196 HEAD AND NECK 



the fila of the posterior nerve-roots enter the medulla spinalis 

 (O.T. spinal cord) in accurate linear order. There is no 

 corresponding furrow on the anterior part of each lateral half 

 of the medulla spinalis in connection with the emergence of 

 the fila of the anterior nerve-roots ; and it should be noted 

 that these fila emerge over a broad area, which corresponds 

 in its width to the thickness of the subjacent anterior column 

 of grey matter. 



The medulla spinalis is composed of an inside core of grey 

 matter which is surrounded on all sides by an external coating 

 of white matter. 



Grey Matter of the Medulla Spinalis. The grey matter in 

 the interior of the medulla spinalis has the form of a fluted 

 column. When seen in transverse section, it presents the 

 shape of the letter H. In each lateral half of the medulla 

 spinalis there is a comma-shaped mass of grey matter, the 

 concavity of which is directed laterally. The crescents of 

 opposite sides are connected across the median plane by a 

 transverse band, which is called the grey commissure. The 

 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 it 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. This 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 commissure 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 crescentic 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 very blunt at its 

 extremity. Further, its extremity is separated from the surface 

 by a tolerably thick coating of white matter, through which 

 the fila of the anterior nerve-roots pass on their way to the 



