VII.] THE WHITE COLUMNS. 189 



the cavity (which goes to form the true spinal canal) by 

 a median coalescence of the side walls. 



The lateral white columns have on the seventh day 

 increased in size and become less granular, and the lines of 

 junction between them and the anterior columns have now to 

 be arbitrarily selected. The posterior white columns are 

 still much thinner and more granular than the anterior. The 

 nuclei of the white matter are more numerous than before. 



Some of the septa of the white matter can now be traced in the one 

 direction into the grey matter, and in the other direction into the connective 

 tissue around the cord. Whether these are nerve-fibres which have separated 

 irom the remainder of the fibres, to enter the cord at a different point, or are 

 merely trabecuiae of connective tissue, cannot be absolutely determined. The 

 latter view however seems most probable. In the grey matter, the anterior 

 and posterior divisions are better distinguished than at an earlier date. In 

 particular the nuclei of the cells of the posterior division are both smaller and 

 more numerous than those of the anterior. Some of the fibres from the 

 posterior root, after entering the grey matter, quickly pass out again into the 

 posterior column of the white matter. 



In the anterior division of the grey matter, near the entrance of the anterior 

 roots, there is a peculiar and well-marked mass of somewhat triangular cells, 

 with large and distinct nuclei, more deeply stained with carmine than the 

 remainder of the grey matter. This mass of cells is present in the lumbar and 

 sacral regions, but is deficient or very inconspicuous in the dorsal portion of the 

 cord. The nuclei of the whole anterior region of the grey matter have increased 

 in size, and the cells to which they belong (when clearly visible) are usually 

 iound to be angular. 



Around the true spinal canal, the line of separation between the epithelium 

 and the grey matter is sharply defined, but elsewhere is very indistinct. 



By the end of the seventh day, the following important 

 parts of the cord have been definitely established : 



(1) The anterior and posterior fissures. 



(2) The anterior and posterior horns of grey matter. 



(3) The anterior, posterior and lateral columns of white 

 matter. 



(4) The spinal canal. 



As yet, however, the grey mass of the two sides of the 

 cord only communicate by the anterior grey commissure, and 

 the white columns of opposite sides do not communicate 

 at all. The grey matter, moreover, still far preponderates 

 over the white matter in quantity. 



By the ninth day the posterior fissure is fully formed, 

 and the posterior grey commissure has also appeared. 



In the centre of the sacral enlargement this commissure 

 is absent, and the posterior columns at a later period separate 



