280 



PHYSIOLOGY 



from the surface into a special collection of cells such as the posterior root 

 ganglion, or may be one of the mass of cells and interlacing processes 

 making up a central nervous system. All nerves are alike in possessing 

 as their conducting part the continuous strand of protoplasm produced 

 from the nerve-cell and known as the axon or axis cylinder. By specia 1 

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

 fibrils, embedded in a more fluid material (Fig. 102). These neuro- 



FIG. 102. Medullated nerve fibres, showing continuity of the ncuro-fibrils 

 across the node of Ranvier. (BETHE.) 

 a, longitudinal ; 6, 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 histo- 

 logical 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, 



