NEUROGLIA 



199 



surface of the cord, and thus from the epithelium of the embryonic body, 

 the exact method of their development and growth into this secondary 

 position has not been worked out. Neu- 

 roglia cells show a strong tendency to 

 syncytial arrangement, due, as in connec- 

 tive tissue, to their function. 



Neuroglia is found, not only in the 

 central nervous system and nerve cords 

 of most animals, but also in the sensory 

 nerve tissues on the periphery. Its most 

 primitive form can, perhaps, be seen in 

 the " supporting cells " of olfactory and 

 tactile epithelia. Here the cells have no 

 processes, and little resemble neuroglia 

 elements, and it is only by virtue 01 

 their association with sensory nerve cells 

 that they can be called neuroglia at 

 all. 



In the visual tissues these cells show 

 a distinct advance as neuroglia cells 

 over the condition found in the other 

 sense organs. This advance is not 

 marked in the eyes of the lower invertebrates. A mere beginning 

 is represented in the glia cells found in the eyes of planarian worms. 

 In the vertebrate visual tissue, the retina, neuroglia is well developed, 

 almost as much so as in the central nervous tissues. This is easily under- 

 stood when we recall that the retina is only secondarily derived from the 

 surface, being evaginated from the brain, which was in its turn invagi- 

 nated from the dorsal surface of the young embryo. The retinal neu- 

 roglia cell, or radial fiber, as it is called, is considerably branched, and 

 acts as an efficient support to the delicate nerve cells of the retina, from 

 whose embryonic ancestral cells it was also derived (see Fig. 175, B). 



Technic. The general outlines of neuroglia cells may be most satis- 

 factorily seen when the tissue has been prepared by one of the methods 

 of Golgi. After the location of these kinds of cells is known, their cell 

 bodies may be recognized in many ordinary preparations and distin- 

 guished from the ordinary connective-tissue cells in that preparation. 

 It may be distinguished with certainty from these cells in two ways: 

 by the use of special stains, as Mallory's stain for neuroglia, or by a study 

 of its histogenesis. This latter way of determining the nature of the cell 

 is most satisfactory and is not difficult. 



FIG. 1 78. Transection of a nerve con- 

 nective in the leech to show one of the 

 large central neuroglia cells with its 

 fibrillar processes reaching out in all 

 directions to the periphery and act- 

 ing as a support for the numerous 

 nerve fibers that run between them 

 in bundles. (After K. C. SCHNEIDER.) 



