THE MENINGES AXD BLOOD SUPPLY 561 



coats. The three layers composing this investment soon lose their 

 distinctive characteristics, fuse together, and blend with the epi- 

 neurium of the nerve trunks. 



Fluid injected into the subdural space may be readily forced 

 into the lymphatics of these epi- and perineural sheaths and may 

 thus travel to parts quite remote from the central nervous system. 



The subarachnoid space within the cranium is of limited 

 breadth, but within the spinal canal it is much broader and con- 

 tains not only the larger blood vessels which are loosely attached to 

 the surface of the pia, but also the many spinal nerve roots pass 

 downward through this space toward their foramina of exit. 



The subarachnoid space is lined by a thin endothelial layer, 

 its outer wall being formed by the arachnoid, its inner by the outer 

 surface of the pia mater ; its cavity is filled with cerebro-spinal- 

 fluid, which closely resembles, yet differs somewhat in chemical 

 composition from the lymph. This space is in communication 

 through the foramen of Majendie with the central canal of the 

 spinal cord and the ventricular cavities of the brain. It is also 

 thought to communicate with the cerebral ventricles at several 

 other points. 



The spinal portion of the subarachnoid space is crossed by a 

 posterior median septum, the septum posticum, laterally by the 

 ligamentum dentatum, and by several irregular but incomplete 

 septa which, like the ligamentum posticum, connect the pia mater 

 with the arachnoid. 



The ligamentum dentatum is a dense mass of fibrous tissue con- 

 taining few elastic fibres, which, beginning at the lateral surface of 

 the pia as a complete septum, passes, by about twenty-eight serra- 

 tions, across the subarachnoid space, and pushing the arachnoid 

 before it, is attached to the inner surface of the dura mater. The 

 serrations of the dentate ligament do not penetrate the subdural 

 space, for around the point of their attachment the surface of the 

 arachnoid is firmly adherent to the dura mater. Each serration is 

 invested by an endothelial coat continuous with the lining of the 

 subarachnoid space. 



Blood Supply. The blood supply of the central nervous system 

 is derived from vessels which lie within the folds of the pia mater. 

 The larger arteries form an anterior longitudinal group represented 

 in the spinal cord by the anterior spinal artery and its branches, 

 and in the brain by the vessels of the circle of Willis and their im- 

 mediate branches. 

 37 



