154 A MANUAL OF ANATOMY. 



The Arachnoid Membrane. Fig. 20. 



This is a very thin but distinct membrane which lies 

 within the dura and in turn covers the pia. It is connected 

 to the latter by delicate filaments, especially over the con- 

 vexity of the brain, called the subarachnoid tissue, hence 

 it is sometimes described as a part of that membrane (pia). 



The arachnoid passes smoothly over the surface of the 

 pia and does not follow it into the sulci and fissures, except 

 into the great longitudinal fissure, in order to get from one 

 side to the other beneath the falx cerebri ; and into the 

 great transverse fissure, where it has to turn around the 

 tentorium cerebelli. 



Over the convexity of the brain the arachnoid lies 

 closely applied to the pia so that it can with difficulty be 

 separated from it and demonstrated, but on the lower sur- 

 face (base) of the brain it is much thickened and is sepa- 

 rated from the pia in two well-marked localities. 



These spaces within the arachnoid, between the arachnoid 

 and the pia, are called the subarachnoidean spaces, anterior 

 and posterior. 



(i) The anterior subarachnoid space lies at the base 

 of the brain. It reaches forward to the beginning of the 

 great longitudinal fissure, laterally to the inner margins of 

 the temporal lobes of the brain, and posteriorly it covers 

 the pons and medulla, and communicates along the sides 

 of both with the posterior subarachnoid space, and below 

 with the similar cavity surrounding the spinal cord. 



Within the anterior subarachnoid space are contained 

 the medulla, the pons and its peduncles, the cerebral crura, 

 the structures at the base of the brain which form the 

 floor of the third ventricle (see page 222), the basilar, 

 internal carotid arteries, and their branches, especially that 

 arterial anastomosis which forms the circle of Willis. 



