16 ANATOMY OF THE CENTEAL NERVOUS SYSTEM. 



the nervous system, Then in all vertebrates they send peripherally a process 

 which persists until in adult life and which is usually somewhat branched 

 and ends close beneath the pia. There one often meets curious enlargements 

 of the cell-extremity, from which, as in the epithelial cells of the sense- 

 organs, a delicate bristle-like projection rises. In man and the higher 

 mammals the epithelial processes do not appear to reach the periphery in 

 the post-embryonic period. The epithelium of the central canal is ciliated. 



Fig. 4. — A, B, Ganglion-cells; c, Neuroglia-cells ; D, Axia-cy Under; p, Proto- 

 plasmic processes. From the spinal cord. (After Ranvier.) 



But not by any means are all epithelial cells employed in lining the 

 central canal. Through cell-division there arise very many new structures, 

 and one finds that these recede farther and farther from the central canal, 

 with whose wall they often remain in connection through a fine fibre. The 

 branching processes of these cells form a net-work which, in the adult, may 

 permeate the whole substance of the central nervous system, being more 

 dense in some locations than in others. His, their discoverer, called these 



