912 



THE NERVOUS SYSTEM 



Further, it is continuous at intervals with both the pia mater and arachnoid by way of the 

 connective-tissue sheaths of the nerve-roots which are prolonged from the pia and blend with 

 the dura mater in the passage of the nerve-roots through it. The dura is also pierced by the 

 spinal rami of the vertebral arteries, and the connective tissue of the outer walls of these vessels 

 blends with all three of the meninges. The filum terminale of the pia mater extends below the 

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

 there blends with it in line with the coccygeal ligament of the outer surface. 



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

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

 seven segments below the termination of the spinal cord. This extension con- 

 tains 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 



Fig. 718. — The Dura Mater Encephali op the Base of the Cranium. 

 (After Toldt, "Atlas of Human Anatomy," Rebman, London and New York.) 



Position of crista galli 

 Circular sinus 



Process of dura in foramen caecum 



Optic nerve ^ 

 Ophthalmic vein 



Cavernous sinus 



Connection with 

 the rete f oraminis 

 ovalis 



Middle meningeal 

 artery 



Inferior petrosal 

 sinus 



Internal carotid 

 artery 



Bulb of the internal 

 jugular vein 



-— Optic nerve 



Maxillary nerve 



Transverse 



Superficial petrosal 

 nerve 



Mastoid vein 



Vertebral artery 



Fold of dura mater ^ 



Hypoglossal nerve 

 First spinal ner^e 



other region. As each pair of nerve-roots of the cauda equina passes outward, 

 they lie free for a variable distance in a tubular extension of the dura before the 

 latter blends with and contributes 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 arachnoid is quite closely applied to the inner surface of the dura mater. It 

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

 between 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 

 Pacchionian l)odies, and its fluid iHlikcwisoin contact with the blood-vessels passing throughit. 

 It is probably continuous with tlu; lymph-spaces of the norve-roots passing through it, for 

 colored fluids inj(!cted 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. 



