570 



MEMBRANES OF BEAIN AND CORU. 



lu leaving the sknll, the dnra, mater is intimately attached to the 

 margin of the foramen magnum ; but within the vertebral caual it 

 forms a loose sheath around the cord {ihcca), and is not adherent to 

 the bones, which have an independent periosteum. Towards the lower 

 end of the canal, a few fibrous slips proceed from the outer surface of 

 the dura mater to be fixed to the vertebra?. The space intervening 

 between the wall of the canal and the dura mater is occupied by loose 

 fat, by watery areolar tissue, and by a plexus of spinal veins. 



Fig. 394. — The Cranium opened to snow the Falx of the Cerebkuji, and Tentorium 

 OF THE Cerebellum. (Allen Thomson.) 5 



a, right side of the falx cerebri ; a, its anterior narrow jmrt attached to the crista 

 galli ; h, tentorium cerebelli of tlie right side, united to the base of the falx cerebri 

 from 2 to 3, in the line of the sti'aight sinus, and attache<l to the superior border 

 of the petrous bone between 3 and 3' ; V, aperture between the right and left divisions 

 of the tentorium for the isthmus cei-ebri ; 1, 1, the superior longitudinal sinus ; 2, 2, the 

 inferior ; 3, 3, the lateral sinus ; 3, 3', the superior jietrosal sinus ; 3', is close to the 

 anterior clinoid process. 



Opposite each intervertebral foramen the dura-matral theca presents 

 two openings, placed side by side, which give passage to the two roots 

 of the corresponding spinal nerve. It is continued as a tubular pro- 

 longation on the nerve, and is lost in its sheath. Besides this, it is 

 connected with the circumference of the foramen by areolar tissue. 



The fibrous tissue of the dura mater, especially within the skull, is 

 divisible into two distinct layers, and at various places these layers 

 separate from eacli other and leave intervening channels, called sinuses. 

 These sinuses, which have been elsewhere described, are canals for 

 venous l)lood, and are lined with a continuation of the internal mem- 

 brane of the veins. 



The dura mater also sends inwards into the cavity of the skull "three 

 strong membranous processes, or jMvtitions, formed ])y dujilication of its 

 inner layer. Of these, one descends vertically in the median plane, and 



