334 Henry L. Brunei- 



to supplement the action of those muscles. To this end the jaws 

 and other parts have been profoundly modified, the upper lip with 

 its muscle has been developed. The significance of these different 

 modifications in connection with the closing and opening of the 

 mouth must now be considered. 



The most important factor in maintaining a close contact of 

 the jaws during respiration is, of course, the masticatory muscles, 

 which, as shown by Gaupp (20), preserve a constant tension sufficient 

 to hold the jaws together. With the closing of the external nares, 

 however, occurs a special contraction of the masticatory muscles, 

 which thus hold the lower jaw firmly in the sulcus marginalis during 

 the inflation of the lungs. But the masticatory muscles provide for 

 more than the mere adduction of the lower jaw. These muscles 

 include in Rana a masseter major, a masseter minor, a temporalis 

 and a pterygoideus. Of these, the two masseters are simple ad- 

 ductors, the temporalis acts as an adductor and retracts at the same 

 time the lower jaw; the pterygoideus acts as an adductor and pro- 

 tracts the lower jaw. We have then in this arrangement of 

 muscles provision not only for a mere closing of the mouth 

 but also for protraction and retraction of the lower jaw. 

 Such movements, moreover, may be actually observed dur- 

 ing respiration, protraction occurring at the moment of 

 inspiration, retraction immediately after inspiration. Along 

 with protraction occurs also a slight rotation of the lower jaw, whose 

 ventral margin is turned outward by contraction of the submaxillaris 

 and submentalis muscles. 



On observing protraction for the first time during ordinary 

 respiration, I was inclined to consider that movement as a reaction 

 resulting from the pressure of the muscles of the floor of the mouth 

 upon the air confined in the mouth cavity. Such an explanation, 

 however, was shown to be untenable by the fact, that protraction 

 occurs when the mouth is held open so that the air escapes freely. 

 Indeed, if an obstruction, such as a stout cord, be placed between 

 the jaws, vigorous efforts to inspire will be made and protraction 

 will be more pronounced than under normal conditions. At the same 

 time it will become apparent that the protraction of the lower jaw 

 is due to the active working of the masticatory muscles. This con- 

 clusion is confirmed by the study of more energetic respiratory 

 movements of normal type, as for example, in strong inflation of 

 the lungs, or when respiration is resumed after a slight interruption. 



