no 



THE NERVOUS SYSTEM 



border is free, and, together with the dorsum sellae of the 

 sphenoid bone, bounds an aperture through which the mid- 

 brain passes to reach the cerebrum (Fig. 53). 



The Arachnoid is a much more delicate membrane than 

 the dura mater, and it differs still further from the latter in 

 being more intimately related to the brain. 



The Pia Mater, which lies subjacent to the arachnoid, dips 



FIG. 53. The Cranial Blood Sinuses. The left half of the skull and 

 the left cerebral hemisphere have been removed. 



i. Falx cerebri. | 2. Tentorium cerebelli. 



into all the sulci on the surface of the brain, but the arachnoid 

 merely bridges over their margins. Over the various con- 

 volutions the two membranes are closely applied to one 

 another, and in these areas the subarachnoid space is prac- 

 tically obliterated. In certain areas, however, definite 

 intervals exist between the arachnoid and the pia mater. 

 These parts of the subarachnoid space are termed cisternce, and 

 the more important of them are situated on the basal surface 

 of the brain. 



