TllK CRAMAL 1)1' HA MATER 



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nerve-roots through it. The dura is also pierced by the spinal mini of the vertebral 

 arteries, and the connecl i\e tissue of the outer walls of these vessels lilends with all 

 three of the meninges. The lilum terniinale of the ]>ia mater extends below the ter- 

 mination of the spinal cord into the point of the funnel-shaped end of the dura mater, 

 and t here blends with it in line with the coc cyueal ligament of the outer surface. 



The tube of the spinal dura mater varies in calibre with the variations in the diam- 

 eter of the spinal cord. However, the termination of its cavity occurs about seven 

 segments below the termination of the spinal cord. This extension contains the long 

 intra-dural nerve-roots forming the cauda equina. and the calibre of this part, before 

 its sudden contraction, is about as great as that found in any other region. As each 

 pair of nerve-roots of the cauda equina passes outwards, they lie free for & variable 

 distance in a tubular extension of the dura before the latter blends with and con- 

 tributes to the thickness of their sheath. 



The subdural cavity, the space between the dura mater and the arachnoid, is 

 the thinnest of the meningeal spaces. Along the ventral aspect especially, the spinal 

 arai tmoid is quite closely applied to the inner surface of the dura mater. It con- 

 tains a small amount of cerebro-spinal fluid (lymph) which prevents friction between 



Fio. 057. VIEW OF MEMBRANES OF SPIXU. Conn FHOM VENTRAL ASPECT. (Ellis.) 



Spinal dura mater 



Spinal arachnoid 



DORKAL ROOT 

 r/.-.vr/.M/, KOOT 



Lieamentuin dentioulatum 

 Llnea splendena 



the opposing surfaces, and is continuous with the fluid in the like space of the 

 cranial meninges. The space communicates with the venous sinuses of the cranium 

 in the region of the 1'acchionian bodies, and its fluid is likewise in contact with the 

 blood-vessels passing through it. It is probably continuous with the lymph-spaces 

 of the nerve-roots passing through it, for colored fluids injected into it pass into the 

 nerve-roots. The arachnoid is so thin and gauze-like that a ready interchange of 

 fluids between this space and the subarachnoid space is possible by simple filtration. 



The cranial dura mater. The dura mater investing the brain performs a double 

 function it serves as an internal periosteum for the cranial bones and gives support 

 and protection to the brain. In conformity with its double function it consists of 

 two layer.-, easily separable in the child, but closely adhering to each other in the 

 adult . except in occasional localit ies, where there exist small clefts lined with epithe- 

 lium. The large blood sinuses and venous lacuiuc are placed between the two layers 

 and the semilunar ganglia of the trigemini also lie between them. The cranial dura 

 beirins with the adhesion of the spinal dura mater to the periosteum of the foramen 

 magnum, and it forms a sac-like envelope about the entire encephaloii. Consisting 

 of two layers, it is a much thicker membrane than that of the spinal cord. 



The outtr niirfnir of the cranial dura mater when torn away from the cranial 



