22O NORMAL HISTOLOGY. 



certain number of " plasma" cells. The dura mater 

 of the cord does not form the periosteum of the 

 bones forming the spinal canal, and hence in it the 

 looser vascular layer is for the most part wanting. 



The Pia Mater of the brain is a thin connective- 

 tissue membrane, covered on its outer surface with 

 endothelium, and containing an exceedingly abun- 

 dant net-work of blood- and lymph-vessels. Over 

 the surface of the convolutions it forms a single mem- 

 brane containing numerous small lymph-sinuses ; but 

 as it approaches the sulci it is partially separated into 

 two distinct layers, the outer bridging over the sulci, 

 while the inner and more vascular layer dips down 

 to the bottom of them. The space within the sulci, 

 between the two layers, is occupied by numerous 

 larger and smaller lymph-sinuses, which under nor- 

 mal conditions are little more than slits in the con- 

 nective tissue, lined with endothelium ; but they are 

 capable of considerable dilatation when, for any 

 reason, fluid accumulates in the meshes of the pia. 

 These spaces are called sub-arachnoidal lymph-spaces ; 

 the outer layer of the pia having been formerly 

 regarded as a distinct membrane, and called the 

 arachnoid. These sub-arachnoidal lymph spaces, as 

 well as the other numerous lymph-channels of the 

 pia, are in communication with lymph-channels, 

 called pcrivascular lymph-channels, which ensheath 

 certain of the blood-vessels as they enter the brain- 

 substance. 



