THE NERVOUS TISSUES. 



neuroglia, a peculiar form of ectodermic tissue, with, therefore, 

 close relations to the neurogenetic tract. Neuroglia consists of 

 extremely branched elements, or glia- cells, whose numerous pro- 

 cesses break up into 



FIG. 95. 



A 



B 



brush-like bundles of 

 delicate fibrils, which 

 pass in all directions 

 among the nervous ele- 

 ments, filling more or 

 less completely all inter- 

 stices. The body of the 

 glia-cells is frequently 

 stellate, possessing a nu- 

 cleus and staining in- 

 tensely with certain dyes. 



.-pi i r Supporting tissues of nerve-centres : A, extensions of the 



peripheral connective tissue of the pia mater ; B, neuroglia- 

 these neuroglia elements cells, one of which is seen in profile (s). Golgi staining. 



is very striking in Golgi 



silver preparations, where they appear as dark, spider-like figures 

 which send out delicate fibrils in all directions. In the embryo the 

 neuroglia consists of long, delicate fibrils which radiate from the 

 neural tube ; later, these fibres become broken up into shorter 

 threads which interlace and form the supporting tissue. The 

 groundwork surrounding the nerve-fibres within the white matter 

 serves the purpose of covering as well as of support, and replaces 

 the neurilemma 



In addition to the dense reticulum formed by the neuroglia, con- 

 stituting the special sustentacular tissue of the nervous system, pro- 

 longations from the enveloping pia mater in places penetrate within 

 the nervous masses and contribute connective-tissue trabeculae, 

 which form a supporting framework for the blood-vessels. These 

 connective-tissue ingrowths constitute the septa, which in many 

 places, as conspicuously in the spinal cord, accompany the blood- 

 vessels into the nervous substance. The finer ramifications of these 

 partitions fade away in delicate extensions which mingle with the 

 fibrils of the neuroglia-cells. It is evident, therefore, that the sup- 

 porting tissue of the nervous system can no longer be regarded 

 simply as a form of connective tissue, since, in addition to the un- 

 doubted connective tissue present, the larger part is contributed by 

 the peculiar ectodermic structure, the neuroglia. 



THE STRUCTURE OF GANGLIA. 



Along the course of certain nervous cords, such as those consti- 

 tuting the sensory roots of the spinal nerves, the trunks of many of 



