SEC. 2. THE BULB. 



605. The spinal cord, as it ascends to the brain, becomes 

 changed into the more complex bulb, partly by a shifting of the 

 course of the tracts of white fibres, partly by an opening up of 

 the narrow central canal into the wide and superficial fourth 

 ventricle, but chiefly by the development of new grey matter. 



When the anterior, ventral, aspect of the bulb is examined 

 (Fig. 108, C), it will be seen that the anterior columns of the cord 

 are interrupted for some distance in the median line by bundles 

 of fibres (Py. dec} which, appearing to rise up from deeper parts, 

 cross over from side to side and so confuse the line of the anterior 

 fissure. This is the decussation of the pyramids, above which the 

 place of the anterior columns of the spinal cord is taken by two 

 larger, more prominent columns, the pyramids of the bulb (Py.)> 

 which are continued forwards to the hind margin of the pons. 

 On the outer side of, lateral to, each pyramid, lies a projecting oval 

 mass, the olivary body or inferior olive (pi.) separating the pyramid 

 from a column of white matter, the restiform body (R), which, 

 occupying the lateral region of the bulb, when traced backwards 

 appears to continue the line of the lateral column of the cord, and 

 when traced forwards is seen to run up to the cerebellum as the 

 inferior peduncle of that organ. On the posterior dorsal aspect 

 no such decussation is seen. The two posterior columns of the 

 cord diverge from each other, leaving between them a triangular 

 space, the calamus scriptorius, which is the hind part of the 

 lozenge-shaped shallow cavity of the fourth ventricle. As the 

 cord passes into the bulb the posterior column as a whole grows 

 broader, and the division into a median posterior and an external 

 posterior column becomes very obvious and distinct by the 

 appearance of a conspicuous furrow separating the two. At 

 some distance however in front of the point of divergence of 

 the columns or apex of the calamus scriptorius, the furrow 

 becomes less marked, and it eventually fades away. In its course 

 the furrow takes such a line that the median posterior column, 

 forming the immediate lateral boundary of the fourth ventricle, 



