THE STRUCTURE OF THE BRAIN STEM 



365 



In examining successive sections from the spinal cord up through the 

 medulla, the first change which makes its appearance is due to the decussa- 

 tion of the pyramids (Fig. 181). Throughout the spinal cord, fibres have 

 been crossing from one side to the other through the anterior white com- 

 missure, 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 contain all the fibres that in the cord are represented by 

 the direct and crossed pyramidal tracts. 



Funieulus gracilis 

 Funiculus cuneatus 



Sp. root of 5th n. 

 Forniatio reticularis 



Direct cerebellar 

 tract 



Lower end of olivary 

 eminence 



Gracile nucleus 



Cuneate nucleus 



Subst. gel. Eolaiuli 

 Decussation of fillet 



Int. access olivary n. 

 _ Nerve XII. 



1'yramid 



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

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



The next change is due to the ending of the posterior columns (Fig. 182). 

 These are the central ascending branches of dorsal nerve roots, having there- 

 fore 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 

 which 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 

 vsection 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 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 



