GANGLION-CELL AND NEEVE. 17 



cells, which form a part of the frame-work, the Spongioblast ; he designated 

 the incomplete ganglion-cells Neurohlasts. 



When the central nervous system has once passed its first stage of de- 

 velopment practically the same histological relations manifest themselves 

 as one meets in the adult condition. Let us turn our attention briefly to 

 these. 



The whole central nervous system is constructed of connective tissue 

 and nerve-tissue. The first is represented by the sheathes of the numerous 

 vessels, which permeate the organs everywhere as a dense net-work, and by 

 the neuroglia. 



The neuroglia consists of an infinite number of fine fibrillffi, of very 

 different caliber, which permeate the whole central system, and, because of 

 the innumerable intersections, present the appearance of a fine mesh-work. 

 At many of these points of intersection thin plate-cells lie upon the fibers, 

 giving rise to the appearance of neuroglia-fibers (GKafasern) arising from 

 these cells, — Deiter's cells (see Pig. 4, c). 



The neuroglia-net differs somewhat in different parts of the central 

 nervous system, and forms here and there dense accumulations in parts quite 

 devoid of nerve-substance. Thus, a thick layer of nearly pure connective 

 tissue covers the whole surface of the brain and cord and extends a short 

 distance along the nerve-roots in the form of a plug. In the same way 

 there is found on the inner surface of the central nervous system just under 

 the epithelium an especially rich development of neuroglia. The net-work 

 in the gray substance is in some parts denser, in others less dense, than in 

 the white substance. The larger nerve-cells are frequently so encircled that 

 they appear to lie in a fine meshed basket. 



The neuroglia is a peculiar tissue found, as yet, only in the central 

 nervous system, — except that the optic nerve possesses glia. It may be 

 absolutely differentiated from other kinds of tissue by its peculiar reaction 

 to staining when in pathological conditions. Wherever in the central nerv- 

 ous system nerve-substance degenerates through disease, the glia (neuroglia) 

 appropriates the empty space. The replacement with glia has a limit only 

 where its elements are destroyed along with the nerve-substance and where 

 its power of growth is not sufficient to fill up the large deficit. 



It has already been mentioned that the epithelium of the central canal 

 and the ventricles sends long processes into the surrounding nerve-substance. 

 In man these reach the exj;ernal surface only in a few places. These fibers, 

 an instructive figure of which is here presented (Fig. 5), belong naturally to 

 the supporting tissues. 



In Fig. 6 is represented (H) a section through the neuroglia-net of the 

 gray substance of an adiTlt man as it appears after treatment with the 

 Weigert method. 



