THE MEDULLA OBLONGATA. 



229 



posterior surface. The grey matter which surrounds it contains two 

 well-marked groups of nerve-cells ; the anterior of these is the lower 

 part of the nucleus of the hypoglossal or twelfth nerve (n. XIL), the 

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

 the comparatively narrow isthmus which joins the two halves of the 

 spinal cord, a broad raphe now makes its appearance ; this is formed of 

 obliquely and antero-posteriorly coursing fibres, together with some 

 grey matter containing nerve-cells. 



In a section at about the middle of the olive (figs. 259, 260), it will be 

 seen that a marked change has been produced in the form of the 

 medulla oblongata and the arrangement of its grey matter, by the 

 opening out of the central canal into the fourth ventricle. This 

 causes the grey matter which lower down surrounded the central 

 canal to be now spread out at the floor of that ventricle, and the 



FIG. 259. SECTION OF THE ME- 

 DULLA OBLONGATA AT ABOUT 

 THE MIDDLE OF THE OLIVARY 



BODY. (Schwalbe.) -}-. 



/.Ln., anterior median fissure ; n.ar., 

 nucleus arciformis ; p, pyramid ; 

 XIL, bundle of hypoglossal nerve 

 emerging from the surface ; at 6 it 

 is seen coursing between the pyra- 

 mid and the olivary nucleus, o ; 

 f.a.e., external arciform fibres; n.L, 

 nucleus lateralis ; a, arciform fibres 

 passing towards restiform body 

 partly through the substantia gela- 

 tinosa, g, partly superficial to the 

 ascending root of the fifth nerve, 

 aV. ; X., bundle of vagus root, 

 emerging; /. -/-. ,formatio reticularis; 

 c.r., corpus restiforme, beginning 

 to be formed, chiefly by arciform 

 fibres, superficial and deep ; n.c., 

 nucleus cuneatus ; n.g., nucleus 

 gracilis ; t, attachment of the lig- 

 ula ; f.s., funiculus solitarius ; nX. 

 nX'., two parts of the vagus nu- 

 cleus; n.XIf., hypoglossal nucleus ; 

 n.t. , nucleus of the funiculus teres ; 

 n.am. nucleus ambiguus ; ?, raphe ; 

 A, formatio reticularis alba; o', o", 

 accessory olivary nuclei ; o, olivary 

 nucleus; p.o.L, pedunculus olivse. 



collections of nerve-cells from which the hypoglossal and spinal 

 accessory nerves respectively arose now, therefore, lie in a corre- 

 sponding situation. At this level, however, the outer group which 

 corresponds with the nucleus of the spinal accessory in the lower 

 part of the bulb has become the nucleus of the vagus or tenth 

 nerve (n.X.). The nerve-bundles of the roots of these nerves can 

 be seen in the sections coursing through the thickness of the bulb 

 and emerging, those of the hypoglossal (XIL) just outside the pyramids, 

 those of the spinal accessory and vagus (X.) at the side of the medulla 

 oblongata. The two sets of emerging fibres thus appear to subdivide 



p.o.i: 



