508 DAVIDSON BLACK 
It is possible to distinguish two cell groups within the nucleus: 
a dorsal group which is present throughout the nucleus, and a 
group more ventrally situated which is present only in its caudal 
portion. In figure 24, some cells of the latter group may be 
seen lying ventrally, between the fasciculi longitudinales mediales 
‘and below the small dorsal cell groups. The emergent rootlets 
pass ventrad through the fine strands of the commissura 
ansulata. 
Tectum ° 
at ‘Ss SS Tas 
Tectum 
Pe eS \ 
\i 
) 
PZ] ~Telob.b. 
fy 
: N.IL. 
\ ‘Nulll.vent. 
Com.ans. 
23 24 
Fig. 23 Polyodon spathula. Transverse section of brain stem to illustrate 
the relations of the trochlear nerve to the neighboring structures. 
Fig. 24 Outlines of transverse section of brain stem at exit level of oculo- 
motor root. Abbreviations: Coll., colliculus; Com.ans., commissura ansulata; 
N.III., emergent bundles of oculomotor nerve; N.JV., trochlear nerve; 
Nu.II1.dors., dorsal moiety of oculomotor nucleus; Nu.J1/.vent., ventral moiety 
of the oculomotor nucleus; Tr.lob.b., tractus lobo-bulbaris; 7'’r.lob.cb., tractus 
lobo-cerebellaris. Other abbreviations as before. 
In both Amia and Acipenser, dorsal and ventral cell groups 
may be distinguished in the oculomotor nucleus, though in the 
young Lepidosteus specimens described by Theunissen this con- 
dition was not apparent. Among these forms the oculomotor 
complex occupies the most rostral position in Amia, where the 
nucleus lies almost wholly in front of the rostral border of its 
emergent root. In Acipenser the nucleus is somewhat more 
caudally placed while in both Polyodon and Lepidosteus the 
