8 GRAY SUBSTANCE OF THE MEDULLA OBLONGATA. 



fibres enter. The restiforin nucleus (7% r) is very conspicuous and is entirely filled 

 with cells, some of which are quite large, and along its posterior border numerous 

 tufts of fibres and cells are pushed out into the restiform body (r). The post-pyra- 

 midal nucleus has become a fan-like expansion of cells and radiatmg fibres, quite 

 filling the post-pyramidal body (p). Both of these nuclei are traversed by the! 

 arciform fibres, many of which originate from their cells. ' 



(4.) In the sections next above (Figs. 19, 19") we have a decided change m the , 

 form of the central gray substance. The raphe (Fig. 19", R) is now completely 

 formed, and has pushed the central canal (c) somewhat backwards ; the posterior 

 fissure has almost entirely disappeared, and is reduced to a deep sulcus, while the 

 central gray substance is drawn out posteriorly in a very remarkable manner, until 

 it reaches the sulcus. Behind the central canal, and nearly parallel with the sides 

 of the gray substance, are situ.ated two elongated groups of oval and fusiform cells 

 {S), which are continued backwards until they nearly meet at the middle line, 

 forming the nuclei of the highest roots of the spinal accessory nerve. This nucleus 

 is somewhat bifurcated by bundles of longitudinal fibres which pierce its apex (?), 

 and in the anterior spur some remains of the tractus intermedio-lateralis are still 

 persistent. 



The cells of the anterolateral nucleus (B) are very numerous, fiUing nearly the 

 entire antero-lateral columns, and serving to connect the anterior and posterior por- 

 tions of the medulla, by means of the arciform fibres which traverse this nucleus, 

 and in many cases enter its cells, the processes from which pass in every direction 

 transversely as well as longitudinally. This group is also closely related to the 

 olivary bodies, which are now quite fully developed as a compact coil of small cells 

 imbedded in a mass of fibres, situated on the lateral border of the pyramid outside 

 the hypoglossal roots. 



The situation of the olivary bodies (0) with respect to the hj'poglossal roots, 

 constitutes one of the most striking differences between the human medulla and 

 that of most of the mammalia, and is produced by the great development of the 

 pyramids, as well as of the olivaries themselves, in the human medulla, leaving 

 insufiicient room for the hypoglossal roots to pass on the outer side, as is easily 

 done where the development of these bodies is comparatively so slight, as it is, 

 even in animals possessing so distinct olivary convolutions as the carnivora. 



On the inner side of the hypoglossal roots, we find in man a large and elongated 

 group of cells (s) called by Stilling the great pyramidal nucleus, and considered by 

 him, together with the small pyramidal nuclei, noticed above, as the chief source 

 from which the fibres of the pyramidal column proceed. Although some of the 

 transverse bundles by which the pyramids are everywhere pierced, undoubtedly 

 arise from these cells, I entirely agree with Clarke^ in considering this group (the 

 great pyramidal nucleus) as a portion of the olivary column, the peculiar structure 

 of which the cells assume more and more as we ascend, being often found in the 

 upper part of the medulla (Fig. 23"), arranged in a little convolution, evidently of 

 the same nature as the larger olivary lamina with which it is connected. 



' Philos. Trans. 1858, 244. 



