STRUCTURE OF NEUROGLIA. 97 



which it has grown. This separated part of the axis-cylinder dies, and 

 its medullary sheath undergoes a gradual process of disintegration into 

 droplets of myelin, which is known as the Wallerian degeneration (fig. 

 113, A to C), and which in man and mammals begin about two or 

 three days after section of the nerve. Therefore when a peripheral 

 nerve is cut, all the nerve-fibres distal to the point of section must 

 degenerate, because all have grown from and are connected with nerve- 

 cells in or near the nerve centre the afferent fibres with the cells of 

 the ganglion on the posterior root, the efferent fibres with the cells of 

 the anterior horde of the spinal cord. 



If regeneration takes place in the cut nerve, it is effected not by a 

 re-establishment of connection between the degenerated fibres and those 

 of the central stump (which are not degenerated), but by an outgrowth 

 of new fibres from the stump (fig. 113, D) ; these may find their way 

 to the periphery along the course of the degenerated fibres. If they 

 succeed in doing so, the continuity and conducting power of the nerve 

 becomes restored. 



In the brain and spinal cord the nerve-cells and nerve-fibres are 

 supported by a peculiar tissue which has been termed the neuroglia. 

 It is composed of cells and fibres, the latter being prolonged from the 

 cells. Of the fibres some are radially disposed. They start from the 

 fixed ends of the ciliated epithelium-cells which line the central canal of 

 the spinal cord and the ventricles of the brain, and pass in a radial 

 direction, slighting diverging as they proceed, and constantly branching 

 towards the surface of the organ, where they end in slight enlargements 

 attached to the pia mater. The other fibres of the tissue are cell-pro- 

 cesses of the neuroglia- or glia-cells proper (spider-cells). These cells 

 are stellate in shape (fig. Ill, n], and their fine and frequently ramifying 

 processes pass as neuroglia-fibres between the nerve-cells and nerve- 

 fibres which they aid in supporting. 



