4i8 



HISTOLOGY 



(Fig. 424); it contains also stellate neuroglia cells, the processes of which 

 are said to become transformed into a granular substance. 



Ependyma. The ependyma is that part of the neuroglia which lines 

 the central canal. It appears like a simple columnar epithelium, but its 

 cell-like bodies are the ends of strands which primarily extend clear 

 across the spinal cord to the external limiting membrane. A nucleus is 

 generally found in the strand near the central canal, and there may be 

 others further out (Fig. 431). Although in the embryo strands may 

 readily be traced from the central canal to the periphery, in the adult 

 they are generally broken up into stellate cells, or forms retaining a chief 



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. 



Chief process. 



Neuroglia cell of the gray substance of the base 

 of the dorsal column of a human embryo. 



FIG. 431. NEUROGLIA CELLS FROM THE SPINAL CORD. X 280. 



process directed either toward the central canal or the periphery (Fig. 431). 

 All these cells are parts of a general syncytium, as already described. 



The ependymal cells at birth, and for sometime afterwards, possess 

 cilia projecting into the central canal, but in the adult these disappear. 

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

 been found at their bases, but not diplosomes. 



Surrounding the central canal, outside of the ependymal layer, there 

 is a zone of central gray substance, characterized by concentrically ar- 

 ranged neuroglia cells, one of which is shown in Fig. 431. 



BRAIN. 



Development and General Features. If a human embryo of 4 mm. is 

 placed in such a position that the spinal portion of the medullary tube is 



