VII.] THE POSTERIOR FISSURE. 187 



the greater number of nuclei present in the former. The 

 outlines of the cells are more clearly marked and somewhat 

 more angular in shape than they were on the fourth day. 



The distinctions between the several parts of the grey matter are chiefly 

 brought about by variations in the number of nuclei in a given area. Throughout 

 the cord fibres of the grey matter seem to be continuous with the epithelium of 

 the neural canal, but this is much more strongly marked in the posterior than in 

 the anterior region. In the posterior region also, it is still much more difficult 

 to trace the roots of the nerves than in the anterior. 



Of the three columns into which the white matter on 

 each side is divided, the anterior column differs from the 

 posterior in being thicker and also in having wider meshes 

 and fewer granules. The lateral column is the most granular 

 of all and very conspicuous. The minute structure of the 

 white matter remains about the same as on the fourth day. 



Meanwhile an alteration is taking place in the external 

 outline of the cord. From being, as on the fourth and fifth 

 days, oval in section, it becomes, chiefly through the increase 

 of the white matter, much more nearly circular. 



On the seventh day the most important event is the 

 formation of the posterior fissure. 



This is brought about by the absorption of the roof of 

 the posterior of the two parts into which the neural canal 

 has become divided. 



Between the posterior horns of the cord, the epithelium 

 forming the roof of the, so to speak, posterior canal is 

 along the middle line covered neither by grey nor by 

 white matter, and on the seventh day is partially absorbed, 

 thus transforming the canal into a wedge-shaped fissure, 

 whose mouth however is seen in section to be partially 

 closed by a triangular clump of elongated cells (Fig. 59 c.). 

 Below this mass of cells the fissure is open. It is separated 

 from the 'true spinal canal ' by a very narrow space along 

 which the side walls have coalesced. In the lumbar and 

 sacral regions the two still communicate. 



We thus find, as was first pointed out by Lockhart Clarke, 

 that the anterior and posterior fissures of the spinal cord 

 are, morphologically speaking, entirely different. The ante- 

 rior fissure is merely the space left between two lateral 

 downward growths of the cord, while the posterior fissure is 

 part of the original neural canal separated from the rest of 



