194 



THE ESSENTIALS OF HISTOLOGY 



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 an olivary body (fig. 228), it 

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

 the medulla 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 below surrounded the central canal to be now spread 



H.X11 



p n.n r 



FIG. 228. SECTION OF THE MK- 

 IK'LLA OBLONGATA AT ABOUT 

 THE MIDDLE OF THE OLIVARY 



BODY . 



f .l.a. anterior median fissure ; n.ar, 

 nucleus arcifonnis ; p, pyramid ; 

 XII. bundle of hypoglossal nerve 

 emerging from the surface ; at 6 it 

 is seen coursing between the pyra- 

 mid and the olivary nucleus, o ; 

 f.n.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, 

 a. V. ; X, bundle of vagus root, 

 emerging ;/./. formatio reticularis ; 

 <./. corpus restiforme, beginning to 

 lie formed, chiefly by arciform fibres, 

 superficial and deep ; n.c. nucleus 

 cuneatus ; n.g. nucleus gracilis ; 

 t, attachment of the ligula; f.s. 

 fuuiculus solitarius; n.X. n.X'. 

 two parts of the vagus nucleus; 

 n.XII. hypoglossal nucleus; n.t. 

 nucleus of the funiculus teres ; 

 ji.tim. nucleus ambiguus ; r, raphe ; 

 A, continuation of anterior column 

 of cord ; o', o", accessory olivary 

 nuclei ; olivary nucleus ; p,o.l. pe- 

 dunculus olivee. 



out at the floor of that ventricle, and the collections of nerve-cells from 

 which the hypoglossal and spinal accessory nerves respectively arose 

 now, therefore, lie in a corresponding situation. At this level, how- 

 ever, the outer group which corresponds with the nucleus of the spinal 

 accessory in the lower part of the medulla has become the nucleus of 

 the vagus or tenth nerve. The nerve-bundles of the roots of these 

 nerves can be seen in the sections coursing through the thickness of 

 the medulla and emerging, those of the hypoglossal (xn.) just outside 

 the pyramids, those of the spinal accessory and vagus (x.) at the side 

 of the medulla. The two sets of emerging fibres thus appear to sub- 

 divide each lateral half of the medulla into three areas a posterior, a 

 middle, and an anterior. Of these the posterior is chiefly occupied by 

 the grey matter of the floor of the fourth ventricle, and, with fibres 

 which are passing obliquely upwards and outwards towards the cere- 

 bellum, forming its inferior crus (restiform body, c.r.) ; and in addition 

 there is the continuation upwards of the portions of grey matter forming 

 the nuclei of the funiculus gracilis (n.g.), of the funiculus cuneatus 



