624 THE NEEVOUS SYSTEM 



teristics, fuse together, and blend with the epineurium 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 contains 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 cerebrospinal fluid, which 

 closely resembles, yet differs somewhat in chemical composition from the 

 lymph. It contains a few lymphocytes, estimated at five per cubic milli- 

 meter of fluid. This space is in communication through the foramen 

 of Majendie, an opening in the roof of the fourth ventricle, 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 den- 

 ticulatum, and by several irregular but incomplete septa which, like the 

 ligamentum posticum, connect the pia mater with the arachnoid. 



The ligamentum denticulatum is a dense mass of fibrous tissue 

 containing a few elastic fibers, which, beginning at the lateral surface of 

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

 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, branches 

 of the internal carotid and the vertebral arteries. 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 ves- 

 sels of the circle of Willis and their immediate branches. 



Two sets of vessels may be said to be distributed from these sources 



