CENTRAL RELATIONS OF COMPONENTS. 49 



receive from the caudal edge of the cerebellum a cap of 

 the molecular layer, the cerebellar crest. Still farther 

 caudad the two cerebellar crests, immediately after their 

 separation from the cerebellum, fuse in the middle line 

 dorsally of the fourth ventricle (Fig. i8). The fusion is 

 substantial, involving the molecular layer, the now rudi- 

 mentary granular layer and the underlying fibres of the 

 tuberculum acusticum proper. The molecular layer caps 

 the tuberculum for its entire length, i. e., caudad beyond 

 the cephalic end of the lobus vagi and nearly to the caudal 

 tip of the cerebellum. Its extreme caudal tip is shown in 

 Fig. 17. 



The cerebellum we know to be related to the secondary 

 tracts of all of the sensory nerves. It is said by several 

 •authorities to receive direct root fibres from the trigem- 

 inus; but its direct relations to the roots of the acustico- 

 lateral nerves is strikingly characteristic of this system. 



Several large bundles of root fibres from the VIII nerve 

 (Fig. 19, cb. VIII) can easily be followed in the transec- 

 tions up into the cerebellum near its cephalic end. Proba- 

 bly other similar bundles run into the cerebellum from 

 the three lateral line roots, as described by Johnston ('98). 



The VIII nerve and probably also the three lateral line 

 roots send root fibres caudad, thus constituting the spinal 

 VIII tract. These fibres form two close round bundles 

 lying at the periphery of the oblongata. The sensory 

 root of the vagus emerges just dorsally of them (Fig. 17, 

 sp. VIII). Immediately caudad of the level here figured 

 (640) they turn ventrad, forming external arcuate fibres 

 to cross in the extreme ventral portion of the raphe. This 

 decussation occupies the extreme ventral surface of the 

 brain for almost the entire extent of the region of the 

 lobus vagi. 



