WHITE MATTER OF MEDULLA. 2QI 



commissural fibers of the reticular substance are also included in 

 the white substance. 



Substantia Reticularis. Superficially, the white matter is 

 collected into great bundles of fibers, such as the pyramids, lateral 

 column and restiform body; but, in the deep parts of the medulla, 

 the white matter enters into a great network called the substantia 

 reticularis, which has gray matter in its meshes (Figs. 92 and 94). 

 It is continuous, above, with the reticular formation of the pons, 

 and, below, with the fasciculi proprii of the spinal cord. The 

 substantia reticularis contains many scattered fibers, processes 

 of its intrinsic neurones, which form a frequently interrupted and, 

 for the most part, a crossed ascending and descending tract. 

 Transverse and oblique fibers are, likewise, numerous in the retic- 

 ular substance. They are chiefly the arcuate -fibers. The distinct 

 tracts of longitudinal fibers contained in it will be noticed later 

 (page 293). The gray substance of the substantia reticularis 

 is composed of the nucleus lateralis inferior, the eighth to the twelfth 

 cerebral nerve nuclei and the olivary nuclei. 



Raphe (Figs. 92 and 94). The raphe is, primarily, a sagittal 

 lamina of neuroglia derived from the floor-plate of the myelen- 

 cephalon. It lies in the median plane and joins the lateral halves 

 of the medulla together. It is very distinct hi the superior part 

 of the medulla. Below the level of the olive, it is entirely obliter- 

 ated by the fillet and pyramidal decussations. The raphe is 

 pierced transversely by decussating and commissural fibers and 

 is traversed dorso-ventralward by the anterior external arcuate 

 fibers. 



The white matter of the medulla is divided by the direction 

 of its fibers into three classes or systems: (i) Transverse fibers; 

 (2) dorso- ventral fibers; and (3) longitudinal fibers. 



(i) The transverse fibers of the medulla are really more or 

 less oblique hi direction and most of them are arched. They in- 

 clude the fibers of the following: (a) The pyramidal decussation 

 (decussatio pyramidum), (b) the medial fillet in the decussatio 

 lemniscorum, (c) the anterior external arcuate fibers on the sur- 

 face and, also, in the decussatio lemniscorum, and (d) the cere- 

 bello-olivary fibers. The pyramidal decussation (Fig. 95) is lo- 



