10 GRAY SUBSTANCE OF THE MEDULLA OBLONGATA. 



The hypoglossal nuclei {H) are spread out into closely crowded clusters of large, 

 multipolar cells. The vagal nuclei ( V) are considerably developed, and filled with 

 oblong, or obovate cells, the more posterior of which are now closely connected with 

 those of the post-pyramidal nuclei. It is in this posterior cell group (</), developed, 

 as Clarke has already pointed out,^ from the substance of both nuclei, that the first 

 appearance of the auditory ganglion can be traced. The vagus nucleus is here 

 bifurcated, as that of the spinal accessory was to some extent, by large bundles 

 of longitudinal fibres (T). The restiform nucleus (r, ?•) has expanded into a large 

 mass of cells and fibres, nearly filling the entire restiform body and pushing the 

 caput cornu stiU. further forwards. 



The antero-lateral nucleus {B) has diminished in size, a few cell groups alone 

 remaining, crowded between the olivary body and the border of the caput ; these 

 groups are still closely comiected with the arciform fibres and probably serve to 

 co-ordinate distant parts of the medulla. A group formed at least partially from 

 the antero-lateral nucleus, is seen (Figs. 20", 21", n) arranged in a long layer close 

 to the border of the olivary lamina. This group, called by Stilling the accessory 

 olivary nucleus, is evidently similar in structure to the olivary lamina, as has already 

 been pointed out by Clarke. Its position and the number of its layers vary in 

 different sections, as well as in diff'erent specimens, and there seems no reason for 

 considering it as in any wise distinct from the olivary body. 



At the junction of the restiform nucleus and caput, a network of cells and fibres is 

 pushed out, extending to the border of the medulla (Fig. 20", x), where a large group 

 of cells is seen closely connected with the band of external arciform fibres ; several 

 such groups are pushed out, either from the caput or restiform nucleus, and this 

 tendency seems to increase as we ascend, tiU the whole re^iform body is fiUed with 

 a mass of more or less compact cell groups, reaching very nearly to the border of 

 the medulla (Figs. 21", 22"). The decussation along the raphe is very marked, and 

 scattered cells are everywhere fomid mingled with its fibres. The arciform fibres 

 interlace in a much more intricate manner than in the lower mammalia, and are 

 connected with numerous cells and cell groups, which serve either as starting-pomts 

 for new fibres or as co-ordinating centres. 



(7.) In the sections just above (Figs. 22, 22") the principal changes are in the 

 posterior portions of the medulla, the anterior and antero-lateral parts undergoing 

 but little change. The hypoglossal nucleus is still very large and prominent, the 

 nerve-roots (A7) winding in a serpentine course through a part of the olivary 

 lamina, but never entering into communication with it. The great pyramidal 

 nucleus of Stilling (s) is here very distinctly seen, as well as the little elongated 

 lamina (?^) situated just above the olivary body (accessory olivary nucleus of StiUing). 



The vagus nucleus (F) has now reached its maximum development; it appears 

 as a large elongated, pyriform mass, containing a group of densely crowded, obovate 

 cells. From it the vagus roots (X) may now be seen proceeding in several very 

 distinct bundles, traversing the caput cornu, Avhich consists of a compact mass of 

 cells connected together by wa^^ bands of fibres. The apex of the nucleus is quite 



' Philos. Transactions, 1858, and Proceedings of the Royal Society, 186L 



