41 8 SPINAL CORD. 



The exact mode of its formation appears to me to be still 

 involved in some obscurity. 



In the Elements of Embryology the development of the posterior fissure 

 was described in the following way : 



" 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. 246, 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 speak- 

 ing, entirely different. The anterior 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 the cavity (which goes 

 to form the true spinal canal) by a median coalescence of the side walls." 



I confess that I have some doubts as to the complete accuracy of the 

 above statement. 



Kolliker gives a full account of the gradual atrophy of the central canal ; 

 but I do not fully understand his statements with reference to the formation 

 of the posterior fissure, which in fact appears to be only incidentally 

 mentioned. It would seem from his account that a shallow and somewhat 

 wide dorsal fissure is formed to start with, in the human embryo, by two 

 projections of the posterior white horns. On the atrophy of the central 

 canal this furrow becomes narrowed, but Kolliker does not definitely state 

 how it becomes deepened so as to give rise to the permanent dorsal fissure. 



It seems to me probable, though further investigations on 

 the point are still required, that the dorsal fissure is a direct 

 result of the atrophy of the dorsal part of the central canal 

 of the spinal cord. 



The walls of the canal coalesce dorsally, and the coalescence 

 gradually extends ventralwards, so as finally to reduce the 

 central canal to a minute tube, formed of the ventral part of the 

 original canal. The epithelial wall formed by the coalesced 

 walls on the dorsal side of the canal is gradually absorbed. 



The epithelium of the central canal, at the period when its 



