MOTOR NUCLEI IN PHYLOGENY 497 
primitive position on the level of its root exit, and its migra- 
tion has been in the direction of the greatest number of im- 
pulses acting upon it reflexly in accordance with the first concept 
of neurobiotaxis. The long intramedullary, ascending course 
over which the emergent motor facial root passes, marks the 
path along which the nucleus of this nerve travelled during the 
phylogenetic development of this motor nuclear pattern (v. 
Kappers, l. c.). 
The factors operating to produce the slight displacement of 
the motor LX nucleus caudal of the level of its root exit, with 
the resulting formation of its ascending emergent motor root, 
are essentially similar to those discussed in the preceding para- 
graph. For further details of the phylogenetic displacements 
of the motor VIJ-IX nuclei, reference should be had to Kappers’ 
original papers on this subject (1. ¢.). 
The respiratory mechanism in sharks in general may be 
described as consisting of two opposing muscular complexes 
(constrictor group and dilator group) which operate in alternate 
rhythmic opposition and which are segmentally innervated 
from before backwards by the motor components of ‘branchial 
nerves VII, IX, X. The respiratory reflex is initiated chiefly 
by visceral afferent impulses arising in the branchial mucosa, 
which reach the communis center through the visceral sensory 
components of branchial nerves VII-IX—-X (6). Thus, the 
formation of the caudal viscero-motor column by the intimate 
association of the motor VII-IX—X nuclei and the situation of 
this column in close relationship with the communis nucleus, 
furnishes a striking example of the important réle played by the 
peripheral respiratory mechanism in the production of specific 
nuclear (reflex) pattern.® 
6 That the primitive segmental nature of the respiratory centers has not 
become lost by this adjustment is evident from the observations of Hyde (48). 
This author carried out her experiments on the skate, though unfortunately 
she has omitted to identify the species of the animals on which she worked. 
She has demonstrated, however, that the respiratory centers are segmentally 
arranged and bilateral in nature and has indicated physiologically the charac- 
teristic anatomical arrangement of the ascending motor and descending sensory 
facial roots (1. c., p. 247, fig. 3). 
