THE CEREBRO-SPINAL CENTRE. 



285 



(Fig. 20) of the occipital bone. 

 This cerebro-spinal centre con- 

 sists of similar right and left 

 halves, incompletely separated 

 by grooves and fissures. Brain 

 and spinal cord are very soft 

 and easily crushed ; accordingly, 

 both are placed in almost com- 

 pletely closed bony cavities, and 

 are also enveloped by mem- 

 branes which give them sup- 

 port. These membranes are 

 three in number. Externally 

 is the dura mater, tough and 

 strong, and composed of con- 

 nective tissue. The innermost 

 enveloping membrane of the 

 cerebro-spinal centre, in imme- 

 diate contact with the proper 

 nervous parts, is the pia mater, 

 less dense and tough than the 

 dura mater. Covering the out- 

 side of the pia mater is a layer of 

 flat cells ; a similar layer lines 

 the inside of the dura mater, 

 and these two layers are de- 

 scribed as the third membrane 



Why are brain and spinal cord 

 placed in bony chambers? How 

 many membranes also support them? 

 What is the outside membrane 

 named? What are its mechanical 

 properties? Of what is it composed? 

 What is the pia mater? 



