344 Henry L. Bruner 



just at the moment of inspiration, when such an event is especially 

 to be avoided. This effect of the contraction of the submentalis is 

 counteracted by protraction of the lower jaw, whose caudally diver- 

 gent arms are thus forced forward and kept in close contact with 

 the dentary portions of the maxillaria and intermaxillaria and with 

 the upper lip. 



In view of the importance of maintaining a close union of 

 mouth parts during inspiration, the function which I have above 

 attributed to protraction of the lower jaw would appear to be suf- 

 ficient to warrant the occurrence of such a movement in respiration. 

 But protraction of the lower jaw has also another function, which 

 may be readily discovered, if, while the mouth is closed, the lower 

 jaw of a recently killed frog is pushed gently forward on each side 

 by means of forceps. That the closing of the external nares, which 

 follows from this experiment, may result from protraction of the 

 lower jaw in the living animal, is seen if the nerves supplying the 

 submentalis muscle are severed. After this operation, if the closing 

 of the external nares were wholly dependent upon the action of the 

 submentalis, we should expect that such closing would cease altogether. 

 But this is not the case. Elevation of the tuberculum praelinguale 

 is now effected through protraction of the lower jaw, the movement 

 becoming much more pronounced than usual, and thus the external 

 nares may still be closed. Repeated observations on Rana 

 pipiens point to the conclusion that, under conditions simi- 

 lar to the above, the inspiratory movements are uniformly 

 attended by protraction of the lower jaw, and that without 

 such protraction the closing of the external nares does not 

 occur. 



The manner in which this result is effected may be readily- 

 understood if we take into account, a) the provisions for maintaining 

 a constant contact of the two jaws during inspiration, and b) the 

 mode of union of the two halves of the lower jaw at the rostrum. 

 As they approach the median line, both the partes mentales of the 

 dentalia and the two segments of Meckel's cartilage bend upward 

 and the junction of the latter in the tuberculum praelinguale occurs 

 at an acute angle. Now, if protraction occur while the mouth is 

 closed, the tuberculum praelinguale, held in place by the incurved 

 dentary processes of the intermaxillaria, by the upper lip, and per- 

 haps also by the submentalis, can move only in the direction of 

 the palatine processes of the intermaxillaria. For such a movement, 



