THE CENTRAL NERVOUS SYSTEM. 



1023 



The spinal cord, therefore, hangs 

 FIG. 878. 





Pons 



Arachnoid 



Medulla 



the pia, containing the ccrcbro-spinal Jluid. 

 suspended within the tube of dura, 

 surrounded by a cushion of fluid 

 an arrangement well adapted to insure 

 the nervous cylinder against the inju- 

 rious effects of shocks and of undue 

 pressure during changes in the position ( - 

 of the spine. Both spaces, but par- 

 ticularly the subarachnoid, are crossed 

 by fibrous trabeculae and thus imper- 

 fectly subdivided into secondary com- 

 partments, all of which are lined with 

 endothelium. 



The spinal cord is fixed within the 

 loose dural sheath not only by the root- 

 fibres of the spinal nerves that pass 

 between the cord and the outer envelope, 

 but also by two lateral fibrous bands, the 

 ligamenta denticulata, that are continu- 

 ous with the pia along the cord, one on 

 each side. Mesially they are attached 

 between the anterior and posterior root- 

 fibres and externally to the inner surface 

 of the dura by the tips of pointed pro- 

 cesses, about twenty-one in all, that 

 stretch across the subarachnoid space, 

 which they imperfectly divide into a 

 general anterior and a posterior com- 

 partment. The ligaments, covered by 

 prolongations of the arachnoid, extend 

 the entire length of the cord, the first pro- 

 cess being attached to the margin of the 

 foramen magnum, immediately above 

 the vertebral artery as it pierces the dura. 

 The succeeding ones meet the dura 

 between the pairs of spinal nerves, the 

 lowest process lying between the last 

 thoracic and the first lumbar nerve. 

 In the cervical and thoracic region, a 

 median fibrous band, the septum posticum, connects the posterior surface of the cord 



FIG. 879. 



Anterior 

 roots of 

 spinal nerves 



., Dura. 



reflected 



\ Spinal cord, 



covered with 



v '. ,' arachnoid 



. * and pia 



Upper part of spinal cord within dural sheath, which 

 has been opened and turned aside ; ligamenta denticulata 

 and nerve-roots are shown as they pass outward to dura. 



Dural sheath 

 Periosteum 



Spinal cord 



Posterior root 



Ligamentum 

 denticulatum 



Extradural 



areolar tissue 



\nterior root 



Spinal ganglion 



Spinal nerve 



Vertebral artery . 

 Body of fourth cervical vertebra - 



Transverse section of vertebral canal at level of fourth cervical vertebra, spinal cord in position. 



with the dura and partially subdivides the subarachnoid space. Lower, this partition, 



