THE STRUCTURE OF THE BRAIN STEM 



419 



antero-lateral ascending tract, which, passing up in the superior 

 peduncles, bends dorsally round the fourth nerve and then, turning 

 backwards, ends in the superior vermis of the cerebellum. In a section 

 through the upper part of the pons the division into the formatio 

 reticularis or tegmentum and the part made up of transverse and 

 longitudinal fibres, the pedal portion, is well marked (v. Fig. 187). 

 The fourth ventricle has now become constricted to a narrow canal 



4th ventricle 

 Mesenc. root of 5th i 



Postr. long, bundle 



Form, reticularis 

 Nucleus of lateral fillet 



5th nerve 



Valve of Vieussens 



Floor of 4th ventricle 



Supr. cerebeilar 

 peduncle 



Lateral fillet 



Commencing decus- 

 sation of supr. 

 cerebeilar ped. 

 Mesial fillet 



yramids 



FIG. 187. Section across upper part of , pons Varolii of the orang. (CUNNINGHAM.) 



triangular in section and closed above by the valve of Vieussens. It 

 is surrounded, especially on its ventral side, by grey matter containing 

 the cells of origin of the fourth nerve. In the tegmental portion we 

 may distinguish on each side the superior cerebeilar peduncle. Out- 

 side the longitudinal fibres of this peduncle are a number of transverse 

 fibres derived from the corpus trapezoides seen in the previous section. 

 To these fibres is given the name of the ' lateral fillet/ They are on 

 their way to end in the roof of the mid-brain in the posterior corpora 

 quadrigemina. The posterior longitudinal bundle lies near the 

 middle line, immediately under the grey matter of the floor of the 

 fourth ventricle, while the longitudinal fibres of the fillet, now called 

 the mesial fillet, form a distinct mass in the ventral portion of the 

 formatio reticularis. The pedal portion contains the longitudinal 

 fibres of the pyramids, now much increased in amount, cut up into 

 bundles by transverse fibres derived from the middle peduncles of the 

 cerebellum. 



The cerebellum, which covers in the fore part of the fourth ventricle, 



