MOTOR NUCLEI IN PHYLOGENY 269 
the close association of certain trigeminal and facial elements 
within the limits of a common V—VII nucleus is highly significant 
as an indication of the probability that these motor V cells 
represent elements governing the action of the m. pterygoideus 
anterior. 
3. Eye-muscle nerves 
Nerve VI. But little remains to be said concerning the abdu- 
cens nerve, which in most respects differs but little from that of 
reptiles. An interesting series of gradations in the exit level 
of its motor rootlets is shown in figures 15 and 16. In Cacatua, 
Ciconia, and Colymbus’ these emerge rostrad of the exit level 
of the motor VII root; in Spheniscus their emergence is both 
rostrad and caudad of this level, but in Casuaris the primitive 
condition is for the most part retained and most of the abducens 
rootlets in this form emerge caudad of the exit level of the motor 
root. The dorsally placed abducens nucleus is located for the 
most part on, or slightly rostrad of, the motor VII root exit, and 
is somewhat larger than the reptilian abducens cell group, a 
fact which, as Kappers pointed out, is to be correlated with the 
greater complexity of the musculature which it supplies in birds 
(82, 34). 
Nerves III andIV. Inall birds examined the trochlear nucleus 
is in close apposition with and frequently overlapped by the 
caudal end of the oculomotor cell group. The apposition of 
these two nuclei is evidently due to the rostral migration of the 
former, whose cells have been shown by Bok (11) to have their 
ontogenetic origin a considerable distance caudad of the oculo- 
motor cell group. Though the position of both oculomotor and 
trochlear nuclei in relation to the exit levels of the oculomotor and 
motor V roots is subject to considerable variation, the exit level 
of the trochlear root in relation to its nucleus is relatively con- 
stant. In Casuaris, Cacatua, and Ciconia it emerges at the 
caudal end of the trochlear nucleus in Colymbus at the midlevel 
of the nucleus, and in Spheniscus at its rostral end (cf. Kappers, 
34). Thus in no case among birds does the trochlear decussa- 
tion and emergence take place below the level of its own nucleus 
as is commonly observed in lower forms (ef. fig. 14). 
