MOTOR NUCLEI IN PHYLOGENY 499 
It is an observation of very long standing (10) that among 
sharks, during the dilation of the mouth cavity and pharynx 
in inspiration, water may enter either through the mouth, the 
spiracle (Gf present), or even through the gill slits themselves. 
In sharks, as in cyclostomes, the arrangement of the gills per- 
mits of the efficient action of these organs irrespective of the 
direction of the respiratory flow.’ 
It becomes evident, therefore, that in sharks the alternate 
opening and closure of the mouth in synchronism respectively 
with the alternate dilation and constriction of the pharynx, is 
not a sine qua non tor efficient breathing. It follows from this 
that efferent impulses from the motor V nucleus are not neces- 
sarily called forth during each reflex respiratory cycle in the 
same way as are those from the various constituents of the 
caudal viscero-motor column. ‘The rostral position of the motor 
V nucleus in sharks may thus be said to express a function of 
the negative influence of the communis center upon the reflex 
action of the jaw musculature in these forms. 
In this connection the work of Harman (33) on the innerva- 
tion and development of the musculature of the nictitating 
membrane and eye-lids in sharks should also be noted. The 
musculature in question is innervated by the trigeminus and the 
perfection of its development appears to vary inversely with the 
size of the spiracle. Thus, according to this author, in rays where 
the spiracle is of large size, the musculature is absent, while in 
Galeus where the spiracle is absent or minute, the musculature 
of the eye-lid is well developed. Scyllium stands intermediate in 
? Baghoni (7 and 8) evidently does not consider that the inflow of water 
through the external gill openings in selachians is a part of the normal respiratory 
cycle in these animals. However, Couvreur (14) working on Torpedo marmo- 
rata has confirmed this observation and the work of Hyde (48) demonstrates 
beyond a doubt that respiration of a sufficiently efficient nature to prolong life 
indefinitely can be maintained in the skate by the action of the last four gill 
arches alone. This observer also expressly states that the ‘“‘motor nerves con- 
cerned in respiration are the fifth, seventh, ninth and tenth nerves, of which 
the fifth is least important and the tenth most important”’ (I. ¢., p. 240). The 
relative independence of the trigeminal musculature during the respiratory 
cycle has also been noted in Rhina by Darbishire (17). 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 27, NO. 4 
