7:^0 



THE SERVO US SYSTEM 



rougli and is loosely connected by fatty tissue to the ligaments and ])eriosteum 

 which line the neural canal. Some stronger retinacula connect it to the posterior 

 common ligament. Its inner surface, smooth and shining, is separated from the 

 arachnoid by a narrow subdural space. It is connected to the cord by a special 

 develo])ment of the pia mater — the ligamentum denticulatum. It is continuous 

 through the foramen magnum with the supporting layer of the dura mater of the 

 l)rain. It sends tul)ular prolongations around the spinal nerves. These prolonga- 

 tions gradually merge into the connective tissue sheaths (epineurium) of the nerves. 

 Each tube is divided by a septum into two compartments, one for the anterior, and 

 one for the jtosterior root of the spinal nerve. 



The differences Ijetween the dura mater of the brain and cord may be 

 sunnnarised as follows: — The dura mater of the cord does not send processes or 

 infoldings into the fissures of the cord. It represents only the supporting layer 

 of the dura mater of the brain. The periosteal layer is represented by the periosteum 

 of the neural canal, but is separated from the supporting layer bj'' fatty tissue, 

 and by some venous plexuses. Hence this periosteum is only brought into relation 



Fig. 436. — View of the Membranes of the Spinal Cord. (Ellis.) 



Dura mater 



POSTERIOR ROOT 

 ANTERIOR ROOT 



liigamentum denticulatum 

 Linea splendens 



witli the dura mater i)roper by the fact, already noticed (page 672), that the dura 

 mater of the cranium is directly continuous Ijy the medium of the posterior 

 occipito-axial ligament with the periosteum of the neural canal. 



The ARACHNOID is separated from the dura mater by a narrow subdural space, 

 and is comiected to the dura mater only opposite the attachments of the ligamentum 

 denticulatum, to be presently described. It is a thin membrane resembling the 

 cere])ral arachnoid, with which it is continuous. It sends tubular investments 

 along the nerve-roots. These investments eventually become continuous with the 

 perineural sheaths of the nerves. It extends downwards within the thcca as far as 

 the second or third sacral vertebra, forming a common investment to the bundle of 

 nerve-roots constituting the cauda equina. 



The subarachnoid tissue is nnich less ainnidant in the spinal ])orti(in than in 

 the cerel)ral part of the sul)arachiioid space. It ])asses from arachnoid to i»ia mater 

 in the form of delicate threads, which are separated from one another by compara- 

 tively wide intervals. It also gives sujiport to the nerve-roots as they cross the 

 subarachnoid s])ace (fig. 435). An incomplete membranous septum, the septum 

 posticum ( Key and Retzius), passes from arachnoid to pia mater opposite the 



