MOTOR NUCLEI IN PHYLOGENY 495 
in a given species but slight, if any, individual variation in the 
development of this musculature obtains. 
In the absence of disturbing individual peripheral variations 
it is probable that, as Kappers has pointed out, reflex impulses 
passing by way of the well developed fibrae arcuatae dorsales 
and the fasciculus longitudinalis medialis from the acustico- 
lateral areas, exercise no small influence upon the rostral end 
of the somatic motor column and contribute largely to the 
dorsal situation of the cells of this nucleus. Further, the absence 
of ventrally situated motor elements in the spino-occipital 
nucleus in selachians would be in accordance with the compara- 
tively slight development in these forms of the ventral reflex 
pathways, le., tractus octavo-motorius cruciatus ventralis, 
tractus tecto-bulbaris ventralis, etc. (40 and 43). 
Motor vagal nucleus (Nu. mot. N. X.). The trapezius muscle 
in sharks is a well developed structure which receives its inner- 
vation by way of the so-called caudal ascending vagus root. 
This root corresponds to the ‘posterior fasciculus’ of the vagus 
which Owen described as ‘‘representing the nervus accessorius”’ 
(85). This statement has been verified in detail by Furbringer 
who demonstrated once more the connection between this por- 
tion of the vagus and the well developed trapezius muscle in 
Hexanchus, ete. (25). It follows from this that the caudal end 
of the motor vagus nucleus must represent the nucleus acces- 
sorius which here occupies its most primitive position on the 
same plane and in undisturbed continuity with the caudal 
viscero-motor column (fig. 17). 
Among sharks, the extent of the viscero-motor column caudad 
of the exit level of the first spino-occipital rootlets, appears to 
be more or less directly correlated with the degree of develop- 
ment of the trapezius musculature. Among rays, however, 
such direct correlation between these structures may not ob- 
tain. Thus, according to Kapper’s reconstruction (65, fig. 8, 66, 
fig. 12), the viscero-motor column in Raja clavata extends but a 
short distance caudad of the exit level of the first spino-occipital 
nerve. However, this appearance may be due in part to the 
loss of certain occipital segments which, as Furbringer has shown, 
are absent among the higher forms of elasmobranchs (rays). 
