390 A MANUAL OF ANATOMY. 



Beginning above with a pair of curved bone cutters (having a blunt-pointed 

 lower blade), carefully sever all the laminae on both sides from above down- 

 ward, keeping as close to their outer extremities as possible. Remove the 

 laminae, the spinous processes, and connecting ligaments. Clean the posterior 

 surface of the dura. Carefully trace outward the nerve trunks as they pass 

 from the dura to enter the intervertebral foramina. 



Open the dura carefully from above downward. 



The Spinal Dura. Figs. 80, 81. 



This membrane is similar in structure to the cranial dura, 

 with which it is continuous at the foramen magnum. It 

 differs in this point that it does not in the spine form the 

 inner periosteum for the vertebrae as it does for the cranial 

 bones, though it is connected to the periosteum and liga- 

 ments of the former by a loose connective tissue. It extends 

 from the foramen magnum to about the second or third 

 sacral vertebra, beyond which it is prolonged as a cord-like 

 sheath for the filum terminate of the cord, and is attached 

 to the base of the coccyx. 



The dura is pierced by the spinal nerve, around which 

 it is prolonged in the shape of funnel-like sheaths for a 

 short distance, until it blends with the sheaths of the 

 nerves. 



The inner surface of the dura is smooth, as in the brain, 

 and is separated in a similar manner from the arachnoid 

 membrane by the subdural space. The spinal dura does 

 not send processes into the fissures of the cord. 



The Spinal Arachnoid. 



This reaches as low as the dura, to the inner surface of 

 which it is closely applied (but separated from it by the 

 subdural space, which in the normal condition is very nar- 

 row). It surrounds the nerves within the tubular sheaths 

 of the dura and terminates like that membrane by becom- 

 ing continuous with the sheaths of the nerves. 



