IX 



THE NERVOUS SYSTEM 



The nervous system consists of central organs 

 — ^the brain and spinal cord — and peripheral 

 nerves and ganglia. In addition to the pro- 

 tection afforded by the bones of the cranium 

 and vertebral column, the central nervous 

 system is surrounded by a membranous in- 

 vestment which not only serves a protective 

 function but also furnishes a distributing area 

 for blood-vessels previous to their entry into 

 the substance of the nerve masses. 



The membranes, or meninges, of the brain 

 and spinal cord consist of an outer, strong, 

 fibrous investment, the dura 7nater, and a 

 more delicate vascular membrane, the pm 

 mater, closely applied to the surface of the 

 brain and cord. Between these membranes is 

 a scanty representative of the mammalian 

 arachnoid mater. 



89 



