THE MENINGES AND BLOOD SUPPLY 



621 



tissue which contains relatively few elastic fibers but is said to contain 

 neither blood-vessels, lymphatics nor nerves. This thin fibrous mem- 

 brane is covered on either side by a layer of endothelium; that upon 

 its outer surface consists of cells of considerable thickness, which are 

 derived from the lining membrane of the inner wall of the subdural 

 space; the cells upon its 

 inner surface are thinner 

 and are derived from 

 the walls of the sub- 

 arachnoid space. 



Delicate septa - like 

 bands pass from the in- 

 ner surface of the arach- 

 noid to the adjacent por- 

 tions of the pia mater 

 (Fig. 535). These proc- 

 esses are likewise invest- 

 ed by the endothelial 

 lining of the subarach- 

 noid space. A similar 

 investment clothes the 

 processes of the ligamen- 



FIG. 535. SECTION OF THE SPINAL CORD AND ITS 

 MEMBRANES, FROM THE UPPER THORACIC RE- 

 GION. 



a, dura mater; b, arachnoid; c, septum posticum; 

 d, e, f, subarachnoid trabeculae; n, nerve fiber 

 bundles; g, ligamentum denticulatum; I, subarach- 

 noid space. (From Schafer, after Key and Retzius.) 



lum denticulatum of the 



spinal cord which at- 



taches the pia mater spi- 



nalis on either side to the 



adjacent portions of the 



dura mater. Subarachnoid trabeculae support the nerve fibers of the 



dorsal roots. 



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

 a line opposite the dorsal median septum of the spinal cord, forms a 

 fairly definite partition, the septum posticum. In the cervical region this 

 is an uninterrupted septum, but in the thoracic and lumbar regions it 

 is incomplete. 



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

 the superior longitudinal sinus, sends outward many villus-like projec- 

 tions or arach noid tn^t(Pacchionian bodies; granulationes arachnoiifales) 

 which protrude into the venous sinuses to such an extent as often to 

 produce corresponding depressions in the inner surface of the bones of 

 the cranial vault, into which they push, carrying before them a much 



