THE NERVOUS SYSTEM. 559 



outer surface also is a layer of endothelial cells very similar to those found in 

 serous membranes. (2.) The Arachnoid is a much more delicate membrane, very 

 similar in structure to the dura mater, and lined on its outer or free surface by an 

 endothelial membrane. 



Fig. 350. View of the cerebro-spinal axis of the nervous system. The right half of the 

 cranium and.trunk of the body has been removed by a vertical section; the membranes of the 

 brain and spinal cord have also been removed, and the roots and first part of the fifth and ninth 

 cranial, and of all spinal nerves of the right side, have been dissected out and laid separately on 

 the wall of the skull and on the several vertebne opposite to the place of their natural exit from 

 the cramo-spinal cavity. (After Bourgery.) 



(3.) The Pia Mater consists of two chief layers, between which numerous blood- 

 vessels ramify. Between the arachnoid and pia mater is a network of fibrous- 

 tissue trabeculse sheathed with endothelial cells: these sub-arachnoid trabeculw 



