246 DAVIDSON BLACK 
In Cacatua, Ciconia, Colymbus, and Columba the rootlets 
of the abducens nerve make their exit from the brain stem rostrad 
of the exit level of the motor VII root in a manner similar to 
that obtaining among reptiles in Boa and Varanus. On the 
other hand, in Spheniscus, Chrysomitris, and Casuaris, some of 
the abducens rootlets emerge caudal to the lower border of the 
emergent motor VII root and resemble in this respect the mode 
of exit of the abducens rootlets in Alligator (figs. 15 and 16, pp. 
259, 260). 
Nerve V 
In Cacatua the motor V nerve takes its origin within the 
brain stem from three quite distinct cell groups which are here 
distinguished as the dorsal motor V nucleus, the combined 
V-VII motor nucleus and the ventral or chief motor V nucleus. 
The dorsal motor V nucleus forms a small circumscribed 
group of large multipolar cells situated beneath the gray matter 
of the ventricular floor and lateral to the fasciculus longitudi- 
nalis medialis. It is rostral of the abducens nucleus and is more 
dorso-laterally placed than the latter (cf. figs. 6 and 7). The 
radicular fibers arising from this cell group pass laterad and then 
ventro-laterad to emerge at the periphery, ventral to the enter- 
ing sensory trigeminal root. 
From the rostral end of the cell column whose relations as the 
dorsal VII motor nucleus have already been described, a num- 
ber of radicular fibers emerge which take a characteristic indi- 
rect course dorso-medial through the tegmentum to join those 
arising from the dorsal motor V nucleus and to reach the 
periphery in common with the latter. The nucleus from which 
these fibers arise constitutes the combined V—VII cell group of 
the trigeminal motor nuclear complex, or the rostral continuation 
of the dorsal motor VII cell group of the facial motor complex. 
The major part of the motor root of the trigeminal nerve 
in Cacatua arises in a large nucleus which is situated in the ven- 
tro-lateral area of the tegmentum, ventral to the emergent 
fibers arising in the dorsal motor V nucleus. From the ventral 
nucleus the emergent V root fibers pass directly ventro-lateral 
