THE CENTRAL NERVOUS SYSTEM 671 



the ventricles of the brain (the ependyma, as it is called). 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 inter- 

 posed (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, cerebellar 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 differ- 

 ences 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 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. 



In other words, the grey matter developed in the roof of the 

 cerebral vesicles i and III. (Fig. 226) (the grey matter of the cere- 

 bral and cerebellar cortex) comes to overshadow the superficial grey 

 matter hitherto present only in the roof of vesicle II. (in the corpora 

 bigemina). And this cortex formation becomes larger in amount, 

 and, in the case of the cerebral grey matter, more richly convoluted, 

 the higher we ascend, until it reaches its culmination in man. As 

 the anterior cerebral vesicles develop, they spread continually back- 

 ward until at length the cerebral hemispheres cover over, and almost 

 completely surround, the primary fore-brain, and the mid- and hind- 



