CHAPTER XL. 



THE MENINGES OF THE BRAIN. 



CEREBRAL SINUSES. LANDMARKS. 



Diseases of 



The Meninges of the Brain. The dura mater is 

 a tough fibrous membrane which is in contact with the in- 

 ner surface of the cranial bones, and acts as a periosteal 

 covering for them, and which, in addition, affords support 

 and protection to the brain. In accordance with these func- 

 tions it is arranged in two layers, viz., a periosteal and a 

 supporting layer. The supporting layer constitutes the 

 projections of the dura mater into the cavity of the cra- 

 nium, termed the falx cerebri, the falx cerebelli and the 

 tentorium cerebelli. The separation of these layers forms 

 also the cranial sinuses which will be considered later on. 

 The arachnoid forms a well defined limiting mem- 

 brane on the side towards the dura mater the interval 

 between the dura mater and this membrane being termed 

 the subdural space but on its inner surface it fades away, 

 gradually, into what is erroneously termed the suba- 

 rachnoidean space. This so-called space is, in reality, 

 filled with tissue the subarachnoidean tissue formed of 

 fine trabeculse lined by endothelium. The arachnoid does 

 not, like the pia mater, dip into the fissures and sulci of 

 the brain, hence, there is left between them, i.e., between 

 the arachnoid and the pia mater, an interval filled with 

 this subarachnoidean tissue. Posteriorly, between the 

 medulla and the overhanging inferior vermiform process 

 of the cerebellum, this interval or subarachnoidean space 

 is strongly marked, while anteriorly, extending to the 

 commencement of the great longitudinal fissure beneath 

 the pons and crura, and limited laterally, by the tempero- 



