186 THE FIFTH DAY. [CHAP. 



pressed canal come into absolute contact in the middle. So 

 that sections no longer shew an hour-glass cavity, but two 

 more or less elliptical cavities, representing the former term- 

 inal enlargements, one anterior and one posterior, separated 

 by a neck in which the epithelium of the one side is closely 

 applied to that of the other. In other words, the original 

 single canal has been divided longitudinally into an anterior 

 and posterior canal. Of these the anterior will alone remain 

 as the permanent central canal of the spinal cord. In the 

 lumbar region this division has as yet not taken place. 



The anterior white columns have very much increased 

 in quantity ; the posterior white columns have also become 

 distinct, and the two form together a thick covering for the 

 grey matter. The two columns of each side are continuous 

 with each other, but their line of junction is clearly marked ; 

 and on the sixth day there may be seen at this spot a small 

 mass of white matter, differing somewhat from the rest in 

 appearance, which perhaps may be looked upon as the first 

 commencement of the lateral column. The columns of the 

 one side are not continuous with those of the other either 

 posteriorly or anteriorly. In other words, there are as yet 

 no white commissures. 



The anterior ends of the cord on each side of the middle 

 line have commenced to grow downwards. These outgrowths, 

 in which both the white and the grey matter take part, 

 have an important function. They enclose between them a 

 somewhat linear space : the commencement of the anterior 

 fissure. This, which is at first not very deep and rather wide, 

 may be noticed already on the fifth day (L. Clarke) and on 

 the sixth day is very clearly marked. 



Corresponding with these grosser changes, certain histo- 

 logical features make their appearance. Between the an- 

 terior and posterior parts into which the grey matter is 

 divided on each side, or, as we may now call them, the 

 anterior and posterior cornua, there is found a rather lighter 

 band of grey matter in which the nuclei are somewhat more 

 scattered. The anterior cornu exhibits a further division 

 into an outer and upper part, and a lower and inner part, 

 in both of which the nuclei are more numerous than in the 

 intervening mass. The posterior cornu is of considerably 

 darker colour than the anterior, the difference being due to 



