TRI'] MEDULLA ()BLOX( iATA. 147 



through the white matter to the grey matter. These latter fibres, 

 together with those of the group first described, do not arise from 

 the grey matter in this part of the medulla ohlongata, but em-ve 

 round and run backwards longitudinally through the grey matter, 

 thus forming a rounded bundle of fibres (Fig. 99 F). Between these 

 fibres are interspersed small nerve-cells and nuclei which disappear 

 as the fibres approach the white matter. The vagus undoubtedly 

 receives fibres from the grey matter throughout a long course, 

 and again receives a large bundle just before leaving the grey 

 matter. The more exact origin of the various fibres has not been 

 traced. 



6. The 7ivclens magniis (Reissner and Stieda) is a very peculiar 

 group of cells placed on either side, in the most anterior portion of 

 the jf^ar^ commissuralts, that is, immediately underneath the valvula 

 cerebelli. The large cells are arranged in a transverse section in a 

 single row so as to enclose a pear-shaped space on either side, which 

 has its long axis directed from above, downwards and outwards, the 

 nai-rower end being above. In longitudinal section the line of cells 

 is seen to be open in front. The sjiace enclosed by these cells is 

 occupied by a granular ground-substance which contains only few 

 nuclei. Bellonci is of opinion that these nuclei represent the corjjora 

 quadrigeinina posteriora of higher animals. 



White matter. In the hinder part of the mednlla ohlongata the 

 arrangement of the white matter resembles that of the white 

 matter of the spinal cord; fm-ther forwards the white matter of 

 the dorsal siirface commences to pass to either side, and ultimately 

 it forms the outer part of the walls of the fourth ventricle. The 

 fibres of the white matter of the ventral surface are unchanged in 

 direction as they proceed forwards. The fibres of the anterior part 

 of the medidla are thinner than those of the posterior portion 

 (Stieda), according to Reissner they gradually thin as they j)ass 

 forwards. The fibres are nearly all longitudinal, suc^^ transverse 

 and oblique fibres as are present being chiefly in conntction with 

 the various nerve-roots and the commissures. 



The commiMura superior is naturally lost in consequence of the 

 opening of the central canal into the fourth ventricle ; the com^ 

 misaura inferior is increased in the anterior half and decreased in 

 the posterior half of the 'medulla oblongata ; in the latter the 

 fibres become more and more oblique, and decussate very freely; 

 ultimately they seem to be either continued as longitudinal fibres or 

 to join the ganglia. 



L 2 



