MOTOR NUCLEI IN PHYLOGENY 247 
and emerge on the periphery ventral to the entering sensory 
root of this nerve (fig. 7). At its dorso-medial angle the chief 
or ventral motor V nucleus comes into contact with the rostro- 
ventral border of the combined V—VII motor cell column (fig. 
LGD ps 260): 
Fig. 7 Cacatua roseicapilla. Transverse section through the brain stem at 
the exit level of the motor trigeminal root and first abducens rootlet. Same 
magnification as figure 1. An absence of sharp radicular differentiation rendered 
obscure the relations of the mesencephalic V root at its exit level in Cacatua, a 
condition which van Valkenburg also found to obtain in both Ciconia and Chryso- 
mitris (52 and 54). JL.l., secondary cochlear tract; Lob. op., optic lobe; Nw. U.L., 
nucleus of ascending cochlear tract; Nu. V. m.d., dorsal motor V nucleus; Nw. 
V. m.v., ventral or chief motor trigeminal nucleus; Nu. V.s., sensory V nucleus 
with rostral end of the descending trigeminal root; Nu. V.s.p., chief sensory 
trigeminal nucleus (cf. van Valkenburg, 53, figs. 7 and 8, Taf. XIX-XX, Ciconia) ; 
R.V.desc., descending V sensory fibers; R.V.m., motor trigeminal root., R.V.m.d., 
dorsal motor trigeminal rootlets; R.V.s., entering sensory trigeminal fibers; 
R,VI., first emergent abducens rootlet; Tr. b.f., bulbo-tectal tract; Vent., ven- 
tricular cavity. 
The origin of the motor V root in Ciconia differs from that 
in Cacatua chiefly in the absence of fibers corresponding to 
those arising in the combined V—VII motor nucleus of the latter 
form. Motor trigeminal fibers were identified arising in a small 
