670 A MANUAL OF PHYSIOLOGY 



of the cord, and giving off bloodvessels to both, is the pia mater. 

 Between the dura and the pia, separated from the latter by a jacket 

 of cerebro-spinal fluid, is the double layer of the arachnoid. The 

 comparatively coarse septa that run into the nervous substance 

 as if coming off from the pia mater are the main beams in the 

 scaffolding of non-nervous material with which that substance is 

 interwoven, and by which it is supported. The interstices are filled 

 in by a thick-set feltwork of interlacing but unbranched neuroglia 

 fibres, which lie close against the small glia cells, but in the adult at 

 least are perfectly distinct from them. In preparations impregnated 

 by the Golgi method the fibres appear to be processes running out 

 from the attenuated cell-body like the arms of a microscopic crab or 

 spider. But this is a deceptive appearance, as Weigert has shown 

 by means of a special method, in which the neuroglia fibres are 

 alone stained. It is possible, however, that in the embryo the fibres 

 are formed by the cells, and afterwards become detached from them. 



FIG. 237. NEUROGLIA FIBRES AND CELLS (from a human embryo 30 cm. in 

 length). The small cell on the right is from the grey, the other two from the 

 white substance ; Golgi's method (Kolliker). 



The glia fibres are perfectly distinct from the nervous substance 

 proper, but they are not ordinary connective tissue. Indeed, it would 

 appear that no connective tissue of mesoblastic origin exists within 

 the nervous substance ; even the coarse septa, and particularly the 

 one which constitutes the so-called posterior fissure, seem to consist 

 of neuroglia, and not to be processes of pia mater. In the white 

 matter nearly every medullated nerve-fibre is divided from its neigh- 

 bours by glia fibres, which form a wide-meshed network. The 

 network is denser in most parts of the grey substance, though not in 

 all. The neuroglia is present in greatest abundance in the grey 

 matter immediately surrounding the central canal of the cord and 



