440 



THE BRAIN 



angular spaces exist between the arachnoid and the pia 

 mater. In these the subarachnoid tissue can be studied, and 

 it will be seen that these intervals on the surface of the 

 cerebrum serve as communicating 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 trabeculae and are 

 bathed by the subarachnoid fluid. 



Cisternae Subarachnoideales. In certain situations the 

 arachnoid is separated from the pia mater by intervals of 



Vein 



Sub-arachnoid space and trabeculaj 



Dura mater 

 ^7>~~~Subdural space 

 \ Arachnoid 

 Pia mater 



FIG. 175. Diagrammatic section through the Meninges of 

 the Brain. (Schwalbe. ) 



co. Grey matter of cerebral gyri. 



considerable depth and extent. These expansions of the 

 subarachnoid space are termed cisternae Subarachnoideales. In 

 them the subarachnoid tissue is relatively reduced. There 

 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. A beautiful 

 demonstration of these may be obtained by dividing in the 

 median plane, with the scissors, the sheet of arachnoid which 

 is spread over the medulla oblongata and pons, and turning 

 the two pieces gently aside. 



Certain of the cisternoe require special mention. The largest and most 

 conspicuous is called the cisterna cerebello-medullaris (O.T. wagna). It is 

 a direct upward continuation of the posterior part of the subarachnoid space 



