MOTOR NUCLEI IN PHYLOGENY 411 



facial musculature is represented as follows: mm. depressor 

 mandibulae, interhyoideus, cerato-hyoideus externus and sphinc- 

 ter colli (quadi'ato-pectoralis). 



Not only is the facial musculature in urodeles more exten- 

 sively represented than in anurans, but with the exception of 

 the m. depressor mandibulae, all these muscles in urodeles are 

 intimately asssociated with the IX-X musculature and play a 

 relatively important role in the inspiratory movements of the 

 hyo-branchial region. Before the possible significance of this 

 association can become fully apparent, it is necessary to con- 

 sider the arrangement and innervation of the aspirator and 

 inspirator muscles in anurans and urodeles. 



Aspirator musculature. In both anurans and urodeles the 

 depression of the bucco-pharyngeal floor in aspiration is chiefly 

 due to the action of the sterno-hyoid and omo-hyoid muscles.^ 

 These muscles by their aspiratory action also aid in expiration. 

 Owing to the aspirator action of these powerful somatic muscles, 

 the branchial musculature aside from the intrinsic laryngeal 

 complex, plays chiefly an inspiratory role. 



Inspirator musculature. In anurans the chief muscles con- 

 cerned in this action of inspiration are the mm. petro-hyoidei 

 anterior and posterior, innervated respectively by the IX and 

 X nerves. Inspiration is also strongly assisted by the contrac- 

 tion of the mm. genio-hyoidei, hyo-glossi and genio-glossi (N. 

 XII) and in addition is aided somwhat by the tonic action of the 

 mm. submentalis, submaxillaris and subhyoideus (N. V and VII) 

 (v. Gaupp, I.e.). Thus in anurans the chief center from which 

 the respiratory movements of the visceral musculature are di- 

 rected, is the motor IX-X nucleus, which is placed in close re- 

 lation both to the somatic motor center governing the chief 

 aspirators (mm. sterno-hyoidei and omo-hyoidei) and to that 

 of the most important accessory inspirators (mm. genio-hyoidei, 

 hyo-glossi and genio-glossi), but which is relatively far removed 

 from the center concerned in innervating the essentially non- 

 respiratory jaw musculature (i.e., motor V and VII nuclei). 



'■ It is of interest to recall that in ganoids and teleosts the m. sterno-liyoideus 

 is also one of the chief inspirator (i.e., aspirator) effectors (v. 10). 



