CEREBELLUM OF URODELE AMPHIBIA 11 



Reading forward in transverse sections, the tractus thalamo- 

 bulbaris of Necturus is small and compact just caudad of the 

 superficial origin of the V roots, l>dng dorso-laterally of the lemnis- 

 cus and close to the ventricular grey. At the level of the V roots, 

 this tract is crossed by the motor V root and is joined by the 

 fibers of the mesencephalic V root, some of which are coarser than 

 any of these (fig. 11, tr.ih.h.-\-mes.V.). Rostrally of the V roots, 

 the mixed system forms a series of small fascicles of coarse fibers 

 bordering the ventricular grey and extending lateralward from a 

 point dorsally of the lemniscus to the shallow sulcus which marks 

 the medial boundaiy of the area acustico-lateralis (fig. 10). Still 

 farther forward under the recessus lateralis the lenmiscus turns 

 lateralward and these coarse fibered fascicles lie dorsally of it 

 (fig. 9), but separated from it by masses of heavily medullated 

 fibers. In front of the recessus lateralis there is another rear- 

 rangement (fig. 5). Most of the trigeminal fibers separate dor- 

 sally, some to enter the commissura cerebelli, others to ascend 

 into the tectum mesencephaU at a higher level than any other 

 fibers now under consideration. The remaining fibers, chiefly 

 belonging to the tractus thalamo-bulbaris, are immediately joined 

 by coarse fibers of the tractus spino-tectahs, as already described 

 (fig. 4, tr.sp.t. -i-th.b.), and the mixed bundle as thus reconstituted 

 ascends into the mesencephalon, lying deeper than the lemniscus 

 and parallel with it. Its further course has not been studied in 

 Necturus. 



The fibers of the mesencephaUc V root which join the commis- 

 sura veli for the most part separate from it again to enter the 

 tectum mesencephaU of the same side, chiefly for distribution to 

 the roof of the recessus posterior mesencephali, though some of 

 them doubtless cross to the opposite side in the commissure. 



From the preceding account it appears that the fibers of the 

 mesencephaUc root of the V nerve are mingled with two other 

 coarse fibered medullated tracts, the tractus thalamo-bulbaris and 

 the tractus spino-tectalis, and the analysis of this complex is very 

 difficult. I have found no evidence that any fibers of the V roots 

 terminate in the cerebellum. Those which pass through the 

 cerebeUum, including those which cross in the decussatio veU, 



