chap, xvn.] THE MEDULLA OBLONGATA. 147 



the white matter, and forms a distinct accumulation 

 of grey matter in each of these funiculi ; they are 

 respectively the nucleus gracilis and the nucleus 

 cuneatus. The former is the grey matter, in the 

 axis cylinder process of whose ganglion cells the nerve- 

 fibres of the funi<3ulus gracilis originate ; but in the 

 latter only a portion of the nerve-fibres of the funi- 

 culus cuneatus take their origin ; since another part 

 of it joins the restiform body, and so passes into the 

 cerebellum. 



190. In the upper part of the medulla, i.e., in the 

 region of the fourth ventricle, the grey matter forms a 

 continuous mass, the floor of the fourth ventricle (Fig. 

 89). In this region there is a distinct median septum, 

 by which the medulla is divided into two halves ; this 

 is the raphe. It consists of a thin membrane of nerve 

 substance extending from the anterior longitudinal 

 fissure to near the middle line of the floor of the fourth 

 ventricle. This membrane consists of white matter 

 in the shape of bundles of medullated nerve-fibres 

 passing longitudinally, transversely, and obliquely ; 

 and of small masses of grey matter interspersed 

 between the nerve-bundles, and especially at the side 

 of the raphe, where nerve-fibre bundles pass out of 

 it. The grey matter contains multipolar ganglion 

 cells. 



191. In a transverse section through the upper 

 part of the medulla, we find at the side of the 

 pyramid, and a little behind it, but covered on the 

 outer surface by white matter i.e., the bundles of 

 nerve-fibres constituting the fibrse arcuatse externse 

 a plicated lamina of grey matter which constitutes the 

 olivary nucleus, or nucleus dentatus of the olivary 

 body. It extends with its posterior portion into the 

 reticular formation. Continuous with the olivary 

 nucleus, but situated nearer the raphe, is a small 

 lamina of similar grey matter ; this is the accessory 



