564 THE NEKVOUS SYSTEM. 



Formatio Reticularis. Behind the olive and the pyramid is the formatic 

 reticularis. In the medulla oblongata it occupies a position which, to a large 

 extent, corresponds with that of the lateral funiculus in the spinal medulla. Ir 

 transverse section it appears as an extensive area, which is divided into a lateral 

 and a medial field by the fila of the hypoglossal nerve as they traverse the s.ubstanc< 

 of the medulla oblongata to reach the surface. In the lateral portion, which lies 

 behind the olive, a considerable quantity of gray matter, continuous with that ii ; 

 the spinal medulla, is present in the reticular formation ; it is, therefore, called th 

 formatio reticularis grisea. In the medial part, which lies behind the pyramid 

 the gray matter is extremely scanty, and the reticular matter here is termed thi 

 formatio reticularis alba. 



The nerve-fibres which traverse the formatio reticularis run both in a transvers< 

 and in a longitudinal direction. The transverse fibres are the internal arcuate fibres 

 The longitudinal fibres are derived from different sources in the two fields. In th 

 formatio grisea they represent to a large extent the fibres of the lateral funiculus o 

 the spinal medulla, after the removal of the posterior spino-cerebellar and the latera 

 cerebro-spinal tracts. They consist, therefore, of the fibres of the fasciculi rubro 

 spinalis, thalamo-olivaris, spinothalamicus, and spinocerebellaris anterior (antero, 

 lateralis superficialis) of the spinal medulla. In the formatio alba the longitudina 

 fibres are the tract of the lemniscus medialis, the fasciculus tectospinalis, and th 

 medial longitudinal bundle, all of which have already been described. 



Central Canal and the Gray Matter which surrounds it. The central cana 

 as it proceeds upwards through the closed part of the medulla, is gradually force 

 to assume a more dorsal position, owing to the accumulation of fibres on its ventre 

 aspect. (Moreover, the posterior cleft-like part of the cavity of the foetal neur* 

 tube, which becomes obliterated in the spinal medulla by the fusion of its wall 

 remains patent in the medulla oblongata. Hence the central canal in the close 

 part of the medulla oblongata extends backwards to the roof-plate.) First tl 

 decussation of the pyramids, and then the decussation of the medial lemniscu 

 both of which take place in front of the canal, tend to push it backwards; ar 

 the formation of the longitudinal strands in which these intercrossings result (vi; 

 the pyramid and the medial lemniscus), together with the continuation upwards \ 

 the funiculus anterior proprius, leads to a great increase in the amount of tissi 

 which separates it from the anterior surface of the medulla oblongata. In the close 

 part of the medulla oblongata the canal is surrounded by a thick layer of gre 

 matter, which is continuous with the basal portions of the 'anterior and posteri' 

 columns of gray matter of the spinal medulla. This central gray matter is sharp' 

 defined on each side by the internal arcuate fibres, which curve forwards ar 

 medially around it. Finally, the central canal opens on the dorsal aspect of tl 

 medulla oblongata into the cavity of the fourth ventricle. The central mass 

 gray matter which surrounds the canal in the closed part of the medulla oblonga 

 is now spread out in a thick layer on the floor of the fourth ventricle, and in su< 

 a manner that the portion which corresponds to the basal part of the anteri 

 column of the spinal medulla is situated close to the median plane, whilst the pa 

 which represents the base of the posterior column occupies a more lateral positic 

 This is important, because the nucleus of origin of the hypoglossal nerve is plac 

 in the medial part of the floor, whilst the nuclei of termination of the afiere 

 fibres of the vagus, glossopharyngeal, and acoustic nerves lie in the lateral part 

 the floor. The gray matter of the ventricular floor is covered with ependyma. 



Three Areas of Flechsig. In transverse sections, through the upper, open part of 1 

 medulla oblongata, the fila of the hypoglossal and vagus nerves are seen traversing the substai 

 of the medulla oblongata. The nucleus of origin of the hypoglossal is placed in the gray mat 

 of the floor of the fourth ventricle close to the median plane ; the nucleus of the vagus is situa 

 in the gray matter of the ventricular floor immediately to the lateral side of the hypoglos 

 nucleus. From these nuclei the root-bundles of the two nerves diverge from each other as t 

 are traced to the surface and subdivide the substance of the medulla, as seen in transverse secti 

 into the three areas of Flechsig, viz., an anterior, a lateral, and a posterior. 



The anterior area, which is bounded medially by the median raphe and laterally by 

 hypoglossal roots, presents within its limits : (a) the pyramid ; (6) the lemniscus medialis 

 the fasciculus tecto-spinalis ; (d) the medial longitudinal fasciculus ; () the medial access- 

 olivary nucleus ; (/) the arcuate nucleus. 



