CHAP, ii.] THE BRAIN. 945 



are called arcuate or arciform fibres, internal arcuate fibres (Fig. 

 109, 6, f. a. i.) to distinguish them from the external arcuate 

 fibres (f. a. e.) of which we shall speak presently. Fragments 

 of more compact grey matter, also belonging probably to the 

 anterior horn, are seen at intervals in this area, Fig. 109, 6, ac. 

 and elsewhere. We have seen that nearly all the way along 

 the cord the grey matter of the neck of the posterior horn is 

 similarly broken up by bundles of fibres into what we there called 

 the reticular formation (Figs. 98, 99, r. f. p. and r.f. I.); and this 

 area in the bulb though it possesses characters of its own is also 

 called the reticular formation. In the more lateral portion of 

 this formation, the network is more open and irregular, the 

 fibres are finer, and the nerve cells are more abundant than in 

 the median portion where the nerve cells, except in the immediate 

 neighbourhood of the raphe, are less numerous or even absent, 

 and the fibres are coarser. These two parts are sometimes distin- 

 guished as the outer or lateral, and the inner or median formation. 

 In the middle line the fibres distinctly interlace and decussate 

 in an oblique manner, some running nearly vertically in the 

 dorso-ventral plane, thus constituting as we have said a thick 

 raphe, which, however, at its edges gradually merges into the 

 more open network. 



609. Within the area, bounded by the pyramids ventrally, 

 the expanded grey matter dorsally, the raphe in the middle line, 

 and the white matter laterally, certain distinct compact masses 

 of grey matter make their appearance, as we pass upward towards 

 the pons. 



One of the most important of these gives rise to the olivary 

 body, or inferior olive which, as we have seen, projects as an oval 

 mass (Fig. 108, ol.) on each side of the pyramids, reaching from 

 a level which is somewhat higher up than the lower limit of the 

 pyramids, almost but not quite to the pons. The olivary body, 

 as a whole, consists partly of white matter, that is of fibres, and 

 partly of grey matter, sometimes called the olivary nucleus. This 

 latter is disposed in the form of a hollow flask or curved bowl, 

 with deeply folded or plaited walls, having a wide open mouth 

 directed inwards towards the middle line, and forwards towards 

 the pons (Fig. 109, 4, 5, 6, ol). The flask is filled within by 

 white matter, and covered up on its outside with white matter 

 as well as traversed by fibres. The grey matter thus forming 

 this flask-shaped nucleus consists of small rounded nerve cells, 

 lying in a bed of tissue which is partly ordinary neuroglia, and 

 partly a fine nervous network. 



Lying to the median side of the olivary body, immediately 

 dorsal to the anterior pyramid is another small mass of grey 

 matter, in the form of a disc, appearing in transverse sections as 

 a thick bent rod, in some sections consisting of two parts (Fig. 109, 

 4, ol. a). This is the accessor)/ olivary nucleus. A very similar 



602 



