466 



THE NEKVOUS SYSTEM 



nerve elements. They are frequently in intimate relation with 

 the blood vessels, on the walls of which many of the glia fibres, 

 particularly the thicker or mossy cell variety, terminate in ex- 



FIG. 369. NEDKOGLIA CELLS AND FIBRES FROM THE SPINAL CORD OF AN ELEPHANT. 



The letters indicate various types of neuroglia cells. Z, a leucocyte. Beiida's stain. 

 x 940. (After Hardesty.) 



panded plates, which, in some parts, form an almost complete 

 outer membranous coat of the vessel. 



The astrocytes are ontogenetic derivatives of the embryonic 

 ependyma 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 oc- 

 curs throughout the brain and spinal cord, and also in the olfac- 

 tory nerves, the optic chiasm, and the retina of the adult. 



The supporting tissues of the central nervous system include, 

 besides the neuroglia, numerous bands or trabeculae of fibrous 

 connective tissue, which push inward from the pia mater, carrying 

 with them the vascular branches for the supply of the nervous 

 tissues, and which penetrate deeply into the substance of the 

 spinal cord and brain. 



