EPENDYMA. 



123 



canal. It appears like a simple cylindrical epithelium but the cell-like 

 bodies are the ends of strands which may extend clear across the spinal 

 cord to the external limiting membrane. A nucleus is generally found in 

 the strand near the central canal; there may be others further away. 

 Although in the embryo strands from the central canal to the periphery 

 are easily traced, in the adult these are largely broken up, giving rise to 

 cells with chief processes either to the periphery or to the central canal; 

 if the radial strand is lost on both sides, stellate neurolgia cells result. 

 These are shown in Fig. 148. (The figure also shows the neuroglia cells 

 with concentric fibers characteristic of the central gray substance, and a 



From the substantia gelatinosa of a newborn rat. 



Neuroglia cell. 



Central canal. 



Ependymal cells. 



Neuroglia cell of 

 the white substance, 

 from a cat 6 weeks 

 old. 



Concentric neuroglia cell from a cat 

 six weeks old. 



Neuroglia cell of the gray substance of the base 



of the posterior column of a human embryo. 

 FIG. 148. NEUROGLIA CELLS FROM THE SPINAL CORD. X 280. 



neuroglia strand with very numerous delicate processes from the substantia 

 gelatinosa. These processes are said to be transformed into a granular 

 substance. The gelatinous substance contains a few very small nerve 

 cells, a network of fine nerve fibers and occasional stellate neuroglia cells.) 

 The ependymal 'cells' at birth and for some time afterward possess cilia 

 projecting into the central canal. In the adult they have disappeared. 

 It is questionable whether or not they are motile. Single bodies but not 

 diplosomes have been found at their bases. They have been considered 

 to be more like the cilia of the epididymis than like those of the trachea. 

 The neurone theory. Years ago it was thought that the central 

 nervous system was a continuous network of fibers, prolongations of which 



