TISSUES OF SPECIAL FUNCTION. 



101 



cylinder, which at intervals appears to be nucleated. These nu- 

 clei are presumably constituents of a membranous investment or 

 neurilemma ; but the latter is difficult of demonstration because of 

 its thinness and transparency, and its constant presence is not defi- 

 nitely established (Fig. 86). 



Unlike the medullated variety, the gray nerve-fibres frequently give 

 off branches, which join other fibres and constitute true anastomoses. 



Non-medullated fibres are most abundant in the sympathetic 

 nervous system, but occur also in the nerves derived directly from 

 the brain and spinal cord. 



3. Neuroglia. The nerve-cells and fibres of the central nervous 

 system are surrounded and supported by a tissue which is derived 

 from the epiderm, and is called the " neuroglia." It must be re- 

 garded as a variety of elementary tissue having functions similar to 

 the connective tissues, although its origin makes its relations to the 

 epithelial tissues very close. 



Neuroglia consists of cells, the " glia-cells," which vary consider- 



FIG. 87. 



FIG. 88. 



Glia-cells from the neuroglia of the human spinal cord. (Retzius.) 

 Fig. 87. Three cells from the anterior portion of the white matter: a, processes extending to 



the surface of the cord ; b, cell-body ; c, long, delicate process extending far into the white 



matter. 

 Fig. 88. Two cells from the deep portion of the white matter. 



ably in character, and an intercellular substance, which is for the 

 most part soft and homogeneous, resembling in this respect the 

 cement-substance found in epithelium, but which may, here and 

 there, contain a few delicate fibres, possibly derived from the pro- 

 cesses of some of the cells, or possibly of mesodermic origin, and, 

 in consequence, belonging to the connective tissues. 



