THE DISSECTION OF THE BACK 183 



plane anterior to that ligament opposite each vertebral body. 

 Each transverse vein receives large tributaries from the in- 

 terior of the vertebra. Superiorly, each of the main longitudinal 

 channels communicates with the occipital sinus or the basilar 

 plexus within the cranium ; and each of the posterior channels 

 gives off a branch which emerges above the posterior arch of 

 the atlas to join the commencement of the vertebral vein. 

 Opposite the various intervertebral fibro-cartilages the anterior 

 plexus sends off f branches which run towards the inter- 

 vertebral foramina, where they join with corresponding branches 

 of the posterior plexus, to form the intervertebral veins which 

 accompany the corresponding spinal nerves. 



Meninges of the Medulla Spinalis (Fig. 74). The medulla 

 spinalis, like the brain, with which it is continuous, is enveloped 

 by three membranes termed meninges. The most external 

 investment is a strong fibrous membrane called the dura 

 mater; the second, in order from without inwards, is a non- 

 vascular tunic termed the arachnoid '; whilst the third and 

 most internal is the pia mater. These membranes are 

 directly continuous with the corresponding investments of 

 the brain. 



Dissection. The outer surface of the dura mater must now be cleaned. 

 This is effected by removing the loose areolar tissue, soft fat, and posterior 

 intraspinal veins from the vertebral canal. It is necessary, also, to define 

 carefully the numerous lateral prolongations which the membrane gives to 

 the spinal nerves. 



Dura Mater Spinalis (Fig. 74). In the vertebral canal the 

 dura mater constitutes an exceedingly dense and tough fibrous 

 tube, which extends from the foramen magnum above, to the 

 level of the second or third piece of the sacrum below. It is 

 separated from the walls of the vertebral canal and its lining 

 periosteum by an interval, which is filled by loose fat and areolar 

 tissue, and the internal vertebral venous plexus. Even 

 before the membranous tube is laid open, the dissector can 

 readily satisfy himself that it forms a very loose sheath around 

 the spinal medulla and the nerve -roots which form the cauda 

 equina ; in other words, it is very capacious in comparison 

 with the volume of its contents. Its calibre, moreover, is by 

 no means uniform ; in the cervical and lumbar regions it is 

 considerably wider than in the thoracic region, whilst in the 

 sacral canal it rapidly contracts and finally ends by blending 

 with the filum terminate, a fibrous thread which is prolonged 

 ii 12 c 



