506 DAVIDSON BLACK 
Motor trigeminal nucleus and roots (Nu. et rad. mot. V). In 
Polyodon, the motor V nucleus is dorsally situated in the lateral 
area of the ventricular gray, and it les wholly caudal to the 
rostral border of its emerging root (fig. 25 A). In the rostral 
third of its length the nucleus becomes thinned somewhat, 
so that a partial division into two is evident. The fibers aris- 
ing in this nucleus become collected to form a bundle which 
ascends just laterad of the nuclear mass (fig. 21 A). Toward the 
ica 
_- Valyula == 
Fig. 21 Polyodon spathula. Transverse section of brain stem at the exit 
level of the motor trigeminus root. 
Fig. 214 Portion of transverse section at somewhat more caudal level than 
the preceding to illustrate the relations of the ascending motor trigeminal root. 
Fig. 22 Transverse section of brain stem at the exit level of the trochlear 
nerve. Abbreviations: A.cb., auricular lobe of cerebellum; Cb., cerebellum; 
N.IV., trochlear nerves decussating at their emergence; Nu.V.m., motor trigem- 
inal nucleus; R.V.m., emergent motor trigeminal fibers collected into two root- 
lets; R.V.m.asc., ascending fibers of motor trigeminal root. Other abbreviations 
as before. 
