MOTOR NUCLEI IN PHYLOGENY 409 



orbital cartilage; in Triton and Cryptobranchus, with the 

 exception of the absence of the antorbital muscles, the motor 

 distribution of the V is essentially similar to that in Siren. 



It is evident on comparing the motor nuclear pattern in 

 amphibians and sharks that the position of the motor V nucleus 

 in relation to its emergent root is very similar in the two groups 

 (cf. figs. 9, 10 and IIB). It has already been pointed out that 

 ''the rostral position of the motor V nucleus in sharks may be 

 said to express a function of the negative influence of the com- 

 munis center upon the reflex action of the jaw musculature in 

 these forms" (10, p. 499). In amphibians likewise the impor- 

 tance of the trigeminal musculature in respiration is relatively 

 slight, and its function during this act is in most cases chiefly 

 that of tonic contraction (Gaupp, 24). It would thus appear 

 that in both selachians and amphibians the primitive dorsal 

 position of the motor V nucleus approximately on the level of 

 its root exit, is to a large extent an indication of the relative 

 independence of the trigeminal musculature in the respiratory 

 activities of these forms (vide infra). 



In this connection it is of interest to recall that the motor 

 V nucleus in Siren, unlike any other amphibian examined, is 

 situated almost wholly caudad of the exit level of its root. 

 Norris and Bruner have found antorbital muscles developed 

 only in Siren and Amphiuma. These muscles direct the activity 

 of the choanal openings during respiration (Bruner, 13, 14) 

 and their nerve supply is derived from the pterygoid branch 

 of the ramus mandibularis V (Norris, 45, 46). Though these 

 relations may be quite fortuitous, it is not impossible that the 

 caudal situation of the motor V nucleus with reference to its 

 root exit may be correlated with the respiratory function of 

 the antorbital elements. 



Facial 7nusculature. The peculiar differences in the relations 

 of the motor VII nucleus in anurans and urodeles which have 

 been described above, assume a new significance when the func- 

 tional development of the facial musculature is considered in 

 the two orders. 



