THE AKACHNOIDEA. 



671 



Cavum Subarachnoideale. The interval between the arachnoidea and the 

 pia mater receives the. name of the subarachnoid space. It contains the 

 cerebro-spinal fluid, and communicates freely, through certain well-defined 

 apertures, with the ventricular cavities in the interior of the brain (aperturse 

 ventriculi quarti). 



Within the cranium the subarachnoid space is broken up by a meshwork of fine 

 filaments and trabeculae, which connects the two bounding membranes (viz., the 

 arachnoidea and the pia mater) in the most intimate manner, and forms a delicate 

 sponge-like interlacement between them. Where the arachnoidea passes over the 

 summit of a cerebral gyrus, and is consequently closely applied to the sub- 

 jacent pia mater, the meshwork is so dense and the trabeculse so short that it is 

 hardly possible to discriminate between the two membranes. To all intents and 

 purposes they form in these localities one lamina. In the intervals between the 

 rounded margins of adjoining gyri, however, distinct angular spaces exist, where 

 the subarachnoid trabecular tissue can be studied to great advantage. These 



Granulatio arachnoidealis 



Lacuna lateral 



Dura mater - 



Subdural space - 

 Arachnoidea 



ibarach- 



id space- 



" tissue 



mater 



1. 596. DIAGRAM to show the relations of the membranes of the brain to the cranial wall and the cerebral 

 gyri, and also of the arachnoideal granulations to the superior sagittal sinus and the lateral lacunae. 



intervals on the surface of the cerebrum constitute numerous communicating 

 channels which serve 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 sub- 

 arachnoid trabeculse, and are bathed by subarachnoid fluid. 



In certain situations within the cranium the arachnoidea is separated from the 

 pia mater by intervals of considerable width and extent. These expanded portions 

 of the subarachnoid space are termed cisternse subarachnoideales. In these the sub- 

 arachnoid tissue is much reduced. There is no longer a close meshwork; the 

 trabeculse connecting the two bounding membranes take the form of long fila- 

 mentous intersecting threads which traverse the spaces. All the subarachnoid 

 cisterns communicate in the freest manner with each other and also with the 

 narrow channels on the surface of the cerebrum. 



Certain of these cisterns require special mention. The largest and most con- 

 spicuous is the cisterna cerebellomedullaris. It is formed by the arachnoid 

 membrane bridging over the wide interval between the posterior part of the 

 inferior surface of the cerebellum and the medulla oblongata. It is continuous 



