148 MR HAROLD AXEL HAIG ON THE 



The substantia gelatinosa Rolandi is now quite separate from the rest of the grey 

 matter and lies enclosed by white fibres which partly arch round it and partly form the 

 lower limit of the restiform body. The pyramids form marked protuberances anteriorly 

 in the middle line. 



No trace of an olivary body is seen as yet, a feature which is of comparative value ; 

 the nuclei gracilis and cuneatus are well defined, the latter lying now well to the outer 

 side of the former. 



Large nerve-cells are to be made out in the grey matter of the still visible represen- 

 tatives of the anterior cornua. 



The proportion of grey to white matter is as 3 to 5 approx. 



The transverse diameter of the section markedly exceeds the antero-posterior 

 diameter : this feature is now prominent throughout the whole extent of the medulla, 

 until, when the pons is reached, the disproportion becomes less obvious. 



C. At the lower limit of the olivary body. (PL I. fig. 3.) 



A median raphe has now appeared, and the central region is occupied by intercross- 

 ing strands of fibres (internal arcuate fibres), and just dorsal to the pyramids the tract 

 of the fillet may be seen. 



The central canal has opened out on to the floor of the 4th ventricle, the latter being 

 covered by a layer of ependyma. 



The restiform body is now becoming a more obvious feature, and outside this fibres 

 forming a well-marked covering, passing from the pyramid regions round the restiform 

 body towards the dorsal region. 



Two well-defined nuclei have now taken the place of the nuclei gracilis and 

 cuneatus: these are respectively the nucleus of the 12th cranial nerve and the dorsal 

 nucleus of the 10th cranial nerve (n. 12th, n. 10th). 



The olivary body is peculiar in that in section it shows an internal mass of grey 

 matter, loaded with rather large nerve-cells, having the form of a U-shaped fold, which 

 however does not possess any folds of the second order such as are to be seen in the 

 grey matter of the human olivary body. In some regions this olive is open centrally 

 (hilus), but in the lower regions takes the form of a closed oval of grey matter (o.n.). 

 The olive does not form a very marked external projection, and, moreover, especially in 

 the higher regions of the medulla, appears to be further removed from the pyramids. A 

 few of the internal arcuate fibres may be traced from the hilus olivse across the median 

 raphe to the opposite restiform body. 



The rest of the grey matter is of a somewhat scattered aspect : the substantia 

 gelatinosa still forms an obvious mass laterally enclosed by the curved restiform body : 

 some of the masses of grey matter lying ventrally may possibly represent accessory 

 olivary bodies, but some certainly belong to the category of arcuate nuclei. 



Just below the floor of the 4th ventricle in the middle line is a tract of fibres repre- 



(ROY. soc. EDIN. TKANS., VOL. XLVIII., 856.) 



