498 DAVIDSON BLACK 
Abducens nucleus and root (Nu. et rad. N. VI). The charac- 
teristically dorsal position of this nucleus among sharks appears 
to be also a primitive one, and in this situation the nucleus is 
reflexly dominated by impulses from the acustico-lateral area 
which reach the nucleus through the posterior longitudinal 
bundle and also directly by dorsal arcuate fibers (64 and 66). 
The position of the nucleus between the exit levels of the 
motor VII and IX roots seems also to be a primitive character, 
so that the phylogenetic sequence of emergent motor roots in 
this region evidently should read VII, VI, IX. Indeed in the 
primitive form Hexanchus the most caudal fascicle of the 
abducens root emerges on the level of the motor glossopharyngeus 
exit (vide supra, figs. 13 and 17 B). 
Trigeminal nucleus and root (Nu. et rad. N.V.). In all selachi- 
ans the motor trigeminal nucleus lies in a somewhat dorsal posi- 
tion and extends some distance both caudal and rostral of its 
root exit. In the latter respect and in its isolation from the 
motor facial elements, the trigeminal nucleus of sharks differs 
from that in cyclostomes. Indeed, with the exception of birds, 
a greater proportion of the motor V nucleus is placed rostrad 
of the level of its root exit in sharks than in any other group of 
vertebrates. 
In sharks the rostral position of the motor V nucleus, quite 
removed from the caudal viscero-motor column, is of signifi- 
cance in view of the peculiar arrangement of the respiratory 
mechanism in these animals. It has been pointed out already 
that in cyclostomes both inspiration and expiration takes place 
through the gill openings when the animal is attached by its 
mouth to any object. As a corollary to this the vascular gill 
folds in these forms are fixed in such a manner that they can 
act as efficient respiratory organs without regard to the direc- 
tion of the flow of water through the gill openings. The gills in 
sharks consist of closely approximated transverse lamellae which 
also are firmly attached to the sides of the interbranchial septa. 
In the latter respect the gills of sharks and cyclostomes resemble 
one another and differ from those of all other fish. 
