558 THE NEKVOUS SYSTEM 



Although the dura mater is but poorly supplied with blood 

 vessels, it is relatively rich in lymphatics, which open into the 

 subdural and epidural spaces and are continuous with the perivas- 

 cular and perineural lymphatics which leave the cerebro-spinal 

 cavities in company with the cranial and spinal nerves and the 

 larger blood vessels. In this way the lymphatics of the dura mater 

 and its adjacent spaces are in communication with the lymphatic 

 vessels of the eye, nose, ear, and cervical lymphatic nodes. These 

 communications are of special importance as indicating the path 

 followed by certain pathological processes which involve the me- 

 uinges. 



Where the outer surface of the dura is not attached to the sur- 

 rounding bone or connective tissue, it is covered by a thin endothe- 

 lial coat, the lining endothelium of the epidural spaces. Its inner 

 surface is lined by somewhat thicker endothelial cells, forming the 

 wall of the subdural space. 



The arachnoid is a thin membranous sheet which is suspended 

 between the dura and the pia mater. It is composed of a deli- 

 cate areolar tissue which contains relatively few elastic fibres and 

 almost no blood vessels. This thin fibrous membrane is covered on 

 either side by a layer of endothelium ; that upon its outer surface 

 consists of endothelial cells of considerable thickness, which are 

 derived from the lining membrane of the inner wall of the sub- 

 dural space ; the cells upon its inner surface are thinner and are 

 derived from the walls of the subarachnoid space. 



Delicate septa-like bands pass from the inner surface of the 

 arachnoid to the adjacent portions of the pia mater. These proc- 

 esses are likewise invested by the endothelial lining of the sub- 

 arachnoid space. A similar investment clothes the processes of 

 the ligainenlui* dentatum of the spinal cord which attaches the pia 

 mater spinalis on either side to the adjacent portions of the dura 

 mater. 



A fibrous septum passing from the arachnoid to the pia mater, 

 along a line opposite the posterior median fissure of the spinal cord, 

 forms a fairly definite partition, the septum posticum. In the cer- 

 vical region this is an uninterrupted septum, but in. the thoracic 

 and lumbar regions it is incomplete. The lymphatics of the arach- 

 noid membrane communicate with those of the pia mater through 

 this and other septa-like bands which unite the two membranes. 



The cranial arachnoid, in the vicinity of the cranial sinuses, 

 sends outward many villus-like projections or arachnoid villi 



