82 HEAD AND NECK 



above and below than in the thoracic region connect the tube 

 of dura mater to the posterior longitudinal ligament of the 

 vertebral column. No connection of any kind exists between 

 the dura mater and the vertebral arches or ligamenta flava. 



Dissection. The tube of dura mater may now be opened with 

 the scissors. The incision should be carried through the mem- 

 brane in the median plane. Care, however, must be taken not 

 to injure the delicate arachnoid, which is subjacent. 



Cavum Subdurale. The subdural cavity is the capil- 

 lary interval between the dura mater and the arachnoid 

 (Fig. 20). The deep surface of the dura, which is turned 

 towards the space, is smooth, moist and polished. The 

 dissector will notice, upon each side, the series of apertures 

 of exit for the roots of the spinal nerves. They are ranged 

 in pairs opposite each intervertebral foramen. The subdural 

 space is prolonged laterally, for a short distance, upon each 

 of the nerve-roots, and has a free communication with the 

 lymph paths present in the nerves. 



Viewed from the inside of the tube of dura mater, each of the two 

 nerve-roots belonging to a spinal nerve is seen to carry with it a special 

 and distinct sheath. When examined, however, on the outside of the tube 

 of dura mater, they appear to be enveloped in one sheath, because the two 

 sheaths are closely held together, on the outside, by intervening connective 

 tissue which can be removed with a little careful dissection. When that is 

 done, the two tubular sheaths will be seen to remain distinct as far as the 

 ganglion on the posterior root of the nerve. At that point they blend 

 with each other. 



Arachnoidea Spinalis (Fig. 20). The arachnoid, like the 

 dura mater, forms a loose, wide investment for the spinal 

 medulla. Unlike the dura, however, it is remarkable for its 

 great delicacy and transparency. The sac is most capacious, 

 and can be demonstrated most easily towards its lower part, 

 where it envelops the extremity of the spinal medulla and 

 the collection of long nerve-roots which constitute the cauda 

 equina. Make an incision into it, and insert the handle of 

 the scalpel, or, better still, inflate the sac with air by means 

 of a blowpipe. Above, the spinal arachnoid becomes con- 

 tinuous, through the foramen magnum, with the arachnoid 

 membrane of the brain. On each side, it is prolonged upon 

 the various nerve-roots, contributing a tubular sheath to each. 

 It ends, below, at the level of the second sacral vertebra, by 

 fusing with the filum terminale. 



Cavum Subarachnoideale (Fig. 20). The sub-arachnoid 



