NEUROGLIA 145 



of each of these tissues appear to be ontogenetically derived either di- 

 rectly or indirectly from its cells, yet when fully formed they often exist 

 as anatomically distinct elements. 



Occurrence of Neuroglia. Xeuroglia cells and fibers occur in both 

 gray and white matter of the central nervous system, though perhaps 

 more abundant in the latter. The fibers radiate for considerable dis- 

 tances from their glia cells, and thus form a supporting tissue for the 

 nerve elements. They are frequently in intimate relation with the 

 blood-vessels, on the walls of which many of the glia fibers, particularly 



FIG. 160. NEUROGLIA CELL WITH ADJACENT FIBERS FROM THE PINEAL BODY 



OF A YEARLING SHEEP. X 1500. 



the thicker or mossy variety, terminate in expanded plates, which, in 

 some parts, form an almost complete outer membranous coat of the 

 vessel. 



The astrocytes are ontogenetic derivatives of the embryonic epen- 

 dyma cells. From their point of origin around the neural canal they 

 wander to all portions of the central nervous system, and even into the 

 optic and olfactory tracts, which are embryonic outgrowths from the 

 fetal cerebral vesicles. Thus neuroglia occurs throughout the brain 



