190 NERVE-CENTRES. [CH. xvn. 



easily distinguishable by the naked eye. They are called respec- 

 tively white matter and grey matter. 



White matter is composed of medullated nerve-fibres, which 

 differ in structure from the medullated fibres of nerve by having 

 no primitive sheath. 



Grey matter is the true central material so far as regards func- 

 tion ; that is to say, it is the part which receives and sends out 

 nervous impulses; it is characterised by containing nerve-cells 

 and their branches. 



In the brain the grey matter is chiefly situated on the surface, 

 forming what is called the cortex ; the white matter and certain 

 subsidiary masses of grey matter are in the interior. 



Fig. 196. Branched neuroglia-cell. (After Stohr.) 



In the spinal cord, the grey matter is in the interior, the white 

 matter outside. 



In both grey and white matter the nerve-cells and nerve-fibres 

 are supported by a peculiar tissue which is called neuroglia. It 

 is composed of cells and fibres, the latter being prolonged from 

 the cells. Some of the fibres are radially arranged. They start 

 from the outer ends of the ciliated epithelium cells that line 

 the central canal of the spinal cord and the ventricles of the 

 brain, and diverge constantly branching towards the surface of 

 the organ, where they end by slight enlargements attached to the 

 pia mater. The other fibres of the tissue are cell pix>cesses of 

 the ueuroglia or glia cells proper, or spider cells as they are some- 

 timeK termed (see fig. 196). 



Neuroglia is thus a connective tissue in function, but it is not 



