464 THE NEKVOUS SYSTEM 



The latter are very probably produced by the glia cells, of which 

 they were formally considered to be processes. 



The glia cells, as seen in Golgi preparations, are divisable into 

 two distinct types, the ependyma cells and the astrocytes. 



The ependyma cells may be considered as undifferentiated relics 

 of the embryonal cells, from which both glia and true nerve or 

 ganglion cells were presumably developed. These cells line the 

 central canal of the spinal cord and the ventricles of the brain, in 

 which latter organ they also form the covering or outer coat of 

 the telae choroidei. 



The ependyma consists of long nucleated columnar cells whose 

 free ends, in fetal and early life, carry a tuft of cilia ; in adult life 



FlG. 367. A LONG-RATED ASTBOCYTE. 



Golgi's stain. Highly magnified. (After Berkley.) 



they are usually non-ciliated. The attached ends of these cells 

 are embedded in the surrounding gelatinous tissue, and are fre- 

 quently prolonged for some distance as a fine branched process. 

 In this way the ependyma of the spinal cord enters into the for- 

 mation of the substantia gelatinosa centralis, in which the branched 

 processes of its cells ramify in a glia-like manner. In the fetus 

 the filamentous processes extend from the central canal all the 

 way to the periphery of the spinal cord. In the adult the epen- 

 dyma cells are prone to so multiply as to almost occlude the 

 central canal ; their processes have apparently become shorter, 

 and now reach the surface of the spinal cord only at its posterior 

 median sulcus. 



