MOTOR NUCLEI IN PHYLOGENY 249 
this cell group among reptiles in chelonians and in Alligator 
(figs. 15 and 16, pp. 259, 260). 
Nerves III and IV 
The oculomotor nerve in Cacatua arises in a highly developed 
cell complex which, as in all other birds examined, is in close 
contiguity with the trochlear nucleus. Among birds the cell 
groups composing these associated nuclei are demarcated more 
clearly and precisely than is usual in any of the mammalian or- 
ders. This is largely due to the relatively slight development 
of small association elements in this region among birds. 
The trochlear nucleus occupies a dorsal position, lying within 
the periependymal gray upon the dorsal surface of the fasciculus 
longitudinalis medialis (fig. 8). In its rostral portion the 
trochlear nucleus is accommodated within a distinct groove or 
excavation upon the dorsal surface of the latter fiber tract and 
a caudal extension of the pars dorsalis of the median oculomotor 
cell group comes to lie between the nucleus and the median 
sulcus of the ventricular floor (fig. 11, h and 7). 
The trochlear root fibers descend within the nucleus and 
towards the caudal end of the latter become collected into bundles 
which then emerge from its dorso-lateral margin. The radicu- 
lar bundles thus formed pass dorso-lateral within the ventricu- 
lar gray (fig. 8) to reach the superior medullary velum, in 
which the trochlear decussation occurs and from the lateral side 
of which the crossed trochlear roots emerge. 
Within the oculomotor nuclear complex in Cacatua three 
well-marked subsidiary nuclei are to be distinguished which, 
in conformity with Kappers’ descriptions (l.c.), are here termed 
the median, the dorso-lateral, and the accessory cell group, re- 
spectively. The general arrangement of these cell groups is 
illustrated in figure 9, where it will be seen that the median cell 
group is largest in cross-sectional area and is evidently divisible 
into dorsal and ventral portions. The largest elements within 
the oculomotor nuclear complex are found in the dorso-lateral — 
cell group, while the smallest comprise the collection termed the 
