496 DAVIDSON BLACK 
Vetter (95) considers that the trapezius is derived from a 
primitive superficial visceral constrictor system of muscles 
which were innervated by the vagus. In sharks, however, the 
trapezius has lost its visceral constrictor character and, though 
retaining its vagus innervation, has become a muscle of impor- 
tance in the somatic movements of the head and pectoral girdle. 
The relations of the nucleus accessorius accord well with this 
hypothesis for, though it retains the essential visceral character- 
istic of continuity with the caudal viscero-motor column, yet it 
has come to lie in the upper cervical region where it is closely 
related to the general somatic sensory centers receiving afferent 
fibers from the skin area overlying the trapezius. Thus, the 
peripheral change in muscular function has been accompanied 
part passu by a central change in the reflex connections of its 
motor nucleus. In other words, the motor nucleus tends to 
migrate in the direction of the most important centers acting 
upon it reflexly. 
Motor facial and glossopharyngeal nuclei and roots (Nu. et rad. 
mot N. VII-IX). The increased importance of the visceral 
sensibility in the life habits of the selachians, as compared with 
cyclostomes,® is evidenced by the formation in the former ani- 
mals of a communis nucleus in which the visceral sensory com- 
ponents of the facial nerve terminate in common with those of 
the glossopharyngeal and vagus nerves and on the level of the 
latter. The association of the visceral sensory nuclei of these 
nerves to form a common center situated a considerable distance 
caudad of the level of entrance of the sensory VII root, affords 
an example of nuclear association under the influence of similar, 
simultaneous stimulation (vide supra). 
It follows from this, that in selachians, the chief center act- 
ing reflexly upon the motor facial nucleus (viz., the nucleus of 
its own sensory root) is situated at and caudal to the exit level 
of the glossopharyngeus. As a result the motor VII nucleus, 
like that of its sensory root, has also become displaced from its 
* Taste buds among sharks are more numerous than in cyclostomes, though it 
is probable that they are more or less restricted to the pharynx, gills and mouth 
in the former animals (vide Kappers, 68). 
