STRUCTURE OF THE MEDULLA OBLONGATA 193 



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, where, together 

 with the fibres of the direct pyramidal tract, they constitute the promi- 

 nent mass of white fibres which is seen on the front of the medulla 

 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. 22, n.l.) 



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

 increased development of the posterior median and cuneate fasciculi. 

 This causes the posterior cornu (fig. 226, 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 prominence 

 upon the surface of the medulla, which is known as the tubercle of Rolando 

 (B). Grey matter also soon becomes formed within the upward pro- 

 longations of the posterior median column and of the cuneate fasciculus 



FIG. 227. SKCTION OF THE .MEDULLA 

 OBLONGATA IN THE REGION OF 

 THE SUPEKIOU PYKAMIDAL DECUS- 



SATION. 



a.m.f. anterior median fissure; f.n. 

 superficial arciform fibres emerging 

 from the fissure ; py, pyramid ; n.ai: 

 nucleus of the arciform fibres; /.'. 

 deep arciform fibres becoming super- 

 ficial ; o, lower end of olivary nucleus ; 

 o 1 , accessory olivary nucleus ; n.l. nu- 

 cleus lateralis ; /'.;. formatio reticu- 

 laris ; /. ". arciform fibres proceeding 

 ]rom 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. f uniculus cuneatus ; n.g. nucleus 

 gracilis ; f.g. f uniculus gracilis; p.mj. 

 posterior median fissure ; c.c. central 

 canal surrounded by grey matter, in 

 which are, n.XI. nucleus of the spinal 

 accessory, and n.XH. nucleus of the 

 hypoglossal ; s.d. superior pyramidal 

 decussation. 



a.V. 



nM. 



f.a! 



a.m.f. 



(fig. 227, n.g., n.c.), but most of the grey matter of the cornu becomes 

 broken up, by the passage of bundles of nerve-fibres through it, into a 

 reticular formation (f.r), the production 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 conies 

 nearer to the posterior median fissure. The grey matter which sur- 

 rounds it contains two well-marked groups of nerve-cells ; the anterior 

 of these is the nucleus of the hypoglossal or twelfth nerve (n. x//.), the 

 posterior that of the spinal accessory or eleventh (n. xi.). Instead of the 



