266 FROGS, TOADS, AND NEWTS [CH. VIII 



The closure of the nostrils appears to be brought about 

 by the firm closure of the jaws. The pressure of the lower 

 jaw bends up the intermaxillary region of the upper jaw, 

 forcing the alar cartilages against the inner margin of 

 the nostril, temporarily rendered tense by certain muscles 1 , 

 and thus shuts the nostrils externally. Their internal 

 apertures are covered by the two processes A, A (Fig. 44) 

 which arise from the anterior sides of the bases of the 

 anterior cornua of the hyoid, and which are pressed up 

 against them. 



The glottis is normally closed by its 

 own elasticity ; the muscles which effect 

 its opening originate from the bony 

 posterior cornua of the hyoid and are 

 inserted along the lips of the glottis at 

 right angles to the axis of the slit. 



At the end of each act of pulmonary 

 respiration the rapid oscillatory move- 



ments of the throat are resumed with The posterior cor- 

 nua are shaded. 

 the nostrils open. In this way the 



mixture of fresh and expired air left in the mouth and 

 throat is quickly exchanged for air entirely fresh. 



The several phases and actions in pulmonary respira- 

 tion may be summarised as follows : 



1. Aspiration. Nostrils open ; glottis closed ; floor of 

 mouth lowered by sterno- and omohyoids. 



2. Expiration. Nostrils closed; glottis open; lungs 

 compressed by action of sterno- and omohyoids, by 

 abdominal muscles, and by intrinsic elastic tissue. 



1 Bruner, Anat. Anzeig. xv. 



