94 HEAD AND NECK 



postero-median septum passes from the surface of the medulla 

 spinalis to the grey commissure. The bottom of the antero- 

 median fissure is separated from the grey commissure by an 

 intervening strip of white matter which is termed the anterior 

 white commissure. In the grey commissure may be seen the 

 central canal of the spinal medulla. It is just visible to the 

 naked eye as a minute speck. The canal tunnels the entire 

 length of the spinal medulla, and opens above (after having 

 traversed the lower half of the medulla oblongata) into the 

 fourth ventricle of the brain. The portion of the grey com- 

 missure which lies posterior to the central canal is called the 

 posterior commissure ; the portion anterior to it receives the 

 name of anterior grey commissure. 



In each lateral mass of grey matter certain well-defined 

 parts may be recognised. The projecting portions which 

 extend posterior and anterior to the connecting transverse 

 grey commissure are termed the posterior and the anterior grey 

 columns. They can be distinguished from each other at a 

 glance. 



The anterior grey column is short, thick, and its anterior 

 margin is very blunt. Further, its anterior margin is separated 

 from the surface by a moderately thick coating of white matter, 

 through which the fila of the anterior nerve-roots pass on their 

 way to the surface. The thickened anterior margin of the 

 anterior column is termed its head, and the constricted part 

 close to the grey commissure is called the neck. The posterior 

 grey column^ in most localities, is narrow. Further, it is drawn 

 out to a fine edge, which almost reaches the bottom of the 

 postero-lateral sulcus. This sharp edge receives the name of 

 the apex of the posterior column ; the slightly swollen part 

 which succeeds it is the head of the posterior column ; whilst 

 the slightly constricted part adjoining the grey commissure 

 goes under the name of the neck of the posterior column. 



Covering the edge of the posterior column there is a sub- 

 stance which differs in its composition from the general mass 

 of grey matter, and presents a translucent appearance. It 

 is termed the substantia gelatinosa (Rolandi}. 



The grey matter is not present in equal quantity through- 

 out the entire length of the medulla spinalis. Therefore it is 

 necessary that it should be considered in different regions; and 

 it must be understood, when the terms cervical, lumbar, sacral, 

 etc. are applied to different portions of the spinal medulla, 



