MOTOR NUCLEI IN PHYLOGENY 551 
that among these forms abducens nuclear elements occupy a 
more rostral position than obtains in any other group of verte- 
brates yet investigated.!! 
Motor trigeminus nucleus and root (Nu. et rad. mot. N.V). The 
motor V nucleus among teleosts is, with few exceptions, divisible 
into two quite definite sub-nuclei. Of these, one is rostro-dorsal 
in position, while the other is more caudo-ventrally placed. 
Kappers has demonstrated that the chief factor responsible for 
the ventral position of trigeminal elements in teleosts, in con- 
trast to selachians and ganoids, is the neurobiotactic influence 
of the secondary ascending gustatory tract (64, 66). This author 
has also shown that the caudo-ventral cell group of this nucleus 
is even more intimately associated with the secondary gustatory 
tract than are the elements of the dorsal cell group, and is largest 
and most ventrally placed among those forms in which this tract 
is best developed (I. c.). 
A study of the mechanics of respiration among teleosts brings 
out the fact that the different muscles innervated by the motor 
fibers of the trigeminus are by no means equally concerned in 
the respiratory act. In view of the direct bearing which such 
considerations must have upon the question of the arrange- 
ment of the reflex nuclear pattern in these forms, it becomes 
desirable before further discussion to review briefly the recent 
results of investigations in this field. 
The respiratory current in most teleosts is produced chiefly 
by the action of the opercular apparatus in conjunction with the 
maxillary, mandibular and branchiostegal valves in a manner 
to which reference has already been made (Dahlgren, 15; 
Baglioni, 8). 
According to Deganello (18) the principal muscles concerned 
in these movements are the following. In inspiration: (a) m. 
sterno-hyoideus (spinal nerves 1 and 2); (b) m. dilator operculae 
4 It is worthy of note in view of the small size and the lack of compactness of 
the abducens nucleus in Pleuronectidae, despite the relatively great importance 
of the oculomotor apparatus in these forms, that according to Harman (33) the 
external rectus muscle is peculiarly subject to variation among flat-fish and in 
most of these forms showed evidence of considerable reduction. 
