THE DISSECTION OF THE BACK 95 



that those terms apply to the regions to which the nerves of 

 the same name are attached. 



Wherever there is an increase in the size of the nerves 

 attached to a particular part of the medulla spinalis, there a 

 corresponding increase of the grey matter may be noticed. It 

 follows that the districts in which the grey matter bulks 

 most largely are the lumbar and cervical enlargements. 

 The great nerves which go to form the limb plexuses enter 

 and pass out from those portions of the medulla spinalis. In 

 the intervening thoracic region there is a reduction in the . 

 quantity of grey matter, in correspondence with the smaller 

 size of the thoracic nerves. 



The shape of the columns of grey matter, in section, is not 

 the same in all regions. In the thoracic region both columns 

 are narrow, although the distinction between the anterior grey 

 column and the more attenuated posterior grey column is still 

 sufficiently manifest. In the cervical region the contrast 

 between the grey columns is most marked ; the anterior grey 

 column is very thick in comparison with the posterior grey 

 column. In the lumbar region, on the other hand, the 

 difference in the thickness of the two grey columns is not 

 nearly so apparent, owing to a broadening out of the 

 posterior grey column. A section taken from the centre of 

 each region can very readily be recognised by the features 

 mentioned (Fig. 27). 



In the thoracic region of the spinal medulla, more especi- 

 ally in the upper part, there is another character which is 

 very distinctive. A pointed and prominent triangular pro- 

 jection juts out from the lateral aspect of the crescentic mass 

 of grey matter, nearly opposite the grey commissure. It is 

 called the lateral grey column (Fig. 27, B and C). It dis- 

 appears in the cervical and lumbar enlargements generally, 

 but again becomes evident both in the upper cervical and in 

 the lower sacral regions. 



Below the thoracic region the postero- median septum 

 diminishes and the antero-median fissure increases in depth, 

 until, in the sacral region, they are almost equal in depth and 

 the central canal occupies the centre of the medulla spinalis. 

 White Matter of the Medulla Spinalis. The white matter 

 forms a thick coating on the outside of the fluted column of 

 grey matter. It is marked off into three funiculi. The 

 posterior funiculus is wedge-shaped in transverse section, 



