MEMBRANES 



channels for the free passage of the subarachnoid fluid from 

 one part of the brain to another. The larger branches of 

 the arteries and veins of the brain traverse the subarachnoid 

 space; their walls are directly connected with the subarachnoid 

 trabeculse and are bathed by the subarachnoid fluid. 



Cisternae Subarachnoideales. In certain situations the 

 arachnoid is separated from the pia mater by intervals of 

 considerable depth and extent. Such expansions of the 

 subarachnoid space are termed subarachnoid cisterns. In 

 them the subarachnoid tissue is relatively reduced. There 



Subarachnoid space and trabeculae 



""Dura mater 

 -~~~Subdural space 

 \ "Arachnoid 

 Pia mater 



FIG. 140. Diagrammatic section through the Meninges of 

 the Brain. (Schwalbe. ) 



co. Grey matter of cerebral gyri. 



is no longer a close meshwork ; the trabeculae connecting the 

 two bounding membranes take the form of long filamentous 

 intersecting threads which traverse the spaces. The dissector 

 will obtain a beautiful demonstration of the conditions de- 

 scribed by dividing the sheet of arachnoid which is spread 

 over the medulla oblongata and pons, and turning the two 

 pieces gently aside. The division must be made in the 

 median plane with scissors. 



Certain of the cisternse require special mention. The 

 largest and most conspicuous is called \hz l risterna cerebello- 

 medullaris (O.T. magna) (Fig. 141). It is a direct upward 

 continuation of the posterior part of the subarachnoid space 

 of the spinal meninges into the posterior part of the cranium. 

 It is formed by the arachnoid membrane bridging over the 



in 24 a 



