500 DAVIDSON BLACK 
this respect, having a medium sized or small spiracie and a well 
developed nictitating membrane but no eyelid musculature. 
On comparing the reconstruction charts of Raja (66, fig. 12), 
Seyllium (66, fig. 11) and Galeus (64, map C) it appears that in 
Galeus the motor V nucleus extends furthest rostral from the 
frontal margin of the emergent motor VII root, Scyllium comes 
next in order and in Raja the motor V nucleus reaches nearest 
to the frontal margin VII root. (For details consult Kappers’ 
list of topographic relations, 64, pages 119-125). 
There would thus appear to be some relationship existing 
between the position of the motor V nucleus in sharks and the 
degree of development of prespiracular trigeminal eye-lid mus- 
culature, though at present no accurate comparison can be 
made. 
Though the bulk of the motor V nucleus lies in a dorsal posi- 
tion, there is evidence of a tendency toward ventro-lateral dis- 
placement throughout the whole length of the nucleus, for a 
greater number of the large dendrites of its cells extend ventro- 
laterally and the diameter of the nucleus itself is increased in 
this direction (fig. 14). 
The elongation of the dendrites and the increased ventro- 
lateral diameter of the nucleus, bring the motor elements into 
close relationship with the reticular formation surrounding 
both the upper end of the descending sensory trigeminal root 
and the relatively slightly developed secondary ascending 
gustatory tract. 
The influence of the trigeminal cutaneous impulses upon the 
reflex action of the jaw musculature in elasmobranchs must be 
great in view of the ventral position of the mouth. In manipu- 
lating its food, before this can be swallowed, the animal must 
rely almost entirely upon its sense of touch. The slight displace- 
ment of some of the motor V elements and the extension of pro- 
cesses in the direction of the reticular gray about the rostral 
end of the descending sensory V nucleus thus illustrate further 
the first concept of neurobiotaxis (vide supra). 
Oculomotor and trochlearis nuclei and roots (Nu. et rad. Nn. IIT 
and IV). Compared with petromyzonts, the trochlear nucleus 
