TITK MKnrU.A <>I!U>\r;ATA. 147 



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

 to^vther N'ith those of the group first described, do not arise from 

 the grey matter in this part of the medulla ollongata, but curve 

 round and run backwards longitudinally through the grey matter, 

 thus forming a rounded bundle of fibres (Fig. 99 /). 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 nucleus marjnvs (Reissner and Stieda) is a very peculiar 

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

 the pars commi-wraliz, that is, immediately underneath the valvufa 

 cerebeWi. 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 

 narrower end being above. In longitudinal section the line of cells 

 is seen to be open hi front. The space 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 corpora 

 quadrigemina posteriora of higher animals. 



White matter. In the hinder part of the medulla ollongata the 

 arrangement of the white matter resembles that of the white 

 matter of the spinal cord; further forwards the white matter of 

 the dorsal surface 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 medulla are thinner than those of the posterior portion 

 (Stieda), according to Reissner they gradually thin as they pass 

 forwards. The fibres are nearly all longitudinal, such transverse 

 and oblique fibres as are present being chiefly in connection with 

 the various nerve-roots and the commissures. 



The commiissiira superior is naturally lost in consequence of the 

 opening of the central canal into the fourth ventricle; the cow- 

 ii*xv.ra inferior is increased in the anterior half and decreased in 

 the posterior half of the medulla ollongata; in the latter the 

 fibres become more and more oblique, and decussate verv freelv; 

 ultimately they seem to be either continued as longitudinal fibres or 

 to join the ganglia. 



I. 2 



