. 
CRANIAL NERVES OF SILURUS AND MORMYRUS 27 
position. The exact connection with these nuclei cannot, how- 
ever, be found. 
The medial nucleus extends through only six sections (of 25 
micra each) and contains exquisite motor cells. The dorso- 
lateral nucleus is a part of a cell column which extends from some 
sections.in front of the root level until one section caudally from 
it. The most caudal part of this column is separated from the 
frontal part; the caudal part is probably the dorso-lateral II 
nucleus. Its cells have the character of motor cells and fibers 
from it can be traced in a central direction. The cells of the 
dorsal nucleus are larger than those of the ventral. Six of them 
may be found in one section on either side of the raphé (fig. 12). 
The trochlear root leaves the brain between what Franz has 
called the Mormyro-cerebellum and the Ichthyo-cerebellum, i.e., 
between the valvula cerebelli and the corpus cerebelli. The 
extramedullary course of the root is interesting in that it first 
runs dorsad over the Ichthyo-cerebellum and then, 16 sections 
in front of its exit, decussates with the contra-lateral root. This 
decussation lies free in the cranial cavity, that is, outside any 
part of the brain wall or ependymal membrane. 
After the decussation the root shifts ventrally and caudally 
along the cerebellum and leaves the periphery of the brain as 
soon as this is possible, that is, between the Mormyro-cerebellum 
and the oblongata (fig. 14). 
This course, though strange, is not unique in principle. A 
similar relation has been described by Huet*? in Gadus, where it 
was pointed out that this strange course is due to the valvula. 
Here also the great hypertrophy of the Mormyro-cerebellum 
must be considered as its cause. The very frontal place of en- 
trance of this root (still more frontal than in Silurus) may also be 
explained perhaps by the frontal growth of the cerebellum. 
The central course of the IV nerve cannot be traced on account 
of its small size and the intricate net-work of fibers in this region. 
Nevertheless in comparison with the III root and nucleus the 
30 Notes on the trochlear and oculomotor nuclei and the trochlear root in the 
lower vertebrates. Proc. kon. Akad. van Wetensch. Amsterdam, March 23. 
1911, p. 897. 
