8o HEAD AND NECK 



(O. T. spinal cord) and its membranes are in sittt, but it is convenient to 

 describe them at this stage. Indeed, the dissection is one of considerable 

 difficulty, even under the most advantageous circumstances. They form 

 two main longitudinal venous channels, placed one upon each side of the 

 posterior longitudinal ligament of the vertebral bodies, and they are joined 

 by transverse branches which cross the median plane, anterior to that 

 ligament, opposite each vertebral body. Each transverse vein receives 

 large tributaries from the interior 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 oft" branches which 

 run towards the intervertebral foramina, where they join with correspond- 

 ing branches of the posterior plexus, to form the intervertebral veins which 

 accompany the corresponding spinal nerves. 



Meninges of the Medulla Spinalis (Fig. 21). The medulla 

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

 by three membranes, termed menmges. 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. The three 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 the removal of the loose areolar 

 tissue, soft fat, and posterior internal vertebral 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. 21). 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 with 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 below the spinal medulla ; 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 



