134 ELEMENTS OF HISTOLOGY. [Chap. xvi. 



neuroglia lias a very unequal distribution ; but there 

 are certain definite places in which there is always a 

 considerable amount a condensation, as it were, of 

 neuroglia tissue. These places are : underneath the 

 pia mater i.e., on the outer surface of the white 

 matter there most of the neuroglia fibrils have a 

 horizontal direction ; near the grey matter there is 

 a greater amount of neuroglia between the nerve- 

 fibres of the white matter than in the middle parts of 

 this latter ; in the septa between the columns and 

 between the divisions, of columns of white matter ; at 

 the exit of the anterior and the entrance of the 

 posterior nerve roots. 



A considerable accumulation of neuroglia is 

 present immediately around the epithelium lining the 

 central canal j this mass is cylindrical, and is called 

 the central grey nucleus of Kolliker. The epithelial 

 cells lining the central canal are conical, their bases 

 facing the canal, their pointed extremity being drawn 

 out into a fine filament intimately interwoven with 

 the network of neuroglia fibrils. In the embryo and 

 young state, the free base of the epithelial cells has a 

 bundle of cilia, but in the adult they are lost. 



Another considerable accumulation of neuroglia 

 exists in the posterior portion of the posterior grey 

 horns, as the substantia gelatinosa of Rolando. 



173. The white matter (Fig. 85) is composed, 

 besides neuroglia, of medullated nerve-fibres varying 

 very much in diameter, and forming the essential and 

 chief part of it. They possess an axis cylinder and 

 a thick medullary sheath more or less laminated, but 

 are devoid of a neurilemma and its nerve corpuscles. 

 Of course, no nodes of Hanvier are observable. In 

 specimens of white matter of the posterior columns, 

 where the nerve-fibres have been isolated by teasing 

 or otherwise, many fine medullated fibres are met 

 with which show the varicose appearance mentioned 



