BRACHIA OF THE PONS AND PROLONGATION OF THE CRUSTA. 459 



the greater part of those spinal fasciculi of the tegmentum that 

 fill the space between the posterior longitudinal fasciculi and 

 the fillet layer. A slender portion only of them remains 

 anteriorly to the decussation between it and the layers of the 

 fillet. Their disposition in this region is entirely governed by 

 the behaviour of those decussating fibres by which they are 

 interlaced and displaced ; being pushed to the outer side so 

 long as the section of the superior peduncle occupies the median 

 line, and again inwards- as soon as this obtains a lateral posi- 

 tion. 



Fig. 252 exhibits a new phase of the course pursued by 

 the superior peduncle. After complete decussation (and it 

 is indeed, as Stilling represents, total, and not as Arnold 

 subsequently maintained, only partial), the superior peduncle 

 elevates itself so as to project freely outwards and upwards 

 from its previous imbedding into the posterior division of 

 the projection system (fig. 252). Above the greatest convexity 

 of the pons it is still covered by a portion which becomes 

 thick in front of the deep-lying lamina of the fillet S (fig. 

 252, on the left side SA) ; it then becomes, opposite the great- 

 est convexity of the pons, completely free, but is already, at 

 the level of the origin of the fifth nerve, imbedded in the 

 medullary mass of the cerebellum (fig. 253, A) ; it is next 

 covered by the other arms of the cerebellum, and beyond 

 this becomes connected with the grey matter of the corpus 

 dentatum. 



Whereas it forms in the free part of its course the lateral 

 wall of the fourth ventricle, it is, after its coalescence with 

 the medulla of the cerebellum, only a partial constituent of 

 the extensive medullary roof with which the inferior surface 

 of the cerebellum overarches the grey floor of the rhomboidal 

 fossa (fourth ventricle). In their course between the corpora 

 quadrigemina and the cerebellum, the superior peduncles 

 become complicated by the enclosure of the superior medullary 

 velum, or median valve of the cerebrum (valve of Vieussens, 

 figs. 250, 252, F), so as to form, according to the expres- 

 sion of Burdach, the connecting system of the cerebellum. 

 This cerebral velum, the continuation of the peduncle of the 

 corpora quadrigemina (frenulum) deserves the name of pro- 



