28 GRAY SUBSTANCE OF THE MEDULLA OBLONGATA. 



ing from the accessory. A glance at Figs. 38 or 40 will show large bundles of the 

 vagus roots taking this course, and the same is also true in the human medulla, 

 where this course is very evident, though perhaps not quite so distinctly made out 

 as in the sheep. 



These large bundles turning forwards from the vagus roots, appear to be com- 

 posed of fibres which may be divided into three classes according to the course they 

 pursue while passing towards the raphe. 1st. An uppermost layer (Plate XI, Fig. 

 40, D), which passing along the edge of the hypoglossal nucleus for a short distance 

 and then turning outwards, pursue a wavy course among the longitudinal bundles 

 of the antero-lateral and anterior columns ; some of them may be traced to the 

 raphe (c), while many of them are soon lost sight of. 2d. The next set pass along 

 the hypoglossal nucleus, forming a complete marginal border ; the fibres cross the 

 hypoglossal roots with which they frequently interlace, and proceeding onwards ^ 

 stiU along the edge of the nucleus, are sometimes joined by a few fibres from the 

 hypoglossal roots which they accompany to the raphe. (See page 16.) 3d. This set is 

 composed of the deepest lying fibres ; these pass through the hypoglossal nucleus 

 at various depths, many as far back as the middle of the nucleus, and some even 

 through its posterior portion. Some of the fibres composing the two last sets enter 

 cells in the hypoglossal nucleus, and are thereby brought into indirect relation with 

 the hypoglossal roots. (Fig. §8.) 



The further course of the fibres after decussating is not easy to make out, and 

 although I have carefully studied very many specimens in the endeavor to ascertain 

 their destination, I can only briefiy state that while many are immediately lost sight 

 of in the anterior columns, some are seen passing along the raphe anteriorly for 

 some distance, soon, however, curving outwards into the anterior portion of the 

 medulla, probably becoming longitudinal fibres. A few seem to bend backwards, 

 and pass along the raphe posteriorly, either entering the hypoglossal nucleus, or 

 passing still further backwards towards the posterior commissure of the opposite 

 side, along which they may sometimes be traced. 



The posterior commissure, or marginal border of fibres forming the floor of the 

 fourth ventricle, is chiefly derived from the posterior portion of the vagus nucleus, 

 though occasionally reinforced by flbres from the nucleus of the hypoglossal. I 

 have had repeated opportunity of verifying the truth of Clarke's statement quoted 

 above (p. 27), in regard to the connection of cells in the hypoglossal nucleus with 

 fibres derived from the vagus roots, as well in the human medulla as in various 

 animals. 



A very remarkable cell group located just at the point where the marginal 

 bundles passing forwards from the vagus cross the hypoglossal roots, intended 

 apparently to connect the two, in the same manner as the anterior and posterior 

 spinal roots are united, has been described in the section on the hypoglossal roots. 



It is a question of great interest to ascertain, if possible, whether, as is the case 

 with the spinal roots, any of the vagus fibres are directly continuous with those of 

 the hypoglossal, but it is extremely difiicult to decide this point with accuracy. I 

 have repeatedly thought that I could make out a direct continuity between single 



