TISSUES OF SPECIAL FUNCTION. 



109 



central nervous system contains fibrous prolongations of the epi 



thelial cells of the ependyma and central canal of 



the spinal cord (Fig. 97). Fibrous constituents are 



also derived from the areolar tissue which extends 



into the organs of the central nervous system from 



their fibrous investments, the pia mater, in company 



with the vascular supply. 



The central nervous system, then, consists of a 

 small amount of a ground-substance and a great 

 number of cells, most of which possess numerous 

 delicate fibrillar processes which interlace in all 

 directions. Some of these cells are the function- 

 ally active elements of the organs, the nerve-cells. 

 Others belong to the sustentacular tissue, and are 

 probably functionally passive, constituting the in- 

 terstitium. Both kinds of cell are developed from 

 the epiderm, and are therefore genetically closely 

 related to each other. 



4. Nerve-endings. — Nerve-fibres terminate in two 

 ways : first, in free ends lying among the elements 

 of the tissues to which the nerve is distributed ; 

 second, in terminal organs, containing not only 

 nerve-filaments, but cells which are associated with 

 them to form a special structure. The simplest 

 mode of termination consists in a separation of the minute fibrillae 



Fig. 98. 



Ependyma and glia- 

 cells from the spi- 

 nal cord. (Retzius.) 

 a, ependyma cell in 

 the wall of the cen- 

 tral canal ; b, neuro- 

 glia-cell near the 

 anterior fissure of 

 the cord. 



Termination of nerves by free ends. (Retzius.) Nerve-endings among the ciliated columnar 

 epithelium on the frog's tongue. Two goblet-cells, the whole bodies of which are colored 

 black, are represented. The other cells are merely indicated. 



composing the axis-cylinders of the medullated fibres, or the chief 

 bulk of the non-medullated fibres, into a number of delicate fila- 



