SPINAL DURA MATER 



911 



canal, though continuous at the foramen magnum with the outer layer of the 

 cranial dura mater, is not considered a part of the spinal dura mater, from the 

 fact that it is so widely separated from the layer actually investing the spinal 

 cord. Thus, since the cranial and spinal portions of the dura mater differ, they 

 are described separately. 



The spinal dura mater is a fibrous tube with funnel-shaped caudal end which 

 encloses and forms the outermost support of the spinal cord. It consists of but 

 one layer and this corresponds to the inner layer of the cranial dura mater. It 

 begins at the foramen magnum and terminates in the spinal canal at about the 

 level of the second piece of the os sacrum. It is firmly attached to the periosteum of 

 the surrounding bones only in certain localities: — 



(1) The upper end of the tube blends intimately with the periosteum of the margin of the 

 foramen magnum, and thus in this locahty it becomes continuous with the outer layer of the 

 cranial dura mater. Also in this locality it is attached firmly, though less intimately, to the 

 periosteum of the posterior surfaces of the second and third cervical vertebrae. This locahty 

 may be considered the upper fixation-point of the spinal dura mater. (2) It extends laterally 

 and contrilautes to the connective tissue investments of each pair of spinal nerves, and as such 

 it passes into the intervertebral foramina and becomes continuous with the periosteum 

 lining each. (3) Along its ventral aspect the spinal dura mater is attached by numerous proc- 



FiG. 717. — View of Membranes of Spinal Cord from Ventral Aspect. (ElUs.; 



Spinal dura mater- 



Spinal arachnoid 



Dorsal root 



Ventral root 



Ligamentum denticulatum 



Linea splendens 



esses to the posterior longitudinal ligament of the vertebral canal. These attachments are 

 more or less delicate, loose, and irregular, and are easily torn or cut in removing the speci- 

 men. They are stronger and more numerous in the cervical and lumbar regions than in the 

 thoracic. (4) In the space between the dura and the walls of the vertebral canal (epidural 

 cavity) lies the rich internal vertebral venous plexus, and along the lateral aspect the dura is 

 occasionally connected with the periosteum through the tissue of the walls of the vessels of this 

 plexus, especially in case of the vessels which penetrate the dura. Along its dorsal aspect the 

 spinal dura mater is practically free from the wall of the vertebral canal. (5) At its lower 

 and funnel-shaped extremity, opposite the second sacral vertebra, the tube suddenly contracts 

 into a filament extending into the coccyx and breaking up into a number of processes which 

 become continuous with the periosteum of the dorsal surface of the coccyx. This filament 

 is the coccygeal ligament or filum of the dura mater, and its attachment may be considered 

 the lower fixation-point of the spinal dura mater. (See figs. 613 and 715). The extent of the 

 tube is maintained chiefly by means of the two fixation-points, for all the_other attachments 

 are sufficiently loose to permit of the movements of the vertebral column. 



The inner surface of the spinal dura mater appears smooth, but upon closer 

 examination it is found to be connected with the arachnoid by a few delicate sub- 

 dural trabeculse — occasional fine strands of connective tissue bridging the sub- 

 dural space (fig. 725). Along its lateral aspects the inner surface is at intervals 

 quite firmly attached to the pia mater by the dentations of the ligamenta dentic- 

 ulata, which are prolonged through the arachnoid. 



