388 DURA MATER. 



with the pia mater, the mother membranes, which give 

 origin to all the rest of the "body, is the outer one, cover- 

 ing the spinal cord, and continued upon the brain. It 

 is called tlieca vertebralis, and extends from the foramen 

 magnum, to which it adheres, down the spinal canal its 

 whole length, to the sacrum, where it sends off several 

 processes forming sheaths for the sacral nerves. It differs 

 from the cranial portion, at the posterior part of the canal, 

 in not being connected with the bones; but having, inter- 

 posed between it and the bony arches, a soft fatty tissue, 

 which has been compared to the marrow of the long bones. 

 The external surface, at this point, is smooth and covered 

 by a plexus of veins. In front, as just stated, it is attached 

 to the posterior vertebral ligaments. On each side it sends 

 off a tubular process or sheath, for each of the spinal nerves, 

 which extends beyond the intervertebral foramina, and 

 becomes lost on the cellular tissue. 



It has a much greater capacity than the cord it encloses, 

 is larger in its upper and lower portions, and contracted 

 in the middle. Its inner surface is smooth, and presents 

 double rows of openings through which pass the anterior 

 and posterior roots of the spinal nerves; here also is seen 

 the attachment of the ligamenta denticulata, or tooth-like 

 processes, disposed laterally. 



Structure. Its structure is essentially fibrous, having 

 the fibres, which are white, running in various directions, 

 and in some places, as in the brain, separable into two 

 laminas. It is a strong, resisting, inelastic tissue, resem- 

 bling the sclerotic coat of the eye, and, like it, admirably 

 adapted for protecting the spinal marrow and the brain. 



The blood-vessels of the spinal portion of the dura mater 

 are the arteries which come from the vertebral, intercostal, 

 lumbar, and sacral. The veins accompanying the arteries 

 are found to terminate in the two long vertebral sinuses, 

 which extend the whole length of the spine on the back 

 part of the bodies of the vertebrae. 



Tunica arachnoidea (am^, a spider's web, t-5o f , likeness) 

 is the next membrane, after removing the dura mater, and 



