MEMBRANES OF THE NERVE-CENTRES. 



A 



completely closed bony cavities and 

 Are also enveloped by membranes 

 which give them consistency and 

 support. These membranes are 

 three in number. Externally is the 

 dura mater, very tough and strong 

 .and composed of white fibrous and 

 elastic connective tissues. In the 

 cranium this dura mater adheres 

 by its outer surface to the inside of 

 the skull, serving as the periosteum 

 of its bones; this is not the case 

 in the vertebral column, where the 

 dura mater forms a loose sheath 

 around the spinal cord and is only 

 attached here and there to the sur- 

 rounding bones, which have a sep- 

 arate periosteum of their own. The 

 innermost membrane of the cerebro- 

 spinal centre, lying in immediate 

 contact with the proper nervous 

 parts, is the pia mater, also made 

 up of white fibrous tissue inter- 

 ivoven with elastic fibres, but less 

 closely than in the dura mater, so 

 as to form a less dense and tough 

 membrane. The pia mater con- 

 tains many blood-vessels which 

 break up in it into small branches 

 before entering the nervous mass 

 beneath. Covering the outside of 

 the pia mater is a layer of flat 

 closely fitting cells, a similar layer 

 lines the inside of the dura mater, 

 and these two layers are described 

 as the third membrane of the cere- 

 bro-spinal centre, called the arach- 

 noid. In the space between the 

 two layers of the arachnoid is con- 

 tained a small quantity of watery 



157 



FIG. 63. The spinal cord 

 and medulla oblongata. A. 

 from the ventral, and B, from 

 the dorsal aspect ; C to H, 

 cross-sections at different 

 levels. 



