CHAPTER V 



NERVOUS SYSTEM 



INVESTING MEMBRANES. 



1. The most external of the three membranes 

 which enclose the brain and spinal cord is the 

 dura mater. It is a thick, fibrous membrane, 

 which lines the inside of the skull and vertebral 

 column. It dips down in a longitudinal fold be- 

 tween the two lobes of the brain forming theyW.r 

 cerebri. It also makes a transverse fold below the 

 posterior margin of the parietal bone, between the 

 cerebrum and cerebellum. This is called the ten- 

 torium, and in adult cats becomes ossified. 



2. The most internal membrane, the pia mater, 

 is much thinner and less dense, and closely invests 

 the brain and cord. The blood-vessels supplying 

 the central nervous system are contained in this 

 membrane. 



3. The arachnoid is a very delicate membrane, 

 lying between the dura and pia mater. It is 

 closely connected with the dura mater, but does 

 not follow all the folds of the pia mater. It 

 may be seen bridging over the fissures of the 

 brain. 



