w 



THE STRUCTURE OF THE BRAIN STEM ,, ;7 



are formed by a great hypertrophy of the grey matter at the r. 

 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. 



Funiculus gracilip 

 Funiculus cuneatus 



Sp. root of 5th n 

 Foriuatio reticularie, 



Direct cerebellar 

 tract 



Lower end of olivary 

 eminence 



racile nucleus 



Cuneate nucleus 



Subst. gel. Roland! 

 Decussation of fillet 



Int. access olivary n. 

 erve XJI. 



Jj Pyramid 



FIG. 182. Transverse section through medulla of foetus, immediately above pyramidal 

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



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

 the lower limb, end in arborisations round the cells of the nucleus gracilis, 

 while those of the postero- external column, or column of Burdach, of which 

 the majority is formed by fibres from the upper limbs, terminate in the 

 grey matter of the nucleus cuneatus. The cells of these two masses of 

 grey matter of course give off axons, which can carry on the impulses 

 brought to them by the fibres of the posterior columns. These axons speedily 

 leave the dorsal aspect of the medulla, bending round, as the arcuate fibres, 

 to the deeper parts of its structure. Thus nothing is left to take the place of 

 the posterior columns on the posterior aspect of the cord. With the dis- 

 appearance of these columns and the development of the pyramids we get a 

 practical obliteration of the anterior fissure and a displacement of the central 

 canal towards the dorsal surface. A little higher up (Fig. 183) the canal 

 opens out altogether, forming the fourth ventricle, covered on its dorsal 

 surface only by a thin layer of ependyma, a simple epithelium representing 

 all that is left of the dorsal wall of the primitive cerebral vesicle. The 

 appearance of the section is now modified by two structures. In the first 

 place, a new mass of grey matter, consisting of a thin layer shaped like 

 flask with its orifice directed inwards, is developed in the lateral part of the 

 medulla, between the pyramids in front and the tubercle of Rolando behind. 

 This is the olivary body, and has on its inner and dorsal sides two little gre 

 masses which are the accessory olivary bodies. The other feature is the new 



