8 THE MENINGES OF THE BRAIN. 



function. It is covered on both surfaces by endothelium and 

 is separated from the vertebrae by areolar tissue, fat and the plexus 

 of internal vertebral veins. 



THE ARACHNOID OF THE BRAIN. 



(Arachnoidea Encephali.) 



In structure it is a delicate, fibrous, web-like membrane covered 

 externally with endothelium. Internally it is joined to the pia 

 mater by innumerable fibrous trabeculae, the subarachnoid tissue 

 (Fig. 4). The trabeculae are ensheathed and all subarachnoid 

 spaces lined with a single layer of endothelial cells, hence both 

 surfaces are formed of endothelium. Conical elevations of 

 fibrous tissue with their investing endothelium constitute the 

 villi seen on the outer surface.* 



Relations. The arachnoid follows the inner surface of the 

 dura and is prolonged, as a sheath, upon the nerves which pierce 

 it. It does not dip into the sulci of the cerebrum (Fig. 6); but 

 only into the lateral fissure and the longitudinal fissure, and does 

 not reach to the bottom of the latter. From the pia it is sepa- 

 rated by the subarachnoid spaces (cava subarachnoidealia}. The 

 anterior subarachnoid space (Fig. 6) includes the cistema pontis, 

 cisterna interpeduncularis, cisterna chiasmatis and cistema 

 fossae lateralis cerebri. It is located in front of the medulla, pons 

 and mid-brain and between the temporal lobes of the cerebrum. 

 The posterior subarachnoid space (Fig. 6) is located behind the 

 medulla and cerebellum. It embraces the cisterna cerebello- 

 medullaris, a space between the medulla and cerebellum, and the 

 cisterna venae cerebri magnae, situated along the straight sinus 

 and the great cerebral vein. These two, the anterior and posterior, 

 are the largest subarachnoid spaces and they contain much of 

 the subarachnoid fluid. But in the cerebral sulci and fissures 

 there are streams of this fluid which constitute the subarachnoid 

 rivulets. The anterior subarachnoid space has slit-like com- 



* Some authors consider this membrane, as just described above, 

 merely as a visceral layer of the arachnoid, and regard the endothelial 

 lining of the dura mater as its parietal layer. According to such, 

 therefore, the subdural space becomes the arachnoid space. 



