THE DISSECTION OF THE BACK 185 



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 membrane in the 

 median plane. Care, however, must be taken not to injure the delicate 

 arachnoid, which is subjacent. 



Cavum Subdurale. The subdural cave (O.T. space) is the 

 capillary interval between the dura mater and the arachnoid 

 (Fig. 74). The deep surface of the dura, which is turned 

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

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

 of exit for the roots of the spinal nerves. These 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 this 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. 74). The arachnoid resembles 

 the dura mater in forming a loose, wide investment for 

 the spinal medulla. Unlike the dura, however, it is remark- 

 able for its great delicacy and transparency. The sac which 

 it forms 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 arachnoid 

 becomes continuous, at the level of the foramen magnum, with 

 the arachnoid membrane of the brain, whilst, laterally, it is 

 prolonged upon the various nerve-roots, thus contributing 

 to each a tubular sheath. It terminates blindly below at the 

 level of the second or third sacral vertebra. 



Cavum Subarachnoideale (Fig. 74). This term is applied 

 to the wide space between the arachnoid and pia mater. It 

 is occupied by a variable amount of cerebro-spinal fluid, and 



