THE ESSENTIALS OF HISTOLOGY. 



tract (p) from the lateral column of the spinal cord on each side 

 through the root of the anterior cornu and across the anterior median 

 fissure to the opposite anterior column of the medulla oblongata, 

 where, together with the fibres of the direct pyramidal tract, they 

 constitute the prominent mass of white fibres which is seen on the 

 front of the bulb, on each side of the middle line, and is known as the 

 pyramid. By this passage of fibres through the grey matter the tip of 

 the anterior cornu (a) is cut off from the rest and becomes pushed as it 

 were to the side ; in sections a little higher up it appears as an isolated 

 mass of grey matter which is known as the lateral nucleus (fig. 

 258, n.L). 



A change also occurs in the posterior cornu in consequence of the 

 increased development of the posterior column of white matter. 

 This causes the posterior cornua (fig. 257, p c) to be pushed towards the 

 side, the V which they form with one another being thus opened out ; 

 at the same time the tip of the cornu swells out and causes a pro- 

 minence upon the surface of the medulla, -which is known as the 

 tubercle of Rolando (R). Grey matter also soon becomes found 

 within the upward prolongations of the postero-mesial column and of 

 the cuneate funiculus (postero-lateral column, fig. 258, n.g., n.c.). But 



/# ri.rr. 

 f.rn,f. | "> 



FIG. 258. SECTION OF THE MEDULLA 



OBLONGATA IN THE REGION OF 

 THE SUPERIOR PYRAMIDAL DECUS- 



SATION. (Schwalbe.) f. 



a.m./., anterior' median fissure ; /.a., 

 superficial arciform fibres emerging 

 from the fissure ; py., pyramid ; n.a.r., 

 nucleus of the arciform fibres; f.a'., 

 deep arciform fibres becoming super- 

 ficial ; o, lower end of olivary nucleus ; 

 o', accessory olivary nucleus ; n.L, nu- 

 cleus lateralis ; f.r., formatio reticu- 

 laris ; f.a 9 ., arciform fibres proceeding 

 from formatio reticularis ; g, substantia 

 gelatinosa of Rolando ; a. V., ascending 

 root of fifth nerve ; n.c., nucleus cunea- 

 tus; n.c'., external cuneate nucleus; 

 f.c., funiculus cuneatus ; n.p., nucleus 

 gracilis ',f.g., funiculus gracilis ; p.m.f., 

 posterior median fissure ; c.c., central 

 canal surrounded by grey matter, in 

 which are, n.XI., nucleus of the spinal 

 accessory; and n.XII., nucleus of the 

 hypoglossal ; s.d., superior pyramidal 

 decussation (decussation of fillet). 



most of the grey matter becomes broken up, by the passage of bundles 

 of nerve-fibres through it, into a reticular formation (f.r.) the pro- 

 duction of which is already foreshadowed in the upper part of the 

 spinal cord. The central canal of the spinal cord is still seen in the 

 lower part of the medulla oblongata (c.c.), but it comes nearer to the 



