CH. XLI.] MEMBRANES OF BRAIN AND SPINAL CORD. 603 



Membranes of the Brain and Spinal Cord. The Brain and Spinal 

 Cord are enveloped in three membranes (i) the Dura Mater, (2) the 

 Arachnoid, (3) the Pia 

 Mater. 



(1) The Dvra Mater, 

 or external covering, is 

 a tough membrane com- 

 posed of bundles of 

 connective tissue which 

 cross at various angles, 

 and in whose interstices 

 branched connective- 

 tissue corpuscles lie : it 

 is lined by a thin elastic 

 membrane, on the inner 

 surface of which is a 

 layer of endothelial cells. 



(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 mem- 

 brane. 



(3) The Pia Matrr of 

 the cord consists of two 

 layers between which 

 numerous blood-vessels 

 ramify. In that of the 

 brain only the innerof the 

 two layers is represented. 

 Between the arachnoid 

 and pia mater is a net- 

 work of fibrous tissue 

 trabeculae sheathed with 

 endothelial cells : these 

 sub-arachnoid trabeculae 

 divide up the sub-arach 

 noid space into a num- 

 ber of irregular sinuses. 

 There are some similar 

 trabeculae, but much 

 fewer in number, tra- 

 velling the sub-dural 

 space, i.c., the space 

 between the dura matei 

 and arachnoid. 



Fig. 449. View of the eere- 

 bro-dpinal axis of the 

 nervous system. The 

 rifrht half of the cranium 

 and trunk of the body 

 has been removed by a 

 vortical section ; the 

 membranes of the brain 



and xpinal cord have also been removed, and the rootn and Brut part of the fifth nnd 

 ninth cranial, and of all the sjiin.il nerves of the right ride, have been dissected out 

 and laid separately on the wall of the skull and on the several vcrtebne opposite to 

 the place of their natural exit from the cranio-epinal cavity. (After Bourgry.) 



