ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 579 



their connection with cells from which they arose as processes. Such 

 processes from the epithelial cells come to form somewhat irregular 

 bundles between the anterior and posterior horns and in the longi- 

 tudinal fissures. They run often into the pia and unite with it, while 

 from the pia connective-tissue fibrils, usually in association with 

 blood-vessels, run parallel to the former out to the commissural 

 region of the cord. Thus the so-called " pia processes " filling up 

 longitudinal fissures have partly this connective-tissue origin, but are 

 also in part, and usually for the most part, cornified glia-fibres, either 

 direct processes of the nearest " epithelial cells" of the central canal, 

 or fibres which have become independent. As to the amorphous 

 element, the structureless ground-substance or matrix, Dr. Gierke is 

 inclined to refer its origin to the gradual change of the embryonic 

 cells, though he does not exclude the possibility of its being in part 

 excreted by the glia-cells. In the great subsequent growth of grey 

 matter round about the epithelium and substantiva gelatinosa centralis, 

 glia-cells are seen, originating either from those of the inner layer, 

 or more probably from a new modification of embryonic cells. The 

 details as to the modifications of the glia-cells, the development of 

 processes, &c, cannot be summarized. 



In the brain the elements of the neuroglia are essentially similar 

 to those of the spinal cord ; the two types of cells are not, however, 

 so well marked ; the cell-bodies, especially in the molecular layers 

 of the cerebellum, sometimes almost disappear, so that their former 

 position is only indicated by a small knot from which the processes 

 diverge ; while the glia-cells in the grey sheath of the cerebrum 

 exhibit as a result of cornification a peculiar nuclear modification, as 

 the nuclei, instead of shrivelling smaller and smaller, retain their size 

 but lose their sharp contour ; these cells are further peculiar in their 

 very granular appearance. 



Function. — The function of the glia is to surround and protect the 

 nervous elements ; it penetrates every portion of the central nervous 

 system ; the whole network of glia-cells is continuous, the ground- 

 substance and the nervous elements fill up the meshes. Specially 

 differentiated is the glia-sheath which surrounds the whole central 

 organ and separates the inner substance from the pia mater. It is 

 always to be found where the pia covers the surface ; it effects on the 

 one hand the peculiar union between the surface of the nervous organ 

 and the pia, and forms the constant narrow lymph spaces between 

 them, while it is also obviously protective and serves as a sort of 

 basis for the neuroglia network. Its variations and exact histological 

 relations are intimately described, especially as they occur in the pike. 



The cavities of the central nervous organ are surrounded by layers 

 of neuroglia in which few nervous elements occur ; the " granular 

 tissue " often described is produced by cross sections of processes. 

 Round the cavity of the ventricle this tissue is limited by a layer of 

 epithelial-like cells, sometimes finely ciliated, sometimes flattened. 

 They are in close union with the neuroglia and are modified glia-cells. 

 Besides the outer glia-sheath and the inner lining of the central 

 cavities, the glia framework, with its associated network, is described 



2 Q 2 



