CHAP, ii.] THE BRAIN. 943 



help to break up the grey matter (which previously formed the 

 anterior horns) into what is called the reticular formation. We 

 shall return to this presently, but may here call attention to a 

 special development of these decussating fibres which is seen 

 just above the decussation of the pyramids. In a section at this 

 level (Fig. 109, 3) a strand of fibres (supra Py. dec.) may be seen 

 to start chiefly from the gracile nucleus but also to some extent 

 from the cuneate nucleus, to sweep round the central grey 

 matter, and to decussate ventral to this between it and the 

 bottom of the anterior fissure. This is called the superior de- 

 cussation, or, for reasons which we shall see later on, the sensory 

 decussation. 



607. We must now turn to the posterior fissure and its 

 relations to the fourth ventricle. We saw that at the beginning 

 of the pyramidal decussation, the posterior horns had been 

 thrown backwards and outwards so as to increase the posterior 

 columns. The posterior fissure is still of great depth, so that by 

 the increase of breadth and maintenance of depth the posterior 

 column, the lateral limit of which is still sharply marked out 

 by the swollen head of the posterior horn as well as by the highest 

 posterior rootlets of the first cervical nerve, acquires at this level 

 its maximum of bulk. 



From this point forward the depth of the posterior fissure and 

 the dorso-ventral diameter of the posterior columns diminishes. 

 The head of the horn (Fig. 109, 2) is thrown still further outwards 

 into the lateral regions ; developments of grey matter at the base 

 and to some extent at the neck of the horn (of these we shall 

 speak presently) encroach (Fig. 109, 3) dorsally on the white 

 matter of the columns ; and the central grey matter appears to 

 rise dorsally at the expense of the posterior fissure, in coincidence 

 with the development described above as taking place on the 

 ventral side of the canal. 



Still a little further forward, in a section for instance (Fig. 109, 

 4) a little way behind the apex of the calamus scriptorius, the 

 central grey matter, which still forms a rounded mass around the 

 central canal, is brought yet nearer to the posterior fissure. 



In a section yet a little further forward (Fig. 109, 5) carried 

 through the hinder narrow part of the fourth ventricle itself, it is 

 seen that the central canal has opened out on to the dorsal surface, 

 and that the grey matter, which in previous sections surrounded 

 it, is now exposed to the surface on the floor of the ventricle, the 

 median posterior columns being thrust aside. In a still more 

 forward section (Fig. 109, 6) this grey matter in correspondence 

 with the increasing width of the ventricle occupies a still wider 

 area, thrusting still further aside the narrowing upper ends of the 

 two posterior columns. 



During these successive changes, the large wide posterior 

 (both external posterior and median posterior) columns of the 



F. 60 



