1004 



HIM AN ANATOMY. 



constituents of the neuroglia arise. So long as neuroglia is being produced, as in 

 the nervous axis of young ;mim;ils, the large gliogenetic cells are present and directly 

 concerned in the production of additional ribrillae, their cytoplasm becoming pro- 



Mvely less granular and reduced through the various transition phases until in 

 tin- final condition, as the small x/ia <r//s, little more than the nucleus remains. 

 During these changes very many titnilla lose their connection with the cells and, in 

 conjunction with the glia threads still attached to the astrocytes, form an elaborate 

 interlacement in which the neuroglia cells, now reduced and for the most part devoid 

 of ]>: lie scattered at uncertain intervals. 



In all parts of the central nervous system the mature neuroglia consists of 

 essentially the same tissue, the differences presented in certain localities depending 

 largely upon variations in its compactness. Kverywhere the chief part of the sup- 

 porting tissue consists of the intricate felt-work of fibrillae, glia-Jibres, as they are 

 called, which are usually free but to some extent connected with the spider-cells or 

 astrocytes. Where, however, the neuroglia borders the neural tube (the ventricles 

 of the brain and the central canal of the spinal cord) as the ependymal layer, its 

 Arrangement exhibits peculiarities that call for later special mention. 



In the immediate vicinity <>f the neurones the felt-work of the fibrilke is unusually close, so 

 that the cell-bodies and the nx.ts of the processes are surrounded by a protecting sheath, the 

 glia-caf>sule. This diminishes along the demlrites, and after these begin to branch the neuroglia 

 no longer forms a complete special investment. The medullated nerve-fibres within the brain 

 and spinal cord are also provided with delicate ncitrogliar sheaths which replace the neurilemma 

 which on these fibres is wanting. These sheaths are prolonged for some distance on the fibres 



of the roots of the spinal nerves. The fibres of the optic 

 ner\e and of the olfactory tract are accompanied through- 

 out their length by neurogliar sheaths, those of the 

 remaining cranial nerves losing these envelopes shortly 

 after leaving the brain I Rubaschkin). 



I'.eiieath the pia mater the neuroglia is especially 

 dense and forms the external subpial layer that every- 

 where invests the nervous mass, following all the inequali- 

 ties < if its surface. In this manner the pia mater is excluded 

 and, except where its connective-tissue strands accompany 

 the blood-vessels that enter the nervous mass, takes no 

 part in the make-up of the supporting stroma. The 

 subpial layer consists of a dense felt-work of glia-fibres, 

 disposed in various planes, which are partly free and partly 

 the processes of spider cells. Internally the layer fades 

 into the adjoining diffuse neuroglia without demarcation. 

 At the periphery the fibres often exhibit a radial disposi- 

 tion, their outer ends usually being somewhat expanded. 

 Within the white matter the neuroglia, both in its distri- 

 bution and density, is fairly uniform, although special 

 tracts often separate the larger bundles of nerve-fibres. 

 Its arrangement within the gray matter presents less 

 uniformity, since more or less marked condensations 

 occur where the nerve-cells are collected into nuclei, as 

 Conspicuously seen in the inferior olive. 



FIG. 848. 



t nriiro- 



K!I:I siirriiuniliiiK j.iii.il 



(Kubiifih- 



Where the neuroglia borders the neural tube 

 i.illy the central canal of the spinal cord) it 

 constitutes the ependymal layer, the peculiari- 

 : \\hich call for special mention. The imme- 

 diate lining of the tube consists of a single layer of 

 pyramidal epithelial elements, the cpcndynml cells, whose free surfaces or bases look 

 towards the lumen, and the apices towards the MM i ounding nervous tissue. At least 

 during the earlier years in man, and throughout life in many lower mammals, the 

 h cell is beset with a number of hair-like processes that in their 

 relations with the cytoplasm correspond to ordinarv cilia. The pointed distal end of 

 the epeiidvmal cell is prolonged into a conical process that is directly continii'-d 

 into usually a single neurogliar fibre which, after a . our>- of uncertain length becomes 



