534 DAVIDSON BLACK 
The conditions which bring about this complicated course of 
the IV root have been thoroughly investigated, independently by 
V. Franz (23) and by Huet (1. c.).. The latter author has shown 
that the division of the trochlear root into two bundles and their 
subsequent decussation at different levels, only occurs among 
those teleosts having a highly complicated valvula cerebelli 
( im 
 Valvula jo 6 
a) | 
an J 
Fig. 37 Solea vulgaris.- Transverse section of brain stem on the exit level 
of the oculomotor root. Abbreviations: Coll., colliculus; Com.ans., commissura 
ansulata; Fib.prof.tc., fibrae profundae tecti; N.JIJ., oculomotor nerve; 
Nu.III.vent., ventral moiety of oculomotor nucleus; Nu.JV., nucleus troch- 
learis; Tor.long., torus longitudinalis; Tr.mes.cb.s., tractus mesencephalo- 
cerebellaris superior. Other abbreviations as before. 
(e.g., Gadidae and Pleuronectidae). In selachians, where no 
valvula is developed, in ganoids and in teleosts having a relatively 
small and simple valvula (e.g., Lophius), no such complicated 
arrangement of trochlear roots obtains. Thus, from the evidence 
at his disposal, Huet concludes that the action of the valvula 
cerebelli, in producing this split in the decussating trechlear 
roots, is purely a mechanical one due to its growth and the 
consequent folding of the part. 
