58 THE MEDULLA OBLONG ATA AND PONS VAEOLII. 



with the external arched fibres, and a considerable amount in the median septum or 

 raphe. These three structures may therefore next be described. 



The formatio reticularis (figs. 43, 44, 46, f.r.) occupies thewhole of the anterior 

 and lateral areas of the bulb, dorsal to the pyramids and olives respectively. It is 

 thus named on account of the appearance which it presents in a transverse section 

 viewed under a moderate magnifying power. This reticular appearance is 

 caused by the intersection of bundles of fibres belonging to two sets which run at 

 right angles to one another. Those of the one set are longitudinal, and these are 

 intersected by transverse fibres, which pass obliquely from the raphe outwards and 

 somewhat backwards with a curved course towards the funiculus gracilis and f uniculus 

 cuneatus, and the olivary nucleus ; and also in the upper part of the medulla oblongata 

 towards the restiform body. 



In some parts grey matter with nerve-cells enters into the constitution of the 

 formatio reticularis. The cells are especially large and numerous in the reticular 

 formation of the lateral area near the anterior area, where in the lower part of 

 the medulla oblongata is situated a remnant of the anterior horn ; and its grey 

 matter is presumably derived in great measure from the latter. In the anterior or 

 mesial area of the bulb, nerve-cells are mostly absent from the formatio reticularis, 

 and this is therefore sometimes distinguished as the formatio reticularis alba (fig. 48, 

 r.a.), from the other or formatio reticularis grisea (r.g.). 



The longitudinal fibres of the reticular formation of the anterior area comprise 

 at least two sets, viz. : (1) those which occupy the tract nearest to the pyramids and 

 which belong to the tract of the fillet above described ; and (2) those which are pro- 

 longed from the remainder of the anterior column of the spinal cord after the passage 

 of the anterior (direct) pyramidal tract into the outer side of the pyramid. The 

 latter become in the higher levels of the medulla oblongata gradually obscured or 

 replaced by an accumulation of grey matter which resembles that of the grey reticular 

 formation and has been -termed by Roller, nucleus centralis. But some of the 

 longitudinal fibres of the anterior area remain distinct, and become in the upper 

 regions of the medulla oblongata collected into a compact bundle which is known 

 as the posterior or dorsal longitudinal bundle. 



The fibres of the reticular formation of the lateral area are prolonged from the 

 remains of the lateral column after the lateral pyramidal tract and the dorsolateral 

 cerebellar tracts have passed to their respective destinations. Those which are 

 nearest the olivary nucleus mostly belong to the anterolateral cerebellar tract (fig. 44). 

 The fibres of the lateral area are added to as we trace them upwards in sections, the 

 increase being due either to the turning upwards of some of the inner arched fibres 

 or to the accession of fibres which are derived from nerve-cells in the grey matter 

 near the posterior surface, or in the grey reticular formation. 



According to Deiters, the nerve-fibre processes of the nerve-cells of the reticular 

 formation all pass downwards, while their branched processes are directed 

 horizontally. 



The arched or arcuate fibres of the medulla oblongata, which have been more 

 than once alluded to, are the curved fibres which are seen in transverse sections 

 coursing in the plane of the section. From their position they are distinguished into 

 external and internal, or superficial and deep. 



The outer or superficial arched fibres (figs. 44, 46) emerge for the most part 

 from the anterior median fissure, and passing over the pyramids and olives, many 

 of them go to the restiform body. They are added to by deep fibres which come to 

 the surface partly in the groove between the pyramids and olives, partly after passing 

 through the olives, as before mentioned. Traced back in the anterior median fissure 

 they are seen to enter the raphe, and to cross over in it ; after which it is supposed 

 that they may become longitudinal, but their further course is not certainly known. 



