THE STRUCTURE OF THE BRAIN" STEM 



413 



fibres have been crossing from one side to the other through the anterior 

 white commissure, many of them belonging to the pyramidal system. 

 But at the lower border of the medulla we see a large mass of fibres 

 crossing between the anterior columns and the postero-lateral columns, 

 at first cutting off the head of the anterior horn and later on breaking 

 this up altogether, so that the only definite collection of grey matter left 

 in this situation is a small part of the lateral column of grey matter 

 known as the lateral nucleus. In this way are formed the big anterior 

 columns of the medulla, which are known as the pyramids, and 



Funiculus gracil 

 Funiculus cuneatu 



Sp. root of 5th M. 



Formatio reticulari? 



Direct cerebellar 



tract 



Lower end of olivary 

 eminence 



Oracile nucleus 



Cuneate nucleus 



Subst. gel Rolamli 

 Decussation of fillet 



Int. access olivary n 

 Nerve XII. 



Pyramid 



FIG. 183. Transverse section through medulla of foetus, immediately above 

 pyramidal decussation. (CUNNINGHAM.) Stained by Pal-Weigert method. 



contain all the fibres that in the cord are represented by the direct 

 and crossed pyramidal tracts. 



The next change is due to the ending of the posterior columns 

 (Fig. 183). These are the central ascending branches of dorsal nerve 

 roots, having therefore an origin outside the cord. On their way up 

 the cord they send in collaterals to end in the grey matter of the 

 posterior horn. The main mass terminates in the medulla, just above 

 the pyramidal decussation, in two collections of grey matter the 

 nucleus gracilis and the nucleus cuneatus w r hich are formed by a 

 great hypertrophy of the grey matter at the root of the posterior 

 horn. The effect of this development in the dorsal region of the 

 medulla is to push the head of the posterior horn outwards. At the 

 same time this mass of gelatinous substance becomes enlarged, so that 

 in section we have three grey masses from within outwards, the nucleus 

 gracilis, the nucleus cuneatus, and the nucleus of Rolando. 



The fibres of the postero-median column, which are derived 

 chiefly from the lower limb, end in arborisations round the cells of the 



