MOTOR NUCLEI IN PHYLOGENY 533 
closely associated in Solea, so that the nucleus of the latter nerve 
is separated by but a slight interval from the most dorsal part 
of the oculomotor complex. The oculomotor nucleus is situ- 
ated in the mesencephalic ventricular gray, and at its rostral 
end is made up of a quite compact cell group, lying dorsal to the 
fasciculus longitudinalis medialis. As this nucleus is_ traced 
caudad, its dorsal part becomes enlarged, and in addition along 
the raphé, an extensive ventro-median cell group appears, 
whose elements, though separated into bilaterally symmetrical 
groups, give rise apparently to fibers passing to both right and 
left oculomotor nerves. The major portion of this nuclear com- 
plex lies frontal to the rostral border of the emergent III nerve 
but a small part of the ventro-median division of the nucleus 
extends a short distance caudad of this level. The emergent 
radicles converge in the raphé to form several stout bundles, 
which in turn pierce the commissura ansulata in a ventro-lateral 
direction to reach the surface of the midbrain (fig. 37). 
The trochlear nucleus in Solea, is almost a caudal continuation 
of the dorsal part of the oculomotor nucleus, there being but 
two cell-free sections separating the two. The IV nucleus is a 
compact cell group lying upon the fasciculus longitudinalis 
medialis, and extending in this situation approximately over 
the length of the superficial attachment of the oculomotor nerve 
(fig. 40 A). Thus, in figure 37, it is possible to show the emerg- 
ing radicles of N. III, together with the ventral part of nucleus 
N. III and nucleus N. IV, all in one transverse section. 
The course of the trochlear roots caudad to their points of 
emergence is most complicated in Solea. The rootlets become 
separated shortly after their origin into two distinct bundles, 
one of which after bending around the tractus cerebello-mesen- 
cephalicus, decussates with its fellow of the opposite side and 
emerges through the valvula. The second rootlet passes rostrad 
a short distance in the substance of the valvula, and, after de- 
cussating with its fellow at this level, descends between the 
valvula and the tectum, to join the rootlet first described at its 
point of emergence from the brain. 
