492 DAVIDSON BLACK 
The trochlear nucleus lies in a crescentic depression upon the 
dorsal surface of the fasciculis longitudinalis medialis (fig. 15). 
Its large motor cells are more compactly arranged than those of 
the oculomotor nucleus with which it is in contact rostrally. 
The emergent root passes dorsally in the Sylvian gray and, 
after decussation with its fellow of the opposite side above the 
ventricle, emerges dorso-laterally between the cerebellum and 
the midbrain. 
The elements composing the oculomotor nucleus are arranged 
irregularly and are not divisible into definite subsidiary groups 
(fig. 16). The emergent radicles pierce the posterior longitu- 
dinal bundle on their course to the ventral periphery of the 
midbrain. No definite arrangement of crossed oculomotor 
fibers could be determined. No small reticular elements such 
as are characteristically present in higher vertebrates were 
found in association with either oculomotor or trochlear nuclei. 
The sagittal relations of these nuclei and their roots are charted 
in figure 17 C. The oculomotor nucleus occupies a dorsal posi- 
tion on the level of its root exit and extends rostrad of this level 
some distance on the same horizontal plane. The trochlear 
nucleus lies directly behind it but on a slightly more dorsal 
plane. In most selachians the trochlear root emerges from the 
brain some distance behind the level of the caudal end of its 
nucleus (fig. 17 A). In Selache maxima, however, the root 
emerges at the level of the caudal third of its nucleus (fig. 17 C) 
and in Hexanchus at the level of the caudal end of its nucleus 
(fig. 117-3). 
Numerous large cells constituting the nucleus magnocellu- 
laris tecti may be observed in the ventricular gray of the mid- 
brain roof (figs. 15 and 16). Among the cells of this nucleus, the 
radix mesencephalica trigemini takes its origin. As this root 
in all probability is a highly specialized sensory component of 
the trigeminal nerve, it has not been charted (cf. Johnston, 54; 
Van Valkenburg, 93 and 94). 
Fig. 17 Reconstruction charts of motor roots and nuclei. (A), Heptanchus 
(after Kappers). (B), Hexanchus (mod. after Kappers, 66). (C), Selache 
maxima. Signs and abbreviations as before (page 476). 
