

LECTURE XL 



THE MEDULLA OBLONG AT A AND THE TEGMENTUM OF THE PONS. 



GENTLEMEN : In the last lecture we learned how the con- 

 formation of the oblongata was brought about by the peculiar 

 alteration in the relative position of vast tracts of fibres, by the 

 appearances of new nuclei, and by the disappearance of the 

 posterior columns. We have, however, failed to trace a number 

 of tracts upward from the spinal cord, and to these we will now 

 turn our attention. The posterior columns have found their 

 indirect prolongation in the layer of the fillet, and to this layer 

 also have passed those mtrtw fibres which ascended in the antero- 

 lateral columns. The pyramidal tracts of the anterior and 

 lateral columns, now united, lie on the ventral surface as the 

 thick pyramids of the oblongata. The direct lateral cerebellar 

 tract maintains its position on the outer periphery far up into 

 the level of the olivary bodies. At that point its fibres, still 

 gently ascending, begin to pass dorsad to the cerebellum. Here 

 they form the nucleus of "a large bundle which appears at this 

 level, the posterior cerebellar peduncle, or corpus restiforme. 



The restiform body arises externally to the upper extremity 

 of the posterior columns, and at this point is made up principally 

 of the direct lateral cerebellar tract. To this are added the 

 posterior external arciform fibres, which, as you see from Figs. 

 117 and 110 (above and to the left), pass to them from the pos- 

 terior columns along the outer and posterior periphery of the 

 medulla oblongata. Fibres are also added to them from in 



C 1 



front. These are the anterior external arciform fibres, which 

 originate apparently in the layer of the fillet between the 

 olivary bodies, and are really, therefore, continuations of the 

 crossed posterior columns. These fibres approach the surface 

 near the middle line in front, extend around past the pyramids, 

 and run outward and backward to the restiform bodies. They 



(195) 



