THE STRUCTURE OF NERVE FIBRES 



251 



interlacing processes making up a central nervous system. All nerves are alike in possess- 

 ing as their conducting part the continuous strand of protoplasm 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-fibrils are supposed to be continuous throughout the cell and the axis 



FIG. 101. Medullated nerve fibres, showing continuity of the neuro-fibrils across 



the node of Ranvier. (BETHE.) 

 a, longitudinal ; &, transverse section. 



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 conduct or to the character of 

 their parent cell. Thus all the fibres which are given off from and which enter the 

 central nervous system, i.e. the brain 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 sur- 

 rounded 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 break is the node of Ranvier, the intervening 

 portions .of medullated nerve being the internodes. In each internode, lying closely 

 under the neurilemma, is an oval nucleus embedded in a little granular protoplasm. 

 The medullated nerve fibres vary considerably in diameter, the largest fibres being 

 distributed to the muscles and skin, the smallest carrying impulses from the central 

 nervous system to the viscera. The latter all come to an end in some collection of 

 ganglion-cells of the sympathetic chain or peripheral ganglia, the impulses being carried 

 on to their destination by a fresh relay of non-medullated nerve fibres. 



