336 Henry L. Bruner 



the contraction of the masticatory muscles, and thus naturally assists 

 in maintaining a close contact of the two jaws. 



The function of the m. labialis superior will be readily under- 

 stood. When the mouth of Rana is closed by the contraction of 

 the masticatory muscles the rod-like dentary bones fit into the 

 sulcus marginalis; the tuberculum subrostrale, into the fossa sub- 

 rostralis media; the pulvinar subrostrale, into the fossa subrostralis 

 lateralis. With the occurrence of inspiration the lower jaw is ad- 

 ducted and protracted, the partes mentales of the dentalia are held 

 in place by the incurved dentary processes of the intermaxillaria, 

 while throughout its entire length the lower jaw is closely 

 embraced and held in place by the labialis superior. The close 

 contact between the upper lip and lower jaw, which is rendered 

 difficult by the presence of the teeth, is secured partly by the ex- 

 tension of the lip below the teeth, partly through the contraction 

 of the labialis superior, as a result of which the teeth become wholly 

 imbedded in the inner layer of the lip, and the contact of the latter 

 with the lower jaw is made practically continuous. The functional 

 importance of the labialis superior in inspiration may be seen if 

 the rostral portion of the upper lip be cut or removed in a chloro- 

 formed specimen. The animal recovers fully from the operation and 

 ordinary quiet inspiration occurs as usual. Wheu, however, more difficult 

 respiration is attempted, such as strong inflation of the lungs, the 

 mouth can no longer be securely closed and the air escapes between 

 the jaws. By comparison with other specimens which have the 

 upper lip intact, it is found that the power of inflation of the operated 

 specimen is greatly below normal. 



Concerning the nature of the contraction of the labialis superior, 

 observations made on Rana with closed mouth might seem to point 

 to a rhythmical activity, corresponding to the inspiratory movements. 

 Such an inference is at least suggested by the evident motion of the 

 upper lip, which is more pronounced at the rostrum and becomes 

 weaker toward the angles of the mouth. A closer study, however, 

 shows this motion to be a result of the movement of the skeletal 

 parts (intermaxillaria and lower jaw). This explanation is fully 

 justified by the fact that, if the mouth of Rana be held slightly open 

 so that contact of the jaws is impossible, the upper lip remains 

 motionless, although the animal still attempts to breathe and the 

 bucco-pharyngeal movements occur as usual. This observation leaves 

 room for but one conclusion, namely, that the musculus labialis 



