differences, 

 which appear to bear some relation to the nature of the processes they con- 

 duct or to the character of their parent cell. Thus allthe fibres which are 

 given of! from and which enter the centrajjiexs&wwrsystem, i.e. the ^rain and 

 spinal cord, belong to the class known as medullated. In this type the 

 conducting core or axis cylinder is surrounded with a layer of apparently 

 insulating material known as myelin, forming the medullary sheath, or the 

 sheath of Schwann. This sheath consists of a fatty material composed largely 

 of lecithin, and staining black with osmic acid, supported in the interstices 

 of a network formed of a horny substance known as neurokeratin. The 

 medullary sheath is surrounded by a structureless membrane, the primitive 

 sheath or neurilemma. At regular intervals a break occurs in the medullary 

 sheath, the neurilemma coming in close contact with the axis cylinder. This 



