THE CENTRAL NERVOUS SYSTEM 759 



running out from the attenuated cell - body like the arms of a 

 microscopic crab or spider. But according to Weigert this is a 

 deceptive appearance, as he has attempted to show by means of 

 a special method, in which the neuroglia fibres are alone stained. 

 If this is the case, we must assume that in the embryo the 

 fibres are formed by the cells, and afterwards become detached 

 from them. The processes of the typical ' spider ' glia cells are 

 unbranched even when of great length in proportion to the diameter 

 of the cell-body. Other neuroglia cells have branched processes. 

 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 mesodermic origin exists 

 within the nervous substance ; even the coarse septa, and particu- 

 larly 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 neighbours 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 the ventricles of the brain (the ependyma, as it is called), from 

 which long neuroglia fibres pass out radially, giving off branches on 

 their course, and ending in little knobs or enlargements attached to 

 the pia mater. Contrary to the common opinion, the substance of 

 Rolando is poor in neuroglia (Weigert). 



General Arrangement of the White and Grey Matter in 

 the Central Nervous System. (i) Around the central canal, as 

 we have seen, a tube of grey matter sheathed with white fibres is 

 developed. This tube, from optic thalamus to conus medullaris, 

 may be conveniently referred to as the central grey axis or stem, 

 which, in the lowest vertebrates e.g., fishes is much the most 

 important part of the central nervous system. 



(2) On the outer surface of the anterior portion of the neural 

 axis, but not in the part corresponding to the spinal cord, is laid 

 down a second sheet or mantle of cortical grey matter. Between 

 this and the primitive grey stem are interposed (a) the sheath of 

 white fibres that clothes the latter, and connects its various parts, 

 and (b) a new development of white matter (corona radiata, cere- 

 bellar peduncles), which serves to bring the cortex into relation 

 with the primitive axis, and through it with the rest of the body. 



Although there are histological and developmental differences 

 between the cerebral and the cerebellar cortex, we may, for some 

 purposes, classify them together as cortical formations. And we 

 may also include under this head the corpora striata, which, 

 although for descriptive purposes generally grouped with the 

 optic thalami and the other clumps of grey matter at the base 

 of the brain, as the basal ganglia, are to be regarded as cortical 

 in character. As we mount in the vertebrate scale the cortex 

 formation of the secondary fore-brain and hind-brain acquires 

 prominence. 



